Turning Points in Solid-State, Materials and Surface Science: A Book in Celebration of the Life and Work of Sir John Meurig Thomas

Turning Points in Solid-State, Materials and Surface Science A Book in Celebration of the Life and Work of Sir John Meurig Thomas

Photograph of John Meurig Thomas taken at the University of Wales, Swansea, by John D Roberts, California Institute of Technology, in 1992

Sir John Meurig Thomas (right) with Sir Michael Atiyah, O.M. (left), 30th April 2007

Turning Points in Solid-State, Materials and Surface Science A Book in Celebration of the Life and Work of Sir John Meurig Thomas

Edited by Kenneth D.M. Harris School of Chemistry, Cardiff University, Cardiff, UK

Peter P. Edwards Inorganic Chemistry Laboratory, University of Oxford, Oxford, UK

ISBN: 978-0-85404-114-5 A catalogue record for this book is available from the British Library r The Royal Society of Chemistry 2008 All rights reserved Apart from fair dealing for the purposes of research for non-commercial purposes or for private study, criticism or review, as permitted under the Copyright, Designs and Patents Act 1988 and the Copyright and Related Rights Regulations 2003, this publication may not be reproduced, stored or transmitted, in any form or by any means, without the prior permission in writing of The Royal Society of Chemistry, or in the case of reproduction in accordance with the terms of licences issued by the Copyright Licensing Agency in the UK, or in accordance with the terms of the licences issued by the appropriate Reproduction Rights Organization outside the UK. Enquiries concerning reproduction outside the terms stated here should be sent to The Royal Society of Chemistry at the address printed on this page.

Published by The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 0WF, UK Registered Charity Number 207890 For further information see our web site at www.rsc.org

Introduction

CHAPTER 1

Voyages with the Master AHMED H. ZEWAIL Physical Biology Center for Ultrafast Science and Technology, Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, CA 91125, USA

When I arrived in Philadelphia in August of 1969, I knew only of John Meurig Thomas the scientist. For more than three decades since then I have had the privilege of knowing John the scientist, the friend, and the communicator. In each of these dimensions, John is a Master. And he has one more unparalleled fourth dimension – a brilliant memory and a mental hard disk with unlimited storage capacity! Very few scientists are as versatile as John in his cross-linking of different science disciplines, and as cultured as he is in other facets of life – even in sports he was, as a schoolboy, the walking-race champion of Wales, and was also a member of the University of Wales cricket team in 1955. My first encounter with John was in June of 1970 at the Molecular Crystal Symposium organized by Robin Hochstrasser in Philadelphia at the Laboratory for Research on the Structure of Matter (LRSM). Among the stars present were Aleksander Davydov, Don McClure, Jan van der Waals, Hans Christoph Wolf, Wilse Robinson, and others. What John presented was his studies of dislocations in organic crystals and their vital role in determining optical and electrical properties. As a beginner, I was unaware of the totality of his impact in the field, but what impressed me most was his masterful presentation which he delivered with clarity, eloquence, and scholarly intellect. This memorable experience at the conference prompted me to squeeze myself in between the stars (Figure 1) in order to have a picture in the proximity of the world-renowned Davydov and to have a few words with John. Of course, at my level, the discussion was primarily about his enchantment with Egypt which he explored later in 1973 as a Visiting Professor at the American University in Cairo. By the time of the eighth conference in the same series, organized in June of 1977 by Mostafa El Sayed (Figure 2), I was an invited speaker and went to Santa Barbara from Caltech, where I had been appointed as an Assistant 3

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Figure 1

Chapter 1

The conference photo taken in June of 1970 on the steps of the Laboratory for Research on the Structure of Matter at the University of Pennsylvania, Philadelphia. A.S. Davydov is in the first row, second from the left. John Thomas and I are in the second row, seventh and third from the left, respectively. The organizer of the conference, Robin Hochstrasser, is the tallest in the back row, with a smile, next to Peter Rentzepis.

Professor. John was there, also as an invited speaker, and again he delivered a powerful presentation. To this day I can recall the way John presented his work and particularly the way he handled the Chair of his session. In a preemptive strike designed to secure more time for himself he said, ‘‘Mr. Chairman, I am about to finish,’’ meaning he needed another five minutes or more! At this meeting I realized one of John’s most impressive traits – his expansive thirst for knowledge and his resulting interest in broad areas of science in general and scientists in particular. This was certainly true in my case. At the conference, I spoke about the phenomena of ‘‘optical coherence’’ in molecular crystals and the new techniques for direct probing, in a talk titled, ‘‘Optical dephasing and radiationless transitions in molecular crystals.’’ Instantly, John became interested and asked me numerous questions with a display of genuine excitement about the development, even though it was not his area of primary interest. We did not cross paths again for some time, until a meeting at the Royal Society in London in February of 1990. In this discussion meeting, during which John presented his Bakerian lecture (on new crystalline catalysts), I gave a lecture titled ‘‘Femtosecond reaction dynamics,’’ and John again was aware

Voyages with the Master

Figure 2

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The conference photo taken in June of 1977 at a beachfront hotel in Santa Barbara, California. John Thomas and I are in the third row from the back (middle) and third row from the front to the right, respectively. The organizer, Mostafa El Sayed, is in the front row fifth from the left.

of the research in this area. Following the lecture, he invited me for lunch where he told me a story and made a prediction. The story relates to our 1987 publication with Marcos Dantus and Mark Rosker on the direct observation of the transition state with femtosecond time resolution. John discussed the paper with his students at the Royal Institution (RI) (in particular, Kenneth Harris) and described it to them as a historic landmark paper. The prediction was that the work was deserving of the Nobel Prize. John was serious and I trusted his sincerity. But what was so unique was that he actually read the paper and appreciated the value of a contribution that was far from his own field of endeavor. In fact, he zoomed in on the central concept of coherence in observing atomic motions; a difficult concept to grasp, even for some experts. It was at this meeting that I earned an invitation from John, as Director of the RI, to give the Faraday Discourse, enticing me – typical of John! – by mentioning the names of previous speakers from Caltech such as Robert Millikan, George Ellery Hale, Linus Pauling, and Murray Gell-Mann. John and his beloved wife, Margaret, were truly gracious hosts at the RI. With John as Director, the Faraday Discourse on March 22, 1991 was an experience organized in the true tradition of the place and the history it had integrated over time. Even though I knew the former Director, George Porter, for many years,

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this was my first time as an invited lecturer. In the Director’s flat, Margaret arranged a lovely dinner in the company of David and Jill Buckingham, Sir Brian and Lady Pippard, Sir Geoffrey and Lady Wilkinson, and Lord and Lady Dainton (formerly Sir Fredrick Dainton). A few minutes before the lecture I was locked in a small room, literally a ‘‘Faraday cage.’’ But just before caging me, John handed me a postcard of Sir James Dewar of liquid hydrogen fame on the back of which he had written the names of the three Nobelists from Caltech who had been in a similar situation, with John again making his obvious implication! At exactly 9:00 p.m., John and I, in our tuxedos, walked together into the theatre as its double doors opened, and I began the lecture. The Discourse was held in the same place Michael Faraday lectured, and it surely radiated past achievements and displayed a sense of history, and John fitted in well among the previous Directors, Davy, Faraday, Bragg (Sir Lawrence), and Porter. Eadweard Muybridge gave a discourse on ‘‘animals in motion’’, on March 13, 1882, at which T. H. Huxley and the poet Alfred, Lord Tennyson were present. We found his discourse demonstration of a slotted drum which, upon rotation, shows the animated horse in motion. I had thought of relating my discourse to motion, but now to the motion of atoms, with the title, ‘‘Filming in a millionth of a billionth of a second.’’ The theatre was packed and I thought of embarrassing John, but without success. I mentioned that the only way I could explain the full attendance was that the Director must have promised them a discourse by Omar Sharif. John led the audience with a big laugh! Since that time, I have greatly enjoyed both my scientific and personal interactions with John. I have become increasingly aware of his extraordinary ability to look at the big picture of science and humanity and in his genuine interest in popularizing science. His book on Faraday and his writings on Humphry Davy, Lawrence Bragg, and Max Perutz are examples of his devotion to the service of knowledge and his brilliant mastery of the English language – with a strong, attractive Welsh accent! In fact, I have two bulky files loaded with John’s outside-of-science writings. But, John is also a caring fellow scientist. He has written many obituaries and given the eulogies of distinguished scientists to salute their contributions to science and society. I have repeatedly told John to write my obituary in advance as I know it will be exceptional! He is also a cultured man in music, art, and history. John does all of these activities while maintaining passionate interest in his own science with pioneering contributions over six decades of research at the University of Wales, in Bangor and Aberystwyth, University of Cambridge, and the RI. John is distinguished for his innovative and diverse contributions, from solid state chemistry to heterogeneous catalysis, including the study of nanostructures, long before they became popular! He and his co-workers have designed, synthesized, and characterized hundreds of new heterogeneous catalysts. He has also developed and applied a wide range of tools for the study of solids and their surfaces, zeolites, clays, and other analogs. With these techniques, he has elucidated the importance of the structure in the function. The methods involved include high-resolution electron microscopy, electron diffraction,

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synchrotron radiation, solid-state NMR, photoelectron spectroscopy, and computational techniques. At present, with Paul Midgley and others, he is pioneering powerful ex situ techniques, such as nanotomography and nanoholography, for studying solids. Moreover, he has established in situ methods for investigating solid catalysts, the result of which, through his ‘‘single-site’’ solid catalysts, provide strategies that are applicable in the design of new catalysts for a wide range of reactions. Earlier, John was a leader in elucidating the manner in which the surface and bulk properties of crystals are influenced by structural imperfections, notably dislocations. This work on dislocations was critical to the understanding of physical, chemical, and spectral properties of crystals such as graphite, layered minerals, and molecular solids. His interest in diffraction and microscopy turned out to be the first scientific bonding we had. In 1991, the same year I gave the Faraday Discourse, I proposed ultrafast diffraction as a method for structural dynamics. Without delay, in the same year, John wrote a ‘‘News and Views’’ piece in Nature1 titled ‘‘Femtosecond diffraction.’’ Towards the end of the piece, he concluded with the following words: ‘‘If the experiment does indeed prove successful, it will mark the dawn of an important new era . . .’’ It took one decade (2001), and developments over several generations of instruments, to transform a dream into reality, from the exploration of the potential of the approach to the explosion of the applications in real experimental determination of isolated transient molecular structures. In retrospect, what is remarkable about John is his broader vision of the significance of determining structures in the act of change irrespective of the phase they are in. When we reached the condensed phase with ultrafast electron crystallography in 2004, John published in Angewandte Chemie2 an overview pointing out several potential applications including those in heterogeneous catalysis. But, he reached the apex of excitement when it became possible to image in real space with 4D ultrafast electron microscopy using single-electron packets. Because John has followed the trajectory of developments since its naissance, and is himself a pioneer in the applications of microscopy to materials science, he decided to write a 2005 highlight in Angewandte Chemie3 describing the development and the prospects for numerous branching applications. In the same year, 2005, Kenneth Harris and John in a paper published in Crystal Growth and Design4 explored some applications in domains of biological macromolecules and solid-state chemistry. Once again his passion for a new development was sincere and scholarly. After Margaret passed away, John needed his friends as much as ever, especially with the vacuum left behind by Margaret after decades of being together and sharing wonderful events at the RI, Peterhouse, and places all over the globe. And so I was delighted to see him in Cairo on the occasion of his receiving an honorary degree from the American University in Cairo in 2002, and we spent some time reflecting on life and science. To me, time spent with John is never dull, and is always enriching to the intellect and spirit. This tradition of intense and pleasurable discussion continues until today with visits in Cambridge, Pasadena, and other places around the world. The last time

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Figure 3

Chapter 1

A recent photo taken in June of 2006 at the University of Cambridge on the occasion of an Honorary Degree celebration to the Archbishop of Canterbury, The Most Rev. and Rt. Hon. Dr. Rowan Williams (in Divinity) and the author (in Science). John Thomas joined us in the celebration as a member of the faculty dressed in his colorful Scarlet Festal Robe. The Archbishop was born in Swansea into a Welsh-speaking family, making this photo indeed special, as I am surrounded by Wales’ most distinguished men of faith and of science.

I was in Cambridge in the summer of 2006 (Figure 3), we went on a walk and John showed me the hospital where Margaret passed away and the walks that they took together. And that is what life is about, especially when recognizing our fate as noted by Shakespeare’s Prospero: ‘‘We are such stuff as dreams are made on, and our little life is rounded with a sleep.’’ In Margaret’s eulogy in October of 2002, John closed with the words, ‘‘She left the world a better place.’’ In life one meets many people, interacts with some for the sake of mutual benefits, dislikes some for their attitude or personality, but cherishes only a special few for their integrity, professional achievement, and human decency. John is just such a person. Despite his attributes of great value and character, I dislike one thing about John. In his presence one feels a brain memory capacity of kilobytes while his is gigabytes or more. He recalls with lucidity events, names, and stories from long ago as if they happened yesterday, while the rest of us struggle to remember. At a recent meeting of the American Philosophical Society, I asked our mutual friend Jack Roberts, ‘‘Has John been consistent in

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his story telling over the years?’’ Jack answered, ‘‘The trouble is that I do not know because I forget most of the stories!’’ John, you are truly a master of science and humanity. We all wish you a very happy 75th birthday and we expect to celebrate your 100th remembering, John, that Ramses II lived to be almost 100, and you do not yet have even close to his 110 children!

References 1. 2. 3. 4.

J.M. Thomas, Nature, 1991, 351, 694. J.M. Thomas, Angew. Chem., Int. Ed., 2004, 43, 2606. J.M. Thomas, Angew. Chem., Int. Ed., 2005, 44, 5563. K.D.M. Harris and J.M. Thomas, Cryst. Growth Des., 2005, 5, 2124.

Foreword John Meurig Thomas One of John Thomas’s special qualities is his way of making instant, deep connection with another person. I remember well our first meeting: I was dispatched as a Chemical Society lecturer to the Coleg Prifysgol Cymru. By train, of course. And, this being 1975 and the U.K., naturally there was a rail strike the day after my lecture. And the day after that. But I did not mind the enforced stay in Aberystwyth, for there I met John. At the time I was beginning to work in inorganic and surface chemistry; I could not have wished for a better initiation to amphiboles and zeolites than I got in those two days. The contributors to this book celebrate the astounding diversity of John’s work. Let me walk down another footpath, one that passes the leitmotifs I see in John Thomas’s work, here all expressed in unvarnished English verbs: seeing loving complexity trying to understand getting things to go reaching out Seeing: From his earliest studies to this day, John has cared for structure and found ways to probe it with evermore informative tools. Many of us remember those Aberystwythian home-made – yet how sophisticated – stages for X-ray induced photoelectron spectroscopy. Chemistry (and science in general) progresses through experiment, cool logic and wild ideas, with unwarranted yet fecund leaps of the imagination from a hint of a shoulder in a spectrum to reliable knowledge. It is so often a kind of knowing without seeing. But oh, what convinced certainty comes from superimposing a small drawing of a molecule on an in situ transmission electron microscopy (TEM) image of a zeolitic catalyst! Loving complexity: For John it always was more than fearlessness. It was tough love. ‘‘Tough’’ because the polyphasic and intergrown materials whose structure he and his able collaborators untangled did not yield up their rich structures easily. And all those zeolites!

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Trying to understand: Yes, he loves the denumerable near-infinity of silicate networks. But I have in mind something deeper, John Thomas’s proclivity ‘‘on Newtonian wings to soar,’’ to use Humphry Davy’s phrase. It is that palpable desire evident in every Thomas paper, the abiding passion to understand what is behind chemical action. Getting things to go: Now that sounds really Newtonian, but, of course, I refer here to John’s love affair with catalysts. And his green (or shall we call it ‘‘oil-stained’’) thumb for designing catalysts. One of the few words to enter common parlance, catalysis remains a wonder, a metaphor, a bridge twixt science and magic. For thermodynamics just dryly delimits speculation, and by and large chemistry is the science of transformations that refuse to go. Catalysts are adventitious, catalysts are the heroines of serendipity tales. And catalysts are designed. John Thomas is the reigning ringmaster of a catalytic circus, in which various active particles jump, at his command, into tailored nooks and crannies in lattice-works out of his imagination. And there do their handsome, desired breaking and making of bonds. What fun! Reaching out: So he sings. And John Thomas loves to talk. The Welsh gift, the flourish, mixed with his love of science, and the strongest teaching impulse, self-assemble for our good. Faraday wrote ‘‘. . . the generality of mankind cannot accompany us one short hour unless the path is strewed with flowers.’’ Well, John Thomas has tossed bouquets upon bouquets to the world. I think there never was, at least not since the founding spirits, a man so right for the Royal Institution as John Thomas. Everything came together in the discourses, in the Christmas Lectures of those five years. There was no need to be locked up in Faraday’s lab; we could not wait to get to that lectern. And the boy whose physics mistress roused him by talking of Faraday, could stand at Faraday’s bench. . .. The themes I have strolled by cannot capture the spirit of the great scientist we celebrate. The 1795 Davy poem I cited (‘‘Sons of Genius’’) calls his heroes ‘‘Sons of Nature,’’ and has them delighting in ‘‘the train of mild philosophy’’ as well as ‘‘the rough precipice’s broken steep.’’ That is an excellent characterization of John Thomas, for sure. Or shall we just call him the man with infinite zest for catalysis? Roald Hoffmann

Preface Solid materials are ubiquitous in the world around us, and the wide array of materials (ranging from ceramics to polymers, pharmaceuticals to pigments, and catalysts to superconductors) that play a role in enhancing our everyday lives have arisen, at least in part, through the work of solid-state and materials scientists. An important pre-requisite for the development and exploitation of materials for such applications is the elucidation of a fundamental understanding of their properties, and in particular to establish the relationships between a specific property or function of interest, and parameters such as the structure and composition of the material. To advance our understanding of such fundamental properties, and hence to pave the way for future generations of applications, relies heavily on the ingenuity of those scientists who are dedicated to advancing the fundamentals of this field, and their skills in deploying a range of experimental and computational techniques to elucidate increasingly detailed facets of information. Progress is also heavily reliant on the work of those whose research is focused on advancing new aspects of such techniques themselves, with the aim of increasing the power and scope of these techniques to investigate systems in hitherto unexplored regimes of higher resolution, shorter timescales or smaller quantities. There is no doubt that the exploration of solid materials represents one of the most fascinating and rewarding areas of scientific endeavour in the present day, and one of the intentions of this book is to convey some of the excitement that is associated with research in this particular area of science. The aims of this book are two-fold: first, to provide a state-of-the-art survey of some of the most important recent developments across the solid-state, materials and surface sciences, and second (but intimately interwoven with the first), to serve as a tribute to the life and work of Professor Sir John Meurig Thomas, F.R.S., who has made monumental contributions to this field of science throughout his distinguished 50-year career in research, during which he has initiated, developed and exploited many important branches of this field. The subject matter is sub-divided into four main sections: (i) inorganic solid state chemistry, (ii) organic solid state chemistry, (iii) solid catalysts, surface and materials chemistry, and (iv) electron microscopy and its contribution to chemistry and materials science. The selection of these specific areas of the vii

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subject has been dictated in part by the fact that they span some of the most important sub-disciplines of contemporary research in solid-state, materials and surface sciences, but also in part because these areas represent the four primary pillars of research achievement on which the career and reputation of Sir John Meurig Thomas have been built. In addition to presenting a modern survey of a specific area of the subject, each chapter is also intended to project into the future development of the field. Each individual chapter is contributed by an internationally leading expert in the relevant field, and we are particularly delighted that two distinguished Nobel Laureates, Professor Roald Hoffmann and Professor Ahmed Zewail, are among the authors who have contributed to this book. Sir John Meurig Thomas, who is currently Honorary Professor of Solid State Chemistry at the University of Cambridge, is distinguished for his research achievements across a very wide range of areas relating to solid-state, materials and surface sciences. During the last 25 years in particular, his research has been directed primarily towards the elucidation of fundamental principles of heterogeneous catalysis, and the exploitation of these principles to design new catalyst materials for effecting a wide range of chemically and industrially important processes, under environmentally beneficial conditions. As a result of his many pioneering and innovative contributions, he is widely regarded as the pre-eminent scientist in this field in the world today. His seminal contributions in this field encompass: (i) the development of a broad range of new techniques for characterization of heterogeneous catalytic systems, which have led to an unprecedented level of understanding of the role of the active sites in catalytic processes (with particular novelty in his use of techniques to probe heterogeneous catalytic systems in situ, under real operating conditions, during catalytic processes), (ii) the development of a fundamental physico-chemical understanding of solid acid catalysts based on a wide range of structure types (clays, zeolites, aluminophosphates, mesoporous materials and derivatives of these materials), and (iii) the design of new generations of catalysts for carrying out a range of chemically and industrially important transformations under environmentally benign conditions (with particular recent interest in selective hydrogenation reactions, including the introduction of enantioselectivity in many of these reactions, and benign oxidation processes). His research in all of these areas has been distinguished by its originality and creativity, and by considerable productivity, and his contributions, encompassing both the elucidation of fundamental principles and the application of this fundamental knowledge to design and develop new and improved catalytic materials, have represented a substantial contribution towards the advancement of the field of heterogeneous catalysis. In addition to these fundamental achievements, his work has also had a direct impact in underpinning a wide range of industrially important chemical processes. Throughout his recent research focusing on fundamentals and applications of heterogeneous catalysis, and in his earlier, equally wide-ranging investigations of the properties of solid and their surfaces, Sir John Meurig Thomas has held firmly to the belief that ‘‘tools and techniques play at least as important a

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part in the evolution of scientific and technological progress as do ideas and theories’’ (see Chapter 48 of this book). Illustrating this philosophy, and his commitment not only to exploit the existing array of techniques but in addition to initiate, to develop and to adapt new aspects of techniques to explore specific issues, he has made many seminal contributions to the development of new experimental techniques and strategies for the characterization of materials. Some of the highlights from this aspect of his work include: (i) his multi-faceted applications of electron microscopy and electron energy loss spectroscopy to characterize structural and defect properties of materials, (ii) his groundbreaking applications of high-resolution (magic angle spinning) solid state NMR in the study of microporous solids, (iii) his development of the combined use of X-ray diffraction and EXAFS spectroscopy for in situ characterization of catalytic systems and processes, (iv) his development of electron tomography (carried out using a scanning transmission electron microscope), with particular interest in establishing this technique for characterization of the composition, structure and morphology of nanoparticle catalysts on high-area supports, and (v) his applications of computational techniques (often in tandem with experimental studies) for elucidating detailed mechanistic insights into catalytic processes. In addition to his more recent focus on heterogeneous catalysis, it is relevant to recall the many seminal contributions that he made to several other areas of solid-state, materials and surface sciences at earlier stages of his career. For example, at Bangor (1958 – 1969), he explored the chemical consequences of dislocations and other structural imperfections in crystals, developed new techniques of optical and electron microscopy, time-lapse microcinematography, etch decoration, electrical resistance, space-charge limited current, and conductivity-glow measurements to investigate the etching and reactivity of minerals (graphite, molybdenite and calcite) and the excitonic behaviour of molecular crystals such as anthracene, rotator-phase solids and protonic conductors. He introduced the language of dislocation theory into chemistry, which helped rationalize and explain many hitherto unexplained phenomena, such as the rapid diffusionless phase-transitions of certain organic solids (in terms of martensitic transformations), ‘‘anomalous’’ products in solid-state organic photodimerizations (for example in substituted anthracenes) in terms of stacking faults and partial dislocations, and the existence of singlet and triplet exciton traps and charge carrier (electron and hole) traps in terms of specific kinds of dislocations. At Aberystwyth (1969 – 1978), he established the world’s leading group in the solid-state chemistry and surface chemistry of solids, and he pioneered the introduction and exploitation of electron microscopy within the chemical sciences, both through fundamental developments of the technique, and through his seminal demonstrations of the scope and potential for using electron microscopy and related electron-beam techniques (particularly analytical X-ray photoelectron spectroscopy) to reveal details of structural, chemical and electronic properties of solids (including an early report of carbon nanotubes). In this regard, he introduced high-resolution electron microscopy

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as an indispensable tool for (real-space) structural elucidation of sub-picogram quantities of a wide range of minerals and developed analytical electron microscopy (using electron stimulated X-ray emission) to determine local composition and non-stoichiometry with nanometre electron probes. He also discovered many organic reactions that are catalyzed in the inter-lamellar regions of clay minerals, as well as the existence of incommensurate structures in intercalates, and he provided the first direct proof of the existence of staging in the phenomenon of intercalation. Among the new techniques that he developed were photoelectron diffraction, conversion-electron Mo¨ssbauer spectroscopy, dynamic high-resolution electron microscopy, and photo- and electroluminescence to elucidate a range of solid-state and surface phenomena. Through his work on organic crystalline materials during this time (particularly concerning photoreactivity of organic crystals, and the design of crystal structures for specific targeted photochemical reactions), he also laid the foundations of the field of ‘‘crystal engineering’’, which is nowadays a burgeoning area of activity. Reading through the list of these highlights from his research career, it is abundantly clear that many areas of contemporary importance within solidstate, materials and surface sciences stemmed directly from early work that he pioneered, and many of these contributions can be seen as genuine ‘‘turning points’’ within their respective scientific disciplines. During his distinguished career, Sir John Meurig Thomas has held some of the most prestigious scientific appointments in the United Kingdom, including Head of Department and Professor of Physical Chemistry at the University of Cambridge, and subsequently Director and Fullerian Professor of Chemistry at the Royal Institution (an appointment held previously by several other distinguished scientists, including Humphry Davy, Michael Faraday, W.L. Bragg and W.H. Bragg). His achievements in research have been recognized by many prestigious awards, including several of the premier awards of the Royal Society (Bakerian Lectureship and Davy Medal), the Royal Society of Chemistry (including the Faraday Medal, Longstaff Medal and Sir George Stokes Medal), the Linus Pauling Gold Medal for contributions to the advancement of science (the first non-American recipient of this award), the Willard Gibbs Gold Medal of the American Chemical Society, the Semenov Centenary Medal of the Russian Academy of Sciences, and the Giulio Natta Gold Medal of the Italian Chemical Society. He was the first recipient (in 1999) of the American Chemical Society Award for Creative Research in Homogeneous and Heterogeneous Catalysis; the citation for this award was ‘‘For laying down the basic principles for catalytic active site engineering by designing and synthesizing novel, exquisitely tailored solid catalysts and by pioneering the development of techniques for determining active site structures under operating conditions’’. And in recognition of his original contributions to analytical mineralogy and geochemistry, the International Mineralogical Association, in 1995, named a new mineral (Meurigite) in his honour. A full list of his achievements, awards and distinctions can be found in the brief curriculum vitae included on pages xx–xxiv of this book, and a full list of his publications

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(which number more than 1000) can be found at http://www-hrem.msm. cam.ac.uk/people/thomas/ In addition to his immense contributions to scientific research, Sir John Meurig Thomas has also taken a very active and innovative role in promoting and advancing the public appreciation of science, particularly by inspiring the scientific interests of young people. Following in the footsteps of Michael Faraday (one of his predecessors at the Royal Institution, and undoubtedly one of the greatest ever proponents of the popularization of science), Sir John has taken a very active role through his many public lectures and television broadcasts in promoting the awareness and popularization of science among the general public (including the Royal Institution Christmas Lectures in 1987 and numerous lectures at the National Portrait Gallery in London on the lives of famous people from the history of science). He is a uniquely gifted lecturer, with a profound scientific knowledge spanning an exceptionally diverse range of disciplines, and has a deep appreciation of the significance of contemporary scientific progress and its historical context. As an educator, teacher and research mentor, Sir John Meurig Thomas has taught and inspired generations of young scientists, many of whom now hold key academic and industrial positions throughout the world. His appointment as Head of the Department of Physical Chemistry at the University of Cambridge in 1978 led to a transformation of the teaching of the subject of Physical Chemistry in Cambridge, and the research directions that he introduced to Cambridge and the impact that he made at that time are still very much in evidence in the Department of Chemistry in Cambridge today. As an international statesman for science, Sir John Meurig Thomas has held a number of important appointments, including Chairmanship of the CHEMRAWN (Chemical Research Applied to World Needs) Committee of the International Union of Pure and Applied Chemistry. He has also, through his scientific endeavours and other work, been active in promoting the culture and language of Wales both within the United Kingdom and beyond, and through his high international scientific profile he has served in many ways as a cultural and scientific ambassador for Wales. We are particularly grateful to Sir John for contributing a chapter of his own (entitled ‘‘Design and Chance in My Scientific Research’’) to this book. As suggested by its title, this chapter provides a unique account of many of the key ‘‘turning points’’ that shaped his own scientific life, and conveys many fascinating insights into the way in which the career of one of the most distinguished scientists of the present day developed and blossomed. In addition to the reminiscences written by his own hand in this chapter, many of the other chapters of this book also contain personal reminiscences written by those authors who have known and worked with Sir John (as students, colleagues and collaborators) at different stages of his scientific career (including, in the Appendices, some short articles that have been contributed solely for this purpose). These reminiscences and anecdotes provide a fascinating account not only of Sir John the scientist, the researcher and the educator, but also of the personality, the unique character, and the human being. ‘‘An unforgettable

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person’’, to quote the words of Professor John D. Roberts (former Provost and Chairman of the Division of Chemistry and Chemical Engineering, California Institute of Technology) in Appendix 3 of this book, seems to sum up perfectly the impression that is gained by all those who have had the privilege of meeting Sir John. It seems somewhat anomalous that much of our discussion above seems to have referred to Sir John’s career using the past tense, when we all know that, as he approaches his 75th birthday, he is still as active, as energetic and as full of enthusiasm for science as he has ever been. So, while taking this opportunity to celebrate the many successes and achievements of his scientific career to date, we also wish him continued good health and many more productive years at the cutting edge of scientific research in the future. Kenneth D.M. Harris Peter P. Edwards

Contents Curriculum Vitae, Awards and Honours of Sir John Meurig Thomas

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Contributors Introduction Chapter 1

Voyages with the Master Ahmed H. Zewail

3

Section A: Inorganic Solid State Chemistry (Nanoporous Solids, Complex Oxides, Zeolites, Minerals, Non-Stoichiometry, Computation and Modelling) Chapter 2

Chapter 3

Chapter 4

Chapter 5

Chapter 6

Multifaceted Studies of Zeolites and Other Catalytic Materials Anthony K. Cheetham

13

The Deductive Approach to Chemistry, a Paradigm Shift Martin Jansen

22

Future Energy Materials: Three Challenges for Materials Chemistry Peter P. Edwards and Vladimir L. Kuznetsov

51

Structural Diversity and Potential Applications of Metal–Organic Coordination Polymers Jiesheng Chen and Ruren Xu

76

Elucidating Crystal Growth in Nanoporous Materials: The Importance of Microscopy Michael W. Anderson, L. Itzel Meza, Jonathan R. Agger, Martin P. Attfield, Maryam Sho¨aˆee`, Chin B. Chong, Ayako Umemura and Colin S. Cundy

xiii

95

xiv

Chapter 7

Chapter 8

Chapter 9

Contents

Exploration of New Porous Solids in the Search for Adsorbents and Catalysts Paul A. Wright and Wuzong Zhou

123

Concerning the Solid State Packing of [(ButCO2)3M2]2 (l-9,10-anthracenedicarboxylate) Compounds (M = Mo or W) and Other Matters Malcolm H. Chisholm, Matthew J. Byrnes, Ajatshatru Mehta and Patrick M. Woodward

138

High Pressure and High Temperature Oxidation in the IrSr2RECu2O8 Family of Cuprates: The Disordered Multiple Perovskite (A1/3 A 0 2/3)(B1/3 B 0 2/3)O3-x Phases A. J. Dos Santos-Garcı´a, G. Heymann, H. Huppertz and M. A´. Alario-Franco

151

Chapter 10

Melting and Amorphisation G. Neville Greaves

165

Chapter 11

Computer Modelling in Solid-State Chemistry C. Richard A. Catlow, Said Hamad, Devis Di Tommaso, Alexey A. Sokol and Scott M. Woodley

180

Chapter 12

Towards a Catalogue of Designer Zeolites M. M. J. Treacy, M. D. Foster and I. Rivin

208

Chapter 13

Discovering New Crystal Architectures Filipe A. Almeida Paz, Dorota Majda, Robert G. Bell and Jacek Klinowski

221

Chapter 14

Chemical Modulations in Pb–Bi Sulfosalts: A Glimpse at Minerals in Solid-State Chemistry Allan Pring and Cristiana L. Ciobanu

239

Complexity: In the Eye of the Beholder (This Beholder is a Crystallographer) Sven Lidin

250

Chapter 15

Chapter 16

Synthesis and Characterization of Zn-T-Sites in Mazzite David E. W. Vaughan, Ingrid J. Pickering, Graham N. George and Jeffrey R. Shallenberger

258

Chapter 17

Anything Protons Do, Muons Do Better! E. A. Davis

271

xv

Contents

Section B: Organic Solid State Chemistry Chapter 18

Molecular Cohesion and the Structure of Organic Crystals Jack D. Dunitz and A. Gavezzotti

Chapter 19

Aperiodicity in Organic Materials Kenneth D. M. Harris

Chapter 20

From the Synthesis of Acetylenic Natural Products to Seeing the Light with Polymers Andrew B. Holmes, Paul L. Burn, Arno Kraft, Jonathan M. White and Wallace W. H. Wong

Chapter 21

Molecular Recognition within One-Dimensional Channels Mark D. Hollingsworth

Chapter 22

FTIR Study of Short Range Mobility in Some Crystalline Peroxides: Solid-State Rotational Isomerism of CO2 J. Michael McBride and Kevin L. Pate

285

302

334

346

362

Section C: Solid Catalysts, Surface and Materials Science Chapter 23

From ‘Nature’ to an Adventure in Single-Site Epoxidation Catalysis Hendrikus C. L. Abbenhuis and Rutger A. van Santen

Chapter 24

A Comparison between Enzymes and Solid State Catalysts Robert J. P. Williams

Chapter 25

Zeolite Modelling: Active Sites in Different Framework Structures and in Different Crystallographic Positions Joachim Sauer

Chapter 26

Chapter 27

Magnetic Resonance Imaging: A New Window on the Catalyst Operating in the Reactor Environment L. F. Gladden, B. S. Akpa, L. D. Anadon, C. P. Dunckley, M. H. M. Lim, M. D. Mantle and A. J. Sederman Dissociative Chemisorption of Hydrogen Chloride at Cu(110): Atom-Resolved Time-Dependent Evidence for Transient States in the Formation of the ‘‘Final State’’ Stable Chloride Overlayer A. F. Carley, P. R. Davies, K. R. Harikumar, R. V. Jones and M. Wyn Roberts

385

396

441

457

479

xvi

Chapter 28

Chapter 29

Chapter 30

Chapter 31

Chapter 32

Contents

Recent Advances in Single-Site Photocatalysts Constructed within Microporous and Mesoporous Materials Masakazu Anpo and Masaya Matsuoka

492

Structural Organization of Catalytic Functions in Mo-Based Selective Oxidation Catalysts Masahiro Sadakane and Wataru Ueda

507

Designing Active Sites for Surfaces: From Tightly Bound to Loosely Anchored Thomas Maschmeyer

519

Polynuclear Transition Metal Cluster Complexes Containing Tin Ligands: Precursors to New Heterogeneous Nano-Catalysts Richard D. Adams and Burjor Captain Selective Oxidation Using Gold and Gold–Palladium Nanoparticles Graham J. Hutchings

534

550

Chapter 33

Electronic Factors in Hydrocarbon Oxidation Catalysis Jerzy Haber

568

Chapter 34

The Importance of Selectivity in Ammoxidation Catalysis Robert K. Grasselli

577

Chapter 35

The Mysteries of Water in Catalyst Preparation: Solvent or Much More? Michel Che

Chapter 36

Chapter 37

Chapter 38

Solid Acid Microporous H-SAPO-34: From Early Studies to Perspectives Leonardo Marchese, Gloria Berlier and Salvatore Coluccia Strategically Designed Single-Site Heterogeneous Catalysts for Clean Technology, Green Chemistry and Sustainable Development Robert Raja Catalysis by Lewis Acids: Basic Principles for Highly Stereoselective Heterogeneously Catalyzed Cyclization Reactions Mercedes Boronat, Avelino Corma and Michael Renz

588

604

623

639

xvii

Contents

Chapter 39

Recent Advances in XPS of Non-Conductors G. Michael Bancroft, H. W. Nesbitt, V. P. Zakaznova-Herzog and J. S. Tse

651

Section D: Electron Microscopy and its Contribution to Chemistry and Material Science Chapter 40

Chapter 41

Chapter 42

Chapter 43

Electron Microscopy Studies of Structural Modulation in Micro- and Meso-Porous Crystals Osamu Terasaki, Tetsu Ohsuna, Zheng Liu, Yasuhiro Sakamoto, Keiichi Miyasaka, Nobuhisa Fujita, Nozomu Togashi and Shunai Che Extrapolating from Fifty Years of Dislocation Imaging – Reaching into the Core Archie Howie Turning Points in Understanding the Emission of Brilliant Light from Highly Defective GaN-Based Materials and Devices Colin J. Humphreys Electron Tomography: A 3D View of Catalysts and Nanoscale Structures Paul A. Midgley

Chapter 44

Nano and Mesoporous Materials: A Study by HREM Jose´ M. Gonza´lez-Calbet, M. Luisa Ruiz-Gonza´lez and Marı´a Vallet-Regı´

Chapter 45

In Situ Direct Observation at Atomic Scale Twinning Transformations and the Formation of Carbon Nanostructures in WC Pratibha L. Gai, C. C. Torardi and E. D. Boyes

Chapter 46

A Survey of the Bi2O3–MoO3 Binary System Douglas J. Buttrey

Chapter 47

An Investigation of the Surface Structure of Nanoparticulate Systems Using Analytical Electron Microscopes Corrected for Spherical Aberration Rik Brydson and Andy Brown

667

687

698

711

727

745

754

778

xviii

Contents

Closing Chapter Chapter 48

Design and Chance in My Scientific Research John Meurig Thomas

795

Appendices: Tributes to Sir John Meurig Thomas Appendix 1

Tribute to John Meurig Thomas on the Occasion of His 75th Birthday David Buckingham

853

Appendix 2

John Meurig Thomas and the Royal Institution John Waterlow

855

Appendix 3

Sir John Meurig Thomas: An Unforgettable Person John D. Roberts

856

Appendix 4

John Meurig Thomas on His 75th Birthday Ralph Kohn

858

Appendix 5

Remembering a Period of Work with Sir John Meurig Thomas Gilbert Sloan

861

Reflections on John Meurig Thomas on the Occasion of His 75th Birthday Martin Pope

863

Bangor 1966–1969; Aberystwyth 1969–1973; Some Fond Reflections Stan Moore

866

Appendix 6

Appendix 7

Appendix 8

Aberystwyth 1970–1973. Reflections and Lessons Learnt Gari Owen

Appendix 9

Molecular Modelling Input to Organic Solid State and Zeolite Chemistry: Reminiscences (1975-84) S. Ramdas

868

872

Appendix 10 Reflections of a Cambridge Undergraduate Angus Kirkland

876

Appendix 11 Sir John Meurig Thomas Brian Johnson

879

Contents

Appendix 12 Getting the Details Correct David Jefferson

xix

881

Appendix 13 Tribute to Sir John Meurig Thomas on the Occasion of His 75th Birthday Gordon M. Parkinson

883

Appendix 14 Solid State Chemistry and the Edward Davies Chemical Laboratories Bill Jones

885

Subject Index

887

Curriculum Vitae, Awards and Honours of Sir John Meurig Thomas PROFESSOR JOHN MEURIG THOMAS Date of Birth: 15 December 1932, Llanelli, Wales, UK

Present Positions Held:  Honorary Professor of Materials Science, University of Cambridge (2002 – )  Emeritus Professor of Chemistry, Davy Faraday Research Laboratory, Royal Institution of Great Britain, London (since 2002)  Honorary Distinguished Professor of Materials Chemistry, Cardiff University, Wales (2005 – )  Distinguished Visiting Professor of Nanoscience, University of South Carolina, USA (2005 – )  Honorary Distinguished Professor of Materials Chemistry, University of Southampton (2006 – )  Honorary Professor, Graduate School of Engineering, Osaka Prefecture University, Japan (2006 – )  Honorary Professor, State Laboratory of Inorganic Chemistry and Materials Science, Jilin University, China (2007 – ) Positions Formerly Held:    

Director, Royal Institution of Great Britain (1986–91) Director, Davy Faraday Research Laboratory (1986–91) Fullerian Professor of Chemistry, Royal Institution (1988–94) Head, Department of Physical Chemistry, University of Cambridge and Professorial Fellow at Kings College, Cambridge (1978–86) xx

Curriculum Vitae, Awards and Honours of Sir John Meurig Thomas

xxi

 Master (Head) of Peterhouse (College), University of Cambridge (1993– 2002)  Deputy Pro-Chancellor, Federal University of Wales (1991–94)  Professor of Chemistry and Head of Department , University College, Wales, Aberystwyth (1969–78)  Assistant Lecturer, Lecturer, Reader in Chemistry, University of Wales, Bangor (1958–69) National and International Awards: 2007

2005 2004 2003 1999 1997 1996

1995

1994 1992

1989

The International Precious Metal Institute Distinguished Achievement Award ‘‘for pioneering contributions to the field of heterogeneous catalysis using precious metals’’ Sir George Stokes Gold Medal, Royal Society of Chemistry ‘‘for pioneering and innovative electron based nanochemical analysis’’ Guilio Natta Gold Medal, Italian Chemical Society ‘‘for outstanding work in catalysis’’ Linus Pauling Gold Medal, Stanford University ‘‘for contributions to the advancement of science’’ First recipient of American Chemical Society Award ‘‘for Creative Research in Heterogeneous and Homogeneous Catalysis’’ Honorary Medal, Krakow Academy of Knowledge, Poland ‘‘for Distinguished Public Service’’ Semenov Centenary Medal, Russian Academy of Sciences Honorary Medal, Polish Academy of Sciences, Warsaw Longstaff Medal, Royal Society of Chemistry Willard Gibbs Gold Medal of the American Chemistry Society (first British chemist to be honoured in 80 years): ‘‘for pioneering work in solid-state chemistry and materials science ..... His original work (on solids) has led to major advances in the science and technology of absorbents and catalysts’’ New mineral, meurigite, named in his honour by International Mineralogical Association to recognise his pioneering work in geochemistry Davy Medal of the Royal Society (its premier medal in the Physical Sciences) Messel Gold Medal, Society of Chemical Industry, awarded biennially ‘‘for meritorious distinction in science, literature, industry or public affairs’’ Faraday Medal, Royal Society of Chemistry (its premier medal, awarded every three years) Sesquincentenary Medal of the Royal Microscopical Society

xxii

Curriculum Vitae, Awards and Honours of Sir John Meurig Thomas

Other Medals:  Hugo Mu¨ller Medal (1983), Solid-State Chemistry Medal (1978), Tilden Medal (1973), Corday-Morgan Medal (1969) [all from the Royal Society of Chemistry]  The Pettinos Prize (first recipient), American Carbon Society (1969)  Bruce-Preller Prize, Royal Society of Edinburgh (1989)  Silver Medal ‘‘for services to science celebrating 750th Anniversary of the University of Siena’’ (2005) Honorary Doctorates from: Wales (LL.D.); Council of National Academic Awards (D.Litt.); Heriot-Watt, Edinburgh (D.Sc.); Birmingham (D.Sc.); Open University (D.Univ.); Lyon, France (D.Sc.); Complutense, Madrid (D.Sc.); Glamorgan (D.Sc.); Western Ontario, Canada (D.Sc.); Eindhoven, The Netherlands (D.Sc.); Hull (D.Sc.); Surrey (D.Univ); Aberdeen (D.Sc.); American University in Cairo (D.Sc.); Turin, Italy (D.Sc.); Clarkson, USA (D.Sc.); Sydney, Australia (D.Sc.) Honorary Foreign Fellowships or Memberships:  2006, European Academy of Sciences  2005, Mendeleev Chemical Society, Moscow  2004, Accademia Nazionale dei Lincei, Rome  2003, Go¨ttingen Academy of Sciences  1999, Royal Spanish Academy of Sciences  1998, Polish Academy of Sciences  1998, Hungarian Academy of Sciences  1995, Third World Academy of Sciences, Trieste

 1994, Academy of Sciences of Venezuela  1994, Russian Academy of Sciences  1993, Royal Society of Edinburgh  1992, American Philosophical Society, Philadelphia  1991, Engineering Academy of Japan  1990, American Academy of Arts and Sciences, Cambridge  1989, Academia Europaea  1985, Indian National Academy, New Delhi  1981, Indian Academy, Bangalore

John Meurig Thomas holds over forty honorary fellowships in universities and colleges in the UK and elsewhere in the world. A Fellow of the Royal Society since 1977, in 1999 he was elected Honorary Fellow of the Royal Academy of Engineering for work that ‘‘has profoundly added to the science

Curriculum Vitae, Awards and Honours of Sir John Meurig Thomas

xxiii

base of heterogeneous catalysis leading to the commercial exploitation of zeolites through engineering processes’’. Also in 1999 he was made (honorary) Fellow of the Institute of Physics. Served as Government Advisor on the Council on Applied Research and Development (1982-85) at the Cabinet Office, Whitehall, London. Chairman of CHEMRAWN (Chemical Research Applied to World Needs) of the International Union of Pure and Applied Chemistry (1988-92). President of the Faraday Division of the R.S.C., and of the Chemistry Division of the British Association for Advancement of Science; of the London International Youth Science Festival; and a Trustee of the National Science Museum (1990-95) and of the Natural History Museum (1989-91). He is the vice-President of the Cambridge University Musical Society (1994 – ). In 2000, the Electron Microscopy and Microanalysis Society of the Americas held a three-day symposium in his honour at their annual convention in Philadelphia. He has broadcast extensively on radio and television in the UK and abroad, and given numerous popular lectures to lay audiences world-wide and lunchtime lectures at the National Portrait Gallery, London. Named Lectureships Abroad and in the UK: John Meurig Thomas has given over a hundred Named Lectures world wide, including those named in honour of: Rutherford (New Zealand); Van’t Hoff (Royal Academy Netherlands); Helmholtz (Berlin); Darwin (Cambridge); Debye (Utrecht); Pauling (Caltech, Stanford and Oregon); Larmor (Cambridge); Baker (Cornell); Woodward (Harvard and Yale); Pitzer (Berkeley); Krishnan (New Delhi); Bernal (London); Ziegler (Germany); Liversidge (Sydney); Polanyi (Toronto); Sunner (Lund, Sweden); Willard Gibbs (ACS, Chicago); Faraday (RSC, London); Birch (Canberra); Hund (Stuttgart); Watson (Caltech); Drickamer, (Urbana); Taylor (Penn State); Guggenheim (Reading); Rogers (Michigan); Shipley (Clarkson); Oersted (T.U. Denmark); Bakerian (Royal Society, London). In 1986 he was a plenary speaker, along with Professor Ken-ichi Fukui, at the Japan Key Technology event, Tokyo, to honour the sixtieth year of the reign of the Emperor of Japan. In 2006 he was plenary speaker at the Tercentenary Celebrations of the birth of Benjamin Franklin, American Philosophical Society, Philadelphia. Short-term Visiting Professorships:  Scuola Normale Superiore, Pisa  University of Florence  Technical University, Eindhoven

 Texas A and M University  Yale University  Cornell University

xxiv

Curriculum Vitae, Awards and Honours of Sir John Meurig Thomas

 Ecole Nationale Superieure, Paris  Jawaharlal Nehru Centre for Advanced Studies, Bangalore  Max Planck Institute, Mu¨lheim  Weizmann Institute of Science  Universities of Western Ontario, McMaster and Calgary  University of California, Berkeley

 Indiana University  Northwestern University  Pennsylvania State University and Arizona State University  California Institute of Technology  University of Sydney  Ruprecht-Karls-Universita¨t, Heidelberg.

Membership of International Advisory Boards: At various times, John Meurig Thomas has been a member of several International Advisory Boards, including the following:  Science Centre, University of Alexandria, Egypt  Weizmann Institute, Israel  Laboratory of Molecular Sciences, California Institute of Technology, U.S.A.  Beroskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences  EXSELENT (Extremely Selective and Enantioselective Materials for Controlled Sorption and Catalysis), Arrhenius Laboratory, University of Stockholm, Sweden  National Institute of Informatics, Tokyo, Japan In 1991 he was knighted by Queen Elizabeth II for ‘‘services to chemistry and the popularisation of science’’ Awarded Medal of the Honourable Society of Cymmrodorion (London) for services to Welsh culture and British public life (2003) – first scientist to be so honoured since its inception 160 years ago. John Meurig Thomas is the author of over 900 research papers on the materials and surface chemistry of solids, and over 100 review articles on science, education and cultural issues. He is the co-author of 25 patents, two University texts on Heterogeneous Catalysis and a biographical–philosophical study of Michael Faraday, 1991 (Japanese Translation, 1994; Italian Translation, 2007). The full publication list of John Meurig Thomas is available at: http://wwwhrem.msm.cam.ac.uk/people/thomas/

Contributors Miguel A. Alario-Franco Departamento de Quı´ mica Inorga´nica Facultad de Ciencias Quı´ micas Universidad Complutense de Madrid 28040 Madrid Spain

Hendrikus C. L. Abbenhuis Laboratory of Inorganic Chemistry and Catalysis Eindhoven University of Technology PO Box 513 NL-5600 MB Eindhoven The Netherlands

Richard D. Adams Department of Chemistry and Biochemistry The University of South Carolina 631 Sumter Street Columbia SC 29208 USA

Filipe A. Almeida Paz Department of Chemistry CICECO, University of Aveiro Campus Universitario de Santiago Aveiro 3810-193 Portugal

Jonathan R. Agger Centre for Nanoporous Materials School of Chemistry The University of Manchester Oxford Road Manchester, M13 9PL UK

L. D. Anadon Department of Chemical Engineering University of Cambridge New Museums Site Pembroke Street Cambridge, CB2 3RA UK

B. S. Akpa Department of Chemical Engineering University of Cambridge New Museums Site Pembroke Street Cambridge, CB2 3RA UK

Michael W. Anderson Centre for Nanoporous Materials School of Chemistry University of Manchester Oxford Road Manchester M13 9PL UK xxv

xxvi

Masakazu Anpo Department of Applied Chemistry Graduate School of Engineering Osaka Prefecture University 1-1 Gakuen-cho Sakai, Nakaku Osaka 599-8531 Japan

Martin P. Attfield Centre for Nanoporous Materials School of Chemistry The University of Manchester Oxford Road Manchester, M13 9PL UK

G. Michael Bancroft Department of Chemistry The University of Western Ontario Chemistry Building 1151 Richmond Street London Ontario N6A 5B7 Canada

Robert G. Bell Davy-Faraday Research Laboratory The Royal Institution of Great Britain 21 Albemarle Street London, W1S 4BS UK

Gloria Berlier Dipartimento de Chimica Inorganica, Fisica e dei Materiali and NIS Centre of Excellence Universita´ de Torino Via P. Giuria 7 10125 Torino Italy

Contributors

Mercedes Boronat Instituto de Tecnologia Quimica (UPV-CSIC) Universidad Polite´cnica de Valencia Avda. De los Naranjos s/n 46022 Valencia Spain

E. D. Boyes Department of Physics University of York Heslington York, YO10 5DD UK Andy Brown Leeds Electron Microscopy and Spectroscopy Centre Institute for Materials Research SPEME University of Leeds Leeds, LS2 9JT UK

Rik M. D. Brydson Institute for Materials Research School of Process, Environmental and Materials Engineering University of Leeds Clarendon Road Leeds West Yorkshire LS2 9JT UK

Paul L. Burn School of Molecular and Microbial Sciences University of Queensland St Lucia Qld 4072 Australia

xxvii

Contributors

Douglas J. Buttrey Department of Chemical Engineering University of Delaware 326 CLB 150 Academy Street Newark DE 19716 USA

Matthew J. Byrnes Department of Chemistry The Ohio State University Columbus Ohio, 43210 USA

Burjor Captain Department of Chemistry and Biochemistry The University of South Carolina 631 Sumter Street Columbia SC 29208 USA

A. F. Carley School of Chemistry Cardiff University Park Place Cardiff, CF10 3AT UK

C. Richard A. Catlow Department of Chemistry University College London 20 Gordon Street London WC1H 0AJ UK

Michel Che Laboratoire de Reactivite´ de Surface et Structure Universite´ Pierre et Marie Curie 4 Place Jussieu Tour 54-55 75252 Paris Cedex 05 France

Shunai Che School of Chemistry and Chemical Technology Shanghai Jiao Tong University Shanghai PR China

Anthony K. Cheetham Materials Research Laboratory University of California Santa Barbara CA 93106 USA

Jiesheng Chen State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry Jilin University Changchun 130012 People’s Republic of China

Malcolm H. Chisholm Department of Chemistry The Ohio State University 100 W. 18th Ave. Columbus Ohio 43210 USA

xxviii

Chin B. Chong Centre for Nanoporous Materials School of Chemistry The University of Manchester Oxford Road Manchester, M13 9PL UK

Cristiana L. Ciobanu Department of Mineralogy South Australian Museum North Terrace Adelaide South Australia 5000 Australia

Salvatore Coluccia Dipartimento di Chimica Inorganica, Fisica e dei Materiali and NIS Centre of Excellence Universita` di Torino, Via P. Giuria 7 10125 Torino Italy

Avelino Corma Instituto de Tecnologı´ a Quı´ mica (UPV-CSIC) Universidad Polite´cnica de Valencia Avda. de los Naranjos s/n 46022 Valencia Spain

Colin S. Cundy Centre for Nanoporous Materials School of Chemistry The University of Manchester Oxford Road Manchester, M13 9PL UK

Contributors

E. A. Davis Department of Materials Science and Metallurgy University of Cambridge New Museums Site Pembroke Street Cambridge CB2 3QZ UK

P. R. Davies School of Chemistry Cardiff University Park Place Cardiff, CF10 3AT UK

C. P. Dunckley Department of Chemical Engineering University of Cambridge New Museums Site Pembroke Street Cambridge, CB2 3RA UK

Jack D. Dunitz Chemistry Department OCL ETH-Ho¨nggerberg HCI H333 CH-8093 Zu¨rich Switzerland

Peter P. Edwards Inorganic Chemistry Laboratory University of Oxford South Parks Road Oxford OX1 3QR UK

xxix

Contributors

M. D. Foster Department of Physics Arizona State University PO Box 871504 Tempe AZ 85287-1504 USA

Nobuhisa Fujita Department of Physical, Inorganic and Structural Chemistry Arrhenius Laboratory Stockholm University SE-106 91 Stockholm Sweden

Pratibha L. Gai Department of Chemistry University of York Heslington York, YO10 5DD

A. Gavezzotti Dipartimento di Chimica Strutturale e Stereochimica Inorganica University of Milano Via Venezian 21 I-20133 Milano Italy

Graham N. George Department of Geological Sciences University of Saskatchewen 114 Science Place Saskatoon SK S7N 5E2 Canada

Lynn F. Gladden Department of Chemical Engineering University of Cambridge New Museums Site Pembroke Street Cambridge CB2 3RA UK

Jose´ M. Gonza´lez-Calbet Departamento de Quı´ mica Inorga´nica Facultad de Ciencias Quı´ micas Universidad Complutense de Madrid 28040 Madrid Spain

Robert K. Grasselli Department of Chemistry Technische Universita¨t Mu¨nchen D-85748, Garching Germany

G. Neville Greaves Institute of Mathematical and Physical Sciences University of Wales Aberystwyth Aberystwyth Ceredigion SY23 3BZ UK

Jerzy Haber Institute of Catalysis and Surface Chemistry Polish Academy of Sciences ul. Niezapominajek 8 PL-30239 Krakow Poland

xxx

Said Hamad Davy Faraday Research Laboratories and Department of Chemistry University College London 20 Gordon Street London, WC1H OAJ UK

K. R. Harikumar School of Chemistry Cardiff University Park Place Cardiff, CF10 3AT UK

Kenneth D. M. Harris School of Chemistry Cardiff University Park Place Cardiff CF10 3AT UK

G. Heymann Department Chimie und Biochemie Ludwig-Maximilians-Universita¨t Mu¨nchen Butenandtsrasse 5-13 81377 Mu¨nchen Germany

Roald Hoffmann Department of Chemistry and Chemical Biology Cornell University Baker Laboratory Ithaca NY 14853-1301 USA

Contributors

Andrew B. Holmes School of Chemistry The University of Melbourne Bio21 Institute Melbourne Victoria 3010 Australia

Mark D. Hollingsworth Chemistry Department 111 Willard Hall Kansas State University Manhattan KS 66506 USA

Archie Howie Department of Physics University of Cambridge Cavendish Laboratory J.J. Thomson Avenue Cambridge CB3 0HE UK

Colin J. Humphreys Department of Materials Science and Metallurgy University of Cambridge New Museums Site Pembroke Street Cambridge CB2 3QZ UK

H. Huppertz Department Chimie und Biochemie Ludwig-Maximilians-Universita¨t Mu¨nchen Butenandtsrasse 5-13 81377 Mu¨nchen Germany

xxxi

Contributors

Graham J. Hutchings School of Chemistry Cardiff University Park Place Cardiff CF10 3AT UK Martin Jansen Max Planck Institute for Solid State Research Heisenbergstraße 1 D-70569 Stuttgart Germany Brian F. G. Johnson Department of Chemistry University of Cambridge Lensfield Road Cambridge CB2 1EW UK R. V. Jones School of Chemistry Cardiff University Park Place Cardiff, CF10 3AT UK

Vladimir Kuznetsov Inorganic Chemistry Laboratory University of Oxford South Parks Road Oxford, OX1 3QR UK

Sven Lidin Department of Physical, Inorganic and Structural Chemistry Arrhenius Laboratory Stockholm University, SE-106 91 Stockholm Sweden

M. H. M. Lim Department of Chemical Engineering University of Cambridge New Museums Site Pembroke Street Cambridge, CB2 3RA UK

Jacek Klinowski Department of Chemistry University of Cambridge Lensfield Road Cambridge, CB2 1EW UK

Zheng Liu Department of Physical, Inorganic and Structural Chemistry Arrhenius Laboratory Stockholm University SE-106 91 Stockholm Sweden

Arno Kraft Chemistry, School of Engineering and Physical Sciences Perkin Building Heriot-Watt University Edinburgh, EH14 4AS UK

Dorota Majda Department of Chemistry University of Cambridge Lensfield Road Cambridge, CB2 1EW UK

xxxii

M. D. Mantle Department of Chemical Engineering University of Cambridge New Museums Site Pembroke Street Cambridge, CB2 3RA UK

Leonardo Marchese Dipartimento di Scienze e Tecnologie Avanzate Universita` del Piemonte Orientale ‘‘A. Avogadro’’ C. so Borsalino 54 I-15100 Alessandria Italy

Thomas Maschmeyer School of Chemistry University of Sydney Building F11 Sydney NSW 2006 Australia

Masaya Matsuoka Department of Applied Chemistry Graduate School of Engineering Osaka Prefecture University 1-1 Gakuen-cho Sakai, Nakaku Osaka 599-8531 Japan

Ajatshatru Mehta Department of Chemistry The Ohio State University Columbus Ohio, 43210 USA

Contributors

L. Itzel Meza Centre for Nanoporous Materials School of Chemistry The University of Manchester Oxford Road Manchester, M13 9PL UK

J. Michael McBride Department of Chemistry Yale University Box 208107 New Haven CT 06520–8107 USA

Paul A. Midgley Department of Materials Science and Metallurgy University of Cambridge New Museums Site Pembroke Street Cambridge CB2 3QZ UK

Keiichi Miyasaka Department of Physical, Inorganic and Structural Chemistry Arrhenius Laboratory Stockholm University SE-106 91 Stockholm Sweden

H. W. Nesbitt Department of Earth Sciences The University of Western Ontario London Ontario N6A 5B7 Canada

xxxiii

Contributors

Tetsu Ohsuna Department of Physical, Inorganic and Structural Chemistry Arrhenius Laboratory Stockholm University SE-106 91 Stockholm Sweden

Michael Renz Instituto de Tecnologia Quimica (UPV-CSIC) Universidad Polite´cnica de Valencia Avda. De los Naranjos s/n 46022 Valencia Spain

Kevin L. Pate Department of Chemistry Marietta College Marietta Ohio 45750 USA

I. Rivin Department of Mathematics Temple University 1805 North Broad Street Philadelphia PA 19122 USA

Ingrid J. Pickering Department of Geological Sciences University of Saskatchewen 114 Science Place Saskatoon SK S7N 5E2 Canada

Allan Pring Department of Mineralogy South Australian Museum North Terrace Adelaide South Australia 5000 Australia

Robert Raja School of Chemistry University of Southampton Highfield Campus Southampton SO17 1BJ UK

M. Wyn Roberts School of Chemistry Cardiff University Park Place Cardiff CF10 3AT Wales

M. Luisa Ruiz-Gonza´lez Departamento de Quı´ mica Inorga´nica Facultad de Ciencias Quı´ micas Universidad Complutense de Madrid 28040 Madrid Spain

Masahiro Sadakane Catalysis Research Center Hokkaido University North 21 West 10 Kita-ku Sapporo 001-0021 Japan

xxxiv

Contributors

Yasuhiro Sakamoto Department of Physical, Inorganic and Structural Chemistry Arrhenius Laboratory Stockholm University SE-106 91 Stockholm Sweden

Maryam Shoaee Centre for Nanoporous Materials School of Chemistry The University of Manchester Oxford Road Manchester, M13 9PL UK

A. J. Dos Santoa-Garcı´ a Laboratorio de Quı´ mica del Estado So´lido Departamento de Quı´ mica Inorga´nica and Laboratorio Complutense de Altas Presiones Facultad de Ciencias Quı´ mica Universidad Complutense de Madrid 28040 Madrid Spain

Alexey A. Sokol Davy Faraday Research Laboratories and Department of Chemistry University College London 20 Gordon Street London, WC1H OAJ UK

Joachim Sauer Institut fu¨r Chemie Humboldt Universita¨t zu Berlin Unter den Linden 6 D-10099 Berlin Germany

A. J. Sederman Department of Chemical Engineering University of Cambridge New Museums Site Pembroke Street Cambridge, CB2 3RA UK

Jeffrey R. Shallenberger Materials Research Institute Pennsylvania State University University Park PA 16802 USA

Osamu Terasaki Department of Physical, Inorganic and Structural Chemistry Arrhenius Laboratory Stockholm University SE-106 91 Stockholm Sweden

Sir John Meurig Thomas Department of Materials Science and Metallurgy University of Cambridge New Museums Site Pembroke Street Cambridge CB2 3QZ UK

Nozomu Togashi Namiki Precision Jewel Co. Ltd. Tokyo Japan

xxxv

Contributors

Devis de Tommaso Davy Faraday Research Laboratories and Department of Chemistry University College London 20 Gordon Street London, WC1H OAJ UK

C. C. Torardi DuPont Central Research and development Wilmington DE 19880-0356 USA

Michael M. J. Treacy Department of Physics Arizona State University P.O. Box 871504 Tempe AZ 85287-1504 USA

J. S. Tse Department of Physics University of Saskatchewan Saskatoon SK S7N5E2 Canada

Wataru Ueda Catalysis Research Center Hokkaido University North 21 West 10 Kita-ku Sapporo 001-0021 Japan

Ayako Umemura Centre for Nanoporous Materials School of Chemistry The University of Manchester Oxford Road Manchester, M13 9PL UK

Marı´ a Vallet-Regı´ Departamento de Quı´ mica Inorga´nica y Bioinorga´nica Facultad de Farmacia Universidad Complutense de Madrid Pza Ramon y Cajal 28040 Madrid Spain

Rutger A. van Santen Laboratory of Inorganic Chemistry and Catalysis Eindhoven University of Technology P.O. Box 513 NL-5600 MB Eindhoven The Netherlands

David E. W. Vaughan Materials Research Institute Pennsylvania State University 276 Materials Research Laboratory Building University Park PA 16802 USA

Jonathan M. White School of Chemistry Bio21 Institute The University of Melbourne Melbourne Victoria 3010 Australia

xxxvi

Robert J. P. Williams Inorganic Chemistry Laboratory University of Oxford South Parks Road Oxford OX1 3QR UK

Wallace W. H. Wong School of Chemistry Bio21 Institute The University of Melbourne Melbourne Victoria 3010 Australia

Scott M. Woodley Davy Faraday Research Laboratories and Department of Chemistry University College London 20 Gordon Street London, WC1H OAJ UK

Patrick M. Woodward Department of Chemistry The Ohio State University Columbus Ohio, 43210 USA

Paul A. Wright School of Chemistry University of St. Andrews St. Andrews Fife KY16 9ST UK

Contributors

Ruren Xu State Key Laboratory of Inorganic Synthesis and Preparative Chemistry College of Chemistry Jilin University Changchun 130012 People’s Republic of China V. P. Zakaznova-Herzog Department of Earth Sciences The University of Western Ontario London Ontario N6A 5B7 Canada Ahmed H. Zewail Physical Biology Centre for Ultrafast Science and Technology Arthur Amos Noyes Laboratory of Chemical Physics California Institute of Technology Mail Code 127-72 1200 East California Boulevard Pasadena CA 91125 USA Wuzong Zhou School of Chemistry University of St. Andrews Purdie Building St. Andrews Fife KY16 9ST UK

Section A: Inorganic Solid State Chemistry (Nanoporous Solids, Complex Oxides, Zeolites, Minerals, Non-Stoichiometry, Computation and Modelling)

CHAPTER 2

Multifaceted Studies of Zeolites and Other Catalytic Materials ANTHONY K. CHEETHAM Materials Research Laboratory, University of California, Santa Barbara, CA 93106, USA

1 Introduction I believe that I was first introduced to John Thomas by Mike Goringe in the mid-1970s. Mike and I lived in Woodstock at the time, just a few miles north of Oxford, and we both played cricket regularly for the Blenheim Park club on the south lawn of Blenheim Palace. John had come from Aberystwyth to visit the Materials Department at Oxford and Mike invited him to make a guest appearance at the club. Happily, none of us can remember our scores after so many years, but I can remember being vividly struck by the keen intelligence and quick-wittedness of our guest. This was the first of many meetings with John in the 1970s: several in the Chemical Crystallography laboratory at Oxford, where I had joined the faculty in 1974, once at a discourse by John at the Royal Institution in London, and a number of times at Gordon Conferences and other meetings. It was not until about 1980 that we began our first collaboration. I had been working extensively in the 1970s with the recently discovered Rietveld method for analysing powder neutron diffraction data, and John, by then at Cambridge, invited me over to talk about the possibility of applying this powerful tool to some problems in the zeolite area. His enthusiasm and persuasiveness quickly convinced me that this would be an exciting adventure, and thus began a wonderful collaboration that lasted about 15 years and produced over 30 papers. The collaboration was further enhanced when John became the Director of the Royal Institution (RI) in 1986 and persuaded the RI to appoint me to a part-time chair in Solid State Chemistry. The collaboration also survived my move from Oxford to the University of California at Santa Barbara in 1991, and continued actively until about 1996 when our interests began to diverge, John’s 13

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focusing more on practical catalysis and mine on hybrid and magnetic materials. The friendship remained, however, and it therefore gives me great pleasure to dedicate this chapter to John on the occasion of his 75th birthday.

2 Zeolite Science Our collaboration on zeolites, like much of our later work, involved the complementary use of more than one technique. In order to probe the nature of silicon–aluminium ordering in zeolite-A, we used a combination of the Rietveld method with another recently developed tool: magic angle spinning nuclear magnetic resonance (MAS NMR). The application of 29Si MASNMR to aluminosilicate zeolites was pivotal in establishing the credibility of this new tool for addressing problems in the chemistry of materials. Following closely on the heels of the pioneering work of Lippmaa and his colleagues in Tallin on aluminosilicates, we were quickly able to show that the widely studied structure of zeolite-A had some unexpected features. Our first paper showed that it could be rhombohedral rather than cubic, depending upon the Si/Al ratio,1 while a second paper suggested that the Si/Al ordering might be more complex than had hitherto been thought.2 It turned out that we were misled by the 29Si chemical shifts, which spanned a larger range than had previously been thought, but the error was quickly rectified in a joint paper with J.V. Smith,3 and John and I went on to publish our first joint paper in Nature in which we demonstrated the power of the combined neutron and NMR approach to zeolites for the elucidation of not only the Si,Al ordering but also the precise locations of the exchangeable cations in zeolite Tl-A.4 One of the features of the Nature paper that captured people’s imagination was the sheer size of the unit cell of zeolite-A, which, at almost 15,000 A˚3, was the largest structure ever refined by the Rietveld method to that date. Slightly later, we also published similar work on zeolites ZK-4 and Na-Y.5 The success of our neutron work on Si,Al ordering and cation positions in zeolites led us to explore other applications of the Rietveld method in this area. Given the widespread use of zeolites as acid catalysts for hydrocarbon cracking and isomerization reactions, we attempted to use neutrons to establish the precise locations of the active sites in zeolite La-Y, an important commercial catalyst. This was very successful and led to a detailed description of the cation hydrolysis reaction that results in the formation of acid sites in La-Y.6 At about the same time (ca. 1983), the success of molecular modelling in addressing host– guest interactions in the field of pharmaceutical chemistry inspired us to examine what this approach might offer in the context of adsorbed molecules in zeolite cavities. With the able assistance of Ramdas, who later joined British Petroleum, and Paul Betteridge and Keith Davies from Oxford, we were immediately rewarded with some beautiful results on the behaviour of the simple hydrocarbons in a number of zeolites.7 Our early work took full advantage of the powerful advances in computer graphics for representing the van der Waals surfaces of the cavities and channels in zeolites (Figure 1) and utilized contour maps to visualize the potential energy surfaces for adsorbed molecules.

Multifaceted Studies of Zeolites and Other Catalytic Materials

Figure 1

15

A net representation of the van der Waals surface of the sinusoidal and linear channels in zeolite ZSM-5.

Having demonstrated that the computer modelling was able to predict the location and heat of adsorption of a guest molecule in a zeolite cavity, we proceeded to combine the simulations with neutron diffraction studies in order to test the reliability of the calculations. This proved to be successful beyond our wildest dreams, leading to the elucidation of the location of xenon in zeolite rho,8 and a subsequent study of the location of a hydrocarbon (pyridine) in the channel of zeolite K-L.9 The latter result (Figure 2), which relied on the complementary skills of Paul Wright and Andreas Novak, appeared on the front cover of Nature in December 1985 and greatly added to the credibility of computer modelling in the field of zeolites. Indeed, for the first time chemists were able to talk in terms of the precise location of an adsorbed molecule in a zeolitic cavity. If scientific research can ever be thought of in terms of a golfing analogy, then the recollection of this publication is cherished like the memory of one’s first birdie! The work on pyridine in zeolite-L was published just as John was preparing to move from Cambridge to the Royal Institution, and the pace of our collaboration slowed down for a time while we established the infrastructure for materials chemistry and catalysis at the RI. In addition to establishing a powerful capability for doing more sophisticated computer simulations, we also benefited from the arrival of people who knew exactly how to take full advantage of them. One such person was Yashonath, who had done a postdoc with Mike Klein at Penn and was a specialist in Monte Carlo methods. He quickly adapted his codes to handle our complex zeolite problems, and our initial work resulted in a paper in Nature on ‘‘The siting, energetics and mobility of saturated hydrocarbons inside zeolite cages’’ (Figure 3).10

16

Figure 2

Chapter 2

The location of pyridine in the channel of potassium zeolite-L. Note the manner in which the lone-pair on the nitrogen (shown in yellow) interacts with the potassium (shown in blue), while the molecule itself lies close to the wall of the cavity.9

Yashonath helped to train others, too, paving the way for a series of simulation papers on adsorbed species in zeolites,11–14 including some of the first molecular dynamics studies of hydrocarbons in zeolites, which were done in collaboration with colleagues at Shell in Amsterdam.15,16 One last note on our aluminosilicate collaboration should not be forgotten. In 1988 Richard Catlow took up a part-time chair at the Royal Institution, bringing further capabilities to the RI’s computational programme. This led, among other things, to a joint 1990 paper in Advanced Materials that involved the three of us. Together with Julian Gale and Rob Jackson, we had undertaken the daunting task of simulating, for the first time, the behaviour of an organically pillared clay. We chose the analinium–vermiculite system because there was a reasonable crystal structure of this high charge-density material, and we obtained remarkably good agreement between the experimental structure and the energy-minimized model.17

3 Aluminium Phosphates Our joint work on aluminosilicate zeolites during the 1980s took place against a background of increasing interest in aluminium phosphate (AlPO4) molecular sieves, which were first reported by Edith Flanigen and her colleagues at Union

Multifaceted Studies of Zeolites and Other Catalytic Materials

17

18

Carbide in 1982. As the potential of these new materials in catalysis began to emerge, our imagination was captured by a report that SAPO4-34, which adopts the chabazite structure, could catalyze the conversion of methanol to light olefins. This provided us with an opportunity to carry out a beautiful in situ NMR study of this interesting coupling reaction. My student Clare Grey took the lead, and we published a very nice paper in a new journal that John had helped to launch, Catalysis Letters.19 The high level of interest in AlPO4s also inspired us to start some synthetic work in the area using hydrothermal and solvothermal methods. I had some experience in the synthesis and characterization of mixed metal oxides and metal phosphates by conventional solid state reactions, but these lower temperature methods were new to both of us. We pursued our joint effort at both the RI and Oxford with Richard Jones and Anne Chippindale, respectively. This work led to a series of papers20–23 during the period when I was in the process of moving from Oxford to Santa Barbara. I was frankly rather disappointed because we failed to make any new 3-D AlPO4 architectures at the time, but rather made a series of 1-D chains and 2-D layered structures. Although this demonstrated that the aluminium phosphates could exist in a full range of dimensionalities, just as Pauling had shown for the aluminosilicates many decades earlier, the materials had no real utility in terms of catalysis. The disappointment has been mitigated over the years, however, because these four papers have garnered around 100 citations apiece, reminding me once more that we are not always the best judges of our own work. Furthermore, we did subsequently succeed in making 3-D AlPO4 frameworks, both jointly24 and independently. The joint work on AlPO4s had one other exciting outcome. In the mid-1990s, by means of a trans-Atlantic collaboration that involved not only my group and John’s, but also Leo Marchese in Turin and Paul Wright in St. Andrews, we published an exciting paper in Science25 on the interaction between protons and water in a solid acid catalyst (H-SAPO-34). My student Luis Smith did the neutron diffraction work, while Leo made measurements by infrared (IR) spectroscopy. The results gave us a fairly complete description of the formation of hydronium-type ions in the cavities, a result that stimulated a great deal of subsequent theoretical work. Shortly afterwards, we published a paper on the nature of the acid sites in dehydrated H-SAPO-34, again using the same combination of IR spectroscopy and neutron diffraction.26

4 In situ Studies of Metal Oxides and Supported Metal Catalysts John and I began to collaborate on natural gas conversion shortly before I moved to UCSB in 1991. The RI was then nicely equipped with an X-ray diffraction/mass spectrometry facility for studying catalysts in situ under realistic reaction conditions, and we chose to look at the fate of an oxide pyrochlore, Eu2Ir2O7, during the conversion of methane to synthesis gas by partial

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oxidation. The results elegantly confirmed what we had already suspected, which was that reduction by the methane caused the oxide to disproportionate into Eu2O3 and particles of iridium metal; in essence, we had generated a supported metal catalyst in situ. This experiment was followed by a similar in situ experiment at the Daresbury synchrotron, where we studied natural gas conversion by CO2 reforming over Ln2M2O7 pyrochlores (M ¼ Ru, Ir) by energy dispersive X-ray diffraction (XRD).28 The use of the synchrotron source gave much better time resolution and enabled us to study the kinetics, though the diffraction resolution with the energy dispersive XRD approach was not as good as with the constant wavelength method. Complementary studies by temperature programmed reduction under methane, hydrogen, and carbon monoxide revealed that the mechanisms with the ruthenates were subtly different from those with the iridates, with the former showing clear evidence for the formation of an intermediate perovskite phase during the reduction of the pyrochlore. As a side product of the work on supported ruthenium and iridium catalysts for natural gas conversion, we became curious about the relative strengths of different methods by which the sizes of small particles could be estimated. Working with metal particles of diameter 2–3 nm, we undertook a comparative study by X-ray powder diffraction (line broadening), extended X-ray absorption spectroscopy (EXAFS), and transmission electron microscopy.29 Published in 1994, this work was about a decade ahead of its time and was unable

Figure 3

Probability density distribution of methane in sodium zeolite-Y from Monte Carlo calculations at 0 K, 100 K, 170 K, and 298 K.

Multifaceted Studies of Zeolites and Other Catalytic Materials

19

to benefit from the visibility that it might have received in the modern era of nanotechnology and nanomaterials. During one of my summer visits to the RI in the early 1990s, we also carried out an interesting in situ study of gel crystallization during the synthesis of the perovskite PZT, PbZr1
xTixO3, which is an important commercial ferroelectric.30 In addition to discovering a previously unsuspected intermediate fluorite phase and establishing the kinetics of the crystallization, we also found that it was possible to control the particle size and we were able to study the evolution of particle size as a function of processing temperature and time. The interpretation of this data was greatly enhanced by discussions with Jim Speck, one of my colleagues in the Materials Department at UCSB, while the experimental work at the RI benefited greatly from the efforts of ‘‘Raj’’ Natarajan, who subsequently became one of my post-docs in Santa Barbara and is now on the faculty at the Indian Institute of Science in Bangalore.

5 Conclusions Sitting at home in Santa Barbara writing this short chapter for the book in honour of John’s 75th birthday, I am struck by many thoughts and reflections. First, our 15 years of close collaboration between 1981 and 1996 produced a very impressive body of work, with over 30 publications averaging more than 50 citations apiece. It gives me particular satisfaction, even now, to look back at the way in which we brought a number of complementary tools to bear on our problems in a multifaceted manner. We also benefited from the creativity and hard work of some outstanding co-workers, many of whom have been mentioned in the paragraphs above. I have several reasons to be grateful to John. By working closely with him, I learned for the first time the distinction between interesting problems and important problems, for John has an extraordinary knack of spotting the latter. He also influenced the scope of my research in a number of ways. For example, in the years since our joint work on aluminosilicate zeolites, which led to about 18 papers, I have published around 70 further papers on zeolites in my own right. And although my interest in phosphates pre-dates my collaboration with John, in the area of phosphate molecular sieves, where we had a handful of joint papers, I have since published about 50 more. Our collaboration took other forms, too, of course. We organized meetings together, such as the UK–Russia workshop on Heterogeneous Catalysis at Oxford in 1987. We travelled to Gordon Conferences in New Hampshire together, on one occasion being diverted to Montreal where we hired a car to drive down to Plymouth, NH. And in cooperation with Millie Dresselhaus, we jointly launched Current Opinions in Solid State and Materials Science in 1996. Looking back on my own career, I can see that my research went into a higher gear in about 1980 when our collaboration began. There is no way of knowing if it would have done so in the absence of John’s influence, but I can certainly be confident that my career would have followed a quite different trajectory and

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that he has had an enormous influence on my scientific interests and ambitions. Happy birthday, John, and thank you!

References 1. J.M. Thomas, L.A. Bursill, E.A. Lodge, A.K. Cheetham and C.A. Fyfe, J. Chem. Soc., Chem. Commun., 1981, 276. 2. L.A. Bursill, E.A. Lodge, J.M. Thomas and A.K. Cheetham, J. Phys. Chem., 1981, 85, 2409. 3. A.K. Cheetham, C.A. Fyfe, J.V. Smith and J.M. Thomas, J. Chem. Soc., Chem. Commun., 1982, 823. 4. A.K. Cheetham, M.M. Eddy, D.A. Jefferson and J.M. Thomas, Nature, 1982, 299, 24. 5. A.K. Cheetham, M.M. Eddy, J. Klinowski and J.M. Thomas, J. Chem. Soc., Chem. Commun., 1983, 23. 6. A.K. Cheetham, M.M. Eddy and J.M. Thomas, J. Chem. Soc., Chem. Commun., 1984, 1337. 7. S. Ramdas, J.M. Thomas, P.W. Betteridge, A.K. Cheetham and E.K. Davies, Angew. Chem., Int. Ed. Engl., 1984, 23, 671. 8. P.A. Wright, J.M. Thomas, S. Ramdas and A.K. Cheetham, J. Chem. Soc., Chem. Commun., 1984, 1338. 9. P.A. Wright, J.M. Thomas, A.K. Cheetham and A.K. Nowak, Nature, 1985, 318, 611. 10. S. Yashonath, J.M. Thomas, A.K. Nowak and A.K. Cheetham, Nature, 1988, 331, 601. 11. D.E. Akporiaye, S.D. Pickett, A.K. Nowak, J.M. Thomas and A.K. Cheetham, Catal. Lett., 1988, 1, 133. 12. S.D. Pickett, A.K. Nowak, A.K. Cheetham and J.M. Thomas, Mol. Simul., 1989, 2, 353. 13. S.D. Pickett, A.K. Nowak, J.M. Thomas and A.K. Cheetham, Zeolites, 1989, 9, 123. 14. A.K. Cheetham, J.D. Gale, A.K. Nowak, B.K. Peterson, S.D. Pickett and J.M. Thomas, Faraday Discuss. Chem. Soc., 1989, 87, 79. 15. S.D. Pickett, A.K. Nowak, J.M. Thomas, B.K. Peterson, J.F.P. Swift, A.K. Cheetham, C.J.J. den Ouden, B. Smit and M.F.M. Post, J. Phys. Chem., 1990, 94, 1233. 16. A.K. Nowak, C.J.J. den Ouden, S.D. Pickett, B. Smit, A.K. Cheetham, M.F.M. Post and J.M. Thomas, J. Phys. Chem., 1991, 95, 848. 17. J.D. Gale, A.K. Cheetham, R.A. Jackson, C.R.A. Catlow and J.M. Thomas, Adv. Mater., 1990, 2, 487. 18. S.T. Wilson, B.M. Lok, C.A. Messina, T.R. Cannan and E.M. Flanigen, J. Am. Chem. Soc., 1982, 104, 1146. 19. Y. Xu, C.P. Grey, J.M. Thomas and A.K. Cheetham, Catal. Lett., 1990, 4, 251.

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20. R.H. Jones, J.M. Thomas, R. Xu, Q. Huo, Y. Xu, A.K. Cheetham and D. Bieber, J. Chem. Soc., Chem. Commun., 1990, 1170. 21. R.H. Jones, J.M. Thomas, R. Xu, Q. Huo, A.K. Cheetham and A.V. Powell, J. Chem. Soc., Chem. Commun., 1991, 1266. 22. A.M. Chippindale, A.V. Powell, L.M. Bull, R.H. Jones, A.K. Cheetham, J.M. Thomas and R. Xu, J. Solid State Chem., 1992, 96, 199. 23. J.M. Thomas, R.H. Jones, R. Xu, J. Chen, A.M. Chippindale, S. Natarajan and A.K. Cheetham, J. Chem. Soc., Chem. Commun., 1992, 929. 24. A.M. Chippindale, A.V. Powell, R.H. Jones, J.M. Thomas, A.K. Cheetham, Q. Huo and R. Xu, Acta Crystallogr. Sect. C, 1994, C50, 1537. 25. L. Smith, L. Marchese, A.K. Cheetham, J.M. Thomas, P.A. Wright and J. Chen, Science, 1996, 271, 799. 26. L. Smith, A.K. Cheetham, L. Marchese, J.M. Thomas, P.A. Wright, J. Chen and E. Gianotti, Catal. Lett., 1996, 41, 13. 27. R.H. Jones, A.T. Ashcroft, D. Waller, A.K. Cheetham and J.M. Thomas, Catal. Lett., 1991, 8, 169. 28. A.T. Ashcroft, A.K. Cheetham, R.H. Jones, S. Natarajan, J.M. Thomas, D. Waller and S.M. Clark, J. Phys. Chem., 1993, 97, 3355. 29. A.T. Ashcroft, A.K. Cheetham, P.J.F. Harris, R.H. Jones, S. Natarajan, G. Sankar, N.J. Stedman and J.M. Thomas, Catal. Lett., 1994, 24, 47. 30. A.P. Wilkinson, J.S. Speck, A.K. Cheetham, S. Natarajan and J.M. Thomas, Chem. Mater., 1994, 6, 750.

CHAPTER 3

The Deductive Approach to Chemistry, a Paradigm Shift MARTIN JANSEN Max-Planck-Institut fu¨r Festko¨rperforschung, Heisenbergstraße 1, Stuttgart D-70569, Germany

1 Introduction Even today the pillars called analysis and synthesis still constitute the foundation of chemistry. Taken in its general sense, analysis goes far beyond determining chemical compositions, and includes investigating static structures, dynamic behaviour as well as all chemical or physical properties of matter, regardless of whether they might suggest applications or not. Synthesis, on the other hand, comprises all actions that lead to defined chemical compounds. Taken in their etymological senses, these terms appear to be in opposition to each other. In chemistry, however, analysis and synthesis go hand in hand, even in a synergetic manner, thus keeping chemistry on the track of steady and fascinating progress. Two key factors are triggering innovation in chemistry: new methods in analysis, e.g. providing ever better spatial or energetic resolution in determining atomic or electronic structures, respectively, and in synthesis by providing new (classes of) chemical compounds. There are very few scientists that have contributed in an equally innovative manner to both fields. The extremely fruitful efforts by John Meurig Thomas in providing new (single site) heterogeneous catalysts,1 superior with respect to the relevant figures of merit, many even successful in various applications,2 and in understanding how they function, provide such an example. Furthermore, he has employed computational chemistry3 as another tool to create a harmonic and prolific triad, together with the analytical and synthetic experimental counterparts. Opting for such a strategy has been the logical consequence of the self-imposed tasks of developing heterogeneous catalysts for specific reactions in a directed manner and of understanding the catalytic mechanisms on the microscopic level. In order to meet the latter requirement, one has to 22

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23

consider the local reaction sites, which at best exhibit short-range order, as well as the impact of the long range ordered matrix. The resulting challenge to monitor these two features simultaneously, i.e. in exactly identical conditions and at the same spot of the sample, has been ingeniously met by John Thomas in combining diffraction, addressing the long range structure, and spectroscopy as a local probe, in one experiment.4,5 Synthesis and analysis often do not appear to be of equal weight for the chemist. The analytical tools rather serve synthesis, which is at the core of chemistry. Synthesis is in the focus for economic reasons, reflecting the huge share of world market volume as contributed by materials and drugs, for understanding and replicating our physical surroundings, be it biological or inorganic matter, and in particular for knowledge driven basic research, pleasing human curiosity and contributing to human culture. Therefore it is well understandable that control of synthesis has continued to be on the top of the chemical agenda.

2 Chemical Synthesis, Setting the Stage Although, in the chemist’s daily work, synthesis is meant to transform certain starting materials to the desired product compound, in an ultimately puristic sense, it starts from the elements. Thus the number of different elements available, and the multiplicity of bonding options they offer, stake out the territory for synthetic chemistry. That part of the universe accessible to human perception consists of one and the same set of chemical elements, out of which about 86 are stable and can be employed in chemical synthesis. From this number, one limiting factor results immediately, that is the maximal number of P 86 86 possible combinations of element types, which amounts to 86 ð n¼0 n Þ ¼ 2 . From Figure 1, and the inset, it follows that the resulting number of different chemical systems, even when considering only subsets, is beyond the power of our imagination.6 It is interesting to note to what little extent these systems have been investigated, not to speak of being fully explored, so far. The answer to the more important question of how many different compounds can be made, using 86 stable elements, is most probably ‘‘innumerably infinite.’’ It is easy to give the reader a feeling for whether this statement comes close to the truth or not. One might follow Po`lya who generated, and was able to count, the compounds in the binary system of alkanes including all possible isomers, employing graph theory.7 As one can see from Figure 2, using just 24 carbon atoms 14 506 015 different alkanes can be made! A similar scenario develops for extended solids, because of the variability of periodicity, which is in principle unlimited. As a consequence, binary SiC can form an infinite number of stacking variants (polytypes). We regard this as inconceivable, yet this breathtaking plethora of possible chemical compounds represents the true face of chemistry, virtually defining its identity.6 From the numbers and examples given, it is obvious that only a minute portion of the compounds possible, much less than the tip of an iceberg, has been realized, so far. Admittedly, it is

24

Figure 1

Chapter 3

‘‘Mountain of materials,’’6 number of element combinations calculated for a set of up to 86 different elements; the inset displays the number of possible systems containing one to four components (red), and the number of those investigated (green).

definitely hard to perceive these numbers as ‘‘limiting’’; they rather indicate superabundant opportunities for chemistry. Of course, one has to keep in mind the hypothetical character of the above combinatorial considerations. In exploring the chemical world physically, one will sooner or later run into practically insurmountable barriers. One quite obvious limitation results from the total mass of matter available in the universe. However, this aspect would only establish a restriction if one wanted to experimentally investigate all possible chemical systems simultaneously. Further arguments against accessibility of high component systems claim that it is impossible to force more than about seven different elements into a solid compound, via conventional all-solid-state reaction routes.8,9 The intricacies to be managed at running such reactions, in particular in a reproducible fashion, are well recognized.10 As another approach for estimating the highest practically achievable number of constitutional components of a compound, it has been assumed that the limit is set by those observed in nature, or thus far synthetically realized.11–13 This number is said to converge towards a limit of seven. However, by just looking into some pertinent databases we have already found compounds with up to 11 elements: Cs2K4((W3Te(Te2)3(CN)6)2Cl)Cl(H2O)5,14 C30H34AuBCIF3N6O2P2PtW,15 ((C5H5)Re(NO)(PPh3)(ICH2Si(CH3)3)BF4*CH2Cl2.16 Also, the observation that the free enthalpy of formation of a compound, e.g. from the constituting binaries, converges towards zero with an increase in the number of

The Deductive Approach to Chemistry, a Paradigm Shift

Figure 2

25

Results of a graph theoretical enumeration of possible alkanes.

its components, has been used as an argument lending support to the opinion that compounds consisting of a high number of different elements would not exist. However, we regard all these considerations as speculative, defining our present capabilities rather than the potential opportunities. Those former conclusions definitely depend on the procedures applied for synthesis, or conditions prevailing during genesis, e.g. of minerals in the Earth’s mantle. It is our conviction that removing the barriers in solid state synthesis – most commonly impeding transport of matter – by using modern techniques like atom-by-atom deposition17 will open the access to 20 (and higher) component systems. Finally, we do not see any justification for excluding solid solutions,8,11 when discussing limits on the number of elements in a compound. These are regular manifestations of matter, playing an important role in earth sciences and in optimizing specific properties by extrinsic doping. But regardless of what the maximum possible number of constituent elements in a chemical compound precisely happens to be, our rough combinatorial estimates, even if ignoring practical restrictions, certainly give a proper impression of the general magnitude of the problem to be coped with in synthetic chemistry. From the numbers discussed, the overwhelming size of the tasks required to run syntheses in a rational and purposeful manner is quite obvious, and, at the same time they document what wide areas of chemistry have still remained unexplored. Finally, the enterprise of systematically and rationally investigating this treasure will be of a complexity18 challenging and even overpowering the information technologies19 presently available.

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3 The Inductive Approach to Chemistry At the beginning of chemistry as a scientific discipline, the observations were naturally rather incoherent, and also imprecise. Thus they were difficult to place into a consistent rationale, and virtually the only option left was just to document them, and subsequently with growing factual knowledge, to extract some systematics. Applying this inductive approach20 to chemistry has proved to be extremely successful. The early stage of discovery of the elements and the formulation of the periodic system of the elements (PSE) might serve as an example corroborating this view.21 Firstly, all kind of matter was scrutinized for characteristics typically attributed to elements, without knowing any underlying principle or even the number of elements to be expected. In the next step, attempts were made to classify the elements known, with all their greatly varying appearances, by correlating chemical or physical properties, e.g. with their atomic masses. This was a rather arbitrary approach; however, these efforts eventually resulted in establishing the PSE by D. Mendelejew and L. Meyer which has constituted a tremendous breakthrough in chemistry. In particular, the way Mendelejew utilized this arrangement right from the beginning has pointed the way to modern chemistry. Only some decades later, the underlying principles of this ordering scheme were unveiled. Taking a look at various classes of compounds, e.g. oxides, fluorides or intermetallics, and how the knowledge of them has been developing, demonstrates the continuous success of the inductive strategy. The progress in the chemistry of binary and ternary fluorides as documented in a series of reviews22–25 may serve as an example. It is important to understand why chemists have stuck to the inductive and descriptive procedures for so long. Firstly, because it was obviously a successful way to cope with the intriguing fact that the observations were made on a macroscopic level while the underlying mechanisms went on at the atomic scale (which in the beginning was not accessible with the available tools). More importantly, chemists have been correctly convinced that confirmed experimental observations, added to the growing stock of knowledge, will survive for all times, while the interpretations and the models conceived will be subject to change. Following the guideline of inductionism, a wealth of knowledge has been accumulated, providing systematics based on composition, structural principles, and reactivities. However, this approach has left the various classes of compounds in quite different states of maturity and completeness.

4 The Energy Landscape Concept In an attempt to unify all classes of chemical compounds, one may look for a feature shared by all of them. As one such common feature we identify the basic precondition for a compound to exist for a given period of time in a given (equilibrium) geometry. This property is the so-called ‘‘local ergodicity.’’6,26–29 Quite obviously, there must be some gain in binding energy, as compared to the

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same ensemble of unbonded atoms, and moreover, the equilibrium geometry is typically associated with a minimum in binding energy. Since this holds true for each stable chemical compound, and since one can move from one minimum to another by modifying the structures (isomers), or exchanging, adding or removing atoms, one easily arrives at the concept of a landscape of (binding) energy,6,26–30 with the minima associated with stable configurations, cf. Figure 3. This way of projecting all known as well as not yet known chemical compounds onto an energy landscape is providing a sound concept for any attempt to analyze chemistry, in particular the issue of synthesis, on a universal foundation.6,26–30 As a particular strength of our concept, all classes of matter are included on an equal footing, thus artificial trenches are removed, e.g. between chemistry of molecular compounds and extended solids, or between natural and synthetic matter. Furthermore, valuable insights into many implications of chemical synthesis can be extracted without any effort. In the past, the question of whether a target compound would be stable was almost exclusively evaluated based on thermodynamic considerations.31,32 However, kinetic stability alone is already sufficient for a certain chemical structure to be experimentally accessible.30 Since composition and structure of a stable configuration (associated with a locally ergodic region on the energy landscape) are predetermined by natural laws, neither composition nor structure can be subject to any kind of arbitrary tuning or shaping by the chemist, and using the term ‘‘design’’ in the context of developing targets for chemical synthesis is definitely inappropriate.26 As an even more substantial consequence of this view, chemistry can be approached in a deductive way by deriving the structure

Figure 3

Visualization of a section of a multiminima energy landscape, e.g. of chemical matter.

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of the respective energy landscape from ‘‘first principles.’’ Finally, a clear definition of chemistry is provided with almost mathematical stringency: doing chemistry corresponds to exploring the energy landscape associated with chemical matter, thus accumulating the stockpile of all substances capable of existence.

5 Exploration of Chemical Energy Landscapes The first step to be done in synthesis planning is to identify a target compound that is either thermodynamically or at least kinetically stable. Conventionally, this can be achieved by relying on intuition or experience, frequently extrapolating confirmed knowledge, e.g. by assembling structural increments known to represent stable topologies. By now, as a rather common procedure, such ‘‘raw’’ configurations that had been derived through heuristic concepts, are subjected to geometry optimizations on a quantum mechanical basis,33–35 and are thus tested for stability before starting the respective experiments. However, such local approaches suffer from a lack of generality, might be misled by prejudice and thus fail to identify all compounds possible in the system under investigation. In particular, such attempts to predict structures and stabilities of extended solids that might be encountered when entering a yet unexplored system would go astray, as a rule. Dealing with intermetallics in this respect is awfully frustrating. Some of these pitfalls can be addressed appropriately by globally exploring the relevant (part of the) energy landscape, most favourably based on ab initio quantum mechanical energy calculations.

5.1

Computational Approaches

In principle, the configurations and energies corresponding to the ground and excited states of a chemical system can be calculated by solving the respective Schro¨dinger equation. However, it is quite obvious that the number of particles to be considered in trying to cover the full compositional and structural diversity exhibited by an ensemble of atoms of realistic size makes such a straightforward approach intractable, at least by the currently available tools. Instead, for the exploration of landscapes of high complexities, like the energy landscapes of chemical matter under discussion, performing stochastic walks guided by physical (e.g. energy based) or nonphysical cost functions have proved to be an efficient strategy. A variety of well-developed algorithms serving this purpose have become available, the most important approaches being based on Monte Carlo techniques, genetic algorithms or neural networks. Using appropriate cost functions, virtually each of them would, in principle, be suited for global searches of chemical landscapes. In our implementation we have chosen the ‘‘Metropolis Monte Carlo’’ variant of ‘‘simulated annealing.’’27–29,36,37 In a similar way, genetic algorithms have been employed at predicting inorganic crystal structures.38 The specific features of our implementation have been determined both by the basic objective of our efforts, i.e. predicting targets for chemical synthesis,

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and by technical feasibility, considering the performance of algorithms and computer hardware presently available. In marked contrast to the techniques previously employed in computational determinations of crystal structures of already synthesized solids,39 we allow the translational lattices as well as the compositions to be freely varied. These are indispensable requirements to make sure that the whole landscape under inspection is accessible and the full compositional and structural variability is explored. A flowchart of our implementation is displayed in Figure 4. Regrettably, we have to resort to rather general two body potentials for the energy calculations, during the global Metropolis runs.27 This was on one hand forced by the necessity to keep the computational effort needed for full explorations of systems of realistic sizes within bounds, and on the other to introduce as little bias as possible into the random walks. For instance we do not use empirical potentials, optimized for a given compound, because this would give an undesired preference to this, or a closely related, structure. The price to be paid is a certain restriction of our approach to polar

Figure 4

Flowchart for the modular approach to predicting chemical compounds capable of existence. Upper part refers to the global search and preliminary ranking procedures, lower part to local ab initio optimization of particular structure candidates.

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(ionic) compounds, and some preference to the more polar ones among the configurations in the system under investigation. Furthermore, quantitative details of the structures generated, such as lattice constants and total energies, are not very precise, on this level. However, for many examples, we have been able to demonstrate that our approach is rather robust, also with respect to the parameters used in the potential functions: in all systems explored, those compounds already known from experiment have been reproduced in our search runs, and all additional local minima discovered have stayed stable during subsequent local optimizations, using ab initio methods. As the result of the global explorations, assuming T ¼ 0 K and suppressing the zero-point vibrations, a continuous hypersurface of potential energy as a function of the underlying configurations is obtained (see Figure 5, top). If one excludes quantum mechanical effects such as tunneling, all minima identified would correspond to stable configurations. For extended solids, these energy landscapes already give a rather realistic idea of the most stable compounds to

Figure 5

Schematic presentation of the continuous landscape of potential energy, as a function of configuration space (top). Locally ergodic regions, at finite temperature and with vibrations allowed, based on the same configuration space indicated by blue basins (bottom).

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be encountered when investigating that system experimentally, since for solid matter temperature dependence of the enthalpies of formation as well as entropic contributions commonly are too small to let low lying minima disappear, when switching to nonzero and somewhat elevated temperatures. One should however keep in mind that entropically stabilized and nonperiodic configurations will be missed: the first due to focusing only on the minima of potential energy, the second due to using finite simulation cells. Beyond employing our implementation at identifying compounds capable of existence, our approach also offers the opportunity to explore the barrier structure40 of the landscape of potential energy by applying ‘‘lid’’ or ‘‘threshold’’ techniques,41–44 or to spot entropically stabilized regions.45 Also an estimate for the transition probabilities between the minimum regions can be provided. In Figure 6 the results of such an investigation are displayed as a tree graph, including the low lying minima, the heights of the barriers between them, and the number of states associated with the respective minimum. In order to validate the results of the global searches based on ‘‘cheap’’ empirical potentials, and to refine the structures and energies obtained, ab initio local optimizations are subsequently performed for the most promising structure candidates. Using public domain Hartree–Fock and DFT codes, e.g. CRYSTAL-, WIEN-, or VASP-programs, the lattice parameters and the positional parameters are optimized in an iterative process (cf. Figure 4),46 and in order to get insights into the pressure dependence of the total energies of the candidate structures, these iterations are repeated for various fixed volumes. In Figure 7, the results of such an investigation are displayed for Li2CO3.47 The improvements in precision as achieved by switching to ab initio tools are significant; however, the overall structure of the energy landscape reflecting chemical configurations capable of existence remains virtually unchanged. Comparing the results obtained for the lattice constants to the experimental data reveals the well-known bias of the

Figure 6

Schematic illustration of a section of the energy landscape of chemical compounds, including a tree graph representation. M1–M3 are local minima, L(1)–L(4) energy lids applied and the grey areas indicate by their sizes the number of states associated with the respective minima within an energy slice.

32

Figure 7

Chapter 3

E(V) curves for predicted polymorphs of Li2CO3; the modification stable at ambient pressure, and a recently discovered high-pressure polymorph, are marked by (a) and (b) respectively.

methods employed. HF is overestimating, and DFT is partly underestimating the absolute values of the lattice vectors (cf. Figure 8). In numerous instances, and in many aspects, the predictive power of our implementation has been demonstrated. To give an example, among the E(V) curves for the low energy polymorphic structures of Li2CO347 (see Figure 7), the one corresponding to the experimentally known modification (at standard conditions) occurs at lowest energy, and, in addition, a predicted high pressure modification has been found independently in a DAC experiment.48 Although our general approach has proven to meet the objective of predicting targets for (solid state) synthesis in systems of realistic size, it is still lacking the desired generality with respect to the types of chemical bonding addressed. This is due to the type of potentials used for the energy calculations during the simulated annealing searches. The obvious, and in the final analysis only, way to overcome the present limitations is to resort to ab initio methods. Problems of a size typically encountered in inorganic solid state chemistry comprise 4–8 formula units and 2–16 atoms per formula unit, resulting in about 50–150 atoms per unit cell. Bearing in mind that one ‘‘simulated annealing’’ run commonly takes 105–107 total energy calculations until convergence, and about 1000 such runs are required at minimum to get close to a complete set of structure candidates for a given system, the computational resources required increase dramatically, by a factor of 100 per total energy calculation. One way, of course, to cope with the situation is to employ the highest possible computing power and to utilize the most efficient codes, e.g. parallel operating ones. If, in addition, the configuration space to be

The Deductive Approach to Chemistry, a Paradigm Shift

Figure 8

33

Calculated and experimental lattice constants for the lithium halides employing different (HF, DFT) ab initio methods.

searched is restricted to a small part of respective interest, global searches using high level ab initio energy calculations are tractable, as has been shown recently for the polymorphism of SiO249 and the high pressure modifications of CaCO3.50 In order to extend the scope of our tools, we have decided to stick to a two step approach, and to use fast ab initio codes allowing for an exhaustive global exploration of systems of real size, as far as possible, at an acceptable expense of precision, and to perform subsequent local optimizations of selected structure candidates on a high level of precision.51 In pursuing our objective of computationally exploring energy landscapes of chemical compounds, the combinatorial complexity, as pointed out above, has caught up with us. Global runs on a given system can easily end up with some tens of thousands of structure candidates,52 which have to be scrutinized for identical ones and for those already known from experiment. Such a daunting and tedious task can hardly be done by hand. Therefore, we have developed tools for an automated processing of the results.46 In order to avoid arbitrariness or prejudice, either with regard to translational or to rotational symmetry, no symmetry constraints are applied during the global exploration, and the thousands of candidates need to be analyzed regarding possible symmetries. Thus, in a first step, within given tolerances, a conventional unit cell and all symmetry elements present in the structures are elaborated,53 which is followed by an automatic determination of the space group.54 Finally, using a pattern recognition algorithm which allows isostructures to be matched,55 even if they are on a grossly different scale, independently of the functions of the

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constituting atoms, e.g. anionic or cationic, the file of predicted configurations is searched for equivalent ones and the resulting unique set is compared to databases of already known structures. Besides serving this purpose, the pattern recognition algorithm55 is an invaluable tool in screening data files like ICSD56 for related structures, and the structural ‘‘distance’’ between them.57

5.2

Explorations Based on Non-physical Criteria

Although it is clear that energy is the pivotal factor determining the structure of the energy landscape of chemical compounds, and thus their capability to exist, rather efficient non-physical tools have become available for reliably predicting not yet synthesized configurations, in distinct areas of chemistry. These latter approaches of anticipating structures rely on topological features that are known to be stable from experience, and in principle, all concepts in use for classifying structures in chemistry also have some predictive potential, or are at least suited for validating predicted structures.6 The classical crystal chemical rules as formulated by V.M. Goldschmidt58 continue to provide a sound basis for anticipating extended inorganic structures, which may be best demonstrated by the example of the perovskite family. L. Pauling’s ideas on bond-length/ bond-strength relationships59 have been further developed60 and have been used as cost functions in computationally generating structures for prescribed compositions and lattice constants.61 In a similar way, plausible structures can be derived by considering symmetry, space filling62 and types of packings63,64 as well as the restrictions imposed by the intrinsic properties of 3D, e.g. reflected by the space groups65 and Wyckhoff positions66 possible. In the area of intermetallic phases, the well established tools like structure field analysis,67 electron counts,68 and electronegativity (work function) balances69 are providing more general than specific, or practically useful, structural information. All these heuristic approaches suffer from lack of generality and of predictive power because they inherently tend to extrapolate existing knowledge. They most probably will also fail in achieving completeness. In some domains of chemistry, the inductive approach has led to an admirable ability in predicting unknown configurations. In these latter fields, the technique applied to deriving structures is based on linking rigid structural fragments. Such strategies are farthest developed in organic chemistry, where the local stereochemistries of the atom types commonly involved are perfectly known. Among the earliest systematic applications at exhaustively predicting (enumerating) members of families of compounds is the work by Po`lya who, in the example of alkanes, even tackled this task by formulating it mathematically.7 In this context, one might also refer to that part of Leonhard Euler’s work dealing with convex polyhedra which has enabled scientists to immediately enumerate all possible topologies of fullerenes,70 when they were discovered about two centuries later. Such general lines of action also apply to molecular inorganic chemistry of main group elements, however, with significantly less dependable outcomes. The virtually infinite size of extended solids makes it much more intricate to deal with them, based on the concept of structural increments. Historically, the

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building block approach was first applied to solid state structures by deriving the familiar and rather consistent systematics of silicates.71–73 Here again, the criteria of classification have been successfully employed for the purpose of prediction. For silicates, graph theoretical74 as well as graphical procedures75 have been used in enumerating (part of) the plethora of connectivities possible, using the SiO4-tetrahedron as a building block. In the light of the considerations presented here, one notices the limitations of these systematics, since the nearly infinite ability to mix-and-match such building blocks in ever-increasing unit cells is overwhelming. Applying such approaches to coordination polymers has furnished an impressive breakthrough.76,77 Selecting directionally fixed connectivities between bidentate or tridentate linkers, of defined lengths, and a central metal, has allowed correct prediction of open framework structures, including the accessible porosities and the correct translational symmetries. In some instances, the configuration space accessible has been restricted successfully, hardly leaving any path to escape to nondesired configurations, during the experimental realization.78,79 As another tool in the category of non-energy based approaches for systematically generating crystal structures, mathematical enumeration techniques, like graph theory, are continuing to attract attention.74,80–82 Recently, a procedure for enumerating crystalline networks, based on mathematical tiling theory,83 has been suggested.84 As a particular strength of this approach, the complete set of topologies possible is generated, at the boundary conditions given, and recently this procedure has been discussed in the context of enumerating possible zeolites.85 However, mathematical tilings of 3D do not necessarily correspond to stable chemical configurations. Moreover, they do not even show all experimentally possible configurations because the necessary definition of boundary conditions does not contain information on the chemical elements, which would be substantial because most of them show an appreciable variability with respect to chemical bonding, and thus local topology, among others also depending on the partner elements involved. Thus, they do not behave as uniformly as one would have to assume for a mathematically defined object. Nevertheless, (mathematical) enumeration of possible topologies is of significant momentousness in exploring chemical landscapes. Its usefulness strongly depends on the respective system under consideration. For the alkanes, Po`lya’s approach exactly reproduces all possible configurations,7 including isomers, and each individual representative corresponds to a minimum on the energy landscape, at least at 0 K. (At this point, we pass over the otherwise interesting question of to what extent representatives of high molar weight might be experimentally accessible.) However, with decreasing uniformity and rigidity of the stereochemistry of the chemical elements involved, the dependability and predictive power of purely mathematical enumerations will also decrease. Of course, topologies generated by enumeration, as well as those derived along other heuristic concepts (see earlier citations), may serve as valuable starting configurations for (local) computational relaxation, thus checking their kinetic stabilities, and determining their energies of formation, lattice constants, and further relevant physical data. Where applicable, such a

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procedure might speed up searches for synthetic targets. Random walks based on energy as a cost function, on the other hand, may save time because the new configurations obtained are already preselected with respect to energy and kinetic stability. When choosing the route most efficient for the particular purpose, one has to carefully weigh up the advantages and disadvantages of the competing approaches, often having to balance available computing time with accuracy and completeness of the result. The outcome would be strongly dependent on the chemical systems under investigation, and it is quite obvious that one would rarely invest the efforts of computational global optimization in, for example, planning syntheses in organic chemistry.

5.3

Experimental Exploration

The procedure traditionally followed in searching chemical systems for the stable compounds they host has been the experimental one by ‘‘trial and error,’’ based on some more or less rational concepts, which also constitutes the basis of the inductive approach to chemistry. Although such a way of discovering new materials has been rather successful in uncovering new compounds featuring exciting structures, bonding schemes or properties, scientists in this situation have always felt uncomfortable for the obvious lack of control and understanding. However, from a pragmatic point of view, it clearly might appear attractive to let the chemical systems of interest find their stable configurations on the energy landscape on their own. This is the basic idea underlying the ‘‘multisample concept,’’86 or in modern terms the ‘‘highthroughput techniques.’’ Basically, all these strategies have in common a parallel processing of many samples of different compositions. Presently, this field of experimental materials research is in full blossom, and numerous pertinent reviews have become available.87–89 In the context of this essay, it is interesting to examine whether, and under what conditions, high-throughput syntheses really meet the challenging requirement of being suited to exhaustively exploring chemical landscapes. Taking YBa2Cu3O7
x, as an example, a rough estimate6 immediately demonstrates that an intractable number of B1080 (parallel) experiments would be needed to ensure that one encounters the target compound as one of the products, if the unrestricted configuration space of 86 stable elements were to be searched. Irrespective of the precision of this estimate, it is obvious that the discovery of new materials cannot rely upon high-throughput techniques alone. However, the multisample concept of processing many starting material mixtures in parallel has proved its particular strength in optimizing known functional materials, like phosphors90 or heterogeneous catalysts,91 by applying a fine meshed grid of compositions. For the purpose of selecting new materials with desired properties straightforwardly, the single-sample concept has been suggested as a new tool for combinatorial chemistry.8,9,12,13 Here, multicomponent mixtures of solids are subjected to reactions at high temperature, and the products obtained are directly screened, with respect to a predefined property like ferromagnetism or superconductivity, utilizing the same property for the separation of the

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functional material from undesired by-products. However, there are some doubts as to whether the full chemical variability can be explored along such a route because the macroscopic grain sizes and the normally low diffusion coefficients of cations in oxides, for example, will most probably impede all compositions to be realized.

6 Relating ‘‘Configurational’’ and ‘‘Thermodynamic’’ Spaces The energy landscape of chemical compounds as discussed thus far is based on atomic configurations, defined by the types and numbers of atoms involved, and their positional vectors.27–29 The behaviour of real chemical systems, however, is controlled by the thermodynamic variables of state, i.e. by the pressure (volume) and temperature conditions applied, and the concentrations of the constituent elements. In the case of (global) thermodynamic equilibrium, setting these variables to certain values uniquely fixes the state of the system under consideration, i.e. its complete phase content. One should recall that structural information is neither needed for, nor extractable from, such a description. Furthermore, global equilibrium implies that we are considering essentially infinitely long timescales. Since it is our declared goal to provide generally applicable tools allowing for directed chemical syntheses, we can not restrict our considerations to thermodynamically stable entities only, but have to include kinetically stable states of matter that are thermodynamically metastable on finite timescales as well. The equilibrium states are commonly documented in phase diagrams that represent projections onto the space of the thermodynamic variables of state of those parts of the (hyper)spaces of free enthalpy attributed to locally ergodic minimum regions of a given system that represent equilibrium states, at respective p, T, xi conditions (cf. Figure 9). Including metastable states requires us to consider the full (hyper)spaces of free enthalpy for all the phases in a chemical system that are kinetically stable. In principle, the latter can be derived from the configuration space6 by determining all regions on the energy landscape that are locally ergodic on the timescale tobs, for temperature T.28 In particular, this means that lattice vibrations including zero point vibrations need to be taken into account. The lattice vibrations will sample all configurations within the amplitudes of vibration, thus generating an ensemble of configurations among which the system fluctuates, depending on the conditions given. Raising the temperature would enable transport and thus structural transformations to take place, and all configurations that are not kinetically stable will decay. As a result, we encounter discrete, locally ergodic minimum regions comprising a larger number of configurations constituting states that are all populated at the thermodynamic boundary conditions given (cf. Figure 5). Such regions can encompass one or many local minima; they represent a macroscopic thermodynamic ensemble, the state functions of which can be calculated according to the prescriptions of statistical thermodynamics. If applied to crystalline solids,

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Figure 9

Chapter 3

Three component phase diagram at constant temperature and pressure, displaying surfaces of free enthalpies for three different locally ergodic regions. In blue: solid solution A/B/C, red: melt A/B/C, green: surface of free enthalpy for a metastable solid solution.

deriving the free enthalpy associated with a locally ergodic region, starting from the potential energy (Epot), requires only a few, nevertheless intriguing, steps. To the enthalpy, H ¼ Epot+pV, the entropy terms have to be added. In this case, these are basically due to the phonon densities of states, and the configurational contributions, if many local minima contribute to the locally ergodic regions. By evaluating these quantities for various temperatures and pressures, the surface of free enthalpy can be scanned as a function of the thermodynamic variables for a given locally ergodic region of the energy landscape that can be associated with a prototype (ideal) structure.6 A given chemical system will exhibit many different locally ergodic regions, one of them representing the global minimum of the free enthalpy. Two borderline types of behaviour may be considered. For very long observation times, all locally ergodic regions will merge into a globally ergodic region, and thermodynamic equilibrium will be reached. At the opposite extreme, all configurations associated with local minima are assumed to survive for the time of observation, thus constituting metastable states. For the latter scenario, it is possible to determine the free enthalpies associated with locally ergodic regions R as a function of p, T, xi, and thus phase diagrams including metastable states of matter can be constructed. Of course, now the full function GR(p,T,xi) (i ¼ 1
n) restricted to such a region R needs to be considered, and the elegant way of densely

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displaying the information on equilibrium states by projecting G onto the subspace of the variables p, T, xi (i ¼ 1
n) is no longer viable. It should be noted that, as a highly welcome ‘‘spin-off’’, all relevant thermodynamic entities like enthalpies, entropies, and specific heats are supplied. The thermodynamic data generated along this procedure provide a sound basis for predicting and assessing the (meta)stability of targets for chemical synthesis. Moreover, they are suited to complement and validate the conventional phase diagrams. Thus far, the various thermodynamic details known for a given system are fitted together using the elegant CALPHAD approach,92,93 which, however, only affords interpolation and consistency checks of the experimental data. A missing thermodynamically stable compound inevitably leads to a seriously wrong phase diagram. Here, implementing data from computational chemistry, accessible as described above, would improve the reliability significantly. In special cases, the construction of yet unexplored, or experimentally inaccessible, regions of the phase diagrams has turned out to be rather straightforward, employing our approach.97–99 For most of the ternary combinations of the alkali halides, AX/A 0 X, the high temperature regions of the phase diagrams are well investigated while the low temperature parts have proven to be experimentally inaccessible, due to slow kinetics. Here one might encounter miscibility gaps or stable and metastable, respectively, ordered crystalline compounds. Since the contributions from the phonon densities of state to the entropy of reaction between two very much related alkali metal halides, e.g. NaCl and LiCl, can be neglected to first order,97 the configurational entropy constitutes the essential contribution that determines whether a solid solution or an ordered compound is thermodynamically stable. The task has been tackled by first exploring the landscape of the ternary systems AX/ A 0 X globally. The resulting configurations are then examined and attributed to locally ergodic regions, according to their potential energies. Some of these minimum regions represent thermodynamically stable or metastable individual ternary structures, others constitute representative configurations of solid solutions. For the latter, the enthalpies of formation have been calculated, using various supercell descriptions. Finally, as the only contribution to the entropy of reaction, the configurational entropy is computed, and the free enthalpies of the various ordered compounds and solid solutions are obtained. The approach and the results are illustrated in Figures 10 and 11, for two exemplary cases. As one can see, quite reasonable results have been achieved, without using any input from experiment. For NaCl/LiCl, the miscibility gap is modeled properly, also meeting the two experimental points, while for CsI/LiI as another realistic scenario, metastable and stable ternary structures prevail.

7 Exploring Routes of Synthesis For the second of the two steps minimally required to enable directed chemical synthesis, one has to provide synthetic procedures that would allow approaching

40

Figure 10

Chapter 3

Procedure followed in predicting phase diagrams. Global search for structure candidates (top), calculation of DHf based on the supercell approach (middle), and free enthalpies calculated for three temperatures (bottom).

The Deductive Approach to Chemistry, a Paradigm Shift

Figure 11

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Phase diagrams for quasi-binary systems NaCl–LiCl (top) and CsI–LiI (bottom), solidus/liquidus parts from experimental data, treated by the CALPHAD approach, low temperature parts predicted by the procedure displayed in Figure 10.

the target of synthesis, as selected from the pool of configurations capable of existence.6 Such processes are dynamic in nature, and time as well as temperature play a crucial role. Therefore, one can no longer resort to virtual spaces and hypothetical boundary conditions. Instead, appropriate paths of synthesis can only

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be identified on the landscape of free enthalpy associated with all starting material phases, all possible intermediates and the reaction products, including the targeted compound as well as the competing alternatives. In principle, such paths will become available as a spin-off when establishing the landscapes of free enthalpy through global exploration of the energy landscape of chemical systems. However, it is quite obvious that such an enterprise would create a new dimension of complexity, which will probably not be overcome in the near future. Also, in this respect of identifying viable routes allowing for rationally addressing a specific synthetic task, the various fields of chemistry differ conspicuously in maturity. In particular, in organic chemistry, and some related areas of molecular chemistry, one has arrived at almost full control. Typically, the regio-, topo- or enantioselectivities are achieved by functionalizing the starting material molecule, thus introducing reactive sites in an otherwise inert backbone, and allowing one to run the reactions under kinetic control. Although topotactic reactions can be regarded as conceptual counterparts in solid state chemistry, those approaches of molecular chemistry do not apply to the synthesis of extended solids. Here, only in some very limited circumstances can one synthesize a targeted solid compound quite straightforwardly. If one restricts the configuration space and the thermodynamic variables of state accordingly, one might encounter a situation where the target compound is thermodynamically stable, and, according to Gibbs’ phase rule, the only condensed phase that would survive. In this instance, it is sufficient to provide (thermal) activation in order to allow the reaction to proceed to the desired (thermodynamically stable) product. In instances where also the reaction proceeds in thermodynamic equilibrium, the underlying mechanisms, e.g. interdiffusion, chemical transport or crystallization from a molten mixture of starting materials, have been successfully modeled. However, such a strategy would not apply to the synthesis of metastable compounds. The only way to purposefully enter a particular locally ergodic region of the energy landscape is to generate supercritical nuclei of the desired compound, and to let them grow. As judged by its general validity and applicability, this latter stratagem constitutes the counterpart of the control of molecular synthesis by deliberate functionalization. The intriguing processes involved in nucleation100 across the relevant time and length scales, however, will be extremely hard to master: from gaseous, liquid or solid feedstocks of the starting materials, nuclei form while passing through ill defined dynamic states of preorganization and subcritical ensembles. In particular, coping with the population dynamics101 of all the intermediate states involved constitutes a real challenge for computational chemistry, and, in the next step, also for experiment. As another complication, the supercritical nuclei may undergo structural phase transitions while growing. It is quite obvious that gaining rational control of the structure directing steps of nucleation and growth is a task ahead of us of herculean dimensions. What can be done right now, is neither sufficient nor satisfying. One might follow the ideas of W. Ostwald and M. Vollmer concerning the preferred nucleation of metastable compounds from supersaturated feedstocks,17,102–104 or one could vary the external boundary conditions (temperature, pressure or degree of

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supersaturation) during nucleation. Such measures would allow the outcomes of the experiments to be influenced, but cannot be regarded as tools of control. Approaches presently within reach are based on seeding, either homogeneously or heterogeneously. In particular, heterogeneous seeding appears to be promising. For a target compound predicted to be metastable, one could inspect the low indexed and most densely occupied lattice planes and look for known crystalline compounds displaying a lattice plane that matches one of those of the target compound, with respect to lattice spacings as well as polarities.6,105 If such a compound has been identified, the target compound could be synthesized by epitaxial growth (heterogeneous nucleation), and the material obtained could be harvested and used in further experiments for homogeneous seeding.

8 The Deductive Approach to Discovery of Materials Exhibiting Desired Properties The impetus of chemical synthesis is nourished from two principally different sources. On one hand, there is the academic world, striving for gain in knowledge, on the other the industrial one, interested in creating competitive products of high added value. Therefore, it is well understandable that, driven by the prospects of potential industrial applications, there has always been a strong demand for high efficiency in improving known and discovering new materials. Being able to intentionally generate new materials with predefined properties seems to be a particularly strong desire, as indicated by the wordings ‘‘tailoring’’ or ‘‘designing’’ materials or drugs, frequently used in this context. However, as explained in the previous chapters, all chemical configurations capable of existence are predetermined by natural laws, with respect to all structural details and to their properties, as well. Thus, the only action that can be taken by the chemist or materials scientist is to discover a material or to search a chemical system for useful materials. This can be done experimentally or computationally. Inevitably, the invention of any new material proceeds through the discovery of at least a metastable compound, which can be examined for certain properties in a subsequent step. Thus, what we call the ‘‘rational’’ or ‘‘deductive’’ approach to materials discovery proceeds in a oneway fashion, from identifying a (meta)stable compound to its properties.106 Here, we restrict ourselves to addressing computationally assisted materials discovery (see Figure 12). The first part is related to predicting structures and stabilities of new solids by the procedures that have been extensively discussed in Chapter 11. Next, based on the structures identified, all relevant properties can in principle be calculated or at least estimated.107 It has to be admitted, however, that the accuracy currently achievable varies for different properties, and in certain cases, like superconductivity, the respective property cannot be predicted at all, up to now. Of course, this fundamental scheme need not be followed step by step. Instead, one might take short cuts, and, for instance,

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Figure 12

Chapter 3

Rational approach to materials synthesis (flowchart).

directly use the raw configurations, as obtained from the global search, for property screening. As another way of accelerating the process of realizing a specific property, the desired score has been included next to energy as a second rank cost-function during the random search of a selected configurational space.108 However, we regard dropping stable compounds, just because they lack the desired property, an unfortunate step. We rather prefer to collect all stable configurations and to add them to the thesaurus of chemical compounds. This would feature the advantage of having the configurations available at screening for another property that might become relevant at a later time. This approach as outlined above is rather fundamentalistic, and structured for long term and general issues. In many instances there is some pressure on developing a material with a certain property profile, within a time limit. The directedness needed might be achieved by resorting to analogies, data mining or optimizing known systems.

9 The Quintessence Representing the multitude of all known and still unknown chemical compounds on an energy landscape points the way to a deductive treatment of chemistry, quite in contrast to the inductive approach thus far preferably followed in this discipline. A rather simple scenario results if one resorts to the hypothetical conditions of T ¼ 0 K, and the zero-point vibrations suppressed. Then for each imaginable configuration the energy can be calculated. The resulting continuous (hyper)surface of potential energy is directly related to the configuration space, and each minimum of the landscape corresponds to a stable configuration at T ¼ 0 K, and vice versa. Admitting finite temperature and pressure, i.e. under

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realistic conditions, for a given observation time all unstable configurations will decay, while the (meta)stable ones constitute locally ergodic regions, corresponding to a particular macroscopic thermodynamic state. Depending on the thermodynamic boundary conditions applied, one of these regions corresponds to the thermodynamically stable state of the system under consideration, while the (numerous) remaining minimum regions represent metastable ones, exhibiting a wide spread of life times. Applying the procedures of statistical thermodynamics, free enthalpies for each locally ergodic region can, in principle, be derived, and phase diagrams including metastable states can be constructed. Such physically realistic energy landscapes, exhibiting numerous locally ergodic regions, offer a firm foundation for dealing with virtually all aspects of chemistry, on a rational basis. Since the sufficient and necessary precondition for any chemical compound to exist is that it belongs to a locally ergodic region, without any exception all manifestations of chemical matter are covered, and consequently the diverse fields of preparative chemistry are unified and can be dealt with on a comparable footing. The versatility of the energy landscape concept is becoming immediately obvious, when comparing the tools developed for its exploration. Ranking them according to the degree of control they provide, and the correctness of the underlying principles, one would mention, in ascending order, (1) experimental exploration by trial-and-error, (2) experimental exploration based on analogies and other heuristic concepts, (3) structure prediction based on crystal chemical rules, (4) local computational optimization of starting configurations as derived by analogy and other empirical knowledge, (5) computational structure determination of already synthesized compounds, utilizing experimental input (e.g. lattice constants or composition), (6) global optimization using non-physical cost functions, (7) structure prediction by mathematical enumeration and tiling, (8) global optimization with empirical energy as the only cost function, and (9) global optimization at the ab initio level. Each of these general approaches has its own justification. To which one a scientist gives preference, he will decide guided by pragmatism. To give two examples, an unexplored intermetallic system would probably be approached experimentally by systematically scanning a certain field of parameters, while for oxoborates one would prefer using enumeration techniques relying on trigonal-planar or tetrahedral building blocks. However, all concepts that do not rely on the correct physical description using energy as the only parameter of control, principally suffer from bias, in various respects. On the other hand, the serious weak spot of a fully physical description is also quite obvious; it is the giant complexity of the chemical world that cannot be coped with satisfactorily, at least using the tools currently available. However, the energy landscape of chemical matter inherently offers two ways of reducing complexity. Firstly, the minima develop a hierarchy with respect to stability. The low lying ones, if also surrounded by high barriers, will be discovered first, experimentally as well as computationally, while the less stable, shallow minima will be much more intricate to explore. This will constitute a big challenge for future work in improving the respective theoretical and experimental tools. Secondly, the configuration space can be easily

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divided into subspaces of interest. However, even for an elemental or binary system, its full exploration with high accuracy would embody a tremendous effort. It is our conviction that the concept of the energy landscape of chemical matter, based on a correct physical description and thus pointing the way towards treating chemistry deductively, will be the most sustainable approach to successfully address the issue of synthesis planning.

References 1. J.M. Thomas, R. Raja and D.W. Lewis, Angew. Chem., Int. Ed., 2005, 44, 6456. 2. J.A. Ballantine, J.H. Purnell and J.M. Thomas, European Patent 31252 (27.07.1984). 3. D.W. Lewis, D.J. Willock, C.R.A. Catlow, J.M. Thomas and G. J. Hutchings, Nature, 1996, 382, 604. 4. J.W. Couves, J.M. Thomas, D. Waller, R.H. Jones, A.J. Dent, G.E. Derbyshire and G.N. Greaves, Nature, 1991, 354, 465. 5. J.M. Thomas and G.N. Greaves, Science, 1994, 265, 1675. 6. M. Jansen, Angew. Chem., Int. Ed., 2002, 41, 3747. 7. G. Po`lya, Z. Kristallogr., 1936, 93, 415. 8. J. Hulliger, M.A. Awan, B. Trusch and T.A. Samtleben, Z. Anorg. Allg. Chem., 2005, 631, 1255. 9. J. Hulliger, L. Dessauges and T.A. Samtleben, J. Am. Ceram. Soc., 2006, 89, 1072. 10. A.A. Vertegel, K.V. Tomashevich, Yu.D. Tretyakov and A.J. Markworth, Mater. Lett., 1998, 36, 102. 11. F.J. DiSalvo, Pure Appl. Chem., 2000, 72, 1799. 12. J. Hulliger and M.A. Awan, Chem.— Eur. J., 2004, 10, 4694. 13. J. Hulliger and M.A. Awan, J. Comb. Chem., 2005, 7, 73. 14. M.N. Sokolov, P.A. Abramov, A.L. Gushchin, I.V. Kalinina, D.Y. Naumov, A.V. Virovets, E.V. Peresypkina, C. Vicent, R. Liusar and V.P. Fedin, Inorg. Chem., 2005, 44, 8116. 15. P.K. Byers and F.G.A. Stone, J. Chem. Soc., Dalton Trans., 1991, 1, 93. 16. C.H. Winter, W.R. Veal, C.M. Garner, A.M. Arif and J.A. Gladysz, J. Am. Chem. Soc., 1989, 111, 4766. 17. D. Fischer and M. Jansen, J. Am. Chem. Soc., 2002, 124, 3488. 18. P. Coveney and R. Highfield, Frontiers of Complexity. The Search for Order in a Chaotic World, Ballantine Books, Inc., New York, 1995. 19. M.R. Garey and D.S. Jahnson, Computers and Intractability, W.H. Freeman and Co, San Francisco, 1979. 20. F. Bacon, Novum Organum, in the Philosophical Works of Francis Bacon, Routledge, London, 1905. 21. R.M. Cahn, Historische und philosophische Aspekte des Periodensystems der Elemente, HYLE Publications, Karlsruhe, 2002.

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22. R. Scholder and W. Klemm, Angew. Chem., 1954, 66, 461. 23. R. Hoppe, High Oxidation States in Fluorine Chemistry, in Inorganic Solid Fluorides, ed. P. Hagenmuller, Academic Press, 1985, p. 275. 24. B.G. Mu¨ller, Angew. Chem., 1987, 99, 1120. 25. W. Massa and D. Babel, Chem. Rev., 1988, 88, 275. 26. M. Jansen and J.C. Scho¨n, Angew. Chem., Int. Ed., 2006, 45, 3406. 27. J.C. Scho¨n and M. Jansen, Angew. Chem., Int. Ed., 1996, 35, 1286. 28. J.C. Scho¨n and M. Jansen, Z. Kristallogr., 2001, 216, 307. 29. J.C. Scho¨n and M. Jansen, Z. Kristallogr., 2001, 216, 361. 30. M. Jansen, Wege zu Festko¨rpern jenseits der thermodynamischen Stabilita¨t, in Vortra¨ge. N 420. Hrsg.: Nordrhein-Westfa¨lische Akademie der Wissenschaften, Westdeutscher Verlag, Opladen, 1996. 31. R. Hoppe, Fortschr. Chem. Forsch., 1966, 5, 213. 32. W.E. Dasent, Non-Existent Compounds – Compounds of Low Stability, Dekker, New York, 1965. 33. S. Bo¨cker and M. Ha¨ser, Z. Aorg. Allg. Chem., 1995, 621, 258. 34. P. Kroll and R. Hoffmann, Angew. Chem., 1998, 110, 2616. 35. A.Y. Liu and M.L. Cohen, Science, 1989, 245, 841. 36. J.C. Scho¨n and M. Jansen, Predicting structures of compounds in the solid state by the global optimization approach, in Pauling’s Legacy-Modern Modelling of the Chemical Bond, ed. Z.B. Maksic and W.J. Orville-Thomas, Elsevier, Amsterdam, 1999, 103. 37. J.C. Scho¨n and M. Jansen, Structure prediction and determination of crystalline compounds, in Inorganic Chemistry Highlights, ed. G. Meyer, D. Naumann and L. Wesenmann, Wiley-VCH, Weinheim, 2002, 55. 38. S.M. Woodley, P.D. Battle, J.D. Gale and C.R.A. Catlow, Phys. Chem. Chem. Phys., 1999, 1, 2535. 39. C.M. Freeman and C.R.A. Catlow, J. Chem. Soc., Chem. Commun., 1992, 2, 89. 40. M.A.C. Wevers, J.C. Scho¨n and M. Jansen, J. Phys. A: Math. Gen., 2001, 34, 4041. 41. P. Sibani, J.C. Scho¨n, P. Salamon and J.O. Andersson, Europhys. Lett., 1993, 22, 479. 42. J.C. Scho¨n, H. Putz and M. Jansen, J. Phys.: Condens. Mater., 1996, 8, 143. 43. J.C. Scho¨n, Beri. Bunsen-Ges. Phys. Chem., 1996, 100, 1388. 44. M.A.C. Wevers, J.C. Scho¨n and M. Jansen, J. Phys.: Condens. Mater., 1999, 11, 6487. 45. J.C. Scho¨n, M.A.C. Wevers and M. Jansen, Z. Anorg. Allg. Chem., 2004, 630, 156. 46. J.C. Scho¨n, Z. Cancarevic and M. Jansen, J. Chem. Phys., 2004, 121, 2289. 47. Z. Cancarevic, J.C. Scho¨n and M. Jansen, Z. Anorg. Allg. Chem., 2006, 632, 1437. 48. A. Grzechnik, P. Bouvier and L. Farina, J. Solid State Chem., 2003, 173, 13.

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49. R. Martonak, D. Donadio, A.R. Oganov and M. Parrinello, Nat. Mater., 2006, 5, 623. 50. A.R. Oganov, C.W. Glass and S. Ono, Earth Planet. Sci. Lett., 2006 241, 95. 51. K. Doll, J.C. Scho¨n and M. Jansen, Phys. Chem. Chem. Phys., submitted. 52. Z. Cancarevic, J.C. Scho¨n and M. Jansen, Phys. Rev. B, 2006, 73, 2241141. 53. R. Hundt, J.C. Scho¨n, A. Hannemann and M. Jansen, J. Appl. Crystallogr., 1999, 32, 413. 54. A. Hannemann, R. Hundt, J.C. Scho¨n and M. Jansen, J. Appl. Crystallogr., 1998, 31, 922. 55. R. Hundt, J.C. Scho¨n and M. Jansen, J. Appl. Crystallogr., 2006, 39, 6. 56. ICSD-Fiz-Karlsruhe, Inorganic Crystal Structure Database, 2005, http:// icsdweb.fiz-karlsruhe.de 57. A. Hannemann, J.C. Scho¨n and M. Jansen, unpublished. 58. V. M. Goldschmidt, Naturwissenschaften, 1926, 14, 477. 59. L. Pauling, J. Am. Chem. Soc., 1929, 51, 1010. 60. I.D. Brown, Acta Crystallogr., Sect. A, 1973, 29, 266. 61. J. Pannetier, J. Bassas-Alsine, J. Rodriguez-Carvajal and V. Caignaert, Nature, 1990, 346, 343. 62. F. Laves, Theory of Alloy Phases, American Society for Metals, Cleveland, 1956. 63. M. O’Keeffe and B.G. Hyde, Structure and Bonding, Springer, Berlin, 1985, 61, 77. 64. A. Vegas and M. Jansen, Acta Crystallogr., Sect. B, 2002, 58, 38. 65. U. Mu¨ller, Acta. Crystallogr., Sect. B, 1992, B48, 172. 66. I.D. Brown, Acta Crystallogr., Sect. B, 1992, B48, 553. 67. P. Villars, K. Mathis and F. Hulliger, Environment Classification and Structural Stability Maps, in The Structure of Binary Compounds, ed. F.R. de Boer and D.G. Pettifor, Elsevier Science Publishing B.V., 1989, 1. 68. W. Hume-Rothery, R.E. Sallman and C.W. Haworth, The Structure of Metal and Alloys, Institute of Metals, London, 1969. 69. A.R. Miedema and A.K. Niessen, Cohesion in metals—transition metal alloys, in Cohesion and Structure, ed. F.R. de Boer and D.G. Pettifor, North-Holland, Amsterdam, 1988, vol 1. 70. P.W. Fowler and D.E. Manolopoulos, An Atlas of Fullerenes, Dover Publications, 2007. 71. F. Liebau, Structural Chemistry of Silicates, Springer, Berlin, 1985. 72. J.V. Smith, Chem. Rev., 1988, 88, 149. 73. S. V. Krivovichev, Cryst. Rev., 2004, 10, 185. 74. S.J. Chung, T. Hahn and W.E. Klee, Acta Crystallogr., Sect. A, 1984, 40(Suppl. S), C212–C212. 75. F.C. Hawthorne, Acta Crystallogr., Sect. A, 1983, A39, 724. 76. O.M. Yaghi, M. O’Keeffe, N.W. Ockwig, H.K. Chae, M. Eddaoudi and J. Kim, Nature, 2003, 423, 705.

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77. C. Mellot-Draznieks, S. Girard, G. Ferey, J.C. Scho¨n, Z. Cancarevic and M. Jansen, Chem.–Eur. J., 2002, 8, 4103. 78. O.M. Yaghi, H. Li, C. Davis, D. Richardson and T.L. Groy, Acc. Chem. Res., 1998, 31, 474. 79. G. Ferey, Science, 2005, 309, 2040. 80. G. Thimm, Z. Kristallogr., 2004, 219, 528. 81. G. Thimm and B. Winkler, Z. Kristallogr., 2006, 221, 749. 82. M.M.J. Treacy, I. Rivin, E. Balkovsky, K.H. Randall and M.D. Foster, Microporous Mesoporous Mater., 2004, 74, 121. 83. H. Heesch, U¨ber Raumteilungen, Nachr. Ges. Wiss. Go¨ttingen, 1934, 35. 84. O.D. Friedrichs, A.W.M. Dress, D.H. Huson, J. Klinowski and A.L. Mackay, Nature, 1999, 400, 644. 85. J.M. Thomas and J. Klinowski, Angew. Chem., Int. Ed., 2007, 46, 7160. 86. J.J. Hanak, J. Mater. Sci., 1970, 5, 964. 87. J.R.G. Evans, M.J. Edirisinghe, P.V. Coveney and J. Eames, J. Eur. Ceram. Soc., 2001, 21, 2291. 88. R.B. van Dover and L.F. Schneemeyer, Macromol. Rapid. Commun., 2004, 25, 150. 89. H. Koinuma and I. Takeuchi, Nat. Mater., 2004, 3, 429. 90. J. Wang, Y. Yoo, C. Gao, I. Takeuchi, X. Sun, H. Chang, X.–D. Xiang and P.G. Schultz, Science, 1998, 279, 1712. 91. T. Zech, J. Klein, S.A. Schunk, T. Johann, F. Schu¨th, S. Kleiditzsch and O. Deutschmann, High Throughput Analysis: A Tool for Combinatorial Materials Science, ed. R.A. Potyrailo and E.A. Amis, Kluwer Academic/Plenum Publishers, 2003, 491. 92. N. Saunders and A.P. Miodownik, CALPHAD: A Comprehensive Guide, Pergamon, Oxford, New York, 1998. 93. Examples for further approaches to the calculation of phase diagrams can be found in references 94 to 96. 94. D. de Fontaine, MRS Bull. August 1996, 15. 95. G. Kern, G. Kresse and J. Hafner, Phys. Rev. B., 1999, 59, 8551. 96. M. Yu. Lavrentiev, N.L. Allan, G.D. Barrere and J.A. Purton, J. Phys. Chem. B., 2001, 105, 3594. 97. J.C. Scho¨n, I.V. Pentin and M. Jansen, Phys. Chem. Chem. Phys., 2006, 8, 1778. 98. I.V. Pentin, J.C. Scho¨n and M. Jansen, J. Chem. Phys., 2007, 126 124508. 99. J.C. Scho¨n, I.V. Pentin and M. Jansen, J. Phys. Chem. B, 2007, 111, 3943. 100. J. Bernstein, R. Davey and J.–O. Henck, Angew. Chem., Int. Ed., 1999, 38, 3440. 101. M. Santoro, J.C. Scho¨n and M. Jansen, Phys. Rev. E, submitted. 102. W. Ostwald, Z. Phys. Chem., 1897, 22, 289. 103. M. Volmer, Kinetik der Phasenbildung, Dresden, Steinkopff, 1939. 104. D. Fischer and M. Jansen, Angew. Chem., Int. Ed., 2002, 41, 1755.

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105. J.C. Scho¨n, T. Dinges and M. Jansen, Z. Naturforsch., B, 2006, 61, 650. 106. M. Jansen and J. C. Scho¨n, Nat. Mater., 2004, 3, 838. 107. S.M. Arnold, MRS Bull., 2006, 31, 1013. 108. A. Franceschetti and A. Zunger, Nature, 1999, 402, 60.

CHAPTER 4

Future Energy Materials: Three Challenges for Materials Chemistry PETER P. EDWARDS AND VLADIMIR L. KUZNETSOV Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, OX1 3QR, UK

1 Prologue: Personal Perspective by P.P. Edwards I first met Professor John Meurig Thomas on Wednesday, 4th of July 1979, shortly after I arrived at the University Chemical Laboratories at Cambridge. I had recently been appointed to a University Demonstratorship in Inorganic Chemistry by Professor Jack Lewis, whose support in offering me that position enabled my first step towards an academic career. I still vividly remember that first meeting. John instantly relayed to me his admiration for my research activities at Cornell; indeed, such was the high level of glowing praise in the first few minutes that the thought did cross my mind as to whether he had gotten this Edwards mixed up with another Edwards – the highly distinguished physicist, Professor Sam F. Edwards of the Cavendish Laboratory! But no, John was indeed talking of ‘‘Edwards the Chemist’’ as he recounted my research on metal–ammonia solutions and the metal–insulator transition. I feel sure that this snapshot of my own first experiences with John will draw resonances with all colleagues in this volume, for that episode reflects one of his most endearing gifts; namely, his extraordinary, natural ability to make one feel that one’s own research is unquestionably the most outstanding in the field (even though he may be a participant himself in that field!). To simply label this as a gift does not do justice to John’s munificence in that regard – this is a gift honed by his prodigious memory, given credence by his vast command of the scientific literature and, above all, it reflects his genuine desire to support and encourage others. And such praise, delivered always with a command of

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the English language second-to-none, and this ‘‘despite English being his second language’’! (Professor A.D. Buckingham, this volume). So began our long friendship and fruitful association in many areas of research. Our initial collaborations in the early 1980s centred on the study of alkali metal clusters incarcerated within zeolite hosts.1–3 We were both united in our zeal for what at the time we termed condensed matter chemistry, and we had both been touched by the deep insights and enthusiasms of members of the Baker Laboratory, Cornell University where we had, at different times, worked earlier. In the early 1980s John and colleagues had achieved a series of breakthroughs in the real-space imaging of zeolites by high resolution electron microscopy, coupled with their major advances in the structural elucidation of this important class of solids.1 My own interests centred primarily on the nature of the metal–insulator transition,4 divided metals and excess electrons in metal solutions. Coupled with the arrival in Cambridge of an advanced Electron Spin Resonance (ESR) spectrometer (another development which linked us to colleagues at Cornell) we were ideally placed to examine the intriguing reports in the literature concerning alkali metal ion clusters, e.g. Na431 effectively trapped within the intrazeolite cavities. There collaborative studies led to the discovery of a large family of ionic clusters – both paramagnetic and diamagnetic – and a model which united the ‘‘dissolution’’ of alkali metals in both liquid polar solvents, and dehydrated zeolites.1 In Figure 1 we show a schematic representation of the ionic Na431 and the hypothetical Na8 neutral cluster, within zeolite Y, with the accompanying colour changes (white - pink - blue) as more and more alkali metal is ‘‘dissolved’’ within the zeolite (in essence, a solid solvent). John has also always been closely involved with the development of what we now term materials chemistry and we take the opportunity here to offer some personal reflections on the evolution of the subject and its importance and impact in the field of energy materials.

2 Materials Chemistry: Awakening During the past two decades, materials chemistry has attracted worldwide interest as a new and important discipline,6 reflecting the confluence of the streams of chemistry, materials science, physics and engineering. (The interface between chemistry, biology and materials science has meant that much has now been learnt from the principles of structure–function relation in mineralised biological materials; for brevity this interfacial area is not covered here.) The emergence of the subject has resulted from the gradual evolution of chemistry in materials science,6c,7 most notably by the development of modern high technologies. However, the discovery of totally new and unexpected materials and physical phenomena have signalled a step-change in the development of, and attitudes to, materials chemistry. No more so, in our view, than the discovery of high-temperature superconductivity in ceramic cuprates.8

Future Energy Materials: Three Challenges for Materials Chemistry

Figure 1

53

Schematic representation of the formation of the alkali clusters Na431 and Na8 located within the alpha and supercages of Na1-zeolite Y. Samples are also shown of dehydrated zeolite Y, with increasing sodium (metal) concentration. Taken from Emsley and Edwards.5

So it was in 1986 with the breathtaking discovery8 by J.G. Bednorz and K.A. Mu¨ller of high-temperature superconductivity in the La–Ba–Cu–O system – surely one of the greatest ever scientific discoveries. It is interesting to reflect here on the impact of this particular discovery on the development of materials chemistry. From its genesis in 1911 up to 1987, superconductivity was not a property of interest to the vast majority of solid-state chemists. Materials exhibiting this remarkable natural phenomenon were deemed to be in the realm of physicists, materials scientists and engineers. In addition the theory of superconductivity was invariably couched in terms of wave vectors, phonons, reciprocal space, etc.; chemists, of course, are much more at home in ‘‘realspace’’ models, rather than those required in reciprocal or momentum space!9 Bednorz and Mu¨ller’s epoch-making advance changed all this; the phenomenon of superconductivity now truly entered the field of chemistry in a dramatic and unexpected fashion. In our view this was a pivotal event in the true dawning of the field of materials chemistry. For only through a detailed knowledge of the science of preparative solid state chemistry could pure, single-phase materials be synthesised and studied in a definitive fashion. Thus,

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the use (and development) of sol–gel routes, chimie douce chemistry, electrochemistry, microwave chemistry, combustion or self-propagating methods, have been employed in the quest for pure, single-phase superconducting materials. Here, chemists made substantial and often unique contributions to the development of the subject. However, parenthetically, one must note that by far the vast majority of new oxide superconductors were discovered by solid state physicists (with a most notable exception being the mercurocuprate superconductors).10 One of the many quite unexpected and remarkable features to emerge from the discovery of ‘‘High Tc’’ materials is the apparent degree of chemical complexity at first presented by these compounds – for example the highest Tc oxide11 HgBa2Ca2Cu3O71d is synthesized12 from four constituent oxides! Equally unexpected was the unprecedented degree of ‘‘chemical control’’ of High Tc now extant13 – as exemplified by the so-called septenary cuprates (Tl1
yPby)Sr2(Ca1
xYx)Cu2O7. In Figure 2 we show the composition dependence of Tc in the septenary cuprates, where small changes in chemical composition can transform an antiferromagnetic insulator to a high-temperature superconductor (Tc Z 100 K), to a metallic, but non-superconducting oxide. It is important to stress also that these complex septenary phases can be

Figure 2

The chemical control of high-temperature superconductivity. The temperature and composition dependence of the superconducting transition temperature in the (Tl1
yPby)Sr2(Ca1
xYx)Cu2O7 septenary system. Original data from Liu et al.13a redrawn by W.Y. Liang (Cavendish Laboratory).

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synthesised as phase pure materials across the entire 3-D compositional range of chemical stoichiometry.13 And all of this against the backdrop of the absence (even today) of a universally accepted theory of the phenomenon of hightemperature superconductivity.8,14 One should perhaps draw an analogy here with the great traditions of chemistry exploring the far reaches of the Periodic Table – prior to the advent of any accepted atomic theory of the chemical elements. Another development, highlighted by J.M. Thomas,1 has been the possibility of major advances catalysed by new (or improved) instrumentation, noting ‘‘the growth of chemistry depends at least as much on the availability of tools and techniques as it does on concepts and theories’’. The High-Tc era surely was pivotal in the realisation of the importance of determining structure in real space – by electron microscopy – and not only in reciprocal space by electron diffraction. Importantly, J.M. Thomas had earlier laid the foundations for these advances by significant breakthroughs in the real-space imaging of zeolites by high-resolution electron microscopy. An interesting example of this approach in the field of superconductivity is given in Figure 3 which shows the result of a high resolution electron microscopy (HREM) investigation and model image simulation of the High-Tc superconductor Bi21xSr2Ca1
x Cu2O81d; the recorded HREM image is the tip of a crystal just one lattice vector wide, clearly showing in cross section the metal layers in the order (Bi– Sr–Cu–Ca–Cu–Sr–Bi).15 These two examples drawn from our own contributions to the field of High-Tc hopefully reveal just how closely chemistry from

Figure 3

HREM image of the tip of one crystal a single c-lattice vector wide together with a calculated image (left) and derived crystal structure inset. The image clearly shows in cross section the metal layers in the order (Bi–Sr–Cu–Ca– Cu–Sr–Bi); from W. Zhou et al.15

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1987 became woven into the science of materials – we would propose that this is an object lesson in the development of materials chemistry.

3 Materials Chemistry: Intellectual Foundation of the Field Over the past 20 years the intellectual foundation of the field we now recognise as materials chemistry has begun to take shape and the area has achieved international recognition as a forefront area of modern science. C.N.R. Rao6d,16 has defined the subject as follows: ‘‘materials chemistry deals with structure, response and function and has the ultimate purpose of developing novel materials or understanding structure/property relations and phenomena related to a wide range of materials. Structure and synthesis are integral parts of the subject, and they are fully utilised in the strategies for tailor-making materials with desired and controlled properties. What distinguishes the subject from pure solid state chemistry is the ultimate materials objective’’. As noted in Section 2, the origin of materials chemistry results from the evolution of chemistry in materials science, catalysed by the discoveries and advances in High-Tc superconductors and other materials (e.g. C60 – Buckminsterfullerene) and the development of modern techniques and technologies. The broad field of materials chemistry has been routinely defined around the understanding and control of three basic elements: 1. The structure and composition of materials, encompassing the constituent chemical elements and their atomic arrangements over a wide range of length scales. 2. The synthesis of materials; the process by which the particular arrangement of atoms in the material is achieved. 3. The properties of materials; be they electronic, magnetic, optical, dielectric, thermal, adsorptive or catalytic. In our view – the basic theme of this commentary – we now add to this list another critically important element in the make up of the developing field, namely; 4. The performance of materials. These four elements hold close similarities with the now-established benchmark definition of materials science itself.7 This is a clear reflection of the fact that chemists, materials scientists and physicists are beginning to be drawn into a healthy mutual alliance centred around the science and technology of materials. But what distinguishes materials chemistry from its sister multidisciplines is the fact that the basis of materials chemistry of course has its roots in the chemistry of the elements – we label this here as The Periodic Table of Materials.

Future Energy Materials: Three Challenges for Materials Chemistry

Figure 4

57

The four elements of materials science and engineering7 together with a representation of the Periodic Table of the elements as backdrop: here the height of the elemental columns is a measure of each element’s electronegativity (adapted from F.J. DiSalvo19). The aim here is to show the confluence of materials chemistry and materials science.

Chemical periodicity and the Periodic Table find a natural interpretation in the detailed electronic structure of the atom.17 Given that the chemical and physical properties of a material derive from the constituent atoms’ electronic configuration, and especially the configuration of its least highly bound electrons, it follows that ultimately the properties of any material must necessarily be interpreted in terms of the electronic structure of its constituent atoms.18–20 Thus, the concept of chemical periodicity, central to the study of inorganic chemistry,17 may in future find a place in the systematisation and rationalisation of chemical and physical properties of materials – the materials chemistry of the elements – as a natural focal point for the disciplines of chemistry, physics, materials science and engineering.17–21 Figure 4 is an attempt to illustrate the confluence of the various elements of the Periodic Table of Materials. We now give a brief overview of three areas of energy materials in which we attempt to illustrate how materials chemistry can impact upon the intrinsic performance of these materials.

4 Energy Materials Energy is now established as a global area of critical research where innovative materials chemistry will play a pivotal role in meeting the needs of the

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future. Energy generation, consumption, storage and security of supply will continue to be major drivers for the subject. There exists, in particular, the urgent need for new materials for next generation energy-generating and energy-storage devices. Many limitations on, for example, hydrogen storage, fuel cells and photovoltaics are mainly materials and materials chemistry limited. We highlight three areas of activity where materials chemistry is already a significant element of world-wide activities.

4.1

Transparent Conducting Oxides

Electrical conduction in a transparent solid is the rarest form of conductivity.24 The seemingly contradictory properties of close-to ‘‘metallic’’ conductivity in a material simultaneously exhibiting almost complete ‘‘nonmetallic’’ or insulating optical transparency, form the basis of numerous critical applications in contemporary and emerging energy technologies.25–27 In many respects, the basic underpinning materials physics can be reasonably well set out.28–31 Thus, the high conductivity of the prototypical transparent conducting oxide (TCO) SnO2-doped In2O3 (ITO) derives from the presence of shallow donor or impurity states located close to the host (In2O3) conduction band, the donor states produced via chemical substitution of Sn41 for In31 or by the presence of oxygen vacancy impurity states28 in In2O3
x (Figure 5). At room temperature the proximity of such impurity states to the host conduction band ensures facile thermal ionisation into the band, developing, ultimately, a degenerate, itinerant electron gas of current-carrying electrons which also gives rise to far-infrared (Drude-like) absorption and high electrical conductivity, but at the same time the fundamental host band gap is left intact, i.e. the electrically conductive material remains optically transparent in the visible region. For doping levels above that set by the Mott criterion for metallisation32 (i.e. n4nc B1018–1019 cm
3 for ITO), one can consider ITO to have a full valence band and a host 5s conduction band partially filled by a degenerate free-electron gas. Within the free-electron framework, the optical and transparent properties are described quite well30 by a simple Drude model (the addition of a strongly frequency-dependent electron scattering time t for Fermi surface electrons provides a secondary level of sophistication; see below). Conduction electron scattering in ITO thin films occurs from a number of different scattering centres such as impurities, phonons, defects. However, the most dominant process originates from the presence of the very impurity ions responsible for the doping, and this sets the scope for any interpretation of the limiting performance of such materials.29 A calculation29 of electrical resistivity due to this ionised impurity scattering provides an important lower limit to the attainable intrinsic resistivity (equivalently an upper limit to the conductivity) of a TCO material; this is a key performance indicator. The results of such calculations are shown as a function of carrier concentration in Figure 6. The various data points are experimental values taken from the literature for In2O3, ZnO and SnO2 doped systems and

Future Energy Materials: Three Challenges for Materials Chemistry

Figure 5

59

Schematic energy-band model for SnO2 doped In2O3 for small x (insulating) and large x (metallic) materials. Taken from P.P. Edwards et al.30 as modified from the work of Fan and Goodenough.28

the solid line represents a calculation29 of electrical conductivity due to ionised impurity scattering. This sets the upper performance limit to the attainable conductivity at any particular carrier concentration. The similar behaviour observed in all three different systems strongly supports the idea that this impurity scattering mechanism dominates the electrical resistivity (conductivity) in all three cases. In an attempt to enhance the electrical performance by moving to higher conductivities, the films need to be doped to increase the carrier concentration n (Figure 6). However, increases in n will ultimately lead to a degeneration of the optical performance of the material. Specifically, although low resistivity (high conductivity) is highly desirable for the performance of the materials and associated devices, the free-electron density in ITO cannot be increased beyond 2  1021 cm
3 without pulling the plasma frequency into the red end of the visible spectrum – making it highly reflecting.30,31 Furthermore, any increase in the film thickness causes reduced transparency due to a finite skin depth, d, emerging as the electrical conductivity increases with doping.31 However, above the plasma frequency for nE1021 cm
3, we find that increases in the electron mobility, me, cause increases in d, and

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Figure 6

Chapter 4

Literature data for the three oxide systems In2O3, ZnO and SnO2 chosen as displaying high electrical conductivity at any particular carrier concentration. The theory plot ( — ) is from the results of the calculation of conductivity (resistivity) due to ionised impurity scattering. Adapted from J.R. Bellingham et al.29

(approximately) d p me. This leads to the important conclusion that if me in TCOs can be increased/maintained above the present state-of-the-art values of around 50 cm2 V
1 s
1 to values of around 100 cm2 V
1 s
1, then very substantial increases in transparency are possible whilst not sacrificing high electrical conductivity. The clear message is that electron mobility increases will have a significant impact on the key performance characteristics of the power efficiency of light emitting diodes and related devices. The materials physics of the problem can be captured in the following way.30,31 For ITO in the limit of high electron density (i.e. n\1021 cm
3) and high mobility (i.e. me ¼ etm*\20 cm2 V
1 s
1) then the plasma frequency, op, can be written
1=2 ne2 op ffi e1 e0 m where eN is the high frequency dielectric pffiffiffi constant and e0 is the vacuum permittivity. Hence in this limit op / n and thus is determined almost exclusively by n and not t (i.e. the electron mobility, me, recalling that me ¼ et0/m*). This means that for a film to be non-reflective to light of free space l0 (i.e. opope/l0; where l0 is the dc wavelength of light) the electron density n must satisfy 4p2 e0 m ; no m0 e2 l20

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61

which for ITO yields the criterion nðcm
3 Þo1:6  1021 =l0 Therefore for efficient transmission of the whole visible spectrum (including red light of free space wavelength up to 780 nm) the electron density should not exceed 2.6  1021 cm
3. To allow a suitable ‘‘safety margin’’ in our performance criteria, we consider a plasma wavelength of just over 1 mm (i.e. in the near-infrared) in which case the tolerable free electron density (nmax) is ca. 1.5  1021 cm
3. Clearly, values of n>nmax are severely detrimental to the performance transparency of the films owing to the plasma edge creeping into the red part of the visible spectrum. The power reflection coefficient R at an interface between a thick ITO layer and free space can be calculated from the expression;  pffiffiffiffiffiffiffiffiffiffi  eðoÞ
12   RðoÞ ¼ pffiffiffiffiffiffiffiffiffiffi   eðoÞ þ 1 An exact calculation30,31 of R using the full Drude formula for e(o) is shown in Figure 7 as a function of n for various incident wavelengths in the visible spectrum. For frequencies below the plasma frequency, R is close to 1 (i.e. it is very reflective) with a small amount of electromagnetic absorption within the material (approximately equal to (1
R)). These results have important consequence if high transparency films with very low sheet resistance are required. This is therefore best achieved by increasing the electron mobility in

Figure 7

The power reflection coefficient at the interface between a thick TCO layer and free space as a function of free electron density n for various incident wavelengths in the visible spectrum. Taken from P.P. Edwards et al.30

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preference to n, since increasing n leads to increasing conductivity and a proportionate reduction in the skin depth as the plasma frequency is increased. This significant result illustrates the origin of me, so as to maximise this quantity. Importantly, the intrinsic limit of me appears to be set around 90 cm2 V
1 s
1 as a result of ionised impurity scattering.29 In summary, therefore, the task before the materials chemists in an attempt to enhance the intrinsic performance of TCOs is centred around the following three performance criteria for new materials: 1. Maintain the condition of a wide band gap (43.5 eV) and low inter-gap absorption. 2. Maintain the condition of moderate electron densities (n \1021 cm
3) – but certainly not higher than this value. 3. Maintain the highest possible carrier mobility; ideally, close to, or above, 100 cm2 V
1 s
1. Thus, the materials chemistry of such systems now has to be closely allied with materials physics to understand how electron mobilities can be understood, tuned and thereby enhanced. It now appears that such performance limits are regularly being approached in known materials and this fact29 sets the scene for major activities in materials chemistry if new systems are to be discovered which optimise/enhance properties of these important technological materials.

4.2

Thermoelectric Materials: Optimisation Challenges

Thermoelectric devices are unique heat engines, in which charge carriers serve as the working fluid.33,34 They offer a reliable, fully solid-state means of cooling and electrical power generation. One of the important aspects of thermoelectric power generation technologies is the ability to provide electrical power from heat gradients, which could be used in the recovery of large amounts of waste heat and harvesting it into usable electrical energy. However, relatively low energy conversion efficiency (typically around 5%) of thermoelectric materials limits their performance and range of applications to niche areas in refrigeration and power generation technologies. Although ingenious engineering approaches could extend the scope of applications, the real economic impact of this technology depends on the availability of more efficient thermoelectric materials. For a given temperature range of operation the thermoelectric performance of a material is determined by its thermoelectric figure-of-merit Z ¼ a2s/k, where a is the Seebeck coefficient, s is the electrical conductivity and k is the thermal conductivity (k ¼ ke+kL, where ke and kL are the electronic and lattice contributions, respectively). Generally, the transport coefficients of a semiconductor material are more complex than, for example, its optical or magnetic properties, which are very closely related to the band structure. All established thermoelectric materials possess a complex energy-band structure; their degree of degeneracy is high and invariably several types of bonding are involved

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together with several co-existing carrier scattering mechanisms. All this, together with often strong anisotropy of the charge and heat transport properties and the limitations of existing theoretical models, makes any theoretical predictions regarding the thermoelectric figure-of-merit prohibitively difficult. However, several useful conclusions on the directions in search for novel improved materials can be drawn from the classical Fermi–Dirac statistics and a simplified model of electron scattering. In this approximation the figureof-merit Z can be expressed in terms of the fundamental transport parameters through the so-called materials parameter b: 3=2

Z / b¼mðm* Þ

kL

where m is the carrier mobility and m* is the density of state effective mass.33,34 Therefore, to maximise the Z value, a material with large effective masses of charge carriers, high carrier mobility and a low lattice thermal conductivity is required. Although significant improvements in the properties of state-of-the-art thermoelectric materials have been achieved over the last 30 years, the maximum value of the dimensionless figure-of-merit ZT for the best thermoelectric materials has remained around unity over the temperature range of 100–1200 K (Figure 8), even though thermodynamics does not place any upper limit on ZT. The major problem with increasing the performance of thermoelectric materials is a close interrelation of all parameters that determine the figureof-merit, which makes it impossible to control these variables independently. For example, both the Seebeck coefficient and electrical conductivity depend on

Figure 8

Temperature dependence of the dimensionless figure-of-merit ZT of some n-type thermoelectric materials. The figure was compiled using various literature sources.35–37

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Chapter 4

the carrier density. However, while maximisation of the Seebeck coefficient requires a reduction in carrier concentration, the electrical conductivity is, to the first approximation, directly proportional to carrier concentration. While the numerator a2s in the expression for Z can be maximised through optimal doping (typically around 1019–1020 carriers/cm3), the reduction in the lattice thermal conductivity is limited by the ability to reduce its lattice contribution to a minimum value. The remaining contribution to the thermal conductivity is electronic and proportional to the electrical conductivity (kepTs) according to the Wiedemann–Franz law. An attempt to maximise the materials parameter m(m*)3/2/kL also inevitably leads to a compromise concerning the value of carrier mobility m and effective mass m* since these two transport parameters tend to be inversely proportional to each other. The optimisation of these contradictory requirements represents the major challenge for materials chemists in the development of new thermoelectric materials with maximum dimensionless figure-of-merit ZT. The goal for a high-performance thermoelectric material is to maintain the typical electronic properties of a conducting crystal while, at the same time, having the thermal conductivity that is characteristic of an amorphous solid. There are several ways to effectively reduce the lattice thermal conductivity and simultaneously maintain and optimise the electronic structure of a material. One of the traditional ways to reduce the lattice thermal conductivity of a compound is to increase the point defect scattering of phonons by formation of solid solutions. This method has been successfully employed over the last 40 years for the development of all state-of-the-art thermoelectric materials. However, this approach has serious limitations since a high concentration of atomic point defects also inevitably leads to a decrease in the electrical conductivity and carrier mobility.34 One of the recently developed approaches to identify materials with high thermoelectric performance is to search for solids that allow freedom to modify and tailor their crystal structure. Among the most promising novel thermoelectric materials are two classes of inclusion compounds with large voids in their crystal structures, which are occupied by loosely bound atoms. The loose bonds between the filling atoms and the oversized voids cause anharmonic vibrations of the former, which greatly lowers the lattice thermal conductivity value while weakly affecting the electronic transport. This results in a combination of high electrical conductivity typical of crystalline materials and, at the same time, a very low lattice thermal conductivity typical of an amorphous solid.38–40 The temperature dependence of ZT values of two n-type materials that belong to skutterudite (Yb0.2Co4Sb12) and clathrate (Ba8Ga16Ge30) families of inclusion compounds are presented in Figure 8. Another promising class of novel materials with potentially large ZT values is nanostructured materials such as quantum dot superlattices and quantum wire arrays.35,41,42 In these materials the new variable of size becomes available and it is possible to change dramatically the density of electronic states, allowing new opportunities to vary a, s and k independently. In addition, the introduction of many interfaces offers the opportunity to increase phonon

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65

scattering more than electron scattering so that the electrical conductivity is not decreased much while the thermal conductivity is much reduced by interface scattering processes. Significant improvement of the thermoelectric figure-ofmerit ZT in such materials is expected when the structural parameters are carefully tuned to increase the carrier density of states while decreasing the thermal conductivity due to phonon-boundary scattering or phonon spectrum modification. This opens up new possibilities in designing novel materials with enhanced thermoelectric performance.

4.3

Hydrogen Storage Materials

Reducing or eliminating our dependency on petroleum in transportation system is a major objective worldwide. The combination of hydrogen and fuel cells in particular represents a key enabling technology for a future sustainable energy economy which has the potential to revolutionise our energy system offering cleaner, more efficient alternatives to today’s energy technologies (fuel cells are projected to have an energy efficiency twice that of internal combustion engines).43 However, current hydrogen storage systems for vehicular transportation are generally inadequate to meet the necessary driving range requirements of some 300 miles (500 km) without significant intrusion into cargo or passenger space. For this reason effective hydrogen storage is viewed as one of the most critical barriers to the widespread use of hydrogen fuel cells as an energy carrier.44,45 Typically 4–7 kg of hydrogen need to be stored on board a vehicle to come close to the automotive requirements of a range of some 500 km as a benchmark. There are four major options for on board hydrogen storage systems, variously: (1) compressed hydrogen (typically at pressures between 35–70 MPa at room temperature); (2) cryogenic liquid hydrogen (operating at 20–30 K at pressures of 0.5–1 MPa); (3) solid state absorbents, ranging from metal hydrides and complex hydrides to high surface area porous materials; and (4) hybrid solutions, incorporating at least two of the above technologies. These various options are described in detail elsewhere;43,45 here we are concerned solely with the issue of solid state hydrogen storage materials. The objective44–49 is to tune a materials’ properties to obtain reversible hydrogen storage materials with properties between those of the cryogenic adsorbents (typically which have hydrogen binding enthalpies of between 4–20 kJ mol
1 H2) and the intermetallic and complex hydrides which have bond enthalpies of between 30–55 kJ mol
1. An ideal solid state hydrogen storage material should, for economic, environmental and user-friendly reasons satisfy the following performance criteria:44 (i) Hydrogen storage capacity; probably a minimum of 6.5 wt% of abundance of hydrogen and at least 65 g L
1 of hydrogen available from the material.

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(ii) Heat of formation (DH) must be reduced as low as thermodynamically possible. An ideal hydrogen storage material releasing hydrogen via thermal decomposition should be (possibly, only slightly) thermodynamically stable at ambient conditions (1 atm, 25 1C). The desorption should not be associated with a substantial heat release. For this reason, and the necessity for incorporation into a PEM fuel cell for vehicular applications, therefore, we would require; (iii) Operating temperature of less than 70 1C; in essence, this would require only a small amount of heat to evolve molecular hydrogen. (iv) Reversibility of the thermal absorption/desorption cycle. Of course, there are other key factors of cost, toxicity, confinement, etc., but we focus here on the points above since they illustrate some of the major initial materials chemistry challenges. A summary of existing volumetric hydrogen densities and gravimetric hydrogen contents for various hydrogen storage materials is illustrated in Figure 9. We note that there is, as yet, no material known to meet simultaneously all of these, and other important criteria. Within Figure 9, we have also attempted to highlight – in ‘‘broad-brush’’ terms – which of the three chemical species H0, H– and H1 (in the extreme) is potentially the better source of hydrogen; at least for the thermally activated process for hydrogen generation from the storage material. Simple atomic-number-based calculations reveal the obvious fact that only the light chemical elements can be strictly entertained44 if criterion (i) is to be met (this conclusion is clearly relaxed for stationary applications where FeTiH1.7 and multiphase Ti–Zr–V–Fe–Ni systems are highly effective stores; such a relaxation of wt% targets also forms the basis of current hybrid solutions to onboard storage of hydrogen). Accepting these multi-various performance requirements for the perfect storage material now allows us to set objectives for the materials science research as target values for a breakthrough storage compound. Currently, four major approaches are being pursued to achieve those design/performance parameters: 1. A combinatorial, high throughput approach designed to search for materials derived entirely from multinary combinations from ‘‘The Light Periodic Table’’ not previously investigated (also, taking guidance from the story of High-Tc (Figure 2)) where entirely new phases are observed beyond binary and ternary systems.50 2. The destabilisation of existing materials through alloy formation or more complex reaction schemes (see below): the aim here is to attempt to modify the thermal characteristics of materials, while at the same time to maintain a high weight percent for hydrogen storage.51 3. The cryo-adsorption of hydrogen on high-surface-area materials (e.g. activated carbons, zeolites or metal–organic frameworks).52

Future Energy Materials: Three Challenges for Materials Chemistry

Figure 9

67

The gravimetric and volumetric densities and corresponding specific energy and energy density of various hydrogen storage materials, compressed/liquid hydrogen and Li-ion batteries. The temperature data under the composition of some materials indicate the equilibrium temperature for hydrogen pressure 1 bar.

