Studies in Surface Science and Catalysis 62 CHARACTERIZATION OF POROUS SOLIDS II
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Studies in Surface Science and Catalysis Advisory Editors: B. Delmon and J.T. Yates
Vol. 62
CHARACTERIZATION OF POROUS SOLIDS II Proceedingsof the IUPAC Symposium (COPS 11). Alicante, Spain, May 6- 9 , 1 9 9 0 Editors
F. Rodriguez-Reinoso Departamento de Quimica lnorgdnica e Ingenieria Quimica, Universidad de Alicante, Apartado 99, Alicante, Spain
J. Rouquerol Centre de Thermodynamiqueet de Microcalorimetrie, CNRS, 7 3003 Marseille, France K.S.W. Sing Department of Chemistry, Brunel University, Uxbridge, Middlesex UB8 3PH, U.K. and
K.K. Unger lnstitut fur Anorganische Chemie und Analytische Chemie, Johannes Gutenberg-Universitat,0-6500Mainz, F.R.G.
ELSEVIER
Amsterdam - Oxford - New York -Tokyo
1991
ELSEVIER SCIENCE PUBLISHERS B.V. Sara Burgerhahstraat 25 P.O. Box 2 1 1, 1000 AE Amsterdam, The Netherlands Distributors for the United States and Canada:
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Library o f Congress Cataloging-in-Publication Data
IUPAC Symposium. C O P S (2nd : 1990 : Alicante. Spain) Characterization of porous solids I1 : proceedings of the IUPAC Symposium, C O P S 11. Alicante. Spain. May 6-9. 1990 I editors, F. Rodriguez-Reinoso ... [et al.1. p. cm. -- (Studies in surface science and catalysis ; 62) Includes bibliographical references and indexes. ISBN 0-444-88569-2 1. Porous materials--Congresses. I. Rodrjguez-Reinoso. F., 194111. International Union of PGre and Applied Chemistry. 111. Title. IV. Title. Characterization o f porous solids 2. V. Title: Characterization of porous solids two. VI. S e r i e s . TA418.9.P6196 1990 620.1'16--d~20 91- 10354 C1P
ISBN 0-444-88569-2
0 Elsevier Science Publishers B.V., 199 1 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Science Publishers B.V./ Physical Sciences & EngineeringDivision, P.O. Box 330,lo00 AH Amsterdam, The Netherlands. Special regulationsfor readers in the USA -This publication has been registered with the Copyright Clearance Center Inc. (CCC), Salem, Massachusetts. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the USA. All other copyright questions, including photocopying outside of the USA, should be referred to the publisher. No responsibility is assumed by the Publisherfor any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. Although all advertising material is expected to conform to ethical (medical) standards, inclusion in this publication does not constitute a guarantee or endorsement of the quality or value of such product or of the claims made of it by its manufacturer. This book is printed on acid-free paper. Printed in The Netherlands
V
CONTENTS
Preface
XI11
Characterization of porous solids: an introductory survey K.S.W. Sing
1
Simulation of adsorption in model microporous graphite D. Nicholson
11
Theory of adsorption in micropores Z. Tan and K.E. Gubbins
21
Sorption of gases on microporous solids: pore size characterization by gas sorption S.W. Webb and W.C.Conner
31
Analysis of the percolation properties of a real porous material G. Mason and D.W. Mellor
41
The five types of porous structures and their hysteresis loops V. Mayagoitia
51
Model study of the combined effect of heteroporosity of macroscopic heterogeneity gas relative permeability of porous solids N.K. Kanellopoulos, J.K. Petrou and J.H. Petropoulos
61
Percolation theory of capillary hysteresis phenomena and its application for characterization of porous solids A.V. Neimark
67
Modelling of mercury intrusion and extrusion M. Day, I.B. Parker, J. Bell, M. Thomas, R. Fletcher and J. Duffie
75
Wetting phenomena in porous solids: Mechanisms and models A. Winter
85
The contact angle of liquids in porous media U. Demlehner
97
The main principles of modelling of porous solids. Models of systems with needle-like particles A.P. Karnaukhov
105
Adsorption-desorption hysteresis in porous networks D.K. Efremov and V.B. Fenelonov
115
VI
The determination of the pore size distribution of porous solids using a molecular model to interpret nitrogen adsorption measurements C.A. Jessop, S.M. Riddiford, N.A. Seaton, J.P.R.B. Walton and N. Quirke
123
Standardisation, reference materials and comparative measurements for surface area and pore characterisation E. Robens and K.-F. Krebs
133
Fractal characterization of the porosity of organic tissue by interferometry M. Sernetz, H.R. Bittner, P. Bach and B. Glittenberg
141
Determination of surface properties of porous solids K.S. Birdi, D.T. Vu, S.I. Andersen, A. Winter, H. Topsrae and S.V. Christensen
151
A new apparatus for continuous adsorption. Application to the characterization of microporous solids H. Ajot, J.F. Joly, F. Raatz and C. Russmann
161
A new mercury intrusion-retraction simulator used as a means for the characterization of porous materials C.D. Tsakiroglou and A.C. Payatakes
169
Film surface area measurements for microporosity and surface roughness analysis G.P. Johnston, D.M. Smith, A.J. Hurd and P. Pfeifer
179
Some problems about gas adsorption isotherm measurements by automated procedures in manometric devices J.L. Ginoux and L. Bonnetain
189
Morphological influences on unsteady gas diffusivities in porous solids W. C. Conner, S.W. Webb, P. Buckley, S.V. Christiansen, G. Parthun, J.A. Hansen and H. Topsrae
199
Textural characterization of ultrafiltration membranes by thermoporometry and liquid flow measurement J.F. Quinson, N. Nameri and B. Bariou
209
Characterization of the surface fractal dimension of evaporated silver and gold films through adsorption isotherm measurements J. Krim and V. Panella
217
Influence of pore structure parameters on the intraparticle pressure change during adsorption S.E. Scholl and A.B. Mersmann
225
VII
Neutron scattering investigation of adsorption processes in model porous systems J.D.F. Ramsay and R.G. Avery
235
Small angle and ultra-small angle scattering techniques for characterization of porous materials J.C. Dore and A.N. North
245
Gel-precipitated oxide gels with controlled porosity-determination of structure by small angle neutron scattering and adsorption isotherm measurements J.D.F. Ramsay, P.J. Russell and S.W. Swanton
257
Small-angle neutron scattering study of fumed silica powder compaction A.J. Hurd, G.P. Johnston and D.M. Smith
267
The determination of permeability and binary gas diffusion coefficients in novel forms of porous carbons S.B. Bhowmik, S.P. Waldram, R. McMurray and S.R. Tennison
273
Pore-size analysis for permeability estimation in porous material T. Sat0
283
The effects of pore and particle geometry on NMR diffusion measurements in adsorbed liquids S . Bahceli, A.R.S. Al-Kaisi, K. Krynicki and J.H. Strange
293
Pore size analysis of wet materials via low-field NMR D.M. Smith and P.J. Davis
30 1
Characterization of microporosity and surface homogeneity by the study of argon and nitrogen isotherm crossing and measurement of differential enthalpies of adsorption J.M. Martin-Martinez, F. Rodriguez-Reinoso, Y. Grillet, F. Rouquerol and J. Rouquerol
311
Adsorptive properties of activated carbons prepared from kevlar J.J. Freeman, F.G.R. Gimblett, R.A. Hayes, Z. Mohd. Amin and K.S.W. Sing
319
Modification in porous texture and oxygen surface groups of activated carbons by oxidation M. Molina-Sabio, M.A. Muiiecas-Vidal and F. Rodriguez-Reinoso
329
Adsorption of methanol and water by charcoal cloth A.M. Gonplves da Silva, M.M.L. Ribeiro Carrott, P.J.M. Carrott and M.M. Brotas de Carvalho
341
VIII
Influence of coal preoxidation and reactive gas flow rate on textural properties of active carbons J.A. Pajares, J.J. Pis, A.B. Fuertes, A.J. PCrez, M. Mahamud and J.B. Parra
347
Evaluation of microporosity in steam activated brown coal humic acids chars T. Siemieniewska, K. Tomkow, J. Kaczmarczyk, A. Albiniak, Y. Grillet and M. FranGois
357
Induced porosity in activated carbons by catalytic activation A. Linares-Solano, M. Almela-Alardn, C. Salinas-Martinez de Lecea, M.J. Muii6z-Guillena and M.J. IllPn-G6mez
367
Characterization of activated carbon: an approach to the activation process by SAXS and optical microscopy J.M. Guet, Q. Lin, A. Linares-Solano and C. SalinasMartinez de Lecea
379
Dynamic micropore structures of micrographitic carbons during adsorption K. Kaneko, T. Suzuki, Y. Fujiwara and K. Nishikawa
389
Characterization of the porosity of activated charcoals by adsorption from solution J. FernPndez-Colinas, R. Denoyel and J. Rouquerol
399
The porosity of textile fibre surfaces A. McInally, R.R. Mather and K.S.W.Sing
409
Further comments on low pressure hysteresis in activated carbons: effect of preparation method F. Rodriguez-Reinoso, J.M. Martin-Martinez, A. Linares-Solano and R. Torregrosa
419
Multi-stage micropore filling of N, and Ar by microporous carbon fibers K. Kakei, S. Ozeki, T. Suzuki and K. Kaneko
429
Porous structure of synthetic active carbons N.T. Kartel, A.M. Puzy and V.V. Strelko
439
Evaluation of microporosity in activated carbons with high ash (Cr20,) content M.A. Martinez-Shchez, J.M. Martin-Martinez, A.C. OrgilCsBarcel6, F. Rodriguez-Reinoso and M.J. SellCs-PCrez
449
Influence of coal oxidation on coke porosity J.J. Pis, R. MenCndez, J.J. Lorenzana, A.J. PCrez, H. Marsh and E. Romero
459
IX
Comparative studies of the microporous structure parameters evaluated from the adsorption isotherms of various adsorbates on activated carbons M. Jaroniec, J. Choma, F. Rodriguez-Reinoso and J.M. Mart in-Mart inez
469
Estimating micropore sizes in activated carbons from adsorption isotherms B. McEnaney and T.J. Mays
477
A comparative study of the porous structure of active carbons using benzene and water adsorption, inmersion calorimetry and liquid chromatography K.H. Radeke and P. Briickner
491
Mercury porosimetry of porous glass and active carbon preloaded with n-decane or water H. Lentz and Y. Zhou
499
Sorption of hydrocarbons in silicalite-1 and Nay zeolites J.A. Hampson, R.V. Jasra and L.V.C Rees
509
How can an adsorption system show phase transition. A case study on the adsorption of p-xylene in ZSM-5 D. Pan and A.B. Mersmann
5 19
Crystallochemical structure of zeolite micropores and adsorptionenergetic characteristics G.U. Rakhmatkariev, A.A. Isirikjan
525
Sorption of argon and nitrogen on network types of zeolites and aluminophosphates H. Reichert, U. Muller, K.K. Unger, Y. Grillet, F. Rouquerol, J. Rouquerol and J.P. Coulomb
535
Porosity of silicas: comparison of nitrogen adsorption and mercury penetration D.R. Milburn, B.D. Adkins and B.H. Davis
543
Characterization and stability of porous structure of titanium-silicalite by sorption methods G. Lmfanti, F. Genoni, M. Padovan, G. Petrini, G. Trezza and A. Zecchina
553
Study of the pore network of dealuminated faujasites by water vapor adsorption M.H. Simonot-Grange, A. Elm’Chaouri, M. Nafis, G. Weber, P. Dufresne, F. Raatz and J.F. Joly
5 65
Estimation of pore structure parameters for silica and carbon sorbents by macromolecular adsorption N.A. Eltekova and Yu.A. Eltekov
575
Formation of secondary pores in zeolites during dealumination: influence of the crystallographic structure and of the %/A1 ratio H. Ajot, J.F. Joly, J. Lynch, F. Raatz and P. Caullet
583
Vacuum thermal stability and textural properties of attapulgite J.M. Cases, Y. Grillet, M. Franqois, L. Michot, F. Villieras and J. Yvon
591
Characterisation of porous SiO2-Al20, sol-gels: model heterogeneous catalysts P.A. Sermon, T.J. Walton, M.A. Martin Luengo (Yates) and M. Yates
599
Effect of La(II1) on the thermal stability of Al-pillared montmorillonite J.M. Trillo, M.D. Alba, R. Alvero, M.A. Castro, J. Poyato and M.M. Tobias
607
Evolution of porosity during conversion of n-alumina to a novel porous a-alumina fibre M.H. Stacey
615
Evolution of the texture and the thermic stability of a pilc-A1 with varying dialysis time C. Pesquera, F. Gonzalez, I. Benito and S . Mendioroz
625
Microstructure of ex-hydroxide magnesium oxide & products of rehydration M.M.L. Ribeiro Carrott, P.J.M. Carrott, M.M. Brotas de Carvalho and K.S.W. Sing
635
Texture and surface properties of supported metallic oxide catalysts: Na-doped, titania and alumina-supported vanadia M. del Arco, E. Hernandez, C. Martin, I. Mateos and V. Rives
645
Sorption of water vapour by partially decomposed calcium hydroxide K.S.W. Sing, C.R. Theocharis and D. Yeates
653
Texture and sintering of zirconium dioxide-yttrium oxide ceramics A.J. Lecloux, S . Blacher, P.-Y. Kessels, P. Marchot, J.L. Merlo, F. Noville and J.P. Pirard
659
The porosity and permeability of macrodefect free cements K.S.W. Sing and M. Yates
669
XI
An appraisal by M.I.P. of the changes induced in the microstructure of complex sulfide ores by reactive thermal treatments in H2 and N2 M. Fatemi-Sadr and P. Bracconi
677
The adsorption of water vapour by microporous solids P.J.M. Carrott, M.B. Kenny, R.A. Roberts, K.S.W. Sing and C.R. Theocharis
685
Porosity of ancient Egyptian mortars J. Ragai, K.S.W. Sing and M. Yates
693
The porous structure of polymeric sorbents of different nature L.D. Belyakova
70 1
Determination of spatially resolved pore size information B. Ewing, P.J. Davis, P.D. Majors, G.P. Drobny, D.M. Smith and W.L. Far1
709
The influence of porous structure and external morphology on the activity of catalyst spheres prepared by the sol-gel method A.Q.M. Boon, C.J.G. van der Grift, A.J.W. van Veldhuizen and J.W. Geus
717
Characterization of porosity and pore quality in sedimentary rocks M.E. Cather, N.R. Morrow and I. Klich
727
Surface characterization of an upper-permian carbonate rock by N2 adsorption P.J. Mnller, P. Frykman, N. Stentoft and Chr.B. Koch
737
The adsorption of sulphur by macroporous materials L. Daza, S . Mendioroz and J.A. Pajares
747
The differences in the adsorption processes in micro and supermicropores 0. Kadlec
759
Author Index
77 1
Keyword Index
775
Studies in Surface Science and Catalysis (other volumes in the series)
779
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XI11
PREFACE
Since 1958, when the first major conference on porous solids was held at Bristol, U.K., considerable progress has been made in the development and characterization of porous materials. The subsequent international symposia held in 1978 (at Neuchfttel, Switzerland) and in 1983 (Milan, Italy) were well supported and led to the decision to arrange further symposia at regular intervals. As a result the first IUPAC Symposium on the Characterisation of Porous Solids (i.e. Cops I) was held at Bad Soden, F.R.G., in 1987, after which it was decided to hold COPS I1 at Alicante, Spain, in 1990. Following the success of COPS I, the Scientific Committee wanted to encourage a wide range of scientists and technologists to participate in COPS I1 and to provide them with the opportunity to authoritatively assess the progress which had been made in theoretical, experimental and applied research. The Symposium was organised by Professor F. RodriguezReinoso and his colleagues of the Departamento de Quimica Inorghica e Ingenieria Quimica. It consisted of a plenary lecture by Professor K.S.W. Sing, 153 oral and poster presentationsmd an extensive exhibition of equipment. It brought together 222 participants from 29 countries. This volume contains 82 of the papers which were selected and deemed worthy of publication. The organizers wish to express their special thanks to IUPAC for sponsoring the meeting and to the Ministerio de Educaci6n y Ciencia, Universidad de Alicante and Repsol Petr6leo for its generous support which made it possible to hold COPS I1 at Alicante. It has been decided that COPS 111 will be held at Marseille, France in 1993. F. Rodriguez-Reinoso, J . Rouquerol, K.S. W. Sing and K.K. Unger
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F. Rodriguez-Reinosoet al. (Editors), Characterization of Porous Solids II 1991 Elsevier Science Publishers B.V., Amsterdam
CHARACTERIZATION OF POROUS SOLIDS: Kenneth S.W.
A N INTRODUCTORY SURVEY
Sing
Department o f Chemistry, B r u n e l U n i v e r s i t y , Middlesex, UB8 3PH, U n i t e d Kingdom.
Uxbridge,
BACKGROUND The widespread i n t e r e s t in porous solids i s well i l l u s t r a t e d by t h e multifarious n a t u r e o f t h e c o n t r i b u t i o n s t o t h i s volume. Much o f t h e w o r k r e p o r t e d was u n d e r t a k e n o n materials o f technological importance such as adsorbents, c a t a l y s t s a n d constructional materials a n d t h e solids s t u d i e d include carbons, oxides, cements, clays, polymers, zeolites a n d metal films.
In view o f t h i s wide d i v e r s i t y o f interest, it i s p e r t i n e n t t o a s k whether such a b r o a d l y based symposium i s l i k e l y t o b e u s e f u l from a scientific standpoint. T h e f i r s t IUPAC Symposium o n t h e Characterization o f Porous Solids was h e l d in 1987 (i.e. COPS I, Elsevier, 1988) a n d it was e v i d e n t In h i s i n t r o d u c t o r y then t h a t t h e r e was a need f o r f u r t h e r systematic w o r k . paper, E v e r e t t d r e w attention t o some o f t h e o u t s t a n d i n g problems i n c l u d i n g the important requirement o f p r e d i c t i n g technological performance from t h e r e s u l t s o f characterization measurements. T h i s became a l l t h e more u r g e n t w i t h t h e development o f advanced materials a n d shape selective catalysts which r e q u i r e t h e application o f sophisticated characterization techniques. T h e way was t h e r e f o r e p r e p a r e d f o r t h e COPS I I Symposium t o b e h e l d in 1990 a n d t h e o p p o r t u n i t y was t h e n t a k e n t o review t h e status o f t h e more traditional techniques s u c h as a d s o r p t i o n a n d fluid penetration alongside t h e newer experimental techniques a n d computational procedures (e.g. small angle scattering, computer simulation a n d molecular modelling). This introductory s u r v e y i s n o t designed t o p r o v i d e a systematic appraisal o f t h e w o r k described here, but r a t h e r t o set t h e scene f o r these Proceedings of an important symposium. TERMINOLOGY AND MODEL SYSTEMS
'
The ubiquity o f porous materials has led t o confusion in t h e usage o f such terms as 'micropore', 'macropore', ' t o t a l p o r e volume' a n d ' i n t e r n a l area'. In t h e IUPAC classification o f p o r e size, t h e micropore w i d t h i s t a k e n to n o t exceed about 2 nm ( 2 O R J . t h e mesopore w i d t h t o be in t h e r a n g e 2-50 nm In r e c e n t years a n d t h e macropore w i d t h t o b e above about 50 nm ( 0 . 0 5 pm). these d e f i n i t i o n s have s e r v e d u s well, especially in t h e c o n t e x t o f gas adsorption a n d m e r c u r y porosimetry, but it i s becoming i n c r e a s i n g l y clear t h a t some refinements a r e r e q u i r e d a n d t h a t account should b e taken o f p o r e shape. It i s a p p a r e n t t h a t t h e p o r e s t r u c t u r e s o f many systems o f technological importance (e.g. building materials) a r e made up o f cracks, cavities a n d channel n e t w o r k s o f v a r y i n g size, shape a n d c o n n e c t i v i t y . On the other hand, p o r e s t r u c t u r e s can now b e p r e p a r e d w h i c h a r e remarkably u n i f o r m a n d correspond f a i r l y closely t o model systems.
2 Zeolitic s t r u c t u r e s o f high S i / A I r a t i o a r e generally q u i t e d i f f i c u l t t o synthesise in t h e form o f l a r g e c r y s t a l s . It i s t h e r e f o r e n o t e w o r t h y t h a t Unger a n d h i s co-workers have been able t o synthesise l a r g e c r y s t a l s o f T h i s has enabled Reichert e l a l t o g a i n a ZSM-5, Silicalite 1 a n d ZSM-48. much i m p r o v e d u n d e r s t a n d i n g o f t h e i n t r i n s i c p r o p e r t i e s o f these zeolites t h a n was f o r m e r l y possible. Molecular seive carbons can now b e p r e p a r e d from v a r i o u s polymeric p r e c u r s o r s . High-resolution electron microscopy has revealed t h a t t h e pores a r e predominanently slit-shaped. O t h e r systems w h i c h e x h i b i t slit-shaped pores a r e t h e p i l l a r e d clays a n d c e r t a i n inorganic oxides p r o d u c e d by t h e controlled thermal decomposition o f p a r e n t h y d r o x i d e s s u c h as Ca(OHI2 a n d Mg (OH) 2 . K a r n a u k h o v has classified p o r o u s solids as spongy a n d corpuscular. Many c o r p u s c u l a r systems a r e unconsolidated o r o n l y weakly aggregated. If t h e area o f contact between a n assemblage o f g l o b u l a r p a r t i c l e s i s small t h e system w i l l behave in some ways as a non-porous powder (e.g. w i t h respect t o gas adsorption). If t h e powder i s subjected t o compaction o r heat treatment it w i l l t e n d t o u n d e r g o a n i r r e v e r s i b l e change. T h e weakly-bonded aggregate i s t h u s c o n v e r t e d i n t o a more compact agglomerate w i t h a well-defined p o r e s t r u c t u r e . Systems o f t h i s t y p e a r e discussed by Karnaukhov, Mason, Ramsay a n d others. D u b i n i n a n d h i s co-workers f i r s t suggested t h a t micropores should b e sub-divided i n t o t w o groups, w h i c h a r e now usually termed ultramicropores a n d supermicropores. Ultramicroporous solids ( o f p o r e w i d t h < ca 0.7 nm) a r e l i k e l y t o e x h i b i t molecular sieve properties, whereas supermicroporous solids g e n e r a l l y have l a r g e r i n t e r n a l areas a n d p o r e volumes w h i c h a r e accessible t o a w i d e r r a n g e o f a d s o r p t i v e molecules. If these somewhat inelegant terms a r e t o b e retained it would b e desirable t o define t h e ranges o f size more p r e c i s e l y in relation t o p o r e shape (e.g. s l i t s a n d c y l i n d r i c a l channels). It i s obvious t h a t as t h e p o r e w i d t h i s r e d u c e d a n d approaches molecular dimensions so t h e absolute magnitude o f t h e p o r e volume becomes more d i f f i c u l t t o evaluate. For t h i s reason it has been recommended t h a t t h e t e r m effective pore volume should b e employed a n d t h e operational p r o c e d u r e used for i t s evaluation c l e a r l y specified.
The COPS-I Symposium (Elsevier, 1988) p r o v i d e d t h e f i r s t o p p o r t u n i t y for a n e x t e n s i v e discussion o f t h e r o l e o f f r a c t a l analysis in t h e characterization o f t h e t e x t u r e o f solids. A l t h o u g h some aspects a r e open t o c r i t i c i s m t h e r e i s l i t t l e d o u b t t h a t f r a c t a l geometry has been shown t o b e a useful tool in t h e analysis o f data obtained w i t h porous solids o r r o u g h surfaces. T h e studies by K r i m a n d Panella, Johnston e t al, Dore a n d N o r t h a n d Sernetz a n d h i s co-workers i l l u s t r a t e t h e application o f f r a c t a l geometry for t h e analysis of v a r i o u s t y p e s o f experimental data obtained w i t h r o u g h surfaces a n d p o r o u s materials. A t t h e v e r y least, t h e proponents o f f r a c t a l analysis can j u s t i f i a b l y claim t h a t t h e approach p r o v i d e s a systematic basis for t h e analysis o f experimental data obtained w i t h s t r u c t u r a l l v complex nlHerials. U n f o r t u n a t e l y , t h e r e s u l t s o f t h e analysis a r e ofte; difficult to interpret! ADSORPTION Experimental Techniques T h e measurement o f a d s o r p t i o n a t t h e g a s / s o l i d i n t e r f a c e continues t o b e one o f t h e most p o p u l a r techniques f o r t h e s t u d y o f microporous a n d
3 It i s n o t s u r p r i s i n g t h e r e f o r e t h a t many papers in t h i s mesoporous solids. symposium a r e concerned w i t h t h e determination a n d i n t e r p r e t a t i o n o f gas adsorption data.
Great advances have been made in t h e development o f automated equipment f o r a d s o r p t i o n isotherm measurements, but it i s n o t always easy t o o b t a i n reliable data. Robens a n d Krebs stress t h e d e s i r a b i l i t y o f c a l i b r a t i n g new i n s t r u m e n t s w i t h t h e a i d o f reference materials a n d Ginoux a n d Bonnetain also d r a w a t t e n t i o n t o some o f t h e l i k e l y sources o f e r r o r in isotherm measurements. The papers by Conner, Kaneko, Rouquerol, U n g e r a n d t h e i r co-workers u n d e r l i n e t h e importance now attached t o t h e determination o f p h y s i s o r p t i o n isotherms a t v e r y low levels o f surface coverage o r fractional micropore filling, i.e. in t h e r e g i o n o f v e r y low p / p o . Such high resolution a d s o r p t i o n (HRADS) measurements have been shown t o b e especially u s e f u l f o r t h e characterization o f the a d s o r p t i v e p r o p e r t i e s o f zeolites, aluminophosphates a n d molecular sieve carbons. Another b e n e f i t o f automated instrumentation i s t h a t t h e detailed course o f a n isotherm can b e established o v e r a n y pre-selected r a n g e o f p/po. Equipment o f t h i s t y p e o p e r a t i n g in t h e mode o f continuous flow was f i r s t used by Rouquerol a n d h i s co-workers in conjunction w i t h microcalorimetry The results o f f o r s t u d y i n g changes in state o f t h e adsorbed phase. continuous a d s o r p t i o n measurements a r e also r e p o r t e d h e r e by Ajot e t al. Micropore F i l l i n q It is now generally agreed t h a t p h y s i s o r p t i o n w i t h i n t h e n a r r o w e s t micropores ( i .e. t h e ultramicropores) does n o t i n v o l v e monolayer formation, but instead takes place p r e f e r e n t i a l l y a t v e r y low p / p o ( i n i t i a l l y a r o u n d p / p o / v T h i s process i s associated w i t h enhanced adsorbent-adsorbate interactions a n d r e s u l t s in a n appreciable d i s t o r t i o n o f t h e a d s o r p t i o n isotherm. T h e mechanism o f p h y s i s o r p t i o n in t h e wider micropores ( i .e. t h e supermicropores) i s much less well understood, but appears t o i n v o l v e cooperative adsorbate-adsorbate interactions so t h a t a d s o r p t i o n takes place a t somewhat h i g h e r p / p o (-0.01-0.2) by an assemblage o f molecules, i.e. giving quasi-multilayer formation. In t h i s connection it i s o f i n t e r e s t t o n o t e t h e f i n d i n g s o f Nicholson a n d T a n & Gubbins. These two p a p e r s deal w i t h a d s o r p t i o n in model slit-shaped pores w i t h i n a g r a p h i t i c s t r u c t u r e ; t h e former by t h e application o f g r a n d canonical emsemble simulation t o follow t h e a d s o r p t i o n o f a r g o n a n d t h e l a t t e r by t h e use o f meanf i e l d density-functional t h e o r y t o model t h e behaviour o f methane a n d ethane. These studies appear t o s u p p o r t t h e view t h a t favourable circumstances e x i s t f o r t h e filling o f pores o f p a r t i c u l a r dimensions (in relation t o t h e molecular diameter) a n d p o i n t t h e way f o r f u r t h e r w o r k .
T h e q u e s t i o n o f t h e v a l i d i t y o f t h e Dubinin-Radushkevich ( D R ) equation continues t o a t t r a c t a good deal of attention. Many a u t h o r s s t i l l use t h e DR p l o t f o r t h e assessment o f t h e micropore volume whilst o t h e r s a r e more cautious in t h e i r i n t e r p r e t a t i o n o f t h e d e r i v e d values o f micropore volume a n d p o r e width. Confirmation i s p r o v i d e d in a p a p e r by Rodriguez-Reinoso a n d h i s co-workers t h a t excellent agreement can b e obtained between t h e values o f micropore volume obtained by extrapolation o f DR p l o t s a n d t h e corresponding a -plots p r o v i d e d t h a t c e r t a i n conditions a r e f u l f i l l e d - namely t h a t t h e microposre size d i s t r i b u t i o n i s n o t too broad. T h e Alicante scientists also draw a t t e n t i o n t o t h e d i f f i c u l t y o f o b t a i n i n g suitable non-porous reference materials when dealing w i t h microporous carbons h a v i n g high a s h contents. Kaneko a n d h i s co-workers have n o t e d t h a t some DR p l o t s appear t o e x h i b i t a succession o f linear regions. These features a r e i n t e r p r e t e d in terms of a multistage mechanism o f micropore filling, i.e. an extension o f t h e p r i n c i p l e s o f p r i m a r y a n d cooperative micropore filling.
4 As McEnaney a n d Mays p o i n t out, t h e simple DR equation i s based o n t h e assumption t h a t t h e micropore s t r u c t u r e i s homogeneous, i.e. t h a t a l l t h e micropores in t h e adsorbent g i v e t h e same characteristic a d s o r p t i o n potential, E Since t h e equation has a v e r y general mathematical form, t h i s requirement'cannot b e tested by simple inspection o f t h e DR p l o t a n d t h e r e i s l i t t l e d o u b t t h a t most microporous solids a r e s t r u c t u r a l l y heterogeneous.
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To overcome t h i s problem Dubinin, McEnaney, Stoeckli a n d Kadlec have proposed generalised forms o f t h e DR equation w h i c h in p r i n c i p l e should b e applicable t o heterogeneous microporous solids. In practice, t h e main problem in a d o p t i n g t h i s approach i s t o a r r i v e a t a u n i q u e solution for t h e p r o b a b i l i t y d e n s i t y f u n c t i o n o f E a n d hence t h e micropore size d i s t r i b u t i o n . These aspects a r e discussed in &me detail by McEnaney a n d Mays.
As mentioned earlier, a d s o r p t i o n microcalorimetry i s a n invaluable technique f o r s t u d y i n g t h e thermodynamic p r o p e r t i e s o f adsorption systems. The paper by Martin-Martinez e t a l p r o v i d e s a good example o f how adsorption e n t h a l p y measurements can y i e l d a clearer understanding o f the mechanisms o f micropore filling a n d surface coverage. A n improved isosteric method has been developed by Rees a n d h i s co-workers. T h e i r measurements have revealed energetic heterogeneity in t h e adsorption o f ethane a n d propane by Silicalite I . Jessop e t a l have developed a novel p r o c e d u r e f o r computing t h e p o r e size d i s t r i b u t i o n from n i t r o g e n isotherm data. T h e method i s based o n t h e application o f mean-field t h e o r y f o r t h e calculation o f a set o f isotherms c o r r e s p o n d i n g t o pores o f g i v e n w i d t h a n d it i s claimed t h a t t h e molecular model p r o v i d e s a realistic representation o f t h e adsorbed fluid in pores of a l l sizes a n d t h a t t h e method can t h e r e f o r e b e used f o r b o t h micropore a n d mesopore analysis. A limited number o f comparisons have been made w i t h more conventional methods o f p o r e size analysis, but it i s p r o b a b l y too e a r l y t o judge t h e success o f t h i s i n t e r e s t i n g approach. A d s o r p t i o n Hysteresis It i s well k n o w n t h a t c a p i l l a r y condensation in mesopores i s generally associated w i t h hysteresis. Progress has been made in linking t h e characteristic shapes o f c e r t a i n h y s t e r e s i s loops w i t h t h e n a t u r e o f t h e p o r e s t r u c t u r e , but much remains t o be done t o e x p l a i n t h e mechanisms of mesopore filling a n d emptying. T h e p a p e r s by Mayagoitia, Neimark a n d Efremov a n d Fenelonov o n t h e r o l e o f porous n e t w o r k s show how f u r t h e r p r o g r e s s can b e made by t h e systematic computer-assisted analysis o f a number o f c a r e f u l l y selected model systems. The fundamental question o f whether t o adopt t h e adsorption o r desorption b r a n c h of t h e h y s t e r e s i s loop f o r mesopore analysis remains unresolved. Indeed, it seems l i k e l y t h a t t h e r e i s no simple answer, but t h a t t h e computational p r o c e d u r e should b e g o v e r n e d by t h e p o r e geometry a n d n e t w o r k configuration.
F o r many years it was t h o u g h t t h a t a n y h y s t e r e s i s appearing before t h e onset o f c a p i l l a r y condensation was t h e r e s u l t o f slow e q u i l i b r a t i o n o r inaccurate measurements. It i s now known, however, t h a t t h e r e a r e two t y p e s of low-pressure h y s t e r e s i s which a r e associated w i t h p a r t i c u l a r systems. The f i r s t i s a well-defined h y s t e r e s i s loop appearing a t p/po/v 0.1 a n d g i v e n f o r 'This loop has been s t u d i e d example by N2 isotherms o n HZSM-5 a t 77K. in some detail by MUller a n d Unger, who a t t r i b u t e it t o a phase transformation T h e p r e s e n t paper by Pan a n d Mersmann ( l i q u i d - l i k e t o solid-like s t r u c t u r e s ) . p r o v i d e s a somewhat d i f f e r e n t explanation based o n a combination o f localized a d s o r p t i o n o n a r a n g e o f surface sites a n d i n t e r a c t i o n between adsorbed molecules.
5 The second t y p e o f low-pressure h y s t e r e s i s extends down t o much lower p r e s s u r e a n d i s d u e t o e i t h e r a n i r r e v e r s i b l e change in t h e adsorbent (e.g. swelling o r surface chemical change) o r t o t h e slow passage o f molecules t h r o u g h v e r y n a r r o w p o r e entrances o r between small aggregated Rodriguez-Reinoso a n d h i s co-workers now r e p o r t new r e s u l t s particles. w i t h microporous carbons, w h i c h reveal t h a t t h e development o f low-pressure hysteresis i s dependent o n t h e atmosphere ( C O z o r a i r ) in which t h e carbons The a u t h o r s o f f e r t h e t e n t a t i v e explanation t h a t t h e a r e activated. appearance o f t h i s t y p e o f h y s t e r e s i s i s associated w i t h t h e development o f d i f f e r e n t surface s t r u c t u r e s . Adsorption o f Water Vapour A number o f papers in t h i s volume a r e concerned w i t h t h e adsorption o f water vapour, which is o f g r e a t importance in t h e c o n t e x t o f gas separation o r r e s p i r a t o r y protection. Since a c t i v a t e d c a r b o n f i l t e r s have a low a f f i n i t y f o r water vapour, v e r y l i t t l e p o r e b l o c k i n g o c c u r s a t low r e l a t i v e h u m i d i t y . However, i f t h e r e s p i r a t o r i s used in a humid atmosphere o r t h e c a r b o n p r e v i o u s l y exposed t o water vapour, t h e adsorption e f f i c i e n c y i s seriously impaired. The w o r k o f C a r r o t t e t a l has revealed t h a t Silicalite i s in e f f e c t more h y d r o p h o b i c t h a n a n y microporous carbon s t u d i e d so far, since it has both low a f f i n i t y a n d low capacity f o r water vapour. It i s suggested t h a t t h e low water capacity i s d i r e c t l y related t o t h e t u b u l a r n a t u r e o f t h e i n t r a c r y s t a l l i n e channels in Silicalite a n d t h a t a thin l a y e r o f hydrogen-bonded water molecules can more easily form w i t h i n t h e slit-shaped pores o f a c t i v a t e d carbons. A d s o r p t i o n f r o m Solution A d s o r p t i o n from solution measurements have been employed f o r many years t o characterize i n d u s t r i a l adsorbents, but t h e data obtained a r e o f t e n Rouquerol a n d h i s co-workers have now made a difficult to interpret. systematic s t u d y o f a series o f a c t i v a t e d charcoals in w h i c h t h e r e s u l t s of adsorption from solution a r e compared w i t h data obtained by gas a d s o r p t i o n B y a d a p t i n g t h e @ -method, t h e y have shown t h a t a n d immersion calorimetry. t h e adsorption o f benzene from ethanol solutio8 is comparable w i t h t h a t o f n i t r o g e n from t h e gas phase a n d t h a t t h e a d s o r p t i o n from solution data obtained w i t h p r o b e molecules o f d i f f e r e n t shape p r o v i d e a u s e f u l means o f s t u d y i n g t h e enlargement o f mciropore entrances. Another i n t e r e s t i n g s t u d y o f solution a d s o r p t i o n r e p o r t e d h e r e i s t h a t o f Eltekova a n d Eltekov o n t h e a d s o r p t i o n o f macromolecules by mesoporous It i s e v i d e n t from t h i s w o r k t h a t t h e a d s o r p t i o n o f these carbons a n d silicas. large solute molecules can b e optimised by c o n t r o l o f t h e p o r e s t r u c t u r e a n d it is tempting t o suggest t h a t 'micropore filling' e f f e c t s should b e t a k e n i n t o account. F L U I D PENETRATION AND FLOW As E v e r e t t has p o i n t e d o u t (see COPS I , Elsevier, 1988, p.7). t h e d e n s i t y o f porous solids i s n o t a s t r a i g h t f o r w a r d concept. A problem o f i n t e r p r e t a t i o n arises when t h e volume occupied by a g i v e n mass o f solid appears t o b e dependent o n t h e fluid (gas o r l i q u i d ) displaced. This disparity is indicative o f differences in t h e degree o f penetrationof t h e f l u i d s i n t o t h e p a r t i c u l a r pore s t r u c t u r e a n d may b e t h e r e s u l t o f e i t h e r molecular s i e v i n g o r t h e effects o f capillarity. Wetting behaviour i s o f t e n discussed in terms o f contact angle measurements, but t h e paper by Demlehher draws a t t e n t i o n t o t h e d i f f i c u l t y o f
6 obtaining agreement between contact angles determined by d i f f e r e n t methods. A way o f a v o i d i n g t h e contact angle problem i s discussed in t h e paper by Winter, which deals w i t h w e t t i n g a n d displacement o f liquid in single pores a n d c a p i l l a r y networks. Another problem i s t h e swelling which occurs when porous polymers a r e immersed in organic l i q u i d s o r even subjected t o vapour However, Belyakova has f o u n d t h a t t h i s may be minimised by t h e sorption. choice o f a d s o r p t i v e a n d c o n t r o l o f p / p o . M e r c u r y Porosimetry M e r c u r y porosimetry i s featured in many o f t h e c o n t r i b u t i o n s t o t h i s volume. Indeed, it i s now one o f t h e most popular methods available f o r t h e characterization o f a wide r a n g e o f porous materials a n d t h e d e r i v e d p o r e The method i s sizes a r e o f t e n quoted in t h e patent a n d technical literature. based o n t h e non-wetting n a t u r e o f m e r c u r y a n d t h e application o f t h e Washburn equation. T h e volume o f m e r c u r y p e n e t r a t i n g i n t o a porous solid i s determined as a f u n c t i o n o f t h e applied pressure, which i s assumed t o be d i r e c t l y related t o t h e p o r e width. In spite o f t h e g r o w i n g p o p u l a r i t y o f m e r c u r y porosimetry a n d t h e ready availability o f excellent automated equipment, t h e i n t e r p r e t a t i o n o f t h e m e r c u r y i n t r u s i o n - e x t r u s i o n data i s s t i l l f a r from clear. The values o f surface tension a n d contact angle which must b e i n s e r t e d in t h e Washburn equation a r e s t i l l u n c e r t a i n - as a r e t h e limits o f applicability o f t h e equation itself. Other problems include t h e r e v e r s i b l e o r i r r e v e r s i b l e deformation o f t h e p o r e structure, which undoubtedly occurs w i t h some corpuscular o r weakly agglomerated systems.
Many d i f f e r e n t explanations have been proposed f o r t h e appearance o f i n t r u s i o n - e x t r u s i o n hysteresis which appears t o b e a u n i v e r s a l feature o f m e r c u r y porosimetry. The paper by Day e t al helps t o p r o v i d e a b e t t e r u n d e r s t a n d i n g o f t h i s phenomenon a n d also t h e related i r r e v e r s i b l e entrapment o f mercury. The I C I scientists have extended a n d improved t h e By n e t w o r k model approach o r i g i n a l l y used by Haynes, Mann a n d Conner. computer simulation o f a three-dimensional n e t w o r k it i s possible t o model t h e pathways o f advancing a n d receding m e r c u r y threads a n d explore t h e effects o f b l o c k i n g a n d k n o c k i n g o u t pores. The w o r k i s s t i l l in progress, but t h e comparisons w i t h real systems made so f a r indicate t h a t a mechanism i n v o l v i n g t h e spontaneous nucleation o f t h e m e r c u r y meniscus a t t h e s t a r t o f e x t r u s i o n i s untenable a n d t h a t some form o f a i r seeding i s p r o b a b l y essential. C a r e f u l experimental w o r k in t h e IC I laboratories has confirmed t h a t a high level o f r e p r o d u c i b i l i t y can b e achieved in p a r t i a l intrusion, scanning a n d r e c y c l i n g experiments. Lentz a n d Zhou have c a r r i e d o u t a n i n t e r e s t i n g investigation o f t h e effect on m e r c u r y i n t r u s i o n o f p a r t i a l l y filling t h e pores w i t h another liquid. T h e y explain t h e i r r e s u l t s by postulating a change in t h e contact angle, but t h i s explanation i s open t o question in view o f t h e complexity o f t h e p o r e s t r u c t u r e s s t u d i e d so f a r . However, it should b e r e w a r d i n g t o c a r r y o u t more w o r k of t h i s t y p e w i t h c a r e f u l l y selected systems. Davis a n d h i s co-workers have extended t h e i r investigations o f well-defined porous silicas. They r e p o r t f a i r l y good agreement between t h e p o r e volumes a n d p o r e size d i s t r i b u t i o n s determined by m e r c u r y porosimetry a n d n i t r o g e n adsorption, but lack o f agreement between t h e corresponding surface areas. ( T h e l a t t e r values calculated from t h e m e r c u r y i n t r u s i o n c u r v e s a r e These a n d o t h e r appreciably h i g h e r t h a n t h e corresponding BET-areas) r e s u l t s u n d e r l i n e t h e u r g e n t need f o r more fundamental w o r k t o p r o v i d e a more r i g o r o u s basis f o r t h e i n t e r p r e t a t i o n o f m e r c u r y porosimetry data.
.
F l u i d Flow T h e r a t e o f movement o f f l u i d s i n t o a n d t h r o u g h porous media i s o f g r e a t importance in a g r i c u l t u r e , c i v i l engineering, catalysis a n d separation As Conner e t a l p o i n t out, many attempts have been made t o technology. correlate permeability ( o r t r a n s p o r t resistance) w i t h t h e morphology o f a However, it i s n o t s u r p r i s i n g t o find t h a t no simple c o r r e l a t i o n porous solid. can b e f o u n d between t h e t r a n s p o r t p r o p e r t i e s a n d t h e p o r o s i t y as s t u d i e d by Another complication i s t h a t gas a d s o r p t i o n o r m e r c u r y porosimetry. adsorption k i n e t i c s a r e notoriously d i f f i c u l t t o model a t t h e molecular level. Thus, a l t h o u g h gaseous d i f f u s i o n in zeolites a n d molecular sieve carbons has been widely studied, t h e data in t h e l i t e r a t u r e show many anomalies a n d inconsistencies. The problems encountered in experimental permeability studies a r e Quinson e t al; discussed in a number o f papers (e.g. Sato; Bhewmik e t al; A n unexpected development o f high permeability in Sing a n d Yates). porous p l u g s o r membranes i s o f t e n t h e r e s u l t o f uneven macropore o r c r a c k formation during manufacture, storage o r operation (e.g. dimensional changes In t h e i r s t u d y o f model systems, Kanellopoulos a n d h i s o f membranes). co-workers discuss t h e effects o n gas permeability o f d i f f e r e n t forms o f n e t w o r k heterogeneity. It appears from t h i s a n d o t h e r studies t h a t similar changes in permeability a n d percolation thresholds may o r i g i n a t e in q u i t e d i f f e r e n t ways a n d h i g h l i g h t s t h e need f o r caution in t h e i n t e r p r e t a t i o n o f permeability data. A n a l t e r n a t i v e approach i s p r e s e n t e d in t h e paper by Mason a n d Mellor, which follows t h e e a r l i e r w o r k by Mason (see COPS I ) o n In t h e i r p r e s e n t paper, a t t e n t i o n i s g i v e n t o percolation a n d n e t w o r k theory. beds o f packed spheres a n d it i s c o n c l u d e d t h a t s u c h systems can b e t r e a t e d However, as n e t w o r k s a r r a n g e d in t h e form o f t h e 3-D diamond lattice. simulation o f drainage a n d imbibition appears t o indicate t h a t t h e b o n d a n d c a v i t y sizes a r e n o t randomly d i s t r i b u t e d t h r o u g h o u t t h e n e t w o r k . Mass t r a n s p o r t The role o f t h e p o r e s t r u c t u r e in mass t r a n s p o r t in adsorbents a n d catalysts i s discussed in t h e papers by Scholl a n d Mersmann a n d Boon e t al. In t h e former s t u d y , which i n v o l v e s modelling t h e a d s o r p t i o n kinetics, allowance i s made f o r t h e e f f e c t o f v a r i a t i o n o f total p r e s s u r e o n concentration a n d temperature p r o f i l e s w i t h i n a spherical p a r t i c l e a n d t h u s simulate t h e T h e o t h e r s t u d y by Boon e t a l i s conditions o f p r e s s u r e swing adsorption. concerned w i t h t h e behaviour o f porous oxide-based c a t a l y s t spheres p r e p a r e d by t h e sol-gel method. A l t h o u g h t h e y deal w i t h v e r y d i f f e r e n t systems a n d circumstances, these t w o papers b o t h d r a w a t t e n t i o n t o t h e importance o f macroporosity in d i f f u s i o n c o n t r o l a n d mass t r a n s p o r t . M I SC ELLAN EOUS TECH N IQUES Microscopy A l t h o u g h t h e y do n o t appear t o occupy a prominent place in t h e p r e s e n t volume, microscopic techniques continue t o p l a y a v i t a l r o l e in t h e Thus, confidence can b e gained in characterization o f many porous materials. the i n t e r p r e t a t i o n o f adsorption o r flow data if independent evidence can b e obtained o f p o r e shape o r t e x t u r e u n i f o r m i t y . T h e paper by Pis e t a l p r o v i d e s a good example o f t h e application o f optical microscopy. In t h i s case,image-analysis has been used t o p r o v i d e a q u a n t i t a t i v e evaluation o f t h e number, size a n d shape o f pores in cokes The r e s u l t s a r e compared w i t h t h e p r o d u c e d by p r o g r e s s i v e oxidation. m e r c u r y i n t r u s i o n data a n d t h e t w o techniques shown t o b e complementary.
8 The use o f thin section analysis a n d fluorescent microscopy f o r t h e s t u d y o f sedimentary oil-bearing r o c k s i s described in t h e paper by Cather e t al. H i g h resolution electron microscopy i s f e a t u r e d in many o f t h e papers presented here. A l t h o u g h TEM i s n o t easy t o apply, it has been used successfully t o s t u d y micropore a n d mesopore shape in s u c h d i v e r s e systems as modified zeolites (e.g. in t h e w o r k o f A j o t e t a l l , alumina f i b r e s (by Stacey) a n d t h e thermal decomposition p r o d u c t s o f Mg(OH)2 ( R i b e i r o C a r r o t t e t a l l . T h e successful outcome o f these a n d o t h e r studies has t o a l a r g e e x t e n t depended o n t h e c a r e f u l a t t e n t i o n g i v e n t o thin sectioning o r o t h e r forms o f T h e application o f SEM i s o f course less demanding a n d sample preparation. i s o f p a r t i c u l a r value f o r t h e i n v e s t i g a t i o n o f p a r t i c l e / c r y s t a l shape a n d aggregate s t r u c t u r e . SEM i s now g e n e r a l l y r e g a r d e d as an extremely u s e f u l a n c i l l a r y tool f o r s t u d y i n g t h e morphology a n d secondary p o r e s t r u c t u r e o f zeolites, oxides a n d carbons a n d o f multicomponent systems such as cements. Small A n g l e S c a t t e r i n g T h e use o f small angle s c a t t e r i n g techniques f o r s t u d y i n g porous solids i s well established, but it i s o n l y in r e c e n t years t h a t t h e i r full potential has been appreciated. Several p a p e r s in t h e p r e s e n t symposium i l l u s t r a t e t h e application o f small angle n e u t r o n (SANS) a n d X - r a y (SAXS) scattering. Ramsay a n d A v e r y have c o n t i n u e d t o a p p l y SANS in t h e i r studies o f porous oxides: in t h e i r p r e s e n t paper t h e y u t i l i s e H 2 0 / D 2 0 m i x t u r e s t o investigate mechanisms o f p o r e filling a n d conclude t h a t s i g n i f i c a n t differences a r e apparent between t h e state o f adsorbed water in mesoporous silicas a n d microporous ceria. In another paper, Ramsay a n d h i s co-workers r e p o r t t h e f i n d i n g s o f e x t e n s i v e SANS a n d adsorption studies o f a r a n g e o f h y d r o u s oxide gels in a polymer m a t r i x . Stacey has used SANS along w i t h gas adsorption a n d TEM t o investigate The t h e development o f p o r o s i t y in alumina f i b r e s made by Sol-gel methods. SANS p a t t e r n s were a n j m n e t r i c a n d t h i s together w i t h o t h e r evidence indicated O t h e r SANS studies a r e t h a t t h e pores were s t r o n g l y a x i a l l y aligned. r e p o r t e d by Dore a n d N o r t h a n d H u r d e t al. The w o r k by t h e former a u t h o r s i n v o l v e d an i n v e s t i g a t i o n o f H O/D,O in p o r o u s silica a n d oil-bearing rocks. It is e v i d e n t t h a t t h e f r a c t a l dimensionality as calculated from t h e s c a t t e r i n g data i s d i f f e r e n t f o r n e u t r o n s a n d X-rays. This difference is attributed to t h e presence o f 'occluded pockets' in t h e i n t e r f a c i a l r e g i o n in giving d i f f e r e n t e f f e c t i v e roughness factors f o r n e u t r o n s a n d X-rays. Spectroscopic a n d o t h e r methods O f t h e numerous techniques r e f e r r e d t o in t h i s volume a n d n o t discussed so far, special mention must b e made o f F T l R a n d NMR. These techniques may b e a p p l i e d in many d i f f e r e n t ways a n d f o r v a r i o u s reasons i n c l u d i n g s t u d y o f t h e p o r e s t r u c t u r e a n d t h e p r o p e r t i e s o f adsorbed o r occluded material. F T l R i s especially u s e f u l f o r t h e characterization o f surface species a n d t h e state o f adsorbed molecules. A s t u d y o f s u p p o r t e d o x i d e catalysts i n v o l v i n g F T l R a n d a d s o r p t i o n measurements i s r e p o r t e d by Rives a n d h i s co-workers a n d t h e use o f F T l R as a n a n c i l l a r y technique i s r e f e r r e d t o in several o t h e r papers. NMR measurements have been c a r r i e d o u t in t h e c o n t e x t o f image analysis (NMRI) o f p o r e s t r u c t u r e s (Ewing e t al), determination o f d i f f u s i v i t i e s o f adsorbed species (Bahceli e t al) a n d t h e p o r e s t r u c t u r a l analysis o f wet materials (Smith a n d Davis). Such techniques as F T l R a n d NMR have t h e g r e a t advantage t h a t t h e y impose v e r y l i t t l e p e r t u r b a t i o n o n t h e system. In c o n t r a s t techniques s u c h as thermoporometry may i n d u c e s t r u c t u r a l changes. T h i s method, which i s
9 based o n t h e relation between p o r e size a n d t h e f r e e z i n g p o i n t o f c a p i l l a r y condensate, i s f e a t u r e d in t h e paper by Quinson e t a l o n t h e t e x t u r e o f polycarbonate membranes. CONCLUSIONS AND RECOMMENDATIONS
It i s g r a t i f y i n g t o see t h a t t h e characterization o f porous solids i s now a t t r a c t i n g t h e attention o f many d i s t i n g u i s h e d mathematicians, scientists a n d technologists a n d t h a t steady p r o g r e s s i s b e i n g made in modelling t h e behaviour o f idealised p o r e s t r u c t u r e s a n d in a p p l y i n g new a n d improved experimental techniques. On t h e o t h e r hand, it is e v i d e n t t h a t a number o f It i s hoped t h a t t h e following general fundamental problems remain unsolved. recommendations w i l l help t o p o i n t t h e way f o r w a r d in p r e p a r a t i o n f o r COPS I l l . 1. E v e r y e f f o r t should b e made t o a p p l y experimental techniques in a complementary manner r a t h e r t h a n t o t e s t t h e r e s u l t s o f one p r o c e d u r e against those o f another. T h e main a t t r i b u t e s o f t h e most p o p u l a r techniques s u c h as gas adsorption a n d m e r c u r y porosimetry a r e already well k n o w n a n d it i s equally important t o recognise t h e i r limitations. 2. T h e r e i s an u r g e n t need f o r t h e f u r t h e r development a n d p r o d u c t i o n o f a range o f well-defined porous adsorbents a n d membranes. A t t e n t i o n should b e g i v e n t o t h e uniformity o f p o r e size a n d shape a n d t o mechanical a n d thermal s t a b i l i t y .
The lead t a k e n by Robens a n d o t h e r s in t h e d i s t r i b u t i o n o f information a n d reference materials should b e encouraged a n d s u p p o r t e d by the a p p r o p r i a t e national a n d i n t e r n a t i o n a l organizations.
3.
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F. Rodriguez-Reinoso et al. (Editors), Characterization of Porous Solids II 0 1991 Elsevier Science Publishers B.V., Amsterdam
11
SIMULATION OF ADSORPTION IN MODEL MICROPOROUS GRAPHITE
DAVID NICHOLSON Department of Chemistry, Imperial College of Science, Technology and Medicine, London SW7 2AY, U.K.
SUMMARY Grand ensemble Monte Carlo simulations have been carried out for Lennard-Jones models of Ar and N, in model graphite pores. The potentials were initially validated using plane surface experimental data for graphite. The pore model consisted of graphite 0001 planes separated by integer multiples of the graphite interplanar separation (D=0.3375nm) with H=3.0,4.0 and 5.OD where H is the C-centre to centre distance across the pore. Isotherm and isosteric heat curves are reported which support the view that intermolecular cooperative interactions are important in micropore filling. Zero coverage heats show little enhancement above the plane surface values, but cooperative effects produce quite substantial increases in pq,, at higher coverage. INTRODUCTION Nitrogen and Argon are both used extensively as adsorptives for the characterisation of porous materials, particularly at 77.5K. In spite of the ubiquity of such measurements, many fundamental questions remain to be answered in regard to these adsorption systems. Computer simulation can be a valuable technique to this end because it affords the opportunity to elucidate the behaviour of clearly defined model systems and relate this to experimental observation.
Provided that the
simulation can be based on reliable premisses, it may then be possible to restrict the types of model which are feasible, even though exact emulation of experimental observation may be difficult. Adsorption in micropores is of particular interest in this context because, whilst the importance of micropores is now widely recognised, it is difficult to obtain simple well characterised microporous adsorbents, of indeed to be confident that such materials have been discovered, given the difficulfy and complexity of interpretation. The present study is based on three assumptions: (i) Properly executed simulations give a valid account of the statistical mechanics of the system under investigation. (ii) Potential functions which are able to reproduce the main features of the bulk adsorptives and of adsorption on plane surfaces, can be carried over to pore models. (iii) Micropores in graphite are formed by separation of 0001 graphite planes held apart at integer multiples of the interplanar spacing in graphite (taken as 0.3375nm here).
12 The adsorptives were modelled using the 12-6 potential model with the parameters given in Table 1, where
E,,
o, and E
~
om . are the potential well depths and hard sphere diameters for the
adsorbate and adsorbate adsorbent potentials respectively. Clearly this is less satisfactory for N, than for Ar since, in spite of the fact that free rotation is to be expected to reduce the quadrupole effects in the bulk phase above 40K, it is not necessarily the case that this also applies to the adsorbate. This reservation seems to be borne out by the results reported below. Nevertheless it has proved valuable here to be able to compare two similar models which differ only in their molecular parameters. Because of its particular combination of molecular size and interactions the Ar-graphite system is difficult to model with precision at the temperature of interest. A useful touchstone for evaluating model potentials is provided by the liquid to incommensurate solid transition exhibited by this system at 77.5K. It is also of interest in the present context to know how porosity affects this transition.
TABLE 1. Properties of the model adsorptives a
(E,/k)/K
oJnm
Ar
120 95.2
0.3405 0.375
N,
62.5 52.1
2.0 2.0
0.96 1.0
0.4 1.23
For nitrogen a superficially similar transition is observed but here the larger size and weaker intermolecular interactions of the nitrogen molecules ensures that the adsorbate on graphite exists as commensurate structures near to monolayer coverage on graphite, and the transition is from a liquid
to a commensurate solid state. PORE MODEL AND SIMULATION METHODS The dimensions of the pore models studied are listed in table 2. Here H is the distance between C-centres in opposite planar pore walls, D is the separation of the graphite planes (taken as 0.3375nm) and H'=H-D. In keeping with the integer separation model adopted here it was assumed that the ABAB stacking of nonporous graphite would be maintained for the pore structure. It would of course be of some interest to examine this assumption further, especially for the smallest pore size. A notable feature of table 2 is the difference in the number of hard sphere diameters inside the pore for each adsorbate. In the 12-6 model the interlayer spacing in an fcc lattice is very close to o, so that whereas an integer number of layers of Ar is readily accommodated in this packing this is not the case for N,.
Schoen and co-workers (1) have noted interesting effects in the singlet distribution
functions arising from imperfect accommodation in pores of different widths whereby a new layer
13 gradually ‘squeezes in’ as the pore width is increased. The present work shows how this behaviour affects the shape of adsorption isotherms. In all the pore models studied a double minimum is retained in the adsorbent potential field, but overlap effects produce an attractive field even at the pore centre. The simulations were carried out in the grand (v,V,T) ensemble. The molecular interactions were cut off at the surface of a cylinder, with axis normal to the graphite planes, of radius 3.50, centred on a ’trial’ molecule. Long range corrections, using a mean field assumption (4) and frequently updated
TABLE 2. The Pore Model
II
H/D
H/nm H’/nrn H’/aA, H’/o,,
5.0 4.0 3.0
1.688 1.350 3.965 3.600 1.350 1.013 2.975 2.701 1.013 0.675 1.982 1.800
stacking
AB AA AB
II
densities, from the singlet distribution across the pore, were applied at each step. The validity of this procedure was verified by carrying out a few simulations with cutoff at 5.00. The pressure was calculated from the corrected chemical potentials assuming ideal gas behaviour for the vapour phase. Various initial configurations were investigated, including pores which had been filled at very high pressures; commensurate and incommensurate states and empty pores. Hysteresis was observed in the H=5.OD and in the H=3.OD pores for Ar, but not for N., observed at H=7.000,,
In previous work with N, hysteresis was
but not at H=5.000a, (=5.55D) (2,3). Once the existence of a stable and
reversible configuration had been established, subsequent runs were initiated by readjusting the pressure to the desired value and permitting filling or emptying to proceed until a new converged region was reached. The length of simulation run depended on pressure, degree of filling and initial configuration; averages were taken over at least 1 . 5 ~O6 1 configurations; uncertainties in isotherm points is ca 0.05% but can be as high as 5% in the q , values in the region of the maxima since these are calculated as fluctuations (4).
RESULTS FOR PLANE SURFACES The adsorbate-adsorbent potential was modelled on the basis of summation over 12-6 potentials using the familiar truncated Fourier expansion representation (5). The height of the surface barriers was modified, as described in detail elsewhere (6,7,8), by introducing a parameter h, such that 1=-1gives a smooth surface, and hzl raises the surface barriers by a factor of (l+h) compared to the corrugation from an unmodified 12-6 potential. A second adjustable parameter Q was introduced to allow for the possibility of repulsion between adsorbate atoms in the adsorbed layer adjacent to the
14 wall. A fuller discussion of these parameters has been given elsewhere (8); the values used in this work are summarised in table 1, li
Fig. 1. Adsorption on a planar 0001 graphite surface at 77.5K. Experimental data ( + +) and simulation results (...O...)for argon (left hand panel)and for nitrogen (right hand panel). Surface coverage, 8, is in units of close padted incommensurate monolayers for the argon and in u n i t s of c o m m e n s u r a t e monolayers for the nitrogen.
111
9 Ra
Fig. 1 shows the plane surface isotherms for the system studied. It is to be noted that whereas the experimental liquid-incommensuratesolid transition for Ar (9,lO) is well reproduced by the simulation, the strength of the liquid-commensurate solid transition in N, (10,ll) is greatly overstated. For the Ar simulation the potential was readjusted to compensate for corrugation effects so as to produce close agreement between the heats at zero coverage from simulation and from experiment (8). This was not done for N, since these heats were already high (pq,,(O=O)=14.1 from simulation compared to an estimated value of 13.1 from experiment). It is probable that improvement could be achieved for the N, by resort to a diatomic model with quadruples (12). The accurate reproduction of transitions over
a very small range of coverage affords a particularly stringent test of the potential functions and highlights the sensitivity of adsorption isotherms to changes in the interaction energy as noted in earlier work (7,8). ADSORPTION IN MODEL MICROPORES The adsorption isotherms for Ar, plotted as coverage versus pressure, are shown in Figs. 2, 3 and 4 and those for N, in Figs. 5 and 6. In this form comparison with the plane surface simulation can readily be made. It is clear that pore structure has less effect on the nitrogen adsorption than on the argon adsorption; for the latter the pore filling pressure is shifted by roughly an order of magnitude for each pore width. For N, the shift is approximately half of this, reflecting the weaker interaction between N, molecules compared to that for Ar (table l ) , and is one indication of the importance of cooperative interactions in pore filling. The Ar isotherm for H=5.OD (Fig. 4) has an essentially type IV character and follows the plane surface isotherm very closely up to 8-0.83 at this point it branches, the lower branch has a transition, similar to that observed on the plane surface, from a liquid-like to an extremely stable
15 1.o
0.8
-
0.6
-
0.4
-
8
0.03
Fig. 2. Ar adsorption in a model pore with H=3.OD, H’=l.980 at 77.5K. The inset shows the initial Henry’s law region. Coverage is in units of incommensurate monolayers on the plane surface
0.02
0.01
0.00 0.0 0.5 1.0
I
0.2
-
-
-
0.0
.,.
1.5 2.0 2.5 3.0
__________-___----0.5
0.0
1.0
1.5
2.0
2.5
I
a
3.0
3.5
Fig. 3. Ar adsorption in a model pore with H=4.OD, H’=2.97o at 77.5K. The dotted line shows the adsorption on a plane surface. Coverage is in units of incommensurate monolayers on the plane surface
2.5
I ~‘~/atnr
2.0
1.5
e 1.0
0.5
0.0
Fig. 4. Ar adsorption in a model pore with H=5.OD, H’=3.98o at 77.5K. The dotted line shows the adsorption on a plane surface. Coverage is in units of incommensurate monolayers on the plane surface
+ .....
e&..
P
1 , 5
f5
25
35
Io * P / ~ ~
45
16 solid-like monolayer, (as judged by the very low free energy calculated from the pressure virial (4)). There is then a transition to the filled state. The upper branch remains stable over a wide range of pressure. The thermodynamic transition pressure could be determined by Gibbs ensemble 'pore-pore' simulations (13). The N, isotherm in the H=5.OD pore is also type IV, the plane surface isotherm is again followed to the pre-transition coverage, where the adsorbate forms stable monolayers on the
two surfaces but no sharp liquid-solid transition occurs. In contrast to the Ar isotherm, adsorption beyond the monolayer then continues up to 8-1.2 before the whole pore fills. The departure from the plane surface behaviour at this stage is mainly attributable to an incipient layer in the centre of the pore which is manifested in the singlet distributions as a weak double maximum, whereas Ar shows singlet distribution function maxima only at the pore walls up to the point at which filling occurs. The differences in isotherm shape are therefore related to the inability of the pore to accommodate an exact number of fcc layers of N, within the pore width chosen for the model. A striking difference between these two isotherms is the complete absence of hysteresis for the nitrogen. This strongly suggests that hysteresis is largely an artefact of the simulation; its occurrence for Ar being related to the much stronger Ar-Ar interactions, and to the well-known inability of simulation to produce crystalline structures from disorganised starting configurations. For N, the interactions are much weaker and fcc structures within the pore are not possible.
Fig. 5. Adsorption of 12-6 nitrogen at 77.5 in model graphite pores. H=4.OD, H'=2.70 (0); H=5.OD, H'=3.& (A); plane surface (...O...)
Fig. 6. Adsorption of 12-6 nitrogen in a model graphite pore with H=3.OD, H'=l.80. The inset shows the initial Henry's law region.
17 In the H=4.OD pore the isotherms change their character, that for N, is close to type I, departure from the plane surface isotherm occurs when 8> 0.5 and there is again a remaining vestige The stronger interactions between Ar atoms lead to a much more of the monolayer transition at 8-1 .l. rapid rise above the plane surface adsorption in this isotherm (Fig. 3). No clear filling transition is apparent for either adsorbate at H=4.OD but the concave shape of the Ar isotherm to the 8 axis associated with adsorption well above the plane surface level is strongly indicative of the way in which pore structure acts to enhance the effects of intermolecular interactions. In the smallest pore studied (H=3.OD) a strong transition is observed (Figs. 4 and 6), but this is now a sub-monolayer transition akin to that which occurs at these low coverage in larger pores and on plane surfaces, but usually masked, as it is here for the H=4.0 and 5.OD pores, by the small pressure scale over which it can be resolved. This is illustrated by exhibiting the initial Henry's law region for the H=3.OD isotherms. Adsorption on the plane surface is now quite negligible over the pressure scale of these graphs. The Ar isotherm again exhibits a much sharper transition than that for N, and it is possible that a narrow hysteresis loop exists in this region. As before, no hysteresis loop was found for the N, isotherm where the transition is also much more gradual. The Ar isotherm fills to a complete monolayer between p-2.5~10.~ atm and 2 0 ~ 1 atm. 0 ~ Adsorption occurs even more .~ slowly above the transition for N, than for Ar and is not complete until ~ - 1 0 atm. At the next pore size (H=2.OD), according to the model investigated here, potential overlap is such that Ar is only weakly attracted, and N, is excluded under normal pressures. It is worth noting that a similar model, based on expanded graphite spacings (14), would accommodate both adsotptives. The isotherms can alternatively be displayed as fractional filling plotted against relative pressure. The relative pressure at which pore filling occurs is similar for both adsorptives for the H=3.OD and H=4.OD pores, being approximately 7 ~ 1 0and . ~ 2 ~ 1 0respectively. .~ At H=5.OD however the relative pressures of the filling transition are O.O2(N,) and 0.002(Ar). There is ambiguity about the definition of fractional filling because of the uncertainty concerning the state of the adsorbate. Argon
liq. 1.05 1.13
sol.
liq.
sol.
liq.
sol.
0.909 1.03
0.894 1.04
0.903
0.902 1.21
0.965 1.22
0.968
to be solid-like, both because of the temperature (T*=0.67) and because of the very nearly exact is expected accommodation of the fcc lattices. For N, the Situation is less clear, the liquid state would seem to be the more probable at this temperature, however singlet distribution functions (Fig. 7) show that ordered layering occurs, suggesting that a solid-like state exists within the pores, even though the
18 layers are not always complete. The values of fractional filling, W/W, within the filled pores, show that neither the liquid, nor the solid hypothesis is entirely satisfactory: solid densities are not reached, but densities are in excess of those of the bulk liquid - especially for N., No significant trend with pore size is apparent in table 3 for Ar, but the N, density increases in the DNo smaller pores.
'I 6 .
Fig. 7. Singlet distribution functions for n-n at a pressure of 0.02 atm (P/PO=0.016). The full line is for the H=5.OD pore, the p ( l ) triangles for the plane surface.
,
2 .
0 .
I
-2 0.5
t.0
1.5
20
25
30
3.5
I
4.0
z/a
The isosteric heat curves are shown in Figs. 8 and 9. The plane surface curves exhibit characteristic maxima with cusped minima near to the transition, as observed in experiment and 40
35-
7
25
5
30-
3:
. I
25
.
lo ~
--
*.-*.-*
f
20
% -
Fig. 8. lsosteric heats of adsorption for argon in model graphite pores at 77.5K plotted against coverage, 8. H=3.OD (v);H=4.OD (0); H=5.OD (A); plane surface ( 0 )
10
Fig. 9. lsosteric heats of adsorption for 12-6 nitrogen in model graphite pores plotted against coverage (commensurate monolayer units). Symbols as in Fig. 8.
19 discussed elsewhere (7). Up to a coverage of 8=0.8,the heats for the 5.OD pores show only minor departure from the plane surface data, but the maximum is stronger. In the 4.OD pore initial slopes are noticeably steeper; it is likely that maxima also occur here but if so they were not resolved in these simulations. The increasing importance of intermolecular interactions is demonstrated more clearly in the heats for the smallest (H=3.OD) pores which have high initial slopes and are displaced well above the plane surface curve. The enhancement of the initial heat (Sq,,(e=O)) is small in the H=3.OD pores and negligible for the larger pores (table 4) TABLE 4. lsosteric heats of adsorption at zero coverage Plane HID= surface
5.0
4.0
3.0
Ar enhancement
15.03
15.2 1.01
15.50 1.03
17.00 1.13
N* enhancement
14.10
14.28 1.01
14.62 1.04
16.35 1.16
DISCUSSION AND CONCLUSIONS The isotherms and heat curves reported here differ in many respects from those normally associated with experimental results (14,l 5) from adsorption in graphite micropores. Typically these give fairly smooth type I isotherms, and heat curves which decrease rapidly from an initial maximum; the latter may show inflections but do not have maxima at high filling.
One reason for these
differences may be the difficulty in making measurements at sufficiently high resolution, even with present day equipment; another could stem from the inevitability of pore size distributions in experimental materials. The model examined here also suffersfrom several defects and uncertainties,
thus, even if the basic tenet of integer spacings is accepted, there is uncertainty about the graphite plane spacings (14) and the role played by graphite edge planes which could be very significant, as could wedge rather than parallel geometry. Nevertheless a number of observations may be made which have consequences both for future simulation studies as well as for the interpretation of experimental data: (i) The simulation results emphasise again (7,8)the extreme sensitivity of isotherms (plotted in the usual way as adsorption versus pressure) to small modifications in the interaction potentials. Qualitative differences,such as sharpness of a transition as well as quantitative differences may result from such changes. (ii) The mechanism of micropore filling is responsive to these changes in two ways: firstly even quite a small increase in the potential at a wall, due to overlap from the potential at the opposite wall
20 significantly alters the adsorption at a given pressure; this effect is amplified by the consequent increase in the adsorbate intermolecular field acting initially in a lateral direction. In the present work this is clearly seen in the difference between the 5.OD and the 4.OD pores; in the former overlap effects are insufficient to perturb the normal monolayer formation process, it is only when second layer adsorption begins that the influence of pore structure is evident; at this stage second layer molecules from opposite pore walls can interact strongly with each other and pore filling occurs. In the smaller pores both overlap effects and adsorbate interactions from the opposite wall can occur simultaneously and reinforce one another leading to a cooperative process. These phenomena are manifested in the change in initial slope and final maximum of the differential enthalpy curves, especially those for the smallest pore. It is possible that some of this 'intermolecular enhancement' is seen in experimental data for real materials with size distribution as high initial isosteric heats; if so its presence would be difficult to distinguish from surface heterogeneity enhancement. In any case no other feature of the present model can account for the high initial heats observed experimentally. (iii) Differences between Ar and N, as a probe are shown to be manifested in ways other than mere size effects. These come about firstly because the cooperative effects referred to above are magnified as E increases and secondly because of the more subtle influence of perfect or imperfect accommodation of the molecules by the pore. ACKNOWLEDGEMENTS I wish to thank the University of London Computer Centre for a generous allowance of computer time and Dr. N.G. Parsonage and Prof. W.A. Steele for helpful discussions.
REFERENCES 1 2 3 4 5 6 7 8
9 10 11 12 13 14 15 16
M.Schoen, D.J.Diestler and J.H.Cushman, J.Chem.Phys. 87, 5464 (1987). J.P.R.B.Walton and N.Ouirke, Mol. Simulation. 2,361 (1989). N.A.Seaton, J.P.R.B.Walton and N.Quirke, Carbon, 27, 853 (1989). D.Nicholson and N.G.Parsonage, "Computer simulation and the statistical mechanics of adsorption", p.97, Academic Press (London,New York) (1982). W.A.Steele, Surface Sci. 82, 817 (1973). D.Nicholson, L.A.Rowley and N.G.Parsonage, Mol. Phys. 44,629, (1981). D.Nicholson and N.G.Parsonage, J. Chem. SOC..Faraday Trans.2, 82, 1657 (1986). D.Nicholson, R.F.Cracknell and N.G.Parsonage, Mol. Simulation, in press (1990). Y.Grillet, FRouquerol, J.Rouquerol, J. Chim. Phys. 2,179 (1977), J.Coll. and Interf. Sci. 70, 239, (1979). Y.Lahrer, J.Chem. Phys. 68,2257, (1978). D.M.Butler, G.B.Huff, R.W.Toth and G.A.Stewart, Phys. Rev. Lett. 35,1718, (1975). J.Talbot, D.J.Tildesley and W.A.Steele, Mol. Phys. 1331 (1984). A.Z. Panagiotopoulos. Mol. Phys. 62,701, (1987). K.Kakei, S.Ozeki, TSuzuki and K.Kaneko, J. Chem. SOC.Faraday Trans. S, 371 (1990). K.Kaneko, T. Suzuki, K. Kakei, Langmuir, 5,879,(1989). D.Atkinson, P.J.M.Carrot, Y.Grillet, J.Rouquerol and K.S.W.Sing, Fundamentals of Adsorption p.89, ed. A.I.Liapis, (Engineering Foundation, New York) (1987).
s,
21
F. Rodriguez-Reinoso et al. (Editors), Characterization of Porous solids II 0 1991 Elsevier SciencePublishersB.V., Amsterdam
THEORY OF ADSORPTION IN MICROPORES Ziming Tan and Keith E. Gubbins School of Chemical Engineering, Cornell University Ithaca, New York 14853, U.S.A. ABSTRACT We test three theories for adsorption and capillary condensation in pores against computer simulation resulcs. They are the Kelvin equation, and two forms of density functional theory, the local density approximation (LDA) and the (nonlocal) smoothed density approximation (SDA); all three theories are of potential use in determining pore size distributions for mesoporous solids, while the LDA and SDA can also be applied to microporous materials and to surface area determination. The SDA is found to be the most accurate theory, and has a much wider range of validity than the other two. The SDA is used to study the adsorption of methane and methane-ethane mixtures on models of porous carbon in which the pores are slit-shaped. We find that an optimum pore size and gas pressure exists that maximizes the excess adsorption for methane. For methaneethane mixtures we show the variation of selectivity with pore size and temperature. INTRODUCTION Although adsorption data and mercury porosimetry are widely used to characterize porous materials [l], the classical methods for interpreting such data rely on equations that are more than 40 years old, and are of uncertain validity, particularly for micropores and small mesopores.