4. A hybrid solution combining low DH hydride approaches with a (moderately) high pressure compressed hydrogen design (e.g. operating at 30 MPa with a TiCrMn or related alloy).53 We use the second path, that is material destabilisation, to illustrate how this key performance parameter, Tdec, representing the temperature of thermal decomposition of a hydrogen storage material, can be modified; this is one of the most important practical parameters connecting both the thermodynamic and kinetic aspects of these materials. This leads us to a brief discussion on how Tdec, the decomposition temperature, may be understood and consequently tuned to optimise one performance parameter for a hydrogen storage material.44,54 Grochala and Edwards44 have presented thermodynamic arguments for understanding how the values of Tdec – certainly those for binary hydrides

68

Figure 10

Chapter 4

A correlation of the temperature, Tdec, at which thermal decomposition of binary hydrides MH2 to the constituent elements proceeds, and the corresponding standard redox potential of the Mn1/M0 redox pair in acidic aqueous solutions, E0. The ranges of the working temperatures for prototypical fuel cells are also shown. The E0 values for H2/2H
and H0/H
redox pairs are indicated. Taken from Grochala and Edwards.44

(MHn) – might correlate with the E1 value for the corresponding Mn1/M1 pair in aqueous solutions. Such a correlation is presented in Figure 10, where we show a plot of the experimental Tdec versus E1 data for a wide variety of binary hydrides. As one can discern from Figure 10, there is an excellent correlation between Tdec and E1 for a wide range of (seemingly) chemically disparate metal hydrides. This simple empirical correlation, having its roots in the thermodynamics of MHn formation, forms almost a ‘‘sorting map’’ of experimental systems. One can clearly see also the importance of Tdec in matching the operating temperatures of different types of H2 fuel cells, ranging from alkaline, through polymer electrolyte membrane, to solid oxide and molten carbonate cells. The monotonic behaviour of the Tdec versus E1 relationship for the binary

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hydrides has been rationalised on the basis of thermodynamic and theoretical arguments.44 Given the importance of Tdec, and its ‘‘matching’’ to the various operating temperatures of fuel cells, the notion (2 above) arises of ‘‘destabilising’’ high weight percent stores in an attempt to reduce decomposition temperatures of hydrogen storage materials.51 Many hydrides of the light chemical elements have DH values larger than the desired range of ca. 30–60 kJ mol
1 H2. Experimental and theoretical work is currently underway on destabilised systems by mixing chemical hydrides with other compounds. Destabilisation enables one to precisely modify the thermodynamics of dehydrogenation processes by substituting an energetically unfavourable reaction with another reaction involving the formation of different compounds in the dehydrogenated state. The formation of these compounds reduces the enthalpy of the dehydrogenating reaction and lowers the dehydrogenation temperature. An example55,56 of such an approach is illustrated in Figure 11 for the LiBH4+MgH2 destabilised system exhibiting decreases in the enthalpy of decomposition and the decomposition temperature by 23 kJ mol
1 H2 and 240 K, respectively, compared to pure LiBH4.56,57 In Figure 9 different colours are used for different hydrides to distinguish the dominant charge of hydrogen atoms. This general approach could facilitate the choice of pairs of hydrides for development of novel destabilised systems. Chemical reactions in the destabilised hydrogen storage systems often involve the formation of intermediate compounds, for example Li4BH4(NH2)3 in the LiBH4+LiNH2 system.58,59 Such intermediate compounds could significantly improve the kinetics of the dehydrogenation process. The number of possible destabilisation reactions significantly exceeds the number of existing chemical hydrides which offers a promising avenue for developing a viable hydrogen storage system. A number of destabilisation reactions have already been studied; some of the results are presented in Figure 12.

Figure

11

Reduction in the decomposition temperature of destabilised LiBH4+MgH2 system by 240 K compared to pure LiBH4.56

70

Figure 12

Chapter 4

Effect of destabilisation on gravimetric hydrogen density and hydrogen decomposition temperature (1 bar).60–64

This suggests that the most promising candidate for a destabilised system meeting the mobile storage requirements is ammonia–borane (Figure 12). Another key performance challenge for many destabilised systems is to achieve the reversibility of hydrogen storage, a particularly difficult challenge. In summary, stabilising the dehydrogenated state of a hydrogen storage material reduces the enthalpy of dehydrogenation, thereby increasing the equilibrium hydrogen pressure. Using this approach, the key performance parameter of Tdec can potentially be tuned to an extent finer than would be possible with individual materials. The strategy of ‘‘chemically controlling’’ Tdec by using alloying elements to form stable compounds or alloys upon dehydrogenation opens up real possibilities for increasing the equilibrium pressures of hydrogen-rich but strongly bound hydrides. We have focused on LiBH4 in combination with MgH2 in Figure 11, but as highlighted in Figure 12, a large number of destabilised reactions have already been studied. Recent developments with ammonia borane, NH3BH3, also highlight the great potential of this technique when applied to high weight percent, molecular compounds. Maintaining high weight percent hydrogen storage, whilst reducing Tdec and enhancing reversibility, will represent a major milestone in the area of materials

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chemistry; indeed this is widely recognised as a potential ‘‘show-stopper’’ for the transition to a hydrogen economy.

5 Epilogue: Materials Chemistry, a New Interdiscipline It is clear that materials chemistry is certainly now a multidiscipline in its own right – rich in opportunities and challenges, and no more so than in the area of energy materials research. We have outlined how the tetrahedron of synthesis/structure/properties/ performance (Figure 4) outlines a good basis for a description of, and a basis for, the continued development of the subject. We have attempted to illustrate the importance of a basic understanding of the fundamental science in three areas of energy materials, transparent conducting oxides, thermoelectric materials and hydrogen storage materials, which provides deep insights into the intrinsic performance limits of these systems. To take one example, the optical and electronic properties of transparent conducting oxides are crucial in limiting properties – and hence performance – of these materials. This example, hopefully, also illustrates that relatively simple models (here the free electron gas model of Drude) can be extremely useful in establishing performance limits. One should add that, despite the high sensitivity of a material’s properties on processing issues and conditions (grain boundaries, etc.), a material’s electronic structure is arguably the most important factor for understanding the unique interplay between chemical, electronic, magnetic, thermodynamic and kinetic properties of energy materials. In our discussion we have concentrated on performance as a new dimension for the emerging field, but of course implicit in this term are economic, environmental and other key societal factors (public acceptability of a new energy vector, e.g. hydrogen, etc.). The science-to-social need is a crucial part of the subject’s development; the place of materials research in the ‘‘economy’’ and well-being of society will be analysed as never before.7 Nevertheless, the core theoretical/experimental basis of the field is embedded in the tetrahedron shown in Figure 4, with the underlying parentage of the Periodic Table, as a key distinguishing feature of materials chemistry. The search for new materials must always be underpinned by such theoretical frameworks if new materials with improved combinations of properties are to be discovered – and utilised. The (part) title of our volume is centred around the descriptor ‘‘Turning points . . . ’’, the combination of words which is defined by the Oxford Dictionary of Current English65 (admittedly, only John’s second language) as ‘‘Crisis : or prove the contrary of what was intended’’. This was clearly not what was intended! Indeed, the outcome of this marvellous collection of articles honouring John Meurig Thomas is a vivid illustration of the corresponding definition of ‘‘crisis’’; defined from that same tome as ‘‘the decisive moment’’. We certainly

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do believe this is a decisive moment in the evolution and expansion of the subject into major new fields. John, in this same year as the 200th anniversary of Humphry Davy’s discovery of K and Na, and also the 150th anniversary of Michael Faraday’s pronouncement on gold in its metallic, divided state, we wish you a happy 75th birthday!

References 1. For an overview and personal account, see P.P. Edwards, P.A. Anderson and J.M. Thomas, Acc. Chem. Res., 1996, 29, 23. 2. P.P. Edwards, M.R. Harrison, J. Klinowski, S. Ramdas, J.M. Thomas, D.C. Johnson, and C.J. Page, J. Chem. Soc., Chem. Commun., 1984, 982. 3. M.R. Harrison, J. Klinowski, J.M. Thomas, D.C. Johnson, C.J. Page and P.P. Edwards, J. Solid State Chem., 1984, 54, 330. 4. The Metallic and Nonmetallic States of Matter, ed. P.P. Edwards and C.N.R. Rao, Taylor and Francis, London, 1984. 5. J. Emsley and P.P. Edwards, New Sci., 1987, 9, 32. 6. (a) Chemistry of Advanced Materials, ed. C.N.R. Rao, Blackwell, Oxford, 1994; (b) L.V. Interrante and M.J. Hampden-Smith, Chemistry of Advanced Materials, Wiley-VCH, New York, 1998; (c) X. Xiao, MRS Bull., 1996, 5; (d) C.N.R. Rao, Encyclopaedia of Physical Science and Technology, 3rd edn, Academic Press, Boston, USA, vol. 9, 2002, 181. 7. M.C. Fleming, Annu. Rev. Mater. Sci., 1999, 29, 1. 8. (a) J.G. Bednorz and K.A. Mu¨ller, Z. Phys. B., Condens. Matter, 1986, 64, 189; (b) J.G. Bednorz and K.A. Mu¨ller, Europhys. Lett., 1987, 3, 379; (c) K.A. Mu¨ller, J. Phys. Condens. Matter, 2007, 19, 251002; (d) A. Cho, Science, 2006, 314, 1072. 9. P. Day, J. Phys., 1999, 100. 10. S.N. Putilin, E.V. Antipov, O. Chmaissem and M. Marezio, Nature, 1993, 362, 226. 11. A. Schilling, M. Cantoni, J.D. Guo and H.R. Ott, Nature, 1993, 363, 56. 12. G.B. Peacock, I. Gameson, M. Slaski, W. Zhou, J.R. Cooper and P.P. Edwards, Adv. Mater., 1995, 7, 925. 13. (a) R.S. Liu, P.P. Edwards, Y.T. Huang, S.F. Wu and P.T. Wu, J. Solid State Chem., 1990, 86, 334; (b) R.S. Liu and P.P. Edwards, in Synthesis and Characterization of High Temperature Superconductors, ed. J.J. Pouch, S.A. Alterovitz, R.R. Ramafsky and A.F. Hepp, Trans. Tech. Publication Ltd, Switzerland, Mater. Sci. Forum, 1993, 130–132, 435. 14. A.S. Alexandrov and P.P. Edwards, Physica C, 2000, 331, 97. 15. W. Zhou, A.I. Kirkland, K.D. Mackay, A.R. Armstrong, M.R. Harrison, D.A. Jefferson, W.Y. Liang and P.P. Edwards, Angew. Chem. Int. Ed. Engl., 1989, 28, 810. 16. C.N.R. Rao, J. Mater. Chem., 1999, 9, 1.

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17. N.N. Greenwood and E.A. Earnshaw, Chemistry of the Elements, 2nd edn, Butterworth-Heinemann, Oxford UK, 1997. 18. A.H. Cottrell, Introduction to the Modern Theory of Metals, The Institute of Metals, London, 1988. 19. See also: F.J. DiSalvo (a) Science, 1990, 24, 649; (b) Solid State. Commun., 1997, 102, 79. 20. (a) R. Hoffmann, Solids and Surfaces: A Chemist’s View of Bonding in Extended Structures, VCH Publishers, Weinheim, Germany, 1988; (b) R. Hoffmann, An unusual state of matter, The Metallic State, University of Central Florida Press, Orlando, FL, 1987, 101. 21. H. Ehrenreich, Science, 1987, 235, 1029. 22. Advanced materials for energy storage, MRS Bull., 1999, 24. 23. Foresight review of how science and technology could contribute to better energy management of the future, Foresight Programme of the Office of Science and Innovation (UK), available online from: http://www.foresight. gov.uk/Energy/Reports/Mini_Energy_Reports/Energy.html 24. C. Kilic and A. Zunger, Phys. Rev. Lett., 2002, 88, 95501. 25. I. Hamberg and C.G. Granqvist, J. Appl. Phys., 1986, 60, R123. 26. C.G. Granqvist and A. Hultaker, Thin Solid Films, 2002, 411, 1. 27. Basic Research Needs for Solar Energy, Report of the Basic Energy Sciences Workshop on Solar Energy Utilization, 2005, Office of Science (USA), available online from: http://www.sc.doe.gov/bes/reports/list.html 28. J.C.C. Fan and J.B. Goodenough, J. Appl. Phys., 1977, 48, 3524. 29. J.R. Bellingham, W.A. Phillips and C.J. Adkins, J. Mater. Sci. Lett., 1992, 11, 263. 30. P.P. Edwards, A. Porch, M.O. Jones, D.V. Morgan and R.M. Perks, Dalton Trans., 2004, 2995. 31. A. Porch, D.V. Morgan, R.M. Perks, M.O. Jones and P.P. Edwards, J. Appl. Phys., 2004, 95, 4734. 32. P.P. Edwards and M.J. Sienko, Phys. Rev. B, 1978, 17, 2575. 33. A.F. Ioffe, Semiconductor Thermoelements and Thermoelectric Cooling, Infosearch, London, 1957. 34. H.J. Goldsmid, Electronic Refrigeration, Pion, London, 1986. 35. For comprehensive reviews of thermoelectric materials, see (a) T.M. Tritt (ed), Semiconductors and Semimetals, Academic Press, New York, 2001, 69; (b) CRC Thermoelectrics Handbook: Macro to Nano, ed. D.M. Rowe, CRC Press, Taylor & Francis Group, Boca Raton, London, New York, 2006. 36. G.S. Nolas, M. Kaeser, R.T. Littleton IV and T.M. Tritt, Appl. Phys. Lett., 2006, 77, 1855. 37. A. Saramat, G. Svensson, A.E.C. Palmqvist, C. Stiewe, E. Mueller, D. Platzek, S.G.K. Williams, D.M. Rowe, J.D. Bryan and G.D. Stucky, J. Appl. Phys, 2006, 99, 023708. 38. G.S. Nolas, J.L. Cohn and G.A. Slack, Phys. Rev. B, 1998, 58, 164. 39. A. Bentien, M. Christensen, J.D. Bryan, A. Sanchez, S. Paschen, F. Steglich, G.D. Stucky and B.B. Iversen, Phys. Rev. B, 2004, 69, 045107.

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40. V.L. Kuznetsov, L.A. Kuznetsova, A.E. Kaliazin and D.M. Rowe, J. Appl. Phys., 2000, 87, 7871. 41. L.D. Hicks and M.S. Dresselhaus, Phys. Rev. B., 1997, 47, 12727. 42. M.S. Dresselhaus, Y.-M. Lin, S.B. Cronin, O. Rabin, M.R. Black and G. Dresselhaus, in Semiconductors and Semimetals, vol. 71, ed. T.M. Tritt, Academic Press, New York, 2001. 43. (a) P. Hoffman, Tomorrow’s Energy: Hydrogen, Fuel Cells and the Prospect for Cleaner Planet, The MIT Press, Cambridge, MA, 2002; (b) National Hydrogen Energy Roadmap: US Department of Energy, November 2002, available online from: http://www1.eere.energy.gov/hydrogenandfuelcells/ pdfs/national_h2_roadmap.pdf; (c) S.G. Chalk and J.F. Miller, J. Power Sources, 2006, 159, 73. 44. W. Grochala and P.P. Edwards, Chem. Rev., 2004, 104, 1283. 45. For an excellent recent review of the motivation for hydrogen as a fuel in vehicular fuel cell applications, see R.V. Helmolt and U. Eberle, J. Power Sources, 2007, 165, 833. 46. L. Schlapbach (guest ed), Mater. Res. Bull., 2002, 675 and accompanying papers. 47. E. Tzimas, C. Filiou, S.D. Peteves and J.-B. Veyret, Hydrogen Storage: State-of-the-Art and Future Perspective, Mission of the Institute for Energy, European Commission, Directorate General Joint Research Centre, The Netherlands, ISBN 92-894-6950-1, 2003, available online from: http:// ie.jrc.cec.eu.int/publications/scientific_publications/2003.php 48. G. Sandrock and R.C. Bowman Jr., J. Alloys Compd., 2003, 794, 356. 49. F. Schu¨th, B. Bogdanovic´ and M. Felderhoff, Chem. Commun., 2004, 2249. 50. For a description of a current UK research programme, see D. Nikbin, Fuel Cell Rev., March 2006, 15. 51. For a recent overview of destabilised metal hydrides see S.V. Alapati, J.K. Johnson and D.S. Scholl, J. Phys. Chem. B, 2006, 110, 8769. 52. L.C. Rowsell and O.M. Yaghi, Angew. Chem., Int. Ed., 2005, 44, 4670. 53. Y. Kojima, Y. Kwaai, S. Towata, T. Matsunaga, T. Shinozawa and M. Kimbara, J. Alloys Compd., 2006, 419, 256. 54. W. Grochala and P.P. Edwards, J. Alloys Compd., 2005, 31, 404. 55. J.J. Vajo, S.L. Sheith and F. Mertens, J. Phys. Chem. B, 2005, 109, 3719. 56. G.L. Olson and J.J. Vajo, DoE 2006 Hydrogen Program Annual Review, Washington, DC, May 2006, 16. 57. M. Aoki, K. Miwa, T. Noritake, G. Kitahara, Y. Nakamori, S. Orimo and S. Towata, Appl. Phys. A, 2005, 80, 1409. 58. P.H. Chater, W.I.F. David, S.R. Johnson, P.P. Edwards and P.A. Anderson, Chem. Commun., 2006, 2439. 59. G.P. Meisner, M.L. Scullin, M.P. Balogh, F.E. Pinkerton and M.S. Meyer, J. Phys. Chem. B, 2006, 110, 4186. 60. Y. Nakamoto and S.-I. Orimo, J. Alloys Compd., 2004, 370, 271. 61. J. Lu, Z.Z. Fang and H.Y. Sohn, Inorg. Chem., 2006, 45, 8749. 62. Y.W. Choa, J.-H. Shima and B.-J. Lee, CALPHAD, 2006, 30, 65.

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63. S.V. Alapati, J.K. Johnson and D.S. Sholl, J. Phys. Chem. B, 2006, 110, 8769. 64. S.V. Alapati, J.K. Johnson and D.S. Sholl, Phys. Chem. Chem. Phys., 2007, 9, 1438. 65. Compiled by F.G. Fowler and H.W. Fowler, The Oxford Pocket Dictionary of Current English, 4th edn, Clarendon Press, Oxford, 1942.

CHAPTER 5

Structural Diversity and Potential Applications of Metal–Organic Coordination Polymers JIESHENG CHEN AND RUREN XU State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, People’s Republic of China

Through his academic career, Sir John Meurig Thomas has realized great achievements and contributions in a variety of areas. One of his main interests has been in solid materials which find applications in catalysis. Zeolites, a class of microporous crystalline compounds, are typical solid materials which have been widely used in catalysis as well as in adsorption, ion-exchange and separation. Sir John’s pioneering work1 in electron microscopic imaging of zeolites shed an enormous amount of light on the structures of these intriguing materials, and nowadays electron microscopies, especially transmission electron microscopy (TEM), have become important techniques for the elucidation of microstructures of known and unknown zeolite materials. Sir John, in association with C.R.A. Catlow and G. Sankar at the Royal Institution of Great Britain (RIGB) and a few other co-workers, also extensively used synchrotron radiation sources (mainly X-ray) to investigate the structures and active catalytic sites of zeolitic materials. Conventional zeolites are aluminosilicates with frameworks composed of Si, Al and O, and in the 1980s microporous aluminophosphates (AlPOs) and their derivatives were also synthesized. From the late 1980s, Sir John was involved in studies on synthesis, structural characterization and catalytic properties of new AlPOs and substituted AlPOs. His research work in this area carried out in collaboration with one of us (Ruren Xu) resulted in a number of highly cited publications.2,3 The other author of the current article (Jiesheng Chen) used to work at RIGB as a postdoctoral research fellow under Sir John from 1990 to 1994, and his research work was mainly focused on the exploration of substituted AlPO materials with new catalytic properties. At that time, not only J.S. Chen 76

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but also a number of other colleagues such as Y. Xu, R. Bell, P.A. Wright, S. Natarajan, G. Sankar, L. Marchese and R. Raja at RIGB were also involved in this research area. It should be particularly mentioned that one of the colleagues at that time, R.H. Jones, played an important role in the structural analysis of new AlPO materials. In fact, aluminophosphate compounds show vast diversity in structure as well. Not only three-dimensional (3D) frameworks but also 2D sheet and 1D chain structures of aluminophosphates4,5 have been obtained in the past, and the topologies and building-unit connections for 3D aluminophosphates vary enormously. Apart from conventional zeolites and microporous AlPOs, many other openframework compounds composed of elements other than Si, Al and P have also been synthesized, and their structural features are equally diverse. Notably, from the late 1990s a new class of porous crystalline compounds whose frameworks consist of metal centres and organic linkers have been discovered. These compounds are usually designated metal–organic framework (MOF) materials or metal–organic coordination polymers (MOCPs) in general.6,7 Three-dimensional framework MOCPs usually contain various channels, and these channels differ from those of zeolites in shape, size and adsorption properties. Many as-synthesized MOF materials accommodate guest molecules in their channels or pores, and these guest molecules may be the solvent of the synthetic system or templates used as for the synthesis of zeolites. The thermal stability of MOFs is lower than that of inorganic framework porous materials, and therefore their applications in conventional high-temperature catalysis are limited. However, in non-conventional fields the applications of MOFs have been showing great potential.

1 d-Block Metal Coordination Polymers Multidentate ligands containing two or more carboxylate groups on a benzene or a larger aromatic ring may easily connect metal ions into 3D coordination polymers. In a mixed solvent system of water and ethanol, Williams and coworkers synthesized a 3D framework porous compound [Cu3(TMA)2(H2O)3]n using benzene-1,3,5-tricarboxylic acid (TMA or BTC) and copper ions as reactants.8 This compound consists of [Cu2(O2CR)4] (R stands for aromatic ring) structural units which are interconnected to form a 3D channel system. The as-synthesized compound occludes guest water molecules in the channels, but these molecules may be removed through thermal treatment, or may be replaced by other guest molecules such as pyridine. This porous coordination polymer is thermally stable up to about 240 1C. 1,3,5,7-Adamantane-tetracarboxylic acid (denoted ATC) has four carboxylate groups and is also an ideal multidentate ligand. The reaction of this ligand with Cu21 under hydrothermal conditions (190 1C) results in a coordination polymer Cu2(ATC)
6H2O. This compound possesses large channels, and the guest molecules may be driven out without apparent change of the host framework. The compound exhibits excellent microporous adsorption

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0

00

properties. The reaction of 4,4 ,4 -benzene-1,3,5-triyl-tribenzoic acid (denoted BTB) and cupric nitrate in a mixed solvent of ethanol, dimethylformamide (DMF) and water at 65 1C for 1 day gives rise to an interwoven coordination polymer Cu3(BTB)2 (H2O)3
(DMF)9(H2O)2.10 This compound possesses channels with guest DMF and water molecules inside. Upon removal of the guest molecules, the compound exhibits excellent adsorption properties. The other common donor coordination atom besides O is N. Many aromatic rings may contain one or more N heteroatoms. These aromatic rings may interconnect one another to form larger ligands, and more than one N atom in the compound may participate in coordination to metal ions. 4,4 0 -Bipyridine (bpy) is an N-containing bidentate ligand widely used to form coordination polymers. For example, bpy has been used as a ligand and a series of framework compounds containing guest anions and water molecules have been prepared in the reaction system of copper ions and AF6-type anions (A ¼ Si, Ge and P). The framework structures of these compounds may be controlled by varying the guest anions.11 The framework geometry of porous 3D framework compounds formed by some N-containing ligands and metals is affected by the guest species in the channels to a great degree. Biradha and Fujita used 1,4,6-tris(4-pyridyl)triazine (denoted TPT) as the ligand, and synthesized the framework compounds [(ZnI2)3(TPT)2
5.5C6H5NO2] and [(ZnI2)3(TPT)2
5.5C6H5CN] from the solution of ZnI2 and nitrobenzene or cyanobenzene.12 It was discovered that in the presence of the guest nitrobenzene or cyanobenzene molecules, the whole framework of the host is swollen, whereas when the guest molecules are removed, the host framework apparently shrinks. The guest species in this compound may also be replaced by other molecules. Lin et al. used an axially chiral bridging ligand (R)-6,6 0 -dichloro-2,2 0 dihydroxy-1,1 0 -binaphthyl-4,4 0 -bipyridine, L, to construct homochiral porous MOFs.13 They obtained crystals of [Cd3Cl6L3]
4DMF
6MeOH
3H2O by slow diffusion of diethyl ether into a mixture of the ligand and CdCl2 in MeOH/DMF. The structure of this compound is a noninterpenetrating 3D network with very large chiral channels of B1.6  1.8 nm cross-section. It was revealed that the compound contains 54.4% void space that is accessible to guest molecules, and X-ray powder diffraction demonstrated that the framework structure was maintained upon removal of all the solvent molecules. Interestingly, Ti-(OiPr)4 can react with the chiral dihydroxy groups in the channels of the compound to afford a Lewis acidic material which is catalytically active for the addition of ZnEt2 to aromatic aldehydes to form chiral secondary alcohols. Some aromatic cyclic molecules not only contain N heteroatoms on the aromatic ring but also are attached by carboxylate group(s) on one or more of the carbon atoms. Therefore, these ligands can form coordination polymers through coordination of not only their N but also their carboxylate groups. Zhao et al. used pyridine-2,6-dicarboxylic acid (H2dipc) as ligand and synthesized a coordination polymer with an empirical formula [Ln(dipc)3Mn1.5(H2O)
nH2O] from Mn21 and rare-earth metal ions Ln31 (Ln ¼ Pr, Gd, Er) under

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14

hydrothermal conditions. This polymeric compound possesses 1D channels with a channel diameter of 0.6 nm, and the guest water molecules are distributed in the channels. Structural analysis indicates that these guest water molecules may be removed from the channels without collapse of the framework, and therefore, this polymer may be used as a microporous crystal molecular sieve. Because there are unpaired electrons on the framework metal ions, the compound also exhibits magnetic properties. Chiral porous coordination polymers have great application potential because they may be used as chiral catalysts, and it has been continually attempted to introduce chirality into coordination polymers with various approaches. A simple strategy is to use chiral ligands, and after the formation of coordination polymers the chirality is automatically introduced into the polymer with the ligand. Quitenine (6 0 -methoxyl-(8S,9R)-cinchonan-9-ol-3-carboxylic acid (HQA)) is a chiral ligand molecule. If quitenine is reacted with Cd(OH)2 in a hydrothermal system containing racemic 2-butanol, a host–guest coordination polymer Cd(QA)2 with chiral channels crystallizes.15 In the chiral channels of this compound there exist chiral 2-butanol guest molecules, suggesting that the chiral channels are able to separate the guest enantiomers. 9,9-Diethyl-2,7-bis(4pyridylethynyl)fluorene, and chiral 9,9-bis[(S)-2-methylbutyl]-2,7-bis(4pyridylethynyl)fluorene were also used to react with copper nitrate to prepare coordination polymers with grid channels. If the ligands are chiral, the coordination polymers obtained are also chiral.16 Another method of preparing chiral coordination polymers is to use special guest molecules as templates which may also induce the formation of chiral coordination polymers. Kepert et al. used ethylene glycol and propylene glycol as templates to induce the formation of metal benzene-tricarboxylate porous coordination polymers, the framework of which exhibits chirality.17 Through mixing of N-containing and O-containing ligands in the same reaction system, porous coordination polymers with two or more ligands may be obtained. Under this circumstance, each ligand contains either N or O, but the whole coordination polymer contains both N and O. Through solvothermal reaction (150 1C) of a 1D chain coordination polymer [Co(bpdc) (H2O)2]
H2O (bpdc ¼ bipyridine dicarboxylic acid) in a DMF solution, the chain structure is converted to a porous 3D framework compound [Co3(bpdc)3bpy]
4DMF
H2O.18 The solvent water and DMF molecules are accommodated in the channels as guests. It is interesting that the conversion from 1D to 3D is reversible. The channels of 3D framework compounds are shape-selective, and they may act as ideal sites for shape-selective catalysis. After reactions, the products may be isolated through reversible conversion of the host framework to the 1D framework.

2 Adsorption and H2 Storage Properties of MOFs The as-synthesized metal–organic framework compounds usually contain guest species such as water and other solvent molecules in their pores and/or

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channels. Upon thermal treatment, these guest molecules may be driven out of the framework structures of the MOFs, and in some cases the framework structures of the thermally treated MOFs are maintained. The stable MOFs after removal of guest species are porous and may adsorb a variety of other molecules that are smaller than the pore size of the MOFs. H2 is a clean energy source, and the use of H2 in fuel-cell operation is very promising. However, the storage of H2 in high capacity (both in weight and in volume) is still rather challenging. Recently, the use of MOFs as H2 storage media has been extensively investigated, and it has been found that some MOFs exhibit significant H2 storage capacities. So far as is known, most of the reported MOFs with H2-storage capacities are composed of Zn(II) or Cu(II) and organic linkers.19 Yaghi and co-workers investigated, in detail, the adsorption properties of their MOF compounds composed of Zn(II) and multicarboxylate linkers. Type I isotherms typical for zeolites and related microporous materials have been observed for MOF-2, MOF-3 and MOF-5. After evacuation, MOF-5 possesses a free pore volume of 55–60%. Metal coordinative unsaturation is present in the channels of MOF-4, and as a result, a more complex sorption processes may occur as observed for ethanol uptake in MOF-4. It was reported that MOF-5 adsorbed hydrogen up to 4.5 wt% at 78 K and 1.0 wt% at room temperature and a pressure of 20 bar.20 Inelastic neutron scattering spectroscopy indicates the presence of two binding sites associated with hydrogen binding to zinc and the BDC linker, respectively. It was also found that the topologically similar isoreticular IRMOF-6 and -8 with cyclobutylbenzene and naphthalene linkers, respectively, exhibit approximately double and quadruple the adsorption capacity of MOF-5 at room temperature and 10 bar. Following the first report in H2 adsorption by MOFs, Yaghi et al. have also tested the H2 uptake properties of other MOF compounds, among which is MOF-505 with an as-synthesized composition of [Cu2(bptc)(H2O)2(DMF)3(H2O)] where bptc stands for 3,3 0 ,5,5 0 -biphenyltetracarboxylate and DMF for N,N-dimethylformamide. Upon removal of the guest molecules, this compound takes up 2.47 wt% H2 at 77 K and 760 Torr. A chiral zinc–organic framework compound [Zn2(L)]
4H2O has been synthesized through hydrothermal reaction of ZnCl2 and 4,4 0 -bipyridine-2,6,2 0 ,6 0 tetracarboxylic acid (H4L).21 Structural analysis indicates that the framework of the compound is a five-connected network with a 4466 topology comprising Zn(II) bound to the L4 anion. The chirality is generated by the helical chains of hydrogen-bonded guest water molecules in the channels of the compound, and removal of these guest water molecules from the crystal leads to a porous compound Zn2L which is thermally stable and chemically inert. The BET surface area of the guest-removed Zn2L is 312.7 m2 g1 and the compound also exhibits significant gas storage capacities for H2 (1.08 wt% at 4 bar and 77 K) and for methane (3.14 wt% at 9 bar and 298 K). The adsorption behaviour of Zn2L towards other organic solvent vapours such as benzene, chloroform and toluene has also been investigated, and reveals that the adsorption is dominated by adsorbate–adsorbate or adsorbate–adsorbent interactions.

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Garberoglio et al. have modeled adsorption of light gases including H2 on a number of MOF materials using molecular simulations.22 Good agreement between simulations and experiments is observed for some cases but very poor agreement also exists in other cases. Their calculations indicate that at room temperature none of the tested materials is able to store significant amounts of hydrogen for use in fuel-cell vehicles. Nevertheless, IRMOF-14 exhibits very high H2 uptake at 77 K, and the total uptake of this material may reach 15 wt%. Yang and Zhong also performed Monte Carlo simulation and density functional theory calculations on adsorption of hydrogen in MOF-505 to provide insight into molecular-level details of the underlying mechanisms.23 Their calculation results show that metal–oxygen clusters are preferential adsorption sites for hydrogen, and the strongest adsorption of hydrogen is found in the directions of coordinatively unsaturated open metal sites. The H2 storage capacity of MOF-505 at room temperature and moderate pressures is predicted to be low. Cu3(TATB)2(H2O)3 (TATB stands for 4,4 0 ,400 -s-triazine-2,4,6-triyltribenzoate) is an interesting interweaving MOF.24 Despite the interweaving feature of its framework, the MOF compound exhibits N2 and H2 adsorption capacities after removal of the guest water molecules in the channels. Although the surface area (3800 m2 g1) of this MOF material is lower than those of IRMOF-20 and MOF177 (4346 and 4526 m2 g1, respectively), this compound has greater hydrogen adsorption capacity (1.9 wt% versus 1.356 and 1.25 wt%, respectively for the latter two at 77 K and 760 Torr). Hydrogen uptake has also been compared with another two interpenetrating MOFs, IRMOF-9 and IRMOF-13. Cu3(TATB)2 possesses a higher hydrogen uptake than either (1.17 and 1.73 wt%, respectively). The high H2 uptake has been attributed to the presence of accessible unsaturated metal centres and the existence of pores and channels in a size range well suited to the dihydrogen molecule. Long and coworkers reported the synthesis and hydrogen storage of a unique MOF compound [Mn(DMF)6]3[(Mn4Cl)3(BTT)8 (H2O)12]2
42DMF
11H2O
20CH3OH with exposed Mn21 sites.25 The 3D MOF compound, which crystallizes from a methanol solution containing MnCl2
4H2O and DMF at 70 1C, is composed of Mn21 cations and 1,3,5benzenetristetrazolate (BTT) with solvent molecules occluded in the pores of the framework. After removal of the solvent molecules, the structure remains intact, and shows a total H2 uptake of 6.9 wt% at 77 K and 90 bar. It has also been revealed that the compound exhibits a very high isosteric heat of adsorption (maximum 10.1 kJ mol1) which is related to H2 binding at coordinatively unsaturated Mn21 centres in the framework. Kitagawa et al.26 synthesized a yellow compound with the composition [Zn(TCNQ)bpy]
6MeOH, where TCNQ stands for 7,7,8,8-tetracyano-p-quinodimethane, by reacting Zn(NO3)2
6H2O with LiTCNQ and bpy in MeOH. The Zn ions in this compound are octahedrally coordinated to the four cyanide nitrogen atoms of TCNQ in the equatorial plane and the two nitrogen atoms of the bpy at the axial sites. The Zn ions are linked by TCNQ molecules to give a 2D corrugated layer in the ab plane, whereas the bpy ligands bridge the Zn ions in adjacent layers to form a 3D pillared layer structure. There exist 2D channels

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in the compound with a void space of about 50.7%. Charge balance of the framework requires the charge number of the TCNQ in the compound to be 2, and therefore, the pore surface of the open framework is full of strong donor sites (TCNQ2), which may interact with guest molecules. When the compound is immersed in benzene, the guest MeOH can be exchanged with benzene within 10 s leading to a new material which is red. The crystal structure varies accordingly, but the coordination environment around the Zn ion remains. Other guests such as toluene, ethylbenzene, anisole, benzonitrile, and nitrobenzene may also be exchanged for MeOH in the compound. The colour of the crystals is specific to the guest, and the guest exchange is reversible upon removal/accommodation of the guests. However, it should be pointed out that this compound does not possess the permanent porosity characterized by sorption measurements. MIL-100 and MIL-101 are two chromium carboxylate coordination polymers built up from trimers of metal octahedra and di- or tricarboxylic acids. These two compounds possess giant-pore systems and high surface areas, and they adsorb large amounts of hydrogen at 77 K, with a capacity close to 6.1 wt% for MIL-101 as well as the highest heat of adsorption (10 kJ mol1) at low pressure. Very large pores are not as effective for H2 storage as small pores, and therefore it is believed that the relatively smaller pore system in MIL101 accounts for the high H2 uptake capacity of this MOF material.27 From a similar reaction system, Fe´rey et al. synthesized a new 3D chromium(III) naphthalene tetracarboxylate, Cr3O(H2O)2F(C10H4(CO2)4)1.5
6H2O (MIL-102), from an aqueous mixture of Cr(NO3)3
9H2O, naphthalene1,4,5,8-tetracarboxylic acid, and HF. The structure of MIL-102 is 3D and consists of trimers of trivalent chromium octahedra and tetracarboxylate moieties. In MIL-102 there are small 1D channels filled with water molecules, which interact through hydrogen bonds with terminal water molecules and oxygen atoms from the carboxylates. MIL-102 exhibits a hydrogen storage capacity of about 1.0 wt% at 77 K, and it also adsorbs CO2, CH4 and N2.28 It has also been reported that metal–organic framework compounds may be used as host materials for drug delivery. The adsorption and delivery of a model anti-inflammatory drug, ibuprofen, by MIL-100 and MIL-101 have been demonstrated.29 The solvothermal reaction of zinc nitrate, sodium hydroxide and 1,3-benzenedicarboxylic (m-BDC) acid yielded a heterometallic MOF compound, [ZnNa(mBDC)2]
NH2(CH3)2 (MOF-CJ2),30 which contains zinc(II) and sodium(I) as the metal centres and the dicarboxylate as the coordination linkers. It is unusual that the alkali metal sodium cation, which normally functions as a charge-balancing species in framework compounds, is incorporated into the framework structure of MOF-CJ2. The framework of the compound consists of M-O-C (M ¼ Na and Zn) rods formed by alternating six-coordinated Na(I) centres and four-coordinated Zn(II) centres. These rods are further linked by the m-BDC ligands to form primitive cubic (pcu) rod packing. This framework is the first open-framework heterometallic MOF structure based on the assembly of infinite rod building units.

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3 Metal–Organic Coordination Polymers Involving 4f Metals Although a series of 3d–4f heterometallic complexes with discrete structures have been obtained through conventional self-assembly reactions in solution, the synthesis of coordination polymer compounds, especially 3D framework 3d–4f heterometallic coordination polymers, has been less successful. Lanthanide ions behave as hard acids and they prefer oxygen to nitrogen donors in coordination, while d-block metal ions are borderline acids, having a tendency to coordinate to N-donors as well as O-donors. Therefore, a feasible approach to construct 3d–4f heterometallic frameworks is self-assembly of mixed metal ions and proper ligands containing mixed-donor atoms. In this regard, elaborately designed ligands with N-donor and O-donor atoms have been employed to generate 3d–4f heterometallic coordination polymers. Using appropriate ligands with mixed-donor atoms, we successfully synthesized three isostructural 3d–4f heterometallic compounds [Ln2(H2O)4M2(H2O)2(QA)5]
nH2O (H2QA¼ quinolinic acid; Ln ¼ Gd and Dy, M ¼ Ni and Co) from a hydrothermally pretreated reaction system. These compounds show an interesting 3D open-framework topology with 1D chairlike channels which are occupied by noncoordinating water molecules. It is obvious that the use of ligands with mixed-donor atoms is a key strategy in the crystallization of 3d–4f frameworks.31 If the reaction conditions are controlled properly, homometallic lanthanide coordination polymer compounds can also be obtained. Under hydrothermal conditions we synthesized four homochiral porous lanthanide phosphonates, [Ln(H2L)3]
2H2O, (H3L represents (S)-HO3PCH2-NHC4H7CO2H, Ln ¼ Tb, Dy, Eu and Gd) using chiral N-(phosphonomethyl)proline as the linking ligand. These compounds are isostructural, and they possess a 3D supramolecular framework (Figure 1) built up from 1D triple-strand helical chains. Each of the helical chains consists of phosphonate groups bridging adjacent Ln(III) ions. The helical chains are connected through hydrogen bonds to form 1D tubular channels in which helical water chains are located. Upon removal of the water chains, the compounds exhibit adsorption capacities for N2, H2O, and CH3OH molecules.32 The successful preparation of the four solid compounds provides valuable information for further construction of other homochiral porous lanthanide phosphonate frameworks. Such chiral porous materials hold promise in applications such as enantioselective separation and heterogeneous asymmetric catalysis. The preheating and cooling-down synthetic approach not only reduces the possibility of involvement of coordination water molecules in the compounds but also improves the solubility of aromatic carboxylic acids and therefore the formation of single crystals suitable for structural analysis. Through this approach, four new rare-earth coordination compounds, [Eu(NDC)1.5 (DMF)2], [Nd2(NDC)3(DMF)4]
H2O, [La2(NDC)3(DMF)4]
0.5H2O, and [Eu(BTC)(H2O)], where NDC stands for 1,4-naphthalenedicarboxylate and

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Figure 1

Chapter 5

Space filling structure of [Ln(H2L)3]
2H2O viewed along the c axis.

BTC for 1,3,5-benzenetricarboxylate, have been prepared. The former three compounds possess similar 2D structures, in which the NDC ligands link M(III) (M ¼ La, Nd and Eu) ions of two adjacent double chains constructed by NDC ligands and dinuclear M(III) building units, whereas in the fourth compound, the Eu(III) ion is seven-coordinated by O atoms from six BTC ligands and one terminal water molecule in a distorted pentagonal bipyramidal coordination environment. The Eu-containing compounds exhibit strong red luminescence upon 355 nm excitation. The Nd-containing material shows interesting emissions in the near-IR region, and yellow (580 nm) pumping of this compound results in UV and intense blue emissions through an up-conversion process.33

4 Uranyl–Organic Coordination Polymers The vast majority of MOCPs synthesized so far involve the d-block metals. Whereas 4f metals were also reported to form such assemblies, the 5f metals have been used less commonly as centres for the assembly of organic-bridged coordination networks. Nevertheless, it has proved that 5f metals (at least uranium) are able to form various coordination polymer compounds with suitable ligands.34 A 3D metal–organic framework composed of uranyl and adipate (UO2(C6H8O4)),35 is constructed via assembly of [(UO2)2O8] dimers through flexible adipate linker species. This compound is unusual because other uranyl– aliphatic dicarboxylates such as glutaric, pimelic, suberic, azelaic and sebacic

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85

acids all result in 2D layered topologies consisting of either uranyl dimers, or 1D chains of edge shared hexagonal bipyramids.36,37 It is thought that the rotation around C–C bonds within the adipate backbone allows for the 3D arrangement of the compound. Unlike in the case for d-block and 4f metal coordination polymers, preparation of templated uranyl-containing MOFs has proven unsuccessful using an aliphatic carboxylate as the linker and a N-containing organic molecule as the template. Instead, what normally happens is that both the carboxylate anion and the N-containing molecule directly coordinate to the uranyl centres. For example, UO2(C6H8O4)(C10H8N2) (C6H8O4 stands for adipate and C10H8N2 for 4,4 0 -bipyridine) is a uranyl compound with a structure consisting of chains formed by the adipate groups and the uranyl units along the c axis. These chains are linked together through 4,4 0 -bipyridine groups to form sheets which are arranged in a three-way interpenetrating manner. Cahill et al. have tried the construction of U(VI) compounds using multiple linker species, and a number of new compounds with various topologies have been obtained. In these compounds, the alipathic carboxylates are always linker species whereas the aromatic pyridyl species act as either linkers or non-coordinating charge balancing guests. The hydrothermal reactions38 of UO2(NO3)2 with 1-oxo-4-cyanopyridine (ocpy) and ethyl (S)-lactate lead to the formation of two chiral 2D uranyl compounds, uranyl-bis(1-oxo-4-pyridylcarboxylate) UO2(opyca)2 and homochiral uranyl-bis[(S)-lactate]. The structure of UO2(opyca)2 has a distorted pentagonal bipyramid as the local coordination geometry around each U atom centre, defined by seven oxygen atoms from two carboxylate groups, two N-oxide and two oxo groups. The local coordination geometry around each U(VI) centre in uranyl-bis[(S)-lactate] is similar to that found for UO2(opyca)2 but in this compound there are two crystallographically independent U(VI) centres. At room temperature both compounds emit green light typical of uranyl emission. Furthermore, the two compounds show nonlinear optical properties, and the second harmonic generation coefficients of the powder samples are 0.4 and 0.1 times that of urea, respectively. The uranyl-bis[(S)-lactate] compound also exhibits ferroelectric effects. Through a hydrothermal technique, Thuery successfully synthesized two new uranyl-organic polymeric compounds39 using citrate (cit) and tricarballylate (tca) as structure linkers. The compounds are either 3D ([(UO2)2(Hcit)2]2) or 2D (uranyl-tca). O’Hare et al. reported40 the synthesis of several interesting uranyl–organic framework compounds, in which the uranyl units are connected by bidentate dicarboxylate anions such as succinate, glutarate and isophthalate. Some of the as-prepared compounds contain cavities in which water and organic species are accommodated. However, the thermal stability of the compounds has not been demonstrated. The same research group also synthesized and structurally characterized41 four new uranium isonicotinate framework compounds which crystallize in the UO2(CH3CO2)
2H2O–HF– isonicotinic acid system. The dimensionality of the hydrothermally obtained uranyl isonicotinates varies from zero UO2(C5H5NCO2)(CH3CO2)2, to one

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[UO2F2][C5H5NCO2] and [UO2F3][C5H6NCO2]
0.5H2O, and two [UO2F2]2 [C5H5NCO2]
H2O. The material UO2(C5H5-NCO2)(CH3CO2)2 is composed of UO8 hexagonal bipyramids in which the uranyl centre is coordinated by two acetate and one isonicotinate ligands. The 1D compound [UO2F2][C5H5NCO2] consists of chains of edge-sharing [UO3F4] pentagonal bipyramids with isonicotinate ligands, whereas the other 1D compound, [UO2F3][C5H6NCO2]
0.5H2O, is constructed from edge-sharing chains of [UO2F5] pentagonal bipyramids and a hydrogen-bonding network of isonicotinic acid. The material [UO2F2]2[C5H5NCO2]
H2O contains uranium oxyfluoride layers consisting of edge-sharing dimers of UO3F4 and the corner-sharing UO3F4 chains. It seems that the fluoride ions play an important role in the construction of these uranyl isonicotinate compounds. 3,4-Pyridinedicarboxylic acid (3,4-pydaH2) and 2,4-pyridinedicarboxylic acid (2,4-pydaH2) have also been used to react with uranyl cations under hydrothermal conditions and two new uranium coordination polymers (UO2)3(m3-O)(m3-OH)2-(3,4-pydaH)(3,4-pyda)0.5 and [(UO2)3(m3-O)(m3-OH) (m2-OH)(2,4-pyda)(H2O)2]
H2O have been prepared.42 The former compound features a wave-like 2D layer constructed by 1D UO(OH) ribbons and the 3,4-pyda ligands, whereas the latter compound consists of a planar ‘‘openlayer’’, which is constructed by the UO(OH) polyhedra and the 2,4-pyda ligands. The photoluminescent properties of the compounds depend on both the ligands and the structural features of the inorganic building units. UO2(pdc)(H2O) (pdc stands for pyridine-2,6-dicarboxylic acid) is a helical chiral compound formed by coordination of uranyl units by pdc molecules.43 The material is thermally stable up to at least 350 1C and exhibits considerable adsorption capacity for water and methanol upon removal of the guest species in the microporous channels. Although the structure of this compound was previously described by Immirzi44 and its luminescence was discussed by Thuery and co-workers,45 the existence of disordered water molecules in this compound was not noticed. It is of interest that the helical arrangement (Figure 2) of the uranyl ions and the pdc molecules generates a nano-channel with a diameter of about 6.36 A˚. The void volume of the channels is estimated to be 20.4% of the total volume of the compound. The compound does not adsorb ethanol after dehydration, probably because the pore size is not large enough to allow the penetration of ethanol molecules after dehydration, which leads to the shrinkage of the crystal structure and consequently the puckering of the helical channels. From a hydrothermal reaction system containing uranyl ions and carboxylic acids, two new uranyl-organic compounds [(UO2)3O(OH)3(NA)2]
H3O1 (HNA ¼ nicotinic acid) and (UO2)2(phen)2(BTEC) (H4BTEC ¼ 1,2,4,5benzenetetracarboxylic acid, phen ¼ 1,10-phenanthroline) were crystallized.46 The former compound consists of infinite helical polyoxouranium ribbons built up from trinuclear [(UO2)3O(OH)3]1 units and NA ligands, whereas the latter is composed of 2D sheets joined together through p–p interactions. Both compounds show intense emission under excitation of UV light. Both homometallic and heterometallic uranyl-organic polymers can be prepared by using pyridine-dicarboxylate (pdc) as a linker.47 A homonuclear

Metal–Organic Coordination Polymers

Figure 2

87

View of the 3D structure of UO2(pdc)(H2O) (a) along the c axis, (b) a single channel formed by each single-stranded helix and (c) the single-stranded helix along the a axis.

uranyl ‘‘end-member’’ UO2(C5H2N2O4)(H2O) with a layered structure has been crystallized in the presence of pdc molecules and uranyl ions, and interestingly, the uranyl cation is bound to both carboxylic groups and the N/O sites. If Cu(II) centres are added into the reaction system, a heterometallic Cu(II)- and U(VI)-containing compound (UO2)Cu(C5H2N2O4)2(H2O)2 in which the uranyl centres remain coordinated to O-donor sites is formed. It is noted that the luminescent property appears to be influenced by the Cu(II) centre as UO2(C5H2N2O4)(H2O) is highly luminescent but (UO2)Cu(C5H2N2O4)2(H2O)2 is not. The presence of Cu(II) cations may quench the photoluminescence of the uranyl units. A similar phenomenon has been observed in the uranyl-4,5imidazoledicarboxylic acid (4,5-idca) system. The uranyl ‘‘end-member’’, (UO2)(C5H2N2O4), also shows N/O coordination and chelation. Addition of Cu(II) to the reaction system leads to two higher-dimensional compounds (UO2)2(C5N2O4H)2(C5N2O4H2)4Cu3(H2O)2
2H2O and (UO2)2 (C5H2N2O4)2 (OH)2Cu
2H2O. The former contains two distinct coordination geometries for the Cu(II) sites (square planar and octahedral coordination), whereas the latter contains sheets of [(UO2)2O8] dimers linked through dicarboxylate groups that are also coordinated to Cu(II) centres in square planar geometry. The uranyl cation can also form heterometallic polymeric compounds in combination with Pb(II) centres through the use of 2,6-pyridinedicarboxylic acid (H2pdc). In this case, two 3D structures [(UO2)(C7H3NO4)2Pb2(C2O4) (H2O)2] and [(UO2)(C7H3NO4)2Pb
2H2O] have been prepared.48 Both compounds contain edge-sharing uranyl and Pb(II) polyhedra. The first compound, [(UO2)(C7H3NO4)2Pb2(C2O4)(H2O)2], consists of uranyl hexagonal bipyramids edge-sharing with Pb(II)O6 octahedra to form trimers, which are linked by

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2,6-pydc into sheets. Interestingly, these sheets are connected through oxalate ligands presumably formed from decarboxylation of the pydc linker.49 The second compound, [(UO2)(C7H3NO4)2Pb
2H2O], shows similar local geometry about the metal centres. Using Zn(II) acetate and uranyl acetate as reactants, we successfully prepared a novel 3D coordination polymer comprising inorganic U–O–Zn-clustered double sheets and organic ligands (ZnO)2(UO2)3(NA)4(OAc)2 (HNA stands for nicotinic acid; HOAc for acetic acid) from a hydrothermal reaction system.50 X-Ray diffraction reveals that the structure of the compound is 3D (Figure 3) with rich coordinations, including eight-coordinate hexagonal bipyramidal U(1) cations, seven-coordinate pentagonal bipyramidal U(2) cations, six-coordinate octahedral Zn(1) cations, and five-coordinate trigonal bipyramidal Zn(2) cations. Thermogravimetric analysis indicates that the coordination polymer is rather thermally stable (up to 400 1C), due to the formation of U–O–Zn double layers that solidifies the flexible organic ligands. It is interesting to note that the compound shows semiconducting properties and it may be regarded as a 2D semiconductor material. Under illumination without external electric field, the surface photovoltage spectrum (SPS) of the compound shows two main response bands at approximately 350 and 463 nm. Furthermore, photocurrent has also been observed for the compound. The successful synthesis of (ZnO)2 (UO2)3(NA)4(OAc)2 and the discovery of its unusual physical properties suggest that it is possible to prepare new semiconducting materials based on coordination polymer compounds containing actinides.

Figure 3

The 3D framework of (ZnO)2(UO2)3(NA)4(OAc)2 viewed along the [100] direction.

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A zinc(II)-containing uranyl–organic coordination polymer (UO2)2 (m2-OH)(pdc)2Zn(bpy)(OAc)2(H2O)9 (H2pdc ¼ pyridine-2,6-dicarboxylic acid, bpy ¼ 4,4 0 -bipyridine, HOAc ¼ acetic acid)51 has also been synthesized. In this compound, there exist voids generated by the stacking of the (UO2)2 (m2-OH)(pdc)2(OAc)Zn(bpy) chains, and these voids are occupied by water chains each consisting of a cyclic water tetramer and a water pentamer. It is believed that these infinite water chains play an important role in the stabilization of the crystal structure. It has also been demonstrated that uranyl and the d10 silver(I) ion are able to form heterometal–organic coordination polymers.52 For example, we successfully obtained [Ag(bipy)(UO2)(bdc)1.5] (bipy ¼ 2,2 0 -bipyridyl, bdc ¼ 1,4-benzenedicarboxylate) and [Ag2(phen)2UO2(btec)] (phen ¼ 1,10-phenanthroline, btec ¼ 1,2,4,5-benzenetetracarboxylate) through hydrothermal assembly of the metal sources with mixed ligands. Both compounds feature a 2D network with p–p overlap interactions between the aromatic fragments in the neighbouring layers. In the structure of [Ag(bipy)(UO2)(bdc)1.5], the uranyl units are connected by bridging bdc ligands to produce chains. These chains are linked by additional bdc groups to form a 2D network with [Ag(bipy)]1 subunits. The structure of [Ag2(phen)2UO2(btec)] is composed of 2D layers with Ag–UO8–Ag trinuclear cores linked by bridging btec ligands. These two compounds are insoluble in water and they show photocatalytic degradation performance superior to that of commercial TiO2 (Degussa P-25) when tested on nonbiodegradable rhodamine B (RhB) as model pollutant. It is remarkable that [Ag(bipy)(UO2)(bdc)1.5] also shows photocatalytic activity under visible-light irradiation. On the basis of the monitored intermediate species and the final mineralized products, the relationship between the structure of the photocatalysts and the photocatalytic activity has been elucidated, and it is believed that the photodegradation of RhB in aqueous solution catalyzed by the two uranyl– Ag compounds involves photoexcitation of the uranyl centres and molecular oxygen. In combination with divalent nickel cations, uranyl species may be assembled to form microporous frameworks under the assistance of organic ligands. For instance, a uranium–nickel–organic hybrid compound53 with micropores [Ni2 (H2O)2(QA)2(bpy)2U5O14(H2O)2(OAc)2]
2H2O (HOAc ¼ acetic acid; bpy ¼ 4,4 0 -bipyridine; H2QA ¼ quinolinic acid) has been crystallized from a hydrothermal system, and this compound exhibits photocatalytic activity for the degradation of methyl blue as a model pollutant. The structure of the compound is a 3D network (Figure 4) constructed from the polyoxouranium ribbons and Ni metal–organic layers through sharing QA ligands, in which the polyoxouranium ribbons are composed of a UO8 hexagonal bipyramid and two different UO7 pentagonal bipyramids. There are channels in the framework structure, and these channels are occupied by disordered water molecules. It has also been proved that the compound is thermally stable, and upon dehydration, the material reabsorbs water molecules. Nevertheless, this metal– organic-framework material shows no N2-adsorption capacity, probably because the pore size is not large enough to allow the penetration of N2 molecules.

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Figure 4

Chapter 5

View of the structure of [Ni2(H2O)2(QA)2(bipy)2U5O14(H2O)2(OAc)2]
2H2O along the b (left) and a (right) axes.

5 p-Block Metal Coordination Polymers p-Block metal coordination polymers are relatively rare. This class of coordination polymer compounds reported so far are mainly constructed from metal centres of Pb and Al. [Pb(dimb)(DMF)(NO3)2] is a polymeric compound with a 1D zigzag chain structure, whereas [Pb(dimb)(SCN)2] (dimb ¼ 1,3-bis(imidazol-1-ylmethyl)-benzene) possesses a 2D corrugated network composed of 24-membered M2L2 metallocyclic rings. The structure of [Pb(bimb)1.5 (NO3)2](DMF) [bimb ¼ 4,4 0 -bis(imidazol-1-methyl)-biphenyl] contains a 1D infinite noninterpenetrated molecular ladder with cavities, and DMF molecules fill the channel formed by two adjacent ladders. All three complexes exhibit third-order nonlinear optical (NLO) properties.54 The hydrothermal reaction of 5-hydroxyisophthalic acid (H3L) with lead and nickel ions results in a heterometallic polymer Pb6Ni(m3-OH)8(La)2 containing heptanuclear [Pb6Ni(m3-OH)8]61 units and a homometallic compound [Pb(Lb)(H2O)]
(H2O)0.25 consisting of 1D lead oxide chains,55 where La and Lb stand for 4,6-dinitro-5-hydroxyisophthalate and 2,4-dinitro-5-hydroxybenzoate, respectively. Obviously, the ligands are formed from in situ hydroxyl directed dinitration of the L molecules during the hydrothermal reaction. Assembly of 4,4 0 -bipyridine N,N 0 -dioxide (bpno) and lead(II) nitrate with or without dicyanamido ions (dca) in aqueous solution leads to two coordination polymer compounds Pb2(bpno)4(dca)2(NO3)2Pb2(bpno)4(NO3)4
5H2O and Pb(bpno)(NO3)2
H2O.56 Both compounds consist of dinuclear Pb(II) units bridged by bpno ligands, and the former material possesses a bilayered rhombus grid whereas the latter has a monolayered rhombus grid. Upon excitation by UV light at room temperature, the two compounds emit intense visible light at wavelengths 616 and 617 nm, respectively. Through use of different dicarboxylate linkers and nitrogen-containing ligands (NCLs), a variety of lead(II) materials with rich architectures have been synthesized hydrothermally or solvothermally.57 On the basis of synthesis and structural characterization, we have elucidated the effects of ligands and solvents on the construction of the coordination polymers. In addition, the photoluminescence and nonlinear optical properties of these compounds have

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been investigated in detail. It has been found that the NCLs, the organic acids, and the solvents all affect the network structures of the coordination polymer products. The geometry and size of the NCLs which provide potential supramolecular recognition sites for p–p stacking interactions, are essential in determining the structures of the final metal assemblies. Through changing solvents, the cis and trans conformations of H2CHDC are separated completely, and the large PbII wheels ([Pb8(cis-CHDC)8]) with bridging carboxylate oxygen atoms are isolated successfully. The photoluminescence and nonlinear optical properties of the compounds indicate that they may be good candidates for luminescent materials. Loiseau and co-workers described58 the hydrothermal synthesis and structural characterization of an interesting aluminium 1,4-benzenedicarboxylate coordination polymer Al(OH)[O2CC6H4CO2]
[HO2CC6H4CO2H]0.70 (designated MIL-53 as (Al)). The 3D framework of this compound is built up of infinite chains of corner-sharing AlO4(OH)2 octahedra, which are interconnected by the 1,4-benzenedicarboxylate groups, creating 1D rhombic-shaped tunnels. Disordered 1,4-benzenedicarboxylic acid molecules are trapped inside these tunnels, but these guest molecules are removable through simple heating, leading to a nanoporous open-framework (MIL-53 ht (Al)) with empty pores. This solid shows a surface area of 1590 m2 g1 and is thermally stable up to 500 1C. It is interesting to note that after removal of the occluded 1,4-benzenedicarboxylic acid molecules, the compound reversibly adsorbs water molecules which interact with the framework of the compound through H-bonds. MIL-96, an aluminium trimesate Al12O(OH)18(H2O)3(Al2(OH)4)[btc]6
24H2O, was also prepared59 under hydrothermal conditions in the presence of 1,3,5-benzenetricarboxylic acid (trimesic acid or H3btc). The structure of MIL-96 features a 3D framework consisting of trinuclear oxo-bridged aluminum clusters and infinite chains of AlO4(OH)2 and AlO2(OH)4 octahedra which form a honeycomb lattice based on 18-membered rings. The 3D framework of MIL96 possesses three types of cages, two of which have pore volumes of 417 and 635 A˚3, respectively. The third type has a smaller pore volume and contains disordered octahedral aluminium Al(OH)6. This material is able to adsorb both carbon dioxide and methane at room temperature and hydrogen at 77 K.

6 Perspectives MOCPs show remarkable structural diversity, and in principle it is possible to design the framework structures of these compounds and to synthesize them rationally. It is also envisaged that through using ligands with functional groups, MOCPs with various properties may be achieved. Although the thermal stability of metal–organic compounds is generally lower than conventional zeolite materials, the lower framework densities and higher surface areas, the framework flexibility and the ease of topological adjustment for MOCPs render this novel class of materials very attractive as efficient adsorbents, as gas storage media, and as catalysts under mild conditions. The intrinsic properties

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(such as magnetism and luminescence) of the metal centres in a coordination polymer may also lead to the appearance of new functions associated with the polymeric compound. It is anticipated that MOCP materials will play increasingly important roles in various application fields.

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50. W. Chen, H.-M. Yuan, J.-Y. Wang, Z.-Y. Liu, J.-J. Xu, M. Yang and J.-S. Chen, J. Am. Chem. Soc., 2003, 125, 9266. 51. Y.-S. Jiang, G.-H. Li, Y. Tian, Z.-L. Liao and J.-S. Chen, Inorg. Chem. Commun., 2006, 9, 595. 52. Z.-T. Yu, Z.-L. Liao, Y.-S. Jiang, G.-H. Li and J.-S. Chen, Chem.–Eur. J., 2005, 11, 2642. 53. Z.-T. Yu, Z.-L. Liao, Y.-S. Jiang, G.-H. Li, G.-D. Li and J.-S. Chen, Chem. Commun., 2004, 1814. 54. B. Sui, W. Zhao, G. Ma, T. Okamura, J. Fan, Y.-Z. Li, S.-H. Tang, W.-Y. Sun and N. Ueyamac, J. Mater. Chem., 2004, 14, 1631. 55. X. Li, R. Cao, Z. Guo and J. Lu, Chem. Commun., 2006, 1938. 56. Y. Xu, D. Yuan, L. Han, E. Ma, M. Wu, Z. Lin and M. Hong, Eur. J. Inorg. Chem., 2005, 2054. 57. Y. Yang, G.-D. Li, J.-J. Cao, Q. Yue, G.-H. Li and J.-S. Chen, Chem.–Eur. J., 2007, 13, 3248. 58. T. Loiseau, C. Serre, C. Huguenard, G. Fink, F. Taulelle, M. Henry, T. Bataille and G. Fe´rey, Chem.–Eur. J., 2004, 10, 1373. 59. T. Loiseau, L. Lecroq, C. Volkringer, J. Marrot, G. Fe´rey, M. Haouas, F. Taulelle, S. Bourrelly, P.L. Llewellyn and M. Latroche, J. Am. Chem. Soc., 2006, 128, 10223.

CHAPTER 6

Elucidating Crystal Growth in Nanoporous Materials: The Importance of Microscopy MICHAEL W. ANDERSON, L. ITZEL MEZA, JONATHAN R. AGGER, MARTIN P. ATTFIELD, MARYAM SHO¨AˆEE`, CHIN B. CHONG, AYAKO UMEMURA AND COLIN S. CUNDY Centre for Nanoporous Materials, School of Chemistry, The University of Manchester, Oxford Road, Manchester M13 9PL, UK

1 Introduction A picture is worth a thousand words or a micrograph is worth a thousand pieces of data. Microscopy has always had the distinct scientific advantage of the power of persuasion, which, of course, is a double-edged sword. Nonetheless, this is a significant advantage when deployed correctly and effectively. This is illustrated eminently in the field of nanoporous materials where microscopy has aided scientists to better understand the complexities of structure and crystal growth. With the advent of high-resolution electron microscopy in the late 1970s and early 1980s scientists were able to see for the first time, with their own eyes, the pore structure of zeolites which is the seat of their incredible adsorption, cation exchange and catalytic properties. Properties which were known by scientists such as R.M. Barrer1 and D.W. Breck2 for decades previously and yet to be able to see the pore architecture was, at the time, awe-inspiring. I (MWA) remember an undergraduate lecture at the University of Edinburgh delivered by the late Barrie Lowe, a zeolite chemist with a fascination for zeolite synthesis. He was giving a final year lecture in 1980 on solid-state chemistry and Many of the foundations of electron microscopy in nanoporous materials were laid down in the early 1980s by the Cambridge group headed by J.M. Thomas. These principles have been carried forward and in this paper we discuss some of the latest results from atomic force microscopy and high-resolution electron microscopy and put these into a historical perspective.

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was relishing the opportunity to tell us about his personal fascination with zeolites. But he was particularly excited that day because he had just read a paper3 from the Cambridge group, headed by J.M. Thomas, which showed a high-resolution electron micrograph of zeolite A revealing the pore structure and cage units (see Figure 1). It brought to life the subject that Lowe had been studying himself for much of his career.4 Indeed that early paper by Bursill et al.3 is remarkable in that they had chosen a particularly difficult zeolite to image, the high aluminium content rendering the zeolite particularly susceptible to beam damage in the electron microscope. Indeed much of that paper concerned the fragility of the structure in the electron microscope and techniques to circumvent these problems. More recently with the development of much higher accelerating voltages, HREM may be applied to highly beam sensitive systems such as zeolite A with greater ease. Figure 2 shows an HREM image of a complete nano-crystal of zeolite A recorded at 1 MeV accelerating voltage. At such high energy, the cross-section for capture of the electron is substantially diminished and consequently the sample is less susceptible to beam damage. This image, recorded by the group of Osamu Terasaki (currently at the University of Stockholm and a previous colleague of J.M. Thomas) shows the remarkable clarity in the image, in particular at the surface of the crystal which is beginning to reveal the important surface structure relevant to crystal growth processes. The early paper on zeolite A also opened a window to the study of defects and intergrowths in zeolites. This latter ability of microscopy techniques to study non-periodic effects, such as defects, is crucial, as the crux of

Figure 1

Early transmission electron micrograph with simulation of the (001) zone axis of zeolite Na-A by Bursill et al.,3 reproduced with permission from the authors.

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Recent transmission electron micrograph of the (001) zone axis of nanocrystalline zeolite Na-A recorded at 1 MeV accelarating voltage. Courtesy of Osamu Terasaki.

real solid-state chemistry, stucture and function, is often governed by imperfections. This was to be the theme of many electron microscopy studies over the next decades with much of the early work emanating from the Cambridge group.5–15 But it was the enthusiasm of Barrie Lowe in his final year undergraduate lecture which inspired MWA to pursue a PhD in zeolite chemistry and indeed, perhaps to the annoyance of the group in Edinburgh, not to do a PhD in Edinburgh but to move to Cambridge where he completed his PhD under the guidance of J.M. Thomas. A quarter of a century later MWA finds his research on the crystal growth of nanoporous materials relying heavily on microscopy techniques. Still high-resolution transmission electron microscopy, but even more key is the use of atomic force microscopy (AFM), a technique which had not been invented in 1980.16,17 With AFM it is possible to measure with precision the surface topography with sub-nanometre resolution. Figure 3 shows the surface topography of zeolite A recorded on an AFM integrated with a high-power inverted optical microscope. This has the ability to position the AFM tip on a desired crystal facet and also, as seen in Figure 3, to superimpose the AFM image onto the optical micrograph. This technique has been invented in particular for studying biological samples, however, it is also particularly useful for crystal growth studies. Square terraces are immediately apparent on the (100) crystal surface of zeolite A and the terrace height can be measured accurately to be 1.2  0.1 nm. AFM also has the distinct advantage

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AFM of zeolite A showing surface topology superimposed on optical micrograph of crystal. Terraces are 1.2 nm high and illustrate layer-by-layer crystal growth.18

that it may be performed in solution under conditions suitable for crystal growth. Such images, as discussed later, are therefore helping to reveal the nature of fundamental crystal growth processes.

2 Crystal Growth: Learning from Defects and Intergrowths The most important aspect of a nanoporous crystal, with pore diameter of dimensions from sub-nanometre up to about 1.4 nm, is the exceedingly high surface area and confined geometry of the pore architecture. However, in addtion to these geometrical constraints, the high electrical fields generated within these nano-dimensioned cavities are responsible for many of their important industrial applications. Consequently, understanding the details of this crystal architecture is paramount. The crystals form this special structure during nucleation and crystal growth from a solvo-thermal synthesis. For zeolites, the solvent is normally water and the temperatures required range from room temperature to 200 1C. Typical zeolite crystals are relatively small with sizes ranging from ca. 100 nm to 200 mm. Similar to all crystals, zeolites incorporate defects during growth. These defects are extremely well defined and, because of the complex architecture of the zeolite framework, the nature of these defects is often unique to a particular zeolite structure. Furthermore, owing to the strong covalent bonding of the framework, these defects will not readily migrate and anneal and as a conseqence

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they provide a signature of the crystal growth mechanism. In other words, any growth mechanism postulated for a zeolite must be able to describe not only the development of the regular crystalline framework but also the incorporation of specific aperiodic defect structures.19 Defects can be either local or extended. In order to characterise the former, usually a short range spectroscopic approach such as infra-red or nuclear magnetic resonance spectroscopy is suitable. However, for extended defects, microscopy methods are ideal. An illustration of the extreme complexity of extended defect structures in zeolites was illustrated early on by the Thomas group (see Figure 4) in

Figure 4

HREM of zeolite L showing co-incidence boundary caused by a rotation of the zeolite structure,12 adapted with permission from the authors.

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zeolite L. A curious coincidence boundary is formed by the rotation of one layer of the zeolite L structure by 32.2 1. The resulting structure exhibits coincidence of pores running in the (001) direction with a superlattice repeat O13 times that of the original. As the new tunnel structure passes the full length of the crystal, this can be seen easily in projection in the high-resolution electron microscope. Other defects seem at first sight more conventional in nature such as the screw dislocation observed in zeolite A. The dislocation is most prominently revealed in the AFM images of the (100) surface, see Figure 5, which show spiral terraces.20 Such spiral growth is a conventional manner for crystal growth in dense phase materials, with the centre of the spiral located at a screw dislocation running through the crystal. In zeolites the phenomenon is harder to understand as a spiral around the screw dislocation would require a diplacement Burger vector of tens of a˚ngstroms in order for the structure to reconnect without dangling bonds. The associated strain could be accommodated by a mesoscopic void running through the crystal, around which the screw dislocation is wound. Indeed there are indications from the AFM images in Figure 5 that such a mesoscopic void exists. The size of this void is difficult to judge as the hole observed by AFM is a convolution of the AFM tip shape and the void. Interestingly, however, Slater et al.21 demonstrated in a theoretical examination that a screw dislocation could be accommodated with little strain without any mesoscopic void. This is also shown in Figure 5 whereby a twist in the structure allows reconnection and the strain is relieved within a few unit cells. Another good example whereby the defect and intergrowth structure in a nanoporous material could be usefully controlled is the titanosilicate ETS-10.22,23 Figure 6 shows a HREM image of ETS-10 which is a wide-pore three-dimensional channel structure. It is a particularly interesting material for a number of reasons. First, unlike an aluminosilicate zeolite where each aluminium in the structure has a one minus charge, the octahedrally coordinated titanium in ETS-10 has a two minus charge. As a consequence, ETS-10 has a very high affinity for divalent charge-balancing cations which makes it particularly attractive for removal of heavy metal contaminants from aqueous environments. Second, ETS-10 exists naturally as a random intergrowth between two end-member polymorphs. As can be seen from Figure 6, the stacking of the main channels from the bottom of the micrograph to the top is randomly to the left and right. One ordered polymorph, termed Polymorph A,22,23 has an ordered arrangement left/right/left/right . . . (see Figure 6). If this polymorph could be synthesised it would have a large spiral channel running through the structure. Synthesis of such chiral structures in microporous oxides is elusive and requires a much deeper understanding of the growth mechanism. How ETS-10 grows is partly revealed by the presence of extended defects in ETS-10, the double pores in Figure 6. These double pores must be incorporated as a result of a layer-by-layer growth process. Therefore, in order to affect the nature of a subsequent layer it will be necessary to prime the growing surface which either encourages the desired next layer or prevents the unwanted layer forming.

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AFM of (100) surface of zeolite A (lower four images showing spiral growth.20 The upper structures illustrate possible screw dislocations determined theoretically.21

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Upper image structure of polymorph A of ETS-10 with the spiral channel cut from the framework. The HREM image of ETS-10 below shows a random intergrowth and defects.22,23

3 Faujasitic Structures How crystals nucleate and grow is a problem that has challenged scientists for many years: how order is created from disorder; the driving forces involved; the quest for crystalline perfection.24–26 In many ways nucleation and crystal growth should not be considered as separate phenomena, however, for practical reasons it is useful to do so. The techniques at our disposal to follow the nucleation and crystal growth stages are substantially different and therefore there is often a visible seam between our perception of the two processes. In terms of crystal growth, the advent of scanning probe microscopies16 (SPM) and in particular AFM17 has permitted the detailed

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observation of nanometre-sized events at crystal surfaces. This is often possible under in situ crystal-growth conditions as the technique can be operated to observe surfaces under solution. Real-time images of growing crystals have revealed terrace growth, spiral growth, the inclusion of defects and the occlusion of foreign particles in a wide variety of growth studies.27,28 We have recently recorded the first ever in situ images of the alteration of a zeolite surface under a variety of supersaturation conditions.29 By measuring real-time micrographs at a range of temperatures, the free-energy for individual growth processes can be determined. To date most of these crystal growth studies have been on dense phase ionic crystals, such as calcite, or molecular crystals, such as proteins and viruses. There has been a modest amount of work performed on nanoporous crystals such as zeolites and zeotypes of which we have been at the forefront.30–39 The reason for this is two-fold: first, often the most interesting open-framework structures can only be crystallised as micron-sized crystals, making observation by AFM demanding; second, there has been a recent emphasis within the community on making new materials rather than on understanding formation. In our view, this is an oversight which is clear by the vast amount of new information forthcoming on understanding crystal growth in macromolecular systems which is helping to address problems such as: overcoming crystal size limitations; improving crystal purity; controlling intergrowth structures and controlling crystal habit. In open-framework materials a better understanding of the crystal growth processes will lead to new methodologies to control similarly important crystal features. But furthermore it could lead to both new structures and also more cost-effective routes to existing but prohibitively expensive known structures. Figure 7 illustrates an interesting example of how materials with similar framework topologies grow via different crystal growth mechanisms. The crystal system is the industrially important faujasite (FAU) topology. The archetypal synthetic zeolite in this class is zeolite Y, used in the catalytic cracking of petroleum fractions. Zeolite Y is an aluminosilicate cubic framework structure typically synthesised from an alkaline aluminosilicate gel at elevated temperature. However, the framework structure is amenable to replication with a variety of framework elements. One example discovered by Gier and Stucky40 in 1991 was the zincophosphate analogue of the FAU structure. The synthesis was carried out at very low temperature, ca. 4 1C, from an initially clear solution. Similar to a zeolite, the zincophosphate has an anionic framework which must be counterbalanced by extra-framework cations. This synthesis was further tailored by the group of Prabhir Dutta34 who were attempting to control the synthesis conditions in order to synthesise perfect crystals. They adopted a method that relied on the preparation of micron sized reaction vessels inside reverse micelle surfactant phases. The micro-water droplets acted as micro-reactors. Using this method they were able to synthesise crystals of very high quality. Figure 7 compares the surface topology of a typical conventional aluminosilicate zeolite Y synthesis, which we reported,30 with that of a zincophosphate with FAU structure synthesised in a reversemicelle system.34 In both cases growth terraces are observed indicating a

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AFM images of the (111) surface of the FAU structure. Top image aluminosilicate zeolite Y. Lower image ZnPO-X synthesised in a reversemicelle condition, reproduced by courtesy of the author.34

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layer-by-layer growth process. Also the terraces in both systems are approximately triangular, reflecting the three-fold symmetry along the (111) direction, with terrace height ca. 1.5 nm. The height is indicative of an FAU sheet, a fundamental structural unit of the FAU framework. However, the terraces differ in one very important aspect, namely the orientation of the triangular terraces with respect to the triangular (111) facet of the crystal. In the conventional aluminosilicate zeolite Y, the terraces are rotated 60 1 with respect to the crystal facet. In the zincophosphate system synthesised in reverse micelle the terraces have coincident orientation with the crystal facet. The orientation of these facets will be a direct result of the growth mechanism on this facet that must be substantially different in order to effect the orientation change in the terrace. In order to decipher the reason for this difference will require modelling of the mechanism, similar to that shown later for zeolite A, but hitherto this remains a matter for debate.

4 Zeolite A Transformations Zeolite A is one of the most important commercial zeolites as it is used in washing powders as a water softening agent and desiccant. AFM is beginning to reveal some of the molecular events that occur during crystal growth.31,36 Figure 8 illustrates a typical AFM topography measured on the (100) surface of zeolite A. This particular micrograph has been chosen because it exhibits many different features on the one crystal, however, the occurence and abundance of the different features is a function of the crystal growth conditions such as temperature, supersaturation, pH, starting reagents, etc. First, the (100) surface exhibits square terraces with the edges of the terraces aligned with the (100) direction of the crystal. The principal step height of these terraces is 1.2  0.1 nm, equivalent to half a unit cell. The well-defined terrace topology, both height and straight terrace edges, indicates that certain surface structures have lower surface free-energy and are consequently more stable. It should be pointed out that the AFM of crystals such as that shown in Figure 8 are recorded on crystals that have been removed from the growing medium and washed. Consequently the topologies observed will be the stable structures which, although very important for the growth mechanism, will not give the full picture of crystal growth. Also shown in Figure 8 are curved growth fronts. These occur when two growing terraces merge creating a kink site at the point of union. This kink site becomes the preferred growth point and a curved growth front ensues until a new rectangular growth front is created. This has been verified by modelling studies some of which are given in a later section of this chapter. The morphology of the terraces, curved growth fronts and density of separate growth nuclei yield the relative rates of surface nucleation, growth at edge sites and growth at kink sites. Also shown in Figure 8 is a preponderance of pyramid structures. This is a very common feature in zeolite A growth and these can be easily seen in scanning electron micrographs as the height of the pyramids can be tens of nanometres. Careful inspection of the AFM micrographs shows that there is a defect at the centre of each pyramid

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AFM of (100) surface of zeolite A showing surface topology illustrating square terraces, curved growth fronts, pyramids and defects.20

which causes the terraces all to nucleate at the same lateral position on the (100) surface. These defects are removed preferentially during dissolution experiments indicating that they are structurally less stable. This is an important part of the growth mechanism in zeolite A, especially in crystals that are grown from nutrient which has not been carefully filtered in order to remove fine impurities. Indeed, when reaction mixtures are filtered, the resulting crystals do not exhibit these pyramids and the surface topology does not indicate the presence of defects. This suggests that the defects may well be associated with foreign particles. Finally, Figure 8 shows substantial crack defects in zeolite A, another feature which is typical when there are impurities in the starting nutrient. Zeolite A does not only grow layer-by-layer but also, as pointed out previously, via spiral growth as shown in Figure 5 although this is less common. Figure 9 shows a series of atomic force micrographs which were recorded under a solution of sodium hydroxide. Each image takes a few minutes to record which is, consequently, the time-scale for measuring kinetic processes via this method. Although there are some more modern methods to record atomic force micrographs at much higher frequency, these are not very easy to adapt to observing micron-sized crystals under solution. Initially in Figure 9 we observe square terraces which are 1.2 nm in height.36 As the dissolution proceeds the terrace withdraws across the zeolite surface. At short times the first parts of the terrace to withdraw are at the holes in the terraces. These are essentially kink sites, that were shown from the growth studies to be the preferred sites for

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4.3  4.3 mm2 deflection AFM images of (100) surface of zeolite A crystal dissolving under 0.5 M NaOH.36

growth. Consequently, it is not surprising that these are the first sites to be dissolved. As the terraces recede across the surface the terrace height is only 0.9 nm in height, in other words, not the full height of the half unit cell. Also observed is a ‘‘shadow’’ of the withdrawing terrace which retains the original shape of the terrace and this is only 0.3 nm in height. The terrace is therefore being removed in a two-step process. First, by correlated removal by terrace retreat of a 0.9 nm step followed by an uncorrelated removal of a 0.3 nm step. The question then arises as to the nature of the units associated with these different step heights. Zeolite framework materials are constructed from units of well-defined structure, and it is therefore reasonable to expect that the different heights observed in the atomic force micrographs correlate with well-defined parts of the crystal structure. Zeolite A is composed of sodalite cages, truncated octahedra, connected through double four-rings. The height of one sodalite cage is approximately 0.9 nm. These sodalite cages are capped off

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with a single four-ring which is associated with another 0.3 nm step. It is impossible to say whether these are indeed the terminations of the crystal, however, there is strong evidence from theoretical calculations that indeed these are the lowest energy structures at the surface of the zeolite. Therefore, we can understand why the 0.3 nm step is removed in an uncorrelated fashion whereas the 0.9 nm step is removed in a correlated fashion. Figure 10 shows

Figure 10

Schematic illustration of the dissolution process in zeolite A.36

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schematically the removal of different units. As can be seen, the single four-rings are not connected to one another. Consequently, it is not surprising that they are removed in an uncorrelated fashion. As the four-rings are not connected to one another it is not necessary for one of the single four-rings to be removed before its neighbour is removed. On the other hand, the sodalite cages are connected to one another and consequently it is necessary for one sodalite cage to be removed before the next sodalite cage can be removed. What it is not possible to say from the atomic force micrographs is whether the original surface is terminated with the single four-ring or with the sodalite cage. Either explanation would fit with the experimental data. However, again from theoretical calculations, it can be shown that the lowest energy structure is based on the single four-ring. In order to address this question experimentally it will be necessary to turn to high-resolution electron microscopy such as the image shown in Figure 2. If the quality of the electron micrographs are sufficient at the surface, and with the proviso that the images show the full projected profile at the surface, then it should be possible to glean information about the surface terminations. Indeed this has already been done for zeolite L,41 zeolite Y42 and zeolite beta.43 Another very interesting feature that is observed upon the dissolution of zeolite A can be seen in Figure 11. As the terrace withdraws it eventually breaks up into small nano-squares which are very mono-disperse in size. They have dimensions approximately 90  90  0.9 nm3. These nano-squares persist for quite a long time before finally dissolving suggesting that they are in some ways stabilised at this particular size. It is unclear why this should be the case and it will be necessary to determine theoretically both the entropic and enthalpic components of the process of dissolution to shed some light on this very curious phenomenon. Further, in Figure 10 when the final nano-square is removed from the top of the pyramid it is possible to see the underlying defect at the centre of the pyramid as described previously.

5 Effect of Supersaturation: Silicalite System Silicalite is another very important zeolite system. It is the purely siliceous end member of the MFI system of which ZSM-5 is perhaps the most important material. Figure 12 shows a scanning electron micrograph of two substantially different types of crystals. The smaller crystals with round ends are synthesised at relatively low temperatures and the synthesis is also stirred. The much larger crystals that are boat-shaped are synthesised at high temperatures without stirring. Despite belonging to the same crystal system, clearly these two crystal habits are substantially different. The aspect ratio of the crystals is different and the facets that are exposed, especially at the ends of the crystals, are also different. Spectroscopic techniques also show that the smaller crystals exhibit far fewer defects than the larger crystals. Consequently, if we can understand something about the crystal growth mechanism in this system it should be possible to say something about the different morphologies of these two

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1 mm  1 mm AFM images showing dissolution of zeolite A in mother liquor diluted to 67%.

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SEM images of different morphologies of silicalite achieved by different synthetic conditions. Scale bar 10 mm for the lower images synthesised at high temperature and 5 mm for the upper images synthesised at low temperature.20

synthetic methods. The structure of silicalite is formed by connecting structural chains composed principally of five-membered rings. These chains are chiral in nature spiralling either in a left- or right-handed manner. As with any structural enantiomorphic pair they can be connected to one another either through a mirror plane or via a centre of inversion. The full structure for silicalite is formed by connecting the chains in one direction via the mirror-plane and in the orthogonal direction by an inversion centre. However, this leads to the possibility of mistakes forming which are very common in this particular structure. Indeed much of the work from the Cambridge group early on was involved with research into intergrowths formed by such mistakes. The surface topology of silicalite reveals a plethora of different surface features which all give clues as to the nature of the growth mechanism.29 Figure 13 shows a variety of atomic force micrographs recorded on the larger boat-shaped crystals. Figure 13a shows a large composite picture of a substantial area of the (010) surface of silicalite. Different terrace structures can be seen in different parts of the crystal. For instance, towards the left-hand side of the figure and towards the centre of the figure, terraces are observed with more or

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less straight edges. The edges of these terraces run parallel to one of the principal crystallographic directions. The height of the terraces is equivalent to tens of unit cells, in other words they are not equivalent to a specific unit of the structure of silicalite. Towards the right-hand side of Figure 13a the terraces are circular in nature and the terrace height is now equivalent to one unit cell, that is 1.0 nm. In Figure 13b some finer features are also observed emanating from the large defect structure clearly apparent in the image. This seems to act as a nucleation point for terrace growth in much the same way as the defect sites in zeolite A which caused the pyramidal growth. Indeed this seems to be a common feature in the growth of zeolites that defects and imperfections act

Figure 13

AFM micrographs showing the surface topography of silicalite. The insets in each case show the crystal orientation and the area scanned. (a) 27.0  7.2 mm2 amplitude image of the (010) face, (b) 3  3 mm2 amplitude image of the (010) face, (c) 6 6 mm2 amplitude image of the (010) face, (d) 7  7 mm2 amplitude image of the (100) face, (e) 7.5  7.5 mm2 amplitude image of the (100) face.20

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as nucleation centres for growth. Of course this is not surprising as it is also a common feature of crystal growth in dense phase systems. These large silicalite crystals we know have a very large concentration of defects and this is likely as a result of incorrect stacking of the growth units. We have shown that these intergrowth structures caused by stacking sequence imperfections hinder the growth but can be healed by an over-growth of the defect.35 This slowing down of the growth process causes terraces to stack up resulting in these very high features observed on the surface of the large crystals. The atomic force micrographs shown in Figure 13 were recorded on crystals that had been removed at the end of the synthesis from the nutrient. At this point in the crystallisation all the silica nutrient has been exhausted and the supersaturation is consequently very low. One way to address this issue of supersaturation would be to record the AFM under in situ conditions. However, silicalite is grown normally at quite high temperatures well above 100 1C. This is beyond the easily attainable temperature for AFM under solution. Furthermore, the typical synthesis uses a gel which is opaque to the laser used in the AFM in order to determine tip deflection. As a result it is not easy to record measurements in situ for the silicalite system under these conditions. We have used an alternative approach in these circumstances in order to achieve meaningful information over a variety of supersaturation conditions. Syntheses have been performed under continuous flow conditions whereby the supersaturation is maintained at a predefined level. The synthesis can then be stopped at a particular point and the AFM recorded ex situ. This has been done for a series of samples some of which are illustrated in Figure 14. The scanning

Figure 14

(a) and (b) show levels of supersaturation and crystal length during crystallisation. Samples were extracted in the two highlighted regions. (c) shows SEM images as the crystal grows.

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electron micrographs are all shown to the same absolute scale and illustrate that the length of the crystals is increasing linearly with time. Also shown in Figure 14 is a normal supersaturation profile as a function of time during the crystal growth. At short times the supersaturation level is low as amorphous nutrient is dissolved into solution. During this regime the crystal growth is very slow. As the supersaturation increases it eventually reaches a constant and high level at which time the crystals grow at a constant rate. At the end of the synthesis all nutrient is consumed, the supersaturation level falls and the crystals cease to grow. Using our ex situ method for growth we can prepare crystals either in a regime of higher supersaturation or in a regime of low supersaturation. The results of the AFM on a complete series of crystals prepared in this manner are shown in Figure 15 and the results are quite striking. The left side of Figure 15 shows the (010) face and the right side shows the (100) face. At short times, Figures 15a–c, the atomic force micrographs are recorded as the supersaturation level is increasing. From Figures 15d–h the supersaturation level is high. Then the supersaturation level is allowed to drop as the crystals equilibrate with the growing solution. Figure 15i is therefore recorded at low supersaturation. Nutrient flow is then switched on again so that from Figures 15j to 15m the supersaturation level is once again high. Finally the nutrient is switched off once again such that the supersaturation level decreases. The final atomic force micrograph, Figure 15n, is then recorded at low supersaturation. There is a stark contrast between the atomic force micrographs recorded at low and at high supersaturation. At high supersaturation there are a plethora of nucleation sites observed covering both crystal facets. In other words the nucleation density is very high. At low supersaturation the nucleation density is very low, however, the terraces have continued to spread across the surface of the facets resulting in well-defined terrace topography. This indicates that the first process to be switched off as the supersaturation drops is the surface nucleation. As the free-energy of a clean surface will be lowest, the surface nucleation events will be the most energetically unfavourable, requiring the highest supersaturation. This issue is addressed again in the section on modelling of crystal growth where we are able, in the computer, to mimic these changes in supersaturation. However, this work illustrates how fundamental growth processes can be individually switched on and off. This is the beginning of the type of control that will be required in order to control intergrowth structures.

6 MOFs Metal–organic frameworks (MOFs) are a rapidly emerging class of hybrid nanoporous materials. Their attraction derives from inclusion of both inorganic and organic components within the framework. The latter may be modified to allow for the design of materials with specific functionalities, properties and structure.44 Certain MOFs are finding potential for commercial application particularly in the field of selective gas adsorption. A particularly

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AFM images (a)–(n) on the left of (010) face and (a)–(n) on the right of (100) face of silicalite. The images have different scales as the crystal grows.

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attractive prospect is the use of MOFs for hydrogen storage. However, industrial usage will benefit from the ability to control defects and crystal habit. Such control requires fundamental understanding of the crystal growth processes involved. One MOF of particular interest is Cu3[(O2C)3C6H3]2(H2O)3, HKUST-1,45 the structure of which is shown in Figure 16a. Micrographs of (111) faces of HKUST-1 exhibit multi-nucleated growth, as shown in Figures 16b and c. The nature of the spirals is complex – multiple, interpenetrating spirals are often observed and these may grow in the same direction or opposing directions. The latter phenomenon produces topography similar to layer growth. Spirals adopt the trigonal symmetry of the growing face. Detailed cross-sectional analyses reveal the predominant step height to be 1.5 nm, with additional step heights of 0.8, 2.2 and 3.0 nm (all 0.1 nm). These step heights correspond to integer multiples of the 0.76 nm d222 spacing. Whereas the separation between steps on the surface of zeolites decreases towards the edge of the crystal, that of HKUST-1 remains constant. This is indicative of non-diffusion limited growth, consistent with the low viscosity of the synthesis medium. Kink site growth does not predominate in this material. This is evidenced both by the transition of the spiral shape from triangular to circular, Figure 16b, and the persistence of terrace coalescence cusps, lower right portion of Figure 16c. Faulting of the crystal surface is often observed, evidenced by streaking in the micrographs (see Figure 17). The terrace structure is invariably commensurate across these faults, indicative of faulting occurring after crystallisation is complete. Such faulting highlights the fragility of this material and may be a consideration for its usage in certain applications. This AFM study of HKUST-1 is the first to be reported for a MOF material and provides insight into its crystal growth process.

7 Modelling The ultimate goal from this wealth of new information which has been derived from AFM is to establish the growth mechanism and to determine activation energies and rate constants for fundamental processes.31 Armed with this new information it is hoped that it will be possible to control crystallisation with precision hitherto impossible. This would allow, for instance, the preparation of nanoporous materials in the absence, or with substantially reduced amounts, of expensive templating agents. The question then arises how to extract the maximum possible information from the atomic force micrographs. A process of modelling is necessary. This can work from the bottom up or from the top down. Working from the bottom up requires the calculation of fundamental rate processes utilising either ab initio or molecular dynamics calculations to determine reaction pathways. There has been considerable success recently using such an approach for the molecular crystal system urea.46 Fundamental rate constants were determined for particular attachment processes and these rates used in turn to calculate crystal morphologies. The use of similar methods

Figure 16

(a) The structure of HKUST-1 viewed along (100) and AFM amplitude images of (111) facets of HKUST-1 showing (b) a 6  6 mm2 image of a double spiral and (c) an 8.5  8.5 mm2 image of a sextuple spiral.

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AFM image (8.0  8.0 mm2) of (111) face of the metal–organic framework HKUST-1 showing spiral growth and linear defects.

for covalently bonded crystals such as framework nanoporous materials will be substantially harder because of the complexity of the system.47 Nevertheless, inroads have been made into the topic with the establishment of surface free energies for realistic growth sites. The problem is compounded for zeolites by the presence of both water and cations in the system and, consequently, most of the initial work has been done on silica frameworks. If the rate constants can be determined reasonably accurately then it should be possible to use that information to calculate not only crystal morphologies but also surface topologies. At present, in the absence of such information we are utilising a top-down approach. In this method we start from the experimental information, that is, crystal morphology from electron microscopy and surface topologies from AFM. We then develop models based upon a simplified reaction mechanism to which we assign fundamental growth rates. Using a three-dimensional model we then compute crystal morphology and surface topology and compare this with the experimental data. As the calculation is very rapid, at least for crystals with a size of about half a micron, the process can be iterated until a unique set of fundamental reaction rates gives a solution that matches the experimental data. Figure 18 shows an example of a three-dimensional calculation of both crystal morphology and surface topography using this top-down approach. The calculation in essence is suitable for a cubic system such as zeolite A or sodalite. The structure of the zeolite is broken down into fundamental units

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Figure 18

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Modelling of crystal growth in zeolite A. The three-dimensional model can simulate both crystal morphology and surface topology in order to extract fundamental growth rates by comparison with AFM and SEM data.48

such as cages or ring structures which are likely to be the lower energy structures present at the surface of the crystal. A matrix is then developed for the connectivity of these units and rates applied to those linkages depending upon the structural neighbourhood. The accuracy of the calculation will depend to some extent on how the local structure is differentiated. Considering only first nearest neighbours means that only surface topography can be calculated and not crystal morphology. Going to second nearest neighbours

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allows crystal morphology also to be determined. It is also possible to change probabilities, or rates, as the calculation progresses and this will mimic changing supersaturation in the reaction mixture (similar to the situation that was observed in silicalite). Figure 18 shows three calculations with substantially different output both for surface topology and crystal morphology. Depending upon the fundamental rate processes either (100) facets are predominant or (110) facets become predominant. As all the rate processes become similar then the crystal morphology becomes more or less spherical. Similarly, as kink site growth becomes predominant over edge site growth then the surface terraces become more square in nature. We are then able to compare these theoretical calculations with the experimental data and iterate the process until a suitable match is found. In principle it should be possible to marry the top-down and bottom-up approaches with a unifying set of rate constants. At that point we should have a very good model, from fundamental principles, of how these nanoporous materials grow.

Acknowledgement We would like to acknowledge the assistance of R.J. Plaisted for help with the continuous flow experiments. We also thank EPSRC and ExxonMobil for funding.

References 1. R.M. Barrer, Hydrothermal Chemistry of Zeolites, Academic Press, London, 1982. 2. D.W. Breck, Zeolite Molecular Sieves, Wiley, 1974. 3. L.A. Bursill, E.A. Lodge and J.M. Thomas, Nature, 1980, 286, 111. 4. B.M. Lowe, Zeolites, 1983, 3, 300. 5. L.A. Bursill, J.M. Thomas and K.J. Rao, Nature, 1981, 289, 157. 6. M. Audier, J.M. Thomas, J. Klinowski, D.A. Jefferson and L.A. Bursill, J. Phys. Chem., 1982, 86, 581. 7. J.M. Thomas, G.R. Millward, S. Ramdas, L.A. Bursill and M. Audier, Faraday Discuss. 1981, 72, 345. 8. J.M. Thomas, S. Ramdas, G.R. Millward, J. Klinowski, M. Audier, J. Gonzalez-Calbet and C.A. Fyfe, J. Solid State Chem., 1982, 45, 368. 9. J.M. Thomas, G.R. Millward, S. Ramadas and M. Audier, ACS Symp. Ser., 1983, 218, 181. 10. G.R. Millward, S. Ramdas, J.M. Thomas and M.T. Barlow, J. Chem. Soc., Faraday Trans. 2, 1983, 79, 1075. 11. G.R. Millward and J.M. Thomas, J. Chem. Soc., Chem. Commun., 1984, 77. 12. O. Terasaki, J.M. Thomas and S. Ramdas, J. Chem. Soc., Chem. Commun., 1984, 216. 13. O. Terasaki, J.M. Thomas and G.R. Millward, Proc. R. Soc. London, Ser. A, 1984, 395, 153.

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14. G.R. Millward, S. Ramdas and J.M. Thomas, Proc. R. Soc. London, Ser. A, 1985, 399, 57. 15. G.R. Millward, J.M. Thomas and R.M. Glaeser, J. Chem. Soc., Chem. Commun., 1985, 962. 16. G. Binnig, H. Rohrer, C. Gerber and E. Weibel, Phys. Rev. Lett., 1982 49, 57. 17. G. Binnig, C.F. Quate and C. Gerber, Phys. Rev. Lett., 1986, 56, 930. 18. Recorded on a NanoWizard AFM from JPK by the authors. 19. M.W. Anderson, J.R. Agger, N. Hanif, O. Terasaki and T. Ohsuna, Solid State Sci., 2001, 3, 809. 20. L.I. Meza, PhD thesis, The University of Manchester, 2006. 21. A.M. Walker, B. Slater, J.D. Gale and K. Wright, Nat. Mater., 2004, 3, 715. 22. M.W. Anderson, O. Terasaki, T. Ohsuna, A. Philippou, S.P. MacKay, A. Ferreira, J. Rocha and S. Lidin, Nature, 1994, 367, 347. 23. M.W. Anderson, O. Terasaki, T. Ohsuna, P.J. O’Malley, A. Philippou, S.P. MacKay, A. Ferreira, J. Rocha and S. Lidin, Philos. Mag. B, 1995 71, 813. 24. G.Z. Wulff, Kristallogr. Kristallgeom., 1901, 34, 949. 25. J.W. Gibbs, Collected Works, Longman, New York, 1928. 26. W.K. Burton, N. Cabrera and F.C. Frank, Philos. Trans. R. Soc. London, Ser. A, 1951, 243, 299. 27. A. McPherson, A.J. Malkin and Y.G. Kuznetsov, Annu. Rev. Biophys. Biomol. Struct., 2001, 29, 361. 28. C.M. Zaremba, A.M. Belcher, M. Fritz, Y.L. Li, S. Mann, P.K. Hansma, D.E. Morse, J.S. Speck and G.D. Stucky, Chem. Mater., 1996, 8, 679. 29. J.R. Agger, L.I. Meza, C.S. Cundy, R.J. Plaisted and M.W. Anderson, Stud. Surf. Sci. Catal., 2005, 158, 35. 30. M.W. Anderson, J.R. Agger, J.T. Thornton and N. Forsyth, Angew. Chem., Int. Ed., 1996, 35, 1210. 31. J.R. Agger, N. Pervaiz, A.K. Cheetham and M.W. Anderson, J. Am. Chem. Soc., 1998, 120, 10754. 32. M.W. Anderson, J.R. Agger, N. Pervaiz, S.J. Weigel and A.K. Cheetham Proc. 12th Int. Zeol. Conf., Baltimore, 1998, MRS, ed. M.M.J. Treacey et al., pp 1487. 33. M.W. Anderson, N. Hanif, J.R. Agger, C.-Y. Chen and S.I. Zones, Stud. Surf. Sci. Catal., 2001, 135, 141. 34. R. Singh, J. Doolittle Jr., M.A. George and P.K. Dutta, Langmuir, 2002, 18, 8193. 35. J.R. Agger, N. Hanif, C.S. Cundy, A.P. Wade, S. Dennison, P.A. Rawlinson and M.W. Anderson, J. Am. Chem. Soc., 2003, 125, 830. 36. L.I. Meza, M.W. Anderson, J.R. Agger, Chem. Commun., 2007, 2473. 37. S. Sugiyama, S. Yamamoto, O. Matsuoka, H. Nozoye, J. Yu, Z. Gaugshang, S. Qiu and O. Terasaki, Microporous Mesoporous Mater., 1999, 1, 28. 38. S. Dumrul, S. Bazzana, J. Warzywoda, R. Biederman and A. Sacco Jr., Microporous Mesoporous Mater., 2002, 54, 79.

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39. S. Bazzana, S. Dumrul, J. Warzywoda, L. Hsiao, L. Klass, M. Knapp, J.A. Rains, E.M. Stein, M.J. Sullivan, C.M. West, J.Y. Woo and A. Sacco Jr., Stud. Surf. Sci. Catal., 2002, 142A, 117. 40. T.E. Gier and G.D. Stucky, Nature, 1991, 349, 508. 41. T. Ohsuna, B. Slater, F. Gao, J. Yu, Y. Sakamoto, G. Zhu, O. Terasaki, D.E.W. Vaughan, S. Qiu and C.R.A. Catlow, Chem.–Eur. J., 2004 10, 5031. 42. O. Terasaki, T. Ohsuna, V. Alfredsson, J.O. Bovin, S.W. Carr, M.W. Anderson, D. Watanabe, Stud. Surf. Sci. Catal., 1994, 83, 77. 43. B. Slater, C.R.A. Catlow, Z. Liu, T. Ohsuna, O. Terasaki and M.A. Camblor, Angew. Chem., 2002, 114, 1283. 44. M.J. Rosseinsky, Microporous Mesoporous Mater., 2004, 73, 15. 45. S.S.Y. Chui, S.M. Lo, J.P.H. Charmant, A.G. Orpen and I.D. Williams, Science, 1999, 283, 1148. 46. S. Piana, M. Reyhani and J.D. Gale, Nature, 2005, 438, 70. 47. B. Slater, C.R.A. Catlow, Z. Liu, T. Ohsuna, O. Terasaki and M.A. Camblor, Angew. Chem., Int. Ed., 2002, 41, 1235. 48. C.B. Chong, PhD thesis, The University of Manchester, 2007.

CHAPTER 7

Exploration of New Porous Solids in the Search for Adsorbents and Catalysts PAUL A. WRIGHT AND WUZONG ZHOU School of Chemistry, University of St Andrews, Purdie Building, North Haugh, St. Andrews, Fife, KY16 9ST, UK

At the beginning of the 1980s, the field of ordered microporous solids, with its well developed applications in adsorption and acid catalysis, was dominated by aluminosilicate zeolites. These included materials prepared via fully inorganic gel syntheses (such as zeolites A, L, Y and mordenite) as well as more siliceous zeolites prepared with readily available organic bases that acted as structure directing agents (SDAs). The most notable of these high silica zeolites were the large pore zeolite Beta and the medium pore ZSM-5, both of which offered improved properties over existing acid catalysts. The high silica materials are stable structures with high acid strength. Furthermore, the medium pores of ZSM-5 give excellent product selectivities, particularly in the conversions of monoaromatics, and Beta, the first high silica zeolite with a three-dimensionally connected pore system, finds increasing application in the conversions of larger molecules, where it provides an alternative to zeolite Y. Since the early 1980s, there has been a tremendous development of the structural types of ordered porous solids. The novel structural chemistry possessed by some of these offers improved properties in catalysis, particularly in selective oxidation, and in adsorption, where very large pore volumes have been observed. Furthermore, crystalline microporous solids (pore sizes 3–20 A˚) were joined in the early 1990s by ordered mesoporous solids (pore sizes 420 A˚), described in the second part of this chapter. Throughout, we will discuss recent developments in the synthesis of novel structure types (micro- and mesoporous) and their impact (potential or actual) in adsorption and catalysis.

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1 Developments in the Synthesis and Study of Crystalline Microporous Solids There has been tremendous activity in the synthesis of novel crystalline microporous solids, which can be traced to key developments that include: expansion of the range of inorganic composition of the frameworks, the rational design of ‘templating’ organics and the development of organic–inorganic hybrid solids with permanent porosity. In parallel with all of these, structural methods have been devised that are sufficiently powerful to determine the structures of materials frequently synthesised as microcrystalline powders that are not amenable to study by conventional laboratory single crystal X-ray diffraction. The development of electron microscopy and synchrotron X-ray diffraction, both single crystal and powder, have been invaluable here, frequently augmented by neutron diffraction, solid state NMR and computer simulation to establish structural details. Sir John Meurig Thomas has made major contributions in all these areas, both at Cambridge and at the Royal Institution. Early work on electron microscopy, for example, demonstrated the power of high resolution transmission electron microscopy to identify structure and microstructure in the important zeolite catalyst ZSM-5.1 Electron microscopy has since developed into an indispensable tool, both in structure solution and in the visualisation of structural intergrowths. A recent example from our own laboratory is in the imaging of zeolite Beta,2 where the visualisation of ‘double pore’ defects strongly supports a model of layer-by-layer growth for this important large pore catalyst (in this case large pore refers to pores bounded by 12 cations and 12 oxygen atoms, with a free diameter of ca. 8 A˚). Pioneering studies using the other methods to investigate the structure of zeolites with adsorbed molecules and in situ under catalytic conditions have opened the way to a much deeper understanding of microporous adsorbents and catalysts.

1.1

An Expanded Compositional Range

The widening of the compositional range from aluminosilicate zeolites and their silica polymorphs to the much greater diversity we see now was prompted by the discovery of the aluminophosphates by Flanigen and co-workers in the early 1980s.3 They showed that tetrahedrally coordinated AlPO4 frameworks are readily prepared with structures that are in some cases identical to those of zeolites (A, X, Y and chabazite, for example) but also exhibit topologies not observed as silicates, such as the very large pore VPI-5,4 which has channels ca. 1 nm in free diameter. Their chemistry can be expanded by framework substitution during synthesis, for example (i) of metal cations such as magnesium, cobalt or manganese for aluminium or (ii) of silicon for phosphorus or in a coupled substitution for aluminium and phosphorus. These substitutions impart important catalytic functions to these solids, including those of selective oxidation and acid catalysis, and it is in these areas that the group of Thomas has made a major and sustained contribution.

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In most cases the acidity is not as strong as observed for zeolites of similar structure. Indeed, this has been made use of in the application of the acidic silicoaluminophosphate (SAPO) form of chabazite, SAPO-34, for which the combination of acidity and pore shape gives an active and selective catalyst for the methanol-to-olefins reaction that currently shows promise for the production of ethylene and propylene precursors for polyolefins.5 Here the deactivation rate is slower than its aluminosilicate analogue because of its lower acidity. By contrast, MgAPO-36, the structure of which was solved at the Royal Institution by a combination of electron microscopy, powder diffraction and computer modelling,6 possesses acidity comparable in strength with that of zeolites. While some AlPO4-based solids do therefore possess combinations of acidity and pore size giving catalysts of industrial promise they are unlikely to supplant zeolites as industrial shape selective solid acid catalysts. However, as Thomas and workers at Shell7 independently recognised, the chemical properties of their frameworks offer interesting possibilities for selective oxidation catalysis. This catalytic behaviour results from the M21 " M31 redox couples, as subsequently demonstrated and innovatively exploited by Thomas and co-workers at the RI and Cambridge, and described elsewhere in this book. In addition to aluminophosphates, other framework chemistries have been expressed in microporous solids that have found catalytic application. The titanosilicate analogues of zeolites, such as titanosilicalite-1 (TS-1),8 the titanium-containing silica version of ZSM-5, and the large pore Ti-Beta, have been found to have exceptional properties as selective oxidants, and stannosilicate Beta has also been shown to be an excellent catalyst for fine chemical conversions.9 The ability of the titanium and tin to coordinate peroxides and thereby activate them for reaction with hydrocarbons is essential for their selective activity. The inclusion of tetrahedral cations other than silicon or aluminium can also have the effect of changing the structure that crystallises. Germanium, for example, favours the formation of 3MRs and 4MRs, and as a result many new germanosilicates have been prepared, including the recently discovered extra-large pore germanosilicate, ITQ-33.10 In all these cases, framework cations are tetrahedrally coordinated. There is a growing family of microporous solids, however, that include cations with different coordination into the framework. In the titanosilicate ETS-10, for example, while the silicon is tetrahedral the titanium is permanently octahedrally coordinated, within chains of corner-sharing octahedra.11 Although in this material the titanium is not active as a selective oxidation catalyst, it does possess interesting electronic and optical properties. Other porous solids with mixed coordination include metal phosphates, such as the nickel phosphate VSB-5.12

1.2

Designer Templates

For tetrahedrally coordinated silicates or aluminophosphates, the majority of new structures are now prepared using organic amines or cations as SDAs, or organic templates. They control the nucleation and growth of open frameworks

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and stabilise them with respect to denser phases. Whereas the early studies concentrated on the use of relatively simple and readily available species, the last 20 years have seen the use of increasingly complex molecules as templates. In this approach, the final position of the organic species in the pore space is governed by short range, non-bonding interactions that are well modelled computationally by interatomic potentials. The close agreement between the predicted location of templates (on the basis of combined Monte Carlo-Simulated Annealing routines) and that determined using diffraction supports the accuracy of these simulations. Modelling can be used reliably to predict template positions where they cannot be determined by experiment: conversely, it means that templates can be designed for hypothetical structures that may be energetically feasible but have not yet been prepared. The use of programs such as ZEBEDDE (Zeolites By Evolutionary De-novo Design),13 in which potential templates are grown computationally within the pores of a hypothetical structure, remains an attractive approach. The empirical approach of the use of alkylammonium SDAs, rationally designed on the basis of charge, stability, size and shape and synthesised via a wide range of organic routes, has proved highly successful in the synthesis of microporous silicates by the groups of Casci (at ICI), Zones (Chevron), Corma (Valencia) and at Mobil and Mulhouse, leading in each case to a family of high silica or pure silica zeolitic materials (named NU-n, SSZ-n, ITQ-n, ZSM/ MCM-n and IM-n, respectively). We have ourselves collaborated with the group of Suk Bong Hong, in Korea, examining the structures of the TNU-n series of silicates. Using the diquaternary template bis-N-methylpyrrolidiniumbutane, for example, several zeolites can be prepared by carefully adjusting the synthetic conditions.14 The structure of one of these solids, TNU-9, was recently solved in a multinational collaboration, including a state-of-the-art combination of electron microscopy from the group of Terasaki in Stockholm and X-ray powder diffraction analysis by researchers at ETH, Zurich.15 TNU-9 is the most complex zeolite known. The framework contains 24 symmetrically distinct tetrahedral cation sites and two different medium pore 10MR channel systems running down the b axis, linked perpendicularly and running between identical but asymmetric silicate sheets that upon stacking give two distinct intersheet regions. This pore structure is likely to give interesting shape selectivities in catalytic reactions. The role of the diquaternary cation as a structure directing agent has been investigated by modelling, revealing four different sites for a single type of SDA.16 It is likely that other, as yet unknown, structures will have sets of sites of different geometry and size within their pore systems, and these could be directed by mixtures of two or more template molecules. This ‘co-templating’ approach is therefore a promising route to new solids. The use of more complex ‘designer’ templates has also been a profitable one for the synthesis of novel aluminophosphate structures. The remarkable metallo-aluminophosphate DAF-1, for example, was prepared at the Royal Institution in 1992 using the decamethonium ion, ((H3C)3N1(CH2)10N1(CH3)3), and was the first example of a structure with two parallel, distinct, large pore and interconnected channel systems (Figure 1).17 Continued research into the

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Figure 1

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Several novel aluminophosphate-based frameworks have been prepared by the use of complex organic templates: (left) DAF-1; (right, above) STA-2, showing also the position of the diquaternary template in the cages; (right, below) STA-7, which possesses two cages of different sizes (key: green spheres, aluminium; purple spheres, phosphorus; red, oxygen).

synthesis of novel aluminophosphate structures at St Andrews using diquaternary, triquaternary and also azamacrocyclic templates has given a rich variety of structures. Figure 1 also shows STA-2 and STA-7, two novel small pore structures with three-dimensionally connected pores prepared only as aluminophosphates.18 Suitable framework substitutions give catalysts active in methanolto-olefin or selective oxidation reactions mentioned previously. STA-7, for example, is a small pore aluminophosphate related to SAPO-34 but with two different cage sizes, rather than one. The structure is only synthesised as the catalytically active and stable silicoaluminophosphate variety by choosing two template molecules of different sizes, 1,4,7,11-tetraazacyclotetradecane (cyclam) and tetraethylammonium, one to stabilise each of the cage types.

1.3

Microporous Organic–Inorganic Hybrids – MOFs

If there has been a steady increase in the chemical and geometrical range of microporous inorganic solids, then this has been as nothing by comparison with the number of porous hybrid framework solids that have recently been prepared. Originally described as coordination polymers, they are more widely

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known as metal–organic frameworks (MOFs). Early examples include the aluminium methyl phosphonates AlMePO-a and b, Al2(CH3PO3)3, similar to AlPO4s but with their pores lined by fast-rotating methyl groups (Figure 2).19 As a consequence of their organic lining these solids have adsorption properties quite different from purely inorganic AlPO4s, and adsorbed molecules have much higher re-orientational mobilities within the pores, clearly shown by deuterium NMR.20 Further microporous phosphonate solids have recently been prepared, including a large pore nickel bisphosphonate (Figure 2)21 that consists of helical edge-sharing chains of octahedrally coordinated metal cations, cross-linked by N,N 0 -piperazinebismethylenephosphonate groups. While these phosphonates are of interest, by far the most well known and varied family of MOFs is that of metal di- and tri-carboxylates. The best known are those of Yaghi (MOF-n, especially MOF-522), those prepared at the

Figure 2

Selected microporous organic–inorganic hybrid solids, or MOFs: (a) the b-polymorph of Al2(CH3PO3)3, with octahedral and tetrahedral aluminium in green and phosphonate tetrahedra in purple, (b) Ni2(H2O)2 O3PCH2NC4H8NCH2PO3 with NiO5N octahedra green and phosphonate tetrahedra yellow, (c) MIL-53, in which corner-sharing chains of MO6 octahedra (M ¼ Cr, Al, Fe, Ga), in yellow, are linked by terephthalate groups, (d) HKUST-1, or copper benzene-tricarboxylate, in which dimers of CuO5 square pyramids (blue) are linked by benzene-1,3,5-tricarboxylate (trimesate) groups.

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Institute of Lavoisier in Versailles by the group of Fe´rey (MIL-n, especially MIL-53,23 -100, 10124), and the copper trimesate HKUST-1,25 all with stunningly beautiful crystal architectures (MIL-53 and HKUST-1 are represented in Figure 2). Many others continue to be prepared. Most work on these solids has concentrated on their adsorption properties and in particular the very large pore volumes they exhibit. Hydrogen storage properties have received the most attention, but applications involving the adsorption of methane and carbon dioxide are also possible. Relatively few catalytic studies using these solids have appeared, but it seems likely that they could find use, suitably functionalised, in low temperature solid/liquid phase reactions.

2 Mesoporous Solids: From Silica Supports to Porous Single Crystal Metal Oxides The discovery of ordered mesoporous silicates by Mobil scientists in 199226,27 opened a new research field in porous solids. These materials possess regular pores in the 1.5–10 nm range and exhibit remarkable structural features that heralded a new generation of adsorbents and catalysts. The Cambridge group released its first report in the field in 1995 and has made an important contribution to the synthesis and modification of mesoporous silicas. In particular, Sir John Meurig Thomas focused on the development of the porous materials into real catalysts and also published a number of important papers in HRTEM (High Resolution Transmission Electron Microscopy), STEM (Scanning Transmission Electron Microscopy) and electron tomography of catalytically active nanoparticles inside the mesopores.28 In the last 15 years, the family of mesoporous solids has been extended from silica into metal and metal oxide compositions, and wide-ranging applications can be expected in the future.

2.1

Mesoporous Silicas

The first and simplest phase is MCM-41, which contains a hexagonal array of cylindrical mesopores. Its pore structure can readily be observed by TEM along the pore direction and perpendicular to the pores.29 However, the diverse forms of liquid crystals of surfactants also lead to different pore dimensions and geometries. The structure of SBA-15,30 for example, looks similar to that of MCM-41, but the former also has smaller mesopores connecting the principal pores, resulting in a 3-dimensional rather than a 1-dimensional pore system. This feature is important in the later development of non-silicate porous crystals. Among the other mesoporous solids prepared by the group of Stucky in Santa Barbara, SBA-2 contains spherical cages in a hexagonal close packed (hcp) arrangement. In fact, our TEM studies revealed that irregular intergrowths of hcp and cubic close packed (ccp) components commonly exist in SBA-231 and we related this phenomenon to the intergrowths of the FAU (ccp) and EMT (hcp) polytypic framework types observed in faujasite-type zeolites.

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(Three letter codes FAU and EMT refer to zeolite framework types seen, for example, in faujasite and EMC-2.32) More cage-containing mesoporous silicates have been developed. Their pore sizes can be easily tuned and their thermal stability has been greatly improved. Two important phases are SBA1633 and FDU-12.34 SBA-16 has a body-centred cubic structure (space group Im 3m). FDU-12 is essentially face-centred cubic (space group Fm3m) with a ccp arrangement of spherical cages, although, like SBA-2, an intergrowth of domains with ccp and hcp is often seen.

2.2

Modified Mesoporous Silicates for Catalysis

Immediately after the first report of mesoporous silicas, it was realised that these materials have high potential for application in catalysis and gas separation due to their large uniform pores. However, two disadvantages must be overcome. First, the composition of pure silica indicates that the materials lack chemical activity. Second, almost all the mesoporous silicas templated by organic surfactants have amorphous walls, which are normally less stable than crystalline materials. As a result, modifications of the mesoporous silicas have been investigated extensively over the last 15 years. The classic method of increasing chemical activity of silica-based materials is by doping with elements of oxidation state lower than 4+. For example, a reasonably large amount of aluminium can be added into the silica wall of MCM-41.35,36 However, the stability of the materials is greatly reduced. In fact, many other elements can be used to substitute Si in these mesoporous materials and almost all of the processes inevitably reduce the stability.37 Another way to turn mesoporous silicas into catalysts is by loading catalytic nanoparticles into the pores. An early example is introducing bimetallic nanoparticles of Ru and Ag3Ru10 into mesoporous MCM-41 with a pore diameter of 3 nm, in a project led by Johnson and Thomas.38,39 The materials were tested as catalysts in a reaction of the hydrogenation of hex-1-ene to hexane. A high selectivity (in excess of 99%) and a high turnover frequency of at least 6300 mol hexane per mol [Ag3Ru10] per hour were observed. Although the nanoparticles were not strictly ordered in the pores, they are all in contact with the inner surface of the pores which are themselves ordered in a hexagonal array. Therefore, the nanoparticles can be imaged with different image contrast from the silica background in the HRTEM images and more significant contrast in annular dark-field high-resolution electron microscopic images. Extended X-ray absorption fine structure (EXAFS) confirmed the bimetallic composition of the nanoparticles, which were firmly anchored inside the siliceous mesopores. When the metallic nanoparticles (Ru10 or Ru6) were partially ordered in the mesopores of MCM-41, as we demonstrated in 1998,40 one may observe the corresponding SAED patterns and therefore estimate the average inter-particle distance (Figure 3). The ruthenium clusters, which possess high activity as hydrogenation catalysts, were well separated inside the pores with an average

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Figure 3

131

Left: STEM bright-field image of MCM-41 loaded with [H2Ru10 (CO)25][PPN]2 showing highly regular features along the pore axis, with its Fourier transform (inset). Right: Van der Waals surface interactions of two [H2Ru10(CO)25]2– and two PPN1 molecules packing along a single mesopore.

distance (2.95 nm) determined by the dimensions of the precursor complex salts, e.g. [Ag3Ru10C2(CO)28Cl][PPN]2 and [Ru12C2Cu4(CO)32Cl2][PPN]2. Accordingly, the HRTEM images show a ‘rosary’ pattern along the mesopores. Furthermore, our subsequent STEM dark-field Z-contrast images revealed nanoparticles in the mesoporous silicas much more clearly.41 When larger nanoparticles are loaded into mesopores, atomic images can be directly observed, even when a mesoporous silicate without long range order is used as a support. This was demonstrated in our report of Co and NiPd nanoparticles used as highly effective, cheap, recyclable and industrially-viable catalysts for the hydrogenation of a range of nitro-substituted aromatics under mild conditions.42 The group of Thomas developed single site catalysts based on the mesoporous silicas, which are discussed elsewhere in this book. The above examples concern the post-synthesis introduction of metal nanoparticles. Recently, a new method, the so-called true liquid crystal templating method, was developed. The addition of metal-containing precursors to the synthetic system for mesoporous silicas prior to sol–gel condensation results in the successful incorporation of nanoparticles of RuO2 in MCM-41. The resulting materials are effective catalysts for alkene hydrogenation43 and water oxidation.44,45 Similarly, many transition metals nanometres in size (e.g. Pd, Au, Ir, Pt, Fe, Co, Cr)46 as well as bimetallic nanoparticles, such as PtCo, PdAu,47 have been introduced into mesoporous silicas by this route. Finally, the extra-large pore SBA-15, suitably functionalised, has been found to be an excellent support for immobilising enzymes,48 a possibility foreseen by

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Sir John Meurig Thomas in his far-sighted commentary on the possibilities of mesoporous silicas written soon after they had first been reported.49

2.3

Non-Silicate Mesoporous Oxides

All these ordered mesoporous silicates were fabricated by using liquid crystalline arrangements involving micelles of organic surfactants as templates, in the so-called soft templating method. This method can also be used to prepare nonsilicate mesoporous transition metal oxides. In fact, almost all transition metal oxides can be made in this porous form if a suitable precursor is used and the materials may give different properties to meet the requirement for a particular application. However, the walls of these materials are amorphous, since the organic surfactants cannot be maintained at the high temperature needed for crystallisation of the oxides. On the other hand, if mesoporous silicas are used as templates, in the so-called hard templating method, we can make porous crystals of oxides. The first demonstration of the hard-templating methodology was given by Korean scientists in 1996,50 when they tried to image the channels of mesoporous silica by filling it with platinum. This method has now been developed to make nanowires and many porous crystals of transition metal oxides. Our first specimen of porous single crystals of metal oxide, Cr2O3 templated by SBA-15, was synthesised and characterised in 2003 in collaboration between Zhou and He’s group in Fudan University, Shanghai.51 These materials are of great interest because they can be regarded as self-supported nanoscale catalysts and have a high potential for catalytic application. The general route for producing porous crystals of oxides using mesoporous silicas as templates is by introduction of a metal-containing precursor into the silica pores, followed by its thermal decomposition and the subsequent crystal growth of the metal oxide inside the pores upon continued heating. A crucial step in the above process is the impregnation of the precursor. Several methods have been developed, the so-called surface modification ‘‘two solvents’’ method, the evaporation method and the solid–liquid method. In the surface modification method, the inner surface of the mesoporous silica template is functionalised via aminosilylation of the surface silanols and then a selected heteropolyacidic precursor (e.g. H2Cr2O7 for Cr2O3 and H3PW12O40 for WO3) is anchored.51,52 The functionalised surface is positively charged and the metal-containing ions are negatively charged (heteropolyacidic anions). Therefore, the driving force for migration of the precursor is mainly ionic attraction. In the ‘‘two solvents’’ method, a suspension of mesoporous silica in dry hexane is mixed with an aqueous solution of metal nitrate, e.g. Cr(NO3)3
9H2O. The precursor molecules will then move into the pores during overnight stirring.53 In the evaporation method, the mesoporous silica template is mixed with a metal nitrate in ethanol. It was thought that the nitrate precursor entered the pores during the evaporation of ethanol by capillary action. Very recently, we found the nitrate precursor did not enter the pores during the evaporation of ethanol and the migration actually took place during

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the thermal treatment when the nitrate melted. We then used a solvent-free solid–liquid method to introduce nitrates and high quality porous crystals of metal oxides were obtained.54,55 After the nitrate precursor was loaded, it decomposed during stepwise thermal treatment. The sequence of decomposition inside the mesopores was different from that without the mesoporous silicate. A confinement effect of pores, as a nanoreactor, is obvious. For example, when Cr(NO3)3
9H2O was used as the precursor, the formation temperature of Cr2O3 crystals inside the pores was 350 1C, while the corresponding temperature without mesoporous template was 400 1C. At 350 1C in the latter case, formation of Cr2O5 was observed. By way of comparison, Co(NO3)2
6H2O decomposed inside the pore to form intermediate crystalline phases of Co(NO3)2
2H2O and CoNO3(OH)
H2O, and finally Co3O4 crystals at 150 1C. Without using the mesoporous template, only

Figure 4

TEM examination of a porous single crystal of Cr2O3 viewed down two principal zone axes. (a) TEM image showing mesopore structure along the [1 1 1] direction of the KIT-6 related cubic unit cell and (b) the corresponding SAED pattern indexed onto the rhombohedral unit cell of Cr2O3. (c) TEM image showing the mesopore structure along the [1 0 0] zone axis of the KIT-6 related unit cell and (d) corresponding HRTEM image on the [2 2 -1] zone axis of the Cr2O3 unit cell.

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Co(NO3)2
4H2O crystallised and heating Co(NO3)2
6H2O gave different intermediate phases of a mixture of Co(NO3)2
4H2O and Co(NO3)2.56 TEM examinations of the porous crystals of these oxides (Cr2O3, Co3O4, NiO, WO3, etc.) indicate that the original pore systems of the mesoporous silicates can be replicated perfectly. Consequently, the morphology of the porous oxides templated by SBA-15 is an array of nanorods connected by some small bridges51 and that templated by KIT-6 is wave-shaped nanowires.53 If SBA-16 and FDU-12 were used as templates, we can expect the structures of the porous oxides to be three-dimensional arrangements of solid nanospheres connected to each other by some very short nanorods.54 The beauty of these porous crystals of transition metal oxides is that the whole particles are single crystals with a three-dimensional regular mesopore network as shown in Figure 4. The novel structures of the porous crystals of transition metal oxides imply that the new materials can be developed into self-supported nanoscale catalysts with activities comparable to those of nanoparticle catalysts but can also give shape selectivity due to the regular mesopores. It is also expected that the materials may have interesting physical properties in magnetism and gas adsorption.

3 Conclusion The nanoporous solids we have described offer a range of properties far beyond those of the zeolites known at the beginning of the 1980s. It seems to us that the inspiration for this remarkable expression of form and function lies in their exquisite nanoscale structure, the ‘architecture of the invisible’, so memorably elaborated by JMT in a 1993 commentary of the role of 80 years of diffraction in chemistry.57 Diffraction is one of many techniques he has championed in his studies of catalysts at Cambridge and the Royal Institution, in an endeavour we, like many others, are grateful to have shared with him.

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7. B. Kraushaar-Czarnetzki, W.G.M. Hoogervorst and W.H.J. Stork, Stud. Surf. Sci. Catal., 1994, 84, 1869. 8. M. Taramasso, G. Perego and B. Notari, U.S. Patent, 1983, 4, 410; C. Perego, A. Carati, P. Ingallina, M.A. Mantegazza and G. Bellussi, Appl. Catal. A, 2001, 221, 63. 9. A. Corma, L.T. Nemeth, M. Renz and S. Valencia, Nature, 2001, 412, 423; M. Renz, T. Blasco, A. Corma, V. Fornes, R. Jensen and L. Nemeth, Chem.–Eur. J., 2002, 8, 4708. 10. A. Corma, M.J. Diaz-Cabanas, J.L. Jorda, C. Martinez and M. Moliner, Nature, 2006, 443, 842. 11. M.W. Anderson, O. Terasaki, T. Ohsuna, A. Philippou, S.P. Mackay, A. Ferreira, J. Rocha and S. Lidin, Nature, 1994, 367, 347. 12. N. Guillou, Q. Gao, P.M. Forster, J.-S. Chang, M. Nogues, S.-E. Park, G. Fe´rey and A.K. Cheetham, Angew. Chem., Int. Ed., 2001, 40, 2831. 13. D.W. Lewis, D.J. Willock, C.R.A. Catlow, J.M. Thomas and G.J. Hutchings, Nature, 1996, 382, 604. 14. S.B. Hong, E.G. Lear, P.A. Wright, W.Z. Zhou, P.A. Cox, C.H. Shin, J.H. Park and I.S. Nam, J. Am. Chem. Soc., 2004, 126, 5817. 15. F. Gramm, C. Baerlocher, L.B. McCusker, S.J. Warrender, P.A. Wright, B. Han, S.B. Hong, Z. Liu, T. Ohsuna and O. Terasaki, Nature, 2006, 444, 79. 16. S.B. Hong, H.-K. Min, C.-H. Shin, P.A. Cox, S.J. Warrender and P.A. Wright, J. Am. Chem. Soc., 2007, in Press. 17. P.A. Wright, R.H. Jones, S. Natarajan, R.G. Bell, J.S. Chen, M.B. Hursthouse and J.M. Thomas, J. Chem. Soc., Chem. Commun., 1993, 633. 18. G.W. Noble, P.A. Wright and A. Kvick, J. Chem. Soc., Dalton Trans., 1997, 4485; P.A. Wright, M.J. Maple, A.M.Z. Slawin, V. Patinec, R.A. Aitken, S. Welsh and P.A. Cox, J. Chem. Soc., Dalton Trans., 2000, 1243. 19. K. Maeda, J. Akimoto, Y. Kiyozumi and F. Mizukami, Angew. Chem., Int. Ed. Engl., 1995, 34, 1199; K. Maeda, J. Akimoto, Y. Kiyozumi and F. Mizukami, J. Chem. Soc., Chem. Commun., 1995, 1033. 20. J. Gonzalez, R.N. Devi, P.A. Wright, D.P. Tunstall and P.A. Cox, J. Phys. Chem. B, 2005, 109, 21700. 21. J.A. Groves, S.R. Miller, S.J. Warrender, C. Mellot-Draznieks, P. Lightfoot and P.A. Wright, Chem. Commun., 2006, 3305. 22. H. Li, M. Eddaoudi, M. O’Keeffe and O.M. Yaghi, Nature, 1999, 402, 276. 23. F. Millange, C. Serre and G. Fe´rey, Chem. Commun., 2002, 822; T. Loiseau, C. Serre, C. Huguenard, G. Fink, F. Taulelle, M. Henry, T. Bataille and G. Fe´rey, Chem.–Eur. J., 2004, 10, 1373. 24. G. Fe´rey, C. Mellot-Draznieks, C. Serre and F. Millange, Acc. Chem. Res., 2005, 38, 217. 25. S.S.Y. Chui, J.P.H. Charmant, A.G. Orpen and I.D. Williams, Science, 1999, 283, 1148. 26. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature, 1992, 359, 710.

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27. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins and J.L. Schlenker, J. Am. Chem. Soc., 1992, 114, 10834. 28. J.M. Thomas, O. Terasaki, P.L. Gai-Boyes, W.Z. Zhou and J. GonzalezCalbet, Acc. Chem. Res., 2001, 34, 583. 29. C. Cheng, W.Z. Zhou, D.H. Park, J. Klinowski, M. Hargreaves and L.F. Gladden, J. Chem. Soc., Faraday Trans., 1997, 93, 359. 30. D.Y. Zhao, J.L. Feng, Q.S. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka and G.D. Stucky, Science, 1998, 279, 548. 31. W.Z. Zhou, H.M.A. Hunter, P.A. Wright, Q. Ge and J.M. Thomas, J. Phys. Chem., 1998, 102, 6933. 32. C. Baerlocher, W.M. Meier and D.H. Olson, ‘Atlas of Zeolite Framework Types’, 5th edn, Elsevier, Amsterdam, 2001. 33. D.Y. Zhao, Q.S. Huo, J.L. Feng, B.F. Chmelka and G.D. Stucky, J. Am. Chem. Soc., 1998, 120, 6024. 34. J. Fan, C.Z. Yu, F. Gao, J. Lei, B.Z. Tian, L.M. Wang, Q. Luo, B. Tu, W.Z. Zhou and D.Y. Zhao, Angew. Chem., Int. Ed., 2003, 42, 314; J. Fan, C.Z. Yu, J. Lei, Q. Zhang, T.C. Li, B. Tu, W.Z. Zhou and D.Y. Zhao, J. Am. Chem. Soc., 2005, 127, 10794. 35. Z. Luan, C. Cheng, W.Z. Zhou and J. Klinowski, J. Phys. Chem., 1995, 99, 1018. 36. Z. Luan, H. He, W.Z. Zhou, C. Cheng and J. Klinowski, J. Chem. Soc., Faraday Trans., 1995, 91, 2955. 37. C. Cheng, H. He, W.Z. Zhou, J. Klinowski, J.A. Gonclaves and L.F. Gladden, J. Phys. Chem., 1996, 100, 390. 38. D.S. Shephard, T. Maschmeyer, B.F.G. Johnson, J.M. Thomas, G. Sankar, D. Ozkaya, W.Z. Zhou, R.D. Oldroyd and R.G. Bell, Angew. Chem., Int. Ed. Engl., 1997, 36, 2242. 39. D.S. Shephard, W.Z. Zhou, T. Maschmeyer, J.M. Matters, C.L. Roper, S. Parsons, B.F.G. Johnson and M. Duer, Angew. Chem., Int. Ed., 1998, 37, 2719. 40. W.Z. Zhou, J.M. Thomas, D.S. Shephard, B.F.G. Johnson, D. Ozkaya, T. Maschmeyer, R.G. Bell and Q. Ge, Science, 1998, 280, 705. 41. D. Ozkaya, W.Z. Zhou, J.M. Thomas, P. Midgley, V.J. Keast and S. Hermans, Catal. Lett., 1999, 60, 113. 42. R. Raja, V.B. Golovko, J.M. Thomas, A. Berenguer-Murcia, W.Z. Zhou, S. Xie and B.F.G. Johnson, Chem. Commun., 2005, 2026. 43. N.C. King, C. Dickson, W.Z. Zhou and D.W. Bruce, Dalton Trans., 2005, 1047. 44. M.J. Danks, H.B. Jervis, M. Nowotny, W.Z. Zhou, T.A. Maschmeyer and D.W. Bruce, Catal. Lett., 2002, 82, 95. 45. N.C. King, C. Dickinson, W.Z. Zhou and D.W. Bruce, Dalton Trans., 2005, 1027. 46. N.C. King, R.A. Blackley, W.Z. Zhou and D.W. Bruce, Chem. Commun., 2006, 3411.

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CHAPTER 8

Concerning the Solid State Packing of [(ButCO2)3M2]2 (l-9,10-anthracenedicarboxylate) Compounds (M = Mo or W) and Other Matters MALCOLM H. CHISHOLM, MATTHEW J. BYRNES, AJATSHATRU MEHTA AND PATRICK M. WOODWARD Department of Chemistry, The Ohio State University, Columbus, Ohio 43210, USA

1 Introduction Upon completion of my PhD with Professor D. C. Bradley, FRS, at Queen Mary College (QMC), London University I (M.H.C.) took up an appointment as a postdoctoral fellow with Professor H. C. Clark at the University of London Ontario. The move from London to London, Ontario in 1969 was very interesting for its contrasts. Ontario then had some very strange provincial rules, no doubt as a result of a Scottish Presbyterian influence. For example a group trip to a local pub on a Friday evening was a segregating affair. There was just one woman in the Clark group of 10 or 12 and the pub had two types of bars: one for ladies and escorts and the other for men only. A lady could only have three male escorts! Nor could one in those days stand to drink a beer or move from one table to another table with a drink in one’s hand. The barman was required to move the drink. These limitations aside the group’s comradeship and the chemistry were great and several of my immediate lab mates went on to have distinguished careers in academe (Professors Richard J. Puddephatt, Hideo Kurosawa and Kenji Itoh) or industry (Dr Leo E. Manzer) to name just those with whom I have maintained steady contact. The world seemed larger in those days and Canada more remote. When visiting speakers came to lecture at 138

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the newly established Chemistry Department at the University of Western Ontario (UWO) they often gave two or three lectures and stayed for several days. This, of course, allowed for more socializing with the visitors, and the students and postdoctoral fellows in addition to the academic members of staff got to participate in discussions of chemistry and other matters. This was a very thrilling and stimulating experience and one that was rather different from that at London University where I had rarely spoken with visiting speakers – though I do recall one day at QMC meeting the late Henry Gillman and being totally amazed as he recounted his discussions with Grignard. This had seemed to me as a 21 year old like listening to someone who had known Moses. To the great credit of the chemistry department at UWO they maintained a very distinguished visiting lecture series and thinking about this now I realize how greatly I benefited and was influenced by this and how much it must have cost them financially. But it was surely worth every penny for the seminar series in any department is the window of engagement with the outside world and in many ways just as influential as the written words that appear in the literature. It was during this time that I got to meet Henry Taube, Harry Gray, Daryle Busche, Jay Kochi, Malcolm Green and the man we honour in this volume, Professor Sir John Meurig Thomas. A truly great lecture requires that its form be every bit as good as its content. Indeed without a good delivery the information is most often lost on the audience due to inattention, boredom, and sleep. Circa 1970, John’s star had already risen and he was publishing both prolifically and profoundly and seemed to continually command the attention of the reviewers and editors of Nature. As I recall, his lecture at UWO was on the imperfections and dislocations within crystals and their significance in determining reactivity. Though this subject matter was far removed from my own research interests at that time, which dealt largely with the organometallic chemistry of cationic complexes of platinum(II), I do well recall his engaging and inspirational delivery. This has, of course, become a trademark of all John’s lectures. His selection of a word and crafting of a phrase together with impeccable pace and timing provide for a near theatrical experience of the highest order. The second time I met John was at a meeting I organized in Bloomington, Indiana in 1982 shortly after I had moved to Indiana University from Princeton University. This conference was jointly sponsored by the inorganic divisions of the American Chemical Society, the Royal Society of Chemistry (UK), and the Canadian Institute of Chemists, as it was then called, with additional funding from numerous agencies and industries. The conference theme and its published volume and proceedings was titled ‘‘Inorganic Chemistry: Toward the 21st Century.’’ The emphasis was forward looking and I must say that the turn of the millennium in 1982 seemed just as far away as Orwell’s ‘‘1984’’ had when read at school ca. 1960. To this symposium were sent 20 speakers from the USA, and 10 from each of the UK and Canada and John was amongst the 10 from the UK contingent. Aside from the representatives from the USA, the other speakers were selected by their countries and I had no hand in the selection process. John was by this time Professor of Physical Chemistry in the

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University of Cambridge and when I saw his name on the UK list I wondered how this interloper had been selected. Surely this was a sign of weakness in the folds of UK inorganic chemistry. John’s lecture was, however, perfectly appropriate and highlighted the power of high resolution electron microscopy to the study of the surfaces of inorganic materials. This has been one of his continuing interests along with pioneering the applications of numerous analytical techniques to problems of solid state inorganic materials and catalysis so this talk was truly well placed and influential. The Department of Chemistry prevailed upon John to stay a little longer in order to give another lecture. This he did with great effect and he returned to an earlier theme relating organic reactions within the solid state to their intermolecular ordering. Since that time I have often had the opportunity to host John for a lecture and, indeed, to enjoy his company in more social settings. There can hardly be a more entertaining raconteur or dinner companion whether the subject be related to science, politics or trivia. For my part as a synthetic chemist, my area of expertise lies completely far afield from John’s though we share many common interests in reactivity and catalysis. Synthetic chemists have to rely on applying physical and analytical methods to know what they have made and in this regard the physical, analytical and theoretical chemists are much more clever. However, the value of synthesis in chemistry cannot be denied. To paraphrase the late Sir Geoffrey Wilkinson: ‘‘If you can’t make it, you can’t measure it.’’ It is also probably true that while the study of reaction mechanisms has led to our understanding of catalysis, the most significant discoveries in catalysis were made by serendipitous synthesis. Similarly new modes of chemical bonding, as in the discovery of sandwich metal complexes, dinitrogen and dihydrogen complexes and MM multiple bonding, were recognized only after synthesis. Probably the most powerful method of determining what you have made these days is single crystal X-ray crystallography. For relatively small molecules this can all be done in one day, from data collection to full refinement and with good data there can be no question concerning the arrangement of the atoms with respect to each other. All other spectroscopic techniques cannot compete in this regard and as stated by the late Professor Cotton may be viewed merely as ‘‘sporting techniques’’. However, single crystal X-ray crystallography does still require the preparation of single crystals and sometimes this proves too difficult for even a talented synthetic chemist. Sometimes the crystals are too small, too thin, or whisker-like for even modern instrumentation or synchrotron sources. Sometimes a chemist may know the overall molecular structure of the subunits, as for example with an aromatic ring, a porphyrin or a paddlewheel carboxylate, M2(O2CR)4, having a virtual D4h M2(O2C)4 core of the type shown in I below, but not how these units are connected or arranged in space in the solid state. Yet it is often the molecular connectivity that is important in determining the physical properties of the material in the solid state – the magnetism, conductivity, optical properties, etc. Fortunately recent developments in the applications of powder X-ray diffraction (PXRD), as pioneered by one of the editors in this volume along with others, can greatly assist in clarifying these matters and this forms the scientific portion of this article.1

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Concerning the Solid State Packing

C O

O

M O

O

C

C O M

O C

O I

O

2 Dicarboxylate Linked M2 Quadruply Bonded Complexes When two MM quadruply bonded complexes are linked by a dicarboxylate group, the M2 centres may be electronically coupled as a result of the M2 d orbital combinations mixing with the ligand bridge p-system. The two CO2 units act as alligator clips: they link the M2 units together covalently and electronically couple them via the p-system. The key orbital interactions are shown in II and III below for the oxalate bridge.2

II (b3u in D2h symmetry)

III (b1g in D2h symmetry)

Of the two orbital interactions shown, II is the most important because of orbital energy and overlap considerations. [The filled CO2 p orbitals lie roughly 6 eV lower in energy than the M2 d orbitals]. Consequently metal d to bridge p* bonding occurs, splitting the energy of the two M2 d combinations and leading to the preference of a planar oxalate bridge. The free oxalate dianion, however, has a twisted D2d structure which minimizes electrostatic repulsion between the oxygen atoms. As a consequence, of those two opposing forces the planar structure is favoured by only a modest amount, B4–9 kcal mol–1, based on electronic structure calculations on model compounds in the gas phase. Thus in solution, there exists a Boltzmann distribution of rotamers with respect to the dihedral angle between the two CO2 units of the oxalate bridge. The electronic coupling of the two M2 centres is a maximum for the planar D2h structure and is completely lost in the D2d structure. The oxalate bridge thus acts as a molecular rheostat with Y ¼ 0 (D2h) being fully on and Y ¼ 901 (D2d) being off. As a consequence, the oxalate-bridged complexes show thermochromism in solution but not in the crystalline state. The colour arises from the metal d to bridge oxalate p* transition.

142

Figure 1

Chapter 8

Electronic absorption spectra of [(ButCO2)3W2]2(m-O2CCO2) at 2, 50, 100, 150, 200, 250 and 300 K in 2-methyltetrahydrofuran solution. Reproduced from Ref. 2 with permission.

The electronic absorption spectrum of the complex [(ButCO2)3W2]2 (m-O2CCO2) in 2-methyltetrahydrofuran is shown in Figure 1. The room temperature absorption maximum, lE700 nm with eE36 000 M
1 cm
1 shifts smartly to the red on cooling and sharpens in intensity to eE150 000 M
1 cm
1 at 2 K. The prominent vibrational features seen to shorter wavelength (higher energy) arise from the coupling of the electronic wave function with the totally symmetric vibrations of the oxalate ligand in the photoexcited state. As can be anticipated from the description of the LUMO, placing an electron in a CO antibonding orbital and a C–C bonding orbital produces a significant change in C–C and C–O distances. In contrast to the spectral features described above, the electronic absorption spectra of the molybdenum and tungsten oxalate-bridged complexes are quite different when recorded either as a Nujol mull or in the solvents toluene or water. The latter spectra are remarkably similar and are shown in Figure 2. This puzzling observation was shown to arise because in toluene and in water the compounds do not truly dissolve: they form colloidal suspensions or nanoparticles due to the association of units in solution via intermolecular M2  O bonds (vide infra).3 Despite repeated attempts, crystals of these complexes suitable for single crystal diffraction studies could not be obtained because they formed fibrous

Concerning the Solid State Packing

Figure 2

143

Electronic absorption spectra of [{(tBuCO2)3W2}2(m-O2C2O2)] measured in water, Nujol mull, and toluene. Reproduced from Ref. 3 with permission.

needle-like crystals. Therefore, we turned to X-ray powder diffraction (XRPD) methods in hopes of elucidating the solid state packing schemes of these molecules. Data were collected by using a Bruker D8 diffractometer equipped with an incident beam Ge monochromator, a spinning capillary stage and a position sensitive detector. The air-sensitive samples were ground in a mortar and pestle and were sealed in 0.7 mm diameter capillaries inside a glove box. Patterns were indexed using the CRYSFIRE4 suite of indexing programs. Direct space structure solution was performed based on prior knowledge of molecular connectivity and geometry using the simulated annealing minimization algorithms incorporated in the program DASH.5,6 The solution reveals the molecular packing of molecules and the dihedral angle between the two CO2 planes of the bridging oxalate. With these limitations in mind, the drawing shown in Figure 3 indicates the salient features of the structure of the [(tBuCO2)3Mo2]2(m-O2CCO2) molecule in the solid state. Each unit contains a near planar central oxalate bridge where the O2C–CO2 dihedral angle is 15  31. Each molecule is connected to its nearest neighbours via weak intermolecular dative oxygen to metal bonds. These intermolecular Mo  O interactions involve both certain pivalate oxygen atoms and the oxygen atoms of the oxalate bridge. The axial ligation shown in Figure 3 leads to infinite chains of Mo4 containing molecules in the solid state and is a more complex variation of the packing of Mo(O2CR)4 compounds in the solid state that form laddered structures based on the coordination packing shown in IV below.

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Figure 3

Chapter 8

Intermolecular Mo–O interactions in [(ButCO2)3Mo2]2(m-O2CCO2), which run parallel to the b-axis. All intermolecular Mo–O distances are 2.9(1) A˚. The tert-butyl groups have been omitted for clarity, and the colour scheme is Mo ¼ green, O ¼ red and C ¼ grey. Reproduced from Ref. 2 with permission.

It should be noted that the molecular geometry of the gas phase calculated structure for the model compound [(HCO2)3Mo2]2(m-O2CCO2) remains present in this solution, where tert-butyl groups have replaced the H atoms in the C–H bond. Only the dihedral angle between the two CO2 planes of the oxalate ligand were allowed to vary in the solid state structure determination.

Based on a knowledge of this solid state structure for the oxalate-bridged Mo4-containing compound, we can reasonably understand why the electronic absorption spectra in the solid state (recorded in a Nujol mull or as a colloid) are so very different from the spectrum collected in tetrahydrofuran solution where each molecule is isolated. The change in the electronic absorption spectra

Concerning the Solid State Packing

145

upon association of one molecule with its nearest neighbours is reminiscent of that seen for aromatic molecules that show concentration dependent p–p stacking in solution.7 Subsequent to this work we found very similar spectroscopic properties for p-terephthalate-bridged MM quadruply bonded dimers of dimers. The complexes also associated in the solid state by way of intermolecular M2 to carboxylate oxygen bonds to form laddered structures.8

3 The 9,10-Anthracenedicarboxylate-Bridged Compounds In contrast to the oxalate- and terephthalate-bridged compounds, which maintain the same colour to the eye in the solid state and in solution, the related 9,10anthracenedicarboxylate complexes [(ButCO2)3M2(C14H8-9,10-(CO2)2) where M ¼ Mo or W] appear very different.9 As shown in Figure 4, the molybdenum compound is yellow as a powder and intensely red in THF (tetrahydrofuran) solution while the tungsten complex is a pink-red powder that forms a green-blue solution. As expected from the obvious differences in appearance as judged by the eye, the electronic absorption spectra recorded as Nujol mulls and in solution are vastly different. For example, the tungsten complex which has an absorption centered at 550 nm in the Nujol mull shows a band maximum B750 nm in THF solution, which is responsible for its blue-green colour. We were naturally curious concerning the origins of this effect and being unable to grow single crystals, we once again looked to gain insight concerning the molecular packing through the aid of PXRD. Direct space structure solution of molecular solids from powder data is particularly well suited to structures that contain only one crystallographically unique molecule per unit cell and to molecules where a large part of the electron

Figure 4

The solids and their solutions as they appear to the eye. The tungsten complexes are shown on the left and the molybdenum on the right.

146

Figure 5

Chapter 8

View of the minimum energy twisted D2 (y ¼ B541) formate structure [{M2(O2CH)3}2(m-9,10-C14H8(CO2)2)] (where M ¼ Mo, W).

density originates from a well-defined structural unit such as a planar ring, a rigid rod, and/or a heavy atom.1 The molecules under consideration are thus ideal; they have a well-defined M2(O2C)4 unit for which there are now numerous well refined structures10 as well as the planar anthracene unit. Electronic structure calculations on the model compound [(HCO2)3Mo2]2(m-9,10-(CO2)2C14H8) indicated a minimum energy structure having D2 symmetry.9 The gas phase structure of the model compound is shown in Figure 5. The nonplanar structure of the bridge represents a compromise of steric and electronic factors. Even though M2 d bridge p* interactions favour a planar-bridged structure, which maximizes electronic coupling across the bridge, the peri CH  O interactions of the 9,10-anthracenedicarboxylate prevent this from being realized. Indeed, for the model compounds, the planar D2h structure is calculated to be B12 kcal mol–1 higher in energy than the D2 structure where the dihedral angle between the CO2 and the anthracene planes is 541. This geometry for the central portion of the molecule is essentially the same as that seen in the solid state structures of 9,10-anthracendicarboxylate esters. The model compound shown in Figure 5 thus represented the unit employed in the structural refinement where the C–H moieties were replaced by an idealized C–CMe3 unit. The PXRD data for the molybdenum and tungsten complexes showed these two compounds to be isomorphous. The molybdenum diffraction data were superior and are shown in Figure 6. Based on the positions of the first 13 peaks in the diffraction pattern the unit cell was determined to be monoclinic with cell dimensions: a ¼ 21.3662 A˚, b ¼ 5.6757 A˚, c ¼ 12.6829 A˚, and b ¼ 106.981. The systematic absences were consistent with a primitive cell, leading to possible space group symmetries of P121, P1m1, and P12/m1. Peak intensities were extracted (w2 ¼ 2.332) using the whole pattern fitting routine in DASH, which is based on the Pawley method.

147

Intensity

Concerning the Solid State Packing

3

Figure 6

8

13

18

23

28

33 2 theta

38

43

48

53

58

Powder diffraction data for [(tBuCO2)3Mo2]2(m-9,10-C14H8(CO2)2).

As with the oxalate-bridged molecules, the structural determination was carried out using the global optimization approach within DASH. In this case there are three variables to describe the position of the molecule within the unit cell, three variables to define the orientation of the molecule, six torsional degrees of freedom associated with the Me3C groups and one torsional degree of freedom associated with the twist about each C–CO2 bond of the anthracene ring. This reduces the number of variables from 194 to 14. In the light of the relatively small number of observed reflections, this reduction in the number of variables is essential to extract a meaningful structure from the data. The profile w2 was found to be 6.4 when the space group symmetry was assumed to be P2, which is a very good fit for the data. When the space group symmetry was taken to be P2/m or Pm, the profile w2 value increased dramatically, to 29 and 57, respectively. Hence P2 was confirmed as the correct space group. As a final step in the structure determination, rigid-body Rietveld refinement was carried out on the molecule. The orientation and location of the molecule and anthracene torsion angles were refined. The torsion angles of tert-butyl groups were kept fixed during refinement. Figure 7 shows the molecular packing of the structure. In contrast to the structures of the oxalate and p-terephthalate-bridged complexes which form laddered structures due to intermolecular M2  O interactions, the present structure contains ‘‘isolated’’ molecules. The shortest intermolecular Mo–O distance in a neighbouring molecule is B3.6 A˚ which is far too long to invoke even the weakest of interactions. Indeed, the packing of the molecules is more influenced by hydrocarbon interactions than by Mo2  O

148

Figure 7

Chapter 8

Space filling representation of the crystal packing of [(tBuCO2)3Mo2]2(m9,10-C14H8(CO2)2) showing the layer of molecules viewed down b-axis. Atomic colour scheme: Mo ¼ purple, O ¼ red, C ¼ grey, H ¼ white.

intermolecular attractive forces. The dihedral angle between the Mo2O2C and the anthracene C14 planes is 551, as expected from both the calculations on the model compound and the known structures of 9,10-diethers of anthracene.

4 Concluding Remarks Powder X-ray diffraction can be used to reliably establish the gross features of the packing of these oxalate and 9,10-anthracenedicarboxylate linked MM quadruply bonded compounds in the solid state. The structures presented here do not represent a full structural determination as all the M–M, M–O, O–C and C–C distances were estimated from electronic structure calculations rather than

Concerning the Solid State Packing

149

directly determined using crystallographic methods. Nevertheless, the distances and angles are all quite reasonable, based on related single crystal structural studies. The lack of significant M2  O intermolecular interactions in the solidstate structure is in marked contrast to what will exist in THF solutions where metal–oxygen THF bonds along the M–M axis will be present. Given that the colour of these compounds arises from metal to bridge charge transfer it is not surprising that a ‘‘gas phase’’ colour should differ from that in solution. As has been shown for the oxalate-bridged complexes, these complexes show marked solvatochromism.11 The stabilization of the positive charge on the metal by the donor THF molecules which arises on photoexcitation into the MLCT produces a marked bathochromic shift. This we propose is principally responsible for effects described herein for the anthracenedicarboxylate-bridged compounds and shown in Figure 4. Furthermore, it is becoming more and more apparent that the application of direct space methods to the determination of the ordering of molecules in the solid state will become a common practice for chemistry in years to come. Finally, I should like to state that it is a great pleasure to contribute to this volume which celebrates the occasion of the 75th birthday of Sir John Meurig Thomas. We have surely all learned a great deal from John. In his research he has been both prolific and profound. In his teaching and promotion of science to the public he has been a true evangelist. In his company we have enjoyed his generosity, his humanity, and his humor.

Acknowledgments We thank the National Science Foundation for support of this work.

References 1. K.D.M. Harris and E.Y. Cheung, Chem. Soc. Rev., 2004, 33, 526. 2. B.E. Bursten, M.H. Chisholm, R.J.H. Clark, C.M. Hadad, S. Firth, A.M. Macintosh, P.M. Woodward, P.J. Wilson and J.M. Zaleski, J. Am. Chem. Soc., 2002, 124, 3050. 3. M.H. Chisholm and N.J. Patmore, Inorg. Chim. Acta, 2004, 357, 3877. 4. CRYSFIRE was written by Robin Shirley and can be obtained free of charge from http://www.ccp14.ac.uk/tutorial.htm 5. DASH was written by W.I.F. David and K. Shankland and can be purchased from the Cambridge Crystallographic Data Centre. See www.ccdc.cam.ac.uk for more details. 6. W.I.F. David, K. Shankland and N. Shankland, Chem. Commun., 1998, 931. 7. J.N. Murrell, in The Theory of the Electronic Spectra of Organic Molecules, Methuen and Co. Ltd., London, 1963. 8. B.E. Bursten, M.H. Chisholm, R.J.H. Clark, S. Firth, C.M. Hadad, P.J. Wilson, P.M. Woodward and J.M. Zaleski, J. Am. Chem. Soc., 2002, 124, 12244.

150

Chapter 8

9. M.J. Byrnes, M.H. Chisholm, D.F. Dye, C.M. Hadad, B.D. Pate, P.J. Wilson and J.M. Zaleski, Dalton Trans., 2004, 523. 10. Multiple Bonds between Metal Atoms, ed. F.A. Cotton, C.A. Murillo and R.A. Walton, 3rd edn, Interscience, New York, 2005. 11. M.H. Chisholm and N.J. Patmore, Inorg. Chim. Acta, 2004, 357, 3877.

CHAPTER 9

High Pressure and High Temperature Oxidation in the IrSr2RECu2O8 Family of Cuprates: The Disordered Multiple Perovskite (A1/3 A 02/3) (B1/3 B 02/3)O3
x Phases A. J. DOS SANTOS-GARCI´A,1 G. HEYMANN,2 H. HUPPERTZ2 AND M. A´. ALARIO-FRANCO1 1

Laboratorio de Quı´ mica del Estado So´lido, Departamento de Quı´ mica Inorga´nica, and Laboratorio Complutense de Altas Presiones, Facultad de Ciencias Quı´ mica, Universidad Complutense de Madrid, 28040 Madrid, Spain; 2 Department Chemie und Biochemie, Ludwig-MaximiliansUniversita¨t Mu¨nchen, Butenandtstrabe 5-13, 81377 Mu¨nchen, Germany

1 Introduction In previous studies we have performed a wide investigation of the structure, microstructure and magnetic properties of a new family of cuprates, IrSr2RECu2O8 (Ir-1212)1,2 where RE is a rare earth cation, prepared at high pressure (HP) and high temperature (HT). This is an interesting family of materials since in many of its members there coexist magnetic and superconducting properties. Structurally, these compounds show the well known M-1212 type structure,3 a perovskite triple superstructure characteristic among many others of YBCO, and the well known ruthenocuprates, of general formula RuSr2RECu2O8 (Ru-1212).4,5 When the synthesis conditions are far from optimal and, in particular, under oxidizing conditions, another perovskite phase is usually obtained as an 151

152

Chapter 9

impurity. In this way, SrRuO3 is often quoted to appear in the synthesis of the Ru-1212 materials.6,7 In the case of the Ir-1212 materials, by controlling the synthesis, we have been able to isolate a new perovskite phase as a single one. This is a disordered version of the usual M-1212 structure in which both the big (Sr and RE) and the small (Cu and Ir) cations are randomized in their respective sites. A certain oxygen substoichiometry has also been observed. We have determined that the average symmetry of this novel phase changes with the rare earth, being cubic for RE ¼ Sm and orthorhombic for the remaining ones (Nd, Eu, Gd and Tb). As this type of ‘‘simple’’ perovskite is not that common, and in view of the importance of the M-1212 type structure, we have made a detailed structural, microstructural and magnetic study of these disordered multiple perovskites. This has also allowed us to ascertain the true unit cell and symmetry of the different novel phases as well as to analyze the influence of the disordering on the magnetic properties.

2 Experimental Samples were prepared at HPs (r80 kbar) and HTs in a Belt type press, located in the Laboratorio Complutense de Altas Presiones–UCM Madrid,8 while those requiring higher pressures (480 kbar) were made in a Walker-type multianvil pressure module9,10 installed at the Ludwig-Maximilians-Universita¨t Mu¨nchen.11 Adequate amounts of a mixed oxide precursor SrCuO2, prepared beforehand by solid state reaction, together with IrO2, CuO, RE2O3 (or Tb4O7) and SrO2 (AR, Sigma Aldrich) were thoroughly mixed inside a glove box. Then, the mixture was pressed on a platinum capsule (for RE ¼ Sm, Eu and Gd) or a BN crucible (for RE ¼ Nd and Tb) and treated at HPs and HTs according to: SrCuO2 þ 1=2RE2 O3 þ IrO2 þ CuO þ SrO2 ! IrSr2 RECu2 O9
d

ð1Þ

The presence of SrO2 makes a more oxidizing reaction atmosphere so the oxygen content is higher than the expected value of 8 for Ir-1212 (see below). Samples were characterized by X-ray powder diffraction (XRD) on a Philips X’Celerator diffractometer (CuKa1-radiation, l ¼ 1.54056 A˚). The XRD patterns were refined with the Rietveld procedure using the Fullprof_Suite program.12 High-resolution TEM images and selected area electron diffraction (SAED) were performed on Jeol JEM 3000EX and 200KV and Philips CM 200 FEG microscopes. Cationic compositions were checked by EDS (Link Pentafet 5947 Model, Oxford Microanalysis Group) by in situ observations in the electron microscope. The oxygen content was determined by thermogravimetric analysis performed on a homemade system based on a Cahn D-200 electrobalance. Magnetic susceptibility measurements were performed over the temperature range 1.9–300 K, using a Squid Quantum Design XL-MPMS magnetometer.

153

High Pressure and High Temperature Oxidation

3 Results and Discussion 3.1

Synthesis

Table 1 shows the optimal conditions (pressure, temperature and time) required for the synthesis of both type of materials: ordered Ir-1212, i.e. IrSr2RECu2O8 and the novel disordered perovskite materials, of general formula (Sr2RE) (Cu2Ir)O9
d. It can be seen that keeping constant the optimum pressure used to obtain the IrSr2RECu2O8 compounds, at higher temperatures one is able to obtain the corresponding disordered (Sr2RE)(Cu2Ir)O9
d new phases. It is clear that the temperature and the oxidizing conditions are crucial in the disordering process.

3.2

The Chemical Composition

Cationic ratios were determined by EDS spectra on several crystals. The normalized average value obtained from all the samples corresponds to the 1212 nominal composition, i.e. IrSr2RECu2Ox. Furthermore, the thermal decomposition of the pure Gd sample, made in air at temperatures up to 950 1C, gives as final products CuO and the corresponding double perovskite, i.e. Sr2GdIrO6. On the other hand, the oxygen content was obtained by thermogravimetric analysis from the direct reduction of the gadolinium single phase, (Sr2Gd)(Cu2Ir)O9
d. The total reduction of this material to Cu, Ir, Gd2O3 and SrO, under a reducing atmosphere (0.2 atm He–0.3 atm H2) and heating up to 625 1C, at a rate of 6 1C min
1, allows one to determine a weight loss corresponding to 9 – d ¼ 8.82. Table 1

Optimal pressure, temperature and time conditions required for the synthesis of both types of materials: ordered and the novel, disordered multiple perovskite.

RE rRE /A˚ P/kbar T/K t/min

VIII

31

1

Nd

ABO3 (Sr2RE)(IrCu2)O9
d Sm Eu

Gd

Tb

1.109 115 1373–1473 20

1.099 60 1673 90

1.053 60 1673 90

1.040 92 1673 30

1.053 60 1393 30

1.040 92 1373–1473 20

1.066 30 1575 60 Ir-1212 IrSr2RECu2O8

VIII rRE31/A˚1 P/kbar T/K t/min 1

1.109 – – –

1.099 60 1373 30

Shannon and Prewitt data from Ref. 29.

1.066 30 1173 35

154

Chapter 9

Therefore, the chemical composition of these samples can then be written as (Sr2RE)(Cu2Ir)O8.82, for the Gd material. This is an important oxygen excess with respect of the original IrSr2RECu2O8 sample, and rather close to the oxygen content of a triple perovskite: A3B3O9.

3.3

The Average Structure

X-ray diffraction patterns can be indexed as a cubic perovskite for the Sm sample, Figure 1a, and as an orthorhombic perovskite for the remaining ones, corresponding to Nd, Eu, Tb and Gd (Figure 1b shows the pattern corresponding to Gd). In order to refine them, by the Rietveld procedure, we used the SrTiO313 cubic perovskite structure for the Sm sample while orthorhombic SrRuO314 was used for the Gd sample as starting models. The A site of these single perovskites was fully occupied at random by 2 Sr and 1 RE, while 2 Cu and 1 Ir were placed in the B sites. In this structural model, such a situation was treated by assigning to the different atoms the same x,y,z site and the same displacement parameters Uij. One of the atoms (Sm/Ir) was then given a site occupancy of k (1/3) and the other one (Sr/Cu) a site occupancy of 1
k (2/3).

Figure 1a

Rietveld refinement fit of the X-ray diffraction pattern for (Sr2Sm) (Cu2Ir)O9
d. The ‘‘impurity’’ observed corresponds to the ordered IrSr2SmCu2O8.

155

High Pressure and High Temperature Oxidation

As there was a strong correlation between the site occupancy and the displacement factor, it was better to refine them in alternating cycles of refinement until they converged. Since there also existed a number of oxygen vacancies, the corresponding oxygen site occupancy was varied freely until the final value was reached.15 The results of the refinement can be seen on Figures 1a and b; Table 2 gives the refined cell and atomic parameters as well as the fit agreement factors obtained for (Sr2RE)(Cu2Ir)O9-d with RE ¼ Gd and Sm. Taking together the analytical data and the crystal structure refinement, it appears that the oxidation of the ordered IrSr2RECu2O8 1212-type structure to the disordered (Sr2RE)(Cu2Ir)O9
d one leads to the conversion of the vast majority of the copper pyramids [Cu–O5] to octahedra [Cu–O6]; at the same time, Ir and copper are randomly distributed in the octahedra. Concomitantly, Sr and RE ions randomize in the resulting cubo-octahedral positions. In both of these polyhedra there are some oxygen vacancies; for the oxygen content of the Gd case these amount to B3.7% while B1.7% was found in the Sm case. This may be related to the differences in the average structures; the Sm one, the closer to stoichiometric, is cubic while the Gd one is orthorhombic. However,

Figure 1b

Rietveld refinement (Sr2Gd)(Cu2Ir)O9
d.

fit

of

the

X-ray

diffraction

pattern

for

156

Table 2

Chapter 9

Unit cell parameters and crystallographic sites, as well as fit agreement factors, obtained from the Rietveld refinement of (Sr2RE) (Cu2Ir)O9-d with RE ¼ Gd and Sm.

a/A˚ 5.542(1) Atom

(Sr2Gd)(Cu2Ir)O9
d b/A˚ c/A˚ 5.540(1) 7.8472(7) Wyckoff Position x

Sr Gd Cu Ir O1 O2

4c 4c 4b 4b 8d 4c


0.015
0.015 0.5 0.5 0.889
0.008

(1) (1) (8) (1)

(S.G. Pbnm) V/A˚3 240.95(9) Y

Rwp 0.0915 z

Rp 0.0693 Uiso/A˚2

w2 1.20 Occ

0.011 (2) 0.011 (2) 0.0 0 0.371 (9) 0.53 (1)

0.25 0.25 0 0 0.176 (4) 0.25

0.9 (1) 0.9 (1) 0.843 0.843 0.05 0.05

0.658 0.342 0.671 0.329 1.89 1

(S.G. Pm3m)

a/A˚ 3.923(1) Atom

(Sr2Sm)(Cu2Ir)O9
d b/A˚ c/A˚ 3.923(1) 3.923(1) Wyckoff Position x

V/A˚3 60.389(1) Y

Rwp 0.125 z

Rp 0.0713 Uiso/A˚2

w2 4.82 Occ

Sr Sm Cu Ir O

1a 1a 1b 1b 3c

0 0 0.5 0.5 0.5

0 0 0.5 0.5 0

0.374 0.374 0.377(2) 0.377(2) 0.05

0.583 0.292 0.694 0.347 2.946

0 0 0.5 0.5 0.5

we have not seen any evidence of oxygen vacancy ordering and, as discussed below, the cell observed by ED and TEM is always orthorhombic. Presumably this oxygen deficiency could be eliminated/increased if somewhat stronger/ softer oxidizing conditions were to be used. It is especially relevant that both the ordered and disordered phases have exactly the same cation stoichiometry although different oxygen content. We are then not really dealing with a simple order–disorder change,16 like in a phase transition; it is in fact a chemical oxidation process accompanied by a disordering of both metal sublattices of the triple perovskite cell. Figure 2 shows the relation between both structures. This is indeed reminiscent of several oxygen intercalation–deintercalation processes, such as Ca2Mn2O5 to CaMnO317,18 or indeed YBa2Cu3O6 through Y2Ba4Cu3O13 to YBa2Cu3O7.19 However, the present case has the added interest of the disordering of the cations present in both the A and B perovskite positions, which take place with the oxidation. Disordering of the big cations (i.e. A-cations) is present in, for example, (LaSr)CuGaO520,21 or (LaSr)CuAlO5.22,23 Yet, in these cases the B- and B 0 -cations are distinctly ordered, unlike the cases reported here. A somewhat related case is that of synthetic isolueshite (Na0.75La0.25)(Nb0.5Ti0.5)O3 where the cations are also disordered, although in that case there are two different A sites in the space group Cmcm.24 The closest cases are, however, those of the disordered double perovskites BaLaFeMoO625 and CaBaZrGeO625 which are cubic. On the other hand, SrLaCaRuO626 and SrLaFeCuO627 are

157

High Pressure and High Temperature Oxidation

orthorhombic due to the tilt octahedra in the absence of cation ordering and show the CaTiO3 structure (space group Pnma). Table 3 gives the principal interatomic distances and angles for the Sm and Gd compounds. The observed bond lengths are of course the average of the Cu–O and Ir–O distances in the disordered material; we assume that the Ir–O environment is constant, [Ir–O6], and that changes in the average are due to changes in the Cu–O environment, from [Cu–O5] to [Cu–O6]. It is also interesting to make a comparison between the distances in the ordered and disordered Gd phases. The equatorial octahedron/pyramid B)

A)

Ir Sr

A (Sr/ TR)

Cu RE + O2

Cu

Ir

B (Cu/Ir)

Figure 2

Schematic structural representation of the oxidation of ordered IrSr2RECu2O8 to disordered (Sr2Sm)(Cu2Ir)O9
d.