The most important
of these equations are those of Brunauer, Emett and Teller (BET), Kelvin, and Dubinin and Radushkevitch (DR) and their modified forms [l]. The BET equation neglects adsorbate-adsorbate interactions, heterogeneity of the surface, and variations in properties of adsorbed layers after the first; nevertheless, it usually gives a good account of low pressure adsorption, especially for nonporous materials.
The Kelvin equation assumes (a) the vapour phase is ideal, (b) the
liquid phase is incompressible,with a molar volume that is negligible compared to the gas, and (c) the system is large enough for the surface tension to be a useful concept.
Assumptions (a) and (b) will lead to significant errors at
higher temperatures, especially as the capillary critical point is approached, while approximation (c) will lead to increasing errors as the pore size decreases.
Thus, molecular dynamics simulations of small drops of Lennard-
Jones molecules [ 2 ] have shown that the surface tension
-y
departs significantly
from its bulk liquid value for drop diameters below about 140 (u- molecular diameter), and for drop diameters below 7-8u surface tension ceases to have any
22
meaning (e.g., the various thermodynamic equations involving -y are inconsistent). A similar breakdown occurs in using the Kelvin equation for pores whose diameters are in this region [ 3 1 . The DR equation introduces a single adjustable parameter to characterize the pore-fluid system, and is essentially empirical in nature. Statistical mechanics provides a more reliable and general approach to interpretation of adsorption and porosimetry experiments. At the present time the two most promising approaches are density functional theory and direct molecular simulation (Monte Carlo or molecular dynamics).
The simulation
approach [ 4 ] has the advantage that the statistical mechanical equations are solved exactly for the prescribed model of the pore geometry and intermolecular interactions; it is relatively easy to incorporate surface structure and heterogeneity and a variety of pore geometries and irregularities. The principal disadvantage is cost; a simulation for a single state point usually takes one to several hours on a fast computer.
The density functional theory [ 5 ]
calculations are faster by about one to two orders of magnitude, and provide both more detailed insight and higher accuracy than the classical methods currently in use.
Two principal forms of the theory exist, a local and a nonlocal form
(these terms are defined in the following section), the nonlocal form being the more accurate. The nonlocal theory gives a good description of adsorption and phase transitions in slit pores of all widths, and of cylindrical pores for pore radii down to about 1.60; it describes all six classes of isotherms [l], including step-like ones (class VI), and is good for both subcritical and supercritical temperatures. Its principal limitations are its failure for very narrow cylindrical pores (it does not predict the correct one-dimensionallimit), and its failure to predict the solid-liquid transition for the adsorbate. It has
so
far only been applied to pores of simple geometry having smooth
structureless walls; for more complex pores it is not yet clear whether the theory will offer major advantages over direct simulation methods.
The local
form of the theory has been used recently by Seaton et al. [ 6 ] to obtain pore size distributions from nitrogen isotherm data on carbons. In this work, following a brief description of the density functional theory (Sec. 2 ) ,
we report tests of the theory and of the Kelvin equation against
computer simulation results (Sec. 3 ) .
We also describe (Sec. 4 ) an application
of the density functional theory to the adsorption of methane and methane-ethane mixtures in model carbon pores. MEAN FIELD DENSITY FUNCTIONAL THEORY The fluid-in-pore system is treated as an inhomogeneous fluid at fixed temperature T and chemical potential solid walls of the porous material.
p
in an external field v(r) exerted by the
For this choice of independent variables,
(p,T,V), the appropriate free energy that must be minimized at equilibrium is
23
the grand potential, 0
=
-pV+rS, where p is pressure and S is surface area.
The procedure in density functional theory [S] is to introduce a grand potential functional n[p(r)]
that has the properties that it is uniquely defined once the
density profile in the porous material, p(r),
is defined, and has its minimum
value at equilibrium. We must now write an approximate expression for n[p(r)], and minimize it with respect to p(r) to find the equilibrium density profile. In the mean field approximation, the grand potential functional can be written in the form [3,5,7,8]:
The first two terms on the right side of this equation represent the contribution to the Helmholtz energy due to the short range repulsive intermolecular potential between the fluid molecules, the third term is the corresponding contribution to the Helmholtz energy due to the long range intermolecular potential, ulong(r), in the mean field approximation (setting the pair correlation function in this term equal to unity), and the last term is the contribution from the external field v(r) due to the solid. In the first two terms a(r) is the Helmholtz energy density at the point r in the pore, aid being the ideal gas part and acon the (excess) configurational part due to (repulsive) intermolecular forces.
The
ideal gas part is exactly local, i.e. it can be calculated as the Helmholtz energy density of a uniform fluid whose density is the same as that of the nonuniform fluid at the point r , i.e. p(r).
The second term on the right
contains aconrwhich is nonlocal; i.e. it depends not only on the local density p ( r ) at r in the pore, but also on the density at neighboring points around r. Much attention has been paid to this term in the last few years by theorists,
and current theories can be divided into two forms: (a) the local density approximation (LDA), in which aeon is treated locally, i.e. to the local density p(r);
-p
is simply set equal
and (b) a nonlocal smoothed density approximation
(SDA), in which acOn is calculated as the value for a uniform fluid whose molecules interact with repulsive forces only, andwhose density is some smoothed value p(r).
In both the LDA and SDA this uniform fluid of purely repulsive
molecules is approximated by a fluid of hard spheres of diameter d.
This
approximation is known to be quite accurate, provided d is chosen suitably; often the Weeks-Chandler-Andersenformula is used [9]. In the SDA the smoothing of the density around the point r of interest is intended to account for the effects of the large density gradients that exist in small pores, and is found to work well provided the recipe to calculate
7
is chosen to give an accurate account
of the properties of the uniform fluid. Several such recipes exist [7]. In our
24
work we have chosen to use the one due to Tarazona [ 8 ] , in which 7 is calculated by comparing the first few terms in the virial expansion of the direct correlation function for hard spheres with those from the known Percus-Yevick result. The resulting theory is both tractable and reasonably accurate. It has been extended to mixtures by Tan et al. [lo]. COMPARISON OF THEORIES AND SIMULATION We first compare our SDA results for the excess adsorption per unit of surface area with the grand canonical Monte Carlo (GCMC) simulation results of van Megen and Snook (vMS) [ll]. Calculations were carried out for a LennardJones (LJ) fluid with parameters modeling ethylene in a slit-like carbon pore with a 1 0 - 4 - 3 potential for the solid-fluid potential [12] (see next section). The excess adsorption per unit area, rs, is defined as
where
p(z)
width.
is the density profile, pb is the bulk density, and H is the pore
In Fig. 1 is shown ps
(supercritical) and H* (H/u,)
- 5.
(r$J
Here u1 and
c1
-
s
=
1.35
are the LJ parameters for
ethylene. An adsorption isotherm for a subcritical temperature, T" H"
-
vs p*b ( ~ ~ for 2 ~T*) (kT/a,)
=
0.95, and
10 is shown in Fig. 2. In the SDA results in Figs. 1 and 2 a temperature
I
T' = 1.35
1
0 GCMC (this work)
I
01 0
A GCMC (van Megen and Snaak)
o GCMC (van Megen and SnOOk)
- SDA
- SDA
0 .I
0.2
0.3
0.4 I
0
I
0
0.2
4
I
0.4
0.6
0.8
1.0
P/P"
Fig. 1 Adsorption isotherm for ethylene in carbon pores at a supercritical temperature, ' T 1.35.
-
Fig. 2 Adsorption isotherm for ethylene in carbon pores at a subcritical isotherm, T* = 0.95. The metastable regions predicted by the SDA are included.
25 dependent hard sphere diameter was used [12]. Good agreement between the SDA and computer simulation is found in both cases. In Fig. 3 , we compare the LDA, SDA, and Kelvin equation with molecular dynamics (MD) simulation results for LJ
fluids in cylindrical pores using the results of Peterson et al. [3]. The Kelvin equation is
where po is bulk fluid vapor pressure, y and vL are the surface tension and molar volume of the liquid, N is Avogadro's number, k is Boltzmann's constant, and R is pore radius. Calculations were carried out for a LJ fluid with Ar parameters in a cylindrical pore with a CO, solid wall. F o r the temperatures shown, the SDA results are in reasonable agreement with the simulation.
The LDA gives
noticeably poorer predictions, but better than the Kelvin equation in general. The Kelvin equation is much poorer at the higher temperature, as expected. We note that in the SDA calculations shown in Fig. 3 (taken from Peterson et al. [3]) the hard sphere diameter was taken to be independent of temperature; somewhat better results are to be expected if the temperature dependence is accounted for [12]. RESULTS FOR ADSORPTION IN CARBON PORES We report here results for LJ fluids and mixtures in a model pore of slitlike geometry.
Following our earlier work [12,13,14],the fluid-fluid pair
interaction was described by a cut-and-shiftedLJ potential. The fluid potential parameters [15] chosen to model methane (1) and ethane (2) were: u1
=
cl/k = 148.1K,
0.381nrn
u2
=
0.395nrn,
c2/k
=
243.0K
The 10-4-3model was used for the solid-fluid potential [15]:
with parameters modeling a carbon graphite surface [15]: us
=
0.340nm,
a,/k
=
28.OK,
A
=
0.335nm,
ps
=
114nn1-~.
26 I/' i ; T' = 0.7
R'
Fig. 3 Capillary condensation conditions for IJ Ar in a GO2 cylinder at T* 0 . 7 (left) and 0.85 (right).
-
The cross-parameters, osf and e s f , were calculated using the Lorentz-Berthelot rules. Pure Methane We have examined the excess adsorption per unit of volume of pure methane, which is defined by
In Fig. 4 are displayed the excess adsorption isotherms,
r,,
for H
=
1.9 nm at
temperatures T from 200 K, which is near the bulk fluid critical temperature, to 296 K.
Each isotherm exhibits a maximum. The isotherms are very similar in
shape for the temperatures shown. for We are particularly interested in how the maximum excess adsorption, rvm, versus H. an isotherm varies with pore width, H. In Fig. 5 , we have plotted rvm The results show that for each temperature there is an optimum pore width that maximizes the adsorption. At H" = 1 . 6 4 , rvm falls to zero. For pore sizes below this value we found no adsorption in the pore. Methane/Ethane For the binary mixture, we focussed on the selectivity of component 2 (ethane) relative to component 1 (methane), which is defined as
21
1.5
I.o
0.5
L’ -60
0
Fig. 4 Adsorption per unit pore volume; methane in carbon pores for H = 1.9 nm.
..-- -
I II I 0-
I
I
I
I
z96 K
-- ----__
I
I
Fig. 5 The maximum excess adsorption of methane in carbon pores, rm, vs H.
6.0
,'
H+
I Z - ;1. ' ~ 3
T' = 2.0 (296)
' ,
: e(30.56)
.I , Yb,CH4 = Os ! '"'..~ '\ I ii' ....... --._ ; -._-.. ! 8 - ;: ,. -, \'....... '.C '\
-- - _ -237K -_
'.
I
....... .......
;1:
..__ .............
i!
s
.;I'
I
i /
......252..........
- --- . - -.-. ....................... -._ -
--- -- --- - - - - -26'-
-226-
I
4-,
--
326 3.5 -
I
20
40
60
30
40
Fig. 7 Selectivity of ethane at fixed bulk mole fraction for H 3.05 nm for several temperatures.
Fig. 6 Selectivity of ethane versus pressure at T 296 K and H 3.05 nm for several bulk gas compositions.
-
20
10
0
-
-
where ybl is the mole fraction of methane in the bulk phase, and x1 is the overall mole fraction of methane in the adsorbate phase, given by
In Fig. 6 are displayed the selectivity versus the bulk pressure p for different bulk mole fractions for a fixed pore size, H
=
3.05nrn.
For a solution with high
concentration of methane, (e.g., ybl = 0 . 9 at this temperature), the isotherm passes through a maximum and levels off as the pressure increases.
This type
of S-p isotherm is typical for a fluid at supercritical conditions (we note that the capillary critical point depends on the bulk mole fraction).
For results
at low concentration of methane (e.g.,ybl= 0.1 at this temperature), however, the isotherm exhibits a second maximum.
This type of isotherm seems to occur
when the temperature is near the capillary critical point. The result shown in Fig. 6 is for a rather large pore.
For the ybl values
indicated above, the bulk critical temperatures are 301, 2 6 2 , and 206 K, respectively. The corresponding capillary critical points are shifted to lower values as the pore size is decreased [ 3 ] .
We therefore expect that at a smaller
H value, e.g., 1.0 nrn, S-p isotherms for T
=
296
K will fall into the first type
discussed above for most yhl values.
Fig. 6 Selectivity of ethane versus pressure at T 296 K and H 3.05 nm for several bulk gas compositions.
-
-
Fig. 7 Selectivity of ethane at fixed bulk mole fraction for H 3.05 nm for several temperatures.
-
29
In Fig. 7 we show isotherms for several temperatures from supercritical to sub-critical at fixed bulk mole fraction and pore size. At high temperatures (296 and 326 K) the isotherms are of the first type, passing through one maximum. As the temperature is lowered (252 and 267 K) , the second maximum develops, showing the second type of isotherm. Finally at sub-critical temperatures (237 K) a gas-liquid phase transition occurs. CONCLUSION The SDA form of density functional theory gives generally good results, particularly when the temperature dependence of the hard sphere diameter is accounted for.
It is much superior to both the LDA and the Kelvin equation,
giving a good description of slit-like pores
for all pore widths and
temperatures, and of cylindrical pores for radii down to about 1 . 6 ~ .The Kelvin equation fails for small pores and also for higher temperatures, especially near the capillary critical point.
The SDA should provide a powerful tool for
interpreting adsorption data to characterize pore size distributions and surface area. Since it can describe the whole isotherm over a wide range o f temperatures (in contrast to the Kelvin equation) it should allow a more complete and reliable characterization of porous materials. The calculation of the supercritical adsorption of LJ methane in carbon pores suggests that there exists an optimum pore size for methane adsorbed in porous carbon. At a fixed temperature, the maximum excess adsorption per unit of pore volume passes through a global maximum for a particular pore width.
The
selectivity isotherm for methane-ethane mixtures shows different shapes when the temperature changes. At high temperatures, it passes through a maximum. When the temperature is near the capillary critical one, a second maximum appears. As the temperature is further lowered, phase transition occurs. ACKNOWLEDGMENT We thank the Gas Research Institute and National Science Foundation (grant no. CTS-8914907) for support of this work.
30 REFERENCES
10
11
12 13 14
15
S.J. Gregg and K.S. W. Sing, Adsorption, surface area and porosity, Academic Press, Landon (1982). S.M. Thompson, K.E. Gubbins, J.P.R.B. Walton, R.A.R. Chantry and J . S . Rowlinson, A molecular dynamics study of liquid drops, J. Chem Phys. 81, 530 (1984) . B.K. Peterson, K.E. Gubbins, G.S. Heffelfinger, U. Marini Bettolo Marconi and F. van Swol, Lennard-Jones fluids in cylindrical pores: Nonlocal theory and computer simulation, J. Chem. Phys., 88, 6487 (1988). M.P. Allen and D.J. Tildesley, Computer simulation of liquids, Clarendon Press, Oxford (1987). R. Evans, The nature of the liquid-vapour interface and other topics in the statistical mechanics of non-uniform, classical fluids, Adv. Phys., 28, 143 (1979). N.A. Seaton, J.P.R.B. Walton and N. Quirke, A new analysis method for t h e determination of the pore size distribution of porous carbons from nitrogen adsorption measurements, Carbon, 27, 853 (1989); C.A. Jessop, S.M.Ric?diford, N.A. Seaton, J.P.R.B. Walton and N. Quirke, The determination of the pore size distribution of porous solids using a molecular model to interpret nitrogen adsorption measurement, paper presented at IUPAC Symposium on Characterization of Porous Solids, Alicante, Spain, May 6-9, 1990. C.G. Gray and K.E. Gubbins, Theory of molecular fluids, Vol. 2, Ch. 8, Clarendon Press, Oxford, in preparation (1991). P. Tarazona, Free-energy density functional for hard spheres, Phys. Rev. A, 31, 2672 1985); some of the equations in this paper are incorrect; corrected versions are in P. Tarazona, U. Marini Bettolo Marconi and R. Evans, Phase equilibrium at fluid interfaces and confined fluids: Nonlocal versus local density functionals, Mol. Phys., 60, 573 (1987). J.D. Weeks, D. Chandler and H.C. Andersen, Role of repulsive forces in determinig the equilibrium structure of simple liquids, J. Chem, Phys., 54, 5237 (1971). Z. Tan, U. Marini Bettolo Marconi, F. van Swol and K.E. Gubbins, Hardsphere mixtures near a hard wall, J . Chem. Phys., 90, 3704 (1989). W. Van Megen and I.K. Snook, Physical adsorption of gases at high pressure, I. The Critical Region, Mol. Phys., 45, 629 (1981); Physical adsorption of gases at high pressure 111. Adsorption in slit-like pores, Mol. Phys., 54, 741 (1984). Z. Tan and K.E. Gubbins, Adsorption in carbon micropores at supercritical temperatures, J . Phys. Chem., 94, 6061 (1990). 2 . Tan, K.E. Gubbins, F. van Swol and U. Marini Bettolo Marconi, Mixtures confined to narrow slit pores: Computer simulation and theory, Proc. Third Internat. Conf. on Fund. Ads., Sonthofen, FRG, in press (1990). 2 . Tan, F. van Swol and K.E. Gubbins, Lennard-Jones mixtures in cylindrical pores, Molec. Phys., 62, 1213 (1987). W.A. Steele, The physical interactionof gases with crystalline solids, Surf. Sci., 36, 317 (1973); The interaction of gases with solid surfaces, Pergamon, Oxford (1974).
F. Rodriguez-Reinoso et al. (Editors), Characterization 0fPorou.s Solids II 1991 Elsevier Science Publishers B.V., Amsterdam
31
SORPTION OF GASES ON MICROPOROUS SOLIDS: PORE SIZE CHARACTERIZATION BY GAS SORPTION STEVEN W. WEBB and W. CURTIS CONNER, Department of Chemical Engineering, University of Massachusetts, Amherst, MA, 01003 USA ABSTRACT Characterization of micropores in zeolite crystals may be performed by automated, dynamic, high resolution adsorption. Of the systems considered only nitrogen over ZSM-5 silicalite at 77 K shows an anomalous hysteresis/transition in the micropore sorption isotherm. The presence of aluminum, steaming of the zeolite or thc use of argon @ 77 K or C02 @ -6O'C eliminates the transition and hysteresis. Framework aluminum tends to reduce pore volume and to broaden the pore size distribution. Steaming reduces pore volume, broadens pore size and generates a significant amorphous phase, presumably largely aluminum. The measured pore volume, but not necessarily the pore dimensions, depend on the equilibration time during adsorption. INTRODUCTION It is well known that zeolites enhance selectivity based on the size of their intracrystalline pores. Zeolite crystals exclude or capture molecules based on the ratio of molecule size to pore size. Measurement of pore size by crystal size (e.g., X-ray diffraction) fails to account for the influence of the dynamics of the crystal structure, the dynamics of the sorbing molecules or the interaction between zeolite pore and sorbed molecule. The crystals and/or sorbed phase after sorption may be structurally different from the bulk phase and/or unfilled zeolite. The pore sizes determined in an X-ray analysis may be different from those present during sorption. Thus, it is preferred to study zeolite morphology by a combination of structural and sorption analysis. In this manner, it is possible to study both the state of the zeolite crystals and the state of the sorbed phase and to infer how these influence the amount of sorption of gas phase molecules and the effective micropore size. Solids characterization may be performed by Si29 NMR or XRD. With Si29 NMR, changes in the crystal dimensions due to sorption have been observed (refs. 1-3). Sorbed phase characterization can be studied by volumetric sorption. Thermodynamically simple molecules (e.g., spherical and small) at low temperatures arc used to study pore volume and sizes. A novel, automated, low pressure sorption instrument (Omicron Tech., Berkcley Heights, NJ, USA) has greatly simplified the analysis of zeolites. The technique has recently been named High Resolution Adsorption (HRADS). PORE SIZE CHARACTERIZATION BY HRADS Pore sizes are determined by the amount of sorption of a gas at pressures far less than its saturation pressure. At equilibrium, the free energies of the gas and pore-condensed phase must be equal. The free energy of the sorbed phase depends on the surface energy and the interaction between the pore walls and sorbed molccules. The free energy of the gas is related to its pressure. Sorption in micropores requires very low relative pressures, p/po, attained either with low partial pressures or with high PO. Experimentally, either case proves more difficult than normal mesopore adsorption since quantitative high vacuum is difficult to control and at high po increasing the total amount of sorption is reduced. Also, at low pressures heat transfer is poor and maintcnance of thcrmal equilibrium becomes more difficult. The use of an automated instrument overcomes some of these problems.
Adsorption may c o n f i i the pore size distribution (PSD) and the pore types in the zeolite crystal identified by XRD. However, the crystalline morphology of zeolites during adsorption is not static. The framework may flex or deform with accumulation of a sorbed phase. Structural alterations in the crystal due to sorption have obvious implications for the performance of zeolites in separation/catalysis. Sorption in pure ZSM-5 (i.e., no amorphous phase) is Type 1. However with some adsorbents, transitions (i.e., abrupt change in isotherm slope) and hysteresis, (i.e., sorption and desorption isotherms which do not overlay) have been observed over ZSM-5. The cause of hysteresis and transitions may include, 1) Sorbent: a configurational relaxation of the sorbed phase which depends on the concentration of sorbed molecules; 3-D or 2-D "freezing" or surface phase splitting which alters the density of the sorbed phase. 2) Crystal: a crystallinerearrangement, flexing, or swelling of the crystal structure. 3) Structural: a multimodal PSD or pore network effectsrelated to pore fillinglemptying. Hysteresis in micropore sorption was originally claimed to be due to swelling of the micropores that traps sorbed molecules (ref. 4). Evidence for pore swelling was found (ref. 5) by observing hysteresis in particle volume, conductivity, as well volume adsorbed. This early work was done with microporous carbon, a soft, amorphous solid. With rigid oxide crystals, swelling is not be expected. However, at large length scales the crystal network can flex resulting in distortion of the pore shapes and effectivesizes. Flexing of zeolite crystals has been observed in recent Si29 NMR results (refs. 2-3). Zeolites can undergo crystalline rearrangement. For instance, high silicon ZSM-5 and ZSM-11 undergo a monoclinic-orthorhombic transition at temperatures of 67 and 47°C respectively (ref. 3). Crystalline rearrangement during sorption will alter the pore sizes and could lead to an abrupt transition in the isotherm. However, this effect is equilibrium driven and should occur at a single point (pressure and temperature). Therefore, while a transition in the isotherm can be explained by a crystalline rearrangement, hysteresis cannot. Molecular rearrangement in the gas phase is essential to configurational diffusion in the confined pore space. If it occurs fast, relative to the rate of change of pressure, pore diffusion will be fast and not limiting. If it is slow, then equilibrium will not be maintained and the data will be meaningless. Very low pressures (P), large molecules (low Dconfigurational) and large crystals (Rcrystal) will require very slow pressure changes. For this reason, static sorption (dP/dt = 0) and finite equilibration times must be used for zeolite sorptions.
Pressure changes must also be much slower than the phase transformation rate. If pressure is increased too quickly, non-equilibriumcondensation (i.e., spinodal) may occur. The sorbed phase will be of lower density than is thermodynamicallyfavored. This effect may be due to insufficient freedom in the micropores to permit facile molecular relaxations necessary to create an equilibrium sorbed phase within normal observable times. At some degree of excess pressure (pressure deviation from the thermodynamic phase transition), the metastable condensed phase may relax. Relaxation of molecules in the sorbed phase may cause a change in density which will abruptly alter the micropore filling and produce a transition. For instance, during increasing pressure the density of the sorbed phase may abruptly increase as the sorbed phase goes from a metastable "liquid" to a more stable "ice". During desorption, an amount of under pressure will be required to reform the "liquid phase. Molecular relaxation can be so constrained in the narrow zeolite pore that a condensed, non-equilibrium phase is "frozen" in the pore. This type of transition is analogous to a spinodal decomposition; a spontaneous rearrangement of non-equilibrium phases to stable equilibrium which occurs at a certain amount of excess pressure. Spinodal rearrangement is facilitated by nucleation. For highly localized sorption (e.g. H-bonding or aluminum
33
sites in the lattice), rearrangement may be so fast as to preclude spinodal effects and eliminate the observed transition. Freezing of condensed nitrogen and argon has been reported (refs. 5-6) by microcalorimetric studies over graphitized carbons. Abrupt transitions in sorption isotherms were coincident with increases in the isosteric heat of adsorption. A surface phase transition is claimed. A transition during sorption of p-xylene was observed over ZSM-5 at 70°C by Olsen (ref.7) which they also ascribed to an ordered packing of sorbed molecules. As this represents a ciensification of adsorbed species compared to the liquid, it should be a distinct phase and there would be an associated phase transition for its forrnation.either from the gas or from the liquid states. The spinodal phenomena could explain both hysteresis and the transition. The effect should be experimentally observable if the relaxation time is similar to the experimental times. For zeolite powders, gas phase transport times will be of order 0.1-10 seconds, far less than experimental static equilibration times (> 10 minutes). Sorbed phase relaxation times are unknown but may be large enough to be observed. SORFTION IN ZEOLITES: QUANTITATIVE THEORY Sorption in ZSM-5 particles using nitrogen at 77 K was reported by Unger and Muller (ref.8). Hysteresis was observed at a relative pressure of around 0.1 and spanned a pressure range of 0.05 p/po. At this pressure, most of the micropores are filled. Since the crystals were large, the effect of interparticle surface sorption is minimized. They found that hysteresis was sensitive to three factors: 1) Aluminum and/or cation content: only silicalite (Si/Al> 500) showed hysteresis.
2) Tempcrature: higher temperature (90 K) caused the transition to move to a lower relative pressure.
3) Polarizability of the sorbing gas: argon has no permanent dipole and showed no hysteresis. Venero and Chiou (ref. 9) measured sorption isotherms over ZSM-5, CaA and NaY zeolites using both nitrogen at 77K and argon at 86K. They found that argon gave more accurate predictions of pore sizes of physical mixtures of zeolites than nitrogen. Sorption in micropores can occurs by condensation (refs. 10-11). rather than by multilayer physisorption. Condensation in pores less than 20 8, corresponds to less than 5 sorbent molecules between the pore walls. The Kelvin model is inappropriate for modelling sorption since an equilibrium phase, with continuum properties of surface tension and molar volume, does not exist. The critical parameter controlling the sorption isotherm in micropores is the ratio of pore size/molecule size. The effect of packing of sorbed molecules may be important. The volume of sorbed molecules alone will underestimate the pore volume simply due to the manner in which the sorbed molecules pack in the condensed phase. Unlike a bulk liquid phase in which free fluctuations produce a single phase density, the condensed phase in a micropore is constrained to few configurations leading to many possible densities. The particular density of the sorbed phase will depend on how the phase was assembled during micropore filling. These details of sorption are of no consequence for conventional analysis of meso and macropore sorption. One of the simplest quantitative models was proposed by Horvath and Kawazoe (ref.12) developed for adsorption in active carbons. It is employed in these studies to compare different zeolites, but, recognizing thc differences between active carbons and zeolites, it is only a qualitative measure of pore dimensions. This method (denoted "H-K) is based on statistical thermodynamics of the adsorbed gas molecules on surfaces. They use a 106 Lennard-Jones potential model to relate the free energy of a sorbed gas molecule to the distance between the gas molecule and solid surface. The smallest pore size is constrained by the diameter of the sorbent molecule (e.g., for nitrogen: 3.65 8,). Sensitivity increases with decreasing pore size. The comparison between the pore size predicted by the Kelvin and H-K theories is shown below in figure 1.
34
Pore 2.0 Size
(nm)
1.5
.o
1
0.5 0.0
1 0 . ~.w4 1 0 . ~
lo-'
Relative Pressure, p/p,
ion
Figure 1: Horvath-Kawazoe vs. Kelvin Equation Quantitative Relationship Between Equilibrium Pressure and Micropore Diameter EXPERIMENTAL We studied six (6) zeolite powders, the first four of which were supplied by Haldor Tops@, Denmark and the last two were supplied by Mark Davis of Virginia Polytechnic Institute and by Union Carbide Corp. 1) ZSM-5; silicalite with Si/A1=500, 2) a higher aluminum ZSM-5 zeolite with Si/A1=36, 3) a "mildly" steamed ZSM-5 zeolite with Si/A1=43, 4) a "severely" steamed ZSM-5 zeolite with Si/Al=108
5 ) ZSM-11 6) VPI-5; an aluminophosphate (ref. 14). The crystal sizes were unknown;particle sizes were less than 1 micron. The solids were prepared by drying under vacuum at 350°C for 12 hours at torr . Sorption was studied using an Omnisorb 360 automated sorption instrument (Omicron Technology, USA). By performing a dead volume correction with helium, instrument software calculates the amount sorbed;using the H-K model, the pore size distribution is calculated. Sorbents were: nitrogen at 77 K (liquid nitrogen, p e l atm.), argon at 77 K (liquid nitrogen, po=200 torr), and carbon dioxide at -58°C (dry ice-acetone, p p l atm.). Isotherms were collected in two ways: (1) statically from to 0.1, followed by dynamic from 0.1 to 0.3 p/po and (2) dynamic ad/desorution from 0.01 to 1.0 and back to 50 tom. Static sorption is used to study micropore sizes. Dynamic sorptions explore mesopores and hysteresis in sorption in a quasi-equilibrium manner. The static method base case was 1.5 scc ( 5 minutes add time at 0.3 sccm) of gas charged to the system (volume -47 SCC)followed by an 8 minute equilibration per point. The gas charge determines the resolution and pressure range of the static isotherm. If the sample is weakly sorbing, then a smaller charge is required to resolve thc micropore pressure range. The base case corresponds to a characteristic time of 8+5 =13 minutes per static point. This time is considerably longer than the expected gas phase diffusion time and may be longer than the relaxation time for molecules in the condensed phase. Nevertheless, the equilibration time was varied to look for a phase relaxation influence. The base case dynamic isotherm was collected at 0.3 sccm (47/0.3=150 minute characteristic time). Different flow rates were used to test for a dynamic effect on the transition (which occurs outside the micropore filling pressure range).
35 Desorptions are limited to -50 torr with the current instrument configuration. At low sorbate pressures during desorption, the amount of sorbate leaving the solid becomes so low that the outlet valve is unable to maintain the set flow rate and the analysis fails. RESULTS AND DISCUSSION Nitrogen Isotherms at 77 K Dynamic nitrogen isotherms are shown in Figure 2. Calculated micropores volumes are listed in Table 1. The presence of aluminum causes a reduction in microporosity. Steaming causes a further drop in microporosity perhaps by creation of an amorphous aluminum phase. Steaming, used to reduce framework aluminum, results in the creation of a significant amorphous phase and the redistribution of the crystalline phase to smaller pore sizes. Figure 3 shows the very low pressure (nitrogen at 77 K) region of sorption ("static" base case). The silicalite sorption profile is identical to that reported by Unger and Muller (ref.8). The isotherms and pore size distributions of all three ZSM-5 zeolites are similar. Hysteresis is observed for the silicalite. Our results agree with Unger and Muller (ref.8) and show that the presence of aluminum and/or steaming of the ZSM-5 eliminates hysteresis with nitrogen. PSD's (H-K model) are shown in Figure 4; mean sizes (volume basis) are shown in Table 2. The silicalite ZSM-5 and its higher aluminum companion show peak pore sizes at 5 and 10 A. The ZSM-11 has a sharp peak at 7.4 8,. Both the mildly and severely steamed zeolites have featureless PSD's indicating that most of the micropores are destroyed by steaming. The VPI-5 aluminophosphate has a broad size distribution and mean size of 15.5 A. This compares with a size determined by XRD of 12.6 8, (ref. 13).
'
@ 150 M
v
38 100 e,
-5
>
50
Si/A1>500; UCC)
8
QY
v)
1
aii
ZSM-5 ( s k e d ; Si/A1=43) 0 . - , . I I . , . I 0.0 0.2 0.4 0.6 0.8 1.0 Relative Pressure (p/pO)
-
50
P
0.0
0.2 0.4 0.6 0.8 Relative Pressure (p/pO)
Figure 2: Nitrogen Full Ad/Desorption Isotherms for Selected Zeolites at 77 K
1.0
36
200 175 150 125 100 75 50 25
5 (Si/Al=108; sev. stmed)
0 10-1 loo Relative Pressure (p/po) >ow Pressure Nitrogen Isotherms for Selected Zeolites at 77 K
Figure
Areon Isotherms @ 77K All isotherms are Type I with no hysteresis or transitions. Lack of hysteresis was also found (ref. 12) with argon isotherms at 87 K. The lack of transitions and hysteresis may be due to the greater stability of the condensed phase with argon at this temperature. Micropore volumes (Table I) are ordered the same as nitrogen. Figure 4 shows the low pressure argon isotherms. Venero and Chiou (ref. 9) found that argon (87 K) providcd more accurate pore size discrimination than nitrogen. We find that argon does not discriminate between these zeolites as well as nitrogen does. TABLE 1 Micropore Volumes at Various Temperatures and Sorbenls (measured at 0.3 p/p,)
Zcolite
Sorbcnt
ZSM-5 nitrogen' VPI-5 nitrogen ZSM-5 nitrogen ZSM- 11 nitrogen ZSM-S/Steamed niuogen ZSM-5 argon2 ZSM-5 argon ZSM-S/Steamed argon VPI-5 co23 ZSM-5
co2 COZ co2
ZSM-5 ZSM-11 ZSM-SIS~CZUTIC~ C02
Tempcrature (K) 77 77 77 77
Si/A1 Ratio
77
43 500 36 43
77 77 77 21s 215 215 215 215
(dim) 500
__
36 _.
__
500 36
__
43
Micropore Volume (cc/g) 0.227 0.196 0.195 0.112 0.104 0.207 0.153 0.041 0.215 0.172
0.128 0.108 0.096
l:based on condcnsed phase (liquid) dcnsity of 0.818 gramlcc or 0.00156 cc-liquidlcc-gas 2: based on condensed phase (solid) density of 1.477 grarnlcc or 0.001207 cc-solidlcc-gas 3: based on condensed phase (solid) density of 1.265 gxarnlcc DI 0.001550cc-solidlcc-gas
37 TABLE 2 Pore Size Averages and Pore Volume Using the Horvath-Kawazoe Model with Nitrogen at 77K (0.3 cc step with 8 minutes equilibrate/ pt.) Zeolite Si/A Pore Average Ratio Volume (cc/g) Pore Size (A) ZSM-51 500 0.227 1.4 ZSM-11 __ 0.112 7.4 ZSM-5 36 0.195 9.0 ZSM-S/Steamed 43 0.104 12.6 VPI-5 __ 0.196 15.5 1:
bimodal distribution with peaks at 11 and 5.4 8,
-d
-E 0 v)
12
-
10
w 3
.M
U
O
6
E
4
s
ZSM-5 (Si/Al=36) ZSM-I1 (UCC) ZSM-5 (Si/AI=108) sev. stmcd
.$? 2
so
0.4
0.9
1.4
Pore Size (nm)
1.9
Figure 4: Calculatcd Micropore Size Distributions for Selected Zeolites (Nitrogen at 77 K) Argon Isotherms 0 77K Figure 5 shows the complete argon isotherms. All isotherms arc Type I with no hysteresis or transitions. Lack of hysteresis was also found by Ungcr and Muller [ref. 81 with argon isotherms at 87 K. The lack o l transitions and hysteresis may be due to the greater stability of the condensed phase with argon at this temperature. Micropore volumcs (Table 1) are ordered the same as nitrogen. The reason for the diffcrences between the pore volumes for ZSM-5 and ZSM-11 are unknown but are undoubtedly due to some difference between the sample morphology. Careful inspection of the curves for ZSM-5 in figure 5 show that the desorption branch docs not meet the adsorption branch. We are convinced that this is an cxperimental/analytical artefact. It is not real. Figure 6 shows the low pressure argon isotherms. Venero and Chiou [ref 91 found that argon (87 K) provided more accurate pore size discrimination than nitrogen. We find that argon does not discriminate between these zeolites as well as nitrogen does.
2 E 200j
200
.bO
3 0
150
3 P
175 150 125 100
ZSM-5 (SdAk36)
z ;;
50
2 2 5
0 0.0 0.2
0.6
0.4
0.8
Relative Pressure (p/pO)
Figure 5: Argon Addcsorption Isotherms for Selected Zeolites at 77 K
3, amed)
lo-*
1.0
10-1
loo
Relative Pressure (p/po) Figure 6: Static, Low Pressure Argon Isotherms for Selected Zeolites at 77 K
Carbon Dioxide Isotherms 0 215K Figure 7 shows the addesorptions to 0.3 p/po. The VPI-5 zeolite, which has the largest transition pressure and largest pores, shows pronounced hysteresis. The micropore volumes are shown in Table 1. The increase in transition pressure with higher sorption temperature provides incentive to study micropores with higher temperature sorbates. The micropore volume trend is maintained for nitrogen, argon and carbon dioxide. At the higher temperature (where relaxation and transport processes are presumably faster) there is no increase in micropore volume. Thus, activated sorption, gas diffusion and phase relaxation in the micropores is either much longer, or shorter, than the base case experimental time. This is welcome confirmation of the ability of the static volumetric technique to produce consistent isotherms. 150
13 loo
SM-5 (Si/Ab500) ZSM-5 (Si/A1=36) ZSM-11
50
-5 (Si/A1=43) smcd
P 0.0
0.1 0.2 0.3 Relative Pressure (p/po)
0.4
Figure 7: Ad/Dcsorption Isotherms for Carbon Dioxide for Selected Zeolites at -6O'C
39 AdsorDtion Dvnamics and Hvstercsis Only two isotherms showed hysteresis; silicalite with nitrogen at 77 K and VPI-5 with carbon dioxide at 215K. The VPI sample did not show a pronounced transition and therefore, its non-ideal isotherm is perhaps attributable to a residual small pore amorphous phase. This would be an artifact of the synthesis and has little to do with micropore adsorption. Figure 8 shows dynamic isotherms for various experimental times for ZSM-5 silicalite powder with nitrogen at 77 K. and Figure 9 shows the calculated H-K PSD's from these isotherms. The zeolite pore sizes are not afrcctcd by experimental dynamics. While some resolution is lost, the size distributions are insensitive to expcrimcntal time. The zeolite pore sizes are not affected by experimental dynamics but the pore volumes are effectcd. These rcsults provide confidence in the ability of HRADS to quantitatively size micropores. h
10 1
%
-
Pu 200 v
38
8 min. equil. + 0.2 sccm
100
v1
-z P
0 0.00 0.05 0.10 0.15 0.20 0.25 0.30
Relative Pressure (p/po) Figure 8: Sorption over Silicalite at Varying Experimental Times (Nitrogcn at 77 K)
0.0
0.5
1.0 1.5 Pore Size (nm)
2.0
2.5
3.0
Figurc 9: Porc Size Distribution of Silicalitc at Varying Equilibration Times (Nitrogcn at 77K)
However, it is observed that the Gumulative effect of instrument time is significant and influences the total
d This . means that the amount sorbed dcpends on how the micropores are filled. Longer equilibration time during micropore filling results in less gas sorbed (a more dcnse condcnsed phase) which is an indication of a significant and observable phase relaxation dynamic. Apparently, over 30 minutes is required between data points during micropore filling to achieve equilibrium. Slower flow rate of gas during dynamic sorplion results in more gas sorbed (a lower condensed phase dcnsity, which is counterintuitive) and deviation from Type I behavior. However, the pressure at which the transition occurs is insensitive to experimental times. Pressure range remains 0.15-0.25 and volume change remains constant. The proposcd spinodal relaxation occurs much faster than the experimental times observcd and is indcpcndent of thc dcnsity or thc sorbed phase. The same dynamic experiments were performed with a powder ZSM-5, Si/A1=36. This zcolitc docs not give a transition. The same trends were observed. The dynamics of the adsorption experiment arc important in determining pore volume; however, pore size is relatively insensitive to cxperimcntal dynamics. Howcver, we €ound that with this zeolite, equilibration times of 15-30 minutes produce identical isotherms. Phase relaxation in this solid is apparently faster than in silicalite. The presence of significant aluminum in the framework may incrcase relaxation (perhaps by providing more nucleation sites) and help stabilize thc sorbed phase. This would bc consistent with the influence of aluminum on hysteresis.
40 CONCLUSIONS Nitrogen isotherms at 77 K reproduce the results of Unger and Muller (ref. 8). Only silicalite/nitrogen demonstrates hysteresis. The presence of aluminum and steaming of the zeolite eliminate hysteresis. Nitrogen sorption at 77 K is capable of accurately determining micropore size. ZSM-5 has a bimodal pore size distribution; ZSM-11 has a single sharp peak at 7.3 A. The VPI-5 aluminophosphate has a much larger pore size, 15.5 A, which is comparable to that measured by x-ray diffraction. Argon isotherms at 77 K are qualitidlively similar to nitrogen. Hysteresis is not observed which indicates that increased stability of the condensed phase by sorbing structurally simple gases eliminates hysteresis. Argon isotherms can distinguish pore size diflerenccs as wcll as nitrogen. Carbon dioxide isotherms at -65°C are without hysteresis. Transition pressures are much Ilighcr than with nitrogen and therefore more easily resolved without resorting to high vacuum conditions. The CO2 isotherms can discriminate between the different zeolites tested. C02 shows hysteresis only with VPI-5 large pore size solid which is probably an artifact of that particular solid. Pore size distribution is not greatly influenced by experimental times used in the adsorption. Thus, HRADS is a good technique for micropore size analysis. Volume sorbed depends on experimental times, thus, estimation of pore volume by HRADS may not be reliable.Condensed phase relaxation or a spinodal decomposition in the micropores during adsorption is the probable cause of hysteresis in sorption over high silica ZSM-5. Localized sorption, encouraged by the presence of aluminum, eliminates the hysteresis by decreasing the phase relaxation time and preventing the formation of a metastable state. ACKNOWLEDGEMENTS This work was supported by the Petroleum Research Fund of the America1 Chemical Socicty under grant under grant 22916-ACS. REFERENCES 1. C. Fyfe, G. Kennedy, C. De Schutter, and G. Kokotailo, I. Chem.Soc., Chem. Comm., .54, (1 984) - , 2. G.T. Kokotailo et al., Proceedings of the Seventh International Zeolite Conference, KodanshdElevier, Tokyo, pp. 361, 1986 3. W.C. Conner, P. Vincent, P. Man, and J.Fraissard, Catalysis Letters 4(1) (1990) 75. 4. J.C. Arne11 and H.C. McDermott, Proceedings of the 2nd International Congress on Surlace Activation, 11, pg.122, Butterworth, 1957. 5. J. Rouquerol, S . Partyka and F. Rouquerol, J. Chem.Soc., Far. Trans. I 7 3 (1977) 306-314. 6. Y. Grillet, F. Rouquerol and J. Rouqerol, J. Col. and Int. Sci. 20(2) (1979) 239-244. 7. D.H. Olsen, G.T. Kokotailo and J.L. Lawton, J.Physica1 Chemistry 85 (1981) 2238-2243. 8. K.K. Unger and U. Muller, "Characterization of Porous Solids" K.K.Unger (Ed.) Elscvicr, \ - -
1 088
9.
10. 11. 12. 13.
A. Venero and J. Chiou, Charactcrization of Zeolites by Gas Adsorption at Low Pressures, unpublished, Omicron Technology Corporation, Berkeley Heights, NJ, USA, 1988. M.M. Dubinin, J. Colloid and Interface Science, 23 (1967) 487-499. E.G. Derouane, J.M. Andre, and A.A. Lucas, J.Catalysis, 110 (1988) 58-73. G. Horvath and K. Kawazoe, J. of Chem. Eng. Japan, 16(6) (1983) 470-475. M.E. Davis, C. Montes, P.E. Hathaway, J.P. Arhancet, D.L. Hasha and J.M. Garccs, Physicochemical Propeties of VPI-5, submitted to J.Am.Chem.Soc.(l989).
F. Rodriguez-Reinosoet al. (Editors), Characterization of Porous Solids I1 0 1991 Elsevier Science Publishers B.V., Amsterdam
41
AHALYSIS OF TEE PERCOLATIOI PROPERTIES OF A REAL POROUS HATERIAL Geoffrey KASOB’ and David W. KELLOR’
’
Department of Chemical Engineering, Loughborough University of Technology, Loughborough, L e i c e s t e r s h i r e , England.
BP Research Centre, Chertsey Road, Sunbury-on-Thames,
Kiddlesex, England.
WHHABY A packing of 3,367 equal s p h e r e s f o r which t h e c e n t r e c o o r d i n a t e s were accurately known w a s d i s s e c t e d i n t o 14,870 i r r e g u l a r t e t r a h e d r a l pores. The drainage and imbibition curvatures a s s o c i a t e d with t h e s e pores were calculated using t h e Haines insphere approximation. The network of interconnections of t h e t e t r a h e d r a w a s calculated. Drainage and imbibition of t h e network were simulated using a percolation model. Residual entrapment w a s not modelled. I t is concluded t h a t t h e b e s t regular lattice approximation t o t h e real i r r e g u l a r network is t h e diamond lattice. However, n e i t h e r t h e bond sizes, nor t h e c a v i t y sizes are randomly s i t u a t e d on t h e network and t h e e f f e c t of t h i s non-randomness is t o s i g n i f i c a n t l y s h i f t t h e percolation threshold.
IIITBODUCTIOII The a p p l i c a t i o n of percolation theory t o t h e behaviour of f l u i d s i n porous media h a s led t o increased understanding network-related
effects.
of
drainage-imbibition
h y s t e r e s i s and o t h e r
The primary outcome h a s been t h e r e a l i s a t i o n t h a t f o r
many of t h e processes t h a t o c c u r i n a pore space, such as drainage-imbibition, desorption-adsorption
and mercury porosimetry, t h e network of connections is of
paramount importance ( r e f s 1, 2).
So important, i n f a c t , t h a t a l m o s t any network
w i l l e x h i b i t most of t h e f e a t u r e s sought t o be modelled.
A s a result work h a s
tended t o concentrate, n o t so much on understanding t h e actual network s t r u c t u r e o f real
porous
materials, but
on
including
increasingly
mechanisms so a s t o model decreasingly important e f f e c t s .
sophisticated
conditional
I t h a s become customary
i n computer modelling of percolation behaviour t o use lattice network structures r a t h e r than random s t r u c t u r e s . probably
because
of
the
ease
The cubic
of
labelling
programmes as three-dimensional a r r a y s .
lattice has been
the
t h e m o s t popular,
interconnections
in
computer
B u t , i n t h e background, The Great Question
of percolation theory ( r e f . 3 ) remains unanswered - Vhat is t h e n e t w o r k structure
of real materials and which, if any, of t h e regular lattice s t r u c t u r e s m o s t c l o s e l y m o d e l s it?
W e r e p o r t here a n a n a l y s i s of t h e network s t r u c t u r e of a real porous
material, a l b e i t only a random packing of equal s p h e r e s , and show t h a t , from a
percolation s t a n d p o i n t , it is best modelled by t h e diamond lattice s t r u c t u r e . The Great Assumption of percolation modellers is t h a t some property (usually “pore radius” f o r drainage-imbibition)
of t h e bonds (or sites) of t h e network is
42
randomly distributed across the structure.
This assumption is made both because
it s e e m s reasonable, and also because the behaviour of models using it seem to closely follow the behaviour of real systems.
But there is little evidence,one way
or the other, concerning its validity, although it is known that for the pores in a porous material both the site sizes and bond sizes cannot simultaneously be Ye also report here the percolation properties of
randomly distributed (ref. 4 ) .
our real pore network and show that, although the correlation between adjacent bond (and site) sizes appears small, the effect on the percolation threshold is large.
It may well be that it is not enough to nominate a lattice, and a bond or site distribution, but a bond (and site) correlating factor may also be needed to fully describe a pore network with regard to its percolation behaviour. DluYSIs
In 1960 Finney reported the measurement of the coordinates of 3,367 spheres in a random sphere packing (ref. 5).
The purpose was to see if this structure was a
practical model of liquid structure.
These sphere centre coordinates have been
used by other workers to model the structure of liquids and glasses but, so far, noone s e e m s to have used them as the model of the pore structure of a porous material. In order to sub-divide the void space of a random sphere packing, some kind of irregular individual pore has to be defined.
For various reasons we have used the
irregular tetrahedron defined by four adjacent sphere centres as the unit cell of the pore space.
Such a unit cell has a sphere at each vertex and has four
windows, one on each face, and a void space in the centre.
The cell has four
connections (the faces) to the neighbouring cells and these connections are the windows (or constrictions) in the pore space.
These constrictions dominate
Imbibition is determined by wider parts of the pore space.
drainage behaviour.
For modelling the imbibition of wetting fluids the interior of the tetrahedral cell gives the broadest part of the pore. The tetrahedral cell is thus a sensible subdivision into pores when capillary properties are to be modelled. The division of the sphere packing
into tetrahedra has
the practical advantage that only
“neighbouring spheres” have to be found, and, in a mathematical sense, this is unambiguous.
The division, whilst technically easy, does require considerable
computational effort (ref. 6), and will be described elsewhere.
CAPILLARY PBOPGBTLGS OF FQRES Because imbibition and drainage behaviour are to be modelled, we require the This involves calculating
capillary properties of the individual tetrahedral pores. the
curvature of
menisci
in
non-axisymmetric, converging-diverging pores
something that cannot yet be done
-
-
and consequently we have fallen back on the
43 "Haines insphere" probably
(ref.?)
involves
t o g i v e meniscus curvature.
roughly
equal
proportional
error
This is imprecise in
the
window
but
menisci
(associated with t h e bonds of t h e network and drainage) and t h e c a v i t y menisci (associated with t h e s i t e s , and hence imbibition). The coordinates of t h e Finney packing gave 14,870 t e t r a h e d r a l pores. window
The
(bond) meniscus curvature d i s t r i b u t i o n w a s calculated, using t h e i n s p h e r e
approximation and is shown i n Figure 1. t h e bonds of t h e network.
There were 30,719 windows and t h e s e were
Likewise, t h e imbibition curvatures, one f o r each pore,
were calculated and t h e i r curvature d i s t r i b u t i o n is shown i n Figure 2.
O OZ5
I
C"W0t"W
F i g u r e 1. D i s t r i b u t i o n of meniscus c u r v a t u r e s f o r t h e 30,719 windows i n t h e network c a l c u l a t e d u s i n g t h e Haines i n s p h e r e a p p r o x i r a t i o n .
curvature
F i g u r e 2. D i s t r i b u t i o n of meniscus c u r v a t u r e s f o r t h e 14,870 s i t e s i n the network c a l c u l a t e d u s i n g t h e Haines i n s p h e r e approximation.
THB DETVOBI[ Each t e t r a h e d r a l pore was numbered.
Then, for each pore i n t u r n , t h e numbers
of each of t h e neighbouring pores were evaluated. ( r e f . 8).
Full d e t a i l s are given elsewhere
This a r r a y s t o r e d t h e s t r u c t u r e of t h e network.
A convention w a s
44 adopted in which the outside of the packing was numbered as tetrahedron zero and consequently,when drainage was simulated, the outside of the packing network could readily be identified. DRAIBAGE A computer programme was written to simulate drainage of the packing.
The
Rules for Drainage were:
Rule 1 : Rule 2:
I n order t o drain, a cell must be connected to at least one immediate neighbour which is already empty of wetting fluid, In order t o drain from an erpty neighbouring cell, the "current curvature" nust exceed the critical curvature (given by the Haines insphere) of the face which connects the cell to its empty neighbour.
It should be noted that these rules do not permit entrapment of liquid by disconnection of the continuous liquid phase. The packing starts full of fluid with the current curvature set to zero.
The
current curvature was then incrementally increased and the total volume (and number of cells) drained at that curvature was calculated using the Rules for Drainage. Access to the packing
WAS
assumed to be over the entire outer surface (the pore
numbered zero was taken to be initially empty).
The results are shown in Figure 3 .
curvature
Figure 3 . Volume and Number Fraction Full against meniscus curvature for drainage of the packing from the outside surface. The similarity of these curves indicates that there is little correlation between cell volume and drainage curvature for the individual cells. Note also that This is because the packing there is no sharp percolation threshold. is small relative to its surface area. Two main conclusions can be drawn from Figure 3 .
The first, and most obvious,
is that the Number Fraction emptied corresponds closely with the Volume Fraction
emptied.
This indicates that there is no correlation between pore drainage
curvature and pore volume.
The lack of such a correlation is not intuitively
obvious but is obviously relevant to percolation modellers because "number emptied" is usually the variable calculated.
The second conclusion is that even though the
45
packing contains over 14,000 pores, it is still far too small. is the lack of any sharp percolation threshold.
The indicator here
The rounded breakthrough between
curvature 4 and 7 is caused by the sample being too small. precise threshold posed an interesting problem.
How to find the
The usual solution would be to
increase the size of the network or adopt repeating boundary conditions.
But this
was not possible in our case because the source packing was of fixed size and had no regular lattice structure. access to the outside.
So we adopted the alternative approach and limited
For the initial drainage simulation all the faces on the
outside were taken to be accessible.
There were 1958 of them.
Let us define a
"sample size ratio" (S,)
S, = total number of tetrahedral pores/number of faces accessible which, for the initial simulation was
S,
14870 / 1958 = 7.6
=
Ideally S,
would be infinite or, at least, very large.
increased so
the 1,958 accessible faces was decreased.
The 14,870 could not be Faces on the outside of
the packing were randomly selected and considered to be non-accessible. drainage simulation was re-run. Sample Sample Sample Sample Sample
A
B C
D E
1958 982 187 22 9
accessible accessible accessible accessible accessible
The
There were 4 repeat simulations (B to E ) faces faces faces faces faces
S, = SR = S, = S, = S, =
7.6 15.1 79.5 675.9 1652.2
In effect, Sample E is more that 200 times larger than A and is equal to around 3x10'.
tetrahedral pores.
The results are shown in Figure 4.
Of course having
such a limited number of access points (9 for Sample E) gives rise to statistical lumpiness near breakthrough but the clear fact is that the Sample D gives a much better developed percolation threshold than Sample A . The curves shown in Figure 4 are for a simulated drainage process. In normal percolation variables the curves would show the accessible number fraction of bonds (or sites) plotted against the probability of a bond being available.
Since we
know the number frequency of the bond meniscus curvatures it is easy to transform the variable called "meniscus curvature" into "probability of a bond being larger than a particular meniscus curvature".
We can now plot the percolation graph for
the number of accessible sites (not bonds, note, because pore volume relates to sites in drainage (ref.2)) on a percolating bond network (Figure 5).
46
Figure 4 . Fraction emptied during drainage using limited access to the outside of the packing. The restriction of the number of windows on the surface of the packing through which the non-wetting phase enters sharpens up the percolation threshold.
Drainage of the bond network plotted in conventional Figure 5. percolation variables. This Figure is similar to Figure 4 but with the transformation of the x-axis into probability. Figure 5 shows that the critical percolation threshold (p,.*) for this network is pr- = 0.51 (*O.Ol).
This is a surprising value, corresponding approximately to the
threshold value for a 2-D square lattice.
Using the approximation that in 3-D, at
the percolation threshold, about 1.5 bonddnode have to percolate (ref. 9), gives an expected value of pEP = 0.375. than expected.
So the actual percolation threshold is much larger
There could be two explanations:
this actual network is not
regular, and the bonds are not necessarily situated at random.
To test the
significance of the bonds not being sited at random, the bond sizes were randomly re-assigned across the whole network and the drainage simulation was repeated. Now, the percolation threshold was p c P = 0.38 (f0.01) (Figure 6 ) , which was significantly different to 0.51 for the real packing.
The percolation threshold of
47
0.38 is close to that of the 3-D diamond lattice (0.3903 (ref. 101, a structure with
four bonds meeting at each site.
So there is evidently sufficient correlation
between adjacent bonds in the real packing to significantly shift the percolation threshold.
08
Y
e
\
0 6
o4 z
,
,
Randamired , Sample E ,
c
E
,
0 2
- _- - _ _
00 01
02
03
0 4
05
06
07
08
09
Probability
Figure 6 . A repeat of the drainage simulation using the same network of connections as Figure 5 but with the bond sizes randomly reassigned to give a completely random structure. Row the percolation threshold occurs at the expected value indicating that the real network does not have the bond sizes sited at random. The conclusions for drainage of the real network are: the the the iii) the
i) ii)
2-D square lattice gives the correct percolation threshold, network of connections can be reasonably well approximated by diamond lattice, bond sizes cannot be assigned at random.
These conclusions give percolation modellers three options: Use the square lattice and assume that bonds are situated at random, Use the diamond lattice and find out how adjacent bonds should be correlated, c) Use the Bethe lattice with bonds situated at random and with the bondlnode ratio chosen to give the correct percolation threshold.
a) b)
Option b) has the disadvantage that it cannot currently be done! Option a) will be good for some predictions but is only two-dimensional and will certainly break down if two phase permeabilities are calculated.
Also,
the square network
only approximates the percolation threshold and only certain percolation properties are known for this lattice. percolation threshold
and
Option c) has the advantage of flexibly matching the giving
analytic functions (ref. 11,
disadvantage of using an unreal network. time will tell.
but
has
the
Which option is best in practice only
48
IHBIBITIOI
Imbibition is closely related to drainage: the network remains the same but now it is the number of accessible sites on the site tree that is required.
The
meniscus imbibition curvature for a tetrahedral pore can be approximated by the Haines insphere and Figure 2 showed the curvature frequency distribution. The Rules for Imbibition were:
Rule 1 : Rule 2:
In order to fill, a cell Rust be connected to at least one ianediate neighbour which is already filled with vetting liquid, For a pore to f i l l from a filled neighbouring cell, the "current curvature" must be below the critical curvature (given by the Haines insphere) of the body of the pore,
Again, these rules preclude any residual entrapment of the non-wetting phase.
A computer programme was written incorporating these Rules and the imbibition of the packing was simulated.
There were six simulations, one of the disaggregated
set, and five others (A to E) in which access to the outside of the packing was restricted in order to sharpen the percolation threshold.
The number of access
cells was made to match the number for the drainage simulation and consequently the SR values for Samples A
- E are identical to the values previously tabulated for
drainage. The results, in terms of meniscus curvature, are shown in Figure 7. curvature threshold associated with imbibition is 5.85 (i0.05).
The
Again, as in
drainage, the capillary pressure can be related to the probability of a tetrahedral pore filling and Figure 7 can be transformed into conventional percolation variables, this time the fraction of accessible sites on the site tree.
0
2
in
12
Figure 7. Imbibition of the real pore network in terms of meniscus curvature. Bate that the network needs restricted access to the invading phase if it is to show a pronounced percolation threshold. Transforming Figure 7 into the conventional percolation variables gives Figure 8 , from which it can be seen that the percolation threshold is 0.32 (f0.01).
The
percolation threshold of the 2-D square lattice for site percolation is 0.59
49
(ref. 9 ) ,
so,
unlike drainage (involving bonds), the 2-D lattice is a very poor
approximation for imbibition.
The site percolation threshold of the 3-D diamond
lattice (ref. 9) is 0.43,which is also widely different.
Could it be that, like the
window radii, the cavity radii are not randomly situated on the lattice?
The site
network was randomised by rearranging the site sizes on the same network of connections and the imbibition simulation re-run for the Sample E condition. results are shown on Figure 9.
The
Bow the percolation threshold has moved to
0.44 (iO.Ol), virtually the value for the diamond lattice, thus confirming that the
real network approximates to the diamond lattice, and that the sites are not situated at random.
3 Prabobiliiy
Figure 8. Imbibition of the real network in terms of conventional percolation variables. Note that the percolation threshold for this, (the site problem), is significantly different to the expected value of 0.43 (ref. -9) for the diamond lattice.
,I/,
0 0 0 1
0 2
03
0 4
05
06
,
,
,
07
08
09
n
Probobility
Figure 9. Imbibition (the site problem) of the network with the site sizes re-distributed at random on the same network. The effect of having the sites randomly distributed is to move the percolation This means that the sites in threshold to that of the diamond lattice. the real network are not distributed randomly.
50 The conclusions f o r imbibition are t h a t : i) t h e 2-D square l a t t i c e g i v e s t h e wrong p e r c o l a t i o n t h r e s h o l d , i i ) t h e network c a n be r e a s o n a b l y approximated by t h e diamond l a t t i c e , i i i ) t h e c a v i t y s i z e s are not d i s t r i b u t e d a t random b u t are s u f f i c i e n t l y correlated t o s i g n i f i c a n t l y s h i f t t h e percolation threshold.
coIcLusIoIs For percolation s t u d i e s , t h e regular diamond lattice is a good approximation t o t h e irregular network of pores i n a random packing of s p h e r e s . However, t h e c o n s t r i c t i o n s
(bonds) which dominate drainage are not randomly
s i t u a t e d i n t h e real network but are c o r r e l a t e d such t h a t " l i k e s repel" and t h i s s i g n i f i c a n t l y s h i f t s t h e percolation t h r e s h o l d , making t h e network harder t o d r a i n . There is a similar non-randomness
i n t h e d i s t r i b u t i o n of t h e c a v i t i e s (sites) which
determine imbibition but, i n t h i s case, it is t h e r e v e r s e c o r r e l a t i o n and it makes t h e network easier t o f i l l .
ACKIOVLEDGEICBITS W e thank Professor J.L.
Finney, Birkbeck College, f o r permission t o u s e h i s
s p h e r e centre coordinates, and
Dr
A.C.
Vright,
University
of
Reading,
for the
resolution of t h e Finney coordinates i n t o t e t r a h e d r a .
REFEJZEICES 1
G. Mason, Determination of the Fore Size Distributions and Fore Space In terconnectivity of Vycor Pomus Glass f m m Adsorption-Desorption Hysteresis Capillary Condensation Isotherms, Proc. Roy. Soc., 4156 (1988) 453-486.
2
G. Mason, Site and Bond Fractions on Bethe Trees, Powder Technology, 39 (1984) 21-28.
3
G. Mason, Porous Haterials and Fercolation Theory, i n K.K. Unger et al. (Eds), C b a r a c t e r i s a t i o n of Porous S o l i d s , Blsevier, Amsterdam, 1988, pp 323-332.
4
(Y. L i ) Yu, V.G. Laidlaw and I.C. Vardlaw, Sensitivity of Drainage and Imbibition to Pore Structures as Revealed by Computer Simulation of Displacement P m e s s , Advances i n Colloid and I n t e r f a c e Science, 26 (1986) 1-68.
5
J.L. Finney, Random Fackings and the Structure of Simple Liquids. I The Geometry of Random Close Packing, Proc. Roy. Soc., 319A (1970) 479-494; also, J.L. Finney, Random Fackings and the Structure of the Liquid State, PhD Thesis, University of London, 1968.
6
A.C. Vright, Personal communication.
7
V.B. Haines, Studies on the Fhysical Pmperties of Soil, J. Agric. Sci., 1 7 (1927) 264-290.
8
D.V. Mellor, Random Close Packing of Equal Spheres; Structure and Implications for Use as a Xadel Fvmus Xedium, PhD Thesis, Open University, 1985.
9
J.M. Ziman, Had& of D i s o r d e r : The T h e o r e t i c a l Physics of D i s o r d e m d Systems, Cambridge University P r e s s , 1979.
Borogenaauely
10 F.A.L. Dullien, Porous Hedia: F l u i d Transport and Pare S t r u c t u r e , Academic P r e s s , Bew York, 1979.
51
F. Rodriguez-Reinosoet al. (Editors),Characterization of Porous Solids II 0 1991 Elsevier Science PublishersB.V., Amsterdam
THE FIVE TYPES OF POROUS STRUCTURES AND 'THEIR HYSTERESIS LOOPS VICENTE MAYAGOITIA Departamento de Quimica Universidad Autonoma Apartado Postal 55-534, Mexico 13, D. F., MQxico
Metropolitana-Iztapalapa
ABSTRACT Porous materials are classified within five types according
to
the
relative positions of their site- and bond- size distributions. l'his leads to a better understanding of the morphological aspects of the porous medium as well as an assessment of the different mechanisms arising during capillary condensation and evaporation. For each one of these types of materials, relevant char-acteristicscan be recognised in their hysteresis loops. I NTRODUC'TION
The classification of adsorption hysteresis loops has been always stated in Lerms of the appearance of these curves, e.g. their shape or extension. Among the most important classifications, that of de Boer (ref. 1 ) i s based 011
a combination between the steep or sloping character of the adsorption and
desorption branches, while Everett's classification (ref. 2 ) emphasizes the extent of the region of relative pressures at which hysteresis occurs. A classification adopted by the IUPAC (ref. 3 ) considers four types of loops, vliich are identified according to the slope of the boundary curves. I t has been intended, a posteriori, t o relate these shapes of hysteresis loops to some processes of filling by capillary condensate or evaporation of the liquid held
in a pore, and in order to justify the existence of these
mecharijsms. several models of the pore geometry have been consjdered. The shape of a hysteresis loop is influenced by many factors, porous structure being the dominant one of them and for this reason i t appears as the basic property of our classification. Moreover, instead of looking for types of hysteresis loops, we prefer to define types of porous structures. What we propose is the folfowing:
- first, to classify the porous materials according to the most relevant characteristics which define their morphology, or the precise seqaence of element sizes throughout the network, statistically expressed, follcwed by
- air investigation about the relationship between the geometrical properties of the porous network and the possible mechanisms for vapour-liquid and
52
liquid-vapour phase transitions to take place in it, and then finally
- to predict the shapes of the hysteresis loops produced by each type, in such a way that the most important features of these loops can be explained satisfactorily in terms of cohesive and adhesive interactions and
the
statistical properties of the porous network. However, before proceeding with the above sequence, it would be better to start with an analysis of both the classical vision and the newest aspects of fluid transitions in pore space. THE MAIN CAUSES OF ADSORPTION HYSTERESIS Even if real structures may be very complicated in shape, in this work very simple geometries will be mentioned exclusively. Some authorities in this field (ref. 4). have recommended to lose generality in order to gain clarity. Everett (ref. 5). in his remarkable review of adsorption hysteresis, has extensively discussed about possible explanations for the existence of such phenomena. In the present contribution only some of the most common causes of hysteresis have been considered, although it has to be recognized that other factors can be more determinant at some particular conditions, e.g. the failure of the liquid phase in an extreme state of stress (ref. 6 ) or the contact angle hysteresis within the isotherms (ref. 7). Katz (ref. 8 ) pointed out that any convenient theory of adsorption must take into account the two following causes of hysteresis in pores: a delay in the formation of hemispherical menisci and the impossibility to have a liquid-vapour transition inside an element in which a liquid-vapour meniscus
*
is absent
.
In reality, the interaction between voids and the capillaries linking them is a little more complex, and is present in condensation as well as in evaporation. Consider that the porous medium can be visualized as a COMeCted network of alternated elements, the sites, or voids, and the bonds or necks. Then, if the connectivity, C, is the number of bonds meeting at a site, each void possesses C entries. Sites "opened at several poles" are the counterpart of what is very popular for bonds: "bonds opened at both ends". This last characteristic drastically alters the behaviour of phaseI
transitions. Let us assume, for the sake of simplicity, that the bonds lead directly to the free vapour phase. During condensation then, a sequential Foster (ref. 9 ) and Cohan (ref. 10) principally contributed to establishment of the delayed meniscus theory.Everett and Haynes (ref. 4). Broekhoff and de Boer (ref. 11) made very substantial contributions to understanding of these phenomena. Kraemer (ref. 12) and McBain (ref. developed the ink bottle theory.
the and the 13)
53
filling, according to
their size, can arise for bonds, following the
cylindrical geometry. In this way, bonds fill on their own. On the other hand, it is impossible for a site to fill on its own unless the C bonds have been previously filled. The requirement to fulfill in this case is clear: all these bonds must possess radii in such a way that r1, r2 ,.... r C < R / 2 .
If
several bonds remain unfilled, the meniscus located in the pore lacks continuity, and its advancement to fill completely the site with condensate is impossible, even if this element is in a saturated state. In the event of only one of the bonds being empty, the meniscus can advance straightforwardly into the site to fill it together with the remaining empty bond. Now, if one of the bonds possesses a radius r1 equal to that of the site, this fills reversibly only if r2 , . . . rC < R / 2. For any other case condensation in the site is controlled by the biggest bond among the remaining r2 to rC. Anyway, an hysteretic behaviour would always be inherent to bonds labeled as r2,. . rC. Condensation and evaporation in porous networks obey the above arguments, but are complicated because of the possibility of many different menisci paths within the network. Quinn and McIntosh (ref. 14) were the first to stress the importance of this pore- blocking effect during evaporation. Everett (ref. 15) and Barker (ref. 16) gave an explanation of the fundamental aspects of it. More recently, assisted and hindered transitions arising from cooperative effects all along the network have been pointed out by Morioka and Kobayashi (ref. 171, and by Mayagoitia et al. (ref. 18). Cooperative behaviour during condensation seems to be the rule rather than the exception. The consideration of all these mechanisms leads to the conclusion that the morphology, or the precise sequence of element sizes throughout the network,
". . .controls,.. to
a
major
. . . the
extent,
condensation-evaporation
characteristics"(ref. 19). FORMER TYPES OF POROUS STRUCTURES As Everett
(ref. 19) noted, the so-called pore- size distribution
"
...
involves, in fact, two statistical functions rather than one". These two statistical functions are the site and the bond- size distributions. With respect to the overlap between these distributions, three situations are possible (ref. 20): I
- a zero or very low overlap, in which case the sizes of sites and bonds are notably different. The sizes of elements are disposed across the network completely at random. Types I to 111. t
-
- a large overlap causes a structuration of the elements in the network. A
*
This structuration has been observed recently by means of Monte Carlo methods (ref. 21).
54
size- segregation effect arises.There form regions of big e1ements:big sites and bonds linked together, and somewhere apart there lie regions constituted of smaller elements reunited. Type IV.
- an overlap tending to completeness. The size- segregation effect is so strong that the network is broken into a collection of "homotatic" regions, each of them possessing sites and bonds of the same size, the bonds of which
*
behave in practice as independent . Type V. Dealing
again with
a
situation of
nearly
zero
overlap,
the
two
distributions could lie very far apart or, conversely, very close to each other. Three situations still arise:
- the distributions are
so
far apart, as to avoid any interaction between
sites and bonds during capillary condensation, Type I,
- there exists an intermediate situation in which bond-site interactions are moderate, Type 11, or finally
- the distributions lie so close to each other that even before the onset of the independent filling of bonds, all the sites are already, by virtue of their
size,
in
a
state
of
supersaturation,
i.e.
are
eligible
for
condensation, and the transition depends only on the state of their bonds. Type 111. REQUERIMENTS FOR THE PREDICTION OF HYSTERESIS CURVES In principle, an estimation of the hysteresis and scanning curves from a twofold size-distribution and connectivity is possible. The aspects that appear to be absolutely unavoidable to deal with are the following:
- a critical analysis of the morphology of the adsorbent, allowing a proper treatment of the interactions between the elements of the network. i.e., low overlapped structures are fully random media, consequently pore-blocking effects are very important to consider. On the other hand, for structures displaying an overlap tending to completion, pore blocking is absent,
so
that
it would be a serious error to incorporate percolation relationships in the treatment (ref. 22).
-
all kind of possible interactions between the elements of the network,
assisted or hindered, must be envisaged, and we draw attention specially to the cooperative phenomena pccurring during capillary condensation, a subject that has been very scarcely treated (ref. 18, 20).
*
the
analysis performed
should
be
the
most
precise,
then
domain
The real impossibility of having rigorously the same size for sites and bonds in the same region is not to be considered as a serious problem, as long as the bonds completely control the condensation- evaporation characteristics.
55 complexions,rendering the state (empty or full with capillary condensate) of both sites and bonds, in terms of their size, can be represented.
- adsorbate/adsorbent interaction is to be taken into account by means of an adsorption potential that not only leads to the development of an adsorbed layer but
that also modifies drastically the Kelvin equation and
the
conditions of capillary condensation and evaporation to take place (ref. 11).
If one could ignore the influence of this potential the uncorrected Kelvin equation would lead to a critical radius of curvature, Rc. Kelvin equation renders a value R as well as another value R
2
instead of Rc,
in place of R
C
The corrected
for a spherical geometry,
/ 2 for a cylindrical geometry,
both as functions of the relative pressure. Table 1 presents some comparative values for the condensation of nitrogen at 77 K. TABLE 1. Influence of the adsorption potential on the condensation of nitrogen at 77 K in sites (hollow spheres), R1, and bonds (hollow cylinders), R2.
I
1 I
II
**
**
Critical radius
True condensation radius
True condensation radiui
20
38
25
40
65
41
60
91
56
80
115
70
100
139
84
200
256
147
300
368
208
as defined by the non-corrected Kelvin equation
The new parameters, R 1 and R2, must replace the former ones in all the expressions describing interactions during capillary condensation, evaporation and scanning (ref.20.22).
- finally,in order to represent the hysteresis and scanning curves for a I
particular adsorbate/adsorbent pair, it is required to be acquainted with all the relevant information about the .nature of both components and their interaction, as well as for all the difficulties involved in the definition of such a system.
56
CALCULATION OF HYSTERESIS LOOPS A very complex method of calculation, taking into account all these five
remarks is being tested by us. and constitutes perhaps the most complete approach to the investigation of the texture of a porous material and the behaviour that a condensable fluid is undergoing in it.
This method is
probabilistic (analytic) in nature. First of all, the twofold distribution and a value for C are imposed. For a given relative pressure, values of R
1 and R2, as well as the thickness of the adsorbed layer for all sizes of sites
and bonds are calculated. With the relative pressure kept fixed again, the degree of filling for sites and bonds of every size is calculated by means of eqns. (24) to (39) of (ref. 2 0 ) and ( 1 ) to (51) of (ref. 22) and for all kind of
envisaged
processes:
ascending
and
descending boundary
curves
and
scanning. Afterwards the overall degree of filling is calculated by means of eq. (52) of (ref.22), but in a more refined manner as the adsorbed layer has been taken into account. The use of this method provides adsorption-desorption curves very similar to those observed for real porous materials. Figs. 1-8 show theoretical isotherms for the adsorption of nitrogen at 77K in different types of porous structures. For instance in cases labelled as types I ,
I1 and
I11
(i.e. those
corresponding to zero overlap), a common characteristic is a very steep descending boundary curve (see Figs. 1-41. As in these structures the sizes of the void elements are disposed throughout the network totally at random, the porous medium (initially saturated with condensate) is invaded by vapour at a percolation threshold, so that the reason for the abrupt fall of the descending curve. From type I to type 111, the hysteresis loop decreases in width, while at the same time the adsorption layer becomes more important. For a type 111,
the slope of
the adsorption branch, within a great
extension of the hysteresis loop, is higher than that corresponding to the desorption branch. Here cooperative phenomena during adsorption are more intense than during desorption. This has not been mentioned by previous authors. A comparison between Figs. 3 and 4 shows that, other structural parameters
being constant, a variation of the connectivity (which drastically alters the I
shape of the isotherm for type I structures) does not influence significantly the appearance of the isotherm again for type I 1 1 structures.Experimenta1 curves for the adsorption of vapour at 298 K in model mesoporous carbons (ref. 2 3 ) consisting of monosized spheres forming a regular array resemble closely to those found in figs. 3 and 4. Very complicated calculations are involved in type IV structures that it
51
VERALL EGREE F FILLING
I 1
VERALL lEGREE F FILLING
I
RELATIVE PRESSURE
RET-ATLYE PRESSURE
Fig. 1. Type I structure.