Table 3

Principal interatomic distances and angles obtained from the refinement of the Gd and Sm disordered compounds. Distances/A˚

Angles/degrees (Sr2Gd)(Cu2Ir)O9
d

Cu/Ir–O(1)  4 Cu/Ir–O(2)  2 Gd/Sr–O(1)  8 Gd/Sr–O(2)  4

1.97 1.96 2.78 2.77

Cu/Ir–O Gd/Sr–O

(Sr2Sm)(Cu2Ir)O9
d 1.96 O–Cu/Ir–O 2.77 O–Cu/Ir–O

(1) (1) (1) (1)

O(1)–Cu/Ir–O(2) O(2)–Cu/Ir–O(2) O(1)–Cu/Ir–O(1)

95.6 180 180

90 180

158

Chapter 9

distances in the ordered material (Ir–O ¼ 1.98 A˚ and Cu–O ¼ 1.92 A˚, average 1.95 A˚) and pseudo-octahedron equatorial distances in the disordered one (Ir/Cu–O ¼ 1.97 A˚) are rather close. However, the conversion from the vast majority of square planar pyramids in the ordered phase to pseudo-octahedral groups in the oxidized disordered one makes the axial distances (i.e. Cu–O ¼ 2.30 A˚ and Ir–O ¼ 1.82 A˚, average 1.96 A˚) in ordered Ir-1212 become equal in the disordered phase, (i.e. Ir/Cu–O ¼ 1.96 A˚). When Cu12 occupies a site with tetragonal symmetry, the eg band will be split into two sub-bands due to the strong Jahn–Teller effect in these orbitals. If, as we explained above, the conversion of the square copper pyramids is to copper octahedra, the number of Cu12 ions that occupy a site with tetragonal symmetry are considerably diminished, and therefore the Jahn–Teller effect is not so important. In fact, the pseudo-octahedron is almost regular, ra/e ¼ 1.96/1.97 E1, in the disordered phases. It is also interesting to consider the octahedron tilt angle in these disordered perovskites. This can easily be done with the program SPUDS.28 However, one needs the oxidation state of the cations. As we have two different transition metals Ir and Cu in the B position, one has to guess their respective oxidation states. Two main possibilities appear to be Ir15 and Cu12 or Ir14 and Cu13, and indeed any intermediate situation. The corresponding tilt angles are for the Gd case F ¼ 14.71 and 13.01, respectively, and in the Sm case F ¼ 14.51 and 12.71, respectively. One can see that the charge distribution does not affect too much the tilt angle (o12%). Once we have characterized this new family of orthorhombic disordered perovskites, it is worth trying to see their respective tolerance factors (t-factors). For that we need the ionic radii of the corresponding RE cations in 12-fold coordination. It so happens that there are only available data29 for La, Ce, Nd and Sm(III ) and, as we have prepared the compounds corresponding to Nd, Sm, Eu, Gd and Tb rare earth ions, a simple linear extrapolation of the Shannon and Prewitt data for the experimentally known coordination was performed, giving an interesting plot of the lanthanide contraction for the XII rRE13 ions (Figure 3). Notice that the experimental Shannon–Prewitt data known in 12-fold coordination follow a higher contraction than those obtained by extrapolation. . .! In any event, though, the t-factors of all the compounds established on the basis of either set are rather similar, e.g. tNd exp ¼ 0.954 vs. tNd extrapolated ¼ 0.963, tSm exp ¼ 0.95 vs. tSm extrapolated ¼ 0.96. On the basis of these values, it is not surprising that these disordered multiple perovskites are not cubic but orthorhombic. Interestingly, it was common to obtain in the same sample – and even in the same crystal! (see below) – both ordered (1212) and disordered ((Sr2RE)(Cu2Ir)O9
d) phases. This is a clear indication that the synthesis conditions are close to the ‘‘ideal equilibrium’’: IrSr2RECu2O8+(1
d)/2O2 - (Sr2RE)(Cu2Ir)O9
d. However, we have not yet succeeded in making the back reaction, i.e. to reduce the disordered phase to the ordered one. Another interesting point worth mentioning is that the XRD patterns corresponding to

159

High Pressure and High Temperature Oxidation 1.42 Shannon & Prewitt Extrapolated

1.40 1.38

La

1.36

Nd

XIIrRE+3

1.34 Sm

1.32 1.30

Tb

1.28 1.26

Er

1.24 1.22 1.20

Lu

1.18 0

2

4

6

8

10

12

14

f electrons

Figure 3

A plot of the XII coordinated radii of the trivalent lanthanide cations (see text for details).

h/2 k/2 0

100p

010p

[001]p

Figure 4a

Electron diffraction pattern of (Sr2Gd)(Cu2Ir)O9
d along the perovskite [001]p zone axis. Two twofold superstructures are present. Also there are spots at h/2 k/2 0 (h and k odd). See text for details.

the other RE (data not shown, i.e. Nd, Eu and Tb) show as the main impurity iridium metal. This phenomenon is indicative of an excess temperature that decomposes the samples. In spite of the presence of the impurities, these disordered phases can be obtained as main phases, although the poor quality

160

Chapter 9 100p 2ap

010p [001]p

√2ap

√2ap 100 p 010p 2ap [001]p

Figure 4b

ap

A high resolution electron micrograph showing a three-dimensional microdomain texture. The long c axis of the supercell (cl ¼ l2ap E 7.8 A˚ 0 ) is randomly distributed in the three space orientations. The Fourier transform, shown as inset, corresponds to the pattern in Figure 4a.

of the corresponding XRD patterns makes it difficult to refine them from powder diffraction data.

3.4

The Microstructure

Although the ideal, or Pm3m aristotype, perovskite structure is cubic, it has been shown that the majority of synthetic simple ABX3 perovskite group compounds are distorted derivatives resulting from: (1) rotation or tilting of distortion-free BX6 polyhedra (more common situation); (2) first order Jahn–Teller distortion of BX6 octahedra; (3) second order Jahn–Teller effects on A- and B-cation polyhedra, reflecting mixing of molecular orbitals and/or lone pair effects. Although these distortion phenomena are frequent enough, their observation is somewhat complicated, especially from X-ray powder diffraction. In view of past experience concerning the true symmetry of these perovskites,30–34 we have performed a microstructural study by means of SAED and high resolution electron microscopy (HREM). The electron diffraction patterns corresponding to the whole series – including the Sm case – show clear evidence of the presence of the so-called diagonal cell BO2ap  O2ap  2ap. This superstructure arises as a consequence of the octahedral tilt around the unit cell axes in order to achieve the lowest energy for the crystal to accommodate a small A-cation for the 12-fold site within a BX6 polyhedral framework. In the electron diffraction pattern corresponding to the Gd sample in Figure 4a, the strong spots can be indexed on the basis of a [001]p perovskite zone axis.

161

High Pressure and High Temperature Oxidation c)

b)

7.8 Å

11.6 Å

2ap

3ap B

10 nm

Figure 5

A

a)

(a) Electron micrograph of a crystal showing an intergrowth of the ordered IrSr2TbCu2O8 (c E 3ap) and disordered (Sr2Tb)(Cu2Ir)O9
d (c E 2ap). (b) Fourier transform of same showing the presence of the two cells. (c) Higher resolution image showing the regularity of the intergrowth boundary.

Besides, the weak spots at h/2 k/2 0, where h and k are integers, suggest the presence of the indicated diagonal cell. Also the spots present at h/2 0 0 and 0 k/2 0, together with the corresponding HREM shown in Figure 4b indicates a microdomain texture, in which the c ¼ 2ap axis of the diagonal cell is distributed at random in the three space directions. It is worth mentioning that a low tilt angle, such as observed here – see above – favours the presence of a microdomain texture (see the discussion of this part in Ref. 30). A very interesting microstructural situation can be seen in Figure 5a, corresponding to a crystal of the Tb-iridocuprate, by no means exceptional, in which both the ordered IrSr2TbCu2O8 and the disordered (Sr2Tb)(Cu2Ir)O9
d phases coexist and are joined along a rather regular boundary. The Fourier transform for the whole image, Figure 5b, shows the cord ¼ 3a periodicity of the ordered phase (region A), together with the cdisord ¼ 2a periodicity from the disordered phase (region B). In Figure 5c, at higher resolution, it can be seen that both phases join at the boundary in a rather smooth way. This is certainly due to the fact that both have the topology of a perovskite and very close metric as indicated by their lattice parameters (Table 2). Yet, this seems to have

162

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important implications concerning the oxygen stoichiometry of the system. The fact that both end members of a hypothetical ‘‘solid solution’’, IrSr2RECu2O8 and (Sr2RE)(Cu2Ir)O9
d, coexist in the same crystal seems to indicate that we have an intergrowth phase mixture rather than a non-stoichiometric continuous solid solution. Changes in oxygen content appear to modify the relative proportions of the end members.

3.5

Magnetic Properties

As expected, the random distribution of the Ir/Cu and Sr/RE cations in their respective crystallographic sites makes less likely the presence of long range magnetic interactions. In fact, paramagnetic behaviour is to be expected for these phases. In this sense, Figure 6 shows the AC susceptibility measurement of both ordered and disordered Gd phases. The shoulder observed at B15 K in the ordered phase, reported elsewhere,2 does indicate an overall ferrimagnetic order between Gd and Ir sublattices below 15 K. On the other hand, all the disordered phases that we have prepared show a not unexpected paramagnetic behaviour. This is somewhat deceptive compared to the rich magnetic behaviour shown by the ordered 1212 phases.

IrSr2GdCu2O8

0.12 χ (emu mol-1)

0.10

χ (emu mol-1)

0.10

0.08

0.06

(Sr2Gd)(Cu2Ir)O9-δ

0.05

0.00 50 T (K)

0

0.04

100

0.02

0.00 0

50

100

150

200

250

300

T (K)

Figure 6

Magnetic susceptibility (AC mode) as a function of temperature of the ordered and disordered phases. Inset shows an enlarged section of the magnetic phase transition region in the ordered phase.

High Pressure and High Temperature Oxidation

163

4 Conclusions Searching for the optimal synthesis conditions for the iridates IrSr2RECu2O8, we have been able to isolate a new disordered perovskite in which both the A and B positions are multiply occupied, (Sr2RE)(Cu2Ir)O9
d. Although the average structure seems to change with cation size, the true structure is common for all of these materials and has orthorhombic symmetry (space group Pbnm, cell BO2ap  O2ap  2ap). This structure is, on the other hand, distributed at random in microdomains. This order–disordering HP and HT chemical oxidation reaction is a remarkable one which exhibits interesting aspects concerning the differences between a solid solution and a phase mixture. A final point worthy of note is that, usually under pressure one gets a more ordered phase and a lower symmetry. Here, however, the oxidation reaction – as opposed to a phase transition – leads to an increase in the symmetry and a cubic disordered phase.

Acknowledgements We would like to thank financial support from CICYT, programa MAT200401641, Comunidad Auto´noma de Madrid, programa MATERYENER, PRICYT S-0505/PPQ-0093 (2006), Fundacio´n Areces, Programa Fı´ sica de Bajas Temperaturas (2003) and the European Science Foundation within the COST D30 network (D30/003/03). We also thank Dr J. Romero de Paz, Dr J. M. Gallardo-Amores and A. Go´mez-Herrero for technical assistance.

References 1. A.J. Dos Santos-Garcı´ a, PhD Thesis, Universidad Complutense de Madrid, Spain, 2007. 2. A.J. Dos Santos-Garcı´ a, M.H. Aguirre, E. Mora´n, R. Sae´z-Puche and M.A´. Alario-Franco, J. Solid State Chem., 2006, 179, 1275. 3. H. Shaked, P.M. Keane, J.C. Rodriguez, F.F. Owen, R.L. Hitterman and J.D. Jorgensen, Crystal Structure of the High-TC Superconducting CopperOxides, Elsevier Science BV, Amsterdam, The Netherlands, 1994. 4. L. Bauernfeind, W. Widder and H.D. Braun, Physica C, 1995, 254, 151. 5. R. Ruiz-Bustos, J.M. Gallardo-Amores, R. Sa´ez-Puche, E. Mora´n and M.A´. Alario-Franco, Physica C, 2002, 382, 395. 6. A. Hassen, J. Hemberger, A. Loidl and A. Krimmel, Physica C, 2003, 400, 71. 7. S. Malo, D. Ko, J.T. Rijssenbeek, A. Maignan, D. Pelloquin, V.P. Dravid and K.R. Poeppelmeier, Int. J. Inorg. Mater., 2000, 2, 601. 8. http://www.ucm.es/info/labcoap/index.html 9. D. Walker, M.A. Carpenter and C.M. Hitch, Am. Mineral., 1990, 75, 1020. 10. H. Huppertz, Z. Kristallogr., 2004, 219, 330. 11. http://www.cup.uni-muenchen.de/ac/hupertz/

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12. J. Rodrı´ guez-Carvajal, Phys. B, 1993, 192, 55. 13. R.S. Roth, J. Res. Natl. Bur. Stand. (US), 1957, 58, 75. 14. C.W. Jones, P.D. Battle and P. Lightfoot, Acta Crystallogr., Sect. C, 1989, C45, 365. 15. W. Massa, Crystal Structure Determination, 2nd edn, Springer-Verlag, Berlin, 2004. 16. S.A.T. Redfern and M.A. Carpenter, Transformation Processes in Minerals, Reviews in Mineralogy and Geochemistry, vol. 39, Mineralogical Society of America, Washington, DC, 2000. 17. K.R. Poeppelmeier, M.E. Leonowicz, J.C. Scanlon, J.M. Longo and W.B. Yelon, J. Solid State Chem., 1982, 45, 71. 18. K.R. Poeppelmeier, M.E. Leonowicz and J.M. Longo, J. Solid State Chem., 1982, 44, 89. 19. C. Chaillout, M.A. Alario-Franco, J.J. Capponi, J. Chenavas, P. Strobel and M. Marezio, Solid State Commun., 1988, 65, 283. 20. J.T. Vaughey, R. Shumaker, S.N. Song, J.B. Ketterson and K.R. Poeppelmeier, Mol. Cryst. Liq. Cryst., 1990, 184, 335. 21. J.T. Vaughey, J.B. Wiley and K.R. Poeppelmeier, Z. Anorg. Allg. Chem., 1991, 327, 598. 22. J.B. Wiley, L.M. Markham, J.T. Vaughey, T.J. McCarthy, M. Sabat, S.J. Hwu, S.N. Song, J.B. Ketterson and K.R. Poeppelmeier, in Chemistry of High-Temperature Superconductors II, ed. D.L. Nelson and T.F. George, Symposium Series No. 377, American Chemical Society, Washington, 1988, 304. 23. J.B. Wiley, M. Sabat, S.J. Hwu, K.R. Poeppelmeier, A. Reller and T.J. Williams, J. Solid State Chem., 1990, 87, 250. 24. S.V. Krivovichev, A.R. Chakhmouradian, R.H. Mitchell, S. Filatov and N.V. Chucanov, Eur. J. Mineral, 2000, 12, 597. 25. T. Nakamura and J.H. Choy, J. Solid State Chem., 1997, 20, 233. 26. J.P. Attfield, P.D. Battle, S.K. Bollen, S.H. Kim, A.V. Powell and M. Workman, J. Solid State Chem., 1992, 96, 344. 27. G. Blasse, J. Inorg. Nucl. Chem., 1965, 27, 993. 28. M.W. Lufaso and P.M. Woodward, Acta Crystallogr., Sect. B, 2001, B57, 725. 29. R.D. Shannon and C.T. Prewitt, Acta Crystallogr., Sect. A, 1976, A32, 751. 30. A. Vegas, M. Vallet-Regı´ , J.M. Gonza´lez-Calbet and M.A´. Alario-Franco, Acta Crystallogr., Sect. B, 1986, B42, 167. 31. A. Va´rez, F. Garcı´ a-Alvarado, E. Mora´n and M.A´. Alario-Franco, J. Solid State Chem., 1995, 118, 78. 32. M. Aguirre, R. Ruiz -Bustos and M.A´. Alario-Franco, J. Mater. Chem., 2003, 13, 1156. 33. M. Vallet-Regi, J.M. Gonzalez-Calbet, J. Verde and M.A. Alario-Franco, J. Solid State Chem., 1985, 57, 197. 34. M.A. Alario-Franco, J.-C. Joubert and J.-P. Le´vy, Mater. Res. Bull., 1982, 17, 733.

CHAPTER 10

Melting and Amorphisation G. NEVILLE GREAVES Centre for Advanced Functional Materials and Devices, Institute of Mathematical and Physical Sciences, University of Wales Aberystwyth, Aberystwyth, Ceredigion SY23 3BZ, UK

While John Meurig Thomas was not the first person to interest me in microporous crystals, he was certainly the most insistent that these were ideal candidates for the new combinations of X-ray techniques I was developing in the late 1980s with non-crystalline materials in mind. Shortly before he moved from Cambridge to the Royal Institution (RI) he visited my office at the Synchrotron Radiation Source and simply enthused over what X-ray spectroscopy, diffraction and scattering could bring to feed his passion for watching zeolites form and catalysis happen. It needed the new X-ray detectors and geometries that were being commissioned by my Materials Science Group at Daresbury Laboratory. A most fruitful collaboration resulted between ourselves and the groups at the RI that John and also Richard Catlow were establishing, and a sizeable body of novel in situ materials chemistry was born, both in catalysis1 and in other areas.2 The starting point was the first experiment coupling in situ X-ray spectroscopy with X-ray diffraction, which Nature highlighted as the ‘‘Daresbury Double’’.3 Melting, however, is the subject of this birthday contribution. The connection with microporous catalysts is the inherent instability of zeolites and their tendency at high temperatures and pressures to amorphise into new and exciting glasses. The same combined X-ray techniques that proved so essential in the early 1990s for charting the synthesis of zeolites have proved just as useful in following their destruction. Amorphisation of minerals4 turns out to be a rather special case of melting, while melting per se remains one of the major unsolved problems in physics.5 For instance, melting and freezing are traditionally envisaged as opposite sides of the same first order phase transition, defined by a critical point in temperature and pressure. Yet there is increasing evidence that melting is preceded by progressive internal disordering within the period lattice.6 Freezing, on the other hand, is a non-equilibrium 165

166

Chapter 10

kinetic phenomenon leading to the fascinating supercooled state out of which glasses can emerge if the liquid is sufficiently viscous7 or crystals if it is not. Indeed the extent to which a liquid supercools before it crystallises is dependent on how it is contained. If it is not, which is possible using levitation furnaces,8,9 glasses can be formed that would be precluded if cooled from a crucible. Melting and also amorphisation therefore reside at the cross roads of the liquid, glassy and crystalline states – a burgeoning area in the physical sciences.10

1 Melting 1.1

Clausius–Clapeyron Relation

The physics and chemistry of melting started in the mid-19th century with the Clausius–Clapeyron relation that defines first order phase transitions, viz: dT DV ¼ dP DS

ð1Þ

Where melting is concerned, Equation (1) defines the boundary between liquid (L) and crystalline (C) phases, where DV ¼ VL–VC and DS ¼ SL–SC are the respective differences in molar volume and entropy. Simple close-packed materials like metals and alkali halides expand on melting at ambient pressure and become disordered, so DV and DS are both positive as is the slope of the m melting curve, dT dP .

1.2

Melting Curve of Alumina

Alumina provides a striking example of melting and of the versatility of new experimental methods. The impressive experiments of Shen and Lazor,11 which are included in Figure 1a, follow the rise in the melting point from 2323 K at ambient pressure up to 3675 K at 25 GPa. They are bounded in Figure 1a by two schemes for melting adopted in the molecular dynamics (MD) simulations of Ahuja et al.12 No less impressive is the huge increase in the molar volume, DV, measured at the melting point (Figure 1b), results obtained with an aerodynamic levitation furnace, where the density of the crystalline state has been determined from in situ X-ray diffraction as the melting point, Tm, is approached.13 The densities of the supercooled and molten states are derived from high speed camera images of the changing diameter of levitating drops.14 Given that the increase in entropy on melting DS ¼ LF/Tm, where LF is the latent heat of fusion, the melting curve predicted by the Clausius–Clapeyron relation (Equation (1)) at ambient pressure is plotted in Figure 1a. The agreement is surprisingly good considering the inevitable inaccuracies in these difficult experiments, but also the fact that the local structure of alumina changes on melting. Structure factors, S(Q), for liquid alumina were first measured using X-rays in 1997 by Ansell and co-workers who reported a sharp drop in coordination number compared to corundum.15 These groundbreaking experiments using an

167

Melting and Amorphisation 40 (a)

4500

(b)

4000

36 dTm /dP=∆V/∆S

3500 3000 2500

Molar Volume / cm3

Melting Temperature / K

Al2O3

38

Al2O3

34 ∆V

32 30 28

2000

MD Ahuja et al 1998 Expt. Shen & Lazor 1995

26

Tm

1500 24 −5

0

5

10 15 20 25 30

Pressure / GPa

Figure 1

0

1000

2000

3000

T/K

Melting corundum. (a) Melting curve combining the experimental measurements of Shen and Lazor11 with the results of MD calculations published by Ahuja et al. for one and two phase systems.12 (b) Stepwise increase in molar volume on melting obtained from in situ X-ray diffraction13 and the imaging of molten drops.14 The prediction of the Clausius–Clapeyron relation (Equation (1)) using LF ¼ 110 kJ/mol and Tm ¼ 2323 K and the increase in molar volume, DV, at ambient pressure shown by the red line in (a). Insert in (b): the network-like structure of molten alumina obtained from empirical potential structure refinement analysis of the experimental neutron structure factor.9

aerodynamic levitation furnace were followed by neutron S(Q) experiments9 where the three-dimensional structure was modelled using the empirical potential structure refinement method of Alan Soper16 incorporating the density measurements of Glorieux.14 The coordination numbers of AlO6 and OAl4 in corundum fall to almost AlO4 and OAl2.7 on melting, resulting in the quasi-network structure illustrated by the inset in Figure 1b. The Al–O distance reduces at Tm from 1.93 A˚ in corundum at 2170 K17 to 1.76 A˚ in molten alumina.9 At the same time the average separation between atoms, r, which parameterises the Debye Model – vide infra – increases from 2.04 to 2.23 A˚. Accordingly liquid alumina is more similar to the network structure of silica than the more closely packed structure of a-Al2O3. Nevertheless at the melting point of alumina both the free energy of the crystal and the liquid should be identical if the Clausius–Clapeyron relation (Equation (1)) is to apply.

1.3

Lindemann’s Melting Rule

Virtually all accounts of melting over the last century come back to Lindemann’s melting rule18 which had its origins in Einstein’s vibrational model of specific

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heat. Lindemann originally proposed that melting occurs when atoms oscillating about their mean positions begin to collide with one another – a kinetic rather than a thermodynamic description. In the form later developed by Gilvarry,19 melting is predicted to occur when the incoherent root
mean 1=2square displacement of the average atoms about their mean positions u2 exceeds a certain fraction of their average separation,  r. In terms of the Debye model20     
2 YD 1 YD  B Y2D F u ¼ 9 h2 T=mk þ ð2Þ 4 T T  is the mean atomic mass, YD is the Debye temperature, kB is where m Boltzmann’s h is Planck’s constant divided by 2p. F YTD ¼ R YD =T zdz constant and  T YD 0 ez
1 approaches unity when T4YD and the zero point motion 9 h2  1=3   4mkB YD becomes a minor correction. Therefore taking rm ¼ VA , where VA is the average atomic volume, the melting temperature Tm is given by  2D =9h2 ¼ L2 kB mY  2D r2m2 =9h2 ð3Þ Tm ¼ om2m 4kB mY
 r2m . For many simple crystalline materials L E 0.1 at where L2 ¼ u2m = the melting temperature, Tm,21 in which case inserting L ¼ 0.1 in Equation (3) provides an empirical law for predicting Tm. The prerequisite, though, is that the crystalline systems are ‘‘Debye-like’’, which is generally meant to mean that the Debye frequency, oD ¼ kB YD = h, approximately aligns with the top of the measured vibrational density of states (VDOS).22 For a-Al2O3, for instance, n D ¼ oD =2p equals 22 THz, which is in the vicinity of the optic modes measured by inelastic neutron scattering23 and from Equation (3), L ¼ 0.1. For quartz, on the other hand, despite the stronger chemical bond, nD equals 10 THz, falling in the bottom half of the VDOS, and L ¼ 0.2 at the melting temperature. For zeolites, where Si–O and Al–O bond strengths are similar, these discrepancies become even more exaggerated. Nevertheless for crystalline systems, like metals and alkali halides and oxides that are reasonably close packed, the harmonic Debye model offers a global structure-independent description of the dynamics and Lindemann’s rule (Equation (3)) is surprisingly predictive.21,24 Of course, as melting is approached the vibrational energy rises through the asymmetric interatomic potential leading to anharmonic vibrations. Nevertheless, if the expansion coefficient a remains linear with temperature, thermodynamic properties can still be successfully described by assuming that vibrations remain harmonic as interatomic separations increase with temperature – the quasi-harmonic approximation. In this approximation, at temperatures above YD, quantities like the specific heat at constant volume, the Gru¨nheisen parameter and also aK, the product of the expansion coefficient and the bulk modulus K, should all be temperature independent.22 Alumina conforms well to the quasi-harmonic approximation as Tm is approached. 1=3 In the Debye model oD / v=VA ,20 where v is the Debye speed of sound. Accordingly, frequencies of the harmonic (acoustic) modes are directly related to the bulk and shear moduli, K and G, of the crystal. Gilvarry appealed to this connection between kinetics and thermodynamics19 in order to develop

169

Melting and Amorphisation

Equation (3) into an expression for the melting curve that complements the Clausius–Clapeyron relation (Equation (1)):   dTm Tm dK
1 ð4Þ ¼ dP K dP dK=dP can be obtained in the quasi-harmonic approximation relation from the slope of lnK versus ln r, if the temperature dependences of the density r and of the bulk modulus K are known as Tm is approached. Virtually the same melting curve expression (Equation (4)) is obtained from Poirier’s dislocation melting model.21 Indeed Equation (4) also emerges from Lennard-Jones and Devonshire’s atomic disordering theory of melting.25 Both these models overcome a common criticism of Lindemann’s melting rule that, unlike the Clausius– Clapeyron law which is based on the coexistence of liquid and crystal each with the same free energy along the phase transition boundary, Equation (3) is only tied to the properties of the crystalline state. For both the dislocation model21 and the atomic disordering model25 however, disordered as well as periodic components are present at the melting point and Lindemann’s rule is returned. Accordingly at the melting point Equation (4) indicates KC

dKL dKC
KL ¼ KC
KL ¼ DK dP dP

ð5Þ

As the high frequency bulk modulus of a solid KN, which can be obtained from Brillouin scattering or inelastic X-ray scattering, approximately equals the static bulk modulus K, KL for the liquid can be approximated from measurements of the respective transverse and longitudinal sound velocities, vT and vL , viz: K ¼ C11
4C44/3, where C11 ¼ rv2L and C44 ¼ rv2T . In particular KC 4 KL so the step change in specific volume DV at the melting point (Figure 1b) is also accompanied by a step change in bulk modulus DK in the opposite sense.
 Returning to Equation (3), the total mean square displacement u2 is
h m2 i Q 2 = 3 contained in the Debye–Waller factor familiar in crystallography ,
2 e where Q is the scattering wave vector. At modest temperatures u is readily measured from the decrease in intensity of
the  Bragg peaks compared to the background of thermal diffuse scattering. u2 is also contained in for crystals and glasses the intermediate scattering function, F(Q, t), in the limit as Q-0 and t -N. F(Q, t) can be obtained by inelastic X-ray scattering experiments26 and also from MD simulations from the transform of the van Hove space–time correlation function. As a result, excellent opportunities are now emerging to explore Lindemann’s rule not just by extrapolating Equation (2) to Tm using room temperature densities and elastic constants – the harmonic approximation – but also by looking for departures from Equation (2) as elastic constants soften in the vicinity of the melting
temperature.  At temperatures close to melting, u2 becomes more difficult to extract from 24 diffraction experiments because
2 of anharmonic factors. However, when these are correctly accounted for, u is found to rise above the value predicted by the harmonic Lindemann rule (Equation (3)).24 Our preliminary results for

170

Chapter 10

corundum are reproduced in Figure 2a where synchrotron radiation powder diffraction patterns from levitated spheres have analysed by the Rietveld
been 1=2 method to obtain the Lindemann ratio L ¼ u2 =r.13 Some softening of the elastic constants K and G as melting is approached is accompanied by a significant increase in thermal diffuse scattering. Analogous disordering of the periodic structure as T-Tm has been reported recently in fascinating studies of colloidal crystals.27 These beautifully imaged premelting phenomena have been specifically correlated with extended defects in the close-packed periodic colloidal structures – notably grain boundaries but also dislocations. It is interesting to note that the softening of elastic constants as Tm is approached is also inherent in the supercooled state. Mode coupling theory, for example, predicts that above the glass transition the speed of the slow a processes of the glassy state accelerate towards the speed of the fast vibrational b processes, the two becoming indistinguishable once the classical liquid state is reached.10,28 This effect is accompanied by an increase in translational diffusion,29 particularly for fragile liquids, of which molten alumina appears to be an extreme example.10,30 This is the same temperature range where the harmonic and quasi-harmonic approximations (Figure 2a).
divide 1=2 Also included in Figure 2 are values for m2 = r which we have estimated from the remarkable X-ray inelastic scattering data of liquid alumina, recently published by Sinn et al.31 The downward discontinuity in the bulk modulus at the melting point DK (Equation (5)) is matched by an upward discontinuity in

0.22 0.20

0.6

α-Al2O3 Lindemann Law (a) α-Al2O2 XRD Debye-Waller Factors Liquid Al2O3

(b)

0.5

0.18 Crystal Liquid

0.4

Al2O3

0.14

<µ2>1/2

<µ2>1/2/

0.16

0.12

MD 0.3

0.10 0.2 0.08 0.06 0.04

Tm 0.0

0.02 1000

1500

2000 T/K

Figure 2

Tm

0.1

2500

3000

−1

0

1

2

3

4

5

6

7

Temperature

Mean square displacement Tm. (a)
1=2above and below the melting point, Preliminary results of m2 = r for crystalline alumina up to Tm.13 Data for 31 liquid alumina13 estimated from inelastic X-ray scattering
data. (b) 1=2 LaViolette and Stillinger’s Lennard-Jones MD simulations of m2 versus temperature across the melting point.32

Melting and Amorphisation

171


2 1=2 m (Figure 2a). A similar picture appeared in early MD simulations by LaViolette and Stillinger.32,33 Using an interatomic derived from the
potential 1=2 Lennard-Jones potential, they calculated L ¼ m2 =r ¼ 0:5 in the liquid state, rather larger than what is observed when alumina melts. More recent Lennard-Jones MD calculations by Luo and co-workers account for anharmonicity at the melting point34 and yield a Lindemann ratio L of 0.116 increasing to 0.143, virtually matching the behaviour in alumina shown in Figure 2a. These authors also follow the T–P melting curve for a LennardJones system, and confirm that L remains virtually constant with increasing pressure, therefore establishing that the Lindemann rule (Equation (3)) applies, not just at ambient pressure, but at high pressures too.

2 Amorphisation 2.1

Negative Melting Curves

Melting curves are not always positive, ice being the familiar example. DVo0 DV and DS40 and so from Equation (1), dT dP ¼ DS o0. Predicting the depression in the melting point of ice with pressure by the Thomsons was one of the first successful applications of thermodynamics. Now 150 years on it is the riches of the physics of crystalline, supercooled and glassy water that are attracting attention.35,36 Critical in this renaissance of the thermodynamics of water have been the groundbreaking experiments of Mishima and colleagues who showed how the low temperature hexagonal phase of ice could be amorphised to a glass at 77 K under 1 GPa of compression – the critical T–P point intersecting the extrapolated negative melting curve.37 Moreover, by reducing the pressure, a second glassy phase was discovered together with a reversible first order phase transition between the two viz: from a high density amorphous (HDA) phase to a low density amorphous (LDA) phase.38 In addition to the difference in molar volume between these two glassy states or polyamorphs, DV ¼ VHDA
VLDA, a difference in entropy, DS ¼ SHDA
SLDA, is also expected, the LDA phase being the more ordered. Accordingly, a decompressive HDA–LDA liquid–liquid phase transition should be exothermic, with a stepwise decrease in density and entropy. If DV ¼ VHDA
VLDAo0 and DS ¼ SHDA
SLDA40, Equation (1) predicts that the characteristic temperature of this transition will fall with increasing pressure, like the melting curve of ice, but now originating below Tm from a critical point in the supercooled region.35 The existence of more than one supercooled amorphous phase – now generally referred to as polyamorphism39 – has been advanced as the destabilising factor that might trigger the amorphisation of other crystalline systems under thermobaric stress – not just ice.40 Indeed, after the amorphisation of ice was first discovered37 an analogous transformation in quartz was soon reported, a-SiO2 amorphising to a glass under the rather larger pressures of 30 GPa.41 Amorphisation, or low temperature vitrification, now embraces many minerals, destabilisation being promoted not just through pressure but also for

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Chapter 10 4

high density phases through decompressive stress. In either case, the route to an alternative stable crystalline phase is kinetically hindered by an amorphous intermediate phase.

2.2

Zeolite Amorphisation

It was this background of crystalline destabilisation under thermobaric stress that led to our own studies of the collapse of zeolites.42,43 These experiments benefited from the combined synchrotron radiation techniques originally exploited in studying the synthesis of catalytic microporous materials.1 Crystalline microporous materials like Na zeolite A and Na zeolite Y are extremely resilient, considering their low density structures. Nevertheless once calcined they eventually succumb to thermobaric stress, their filigree periodic low density structures amorphising with an abrupt reduction in volume, DVA, as illustrated in Figure 3a. The Debye–Scherrer pattern disappears abruptly over a narrow range of pressure and/or temperature, the process being irreversible – certainly over periods of months. For instance, at ambient temperature zeolite collapse occurs around 3–4 GPa (PA) and at ambient pressure at around 1100 K (TA) which defines the ‘‘negative amorphisation curve’’ shown in Figure 4a for zeolite A. However, the low temperature melting of zeolites is more subtle. The negative amorphisation curve defines a liquid–liquid transition between LDA and HDA phases, but zeolite collapse in practice involves excursions into a region of negative pressure,44 as Figure 4 illustrates. The decrease in molar volume DVA on amorphisation in Figure 3a is defined by P1 and T1, where collapse commences, and by P2 and T2, where collapse is completed. Adding these to the T–P diagram in Figure 4a establishes experimental boundaries for a zone of instability either side of the negative amorphisation line defined by TA,PRP
TRT,PA, with T1,PRP
TRT,P1 defining the thermobaric limits for zeolite stability and T2,PRP
TRT,P2 the limits beyond which vitrification appears irreversible. In the Ponyatovsky–Barkolov model for amorphisation,40 the LDA–HDA liquid–liquid phase transition across the TA,PRP
TRT,PA negative amorphisation curves between the low and high density phases that DV A are believed to destabilise the periodic lattice is defined by dT dPA ¼ DS , where DS ¼ SLDA
SHDAo0 and DV ¼ VLDA
VHDA40. This is bounded by the respective spinodal limits defined by d2G/dc2 ¼ 0, where c, for example, is the concentration of the LDA phase.10,45 These limits for the LDA and HDA phases are shown by the dashed lines in Figure 4a and are in reasonable alignment with the experimental T1,PRP
TRT,P1 and T2,PRP
TRT,P2 boundaries determined from Figure 3a.42 Figure 4 includes important new ab initio 0 K computer simulations which model the amorphisation processes in zeolites induced by compression but also by decompression.46 With increasing pressure these calculations reveal a displacive order–disorder transition (marked II–III). This is identified by an abrupt decrease in volume close to 3 GPa and is associated with a narrowing of

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T/K 1050

(a)

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100 90

16 kJ/mol

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110

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80 70

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TA

PA

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T2

60

50 50 40

0 40 0

Figure 3

Pint / GPa

Molar Volume / cm3

1100

zeolite A

5 P / GPa

10

2 P / GPa

β / GPA21

110

1000

4

Amorphisation of zeolite A. (a) Stepwise decrease in volume DVA under pressure at 1 mPa s
1 (left) and temperature at 30 mdeg s
1(right). T1 and P1 define the start and T2 and P2 the end of zeolite collapse while TA and PA define the turning points. These are used to identify the boundaries between zeolite, LDA and HDA phases in Figure 4. A DTA scan is shown together with the enthalpy changes DH taken from Ref. 53 for zeolite A, nepheline (LDA) and high density glass (HDA). (b) Dependence of macroscopic compressibility b (right) and the internal pressure Pint (left) on applied hydrostatic pressure during compressive amorphisation, measured from changes in the zeolite diffraction pattern. Taken from Refs. 42 and 44.

the Si–O–Al bridging angle. In all other respects the crystalline phase II and the amorphous phase III are toplogically equivalent and the phase transition is found to be reversible but with hysteresis (Figure 4b). Interestingly, this first order transition lies close to our experimental T1,PRP
TRT,P1 line extrapolated to 0 K. We therefore associate zeolite A at ambient pressure with Peral and I´n˜iguez’s phase II and our LDA phase with their amorphous phase III. At higher pressures of around 5 GPa the simulations reveal a subsequent first order topologically disordering transition, now between two amorphous phases labelled III and IV. At this point the double four-fold rings – the smallest of the secondary building units of the zeolite A structure – collapse with a further reduction in the bridging oxygen bond angle.46 The III–IV transition is reversible but the hysteresis bypasses the amorphous phase III eventually recovering the reference zeolite close to
4 GPa. Our experimental negative amorphisation curve TA,PRP
TRT,PA extrapolated to 0 K falls close to the III–IV transition from which we can associate our HDA phase with Peral and I´n˜iguez’s topologically disordered phase IV. This phase is maintained to

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0

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-2

6

8

10

(b)

Figure 4

Comparison between the various thresholds in the collapse of zeolite A. Upper frame: experiment.10,42,44 The limits T1, P1, T2, P2, TA and PA for thermal and pressure-induced amorphisation are taken from Figure 3a. The dashed lines are the spinodal limits calculated from the model of Ponyatovsky and Barkolov.40 The dashed lines at negative pressures refer to the decompression at the start of collapse (blue) and the turning point (green) illustrated in Figure 3a. Lower frame: ab initio 0 K computer simulations of volume vs. pressure.46 First order discontinuous transitions II–III and III–IV are reversible via IV* and IV** to the reference zeolite I. The vertical arrows in (b) follow extrapolations of the experimental data to 0 K in (a), and associate the zeolite at positive pressure and the amorphous LDA and HDA phases with II, III and IV, respectively.

pressures of 10 GPa at the point TRT,P2 where experimentally we find all of the zeolite has amorphised.42 An exciting outcome of these simulations46 is that, not only are the zeolite– LDA and LDA–HDA transitions analysed from experiment (Figure 4a)42

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replicated (Figure 4b), but also these are abrupt in the 0 K calculations and well-separated thermodynamically. In particular the LDA or phase III shares the topology of the zeolite. We have already drawn attention to the fact that the LDA phase should be an ordered or perfect glass,10,42,43,47 equivalent to the ideal melt-quenched glass predicted by Kauzmann48 with entropy equal to the equivalent crystal at some finite glass transition temperature, TK. Synthesising a perfect glass from a crystalline precursor rather than from a supercooled liquid avoids the interruption of recrystallisation that has so far precluded reaching this low entropy LDA glass by conventional cooling from the melt.10,49,50 The alumino-silicate melts equivalent to a conventional HDA alumino-silicate melt are the feldspars. The entropy difference between molten and crystalline states is the configurational entropy, Sc, acquired when a crystal melts and for which the extra configurations are mainly responsible for the increase in specific volume, DV at Tm (Figure 1b). If DCP(T) is the difference in specific heat between R T the supercooled liquid and the crystal at a given temperature, Sc ¼ TmK DCP d ln T. So, if LDA is a perfect glass Sc ¼ DS ¼ SHDA
SLDA, the difference in volume between the HDA and LDA phases, DV ¼ VHDA
VLDA, should be given by Equation (1) DV ¼ DS

dT dP

Taking the measured value of Sc ¼ DS for nepheline (15 J mol K
1)51 and the slope of the negative amorphisation curve from Figure 4(a) dT=dP ¼
210
7 K Pa
1 gives VLDA
VHDA ¼ 3 cm3 or 12% of DVA ¼ Vzeolite
VHDA from Figure 3a. This is in reasonable agreement with Peral and I´n˜iguez’s MD calculations (Figure 4b), where VIII
VIV is 14% of the total volume change between the ambient zeolite phase II and the topologically disordered phase IV. The increase in entropy between the LDA and HDA phases is reflected in the endothermic step in CP (Figure 3a). Finally we turn to the unusual T–P behaviour in zeolite collapse that occurs at negative pressures (Figure 4a) where the MD calculations indicate the reversibility of the amorphisation processes (Figure 4b).46 Experimentally we find that thermal collapse is accompanied by a sharp rise in the expansion coefficient of the residual crystalline fraction while pressure-induced collapse is accompanied by a lowering in the compressibility42 – in either case the remaining zeolite is stretched signifying negative internal pressures, as shown in Figure 3b. Dramatic evidence for decompression can be seen in micrographs of recovered material (see the inserts in Figure 3b). In the model of Cohen et al.52 for displacive amorphisation, long range order is destroyed because domain nucleation overwhelms growth. If nucleation is of higher density than the precursor crystal and randomly distributed, then intervening periodic zones should suffer decompression on average. The internal pressure is estimated in Figure 3b from the difference in compressibility b with the ambient value and registers approximately
2 GPa at P1, the point at which collapse starts to accelerate, and around
4 GPa at P2 by the time the ruby calibrant in the diamond anvil cell has reached 4 GPa and collapse is complete.44 In the same

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way, for temperature-induced collapse, the increase in thermal expansion coefficient and the ambient compressibility enable the internal pressure in the crystalline fraction to be estimated. These negative internal pressures at the start of collapse TRT,P1 and T1,PRP and at the turning point TRT,PA and TA,PRP are shown connected by the dashed blue and green lines respectively in Figure 4a. Extrapolated to 0 K these extend over reference I in the computer simulations at negative pressures in Figure 4b.46 Because experimentally thermobaric stress is applied sequentially, we conclude that the zeolite–LDA transition is progressively achieved through a process of compression and decompression followed by recompression, until full transformation to the LDA phase (III) is achieved. The dynamics of zeolite collapse should therefore be controlled by the viscosity of the LDA phase, as it is gradually accumulated.10,42 Thermally-induced zeolite collapse to the LDA phase is accompanied by a sharp exotherm in CP which anticipates the endothermic step associated with the LDA–HDA transition (Figure 3a).42 This exotherm confirms that the zeolite has a higher enthalpy than the amorphous phase it transforms into. The enthalpies of anhydrous zeolites, glasses and feldspar crystals have been meticulously catalogued by Navrotsky and Tian in a comprehensive study and decrease in that order.53 Setting the enthalpy of the LDA phase equal to that of nepheline, the different enthalpies for zeolite A, LDA and HDA are sketched in Figure 3a, signifying a drop of 25 kJ mol
1 for the zeolite-LDA transition. Taking the average decompression, DP, between the applied temperatures T1 and TA of 0.3 GPa (Figure 3a) and the mean molar volume, V, for the zeolite– LDA system of 78 cm3 (Figure 4a), V DP experiences a similar drop of 23 kJ mol
1. Given that DH ¼ TDS + VDP, this suggests that DS E 0. Little entropy change between the zeolite and LDA phases is consistent with the displacive nature of the II–III transition46 (Figure 4b) and the ‘‘perfect glass’’ label we have used for the LDA phase.10

3 Amorphisation and Double Well Potentials Displacive phase transitions in silicates – like a to b quartz – can be modelled on the dynamics of low frequency rigid unit modes between adjacent tetrahedra.54 Equilibrium positions are paired through double well potentials associated with very soft modes. We have recently detected strong nondispersed features by inelastic neutron scattering at very low frequencies (4  1011 Hz) in zeolites and also in glasses including silica,43 which we have attributed to the librational modes responsible for the destabilisation of microporous crystals as well as the unusual low temperature thermal properties of glasses.55 In amorphised material these modes should promote the transformation of LDA into HDA phases and now, considering Figure 4, the reversibility of these first order phase transitions. The recent observation of similar sub-terahertz (THz) features in quartz and corundum13 encourages us in the view that they may also lie at the heart of normal melting and freezing. This

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would put the emphasis of melting on twisting rather than on the stretching criterion that forms the basis of the Lindemann rule.19 At lower temperatures, librational modes in standing wave configurations can promote network distortion at metal sites in zeolites, which has been advanced as an explanation of their catalytic activity56 – as observed in Ni exchanged zeolite Y,57 one of the first discoveries to be made by John, myself and our colleagues at Daresbury Laboratory and the Royal Institution from combining X-ray spectroscopy and X-ray diffraction.

Acknowledgements Steve Fearn, Louis Hennet, Jorge I´n˜iguez, David Keen, Chris Martin, Florian Meneau, Alexandra Navrotsky, Irina Pozdnyakova, Sabyasachi Sen and Martin Wilding are thanked for stimulating discussions. The support of the Higher Education Funding Council of Wales is acknowledged through the Centre for Advanced Functional Materials and Devices, as is Science and Technology Facilities Council for providing access to the Synchrotron Radiation Source.

References 1. J.M. Thomas and G.N. Greaves, Science, 1994, 265, 1675. 2. W. Bras, G.E. Derbyshire, A.J. Ryan, G.R. Mant, A. Felton, R.A. Lewis, C.J. Hall and G.N. Greaves, Nucl. Instrum. Methods Phys. Res., Sect. A, 1993, 326, 587. 3. J.W. Couves, J.M. Thomas, D. Waller, R.H. Jones, A.J. Dent, G.E. Derbyshire and G.N. Greaves, Nature, 1991, 354, 465. 4. P. Richet and P. Gillet, Eur. J. Mineral., 1997, 9, 907. 5. P.W. Anderson, Basic Notions of Condensed Matter Physics, Benjamin, London, 1984. 6. P.N. Pusey, Science, 2005, 309, 1198. 7. P.G. Debenedetti and F.H. Stillinger, Nature, 2001, 410, 259. 8. L. Hennet, C. Landron, J.-P. Coutures, T.E. Jenkins, C. Aletru, G.N. Greaves, A. Soper and G.E. Derbyshire, Rev. Sci. Instrum., 2000, 71, 1745. 9. C. Landron, L. Hennet, T.E. Jenkins, G.N. Greaves, J.P. Coutures and A.K. Soper, Phys. Rev. Lett., 2001, 86, 4839. 10. G.N. Greaves and S. Sen, Adv. Phys., 2007, 56, 1. 11. G.Y. Shen and P. Lazor, J. Geophys. Res., 1995, 100, 17699. 12. R. Ahuja, A.B. Belonoshko and B. Johansson, Phys. Rev. E, 1998, 57, 1673. 13. G.N. Greaves, M.C. Wilding, S. Fearn, Q. Vu Van, L. Hennet, I. Pozdnyakova and O. Maje´rus, 2007, unpublished results. 14. B. Glorieux, F. Millot, J.-C. Rifflet and J.-P. Coutures, Int. J. Thermophys., 1999, 20, 1085.

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15. S. Ansell, S. Krishnan, J.K.R. Weber, J.J. Felton, P.C. Nordine, M.A. Beno, D.L. Price and M.-L. Saboungi, Phys. Rev. Lett., 1997, 78, 464. 16. A.K. Soper, Chem. Phys., 2000, 258, 121. 17. N. Ishizawa, T. Miyata, I. Minato, F. Marumo and S. Iwai, Acta Crystallogr., Sect. B, 1980, 36, 228. 18. F.A. Lindemann, Phys. Z., 1910, 11, 609. 19. J.J. Gilvarry, Phys. Rev., 1956, 102, 308. 20. J.M. Ziman, Principles of the Theory of Solids, Cambridge University Press, Cambridge, 1965. 21. J.-P. Poirier, Introduction to the Physics of the Earth’s Interior, Cambridge University Press, Cambridge, 2004. 22. O.L. Anderson, Equations of State of Solids for Geophysics and Ceramic Science, Oxford Monographs on Geology and Geophysics, Oxford University Press, Oxford, 1995. 23. C. Rambaut, H. Jobic, H. Jaffrezic, J. Kohanoff and S. Fayeulle, J. Phys.: Condens. Matter, 1998, 10, 4221. 24. C.J. Martin and D.A. O’Connor, J. Phys. C: Solid State Phys., 1977, 10, 3521. 25. J.E. Lennard-Jones and A.F. Devonshire, Proc. R. Soc. London, Ser. A, 1939, 170, 464. 26. T. Scopigno, G. Ruocco, F. Sette and G. Monaco, Science, 2003, 302, 849. 27. A.M. Alsayed, M.F. Islam, J. Zhang, P.J. Collings and A.G. Yodh, Science, 2005, 309, 1207. 28. W. Go¨tze, J. Phys.: Condens. Matter, 1999, 11, A1. 29. F.H. Stillinger and J.A. Hodgdon, Phys. Rev. E, 1994, 50, 2064. 30. G. Urbain, Rev. Int. Hautes Temp. Re´fract., 1982, 19, 55. 31. H. Sinn, B. Glorieux, L. Hennet, A. Atlas, M. Hu, E.E. Alp, F.J. Bermejo, D.L. Price and M.-L. Saboungi, Science, 2003, 299, 2047. 32. R.A. LaViolette and F.H. Stillinger, J. Chem. Phys., 1985, 83, 4079. 33. F.H. Stillinger, Science, 1995, 267, 1935. 34. S.-N. Luo, A. Strachan and D.C. Swift, J. Chem. Phys., 2005, 122, 194709. 35. H.E. Stanley, S.V. Buldyrev, G. Franzese, N. Giovambattista and F.W. Starr, Philos. Trans. R. Soc. London, Ser. A, 2005, 363, 509. 36. P.G. Debenedetti, J. Phys.: Condens. Matter, 2003, 15, R1669. 37. O. Mishima, L.D. Calvert and E. Whalley, Nature, 1984, 310, 393. 38. O. Mishima, J. Chem. Phys., 1994, 100, 5910. 39. M.C. Wilding, M. Wilson and P.F. McMillan, Chem. Soc. Rev., 2006, 35, 964. 40. E.G. Ponyatovsky and O.I. Barkolov, Mater. Sci. Rep., 1992, 8, 147. 41. R.J. Hemley, A.P. Jephcoat, H.K. Mao, L.C. Ming and M.H. Manghnani, Nature, 1988, 334, 52. 42. G.N. Greaves, F. Meneau, A. Sapelkin, L.M. Colyer, ap I. Gwynn, S. Wade and G. Sankar, Nat. Mater, 2003, 2, 622. 43. G.N. Greaves, F. Meneau, O. Maje´rus, D. Jones and J. Taylor, Science, 2005, 308, 1299.

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44. G.N. Greaves and F. Meneau, J. Phys.: Condens. Matter, 2004, 16, S3459. 45. F. Meneau, Ph.D. Thesis, Studies of Amorphisation in Zeolites, University of Wales, Aberystwyth, 2003. 46. I. Peral and J. I´n˜iguez, Phys. Rev. Lett., 2006, 97, 225502. 47. F. Meneau and G.N. Greaves, Nucl. Instrum. Methods Phys. Res., Sect. B, 2005, 238, 70. 48. W. Kauzmann, Chem. Rev., 1948, 43, 219. 49. J. Zarzycki, Glasses and the Vitreous State, Cambridge University Press, Cambridge, 1991. 50. P.G. Debenedetti and F.H. Stillinger, Nature, 2001, 410, 259. 51. M.J. Toplis, D.B. Dingwell, K.-U. Hess and T. Lenci, Am. Mineral., 1997, 82, 979. 52. M.H. Cohen, J. I´n˜iguez and J.B. Neaton, J. Non-Cryst. Solids, 2002, 307–310, 602. 53. A. Navrotsky and Z.-R. Tian, Chem.–Eur. J., 2001, 7, 769. 54. M. Dove, Am. Mineral., 1977, 82, 213. 55. W.A. Phillips, Amorphous Solids: Low Temperature Properties, Springer, Berlin, 1981. 56. K.D. Hammonds, H. Deng, V. Heine and M.T. Dove, Phys. Rev. Lett., 1997, 78, 3701. 57. E. Dooryhee, A.T. Steel, P.J. Maddox, J.M. Thomas, C.R.A. Catlow, J.W. Couves, G.N. Greaves and K.P. Townsend, J. Phys. Chem., 1991, 95, 1229.

CHAPTER 11

Computer Modelling in SolidState Chemistry C. RICHARD A. CATLOW, SAID HAMAD, DEVIS DI TOMMASO, ALEXEY A. SOKOL AND SCOTT M. WOODLEY Davy Faraday Research Laboratories and Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, UK

1 Introduction Model building goes back to the beginning of scientific thought; and computer modelling is simply the application of contemporary technology to this core scientific activity. Modelling now pervades all scientific disciplines and is applied on almost all the length and time scales used in present-day science. Applications in chemistry and materials sciences have been particularly successful and this article will focus on the role of modelling techniques in solid-state chemistry, where the range and applicability for the techniques has developed enormously in the last 30 years and whose importance was realised in the early days of the field by Sir John Meurig Thomas.1 We cannot in an article of this length survey adequately what has become a major field of contemporary chemical and materials sciences; but we hope to show how the field has developed, to outline some of its major achievements, and to indicate the range and excitement of recent applications. We will concentrate on modelling at the atomic and molecular level while acknowledging the growing importance of modelling at larger length and longer timescales.

2 Motivation and Background We have noted that modelling is an indispensable scientific tool, but why do we need models? In contemporary physical, including solid-state, sciences, 180

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modelling is used for four main reasons:
To gain insight and understanding of complex systems, for example, catalytic processes and the mechanisms of crystal growth and nucleation.
To derive numerical data, for example, the formation energies of defects in solids and the binding energies of molecules to surfaces.
To obtain information on systems that may be very difficult or inaccessible to experimental study, for example, materials under extreme conditions of temperature and pressure in planetary interiors.
To predict new systems and phenomena, for example, new, as yet un-synthesised crystal structures. These categories are neither comprehensive nor mutually exclusive, but they are useful and we may find many examples in solid-state chemistry of all four categories, with an increasing emphasis in recent work on predictive applications. The earliest applications in solid-state chemistry in the 1960s and 1970s concerned calculations of lattice and defect energies and a still useful review of this earlier work is available in Ref. 2. The importance of this work is that it showed that for ionic and semi-ionic solids, using carefully parameterised Born model potentials, it was possible to achieve quantitative agreement with experiment. The impact in the field of the physics and chemistry of defective solids was particularly marked, and in the early 1980s modelling methods rapidly became a routine adjunct to experimental studies. An important landmark was the development by Norgett of the HADES code (see Ref. 3), which was based on the approach originally proposed by Mott and Littleton4 and which allowed calculations on defect formation and migration energies to be undertaken in a straightforward and automated manner. By the late 1980s the field had become mature and well established, but the importance of calculations on defects and impurities in solids remains strong in the contemporary field although such calculations now often make use of quantum mechanical (QM) methods as discussed later in this article. From these early foundations the field rapidly developed in a number of different directions. Modelling of defects rapidly engaged with the structural problems posed by non-stoichiometric solids and by fast ion conduction. Lattice energy calculations moved from simple ionic solids to far more complex systems, where a particularly fruitful field developed in the modelling of silicate minerals.1,5 Modelling studies embraced amorphous as well as crystalline solids and surface in addition to bulk properties. These and other achievements will be discussed in greater detail below. The vitality of the field has relied on three main factors: first the continuing development in methods and algorithms; second, the continuing exponential growth in computer power; and third and most importantly, the engagement with experiment; and it is in this latter respect that John Meurig Thomas has made such significant contributions to computational solidstate chemistry.

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3 Methods Our account here is brief as the field has been very extensively reviewed in recent years (see, for example, Refs. 6 and 7). Earlier work was largely based on interatomic potential (IP) based methods, which do not attempt to solve the Schro¨dinger equation for the system studied, but rather use parameterised functions representing the interaction energy between pairs or larger numbers of atoms or ions. Such potentials may be implemented in static lattice or energy minimisation methods (as in the defect and lattice energy calculations referred to above), Monte Carlo methods or molecular dynamics techniques. Potentials may be parameterised by empirical fitting procedures or by direct calculation of the interaction energy using theoretical methods. Good quality potentials are now available for many inorganic and molecular materials and there is a wide range of excellent general-purpose software. Applications using these techniques continue to make an important contribution to the field. Despite their versatility, potential based methods are, however, limited in their scope. They cannot model problems and properties that depend directly on electronic structure, for example, reactivity and spectroscopy; and there may sometimes be uncertainties about the extent of transferability of potential parameters. The last 10 years have, however, seen an explosion in the application of electronic structure methods. The vanguard of these developments has been in density functional theory (DFT), which rests ultimately on the pioneering work of Hohenberg, Kohn and Sham,8,9 and which allows calculations of ground-state energies and electron densities for molecules and solids with a reasonable level of accuracy using affordable computer resources. Particularly attractive aspects of DFT are the relatively low scaling, that is, the calculation time increases less dramatically with system size than with the more traditional Hartree–Fock (HF) methods. The methods are, therefore, applicable to increasingly large and complex systems and the range of applications is set to expand with the growth in ‘‘Order N’’ techniques, that is, methods for which the computational requirements scale linearly with system size. The majority of recently published electronic structure calculations on solids have therefore employed DFT, but HF methods continue to have a significant role in the field; moreover, there has been a considerable growth in hybrid approaches which blend ‘‘exact’’ HF exchange with the DFT approach. We refer the reader to the reviews cited above for details. Electronic structure methods can be implemented in a number of ways. The most popular in recent applications in solid-state science is to use three-dimensional periodic boundary conditions (PBC), with defects, impurities and sorbed species, as well as surfaces and interfaces, being treated using a super-cell. Such methods have many technical advantages and the plane wave pseudopotential approach has been particularly effective; moreover, the method may be employed in dynamical simulations either in straightforward (if expensive) adiabatic procedures or using the more ingenious fictional electron dynamics of the celebrated Car–Parrinello approach.10 The use of PBC does, however, become more problematic when studying, for example, large sorbed

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molecules and complex defects, where a better approach is based on an old idea in computational chemistry and physics in which the system is partitioned into the region of interest, for example, the defect or sorbed molecule, and the surrounding (embedding) matrix: the former is treated at a high QM level; the latter is described using more approximate level, typically IP based methods, as it is usually only necessary to describe the electrostatic and steric constraints of this region. A number of technical developments have given this method much greater reliability and applicability in recent years and we may anticipate that embedded cluster methods will make a major contribution to the field in the future. We conclude this section with the following reflections and comments on the present status of computational methodologies in solid-state science: (i) There will be a continuing role for IP based methods: even in the era of O(N) DFT and petaflop computing, such methods will be the only way to tackle many of the complex problems and systems posed by condensed matter science. Moreover, there are many problems, for example the modelling of bulk structures and defects where, given good quality potential models, these methods are probably the most appropriate and accurate. (ii) DFT is an approximate method. There is no exact exchange-correlation functional and the common use of the term ‘‘first principles’’ in describing DFT calculations, although not incorrect, has had a tendency to be misleading and to imply that they are exact. DFT has made a huge contribution to the field, but its role must be kept in perspective. (iii) A multi-technique approach is generally needed for the problems addressed by the solid-state chemist; in particular, it is often necessary to combine potential based and QM methodologies. This feature will become apparent in the later sections of this article.

4 Achievements Earlier, we attempted to describe the early development of the field; here we shall try to summarise some of the main achievements of computational solidstate chemistry since its genesis in the 1970s. We highlight the following main areas.

4.1

Crystal Structure Modelling and Prediction

One of the most fundamental challenges in theoretical solid-state science is the prediction of crystal structures. And indeed in a notorious ‘‘News and Views’’ article in Nature in 1988, John Maddox11 commented that ‘‘One of the continuing scandals in the physical sciences is that it remains in general

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impossible to predict the structure of even the simplest crystalline solids from a knowledge of their chemical composition’’. Maddox’s provocative remarks were not true when they were first made 20 years ago; and indeed structure prediction is one of the great success stories of the field. Early developments were based on lattice energy calculations, which in the 1980s were then coupled with energy minimisation methods. Considerable success was enjoyed in modelling oxides and silicate structures,1,5,12 and some of the greatest successes were achieved in the field of microporous aluminosilicates (zeolites).13 Of particular note was the successful modelling of the polymorphs of zeolite beta14 and the monoclinic distortion of the pure silica pentasil zeolite, silicalite – a subtle structural feature that was accurately reproduced by the lattice energy calculations of Bell et al.15 These calculations are, however, ‘‘modelling’’ and not ‘‘prediction’’. They take (possibly approximate) structures, which are refined by minimisation. Their success, however, validates the methods and potentials, and such calculations are a necessary precursor to further modelling studies. The methods may also be of real value in refining approximate or inaccurate structures as shown by the elegant work of Shannon et al.16 on zeolite nu, which was essentially solved using minimisation methods. The major challenge is, of course, prediction and here there have been substantial developments in the last 10 years. The problem is essentially one of devising procedures to explore the energy landscape quickly and effectively so that the low energy regions, in which the stable structure(s) lie, can be located. A number of procedures are available of which the following are the most widely used. Simulated annealing (SA), which explores energy landscapes by undertaking a molecular dynamics or Monte Carlo simulation, initially at high temperatures; subsequent cooling (or annealing) allows the system to freeze into low energy regions. The approach has been used to great effect in structure solution (see, for example, Ref. 17) and in a number of elegant, predictive studies of inorganic solids by Scho¨n and Jansen.18,19 It is a straightforward and robust procedure. If it has a weakness, it is that the exploration of the energy landscape necessarily starts from a single point and it is possible that not all low energy regions may be accessed. To minimise this danger, multiple runs with different initial parameters must be undertaken, as in the recent work of Hamad et al.20 on nanoparticle structure prediction. Genetic algorithm (GA) methods, or more generally evolutionary algorithm (EA) methods, avoid the difficulty of the single starting point by setting up a population of structures, where typically different random arrangements of atoms make up each candidate structure in the initial population. A cost function is specified, which must be a rapidly computable ‘‘figure of merit’’ for the structure: it may be a simple function depending on bond lengths and coordination numbers or a crude estimate of the lattice energy. The algorithm works by mimicking Darwinian (or in some cases Lamarckian) evolution. The system passes through many (typically, several hundred) generations. The better structures, as measured by the cost function, pass directly to the next generation (elitism); while a proportion of these structures are chosen for

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‘‘breeding’’ (with selection biased towards better structures) in which each pair of structures generates offspring by a process in which they can exchange information and undergo mutations. The latter procedure ensures that the population retains its diversity while it evolves towards structures with optimum cost functions. At the end of the GA, the better structures are subjected to standard full lattice energy minimisation. The viability of this approach in structure prediction has been shown in a series of studies of Woodley and coworkers who have applied the techniques successfully to both dense21 and microporous22 oxide structures; in the latter case, it is necessary to specify ‘‘exclusion regions’’ in order to model the microporosity of the system. We also note that GA methods have been successfully used in structure solution23 where the cost function is now the crystallographic ‘‘R-factor’’, and that EA methods are widely and generally applicable to problems in structure prediction and solution in chemistry. Application of topological methods, for which there is a long history in crystallography. The approach is most appropriate and successful for the case of microporous structures of which there have been several studies, most notably of Smith,24 Treacy et al.25 and recently, Bell, Foster and Klinowski.26,27 The latter studies use combinatorial tiling theory to enumerate networks systematically; the resulting topologies are then used to generate pure silica structures whose energies are then calculated using lattice energy minimisation. This feature is important: as well as generating new structures, it is essential to test their thermodynamic feasibility. Bell et al. have predicted a number of new, stable, but at present hypothetical structures; an example is shown in Figure 1. The challenge is now to synthesise these new materials. Molecular packing methods, which have been widely used in structure prediction of molecular crystals; the different modes of molecular packing are explored in a systematic manner and candidate structures are then subjected to energy minimisation. The method has been very widely used and automated in recent years by Price.28 A very nice example of their recent work is the prediction of a new crystalline polymorph of the pharmaceutical compound, 5-fluorouracil, which was subsequently crystallised from an anhydrous solvent. A detailed account of structure prediction for inorganic materials is given in Ref. 29. Here, it will suffice to say that the field has responded well to Maddox’s challenge of 20 years ago.

4.2

Structures of Amorphous Solids

The need for modelling tools in the structural chemistry of amorphous materials is probably even greater than with crystalline solids owing to the greater difficulty in obtaining unambiguous models from experiment. The most widely used procedure is the ‘‘melt–quench’’ approach in which the melting of an appropriate crystalline solid is simulated using molecular dynamics; the melt is then rapidly cooled so as to freeze into an amorphous structure. This procedure, of course, mimics the real way in which glasses are prepared, although the

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Figure 1

Framework structures of some of the uninodal hypothetical zeolites derived from tiling theory: (a) structure 1_11, derived from one of the simple tilings, has a very low framework density. The cages are shown more schematically in (b), illustrating the openness of the structure; (c) large-pore structure 1_71 has unidirectional 12-ring channels; and (d) tetragonal structure 1_14 has elongated 12-ring channels, which run in orthogonal directions, intersecting to form a 3-D pore system.

speed of the simulated quench (which will, at most, be a few nanoseconds) is many orders of magnitude greater than that of a real quench. Nevertheless, the method has proved to be of great value. There have been many studies of amorphous silica (see, for example, Ref. 30), which have generated models that agree well with experimental scattering data. The field has been extended to include silicates31 and most recently bioglasses.32 Modelling methods are now established tools in the science of amorphous solids.

4.3

Surface Structures and Properties

As with amorphous systems, there is a strong need in surface structural studies to have complementary information from modelling; and indeed simulation tools have become quite standard and routine in contemporary surface science. Pioneering work was undertaken in the late 1970s by Tasker, who established

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the effectiveness of surface structure and energy calculations based on 2D PBC.33 Tasker also made a substantial contribution to the field by his classification of ionic surfaces in terms of the electrical dipole moment in the repeat unit perpendicular to the surface and his demonstration that systems in which such dipoles are non-zero are intrinsically unstable. The field of oxide surface modelling progressed rapidly in the 1980s with the widespread demonstrations of surface rumpling and relaxation effects. A particularly notable study concerned the 0001 surface of alpha-Al2O3,34 which predicted very large inward relaxation of the top layer of Al31 ions. These predictions used IP based calculations and were subsequently supported by DFT calculations; they were verified by impressive grazing angle incidence, X-ray diffraction studies.35 The field developed in two directions in the 1990s: IP methods were applied to systems of growing complexity, for example, carbonates and silicates; and DFT techniques were increasingly (and successfully) applied to both oxide and metallic surfaces. There was also growing interest in the adsorption of water on a range of systems including calcite and TiO2. The (110) surface of TiO2 has proved to be a particularly fertile field for joint computational/experimental studies; Ref. 36 provides a good review. The contemporary field is interacting increasingly strongly with the complex issues posed by reactivity (see below), growth and dissolution (see, for example, the collection of articles in Ref. 37), where modelling has acquired a truly predictive capacity both qualitatively and quantitatively.

4.4

Defect Structures and Energies

As discussed above, many of the earlier successes of the field concerned the chemistry of defective solids, where modelling was shown to be able to make quantitative predictions and to provide valuable qualitative guidance and insight. The following features deserve particular note: (i) IP based calculations using the Mott–Littleton method4 on ionic and semi-ionic solids with closed-shell ions are able to give accurate values of the formation and migration energies of point defects, impurities and defect clusters.2 Indeed, for these systems there is no convincing evidence that superior results can be obtained using QM (including DFT) methods. (ii) Mott–Littleton methods have proved very powerful in unravelling the complex defect structures of non-stoichiometric compounds, such as TiO2x,38 UO21x39 and Fe1xO40 and have proved highly complementary to experimental investigations using diffraction and microscopy techniques. (iii) Molecular dynamics techniques have been of great value for modelling systems showing high ionic conductivity (fast-ion or superionic conductors), as shown by earlier work on fluorite-structured halides and Li1 conducting solids (see Ref. 41 for a review) and more recent studies of oxygen ion conducting materials.42 Such simulations have achieved

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good agreement with measured transport coefficients and have provided valuable insights into ion transport mechanisms. DFT methods will make a growing contribution to the field, with calculations commonly performed on three dimensional periodic defect supercells, although we note the caveat on ionic systems in (i) above. Embedded cluster methods in which a quantum mechanically described cluster containing the defect is embedded in an IP description of the surrounding lattice are perhaps the more appropriate technique and have been used effectively in, for example, recent studies of hydrogen containing defects in silicate minerals.43 The majority of calculations on defect structures have concerned point defects; but very useful studies have been reported on grain boundaries, shear planes and dislocations. Studies of both point and extended defects will remain an important and active area of the field.

4.5

Sorption

This wide-ranging field has been reviewed extensively in recent years (see, for example, the chapters of Smit and of Auerbach in Ref. 44). Broadly speaking, the field can be divided into the following kind of studies. (i) Qualitative investigations of sorption sites, where straightforward energy minimisation methods proved useful, as in the early studies of Wright and co-workers using combined minimisation/neutron scattering techniques to locate sorbed molecules in zeolites.45 Somewhat more sophisticated techniques were developed by Freeman et al.46 based on a crude Monte Carlo method, which was combined with minimisation methods to locate low energy sites in microporous materials, a good illustration of which was their early study of butane in the zeolite ZSM-5. These methods are now routine and very widely applicable. (ii) Detailed, quantitative modelling of sorption isotherms and thermodynamics, which are usually based on the application of Grand Canonical Monte Carlo (GCMC) techniques. Such calculations are critically dependent on the availability of high quality IPs for the host–sorbate interactions; but when such parameters are available, good quantitative agreement with experiment can be achieved, as discussed by Smit in Ref. 44. (iii) Applications of MD techniques to modelling the diffusion of sorbed species. Such studies have enjoyed considerable success. They have revealed valuable insight regarding migration mechanisms and in some cases quantitative agreement with experiment. The article of Auerbach, referred to above in Ref. 44, provides a good account of the state of the art in this field. The work referred to above has concerned physisorption and has employed IP based techniques, which are most appropriate for such studies. Indeed it should

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be noted that physisorption is often dominated by dispersive interactions, which can, in general, not be adequately modelled with DFT. Modelling of chemisorption will be addressed in the next section.

4.6

Reactivity

We again address a very extensive field, which has been particularly active in recent years, and where DFT methods have proved effective and robust. We confine our attention to applications in heterogeneous catalysis, and focus on recent work on microporous and oxide catalysts, while noting the very extensive and successful application to the field of metallic catalysts. Good illustrations of the current status of the field are found in our recent article,47 which highlighted work in two areas: (i) Metal substituted microporous oxidation catalysts, in particular the widely studied TS-1 catalyst, which is a titanium substituted silicalite material, widely used in industrial partial oxidation catalysis. A combination of computer modelling studies and X-ray absorption spectroscopy48 has successfully elucidated detailed models for the active sites in these catalysts and has provided valuable insights into reaction mechanisms. Indeed this work nicely illustrates the complementary nature of computation and synchrotron based experiment in this field. (ii) Methanol synthesis catalysis, in particular the conversion of syngas (CO–CO2–H2) to methanol using the ZnO–Cu catalyst, where modelling has been able to identify active sites on the ZnO surface and to propose a plausible catalytic cycle (see Figure 2); see Ref. 49. Such calculations necessarily require QM techniques. The work on the Cu–ZnO catalyst employed the embedded cluster techniques referred to above. Such methods should have a wide range of applications in catalytic science as discussed in more detail in Ref. 47.

4.7

Synthesis, Nucleation and Growth

Among the most challenging fields of application of modelling tools are those relating to the understanding of solid-state synthesis and to the guidance of synthetic strategies. Good examples of both categories of application are found in the field of zeolite science. Zeolites are synthesised hydrothermally with organic templates being commonly added to the synthesis gel in order to direct the synthesis towards specific microporous architectures. A number of modelling studies have sought to elucidate the fundamental condensation processes occurring in the synthesis gel. The most recent is the work of Mora-Fonz et al.50 who applied DFT methods to study the condensation reactions of small silica clusters. Their work highlighted the role of pH, as condensation was shown to require anionic clusters which are only stable under conditions of high pH.

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Figure 2

Proposed catalytic cycle for conversion of CO–H2 to methanol.

Figure 3

DAF-5 structure containing de novo designed template.

The study also showed that formation of rings – particularly those containing four Si atoms – is highly favoured – a finding that is again very relevant to zeolite synthesis. The challenge of guiding zeolite synthesis was met by Lewis and Willock who developed methods for computational design of templates for the synthesis of specific microporous architectures. The ZEBEDDE code51 uses de novo design techniques to ‘‘grow’’ template molecules computationally within the target microporous host. The most successful application of this technique is illustrated in Figure 3, which shows the computationally designed template for synthesis of the DAF-5 host.52 This template succeeded in synthesising the target material, phase pure and in a few hours, in contrast to earlier syntheses,

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which required lengthy periods and produced a multi-phase sample. This work remains one of the nicest examples of computational materials design.

5 Recent Case Studies The previous sections of this article have, we hope, given a general indication of the range and scope of computational solid-state chemistry. Here we attempt to give an indication of its current status with four brief descriptions of recent topical applications.

5.1

Nanocluster Structures and Energies

As discussed earlier, global optimisation techniques have commonly been employed to generate approximate, or ‘‘sensible’’, structures, which may be subject to further refinement, and the achievements of this approach in crystal structure prediction were briefly summarised in the previous section. Here, we concentrate on the challenges posed by the prediction of the structures of nanoparticles – a topic of growing importance in solid-state science and one that poses many challenges. For particles whose size is greater than tens or hundreds of nanometres, the structure is likely to resemble relaxed fragments cut straight from the bulk phase. But for smaller clusters the structure may be remarkably different, as illustrated in Figure 4. If we are to determine the structures adopted by these small inorganic particles, we clearly require different experimental techniques from those used to determine the structure of the bulk or large particles. Calculation of the properties of model clusters (infrared spectra, for example,

Figure 4

GM structure, found for MgO53 (left) and ZnO54 (right).

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Intensity (km/mole)

1000 800 600 400 200 0

0

300

600

900

1200

(ZrO2)7

Frequency (cm-1)

Figure 5

Calculated infrared spectra for two zirconia clusters.

-31

1

3

5

-31.4

12

4 13

-31.8 -32.2

14

-32

7

3

4

Figure 6

5

6

7

11

1

2 3

-34 6

12

4 5

-36

13 -38

14

15 -32.6

10

8 11

2

6

9

10 Binding energy (eV/n)

7

-30.6

-30

9

8

-30.2

15 8

9

10 11 12 13 14 15

-40

1

3

GM structures and formation energies of ZnS

5

56

7

9

11

13

15

57

and TiO2.

see Figure 5) may assist the structure solution; but before the properties of any model cluster can be computed, we first need to predict its structure. Clusters are postulated to adopt the configuration with the lowest energy of formation, that is, the global minimum (GM) structure. Relaxing fragments cut from the bulk phase with the correct number of ions and stoichiometry will not necessarily generate the GM structure. Hence, global optimisation methods, particularly EAs (see Hartke54 for a more detailed review) are used to search the potential energy landscape for low energy isomers for each cluster size. Two examples of GM structures found by the application of this approach55,56 are shown in Figure 6. Ideally, the model used to define the formation energy for inorganic particles should include electronic effects; however, as many isomers need to be considered, QM calculation of energies and forces is often too computationally expensive. QM approaches also require a good starting configuration; otherwise the calculation is problematic (for example, the self-consistent field cycle may fail to converge). Although global optimisation searches on the QM energy landscape have been applied to some specific small cluster sizes,57 atomistic models (employing IPs) have been traditionally used to select the key structures to be refined using a QM approach. As the order of stability may change upon switching models, EAs are employed

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to generate a set of low energy configurations, rather than just the GM defined by IPs. The focus now is on the structure of small-sized particles formed by laser ablation of II–VI compounds. Clusters with a complete range of sizes and with no one particular size dominant can be produced from ZnO.58 However, ZnS clusters, in contrast, are dominated by an abundance of (ZnS)131, and possibly (ZnS)341. The ‘‘magic number’’, n ¼ 13, is also found for clusters produced by laser ablation of CdS and CdSe.59 It is widely held that magic numbers indicate particularly stable clusters. In particular, it has been established that for alkali halides, a cuboid fragment, particularly if it has square faces, carved from the bulk, is more stable than the same structure with two ions added or removed; examples are shown in Figure 4. The relative stability of the clusters therefore accords with ‘‘magic number’’ behaviour. However, for ZnS this simple approach fails: the relative stabilities of GM structures, shown in Figure 6, would suggest incorrectly that n ¼ 12 is more likely to be a magic number for ZnS. A comparison of structures adopted for (ZnS)13 and (ZnO)13 may shed light on why n ¼ 13 is a magic number for ZnS and not ZnO. There have been many structures proposed for (ZnS)13, as shown in Figure 7, which also reports calculated energies obtained using IP techniques. Three can be obtained by relaxing an appropriate fragment cut from the rock salt structure (which was shown for (MgO)13 to have the lowest energy60) and the two bulk phases of ZnS, respectively. During the geometry relaxation of the wurtzite fragment, the double layer structure inflates, which results in the creation of a bubblestructured cluster. As might be expected, for larger sized clusters, the wurtzite fragment has a lower formation energy than zinc blende, which is the more stable polymorph for ZnS. By simulating the process of annealing with MD, two lower energy isomers were generated;58 one taking the form of an ‘‘ashtray’’ (containing one octagon and reported as the GM structure for (Mg21O2)1360) and another, ‘‘basket 1’’. To date, the lowest energy structure

Bubble 1 -32.45

Ashtray -32.43

Wurtzite -32.40

Bubble 2 -32.35

Bomb 1 -32.17

Basket 1 -32.38

Basket 2 -32.40

Rock-salt -32.06

Zinc-Blende -31.97

Bomb 2 -32.05

Figure 7

Key low energy structures for (ZnS)13, with energies per formula unit in electron volts.56

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for (ZnS)13 is a bubble composed of hexagons and squares, and labelled ‘‘bubble 1’’ in Figure 7, which was found61 by an approach popularised by Wales, called Monte Carlo Basin Hopping,62 and also using a GA within a multi-stage approach.55 During the GA search, the possible location of each ion is constrained to a predefined grid, after which the better candidates are relaxed in the second stage in continuous space. This approach readily generated the GM and the next five lowest energy metastable structures, including another basket and another bubble. However, before modifications were made to the search algorithm, the dense-like structures, ‘‘bombs 1 and 2’’, were not found. The formation of a dense cluster (as opposed to a bubble or basket as its interior contains a cation), shown in Figure 7, has been considered59 as a key reason for the abundance of n ¼ 13 sized clusters. The central cation is 4-coordinated to 4-coordinated anions on the surface of the cluster. In order to encourage the GA to find such structures, a Zn ion or a [ZnS4]6 tetrahedron was fixed within a shell of grid points. Applied to clusters of size n ¼ 13, the four dense structures (two pairs of enantiomeric configurations) were then found, but just as importantly no dense structures were found for clusters of sizes 11, 12, 14 and 15. If (ZnS)13 does adopt a dense structure, whereas clusters of other sizes are hollow, then this could account for the first magic number of ZnS. If the nucleation of (ZnS)n can be envisaged as a process of growing bubbles, then further growth from (ZnS)13 will be kinetically and thermodynamically hindered, as there are no stable, next sized, dense clusters. Note, however, that the dense clusters are not the GM structure for n ¼ 13. The models shown in Figure 7 were used to investigate the (MX)13 structures for a range of II–VI compounds, M ¼ Zn, Cd and X ¼ O, S, Se, Te, using high quality QM calculations. Bubble 1 is found to be the GM configuration for (MX)13, except for (ZnO)13 and (CdO)13, where basket 2 and the rock salt clusters, respectively, are favoured. The latter is not too surprising, as larger ring-like GM structures were reported for (ZnO)n63 than for (ZnX)n, where X ¼ S, Se and Te. Remarkably, (CdS)13 has four configurations within the range of typical thermal excitations. All four can be expected therefore to dominate the population of (CdS)13 and 13 is a magic number for this compound. Although with fewer configurations, a similar uncertainty in the GM configuration is found for (ZnS)13. In contrast, for the other materials the GM configuration is unique. Both enantiomeric configurations for bomb 2 were found to be unstable for (ZnO)13, for which the configurations relaxed to the enantiomeric configurations for the bomb 1 structure. Thus, there is a smaller number of low energy stable structures for (ZnO)13, which does not have 13 as a magic number (and likewise for (CdO)13). To summarise, there is a diversity of low energy stable (MX)13 configurations for certain compounds, which could prove to be the major factor behind the appearance of islands of stability of the cluster sizes that have been observed.58,59 Although the formation energy of the GM for cluster n ¼ 13 is not lower than that found for the GM of similar sized clusters, the kinetic barrier for cluster nucleation could be an alternative explanation.

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5.2

195

Pre-Nucleation Phenomena and Polymorphism

Polymorphism, that is, the ability of a molecule to pack in different crystal structures, is an issue of great importance for the pharmaceutical industry. Since the physical and chemical properties of a molecular crystal depend strongly on the crystal packing, drugs can only be patented for a particular polymorph. The emergence of a more stable polymorph during the process of drug manufacturing can have dramatic consequences, as happened in the case of the anti-HIV pharmaceutical ritonavir,64 which had to be withdrawn from the market, causing enormous financial losses. Here, we show how molecular dynamics (MD) simulations have shed light on the polymorphism of the important molecular crystal, 5-fluorouracil, which has been used in anti-tumour treatments since 1957. For many years, the only polymorph known was ‘‘form I’’, shown in Figure 8, in which the main characteristic is the presence of regions with close F  F interactions. But a recent computational study65 predicted the existence of a very stable polymorph, ‘‘form II’’, which comprises chains of doubly hydrogen bonded molecules. This study was followed by an extensive polymorph screening, and the predicted polymorph was crystallised from only one of the solvents used, pure nitromethane. All other solvents (including nitromethane that had been exposed to the atmosphere) induce the growth of form I. There was no direct explanation of this behaviour, but solvation effects are clearly crucial, so we proceeded to study the relationship between solvation and polymorphism using MD.66 All the MD simulations were performed on the HPCx terascale facilities at Daresbury Laboratory, using the code DL_POLY2.67 The unit cell consisted of 1550 water molecules, modelled with the SPC potential, and 16 5-fluorouracil molecules, modelled with the DREIDING force field.68 The system was kept at ambient pressure and temperature using the Nose´-Hoover thermostat and barostat, with parameters between 0.1 and 0.4 ps. The timestep was 0.75 fs, and the cell size was around 37 A˚. After an initial equilibration period of 50 ps, we simulated the system for 4 ns. We found that 5-fluorouracil molecules have hydrophobic (F atom) and hydrophilic (O atoms) regions. The simulations show

Figure 8

Form I (left) and form II (right) of 5-fluorouracil. Hydrogen bonds are represented by dotted lines.

196

Figure 9

Chapter 11

Left: snapshot showing a single hydrogen-bond formed between 5-fluorouracil molecules in aqueous solution. Right: snapshot of a chain of three doubly hydrogen bonded 5-fluorouracil molecules, formed in pure nitromethane solution.

frequent F  F interactions in aqueous solution, as a consequence of the weak hydration of this hydrophobic end of the molecule. There are also C–O  H–N hydrogen bonds between the molecules, but doubly hydrogen bonded chains of the type appearing in form II are rarely observed. A closer examination of the dynamics of the system shows that doubly hydrogen bonded dimers are not observed because the interaction between water molecules and 5-fluorouracil is very strong, which favours solute–solvent, rather than solute–solute interactions (see Figure 9). The formation of the nuclei which could lead to the growth of form II is therefore very unlikely, since the clusters formed will mostly contain singly hydrogen bonded molecules, as well as close F  F interactions. It is thus clear that our simulations suggest that crystallisation from aqueous solution will yield form I, in which such close interactions and H-bonds are observed. We also performed simulations of 5-fluorouracil in pure nitromethane solution, using 480 nitromethane and 16 5-fluorouracil molecules, with a unit cell size and simulation parameters similar to those of the previous simulation. Our simulations showed that the interaction between 5-fluorouracil and nitromethane is relatively weak, which in turn induces the formation of doubly hydrogen bonded chains. It is therefore clear that nitromethane will favour the crystallisation of form II, which has these chains as building units. Our last set of simulations was aimed at providing an understanding of the reasons why form II can only be grown from pure nitromethane solutions. We modelled the presence of water impurities by introducing 64 water molecules in a unit cell with 496 nitromethane molecules and 16 5-fluorouracil molecules. The number of water molecules is of the order of magnitude expected in un-purified solutions, due to the hygroscopic nature of nitromethane. The simulations show that water molecules interact strongly with 5-fluorouracil molecules, forming clusters around them, and that even this small number of water molecules is enough to inhibit the formation of doubly hydrogen bonded chains. Consequently form I will be the polymorph that is most likely to be grown from contaminated nitromethane solutions, in agreement with the experiment.

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Our three sets of MD simulations provide a rationalisation at the atomic level of the experimental observations relating to the polymorphism of 5-fluorouracil. They have given very valuable insights into the pre-nucleation processes that take place in its crystallisation from solution, and into the important role that solvation plays in controlling the polymorphic outcome.

5.3

Defects in Semiconducting Oxides: ZnO

ZnO is widely used in catalysis, electrical devices, optoelectronics and pharmaceuticals, which often depend crucially on the defect properties of this versatile material. The nature of the intrinsic defects in ZnO, however, remains elusive, and, so far, there is no unambiguous assignment of experimental data to particular defect species (with the possible exception of the positron annihilation spectroscopic evidence for Zn vacancies – see Refs. 69–71 and references therein). Theoretical work is therefore essential to complement the extensive body of experimental data. We have investigated first the intrinsic point defects in ZnO and second, H, N, P, Li, Fe, Cu, Al and In impurity centres.72 Atomic and electronic structures as well as defect formation energies have been obtained for the main oxidation states of all the defects using our embedded cluster, hybrid QM–molecular mechanical approach to the treatment of localised states in ionic solids.73,74 In contrast to a large number of recent periodic density functional calculations, we are able to employ in these studies significantly more accurate QM methods, for example, those based on hybrid exchange-correlation density functionals, which allows us to approach the limit of chemical accuracy in the energetics of defect formation, which are given in Table 1, where we also report energies calculated using the classical Mott–Littleton approach.4 The results show that oxygen Frenkel pairs have low energies of formation; moreover, we note that O interstitials have the lowest energies of formation among all intrinsic defects, which suggests their dominance under oxidising conditions. However, both Zn interstitials (2.2 eV) and O interstitials (1.7 eV) have similar, relatively low energies of formation. Moreover, the Schottky pairs are slightly more favourable than oxygen Frenkel pairs. Hence, the dominant defect species should be determined by the sample history and working conditions. Next, based on the calculated values of the vertical ionisation potential and electron affinity, we have determined defect levels in the band gap of ZnO; the results are illustrated in Figure 10. With these calculations we have been able to explain the following experimentally observed phenomena:
We propose that the neutral and singly positively charged Zn interstitial defect is responsible for E1 and E3 (majority) donor bands from electrical measurements.
Zn vacancies are proposed as the majority acceptor in agreement with experimental assignment based on positron annihilation spectroscopic and other studies. This defect is found to be stable in five charge states.

198

Table 1

Chapter 11

Calculated energies (eV) for the defect pair formation in ZnO. Pure MM energies obtained using the Mott–Littleton approach and interatomic potentials are shown for comparison for relevant charged defects.

Defect Reactions

QM/MM Energy

MM Energy

Frenkel pair in O sublattice OXO + VXi - VXO + OXi OXO + VXi - VdO + O 0 i OXO + VXi - VddO + O00 i

6.948 7.964 8.868

8.797

Frenkel pair in Zn sublattice ZnXZn + VXi - VXZn + ZnXi ZnXZn + VXi - V0Zn + Zndi ZnXZn + VXi - V00 Zn + Znddi ZnXZn + VXi - VXZn + Znddi + 2e 0

12.002 9.637 7.380 9.463

7.501

Schottky pair ZnXZn + OXO - VXZn + VXO + ZnO (s) ZnXZn + OXO - V0Zn + VdO + ZnO (s) ZnXZn + OXO - V00 Zn + VddO + ZnO (s)

8.872 7.645 6.896

6.749

Exciton recombination at this defect species is proposed as a source for the main photoluminescence bands: ultraviolet (an acceptor level at 3.2 eV in a donor to acceptor photoluminescence (DAP) transition); green (a triplet level at 2.5 eV), and red (at 1.9–2.0 eV).
The neutral O interstitial in a split-interstitial peroxy configuration (at 2.8 eV) should also contribute to blue and green luminescence by an exciton recombination and DAP transition from donor Zn interstitials.
O vacancies could not be a source of green luminescence, but could contribute to near-gap (UV) and red-orange luminescence bands (at 2.1 eV and below) via the exciton recombination mechanism. The corresponding surface species, however, may contribute to the luminescence of freshly prepared samples.
Our calculations confirm that Cu, which is stable in ZnO in two charge states, is an efficient electron scavenger. The singly negatively charged Cu impurity is proposed as an E4 donor. Calculated defect levels (at 2.7 and 0.55 eV) are in good agreement with experiment, which established Cu as a distinct source of green luminescence from ZnO. Our calculations on impurities show their donor and acceptor properties in agreement with experiment where available. Curiously, our calculations suggest that a number of such extrinsic defects, for example Li and N, can also contribute to the green luminescence band, the origins of which caused much controversy in the literature.

Computer Modelling in Solid-State Chemistry

Figure 10

199

Calculated energy levels (eV) of intrinsic and extrinsic defects in ZnO.

These studies are currently continuing on other extrinsic defects and defect complexes.

5.4

Enantioselectivity in Ruthenium(II) Hydrogenation Catalysts

This section highlights the role of computational methods in catalytic science and concerns one of the most significant developments in the synthesis of

200

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enantiomerically pure alcohols, namely the discovery by Noyori and co-workers of highly efficient Ru(II)-amine-based complexes for the hydrogenation of prochiral ketones.75 Among the best catalysts for carbonyl hydrogenation developed by Noyori are ternary ruthenium complexes made up of (phosphane)n and diamine, and a Ru(II) centre.75 The experimental evidence shows how subtle modifications of these organic ligands, especially the substituents of the phosphorus, produce very significant changes in the enantioselectivity. One example is reported in Scheme 1: the acetophenone is reduced to (R)-phenylethanol with an enantiomeric excess (ee) of 99% if the reaction is catalysed by trans-Ru(H)2(S,S-dpen)(S-xylbinap) (1), and the ee is drastically reduced to 80% when the reaction is promoted by trans-Ru(H)2(S,S-dpen)(S-tolbinap) (2). The position of the methyl groups in the aryl substituents at the phosphorus atom is the only structural difference between 1 and 2. Here, we report a computational study on the reduction of acetophenone catalysed by trans-Ru(H)2(S,S-dpen)(S-xylbinap). The aim is to provide a theoretical characterisation of the factors controlling the enantioselectivity in the Ru(diphosphine)(diamine) class of catalysts. To model the approach of the ketone to the catalyst, we have applied a geometry optimisation technique where, starting from separate non-interacting reactants, at each stage the geometry of the system is optimised with respect to the constraint, namely the (Ru–)H  C(¼O) distance.76 In fact, a computational study on a model Ru(diphosphine)(diamine) catalyst77 has shown that (Ru–)H  C(¼O) is the ‘‘pseudo’’ reaction coordinate for the metal–ligand OH

O +

H2

Ru(II) complex

CH3

Ar2 P

H

CH3

H2 N

H

Ru P Ar2

Scheme 1

H

N H2

H

(S)-xylbinap: Ar = 3,5-(CH3)2C6H3

(1)

(S)-tolbinap: Ar = 4-CH3C6H4

(2)

Hydrogenation of acetophenone to phenylethanol catalysed by Ru (diphosphine) (diamine) complexes.

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Relative Energy - 9 Ang [kJ/mol]

40,0 30,0

P

P

P

Ru

20,0

O N

N

10,0

Q1

P

P

Ru ON

N

P

P

P

Ru O N

Q2

Ru N

Q3

O N

N

Q4

0,0 Q1 Q2 Q3 Q4

-10,0 -20,0 -30,0 1,0

Figure 11

2,0

3,0

4,0 5,0 6,0 d (O=)C---H(-Ru) [Ang]

7,0

8,0

9,0

Electronic energy variation of the system [trans-Ru(H)2(S,S-dpen)(S-xylbinap)+acetophenone] along the [(Ru-)H  C(¼O)] internuclear distance for each possible approach (Q1, Q2, Q3, Q4). Values computed at the DFT– PBE level of theory.

bifunctional catalysis, the mechanism involved when the reduction of a ketone is promoted by Noyori-type complexes.78 Figure 11 shows the energy variation of the system [1+acetophenone] as a function of (Ru–)H  C(¼O) for each possible approach of the acetophenone on the active sites (Ru–H, N–H) of the catalyst [Q1 and Q4 give (R)-phenylethanol; Q2 and Q3 give (S)-phenylethanol]. All four pathways show a maximum energy centred at approximately 2 A˚, which corresponds to the transition state structure for the H-transfer acetophenone–phenylethanol reaction (Hydrog. TS). The relative energies of the Hydrog. TS structures are: 7.61 kJ mol1 for Q1, 9.58 kJ mol1 for Q2, 25.70 kJ mol1 for Q3 and 35.36 kJ mol1 for Q4. Since the reaction will proceed mostly through the lowest energy saddle point, the Q1 approach is by far the most favourable kinetically. The four alternative pathways display very different energetic trends. In particular, Q1 displays a double-well energy profile and two distinct minima at 3.75 A˚ (INT-I) and 2.5 A˚ (INT-II), while along Q2, a single minimum is located at 3.75 A˚ (INT-I). The structure of the transition state (TS) and of the freely optimised minima along Q1 and Q2 are reported in Figure 12 and described in Table 2. For Q1, the intermediate INT-I corresponds to the situation where the acetophenone is outside the ‘‘pocket’’ made by the bulky aryl groups of the catalyst 1 and where the phenyl group of the acetophenone is approximately on the plane defined by the C–CQO atoms (in Table 2, g ¼ 4.51). In the intermediate INT-II, the phenyl group of the acetophenone rotates (in Table 2, g changes from 4.51 in INT-I to 19.81 in INT-II) in order to enter into the

202

Figure 12

Chapter 11

Minima (INT-I, INT-II for Q1 and INT-I for Q2) and transition state (TS) structures for acetophenone entry in the active sites of the transRu(H)2(S,S-dpen) (S-xylbinap) catalyst as computed by the DFT-PBE method.

‘‘pocket’’ of the catalyst 1. Table 2 shows that the torsional angle of the phenyl group g is the only structural parameter that changes significantly on going from INT-I to INT-II. In particular, the out-of-plane angle t in INT-I and INT-II indicates that the carbonyl carbon is still sp2 in character. In Table 2, the relative energies of the intermediates INT-I and INT-II indicate an extra stabilisation of 9.78 kJ mol1 when the acetophenone enters into the ‘‘pocket’’ (INT-II). We explain the larger stabilisation of INT-II in terms of electronic effects (formation of stronger XH–p hydrogen bonds, where X ¼ N and C, when the acetophenone enters into the ‘‘pocket’’) and steric effects (the minimum CH3  H3C distance decreases from 2.36 A˚ in INT-I to 2.48 A˚ in INT-II). In the Hydrog. TS structure, the torsional angle of the phenyl group (g ¼ 20.41) is the same as in the intermediate INT-II (g ¼ 19.81). This result indicates that along Q1 the formation of a stable intermediate (INT-II) is induced by the rearrangement of the phenyl group [rotation along C–C(¼O)] in order to have a conformation closer to that of Hydrog. TS. For the Q2 approach, the minimum INT-I in Figure 12 is analogous to the intermediate INT-I along Q1 and corresponds to the situation where the acetophenone is outside the ‘‘pocket’’. No other intermediates have been located along Q2 and in Hydrog. TS the conformational structure of the acetophenone is very different from the conformation in INT-I. In particular, the torsional angle g changes drastically from 4.01 in INT-I to 41.31 in Hydrog. TS (see Table 2). Therefore, when the reaction proceeds along the Q2 pathway, there is an extra energy cost to go from the reactant-complex INT-I to Hydrog. TS associated with the torsion of the phenyl group. In fact, at

Q2

Q1

r(CH1)

3.76 3.65 2.00 3.77 1.90

DE

13.23 23.01 7.61 17.50 9.58

1.72 1.72 1.78 1.72 1.82

1.71 1.70 1.67 1.70 1.66

2.20 2.21 2.18 2.20 2.19

2.25 2.25 2.27 2.25 2.27

r(RuP) 1.02 1.02 1.04 1.02 1.04

r(N–H)

1.23 1.23 1.26 1.24 1.26

r(CO)

3.42 3.79 1.95 3.52 1.93

1.0 1.1 16.2 1.5 20.4

t

r(OH)

r(RuN)

r(RuH1) r(RuH2)

Acetophenone

trans-Ru(H)2(S,S-dpen)(S-xylbinap)

4.5 19.8 20.4 4.0 41.3

g

Energetic and structural characterisation of the minima (INT-I, INT-II for Q1 and INT-I for Q2) and transition state (TS) structures for the trans-Ru(H)2(S,S-dpen)(S-xylbinap)-catalysed acetophenone reduction as computed by the DFT–PBE method. Energies in kJ mol1, distances in A˚ and angles in degrees. DE: electronic energy difference with respect to energy at 9 A˚ separation; t: out-of-plane bending of the carbonyl carbon; g: torsional angle of the phenyl group along the C–C(¼O) bond.

INT-I INT-II TS INT-I TS

Table 2

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the DFT-PBE level, the energy difference between the conformation of the acetophenone with g ¼ 4.01 and the conformation with g ¼ 401 is approximately 9 kJ mol1. Since the difference between the activation energy for the H-transfer process along Q1 (DEaQ1 ¼ 15.40 kJ mol1) and along Q2 a (DEQ2 ¼ 27.08 kJ mol1) is close to 9 kJ mol1, we argue that in the transRu(H)2(S,S-dpen)(S-xylbinap)-catalysed acetophenone hydrogenation the high enantioselectivity for the R-alcohol is associated with the existence of a stable intermediate (INT-II) along the Q1 reaction pathway, where the acetophenone has the same conformation as in the Hydrog. TS structure. The formation of this intermediate is hindered for the competitive pathways. Further details of this work are given in reference 79.

6 Conclusions Computational methods now provide us with some of the most powerful tools for studying matter at the atomic and molecular level. The techniques are at their most powerful when used in conjunction with experiment and their growing predictive power will allow them to lead and not to follow experimental studies. The continuing growth in computer power, with the advent of the ‘‘petaflop’’ era, together with developments and innovations in techniques, offer an exciting future for the field.

Acknowledgements We are grateful to Sir John Meurig Thomas and many other colleagues for their contributions to the work summarised in this article. The work has been supported by several grants from EPSRC, EU, ICI and Johnson Matthey.

References 1. C.R.A. Catlow, J.M. Thomas, S.C. Parker and D.A. Jefferson, Nature, 1982, 295, 658. 2. Lecture Notes in Physics, ed. C.R.A. Catlow and W.C. Mackrodt, vol 166, Springer, Berlin, 1992. 3. C.R.A. Catlow, R. James, W.C. Mackrodt and R.F. Steward, Phys. Rev. B, 1982, 25, 1006. 4. N.F. Mott and M.J. Littleton, Trans. Faraday Soc., 1938, 34, 485. 5. C.R.A. Catlow and G.D. Price, Nature, 1990, 347, 243. 6. Computational Materials Science, ed. C.R.A. Catlow and E.A. Kotomin, NATO series III, IOS Press, Amsterdam, 2003, 187. 7. C.R.A. Catlow, R.G. Bell, F. Cora, S.A. French, B. Slater and A.A. Sokol, Annu. Rep. Prog. Chem. Sect. A, 2005, 101, 513. 8. P. Hohenberg and W. Kohn, Phys. Rev. B, 1964, 136, 864. 9. W. Kohn and L.J. Sham, Phys. Rev. A, 1965, 140, 1133.

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CHAPTER 12

Towards a Catalogue of Designer Zeolites M. M. J. TREACY,a M. D. FOSTERa AND I. RIVINb a

Department of Physics, Arizona State University, P.O. Box 871504, Tempe, AZ 85287-1504, USA; b Department of Mathematics, Temple University, 1805 North Broad Street, Philadelphia, PA 19122, USA

1 Introduction John Thomas has made many important contributions to the field of zeolites. His research interests have spanned the field, from synthesis and applications, to structure characterisation. His passionate interest in transmission electron microscopy as a characterisation tool in solid state chemistry, and his early realisation of its importance for the study of defects in materials,1–3 led to a number of influential discoveries and at least one well-known aphorism.4 His influence has been far-reaching. The study of defects in zeolites by transmission electron microscopy (TEM) was an area of research of one of us (MMJT) when employed at the Exxon Corporate Research Laboratory in the 1980s, and John’s visits were always enjoyed and his wisdom valuable. The present work, although apparently unrelated to TEM, can be traced back directly to those days. The study of periodic defects and their permutations to produce new topologies is central to this work, and it is a pleasure to dedicate this article to John. At present, each new zeolite framework discovery is essentially a serendipitous event. Although skilful synthesis chemists know how to choose productive areas within the synthesis phase space, the final product is seldom known in advance. The material structure, if new, is determined usually by a herculean structure refinement involving a combination of X-ray, neutron and electron scattering, and sometimes using connectivity data gleaned from nuclear magnetic resonance. The Structure Commission analyses the data supporting the newly proposed topology, and if approved, the framework joins the ‘‘Zeolite Hall of Fame’’ i.e. the Atlas of Zeolite Framework Types. 208

Towards a Catalogue of Designer Zeolites

209

Occasionally, the new framework will exhibit potentially useful sorption or catalytic properties, and will be scrutinised for commercial usefulness. It is fair to say that, at present, the rate of discovery of zeolite frameworks that are uniquely useful to society is distressingly low. Given the huge economic benefits that are potentially available, a more rational approach to zeolite synthesis is needed. We need to know the frameworks in advance of synthesis, so that we can model their properties. The potentially useful frameworks then become synthetic targets and serendipity plays less of a role. This scenario involves two crucial assumptions; the first is that we can predict and model new zeolite frameworks; the second is that synthesis chemists can make framework materials on demand. Much progress has been made over the past 50 years on the first step, with much being owed to the early pioneers, A.F. Wells and J.V. Smith.5–8 For a more detailed review of the earlier work see the introduction to Ref. 9. The targeted synthesis step is known to be a hard one, but there are no a priori reasons that it should be impossible. However, before we reach this stage, we need a database of potential frameworks whose physical and chemical properties are estimated. This database, or catalogue, would be the vade mecum for the synthesis chemist. It would contain details of the topology, the likely unit cell dimensions and the pore sizes. It would be an interactive resource, allowing one to explore framework composition, visualise molecules passing in and out, and ideally would suggest template molecules for synthesis. We have begun to create such a catalogue.9–11 It can be viewed and searched at http://www.hypotheticalzeolites.net. It is presently just one step towards a future turning point in zeolite chemistry where useful zeolites can be identified and then synthesised. In this article, we explain how the frameworks are generated at present, and outline the mathematical challenges that lie ahead before the promise can be fulfilled.

2 Zeolites as Graphs Zeolites are crystals, which means that their idealised structures are invariant with respect to one of the 230 crystallographic space groups. It is usual to think of the unit cell as the basic crystallographic tile that fills space. However, each space group has a fundamental domain that is usually smaller in volume than the unit cell. The unit cell itself can be constructed by translating, rotating and mirroring copies of the fundamental domain, and tiling those copies. These copying operations are the space group symmetry elements, and are distinct from the translation operations that are common to all space groups. The fundamental domain cannot be further subdivided without altering the space group symmetry. All the information necessary to understand a zeolite framework can be expressed in terms of the atoms contained in this fixed fundamental domain. Although actual zeolites can have complicated and varied compositions, we here concern ourselves only with the framework, ignoring guest atoms in the

210

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channels and cages, and assuming a pure tetrahedral TO2 composition. In zeolites, the tetrahedral ‘‘T-atoms’’ tend to be typically silicon, aluminium and phosphorus. In our treatment, we simplify further by omitting oxygen atoms and treat T-atoms generically to produce a four-connected framework. The next step is to examine the network fragment inside the fundamental domain, and to specify how it connects to the neighbouring images of the fundamental domain. Figure 1 depicts the fundamental region and the unit cell for the FER framework, which has four unique basis T-atoms. The figure shows how the four basis atoms connect to the adjoining environment, which comprises images of the basis atoms, to build the framework. Each T-atom in a fundamental domain D is a unique basis atom, and is connected to four other atoms, either in D itself or in some image gD of one of the basis T-atoms, where g is an element of the crystallographic group. This information can be thought of as a degree four graph with ‘‘coloured’’ edges (connections) – the colours being the group elements g. The number of colours, when described this way, is infinite, but it is not an unreasonable assumption that only side-pairing transformations g are of interest where we concern

b

4

3 2 1 c

Figure 1

a

The FER framework (space group Immm, No. 71) contains four unique (basis) T-atoms, which are shown here as blue tetrahedra with labels 1–4. These basis atoms lie within the fundamental region, which is outlined. The T-atoms all lie on sites. T1 lies on the edge (x, 0,0), Wyckoff site e. T2 lies on the face (x, 0, z), Wyckoff site m. T3 resides inside the fundamental region on the general site (x, y, z), Wyckoff site o. T4 lies on the face (x, y, 0), Wyckoff site n.

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211

ourselves only with connections between adjacent images of the fundamental domain. For the p unique T-atoms, we perform a combinatorial symmetryconstrained inter-site bonding search (SCIBS) over all permutations of sites and all combinations of bond colours that generate degree four graphs. The graphs that are missed because of the adjacency assumption will be recovered when we examine subgroups of the space group, but not necessarily with the same count of basis T-atoms. The ‘‘coloured’’ graph for FER is represented diagrammatically in Figure 2. In this representation, each of the four basis T-atoms has four outward connections to images of the basis atoms. A connection made through the identity operator (x,y,z) represents a direct connection to another basis T-atom. The number of inbound connections is determined by the symmetry of the T-atom’s resident site, and can contain redundant information. For example, the connection 1 (x,y,z) 2 automatically implies the reverse connection 2 (x,y,z) 1. An efficient representation of the coloured graph is listed to the right of the figure. The coloured graph contains no information about the unit cell dimensions, the location of the T-atoms or about the T–T bond distances. It contains topological information only. Thus, not all degree four graphs can necessarily be realised as zeolites. Lists of all degree four graphs are generated. Simple labelling permutations are redundant. However, some space groups, particularly those with mirror symmetry elements, can create degenerate graphs that are not simple permutations of labels. These isomorphic graphs can be computationally costly to detect, and are not filtered at this stage.

Figure 2

A diagrammatic representation of the coloured graph for FER in space group Immm. The operator generating the bond is the ‘‘colour’’ of that edge, or connection, and is applied to the bonded basis T-atom (the atom that is being pointed at by the arrow) to generate the connected image. Each of the four basis T-atoms has four outward connections, including connections to themselves. However, the number of inbound connections can vary since all connections are to images of these sites. The operator (x, y, z) is the identity operator and so represents a direct bond between basis atoms. A more compact description is presented on the right, with redundant connection data removed.

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Since we are interested in identifying the chemically viable subset of these graphs, we need to embed them into Cartesian space – that is, to try to turn them into tetrahedral silicates. As a goodness of fit, we examine the energy of formation of these graphs with respect to perfect SiO4 tetrahedra. To do this with reasonable computational speed, we use an empirical potential created by Boisen, Gibbs and Bukowinski (BGB)12,13 that works well for regular tetrahedral siliceous materials. Oxygen atoms are initially placed at the midpoints of the T–T linkages, with appropriate crystallographic constraints applied (such as when an oxygen sits on a mirror plane). We use a modified form of their potential, mBGB, that inhibits the breaking of T–O–T linkages since these are fixed for each graph. Although the energy for the mBGB modification deviates significantly from the normal BGB energy for large distortions from regular tetrahedral SiO4 geometry, it agrees exactly for small distortions. Since we are interested mainly in structures that are close to being regular tetrahedral, this mBGB modification is well suited to our problem. The graphs are embedded using a variety of Monte Carlo methods, mostly simulated annealing. The procedures used have been described in more detail elsewhere,9,10,14 but are also evolving with time.

3 Present Status In principle, the SCIBS method is bounded, generating a complete set of graphs. Given a space group and the number of basis T-atoms, then the number of adjacent degree four graphs is finite and enumerable. In practice, the subsequent embedding step may inadvertently discard a viable graph. Tests performed on example data sets reassure us that the methods used do not discard many viable frameworks, if any. The price paid for this completeness is that there is a combinatorial explosion of graphs as the number p of basis T-atoms increases. Figure 3 presents log– linear plots of the number of graphs as a function of p for five space groups. The growth in number of graphs is clearly exponential in p. For space group Pnma (No. 62) there are already 107 graphs for p ¼ 3 basis atoms. It is a sobering fact that the well-known zeolite framework MFI occurs in this space group with p ¼ 12 basis T-atoms. Extrapolating the line, we find that there will be a staggering B1025 graphs to examine in order to identify the MFI framework. Clearly, an efficient method for further filtering (or simply not generating) the unfeasible graphs is needed. It is also found that the fraction of viable frameworks decreases rapidly with p. Consequently, the rate of discovery of new frameworks decreases dramatically as p increases. Nevertheless, to date, we have refined over 3.5 million low energy graphs. A large fraction of these are duplicates since the same topology can recur in different space groups, or even within the same space group with reoriented and enlarged cell volumes. For example, the LTL topology occurs in P6/mmm (No. 191) for all p Z 2. We have not yet identified unambiguously the unique topologies in our database. For example, we know that quartz recurs about 100 times, and

213

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number of graphs, N

108

106

104 230 227 191 63 62

102

100 0

Figure 3

1

2 3 4 5 number of basis T-atoms, p

6

Plot showing the combinatorial explosion of degree four graphs as the number p of basis T-atoms increases, for five space groups. At present, the practical upper limit for the annealing step is about 108 graphs. Thus, we have been able to explore up to p ¼ 6 for some high-symmetry space groups, but only p ¼ 2 for others. Not all space groups support uninodal (p ¼ 1) graphs, such as P6/mmm, No. 191 for which p Z 2.

cristobalite is also a commonly occurring topology. By examining coordination sequences out to the 16th shell, we conservatively estimate that there are at least 100 000 unique topologies present. In parallel with our studies, other groups have made important progress in zeolite database generation using different strategies. Each method, of course, has its pros and cons. Earl and Deem15 have constructed a similar database of zeolite frameworks by using an adaptive simulated annealing approach. At present, it is hosted on our web site in parallel with our database. They start by selecting a space group and a number of basis T-atoms, which are all placed in the general site at random starting configurations. During the anneal, basis T-atoms can merge if the energy is lowered by the merger according to a cost function that is optimised for tetrahedral frameworks. After many such runs, a collection of low energy frameworks is found, some being found many times. An attractive feature of this approach is the analogy to thermodynamic equilibrium, in that a cost of formation and the entropy of the system are allowed to determine the resulting frameworks. This has the advantage over our approach in that the graph formation and the embedding are occurring in the same step, with the annealer rapidly rejecting implausible (i.e. high energy) graphs early in the process. However, a possible disadvantage is that some of the narrower local minima could be missed, preferring to seek out the global minimum – i.e. quartz and cristobalite dense phases are probably found frequently. Nevertheless, the Earl and Deem database has a comparable number of entries to ours. At the time of writing, the two databases, both being new, have not yet been compared or merged. Friedrichs et al.16 have used an alternative approach that takes advantage of the fact that zeolites are built from polyhedral units. By tiling combinations of

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pre-assembled polyhedra, they identify the space-filling forms. In their approach, the space group and basis T-atoms must be identified after the fact. Friedrichs has developed a powerful computational tool, SYSTRE,17,18 that uniquely identifies each topology by making use of a partial embedding of each graph in order to identify the maximum topological symmetry. Many of the frameworks discovered are mesmerising. We show one of them in Figure 4, chosen for its large pore size. It is labelled as framework number 229_5_8058871 (i.e. space group 229, or Im 3m, five basis T-atoms, graph number 8058871). It has enormous cavities capable of holding a sphere 24.7 A˚ in diameter. Spheres up to 10.8 A˚ diameter can diffuse freely through the framework.

4 The Turning Points: Past, Present and Future The graph in Figure 5 reveals clearly the rapid growth in the number of hypothetical zeolites in recent years. The number of zeolite frameworks that

Figure 4

The final embedding of graph number 229_5_8058871. It occurs in space group Im3m (No. 229) with five basis T-atoms. It contains large cavities, capable of supporting spheres 24.7 A˚ in diameter, which are shown here in yellow. Spheres up to 10.8 A˚ diameter can diffuse freely through the framework.

215

Towards a Catalogue of Designer Zeolites 200

3.0

Hypothetical zeolites Known

150

2.5 2.0

100

1.5 1.0

50

0.5

Hypothetical (millions)

Known zeolites

0.0

0 1980

1990

2000

2010

Date

Figure 5

Graph showing the increase with time of the number of known zeolites (circles) and the number of hypothetical zeolites (squares). The hypothetical count is restricted to those entries in our database. The database of Deem and co-workers has a comparable number. Prior to 2003, the number of hypothetical frameworks known was undoubtedly fewer than 10 000, and probably closer to 1000.

have been assigned three-letter framework codes by the Structure Commission of the International Zeolite Association is growing exponentially, but appears sedate compared to the rapid explosion in hypothetical zeolites. The present growth in number of hypothetical structures is traceable to a number of developments, which are enumerated here. 1. The demonstration that systematic permutations and combinations of linkages between building units (sheets and cages) leads to new zeolite topologies. These discoveries were driven most notably by Wells5,6 and Smith,7,8 although others had remarked that recurrent twinning and the systematic introduction of defects produced new topologies.1,19 Later, Friedrichs et al.16 formalised the mathematics underpinning the threedimensional space-filling tilings of polyhedra. Independently, Treacy et al. and Joachim-Klein (for uninodal graphs) developed the SCIBS method for enumerating connections between tetrahedral units.9,10,20,21 2. The early transmission electron microscopy work of John Thomas and his colleagues provided important proof of the existence of crystallographic defects in zeolites, most notably in MFI, LTL and FAU materials. This affirmed that the elusive hypothetical framework topologies could exist, at least locally within another related structure.1 Importantly, they demonstrated that the hitherto hypothetical ‘‘Breck’s Structure 6’’,19 or ‘‘hexagonal faujasite’’, exists at the local level in the neighbourhood of planar defects within some synthetic cubic faujasites. The elusive hexagonal synthetic target was later synthesised in pure form by Delprato et al.,22 and is now known as the EMT framework. 3. The development of computerised structure embedding methods has been vital. The original distance least squares program DLS76 of Baerlocher

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et al. is still a standard tool in zeolite structural studies. Advances in computerised embedding methods, such as simulated annealing, have also been crucial. Naturally, the constant growth in computer performance has been highly enabling, in this, and in all areas of science. The development of efficient and accurate empirical potentials24 has proven important for the final embedding stage. Ab initio methods, although accurate, are computationally costly. The empirical potential of Boisen et al.12,13 has been valuable to the authors’ SCIBS method. The general utility lattice program (GULP)25 has proven useful in this research. 4. The solution to the graph uniqueness problem by Friedrichs and O’Keeffe has eliminated a tricky graph-theoretic barrier for the growth of zeolite framework databases.17 5. The development of the internet allowing access to important research results and to vast databases has been important. Given the frequent updates, a physical paper catalogue, or even a periodically distributed disk of hypothetical zeolites, would be unmanageable. It is clear that these databases are still in their infancy, and future efforts will be to render them as complete as is practically possible. Technically, the number of zeolitic graphs is infinite. However, in practice the list of chemically realisable frameworks will be finite. Future advances will undoubtedly be enabled by developments in the mathematics relating to crystallographic graphs, as well as developments in computational algorithms. Some of the outstanding computational, combinatorial and probabilistic questions can be summarised as follows: Question 1. How does the number of isomorphism classes of coloured graphs depend on their size and the group? In other words, how many different zeolite frameworks are there with a fixed symmetry group and with a given number of crystallographically unique T-atoms? Question 2. How does one generate all coloured graphs up to a certain size, taking care to generate each isomorphism class exactly once? Questions 1 and 2 are closely related, since enumeration and generation methods are often closely related. One quite promising, and insufficiently explored, approach is to develop a complete ‘‘transformation grammar’’ on degree four graphs, whereby existing graphs (preferably realisable frameworks) form the starting point for new families of graphs. This is related to the setup used by Sleator et al.,26 who demonstrated a way to compactly encode all the graphs described by words of length m in a graph grammar. This gives both tight upper bounds on the number of such graphs, and furthermore, an efficient method of constructing them. A completely different (and independent) approach, which has recently been found very useful in robotics (but is, in some sense, closely related to the approach of Ref. 26.) is the use of CAT(0) complexes to find short paths in combinatorial configuration space.27 Question 3. Is there a way to determine whether a framework is a suitable candidate for a physically occurring structure based strictly on its topology?

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217

A structure is considered suitable if it admits a low energy embedding in threedimensional space. In turn, the energy is determined by the various Coulomb forces and the constraints of quantum mechanics, and is quite expensive to compute. Finding an embedding of minimal energy is more difficult yet. Finding a simple combinatorial action that is well correlated with the physical properties is of considerable interest to both mathematics and chemistry. There are some simple filters we already use, such as constraints on the topological density.10 An idea that we are starting to explore is to use the spectral properties of the graph.28 Since the framework is infinite, computing them is tricky, especially since the obvious idea of looking at bigger and bigger pieces of the framework quickly becomes prohibitively expensive since the complexity of computing eigenvalues and determinants grows cubically with the number of vertices. Luckily, there are ‘‘closed form’’ ways of defining graph determinants and spectral zeta functions, which were recently worked out in the context of geometric group theory and von Neumann algebras.29,30 This should allow us to investigate the relationship between the regularised logarithm of the determinant of the graph Laplacian and the energy. It is known, due to the work of Kenyon,31 that in the planar case this is related to the Gibbs free energy of the framework, and so there is at least a philosophical reason to expect some level of success. There is also the possibility of exploring the connection of these questions to hyperbolic geometry, in the spirit of Refs. 32 and 33. Question 4. How do we find an optimal (or quasi-optimal) embedding of a framework rapidly? This has two aspects: first, how to quickly compute the energy and (if possible) its derivatives, and secondly how to optimise the energy over the space of the configurations of T-sites. In order to evaluate the energy function, the first observation is that the potential is the sum over pairs of sites that are near each other, and so there is a natural way to evaluate this – Greengard’s algorithm34 for fast evaluation of Coulomb-type potentials. A completely different (and somewhat faster, though somewhat less accurate idea) is to use dynamic Voronoi diagrams.35 We can quickly compute and keep updated a Voronoi diagram of our sites, and computing interactions with only the nearby sites gives a simple linear time algorithm to compute a good approximation to the energy. Which of these algorithms is preferred depends in part on the optimisation scheme used. For local optimisation (for example, the conjugate gradient method), it is useful to be able to compute very good approximations to the gradient of the energy with respect to the positions of the sites, and the best method for that appears to be automatic differentiation.36 Our experiments indicate that finite differencing is unstable in these applications, and symbolic differentiation is prohibitively expensive. However, it is important that the underlying algorithm be amenable to automatic differentiation, and we will be investigating this. For global optimisation, there are (at least) two possibilities – a version of the simulated annealing algorithm (which has been our workhorse to date) and also a tree optimiser. Question 5. What will be the physical properties of a hypothetical zeolite framework? In particular, what are the pore shapes and sizes? This question comes down to some quite subtle questions in computational geometry. To

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determine the size of the largest sphere that can fit into the pores of the structures, it seems that Delaunay tessellations provide the most convenient tools,35,37 although the correct object – the Apollonian tessellation – is quite hard to compute, even though algorithms are known.38 In any event, the important question is whether a zeolite framework can ‘‘accommodate’’ a given organic molecule, or a piece thereof. This requires much more sophisticated technology, and is related to the work of Edelsbrunner on a-shapes,39 and of Ghrist on sensor networks and homology.40 John Thomas and his colleagues41 have made important inroads in this problem with the ZEBEDDE program, which can assemble potential template molecules within the confines of the zeolite pores. At present, the authors are exploring methods to estimate the absorption capacity of zeolites by packing spheres of known radii into the voids. Figure 6 shows graphically the outcome when spheres with the nominal diameters of He, Ne, Ar, Kr and Xe are packed into the MFI framework. However, the ultimate computational goal is to pack non-spherical molecules efficiently and thermodynamically realistically, into the frameworks. The recent discovery that real zeolite frameworks are flexible (i.e. the tetrahedral linkages within the framework can flex cooperatively to expand or shrink the unit cell within a certain range, or ‘‘window’’) is potentially important, and may provide the key to rapidly identifying viable frameworks.42

Figure 6

Sphere packing results for the MFI framework. From left to right, the packed spheres and their nominal van der Waals radii are: He (1.40 A˚), Ne (1.54 A˚), Ar (1.88 A˚), Kr (2.02 A˚) and Xe (2.16 A˚). Assuming an oxygen radius of 1.32 A˚ and a silicon radius of 0.9 A˚, we find that we can pack 64.4, 39.5, 24.8, 20.6 and 17.2 atoms per unit cell, respectively.

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219

At present, about 50% of all new structures presented to the Structure Commission of the International Zeolite Association were already present in our database. This is already a strong indicator that the database is a zeolitic gold mine. However, the tools needed to reliably predict the useful frameworks are still primitive. At the time of writing, these newly emerging databases are more ‘‘postdictive’’ than they are predictive. Future developments in the mathematics of computational geometry and algorithm design will surely turn these databases into tempting wish lists of possibly useful zeolites. However, it will take additional developments in the techniques of targeted synthesis before the promise of a designer catalogue can become reality.

References 1. M. Audier, J.M. Thomas, J. Klinowski, D.A. Jefferson and L. Bursill, J. Phys. Chem., 1982, 86, 581. 2. G.R. Millward, S. Ramdas, J.M. Thomas and M.T. Barlow, J. Chem. Soc., Faraday Trans., 1983, 79, 1075. 3. O. Terasaki, J.M. Thomas and G.R. Millward, Proc. R. Soc. London, Ser. A, 1984, 395, 153. 4. J.M. Thomas, Chem. Br., 1970, 6, 60. 5. A.F. Wells, Three-Dimensional Nets and Polyhedra, Wiley, New York, 1977. 6. A.F. Wells, Further Studies of Three-Dimensional Nets, American Crystallographic Association, Monograph No. 8, Polycrystal Book Service, Pittsburgh, 1979. 7. J.V. Smith, Chem. Rev., 1988, 88, 149. 8. J.V. Smith, in Zeolites: Facts, Figures, Future, vol 49A, ed. P.A. Jacobs and R.A. van Santen, Elsevier Science Publishers, Amsterdam, 1989, 29. 9. M.M.J. Treacy, K.H. Randall, S. Rao, J.A. Perry and D.J. Chadi, Z. Kristallogr., 1997, 212, 768. 10. M.M.J. Treacy, I. Rivin, E. Balkovsky, K.H. Randall and M.D. Foster, Microporous Mesoporous Mater., 2004, 74, 121. 11. M.D. Foster and M.M.J. Treacy, 2005, http://www.hypotheticalzeolites.net/ 12. M.B. Boisen Jr. and G.V. Gibbs, Phys. Chem. Miner., 1993, 20, 123. 13. M.B. Boisen Jr., G.V. Gibbs and M.S.T. Bukowinski, Phys. Chem. Miner., 1994, 21, 269. 14. S.A. Wells, M.D. Foster and M.M.J. Treacy, Microporous Mesoporous Mater., 2006, 93, 151. 15. D.J. Earl and M.W. Deem, Ind. Eng. Chem. Res., 2006, 45, 5449. 16. O.D. Friedrichs, A.W.M. Dress, D.H. Huson, J. Klinowski and A.L. Mackay, Nature, 1999, 400, 644. 17. O.D. Friedrichs and M. O’Keeffe, Acta Crystallogr., Sect. A, 2003, A59, 351. 18. M.D. Foster and M.M.J. Treacy, http://www.gavrog.org//, 2006.

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19. D.W. Breck, Zeolite Molecular Sieves. Structure, Chemistry and Use, Wiley, New York, 1974. 20. M.M.J. Treacy, S. Rao and I. Rivin, in Proceedings of the 9th International Zeolite Conference, Montreal 1992, vol 1, ed. R. von Ballmoos, J.B. Higgins and M.M.J. Treacy, Butterworth-Heinemann, Stoneham, MA, 1993, 381. 21. H.-J. Klein, in 10th International Conference on Mathematical Modelling and Scientific Computing, July 1995, Boston, vol 6, ed. X.J. Avula and A. Nerodi, Principia Scientia, St. Louis, 1996, 940. 22. F. Delprato, L. Delmotte, J.L. Guth and L. Huve, Zeolites, 1990, 10, 546. 23. C. Baerlocher, A. Hepp and W.M. Meier, DLS-76 – A Program for Simulation of Crystal Structures by Geometric Refinement, ETH Report, Zurich, 1977. 24. M.J. Sanders, M. Leslie and C.R.A. Catlow, J. Chem. Soc., Chem. Commun., 1984, 1271. 25. J.D. Gale, J. Chem. Soc., Faraday Trans., 1997, 93, 629. 26. D.D. Sleator, R.E. Tarjan and W.P. Thurston, SIAM J. Discrete Math., 1992, 5, 428. 27. R. Ghrist and V. Peterson, Adv. Math., 2007, 38, 302. 28. D. Jakobson and I. Rivin, Forum Math., 2002, 14, 147. 29. R.I. Grigorchuk and A. Zuk, The Ihara zeta function of infinite graphs, the KNS spectral measure and integrable maps, in Random Walks and Geometry, Walter de Gruyter GmbH & Co. KG, Berlin, 2004. 30. D. Guido, T. Isola and M.L. Lapidus, Technical Report arxiv.org:math.OA/ 0608229, 2006. 31. R. Kenyon, Invent. Math., 2002, 150, 409. 32. I. Rivin, Ann. Math. (ser. 2), 1994, 139, 553. 33. I. Rivin, Adv. Appl. Math., 2003, 31, 242. 34. J. Carrier, L. Greengard and V. Rokhlin, SIAM J. Sci. Statist. Comput., 1988, 9, 669. 35. A. Okabe, B. Boots, K. Sugihara and S.N. Chiu, Spatial Tessellations: Concepts and Applications of Voronoi Diagrams, 2nd edn, Wiley Series in Probability and Statistics, Wiley, Chichester, 2000. 36. A. Griewank, Evaluating Derivatives. Volume 19 of Frontiers in Applied Mathematics. Principles and Techniques of Algorithmic Differentiation, vol 19, Society for Industrial and Applied Mathematics (SIAM), Philadelphia, PA2000. 37. M.D. Foster, I. Rivin, M.M.J. Treacy and O. Delgado Friedrichs, Microporous Mesoporous Mater., 2006, 90, 32. 38. F. Aurenhammer, SIAM J. Comput., 1987, 16, 78. 39. H. Edelsbrunner, Geometry for modeling biomolecules, in Robotics: The Algorithmic Perspective, A.K. Peters, Ltd, Natick, MA, 1998. 40. V. de Silva and R. Ghrist, Int. J. Robot. Res, in press. 41. D.J. Willock, D.W. Lewis, C.R.A. Catlow, G.J. Hutchings and J.M. Thomas, J. Mol. Catal. A, 1997, 119, 415. 42. A. Sartbaeva, S.A. Wells, M.M.J. Treacy and M.F. Thorpe, Nat. Mater., 2006, 5, 962.

CHAPTER 13

Discovering New Crystal Architectures FILIPE A. ALMEIDA PAZ,1 DOROTA MAJDA,2,3 ROBERT G. BELL4,5 AND JACEK KLINOWSKI3 1

Department of Chemistry, CICECO, University of Aveiro, Campus Universita´rio de Santiago, Aveiro, 3810-193, Portugal; 2 Department of Chemistry, Jagiellonian University, ul. Ingardena 3, 30–060 Krako´w, Poland; 3 Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, UK; 4 Davy-Faraday Research Laboratory, The Royal Institution of Great Britain, 21 Albemarle Street, London W1S 4BS, UK; 5 Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, UK

1 Introduction The contrast between the small number of physical laws and the enormous number of structures to which these laws give rise is an intriguing aspect of the natural world. The handful of rules of chemical bonding results in many thousands of crystalline inorganic compounds with a bewildering number of different structures, even when only a few chemical elements are involved. For example, over a half of all known minerals are silicates and aluminosilicates with distinct structures, although their frameworks are built only of silicon, oxygen and aluminium. Enumeration of networks of atoms in inorganic structures is a matter of considerable interest, but a formidable task for a scientist. Apart from the pure academic interest in the problem, there are two reasons why such enumeration is important. First, a ‘‘library’’ of well-characterised, chemically feasible, hypothetical structures would facilitate design strategies that would ultimately lead to their syntheses. Second, X-ray, neutron and electron diffraction patterns calculated for such structures would be a great boon in determining the atomic coordinates of newly prepared open-structured materials: it would simply entail a straightforward comparison of the experimentally obtained pattern with the database. 221

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2 Crystalline Molecular Sieves Derivation of chemically viable hypothetical networks is particularly desirable for crystalline microporous molecular sieves, of which there are now 176 recognised structure types, with several new ones being added to the list every year (Table 1).1 Crystalline molecular sieves are a class of porous crystalline open-framework solids which includes aluminosilicates (zeolites), aluminophosphates and related materials.2 Zeolites are built from cornersharing SiO4 and AlO4 tetrahedra linked by the apical oxygen atoms to form frameworks of high internal surface area with regular channel systems and cavities of molecular size (Figure 1). Other elements, such as Ga, Ge, B and Fe can substitute for Si and Al. The net negative charge of the framework, equal to the number of the constituent aluminium atoms, is balanced by exchangeable cations located in the channels which normally also contain water. The name ‘‘zeolite’’ (from the Greek zeo ¼ to boil and liyos ¼ stone) was coined to describe the behaviour of the mineral stilbite which loses water on heating and thus seems to boil. There are ca. 40 identified zeolite minerals and more than 130 purely synthetic species.1 All known zeolites, both natural and synthetic, contain channels circumscribed by tetrahedrally coordinated Si or Al atoms. Microporous silicates with windows of insufficient size to sorb water molecules reversibly are known as clathrasils.3 The AlPO4 molecular sieves, the porous crystalline equivalents of aluminium phosphate, are built from alternating AlO4 and PO4 tetrahedra.4 Incorporation of an Si source into AlPO4 gives silicoaluminophosphates (SAPO) and the incorporation of a metal (Me) into AlPO4 and SAPO gives the MeAPO and MeAPSO sieves, respectively. Other zeolite-related structures with novel compositions such as zincosilicates, gallophosphates, aluminoarsenates, galloarsenates and beryllophosphates have also been reported. Of the 44 recognised AlPO4 and related structures, 19 have the framework topologies of known zeolites, and the rest are novel structures (Table 1). Some microporous aluminophosphates contain wider channels (410 A˚) than any known zeolite. Open-structure aluminosilicates are centrally involved in the catalytic conversions of the petrochemical industry5 (in cracking, hydrocracking, alkylation, isomerisation and de-hydroisomerisation). Framework-substituted, openstructure AlPO catalysts are also important.6 For example, a SAPO-347 catalyst has been commercialised for the acid-catalysed dehydration of methanol to yield ethene and propene for the polymer industry. Transition-metal, framework-substituted AlPOs exhibit a wide variety of catalytic action in selective oxidations8 (such as the aerial oxidation of cyclohexane to adipic acid), the in situ production of hazardous reagents (such as hydroxylamine from NH3 and air), the conversion of cyclohexanone, NH3 and air to e-caprolactam and nylon-6,9 and the generation of organic chemicals of value as pharmaceuticals. The annual industrial consumption of zeolites is ca. 550,000 tonnes, of which 135,000 tonnes are used in catalysis, 375,000 as ion exchangers in detergent powders and 40,000 as sorbents. The topology of zeolitic, AlPO4 and related structures has been reviewed.10

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Table 1

Microporous zeolitic and related molecular sieves. Silicates and phosphates

Silicates AFG BEA BCT BIK BOG BRE CAS CDO CFI –CHI CON DAC DDR DOH DON EAB EDI EMT EON EPI ESV ETR EUO FAR FER FRA

GIU GME GON GOO HEU IFR IHW IMF ISV ITE ITH ITW IWV JBW KFI LIO –LIT LOV LTL LTN MAR MAZ MEI MEL MEP MER

MFI MFS MON MOR MOZ MSE MTF MTN MTT MTW MWW NAB NAT NES NON NSI OBW OFF –PAR PAU PHI –RON RRO RSN RTE RTH

RUT RWR SFE SFF SFG SFH SFN SGT SSY STF STI STT SZR TER THO TOL TON TSC TUN UFI VET VNI VSV –WEN YUG

ABW AFI ANA AST BPH CAN CGS CHA ERI FAU GIS LAU LEV LOS LTA MSO OWE RHO SOD

Phosphates

Germanates

Others

ACO AEI AEL AEN AET AFN AFO AFR AFS AFT AFX AFY AHT APC APD ATN ATO ATS ATT ATV AWO AWW CGF –CLO DFO EZT

ASV BEC IWR IWW SOS UOZ UTL

CZP DFT OSO RWY WEI

NPO OSI PON SAO SAS SAT SAV SBE SBS SBT SFO SIV UEI USI VFI ZON

3 Structural Description of Molecular Sieves Si, Al and P atoms (known as T-atoms) in molecular sieves occupy fourconnected vertices of a three-dimensional net, and the oxygen atoms occupy two-connected positions between the four-connected vertices. Given that each oxygen lies between two T-atoms, the topology of the framework may be considered simply in terms of the connectivity of the T-atoms. Thus each T-atom is treated as a vertex of a three-dimensional net, and each vertex lies at the intersection of four T–T edges. Such a net is said to be four-connected. If all vertices in a net are topologically identical, the net is described as uninodal. Binodal, trinodal, etc. nets are those with two, three, etc. topologically distinct types of vertices. The diversity of known structures is such that topological classification of structures is necessarily based on subunits of linked tetrahedra, known as secondary building units (SBU) (Figure 2).11 The SBU’s are the smallest number of simple units from which all known structures can be built. Polyhedral cages, the larger building blocks, are each composed of a handful of SBU’s and can in turn be combined to form an infinite framework. For example, the

224

Figure 1

Chapter 13

Framework structures of zeolites of structure types SOD (sodalite), LTA (zeolite A), FAU (faujasite), CAN (cancrinite), KFI (zeolite ZK-5) and RHO (zeolite Rho). The positions of tetrahedral atoms are at the crossings of the straight lines which symbolise T–T linkages. Oxygen atoms (not shown) lie approximately halfway between the T-atoms. The types of cages involved in each structure are represented by polyhedra which have been shrunk towards their centres. Sodalite (structure type SOD) is formed by direct face sharing of four-membered rings in the neighbouring truncated octahedra, more correctly described as tetrakaidodecahedra (also known as ‘‘sodalite cages’’ or ‘‘b-cages’’). Zeolite A (structure type LTA) is formed by linking the sodalite cages through double four-membered rings. Faujasite (structure type FAU) is formed by linking the sodalite cages through double six-membered rings. Cancrinite is formed by direct linking of 11-hedra (‘‘e-cages’’ or ‘‘cancrinite cages’’). Other polyhedra are the ‘‘a-cage’’ (26-hedron of type I); double eight-membered ring; double six-membered ring (hexagonal prism) and the 18-hedron (‘‘g-cage’’). Exchangeable nonframework cations are not shown for clarity.

truncated octahedron (or b-cage), may be linked directly to other b-cages to form the SOD structure type, via double four-membered rings to form zeolite A (LTA structure type), or via double six-membered rings to form the zeolitic mineral faujasite (FAU) (Figure 1). A periodic network of tetrahedral atoms can be described in terms of the ‘‘circuit symbol’’ of each T vertex. Each T vertex participates in six Ti–T–Tj angles, and for each angle there is a shortest circuit of edges T–Ti


Tj–T. The set of six numbers forms the circuit symbol of the vertex,12 which reflects the degree of compactness of a net. A simplified form of the circuit symbol is a ‘‘loop coordination’’: a graph showing only the number of three- or fourmembered rings in which a given T-atom participates (Figure 3).

Discovering New Crystal Architectures

Figure 2

225

Secondary building units (SBU’s) found in molecular sieve structures. Numbers show in how many structure types a given SBU appears.

A powerful description of a net involves the concept of ‘‘coordination sequence’’ (Figure 4).13 Thus, in a four-connected network each T-atom is connected to N1 ¼ four neighbouring T-atoms through oxygen bridges. These are then linked to N2 T-atoms in the next shell, in turn connected to N3 T-atoms, etc., including each T-atom only once. For example, the coordination sequence for FAU is 4, 9, 16, 25, 37, 53, 73, 96, 120, 145. Although the coordination sequence for each kind of T-atom is not completely unique to a given structure, and occasionally distinct structures (such as LTA and RHO) have the same coordination sequence, it is a very useful guide since structures with different coordination sequences are guaranteed to be different. Framework density (FD) is defined as the number of T-atoms per 1000 A˚3, while topological density, r10, defined so that 1000r10 is the number of T-atoms in the first 10 coordination shells of a given T-atom,14–16 also reflects the degree of compactness of a net. Recent structural descriptions of molecular sieves use the concept of nodal surfaces,17 equipotential surfaces and triply periodic minimal surfaces (TPMS),

226

Figure 3

Chapter 13

Loop configurations:1 solid lines represent T–O–T linkages, dotted lines represent non-connected T–O bonds found in interrupted frameworks.

surfaces with zero mean curvature at all points.18 These concepts are fundamentally different from the ‘‘conventional’’ approaches described above in that they do not consider chemical bonds and bond angles but treat structures as an assembly of atoms ‘‘decorating’’ an infinite surface.

4 Non-Systematic Structural Enumeration Enumeration of zeolitic structures originates in the work of Wells.12,19 Early enumerations were derived by empirical methods, and new structures predicted by linking together structural subunits in new ways, either by building models or by computer simulation. As the T–O distances in all known zeolites are in the 1.58–1.78 A˚ range (so that all T–T edges are close to 3.1 A˚) and the T–O–T angles are in the 130–1601 range, models can be built using identical sections of plastic tubing attached at each end to tetrahedral nodes.

Discovering New Crystal Architectures

Figure 4

227

Graphical representation of the calculation of the coordination sequence for the site marked with an arrow in a two-dimensional five-connected plane net.

However, of the many structures which are generated, only some will be ‘‘chemically reasonable’’.20 Topology takes no account of chemistry and the basic requirement for chemically realistic structures is that bond lengths and angles are within a certain acceptable range. O’Keeffe16 described as ‘‘realisable’’ nets which can be realised geometrically with each vertex having only four equidistant nearest neighbours and with the T–T linkages corresponding to the edges of the net, and suggested that the number of such uninodal nets is finite and amounts to ‘‘some hundreds’’. For the known zeolites and zeolite-type materials, the values of framework density, FD, range from 12.5–25.1. The magnitude of FD depends on the type and relative number of the smallest rings.21 The frameworks of lowest density have a maximum number of four-membered rings, and the minimum framework density increases with the size of the smallest rings. Brunner22 considered the likelihood of preparing highly siliceous frameworks of a given topology in terms of the loop configuration. By examining tetrahedral structures with respect to the smallest rings, bond angles, symmetry, loop configuration and framework density,23 he concluded that the lowest density is obtained for structures of high symmetry. Aware of the practical importance of nets with very open frameworks (FDo12) and wide channel openings (containing a maximum of three- and four-rings), Barrer and Villiger24 were the first to find a chemically realistic net with wide unidimensional channels. Meier25 and Hansen26 derived a series of low-density nets, while Smith and co-workers described a large number of hypothetical stereochemically realisable structures.27,28 One of their interesting results was the prediction28 of the net with 18-rings (net 81(1) in the original paper) which was subsequently identified in the aluminophosphate VPI-5 (VFI structure type).29

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O’Keeffe and co-workers derived many new structures using empirical computer search algorithms.14,16 A point was moved in small increments throughout the asymmetric unit of the unit cell of all the cubic, hexagonal, tetragonal and orthorhombic space groups in turn. All the equivalent points in the cell generated by the group-symmetry operations were then identified. The topology of the net defined by the four nearest neighbours of the initial point was then characterized by its coordination sequence. Most of these nets were new. Treacy et al.30 described a computer method for generating periodic fourconnected frameworks. Given the number of unique tetrahedral atoms and the crystallographic space group type, the algorithm explored all combinations of connected atoms and crystallographic sites, seeking the four-connected graphs. This database, which lists hypothetical structures enumerated by exploring all combinations of connected atoms and crystallographic sites, contains no fewer than 933,672 structures,31 including the results of an elegant topological search for the framework of ZSM-10.32 Hyde and co-workers34 have shown that many low-density frameworks are related to periodic minimal surfaces. The results were considered in the light of framework densities of highly siliceous zeolites, clathrasils and dense silicates in order to separate the roles of geometry and chemistry in determining framework topology. Using an analogy with three-connected networks of hyperbolically curved single sheets, Fogden and Jacob33 described a method for construction of frameworks corresponding to their interconnected, doublesheet relatives, and gave models of hypothetical frameworks fitting the triply periodic minimal surfaces P,35 D36 and G.36 The channels and cavities in these structures are significantly larger than those in known zeolites.

5 Systematic Enumeration Using Tiling Theory The work on enumeration described above has been very useful, but suffers from the drawback of being non-systematic: it is never quite certain that all the structural possibilities have been considered. Our approach,37 which takes care of this problem, is based on advances in combinatorial tiling theory.38–41 All periodic tilings of the Euclidean plane, the sphere and the hyperbolic plane have been classified earlier,40,42 and algorithms, which enumerate and permit the visualisation of all possible topological types of tilings for each two-dimensional symmetry group with 1, 2, 3, etc. kinds of inequivalent vertices, developed.42 We could therefore address the three-dimensional case, which has direct applications to structural chemistry. We define a tiling as a periodic subdivision of space into bounded, connected regions without holes, which we call tiles. If two tiles meet along a surface, we call the surface a face. If three or more faces meet along a curve, we call the curve an edge. Finally, if at least three edges meet at a point, we call that point a vertex. A network is thus formed by the vertices and edges. The configuration of edges, faces and tiles around a given vertex can be described by what is known as the ‘‘vertex figure’’, obtained by placing the centre of a small notional sphere at the vertex and

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considering the tiling of that sphere formed by the intersections with the different tiles touching that vertex. The starting point is to associate with each type of periodic tiling a unique ‘‘Delaney symbol’’.39,40 This is arrived at by breaking the tiling down into simplices using a barycentric subdivision. Any (n+1) points in n-space which do not lie in an (n – 1)-dimensional space are the vertices of an n-dimensional simplex. A simplex in two dimensions is thus a triangle, and in three dimensions, a tetrahedron. A tiling is uniquely described by a Delaney symbol, and a unique Delaney symbol describes one, and only one, tiling. As it would be difficult to visualise how tilings are generated in 3-D Euclidian space, we shall use a two-dimensional example. Consider a two-dimensional tiling composed of squares and octagons (Figure 5). The central point of each of the tiles is connected with dashed lines to the centres of the edges, and with dotted lines to the vertices, producing six different kinds of triangles, labelled A–F. The Delaney symbol for the tiling is constructed by specifying from which kind of original tile the different kinds of triangles originate, and how they are linked together (Figure 6).39,40 For example, triangle A comes from the octagonal tile (with eight edges), is adjacent to triangles B, D and B in that tile, and the original vertex belongs to three surrounding tiles. The Delaney symbol for the whole tiling is thus BDB83 ACA83 DBF83 CAE83 FFD43 EEC43 where each group of characters contains information about one class of triangles in the barycentric subdivision: group 1 for class A, group 2 for class B, etc. The letters refer to the neighbours across dashed, dotted and solid lines, respectively, of this class of triangles, while the numbers are the same as those inside the rectangles in Figure 5. The classification of all periodic tilings of a given kind then reduces to the enumeration of the corresponding Delaney symbols, which is equivalent to ‘‘mutating’’ the inorganic gene. This approach can be used in any number of dimensions by dividing polytopal tiles into simplices. We have so far described all possible Euclidean uni-, bi- and trinodal tilings based on ‘‘simple’’ vertex figures (tilings with vertex figures which are tetrahedra) and all ‘‘simple’’ and ‘‘quasi-simple’’ uninodal tilings with vertex figures containing up to six extra edges38 (in ‘‘quasi-simple’’ tilings the vertex figures are derived from tetrahedra, but contain double edges). There are exactly 9 and 117 topological types of four-connected uninodal43 and binodal44 nets, respectively, which are based on ‘‘simple’’ tilings. In addition, there are at least 157 additional uninodal nets derived from ‘‘quasi-simple’’ tilings.43,45 For example, zeolitic structure types SOD, LTA, RHO, FAU, KFI and CHA are all based on ‘‘quasi-simple’’ tilings (Figure 1). Although we originally claimed that there are exactly 926 trinodal structures based on simple tilings,44 it turns out that we had mistakenly dismissed a further 412 structures. Known zeolitic structure types involve n-nodal structures with n up to 12, found in zeolite ZSM-5 (MFI structure type), one of the most complex inorganic structures known. Given the vast number of possible combinations

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Figure 5

Barycentric subdivision of a two-dimensional tiling composed of squares and octagons.

Figure 6

Graphical representation of the Delaney symbol for the tiling shown in Figure 5.

of various vertex figures, MFI is not expected to be found among the first few billion structures enumerated. All the same, the method is systematic and exhaustive, and will eventually deliver the ZSM-5 structure. Further, only a small fraction of the many structures will be chemically feasible (with bond lengths and angles within an acceptable range).20 Given a realisable hypothetical structure, a least-squares fit leading to optimal atomic positions can then be performed by computer.

6 Chemically Feasible Zeolitic Structures We have used computational chemistry methods to identify the most chemically plausible hypothetical frameworks. Aware of the fact that zeolites are

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formally derived from silica, we have treated the structures as silica polymorphs with the chemical formula SiO2. Silicon atoms were inserted at each vertex of the enumerated networks, and a bridging oxygen was placed between each pair of neighbouring Si atoms, separated by a typical Si


Si distance. An energy minimisation program46 then calculated the framework energy relative to a-quartz, the most stable form of the mineral, and the framework density (the number of tetrahedral atoms per 1000 A˚3) for each structure. These were then compared with the corresponding values for known zeolite frameworks, also treated as silica polymorphs. Figure 7 gives the plot of framework energy relative to a-quartz, EF, vs. the framework density, FD, for all known zeolites treated as silica polymorphs. We excluded the four non-silicate structure types which substantially deviate from the rest: WEI (calcium beryllophosphate), CZP (sodium zincophosphate), OSO (potassium beryllosilicate) and RWY (gallium germanium sulphide). The line of best fit has the formula y ¼ –1.436x+40.094, where x is framework density (FD) and y is DEquartz. The important conclusion from the figure is that for all silicate zeolites, the framework energy relative to a-quartz is below 30 kJ mol1. The Cerius2 software suite47 was used for visualising and manipulating the structures and for calculating free volumes, space group symmetry and other

Figure 7

Framework energy, EF (kJ mol1), with respect to a-quartz, vs. framework density (Si atoms per 1000 A˚3) for (a) all known zeolitic structure types. The equation of the straight line (in blue) is EF ¼ –1.436 FD+40.094 and was calculated from a total of 154 points (labelled structures were left out); (b) hypothetical uninodal structures and (c) hypothetical binodal structures.

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parameters. In addition to calculating the energetics of the hypothetical structures, it is important to compare the calculated values with the values for all known zeolite frameworks. Thus all relevant properties were also calculated for the purely siliceous forms of all known zeolite topologies. Lattice energies were calculated relative to a-quartz, the most stable form of the mineral at ambient temperature. The ‘‘accessible volume’’ was determined by tracing out the volume by the centre of the probe molecule as it follows the structure contours, but with the extra requirement that the probe must enter the unit cell from the outside via sufficiently wide pores or channels. The accessible volume gives an indication of the space available within each structure for applications in molecular sieving and catalysis. The calculations of the accessible volume were performed using the Free Volume module of the Cerius2 package, which applies the Connolly method48 consisting of ‘‘rolling’’ a probe molecule with a given radius over the van der Waals surface of the framework atoms. We have used a probe molecule with a radius of 1.4 A˚ (such as water) and 1.32 and 0.9 A˚ for the radii of O and Si atoms, respectively. Enumeration of chemically realisable frameworks containing large amounts of internal space (i.e., those containing channels and/or voids) is of particular interest, because such materials can act as ‘‘microreactors’’ containing implanted catalytically active groups or encapsulated transition-metal

Figure 8

Structures enumerated and subsequently synthesised. (a) RWY50 (our structure 1_1);51 (b) NPO52 (our structure 1_88);51 (c) BCT53 (our structure 1_211)51 and (d) UFI54 (our structure 3_835).

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49

complexes. The crucial structural parameters here are the amount of void volume and its accessibility (whether or not a molecule can enter the structure from the outside), which we have calculated for all the hypothetical structures. The accessible volume for known zeolites is in the range of 0–28 A˚3 per Si atom. Our search for more of these continues. When our original paper37 was published, the database maintained by the International Zeolite Association contained 121 recognised structure types, and the current number is 176. The 55 new structures can, in principle, all be

Figure 9

Hypothetical uninodal zeolites with 12-membered rings. Our structures: (a) 1_71; (b) 1_73; (c) 1_89.

Figure 10

Hypothetical binodal zeolites with 12-membered rings. Our structures: (a) 2_51; (b) 2_53.

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obtained using our method. Among the structures synthesised since 1999 we have specifically described structure types RWY50 (our structure 1_1)51, NPO52 (our structure 1_88)51, BCT53 (our structure 1_211)51 and UFI54 (our structure 3_835) (Figure 8). As mentioned above, we are particularly interested in ‘‘low energy’’ and ‘‘open framework’’ materials. In a series of papers37,43–45,55 we have described many such materials. Here, we concentrate on new results concerning zeolites with wide channels (i.e., 12-membered or larger). In Figures 9–12 we illustrate such new hypothetical structures. All are chemically feasible according to our

Figure 11

Hypothetical trinodal zeolites with 12-membered rings. Our structures: (a) 3_660; (b) 3_818; (c) 3_681; (d) 3_772; (e) 3_757 and (f) 3_934.

Discovering New Crystal Architectures

Figure 12

235

A hypothetical trinodal zeolite with 16-membered rings (our structure 3_971).

criteria, and we believe that most of them will be synthesised very soon. Note in particular the astonishing 16-membered channel structure shown in Figure 12. This material has very low framework density (FD ¼ 11.32 Si atoms per 1000 A˚3) and framework energy of 28.26 kJ mol1 with respect to a-quartz.

References 1. C. Baerlocher, W.M. Meier and D.H. Olson, Atlas of Zeolite Structure Types (updates on http://www.iza-structure.org/), Elsevier, London, 2001. 2. R.M. Barrer, Zeolites and Clay Minerals as Sorbents and Molecular Sieves, Academic Press, London, 1978; D.W. Breck, Zeolite Molecular Sieves: Structure, Chemistry and Use, Wiley, London, 1974; R. Szostak, Molecular Sieves: Principles of Synthesis and Identification, Van Nostrand Reinhold, New York, 1989. 3. F. Liebau, H. Gies, R.P. Gunawardane and B. Marler, Zeolites, 1986, 6, 373. 4. S.T. Wilson, B.M. Lok, C.A. Messina, T.R. Cannan and E.M. Flanigen, J. Am. Chem. Soc., 1982, 104, 1146. 5. A. Corma, M.J. Diaz-Cabanas, J.L. Jorda, C. Martinez and M. Moliner, Nature, 2006, 443, 842. 6. J.M. Thomas, R. Raja and D.W. Lewis, Angew. Chem. Int. Ed., 2005, 44, 6456. 7. S.T. Wilson, Abstr. Pap. Am. Chem. Soc., 2006, paper 231–INORG; L. Smith, A.K. Cheetham, L. Marchese, J.M. Thomas, P.A. Wright, J. Chen and E. Gianotti, Catal. Lett., 1996, 41 13. 8. J.M. Thomas and R. Raja, Top. Catal., 2006, 40, 3. 9. J.M. Thomas and R. Raja, Proc. Natl. Acad. Sci. U.S.A., 2005, 102, 13732.

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10. J.V. Smith, Chem. Rev., 1988, 88, 149. 11. W.M. Meier, 1st International Zeolite Conference, London, 4–6 April 1967, 1968. 12. A.F. Wells, Further Studies of Three-Dimensional Nets, American Crystallographic Association Monograph No. 8, Polycrystal Book Service, Pittsburgh, 1979. 13. G.O. Brunner and F. Laves, Wiss. Z. Tech. Univ. Dresden, 1971, 20, 387; W.M. Meier and H.J. Moeck, J. Solid State Chem., 1979, 27, 349. 14. M. O’Keeffe, Acta Crystallogr., 1992, A48, 670; M. O’Keeffe and S.T. Hyde, Z. Kristallogr., 1996, 211, 73. 15. M. O’Keeffe, Acta Crystallogr., 1995, A51, 916. 16. M. O’Keeffe and N.E. Brese, Acta Crystallogr., 1992, A48, 663. 17. S. Brenner, L.B. McCusker and C. Baerlocher, J. Appl. Crystallogr., 1997, 30, 1167; S. Brenner, L.B. McCusker and C. Baerlocher, J. Appl. Crystallogr., 2002, 35, 243; L.B. McCusker, C. Baerlocher, R. Grosse-Kunstleve, S. Brenner and T. Wessels, Chimia, 2001, 55, 497. 18. S. Andersson, S.T. Hyde, K. Larsson and S. Lidin, Chem. Rev., 1988, 88, 221; S. Andersson, S.T. Hyde and H.G. von Schnering, Z. Kristallogr., 1984, 168, 1; A.L. Mackay, Philos. Trans. R. Soc. London, Ser. A, 1993, A442, 47. 19. A.F. Wells, Three-Dimensional Nets and Polyhedra, Wiley, New York, 1977; A.F. Wells, Structural Inorganic Chemistry, 5th edn, Oxford University Press, Oxford, 1984. 20. R. Gramlich-Meier and W.M. Meier, J. Solid State Chem., 1982, 44, 41. 21. G.O. Brunner and W.M. Meier, Nature, 1989, 337, 146. 22. G.O. Brunner, Zeolites, 1993, 13, 592. 23. G.O. Brunner, Zeolites, 1993, 13, 88. 24. R.M. Barrer and H. Villiger, Z. Kristallogr., 1969, 128, 352. 25. W.M. Meier, Pure Appl. Chem., 1986, 58, 1323. 26. S. Hansen, Naturwissenschaften, 1990, 77, 581; S. Hansen, Nature, 1990, 346, 799. 27. J.V. Smith, Am. Mineral., 1977, 62, 703; J.V. Smith, Am. Mineral., 1978, 63, 960; J.V. Smith, Am. Mineral., 1979, 64, 551; J.V. Smith and J.M. Bennett, Am. Mineral., 1981, 66, 777; J.V. Smith, Z. Kristallogr., 1983, 165, 191; J.V. Smith and J.M. Bennett, Am. Mineral., 1984, 69, 104; J.M. Bennett and J.V. Smith, Z. Kristallogr., 1985, 171, 65; J.J. Pluth and J.V. Smith, Nature, 1985, 318, 165; F.C. Hawthorne and J.V. Smith, Can. Miner., 1986, 24, 643; F.C. Hawthorne and J.V. Smith, Z. Kristallogr., 1986, 175, 15; J.V. Smith and W.J. Dytrych, Z. Kristallogr., 1986, 175, 31; F.C. Hawthorne and J.V. Smith, Z. Kristallogr., 1988, 183, 213; J.W. Richardson, J.V. Smith and J.J. Pluth, J. Phys. Chem., 1989, 93, 8212; J.V. Smith, ACS Abstr., 1993, 205, 157-IEC; K.J. Andries and J.V. Smith, Acta Crystallogr., 1994, A50, 317; S.X. Han and J.V. Smith, Acta Crystallogr., 1994, A50, 302; S.X. Han and J.V. Smith, Acta Crystallogr., 1999, A55, 332; S.X. Han and J.V. Smith, Acta Crystallogr., 1999, A55, 342; S.X. Han and J.V. Smith, Acta Crystallogr., 1999, A55, 360.

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28. J.V. Smith and W.J. Dytrych, Nature, 1984, 309, 607. 29. M.E. Davis, C. Saldarriaga, C. Montes, J. Garces and C. Crowder, Nature, 1988, 331, 698. 30. M.M.J. Treacy, K.H. Randall, S. Rao, J.A. Perry and D.J. Chadi, Z. Kristallogr., 1997, 212, 768; M.M.J. Treacy, I. Rivin, E. Balkovsky, K.H. Randall and M.D. Foster, Microporous Mesoporous Mater., 2004, 74, 121; M.M.J. Treacy, M.D. Foster and K.H. Randall, Microporous Mesoporous Mater., 2006, 87, 255. 31. M.D. Foster and M.M.J. Treacy, Hypothetical Zeolites: Enumeration Research (updates on http://www.hypotheticalzeolites.net/), 2004. 32. M.D. Foster, M.M.J. Treacy, J.B. Higgins, I. Rivin, E. Balkovsky and K.H. Randall, J. Appl. Crystallogr., 2005, 38, 1028. 33. A. Fogden and M. Jacob, Z. Kristallogr., 1995, 210, 398. 34. S.T. Hyde, Acta Crystallogr., 1994, A50, 753; S.T. Hyde, B.W. Ninham and Z. Blum, Acta Crystallogr., 1993, A49, 586. 35. P.J.F. Gandy and J. Klinowski, Chem. Phys. Lett., 2000, 322, 579. 36. P.J.F. Gandy, D. Cvijovic 0 , A.L. Mackay and J. Klinowski, Chem. Phys. Lett., 1999, 314, 543. 37. O. Delgado Friedrichs, A.W.M. Dress, D.H. Huson, J. Klinowski and A.L. Mackay, Nature, 1999, 400, 644. 38. O. Delgado Friedrichs, Discret. Comput. Geom., 2001, 26, 549. 39. A.W.M. Dress, Springer Lect. Notes Math., 1985, 1172, 56. 40. A.W.M. Dress, Adv. Math., 1987, 63, 196. 41. A.W.M. Dress, D.H. Huson and E. Molna´r, Acta Crystallogr., 1993, A49, 806. 42. D.H. Huson, Geometriae Dedicata, 1993, 47, 269. 43. M.D. Foster, O.D. Friedrichs, R.G. Bell, F.A.A. Paz and J. Klinowski, J. Am. Chem. Soc., 2004, 126, 9769. 44. A. Simperler, M.D. Foster, O.D. Friedrichs, R.G. Bell, F.A.A. Paz and J. Klinowski, Acta Crystallogr., 2005, B61, 263. 45. M.D. Foster, O. Delgado Friedrichs, R.G. Bell, F.A.A. Paz and J. Klinowski, Angew. Chem. Int. Ed., 2003, 42, 3896; M.D. Foster, A. Simperler, R.G. Bell, O.D. Friedrichs, F.A.A. Paz and J. Klinowski, Nat. Mater., 2004, 3, 234. 46. J.D. Gale, J. Chem. Soc., Faraday Trans., 1997, 93, 629. 47. Cerius2, v. 4.0, Molecular Simulations Inc., San Diego, 1999. 48. M.L. Connolly, J. Am. Chem. Soc., 1985, 107, 1118. 49. J.M. Thomas, R. Raja, G. Sankar and R.G. Bell, Nature, 1999, 398, 227. 50. N.F. Zheng, X.G. Bu, B. Wang and P.Y. Feng, Science, 2002, 298, 2366. 51. A. Simperler, M.D. Foster, R.G. Bell and J. Klinowski, J. Phys. Chem., 2004, B108, 869. 52. S. Correll, O. Oeckler, N. Stock and W. Schnick, Angew. Chem. Int. Ed., 2003, 42, 3549. 53. W.A. Dollase and C.R. Ross, Am. Mineral., 1993, 78, 627.

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54. C.S. Blackwell, R.W. Broach, M.G. Gatter, J.S. Holmgren, D.Y. Jan, G.J. Lewis, B.J. Mezza, T.M. Mezza, M.A. Miller, J.G. Moscoso, R.L. Patton, L.M. Rohde, M.W. Schoonover, W. Sinkler, B.A. Wilson and S.T. Wilson, Angew. Chem. Int. Ed., 2003, 42, 1737. 55. D. Majda, R.G. Bell, O. Delgado Friedrichs and J. Klinowski, Proceedings of the XIII Zeolite Forum, Polanczyk, Poland, 10–13 September, 2006, 75.

CHAPTER 14

Chemical Modulations in Pb–Bi Sulfosalts: A Glimpse at Minerals in Solid-State Chemistry ALLAN PRING AND CRISTIANA L. CIOBANU Department of Mineralogy, South Australian Museum, North Terrace, Adelaide, South Australia 5000, Australia

1

Introduction

Mineralogy and chemistry share a common history and in many ways it can be rightly said that chemistry, particularly inorganic and solid-state chemistry, developed from mineralogy. Minerals are, after all, inorganic products formed by geological processes. Indeed most of the chemical elements, apart from the gases, were discovered during the analysis of minerals. Great chemists such as Karl Scheele (1742–1786), Joseph Louis Gay-Lussac (1778–1850), Jo¨ns Jacob Berzelius (1779–1848) and Humphrey Davy (1778–1829) are all equally famous for their mineralogical contributions. The minerals, scheelite (CaWO4), gaylussite (Na2Ca(CO3)2
5H2O), berzelianite (Cu2Se), berzeliite (Ca,Na)3(Mg,Mn)2(AsO4)3 and davyne (Na,Ca,K)8Al6Si6O24(Cl,SO4,CO3)23 are testament to the prominence of these chemists in mineralogy. There is also avogadrite (K,Cs)BF4 named for Amadeo Avogadro (1776–1856), vauquelinite (Pb2Cu(CrO4)(PO4)(OH) for Louis Vauquelin (1763–1829) and meurigite [K(H2O)2.5][Fe3+8(PO4)6(OH)7 (H2O)4] named for Sir John Meurig Thomas. The modern concept of a mineral as a naturally occurring inorganic compound was formulated late in the eighteenth century, when mineral descriptions approaching today’s standards began to appear.1 There was rapid growth in the number of known mineral species throughout the nineteenth century, driven in part by a growing need to develop methods to systematically identify 239

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minerals and describe their properties for industrial purposes. The rapid discovery of new elements was largely a result of improved mineral analysis. Important developments in mineralogy paralleled those in chemistry and also drove new research directions in crystallography, leading to understanding of the structure of matter. By the time of the discovery of X-rays by Ro¨ntgen in 1895, the number of recognised minerals had reached almost 800.2 The work of Max von Laue and W.H. Bragg and W.L. Bragg launched the field of crystal structure analysis in 1912, and the knowledge of the precise positions of atoms in crystal structures and the distances between them gave structural meaning to the chemical formula of many minerals and a new basis for their classification. Hence, the concept of minerals being the salts of hypothetical acids was banished. It is an interesting footnote in the history of science that the great English amateur crystallographer, William Barlow, and the chemist, William Pope, had correctly deduced the crystal structures of NaCl, KCl, CsCl, ZnS and several other simple compounds. This work was based solely on ideas of close-packing and symmetry that were developed some years before the discovery of X-ray diffraction.3 It was, however, W.L. Bragg and his group in Manchester, Goldschmidt in Oslo and Pauling in Pasadena who made significant contributions to crystal chemistry during the period after World War I, and laid the foundations for modern crystal chemistry in broad terms. It can rightly be said that the advances in X-ray crystallography in the first half of the twentieth century and the development of microanalytical techniques such as the electron microprobe around 1960 revolutionised mineralogy and solid-state chemistry. The widespread use of such techniques greatly enlarged the understanding of the chemical variation between and within minerals. The electron microprobe for the first time brought a high level of spatial resolution to chemical analysis and showed the inhomogeneous nature, at the microscopic scale, of many minerals. This made possible the development of many new applications for petrology and ore geology. Analytical limitations are steadily decreasing and modern technology is able to detect internal crystalline order at better than nanoscale resolution. The development of high resolution transmission electron microscopy (HRTEM) in the 1970s revealed that the ultra-microstructure of minerals can be quite complex and that chemical substitutions via solid solution and non-stoichiometry via point-defect mechanisms were not the only means of chemical adaptability revealed by minerals.w

w

The first HRTEM papers on minerals started to appear while one of us (AP) was an undergraduate at Monash University in the mid-1970s. It was the work on the rock-forming silicates that John Thomas was conducting with David Jefferson and others that prompted AP to join the Thomas group in Cambridge as a Ph.D. student in 1980. At the time the Thomas group had around 40 members, who pursued active research in zeolites, chain and layer silicates, as well as a variety of catalysts, and solid-state organics. AP’s Ph.D. project investigated natural and synthetic Ba and Cs compounds to assess their potential for nuclear waste immobilisation using HRTEM.

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2

241

Modular Crystallography

Running parallel with increasing progress in transmission electron microscopy in the 1970s were major developments in mineralogy that recognised the modularity of crystal structures of complex minerals, such as the lead bismuth sulfides.4,5 The ideas behind modular crystal chemistry have their roots in the work of Baumhauer.6 He introduced the concept of polytypism in order to explain the many structural modifications of SiC, formed by different stacking sequences of layers that are identical both structurally and chemically. Magne´li7 introduced the concept of a homologous series to describe groups of compounds that have variable chemistry but can be characterised by a general formula using a structural operator to derive one member from another. This concept is now applied to many groups of minerals and in particular sulfosalts such as the lillianite series5,8 that will be discussed here. The somewhat parallel idea of polysomatism was introduced by Thompson9,10 in order to explain the relationship between structures that are based on ordered intergrowths of two or more structurally and chemically distinct types of units or modules. This is different to the polytypical approach that requires identical crystal–structural units or to the homology concept that uses a common structural operator for all minerals in a group. One of the best-known examples of polysomatism is the biopyribole mineral group.11 In modular crystallography, families of structures are generated by stacking structural units in different ways with the central requirement being that the energy of the interface between such component units is relatively low. The most extensive modular structural families are those whose individual modules have a very low residual electrostatic charge and consequently low surface energies. Structural disorder due to errors in the stacking sequence have been observed in nearly all modular series, particularly in synthetic compounds that are obtained in experiments where conditions can be far from equilibrium. Can structural disorder also be observed in minerals formed in geologic environments where cooling rates are orders of magnitude longer than in experimental runs? Can minerals, in particular sulfides, preserve disorder, when we know that atomic diffusion rates can be significant even at temperatures as low as o200 1C?z Several HRTEM studies on synthetic compounds formed in the Pb–Ag–Bi sulfide system – analogues to minerals from the lillianite series – have shown extensive disorder in samples prepared either by quenching or annealing.13,14 In the following sections we will discuss the same type of compounds, but those formed instead as minerals in a natural occurrence. The idea behind this approach is to see in what ways the natural specimens compare to the synthetic ones. In particular, could disordered intergrowths of different block sizes in z

AP started to research these complex sulfosalt minerals when he returned to Australia in 1983 and for a number of years concentrated on the Pb-As-S minerals, which occur in well-formed crystals. In this system, however, disorder proved to be rather rare, although stacking disorder in the polytypes of baumhauerite (named appropriately for Baumhauer) is not uncommon.12 CC came to Adelaide in 2005 on a research fellowship to undertake HRTEM studies on sulfosalt minerals.

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these minerals provide an alternative mechanism to solid solution to explain their chemical flexibility, and is this possibly linked to long-range compositional modulations?

3

The Lillianites

The lillianite homologous series is a group of Pb–Bi(Ag) sulfosalts that have structures based on the ordered intergrowth of a ‘galena-like’ motif cut parallel to (110)PbS. The different blocks correspond to individual homologues having a ‘chemically twinned’ arrangement consisting of chains of MS6 octahedra that are linked by bi-capped trigonal prisms of PbS612 along mirror planes (Figure 1).15–18 Distinct lillianite homologues in the series differ in the width of the galena-like slabs. This can be expressed by N, the number of metal octahedra in the chains that run diagonally across individual galena-like slabs and parallel to (110)PbS. Homologues are denoted N1,N2L, where N1 and N2 are the number of metal sites in the two alternating slabs. The general formula for the lillianite series is PbN12xBi21xAgxSN12, where N ¼ (N1 + N2)/2 and x is the Ag + Bi ¼ 2Pb substitution coefficient, with xmax ¼ (N2)/2. The lillianite group contains both orthorhombic (N1 ¼ N2) and monoclinic (N1 a N2) members, including lillianite (4,4L) (Pb3Bi2S6) and vikingite (4,7L) (Pb8Ag5Bi13S30) (see Table 1 for full list of natural members of the series). The

Figure 1

Structural diagram of lillianite showing the atomic arrangement. The chemically twinned PbS(110) like units are arranged about the twinned plane. The metal sites on the mirror plane are in bi-capped trigonal prismatic co-ordination. The different homologues in the series are generated by variation in the length of the octahedral chains between twin planes. In this case N ¼ 4.