Fig. 2. Type I1 structure.
1
1VERALL IEGREE
I F FILLING
7 i
b
RELATIVE PRESSURE
Fig. 3. Type I11 structure with C = 3.
oh
RELATIVE PRESSURE
F-g. 4. Type111 structure w i t h C = 6.
Comparison of some adsorption hysteresis cycles, calculated from twofold distributions, f o r several types o f porous structures.
58
. . OVERALL DEGREE OF FILLING
o
OVERALL DEGREE OF FILLING
.
.. . . e
0
RELATIVE PRESSURE
1
Fig. 5. Type V. Small pores.
0
RELATIVE PRESSURE
1
Fig. 7. Type I. Ascending scanning curves.
0
RELATIVE PRESSURE
o
1
Fig. 6. Type V. Big pores
0
RELATIVE PRESSURE
1
Fig. 8. Type 11. Ascending scanning curves.
Comparison of some adsorption hysteresis cycles, calculated from twofold distributions, f o r several types of porous structures.
59 has been impossible for the moment to obtain adsorption isotherms. Fig. 5 corresponds to a type V (overlap between size distributions tends to completeness) material, made of small pores. There exists an initial plateau at the upper part of the descending boundary curve. This is not at all due to the existence of a pore-blocking effect, since the hypothesis employed have nothing to do with this phenomenon. The plateau is better understood in terms of the delayed meniscus theory, so that this effect gains a great importance in the characterization of the porous medium. Fig. 6 is an isotherm for a solid (type V) made of big pores, which closely resembles the adsorption isotherms found f o r globular carbon samples, where the particles are partially coalesced (ref. 2 3 ) . Scanning curves were also calculated for types I and I 1 and the results are shown in Figs. 7 and 8 . It is expected that an analysis of the scanning curves corresponding to real materials, will extend and complement the textural information obtained from the boundary curves. The method of calculation here outlined constitutes a powerful tool for the determination of the textural properties of a porous solid. NEW CLASSIFICATION OF POROUS MATERIALS The above results and discussion seem to confirm the appropriateness of a classification published elsewhere (ref. 2 0 ) . However it is necessary to stress the importance of the adsorption potential
(amerely
in the
development of the thickness of the adsorbed layer). We have also learned from our Monte Carlo results to explore the porous morphology in relation with the intensity of the overlap. Consequently, this classification can be improved on the basis of the following remarks: TYPE I. A material should be considered as such if there is not overlap at all and if i t there is a span of radii in which there are no elements between RZ(RBB) and RI(RSs). (RBB denotes the biggest bond, while Rss is the smallest
site).
TYPE 11. This is the general case of low overlap, meaning for this a value as the network has not been yet
of such parameter between 0 and 30 %, structurated appreciably.
A material of reduced overlap having practically all the I elements within a span of radii between R (R 1 and R (R 1 (R is the 2 SB I BS SB TYPE 1 1 1 .
size of the smallest bond and R
BS
is that of the biggest site).
TYPE IV. A network having a significant overlap. TYPE V. A situation in which overlap is larger than 85 %.
60
CONCLUSIONS The arguments relating a porous structure and its morphology to mechanisms
of phase transitions
-
specially vapour-liquid transitions - reveal to be
highly consistent. A previous clasification of porous structures within five types was improved by considering recent results of Monte Carlo estimations of porous morphology and the role of the adsorption potential.
ACKNOWLEDGEMENT This work was supported by the National Council of Science and Technology of Mexico (CONACyT).
LITERATURE CITED 1
2 3 4 5 6
J. H. de Boer, in D. H. Everett and F. S. Stone (Eds.1, Structure and Properties of Porous Materials, Colston Papers, Vol. 10, p. 90, Butterworths, London, 1958. D. H. Everett, in E. A Flood (Ed.1, The Solid-Gas Interface, Vol 2, p. 1059, Marcel Dekker, New York, 1967. K. S. W. Sing, D. H. Everett, R. A. W. Haul, L. Moscou, R. A. Pierotti,J. Rouquerol and T. Siemieniewska, Pure and Appl. Chem., 57 ( 4 ) (1985) 612. D. H . Everett and J. M. Haynes, J. Colloid & Interface Sci. 38 (1972) 125. D. H. Everett, in E. A. Flood (Ed.1, The Solid-Gas Interface, Vol 2, pp. 1055 - 1113, Marcel Dekker, New York, 1967. C. G. V. Burgess and D. H. Everett, J . Colloid Interface Sci., 33 (1970) 611.
7 8
9 10 11 12 13 14 15
16
17 18
R.Zsigmondy, Z . Anorg. Allgem. Chem., 7 1 (1914) 356. S . M. Katz, J. Phys. Chem., 53 (1949) 1166. A. G. Foster, Trans. Faraday SOC., 28 (1932) 645. L. H. Cohan, J. Am. Chem. SOC., 66 (1944) 98. J. C. P. Broekhof-fand J . H. de Boer, J. Catalysis, 9 (1967) 15. E. 0. Kraemer,in H. S . Taylor (Ed.1, A Treatise on Physical Chemistry p. 1661, New York, 1931. J. W. McBain, J. Am. Chem. SOC.,57 (1935) 699. H. W. Quinn and R. Mc Intosh, in J. H. Schulman (Ed.1, Surface Activity, Vol. 2, p. 122, Butterworths, London, 1957. D. H. Everett, in D. H. Everett and F. S . Stone (Eds.1, Structure and Properties of Porous Materials, Colston Papers, Vol. 10, p. 117, Butterworths, London, 1958. J. A. Barker, in D. H. Everett and F. S . Stone (Eds.1, Structure and Properties of Porous Materials, Colston Papers, Vol. 10, p. 125, Butterworths, London, 1958. Y. Morioka and J. Kobayashi, J. Chem. SOC.Jpn.. 2 (1979) 157. V. Mayagoitia, F. Rojas and I. Kornhauser, J. Chem. SOC., Faraday Trans.
1, 8 1 (1985) 2931. 19 D. H. Everett, in E. A Flood (Ed.1, The Solid-Gas Interface, Vol 2, p. 1083, Marcel Dekker, New, York, 1967. 20 V. Mayagoitia, F. Rojas and I. Kornhauser, J. Chem. SOC.Faraday Trans. 1, 84 (1988) 785. 2 1 M.J. Cruz, V. Mayagoitia and F. Rojas, J. Chem. SOC.Faraday Trans. 1, 8 5 ( 8 ) (1989) 2079. 22 V. Mayagoitia, B. Gilot, F. Rojas and- I. Kornhauser, J . Chem. Soc., Faraday Trans. 1, 84 (1988) 801. 23 F. Hojas, Ph. D. Thesis, University of Bristol, England, 1982.
61
F. Rodriguez-Reinosoet al. (Editors), Characterization of Porous Solids XI 1991 Elsevier Science Publishers B.V., Amsterdam
MODEL STUDY OF THE COMBINED MACROSCOPIC HETEROGENEITY GAS SOLIDS
EFFECT OF HETEROPOROSITY OF RELATIVE PERMEABILITY OF POROUS
N.K.Kanellopoulos,J.K.Petrou and J.H.Petropou1os Physical Chemistry Laboratory Nuclear Research Aghia Paraskevi Attiki,Greece.
Center,
15310
S-ry
A model study of the combined effect of macroscopic heterogeneity and heteroporosity on the relative gas permeability of a porous solid, as a function of the fraction of pore volume occupied by a foreign sorbate, is reported. The heteroporous solid was modelled as a regular capillary network with randomly varying capillary radius, characterized by the radius distribution and the structure of the network, notably network connectivity. Macroscopic heterogeneity was introduced by allowing the local porosity of the solid to vary along or across the axis of permeation. Model calculations were performed for various macroscopic and microscopic parameter nalues, in order to obtain a realistic assessment of the relative importance of the respective effects and the way in which they combine to produce the final observable result. Introduction In previous work [1-41 it was shown that the relative permeability of porous solids is an important source of information about their pore structure. For the simulation of the pore structure, a network model has been employed, consisting of a regular array of nodes joined together by cylindrical capillaries of randomly varying radius r. The model is completely characterized by the capillary radius probability distribution f(r) and the connectivity of the network , n ~ , given by the number of capillaries meeting at each node. However, in practice, exploitation of relative permeability measurements is hindered by macroscopic nonhomogeneities, in the form of e.g. non-uniform porosity produced in the common It has been demonstrated [ l ] that pelletization procedures[S] the effect of macroscopic heterogeneity on the initial slope of the relative permeability curves can be very significant and hence cannot be ignored in practice. In addition, it has been shown that the effectiveness factors of catalysts formed by compression can be [6]. greatly affected by macroscopic heterogeneities However, a realistic model capable of representing the behavior of a mesoporous non-homogeneous pellet over the full range of relative permeability is, as yet lacking. Such a model is presented here. Macroscopic inhomogeneity is represented by making the the local porosity E of the pellet (in the form of a slab) a function of the normalized spatial coordinate o
.
62
non-homogeneity] or along one lateral direction y [radial non-homogeneity]. In the former case, q=x, with x=O and x=l at the upstream and downstream faces of the porous pellet, respectively (all axial distances are thus normalized with respect to the thickness of the pellet). In the latter case , q=y, O
= €0 (l+k1q+k292)
(1)
where EO = e(q=O) and kl, kZ=const. If we denote by em, the maximum (if E ~ > E ~or) minimum (if E ~ < E ~value ) of E , E ~ / E ~ , affords a convenient measure of the degree of heterogeneity. The observed overall porosity of the pellet is given by
We introduce the normalized radius p=r/rv, where rv is the middle radius. The corresponding probability distribution is given by
-
f(p)dp=f(r)dr
(3)
and is defined between lower and upper limits p=pa and pb respectively. The maximum breadth of f(p) is given by Zo=pb-pa A "homoporous" ("heteroporous")solid is defined by o=O(o>O) and the value of a affords a convenient measure of the degree of heteroporosity. For simplicity, it is assumed that the variabilfty of local porosity results in spatial variation of rm only; 1.e. we have E=
L,
ll Km2
where Lv is the total pore length per unit volume of medium and is the second moment of f(p) defined by
(4) porous
J Pa In eq. ( 4 ) L , and are constant. The relative gas permeability of a porous solid, PRI is defined as the gas permeability of the pellet measured in the presence of a foreign sorbed vapour, occupying a certain fraction (vs) of the pore volume, normalized with respect to the gas permeability in the absence of foreign sorbate [l-41: Here, h
PR'Pg --,-
(G, ) /Gg( 0 )
(6)
where Pgrvs represent the observed overall values, which can be calculated here from the corresponding local values Pg(vs,q) and v,(q). At any location, the sorbed vapor is present as (1) condensate completely filling the smaller "subcritical" pores of radius KrK, leaving an effective open core radius r-t.The parameters KK and t
63
depend on the relative vapor pressure p/po used in the particular experiment as follows [l-41
-t(q) -- t
rm(q)
-
c21/3 rm(q) ( ~ ~ ( P / P o )
(7)
where C1=-0.477 nm, C2'0.2 18 nm3 are values representative of N2 at 77 K. The local pore volume fraction occupied by foreign sorbate is given by
; and Pac'PKf
where pac=pa, if PK'Pa
if PK>Pa-
The overall fractional pore volume of porous medium occupied by foreign sorbate is given by
Here, Pg (v?g) is calculated in the Knudsen regime , by the effective medium approximation (EMA), namely [4]. This treatment yields
.
where g( q)=p rm3 (p-t) ( p=const ) represents the conductance of a pore in the actual network in the Knudsen regime (neglecting effects of finite pore length) and gM is the corresponding conductance of a pore in the effective medium network (composed of pores of equal conductance); g# is proportional to Pg!vs) The observed overall permeability coefficient Pg is given in terms of the corresponding local values Pg by
.
(a) in the case of axial heterogeneity (q=x)
L
J
64
(b) in the case of radial heterogeneity (q=w)
0
,The effect of the heterogeneity and heteporosity on the PR(vS) curves, over the whole range of vs, is illustrated in a Figure 1. The heteroporoustypical set of results of heterogeneous curves (denoted by HH) are compared with the corresponding homoporous-heterogeneous (OH), heteroporoushomogeneous (HO) and homoporous-homogeneous ( 0 0 ) media. The similarity of the effects of axial (radial) heterogeneity and the low (high) network connectivity nT should be noted. These cases present the characteristics of a serial (parallel) pore array, where narrow (wide) pores dominate [ 2 ] . Thus, the respective PR(vs) curves lie below (above ) that corresponding to the 00 case. In addition, as in the case of connectivity [ 2 ] , the effect of heterogeneity is more(1ess) profound in the lower (upper) parts of the curves. Thus significant shifts of the percolation thresholds can be caused by heterogeneity effectcs. Finally it is noteworthy that, although the combined effect of heteroporosity and heterogeneity is usually cumulative, there important exceptions to this. A striking case in point is afforded by the crossing of the OHX and HHX curves at n ~ = 4 ;at higher vs the deviation caused by the heterogeneity alone is greater than the combined effect of both the heterogeneity and the heteroporosity porosity. Figure 1 shows that the dexiations between the uniform and the non-unizorm porosity P R ( V ~ )curves are larger (smaller) at high {low! vs. This indicates that the effect of the inhomogeneity is more (less) evident in cases where the condensation (adsorption) mode of sorption is more predominant. This effect is shown more clearly in Figures 2 and 3 for the hypothetical cases of pure condensation and pure adsorption respectively.
1
\\ t
O2
Pa
0
Fig.1: Results of model relative permeability calculations representative of N2 at I 1 K for homoporous-homogeneous ( 0 0 ) , heteroporous-homogeneous (HO), homoporous-heterogeneous (OH) and heteroporous-heterogeneous (HH) media of slab geometry. For the homoporous medium rm=3.4 nm and for the homogeneous medium ~=0.40. The cases of heteroporosity and heterogeneity are reperesented respectively by the T ( p ) and ~ ( q ) functions shown in the insets. Values of salient parameters: a=0.3 nT=4(---),18(-- - ) , &0=.23, kl=1.5,k2=0 ( E O / E ~ ~ ~ = ~ . ~
h
rm(l)/rm(0)=1.58,~=0.40)),
Lv=.02/n,X(Y) denote cases of axial and (radial) heterogeneity.
65
In Figure 2 it is illustrated that homoporous heterogeneous in the axial direction ( O H X ) are steeper than the corresponding heteroporous heterogeneous curves HHX. This is attributed to the fact that pore blocking by condensation is more efficient in the former case , since it takes place only in the smaller porosity section of the plug; on the contrary, for the case of heteroporous heterogeneous system ( H H X ) , the pore blocking is less efficient, since small pores are blocked along the whole x-axis. The opposite holds for the case of y-inhomogeneities. The homoporous heterogeneous curves ( O H Y ) are less steep than the corresponding heteroporous heterogeneous ( H H Y ) curves and present values at the percolation threshold. This is due to higher V ~ F the fact that for the homoporous heterogeneous case (OH) at the percolation threshold all the pores are blocked except for the largest pores at the largest porosity section, whereas for the heteroporous heterogeneous case at the percolation threshold several large pores are open along the y-axis. Figure 3 shows that for the case of homogeneous plugs the effect of heteroporosity is important only for the low connectivities. The heteroporous homoporous ( H O ) curve is lower, since the constriction caused by the smaller core radius pores cannot be by-passed, due to the low connectivity [ 7 ] . The heteroporous heterogeneous ( H H X ) curves are lower than the coresponding homoporous heterogeneous ( O H X ! curves due to the constriction effect of the smaller pores in the heteroporous case. This explains the crossing of the low connectivity HHX and OHX curves of Figure 1. The homoporous heterogeneous ( O H X ) curve at n ~ = 4is higher than heteroporous heterogenous ( H H X ) in the initial pure adsorption portion of the curve in agreement with Figure 3; and the opposite holds in the lower section of the curves, where the pure condensation mode of sorption is predominant, in agreement with Figure 2.
0.1
f
0,s
PR
08
v,
-
t
0 8' vs
Fig.2: PR curves as in Fig.1 for the hypothetical case of pure condensation.
-
Fig.3: PR curves as in Fig.1 for the hypothetical case of pure adsorption.
66
References 1 N.K.Kanellopoulos, J.H.Petropoulos and D.Nicholson,Effect of Pore Structure and Macroscopic Non-homogeneity on the Relative Gas permeability of Porous Solids, J.Chem.Soc., Faraday Trans. 1, 81 (1985) 1183 2 N.K.Kanellopoulos and J.K.Petrou, Relative Permeability of parallel and serial capillary models with various radius distributions,J Membrane Sci.,35 (1987) 21 3 D. Nicholson and J.H.Petropoulos, Gas relative permeability in the capillary network model, J.Chem.Soc., Faraday Trans. 1,80 (1984) 1069. 4 J.H.Petropoulos, J.K.Petrou and N.K.Kanellopoulos, Explicit relation between relative permeability and structural parameters in stochastic pore networks, Chem.Eng. Sci.,44 (1989) 2967 5 C.G.Goetze1, Treatise on Powder Metallurgy, Interscience, New York, 1949 ; Vol.1, Chaps. 8,9. 6 S.Kasaoka and Y. Sakata, Effectivess factors for nonuniform catalyst pellets, J. of Chem.Engr. of Japan, Vol. 1, 2 (1968) 138. 7 N.K.Kanellopoulos, J.K.Petrou and J.H.Petropoulos, Realistic modelling of the interaction of vapors with densely packed spherical particles, Part 11: Relative permeability, J. Colloid and Inter.Sci., 96,1,(1983) 101.
F. Rodriguez-Reinosoet al. (Editors), Characterization of Porous Solids I1 1991 Elsevier Science Publishers B.V., Amsterdam
PERCOLATION THEORY OF CAPILLARY HYSTERESIS PHENOMENA AND APPLICATION FOR CHARACTERIZATION OF POROUS SOLIDS
67
ITS
A.V. NEIMARK Institute of Physical Chemistry of the USSR Academy of Sciences. Moscow (USSR) ABSTRACT
A statistical theory of capillary hysteresis phenomena in porous media has been developed. The analysis is based on percolation theory and pore space network models. New methods for computation of porous structure parameters are proposed as application. The main results are: - percolation theory of adsorption hysteresis in mesoporous materials with hysteresis loops of H1 and H2 type by IUPAC classification and corresponding methods of pore size distribution computation; - theory of cooperative capillary condensation in stochastic channels network based on three-component bond percolation problem; - theory of hysteresis loop scanning isotherms of adsorption and desorption in stochastic cavities and throats network based on mixed bond-site percolation problems. INTRODUCTION
The presence of adsorption hysteresis is the special feature of all adsorbents with a rnesopore structure. The adsorption and desorption isotherms differ appreciably from one another and form a closed hysteresis loop. According to the IUPAC classification four main types of hysteresis loops can be distinguished: H1, H 2 , H3 and H4 (ref. 1). Experimental adsorption and desorption isotherms in the hysteresis region provide information for calculating the structural characteristics of porous materialsporosity, surface area and pore size distribution. Traditional methods for such calculations are based on the assumption of an unrelated system of pores of simple form, as a rule, cylindrical capillaries. The calculations are based on either the adsorption or the desorption isotherm, ignoring the existence of hysteresis in the adsorption process. This leads to two different pore size distributions. The question of which of these is to be preferred has been the subject of unending discussion. In this report a statistical theory of capillary hysteresis phenomena in porous media has been developed. The analysis is based on percolation theory and pore space networks models, which are widely used for the modeling of such processes by many authors (refs. 2-10). The new percolation methods for porous structure parameters computation are also proposed.
68
RESULTS AND DISCUSSION Cooverative character of adsorvtion vhenomena. In real materials the pores are connected to one another and form a three-dimensional network. The interconnection of pores accounts for the cooperative character of adsorption phenomena. In capillary condensation the effect of the initiation of condensation in the wide pores appears after condensation in the narrow pores adjacent to them. The delay in desorption from the wider pores is stipulated by its blocking by the narrower ones. These cooperative effects cannot be allowed for by a model of unrelated pores: the requirements for filling or emptying of a given pore depend not only on its own characteristics but on the characteristics of adjacent pores as well. The influence of interconnection effects is diagrammatically illustrated on the example of a simple system consisting of one wide capillary of radius p p and two capillaries of radius p1 (see Fig. 1): Capillary condensation in cylindrical capillary of radius p occurs at one value of relative pressure x+ (x=p/ps) and desorption at another value of relative pressure x-. The values x+ and x- depend on pore radius p , moreover x - ( p ) > x + ( p ) . In this inequality the capillary hysteresis on the level of one capillary is displayed. It is conditioned by the difference of the mechanisms of capillary condensation and desorption. Capillary condensation occurs by means of spontaneous filling at the moment of the loss of adsorption film stability on the internal surface of capillary. This process is not reversible. Desorption occurs at the moment of equilibrium meniscus formation on the open end of capillary.
Fig. 1. Hysteresis loops in a model system of three capillaries.
69
In the system of unrelated pores the adsorption and the desorption isotherms form two hysteresis loops (Fig, la). In the case the wide capillary is connected with the narrow one the second loop disppears (Fig. lb). After the condensation in the narrow capillary in the place of capillaries intersection the equilibrium meniscus is forming and condensation in the wide capillary occurs reversibly at such relative pressure x = x - ( p 2 ) as its emptying under desorption. In the other situation, when the wide capillary is blocked by narrow ones (Fig. lc) , the hysteresis loop transforms essentially. In this case the adsorption process is going on as in the previous one, but the desorption process is quite different. The desorption from the wide pore occurs only after the emptying of blocking narrow pores at x = x - ( p l ) . On this simplest example we see that interconnection effects have essential influence on capillary condensation and desorption processes, and on the shape of hysteresis loop. Ought to remark, that in the literature the main attention was attracted to the blocking effects under desorption, but the effects of capillary condensation's initiation were avoided. Usually the authors assume that the condensation in the network of pores occurs as in the system of unrelated pores (Ref. 1,8). Both these effects displace the isotherms of adsorption and desorption towards smaller relative pressure as compared with the system of unrelated pores. Hence it follows: 1) the pore size distribution, calculated in the frameworks of unrelated pores model, gives the decreased values of pore radii; 2 ) the distribution obtained on the bases of adsorption isotherm, differs from the distribution obtained on the bases of desorption isotherm. Cooperative effects can be taken into account by means of network models, reflecting the special features of the pore structure more fully, than a system of unrelated pores. Network models of Pore structure. It is useful to distinguish two different network models. The first one used for the adsorbents exhibiting a hysteresis loop of type H1 is the network of channels. The second one used for the adsorbents exhibiting a hysteresis loop of type H2 is the network of cavities and constrictions. These models are the particular cases of more common model of the network of cavities and channels. In the first case we suppose that the pore space consists of intersecting channels of different sizes and the main pore volume
is concentrated in the networks bonds (Fig. 2a). In the second case we suppose that the main pore volume is concentrated in the network sites imitating the pore cavities connected by more narrow pore constructions (Fig. 2b). V
Fig. 2. Hysteresis loops of type H1 (a) and of type H2 (b) and corresponding pore space models: network of channels (a) and network of cavities and constrictions (b). Some results in Dercolation theory of adsomtion hysteresis. By means of percolation theory it is shown that the difference in the properties of the hysteresis in the adsorbents characterized by the loops of type H1 and type H2 can be explained in the frameworks of these models(refs. 5-7, 11). The particular attention is spared to the scanning isotherms. The course of isotherms scanning loop of type H1 is quite different from the course of isotherms scanning loop of type H2. In the first case the scanning isotherms form closed loops inside the main loop (Fig. 2a). In the second case the scanning isotherms of desorption, starting from the main adsorption branch, that finish in the point A of the beginning of hysteresis. On their turn the scanning isotherms of adsorption, starting from the main desorption branch, finish in the point C of the end of hysteresis. Analogous behavior is typical for isotherms scanning internal hysteresis loops as well. This difference was explained by the peculiarities of porous structure, which are taken into account in the network models mentioned above. The theory of cooperative capillary condensation in stochastic network of channels is developed. The corresponding mathematical problem is reduced to a three-component bond percolation problem. At a given relative pressure x in the network of channels three types of bonds are distinguished: subundercritical channels of equivalent size p < p + ( x ), intermediate- of size p + ( x )< p < p - ( x ) and overcritical - of size p > p - ( x ) . Here functions p + ( x ) and p - ( x ) determine the equivalent sizes of pores in which the capillary condensation and desorption are observed at relative pressure x .
71
The problem of design of the isotherms scanning the loop of type H2 is reduced to mixed bond-site percolation problem (ref. 6). The special methods for calculating the pore size distribution in adsorbents having loops of type H1 and H2 are suggested (refs. 5, 7). Their principal innovation is that they employ simultaneously information obtained experimentally from both the adsorption and the desorption branches of the isotherm. These methods are used in new versatile software for characterization of porous solids (ref. 12). The detailed description of percolation method for the interpretation of hysteresis loop of type H1 is given below. Percolation method for calculatinq the Dore size distribution (loor,
H1) The problem on desorption from the network of channels along the main desorption branch is the classical problem on bond percolation. In the network two types of bonds are distinguished: overcritical - of pore p > p - ( x ) and undercritial - of size p < p - ( x ) . At the process of pressure reduction down to a given value x the desorption occurs not from the all undercritical pores (as would be in a system of unrelated channels) but from only those pores which are forming connected system of undercritial pores looking on the external surface of the sample. The point E of the transition from the gently sloping section of desorption isotherm to the sharp one corresponds to the percolation transition - to the forming of connected system of overcritical pores and the beginning of desorption from the sample's volume. In the point E the portion of overcritical bonds in the network is equal to the percolation threshold p c . The portion Q-(x) of channels, got free of capillary condensate at given value x, is determined by the connectivity function of the network:
Here p,(p-(x)) is the portion of overcritical channels of size p > p (x). The connectivity function Q,(p) is to be calculated by means of percolation theory. The model of network of channels proposed that equivalent sizes and other geometrical characteristics of pores are not correlated. This assumption produced the following equation between isotherms of adsorption V+(x) and desorption V-(x) at x<xB ( x B
72
corresponds to the beginning of capillary condensation).
Equation ( 2 ) permits to calculate the portion Q-(x) of free pores at desorption. On the base of the comparison of this value obtained from experimental data with the theoretical value (1) we can determine the portion p,(p-(x)) of overcritical pores. So the integral pore size distribution function is equal to
where QC(-') is the function inverse to the connectivity function
Q,(P)
-
This equation constitutes the basis of the percolation method for calculating the mesopores size distribution. The main theoretical problem is the determination of the connectivity function Q,(p) for given network. For a three-dimensional network we have obtained interpolation formulae which describe with required accuracy the course of connectivity function over the whole range of p : for O
2pc0-54(p-pc) 0*46
for pc
1
(4)
The percolation threshold pc is in general related to the coordination number z of network by the following approximate formulae: p p l .5/z
(5)
By using the interpolation formulae (4) for Q,(p) we obtain for the integral pore size distribution function the final equation:
73
In calculating the mesopores size distribution by the percolation method the only free parameter is the coordination number z of pore network. If there is no independent information about the special topology of the pores, the choice of a coordindation number of six is the most reasonable. The typical example of pore size distribution is presented on Fig. 3 for a sample of silica gel of medium porosity exhiting the classical hysteresis loop of type H1. Curve 1 shows the volume differential pore size distribtuion calculated from the adsorption and desorption isotherms of nitrogen by using equation ( 6 ) by Dubinin's method from the adsorption isotherm (curve 2) and the desorption isotherm (curve 3 ) using the model of unrelated pores.
Fi.g. 3 . Differential size distribution functions (N2/Si02,loop H1) , 1-percolation method, 2 - standard method (adsorption branch), 3- standard method (desorption branch). Finally we should mention yet another advantage of the percolation method: it avoids the necessity of using either the socalled absolute isotherms or the t-curves of the Boer (ref. l), wich occur in any of standard methods of calculation. In the percolation method the absolute isotherm is replaced by the initial part of the adsorption branch of the isotherm, V + ( x ) , which
74
indisputably reflects the contribution of reversible adsorption processes more faithfully than does the absolute isotherm, which is obtained by averaging experimental data for non-porous samples. REFERENCES 1 S.J. Gregg, K.S.W.Sing, Adsorption, Surface Area and Porosity, Academy Press, London (1982). 2 G.C. Wall, R.J.C. Brown, J. Coll. Int. Sci., 82, 36 (1981). 3 L.I. Kheifets, A.V. Neimark, Multiphase Processes in Porous Media, Khimia, Moscow (1982). 4 G. Mason, J. Coll. Int. Sci., 88, 36 (1982). 5 A.V. Neimark, Rep. Acad. Sci. USSR (Phys. Chem.). 273, 867 (1983). 6
7 8
A.V. Neimark, Colloid J. (USSR), 46, 813 (1984). A.V. Neimark, Russian J. Phys. Chem. 60, 1045 (1986). G. Mason, Characterization of porous solids (COPS-I), 1988, p. 223.
M. Parlar, Y.C. Yortsos, J. Coll. Int. Sci., 124, 162 (1988). 10 M. Ianuka, J. coll. Int. Sci., 127, 35, 48 (1989). 11 A.V. Neimark, The theory of capillary hysteresis phenomena and methods of calculation of mesoporous structure characteristics, D.Sc. Thesis. Moscow State University (1987). 12 A.V. Neimark, A.B. Rabinovich, L.I. Kheifets, Preprints of COPS11. Poster 31. Alicante, (1990).
9
75
F. Rodriguez-Reinosoet al. (Editors),Characterization 0; Porous Solids I1 0 1991 Elsevier Science PublishersB.V., Amsterdam
MODELLING OF MERCURY INTRUSION AND EXTRUSION M Day*, I B Parker, J Bell, M Thomas, R Fletcher, J Duffie ICI Chemicals & Polymers Ltd. Wilton, Cleveland TS6 W E , UK ABSTRACT Experimental observation of mercury intrusion, extrusion and entrapment are reported for two unimodal and two bimodal materials. The results are discussed in terms of cubic network models with capillary channels assigned at random from a log normal distribution. Criticality is determined by the Washburn equation and two mechanisms for filling and emptying the networks are considered. INTRODUCTION Mercury porosimetry is widely used to determine the pore size distributions of porous solids. The relationship between pressure and intruded pore diameter is described by the Washburn equation. The extrusion of mercury as the pressure is lowered after intrusion reveals two general phenomena, namely hysteresis between intrusion and extrusion, and the entrapment of mercury when the pressure is reduced to one atmosphere.
It is now believed that these are dependent on the topology and
pore geometry of the material and not simply on pore size distribution (1, 2, 3, 4 ) .
The present project was designed to provide a better understanding of the mechanism of entrapment and hysteresis. To this end, mathematical models of various hypothetical networks of different connectivity have been developed. A systematic investigation of experimental intrusion-extrusion and entrapment
has also been carried out on four well characterised rigid mesoporous and macroporous solids, with either uni-and bimodal pore size distributions.
In
addition, the mechanism of thread breakage in single 15 um glass capillaries has been studied. * Address for correspondence METHODS AND RESULTS Techniques and Materials Micromeritics 9220 mercury porosimeters were used, capable of generating pressures up to 4 1 4 MN/m2, using programmable pressure tables, and employing "dwell times" from < 30-3600 seconds at each intrusion or extrusion pressure. Four rigid porous solids A, B, C, D have been examined.
A,
(an alumina),
C and D , (silica aluminas) were derived from our laboratories, and B was a Shell silica coded
S 980
G 2 3.
They were dried in air in an oven at llO°C
16 overnight, then evacuated in the porosimeter to 2 N/m2 for half an hour. The porosimeter was programmed to carry out four consecutive intrusion/extrusion cycles.
In order to generate partial intrusionlextrusion
loops, intrusion pressures up to the desired level were programmed, followed by extrusion to 200 kN/m2, then reintrusion to the original intrusion level. In complementary experiments a 'macro" capillary, id=15 wn width, length 20 nun was designed to fit into one of the ports of the porosimeter for visual observation of intrusion, extrusion, and entrapment via thread breakage at pressures up to 200 kNlm2. Experimental Results Table 1 reports the total pore volume of samples A , B, C, D, together with the maximum entrapment, and mean pore diameters derived using the Washburn equation, and assuming 7
=
485 dyn/cm and 8 = 140 degrees, where 7 is the
surface tension, and 0 the angle of contact of mercury at 25°C. Table 1 Sample A
Total intrusion mllg
Max entrapment % total intrusion
0.52 0.85 1.50
B C D
0.59 A and
Mean pore diam wn
18 16
0.011 0.049
32 30
0.300 0.010
1.50 0.10
B are clearly unimodal with narrow distributions of pore sizes:C and
D are bimodal, with two different separations between the two sets of pores.
Hysteresis and entrapment were observed in all four samples. The first extrusion led to entrapment of the order of 20% total intrusion, and second intrusion starting from the entrapped condition, followed the analytical form of first intrusion and was reproducible over at least four cycles for a given "dwell time" per pressure point. Table 2 reports intrusion and entrapment for each cycle for sample A at two pressures. The effect of "dwell time", and hence total run time on the envelope of first intrusion/extrusionwas determined for "dwell times" from 30-3600 seconds, on both samples A . B .
Table 3 reports the total intrusion and
entrapment for each total run time. The behaviour of intrusion and extrusion in the first cycle as a function of the extent of intrusion was determined on samples A , C. D, and compared with the total envelope data.
The effect of partial intrusion was to narrow
the hysteresis loop and decrease entrapment. The data are shown in Fig (1,2).
77 TABLE 2 ( R e p r o d u c i b i l i t y of I n t r u s i o n , Cycle number 1 2
vol. m l / g p = 158.6
5 Dwell t i m e
-
intruded a t 27.4 MN/m’ 0.107 0.110 0.110 0.112 0.113
0. 380 0. 380 0.380 0. 380 0.380
3 4
Sample A)
30 s e c s
TABLE 3 ( E f f e c t of D w e l l Time i n Seconds) Sample B
Sample A Dwell
Dwell Volume. m l / g Intruded Entrapped
time
0. 516 0. 509 0.535 0. 519 0. 514
30 I20 300 600 3600
Volume, m l / g Intruded Entrapped
time
0.080 0.072 0.086 0.071 0.069
30
0.837
3600
0.860
0.137
0.144
TABLE 4 ( E f f e c t of P a r t i a l I n t r u s i o n )
Pressure MN/ m’
Sample A
117.2 131.0 144.8
Sample C
Sample D
TABLE 5 Mechanism
158.6 241.4 413.8 5. 5 34.5 41. 4 55.2 103.4 413.8 34.5 103.4 206.9
Intrusion volume mli g
Entrapment volume mll g
Width of l o o p MN/ m’
0.078 0. 151 0. 274 0. 360 0. 527 0. 556 0.573 0. 815 I. 073 1. 313 1.475 1. 513
0. 031 0.040 0.062 0.071 0.094 0.099
not p a r a l l e l 0.48 0.75 0. 82 0. 88
0.430
not p a r a l l e l
I. 10
0.500 0.520 0. 523 0.520 0. 522 0.127 0.152 0.176
0. 363 0.589
0.21 0.41 s e e Fig 2
( N o d e l R e s u l t s on Network A) Size of Network
SD ofPSD
Fraction Knocked
Z
our
Fraction Blinded Pores
Hyster -esis Loop
WidtP kNlm
Entraprent
ReIntrusion Path
0. 0 0
N N
0 0 18 24 0
23 46 57 71
E E
11
55 66 71 63 78 48 56 62 72 18
z
Pores
I
30X30X30
I1
30X30X30
0. 00 0. 50 0.83 0. 97
18 18 18 18 18 18 18 18 18 18 36 36 36 36
W
0.00 0. 00
P P/ w
0.00
n. .. nn _.
0. 50 0. 67
0. 00
0 . 00
P
0. a3
0. 00 0. 50 0. 67 0. 00 0. 00
P PI w
0 . 00 0 . 00 0.00 0 . 33
0. 50 0. 67
Shape o f h y s t e r e s i s l o o p
0. 00
as
24 34 09 20 0 12 24 43 110
P
P P P
0. 0 0 0.00
W=WEDGE
P=PARALLEL
P a t h of r e - i n t r u s i o n 1
Along f i r s t i n t r u s i o n = I
2
Along e x t r u s i o n
3
In between i n t r u s i o n and e x t r u s i o n
4
J o i n s i n t u s i o n t o w a r d s t h e t o p of i n t r u s i o n p a t h = I-TOP
5
J o i n s i n t r u s i o n h a l f way up t h e i n t r u s i o n p a t h
=
E
I
I I/EALF
APPROX E I- TOP APPROX E
I/EALF I-TOP INTERHED I
p a r a l l e l o r wedge i s a q u a l i t a t i v e d e s c r i p t i o n f r o m t h e graph6 of
i n t r u s i o n l e x t r u s i o n v e r s u s log p o r e d i a m e t e r . N=NO LOOP
34
E =
INTERMED
=
IlHALF
79
FIG 2 1.6
,
PARTIAL INTRUSION DATA SAMPLE C 7
__-
41 3.7
_ _ _ _ _ _ 103.4 1.2
-
1.1
o"
0.7
>
0.6
55.2
_ _ _ 41.4
34.5 E E J.J
5
7 1 9 7
LOG10 PRESSURE (Pa)
PARTIAL INTRUSION DATA 0.6
0.5
0.4
r: 0.2
0.1
0
80
THE MODEL Network Definition The three-dimensional model network is based on a cubic lattice.
The
lengths of the pores are all equal to one unit and the radii of the pores are allocated as random values from a log-normal distribution using a NAG library routine. A pseudo-random generator is used with a fixed seed s o that reproducible runs can be achieved.
Reduced connectivity was achieved by
randomly deleting pores from a network by "knock-out",and blind pores were created by blocking access to a node at one end of the pore. A network containing 30 x 30 x 30 segments was used. Algorithm Details Intrusion and extrusion were assumed to be determined by the Washburn equation. Intrusion.
The lattice is scanned to find all the pore critical pressures.
These are ordered in ascending size and the pressure increased in steps to achieve each critical pressure in turn. equivalent pore is investigated to
see
As each pressure is reached the if Hg exists at either end.
If not,
the next pressure is taken. If Hg at the entrance is trapped, the next pressure increment is made with no further action.
Filling occurs when the
mercury has a continuous path to the outside. When this occurs both end nodes are checked for other super critical pores. All empty pores of greater diameter connected to this critical pore are filled. All pores containing trapped mercury encountered during this procedure are now flagged as no longer trapped. An air seed is flagged when mercury is present at both ends of the critical pore.
No air seeds are created above their critical pressure
as we do not model the sequence of filling, only the logic.
The scanning
process continues until the designated proportion of the network is filled. Extrusion.
The pressure is dropped in the reverse of the sequence for
incrementation. The critical pore is examined. If no Hg exists at one end, and a liquid path exists to the outside, then that pore is emptied. Pores connected to the emptying end are further examined for the possibility of emptying because new Hg/air interfaces have been created. Mechanisms Preliminary capillary experiments indicated that where filling takes place from both ends and a seed is trapped within the capillary, emptying will take place, but when filling is from one end, reduction of applied pressure alone is not sufficient to cause the capillary to empty. Two theoretical mechanisms of intrusion/extrusionin the network were
considered, as follows:-
81
tlb 3
NETWORK A
10
0
6.6
7
6.8
7.2
7.6
7.4
-778.2 8.4
8
7.8
8.6
LOGlO PRESSURE (Pa)
FIG 4
'lo 100
80
MECHANISM II: EFFECT NETWORK A
3 T-f-i-inirusion
-
~
r
--1 ,
1 follows intrusion
-1
L-_ -
OF KNOCKOUT
I
z
$
.-,-------
70
3
5
601
I
5
50
2
40-
n
30 20 10
0
1
-
No K n o c k o u t
-
-
I
2 in 6 K n o c k o u t
I1 II
4I !
6
I
I
I I
1
6.4
I
I
6.8
I
I
7.2
I
I
7.6
LOGlO PRESSURE (Po)
I
,k-8
. . --I--
8.4
8.8
82
FIG 5
MECHANISM II: EFFECT OF KNOCKOUT NETWORK A
60
50
10
0
I
!
I
I
1
I
I
I
I
I
4:
---7---r-1 8.4 8.8
a
7.6
7.2
6.8
6.4
LOGlO PRESSURE (Pa)
110 100-
90
-
ao
-
$
70-
$
60-
3
s 50
M
30 40
20
-.-.
.,L.a.*.. " YL.A...
.,
.*.*r;UYI
,;".: :.
2 .
Y L . ~ V " "-.).-I Y Y ~Y
, , , , u " * * *
I ._
-------
1st R e - i n t r u s i o n / E x t r u s i o n 2nd R e - i n t r u s i o n / E x t r u s i o n
i
I'
~
I
.......--.--.
i
-
10 -
0
I
6.6
1
6.8
!
~
I
---_--
- 1st I n t r u s i o n / E x t r u s i o n
I
3
4 IN 6 KNOCKOUT
I
7
I
I
7.2
I
I
7.4
,
r
7.6
,
1 - - 7 7 r - T - i -
7.0
a
0.2
0.4
0.6
83
I
A mechanism which proposes the spontaneous nucleation of the mercury
meniscus at the start of extrusion. path exists to the outside. their
own
Pores always empty if a continuous
Pores which fill at pressures other than
Pc will be trapped.
Pores which fill at their critical
pressure have at least one path to the outside bath consisting of larger diameter pores.
If a pore fills at a higher pressure than its own
critical pressure, then all paths to the outside have at least one pore of smaller radius. own
I1
These will empty at higher pressures to the pores
critical pressure and leave this pore trapped.
A mechanism (multiseed) in which the presence of a disturbing influence
is assumed necessary to encourage the mercurylair interface to be created.
Following Mann ( 3 ) , the idea of a single interface, or an air
seed interface with two menisci which exist within any pore filled from both ends at its critical pressure, and which persists because of the high pressure of trapped gas, was considered. Both ends of the pore have to be investigated separately.
If no filled path exists to the
outside, the whole set of pores visited are flagged as trapped and do not take further part in the extrusion process. Results of Modelline, The two mechanisms were applied to networks based on the mean pore radius of sample A derived from intrusion data and two unimodal distributions of pore radii including one similar to that of sample A , with or without a reduction in connectivity via blinding or knocking out pores, running three intrusion/extrusioncycles per network.
This does not imply that the network
represents sample A , and so will be referred to as network A .
In each case
the cycles were examined for hysteresis, the extent of hysteresis, the amount of entrapment, and whether re-intrusion followed the path of extrusion or first intrusion, as a function of connectivity. Results are reported in Table 5 and Figs 3 , 4 , 5 , 6 . Discussion and Conclusions The experimental findings can be summarised as follows. Hysteresis between intrusion and extrusion was reproducible in all cases, and there was no significant time dependence. observed in all cases
:
Entrapment at the end of first extrusion was
it was reproducible and permanent in the time scale
of the experiment. Re-intrusion has the analytical form of first intrusion in all cases.
Partial intrusion resulted in a lower level of entrapment and
narrowing of hysteresis compared to full instrusion. The level of entrapment and the width of hysteresis were proportional to the extent of intrusion. Discussion of modelling is confined to network A .
It was found that a
decrease in contact angle will generate hysteresis, but not entrapment, in any system, and has not been considered further. Mechanism I did not predict
84 hysteresis for a network with connectivity of six, and required a minimum knock-out of 86% of pores to generate hysteresis in which intrusion and extrusion were separate and parallel.
In all cases, re-intrusion followed
the path of extrusion and so did not accord with experimental observations. Mechanism I1 predicted partial hysteresis for a network with full connectivity. From zero knock-out to 50%, re-intrusion followed first cycle intrusion, as observed experimentally, but as knock-out increased above 50%, the path of re-intrusion tended to move back towards the path of extrusion. Partial instrusion resulted in a narrowing of the hysteresis l o o p and a decrease in the level of entrapment, as observed experimentally. REFERENCES 1 2 3 4
D H Everett, "Characterisationof Porous Solids", p 229. Society of Chemical Industry, 1979. W C Conner, J Catal 83, 336, 1983 R Mann, Chem Eng Science 34, 1203, 1979 N C Wardlaw, Powder Technology 29, 127, 1989 This is a collaborative project between Catalysis Research Centre (Billingham), Analytical and Physical Sciences Group (Runcorn), and Petrochemicals Group (Wilton) of ICI Chemicals and Polymers Ltd.
ACKNOWLEDGEMENTS The authors wish to acknowledge valuable discussions with Prof K S W Sing, Brunel University, and Dr D Nicholson, Imperial College, throughout this project .
85
F. Rodriguez-Reinoso et al. (Editors), Characterization of Porous Solids I1 0 1991 Elsevier Science Publishers B.V., Amsterdam
WETTING PHENOMENA IN POROUS SOLIDS: MECHANISMS AND MODELS
A. Winter Geological Survey of Denmark, Thoravej 8, DK-2400 Copenhagen NV, Denmark
ABSTRACT The purpose of this paper is to describe physical mechanisms and mathematical models of disordered porous solids relevant for studies of wetting phenomena in such media. Two distinct levels of investigation are considered: a. the level of a single pore, b. the level of a network of capillaries. Investigations of wetting phenomena at the single pore level include a description of two different wetting regimes. Furthermore, a criterion for stability of thin wetting films is described. The class of mathematical models of porous solids pertaining to the level of a network of capillaries is that of fractal constructs. Applicability of such models to description of porewall roughness is discussed. WETTING PROBLEM: OPEN AND CONFINED GEOMETRIES Wetting phenomena in porous solids involve three coexisting phases: fluid (liquid A or gas), liquid B and solid. In order to describe the interfacial properties of such systems one needs three interfacial tensions, 71v,ysl, and ysv,pertaining to the liquid-fluid-, solid-liquidand solid-fluid interfaces, respectively. An important parameter in studies of wetting phenomena is the spreading coefficient,
S, defined as follows
s = ysv
- Ysl - Ylv
(1)
Two different situations may occur, one where S is negative and another with S being positive. In the former case the liquid wets the underlying solid partially, i.e. a liquid drop placed on the solid does not spread spontaneousIy and the contact line, separating the three coexisting phases, makes a well-defined angle, 0 , with the substrate. In the second case (S > 0), the liquid spreads spontaneously on the solid. More specifically, a thin liquid film separating the fluid phase and substrate is created ( 0 = 0). According to Antonow’s rule (ref. ll), the solid-fluid interfacial tension is given by the following expression
Ysv
= Ylv
+ Ysl
(2)
or, equivalently, S = 0. It should be noted that Antonow’s rule is sometimes violated. This may happen, for instance, when the three-phase system under consideration is described by two order parameters (ref. 11).
86
Experimental investigations of configurations of the wetting- and nonwetting phases in square-sectional capillaries have shown that the wetting fluid tends to occupy the four corners of the capillary. On the other hand, the center of the capillary is filled by the nonwetting phase, see Figure 1. Triangular - Like Channel
a
C
t Brine
I
(Porewal I) phase 1
I
d
Fig. 1. Configuration of the "oil-water'' interface in a square-sectional capillary tubing: (a) 45' plane. (b) and (c) 90° plane (Figure l(b)) shows the case of strong adverse pressure: only corners of the capillary are wetted; Figure l(c) shows the case of weak adverse
pressure: porewalls are entirely wetted). The close-up l(d) shows the thin film (dotted) consisting of N molecules surrounded by the bulk aqueous phase consisting of N-M molecules. An important question is to what extent the wetting phase is present outside the four corners. This problem has been treated theoretically by Joanny and de Gennes (ref. 7). They assumed that the wetting phase is influenced by two kinds of forces: van der Waals forces favoring the existence of a wetting film and the adverse pressure, A p , opposing its formation. In the case of a horizontal capillary shown in Figure 1, the adverse pressure is identical to the capillary pressure, i.e. the difference in pressure, A p c , between the wetting- and nonwetting phases.
Thus, according to the Laplace equation APC = rY
(3)
where Ape is the capillary pressure, y is the interfacial tension between oil and water phases and r is the mean curvature of the interface. In particular, Joanny and de Gennes (ref. 7) have shown that the wetting film extends beyond the four corners of the capillary provided that the adverse pressure exceeds certain critical level, ApCr,given by the following expression
where S is the spreading coefficient and A is the Hamaker constant. Theoretical underpinnings of the problem of presence or absence of thin wetting films in square-sectional capillaries are given in the subsequent section. STABILITY O F THIN WETTING FILMS AND THICKNESS TRANSITIONS Thin wetting films can be considered as phases extending in two lateral dimensions only. Consequently, most of their properties are controlled by forces originating from molecules residing in the ambient, three-dimensional bulk phases. In particular, this is always the case when thickness of the wetting film is smaller than the range of intermolecular interactions. Such interactions typically extend from a few to hundreds of atomic diameters. In the case shown in Figure 2a the chemical potential of the a m , p 3 ~ is, the same as that in the 3D film phase under the same conditions. On the other hand, in the case where there are overlapping force fields originating from the two interfaces of the thin film (cf. Figure 2b), the chemical potential deviates from its value in the 3D phase. More precisely,
~ ~ f (=hP )~ + D pez(h)
(5)
where p e z is the chemical potential that a molecule residing in the 2D film phase has in excess (or deficiency) as compared with its counterpart placed in the 3D phase. The properties of the overlapping force fields may vary from one case to another depending on their origin. Consequently, the excess chemical potential, pez , can be influenced by forces underlying adsorption at the film surfaces, dispersion forces or electric forces acting between charged film surfaces. It should be noted that the excess chemical potential of the film phase is related to the so-called disjoining pressure, II(h) , introduced by Derjagin and his co-workers. It is defined by the following formula (ref. 5)
88
(6)
k ( h ) = - v H( h )
where v the volume per molecule in the 3D phase, i.e. in the infinitely thick film. The problem of dependence of the disjoining pressure function on film thickness has been studied by Dzyaloshinskii, Lifshitz and Pitaevskii (ref. 6).
phase 1 phase 1 3D
phase 2 phase 2 a
b
Fig. 2. Schematic cross-section of a thin film between two bulk phases: (a) the force fields of the two film interfaces do not reach each other, (b) overlapping of the force fields originating from the two film interfaces. Consider now a thin film configuration shown in Figure 2. The thin wetting film 3 is assumed to consist of a one-component liquid squeezed between two plane-parallel 3D phases 1 and 2 representing a porewall and nonwetting liquid, respectively. Let us assume that the positions of the dividing surfaces have been chosen according to the usual conventions (ref. 4). The wetting film is assumed to consist of N molecules. Its thickness can be determined from the following expression
h = (v/A)N
(7)
where v is the volume per molecule in the infinitely thick (3D) film phase and A is film area. In order to formulate the stability conditions for the thin film phase, assume that it M molecules. The chemical potential of the M-N molecules, exterior to the film phase, is denoted by p . The Gibbs free energy of the entire system, G, , is given by the following expression assuming that M >> N: is a part of a larger system containing
G,(N) = ( M - N ) p
+ G ( N )+ constant
where G (N)is the Gibbs free energy of the wetting film. The Gibbs free energy of a finite size, one-component film phase of N molecules can be stated as follows (refs. 8J2)
89
where 1130 is the Gibbs free energy which the finite-size film phase would have had it were a part of a 3D phase of the same composition and @ ( N )is the excess free energy of the
film due to the work associated with the formation of its two interfaces. The system will be in stable thermodynamic equilibrium when its Gibbs free energy fulfils the conditions (dG,/dN)~,,,,,t = 0 and (d2Gs/dN2)A=const > 0. Combining equations (5), (7) and the stability criteria stated above, one can derive the following conditions for the stable equilibrium
P f ( h )= P
and d t”f(h)/dh> 0
(11)
where p is the chemical potential of the contacting 3D phase. If only eq. (lo), is satisfied, but dPf(h)/dh <0
(12)
the film is said to be in unstable thermodynamic equilibrium. It should be noted that equation (10) defines thickness at which a thin wetting film is in the thermodynamical equilibrium. Moreover, for a given value of the chemical potential, p , in the ambient 3D phase and depending on type of nonlinearity associated with the prespecified p f ( h ) dependence, a number of solutions to eq. (10) varies. Let us consider now the case of p f ( h ) dependence shown in Figure 3. The ascending parts of p f ( h ) correspond to a thin film being in a stable thermodynamic equilibrium for h < h,,, and h > hmin. On the other hand, the thickness range corresponding to the descending portion of the p f ( h ) , results in unstable thermodynamic equilibrium of the is termed a -film and its wetting film. In the stable case, the thinner film ( h 5 h,,,) thicker counterpart ( h 2 h,i,) is termed /? -film. Thus, the wetting film can appear in two thickness states, each representing a distinct 2D phase. One concrete example of a situation with the sigmoidal p f ( h ) dependence is when the dominating components of the disjoining pressure are those representing the molecularand ionic-electrostatic molecular interactions, i.e. II(h)= A / h 3
+B/h2
(13)
where A is the Hamaker constant and B is another constant representing the ionicelectrostatic forces. In this case, the augmented Young-Laplace equation is as follows (cf. eq.(3)
90
where I' = 1/R1+ 1/Rz (R1 and R2 are the orthogonal radii of curvature). In the center of the tubing to the following form
2 = 0 and the Young-Laplace equation (14) degenerates
h3 - ( B / A p ) h+ A / A P = 0 where A P = AP, - y/Rz
hmax
hmin
h
Fig. 3. Portion of a thin film chemical potential isotherm. cc -film and /3 -film of different thickness coexist at p = p e . h,,, and hmin are the equilibrium thin film thickness correspondinq to pmaz and pmin , respectively. Equality of the two hatched areas follows from Maxwell s rule. The above considerations indicate that presence or absence of thin wetting films in square-sectional capillaries can be explained by different values of the excess chemical potential of the wetting film in the two cases. Such changes of the excess chemical potential also appear in certain enhanced oil recovery schemes. This problem is taken up in more details in the subsequent section. MOBILIZATION PROCESSES AND WETTING: SINGLE PORE LEVEL This section describes an experimental study of mobilization of a drop trapped in a square-sectional capillary by a microemulsion slug (refs. 1, 2). Its main purpose is to give a concrete example of a situation where thin film phases appear in a complex industrial process (here: enhanced oil recovery processes at the microscale). The sequence of events observed during mobilization of entrapped drops of a non- wetting phase in a square-sectional capillary is as follows (cf. Figure 4): 1. The front of a microemulsion slug is approximately 25 microns from the back of the drop. The trailing interface of the nonane suddenly retracts and contacts the slug of microemulsion. This is probably due to a film of brine surrounding the front of the
91
slug. The high concentration of the surfactant in this film seems to be responsible for a steep change in the interfacial tension between the wetting- and nonwetting phases in the rear part of the drop. The drop jumps backwards toward the slug and returns to a stable configuration (cf. Figures 4-1and 4-2). 2. The rear interface of the drop is locally ruptured by the front of the microemulsion. The observed phenomena (strong rippling and rapid expansions and contractions of the interface) are probably caused by local changes in the interfacial tension (Marangoni effect) (cf. Figure 4-3).
3. Marangoni effect initiates internal circulation inside the drop. In particular, isolated parts of the drop, close to its rear interface, become emulsified. Consequently, an internal interface separating the emulsified and nonemulsified parts of drop is created. The rolling motion inside the drop is responsible for the transport of its non-emulsified parts to the surface (cf. Figure 4-4). 4. At the final stage of mobilization the whole drop becomes emulsified. The microemul-
sion slug bypasses the drop which snaps-off through the constriction (cf. Figures 4 5 and 4-6). The sequence of events described above is by no means unique: it is extremely sensitive to a composition of the microemulsion. Nevertheless, it provides an excellent illustration of complexity of physicochemical mechanisms governing stability and thickness transitions in thin film phases. WETTING PHENOMENA IN POROUS SOLIDS This section extends our discussion of wetting regimes in a single pore to the level of a network of pores. As the porous solids are structurally extremely complex, this section focuses only on one property affecting wetting phenomena: fractal nature of porewall roughness. A number of recent papers support the hypothesis that, at least in some cases, roughness is indeed fractal (ref. 9). As a concrete example of a model porous medium consider a geometrical construct shown in Figure 5 . Its cross-section consisting of polygonal grains and square-sectional pores in is shown in Figure 6. The porespace is a prespecified range of cutoffs, lmin5 1 5 I,,,, assumed to be initially entirely filled with a wetting phase. The oil phase migrating from the source rock into the reservoir zone displaces the resident wetting fluid. The natural question to be posed is whether wetting films extend beyond the corners of capillary tubes. Assume that the adverse pressure, favoring presence of wetting films, is represented by the van der Waals dispersion forces. In that case all tubes for which the capillary pressure is smaller than the critical pressure given by eq. (4)will have wetting films entirely covering the porewalls. In the remaining tubes wetting films will be retained only in the corners (cf. Figure 6). The above hypothesis concerning distribution of the wetting phase in a fractal porous solid has been tested by considering a capillary pressure measurement using the porous plate method. In capillary pressure measurements the porous sample is initially totally filled with the water phase. A nonwetting phase ( e g oil) is then gradually injected into the sample. As the injection pressure increases, more and more water is removed from the solid. At each step of the experiment the wetting phase appears in one of the two regimes shown in in Figure 1. It seems reasonable to assume that at the end of the desaturation
92
Fig. 4. Enhanced oil recovery at the microscale: mobilization of a trapped nonane drop by a microemulsion slug (modified after (ref. 1)). Detailed explanation of the 6 stages of the mobilization process are given in the text. process all (or almost all) pores retain the wetting phase only in corners of capillary tubes. However, in the case of extremely smooth pores the wetting phase in thin films connecting the corners must also be considered.
93
A simple derivation shows that for the class of fractal porous media shown in Figure 5, the capillary pressure, P,, and the saturation of the wetting phase, S, , are linked by the following relation (cf. Winter, in preparation)
where D is the fractal dimension of the porewalls characterizing their roughness and S, is the part of the wetting phase remaining ineorners of tubings.
Fig. 5 . Three-dimensional view of the fractal construct used as a model porous medium. By fitting the results of capillary pressure experiments (cf. Table I) to the above expression one gets the following values of the fractal dimension for the two cases: D1 = 2.40 and D2 = 2.29. The correlation coefficients are 1.00 and 0.99, respectively. The shape of the distribution function of pore sizes is, of course, a crucial parameter controlling wetting regimes in fractal porous media: depending on its properties, the fraction of pores in one of the two wetting regimes varies affecting many important properties of the medium, such as, e.g. its ability of fluid transport.
94
a
b
Fig. 6. Cross-section through a fractal porous medium showing the configuration of the thin film phase in the pores. Only pores with sizes below the critical threshold level are entirely wetted. In the remaining pores the presence of the film phase is controlled by the magnitude of the adverse pressure: (a) the porewalls are entirely wetted; (b) the film phase appears in corners only.
95
FINAL REMARKS The fundamental mechanisms governing wetting phenomena in disordered porous solids have been described. At the level of a single pore, a square-sectional-pore has been used both as an experimental and theoretical tool in wettability studies. At the level of a network of pores, a fractal porous network has been introduced to describe roughness of porewalls. A fractal dimension associated with this network has been found from the capillary pressure curve. Thus, two parameters emerge as the decriptors of wetting phenomena in a complex porous network: (a) at the single pore level: the exponents in monomials in the film thickness descibing the disjoining pressure isotherms (cf. eq. (13). These exponents charaterize solidliquid interactions and do not change as the configuration of liquids in the pore space varies (ref. 13). At the level of a network of pores: the fractal dimension describing roughness of porewalls. (b) More work, combining information derived from thin film physics at the single pore level and utilizing fractal geometry at the network level, is necessary to assess the applicability of the approach described in this paper to evaluation of wetting phenomena in real life industrial processes.
ACKNOWLEDGEMENTS This work was supported by the EC Research Contract No. RIlB-0290-C(AM).
Saturation
PC
Saturation
PC
(fraction)
psi
(fraction)
psi
0 2 3.97 7.96 15.9 31.2 64 160
1 0.163 0.098 0.063 0.043 0.027 0.016 0.007
1
0.155 0.100 0.069 0.047 0.033 0.020 0.011
0
0 3.97 7.96 15.9 31.2 64 160
Table I. Results of capillaly pressure measurements. Only the 6 last measurements, corresponding to water saturations smaller than 0.069, have been used in the computations of the fractal dimension.
96
REFERENCES 1. Arriola, A., Ph.D. Thesis, University of Kansas, 1983.
2. Birdi, K.S., Vu, D.T. & Winter, A., Proceedings of the IV-th European Symposium on Enhanced Oil Recovery, Hamburg, October 27-29, (1987) 945. 3. Birdi, K.S., Vu, D.T. and Winter, A., Experimental Studies of Mobilization Mechanisms in Square-sectional Capillaries, (in preparation). 4. Croxton, C., Introduction to Liquid State Physics, J. Wiley and Sons, 1975. 5. Derjagin, B.V., Churaev, N.V., Muller, V.M., Surface Forces, Plenum Publishing Corporation, New York, 1987. 6. Dzyaloshinskii, I.E., Lifshitz, E.M., Pitaevskii, L.P., Sovjet Physics JETP, vol. 37, (1960) 161. 7. Joanny, J.F., de Gennes, P.G., C.R. Acad. Sc. Paris, t. 299, serie I1 no. 10 (1984) 605. 8. Kashchiev, D., Surface Science, vol. 225, (1990) 107. 9. Katz, A.J., Thompson, A.H., Physical Review Letters, vol. 54, no. 12, (1985) 1325. 10. Lenormand, R., Zarcone, C. & Sarr, A., J. Fluid Mechanics, vol. 135, (1983) 337. 11. Rowlinson, J.S., Widom, B., Molecular Theory of Capillarity, Oxford University Press, 1982. 12. Rusanov, A.I., Phasengleichgewichte und Grenzflaechenerscheinungen,Akademie Verlag, Berlin, 1978. 13. Toledo, P.G., Novy, R.A., Davis, H.T. and Scriven, L.E., International Workshop on "Indirect Methods for Estimating the Hydraulic Properties of Unsaturated Solids", Riverside, CA, October 11-13, (1989). 14. Winter, A., in: Phase Transitions in Soft Condensed Matter, T. Riste & D. Sherrington (eds.), pp. 237-243, Plenum Publishing Corporation, New York, (1989) 237. 15. Winter, A,, International Journal of Physicochemical Hydrodynamics, vol. 9, no. 3-4, (1987) 589. 16. Winter, A., in: Mathematics of Oil Recovery, P. King (ed.), to be published by Oxford University Press. 17. Winter, A., Stability of Thin Wetting Films and Wettability Reversal in Reservoir Rocks, presented at the Symposium on findamentals of Fluid Transport in Porous Media organized by Institut Francais du Petrole, May 14-18, 1990, Arles, France (in preparation).
F. Rodriguez-Reinosoet al. (Editors), Characterization of Porous Solids I1 0 1991 Elsevier Science Publishers B.V., Amsterdam
97
THE CONTACT ANGLE OF LIQUIDS IN POROUS MEDIA U. Demlehner Wacker-Chemie GmbH, Dept. F/PK 20, D-8263 Burghausen, FRG
SUMMARY We developed a method to determine the contact angle of a liquid penetrating a porous medium by a non-linear fit ofthe continuously measured liquid uptake over a period of some minutes. This method makes it possible to measure the contact angle without any additional assumptions on completely wetting reference liquids. The method was applied to porous glass frits with water, toluene, and different silicone oils. The measured contact angles ranging from 60' to 80' are much higher than the equilibrium contact angles on glass. INTRODUCTION The determination of the contact angle between a liquid and a solid material is of fundamental importance for the characterization of the wetting behaviour of the solid's surface. There exist a plenty of methods to measure the contact angle if the solid surface is non-porous and smooth, since the contact angle may then be measured by optical methods and in an equilibriumstate (ref 1). The determination ofthe contact angle between powder particles with irregular surfaces and a liquid is much more difficult. The powder may be compressed to a tablet showing a macroscopically smooth surface. Experiments and theory have shown that the contact angles measured on the outer surface of compressed powder cakes do not represent thermodynamically consistent equilibrium values (ref. 1). Methods for the determination of the contact angle between a liquid and the inner surface of a powder cake base in most cases on the capillary force which drives the liquid into the powder cake. The counter pressure to balance this capillary pressure is determined with the Bartell method (refs. 1,2,3). A widely used technique is the imbibition method based on the Washburn equation (refs. 1,4). Using this method, the rate ofcapillary penetration is measured either by observing the raise ofthe liquid penetrating the powder cake or by continuously registering the weight ofthe imbibed liquid (reE 5). Ifthe evaporation ofthe liquid is negligiblethe weight loss ofthe liquid reservoir may alternativelybe registered (ref 6). A premise for the imbibition method is the preparation of at least two totally identical powder plugs, one for the measurement with a reference liquid and the other one for the measurement with the liquid under investigation. From the experimental standpoint it is virtually impossible to prepare two powder cakes with identical pore system properties. The reference liquid has to be a completely wetting liquid with a contact angle of 6 = 0' (cos 6 = 1). Since the contact angle between this reference liquid and the powder surface cannot be measured by a direct method, the assumption 0 = 0' can never strictly be proofed. Furthermore, a contact angle measured by the imbibition method is an advancing contact angle and, therefore, not identical with the corresponding equilibrium contact angle.
98
The arguments listed above show that an imbibition method has to be developed, which makes the reference liquid unnecessary and which allows the determination of the contact angle with one sample of a
porous substrate and one liquid. This very method is an absolute method, i. e., no additional assumptions concerning the contact angle are necessary. THEORY Our approach uses the fact that the Washburn equation is an approximate solution for the liquid imbibition into a vertical capillary (refs. 1 , 4 , 7 , 8 , 9 ) . The equation of motion for a liquid raising in a vertical capillary with constant radius ris eq. 1, where his the height ofpenetration at time f, g the gravity constant, 71 the dynamic viscosity, p the gravimetric density and 0 the surface tension ofthe liquid.
If the porous body is modelled by a bundle of vertical capillaries with equal and constant radius r, the height ofpenetration h may be replaced by the liquid uptake per unit area p ofthe bundle (eq. 2). CL
= PPh This model may be extended to any porous body with constant cross section and porosity p , if r is
considered as an equivalent capillary radius or, simply, as an unspecified geometrical parameter. The differential equation for the liquid uptake per unit area p of a porous body is now eq. 3 with the parameters b and pm being defined by eq. 4 and 5. The dynamic viscosity q was replaced by the kinematic viscosity u using u=q / p .
The second term in eq. 3 is negligible for short time t and, consequently, low liquid uptake p. The solution for this limiting case is the Washbum equation (eq. 6). The physicochemicalparameters of the liquid and the porous body may be grouped to a system parameter w (eq. 7). This liquid uptake coefficientw is a measure for the rate of imbibition in the system liquid/porous body. It has been shown that the predictions of eq. 7 regarding the dependance ofthe liquid uptake coefficient w on the physicochemical parameters u and u are experimentally observed (ref. 6) . This may be considered as an indication, that the the capillary bundle model is an appropriate model for aporous body.
99
1
rpp2acos6
w = (
2u
(7)
Inspection of eq. 6 shows that it is not possible to determine the contact angle 8 without knowing the geometrical parameter r. This property of the equation was stressed in the Introduction. Since the Washburn equation is the solution for short time t, eq. 6 cannot be used to describe the liquid imbibition for t-m; the Washburn equation predicts p-00 for 1-00. This physical senselessprediction has been the subject ofmuch discussionwhich ignored the limiting character ofeq. 6 (ref. 10). The correct solution of eq. 3 for long time t is eq. 8. Inspection of this equation shows that p - p w for Consequently, the parameter pw is the final liquid uptake ofthe porous body after infinite time when
t-00.
the liquid has come to rest.
t
=
-1 b [p+p,ln
[1-2-]
If the In-term of eq. 8 is expanded into a first order series, the Washburn equation (eq. 6 ) is the result proving the connection between the liquid uptake coefficientw and the parameters b and pw (eq. 9).
w
=
(9)
[2bpm]1'2
The correct solution ofthe imbibition equation (eq. 8) opens the way to the simultaneous determination of the contact angle 0 and the geometrical parameter r. Eq. 8 may be fitted by standard non-linear regression
techniques if enough data points can be provided. The fit parameters b and pm allow the computation of 6 and r.
t
=
"
-b
p + p +p,ln O
[
pz2J]
I--
- to
Since we cannot start measuring exactly at t - 0 and p=O, we actually use eq. 8 with a minor modification (eq. 10). The parameters to and po serve to compensate the time offset at the start of the measuring process. It should by noted, that we now use t and p to designate our experimental coordinate system, i. e., t designates the time from the start of data sampling, not the time from the first contact between the liquid and the porous body. An alternative approach was proposed by Hilbig and Girlich (ref. 7). They used a second order series
expansion ofeq. 8 which leads to the linear regression equation shown in eq. 11with our notation. Despite its simplicity, the approach ofHilbig and Girlich does not appear to have found much recognition yet.
EXPERIMENTAL Toluene was of analytical grade and used without further purification. Water was deionized by an ionexchanger. The silicone oils (dimethylpolysiloxaneswith trimethylsilyl-endgroups) being of technical grade are manufactured by Wacker-Chemie GmbH, Munich (FRG).
100
Sintered glass frits were used as model I
porous bodies. 1500 g of lead free soda glass
I gloss frit
spheres (Dragonit 25, Dragon-Werk, FRG) with average diameter 45 - 70 pm were poured
sample support
into an alumina mould (160 x 130 x 90 mm3). The mould was covered with an alumina plate which fitted exactly into the mould. In a
liquid
___ electronic balance
--
computer
-
&& Apparatus for the imbibition measurements
chamber furnace the mould was heated to 640 'C with a heating rate of 3.5 Wmin, held for 120 min, and cooled to room temperature with a cooling rate of 10 'C/min. The samples with
constant
cross
section
(120x30x30mm3) were cut from the obtained body with a diamond circular saw. They were carefully cleaned with chromic acid, water and acetone. The glass fits were dried in a vacuum drying oven at 110 'C/50 mbar for 20 h before they were used for imbibition experiments. The apparatus for the measurement ofthe liquid imbibition into the glass fits has been described in detail elsewhere (ref. 6). A schematic sketch is shown in Fig. 1.Usually, the data acquisition frequencywas 1- 2 data
points per second. This means, that a typical experiment which lasts for about 10 - 15 min until the glass frit is saturated by the liquid, consists of cu. 400 data points. The data acquisition frequency was lowered in experiments with high viscosity liquids having a low imbibition rate. RESULTS AND DISCUSSION
Fit Procedure Tests have shown, that optimal performance of the fit may be achieved by a three step technique using standard linear and non-linear regression algorithms (ref. 11): Step 1: Linear regression of the Washburn equation (eq. 6) for the time range 10 s I 100 s of the imbibition measurement. The result ofthis step is the liquid uptake coeficient w. Step 2: Non-linear regression (Levenberg-Marquardt method) of the correct solution (eq. 10)
holding to and poconstant at 0. In order to fit the data by a non-linear regression, estimates ofthe parameters
b and pw have to be supplied. The estimate for the parameter b is computed by means of eq. 9 assuming an arbitrary value for the final liquid uptake pm. Our tests have shown, that the convergence of the non-linear fit is not deteriorated by a rather poor estimate of pw. We actually multiply the value of the liquid uptake at the by a factor oftwo or three and use this value as an estimate for pw. stop ofthe measurement pstop Step 3:
Non-linear regression (Levenberg-Marquardt method) ofthe correct solution (eq. 10)with all
were computed in step 2 and the estimates for to and poare parameters to vary. The estimates for b and pLm supposed to be 0.
101
Tests have shown that it is not possible to combine steps 2 and 3 into a single non-linear regression fit. The convergence ofthis four parameter fit is very poor; in many cases, the fit did not converge at all. Steps 2 and 3 utilize all the measured data points with t 10 s, since the very first data points should be rejected. The determination of the porosity p deserves special mention. This parameter
30
entered the theory of the imbibition process in
cN- 7
E
3 Y
eq. 2 as a proportionality factor connecting the 20
height h of the liquid in the porous body with
i
the liquid uptake per unit area p. This
0)
1 0
c
n
m ._
derivation makes clear that the porosity to be
10
considered here is defined as the ratio between
.-
U 3 1
the volume of the imbibed liquid and the 0
100
200
300
400
5
1
Time t [s]
geometrical volume of the porous body being the cross section ofthe body multiplied with the penetration height of the liquid. If the
Measured and fitted data
imbibition process is followed until the porous body is saturated with the liquid, the plot of p vs. f shows a point, where the slope of the curve
suddenly decreases. This point with the coordinates (tst/psat)is called the saturation point of the porous body. The porosity p may be computed from the saturation liquid uptake psatund the height of the porous body by an appropriate transformation of eq. 2. Alternatively, any combination of p and h may be used for this purpose. Inspection of Fig. 2 reveals that the applicability of the Washburn equation (eq. 6) is limited to data points with 10 s t c 100 s. The correct solution of the imbibition equation (eq. 10) fits the measured data (
points perfectly. Measured Contact Anales Table 1shows some values for the equivalent radius rand the contact angle 6'.
We used three groups ofliquids to test our approach for the determination of 6'. The first liquid was water being a high surface tension liquid (0 = 72 mN/m), while toluene and the silicone oils are low surface tension liquids (U= 28 mN/m (ref. 12) and c = 18 mN/m, respectively). The silicone oils AK 10, AK 50 and AK 300 are poly(dimethylsiloxanes) with trimethylsilyl-endgroups. They had viscosities at 25 "C of v = 7 mm2/s, Y = 35 mm2/s, and V
= 250 mm2/s, respectively.
All these liquids are wetting liquids for glass, i. e., the equilibrium contact angle on a smooth glass surface is eeq= 0 (ref. 13). It is evident, that the measured contact angles are far apart from this equilibrium contact angle eeq.They are essentially identical for all tested liquids. Discussion Our approach to the determination of contact angles between the capillary surface of porous bodies and liquid penetrating the capillaries makes it possible to measure the contact angles without any assumption on
102
TABLE 1 Measured values (av. = average for the respectiveliquid) Liquid
W
[kg/m2min’n]
I
P
Water Water Water
r
0.25 0.27 0.26
4.3 9.5 1.0
Water (av.) Toluene Toluene Toluene Toluene Toluene Toluene Toluene
10.7 9.99 11.7 11.9 10.3 10.0 9.81
110 144 27 1 408 124 118 107
0.27 0.26 0.27 0.31 0.26 0.22 0.25
3.9
3.3 2.7 2.1 3.6 3.9 3.8
Toluene (av.) SiliconeAK 10 SiliconeAK 50 SiliconeAK 3 00 Silicone (av.)