L L 4,7 L 4,8 L 7,7 L 5,9 L

4,4

L Disordered

11,11

4,4

a ¼ 13.54 a ¼ 7.08 a ¼ 13.60 a ¼ 13.35 a ¼ 13.71 a ¼ 13.46 a ¼ 13.46

b ¼ 20.45 b ¼ 19.57 c ¼ 25.25 c ¼ 26.54 b ¼ 31.21 c ¼ 30.18 b ¼ 44.04

c ¼ 4.10 c ¼ 8.27 b ¼ 8.22 b ¼ 4.09 c ¼ 4.13 b ¼ 4.10 c ¼ 4.09

b ¼ 93.4

b ¼ 107.2 b ¼ 95.6 b ¼ 92.8

Bbmm P21/c P2/a C2/m Bbmm C2/m Bbmm

0 0 0 0 0 0 0

12 0 12 0 0 15 0 16 18 0 0 18 0 26

Pb3Bi2S6 PbAgBi3S6 Pb8Ag5Bi13S30 Pb6Ag7Bi15S32 Pb6Bi2S9 Pb10Ag7Bi15S36 Pb15Ag12.5Bi20.5S52 PbAgBi3S6 to Pb3Ag1.5Bi3.5S9

Space Group

Lillianite Gustavite Vikingite Treasurite Heyrovskyite Eskimoite Ourayite Schirmerite

Angle (1)

Diagnostic Reflection

Cell Repeats (A˚)

Formula

Mineral

Structure

Summary of compositional and structural data for the members of the lillianite homologous series. (After Makovicky.5)

Table 1

Chemical Modulations in Pb–Bi Sulfosalts 243

244

Chapter 14

substitution Ag + Bi ¼ 2Pb results in additional members when over half of the available Pb sites are occupied by Ag and Bi. Thus gustavite, even though it has the same homologue notation as lillianite, i.e. (4,4L), is different in composition, i.e., PbAgBi3S6. Chemical analyses of many lillianite homologues give nonintegral values for N and this suggests that the differences might be due to variation in the frequency of the ‘chemical twinning’ (that is block-width disorder). In their extensive HRTEM study on synthetic compounds from the lillianite series, Skowron and Tilley14 examined both quenched and annealed material and found several new homologues that are not yet described from natural specimens, including 4,5L, 7,8L and 8,8L. They also found that the temperature of annealing was important in stabilising each homologue. At their higher annealing temperature (700 1C) the phases were dominated by homologues with N ¼ 4 or 7 blocks, either ordered or disordered, whereas a wider variety of homologues was stable at the lowest temperature they considered for the experiments (500 1C). They also found disordered intergrowths involving different N-sized blocks inserted in an otherwise perfectly ordered matrix; for example, pairs of 8,8 blocks in 7,7L. Skowron and Tilley14 did not find synthetic phases corresponding to the minerals eskimoite (5,9L) or ourayite (11,11L), thus again highlighting those differences that exist between the behaviour of this system experimentally and in nature. Pring et al.8 confirmed the existence of disordered intergrowths of lillianite/gustavite-like blocks (N ¼ 4) and heyrovskyite-like (N ¼ 7) structural blocks. One disordered sequence, examined in detail, gave an average homologue number N ¼ 4.92 corresponding to a composition of Pb1.52Bi3.2Ag1.2S6.92. An axial next-nearest neighbour Ising model was used to follow the fluctuations in the average homologue number N across the crystal. This yielded compositional fluctuations of the order of 70–170 A˚ over an 1800 A˚ region of the crystal, with a 220 A˚ lamella of ordered vikingite (Figure 2). This extensive disorder suggested that it might be linked to some form of compositional modulation. Vikingite (4,7L) occurs in macroscopically well-ordered states, so it is clear that the sequence 4,7 is more stable than a random intergrowth of four and seven ribbons having the same bulk composition. Detailed examination of the sequence of slabs in Figure 2 reveals some evidence of partial ordering.8 There are 163 slabs in the image; 113 slabs have N ¼ 4 ribbons and 50 slabs have N ¼ 7 ribbons. The sequence 7,4,4,4 or 4,4,4,7 occurs 20 times in the image, including a section where the sequence is repeated four times. The 7,7 units generally only occur in single strips although there is a sequence of three such units on the left-hand side of Figure 2. The sequence 4,4,7,7 occurs eight times in the image while the alternative 7,4,7,7 occurs only twice. The sequence 4,4,7,7 is compositionally equivalent to vikingite, whereas the 7,4,7,7 sequence represents a composition richer in Pb. Trends in the randomness of the gustavite-vikingite intergrowth were evaluated and the dominant slab sequence was found to be 4,4,4,7 and 4,4,7,7, suggesting that some longer-period homologues may be stable. One can argue that the ordered long sequences 7,4,4,4, are also more stable than a random intergrowth of four and seven ribbons having the same bulk

Chemical Modulations in Pb–Bi Sulfosalts 77 44 44

77 44 44 44 47 47

47 44 47 44 44 47 44 44 44 44 47

245 47 44

50 Å

200 Å

Figure 2

Image showing the disordered intergrowth of 4,4L, 4,7L and 7,7L blocks (denoted as N1 + N2 sums) projected down the [001] direction. From the weighted average of homologue numbers for the images it is possible to calculate the composition of the disordered area, Pb1.52Bi3.2Ag1.2S6.92 where Naver = 4.92.

composition, but probably less stable than those formed by exsolutions of vikingite (7,4) in gustavite (4,4). Thus, the long-period ordered intergrowths probably represent an intermediate stage in a diffusion-controlled exsolution process from a not quite homogeneous unknown precursor with Naver E 5. This concept is in agreement with the observed structural faults. Figure 3 shows a HRTEM image of one of the more complex defects found by Pring et al.8 In this defect, the unit sequence 4,7,4,7,7,7,4,4,4,4,7,4 is transformed by the ‘jogging’ of three twin planes to give the sequence, 4,7,4,4,7,4,7,4,7,4,7,4. In this defect, the number of twin planes is conserved across the defect and thus the average value of N (and hence the composition) does not change. This was a common feature found in all defects reported by Pring et al.8 and also by Skowron and Tilley.14 These defects do not operate as a mechanism for compositional variation, but rather locally change block width. A long-range compositional modulation in a mineral could provide interesting clues into the nature of its formation. Is this related to periodic variations in the composition of the mineralising fluid that in turn can be linked to selforganising phenomena that arise due to fluctuations, instability and chaotic behaviour of one or the other parameters controlling that system?19 Or are they related to fluctuations in supersaturation associated with fluid flow and volume that have also been shown to lead to compositional zoning in minerals grown under hydrothermal conditions?20 Are these mineral systems useful models for the production of crystals that process gradients in physical properties? To test these ideas of long-range compositional variation via block-width modulation in these Pb–Ag–Bi sulfide minerals, we needed to examine a much longer structural sequence than the one presented above. In order to do this it is necessary to prepare an ion-thinned foil in the correct crystallographic

246

Chapter 14

47

47

77

44 47

44 44 47

47

47

47

50 Å

Figure 3

HRTEM image of crystal defects in which the chemical twin planes are altered (or jogged) to accommodate change in block width. The sequence 4,7,7,7 4,4,4,4 is transformed by the ‘jogging’ of three twin planes to give the sequence, 4,4,7,4 7,4,7,4.

orientation, with the stacking sequence running parallel to the edge of the holes. This proved to be not a trivial task for an opaque mineral that is impossible to orient before ion-beam thinning, but we finally obtained a suitable sample; this was a crystal of the mineral eskimoite, the 5,9L homologue (Pb10Ag7Bi15S36) and an Ag-substituted polymorph of the more widespread 7,7L homologue heyrovskyite (Pb24Bi4S36). The eskimoite as well as the disordered lillianite/ gustavite crystal above, both came from the same hand specimen, which originated at the Ivigtut cryolite deposit in southern Greenland. A number of overlapping HRTEM images were obtained to give a lattice image mosaic of over 80,000 A˚ of the eskimoite crystal edge. This was then analysed to establish the stacking sequence of the various homologue blocks. A portion of the crystal is shown in Figure 4, and the disorder is clearly visible, however many of the defects represent antiphase boundaries, in which the stacking sequence simply reverses 9,5,9,5,9,5|5,9,5,9,5,9,5 or 5,9,5,9,5,9|9,5,9,5,9,5,9. There is, however, some genuine block-width disorder, such as extra pairs of N ¼ 9 blocks and extra pairs of N ¼ 5 blocks. These often occur in groups where the defect pairs are separated only by short sequences of ordered material (1–5 unit cells). These block-width errors appear to be largely self-correcting, as a 5,5 block inserted in an ordered eskimoite sequence (5,9,5,5,5,9) is often followed at a short distance away by the insertion of a 9,9 block in the same matrix (5,9,5,9,9,9,5,9). Over the length of the edge measured that contains some 2698 unit cells there are 94 additional nine units giving an average N value of 7.07 corresponding to a composition of approximately Pb2.57Bi3.75Ag1.75S9.07; this is close to the composition of stoichiometric eskimoite (Table 1). There are three regions

247

Chemical Modulations in Pb–Bi Sulfosalts

1000 Å

300 Å

Figure 4

HRTEM images of eskimoite crystal edge projected down the [010] direction. Above is a small segment of the 80,000 A˚ long crystal edge at low magnification. Note the distribution of defects in the image. Below is a higher resolution image of the same region of the crystal with a 5,5,9,9 defect marked (arrow) and other defects clearly visible in the crystal.

N=9

0

300

600

900

1200

1500

1800

2100

2400

2700

N=5

Figure 5

Schematic diagram showing the distribution of extra N ¼ 5 and N ¼ 9 blocks in the eskimoite crystal. The horizontal axis represents the number of unit cells in the sequence from one end. Note that there are more N ¼ 9 defects than N ¼ 5 defects and also that the N ¼ 5 defects tend to be always closely associated with N ¼ 9 defects. Note also that the distribution of extra N ¼ 9 defects is uneven through the crystal resulting in compositional zoning.

of the crystal with over 200 unit cell repeats that have no extra 9,9 or 5,5 blocks, indicating some modulation in the density of block-width defects with variable distance in the crystal (Figure 5). A detailed statistical analysis of the sequence is currently in progress. Based on the above, the compositional modulation in the eskimoite crystal is minor. This is because, even though the 5,5 and 9,9 ‘defects’ are commonly present as extra units, inserted at intervals of variable length in the ordered 5,9 parent matrix of the crystal, their combination from one interval to another generally preserves the composition along the sequence. There is, nonetheless, a

248

Chapter 14

certain periodicity in the insertion and relative abundance from one region to another of such ‘defect’ blocks, with inverse chemical effects in the eskimoite crystal and this can be considered to map a rhythmic zonation at the lattice scale (Figure 5). The alternation of (5,5) and (9,9) pairs over sequences of 100–150 A˚ show a compositional loop between Ag-poor (5,5) and Ag-rich (9,9) modules with amplitude that is relevant only at this scale. Such compositional repeats can be interpreted as the expression of medium-range chemical oscillations that are encoded within the the longer range structural modulation. The crystal-structural modularity of sulfosalts, such as the example presented here, is well-suited to lock in subtle chemical variations if there is a coupling between such oscillations and the rate of atom ordering within coherent structural units at the lattice scale. Based on experiments that produced oscillatory zoning in crystals, the conditions required for the appearance of this selfpatterning phenomenon in hydrothermal systems is a non-stirred solution, as occurs in closed cavities in mineralising systems.21 So, although the inlet solution remains unchanged during the crystal formation, the crystallisation process itself may induce chemical oscillations in the same solution by a feedback effect, coupling the rate of diffusion to the induced changes in the respective fluid. It is interesting to contrast the difference in the states of order in the crystals with N ¼ 4 and 7 blocks, both synthetic and natural, with the N ¼ 5 and 9 blocks in eskimoite. The eskimoite homologue (5,9L) was not one of those noted by Skowron and Tilley,14 although they did find a number of other homologues with N ¼ 5 blocks. This suggests that the stability of the 5,9L homologue is linked to its high Ag and Bi contents, especially when compared to its polymorph heyrovskyite (7,7L); the latter is known to have limited compositional field with respect to Ag in nature.22 It is also worth noting, when considering the relative stabilities of the various block sizes, that eskimoite (5,9L), vikingite (4,7L) and gustavite (4,4L) are all intergrown in specimens from Ivigtut. Examination of the distribution of the lillianite minerals in nature and in synthetic studies indicate that N ¼ 4 and N ¼ 7 are more stable over a larger range of temperature and composition than other homologues. There is also a ‘mineral’ in nature, ‘schirmerite’, which is believed to be a disordered intergrowth of N ¼ 4 and N ¼ 7 blocks. The mechanisms for compositional zoning on a range of scales from the atomic to the macro can be subtle in minerals, as nature is able to anneal and transform on timescales unimaginable to the laboratory. It is clear that there is so much that solid-state chemists and materials scientists can still learn about the subtleties of crystal chemistry by looking at minerals. J.S. Anderson once mused that solid-state chemists should set up long-term annealing experiments and will them to their grandchildren!

Acknowledgements AP wishes to thank Professor John Meurig Thomas for introducing him to HRTEM. For the current work, thanks are due to Dr D.A. Jefferson of the

Chemical Modulations in Pb–Bi Sulfosalts

249

Department of Chemistry, University of Cambridge, for access to HRTEM facilities, Dr Ole Petersen of the Geological Museum, Copenhagen, for the sample of the gustavite-cosalite-galena-bearing mineral suite from Ivigtut, and Mr D. Ware for assistance in preparation of the ion-thinned specimens. The experimental work for this study was undertaken while one of us (AP) was a visiting fellow commoner at Trinity College, Cambridge and, thanks are extended to the master and fellows of that college for their hospitality and financial support. The financial support of the Australian Research Council is also gratefully acknowledged.

References 1. A.G. Bulakh, A.A. Zolotarev and S.N. Britvin, Neues Jb. Miner. Monat., 2003, 446. 2. B.J. Skinner and H.C.W. Skinner, Mineral. Rec., 1980, 11, 333. 3. L. Pauling, in: Structure and Bonding in Crystals, M.A. O’Keefe and A. Navrotsky (ed), Academic Press, New York, 1981, Vol. 1, 1. 4. Y. Take´uchi, in: Volcanism and Ore Genesis, T. Tatsumi (ed), University of Tokyo Press, Tokyo, 1970, 395. 5. E. Makovicky, Fortschr. Mineral., 1981, 59, 137. 6. H. Baumhauer, Z. Kristallogr., 1915, 55, 249. 7. A. Magne´li, Acta Crystallogr., 1953, 6, 495. 8. A. Pring, M. Jercher and E. Makovicky, Mineral. Mag., 1999, 63, 917. 9. J.B. Thompson Jr., Am. Mineral., 1970, 55, 292. 10. J.B. Thompson Jr., Am. Mineral., 1978, 63, 239. 11. D.R. Veblen, Am. Mineral., 1991, 76, 801. 12. A. Pring, Schweiz. Miner. Petrogr., 2001, 81, 69. 13. A. Skowron and R.J.D. Tilley, Chem. Scripta, 1986, 26, 353. 14. A. Skowron and R.J.D. Tilley, J. Solid State Chem., 1990, 85, 235. 15. E. Makovicky and S. Karup-Møller, Neues. Jb. Miner. Abh., 1977, 130, 264. 16. E. Makovicky, Neues. Jb. Miner. Abh., 1977, 131, 187. 17. E. Makovicky and S. Karup-Møller, Neues. Jb. Miner. Abh., 1977, 131, 56. 18. E. Makovicky, in Modular Aspects of Minerals, S. Merlino (ed), Eo¨tvo¨s University Press, Budapest, 1977, 237. 19. F. Di Benedetto, G.P. Bernardini, P. Costagliola, D. Plant and D.J. Vaughan, Am. Mineral., 2005, 90, 1384. 20. A. Putnis, M. Prieto and L. Fernandez-Diaz, Geol. Mag., 1995, 132, 1. 21. A.J. Reeder, R.O. Fagioli and W.J. Meyers, Earth Sci. Rev., 1990, 29, 39. 22. E. Makovicky, W.G. Mumme and B.F. Hoskins, 1992, Can. Mineral., 1992, 29, 553.

CHAPTER 15

Complexity: In the Eye of the Beholder (This Beholder is a Crystallographer) SVEN LIDIN Department of Physical, Inorganic and Structural Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91, Stockholm, Sweden

1 Complexity Complexity is something of a buzzword in structural science, and we all love to present our findings as complex since this not only puts the findings in a glorified light, but the glory also rubs off on the finder who must have been devilishly clever to solve the complex problem, hence our part of our fascination with complexity, real or imagined. But what is structural complexity really? A common definition deals with the number of parameters needed to describe a solution, but an equally valid interpretation is to consider complexity to be a measure of the difficulty of a structural problem.

1.1

Complexity in Structural Solution

Looking back on the history of structural solution by X-ray diffraction, we find that this complexity is a function of time. The very first structural solutions were found using symmetry considerations and trial-and-error methods, and complexity would emerge as soon as the number of possible solutions went beyond what was testable with the computational capabilities of the time. The notion of complexity changed with the introduction of the Patterson function in the 1930s that allowed the solution of structures with a small number of independent atoms, alternatively, with a small number of relatively heavy atoms to start phasing. The introduction of the Patterson function was clearly a turning point in structural science. The next important step in materials 250

Complexity: In the Eye of the Beholder

251

crystallography and small molecule crystallography was the advent of direct methods developed by Hauptman and Karle in a series of papers starting in the early 1950s. This method allows for the solution of rather large structures, provided certain conditions are met. This time complexity changed meaning in a more subtle way. Perhaps the most common reason direct methods fail (barring bad samples and errors in the analysis) is problems with pseudosymmetry. Particularly difficult cases include low symmetry superstructures in high symmetry systems where automated procedures are poor at picking up subtle symmetry imbalances, and the choice of origin becomes crucial. Quite often, the only recourse is to work in P1 to solve the problem, and then revert to the proper space group, and this method is by no means foolproof.

2 Incommensurability An interesting turning point in inorganic chemistry was the introduction of the concept of incommensurability. Although this phenomenon was recognized very early in crystallography, it did not attract a lot of attention until the work of deWolff on NaCO3.1 This seminal work marked the starting point of a sequence of papers, mainly from the Netherlands and Japan, dealing with various aspects of aperiodic crystallography. While progress was rapid, acceptance was still slow in a rather conservative crystallographic community. Readily affordable area detectors, and the increased use of electron diffraction provided an ever-increasing number of examples of incommensurability, but the real breakthrough in the larger community came with the software system JANA2 developed by Vaclav Petricek and his co-workers in Prague. With this splendid tool, suddenly the world of aperiodic analysis became available to the non-experts. At about this time I had the good fortune to be called for an interview for a position, where one of the referees was Sir John Meurig Thomas. One of the research projects I presented was on super-structure ordering in intermetallics. I had successfully completed some work, while other parts eluded me due to my limited knowledge of aperiodicity. When pressed by Sir John on how to proceed with these issues, I produced a somewhat lame ‘‘that would require aperiodic analysis, of which I have no command.’’ I got the expected reply ‘‘perhaps it’s time to take that command.’’ The thought had certainly crossed my mind before, but this contact provided the spark I needed to start the ignition.

2.1

Charge Flipping

I have been lucky to be involved in this branch of structural science that has enjoyed a tremendous growth over the last few years, and very recently I have had the privilege to watch a new turning point being reached – the introduction of charge flipping.3 While refinement of small amplitude modulations has long been an easy task, large scale modulations have provided a real challenge. The

252

Chapter 15

correct initial phasing of the modulation functions has been a non-trivial task, and for complicated structures it has been outright difficult, and very timeconsuming, since the correctness has sometimes been difficult to ascertain without proceeding quite far in the refinement work. Many blind alleys may have to be negotiated before the proper solution is found. Certainly there has been work on multidimensional direct methods and the Patterson function still applies, but for the more demanding cases it has been down to some trialand-error work or lucky guessing. So what is charge flipping? This remarkable, and very robust method works in a way that is reminiscent of several earlier methods, but the application of the procedure to ab initio structural solution had not been tried previously. The recipe is extremely simple: assign random starting phases to a data set, compute the Fourier transform, identify any part of the electron density map that has a charge below a certain positive threshold value, and change the sign of that charge. Calculate the phases associated with this charge-flipped electron density map, and use them together with the experimental amplitudes to generate a second iteration of the electron density map, etc. The most remarkable feature of this procedure is that it works. Sometimes it works quickly, but quite frequently it takes several hundred cycles to obtain convergence. An important feature of the procedure is that it operates in P1. For the method to work, it appears that it must be allowed to sample a large part of the configurational phase space.

2.2

A Practical Example

What then are the advantages of this method compared to direct methods? It is model independent, symmetry independent and dimensionally independent, meaning it can be used to phase an incommensurately modulated structure in its entirety. Because of the symmetry independence, it is also impervious to pseudo-symmetry. The main remaining problems are incomplete data (completeness to a good resolution is a prerequisite) and bad data (twinning is still an issue). To illustrate the power of the method, I will review an incommensurate case that we solved a few years ago using trial-and-error phasing and a great deal of huffing and puffing, and where the strength and ease of charge flipping becomes apparent. The case in question is the high temperature polymorph of Zn3 xSb2.4 The published structure of this incommensurately modulated phase contains six independent Sb positions and 18 Zn positions. Five of the Sb positions are weakly modulated, while the sixth shows a large, sawtooth-like displacement along the modulation direction. Only one of the Zn atoms shows full occupancy, while the others exhibit more or less erratic occupational behaviour. Solution and refinement of this structure was difficult because although a correct initial phasing was soon found, there was little progress in terms of refinement R-values, the obvious reason for this being the nonharmonic behaviour of the strongly modulated atomic positions. A reasonable fit was first achieved only when all pertinent positions were treated with

253

Complexity: In the Eye of the Beholder

non-harmonic modulation functions, i.e. step functions and sawtooth-like displacements. The cause for all this anharmonicity is the peculiar nature of the compound: the Sb atoms form a rather rigid sub-lattice of icosahedra interpenetrating along the a-axis (Figure 1). This leads to three distinct Sb–Sb distances, vertex-to-vertex, vertex-to-centre and centre-to-centre. While the vertex-to-centre distances are not too different from the vertex-to-vertex distances, the centreto-centre distances are significantly different. In fact, while vertex-to-vertex distances are typical for Sb3 –Sb3 contacts, the centre-to-centre distances are closer to, albeit a little longer than, those expected for Sb42 . This inherent frustration in the structure is resolved by alternating short Sb42 units and lone Sb3 , the proportion being given by the ratio between their ideal distances and the geometrical constraints of the icosahedral arrangement. This is the cause of the non-stoichiometry of the compound. It is also the cause of the modulation. What complicates matters is that the positions of the Zn atoms are at the centres of the tetrahedral interstices of the Sb sub-lattice. The occupancy follows the

Figure 1

Part of the Sb network in ht-Sb2Zn3 x. Note how Sb positions in the centre of the column of pentagonal antiprisms (grey spheres) alternate between exhibiting long distances corresponding to Sb3 and short distances corresponding to Sb42 units.

70 60

Residua

50 40 30 20 10 0 1

Figure 2

11

21

31

41

51 61 Iterations

71

81

91

101

Convergence of charge flipping for the structural solution of Sb2Zn3 x.

254

Chapter 15

(a)

2.0

x3=0.543,

x4

x2=0.747

1.6

1.2

0.8

0.4

0.0 -0.40 -0.20 (b) 2.0

0.00

0.20

x1

x3=0.042, x2

0.40

=0.748

x4 1.6

1.2

0.8

0.4

0.0 -0.40

-0.20

0.00 0.20

Figure 3

x1

0.40

Electron density map of the strongly modulated Sb position. (a) Density from published model (sawtooth modulation indicated in red). (b) Density obtained from charge flipping.

255

Complexity: In the Eye of the Beholder (a) 2.0

x3=0.847,x

2=0.503

x4

1.6

1.2

0.8

0.4

0.0 -0.20

0.00 0.20

(b) 2.0

0.40

x1

0.40

x1

x3=0.651, x2=0

.494

x4 1.6

1.2

0.8

0.4

0.0 -0.20

0.00 0.20

Figure 4

2

Electron density map of the position Zn . (a) Density from published model (sawtooth modulation indicated in red). (b) Density from charge flipping.

256

Chapter 15

(a) 2.0

x3=0.308, x2=0

.903

x4 1.6

1.2

0.8

0.4

0.0 -0.20

0.00 0.20

(b) 2.0

0.40

x1

0.40

x1

x3=0.193,x2=

0.097

x4 1.6

1.2

0.8

0.4

0.0

-0.20

Figure 5

0.00

0.20

Electron density map of the position Zn3. (a) Density from published model (sawtooth modulation indicated in red). (b) Density from charge flipping.

Complexity: In the Eye of the Beholder

257

normal rule that face-sharing tetrahedral interstices cannot be occupied simultaneously, and further, Sb-tetrahedra that contain the short Sb–Sb contact from the Sb42 unit as an edge are too small to host an interstitial Zn. The result is a Zn occupancy that is erratically jumping from one interstitial position to another in a fashion that is rather difficult to model. Incomplete models lead to high residual electron densities, high R-values, and doubts in the mind of the crystallographer that the solution is correct. I recently attempted a de novo solution of the structure from charge flipping using the software ‘‘Superflip’’ developed by Lukas Palatinus.5 Convergence of the procedure was reached after about 80 cycles (Figure 2) and the solution from charge flipping is quite complete: the six Sb atoms are clearly distinguished from the Zn positions, and one of the Sb atoms shows the expected, strongly modulated behaviour (Figure 3). For Zn, 13 positions are found in the initial search of the electron density map. More importantly, the occupational modulations are clearly evident for all Zn positions. Examples are given in Figures 4 and 5. This is very valuable information since the solution is model independent. No prejudice concerning the occupational modulation will affect the electron density. A comparison between the electron density for a few of the atomic positions in the two models clearly shows that they are very similar, not only in general, but in detail, and thus that, charge flipping can yield not only qualitatively correct solutions, but quantitative agreement, even for complex cases.

3 Conclusion I hope I have been able to convey the advantages of charge flipping for incommensurate structure analysis. It should however be pointed out that the method is very generally applicable, and that my experience is that it may be successfully applied to many cases where other methods fail. The simplicity of the method and the ease of use of software available in the public domain will certainly make this new tool widespread and appreciated. My only concern is that the frontiers of complexity are bound to move again, leaving most of my work in the despised area of simplicity.

References 1. P.M. De Wolff, Acta Crystallogr., 1972, A28, S101. 2. V. Petricek, M. Dusek and L. Palatinus, Jana2000 Institute of Physics, Academy of Sciences of the Czech Republic, Praha, 2005. 3. G. Oszlanyi and A. Suto, Acta Crystallogr., 2004, A60, 134. 4. M. Bostrom and S. Lidin, J. Alloys Compd., 2004, 376, 49. 5. L. Palatinus and G. Chapuis, Superflip – computer program for solution of crystal structures by charge flipping in arbitrary dimensions, 2006, http:/ superspace.epfl.ch/superflip.

CHAPTER 16

Synthesis and Characterization of Zn-T-Sites in Mazzite DAVID E. W. VAUGHAN,a INGRID J. PICKERING,b GRAHAM N. GEORGEb AND JEFFREY R. SHALLENBERGERa a

Materials Research Institute, Pennsylvania State University, University Park, PA 16802, USA; b Department of Geological Sciences, University of Saskatchewen, 114 Science Place, Saskatoon, SK, Canada, S7N 5E2

1 Introduction The substitution of divalent metal ions into zeolite framework tetrahedral positions (T-sites) is well established in ALPO zeolites,1 and several zincosilicate zeolites2–4 are known, including Zn-‘‘silicalite’’ (MFI).5 Reports of zinc substitutions into aluminosilicate zeolites are relatively rare and are mainly reported in the patent literature. Vaughan and Strohmaier prepared Zn-aluminosilicate mazzite (MAZ)6 and Linde-L (LTL)7 and Araya and Creeth8 made faujasite (FAU-X,Y), offretite (OFF) and gismondine (GIS). The latter authors also substituted zinc into these frameworks by secondary synthesis. Zn-divalent cation substitution increases the charge density of the framework, and hence the cation exchange capacity of the zeolite, and provides for a more reactive T-site. The former property can be used to improve the sorption or catalytic selectivity of the zeolite, as demonstrated for improved O2/N2 separation using Zn substituted low ratio FAU.9 The latter property facilitates the creation of reactive framework metal sites, the extraction of which by mild acid treatments can create lattice vacancies. These lattice ‘‘hydroxyl nests’’ can be filled with Si to produce higher stability, moderate acidity, higher Si/Al ratio zeolites. When Fe, Ni or Zn are the substituents, the zeolite will scavenge sulfur containing molecules (H2S, COS, mercaptans, etc.) providing useful materials for pollution abatement and gas purifications. In addition to expanding the compositional range, interest in such ZnT-atom substitutions in the MAZ structure10 was stimulated by the possibility 258

Synthesis and Characterization of Zn-T-Sites in Mazzite

Figure 1

259

Comparison of the MAZ and the theoretical ‘‘omega’’ structure.

that they may initiate the crystallization of the closely related theoretical structure ‘‘omega’’ proposed by Barrer and Villiger,11 shown in Figure 1. Both structures are built from identical columns of gmelinite cages linked through common 6-rings, but in MAZ the columns are related by a 63 rotation rather than the normal 6 rotation in ‘‘omega.’’ The two structures have similar theoretical powder X-ray diffraction patterns (PXRD), ‘‘omega’’ being differentiated by additional weak peaks at 11.651 (001) and 22.801 (221) 2y. These peaks were not observed in PXRD patterns of our samples. MAZ is one of several stable 12-ring channel zeolites of interest in catalysis,12–14 an interest strengthened by the commercialization of the LTL zeolite for the conversion of paraffins to aromatics.15,16 A major problem in studies of this kind is the issue of differentiating between cations in the framework and interstitial cations in exchange sites. Ion exchange with other cations may remove some exchangeable Zn21 but the possibility of cations located in ‘‘locked-in’’ sites (small cages or prisms) complicates the differentiation. More than one T-site and three T-atoms further complicate the MAZ characterization, particularly for NMR analyses. Ristic et al.17 characterized Zn-APO-50 (AFY) using Rietveld analysis of PXRD, 31P-NMR data, and extended X-ray absorption fine structure (EXAFS)18 but failed to locate extra-framework ions. Hunsicker et al.19 used X-ray photoelectron spectroscopy (XPS) to characterize Zn-aluminosilicate FAU with Si/(Al+Zn) framework ratios near unity, indicating that zinc is distributed between cation and framework sites.

2 Experimental 2.1

Sample Preparation

Two Zn-aluminosilicate MAZ samples and a non-Zn conventional aluminosilicate MAZ were synthesized using the general methods described for the synthesis of transition metal containing MAZ (ECR-22D)5,20, using the tetramethylammonium (TMA) cation as the template. Attempts to make Zn-MAZ with other templates known to promote the crystallization of MAZ in the

260

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absence of Zn (choline, bis-dihydroxydiethyl-dimethylammonium22) also co-crystallized FAU, SOD and GIS. Maz-1 was made from a gel having a Si/(Al+Zn) ratio of 4.44 and a composition: 0.95 TMA2O: 4.9 Na2O: Al2O3: 1.6 ZnO: 16 SiO2: 238 H2O: (3.2 NaNO3: 0.5 Na2SO4). (This is similar to Example 2 of Ref. 5, but replacing NiCl2 with the same moles of Zn(NO3)2.) The gel was made by mixing 25 g of 25% TMAOH (SACHEM) with 130.6 g of N-Sil sodium silicate (PQ Corp.), followed by 10.4 g of ‘‘seed solution,’’23 24.6 g of Na-aluminate solution (15.5% Al2O3, 14% Na2O), 17.84 g of Zn(NO3)2
6H2O dissolved in 15.5 g H2O, and 4 g of aluminum sulfate (17H2O ) dissolved in 6 g H2O. After thorough mixing in a blender, a 30 g sample was placed in a 45 ml Parr Teflon lined ‘‘acid-digestion bomb’’ and reacted for 15 h at 140 1C. The MAZ product was filtered, washed on a vacuum filter and dried at 110 1C. The product comprised very thin lathlike crystals (Figure 2) and chemical analysis gave an oxide composition: 0.182 TMA2O: 1.33 Na2O: Al2O3: 0.546 ZnO: 6.43 SiO2. (The TMA value was derived from the TGA weight loss between 500 1C and 600 1C.) The Si/ (Al + Zn) ratio was 2.53; assuming that all the Zn21 is in the framework, this represents a T-site occupancy of 6%. Additional samples run in 125 ml Teflon bottles at 100 1C yielded MAZ samples after 3 and 5 days. Maz-2 was made from a gel having a Si/(Al + Zn) ratio of 6.14 and a composition: 0.45 TMA2O: 5.75 Na2O: Al2O3: 0.16 ZnO: 13.27 SiO2: 314 H2O. The gel was made by mixing 79.2 g of N-Sil with 4 g of TMABr dissolved in 10 g of H2O, 1 g of seeds, 20 g of sodium aluminate solution (14.5% Al2O3, 14.2% Na2O), and 1.32 g of ZnSO4
7H2O dissolved in 15 g of H2O. Fifteen grams of this gel were reacted in a 23 ml Parr ‘‘bomb’’ for 90 h at 140 1C, followed by vacuum filtration

Figure 2

SEM images of Maz-1 (left); Maz-2 (right). Bar ¼ 1 mm.

Synthesis and Characterization of Zn-T-Sites in Mazzite

261

and washing with H2O. The MAZ product comprised thin lath-like crystals (Figure 2) with a composition: (0.185 TMA2O): 0.97 Na2O: Al2O3: 0.175 ZnO: 6.63 SiO2. (The TMA value was derived from the TGA weight loss between 450 1C and 600 1C.) The Si/(Al + Zn) ratio was 3.05, representing a Zn-T-site occupancy of 2%. An identical 87 h experiment gave a similar high purity MAZ product. Experiments run at 100 1C were contaminated with GIS. Maz-3 is a nano-crystal aluminosilicate Na,TMA-MAZ made using the method described in detail elsewhere.24 The analyzed Si/Al ratio was 2.68. Zn-montmorillonite was made by first forming a gel by diluting 15 g of HS-40 colloidal silica with 5 g of de-ionized water, mixing in a solution of 0.68 g NaOH dissolved in 8 g de-ionized water, followed by 5.64 g of NaNO3 and 9.94 g of Zn(NO3)2
6H2O dissolved in 20 g de-ionized water, giving a composition: SiO2: 0.36 ZnO: 0.34 Na2O: 20H2O. Ten grams of this gel were reacted in a 23 ml Parr ‘‘bomb’’ at 175 1C for 23 days, at which time PXRD and scanning electron microscopy (SEM) showed the product to be excellent montmorillonite but of very small crystal size (very similar to the commercial synthetic Laporte Laponiter montmorillonite). This was used as a standard for octahedral Zn21 in the XPS experiments. When the zinc nitrate was replaced with the corresponding sulfate, willemite (Zn2SiO4)25 was the main product together with minor montmorillonite.

2.2

Analytical Procedures

All samples were evaluated by PXRD using a Siemens D500 diffractometer (y/y, CuKa radiation, Bragg–Brentano geometry, MDI Jade 7 software). Elemental analysis was by ICP-AES after fusing the zeolites at 1050 1C in a solid mixture of 90% lithium tetraborate and 10% lithium carbonate, followed by dissolution in dilute nitric acid. SEM images were obtained on a Hitachi S3500-N SEM after coating the samples with gold. Thermogravimetric analyses (TGA) were carried out in air from 25 1C to 1000 1C at 10 1C min-1 on a Thermo-Electron 2050 instrument. X-ray absorption spectroscopy was carried out on Beamline 7-3 at the Stanford Synchrotron Radiation Laboratory with the SPEAR storage ring at 3 GeV and 70–100 mA. The beamline configuration consisted of a double crystal monochromator with Si(220) crystals, an upstream vertical aperture of 1 mm, and no focusing optics. Harmonic rejection was achieved by detuning one monochromator crystal to 50% off peak. Mazzite samples were packed neat into a 1 mm path length plate with Mylar tape for windows. Measurements were made in transmission with N2-filled ion chambers and the sample was held at approximately 15 K in a liquid helium flow cryostat. The spectrum of a zinc foil was collected simultaneously with that of each sample; the first energy inflection of the foil was assumed to be 9660.7 eV. Data collection was carried out using the program XAS_COLLECT.26 Data analysis used the program suite EXAFSPAK (http://ssrl.slac.stanford. edu/exafspak.html). The EXAFS spectra were analyzed using full multiple

262

Chapter 16

scattering phase and amplitude functions calculated using the program FEFF7.27,28 Coordinates were taken from the crystal structure for MAZ10 with the coordinates of the first shell oxygens adjusted radially to give the ZnO distance of 1.94 A˚ observed in preliminary fits. The scale factor was refined according to coordination numbers of 6 for aqueous zinc sulfate,29 and fixed at 1.32. The nominal threshold value, E0, was fixed at 9680 eV and then a correction to the threshold value DE0 was initially refined to be 10.4 eV and fixed thereafter. Paths were examined for significance and three single scattering paths (ZnO, Zn


Si (next nearest neighbour) and Zn


Si4R (across the 4-ring) were kept together with the triangular path ZnOSi


Zn. The 4-leg ZnOSiOZn path was considerably lower in amplitude and was not significant. In all of the refinements the degeneracy of the 3-leg path was constrained to be twice that of the 3.2 A˚ Zn


Si path and their Debye–Waller factors (measures of static and thermal disorder) were constrained to be the same. In order for the best comparison, the Debye–Waller factors of the outer shells were refined for sample Maz-1 and then these values were copied and fixed for the other sample. In both cases the coordination numbers and interatomic distances were refined. XPS was performed on a Kratos Analytical Axis Ultra instrument utilizing monochromatic Al Ka X-rays (hn ¼ 1486.6 eV). The binding energy was calibrated using sputter cleaned copper (932.7 eV) and gold (84.0 eV) foils. The samples were dusted onto 3Mt double sided adhesive tape. Sample charging was controlled through the use of a low energy electron flood gun. Choosing a reference peak to charge reference to is difficult in these specimens. The carbon peak is the typical choice, although some specimens displayed evidence of two hydrocarbon peaks being present at different potentials. Most likely these are due to hydrocarbons from the tape and TMA trapped within the gmelinite cages of the MAZ. (It is also possible, but rare, for TMA to occupy both channel and cage sites.) Charge correction was done by shifting the silicate peak to 103.38 eV.19 The presence of Na and Zn are expected to lower the Si 2p binding energy, so attention was paid to absolute differences in peak positions which are unaffected by the choice of charge reference. The elemental compositions were determined by applying relative sensitivity factors to the integrated peak areas after subtracting linear backgrounds. All measurements were performed at a takeoff angle of 901 with respect to the sample surface plane.

3 Results and Discussion For all the Zn21 to be in framework T-sites, Na1 and TMA1 cations must balance the MAZ framework charge deficiency. As (Na + TMA) ¼ (Al + 2Zn) (approximately) in both Zn-MAZ samples, Zn must be located almost completely in framework sites. The morphologies of both Zn-MAZ samples (Figure 2) were unusual in that they formed very thin (o20 nm) and narrow (100– 200 nm) lath-like crystals. We cannot ascribe this to the presence of zinc as the zinc-free Maz-3 sample had identical morphology to Maz-2. Sulfate is an

Synthesis and Characterization of Zn-T-Sites in Mazzite

Figure 3

263

Powder X-ray diffraction patterns of Zn-aluminosilicate Maz-1 and Maz-2 compared to the nano-crystal aluminosilicate Maz-3 sample.

unlikely cause as previous numerous aluminosilicate MAZ syntheses in the presence of sulfate24 produced agglomerates of bundled larger needles or rods, similar to those observed by others.30 Similarly, nitrate, used in the Maz-1 synthesis but not the Maz-2 and -3 preparations, cannot be the cause. The small crystal dimensions are reflected in the X-ray diffraction pattern peak broadening shown in Figure 3. Indexing of the PXRD patterns gave unit cell values (hexagonal, P63/mmc (#194)) for Maz-1, a ¼ 18.240 A˚, c ¼ 7.672 A˚, V ¼ 2210.7 A˚3; Maz-2, a ¼ 18.253 A˚, c ¼ 7.661 A˚, V ¼ 2210.4 A˚3; Maz-3, a ¼ 18.233 A˚, c ¼ 7.664 A˚, V ¼ 2206.7 A˚3. The unit cell similarities, despite the inclusion of Zn in T-sites in samples Maz-1 and -2, reflects the flexibility of the framework; the longer Zn–O bond length being compensated by changes in bond angles, confirmed by the EXAFS results. The Maz-1 and -2 TGA analyses are similar (Figure 4) and show a peak at B120 1C indicative of water loss from surfaces of these thin crystals, a second peak at B200 1C representing water loss from the 12-ring channel, and a third peak at B600 1C indicative of burn-off of the TMA template trapped in the gmelinite cages. The lack of a peak between 350 1C and 500 1C, characteristic of de-hydroxylation of T-site species, suggests the absence of octahedral or pentahedral hydroxylated Zn in T-sites. The total weight loss is 16% wt. Zinc K near-edge spectra of the two preparations of Zn-MAZ (Maz-1 and -2) are shown in Figure 5 compared with dilute zinc sulfate. The Zn-MAZ spectra are shown to closely resemble each other, and to differ substantially from that of the Zn21 solution, which is hex-aqua. The Zn K-edge EXAFS spectra,

Chapter 16 100

0.1 0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0

Deriv. Weight TGA

0

100

200

300

400

90

500

600

700

800

900

Weight (%)

Deriv. Weight (%/°C)

264

80 1000

Temperature (°C)

Figure 4

Thermogravimetric analysis of the Maz-2 sample.

Figure 5

Zinc K X-ray absorption near-edge spectra of aqueous zinc sulfate solution and zinc MAZ samples.

together with the corresponding Fourier transforms, are shown in Figure 6 for the Zn-MAZ samples. The Fourier transforms are dominated by first shell interactions at around 2.0 A˚ but also show some backscattering at 3–3.5 A˚ and a smaller peak just above 4 A˚. The results of multiple scattering fit analyses of the EXAFS are shown in Table 1 and Figure 6. There are no significant differences between the EXAFS fits of the two different Zn-MAZ samples (Table 1); in all cases the values obtained for a given parameter are within 3 e.s.d.s of each other. The first shell fits well in all cases to 4 ZnO at 1.94–1.95 A˚, in exact agreement with the EXAFS data of Araya and Creeth8, who also made Zn-aluminosilicate FAU, and the average Zn–O bond length found by Rietveld analysis of PXRD data

Synthesis and Characterization of Zn-T-Sites in Mazzite

Figure 6

Table 1

265

Zn K-edge EXAFS (a) and corresponding Fourier transforms (b) for zinc in Maz-1 and Maz-2 samples. The Fourier transforms have been phasecorrected for first shell Zn–O. Data are shown as solid lines and the best fit as a dashed line. The parameters determined by the best fit are shown in Table 1.

Results of EXAFS curve-fitting of Zn-MAZ samples.a

ZnO Zn


Si ZnOSi


Znb Zn


Si4Rc

N R s2 N R s2 R N R s2

Maz-1

Maz-2

3.9(2) 1.944(2) 0.0052(3) 3.5(1.2) 3.175(8) 0.011(2) 3.40(2) 1.4(9) 4.41(2) 0.006(4)

4.1(3) 1.935(3) 0.0055(5) 5.3(1.0) 3.182(8) 0.011d 3.37(2) 1.6(8) 4.40(3) 0.006d

Coordination numbers, N, interatomic distances, R (A˚) and Debye–Waller factors, s2 (A˚2). Three times the estimated standard deviation in the last digit(s) is shown in parentheses after the value, and is equivalent to the 99% confidence limit. This value is a measure of the precision of the fit. The value of the accuracy is in general somewhat higher than the precision and is typically 20% for N and 0.02 A˚ for R. b For this shell, N, which in this case is the path degeneracy, is twice that of Zn


Si and s2 is constrained to an identical value to that of Zn


Si. c Interaction across the four-ring. d For Maz-2 these values were held constant at the values obtained for Maz-1. a

for Zn-phosphate-FAU.31 A search of the Cambridge Crystallographic database for zinc coordinated by 4, 5 and 6 oxygens yielded mean interatomic distances of 1.96  0.03, 2.04  0.02 and 2.10  0.02 A˚, respectively, further confirming the assignment of 4-coordinate zinc. The second shell at 3–3.5 A˚ was modeled as a combination of a 2-leg Zn


Si path and the 3-leg ZnOSi


Zn path (having twice the degeneracy of the 2-leg path). The distances of these two paths were allowed to independently float in the fits. Combining the ZnO and

266

Chapter 16

Zn


Si observed distances, together with an idealized value of 1.60 A˚ for the SiO distance, a 3-leg path length of 3.36 A˚ is obtained, in very good agreement with the refined values of 3.40 and 3.37 A˚ (Table 1). The same 2-leg distances yield a mean ZnOSi angle of 1271, much more acute than the average angle of 1501 obtained from the MAZ crystal structure of Galli10 using a similar approach. Presumably this is due to the substitution of the substantially bigger Zn21 cation for the Al31 (typical Al–O bond length of 1.74 A˚), a relative expansion of 11.5%, while the Zn


Si distance only expands 1.4% over the crystal structure mean distance of 3.13 A˚. The third shell fits to a Zn


Si4R interaction at 4.40–4.43 A˚ and is essentially identical to the mean distance across a 4-ring in the MAZ structure. The core level photoemission spectra are shown in Figure 7 and the data are summarized in Tables 2 and 3 along with the comparable FAU data from Hunsicker et al.,19 who compared Zn21 in a zinc exchanged aluminosilicate FAU (cationic, Table 2) with a directly synthesized Zn-aluminosilicate FAU (tet) together with a sample of the latter post-treated with an alkali zinc oxide solution (mix). The octahedrally coordinated Zn montmorillonite reference material has lower Zn 2p3/2 and Zn 3d binding energies compared with the MAZ samples and the Hunsicker cationic, tetrahedral and mixed coordination samples. The Maz-1 Zn peak positions are consistent with the tetrahedral FAU (tet) sample previously reported by Hunsicker. Maz-2 has a broader Zn 2p3/2 peak and is centred at a lower binding energy, between that for Maz-1 and the

6×104 Zn-montomorillonite BE⫽1022.03 eV FWHM⫽1.65 eV

Counts per sec

5×104

4×104 MAZ-2 BE⫽1022.23 eV FWHM⫽2.04 eV MAZ-1 BE⫽1022.58 eV FWHM⫽1.88 eV

3×104

2×104

1028

1026

1024

1022

Binding energy (eV)

Figure 7

Zn core level photoemission spectra.

1020

1018

267

Synthesis and Characterization of Zn-T-Sites in Mazzite

Table 2

Core level photoemission data for Maz-1 and Maz-2 compared to published work. Maz-2

Maz-1

Zn-montmorillonite

Cationic

Tet

Mix

From Ref. 19 Zn 2p3/2 Zn 3d Zn 2p3/2 –Zn 3d Si 2p Al 2p

1022.23 –

1022.58 11.5 1011.08

1022.03 11.04 1010.99

1023.68 12.56 1011.12

1022.62 11.47 1011.15

1022.93 11.67 1011.26

103.38 74.98

103.38 75.08

103.38 73.78

74.99

103.38 75.11

103.38 75.32

1.85

1.8

1.84

Zn 2p3/2 FWHM (eV) 2.04

Table 3

1.88

1.65

XPS chemical analyses for Zn-MAZ samples (in atom%). Maz-2

Maz-1

Cationic

Tet

Mix

9.4 1.4 54.8 7.3 19.7 7.4 2.24

10.6 3.7 51.6 10.3 16.3 7.5 1.47

From Ref. 19 Na Zn O C Si Al Si/(Al + Zn)

6.1 0.4 55.9 14.4 17.6 5.7 2.89

7.5 1.9 54.3 14.7 17.1 4.6 2.63

4.6 1.2 54.2 11.3 21 7.7 2.34

Zn-montmorillonite, possibly indicating some Zn21 hydroxylated framework or cation component. The XPS compositional data are quite different to the compositions obtained by bulk chemical analysis (ICP-AES); for Maz-1 Na, Zn and Si values are all high compared to Al, and for Maz-2 Na is high and Zn and Si values are low. The inconsistencies are reflected in the XPS Si/(Al+Zn) ratios; for Maz-1 it is 4% high and for Maz-2 it is 5% low.

4 Conclusions The combined analyses are in broad agreement that zinc is incorporated into framework tetrahedral sites but are somewhat in conflict for the low zinc containing sample as to the extent of incorporation. This may be due to the surface sensitivity of XPS, in contrast to the bulk sensitivity of EXAFS and chemical analysis. The bulk chemical analysis and EXAFS confirm that the zinc in these samples is in the framework tetrahedral sites, although the T-site zinc occupancies, assuming all the zinc is tetrahedral, are low (6% and 2%), as were those reported by others.8,19 The Na1 and TMA1 balance the net negative charge on the framework derived from replacement of Si41 by Al31 and Zn21.

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The EXAFS data show that the longer Zn–O bond length is compensated by a more acute Zn–O–Si bond angle (1271 compared to 1501 for the aluminosilicate Si–O–Al bond angle) and this is reflected in the similar unit cell values for the zinc-aluminosilicates (Maz-1 and -2) and aluminosilicate (Maz-3) samples. The XPS data for Zn are comparable to those obtained by Hunsicker et al.19 for ZnFAU, who noted that, depending on sample treatments, the Zn21 may be distributed between framework and cation (or hydroxylated framework) sites. These XPS data establish a mutual confirmation for tetrahedral Zn21 in the two structures. Although these results demonstrate Zn in the T-sites, one notes that in the case of lower Zn input (Maz-2) in the gel, all the Zn is incorporated into the product, but in the higher Zn loading (Maz-1) only about 75% of the Zn21 in the starting gel is incorporated into the product. This may indicate that for aluminosilicate zeolites, toleration for Zn in T-sites is limited to low values, B6% in the case of MAZ.

Acknowledgments Two co-authors (DEWV and IJP) have had a long, stimulating and enjoyable association with Sir John Meurig Thomas. DEWV first met him in 1963 in the University Club (Penn State) ‘‘pool hall,’’ where he and another South Walian regularly won free beers from the local pool cognoscente. John was also a founding member of the Penn State Cricket Club. (That summer he also made a classic film of the reactivity of transition metal particles and defects on graphite oxidation.) Our friendship continued after we both returned to the UK, and our common interest in what are now called nano-materials, and catalysis, was strengthened by scientific collaborations, consulting and family friendships. IJP, now a Canada Research Chair, earned her Ph.D. as his first graduate student at the Royal Institution and expresses her heartfelt gratitude to him for his guidance and tutelage during her formative years. DEWV is supported in part by the Penn State Materials Research Institute (PSMRI) and the Penn State MRSEC under NSF grant DMR 0213623. GNG and IJP are supported by the Canada Research Chairs program, the Province of Saskatchewan and NSERC Canada. Portions of this research were carried out at the Stanford Synchrotron Radiation Laboratory, a national user facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences. The SSRL Structural Molecular Biology Program is supported by the Department of Energy, Office of Biological and Environmental Research, and by the National Institutes of Health, National Centre for Research Resources, Biomedical Technology Program. We thank Dr Maria Klimkiewicz (PSMRI) for the SEM images.

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CHAPTER 17

Anything Protons Do, Muons Do Better! E. A. DAVIS Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge, CB2 3QZ, UK

1 Introduction The simulation of hydrogen by positive muons has proved to be extremely valuable in the identification of potential sites for hydrogen in semiconductors and insulators. Although the muon has a mass one-ninth that of the proton, its interaction with the host lattice, both electronically and chemically, is virtually identical to that of a proton. During its 2.2 ms lifetime (experiments are frequently undertaken over a timescale of up to 10 lifetimes), the muon can diffuse, interact with, and adopt positions in the lattice that protons themselves would occupy. If the temperature is sufficiently low, muons can capture electrons to form muonium (Mu ¼ m1+e
) – effectively a light isotope of hydrogen. The reduced mass of muonium is within 5% of that of hydrogen and so its Bohr radius and ionization energy are essentially the same as those of hydrogen. Just as for donors or acceptors in doped semiconductors, the ionization energies of hydrogen or muonium incorporated into materials with a high relative permittivity are reduced from their vacuum-state value of 13.6 eV – sometimes by several orders of magnitude, leading to extended ground-state wave functions or orbitals. Confirmation of shallow states associated with muonium in several II–VI semiconductors, in at least one III–V, and in several large-bandgap oxides, has recently been obtained from muon spin rotation experiments. Prior to these discoveries, it had been assumed that the electronic states associated with muonium implanted into semiconductors always lay deep in the energy gap of the host material. The breakthrough came after judicious recognition of a beating in the spin precession signal corresponding to transitions between muonium levels whose degeneracy is split in a magnetic field. A 271

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Fourier transform of the raw data revealed line spectra from which a hyperfine constant could be extracted. The relevance of this work to the semiconductor community – including those interested in commercial development of devices – is the need to understand the behaviour of hydrogen in semiconductors. Whether added intentionally (for example, to activate or passivate dopants) or incorporated unavoidably perhaps during processing, the role played by hydrogen is always significant but not easy to predict or determine. This applies not only to its atomic position in the host lattice but also to the location of its associated electronic states in the energy gap. Muonium spectroscopy has been particularly successful in answering both these questions for many semiconductors. Another field to which such studies contribute is that of the choice of new materials for gate dielectrics. Silicon dioxide, the traditional insulator used for this purpose, is facing limitations in new applications, for example, those involving ‘flexible electronics’. Insulators with large band gaps and high relative permittivities are being sought from new families of oxides. If hydrogen forms shallow donor centres in any of these, this fact alone makes them unsuitable candidates. Already it has been demonstrated that muonium (and by implication hydrogen) forms a shallow donor level in ZnO and many other oxides. There are several reasons for the success of the muon experiments over similar attempts using hydrogen itself. One of these is the extraordinary sensitivity of the techniques available, by which one can essentially ‘see’ individual muons implanted one at a time. In contrast, in order to be visible, hydrogen has to be incorporated in relatively large concentrations, leading to problems associated with solubility, interactions, chemical reactions, etc. Another and more fundamental aspect is that hydrogen frequently forms what is known as ‘a negative-U system’, which means in essence that neutral states of hydrogen dissociate into positively and negatively charged states, making the neutral state inaccessible under thermal equilibrium conditions. Such conditions do not apply in the muon experiments, a situation that permits observation of both the ionized (charged) and unionized (neutral) states.

2 Experimental Techniques The majority of experiments have been undertaken using the ISIS pulsed muon source at the Rutherford Appleton Laboratory, UK (see Figure 1), or the TRIUMF continuous source in Vancouver, Canada. Muons are produced from the decay of pions, these being generated when a target of graphite or other light element is bombarded with high-energy protons from an accelerator. The muons have an initial energy B4 MeV and are 100% spin polarized. This polarization is retained during their thermalization within a sample and it is the subsequent evolution of this with time that forms the basis of muon spin relaxation or muon spin rotation experiments. The technique of transverse-field spin rotation involves applying a magnetic field perpendicular to the direction

Anything Protons Do, Muons Do Better!

Figure 1

273

Muon beam lines at the Rutherford Appleton Laboratory (courtesy of the Rutherford Appleton Laboratory).

of the incoming beam of muons (transverse to their spin) and monitoring the resulting precession signal via the emission of positrons that are emitted preferentially in the direction of the spin at the moment of the muon’s radioactive decay. For bare muons this is simply the Larmor frequency but for muonium several frequencies are observed. The resulting spectra can be analysed to determine the components of the hyperfine coupling constant between electron and muon, and to infer the nature of the trapping sites for muonium in the lattice. In order to illustrate the kind of information that can be obtained using the techniques of muon spectroscopy as applied to semiconductors, two examples will be described in some detail. The first is silicon, a material in which the electronic levels associated with muonium (and by inference, hydrogen) lie deep in the gap but for which there are two distinct sites. The second is zinc oxide in which the levels are shallow.

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3 Deep Centres: Silicon Early transverse-field muon spin rotation experiments on crystalline silicon at low temperatures1 revealed evidence for the existence of two paramagnetic sites for the neutral state of muonium Mu0 (see Figure 2). The first is identified by a pair of lines whose splitting yields a hyperfine coupling constant (between the muon and its bound electron) that is approximately half the value for vacuum-state muonium or muonium in SiO2. These lines, labelled MuT0 in Figure 2, are associated with muonium in a site (T) that has tetrahedral symmetry, namely the open cages in the silicon structure. The reduced hyperfine constant results from the electron spending some of its time on the surrounding atoms. Rapid diffusion of this centre between equivalent sites is inferred from the linewidths. The second site, characterized by a pair of lines at lower frequencies (see Figure 2), has axial symmetry along the o1114 directions and has been identified2–4 as a bond-centred (BC) site, Mu0BC. For this centre the isotropic component of the hyperfine tensor (the contact term) is small, the electron wave function having a node at the muon and maximum amplitude on the silicon antibonding orbitals.5 The silicon atoms relax outwards by more than 20% of the bond length to accommodate the muon. Finally, we see in Figure 2 a signal labelled m1 associated with bare muons that have not formed muonium.

Figure 2

(a) Muon spin rotation spectra from Si at 77 K compared with that of fused quartz (SiO2) at room temperature in a magnetic field of 100 gauss. The pairs of lines at high frequency arise from muonium in a tetrahedral cage site. The splitting of these lines reflects (inversely) the magnitude of the hyperfine coupling constant, which has a value close to that of vacuum-state muonium or muonium in quartz, but is approximately half as large in silicon. The pair of lines at 40–50 MHz (for Si only) is associated with bond-centred muonium. The very-low-frequency lines correspond to diamagnetic muonium. (b) Sites associated with muonium in the silicon lattice (from Refs. 1 and 26).

Anything Protons Do, Muons Do Better!

Figure 3

275

(a) Configurational-coordinate diagram for muonium states in Si. (b) Electronic energy levels associated with muonium in the T and BC sites (from Ref. 6).

As the temperature is raised, both Mu0T and Mu0BC ionize above about 230 K and 130 K respectively with activation energies of a few tenths of an electronvolt. Such measurements and related studies yield the depths of the levels associated with these centres within the energy gap of silicon. An estimate of the depth of the donor level associated with Mu0BC is 0.21 eV below the conduction band edge6,7 – considerably deeper than those of typical shallow dopant levels. This energy is denoted by E0/+ BC in Figure 3b. The energy to place a second electron on the bond-centre site is likely to be high and the level associated with Mu
BC is predicted to be in the conduction band. In contrast MuT can accommodate a second electron, yielding an acceptor level in the gap at a depth below the conduction band edge denoted by E
/0 T in Figure 2b. Note that if, as shown, the acceptor level lies below the donor level, the levels comprise a negative-U system. The schematic configurational-coordinate diagram illustrated in Figure 3a has been proposed6 to account for experimental findings on the various charge states and sites of muonium in silicon. It should be equally applicable to hydrogen in silicon, apart from slight differences related to the higher zeropoint energy of muon. The solid curves represent states that are active in intrinsic and p-type material – namely the neutral and positively charged states of MuBC and the neutral state of MuT. The negatively charged state of Mu0T (dotted curve) is observed in n-type silicon. The various possible charge and site conversions that occur can be followed on this diagram. Note that e
c refers to an electron at the bottom of the conduction band; addition to the curves of the energy of one or two of these electrons permits easier comparison of ionization/ barrier energies.

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The ground state of the system is neutral muonium in the bond-centre site, Mu0BC. Conversion to Mu0BC from the neutral tetrahedral (T) site, Mu0T, involves surmounting a barrier of height B0.39 eV but with a net gain in energy of B0.2–0.3 eV. The reverse process is therefore unfavourable. Mu0BC can ionize to Mu+ BC by electron emission or hole capture, or it can capture an electron (in n-type silicon) by surmounting a barrier of height B0.34 eV, in which case a site change to Mu
T occurs and some of this energy is recovered. Theoretical predictions8 concerning the location of isolated hydrogen in semiconductors involve calculations of the total energy when it is located at various high-symmetry sites. Most studies find the bond-centre location to be the most stable. With regard to the charged states, H1 in the BC site is found to be even more stable than H0, which is perhaps not surprising as the nonbonding state in the gap is then empty. The positively charged BC state lies more than 1 eV below that of the T site, the latter having a local maximum on the total energy surface for H1. The situation for H
is the reverse of that for H1: the T site is favoured over the BC site by about 0.5 eV. These predictions are borne out by the muonium results. The relative proportions of H in the various charge states under equilibrium conditions can be determined by calculations of their formation energies with respect to the Fermi level. It is found that H0 is not stable for any position of the Fermi level. The variation in energy of H in the three charge states with their position in the silicon lattice9 is shown in Figure 4. From these theoretical results, one can see that H0 has a minimum at the BC site, H
at the T site, and H1 at a site about half-way between the BC and P sites. The average of the H1 and H
minima lies below the minimum of H0, meaning that the reaction

Figure 4

Calculated variation of the energies of the three charge states with position between the BC and T sites for optimally relaxed Si atom positions. For H0 and H1 the energies include those of one or two electrons, respectively, at the bottom of the conduction band (from Ref. 9).

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2H - H +H is exothermic. A consequence of this negative-U situation for H in Si is that the neutral state cannot be observed experimentally without departure from equilibrium conditions, requiring, for example, illumination of the sample. Muon studies do not have this limitation, the various sites being populated in accordance with the kinetics of the implantation process. 0

1




4 Shallow Centre: ZnO ZnO is a semiconductor that is invariably found to have n-type conductivity. The theoretical prediction10 that this could arise from hydrogen impurity acting as a shallow donor state was quickly confirmed by muon implantation studies11,12. Below 40 K, a distinctive beating of the muon precession signal provides the required signature (Figure 5a). In the case of a small hyperfine constant, one can easily reach the so-called Paschen–Back regime in moderate field. Then a triplet of lines is seen in a Fourier transform of the raw data, the central one of which corresponds to the bare muon (Figure 5b). The separation of the two satellite lines provides a direct measure of the hyperfine coupling constant between the muon and the electron. For ZnO this is 500  20 kHz, which is 0.011% of the free-muonium value of 4463 MHz, immediately indicating a small electron spin density at the site of the muon and an extended wave

Figure 5

Muonium spin precession signal for a ZnO powder sample at 5 K. The upper plot (a) is the raw time-domain spectrum (corrected for the muon decay) while the lower plot (b) is the corresponding frequency spectrum. The central line corresponds to the Larmor frequency of the bare muon (ionized muonium) and the two symmetrically disposed satellites are associated with muonium. The dotted curve is a theoretical fit using a powder-pattern lineshape. (From Refs. 11 and 27.)

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function associated with a shallow donor state. Additional measurements13 in longitudinal magnetic fields have provided further evidence for these findings. Studies of the temperature dependence of the lines reveal that, as the temperature is raised, the central line increases in amplitude at the expense of the satellites. This is to be expected for ionization of the donor state. Arrhenius plots produce activation energies for the central (diamagnetic) line of B26 meV and of the satellite (paramagnetic) lines of B33 meV. The results imply that muonium in ZnO ionizes above B40 K with an activation energy of about 30  3 meV. Investigations by others have yielded a variety of possible values for the depth of hydrogen donors in ZnO. Hall effect studies14 have been analysed in terms of the ionization of two donors having energies of 31 meV and 61 meV, with the authors preferring to associate the lower of these two values with hydrogen donors. Hofmann et al.15, as part of their ENDOR investigations, also made Hall effect measurements and found two activation energies with values 35 and 66 meV. Early photoluminescence data16 gave 52 meV as the depth of a hydrogen donor. More recent photoluminescence studies17 suggest a lower value of 40 meV. Assuming for the sake of simplicity an isotropic centre, the effective Bohr radius a* can be obtained directly from the hyperfine constant A*, since this scales as the third power of the radius: a ¼ a0 ðA0 =A Þ1=3 where A0 is the free-muonium hyperfine constant and a0 is the Bohr radius. Taking A0 ¼ 4463 MHz and a0 ¼ 0.053 nm, we find a* ¼ 1.1 nm. This can be compared with the value estimated from a ‘hydrogenic’ model: a ¼ a0 eðme =m Þ where e is the relative permittivity and m* the effective mass for electrons in ZnO. Using the values e ¼ 8 and m*/me ¼ 0.24, gives a* ¼ 1.7 nm, in fair agreement with the value deduced from the data. In a similar vein we can estimate the ionization energy from the hyperfine constant: I  ¼ I0 ðA =A0 Þ1=3 =e where I0 is the Rydberg ¼ 13.6 eV. This yields I* ¼ 51 meV, which can be compared with the ‘hydrogenic’ value, given by I  ¼ I0 ðm =me Þ=e2 of 50 meV. The agreement is good, bearing in mind the assumptions in the hydrogenic model – no central-cell corrections for example. The experiments described above were made on powder samples of 99.999% purity from Alfa Aesar. Single-crystal studies have also been undertaken.13 Spectra taken at different orientations, y, of the crystallographic c-axis with

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respect to the muon beam reveal a shift of the central peak that is consistent with the centre being anisotropic. The frequency dependence is given by Dn ¼ A þ

D b ð3 cos2 y
1Þ 2

where A* is the isotropic part of the hyperfine tensor and D is a dipolar term. These data yield a very similar value of A* (namely 490  10 kHz) to the powder sample studies but confirm an anisotropy of the centre with D ¼ 260  20 kHz. From the angular dependence one can deduce that the symmetry axis of the centre is parallel to the c-axis. Of the possible sites that hydrogen (muonium) can occupy in the wurtzite lattice, a BC site has been shown theoretically to have the lowest energy18. Experimentally, it is difficult to distinguish between this site and the site antibonding to oxygen along the c-axis. Shimomura et al.19 have also made muonium studies on single crystals of ZnO and claim to have identified two distinct shallow centres, one associated with each of the above sites.

5 Shallow versus Deep The reason why muonium (and by implication hydrogen) acts as a shallow centre in some materials and as a deep centre in others is intriguing20. It appears that the electron affinity of the host is a crucial parameter, as revealed by the plot shown in Figure 6. Here the muonium hyperfine constant, relative to the vacuum-state value, is plotted versus electron affinity for a variety of materials, from insulators such as SiO2 and diamond on the left (for which A*/A0 is close to unity) to semiconductors in the middle of the plot. The five semiconductors with an effectively zero (on this scale) hyperfine constant are those in which muonium forms shallow centres. It is evident that the ‘dilation of the wavefunction’ from atomic-like to extended occurs rather suddenly on this plot, corresponding to an electron affinity of about 3.7 eV. The concept of the electron affinity of the host being the all important factor influencing the deep to shallow transition is implicit in the band-offset diagrams proposed by Van de Walle21. An example is shown in Figure 7 for a few materials. In this diagram, Ec represents the energy of the bottom of the conduction band and Ev the top of the valence band, both plotted on an absolute energy scale, i.e. with respect to the vacuum level. The dashed line marked +/
gives the energy at which the formation energies of the positively and negatively charged states of hydrogen are equal. This represents the position at which the Fermi level would be pinned in a negative U system. If this level lies in the conduction band (as in ZnO and InN22) hydrogen forms a shallow donor level. If the level lies in the band gap (as for the other three materials) then hydrogen forms a deep centre. The predictive nature of this model is currently being tested for other semiconductors, in particular oxides other than ZnO.23,24 Peacock and Robertson25 have questioned whether the hydrogen level really does lie at a constant depth below the vacuum level. A subtle point relevant to

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Figure 6

Plot of the normalised muonium hyperfine constant versus electron affinity (from Ref. 20).

Figure 7

Band-offset diagram embracing several semiconductors (from Ref. 22).

this question is that the donor state actually lies at the level +/0, i.e. the energy through which the Fermi level would pass when the centre ionizes, changing from the neutral to the positively charged state. This level differs by U/2 from the +/
level and so, even if the +/
level is invariant, the +/0 level is not

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281

expected to be so. Under thermal equilibrium conditions the Fermi level is pinned at +/
(for a negative U system). Such conditions do not apply in the muonium experiments and so we are able to explore the higher +/0 level directly, without the need to illuminate the sample to reveal the neutral state, as required for, say, ENDOR experiments using hydrogen itself.15

Acknowledgements The author wishes to acknowledge fruitful and enjoyable collaboration with several colleagues, namely S F J Cox (Rutherford Appleton Laboratory and University College London), P J C King, J S Lord and S P Cottrell (Rutherford Appleton Laboratory), J M Gil, H Alberto, R Vila˜o, J Piroto Duarte and N Ayres de Campos (Coimbra University, Portugal) and R Lichti (Texas Tech University, USA). As a physicist it is a pleasure to have been invited to contribute this chapter to a book in honour of a distinguished chemist. Sir John Meurig Thomas’ interests are of course much wider than this label might imply – a fact that has been made very evident to me over the past few years in the Department of Materials Science and Metallurgy in Cambridge where fate brought us together and where we have had many coffee-time discussions on topics beyond the boundaries of our respective disciplines. Many thanks for your friendship John, and, of course, ‘happy returns’.

References 1. J.H. Brewer, K.M. Crowe, F.N. Gygax, R.F. Johnson, B.D. Patterson, D.G. Fleming and A. Schenck, Phys. Rev. Lett., 1973, 31, 143. 2. R.F. Kiefl and T.L. Estle, in: Hydrogen in Semiconductors, J.I. Pankove and N.M. Johnson (eds), Academic Press, New York, 1991, 547. 3. S.F.J. Cox, Philos. Trans., 1995, 350, 171. 4. R.F. Kiefl, M. Celio. T.L. Estle, S.R. Kreitzman, G.M. Luke, T.M. Riseman and E.J. Ansaldo, Phys. Rev. Lett., 1988, 60, 224. 5. S.F.J. Cox and M.C.R. Symons, Chem. Phys. Lett., 1986, 126, 516. 6. S.R. Kreitzman, B. Hitti, R.L. Lichti, T.L. Estle and K.H. Chow, Phys. Rev, 1995, B51, 13117. 7. R.L. Lichti, K.H. Chow, S.F.J. Cox, J.M. Gil, D.L. Stripe and R.C. Vila˜o, Physica B., 2006, 376–377, 587. 8. J.I. Pankove, Appl. Phys. Lett., 1978, 32, 812. 9. N.M. Johnson, C. Henry and C.G. Van de Walle, Phys. Rev. Lett., 1994, 73, 130. 10. C.G. Van de Walle, Phys. Rev. Lett., 2000, 85, 1012. 11. S.F.J. Cox, E.A. Davis, S.P. Cottrell, P.J.C. King, J.S. Lord, J.M. Gil, H.V. Alberto, R.C. Vila˜o, J. Piroto Duarte, N. Ayres de Campos, A. Weidinger, R.L. Lichti and S.F.C. Irvine, Phys. Rev. Lett., 2001, 86, 2604.

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12. J.M. Gil, H.V. Alberto, R.C. Vila˜o, J. Piroto Duarte, P.J. Mendes, L.P. Ferreira, N. Ayres de Campos, A. Weidinger, J. Krause, E.A. Davis, S.P. Cottrell and S.F.J. Cox, Phys. Rev., 2001, B64, 075205. 13. H.V. Alberto, R.C. Vila˜o, J. Piroto Duarte, N. Ayres de Campos, R.L. Lichti, E.A. Davis, S.P. Cottrell and S.F.J. Cox, Hyperfine Interact., 2001, 136/137, 471. 14. D.C. Look, D.C. Reynolds, J.R. Sizelove, R.L. Jones, C.W. Litton, G. Cantwell and W.C. Harsch, Solid State Commun., 1998, 105, 399. 15. D.M. Hofmann, A. Hofstaetter, F. Leiter, H. Zhou, F. Henecker, B.K. Meyer, S.B. Orlinskii, J. Schmidt and P.G. Baranov, Phys. Rev. Lett., 2002, 88, 045504. 16. D.C. Reynolds and T.C. Collins, Phys. Rev., 1969, 185, 1099. 17. D.C. Look, C. Coskun, B. Clafin and G.C. Farlow, Physica, 2003, B340–342, 32. 18. E.V. Lavrov, J. Weber, F. Bo¨rrnert, C.G. Van de Walle and R. Helbig, Phys. Rev., 2002, B66, 165205. 19. K. Shimomura, K. Nishiyama and R. Kadono, Phys. Rev. Lett., 2002, 89, 255505. 20. S.F.J. Cox, J. Phys.: Condens. Matter., 2003, 15, R1727. 21. C.G. Van de Walle and J. Neugebauer, Nature, 2003, 423, 626. 22. E.A. Davis, S.F.J. Cox, R.L. Lichti and C.G. Van de Walle, Appl. Phys. Lett., 2003, 82, 592. 23. S.F.J. Cox, J.S. Lord, S.P. Cottrell, J.M. Gil, H.V. Alberto, A. Keren, D. Prabhakaran, R. Scheuermann and A. Stoykov, J. Phys.: Condens. Matter, 2006, 18, 1061. 24. S.F.J. Cox, J.L. Gavartin, J.S. Lord, S.P. Cottrell, J.M. Gil, H.V. Alberto, J. Piroto Duarte, R.C. Vila˜o, N. Ayres de Campos, D.J. Keeble, E.A. Davis and M. Charlton, and D.P. van der Werf, J. Phys.: Condens. Matter, 2006, 18, 1079. 25. P.W. Peacock and J. Robertson, Appl. Phys. Lett., 2003, 83, 2025. 26. E.A. Davis, J. Non-Cryst. Solids, 1996, 198–200, 1. 27. E.A. Davis, in: Zinc Oxide – A Material for Micro- and Optoelectronic Applications, N.H. Nickel and E. Terukov, (eds), Springer, Dordrecht, The Netherlands, 2005, 115.

Section B: Organic Solid State Chemistry

CHAPTER 18

Molecular Cohesion and the Structure of Organic Crystals JACK D. DUNITZa AND A. GAVEZZOTTIb a

Chemistry Department OCL, ETH-Ho¨nggerberg HCI H333, ETH-Zurich, CH-8093 Zurich, Switzerland; b Dipartimento di Chimica Strutturale e Stereochimica Inorganica, University of Milano, Via Venezian 21, I-20133 Milano, Italy

1 Some History Among John Meurig Thomas’s scientific interests, too many and too multifarious to be listed here, a major preoccupation is with the forces that guide and preserve atomic and molecular architectures in the solid state. The analysis of known solid-state structures in terms of ‘‘non-bonded’’ forces, the design of solids with desired properties and reactivities, the prediction and preparation of new phases – these are all areas that have attracted Sir John’s attention and where he has made far-reaching contributions. In deference to Sir John’s enduring concern for the history of science, for how its past has influenced its present and its foreseeable future, we shall attempt here to outline the development of theoretical models of intermolecular forces with special reference to the organic solid state and point to possible new directions. The notion that macroscopic bodies are made of atoms and molecules that attract one another is as old as antiquity, even if it was expressed in very different terminologies to those in our modern vocabulary. In his Opticks, Isaac Newton wrote: ‘‘There are therefore Agents in Nature able to make the Particles of Bodies stick together by very strong Attractions. And it is the Business of experimental Philosophy to find them out.’’1 Newton was probably thinking of solids when he wrote those memorable lines; and the notion that solid matter is made of small particles sticking together so as to produce a compact arrangement came to the mind of natural philosophers very early on, long before the quantitative assessment of molecular properties in terms of chemical bonding, shape and structure became possible: ‘‘As for solids . . . in order that among their 285

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molecules or particles be made such a tight binding as is cause of their compactness, it is necessary that they may be located in certain given ways, which are appropriate for cohesion to exert its energy’’;2 ‘‘The attractive forces acting between the atoms will cause the portions of space which they respectively appropriate . . . to be in contact with one another, at the maximum number of points; as a result . . . the molecules themselves will also pack closely together’’.3 An early recognition that molecules exert attractive forces on one another also in the gaseous state arose in the latter part of the 19th century and has come down to us mainly from van der Waals’s interpretation of experimental deviations of real gases from ideal gases. In the van der Waals equation: P¼

RT a
2 Vm
b Vm

ð1Þ

relating the volume of a real gas to the applied pressure, the quantity b is related to the finite volume of the molecules (assumed to be zero in the ideal gas equation), and a is a factor arising from the virial of intermolecular attractive forces, that leads to reduction in pressure. This quantitative relationship came out of accurate experimental observations and modern chemical thinking, although the nature of the molecular entities and of the forces acting among them was only approximately known; still today we talk about van der Waals molecular volumes and van der Waals radii, and also about van der Waals forces without defining too closely what they mean. As a young man, even before his astonishing series of papers in 1905, Albert Einstein was occupied with the problem of intermolecular interactions – in addition to his other interests. His first published paper,4 written when he was 21 years old, is concerned with intermolecular forces in liquids. Although many eminent physicists and chemists at the time were still reluctant to accept that molecules really exist, Einstein seems to have had no doubts about this. He assumed that the potential between two molecules is of the form P ¼ P1
c1 c2 fðrÞ

ð2Þ

where f(r) is a universal function of the intermolecular distance and the constants c1 and c2 depend on the molecular species, being sums of ci values for the constituent atoms. Without going into detail, we note that by fitting to experimental values of measurable properties, such as surface tension (capillarity) and compressibility of common organic liquids, Einstein estimated atomic c values in arbitrary units for several elements, for example: cH ¼ 1:6; cC ¼ 55; cO ¼ 46:8; cCl ¼ 60; cBr ¼ 152; cI ¼ 198 Although the paper is today of no more than historical interest, the molecular c’s may be seen to be somewhat analogous to molecular polarizabilities in the much later London expression for the dispersion force between two molecules and the atomic c’s to atom polarizabilities. From a modern compilation,5 atomic polarizabilities a in A˚3 units are: aH ¼ 0:36; aC ¼ 1:44; aO ¼ 0:92; aCl ¼ 1:62; aBr ¼ 2:02; aI ¼ 2:65

Molecular Cohesion and the Structure of Organic Crystals

287

Indeed, apart from the values for hydrogen, there is even a rough proportionality. In 1912, following Max von Laue’s intuition, came the discovery of X-ray diffraction by crystals. In the course of a discussion with Peter Paul Ewald, it seems to have become clear that the distances between atoms in crystals should be of the same order of magnitude as the wavelength of X-rays and hence that interference phenomena might be expected to occur. Soon after the experimental verification by Walther Friedrich and Paul Knipping, the news traveled to Cambridge where the Braggs, William Henry and William Lawrence, father and son, used the X-ray diffraction patterns produced by simple ionic crystals to determine their internal atomic arrangements. The first analyses were of alkali halide crystals, which were shown to be built from alternating patterns of cations and anions. Today we take this so much for granted that it may be hard to imagine how difficult it was for contemporary chemists to accept such ideas. Even as late as 1927, Henry E. Armstrong wrote in Nature:6 ‘‘Prof. W. L. Bragg asserts that in sodium chloride there appear to be no molecules represented by NaCl. The equality in number of sodium and chlorine atoms is arrived at by a chess-board pattern of these atoms: it is a result of geometry and not of a pairingoff of these atoms. . . . Chemistry is neither chess nor geometry, whatever X-ray physics may be. . . . It were time that chemists took charge of chemistry once more and protected neophytes against the worship of false gods; at least taught them to ask for something more than chess-board evidence’’. As the systematic determination of crystal structures progressed, the atomic arrangements in simple ionic compounds were found to be understandable in terms of a few simple rules associated with the names of Ewald himself, Max Born, Fritz Haber, Erwin Madelung, and Kasimir Fajans; and, of course, the Braggs themselves, who led the way in the systematic determination of crystal structures. While simple ionic crystals, such as the alkali halides, are somewhat limited in the types of crystal structure they can adopt, this is certainly not the case for more complex minerals, such as mica KAl3Si3O10(OH)2 or zunyite, Al13Si5O20(OH)18Cl, for example. In 1929, Linus Pauling7 formulated a set of rules that was successful not only in testing the correctness of known structures but also in predicting unknown ones. As Pauling himself remarked, these rules are neither rigorous in their derivations nor universal in their application but they have proved remarkably successful. Pauling’s second rule states essentially that electrostatic lines of force stretch only between nearest neighbours. In his 1937 book on the structure of minerals,8 W. L. Bragg wrote: ‘‘The rule appears simple. But it is surprising what rigorous conditions it imposes on the geometrical configuration of a silicate. . .To sum up, these rules are the basis for the stereochemistry of minerals.’’ As far as the metallic state is concerned, the arrival of X-ray diffraction soon showed that the arrangements of atomic nuclei in metallic crystals led to a simple model in terms of close-packed structures of atomic spheres with high coordination numbers: cubic and hexagonal close packing with coordination number 12 and body-centered packing with coordination number 14. Of course, a more refined quantum-mechanical description reveals that the electron density is smeared out into an electronic sea where

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atoms are no longer spherical, but the simple model of close-packed spheres preserves its fascination and its didactic value besides its predictive power. Questions about how organic molecules attract or repel one another took longer to be asked and even longer to be answered in a proper scientific, quantitative manner. There are several reasons for this. In the first place, most simple organic compounds are liquids at normal temperature and pressure, so that they are more difficult to obtain and to study in the crystalline state. The low vaporization temperatures of the liquids and the low melting points of most organic crystals indicate that cohesive forces among organic molecules are only a small fraction of those operative in typical inorganic salts and metals. Then again, the exact shapes and sizes of organic molecules were still unknown in those early days or at best matters for informed speculation. Most of the early X-ray crystallographic studies of organic compounds were concerned with the determination and systematization of molecular shapes and sizes, with an emphasis on interatomic distances and angles; crystal packing matters were largely ignored. In contrast to simple inorganic compounds, which tend to form high-symmetry crystals, organic crystals typically crystallize in systems of lower symmetry: orthorhombic, monoclinic or triclinic. While sodium chloride forms cubic crystals and its structure could be derived by applying symmetry rules, crystals of anthracene, examined in 1920,9 were monoclinic and thus intractable by the methods then in use. There were exceptions, of course, and it is interesting and perhaps not merely coincidence that two of the earliest examples of successful molecular structure determination involved high-symmetry crystals and correspondingly high-symmetry molecules. One was the 1923 analysis of hexamethylenetetramine,10 C6H12N4. The crystals are cubic, space group I
4m3 with two molecules per unit cell. Symmetry considerations require that the four nitrogen atoms occupy vertices of a regular tetrahedron and the six carbon atoms vertices of a regular octahedron, enough to establish the cage structure of the molecule. The other example is the 1928 analysis of hexachloro- and hexabromocyclohexane;11 the crystals are cubic, space group Pa3, with four molecules in the unit cell. Here, application of symmetry arguments led to the establishment of the chair form of the cyclohexane ring with the substituents in equatorial positions. In those early days, and indeed until much later, the main information to be derived from the crystal structure analysis of an organic compound was about the molecular constitution and conformation, with approximate values of interatomic distances and angles from which often far-reaching conclusions about the bonding details were drawn. One intermolecular linkage that was early recognized in organic crystals as well as in inorganic ones was the hydrogen bond. Because of their low electron density, hydrogen atoms are difficult to locate with X-rays. Thus, hydrogen atoms were generally located by stereochemical model considerations, that is, by informed guessing. Often they were just left out of the structural description altogether so that in many early pictorial representations of the crystal structures there appeared to be empty regions of space between the molecules. However, even if the locations of hydrogen atoms in crystal structures needed to be guessed rather than located

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experimentally, by the late-1930s, hydrogen bonds had been recognized as an important kind of structural glue, not only in ice and solid hydrogen fluoride but also in simple carboxylic acids and amides, where O–H. . .O and N–H. . .O hydrogen bonds were assigned a dominant role in controlling the molecular arrangements found in crystals. Otherwise, little attention was given to questions of what holds the molecules together in a given crystal structure. Often it seemed to be mere empty space. However, molecular wave functions extend to infinity, so that space is never quite empty. As far as the physics of intermolecular interactions is concerned, what matters is the nature and strength of the electromagnetic fields produced by the electrons and nuclei. The strong Coulombic field exerted by the highly polar cations and anions in minerals was comparatively easy to study, and atomic cohesion in such crystals seemed to present no fundamental problems. Similarly, as an obvious extension, it was soon shown that the average interaction energy between molecules with a permanent dipole moment, e.g. water, is attractive. The nature of the cohesive forces among neutral unpolar molecules remained elusive. As an extreme example consider solid argon, for which no theory based on classical mechanics and electrostatics could possibly reproduce the lattice energy. The mysterious missing term, the dispersion energy, could only be understood after the advent of quantum mechanics. No one better than London himself has expressed the underlying source of dispersion forces:12 ‘‘These very quickly varying dipoles, represented by the zeropoint motion of a molecule, produce an electric field and act upon the polarisability of the other molecule and produce there induced dipoles, which are in phase and in interaction with the instantaneous dipoles producing them . . . we may imagine a molecule in a state k as represented by an orchestra of periodic dipoles mkl which correspond with the frequencies nkl ¼ (El
Ek)/h of (not forbidden) transitions to the states l. These ‘oscillator strengths’, mkl, are the same quantities which appear in the ‘dispersion formula’ which gives the polarisability of the molecule in the state k when acted on by an alternating field of the frequency n.’’ The theoretical treatment shows that the leading term in these forces decays as the inverse sixth power of intermolecular distance. It is thus obvious that in order for these weak, short-range forces to exert their action as efficiently as possible, molecules must be in close contact, bumps into hollows, with as little empty space as possible. It was A. I. Kitaigorodski’s great achievement, starting from a critical survey of organic crystal structures coming to be known in sizeable numbers by the early 1960’s, to put these concepts on a systematic quantitative footing.13

2 More Recent Times 2.1

The Atom–Atom Method

The simplest approach for estimating the potential energy of a pair of molecules or of an assembly of molecules, as in a crystal, goes under the name of

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atom–atom potentials; the basic assumption here is that the centres for the evaluation of the potential coincide with the nuclear positions. Such an approach is an extension of the methods that were being developed around mid-century for conformational analysis of organic molecules by Westheimer,14 Hill,15 Dunitz and Schomaker,16 Hendrickson,17 Wiberg,18 Bartell et al.,19 Lifson and Warshel,20 Allinger et al.,21 Scott and Scheraga22 and others (for a critical review see Ref. 23). The emphasis there was on the role of nonbonded interactions or ‘‘steric effects,’’ as they were often called, in dictating the variation in interatomic distances and angles and conformations of organic molecules. While ‘‘packing effects’’ could be invoked to explain almost any unusual observation in the solid state, this was clearly not the case for observations on isolated molecules. The need for ‘‘non-bonded’’ terms arose when it was realized that the gas-phase conformations of organic molecules could not always be explained on the basis of standard bond lengths and angles, using only bond-stretch, bond-angle-bending and torsional force-field terms, parameterized from vibrational spectroscopic data. For example, the stretched C–C bonds in cyclobutane could only be explained by postulating that there is a strong repulsion between the atoms across the diagonals of the four-membered ring.16 This and other evidence revealed that interactions between formally non-bonded atoms need to be included in any attempt to calculate the equilibrium geometry and energy of a molecule. The idea behind molecular mechanics, namely that the atoms of a given molecule interact by some sort of attractive–repulsive potential found an obvious extension in the evaluation of potentials and forces between atoms in different molecules, and thus opened the way to the study of condensed phases. By the mid-1960s, such non-bonded interactions were being used by the Kitaigorodski school,24 especially by Kira Mirsky, as a basis for calculating lattice energies of molecular crystals. In a series of papers that quickly became citation classics, D. E. Williams25 was able to parameterize a consistent set of intermolecular potentials for crystals of organic molecules containing C, H, N, O, F and Cl atoms. The basic assumption, both in the intra- and in the intermolecular approach, is that the interaction potential Vij between a pair of atoms i and j which are not bound through a proper chemical bond, or more appropriately, which do not in any way share an interaction mediated by the intervening electrons (as, for example, in pairs of atoms chemically bound to the same atom), depends only on the interatomic distance Rij, and can be expressed in the form of a sum of repulsive and attractive terms, for example:
n
1 Vij ¼ A expð
BRij Þ
CR
6 ij þ D Rij þ qi qj Rij þ   

ð3Þ

with A, B, C, D, . . . empirical parameters and q’s formal atomic charges. For a crystal, the lattice energy is obtained by an appropriate summation over all such terms. The main reason for the almost instantaneous popularity and success of this model is that it was readily applicable even with the modest computer resources of those times. Since then it has been continuously developed and used in countless applications for estimates of lattice energy, for

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example in assessing the relative energies of crystal polymorphs and in crystal structure prediction exercises. The widely used OPEC (organic potential energy calculations)26 allows one to estimate the potential energy increase resulting from molecular rotations or translations in a crystal and has been much used in studies of solid-state chemical reactions. The continuing success of this simple model rests upon the very high results/parameters ratio and on its ease of application, even to extensive molecular dynamics simulations. The atom–atom potential method is still a vital tool in modern molecular simulation and will remain so for a long time. All atom–atom pair potential curves share some characteristic common features (Figure 1). The deeper the minimum, the sharper the curvature at the turning point; the anharmonicity of the curve is such that when an interatomic distance is decreased below its equilibrium value, the energy rises more steeply than when the distance is increased. For distances slightly smaller than the turning point (Req) the interaction term may be stabilizing, i.e. the potential energy is still negative, but the interatomic force is repulsive, not attractive. In descriptions of molecular packing in crystal structures, the emphasis is usually on short intermolecular distances or ‘‘contacts.’’ Insofar as these typically correspond to local repulsive forces between the atoms concerned, and the system is in equilibrium (no net force), the repulsive forces must be balanced by attractive forces arising from interactions between more distant atom pairs in the different molecules. Thus, in attempts to describe and systematize intermolecular interactions in crystals, the usual, natural preoccupation with short interatomic contacts as the main structural glue may be misleading. Since hydrogen atoms are usually the ones on the peripheries of organic molecules, they are the ones that are typically invoked in intermolecular contacts. Indeed, C–H. . .X contacts in organic crystals are almost unavoidable. Uncritical interpretations of the significance of such C–H. . .X contacts

Figure 1

A typical interatomic model potential as a function of interatomic distance (A˚). For the usual atoms of organic chemistry, one energy unit in the graph may be of the order of 0.5
1 kJ mol
1.

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can lead to questionable conclusions, for example, that they necessarily correspond to structurally significant although weak intermolecular hydrogen bonds. Undoubtedly, O–H. . .O and N–H. . .O interactions can correspond to very strong hydrogen bonds and play an important if not decisive role in stabilizing structures in which they occur. However, the extrapolation to weaker and weaker types of interaction needs to be done with much more care than is often allotted to the task. If you search almost any organic crystal structure for C–H. . .X contacts (where X naturally depends on which other elements are present on the molecular periphery), then you are likely to find them. Experimental support for specific bonding atom–atom interactions between neighboring molecules has been inferred on the basis of intermolecular bondpaths (in the Bader sense) observed in electron-density distributions in crystals. The matter has been carefully analyzed by Gatti.27 The question is whether such bond paths arise as the result of intermolecular bonding interactions or merely from residual overlapping of charge clouds of atoms in neighboring molecules. For strong intermolecular O–H. . .O and N–H. . .O hydrogen bonds the former may well be the case. But even in the case of such hydrogen bonds, Spackman28 has shown that to a good approximation experimental bond paths are close to those produced by simple addition of non-interacting overlapping spherical atom electron densities. Experimental bond paths have reportedly been observed for intermolecular H. . .H interactions between phenyl groups in crystalline salts of tetraphenylborates.29 We do not believe that such observations should be regarded as evidence for specific atom–atom interactions in crystals. More likely, they arise as a result of the inevitable H. . .H contacts that arise between phenyl groups of different molecules in crystals of tetraphenyl compounds. Notwithstanding these more philosophical issues, the atom–atom potential method has been and is still one of the pillars of computational molecular simulation. Its power was appreciated by John Meurig Thomas quite early in his career during the period when he was interested in solid–solid phase transformations and photochemical reactions in organic crystals. In particular, this approach was used by Thomas and his collaborators to derive molecular packings and lattice energies of organic crystals from incomplete and sometimes fragmentary experimental data. We take here two examples out of many. On cooling crystals of pyrene to below 120 K they often shatter. From electron diffraction patterns of small regions of cooled crystals the unit cell dimensions of the low-temperature phase (pyrene II) could be measured and, in the absence of reliable diffraction intensities, its structure was derived through the use of atom–atom potential calculations.30 Many years later, the essential correctness of the proposed structure was confirmed from neutron powder diffraction data from a deuterated sample at 4.2 K.31 Then there was the question of the structure and dynamics of a new, triclinic phase of anthracene (II), produced by shearing stress at ambient temperature. As in the pyrene example, the existence of the new phase was detected from the electron diffraction pattern of a tiny crystallite containing both transformed and

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untransformed regions. In the absence of diffraction intensities, the crystal structure of the new form (triclinic, P1, Z ¼ 2) was derived by the atom–atom potential method.32 In this case, as a notable extension of the method, lattice dynamical calculations were used to compute the infrared and Raman active vibrations of both crystal modifications. The atom–atom calculations suggested that other anthracene structures, comparable in energy with the stable anthracene I, can be generated from the stable structure by motion along slip planes. Indeed, evidence for another metastable anthracene phase formed by vapour growth has been adduced from X-ray diffraction measurements33 but it is not the same as the phase generated by shear. In any case, the evidence for the new polymorph is questionable.34 In spite of the popularity and strengths of the atom–atom approach there are obvious weaknesses. The constants A, B, C, . . ., as well as the ‘‘atomic charges’’ qi, need to be assigned individually for each kind of atom pair, and this cannot be done by theoretical considerations. The adherence of the mathematical model to physical reality is intrinsically weak, and the required parameters must be obtained by the ad hoc expedient of fitting to a large amount of experimental data, mainly statistical data on atom–atom distances of closest contact between molecules in available crystal structures and on heats of sublimation.35 There are no constraints on the values of these constants as long as they properly carry out their job. This manner of fitting a model to experimental data has its dangers, especially that of encouraging belief in the physical reality of the model, even when it is has no underlying theoretical basis. Thus, although there have been attempts to recover the physics of the interaction a posteriori by identifying the various terms in the analytical power expansion with real effects in terms of basic phenomena, this can only be misleading. For example, although the exponential term behaves as a repulsion energy, it has no counterpart in terms of fundamental theories. Similarly, the R
6 term behaves like the leading term of dispersion energy, but the proportionality constant is just a black-box number. Even the R
1 term, formally representing the Coulombic interaction between atoms is far from giving a faithful reproduction of the actual Coulombic interaction between molecular charge distributions. Very wisely, Kitaigorodski himself expressed the opinion that ‘‘. . . the atom–atom potential method represents a variational treatment using . . . A’s, B’s and C’s as variational parameters. Clearly, there is no reason to attach any physical meaning to the parameters . . . there is no reason to regard the sum of the
AR
6 terms as the dispersion energy . . . and the sum of qq 0 r
1 terms as the electrostatic energy’’.36 The atom–atom potential method is an expert system but it has no basis in fundamental physics.

2.2

Distributed Charge Methods

The assumptions embedded in the atom–atom approach to intermolecular energies are indeed far from physical reality; for example, negative charges

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may be placed at the positions of atomic nuclei! In more elaborate descriptions, the charge distribution is modeled by a set of distributed multipoles, designed to fit the electrostatic field of the molecule as computed by some quantummechanical procedure. The Coulombic contributions to the intermolecular potential energy can then be calculated as sums of multipole–multipole interaction terms; the missing dispersion, polarization and repulsion terms must be evaluated by some additional procedures. This approach, originally developed by Anthony Stone,37 has been extensively exploited for molecular crystals by Sally Price and co-workers.38 Molecular charge distributions from good quality wave functions can of course be obtained from ab initio molecular orbital calculations, or even from electron density studies in accurate X-ray diffraction analyses at very low temperature. These charge distributions can then be expanded using a linear combination of terms including radial functions and spherical harmonics. Intermolecular Coulombic energies between two approaching charge distributions can then be calculated by direct integration.39,40 These energies are as accurate as the wave function is.

2.3

Penetration Energy

An important limitation of models involving localized point charges or distributed multipoles to represent the charge density distribution around the atomic nuclei in a molecule is the neglect of penetration energy. When electron densities of neighbouring molecules overlap there is a destabilizing Coulombic energy contribution from electron–electron repulsion but there is also a stabilizing contribution from the interaction between the electron density of each molecule with the positive nuclear charges of the other. The balance between these two opposite Coulombic energy contributions depends on fine details of the overlap. For the overlap between charge distributions of neighbouring molecules at normal intermolecular separations, the energy balance usually corresponds to a small overall stabilization. A simple example may demonstrate the dramatic difference between energies derived from a point-charge model and from interactions between delocalized electron densities. Figure 2a shows the Coulombic energy for the approach of two pseudoatoms each with nuclear charge +7.0 and electron charge –7.2. In the point-charge model, the electron charges are assumed to be localized at the nuclei, to yield two localized net charges of
0.2. The result is, as expected, repulsion at all distances; negative charges repel one another. In the delocalized model, each electron charge of
7.2 is spread over 1782 points in accordance with an exponential decrease with distance out to 2.6 A˚ from each nucleus. The interaction energy is identical to that of the point-charge model at large internuclear separation. However, in contrast to the point-charge model, the interaction energy goes through a turning point and at shorter internuclear separation it becomes strongly stabilizing. Figure 2b shows the corresponding energies when one of the atoms has net charge
0.2 (+7.0,
7.2), as before,

295

Molecular Cohesion and the Structure of Organic Crystals a)

b)

30

10

20

Coulombic energy, kJ/mol

Coulombic energy, kJ/mol

25

15 10 5 0 point-charge integration

-5 -10

0

-10

-20 point-charge integration

-30

-15 -40

-20 2

Figure 2

3

4 5 distance

6

2

3

4 5 distance

6

(a) Coulombic energy (kJ mol
1) of two pseudoatoms each with nuclear charge +7 and electron charge
7.2, approaching at a variable distance. Squares: point-charge energy for a net charge of
0.2 at the nuclear location; triangles: negative charge distributed over 1782 points around the nucleus. (b) As before, but one atom with electron charge
7.2, the other with electron charge
6.8, ie opposite net charges.

and the other has opposite net charge +0.2 (+7.0,
6.8). Here the pointcharge model gives, as expected, stabilization at all distances, while the delocalized model produces a more complex behaviour. Here the interaction energy is stabilizing at about 6 A˚ internuclear distance, but the force becomes slightly repulsive at internuclear separation between about 5 and 3 A˚, then strongly attractive in the region 3 to 2 A˚. Although this is only a computational experiment on pseudoatoms, what a difference! It is clear that inferences based on localized charge models may be right or wrong, depending on features of the outer regions of the molecular electron densities, which play no part in pointcharge or distributed multipole models. We should then not be entirely surprised that theories of molecular packing based on point-charge models have led to pictures of attractive chlorine– chlorine interactions (the so-called ‘‘chloro effect’’) and also of repulsive halogen–halogen interactions. In another extreme example, the Coulombic energy of the hexachlorobenzene crystal is calculated by the point-charge model with a charge of +0.1 on carbon to be destabilizing (+18 kJ mol
1), but it is correctly calculated to be stabilizing (
40 kJ mol
1) by the delocalized Pixel model described below. The reason why the atom–atom method has been so successful is that such gross deficiencies of the point-charge Coulombic model are damped if not entirely corrected by the parameterization of the other terms in the potential; two errors may cancel, but the real physics of the situation becomes obscure.

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2.4

Chapter 18

The Pixel Method

Nowadays, with computers that are able to carry out calculations at the teraflop per second level, the time has come to move to fully delocalized models of the electron density even in semi-empirical methods. The SCDS (semi-classical density sums) or Pixel approach41 uses an accurate electron density for the individual molecules and then estimates the interaction energy due to reorganization of the electron density in the extended system by introducing separate polarization, dispersion, charge transfer and repulsion terms. The electron density of the molecule is obtained by some standard quantum-mechanical calculation and sampled on a grid containing about 106 pixels, and is then contracted into super-pixels each containing n  n  n original steps, where n is typically 3, 4 or 5. Pixels containing less than some minimum charge of about 10
6 electrons are discarded as insignificant, and the pixel contents are renormalized to balance the sum of the nuclear charges. In this way, the molecular density ends up being described by some 10,000 pixels. Clusters of molecules or crystals are then built by simple juxtaposition of the Pixel electron densities of individual molecules. The Coulombic energy of the system (ECOUL) is calculated by direct summation over pixel–pixel, pixel–nucleus, and nucleus–nucleus Coulomb interactions: this is the change in Coulomb energy that occurs on forming the molecular cluster from the molecules at infinite separation. If there were no reorganization of the molecular charge distribution on going from the isolated molecules to a molecular cluster, the Coulomb energy calculated in this way would be entirely correct. However, on passing from the isolated molecule to a cluster of molecules, the molecular charge distributions of the individual isolated molecules are slightly changed by their interactions with the charge distributions of other molecules. This charge reorganization requires the introduction of correction terms in the model, terms which cannot be calculated in a rigorous manner but can be estimated with good accuracy with the help of a few reasonable assumptions based on physical principles. They are the polarization energy (EPOL), the dispersion energy (EDISP) and the repulsion energy (EREP). This partitioning of the intermolecular energy is semi-empirical and hence to some extent arbitrary. However, the separate terms can be calibrated for some representative systems by comparison with more accurate but more time-consuming methods, such as Stone’s IMPT (Inter-Molecular Perturbation Theory).42 Although this way of partitioning the interaction energy is not rigorous, it corresponds to well established concepts in structural chemistry and the results turn out to agree largely with chemical expectations. For example, polarization terms (EPOL) turn out to be important in hydrogen-bonded systems where they compensate, so to speak, for neglect of the large reorganization of the molecular density distributions in such systems. Dispersion energies (EDISP) are large in contacts among hydrocarbon molecules, especially aromatic ones, and are relatively less important in chemical environments where contacts between atoms of different electronegativity occur. Obviously there is no possibility for allowing antisymmetrization of the wave function in this simple repetition of molecular charge densities. One effect of

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antisymmetrization would be to remove charge density from overlapping regions, thus increasing the nucleus–nucleus repulsion. The Pixel repulsion term is calibrated as roughly proportional to the overlap integral between individual molecular densities, as a partial replacement for the missing effects of Pauli exclusion. In spite of its many approximations and assumptions, Pixel is able to reproduce the sublimation enthalpies of organic crystals43 and mimics the ab initio results for molecular clusters with considerable accuracy44 at a small fraction of the computational cost. For example, one can run the lattice energy of a molecule such as anthraquinone in about 10 min on an ordinary PC, and parallelization even makes it possible to optimize lattice energies with respect to lattice parameters and molecular orientation.

2.5

The Ab initio Approach

In principle, if we had a powerful enough computer, we could calculate the lattice energy of any given crystal structure from quantum mechanics. In principle! Take benzene as an example. First we would calculate the energy of a benzene molecule at some suitable ab initio level (say MP2/6**g(d,p) – it is not necessary for the non-specialist to know exactly what these symbols mean), making sure that we had reached the energy minimum in the multiparameter space defining the nuclear coordinates. This calculation would yield a numerical value of the energy, around
231.518 a.u. (atomic units), equivalent to
607836.6 kJ mol
1. This is not the energy of formation of a benzene molecule from six carbon and six hydrogen atoms in their ground states; it is the energy of formation of a benzene molecule from six nuclei of charge +6, six nuclei of charge +1, and 42 electrons at infinite separation. The energy of a benzene dimer, calculated in the same way, with minor modifications to take care of technical problems, would come out at around
463.041 a.u., very nearly twice the above value. The small difference:
463.041
2(
231.518) ¼ –0.005 a.u. or
13 kJ mol
1 would be the calculated energy of the benzene dimer with respect to two separated benzene molecules, i.e., the cohesive energy of the benzene dimer (at 0 K without zero-point-energy, inclusion of which would only complicate the argument without adding anything of importance). The calculation yields this number and this number alone, so there is no point in arguing about to what extent the small stabilization of the dimer with respect to two monomers arises from Coulombic interactions between the separate charge distributions or from polarization or from dispersion or from anything else. At the fundamental ab initio level these would be meaningless questions. What applies to the dimer would apply equally to an assembly of benzene molecules, and in particular to the particular assembly that corresponds to the benzene crystal. As far as ab initio methods are concerned, the benzene crystal is just about the limit of what is practicable today.45 For assemblies of much larger molecules, ab initio methods are still on the horizon: some attempts at simulating the dispersion energy terms have appeared.46

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In terms of the basic physics, the only energy terms that appear in the ab initio calculation are Coulombic: the Coulombic repulsion between the nuclei and between the electrons, and the Coulombic attraction between the nuclei and the electrons. According to the Hellman–Feynman theorem, the force on a nucleus can be calculated from the charge distribution as if it were classical, i.e. by using Coulomb’s law. If the exact wave function were known for any molecule or assembly of molecules in its equilibrium configuration, the bonding could then be analyzed in purely Coulombic terms. Thus, in principle, if the correct electron density were available, a purely Coulombic calculation would suffice to yield the correct energy. In practice, such a calculation is not practicable today for an extended system such as a crystal of moderate complexity. The difficulty is not that Coulomb’s law breaks down; it is that we do not have the correct electron density. Allowance for the reorganization of the electron density on going from the isolated molecule to the dimer and to larger molecular assemblies would require very large basis sets and extensive optimization of the many variables needed to describe the systems. At present we cannot do this for molecular crystals of even modest complexity. Here the newly developed Pixel method provides a compromise that offers sufficient accuracy for most purposes with the additional bonus of (or at the price of, depending on one’s outlook) introducing separate polarization, dispersion, charge transfer and repulsion terms.

3 A Future Challenge Prediction is a risky business, especially when it concerns events whose outcome is still uncertain. Crystal structure prediction (CSP) is an exercise in which unknown crystal structures of organic molecules are deduced (or guessed) from the information expressed by molecular connectivity alone. We have at our disposal a vast library of known crystal structures of organic and organometallic compounds in the Cambridge structural database (CSD, with almost 400,000 entries as of January 2007).47 Information from this source, together with all too scarce thermodynamic data48 has been utilized in the construction and parameterization of atom–atom force fields34 that have been supplemented, more recently and to a limited extent, by the use of first principles quantum-chemical calculations. All these have provided a consistent and robust body of knowledge for the understanding of known crystal structures and for the computer simulation of organic condensed phases.49 The next development must involve the transition from reassuring post-diction towards reliable prediction of organic crystal structures and energies. The accomplishment of this task is, however, confronted with numerous practical and conceptual obstacles, as has been demonstrated by the results of recent objective tests.50 The problem is not that of generating a sufficiently large library of possible periodic arrangements; typically, for any given molecule, a large collection of computational crystal structures can be produced without difficulty, often in a matter of minutes with modern computers. Usually, the

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experimental structures are to be found among the computational ones, but not necessarily among those with the most stabilizing lattice energy. There are several possible reasons for this. Energy differences between crystal polymorphs are extremely small, a matter of a few kilojoules per mole. The same holds for different possible crystal structures of a given molecule. Typically, a computational search produces many different periodic arrangements within a narrow window of lattice energies, with unknown energy barriers between them. Besides, since entropic factors are difficult to estimate accurately in computations, the ordering of free energies at normal temperature and pressure becomes even more uncertain. To complicate matters further, the crystallization process is under kinetic rather than thermodynamic control. Relative rates of nucleation and growth of different crystal phases determine which crystal phase will be produced under the conditions of crystallization. At present, it is virtually impossible to properly account for the dynamics of crystallization processes involving solvent–solute interactions. Once formed, a thermodynamically unstable crystal form can indeed, in principle, transform to a thermodynamically more stable form, but solid–solid phase transitions tend to be slow. A metastable crystal form can persist for a long time, indeed practically forever. Thus, the experimentally found crystal structures of a given compound need not be those of minimal free energy. Besides these problems, the force fields employed in the computations may not be sufficiently accurate, and the search for possible periodic molecular arrangements may not be sufficiently exhaustive. How successful can one expect computational methods to be that essentially neglect temperature and time? At present, success in crystal structure prediction is very limited, even for rigid molecules, and it is virtually zero for flexible molecules, where conformational adjustment is coupled to intermolecular aggregation requirements. However, as is usual in science, work is in progress and new developments can be expected.51–53

References 1. I. Newton, Opticks, Dover, New York, 1952. (Based on the Fourth Edition, London, 1730, p. 394.). 2. G. Brugnatelli, Trattato Delle Cose Naturali E Dei Loro Ordini Conservatori, Pavia, Tipografia Tizzoni, 1837, 1, 34. 3. W. Barlow and W.J. Pope, J. Chem. Soc. Trans., 1906, 89, 1675. 4. A. Einstein, Ann. Phys., 1901, 309, 513. 5. K.J. Miller, J. Am. Chem. Soc., 1990, 112, 8533. 6. H.E. Armstrong, Nature (London), 1927, 120, 478. 7. L. Pauling, J. Am. Chem. Soc., 1928, 51, 1010. 8. W.L. Bragg, Atomic Structure of Minerals, Cornell University Press, Ithaca, New York, 1937. 9. W.H. Bragg, Proc. Phys. Soc., 1921, 34, 33. 10. R.G. Dickinson and A.L. Raymond, J. Am. Chem. Soc., 1923, 45, 22. 11. R.G. Dickinson and C. Bilicke, J. Am. Chem. Soc., 1928, 50, 764.

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12. F. London, Trans. Faraday Soc., 1937, 33, 8. 13. A.I. Kitaigorodski, Org. Kristallokhimiya, Translated from Russian in Organic Chemical Crystallography, Consultants Bureau, New York, 1961; A.I. Kitaigorodski, Molecular Crystals and Molecules, Academic Press, New York, 1973. 14. F.H. Westheimer, J. Chem. Phys., 1947, 15, 252. 15. T.L. Hill, J. Chem. Phys., 1948, 16, 938. 16. J.D. Dunitz and V.J. Schomaker, Chem. Phys., 1952, 20, 1703. 17. J.B. Hendrickson, J. Am. Chem. Soc., 1961, 83, 4537. 18. K.B. Wiberg, J. Am. Chem. Soc., 1965, 87, 1070. 19. E.J. Jacob, H.B. Thompson and L.S. Bartell, J. Chem. Phys., 1967, 47, 3736. 20. S. Lifson and A. Warshel, J. Chem. Phys., 1968, 49, 5116. 21. N.L. Allinger, M.T. Tribble, M.A. Miller and D.H. Wertz, J. Am. Chem. Soc., 1971, 93, 1637. 22. R.A. Scott and H.A. Scheraga, J. Chem. Phys., 1966, 44, 3054. 23. E.M. Engler, J.D. Andose and P.V.R. Schleyer, J. Am. Chem. Soc., 1973, 95, 8005. 24. A.I. Kitaigorodski, in Advances in Structure Research by Diffraction Methods, vol 3, R. Brill and R. Mason (eds), Pergamon Press, Oxford, 1970, 173. 25. D.E. Williams and T.L. Starr, Comput. Chem., 1977, 1, 173; L.-Y. Hsu and D.E. Williams, Acta Crystallogr., 1980, A36, 277; S.R. Cox, L.-Y. Hsu and D.E. Williams, Acta Crystallogr., 1981, A37, 293; D.E. Williams and S.R. Cox, Acta Crystallogr., 1984, B40, 404; D.E. Williams, J. Comput. Chem., 2001, 22, 1154. 26. A. Gavezzotti, OPEC, Organic Packing Energy Calculations, University of Milano, 1973–1997. See also A. Gavezzotti and M. Simonetta, Chem. Rev., 1982, 82, 1. 27. C. Gatti, Z. Kristallogr., 2005, 22, 399. 28. M.A. Spackman, Chem. Phys. Lett., 1999, 301, 425. 29. C.F. Matta, J. Hernandez-Trujillo, T.-H. Tang and R.F.W. Bader, Chem.– Eur. J., 2003, 9, 1940. 30. W. Jones, S. Ramdas and J.M. Thomas, Chem. Phys. Lett., 1978, 54, 490. 31. K.S. Knight, K. Shankland, W.I.F. David, N. Shankland and S.W. Love, Chem. Phys. Lett., 1996, 257, 490. 32. C.M. Gramaccioli, G. Filippini, M. Simonetta, S. Ramdas, G.M. Parkinson and J.M. Thomas, J.C.S. Faraday II, 1980, 76, 1336. 33. B. Marciniak and V. Pavlyuk, Mol. Cryst. Liq. Cryst., 2002, 373, 237. 34. J. van der Streek and S. Motherwell, Acta Crystallogr., 2005, B61, 504. 35. A. Gavezzotti and G. Filippini, J. Phys. Chem., 1994, 98, 4831. 36. A.J. Pertsin and A.I. Kitaigorodski, The Atom–Atom Potential Method, Springer-Verlag, Berlin, 1987, Chapter 3. 37. A.J. Stone, Chem. Phys. Lett., 1981, 83, 233.

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38. D.J. Willock, S.L. Price, M. Leslie and C.R.A. Catlow, J. Comput. Chem., 1995, 16, 628; S.L. Price, J. Chem. Soc., Faraday Trans., 1996, 92, 2997; T. Beyer, G.M. Day and S.L. Price, J. Am. Chem. Soc., 2001, 123, 5086. 39. T.S. Koritsanszky and P. Coppens, Chem. Rev., 2001, 101, 1583. 40. A. Volkov and P. Coppens, J. Comput. Chem., 2004, 25, 921. 41. A. Gavezzotti, J. Phys. Chem., 2003, B107, 2344. 42. A.J. Stone, The Theory of Intermolecular Forces, Clarendon Press, Oxford, 1996 (reprinted with corrections, 2000). 43. A. Gavezzotti, Z. Kristallogr., 2005, 220, 499. 44. A. Gavezzotti, J. Chem. Theor. Comput., 2005, 1, 834. 45. W.B. Schweizer and J.D. Dunitz, J. Chem. Theor. Comput., 2006, 2, 288. 46. R. Dovesi, M. Causa`, R. Orlando, C. Roetti and V.R. Saunders, J. Chem. Phys., 1990, 92, 7402. 47. Available through the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, England (www.ccdc.cam.ac.uk). 48. For a compilation of experimental sublimation enthalpies see: J.S. Chickos and W.E. Acree, J. Phys. Chem. Ref. Data, 2002, 31, 537. 49. A. Gavezzotti, Molecular Aggregation, Structure Analysis and Molecular Simulation of Crystals and Liquids, Oxford University Press, Oxford, 2007. 50. G.M. Day, W.D.S. Motherwell, H. Ammon, S.X.M. Boerrigter, R.G. Della Valle, E. Venuti, A. Dzyabchenko, J.D. Dunitz, B. Schweizer, B.P. van Eijck, P. Erk, J.C. Facelli, V.E. Bazterra, M.B. Ferraro, D.W.M Hofmann, F.J.J. Leusen, C. Liang, C.C. Pantelides, P.G. Karamertzanis, S.L. Price, T.C. Lewis, H. Nowell, A. Torrisi, H.A. Scheraga, Y.A. Arnautova, M.U. Schmidt and P. Verwer, Acta Crystallogr., 2005, B61, 511. 51. P. Raiteri, R. Martonak and M. Parrinello, Angew. Chem., Int. Ed., 2005, 44, 3769. 52. A.R. Oganov and C.W. Glass, J. Chem. Phys., 2006, 124, 244704. 53. M.A. Neumann and M.-A. Perrin, J. Phys. Chem., 2005, 109, 15531.

CHAPTER 19

Aperiodicity in Organic Materials KENNETH D. M. HARRIS School of Chemistry, Cardiff University, Park Place, Cardiff CF10 3AT, UK

1 Introduction The most significant turning point in my scientific career was without a doubt being given the opportunity (in 1985) to join the research group of Sir John Meurig Thomas at the University of Cambridge to study under his supervision for a PhD degree. Not only did this opportunity provide a springboard to transform childhood scientific dreams into reality, but one could not have wished for a more erudite, enthusiastic, encouraging and supportive supervisor at this formative stage of one’s entry into scientific research. During this time, every meeting with ‘‘JMT’’ (whether in his office, on a train between Cambridge and London, at lunch in the graduate centre, or joining him on a 2-minute walk while he posted a letter) was a source of new ideas and inspiration, and one was always left feeling scientifically enriched by the experience. His unbounded enthusiasm for science was inspirational, and his ability to provide wise guidance and perpetual encouragement were such that those working in his research group were continuously instilled with a tremendous feeling of optimism and excitement for their research. Furthermore, research discussions with him were not only occasions to learn from his seemingly colossal intellect and to progress with one’s research through deep contemplation and uninhibited debate, but they were also interesting (and often entertaining) occasions, punctuated as they typically were by relevant anecdotes (which, whenever the opportunity arose, reflected a passionately Celtic perspective), historical asides and his characteristic ability to blend seriousness and good humour. And just as impressive as the depth of his knowledge in his own field of specialization was the breadth of his knowledge across the full spectrum of scientific disciplines – thus, in the course of our discussions on the topics of my own research 302

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work, he succeeded in introducing me to fields as diverse as fractal geometry, molecular electronics, the structural biology of proteins and viruses, and the pioneering work of Ahmed Zewail in the development of femtosecond spectroscopy, to list just a few. In addition to the profound interest that he showed in our research work and the great motivational drive that he exerted in encouraging us to generate new knowledge and deeper understanding in our research, it was very clear that he also cared deeply about each person in his research group as an individual, and he took great interest in, and responsibility for, their education and scientific development. In every meeting with JMT, he was a continual source of new ideas, but he was also fully supportive in encouraging us to develop our own scientific thoughts and opinions. In this way, he cultivated in his research group a fertile breeding ground for new scientific ideas to germinate, and he provided the support, encouragement and resources to enable these ideas to be brought to full fruition. Certainly, I had the over-riding impression during my time in his research group that we were members of a team that was leading the way at the cutting edge of research in the field, and that we were being led in this important quest by the pre-eminent scientist in the field. It really was a privilege to have been given the opportunity to spend a period of time within this environment and it has been a continued privilege to maintain my scientific collaboration with him to the present day, and hopefully long into the future. A few months after beginning my PhD research, JMT suggested during one of our regular meetings that I should start some research on structural properties of urea inclusion compounds, in addition to the project (on solid state photodimerization reactions) that I was currently undertaking at that time. Mark Hollingsworth, who had just joined the research group as a postdoc studying photochemical reactions of organic molecules (diacyl peroxides) within zeolites, had been given similar encouragement to study the same reactions in urea inclusion compounds, and the aim was for Mark and myself to work in tandem on studies of photochemical and structural properties of these materials, respectively. In this way, a very fruitful collaboration and friendship was started, and I learned greatly from my interaction with Mark during this time. Such is the rich and diverse array of properties that has been found to be exhibited by urea inclusion compounds, and the seemingly endless opportunity to exploit these materials to gain new facets of fundamental understanding of the nature of the organic solid state, that we have both continued to carry out research on urea inclusion compounds ever since. Soon after recording our first X-ray diffraction photograph of a urea inclusion compound, it was clear that these materials exhibit some very interesting diffraction properties, which we could understand on the basis that they are incommensurate materials. Early thoughts on general aspects of the structural properties of these materials, which evolved through many discussions on this subject with JMT, are summarized in Ref. 1, while specific details relating to the diacyl peroxide/urea inclusion compounds were published subsequently.2 Another important collaboration during this time was with Andrew Rennie, a mathematics undergraduate in Cambridge who had written to JMT (in his

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position as Director of the Royal Institution) to enquire if he could carry out a summer research project there. I felt greatly honoured that JMT asked me to supervise Andrew in this project, which led to the development of our mathematical model of incommensurate versus commensurate behaviour in onedimensional inclusion compounds,3–5 which is described in more detail in Section 2.4. From these early beginnings started a long fascination with aperiodic crystalline materials (of which incommensurate solids are a subset) that has continued to the present day. Aperiodic crystals may be defined, in general terms, as materials that lack three-dimensional translational periodicity (and are thus distinct from conventional crystals), but yet have aspects of long-range order that give rise to sharp Bragg reflections in their X-ray diffraction patterns. This article is devoted to surveying two aspects of aperiodicity in organic materials. The first part of the article is focused on the concept of incommensurateness in one-dimensional inclusion compounds, and is illustrated mainly by examples from our research on urea inclusion compounds (covering work that ranges in time from my PhD studies with JMT to the present day). The second part of the article is focused on the concept of quasicrystalline materials, and particularly concerns the development of a strategy for the design of a quasicrystal based on discrete organic molecular building units. The aim of both parts is to raise and discuss key concepts within these themes, rather than to provide a comprehensive overview of the field.

2 Incommensurate Materials 2.1

Introduction to the Structural Classification of Commensurate/Incommensurate Inclusion Compounds

From the chemical viewpoint, solid inclusion compounds are composed of two chemically distinguishable substructures: the host and guest substructures. In many cases, the guest molecules are disordered within the host structure, but when the guest molecules are ordered (at least sufficiently well ordered that an average lattice periodicity can be defined), an important concept is the degree of structural registry between the two periodic (host and guest) substructures. Here we focus only on the simplest case in which the host substructure is a periodic one-dimensional tunnel, within which the guest molecules are ordered with a well-defined periodicity along the tunnel axis (c-axis) (see Figure 1). The periodic repeat distance of the guest molecules along the tunnel is denoted cg and the periodic repeat distance of the host substructure along the tunnel is denoted ch. From the structural viewpoint, the degree of registry between these two periodic substructures is assessed by considering the relationship between cg and ch (we shall discuss later some physical properties of inclusion compounds that depend on the degree of structural registry between the host and guest substructures). Conventionally, the ratio cg/ch is used to subdivide these

305

Aperiodicity in Organic Materials ch

ch

cg

Figure 1

ch

cg

ch

cg

ch

ch

cg

ch

cg

ch

cg

ch

ch

cg

Schematic representation of a tunnel inclusion compound viewed perpendicular to the tunnel axis. The definitions of cg and ch are shown.

materials into two categories: commensurate and incommensurate. In the classical definition, the inclusion compound is assigned as commensurate if cg/ch is a rational number and incommensurate if cg/ch is an irrational number. However, since experimental measurements of cg and ch can never be made with infinitely high precision, a more practical definition is to assign an inclusion compound as commensurate if the ratio cg/ch is sufficiently close to a rational number with low denominator. Thus, for a commensurate system, sufficiently small integers p and q can be found such that pch E qcg, whereas if no sufficiently small integers p and q can be found to satisfy this relationship, the inclusion compound is incommensurate. The values of cg and ch can be determined from appropriate diffraction studies. For example (Figure 2), in the case of an incommensurate system, a single crystal X-ray diffraction rotation photograph recorded for the crystal rotating about the tunnel axis shows separate sets of layer lines from the host and guest substructures, and the values of cg and ch can be determined from the spacing of the layer lines in each set. As evident from Figure 2, the zero layer line is common to both substructures, but there is no coincidence of any other higher-order layer lines from the host and guest substructures. For a commensurate inclusion compound on the other hand, separate layer lines due to the host and guest substructures cannot be distinguished, and all layer lines are described by a single common periodicity along the c-axis. We now consider in more detail some consequences of the incommensurate/ commensurate nature of a tunnel inclusion compound in terms of diffraction and structural properties, focusing in particular on the case of urea inclusion compounds. Subsequently, we discuss the implications in terms of energetic and vibrational properties.

2.2

Introduction to Urea Inclusion Compounds

As all the examples discussed in this part of the article concern urea inclusion compounds,6–10 it is relevant to give a brief introduction to these materials here. In ‘‘conventional’’ urea inclusion compounds, the host structure1,11 is constructed from a hydrogen-bonded arrangement of urea molecules, within which there are linear, parallel tunnels (Figure 3). The diameter of the urea tunnel varies between ca. 5.5 A˚ and 5.8 A˚ as a function of position along the tunnel,12 and is suitable for

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(H, K, 3, 0) (H, K, 0, 4) (H, K, 0, 3) (H, K, 2, 0) (H, K, 0, 2) (H, K, 1, 0) (H, K, 0, 1) (H, K, 0, 0)

Figure 2

Single crystal X-ray diffraction rotation photograph for an incommensurate tunnel inclusion compound (the 1,9-diiodononane/urea inclusion compound), recorded with the single crystal rotating about its tunnel axis. The layer lines (horizontal) from the host component are indicated by red arrows and the layer lines from the guest component are indicated by yellow arrows. In this case, the guest layer lines contain both discrete scattering (sharp spots) and diffuse scattering. The fact that separate sets of layer lines are observed for the host and guest components is a consequence of the incommensurate relationship between ch and cg. Indexing of the layer lines is shown on the right hand side (see Section 2.3 for a definition of the Miller indices (H, K, L, M)).

accommodating guest molecules based on a sufficiently long n-alkane chain, with only limited substitution of the n-alkane chain permitted. The urea tunnel structure is stable only when the tunnels are filled with a dense packing of guest molecules (removal of guest molecules from urea inclusion compounds leads to the instantaneous collapse of the ‘‘empty’’ tunnel structure to form a structure of higher density – the well-known crystal structure of ‘‘pure’’ urea, which does not contain empty tunnels). A wide range of different types of guest molecules have been shown to form urea inclusion compounds, and the vast majority of these inclusion compounds have the same urea host structure at ambient temperature. Such cases are called ‘‘conventional’’ urea inclusion compounds and are characterized by: (i) a hexagonal host tunnel structure (space group P6122 or P6522), (ii) an incommensurate relationship between the periodicities of the host and guest substructures along the tunnel axis (Figure 2), and (iii) substantial dynamic disorder (reorientation about the tunnel axis) of the guest molecules at ambient temperature. In most cases, order–disorder phase transitions occur at sufficiently low temperature, and are associated with a distortion of the host tunnel (to a lower symmetry than hexagonal) and a concomitant decrease in the reorientational motion of the guest molecules. A wide range of fundamental physicochemical properties are found to be exhibited by urea inclusion compounds, and

Aperiodicity in Organic Materials

Figure 3

307

Structure of the hexadecane/urea inclusion compound at ambient temperature, showing nine complete tunnels (with van der Waals radii) viewed along the tunnel axis. The guest molecules have been inserted into the tunnels illustrating orientational disorder (which is known, from X-ray diffraction data and spectroscopic investigations, to exist at ambient temperature).

there has been much interest (by several research groups) in understanding the incommensurate structural properties,1–3,13–15 the order–disorder phase transitions,16–19 dynamic properties (particularly concerning molecular motion of the guest molecules),20–25 properties relating to one-dimensional confinement,26–29 host–guest chiral recognition,30–33 chemical reactions,34 the control of crystal morphology,35,36 and ferroelastic properties.37,38

2.3

Diffraction Properties and Structural Aspects

The diffraction properties and structural properties of a commensurate inclusion compound are similar to those of a conventional crystal. The periodicities of both the host and guest molecules are described by a common threedimensionally periodic lattice and the symmetry of the structure is described by a conventional three-dimensional space group. All diffraction maxima in the diffraction pattern are described by a single three-dimensionally periodic reciprocal lattice, and are indexed by three integer Miller indices (H, K, L). Failure to be able to account for the positions of all maxima in the diffraction pattern of an inclusion compound by a single three-dimensionally periodic reciprocal lattice (or the need to employ non-integer Miller indices) often provides the first indication that the structure may be incommensurate.

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For a tunnel inclusion compound in which the host and guest substructures are incommensurate along the tunnel (c-axis), the two substructures are usually commensurate in other directions by virtue of the fact that the guest molecules are constrained to occupy the host tunnels. Thus, when the structure is projected on to a plane perpendicular to the tunnel axis, there is a commensurate relationship between the host and guest substructures, and they thus have a common a*b* reciprocal lattice plane. Each substructure in the inclusion compound can be considered in terms of an incommensurately modulated ‘‘basic structure’’. The basic structure has threedimensional lattice periodicity and has crystallographic symmetry described by a three-dimensional space group. The incommensurate modulation to a given basic structure represents the structural perturbations that arise from its interaction with the other substructure. Thus, the basic structure itself is a hypothetical entity, and represents the structure to which the real substructure would relax if there were no interaction with the other substructure (i.e. in the absence of host–guest interaction). Clearly, the incommensurate modulations in one subsystem have the same periodicity as the basic structure of the other subsystem, and the incommensurate inclusion compound can be considered as an intergrowth of two incommensurately modulated substructures. To illustrate the concept of an incommensurate modulation, Figure 4 shows a schematic case of a displacive modulation in a one-dimensional array of atoms (which could, for example, represent the guest substructure in a tunnel inclusion compound). The period of the basic structure is denoted cb and the period of the modulation is denoted cm. Clearly the modulation is incommensurate if the ratio cb/cm is irrational.

(a) z cb

(b) d(z)

cm

z

(c) z

Figure 4

Schematic illustration of a one-dimensional structure (an array of atoms) containing an incommensurate displacive modulation: (a) the one-dimensional basic structure with periodicity cb, (b) the modulation function, which in this example is a sinusoidal wave with wavelengh cm (the value of d(z) indicates the displacement along z of an atom located at position z in the basic structure), and (c) the actual modulated structure (the modulation is incommensurate if cm/cb is irrational).

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Aperiodicity in Organic Materials 1,7,13,14

Returning to urea inclusion compounds, the lattice describing the periodicity of the basic host structure is {ah, bh, ch} and the corresponding reciprocal lattice is {ah*, bh*, ch*}. The lattice describing the periodicity of the basic guest structure is {ag, bg, cg} and the corresponding reciprocal lattice is {ag*, bg*, cg*}. The vectors ch and cg are parallel to each other and are directed along the tunnel axis. As a consequence of the incommensurate relationship between ch and cg, the complete diffraction pattern from the inclusion compound cannot be rationalized on the basis of a single three-dimensionally periodic reciprocal lattice. Thus, it is not possible to express the positions (S*) of all diffraction maxima within the diffraction pattern in terms of a linear combination S
¼ Ha
1 þ Ka
2 þ La
3 of three reciprocal lattice vectors {a1*, a2*, a3*} with integer coefficients H, K and L. For an incommensurate tunnel inclusion compound, one additional reciprocal lattice vector (a4*) is required in order to have integer indexing of all maxima in the diffraction pattern. Thus, S
¼ Ha
1 þ Ka
2 þ La
3 þ Ma
4 with integer coefficients H, K, L and M. In practice, one possible choice for the set of reciprocal lattice vectors {a1*, * a2 , a3*, a4*} for a urea inclusion compound is {ah*, bh*, ch*, cg*}, recalling that the host and guest substructures have a common a*b* reciprocal lattice plane, and hence ah* ¼ ag* and bh* ¼ bg*. The diffraction maxima, which are indexed by the four integer Miller indices (H, K, L, M), can be subdivided as follows: (i) M ¼ 0: ‘‘main reflections’’ from the host substructure, which primarily contain information on the basic host structure, but also contain information on the incommensurate modulations within the guest substructure (note that these modulations have the same periodicity as the basic host structure). (ii) L ¼ 0: ‘‘main reflections’’ from the guest substructure, which primarily contain information on the basic guest structure, but also contain information on the incommensurate modulations within the host substructure (note that these modulations have the same periodicity as the basic guest structure). (iii) L a 0 and M a 0: ‘‘satellite reflections’’ that arise due to the intermodulations of the two substructures. Clearly the (H, K, 0, 0) reflections are a superposition of main reflections from both substructures, representing the common a*b* reciprocal lattice plane discussed above. We note that the satellite reflections with L a 0 and M a 0 are typically very weak in comparison with the other types of reflections in the diffraction pattern. A normal powder X-ray diffraction pattern of a urea inclusion compound (Figure 5) shows no discernible evidence for these satellite reflections (note that reflections of the type (H, K, 0, M) are also significantly weaker than

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Chapter 19

Intensity (a.u.)

m

m h h g

m

h gg h

h

h h

10

Figure 5

20

30

40

2θ

Powder X-ray diffraction pattern of the 1,8-dibromooctane/urea inclusion compound. Peaks labelled ‘‘m’’ are of the type (H, K, 0, 0), peaks labelled ‘‘h’’ are of the type (H, K, L, 0), and peaks labelled ‘‘g’’ are of the type (H, K, 0, M).

those of the type (H, K, L, 0), in part as a consequence of the dynamics20–25 of the guest molecules). To observe the reflections with L a 0 and M a 0, and thus to provide direct evidence for the incommensurate inter-modulation of the host and guest substructures, one-dimensional scans through reciprocal space have been recorded for a single crystal of the 1,10-dibromodecane/urea inclusion compound using synchrotron X-ray radiation (on Station 16.3 at Daresbury Laboratory). A typical scan of this type is shown in Figure 6, and was carried out as a function of L for fixed H and K, with the specific H and K chosen such that the scan passed through main reflections (H, K, 0, M) of the guest substructure but did not pass through main reflections (H, K, L, 0) of the host substructure (note that although the host and guest substructures are commensurate in projection onto the plane perpendicular to the tunnel axis, the guest substructure in the 1,10-dibromodecane/urea inclusion compound39 is a superstructure of the host substructure in this plane). This scan reveals direct evidence for the existence of satellite reflections with L a 0 and M a 0, which, as expected, are substantially weaker than the other types of reflections in the diffraction pattern. Given that four reciprocal lattice vectors are required to describe the positions of all maxima in the diffraction pattern, we now consider how this situation transforms to describe the real structure in direct space. For urea inclusion compounds, the structural periodicity in direct space requires four lattice vectors and the symmetry of the composite inclusion compound can be described using a four-dimensional space group.1,13 Thus, in general, for a material that is incommensurate in d dimensions, the symmetry can be described in a (3+d)-dimensional space group.40–44 Here we focus only on the case, exemplified by urea inclusion compounds, in which d ¼ 1. We note that only a subset of all (3+d)-dimensional

311

Aperiodicity in Organic Materials 20 (H, K, 0, 1) 15

Intensity

Satellite (H, K, 1, –1) Satellite (H, K, 1, 1)

10

5 c∗ g 0

c ∗h

0

1 1

Figure 6

2 2

3

4

L 3 5 M

One-dimensional scan through reciprocal space for the 1,10-dibromodecane/ urea inclusion compound parallel to the cg* direction (or ch* direction, which is parallel to cg*), for fixed values of H and K. The horizontal axis is labelled both according to the guest reciprocal lattice vector cg* (Miller index M) and the host reciprocal lattice vector ch* (Miller index L). Note: ch* ¼ 1.65 cg* for 1,10-dibromodecane/urea. Two satellite reflections (H, K, 1, 1) and (H, K, 1, 1) are observed in this scan. Note that, because of the particular H and K values chosen for this scan, there is no main reflection from the host substructure at (H, K, 1, 0).

space groups are actually suitable for describing the symmetry properties of a material that is incommensurate in d dimensions, as some stringent conditions exist for a higher-dimensional space group to be suitable in this regard. Thus, not all symmetry operators in (3+d)-dimensional space are allowed for incommensurate crystals, and in particular, symmetry operators that mix the coordinates corresponding to three-dimensional space with the additional coordinates arising because of the incommensurateness are forbidden. As a consequence, for materials (such as urea inclusion compounds) that are incommensurate in one dimension, only 775 of the 4895 four-dimensional space groups satisfy these stringent conditions. It is important to recall that, while the symmetry of an incommensurate onedimensional inclusion compound can be completely described using an appropriate four-dimensional superspace group, the real material is a three-dimensional entity that exists in the same three-dimensional world that we live in. Thus, the real incommensurate material can be regarded as an appropriate three-dimensional section through the four-dimensional space that is required to define the symmetry of the system. The Fourier transform of the four-dimensional direct space is a four-dimensional reciprocal space, in which each diffraction maximum is described by four integer Miller indices (H, K, L, M) as discussed above. Again, the actual experimental diffraction pattern is measured in three-dimensional space, and corresponds to an appropriate projection of the four-dimensional reciprocal space onto three-dimensional space. It is relevant to note that, upon Fourier transformation, a three-dimensional section through four-dimensional

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direct space transforms as a three-dimensional projection of the corresponding four-dimensional reciprocal space. The methods of superspace symmetry have been used to derive the fourdimensional superspace groups that are applicable in the case of incommensurate urea inclusion compounds,13 and superspace descriptions for the specific cases of the heptadecane/urea,15 octane/urea,45 octanedioic acid/urea46 and suberic acid/urea47 inclusion compounds have been reported. It is important to emphasize the benefits of understanding the symmetry properties (and structural properties) of the composite inclusion compound in a four-dimensional superspace group, rather than restricting the structural description of such materials at the level of the separate basic host and basic guest structures. Knowledge of the structural properties of the separate basic structures contains no information on the modulations within each subsystem in the real inclusion compound. Even though the modulations may in some cases represent rather small structural perturbations, they may nevertheless have an important bearing on properties of the inclusion compound.

2.4

Energetic Aspects

As discussed in several other chapters of this book, the structure of any crystalline material arises as a consequence of the drive to attain an energetically favourable arrangement of the constituent molecules, and thus the observed structure arises as a consequence of the underlying energetic properties of the system. Similarly, to obtain a fundamental understanding of the incommensurate versus commensurate nature of a one-dimensional inclusion compound, it is necessary to understand the energetic properties of the system. Thus, the structural definition of incommensurate versus commensurate systems, based on the rationality of the ratio cg/ch, is merely a consequence of the underlying energetic properties, and it is essential to understand the energetic factors that lead to incommensurate or commensurate behaviour in these materials. To obtain a more fundamental understanding of this issue, a commensurate/ incommensurate classification that reflects a division in the energetic ‘‘behaviour’’ of one-dimensional inclusion compounds has been developed.3 We begin by considering the key definition of commensurate/incommensurate behaviour within this theoretical approach. For a given value of guest periodicity cg, we consider the fluctuation in the average host–guest interaction energy per guest molecule as the guest substructure is moved along the tunnel, keeping cg fixed. If the fluctuation is sufficiently small (i.e. less than e, where e is a physically relevant energy quantity for the system of interest), the inclusion compound is considered to exhibit incommensurate behaviour (Figure 7a). In principle, for an incommensurate system in which the tunnel has infinite length, the fluctuation is exactly zero. On the other hand, if the fluctuation is sufficiently large (i.e. larger than e), the inclusion compound is considered to exhibit commensurate behaviour (Figure 7b). In the commensurate case, energetic ‘‘lock-in’’ between the host and guest substructures will occur for a specific position of the

313

Aperiodicity in Organic Materials <Ehg>

(a)

+ε

λ

−ε

<Ehg>

(b)

+ε λ −ε

Figure 7

Schematic illustration of the fluctuation in the average host–guest interaction energy (oEhg4) as a function of the position (l) of the guest substructure relative to the host substructure in a one-dimensional inclusion compound for (a) incommensurate behaviour and (b) commensurate behaviour.

guest substructure relative to the host substructure. For the incommensurate case, the energy of the inclusion compound is essentially independent of the position of the guest substructure relative to the host substructure. It is important to note that, even for an inclusion compound for which the optimal value of cg corresponds to incommensurate behaviour, the fluctuation in host– guest interaction energy for a single guest molecule translated along the host tunnel can be large – the important issue underlying the definition of an incommensurate material is not the fluctuation in the host–guest interaction energy for a single guest molecule but rather the fluctuation in the host–guest interaction energy averaged over the complete set of guest molecules along the tunnel. With this basic definition of the energetic distinction between incommensurate and commensurate systems, methodology has been developed5 for applying these concepts to predict structural properties of one-dimensional inclusion compounds from knowledge of potential energy functions for the inclusion compound (assuming that the host structure is known and ch is fixed). Fundamental to this approach is the definition of an appropriate energy expression – the ‘‘characteristic energy’’ of the inclusion compound – that directly indicates the relative energetic favourability of inclusion compounds with different guest periodicities. The ‘‘characteristic energy’’ defined as: ! ! n1 X 1 1 ^ nÞ ¼ inf Eða; Eh ðka þ lÞ þ E^guest ðaÞ þ E^intra a l n k¼0

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In this expression, n is the number of guest molecules in the host tunnel, a is the ratio cg/ch (representing a scaled guest periodicity), the first guest molecule is located at position t ¼ l along the tunnel, Eh(t) is the host–guest interaction energy for an individual guest molecule at position t along the tunnel, E^guest (a) is the guest–guest interaction energy per guest molecule when the scaled guest periodicity is a, and E^intra is the intramolecular potential energy of the guest molecule (it is assumed that all guest molecules adopt the same conformation inside the tunnel, as required for a strictly periodic system). As elaborated fully elsewhere,3,5 a critical feature of the definition of characteristic energy is the factor 1/a, which ensures that the characteristic energy refers to energy per unit length of tunnel, rather than energy per guest molecule. The potential energy functions Eh(t), E^guest (a) and E^intra may be readily computed using appropriate potential energy parameterizations for the inclusion compound of interest. The optimum guest structure for the inclusion compound corresponds to minimum characteristic energy, and methodology has been developed3,5 to allow the following structural properties to be predicted for the inclusion compound of interest: (i) the optimum value of the guest periodicity cg, (ii) whether the optimum value of cg corresponds to commensurate or incommensurate behaviour, and (iii) the optimum conformation of the guest molecules within the host structure. The method has been applied successfully to predict structural properties of alkane/urea inclusion compounds48 in good agreement with experimental results. The method has also been used to rationalize unusual conformational behaviour in a commesurate inclusion compound.49 In Section 2.1, our structural classification of a commensurate material was based on the ability to find sufficiently small integers p and q such that pch E qcg, with the material otherwise classified as incommensurate. Relevant questions in this regard are how small is ‘‘sufficiently small’’ and what do we mean by ‘‘approximately equal to’’? The mathematical analysis in Ref. 3 approaches the definition of commensurate/incommensurate systems purely from the viewpoint of the energetic behaviour of the material, but this approach is found to lead to excellent agreement with the commonly applied practical criterion that a one-dimensional inclusion compound is commensurate if and only if sufficiently small integers p and q can be found such that cg/ch is approximately equal to p/q. However, the energetic definition is successful in giving mathematical rigour to the somewhat vague terms ‘‘sufficiently small’’ and ‘‘approximately equal to’’, and provides a more fundamental approach for understanding the basis of incommensurateness in such materials.

2.5

Some Physical Properties Relating to Incommensurateness

Incommensurateness in a solid inclusion compound can have important consequences for some physical properties of the material, including: (i) the distribution of guest molecule environments (with implications for issues such as host–guest chiral recognition, chemical reactions and inhomogeneous broadening of spectral lines), (ii) vibrational properties, and (iii) the possibility of

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activationless transport of guest molecules through the tunnels. This section discusses some issues relating to each of these aspects.

2.5.1

Distribution of Guest Molecule Environments

An important consequence of an incommensurate relationship between the host and guest substructures in an incommensurate inclusion compound is that, in principle, each guest molecule within a given tunnel samples a slightly different local environment with respect to the host structure. For a commensurate material, on the other hand, the guest molecules in the tunnel sample only one or a small number of different local environments within the host structure. Clearly several chemical and spectroscopic properties of the guest molecules may reflect these differences in the distribution of local environments for commensurate and incommensurate systems. For example, if a chemical reaction of the guest molecules occurs for which the mechanism is highly sensitive to the local environment, the product obtained from the reaction may be critically dependent on the exact location of the guest molecule within the host structure. Such a reaction occurring in an incommensurate inclusion compound would be expected to lead to a product distribution that reflects the distribution of local environments of the reactant guest molecules. Another scenario would be for the reaction of each guest molecule to lead to the same product, but for the rate of reaction to depend on the exact location of the guest molecule within the host structure. In this situation, the incommensurate nature of the material would lead to a distribution of rate constants for the reaction, and so-called ‘‘dispersive kinetics’’.50 Another physical manifestation of incommensurateness arises if the spectroscopic properties of the guest molecules are influenced strongly by their local environment. In this situation, the spectroscopic properties of the guest molecules would be expected to depend critically on the exact location of the guest molecule with respect to the host structure, and a distribution of spectroscopic responses would be observed for an incommensurate inclusion compound. For such materials, inhomogeneous broadening of spectral lines would be expected, leading inter alia to the opportunity to carry out ‘‘hole-burning’’ types of experiments. Another aspect for which the distribution of guest molecule environments can have important implications concerns the question of host–guest chiral recognition, which is relevant for cases in which the host tunnel and guest molecules are both chiral. In addition to the prospects for exploiting the chirality of the host structure through chemical reactions of the guest species, applications based on separation of the two enantiomers of a chiral guest may also be envisaged. Clearly, the extent of host–guest chiral recognition depends on the extent to which the host–guest interaction energies are different for the two enantiomers (R and S) of a chiral guest molecule within a given enantiomorph of the chiral host tunnel. However, it is important to recall that host– guest interaction depends on the position (z) of the guest molecules along the tunnel, and the relative energetic preferences for the two enantiomers of the guest may vary as a function of z. This issue becomes particularly pertinent for

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incommensurate systems, recalling that each guest molecule within the tunnel experiences a different local environment with respect to the host structure (and hence samples a different part of the host–guest interaction potential). For this reason, investigations of host–guest chiral recognition in incommensurate inclusion compounds must give due consideration to the way in which the host-guest interaction energy varies as a function of z. As discussed in Section 2.2, the urea tunnel structure is chiral, with the tunnel constructed from a spiral hydrogen-bonded arrangement of urea molecules. A given single crystal of a urea inclusion compound either contains only righthanded spirals (space group P6122) or only left-handed spirals (space group P6522). There is clearly considerable potential to exploit this chirality in the properties and applications of these materials, although an important factor is the extent to which the interaction between a host tunnel of a given chirality (e.g. P6122) differs for the two enantiomers (R and S) of a chiral guest molecule. In spite of the potential for exploiting the chirality of urea inclusion compounds, virtually all reported studies of urea inclusion compounds have focused on achiral guest molecules (an important exception is the work of Schlenk30,31 which demonstrated experimentally that urea inclusion compounds are indeed able to exhibit some degree of chiral recognition). To investigate fundamental aspects of host–guest chiral recognition in urea inclusion compounds, we have carried out computational studies of host–guest interaction for chiral 2-bromoalkane32 and 2-hydroxyalkane33 guest molecules. For each guest molecule considered, both R and S enantiomers were studied, and for each enantiomer the following conformations of the end-group containing the Br atom or OH group were considered: (i) Br/OH trans and CH3 group gauche (denoted by subscript t); (ii) Br/OH gauche and CH3 group trans (denoted by subscript g). Thus, for each guest molecule, the following four different conformation/enantiomer combinations were considered: Sg, St, Rg and Rt. In each case, the host–guest interaction energy was determined as a function of the position (z) of the guest molecule along the host tunnel (within the unique range 0 r z r ch/6). Representative results for the 2-bromotetradecane/urea and 2-hydroxytridecane/urea inclusion compounds are shown in Figure 8. The results of these studies32,33 show that for all 2-bromoalkane guest molecules considered, the Br trans conformation is preferred over the Br gauche conformation at all positions along the tunnel for both enantiomers (in contrast, the Br gauche conformation is preferred for isolated 2-bromoalkane molecules). Furthermore, in this conformation, the R enantiomer is preferred over the S enantiomer at all positions along the tunnel of the P6122 host structure (with the exception of 2-bromoundecane, for which some regions of the tunnel have a slight preference for the S enantiomer). On taking into account the incommensurate nature of the 2-bromoalkane/urea inclusion compounds (i.e. on carrying out an appropriate averaging of the host–guest interaction energy as a function of z), an overall excess of the R enantiomer within the P6122 host structure is predicted in all cases. For 2-hydroxyalkane/ urea inclusion compounds, on the other hand, a substantially different picture emerges. In general, the OH gauche conformation is preferred over the OH

317

Aperiodicity in Organic Materials z/Å

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Figure 8

Computed host–guest interaction energies to probe host–guest chiral recognition in urea inclusion compounds. The computed host–guest interaction energy is shown as a function of position (z) along the tunnel for a single guest molecule of (a) 2-bromotetradecane and (b) 2-hydroxytridecane in the P6122 urea host structure. In each case, the results are shown for both the R and S enantiomers of the guest molecule in both the Br/OH trans and Br/OH gauche conformations.

trans conformation. For some alkane chain lengths, there is a marked preference for the R enantiomer over the S enantiomer, whereas for other chain lengths, the host–guest interaction energy is lower for the R enantiomer in some regions of the tunnel and lower for the S enantiomer in other regions (see the results for 2-hydroxytridecane/urea in Figure 8b). In these cases, when the results are considered over all positions of the guest molecules within the host tunnel, it is predicted that no substantial enantiomeric excess would be observed in the inclusion compound.

2.5.2

Vibrational Properties

As discussed in Section 2.3, the observation of satellite reflections with L a 0 and M a 0 in the X-ray diffraction pattern of a one-dimensional inclusion compound provides direct experimental evidence for the incommensurate nature of the material. Another opportunity to obtain direct experimental

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evidence of incommensurateness arises from consideration of vibrational properties. In particular, as now discussed, the energetic reasons that underlie incommensurate behaviour in a solid inclusion compound have a direct manifestation in terms of the acoustic vibrational modes of the material. First, we recall that conventional crystals (including commensurate inclusion compounds) have three translation invariances (i.e. a translation of the crystal with no change of energy), corresponding to translation along the x, y and z axes in three-dimensional space. An incommensurate one-dimensional inclusion compound, on the other hand, has four translation invariances. The extra translation invariance is an internal translation invariance, and corresponds to the shift of the guest substructure relative to the host substructure along the incommensurate direction (as discussed above, the energy of an incommensurate inclusion compound is, in principle, independent of the shift of the guest substructure relative to the host substructure along this direction). There is an acoustic phonon corresponding to each translation invariance in a crystal, and therefore an incommensurate one-dimensional inclusion compound should have four acoustic phonons and a commensurate inclusion compound should have three acoustic phonons. The additional acoustic mode in the incommensurate system is called the ‘‘sliding mode’’, and observation of the sliding mode can be taken as direct experimental evidence for incommensurate behaviour of the inclusion compound. Unfortunately, the converse is not true, as there are experimental reasons that the sliding mode may be difficult to detect, even if the material is incommensurate. With this motivation, Brillouin scattering investigations51 of the heptadecane/urea inclusion compound have provided evidence for a fourth acoustic mode, assigned as the sliding mode, thus substantiating the incommensurate nature of this inclusion compound. However, we note that Brillouin scattering investigations for other urea inclusion compounds have not been able to observe the sliding mode.52–54

2.5.3

Molecular Transport Processes

Many different types of solid inclusion compounds (e.g. zeolites) have found important applications in molecular separation processes, based on the fact that the host structure displays selectivity with regard to the incorporation of guest molecules of differing size and shape. In the case of urea inclusion compounds, however, the fact that the ‘‘empty’’ urea tunnel structure is unstable limits the opportunity to develop analogous types of applications based on molecular adsorption and/or molecular separation. Nevertheless, a process for achieving guest exchange in urea inclusion compounds, by a mechanism that does not proceed via the empty host tunnel structure, has been identified and demonstrated27 to occur successfully. This process involves net transport of guest molecules in one direction along the tunnel in the urea host structure by inserting ‘‘new’’ (thermodynamically more favourable) guest molecules at one end of a crystal of a urea inclusion compound (e.g. by dipping the crystal into the liquid phase of the new guest), with the ‘‘original’’ guest molecules expelled from the other end of the crystal (Figure 9). This mechanism

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Figure 9

319

Schematic illustration of guest exchange in a single crystal of a urea inclusion compound. The original guest molecules (green) are replaced by new (energetically more favourable) guest molecules (blue) by dipping one end of the original crystal into the liquid phase of the new guest molecules.

for guest exchange satisfies the requirement that the tunnels remain fully occupied at all stages throughout the guest exchange process, but with the actual identity of the guest molecules changing as a function of time during the process. Single crystal X-ray diffraction studies for a crystal that has undergone partial guest exchange demonstrate incontrovertibly that the transport of guest molecules occurs inside the tunnels (Figure 10). An important feature underlying the idea of carrying out such guest transport processes within an incommensurate inclusion compound was the fact that the energy of an incommensurate inclusion compound is essentially independent of the position of the guest substructure relative to the host substructure (see Section 2.4), suggesting the possibility of activationless translation of the guest substructure along the host tunnel. However, the effects at the ends of the tunnel must also be considered, as there may be a significant activation associated with the entry of a new guest molecule at one end of the tunnel and/or the expulsion of an original guest molecule from the other end of the tunnel. To obtain deeper insights into such guest exchange processes in urea inclusion compounds, we have used confocal Raman microspectrometry as an in situ probe (Figure 11), demonstrating55 that this technique can yield information on the spatial distribution of the original and new guest molecules within the crystal, and details of how the spatial distribution of the original and new guest molecules varies as a function of time during the process. Our work in this area has focused primarily on the system comprising the 1,8-dibromooctane/urea inclusion compound as the original crystal, and pentadecane as the new type of guest molecule. Analysis of the Raman microspectrometry data has focused55 on studying the variation of the intensity of the C–Br stretching band of the original 1,8-dibromooctane guest molecules (for the predominant trans end-group conformation), as a function of position in the crystal and as a function of time (Figure 12). Clearly,

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Figure 10

Single crystal X-ray diffraction rotation photographs for (a) the 1,8dibromooctane/urea inclusion compound, and (b) the same single crystal after partial exchange with pentadecane guest molecules (ex situ measurement). The X-ray diffraction photograph shown in (b) provides clear evidence for the presence of both 1,8-dibromooctane and pentadecane guest molecules inside the single crystal (characteristic guest layer lines are labelled with red and green arrows respectively).

Raman laser scanned along the crystal

Reservoir containing liquid pentadecane

Y X Z Sealing system

Figure 11

Original single crystal of 1,8-dibromooctane/urea

Schematic illustration of the experimental assembly for in situ Raman microspectrometry to probe guest exchange in a urea inclusion compound, comprising the single crystal of the urea inclusion compound (green), initially containing 1,8-dibromooctane guest molecules, attached to a reservoir containing liquid pentadecane (blue).

321

Aperiodicity in Organic Materials 18 hours

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In situ time-resolved and spatially resolved monitoring of guest exchange in a single crystal of a urea inclusion compound, using Raman microspectrometry. The Raman micrographs were recorded during transport of pentadecane molecules into and along the tunnels, displacing the guest molecules (1,8-dibromooctane) originally present. The probed region shown represents only part of the crystal, and the transport of guest molecules occurs from left to right (the tunnels run horizontally in the micrographs shown). Regions coloured blue are rich in pentadecane, and regions coloured green are rich in 1,8-dibromooctane. The time taken to record each micrograph was ca. 28 minutes; the three micrographs shown were recorded (a) 18 h, (b) 29 h, and (c) 40 h after commencement of the guest exchange process.

such data provide access to quantitative information on kinetic and mechanistic aspects of the transport of guest molecules through the host tunnel structure during the guest exchange process. For example, at ambient temperature, the progress of the transport process shows a linear variation with time, and occurs at a rate in the range ca. 70–100 nm s1 (the rate of movement of the centroid of the sigmoidal distribution shown in Figure 13a), which corresponds to between about 30–50 new guest molecules entering the tunnel per second. Furthermore, our in situ studies employing confocal Raman microspectrometry56 have revealed that the guest exchange process is associated with significant changes in the conformational properties of the original (1,8-dibromooctane) guest molecules, corresponding to a significant increase in the proportion of 1,8-dibromooctane guest molecules with the gauche end-group conformation within the ‘‘boundary region’’ between the original and new guest molecules (Figure 13b). We now consider the physical basis for the observation that the guest transport process occurs at constant rate. Displacement of the complete set of guest molecules along the tunnel relies upon insertion of new guest molecules at one end of the tunnel and expulsion of the original guest molecules at the other end of the tunnel. As translation of the complete guest substructure along the tunnel in an incommensurate inclusion compound should approximate to activationless transport, the rate limiting step of the guest exchange process must correspond either to the entry of new guest molecules or the expulsion of the original guest molecules at the two ends of the tunnel. Both of these interfacial processes are expected to exhibit zeroth order kinetics, from which the rate of the overall guest exchange process is expected to be independent of

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Figure 13

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Variation in the intensity of the C–Br stretching vibration as a function of position along the crystal for (a) C–Br bonds in the trans conformation and (b) C–Br bonds in the gauche conformation during exchange of 1,8dibromooctane guest molecules by pentadecane guest molecules. The sigmoidal distribution in (a) reflects the replacement of 1,8-dibromooctane guest molecules (right side) by pentadecane guest molecules (left side), with the guest transport process occurring from left to right. The data in (b) provides clear evidence for a local increase in the proportion of 1,8dibromooctane guest molecules in the gauche conformation within the ‘‘boundary region’’ (shown by blue vertical lines).

time, as observed experimentally. These qualitative concepts are currently being embodied within the development of a rigorous kinetic model to describe the guest transport process in these materials. At this stage, several fundamental aspects relating to such guest exchange processes remain to be understood, and a variety of techniques are currently being employed for this purpose. Clearly, the development of an understanding of the fundamentals of the guest exchange process in such materials is a prerequisite for developing and optimizing a range of potential applications in molecular separation, based for example on discrimination of molecular size, shape and chirality.

2.6

Concluding Remarks

In spite of the apparent structural simplicity of inclusion compounds based on one-dimensional tunnel structures, it is clear that these materials display several properties of fundamental physico-chemical significance that will continue to challenge researchers in this field for years to come. Conceptually, the diffraction properties of incommensurate materials and the corresponding structural descriptions in direct space extend beyond the normal crystallographic principles encountered for conventional crystals; clearly an important aspect for future endeavour is to obtain a deeper understanding of the ways in which the physical properties of these materials are influenced by their incommensurateness, and ultimately to find strategies to exploit these properties.

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3 Quasicrystalline Materials 3.1

Introduction to Quasicrystals

At the time that I joined JMT’s research group in Cambridge in the mid1980s, it seemed that many of the most important developments in the physical sciences were happening in the solid state, and it really felt like an exciting time to be entering this field of science. Of course, JMT himself was responsible for many of these developments and several seminal advances were being made within his research group. Elsewhere, two particularly revolutionary discoveries were made around this time – high-temperature superconductivity57 (for which there was much interest locally, with Peter Edwards, then in Cambridge, spearheading the U.K. effort in this new field)58,59 and quasicrystals.60 The discovery of quasicrystals was made in 1984, with the observation by Schechtman and co-workers60 that certain metal alloy materials (typified by AlxMny) can exhibit diffraction patterns with 10-fold symmetry. This observation was apparently contradictory, as the diffraction patterns of these materials comprise sharp Bragg-like reflections, characteristic of ordered crystalline materials, but yet it had long been known61 that 10-fold or 5-fold symmetry is impossible in a crystalline material with long-range periodic order. This issue was resolved62–64 by recognizing that certain quasiperiodic tilings (e.g. the Penrose tiling)65 have diffraction patterns that contain sharp Bragglike reflections based on a 10-fold symmetric reciprocal space,66 resembling those observed for the metal alloy materials. Such tilings67–69 are constructed from a set of geometrically well-defined tiles, assembled according to welldefined rules, but do not have translational periodicity. Subsequently, a wide range of other materials have been discovered to exhibit diffraction patterns with 10-fold symmetry, and their properties have been investigated experimentally and theoretically.70–74 The term ‘‘quasicrystal’’ is now widely used for such materials. To a large extent, structural rationalization of quasicrystals has focused on pursuing analogies to the Penrose tiling,71–73,75 although other quasiperiodic models have also been proposed (such as the cluster model73,76–78 based on a quasiperiodic ‘‘coverage’’, rather than quasiperiodic ‘‘tiling’’), and symmetry properties have been rationalized in higher-dimensional superspaces70,79–83 employing similar principles to those used to describe incommensurate materials (see Section 2.3). In spite of the huge interest in quasicrystalline materials during the last 20 years or so, the examples reported during this time were dominated by metal alloy materials, and there was no report of a molecular quasicrystal. Recently, however, we reported the first proposal of a quasicrystalline material in which the building units are organic molecules, based on similar design principles to those that are employed in the design of crystalline molecular materials – an endeavour called ‘‘crystal engineering’’. We now give a brief description of crystal engineering (Section 3.2), before describing some key elements of our strategy for the design of a molecular quasicrystal (Section 3.3).

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Concepts of ‘‘Crystal Engineering’’

There is currently much interest in understanding the fundamental factors that control the observed structural properties of crystalline organic materials as such knowledge is an essential pre-requisite for the design of molecular crystals for specific applications. The name crystal engineering84–89 is used to describe this area of activity (although the term is just as frequently misused in contexts that do not actually involve any real element of crystal design). Proper rationalization of the factors that control the structural properties of such materials is often far from straightforward, as an experimentally observed crystal structure arises from the subtle inter-play of several different types of intermolecular interactions of comparable strengths. Nevertheless, when one specific interaction (or a small number of interactions) has a dominant role in directing the structure, it may be possible to reach a reliable rationalization of the observed arrangement of molecules in the crystal, and hence to exploit such understanding as the basis for the reliable a priori prediction of the structural properties of other (related) materials. Most successful crystal engineering strategies have exploited this approach, particularly by focusing on hydrogen bonding as the basic constructional element.90,91 The success of this approach is based on the fact that the hydrogen bond is generally more geometrically discriminating than the other types of intermolecular interaction that arise in organic molecular crystals. As discussed in Section 3.3, our work to design a quasicrystalline molecular material has applied such crystal engineering principles. It is relevant to note the pioneering contributions made by JMT, particularly during the 1970s and early 1980s, towards the birth of the field of crystal engineering through his work on photoreactivity of organic crystals,85 and specifically on the design of crystal structures for specific targeted photochemical reactions. Indeed, some of his earliest papers in this field are the first to use the term ‘‘crystal engineering’’,86,87 and the great Russian physical chemist A.I. Kitaigorodsky, who himself made major contributions towards deriving a fundamental understanding of the structures of organic molecular crystals, describes JMT in one of his seminal review articles92 as ‘‘one of the pioneers of this particular field’’.

3.3

The Design of a Molecular Quasicrystal

Our strategy for the design of a molecular quasicrystal93 has used the Penrose tiling (Figure 14) as the basic structural template, with the aim of positioning a molecule at each node (vertex) of the tiling, and with each molecule forming a robust intermolecular linkage to each neighbouring molecule (along the lines between adjacent nodes on the tiling). It is important to emphasize that, within this design strategy, the molecules represent the nodes of the Penrose tiling and do not represent the tiles themselves. The two types of tile (thick rhombus and thin rhombus) within the Penrose tiling are instead represented by the regions of space between molecules (see below). Clearly, the intermolecular linkages

Aperiodicity in Organic Materials

Figure 14

325

A Penrose tiling, with one example of each of the seven different types of node indicated by a red circle.

should be strong and directional (linear), such that aggregation of the molecules in the proposed tiling arrangement is energetically favourable. As highlighted in Figure 14, a Penrose tiling contains seven different types of node, and our aim was to design a set of molecules with the same geometric properties as each of these nodes. The angles between the lines at each node are integer multiples of 36 1, and we therefore require a molecular core that has 10-fold symmetry (or at least approximate 10-fold symmetry), together with linear intermolecular linkages that are oriented in the same way as the lines that emanate from each node in the tiling. In designing an appropriate molecular core, it is important to emphasize that molecules with 10-fold symmetry are exceptionally rare, if not unprecedented, but the molecule shown in Figure 15a (C30H10; 10,5-coronene) has been shown to be an appropriate candidate. To date, however, there has been no reported synthesis of this molecule. In our design strategy, the intermolecular linkages (representing the lines between nodes on the tiling) are formed by substituents on the 10,5-coronene ring of the type (Figure 15b) –(CRC)n–CO2H (n ¼ 0, 1, 2, . . .), based on the following design elements: (i) the substituents are linear, (ii) the intermolecular linkage formed between two such substituents is the well-known carboxylic acid dimer motif (Figure 15c) comprising two strong O–H  O hydrogen bonds and is such that the two substituents are collinear with each other, and (iii) there is scope to vary, and hence to optimize, the length of the substituent by changing n (in practice, we have found that n ¼ 1 is optimal; see below). For n ¼ 1, the distance between the centres of two molecules linked through the hydrogen bonding arrangement shown in Figure 15c is ca. 21.37 A˚.

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Figure 15

(a) 10,5-coronene (C30H10), (b) an example of a molecule from the C30H10m(CCCO2H)m family (m ¼ 5), and (c) the linear interaction between two –CRC–CO2H substituents via the carboxylic acid dimer motif, which involves two O–H  O hydrogen bonds.

Figure 16

The seven types of node in a Penrose tiling, and the corresponding molecule from the C30H10m(CCCO2H)m family. The red ‘‘star’’ indicates the part of the tiling that is actually ‘‘occupied’’ by the molecule shown. The relative frequencies of occurrence of each of the seven types of node in a Penrose tiling are indicated, where t is the ‘‘golden ratio’’ [(1+O5)/2 E 1.618].

There are seven different types of node in the Penrose tiling,67–69,94 each characterized by a different local geometry. Figure 16 shows each type of node together with the molecular representation based on the 10,5-coronene core and the relevant arrangement of –CRC–CO2H substituents. All of these molecules are members of the general family C30H10m(CCCO2H)m, with m ¼ 3–7. The linear hydrogen-bonded linkages between neighbouring molecules represent the lines between adjacent nodes in the tiling, and create the two types of tile (thick and thin rhombuses) as the region between groups of four molecules (Figure 17). We note that the optimal length (n ¼ 1) of the substituents is dictated by properties of the thin rhombus (Figure 17). Thus, for n ¼ 0, the molecular cores across the short diagonal of the thin rhombus would give rise to severely unfavourable repulsive interactions. For n ¼ 1 (Figure 17a), the shortest H  H

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Figure 17

327

(a) The thick rhombus and thin rhombus in the molecular representation of a Penrose tiling, generated from groups of four molecules from the C30H10m(CCCO2H)m family interacting through hydrogen bonding in the carboxylic acid dimer motif. (b) The molecular quasicrystal constructed from the Penrose tiling shown in Figure 14 (showing exactly the same region of the tiling as Figure 14). Examples of the thick rhombus and thin rhombus are shaded blue and green respectively.

distance across the short diagonal of the thin rhombus is ca. 2.3 A˚, which is close to the optimal van der Waals contact distance for a non-bonded H  H interaction. Values of n Z 2 would lead to an unfavourably low density of molecular packing in the plane. As our proposed molecular quasicrystal is composed of seven different molecules, it is thus analogous to a multi-component co-crystal material. For an infinite Penrose tiling, the seven different types of node occur in well-defined frequency ratios, as shown in Figure 16, and the relative frequency of occurrence of each of the seven types of molecule in the quasicrystal should match these ratios. Starting from the Penrose tiling shown in Figure 14, the corresponding molecular quasicrystal constructed using the strategy discussed above (after minimization of the intermolecular potential energy) is shown in Figure 17b, and the X-ray diffraction pattern calculated for this molecular quasicrystal is shown in Figure 18. Clearly, the diffraction pattern has sharp Bragg-like maxima, and the positions and intensities of these diffraction maxima define a 10-fold symmetric reciprocal space, as found for established classes of quasicrystals based on metal alloys. In the course of our work, we have generated a large number of finite sections of Penrose tilings and constructed the corresponding molecular quasicrystals. The diffraction patterns obtained in each case are essentially indistinguishable from each other, provided the sampled section of tiling used to calculate the diffraction pattern is sufficiently large. Patterson maps corresponding to these molecular quasicrystals are also essentially indistinguishable, provided again that a sufficiently large section of the tiling is sampled. These observations follow directly from well-established

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Chapter 19

X-ray diffraction pattern calculated for the molecular quasicrystal shown in Figure 17b. Note: a denotes the distance between the centres of adjacent molecules (after relaxation) and has the value a ¼ 21.37 A˚.

properties of Penrose tilings,65,67–69 in particular the fact that any region of a given Penrose tiling can be found to exist in any other (infinite) Penrose tiling. Future extensions of the design strategy to generate a structurally more diverse range of molecular quasicrystals based on generalized Penrose tilings94 will lead to well-defined differences between the diffraction patterns and Patterson maps of different molecular quasicrystals (generalized Penrose tilings involve other types of node, all of which are represented by molecules within the C30H10m(CCCO2H)m family, in addition to the seven types of node of the standard Penrose tiling). The primary challenge for preparation of the quasicrystal from the seven molecular components, for example in a crystallization experiment, will be to direct the crystallization towards the desired quasicrystal instead of other competing processes, such as the formation of crystalline phases comprising a single type of molecule, or co-crystals comprising two or more types of molecule. At present, none of the individual molecules shown in Figure 16 have been synthesized and their crystal structures are unknown, and we are therefore unable at this stage to assess the relative energetic properties of the molecular quasicrystal versus such crystalline phases. Finally, we note that the design shown in Figure 17b is a two-dimensional quasicrystalline molecular array, and to extend this concept to construct a three-dimensional quasicrystal implies appropriate stacking of these two-dimensional sheets. In addition to the aim of achieving the experimental realization of a three-dimensional molecular quasicrystal, the study of two-dimensional quasiperiodic molecular arrays (such as that shown in Figure 17b) adsorbed

Aperiodicity in Organic Materials

329

on appropriate substrates would also be interesting, particularly from the viewpoint of controlling and understanding surface structural properties.

3.4

Future Directions

The successful strategy to design an energetically stable quasicrystal constructed using organic molecules as the building units represents the first proposed example of a molecular quasicrystalline material. The proposed molecular quasicrystal has arisen through rational design principles, based on the central idea of identifying discrete molecular building units that promote strong intermolecular interactions in well-defined local geometric arrangements that correspond to the local geometries of nodes in the Penrose tiling. Our strategy provides a basis not only for the realization of quasicrystalline molecular materials based on the standard Penrose tiling, but can also be extended directly, by appropriate selection of other well-defined sets of molecular building units within the C30H10m(CCCO2H)m family, to design molecular quasicrystals based

Figure 19

Proposal of a new macromolecular quasicrystal, based on the 10,5coronene core (a) representing the nodes in a Penrose tiling and linear tetraethynyl linkages representing the lines between adjacent nodes (b).

330

Chapter 19

on generalized Penrose tilings. Further generalizations of our strategy to design other types of quasiperiodic molecular material, including those based on other ‘‘forbidden’’ symmetries such as 7-fold or 8-fold, may also be readily envisaged. Another future direction95 concerns the design of a macromolecular quasicrystalline array, in which the hydrogen bonded linkages between nodes are replaced by covalent linkages. In this regard, our present focus is directed towards the material shown in Figure 19 containing linear covalent tetraethynyl linkages, for which computational investigations are currently underway to explore the electronic properties. At least at a superficial level, the resemblance to graphene sheets96 may be noted.

Acknowledgements As discussed in Section 1, I am grateful to Professor Sir John Meurig Thomas for introducing me to the fascinating subject of urea inclusion compounds (as well as to many other areas of research in solid state science) and for the many aspects of help and guidance that he has given to me over the years. I am also grateful to the members of my research group and research collaborators who have made substantial contributions to our research in the specific areas covered by this article (particularly Mao-Hsun Chao, Arnaud Desmedt, Franc¸ois Guillaume, Mark Hollingsworth, Benson Kariuki, Andrew Rennie, Javier Martı´ -Rujas, Lily Yeo, Zhongfu Zhou and others mentioned in the references cited). Fang Guo is thanked for help in the preparation of figures for this article.

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14. R. Lefort, J. Etrillard, B. Toudic, F. Guillaume, T. Breczewski and P. Bourges, Phys. Rev. Lett., 1996, 77, 4027. 15. T. Weber, H. Boysen, F. Frey and R.B. Neder, Acta Crystallogr., 1997, B53, 544. 16. N.G. Parsonage and R.C. Pemberton, Trans. Faraday Soc., 1967, 63, 311. 17. K.D.M. Harris, I. Gameson and J.M. Thomas, J. Chem. Soc., Faraday Trans., 1990, 86, 3135. 18. L. Yeo, B.M. Kariuki, H. Serrano-Gonza´lez and K.D.M. Harris, J. Phys. Chem. B, 1997, 101, 9926. 19. H. Le Lann, C. Odin, B. Toudic, J.C. Ameline, J. Gallier, F. Guillaume and T. Breczewski, Phys. Rev. B, 2000, 62, 5442. 20. H.L. Casal, D.G. Cameron and E.C. Kelusky, J. Chem. Phys., 1984, 80, 1407. 21. K.D.M. Harris and P. Jonsen, Chem. Phys. Lett., 1989, 154, 593. 22. A. El Baghdadi, E.J. Dufourc and F. Guillaume, J. Phys. Chem., 1996, 100, 1746. 23. F. Guillaume, C. Sourisseau and A.-J. Dianoux, J. Chim. Phys. (Paris), 1991, 88, 1721. 24. S.P. Smart, F. Guillaume, K.D.M. Harris and A.-J. Dianoux, J. Phys.: Condens. Matter, 1994, 6, 2169. 25. P. Girard, A.E. Aliev, F. Guillaume, K.D.M. Harris, M.D. Hollingsworth, A.-J. Dianoux and P. Jonsen, J. Chem. Phys., 1998, 109, 4078. 26. M.D. Hollingsworth and N. Cyr, Mol. Cryst. Liq. Cryst., 1990, 187, 135. 27. A.A. Khan, S.T. Bramwell, K.D.M. Harris, B.M. Kariuki and M.R. Truter, Chem. Phys. Lett., 1999, 307, 320. 28. M.-H. Chao, K.D.M. Harris, B.M. Kariuki, C.L. Bauer and B.M. Foxman, J. Phys. Chem. B, 2002, 106, 4032. 29. S.-O. Lee, K.D.M. Harris, P.E. Jupp and L. Yeo, J. Am. Chem. Soc., 2001, 123, 12913. 30. W. Schlenk, Justus Liebigs Ann. Chem., 1973, 7, 1145, 1156, 1179, 1195. 31. R. Arad-Yellin, B.S. Green, M. Knossow and G. Tsoucaris, in Inclusion Compounds, vol. 3, J.L. Atwood, J.E.D. Davies and D.D. MacNicol (eds), Academic Press, New York, 1984, 263. 32. L. Yeo and K.D.M. Harris, J. Chem. Soc., Faraday Trans., 1998, 94, 1633. 33. L. Yeo and K.D.M. Harris, Mendeleev Commun., 2004, 14, 263. 34. M.D. Hollingsworth, K.D.M. Harris, W. Jones and J.M. Thomas, J. Inclusion Phenom., 1987, 5, 273. 35. S.-O. Lee and K.D.M. Harris, Chem. Phys. Lett., 1999, 307, 327. 36. N.E. Kelly, S.-O. Lee and K.D.M. Harris, J. Am. Chem. Soc., 2001, 123, 12682. 37. M.E. Brown and M.D. Hollingsworth, Nature, 1995, 376, 323. 38. M.D. Hollingsworth, M.L. Peterson, J.R. Rush, M.E. Brown, M.J. Abel, A.A. Black, M. Dudley, B. Raghothamachar, U. Werner-Zwanziger, E.J. Still and J.A. Vanecko, Cryst. Growth Des., 2005, 5, 2100. 39. K.D.M. Harris, S.P. Smart and M.D. Hollingsworth, J. Chem. Soc., Faraday Trans., 1991, 87, 3423.

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CHAPTER 20

From the Synthesis of Acetylenic Natural Products to Seeing the Light with Polymers ANDREW B. HOLMES,a,b PAUL L. BURN,c ARNO KRAFT,d JONATHAN M. WHITEa AND WALLACE W. H. WONGa a

School of Chemistry, Bio21 Institute, The University of Melbourne, Melbourne, Victoria 3010, Australia; b Department of Chemistry, Imperial College, South Kensington, London, SW7 2AZ, UK; c School of Molecular and Microbial Sciences, University of Queensland, St Lucia, Qld. 4072, Australia; d Chemistry, School of Engineering and Physical Sciences, Perkin Building, Heriot-Watt University, Edinburgh EH14 4AS, UK

1 Introduction In 1972 Andrew Holmes was appointed as a University Demonstrator (University Assistant Lecturer) in the Department of Organic, Inorganic and Theoretical Chemistry at the Cambridge University Chemical Laboratory in Lensfield Road. Working with Professor Ralph Raphael, he and Nick Wellard developed the synthesis of conjugated cis-enynes; application of this method to marine natural products unexpectedly produced a polymer that led to a stimulating interaction with Professor John Meurig Thomas and ultimately Richard Friend in the Cavendish Laboratory. This chapter is a personal historical account of the work in Cambridge which led to the serendipitous discovery of light emitting polymers and the formation of Cambridge Display Technology.

2 Conjugated cis-Enynes and Polydiacetylenes In 1971 Witkop and Daly reported the structure of the spirocyclic piperidine alkaloid histrionicotoxin 1.1 (Scheme 1), which had been isolated from skin extracts of the Colombian poison arrow frog Dendrobates histrionicus.1 334

335

Synthesis of Acetylenic Natural Products Cl O

O NH

OH

cis-Maneonene-A 1.2

Histrionicotoxin 1.1

Scheme 1

OH

The frog venom alkaloid histrionicotoxin 1.1 and the Hawaiian marine natural product cis-maneonene A 1.2.

SiMe3 H , Lindlar catalyst 2

OH

methanol 2.1

SiMe3

Br

65%

Bu4NF

2.2

OH 2.3

SiMe3

Scheme 2

The synthesis of a model cis-enyne 2.3 for histrionicotoxin.

In addition to its intriguing structure (including the presence of two pendant cis-enyne side chains) this alkaloid exhibited fascinating properties as a selective inhibitor of the nicotinic acetyl choline receptor, and there was a real demand for its synthesis, especially as a ban had been placed on the export of frog skins from Colombia. In addition, frogs reared in captivity did not secrete this particular compound, suggesting that its origin was dietary. We commenced work on the synthesis of histrionicotoxin. In the intervening years, the molecule has been synthesised by a number of research groups2–4 including, ultimately, our own.5,6 Our approach, developed in collaboration with Professor Ralph Raphael, to a model compound 2.3 containing a cis-enyne side chain is shown in Scheme 2.7 Partial catalytic hydrogenation of the less hindered (internal) triple bond afforded selectively the silylated enyne that was deprotected with fluoride ion to afford the required model compound 2.3. Having developed a suitable method for the introduction of the cis-enyne grouping we searched the literature for other natural products containing this side chain. Our attention was drawn to the family of maneonenes isolated from the Hawaiian red alga Laurencia nidifica by Erickson and colleagues.8 The attraction of cis-maneonene A 1.2 (Scheme 1) was that it had not been previously synthesised and, there being no further supplies of the natural material in existence, that a trip to Oahu would ultimately be necessary to collect further samples of the natural material to compare with our synthetic material.

336

Chapter 20

O

O

CHO

51%

3.1

O

H Br

OH

O

0.1 mm Hg 30 min

SiMe3 Me2NC(Cl)=CMe2 CH2 Cl2, 0 °C 59%

O

20%

O

Scheme 3

SiMe3

OMe

3.3 SiMe3

Cl

Bu4 N+ F -

cis-maneonene A

THF, 25 °C 30 min, 40% Br

3.5

End stages of the synthesis of cis-maneonene A 1.2.

OH

O

Recrystallise from aq. MeOH

Blue-black metallic like material

SiMe3

OMe

H Br

H Br

O

Br

3.4

O

O

SiMe

OMe

3.2 (major diasteroisomer)

200 °C

OH

hexane-MeOH-EtOAc

MgBr2, ether -78 °C

OMe

Br

O

H2, Lindlar catalyst

SiMe3

Li O

OH

3.2 1. Bu4N+F- in THF, 25 °C 2. Recrystallise from EtOAc-petroleum OH

O

on standing in light (or heat, or γ-rays) O

H Br

OMe

Scheme 4

4.1

Desilylation of the silylated diacetylene to produce the diyne 4.1.

The last stages of our synthesis of cis-maneonene A 1.2 are summarised in Scheme 3. The partial catalytic hydrogenation sequence worked perfectly to introduce the cis-enyne functionality (3.3), and the remaining steps were completed in short order through the intermediates 3.4 and 3.5. In order to know the relative stereochemistry of the hydroxy-substituent in the intermediate diacetylene 3.2 we attempted to grow crystals suitable for X-ray analysis by recrystallisation from aqueous methanol. Surprisingly, we obtained blue-black crystals with a shiny metallic appearance on the surface (Scheme 4 and Figure 1). We subsequently discovered through the collaboration about to be described that if the silylated diyne was desilylated with fluoride in the absence of light, a colourless crystalline product was obtained that was the free diacetylene 4.1.

Synthesis of Acetylenic Natural Products

Figure 1

337

Optical microscope images of (a) the colourless crystals of the diyne 4.1 and (b) the blue-black crystals described in Scheme 4 (the latter using plane polarised light).

Exposure of this material to light or heat or gamma irradiation afforded the coloured crystals which eventually turned a jet black colour. The colourless crystals are illustrated in Figure 1 together with a view of the coloured crystals through an optical microscope using polarised light. The usual reaction of a synthetic organic chemist at this stage would have been to discard the unwanted material and to proceed to completion of the synthesis in hand. However, we were blessed by having Professor John Meurig Thomas as Head of the Department of Physical Chemistry. Not only was he a world leader in the properties of solid-state materials, but also he engaged in lively outreach to all his colleagues, including the synthetic organic chemists. So we approached him and explained what we had done, showed him the crystals and asked his advice. We remember him taking us aside and explaining that we had probably inadvertently prepared an ordered poly(diacetylene), which exhibited interesting non-linear optical properties and the phenomenon of pleochroism or anisotropy in the behaviour of the crystals to the transmission of polarised light. He pointed us to William Jones (presently Head of the Department of Chemistry in Cambridge) and Gordon Parkinson, who kindly helped us acquire the image shown in Figure 1b. This clearly shows the rosy purple appearance of the crystals through transmitted polarised light and the essential transparency of the same crystals when turned through an angle of about 901 through the long axis. At that time we were also strongly influenced by two sabbatical visitors in Cambridge, Craig Eckhardt from the University of Nebraska-Lincoln and J. Michael McBride from Yale. Through many discussions and suggestions from all the above-mentioned people, we were encouraged to follow up this observation through investigation of the properties of the diyne 4.1. The original sample had been prepared by Clive Jennings-White as part of his PhD project. He was succeeded by David Kendrick who completed the synthesis of the maneonenes (Scheme 3) and showed that crystals of the freshly prepared diyne 4.1 were colourless.9 The original crystals were analysed

338

Figure 2

Chapter 20

Chem-3D representation of (a) X-ray crystal structure of diyne 4.1 and (b) crystal packing.

by Dr (now Professor) Paul Raithby, but the samples and full X-ray data have not survived the passage of time. Samples of the silylated diyne were also prepared by Guy Pooley during his PhD project and these have recently been reconverted into the diyne in collaboration with Dr Wallace Wong in Melbourne; the X-ray analysis of these freshly prepared crystals was carried out by Jonathan White in Melbourne.10 The X-ray structure is illustrated in Figure 2a and the unit cell (originally analysed by Professor McBride) is shown in Figure 2b. Analysis of the unit cell (Figure 2b) of the crystals of the diyne 4.1 clearly shows the orientation of the diacetylene units in the ‘‘ladder-like’’ arrangement proposed by G. Wegner to explain the phenomenon of polymerisation of ordered diacetylenes in the solid state.11 The mechanism of this free radical-induced process has subsequently been exhaustively analysed by Sixl and is thought to involve alkynyl carbene intermediates.12 We therefore followed up the original observation by monitoring the solid-state polymerisation of the diyne 4.1 under gamma irradiation. Working with a PhD student, Joan Pennington, and with Bill Jones and Gordon Parkinson, we analysed Weissenberg photographs of the behaviour of the diyne as a function of time to try and correlate the original order in the monomeric crystal with that in the developing polymer. We received financial support from the British Technology Group whose programme manager Dr Ken Hills suggested that we engage with other collaborators to follow up the non-linear optical properties of the polydiacetylenes. Dr Hills also drew our attention to the physics group led by Richard Friend in the Cavendish Laboratory, and their use of polyacetylenes in field effect transistors.

3 Light Emitting Polymers – a Shift in Direction Following the recommendation from Dr Hills we eventually made contact with Richard Friend and were able, in 1988, to write a joint grant proposal for a new UK Science and Engineering Research Council (now Engineering and Physical Sciences Research Council) call for research under the ‘‘Molecular Electronics Initiative’’. The proposal called for the study of polydiacetylenes and

339

Synthesis of Acetylenic Natural Products

poly(arylene vinylene)s in applications for optoelectronics. Paul Burn was appointed to a postdoctoral position in chemistry and joined the Cavendish team of Jeremy Burroughes and Donal Bradley. Adam Brown (physics) and Arno Kraft (chemistry) joined the team in the following year. The project called for the synthesis of diacetylenes and poly(p-phenylene vinylene) (PPV, 5.6) the synthesis of which was developed and improved in Lensfield Road (Scheme 5). In investigating the properties of this PPV as an insulating material for field effect transistors, Jeremy Burroughes observed that it emitted green-yellow light when sandwiched between charged electrodes. This caused much excitement as, although the electroluminescence (EL) of thin films of conjugated organic molecules had by then been well established by Tang and Van Slyke,13 no-one had expected this phenomenon in conjugated polymers. The team rapidly turned its attention to the field of polymer EL and for about a year had opportunities to exploit a field of research without competition from other groups. The first results were communicated in 1990 after much effort to protect them in patent applications.14

S

Cl

S

CH2Cl

ClCH2

S

Cl

5.2

5.1 Base

Cl

Quinomethide intermediate (detectable by UV/VIS)

S 5.3

OMe MeOH 5.5

n

S

Cl

Heat, H+ Heat

5.6

Scheme 5

n

Wessling sulfonium route to PPV 5.6.

5.4

n

340

Chapter 20

The phenomenon of organic and polymer EL is best understood by consideration of the simplified Jablonski diagrams in Figure 3.15 Photoexcitation can excite an electron from the highest occupied molecular orbital (HOMO, the valence band) to the lowest unoccupied molecular orbital (LUMO, the conduction band). Radiative decay from the first excited singlet state leads to fluorescence. The HOMO–LUMO energy gap determines the wavelength of light emitted and the fraction of photons emitted as a function of those absorbed is a measure of the photoluminescence (PL) efficiency. This figure can reach about 60% for the most efficient solid-state materials. PL in solid organic films is generally a much less efficient process than in dilute solution where the excited entities are kept well separate and have no efficient nonradiative pathway by which to decay. Singlet lifetimes are generally of the order of nanoseconds. In the EL experiment (Figure 3b) charge injection at the anode leads to negatively charged species (radical anions or negative polarons) at the interface of the polymer with the cathode and to radical cations (positive polarons) at the interface with the anode. When the organic material is a thin film of the order of 100 nm, the strong electric field created by the applied bias voltage across the electrodes draws the charged species to the oppositely charged electrode by a hopping mechanism from chain to chain. Charge annihilation on the same conjugated segment results in the formation of an exciton, which as a singlet can produce fluorescence, as in the PL experiment. EL is therefore the consequence of fluorescence induced by double charge injection and charge annihilation. The typical Cambridge device configuration illustrated in Figure 4 is representative of all polymer light emitting devices (LEDs) fabricated up to the present day. (a)

LUMO

hν

photon

HOMO Singlet state

(b) Cathode (Ca, Li, Al etc) Electron injection Conduction band (LUMO) Hole injection Valence band (HOMO)

Figure 3

Radical anion (negative polaron)

Singlet state

photon Radical cation (positive polaron)

Anode (ITO)

Excitation of and emission from a conjugated species by (a) photoluminescence and (b) electroluminescence.

341

Synthesis of Acetylenic Natural Products * PPV

Al, Mg or Ca cathode

* n

External circuit

Indium tin oxide (ITO) anode

glass substrate

Figure 4

A typical device configuration for a polymer LED. OC6H13 MeO *

*

*

* n OMe

6.1

Scheme 6

OC6H13 CN

* m

C6H13O

NC C6H13O

* n

R

6.2

R

n

6.3

Tunable copolymers 6.1, cyano-PPV 6.2 and polyfluorene 6.3.

The early Cambridge work showed that the wavelength of light emitted was essentially determined by the HOMO–LUMO gap of the luminescent polymer. The first PPV polymers 5.6 were prepared by the Wessling route (Scheme 5).16 Variation of substituents and the use of the Gilch dehydrohalogenation procedure17 on derivatives of the bis-chloromethyl benzene 5.1 carrying solubilising side chains led to solution-processible conjugated polymers that essentially emitted over much of the visible spectrum. An important contribution was the recognition that statistical copolymers (e.g. 6.1, Scheme 6) could be used to tune this emission; this observation lies at the heart of all commercial light emitting polymer technology.18,19 Many factors determine the efficiency of polymer LEDs. These include the efficiency of charge injection, the mobility of charges in the device (and the balance of mobility of charges), the percentage of electronhole recombinations that leads to fluorescence and the efficiency of out-coupling of emitted light.15,20 In respect of the barrier to charge injection, it was discovered that the solution-processible cyano-substituted PPV derivative 6.2 (Scheme 6) exhibited a significantly higher electron affinity than the earlier generations of PPV derivatives and enabled efficient negative charge injection using aluminium electrodes, resulting in a 10-fold enhancement in device efficiency.21 This material formed one of the very early polymers used in a bulk polymer heterojunction photovoltaic device to reverse the LED function of the active polymer layer and harness light and turn the energy into electricity.22

4 Polyfluorenes and Analogues The poly(arylene vinylene)s have a HOMO–LUMO gap that is not large enough to generate blue luminescence. This problem has been solved by the

342

Chapter 20 *

* *

* Si C8H17

C8H17 7.1

Scheme 7

n

Si C6H13

C6H13

n

7.2

Poly(2,7-dibenzosiloles) 7.1 and poly(3,6-dibenzosiloles) 7.2.

use of polyfluorenes (e.g. 6.3, Scheme 6) that have become the mainstay of most commercial light emitting polymers.23,24 However, it has emerged that polyfluorene can develop a long wavelength emission (around 530 nm) that has been attributed to the presence of fluorenone defects.25 These can be minimised by purification techniques26 or, as we have shown, by simply replacing the carbon atom at C-9 in fluorene by a silicon atom as in the 2,7-polydibenzosiloles 7.1 (Scheme 7).27

5 Triplet Emitters and Phosphorescence Reference to Figure 3b indicates that the spin state of the recombining charged species is statistical and it can be argued that this should generate three times as many triplet excited states as singlets. The triplet states are much longer-lived than singlets and are not efficiently emissive because they have a greater chance of non-radiative decay than fluorescence. This problem was first addressed in small molecule EL by Forrest and Thompson and their colleagues who showed that incorporating phosphorescent organometallic complexes could result in efficient energy transfer (Dexter transfer) from the organic host to the phosphorescent guest, leading to very large improvements in device efficiency.28 The energy transfer must take place from a higher energy triplet host to a lower energy phosphorescent guest, and this technology is likely to have applications in all red organic LEDs. The opportunity exists to apply these principles to polymer LEDs as well. The phosphorescent guest could be simply blended with the polymer host, although there may be problems with solution processibility of the phosphorescent small molecule, or covalently attached to the polymer host. We and others have addressed these opportunities. We have covalently linked the polymer host conjugatively to the phosphorescent ligand complex as in the fluorene derivative 8.1 and also tethered the iridium(III) complex non-conjugatively to the 9-position of the fluorene as in the complex 8.2 (Scheme 8).29,30 Both materials produce devices that are more efficient than the corresponding blended material, but the more efficient material was the non-conjugated tethered complex 8.1 with a long (eight carbon atoms) tether. Such approaches require the preparation of high triplet energy polymer hosts, and the poly(3,6dibenzosilole) 7.2 illustrates one such opportunity.31 However, even higher

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Synthesis of Acetylenic Natural Products

S C8H17 C8H17

N O

S Ir N O

C8H17 C8H17

8.1

S

N

S Ir

O

N O H17C8

H17C8

8

H17C8

C8H17 n

m

C8H17 m

8.2

Scheme 8

Solution-processible polymeric phosphorescent materials 8.1 and 8.2.

triplet energy host materials will be required for blue electrophosphorescent guests.

6 Future Prospects Cambridge Display Technology was founded by the inventors of light emitting polymers in partnership with the University of Cambridge and Cambridge Research and Innovation Limited. The company has developed three research centres and a joint venture with Sumitomo.32 These efforts have produced light emitting polymers that emit over the full range of the visible spectrum and that have device lifetimes suitable for commercialisation in full colour flat panel displays such as are seen in mobile telephones, hand held personal organizers and laptop computers. Extrapolated lifetimes (in hours) for polymer devices

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under constant current driving conditions from an initial brightness of 100 cd m 2 are: Red – 400,000 h (10 cd A 1); Green – 600,000 h (14 cd A 1); Blue – 330,000 h (9 cd A 1).33 When added to the potential of fabricating these devices by inkjet printing techniques, prospects for commercialisation are indeed very good. While some early products are already in the marketplace, the growing success and improvements in liquid crystal displays will continue to provide a focus for improvement and optimisation. However, it is already very clear that organic luminescent materials will have applications in displays, in solid-state lighting and in the reverse process of organic based solar cells that are expected to contribute to the problem of sustainable power generation from low cost large area devices.

Acknowledgement We acknowledge with grateful thanks the personal inspiration and encouragement provided by Professor Sir John Meurig Thomas throughout this project, and it is a pleasure to dedicate this article to him. None of this work would have been possible without the long-term collaboration with Professor Sir Richard Friend and colleagues in the Cavendish Laboratory, and we appreciate their inspiration, patient teaching and warm friendship over many years. We acknowledge with thanks the contributions of our many co-workers on this project and the generous financial support provided by the EPSRC, the European Commission, the Australian Research Council, CSIRO, the University of Melbourne and the Victorian Endowment for Science Knowledge and Innovation, and Cambridge Display Technology Limited.

References 1. J.W. Daly, I. Karle, C.W. Myers, T. Tokuyama, J.A. Waters and B. Witkop, Proc. Natl. Acad. Sci. U.S.A., 1971, 68, 1870. 2. S.C. Carey, M. Aratani and Y. Kishi, Tetrahedron Lett., 1985, 26, 5887. 3. G. Stork and K. Zhao, J. Am. Chem. Soc., 1990, 112, 5875. 4. M.S. Karatholuvhu, A. Sinclair, A.F. Newton, M.-L. Alcaraz, R.A. Stockman and P.L. Fuchs, J. Am. Chem. Soc., 2006, 128, 12656. 5. G.M. Williams, S.D. Roughley, J.E. Davies and A.B. Holmes, J. Am. Chem. Soc., 1999, 121, 4900. 6. E.C. Davison, M.E. Fox, A.B. Holmes, S.D. Roughley, C.J. Smith, G.M. Williams, J.E. Davies, P.R. Raithby, J.P. Adams, I.T. Forbes, N.J. Press and M.J. Thompson, J. Chem. Soc., Perkin Trans. 1, 2002, 1494. 7. A.B. Holmes, R.A. Raphael and N.K. Wellard, Tetrahedron Lett., 1976, 1539. 8. S.M. Waraszkiewicz, H.H. Sun, K.L. Erickson, J. Finer and J. Clardy, J. Org. Chem., 1978, 43, 3194. 9. A.B. Holmes, C.L.D. Jennings-White and D.A. Kendrick, J. Chem. Soc., Chem. Commun., 1983, 415.

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10. The crystallographic data has been deposited with the Cambridge Crystallographic Data Centre (ref. CCDC 648037). 11. G. Wegner, Z. Naturforsch., 1969, 86, 824. 12. H. Sixl, Adv. Polym. Sci., 1984, 63, 49. 13. C.W. Tang and S.A. Van Slyke, Appl. Phys. Lett., 1987, 51, 913. 14. J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burn and A.B. Holmes, Nature (London), 1990, 347, 539. 15. A. Kraft, A.C. Grimsdale and A.B. Holmes, Angew. Chem., Int. Ed., 1998, 37, 402. 16. R.A. Wessling, J. Polym. Sci., Polym. Symp., 1985, 72, 55. 17. H.G. Gilch and W.L. Wheelwright, J. Polym. Sci., Part A1, 1996, 4, 1337. 18. P.L. Burn, A.B. Holmes, A. Kraft, D.D.C. Bradley, A.R. Brown, R.H. Friend and R.W. Gymer, Nature (London), 1992, 356, 47. 19. P.L. Burn, A.B. Holmes, A. Kraft, D.R. Baigent, D.D.C. Bradley, A.R. Brown, R.H. Friend, R.W. Gymer, A.B. Holmes and R.W. Jackson, J. Am. Chem. Soc., 1993, 115, 10117. 20. R.H. Friend, R.W. Gymer, A.B. Holmes, J.H. Burroughes, R.N. Marks, C. Taliani, D.D.C. Bradley, D.A. Dos Santos, J.L. Bre´das, M. Lo¨gdlund and W.R. Salaneck, Nature (London), 1999, 397, 121. 21. N.C. Greenham, S.C. Moratti, D.D.C. Bradley, R.H. Friend and A.B. Holmes, Nature (London), 1993, 365, 628. 22. J.J.M. Halls, C.A. Walsh, N.C. Greenham, E.A. Marseglia, R.H. Friend, S.C. Moratti and A.B. Holmes, Nature (London), 1995, 376, 498. 23. U. Scherf and E.J.W. List, Adv. Mater., 2002, 14, 477. 24. I.D. Rees, K.L. Robinson, A.B. Holmes, C.R. Towns and R. O’Dell, MRS Bull., 2002, 451. 25. E.J.W. List, R. Gu¨ntner, P. Scanducci de Freitas and U. Scherf, Adv. Mater., 2002, 14, 374. 26. M.R. Craig, M.M. de Kok, J.W. Hofstraat, A.P.H.J. Schenning and E.W. Meijer, J. Mater. Chem., 2003, 13, 2861. 27. K.-L. Chan, M.J. McKiernan, C.R. Towns and A.B. Holmes, J. Am. Chem. Soc., 2005, 127, 7662. 28. C. Adachi, D.F. O’Brien, M.E. Thompson and S.R. Forrest, J. Appl. Phys., 2001, 90, 5048. 29. A.J. Sandee, C.K. Williams, N.R. Evans, J.E. Davies, C.E. Boothby, A. Ko¨hler, R.H. Friend and A.B. Holmes, J. Am. Chem. Soc., 2004, 126, 7041. 30. N.R. Evans, L.S. Devi, C.S.K. Mak, S.E. Watkins, S.I. Pascu, A. Ko¨hler, R.H. Friend, C.K. Williams and A.B. Holmes, J. Am. Chem. Soc., 2006, 128, 6647. 31. K.L. Chan, S.E. Watkins, C.S.K. Mak, M. McKiernan, C.R. Towns, S.I. Pascu and A.B. Holmes, Chem. Commun., 2005, 5766. 32. Cambridge Display Technology Limited – http://www.cdtltd.co.uk/. 33. D. Fyfe, All Plastic OLED Displays – Hype Revived?, Plenary lecture presented at Plastic Electronics Conference, Frankfurt, Germany, October 24–25, 2006.

CHAPTER 21

Molecular Recognition within One-Dimensional Channels MARK D. HOLLINGSWORTH Department of Chemistry, 111 Willard Hall, Kansas State University, Manhattan KS 66506, USA

1 Introduction Just as it was a great privilege to work as a postdoctoral research associate with John Meurig Thomas (JMT throughout this chapter), it is now a great honour to contribute to this book that celebrates his tremendous enthusiasm for science and his many years of leadership in the field of solid-state chemistry and materials science. JMT has influenced my career and outlook on science in too many ways to enumerate here, so this chapter will restrict itself to tracing some of the origins of a long-standing research program in molecular recognition to my postdoctoral work with him. I had started my graduate research with Mike McBride in solid-state organic photochemistry in 1980, the year that Mike had spent a sabbatical semester with JMT in Cambridge. In 1983, when I wandered into Mike’s office to ask what he thought I should do after I finished my PhD, his answer went something like, ‘‘Why not do a post-doc with John Thomas in Cambridge? He is one of the brightest lights in British science, and it would broaden your horizons.’’ This was great advice. Mike was well aware of JMT’s elegant work in both solid-state organic and inorganic chemistry, and the more I read about this unfamiliar side of my field of research, the more intrigued I became. During my PhD work, we had been so wrapped up with ‘‘seeing’’ the fragments rearrange in organic crystals during solid-state reactions1–4 that the prospect of ‘‘seeing’’ atoms with electron microscopy 5–8 was tantalizing, to say the least. By 1983, JMT’s reach in the field was already quite long. As I was digging into his work, I found that JMT had already published articles in over 100 journals, including 25 in Nature, half of which had been published in the previous five years. His work in solid-state organic chemistry and crystal 346

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347

engineering, especially with Bill Jones, was much more familiar to me than his work on zeolites, but throughout there were many common threads17–21 with the work that I was doing in the McBride group on the role of local stress in solid-state reactions. It is appropriate here to thank JMT for his tremendous patience with me as I tried to finish my dissertation and begin my work in his group. The grant from the Science and Engineering Research Council that was to fund my research had started early in 1984, but the best I could do was to show up for a month in September of 1984 to overlap with Charis Theocharis, who was wrapping up his work in the group. The plan was to return to Yale to finalize my dissertation and be back in Cambridge by January. Unfortunately, I learned the hard way that it is essential to ‘‘dock’’ before you ‘‘postdoc.’’ When I returned to Yale in October of 1984, the threads that held my dissertation together began to fray, and I found that much of what I had written was wrong! Two months turned into thirteen, and I finally made it to Cambridge for good in November 1985. Luckily, by then I had my own funding, so the fact that JMT’s grant was almost finished was not as much of an obstacle. Upon reviewing our correspondence from 1983 to 1985, I am simply amazed at how gracious and understanding JMT was about my struggle with my dissertation as well as my ever-unrealistic projections of when I might eventually finish it. My loss was that JMT moved from Cambridge to the Royal Institution in the summer of 1986, so I was not able to work with him as much as I would have otherwise. Soon after my arrival, I set about trying to use electron paramagnetic resonance (EPR) to observe radical pairs by photolysing long-chain peroxides in channel-type zeolites such as silicalite (Scheme 1). From work in the McBride group, we knew that the zero-field splitting in EPR spectra of triplet radical pairs could provide exquisitely accurate distances between the radical centres and that hyperfine splittings (hfs) could provide information on dynamics,1–4 so we were optimistic that this work would complement the other research in JMT’s group on intercalation and reactions of organic guests in zeolites and clay minerals. When, in the first EPR experiments, we found only isolated radicals upon photolysis of diundecanoyl peroxide (UP) in silicalite, our attention turned immediately to urea inclusion compounds (UICs), which we thought would be more controllable analogs of the channel-type zeolites. My PhD research had focused on the local stress that is generated in solid-state photofragmentation reactions of peroxides. By using the asymmetric stretching mode of CO2 as a probe, we had shown that reaction-generated stresses could be as large as

Scheme 1

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20 kbar (B20,000 atmospheres) in certain single crystals of peroxides.1–4 Because the rigid framework of the urea channels and the loose packing of guests within them constrained the reaction-induced stress to be transmitted more or less along the channel axis, this seemed like an ideal way of turning this terribly complicated three-dimensional (3-D) problem of anisotropic stress into a 1-D one. The 1-D channel systems were complicated enough, however. At liquid nitrogen temperatures, light UV photolysis of UP/urea gave a clear signal from a triplet radical pair that had a maximum zero-field splitting when the needle axis of the crystal (the channel axis) was aligned along the magnetic field. However, the hfs from eight nearby protons made these spectra horribly complex and not amenable to any simple analysis.22 Furthermore, the temperature dependence of the zero-field splitting was anomalous, since it appeared that the distance between radicals was changing as a gradual function of temperature from 20 to 190 K instead of in discrete steps, as one might have expected from related studies on molecular crystals. The confusion over these deceptively simple spectra of unlabeled radicals was resolved by employing diundecanoyl peroxide that had been labeled with deuterium in the a and b positions (from B. E. Segmuller1). With their collapsed hfs, the much simpler spectra from these crystals showed that between 35 and 190 K, there were at least three discrete radical pairs that existed in equilibrium with each other.23 The symmetry averaged projections of the inter-radical vector along the channel axis increased from 6.8 A˚ (Pair 1) to 8.5 A˚ (Pair 2) to 9.5 A˚ (Pair 3) as the temperature was raised. Pair 2, in which the inter-radical separation approximated the sum of the van der Waals radii of the fragments, was enthalpically the most stable, but Pair 3 predominated at the highest temperatures. Although the radicals in this pair appeared to be beyond van der Waals contact, Pair 3 was favoured by entropy at the highest temperatures because of the increased rotational freedom that occurs in that species. The rates of collapse of radical pairs, which were highly dispersive, were influenced appreciably by substituents.22 Both 5-bromopentyl radicals (from 6-bromohexanoyl peroxide/urea or 6-BrHP/urea)) and 3-methyloctyl radicals (from 4-methylnonanoyl peroxide/urea) decayed more slowly than the decyl radicals generated in UP/urea, suggesting either that the mid-chain methyl or the terminal bromine could act as ‘‘anchors’’ or that these provided steric barriers to the escape of CO2 from the radical pair cage. One question that arose, however, was whether the rates of collapse could be influenced by terminal substituents in neighboring guest molecules. There was ample precedent for remote substituent effects in the photoreactions of single crystals of peroxides,1,4,24,25 so intermolecular substituent effects seemed likely. Because of the tremendous sensitivity of EPR, it also seemed likely that mixed UICs containing terminally substituted peroxides as dilute impurities (say 2%) in the presence of a,o-disubstituted alkanes (98%) could be used to study the influence of intermolecular environment on the structure and kinetics of radical pairs (Scheme 2). Such ‘‘solvent effects’’ would be very specific since the guest molecules were constrained to lie in the same urea tunnel and within van der

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Scheme 2 Waals contact. The 1-D urea tunnels seemed ideal for isolating interactions between terminal functional groups, and EPR of radical pairs seemed particularly well suited to studying their influence on solid-state reactions. Although I made preliminary attempts to detect radical pairs in mixed UIC crystals of 6-BrHP and 1,10-dicyanodecane during the summer of 1986, it soon became apparent that I had no idea of whether the incorporation of the minor guest would be random or not. As I was starting my independent academic career in 1987, it dawned on me that the 1-D channels of urea were an especially good way of understanding molecular recognition and that cross polarization magic angle spinning (CP-MAS) nuclear magnetic resonance (NMR) might be ideal for probing these functional group interactions. This research program appeared to have much broader implications for the field of crystal engineering and solid-state chemistry than the original study of substituent effects on the collapse of radical pairs. After Kurt Zilm had joined the faculty at Yale, I had become familiar with some aspects of solid-state NMR,26,27 but I had gained a much more detailed knowledge of this technique while in Cambridge. In the early 1980s, JMT and his colleagues had been pioneers in the use of solid-state NMR to study aluminosilicates,28–34 and Kenneth Harris, who was JMT’s graduate student and my closest colleague in Cambridge, had been studying guest dynamics of perdeuterated hexadecane in urea with 2H NMR.35 Just as with the zeolites, where the large chemical shift dispersion of 29Si and 27 Al could be used to distinguish different sites, 13C, 15N and 19F MAS NMR seemed well suited to ascertain and quantify the local environments of terminal functional groups. Because the UICs were typically incommensurate solids, the guest molecules should reside in a multitude of roughly equivalent host environments.36–38 Extensive spectroscopic work had shown that linear hydrocarbons and many of their terminally substituted analogs exhibited high amplitude librations and/or rapid rotations about the channel axes of these inclusion compounds.39–46 Such rapid motions suggested only modest interactions between the hosts and guests, so the guests were expected to give rise to sharp lines in the CP-MAS spectra. This was certainly the case for a,o-disubstituted alkanes, whose terminal carbons typically gave sharp singlets and whose chemical shifts were relatively insensitive to chain length in the UICs that were thought to be incommensurate. Although the original idea had been to study the interactions between guests in mixed UICs, the NMR study was better suited to channel inclusion compounds containing unsymmetrically substituted guests. In such systems, where H and T are the head and tail of the guest, respectively, the end-group parity

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could be ‘‘head-to-head’’ (HH), ‘‘tail-to-tail’’ (TT) or ‘‘head-to-tail’’ (HT). Unlike other studies of molecular recognition in the solid state, this 1-D channel system isolated the functional groups of interest as pairs in a system that gave essentially the same crystal packing for a wide variety of chain lengths and functional group identities. Because of the orientational averaging in these crystals, it seemed reasonable that the resonances for terminal functional groups in HH and HT pairs could be resolved and that it would therefore be possible to measure the populations of these species. With a statistical distribution of guest orientations, the ratio of (HH+TT) and 2(HT) would be unity, and the peak ratio for a given resonance would be 1:1. To the extent that guest–guest and/or host–guest interactions biased the orientation of guests in the growing crystal, however, the guests should pack in a non-random fashion, and this would be reflected in the peak intensities. For the UICs, this became a study of molecular recognition during crystal growth. Because the UICs containing guests with very short chains are unstable, it was necessary to use guests with a minimum of six or so carbons in their chains. However, once these guest molecules were incarcerated within the urea tunnel, the rigid, hydrogen-bonded host framework effectively prevented them from undergoing either end-for-end reorientation or ‘‘reptation’’ past each other to a new location (and recognition site) in the channel. In many cases, such as methyl 10-undecenoate/urea (Figure 1), the chemical shift dispersion of terminal functional groups was adequate to resolve the HH and HT pairs, but the bias (2(HT)/(HH+TT) ¼ 1.3) was fairly small. The similarity in molecular recognition for this unsaturated ester and its saturated analog (methyl undecanoate, where 2(HT)/(HH+TT) ¼ 1.27)47 suggested the preferential adsorption of the ester (a hydrogen bond acceptor) onto the {0001} growth faces of this crystal. In other cases, such as cyanoalkanes, both the site splitting and the bias were much larger.23 With the nitrile carbons and other nitrogen-containing systems, however, it was necessary to distinguish site splittings (i.e. HH versus HT) from residual dipolar interactions between 13C and 14N that were not completely averaged by the MAS.48 Such residual dipolar interactions are typically manifested as 2:1 doublets, which was exactly the ratio observed for 1-cyanodecane/urea! With the incommensurate UICs in which guest– guest interactions were relatively weak, large amplitude guest motions are sufficient to average these interactions and give narrow singlets for each type of site. Initially, however, this was the source of great confusion, particularly because other authors had erroneously interpreted band doubling in related spectra in terms of residual dipolar interactions49,50 but also because our first dinitrile/urea ‘‘standard’’ (sebaconitrile/urea) was a commensurate structure in which the host and guest were hydrogen bonded to each other.51 Such hydrogen bonding effectively quenches the guest motions and gives rise to significant residual dipolar interactions, which can be distinguished from site splittings by their inverse dependence on magnetic field (or by labeling with 15N).

Molecular Recognition within One-Dimensional Channels

Figure 1

351

13

C CP-MAS NMR spectrum (50.3 MHz) of methyl 10-undecenoate/urea, showing band doubling for the methoxy carbon and the two alkenyl carbons, but not for the carbonyl. Chemical shifts for HH and TT pairs were established with spectra of dimethyl sebacate/urea and with 1,8-nonadiene/urea, which gave sharp singlets for the resonances from terminal functional groups. In this crystal, grown from methanol, there is a small bias towards HT alignment of guests.

2 A Scale of Functional Group Interaction Energies? Although the original goal of this work had been to study the kinetic control of guest orientations and occupancies during crystal growth, it soon became apparent that by achieving thermodynamic control of guest orientations, this method could provide an empirically determined set of rules that could guide researchers in their attempts to design new systems such as polar crystals and supramolecular assemblies. In most molecular crystals, each molecule is surrounded by six nearest neighbours, so the lattice sums that determine the overall crystal packing typically contained several hundred terms, only a fraction of which were known accurately, if at all. When designing new materials (or other supramolecular systems), however, chemists needed the answers to simple questions: Which contacts are reliable? When there is a choice between two or more interactions, which way will it go? What is the energetic bias? By isolating functional groups as pairs in a 1-D channel that approaches the spatial constraints of a molecular crystal, while exhibiting the equilibrium control of partitioning that is characteristic of liquids and gases, the goal of this research program was to answer the simple question of ‘‘What sticks to what, and by how much?’’ Equilibrium control of the guest

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orientations in 1-D channels seemed to be a powerful way of developing a selfconsistent scale of interaction energies for a variety of functional group pairs, and the hope was that it would complement the work of Peggy Etter and others on hydrogen bond preferences.52–54 But how to achieve equilibrium and transferability of interactions? Because of the chain length and stability problems mentioned above, thermodynamic control was not possible for the UICs, so we soon turned our attention to channel inclusion compounds of perhydrotriphenylene (PHTP; Scheme 3). Since the mid-1960s, Mario Farina and his co-workers had studied the inclusion properties of this host and had shown that PHTP, like urea, typically formed incommensurate inclusion compounds in which the guests were packed within van der Waals contact of each other.55–59 The crystal packing in PHTP inclusion compounds containing linear guests was essentially isomorphous, and the channels were separated by 14.3 A˚, so interchannel ordering of guests, which we had observed in certain UICs,60–62 was insignificant. Combined with the relative stability of PHTP inclusion compounds containing short chain guests, the wider (B5.7–6.7 A˚) and more flexible channels of PHTP gave us a fighting chance to observe end for end exchange of the guest molecules within the tunnels. The channel walls of PHTP were also quite ‘‘smooth,’’ and were composed of simple hydrocarbons, so this system minimized specific host–guest interactions (Figure 2). Just as with urea, the 13C CP-MAS NMR spectra of dozens of unsymmetrically substituted alkanes (X(CH2)nY) included in PHTP showed asymmetric band doubling for terminal functional groups and adjacent carbons, but none for internal carbons.63 And, once again, aliphatic nitriles exhibited a significant bias towards head-to-head alignment (Figure 3a–c), this time with a 4:1 ratio of (HH+TT)/(2HT). Although we had long sought equilibrium control of guest partitioning, we were still surprised (and obviously delighted) by the spectrum of 1-cyanopentane/PHTP (Figure 3d), whose nitrile carbon region exhibited a single, broad Lorentzian line between the peaks we had observed for HH and HT pairs in higher homologs. As temperature dependence studies of numerous guests have now demonstrated, 1-cyanopentane undergoes end-for-end exchange that is rapid enough to average the HH and HT resonances to a single line at room temperature (Scheme 4). With the higher homolog, 1-cyanohexane, coalescence occurs near 320 K; analysis of the spectra for this exchanging system showed that for a two-state system (with energies EHT and (EHH+ETT)/2), enthalpy favours the ‘‘symmetric’’ state by 5.2(3) kcal mol
1. Entropy disfavours

Scheme 3

Molecular Recognition within One-Dimensional Channels

Figure 2

353

Channel axis view of the van der Waals surface of an inclusion compound of PHTP containing an arbitrarily oriented linear hydrocarbon. Coordinates are from Ref. 56.

the same state by 14(3) cal mol
1 K
1, presumably because of restricted motional freedom for the CN    NC pairs, which are thought to be overlapped significantly and tethered by dipole–dipole interactions.64 As outlined in Table 1, the use of guests with short chain lengths in PHTP provides a general strategy for studying many other functional group pairs. The principal criteria are isomorphous packing in a minimally interacting system that allows equilibrium control of partitioning between functional group pairs. This work is not without its difficulties, however. The rates of end-for-end exchange of guests are dramatically dependent on chain length and substituents, as are the residual dipolar interactions that complicate interpretation of these spectra. Unpublished NMR studies in our laboratory by Ulrike WernerZwanziger show that the nitrile peaks in 1-cyanoheptane/PHTP show no sign of broadening and coalescence even at 358 K, so this constrains the chain length of the guest quite significantly.65 In those systems with longer chain guests, site exchange can occur, however, most likely through a reptation mechanism that does not achieve true equilibrium.

354

Figure 3

Chapter 21

13

C CP-MAS NMR spectra (50.3 MHz) of the nitrile region of nitrile/PHTP inclusion compounds. The spectrum of the symmetric dinitrile (f) establishes the chemical shift of the CN    NC pair, whereas that of 7-bromoheptanenitrile (e) helps demonstrate that the upfield resonances in a–c arise from CN  Me pairs. Rapid end-for-end exchange in 1-cyanopentane/PHTP produces a single Lorentzian line (d). At higher temperatures, the bands for 1-cyanohexane/PHTP collapse to a broad singlet whose position shifts with increasing temperature. Adapted from Ref. 63 with permission from the American Chemical Society.

In systems with the potential for strong guest–guest interactions, such as 6-aminohexanenitrile/PHTP, band doubling again occurs in the nitrile region (Figure 4). However, the doubling arises not from site differences between HH and HT pairs, but from residual dipolar interactions that are not averaged by guest motion. In this system, the guests appear to exist almost exclusively as HT

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Scheme 4

Table 1

Rationale for using PHTP and analogs to measure functional group interactions.

Requirements

Approach

1. Isomorphous replacement of substituents 2. Isolation of functional group pairs within van der Waals contact 3. Minimal bias for specific guest positions in channel by host

Use host–guest system in which host structure controls the packing Use channel inclusion compounds

4. Quantitative evaluation of partitioning 5. Transferability of interaction strengths a. Equilibrium, not kinetic control of guest orientations b. DH (not DG) differences required 6. Adequate sensitivity, dynamic range and spectral windows 7. Accurate structural characterization of functional group pairs 8. Independent probes of interaction strengths

Use incommensurate channel inclusion compounds in which the host is less stable without the guest MAS NMR of 13C, 15N, 29Si and 19F shows site differences and occupancies Use short chain length guests in PHTP to allow end-for-end exchange of guests Measure partitioning at different temperatures Use labeled guests (typically 15–90%) Use a range of spectroscopic and diffraction methods to assess pairwise geometries in PHTP and more tractable UICs Use other spectroscopic probes such as IR and Raman

pairs, so the guest molecules behave as a hydrogen bonded polymer whose motions are relatively frozen (Scheme 5). Although equilibrium control of guest partitioning in 1-D inclusion compounds is necessary for measuring the energetic bias for certain pairwise interactions, the kinetic control of guest orientations that normally occurs with larger guests has important implications for materials design. Under kinetic control, guest incorporation may be treated with a Markov chain formalism,66–68 and the same sort of molecular recognition processes described above can give rise to polar arrangements of guests. For unsymmetric guests, the relative energetics of these three types of contacts (and in particular, the

356

Figure 4

Chapter 21

13

C CP-MAS NMR spectrum of 6-aminohexanenitrile/PHTP at 75.3 MHz. The 2:1 doublet near 120 ppm arises from unaveraged residual dipolar interactions between 13C and 14N in this extensively hydrogen-bonded system, not from site differences btween H–H and H–T sites.

Scheme 5 difference between HH and TT pairs) dictate the orientation of guests emerging from opposite ends of these crystals. For 1-(4-nitrophenyl)piperazine in PHTP, for example, strong –NO2    H–N– interactions, weaker –N–H/H–N– interactions and unfavorable –NO2/O2N– interactions give rise to a self-correcting crystal growth mechanism in which opposite ends of the crystal are decorated with nitro groups. As shown by the extensive work by Hulliger and colleagues on these systems,69–77 this appears to be a general phenomenon, and it is directly related to the broader problem of sector-dependent generation of polarity formation in both solid solutions and pure crystals.78–82 Structure solution of 1-(4-nitrophenyl)piperazine/PHTP required a heroic effort by some of the world’s top crystallographers, who used diffuse X-ray scattering and models of disorder to create an exquisitely detailed picture of the disorder in this system.83,84

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3 Prospects and Conclusions Coupled with the generality of the Markov chain model, the empirical method for assessing pairwise functional group interactions described here should facilitate a rational approach for forming polar inclusion compounds. Although 13C and 15N NMR studies will always require a significant synthetic effort to prepare isotopically labeled compounds, the much higher sensitivity of 1 H NMR and especially 19F NMR make them especially well suited to efficient measurement of interactions within functional group pairs. Alternative strategies, developed by Kenneth Harris and colleagues, utilize the differential incorporation of symmetrically disubstituted guests with different chain lengths as a way of separating the contributions of host–guest and guest–guest interactions in 1-D channel systems.85–87 The most significant challenge with this research is the determination of the local structures of functional group pairs. Although comparisons can be made with certain commensurate structures,23,51,62,88–90 which are often interesting in their own right,91,92 obtaining such structures is difficult, at best, in the incommensurate systems containing dynamically averaging guests.93 Nevertheless, we are inspired by the great courage that John Meurig Thomas has shown through his career as he has pushed the limits of diffraction and spectroscopy to sort out the structural details of so many disordered solids.94–99 Thank you, John, for showing us what is possible and for inspiring so many others to follow your lead.

Acknowledgments I would like to thank A. R. Palmer, N. Cyr, K. D. M. Harris, U. WernerZwanziger, M. E. Brown, J. Huang and J. W. Zwanziger for their help with this work, which was supported by the Natural Sciences and Engineering Research Council of Canada, the National Science Foundation (CHE-9423726, CHE-0096157), and the donors of the Petroleum Research Fund of the American Chemical Society (Nos. 24396-AC4 and 43708-AC10).

References 1. J.M. McBride, B.E. Segmuller, M.D. Hollingsworth, D.E. Mills and B.A. Weber, Science (Washington, D.C.), 1986, 234, 830. 2. J.M. McBride, Acc. Chem. Res., 1983, 16, 304. 3. M.D. Hollingsworth, J.A. Swift and B. Kahr, Cryst. Growth Des., 2005, 5, 2022. 4. M. D. Hollingsworth and J. M. McBride, in Advances in Photochemistry, vol 15, D.H. Volman, G.S. Hammond and K. Gollnick (eds), WileyInterscience, New York, 1990, 279. 5. M. Beer, R.W. Carpenter, L. Eyring, C.E. Lyman and J.M. Thomas, Chem. Eng. News, 1981, 59, 40.

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6. G.M. Parkinson, W. Rees, M.J. Goringe, W. Jones, S. Ramdas, J.M. Thomas and J.O. Williams, Conf. Ser. – Inst. Phys., 1978, 41, 172. 7. J.M. Thomas, New Sci., 1980, 87, 580. 8. J.M. Thomas and D.A. Jefferson, Endeavour, 1978, 2, 127. 9. S. Ramdas, W. Jones, J.M. Thomas and J.P. Desvergne, Chem. Phys. Lett., 1978, 57, 468. 10. H. Nakanishi, W. Jones, J.M. Thomas, M.B. Hursthouse and M. Motevalli, J. Phys. Chem., 1981, 85, 3636. 11. H. Nakanishi, W. Jones, J.M. Thomas, M.B. Hursthouse and M. Motevalli, J. Chem. Soc., Chem. Commun., 1980, 611. 12. H. Nakanishi, G.M. Parkinson, W. Jones, J.M. Thomas and M. Hasegawa, Isr. J. Chem., 1980, 18, 261. 13. H. Nakanishi, W. Jones and J.M. Thomas, Chem. Phys. Lett., 1980, 71, 44. 14. H. Nakanishi, W. Jones, J.M. Thomas, M. Hasegawa and W.L. Rees, Proc. R. Soc. London, Ser. A, 1980, 369, 307. 15. W. Jones, H. Nakanishi, C.R. Theocharis and J.M. Thomas, J. Chem. Soc., Chem. Commun., 1980, 610. 16. W. Jones, S. Ramdas, C.R. Theocharis, J.M. Thomas and N.W. Thomas, J. Phys. Chem., 1981, 85, 2594. 17. J.M. Thomas and J.O. Williams, Chem. Commun., 1967, 432. 18. D. Goode, Y. Lupien, W. Siebrand, D.F. Williams, J.M. Thomas and J.O. Williams, Chem. Phys. Lett., 1974, 25, 308. 19. W. Jones, J.M. Thomas and J.O. Williams, Philos. Mag., 1975, 32, 1. 20. S.E. Morsi, J.M. Thomas and J.O. Williams, J. Chem. Soc., Faraday Trans. 1, 1975, 71, 1857. 21. J.M. Thomas, J.O. Williams, J.P. Desvergne, G. Guarini and H. BouasLaurent, J. Chem. Soc., Perkin Trans. 2, 1975, 84. 22. M.D. Hollingsworth, K.D.M. Harris, W. Jones and J.M. Thomas, J. Inclusion Phenom., 1987, 5, 273. 23. M. D. Hollingsworth and K. D. M. Harris, in Comprehensive Supramolecular Chemistry, vol 6, D.D. MacNicol, F. Toda and R. Bishop (eds), Elsevier Science Ltd., Oxford, 1996, 177. 24. M.D. Hollingsworth and J.M. McBride, J. Am. Chem. Soc., 1985, 107, 1792. 25. J.M. McBride, S.B. Bertman and T.C. Semple, Proc. Natl. Acad. Sci. U.S.A., 1987, 84, 4743. 26. L.B. Alemany, D.M. Grant, R.J. Pugmire, T.D. Alger and K.W. Zilm, J. Am. Chem. Soc., 1983, 105, 2133. 27. L.B. Alemany, D.M. Grant, R.J. Pugmire, T.D. Alger and K.W. Zilm, J. Am. Chem. Soc., 1983, 105, 2142. 28. C.A. Fyfe, G.C. Gobbi, J.S. Hartman, J. Klinowski and J.M. Thomas, J. Phys. Chem., 1982, 86, 1247. 29. C.A. Fyfe, J.M. Thomas, J. Klinowski and G.C. Gobbi, Angew. Chem., Int. Ed. Engl., 1983, 22, 259.

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30. C. A. Fyfe, G. C. Gobbi, J. Klinowski, A. Putnis and J. M. Thomas, J. Chem. Soc., Chem. Commun., 1983, 556. 31. J.M. Thomas, J. Klinowski, S. Ramdas, M.W. Anderson, C.A. Fyfe and G.C. Gobbi, ACS Symp. Ser., 1983, 218, 159. 32. U. Selvaray, K.J. Rao, C.N.R. Rao, J. Klinowski and J.M. Thomas, Chem. Phys. Lett., 1985, 114, 24. 33. J. Klinowski and J.M. Thomas, Endeavour, 1986, 10, 2. 34. X. Liu, J. Klinowski and J.M. Thomas, Chem. Phys. Lett., 1986, 127, 563. 35. K.D.M. Harris and P. Jonsen, Chem. Phys. Lett., 1989, 154, 593. 36. F. Laves, N. Nicolaides and K.C. Peng, Z. Kristallogr., 1965, 121, 258. 37. H.U. Lenne, H.C. Mez and W. Schlenk Jr., Justus Liebigs Ann. Chem., 1970, 732, 70. 38. K.D.M. Harris and J.M. Thomas, J. Chem. Soc., Faraday Trans., 1990, 86, 2985. 39. H.L. Casal, D.G. Cameron and E.C. Kelusky, J. Chem. Phys., 1984, 80, 1407. 40. H.L. Casal, D.G. Cameron, E.C. Kelusky and A.P. Tulloch, J. Chem. Phys., 1984, 81, 4322. 41. K. Takemoto and N. Sonoda, in Inclusion Compounds, vol 2, J.L. Atwood, J.E.D. Davies and D.D. MacNicol, (eds), Academic Press, New York, 1984, 47. 42. H.L. Casal, J. Phys. Chem., 1985, 89, 4799. 43. M.S. Greenfield, R.L. Vold and R.R. Vold, J. Chem. Phys., 1985, 83, 1440. 44. F. Imashiro, T. Maeda, T. Nakai, A. Saika and T. Terao, J. Phys. Chem., 1986, 90, 5498. 45. J.I. Lauritzen Jr., J. Chem. Phys., 1958, 28, 118. 46. R.J. Meakins, Trans. Faraday Soc., 1955, 51, 953. 47. M.D. Hollingsworth and N. Cyr, Mol. Cryst. Liq. Cryst., 1990, 187, 135. 48. A. Naito, S. Ganapathy and C.A. McDowell, J. Magn. Reson., 1982, 48, 367. 49. M.D. Hollingsworth and N. Cyr, J. Chem. Soc., Chem. Commun., 1990, 578. 50. M. Okazaki, A. Naito and C.A. McDowell, Chem. Phys. Lett., 1983, 100, 15. 51. M.D. Hollingsworth, B.D. Santarsiero and K.D.M. Harris, Angew. Chem., Int. Ed. Engl., 1994, 33, 649. 52. M.C. Etter, Acc. Chem. Res., 1990, 23, 120. 53. T.W. Panunto, Z. Urba˜nczyk-Lipkowska, R. Johnson and M.C. Etter, J. Am. Chem. Soc., 1987, 109, 7786. 54. K.D.M. Harris and M.D. Hollingsworth, Nature (London), 1989, 341, 19. 55. G. Allegra, M. Farina, A. Immirzi, A. Colombo, U. Rossi, R. Broggi and G. Natta, J. Chem. Soc. B, 1967, 1020. 56. A. Colombo and G. Allegra, Rend. Accad. Naz. Lincei, 1967, 43, 41. 57. M. Farina, in Inclusion Compounds, vol 2, J.L. Atwood, J.E.D. Davies and D.D. MacNicol (eds), Academic Press, New York, 1984, 69. 58. M. Farina, G. Allegra and G. Natta, J. Am. Chem. Soc., 1964, 86, 516.

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85. K.D.M. Harris, P.E. Jupp and S.-O. Lee, J. Chem. Phys., 1999, 111, 9784. 86. S.-O. Lee, K.D.M. Harris, P.E. Jupp and L. Yeo, J. Am. Chem. Soc., 2001, 123, 12913. 87. S.-O. Lee, K.D.M. Harris, P.E. Jupp, L. Elizabe and S. Swinburn, Mol. Cryst. Liq. Cryst., 2001, 356, 517. 88. M.E. Brown, J.D. Chaney, B.D. Santarsiero and M.D. Hollingsworth, Chem. Mater., 1996, 8, 1588. 89. M.D. Hollingsworth, U. Werner-Zwanziger, M.E. Brown, J.D. Chaney, J.C. Huffman, K.D.M. Harris and S.P. Smart, J. Am. Chem. Soc., 1999, 121, 9732. 90. M.D. Hollingsworth, M.L. Peterson, K.L. Pate, B.D. Dinkelmeyer and M.E. Brown, J. Am. Chem. Soc., 2002, 124, 2094. 91. M.E. Brown and M.D. Hollingsworth, Nature (London), 1995, 376, 323. 92. M.D. Hollingsworth, M.L. Peterson, J.R. Rush, M.E. Brown, M.J. Abel, A.A. Black, M. Dudley, B. Raghothamachar, U. Werner-Zwanziger, E.J. Still and J.A. Vanecko, Cryst. Growth Des., 2005, 5, 2100. 93. A. Nordon, E. Hughes, R.K. Harris, L. Yeo and K.D.M. Harris, Chem. Phys. Lett., 1998, 289, 25. 94. K.D.M. Harris, A.R. George and J.M. Thomas, J. Chem. Soc., Faraday Trans., 1993, 89, 2017. 95. M.J. Jones, K.D.M. Harris, G. Sankar, T. Maschmeyer and J.M. Thomas, J. Chem. Soc., Faraday Trans., 1996, 92, 1043. 96. L. Elizabe, L. Yeo, K.D.M. Harris, G. Sankar and J.M. Thomas, Chem. Mater., 1998, 10, 1220. 97. J.M. Thomas and G. Sankar, J. Synchrotron Radiat., 2001, 8, 55. 98. G. Sankar, J.M. Thomas, C. Richard and A. Catlow, Top. Catal., 2000, 10, 255. 99. J.M. Thomas, Chem.–Eur. J., 1997, 3, 1557.

CHAPTER 22

FTIR Study of Short Range Mobility in Some Crystalline Peroxides: Solid-State Rotational Isomerism of CO2 J. MICHAEL McBRIDEa AND KEVIN L. PATEb a

Department of Chemistry, Yale University, Box 208107, New Haven, CT 06520-8107, USA; b Department of Chemistry, Marietta College, Marietta, Ohio, 45750, USA

1 Introduction1 I owe a great deal to organic peroxides – they helped me earn my PhD, find and keep my academic position, and conduct a long series of research projects on solid-state organic chemistry with outstanding collaborators.2 They were also responsible for my meeting a loyal and inspiring friend in John Meurig Thomas. Azoalkanes and desperation had introduced me to solid-state chemistry. After beginning my PhD work with Paul D. Bartlett at Harvard with a preliminary, solution-phase peroxide project,3 I was hoping to study the coupling of triplet-state radical pairs within a solvent cage. I planned to show that a pair of radicals generated from a precursor with chiral carbons would undergo more randomization by rotation when their coupling was slowed because their azoalkane precursor had decomposed from the triplet, rather than the singlet, photoexcited state, and I hoped that quantitative evaluation of the phenomenon would allow estimating the rate of interconversion between triplet and singlet states. To my chagrin, the radicals from the azoalkane I had painstakingly synthesized for this study, whatever their electronic state, were so hindered and slow to couple that they failed to react within the lifetime of the fluid solvent cage. In the spring of 1966, desperate to complete a respectable thesis and begin 362

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teaching at Yale in the fall, I decided to force cage collapse by conducting the photolysis in frozen solutions, where mobility would be minimized.4 Even if the result would be irrelevant to the question of electron spin correlation, it would say something about reactions in rigid media. Most of my subsequent research over more than 40 years has focused on how a rigid environment can control the mobility of molecular fragments. At first I had naively assumed that the primary effect of freezing a solution was to make it more viscous, but reading the seminal publications of Schmidt and Cohen on solid-state photodimerization5 soon convinced me that there were more subtle effects to study in crystal chemistry. Our early work at Yale focused on pure azoalkane crystals. We learned X-ray diffraction in order to determine the initial atomic coordinates for the reacting molecules and their neighbours, and we studied the nature and stereochemistry of products to infer how the intermediate radical pairs had come together, but we were particularly intrigued by the possibility of studying the reaction intermediates directly by low-temperature electron paramagnetic resonance spectroscopy (EPR). During my PhD research I had used powder EPR to observe radical pairs in my azoalkane,6 but hearing Clyde Hutchison describe his comprehensive single-crystal work with Gerhard Closs on details of a photochemical carbene mechanism convinced me that this technique deserved much wider application.7 I met Hutchison, as well as Gerhardt Schmidt and Mendel Cohen, at the Second International Symposium on Organic Solid-State Chemistry in Rehovot during ‘‘Black’’ September of 1970. This was surely the most influential meeting I ever attended, in part because it convinced me that there was an international community interested in the type of research I wanted to do. It seemed to me that within my research lifetime simple organic reactions were likely to become as easy to study by quantum mechanics as by experiment, but that comparable understanding of the influence of medium on reaction mechanism was much further in the future. Thus there would be continuing need for reliable experimental results on medium effects. Where better to study such effects than in single crystals, where the reaction cavity is uniform and precisely defined? As in the case of structural studies by X-ray diffraction, crystals would be useful for detailed mechanistic studies by EPR not only because they enable the necessary spectroscopies, but also because they provide well-defined structures for investigation. Preliminary EPR studies had shown that, contrary to my intuition, molecular fragments within a pure crystal do not fly every which way upon photolysis, but rather that they lodge in a series of intermediate structures, each as well defined geometrically as those determined by X-ray diffraction for the starting material. The underlying goal of all our subsequent research on solid-state chemistry has been not only to develop qualitative ideas about what makes solid reactions special, but even more to determine enough geometric and energetic detail about these reaction sequences to undergird, and provide critical tests of, a new level of detailed theoretical simulation of reactions in condensed media. In favourable cases, the zerofield splitting from electron–electron magnetic interaction in

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single-crystal EPR allowed determination of the length of radical–radical vectors within a few thousandths of an A˚ngstrom, and their orientation in the crystal frame within a degree or two. Anisotropy of the g- and nuclear hyperfine splitting tensors supplied analogous information about the orientation of individual radicals.8 Such items of information would not, by themselves, suffice to establish atomic trajectories for the reacting molecules, but they should be more than sufficient to identify reliable computer simulations of such trajectories. The solid-state photochemistry of azoalkanes had proven relatively simple, because the radical pairs formed by loss of a single nitrogen molecule between two radicals ultimately collapsed in the crystal cage by coupling or H-atom transfer, so we decided to study diacyl peroxides, where either one or two CO2 molecules could be lost between two radicals, and greater mechanistic diversity was possible. Previous reports of observing methyl–phenyl (MP) or phenyl– phenyl radical pair intermediates in diacyl peroxides were encouraging,9 and our own work prospered, but most radical pairs were stable only below liquid nitrogen temperature. Might it be possible to prepare radical pairs that survive at much higher temperature? We reasoned that although it would be difficult to immobilize radicals as small as methyl or phenyl, bulky radicals separated by two CO2 molecules might survive at much higher temperatures. So we studied pairs of 2,2,2-triphenylethyl and triptycyl radicals, but discovered that they also found ways to react at low temperature.10 It then occurred to us that solid-state immobility might relate not so much to the size of radicals as to the efficiency with which they pack in the crystal lattice. For example, a long-chain alkyl radical, despite its small cross section, might be relatively immobile if it were efficiently packed. Studying such simple systems was also attractive because, lacking charge, polarity, and complex functionality, they seemed well suited for ultimate study by computational simulation. Thus we embarked on a 20-year study of long-chain diacyl peroxides that kept 10 collaborators busy and generated some 3700 dissertation pages.11

2 Meeting John Meurig Thomas In 1980, as we were beginning to study long-chain peroxides, John Meurig Thomas, whom I will presume to refer to below as John, hosted me for a formative sabbatical term in Cambridge. Several years earlier Mendel Cohen had focused my attention on J.M. Thomas of Aberystwyth, who was building on impressive accomplishments with inorganic solids to become a powerful force in solid-state organic chemistry. Cohen and I were particularly impressed by Thomas’s work using optical and transmission electron microscopy (TEM) to suggest a crucial role for defects in several solid-state organic reactions.12 In a 1975 paper with S.E. Morsi and J.O. Williams, he had described studies of dibenzoyl peroxide, one of our favourite materials at that time.13 John and I established a correspondence, and we met first in New York, then at the 1978 Brandeis ICCOSS conference, just as he was preparing to move to Cambridge.

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He graciously invited me to consider spending a sabbatical leave in Cambridge. Within a year and a half I appeared on his laboratory doorstep. We had planned to collaborate on studying organic peroxide crystals by cryogenic TEM. Many hours of peering through the dark over the expert shoulders of Gordon Parkinson convinced me that our particular samples would be difficult for anyone to study by TEM, and impossible for me at Yale. But the hospitality and intellectual stimulation from John and his collaborators set me on a fruitful new research track. I clearly remember the fascinating subjects we discussed while John led me to luncheon at Kings College on a January day early in my sabbatical. Walking from Lensfield Road up Tennis Court Road we discussed the possibility of using IR spectroscopy to monitor reaction-generated local stress in crystalline diacyl peroxides. Turning into Pembroke Street he offered to loan me a translation of the 14th century Welsh bard Dafydd ap Gwilym – and a pamphlet suggesting that Prince Madoc ap Owain Gwynedd had established a Welsh Indian tribe in America in the late 12th century. In Free School Lane we paused at the entrance to the Old Cavendish Laboratory to pay homage to James Clerk Maxwell, J.J. Thomson, William Lawrence Bragg, and the others who had made it a well-spring for physics, chemistry, and biology over more than a century. Thus I first learned of John’s scholarly interest in the origins of our science and of his respect for the accomplishments and humanity of our scientific forbearers, a respect that subsequently became so clear in numerous eloquent lectures and publications on Michael Faraday and John’s many other friends and heroes. We then exercised his prerogative by marching straight across the lawn at Kings. This was heady stuff and a suitable introduction to diverse life in John’s intellectual and cultural fast lane. I subsequently followed up on each of the topics we had discussed, but the one most relevant here is using Fourier Transform Infrared Spectroscopy (FTIR) to study peroxide decomposition.

3 FTIR of CO2 Single-crystal EPR spectroscopy is a reliable workhorse for studying the motion and reaction of radical pairs in exquisite geometric detail, as studies of long-chain diacyl peroxides culminating in Michael Biewer’s comprehensive dissertation have subsequently underlined.11g But insensitivity to diamagnetic intermediates, the great strength of EPR for studying radical intermediates at very low concentration, is also a potential liability. EPR gives only indirect information about diamagnetic intermediates and products, and it seemed possible that it was revealing only minor side reactions in the overall photolytic decomposition scheme. Using isotope dilution and low conversion to prove that the dominant products arose from the intermediates studied by EPR was very laborious.14

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Bonnie Whitsel’s EPR studies of acetyl benzoyl peroxide (ABP) suggested that the local pressure generated by fragmenting a molecule within an otherwise pure crystal could have dramatic influence on subsequent reactions. Despite the possibility that such phenomena should be pervasive in solid-state reactions, this stress was necessarily very local and difficult to measure in a lightly damaged crystal.15 But in 1968, Riepe and Wang at Yale had used IR frequency shifts to study the possibility that CO2 was stressed in the active site of carbonic anhydrase.16 The high oscillator strength and unusual frequency of CO2 asymmetric stretching made the sensitivity of FTIR comparable to that of EPR for studying intermediates at low concentration. In the IR spectra of CO2 might chance have provided both an ideal probe of local stress within reacted sites in diacyl peroxides and a means to supplement EPR spectroscopy by revealing diamagnetic reaction intermediates? John Meurig Thomas played two key roles in helping us to realize the potential of this technique. First, he encouraged the project begun in preliminary trials during my term in Cambridge; second, he made substantial sacrifices to allow Mark Hollingsworth the time to complete a remarkable thousand-page Yale dissertation on this subject.11b Mark is one of very few individuals I know who work as hard and effectively as John Meurig Thomas. In early 1983, as he was starting to wrap up his experimental work on applying FTIR spectroscopy of CO2 as a probe of stress and mechanism in crystalline diacyl peroxides, Mark determined to spend his postdoctoral years in Cambridge. John promptly accepted Mark’s application and arranged funding to begin in early 1984. Mark decided that it was critical to do a few more experiments, and by early 1984 he had only begun analyzing his FTIR spectra and writing his dissertation full time. At the same time Simon Kearsley came to Yale from Cambridge and began to apply molecular mechanics to analyze the peroxide EPR data we had been accumulating.17 The more Mark analyzed and wrote, the more treasure he discovered. Despite my repeated, if half-hearted, threats to cut off his support and force him to Cambridge, he kept analyzing and writing diligently for a year and a half, arriving in Cambridge only in late 1985. The extraordinary sympathy and patience of John Meurig Thomas during this productive but trying period became as legendary at Yale as Mark’s dissertation itself, which won the ACS prize for the best of the year. As a tribute to John and to acknowledge his patience, the balance of this chapter will describe a project that could never have occurred but for the time he made available to Mark in 1984–1985. If the reader is less patient than John and chokes on the mass of seemingly trivial quantitative detail in the following account, let him turn to the Summary and Apologia at the conclusion of this chapter.

4 Isotopic CO2 as a Probe of Mechanism As his analysis progressed, Mark became increasingly confident about tiny spectral blips that I suspected were simply residual noise in his after–before

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FTIR Study of Short Range Mobility in Some Crystalline Peroxides

difference spectra. The fraction of peroxide molecules that had been photolyzed in his experiments was very small, typically less than 1 in 2000, so only CO2’s unique asymmetric stretching frequency allowed the discernment of any product peaks at all among the forest of strong absorptions from undamaged matrix molecules. In the fourth and final appendix to his thesis, Mark interpreted apparently negligible blips as signals from natural abundance 18OQCQO, which would represent less than one molecule in 100,000. In a few cases there was a tiny blip at the 18O-mass-corrected position corresponding to a normal CO2 peak, but in most cases there appeared instead a pair of peaks, hardly larger than the noise, flanking that position and split by as much as 2 cm
1. Mark attributed this splitting to two antiparallel orientations of 18OQCQO in unsymmetrical lattice sites. Although the intermolecular forces on the two ends of such a molecule necessarily balance at the equilibrium structure, the intermolecular force constants could differ, creating a difference in the asymmetric stretching frequency between OQCQ18O and 18 OQCQO.18 Nearly a decade later Kevin Pate decided to test Mark’s suggestion that this difference would allow studying end-for-end rotation of 18OQCQO in a suitably labeled crystalline sample.1 Kevin studied three peroxides with which our group had long experience: ABP, di-11-bromoundecanoyl peroxide (BrUP), and 11-bromoundecanoyl decanoyl peroxide (BrUDP).

O ABP

H3C

O

O O

O Br

O

O

Br O

BrUP

O Br

O BrUDP

O

CH3 O

5 ABP: End-for-End Rotation and CO2 Interchange In previous EPR investigations Whitsel and Merrill had studied two kinds of radical pairs in photolyzed single crystals of ABP: the methyl-benzoyloxyl radical pair (MB) from loss of one CO2 molecule, which decays with a half-life of 1 min at about 76 K (Ea ¼ 5.7 kcal mol
1); and the MP radical pair from loss of two CO2 molecules, which decays at this rate at about 79 K (Ea ¼ 7.7 kcal mol
1).15,19 Both MP and MB structures are structurally welldefined, but selective 17O labeling showed that the benzoyloxyl radical of MB undergoes exchange of oxygens, presumably by rotation of its
CO2 group, with a half-life of 1 min at about 72 K (Ea ¼ 4.7 kcal mol
1).20

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Chapter 22 O

O C O

C CH3

O

76K O

MB

H3C

O

O O

O H3C

O

O O

CH3

MP O O C C O O

O C O 79K

O C O H3C

Hollingsworth had studied three isotopic variants of ABP using FTIR to show that photolysis of a single crystal at 90 K, at which temperature all radical pairs collapse, generates a single CO2 geometry in sites that have lost only the acetoxyl CO2 and formed methyl benzoate (MB), but it generates two different pairs of CO2 molecules corresponding to differently structured sites that include toluene product.11b,18 In the present work we attempted to study ABP by FTIR at lower temperatures, where the radical pairs would be stable, and end-for-end rotation of CO2 might be frozen. This work proved challenging, because in many experiments the growth of a broad, unidentified peak21 obscured the sharp CO2 peaks of interest. One particular crystal provided much of the information we sought. It was grown using ABP that had been prepared by reacting acetyl chloride with peroxybenzoic acid, which itself had been prepared by reacting 90% H2O2 with a commercial sample of 99% carbonyl 13C labeled benzoic acid. This ABP crystal contained 1% (natural abundance) 13C in its acetyl CO2, 99% 13 C in its benzoyl CO2, and 4–7% 18O in its benzoyl carbonyl group.22 Thus, in Figure 1, OQCQO FTIR signals (in the region 2335–2350 cm
1) originate from the acetyl group of ABP, OQ13CQO FTIR signals (2270–2285 cm
1) originate from the benzoyl group, and 18OQ13CQO FTIR signals (near 2260 cm
1) originate from benzoyl groups with 18O in the carbonyl position. The blue curve of Figure 1, which was measured at 17 K after 15 min of photolysis at that temperature, shows five principal CO2 peaks. (Asymmetric stretching frequencies for the CO2 pairs in various isotopic permutations are assigned in the Appendix.) Some of these five peaks shift when the crystal is warmed successively to a specified temperature (45 K, green; 70 K, orange; 90 K, red) for about 30 s before cooling again to 17 K for measurement. Comparison of these spectra with those from an unlabeled crystal of ABP showed that the five peaks represent two isotopically mixed 13CO2–12CO2 pairs, whose MP radical pairs decay to toluene, and an isolated 12CO2, whose MB radical pair decays to MB. The isolated peak for the CO2 molecule with the MB radical pair shows no change whatever after warming to 90 K. This hints that in most cases this

FTIR Study of Short Range Mobility in Some Crystalline Peroxides

Figure 1

369

Asymmetric stretching FTIR spectrum (cm
1) for CO2 pairs in a crystal of ABP that is 99% labeled with 13C in the carbonyl position of its benzoyl group and B5% labeled with 18O in the carbonyl oxygen of the same group. The crystal was photolyzed at 17 K and then measured at 17 K with brief annealing at the indicated temperatures. The top frame shows peaks of unlabeled CO2 from the acetyl group. The bottom frame shows peaks of 13CO2 from the benzoyl group. The inset shows peaks of 18OQ13CQO from the benzoyl group. The spectra are offset in frequency according to the differences among the corresponding isotopomers in the gas phase. There is a persistent peak for a single CO2 in the presence of methyl benzoate (MB). Two pairs of CO2 molecules in the presence of methyl/phenyl radical pairs (MP and MP 0 ) decay at different temperatures to a single pair of CO2 molecules in the presence of toluene (T). Subscripts a and b distinguish peaks that derive from acetyl and benzoyl groups, respectively. See Figure 2 for more detailed spectra of 18 OQ13CQO.

radical pair has already decayed at 17 K, so that the trapped MB radical pair, whose decay at 76 K was studied by EPR, may represent a minor pathway. EPR experiments down to 4.2 K failed to find the hypothetical rapidly decaying MB pair.14b It is possible, but unlikely, that collapse of the MB radical pair would fail to shift the frequency of the adjacent CO2 by at least 0.2 cm–1.

370

Chapter 22

The CO2 pairs formed with the MP radical pairs are more interesting, because one of the molecules is isotopically labeled. At 17 K (blue) the major pair has a low-frequency peak for the acetyl-derived CO2 (MPa) and a highfrequency peak for benzoyl-derived 13CO2 (MPb). The minor pair has the opposite, a high-frequency peak from the acetyl-derived CO2 (MP 0 a) with a low-frequency peak from benzoyl-derived 13CO2 (MP 0 b). Upon brief warming to 45 K (green), the minor MP 0 peaks disappear, presumably because the adjacent radical pair collapses to form toluene. The resulting new CO2 pair has a high-frequency peak from acetyl-derived CO2 (Ta) together with a low-frequency peak from benzoyl-derived 13CO2 (Tb). Note that the subtle 0.2 cm–1 shift of the low-frequency peak from MP 0 b to Tb is clearly evident. The minor radical pair associated with MP 0 and its decay below 45 K have not been observed by EPR. Subsequent brief warming to 70 K (orange) decreases the MPa peak slightly and increases Ta, showing that the major MP pair is beginning to convert to the same T species formed below 45 K from the minor MP 0 pair, although most of the major MP survives briefly at this temperature. In this case the frequency shifts are much more dramatic: acetyl-derived CO2 shifts by 8.2 cm
1 (MPa to Ta), benzoyl-derived 13CO2 by 5.2 cm
1 (MPb to Tb). This transition is surely the same one observed by EPR at 79 K.23 Warming to 90 K causes the major MP species to complete its conversion to the toluene product. More interestingly, two new peaks appear which differ from those of the previous T pair solely in respect to which site contains 13CO2. The new ‘‘exchanged’’ pair, TE, has a low-frequency peak from acetyl-derived CO2 (TaE) with a high-frequency peak from benzoyl-derived 13CO2 (TbE). The small TbE peak in the orange 70 K spectrum suggests that a small amount of CO2 site-exchange may arise during conversion of the major MP species to toluene. It is remarkable that when Hollingsworth photolyzed ABP at 90 K he observed a second CO2 pair in addition to the one seen in the present work, in which photolysis at 17 K was followed by warming to 90 K. However it may not be surprising that lattice control over product structure becomes less stringent at higher photolysis temperature. The 18OQ13CQO region of the spectrum shows end-for-end rotation of the MPb molecule. In the 17 K spectrum there is a single peak at 2260.2 cm
1, slightly lower than the 2260.45 cm
1 expected for the change in reduced mass from OQ13CQO. The corresponding spectra after warming to 45 and 70 K show this peak at about half its initial intensity together with a new peak of the same intensity (rot MPb) on the other side of the expected frequency. Both peaks disappear when MP converts to T at 90 K. In a separate experiment the rate of end-for-end rotation at 23 K was measured by monitoring these two peaks at 17 K after successive periods of warming to 23 K (Figure 2). Together with the previous EPR results, these observations establish the following kinetic scheme for the decomposition mechanism of photolyzed ABP.

371

FTIR Study of Short Range Mobility in Some Crystalline Peroxides O O H3C

O O

O O C C CH3

O

hν 17K

C CH3

O O

O

O O

MP'

MP

partial radical-pair collapse?

25K rotation of CO 2

T

45K radical-pair collapse 72K -CO 2 rotation

73K radical-pair collapse

O

90K CO 2 -pair exchange

Figure 2

MB

76K radical-pair collapse

O

End-for-end rotational equilibration at 23 K of the benzoyl-derived CO2 in the MP pair of ABP. The peak at 2260.8 cm
1, due to 18O in the peroxylderived end of the molecule, grows at the expense of the peak at 2260.1 cm
1, due to 18O in the carbonyl-derived end of the molecule. The inset, a plot of the logarithm of the difference in peak height against time, shows good firstorder kinetics over more than 90% decay towards equilibrium with a halflife of 2.2 min.

372

Chapter 22

6 BrUDP: Rotation, Site Exchange, and Preservation of Asymmetry EPR studies have shown that in long-chain diacyl peroxides there is no analogue of the singly decarboxylated MB radical pair of ABP. Both acyloxyl radicals decarboxylate upon photolysis to generate pairs of alkyl radicals separated by a pair of CO2 molecules. Peroxides with a wide variety of chain lengths and terminal substituents occur in two local crystal packing motifs,24 each of which has its own characteristic kinetic scheme.25 For quantitative study of lattice influence on reaction mechanism it is particularly valuable to compare crystals with the same local packing motif in the vicinity of the reacting group, so that any dramatic structural differences are remote ones. Such cases should provide good tests for the ability of a computer simulation to predict the direction of a structural or kinetic change in response to a subtle packing change, analogous to the study of substituent effects in solution chemistry. For example one can compare the photolytic mechanism in BrUP with that in BrUDP. Figure 3 compares the packing of these two crystals. On the left are five molecules from a central layer of BrUP, which is shown in Br  Br contact with three molecules in each of the two adjacent layers. On the right are the analogous five molecules in a central layer of BrUDP, which is shown in Br  Br contact with three molecules in the layer below and in CH3  CH3 contact with three molecules in the layer above. The packing similarity is obvious.

Figure 3

Crystal packing comparison of BrUP (left) with BrUDP (right) with hydrogen atoms omitted. Portions of three successive layers are shown. The surroundings of reaction centres in the middle layer (red line) are isostructural for the two crystals within the blue lines (to 10 A˚ in the direction of the decanoyl chain, to 30 A˚ in the direction of the bromoundecanoyl chain). Coordinates from Refs. 11g and 24.

FTIR Study of Short Range Mobility in Some Crystalline Peroxides

373

The red line in Figure 3 shows the centre of the layer, where CO2 pairs and radical pairs will be generated. The blue lines delimit the region of close structural analogy between BrUP and BrUDP. Up to 9.8 A˚ in one direction from the reaction centre, and 29.5 A˚ in the other direction, the packing is virtually identical. The RMS difference in distance to corresponding nonhydrogen atoms within 4 A˚ of the oxygens in BrUP and BrUDP is 0.09 A˚. The RMS difference for such atoms between the two chains of BrUDP is 0.04 A˚. The isostructural range is even longer, 11 and 40 A˚, when measured along the slanted alkane chains, which transmit the strongest mechanical influence. Comparing the structure and kinetics of corresponding intermediates between BrUP and BrUDP reveals the influence of subtle lattice differences. Reaction sites in BrUP have twofold symmetry, which is only approximate in BrUDP. This means that when the two members of a pair of molecular fragments within a reaction site undergo different motion, as they almost invariably do, two symmetry-related structures will be generated in BrUP, but in BrUDP there is the possibility for two different structures related by only approximate symmetry. Thus one can study subtle lattice influence not only by comparing separate experiments on BrUP and BrUDP, but also by comparing the pseudosymmetric pairs of intermediates within a single experiment on BrUDP. The spectra of Figure 4 provide the basis for the latter kind of comparison. The two 18O-labeled BrUDP samples for Figure 4 were prepared by reacting an unlabeled peroxyacid with the complementary 18O-labeled acid chloride. Since the acid chlorides were prepared from carboxylic acid that had been labeled by treating acid chloride with H218O, only half of their molecules contained 18O label. Thus each sample contained 50% of unlabeled BrUDP and 50% of BrUDP that was 18O-labeled in the carbonyl group of either the 11-bromoundecanoyl chain (Figure 4a) or the decanoyl chain (Figure 4b). Obviously peaks deriving from unlabeled BrUDP are identical between 4a and 4b. Consider the three low-temperature spectra in Figure 4 (18, 25 and 46 K in 4a; 17, 25 and 44 K in 4b). A few minor peaks (see below) disappear before 46 K, but five significant peaks persist in each sample and are denoted by vertical lines. All five correspond to the lowest-temperature CO2 pair observed by Hollingsworth in unlabeled BrUP, which showed a very strong peak at 2346.6 cm
1, and a very weak peak at 2327.9 cm
1.26 These BrUP peaks result from strong (9.0 cm
1) coupling between a CO2 with a high intrinsic frequency (2339.4 cm
1) and an adjacent CO2 with a low intrinsic frequency (2335.1 cm
1). The strong peak corresponds to in-phase vibration of the pair of parallel molecules, and the very weak peak to their out-of-phase vibration. In BrUDP there are two pseudosymmetric CO2 pairs of this type, which we call 1a and 1b. Pair 1a has intrinsic frequencies of 2339.3 and 2335.3 cm
1 (coupling 8.3 cm
1); Pair 1b has intrinsic frequencies of 2338.3 and 2336.4 cm
1 (coupling 8.5 cm
1). They give the same strong in-phase peak at high frequency (black line in Figure 4, 2345.8 cm
1), but their out-of-phase peaks are too weak to discern in Figure 4.

374

Figure 4

Chapter 22

FTIR spectra for CO2 asymmetric stretching (2310–2350 cm
1) in photolyzed BrUDP measured at 17 K after photolysis at that temperature and brief annealing at the colour-coded temperature. In each frame, the carbonyl oxygen atom indicated in red was 50% 18O labeled. Vertical lines are drawn at the frequencies for pseudosymmetrically related signals of the initial CO2 pairs P1a (blue) and P1b (pink). Note that P1a peaks disappear at slightly lower temperature than P1b peaks. Note also that while the low-frequency (18OQCQO) peaks for P1a and P1b are single, those for R1 and R2 are doubled because of end-for-end rotational equilibration.

FTIR Study of Short Range Mobility in Some Crystalline Peroxides

375

Isotopically mixed Pair 1a gives the peaks marked by blue vertical lines in Figure 4, while isotopically mixed Pair 2b gives the peaks marked by pink lines. Since the high-intrinsic-frequency unlabeled CO2 of isotopically mixed Pair 1a (blue, 2342.2 cm
1) appears in Figure 4b, and not in Figure 4a, it is clear that this molecule comes from the bromoundecanoyl carbonyl group, which is unlabeled in Figure 4b but labeled in Figure 4a. By the same reasoning the high-intrinsic-frequency unlabeled CO2 of isotopically mixed Pair 1a (pink, 2341.4 cm
1) appears in Figure 4a and originates from the decanoyl carbonyl group. That is, Pair 1a and Pair 1b are related by pseudosymmetry in the BrUDP lattice, and correspond to Pair 1 in BrUP. As expected these three pairs are similar in their intrinsic frequencies and in their coupling constants. The differences in these parameters demonstrate the influence of structural dissimilarity due to differences in packing 10 A˚ or more from the reaction site. There are other similarities between Pair 1a and Pair 1b of BrUDP. The 18 OQCQO peaks of the isotopically mixed pairs are offset from the expected average positions, because the orientation of the carbonyl oxygen in the starting carbonyl group persists in these molecular pairs. Only one of the end-for-end rotational isomers appears for the low-frequency 18OQCQO (2315.9 cm
1 for Pair 1b in Figure 4a, 2314.8 cm
1 for Pair 1a in Figure 4b). In each case the rotamer formed is the one that gives the lower frequency. The high-frequency 18OQCQO signals (2318.4 cm
1 for Pair 1b in Figure 4b, 2318.8 cm
1 for Pair 1a in Figure 4a) are even more interesting. For both peaks there is a partner about 1/4 as large at lower frequency that corresponds precisely to expectation if 20% of the molecules rotate end-for-end during formation of the high-frequency member of each pair. Note that this partial rotation must have occurred during formation of the pairs, since no further rotation occurs as long as the pairs survive. Precisely the same behaviour is observed for Pair 1 in BrUP. With carbonyl labeling, the low-intrinsic-frequency 18OQCQO appears exclusively as the low-frequency rotamer, and the high-intrinsic-frequency 18OQCQO appears four times more often as the high-frequency rotamer than as the low-frequency rotamer. In the case of BrUP, this observation was confirmed by independent analysis of a peroxy-labeled crystal, which revealed the high-frequency to lowfrequency rotamers in a 20:80 ratio, exactly the opposite of what was observed in the carbonyl-labeled crystal. To summarize, in all of these three versions, Pair 1 of BrUP and Pairs 1a and 1b of BrUDP, one CO2 is formed at 17 K without end-for-end rotation, and the other is formed with 20% end-for-end rotation, but there is no subsequent rotation up to about 48 K. Despite these mechanistic similarities, there is a kinetic difference between pseudosymmetry-related Pairs 1a and 1b of BrUDP. In both Figures 4a and 4b it is clear from the bands due to the unlabeled CO2 in isotopically mixed pairs (the strong doublet between 2339 cm
1 and 2345 cm
1) that Pair 1a (blue) decays at a lower temperature that Pair 1b (pink). This difference is very subtle (481 versus 501), but the 21 difference is completely reliable since the processes being compared are observed simultaneously in the same crystal. One could not

376

Chapter 22

be confident of so subtle a difference measured for processes in two different crystals. The difference in decay temperatures shows clearly, if not surprisingly, that the pseudosymmetry-related Pairs 1a and 1b of BrUDP do not equilibrate with one another even at the temperature at which they acquire sufficient mobility to decay irreversibly to a new structure. EPR studies of BrUDP by Biewer showed that the decay of Pair 1, the initial radical pair, at these temperatures generates a radical pair in which the radical carbons have been displaced by rotation about the next-adjacent carbon– carbon bonds of otherwise immobile alkyl chains.11g Rotation in the radical from the bromoundecanoyl chain is by about 421; rotation in the radical from the decanoyl chain is by about 1601. The spectra of Figure 4 show that both Pair 1a and Pair 1b decay in this process to the same product mixture, not to pseudosymmetry-related mixtures. That is, the product structure is determined by the lattice, not by the arrangement of the starting fragments. Isotopically mixed pairs from the dominant species in this mixture, Pair R1, show a strong doublet centred at 2327.3 cm
1 in Figure 4a and a strong barely-resolved doublet centred at 2318.4 cm
1 in Figure 4b. That both of these signals are symmetrical doublets shows that 18OQCQO molecules in both sites have undergone complete end-for-end rotational equilibration. But the paired CO2s have not undergone complete equilibration by exchanging positions with one another, which would have made the spectra of 4a and 4b identical. Instead, about 75% of the 18OQCQO molecules in the high-frequency position of this pair derive from the original bromoundecanoyl chain, and only about 25% from the original decanoyl chain. There are similar but weaker doublet signals at slightly higher frequency, which we attribute to a Pair R2, probably related by pseudosymmetry to Pair R1. The minor peaks observed below 30 K (and referred to above) represent two intermediates on a secondary pathway that also leads to species R1 at 25 K, whereas normal formation of R1 from Pairs 1a and 1b requires warming to nearly 50 K. A remarkable feature of this low-temperature conversion to R1 is that it occurs with only slight end-for-end rotation of 18OQCQO. Rotational equilibration for this R1 was found to begin at 42 K, so it is not surprising that equilibration has occurred in R1 pairs that are formed from Pairs 1a and 1b at 50 K.

7 Summary and Apologia The current results increase our knowledge about the scope of motion available to intermediates during reactions within molecular crystals. At the very least they validate Hollingsworth’s conjecture that FTIR can measure end-for-end rotation of 18OQCQO in an unsymmetrical lattice site. They show that such rotation sometimes occurs, at least to a limited extent, upon initial CO2

FTIR Study of Short Range Mobility in Some Crystalline Peroxides

377

formation. More often the initial CO2 molecules retain specific orientation with respect to the starting material, and end-for-end rotation occurs upon subsequent warming, at 23 K for the MPb molecule in ABP, and at 42 K for the R1B molecule in BrUDP. These results also support other qualitative conclusions: that it is possible, but not easy, for a pair of adjacent CO2 molecules to exchange positions, that such exchange can occur during a structural transformation, even when it does not occur at the same temperature after the transformation, that a structural relaxation that carries a starting material into two pseudosymmetrically related product structures is sufficiently irreversible that the two product structures cannot equilibrate with one another. Reaching these qualitative conclusions required a great deal of synthetic, experimental, and analytical effort. The data referred to in this chapter is the tip of the iceberg of detailed quantitative spectroscopic and kinetic information on diacyl peroxides recorded in the dissertations cited. Publishing all these numbers would be like publishing diffraction intensities for X-ray structural determination of the ribosome. Our goal has been to develop a comprehensive understanding, in structural and kinetic detail, of how a matrix controls all the atomic trajectories involved in a solid-state reaction sequence. Our experimental results provide very suggestive, but only partial, glimpses of these trajectories. Our conclusions thus far, while they demonstrate clearly that there are detailed reaction mechanisms worth knowing, probably do not by themselves justify the enormous amount of effort devoted to the project by such talented students. We hope to be vindicated in the future, when our data may anchor and validate a computational simulation of the atomic trajectories for these reactions. We chose these systems with an eye to making them as easy as possible to simulate. If a reliable simulation cannot be developed for such simple systems, it is difficult to see how one could believe simulations of more complex processes like enzyme catalysis. Simon Kearsley’s preliminary simulation for ABP showed that achieving this goal should be possible.17 Thus we have chosen to honour the 75th birthday of the pioneer John Meurig Thomas not by presenting a completed project, but rather by inviting a new generation of experimental and computational chemists to use these and other data to develop a comprehensive picture of what controls reactivity in organic solids.

Acknowledgments In addition to thanking John Meurig Thomas for his patient encouragement of this work and for the long personal friendship between his family and that of JMM, we would like to acknowledge the collaborators cited in the references, in particular Mark Hollingsworth and Michael Biewer. We are grateful for financial support from the Mechanics Division of the Office of Naval Research and from the National Science Foundation.

378

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Appendix Tables 1 and 2 show the asymmetric stretching frequencies (cm
1) for CO2 pairs in crystalline ABP and BrUDP/BrUP, respectively. Frequencies in bold face were observed experimentally. Frequencies in italics were not observed directly, because of inappropriate labeling or interference by stronger peaks, so they were calculated from the ‘‘intrinsic’’ frequencies for the two unlabeled CO2s and the coupling (cm
1) between them, with appropriate correction for the effect of isotopic substitution on reduced masses. Observed frequencies agree with calculated frequencies to within 0.1 cm
1 for Table 1 and to within 0.2 cm
1 for Table 2. Hi and Lo denote the higher and lower frequencies for a particular pair whose isotopic composition is given in parentheses with the occupant of the high-intrinsic-frequency site cited first. For example, ‘‘Lo (13/ 18-12)’’ denotes the average position of the lower-frequency peak for a pair in which OQ13CQ18O occupies the higher-intrinsic-frequency site, and the other site is occupied by normal CO2. Since in this case 18O end-for-end isomerism is involved, the separation of the doublet that flanks this average peak position is reported as ‘‘Hi 18 split.’’ Table 1

FTIR frequencies for CO2 pairs in ABP.

Species Decay Temperature (K)

MP

MP 0

T

40

80

Hi (intrinsic) Lo (intrinsic) Coupling Hi source

2343.4 2339.1 1.2 Benzoyl

2345.7 2338.3 4.6 Acetyl

2347.2 2338.2 5.0 Acetyl

Hi (12-12) Lo (12-12)

2343.7 2338.8

2347.8 2336.0

2349.4 2335.8

Hi (12-13) Lo (12-13)

2343.5 2273.6

2346.1 2272.7

2347.6 2272.5

Hi (13-12) Lo (13-12)

2339.3 2277.9

2338.6 2279.8

2338.6 2281.1

Hi (12-13/18) Lo (12-13/18) Lo 18 split

2343.4 2256.3

2345.9 2255.4 0.3

2347.5 2255.2 1.0

Hi (13/18-12) Lo (13/18-12) Hi 18 split

2339.1 2260.5 0.6

2338.6 2262.4

2338.5 2263.9 1.8

Hi (18/18) Lo (18/18) Hi 18 split Lo 18 split

2326.7 2321.9

2330.9 2319.2

2332.4 2319.0 1.5 0.8

490

FTIR Study of Short Range Mobility in Some Crystalline Peroxides

Table 2 Species

379

FTIR frequencies for CO2 pairs in BrUDP and BrUP. BrUDP BrUDP BrUP 1a 1b 1

BrUDP R1

BrUP R1

BrUDP R2

BrUP R2

Hi (intrinsic) 2339.3 Lo (intrinsic) 2335.3 Coupling 8.3 Hi source BrU

2338.3 2336.4 8.5 D

2339.4 2344.2 2345.5 2335.1 2335.2 2336.4 9.0 0.0 1.5 — B80% D —

2345.4 2336.6 1.0 Mostly BrU

2342.1 2335.6 1.2 —

Hi (12-12) Lo (12-12)

2345.8 2329.0

2345.8 2328.6

2346.5 2344.2 2327.9 2335.1

2345.6 2336.2

2345.6 2336.6

2342.4 2335.4

Hi (12-13) Lo (12-13)

2340.3 2268.9

2339.3 2270.0

2340.6 2344.2 2268.7 2269.9

2345.5 2271.1

2345.5 2271.3

2342.2 2270.2

Hi (13-12) Lo (13-12)

2336.4 2272.8

2337.5 2272.0

2336.4 2335.2 2272.8 2278.7

2336.4 2280.0

2336.7 2279.8

2335.6 2276.6

Hi (12-18) Lo (12-18) Lo 18 split

2342.2 2315.5 1.4

2341.4 2316.2 0.6

2342.8 2344.2 2314.9 2318.4 1.0 0.4

2345.7 2319.5 0.8

2345.4 2319.8 0.7

2342.2 2318.6 0.6

Hi (18-12) Lo (18-12) Hi 18 split

2339.4 2318.2 1.3

2340.3 2317.7 1.3

2339.8 2335.2 2317.8 2327.3 1.1 1.3

2336.7 2328.3 1.1

2336.6 2328.2 1.1

2335.7 2325.3 2.1

References 1. Sections 1–4 are by JMM, Sections 5–7 are taken in part from K.L. Pate, Probing Molecular Mobility of Reaction-Generated CO2 in Photolyzed Acyl Peroxide Single Crystals: An FT-IR Investigation, Dissertation, Yale University, 2000. 2. M.D. Hollingsworth, J.A. Swift and B. Kahr, Cryst. Growth Des., 2005, 5, 2022. 3. P.D. Bartlett and J.M. McBride, J. Am. Chem. Soc., 1965, 87, 1727. 4. P.D. Bartlett and J.M. McBride, Pure Appl. Chem., 1967, 15, 89. The inspiration for trying a viscous medium came from supervising the senior research project of Harvard undergraduate Alan Dafforn, who, as a junior, had worked with John Lombardi on a project involving phosphorescence depolarization with A.C. Albrecht of Cornell. J.R. Lombardi, J.M. Raymonda and A.C. Albrecht, J. Chem. Phys., 1964, 40, 1148. 5. M.D. Cohen and G.M.J. Schmidt, J. Chem. Soc., 1964, 1996. 6. My interest in EPR had begun in graduate school, in part because Jim Vincent, my roommate, and his advisor August Maki were among the first to observe photoexcited triplet states by EPR. J.S. Vincent and A.H. Maki, J. Chem. Phys., 1963, 39, 3088. 7. C.A. Jr. Hutchison, Pure Appl. Chem., 1971, 27, 327. 8. J.M. McBride, M.W. Vary and B.L. Whitsel, ACS Symp. Ser. (Org. Free Radicals), 1978, 69, 208. In principle electron-nuclear double resonance

380

9.

10.

11.

12. 13. 14.

15. 16. 17.

18.

Chapter 22

spectroscopy (ENDOR) would provide additional information about radical positions with respect to surrounding molecules,7 but exploiting ENDOR in these systems proved to be beyond our experimental grasp. A.V. Zubkov, A.T. Koritsky and Ya.S. Lebedev, Dokl. Akad. Nauk USSR, 1968, 180, 1150; H.C. Box, E.E. Budzinski and H.C. Freund, J. Am. Chem. Soc., 1970, 92, 5305; V.I. Barchuk, A.A. Dubinsky, O. Ya. Grinberg and Ya.S. Lebedev, Chem. Phys. Lett., 1975, 34, 476. D.W. Walter and J.M. McBride, J. Am. Chem. Soc., 1981, 103, 7069, 7074. Triptycyl radical pairs collapsed by reacting with a common chloroform molecule in the solvated crystal, unpublished work by C. Reichel and B. Kahr. In addition to Ref. 1, these dissertations include: (a) B.E. Segmuller, Diundecanoyl peroxide: EPR study and product analysis, Dissertation, Yale University, 1982; (b) M.D. Hollingsworth, IR studies of CO2 dimers as a probe of local stress in solid state reactions, Dissertation, Yale University, 1986; (c) D.E. Mills, Bis(11-bromoundecanoyl) peroxide: remote substituent effects on a solid-state reaction, Dissertation, Yale University, 1986; (d) X.W. Feng, Mechanistic ESR study in the solid-state radical reactions in crystal of bis(11-bromoundecanoyl) peroxide and its analogues, Dissertation, Yale University, 1989; (e) S.B. Bertman, Solid substituent effects: systematic investigation of the influence of molecular interaction on bulk properties of organic solids, Dissertation, Yale University, 1990; (f) R.L. Carter, Solid state chemistry Part I: mechanistic studies on the decomposition of didecanoyl peroxide, Dissertation, Yale University, 1993; (g) M.C. Biewer, Investigation of radical motion in the single crystal photolytic decomposition mechanism of (11-bromoundecanoyl) (decanoyl) peroxide, Dissertation, Yale University, 1995. M.D. Cohen, Z. Ludmer, J.M. Thomas and J.O. Williams, Proc. Roy. Soc. A, 1971, 324, 459. S.E. Morsi, J.M. Thomas and J.O. Williams, J. Chem Soc., Faraday Trans. 1, 1975, 71, 1857. (a) A.B. Jaffe and J.M. McBride, Solid Org. State, 1974, 4, 16; (b) B.L. Whitsel, The kinetics and structure of radical pairs: acetyl benzoyl peroxide, Dissertation, Yale University, 1977. N.J. Karch, E.T. Koh, B.L. Whitsel and J.M. McBride, J. Am. Chem. Soc., 1975, 97, 6729; cf. Ref. 14b. M.E. Riepe and J.H. Wang, J. Biol. Chem., 1968, 243, 1779. S.K. Kearsley, and J.M. McBride, Mol. Cryst. Liq. Cryst. Inc. Nonlinear Opt., 1988, 156, 109. Despite the preliminary success of Kearsley, thus far only he and Gavezzotti have used any of our experimental data for their intended purpose of formulating and testing theories for solid-state reaction mechanism. A. Gavezzotti and R. Bianchi, Chem. Phys. Lett., 1986, 128, 295. M.D. Hollingsworth and J.M. McBride, in Advances in Photochemistry, vol 15, D. Volman, G.S. Hammond and K. Gollnick (eds), 1999, 279–379, 345.

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19. Temperatures are noted as ‘‘about’’ because it is difficult to measure the temperature of a small crystal cooled by a stream of cold gas or by mounting on a cooled salt plate. Absolute values of temperature are probably accurate to within a few degrees, and relative temperatures for either EPR or FTIR within about one degree. 20. R.A. Merrill, The benzoyloxyl radical, a 2B2 ground state, Dissertation, Yale University, 1986. cf. J.M. McBride and R.A. Merrill, J. Am. Chem. Soc., 1980, 102, 1723. 21. As noted by Hollingsworth, the broad peak was particularly troublesome when using older ABP crystals, even those stored in a darkened cold room. He suggested that prior decomposition may be sufficient to generate linebroadening stress fields in older crystals. 22. The commercial benzoic acid was prepared from 13C-enriched CO2, which is fortuitously enriched in 18O. 23. The 91 temperature difference is attributable to systematic differences in temperature measurements and to the time required to warm and cool the FTIR sample holder. 24. J.M. McBride, S.B. Bertman and T.C. Semple, Proc. Natl. Acad. Sci. U.S.A., 1987, 84, 4743; J.M. McBride, S.B. Bertman, D.Z. Cioffi, B.E. Segmuller and B.A. Weber, Mol. Cryst. Liq. Cryst. Inc. Nonlinear Opt., 1988, 161, 1. cf. Ref. 11e. 25. J.M. McBride, B.E. Segmuller, M.D. Hollingsworth, D.E. Mills and B.A. Weber, Science, 1986, 234, 830; X.W. Feng and J.M. McBride, J. Am. Chem. Soc., 1990, 112, 6151. cf. Refs. 11c,d,f. 26. M.D. Hollingsworth and J.M. McBride, Chem. Phys. Lett., 1986, 130, 259.

Section C: Solid Catalysts, Surface and Materials Science

CHAPTER 23

From ‘Nature’ to an Adventure in Single-Site Epoxidation Catalysis HENDRIKUS C. L. ABBENHUIS AND RUTGER A. VAN SANTEN Laboratory of Inorganic Chemistry and Catalysis, Schuit Institute of Catalysis, Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven, The Netherlands

1 Introduction: ‘Provoked by Nature’ Sir John Meurig Thomas’s research over the past 20 years has focused on the design, preparation and testing of new solid catalysts. He has been conspicuously successful in modifying nanoporous materials so as to place isolated, single-site active centres on their large internal areas. As such, it should be no surprise that our research, aimed at detailed molecular understanding of catalytic events created synergy, adding to our relationship. Especially so in the summer of 1994 when we started an experimental work with the aim of using organometallic building blocks to construct heterogeneous catalysts, through lego-like chemistry, that would render catalytic materials with controlled pore size and molecularly defined, isolated active sites. Soon, we became deeply involved in the topic of titanium-mediated catalytic epoxidation and consequently titanium-grafted silicas (Figure 1).1 So was Sir John Meurig Thomas.2 In the fall of 1995, Sir John published in Nature on the direct grafting of organometallic synthons onto the inner walls of mesoporous silica MCM-41.3 This generated a shape-selective catalyst with a large concentration of accessible, well-spaced and structurally well-defined active sites. Specifically, attachment of a titanocene-derived catalyst precursor to the pore walls of MCM-41 produced a catalyst for the epoxidation of cyclohexene and more bulky cyclic alkenes. 385

386

Chapter 23

OH T SiO O Si

a) a

Figure 1

OH

Si O

OH Ti SiO

Ti OSi

SiO

OSi

O Si

O Si

b

c

Environment of the terdentate chelated titanium sites (type b) in surfacegrafted TiTMCM4l catalysts (blue ¼ Ti, yellow ¼ Si, red ¼ O, white ¼ H) and relation with possible framework titanium sites in titanosilicates: (a) bipodal site, (b) tripodal (open lattice) site and (c) tetrapodal (closed lattice) site.

In doing so, Sir John took a shortcut compared to our approach that provoked ample discussion. For instance, were these catalysts really stable? Why did they only work with anhydrous organic peroxides? Was there a difference with just titanium on silica as employed industrially in Shell’s SMPO (Styrene Monomer Propylene Oxide) process for propylene oxide? How to make molecularly defined catalytic materials? What was the relationship with microporous titanium silicalite (TS-1) that performed so beautifully with aqueous hydrogen peroxide? Indeed, Nature publications like Sir John’s should provoke and inspire and this was certainly so for us. In fact, it stimulated experimental work that brought excitement and which is still not finished to date. In this account, we will discuss our work following the original questions.

2 Are Silica-Grafted Titanate Catalysts Really Stable? When we started our search in 1994 for non-leaching heterogeneous liquidphase oxidation catalysts, detailed studies of how the catalytically active metal species are bonded to the support were rare. As today, usually more attention was paid to the performance of the catalyst, rather than to the fundamental question of whether the catalyst is truly heterogeneous or not. In fact, many catalysts consisting of a metal oxide on an inert carrier owe their catalytic

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387

activity to rapid leaching of the metal from the surface to form active homogeneous catalysts, a fact that the designers of the original catalysts clearly did not have in mind. In order to approach this problem at a molecular level, we first reported in 1997 on epoxidations catalyzed by model systems, titanium silsesquioxanes. Soon, Sir John and co-worker Thomas Maschmeyer followed with their own findings, as did leading industrial researchers from Shell!1 In these silsesquioxanes, the titanium site is incorporated via spatially oriented siloxy bonds (Ti–O–Si) which structurally resemble surface sites that have been purportedly identified on silica surfaces (Figure 2). Heterogeneous titanium catalysts are very important in oxidation processes but at the same time have been reported to undergo some leaching in liquid-phase applications. For instance, the highly active Shell titanium/silica epoxidation catalyst used is the SMPO process for propylene oxide, becomes only truly heterogeneous after a certain time on stream.4 Similar materials, that were reported as the result of grafting silica or MCM-41 mesoporous silica with titanium derivatives, or even novel titanosilicates, might therefore be only partially heterogeneous when applied in liquid-phase oxidation reactions. With silsesquioxane model systems, we proved that liquid-phase, silicasupported titanium epoxidation catalysts are predicted to be accessible and active in many cases but will be truly heterogeneous only when stringent conditions are met.5 Particularly, the titanium site should be incorporated in a silanol nest rendering at least terdentate silanolate coordination. These findings are consistent with a mechanism for alkene epoxidation in which

Figure 2

Incompletely condensed silsesquioxanes (POSS) suitable for modeling terdentate (a), bidentate (b) and monodentate (c) silanolate chelation and examples of titanium derivatives.

388

Figure 3

Chapter 23

Part of the epoxidation pathway proposed by Clerici and Sheldon in which the peroxide reacts with the titanium site to form an active species for epoxidation.

reversible hydrolysis of a titanium siloxy function occurs. This supports the now firmly established mechanism of heterogeneous alkene epoxidation by titanium silicalites proposed by Clerici, Ingallina, Sheldon and co-workers (Figure 3).6 Still, the debate on catalyst stability has not ended. Clearly, denticity of siloxy chelation is the factor that determines whether a titanium silsesquioxane catalyst is stable in anhydrous media. For silica-supported titanium sites, this statement is further collaborated by computational work, for instance, of Hillier et al.7 However, when a wet medium for the epoxidation reaction is used, the denticity is not the only factor that determines the stability of the catalyst. In addition, the intramolecular surrounding of the titanium site should then be hydrophobic enough to assist in further protecting this site against irreversible hydrolysis.

3 Modeling Shell’s SMPO Process for Propylene Oxide As Sir John’s titanium-grafted MCM-41 material contains mainly robust tripodally anchored titanium sites, it should follow that the catalyst should not leach titanium in aprotic liquid-phase epoxidation catalysis. After synthesis, the coordination sphere of titanium was completed by a cyclopentadienide ligand. At the start of epoxidation with organic peroxide, this Cp ligand is gradually displaced by alkoxide, for instance t-butoxide if TBHP (tert-butylhydrogen peroxide) is employed as the oxidant. As a result, the catalysis kinetics exhibit an induction period before second order kinetics are observed. Replacing Cp with hydroxyl renders very active catalysts that display truly second order kinetics from the start of the epoxidation (Figure 4).8

4 How to Make Molecularly Defined Catalytic Materials? In the mid-1990s, the major hurdle in the use of silsesquioxanes, not even to mention the impossibility to turn them into heterogeneous catalysts, was the

From ‘Nature’ to an Adventure in Single-Site Epoxidation Catalysis

Figure 4

389

Epoxidation kinetics of TiCp (left) and TiOH (right) POSS catalysts revealing displacement of Cp.

long preparation time (ranging from a few weeks to 36 months) and the limited scope of the organic side groups on the silicon atoms (non-reactive groups unsuitable for ligand immobilization). Since then, new developments and ideas have shortened the preparation times and broadened the scope considerably. Starting with Thomas Maschmeyer, the use of high-throughput experimentation and synthesis robots have accelerated the optimization of synthesis conditions.9 Recently, base-catalyzed polycondensation reactions have proven to be an excellent way to prepare large quantities of silsesquioxanes. Parallel and in synergy with Joe Lichtenhan we have applied for patents on the preparation of completely condensed and incompletely condensed silsesquioxanes with isobutyl and iso-octyl side groups that can be prepared on large scales in a short time. Until recently, functionalization of silsesquioxane silanols has been limited to either corner-capping of trisilanols with a trihaloorganosilane moiety, leaving no further reactive silanol groups, or reaction of the trisilanol with mono- or di-haloorganosilane reactants, leaving two or one silanol groups, respectively. In the first case, a large number of possible side groups can be introduced, ranging from simple alkyl groups to reactive alcohols, amines and alkenyl groups. These groups allow the silsesquioxane cores to be included in polymeric materials. Furthermore, there was substantial interest in octafunctional silsesquioxanes where all the side groups on the silicon atoms are identical and reactive. In these cases the functionality ranges from alkyls, alcohols, amides and carboxylates to halides, nitrates and phosphanes.10 These can even be used as building blocks for dendrimers, as shown by ColeHamilton et al. for use in catalytic hydroformylation reactions.11 The latest development involves the synthesis of functionalized silsesquioxane trisilanols. With these, homogeneous catalysts like our titanium-based epoxidation catalysts can be straightforwardly converted into molecularly defined catalytic materials. Clearly, silsesquioxane-derived metal catalysts should no longer be regarded as chemical curiosities. They provide new catalysts with both homogeneous and heterogeneous applicability. Most interestingly, it is now firmly established that

390

Figure 5

Chapter 23

Graphical representation of the adsorption of a silsesquioxane titanium complex in the 30 A˚ pores of all-silica MCM-41.

the steric and electronic properties of silsesquioxane silanolate ligands render metal centres more Lewis acidic than conventional alkoxide or siloxide ligands do. This concept has been exploited in newly developed catalysts beyond epoxidation as for example alkene metathesis, polymerization and Diels–Alder reactions of enones. Other applications are envisioned in the near future. In fact, we believe so strongly in the inherent nanotech innovation that we recently founded the company ‘Hybrid Catalysis’ to further commercialize silsesquioxane derived catalytic materials. With homogeneous, active epoxidation catalysts in hand, we started to work on their immobilization. An exciting first finding was that this could be achieved by exploiting the strong adsorption of complex 1 in all-silica MCM-41 channels (Figure 5).12 The resulting self-assembled materials are active, truly heterogeneous and recyclable catalysts for alkene epoxidation in the liquid phase. Essential for an irreversible adsorption of the silsesquioxane complex proved the use of somewhat hydrophobic, aluminium-free MCM-41. Somewhat disappointing was the finding that none of the epoxidation catalysts developed so far were active in applications with aqueous hydrogen peroxide, as was the case for the Shell SMPO catalyst. Clearly, just having a robust, accessible titanium site was not enough, nor was every combination of just such a site with any support.

5 Why Water Kills the Cat For us, this was a puzzle that needed to be solved soon! Solvay was by then funding our research and as a major producer of hydrogen peroxide they were not too pleased with catalysts that refused to work with their oxidant. Little help came from the observation that few catalysts have been truly efficient in alkene epoxidation with aqueous hydrogen peroxide. Development of such catalysts was, and still is, so important since with regard to desirability, this oxidant comes second only to oxygen itself.13 Still rather unbeatable, the best catalyst in this field is the synthetic titanium containing zeolite, titanium silicalite-1 (TS-1),14 which is active for a wide range of oxidation

From ‘Nature’ to an Adventure in Single-Site Epoxidation Catalysis

Figure 6

391

Synthesis of three-dimensionally netted polymeric catalysts, these bioinspired catalytic ensembles are indeed active with aqueous hydrogen peroxide.

reactions, including epoxidation.15 For TS-1, activity seems to originate from a combination of a robust active Ti(OSi)n site (n ¼ 3, 4),16 and its location in a hydrophobic channel or cavity in the MFI (ZSM-5) structure.17 The resulting catalytic ensemble prevents poisoning of the active site by water as well as unproductive decomposition of the oxidant. Through our previous work on homogeneous epoxidation catalysis, we knew that we had robust silsesquioxane titanium derivatives in hand. The next challenge was to convert these compounds into materials that would add a hydrophobic environment, if not to dream of defined pores and cavities, to the active site (Figure 6). Profiting from advances in silsesquioxane ligand synthesis, we were able to make titanium derivatives with a function suitable for ligand tethering. Such a function was provided by a vinyl containing silsesquioxane ligand that could be grafted on a commercially available silicone system through hydrosilylation. Subsequently, we were very excited when we found that these catalytic materials could indeed be used in heterogeneous epoxidation with aqueous hydrogen peroxide.18 These first results demonstrated that grafting of functionalized titanium silsesquioxanes on polysiloxanes provided a way to realize the formation of a catalytic ensemble that was capable of performing epoxidation with aqueous hydrogen peroxide. Clearly, the entire system was capable of outperforming the sum of its parts; it was the synergy between active site and its environment that allowed our hybrid catalysts, and likewise TS-1, or even metalloenzymes to achieve their desirable performance. Having said so much, our hybrid catalysts were relatively simple, amorphous materials that did not compete in activity with TS-1. The next challenge was how to control the porosity of our materials.

392

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6 The Quest for Hybrid Catalysis Catalyst immobilization as shown above, either involving inorganic solids or polymer supports are the two most commonly used heterogenization procedures. They each have some advantages over the other, and provide routes to catalytic materials that can meet special demands in chemical reaction engineering.19 Inorganic supports often possess rigid porous structures and high specific surface areas and are easy to separate, while organic polymers often make the catalyst behave more homogeneously because of the better compatibility with organic reaction media. Considering this fact and our previous results for the immobilization of titanium silsesquioxanes, we proposed combining these two methods to synthesize porous inorganic solid-supported polymeric silsesquioxane films. The resulting hybrid catalyst should have several merits: facilitated diffusion of reactant molecules, better accessibility of the active sites due to the rigid porous structure of the inorganic support, and improved compatibility between host and guest species and organic solvents arising from the specific properties of the organic polymer films. Of course there is a catch in any project aimed at making complex materials. First, it is necessary to avoid pore blockage by the polymer since an open porous structure is important for the accessibility of the active sites, so appropriate choices of inorganic support and the polymerization method should be made. The second challenge is to increase the stability of the polymer on the inorganic support. Ordered mesoporous silica provides an ideal support as the ordered open mesopore systems with uniform pore diameters are beneficial for diffusion and dispersion of guest species and hence will reduce the possibility of pore blockage. There have been several previous investigations of the synthesis of organic polymers inside mesoporous silica. The resulting hybrid composites showed some interesting physical properties due to the confinement of the nanopores.20 However, in most cases, the entire pore volume of the mesoporous silica was filled by the incorporated polymer.21 Therefore, for retaining a porous structure after the polymer incorporation, an efficient immobilization method needs to be developed. Here, we got inspiration from Ryoo et al. who reported recently that functional polymer–silica composites with well-defined mesoporosity could be obtained by in situ polymerization of monomers adsorbed on the pore walls of mesoporous silica SBA-15.22 A key step to avoid pore blockage was to form a uniform monomer film on the pore wall surface of SBA-15 before polymerization by selectively removing the solvent used for the impregnation of monomers. SBA-15 is probably one of the best supports for the synthesis of porous organic polymer–silica composites.23 In addition to the ordered two-dimensional hexagonal mesoporous structure similar to MCM-41, SBA-15 presents the advantage of larger pore diameters (47 nm) than the MCM-41 used previously for the immobilization of our catalysts. This is important for efficient diffusion and dispersion of POSS-containing monomers inside the nanopores and for reducing the risk of pore blockage. (The titanium silsesquioxanes have an approximate diameter of 1.5 nm, roughly half the pore diameter of MCM-41.)

From ‘Nature’ to an Adventure in Single-Site Epoxidation Catalysis

Figure 7

393

Schematic presentation of the procedure for the synthesis of SBA-15supported hybrid catalysts.

Moreover, for most SBA-15 materials, there are complementary mesopores/ micropores inside the mesopore walls connecting the mesopores in a threedimensional porous structure, which can help the polymer film interconnect into a three-dimensional network, interpenetrate with the silica framework, and thus increase the stability of the polymer on the silica support. Not to forget the context with TS1 and hydrophobic pores, Ryoo et al. reported the synthesis of ferrocene-functionalized mesoporous polymer–silica nanocomposite materials that exhibited high activity and selectivity towards catalytic hydroxylation of phenol, which was once more attributed to the hydrophobic nature of the supported polymer surface (Figure 7).24 Using the lessons learned in previous studies, we demonstrated the successful combination of two commonly used immobilization methods by integrating titanium silsesquioxanes into an SBA-15-supported polystyrene film by in situ copolymerization. The resulting hybrid materials have highly ordered mesoporous structures and proved to be very active heterogeneous catalysts for

394

Chapter 23 25

alkene epoxidation with TBHP or aqueous hydrogen peroxide. Fusion of the silica support (highly porous structure, hydrophilic surface) and of the polystyrene and POSS (Polyhedral Oligomeric Silsesquioxanes) (hydrophobic network) into one entity makes the hybrid materials behave as interfacial catalysts in epoxidation with aqueous peroxides and leads to much higher activity than their homogeneous counterparts due to the hydrophobic environments around the active centres.

7 A Word in Retrospect In the summer of 1995, we were obviously provoked, triggered and inspired by one of Sir John’s publications in Nature. For us, this marked the start of an adventure in epoxidation catalysis that has still not ended today. We have come a long way starting with modeling the active sites of proposed heterogeneous catalysts using silsesquioxane chemistry. From that, Lego-like chemistry and methodology were developed for hybrid precision catalysts. At present, the toolbox for modifying nanoporous materials so as to place isolated, single-site active centres on their large internal areas has become available. As such, the start of new adventures is marked with a technology push for the application of advanced catalysts aimed at sustainability, cascade reactions, rapid lead finding and industrial implementation.

References 1. M. Crocker, R.H. Herold, A.G. Orpen and M. Overgaag, J. Chem. Soc., Dalton Trans., 1999, 21, 3791. 2. R.D. Oldroyd, J.M. Thomas, T. Maschmeyer, P.A. MacFaul, D.W. Snelgrove, K.U. Ingold and D.D.M. Wayner, Angew. Chem., Int. Ed. Engl., 1996, 35, 2787. 3. T. Maschmeyer, F. Rey, G. Sankar and J.M. Thomas, Nature (London), 1995, 378, 159. 4. H.P. Wulff, US Pat., 3 923 843, 1975 (Chem. Abstr., 84, 89977d); H.P. Wulff and F. Wattimena, US Pat., 4 021 454, 1977 (Chem. Abstr., 87, 22393d). 5. H.C.L. Abbenhuis, S. Krijnen and R.A. van Santen, Chem. Commun., 1997, 331. 6. U. Romano, F. Esposito, F. Maspero, C. Neri and M.G. Clerici, La Chimici L’industria, 1990, 72, 610; E. Ho¨ft, H. Kosslick, R. Fricke and H.-J. Hamann, J. Prakt. Chem., 1996, 388, 1; H.X. Gao, G.X. Lu, J.S. Suo and S.B. Li, Appl. Catal., 1996, 138, 27; P. Ingallina, M.G. Clerici, L. Rossi and G. Bellussi, Stud. Surf. Sci. Catal., 1994, 92, 31; R.A. Sheldon, J. Mol. Catal., 1980, 7, 107 and unpublished results from Schram and Van Broekhoven cited therein. 7. D. Tantanak, M.A. Vincent and I.H. Hillier, Chem. Commun., 1998, 1031. 8. H.C.L. Abbenhuis et al., Dalton Trans., accepted for publication.

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9. P.P. Pescarmona, J.C. Van der Waal, I.E. Maxwell and T. Maschmeyer, Angew. Chem., Int. Ed., 2001, 40, 740. 10. R.W.J.M. Hanssen, R.A. van Santen and H.C.L. Abbenhuis, Eur. J. Inorg. Chem., 2004, 4, 675. 11. L. Ropartz, K.J. Haxton, D.F. Foster, R.E. Morris, A.M.Z. Slawin and D.J. Cole-Hamilton, J. Chem. Soc., Dalton Trans., 2002, 4323. 12. S. Krijnen, H.C.L. Abbenhuis, R.W.J.M. Hanssen, J.H.C. Van Hooff and R.A. van Santen, Angew. Chem., Int. Ed., 1998, 37, 356. 13. G. Strukul, Catalytic Oxidations with Hydrogen Peroxide as Oxidant, Kluwer, Dordrecht, 1992. 14. G. Perego, M. Taramasso and B. Notari (SNAM Progetti S. p. A.) BE 886812, 1981 (Chem. Abstr., 1981, 95, 206272k). 15. B. Notari, Catal. Today, 1993, 18, 163. 16. S. Bordiga, S. Coluccia, C. Lamberti, L. Marchese, A. Zecchina, F. Boscherini, F. Buffa, F. Genoni, G. Leofanti, G. Petrini and G. Vlaic, J. Phys. Chem. B, 1998, 102, 6382. 17. B. Notari, Adv. Catal., 1996, 41, 253. 18. M.D. Skowronska-Ptaskinska, M.L.W. Vorstenbosch, R.A. van Santen and H.C.L. Abbenhuis, Angew. Chem., Int. Ed., 2002, 41, 637. 19. For recent reviews on immobilization of homogeneous catalysts, please see: N.E. Leadbeater, M. Marco, Chem. Rev., 2002, 102, 3217; C.A. McNamara, M.J. Dixon and M. Bradley, Chem. Rev., 2002, 102, 3275; Q.H. Fan, Y.M. Li and A.S.C. Chan, Chem. Rev., 2002, 102, 3385; C.E. Song and S. Lee, Chem. Rev., 2002, 102, 3495; D.E. De Vos, M. Dams, B. F. Sels and P.A. Jacobs, Chem. Rev., 2002, 102, 3615; P. Mastrorilli and C.F. Nobile, Coord. Chem. Rev., 2004, 248, 377. 20. C.G. Wu and T. Bein, Science, 1994, 264, 1757. 21. T.Q. Nguyen, J.J. Wu, V. Doan, B.J. Schwartz and S.H. Tolbert, Science, 2000, 288, 652. 22. M. Choi, F. Kleitz, D. Liu, H.Y. Lee, W.S. Ahn and R. Ryoo, J. Am. Chem. Soc., 2005, 127, 1924. 23. D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka and G.D. Stucky, Science, 1998, 279, 548. 24. A. Dubey, M. Choi and R. Ryoo, Green Chem., 2006, 8, 144. 25. L. Zhang, H.C.L. Abbenhuis, G. Gerritsen, N. NiBhriain, P.C.M.M. Magusin, B. Mezari, W. Han, R.A. van Santen, Q. Yang and C. Li, Chem.–Eur. J., 2007, 13, 1210.

CHAPTER 24

A Comparison between Enzymes and Solid State Catalysts ROBERT J. P. WILLIAMS Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QR, UK

1 Introduction The major interests of John Meurig Thomas have been, and still are, in solid state materials, especially in their structural features, and in the synthesis of catalysts from solid state porous oxides containing metal ions, see Figure 1.1 Now a feature of these catalysts is that they are rarely selective and considerable rate enhancement often requires high pressures or high temperatures. They do not have the power of metallo-enzymes.2 Enzymes have to be very active and selective in their reactions since they are taken from biological cells in ambient conditions. In cells a multitude of sensitive reagents are present, many very aggressive, e.g. O2 and H2O2. By way of further contrast a solid state catalyst is employed in a reaction of individually selected reactants and their product control needs only to be modest. This raises the question as to why there is this big difference in the ability to catalyse. Comparison is made difficult however by the way rate enhancement and selectivity have been examined and the theoretical treatments employed to explain them. When we have tackled these two points we can see what common factors there are and what are the major advantageous features of enzyme action. Similar difficulties are apparent when we compare molecular catalysts3 with enzymes. I shall deliberately select examples from John’s research to illustrate the comparison. (From the outset I stress that there has been much input to its contents from John himself.)

1.1

The Use of Rate Expressions

Two approaches will be described here in very simplified conceptual equations.4 In the earliest analyses of rates of reaction the expression used was rate ¼ k½reactants
n 396

ð1Þ

A Comparison between Enzymes and Solid State Catalysts

Figure 1

397

Three-dimensional representation of the pore structure of AlPO-18, AlPO-36 and AlPO-5 (pore apertures are, respectively, 3.8 A˚, 6.57.5 A˚; and 7.3 A˚). The size of the atoms in the structures corresponds to their van der Waals radii. A metal ion can be substituted for an Al31 ion or a metal ion complex can be attached to or included in the framework. From Thomas et al.25 with permission.

where k is a rate constant and [reactants]n can be expressed by product functions for any number of reactants. The temperature dependence of k was given by k ¼ CPeDG/RT where the constants related to collision rate, C, and orientation factors, P, in the collision. For a catalyst reaction in solution an extra concentration term is that of the catalyst. In a catalysed unimolecular reaction, (now bimolecular) only the value of DG is reduced. For a catalysed bimolecular reaction in solution an improbable three molecule collision must occur, making it extremely unlikely. The rate of reaction on collision is increased again by reduction of DG. Solid catalysts can be described in a similar way replacing the concentration dependence of the catalyst by sites on the surface. An alternative approach stresses the prior binding of reactants to the catalyst such that in the bound state the reaction is unimolecular. The simplest rate constant expression becomes 0

rate ¼ Kk ¼ KeDG =RT

ð2Þ

where K is an equilibrium constant (equilibrium holds provided on/off binding rates are faster than reaction rates (see ‘‘Note’’ at the end of the article)). There are

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also two separate considerations of orientation and of exponential temperature dependent factors in K and k. A clear advantage of binding is that it increases the probability of reaction and DG values can also be reduced. The difficulties for any comparison we meet are in the ability, even the wish, to follow reactions in sufficient detail to obtain the parameters in either equation in all cases. Experimental approaches using the different catalysts, Table 1, have had different objectives too so that the rate enhancements achieved are again difficult to compare in terms of factors in Equations (1) or (2). Often examination of a molecular or a solid state catalyst activity stops with knowledge of the yield of a product in a given time. Selectivity study is not necessary since the reaction and catalysts are chosen so as to yield a definite product and, unless enantioselectivity is required, even a degree of mixture of products can be satisfactory. Enzymes have been subjected to much more detailed experimental kinetic appraisal as their products and their purity are almost taken for granted as 100%. Another problem for comparison is that while enzymes operate in aqueous solution at pH ¼ 7 and ambient temperature/pressure, other catalysts do not have any such limitations to their use. In an earlier paper5 we have recognised these difficulties and proposed that there were three kinds of safe topics for comparisons between the catalyst classes. (a) The metal ions chosen in man-made catalysts as opposed to those selected by nature in enzymes for a given reaction. (b) Given the selected metal ion, the choice of donor atoms of ligands as binding groups. Table 1

Principal types of catalysts.

Name or classification

Properties

Homogeneous 1.

Molecular systems

2.

(Heterogenized variants) Enzymes

3.

(Heterogenized variants) Heterogeneous solid systems Nanoporous solids Dendrimers Metallic and conducting

Single atom/ion site (not constrained) (a) Soluble small molecule active in solution (b) Small molecule attached by flexible-linker to a surface Soluble large proteins active usually in water including proteins such as antibodies modified for catalytic purposes Membrane catalysts including enzymes Continuous solids such as oxides and sulphides Open frameworks including immobilised atoms, complexes or nanoparticles of metals Metals, alloys, metal conductors

Note: The active site atoms in all the cases except the first may be constrained, see text.

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399

(c) The manner in which the complexes of (a) and (b) were held in a framework, that is the overall structure especially that of the catalytic region. The comparison of catalysts had to be restricted more or less to single sites containing one metal ion in order to remove participation of the frameworks in the catalytic acts. We left to one side catalysts with adjacent sites or distant non-adjacent sites as their properties could depend so much on the nature of the very different frameworks. Here we shall explore these cases further. As an entry into the topic we shall take a step backward at first and look at some common ground associated with the above three points of comparison, summarizing them in Sections 2 and 3 so as to introduce the energetics as used particularly in studying enzymes following elaboration of Equation (2). While we look at the specific nature of enzyme frameworks we shall refer to the frameworks of other types of catalyst, especially solids (Table 1).

2 The Concepts of the ‘‘Active Site’’ and the ‘‘Active Region’’ It is commonly assumed that catalysts have active ‘‘sites’’. Somewhat curiously the concept of the catalytically active site first entered the field in the description of heterogeneous metal catalysis in the 1920s when H.S. Taylor proposed that topographical irregularities, such as steps and kinks at otherwise flat metal surfaces, were likely to exhibit enhanced reactivity compared with sites at flat surfaces. We shall see later that, as applied to metals, it is a difficult definition to analyse in structural detail. The concept of an active site in small molecule catalysts generally and in the field of biological (chiefly enzymatic) catalysts was discussed in the early 1930s and is much easier to appreciate. It referred to a small centre of possible binding to and of attack by the catalyst, e.g. a metal ion. The small molecules were usually of known outline structure even at that time and often the sources of activity were clear. The binding and the attacking site were seen as one and the same, see Figure 2. As a result, and with increased sophistication in the development of the small molecular metal-containing catalysts, Figure 2,6 together with more detailed knowledge of their structures and the energetics of their mechanism, much of the application of the active site principle has been limited to action of the metal elements as will be familiar to any scientist interested in the theory of these catalysts.3,4 When molecular catalysts have considerable molecular weight they can be compared directly with enzymes, while when these small molecular catalysts are embedded in or attached to surfaces they can be compared with heterogeneous solid catalysts. We turn next to the idea of an active ‘‘site’’ in an enzyme. A dramatic advance in our understanding of the nature of active regions (not just attacking sites as we shall see) in enzymes occurred in the 1950s–1960s period, when the atomically resolved structures, first of a protein, myoglobin, Figure 3,7 and then of lysozyme, an enzyme, were determined by X-ray

400

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[Ru(OAc)2((S)-binap)]

[Ru(OAc)2((R)-binap)]

Figure 2

R3

COOH

R2

R1

H2

R3

COOH

R2

R1

COOR1

COOR1

NHCOR2

NHCOR2

Stick models depicting the R and S BINAP enantioselective hydrogenation catalysts (see text) devised by Noyori et al.6 Note the steric hindrance in the binaphthalene ligand contributing to constraint and enantioselectivity. The metal ion at the centre binds and activates reaction.

crystallography.8 Immediately the three-dimensional details of the oxygen binding site of myoglobin revealed a possible first step of catalysed oxygen reactions, haem iron binding, but the site was complicated by the further interaction of O2 with other protein groups. A similar situation arose with the well-defined substrate-binding cleft (active region) of lysozyme, that exists in the folded structure of its 128 amino acids, which apparently allowed the visualisation of a defined active ‘‘site’’ of two attacking carboxylates. The function of these, lysozyme’s, non-metal attacking groups, was aided however by the sterically restricting and binding side chains located nearby. Only when consideration was given to them (together with the attacking groups) was it made possible to formulate a plausible catalytic mechanism for lysozyme’s mode of action and to demonstrate its selectivity in terms of complex kinetics. Somewhat distantly, these were related to Equation (2). These structures brought into focus the fact that the ‘‘active site’’ of an enzyme could be a much more complicated structure (region) than had been conceived from small complex molecule or metal (solid state) structures since it involved not just active attacking sites but considerable areas of the matrix surface at least in binding. The substrates of lysozyme are in fact large polysaccharides. The surface and the matrix of the protein were proved later to be far from static so that the catalytic act involves in addition a dynamic cycling of not just local groups near the site but of more remote regions and indeed of the whole matrix.9 This makes the energetics of the lysozyme action difficult to describe.

A Comparison between Enzymes and Solid State Catalysts

401

The allosteric switch in haemoglobin, a tetramer of myoglobin-like units, also shows this mobility during O2 binding. In many of the immediately following sections we shall avoid discussion of mobility in enzymes as there is little here in common with other catalysts. Now lysozyme’s attacking groups are not metal ions, so that any comparison with small metal complexes and solid state metal ion lattice catalysts (the major number of catalysts) is not easily made. We shall therefore use metal enzymes, Table 2, as the basis of discussion and for any comparison with solid state and molecular catalysts. We shall take the structure of myoglobin, Figure 3, a haem iron containing protein, as one example which, though not catalytically active, can be made into an enzyme by mutation.10 We shall also reduce the complexity of the discussion by referring most frequently to small molecular substrates, such as H2, O2, N2, CH4, H2O, which we note are almost invariably activated by metal-containing catalysts. Now, on the basis of the early studies, mostly those of enzymes, different ideas as to the way in which active regions catalyse reactions were developed (Table 3). Our concern will be to illustrate these general ideas and to introduce others while we examine enzymes in some detail. Note that Figures 1–3 illustrate the very different nature of frameworks in three different classes of catalyst. We restrain from discussing the fourth, conducting frameworks, until Section 9.

Figure 3

A protein like myoglobin, shown here, contains a haem group, which is the open-sided active site for the binding of oxygen (after Kendrew). (Strictly speaking, myoglobin is not an enzyme, but, by genetic mutation,10 it can be converted into one). The key point here is that this well-defined structure illustrates well the manner in which a spatially small active centre is embedded within a large a-helical proteinaceous matrix.

402

Chapter 24

Table 2

Examples of metallo-enzymes.

Class of Catalysis

Example of active site Mn1

Acid/base hydrolysis Electron transfer Oxidation (O2) Oxidation (H2O) Oxidation (H2O2, RO2H) Hydrogenation Group transfer (–CH3) Group transfer ðOPO2 3 Þ Group transfer (CO)

Zn (Mg, Co, Ni, Ca) Fe, haema (Fe), Cu Fe, haema (Fe), Cu Moa, Wa, Mn Se, Fe, haema (Fe) Ni, Fe Co (B12)a Mg Ni (F-430)a

Note: Organic side-chains and metal ions can be substituted. The organic side chain substitution is usually done by gene mutation but metal ion substitution is done by direct exchange and in fact this can be done for S and Se. a In these cases the metal ion is in a metal complex.

Table 3

Active site concepts.

1. Basic idea of a catalytic ion or complex (19th century) 2. Taylor’s concept of local regions of heightened activity on metal surfaces. Parallel views of special points in molecules and on enzyme surfaces (1920–1940) 3. Haldane’s suggestion of steric strain on binding of a substrate (1935) 4. Pauling’s proposal that the binding of a substrate by the catalyst changed its structure to one close to that of the transition state for reaction, see Figure 6 (1945) 5. Koshland’s idea of induced fit whereby the enzyme (catalyst) enclosed itself around the substrate (1958) 6. The proposal of Gray, Malmstro¨m and Williams that the matrix or framework of a catalyst induced constraints on the catalyst activating its attacking active site (1960–2002) 7. The discovery that many catalytic acts required mobility through NMR and other methods (1970 to today)

3 Survey of Single Sites of Metal Ions in Catalysts Elsewhere we have provided a comparative description of single site metal catalysis.5 Rather than repeating our earlier analysis here we summarise those properties which show similarities and differences between the three groups of catalyst, leaving to one side conductor and cluster catalysts and any discussion of mobility until later.

3.1

The Choice of Metal Atom or Ion

In the case of enzymes, choice is restricted to the environmentally available elements mainly Mg, V, Mn, Fe, Co, Ni, Cu, Zn in oxidation states I, II or III and Mo and W in oxidation states IV or VI.5 For all other classes of catalyst the choice is more open, that is from all elements of the Periodic Table. The states

403

A Comparison between Enzymes and Solid State Catalysts H(O)CC2H4R

CO

H

CO

Co H2

CH2

CO

CO

CHR CO

R CH2 O

CH2

CH2 CHR CO H Co

CO

CO Co CO

CO

CO

R

CO

CO

CH2 CO CO

Figure 4

CH2 CO Co

CO

CO

The hydroformylation reaction, which converts an alkene CH2QCHR into an aldehyde in the presence of H2 and CO, is catalysed by the homogeneous, molecular entity HCo(CO)4 as outlined here (After Leeuwen, Ref. 20a, p. 127). Note the movements of the substrate on the metal.

of the metal ions of the first transition series when the oxidation state is II or III are usually high-spin. In special cases low-spin Fe, Co and Ni are made by insertion in synthesised chelates, haem, vitamin B12 and F-430, i.e. in coenzymes. In the second and third row transition metal series of molecular and solid state catalysts, the states of the metal atoms or ions are virtually all low-spin with definite kinetic advantages. In molecular catalysts, the oxidation state zero is also common, for example in carbonyls, Figure 4. This state is not available to enzymes. Thus enzymes must have very refined metal ion complexes to match the catalytic powers of the other groups of catalyst.

3.2

The Choice of Ligand Atoms

The ligand atoms for binding metal ions are restricted in enzymes to the biological protein donors O, N and S with a very rare C-ligand, but they can be in unsaturated rings in coenzymes, see Figure 7 later. Solid state catalysts have binding atoms, which are largely restricted to the O of framework oxides, see Figure 1, or those of synthesised bound molecular complexes. However, there is also the possibility of using C and S in carbides and sulfides and a greater variety has been introduced with the synthesis of soft solid matrices. Finally it is possible to synthesise solids with other donors by replacement of the atoms of the framework. Molecular complexes can have a great variety of O, N, S and C donors, of different saturated or unsaturated ligands, note CO and cyclopentadiene, see Figures 2 and 4. They can also have P- or even As-donors, especially phosphines. Note that molecular units can be attached to surfaces

404

Chapter 24

of, or put inside, certain solid frameworks. Variety is again most limited in enzymes.

3.3

The Nature of Metal Ions Plus Ligands

The structure of the unit formed from (1) and (2) is frequently an open-sided cavity, see Figures 1, 2 and 3. It can then select by exclusion so that enantiomorphic substrates can be separated. Selection by binding, see also frameworks, is not great except in enzymes where there is greater need for it. The symmetry of the metal ion site can also be selected opposite function in enzymes, (see below), but less so in molecular catalysts and is considerably limited in solid state, catalysts. We stress the advantage of the nature and the properties of the folding of proteins below.

3.4

The Solvent and the Thermal Stability

The solvent for enzymes is usually water at pH ¼ 7. Solvents for molecular and solid state catalysts can be chosen at will and solids can be used with gaseous reactants. The separation of products is simple when using solids but not in the other two cases. Moreover while enzymes give but one product other catalysts give mixtures requiring separation. Thermal stability is only good for solid state catalysts.

3.5

The Substrates

There are limitations to enzyme substrates as they must be soluble in water. No such restriction applies to the other catalysts.

4 The Frameworks Holding Active Sites In this article I shall assume that the reader is familiar with the basic features of the frameworks of all the four classes of catalysts, molecular and enzymic (homogeneous) and non-conducting and conducting solids (heterogeneous), Table 4, see earlier figures. As pointed out above, it was conventional to consider catalysis in all classes as involving an active metal ‘‘site’’ without much reference to these frameworks. We have seen already however how the attack can be refined by the way in which the framework holds the metal ion. It can be made to be open-sided or with an easily displaced simple ligand in all kinds of catalyst. The most conventional sites are of incomplete octahedral or tetrahedral symmetry. Elsewhere5,11 we have explained that a very stable ground state is not likely to be the most advantageous for catalysis. It should be noted then that the framework can force upon a metal an unlikely coordination structure and energy. Such constrained (entatic) states are seen in many enzymes and have been shown to match catalytic needs, Figure 5,11 and they are not

A Comparison between Enzymes and Solid State Catalysts

Table 4

405

Some commonly used solid heterogeneous catalysts.

Catalyst

Reaction Catalysed

Finely divided nickel

Hydrogenation of fats/unsaturated naturally occurring molecules Synthesis of ammonia (from N2 and H2)

Iron (with potassium promoter) Cobalt-based alloys supported on oxides Pt/Al2O3 La31-exchanged zeolite-Y Pt on H1-zeolite-Y H1-ZSM-5

TS-1 (Ti silicalite)

Fischer-Tropsch synthesis of alkanes, alkanols or alkenes from ‘‘synthesis gas’’ (CO+H2 mixture) Reforming of hydrocarbons, i.e. production of alkanes from linear ones Catalytic cracking of hydrocarbons Hydroisomerisations (i) Isomerisations, e.g. of but-1-ene to 2-methyl propene (ii) Catalytic dewaxing (conversion of linear alkanes to branched ones) (iii) Alkylation of aromatics, e.g. production of ethyl benzene from benzene and ether

(i) Ammoximation of cyclohexanone (in the production of e-caprolactam)

(ii) Oxidation of benzene to phenol with H2O2 Ti(IV) in SiO2 Zirconocene alkyls (on oxide supports)

Epoxidation of alkenes with ROOH (i) Stereoregular polymerisation of alkenes (ii) Selective hydrogenation of aromatics

SAPO-11 and SAPO-34 Acidic clays

Dehydration of methanol (selectively) to light olefins (i) Alkylation of aromatics and other unsaturates (ii) Synthesis of ethyl acetate from glacial acetic acid and ethane

unknown in the other classes of catalysts. It is a particular feature of enzymes in that their protein folds readily give rise to such sites. There are some common features of the frameworks as they all can be designed to be of enantiomorphic selectivity or to have cavities excluding ligands of certain sizes and shapes while allowing others to the active site. It is within the cavity where we see further the advantages of enzymes since a folded protein with some twenty different side chains of its amino acids and a hundred or so folds can create binding cavities of a very great variety. (Each unit in the protein is an optically active amino acid.) Moreover, these cavities differ from those in other classes of catalyst in two respects. The side-chains in combination give almost specific binding properties and they have a limited but potentially useful mobility, see later. Returning to the case of lysozyme, we saw that the binding site extended over a considerable distance so that we could well consider an active region where binding itself can enhance catalysis. Two substrates can then be aligned for selected and specific reaction. In order to see the advantages of enzymes so produced in greater detail we need to increase

406

Figure 5

Chapter 24

The structure of the copper protein, plastocyanin. It is a catalyst for electron transfer. Note that the protein is a relatively rigid b-sheet (see arrows) and that this helps to constrain the structure and electronic state of the copper enabling electron transfer (see insert). The metal ion here is enclosed. The figure is reproduced from Ref. 11c.

somewhat the sophistication of our approach using transition state theory. Although this theory is commonly used in the literature of enzyme action, it is somewhat less employed in that of molecular catalysts and unfortunately even less so in the analysis of solid state catalyst action. So as to have a comparative outline of the catalysis we have compiled Table 5 for reference.

5 Free-Energy Diagrams and Their Value in Interpreting Catalytic Phenomena As indicated in the introduction there are two different approaches to catalysis represented in outline by Equations (1) and (2).4 In the case of Equation (1) we can only ask about collision frequency, orientation and energetics of the collisional state between catalyst and reactant. Equation (2) considers the alternative that there is equilibrated binding between them. Now as stated in the bound condition there can be a series of steps in which the substrate (reactant) passes through intermediates all in energised steps, Figure 6. Using Eyring transition state theory4 we can treat each bound intermediate as being in

407

A Comparison between Enzymes and Solid State Catalysts

Table 5

Features of individual active sites. Molecular

Enzyme

Solid State

Selection of metal

All possible

All possible

Structure (a) (b) Spin state Flexibility Selectivity Cooperativity between sites Solvents Substrates Chemical modification

Open-sided Sterically hindered Usually low-spin Modest High Not common

Environmentally available Open-sided Constrained Frequently high-spina Considerable Very high Frequent

Stability

0–100 1C

a

Usually organic Very versatile Different ligands

Usually water Water soluble Mutation metal substitution (see lattice) 0–100 1C

Open-sided Often constrained Usually high-spina Low Modest Not common Versatile Very versatile Metal substitution Limited matrices Very wide temperature range

(1) Low-spin complexes can be incorporated with Co, Ni and Fe in porphyrins. (2) Clusters and metal catalysts are excluded due to the ill-defined nature of their active sites, Section 10.

a given free energy state. As mentioned earlier, to calculate the rate of a step the transition between any bound (intermediate) state is treated as an equilibrium between it and the top of a barrier, the transition state, when the actual rate step is reduced to a (simple) vibronic change from one side of the barrier to the other. We outline the simple approach we shall use here while later we give reasons for believing the real situation is more complicated. Notice that one of the steps will be overall rate-limiting and that we assume that on- or off-rates are not rate-limiting. As stated in the Introduction, catalysts are agents that accelerate specific reactions and, by definition, under ideal conditions, their state at the termination of a conversion is the same as that at their commencement. There can be no overall change in the free energy of the catalyst but it can cycle during reaction. In a real situation, depicted as in Figure 7, there is a series of states, each rate being given by Equation (2). The reactants and the catalyst are intimately combined in binding region(s) in the series of states. Both experience changes of internal free energy in the processes that can lead to the formation of transition states (T) and of intermediates (I). A catalyst may be seen to undergo a cycle of energy and conformational changes as it goes through a series of states, an example is given in Figure 7,12 which we have dissected into parts in Figure 6. Whereas the intermediates In have readily measurable lifetimes, ranging from minutes to as short as 106 to 1010 s, the transition states Tn are so short (of the magnitude of the time of a bond vibration (1013 s)) that their lifetimes are, in general, not usually experimentally accessible, but see Ref. 13. The separation of the catalytic site from the reactants allows us to focus on the structure and free energy of the states of the attacking groups alone as well as on those of the possible intermediates, I, and of the transition states, T, of the substrates

408

Chapter 24

A T T

T

T ∆G

I I

I Reactants Enters

Product leaves Reaction Coordinate

B

∆G

T

T

I Product leaves

Reactants Enters Reaction Coordinate

Figure 6

(A) Free-energy profile as a function of reaction coordinate depicting changes associated with the formation of bound intermediates (I) and transition states (T) that are involved in the course of the overall reaction. The steeply rising (dashed) curve indicates the situation that prevails when the reactants cannot even form a bound intermediate, because of the inability of reactants to enter the framework of the catalyst. The fourth step is the activation of substrate leaving. (B) The free-energy profile of the catalyst only but which contributes to the profile of (A). The selectivity of a catalyst for a particular product is governed mainly by the relative heights of transition-state maxima, and the ability of the catalyst to prevent leaving of intermediates. Note that, the maxima and minima coincide closely with those for the overall reaction, but this is not a necessity. The catalyst may have atoms in its active site with specially constrained geometric or electronic states even in its substrate-free condition, see (—) on ordinate in figure B due to the mode of binding in its framework. Passage over barriers (T) can therefore be strongly facilitated thus contributing to the lowering of those in (A) during catalytic action.

with their chemical consequences. (Note that the framework can be intimately involved in the initial and intermediate states of the attacking groups in all the intermediates as seen in the case of lysozyme.) While structures of I are often determinable by experiment, especially in enzyme reactions, as are their free

409

A Comparison between Enzymes and Solid State Catalysts N2O + CO

N2 + CO2 Ts

CO CO N2O 6D Fe+

47.2 0.9 14.9

TS

CO

N2

61.8

0.9

CO 22.9

86.7

30.6 TS

CO

47.8 61.4

N O C Fe

N2 Fe+ + N2O

FeO+ + N2 N2 FeO+ + CO

Figure 7

6D Fe+ N2 CO2

Fe+ + CO2

The computed profile of the overall reaction: N2O+CO-N2+CO2, when divided into two discrete steps (Fe1+N2O-FeO1+N2 and FeO1+COFe1+CO2) at a gas-phase single-site catalyst such as Fe1 (from Ref. 12).

energies, it is theoretical calculation that is most valuable in deriving probable structures and energies of intermediates in many cases and in all cases of transition states, T.14 We also observe that the diagram allows for mobility of the catalyst structure so that we are considering a series of structures as well as energies of intermediates in the cycle. A major feature of the active region, other than the activation of the substrate(s) by reducing the energies required to reach the transition state, is that when there is more than one substrate the site can concentrate the substrate species so that they are in close proximity and align them correctly for reaction. In effect, this is an entropy reduction in the bound states relative to the free substrates. It is for this reason, and in consideration of other entropy factors, that we plot the free energy, DG, on the ordinate rather than energy DE. Binding of any substrate in itself has other features in that, if there are two points of attachment, the substrate may be strained although these points may be remote from the bond to be attacked, see Table 5. Increasing the number of points of attachment or of repulsion (no binding, i.e. in a collisional process, see Equation (1)) to a relatively rigid surface of any substrate will increase selectivity and strain, but it also gives rise to enantioselectivity, as pointed out by Ogston.15 When there are two substrates the active region may have two adjacent attacking sites. These considerations make it clear that the framework

410

Chapter 24

of a molecular catalyst and the matrix of solid state catalysts and that of enzymes especially can play a very decisive role in the catalytic act. While we have conceptually separated the active site attacking groups from the binding sites, they are obviously linked. The idea of an active region is, therefore, a composite cooperative one and this conclusion will be developed in subsequent sections of this article. It is here that the subtle nature of enzyme catalysis rests. We need to make it clear that as much as a catalyst activates a reaction, it is not usually the case that the product is a thermodynamic end point. Most usually, a catalyst is required to selectively produce a simple ‘‘intermediate’’ product so that catalysis has to stop in a chosen way. Almost invariably, an enzymatic catalyst is also required to be capable of selecting (with high precision) its actual substrate (reactant) from among a wide range of compounds. Overall, selectivity is in accepting as well as processing. Homogeneous (small molecule) as well as heterogeneous catalysts (in laboratory or industrial contexts) have difficulty in stopping reactions at given stages of conversion. Of course enantiomorphic selection is sometimes required of all catalyst classes. Only occasionally, as in shape-selective conversions involving nanoporous catalysts (a special interest of John Meurig Thomas, see below), does it exercise selectivity of substrate in a similar manner to that of an enzyme, and the degree of selectivity is normally not great in the activity of a molecular catalyst. The fidelity attainable in enzymatic catalysts often exceeds 1 in 105, whereas in non-enzymatic catalysis (homogeneous or heterogeneous) selectives seldom exceed 1 in 102 except in enantioselective processes. It is the enzyme framework which is so important here.

5.1

Complications in Transition State Theory

The basis of Figure 6 is transition state theory of reactions as outlined by Eyring and Evans. This theory is often illustrated by the uncatalysed reaction H2 þ D , DH þ H in which reactants proceed up a three-dimensional energy hill towards a pass which is the single transition state T. The T-state pass is crossed by a single vibrational disruption of the energised H2 in the presence of D, giving D  H  H, and then allowing DH to form on leaving the pass and to run down the opposite side of the energy hill to DH and H. This simplistic account will not describe a real passage of reactants of a complicated structure to products, since the reactants and products have many degrees of freedom and can reach transition states of somewhat varying energy, limited by Boltzmann factors, see Equations (1) and (2). Moreover, as they approach the transition state, there can be internal redistribution of energy within the participating molecules so that a multitude of equilibrated energy valleys and approaches to the T-states (now plural) can be envisaged. Lastly there is no guarantee that equilibrium of states is approached in a given flow of reaction via intermediates. Undoubtedly, there should be a more elaborate picture but it lends itself only to

A Comparison between Enzymes and Solid State Catalysts

411

a more complex mathematical analysis and no pictorial representation. We shall therefore use the simple diagram, Figure 6, trusting the reader to set aside all the aforementioned complications, at least initially, for this allows us to compare the energetics of the different classes of catalyst at a first level of approximation.

5.2

Computational Approaches to Active Sites

Whereas considerable advances have recently been made in the experimental (in situ) determination of the structures of all types of catalysts, there has also been a burgeoning in the deployment of density functional theory (DFT), giving quantitative insights into the energetics of individual steps that take place at such catalysts (see Figure 7). These calculations14 include not just the transformation of the substrate but also the cyclic changes of the catalysts. The number of atoms in such calculations can now be well over a hundred. It has become fashionable of late to invoke the results of DFT calculations in examples of molecular and enzymic (homogeneous) catalysts. Whilst this is a reflection of the power of computational approaches, there are dangers in giving them too much weight, especially when the errors involved in such calculations (10 kJ mol1) or the assumptions made in order to make the calculations manageable, are not emphasised. Now while we can see that the real situation may be very complicated and computerised searches may be helpful, we draw attention to the work of Marcus16 and others, since their basic algebraic descriptions of the steps of electron and atom, especially hydrogen, transfer have proved remarkably successful. We shall devote three sections, one to the theories of electron transfer, Section 11.3, the second on proton flow (Section 11.4) and the third on ion or atom transfer (Section 11.5) as these steps are of considerable importance in many catalysed processes. Note that the substrates considered here are extremely simple in themselves but their movements are not. Turning to practical examples, we shall assume familiarity with the simplest catalysts with a single metal ion ‘‘site’’1,3,5 and consider the cases of more complex catalytic ‘‘regions’’ where clearly the framework is essential. Under each major heading, we describe molecular catalysts very briefly as their ligand framework is much less important than in enzyme and solid state catalyses.

6 Catalysts Using Adjacent Sites 6.1

Molecular Catalysts

It is not easy to synthesise small molecular catalysts with adjacent sites and it becomes even more difficult to synthesise them with non-adjacent sites, see Section 7. One example of adjacent site catalyses with a single metal ion is that of a metal ion complex bound in a unit with cyclodextrins for use in both hydrolytic and oxidative chemistry. Here one site is a metal and the second is an organic centre for binding substrates (see Ref. 32). Recently the investigation of

412

Chapter 24

such possibilities has been intensified. An example of two metal ion adjacent sites is the insertion catalyst composed of a pair of Cr metal centres linked together by a flexible chain,17 and Cowie and co-workers have initiated a series of studies using heavy metal complexes.18 It is also possible to use maquettes to bind two adjacent sites as in the peptides which bind iron porphyrins.19 These are unusual examples as can be seen by consulting a text on molecular catalysts, which is overwhelmingly devoted to single metal element sites.20

6.2

Enzymes

By way of contrast most metal enzymes use adjacent site catalysis2,5 amongst which we note especially those with one metal and one or more organic sidechains. We have in mind as clear examples many zinc and other cationic hydrolytic enzymes such as proteases, nucleases and saccharases. There is also a huge variety of oxidases and peroxidases, Table 6, where the organic protein framework binds and helps to activate the substrate while the metal activates oxygen, see Figure 2, or hydrogen peroxide. In the peroxidases, peroxide activation is at the Fe31 haem while the substrate binds some 5–7 A˚ away in the matrix.21 Electron transfer generates the substrate radical which is then attacked by water to give a hydroxylated species. The catalyst is cycled first via oxidation of the iron and the porphyrin ligand and to the MO state sometimes then by reduction on oxidation of the substrate. There is a huge variety of enzymes which may insert O2, O or OH into a multitude of selected organic molecules or generate energy by remote oxidation of substrates while giving 2H2O, Table 6. Substrate selection is based on preferential binding of some, and exclusion of other, molecules from the active site cavity, and the substrate is bound in an exact regio- and enantio-selective specific manner by the organic side-chains of the protein at a nearby site. The sophistication of these catalysts based on protein matrices is far beyond that of any man-made inorganic catalyst. The activation of oxygen is by special constrained metal ion sites involving almost invariably Fe21, as a simple ion or as haem, or Cu1, or both at adjacent sites. Examples are, cytochrome P-450, given in Figure 8 (top) and cytochrome oxidase. We shall not describe in detail the iron or haem iron sites involved in direct bound oxygen attack on substrates here as they are expertly described by Poulos et al.,22 who show the series of intermediates also seen in peroxidases. The mechanisms are very well documented and often have a preferred order of addition such that only the substrate bound enzyme allows the iron to be reduced, then oxygen to bind while the enzyme side-chains are protected by the substrate, before O2 is partially reduced. The partial reduction of the metal and oxygen to MO+H2O is achieved by long-range electron transfer from non-adjacent sites, Section 11.3. The overall process seen in products is always apparently due to two one-electron oxygen or hydroxyl transfer steps where for example the bound substrate and oxygen are retained until product release. Transient radicals often include porphyrin or amino-acid side chains in the matrix but

A Comparison between Enzymes and Solid State Catalysts

Table 6

413

Some oxidative enzymes employing two adjacent sites.

Enzyme

Metal

Reaction

(a) Using O2 Cytochrome P-450

Fe(haem)21

Hydroxylation. Epoxidation of saturated carbon compounds Hydroxylation of methane Regio-specific oxidation of lipids Introduction of C(9) unsaturation Galactose to a hexadialose Oxidation to dihydroxynaphthalene 5-membered ring expansion

Methane oxidase Lipoxygenase Stearyl desaturase Galactose oxidase Naphthalene 1-2 dioxygenase Cephalosporin synthetase (b) Using H2O2 Peroxidase Chitinase Hydroperoxidase

Fe2O Fe21 Fe2O Cu1 Fe21 Fe21 Fe(haem)31 Fe(haem)31, Mn31 Fe(haem)31 Vanadyl 51

Oxidation of phenols Oxidation of chitin Oxidation of CN, Cl Oxidation of Cl

Note: One site is the organic matrix adjacent to the bound metal ion site.

radicals are rarely released. These enzymes, Figure 8 (top), often utilise mobility at and around the active site to control ordered addition of O2 and substrate – a very different mechanism to that in solid state catalysis. This type of mechanism using MO species derived from O2 is also seen on metal surface catalysts. (The reader may come across the term co-enzyme in the context of these and other biochemical catalysts. The co-enzyme is a small molecular unit or complex which may be immobile in the enzyme, as iron porphyrins are, or can be a mobile small unit which carries groups such as –H, –CH3 or –NH2 or just electrons. Compare the use of molecular complexes bound to or in solid state frameworks. In the case of cytochrome P-450 a pyridine derivative is the small coenzyme, NADH, which carries reducing equivalents to the enzyme from a non-adjacent distant site to the haem iron. We refer to this part of the activity of the enzyme in Section 7.1.) In the examples of the use of oxygen reduction to water in energy transduction employing enzymes such as cytochrome oxidase there is a long chain of electron- and proton-transfer centres making wiring circuits of both particles Section 11. The long-range electron transfer is also managed in all kinds of catalysis using electrodes to supply electrons. Enzymes attached to electrodes are used as sensors.

6.2.1

A Rearrangement Catalyst Related to Oxidative Enzymes

A strange case of radical intermediates in a rearrangement reaction is that catalysed by coenzyme vitamin B12. Here radical Hd atom transfer occurs between a cobalt–adenosyl complex and the substrate via an adenosyl free radical intermediate where the adenosyl anion is initially bound to the cobalt, Figure 9. On substrate binding to the matrix, not the cobalt, the anion is

414

Figure 8

Chapter 24

The enzyme cytochrome P-450 (top) which acts as a hydroxylating agent using molecular oxygen. To do so it must cycle through intermediates, different constrained states of the iron. The substrate binds before the oxygen in a protective manner. The bound oxygen is reduced first to water and an iron oxene which is the hydroxylating agent. The reaction scheme (bottom) illustrates some of the conformational changes as seen in oxygenbinding proteins such as haemoglobin. Note there is a non-adjacent site for electron supply. The cycling of the catalyst can be considered separately from the processes of the substrate reaction; see Figure 6. The figure is adapted from Ref. 3.

A Comparison between Enzymes and Solid State Catalysts

Figure 9

415

Coenzyme B12. For the reaction pathway of rearrangement of methylmalonyl CoA, see Ref. 48a. Note that there is bond-breaking of R, the adenosyl ligand, at the cobalt with redox-state change, but the metal ion remains in a low-spin state. The cobalt–carbon bond is held in a constrained manner so as to facilitate ‘‘break’’.23

released homolytically as a R – CHd2 radical and a Co21 ion.22 (We could call this conjoint metal/ligand catalyst as in the subsequent steps it is the CHd2 which acts in H atom transfer23). The cobalt is always in a low-spin state. We include this case here as an illustration for it is clearly an example of adjacent site catalysis with a ligand bound metal ion where the ligand becomes a second adjacent site attacking agent. Before proceeding we stress the remarkable sophistication of these enzymes. Substrates are held in alignments suitable for reaction, small mobility is allowed so that relaxation energies necessary for the passage through intermediates are kept small, and attacking groups, metal ions and others, are in usual states reducing DG of the catalyst itself (Figure 6). We cannot find similar features in molecular or solid state catalysts.

6.2.2

Hydrolytic Enzymes

One further illustration of the value of the framework in enzyme catalysis is provided by the zinc enzyme carbonic anhydrase. The framework provides: (1) A constrained (entatic) zinc ion site, slightly adjustable,24 suitable for catalysis (Figure 10a). (2) A deep cavity for the entry of substrates, restricted to CO2 and H2O appropriately positioned, for details see Ref. 48b.

416

Figure 10

Chapter 24

The enzyme carbonic anhydrase which holds zinc at the active site in a constrained state (top), but such that it can move through different states by small conformational changes (bottom). The two substrates enter the site via a narrow selective channel. The probable scheme of reaction at the zinc is shown as a cycling through coordination number changes.24

(3) An adjacent base, an imidazole, to assist attack. (4) A catalysed pathway for proton movement to the active region, see Section 11.4. (5) Very limited but useful mobility to allow adjustments as the reaction passes through intermediates. Compare the even more limited movement allowed in the copper enzyme, Figure 4, both are b-barrel structures, and the greater mobility in the helical enzymes, see Figures 3 and 8b.

A Comparison between Enzymes and Solid State Catalysts

417

The extremely effective design features allow highly heightened activity and great selectivity where the structure has arisen through repeated mutation of amino acids to attain an optimal catalyst. This is probably the simplest example of the result of ‘‘design’’ in enzymes. It has been very difficult to find a good molecular catalyst for this cyclic reaction.24

6.3

Solid State Catalysts

An example of an adjacent-site effect in heterogeneous catalysis (as already mentioned) occurs in the oxyfunctionalisation of n-hexane within the chabazitic cage of a MIIIALPO-18 (or MIII ALPO-34) nanoporous catalyst where MIII is FeIII,25 see Figure 1. The known regioselective attack of terminal methyl groups of a linear alkane (in the presence of O2 and a MIIIALPO catalysts), can occur sequentially at each end of n-hexane when the catalyst has two active sites separated spatially by 5.4 A˚, the same distance that separates the two terminal CH3 groups of n-hexane, Figure 11. Adipic acid is therefore produced as the favoured product of this aerial oxidation.25 This catalyst is quite rigid and one or other of the iron ions can attack any available substrate so that it has only a small degree of selectivity. While ALPO-31 retains the substrate as it passes through intermediates only releasing the desired product, Fe-ALPO-5 with a

Figure 11

The nanoporous solid AlPO-5 becomes a powerful oxidation catalyst when a few of the Al31 ions in the framework are substituted by transition metal ions in high oxidation states (such as Fe31, Co31 and Mn31) and tetrad hedral coordination. Free radicals, such as C6H11 are readily formed when cyclohexane enters the pores (diameter 7.3 A˚) of this open-framework catalyst. A series of reactions of cyclohexane lead to the formation of a number of partially oxidised products. If FeAlPO-31 is used as a catalyst in place of FeAlPO-5, a much larger yield of adipic acid is obtained. Note the rigidity of the lattice may constrain the geometry and electronic states of the cation site but allows little mobility (after Thomas et al.25).

418

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Figure 12

The catalytic pathways of sequential hydrogenation of benzene and stereoregular polymerisation of alkenes at the zirconocene-type cation, CpZr(CH3)21 bound to an alumina surface pre-treated with sulfuric acid Cp is cyclopentadienyl (after Marks et al.26).

larger cavity of 7.3 A˚ releases the first product of oxidation of cyclohexanol (Figure 11). Much of the difference in selectivity is based upon restrictions on entry and leaving. The basic framework has little sophistication in its structure when compared with an enzyme. Finally a sophisticated solid state catalyst of a molecular complex on a zirconium oxide surface is shown in Figure 12 where the surface oxide (converted to –OH) undoubtedly act as adjacent sites to the complex.26

7 Catalysts with Non-Adjacent Sites 7.1

General Considerations: Molecular and Enzyme Catalysts

There are two major classes of non-adjacent sites amongst molecular, enzymic and solid state catalysts, while we still exclude discussion of conductors. The two are catalysis by (a) two immobile sites separated by at least several A˚ngstroms and (b) two mobile sites separated initially by several A˚ngstroms but which come together so as to be together during catalysis. The second group will be described in Section 8.2. The case of two fixed remote sites is readily appreciated by giving an example from enzymes which is an extension of the discussion of the P-450 cytochromes where we have already seen how oxygen and substrate are handled. Electrons are supplied in this enzyme from a remote, non-adjacent site to reduce bound O2 to FeO. Under these conditions introduction of electrons stops at a particular state, for otherwise, many unwanted oxidations could occur. In an alternative to reaction to an oxene and H2O, as in the P-450, the oxygen can be taken down to 2H2O using fourelectron reduction in one-electron steps, the electrons coming from a remote site. This is the function of laccase,27 a copper enzyme (Figure 13, see also Figure 4). Here the second substrate, indicated by ‘‘electrons’’ in Figure 13, is

A Comparison between Enzymes and Solid State Catalysts

Figure 13

419

The enzyme Laccase, non-adjacent site catalyst of oxidation. One electron oxidation occurs at the single copper ion by electron transfer, see Figure 4, from polymerisable monomers. The electrons pass through the protein to the multi-copper site which binds oxygen but does not release it until it is reduced to 2H2O. This requires not only electrons but protons to travel in the matrix, see Section 11. The figure is modified from Messerschmidt et al.48b

bound close to a single copper which acts so as to abstract one electron at a time from a substrate and then passes electrons to a two (or three) copper ion centre. It is this centre which holds the various intermediates from O2 down to 2H2O. Many enzymes work via these chains of electron transfer, some of which allow the capture of the energy difference between the redox potentials of a reductant and an oxidant. Now this is the general function of the fuel cell reaction 2H2+O2 - 2H2O, giving electric power. Here in man-made devices doped metal electrodes act as H2 and O2 catalysts separated by a metal wire. This remarkable parallel with biological energy conversion will lead us to a discussion of electron (and proton) transfer in matrices which in the limits of conduction can be semi-conductors or even metals, see Section 11. It is clear that catalytic enzymes could also be connected to electrodes and this is also the basis of many man-made sensors using catalysts while in organisms the embedding of enzymes in membranes gives rise to parallel properties of sensing by electron transfer. Another way of utilising oxygen is to bring about remote

420

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Figure 14

The molybdenum coenzyme used in the transfer of oxygen atoms from H2O (oxidised) to aldehydes giving ketones. Molybdenum is peculiarly suited to atom transfer catalysis.2

two-electron oxidation of a substrate by it O2 þ 2RCHO ! 2RCOOH where O2 is bound at a non-adjacent site. Molybdenum enzymes for this purpose use a complex, Moco (Figure 14), embedded in a protein.28 Here the oxidation by O2 is not direct but occurs as follows: O2 ðremote siteÞ þ MoIV ðsecond siteÞ gives by electron transfer VI O2 2 ðremote siteÞ þ Mo ðsecond siteÞ

MoVI þ H2 O ! MoO þ 2Hþ MoO þ RCHO ! RCOOH þ MoIV þ O2 2 þ 2H ! H2 O2 at the remote site

Electrons pass from MoIV to O2 via the protein. Many molybdenum enzymes use this mechanism of oxidation in which the solvent for the reaction, H2O, acts much like the solvent of the solid state matrix of the Mn/Ca/O oxidative catalyst for dehydrogenation of propane and butane to benzene and p-xylene, respectively. As mentioned already it is possible to attach molecular or solid state catalysts to electrodes. The reverse reaction of H2O - O2 through the action of light can be connected to the production of H1 and OH– by enzymes in different parts of space,29 oxidation (O2) H2O

hv

e

+

+

recombination + usable energy

reduction (H2)

where " is a positive hole, which is formed away from e. This use of spatially separated catalysts is the essence of photo-energy capture devices. There are many attempts by man to discover parallel devices for clean conversion of light to useful energy.

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421

Molecular catalysts of this kind are little studied, though molecular wires connecting donors and acceptors have been intensely studied in an effort to understand electron transfer in organic matrices.