2.19 1.14 0.626
1
41.0 126 54.8
cos 0
0.24 0.24 0.23
6.5 3.4 7.5
0
“I
[PI
&mZ]
0.33 0.21 0.24
71 78 76
0.261t0.06
754
0.30 0.34 0.51 0.52 0.32 0.39 0.30
73 70 59 58 71 67 73
0.38k0.10
67dO
0.31 0.49 0.46
72 60 63
0.42*0.10
654
the wetting behaviour of a reference liquid. It is essential that enough data points are supplied for the nonlinear fit and that the time range of the experiment spans far beyond the scope ofthe Washburn equation (eq. 6).
Simulation tests have shown, that the ratio between the liquid uptake pstop at the stop of the experiment
, be larger than ca. 0.1 for the non-linear fit to converge with sufficient and the final liquid uptake l ~should precision. This limitation is imposed by the optimization procedure of the non-linear regression. If the ratio pstop/pw is smaller than ca. 0.1, the Washburn equation describes the data points well. This means from the
mathematical standpoint, that the non-linear fit will not converge. In this case the liquid uptake coefficientw can solely be determined. The contact angles measured by the described method are much higher than the equilibrium contact angles of the respective liquids on smooth glass surfaces. This result parallels results ofZografi et af. (refs. 15, 16,17), who showed, that contact angles ofat least partly wetting liquids measured by the imbibition method
are generally higher than the respective equilibrium values. They concluded that those contact angles cannot be expected to be physically correct in the sence ofintrinsic equilibrium contact angles. The advancing meniscus of a liquid raising in a capillary has to find a compromise between two forces: the gravitational force which tends to flatten the meniscus, and the capillary force which tends to adjust the contact angle between the liquid and the capillary walls. It is reasonable to assume, that the observed contact angle has avalue between these two extrema, i. e., the contact angle 0 ofawetting liquid should be larger than the equilibrium contact angle 8- but smaller than 90 *. Inspection ofTable 1reveals that the measured values of 0 indeed fulfill this assumption.
103 It would be expected that the contact angle 0 measured by the imbibition method should become similar to the equilibrium contact angle eeq, ifthe superficialvelocity ofthe liquid meniscus decreases (ref. 14). This may be achieved by using liquids with high viscosity since the liquid uptake coefficient w is proportional to the inverse square root ofthe viscosity (c.f: eq. 7) (ref. 6) and w itselfis proportional to the superficialvelocity (c.
f: eq. 3). Though the comparison between the measured liquid uptake coefficients w for toluene and the silicone oils shows, that the superficialvelocity is changed by a factor of about 20, the observed contact angles
0 are still essentially identical. The experiments with the silicone oils prove that it is not possible to achieve quasi-equilibriumconditions by decreasing the superficialvelocity of the raising liquid meniscus. A further point deserves special discussion. It is obvious that the contact angle measured by the imbibition technique is an average contact angle, i. e., the measured value of 0 is an average for all the capillaries in the porous body having different orientation. It may be argued that this fact jeopardizes the interpretation of the contact angles measured by our technique at all, but the same argument would be valid for the conventional imbibition measurements using the Washburn equation. As far as we know, this point has never been discussed in depth. CONCLUSION Our experiments show that it is possible to determine the contact angle 0 with one sample of a porous body and one liquid without making any assumptions about the contact angle of a reference liquid. Our data indicate that the contact angles 0 ofwater, toluene and silicone oils towards the capillary walls of glass frits are much higher than the respective equilibrium contact angles. The measured contact angles obviously are non-equilibrium (advancing) contact angles and should not be discussed in terms of equilibrium surface energies. At this moment, we do not have a tentative explanation about the finding, that the measured contact angles are essentially identical for all the liquids tested. The results of our study make the conventional surface characterization of porous media (e. g. powder plugs) by measuring the contact angles of a series of liquids with different surface tensions by the imbibition method doubtlid, since the assumption of a completely wetting reference liquid ( 0 = 0 -) may not be justified. Furthermore, our data show, that the measured contact angles have no relevance to equilibrium surface energies, at least on high energy surfaceswetted by the liquids used for the imbibition experiments. ACKNOWLEDGMENTS The preparation ofthe glass frits by Dr. B. Pachaly, Wacker-Chemie GmbH, is gratefully acknowledged. Thanks for technical assistance is due to M. Brummer. REFERENCES
1 2 3 4
5 6 7
Neumann, A. W., Good, R. J.; Sud Colloid Sci. (R. J. Good, R. J. Stromberg, ed.; Plenum: New York) 1979,11,31 Heertjes, P. M., Kossen, N. W. F.; Powder Technol.1967,1(1), 33 White, L. R.; J. Colloid InterfaceSci. 1982,90(2), 536 Washbum, E. W.;Phys. Rev. 1921,17(3), 273 Weber, E., Neumann, A. W.; Giesserei 1969,56(21), 628 Demlehner, U.;Farbe Lack 1989,95(10), 708 Hilbig, G., Girlich, N.; Bauphysik 1984,6(6),214
104
8 9 10 11 12 13 14 15 16 17
Hilbig, G.; Bauphysik1986,8(4), 111 Schindler, B., Sell, P. J.; Chem.-hg.-Tech. 1973,45(9-lo),583 Hoffmann, D., Niesel, K ; A m . Cerum. SOC.Bull. 1988,67(8),1418 Press, W. H., Flannery, B. P., Teukolsky, S . A., Vetterling, W. T.; Numerical Recipes; Cambridge University Press: Cambridge 1986 Adamson, A. W.; Physical Chemistry of Surfaces;4th ed.; J. Wiley 62 Sons: New York 1982; p. 40 Lit. 12, p. 349 Rose, W., Heins, R. W.; J. Colloid Sci. 1962,17,39 Yang,Y. W., Zografi, G.; J. Pharm. Sci. 1986,75(7),719 Yang, Y.-W., Zografi, G., Miller, E. E.; J. CoZZoid Interface Sci. 1988,122(1),24 Yang,Y.-W., Zografi, G., Miller, E. E.; J. Colloid Interface Sci. 1988,122(1),35
F. Rodriguez-Reinosoet al. (Editors), Characterization of Porous Solids II 0 1991 Elsevier Science Publishers B.V., Amsterdam
105
THE MAIN PRINCIPLES OF MODELLING OF POROUS SOLIDS. MODELS OF SYSTEMS WITH NEEDLE-LIKE PARTICLES A. P
. KARNAUKHOV
Institute of Catalysis, Novosibirsk 630090 (USSR) SUMMARY As the result of analysis of literature and development of some new ideas the main principles of porous solids modelling are presented. Two models of the systems with needle-like particles are discribed. Special attention is paid t o the problem of network modelling. INTRODUCTION Porous solids of various origins are of various and complex morphology (ref. I ) . Their structure is corpuscular (ensembles of particles) o r spongy (labyrinth of shannels and cavities). Modelling of these complex systems is necessary for theoretical description and interpretation of their geometrical, sorption, diffusion, mechanical, thermal and electrical properties (ref. 2). Model determination The model of a porous solid is the system of particles or pores approximately reflecting its basic properties; the most important requirement is the geometrical similarity. The model is more simple than a porous solid. The degree of simplification depends on the aims of investigation and the possibilities of an investigator. The simplest models are the systems or the regularly arranged particles o r the pores o f a simple geometrical form and equal size. F o r corpuscular structures these are the models which consist of the regularly arranged globules, round disks, round rods, polyhedrons. F o r spongy structures these are the models of cylinder or inkbottle pores. More complex are the models of irregularly arranged particles o r pores of various sizes. Agreement between the model and the object A real porous solid is substituted by an approximately equivalent model. Equivalence may vary in different properties: geometrical, sorption, physical, etc. To have a model fitted to an ob-
106
ject, the exact identity in any property and an approximate correspondence in all others are postulated. This may be illustrated by evolution of simulation for such a typically globular system as silica gel. In the simplest model 1 of the uniform capillaries (see Fig. I ) , an assumption of the identity of the pore volume V and the surface area A in a sample and a model, was accepted and the pore diameter in model was calculated. It is close to an average diameter in the sample. In a more complex model 2 of capillaries of various sizes, the identity of sorption or mercury intrusion isotherms was taken and thus the pore size distribution was calculated. In the simplest uniform globular model 3 (ref. 3 ) the identity of V and A was assumed and thus coordination number n, diameter of globules D, sizes of necks dn = 2.8 V/A, and voids d were calculated. In the next three globular models 4, 5, 6 (ref. 3) approximate or exact identity of sorption isotherms was accepted. Texture parameters of these models are somewhat different from those for the sample due to the ignorence of interconnections of pores. In the network model 7 of spherical voids and cylindrical necks (refs. 4,5), the identity of sorption isotherms was assumed and the necks' and voids' distributions were determined. Geometrical and physical properties of the sample and the model as well as the distribution of voids are significantly different from each other due to the opposite sign of the curvature of a surface in the sample and in the model. Typical element of model From the very start, the modelling of porous systems was connected with choice of the typical element of a model: the cylindrical capillary, conic pore, inkbottle pore, slit- and wedgeshaped pores. From the contemporary point of view, these models are more suitable to studying the spongy structures. In case of the corpuscular systems it is useful to distinguish the elementpore as the space between particles. In this case, an elementary pore is obtained by cutting the neighbouring particles by planes in the directions, dependent on the geometry of a system. For the globular models 3 , 4 , 6 (see Fig. I ) , secant planes p8SS through the globules centres and points of their contact. As a result, from the models 3,4 regular polyhedrons are cut, and
107
Fig. 1. Evolution of the model of silica gel texture. V,, As -- pore volume and surface area of the sample, a -- adsorption on iQ; v , -- pore volume an surface area of the model, -- adsorption on n -- coordination number of globules packing, dN/dn -- distribution of the number of globules over the coordination number, dNn/dn -distribution of the number of necks over their sizes, z -- coordination of a lattice,/,,N Ntot -- cumulative fraction of necks. The solid curve -- experimental isotherm; the dotted curve -isotherm for the model. D, d, dn, -- diameters of $obule, Pore Y neck, void.
9
3;
v, =Y, As=Am
approxiinate lden-
continuouS
Identity
-'igm 1
108
from the model 6 -- irregular polyhedrons (Fig 2) are cut with elementary pores inside. Fig. 3 illustrates one of the possible elementary pores for the platelet structures, a n d Figures 5 and 8 in last sect.-- the elementary pores for the needle-like structures.
Fig. 2. Models of the elementary pores for the irregular packings of globules. Fig. 3 . Model of the elementary pore for the platelet structures.
Interconnection between the model's elements In the previous classical modelling, the elements mentioned above were supposed to be independent. At present, it is obvious that this assumption leads to a distortion in description of various processes occurring in porous solids, such as desorption of sorbate, intrusion of mercury, filtration, molecular-sieve adsorption. For this reason, modelling of the primary elements may be considered as the first step, which is necessary though insufficient. The second step must be the modelling of interconnection between the elements. At present, it is conducted by the network models, and processes in them -- by percolation theory (refs.4-7). However, so far it was made only for comb5nation of the sphe-
109
rical voids and round windows. Strictly speaking, these network models are appropriate for description of spongy structures only. Their application to the globular systems may lead to mistakes in interpretation o f the adsorption branch as independent capillary condensation in voids due to convex curvature of void surface. As it was shown by Karnaukhov and Kiselev (ref. 8 ) , the capillary condensation in these systems, starting at places of globules' contacts, leads to merging of a condensate in the necks of pores and its following spontaneous propagation to the pore void. F o r this reason, the next development of the problem of modelling seems to be in creation of the network models, in which the elements may be of the other various types of pores. Different degree of elements disordering should be considered in them. F o r instance, in the platelet systems there may be an orientated packing of elements according totheir basis planes, and an irregular one in every layer, in the needle-like systems -- the orientated packing in bundles, and the irregular distribution of the bundles themselves, etc. It is possible that application of various models from those where the elements are completely independent (the models of montmorillonite, ?-Al2O3) to those, in which they are completely interconnected (the models o f porous glasses, xerogels) will allow to explain the origin of not only the type of H2 capillary condensation hysteresis, but other types as well. It may be good to use the irregular network models with the alternating coordination of the lattice (ref. 5 ) , the two-dimensional analogue of which is the lattice given in Fig. 4. Distribution of the coordination numbers in this lattice may be found by independent physical or computer simulation (refs. 9,lO). Ffg. 4. Scheme of the twodimensional irregular network model.
110
Inversion rule Porous solids are the two-phase system: gas o r liquid phase and solid phase. Accroding to inversion rule (refs. l , l l ) , the sum of volume fractions of solid and pores is equal to one: -?+E
= I
.
(1 1
This rule means that the detailed geometry of solid part determines the detailed geometry of porous part and vice versa. The rule of inversion may be efficiently applied to modelling. Due to it, studying of the pores structure may be substituted by studying of the solid’s structure. The particular porous solid may be modelled in one of these systems. Corpuscular solids are more simple and exact to be modelled by the system of particles, the spongy ones -- by the system of pores. The rule of inversion makes it possible to establish the relation between the parameters of particles and those of pores in corpuscular structures (ref. 1) on the basis of equality of the surface of pores and the surface of particles. This equality presents the expression for the pore diameter: K = A V p ? D , (2) dP KPr where K Kpr are the form factors for pores and for particles, P’ V -- volume of pores, density of solid, D -- size of parP ticles. This expression directly points to the fact that the more is the size of particles and the more loose is their packing (deter mining the volume of pores) the more the size of pores is.
p--
Models of systems of the needle-like particles and fibres The morphology of these materials may be divided into two groups. In the first one needles and fibres are orientated along the particle axis (e.g., the needles of C(-Fe2O3, tubular crystals of chrysotile asbestos, fibrous polymers, fibrous carbon, etc.), in the other one there is random packing of particles (e.g., in boehmite, x - A l 2 O 3 , gels of tungsten oxide, zirconium oxide, in many clay minerals, in paper, and filters). According to this two models may be presented. The first one is the model of longitudinal packing of round
111
rods. For three regular packings of these rods with the coordination numbers six ( & = 9.25%), four ( 6 = 21.5%) and three ( & = 39.5%) it is possible to cut three elementary pores (see Fig. 5) and to construct an interpolation curves (solid lines) which are the porosity and pore size versus n (Fig. 6). The diameter of rods is determined from micrographs or from the surface area, quasicoordination number -- from porosity.
n.= 6 (100-
€)%
80
n= 4 60
rn 2
n=3
4
6
Fig. 5 . Models of the elementary pores for the longitudinal regular packings of round rods. Fig. 6. Interpolation curve of the dependence of porosity and relative size of pores d/D on the coordination number for the regular longitudinal packings o f round rods. D is the diameter of the rods, d is the diameter of the circumference inscribed in a pore (ref. 12). 1 and 2 are the experimental values for the cuts of cords and packings of steel rods.
Fig. 6 demonstrates an excellent agreement of theoretical (points 3) and experimental (points 1,2) dependence 6 on n. Experimental values are obtained by study of cord fibres' cuts and buttends.of steel rods' bundles (ref. 12). The maximum difference between pore diameters, determined by mercury penitration and by model was 19%. The model of the longitudinally packed r o d s is an ideal example of the pores independent of each other. The percolation effects should not take place for it. For this reason, traditional
112
methods of capillary condensation and of the mercury porosimetry may be applied to it. In case of occasionally distributed needles one can use a model of the cross-sectionally round rods. In this model their rods are packed in layers, so that their axes are perpendicular in adjacent layers. In every layer the distance between the rods may vary, which leads to changing of the volume of pores and porosity. The parameters of the model are the diameter of rods D and the distance between the axes of rods C expressed in fractions of D. C is calculated from the value of porosity by the curve given in Fig. 7 . The minimum value of porosity Emin = 0.215 corresponds to the dense packing of rods in the layer ( C = 1) . The model of the elementary pore for the packing C = 2, E = 0.6 is given in Fig. 8 .
40
3
2
3
4
5
6
7
Fig. 7. Dependence of the porosity on the distance X between the axes of round rods. There is a relative distance C = X/D on the abscissa axis. Fig. 8. Model of the elementary pore for a regular cross-sectional packing of round rods with C, equal to 2 . The large experimentally measured value of porosity f o r the systems o f the needle-like particles serves as the basis f o r application o f the described model. For the occasional packing of steel rods (ref. 1) the value = 0.61 was obtained, which corresponds to C = 2.0 for the model. In ref. 13 for the needle-
I
113
like structure of ferrous oxide (phase of goethite), the value = 0.64 was obtained (C = 2.2). The model of y-A1203 ( A = 210 m2/g, 6 = 0 . 8 7 ) , studied in (ref. In?), has the parameters of D = 5.7 nm, C = 5.5. The discussed model is one of the least studied. There is no theory of the capillary condensation and mercury intrusion for it. For this reason it is difficult to calculate pore sizes from isotherms of sorption and intrusion by these methods. For this model the percolation effects should be the most prominent, and for it to be revealed it is necessary to construct the correspondent net-work models, because the usual models of spherical voids and round windows are not appropriate in this case. REFERENCES 1 2
3 4 5 6 7 8
9 10 11 12
13 14
A.P. Karnaukhov, in S.J. Gregg and K.S.W. Sing (Eds.), Characterisation of Porous Solids, SCI, London, 1979, p. 301. A.P. Karnaukhov, in Physical and Chemical Principles of Synthesis of Oxide Catalysts, Nauka, Novosibirsk, 1978, p. 231 (in Russ.). A.P. Karnaukhov, Kinet. Katal. 1 2 (1971) 1025, 1235 (in Russ.) C . C . Wall, R.J.C. Brown , J. Coll. Interface Sci. 8 2 ( 1 9 S l ) 141. V.P. Zhdanov, V.B. Fenelonov and D.K. Efremov, ibid. 120 (1987) 218; D.K. Efremov and V.B. Fenelonov, Kinet. Catal. Lett. 40 (1989) 177. A.V. Neimark, Dokl. AN SSSR 273 (1983) 384 (in Russ.). G. Mason, in K.K. Unger, J. Rouquerol, K.S. Sing and €1. Kral (Eds.) Characterisation of Porous Solids, Elsevier, Amsterdam, 1988, p . 323. A.P. Karnaukhov and A.V. Kiselev. Zh. Phis. Khim. 3 .1 (1957) . . . . 2635 (in Russ.). R.V. Zagrafskaya, A.P. Karnaukhov and V.B. Fenelonov, Kinet. Katal. 16 (1975) 1583. R.I. Ayukaev, V.K. Kivran and M.E. Aerov, Dokl. AN SSSR 218 (1974) 66 (in Russ.). L.V. Radushkevich, in hl.Tvl. Dubinin and V.V. Serpinskii (Eds.) Base Problems of Phys. Adsorption, Nauka, Moscow, 1970 , p. 270 (in Russ.). S.F. Grebennikov and V.I. Konovalov, in M.M. Dubinin and V.V. Serpinskii (Eds.) Adsorption and Porosity, Nauka, Moscow, 1976, p. 6 3 (in Russ.). K.A. Dadayan, R.V. Zagrafskaya, A.P. Karnaukhov and V.B. Fenelonov, Kinet. Katal. 1 8 (1977) 1517 (in Russ.). V.A. Dzisko, T.S. Vinnikova, L.M. Kefely and I . A . Ryzhak, Kinet. Katal. 7 (1966) 859 (in Russ.).
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F. Rodriguez-Reinosoet al. (Editors),Characterization of Porous Solids 11 0 1991 Elsevier Science Publishers B.V., Amsterdam
115
ADSORPTION-DESORPTION HYSTERESIS IN POROUS NETWORKS D.K.EFREMOV and V.B.FENELONOV Institute of catalysis, Novosibirsk, USSR.
SUMMARY As a result of computer experiments with model porous networks, the factors ( other than pore shape and size distribution ) determining the form of the adsorption-desorption hysteresis loop have been elucidated.
INTRODUCTION At present, quantitative information about pore structures of catalysts and adsorbents with pore sizes ranging from 1.5 to 50 nm is usually obtained from analysis of isotherms of adsorption and desorption in the region of a capillary-condensation hysteresis. It is often assumed that the form of the hysteresis loop is determinated primarily by the pore shape and their Size distribution [ 1,2 I . The objective of this work was to elucidate factors (other than pore shape and size distribution) determininig the form of hysteresis loops. MODEL POROUS SOLIDS This work have been carried out by the method of numerical adsorption experiments on model porous network, a mesoporous fragment of wich is shown in Fig. 1. This model porous space consists of spheroidal cavities ( voids ) and cylindrical necks between these voids. Such a model has allowed us to correctly evaluate volumes of all arising configurations of the adsorbate under different relative pressures of sorbate vapours, P/p,. Generation of the model pore space in computer memory was initiated by introducing density function of radius distribution of spheroids, f ( r ) , which was determined for size region ( A,B ) (see Fig.1 ) and which satisfied the condition : B
J f(r) dr = 1 A
116 0
PORE RADIUS. (A1 Fig. 1 . Density distribution functions f and g of model porous spaces, a fragment of which is shown in the upper part of figure.
Further the subroutine generating random numbers uniformly distributed over the interval ( 0 , l ) was transformed to obtain random numbers over the interval ( A , B ) with the distribution density f ( r ) . The radii so obtained were successively memorized in cells of a three-dimensional numerical N*N*N-array with even coordinates. Then the values of radii of cylinders that conneoted spheroids were introduced into the cells of the same array having two even and one odd coordinates. To obtain a desirable type of distribution of cylinder radii, a random procedure was used. This procedure satisfied the following requirements: (I) radii of all cylinders lie within the segment ( C , D ) , where C < O . 5 * A and D < 0 . 5 * B ; (11) radius of each cylinder does not exceed half radius of the smallest of two connected spheroids : (111) the distance between the centers of neighbouring connected spheroids is equal to 2 * ( B + D ), which precludes the possibility of '*overlapping**of spherical menisci after
117
irreversible condensation in the connecting cylinder. To pass from a simple cubic lattice with a constant coordination number (connectivity) equally 6 to a randomized lattice with average coordination number Z < 6 we have used a random procedure, which substitutes zero for part of cylinder radii in such a way that isolated pore clusters a r e not formed. Density distribution functions used for generating spheroid and cylinder radii in all cases considered below, are shown in Fig.1. In all cases a model porous solid was assumed to have macropores with surface area A,,, which were not filled in model experiments even at a limiting value of P/ Po = 0.99. The contribution of multimolecular adsorption A,,** was added to the adsorbate volume in mesopores at each P o o . In a number of cases the existence of micropores with volume V, filled at very low P o o was assumed. Then the V, value was added to the current volume of the adsorbate. In numerical experiments, changes of the form of isoterms in the region of hysteresis loops were studied as function of: (1) Size of lattice N (where [(N-1)/2 l 3 is amount of spheroidal voids ) , ( 2 ) Connectivity of the lattice (average coordination number)
z, (3) Proportion of the volume of micropores, V, ,in the total pore volume V, ( parameter V,/Vo ), ( 4 ) Proportion of the maoropore surface area, Be,, of the total surface area ,Ao ( parameter A,,/Ao 1. ADSORPTION EXPERIMENT
First, the adsorption and capillary condensation of nitrogen at
77 K was modelling. It was assumed that prior to irreversible capillary condensation the surface of a porous solid is uniformly covered with an adsorption film with the thickness t = 0.354
*
1/3
(
5 / b(P/P0)
)
(here and below all linear ciimensions are given in n m ) . For the calculation of adsorbate configurations at the junctions of spheroids and cylinders,spherical and cylindrical rings with the thickness t were "pasted together" by suitable toroidal-shaped sectors with the sectional radius t. In accord with Kelvin equation it was assumed that cylindrical pores with
118
the radius rc are filled with the condensate if
rc -
t
<
4.7 /
Ln(Po/P)
A hemispherical meniscus with the radius
rk
= 9.4 /
In (Po/P)
was assumed to form at the boundary between filled cylindrical and empty spheroidal pores. The surface of the corresponding geometric configuration of "liquid" was interpreted as "pasted together" from three parts: (1) an interior surface of a spherical ring with a round window, (11) a surface of a suitable toroidal sector with radius t , (111) a surface of a spherical sector which touches the toroidal sector. Spheroidal cavities were considered to be filled provided that
r, - t <
9.4 /
Ln(Po/P) ,
where r, is the spheroid radius. Volumes of the "liquid" were estimated at each P/Po successively for all lattice elements and were summed up. DESORPTION EXPERIMENT To calculate desorption branches of isotherms for each PIPo, a preliminary procedure of percolation "probing1*of the lattice was carried out. The cylinders directly connected with the external surface were assumed to be free from condensate if
re -
t
<
9.4 /
Ln(Po/P)
.
Adjacent spheroids were also assumed to be free from condensate. To indicate the percolation availability, all radii in the corresponding memory cells were multiplied by "-I** . Such an analysis was extended layer-by-layer inside the lattice in all possible directions. Further volumes of the remainder adsorbate were calculated.Then the P/P, was decreased and procedure probing was repeated. Or, in more detailed way. A n emptied spheroid on one of the facets of the model porous cube was chosen ( f o r example, on the bottom facet) and then the Kelvin condition was checked for the
119
cylindrical neck adjacent to the spheroid from the interior. If it is justified, the void lying in the next layer was considered emptied. Then another cylinder in that direction was checked, and so on until the Kelvin condition indicated the stop of the meniscus in the next cylindrical neck. This procedure was repeated for each void of the external layer and for each corresponding ohain of voids and necks, and the total number of the emptied voids was smarized. After that the same prosedure was applied to the upper facet of the model porous solid. And the following cyclic algorithm of probing began functioning: Each horizontal chain of voids and necks was studied from one facet to the opposite one, since then the emptied voids could meet in any place (as the result of the previous vertical sounding). Such investigation was carried out succesively for each of the six facets until during the following cycle of all the six facets the sum of the emptied voids appeared to be as in the previous cycle. It mean the establishment of an equilibrium under given relative pressure in the model experiment. It should be added that when calculating desorption branches of model isotherms, the voids emptied at the previous desorption stage were **memorized** and under the next P/Po it was possible to begin straight with the cyclic sounding. It greatly accelerated the calculations. The above mentioned procedure assures the quickest assessment of the model desorption process. In particular, f o r the lattice consisting of hali a million of voids and of two million necks the desorption branch comprising thirty points is estimated in ninety six minutes with the speed of calculations being three and a half million operations per second. RESULT AND DISCUSSION Results of model calculation of isotherms of adsorption and desorption of nitrogen on model porous solids parameters listed in Table 1 are shown in Fig. 2. The isotherms in Fig. 2 are numerated as the corresponding model porous solids in the upper line of the table. All isotherms were normalized by limiting values of adsorption f o r P/p, = 0.99. The model porous solid 1 and the isotherm 1 are results of averaging of 40 calculations.
120
c. condensation
0.
:
0.0
V I
0.2
0.4
0.6
3 4
0.8
1.0
RELATIVE PRESSUE Fig. 2a. Calculated isotherms: influence of connectivity, Z, and size of lattice, N (structural parameters of corresponding model porous solids are given in the Table 1 ) .
As shown by calculations, the form of hysteresis loops of isotherms can considerably change even at a constant morphology and practically the same size distribution of mesopores of model porous solids. NPAC Broad hysteresis loops which, according to classification, belong to type H2 are characteristic of large lattice (isotherms 2-4 in Fig. 2a). As has been reported, e.g., in [ 4,5 1 ,, such hysteresis loops are caused by percolation-blocking effects leading to the appearance of the desorption "plateau" after saturation and to a rather sharp desorption "knee" after achieving the percolation threshold. A decrease in Z (isotherms 3,4) is accompanied by an increase of percolation effect, i.e., the "desorption threshold" shifts to the left,but the form of the hysteresis loop remaining unchanged. In turn, a decrease in size of the lattice N (isotherm 1 ) makes these effects weaker, and the form of the hysteresis loop closer to that of type HI. The hysteresis loops for intermediate values of N (not shown in the
121
Q
0
0 .o
0.2
I
8
0.4 0.6 0.8 RELAT IYE PRESSURE
1. 0
Fig. 2b. Calculated isotherms: influence of parameters (Vm/Vo) and (A,,'% ) (structural parameters of corresponding model porous solids are given in the Table 1 ) . figure) are located between loops 1 and 2, being characterized by a gradual decrease of the region of plateau and by smoothing the desorption knee with decreasing value N. The influence of parameters Vm/Vo and Ae,/Ao is shown in Fig. 2b. A s micropores appear, the hysteresis loops of type H2 transfer to type H4 (isotherms 3,7,8 in Fig. 2b), while with increasing contribution of adsorption on the "external surfaceft (isotherms 3.5,6) hysteresis loops approach type H3. Qualitatively the same results have been received by us also for tree-shaped Bethe-structures. In conclusion we would like to say same words about three-dimensional model lattice imitating the porous glass, that overlapping spheroidal voids which are is on consisting of connected with each other by circular volumeless windows. We have developed thermodinamics correlation, permitting to determine the moment of irreversible filling of such voids with holes by
122
TABLE 1 .
Structure parameters of model porous solids Number of model porous
parameters
1
4
z
6.0
N V m 4 J
x
4
2
3
4
solid
5
6
7
0
4.0
4.0
4.0
4.0'
4.0
7
6.0 151
151
2.5 151
0
0
0
0
0
0.05
0.05
0.05
0.05
0.35
151
151 151 0 0.6 0.85 0.05
151 0.9 0.05
capillary condensate. On the basis of these correlations a number of computer adsorption experiments has been conducted, which took into account the effects of cooperative capillary condensation and all the regularities of the form changing of hysteresis loops have been confinned. These results will be presented in more details at the Internation Symposium in Moscow in February, 1991. REFERENCES 1. S.J. G r e g and K.S.W. Sing, Adsorption, Surface Area and Porosity, Aoademic Press, London, (1 982) . 2. IUPAC Manual of Symbols and Terminology, Appendix 2, P.l, Colloid and Surface Chemistry, Pure Appl.Chem. 31, 578 (1972). 3 . J. Bonnetain, J.L. Ginnoux, in Adsorption at the Gas-Solid and Liquid-Solid Interface, J. Rouquerol, K.S.W. Sing (eds.), Elsevier (1 982) . 4. G.C. Wall, R.J.C. Brown, J. Colloid Interface Sci., v.82, p.141 (19811. 5. V.P. Zdanov, V.B. Fenelonov, D.K. Efremov, J. Colloid Interface Sci., v.120, p.218 (1987). 6. N.K. Kanellopoulos, J.K. Petrou, J.H. Petropoulos, J. Colloid Interface Sci.,v.96, p.90 (1983).
F. Rodriguez-Reinoso et al. (Editors), Characterizationof Porous Solids II 1991 Elsevier Science Publishers B.V., Amsterdam
123
THE DETERMINATION OF THE PORE SIZE DISTRIBUTION OF POROUS SOLIDS USING A MOLECULAR MODEL TO INTERPRET NITROGEN ADSORPTION MEASUREMENTS
C.A. JESSOP, S.M. RIDDIFORD, N.A. SEATON*, J.P.R.B. WALTON and N. QUIRKE BP Research, Sunbury Research Centre, Chertsey Road, Sunbury-on-Thames, Middlesex TW16 7LN, United Kingdom. *Current address: Dept. of Chemical Engineering, University of Cambridge, Pembroke Street, Cambridge CB2 3RA, United Kingdom.
SUMMARY The pore size distributions of several types of porous solid have been determined from their nitrogen adsorption isotherms using a new analysis method (ref. 1). In contrast to earlier thermodynamic methods (which break down at small pore sizes), the new approach is based on a molecular model of nitrogen adsorption in a pore. The development of this technique means that, for the first time, the distribution of pore sizes may be calculated over both the mesopore and micropore size ranges using a single analysis method. The use of the new method is illustrated with a series of results obtained from carbon, silica and alumina samples. Its predictions are compared with those obtained using established analysis methods.
INTR0DUCI'I 0N The analysis of nitrogen adsorption isotherms is a standard method for the determination of the pore size distribution (PSD) of porous solids. Although the accurate measurement of the isotherm is a matter of routine, its inversion to yield a reliable PSD is much more difficult. Several analysis methods (refs 2-4) have been developed starting from the Kelvin equation, which relates the size of a pore to the pressure at which capillary condensation occurs for the fluid (in this case nitrogen) within it. These methods also require a description of the adsorbed layers which form on the pore walls prior to condensation. This is determined from an isotherm measured on a non-porous solid of the same chemical nature - the so-called t-curve. The Kelvin equation is derived from thermodynamic considerations and is hence exact in the limit of large pores, but it becomes progressively less accurate as the pore size decreases and breaks down completely (refs. 5 and 6) when the pore becomes so small that the molecular structure of the fluid becomes significant. It is therefore certain that these methods will fail for pore sizes in the micropore range (and we have in fact determined from our molecular model that the Kelvin equation breaks down well before this range is reached - see below). In any case, rather general considerations (ref. 5) reveal that, as the pore size W is decreased past a critical value WC,the jump in the individual pore isotherm
124
associated with capillary condensation disappears. Pores with a size smaller than WCdo not exhibit capillary condensation, but instead fill continuously as the pressure is increased. For nitrogen in porous carbon, WC is roughly correspondent with the conventional boundary between micropores and mesopores at 20 8, (ref. 1). Our results (ref. 1) indicate that the lower limit of applicability of analysis methods based on the Kelvin equation is about 90 A. The analysis of the micropore size distribution is even more problematical, because, as noted above, these sizes are too small for thermodynamic techniques to be applicable. The Micropore method of Mikhail et al (ref. 7) is an attempt to develop an analysis by using the t-curve to calculate the thickness of the adsorbed film on each wall of the pore; the pore is filled when the films coalesce. Unfortunately (ref. I), interactions between the molecules in one film and those in another are ignored in this approach, as is the way in which the adsorption energy is enhanced when the pore walls are close together. Both of these important effects cause the adsorbed film to be thicker than on the isolated surface represented by the t-curve. Their neglect means that this analysis method severely underestimates the pressure at which a micropore fills, and, although a number of semi-empirical approaches have appeared, it seems that no established method is based on a correct picture of micropore filling (ref. 8). In any event, the notion that each analysis method is only applicable over a limited range of pore sizes is clearly unsatisfactory in general, because of the inconsistencies which arise when they are applied to adsorbents which contain pores in both size ranges. Recently, a new analysis method has been developed (ref. 1) which is based on a molecular model for nitrogen adsorption in a pore. The model gives a realistic representation of the phase behaviour of nitrogen in pores of all sizes, whilst reproducing the Kelvin equation in the limit of large pores. Because the model has this molecular basis, it automatically incorporates the correct physics of adsorption in pores, such as the buildup of adsorbed layers on the walls, capillary condensation and the crossover to pore filling as the pore size is decreased past WC. Using this single method, the PSD can, for the first time, be determined for the entire range of pore sizes. In this paper, the new method is described briefly, its use in the determination of the PSDs of several different samples is presented and the results are compared with those of established approaches. THE ANALYSIS METHOD
The adsorption isotherm of a porous solid is the aggregate of the adsorption isotherms of the individual pores which make up the pore structure (if it is assumed that adsorption on the external surface of the solid is negligible). The number of moles adsorbed at pressure P, N(P), is given by
125
where p(P,W) is the molar density of nitrogen at pressure P in a pore of width W (i.e. the individual pore isotherm) andf(W) is the pore size distribution - that is, dV/dW, the distribution of pore volume V as a function of pore width. The essence of our analysis method is
*
the use of a molecular model for nitrogen adsorption to calculate the individual pore isotherm P(P,W),
*
the parameterisation off(W) by the adoption of a sufficiently flexible functional form forfand
*
the variation of the parameters until a good fit of Eqn (1) to the experimental isotherm is achieved.
Each element of the method can be considered as being more or less independent. In the work described in this paper, the molecular model known as mean-field theory (briefly described in the following section) is used,fis parameterised as the sum of two lognormal distributions (ref. l), and the variation of the parameters is performed using a quasi-Newton minimisation algorithm (ref. 9). Modifications could be made to the method in any of these areas; for example, it might be appropriate to use a more sophisticated (if computationally expensive) model such as molecular simulation (ref. 6) or switch to an alternative functional form for$ However, the numerical results forf(W) are not significantly affected by a change in the functional form off, provided it is flexible enough to achieve a good fit to the experimental isotherm (ref. 1). Similarly, the use of a different optimisation algorithm in the fit to the experimental data would not affect the results, as long as it were sufficiently robust.
THE MOLECULAR MODEL Our method for the calculation of p(P,W) is a statistical mechanical approach known as meanfield theory (refs. 1 and 5). In this approach, the properties of the nitrogen within the graphite pore are obtained directly from the forces between the constituent molecules. The parameters of the intermolecular forces are determined by (a) ensuring that the saturation pressure and saturated liquid density of the model fluid are equal to the experimental values for nitrogen at its normal boiling point (77 K, which is the temperature at which the adsorption experiments are carried out) and (b) matching the model adsorption on an isolated surface to the experimental t-curve of de Boer et al. (ref. 10). Having fixed the values for these parameters, the theory is then used to calculate model isotherms for pores of a variety of widths, which are then correlated (ref. 1) as a function of pressure and pore width to yield the individual pore isotherm p(P,W). Mean-field theory is known to become less accurate as the pore size is made very small (ref. 11); even for very small pores,
however, this approach is more realistic than methods based on the Kelvin equation.
126 300
h
-
200
v)
Q
v-
b
0
E
100
0
v
>
0
(a)
0.0
0.4
0.2
20
0.6
40
p,pO
w (4
0.8
60
Figure 1. A microporous carbon. (a) Isotherms. The circles are the experimental data; the line is the theoretical fit. (b) Pore size distribution.
RESULTS The new method is in routine use by BP Research and has been applied to a variety of samples, including carbons, silicas and aluminas. The results for several samples are displayed in Figures 1-5. The two parts of each figure are
(a) the experimental nitrogen adsorption isotherm plotted as points, with the theoretical isotherm [i.e. the best fit of Eqn (1) to the data] as a continuous curve. The amount adsorbed is displayed in cm3 of nitrogen at STP per gram of adsorbent, while PIP0 is the relative pressure, with the saturation pressure of nitrogen at 77 K (i.e. 1 atm.).