7.2

Solid State Catalysts

One example of the involvement of two non-adjacent sites in nanoporous (solid) catalysts is exhibited in the one-step conversion of cyclohexanone to e-caprolactam in the presence of NH3 and O2 (or air). Here the catalyst is a microporous ALPO in which some of the AlIII ions have been replaced by a redox ion (typically MnIII or CoIII) and some by a Lewis acid divalent cation30 such as Mg21, Zn21 or CoII. At the redox centre, the ammonia is converted into hydroxylamine, in situ: MnIII

NH3 þ 12O2 ! NH2 OH This then converts the cycohexanone within the pore of the solid catalyst to the oxine, which is then converted, by the Brønsted catalytic activity of the loosely attached proton associated with the framework divalent ion, to the caprolactam (the precursor of nylon-6). This is an example of what the heterogeneous catalysis community refers to as a bi-functional catalyst.

8 Mobility at Active Sites We have mentioned mobility of catalysts several times. Here we give a more detailed description of minor local mobility and of gross conformational change.

8.1

Minor Mobility of Active Sites

We have assumed until now that the catalyst atoms are largely immobilised: a common view of an active site. In fact, in almost every example, the atoms of the catalyst experience changes of bond length and bond angle to some degree. It is recognition of this fact which allows us to see the way in which catalyst atoms can pass over their transition states in Figure 6, especially where there are many steps in the overall process. Examples of some mobility of the coordination sphere have been described in homogeneous catalysis and enzymes, e.g. coenzymes B12 and cytochrome P-450 reactions. Elsewhere, we have stressed that the zinc ion is an excellent acid catalyst in an initially tetrahedral site in enzymes. Like Co21 (high-spin) Zn21 has no strongly preferred coordination number and in its catalytic cycle change of coordination adjusts all bond lengths and angles (Figure 10). By constraining Ti41 in a tetrahedral site in nanoporous solids, a similar possibility for coordination number change is induced with bond distance changes, see Figure 15 and Figure 12. Mobility on the smaller scale is recognised in other instances, e.g. by the Debye–Waller

422

Figure 15

Chapter 24

On the basis of XAFS analysis and computations using density functional theory, this scheme is proposed for the epoxidation of alkene by alkyl hydroperoxides (HOOR 0 ). Experimental evidence shows that both the Z1 and Z2 intermediates may be formed; and the original (constrained) fourcoordinated Ti active site passes through a six-coordinated state (e.g. the Z2 intermediate) in the course of the epoxidation. The Ti41 state remains as such during the ‘‘acid–base’’ process, wherein the oxygen of the hydroperoxide serves as the base (after Thomas et al.25).

factors in X-ray spectroscopy studies of transition metal ions incorporated into nanoporous silicas and enzymes. The movement of atoms on heterogeneous metal catalyst surfaces is often much more considerable,3 and it has also been seen with nanoparticles of metals, see Section 10. Minor motions such as rotations of groups and vibrations within bonds are included in the approach to the transition state (Figure 6). In Marcus’ theory of hydrogen transfer, that is of a hydrogen atom or ion, the rate of transfer is a product of a temperature dependent vibrational/rotational term and a tunnelling term between neighbouring groups.16 The hydrogen cannot tunnel more than about 0.5 A˚ and the distance between the two potential energy minima involved in the transfer process must then be at, or less than, this distance. The implication is that H-bonding between the centres will assist transfer where the distance between minima is small. However although this H-bonding is often strong in acid–base proton changes it can be very small for C–H bonds. Hence in the second case compression or vibronically induced compression of the separation will aid transfer rate by increasing the tunnelling contribution. The best solution involves perfect alignment of the two substrates for H-transfer as seems to be the case in many enzymic reactions. Here then the mobility of a framework, the solvent of molecular catalysts or the mobility of the matrix of proteins, can aid catalysis much more effectively at low temperature than the rigid matrix of solid catalysts both in terms of compression on binding (binding energy assisting conformational progression to the transition

A Comparison between Enzymes and Solid State Catalysts

423

state) and in low energy vibrations locally of the matrix. The effects have been demonstrated in enzyme studies of hydrogen isotope effects especially in lipoxygenases.31 In solid catalysts such vibrations may be important only at high temperature.

8.2

Mobile Segments and Arms: Allosteric Systems

The design of catalysts which can bring together two sites of binding by mechanical adjustments or by surface diffusion is to be seen in the development of some molecular catalysts and of some surface attached molecular catalysts but it is best observed in a number of enzymes. A simple molecular example has been designed by Breslow and has cyclodextrin movable arms attached to a framework including a porphyrin metal ion catalytic centre.32 As described already, the arms carry the organic substrate to be attached and the porphyrin has either zinc, as an acid–base centre to resemble the enzymes peptidases or esterases, or iron, as an oxidation centre for activating oxygen or hydrogen peroxide to resemble P-450 cytochromes. Guided chemical segment movement of enzymes is common. Even the first enzyme crystallised, lysozyme, closes its groove around the substrate on substrate binding. More extensive movements are seen in group-transfer enzymes and we illustrate these cases with the enzyme phosphoglycerate kinase (Figure 16). The enzyme consists of a hinged pair of domains.33 The two domains bind magnesium adenosine triphosphate, MgATP, and glycerate at a considerable distance apart but binding of both causes the phosphate of ATP to move close to the glycerate and be aligned for transfer. Reaction follows to equilibrium in which this third phosphate of ATP is transferred giving adenosine diphosphate (ADP), and phosphoglycerate. Considerable segmental motions are also observed in the series of enzymes of bioenergetic processes. A general description of such mobility in enzymes is given in Ref. 9. We add a note here to the effect that the unit ATP, a carrier of phosphate, is open to free diffusion from enzyme to enzyme as are many other coenzymes carrying –H, – NH2, –COCH3, etc. so connecting two or more free enzymes which are remote in space. Whether carrier assisted sequential catalysis is a useful possibility in organic chemistry is open to test. It is readily seen however that molecular catalysts, free or bound to surfaces, could be designed to mimic the mobile enzymes. Solid state catalysts as designed at present do not use such massive changes of structure. We stress again the remarkable functionality of enzyme matrices in a variety of mobilities.

9 Summarising Survey of Active Regions in Non-Conductor Catalysts Reverting to our introduction, we note that, in Figure 6, the over-riding concern is to lower the overall activation energy of the reactions, retaining

424

Chapter 24

Figure 16

The open structure of phosphoglycerate kinase, an enzyme which catalyses transfer of phosphate from adenosine triphosphate to glycerate, but it only does so on closing. There are two largely b-domains linked by a hinge of helices. Molecular machines are made by using such motions.33

intermediates as far as possible, while obtaining selectivity of attack. Various ideas using the binding energy of the substrate to lower the activation energy of the intermediates have been described, see Table 3. These ideas initially applied to enzymes are now becoming useful in the description of solid state catalysis where the inorganic matrix plays a role somewhat parallel to that of a protein. There is often inadequate kinetic and structural analysis of binding to allow comparison of enzymes with the other classes of catalyst as fully as we would wish. The more general qualitative similarity between all the three different types of catalyst can be seen in their roles in moving electrons or atoms in the active sites. Some further similarities and differences are: (1) Hydrolytic reactions are catalysed by metal ions, which cannot undergo one-electron steps, in all three classes of catalyst, e.g. Zn21, Zr(IV), Ti(IV). The metal ions are selected by cells for enzymes from a very limited number of elements but the choice in other catalysts is not restricted. (2) The geometry of the cavity and its size are controlled extremely well in enzymes, but nowadays improvingly so in solid state catalysts. Molecular catalysts have less matrix control over selection.

A Comparison between Enzymes and Solid State Catalysts

425

(3) Oxidation is often by one electron steps of the catalytic metal ion implying organic radicals and the series of some or all the intermediates  d O 2 , O2H and H2O. In the best cases only the final products are freed, contrast cobalt as a simple ionic homogeneous catalysis with Fe oxidations in pores and in catalysis by enzymes such as cytochromes P-450. Enzymes control the reactivity of intermediates extremely well. Two electron reactions require control of oxidation states such as MO. (4) The metal ion, by its mode of binding, is often activated in a constrained structural site, open-sided but limited in access, and often in a special electronic state as well as of limited coordination number by matrices of all kinds, including the framework of complex molecular catalysts. In effect the ground state of the metal ion may be close to its transition state in free energy, see Figure 6, so smoothing out its part in the reaction energy profile. Sometimes on reaction, with change in oxidation state, the metal ion undergoes coordination number change (contrast electron transfer in Section 11.3). During some reactions the metal ion cycles through several transition states and here the matrix, especially proteins, can guide the sequence of changes. (5) The matrix binds and retains intermediates as well as controlling reactivity until the required products are produced. While the similarities are qualitative, this must not hide the fact that enzymes are quantitatively superior in almost all the above respects. The differences are: (6) The nanoporous matrix itself cannot be attacked by reagents such as oxygen but the molecular and enzyme matrix can be. To avoid such attack, the enzyme requires a fixed order addition of reactants so that the protein is protected by the first bound substrate often not at the metal, e.g. see P-450 cytochromes. Oxygen is a major menace for molecular catalysts and is often excluded from the reaction vessel. (7) The matrix, especially the protein of enzymes, can align two reactants accurately as to ease the energetics of reaction. (8) In oxidation catalysis the abstraction of H from the substrate is often directly by the metal ion in the nanoporous solid (see Figure 11), but it is by the metal-oxene (MO) in the enzyme (Figure 8, but see co-enzyme B12 rearrangement reactions, Figure 9). (9) The protein matrix may have considerable local flexibility while the nanoporous solid has comparatively little. Protein surfaces adapt to their substrates and the small conformation changes assist binding and reaction by induced fit. (10) The reactants and products of enzyme reactions can be optically active enantioselective in all reaction classes and so can certain molecular catalyst reactions but this selection is not easily gained in solids unless molecular catalytic units are attached. (11) The nanoporous solid can be used at high temperatures and pressures and with gaseous substrates while enzymes and molecular catalysts are of limited temperature stability.

426

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10 Ill-defined Active Sites So far we have been concerned with well-defined metal ion active sites but we must now turn to the less clearly describable catalysis by metal or metal-like ‘‘sites’’ such as atoms in clusters. These clusters may contain more or less strongly linked metal ions or metal atoms. In fact there are many materials where this distinction between metallic and ionic systems is not easily made because the strength of interaction between the partners in the matrix (or with atoms in the support) allows distribution of electrons over all the atoms in it. For example, Re2O7 is metallic but it looks, by formula, as if it should be a typical oxide written with a rhenium charge of +7. RuO2 is an example of a good electron conductor which is not a metal. As the non-metal is made larger in many series of metal ion compounds going, for example, from O to Se and Te the properties change: bulk NiTe is as an alloy, as opposed to NiO, a salt, and even in a small cluster valence electrons need not be localised. Now this difficulty of separating more generalised even bulk properties from localised sites is very clear in doped semi-conductors. There is a gradual change in electrical conductivity as dopants interact more strongly with the matrix. In fact there is a continuum from insulators to metals where electron transfer proteins (metal ions doped in protein matrices) and doped salts (metal atoms doped in oxide lattices, e.g. Li atoms in NiO) and even ‘‘doped’’ materials such as the tungsten bronzes, lie in between. In previous sections we have maintained a description of ‘‘active sites’’ which have strong local charge separation and so their structural limits were closely defined. In this section, this is no longer true, even in enzyme clusters such as Fe4S4 individual atoms have no simple oxidation state.

10.1

Molecular ‘‘Ionic’’ Cluster Catalysts

We know of many examples of molecular clusters such as Au55(PPh3)12Cl634 where it is difficult to describe charge distribution. Charge in some is to a degree localised but in others localisation is not observed. Behaviour in the cluster is either that of a good semi-conductor or of metal-like behaviour. The transition from a non-metal to a metal can be mapped sometimes with the size of the cluster but is often temperature- and composition-dependent. Such a cluster complex may be attached to a soft or hard matrix surface. Many model clusters have been synthesised to resemble enzyme clusters but few are catalytically active.

10.2

Clusters in Enzymes

In enzymes, there are many examples of small clusters of ‘‘ions’’ bound together by sulfide and a few bound by oxide (Table 7).2 A simple one is the hydrogenases, where the active site is multinuclear in an Fe4 or an Fe3Ni centre bound by simple sulfide ligands. The problem with the description of catalytic activity

427

A Comparison between Enzymes and Solid State Catalysts

Table 7

Some classes of multiple atom non-adjacent active sites in enzymes.

Site

Reaction

MoFe7S8/Fe8S7 FenSm/FenSm Ni/Fe/S/FenSm Cu.haem(Fe)/Cu Mn4Ca/chlorophyll Ni/Fe/S/FenSn

N2+6H1+6e-2NH3 General electron transfer Hydrogenase H2-2H1 O2-H2O H2O-O2 CO+CH3-CH3CO–

Note: For many more details see Ref. 2.

here is that it is not known to which metal ions the protons, hydrogen atoms (or molecules) and electrons are bound, all of which may be involved in activity. There are many examples of active FenSm related clusters of n up to 8. Another mixed cluster is that of the nitrogen fixation enzyme, Fe7MoS8(X), where X is isocitrate. It is not known where N2, H2 or any of the many intermediates bind. (The situation is rather akin to the behaviour of bimetallic catalysts like Pd/Ru, which freely hydrogenate alkenes; but we are uncertain, as yet, as to where precisely the H2 and the alkene are bound.) The reactions to form acetate from CO and CH4 by the enzyme, acetyl CoA synthetase, also carbon monoxide dehydrogenase, present a similar problem involving an Ni/Fe/S cluster. These clusters resemble small pieces of known minerals leading to ideas concerning the origin of life. A different type of cluster occurs in the manganese enzyme for the photo-oxidation of water to oxygen. Here the binding of the H2O and the intermediates on the way to O2 are all unknown and may not be describable by simple formulae.

10.3

Solid State Cluster Catalysts

In assessing our degree of understanding of this branch of catalysis it is convenient to work progressively from very small clusters, through to nanoparticle clusters (containing from a dozen to several thousand atoms) and then to the bulk (solid) metals.

10.3.1

Nanoparticles of Various Diameters

When such nanoparticles are supported on a variety of oxides it is clear,35 certainly in the case of Au, that a maximum is exhibited in the catalytic activity as a function of particle size. What is not clear is the precise cause of this effect. One suggestion is that only those Au atoms situated at the edges of the interface between the particle and the underlying support are the active sites. Another suggestion is that particle thickness is a relevant determinant of catalytic activity. As against the last view Freund et al.36 have shown exclusively that monolayer Au islands with a thickness of one or two monolayers on an FeO(111) substratum are found to exhibit identical CO adsorption behaviour

428

Chapter 24

as large Au particles. This demonstrates that particle thickness here plays no significant role in CO adsorption and that, therefore, size effects for the lowtemperature oxidation of CO are not related to quantum size effects. Particle size effects on Pd catalysts for alkene hydrogenation, on the other hand, are real. Thus adsorption of trans-2-pentene on well-defined Pd/Al2O3 model catalysts was shown36 to exhibit site-specific behaviour, which results in a strong increase in hydrogenation activity within the 1–5 nm particle size ranges, in contrast to ethene hydrogenation (see Figure 17). The size effects are explained by the hydrogenation reactions proceeding via di-s-bonded pentene, which is favoured on the (flat) terrace sites of large particles, but p-bonding of ethene. The underlying facts here are that all the exposed atoms in these small particles, as in some instances with bulk metal catalysts (e.g. Cu atoms in methanol synthesis from CO and H2), are catalytically active. Another interesting fact pertaining to the far greater catalytic activity of Pd nanoparticles compared with single crystal (extended) surfaces of the same metal36 – in the hydrogenation of 2-pentene and ethene – is that, in the former, the adsorbed organic reactants have access to weakly bound subsurface hydrogen (not present in the single crystals). It has long been known that minute (nano) particles consisting of two distinct metals do exhibit catalytic performance – activity and selectivity – very different from the performance, under identical experimental conditions, of nanoparticles of the separate constituents. Sinfelt37 was the first to fully investigate this phenomenon on nanoparticles of Pt–Ir and Pt–Re of diameters in the range 10–100 nm in processes such as the isomerisation of alkanes and their hydrogenolysis (rupture of C–C bonds) in the presence of hydrogen. More recent studies38,39 on even smaller nanoparticles (diameter ca. 1 nm) consisting of such noble metal pairs, in the form of naked clusters of, for example, Ru5PtC, Ru10Pt2C2, Pd6Ru6 and Ru12Cu4C2, have demonstrated beyond doubt that much superior catalytic performance results with these catalysts, especially in the hydrogenation of ethylenic bonds. A good example is the hydrogenation of muconic acid to yield adipic acid which occurs very

Figure 17

Particle size effects for alkene hydrogenation catalysed by nanoparticles of Pd (after Dayle et al.36).

A Comparison between Enzymes and Solid State Catalysts

429

rapidly in the presence of such small Ru10Pt2C2 nanoparticle catalysts, in which the individual atom sites are adjacent to one another in a precisely known form. Much more needs to be done, theoretically, to understand the precise origin of this synergy. But in qualitative terms such behaviour is understandable. Thus, Pd6Ru6 (or Pt5RuC) are expected to be very good catalysts for the hydrogenation of alkenes because Pd atoms are known to activate H2 and Ru atoms activate ethylenic bonds. This section illustrates that man has synthesised catalysts of great power, mostly using elements not available to enzymes in large clusters of metal atoms, again not available in enzymes. They are frequently in reactions for which enzymes have not been devised since enzymes catalyse mainly water-soluble substrates. It remains difficult to define precisely active ‘‘sites’’. The next section explores another set of catalysts which cannot be synthesised by cells.

11 Catalytic Functions of Bulk Matrices As stated earlier, generalisations that are valid as interpretations of the catalytic performance of bulk metals and many semi-conductors are elusive. Examples are given in Table 8. One can legitimately argue that the reason why both Pt and Pd are such good and versatile catalysts is because they each exhibit a surface reactivity with an aptitude to form bonds (that are not too strong) with a large variety of the elements that figure in organic molecules: C, O, N, H, S. It is also true that dissociative adsorption of H2 and O2 are facile on these metals, so that, compared to a homogeneous gas-phase reaction involving these reactants, the crucial elements (atomic hydrogen or atomic oxygen) are readily available as mobile entities on the catalyst surface. The potential energy diagram that resembles this dissociation of a diatomic molecule applies equally to other species such as N2 or Cl2. And in the case of iron catalysts, dissociative chemisorption of N2 occurs readily, as proven by numerous experimental studies. Indeed for the synthesis of ammonia from N2 and H2, a good understanding exists of the series of reactions in quantitative terms, see Figure 18, Table 8

Some examples of metallic catalysts.

Catalyst

Reaction Catalysed

Finely divided nickel

Hydrogenation of fats/unsaturated naturally occurring molecules Synthesis of ammonia (from N2 and H2)

Iron (with potassium promoter) Cobalt-based alloys supported on oxides Pt/Al2O3

Fischer–Tropsch synthesis of alkanes, alkanols or alkenes from ‘‘synthesis gas’’ (CO+H2 mixture) Reforming of hydrocarbons, i.e. production of alkanes from linear ones

Note: If placed in Table 6 these catalysts are of low molecular selectivity, not very flexible, of quite high thermal stability and have high electrical conductivity.

430

Chapter 24 N + 3h 75 NH + 2H

93 270

NH2 + H 335

–230 (-100 + 2.55) -130 (-65 + 65)

E* 1- N 1- N 2 2 2 2,ad + 23- H2

12

62

-8

-28

Nad+3Had

Figure 18

110

NHad +2had

NH3

NH2,ad NH3,ad +Had

Potential-energy diagram illustrating the progress of the catalytic synthesis of NH3 on iron (after G. Ertl40).

thanks largely to the work of Ertl40a,40b and others:40c,41 H2 ðgÞ ! 2HðadÞ N2 ! N2 ðadÞ ! 2NðadÞ NðadÞ þ HðadÞ ! NHðadÞ NHðadÞ þ HðadÞ ! NH2 ðadÞ NH2 ðadÞ þ HðadÞ ! NH3 ðadÞ NH3 ðadÞ ! NH3 ðgÞ Under steady-state conditions, the general situation at the surface of the Fe catalyst is understood in terms of the underlying view that all the Fe atoms are catalytically active. (The comparison with the enzyme for the synthesis of NH3, Figure 19, is difficult but the formation of H and N atoms may occur.) Facile dissociation of species, such as N2, O2, and even ethane, also occurs at steps on a metal surface. Even at room temperature it has been found that ethane decomposes exclusively at step edges.42 The step edges are poisoned by the reaction products which grow as carbidic islands. Quite remarkably a metal surface giving an MO species from O2 can be likened to the MO of P-450 and both metals (Cu, Ag) can be used to introduce atomic oxygen into organic molecules – a difficult reaction in homogeneous catalysis.

A Comparison between Enzymes and Solid State Catalysts

431

14

Much theoretical work, involving both extended Hu¨ckel and, more recently, DFT calculations have rationalised much of the behaviour of molecules at metal catalyst surfaces. Only relatively rarely, however, is there a sufficient depth of understanding to be able to indicate which metals or alloys will exhibit superior catalytic behaviour. A further great difficulty with exploration of active ‘‘sites’’ is that at the temperatures used in catalysis the atoms of the metal become mobile, flowing and so rearrange the surface.

11.1

Bulk Semi-Conductor Solids

Many of the semiconductor solid state catalysts used in industry are made from such materials as combinations of Co, Mo and S or other non-metals from boron to oxygen. They are of unknown or uncertain structures and are not just used as pure substances but often as films on surface supports such as alumino silicates or silica. Today there is no simple way of describing their activity. We note that a large percentage of the industrial catalysts of this kind are made empirically so that composition in the active region(s) is uncertain. Deposition on an inert substratum matrix could generate unusual crystal phases. Despite these caveats the work of Ertl et al.40 has shown that a great deal can be understood concerning the activity of well-defined surfaces such as that of RuO2.

11.2

Flow between Sites through Matrices

Two of the most important considerations in catalysis are the movements of electrons and protons. When electron movement is very local as in bond changes then we address the theoretical problems as described in Section 5 while Section 8.1 covers the treatment of hydrogen atom bond changes. An extension of the movement of hydrogen over very short distances was treated in part by Marcus’ theory of vibronically assisted tunneling.16,31 These treatments leave to one side long-range movements of electrons and protons between catalyst centres which are far apart as described below. We have therefore added a section on the long-range flow of these charged species. It is also known that many small or even larger molecular units can flow in matrices. Much of the description of that flow was discussed under channels to active sites which could be rigid or mobile. In the case of mobile channels, it is possible to use energy to control flow through conformational switches as seen in proteins especially. Deliberately in this article we have not stressed earlier long-range access to sites of higher specificity although they can dominate overall rates of reaction.

11.3

Electron Flow

Now the rate of electron transfer from a donor, D, to an acceptor, A, conductor has two terms. log kDA ¼ tunnelling rate þ thermal rate

432

Chapter 24

The relative importance of the two terms depends on the nature of the medium between D and A which provides a conduction orbital or band and the temperature. If the highest occupied orbital of D (HOMO) lies near the conduction band then the electron transfer is readily achieved and similarly electron (hole) transfer is possible if the acceptor takes electrons from the highest occupied orbitals of the matrix. In these cases we expect thermal excitation to be important at room temperature as is well known in doped semi-conductors: logðrateÞ varies as DH=RT where DH is the energy gap between the donor and the conduction band. Tunnelling only becomes important at very low temperatures. If the donor and/or acceptor states lie at a considerable energy from those of the matrix then, at low to modest temperatures, temperature independent tunnelling will become important. It is the relative energies of energy states of the matrix and those of D and A which are important in all cases. We noted in Section 7 that in oxidations electron transfer can be from a remote site where there are two nonadjacent catalytic sites and where the second site is the site of substrate binding. This mechanism is much used by enzymes and in bio-energetics where the reaction is of O2 with reducing agents to give water and they keep control over reaction at the two separate but linked sites, see Figures 8, 13 and 19. We wish now to elaborate upon this long-range tunnelling electron transfer through a matrix as it can be used outside biological systems. For any poor conductor, electron flow tunnelling between two sites, a donor D and an acceptor, A, is described by the Marcus equation.16 In the equation

Figure 19

A schematic diagram of the enzyme nitrogenase, for the reaction N2+3H22NH3. The enzyme has many electron transfer centres and a FeMoCo active site, above. It requires energy from ATP and undergoes cyclic conformational changes. The site of binding of substrates is ill-defined.

433

A Comparison between Enzymes and Solid State Catalysts

in the absence of temperature dependent terms the electron transfer rate is given by:  log kDA varies as CbðrÞ where C and b are constants and r is the distance between sites. b is a property of the matrix and can be found by plotting log kDA against r. We now wish to compare proteins43 with other catalysts which have semi-conductor frameworks. Figure 20 shows that b varies from zero for a metal or extended cluster to about 3.5 for a vacuum.44 A very modest conducting matrix close to an insulator with low-lying D and A states will have a large b. What are the implications for useful transfer between two catalyst sites now represented by D and A in industry? From Figure 20 it is clear that electron transfer is sufficiently fast for catalysis if D and A are separated by 10–15 A˚ where b is close to 1.0. This is the value found for proteins with conventional donors and acceptors such as Cu1/Cu21 and Fe21/Fe31, and we have observed that many enzymes are dependent upon such electron transfer rates. For bc1.0 the matrix inhibits electron transfer but for b{1.0 a large separation of sites up to 4100 A˚ allows fast transfer. For example any doped oxide such as an oxide MO or a Ge:Sb/Ga

14

Si:P

NH3:Li

10

MeNH2:Li Log kDA

EtNH2:Li

NiO:Li

5

Vacuum

Water

Unsaturated molecular chains and metal complexes

Proteins

0 0

Figure 20

10

20 D to A Distance (Å)

30

40

A plot of logkAD (where kAD is the rate of electron transfer across different doped matrices) against donor (D) to acceptor (A) distance. The slope, b, characterizes the energetics of these electron transfer ‘‘semi-conductors’’ from almost b ¼ 0 (the value for a metal) to b ¼ 3.5 for a vacuum. Soft protein materials lie around b ¼ 1.0, while doped metal ion oxides can have b values around unity to zero. The b value for a donor/acceptor is related to the conduction energy of the connection relative to the donor, DE, by the pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi the expression b ¼ C DE; m where m* is the effective mass of the electron (after P.P. Edwards et al.44).

434

Chapter 24

silicate can carry electrons by tunnelling when the donors are not too low in 2þ energy, e.g. Lix Ni3þ x Ni1x O, (b ¼ 0.5) while they can also transfer thermally excited electrons at higher temperatures.44 As stated, separating oxidising and reducing catalytic centres across a distance is the basis of fuel cells. Presently a great effort is being made to devise the corresponding photo-activated electron transfer to improve the use of sunlight. The H2 and O2 from H2O will be allowed to react again across a membrane or at separate electrodes to give non-polluting energy. Much of the theory here has been aided by studies of molecular wires.43

11.4

Proton Transfer

Proton transfer is of equal importance as electron transfer in catalysis. The proton can generate hydrolytic attack locally itself or by leaving from water to give hydroxide as an attacking group. Again it can compensate by long range migration for charge changes in substrates, such as oxygen, either in oxidation or reduction reactions. Local proton transfer can occur by tunnelling but only in 0.3 A˚ steps and is generally less interesting than thermally activated transfer but Marcus’ work shows how the two can be combined.16 In long-range proton transfer there is a required rotatory movement of the carrier atoms, O or N, so as to allow protons to move in from one donor and out to an acceptor (as in the phenomenon of Grotthus conduction). Here the importance of –OH (H2O) or – NH2 (QNH) groups is dominant. The picture is of a Grotthus type flow of protons illustrated by a water channel in Figure 21.45 Here the channel is in a protein but the water can be replaced by other H1 carriers. There is exactly the same possibility in solid state pores. In metals the dissociation of H2 to 2H1+2e allows the possibility of proton migration with an electron in metals such as palladium. The value of such proton flow arises in certain types of man-made and biological fuel cells where proton flow is separated from electron flow. Proton flow separated from that of electrons can be controlled in proteins or other matrices as it does not flow in the same part of a material as the electron. The remarkable versatility of the membrane matrix must not be missed in this respect.46 Enzymes, where proton transfer is important, are very numerous and include many oxidases, hydrogenases and some hydrolases, e.g. carbonic anhydrase. A highly instructive study of an enzyme is that of carbonic anhydrase where catalysed H1 migration is a rate limiting state in the reaction. Minor mutational changes of the channel for CO2 and H2O greatly change the catalysed H1 migration.

11.5

Molecular Flow

Another feature often seen in enzymes and porous solids is the flow of solvent and other molecules in channels.47 Particularly important is the flow of water as its fluctuations and those of H-bond structures of the protein can provide a path for H2O itself but also of H1. The movement of water molecules is exceedingly important in reactions of O2 where H2O is a leaving product often connected to

A Comparison between Enzymes and Solid State Catalysts

Figure 21

435

A schematic representation of a water channel for protons supported by membrane protein helices. Some parts of cross-membrane channels are known to have this type of structure.45

a chain of other H2O molecules and in osmotic control. The movement of water in porous solid catalysts has also been shown to be important. Note again that fluctuations may give rise to many transition states while the only points on Figure 6 open to study are intermediates, unless the kind of femtosecond studies of Zewail13 can be made generally possible. Flow through matrices of larger molecules is well-known in protein pumps and channels leading substrates to compartments. Some use of such channels in other matrices has been found employing man-made resins, zeolites and sephadex.

12 Summary For convenience we have repeatedly divided the review into separate descriptions of active sites and matrices or frameworks and their properties. The fact that the two are interwoven is however clear especially as we chart the degree of complexity of the catalysts. The distinct active site concept, (Table 3) is most readily of value in catalysts which are dependent on a single attacking metal ion although its properties are often energetically set by the matrix. As we have stated, it is convenient to consider active sites simply as catalytic units retained by a matrix so as to bring out properties of the metal ions, and their differences, that interact directly with substrates. This being the case we summarised the major points we have made concerning active sites in the three classes of catalyst: molecular, enzymic, and solid state, in Section 3.

436

Chapter 24

When we have discussed activity more widely, we have stressed that the framework will often bind substrates but also assists attack more than by just increasing bimolecular reaction probability and more than the catalysis that would result from the collision activity with the isolated metal element at the centre of an active region. We have shown that the attacking groups, metal elements, are often of enhanced catalytic strength due to the way they are bound in the matrices. We have termed these ‘‘sites’’ constrained (entatic) towards the transition state of their particular action as discussed in the free energy diagram (Figure 6). We have stressed that in this diagram, and elsewhere, the description of active region energies is pictorial and unsophisticated as it takes no account of the possibility of many routes over barriers. We extended this discussion to catalysts in which the active region is composed of two or more adjacent centres. The involvement of the matrix clearly increases as we proceed to examine adjacent ‘‘site’’ catalysts, which are fixed in structures or relatively flexibly disposed. Here active ‘‘sites’’ become properties of the matrix either directly through the properties of near-neighbour centres both in binding and attacking, or by the conformational mobility of the matrix. The metal ion site and the matrix come more and more to be one catalytic region in local parts of the structure. This stress on the functional value of the matrix is a novel feature of this review. In the examination of combinations of remote non-adjacent regions, which can communicate through the matrix (Section 11) two special cases were considered where transport of electrons and/or protons through matrices were analysed. There are possibilities too for the channelled diffusion of many small molecules, e.g. CO, O2 and H2O, and of even larger substrates. Again diffusion can be on surfaces rather than through matrices. Here the matrix plays an integral part in the catalysis and Figure 20 illustrates, for electron transfer tunnelling, that different matrices of catalysts can be compared. Remote binding sites can be brought to interact by mechanical flow of the two remote centres in a metal bringing them to one site for reaction. By drawing attention to differences between the different catalysts as well as similarities, we stress that such knowledge can help in the design of novel catalysts. The advantages of the protein matrix, for example, are the ease of synthesis using sequential addition of monomers (amino-acids), as happens in cells, allowing small (mutational) adjustment in the final protein by experimenters. Parallel synthesis of soft solid state supports can be achieved in principle with any set of monomers using condensation polymerisation, while control over radical polymerisation is possible but more difficult. Now an easier substitution, by exchange, is of the metal ions at active sites and here while biological synthesis is limited by availability this does not apply to chemical manipulation of enzymes, catalytic solids or to the synthesis of molecular catalysts. There are some ten catalytic metal ions used in living cells and taken from their environment but some sixty possible ones in the Periodic Table. The choice of metal ions is not often applied to enzymes. Thus, lessons derived from the molecular and solid state catalysis can indicate a way to

A Comparison between Enzymes and Solid State Catalysts

437

enhance enzymes; on the other hand, the degree of finesse of enzymes and their versatility is an objective for the design of all other classes of catalyst. At present the control over all kinds of dynamics in protein structures is much more sophisticated than in other matrices and the use of the protein matrix for linking remote catalytic sites to a common purpose is also markedly more advanced. While many enzyme properties are of little value in the synthesis of bulk materials from simple substrates, they indicate possibilities in the synthesis of high added-value products. Can chemists synthesise catalysts as sophisticated as enzymes? Reading the literature has made the author realise how research effort in one area of catalysis is often unaware of advances in others. We hope the novelty of the comparative approach to catalysis used here will draw the attention of others to some useful knowledge from somewhat disparate areas of catalytic activity.

Acknowledgement This article is dedicated to Sir John Meurig Thomas but I must make it clear that it could not have been written without his input. All judgements are my own however. I have also benefited from a long exchange with Dr G.F. Sweigers of the CSIRO Molecular and Health Technology Institute, Melbourne, Australia, who has circulated a monograph on his view of catalytic action. The figures of proteins have been modified slightly and are from Ref. 48, which provides a comprehensive list of metallo-enzyme structures with commentary. The figures of solid state catalysts have been supplied by Prof. J.M. Thomas, see Ref. 2.

Note The simplest approach to enzyme, E, and general kinetics is to write the Michaelis–Menton equation: k1 E + S

k−1

ES

k2

P + E

where S is the substrate, ES is the enzyme/substrate complex, and P is the product. The forward rate constant is k1, substrate off-rate is k1 and product formation or off-rate is k2. The equation for the rate is: Rate ¼

Vmax jSj jSj þ KM

where Vmax is the maximum velocity, and KM is a Michaelis constant KM ¼ k1/k1. KM can be replaced by a simple binding constant if k1 and k14k2. Modification of the equation may be necessary for reasons associated with reaction steps which we shall not discuss. Again the enzyme may not return immediately to the initial state E in which case there is a relaxation term of the enzyme. For a detailed study see Ref. 49. In principle the equation is

438

Chapter 24

applicable to other catalysts and the energy states of ES can be described by transition state theory.

References 1. J.M. Thomas and R. Raja, Stud. Surf. Sci. Catal., 2004, 148, 163. 2. J.J.R. Frausto da Silva and R.J.P. Williams, The Biological Chemistry of the Elements, Oxford University Press, Oxford, 2001, 348. 3. R. Mason, M.W. Roberts, J.M. Thomas and R.J.P. Williams, Catalysis in chemistry and biochemistry (Proc. of Roy. Soc. Disc. Mtg., June 2004), Philos. Trans. R. Soc. London, Ser. A, 2005, 363, 765. 4. P.C. Jordan, Chemical Kinetics and Transport, Plenum Press, New York, 1979. 5. R.J.P. Williams, J. Inorg. Biochem., 2007 (accepted for publication). 6. R. Noyori, C.A. Sandoval, K. Mu¨niz and T. Ohkuma, Ref. 3, 901. 7. J.C. Kendrew, R.E. Dickerson, B.E. Strandberg, R.G. Hart, D.R. Davies, D.C. Phillips and V.C. Shore, Nature (London), 1960, 185, 422. 8. D.C. Phillips, Proc. Natl. Acad, Sci. U.S.A., 1967, 57, 484. 9. C.G. Roberts, Mobility and function of proteins and nucleic acids, Ciba Foundation Symposium, Pitman, London, 1983, 93. 10. Y. Watanabe and T. Ueno, Bull. Chem. Soc. Jpn., 2003, 76, 1309. 11. (a) B.L. Vallee and R.J.P. Williams, Proc. Natl. Acad. Sci. U.S.A., 1998, 59, 498; (b) H.B. Gray, B.G. Malmstro¨m and R.J.P. Williams, J. Biol. Inorg. Chem., 2000, 5, 551; (c) R.J.P. Williams, Chem. Commun., 2003, 1109. 12. D.K. Bo¨hme and H. Schwartz, Angew. Chem. Int. Ed. Engl., 2005 44, 2336. 13. A.H. Zewail, Phil. Trans. Roy. Soc. A, 2005, 363, 315. 14. P.E.M. Siegbahn and M.R.A. Blomberg, Ref. 3, 847. 15. A.G. Ogston, Nature, 1948, 162, 963. 16. R.A. Marcus, Phil. Trans. Roy. Soc. B, 2006, 361, 1445. 17. G.J. Hutchings, J. Catal., 1985, 96, 292. 18. J.N.L. Dennett, M. Bierenstiel, M.J. Ferguson, R. McDonald and M. Cowie, Inorg. Chem., 2006, 45, 3705. 19. J.M. Shiftman, A.M. Grosser, B.R. Gibney, R.E. Sharp and P.L. Dutton, Biochemistry, 2000, 39, 14813. 20. (a) P.W.N. Leeuwen, Homogeneous Catalysis: Understanding the Art, Kluwer, Dordrecht, 2004; (b) G.W. Parshall and S.D. Itell, Homogeneous Catalysis, Wiley-Interscience, New York, 1992. 21. G.I. Berglund, G.H. Carlsson, A.T. Smith, H. Szo¨ke, A. Henriksen and J. Hadju, Nature, 2002, 417, 463. 22. (a) T.L. Poulos, Phil. Trans. Roy. Soc. A, 2005, 363, 793; (b) S.J. Lippard, Phil. Trans. Roy. Soc. A, 2005, 363, 861; (c) K.S. Hewitson, N. Granatino, R.W.D. Welford, M.A. McDonough and C.J. Schofield, Phil. Trans. Roy. Soc. A, 2005, 363, 807.

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23. M.A. Foster, H.A.O. Hill and R.J.P. Williams, Biochem. Soc. Symp., 1970, 31, 187. 24. A.E. Dennard and R.J.P. Williams, in Transition Metal Ions as Reagents in Metallo-enzymes, vol 1, D. Carlin (ed), 1967, 115. 25. (a) R. Raja, J.M. Thomas, M. Xu, K.D.M. Harris, M. Greenhill-Hooper and K. Quill, Chem. Commun., 2006, 448; (b) J.M. Thomas, C.R.A. Catlow and G. Sankar, Chem. Commun., 2002, 2921. 26. (a) H. Ahn, C.P. Nicholas and T.J. Marks, Organometallics, 2002, 21, 1788; (b) L. Li, M.V. Metz, H. Li, M. -C. Chen, T.J. Marks, L. Liable-Sands and A.L. Rheingold, Organometallics, 2002, 124, 12725. 27. B.G. Malmstro¨m and G. Lectner, Curr. Opin. Chem. Biol., 1998, 2, 286. 28. R.J.P. Williams and J.J.R. Frau´sto da Silva, Biochim. Biophys. Res. Commun., 2002, 292, 293. 29. R.J.P. Williams, J. Theor. Biol., 1961, 1, 1. 30. J.M. Thomas and R. Raja, Chem. Commun., 2001, 675. 31. J. Klinman, Phil. Trans. Roy. Soc. B, 2006, 361, 1323. 32. R. Breslow, Chem. Rev., 1998, 98, 1997. 33. H.C. Joao and R.J.P. Williams, Eur. J. Biochem., 1993, 216, 1. 34. M. Haruta and M. Date, Appl. Catal., 2001, 222, 427. 35. D.M. Cox, R.O. Brickman and A. Kaldor, Z. Phys. D, 1991, 19, 353. 36. A.M. Dayle, K. Sh. Shcukhutdinov and H.J. Freund, Angew. Chem. Int. Ed. Engl., 2004, 43, 118. 37. J.H. Sinfelt, Bimetallic Catalysts, Wiley, New York, 1983. 38. J.M. Thomas, R. Raja, B.F.G. Johnson, T.J. O’Connell, G. Sankar and T. Khiyak, Chem. Commun., 2003, 1126. 39. R.D. Adams, B. Captain and L Zhu, J. Am. Chem. Soc., 2004, 126 3042. 40. (a) G. Ertl, Angew. Chem. Int. Ed. Engl., 1990, 29, 1219; (b) T. Zambelli, J. Wintterlin, J. Trost and G. Ertl, Science, 1996, 273, 1688; (c) G.A. Somerjai and A.L. Marsh, Phil. Trans. Roy. Soc. A, 2005, 363, 879. 41. J.T. Yates, Jr., J. Vac. Sci. Technol., 1995, 13, 1359. 42. (a) S. Dahl, A. Logadattir, R.C. Egeberg, J.H. Larsen, I. Chorkendorff, E. To¨rnquist and J.K. Norskov, Phys. Rev. Lett., 1999, 83, 1814; (b) R.T. Vang, K. Horkala, S. Dahl, E.K. Vestergoard, B.S. Clausen, J.K. Norskov and F. Besenbacker, Nat. Mater., 2005, 4, 663. 43. (a) C.C. Page, C.C. Moser, X. Chen and P.L. Dutton, Nature, 1999, 402, 47; (b) J.R. Winkler and H.B. Gray, J. Inorg. Biochem., 1997, 2, 399. 44. P.P. Edwards, H.B. Gray and R.J.P. Williams, Angew. Chem. Int. Ed., (in press 2007). 45. R.J.P. Williams, in The Enzymes of Biological Membranes, vol 47, A.M. Martonosi (ed), 1985, 71. 46. M. Bra¨nden, T. Sanden, P. Brzezinski and J. Widengreen, Proc. Natl. Acad. Sci. U.S.A., 2006, 103, 19766. 47. C. Darnault, A. Volbeda, E.J. Kim, P. Legard, X. Xernede, R.A. Lindahl and J.C. Fortecilla-Camps, Nat. Struct. Biol., 2003, 10, 271.

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48. (a) A. Messerschmidt, R. Huber, T. Parlos and K. Weighardt, Handbook of Metalloproteins, vol I and II, J. Wiley, New York, 2001; (b) A. Messenschmidt, W. Bode and M. Cyglar, M., Handbook of Metalloproteins, vol III, Wiley, New York, 2004. 49. B.P. English, W. Min, A.M. van Oijen, K.T. Lee, G. Luo, H. Sim, B. Cherayil, S.C. Kon and X.S. Xie, Nature Struct. Biol. 2006, 2, 86–92.

CHAPTER 25

Zeolite Modelling: Active Sites in Different Framework Structures and in Different Crystallographic Positions JOACHIM SAUER Institut fu¨r Chemie, Humboldt Universita¨t zu Berlin, Unter den Linden 6, D-10099, Berlin, Germany

1 Introduction Zeolites are not only an important class of industrially used catalysts, they are also a perfect example of the active site concept (‘‘The tortuous tale of the catalytically active site’’).1 The large variety of zeolite structures2 for which three-letter codes (e.g. FAU, MFI) are used can be described as microporous aluminosilicate polymorphs, (M1)m[SiO2]n
m[AlO2
]m, made of corner-sharing TO4 tetrahedra (T ¼ Si, Al–). The negative framework charge, defined by the Al content, is compensated by protons or metal cations. These cations are the origin of the catalytic activity of zeolites and the position of Al in the lattice defines the position of the active site. The proton forms of zeolites, H-zeolites, are solid acids.3 Their Brønsted sites have the proton attached to one of the O atoms of the AlO4 tetrahedron thus forming bridging hydroxyl groups, Si–O(H)–Al at corner-sharing O atoms connecting the AlO4 tetrahedron with a neighbouring SiO4 tetrahedron (Figure 1). Transition metal cations as charge compensating cations are also catalytically active. They are either coordinated to two O atoms of one AlO4 tetrahedron, or to two to four O atoms of several TO4 units, typically within an aluminosilicate ring (Figure 2). The activity and selectivity of zeolite catalysts featuring the same active sites can vary substantially. Active sites in different frameworks (Figure 1) or at different framework positions (Figure 2) may be differently accessible for 441

442

Figure 1

Chapter 25

Zeolitic Brønsted sites – bridging hydroxyl groups in different framework structures.

SIII

I2 SII Z6 MFI

FAU

Type I (Z6 in MFI, SII in FAU): coord. to 3-4 O atoms Type II (I2 in MFI, SIII in FAU): coord. to 2 O atoms

Figure 2

Cu1 sites in MFI and FAU frameworks.33,45

reactant molecules or may accommodate transition structures of the catalytic step differently. Beyond this latter concept of shape selectivity,4,5 also the intrinsic activity of the sites may be different in different frameworks or in different crystallographic positions of a given framework, due to different structural boundaries, different ability of adjusting to structural changes in the course of elementary catalytic processes or different long-range interactions with the surrounding crystal. Hence, knowing the crystallographic position of the active sites is key to understanding the structure activity relation for zeolite catalysts. Since the position of the active site within the zeolite lattice is intimately connected with the crystallographic position of Al in the aluminosilicate framework, understanding the Al siting in zeolite structures is a priority, but remained a challenge till today. X-ray and neutron diffraction methods are of limited use in this

Zeolite Modelling

443

respect, because of the very similar scattering properties of Si and Al. The problem is particularly severe for catalysts with a low Al content (high-silica samples) which usually are the most active ones. Further complications arise from the fact that it is not clear if Al is preferentially occupying certain crystallographic sites or if it is statistically distributed. Substantial progress in differentiating between crystallographic positions and in determining the Al distribution has been made with high-resolution solid state NMR due to magic angle spinning (MAS). In 1979 Gu¨nter Engelhardt and Endel Lippmaa demonstrated that 29Si-MAS-NMR can be used to distinguish SiO4 tetrahedra with one, two, three or four Al neighbours in zeolites.6–8 John Thomas together with his colleagues Jacek Klinowski and Colin Fyfe immediately recognized the high potential of that method for structure determination,9 in particular when used in addition to diffraction techniques and high-resolution electron microscopy. For high-silica zeolites, ‘‘resolving crystallographic distinct tetrahedral sites’’ became possible even for structures with low symmetry such as ‘‘silicalite and ZSM-5’’.10 The activities of the early years (‘‘Every spectrum told us something new, because nothing had been done before . . . ’’)11 are described in the book of Engelhardt and Michel.12 The exceptional resolution of the 29Si-NMR spectra showing more than 10 separated resonances in a 10 ppm range could not be reached by MAS for the quadrupolar 17O and 27Al nuclei. Distinguishing between different positions of 17O13,14 and of 27Al15–17 in the zeolite framework is an achievement of the last decade only, thanks to the development of double rotation (DOR) and multi-quantum (MQ) techniques, respectively. Atomistic modelling, in particular by quantum methods, is indispensable in assigning spectroscopic signals to structural features and, in addition, can provide information about structure and reactivity of active sites that cannot easily be obtained by experiments. By lattice energy minimization we can compare structures of the same framework with the active site in different positions. We can then calculate spectroscopic signatures and see which structures fit best the observations. Moreover, we can calculate adsorption energies, energy barriers and reaction rates for catalytic reactions at the different sites and monitor differences. Discussions with John M. Thomas at the Royal Institution about the details of zeolite structures, in particular the Al distribution and the framework and site specifics of catalytic properties, helped to encourage C. Richard A. Catlow and the author in developing force fields for the atomistic simulation of zeolite frameworks with Al and bridging hydroxyl groups in different positions.18,19 The functional form chosen was that of the ion-pair shell-model potentials20 and parameters for the bridging hydroxyl group have been chosen such that agreement of the average of the predicted active site structures with quantum mechanical results of small cluster models was achieved. Later, refined parameters have been obtained by fitting to quantum chemical data exclusively, first at the Hartree–Fock level,21 later at the density functional theory (DFT) level.22 Further progress was made with the development of hybrid quantum mechanics/ molecular mechanics (QM/MM) methods, which treat a portion of the zeolite including the active site by quantum methods, but the periodic environment by

444

Chapter 25 23

force fields. Today, DFT calculations of the complete periodic structure are feasible, in particular for zeolites with small unit cells and at least for benchmark purposes,24,25 and hybrid QM/QM methods are applied to overcome some limitations of DFT by combining them with wavefunction-based electron correlation calculations for the active site.26,27

2 The FAU and MFI Structures The first lattice energy simulations with the ion-pair shell-model potential for zeolites were made for the faujasite (FAU)18 and silicalite/ZSM-5 (MFI)19 framework structures which represent extreme cases. The former has high symmetry (space group Fd  3m) and only one crystallographic distinct tetrahedral site (T site), whereas the latter has 24 distinct T sites in its monoclinic lowtemperature structure (P21/n) and 12 in the orthorhombic high-temperature structure (Pnma). The two examples also differ in their typical Al content. In MFI (Figure 3) the Si/Al ratio is typically high (15–100), and interactions between Al sites are minor. The question is: are there energetically preferred crystallographic positions for Al insertion and is the resolution of 27Al-MASNMR high enough to distinguish between Al(4Si) sites with different local structures. In the FAU lattice (Figure 4), there is only one crystallographically distinct T site, which means that there is only one possibility to create an isolated Al site, but the Si/Al ratio of FAUs is much lower, around 2.7 in Y-zeolites and as low as 1.3 in X-zeolites. The limiting Si/Al ratio is 1 because corner-sharing AlO4 tetrahedra are unstable (Loewenstein or Al avoidance rule). For realistic Si/Al ratios (between 1.2 and 3) there are different possible substitution patterns with the same relative populations of the Si(0Al), Si(1Al), Si(2Al), Si(3Al), Si(4Al) structural units. They can be calculated from the Si/Al ratio provided that Loewenstein’s rule is obeyed. 29Si-NMR can distinguish between the Si(nAl) sites and the intensities of the respective signals are given by the Si/Al ratio, but the specific Al substitution pattern cannot be deduced. The fundamental question is: is the Al distribution for a given Si/Al ratio random or are there energetic preferences. Before we discuss energetic preferences for crystallographic sites in MFI or for substitution patterns in FAU, we should remember that zeolites are metastable crystalline solids and, therefore, it is doubtful that during the zeolite synthesis the lowest energy structure will form. Many zeolite catalysts are also heavily modified after the initial synthesis process, e.g. by cation exchange and/ or calcination. Do these procedures result in energy minimum structures for the final cation content or is the Al distribution given by the original form? Computational chemistry can be helpful in both situations. It can predict the crystallographic Al site or the Al distribution with the lowest energy, but it can also help to identify non-random, non-equilibrium Al siting and distributions by assigning NMR signals to Al in specific crystallographic sites or to a specific Al distribution.

Zeolite Modelling

Figure 3

445

MFI structure details.

3 Crystallographically Distinct Sites in the MFI Structure In 1982 a 29Si-MAS-NMR spectrum of a highly crystalline sample of silicalite (all-silica form of MFI) was reported showing nine resolved signals (Figure 5, top left).10 Their intensities pointed to 24 crystallographic distinct T sites in agreement with a monoclinic structure. Over the years, even better resolved spectra became available (Figure 5, bottom left).28 The observed chemical shifts are determined by the local structure around the nucleus, but bond distances and bond angles cannot directly be deduced from the shift data. A semiempirical correlation29 between the 29Si chemical shift and the average of the four Si–O–Si angles can be used for calculating chemical shifts from XRD-data (Figure 5 shows an example). This proved useful for removing ambiguities from structure refinements, in particular when only powder diffraction data are

446

Figure 4

Chapter 25

FAU structure. Tetrahedral arrangement of sodalite cages connected by D6R. Top: Primitive cell with Al distribution for Si/Al ¼ 3. Bottom: Two of the four inequivalent O sites with bridging hydroxyl groups (O1 and O3).

available. The correlation can also help to assign 29Si-NMR signals to crystallographic positions, but since the connectivity of T sites is available from two-dimensional NMR,30 this has become the main tool for assigning 29 Si-NMR spectra. Instead of diffraction data, bond angles from lattice energy minimizations employing force fields (or DFT calculations) can be used as input for the correlation, and the agreement with observed chemical shifts can serve as a ‘‘quality check’’ for the structure prediction. It is possible to go a step further, replacing the semiempirical correlation between average bond angle and chemical shift by a quantum mechanical calculation of the chemical shift using either finite cluster models defined around the magnetic nucleus31 or periodic plane wave methods.32 Using the former approach for zeolites MFI, MEI, MTW, TON, FAU the structures predicted by the non-empirical shell-model potential prove as accurate as the diffraction structures when judged on the quality of the 29Si-NMR shifts.31 Figure 5 shows on the right the predicted and observed

Zeolite Modelling

Figure 5

29

447

29 Si-NMR spectra of ZSM-5/silicalite (MFI). Left: Early (top)10 and highly resolved spectrum (bottom)28 of the monoclinic structure (24 T sites). The chemical shifts calculated from the bond angles of the XRD refinement are also shown. Right: Quantum chemically calculated shifts for the simulated orthorhombic structure (12 T sites)31 compared to the observed spectrum.

Si-NMR spectra of the orthorhombic MFI structure.31 The quantum mechanical shift calculation for simulated structures yields the right sequence of lines for the different T sites. The observed spectrum is reproduced with an offset of
0.4 ppm and a standard deviation of 1.5 ppm, better than the one obtained with the conventional approach (shift-bond angle correlation, XRD structure) with a standard deviation of 1.9 ppm. Already, the early study of silicalite/ZSM-5 reported two peaks for Al in T sites, at 54.5 and 56.7 ppm.10 A later MQ study found two peaks at 54.5 and 57.1 ppm (H-ZSM-5) and a third one at 59.4 ppm (Na-ZSM-5).15 The question was whether these peaks belong to Al in two (or three) specific crystallographic sites or whether they are composed of contributions from several sites. Our early lattice energy simulations for Al in MFI (using the empirical shell-model ion-pair potential)19 already showed that the energy differences between Al in different sites are small (a few kilocalories per mole) and that thermodynamics would predict little deviation from a random distribution. Table 1 shows the relative energies for the orthorhombic and monoclinic MFI structures obtained with the DFT-parametrized shell-model potential.16,33 The conclusion remains the same, although the details of the stability sequence have changed.

448

Table 1

Chapter 25
1

Relative energies (kcal mol ) of MFI structures with Al in different T sites.

Orthorhombica

Monoclinicb

T site

Energy

T site

Energy

T site

Energy

T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12

0.0 4.1 4.7 3.0 5.7 8.7 1.3 2.2 2.2 3.1 2.8 3.9

T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12

6.2 5.2 4.5 2.1 0.2 2.5 1.1 3.0 4.9 2.6 2.5 4.0

T13 T14 T15 T16 T17 T18 T19 T20 T21 T22 T23 T24

0.0 4.0 4.5 2.7 6.8 9.8 2.0 2.7 2.5 3.4 3.3 4.1

a b

Ref. 33. Ref. 16.

18,3 24

Figure 6

1 17

12

7 6

4,8

20

Calculated 27Al-NMR chemical shifts for Al in 24 different T sites of the monoclinic MFI structure (lines) compared to observed (MQ-NMR) shifts for differently synthesized MFI samples.16 Open triangles indicate less safely identified shifts.

If the energetic differences are small, it is possible that different synthesis procedures for ZSM-5 zeolites with different (but low) Al content and different cations may lead to different Al substitution patterns, thus increasing the total number of resolved 27Al signals in the MQ-27Al-NMR spectra of the whole set of samples. This strategy indeed resulted in the identification of ten distinct resonances, each found for at least two samples, which extend over a range of 13.6 ppm.16 For a given sample, the number of resonances varied between one and three. It was also shown that the cation (H, Li, Na) had a negligible effect on the 27Al shift. The conclusion that the observed resonances belong to Al in different crystallographic sites is supported by quantum mechanical shift calculations (Figure 6).16 The shifts calculated for Al in the 24 different T sites of the

Zeolite Modelling

449

monoclinic structure extent over an even wider range of 14.1 ppm. The observed shifts fall into this range and comparison of the predicted and observed shift patterns even suggests a partial assignment (Figure 6): the resonance at 53.7 ppm belongs to Al in T8 or T4, the resonance at 54.7 to Al in T6 and the resonance at 62.8 belongs to T17. Of the additional resonances those at 50.0 and 63.6 ppm could be tentatively assigned to T20 and T1 respectively. These results tell us that the Al distribution is not controlled by thermodynamic stability, but depends on the synthesis procedure. It would be very interesting to examine the properties of H-zeolites generated from a series of samples with resonances that extend over the full range. If the ion exchange and calcination procedure would not change the Al positions, we may expect a larger variability of acidic sites than we see now for H-ZSM-5 catalysts.34

4 Brønsted Sites and the Al Distribution in FAU As there is only one crystallographically distinct T site in the FAU lattice, there is only one possibility to create an isolated Al site. However, there are four possible O sites to which the proton can be attached creating four different bridging hydroxyl groups. For more than one Al in the lattice, electrostatic arguments predict that Al atoms assume the largest possible distance for a given Si/Al ratio35 which is known as Dempsey’s rule. This rule makes no mention of the cations that compensate the framework charge due to Al. There is a difference between protons that attach directly to one framework oxygen, thus causing a local deviation from the tetrahedral AlO4
structure, and other cations. For two Al substitutions in the primitive FAU cell (24 T atoms), lattice energy minimizations have been made for all possible substitution patterns.36 The expected decrease of the lattice energy with the Al–Al distance has been found only for global charge compensation (the Al charge of 2  (
1) was compensated by increasing all T site charges by 2/24), but not for local charge compensation when one proton is added to an oxygen atom of each AlO4 tetrahedron. In the latter case, there was a strong preference for Al–O–Si–O–Al pairs in fourmembered rings. This observation has been confirmed by quantum mechanical calculations on double six-membered ring (D6R) models. Al–Si–Al pairs within a four-membered ring were always more stable than isomers with the largest possible Al–Al separation. This is not a peculiarity of the FAU lattice, but was also found for other frameworks featuring D6R secondary building units (offretite, zeolite L, erionite, chabasite, gmelinite).36 For FAUs with low Si/Al ratios (zeolites Y and X) two types of methods have been used to determine substitution patterns that are compatible with the measured NMR intensities for a given Si/Al ratio and obey Loewenstein’s rule. One is a statistical approach, relying on electrostatic energy calculations for achieving non-random distributions. The other uses electrostatic arguments for Si/Al orderings in small building units (largest Al–Al distance) and combines them with crystal symmetry arguments. Studies of the latter category have been

450

Chapter 25

published in 1981–1982, again in parallel by the Cambridge/Ontario37,38 and Berlin/Tallin39,40 teams. For the Si/Al ratios of 3.0 and 2.43, typical of Y-zeolites, just one substitution pattern survived for each Si/Al ratio (see Figure 4, top). To rationalize the dependence of the Brønsted acidity of H-FAUs on the Si/Al ratio two extreme models have been invoked, a local one assuming that the acid strength decreases with increasing number of Al in T sites that are in next-nearest neighbour position with respect to the AlO4 tetrahedron with the Brønsted proton, and a global one assuming that the acid strength depends on the mean (Sanderson) electronegativity of the zeolite. The heterogeneity of Brønsted acidity resulting from the Al distribution has been examined by hybrid QM/MM techniques for two types of materials: highsilica FAU with isolated or paired Al sites (1 or 2 Al per 48 T sites) and Y-type materials with Si/Al ratios of 3 and 2.43 for which we adopt the distribution pattern of Refs. 38 and 40 (see Figure 4). The Brønsted acid strength has been characterized by calculated energies of deprotonation and, as a spectroscopic signature of the different bridging hydroxyl groups, their infrared OH stretching frequencies have been calculated. Even for an isolated Brønsted site with only one crystallographically distinct Al site in the FAU lattice, there are four possible O sites to which the proton can be attached creating four different bridging hydroxyl groups. The preference for O1 and O3 occupation and the assignment of the respective OH groups to high frequency (HF) and low frequency (LF) infrared bands is well-established and reproduced by both force field18 and hybrid QM/force field calculations.41,42 For the 2.43 and 3.0 Si/Al ratios O1:O2:O3:O4 proton occupation patterns of 8:2:4:0 and 7:2:3:0, respectively, have been adopted which are close to the ratios inferred from powder neutron diffraction experiments. Figure 7 shows the calculated OH stretching frequencies as a function of the calculated deprotonation energies.42 The most important observation is that the former are primarily determined by the local structure (O1 or O3) while the latter primarily depend on the number of Al atoms in next-nearest neighbour positions. The overall Si/Al ratio seems to affect the acid strength of a particular site only indirectly, the lower the Si/Al ratio the higher the probability that next-nearest Al neighbours exist.

5 Cu(I) Sites in MFI and FAU Frameworks Cu(I) exchanged zeolites, specifically Cu(I)-ZSM-5 are highly active catalysts for the decomposition and selective reduction of NO in exhaust gas.43 The preferred Cu1 sites in the lattice and the coordination of Cu1 ions in these sites could not be determined by experiments. Simulations for MFI,33 FER44 and FAU45 frameworks succeeded in identifying two different types of sites with different coordination and different reactivity. A three-step computational strategy proved very useful.33 The first step is lattice energy minimizations with Al in all possible T sites (Table 1). In a second step lattice energy minimizations are made with at least 10 different Cu1 extra-framework

Zeolite Modelling

Figure 7

451

OH stretching frequencies and energies of deprotonation of bridging hydroxyl groups in H-FAUs as a function of the Al content.42 ‘‘2Al/3’’ denotes that there is a second Al in next nearest neighbour position and that the Si/Al ratio in the framework is 3. The bars ‘‘O1’’ and ‘‘O3’’ on the right side indicate the range of data for the two types of O positions.

positions for each Al position. This allows for a fast determination of the most favoured sites. Third, for selected structures QM-Pot energy minimizations are made using DFT for embedded clusters large enough to capture the most important interactions between the Cu1 ion and the zeolite framework. In both MFI and FER zeolites sites were found with two-, three-, or fourfold coordinated Cu1 ions. The sites were classified depending on the number of O atoms coordinating the Cu1 ion and its position in the framework, as shown in Figure 8. Type II site copper ions (Figure 2) are coordinated to two O atoms of one AlO4
tetrahedron, either at the channel intersection (I2 site in MFI and FER) or on the walls of the main or perpendicular channels (M2 and P2 sites, respectively in FER). Higher coordinated sites, summarized as type I sites, have one or two additional coordinations to other oxygen atoms within a five- or sixmembered (TO)n ring (Figure 2). Large energetic preferences for one or the other Cu1 site or for Al in special T sites are not found. The occurrence of Cu1 with different coordination numbers is supported by an average coordination number of 2–3 found in EXAFS experiments. The coexistence of the two types of sites also emerged from experimental photoluminescence spectra. While the observed 3d10(1S0)–3d94s1(1D2) excitation spectra show two well-separated

452

Figure 8

Chapter 25

Different Cu1 sites in the MFI framework located in rings of different size on the walls of the main channel (M), the zig-zag channel (Z), or at the channel intersection (I).33

bands, the band splitting almost disappears in the emission spectra. The QM-Pot calculations not only confirmed this observation but also provided an explanation.46 In the ground state, different types of Cu1 coordination cause large variations in the excitation energies. In contrast, in the excited state the coordination differences between type I and type II sites disappear. The type I sites give up their additional coordination and retain only the twofold coordination to the AlO4 tetrahedron, whereas type II sites remain unchanged. The reason is that on excitation the 4s orbital becomes occupied, which is much larger than the 3d orbital, and so the Cu1 ion moves away from the zeolite wall. Thus, because the excited structures are alike for all Cu1 sites considered, the emission energies are also very similar. For Y- and X-zeolites (FAU framework), Cu1 ions may be found in the general cation sites SI (inside the double six-ring), site II (in the plane of a sixmembered aluminosilicate ring), and SIII (above the three annealed fourmembered aluminosilicate rings). Sites II and III are on the wall of the large cage in the FAU structure (Figures 2 and 4). Simulations have shown that Cu1 binds significantly less strongly in sites III than in sites I and II.45 Hence, SIII sites will be only occupied in CuX zeolites with a larger Cu/Al content because not all of the larger number of Cu1 ions can be accommodated in SI/SII sites. Cu1 in SIII sites belongs to type II sites (low-coordination), while Cu1 in SII-FAU sites belongs to type I sites (3–4 coordination in six-membered rings). Whereas in high-silica zeolites (MFI, FER) isolated Al sites will prevail, in Cu(I)-Y zeolites we may find six-membered aluminosilicate rings with one or two Al atoms. They can be distinguished by the stretch frequency of adsorbed CO. Hybrid QM/MM simulations show that the HF (2160 cm
1) and LF CO

453

Zeolite Modelling
1

47

1

bands (2145 cm ) observed for Cu(I)-Y zeolites can be assigned to Cu in SII sites with one and two Al in the six-membered ring, respectively.45 In Cu(I)X zeolites the LF band (2130 cm
1)47 is tentatively assigned to SII sites with the maximum number of Al (three) in the six-membered ring, whereas the HF band47 at 2155 cm
1 is assigned to low-coordinated SIII sites (type I), which are occupied in Cu(I)-X only.45 In high-silica zeolites (MFI, FER) it is hardly possible to distinguish between type I and type II sites by monitoring the CO stretching frequency. After CO adsorption, the structure of the adsorption complexes for type I and type II sites is very similar, Cu1 is coordinated to CO and to two O atoms of the AlO4
tetrahedron. This means that on CO adsorption its coordination to the zeolite framework remains unchanged for type II sites, whereas it loses one or two coordinations to framework oxygen atoms for type I sites.48 Crucial is the question whether the two different types of Cu1 sites in MFI and FER exhibit different catalytic activity. Hybrid DFT/MM calculations have been performed to investigate the influence of the Cu1 ion coordination on adsorption of not only probe molecules such as CO,48 but also substrate molecules such as NO49 and NO2.50 On interaction with the ad-molecule the coordination of the Cu1 ion to the zeolite framework remains unchanged for type II sites. For type I sites, the Cu1 ion prepares for optimum bonding of the ad-molecule by giving up its coordination to one or two framework oxygen atoms and moving away from the channel wall. For this reason the interaction energies with the higher coordinated type I sites in MFI are 6–8 kcal mol
1 smaller (in FER are 11–13 kcal mol
1) than with the two-coordinated type II sites (Table 2). Cu1 in type II sites binds NO (and NO2) even stronger than gas phase Cu1 ions which points to an unusual ‘‘activation’’ of Cu1 by the ‘‘zeolite’’ ligand which involves 3d10 - 3d94s1 promotion.49,50 The main Table 2

NO and CO heats of adsorption on different Cu1 sites in zeolites.a Site Type

Cu1NO ON-CuMFI Cu1CO OC-CuMFI OC-CuFER OC-CuY a

II(I2) I(Z6) II(I2) I(Z6) I(P6) I(SII,1-2Al)

DH0 26.5 28.6 21.4 35.4 33.2 26.3 18.1g 17.9
13.9h

Calculated results from Ref. 49 if not otherwise noted. Heat of adsorption at 300 K if not otherwise noted. Ref. 51. d Ref. 52. e Ref. 53. f Ref. 54. g Ref. 55. h Ref. 45. i Ref. 56. b c

DH300 26.5 19.3 31.1 24.2

Observed b 26.1  1.2 (0 K)c B24d 35.5  1.6 (0 K)e 28.7f; B31g 23.9f 20.5g 19.1–15.5i

454

Chapter 25

difference between the two types of sites is how much heat is released on binding the NO substrate. Adsorption can affect the overall kinetics of the catalytic process by making the apparent barrier much lower than the intrinsic barrier. While the intrinsic barrier may be similar on both types of sites, the apparent barrier will be significantly lower for type II sites and catalytic conversion at these sites will be much faster. The more reactive type II sites are located at the intersection of the straight (main) channel and the zig-zag channel in the MFI lattice. This puts some limits on the location of Al in the MFI framework. Type II sites will occur for Al in T1, T2, T6 or T12, but not for T11, T4 or T10.33

6 Outlook Zeolites are unique in offering distinct crystallographic positions for Al as the central constituent of the catalytically active site. In spite of substantial progress with high-resolution NMR for quadrupolar nuclei, identifying the Al positions in high-silica materials remains a challenge. Atomistic simulations tell us that there is little energetic preference between Al in different sites, and experiments point to different populations for different synthesis procedures. For the example of Cu1 sites in high-silica (MFI, FER) zeolites, using quantum calculations we have shown that the catalytic properties of active sites in different locations can vary substantially. This sets the future agenda: understand the synthesis process at the atomic scale and control the Al distribution.

Acknowledgements I thank all colleagues with whom I had the pleasure to work on the original publications cited in this chapter, many of whom have been also helpful in preparing the figures. I thank the German Research Foundation (DFG) and the Funds of the Chemical Industry (FCI) for support.

References 1. J.M. Thomas, Top. Catal., 2006, 38, 3. 2. C. Baerlocher and L.B. McCusker, Database of zeolite structures, 2007, www.iza-structure.org/databases. 3. A. Corma, Chem. Rev., 1995, 95, 559. 4. S.M. Csicsery, J. Catal., 1971, 23, 124. 5. L.A. Clark, M. Sierka and J. Sauer, J. Am. Chem. Soc., 2004, 126, 936. 6. G. Engelhardt, D. Kunath, M. Ma¨gi, A. Samoson, M. Tarmak and E. Lippmaa, in Workshop on Adsorption of Hydrocarbons in Zeolites, ed. M. Bu¨low, Zentralinstitut fu¨r physikalische Chemie, Berlin, 1979. 7. E. Lippmaa, M. Ma¨gi, A. Samoson, G. Engelhardt and A.-R. Grimmer, J. Am. Chem. Soc., 1980, 102, 4889.

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8. E. Lippmaa, M. Ma¨gi, A. Samoson, M. Tarmak and G. Engelhardt, J. Am. Chem. Soc., 1981, 103, 4992. 9. C.A. Fyfe, J.M. Thomas, J. Klinowski and G.C. Gobbi, Angew. Chem. Int. Ed., 1983, 22, 259. 10. C.A. Fyfe, G.C. Gobbi, J. Klinowski, J.M. Thomas and S. Ramdas, Nature, 1982, 296, 530. 11. H. Koller, Solid State Nucl. Magn. Reson., 1997, 9, ix. 12. G. Engelhardt and D. Michel, High-Resolution Solid-State NMR of Silicates and Zeolites, Wiley, Chichester, UK, 1987. 13. L.M. Bull, A.K. Cheetham, T. Anupold, A. Reinhold, A. Samson, J. Sauer, B. Bussemer, Y. Lee, S. Gann, J. Shore, A. Pines and R. Dupree, J. Am. Chem. Soc., 1998, 120, 3510. 14. L.M. Bull, B. Bussemer, T. Anupold, A. Samoson, J. Sauer, A.K. Cheetham and R. Dupree, J. Am. Chem. Soc., 2000, 122, 4948. 15. P. Sarv, C. Fernandez, J.P. Amoureux and K. Keskinen, J. Phys. Chem., 1996, 100, 19223. 16. S. Sklenak, J. Dedecek, C. Li, B. Wichterlova´, V. Gabova, M. Sierka and J. Sauer, 2007, Angew. Chem. Int. Ed., 2007, 46, 7286. 17. O.H. Han, C.S. Kim and S.B. Hong, Angew. Chem. Int. Ed., 2002, 41, 469. 18. K.-P. Schro¨der, J. Sauer, M. Leslie, C.R.A. Catlow and J.M. Thomas, Chem. Phys. Lett., 1992, 188, 320. 19. K.-P. Schro¨der, J. Sauer, M. Leslie and C.R.A. Catlow, Zeolites, 1992 12, 20. 20. G.V. Lewis and C.R.A. Catlow, J. Phys. C: Solid State Phys., 1985, 18, 1149, 0022–3719. 21. K.-P. Schro¨der and J. Sauer, J. Phys. Chem., 1996, 100, 11043. 22. M. Sierka and J. Sauer, Faraday Discuss., 1997, 106, 41. 23. M. Sierka and J. Sauer, J. Chem. Phys., 2000, 112, 6983. 24. J. Hafner, L. Benco and T. Bucko, Top. Catal., 2006, 37, 41. 25. P. Nachtigall and J. Sauer, in Introduction to Zeolite Molecular Sieves, ed. H. van Bekkum, J. Cejka, A. Corma and F. Schueth, Elsevier, Amsterdam, 2007. 26. C. Tuma and J. Sauer, Chem. Phys. Lett., 2004, 387, 388. 27. C. Tuma and J. Sauer, Phys. Chem. Chem. Phys., 2006, 8, 3955. 28. G. Engelhardt and H. van Koningsveld, Zeolites, 1990, 10, 650. 29. G. Engelhardt and R. Radeglia, Chem. Phys. Lett., 1984, 108, 271. 30. C.A. Fyfe and Y. Feng, Nature, 1989, 341, 223. 31. B. Bussemer, K.-P. Schro¨der and J. Sauer, Solid State Nucl. Magn. Reson., 1997, 9, 155. 32. M. Profeta, F. Mauri and C.J. Pickard, J. Am. Chem. Soc., 2003, 125, 541. 33. D. Nachtigallova´, P. Nachtigall, M. Sierka and J. Sauer, Phys. Chem. Chem. Phys., 1999, 1, 2019. 34. L. Peng, H. Huo, Y. Liu and C.P. Grey, J. Am. Chem. Soc., 2007, 129, 335. 35. E. Dempsey, G.H. Ku¨hl and D.H. Olson, J. Phys. Chem., 1969, 73, 387.

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36. K.-P. Schro¨der and J. Sauer, J. Phys. Chem., 1993, 97, 6579. 37. S. Ramdas, J.M. Thomas, J. Klinowski, C.A. Fyfe and J.S. Hartman, Nature, 1981, 292, 228. 38. J. Klinowski, S. Ramdas, J.M. Thomas, C.A. Fyfe and J.S. Hartman, J. Chem. Soc., Faraday Trans. 2, 1982, 78, 1025. 39. G. Engelhardt, E. Lippmaa and M. Magi, J. Chem. Soc., Chem. Commun., 1981, 712. 40. G. Engelhardt, U. Lohse, E. Lippmaa, M. Tarmak and M. Magi, Z. Anorg. Allg. Chem., 1981, 482, 49. 41. U. Eichler, M. Bra¨ndle and J. Sauer, J. Phys. Chem. B, 1997, 101, 10035. 42. M. Sierka, U. Eichler, J. Datka and J. Sauer, J. Phys. Chem. B, 1998, 102, 6397. 43. M. Iwamoto, H. Yahiro, K. Tanda, N. Mizuno, Y. Mine and S. Kagawa, J. Phys. Chem., 1991, 95, 3727, 0022–3654. 44. P. Nachtigall, M. Davidova and D. Nachtigallova, J. Phys. Chem. B, 2001, 105, 3510. 45. P. Rejmak, M. Sierka and J. Sauer, Phys. Chem. Chem. Phys., 2007, DOI 10.1039/b709192c. 46. P. Nachtigall, D. Nachtigallova´ and J. Sauer, J. Phys. Chem. B, 2000, 104, 1738. 47. J. Datka and P. Kozyra, J. Mol. Struct., 2005, 744–747, 991. 48. M. Davidova´, D. Nachtigallova´, R. Bulanek and P. Nachtigall, J. Phys. Chem. B, 2003, 107, 2327. 49. M. Davidova´, D. Nachtigallova´, P. Nachtigall, J. Sauer, H. Koiszumi and P.B. Armentrout, J. Phys. Chem. B, 2004, 108, 13674. 50. L. Rodriguez-Santiago, M. Sierka, V. Branchadell, M. Sodupe and J. Sauer, J. Am. Chem. Soc., 1998, 120, 1545. 51. K. Koszinowski, D. Schro¨der, H. Schwarz, M.C. Holthausen and J. Sauer, Inorg. Chem., 2002, 41, 5882. 52. A. Gervasini, C. Picciau and A. Auroux, Microporous Mesoporous Mater., 2000, 35–36, 457. 53. F. Meyer, Y.M. Chen and P.B. Armentrout, J. Am. Chem. Soc., 1995, 117, 4071. 54. R. Kumashiro, Y. Kuroda and M. Nagao, J. Phys. Chem. B, 1999, 103, 89. 55. O. Bludsky, M. Silhan, P. Nachtigall, T. Bucko, L. Benco and J. Hafner, J. Phys. Chem. B, 2005, 109, 9631. 56. G.D. Borgard, S. Molvik, P. Balaraman, T.W. Root and J.A. Dumesic, Langmuir, 1995, 11, 2065.

CHAPTER 26

Magnetic Resonance Imaging: A New Window on the Catalyst Operating in the Reactor Environment L. F. GLADDEN, B. S. AKPA, L. D. ANADON, C. P. DUNCKLEY, M. H. M. LIM, M. D. MANTLE AND A. J. SEDERMAN Department of Chemical Engineering, University of Cambridge, Pembroke Street, Cambridge CB2 3RA, UK

1 Introduction Magnetic resonance imaging (MRI) is an emerging measurement technique for the study of chemical reactions in situ within reactor environments,1 and a subject closely related to, and derived from, the work and vision of John Meurig Thomas. The real opportunities in developing MRI for application to the study of heterogeneous catalytic processes occurring in the working reactor derive from the ability of magnetic resonance (MR) techniques to probe both physical and chemical phenomena – this means that, in principle, we can image flow fields inside reactors, measure molecular diffusion coefficients inside catalyst pellets, and spatially map chemical conversion within a reactor. In this chapter, we will focus on fixed-bed reactors. These process units are the workhorse of the chemical industry and typically comprise a cylindrical column packed with millimetre-scale catalyst pellets. Somewhat fortunately, fixed-bed reactors can be studied by MRI at a size-scale from which the results can be scaled up for real process application. This chapter addresses recent developments in MRI that enable us to measure intra-pellet diffusion, chemical reaction and single- and two-phase flow in fixed-bed reactors. Before doing this it is worth putting these different 457

458

Chapter 26 103 ideal

intra-pellet mass transfer limitation

inter-phase mass transfer limitation

Effectiveness factor

1

10−3

10−6

10−9 10−3

1

103

106

109

1012

1015

Thiele modulus

Figure 1

A plot of catalyst effectiveness factor against Thiele modulus. Increasing Thiele modulus identifies the increasing dominance of mass transfer limitation.

types of MR measurements into context; that is, why do we need this array of measurements to study the in situ behaviour of a heterogeneous catalyst? The answer to this is found in Figure 1. This is the type of diagram much more associated with the world of chemical engineering than chemistry. However, it provides a clear explanation of what we need to consider if we are attempting to perform a truly in situ experiment. With reference to Figure 1, let us first define what we are plotting. Effectiveness factor is a dimensionless quantity which is defined as the ratio of the observed rate of reaction to the ‘ideal’ rate that would occur in the absence of mass or heat transfer limitations. The ideal rate is that which characterises the reaction when all reactants have unhindered access to the catalytically active site, and all products can pass unhindered back into the inter-pellet space, and hence the output stream, of the reactor; under these circumstances the effectiveness factor takes the value unity. The horizontal axisffi pffiffiffiffiffiffiffiffiffiffi of Figure 1 is identified as the Thiele modulus which is defined as L k=De , where L is the characteristic dimension of the catalyst pellet (typically its radius), k is the intrinsic rate constant of the reaction and De is the diffusion coefficient of the molecular species moving within the pore space of the catalyst. Thus, for a given chemical reaction and size of catalyst pellet, k and L take constant values, and increasing values of Thiele modulus reflect decreasing access to the catalytically active site. Any concentration gradients that develop within the catalyst during operation (which may then be associated with temperature gradients as a result of spatially varying reaction rates), will normally act to reduce catalytic activity below its ‘ideal’ value. If molecular mobility within the catalyst is hindered, thereby giving rise to loss of

Magnetic Resonance Imaging

459

conversion, the catalytic process is said to be suffering from the effects of mass transfer limitation. As seen from Figure 1, conversion can be decreased by orders of magnitude as a result of intra-pellet mass transfer limitation. Even greater loss of catalytic activity is caused by inter-phase mass transfer limitations, associated with delivering reactants to, and products from, the external surface of the catalyst. As chemists we might naturally assume that each catalyst pellet within the reactor ‘sees’ exactly the same environment (i.e., reactant composition) at its external surface. What MRI clearly shows is that this is not the case – these limitations in getting reactants to, and products away, from the catalyst surface can destroy the performance of an otherwise good catalyst. Thus, we see that while the chemist primarily concerns himself with designing the active site for optimal conversion and selectivity, the observed performance of the working catalyst within a reactor can be modified significantly by intra- and inter-pellet mass transfer processes. Now that we are beginning to develop the toolkit to look at the relevant physical and chemical processes occurring within a reactor, we have the opportunity to design the catalyst and reactor as an integrated unit. MRI is unique in its ability to address chemistry, molecular diffusion and flow, and to achieve these measurements non-invasively and without need for chemical or radioactive tracers. The value of using an imaging technique is that we see how the local chemical and physical processes differ from the associated global characteristics. This is very important because until now, most process models will, of necessity, take single value descriptors of a process and assume that these global values provide an adequate description of the overall process performance. An example of this would be using overall (superficial) gas and liquid velocities through the reactor and assuming that such values are adequate for the characterisation of the flow fields contacting each catalyst pellet within the reactor. Later in this chapter we will show just how wrong that assumption can be (Section 4). The images provided by MRI provide invaluable insight into real catalyst and reactor operation, and these data can be used directly in process design. Moreover, if sufficient care is taken to ensure that quantitative data are acquired with respect to both the chemistry and hydrodynamics within the reactor, this developing field of research provides a wealth of new data which can be used in the validation and development of numerical and theoretical models of the relevant transport and reaction processes. The process models which can then be developed will be based on the true physical and chemical phenomena that exist within the reactor, and will therefore be far more reliable when used in designing catalytic reactors and identifying their optimum mode of operation. In the remainder of this chapter we will summarise the recent developments in MRI applied to heterogeneous fixed-bed catalytic reactors. There have been three main areas of development: (i) chemical mapping, (ii) ultra-fast imaging of flow fields and (iii) solids imaging. The last of these – solids imaging – will not be considered here since its primary area of application in reaction engineering is in understanding the operation of fluidised-bed2,3 as opposed to fixed-bed reactors.

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2 Chemical Mapping Chemical mapping inside catalytic reactors is still in its early stages of development. Initial studies have employed 1H observation because of the high signalto-noise associated with 1H measurements. However, as will be discussed, if chemical mapping techniques are going to be used widely in measuring catalyst performance within reactor environments, then 13C observation is the more likely way forward. In the following sections we discuss the reasons for this, and illustrate some of the work in this area.

2.1

1

H Observation

Yuen et al.4 first demonstrated the nature of the information that can be obtained regarding chemical mapping within a fixed-bed reactor, using the liquid phase esterification of methanol and acetic acid catalysed within a fixed bed of H1-ion exchange resin catalyst (Amberlyst 15, pellet size 600–850 mm) as the model reaction system. Experiments were performed in a fixed-bed reactor of internal diameter 10 mm. A two-dimensional (2-D) slice image through the bed is shown in Figure 2a; the full dataset was recorded as a 3-D image with an isotropic resolution 97.7 mm
97.7 mm
97.7 mm. The reactions were performed at an ambient temperature of 295 K. (a)

(b)

(c)

10%

Figure 2

X

54%

(a) 2-D slice through a 3-D RARE image of a fixed bed of ion exchange resin. The image has an isotropic resolution of 97.7 mm
97.7 mm
97.7 mm. The image slice in which the local volumes are located for the volumeselective spectroscopy study is identified. The image was acquired by saturating the bed with pure methanol. The acquisition parameters were set to exploit T2-contrast such that signal was acquired only from the methanol in the inter-pellet space. Visualisation of mean conversion, X, within selected volumes located within the slice section identified in (a) are shown in (b) and (c) for feed flow rates of 0.025 and 0.05 mL min1, respectively. The local volumes have in-plane dimensions of 1.5 mm
1.5 mm and a depth (image slice thickness) of 500 mm.

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Magnetic Resonance Imaging

In these studies, chemical conversion was determined in situ by measuring the H resonance associated with OH groups present. In practice two such resonances exist associated with chemical species inside and outside the catalyst pellets, respectively. The difference in chemical shift between these intra- and inter-pellet species arises because of the different electronic environment of the molecules inside the catalyst pellets compared to their environment in the bulk fluid in the inter-pellet space. In this work, chemical conversion was determined from the MR signal acquired from species in the inter-pellet space of the bed because the signal from inside the catalyst pellets is also going to be influenced, to an unknown extent, by relaxation time contrast. In addition to possible relaxation contrast effects, there will also be modifications to the chemical shifts of individual species resulting from adsorption onto the catalyst; this may cause peak broadening and reduces the accuracy with which we can determine the chemical shift of the species of interest. As follows from Equation (1) which describes the esterification reaction of methanol and acetic acid to form methyl acetate and water: 1

CH3 OH þ CH3 COOH Ð CH3 COOCH3 þ H2 O

ð1Þ

we see that the chemical shift of the OH resonances in the reaction mixture, dobserved, will be given by: dobserved ¼ xAcOH dAcOH þ xMeOH dMeOH þ nxH2 O dH2 O ;

Sxi ¼ 1

ð2Þ

where di is the chemical shift associated with the pure compound i, and xi is the mole fraction of species i in the mixture; all chemical shifts are referenced to tetramethylsilane (TMS). n is the ‘‘number’’ of 1H species associated with OH groups within the water molecule; the physical interpretation of this parameter has been discussed in detail elsewhere.4 The form of Equation (2) arises because of the phenomenon of 1H fast-exchange5 (i.e., occurring on a timescale o106 s) between OH groups associated with the acetic acid (AcOH), methanol (MeOH) and water (H2O) molecules present within the reaction mixture. In the esterification reaction considered here, as conversion increases so the 1H resonance associated with the OH groups moves to lower chemical shift, referenced to TMS; i.e., towards the 1H chemical shift of pure water. Thus, the value of dobserved provides an accurate measurement of the chemical composition of a given reaction mixture. From this value of the mole fraction of acetic acid within the reaction mixture, the extent of conversion within the system is determined. An upper limit on the error in conversion determined using this approach is B2% for a given set of experimental conditions. Clearly, in this particular example, chemical shift provides an elegant measure of conversion and avoids errors in determining concentrations based on analysis of spectral intensities that may be influenced by line broadening and relaxation time effects. However, this approach can only be used when the spectrum is sufficiently simple that all the spectral resonances can be assigned unambiguously. Figures 2b and c show the results of a volume selective spectroscopy experiment in which spectra were recorded from local volumes of dimension

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1.5 mm
1.5 mm
0.5 mm within the fixed bed; the data acquisition time for each spectrum was 3 min. Each selected volume has been colour-coded according to the conversion within that volume as determined from the localised spectroscopy experiment. These images highlight two generic and important features of local catalyst performance in a fixed-bed reactor. First, within a given transverse slice section through the reactor, there exists a range of
of conversions; in this case a fractional variation in conversion (DX=X) B20% is observed, where X
is the mean conversion calculated from the 10 local volume measurements of conversion. Second, increasing the flow rate through the bed results in a decrease in conversion. This is expected since the faster the reactants flow through the bed, the smaller the contact (or residence) time of the reactants with the catalyst pellets. However, it follows from this observation that if local velocities adjacent to individual catalyst pellets vary then this will cause local variation in catalytic conversion. This heterogeneity in local flow field is indeed observed in fixed-bed reactors and will be illustrated in Section 4.1. Of course, it is not sufficient to simply correlate spatially resolved conversion and flow velocities at a single axial location along the bed. The conversion observed at a given position along the length of the reactor will be the resultant of the interplay of hydrodynamics, intra-pellet diffusion and chemical kinetics for all the reactant species that have moved through the reactor to reach that location. The long-term objective of the work described in this chapter is that by applying these various MR techniques we can obtain a sufficiently good understanding of hydrodynamics, diffusion and reaction in reactors, that we can predict the spatial distribution of conversion within real reactors, and use the numerical tools derived from this knowledge to design new, cleaner and more efficient catalytic processes. Studies of this reaction have recently been extended to acquisition of a 4-D CSI dataset, shown in Figure 3; the grey scale indicates the extent of conversion. In such a 4-D dataset, we have three spatial dimensions of imaging and a fourth spectral dimension. As expected from the volume selective spectroscopy studies discussed earlier, conversion is seen to be heterogeneous within transverse sections through the bed at any position along the direction of superficial flow. Although 1H MRI has been used to map the progress of chemical reactions as described above and in application to the catalytic hydrogenation of a-methylstyrene, as reported by Koptyug et al.,6 1H MRI observation is unlikely to become a generic tool for chemical mapping in catalytic reactors. This is because the 1H nucleus is associated with a narrow chemical shift range and, further, most of the species participating in the reaction will have a large number of 1H resonances associated with them. These characteristics of 1H observation mean that it is often impossible to deconvolve unambiguously the resonances of specific reactant and product species in the resulting 1H spectra. Hence, quantitative conversion and selectivities cannot be determined. The situation is made even worse by the decrease in nuclear spin–spin relaxation times of molecules when they interact with the catalyst surface, which causes broadening, and hence increased overlap, of the 1H resonances. Therefore, it follows that, as used in solid state NMR spectroscopy,7,8 13C observation may

Magnetic Resonance Imaging

Figure 3

463

3-D cutaway image showing the extent of conversion of an esterification reaction occurring within a fixed-bed reactor. The conversion was calculated from the chemical shift of the OH peak in a 4-D chemical shift image. The chemical shift image was acquired with an isotropic spatial resolution of 625 mm. The RARE image of the structure of the bed was acquired at an isotropic spatial resolution of 78 mm. Both datasets have been re-interpolated on to a common array giving an effective isotropic spatial resolution of 156 mm. The direction of flow is in the negative z direction. The grey scale indicates the fractional conversion within the bed.

have potential advantages in studying catalytic systems because the 13C nucleus has a wider chemical shift range than 1H, making the spectral resonances of individual molecular species more easily resolved. Further, there will be fewer carbon environments, and hence spectral resonances, in a 13C spectrum when compared to a 1H spectrum of the same system. However, the disadvantage of using 13C is that its natural abundance is only 1.07% and its NMR sensitivity is lower than that of 1H by a factor of 5870, therefore there is considerable loss of signal-to-noise when employing 13C as opposed to 1H observation. In solid state NMR, which typically uses small, closed, sample volumes (B1 cm3), this decrease in sensitivity and natural abundance is overcome by isotopically enriching the species of interest with 13C. However, this approach is too costly for the larger sample volumes required for flow-through reactor studies. It is for this reason that interest in exploiting polarisation transfer techniques has developed.

464

2.2

Chapter 26 13

C Observation

As far back as the late 1980s it was demonstrated that it is possible to combine polarisation transfer techniques used in NMR spectroscopy with imaging pulse sequences9 thereby enabling the natural abundance 13C signal to be spatially resolved. In theory, a signal enhancement of up to a factor of 4 (i.e., gH/gC, where gi is the gyromagnetic ratio of nucleus i) can be achieved with 13C DEPT.10 In this dual resonance experiment, initial excitation is on the 1H channel. Consequently, the repetition time for the DEPT experiment is constrained by T1H (oT1C); where T1i is the T1 relaxation time of nucleus i. This favourable condition allows increased signal averaging, thereby further improving the signal-to-noise ratio, for a given acquisition time. The 13C DEPT-MRI pulse sequence is a direct combination of the 13C DEPT pulse sequence used in MR spectroscopy with the double phase encoding, orthogonal pair of gradients applied during the third evolution period to introduce spatial resolution into the measurement. As the polarisation transfer from 1H to 13C is non-linear, the final 1H y pulse affects the magnitude of the signal depending on which CHn (n ¼ 1, 2, 3) groups are present. The value of y is therefore chosen to select either all or combinations of CHn groups.10 The resulting spectra are analysed to recover quantitative data characterising the amount of the different chemical species present. To date, 13C DEPT-MRI has been used in two case studies of catalytic processes. Akpa et al.11 employed 13C DEPT-MRI to follow a reaction in which competing etherification and hydration reactions of 2-methyl-2-butene (2M2B) were followed within a fixed bed of H1 ion exchange resin; the resin was the same as that used in the esterification reaction described earlier. The reactants used were 2M2B, methanol and water, and the products of the etherification and hydration reactions are tert-amyl methyl ether (TAME, or 2-methoxy-2-methylbutane) and tert-amyl alcohol (TAOH, or 2-methyl-butan2-ol), respectively. All experiments were performed using a Bruker DMX 300 spectrometer with a 7.0 T vertical magnet equipped with shielded gradient coils providing a maximum gradient strength of 100 G cm1. A birdcage r.f. coil of diameter 20 mm – dual tuned to 300 and 75.5 MHz for the 1H and 13C resonances, respectively – was used. The data were recorded as a 5
7 (x
z) 2-D array of spectra, with the data being averaged in the third, y, direction; the centre of the image volumes were separated by a distance of 2.5 mm in the axial (z) direction. The reaction temperature was 313 K. In Figure 4, spectra from the central column of the array are shown, at each of six axial positions. It is seen that the chemical shift range of 13C gives sufficient spectral resolution that we can follow the loss of reactants and the formation of products without need for spectral deconvolution. In calculating concentration and selectivity from these spectra the spectral resonances of the same carbon group must be compared between species, since the degree of polarisation transfer and hence signal enhancement is dependent on the chemical environment of each specific carbon atom. In the calculation of conversion the CH3 resonances of TAME and TAOH occurring at 7.8 and 8.7 ppm respectively were used, and compared with

465

Magnetic Resonance Imaging 12.5 mm 2M2B Signal intensity (arbitrary units)

TAME 10.0 mm

7.5 mm

TAME & TAOH

TAME TAOH

5.0 mm

2.5 mm

0.0 mm 40

35

30

25

20

15

10

5

0

Chemical shift (ppm relative to TMS)

Figure 4

Spatially resolved 13C DEPT-MRI spectra recorded for the competitive etherification and hydration reactions of 2M2B to TAME and TAOH respectively. Spectra recorded at six positions along the length of the bed are shown, at 2.5 mm intervals. The entrance to the bed is at 0 mm.

any of the CH3 resonances of 2M2B (these appear at 13.4, 17.3 and 25.7 ppm); all chemical shifts are quoted with respect to the 13C resonance of TMS. Selectivity to TAME was quantified by comparing the intensity of well-resolved CH3 resonances of TAME and TAOH, which occur at 25 and 28 ppm, respectively. Analysis of the data shown in Figure 4 showed that over the 15 mm height of the bed for which spectra are shown, conversion increased by approximately 25% while selectivity remained approximately constant at 75–80%. This MR method has now also been successfully applied to investigate alkene hydrogenation in a trickle-bed reactor.12 Trickle-bed reactors are wellestablished in industries with large throughputs such as the petrochemical industry where they are used primarily for hydro-cracking, hydro-desulfurisation, and hydro-denitrogenation. They consist of a fixed bed of catalyst pellets, contacted by a gas–liquid two-phase flow, with co-current downflow as the most common mode of operation. Figures 5 and 6 show 13C DEPT-MRI data recorded for the hydrogenation of 1-octene occurring over a 1 wt.% Pd/Al2O3 catalyst. The reactor was of inner diameter 2.5 cm and the catalyst was loaded to a bed height of 3 cm. By employing 13C observation it was possible to spatially map not only 1-octene and octane species but also the formation of 2-, 3- and 4-octene isomers; this would not have been possible using 1H MR. Figure 5 shows 2-D maps of the 13C DEPT MR datasets recorded along the length of the trickle bed; 13C DEPT spectra are acquired separately for (a) the

466

Chapter 26 (a)

Figure 5

(b)

160

140

120 ppm

160

140

120

100

100

60

40

20 ppm

0

60

40

20

0

13

2-D map of C DEPT-MRI spectra recorded along the length of a trickle bed. Separate acquisitions were made for each of the (a) olefinic and (b) aliphatic regions of the spectrum. The data were acquired with the bed operating at steady state for gas and 1-octene flow rates of 32 and 1.0 mL min1, respectively. The white, horizontal lines indicate the limits of the catalyst packing. Below each 2-D map, the 1-D 13C DEPT NMR spectrum recorded at an axial location just before the reactants reach the catalyst (just above the upper white line) is shown. The peaks at 114 and 139 ppm indicate that only unreacted 1-octene exists within the bed at this location, as expected.

olefinic and (b) the aliphatic regions of the 13C spectrum. In this experiment the gas and 1-octene (liquid) flow rates were 32 and 1 mL min1, respectively. The intensities shown in the 2-D map are those of the spectral peaks in the 13C DEPT spectrum. Any horizontal cut through the 2-D map recovers an individual 13C DEPT spectrum. The spectra shown below each 2-D map were acquired just above the upper white line; this line identifies the interface between the pure catalyst support and the catalyst. Therefore in the olefinic spectrum only two peaks occurring at 114 and 139 ppm with respect to TMS are seen. These peaks are associated with 1-octene. No other peaks are seen at this position in the bed because no reaction has occurred at this point. As the reactants move down the bed (below the upper white line) additional peaks are seen at 124 and 131 ppm indicating the formation of 2-octene. More detailed analysis of the relative intensities of peaks within the olefinic region provide evidence that small amounts of 3- and 4-octene isomers are also formed. Figure 6 shows data recorded for a higher gas flow rate of 64 mL min1 at the same 1-octene flow rate of 1 mL min1. Comparing Figures 5a and 6a it is clear

467

Magnetic Resonance Imaging (c) (a)

160

Figure 6

(b)

140

120 ppm

100

60

40

20 ppm

0

2-D map of 13C DEPT-MRI spectra recorded along the length of a trickle bed. Separate acquisitions were made for each of the (a) olefinic and (b) aliphatic regions of the spectrum. The data were acquired with the bed operating at steady state for gas and 1-octene flow rates of 64 and 1.0 mL min1, respectively. (c) 2-D 1H MR image of the spatial distribution of liquid within the bed. At this higher gas flow rate greater reaction occurs as the reactants contact the catalyst resulting in local vaporisation within the bed identified by the region of zero signal intensity.

that increasing the gas flow rate has significantly influenced the product distribution. In Figure 6a, as reaction progresses along the length of the reactor there is loss of spectral intensity at B114 and B139 ppm indicating the disappearance of 1-octene. As 1-octene is used up, so the intensity of a resonance at B131 ppm increases. This feature is predominantly associated with cis- and trans- 3- and 4-octene, since the resonance at B124 ppm, which is associated with 2-octene isomers, is of significantly lower intensity than that at B131 ppm. It is also seen that the integrated intensity of the olefinic region decreases down the reactor while that of the aliphatic region increases, consistent with octane formation. The absence of any peak appearing at a chemical shift of B35 ppm shows that significant amounts of trans 4-octene are not being produced suggesting that most of the further isomerisation from 2-octene is to 3-octene and not 4-octene. Detailed analysis of the spectra yields quantitative conversion and selectivity data. Figure 6c shows a 2-D 1H image of the spatial distribution of liquid within the bed. The loss of 1H signal intensity immediately the feed encounters the catalyst shows that under these operating conditions significant vaporisation occurs upon reaction; gas phase species are not imaged using the acquisition parameters employed in this experiment. The vaporisation event is also seen as a loss of signal intensity in the 13C data shown in Figures 6a and b.

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Chapter 26

Figure 7 shows recent work in which it was demonstrated that data can be acquired sufficiently fast using 13C DEPT-MRI that reactor start-up and the approach to steady-state operation can be followed. Again 1-octene hydrogenation was the reaction of interest. For the data shown in Figure 7, the gas and liquid flow rates are 30 and 2 mL min1, respectively; corresponding to a mole ratio of 1-octene to hydrogen of 2. The data acquisition time was 15 min. Mole fractions of 1-octene, 2-octene and n-octane along the length of the reactor are shown at three time points; namely, when the reactant (in the absence of hydrogen) is first introduced to the bed (t ¼ 0 min) and then at 22.5 and 82.5 min after introduction of hydrogen. The time associated with a given dataset is the time at the halfway point through the total data acquisition time. From these data the conversion and selectivity to 2-octene and n-octane, as a function of both time and axial position along the bed, are obtained.13 Where are the future developments in this field? The major driver is undoubtedly to increase signal-to-noise in the MR measurement, while at the same time maintaining the quantitative nature of the data, and retaining adequate spatial and spectral resolution. Ideally we wish to acquire 3-D datasets such that chemical conversion and selectivity are mapped within the reactor with the same level of detail as is currently possible for the imaging of hydrodynamics (to be described in Section 4).

(b) Mole fraction, y (-)

Mole fraction, y (-)

(a) 1 0.8 0.6 0.4 0.2 0 0

5 10 15 20 Axial position, z (mm)

25

1 0.8 0.6 0.4 0.2 0 0

5 10 15 20 Axial position, z (mm)

25

Mole fraction, y (-)

(c) 1 0.8 0.6 0.4 0.2 0 0

Figure 7

5 10 15 20 Axial position, z (mm)

25

Time resolved axial composition profiles obtained from 13C DEPT MRI measurements during start-up of the hydrogenation of 1-octene over a fixed bed of 1 wt.% Pd/Al2O3 catalyst, for a 1-octene:hydrogen mole ratio of 2.0. Concentrations of 1-octene (J), 2-octene (&) and n-octane (E) along the length of the bed are shown (a) before start-up and then at (b) 22.5 min and (c) 82.5 min after start-up.

469

Magnetic Resonance Imaging

3 Intra-Pellet Molecular Diffusion Earlier, we identified intra-pellet and inter-phase mass transfer limitations as the major contributing factors to loss in catalyst performance when moving from the ‘ideal’ catalyst to the catalyst pellet functioning within the working reactor. The majority of the work in our laboratory has focussed on the fluid contacting patterns within the reactor, with the longer-term objective of quantifying mass transfer between the inter-pellet space and the catalyst pellets comprising the bed. However, we have recently begun to develop and implement MR pulse sequences to obtain spatially resolved, chemically specific measurements of molecular diffusion within individual catalyst pellets. Figure 8 shows the results of our initial study.14 In this example, equal volumes of methyl ethyl ketone and 2-butanol were mixed and then imbibed within 5 wt.% Ru/SiO2 catalyst pellets; the pellets were approximately spherical and of diameter B4 mm. Data were recorded at 293 K. In Figure 8a, images of the liquid phase within two pellets positioned one above the other in a 5 mm test tube are shown. Figure 8b shows spatially resolved measurements of the molecular diffusion coefficients, acquired simultaneously for the two chemical species. The dotted and solid lines refer to 2-butanol and methyl ethyl ketone, respectively. The values of chemically specific, spatially unresolved molecular diffusion coefficients within the catalyst pellet are shown by the horizontal lines. The chemically resolved molecular diffusion data are acquired from an image slice, of rectangular cross-section, taken through one of the pellets at the position shown by the dashed line in Figure 8a; spatial resolution of the (a)

(b)

Diffusivity (10-9m2/s)

2

1.5

1

0.5

0 -3

Figure 8

-2 -1 0 1 2 Radial distance (mm)

3

(a) Spin-echo image of liquid distribution within two catalyst pellets. The image has in-plane spatial resolution of 20 mm
78 mm. (b) Molecular diffusion coefficients of methyl ethyl ketone and 2-butanol as a function of radial position across the dashed line identified in (a). The data for methyl ethyl ketone and 2-butanol are shown by the solid and dotted lines, respectively. MRI data were acquired with a spatial resolution of 156 mm. The horizontal lines identify the spatially unresolved measurement of the respective diffusion coefficient within the catalyst.

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Chapter 26

diffusion measurement is 156 mm. These values compare with the free diffusion values of 2-butanol and methyl ethyl ketone of 1.18
109 m2 s1 and 1.90
109 m2 s1, respectively. The steep rise in measured diffusion coefficient towards the edge of the profiles shown in Figure 8b corresponds to liquid outside the pellet towards the wall of the test tube. A sharp edge to the pellet is not seen because, in this experiment, the distance over which data were acquired extends over a vertical distance of 625 mm. The profile shown in Figure 8b is therefore influenced by the changing radius of the pellet over this height.

4 Imaging Flow Fields in Reactors Thus far we have considered chemical mapping and chemically resolved transport processes occurring within individual catalyst pellets. The last piece of the jigsaw in understanding how catalyst pellets perform in the reactor environment comes from understanding the way the reactant streams contact the pellet surface, and how this influences transport of molecules into and out of the catalyst pore space.

4.1

Single-Phase Flows in Fixed-Bed Reactors

High resolution MRI studies of fluid flow within packed beds of columnto-pellet diameter ratio typical of narrow fixed-bed reactors were first reported in the mid-1990s. These first studies were performed on beds of column-to-pellet diameter ratio 10–20, and used non-porous packing (i.e., glass spheres). Figure 9 shows 2-D sections through a 3-D volume image of the z component of flow velocity within a fixed bed of non-porous spherical pellets; the +z direction is the direction of superficial flow in the reactor. In this particular example, the superficial flow velocity was 0.56 mm s1 corresponding to a Reynolds number of 2.8, hence flow in much of the bed is dominated by viscous forces, associated with flow velocities less than, or of the order of, the superficial velocity. The most striking characteristic of these images is the extent of heterogeneity in the flow field; a relatively small fraction of the inter-pellet space carries a high percentage of the liquid flow.15,16 Such regions of the bed are associated with high fluid velocities and inertial effects increasingly influence the flow profile.17 On the basis of these images, it is clear that any theoretical analysis of the flow within such a reactor must account for distinct populations of fast and slow moving liquid – channelling (i.e., fast flow regions) is not just occurring at the walls of the bed. As a result of this heterogeneity in flow within the bed, the contact time between feed and catalyst will differ very significantly across the bed; i.e., by up to at least an order of magnitude in regions of the bed characterised by the highest and lowest flow velocities, and this will introduce spatially varying mass transfer characteristics within the bed. This is an excellent illustration of how a single value characterising the flow velocity through the reactor must be a gross approximation to reality; that is, local behaviour is significantly different from global behaviour.

471

Magnetic Resonance Imaging yz xy

y x xz

-2.7 mm s-1

Figure 9

4.2

z

vz

9.0 mm s-1

MR visualisation of water flowing within a fixed bed of spherical glass beads; the beads have no MR signal intensity associated with them and are identified as black voxels. Flow velocities in the z-direction are shown with slices taken in the xy, yz and xz planes. In the xy-image the positions at which the slices in the other two directions have been taken are identified. Voxel resolution is 195 mm
195 mm
195 mm. The glass beads are of diameter 5 mm and are packed within a column of internal diameter 46 mm. Local flow velocities vary by up to an order of magnitude within the bed.

Two-Phase Flow in Fixed-Bed Reactors: The Trickle Bed

The ability to image the distribution of gas and liquid within a reactor has provided strong motivation for developing MRI techniques to study trickle-bed reactors. Figure 10 shows the steady-state distribution of liquid during ‘trickle flow’ within a fixed bed. The trickle-flow regime occurs at relatively low gas and liquid flow rates, and is characterised by a constant spatial distribution of gas and liquid within the reactor. In this experiment, only the liquid is imaged, and therefore both the gas phase and catalyst pellets are associated with zero signal intensity (black on the grey scale). Subsequent image analysis of these data enable us to identify all image pixels associated with a liquid–solid interface, and hence we have what, to date, is the only direct measure of catalyst wetting within such systems.18 These experiments have been used to confirm that depending on the way the feed streams to the reactor are introduced, the spatial distribution of reactant–catalyst contacting may be quite different. Our more recent research has focussed on the development and implementation of ultra-fast MRI techniques to study unsteady-state phenomena. In the context of trickle-bed reactors, this has enabled us to image the hydrodynamic phenomena that occur within the reactor as the liquid flow rate is increased and the bed moves from the trickle flow to the pulsing flow regime. The pulsing regime takes the form of alternate gas- and liquid-rich bands moving along the

472

Figure 10

Chapter 26

Imaging of liquid holdup within a fixed bed of 5 mm diameter glass spheres contained within a column of 40 mm. The data were acquired in a 3-D array with an isotropic voxel resolution of 328 mm
328 mm
328 mm. (a) The original image of trickle flow is first binary gated, so that only the liquid distribution within the image is seen (white); gas-filled pixels and pixels containing glass spheres show up as zero intensity (black). (b) Pixels containing any liquid–solid interface are then identified using image analysis techniques and ‘images’ of surface wetting are produced. Data are shown for liquid and gas superficial flow velocities of 3 mm s1 and 66 mm s1, respectively.

reactor such that at any given location within the catalyst bed, there is a timevarying liquid content. Many industrial reactors operate close to the transition between the trickle- and pulsing-regimes, and relatively little is known with regard to the nature of the transition itself. Thus, previously it has not been possible to validate theoretical and numerical models of the transition upon which reliable reactor designs can be based. To study this hydrodynamic transition we have used ultra-fast 3-D MRI which allows us to investigate the spatial distribution of liquid within the bed as a function of time, and clearly reveals when regions of the bed are associated with a constant gas–liquid distribution (i.e., trickle flow) or if a given region has moved into an unstable flow regime characterised by a rapidly changing gas– liquid distribution.19 MRI has provided the first direct experimental evidence that the transition to the pulsing regime is initiated by the formation of local pulses (or instabilities) within the bed, and that the number of these local pulses increases until reaching a maximum number at which point they grow and merge until a single large pulse is formed which covers the full dimensions of the bed. Further, MR data provide evidence of fluctuations in the liquid films on the catalyst pellets; such fluctuations may be the precursor to the formation of liquid bridges which then lead to the formation of the local pulsatile events within the bed.20

Magnetic Resonance Imaging

473

A typical experiment proceeds by acquiring successive 3-D images of liquid distribution within the bed which takes the form of a column of diameter B45 mm, packed with catalyst or support pellets of dimension 1–3 mm. In this work, a 3-D image with a field of view of 60 mm (x)
60 mm (y)
60 mm (z) was acquired as a data array of size 16
16
32, thereby giving a spatial resolution of 3.75 mm
3.75 mm
1.87 mm. Each 3-D image was acquired in 280 ms, i.e., 3-D images were acquired at rates of 3.6 f.p.s. Six series of eight consecutive 3-D images were acquired for each set of flow rates. The stability of the liquid distribution within the reactor is quantified by calculating the standard deviation in the signal intensity associated with each individual voxel throughout the time series of images acquired, thereby producing a 3-D image or map of standard deviation values. Voxels associated with a constant gas– liquid distribution (i.e., typical of trickle flow) are associated with values of standard deviation B0. In contrast when the gas–liquid distribution in the voxel is unsteady (i.e., a local pulse is occurring), that voxel will be associated with a standard deviation value of Z 1. Typical standard deviation maps are shown in Figure 11. Inspection of Figure 11 shows the location of isolated hydrodynamic instabilities within the bed, identified by red voxels. From such data it is possible to determine the spatial extent of the local pulsing regions as the bed moves from the trickle to the pulsing regime. Figure 12 shows how the number of isolated pulses (i.e., in the maps shown in Figure 11, a pulse is defined as a group of connected voxels characterised by a standard deviation Z 1) changes as liquid velocity through the bed is increased, at a constant gas velocity of 300 mm s1. The exact form of this plot varies depending on the

Figure 11

3-D standard deviation maps, combined with a RARE image of the bed, calculated from data acquired at a constant gas velocity of 75 mm s1 for liquid velocities of (a) 7.0 and (b) 10.0 mm s1. The height of the bed shown is 28 mm. Data are shown for half of the bed volume imaged. The standard deviation maps (3.75 mm
3.75 mm
1.87 mm) have been linearly interpolated to the same resolution as a high resolution 3-D RARE image (175 mm
175 mm
175 mm) of the bed to provide insight as to how local pulsing relates to the structure of the bed.

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Chapter 26 30

Number of pulses

25

20

15

10

5

uLT 0

Figure 12

0

2

4

6 8 uL[mm s-1]

10

12

14

Analysis of 3-D standard deviation maps calculated from data acquired with the bed operating at a constant gas velocity of 300 mm s1. The plot shows the average number of independent liquid pulses identified at each liquid velocity. Liquid ‘pulses’ are only visually observed and detected by conductance measurements at liquid velocities greater than B9 mm s1, when the bed is characterised by 1–2 large liquid instabilities; i.e., when all the small isolated pulses have merged such that distribution of all liquid within the bed is temporally unstable.

nature of the packing elements used. We have defined the liquid velocity at which the maximum number of isolated liquid pulses exists as the transition point, uLT. Thus, in addition to identifying the physical mechanism by which the hydrodynamic transition occurs, MRI has also shown that there is no specific transition point; that is, the transition actually occurs over a range of liquid velocities. Moreover, the nature of the transition can be controlled by selection of the shape and size of the catalyst pellets; so we now identify further factors that influence catalyst effectiveness! The same ultra-fast MRI techniques are now being applied to the study of periodic operation of trickle-bed reactors, in which the reactor is forced to operate under transient conditions in order to exploit the non-linearities associated with sudden changes in one or more variables when compared to operating at the corresponding steady-state condition.21 Given that reactions taking place in trickle beds are often controlled by mass transfer processes, periodic operation offers the possibility of optimising and intensifying reactions in trickle beds by modulating or interrupting the flow of gas or liquid reactants, thereby periodically reducing mass transfer resistances. A typical periodic operation strategy is to cycle the liquid feed rate to the reactor from a constant, high liquid velocity to a constant, low liquid velocity; this is the periodic

475

Magnetic Resonance Imaging signal intensity

5 × noise

noise = gas

pellets

Figure 13

2-D MR image of spatially resolved liquid holdup during periodic operation. The image frame shown is recorded during the high liquid velocity phase of the cycle. A single image frame was acquired in 200 ms. The 2-D image of the structure of the bed was acquired as a data array of 256
256 pixels giving an in-plane resolution of 175 mm
175 mm; it was acquired in B34 min.

strategy we have investigated.22 Although our studies are only in their very early stages, they clearly show the power of MRI to reveal how local liquid– catalyst contact may differ very significantly from the global characteristic of the bed. The fact that MRI can reveal the locally varying reactant–catalyst contacting behaviour suggests that we may now be able to understand far more about the science that underpins the apparent advantages of operating periodically. This knowledge will lead to greater confidence in exploiting these new methods industrially and also in designing more effective periodic operation strategies. Initial results from MRI are shown in Figures 13 and 14. Data were recorded for the reactor operating with a period (i.e., full cycle) of 8 s, with equal times of 4 s spent operating at high and low liquid velocities of 15 and 1 mm s1, respectively. The gas velocity was constant at 75 mm s1. 2-D FLASH images were acquired in 200 ms, in-plane spatial resolution was 351 mm
700 mm, with a 2 mm slice thickness. In Figure 13, the pellets are identified as black pixels, and pixels associated with signal intensity greater than five times that of the noise level are deemed to be associated with the liquid phase. Pixels with signal intensity at or below this critical value are assigned as gas-filled. Figure 14a shows the integrated signal intensity from a time series of images such as that shown in Figure 13; each point in Figure 14 represents the integrated signal intensity from a single 2-D image; i.e., it is proportional to the total liquid holdup in the 2-D cross section, analogous to a conductance measurement of liquid holdup. A smooth drainage profile is observed – this is the global

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signal intensity [a.u.]

(a) 10 8 6 4 2 0 0

2

4

6

8

10

12

14

16

Time [s] (c) signal intensity [a.u.]

signal intensity [a.u.]

(b) 1.6 1.2 0.8 0.4 0

0

2

4

6

8

10 12 14 16

Time [s]

Figure 14

1.8 1.6 1.2 0.8 0.4 0 0

2

4

6

8

10 12 14 16

Time [s]

(a) Integrated liquid holdup during the same periodic operation strategy as used in Figure 13. The continuous line shows the integrated signal intensity (associated with the liquid phase) calculated from a time series of images such as that shown in Figure 13. The dashed line shows the timing associated with the cycling of the liquid feed. Local holdup profiles during periodic operation can vary markedly from the global characteristic as shown in (b) and (c).

drainage characteristic of the bed. To investigate how the local liquid–catalyst contacting characteristics vary within the bed, image analysis algorithms are used to segment the inter-pellet space of the bed into individual ‘flow channels’ or ‘pores’. Typically an image of the type shown in Figure 13 would be segmented into B700 flow channels, which are of dimension comparable to that of the packing elements comprising the bed. These channels are then identified in the 2-D images and the temporal evolution of the local liquid holdup throughout the cycle is calculated. The MRI analysis clearly shows that only some of the channels within the bed are associated with a local liquid– catalyst contacting profile of the form shown in Figure 14a. Other common contacting profiles are shown in Figures 14b and c, which differ markedly from the global characteristic. In ongoing work we are incorporating these data into numerical modelling schemes to investigate the importance of including the correct local liquid–catalyst contacting profile into numerical simulations predicting activity and selectivity during periodic operation.

Magnetic Resonance Imaging

477

5 Summary This has been a whirlwind tour through the world of MRI applied to the study of heterogeneous fixed-bed catalysis. It is an area in which many developments are being made, and in which, in the near future, we can expect to explore the interaction of physical and chemical phenomena occurring within catalysts and reactors using a new generation of in situ techniques. Recent developments in ultra-fast MRI and MR chemical mapping techniques now make MRI a robust tool for studying both hydrodynamics and chemical conversion within reactors. The versatility of the MR technique, to some extent, remains a barrier to its use, in that for quantitative measurement very careful development and implementation of the relevant MR methods are required. However, if such care is taken, the ability of MR to study 3-D optically opaque reactors and give quantitative information on both the chemistry and hydrodynamics within the reactor provides significant opportunity for the reaction engineer to design more effective catalyst–reactor systems as well as to use the MR data to validate and develop numerical and theoretical models of the relevant transport and reaction processes, which can then be used, with confidence, in subsequent process design.

References 1. L.F. Gladden, M.D. Mantle and A.J. Sederman, Adv. Catal., 2006, 50, 1. 2. C.R. Muller, J.F. Davidson, J.S. Dennis, P.S. Fennell, L.F. Gladden, A.N. Hayhurst, M.D. Mantle. A.C. Rees and A.J. Sedreman, Phys. Rev. Lett., 2006, Art. No. 154504. 3. C.R. Muller, D.J. Holland, J.F. Davidson, J.S. Dennis, L.F. Gladden, A.N. Hayhurst, M.D. Mantle and A.J. Sederman, Phys. Rev. E, 2007, Art. No. 020302. 4. E.H.L. Yuen, A.J. Sederman and L.F. Gladden, Appl. Catal., 2002, A232, 29. 5. R.K. Harris, Nuclear Magnetic Resonance Spectroscopy, Longman, Harlow, 1986. 6. I. Koptyug, A.A. Lysova, A.V. Kulikov, V.A. Kirilov, V.N. Parmon and R.Z. Sagdeev, Appl. Catal., 2004, A267, 143. 7. A.G. Stepanov, K.I. Zamaraev and J.M. Thomas, Catal. Lett., 1992, 13, 407. 8. M.W. Anderson and J. Klinowski, Chem. Commun., 1990, 918. 9. H.N. Yeung and S.D. Swanson, J. Magn. Reson., 1989, 83, 183. 10. E.D. Becker, High Resolution NMR: Theory and Applications, Academic Press, New York, 3rd edn, 2000. 11. B.S. Akpa, M.D. Mantle, A.J. Sederman and L.F. Gladden, Chem. Commun., 2005, 2741. 12. A.J. Sederman, M.D. Mantle, C.P. Dunckley, Z. Huang and L.F. Gladden, Catal. Lett., 2005, 103, 1.

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13. C.P. Dunckley, Z. Huang, M.D. Mantle, A.J. Sederman and L.F. Gladden, J. Catal., 2007, submitted. 14. M.H.M. Lim, A.J. Sederman, M.D. Mantle and L.F. Gladden, Appl. Catal., submitted. 15. A.J. Sederman, M.L. Johns, A.S. Bramley, P. Alexander and L.F. Gladden, Chem. Eng. Sci., 1997, 52, 2239. 16. A.J. Sederman, M.L. Johns, P. Alexander and L.F. Gladden, Chem. Eng. Sci., 1998, 53, 2117. 17. M.L. Johns, A.J. Sederman, A.S. Bramley, P. Alexander and L.F. Gladden, AIChE J., 2000, 46, 2151. 18. A.J. Sederman and L.F. Gladden, Chem. Eng. Sci., 2001, 56, 2615. 19. L.D. Anadon, A.J. Sederman and L.F. Gladden, AIChE J., 2006, 52, 1522. 20. L.F. Gladden, L.D. Anadon, M.H.M. Lim, A.J. Sederman and E.H. Stitt, Ind. Eng. Chem. Res., 2005, 44, 6320. 21. P.L. Silveston and J. Hanika, Chem. Eng. Sci., 2002, 57, 3373. 22. L.F. Gladden, L.D. Anadon, C.P. Dunckley, M.D. Mantle and A.J. Sederman, Chem. Eng. Sci., 2007, in press.

CHAPTER 27

Dissociative Chemisorption of Hydrogen Chloride at Cu(110): Atom-Resolved Time-Dependent Evidence for Transient States in the Formation of the ‘‘Final State’’ Stable Chloride Overlayer A. F. CARLEY, P. R. DAVIES, K. R. HARIKUMAR, R. V. JONES AND M. WYN ROBERTS School of Chemistry, Cardiff University, Park Place, Cardiff CF10 3AT, UK

The dissociative chemisorption of hydrogen chloride at Cu(110) has been studied by scanning tunnelling microscopy (STM) with emphasis given to the isolation of transient states in the formation of the chloride overlayer. There is atom-resolved evidence at 295 K for a transition from disorder to an ordered overlayer involving chlorine adatom surface diffusion, nucleation, domain (soliton) formation, surface buckling, step-movement and time-dependent relaxation. It can also be considered as a model system revealing possible structural states present in the dissociative chemisorption of diatomic molecules which can influence reactivity in catalytic reactions.

Some Personal Reminiscenses by Wyn Roberts I have known John for nearly 60 years having been brought up in the Amman Valley just a few miles from John in the Gwendraeth Valley in Carmarthenshire. 479

480

Chapter 27

We went to different schools but with close connections, the schools sharing some staff including the sports/physical education teacher. It was as a consequence of us both being members of the Carmarthenshire County Athletics Team that we first became acquainted but it was as students at University College Swansea that the friendship developed and crystallised. It is well documented that it was through my persuasion that John after graduation, chose to change his research direction from pursuing postgraduate work in steroid chemistry to surface chemistry, initially studying the oxidation of carbon monoxide at carbon surfaces. I had suggested that he surely did not aspire to be a coauthor of ‘‘Part 55 Walden Inversion’’, Professor Shoppee, a very eminent steroid chemist, and his intended supervisor, being already on Part 54! I had the task of talking to Charles Shoppee, explaining that John had second thoughts. But chemistry at Swansea was a very civilised department and Shoppee, a gentleman, agreed that John should transfer his registration for a PhD to physical chemistry under the supervision of Keble Sykes, also my supervisor. However, if the change had not occurred I am certain that John would have become as distinguished in steroid chemistry as he is in solid state and surface chemistry. John focussed initially on the surface structure of carbon and its relevance to chemical reactivity and I took up the challenge of the chemistry of metal surfaces, initially the role of sulfur as a catalyst in the formation of nickel carbonyl. Both our research groups became involved in the application and development of experimental methods in the areas of solid state and surface catalysis. John was ‘‘best man’’ at my wedding in 1957, I was his some 2 years later. In this chapter we illustrate the role that STM has had in revealing the complexities that can be associated with the dissociative chemisorption of diatomic molecules at metal surfaces where both the metal substrate and the molecular fragments are mobile and how these may impact on the observed catalytic chemistry.

1 Introduction The classical approach in the development of models for surface reactions and chemical reactivity have relied heavily on the Langmuir–Hinshelwood (L–H) and Eley–Rideal (E–R) mechanisms.1,2 These have been central to the development and current views in heterogeneous catalysis. L
H : AðgÞ þ BðgÞ ! AðaÞ þ BðaÞ ! ABðaÞ ! ABðgÞ E
R : AðgÞ þ BðgÞ ! AðaÞ þ BðgÞ ! ABðaÞ ! ABðgÞ Assumptions are then made regarding the applicability of one of the accepted adsorption isotherms (e.g. Langmuir) and the reaction rate expressed in terms of the gas phase pressures PA and PB. For the above reactions the rates would

Dissociative Chemisorption of Hydrogen Chloride at Cu(110)

481

be given by the following rate expressions: RL
H ¼

kbA PA bB PB ð 1 þ bA P A þ bB P B Þ 2

and RE
R ¼

kbA PA PB 1 þ bA PA

Various assumptions can also be made regarding the strength of the surface bonding in the chemisorbed states A(a) and B(a), and whether, for example, they are dissociatively chemisorbed, enabling kinetic expressions to be derived providing evidence for kinetic reaction orders to be anticipated. In the case of the E–R mechanism, molecule A is thermally accommodated and chemisorbed while B is an incoming gas phase molecule which forms a complex AB which desorbs as the product. With the advent of surface spectroscopies (X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES)) surface concentrations could be determined directly and models developed based on structures observed by low energy electron diffraction (LEED). This is a static surface science approach. But what is more relevant to extracting meaningful kinetic information on reaction mechanisms at single crystal metal surface is the applications of surface sensitive spectroscopies under dynamic conditions in real time – an approach rarely used.3 The problem we face in heterogeneous catalysis is to be able to pinpoint the active sites under dynamic conditions and well illustrated by our studies of catalytic oxidation at single crystal metal surfaces – ammonia oxidation at Cu(110) and Zn(0001) and propene oxidation at Mg(0001).4,5 With both reactants (NH3 and O2) present simultaneously in the gas phase it became clear that transient precursor states could provide low energy pathways to products (NH(a)). There was no spectroscopic (XPS) evidence for adsorption of the reactants ammonia and oxygen, with transient O
states implicated in the rate-determining step in what was a radical-type mechanism (shown below) analogous to a two dimensional gas reaction.6 O2 ðgÞ ! O2 ðsÞ NH3 ðgÞ ! NH3 ðsÞ 1=2O2 ðgÞ ! O
ðsÞ O
ðsÞ þ NH3 ðsÞ ! NHðaÞ þ H2 OðgÞ O
ðsÞ ! O2
ðaÞ Both O
(s) and NH3(s) are present at 295 K at immeasurably low concentrations, the reactions conforming neither to L–H nor E–R mechanisms, with the formation of the final O2
state shutting down (poisoning) the reaction.7 The mechanism also has implications for theoretical studies where assumptions are

482

Chapter 27

made on the energy parameters to be assumed for the reacting surface species. What should be assumed for the surface transients O
(s) and NH3(s)? It was against this background of the ‘‘final state’’ not being catalytically active that prompted us to search, in the dynamics of dissociative chemisorption, for atom-resolved STM evidence for transient states.8 In this chapter we consider the dissociative chemisorption of hydrogen chloride at Cu(110) at room temperature. There was also further interest in chlorine (as HCl) being used as an additive in industrial catalysis for redispensing and activating catalysts. Was this a consequence of chlorine induced mobility of the catalyst substrate and could it be monitored in real time by STM?

2 Experimental Details An STM developed by Omicrom Vacuum Physik with in situ facilities for XPS and mass spectrometry was used to study the dissociative chemisorption of hydrogen chloride at a Cu(110) surface at 295 K. A tungsten tip was used for STM and AlKa (1486.6 eV) radiation for obtaining XP spectra. The base pressure of the spectrometer was B1  1010 mbar. The Cu(110) crystal was obtained from Metal Crystals and Oxides Ltd. and cleaned by Ar1 bombardment (0.6 keV, B15 mA) followed by annealing in vacuum at 800 K for 30 min. The cleanliness of the sample was checked by both STM and XPS. Surface coverages of chlorine adatoms were determined by analysis of the intensities of the Cl(2s) and Cu(2p3/2) spectra. Hydrogen chloride (99%) was obtained from Argo International and checked for purity mass spectrometrically.

3 Results and Discussion The atomically clean Cu(110) surface (Figure 1) was exposed to hydrogen chloride at a pressure of 1  10
8 mbar at 295 K and the sequential development of the structural features observed by STM. A number of distinct stages have been isolated.

3.1

Disorder and Nucleation

During the initial exposure the surface is mainly disordered but with streaks – black dashes – present (Figure 1). These are attributed to mobile chlorine adatoms undergoing surface diffusion (hopping) during the time the STM tip has moved across it but with the chlorine adatom having moved away when the tip returns to its original position. Very similar images were observed by Wintterlin et al.9 for oxygen adatom diffusion at Ru(0001) at room temperature. Analysis of the Cl(2s) intensity in the XP spectrum (Figure 1) at 268.4 eV binding energy indicates a chlorine adatom concentration of 4.5  1014 cm
2 at

Dissociative Chemisorption of Hydrogen Chloride at Cu(110)

Figure 1

483

Images of the Cu(110) clean surface (a) and after exposure to hydrogen chloride (10 L) at 295 K (b); note the streaks associated with chlorine adatoms undergoing surface diffusion 1 L  10
6 Torr). Also shown is the XP spectrum (c) with a peak at 268.4 eV binding energy assigned to Cl(a), the concentration of which is estimated to be 4.5  1014 cm
2. The adatoms are disordered.

484

Figure 2

Chapter 27

Nucleation of a single chloride domain structure at a defect site at Cu(110) following exposure (11 L) to hydrogen chloride. The majority of the chlorine adatoms are disordered.

this stage but with no obvious structure present. Wintterlin et al.9 have, however, developed a fast STM with an imaging rate of 20 frames per second and have therefore been able to monitor directly the surface hopping (diffusion) of individual oxygen adatoms. The hopping rate is estimated to be 14  3 s
1 with an activation energy of 0.7 eV. Nucleation is seen to be initiated at a defect (Figure 2) with the surface gradually being dominated by domains running in the o00014 direction separated by islands of a c(2  2) structure with also evidence for copper sites present. The Cl(2s) intensity indicates that the chlorine adatom concentration is 4.9  1014 cm
2 and the binding energy at 268.4 eV (Figure 3). With time the domains become well defined, approximately 2.5 A˚ in height and separated from each other by c(22) structures, the domains being 18 A˚ apart (Figure 4). The domain walls (or solitons) are a consequence of competition between the elasticity of the surface adlayer and the underlying substrate copper lattice potential. When the adlayer structure, in this case the c(22) Cl lattice, differs from the Cu(110) surface periodicity the misfit is accommodated by restructuring and the formation of domain walls or ‘‘surface strings’’ running in the o0014 direction. With the highly electronegative chlorine adatoms removing charge from the copper substrate atoms the surface buckles. The zig-zag structure associated with domains (Figures 4 and 7) is a secondary reconstruction within the domains and reminiscent of the herringbone reconstruction of the Au(111) surface, where partial dislocations are present at the ‘‘turns’’ in the herringbone structure. The ordered c(22) structure separating the domains has unit cell dimensions of 5.1 A˚ in the o1104 direction and 7.2 A˚ in the o0014 direction. Similar c(22) structures have been observed10 for chlorine – (from Cl2 dissociation) at Cu(100).

Dissociative Chemisorption of Hydrogen Chloride at Cu(110)

485

Figure 3

Islands of c(22) Cl with domain structures running in the o0014 direction and clean copper sites.

Figure 4

Cu(110) surface with the completely formed domain structures running in the o0014 direction; the domains are 18 A˚ apart and approximately 2.5 A˚ in height.

486

3.2

Chapter 27

Surface Relaxation and the ‘‘Final State’’ Structure

When the chlorine induced surface-reconstructed surface (Figure 4) was left in vacuum for 2 h at 295 K it relaxes to the c(2  2) structure (Figure 5). However, further exposure (4400 L) to hydrogen chloride results in an increase in the chlorine atom concentration to 6.8  1014 cm
2, i.e. well beyond the c(22) monolayer concentration (B5  1014 cm
2), and the development of well defined domain walls of the reconstructed surface (Figure 6). This is a very stable chloride structure, unchanged after heating to 550 K.

3.3

Chlorine Induced Step Movement

In Figure 7 are shown a sequence of images taken every 40 s over a period of 15 min when the Cu(110) surface was exposed to hydrogen chloride at a pressure of 1.5  10
8 mbar at 295 K. At the completion of the exposure analysis, the Cl(2s) intensity indicated a chlorine atom concentration of B6  1014 cm
2. The domain structures covered the entire surface but what is also evident is that there is considerable movement of copper atoms resulting in the coalescing of two surface steps during reconstruction. This is also evident by comparing the line profiles taken across the surface at the beginning and the end of the 15 min exposure (Figure 8). We estimate that between the fourth image and the sixth image the two steps have coalesced involving a movement of copper atoms of 100 A˚ in 80 s, i.e. a rate of about 1 A˚ per second. It is this high mobility of copper substrate atoms induced by chlorine that is the likely driving force in reactivation of heterogeneous catalysts by chlorine on the industrial scale.

4 Surface Reactivity, Transient and Disordered States The dissociative chemisorption of hydrogen chloride at Cu(110) involves the participation of transient states, with finite lifetimes, and precursors of the stable chloride overlayer. It also illustrates, a general principle, of how transient states may well have a role in controlling reaction pathways in catalysis, with significant experimental evidence emerging for well ordered surface structures at metal surfaces being inactive. This was first gleaned from classical surface spectroscopic studies3–7 with the transient O
-state active in catalytic oxidation reactions (e.g. of ammonia, propene, water, etc.) and Iwasawa’s study11 of the oxidation of carbon monoxide at Cu(110) where activity ceased with the formation of the well ordered reconstructed (21) O phase (observed by LEED). Likewise in ‘‘applied catalysis’’ Panov’s group12 concluded that the radical O
species was the oxidant of benzene to phenol and also the active oxygen in the oxidation of butane reported by Wang and Barteau,13 the O2
-state being by comparison inactive. STM has taken us much further, in that it has provided atomically resolved images where catalytic activity can be correlated with surface disordered

Dissociative Chemisorption of Hydrogen Chloride at Cu(110)

Figure 5

487

Relaxation of a domain dominated surface structure when left for 2 h at 295 K with the development of a well ordered c(22) structure (a). The XP spectrum (b) indicates that the chlorine adatom concentration is 4.9  1014 cm
2.

488

Figure 6

Chapter 27

The reconstructed surface after high exposure (400 L) to hydrogen chloride with domain structures running in the o0014 direction (a). The XP spectrum (b) shows the characteristic binding energy of chlorine adatoms at 268.4 eV indicating a concentration of 6.8  1014 cm
2 i.e. well beyond the monolayer (B5  1014 cm
2).

Dissociative Chemisorption of Hydrogen Chloride at Cu(110)

Figure 7

489

Sequence of images taken during the exposure of Cu(110) to hydrogen chloride at 295 K and a pressure of 1  10
8 mbar. Images are taken every 40 s over a total exposure time of about 15 min.

states and inactivity correlated with their transformation to well ordered structures. Examples so far available are the oxidation of ammonia and propene at Cu(110) and Mg(0001),14 the cyclotrimerisation of acetylene to benzene15 at Pd(111), H2–D2 exchange reaction and ethane hydrogenation at Pt(111), both poisoned when CO was introduced into the gas phase and resulting in a well ordered but inactive surface.16 The previous active state was disordered. To progress further the many questions unanswered in the molecular understanding of surface catalysis requires expertise in a wide range of experimental methods that can operate under dynamic conditions and with appropriate theoretical inputs – not easily achieved in a single university laboratory!

490

Figure 8

Chapter 27

Line profiles (a) and (b) taken from two images (1) and (8) (Figure 7); two surface steps have merged resulting in two terraces rather than the original three.

Acknowledgement We are grateful for the support of EPSRC.

References 1. J.M. Thomas and W.J. Thomas, Principles and Practice of Heterogeneous Catalysis, VCH Publishers Inc., Weinheim, 1997. 2. M.W. Roberts and C.S. McKee, Chemistry of the Metal–Gas Interface, Clarendon Press, Oxford, 1997. 3. M.W. Roberts, Chem. Soc. Rev., 1989, 18, 451; Appl. Surf. Sci., 1991, 52, 133; A.F. Carley, P.R. Davies and M.W. Roberts, Catal. Lett., 2002, 80, 25; Philos. Trans. R. Soc. A, 2005, 363, 829; P.R. Davies and M.W. Roberts, Atom Resolved Surface Reactions: Nanocatalysis, RSC Publishing, Cambridge, 2007. 4. C.T. Au and M.W. Roberts, Nature, 1986, 319, 206; J. Chem. Soc., Faraday. Trans. 1, 1987, 83, 2047; General Discussion, p. 2085; A.F. Carley, S. Yan and M.W. Roberts, J. Chem. Soc., Faraday Trans, 1990, 86, 2701. 5. C.T. Au, L. Xing-Chang, T. Ji-An and M.W. Roberts, J. Catal., 1987, 106, 538.

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491

6. A. Boronin, A. Pashusky and M.W. Roberts, Catal. Lett., 1992, 16, 345. 7. C.T. Au, A.F. Carley, A. Pashusky, S. Read, M.W. Roberts and A. Zeini-Isfahan, in Adsorption on Ordered Surfaces of Ionic Solids and Thin Films, ed. E. Umbach and H.-J. Freund, Springer Series in Surface Science Springer, Berlin, Heidelberg, 1993. 8. A.F. Carley, P.R. Davies and M.W. Roberts, J. Chem. Soc., Chem. Commun., 1998, 538. 9. J. Wintterlin, J. Trost, S. Renisch, R. Schuster, T. Zambelli and G. Ertl, Surf. Sci., 1997, 394, 159. 10. C.Y. Nakakura, G. Zheng and E.I. Altman, Surf. Sci., 1998, 401, 173. 11. T. Sueyoshi, T. Sasaki and Y. Iwasawa, Chem. Phys. Lett., 1995, 241, 189. 12. V.S. Chernyavsky, L. Pirutko, A.K. Uriarte, A.S. Kharitonov and G.I. Panov, J. Catal., 2007, 245, 466. 13. D.X. Wang and M.A. Barteau, Catal. Lett., 2003, 90, 7. 14. A.F. Carley, P.R. Davies and M.W. Roberts, Philos. Trans. R. Soc. A, 2005, 363, 829. 15. T.V.W. Janssens, S. Volkening, T. Zambelli and J. Wintterlin, J. Phys. Chem. B, 1998, 102, 6521. 16. M. Montano, K. Bratile, M. Salmeron and G.A. Somorjai, J. Am. Chem. Soc., 2006, 128, 13229; G.A. Somorjai and A.L. Marsh, Philos. Trans. R. Soc. A, 2005, 363, 879.

CHAPTER 28

Recent Advances in Single-Site Photocatalysts Constructed within Microporous and Mesoporous Materials MASAKAZU ANPO AND MASAYA MATSUOKA Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Nakaku, Sakai Osaka 599-8531, Japan

1 Introduction The design of highly efficient and selective photocatalytic systems that work with no loss of energy for applications in reducing global environmental problems or energy issues is one of the most urgent and vital goals in environmentally friendly catalytic research. Recently, investigations to address such concerns using semiconducting TiO2 powdered catalysts have been extensively carried out for such significant applications as the decomposition of atmospheric NOx,1,2 the degradation of organic impurities diluted in water,2,3 and the decomposition of water into H2 and O2.4 Studies elucidating the dynamics and mechanisms behind the photocatalytic reactions have shown that the electrons and holes produced in the conduction and valence bands, respectively, of the semiconducting TiO2 powdered catalysts under UV light irradiation play a major role in these reactions. It has also been shown that with a decrease in the particle size of the TiO2 catalyst to less than 100 A˚, a higher efficiency in the reactions can be observed.5 As the size of the TiO2 particles is reduced below a certain critical dimension, the gap between its highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) starts to increase, leading to an enhancement of the reduction ability of the photoformed electrons in the LUMO as well as the oxidation ability of the photoformed holes in the HOMO. This ‘‘size quantization effect’’ leads to high and 492

493

Recent Advances in Single-Site Photocatalysts

selective photocatalytic reactivity quite different from photoelectrochemical reactions occurring on bulk TiO2 powder5,6 due not only to an electronic modification of the TiO2 catalysts but also to the close existence of the photoformed electron and hole pairs and their balanced contribution to the reactions. Of special interest is the design of ion and/or cluster size catalysts within zeolites or mesoporous materials since these fascinating supports offer unique nano- or meso-scaled pore systems, an unusual internal surface topology, and ion-exchange capacities.2 The transition-metal oxide species within these supports are considered to be highly dispersed at the atomic level and also well-defined catalysts which exist in the specific structure of the support framework. These highly dispersed transition metal oxide species can act as efficient photocatalysts having strong oxidation and reduction abilities as expected from the size quantization effect. In fact, highly dispersed transition metal oxide species, such as Ti, V, Cr, Mo, etc., can induce unique photocatalytic reactions due to the following ligand to metal charge-transfer process:1,2 n+

O2−

hν

(n−1)

O−

These charge transfer excited states, in which the electron-hole pair states are localized in close proximity, were found to play a significant role in various photocatalytic reactions such as the decomposition of NO into N2 and O2,7 the degradation of organic impurities in water,8 the photo-oxidation reaction of hydrocarbons9 and the photoinduced metathesis reaction of alkanes.10 In fact, except for highly dispersed transition metal oxides, isolated transition metal ions such as Cu1 or Ag1 ions within zeolites can induce unique photocatalytic reactions such as the decomposition of NOx (NO or N2O) into N2 and O211,12 due to the following inner shell type transitions:

Cu+ ([Ar]3d10)

Ag+([Kr]4d10)

hν

hν

Cu+ *([Ar]3d94s1)

Ag+ *([Kr]4d95s1)

These highly dispersed transition metal oxides or ions can be regarded as ‘‘single-site photocatalysts’’ since their local structures are atomically and uniformly regulated due to the framework structure of the zeolite or mesoporous materials.7 This chapter deals with the photocatalytic activities of such single site photocatalysts incorporated within the framework structures or cavities of zeolites or mesoporous materials. Their local structures are also discussed based on the results obtained by various in situ spectroscopic techniques such as photoluminescence, electron spin resonance (ESR), X-ray absorption fine structure (XAFS), ultraviolet-visible spectroscopy (UV-Vis), and Fourier transform infrared spectroscopy (FT-IR) analyses. Special attention is focused on the relationship between the local structures of these single site catalysts and their photocatalytic properties.

494

Chapter 28

2 Photocatalysis on Ti-oxide Single-Site Catalysts Anchored in Zeolite Cavities; The Direct Photocatalytic Decomposition of NO and Reduction of CO2 with H2O The development of efficient photocatalytic systems which can decompose NOx directly into N2 and O2 is strongly desired in order to establish clean and environmentally friendly deNOx systems for atmospheric purification. It has been reported that the decomposition reaction of NO can proceed photocatalytically on powdered TiO2 at room temperature (rt) and N2O is produced as the major product.13 To decompose NO directly into N2 and O2, single site Ti-oxide photocatalysts were prepared and their photocatalytic activities were investigated. UV light irradiation of powdered TiO2 and Ti-oxide/Y-zeolite catalysts prepared by ion-exchange (ex-Ti-oxide/Y-zeolite) or impregnation (imp-Ti-oxide/Y-zeolite) methods in the presence of NO led to the evolution of N2, O2 and N2O in the gas phase at 275 K with different yields and product selectivity.2,14 As shown in Figure 1, the yields of the photo-formed products increased linearly against the UV irradiation time and the reaction immediately O2-

Ti-oxide Single Site Photocatalysts

Ti4+ O2-

100

60 Yields /µmol g-Ti-1

Selectivity for CH3OH Formation/%

50

40

30

Light Off

Light On

Light Off

40

N2

80

Light On

20

N2O

60

0 200 400 0 UV irradiation time /min

20

40

10

20

0 3.5

4

4.5

5

5.5

6

0 6.5 O2-

Coordination Number

2NO

hν

O2Ti

hν

CO2 + H2O

Selectivity for N2 Formation / %

O2-

O2-

CH3OH + CH4 N2 + O2

(QE = 0.3 %) (QE = 17.5 %)

O2-

4+

O2-

O2O2-

(QE : quantum yield )

Figure 1

Relationship between the coordination numbers and photocatalytic reactivities of titanium oxides.

495

Recent Advances in Single-Site Photocatalysts

Table 1

The yields of the photoformed products, N2 and N2O in the photocatalytic decomposition of NO at 275 K and their distribution on various Ti-based photocatalysts.

Catalysts ex-Ti-oxide/Y-zeolite imp-Ti-oxide/Y-zeolite TiO2 powder

Ti content (wt% as TiO2) 1.1 1.0

Yields (mmol/g of TiO2 h)

Selectivity (%)

N2

N2O

Total

N2

N2O

14 7 2

1 10 6

15 17 8

91 41 25

9 59 75

ceased when irradiation was discontinued. A comparison of the photocatalytic activity of the Ti-oxide/Y-zeolite catalysts and the widely used bulk TiO2 powdered catalyst was of special interest. And as shown in Table 1, the specific photocatalytic reactivity of the Ti-oxide/Y-zeolite catalysts, which have been normalized for the unit amount of TiO2 in the catalysts, are much higher than that for the bulk TiO2.2,14 Moreover, the selectivity for the formation of N2 strongly depends on the type of catalyst. The ex-Ti-oxide/Y-zeolite exhibited the highest selectivity for the formation of N2 while N2O was the major reaction product for both the bulk TiO2 catalyst and imp-Ti-oxide/Y-zeolite. Ti K-edge XAFS (X-ray absorption near-edge fine structure (XANES) and extended X-ray absorption fine structure (EXAFS)) investigations of ex-Ti-oxide/Y-zeolite show that the Ti-oxide species exist in an isolated state with a tetrahedral coordination (Ti–O coordination number: 3.7, Ti–O atomic distance: 1.78 A˚) as a ‘‘single site photocatalyst’’. On the other hand, XAFS investigation revealed that the Ti-oxide species has an octahedral coordination as a small TiO2 cluster catalyst within the imp-Ti-oxide/Y-zeolite. The relationship between the coordination number of the Ti-oxide species and the selectivity for N2 formation in the photocatalytic decomposition of NO on various type of Ti-oxide based photocatalysts are also shown in Figure 1.7 There is a clear dependence of the N2 selectivity on the coordination number of the Ti-oxide species. From these results, it was shown that a highly efficient and selective photocatalytic reduction of NO into N2 and O2 could be achieved with the ex-Ti-oxide/Y-zeolite which includes the highly dispersed isolated tetrahedral Ti-oxide as the active species. The formation of N2O as the major product was also observed for the bulk TiO2 and imp-Ti-oxide/Y-zeolite catalysts, which include the octahedrally coordinated aggregated Ti-oxide species. As shown in Figure 2, the ex-Ti-oxide/Y-zeolite exhibited a photoluminescence spectrum centered at ca. 490 nm by excitation at ca. 290 nm at 77 K. The photoluminescence spectrum is attributed to the radiative decay process from the charge-transfer excited state to ground state of the highly dispersed Ti-oxide species in tetrahedral coordination, as follows:

( Ti4+ − O2−)

hν

hv'

(Ti3+ − O−)*

496

Figure 2

Chapter 28

Photoluminescence spectrum of (a) the ex-Ti-oxide/Y-zeolite catalyst; its excitation spectrum (EX); and (b–e) the effect of the addition of NO on the photoluminescence spectrum. Measured at 77 K, excitation beam: 290 nm, emission monitored at 490 nm; amounts of added NO: (a) 0.0, (b) 0.2, (c) 0.8, (d) 7.6, (e) 21.3 mmol/g.

On the other hand, the imp-Ti-oxide/Y-zeolite did not exhibit any photoluminescence. These results show that the ex-Ti-oxide/Y-zeolite involves a highly dispersed isolated tetrahedral Ti-oxide species as the ‘‘single site photocatalyst’’ while the imp-Ti-oxide/Y-zeolite consists of an aggregated octahedral Ti-oxide species which does not exhibit any photoluminescence. The addition of NO onto the ex-Ti-oxide/Y-zeolite led to an efficient quenching of the photoluminescence spectrum and the lifetime of the charge-transfer excited state was also found to be shortened, its extent depending on the amount of NO added.14 These results show not only that the tetrahedrally coordinated titanium oxide species may be located at positions accessible to the added NO but also that the added NO easily interacts with the charge-transfer excited state of the species.7,14 Based on these results, the reaction mechanism for the photocatalytic decomposition of NO on the isolated tetrahedral Ti-oxide species could be proposed, as shown in Scheme 1. The NO molecule could adsorb onto the oxide species as weak ligands to form the reaction precursors. Under UV light irradiation, the charge-transfer excited complexes of the oxides, (Ti31–O
)*, were formed. Within their lifetimes, the electron transfer from the trapped electron centre, Ti31, into the p-antibonding orbital of NO takes place and, simultaneously, the electron transfer from the p-bonding orbital of another NO into the trapped hole centre, O
, occurs. These electron transfers led to the direct decomposition of two sets of NO on (Ti31–O
)* into N2 and O2 under UV irradiation in the presence of NO even at 275 K. With the aggregated or bulk TiO2 catalysts, the photo-formed holes and electrons rapidly separate

497

Recent Advances in Single-Site Photocatalysts O2Ti4+ O2- O22NO

O2N2, O2

ground state

h+ (N

O)

O2-

(N

(N

O) hv

Ti4+ O2- O2- O2-

(excitation)

O)

OTi3+

(N

O) e-

O2- O2- O2excited state

Scheme 1

Reaction scheme of the photocatalytic decomposition of NO into N2 and O2 on the Ti-oxide/Y-zeolite catalyst at 275 K.

spatially from each other (with large distances between the holes and electrons), thus, preventing the simultaneous activation of two NO on the same active sites and resulting in the formation of N2O and NO2 in place of N2 and O2. Moreover, the decomposed N and O species react with NO on different sites to form N2O and NO2, respectively. These results clearly demonstrate that the use of zeolites as supports could enable the anchoring of a Ti-oxide species in a highly dispersed state as ‘‘a single site photocatalyst’’ within the zeolite cavities and such tetrahedrally coordinated Ti-oxide photocatalysts are promising candidates for systems to remove toxic NOx compounds from the atmosphere. It was also found that Ti-oxide/Y-zeolite catalysts (ex-Ti-oxide/Y-zeolite and imp-Ti-oxide/Y-zeolite) can act as efficient photocatalysts for CO2 reduction with H2O.15 The photocatalytic reduction of CO2 with H2O into chemically valuable compounds such as CH4 or CH3OH is one of the most desired yet challenging goals in the research of environmentally friendly catalysts, which can simulate artificial photosynthesis. UV irradiation of powdered TiO2 and Ti-oxide/Y-zeolite catalysts in the presence of a mixture of CO2 and H2O led to the evolution of CH4 and CH3OH in the gas phase at 328 K with a good linearity against the UV irradiation time, accompanied by trace amounts of CO, C2H4, C2H6 and O2. The ex-Ti-oxide/Y-zeolite exhibits a high reactivity and selectivity for the formation of CH3OH, while the formation of CH4 was found to be the major reaction on bulk TiO2 as well as the imp-Ti-oxide/Y-zeolite. A clear relationship between the coordination number of the Ti-oxide species and the selectivity for CH3OH formation can be observed (Figure 1), showing the highly efficient photocatalytic reduction of CO2 with H2O into CH3OH using the ex-Ti-oxide/Y-zeolite, which includes the highly dispersed isolated tetrahedral Ti-oxide as the active species.

498

Chapter 28

The reaction mechanism for the photocatalytic reduction of CO2 with H2O was investigated by photoluminescence and ESR analyses.2,15 The addition of H2O or CO2 molecules to the ex-Ti-oxide/Y-zeolite led to an efficient quenching of the photoluminescence as well as shortening of the photoluminescence lifetime, suggesting that the added CO2 or H2O interacts or reacts with the Ti-oxide species in both its ground and excited states. UV irradiation of the anchored Ti-oxide catalyst in the presence of CO2 and H2O at 77 K was also found to lead to the appearance of ESR signals due to the Ti31 ions, H atoms, and carbon radicals.2,15 From these results, the following reaction could be proposed: the CO2 and H2O molecules interact with the excited state of the photoinduced (Ti31–O
)* species and the reduction of CO2 and the decomposition of H2O proceed competitively. Moreover, H atoms and OHd radicals are formed from H2O and react with the carbon species formed from CO2 to produce CH4 and CH3OH. These results clearly demonstrate that single site Ti-oxide photocatalysts incorporated within zeolite cavities can enable such artificial photosynthetic reactions as a CO2 fixation reaction with H2O to produce CH3OH with a high selectivity.2,15

3 Design of Visible Light-Responsive Ti-oxide Single-Site Catalysts Ti-oxide single-site photocatalysts anchored within various zeolites exhibited unique and high photocatalytic activity for various reactions such as the direct decomposition of NO into N2 and O2 or the reduction of CO2 with H2O. However, the isolated tetrahedral Ti41 oxide species, the active site of the Ti-oxide single-site photocatalyst, absorbs UV light of wavelengths below 300 nm since the HOMO–LUMO energy gap of this isolated tetrahedral Ti41 oxide species becomes significantly larger than that of bulk TiO2 due to the size quantization effect. In other words, Ti-oxide single-site photocatalysts cannot utilize the abundant solar energy that reaches the earth, necessitating a UV light source for its use as a photocatalyst. From this viewpoint, photocatalysts that can operate efficiently under both UV and visible light irradiation are the most desired for practical and widespread use. The metal-ion-implantation method has recently been applied to modify the electronic properties of Ti-oxide single-site photocatalysts by bombarding them with high-energy metal ions, leading to the discovery that metal-ion implantation with various transition-metal ions such as V, Cr, accelerated by high electric fields can, in fact, produce a large shift in the absorption band of Ti-oxide single-site photocatalysts towards visible light regions.16 Figure 3 shows the effect of V-ion implantation on the diffuse reflectance UV-Vis absorption spectra of Ti-containing mesoporous materials, Ti/HMS and Ti/MCM-41. Their absorption spectra at around 200–260 nm can be attributed to the charge-transfer absorption process, involving an electron transfer from the O2– to the Ti41 ion of the highly dispersed tetrahedrally coordinated TiO4 unit of these catalysts.16 These spectra shift smoothly towards visible light regions, the extent strongly

499

Recent Advances in Single-Site Photocatalysts

Absorbance / a.u

(a) Ti / HMS

(4) (3) (1)

200

250

(2)

300 350 400 Wavelength / nm

450

500

Absorbance / a.u

(b) Ti / MCM-41

(3)

(2)

(4)

(1)

200

250

300

350

400

450

500

Wavelength / nm

Figure 3

Diffuse reflectance UV-Vis absorption spectra of (a) V-ion-implanted Ti/ HMS and (b) Ti/MCM-41. Amount of implanted V ions: (1) 0, (2) 0.66, (3) 1.3, (4) 2.0 (mmol g-cat
1).

depending on the amount of V ions implanted. These results indicate that the interaction of the implanted V ions with the TiO4 units leads to the modification of the electronic properties of the titanium oxide species within the zeolite frameworks.16 The V K-edge FT-EXAFS spectra of the Ti/HMS catalyst implanted with V ions show that the next neighbours of the V environment are not the same as vanadium-oxide based catalysts (e.g., V2O5) and suggest the formation of tetrahedral titanium oxides having V–O–Ti bonding instead of V–O–V linkages.16 These findings show that the formation of the V–O–Ti bridge structures between the isolated tetrahedral TiO4 unit and implanted V ions affect the electronic structure of the isolated tetrahedral TiO4, leading to a red shift in the absorption spectra of these catalysts.

500

Chapter 28 25 Light on

Light off

N2

Yields/
mol g-TiO2-1

20

15 λ > 390 nm 10

N2O N2

5

λ > 420 nm N2O

0 -1

Figure 4

0

1 2 Time/h

3

Reaction time profiles of the photocatalytic decomposition of NO on Ti/HMS and V ion-implanted Ti/HMS under visible light irradiation (l>390 nm, 420 nm). Amount of implanted V ions: 2.0 mmol/g-cat. The yield of N2(K) and N2O(’) formation on V ion-implanted Ti/HMS; the yield of N2 ( ) and N2O( ) formation on Ti/HMS.

The photocatalytic activity of the V-ion-implanted Ti/HMS and Ti/MCM-41 was investigated for the decomposition of NO into N2 and O2 under visible light irradiation (l>420 nm). As shown in Figure 4, visible light irradiation of the V-ion-implanted Ti/HMS led to the efficient decomposition of NO into N2 and O2, while the unimplanted original Ti/HMS exhibited no activity for the reaction under the same reaction conditions. Moreover, no NO decomposition could be confirmed under UV (l 4 300 nm) or visible light irradiation (l 4 420 nm) on the V-ion-implanted HMS. These results show that ionimplantation is an effective technique for the modification of the electronic properties of Ti-oxide single-site photocatalysts, enabling them to absorb and operate under visible light (l 4 420 nm) as highly efficient photocatalysts.

4 Photocatalysis on Cr-Oxide Single-Site Catalysts Anchored in Zeolite Cavities under Visible Light Irradiation; The Preferential Photocatalytic Oxidation of CO with O2 in the Presence of Excess Amounts of H2 Highly dispersed Mo or Cr oxide catalysts have been shown to exhibit high activity for various photocatalytic reactions such as the photo-oxidation of hydrocarbons17,18 or the photo-induced metathesis of alkanes.6 Recently, Mo or Cr oxide catalysts highly dispersed on mesoporous silica (Mo-MCM-41, Cr-MCM-41) have been reported to exhibit photocatalytic activity for the

501

Recent Advances in Single-Site Photocatalysts

preferential oxidation of CO (PROX reaction) with O2 in the presence of excess amounts of H2.19,20 The PROX reaction has been applied for the removal of CO impurities from H2-rich gas in the development of efficient fuel cell systems with Pt or Rh-loaded catalysts at relatively high temperature. This section deals with the photocatalytic PROX reaction on Cr-MCM-41 under visible or solar light irradiation and the mechanism behind the reaction. The UV-Vis spectrum of Cr-MCM-41 exhibits three distinct absorption bands at around 240, 350 and 460 nm due to the ligand to metal charge transfer transition (LMCT: from O2
to Cr61) of the tetrahedrally coordinated Cr61oxide species.17,20 The local structure of the Cr61-oxide species was also investigated by Cr K-edge XAFS (XANES and EXAFS) measurements. Fourier transform of EXAFS showed only a single peak due to the presence of the neighbouring oxygen atoms (Cr–O) at ca. 0.8–1.5 A˚ without any additional peaks due to the Cr–O–Cr bonds. These results indicate that the Cr61-oxide species exist in a highly dispersed state. Curve-fitting analysis of the Cr–O peaks revealed that the Cr61-oxide species exists in a highly distorted tetrahedral coordination with two shorter Cr¼O double bonds (bond length: 1.59 A˚, coordination number: 2.0) and two longer Cr–O single bonds (bond length: 1.85 A˚, coordination number: 2.1).20 Cr-MCM-41 exhibited a photoluminescence spectrum at 550–800 nm upon excitation at around 500 nm at 293 K. The absorption and emission spectra are attributed to the following charge transfer processes on the Cr¼O moieties of the tetrahedral monochromate species (CrO2
4 ) involving an electron transfer from the O2
to Cr61 ions and a reverse radiative decay from the charge transfer excited triplet state:17,20

[Cr6+ =O2−]

hν

hv'

[Cr5+ −O−]*

The photoluminescence of Cr-MCM-41 was found to be quenched in its intensity by the addition of CO, O2 and H2, indicating that the Cr61-oxide species, in its charge transfer excited triplet state, easily interacts with CO, O2 and H2. The photophysical processes on Cr-MCM-41 in the presence of the quencher molecules can be depicted as follows:20 photoluminescence (kp) [Cr 5+−O−]*

radiationless deactivation (kd) deactivation by quencher (kq)

The Stern–Volmer equation can be obtained for the quenching of the photoluminescence by the quencher molecules by applying a steady-state treatment to the above reaction mechanism, as follows: F0 =F ¼ 1 þ t0 kq ½Q where F0 and F show the yields of the photoluminescence in the absence and presence of the quencher molecules, respectively, and where t0, kq and [Q] are

502

Chapter 28 61

the lifetimes of the charge transfer excited triplet state of the Cr -oxide species in the absence of quencher molecules, the absolute quenching rate constant and the concentration of the quencher molecules, respectively. The F0/F values exhibited a good linear relationship with the concentrations of the quencher molecules. The kq value (l/mol s) for each gas was determined by the slope of the Stern-Volmer plots and were found to increase in the following order: H2 (8.63  105) { CO (5.91  109) o O2 (1.12  1010).20 The photocatalytic preferential oxidation of CO with O2 in the presence of H2 was investigated on Cr-MCM-41 at 293 K. Visible light irradiation (l>420 nm) of the catalyst led to the efficient oxidation of CO into CO2, accompanied by the stoichiometric formation and consumption of CO2 and O2, respectively, as shown in Figure 5. The concentration of the CO gas reached below 8 ppm after light irradiation of 150 min, while the amount of H2 remained almost constant. CO conversion and selectivity reached B100% and 97%, respectively, after visible light irradiation of 150 min. The reaction was also found to proceed efficiently even under solar light irradiation.20 In order to elucidate the reaction mechanism, FT-IR investigations were carried out. Visible light irradiation of Cr-MCM-41 in the presence of CO led to the appearance of a typical FT-IR band at 2201 cm
1 due to the monocarbonyl Cr41 species [Cr41(CO)], accompanied by the formation of CO2.20 The addition of O2 to these systems under dark conditions led to the complete 25 H2

Amounts of gasses /µmol

23

Dark

Light on

~ ~ 8

O2

6

CO2 4

2 CO 0

0

30

60

90

120

150

180

Reaction Time / min

Figure 5

Reaction time profiles of the photocatalytic oxidation of CO with O2 in the presence of H2 on Cr61-MCM-41 under visible light irradiation (l 4 420 nm). (Initial amount of gases: CO: 3.8 mmol; O2: 7.5 mmol; and H2: 24.6 mmol.)

503

Recent Advances in Single-Site Photocatalysts Tetrahedrally coordinated Cr6+oxide species in Cr-MCM-41

4+

Cr6+ O

Reoxidation of Cr reduced-species by O2

O2-

O2-

CO

hv

O

Charge transfer excited triplet state

1/2 O2

O2O2-

*

Cr5+

CO O

Cr4+ O

O-

O

H2

O H2O

2 CO CO2

Scheme 2

Reduction of Cr-oxide species by CO

Complete reaction cycle for the photocatalytic oxidation of CO with O2 in the presence of H2 on Cr61-MCM-41.

disappearance of these FT-IR bands. These results clearly suggest that the Cr61-oxide species reacts with CO in its photo-excited state and is reduced into the Cr41 carbonyl species, while these species are easily oxidized by O2 into the original Cr61-oxide species. From these results, the catalytic reaction cycles on Cr-MCM-41 can be proposed as in Scheme 2. Initially, the tetrahedral Cr61oxide species is photo-excited to its charge transfer excited triplet state and reacts with CO to form CO2 and a photo-reduced Cr41 carbonyl species. The Cr41 oxide species is then efficiently oxidized by O2 and the original Cr61-oxide species is generated. The high CO selectivity can be attributed to the high and selective reactivity of the photo-excited Cr61-oxide species with CO, as indicated by the high quenching efficiency of CO as compared to H2.

5 Photocatalytic Reactivity of the Cu1/ZSM-5 Catalyst for the Decomposition of NO into N2 and O2 This section focuses on investigations on the characteristics of the Cu1 species anchored onto the nano-pores of the ZSM-5 zeolite by in situ photoluminescence, ESR, XAFS and UV-Vis analyses and their reaction with NO under UV light irradiation. Cu1/ZSM-5 and Cu1/Y-zeolite catalysts were prepared by evacuation of the Cu21/ZSM-5 and Cu21/Y-zeolite samples prepared by an ion-exchange

504

Chapter 28 11,12

1

method at 973 K. UV irradiation of the Cu ion catalysts in the presence of NO even at 275 K was found to lead to the formation of N2 and O2, with a good linear relationship between the irradiation time and NO conversion. The formation of by-products such as N2O and NO2 was negligible. Cu1/ZSM-5 exhibited photoluminescence at around 440 nm due to the isolated Cu1 ions under UV irradiation at around 300 nm. The yields of the photocatalytic decomposition reaction of NO greatly depend on the pretreatment degassing temperature of the Cu21/ZSM-5 samples, showing a good parallel relationship with the dependency of the intensity of the photoluminescence due to the Cu1 ions. These results indicate that the photo-excited states of the isolated Cu1 ion as a single site photocatalyst play a significant role in the photocatalytic decomposition of NO.11,12 Moreover, the Cu1/ZSM-5 catalyst exhibits higher photocatalytic activity as compared to the Cu1/Y-zeolite catalyst. Considering that Cu1/Y-zeolite exhibits a typical photoluminescence (ca. 520 nm) due to the Cu1 dimer species, it could be concluded that the photocatalytic reactivity of the Cu1 monomer species is higher than that of the Cu1 dimer species. In fact, the photoluminescence spectrum of the Cu1/ZSM-5 catalyst was quenched more efficiently by the addition of NO than for the Cu1/Y-zeolite, indicating that the photo-excited state of the Cu1 monomer species interacts with NO more efficiently than with the Cu1 dimer species.11,12 The addition of NO onto Cu1/ZSM-5 led to the appearance of FT-IR and ESR signals due to the Cu1–NOd
adduct species. UV irradiation of the Cu1/ZSM-5 catalyst having a Cu1–NOd
adduct species led to a decrease in the intensity of the ESR signal assigned to the Cu1–NOd
species with

O

Cu+ (3d10) O

Cu2+

evac. O

O

reduction NO

Monomer

adsorption

(N O)ad

NOδCu+ δ+ O O

nitrosyl

electron transfer into copper N2+O2 NO

adduct Cu h

Cu+ *(3d94s1) excited state

Scheme 3

+*(3d9

) (N O)ad

electron transfer into -anti-bonding orbital of NO

Reaction scheme of the photocatalytic decomposition of NO into N2 and O2 on the Cu1/ZSM-5 catalyst at 298 K (& denotes an electron vacancy).

Recent Advances in Single-Site Photocatalysts

505

irradiation time, without the appearance of any new signal. After UV irradiation was discontinued, the intensity of the signal returned to its original level. These reversible changes in the ESR signal assigned to the Cu1–NOd
adduct species indicate not only that the Cu1–NOd
species acts as a reaction precursor but also that the photo-induced decomposition of NO proceeds catalytically. Together with the results obtained by in situ photoluminescence, ESR, and FT-IR measurements, the mechanism for the photocatalytic decomposition of NO into N2 and O2 on the Cu1/ZSM-5 catalyst at 275 K under UV irradiation could be proposed, as follows (Scheme 3): electron transfer from the excited state of the Cu1 (3d94s1 state) to a p-anti-bonding orbital of NO and simultaneous electron transfer from the p-bonding orbital of another NO to the vacant electronic state of the Cu1 ion (3d94s0 state) occurs, causing local charge separation and a weakening of the N–O bond of the two NO molecules, thus initiating the decomposition of NO into N2 and O2.11,12

6 Conclusions The activity of various transition metal oxides (Ti, Cr) incorporated within the zeolite framework structures as well as transition metal ions (Cu1) exchanged into zeolite cavities as single-site heterogeneous photocatalysts have been outlined here. These single-site heterogeneous photocatalysts with a coordinatively unsaturated coordination sphere could induce unique photocatalyst reactions such as the direct decomposition of NO into N2 and O2 or reduction of CO2 with H2O. Moreover, an ion-implantation method was shown to be an effective method to prepare visible light-responsive Ti-oxide loaded zeolite photocatalysts. And in the case of Cr-MCM-41, the preferential selective oxidation of CO impurities in H2 by visible light irradiation was demonstrated. It should be emphasized that the use of zeolites as a support made it possible to control the local structure of the highly dispersed transition metal oxides or ions at the atomic level, leading to a precise control of the photocatalytic activity as well as selectivity of a reaction. It can, thus, be seen that zeolitic frameworks offer one of the most promising approaches in designing single-site photocatalysts for the development of effective new systems to reduce and eliminate global air and water pollution by harvesting visible or solar light.

References 1. M. Anpo, Bull. Chem. Soc. Jpn., 2004, 77, 1427. 2. M. Anpo, S. Dohshi, M. Kitano, Y. Hu, M. Takeuchi and M. Matsuoka, Annu. Rev. Mater. Res., 2005, 35, 1. 3. M. Kitano, K. Funatsu, M. Matsuoka, M. Ueshima and M. Anpo, J. Phys. Chem. B, 2006, 110, 25266. 4. M. Kitano, M. Takeuchi, M. Matsuoka, J.M. Thomas and M. Anpo, Catal. Today, 2007, 120, 133.

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5. M. Anpo and M. Che, Adv. Catal., 2000, 44, 119. 6. M. Anpo, T. Shima, S. Kodama and Y. Kubokawa, J. Phys. Chem., 1987, 91, 4305. 7. M. Anpo and J.M. Thomas, Chem. Commun., 2006, 3273. 8. H. Yamashita, K. Maekawa, H. Nakao and M. Anpo, Appl. Surf. Sci., 2004, 237, 393. 9. Y. Hu, N. Wada, K. Tsujimaru and M. Anpo, Catal. Today, 2007, 120, 139. 10. M. Anpo, M. Kondo, Y. Kubokawa, C. Louis and M. Che, J. Chem. Soc. Faraday Trans. 1, 1988, 84, 2771. 11. M. Matsuoka and M. Anpo, Curr. Opin. Solid State Mater. Sci., 2003, 7, 451. 12. M. Anpo, M. Matsuoka, K. Hanou, H. Mishima, H. Yamashita and H.H. Patterson, Coord. Chem. Rev., 1998, 171, 175. 13. H. Courbon and P. Pichat, J. Chem. Soc., Faraday Trans. 1, 1984, 80, 3175. 14. H. Yamashita, Y. Ichihashi, M. Anpo, M. Hashimoto, C. Louis and M. Che, J. Phys. Chem., 1996, 100, 16041. 15. M. Anpo, H. Yamashita, Y. Ichihashi, Y. Fujii and M. Honda, J. Phys. Chem., 1997, 101, 2632. 16. M. Anpo and M. Takeuchi, J. Catal., 2003, 216, 505. 17. H. Yamashita, K. Yoshizawa, M. Ariyuki, S. Higashimoto, M. Che and M. Anpo, Chem. Commun., 2001, 435. 18. S. Higashimoto, R. Tsumura, S.G. Zhang, M. Matsuoka, H. Yamashita, C. Louis, M. Che and M. Anpo, Chem. Lett., 2000, 408. 19. T. Kamegawa, R. Takeuchi, M. Matsuoka and M. Anpo, Catal. Today, 2006, 111, 248. 20. T. Kamegawa, J. Morishima, M. Matsuoka, J.M. Thomas and M. Anpo, J. Phys. Chem. C, 2007, 111, 1076.

CHAPTER 29

Structural Organization of Catalytic Functions in Mo-Based Selective Oxidation Catalysts MASAHIRO SADAKANE AND WATARU UEDA Catalysis Research Center, Hokkaido University, North 21, West 10, Kita-ku, Sapporo, 001-0021, Japan

1 Background and Introduction About one-third of industrial organic products are synthesized via catalytic selective oxidation processes, and continuous efforts have been devoted to the development of high performance catalysts because of the high demand for the effective utilization of chemical resources in response to environmental concerns.1 Solid-state catalysts like mixed metal oxides used for industrial oxidation processes have, generally speaking, been composed of complicated compositions and phase-mixtures so far, but the next generation of catalysts will have to be composed of much more advanced and well-organized materials possessing multi-catalytic functions. In this context, it seems highly important to develop new rationale synthetic methods for solid-state catalysts in order to replace the conventional catalyst preparation method, the so-called mixed-andbaked, which still, however, has various merits in aspects of industrial production, such as cost, easy to obtain starting metal materials, large-scale production, and so forth. On stepping forward to the next generation of catalyst preparation and putting the current preparation systems aside, an example of crystalline Moand V-based complex metal oxide catalyst development recently initiated by Mitsubishi Chemicals is highly informative in terms of structural organization of catalytic functions. The catalyst is a crystalline MoVTe(Sb)NbO mixed oxide which shows high catalytic performance in the propane oxidation to acrylic acid and the ammoxidation of propane to acrylonitrile (AN).2 Although the crystal structure of the catalyst has not yet been fully confirmed, it is 507

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ascertained that the oxide is a layered (orthorhombic) structure with a slab consisting of six- and seven-membered ring units with MO6 octahedra and pentagonal {(M)M5O27} units with an MO7 pentagonal bipyramid and five edge-sharing MO6 octahedra (Figures 1a and b). There are four pentagonal units, six-membered rings, and seven-membered rings in one unit cell which is isotypic with Cu–Nb–O–x (x ¼ Cl, Br, I) and Cs–Nb–W–O systems (Figures 1c and d).3 The main components are Mo and V, and Nb and Te (or Sb) can exist in the structure as minor elements. The Te and Sb are believed to be located in the hexagonal rings.4 Our recent works on this catalyst system evidently show that each element arranged individually in the structure and the elemental network constructed by the structural units (Figure 1) can cooperatively work in the course of the propane oxidation at the same time and accomplish the catalytic cycle effectively. This is a prominent example demonstrating that multi-step reactions such as propane oxidation cannot proceed by the admixture of several active sites working separately, but can proceed over the zone of active site network formed structurally and spatially. There is no doubt that this happened because the structural materials were discovered, and this obviously marks a turning point in solid-state oxidation catalysts. Our basic research revealed at the same time that the evolution of the structural materials is sustained by a new-type formation process of solid-state materials which is completely different from the conventional ‘‘mixed-and-baked’’ method.

Figure 1

Ball and stick (a) and polyhedral (b) presentation of the pentagonal unit and orthorhombic Mo3VOx, a–b plane (c) and b–c plane (d).

Structural Organization of Catalytic Functions

509

We describe here our recent achievements including (1) the formation mechanism of orthorhombic and trigonal Mo–V oxides, both of which contain the same building units (six- and seven-membered rings and pentagonal unit) with different ratios, (2) an outstanding catalytic performance of the orthorhombic Mo3VOx and the trigonal Mo3VOx in the selective oxidation of acrolein to acrylic acid, and (3) propane ammoxidation catalyzed by the pure orthorhombic Mo–V based mixed metal oxide with and without additional metals (Nb, Te or Sb), which allows us to discuss the roles of each element and structural unit.

2 Synthesis, Structural Characterization, and Formation Mechanism of Orthorhombic and Trigonal Mo3VOx5a Both the orthorhombic and trigonal Mo3VOx mixed metal oxides were synthesized by hydrothermal treatment of a mixture of ammonium heptamolybdate (NH4)6Mo7O24
4H2O and vanadyl sulfate VOSO4
nH2O (Mo 50 mmol and 12.5 mmol) in H2O (240 mL) (Figure 2). By controlling the pH of the precursor solution with sulfuric acid, two types of crystalline Mo3VOx with orthorhombic structure and trigonal structure were synthesized at pH ¼ 3.2 and pH ¼ 2.2, respectively. Since the crude materials contained an amorphous phase as a by-product in each case, the materials were washed with an aqueous solution of oxalic acid for removal. Metal composition was determined by ICPAES and Mo/V was found to be ca. 3 for both materials.

Figure 2

Synthesis of orthorhombic and trigonal Mo3VOx.

510

Figure 3

Chapter 29

HRTEM images and selected-area electron diffraction (SAED) patterns (insets) of orthorhombic Mo3VOx viewed along the [001] direction (a) as well as the corresponding simulated contrast calculated for a crystal thickness close to 24 nm and a defocus value Df ¼ 130 nm (b). L and S indicate large and small spots, respectively.

Fundamental structures of the orthorhombic and trigonal Mo3VOx were constructed on the basis of the HRTEM images of each sample. Figure 3 represents HRTEM images along the [001] zone axis of the orthorhombic Mo3VOx. An ordered arrangement of two kinds of white spots (large one and small one indicated as L and S in Figure 3, respectively) was observed in accordance with the structure shown in Figure 1c. The large spot and small spot clearly correspond to the seven-membered and six-membered rings of the octahedra, respectively. The pentagonal rings were surrounded by three seven-membered rings and two six-membered rings. A crystal structure of the trigonal Mo3VOx was constructed under the same assumption. Three large white spots corresponding to the seven-membered ring and two small spots corresponding to the six-membered ring were found in one unit cell (Figure 4c). The pentagonal unit was placed in a position surrounded by three seven-membered rings and two six-membered rings, then producing the structure presented in Figure 4a. The trigonal structure produced was confirmed by analyzing the powder XRD pattern using the Rietveld method. The XRD pattern was well-simulated using the structure (with lattice parameters a ¼ 21.433(3) A˚, c ¼ 4.0045(18) A˚, space group P3 (No.143)) and reasonably converged (Rwp ¼ 12.23%). From the above structural analysis, two Mo3VOx solids are regarded as structure variants with the same structural units as the pentagonal ring, the sixmembered ring, and the seven-membered ring but with different arrangements of them. Then one can easily assume that these kinds of solid materials are constructed by stacking of the pentagonal ring units. This interesting crystal formation scheme could be justified by the following facts. As described above, the orthorhombic or trigonal Mo3VOx were synthesized from the precursor solution with a colour of dark violet, which was instantly obtained by mixing the solutions of ammonium heptamolybdate and vanadyl sulfate. We found that the coloured solution gave characteristic

Structural Organization of Catalytic Functions

511

Figure 4

Polyhedral presentation of trigonal Mo3VOx: a–b plane (a) and b–c plane (b). HRTEM images and SAED patterns (insets) of trigonal Mo3VOx viewed along the [001] direction (c) as well as the corresponding simulated contrast calculated for a crystal thickness close to 20 nm and a defocus value Df ¼ 155 m (d). L and S indicate large and small spots, respectively.

Figure 5

Raman spectra (left) of Mo–V solution (Mo/V ¼ 4, Mo ¼ 0.21 M, pH 3.3) (a), Mo–V solution (Mo/V ¼ 4, Mo ¼ 0.21 M, pH 2.2) (b), aqueous Mo72V30 solution (1 g, 30 mL1) (c), aqueous Mo132 solution (1 g, 30 mL1) (d), and aqueous Mo57V6 solution (1 g, 30 mL1) (e). UV–Vis spectra (right) of Mo–V solution (Mo/V ¼ 4, Mo ¼ 0.42 M, pH 3.3) (linear line) and aqueous Mo72V30 solution (dotted line).

Raman bands at 1000–700 cm1 which were completely different from those of each solution before mixing. Then, the Raman spectrum of the precursor solution before the hydrothermal reaction (Figures 5a and b) was compared with that of the solution of three different polyoxomolybdates,

512

Figure 6

Chapter 29

Polyhedral presentation of Mo72V30{[K10Mo72V30O282(H2O)56(SO4)12]26} (a), Mo132{[Mo132O372(H2O)72(CH3CO2)30]42} (b), and Mo57V6{[H3Mo57 V6(NO)6O183(H2O)18]21} (c). Grey polyhedra represent the pentagonal unit and black balls represent vanadium metal.

Mo72V30{[K10Mo72V30O282(H2O)56(SO4)12]26}, Mo132{[Mo132O372(H2O)72(CH3CO2)30]42}, and Mo57V6{[H3Mo57V6(NO)6O183(H2O)18]21}, all of which exhibit the pentagonal {(Mo)Mo5} unit in the discrete structure. Very similar characteristic Raman peaks were observed at 1000–700 cm1 for the polyoxomolybdates (Mo72V30 (Figure 6a),6 Mo132 (Figure 6b),7 and Mo57V6 (Figure 6c)8), shown in Figures 5c–e, respectively. The UV–Vis spectrum of the solution was also similar to that of Mo72V30 (Figure 5, right), where the IVCT (Inter Valence Charge Transfer) (VIV - MoVI) peak was observed at ca. 510 nm.6 The Mo72V30 polyoxomolybdate has 12 pentagonal {(MoVI)MoVI5} units connected by 30 vanadium (IV).6 From these results, we conclude that the pentagonal {(Mo)Mo5} unit is present in the precursor solution before the hydrothermal reaction and interacts with VO21 cations to form a polyoxo-type species in the solution. In fact, discrete polyoxomolybdates have been prepared from room temperature to 363 K by a reaction of the pentagonal units with molybdenum and other elements.9 The pH in this case is known to be one of the most important factors for this formation. Similarly, the formation of the orthorhombic or trigonal Mo3VOx depended on the pH, so that selection of the solid-state phase is controlled by the pH dependent polyoxo-type species with the pentagonal unit. Hence, assembly of the polyoxo-type species consisting of the pentagonal unit could occur under hydrothermal conditions to form three-dimensional metal oxide solids as illustrated in Figure 7. We believe that the unit assembly under hydrothermal conditions will be employed in many cases, and creates new-type crystal solid materials having high-dimensional networks of catalytic elements, and ultimately brings about extremely high performance catalysts.

3 Crystalline Mo3VOx as a True Catalytic Phase for Acrolein Oxidation As summarized in Table 1, the trigonal Mo3VOx catalyst has an outstanding catalytic performance for oxidation of acrolein to acrylic acid; the conversion

513

Structural Organization of Catalytic Functions

Figure 7

Image of formation of orthorhombic Mo3VOx from the pentagonal unit under hydrothermal conditions.

Table 1

Acrolein oxidation over the orthorhombic and trigonal Mo3VOx catalysts.a

Catalyst

Surface area (m2 g1)

Conversion (%)

Trigonal Mo3VOx Orthorhombic Mo3VOx

15.6 14.9

99.7 99.4

Selectivity (%) b

AA 90.5 93.6

AcAc 3.2 1.5

COx 6.3 4.9

a

Reaction condition: amount of catalyst, 0.45 g diluted by 0.05 g of SiC; gas composition, acrolein/ O2/H2O/N2 ¼ 4.8/7.6/26.9/60.6; total flow, 103.3 mL min1; reaction temperature, 463 K. Both catalysts were calcined at 673 K under nitrogen flow before the reaction test. b AA, acrylic acid. c AcA, acetic acid.

of acrolein was achieved to almost 100% at 463 K and the selectivity to acrylic acid was 90%. The orthorhombic Mo3VOx catalyst shows similar high catalytic performance. The catalytic activities attained over the catalysts are significantly superior to those of Mo–V based oxide catalysts reported in patents and papers; the patent catalysts need much higher reaction temperature (usually more than 500 K)10 in order to obtain similar activity to that of the present Mo3VOx catalysts. The main differences between the patent catalysts and the present Mo3VOx catalysts are the preparation method, the former conventional and the latter hydrothermal, and structure, the former being XRD-disordered materials and the latter crystalline solids. Since the XRD-disordered materials

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have the same elemental composition as the crystalline Mo3VOx, the less active XRD-disordered materials might be constructed with the structural units in a disordered fashion, while the highly active crystalline Mo3VOx is constructed in an ordered fashion, thereby allowing for more active sites on the surface. The seven-membered ring site is likely responsible for the oxidation. As a consequence, the trigonal and/or orthorhombic Mo3VOx are potentially true catalytic active phases in the industrial Mo–V based oxide catalysts for acrolein oxidation to acrylic acid.

4 Catalytic Role of Constituents in Orthorhombic Mo3VOx Based Catalyst for Propane Ammoxidation5b Nb, Te, and Sb are common additional elements for the orthorhombic Mo–V based catalyst in order to enhance catalytic performance. However, the catalysts prepared by the conventional method contain impurities (the most common of which has pseudohexagonal structure), and the true roles of each element have been unclear.11 We have recently achieved the preparation of five single-phase Mo–V based mixed metal oxides having the same orthorhombic structure (Figure 1c), Mo-V-O, Mo-V-Te-O, Mo-V-Sb-O, Mo-V-Te-Nb-O and Mo-V-Sb-Nb-O, by modifying the hydrothermal conditions (Table 2). It has been confirmed that Te and Sb sit in the six-membered rings and Nb substitutes for the Mo and/or V site.3 We tested these single-phase catalysts for the propane ammoxidation to acrylonitrile (AN) and elucidated the roles of the constituent elements in the reaction. Propane conversions and product distributions are compared at ca. 680 K, the temperature at which the least stable Mo-V-O can exist without decomposition (Table 3). All the catalysts were found to be active to the same extent for Table 2

Elemental compositions and surface areas of orthorhombic Mo–V based oxide catalysts. Composition (Mo/V/Te or Sb/Nb) Bulka

Catalysts Mo-V-O Mo-V-Te-O

1.00/0.34 1.00/0.36/0.06

Mo-V-Te-Nb-O

1.00/0.25/0.13/ 0.11 1.00/0.33/0.06

Mo-V-Sb-O Mo-V-Sb-Nb-O a b

1.00/0.33/0.11/ 0.11

Determined by ICP-AES. Determined by XPS.

Surfaceb 1.00/0.23/–/– 1.00/0.23/ 0.08/– 1.00/0.11/0.25/ 0.16 1.00/0.16/ 0.08/– 1.00/0.15/0.19/ 0.20

Calcination

Surface area (m2 g1)

N2, 773 K Air, 553 K + N2, 873 K N2, 873 K

6.5 12.2

Air, 593 K + N2, 873 K N2, 873 K

13.8

19.8

11.2

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Structural Organization of Catalytic Functions

Table 3

Product distribution in the ammoxidation of propane over Mo–V based catalysts. Conversion (%)a

Catalyst

Reaction temperature (K)

C3H8

NH3

Mo-V-O Mo-V-Te-O Mo-V-Te-Nb-O Mo-V-Sb-O Mo-V-Sb-Nb-O

682 683 682 683 682

24.2 47.7 52.7 42.5 33.8

25.2 23.0 33.2 28.7 19.0

a

b

Selectivity (%)b O2

AN

PEN

AcN

COx

51.9 59.9 57.7 58.4 39.6

25.2 38.8 61.0 48.9 46.6

28.4 22.1 15.8 18.4 26.3

5.3 3.5 4.1 4.0 4.1

40.0 35.4 18.2 28.3 22.4

Reaction condition: amount of all catalysts, 0.5 g; total flow rate, 52.5 mL min1; gas composition, C3H8/NH3/O2/He ¼ 5.7/8.6/17.1/68.6 (%). AN, acrylonitrile; PEN, propene; AcN, acetonitrile. The selectivities to hydrogen cyanide, acetic acid, and acrylic acid were less than 1%.

Figure 8

Proposed propane ammoxidation pathways used for calculation of rate constants.

propane ammoxidation. The propane conversions of the Te-containing catalysts, however, tended to be slightly higher than those of the Sb-containing catalysts. Despite the fundamental differences in the catalyst compositions (for data see Ref. 5b), the rates of propane oxidation per unit surface area of the catalysts were close in the entire reaction temperature range of 633–713 K. It is, therefore, rational to assume that Mo and V as common constituent elements largely contribute to the creation of active sites for the activation of propane. On the other hand, product selectivity is strongly dependent on the catalyst compositions. Over the Mo-V-Te-O, Mo-V-Sb-O, Mo-V-Te-Nb-O and Mo-V-Sb-Nb-O catalysts, AN was the major product, whereas overoxidation products such as COx were mainly produced over the Mo-V-O catalyst. In order to quantitatively clarify the role of each element in the course of the propane ammoxidation, we compared five catalysts in terms of kinetics on the basis of the proposed ammoxidation network (Figure 8), where propane is first oxidized to propene, then propene is further converted to AN, and at the same time the direct route from propane to AN and COx was also considered. We calculated the initial reaction rate of propane at 683 K and the activation energy for propane oxidation by fitting product (AN, propene and COx) distribution

b

131 105 131 122 107

0.84 0.82 0.81 0.84 0.86

k1 5.77 8.89 10.37 9.82 7.20

k2 2.46 1.68 0.46 1.23 1.99

k3

13.17 3.79 4.20 5.06 2.34

k4

Relative rate constanta

Reaction conditions: reaction temperature, 683 K; W/F, 0.0012–0.019 gcat min mL1; C3H8/NH3/O2/He ¼ 5.7/8.6/17.1/68.6 (%). Reaction conditions: reaction temperature, 653–683 K; W/F, 0.0012 gcat min mL1; C3H8/NH3/O2/He ¼ 5.7/8.6/17.1/68.6 (%).

Mo-V-O Mo-V-Te-O Mo-V-Te-Nb-O Mo-V-Sb-O Mo-V-Sb-Nb-O

a

9.6 13.8 12.6 12.6 9.6

Catalyst

Acitivation energy (kJ mol1)b

0.02 0.06 0.02 0.02 0.03

k5

0.14 0.12 0.17 0.14 0.11

k6

Initial reaction rates, activation energy for C3H8 conversion, and simulated rate constants of Mo–V based oxide catalysts.

Initial rate (mmol m2 min1)a

Table 4

516 Chapter 29

Structural Organization of Catalytic Functions

517

versus propane conversion obtained under various contact times (for details see Ref. 5b). The results are summarized in Table 4. From these data, it is clear again that Mo and V in the octahedral network in the orthorhombic structure are responsible for the activation of propane. Te or Sb as a third constituting element plays an important role in the allylic oxidation of the formed propene, yielding a dramatic improvement in the AN selectivity. Moreover, the oxidative decomposition of ammonia is also suppressed by Te or Sb. Nb, which seems to have a dilution effect on V located in the network, bringing about a further improvement of the AN selectivity, resulting in the Mo-V-Te-Nb-O catalyst exhibiting the highest AN yield. The cooperative actions of each catalytic element in the course of the ammoxidation of propane are very remarkable and undoubtedly are sustained by the high-dimensional structure.

5 Turning Point in the Synthesis of Selective Oxidation Catalysts The selective oxidation catalysts based on metal oxides have become more and more complicated due to the needs of achieving higher yield and extreme selectivity. While it is inevitable that this trend will continue, at the same time the creation of new types of effective catalysts is becoming more difficult. With conventional preparation methods, it has been very hard to prepare crystalline complex multi-elemental oxide catalysts in pure form, yet they will be very important catalytic materials for the next generation of oxidation catalytic systems. For instance, only the hydrothermal method could produce the crystalline form of Mo3VOx phase materials, as described. As far as the conventional preparation method has to be used in synthesizing complex metal oxide catalysts, precise controls on many preparative factors are absolutely necessary. But they are not always possible. We now need a clear design methodology for catalytically active sites and spatial zones. The unit organization process to produce high-dimensional solid materials as demonstrated here seems promising and is likely to become more important in the evolution of metal oxide catalysts such as zeolite catalysts.

References 1. (a) G. Centi, F. Cavani and R. Trifiro, in Selective Oxidation by Heterogeneous Catalysis, ed. M.T. Twigg and M.S. Spencer, Kluwer Academic/ Plenum Publishers, New York, 2001; (b) B.K. Hodnett, Heterogeneous Catalytic Oxidation, Wiley, New York, 2000. 2. (a) T. Uchikubo, K. Oshima, A. Kayou, T. Umezawa, K. Kiyono and I. Sawaki, Mitsubishi Chem. Corp., Patent EP 529853, 1993; (b) T. Uchikubo, Y. Koyasu and S. Wajiki, Mitsubishi Chem. Corp., Patent EP 608838, 1994; (c) T. Uchikubo, K. Oshima, A. Kayou, M. Vaarkamp and M. Hatano, J. Catal., 1997, 169, 394; (d) H. Tsuji and Y. Koyasu, J. Am. Chem. Soc., 2002, 124, 5608.

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3. (a) H. Tsuji, K. Oshima and Y. Koyasu, Chem. Mater., 2003, 15, 2112; (b) P. DeSanto Jr., D.J. Buttrey, R.K. Grasselli, C.G. Lugmair, A.F. Volpe Jr., B.H. Toby and T. Vogt, Z. Kristallogr., 2004, 219; (c) P. DeSanto Jr., D.J. Buttrey, R.K. Grasselli, C.G. Lugmair, A.F. Volpe, B.H. Toby and T. Vogt, Top. Catal., 2003, 23, 23; (d) R. Ross, B. Kratzheller and R. Gruehn, Z. Anorg. Allg. Chem., 1990, 587, 47; (e) M. Lundberg and M. Sundberg, Ultramicroscopy, 1993, 52, 429. 4. (a) J.M.M. Millet, H. Roussel, A. Pigamo, J.L. Dubois and J.C. Jumas, Appl. Catal. A: Gen., 2002, 232, 77; (b) M. Baca and J.M.M. Millet, Appl. Catal. A: Gen., 2005, 279, 67. 5. (a) M. Sadakane, N. Watanabe, T. Katou, Y. Nodasaka and W. Ueda, Angew. Chem. Int. Ed., 2007, 46, 1493; (b) N. Watanabe and W. Ueda, Ind. Eng. Chem. Res., 2006, 45, 607; (c) T. Katou, D. Vitry and W. Ueda, Catal. Today, 2004, 91–92, 237; (d) D. Vitry, Y. Morikawa, J.L. Dubois and W. Ueda, Appl. Catal. A, 2003, 251, 411; (e) T. Katou, D. Vitry and W. Ueda, Chem. Lett., 2003, 32, 1028. 6. (a) A. Mueller, A.M. Todea, J. Van Slageren, M. Dressel, H. Boegge, M. Schmidtmann, M. Luban, L. Engelhardt and M. Rusu, Angew. Chem. Int. Ed., 2005, 44, 3857; (b) B. Botar, P. Koegerler and C.L. Hill, Chem. Commun., 2005, 3138. 7. A. Mueller, E. Krichemeyer, H. Boegge, M. Schmidtmann and F. Peters, Angew. Chem. Int. Ed., 1998, 37, 3360. 8. A. Mueller, E. Krichemeyer, S. Dillinger, H. Boegge, W. Plass, A. Proust, L. Dloczik, C. Meyer and R. Rohlfing, Z. Anorg. Allg. Chem., 1994, 620, 599. 9. Reviews: (a) A. Mueller, Chem. Commun., 2003, 803; (b) A. Mueller, P. Koegerler and A.W.M. Dress, Coord. Chem. Rev., 2001, 222, 193; (c) L. Cronin, P. Kogerler and A. Mueller, J. Solid. State Chem., 2000, 152, 57; (d) A. Mueller and C. Serain, Acc. Chem. Res., 2000, 33, 2; (e) A. Mueller, P. Koegerler and C. Kuhlmann, Chem. Commun., 1999, 1347. 10. (a) H. Hibst, A. Tenten and L. Marosi (BASF Aktiengesellschaft), Patent EP 774297, 1997; (b) J. Tichyi, Appl. Catal. A, 1997, 157, 363; (c) W.-H. Lee, K.-H. Kang, D.-H. Ko, Y.-C. Byun (Lg Chemical), Patent US 6171998, 2001; (d) M. Tanimoto, D. Nakamura and H. Yunoki (Nippon Shokubai Co. Ltd.), Patent EP 1106248, 2001. 11. (a) J.M. Oliver, J.M. Lopez Nieto and P. Botella, Catal. Today, 2004, 96, 241; (b) T. Blasco, P. Botella, P. Concepcion, J.M. Lopez Nieto, A. Martinez-Arias and C. Prieto, J. Catal., 2004, 228, 362; (c) R.K. Grasselli, J.D. Burrington, D.J. Buttrey, P. DeSanto Jr., C.G. Lugmair, A.F. Volpe Jr. and T. Weingand, Top. Catal., 2003, 23, 5; (d) E.K. Novakova, J.C. Vedrine and E.G. Derouane, J. Catal., 2002, 211, 235; (e) T. Shishido, T. Konishi, I. Matsuura, Y. Wang, K. Takaki and K. Takehira, Catal. Today, 2001, 71, 77; (f) P. Botella, J.M. Lopez Nieto, B. Solsona, A. Mifsud and F. Marquez, J. Catal., 2002, 209, 445; (g) L. Luo, J.A. Labinger and M.E. Davis, J. Catal., 2001, 200, 222.

CHAPTER 30

Designing Active Sites for Surfaces: From Tightly Bound to Loosely Anchored THOMAS MASCHMEYER Laboratory of Advanced Catalysis for Sustainability, School of Chemistry, Building F11, The University of Sydney, NSW 2006, Australia

Our understanding of catalytic sites has increased exponentially over the last two to three decades. It is now possible to speak of ‘designing’ a catalytic site without exaggeration. Advances in experimental and theoretical techniques, for example synchrotron-based in situ investigations or the sheer calculational power now available, already in the humble desktop computer, have made this possible. There were many ‘turning points in catalysis’ along this path, not a few of which were directly or indirectly associated with Prof. Sir John Meurig Thomas. I have had the great fortune and pleasure to be ‘in his orbit’ one way or another since 1995 – a circumstance which influenced me greatly scientifically as well as personally. In the following pages it is my aim to re-visit,w as well as highlight new, exciting developments in the fascinating area that is single site heterogeneous catalysis. Homogeneous catalysts are more easily studied than their heterogeneous counterparts, for the obvious reason that they are single site catalysts dissolved in a well-defined reaction medium. Spectroscopic responses of active sites in heterogeneous catalysts are often masked by the response of the bulk material and it is very difficult to tease out structural information of the active site, even w

The work involving metallocene grafting as well as cobalt acetate and diphenylphosphinoferrocenyl (dppf ) tethering was my first contribution during my stay with Sir John as Australian Bicentennial Research Fellow at the Royal Institution (RI); the second contribution was the carbonyl cluster work discussed here, when EU Marie Curie joint fellow at the RI and Cambridge with Sir John and Prof. Johnson.

519

520

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before catalysis, let alone during catalysis – particularly if the support is amorphous and/or if the active sites are not crystallographically distinct. One way of slightly mitigating this difficulty is to start with a well-defined homogeneous species that can be immobilised in some way on a support surface. With such an approach it is possible to select/design the active site precursor with great confidence regarding its structural features and then to match these to the expected surface reactivity. Approaching the design of heterogeneous catalysts in this way, one is in a much more comfortable position than when starting from, for example, a mixed-oxide species where the structural defects might be expected to be the catalytically active sites. In the mid-1990s the preparation of isolated titanium species supported by silica was a topic of great interest, in part due to the development of zeolite TS-1 (an MFI structure with framework-substituted isolated titanium atoms). Ti ions incorporated into the framework sites of silicalite I and II (i.e. TS-1 and TS-2 respectively, introduced by the Enichem Company)1 as well as into the framework sites of ZSM-12,2 ZSM-48,3,4 zeolite b5 and analogous microporous alumino- (and silico-) phosphates such as ALPO-5, ALPO-11 and SAPO-5 all show remarkable catalytic properties.6,7 However, one disadvantage of these titano-microporous catalysts was, and is, that their pore dimensions are too small to allow access to bulky reactants of the kind that dominate most of the chemical transformations which are of central importance in the fine-chemical and pharmaceutical industries. A significant step forward was reported8,9 with the preparation and use of Ti-MCM-41 in which the Ti is incorporated (during synthesis) in the framework of mesoporous SiO2 having a pore diameter of ca. 30 A˚ (see Figure 1). It appeared that either titanol or titanyl species were the active sites, but it was difficult to prepare them with confidence. By extending the concepts of what has come to be known as interfacial co-ordination chemistry11 and of surface organometallic chemistry,12,13 we grafted titanocene dichloride onto the totally accessible inner surfaces of siliceous MCM-41 in the presence of a base, as schematised in Figure 2. The resulting material, with well-separated, well-defined, high surface concentrations of Ti-containing active sites, exhibited very high catalytic performance. By using such a well-defined organometallic precursor it was possible (a) to predict what would happen chemically during the grafting process and (b) to design the system in such a way that one was confident of having isolated sites which could then be interrogated with the synchrotron radiation technique EXAFS (extended X-ray absorption fine structure) to determine the nature of the site unequivocally. Titanocene dichloride is superior to both TiCl4 and Ti(OR)4 as a grafting reagent since, with both of the latter there is a marked tendency for oligomeric titano-oxo species and/or some anatase by-products to be formed during the grafting. This is circumvented by the titanocene dichloride method as the relatively stable cyclopentadienyl ligands protect the titanium centre and, hence, prevent either dimerisation and/or oligomerisation. Moreover, with TiCl4, the evolved HCl is potentially damaging to the siliceous MCM-41,

Designing Active Sites for Surfaces

Figure 1

521

Illustration of titanocene dichloride diffusing into a channel of MCM-41.10

whereas using our methodology the evolved HCl is scavenged by an amine rendering it harmless. Furthermore, by then looking at this system under in situ reaction conditions, it is possible to study the active site during reaction, thereby following the course of action of the whole reaction (given that the products of the reaction were analysed as well). By following the pre-edge peak in the XANES (X-ray absorption near edge structure) region, it was possible to detect when the co-ordination environment changed from four- to six-fold co-ordination. Together with the EXAFS analysis we could categorically state that the titanium centres were atomically dispersed (as expected since the cyclopentadienyl ligands were to prevent dimeror oligomerisation of the titanium species) and that they were surrounded by four equidistant oxygen atoms before catalysis and by the same four oxygens as well as two additional oxygens at a longer distance during catalysis. The fourco-ordinated state was entirely reversible. There was no evidence of the much shorter titanyl oxygen, neither in terms of an average distance that would be less than the one expected for a Ti–O bond in an oxide (based on crystal structure data), nor in terms of atomic density (i.e. we did find four not three oxygens). The two oxygens at a larger distance during catalysis are clearly due

522

Figure 2

Chapter 30

Illustration of the surface reactions associated with the grafting of titanocene dichloride.10

to the substrates (cf. Figure 2b). Also, inspect Figure 3 to assess the quality of the fit between the model and raw data. Following on from this line of thinking the next step then is not only to use well-defined molecular precursors to generate well-defined sites after calcination, but also to use these as functional elements in their own right. Here, the tethering approach comes into play, where for example a well-defined

Designing Active Sites for Surfaces

Figure 3

523

(a) XANES and (b) EXAFS fits corresponding to isolated titanol surface groups.10

trimeric cobalt acetate species is anchored onto a silica surface derivatised with carboxylate groups (cf. Figure 4).14 This catalyst is very active for the conversion of cyclohexane to cyclohexanol and cyclohexanone, using an organic peroxide. Again, investigation by in situ EXAFS, using the first reported flow cell that allowed for measurement of X-ray diffraction as well as EXAFS during catalysis, simulating a fixed bed reactor, was able to illuminate an unsuspected mode of action of the catalyst (cf. Figure 5). The in situ EXAFS data clearly illustrated that the co-ordination environment of the cobalt centres had changed and the trimer underwent a contraction, due to the loss of acetates which were replaced by bridging oxygen groups

524

(a)

Chapter 30

(b)

CH3 CH3

O O

C O py

py

C

O

O Co

Co

O

C O C O

py

O

C CH3

O O

Co

O

CH3

N Si

O C CH3

Figure 4

(a) A space-filling model of the trimeric cobalt acetate species, clearly showing the central active site and (b) an illustration of the tethered catalysts inside an MCM-41 mesopore.

Figure 5

Structure of the tethered cobalt trimer after immobilisation and during catalysis, as derived from in situ EXAFS, using a flow cell.

(most likely derived from the various oxygenated species associated with the conversion of cyclohexane to the corresponding alcohol and ketone). The next step was not only to use the grafted or tethered species on its own, but also to make use of its surroundings, i.e. to use the fact that the pore in which it sat presented a confinement of the active site, thereby reducing the degrees of freedom available to the system, much like the case in an enzyme (cf. Figure 6). The catalyst of choice here was a chiral amino-derivative of the diphenylferrocenyl palladium dichloride complex (cf. Figure 7). Here it was possible to influence the chemistry to a very significant degree. The reaction of cinnamic acid methyl ester with benzylamine produces two regio-isomers, one of which is

525

Designing Active Sites for Surfaces

Auxilliary Directing Group

Chiral Directing Group

41

M-

MC

Reactant Through-Space Interactions

Catalytic Centre

“Chiral Space”

Figure 6

A schematic illustrating a generic model of the confined chiral aminoderivative of the diphenylferrocenyl palladium dichloride complex catalyst.15

Figure 7

(a) The chiral amino-dppf ligand and (b) complexed and anchored inside MCM-41.

chiral. Therefore, a total of three isomers are obtained and it is possible to probe in just one reaction, both regio- and enantio-selectivity. When tethering the complex onto a silica surface of a solid silica particle the enantioselectivity increased significantly compared to that of the homogeneous counterpart, however, when it was anchored inside a mesoporous channel the increase in selectivity became remarkable, i.e. increasing the branched product from 2 to 60% and the enantiomeric excess (ee) from 65 to 95%! Computational modelling showed that the catalyst effectively ‘curls up’ inside the pore, leading to the reduced degrees of freedom (see Figure 8).16

526

Chapter 30 O +

O

40° C, THF

H2N

cat.

+ P

P Pd

NH

HN R&S

Figure 8

The isomers generated during the allylic amination reaction.

So far, we had looked at complexes of one metal centre; what about those of many centres, such as mono- and bi-metallic carbonyl clusters? Such clusters, especially if charged, would be easily persuaded to stay on a silica, or other oxide, surface. However, by themselves as fully saturated clusters they are not known to exhibit remarkable catalytic properties. This might be a different situation if one were to ‘denude’ them of the carbonyl ligands and create monodisperse nanoparticles, supported on oxide surfaces, using vacuum thermolysis.17 Indeed, it is possible to follow such removal of the carbonyls with in situ infrared spectroscopy (cf. Figure 9). Starting from such well-defined precursors is a much more precise route compared to the more commonly employed deposition of metal salts, followed by their high-temperature reduction with hydrogen. Here, we were able to design in a pre-determined way the size, dispersion and composition of the alloy nanoparticles. Furthermore, we could engineer stability into the particles as the copper was to act as a ‘glue’ (being oxophilic) for the surface anchoring of the

Designing Active Sites for Surfaces

527

Figure 9

FTIR-spectra of a copper–ruthenium cluster being exposed to increasing temperatures under high vacuum, showing clearly the disappearance of the carbonyl groups in the region of 2100–1700 cm
1.

Figure 10

(i) Crystal structure of the bimetallic carbonyl cluster used for (ii) deposition into mesoporous MCM-41 employing vacuum thermolysis and (iii) after silica vitrification. The labels a–d show clearly that even after sintering the particles remain in their original positions.

nobler ruthenium – translating into unmatched stability, even after support vitrification (cf. Figure 10). Again, in situ EXAFS allowed us to follow the transformation of the species from carbonyl cluster to alloy particle with great precision (cf. Figure 10), Figure 11. Finally, we were able to correlate successfully the Ru–Ru distances from our EXAFS analysis with those from computational modeling. The initial

528

Figure 11

Chapter 30

Plot of the atomic density for the (a) Ru and (b) Cu K-edges as a function of temperature, (c)–(f ) the corresponding simultaneous EXAFS refinement after heating to 180 1C, using one common bimetallic model structure, reducing the degrees of freedom to a minimum – and well in excess of what renders the analysis statistically meaningful.18,19

approach based on molecular mechanics did yield good agreement for the distances associated with Cu, but the Ru–Ru distances were too long. When employing ab initio density functional theory (DFT) level calculations, we were able to replicate the main characteristics of the EXAFS, including the ruthenium bond distances – due to the surprising result that the carbide carbon moves out of the cluster to the surface of the particle, illustrating beautifully the power in the combination of experiment and computation (cf. Figure 12).20 Although successful, these methods all rely on either the destruction by oxidation (titanocene dichloride) or vacuum thermolysis (carbonyl clusters) of our catalyst precursors, or need a complex chemical modification (dppf ligand) of a homogeneous complex which we know works well already – such modifications can enhance, but also reduce the enantioselective prowess of a chirally selective catalyst. Hence, the next step in the exploration of the design of active sites for heterogeneous catalysts was to immobilise well-defined homogeneous catalysts without any chemical modification. This was possible by using

Designing Active Sites for Surfaces

Figure 12

529

Modelling unexpected EXAFS results successfully with DFT calculations.

electrostatic interactions which served to anchor the catalyst to a charged surface (aluminosilicate). Now the effort was directed along two main paths: (1) fine-tuning of the support surface in terms of charge and surface area and (2) tuning the solvent to the catalyst and support interaction. It turns out that the energetic differences that control enantioselectivity (B4 kJ mol
1)21 are of a very similar dimension to those responsible for the complex remaining on the surface,22 rather than leaching from it back into solution. As a consequence, solvent effects seem to have a great influence on the enantioselectivity of any such electrostatically immobilised system. Our most successful result in this area so far involves the electrostatic immobilization of a rhodium catalyst, comprised of two monophos ligands and (initially) a cyclooctadiene (COD) group (cf. Figure 13). We found that as the polarity of a solvent increased, the leaching of the complex also increased; furthermore, in low polarity solvents the enantioselectivity was often very poor for the systems we studied (e.g. enantioselective hydrogenation of dimethyl itaconate). Surprisingly, water acts as an excellent solvent for these systems as it is too polar and the catalyst remains happily on the surface of the support, while the catalysis proceeds with very high enantioselectivities, rendering the system as selective and reactive as the homogeneous equivalent, with the added bonus of ease of separation and recyclability. This then paves the way for reactions in which these immobilised catalysts can be used in combination with a second catalyst (e.g. an enzyme) in water as solvent, i.e. an aqueous cascade reaction, thereby using a benign solvent as well as reducing the number of separation steps during synthesis (cf. Figure 14).

530

Chapter 30 20 18

MeOH

16 Rh loss (%)

14 12 10 2-PrOH

8 6

EtOAc

4

CH2Cl2

2

MTBE

0 0.0

0.2

Water 0.4

0.6

0.8

1.0

1.2

ETN

Leaching behaviour of [(monophos)2Rh(COD)]1 immobilised on the mesoporous sponge Al-TUD-1 in various solvents plotted as a function of their polarity.

Figure 13

O N P Rh+ P N O

O O

HO

Al

O

Si

O

Al

O

Si

OH

Acylase I

1-AlTUD-1

Acylase I AM Phosphate buffer

O N H

5 bar H2, S/C = 200

O

H

1-AlTUD-1

O

N H

OH O

0.05 M Intermediate Overall

Figure 14

Conv. = 100% ee= 95% Conv. = 98% ee > 98%

A compartmentalised cascade reaction: [(monophos)2Rh(COD)]1 immobilised on the mesoporous sponge Al-TUD-1, working together with the enzyme acylase I to achieve outstanding conversions and selectivity.

This approach relies on one main condition, i.e. the two catalysts do not interfere with one another – however, this condition may not always be satisfied. Therefore, it would be useful to devise a generic method by which one could encapsulate any catalyst to keep it separate from any other catalyst and thereby circumvent any limitations on cascade reactions imposed by catalyst incompatibility (see Figure 15).

531

Designing Active Sites for Surfaces

Figure 15

Formation of hollow nanocapsules (f ) through successive adsorption of positively (orange) and negatively (blue) charged polyelectrolytes onto colloidal templates (a–d), followed by their dissolution (e–f ).23

OAc

OH CALB, vinyl acetate fast

(R )-enantiomer "100%" Coated Zeolite H-Beta Nanoreactors OH

OAc

CALB, vinyl acetate slow

Figure 16

(S )-enantiomer

DKR of 2-phenyl ethanol, as catalysed by zeolite b and CALB.

One such example, where catalyst incompatibility exists, is that of the dynamic kinetic resolution (DKR) of secondary alcohols. Here, the racemisation catalyst of choice (zeolite b) interacts destructively with the enzymatic catalyst (Candida Antarctica lipase B, CALB) (Figure 16).

532

Figure 17

Chapter 30

Fluorescence micrograph (lex ¼ 450–490 nm) of fluorescence-tagged zeolite (orange) surrounded by a polyelectrolyte capsule (blue).

We encapsulated the zeolite with a polyelectrolyte membrane as described above and were able to record a much improved enantioselective performance (up by 25 percentage points), while maintaining largely undiminished racemisation activity. In order to visualise the zeolite particles inside the polyelectrolyte capsule, we ion-exchanged the zeolites with a fluorescent orange dye, [Ru(bipy)3]21, that is distinct from the inherent fluorescence of the polyelectrolyte capsule (which is blue) (cf. Figure 17).24 This contribution has explored how it is possible to design and prepare reliably various single site heterogeneous catalysts by a number of methods, ranging from calcination to vacuum thermolysis and from tethering via electrostatic immobilisation to encapsulation. In every case a well-defined molecular precursor was used to design the eventual heterogeneous catalytic single site, showcasing the incredible variety of possibilities, challenges and opportunities that lie within this branch of catalysis.

Acknowledgements Clearly the work presented here is the work of many, as indicated by the references – however, above all it has been due in no small measure to the

Designing Active Sites for Surfaces

533

inspirational presence and the direct as well as indirect contributions of Prof. Sir John Meurig Thomas over the years.

References 1. M. Tamarasso, G. Parego and B. Notari, 1983, U.S. Patent 4410501. 2. A. Tuel, Zeolites, 1995, 15, 236. 3. D.P. Serrano, H.X. Li and M.E. Davis, J. Chem. Soc., Chem. Commun., 1992, 745. 4. K.M. Reddy, S. Kaliaguine, A. Sayari, A.V. Ramaswamy, V.S. Reddy and L. Bonneviot, Catal. Lett., 1994, 23, 175. 5. M.A. Camblor, A. Corma and J. Perez-Pariente, Zeolites, 1993, 13, 82. 6. N. Ulagappan and U. Krishnasamy, J. Chem. Soc., Chem. Commun., 1995, 373. 7. A. Tuel and Y. Ben-Taarit, J. Chem. Soc., Chem. Commun., 1994, 1667. 8. P.T. Tanev, M. Chibwe and T.J. Pinnavaia, Nature, 1994, 368, 321. 9. G. Sankar, F. Rey, J.M. Thomas, G.N. Greaves, A. Corma, B.R. Dobson and A.J. Dent, J. Chem. Soc. Chem. Commun., 1994, 2279. 10. T. Maschmeyer, F. Rey, G. Sankar and J. M. Thomas, Nature, 1995, 378, 159. 11. S. Scott and J. M. Basset, J. Mol. Catal., 1994, 86, 5. 12. A. Zecchina and C. Otero-Arean, Catal. Rev. Sci. Eng., 1993, 35, 261. 13. Comprehensive Organometallic Chemistry, ed. G. Wilkinson, F.G.A. Stone and E.W. Abel, Pergamon, Oxford, 1982. 14. T. Maschmeyer, R.D. Oldroyd, G. Sankar, J.M. Thomas, I.J. Shannon, J.A. Klepetko, A.F. Masters, J.K. Beattie and C.R.A. Catlow, Angew. Chem., Int. Ed. Engl., 1997, 36, 1639. 15. B.F.G. Johnson, S.A. Raynor, D.S. Shephard, T. Maschmeyer, J.M. Thomas, G. Sankar, S. Bromley, R. Oldroyd, L. Gladden and M.D. Mantle, Chem. Commun., 1999, 1167. 16. S. Bromley and C.R.A. Catlow (unpublished results). 17. D.S. Shephard, T. Maschmeyer, G. Sankar, J.M. Thomas, D. Ozkaya, B.F.G. Johnson, R. Raja, R.D. Oldroyd and R.G. Bell, Chem. – Eur. J., 1998, 4, 1214. 18. R.W. Joyner, K.J. Martin and P. Meehan, J. Phys. C – Solid State Phys., 1987 20, 4005. 19. P.J. Ellis and H.C. Freeman, J. Synchrotron Rad., 1995, 2, 190. 20. S.T. Bromley, G. Sankar, C.R.A. Catlow, T. Maschmeyer, B.F.G. Johnson and J.M. Thomas, Chem. Phys. Lett., 2001, 340, 524. 21. C.R. Landis and J. Halpern, J. Am. Chem. Soc., 1987, 109, 1746. 22. S. Feldgus and C.R. Landis, J. Am. Chem. Soc., 2000, 122, 12714. 23. E. Donath, G.B. Sukhorukov, F. Caruso, S.A. Davis and H. Mohwald, Angew. Chem., Int. Ed., 1998, 37, 2201. 24. A. Fois, A.F. Masters and Th. Maschmeyer, Catalysis Today, (submitted).

CHAPTER 31

Polynuclear Transition Metal Cluster Complexes Containing Tin Ligands: Precursors to New Heterogeneous Nano-Catalysts RICHARD D. ADAMS AND BURJOR CAPTAIN Department of Chemistry and Biochemistry, University of South Carolina, 631 Sumter Street, Columbia, SC 29208, USA

1 Introduction Tin has been shown to be a useful modifier of platinum group metals for heterogeneous catalysts used for petroleum reforming,1 catalytic hydrogenations2 and dehydrogenations.3 It is believed that the tin helps to anchor metallic nanoparticles onto nanoporous supports in a highly dispersed and uniform manner resulting in catalysts with higher activities.2 Tin ligands have been shown to improve the product selectivities of homogeneous transition metal catalysts.4 Triphenylstannane is known to undergo facile oxidative addition reactions at the metal atoms of unsaturated metal complexes to yield products containing triphenyltin and hydride ligands (Equation (1)).5 Palladium complexes will dehydrogenate stannanes catalytically to yield polystannanes.6 P(C6H11)3 Pt[P(C6H11)3]2 + HSnPh3

H

Pt

SnPh3

P(C6H11)3

534

ð1Þ

535

Polynuclear Transition Metal Cluster Complexes

2 Reactions of Carbido–Pentarutheniumcarbonyl Clusters with Triphenylstannane We have shown that triphenylstannane Ph3SnH is an excellent reagent for introducing phenyltin ligands into polynuclear metal carbonyl complexes. Our studies began with our investigation of the reaction of Ru5(CO)15(m5-C), 1 with Ph3SnH.7 In the presence of ultraviolet (UV) irradiation 1 reacts with Ph3SnH by reaction of the SnH and cleavage of one of the apical-basal Ru–Ru bonds in the cluster of 1 to yield the adduct Ru5(CO)15(SnPh3)(m5-C)(m-H), 2 (Equation (2)). H Ru

Ru Ph3SnH

Ru Ru

C Ru

Ru

Ru

Ru C Ru

hν

ð2Þ

Ru 1

2

SnPh3

The benzene derivative of 1, Ru5(CO)12(C6H6)(m5-C), 3 reacts with Ph3SnH at 68 1C to yield two products: Ru5(CO)11(SnPh3)(C6H6)(m5-C)(m-H), 4 and Ru5(CO)11(SnPh3)2(C6H6)(m5-C)(m-H)2, 5 formed by the addition of one and two equivalents of Ph3SnH which is accompanied by the loss of one and two CO ligands, respectively.7 Both contain square pyramidal Ru5 clusters with one and two SnPh3 groups, respectively (Equation (3)). There was no evidence of the formation of stable open cluster complexes in this reaction, as was found in the reaction of 1 with Ph3SnH. SnPh3 Ru Ru

Ru Ru

C Ru

Ru C6H6

3

Ph3SnH 68 °C

Ru

H

H Ru

C Ru

+

Ru

Ru

Ru

Ru C

Ru

Ru

C6H6 Ph3Sn

Ph3Sn

4

H

C6H6

5

ð3Þ At 68 1C, compound 4 is slowly converted to the new compound Ru5(CO)11 (C6H6)(m4-SnPh)(m3-CPh), 6 by loss of benzene and a shift of one of the phenyl groups from the tin ligand to the carbido carbon atom to form a triply bridging benzylidyne ligand. The tin ligand is transformed into a novel quadruply bridging stannylyne ligand (see Figure 1). The reactions of 1 and 3 with Ph3SnH at 127 1C yielded multiple tin addition products. Ru5(CO)10(SnPh3)(m-SnPh2)4(m5-C)(m-H), 7 was the principal

536

Chapter 31

Figure 1

Diagram of the structure of 6.

Figure 2

Diagram of the structure of 7.

product obtained from the reaction of 1 with Ph3SnH at 127 1C.7 Compound 7 contains five tin ligands. Four of these are SnPh2 groups bridging each edge of the base of the Ru5 square pyramidal cluster (see Figure 2). The fifth tin ligand was a SnPh3 group that was terminally coordinated to one of the ruthenium atoms in the base of the square pyramidal cluster. The SnPh2 groups were formed by the cleavage of one phenyl ring from the Ph3SnH. This phenyl ring was combined with the tin hydrogen atom and was eliminated as benzene. The

Polynuclear Transition Metal Cluster Complexes

537

cleavage of a phenyl ring to yield a bridging SnPh2 ligand was subsequently found to be a preferred pathway at high temperatures and would be the source of a wide variety of high nuclearity transition metal–tin cluster complexes. Dialkyl- and diaryltin groups are known to be effective bridging ligands in polynuclear metal carbonyl complexes.8 The reaction of 3 with an excess of Ph3SnH at 127 1C led to formation of two new high-nuclearity cluster complexes: Ru5(CO)8(m-SnPh2)4(C6H6)(m5-C), 8, and Ru5(CO)7(m-SnPh2)4(SnPh3)(C6H6)(m5-C)(m-H), 9. Both compounds contain square pyramidal Ru5 clusters with four SnPh2 groups bridging the edges of the square base. Like 7, compound 9 contains one terminal SnPh3 ligand. When treated with CO at 45 atm, compound 9 was converted to 7 by replacement of the benzene ligand with three CO ligands.

3 Reaction of H4Ru4(CO)12 with Ph3SnH The tetrahedral tetrahydridotetraruthenium cluster complex H4Ru4(CO)12, 10 reacts with Ph3SnH to give tin containing tetraruthenium cluster complexes in which the Ru4 cluster has been converted into a square planar form.9 Two of these new ruthenium–tin cluster complexes are Ru4(CO)12(m4-SnPh)2, 11 and Ru4(CO)8(m4-SnPh)2(m-SnPh2)4, 12 (see Equation (4)). Both complexes contain two quadruply bridging SnPh stannylyne ligands which produce an octahedral shaped Ru4Sn2 cluster. All of the hydrido ligands were lost in the formation of 10 and 11 mostly by combining with some of the phenyl groups that were eliminated from Ph3SnH to from the SnPh ligands. To form 12, four CO ligands were also replaced by SnPh2 ligands that bridge each of the four Ru–Ru edges of the Ru4 square (see Figure 3).

Figure 3

Structural diagram of compound 12.

538

Chapter 31 Ph Sn

Ru H

H

H

Ph3SnH 125 °C

Ru Ru

Ru

Ph2Sn Ru

Ru

Ru Ru

H

+

Ph Sn Ru

Ru Ph2Sn

Ru Ru SnPh2

Sn Ph

Sn Ph

11

12

H4Ru4(CO)12

SnPh2

ð4Þ

4 Reactions of Rhodium and Iridium Carbonyl Cluster Complexes with Triphenylstannane The reactions of Rh4(CO)12 and Ir4(CO)12 with Ph3SnH have yielded the first rhodium–tin and iridium–tin cluster complexes: M3(CO)6(m-SnPh2)3(SnPh3)3, 13 (M=Rh) and 14 (M=Ir).10 These complexes contain triangular M3 clusters with three bridging SnPh2 ligands and three terminal SnPh3 ligands (see Figure 4). The Rh–Rh bond distances are unusually long. Molecular orbital calculations of 13 have shown that the cluster is stabilized by strong Rh–Sn bonding. The direct Rh–Rh interactions are delocalized and weak, as shown by the contour diagram of the highest occupied molecular orbital (HOMO) of 13 shown in Figure 5. Interestingly, the reaction of 13 with Ph3SnH yielded the complex Rh3(CO)3(SnPh3)3(m-SnPh2)3(m3-SnPh)2, 15 that contains an unprecedented eight phenyltin ligands including the first examples of triply bridging SnPh ligands (see Figure 6).

Figure 4

Diagram of the structure of 13.

Polynuclear Transition Metal Cluster Complexes

Figure 5

Diagram of the HOMO in 13.

Figure 6

Diagram of the structure of 15.

539

5 Dirhenium Cluster Complexes Containing Diphenyltin Ligands that add Platinum and Palladium to the Re–Sn Bonds Reaction of Re2(CO)8[m-Z4-C(H)¼C(H)Bun](m-H) with Ph3SnH yielded the new compound Re2(CO)8(m-SnPh2)2, 16 that contains two SnPh2 ligands bridging two Re(CO)4 groups (see Figure 7).11 The reaction of 16 with Pd(PBut3)2 yielded the bis-Pd(PBut3) adduct Re2(CO)8(m-SnPh2)2[Pd(PBut3)]2, 17 that contains the first examples of Pd(PBut3) groups bridging transition metal–tin bonds, in this first case they are Re
Sn bonds (see Figure 8). Fenske–Hall molecular orbital calculations show that the Pd(PBut3) groups form three-centre two-electron bonds with the

540

Chapter 31

Figure 7

A diagram of the structure of 16.

Figure 8

A diagram of the structure of 17.

neighbouring rhenium and tin atoms (see Figure 9). The decrease in electron density in the vicinity of the unbridged Re–Sn bond explains why these two bonds increase in length when the Pd(PBut3) groups are added to their neighbouring Re– Sn bonds. The mono- and bis-Pt(PBut3) adducts, Re2(CO)8(m-SnPh2)2[Pt(PBut3)], 18 and Re2(CO)8(m-SnPh2)2[Pt(PBut3)]2, 19 were formed when 16 was treated with Pt(PBut3)2.

6 Reactions of Triosmium Carbonyl Cluster Complexes with Triphenylstannane It was shown many years ago that triphenylstannane reacts with Os3(CO)11 (NCMe) to yield the triphenyltin complex Os3(CO)11(SnPh3)(m-H), 20.12 Compound 20 is also obtained from the reaction of Os3(CO)12 with Ph3SnH in refluxing xylene (140 1C), but in addition a new triosmium complex Os3(CO)9(m-SnPh2)3, 21 is also obtained. Compound 21 consists of a central Os3 triangle with three bridging SnPh2 groups, one on each of the three Os–Os bonds (Figure 10).13

541

Polynuclear Transition Metal Cluster Complexes

PH3 Pd Re SnH2

H2Sn Re Pd H3P

Figure 9

Figure 10

The 3ag HOMO of the Re2(CO)8(m-SnH2)2[Pd(PH3)]2 model complex of 17.

A diagram of the structure of 21.

Two new cluster complexes Os3(CO)12(Ph)(m3-SnPh), 22 and Os4(CO)16 (m4-Sn), 23 were formed when the complex 20 was heated to reflux in toluene solvent in the presence of an atmosphere of CO.14 Compound 22 contains three osmium atoms with a triply bridging SnPh group (see Figure 11). Benzene was eliminated in the formation of 22 and the cluster was opened by cleaving two of the Os–Os bonds. One of the phenyl groups was shifted from the tin atom to one of the osmium atoms. Compound 22 was transformed to 23 when a solution in octane solvent was heated to reflux under a CO atmosphere. Biphenyl is a coproduct in this

542

Chapter 31

Figure 11

A diagram of the structure of 22.

Figure 12

A diagram of the structure of 23.

reaction. Compound 23 contains two Os2(CO)8 groups held together by a naked quadruply bridging tin atom having a spiro-type structure for the five metal atoms (see Figure 12). Two other new compounds Os2(CO)6 (m-SnPh2)2(SnPh3)2, 24 and the monoosmium complex HOs(CO)4(SnPh3), 25 were formed from the reaction of 22 with an additional quantity of HSnPh3. Compound 24 contains only two osmium atoms linked by an Os–Os bond and two bridging SnPh2 ligands. Each osmium atom in 24 also contains one terminal SnPh3 ligand. Compound 25 contains only one osmium atom, one SnPh3 ligand and a terminal hydrido ligand cis to the SnPh3 ligand in the sixcoordinate pseudo-octahedral complex (see Figure 13). As observed with 19, the reaction of 21 with Pt(PBut3)2 provided a Pt(PBut3) adduct Os3(CO)9[Pt(PBut3)](m-SnPh2)3, 26 by adding a Pt(PBut3) group across one of the metal–tin bonds (see Scheme 1).13 The osmium and tin atoms lie in a

543

Polynuclear Transition Metal Cluster Complexes

Figure 13

A diagram of the structure of 25.

Ph2 Sn Os

40 °C + Pt(PPh3)4

Os Ph

Ph2Sn

Os

Sn Pt

27

Ph

– "Pt(PPh3)2" Ph2Sn PPh3 120 °C PPh3

Ph2 Sn Os

Os Os

But3P 68 °C + Pt(PBut3)2 SnPh2

– PBut3

21

SnPh2

Pt Os Ph2Sn

Os Os

SnPh2

26

Scheme 1

plane while the Pt(PBut3) group is displaced slightly out of that plane. By contrast the reaction of Pt(PPh3)4 with 21 yielded the complex Os3(CO)9[Pt(Ph)(PPh3)2] (m-SnPh2)2(m3-SnPh), 27 in which a Pt(PPh3)2 was inserted into one of the Sn–C bonds to one of the phenyl groups. The resultant Pt(Ph)(PPh3)2 group is terminally bonded to one of the tin atoms. The platinum atom of the Pt(Ph)(PPh3)2 group has a square planar geometry with the two PPh3 ligands in cis coordination sites. Interestingly, when heated, compound 27 reverts to 21 by expelling the Pt(PPh3)2 fragment.

7 The Activation of the Hydride Complex HOs(CO)4(SnPh3) by Pt(PBut3)2 Because of our successful studies of the interactions of Pt(PBut3) groups with M–M15 and M–Sn bonds,11,13 it was decided to investigate the reaction of compound 25 with Pt(PBut3)2. This reaction resulted in the formation of the compound PtOs(CO)4(SnPh3)(PBut3)(m-H), 28 that can be viewed as a Pt(PBut3) adduct of 25 (see Figure 14).16 There is no significant bonding interaction between the platinum atom and the tin atom in 28, but there is a

544

Chapter 31

Figure 14

A diagram of the molecular structure of 28.

Figure 15

A diagram of the molecular structure of 29.

substantial interaction with the hydrido ligand and a bond was formed between the platinum atom and the osmium atom, Pt–Os ¼ 2.7628(3) A˚. The hydrido ligand is bonded both to the Pt and Os atoms as a bridging ligand, Pt(1)–H(1) ¼ 1.92(6) A˚ and Os(1)–H(1) ¼ 1.95(6) A˚. Interestingly, it was found that 28 readily reacts with phenylacetylene PhC2H to yield the alkenyl complex PtOs(CO)4(SnPh3)(PBut3)[m-HCC(H)Ph], 29 formed by the insertion of the PhC2H into the Pt–H and Os–H bonds to the bridging hydrido ligand with transfer of the hydrido ligand to the phenyl-substituted carbon atom (see Figure 15). The alkenyl ligand is p-bonded to the osmium atom and s-bonded to the platinum atom. Interestingly, compound 25 does not react with PhC2H even at 110 1C. Molecular orbital calculations have shown that there is a low lying orbital on the platinum atom (see Figure 16) that most likely assists in the addition of the PhC2H molecule to the complex and then facilitates its combination with the hydrido ligand.

545

Polynuclear Transition Metal Cluster Complexes

O C

t

Bu 3P

Os

Pt H

Figure 16

SnPh3

Lowest unoccupied molecular orbital (LUMO) of 28 shows large empty atomic orbital on the platinum atom.

8 Synthesis of Platinum Ruthenium Tin Cluster Complexes from the Reaction of Triphenylstannane with Platinum–Ruthenium Complexes The reaction of the closed PtRu cluster complex PtRu5(CO)14(PBut3)(m-H)2 (m6-C), 30 with HSnPh3 yielded two trimetallic PtSnRu cluster complexes, PtRu5(CO)13(PBut3)(m-H)3(SnPh3)(m5-C), 31 and PtRu5(CO)13(PBut3)(m-H)2 (m-SnPh2)(m6-C), 32.17 Complex 31 formed by a simple oxidative addition of the SnH bond of the HSnPh3, but unlike its precursor 30, it has an open structure in which the Pt(PBut3) group bridges an edge of the square base of the square pyramidal Ru5 cluster. It contains a single SnPh3 ligand and three bridging hydrido ligands. When heated to 97 1C, compound 31 is converted to 32 by cleavage of a phenyl group from the SnPh3 ligand and elimination of benzene by its combination with one of the hydrido ligands (see Scheme 2). The PtRu5 cluster then closes and the SnPh2 ligand adopts a bridging coordination across one of the Ru–Ru bonds (see Figure 17).

9 Formation of a Highly Active and Selective Hydrogenation Nano-Catalyst from a Platinum Ruthenium Tin Precursor Complex Thomas et al. have shown that platinum–ruthenium carbonyl cluster complexes can be precursors to high quality bimetallic nanoparticles for catalytic hydrogenations when activated in mesoporous silica.18 Recent studies have shown that tin can have an important modifying effect on the catalytic activity of ruthenium nanoparticles. It is thought that the oxophilic character of the tin assists in the anchoring of the bimetallic tin containing particles to siliceous supports.2 This anchoring effect helps to prevent aggregation/sintering processes that are common on silica and lead to catalyst deactivation. In collaboration with Thomas and Raja, we have recently obtained evidence that a combination of the three metals, platinum, ruthenium and tin, provides as yet the best catalyst for the hydrogenation of dimethylterephthalate (DMT)

546

Chapter 31 PBut3 H Ru But3P

Pt

Ru

C

Ru H

Ru Ru

H

Pt

97 °C Ru

H Ru

C

Ru

−C6H6

H

Ru

Ru SnPh2

SnPh3

32

31

Scheme 2

Figure 17

O MeO C

Diagram of the structure of 32.

O C OMe

dimethyl terephthalate (DMT)

H2

O MeO C

O

H2

C OMe

dimethyl hexahydroterephthalate (DHMT)

HOH2C

CH2OH

1,4−cyclohexanedimethanol (CHDM)

Scheme 3 to 1,4-cyclohexanedimethanol (CHDM), a valuable reagent for the synthesis of copolymers (see Scheme 3).19,20 The trimetallic PtSnRu5 cluster complex PtRu5(CO)15(m-SnPh2)(m6-C), 33 (see Figure 18) was deposited on mesoporous Davison 38 A˚ silica. All of the ligands including the phenyl groups on the tin ligand are easily removed by heating to 200 1C.19 HAADF TEM (high angle annular dark field transmission electron microscopy) images of the silica revealed the presence of

Polynuclear Transition Metal Cluster Complexes

547

Figure 18

The structure of the molecular cluster PtRu5(CO)15(m-SnPh2)(m6-C), 33.

Figure 19

HAADF images of Ru5PtSn nanoclusters on Davison 38 A˚ silica (a) before and (b) after catalysis.

nanoparticles 1–2 nm in size. XEDS (X-ray energy dispersive spectroscopy) analysis of individual particles showed that their composition was precisely the same as that of the precursor complex Ru5PtSn within experimental error. The particles remain small during the catalysis. See the HAADF TEM images of catalyst particles before and after use for the catalytic hydrogenation of DMT to CHDM in Figure 19. Figure 20 shows a comparison of the activity of some different combinations of metals with ruthenium for the hydrogenation of DMT to CHDM. The trimetallic Ru5PtSn clearly exhibits the best conversion and best selectivity of all. This is a reaction of major commercial importance. Eastman prepares over 200 million pounds of CHDM per year for use in the synthesis of copolyesters by a two-step reduction process of DMT.20

548

Figure 20

Chapter 31

A bar chart comparing the activity and selectivity of the Ru5PtSn catalyst with other bi- and trimetallic catalysts for the hydrogenation of dimethylterephthalate.

Acknowledgments We wish to acknowledge the many contributions of Erin M. Boswell, Wei Fu, Robert Raja, Jack L. Smith, Jr., and Lei Zhu to this work. Special thanks to Sir John Meurig Thomas for his valuable contributions to the catalysis work and for his support, encouragement and friendship. With every best wish on his 75th birthday.

References 1. (a) R. Burch, J. Catal., 1981, 71, 348; (b) R. Burch and L.C. Garla, J. Catal., 1981, 71, 360; (c) R. Srinivasan and B.H. Davis, Platinum Metals Rev., 1992, 36, 151; (d) T. Fujikawa, F.H. Ribeiro and G.A. Somorjai, J. Catal., 1998, 178, 58; (e) Y.-K. Park, F.H. Ribeiro and G.A. Somorjai, J. Catal., 1998, 178, 66. 2. (a) S. Hermans, R. Raja, J.M. Thomas, B.F.G. Johnson, G. Sankar and D. Gleeson, Angew. Chem. Int. Ed., 2001, 40, 1211; (b) B.F.G. Johnson, S.A. Raynor, D.B. Brown, D.S. Shephard, T. Mashmeyer, J.M. Thomas, S. Hermans, R. Raja and G. Sankar, J. Mol. Catal. A: Chem., 2002, 182–183, 89; (c) S. Hermans and B.F.G. Johnson, Chem. Commun., 2000, 1955. 3. (a) G.W. Huber, J.W. Shabaker and J.A. Dumesic, Science, 2003, 300, 2075; (b) J.W. Shabaker, D.A. Simonetti, R.D. Cortright and J.A. Dumesic, J. Catal., 2005, 231, 67; (c) M. Guidotti, V. Dal Aanto, A. Gallo, E. Gianotti, G. Peli, R. Psaro and L. Sordelli, Catal. Lett., 2006, 112, 89; (d) R.D. Cortright, J.M. Hill and J.A. Dumesic, Catal. Today, 2000, 55, 213.

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4. (a) M.S. Holt, W.L. Wilson and J.H. Nelson, Chem. Rev., 1989, 89, 11; (b) J.N. Coupe´, E. Jorda˜o, M.A. Fraga and M.J. Mendes, Appl. Catal. A, 2000, 199, 45; (c) W.R. Rocha, J. Mol. Struc. (Theochem), 2004, 677, 133. 5. H.C. Clark, G. Ferguson, M.J. Hampden-Smith, H. Ruegger and B.L. Ruhl, Can. J. Chem., 1988, 66, 3120. 6. P. Braunstein and X. Morise, Chem. Rev., 2000, 100, 3541. 7. R.D. Adams, B. Captain, W. Fu and M.D. Smith, Inorg. Chem., 2002, 41, 5593. 8. D.J. Cardin, in Metal Clusters in Chemistry, vol 1, P. Braunstein, A. Oro, P.R. Raithby (eds), Wiley-VCH, Weinheim, 1999, 48. 9. R.D. Adams, E. Boswell, B. Captain, Faraday Trans., 2007 (in press). 10. R.D. Adams, B. Captain, J.L. Smith Jr., M.B. Hall, C.L. Beddie and C.E. Webster, Inorg. Chem., 2004, 43, 7576. 11. R.D. Adams, B. Captain, R.H. Herber, M. Johansson, I. Nowik, J.L. Smith Jr. and M.D. Smith, Inorg. Chem., 2005, 44, 6346. 12. K. Burgess, C. Guerin, B.F.G. Johnson and J. Lewis, J. Organomet. Chem., 1985, 295, C3. 13. R.D. Adams, B. Captain and L. Zhu, Organometallics, 2006, 25, 2049. 14. R.D. Adams, B. Captain and L. Zhu, Organometallics, 2006, 25, 4183. 15. (a) R.D. Adams, B. Captain, W. Fu, M.B. Hall, J. Manson, M.D. Smith and C.E. Webster, J. Am. Chem. Soc., 2004, 126, 5253; (b) R.D. Adams, B. Captain, W. Fu and M.D. Smith, J. Am. Chem. Soc., 2002, 124, 5628; (c) R.D. Adams, B. Captain and L. Zhu, Organometallics, 2006, 45, 430; (d) R.D. Adams, B. Captain, W. Fu, P.J. Pellechia and M.D. Smith, Angew. Chem. Int. Ed., 2002, 41, 1951; (e) R.D. Adams, B. Captain, W. Fu, P.J. Pellechia and M.D. Smith, Inorg. Chem., 2003, 42, 2094; (f) R.D. Adams, B. Captain, W. Fu, P.J. Pellechia and L. Zhu, Inorg. Chem., 2004, 43, 7243; (g) R.D. Adams, B. Captain, M.B. Hall, J.L. Smith Jr. and C.E. Webster, J. Am. Chem. Soc., 2005, 127, 1007; (h) R.D. Adams, B. Captain, P.J. Pellechia and J.L. Smith Jr., Inorg. Chem., 2004, 43, 2695. 16. R.D. Adams, B. Captain and L. Zhu, J. Am. Chem. Soc., 2006, 128, 13672. 17. R.D. Adams, B. Captain and L. Zhu, Inorg. Chem., 2005, 44, 6623. 18. J.M. Thomas, B.F.G. Johnson, R. Raja, G. Sankar and P.A. Midgley, Acc. Chem. Res., 2003, 36, 20. 19. A.B. Hungria, R. Raja, R.D. Adams, B. Captain, J.M. Thomas, P.A. Midgley, V. Golvenko and B.F.G. Johnson, Angew. Chem. Int. Ed., 2006, 45, 4782. 20. S.R. Turner, Polym. Sci., 2004, 42, 5847.

CHAPTER 32

Selective Oxidation Using Gold and Gold–Palladium Nanoparticles GRAHAM J. HUTCHINGS School of Chemistry, Cardiff University, Main College, Park Place, Cardiff CF10 3AT, UK

1 Introduction The new field of oxidation catalysis based on gold nanoparticles is the focus of this article. Until recently gold has been overlooked as a key component of both homogeneous and heterogeneous catalysts. However, the observation that nanocrystalline gold supported on oxides is an effective catalyst for low temperature carbon monoxide oxidation has provided the impetus for researchers to open up new directions in oxidation catalysis, namely selective oxidation of alkenes and alcohols. In this paper, the use of gold and gold–palladium nanoparticles as catalysts for both the direct formation of hydrogen peroxide and the selective oxidation of alcohols will be described. The key features of the catalysts will be discussed and the future prospects for gold-based nanoparticles in oxidation catalysis will be discussed. It is a pleasure to provide an article for this publication in honour of the 75th birthday of Sir John Meurig Thomas. The paper is based on a Franc¸ois Gault lecture given by the author on behalf of EFCATS during 2007; this is considered appropriate as Sir John was elected as the first EFCATS Franc¸ois Gault Lecturer in 1996 in recognition of his outstanding achievements in catalysis. The subject of this paper relates to catalysis by gold. Such a topic seems counter-intuitive to many scientists and the concept of gold being involved in 550

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some way in rapid chemical processes is at odds with the view of gold held by the public at large. Gold, after all, is used in jewellery to fashion items of great beauty and durability. Gold has been the element of commerce for millennia due to its chemical immutability and ability to withstand oxidation, particularly during fires. Gold is also used in dentistry due to its malleability and inertness. Hence, for many chemists gold is the least interesting element in the Periodic Table, after all the chapters in inorganic textbooks dealing with gold usually have the least pages; while for most non-scientists it is the object of desire. Perhaps the position of gold in the Periodic Table should have alerted researchers to an apparent anomoly, since it is surrounded by elements that are used in a broad range of catalytic processes including oxidation. Hence, there should not have been too much surprise when it was found that gold was active, but this was not the case and, indeed, some argued that it was the impurities present in gold that led to the observed activity. It was the chemical inertness of gold on the macroscopic scale that limited investigations into its use as a catalyst, since it is only when gold is present as nanocrystals or cations that the activity is perceived. This new discovery opens up the possibility of gold acting as an effective oxidation catalyst and recent studies are confirming this to be the case. It was the discovery in the 1980s that finely divided supported nanoparticles of gold could act as catalysts for reactions at low temperatures that changed this perception of gold as the inert element; indeed the observation that gold, and gold containing alloys, can be the best catalyst for a wide range of reactions has to be considered as one of the most fascinating current topics in chemistry. This has heralded an explosion of interest in gold catalysis (Figure 1) and consequently, a large number of experimental and theoretical studies are being undertaken to try to elucidate the nature of this interesting catalytic activity. This recent research has been reviewed by Haruta,1–5 Bond and Thompson,6,7

Figure 1

Percentage of publications on gold catalysis normalised against all chemical publications.12

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Chapter 32 8

9

10

11

Bond et al., Thompson, Freund and co-workers, Cortie, Hashmi,12 Hutchings,13–15 Hutchings and Scurrell16 and Hashmi and Hutchings.17 In this article, there is no need to go over the major ground covered in these extensive reviews, but rather the use of supported gold and gold–palladium catalysts for selective oxidation will be explored. In particular, the use of Au/C for the selective oxidation of glycerol, a feedstock that is receiving great current interest as a biorenewable material, will be used as a starting point, and subsequently the use of supported gold–palladium alloys for the direct synthesis of hydrogen peroxide and the rationale for the discovery that these catalysts are exceptionally efficient for the oxidation of alcohols will be used as the background for consideration of where future developments in gold catalysis can be expected.

2 Targets for Oxidation Reactions Oxidation reactions using gas or liquid phase reagents to produce valuable intermediates or products for the chemicals industry remains a major area of research interest.17 At present, major interest in oxidation catalysis remains focused on the products of the bulk chemical industry, e.g. the oxidative dehydrogenation of alkanes to alkenes and the epoxidation of alkenes are intensely researched fields. However, there are major challenges that remain, and personally these are classified as dream reactions. In this category one would place the direct oxidation of methane to methanol, a reaction that has fascinated catalysis researchers for decades.17 The reason for this fascination is three-fold, (a) if this reaction could be achieved at realistic conversions with high selectivity, the discovery would revolutionise the chemical industry, (b) the reaction looks deceptively simple since all four C–H bonds in methane are equivalent and (c) we know it is possible as nature has already figured it out and methane monooxygenase readily carries out this reaction at ambient conditions. The much more demanding direct oxidation of long chain alkanes to primary alcohols, a reaction made more demanding as now regioselectivity as well as chemoselectivity are crucially important, and the direct hydrogenation of oxygen to form hydrogen peroxide exclusively would also be placed in the category of dream reactions. However, there is immense scope to identify new catalytic uses for oxidation catalysis. This opportunity exists as there are many oxidation processes operated in the fine chemicals industry, but unfortunately many reactions are still carried out using stoichiometric oxygen donors often with particularly non-green components. We can anticipate new developments in the future and this is an area of research which promises great opportunities for anyone entering the field of oxidation catalysis. Perhaps one of these areas will be the expansion of oxidation into everyday life and bring it into closer contact with society as a whole. This is the challenge presented by catalytic washing. Initially, this concept was suggested for the washing of fabrics using di-manganese complexes that activated molecular oxygen,18 and although this was far from a commercial success it still represents a fascinating opportunity for research.

553

Selective Oxidation Using Gold and Gold–Palladium Nanoparticles

3 Au/Carbon as a Selective Catalyst for Glycerol Oxidation Amongst the potential new uses of catalytic oxidation technology, the oxidation of alcohols and polyols to chemical intermediates represents a demanding target. Supported platinum and palladium nanoparticles are generally acknowledged as effective catalysts for the oxidation of polyols, for example, in carbohydrate chemistry for the oxidation of glucose to glucinic acid. Glycerol is a highly functionalised molecule that is readily available from biosustainable sources, for example, it can be obtained as a by-product of the utilisation of rape seed and sunflower crops. This makes glycerol a particularly attractive starting point for the synthesis of intermediates, and a large number of products can be obtained from glycerol oxidation (Figure 2). One of the key problems is the potential complexity of the products that can be formed and so control of the reaction selectivity by careful design of the catalyst is required. Glycerol oxidation, in aqueous solution, has been extensively studied and in general, palladium catalysts were found to be more selective than platinum, but in all these previous studies using Pt and Pd, mixtures of most of the potential products were formed in addition to non-selective products such as formic acid and carbon dioxide. Hence, glycerol has remained a challenging starting point for the synthesis of chemical intermediates. The field of selective alcohol oxidation was opened up by the seminal studies of Rossi, Prati and co-workers19–22 demonstrating that supported gold nanoparticles can be very effective catalysts for the oxidation of alcohols, including diols. Recently, we extended these studies to show that Au supported on graphite (1 wt% Au/C) can oxidise glycerol to glycerate with 100% selectivity using dioxygen as the oxidant under relatively mild conditions (Table 1).23,24 NaOH was added as a base since, in the absence of NaOH, no glycerol O HO OH O

OH HO

OH

OH HO

OH TARAC

OH GLYA O HO

O

O

OH

HO

OH DHA

O

O

O

GLY

HO

OH O HYPAC

HO

OH O

O MESAC CO2

Figure 2

Reaction scheme for the oxidation of glycerol.

1 wt% Au/activated carbon 1 wt% Au/graphite 1 wt% Au/graphite 1 wt% Au/graphite 1 wt% Au/graphite 1 wt% Au/graphite 1 wt% Au/graphite 1 wt% Au/graphite 1 wt% Au/graphite 1 wt% Au/graphite 1 wt% Au/graphite

3 3 6 6 3 3 3 3 6 6 6

b

60 1C, 3 h, H2O (and 20 ml), stirring speed 1500 rpm. 220 mg catalyst. c 217 mg catalyst. d 450 mg catalyst.

a

12 12 12 12 6 6 6 6 6 6 6

Catalyst

Po2 (bar)

NaOH (mmol) 12 12 12 24 12 6 6 12 6 12 0

Glycerol/metal (mol ratio) 538b 538b 538b 538b 540c 540c 214d 214d 214d 214d 214d

Oxidation of glycerol using 1 wt% Au/C catalysts.a,24

Glycerol (mmol)

Table 1

56 54 72 58 56 43 59 69 58 91 0

Glycerol conversion (%) 100 100 86 97 93 80 63 82 67 92

Selectivity (%) Glyceric acid 0 0 2 0 0 0 0 0 0 0

Glyceraldehyde

0 0 12 3 7 20 12 18 33 6

Tartronic acid

554 Chapter 32

Selective Oxidation Using Gold and Gold–Palladium Nanoparticles

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conversion was observed. In addition, the carbon supports were also inactive for glycerol oxidation under these conditions, even when NaOH was present, which is an essential feature for an effective catalyst support. For all the data presented in Table 1, the carbon mass balance was 100% indicating that, under these conditions, supported Au/C catalysts are extremely selective for this reaction and no C1 or C2 by-products were detected, which was not the case with Pt or Pd catalysts.23–27 In addition, it is apparent that the selectivity to glyceric acid and the glycerol conversion are very dependent upon the glycerol/ NaOH ratio. In general, with high concentrations of NaOH, exceptionally high selectivities to glyceric acid can be observed. However, decreasing the concentration of glycerol, and increasing the mass of the catalyst and the concentration of oxygen, leads to the formation of tartronic acid via consecutive oxidation of glyceric acid. Interestingly, this product is stable with these catalysts. It is apparent that, with careful control of the reaction conditions, 100% selectivity to glyceric acid can be obtained with 1 wt% Au/C. For comparison, the supported Pd/C and Pt/C always gave other C3 and C2 products in addition to glyceric acid and, in particular, also gave some C1 by-products. In a final set of experiments, catalysts with lower Au concentrations were investigated. For catalysts containing 0.25 or 0.5 wt% Au supported on graphite, lower glycerol conversions were observed (18% and 26% respectively as compared to 54% for 1 wt% Au/graphite under the same conditions) and lower selectivities to glyceric acid were also observed. The previous studies for diol oxidation by Rossi, Prati and co-workers19–22 also showed that the conversion is dependent on the Au loading upon the support. This is considered to be due to a particle size effect of the Au nanoparticles on the support. Gold is known to be a highly effective catalyst for the oxidation of CO,1–17 and it has been shown that the activity is highly dependent on the particle size, and the optimum size is ca. 2–4 nm. However, most interestingly, the Au supported catalysts that were selective for glycerol oxidation comprised Au particles as small as 5 nm and as large as 50 nm in diameter. The majority, however, were about 25 nm in size and were multiply twinned in character. Decreasing the loading to 0.5 wt% or 0.25 wt% did not appreciably change the particle size distribution; the particle number density per unit area was observed to decrease proportionately however, which may be correlated to the decrease in glycerol conversion and selectivity to glyceric acid. The catalysts that were active and selective for glycerol oxidation were not found to be active for the CO oxidation reaction. Consequently, we concluded that different active sites are involved in these two contrasting reactions. Recently, we have used cyclic voltammetry to study the Au catalysts supported on graphite,27 since in this case the support is conducting and this very incisive technique can be used. A set of CV experiments were carried out with the Au/graphite catalysts in the presence of glycerol, air and NaOH, thereby studying the behaviour in situ under reaction conditions (Figure 3). In the forward potential sweep, all catalysts showed a broad signal associated with the electrooxidation of glycerol at ca. 0.9–1.3V (labelled C) and a narrower feature on the reverse sweep (labelled D). Peak D corresponds to the situation in which

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Chapter 32 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

5000

5000

D

4500

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Current Density (µA cm-1)

a 4000

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3500

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C

2500

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2000

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F 1000

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E (Pd/H Reference) / Volts 0.6

0.8

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1.4

1.6 4000

4000

D 3500

3500

b 3000

Current Density (µA cm-1)

3000

2500

2500

C 2000

2000

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F 1000

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-500 0.0

0.2

0.4

0.6

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1.0

1.2

1.4

1.6

E (Pd/H Reference) / Volts

Figure 3

Cyclic voltammetry of Au/graphite catalysts in aqueous NaOH ð0:5 mol=lÞ and glycerol ð0:5 mol=lÞ. (a) 0.25 wt% Au/graphite; (b) 0.5 wt% Au/ graphite.15

the gold surface is being stripped of bulk oxide leaving behind only the Au–OH species (peak A) with a minimal amount of molecular fragments adsorbed (since these have been oxidised during the previous positive potential sweep). This situation leads to peak D being the most intense and the catalyst being in its most active state. Peak C corresponds to the same situation although the relative amounts of strongly adsorbed molecular fragments is increased (since these have not yet been oxidised) and hence a smaller concentration of Au–OH

Selective Oxidation Using Gold and Gold–Palladium Nanoparticles

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species due to site-blocking via glycerol decomposition. Both of these factors lead to peak C being smaller than peak D. This behaviour also emphasises the poisoning effect on the reaction of bulk gold oxides which quench reaction at potentials >1.3 V on the forward sweep and also down to 1.1 V on the negative sweep due to hysteresis in the ‘‘irreversible’’ formation/desorption of the bulk oxide phase.28 This suggests that there should be a strong correlation between activity and the relative intensities of peaks C and D. This proposition is explored below. In addition, it should be noted that the 0.25% and 0.5% Au/C catalysts gave rise to two less intense peaks labelled E (0.38 V) and F (1.0 V). For the active catalyst displaying total specificity to glycerate (1 wt% Au/ graphite) peaks E and F are both absent and we also considered this to be a key finding. Furthermore, current density positive of 1.3 V associated with the electrooxidation of strongly adsorbed glycerol fragments increases in the order: 1% Au/C o 0.5% Au/C o 0.25% Au/C o 1% Au/C (inactive) In this way, the CV study, however, revealed differences between all the four catalyst samples we investigated. In particular, two features were identified that appeared to correlate with catalyst activity: (a) the relative intensities of specific peaks observed in the CV and (b) the amplitude of the current density at >1.3 V. Therefore, in Figure 4, these two parameters are plotted versus catalyst activity, namely (i) the ratio of current densities (j) of peak C/peak D and (ii) the ratio of current density at 1.6 V to the current density at 1.15 V (peak C). Both these parameters express the rates of surface blocking (poisoning) relative to oxidation by adsorbed Au-OH species. Inspection of Figure 4 demonstrates a smooth correlation between activity and both of these parameters, and we considered that this observation may have significance in the design of

Figure 4

Plot of current density at 1.6 V ( j1.6)/current density at 1.15 V ( j1.15) and ratio of peak C/peak D versus percentage conversion for various supported Au/C catalysts used for glycerol oxidation.15

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Chapter 32

improved oxidation catalysts and that the use of CV as a diagnostic method should be encouraged.

4 Selective Oxidation Using Gold–Palladium Alloy Catalysts 4.1

Direct Synthesis of Hydrogen Peroxide

The direct synthesis of hydrogen peroxide from the oxidation of molecular hydrogen by molecular oxygen is considered to be of immense current interest. Hydrogen peroxide is a noted green oxidant that is useful in many large-scale processes such as bleaching and as a disinfectant. Its use in the fine chemical industry accounts for a much lower consumption but since it is viewed as a green oxidant with water being the only by-product, it is recognised to have significant potential in chemical synthesis, particularly in the production of propene oxide using the microporous redox catalyst titanium silicalite TS-1.29,30 In addition, it is considered possible that hydrogen peroxide can replace stoichiometric, i.e. non-catalytic, oxygen donors in a number of processes. At present, hydrogen peroxide is produced by the sequential hydrogenation and oxidation of an alkyl anthraquinone,31 a process that is only economic at a large scale. In contrast most uses of hydrogen peroxide require relatively small amounts. Hence there is a significant mismatch between the current scales of production and usage. The direct small-scale production of hydrogen peroxide at the site where it is used would offer many advantages, not least that this will negate the need to transport concentrated solutions of hydrogen peroxide. At present, no commercial process exists for the direct formation of H2O2, but there has been significant interest in this reaction in industrial laboratories for over 90 years.32 Indeed, Degussa-Headwaters have announced recently that they will commercialize a direct route to produce hydrogen peroxide that can be used in the production of propene oxide.33 Until very recently the catalysts used in these investigations have been based on Pd, and since many researchers have concluded that it is important to try to achieve the highest rate of product formation most of these earlier studies used H2/O2 mixtures in the explosive region; solutions of over 35 wt% hydrogen peroxide have been made by reacting H2/O2 over Pd catalysts at elevated pressures.34 However, operating in the explosive region with H2/O2 mixtures can be considered extremely dangerous, and more recently, studies have concentrated on carrying out the reaction with dilute H2/O2 mixtures well away from the explosive region.35,36 In our initial papers concerning the direct synthesis of H2O2 we showed that catalysts based on Au–Pd alloys supported on alumina can give significant improvements in the rate of hydrogen peroxide formation when compared with the Pd only catalyst.37,38 A detailed study of Au–Pd catalysts supported on alumina showed that only a relatively small amount of palladium is required to achieve a significant enhancement in the rate of hydrogen peroxide synthesis

Selective Oxidation Using Gold and Gold–Palladium Nanoparticles

Figure 5

559

Au(4d) and Pd(3d) spectra for a 2.5 wt% Au–2.5 wt% Pd/TiO2 catalyst after different heat treatments (a) uncalcined, (b) calcined at 200 1C in air, (c) calcined at 400 1C in air and 500 1C in hydrogen.40

(Figure 5). Addition of Pd to Au significantly enhances the catalytic performance for the synthesis of H2O2, and moreover, it is interesting to note that there is an optimum Pd–Au composition (Pd/AuE1:5) where the rate of H2O2 production is much higher than for the pure Pd catalyst, which in itself is significantly more active than pure gold. It is apparent that the addition of Pd to Au also enhances the selectivity to H2O2, and the enhanced rate of H2O2 synthesis is achieved at lower H2 conversion. We decided to study the effect of storage on catalyst activity as, typically, catalyst performance declines with storage for many catalysts. This is well known for catalysts that are used for CO oxidation. We stored a sample of the most active 2.5 wt% Au–2.5 wt% Pd/Al2O3 catalyst, which had been previously calcined at 400 1C, in a sealed container in the dark for 12 months at ambient conditions. Re-evaluation of the catalytic performance of this catalyst showed that under the standard reaction conditions, the activity of the sample had increased from 15 mol of H2O2/h/kgcat to 52 mol of H2O2/h/kgcat,39 representing a dramatic increase in productivity. A detailed scanning transmission electron microscopy-X-ray energy dispersive spectroscopy (STEM-XEDS) analysis of the fresh and the aged Au–Pd catalysts showed that the particle size of the gold–palladium alloy particles increased on storage, through sintering, and these larger particles are associated with the enhanced activity observed. This confirms that for the direct synthesis of hydrogen peroxide, small gold–palladium alloy nanocrystals are not preferred. This study was also a remarkable demonstration of the wide variation in active sites that can be observed with gold catalysts, since the catalysts that are active for CO oxidation

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decrease in activity with storage whereas those active for the synthesis of H2O2 increase in activity. To date the highest activities we have reported39–41 have been with TiO2supported catalysts. In general, Au–Pd/TiO2 catalysts give rates of hydrogen peroxide synthesis that are three times higher than the fresh Au–Pd/Al2O3 catalysts. The reaction time and amount of catalyst used are important variables in the direct synthesis reaction since there are a number of competing processes that lead to the decomposition of hydrogen peroxide, even at 2 1C. The effect of increasing reaction time in the autoclave is shown in Table 2 for the 2.5% Au/2.5% Pd/TiO2 catalyst calcined at 400 1C. Separate experiments were conducted for each reaction time and so the data present the amount of H2O2 formed as an average over the reaction period. It is apparent that the yield of H2O2 increases steadily with reaction time and consequently the rate of formation decreases with increased reaction time, and under these reaction conditions it was concluded that 30 min gives a reasonable compromise between rate and overall yield of H2O2 for the purposes of comparing the catalytic performance of these catalysts. Of course the optimal conditions will vary with other conditions such as temperature and pressure, and so it can be reasonably anticipated that at higher temperatures and reaction pressures much shorter reaction times will be preferred. This means that the direct synthesis method using diluted reactants, that avoids the potential for explosion hazards, will only produce relatively dilute solutions and hence the methodology may be better suited to in situ utilisation in chemical syntheses in which the H2O2 is used as soon as it is formed. In this scenario, optimum use can be made of the very high initial rates of H2O2 formation displayed by the catalysts (Table 2). For the 2.5% Au/2.5% Pd/TiO2 calcined at 400 1C, the initial rate after 5 min reaction time is >100 mol kgcat
1 h
1 with a hydrogen selectivity of >90%; consequently if the hydrogen peroxide can be selectively transferred to a substrate then this process would be exceptionally efficient. One of the key factors that must be considered for heterogeneous catalysts operating in three phase systems is the possibility that active components can leach into the reaction mixture, thereby leading to catalyst deactivation or, in the worst case, leading to the formation of an active homogeneous catalyst. To demonstrate that the TiO2 supported catalysts function as wholly heterogeneous catalysts, an experiment was carried out using a supported gold catalyst Table 2

The influence of reaction time on the conversion of H2 and selectivity to hydrogen peroxide synthesis.a,40

Reaction time (min)

Productivity (mol-H2O2/h/ kgcat)

H2 conversion (%)

H2O2 selectivity (%)

H2O2 yield (%)

5 30 120

114 66 17

8 44 93

93 60 29

7.6 26.4 27.6

a

2.5% Au–2.5%Pd/TiO2 catalyst (20 mg) calcined at 400 1C.

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(calcined at 400 1C) at 2 1C and the yield of H2O2 was determined. Following this reaction, the gold catalyst was removed by careful filtration and the solution was used for a second experiment using O2/H2. No further H2O2 was formed and this confirms that the formation of hydrogen peroxide involves the gold–palladium alloy acting as a wholly heterogeneous catalyst. Detailed structural characterisation has been carried out using X-ray photoelectron spectroscopy (XPS) (Figure 5)40 and transmission electron microscopy (Figure 6).41 Figure 5 shows the combined Au(4d) and Pd(3d) spectra for a 2.5 wt% Au/2.5 wt% Pd/TiO2 catalyst after different heat treatments. For the uncalcined sample, which exhibits the highest rate of H2O2 production that we have observed to date for a titania-supported catalyst, there are clear spectral contributions from both Au and Pd leading to severe overlap of peaks. After heat treatment at 200 1C there is a dramatic decrease in the intensity of the Au(4d) peaks, and after calcination in air at 400 1C followed by reduction in H2 at 500 1C, the intensity of the Au(4d3/2) feature is below detection limits. The uncalcined samples, although more active, were very unstable and leach Au and Pd into solution during reaction, and these cannot be reused. However, catalysts calcined at 400 1C were stable and active. Bulk analysis of the calcined catalysts confirmed an overall Au:Pd ratio of 1:1, and hence we concluded that the XPS results were revealing a core–shell structure for the Au–Pd nanoparticles with a gold rich core. Subsequent detailed transmission electron microscopy confirmed this to be the case.41 We have subsequently found that Au–Pd catalysts supported on Al2O3, Fe2O3 and TiO2 all give this core–shell structure for the gold–palladium alloy particles and high activity and selectivity can be achieved for the direct synthesis of hydrogen peroxide with these catalysts.39–42

4.2

Oxidation of Alcohols

It is well known that metals supported on oxides can be effective catalysts for the oxidation of alcohols to oxides. In the earlier studies in which supported gold nanocrystals were used as catalysts for the oxidation of alcohols, the addition of base was essential for the observation of activity.19–24 Recent studies have shown that the addition of base is not required for the oxidation under mild solvent-free conditions, and this is a major advantage. In particular, two recent studies have shown that supported metal nanoparticles can be very effective catalysts for the oxidation of alcohols to aldehydes using O2 under relatively mild conditions. Kaneda and co-workers43 have shown that hydroxyapatite-supported Pd nanoclusters (Pd/HAP) give very high turnover frequencies for the oxidation of phenylethanol and benzyl alcohol but show very limited activity in octan-1-ol oxidation. Corma and co-workers44 have shown that the addition of Au nanocrystals to CeO2 converts the oxide from a stoichiometric oxidant to a catalytic system with turnover frequencies similar to those of Kaneda and co-workers.43 As we have shown in the previous section, supported Au–Pd alloys are very efficient catalysts for the direct

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Chapter 32

Pd-Lα

O-Kα

Ti-Kα

Au-Pd-Ti

Figure 6

Montage showing the annular dark field (ADF)-STEM image of a bimetallic particle and the corresponding multivariate statistical analysis (MSA) processed STEM-XEDS maps of the gold M2, palladium La, oxygen Ka, and titanium Ka signals. Also shown is a reconstructed MSA filtered Au–Pd–Ti composition map (Ti ¼ red, Au ¼ blue, and Pd ¼ green).41

563

Selective Oxidation Using Gold and Gold–Palladium Nanoparticles

100

100

90

90

80

80

70

70

60

60

50

50

40

40

30

30

20

20

10

10

0

Benzaldehyde selectivity/%

Benzy alcohol conversion/%

synthesis of hydrogen peroxide from the oxidation of H2 by O2 at low temperatures. Hydroperoxy species are considered to be involved in H2O2 formation and since hydroperoxy species are known to be key reagents/intermediates in the oxidation of alcohols,44 we reasoned that the catalysts active for hydrogen peroxide synthesis would also be effective for the oxidation of alcohols, and subsequently41 we demonstrated that TiO2-supported Au–Pd alloy nanocrystals gave significantly enhanced activity for alcohol oxidation under solvent free conditions with O2. When compared with mono-metallic supported Au44 and Pd43 the Au–Pd catalyst nanocrystals give turnover frequencies enhanced by a factor of ca. 25. The TiO2 supported Au–Pd catalysts were investigated for the oxidation of benzyl alcohol at 100 1C using O2 as oxidant in the absence of solvent (Figure 7). It is clear that the Au–Pd/TiO2 catalysts are very active for this reaction and the selectivity to benzaldehyde was Z 96% and only benzyl benzoate was observed as a by-product. Carbon mass balances were 100% and no carbon oxides were formed. The effect of adding Au to a Pd/TiO2 catalyst is clearly apparent in these studies. Although the Pd/TiO2 catalyst exhibited high initial activity, and the addition of Au decreases the activity, the Au–Pd/TiO2 catalyst retains high selectivity to benzaldehyde at high conversion, a feature that is not observed with the supported Au and Pd catalysts. One of the key factors that must be considered for heterogeneous catalysts operating in three phase systems is the possibility that active components can leach into the reaction mixture, thereby leading to catalyst deactivation or, in the worst case, to the formation of an active homogeneous catalyst. Kaneda and co-workers43 and Corma and co-workers44 had previously demonstrated that supported Pd and Au monometallic catalysts were highly

0 0

2

4

6

8

10 12 14 16 18 20 22 24 Time/h

Figure 7

Benzyl alcohol conversion and selectivity in benzaldehyde with the reaction time at 373 K, 0.1 MPa O2 pressure: (’) Au/TiO2, (K)Pd/TiO2, (m) Au-Pd/ TiO2; solid symbols – conversion, open symbols – selectivity.41

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effective for the oxidation of 1-phenylethanol under solvent free conditions at 160 1C with 1 atm oxygen pressure. Under these conditions the Pd/HAP and Au/CeO2 catalysts gave TOFs of 9800 and 12500 h
1 for 1-phenylethanol as substrate. With the Au–Pd/TiO2 alloy catalyst a TOF of 269000 h
1 was obtained. This is a significantly higher activity than that reported to date for selective alcohol oxidation. Hence these studies showed that supported Au–Pd catalysts, prepared by a relatively simple impregnation procedure, are very effective catalysts for the selective oxidation of a range of straight chain, benzilic and unsaturated alcohols, in particular primary alcohols, in addition to being effective for the direct synthesis of hydrogen peroxide.

5 Future Prospects for Catalysis by Gold In a recent review17 the extensive literature on both heterogeneous and homogeneous catalysis by gold has been discussed. Gold is therefore finding numerous applications in all facets of catalysis. Once gold was considered as the last element one would select as a component of a catalyst, whereas now, to many it appears to be the first choice; this is indeed a remarkable reversal in fortunes. However, this has yet to be translated into commercial technology and this has to be the goal for the future, since then the lasting legacy of this early work on catalysis by gold will have been secured. Recently, Thompson and co-workers45 have reviewed the commercial prospects for gold catalysis and they have identified many targets. There are a number of areas in which catalysis by gold can be expected to advance. First, at present there is very limited mechanistic understanding of how supported gold nanocrystals or cations achieve the remarkable activities and selectivities. This, therefore, represents an area where a number of key studies are now required, particularly using in situ spectroscopies together with key experiments aimed at unraveling the kinetics of the reactions. At present, the emphasis has been on reaction discovery since this has provided such rich rewards. This is a challenge that has been recognised in the latest commentary on catalysis by gold by Thomas and Edwards.46 As demonstrated by the work discussed on the direct synthesis of oxygen to form hydrogen peroxide, supported gold catalysts are highly effective for selective hydrogenation after early work by Bond and co-workers47 showed that small gold nanocrystals were effective for the selective hydrogenation of dienes, and subsequent studies by Baillie et al.48 showed that ZnO-supported gold crystallites were highly selective for the challenging selective hydrogenation of a,b-unsaturated aldehydes to yield the unsaturated alcohol. Hydrogenation using gold catalysis has recently been reviewed by Claus.49 Interestingly, selective hydrogenation is much less well studied and hence major opportunities to discover new uses for gold in this field exist. Very recently, Corma and Serna50 have demonstrated the regioselective reduction of a nitro group when other reducible functions are present using gold nanoparticles supported on TiO2 or Fe2O3. The chemoselective hydrogenation of functionalized nitroarenes with

Selective Oxidation Using Gold and Gold–Palladium Nanoparticles

565

H2 under mild reaction conditions was demonstrated providing a previously unknown route for the synthesis of the industrially important cyclohexanone oxime from 1-nitro-1-cyclohexene. This demonstrates the clear potential that is offered by gold for innovation in selective hydrogenation. It is clear that new reactions involving gold for selective hydrogenation will be discovered in the near future. However, the clear advantage offered by gold– palladium alloys for oxidation reactions also needs to be exploited. With respect to the direct synthesis of hydrogen peroxide, there are two facets where we can expect advances. First the major problem in the direct synthesis reaction is that any catalyst that is effective for the selective hydrogenation of dioxygen to hydrogen peroxide is also an effective catalyst for the over-hydrogenation to form water. This is a particular problem with mono-metallic Pd catalysts. Hence we can expect innovations in this area to try to control the overhydrogenation to give improved hydrogen peroxide selectivity. This can be expected to result from an understanding of the reaction mechanism coupled with improved preparation methodology. Secondly, the current gold–palladium catalysts have been shown to give very high rates for the formation of hydrogen peroxide in the absence of acid stabilisers and promoters. This can be exploited to develop reaction methodologies in which the hydrogen peroxide is captured in situ to generate selective oxidation products without the need to isolate the hydrogen peroxide as an intermediate. In summary, it is clear that supported gold catalysts can give exceptional performance for a wide range of redox reactions. This outstanding catalytic activity had remained undiscovered for many years, but now it is widely recognized.1–17 One wonders what new discoveries we now await.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

M. Haruta, Catal. Today, 1997, 36, 153. M. Haruta and M. Date, Appl. Catal. A, 2001, 222, 427. M. Haruta, CATTECH, 2002, 6, 102. M. Haruta, Chem. Record, 2003, 3, 75. M. Haruta, Gold Bull., 2004, 37, 27. G.C. Bond and D.T. Thompson, Catal. Rev. Sci. Eng., 1999, 41, 319. G.C. Bond and D.T. Thompson, Gold Bull., 2000, 33, 41. G.C. Bond, C. Louis and D.T. Thompson, Catalysis by Gold, Imperial College Press, 2006. D.T. Thompson, Appl. Catal. A, 2003, 243, 201. R. Meyer, C. Lemaire, S.h.K. Shaikutdinov and H.-J. Freund, Gold Bull., 2004, 37, 72. M.B. Cortie, Gold Bull., 2004, 37, 12. A.S.K. Hashmi, Gold Bull., 2004, 37, 51. G.J. Hutchings, Gold Bull., 1996, 29, 123. G.J. Hutchings, Gold Bull., 2004, 37, 37. G.J. Hutchings, Catal. Today, 2005, 100, 55.

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16. G.J. Hutchings and M.S. Scurrell, CATTECH, 2003, 7, 90. 17. A.K.S. Hashmi and G.J. Hutchings, Angew. Chem., Int. Ed., 2006, 45, 7896. 18. R. Hage, J.E. Iberg, J. Kerschner, J.H. Koek, E.L.M. Lempers, R.J. Martins, U.S. Racherla, S.W. Russell, T. Swarthoff, M.R.P. van Vliet, J.B. Warnaar, L. van der Wolf and B. Krijnen, Nature, 1994, 369, 637. 19. L. Prati and M. Rossi, J. Catal., 1998, 176, 552. 20. F. Porta, L. Prati, M. Rossi, S. Colluccia and G. Martra, Catal. Today, 2000, 61, 165. 21. C. Bianchi, F. Porta, L. Prati and M. Rossi, Top. Catal., 2000, 13, 231. 22. L. Prati, Gold Bull., 1999, 32, 96. 23. S. Carrettin, P. McMorn, P. Johnston, K. Griffin and G.J. Hutchings, Chem. Commun., 2002, 696. 24. S. Carretin, P. McMorn, P. Johnston, K. Griffin, C.J. Kiely and G.J. Hutchings, Phys. Chem. Chem. Phys., 2003, 5, 1329. 25. H. Kimura, A. Kimura, I. Kubo, T. Wakisaka and Y. Mitsuda, Appl. Catal. A, 1993, 95, 143. 26. P. Gallezot, Catal. Today, 1997, 37, 405. 27. S. Carretin, P. McMorn, P. Jenkins, G.A. Attard, P. Johnston, K. Griffin, C.J. Kiely and G.J. Hutchings, ACS Symp. Ser. (Feedstocks for the Future), 2006, 921, 82. 28. M.A. Schneeweiss, D.M. Kolb, D. Liu and D. Mandler, Can. J. Chem., 1987, 75, 1703. 29. L.Y. Chen, G.K. Chauh and J. Jaenicke, J. Mol. Catal. A, 1999, 132, 281. 30. W. Lin and H. Frei, J. Am. Chem. Soc., 2002, 124, 9292. 31. H.T. Hess, in Kirk-Othmer Encyclopaedia of Chemical Engineering, vol. 13, I. Kroschwitz and M. Howe-Grant (eds), Wiley, New York, 1995, p. 961. 32. H. Henkel and W. Weber, US Pat., 1108752, 1914. 33. Degussa Headwaters builds peroxide demonstrator, Chem. Eng., 2005, 766, 16. 34. L.W. Gosser and J.-A.T. Schwartz, US Pat., 4772458, 1988. 35. J. van Weynbergh, J.-P. Schoebrechts and J.-C. Colery, US Pat., 5447706, 1995. 36. B. Zhou and L.-K. Lee, US Pat., 6168775, 2001. 37. P. Landon, P.J. Collier, A.J. Papworth, C.J. Kiely and G.J. Hutchings, Chem. Commun., 2002, 2058. 38. P. Landon, P.J. Collier, D. Chadwick, A.J. Papworth, A. Burrows, C.J. Kiely and G.J. Hutchings, Phys. Chem. Chem. Phys., 2003, 5, 1917. 39. J.K. Edwards, B.E. Solsona, P. Landon, A.F. Carley, A. Herzing, M. Watanabe, C.J. Kiely and G.J. Hutchings, J. Mat. Chem., 2005, 15, 4595. 40. J.K. Edwards, B.E. Solsona, P. Landon, A.F. Carley, A. Herzing, C.J. Kiely and G.J. Hutchings, J. Catal., 2005, 236, 69. 41. D.I. Enache, J.K. Edwards, P. Landon, B. Solsona-Espriu, A. F. Carley, A.A. Herzing, M. Watanabe, C.J. Kiely, D.W. Knight and G.J. Hutchings, Science, 2006, 311, 362.

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42. B.E. Solsona, J.K. Edwards, P. Landon, A.F. Carley, A. Herzing, C.J. Kiely and G.J. Hutchings, Chem. Mater., 2006, 18, 2689. 43. K. Mori, T. Hara, T. Mizugaki, K. Ebitani and K. Kaneda, J. Am. Chem. Soc., 2004, 26, 10657. 44. A. Abad, P. Concepcio´n, A. Corma and H. Garcia, Angew. Chem. Int. Ed., 2005, 44, 1596. 45. C.W. Corti, R. Holliday and D. Thompson, Top. Catal., 2007, 44, 331. 46. J.M. Thomas and P.P. Edwards, Angew. Chem., Int. Ed., 2007, 46, 5480. 47. P.A. Sermon, G.C. Bond and P.B. Wells, J. Chem. Soc., Faraday Trans. 1, 1979, 75, 385. 48. J.J. Bailie and G.J. Hutchings, Catal. Commun., 2001, 291. 49. P. Claus, Appl. Catal., A, 2005, 291, 222. 50. A. Corma and P. Serna, Science, 2006, 313, 332.

CHAPTER 33

Electronic Factors in Hydrocarbon Oxidation Catalysis JERZY HABER Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Krakow, Poland

Oxygen is one of the most interesting elements playing a fundamental role in catalysis, because on the one hand it is a component of the most widely used type of catalysts – oxides, and on the other hand it is the reactant in one of the most important types of chemical processes – oxidation.1–3 The attack of oxygen on the hydrocarbon molecule is the easiest route to functionalize this molecule, and selective oxidation processes, in which hydrocarbon molecules are oxygenated to form alcohols, aldehydes or acids, are the basis of the modern petrochemical industry. They may be divided into vapour or liquidphase reactions, which are catalyzed by solid oxide catalysts and are carried out as heterogeneous catalytic processes, and reactions in the liquid phase, catalyzed by transition metal organometallic complexes or by enzymes, which are commonly realized as homogeneous catalytic processes, but efforts are undertaken to immobilize the catalysts and turn the processes heterogeneous. The process of oxidation of a hydrocarbon molecule must begin with the activation of the C–H bond. Realization of this first elementary step is particularly challenging, because it must be achieved in the presence of many constraints.4 The C–H bonds in the initial reactant are usually stronger than those in the intermediates, which are the required products. This makes these intermediates prone to rapid further oxidation to thermodynamically stable CO2 and renders the C–H bond activation rate determining and the selectivity kinetically controlled. In homogeneous liquid-phase catalytic processes the initiation consists of the transfer of one electron between the hydrocarbon and the metal complex

568

569

Electronic Factors in Hydrocarbon Oxidation Catalysis

followed by abstraction of a proton and formation of a radical: P:Mþn þ RH ! P:Mþðn
1Þ :RHþ P:Mþðn
1Þ RHþ ! P:Mþðn
1Þ þ R þ Hþ where P denotes an organic complex, e.g. the porphyrin ligand, and R the hydrocarbon radical, starting the chain reaction. Facility of the radical formation along this pathway will depend on the redox potential of the metalloporphyrin, and the strength of the C–H bond of the reactant. The redox potential of the metalloporphyrin may be modified by the choice of the metal, selection of the axial ligand and introduction of substituents into the porphyrin ligand. As an example Figure 1 shows the dependence of the yield of cyclooctanone in the reaction of cyclooctane with dioxygen in the presence of manganese porphyrin catalysts as a function of their half-way reduction potential.5 The second possibility involves generation of alkyl radicals as a result of the abstraction of a hydrogen atom from the hydrocarbon molecule by a coordinated dioxygen, i.e. a superoxo or peroxo metal complex, which has electrophilic properties: P:Mþðn
1Þ þ O2 ! P:Mþn
O
O P:Mþn
O
O þ RH ! P:Mþn
O
OH þ R

30

25

C-one+C-ol, %

Yield, %

20 C-one, % 15

10

C-ol, %

5

0 -0.25

Figure 1

-0.15

-0.05

0.05 E1/2 [V]

0.15

0.25

Yields of cyclooctanone and cyclooctanol in the presence of manganese porphyrin as catalyst as a function of its half-way potential modified by introduction of substituents.5

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Chapter 33

or direct oxidation by a reactive high valent oxometal species P:Mþðnþ1Þ ¼ O þ RH ! P:Mþðnþ1Þ
OH:R ! P:Mþn þ ROH In the case of heterogeneous catalytic oxidation, in the liquid or gas phase, the hydrocarbon molecule is activated by the cleavage of the C–H bond on adsorption at the surface of the solid. The transfer of electrons from an adsorbed hydrocarbon molecule, which behaves as a redox pair, into an oxide with semiconducting properties can take place spontaneously only if the redox potential of this pair is situated above the Fermi level of the solid and above the bottom of the conductivity band, and the extraction of electrons from the solid can take place when the redox potential is located below the Fermi level and below the top of the valence band. This is illustrated in Figure 2.6 The probability of these processes is a function of the density of states in the conductivity and valence band respectively at the potentials corresponding to the redox potential of the adsorbed species. The relative positions of the energy levels in the solid and the redox potential of the reacting molecules may be adjusted by (a) formation of one or more oxide/oxide interfaces with such values of the contact potential that the energy levels in the solid will shift to the optimum position, (b) doping of the oxide with altervalent ions, which will shift the Fermi level, (c) generation of surface defects, which will create a broad distribution of surface electronic

Hydrocarbon

Hydrocarbon

Eredo

Eredo Hydrocarbon E

E

E Eredo

Eredo Oxygen

Eredo Oxygen

Eredo Oxygen

Catalytic oxidation of hydrocarbon molecule can proceed

Figure 2

Catalytic oxidation of hydrocarbon molecule cannot proceed, because the molecule is not activated

Catalytic oxidation of hydrocarbon molecule cannot proceed, because the catalyst is not reoxidized

Energy diagram of the electron transfer from a hydrocarbon molecule to oxygen in the heterogeneous catalytic oxidation mediated by an oxide of semiconducting properties.6

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states mediating the exchange of electrons between the reacting adsorbed molecule and the oxide catalyst, or (d) application of an external potential. The distribution of electrons in the reacting molecule may also play an important role and the possibility that different orbitals participate in the interaction with the solid surface depending on the way the molecule becomes adsorbed. Experimental evidence from different surface sensitive techniques reveals the presence at oxide surfaces of alkoxy and OH groups indicating that both parts of the cleaved C–H bond become bonded to the surface oxide ions.7 The quantum-chemical calculations with the DFT (density functional theory) method for a methane molecule interacting with a vanadium oxide cluster V3O12H9 as a model mimicking a fragment of the oxide surface showed indeed that the most probable is such reaction pathway, in which both fragments of the dissociating C–H bond become attached to the surface oxide ions.8 The proton forms an OH group, the hydrocarbon fragment forms an alkoxy group and two electrons become injected into the conductivity band of the oxide:

The reaction proceeds usually by the Mars–van Krevelen mechanism, in which the molecule is oxidized by the catalyst, which is then reoxidized by gas phase dioxygen. The catalyst must thus be able to undergo easily the change of oxidation state of its cations. Since transition metal cations in oxides are able to change the electronic state in a wide spectrum, these oxides are components of all active and selective catalysts. The oxidation of a hydrocarbon molecule at the surface of an oxide catalyst involves thus the operation of two redox couples: R
CH3 þ 2O
2 ! R
CHO þ H2 O þ 2VO þ 4e
O2 þ 2VO þ 4e
! 2O
2 (where VO denotes an oxygen vacancy) of which the first injects electrons into the oxide catalyst, whereas the second one extracts them from the oxide. In the first redox couple the hydrocarbon molecule is activated by the cleavage of the C–H bond, which is usually the rate determining step in the hydrocarbon oxidation processes, followed by addition of nucleophilic oxygen. As an example, the exchange of electrons with catalysts composed of vanadia supported on titania in liquid phase heterogeneous oxidation will be discussed. Scanning tunneling microscopy shows that heating in vacuum generates surface oxygen vacancies on the surface of the rutile monocrystal.9 By using cyclic voltammetric experiments it has been possible to show that these vacancies are active sites mediating the transfer of electrons in the oxidation of water and evolution of oxygen, the rate being proportional to the surface concentration of

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Chapter 33 0.15

A1

A2

0.10

0.00 0.2

0.6

1.0

-0.05

C3

1.4

1.8

2.2

E [V]

?

j [mA . cm-2]

A3 0.05

-0.10 C2

Figure 3

V5+ / V4+

C1 V4+ / V3+

-0.15

Stationary cyclic voltammetry curves for the rutile electrode reduced in hydrogen and then heated with V2O5 in argon atmosphere at 730 K.10

these vacancies. Vanadium ions deposited at the surface of rutile generate local energy levels (Figure 3) below the conductivity band edge (Figure 4) and mediate the electrocatalytic oxidation of many organic molecules, catechol being an example, pure rutile surface being inactive.10 The rate of the oxidation reaction to quinone (Figure 5) is proportional to the number of vanadium ions present at the surface11 indicating that these ions play the role of active centres in the catalytic oxidation. V14 ions diffuse into the subsurface layer and can be reduced to V13 oxidation state due to charge compensation by protons incorporated into the surface layer, but only the outermost vanadium ions can be oxidized to V15 oxidation state as a result of chemisorption of oxygen. Thus, the position in the energy spectrum of the solid and the ability to change the valence state makes it possible for vanadium ions to mediate the electron transfer between the reacting organic molecules and the catalyst and the oxidation of these molecules takes place, the catalyst mediating the flow of electrons from the organic molecule to dioxygen, which cannot take place directly. A second example is the oxidation of lower alkanes with dioxygen over the vanadium phosphate catalyst to form acids and anhydrides. The rate of oxidation depends on the position of the Fermi level determining the nucleophilicity of the surface as expressed by the binding energy of O1s electrons determined by XPS (X-ray photoelectron spectroscopy) (Figure 6).12 Transition metal oxides are nonstoichiometric compounds (Bertholides) with composition depending on the equilibrium between the lattice and its constituents in the gas phase. This is a dynamic equilibrium, in which the rate of dissociation of the oxide lattice and evolution of oxygen in the form of O2 molecules is equal to the rate of its incorporation from the gas phase into the surface layer of the solid. In the process of dissociation the lattice oxide ions must be extracted from the surface, electrons must be injected into the solid,

573

Electronic Factors in Hydrocarbon Oxidation Catalysis 0.5 mol dm-3 H2SO4 E

E [V]

-0.5

-0.5

EC

0.0

V4+/V3+ EF

0.5

V5+/V4+

0.0 EC EF

1.0

0.5 1.0 1.5

2.0

2.0

2.5

2.5 3.0

λ λ

1.5

EV

e-

Eox E0 redox Ered

e-

3.0 EV

OH

O

OH

Figure 4

O + 2H+ + 2e-

Energy diagram of TiO2 doped with vanadium ions as catalyst in electrocatalytic oxidation of catechol.

j [mA . cm-2]

0.6 0.5

ja

0.4

jc

0.3 0.2 0.1 0.0 0.0

0.5

1.0

1.5

2.0

2.5

Q [mC . cm-2]

Figure 5

Anodic ( ja) and catodic ( jc) current density due to oxidation of catechol to quinone as a function of the surface concentration of vanadium ions.

and oxygen atoms must recombine to form molecules and desorb as dioxygen. The reverse series of elementary steps takes place upon incorporation. These elementary steps result in surface equilibria: 2

O2
$O
ðchemÞ þ VO $O2ðchemÞ $O2ðchemÞ $O2ðadsÞ $O2ðgasÞ

574

Chapter 33 3

C4H10 W.104, mol/h.m2

2 VPO

C5H12 1 VPO C3H8 VPO 0 531.0

Figure 6

531.5 B.E. O 1s, eV

532.0

Rate of the oxidation of propane, butane and pentane as a function of the binding energy of O1s electrons in the (VO)2P2O7 catalyst modified by doping.12

so that the surface of an oxide is always populated with different oxygen species. The surface coverage by these species depends on oxygen pressure in the gas phase, the rate constants of adsorption and chemisorption (transfer of electrons between adsorbed oxygen molecules and the solid), the rate constant of recombination of oxygen ions with surface oxygen vacancies and the dissociation pressure of the oxide.13 The different oxygen species present at the oxide surface have different reactivities and may react with other reactants, adsorbed at the surface, along different pathways. Thus, the selectivity of the reaction will strongly depend on the relative coverages of the surface by these species. Lattice O2
ions exposed at the surface are nucleophiles and are usually
responsible for selective oxidation, whereas O
2(chem) and O(chem) are electrophiles and lead to the formation of radicals, starting the chain reaction, which in the gas phase eventually results in total oxidation.14 They are equivalent to ‘‘hot oxygen atoms’’ described by Roberts et al.15 and their presence may be detected by the same procedure, using probe molecules, which undergo combustion, but are known not to react with lattice oxide ions. Thus, different oxygen species present at the oxide surface compete for hydrocarbon molecules. Moreover, a competition also exists between hydrocarbon molecules and surface oxide vacancies for the electrophilic O
(chem) species, which can either be captured by hydrocarbon molecules to form

Electronic Factors in Hydrocarbon Oxidation Catalysis

575

products of electrophilic oxidation or be incorporated into the surface by reaction with oxygen vacancies VO. Two parallel reaction pathways followed by a hydrocarbon molecule when adsorbed at the surface of an oxide catalyst, its selective oxidation (allylic oxidation in the case of olefins) by surface lattice oxide ions (nucleophilic oxidation) or its destructive oxidation by reactive
oxygen species O
(chem), O2(chem) (electrophilic oxidation), are described by different rate equations, but are coupled together by the equation describing the generation and annihilation of surface oxygen vacancies: O
2 # O
13 V
+O . The contributions from these two pathways depend on the O (chem) relative values of kn, ke, kinc, kdiss, kchem and kads. The first two rate constants depend on the nature of the hydrocarbon molecule, whereas the rate constants describing the transformation of surface oxygen species kinc, kdiss, kchem and kads are characteristic of the oxide. Measurements of the rate of homomolecular isotopic exchange of oxygen permits comparison of the population of the surface of different oxides by electrophilic oxygen.16 The consequence of the dynamic interaction of the catalyst with the gas phase is the adaptability of the oxide to changes in external conditions. The surface of an oxide catalyst may respond to changes in composition of the reacting catalytic mixture resulting in changes of the redox potential of the gas phase in three ways:  the steady-state degree of reduction of the catalyst surface may change, the surface defect equilibria at the oxide surface and in the bulk may be shifted, and changes of concentration of a given type of site involved in the catalytic transformation may cause changes in catalytic properties;  when the concentration of defects at the oxide surface surpasses a certain critical value, ordering of defects or formation of a new bidimensional surface phase may occur resulting often in a dramatic change of catalytic properties;  when a redox mechanism operates in the catalytic reaction, the ratio of the rate of catalyst reduction and its reoxidation may be different for various phases and hysteresis in the dependence of catalytic properties on the composition of the gas phase may appear, these properties being then strongly influenced by the type of pretreatment.

References 1. A. Bielan˜ski and J. Haber, Oxygen in Catalysis, Marcel Dekker, New York, 1991. 2. G. Centi, F. Cavani and F. Trifiro, Selective Oxidation by Heterogeneous Catalysis, Kluwer Academic/Plenum Press, New York, 2001. 3. R.A. Sheldon and R.A. van Santen (eds), Catalytic Oxidation, World Scientific, Singapore, 1995. 4. J.A. Labinger, J. Mol. Catal.: A Chem., 2004, 220, 27.

576

Chapter 33

5. J. Poltowicz and J. Haber, J. Mol. Catal.: A Chem., 2004, 220, 43. 6. J. Haber, Stud. Surf. Sci. Catal., 1997, 110, 1. 7. F. Finocchio, G. Busca, V. Lorenzelli and R.J. Viley, J.Catal., 1995, 151, 204. 8. E. Broczawik, J. Haber and W. Piskorz, Chem. Phys. Lett., 2001, 333, 332. 9. N. Spiridis, J. Haber and J. Korecki, Vacuum, 2001, 63, 99. 10. J. Haber and P. Nowak, Top. Catal., 2002, 20, 75. 11. J. Haber, P. Nowak and P. Zurek, unpublished results. 12. V.A. Zazhigalov, J. Haber, J. Stoch and E.V. Cheburakova, unpublished results. 13. J. Haber and W. Turek, J. Catal., 2000, 190, 320. 14. J. Haber, ACS Symp. Ser., 1996, 638, 20. 15. A.F. Carley, P.R. Davies and M.W. Roberts, Phil. Trans. Roy. Soc. A, 2005, 363, 829. 16. J. Haber and B. Grzybowska, J. Catal., 1973, 28, 489.

CHAPTER 34

The Importance of Selectivity in Ammoxidation Catalysis ROBERT K. GRASSELLIa,b a

Center for Catalytic Science and Technology, University of Delaware, Newark, DE19716, USA; b Department of Chemistry, Technische Universita¨t Mu¨nchen, D-85748, Garching, Germany

1 Background I have had the pleasure to meet Sir John about 30 years ago when I was at SOHIO (Cleveland, OH, USA), and where he was invited to give a series of lectures. A few years later, at the Discussions of the Faraday Society in Nottingham, England (1981), he came to my rescue when I proposed in my lecture surface shear structures as being responsible for the superior performance of our new multiphase-multicomponent ammoxidation catalysts. On our mutual train ride from Nottingham, destinations Oxford and Cambridge, Sir John’s persuasive account of his instrumental facilities convinced me to switch my plan of having the catalyst samples, brought with me for Sir Peter Hirsch of Oxford to examine in his HREM, that we lacked in Cleveland at that time, to Cambridge for analysis. I delivered my lecture at Oxford, never mentioning my samples and headed for Cambridge. From there on, our scientific cooperation and my esteem for Sir John both as a scientist and a person has flourished unabated. Selectivity is of utmost importance in heterogeneous oxidation catalysis as the cost of feed materials continues to escalate. In commercial processes, selectivity at acceptably high conversions is imperative. Heeding this premise we proposed, some forty years ago, the concept of site isolation, defining one of the key requirements needed to achieve selectivity in oxidation catalysis. This concept retains its usefulness in the conceptual design of new selective oxidation catalysts and successfully describes the selectivity behaviour of the currently leading contender, MoV(Nb,Ta)(TeSb)O system, for the ammoxidation of 577

578

Chapter 34

propane to acrylonitrile. The system is comprised of at least two crystalline phases, orthorhombic Mo7.8V1.2NbTe0.94O28.9 (M1) and pseudo-hexagonal (Mo4.67V1.33Te1.82O19.82) (M2), wherein the M1 phase is the key paraffin activating and ammoxidation catalyst, its active centres containing all key catalytic elements V51, Te41, Mo61, properly arranged to transform propane directly to acrylonitrile. Four Nb51 centres, each surrounded by five molybdenum–oxygen octahedra, isolate the active centres from each other, preventing overoxidation to COx. Symbiosis between the M1 and M2 phases is observed at commercially interesting high propane conversions, provided the two phases are finally divided, thoroughly mixed and in nano-scale contact with each other. The M2 phase serves as a cocatalyst to the M1 paraffin activating phase, converting desorbed propylene intermediate effectively to acrylonitrile in a phase cooperation mode.

2 Introduction Selectivity is of utmost importance in industrial catalytic processes with the feed hydrocarbons becoming less abundant and thereby more expensive. In successfully devising catalysts of commercial importance, selectivity of the desirable product must be achieved at reasonably high conversions. Industrial researchers recognized this requirement already in the forming years of heterogeneous selective oxidation catalysis. Thus, as early as 1963 Callahan and Grasselli1 put forward their hypothesis of site isolation. This hypothesis states that oxidation catalysts become selective when the number of reacting surface oxygens at the active centres is limited and these centres are spatially isolated from each other. This hypothesis has been verified on many catalytic systems since its inception and served their originators well in the discovery of an array of catalytic solids for the oxidation and ammoxidation of light olefins that have successfully been commercialized (seven generations of propylene to acrylonitrile ammoxidation catalysts and five oxidation of propylene to acrolein and acrylic acid catalysts).2 The site isolation hypothesis can now be extended to include also paraffin conversion catalysts such as MoV(Nb,Ta)(Te,Sb)O and is the subject of this contribution. Although the well known SOHIO/BP process2,3 for the direct ammoxidation of propylene to acrylonitrile is very efficient giving 80+% acrylonitrile yield on commercial scale,4 there is currently a substantial incentive, because of the large price differential between propane and propylene, to discover an effective propane catalyst so that future ammoxidation processes would be paraffin based. Promising catalyst candidates are promoted VSbO and MoVNb(Te,Sb)O systems, with the latter holding a substantial edge over the former.5–7 Site isolation is also central for achieving good selectivity in the epoxidation of propylene to propylene oxide using TS-1 and hydrogen peroxide,8 the hydroxylation of phenol to p-dihydroxybenzene using also TS-1 and hydrogen peroxide,9 and the hydroxylation of benzene to phenol using Fe-ZSM-5 and

The Importance of Selectivity in Ammoxidation Catalysis

579

10,11

N2O. The desired selectivity is attained only after the respective active sites are sufficiently isolated from each other in the framework structure to prevent undesirable side reactions. Similarly, site isolation also plays a central role in achieving superior selectivity levels in reactions studied by J.M. Thomas and his group. Exemplary are the oxidation of cyclohexane/cyclohexanone using FeAlPO-5 or FeAlPO-31 as catalysts to produce adipic acid,12 n-hexane oxidation using CoAlPO-18 to produce adipic acid,13 and the ammoximation of cyclohexane with ammonia and dioxygen to produce e-caprolactam using MgMnAlPO-5 as catalyst.14,15 All of these catalysts are bi-functional in nature, and it is imperative in the design of the catalysts that the two respective, differing catalytic functions be spatially separated from each other (site isolation) to achieve the desired product selectivity.

3 Experimental The methods employed for the preparation, evaluation and optimization of MoV(Nb,Ta)(Te,Sb)O catalysts and for their structure determinations have been described earlier.16 Further details pertaining to the solutions of the M1 and M2 structures are found in references.17–19 The preparation, compounding and catalytic testing of M1/M2 physical mixtures is described in Ref. 20.

4 Results and Discussion Over the past forty years, the site isolation hypothesis1 has been extended from the original CuO catalyst to include V2O5/KVO4, Bi9PMo12O52, (K,Cs)(Ni,Co,Mg)(Fe,Ce)(Sb,P)BiMoO, USb3O10, FeSbO, VSbO, (VO)2P2O7, and now also the MoV(Nb,Ta)(Te,Sb)O systems and has recently been reviewed.6,21 In these systems, high product selectivity can be explained on the basis that the reactive oxygens at the respective active centres are limited in number and that the active centres are spatially separated from each other. The desired site isolation on the surface of the respective solid catalyst can be achieved either by partial reduction (CuO), by breaking up interconnecting M–O–M–O–M chains (V2O5/KVO4), by phosphate ‘‘fences’’ ((VO)2P2O7) or by the structural makeup of the solid (remaining examples above). While selectivity is of utmost importance in oxidation catalysis, driving innovation by itself, it is not sufficient if searching for a commercially viable catalyst. Selectivity by itself cannot be bottled or put into drums and sold! Selectivity must be coupled with sufficient activity in a catalyst so that reasonable product yields are achieved. It is much easier to obtain high catalyst selectivity at low conversions than at high conversions; but that is commercially an impractical result. Therefore, it is important to discover catalysts having high selectivity also at high conversions. All of the above listed catalysts, obeying the site isolation concept, fall into this category. It is well known by now that most heterogeneous catalysts effective for paraffin activation and subsequent selective oxidation or ammoxidation

580

Chapter 34

contain vanadium as the key paraffin activating element, be it VSbO, MoVNb(Te,Sb)O or AlVON.5–7 In order to achieve selectivity at reasonable paraffin conversions, it is necessary to structurally isolate the paraffin activating vanadium centers from each other. Only V51 sites activate paraffins, V41 sites are ineffective for paraffin activation.5–7,16,21 At present, the MoVNbTeO system gives the highest acrylonitrile yields (B62%);5–7 MoVNbSbO is a close second (B55+%).6,22 The two systems are structurally very similar. Catalytically both systems employ (V51¼O 2 41Vd–Od) moieties as their paraffin activating sites, located in their respective active centres. In the first system Te41–O sites, also located in the active centre, are the a-H abstracting moieties activating the chemisorbed propylene intermediate once formed, while in the second Sb31–O sites perform this function.5–7 The MoVNbTeO system originally discovered by the Mitsubishi Company23 is comprised of three crystalline phases: orthorhombic Mo7.8V1.2NbTe0.94O28.9 (M1), pseudo-hexagonal Mo4.67V1.33Te1.82O19.82 (M2) and a trace of monoclinic TeMo5O16.5–7 The key paraffin conversion phase is M1 (Figure 1). This phase contains active centres comprised of an assembly of five metal oxide octahedra (2V510.32/Mo610.68, 1V410.62/Mo510.38, 2Mo610.5/Mo510.5) and two tellurium-oxygen sites (2Te410.94), which are stabilized and structurally isolated from each other (site isolation) by four Nb51 sites, each surrounded by five molybdenum-oxygen octahedra. These centres contain all necessary key elements within bonding distance of each other to effectively convert propane to acrylonitrile without need of desorption of reaction intermediates. The V51¼O sites activate the propane by abstracting a methylene hydrogen, the Te41–O sites abstract the a-H of the chemisorbed propylene once formed and the adjacent O¼Mo61¼NH sites insert NH into the chemisorbed p-allylic intermediate forming the acrylonitrile precursor as shown in Figure 1. Based on these proposed catalytic steps, derived on the basis of sound classical organic chemistry and in depth knowledge of the M1 structure, a complete reaction mechanism can be written which is recorded in the literature16 but will not be further elucidated here because of space limitations. To attain selectivity, the catalytically active centres of M1 are spatially isolated from each other by Nb51–5Mo51/41 pentagonal bipyramids as illustrated in Figure 2. This arrangement of active centres on the surface favours acrylonitrile selectivity and minimizes unwanted overoxidation to COx. A statistical analysis of the various elemental distribution probabilities at the active centres of M15 predicts that a maximum acrylonitrile selectivity of 81% is attainable using M1 as the catalyst. Thus far the maximum experimentally obtained acrylonitrile selectivity is 72%.5,6 A possible explanation for the difference is that some propylene desorbs from the M1 active centres before it is fully converted to acrylonitrile, begins to migrate, encounters other unoccupied V51 sites and combusts to COx. Experimentally, some unconverted propylene is observed at high throughputs24,25 whereas symbiosis between M1 and M2 phases leading to improved acrylonitrile yields is observed at high propane conversions.5–7,16 The optimum acrylonitrile yield is obtained

581

The Importance of Selectivity in Ammoxidation Catalysis

8

6

11 10 5

9

3 10

5

8

9

6 12

1

11

7

4 2 7

4 12

1

11

5

6

8

10 9

3 6

5

9

10

8 11 M1: V4+0.26 / Mo5+0.74 M2: V4+0.62 / Mo5+0.38 M3: V5+0.42 / Mo6+0.58

M4: Mo6+0.5 / Mo5+0.5 M5, 6, 8,10: Mo6+1.0 M7: V5+0.32 / Mo6+0.68

O

O

V5+

O O

M7

Mo

M4

O

Figure 1

NH 6+

O

Nb5+ O

O M9

O

O O O O

M9: Nb5+1.0 M11: Mo5+1.0 M12: Te4+0.94

O O O

4+

Te O

M1

M5 O

O

M7 O

O O O O

Nb5+ O O

NH 6+

M4

O Te4+

Mo

O O

6+

Mo

V4+

O

M1

O

6+

Mo

M5

O

M9

Catalytically active centre of Mo7.8V1.2NbTe0.94O28.9 (M1) phase in [001] projection and ChemDraw illustration of the active centre.5

with catalysts comprised of about 60% M1 and 40% M2 (Figure 3). This experimental result suggests a cooperative effect (symbiosis) between the phases. Symbiosis between M1 and M2 phases, cooperating with each other, is observed when the two phases are prepared together in one container (optima of Figure 3). Separately prepared phases, co-mingled after preparation as physical mixtures exhibit also symbiosis, however, only if the precursor phases are in the size range of 5 mm, or lower.6,20 The observed catalytic results of a 50 wt% M1/50 wt% M2 physical mixture (4:1 on surface area basis) are shown in Figure 4. The symbiotic effect is clearly observed, particularly at the higher conversions, as expected.5,6 At conversions below 5% there is no enhancement in acrylonitrile selectivity or yield of the physical mixture over that of the M1 phase alone. However, at higher conversions the enhancement is substantial as

582

Figure 2

Chapter 34

Site isolation: four catalytically active centres on M1 surface in [001] projection.16

80 70

AN Yield (mole%)

60

N

50 Ta

40 30 20 10 0 0

20

40

60

80

100

% orthorhombic phase

Figure 3

MoV(Nb,Ta)TeO system. Acrylonitrile yield versus % orthorhombic phase (M1).7

583

The Importance of Selectivity in Ammoxidation Catalysis 50

Selectivity to Acrylonitrile (%)

45 40

symbiosis

35 30 25 20 15 Legend:

10 5

=

5 µm

M1:

M1 = M1 +M +

M1 +M

250-425 µm

M2 M1 +M

, ,

=

M1 +M

0 0

10

20

30

40

50

60

Conversion (%)

Figure 4

Symbiosis of M1/M2 physical mixtures in propane ammoxidation.7,20 Legend: }, Pure M1 phase; ’, M1 and M2 mixed as particles (250–425 mm);
, M1 and M2 mixed as powders (B5 mm) and pressed; ~, M1 and M2 mixed as powders, pressed and reheated at 550 1C for 1 h; m, M1 and M2 mixed as powders, pressed and reheated at 600 1C for 1 h. The physical mixtures consisted of 50 wt% M1/50 wt% M2, corresponding to a surface area ratio of 4:1. Reaction temperature ¼ 380 1C; propane/ammonia/oxygen/ argon ¼ 6.1/7.0/18.0/70.2; space velocity ¼ 3.3–26.3 N cm3/(min g).

revealed in Figure 4. One can reason that at the lower conversion the classical site isolation for M1 dictates the selectivity,5,6,16 while at higher conversions a substantial amount of propylene that forms on the M1 phase desorbs before it can be directly converted to acrylonitrile at the first encountered active centre, and leaves the centre without encountering and adsorbing on a new unoccupied V51 site which would lead to combustion. Instead, the ultimate proximity of the M2 phase allows the desorbed propylene to interact with its surface rather than the M1 surface from where it originated. Since the M2 phase does not possess any V51 centres (only benign V41 centres), but ample Te41 centres (more than on the M1 surface) it activates the olefin effectively and converts it to acrylonitrile. It is also known that the M2 phase is more efficient for the conversion of propylene to acrylonitrile than is M1.20,24,25 Based on the above studies, a reaction network involving the M1 and M2 phases can be proposed as illustrated in Figure 5. A schematic of M1 and M2 surface active centre distributions, as derived from the statistics of the respective phase structures, 5,6 is shown in Figure 6. The schematic illustrates that at low throughput (mild reaction conditions) the M1 phase suffices to convert propane to propylene. At high conversions (demanding reaction conditions)

584

Chapter 34 M1 k3 M1 k5 H2 C

H 3C

M1 M1 CH 3

k1

H C

M1 CH3 a

H 3C

k2

M1 CH 2 a

k4

H 2C

H C

k6 CN

M2

COx

k7

k8

k1 = rate determining k 2 > k1 k3 << 1/5 k2 k4 > k1< k2 k5 << 1/5 k2 k6 <<< k4

Figure 5

H 3C

H C

k7 <<< k4 k8 ~ k4 > k1 k9 < k8 k10 < 1/5 k2 k11 > k8 > k9 k12 < k10

M1 H3C

H C

k9

M1 k

M2

M2

k11

k12

CH 2

10

Proposed propane ammoxidation reaction network using M1 and M2 catalysts.5,6

Mild Operating Conditions (Low Throughput and Low Conversion) AN

AN C3=

C30

AN

NH3 O2

COx

AN M1

C3=

AN M2

Demanding Operating Conditions (High Throughput and High Conversion) AN

C3=

AN

AN

C3=

AN

AN

AN

C30 NH3

For C30 Active Inactive Waste

O2

COx

Figure 6

AN M1

C3=

AN

C3=

For C3= Active Active Waste

M2

Schematic of propane ammoxidation over MoV(Nb,Ta)(Te,Sb)O catalysts.5,6

The Importance of Selectivity in Ammoxidation Catalysis

585

cooperation of the phases (symbiosis) sets in, with the M2 phase becoming a cooperative (mop-up) phase to the M1 phase. Since symbiosis was demonstrated not only for M1/M2 mixtures prepared together in one container but also when prepared separately and post commingled as a physical mixture, provided the particle size of the phases was small enough (r5 mm), it opens up the opportunity to alter the compositions independently in an effort to improve the catalytic performance of either and/or both of the phases. This is the subject of current studies.

5 Conclusions Selectivity in heterogeneous oxidation catalysis is paramount with feed materials for commercial petrochemical processes becoming less abundant and ever more expensive. The desired selectivity behaviour of many heterogeneous oxidation and ammoxidation catalysts can be explained on the basis of the site isolation hypothesis originally proposed some forty years ago by Callahan and Grasselli.1 This includes many model (e.g., CuO; V2O5/KVO4), as well as well known commercial catalysts (e.g., Bi9PMo12O52, USb3O10, FexSbyOz, (VO)2P2O7, (K,Cs)(Ni,Co,Mg)(Fe,Ce)(Sb,P)BiMoO, and now the leading commercial candidate for propane ammoxidation MoV(Nb,Ta)(Te,Sb)O to acrylonitrile). The concept is a useful tool in catalyst design. The optimized MoVNbTeO catalyst system discussed here is comprised of three crystalline phases: Mo7.8V1.2NbTe0.94O28.9 (M1), pseudo-hexagonal Mo4.67V1.33Te1.82O19.82 (M2) and a trace of TeMo5O16, wherein the M1 phase is the key paraffin activating and acrylonitrile forming phase. The active centres of M1 as proposed16 contain all key elements V51, Te41 and Mo61 properly spaced and within bonding distance of each other, to effectively transform propane directly to acrylonitrile under ammoxidation conditions. The active centres are spatially separated from each other by four Nb51 pentagonal bipyramids (each surrounded by five Mo–O octahedra), which provides for the necessary site isolation, consistent with our isolation concept.1,2 At high propane conversions, symbiosis between the M1 and M2 phases is observed provided the two phases are finely divided (B5 mm),20 thoroughly mixed and in intimate contact with each other (nano-scale). Under these conditions enhanced selectivity and yields of acrylonitrile are obtained as a result of phase cooperation, a concept also well known in oxidation catalysis.2,21 With the demonstration that symbiosis can be obtained when the two phases are prepared separately and mixed later, the two phases can be doped independently in an effort to improve the overall catalytic performance of the system. While selectivity is a key requirement for successful selective oxidation and ammoxidation catalysis, it is not sufficient by itself and must be coupled with acceptable activity, particularly when designing and formulating commercially feasible catalysts. Selectivity and activity must be optimized and balanced

586

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simultaneously in a catalytic composition. The mutual attainment of optimal selectivity and activity might be helped by employing the described concepts of site isolation1 and phase cooperation21 in the design of new catalysts.

Acknowledgement The author thanks the Alexander von Humboldt Foundation for partial financial support of the work reported here.

References 1. J.L. Callahan and R.K. Grasselli, AIChE J., 1963, 9, 755. 2. R.K. Grasselli, Top. Catal., 2002, 21, 79. 3. R.K. Grasselli, Handbook of Heterogeneous Catalysis, vol 5, G. Ertl, H. Kno¨zinger and J. Weitkamp (eds), 1997, 2303; ibid., Proceedings of DGMK Conference, Hamburg, Germany, 2001, 147; ibid., La Chimica e l’Industria, 2001, 83, 25. 4. R.K. Grasselli, Catal. Today, 1999, 49, 141. 5. R.K. Grasselli, D.J. Buttrey, P. DeSanto Jr., J.D. Burrington, C.G. Lugmair, A.F. Volpe Jr. and T. Weingand, Catal. Today, 2004, 91–92, 251. 6. R.K. Grasselli, Catal. Today, 2005, 99, 23. 7. R.K. Grasselli, D.J. Buttrey, J.D. Burrington, A. Andersson, J. Holmberg, W. Ueda, J. Kubo, C.G. Lugmair and A.F. Volpe Jr., 2006, 38, 7. 8. G. Belussi and M.S. Rigutto, in: Advanced Zeolite Science and Applications, Stud. Surf. Sci. Catal., vol 85, J.C. Jansen, M. Stoecker, H.G. Karge and J. Weitkamp (eds), Elsevier, Amsterdam, 1991, 177. 9. G. Belussi and C. Perego, Handbook of Heterogeneous Catalysis, vol 5, G. Ertl, H. Kno¨zinger and J. Weitkamp (eds), 1997, 2329. 10. G.I. Panov, A.S. Kharitonov and V.I. Sobolev, Appl. Catal., 1993, 98, 1. 11. E.V. Starkov, K.A. Dubkov, L.V. Pirutko and G.I. Panov, Top. Catal., 2003, 23, 137. 12. J.M. Thomas, R. Raja, G. Sankar and R.G. Bell, in Proceedings of 12th Int. Congr. On Catalysis, Granada, Stud. Surf. Sci. Catal., vol 130, A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (eds), Elsevier, Amsterdam, 2000, 887. 13. R. Raja, G. Sankar and J.M. Thomas, Angew. Chem. Ind. Ed., 2000, 39, 2313. 14. J.M. Thomas and R. Raja, Proc. Nat. Acad. Sci., 2005, 102, 13732. 15. J.M. Thomas, Top. Catal., 2006, 38, 3. 16. R.K. Grasselli, J.D. Burrington, D.J. Buttrey, P. DeSanto Jr., C.G. Lugmair, A.F. Volpe Jr. and T. Weingand, Top. Catal., 2003, 23, 5. 17. P. DeSanto Jr., D.J. Buttrey, R.K. Grasselli, C.G. Lugmair, A.F. Volpe Jr., B.H. Togy and T. Vogt, Top. Catal., 2003, 23, 23. 18. P. DeSanto Jr., D.J. Buttrey, R.K. Grasselli, C.G. Lugmair, A.F. Volpe Jr., B.H. Togy and T. Vogt, Z. Krist., 2004, 219, 152.

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19. E. Garcia-Gonzalez, J.M. Lopez Nieto, P. Botella and J.M. GonzalezCalbet, Chem. Mater., 2002, 14, 4416. 20. J. Holmberg, R.K. Grasselli and A. Andersson, Appl. Catal. A: Gen., 2004, 270, 121. 21. R.K. Grasselli, Top. Catal., 2001, 15, 93. 22. C.G. Lugmair, J. Zysk and R.K. Grasselli, U.S. Patent Appl. 2005/0054869 A1. 23. M. Hatano and A. Kayo, European Patent 318,295, 1988; T. Ushikubo, K. Oshima, A. Kayou, T. Umezawa, K. Kiyono and I. Sawaki, European Patent 529,853, 1992; T. Ushikubo, K.Oshima, A. Kayou, T. Umezawa, K. Kiyono, I. Sawaki and H. Nakamura, US Patent 5,472,925, 1995; H. Hinago, S. Komada and A.K. Kogyo, US Patent 6,063,728, 2000. 24. J. Holmberg, R.K. Grasselli and A. Andersson, Top. Catal., 2003, 23, 55. 25. W. Ueda, D. Vitry and T. Katou, Catal. Today, 2005, 99, 43.

CHAPTER 35

The Mysteries of Water in Catalyst Preparation: Solvent or Much More? MICHEL CHE Institut Universitaire de France, Laboratoire de Re´activite´ de Surface, Universite´ Pierre et Marie Curie - Paris 6, 4 Place Jussieu, 75252 Paris Cedex 05, France

1 Introduction A hundred years ago, in the single and miraculous year 1905, Einstein wrote five papers1–5 that shook the world and changed the face of physics. They were at the origin of the World Year of Physics 2005 (Einstein in the 21st century), which led the French CNRS (National Centre for Scientific Research) to publish a special issue,6 in which the 10 great enigmas of physics were identified. It is striking that the first one, expressed as ‘‘the mysteries of water’’, could have been ranked also as N1 1 for biology and chemistry, so overwhelming is the role of water in life processes and in the genesis of intelligent materials, among which catalytic systems can be included.7 In view of the importance of catalysis and catalysts in the production of chemicals, it is not surprising that the American Chemical Society decided to establish an award in 1999 for ‘‘creative research in homogeneous and heterogeneous catalysis’’. In selecting Sir John Meurig Thomas as first recipient ‘‘for having laid down the basic principles for catalytic site-engineering by designing and synthesising novel, exquisitely tailored solid catalysts’’, the American Chemical Society beautifully underlined three features of Sir John’s research: its conceptual nature, its innovating character and its elegance. It is important to underline that Sir John largely contributed to the development of physical techniques especially designed for determining active site structures under operating conditions. 588

The Mysteries of Water in Catalyst Preparation

589

The present contribution deals with two topics, catalyst preparation and water,8 which always have been dear to Sir John (see for instance Refs. 9 and 7, respectively). Supported catalysts constitute the most important class of catalytic systems, because of the variety of reactions they are able to promote.10 Several strategies exist to design and synthesize such catalysts and Sir John has reviewed one of them, the integrated strategy, for active site-engineering in micro- and mesoporous solids.11 When large quantities of catalysts are required, some strategies however become difficult or impossible to apply. The present lecture concerns such types of catalysts, particularly oxidesupported ones, produced at the industrial scale from conventional oxides (alumina, silica, zeolites) using water-soluble transition metal complexes as precursors of the catalytically active phase. It is noteworthy that the second and third enigmas of physics identified by the CNRS6 are the untractable turbulence and the obscure nature of glass, both involved when catalysts are to be prepared in aqueous solution at the laboratory scale.

2 Molecular Approach to Supported Catalyst Preparation In recent years, major advances have been made in physical techniques12 particularly for the in situ characterization of catalysts, either during preparation or in working conditions.13–15 A molecular approach based on the use of such techniques and of transition metal elements has been developed in our laboratory and applied to the study of oxide-supported catalysts.16 The role of transition metal complexes is threefold16: (i) transition elements generally have remarkable catalytic properties, (ii) their partly filled d orbitals provide them also with unique optical and magnetic properties, and (iii) such properties followed by suitable spectroscopies strongly depend upon the immediate environment of the transition metal elements which can thus function as probes of their own interactions with the oxide support in the course of preparation, thus providing very valuable information on the role played by water. The preparation of catalysts10 involves several stages called unit operations in industry17 or preparation steps in the laboratory, such as impregnation, elimination of the solvent, drying, calcination, sulfidation and reduction. Industry and laboratory procedures are the same and only differ by the quantities of catalysts prepared, typically of the order of the ton and gram respectively. Therefore, we shall use either unit operation or preparation step in what follows. This contribution will consider the deposition methods most frequently used in the industry, i.e., impregnation and equilibrium adsorption. The catalysts obtained are characterized both at the molecular level by spectroscopies such as diffuse reflectance UV–Visible, NMR and EXAFS, and at the macroscopic level by techniques such as TEM, TPR and XRD.

590

Chapter 35

3 The Versatile Role of Water 3.1

Water as Ionizing and Dissociating Solvent

Despite numerous studies, it is still a challenge for theoretical chemists to explain the three anomalies of water simultaneously6: (i) its strong cohesion (high fusion and high boiling temperatures), (ii) its large expansion below 4 1C and upon solidification, and (iii) its high dielectric constant (e ¼ 80). The last property makes water an ionizing and dissociating solvent. It is the most widespread and safest solvent used for the preparation of industrial catalysts, all the more as it is largely available, cheap and environment-friendly.

3.2

Water as r Donor–p Donor Ligand and/or Acid–Base Entity

The early stages of reforming catalyst preparation start by the deposition of platinum on g-alumina from a hexachloroplatinic acid precursor solution.18 The diluted commercial aqueous solutions of hexachloroplatinic acid (say 5
103–7
102 M in Pt), possibly containing added HCl but not modified otherwise, can be conveniently investigated by 195Pt NMR.19,20 From the NMR spectra and the chemical shifts of the various peaks ([PtCl6]2 is taken as reference), the speciation21 of platinum can be easily followed.19 Two types of reaction can be evidenced depending on pH and the concentrations of HCl and hexachloroplatinic acid: (i) substitution of chlorides ½PtCl6 2 þ H2 O ! ½PtCl5 ðH2 OÞ þ Cl 0 ppm

ð1Þ

504 ppm

½PtCl5 ðH2 OÞ þ H2 O ! ½PtCl4 ðH2 OÞ2  þ Cl

ð2Þ

1016 ppm

and (ii) acido–basic reactions ½PtCl5 ðH2 OÞ ! ½PtCl5 ðOHÞ2 þ Hþ

ð3Þ

640 ppm

½PtCl4 ðH2 OÞ2  ! ½PtCl4 ðOHÞðH2 OÞ þ Hþ

ð4Þ

½PtCl4 ðOHÞðH2 OÞ ! ½PtCl4 ðOHÞ2 2 þ Hþ

ð5Þ

1167 ppm

In these reactions, because of the two electron doublets on oxygen, water acts as s donor and p donor ligand22 in the platinum(IV) complexes (Reactions (1) and (2)), and/or as acid–base entity (Reactions (3)–(5)).

591

The Mysteries of Water in Catalyst Preparation

3.3

Water as Viscous Solvent Able to Favour Ion Pairs in the Presence of a Charged Surface23

When solid oxides are put in aqueous solutions, surface hydroxyl groups are formed which behave as amphoteric species: they may be protonated (reverse of Reaction (6)) or deprotonated (Reaction (7)), according to the solution pH: ! SOH þ Hþ SOHþ 2

Ka1

ð6Þ

SOH ! SO þ Hþ

Ka2

ð7Þ

Therefore, a pH-dependent surface charge will develop. In particular, there will be a value of pH ¼ (pKa1 + pKa2)/2 for which the total electric charge of the surface is zero: the point of zero charge (PZC). At lower pH values, the surface bears a global positive charge, able to attract negatively charged complexes such as [PtCl6]2– and at higher pH, a global negative charge, able to attract positively charged complexes such as [Ni(NH3)6]21 by mere electrostatic adsorption. Typical PZCs are 2–2.5 for silica, 8–8.5 for alumina and 4.5–6.5 for titania. Figure 1 illustrates the various adsorption models (I–IV) of a transition metal complex in an aqueous solution in contact with an oxide surface together with the corresponding pertinent types of chemistry.24 It corresponds to the case of a positively charged complex in contact with a negatively charged surface at a pH 4 PZC of the corresponding oxide. In aqueous solution and in the absence of any oxide, a transition metal complex

AA-

AL

A-

L

OH

M L

OH

S

L

OH

AL

L

L

A-

A-

M

L

L

L

O- O-

II

L

L M

OH

L

L

L

L L

M

I

L

L

OH OH

III

L M

O- O-

Electrochemistry Supramolecular Chemistry

IV

Mixed phase

Support

Coordination Chemistry Geochemistry

Figure 1

Various adsorption models (I–IV) of a transition metal complex initially in aqueous solution upon contact with an oxide surface and depending on experimental conditions.24 The figure corresponds to the case of a positively charged complex in contact with a negatively charged surface at a pH 4 PZC of the corresponding oxide. Typical examples are aqueous solutions of octahedral [Ni(NH3)6]21 or square planar [Pd(NH3)4]21 precursor complexes put in contact with silica at pH 4 PZC of SiO2. The corresponding pertinent types of chemistry are also given.

592

Chapter 35

enjoys rotational and translational freedom which becomes much more restricted upon electrostatic adsorption on a charged surface (model I) because of the viscosity of water. Even in dilute electrolyte solutions, many properties of water are strongly modified in the immediate vicinity of a charged interface, especially its density. This effect has chiefly been studied by electrochemists interested in charged metallic surfaces: water close to a silver electrode bearing a surface charge of 0.25 C/m2 reaches a density of 2.0,25,26 i.e., close to that of ice VII which is formed at a pressure of 2.2 GPa (22,000 atmospheres). This phenomenon is called electrostriction. In the first layer of water molecules, the interaction of water dipoles with the surface electric field can be an order of magnitude stronger than hydrogen bonds (which are themselves an order of magnitude higher than kT at room temperature). Thus, the electric dipoles of the water molecules are strongly oriented, in the direction of the surface or away from it depending on the sign of the surface charge. As a consequence, the electric permittivity e falls down from ebulk ¼ 78, to values as low as 2.5.27 It is expected that ion pairing should be favoured in the interface region with respect to the bulk solution. Furthermore, transition metal ions located in this region will have a strongly decreased mobility as though they were imbedded in a glassy matrix.28 Thus, water close to charged surfaces becomes a viscous solvent favouring the formation of ion pairs.

3.4

Water as Ligand and/or H-Bond Intermediate

An intermediate situation between electrostatic adsorption (Figure 1, model I) and inner sphere complex formation (model III) is the formation of an outer sphere complex (model II) where the original ligands remain coordinated to the metal ion but form weak specific bonds with the surface, for instance hydrogen bonds. This is closely related to the field of supramolecular chemistry.29 Unless the precursor complex bears no charge (molecular complex) and/or the surface is neutral (pH ¼ PZC), this supramolecular interaction is accompanied by an electrostatic interaction as described earlier. Outer sphere complex formation can be followed in the [PtCl6]2–/silica system.30 195Pt NMR spectra indicate that the main species, at the liquid–solid interface, are [PtCl5(H2O)]– and [PtCl4(H2O)2]. By comparison with the [PtCl5(H2O)]–/crown ether 18-C-6 model system, which has been studied by several techniques (particularly X-ray diffraction) and shows a very similar 195 Pt NMR shift,31 it has been proposed that the [PtCl5(H2O)]– complex is bonded to the surface of silica by means of a solvent molecule and hydrogen bonds (Figure 2). The three oxygens involved belong either to three of the six oxygens of the crown ether in the model system or to three oxygens of (Si–O)6 cycles, denoted S6R, at the silica surface. It seems that there is a molecular recognition phenomenon between the outer sphere Pt complex and the oxide support, and that the solvent has an important role in this three-partner (precursor complex–solvent–oxide support) system.30

593

The Mysteries of Water in Catalyst Preparation

Cl

Cl Cl

Cl PtIV

494 ppm Cl

H

O

H

Figure 2

493 ppm

Cl

Pt

O

H

O O

O

H

O

O O

S6R H

Cl

Cl

O

O

O

IV

H

O

18-C-6

Cl

Cl

Si

Si H

Si

O

O

Si

O

H

O

Si

Si

Outer sphere complex formed between [PtCl5(H2O)]– and the crown ether 18-C-6 surface model after Ref. 31 and extrapolation to the cycle S6R of the silica surface, both characterized by 195Pt NMR with chemical shifts of 49431 and 493 ppm30 respectively.

While the complex is the guest, the oxide support acts as the receptor, the basic phenomenon being here self-assembly, based on hydrogen bonding and van der Waals interactions. This type of specific interaction falls into the so-called heterosupramolecular chemistry;32 in this context, the [PtCl5(H2O)]–/silica system can be seen as a non-covalent heterosupramolecular assembly while water behaves as ligand and/or H-bond intermediate.

3.5

Water as Labile and Weak r Donor–p Donor Ligand

Catalytically active transition metal ions are generally held to the oxide support by sufficiently strong iono-covalent bonds (model III) (Figure 1) to withstand catalytic reaction conditions (usually flow of reactants, high temperature and high pressure). The bonding of the transition metal precursor complex to the oxide surface corresponds to the so-called grafting process,33 i.e., the substitution of the ligands by surface oxygens leading to the formation of an inner sphere complex. The grafting process would correspond to the following reaction: f2  SiO ; ½NiðNH3 Þ4 ðH2 OÞ2 2þ g ! ½NiðNH3 Þ4 ð SiOÞ2  þ 2 H2 O

ð8Þ

Reaction (8) shows that the two water molecules are the targets for grafting to occur by ligand substitution. In practice, this however may not happen because both water and ammonia are kinetically labile. To make sure that substitution indeed occurs via Reaction (8), the following conditions should be met by the precursor Ni(II) complex in aqueous solution:34,35 (i) The complexation equilibria in the aqueous phase should be such as to start with almost 100% of nickel as one complex of well-defined

594

Table 1

Chapter 35 21

Formation constants of complexes [Ni(H2O)62x(en)x] in water solution at 25 1C (horizontal lines of data correspond to different references given in Ref. 34).

log b1

log b2

log b3

5.47 7.45 7.54 7.60

11.69 13.68 13.94 14.08

16.76 18.02 18.39 19.13

Table 2

Water exchange rates k1 at 25 1C, measured by

17

O NMR.34

Complex

k1 (s1)

[Ni(H2O)6]21, pH 5.5 [Ni(H2O)5(NH3)]21 [Ni(H2O)4(NH3)2]21 [Ni(H2O)3(NH3)3]21 [Ni(H2O)6]21, pH 6 [Ni(H2O)4(en)]21 [Ni(H2O)2(en)2]21

3.6
104 2.5
105 6.1
105 2.5
106 4.4
104 4.4
105 5.4
106

stoichiometry rather than with a mixture of different complexes. This is a thermodynamic condition. Table 1 illustrates that the formation constants of Ni(II) complexes involving water and ethylenediamine (en ¼ NH2–CH2–CH2–NH2) are well separated and respond to this criterion. (ii) The ligands that are to be substituted in the grafting process should be sufficiently labile while ‘‘spectator ligands’’ should be inert. This condition has thermodynamic as well as kinetic aspects. From a kinetic point of view, it is well known that substitution of some aqua ligands for polydentate ligands, such as ethylenediamine, in octahedral complexes increases the lability of the remaining aqua ligands34 as evidenced by Table 2. It is seen that replacement of two water molecules by the bidentate en ligand increases the exchange rate of water k1, each time, by an order of magnitude. This lability has some practical consequences. It allows in particular to graft Ni(II) ions by 1, 2 or 3 bonds to the silica surface when the precursor complexes contain 1, 2 or 3 labile water ligands, i.e., [Ni(en)(dien)(H2O)]21, [Ni(en)2(H2O)2]21 and [Ni(dien)(H2O)3]21 respectively, (dien ¼ NH2–CH2– CH2–NH–CH2–CH2–NH2 is the tridentate ligand diethylenetriamine). Figure 3 gives the structures of the supported complexes, deduced from EXAFS and UV–Visible data, after grafting of these precursor complexes to the surface of silica.36 The previous results surprisingly observed with amorphous silica, which is formed of randomly distributed –[Si–O]n– cycles, with n Z 337 (Figure 4),

595

The Mysteries of Water in Catalyst Preparation [Ni(en)(dien)(H2O)1]2+

[Ni(dien)(H2O)3]2+ [Ni(en)2(H2O)2]2+

A

A

A

A

A

A

O Si

=

O

A

Si

or

Si

(strained) Silica

Figure 3

Models for molecular recognition with interactional complementarity (V. Marchi-Artzner, F. Artzner, O. Karthaus, M. Shimomura, K. Ariga, M. Kunitake, J.-M. Lehn, Langmuir, 1998, 14, 5164) between Ni(II) amine complexes ([Ni(en)(dien)(H2O)]21, [Ni(en)2(H2O)2]21 and [Ni(dien) (H2O)3]21) and surface sites of silica after Ref. 36. The complementarity is assumed to result from cooperative formation of covalent bonds between Ni and Si–OH ligands, and hydrogen bonds joining H of amine ligands and acceptor groups on the silica surface.

Si

O OH O Si Si Si OH O 3

O O

Si

O O

6 O

O

Si 4 OHSi O O Si O O

O

Si OH

SiOH

O

SiOH

O

O

Si OH

OH

O

O

OH Si

O

O Si OH 5

O 8

OH Si

O O

O

Si

O

5

OH Si OH

7

5 OH Si

Si OH

O

O

O

8 Si OH OH Si

O

OH Si

O

Si

O

O 3

Si O OH Si

6

O O Si

Si

O O

O Si

OH

O

O Si

O

O

O

5

O O

O Si

3 O OH O Si O Si

4

O

O

Si OH

O

OH

Si

Si O

Si

Si OH Si

O

Si

6

Si

O

O

Si

7

O Si

O

OH

O

Si

Figure 4

A model of the amorphous surface of silica after Ref. 37.

suggest that transition metal complexes can act as probes towards specific adsorption sites of silica, hence leading to a similar phenomenon encountered in bio- and supramolecular chemistry, referred to as molecular recognition,36 already mentioned for outer sphere complexes (Section 3.4).

596

Chapter 35

Because of its two electron doublets on oxygen, we have seen that water behaves as a s donor–p donor ligand (Section 3.2). The question now arises as to what type of ligand belongs the surface groups able to graft transition metal complexes to oxides? Pure cases of inner sphere complex formation are scarce since other phenomena such as dissolution–reprecipitation (see Section 3.6) may occur in parallel. The clearest instances are observed when ‘‘spectator’’ ligands are inert to substitution, either because of the chelate effect (cis-[Ni(en)2(H2O)2]21 on various supports34) or because of high crystal field activation energy ([Co(NH3)5(RO)]x1 on Z-Al2O338, RO ¼ OH, H2O or alcohol). After deposition from the liquid phase and drying at room temperature, the cis-[Ni(en)2(H2O)2]21 complex with its two labile H2O ligands binds to SiO2, g-Al2O3 or Y zeolite to lead to cis-[Ni(en)2(SO)2]x1. On the basis of their crystal field parameter D, oxide supports (in bold) form a spectrochemical series which can be introduced into the ligand spectrochemical series, showing that surface groups of oxides are weak ligands:22,34 DCl oDAlO oDZO oDSiO oDH2 O oDNH3 oDen In summary, the results presented above show that water is a labile and weak s donor–p donor ligand, which can be used to target the grafting process of transition metal complexes to oxide surfaces, by one, two or three ionocovalent bonds to the surface of oxides whose surface groups appear to be ligands weaker than water.

3.6

Water as Reactant or Reaction Product

Many reports have been published on the formation, during the deposition step itself, of mixed phases or molecular species, containing both the transition metal of the precursor complex and an element from the support. Typical examples are the formation of the mixed phase nepouite, Si2Ni3O5(OH)4, for the Ni(II)/SiO2 system39 (Figure 1, model IV) or the molecular heteropolyanion, Al(OH)6Mo6O183–, for the Mo7O246–/g–Al2O3 system.40 The phenomenon is quite general, since it occurs upon contact of conventional oxides (silica, alumina) with aqueous solutions of precursor complexes, be they cationic, e.g. [Ni(NH3)6]21, or anionic, e.g. Mo7O246–, and for different methods of deposition (impregnation, equilibrium adsorption, deposition–precipitation). Although there has been no systematic study on this aspect, the contact time between the solution and the support seems to play an important role. The formation of such mixed phases and/or molecular entities is all the more important as the contact time increases. For instance, after an exchange process of 500 h, nickel is found to be essentially as a nickel silicate deposited on silica.41 It turned out that this came as no surprise to the scientific community of geochemists. They have been studying for years the alteration of primary

597

The Mysteries of Water in Catalyst Preparation

minerals on weathering, i.e., those changes brought about by climatic factors, including exposure to the soil solution. Dissolution followed by neoformation of secondary phases usually results. Of course geochemistry is most often concerned with ‘‘geological’’ timescales, but this is not necessarily so. Many relevant data are to be found in Ref. 42. To give orders of magnitude for support oxides, the half-life of a surface Al is 669 days (5.8
107 s) for a-Al2O3, but only 44 days (3.8
106 s) for d-Al2O3.43 As discussed below, the presence of transition metal complexes may speed up dissolution phenomena. In the case of the [MgII(H2O)6]/SiO2 system, the timescale for dissolution–reprecipitation was typically a few tens of hours, or 105 s,44 i.e., an order comparable to the duration of unit operations in catalyst preparation.17 Furthermore, catalyst preparation often includes a drying step generally performed at temperatures between 90 and 120 1C. In this temperature range, some water solution may remain in the porosity for a significant lapse of time before drying is completed, and of course dissolution reactions will be speeded up with respect to room temperature.45 In all those geochemistry processes, water plays a dominant role, first as reactant and second as transport agent (see Section 3.7). Let us now consider the case of g-alumina, the oxide support most frequently used in the industry to prepare supported catalysts. Hydration of this oxide has often been reported to occur via the transformation of the surface into aluminium hydroxide-like layers, but there was little evidence to support this hypothesis. It has now been shown46 by XRD, TEM, electron diffraction and solubility studies that a second process of hydration takes place that involves the dissolution of alumina and subsequent precipitation of well-shaped Al(OH)3 particles from supersaturated alumina aqueous solution. This process can be observed at a macroscopic scale (XRD, TEM) at pH 5 (Figure 5) and

200 nm

002 112 110

JCPDS # 76-1782

Figure 5

TEM image of g-alumina after suspension in distilled water for 880 h at pH 5. The insert shows the electron diffraction pattern obtained for the large particle circled in the centre of the picture. The gibbsite structure is in agreement with the XRD results at this pH.46

598

Chapter 35

above, provided that the contact time between alumina and water exceeds 10 h.46 In the reaction: g-Al2 O3 þ 3 H2 O ! 2AlðOHÞ3 ðbayerite; gibbsiteÞ

ð9Þ

and in geochemistry terms, alumina is the primary mineral while bayerite and gibbsite are the secondary minerals, while water is a chemical reactant. Let us now consider the same system but in the presence of a molybdenum salt, say ammonium heptamolybdate, often used to prepare hydrodesulfurization catalysts. The deposition of Mo on g-Al2O3 by the equilibrium adsorption method has been studied by 95Mo NMR, Raman and IR.40 Spectroscopic data converge to indicate that a previously unrecognized species, the Anderson-type heteropolymolybdate, Al(OH)6Mo6O183–,47 plays a major role in this type of synthesis as it is quantitatively formed in the solution within a few hours, by reaction of the heptamolybdate with dissolved aluminic species. This results in a considerable increase of alumina solubility in conditions generally thought to be non-aggressive. Furthermore, this species is also evidenced by proton cross polarization (CP) 27Al NMR on the surface of the supported catalysts after deposition of molybdenum, although it is harder to observe than in the liquid phase, because of the presence of the Al atoms of the support. A parallel is drawn with a well-known concept from geochemistry, i.e., ligand-promoted oxide dissolution. Figure 6 gives a schematic representation of the ligandpromoted dissolution of alumina together with the corresponding 27Al NMR spectra.40 In more general terms, those phenomena, i.e., the dissolution of the oxide support followed by reprecipitation of a mixed species, containing elements of both the support (Al) and the precursor (Mo) require the formation of bonds such as Al–O–Mo which can be accounted for by the following types of reaction:48 SOH þ H2 OM ! SOHM þ H2 O

ð10Þ

SOHþHOM ! SOM þ H2 O

ð11Þ

where no attempt is made to indicate the complete coordination sphere; only the pertinent ligands around S (Al, Si, etc.) representing the support element and M (Ni, Mo, Pd, Pt, etc.) representing the metal of the precursor are given. The dissolution of the oxide support leads to support monomers which can react with the precursor complex to form mixed entities. Olation (Reaction (10)) and oxolation (Reaction (11)) are heterocondensation processes in which a hydroxy or an oxo bridge between S and M are formed respectively with water as reaction product. Because it has two lone pairs, water has been known for long to be a bridging ligand, as are OH groups and oxide ions which can form bonds with both S and M. In Reaction (10) and for some specific cases, it has been observed that upon bonding to the hydroxyl

The Mysteries of Water in Catalyst Preparation

Figure 6

599

Schematic representation of the ligand-promoted dissolution of alumina in an aqueous solution containing heptamolybdate following equilibrium adsorption.40 The 27Al NMR liquid-state spectra of the filtrates obtained after 168 h at various pH are presented on the left top, while the 27Al CP-MAS NMR spectra of g-Al2O3 alone and with 6 and 8 wt.% Mo prepared at pH 4 after 168 h are shown on the right top.

group water forms a m-(H3O2) bridging ligand, from which water can be eliminated as reaction product.49 When ligands such as NH3 or en, with only one lone pair per N are found in the coordination of the metal centre, then heterocondensation Reactions (10) and (11) are no longer possible.16,50

3.7

Water as Transport Agent

One of the important aims of geochemists is to understand how minerals are formed and to study the circulation of the elements in nature, on the basis of the properties of their atoms and ions.51 As emphasized earlier, water plays a dominant role particularly to transport matter as molecular entities after dissolution of minerals by processes described above. If proper conditions are met, reprecipitation can then occur with the formation of neophases. There are many examples which can be found in nature and this has been reviewed by Stumm.42 As far as catalyst preparation is concerned, the role and the importance of water as transport agent have been elegantly demonstrated by Clause and co-workers in the formation of hydrotalcites on leaving alumina for 2 weeks at 333 K in contact with 10–2 M aqueous solutions of Co(II), Ni(II) or Zn(II) at

600

Figure 7

Chapter 35

Experiment evidencing the process of dissolution–reprecipitation occurring in the preparation of oxide-supported catalysts: it illustrates the role of water as transport agent (see text).

pH ¼ 9.52 Figure 7 gives a schematic representation of the experiment, whose key element is a dialysis membrane. At the beginning, the membrane containing only the alumina support in the form of spheres was introduced into water in the beaker. The solution containing the Co(II), Ni(II) or Zn(II) aqua complexes was then introduced drop-wise into the beaker. After 2 weeks at 333 K, the precipitates formed both inside and outside the membrane were recovered separately and analyzed. They were found to be the same: a hydrotalcite containing the two elements originally separated by the dialysis membrane. This evidences that alumina has dissolved to produce [Al(OH2)6]31 able to cross the membrane to form hydrotalcite outside the membrane. The aqua complexes were able to cross the membrane in the opposite direction to form hydrotalcite inside the membrane. Examples of water as transport agent are now frequently found in the literature on supported catalyst preparation.

4 Conclusions: Still on Water, and its Surface I remember reading Sir John’s comments on the book Faraday: The Life by James Hamilton, Harper Collins, pp. 465+XXI, 2002 with great attention. They demonstrated that he loved accuracy, science and comprehensiveness. He entitled his comments: ‘‘Mozart – but only a little of his music’’. I do hope that Sir John will read my contribution with the same critical eyes and tell me its weaknesses. I am sure he will come up with some most valuable comments, leading to future collaboration. I am looking forward to it.

The Mysteries of Water in Catalyst Preparation

Figure 8

601

A physico–chemical promenade on the Seine river: water as a moving surface.

After this little physico–chemical promenade between water and oxide surfaces, I cannot refrain from remembering the moments of pure pleasure we all had ‘‘on’’ the Seine river with Lady Margaret and Daniele on 22 May 1993 (Figure 8). This perhaps was the motivation to organize the banquet of the 13ICC in Paris in 2004 on the Seine river. I am also delighted with the title of the book in honour of Sir John. I remember inviting him to deliver the opening lecture at the very First EuropaCat Congress on Catalysis in Montpellier (France) in 1993, and proposing the title ‘‘Turning Points of Catalysis’’! His lecture led to two reviews published in Angew. Chem., Int. Ed. Engl., 1994, 33, 913 (Turning Points of Catalysis) and Bull. Soc. Chim., 1994, 131, 463 (Landmarks in the evolution of heterogeneous catalysts).

References 1. A. Einstein, A heuristic point of view concerning the production and transformation of light, Ann. Phys., 1905, 17, 132. 2. A. Einstein, A new determination of molecular dimensions, Ann. Phys., 1906, 19, 289. 3. A. Einstein, On the movement of small particles suspended in stationary liquids required by the molecular-kinetic theory of heat, Ann. Phys., 1905, 17, 549. 4. A. Einstein, On the electrodynamics of moving bodies, Ann. Phys., 1905, 17, 891.

602

Chapter 35

5. A. Einstein, Does the inertia of a body depend on its energy content?, Ann. Phys., 1905, 17, 639. 6. Le Journal du CNRS, 181 (February 2005) 12. 7. J.M. Thomas and W.J. Thomas, Principles and Practice of Heterogeneous Catalysis, VCH-Verlag, Weinheim, 1997. 8. These topics are two of those developed during my Franc¸ois Gault lectureship (2004–2006). 9. T. Maschmeyer, F. Rey, G. Sankar and J.M. Thomas, Nature, 1995, 378, 159. 10. G. Ertl, H. Kno¨zinger and J. Weitkamp (eds), Handbook of Heterogeneous Catalysis, VCH-Verlag, Weinheim, 1997. 11. J.M. Thomas, Angew. Chem. Int. Ed., 1999, 38, 3588. 12. B. Imelik and J.C. Ve´drine (eds), Catalyst Characterization: Physical Techniques for Solid Materials, Plenum Press, New York, 1994. 13. Issue on Catalyst characterization under reaction conditions, Top. Catal., 1999, 8, 1. 14. H. Topsoe, Stud. Surf. Sci. Catal., 2000, 130A, 1. 15. B.M. Weckhuysen, Chem. Commun., 2002, 97. 16. M. Che, Stud. Surf. Sci. Catal., 1993, 75A, 31 and 2000, 130A, 115. 17. J.F. Lepage et al., Catalyse de Contact, Technip, Paris, 1978. 18. B. Shelimov, J.F. Lambert, M. Che and B. Didillon, J. Catal., 1999, 185, 462. 19. B. Shelimov, J.F. Lambert, M. Che and B. Didillon, J. Am. Chem. Soc., 1999, 121, 545. 20. B. Shelimov, J.F. Lambert, M. Che and B. Didillon, J. Mol. Catal. A, 2000, 158, 91. 21. For the definition of speciation see D.M. Templeton, F. Ariese, R. Cornelis, L.-G. Danielsson, H. Mantau, H.P. Van Leeuwen and R. Lobinski, Pure Appl. Chem., 2000, 72, 1453. 22. C. Lepetit and M. Che, J. Mol. Catal., 1995, 100, 147. 23. J.F. Lambert and M. Che, Stud. Surf. Sci. Catal., 1997, 109, 91. 24. K. Dyrek and M. Che, Chem. Rev., 1997, 97, 305. 25. M.F. Toney, J.N. Howard, J. Richer, G.L. Borges, J.G. Gordon, O.R. Melroy, D.G. Wiesler, D. Yee and L.B. Sorensen, Surf. Sci., 1995, 335, 326. 26. I. Danielewicz-Ferchmin and A.R. Ferchmin, J. Phys. Chem., 1996, 100, 17281. 27. I. Danielewicz-Ferchmin, J. Phys. Chem., 1995, 99, 5658. 28. V. Bassetti, L. Burlamacchi and G. Martini, J. Am. Chem. Soc., 1979, 101, 5471. 29. J.M. Lehn, Supramolecular Chemistry, VCH, Weinheim, 1995. 30. S. Boujday, J. Lehman, J.F. Lambert and M. Che, Catal. Lett., 2003, 88, 23. 31. D. Steinborn, O. Gravenhorst, H. Hartung and U. Baumeister, Inorg. Chem., 1997, 36, 2195. 32. X. Marguerettaz, A. Merrins and D. Fitzmaurice, J. Mater. Chem., 1998, 8, 2157.

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33. For the definition of grafting, see F. Averseng, M. Vennat and M. Che, Handbook of Heterogeneous Catalysis, G. Ertl, H. Kno¨zinger, F. Schu¨th and J. Weitkamp (eds), VCH-Verlag, Weinheim, 2007. 34. J.F. Lambert, M. Hoogland and M. Che, J. Phys. Chem. B, 1997, 101, 10347. 35. E. Marceau, X. Carrier, M. Che, O. Clause and C. Marcilly, Handbook of Heterogeneous Catalysis, G. Ertl, H. Kno¨zinger, F. Schu¨th and J. Weitkamp (eds), VCH-Verlag, Weinheim, 2007. 36. S. Boujday, J.F. Lambert and M. Che, Chem. Phys. Chem., 2004, 5, 1003. 37. S.H. Garofalini, J. Non-Cryst. Solids, 1990, 120, 1. 38. Y. Chen, J. Hyldtoft, C.J.H. Jacobsen, D.H. Christensen and O.F. Nielsen, Spectrochim. Acta, 1994, 50A, 1879. 39. P. Burattin, M. Che and C. Louis, J. Phys. Chem. B, 1997, 101, 7060. 40. X. Carrier, J.F. Lambert and M. Che, J. Am. Chem. Soc., 1997, 119, 10137. 41. M. Houalla, F. Delannay, I. Matsuura and B. Delmon, J. Chem. Soc., Faraday Trans. I, 1980, 76, 2128. 42. W. Stumm, Chemistry at the Solid-Water Interface: Processes at the Mineral-Water and Particle–Water Interface in Natural Systems, Wiley, New York, 1992. 43. B. Wehrli, E. Wieland and G. Furrer, Aquatic Sci., 1990, 52, 52, quoted in Ref. 42. 44. J.B. d’Espinose de la Caillerie, M. Kermarec and O. Clause, J. Phys. Chem., 1995, 99, 17273. 45. C. Louis, Z.X. Cheng and M. Che, J. Phys. Chem., 1993, 97, 5703. 46. X. Carrier, E. Marceau, J.F. Lambert and M. Che, J. Colloid Interface Sci., 2007, 308, 429. 47. For a description of the structure and properties of polyoxometalates, see the review: P. Gouzerh and M. Che, L’Actualite´ Chimique, 2006, 298, 9. 48. C.J. Brinker and G.W. Scherer, Sol-Gel Science, Academic Press, New York, 1990, 26. 49. M. Ardon, A. Bino and K. Michelsen, J. Am. Chem. Soc., 1987, 109, 1986. 50. J.Y. Carriat, M. Che, M. Kermarec, M. Verdaguer and A. Michalowicz, J. Am. Chem. Soc., 1998, 120, 2059. 51. V.M. Goldschmidt, Geochemistry, Clarendon Press, Oxford, 1954. 52. J.B. d’Espinose de la Caillerie, M. Kermarec and O. Clause, J. Am. Chem. Soc., 1995, 117, 11471.

CHAPTER 36

Solid Acid Microporous H-SAPO-34: From Early Studies to Perspectives LEONARDO MARCHESE,a,* GLORIA BERLIERb AND SALVATORE COLUCCIAb a

Dipartimento di Scienze e Tecnologie Avanzate and Centro Interdisciplinare, Nano-SISTEMI, Universita´ del Piemonte Orientale, Via Bellini 25G, 15100 I-Alessandria, Italy; b Dipartimento di Chimica, Inorganica, Fisica e dei Materiali and NIS Centre of Excellence, Universita` di Torino, Via P. Giuria 7, 10125 Torino, Italy

1 Introduction In the early 1990s, a group of researchersw working at the Royal Institution of Great Britain, London, grouped around the leading figure of Sir John Meurig Thomas, was enthusiastically involved in the study of H-SAPO-34, a silico–aluminophosphate (SAPO) with the same structure as natural chabazite (Figure 1). Sir Thomas was kindly named within the group with the acronym JMT, as he used to sign messages or the front page of relevant papers that he passed to the co-workers. Different approaches and techniques, both experimental and theoretical (thanks to the strong connection with the research group of Prof. Catlow,z who was also working at the Royal Institution), were employed to study the nature of the Brønsted acidity of H-SAPO-34 and other structure-related materials, SAPO-18 and MeAPO-18, which were found to be highly selective in the conversion of methanol to light olefins. The strength of the experimental studies developed at the Royal Institution was that the expertise of the researchers who joined at that time the JMT group w

In the period 1992–1994 the group was formed by P. Wright, J. Chen, G. Sankar, S. Natarajan, L. Marchese and S.M. Bradley. z Among others, J.D. Gale, D.W. Lewis and G. Sastre were working with Prof. Catlow in the title topic.

604

Solid Acid Microporous H-SAPO-34

Figure 1

605

Schematic representation of: (A) the methanol conversion to C2 and C3 olefins inside the chabazite cage of H-SAPO-34 and (B) four possible configurations for Brønsted sites. In (A) yellow and red sticks represent Al/P and O atoms, respectively. The presence of Brønsted sites pointing in the large cage (small grey spheres) is related to the isomorphous substitution of Si atoms (green spheres). In (B) atoms are designated by the following colours: yellow spheres represent Al, blue ones Si and P, red and white O and H respectively. O1 belongs to two 4-T and one 8-T rings, connecting the two 6-T rings of the double 6-T ring layers; O2 belongs to one 4-T, one 6-T and one 8-T ring; O3 belongs to two 4-T and one 6-T ring; O4 belongs to one 4-T and two 8-T rings, forming the bridge between two double 6-T rings. The orientation of the proton with respect to the framework has only a pictorial meaning. Reprinted with permission from Ref. 25. Copyright 2007, American Chemical Society.

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ranged in several fields of solid-state chemistry. Synthesis of microporous molecular sieves and their characterization using a large variety of techniques (X-ray diffraction, solid-state NMR, EXAFS/XANES, thermogravimetric analysis, Fourier transform infrared) were well developed within the group along with catalytic tests. The investigations were thus devoted to understanding the catalytic performance of the materials, trying to pin down the influence of structure and chemical composition on the catalytic performances. One fundamental hint towards the understanding of the acidity of SAPOs and zeolites came from the elegant work of the group of Prof. Barthomeuf, who rationalized the mechanism of silicon insertion in ALPO frameworks, and the trends governing the number and strength of Brønsted acid sites produced in this way.1 These studies, along with the emerging interest for H-SAPO-34 as a commercial catalyst, drove the work of the JMT group in the search for new acid molecular sieves.2–4 The use of zeolites and zeotypes as catalysts for hydrocarbon conversion represents an important tool of the modern chemical and petrochemical industry. The introduction of microporous solid acids for gasoline conversion5 and other processes such as methanol transformation to light olefins (methanol-to-olefin (MTO))6 or gasoline (methanol-to-gasoline (MTG)) resulted in great advantages in terms of safety and waste reduction. The methanol-to-hydrocarbon technology was primarily regarded as a powerful method to convert coal into high-octane gasoline, and was then expanded to the formation of other fuels and chemicals in general. Depending upon the employed conditions and, more importantly, upon the catalyst used, the process can be modulated in order to have a high selectivity to light olefins, resulting in the MTO process.6 Among others, H-SAPO-342,7–9 has shown peculiar selectivity towards ethylene and propylene formation (mixture of C2 and C3 products higher than 80%) with the flexibility of altering the ratio between the two olefins by varying the reactor conditions.6 This catalyst is thus central to the industrial application of MTO processes, being used in the UOP/ Norsk Hydro MTO technology, which is currently under commercialization.7,10 The different selectivity of small-pore H-SAPO-34 with respect to medium-pore zeolite H-ZSM-5, successfully employed for MTG processes, has been explained in terms of shape selectivity and of acid strength. In fact, H-ZSM-5 catalysts oligomerize light olefins giving higher molecular weight olefins or aromatics, while the reaction stops at low olefins on H-SAPO-34. Even if one could explain this trend in terms of product diffusion (H-SAPO-34 has a cage structure rather than a channel structure as in ZSM-5), it is likely that the high selectivity of H-SAPO-34 to C2–C4 hydrocarbons is also related to the medium acid strength of the catalyst.9 When the activity in MTO of H-SAPO-34 and its zeolitic homologue H-chabazite is compared, in fact, the faster deactivation on H-chabazite is acknowledged,11 this process being related to coke formation as a consequence of consecutive reactions between light olefins and strong Brønsted sites. Sir J.M. Thomas in one of the early papers devoted to this subject elegantly commented that: ‘‘There is a class of heterogeneous catalysts in which essentially all the atoms of the bulk of the solid participate directly or are implicated

Solid Acid Microporous H-SAPO-34

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indirectly in the key catalytic processes of the overall reaction. Well known examples are zeolites, the silicoaluminophosphates (SAPOs) etc.12 There is little doubt that one of the keys of the methanol conversion is the loosely bound extraframework proton, which facilitates the formation of intermediate carbocations and other transitory species’’.13 It may be easily recognized that the local structure of Brønsted acid sites in zeolites and SAPOs is similar, being constituted by Si–O(H)–Al units, however the acidity of these materials may be largely different, even in the case of structurally similar systems. The acidity depends on several factors, such as bond angles and bond lengths, the electrostatic potential around the acid centres and within the zeolite cages, etc. Among these factors, the atomic environment around the acid centres plays a relevant role. The first coordination sphere of T atoms around the aluminium is formed exclusively of Si atoms in zeolites, whereas Si and P can be present in SAPOs.1,14–16 In addition, the formation of Si islands in SAPOs (Figure 2A) leads to an even larger number of acid site environments;17 this makes the acidity of SAPOs more tunable than that of zeolites, providing that the Si insertion can be controlled.16,18 This chapter deals with the issues related to understanding the Brønsted acidity of H-SAPO-34 and to the possible strategies to improve the catalyst lifetime. The early studies related to this topic will be reviewed, with particular attention to the results obtained by Sir J.M. Thomas and his collaborators. Even if many years have passed since the early studies, these topics are still hot in catalysis, due to the importance of natural gas conversion for green chemistry, but also for the scientific debate on the mechanisms of acid-catalysed reactions in solid microporous materials. Recent results concerning a new methodology for a quantitative determination of acid sites with slightly different acidity will be resumed. On the basis of these new results, future perspectives of the title subject will be discussed, focussing on the potentiality of novel synthesis approaches, combined with feedback from careful characterization and theoretical predictions.

2 The Early Stages: Understanding the Brønsted Acidity of H-SAPO-34 and its Catalytic Performance for MTOs Among the first studies devoted to the investigation of H-SAPO-34 performance in MTO reactions, we mention the elegant paper by Xu et al. where magicangle-spinning NMR, gas chromatography and FTIR studies were combined to get some insight into the reaction mechanism of acid-catalysed methanol transformations.9 It was proposed that the reaction proceeds through a sequence of consecutive steps passing from dimethyl ether, alkenes, alkanes and finally aromatics. The reaction could be stopped at olefins by optimizing flow rates. The authors concluded that high selectivity to olefins is related to its medium acidity, more than to the small size of the pores, which can only play

608

Figure 2

Chapter 36

(A) Planar scheme for the Si, Al and P distribution in a SAPO network: (a) isolated Si, (b) 5Si island, (c) 11Si island and (d) Si–Al phase with 1 Al in the siliceous island. Reprinted with permission from Ref. 16. Copyright 2005 Annual Reviews. 29Si-MAS-NMR of SAPO-34 prepared in the presence of morpholine (B) and TEAOH (C) at increasing Si concentration (wSi). (B) wSi: 0.10 (a), 0.14 (b), 0.16 (c), 0.18 (d), 0.23 (e) and 0.33 (f). (C) wSi: 0.10 (a), 0.10 (b), 0.11 (c), and a chabazite with Si/Al ¼ 2.34 (d) is also reported for comparison. Adapted from Ref. 17.

Solid Acid Microporous H-SAPO-34

609

an important role in controlling the product distribution (in relation to the limiting product diffusion rate). In that period, small-pore zeolites such as chabazite, erionite, ZSM-34 were tested as catalysts for MTO, since the pore openings consisting of eight T-membered rings were regarded as just fitting the size of small olefin molecules (Figure 1). Indeed those catalysts gave considerably high selectivity to ethylene and propylene but also rapid deactivation of the catalyst performance, as a result of coke formation inside the cavities. Different methods were thus employed to limit the deactivation, by trying to reduce acid strength or density. One of the ways followed to this aim was to synthesize SAPO and ALPO catalysts with small amounts of transition metal ions, like nickel,13 Zn, Mg2 or cobalt,2,19 and this led to the formation of loosely bound extraframework protons related to the presence of Me21(OH)P51 groups.2,20–23 The impressive selectivity to ethylene (nearly 100%) of Ni-SAPO-3424 was related to the modification of the acidity of the zeolite by nickel introduction.13 Structural effects were also investigated by comparing the catalytic performance of different SAPO structures.4 The authors rationalized the activity and selectivity of the diverse catalysts in terms of the strength of Brønsted sites and cavity dimensions. The understanding of the catalytic performances of H-SAPO-34 and related materials in MTO was not only related to direct measurements of the catalytic activity. In the early 1990s, the group of Sir J.M. Thomas at the Royal Institution was studying the nature of Brønsted sites in solid microporous materials. In those days, one fundamental step towards the understanding of the acidity of SAPOs and zeolites came from a massive, independent work of D. Barthomeuf and her group, who rationalized the mechanism of silicon insertion in ALPO frameworks. A topological approach, which took into account both the structure topology and the Al content, was first developed for several zeolites12 and subsequently applied to SAPOs.1 With the aid of planar schemes for representing the distribution of Si and Al in zeolites and Si, Al and P in SAPO networks (Figure 2A), Barthomeuf proposed that the occupation by Al, Si and P atoms of the first and second shell of T atoms around a central Si governs the acid strength through the electronegativity of atoms and the connectivity of tetrahedral units. The acid strength increases as the first shell contains more Si and as the second shell contains more P. The presence of Si islands leads to the formation of stronger acid sites by forming Si(nAl) species (no4) at the border of the islands. The number of these acid sites and of the n value depends on the dimension of the islands, which is related to the Si concentration and to the structure topology. A correlation between acid strength and 29Si MAS NMR chemical shift as the environment of Si in the first and second shell of the tetrahedral unit becomes richer in silicon was finally proposed. These results were lately supported by a theoretical study of Sastre et al. (vide infra). The role of the structure directing agent (SDA) in governing the Si distribution in SAPOs was also investigated. Again the work of Barthomeuf and her group is exemplary; they described, using 29Si-MAS-NMR spectroscopy, the Si distribution in SAPO-34 prepared in the presence of morpholine or

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tetraethylammonium hydroxide (TEAOH) as SDAs (Figures 2B and C). The model proposed for SAPO-34, subsequently generalized to other SAPOs, suggests that the Si distribution depends on the number of framework charges which is influenced both by the number of SDA molecules embedded within the cages and by the Si atoms introduced within the framework.17 Keeping in mind the models proposed by Barthomeuf, the effect of Si incorporation in different ALPO structures was investigated at the Royal Institution by combining different techniques, namely 29Si-MAS-NMR (Figure 3A) and FTIR spectroscopy (Figures 3B and C).3 The authors confirmed that at low Si content (xSi E 0.10, in agreement with Barthomeuf previsions), H-SAPO-34 presents essentially isolated Si centres. However, it has been

Figure 3

Results from combined 29Si-MAS-NMR (A) and FTIR (B,C) study showing that SAPO-34 (wSi ¼ 0.10) displays essentially isolated Si centres (with 29Si chemical shift at
92 ppm), and bridged Si–(OH)–Al hydroxyls at 3626 and 3600 cm–1 (B). In SAPO-18 materials with lower silicon fraction (wSi ¼ 0.047, curve b and 0.087, curve c) Si islands are instead formed, as testified by the NMR signals at d ¼
111 ppm for Si(OSi)4 ,
96,
100 and
105 ppm for Si(OSi)n(OAl)4–n species. Accordingly, the concentration of Brønsted acid sites in SAPO-18 is much lower than SAPO-34 (C). Panel (B) shows the spectra of (a) ALPO-18, (b–e) SAPO-18 with different silicon concentration and (f ) SAPO-34. Reprinted with permission from Ref. 16. Copyright 2005, Annual Reviews.

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recently reported, thanks to the improved performance of cross polarization (CP) techniques, that small Si islands are present also in H-SAPO-34 samples where these species represent less than 10% of the total Si.25 The work by Chen et al. showed that Si islands are formed in H-SAPO-18 catalysts at very low Si content (Figure 3A). This implies a smaller concentration of Brønsted sites in SAPO-18, e.g. larger amount of silicon islands, as testified by the normalized intensity of the infrared (IR) bands related to bridged Si(OH)Al hydroxyls, adsorbing at 3626 and 3600 cm–1 (Figures 3B and C). FTIR spectroscopy has often been used for the characterization of Brønsted acidity, either by the analysis of the vibrational modes of the Si(OH)Al bridged hydroxyls (see for instance Figure 3B) or by use of probe molecules, as detailed in the next paragraph.26–31 In the early 1990s, this technique was applied to get more insight into the nature of Brønsted sites of H-SAPO-34, by studying the spectroscopic features of adsorbed water.32 Being a relatively weak base, H2O can be used for assessing the Brønsted acidity of zeolites, even if the shape of the bands is modified by the presence of strong Fermi resonance effects.27,29 The FTIR spectra obtained at increasing H2O coverage (Figure 4A) were first interpreted as due to H3O1 oxonium ions H-bonded to oxygen atoms of the framework because all the six IR vibrations predicted for such species were identified.32 However, Pelmenschikov and van Santen33 proposed that the bands attributed to H-bonded H3O1 could also be interpreted in terms of Brønsted hydroxyl groups H-bonded to H2O whose band shape is strongly modified by Fermi resonance effects. The nature of the species formed when water interacts with Brønsted acid sites in H-SAPO-34 was further investigated by structural14 and computational34 approaches. A combination of an FTIR and neutron diffraction study showed that H2O adsorption on H-SAPO-34 led to the formation of both 1:1 H-bonded complexes and H3O1 species (Figure 4B), the latter being the result of a proton transfer reaction from the most acidic protons to the H2O molecules.14 However, ab initio simulations of large clusters, which included fragments of the H-SAPO-34 framework and water molecules, suggested that only a protonated water cluster H3O1 . 2H2O is stabilized by a proper arrangement of proton sites within the micropores of the solid (Figure 4C).34 The discrepancy of the results could be interpreted by limitations of both techniques: the possible formation of clusters of adsorbates within the zeolites even at low coverages should be avoided from the experimental point of view, while proper zeolitic structures and their possible ‘solvent’ effects on the adsorbed molecule should be accounted for from the computational point of view. Finally, a quantitative description of the Brønsted acidity in H-SAPO-34 came from the contribution of Smith et al.35 On the basis of FTIR and neutron diffraction experiments, the authors assigned the two IR bands of H-SAPO-34 at 3630 and 3600 cm–1 (see Figure 3B, and following paragraph) to Si(OH)Al bridged hydroxyls with different acid strength related to two well-defined crystallographic positions. The high frequency band at 3630 cm–1 (named A species, OHA) and the low frequency component at 3600 cm–1 (OHC) were assigned to loosely bonded protons on O4 and O2 structural configurations,

612

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Absorbance [a.u.]

(H 2O) n·(H 3O) +

1353 [2γOH···O ]

0

(A)

(B)

Figure 4

[γOH···O ] 795 765

1

3600 3625 [νOH] 3500 3000

2700 [2δOH···O] νOH···O

250 0

200 0

1353 [δOH···O]

150 0

75 0

Wavenumbers [cm-1]

(C)

(A) FTIR spectra of increasing doses of H2O adsorbed on H-SAPO-34. Dotted lines correspond to high H2O dosages, H1/H2O 4 1, which lead to the formation of (H2O)n(H3O)1 clusters. (B) Ball and stick representation of the structure of hydrated H-SAPO-34, showing both the hydronium ion and H-bonded water molecules, proposed on the basis of powder neutron diffraction and IR spectroscopy. (C) Alternative model proposed on the basis of ab initio simulations: two water molecules hydrogen-bonded to H3O1 in the 8-T ring of H-SAPO-34. (A) and (C) reprinted with permission from Ref. 29, copyright 1999, Elsevier; (B) reprinted with permission from Ref. 14, copyright 1996, Science.

respectively (Figure 1B).35 The acidity of these sites was estimated by measuring the shift of the corresponding vibration upon CO adsorption (see below for further details), indicating the different strength of the sites. A third family of acid sites absorbing at 3625 cm–1 (OHB) was also found, but its nature was not discussed in terms of crystallographic positions. When monitored by CO adsorption, the acidity of this site (present as a minor fraction) was found to be very high. Recent work on chabazite-related SAPO materials has thrown some light on the nature of this site, as detailed in the following.25

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3 Insight into the Nature of Brønsted Sites by Computational Techniques It has often been recommended, for monitoring the acidity of catalysts, to use small and weakly interacting probe molecules, such as CO, H2, N2, etc., which induce only a mild perturbation of the catalytic sites.28–30 Nowadays, notwithstanding the above guidelines for experimental determination of relative acidity, the use of computational methods for the determination of acidity differences is becoming increasingly present. The modelling of Brønsted acidity in SAPOs by computational tools is, in fact, becoming more and more relevant.15,36–42 Many studies have been reported, trying to pin down the relations between acid strength and short- and long-range structural factors, such as SiOAl angles or Si–O, Al–O distances,43,44 flexibility of the framework45 and electrostatic potential generated inside the cavities.46–48 Many energy contributions, such as the electrostatic potential, electric field, etc., can now be taken into account in the calculations for determining the vibrational frequency of Brønsted sites and the energy required to elongate the OH bond.49 An example of the level of reliability of the computational tools are the results which describe the correlation between O–H stretching frequency of Brønsted acid centres in zeolites and SAPOs and the gradient norm of the electrostatic potential, that allowed the vibrational properties of microporous solids with a large variety of structure and composition to be described.48 Theoretical studies were also performed to verify the results reported by the group of Barthomeuf on the basis of the topological approach.1,12 Sastre et al. investigated the Si island formation and confirmed that siliceous patches within both SAPO-5 and SAPO-34 are energetically stable, and are more favourable in SAPO-5 where they form at lower silicon concentration.50 These authors reviewed carefully the literature concerning both experimental and theoretical results on the mechanisms of Si substitutions in SAPOs. One of the main results of the theoretical work by Sastre et al. is the estimation of the deprotonation energies of different acid sites, which fully supported the model that indicated that the acidity is greater at the edges of Si islands.15 A correlation between acidity and island dimension was established by considering 5Si and 8Si islands in SAPO-5 and SAPO-34. In addition, a structure-dependence on the formation energy of 8Si islands was found, and this showed that larger islands were more favourable in SAPO-5 because of its more flexible framework structure. Subsequently, these studies were extended to SAPO-11, SAPO-18 and SAPO-31.51

4 Recent Results: Revisiting the Nature of the Acidity in Chabazite-Related SAPOs Concerning the experimental techniques, acid strength of Brønsted sites can be monitored by using probe molecules with basic character, such as ammonia or

614

Chapter 36 31

pyridine, by the analysis of their spectroscopic features or by measuring the temperature at which these molecules are desorbed (TPD measurements).3 However, these techniques are not able to discern among families of Brønsted sites with small differences in acid strength, that could have serious consequences on activity, selectivity and lifetime of the catalysts. In this case the use of probe molecules with weaker basicity is compulsory. One of the most sensitive experimental techniques for this purpose is FTIR spectroscopy.14,16,26,29,35,52,53 This technique is based on the analysis of the IR spectra in the OH stretching (nOH) region and on their perturbation upon adsorption of small molecules (N2, CO, C2H4, etc.), which act as weak bases (B) forming H-bonded adducts (OH  B) with the Brønsted sites. The measure of the DnOH shift upon formation of the OH  B adducts represents a classical method to estimate the acidity of Brønsted sites.28,54 In a recent work, FTIR spectroscopy of adsorbed CO and C2H4 was used to compare the strength and distribution of Brønsted sites in H-SAPO-34 samples with similar chemical composition but prepared with different SDAs.25 A chabazite-related SAPO, CAL-1, recently synthesized adopting an original procedure using a lamellar ALPO as precursor,55,56 and having a high Si loading, was also studied. A detailed analysis of the FTIR spectra in the OH stretching region, and of the downward shift of the OH bands upon CO and C2H4 adsorption (Figures 5A and 6B, respectively), evidenced the presence of three distinct acid sites (named as OHA, OHB and OHC) absorbing at 3631, 3617 and 3600 cm–1 respectively as average values. A relation between width of the OH bands and Si content of the chabaziterelated samples was observed, as exemplified in Figure 5A, where the spectra obtained on a sample prepared with triethylamine – H-SAPO-34 (E), wSi ¼ 0.14 – are compared with those of CAL-1 (wSi ¼ 0.23). While OHA and OHC are clearly resolved in H-SAPO-34 (E), where the contribution of OHB is small, CAL-1 shows broader features, and a larger concentration of the OHB component. The 29Si-CP-MAS-NMR spectra of the two samples (Figure 5B) shows that CAL-1 is characterized by a large amount of Si islands, while only a very small amount of Si patches, likely formed by ca. 10 Si atoms,57 is present in HSAPO-34 (E). Notice that 29Si-MAS-NMR, e.g. without CP, was not able to detect the presence of a minor fraction of small Si patches in similar H-SAPO34 (see above and Figure 3A). The estimation of the acid strength of the three distinct sites was made by performing careful multi-curve fitting also of the broad and intense absorption (Figure 5A) related to the formation of H-bonded OH  CO adducts. The following scale of acidity was found: OHB 4 OHA 4 OHC, each component being downward shifted upon CO adsorption by 331 cm–1 (OHB), 276 cm–1 (OHA) and 150 cm–1 (OHC) (average values). The shift measured for OHB is very large when compared to the usual values measured for isolated sites in SAPO structures,25 which are normally less acidic than those of aluminosilicates with analogous structure.15,39,41 This value resembles that measured upon CO adsorption on protons of the synthetic aluminosilicate with chabazite structure, H-SSZ-13 (316 cm–1).58

Solid Acid Microporous H-SAPO-34

Figure 5

615

(A) FTIR and (B) 29Si-CP-MAS-NMR spectra of as prepared H-SAPO-34related samples with different Si content. Top: CAL-1 (synthesized adopting an original procedure with a lamellar ALPO as precursor and hexamethyleneimine (HMI) as SDA), wSi ¼ 0.23, bottom: H-SAPO-34 (E) prepared with triethylamine, wSi ¼ 0.14 (bottom spectra). (A) FTIR spectra outgassed at 773 K before CO dosage (short-dotted curve) and at the highest CO coverage (PCO D 20 torr, dashed-dotted line). Bands obtained by threepeaks best fitting procedure are reported in full line with the corresponding assignment (OHA is transformed to OHA 0 upon CO adsorption and so on). The position of nOH for free OH and OH  CO adducts of H-SSZ-13, with the corresponding DnOH, is reported for comparison, from Ref. 60. Reprinted with permission from Ref. 25. Copyright 2007, American Chemical Society.

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The consistency of the curve fitting procedure was checked following the work by Makarova et al.54 who found a linear correlation between the intensity growth (and band broadening) of the Brønsted OH bands of H-ZSM-5 and the DnOH shift for OH  B adducts, in agreement with hydrogen-bonding theory. The linear correlation DA/A0 ¼ 0.018DnOH/cm
1 was reproduced and a good agreement was observed for the OHA and OHB data measured on the chabazite-related samples (Figure 6A).25 The data related to OHC species could be corrected by assuming that they are tilted towards the centre of the six rings of the hexagonal prism where they H-bond to framework oxygens (O2 in Figure 1B).25,35 The acidity of the three families of Brønsted sites in chabazite-related SAPO samples was also monitored by probe molecules with different basicity. As a case study we report the results obtained on a H-SAPO-34 (T) sample prepared with TEAOH upon ethylene adsorption (Figure 6B). In agreement with the higher basicity of the probe with respect to CO, a larger shift of the OHA, OHB and OHC bands is observed. As a consequence, the low frequency component of the broad band due to OHB  C2H4 adducts (indicated by an arrow in Figure 6B) can be better appreciated. Also these data show a good agreement with the DA/A0 versus DnOH plot (Figure 6A, light grey hexagons). As far as the nature of OHB sites is concerned, it was suggested that they are located at the borders of Si islands, where they feel a chemical environment similar to that present in zeolites. This hypothesis is in agreement with both the large downward shift upon CO adsorption, and 29Si-CP-MAS-NMR results (Figure 5B). To our knowledge, this is the first report of the presence of such strong acid sites in SAPO materials, confirming the results coming from theoretical calculations,15 that foresaw an acidity comparable to zeolites for sites formed at the borders of silicon-rich regions, in agreement with the topological models proposed by Barthomeuf.

5 Future Perspectives: Catalysts with Modified Morphology and Designed Distribution of Brønsted Acid Sites One of the main drawbacks of the MTO process is related to the fast deactivation of the catalyst, due to coke formation as a consequence of consecutive reactions between light olefins and strong Brønsted sites. This process is due to the presence of strong acid sites and to the limited diffusion of reactants/ products within the chabazite cages. As a consequence, new approaches are needed to prepare catalysts with finely tuned distributions of Brønsted sites and properly designed morphology. ALPO and SAPO molecular sieves are generally prepared by sol-gel processes and hydrothermal synthesis. H-SAPO-34 samples prepared by conventional routes very often present rhombohedral particles with size of the order of microns (Figure 7a), that do not facilitate the mass transfer during catalytic reactions.

Solid Acid Microporous H-SAPO-34

Figure 6

617

(A) Plot of the increase of the intensity DA/A0, (A and A0: intensity of the perturbed and unperturbed OH bands, respectively) versus the DnOH shift, as a consequence of the formation of OH  B adducts in H-ZSM-5 ( ): (1) O2, (2) N2, (3) CO, (4) C2H4. ,, J and &: data obtained upon CO adsorption on H-SAPO-34 prepared with different SDA;B on CAL-1. Full symbols ., K, ’ andE: OHC data corrected by assuming an increase of the extinction coefficient caused by hydrogen bonding with framework oxygen atoms. Circled ,, J, & and B: OHC values before correction of the extinction coefficient. Light grey hexagons: corrected data obtained upon C2H4 adsorption on sample H-SAPO-34 (T), prepared with TEAOH. (B) FTIR spectra of sample H-SAPO-34 (T) outgassed at 773 K before C2H4 dosage (short-dotted curve) and at the highest C2H4 coverage (PC2H4 D 20 Torr, dashed-dotted line). Bands obtained by three-peaks best fitting procedure are reported in full line. Reprinted with permission from Ref. 25. Copyright 2007, American Chemical Society.

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Figure 7

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Scanning electron micrographs of (a) SAPO-34 prepared form conventional gel procedures, (b) ALPO-kan, (c) CAL-1 and (d) detail at high magnification of CAL-4 prepared using layered ALPO-kan precursor.

Recently, a new synthetic approach was proposed, based on the use of lamellar aluminophosphate ALPO-ntu, with the structure of the siliceous kanemite, also referred to as ALPO-kan (Scheme 1y).55,56 It was shown recently that ALPO-kan with intercalated butylamine can be transformed into CAL-1 and CAL-2 (CAL ¼ CAmpinas-ALessandria) with structure analogous to SAPO-34, and into CAL-4 with structure analogous to SAPO-44, depending on the SDA used and the synthesis conditions (Scheme 1). This synthetic strategy has been discussed in a recent review.56 The synthesis procedure based on ALPO-kan, which is composed of disordered thin blades of various shapes and dimensions (Figure 7b), has a strong influence on the particle morphology. CAL-1 is, in fact, formed of randomly distributed lamellae and some small rhombohedral particles formed by the pilling and condensation of lamellae (Figure 7c). This morphology can be observed on other samples prepared from the same precursor (see for instance the high magnification image of CAL-4, Figure 7d). The resulting crystallites are also characterized by lower density with respect to H-SAPO-34 prepared in the classical way.56 The layered-type morphology should affect diffusion and mass transport properties since acid sites are more accessible than in larger, compact particles. Thus, reactions are expected to be y

H.O. Pastore, personal communication.

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619

Scheme 1 facilitated and possibly lead to higher conversions when using materials prepared in the proposed way. In relation to the acidity, the synthesis of SAPOs from lamellar ALPO-kan allows the production of materials with a wide range of Si content. The different Si distribution obtained by this way has been explained by a reaction mechanism involving the attack of the silicate anions to different Al sites in ALPO-kan, so that the resulting chabazite structure is characterized by a more heterogeneous (that is, disorganized) configuration of the hydroxyl groups in CAL-1 in comparison with the ones in SAPO-34.25,56 This new approach can be used to prepare catalysts with the desired distribution of Brønsted sites in terms of strength and density, that is to minimize the strongest Brønsted sites probably responsible for the fast deactivation of the catalyst.

6 Personal Remarks A unique feeling captures us when Sir John speaks about Science: we are all contemporaries. We live with Galileo, Newton, Faraday, Lavoisier, Volta and the others, the excitement of their everyday work, success and disillusion. This is an ingredient of the everlasting ability of Sir John to guide generations of scientists to the frontiers of the most demanding and demanded research goals. And indeed we belong to three different generations enriched by the contact

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with the knowledge, creativity and humanity of Sir John. The first time we met in Torino is vivid in our memory, having a light lunch on the river bank and enjoying the view of the Valentino Park and the Mole Antonelliana in the company of unforgettable sweet Margaret. We wish to remember the enthusiasm of Sir John for science and the wonderful and stimulating periods that some of us spent at the Royal Institution working with him. His passion for the history of science, which he popularizes enthusiastically both in public conferences and by innumerable anecdotes, is admirable. Our first joint paper was written in the period when Sir John jumped from one cab or train to another, for he was deputy pro-chancellor of the Federal University of Wales, lived in Cambridge and spent a few days per week at the Royal Institution in London as professor of chemistry. The correction of the manuscript of FTIR of water adsorbed on H-SAPO-34 and the related discussions were made on both cabs and metro lines, for lack of time and for the urgency of the divulgation of the results. There are several anecdotes and facts which characterize our scientific collaboration with Sir John, and exemplify his attitude towards science. Some of them remain vivid in our mind. ‘‘The best paper is always the next one’’, in replying to a comment of one of us, or ‘‘All is marvellous in science, not all is useful’’, in replying to a proposal of work, are just two examples of his way of thinking.

References 1. D. Barthomeuf, Zeolites, 1994, 14, 394. 2. J. Chen and J.M. Thomas, J. Chem. Soc. Chem. Commun., 1994, 603. 3. J.S. Chen, P.A. Wright, J.M. Thomas, S. Natarajan, L. Marchese, S.M. Bradley, G. Sankar, C.R.A. Catlow, P.L. Gaiboyes, R.P. Townsend and C.M. Lok, J. Phys. Chem., 1994, 98, 10216. 4. J. Chen, P.A. Wright, S. Natarajan and J.M. Thomas, Stud. Surf. Sci. Catal., 1994, 84, 1731. 5. B.W. Wojciechowski and A. Corma, in Catalytic Cracking: Catalyst, Chemistry and Kinetics, Marcel Dekker Inc., New York, 1986. 6. M. Sto¨cker, Micropor. Mesopor. Mat., 1999, 29, 3. 7. J.Q. Chen, A. Bozzano, B. Glover, T. Fuglerud and S. Kvisle, Catal. Today, 2005, 106, 103. 8. J.S. Chen and J.M. Thomas, Catal. Lett., 1991, 11, 199. 9. Y. Xu, C.P. Grey, J.M. Thomas and A.K. Cheetham, Catal. Lett., 1990, 4, 251. 10. S.T. Wilson, P.T. Barger and T.M. Reynolds, US Patent, 1999, 5912393. 11. S. Nawaz, S. Kolboe, S. Kvilse, K.P. Lillerud, M. Sto¨cker and H. Øren, Stud. Surf. Sci. Catal., 1991, 421. 12. D. Barthomeuf, J. Phys. Chem., 1993, 97, 10092. 13. J.M. Thomas, Y. Xu, C.R.A. Catlow and J.W. Couves, Chem. Mat., 1991, 3, 667.

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14. L. Smith, A.K. Cheetham, R.E. Morris, L. Marchese, J.M. Thomas, P.A. Wright and J. Chen, Science, 1996, 271, 799. 15. G. Sastre and D.W. Lewis, J. Chem. Soc. – Faraday Trans. 1, 1998, 94, 3049. 16. H.O. Pastore, S. Coluccia and L. Marchese, Ann. Rev. Mater. Res., 2005, 35, 351. 17. R. Vomscheid, M. Briend, M.J. Peltre, P.P. Man and D. Barthomeuf, J. Phys. Chem., 1994, 98, 9614. 18. M. Briend, R. Vomscheid, M.J. Peltre, P.P. Man and D. Barthomeuf, J. Phys. Chem., 1995, 99, 8270. 19. J.S. Chen, G. Sankar, J.M. Thomas, R.R. Xu, G.N. Greaves and D. Waller, Chem. Mat., 1992, 4, 1373. 20. L. Marchese, J.S. Chen, J.M. Thomas, S. Coluccia and A. Zecchina, J. Phys. Chem., 1994, 98, 13350. 21. L. Marchese, E. Gianotti, N. Damilano, S. Coluccia and J.M. Thomas, Catal. Lett., 1996, 37, 107. 22. L. Marchese, G. Martra, N. Damilano, S. Coluccia and J.M. Thomas, Stud. Surf. Sci. Catal., 1996, 101, 861. 23. P.A. Barrett, G. Sankar, C.R.A. Catlow and J.M. Thomas, J. Phys. Chem., 1996, 100, 8977. 24. T. Inui, S. Phatanasri and H. Matsuda, J. Chem. Soc. Chem. Commun., 1990, 205. 25. G.A.V. Martins, G. Berlier, S. Coluccia, H.O. Pastore, G.B. Superti, G. Gatti and L. Marchese, J. Phys. Chem. C, 2007, 111, 330. 26. C. Morterra and G. Magnacca, Catal. Today, 1996, 27, 497. 27. C. Paze, S. Bordiga, C. Lamberti, M. Salvalaggio, A. Zecchina and G. Bellussi, J. Phys. Chem. B, 1997, 101, 4740. 28. H. Kno¨zinger and S. Huber, J. Chem. Soc. – Faraday Trans. 1, 1998, 94, 2047. 29. S. Coluccia, L. Marchese and G. Martra, Micropor. Mesopor. Mat., 1999, 30, 43. 30. K.I. Hadjiivanov and G.N. Vayssilov, Adv. Catal., 2002, 47, 307. 31. G. Busca, G. Martra and A. Zecchina, Catal. Today, 2000, 56, 361. 32. L. Marchese, J.S. Chen, P.A. Wright and J.M. Thomas, J. Phys. Chem., 1993, 97, 8109. 33. A.G. Pelmenschikov and R.A. van Santen, J. Phys. Chem., 1993, 97, 10678. 34. V. Termath, F. Haase, J. Sauer, J. Hutter and M. Parrinello, J. Am. Chem. Soc., 1998, 120, 8512. 35. L. Smith, A.K. Cheetham, L. Marchese, J.M. Thomas, P.A. Wright, J. Chen and E. Gianotti, Catal. Lett., 1996, 41, 13. 36. J.D. Gale, C.R.A. Catlow and A.K. Cheetham, J. Chem. Soc. Chem. Commun., 1991, 178. 37. J.D. Gale, C.R.A. Catlow and J.R. Carruthers, Chem. Phys. Lett., 1993, 216, 155. 38. R. Shah, J.D. Gale and M.C. Payne, J. Phys. Chem., 1996, 100, 11688. 39. R. Shah, J.D. Gale and M.C. Payne, Chem. Commun., 1997, 131.

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40. J. Sauer, H. Horn, M. Haser and R. Ahlrichs, Chem. Phys. Lett., 1990, 173, 26. 41. J. Sauer, K.P. Schro¨der and V. Termath, Collect. Czech. Chem. Commun., 1998, 63, 1394. 42. M. Elanany, M. Koyama, M. Kubo, P. Selvam and A. Miyamoto, Micropor. Mesopor. Mat., 2004, 71, 51. 43. K.P. Schroder, J. Sauer, M. Leslie, C.R.A. Catlow and J.M. Thomas, Chem. Phys. Lett., 1992, 188, 320. 44. Y. Jeanvoine, J.G. Angyan, G. Kresse and J. Hafner, J. Phys. Chem. B, 1998, 102, 5573. 45. R.A. van Santen, A.J.M. Deman, W. Jacobs, E.H. Teunissen and G.J. Kramer, Catal. Lett., 1991, 9, 273. 46. P. Ugliengo, A.M. Ferrari, A. Zecchina and E. Garrone, J. Phys. Chem., 1996, 100, 3632. 47. G. Sastre, D.W. Lewis and C.R.A. Catlow, J. Phys. Chem., 1996, 100, 6722. 48. D.W. Lewis and G. Sastre, Chem. Commun., 1999, 349. 49. G. Sastre, D.W. Lewis and A. Corma, Phys. Chem. Chem. Phys., 2000, 2, 177. 50. G. Sastre, D.W. Lewis and C.R.A. Catlow, J. Phys. Chem. B, 1997, 101, 5249. 51. G. Sastre, D.W. Lewis and C.R.A. Catlow, J. Mol. Catal. A: Chem., 1997, 119, 349. 52. A. Zecchina and C.O. Arean, Chem. Soc. Rev., 1996, 25, 187. 53. B. Onida, Z. Gabelica, J.P. Lourenco, M.F. Ribeiro and E. Garrone, J. Phys. Chem. B, 1997, 101, 9244. 54. M.A. Makarova, A.F. Ojo, K. Karim, M. Hunger and J. Dwyer, J. Phys. Chem., 1994, 98, 3619. 55. A. Albuquerque, S. Coluccia, L. Marchese and H.O. Pastore, Stud. Surf. Sci. Catal., 2004, 154, 966. 56. H.O. Pastore, L.G. Pedroni, G.A.V. Martins, G.B. Superti, E.C. de Oliveira, M. Strauss, G. Gatti and L. Marchese, Micropor. Mesopor. Mat., 2007, 111, 3116. 57. P.P. Man, M. Briend, M.J. Peltre, A. Lamy, P. Beaunier and D. Barthomeuf, Zeolites, 1991, 11, 563. 58. S. Bordiga, L. Regli, C. Lamberti, A. Zecchina, M. Jorgen and K.P. Lillerud, J. Phys. Chem. B, 2005, 109, 7724.

CHAPTER 37

Strategically Designed SingleSite Heterogeneous Catalysts for Clean Technology, Green Chemistry and Sustainable Development ROBERT RAJA School of Chemistry, University of Southampton, Highfield Campus, Southampton, SO17 1BJ, UK

1 Introduction Eleven years ago, on July 1, 1996, I had the rather unexpected and pleasant opportunity of meeting Sir John Meurig Thomas (JMT) at the 11th International Congress on Catalysis in Baltimore, USA. I was a PhD student at the National Chemical Laboratory (India) and due to the unforeseen illness of my PhD supervisor, I was asked (coerced!) to deliver the lecture in his place. Sir John (whom I did not know at that time) was the first to ask me some revealing questions about zeolite mimics of enzyme catalysts (ZEOZYMES) after my lecture, but, when he asked me a penetrating question about the nature of the active site, I did not have a clue and answered that ‘‘this was not on the list of questions that my PhD supervisor had given me!’’. This response clearly amused him, but this was the beginning of a long-lasting and fruitful collaboration in our fascinating journey into the realms of single-site heterogeneous catalysis. After completing my PhD I was awarded an 1851 Exhibition to work with JMT at the Davy Faraday Research Laboratory (DFRL) of the Royal Institution of Great Britain (1997–1999) in London. It was here, in the basement of the DFRL (adjacent to Michael Faraday’s laboratory and workshop) that we designed and built our unique, state-of-the-art, robotic high-pressure catalytic 623

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reactor with online analytical capabilities. By the summer of 1999, JMT was keen that I move to Cambridge (he was the Master of Peterhouse at that time) and introduced me, on one eventful train journey from London King’s Cross, to Brian Johnson, who was the Head of Inorganic Chemistry. Sir John and Lady Margaret Thomas made us feel welcome in Cambridge and our relationship, both scientific and personal, began to blossom. There were times when we were working on many parallel research topics and needed to be in touch continuously (Lady Thomas, who was always a charming and welcoming host, would often joke that it would be better for me to move in with them) – JMT would pick the most exquisite of places (the Deer Park and Fellows Garden at Peterhouse, The Orchard at Grantchester, Wandlebury and the Perse Cricket grounds, just to name a few) to stimulate these scientific discussions. The focal theme of our research in the past 10 years of our collaboration, has been the discovery, design and synthesis of novel nanoporous and mesoporous solid materials for application as single-site heterogeneous catalysts in chemical, pharmaceutical, fine-chemical and environmental technologies, for selective oxidations, selective hydrogenation and asymmetric syntheses, using environmentally benign reagents, energy-renewable feedstocks and sustainable processes. The approach that we have adopted1–3 embraces the principles and practices of solid-state chemistry, augmented by lessons derived from enzymology, as well as computational chemistry. The dominant issue in our pursuit of green chemistry and clean technology was the need to have a widely applicable strategic principle that offers adequate scope for the rational de novo design of single-site heterogeneous catalysts. This was derived from a better understanding of the atomic and molecular basis of catalytic action by porous inorganic solids, based on the structural understanding of the nature of the active sites, the mode of activation of catalyst precursors and the secrets of their selectivity and durability. This entailed the design and synthesis of novel inorganic solids, especially those with open structures where the active sites are atomically engineered and fully characterized, the microporosity and mesoporosity of which confer attractive and adaptable adsorptive and catalytic properties. In this way we have succeeded in designing a range of new catalysts to effect, inter alia, shape-selective, regioselective, bifunctional and enantioselective catalytic conversions. In particular, large fractions of these catalysts are ideally suited for the era of clean technology in which single-step and/or solvent-free processes abound, and in which benign oxidants such as air or oxygen and inexpensive nanoporous materials are employed.4–8 We have had many successes, both academically and in the industrial context, and I would like to share a few experiences that gave us both immense satisfaction. A range of nanoporous and nanoparticle catalysts, designed, synthesized, characterized and tested, that meet most of the stringent demands of sustainable development and responsible (clean) technology will be described in this chapter. Some specific examples which highlight the industrial and commercial significance of our joint successes in the areas of bulk, fine and pharmaceutical chemistry that are illustrated here include: (a) the

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production of adipic acid (the precursor for Nylon-6,6, polyamides and polyurethanes) from cyclic alkanes, avoiding the use of concentrated nitric acid and the production of greenhouse gases such as nitrous oxide (N2O); (b) the singlestep production of e-caprolactam (precursor of Nylon-6) using a mixture of air, ammonia and a bifunctional microporous solid, without the use of oleum and hydroxylamine sulfate; (c) the bromine-free synthesis of p-xylene from terephthalic acid; (d) conversion of muconic acid to adipic acid using atomically engineered, anchored bimetallic nanoparticle catalysts and (e) the asymmetric synthesis of high-value pharmaceutical intermediates (such as methyl mandelate) using constrained chiral catalysts.

2 Conventional Industrial Methods of Manufacturing Nylon-6, Nylon-6,6 and Terephthalic Acid Most commercial processes for the production of Nylon-6 from cyclohexane as a feedstock involve multiple steps using a homogeneous cobalt naphthenate catalyst and entail the nitric acid oxidation of K–A (mixture of cyclohexanol and cyclohexanone) oil to adipic acid (see Figure 1). The environmentally undesirable features of this multi-step mode of manufacture are the generation of voluminous (5 % of total global N2O emissions in 2000) quantities of N2O (a greenhouse gas that also depletes the ozone layer), massive consumption of concentrated nitric acid (which is used as a stoichiometric oxidant) and several downstream processing disadvantages associated with many of the multiple steps (use of CrIII salts and their problematic disposal).9 The industrial production of adipic acid accounted for emissions of 13.2 million metric tons of CO2 in 2005. In 1999, the worldwide annual production of adipic acid was 2.5 million metric tons and since that time it has increased further. The main use of adipic acid is in the preparation of Nylon-6,6 fibres and resins. Large amounts are also converted to esters for use in plasticizers, lubricants and in a variety of polyurethane resins. Adipic acid is also added to gelatines and jams as an acidulant, and to other foods as a buffering or neutralizing agent. Other uses include adhesives, insecticides, tanning and drying agents, paper manufacture and in the textile industry. Nylon-6, on the other hand, is the polymerized product of e-caprolactam, which is currently manufactured in two distinct ways, each being a two-step process, and one of them (a) involving extremely aggressive reagents (Scheme 1) such as hydroxylamine sulfate, oleum and sulfuric acid.10 Moreover the former method of manufacture generates four times the mass of e-caprolactam as ammonium sulfate, a costly waste product to render neutral or useful. Nylon-6 is currently used for a wide variety of purposes: in the textile industry (lingerie, hosiery, sportswear, leisurewear, fashion wear, linings, parachutes, umbrellas, tents and sleeping bags), floor coverings (carpets and rugs), industrial yarns (tyres, conveyor belts, ropes, nets, fishing lines and tarpaulins), engineering

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Figure 1

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Conventional methods of producing K–A oil and adipic acid.

plastics (automotive air-inlets, engine covers and aircraft windows) and in films (food and industrial packaging).11 Notwithstanding the fact that ammonium sulfate is a low-value fertilizer, it is a major disadvantage in the conventional processes employed to date as it is produced in large volumes (nearly 4 kg for every kg of e-caprolactam produced); disposal therefore is a problem. Also, it affects the purity of e-caprolactam requiring expensive vacuum distillation and vacuum crystallization followed by treatments with ion-exchange resins. Even as a fertilizer, it has a lower output of nitrogen (21–22%) (hence requiring larger volumes for efficiency) and has a high acidity (requiring more lime to keep the soil neutralized). In the end, profitability for caprolactam is dependent upon the amount of ammonium sulfate produced as a by-product.11–13 Nylon-6,6 begins as a polymer whereas Nylon-6 is made from caprolactam, a monomer, where the internal (hydrogen) bonding is weak. Carpets made from Nylon-6 can be depolymerized into its precursor, caprolactam, which, in turn,

Strategically Designed Single-Site Heterogeneous Catalysts

Scheme 1

627

Two industrial methods of producing e-caprolactam.

can be repolymerized and made into Nylon-6. The entire process recovers more than 99% of the energy and materials used to make Nylon-6 carpet yarn (closed-loop recycling). In contrast, Nylon-6,6 is a condensation polymer formed by combining hexamethylene diamine (HMD, the first ‘‘6’’) with adipic acid (the second ‘‘6’’), which make separation and recycling difficult, due to a greater degree of hydrogen bonding and maximum alignment between molecular chains. Best efforts result in down-cycling Nylon-6,6 carpet fibre into lowperformance applications, such as carpet backing. All Nylon-6,6 products eventually end up in landfill sites or incinerators. In Europe, recycling of fibres and plastics is of increasing importance, therefore Nylon-6 manufacture, on this score, is preferred over that of Nylon-6,6 or other (equally recalcitrant) variants. Terephthalic acid is industrially produced14 by a multi-step, liquid-phase air oxidation of p-xylene in the presence of soluble manganese and cobalt acetate catalysts to form p-toluic acid, which is subsequently converted, under quite demanding reaction conditions (at least 175–225 1C; 1500–3000 kPa) in the presence of a bromide and/or bromine salts (Brd and dCH2CO2H radicals in acetic acid) to form crude terephthalic acid (Figure 2). The highly corrosive bromine–acetic acid environment coupled with the high exothermicity of the reaction necessitates the use of expensive titanium-lined equipment.15 Crystalline crude terephthalic acid is collected as wet cake and dried. It is purified by dissolving in hot water/solvents under pressure and selectively hydrogenating the contaminants catalytically. Downstream processing and purification are quite intense and expensive, as the homogeneous catalyst needs to be recovered

628

Chapter 37 CH3 Multi Step Process p-xylene Homogeneous catalysts

CH3 Co/Mn acetate acetic acid solvent

175-225°C 1500-3000 KPa

CH3

H

O

HO

O

.

O2, Br p-toluic acid H2O, HO

HBr

O

4-formylbenzoic acid

HO

by-products

+

Titanium-lined equipment

O

terephthalic acid HO

Figure 2

Highly corrosive bromine-acetic acid environment; highly exothermic reaction

O

Solvent evaporation Disposal of bromine salts High solvent loss Catalyst recovery and recycle Purification step

Current industrial method for the oxidation of p-xylene.

and the bromide salts need to be disposed.16 Terephthalic acid is primarily used in the manufacture and production of polyester fibres, films, polyethylene terephthalate solid-state resins and polyethylene terephthalate engineering resins. In 1993, the worldwide production was estimated to be 17–21 billion kg.

3 Environmentally Benign Production of Nylon-6, Nylon-6,6 and Terephthalic Acid The design strategy for synthesizing a single-site heterogeneous catalyst for the ‘‘N2O-free’’, aerobic oxidation of cyclohexane to adipic acid involved assembling a transition metal in high oxidation state (such as FeIII) within a MAlPO (metal-substituted aluminophosphate) structure in which the pore aperture is significantly smaller than that of FeAlPO-5 (7.3 A˚ diameter)17 so as to create a constrained environment for the cyclohexane oxidation and thus modify the selectivity of the reaction. The framework structure of the microporous catalyst that we targeted here was the one-dimensional channel system, AlPO-31.18 Because of the more puckered inner walls, compared with AlPO-5, there is a

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smaller diameter of pore, 5.4 A˚, which introduces the very kind of constrained environment that is required to enhance the product selectivity of the reaction. What we capitalized on was shape selective catalysis (i.e. ‘‘product shape selectivity’’ – only those products with appropriate molecular dimensions may diffuse easily out of the pores, whereas larger ones formed in the course of the reaction are trapped inside) using a carefully designed microporous solid. The free-radical oxidation that ensues, proceeds in a highly localized manner, in the restricted environment of the catalytically active sites where the FeIII ions are located. We may envisage18 the free radical and molecular intermediates (cyclohexanol, cyclohexanone formed from cyclohexyl hydroperoxide or 2hydroxycylohexanone and 1,2-cyclohexanedione) to be held in the vicinity of the active site, until oxidation proceeds further to yield the more mobile (and desired) linear product (adipic acid; see Figure 3). Our aim,19 when we embarked on the task of devising a low-temperature, solvent-free benign method of synthesizing e-caprolactam, was to identify a family of single-site catalysts that could be controllably tuned to yield maximal efficiencies with regard to the production of (i) hydroxylamine (NH2OH) under in situ conditions and (ii) e-caprolactam. It is a method that utilizes the notion of a bifunctional, open-structure catalyst, and one of its supreme practical advantages is that, unlike the conventional method described in Section 2 which generates massive quantities of a low-grade waste product, ammonium sulfate, it yields no waste. The single-site bifunctional nanocatalysts that we have designed20 (see Figure 4) perform well in consecutively converting cyclohexanone to its oxime and

Figure 3

N2O-free route for the one-step production of adipic acid from cyclohexane.

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Figure 4

Chapter 37

Single-step, solvent-free route to e-caprolactam using molecular oxygen.

e-caprolactam because hydroxylamine (NH2OH) is readily formed in situ inside the pores from NH3 and O2 at the MIII redox active site. The NH2OH converts cyclohexanone to its oxime both inside and outside the pores, and, likewise, at the Brønsted (MII) active sites, cyclohexanone–oxime is isomerized to e-caprolactam inside the nanopores of the catalyst. We found that increasing the concentration of Brønsted (MII) active sites enhances the selectivity for e-caprolactam and these catalysts exhibit excellent shape and substrate specificity (see Figure 4). The ‘‘imine’’ mechanism postulated for TS-1 (see Scheme 1b) can be categorically ruled out as there was no evidence for the formation of the C6H10¼NH intermediate or peroxy dicyclohexylamine (PDCA) by-products. Our approach1 to the design of suitable catalysts for the ‘‘bromine-free’’ oxidation of p-xylene to terephthalic acid relies on the use of single-site catalysts where the active sites are CoIII or FeIII ions in framework sites substituting for AlIII ions. These catalysts operate under solvent-free conditions without the need for corrosive activators and initiators such as bromine or bromide salts. Their mode of operation21 relies on shape-selective free-radical processes of a spatially constrained kind, where we found that assembling up to 10 atom percent of transition metal ions in the framework of the molecular sieve suppresses the formation of 4-formylbenzoic acid and increases the selectivity for terephthalic acid. As in the case of CoIIIAlPO-18, containing ca. 10 atom percent of CoIII ions in the framework,22 one might envisage the oxyfunctionalization occurring

Strategically Designed Single-Site Heterogeneous Catalysts

Figure 5

631

Bromine-free route for the aerobic oxidation of p-xylene to terephthalic acid.

simultaneously at both methyl ends of p-xylene, which could well be the reason for the higher selectivity of the MnIIIAlPO-36 (0.10) catalyst for terephthalic acid (see Figure 5), compared to MnIIIAlPO-36 (0.04). Contrary to earlier observations,14,15 where it was shown that this reaction proceeds in a sequential manner via the formation of p-tolualdehyde and p-toluic acid, thereby requiring the use of the corrosive bromide activators (for activating the second methyl group), our detailed kinetic experiments have revealed the simultaneous oxidation of both methyl groups in p-xylene to produce terephthalaldehyde and terephthalic acid (Figure 5).

4 Adipic Acid from Sustainable Resources Using Atomically Engineered Nanoparticle Catalysts Industrially desirable chemical products such as adipic acid (synthesized from fossil-fuel-based starting materials such as benzene) may be produced in good yields and selectivities from renewable plant-based feedstocks, which can be derived biocatalytically from D-glucose. Because of the need23–25 to expedite the greater utilization of chemical feedstocks derivable from plant or other replenishable sources, research efforts are gradually being focussed on developing high-performance catalysts that yield desirable industrial products in an environmentally benign fashion. It has been shown by Draths and Frost26 that

632

Chapter 37

microbe-catalyzed conversions of simple, plentiful and renewable carbohydrates, such as D-glucose, offer potentially attractive new routes to desirable materials such as adipic acid. In particular, new biosynthetic pathways, involving appropriate genetic manipulation, so as to convert D-glucose into cis,cismuconic acid, have already been worked out. This feedstock is currently synthesized from D-glucose under controlled fermentation conditions using a microbial catalyst under aerobic conditions. A large-scale manufacturing process of this feedstock would be somewhat similar to the microbial synthesis of L-lysine from D-glucose under fermentation conditions. Given the current purchase price of resin-grade adipic acid, it is quite obvious that the estimated cost of synthesizing adipic acid from carbohydrates requires significant cost reduction.26 However, it should be noted that this current price of resin-grade adipic acid does not reflect the problems (and recently introduced legislation) associated with the generation of N2O as a by-product and the use of benzene as a starting feedstock. We have found that bimetallic nanoparticles anchored within silica nanopores display high activities and selectivities in a number of single-step (and often solvent-free) hydrogenations at low temperatures due to the synergy of the two monometallic constituents. Good selective performance is observed27 in the hydrogenation of a wide range of aromatics, olefins, cyclic polyenes and of benzoic acid to cyclohexane carboxylic acid (again, relevant in the production of Nylon-6). By capitalizing on the synergistic aspects of our designed bimetallic nanoparticle catalysts, we were able to convert muconic acid, derived from glucose, to adipic acid (Figure 6). This novel route28 also avoids the use of concentrated nitric acid as the oxidant and the generation of large volumes of ‘‘greenhouse gases’’ such as N2O. It can be seen from Figure 6 that one of our atomically engineered nanoparticle catalysts, Ru10Pt2, is exceptional in its selectivity to other bimetallic nanocatalysts, but more interestingly, it is also superior to industrially used monometallic supported catalysts such as Pt and Rh. This augurs well for the future use29 of high-area, thermally stable, nanoparticle catalysts in a wide range of hydrogenations that may be effected to yield desirable chemical products from plant-crop sources. It is of relevance to point out that D-glucose may also serve as a source of hydrogen as recently demonstrated.26

5 High Performance Non-Phosphine-Based Constrained Chiral Catalysts for the Production of Methyl Mandelate Enantiomerically enriched a- and b-hydroxy carboxylic esters are valuable reagents for optical resolution and are important intermediates in the preparation of pharmaceuticals and agrochemicals.30,31 Esters of mandelic acid, for example, are used as precursors in the synthesis of pemoline, a CNS stimulant and in the production of artificial flavours and perfumes. Methyl mandelates

Strategically Designed Single-Site Heterogeneous Catalysts

Figure 6

633

A catalyst to convert sugar to nylon: bimetallic nanoparticle catalysts for the production of adipic acid from sustainable resources.

are prepared by direct esterification of mandelic acid with alcohols in the presence of sulfuric acid.32 This method of direct esterification with strong acids not only has the disadvantages associated with the corrosive nature of the acid, but is usually accompanied by side-reactions such as carbonization, oxidation and etherification, which decrease the overall selectivity and enantiopurity.32,33 In addition, expensive downstream processing and post-treatment pollution problems make the process commercially and environmentally unattractive. Even though enzymes are currently more extensively used industrially than asymmetric transition-metal complexes for enantioselective catalytic conversions involving pharmaceuticals and agrochemicals, the latter are of growing importance. Biological processes using microorganisms for asymmetric reduction have inherent problems of stability of the enzyme and the need to maintain an expensive NADH reconstitution system.31 The use of organometallic chiral complexes has, however, overwhelmingly been utilized homogeneously and their heterogeneous asymmetric counterparts have hitherto performed disappointingly so far as their enantioselectivity is concerned, largely because they contained a range of different kinds of active centres, each with its own catalytic selectivity. Moreover, one of the major disadvantages of using chiral phosphines is their high cost (the cost of the ligand, in most applications, far outweighs the cost of the final product) and sensitivity to oxidation, which limits their industrial applicability.

634

Chapter 37 34

Maschmeyer et al. showed that by constraining tethered organometallic chiral phosphines within the nanopores of MCM-41, a significant enhancement in enantioselectivity can be achieved for the allylic amination of cinnamyl acetate with benzyl amine. The synthetic strategy here involved the reaction between (S)-1-[(R)-1,2A-bis(diphenylphosphino)ferrocenyl]ethyl acetate and 3-(methylamino)propyltrimethoxysilane to form a silane functionalized ferrocenyl ligand. The incorporation of palladium dichloride forms the target ferrocenyl precursor. The ferrocenyl precursor was then reacted with the mesoporous silica, MCM-41, to yield the anchored catalyst (see Figure 7B). A striking illustration of the power of the confinement effect in favouring chiral conversions was demonstrated in the case of chiral allylic aminations35 and in the one-step hydrogenation of ethyl nicotinate to ethyl nipecotinate,36 for which the performance of the same active site can be compared in a homogeneous and confined heterogeneous form. Whereas the homogeneous chiral phosphine-based catalyst yields a racemic mixture with no enantioselectivity (ee), the spatially constrained form produces substantial amounts of the desired products and a high value of ee.35,36 The validity of the above strategic principle, boosted by support from the German Chemical Industry,37–39 led us to evaluate the reality of tethering rather inexpensive, diamine asymmetric ligands (Scheme 2) onto the inner walls of non-ordered mesoporous silicas. The argument behind using such an

Figure 7

Enantioselectivity can be induced by tethering chiral phosphine-based catalysts to the inner walls of mesoporous silica (B) or by anchoring diamine asymmetric ligands (see Scheme 2) onto concave surfaces (A). In the case of the former, it is the constraints imposed by the space surrounding the metal centre that facilitates the enantioselectivity, whereas in the case of the latter, it is the restricted access generated by the concavity of the pore that is the principal determinant for the ensuing enantioselectivity.

635

Strategically Designed Single-Site Heterogeneous Catalysts NH2 Ph

Ph O Ph

N

NH2

O

NH2

H2N

Et (S)-(-)-2-aminomethyl1-ethylpyrrolidine

(1R,2R)-(+)-1,2diphenylethylenediamine

N H L-tryptophanbenzyl ester

CH2Ph

PhH2C N

N N

O

O

2,2’-Bis[(4S)-4-benzyl-2-oxazoline

N

H (S)-(+)-1-(2-Pyrrolidinylmethyl)-Pyrrolidine

Scheme 2

Some non-phosphine-based chiral ligands that can be anchored to concave silica surfaces via the covalent or non-covalent tethering approaches.

approach was to substantiate the concept that the diamine ligands tethered at concave silica surfaces would boost the enantioselectivity to a far greater degree, in contrast to having them attached to convex surfaces. The idea is that the confining dimensions of the interior of a pore should be able to restrict the possible orientations that a bulky reactant can assume as it approaches a chiral catalytic centre that is attached to a concave pore wall (see Figure 7A). If a reactant is nudged by its surroundings into the right orientation for a stereospecific reaction, then the reaction should proceed enantioselectively. The ligands shown in Scheme 2 can be anchored either covalently40 to the inner walls of the mesoporous silica using a BF4– anion which is hydrogenbonded to the nitrogen of the amino group in the ligand, or via a non-covalent immobilization method41 that uses a surface-bound triflate (such as CF3SO3–) counter-ion to secure the cationic organometallic catalyst in place. Catalysts prepared via the latter method do not require a brominated trichlorosilane tether for anchoring the ligand and hence are an order of magnitude (about one-eighth the cost) cheaper than the covalently tethered analogues. The methodology is also quick, convenient and results in robust effective catalysts, which make them industrially attractive. The comparative performance of one such catalyst [Rh(COD)-(S)-(+)-1-(2pyrrolidinylmethyl)pyrrolidine]CF3SO3 (ligand shown in Scheme 2), tethered to the inner walls of non-ordered mesoporous silica samples with pore dimensions ranging from 38 to 250 A˚, is shown in Figure 8A. The catalyst’s ability to steer the asymmetric hydrogenation of methyl benzoylformate to methyl mandelate diminished systematically with increasing pore size, with the homogeneous counterpart yielding little or no enantioselectivity. As the size of the

636

Figure 8

Chapter 37

The effect of surface curvature (B) and concavity of the support in facilitating the asymmetric hydrogenation of methyl benzoylformate to methyl mandelate (A), using mesoporous silica-tethered [Rh(COD)-(S)-(+)-1-(2pyrrolidinylmethyl)pyrrolidine]CF3SO3 catalysts.

pore in the mesoporous silica is progressively increased, the support becomes less effective in constraining the reactant’s orientation (Figure 8B). This confinement effect has been independently verified and multiply demonstrated by German, Chinese and Korean chemists.42–44

6 Summary and Future Outlook In summary, the range of new single-site heterogeneous catalysts that we have been instrumental in designing opens up fresh approaches in our pursuit of ‘‘clean chemistry’’ and ‘‘green technology’’ by enabling processes that minimize (i) the consumption of energy and production of waste; (ii) the use of ‘‘unacceptable’’ homogeneous catalysts and the problems associated with product separation and purification, requiring expensive downstream processing; (iii) the use of corrosive, explosive, volatile or non-biodegradable solvents; (iv) the use of aggressive, ecologically harmful caustic oxidants such as peracids, oleum, concentrated nitric acid and other such reagents; and (v) the use of aggressive activators such as molecular bromine and other initiators that result in the liberation of noxious by-products and greenhouse gases, such as N2O, that result in ozone depletion and global warming.

Strategically Designed Single-Site Heterogeneous Catalysts

637

In particular, large fractions of the versatile catalysts described in this chapter are ideally suited for the era of green chemistry and clean technology, where it will be necessary to develop renewable sources of energy, to minimize (or recover) waste, to use atom-efficient processes and more importantly, to design novel catalysts that are highly selective as well as being benign to meet the demands of sustainable development.

References 1. 2. 3. 4. 5. 6. 7. 8. 9.

10. 11. 12. 13. 14.

15. 16. 17. 18. 19. 20. 21.

J.M. Thomas and R. Raja, Chem. Commun., 2001, 675. J.M. Thomas and R. Raja, Annu. Rev. Mater. Res., 2005, 35, 315. J.M. Thomas and R. Raja, Topics Catal., 2006, 40, 3. J.M. Thomas, R. Raja, G. Sankar, B.F.G. Johnson and D.W. Lewis, Chem. Eur. J., 2001, 7, 2973. J.M. Thomas, R. Raja and D.W. Lewis, Angew. Chem., Int. Ed., 2005, 44, 6456. J.M. Thomas, R. Raja, G. Sankar, R.G. Bell and D.W. Lewis, Pure Appl. Chem., 2001, 73, 1087. J.M. Thomas, R. Raja, G. Sankar and R.G. Bell, Nature, 1999, 398, 227. R. Raja and J.M. Thomas, Chem. Commun., 1998, 1841. M.T. Musser, Ullmann’s encyclopedia of industrial chemistry, in Adipic Acid, 5th edn, vol. A1, ed. B. Elvers, S. Hawkins, W. Russey and G. Schulz, Wiley-VCH, Weinheim, 1993, 271. R. Mokaya and M. Poliakoff, Nature, 2005, 437, 1243. A. Cunningham, Greener Nylon: one-pot recipe could eliminate industrial leftovers, Science News, September 17, 2005, vol. 168, p. 179. B. Halford, Greener Nylon, C & EN, News of the Week, September 19, 2005, vol. 83, p. 10. L. Vargas, Material change in nylon manufacture, Business Weekly, September 23, 2005, vol. 426, p. 5. R.J. Sheeman, Ullmann’s encyclopedia of industrial chemistry, in: Terephthalic Acid, Dimethyl Terephthalate and Isophthalic Acid, online edn, Wiley-VCH, Weinheim, 2005, 1. W. Partenheimer, in: Catalysis of Organic Reactions, ed. D.W. Blackburn, Marcel-Dekker, New York, 1990, 321–346. Amoco Chemical Company, Material Safety Data Sheet, Amoco TA-33, Chicago, IL, 1993. R. Raja, G. Sankar and J.M. Thomas, J. Am. Chem. Soc., 1996, 121, 11926. M. Dugal, G. Sankar, R. Raja and J.M. Thomas, Angew. Chem., Int. Ed., 2000, 39, 2310. R. Raja, G. Sankar and J.M. Thomas, J. Am. Chem. Soc., 2001, 123, 8153. J.M. Thomas and R. Raja, Proc. Natl. Acad. Sci. USA, 2005, 102, 13732. R. Raja, J.M. Thomas and V. Dreyer, Catal. Lett., 2006, 110, 179.

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22. R. Raja, G. Sankar and J.M. Thomas, Angew. Chem., Int. Ed., 2000, 39, 2313. 23. P.T. Anastas and T.C. Williamson, Green Chem., 1998, 1. 24. H. van Bekkum and P. Gallezot, Top. Catal., 2004, 27, 181. 25. Road map for Biomass Technologies in the United States, http://usms.nist. gov/roadmaps/ 26. K.M. Draths and J.W. Frost, J. Am. Chem. Soc., 1994, 116, 399. 27. J.M. Thomas and R. Raja, J. Organomet. Chem., 2004, 689, 4110. 28. J.M. Thomas, R. Raja, B.F.G. Johnson, T.J. O’Connell, G. Sankar and T. Khimyak, Chem. Commun., 2003, 1126. 29. S. Hurtley, Renewable nylons, editor’s choice, Science, 2003, 300, 867. 30. Y. Tashiro, T. Nagashima, S. Aoki and R. Nishizawa, US Patent No. 4,224,239, 1980. 31. T. Endo and K. Tamura, US Patent No. 5,223,416, 1993. 32. E. Haslam, Tetrahedron, 1980, 36, 2409. 33. G.D. Yadav and R.D. Bhagat, Clean Techn Environ. Policy, 2005, 7, 245. 34. J.M. Thomas, T. Maschmeyer, B.F.G. Johnson and D.S. Shephard, J. Mol. Catal., 1999, 141, 139. 35. B.F.G. Johnson, S.A. Raynor, D.S. Shephard, T. Maschmeyer, J.M. Thomas, G. Sankar, S. Bromley, R.D. Oldroyd, L. Gladden and M.D. Mantle, Chem. Commun., 1999, 1167. 36. S.A. Raynor, J.M. Thomas, R. Raja, B.F.G. Johnson, R.G. Bell and M.D. Mantle, Chem. Commun., 2000, 1925. 37. R. Raja, J.M. Thomas, M.D. Jones and B.F.G. Johnson, German Patent Filing No. DE 10305946, 2003. 38. J.M. Thomas, B.F.G. Johnson, R. Raja and M.D. Jones, US Patent Filing No. US 10326915, 2004. 39. J.M. Thomas, R. Raja, M.D. Jones and B.F.G. Johnson, European Patent Filing No. EP 4001904, 2004. 40. M.D. Jones, R. Raja, J.M. Thomas, B.F.G. Johnson, D.W. Lewis, J. Rouzard and K.D.M. Harris, Angew. Chem., Int. Ed., 2003, 42, 4326. 41. R. Raja, J.M. Thomas, M.D. Jones, B.F.G. Johnson and D.E.W. Vaughan, J. Am. Chem. Soc., 2003, 125, 14982. 42. M. Heitbaum, F. Glorius and I. Escher, Angew. Chem., Int. Ed., 2006, 45, 4732. 43. H.Q. Yang, J. Li, J. Yang, Z.M. Liu, Q.H. Yang and C. Li, Chem. Commun., 2007, 1086. 44. C.E. Song, D.H. Kim and D.S. Choi, Eur. J. Inorg. Chem., 2006, 15, 2927.

CHAPTER 38

Catalysis by Lewis Acids: Basic Principles for Highly Stereoselective Heterogeneously Catalyzed Cyclization Reactions MERCEDES BORONAT, AVELINO CORMA AND MICHAEL RENZ Instituto de Tecnologia Quimica (UPV-CSIC), Universidad Polite´cnica de Valencia, Avda. de los Naranjos s/n, 46022 Valencia, Spain

1 Introduction Menthol is an important fragance compound used in many consumer products such as cigarettes, chewing gums, toothpastes, pharmaceutical and personal care products. It is produced on a large scale in the fine chemical industry, and has an estimated worldwide consumption of 6000 tons/year.1–3 Of the eight stereoisomers of menthol, only (
)-menthol possesses the refreshing peppermint odour and physiological cooling effect that makes it valuable. Therefore, an asymmetric synthesis has been designed for the industrial process of menthol production (Scheme 1). In the first step, an allylic amine, 1a or 1b, is isomerized

NEt2

I

1a NEt2

III

IV

V

O II 1b

Scheme 1

(R )-2

(R )-3

OH

(R)-4

OH

(R)-5

NEt2

Takasago process for (–)-menthol production (for details see Ref. 1). 639

640

Chapter 38

asymmetrically into an enamine of citronellal (R)-2 in the presence of a Rh/BINAP catalyst with 97% ee. Then, citronellal ((R)-3) is produced by hydrolysis of (R)-2 in aqueous acidic conditions. The cyclization of citronellal ((R)-3) into isopulegol ((R)-4) in the presence of a Lewis acid produces two other stereogenic centres in the adequate configuration and then, a final hydrogenation of the remaining double bond gives the desired stereoisomer of the final product menthol ((R)-5). For several years, the cyclization step has been carried out in the presence of ZnBr2, since the stereoselectivity achieved is higher than that obtained with other Lewis acids like FeCl3, AlCl3, SnCl4 or TiCl4,4 or also NbCl5, TaCl5, InCl35 and Bi(OTf)3.6 However, the stoichiometric amounts of ZnBr2 required may cause waste disposal problems, since it acts as a marine toxin.7 Another critical factor in this process is the hygroscopic character of anhydrous ZnBr2 and the fact that wet ZnBr2 is much less active and selective than the anhydrous form. To eliminate these drawbacks from the industrial process, Takasago has introduced an alternative metal complex, namely aluminium tri-(2,6)-diphenylphenoxide,8 that provides very high stereoselectivities in the cyclization (up to 499.3%). Unfortunately, the catalyst has to be destroyed with NaOH after the reaction and therefore cannot be recovered and re-used. Solid materials like zeolites and molecular sieves,9–12 sulfated zirconia,13 hydrous zirconia or Al2O3–SiO2 mixed oxides14 have been applied as catalysts for the cyclization step. Zn15,16 and heteropoly acid (H3PW12O40)17 have been supported onto silicas, and Zr has been introduced into montmorillonite by cation exchange.18 However, none of these catalysts fulfil the demands for an economically reasonable industrial process. In general, the diastereoselectivity obtained is not high enough and turnover numbers are low. Some of these materials suffer from metal leaching, and some others, especially strong Brønsted acidic materials, lack product selectivity. In a more promising concept, zeolites containing isolated and well-defined Lewis acid centres, such as Sn19 or Zr20 atoms occupying framework positions in Beta zeolite, have been employed in the cyclization step. Using Sn–Beta in a fixed bed continuous reactor, more than 104 reaction cycles for each catalytic site were achieved.19 The catalyst provided a 99% conversion during 48 h, and the diastereoselectivity was 83% in favour of the desired isomer when acetonitrile was employed as solvent, although it dropped to 72% in the absence of any solvent.21 In the case of Zr–Beta catalyst, the diastereoselectivity was as high as 93%, and the presence of a certain amount of water was found to be advantageous both for activity and selectivity. To convert Sn–Beta or Zr–Beta into real heterogeneous substitutes of the homogeneous ZnBr2 or Al(OR)3, the diastereoselectivity has to be improved. In this work, the nature of the catalytically active sites has been determined and the mechanism of the cyclization reaction has been studied by means of density functional theory (DFT) calculations. On these bases, leads are given for designing better catalysts and processes.

641

Catalysis by Lewis Acids

2 Characterisation of the Catalytic Active Sites in Sn–Beta

initial rate BV oxidation in mmol/h/g cat.

When Sn–Beta was introduced as a solid catalyst, its Lewis acidity was attributed to tetrahedrally coordinated tin atoms incorporated into the zeolite framework.21,22 Recently, theoretical23,24 and in situ IR spectroscopic studies25 have shown that the catalytic activity of Sn–Beta zeolite for the Baeyer–Villiger (BV) oxidation and the Meerwein-Ponndorf-Verley (MPV) reduction is related to the presence of partially hydrolyzed tetrahedrally coordinated framework (SiO)3SnOH sites (Site A), fully coordinated Sn(OSi)4 sites (Site B) being much less active for these two reactions. Sites A are formed from sites B by hydrolysis of a Sn–O–Si bridge during the calcination process, so that it is possible to obtain catalysts with different concentrations of sites A and B by changing the calcination procedure. To test the activity of both sites for the cyclization reaction, several Sn–Beta samples subjected to different post-synthesis treatments, and therefore containing different number of partially hydrolyzed (SiO)3SnOH sites, were used to catalyze the BV oxidation of adamantanone with H2O2 and the cyclization of citronellal ((R)-3) into isopulegol ((R)-4). The initial rates for the two reactions were measured and they are compared in Figure 1. There is a linear correlation between both series of data, indicating that the activity for the cyclization also increases with the number of hydrolyzed (SiO)3SnOH sites as was seen for BV oxidations. However, the straight line does not meet the origin, which means that there is a contribution other than that of the hydrolyzed (SiO)3SnOH sites to the catalytic activity of Sn–Beta for the cyclization reaction. Since the parent

25

20

15

10

5

0 0

100

200

300

400

500

600

Initial rate cyclization in mmol/h/g cat.

Figure 1

Correlation of the initial rates of Baeyer–Villiger oxidation of adamantanone with hydrogen peroxide and citronellal cyclization, both reactions catalyzed by a series of samples of Sn-Beta zeolite subjected to different post-synthesis treatments.

642

Chapter 38

system, i.e. all-silica Beta zeolite, has been found to be completely inactive,19 this catalytic activity cannot be related to silanol groups or other defect sites, but we have to associate it to fully coordinated tetrahedral Sn(OSi)4 centres i.e. sites B. Thus, in contrast to the BV oxidation and the MPV reduction, the cyclization of citronellal ((R)-3) into isopulegol ((R)-4) is catalyzed by the two types of Lewis acid centres (A and B) existing in Sn–Beta zeolite.

3 Theoretical Study of the Reaction Mechanism The mechanism of cyclization of citronellal (R)-3 into the desired (
)-isopulegol and the most abundant undesired (+)-neo-isopulegol catalyzed by Sn–, Zr– and Ti–Beta zeolites has been theoretically investigated using DFT methods. Since the activity of partially hydrolized Sites A is higher than that of fully coordinated Sites B, the M(O–SiH3)3OH (with M ¼ Sn, Zr, Ti) cluster models described in the previous work24 were used to simulate the Lewis acid active sites. All calculations were performed using the density functional B3PW91 method,26,27 a LANL2DZ effective core potential basis set for Zr,28 Sn29 and Ti,26 and the standard 6-31G(d,p) basis set30 for C, O, Si and H atoms. The reaction involves a nucleophilic attack of the C¼C double bond to the carbonyl carbon atom, resulting in formation of a new C–C bond and a proton transfer from one methyl group to the carbonyl oxygen atom yielding a hydroxyl group. Both reactions (nucleophilic attack and proton transfer) occur at the same time in a concerted way, i.e. in a cyclic carbonyl–ene reaction involving six atoms and six electrons. The optimized structures of citronellal (R)-3 adsorbed on the Lewis acid site, of the transition state for the reaction, and of the adsorbed isopulegol (R)-4 product are depicted in Figure 2, and the most relevant calculated bond lengths are summarized in Table 1. On all three catalysts, the first step in the mechanism is coordination of the oxygen atom of the carbonyl group in citronellal to the Lewis acid metal centre. As discussed in previous work,31 this interaction consists of an electron density

Figure 2

Optimized structures of (a) citronellal ((R)-3) adsorbed on the Lewis acid site, (b) transition state for the cyclization, and (c) (
)-isopulegol ((R)-4) adsorbed on the Lewis acid site.

643

Catalysis by Lewis Acids

Table 1

B3PW91 optimized values of the most important bond distances (A˚) in the structures involved in the cyclization of citronellal ((R)-3) into isopulegol ((R)-4) catalyzed by Sn–, Zr– and Ti–Beta. The atom labelling is shown in Figure 2. Sn

r(OM) r(CaO) r(CaCb) r(CbCc) r(CcCd) r(CdH) r(HO)

Figure 3

Zr

Ti

(R)-3

TS

(R)-4

(R)-3

TS

(R)-4

(R)-3

TS

(R)-4

2.284 1.228 3.079 1.344 1.504 1.097 3.416

2.113 1.330 1.766 1.423 1.441 1.153 1.727

2.262 1.443 1.539 1.517 1.340 2.482 0.977

2.341 1.228 2.999 1.345 1.504 1.098 3.126

2.178 1.333 1.743 1.427 1.436 1.165 1.673

2.318 1.441 1.539 1.516 1.340 2.478 0.977

2.287 1.222 3.090 1.344 1.504 1.097 3.294

2.062 1.324 1.766 1.423 1.435 1.168 1.662

2.251 1.435 1.540 1.516 1.340 2.524 0.975

Calculated energy profiles for the cyclization of citronellal ((R)-3) into isopulegol ((R)-4) catalyzed by Sn-, Zr- and Ti-Beta zeolites.

transfer from the carbonyl oxygen atom to the metal empty orbitals, and a back-donation from the catalyst to the carbonyl p*CO. This back-donation activates the C¼O double bond and an increase in the positive charge on the carbon atom occurs, making it more susceptible to the nucleophilic attack from the C¼C double bond. In the transition state, the CO bond length increases to 1.32–1.33 A˚, the Ca–Cb distance is about 1.75 A˚, indicating that a C–C bond is being formed, and the H atom, although still bonded to the methyl carbon atom, begins to be transferred to the carbonyl oxygen. The final product is in all three cases a (–)-isopulegol molecule bonded to the Lewis acid metal centre through the oxygen atom of the newly formed hydroxyl group. Although not shown, the mechanism of cyclization of citronellal into (+)-neo-isopulegol is equivalent, and occurs in one step involving similar structures and interactions. The calculated energy profiles for the Sn–, Zr– and Ti–Beta catalyzed cyclization of citronellal into (
)-isopulegol are depicted in Figure 3, and the

644

Chapter 38

Table 2

Eads Eact DE

Calculated adsorption (Eads), activation (Eact) and reaction (DE) energies (kcal/mol) for the cyclization of citronellal into (–)-isopulegol and (+)-neo-isopulegol.

citronellal - (
)-isopulegol

citronellal - (+)-neo-isopulegol

Sn

Zr

Ti

Sn

Zr

Ti


14.3 13.0
10.5


16.1 12.6
10.7


8.6 15.7
10.3


15.4 14.3
2.3


17.3 14.1
2.1


9.8 17.2
2.0

Table 3

Measured stereoselectivity towards (
)-isopulegol (in %) and (N2/N1) values calculated for Sn–, Zr– and Ti–Beta zeolites.

Stereoselectivity (%) (N2/N1)exp (N2/N1)products (N2/N1)TS

Sn

Zr

Ti

72 0.39 0.33 0.75

80 0.25 0.29 0.71

59 0.69 0.31 0.60

calculated adsorption, activation and reaction energies for this process and for the cyclization of citronellal into (+)-neo-isopulegol are summarized in Table 2. In all cases, adsorption of citronellal on the metal Lewis acid centre is exothermic, but this interaction is more stabilizing in Sn– and Zr– than in Ti–Beta. The activation barriers, calculated as energy differences between the transition state and adsorbed citronellal are quite similar for Sn– and Zr–, and slightly higher for Ti–Beta, while the reaction energy, calculated as energy difference between adsorbed product and adsorbed reactant, is similar in the three catalysts. It is important to note, in relation to the stereoselectivity of the cyclization step, that the adsorption and activation energies involved in the formation of (
)-isopulegol and (+)-neo-isopulegol are quite similar, and the main difference between them is the considerably lower stability of the (+)-neo-isopulegol isomer. The stereoselectivity, or relative abundance of both isomers, can be estimated using a Boltzmann distribution:   N2
ðE2
E1 Þ / exp RT N1 where N2 and N1 are the number of molecules of (+)-neo-isopulegol and (
)-isopulegol, respectively, E2 and E1 are their calculated energies, and T the reaction temperature. Table 3 summarizes the stereoselectivity towards (
)-isopulegol experimentally measured for Sn–, Zr– and Ti–Beta zeolites (data from Table 4), the relative abundance of both isomers (N2/N1) obtained from these experimental data, and the values given by the Boltzmann distribution. Experiment indicates that Zr–Beta is the most selective catalyst followed by

645

Catalysis by Lewis Acids

Table 4

Conversions and diastereoselectivity for the cyclization of citronellal ((R)-3) into isopulegol ((R)-4) by different Beta zeolite catalysts. The experimental conditions of Ref. 19 are used in all Tables if no other information is stated. Ratio

Entry

Catalyst

Si/M

Solvent

1 2 3 4 5 6 7 8 9 10 11 12

Sn–Beta Sn–Betaa Sn–Betab Sn–Betac Ti–Beta Ti–Beta Zr–Beta Zr–Betad Zr–Betae Zr–Beta Zr–Beta Zr–Beta

123 123 250 123 65 65 100 100 100 100 116 116

– MeCN MeCN MeCN – MeCN tert-BuOH tert-BuOH tert-BuOH MeCN – MeCN

Conversion (%)

Diastereoselectivity (%)

From Ref.

67 99 97 73 40 35 94 61 36 96 67 89

72 83 85 85 59 57 93 90 86 91 80 87

19 19 19 19 20 20 20 20 – –

a

Sn–Beta sample with initial rates of 602 and 21.9 mmol/h/g cal for cyclization and BV oxidation, respectively. b Sn–Beta sample with initial rates of 453 and 12.1 mmol/h/g cal for cyclization and BV oxidation, respectively. c Sn–Beta sample with initial rates of 281 and 3.3 mmol/h/g cal for cyclization and BV oxidation, respectively. d Pretreatment of the catalyst: drying at 100 1C for 4 h. e Pretreatment of the catalyst: drying at 300 1C for 4 h.

Sn–Beta, and that the selectivity of Ti–Beta towards (
)-isopulegol is considerably lower. If we use the calculated energies of adsorbed (
)-isopulegol and adsorbed (+)-neo-isopulegol as E1 and E2 in the Boltzmann equation, the relative distribution of both isomers is in agreement with experiment for Zr– and Sn–Beta catalysts. However, the value obtained for Ti–Beta, 0.31, is far from the experimental value of 0.69. Another possibility is to relate the product distribution not with the stability of the products, but with the activation energy necessary to form them. In this case, the E1 and E2 values are the calculated energies of the transition states leading to formation of (
)-isopulegol and (+)neo-isopulegol, respectively. In this case, the product distributions obtained for Zr– and Sn–Beta are completely different from the experimental values, but the calculated value for Ti–Beta, 0.60, is close to the experimental value of 0.69. The reason for the different behaviour of the three catalysts can be found in the energy profiles depicted in Figure 3. The energy released by adsorption of citronellal on Sn– and Zr–Beta is larger than the activation energy necessary to convert it into any of the two isomers of isopulegol considered. This means that both competitive reactions (cyclization to (
)-isopulegol and to (+)-neo-isopulegol) will occur, and the relative abundance of both isomers will be determined only by their relative stability (thermodynamic control). On the contrary, the adsorption energy of citronellal on Ti–Beta is not as large as the activation energy necessary for the cyclization reaction to occur. Therefore, the relative

646

Chapter 38

abundance of both isopulegol isomers will not be determined by their stability, but by the activation energy necessary to obtain them (kinetic control). An additional beneficial effect might be due to the size of the metal atom incorporated into the zeolite framework. It has been reported earlier31 that the Sn–O bond is shorter than the Zr–O bond in an M–O–Si bridge with 1.881 and 1.963 A˚, respectively. Consequently, it can be assumed that the zirconium atom is slightly displaced from the original silicon framework position towards the zeolite channel, invading the latter. This restricts the available space around the active centre. In the case of the cyclization it has been shown that a reduced space favours the formation of the desired diastereomer.8 Indeed, when one compares the measured and the calculated N2/N1 values for Sn– and Zr–Beta it seems that there is also a positive steric effect in the Zr case.

4 Influence of Solvents and Water on the Diastereoselectivity For the cyclization of citronellal (3) into isopulegol (4), it has been reported that lower conversions and also lower diastereoselectivities are achieved with Zr–Beta or Sn–Beta samples that have been dried accurately (Table 4). We have found in this work (see Table 5) that when a freshly calcined Sn–Beta catalyst is employed in the cyclization reaction the diastereoselectivity is only 75%, and it improves just by increasing the time period between calcination and reaction. During this period no special treatment is applied to the catalyst, it is just stored at ambient conditions. If the diastereoselectivity of fully coordinated Sn(OSi)4 sites (Sites B) and (SiO)3SnOH centres (Sites A) was quite different, the observed behaviour could be related to the hydrolysis of sites B into A by ambient water. However, it can be seen in Table 4 (entries 4 and 2) that samples with almost exclusively Sn(OSi)4 sites gave selectivities of 85% whereas samples with a significant amount of (SiO)3SnOH centres provided selectivities of 83%. Although it could be demonstrated that water itself has a beneficial effect on the diastereoselectivity, the way of action could not be ascertained. It can be seen in Table 6 that Table 5

Cyclization of citronellal (3) to isopulegol (4) catalyzed by Sn–Beta zeolite samples. The samples have been exposed to ambient conditions during different time periods. Conversions and diastereoselectivity towards the desired isopulegol diastereomer are given for 60 min reaction times.

Entry

Aging (Days)

Conversion (%)

Selectivity (%)

1 2 3 4 5 6

1 2 5 11 26 126

99 99 98 99 99 99

76 78 81 82 83 84

647

Catalysis by Lewis Acids

Table 6

Cyclization of citronellal (3) to isopulegol (4) catalyzed by Sn-Beta zeolite (aged for 29 days) in the presence of different amounts of water. Conversions and diastereoselectivity towards the desired isopulegol diastereomer are given for 30 min reaction times.

Entry

Water amount (mg)

Conversion (%)

Selectivity (%)

1 2 3 4 5

0 40 60 100 200

97 96 96 86 67

82 84 84 84

Table 7

Cyclization of citronellal (3) to isopulegol (4) catalyzed by Sn-Beta zeolite (aged for 2 days) in different amounts of acetonitrile solvent. Conversions and diastereoselectivity towards the desired isopulegol diastereomer are given for 30 min reaction times. Reaction conditions: 600 mg of citronellal and the solvent were mixed and a 50 mg sample of the catalyst was added.

Entry

MeCN (g)

Conversion (%)

Selectivity (%)

1 2 3 4 5

0 0.25 0.75 1.40 3.00

63 79 90 89 93

73.1 77.7 77.7 77.7 77.7

small amounts of water, i.e. 40–60 mg, improve the diastereoselectivity towards the desired product from 82 to 84% (cf. entries 1–3), but higher amounts of water were not of interest since the activity of the catalyst declined. A special influence on conversion and diastereoselectivity has also been reported for the solvent.19 In the absence of any solvent the reaction reached a plateau of below 70% conversion and the diastereoselectivity did not surpass 75% (cf. Table 4, entry 1). The addition of acetonitrile solvent in any amount (Table 7) improved the diastereoselectivity significantly, and increasing amounts of acetonitrile accomplished higher conversions after 30 min reaction time. This effect should be partly due to the better diffusion of substrate and product molecules in the presence of a small solvent molecule. However, as a working hypothesis, competitive coordination of the acetonitrile to the active centre could explain both phenomena. The alcohol product is rather bulky, and the approach of the aldehyde substrate to the active centre to which the alcohol product is still coordinated is difficult. However, a small solvent molecule like acetonitrile could compete favourably with isopulegol, which would leave the reaction centre and diffuse into the zeolite channels. The approach of a new molecule of citronellal would be much easier, and the catalytic cycle could start again. Furthermore, the presence of a small molecule coordinated to the Lewis centre would partially

648

Table 8

Chapter 38

Cyclization of citronellal (3) to isopulegol (4) catalyzed by Sn-Beta zeolite (aged for 126 days) in different nitrile solvents at 80 1C. Conversion and diastereoselectivity towards the desired isopulegol diastereomer are given for 60 min reaction times.

Entry

Solvent

Conversion (%)

Selectivity (%)

1 2 3 4 5

MeCN EtCN n-PrCN iso-PrCN iso-BuCN

99 80a 99 7 27

84.2 84.6 85.4 89.8 82.7b

a

b

Conversions of 93% and 96% have been observed at 90 and 95 1C; the selectivity remained essentially the same. Selectivity towards the cyclization was only 76%.

restrict the space around the reaction centre, not as much as to impede the approach of a substrate molecule, but enough to improve the diastereoselectivity. If this working hypothesis were true, diastereoselectivity would improve with increasing size of the nitrile solvent, and when the solvent was bulky enough to impede the approach of citronellal to the active centre, conversion would decrease. And this is exactly what occurs. Whereas linear propionitrile and butyronitrile guaranteed a smooth reaction (Table 8, entries 2 and 3), branched 2-methylpropanenitrile and 3-methylbutanenitrile as solvents accomplished only low conversion levels after 60 min (Table 8, entries 4 and 5). At the same time the diastereoselectivity was improved in the solvent series acetonitrile, propionitrile, butyronitrile, and 2-methylpropanenitrile in parallel with their molecule size (cf. Table 8, entries 1–4). Therefore, we can conclude that the nitrile solvent coordinates to the catalytic centre, helps to replace the product molecule and restricts the space sufficiently to improve the diastereoselectivity. However, in the case of an alcohol solvent, it is not clear whether the same way of action can be claimed or a different type of interaction has to be expected. The first hint for this doubt is that tert-butanol is quite bulky, comparable to 2-methylpropanenitrile, and the reaction proceeds smoothly. Furthermore, the actions of both solvents could be synergetic. As shown in Table 9, the diastereoselectivity achieved in a mixture of both solvents is higher than the diastereoselectivities obtained in each solvent alone. When the cyclization was carried out in the presence of Zr–Beta and 3.00 g of tertbutanol, a diastereoselectivity of 89.4% was obtained (Table 9, entry 2) whereas 3.00 g of acetonitrile as solvent gave only 88.5% selectivity (Table 9, entry 4). However, any mixture of both solvents resulted in a higher selectivity. It can be concluded that the cyclization of citronellal to isopulegol is very sensitive to small changes in the environment. Water molecules, nitriles and tertiary alcohols can contribute to achieve higher diastereoselectivity. It is very likely, and in one case (nitrile) it has been shown, that the improvement of the diastereoselectivity should be due to steric restriction around the catalytic centre.

649

Catalysis by Lewis Acids

Table 9

Cyclization of citronellal (3) to isopulegol (4) catalyzed by Zr-Beta zeolite (aged for 35 days) in different solvents or solvent mixtures. Conversions and diastereoselectivity towards the desired isopulegol diastereomer are given for a 30 min reaction time. Reaction conditions: 600 mg of citronellal and the solvent were mixed and a 50 mg sample of the catalyst was added.

Entry

MeCN (g)

t-BuOH (g)

Conversion (%)

Selectivity (%)

1 2 3 4 5 6 7 8

0 0 0 3.0 2.3 1.5 0.7 0.8b

0 3.0 5.0 0 0.8 1.5 2.3 2.3

67a 93 83 85 84 81 86 81

80.0 89.4 90.3 88.5 90.0 91.3 91.3 91.3

a b

Catalyst aged for only 2 days. Propionitrile was employed instead of acetonitrile.

5 Beneficial Factors for the Diastereoselectivity The cyclization of citronellal to isopulegol is a Lewis acid catalyzed process and there is no doubt that the main factor governing diastereoselectivity is the nature of the Lewis acid centre. It has been shown, experimentally and theoretically, that zirconium and tin are the most suitable elements for the production of a heterogeneous catalyst. The second important factor is the space available around the catalytically active Lewis-acid centre. This space is mainly determined by the zeolite morphology, but a fine tuning can be obtained by using a combination of small solvent molecules. In the case of Beta zeolite the presence of water, nitriles and tertiary alcohols in adequate concentrations further improve the diastereoselectivity of the reaction. It will be interesting in the future to incorporate zirconium and tin into zeolites with other morphologies and different channel sizes. With subsequent solvent fine-tuning, a cyclization process could be designed to replace the actual industrial production process.

References 1. S. Akutagawa, Top. Catal., 1997, 4, 271. 2. J.O. Bledseo Jr., in Encyclopedia of Chemical Technology (Kirk-Othmer), 4th edn, vol. 23, Wiley-Interscience, New York, 1998, p. 858. 3. K. Bauer, D. Garbe and H. Suburg, Common Fragrance and Flavor Materials, Wiley-VCH, Weinheim, 1997, p. 52. 4. Y. Nakatani and K. Kawashima, Synthesis, 1978, 2, 147.

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Chapter 38

5. C.K.Z. Andrade, O.E. Vercillo, J.P. Rodrigues, D.P. Silveira and J. Braz, Chem. Soc., 2004, 15, 813. 6. E.D. Anderson, J.J. Ernat, M.P. Nguyen, A.C. Palma and R.S. Mohan, Tetrahedron Lett., 2005, 46, 7747. 7. Quest International B. V., Eur. Pat. Appl., 1053974, 1999. 8. T. Iwata, Y. Okeda and Y. Hori, Eur. Pat. Appl., 1225163 A2, 2002. 9. M. Fuentes, J. Magraner, C. de las Pozas, R. Roque-Malherbe, J. Perez-Pariente and A. Corma, Appl. Catal., 1989, 47, 367. 10. G.K. Chuah, S.H. Liu, S. Jaenicke and L.J. Harrison, J. Catal., 2001, 200, 352. 11. N. Go¨tz, K. Ebel, M. Friedrich and U. Mu¨ller (BASF AG) PCT Appl. WO 2004/101480, 2004. 12. P. Ma¨rki-Arvela, N. Kumar, V. Nieminen, R. Sjo¨holm, T. Salmi and D.Y. Murzin, J. Catal., 2004, 225, 155. 13. G.D. Yadah and J.J. Nair, Chem. Commun., 1998, 2369. 14. M. Friedrich, K. Ebel, N. Go¨tz and U. Mu¨ller (BASF AG) PCT Appl. WO 2004/092099, 2004. 15. C. Milone, C. Gangemi, G. Nrei, A. Pistone and S. Galvagno, Appl. Catal., A, 2000, 199, 239. 16. C. Milone, A. Perri, A. Pistone, G. Neri and S. Galvagno, Appl. Catal., 2002, 233, 151. 17. K.A. da Silva, P.A. Robles-Dutenhefner, E.M.B. Sousa, E.F. Kozhevnikova, I.V. Kozhevnikov and E.V. Gusevskaya, Catal. Commun., 2004, 5, 425. 18. J. Tateiwa, A. Kimura and S. Uemura, J. Chem. Soc., Perkin Trans. 1, 1997, 15, 2169. 19. A. Corma and M. Renz, Chem. Commun., 2004, 550. 20. Z. Yongzhong, N. Yuntong, S. Jaenicke and G.-K. Chuah, J. Catal., 2005, 229, 404. 21. A. Corma, L.T. Nemeth, M. Renz and S. Valencia, Nature, 2001, 412, 423. 22. M. Renz, T. Blasco, A. Corma, V. Forne´s, R. Jensen and L. Nemeth, Chem. Eur. J., 2002, 8, 4708. 23. M. Boronat, A. Corma, M. Renz, G. Sastre and P.M. Viruela, Chem. Eur. J., 2005, 11, 6905. 24. M. Boronat, A. Corma and M. Renz, J. Phys. Chem. B, 2006, 110, 21168. 25. M. Boronat, P. Concepcio´n, A. Corma, M. Renz and S. Valencia, J. Catal., 2005, 234, 111. 26. J.P. Perdew and Y. Wang, Phys. Rev. B, 1992, 45, 13244. 27. A.D. Becke, J. Chem. Phys., 1993, 98, 5648. 28. P.J. Hay and W.R. Wadt, J. Chem. Phys., 1985, 82, 270. 29. W.R. Wadt and P.J. Hay, J. Chem. Phys., 1985, 82, 284. 30. P.C. Hariharan and J.A. Pople, Theor. Chim. Acta, 1973, 28, 213. 31. M. Boronat, A. Corma, M. Renz and P.M. Viruela, Chem. Eur. J., 2006, 12, 7067.

CHAPTER 39

Recent Advances in XPS of Non-Conductors G. MICHAEL BANCROFT,a H. W. NESBITT,b V. P. ZAKAZNOVA-HERZOGb,c AND J. S. TSEd a

Department of Chemistry, The University of Western Ontario, Chemistry Building, 1151 Richmond Street, London, Ontario, N6A 5B7 Canada; b Department of Earth Sciences, The University of Western Ontario, London, Ontario, N6A 5B7 Canada; c Institute of Isotope Geochemistry and Mineral Resources, ETH Zurich, 8092 Zurich, Switzerland; d Department of Physics, University of Saskatchewan, Saskatoon, Saskatchewan, S7N 5E2 Canada

1 Introduction It is a great pleasure to be able to present a research paper in honour of John Thomas’s 75th birthday. John had an incredibly important influence on GMB’s early career 35 years ago. It is remarkable that the research reported here is so closely related to the research begun with John in 1971; and it is probably interesting (at least to John) of the circuitous path that GMB and HWN’s research has taken since 1971 to improve the quality of the XPS spectra of silicates 35 years after the initial study. GMB arrived at the chemistry department of the University of Western Ontario (UWO) in 1970, after a very rewarding 6 years at the University of Cambridge using the recently discovered Mo¨ssbauer spectroscopy (or nuclear gamma ray resonance1) to study bonding and structure in inorganic chemistry and geochemistry.2–4 The chemistry department at UWO, with Howard Clark as Head, had a very active visiting professor program. John Thomas came as a visiting professor to the UWO Physical Chemistry group in 1971, mainly because of his strong previous contacts with the senior physical chemistry professor Pat Jacobs. During his 2 week stay at UWO, GMB was fortunate to have several enthusiastic research discussions with John concerning the use of Mo¨ssbauer spectroscopy, and the recently developed XPS (or electron 651

652

Chapter 39 5

spectroscopy for chemical analysis (ESCA) ) and UV photoelectron spectroscopy.6 John had a rather new AEI ES100 photoelectron spectrometer with achromatic Mg Ka and Al Ka sources in Aberystwyth, and was planning to begin solid state Mo¨ssbauer studies; whereas GMB had a working Mo¨ssbauer spectrometer for Fe and Sn studies, and was planning to order a photoelectron spectrometer to begin ultraviolet (UV) and X-ray photoelectron studies. They immediately planned a joint photoelectron project (with his graduate student Ian Adams) using the Aberystwyth spectrometer to perform one of the first photoelectron studies of silicate minerals, using the excellent silicate separates that had been provided by Roger Burns7 for the earlier productive collaborative Mo¨ssbauer studies in Cambridge from 1965 to 1969.2,3 But in addition, John approached Pat Jacobs and Howard Clark with the suggestion that they nominate GMB for the British chemistry awards for chemists under 30 years of age – the Meldola Medal from the Royal Institute of Chemistry, and the Harrison Prize from the British Chemical Society. GMB was fortunate enough to win both prizes in late 1971. These awards had a huge impact on a young scientist’s career! The 1971/1972 collaboration with John and Ian resulted in one short survey paper on silicate minerals8 (and another on studies on a correlation of Mo¨ssbauer and ESCA results on Fe organometallic compounds9). Neither paper was groundbreaking. Indeed, apart from a small difference in Al 2p binding energies for Al in 6 and 4 coordination in the silicate study, ‘‘all other binding energies and linewidths appear to be insensitive to chemical and structural changes’’.8 But these studies showed immediately that XPS of nonconductor compounds and minerals (especially powders) had fundamental problems (mentioned briefly by Siegbahn et al.10), primarily due to the charging (and especially differential charging) of the photoelectron peaks that results in peak shifting, with much broadening and asymmetry.11,12 This led GMB on a career quest to improve XPS resolution on all types of samples, so that bonding and structural information could be readily obtained. Compared to the XPS studies of gases, metals and semiconductors, resolution on non-conductors has shown surprisingly little progress in the last 35 years! (Of course the intensities have increased greatly on modern instruments.) The introduction of monochromatized Al Ka sources13 and electron flood guns12,14 only made a substantial linewidth and lineshape difference on ‘‘perfect surfaces’’ such as smooth polymers15 cleaved alkali halide surfaces,14 and very thin films of non-conductors deposited on metals from solution or vapour deposition.16,17 In these ‘‘ideal cases’’, linewidths under 1 eV could be obtained (e.g., a C ls linewidth of 0.85 eV for the –O–C¼O C ls peak of poly(ethylene teraphthalate)15 (PET)). However, for silicate XPS studies, there has been little improvement. For the early XPS silicate studies with achromatic Mg Ka X-ray sources, the O 1s linewidths were well over 2 eV. Even with this resolution, Yin et al.18 suggested that the even broader O 1s peak from a pyroxene was due to two contributions: one from the structural Si–O–Si oxygens (the so-called bonding oxygens (BO)), and another from the Si–O–M (M ¼ a metal cation) oxygens (the so-called non-bonding oxygens (NBO)) in the pyroxene structure.

653

Recent Advances in XPS of Non-Conductors 19–22

In more recent monochromatic XPS studies, the O 1s and Si 2p linewidths of silicate minerals and glasses remained at about 2 eV, making it difficult to resolve the strongly overlapping O 1s BO and NBO peaks. As a result of this poor resolution, most researchers have used only XPS broadscan intensities to quantify the differential release of cations from the surface of silicates23,24 – very useful for determining the initial mechanisms of mineral dissolution. In contrast, advances in the XPS studies of metals, semiconductors and molecular gas phase compounds has been spectacular, primarily because of the use of monochromatized synchrotron radiation.25,26 Eastman and co-workers, along with several other solid state physicists working at the Aladdin synchrotron, pioneered the use of low-energy, high-resolution photoemission studies to obtain the band structure of metals and semiconductors.27 Using vapour evaporation to deposit thin films of non-conductors on metals, GMB collaborated with Eastman to obtain core linewidths of 0.7 eV with instrumental resolutions of 0.3 eV.28 These studies were the first attempt to resolve and characterize a novel splitting of core level XPS peaks which has been called the electric field gradient splitting (or ligand field splitting), which is proportional to the nuclear quadrupole splitting observed by Mo¨ssbauer spectroscopy.29 Much better resolution was required to resolve this splitting of o0.4 eV, which led GMB to use high resolution He I (21.2 eV) and He II (40.8 eV) photoelectron sources to initially resolve this splitting on low binding Zn 3d and Cd 4d levels in Zn and Cd compounds, respectively,30 with total linewidths of o0.1 eV. The chemical sensitivity of the ligand field splitting to bonding and structure was demonstrated experimentally and theoretically;29,30 and high resolution synchrotron XPS studies extended these studies to higher binding energy BE core levels such as I 4d, Xe 4d and Br 3d in I, Xe and Br compounds respectively31 with total linewidths of o0.15 eV. More importantly for this publication, the core level vibrational splittings (first observed on the C 1s level of CH413) on the Si 2p levels of simple silanes were resolved and characterized.32 These studies showed that a major cause of linebroadening in the Si 2p spectra from solid state non-conductor silicates was due to vibrational splitting in the ion state. Recent gas phase Si 2p and C 1s XPS studies of silanes and hydrocarbons, using the third generation ALS Berkeley synchrotron,33,34 gave total linewidths that are very close to the natural linewidths – e.g. 38 meV for SiH4. Thus, the Si 2p linewidths in gas phase silanes have decreased from over 1 eV to o0.1 eV in the last 35 years, while the Si 2p linewidths in the silicate spectra have remained close to 2 eV! Meanwhile, HWN had begun XPS studies (using laboratory monochromatized Al Ka sources) of semiconductor mineral sulfides and arsenosulfides in the early 1990s.35 Because of the increase in both spectral resolution and surface sensitivity with synchrotron sources, GMB and HWN began a productive collaboration (along with Rudiger Szargan from Leipzig) at the Aladdin synchrotron to determine the surface chemistry of sulfides and arsenosulfides, and the initial mechanisms of oxidation of these minerals.36–38 Total linewidths of B0.4 eV were obtained on these semiconductors using room temperature; and linewidths of o0.3 eV have been obtained on other semiconductor sulfides

654

Chapter 39 39

such as PbS and MoS2 using high resolution synchrotron XPS. On semiconductor Si samples, Si 2p3/2 linewidths of B0.2 eV have been obtained.40 The development of the Kratos Axis Ultra XPS, with a novel magnetic confinement charge neutralizer, offered the prospect of much better resolution for non-conductors.41 For example, the C 1s linewidth on the –O–C¼O peak of PET of 0.69 eV was substantially less than the 0.85 eV quoted earlier15 on an ‘‘ideal’’ polymer sample. HWN headed up a UWO effort (while GMB was the Director of the Canadian Light Source in Saskatoon) to obtain this new Kratos instrument, and with the collaboration of Stewart McIntyre in Surface Science Western (SSW), the Kratos instrument began running at SSW in 2003. The rest of this paper provides a few of the results obtained with this instrument, showing that optimal results for XPS of non-conductors can be obtained (with linewidths equivalent to those collected for analogous semiconductors and metals), making the XPS technique much more useful for surface and structural studies on all silicates.

2 Results and Discussion Nearly all the spectra reported in this chapter were obtained with monochromatized Al Ka radiation from the Kratos Axis Ultra spectrometer on fractured non-conductor mineral surfaces. The optimized overall instrumental width with 10 eV pass energy (photon+electron width) was 0.35 eV. Charge compensation from the novel magnetic charge compensation system was adjusted for every sample to obtain the minimum linewidth. The spectra were fit with between 50% Gaussian–50% Lorentzian to 70% Gaussian–30% Lorentzian lineshapes and Shirley backgrounds, as described in the recent publications from this Kratos instrument in SSW.42–48 Our first study42 focussed on the question: can one obtain the same linewidth on a non-conductor as on an analogous semiconductor? Previous high resolution studies on Si, Ge, Sn, P and As gas phase molecules revealed: first, that the As 3d line had a rather narrow inherent lifetime linewidth (r0.2 eV) with little or no vibrational broadening in the ion state;49 and second, that the total As 3d linewidth for the semiconductor FeAsS was o0.4 eV using high resolution synchrotron radiation.38 We chose to study the As 3d level on fractured surfaces in two analogous minerals: the non-conductor orpiment, As2S3, along with the well-studied semiconductor arsenopyrite, FeAsS.38 The charge compensation system is required only for the non-conductor; and this mineral gave a severe test of the charge compensation system because fracturing yielded an irregular surface. The resulting room temperature As 3d spectra in Figures 1a and b are very satisfying: the As 3d linewidths for both minerals are identical at 0.52 eV, and the lineshapes are identical and well-behaved. In an independent study of the As 3d line in As metal on the same instrument, Mark Biesinger at SSW (personal communication) also obtained an As 3d linewidth of 0.51 eV. These results show that the charge compensation system works extremely well, and gives no observable differential charge broadening. The effect of such

Recent Advances in XPS of Non-Conductors

Figure 1

655

As 3d XPS spectra of (a) FeAsS, (b) As2S3 taken with the Kratos XPS, and (c) the As 3d spectrum taken with an SSX-100-XPS.41

broadening is shown in Figure 1c, which shows an As 3d spectrum of the nonconductor with a modern SSX-100 XPS instrument using a monochromatic Al Ka source and a traditional directional flood gun. This spectrum is much broader (linewidth of 0.8 eV) and gives unexplainable charge broadening in the peak tails. The small peaks in the spectra in Figure 1 are due to surface species, identified more clearly on FeAsS in previous surface sensitive synchrotron XPS studies.38 The major contributions to this width are the instrumental width of 0.35 eV, a ground state phonon broadening of r0.1 eV,39 and the As 3d inherent width of r0.2 eV.49 Using a high resolution synchrotron source with an instrumental width of o0.1 eV at low temperature to remove the phonon broadening, it is expected that the total As 3d linewidth for the non-conductor will be r0.3 eV. But what about the much broader O 1s and Si 2p linewidths for silicates? Figure 2 shows the optimized Si 2p and O 1s spectra for fractured SiO2.42 These spectra show very well-behaved peak shapes and much narrower linewidths than seen previously, but both the total Si 2p and O 1s linewidths of 1.3 and 1.2 eV, respectively, are much broader than the As 3d widths in Figure 1. This Si 2p width is consistent with the broad Si 2p width (1.4 eV) on a thin uniform uncharged SiO2 film on Si metal taken at similar resolution on a synchrotron source.50 The previous high resolution gas phase Si 2p spectra of SiF432a,33 and the close analogue Si(OCH3)432b strongly pointed to unresolved ion state vibrational splitting as the cause of the broadening in the Si 2p spectrum. In Figure 3, we compare the Si 2p spectra of the two tetrahedral SiO4 analogues, Si(OCH3)4 and an orthosilicate olivine. The Si–O bond lengths and Si–O

656

Chapter 39 Si 2p, Qz, vac. frac. FWHM3/2,1/2=1.05eV FWHMtotal=1.30eV

O 1s, Qz, vac. frac. FWHM=1.20eV

(a)

(b)

Intensity

3/2

1/2

Diff. %

106 40 20 0 -20 -40

104

102

100

98

536

534

532

530

534

532

530

40 20 0 106

104

102

100

98

-20 536

Binding energy (eV)

Figure 2

Core level XPS spectra of quartz, SiO2: (a) Si 2p and (b) O 1s.42

symmetric stretch are very similar in the two compounds, as shown at the top of the figure. Figure 3 illustrates the effect of just broadening each Si 2p peak in the vibrational envelope from 0.17 eV in the gas phase Si(OCH3)4 spectrum (Figure 3a) to 0.4 eV for olivine (Figure 3b). The overall Si 2p width with these vibrational fits increases from 1.1 to 1.26 eV – in rather good agreement with the Si 2p width in the mineral olivine and in all other silicates that have been studied.42–44,47,48 The fit to the Si 2p olivine spectrum is rather good: the width is underestimated in the fit by only about 0.05 eV. This is again a strong indication that the Si–O vibrational envelope from the Si–O symmetric stretch causes the broad Si 2p linewidths. The O 1s linewidth is probably also due to unresolved vibrational broadening. The O 1s linewidth (1.20 eV in Figure 3) is always slightly broader than the

657

Recent Advances in XPS of Non-Conductors Si(OCH3)4

Mg2SiO4

Tetramethoxysilane (gas) Si-O Bond length: 0.161Å vs 0.163Å Vibrational Splitting 842 vs 863cm-1

O

Mg

C Vibrational Contributions FWHM = 0.170 eV Splittings = 0.125 eV

110

109

108

Binding Energy(eV)

Figure 3

a

Vibrational Contributions FWHM = 0.40 eV Splittings =0.125 eV

1.10 eV

Counts 111

Mg O Mg O Si O Mg O

C O C O Si O C

Olivine (solid)

107

b

1.26 eV

106

104

103 102 101 Binding Energy (eV)

100

Comparison of: (a) the Si 2p XPS spectra of: the gas phase Si(OCH3)4 analogue taken with an instrumental resolution of B0.15 eV32b and (b) the solid state spectrum of a fractured non-conductor olivine, Mg2SiO4,42 taken with the Kratos XPS with an instrumental resolution of B0.35 eV. The dots give the experimental spectra, and the overall solid line from the fit with 10 vibrational peaks and the Si 2p spin-orbit splitting fits the experimental spectrum rather well in both cases.

component Si 2p3/2 line (1.05 eV), indicating that the number of component vibrational peaks is probably larger for O 1s than for the Si 2p spectra. However, Figure 3 indicates that the O 1s spectra should be more sensitive than the Si 2p spectra to the individual peak linewidths (see below, final paragraph of conclusions). Thus, in Figure 3a, the overall Si 2p width is 1.1 eV, whereas the Si 2p3/2 width is 0.8 eV. Other papers using the Kratos from SSW,44,45 and a recent O 1s study of TiO2,51 give somewhat similar O 1s linewidths. The enhanced resolution is critical to resolve and characterize the O 1s spectra of silicates such as orthopyroxenes (Figure 4) that have both structural BO and NBO.44 In diopside, [Ca(Mg0.8Fe0.2)Si2O6], two resolved peaks are seen

658

Chapter 39 O 1s, Diopside FWHM1,2,3=1.11eV

O 1s, Diopside FWHM1=1.11eV FWHM2=1.41eV FWHM3=1.14eV

Intensity

(a)

(b)

O 1s, Bronzite FWHM1=1.16eV FWHM2=1.51eV FWHM3=1.40eV

1 1 2

3

3 3

Diff. %

(c)

40 20 0 -20 -40 538

536

534

2

2 1

532

530

528

40 20 0 -20 -40 538

536

534

532

530

528

40 20 0 -20 -40 538

536

534

532

530

528

Binding energy (eV)

Figure 4

The O 1s XPS spectra fitted with three equal area contributions of: (a) diopside with the additional constraint that the three widths are equal, (b) diopside with no additional constraint, and (c) bronzite with no additional constraint.

clearly in a silicate mineral for the first time, and these two peaks can be immediately assigned based on previous results: the high energy O peak to the O atoms in the bridging Si–O moiety (Si–O–Si) (BO), and the low energy O peak to the O atoms in the M–O–Si moieties (M ¼ Mg, Ca, Fe) (NBO). The BE for these two peaks are very similar to the BO in quartz, and the NBO in olivine, respectively. The ratio of areas of these two peaks NBO:BO should be 2:1 (there are 4 NBO and 2 BO in the Si2O6 moiety), but a two peak fit with identical linewidths44 gives a ratio of 1.6:1. Examination of the diopside crystal structure52 shows that there are three crystallographically distinct O atoms in the Si2O6 unit with proportions of 2:2:2 for SiOSi, (Mg,Fe)OSi and CaOSi. Two three peak fits are given in Figures 4a and b. In Figure 4a, the linewidths for all peaks were constrained to be equal, and the areas and peak positions were allowed to vary. This fit is satisfactory, and the spectrum can be readily assigned based on the order of electronegativities SicMg>Ca: peak 3 to the O in the SiOSi moiety, peak 2 to (Mg,Fe)OSi and peak 1 to CaOSi. A better fit (Figure 4b) can be obtained when the three areas are constrained to be equal, but the linewidths are allowed to vary. The broader width of peak 2 is expected because the Fe resides with the Mg in the so-called M1 site, and the FeOSi moiety would give a larger O 1s BE than MgOSi because the electronegativity of Fe21 is larger than that of Mg21. A similar three peak fit is given for the O 1s spectrum of a bronzite [(Fe0.2Mg0.8)2Si2O6]44 in Figure 4c. The separation between peaks 1 and 3 is only 1.4 eV, and so the BO (peak 3) and NBO peaks (peaks 1 and 2) are not resolved as for diopside. The Fe21 and Mg21 are known to order on the M2 and M1 sites respectively;52,53 and

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Recent Advances in XPS of Non-Conductors

O 2s

Quartz

5 4 3

1

2

Diopside Intensity

Ca 3p 2 30

26

1 22

18

4

3

5 2? 1

Bronzite 3

4 5

2?

1

5 4

Mg-rich olivine

3

2 1 O 2s 35

Figure 5

30

25

20 15 Binding energy (eV)

10

5

0

The inner valence ‘‘O 2s’’ and outer valence band XPS spectra (VBXPS) of quartz, diopside, bronzite and Mg-rich olivine (from top to bottom).

660

Chapter 39

CaMgSi2O6

4

3

5

Intensity

6 2 Diopside

1

3 5 Mg2Si2O6

4

6 2?

1

Bronzite 20

Figure 6

18

16

14

12 10 8 Binding energy (eV)

6

4

2

0

The VBXPS spectra of diopside (top) and bronzite (bottom), background subtracted. The calculated XPS spectra (solid lines) for the diopside (top) and enstatite end members (no iron) are given above the experimental spectra. The intensity of each state was corrected for cross-section and each band broadened by 0.37 eV to match the instrumental resolution. The calculated spectra were aligned to the experimental energies of peak 6.

together with the electronegativities, we can readily assign peaks 2 and 1 to (Mg,Fe)OSi (Mg,Fe are in M2) and MgOSi (Mg is in M1) respectively. Finally, it is important to note: first, that excellent valence band spectra of these minerals can be obtained; second, that these spectra are very sensitive to the nature of the tetrahedral linkages in the silicate; and third, that theoretical calculations reproduce the experimental spectra very well. It was a pleasure to have a former PhD student (JST) collaborate once again on the theoretical calculations. These spectra are much weaker than the core level spectra, and great stability of the surface charge neutralization is required to obtain the spectra of these minerals shown in Figure 5. The inner valence band consists of relatively intense O 2s peaks that chemically shift with the O 1s peaks. The outer valence band peaks 1 and 2 in the lower three spectra can be assigned to the Fe 3d orbitals split by ligand field splitting. The other peaks 3, 4 and 5 can be assigned qualitatively based on MO considerations for a tetrahedral SiO4 moiety.43

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It is worth noting the chemical sensitivity of these spectra. For example, peak 5 is shifted by about 4 eV from olivine to quartz. High quality calculations of the band structure were performed using pseudopotential density functional theory54,55 and compared with the spectra of these minerals. Using the Gelius approximation,56 and atomic cross-sections,57 the calculated valence band spectra have been obtained for all of these minerals. The agreement with the experimental spectra is excellent as shown for the two pyroxenes of idealized composition (without any Fe21) in Figure 6.44 Six peaks are more apparent in these expanded spectra, and subtle differences between the spectra are reproduced by the calculations. For example, peaks 3 and 4 are shifted to higher energy in bronzite than in diopside; and there is a larger splitting between peaks 3 and 4 in bronzite than in diopside.

3 Conclusions Excellent XPS spectra of non-conductor silicates have been obtained for several different types of fractured silicate minerals using the Kratos Axis Ultra spectrometer. The linewidths and lineshapes are well-characterized, and the linewidths are substantially less than obtainable with other charge compensation systems. The valence band spectra are very sensitive to the structure and chemistry of the silicate mineral, and high quality theoretical calculations reproduce these valence band spectra very well. The O 1s spectra are immediately useful for characterizing the different types of O atoms in silicates, and both the O 1s and valence band spectra should be very useful for characterizing the silicate species in more complex silicate materials such as glasses and zeolites. Already, the O 1s spectra have been extremely useful for determining the silicate species in Pb silicate glasses.47 In addition, it is now possible to use these spectra for detailed surface dissolution studies. For example, our study of the dissolution of olivine shows the first direct evidence for a surface monolayer of Si–O–H species from both the O 1s and valence band spectra combined with theoretical valence band calculations.48 In the future, it will be important to have the Kratos XPS on a high resolution synchrotron beamline to increase the O 1s resolution, and especially to increase the surface sensitivity for surface dissolution studies as has been done on sulfides.37,38 The overall Si 2p linewidths will probably not decrease by more than 0.2 eV because of the large vibrational envelopes and the Si 2p spinorbit splitting of 0.61 eV. For example, for Si 2p, even assuming that an individual peak linewidth is close to the Si semiconductor width of 0.2 eV40 at low temperature, the minimum overall Si 2p linewidth will not be substantially less than the 1.1 eV (shown in Figure 3). But for O 1s, it is expected that the overall O 1s linewidth should decrease to close to the Si 2p3/2 linewidth of 0.8 eV shown in Figure 3a. This better resolution would obviously make another big improvement to the O 1s spectra, and a concomitant increase in the amount of structural and chemical information.

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Chapter 39

Acknowledgements This research was funded by the Natural Science and Engineering Research Council (NSERC) of Canada. We would like to thank the SSW staff at the UWO (especially R. Davidson, M. Biesinger and N.S. McIntyre) for all their help and advice with the Kratos XPS.

References 1. R.L. Mo¨ssbauer, Kernresonanzfloureszrnz von Gammastrahlung in Ir191, Z. Phys., 1958, 151, 1525; H. Frauenfelder, The Mo¨ssbauer Effect, W.A. Benjamin, New York, 1962. 2. G.M. Bancroft, Mo¨ssbauer Spectroscopy. An Introduction for Inorganic Chemists and Geochemists, McGraw-Hill, New York, 1973, 252pp. 3. G.M. Bancroft, R.G. Burns and A.G. Maddock, Geochim. Cosmochim. Acta, 1967, 31, 2219. 4. G.M. Bancroft and R.H. Platt, in Advances in Inorganic Chemistry and Radiochemistry, ed. H.J. Emele´us and A.G. Sharpe, Academic Press, New York, 1972, 15, 59. 5. K. Siegbahn, C. Nordling, A. Fahlman, R. Nordberg, K. Hamrin, J. Hedman, G. Johansson, T. Bergmark, S.-E. Karlsson, I. Lindgren and B. Lindberg, ESCA – Atomic Molecular and Solid State Structure Studied by Means of Electron Spectroscopy, Nova Acta Regiae Soc. Sci. Ser. IV, 1967, 20, 283. 6. D.W. Turner, C. Baker, A.D. Baker and C.R. Brundle, Molecular Photoelectron Spectroscopy, Wiley, New York, 1971, 386pp. 7. R.G. Burns, Mineralogical Applications of Crystal Field Theory, 2nd edn, Cambridge University Press, Cambridge, 1993, 176–186, and references therein. 8. I. Adams, J.M. Thomas and G.M. Bancroft, Earth Planet. Sci. Lett., 1972, 16, 429. 9. I. Adams, G.M. Bancroft, K.D. Butler and J.M. Thomas, J. Chem. Soc., Chem. Commun., 1972, 751. 10. K. Siegbahn, C. Nordling, G. Johansson, J. Hedman, P.F. Heden, K. Hamrin, U. Gelius, T. Bergmark, L.O. Werme, R. Manne and Y. Baer, ESCA Applied to Free Molecules, North Holland, Amsterdam, 1971, 200pp. 11. T.L. Barr, Modern ESCA The Principles and Practice of X-Ray Photoelectron Spectroscopy, CRC Press, Boca Raton, FL, 1993. 12. J. Cazaux, J. Electron Spectrosc. Relat. Phenom., 1999, 105, 155. 13. K. Siegbahn, J. Electron Spectrosc. Relat. Phenom., 1974, 5, 3; U. Gelius, J. Electron Spectrosc. Relat. Phenom., 1974, 5, 985. 14. F.J. Himpsel and W. Steinmann, Phys. Rev. Lett., 1975, 35, 1025; F.J. Himpsel and W. Steinmann, Phys. Rev. B, 1978, 17, 2537. 15. G. Beamson and D. Briggs, High Resolution XPS of Organic Polymers – The Scienta ESCA Database, Wiley, New York, 1992.

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16. I. Adams and G.M. Bancroft, Nature, 1974, 250, 219. 17. G.M. Bancroft, I. Adams, H. Lampe and T.K. Sham, J. Electron Spectrosc. Relat. Phenom., 1976, 9, 191. 18. L.I. Yin, S. Ghose and I. Adler, Science, 1971, 173, 633. 19. K. Okada, Y. Kameshima and A. Yasumori, J. Am. Ceram. Soc., 1998, 81, 970 and references therein. 20. H. Seyama, M. Soma and A. Tanaka, Chem. Geol., 1996, 129, 209 and references therein. 21. H. Seyama, D. Wang and M. Soma, Surf. Interface Anal., 2004, 36, 609. 22. B.M.J. Smets and T.P.A. Lommen, J. Non-Cryst. Solids, 1982, 48, 423. 23. J. Schott and R.A. Berner, Geochim. Cosmochim. Acta, 1983, 47, 2233. 24. J. Schott and R.A. Berner, in: The Chemistry of Weathering, ed. J.I. Drever, D. Reidel Publishing Co., Amsterdam, 1985, pp. 35. 25. G. Margaritondo, Introduction to Synchrotron Radiation, Oxford University Press, Oxford, 1988. 26. G.M. Bancroft and Y.F. Hu, in: Inorganic Electronic Structure and Spectroscopy, ed. E.I. Solomon and A.B.P. Lever, Wiley, New York, 1999, 443. 27. D.E. Eastman, W.D. Grobman, J.L. Freeouf and M. Erbudak, Phys. Rev. B, 1974, 9, 3473 and references therein. 28. G.M. Bancroft, T.K. Sham, D.E. Eastman and W. Gudat, J. Am. Chem. Soc., 1977, 99, 1752. 29. R.P. Gupta, J.S. Tse and G.M. Bancroft, Phil. Trans. Roy. Soc., 1980, 293, 535. 30. G.M. Bancroft, D.K. Creber and H. Basch, J. Chem. Phys., 1977, 67, 4891; D.K. Creber and G.M. Bancroft, Inorg. Chem., 1980, 19, 643. 31. J.N. Cutler, G.M. Bancroft, D.G. Sutherland and K.H. Tan, Phys. Rev. Lett., 1991, 67, 1531; J.N. Cutler, G.M. Bancroft, J.D. Bozek, K.H. Tan, J.S. Tse and G.J. Schrobilgen, J. Am. Chem. Soc., 1991, 113, 9125. 32. (a) J.D. Bozek, G.M. Bancroft, J.N. Cutler and K.H. Tan, Phys. Rev. Lett., 1990, 65, 2757; (b) D.G.J. Sutherland, G.M. Bancroft and K.H. Tan, J. Chem. Phys., 1992, 97, 7918 and references therein. 33. T.D. Thomas, C. Miron, K. Weisener, P. Morin, T.X. Carroll and L.J. Saethre, Phys. Rev. Lett., 2002, 89, 223001. 34. T.D. Thomas, L.J. Saethre, K. Borve and J.D. Bozek, J. Phys. Chem., 2004, 108, 4983. 35. A.R. Pratt and H.W. Nesbitt, Geochim. Cosmochim. Acta, 1994, 58, 827; H.W. Nesbitt and I.J. Muir, Geochim. Cosmochim. Acta, 1994, 58, 4667. 36. H.W. Nesbitt, G.M. Bancroft, A.R. Pratt and M.J. Scaini, Am. Miner., 1998, 83, 1067. 37. A.G. Schaufuss, H.W. Nesbitt, I. Kartio, K. Laajalehto, G.M. Bancroft and R. Szargan, Surf. Sci. Lett., 1998, 411, 321. 38. A.G. Schaufuss, H.W. Nesbitt, I. Kartio, K. Laajalehto, G.M. Bancroft and R. Szargan, Am. Miner., 2000, 85, 1754. 39. J.A. Leiro, K. Laajalehto, I. Kartio and M. Heimonen, Surf. Sci., 1998, 412/413, L918; S. Mattila, J.A. Leiro, M. Heinomen and T. Laiho, Surf. Sci., 2006, 600, 5168.

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40. C.J. Karlsson, F. Owman, E. Landmark, Y.-C. Chao, P. Partensson and R.I.G. Uhrberg, Phys. Rev. Lett., 1994, 72, 4145; S.M. Scholz and K. Jacobi, Phys. Rev. B, 1995, 52, 5795. 41. Kratos web page: http://www.kratos.com. 42. H.W. Nesbitt, G.M. Bancroft, R. Davidson, N.S. McIntyre and A.R. Pratt, Am. Miner., 2004, 89, 878. 43. V.P. Zakaznova-Herzog, H.W. Nesbitt, G.M. Bancroft, J.S. Tse, X. Gao and W. Skinner, Phys. Rev. B, 2005, 2, 205113. 44. V.P. Zakaznova-Herzog, H.W. Nesbitt, G.M. Bancroft and J.S. Tse, Surf. Sci., 2006, 600, 3175. 45. M.C. Biesinger, C. Brown, J.R. Mycroft, R.D. Davidson and N.S. McIntyre, Surf. Interface Anal., 2004, 36, 1550. 46. A.P. Grosvenor, M.C. Biesinger, R.S. Smart and N.S. McIntyre, Surf. Sci., 2006, 600, 1771. 47. K.N. Dalby, H.W. Nesbitt, V.P. Zakaznova-Herzog and P.A. King, Geochim. Cosmochim. Acta (accepted). 48. A.P. Zakaznova-Herzog, H.W. Nesbitt, G.M. Bancroft and J.S. Tse, Geochim. Cosmochim. Acta (submitted). 49. Z.F. Liu, G.M. Bancroft, J.N. Cutler, D.G. Sutherland and K.H. Tan, Phys. Rev. A, 1992, 46, 1688. 50. F.J. Himpsel, F.R. McFeely, A. Taleb-Ibrahimi, J.A. Yarmoff and G. Hollinger, Phys. Rev. B, 1988, 38, 6084. 51. H. Perron, J. Vandenborre, C. Domain, R. Drot, J. Roques, E. Simoni, J.-J. Ehrhardt and H. Catalette, Surf. Sci., 2007, 601, 518. 52. J.R. Clark, D.E. Appleman, J.J. Papike, Miner. Soc. Am. Spec. Pap., 1969, 2, 31; S. Ghosh, Z. Kristallogr., 1965, 122, 81 and references therein. 53. G.M. Bancroft, R.G. Burns and R.A. Howie, Nature, 1967, 213, 1221. 54. J.P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett., 1966, 77, 3865. 55. J.M. Soler, E. Artacho, J.D. Gale, A. Garcı´ a, J. Junquera, P. Ordejo´n and D. Sa´nchez-Portal, J. Phys.: Condens. Matter, 2002, 14, 2745. 56. U. Gelius, Electron Spectroscopy, ed. D.A. Shirley, North Holland, Amsterdam, 1972, 311. 57. J.J. Yeh and I. Lindau, Atomic Data and Nuclear Data Tables, 1985, 32, 1.

Section D: Electron Microscopy and its Contribution to Chemistry and Material Science

CHAPTER 40

Electron Microscopy Studies of Structural Modulation in Micro- and Meso-Porous Crystals OSAMU TERASAKI,a TETSU OHSUNA,a,b ZHENG LIU,a,c YASUHIRO SAKAMOTO,a KEIICHI MIYASAKA,a NOBUHISA FUJITA,a,d NOZOMU TOGASHIe AND SHUNAI CHEf a

Department of Physical, Inorganic and Structural Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden; b CREST, Japan Science and Technology Agency, Kawaguchi, Japan; c National Institute of Advanced Industrial Science & Technology, Tsukuba, Japan; d Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, Japan; e Namiki Precision Jewel Co. LTD., Tokyo, Japan; f School of Chemistry and Chemical Technology, Shanghai Jiao Tong University, Shanghai, PR China

1 Introduction 1982 Marked a turning point for one of us, Osamu Terasaki (OT), to work with Sir John as a Guest Research Fellow of the Royal Society. OT was fascinated with zeolites as containers for making novel arrayed clusters, which would have new properties quite different from bulk crystals, within their cages or channels by self-assembly,1,2 and was attracted by the works of Sir John, especially on high resolution transmission electron microscopy (HRTEM) imaging of LTAw zeolite.3 Up until then, OT had concentrated on transmission electron microscopy (TEM) study of modulated structures in Au-based closed-packed alloys such as Au–Mn and Au–Zn alloys. The application of HRTEM to the study of open-structures itself constitutes a major turning point in solid-state and w

Structure Commission of the International Zeolite Association gives a structural code for each framework type of zeolite by three capital letters, e.g. LTA, LTL, MOR etc.

667

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Chapter 40

materials science. As will become evident below, no other technique can cope with the structural problems posed by mesoporous silicas in which the silica walls themselves are amorphous but the arrangement of the cages, channels and pores is crystallographic, as a ‘‘cavity crystal’’. In 2001, Sir John and OT, together with his other colleagues, wrote a brief review of the power of HRTEM and STEM in elucidating the nature of microporous and mesoporous solids.4 ‘‘Space filling and partition’’ is a very important concept in understanding and explaining the self-assembly mechanism. We are impressed that living creatures are intelligent enough to live peacefully by partitioning spaces like Voronoi cells as shown in Figure 1 (a colour picture of coral from Gotland, Sweden). Soap bubbles fill three dimensional (3d) space freely and will arrange themselves to minimise the total area of the interfaces between the bubbles. When three bubbles meet, a line is formed as the common edge of the bubbles by deforming their spherical shapes. A maximum of four soap bubbles can meet at a point, where four lines are connected with the tetrahedral angle, cos
1(
1/3). This is similar to a zeolite framework structures, where Si atoms take the tetrahedral positions and the lines correspond to the Si–O bondings. For a long time Kelvin’s partition, which is known as the zeolite SOD framework type, was believed to be a solution for the arrangement for which

Figure 1

Coral from Gotland, Sweden, taken from a collection of Prof. Mori, Tohoku University, Sendai, Japan.

Electron Microscopy Studies of Structural Modulation

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the total area of the interfaces between the cells of a uniform volume is a minimum. More than 10 years ago, it was shown that another zeolite framework-type called MEP (melanophlogite, SG: Pm3n), which has two kinds of cells, dodecahedron (12-hedron) and tetrakaidecahedron (14-hedron) of equal volume, surpassed Kelvin’s partition by 0.3% in area.5 This MEP is a gas clathrate type I (type II has Fd  3m structure) and is also classified as one of the tetrahedrally close packed (tcp) structures, whose bondings are exclusively tetrahedral like zeolites. In this review, we outline the advances with specific examples, that begin by recalling relevant facts about the Au–Mn alloy system, which then lead on to structural modulations in mesoporous crystals.

2 Modulations in Structure and Composition of Micro- and Meso-Porous Crystals: Lessons from Long-Period Ordered Structures of the Au-Rich Au–Mn Alloys It is instructive for understanding structural modulations to briefly address an example of the structure A1. Before we go through case studies of porous materials, A1 face centred cubic (fcc) and is known as one of the closest packings of a uniform sphere. In Au–Mn systems at high temperature, the Au-rich alloys form disordered A1 structures across a wide composition range up to about 28 at. % Mn. At low temperature, Mn atoms arrange in ordered fashion on an fcc lattice. Various ordered structures with increasing Mn content such as Au4Mn, Au22Mn6, Au31Mn9, Au3Mn and Au5Mn2 were studied by HRTEM experiments. At 20 at. %, Mn atoms occupy every fifth (420)c and ( 240)c planes, where subscript c refers to the fundamental fcc structure. Therefore the Au4Mn (D1a type) structure is described by the stacking sequence of the (420)c plane, Au-plane and Mn-plane, as . . . Mn Au Au Au Au Mn. . . . Excess Mn atoms from 20 at. % can be accommodated by creating a planar defect as shown by large white arrows in Figure 2a, and at 21.4 at. % Mn, excess Mn atoms from 20 at. % can be accommodated by introducing the planar defects every three Mn atom planes parallel to (420)c. The stacking sequence then becomes . . . Mn Au Au Au Au Mn Au Au Au Mn Au Au Au Au Mn . . . and the Au22Mn6 structure results. The Au22Mn6 structure is regarded as a 1d-modulated structure of Au4Mn. A further increase in Mn content causes another planar defect system parallel to (240)c planes in Au22Mn6, and Au31Mn9 and results in the composition of 22.5 at. % Mn. The Au31Mn9 structure is regarded as a 1d-modulated structure of Au22Mn6, and therefore as a 2d-modulated structure of Au4Mn. Changes in the structures with an increase of Mn content from A1 to Au4Mn (D1a type), from Au4Mn to Au22Mn6 and from Au22Mn6 to Au31Mn9 are simply described in terms of a structural/density modulation.6–8 These structural changes are schematically

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Chapter 40

Figure 2

Structures of Au4Mn (a), Au22Mn6 (b) and Au31Mn9 (c) projected along the [0 0 1] direction and thick squares show their unit cells. Large and small circles lie in the planes z ¼ 0 and z ¼ 1/2 of the A1(fcc) structure, and open and solid circles represent Au and Mn atoms, respectively.

summarised in Figure 2, which was confirmed by both HRTEM images and their corresponding electron diffraction (ED) patterns. Their structural stabilities were discussed based on a pairwise interaction model developed for the Ising spin system on an fcc lattice.9 Whereas, in the foregoing, all compositional changes are described by the stacking sequences of Au- and Mn-planes on a fcc lattice, we now turn to a different situation.

2.1

Case 1. Structural Modulation of Mesoporous Crystals

Since the discovery of silica mesoporous crystals,10,11 different structures have been synthesised using both ionic and non-ionic surfactants. An assembly of oligomeric silica under the influence of surfactant molecules forms mesoporous structures. Two basic formation mechanisms have been proposed: (i) the assembly starts from a pre-formed liquid crystal mesophase12 and (ii) the surfactant and silica assemble cooperatively to produce liquid crystalline arrangements.13 In most cases, a cooperative mechanism seems to be in operation. It is important to understand the inter-relationship between the intermicellar interaction, the silica-condensation rate and rigidity of the micelles, water activity,14 surfactant mobility and the final structure of the mesoporous crystals. Other important concepts that should be considered are: (i) charge matching between surfactant and silica and (ii) surfactant packing parameters. The closest packing of uniform hard spheres is fcc (SG: Fm3m) or hexagonal close packing (hcp, SG: P63/mmc), while the foam with MEP type (SG: Pm3n) is known to be the area-minimised structure. The ‘‘minimal surface-area principle’’ is incompatible with closest packing and favours the MEP type over

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671

fcc/hcp. These structures may be considered to be stabilised by a soft-intermicellar potential and hard-core potential, respectively.15 The competition of two different types of potentials may also produce many different structures with space-groups Fm 3m, P63/mmc, Fd  3m, Pm 3n, P42/mnm and so on, covering the range from dispersed spherical micelle structures to the 2d-hexagonal structures. It is interesting that the fcc structure can be stabilised with a hard-core intermicellar potential even without an attractive potential part10 or with a hard-core intermicellar potential plus soft repulsive tail.16 However, we have also found mesoporous crystals with a ‘‘five-fold symmetry’’ morphology resulting from a multiply twinned particle (MTP) as will be discussed below.17 All the external surfaces of the particles are covered by only {1 1 1} planes, which suggests that the surface-boundary energy for {1 1 1} is substantially smaller than that for other planes. This is explained by speculating that the intermicellar pair potential has an attractive potential term, which has a minimum at the intermicellar distance (similar to a Lenard–Jones potential) in addition to a strong repulsive potential (hard-core). It is then natural to conjecture that the uniform spherical micelles, surfactant and silica are coassembled in this way. A close packing of spheres with radius R forms extra octahedral and tetrahedral pores (corresponding inscribed spheres have radii of 0.414 R and 0.225 R, respectively), which are widely used for producing negative opal structures. It is important for a future discussion of the two formation mechanisms of mesoporous crystals to know what kind of TEM image contrast can be observed due to the existence of the extra pores in the case that silica does not fill these pores formed by the close packing of hard spherical shells. Using software written by one of the authors (TO), TEM images are simulated for three different cases of fcc packing: (A) spherical shells of silica, (B) spherical empty cavities embedded in silica with inter-cavity distance 2 nm (silica wall), and (C) the same as case (B) except that neighbouring cavities contact each other directly. The case (C) closely corresponds to mesoporous silica with Fm 3m that we observed using TEM. It is clear from the simulated images along [1 1 0] that we can clearly distinguish between the three cases, especially between the spherical shell of silica (A) and spherical empty cavities (B and C) as shown in Figure 3.

2.1.1

Not a Quasicrystal but a Multiply Twinned Crystal

Using a gemini cationic surfactant, crystals with fcc structure (Fm3m) were synthesised. We have observed unusual morphologies inconsistent with the point-group symmetry m 3m expected for the Fm3m space-group symmetry. Figures 4a–d show SEM images and corresponding schematic drawings of the crystals with pentagonal dipyramid and icosahedral shapes, respectively. Both regular dodecahedron and rhombic triacontahedron have been observed in bulk quasicrystals.18 However, from HRTEM analyses it was concluded in this case that these crystals are not a quasicrystal but multiply twinned particles with Fm 3m structure.17 The closest packing may be explained by a naı¨ ve

672

Chapter 40 Case A

Case B shell thickness = 1nm

Case C

cavity distance = 2nm

cavity distance = 0nm

a

b

e

f

i

j

c

d

g

h

k

l

FCC crystal of silica shell, radius; (a)5nm, (b)10nm, (c)20nm, (d)40nm, thickness; 1nm boundary roughness; 0.2nm

FCC crystal of spherical cavity radius; (e)4nm, (f)9nm, (g)19nm, (h)39nm boundary roughness; 0.2nm

FCC crystal of spherical cavity radius;(i)5nm, (j)10nm, (k)20nm, (l)40nm boundary roughness; 0.2nm

TEM simulation, defocus; 200nm, thickness; 20nm, acceleration Voltage; 300kV

Figure 3

TEM simulated images of three different fcc packings of spherical shells (Case A), and spherical cavities embedded in amorphous silica (Case B) and (Case C). The images are simulated for the [1 1 0] incidence at the following conditions; defocus value: 200 nm, crystal thickness: 20 nm, acceleration voltage: 300 kV.

Figure 4

SEM images of silica mesoporous crystals with Fm 3m and their schematic representations. Pentagonal dipyramidal (a, b) and icosahedral (c, d) morphologies.

Electron Microscopy Studies of Structural Modulation

673

speculation that surfactant and silica assembled to form spherical micelles cooperatively. It is worth noting that the structure of the product is so sensitive to synthesis conditions that a crystal with Pm 3n symmetry, where a ‘‘minimal surface-area principle’’ is operating, was synthesised by changing the amount of H2SO4 or synthesis temperature within the same synthesis system.19

2.1.2

Modulation in Anionic-Surfactant-Templated Mesoporous Silica (AMS)

Recently, using anionic surfactants and co-structure directing agents (CSDA), one of us (SC) has developed a novel synthesis route for mesoporous silica crystals termed AMS-n.20–24 All the structures synthesised with ‘‘conventional’’ cationic or polymeric surfactants have been reproduced as highly crystalline materials. This was achieved through small variations in the synthesis conditions and in all cases using an anionic surfactant with a CSDA. Interestingly, silica mesoporous crystals with fcc and with MEP type structures were successfully synthesised in the same synthesis system as the AMS series. By careful choice in the synthesis systems of anionic surfactants, such as lauric acid and amino acid derived amphiphiles, together with a CSDA, it is possible to scan ‘‘the phase (synthesis space) diagram’’ for new mesostructures which offers new insights into structural relationships between template and inorganic counterpart.23 Within this system, it has also been possible to synthesise for the first time a new bicontinuous silica mesoporous crystal with D-minimal surface, SG Pn 3m, termed AMS-10.24 Figure 5a shows an HRTEM image of an AMS-2 crystal with SG Pm3n recorded with [0 0 1] incidence. The structure of AMS-2 is the same as the MEP type consisting of two different polyhedra, a 12-hedron with 12 pentagons and a 14-hedron composed of two opposite hexagons and 12 pentagons. A planar fault is observed in the image as marked by a black arrow. For clarity, the unit cells of the MEP type are shown by white squares and those across the fault are shown by three white triangles. The arrangement of the polyhedra and the main pore connectivity through hexagonal windows across the fault plane are shown in Figure 5d. At the fault, a new 15-hedron with 3m (C3v) symmetry, with the three-fold axis parallel to [0 0 1], composed of 3 hexagons and 12 pentagons is introduced. Periodical planar faults are also observed as a structural modulation indicated by black arrows and result in a new 1d–modulated structure based on Pm 3n (Figure 5b). This is similar to the case of Au22Mn6 from Au4Mn (Figure 2) except that the lattice is not fixed in the mesoporous system and that there is a small contraction to form a regular triangular-column as shown in Figure 5d. Extra spots due to the periodic modulation along bc* perpendicular to the (0 1 0)c plane appear to divide the distance to the 002 reflection into 4. At the left side of Figure 5b, another modulation indicated by white arrows can also be observed. At the intersection of the two modulations, we can see the same Pm 3n structure (marked by white square and rhombi, top left in

674

Figure 5

Chapter 40

HRTEM images and corresponding ED patterns from AMS-system with SG Pm3n (a) to 1d-modulated structure (b) and 2d-modulated structure (AMS-9) with P42/mnm (c). A schematic drawing of the micellar arrangement (z coordinates are referred to the unit cell of Pm 3n) and the main pore connectivity through hexagonal windows across the planar defect is shown (d).

Figure 5c) as in the original, but rotated by 601. When the second type of modulation is also introduced periodically, the AMS-9 structure with P42/mnm results (Figure 5c). These observations may suggest that if a synthesis system is well controlled and silica condensed slowly relative to surfactant mobilisation, then structural changes in the liquid-crystal is well preserved as silica mesoporous crystals. These structural variations are hardly observed by diffraction measurements alone and are explained well by the simulated images inserted in Figure 5 [TO’s software]. Intergrowth of Fm 3m and Fd 3m structures will be discussed separately (Y. Sakamoto et al.).

2.1.3

Chiral Mesoporous Crystal Based on 2d-Hexagonal p6mm

We now turn to a novel type of modulation, chiral structures, for which the chiral pitch and handedness are experimentally controllable. Recently, a chiral ordered mesoporous crystal was synthesised by SC using the chiral surfactant N-myristoyl-L-alanine sodium salt (C14-L-AlaS) as a template, 3-aminopropyltriethoxysilane (APES) and N-trimethoxysilylpropyl-N,N,Ntrimethylammonium chloride (TMAPs) as CSDAs and tetraethoxylsilane (TEOS) as an inorganic source.20 An SEM image of the crystal and a schematic chiral channel system are shown in Figure 6. The crystal has a characteristic structure, i.e. a twisted hexagonal rod like morphology which possesses hexagonally ordered channels that run along the long axis of the rods at the same

675

Electron Microscopy Studies of Structural Modulation c

a

tube diameter D t

a 3

2

1

d11 =a/2

d10 =a/ 3

321 d b

Pitch tube direction Projection 10

Figure 6

SEM image of chiral silica mesoporous crystal (a) and schematic drawing of its morphology along the {1 0} direction (b). The channel system is shown in cross section (c) and perpendicular to the tube (d). {1 0} planes are parallel to o1 04 and therefore {10} fringes are observed only at the positions marked by the arrowheads in (b).

time as winding around the central axis of the rods. The positions where {1 0} fringes appear are marked by arrowheads in Figure 6b. TEM combined with image simulations confirmed the above structural model as shown in Figure 7.20 The chiral pitch of the channels inside hexagonal chiral mesoporous crystals was measured from the intermittent period. Between two sets of {1 0} fringes marked by black arrows (Figure 7a and b), the rod has twisted by 601, which means that the distance between two sets of {1 0} fringes is one-sixth of one pitch. {1 1} Fringes are also observed in Figures 7a and b as marked by white arrows. The chirality (possessing the quality of being right or left-handed) was determined from the tilt direction of a channel compared with the direction of incident electrons and the curvature direction of the curved intermitted fringes as viewed in the TEM images.25 The chirality and the morphology of the mesoporous silica are controlled by the stirring rate during the chiral surfactant self-assembly.26 Several interesting points were

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Chapter 40

Figure 7

HRTEM images of the chiral silica mesoporous crystal (a) and tube (c), and the corresponding simulated images (b) and (d), respectively. The positions where {10} and {11} fringes appear are maked by black and white arrows in (a) and (b).

observed: (i) there is a linear relationship between pitch length and rod diameter, (ii) the left/right handedness ratio was ca. 3/1 for the total sample regardless of differences in stirring rate or direction, as long as the rate is faster than 400 rpm. Furthermore, a racemic chiral mesoporous silica with twisted 2d-hexagonal structure has been synthesised by using achiral surfactant SDS.27 Recently, right- and left-handed excess chiral mesoporous silica nanotubes with helical channel walls have been formed by self-assembly of SDS in the presence of (R)-(+)- and (S)-(
)- 2-amino-3-phenyl-1-propanol ((R)-(+)- and (S)-(
)APP) chiral molecules. TEM combined with computer simulations confirmed the presence of ordered chiral channels winding around the central axis of the tubes of B100 nm inner diameter (Figures 7c and d).28

2.2 2.2.1

Case 2. Structural Modulation in Microporous Crystals Si/Al Concentration Modulation in the Framework of Zeolite MOR

Zeolite MOR has SG Cmcm with unit cell parameters a ¼ 18.1 A˚, b ¼ 20.5 A˚, c ¼ 7.5 A˚, and has 1d-channels with elongated 12-membered rings (12MR) of

677

Electron Microscopy Studies of Structural Modulation

6.5  7.0 A˚ size along [0 0 1] (Figure 8a). Selenium has an electron configuration [Ar]3d104s24p4. Various allotropes have two nearest neighbours at approximately 901 from each other, and a Se-chain spiral around the [1 1 1] direction of a primitive cubic lattice is expected. However, in the real structure of trigonalSe, the chains are stretched along the axis so that the angles become larger (1051) and the distance between the nearest neighbouring chains (3.6 A˚) is much larger than that within the chain (2.32 A˚). It is interesting to notice that the 1dchannels of MOR seem to ideally accommodate the isolated spiral chains of Se. Photos of Se containing MOR single crystals (Se@MORs) under optical microscopes with polarized light (above: with electric vector E1 parallel to the c-axis of the crystal 1, below: with electric vector E2 parallel to the c-axis of the crystal 2) are shown in Figure 8b. From the above polarized optical microscope images, we expect that isolated Se-chains were formed in the 1dchannels of MOR. It is easy to see a structural change of Se by the naked eye

c-axis

6.5 x 7.0 Å

Å

Capillary

100
m

Se@MOR MOR (a)

Se-chain (c)

c1

18.07 Å

Se

100
m

20.04 Å

E1

Na

c2 b

E2 a

(b)

Figure 8

c

(d)

MOR single crystals containing Se-chains (Se@MOR). Schematic drawing of MOR single crystal and its channel system (a), polarised microscope images of Se@MOR under two different polarised light conditions (b). Se@MOR sealed in glass capillary for resonant X-ray scattering (c) and the projected atomic arrangement obtained from the resonance X-ray scattering along the channel [0 0 1] (d).

678

Chapter 40

through the change in colour from brown for trigonal-Se to orange for the new Se-chain. Using a Se@MOR single crystal sealed in a SiO2 glass capillary under a He atmosphere (Figure 8c) and synchrotron X-ray radiation, the structure of the Se-chain within a channel was studied by resonant X-ray scattering of Se, and the observed structure was a little more complex than the above and the projection of the structural solution along [0 0 1] is shown in Figure 8d. The detailed results will be published separately.30 During the preparation of these isolated Se-chains in the channels of powdered MOR, we observed an unusual black and white wavy contrast with wave fronts parallel to the (0 1 0) plane as shown in Figure 9. This peculiar contrast was determined as an amplitude contrast due to Se-concentration modulation along the b axis.31,32 From the optical absorption spectra of Se confined in the channels, it was concluded that a finite number, ca, 10–20, of atoms of Se are physisorbed at each electric dipole induced by an AlO4
and associated non-framework cation.33 Consequently, Se atoms can be used to probe the compositional variation of Al in the framework of large pore structures at the unit cell scale by producing contrast in the HRTEM images.34

Figure 9

Unusual black and white contrast in the HREM image of MOR (Si/Al ¼ 10) containing Se at [0 0 1] incidence. Se-chains in the channels are imaged as dark dots and the contrast variation is due to the modulation of the Se content along the channel.

Electron Microscopy Studies of Structural Modulation

2.2.2

679

Modulation in LTL

Zeolite LTL has SG P6/mmm with a ¼ 18.4 A˚ and c ¼ 7.5 A˚ and has a 1d-undulating channel with 12-MR. The framework structure contains two important secondary building units, the cancrinite (CAN) cage and the double hexagonal ring (D6R) alternating to form a column. The structure can be described by a two-dimensional hexagonal arrangement of the columns forming both 8-rings and 12-rings. The crystal morphology is a hexagonal cylinder or plates with various aspect ratios. Three different structural modulations were observed. The first one is a coincidence boundary, which was studied at Cambridge,35 and others are a rotational defect and hole.36 The pffiffiffiffiffia columnar pffiffiffiffiffi coincidence boundary is a super-mesh structure 13  13 which entails a rotation of one part of the crystal on the (0 0 1) plane through 32.21. An illustration of the coincidence boundary was used in a school textbook of inorganic chemistry in Greece (Figure 10). An HRTEM image of the rotational defect is shown in Figure 11a. A group of six CAN columns surrounded one main 12-ring channel rotated 301 as a unit around the c-axis and the schematic drawing of the defect and simulated images are shown in Figure 11b–c, respectively.

2.2.3

Incommensurate Modulation in SSZ-24 via Collective Framework Distortion

The XRD powder pattern and 29Si MAS NMR of the silica zeolite SSZ-24 suggest the presence of an extra feature to the previously proposed AFI type framework structure (SG P6cc or the highest symmetry P6/mcc). We have found by TEM that this feature is a result of incommensurate structural modulation. Any structural modulations are barely observed either in ED patterns or HRTEM images taken with [hkl] incidence, where l is not equal to 0. Figure 12a shows an HRTEM image taken with [0 0 1] incidence and the corresponding ED pattern, where we cannot see the effect of structural modulations. However, extra spots produced from a structural modulation are observed along the c* axis.37 Figure 12b shows an ED pattern taken at room temperature with [0 1 0] incidence. The incommensurate extra spots are observed and can be explained by a modulation with wave vector q(0, 0, a) along the reciprocal lattice vector c* and a B 0.38. We observed the corresponding super-period structure in the HRTEM image. In the case of Au-based alloys, we observed various incommensurate modulations of atomic arrangements on the fcc lattice where the modulation wave vectors were governed by the electron atom ratio or size/shape of the Fermi surface. On the other hand, the modulation in SSZ-24 is associated with the ‘‘soft-mode’’ instability of the silica framework. More precisely, each of the SiO4 tetrahedral units in the framework is very rigid against deformation, while neighbouring units can change their relative configurations easily through bending/rotating around the corner shared O atom. Thus the framework structure may be thought of as a network of multiple hinges, and a displacive

680

Figure 10

Chapter 40

The cover of an inorganic chemistry textbook in Greece.

lattice distortion may occur if the framework topology allows a collective move of the hinges. In the harmonic approximation, such a distortion corresponds to a soft mode in the phonon spectra. By the use of the rigid-unit mode approach,38,39 we have analysed the distribution of such soft modes in

Electron Microscopy Studies of Structural Modulation

Figure 11

681

HRTEM images of a rotational defect observed in LTL taken with [0 0 1] incidence (a) and corresponding schematic drawings of the framework (b) and a simulated image for the specimen thickness of 10 nm, defocus value of
68 nm (c).

reciprocal space. In particular, we found a high-symmetric soft mode at the wave vector q ¼ (0, 0, a) with a ¼ 0.392, which was very close to the observed value of a B 0.38 for the modulation. The modulated structure transforms into a disordered one through electron irradiation or heat treatment. In Figure 13a, an ED pattern taken at 100 1C is shown, where incommensurate extra spots are no longer observed. Instead, there are diffuse streaks stretched along the D-line: k ¼ (0, 0, a), slightly curved lines in the vertical direction to the D-line and also weak but rather sharp lines along the 001 direction through the fundamental 00l (l ¼ even) spots. Both of the first two diffuse streaks seem to coincide with the distribution of lowsymmetric soft modes, as shown in Figure 13b. The third ones, rather sharp streaks passing through 00l (l ¼ even) spots, also have counterparts in the phonon spectra. However, this may result either from the narrow width of the crystal along [100] or from the diffraction effect from the crystal edge because no lines through 00l (l ¼ odd) spots were observed.

2.2.4

Zeolite BEC Overgrown on BEA*

Zeolite Beta is the first large pore zeolite with a 3d-channel system and a high Si/Al ratio. Beta contains disorder in the stacking sequence, which does not significantly affect the accessible pore volume but influences the tortuosity of the pore connectivity. Two different ordered polymorphs A (SG: P4122 or P4322)

682

Figure 12

Chapter 40

An HRTEM image of SSZ-24 taken with [0 0 1] incidence (a), and ED pattern taken with [0 1 0] incidence at room temperature with a schematic drawing to show an incommensurate modulation with q(0, 0, a), a B 0.38 (b).

and B (SG: C2/c) were proposed on the basis of HRTEM images and their corresponding ED patterns. A hypothetical polymorph C (SG: P42/mmc) was also suggested.40 The structure of zeolite Beta polymorph C (BEC) was recently solved and confirmed by a Fourier reconstruction of HRTEM images. Low and high magnification TEM images and a corresponding ED pattern taken with JEM-4000EX (400 kV) from the as-synthesised ITQ-14 are shown in Figures 14a–c, respectively. Figure 14a shows that a new crystal with pillarshape morphology overgrows onto ordinary Beta (BEA*). From Figure 14c, it can be seen that besides the many streaks coming from the mixtures of polmorphs A and B (matrix), there are also new spots just lying on the streaks with a nearly square mesh. It should be noticed that the tips of the crystals vary from crystal to crystal and that some growth steps are observed. From the HRTEM image (Figure 14b) the steps can be seen and are shown by arrows. We can determine the stacking sequence, either polymorph A or B, from the

Electron Microscopy Studies of Structural Modulation

Figure 13

683

An ED pattern of SSZ-24 taken with [0 1 0] incidence at 100 1C (a) and the distibution of the soft phonon modes in the relevant section of the reciprocal space calculated within the rigid-unit-mode model (b).

arrangement of white dots (pores) and they are marked on the HRTEM image. In addition to these polymorphs A and B, a new stacking is clearly observed at the region marked by ‘‘New’’. This is the first experimental demonstration of the real existence of polymorph C in the SiO2 system, which was solved and identified by electron crystallography developed for this study. A beautiful single-crystal pillar overgrown on an ordinary Beta* was used for the structural analysis. The electrostatic potential map was reconstructed from a set of HRTEM images, however the map was too blurred to locate the atoms. The atomic positions were enhanced in the blurred potential map with the support of a Patterson map derived from selected area ED patterns.41,42 Simulated HRTEM images of the polymorphs are inserted in the top left of Figure 14b. Projected framework types of polymophs A, B and C are also shown in Figure 14d. The surface structure, growth mechanism and rate, and the morphology were also discussed in separate papers.43,44

3 Conclusion Here we described TEM studies of various structural modulations in micro- and meso-porous crystals, which started at Cambridge in 1982 and continues to the present. These demonstrate the strength of