127
(b) The pore size distributionf(W) corresponding to the fit of Eqn (l), plotted as dVldW as a function of W. Results are presented for three samples of carbon: one containing predominantly micropores (Figure l),one which is mostly mesoporous (Figure 2), and one with a mixture of micro- and mesopores (Figure 3). In addition, results are presented for single samples of silica (Figure 4) and alumina (Figure 5). Looking at the upper part of each figure, it can be seen that our method is able to give a good fit to the the experimental isotherms for all five samples. In these figures, the PSDs start at the size of the pore which fills at the lowest experimental pressure.
400
300
h
n
+
cn CI
200
0
-in
?
s >
100
0
(4 O’Oo5
-2 r
0.4
0.2
0.0
o.6
P/po
1 .o
o.8
~
0.004
0.003
cn 0.002 Y
3 9
s
0.001
0.000 0
100
200
300
600
700
(b) Figure 2. A mesoporous carbon. (a) Isotherms. The circles are the experimental data; the line is the theoretical fit. (b) Pore size distribution.
128 800
600
400
200
0.0
(4
0.2
0.6 p,pO
0.4
0.8
1 .o
0.06
0.05
-
h 7
0.04-
7
En
% 0 3
0.03:
Y
0.02-
9
z
0.01 0.00
I
Figure 3. A micro/mesoporous carbon. (a) Isotherms. The circles are the experimental data; the line is the theoretical fit. (b) Pore size distribution.
Finally, Figure 6 compares our new method and two established approaches: the Cranston and Inkley method (ref. 3) (which is representative of approaches based on the Kelvin equation) and the Micropore method of Mikhail et al. (ref. 7). (The PSD is plotted in the alternative form of dVldlog1oWvs loglow.) The adsorbent is a carbon which contains both micro- and mesopores. The unphysical nature of the assumptions underlying the Micropore method has been described above. The Cranston and Inkley method, on the other hand, is correct in the limit of large pores. However, the Cranston and Inkley method is based upon a cylindrical model for the single pore, whereas our method uses a slit model (ref. 1) [although we note in passing that the incorporation
129
600
400
200
0.0
0.2
0.4
o.6
(a)
100
PIP0
0.8
200
1 .o
500
Figure 4. A micro/mesoporous silica. (a) Isotherms. The circles are the experimental data; the line is the theoretical fit. (b) Pore size distribution. of other pore models into our method would be straightforward; for example, mean-field theory has been used by Peterson et a1 (ref. 12) to study adsorption in cylinders]. If the Cranston and Inkley method were based on a slit-shaped pore as well, it would be expected to be in exact agreement with our method for large slits. This is because it reduces to the Kelvin equation in this limit, which agrees with our method (ref. 1) for large pores. The replacement of a slit-shaped pore
(of width W) with a cylindrical one (of diameter W) reduces the pressure at which the fluid within it condenses, because of the curvature of the walls. Put another way, the size of a cylindrical pore within which condensation occurs at a given pressure is greater than that of a slit pore. The implication for the present comparison is that the Cranston and Inkley PSD should have the same general shape as that obtained from our method, but should be be shifted to higher pore sizes.
130
0
n nnQ
loo
W(A)
200
Figure 5. A mesoporous alumina. (a) Isotherms. The circles are the experimental data; the line is the theoretical fit. (b) Pore size distribution.
Such a trend is indeed observed in Figure 6, and has been found for other adsorbents having a significant volume of mesopores (ref. 13).
To summarise, our new method is expected to produce PSDs which are more accurate than those obtained using methods which are currently available, because it is based on a molecular model that gives a description of adsorption in pores which is more realistic than that contained in the thermodynamic methods. However, its predictions still - correctly - reduce to those of the thermodynamic methods when the pore size is made very large. A further advantage of our
131 300
h
n +
200
v) c
Q
r
m
g
rn
100
Y
>
0
(4
0.0
0.2
0.4
0.6
ppO
1 .o
0.8
3 ,
0.9
1.2
1.5
1.8
109OW
2.1
2.4
Figure 6. Comparison between methods. (a) Isotherms. The circles are the experimental data; the line is the theoretical fit. (b) Pore size distributions. The plain line is the molecular method PSD, the circles are from the Micropore method and the squares are from the Cranston and Inkley method.
method is that, unlike current approaches, it offers a unified analysis over the complete range of pore sizes, which has not hitherto been possible. ACKNOWLEDGEMENT We thank the British Petroleum Company PLC for permission to publish this work.
132
REFERENCES
1.
N.A. Seaton, J.P.R.B. Walton and N. Quirke, Carbon, 27(1989) 853.
2.
E.P. Barrett, L.G. Joyner and P.H. Halenda, J. Am. Chem. SOC.,73 (1951) 373.
3.
R.W. Cranston and F.A. Inkley, Adv. Catal., 9 (1957) 143.
4.
S . Brunauer, R. Sh. Mikhail and E.E. Bodor, J. Colloid Interface Sci., 24 (1967) 451.
5.
R. Evans, U. Marini Bettolo Marconi and P. Tarazona, J. Chem. Phys., 84 (1986) 2376.
6.
J.P.R.B. Walton and N. Quirke, Molec. Sim., 2 (1989) 361.
7.
R. Sh. Mikhail, S . Brunauer and E.E. Bodor, J. Colloid Interface Sci., 26 (1968) 45.
8.
K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol and T. Siemieniewska, Pure Appl. Chem., 57 (1985) 603.
9.
Numerical Algorithms Group Library mark 11, routine E04JBF.
10.
J.H. de Boer, B.G. Linsen and Th. J. Osinga, J. Catalysis, 4 (1965) 643.
11.
J.P.R.B. Walton and N. Quirke, Chem. Phys. Lett., 129 (1986) 382.
12.
B.K. Peterson, J.P.R.B. Walton and K.E. Gubbins, J. Chem. SOC.Faraday Trans. 11, 82 (1986) 1789.
13.
N.A. Seaton and J.P.R.B. Walton, unpublished work (1988).
F. Rodriguez-Fkinosoet al. (Editors),Characterization of Porous Solids I1 1991 Elsevier Science PublishersB.V., Amsterdam
133
STANDARDISATION, REFERENCE HATERIALS AND COMPARATIVE MEASUREMENTS FOR SURFACE AREA AND PORE CHARACTERISATION
E. Robens',
K.-F. Krebs2
'Institut fur Anorganische und Analytische Chemie der JohannesGutenberg-Universitat, Postfach 3980, D-6500 Mainz (Germany) 2E. Merck, Forschung Reagenzien Chromatographie, Postfach 4119, 0-6100 Darmstadt (Germany)
SUMMARY This paper reviews current activities to standardise measur ng procedures for surface area and pore size determination. Comparative measurements at different laboratories using candida e reference materials revealed that surfaces of highly dispersed materials can be affected remarkably during storing and sample preparation. O n account of the fractal nature of these materials the results depend on the measuring methods. Reference materials of dispersed materials are offered by national and international standardisation administrations and by industrial distributers. The standardisation of measuring methods becomes more and more international (ISO, IUPAC). The survey includes tables of standards for pore size and surface area measurements and distributors of reference materials. INTRODUCTION The most important parameter for the characterisation of the surface texture of dispersed solids are the specific surface area, the specific pore volume, the pore size distribution and the particle size distribution. In addition, the fractal dimension seems to be an useful parameter for dispersed materials (ref. 1 ) . T o facilitate the exchange of data and to compare industrial products two approaches have been taken: Standardisation of measuring methods and instruments and certification of reference materials. COHPARATIVE HEASUREMENTS Comparative measurements at different laboratories using candidate reference materials revealed that surfaces of highly dispersed materials can be significantly affected during storing and sample preparation, for example investigations of S C I , IUPAC
134 and NPL ( r e f .
2) on s u r f a c e a r e a s t a n d a r d s and t h e measurements i n
t h e c o n t e x t o f t h e c h o i c e o f p a r t i c l e s i z e and s u r f a c e a r e a r e f e r e n c e m a t e r i a l s by t h e BCR ( r e f .
31, b o t h u s i n g t h e a d s o r p t i o n
method. I n a r e c e n t work t h e m e r c u r y i n t r u s i o n method was t e s t e d (ref.
4).
TABLE 1: D i s t r i b u t o r s o f r e f e r e n c e m a t e r i a l s f o r d i s p e r s e d products BAM - B u n d e s a n s t a l t f u r M a t e r i a l f o r s c h u n g und - p r u f u n g , U n t e r den E i c h e n 87, D - 1000 B e r l i n 45 BAS - B u r e a u o f A n a l y s e Samples L t d . , Newham H a l l , Newby, GB - M i d d l e s b r o u g h , C l e v e l a n d TS8 9EA. U.K. CTIF - C e n t r e T e c h n i q u e des I n d u s t r i e s de l a F o n d e r i e , 44 Avenue de l a D i v i s i o n L e c l e r c , F - 92310 S e v r e s Duke S t a n d a r d s Co., 445 Sherman Avenue, P a l o A l t o , CA 94306, USA E i d g e n o s s i s c h e M a t e r i a l p r u f u n g s - und V e r s u c h s a n s t a l t f u r I n d u s t r i e Bauwesen und Gewerbe, U n t e r s t r . 1 1 , CH - 900 S t . G a l l e n BCR - Community B u r e a u o f R e f e r e n c e , 200 r u e de l a L o i , B - 1049 B r u s s e l I R S I D - I n s t i t u t de R e c h e r c h e s de l a S i d e r u r g i e F r a n c a i s e , B O P . 64, F - 57210 M a i z i e r e s - l e s - M e t z MBH A n a l y t i c a l L t d . , H o l l a n d House, Queens Road, GB - B a r n e t , H e r t s , EN5 405, U.K. NBL - U.S. D e p a r t m e n t o f E n e r g y , New E r u n s w i c k L a b o r a t o r y , R e f e r e n c e M a t e r i a l s S a l e s , 9800 S. Cass Avenue, B l d g . 350, Argonne, I L 60439, USA NIST - O f f i c e o f S t a n d a r d R e f e r e n c e M a t e r i a l s , US D e p a r t m e n t o f Commerce, N a t i o n a l I n s t i t u t e o f S t a n d a r d s and T e c h n o l o g y , Rm. 8311 C h e m i s t r y B l d g . , G a i t h e r s b u r g , ND 20899, USA NPL - N a t i o n a l P h y s i c a l L a b o r a t o r y . D i s t r i b u t o r : O f f i c e o f R e f e r e n c e M a t e r i a l s , L a b o r a t o r y o f t h e Government Chemist ( L G C ) , B l d g 95 R M A I , Queen’s Road, T e d d i n g t o n , M i d d l e s e x , T W l l OLY, U.K. P a r t i c l e I n f o r m a t i o n S e r v i c e , I n c . , P.0.Box 792, 222 G r a n i t e H i l l Road, Grands Pass, Oregon 97526, USA REHCO - C o u n c i l Committee on R e f e r e n c e M a t e r i a l s , I n t e r n a t i o n a l O r g a n i z a t i o n f o r S t a n d a r d i z a t i o n , 1 , r u e de Varembe, B.P. 56, CH - 1211 Geneve 20 RBS - R e g i n e B r o o k s , P a r i s e r S t r . 5, D - 5300 Bonn 1 SMR-LNE - S e r v i c e des M a t e r i a u x de R e f e r e n c e , 1, r u e G a s t o n Boissier, F 75015 P a r i s Silikose-Forschungsinstitut, H u n s c h e r d s t r . 12, D - 4 6 3 0 Bochum S t a u b f o r s c h u n g s i n s t i t u t des H a u p t v e r b a n d e s d e r g e w e r b l i c h e B e r u f s g e n o s s e n s c h a f t e n e.V., L a n g w a r t w e g 103, 0 - 5300 Bonn 1 S t e i n k o h l e n b e r g b a u v e r e i n , H a u p t s t e l l e f u r S t a u b - und S i l i k o s e bekampfung, F r i l l e n d o r f e r S t r . 351, D - 4300 Essen-Kray T e s t f a b r i c s I n c . , 55 Vandam S t r . , New Y o r k 13, N.Y., USA W a s c h e r e i f o r s c h u n g K r e f e l d e.V., A d l e r s t r . 44, D - 4150 K r e f e l d W i r t s c h a f t s v e r b a n d A s b e s t z e m e n t e.V., A r b e i t s - und U m w e l t s c h u t t , K o l n e r S t r . 102-104, D - 4040 Neun
-
135
REFERENCE RATERIALS
T o control their products and to calibrate their instruments many manufacturers of powders or Porous materials produce standardised materials for their own needs (ref. 5). For public use such reference materials are certified, stored and offered by national and international standardisation institutions and by some commercial distributors (Tab. 1). Disregarding the difficulties in storage a variety of particle size and surface area reference materials are available (Tab. 2 ) and more are under consideration. Lists of the many industrial materials suitable for tests a r e included in ref. 9. T h e Institute for Inorganic and Analytical Chemistry of the University flainz has developed single crystals of zeolites and aluminophosphates which may be used as reference materials in the micropore range (ref. 6).
TABLE 2: Certified reference materials for dispersed matter material
order distributor C see number Tab. 1)
particle diameter vm
A1 pha-A1 umina Alpha-Alumina Alpha-Alumina A1 pha-A1 umina A1 pha-A1 umina A1 pha-A1 umina A1 umi na A1 umina
CRM 169 BCR CRM 1 7 0 BCR fl 11-05/09 NPL M 11-06/10 NPL M 11-07/11 NPL M 11-08/12 NPL CRM 171 BCR 8571 NIST/ASTM CRM 174
BCR
Graphitized carbon black Vulcan FT-G M 11-01 Sterling 3-6 M 11-02
NPL
Bronze
Glass Glass Glass Glass Glass
spheres spheres spheres spheres spheres
Caolin, calcined
1003a 1004a 1017a 1018a 1019a
8570
spezific surface area. )
pore region
m2 g0.104 1 *05 2.1/0.7' 0.3/0.1' 0*1/0.04' 1.0/0.3' 2.95 158.7
NPL
0.06
11 71
NIST 8 - 58 NIST in preparation 1 0 0 - 310 NIST 225 - 780 NIST NIST 760 -2160 NIST/ASTM
10.89 (Continued on p. 136)
136
CRM 165 CRM 166 CRM 167
Latex Latex Latex Pol y s t y r o l Pol y s t y r o l Polystyrol Polystyrol Polystyrol
spheres spheres spheres spheres spheres
1690 1691 1660 1661 1665
BCR BCR BCR
2 223 4 * 821 9.475
NIST NIST NET NIST NIST
0 895 0 269 9.89 29 64 9.94
-
S i 1i c a S i 1i c a Tk 800 Silica Gasil Silica S i 1 i c a / A l umina Quartz Quartz Quartz Quartz Q u a r tz Quartz Quartz Quartz Q u a r tz Quartz Quartz Quartz Quartz Quartz Quart z Quartz Quart z Quartz Quartz
NPL M 12-01 NPL M 12-02 NPL M 11-03 NPL M 11-04 8572 NIST/ASTM 0 + 35-3 50 BCR CRM 066 2.4- 32 BCR CRM 067 160 - 630 BCR CRM 068 14 - 90 BCR CRM 069 1.220 BCR CRM 070 50 - 220 BCR CRM 130 480 -1800 BCR CRM 131 B C R 1400 -5000 CRM 132 0.1 - 3 NPL M 13-02 3 - 40 NPL M 13-03 40 -1000 NPL M 13-04 10 - 100 NPL M 13-05 1 - 10 NPL M 13-06 0.35- 2.5 NPL 66 67 3 - 20 NPL 68 140 - 650 NPL 69 12 - 90 NPL 70 0.5- 90 NPL CRM 172 BCR
T i tania Titania-Rutil
M 13-01 CRM 173
NPL BCR
Tungsten
CRM 175
BCR
Portland-cement
114n
Zirkonia Zirkonia Z irkonia Zirkonia Z irkonia
M 13-07
NPL
M 13-08 M 13-09 M 13-10 M 13-11
NPL NPL NPL NPL
0.1-
mesoporous mesoparaus 152 n o n - p o r o u s 260 mesoporous 291 * 2
2.50
3
8.23 0.181
NIST
0.202+ 10.346' 1 5 15 30 60
Test dust
Laughborough U n i v e r s i t y Carborundum D e l co
Test dirt
Spark P l u g Waschereiforschung
2
* ) S p e c i f i c s u r f a c e a r e a s a r e measured u s i n g t h e n i t r o g e n a d s o r p t i o n method a t 77 K, e x c e p t t h o s e marked by * ) w i t h t h e p e r m e a t i o n method and ) u s i n g t h e Wagner t u r b i d i m e t e r .
137
STANDARDISATION For a variety of measuring methods the sample preparation
and
the measuring procedure and in some cases even instruments have been standardised. Nowadays national standards are being harmonized either in the framework of the European Communities o r at the international level. A list of standardisation committees working in this field is appended (Tab. 3). With few exceptions, e.g. single crystals of molecular sieves, dispersed materials exhibit a fractal nature. As a consequence, the results of investigating the geometric structure depend strongly on the unit size of area used to investigate the surface. In general different conditions of sample preparation o r different measuring methods lead to different results. For that reason, standardisation of these procedures is important. Today standardisation covers several methods of particle size analysis including evaluation and presentation of results. The Blaine test, other flow methods and several methods used to characterise building materials are standardised worldwide. The adsorption method and the evaluation using the two-parameter BET equation has been standardised in several countries for a number
TABLE 3: Standardisation institutes which work on standards for the determination of surface area and pore size AFNOR - Association Francaise de Normalisation, Tour Europe Cedex 7, F-92080 Paris la Defence AIA - Asbestos International Association, 68 Gloucester Place, GB - London W 1 H 3 H L , U.K. ASTM - American Society for Testing Materials, 1916 Race Street, Philadelphia 3, PA, USA BSI - British Standards Institution, BS House, 2 Park Street, G B London WlY 4 A A , U.K. DIN - Deutsches Institut fur Normung, Postfach 1107, D - 1000 Berlin - 30 IS0 - International Organization for Standardisation. Secretary: NASK im DIN, Postfach 1107, D - 1000 Berlin - 30 IUPAC - International Union o f Pure and Applied Chemistry, Bank, 2, Pound Way, GB - Oxford, U.K. VDI - Verein Deutscher Ingenieure e.V., Postfach 1139, D - 4000 Dusseldorf - 1
138
TABLE 4: Standards for surface area and pore size distribution measurements AFNOR X 11-601 X 11-601
X 11-601
74 Determination d e l’aire massique (surface sp6cifique) des poudres par courant de gaz Methode de L e a et Nurse. 85 Determination de l’aire massique (surface specifique) des poudres par adsorption de gaz Methode B.E.T.:Mesure volumetrique par adsorption d’azote B basse temperature. 85 - Variantes de la m6thode d e base.
ASTM C 115 C 204
D 2355 T D 3663 D 3906 D 3908 -
D 3942 D 4222 D 4284 -
D 4365 D 4567
-
D 4641
-
D 4824
-
BS 4359
4359
Wagner turbidimeter. Dito: AASHTO T 98, ANSI A 1.7 68 Standard for the determination of the fineness of Portland cement by air flow. Blaine method. Dito: Federal Test Method Standard 1 5 8 + Method 2101, AASHTO T 153: 65 Determination of isotherms of decolorization. 84 Test Method for Surface Area of Catalysts 85a Test Method for Relative Zen ite Diffraction Intensities 82 Test Method for Hydrogen Chem sorption on Supported Platinum on Alumina Catalysts by Volumetric Vacuum Method 85 Test Method for Determination of the Unit Cell Dimension of a Faujasite-Type Zeolite 83 Test Method for Determination of Nitrogen Adsorption and Desorption Isotherms of Catalysts by Static Volumetric Measurements 83 Test Method for Determining Pore Volume Distribution of Catalysts by Mercury Intrusion Porosimetry 85 Test Method for Determining Zeolite Area of a Catalyst 86 Test Method for Sinqle-Point Determination of the Specific Surface Area of Catalysts Using Nitrogen Adsorption by the Continuous Flow Method 87 Practice for Calculation of Pore S i z e Distributions of Catalysts from Nitrogen Desorption Isotherms 88 Test Method for Determination of Catalyst Acidity by Ammonia Chemisorption 84 Determination of the specific surface area of powders - Part 1. Recommendation for gas adsorption (BET) methods. 71 Determination of the specific surface area of powders - Part 2: Air permeability.
DIN 51 9 1 8 52 102 66 1 2 6 T 1
11.86 Bestimmung der Rohdichte nach der Auftriebsmethode und der offenen Porositat durch Impragnieren mit Wasser, Feststoffe. 8.88 Bestimmung von Dichte, Trockenrohdichte, Dichtigkeitsgrad und Gesamtporositat. 5.75 Bestimmung der spezifischen Oberflache pulverformiger Stoffe mit D u r c h s t r o m u n g s v e r f a h r e n ; Grundlagen, laminarer (continued)
139
66 27 66 31 66 32 66 1 6 0 66 161
Bereich 4.77 -: Verfahren und Gerat nach Blaine 10.73 Best immung der spezifischen Oberflache von Feststoffen durch Gasadsorption nach Brunauer, Emmett und Teller (BET); Grundlagen 7.75 Bestimmung der spezifischen Oberflache von Feststoffen durch Stickstoffadsorption nach Haul und Dumbgen 12.85 Partikelgronenanalyse, Begriffe 12.85 -: Formelzeichen, Einheiten
IUPAC Part 1 Part 1 1
72 Manual of symbols and terminology for physicochemical quantities and units. Appendix 11, Terminology and symbols in colloid and surface chemistry. 76 Terminology in heterogeneous catalysis. 85 Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity.
IS0
R24/SC4 E R24/SC4 E
87 Representation of partical size analysis data. 87 Determination of specific surface area by BET gas adsorption method.
of years and a n IS0 standard is in preparation. German D I N standards are collected in ref. 7. The most comprehensive description of the adsorption method is found in an IUPAC recommendation (ref. 8). Laboratory prescriptions for color adsorption tests exist but no official standards. Extended records o f standards for the characterisation of dispersed materials and of sampling were published recently (ref. 9). In Table 4 an updated list o f standards for the determination of surface area and pore size distribution is presented.
OUTLOOK More national and international standardisation procedures for mercury porosimetry and the derivation of pore size distributions from adsorption isotherms are in preparation. Regarding the weakness of the two-parameter BET model for surface area determination in addition the three-parameter BET equation or improved approximations (ref. 10) should be introduced. T h e more lengthy calculations can be easily managed using a computer. Competitive evaluation methods, like the method of Dubinin, Kaganer and Radushkevic are being discussed. Recently, at the University of Austin, Texas, comparative sorption measurements have begun with the aim of standardising a procedure for the determination of the fractal dimension of dispersed materials (ref. 11).
140
REFERENCES 1
2 3 4
5 6
7
D. Avnir: The Fractal Approach to Heterogeneous Chemistry. Wiley, Chichester 1989. D.H. Everett, G.D. Parfitt, K.S.W. Sing, R. Wilson: The SCI/IUPAC/NPL Project on Surface Area Standards. J . Appl. Chem. Biotechnol. 24 (1974) 199-219. Reference Materials and Methods. Status Report 1979, ECSC-EECEAEC, Brussels-Luxembourg 1979. K. Hinrichsmeyer, S. Abdul-Maula, U , Diederichs, F.S. RostBsv: Q u e c k s i l b e r p o r o s i m e t r i e , Ringversuche an erhartetem Zementstein. Bericht des Instituts fur Baustoffe, Massivbau u * Brandschutz der Technischen Universitat Braunschweig, 3.1988 E. Robens, R. Meyer: Reference materials and standardised measuring methods for powders and porous materials. Powder metallurgy International 13 (1981) 1, 44-45. U. Muller, A . Tissler, K.K. Unger,: Zeolithe, porose Festkorper mit definierten Hohlraumsystemen in molekularen Dimensionen. GIT Fachz. Lab. 32 (1988) 635-641. DIN-Taschenbuch 133: Partikelmefitechnik, 3. ed., Beuth, Berlin
1990
K.S.W. Sing, D.H. Everett, R.A.W. Haul, L . MOSCOU, R.A. Pierotti, J. RouquBrol, T. Siemieniewska: Reporting Phvsisorption Data for Gas/Solid Systems with Special Reference to the Determination of Surface Area and Porosity. Pure & Apple Chem. 57 (1985) 4, 603-619. 9 E. Robens, U4 Muller, K.K. Unger: Normung, Referenzmaterial und Ringversuche zur Oberflachen- und Porenbestimmung. VakuumTechnik 38 (1989), 3-4, 65-75. 10 G.L. Aranovich: Correction of the Multimolecular Adsoption Isotherm. Russian J . Phys. Chem. 62 (1988) 1 1 , 1561-1566. 1 1 Prof. 0. Avnir (Dpt. of Chemistry, University of Austin, Texas, Texas 78712-1167), pers. communication. 8
F. Rodriguez-Reinosoet al. (Editors), Characterization of Porous Solids II 0 1991 Elsevier Science Publishers B.V.. Amsterdam
141
Fractal characterization of t h e porosity of organic tissue by interferometry Y. Sernetz, H.R. Bittner. P. Bach. B. Glittenberg I n s t i t u t f u r Biochemie und Endokrinologie der JLU GiePen Frankfurter Str. 100, D-6300 GIESSEN. Germany
*-:
fractal porcsity, chnmatcgr&y, Limited selfsimilarity
interfarmtry, porcsity of o&c
tissue,
Introduction In spatial networks of cross-linked polymer or protein chains, t h e s t a t i s t i c a l distribution of chain lengths and diameters between cross-linking s i t e s produces a broad distribution of open pores, t h e sizes of which may cover a range of several orders of magnitude. The contiguous liquid s p a c e , a c c e s s i b l e f o r t h e s o l v e n t , defines t h e connected porosity as t h e c u m u l a t i v e i n n e r pore volume. T h e s e networks may be described as two-phase systems, composed of t h e two volumes, polymer m a t r i x a n d liquid ( s o l v e n t ) , e a c h of them contiguous i n i t s e l f , y e t interlaced in a highly complex manner. The interface between those intermingled volumes is a highly folded, fractal surface. The present work uses a method combining size exclusion analysis and microi n t e r f e r o m e t r y (SeSt?, Set791 t o determine t h e p o r o s i t y of t i s s u e s l i k e cornea, hyafine cartilage and soft contact lenses and describes i t in fractal terms. Interferometry of porosity The porosity of gels i s usually determined by equilibrium permeation or size exclusion chromatography by measuring t h e internal volume, accessible to t e s t m o l e c u l e s d e p e n d i n g o n t h e i r s i z e ( m o l e c u l a r w e i g h t M W , r a d i u s MR). T h e cumulative pore volume distribution, however, can also be measured optically. By microinterferometry of specimens, embedded i n s o l u t i o n s of t e s t molecules, t h e d e n s i t y of t h e matrix M , t h e i n n e r volume a n d t h e r e l a t i v e a c c e s s of defined molecules can be determined by measuring t h e optical p a t h difference r between t h e porous specimen of thickness d and t h e embedding medium i n t h e equilibrium
1. Supported by grants Se 315/13-1 and 13-3 of the Deutsche F o r s c h ~ ~
'
haf t.
142
of diffusion [SeSSl. This method is used here for t h e description of natural tissues like cornea a n d h y a l i n e c a r t i l a g e a n d c o n t a c t l e n s e s as porous polymers. The measurements were performed with a microscope i n t e r f e r o m e t e r of t h e MachZehnder type (Fig. 1). In w h i t e l i g h t , t h e l i n e s of zero o r d e r of i n t e r f e r e n c e a r e i d e n t i f i e d , t h e i r displacement i s A (Fig. 3a). In monochromatic light (wavelength X), t h e zero order positions remain unchanged, a n d t h e l o c a t i o n s of h i g h e r o r d e r maxima become visible (Fig. 3b). Now t h e displacement A can be expressed in multiples of t h e d i s t a n c e a of a d j a c e n t i n t e r f e r e n c e fringes: A = x - a . The corresponding p a t h difference r between t h e beam crossing t h e specimen, and t h e reference beam can be calculated as r = x.1. The porous specimen with t h e thickness d can be regarded interferometrically as composed of three independent partitions (Fig. 2): The relative path difference r/d consists of t h e contributions of t h e matrix partition M = d d d , accessible t o neither solute nor t e s t molecules, and t h e partition d d d , accessible t o solute but not t o t e s t molecules. Since t h e specimen c a n be considered t o be interferometrically homogeneous and isotropic, t h e volume ratios a r e equal to t h e thickness ratios ( d d d = M = VM/V). The refractive indices of t h e partitions a r e nM, nL, and ns respectively.
-3
d
......... ,............. ......... ....:::.....
C
J
T .. ... . . ...... ........ ...... .. .. .... ....
Fig. 1: Schematic diagram of t h e rnicrointerferometer of Mach-Zehnder type. In t h e measuring beam t h e specimen of thickness d is immersed in t h e solution of t h e test molecules with refractive index ns. The p a t h difference is measured against t h e reference beam passing t h e solution only.
I
solvent
Fig. 2: Scheme of a porous material with t h e three partitions t h a t can be separated by interferometry, t h e corre sponding thickness fractions d and refractive indices n.
143
Fig. 3: Micrographs of specimen in equilibrium with a solution of t e s t molecules showing t h e course of interference fringes and their ordinals a) cornea i n white b) in monochromatic light (546 nm); c ) cartilage in monochromatic light and light (546 nm), showing t h e distortion due t o t h e cells.
The partition ds/d includes those pores which a r e accessible both t o solute and t e s t molecules, and t h e r e f o r e c a n n o t be d i s t i n g u i s h e d from t h e embedding solution. The ratio between dL and ds depends on t h e size of t h e test molecules and their geometry (their fractal dimension). V a r i a t i o n of t h e c o n c e n t r a t i o n of t e s t molecules i n t h e embedding s o l u t i o n s h i f t s t h e s o l u t i o n ' s r e f r a c t i v e index ns. In a g r a p h of t h e r e s u l t i n g r e l a t i v e optical p a t h difference r/d vs. ns, t h e negative slope yields t h e inaccessible p a r t kna = (dL+dn)/d of t h e specimen (Fig. 4).
Using t e s t molecules of i n c r e a s i n g s i z e l e a d s t o a l a r g e r k n a , e v e n t u a l l y reaching i t s maximum kna = 1 for t e s t molecules bigger t h a n t h e biggest pore. The minimum is reached a t t h e size of t h e smallest pore with kna = M ( t h e matrix content).
144 Method: In t h i s investigation rat cornea was used. The hyaline cartilage was t a k e n from t h e hip joint (acetabulum) of a calf [G1881. The Lunelle soft contact lenses were of t h e ES70 type, made from polymethylmethacrylat (PMMA) and polyvinylpyrrolydon (PVP). The Geaflex a r e a l s o s o f t c o n t a c t l e n s e s c o n s i s t i n g of pure, swollen PMMA. The embedding method is based on the principle of size exclusion chromato-
.
graphy. Proteins i n t h e size range of pores a r e used as molecular-weight standards. W e additionally used polyethylene glycols (PEGS) and present t h e i r results separately, for reasons given in t h e l a s t paragraph. In t h e lower size range, test molecules i n t h e range of 20 t o 100 Dalton (Dz0, NaC1, ammonium s u l f a t , a n d glycerol) were used. By t h e s e molecules below t h e s m a l l e s t pore size a n d t h u s f u l l y a c c e s s i b l e , t h e matrix p a r t i t i o n M c a n be determined, irrespective of dissociation or hydratation. The interferometric technique requires both t h e determination of the refractive indices n and of t h e thickness d. The refractive indices were determined with a precision refractometer (Zeiss). In contrast t o t h e previous measurements with gel spheres or gel films CSe881, where t h e thickness could be determined directly from t h e microscopic image, t h e thickness of t h e biological specimen must be determined in a s e p a r a t e step. This is achieved with a calibrated ocular micrometer a t t h e l a t e r interferometric measuring point by turning t h e embedded s t r i p 90 degrees. To avoid dehydration, sealed chambers were used.
45
1-
LO
-
50
35
r'd
Gycerin
-
30 -
25-
l " ~ ' " " ' " " ' " " ' " " ' ~
1.33
nLrrer
1.31
1.35
1.36
1.37
ns
1.38
Fig. 4: Cornea: The relative p a t h difference r / d vs. refractive index ns of t h e embedding solutions with various concentrations of some test molecules. The negative slopes represent t h e relative inaccessibility knn for the different molecules.
145
In o r d e r t o describe t h e e n t i r e r a n g e of t h e kna vs. M W r e l a t i o n , logistic function (LLF) is fitted t o t h e d a t a , if appropriate. The fitting t h e parameters Q for t h e main pore size, and b for t h e distribution width The LLF is discussed i n more detail elsewhere fSeS9, Bi901, a n outline of for t h e fractal interpretation i s given in t h e paragraph before l a s t .
a logsupplies (Eq. 3). t h e use
Results: The I'/d vs. ns d a t a (Fig. 4) show good accordance with t h e expected linear dependence, though for t h e inhomogeneous cartilage t h e variation i s noticeable. Plotting t h e slopes -kne from Fig. 4 (Table 1) vs. molecular size E (= M W or MR) yields t h e cumulative pore volume distribution (Figs. 5). The measurements with small molecules lead t o matrix fractions of M = 15 % (cartilage, Lunelle). 18 % (cornea). t o M = 29 % (Geaflex). For t h e description of t h e e n t i r e molecular size range, a n LLF is f i t t e d t o cornea a n d cartilage d a t a . The Lunelle d a t a do n o t show t h e required symmetry. Thus, a logarithmic function is used instead of a n LLF. A d d i t i o n a l d a t a o n t h e G e a f l e x c o n t a c t l e n s e s a r e t o o s p a r s e f o r a corresponding fitting (Table 1, Fig. 5c, d). 0
% ,-
Cartilage * /
0
0
I q tlR
109
2
3
4
5
-.I
6
-.5
-.x
-.x
0
1 0
, ,
25
,
,
,
,
.25
75
5
., , , , , , .5
,
,
.75
log , , , ,
I25
I
, 1
,
, ,
,
,
1.25
Fig. 5: Cumulative pore volume distribution, described by t h e inaccessibility k n a a s a f u n c t i o n of t h e m o l e c u l a r w e i g h t M W ( a , c ) a n d a s a f u n c t i o n of t h e hydrodynamic molecular r a d i u s MR (bad), determined i n t e r f e r o m e t r i c a l l y f o r cartilage (a.b). cornea, and contact lenses (c,d) with t e s t molecules ranging from mono- and oligosaccharides t o proteins and excluded dextran ( 0 : PEGS, .:others). LLFs a r e used for fitting to cornea and cartilage.
146
The cornea, and t h e contact lenses show porosities mainly distinguished by t h e mean pore size. This size coincides with the 50% mark of relative accessibility ( k ~ 0 . 5 0or kna= H ( l + M ) ), or Q from t h e LLF fitting (Figs. 5 , Table 2 ) . The values range from 3.9 kD or 1.3 nm (cornea) and 5.9 kD or 1.7 nm (cartilage) t o 490 kD or 8.3 nm (Lunelle). The exponents b, characterizing both steepness and distribution width, a r e determined as 2.11 f 0.2 (vs. MR) and 0.74 f 0.08 (vs. MW) for t h e cornea whereas t h e values 3.5 k 1.1 (vs. MR) and 1.5 f 0.4 (vs. MW) for t h e cartilage can only estimate t h e exponent roughly.
Table I
Test molecule Matrix p o r t i o n
MW
MR
[kDaltonl
[nm]
kn a Cartilage
M = du/d
15 %
NaCl 20 % Ammonium s u l f a t e
-06 .10
.151
Glycerol
*
09
Lunelle
kn a Geaflex
18 %
15 %
(29 %)
.18
-22
.29
.192
.149
kn a
Cornea
kn a
* *
PEG200
0.20
0.4
.265
-248
PEG400
0.40
0.6
.263
*
* *
PEG600
0.60
0.7
.168
*
*
*
PEG1000
1.00
0.9
*
.423
*
*
PEG1540
1.54
1.1
.292
-489
*
PEG6000
6.0
2.3
.460
-490
* *
Cytochrome C
12.5
1.7
(-47 1
*
( .994)
*
Lactalbumin
14.8
1.9
.874
-771
.159
*
PEG2Ok
20
4.3
.32
-632
*
*
Chyrnotrypsinogen
24
2.3
-869
*
*
*
PEG35k
35
5.7
.491
.620
*
*
Ovalbumin
44
3.0
-820
.867
.210
.451
BSA
67
3.4
1.105
-912
- 258
*
J'-Globulin
160
4.7
1.121
.990
.402
.776
Thyreoglobulin
660
8.2
-997
-612
*
Dextran blue
2000
37.4
.997
(1.14)
Dextran
2000
37.4
*
1.02
* * *
*
*
* *
147 Discussion
The cornea, o p t i c a l l y almost homogeneous, is e x c e l l e n t l y s u i t e d f o r t h e interferometric t e c h n i q u e . The s t r u c t u r e of biological specimen c a n complicate measurement, as seen with t h e cartilage, where t h e cells distort t h e interference fringes, decreasing t h e accuracy of r/d values (Fig. 312). Size exclusion techniques depend on t h e precondition, t h a t t h e t e s t molecules do n o t i n t e r a c t with t h e porous m a t r i x . T h i s i s t r u e for t h e p r o t e i n s used. Cytochrome a n d d e x t r a n b l u e , however, h a v e shown t o be u n s u i t e d as t e s t molecules. These molecules show interaction with t h e matrix falsifying t h e r/d vs. ns dependences. Moreover, due t o t h e colour of t h e solutions, t h e measurement of their refractive index can yield incorrect values. However, for t h e sake of being complete, t h e results are included i n table 1. I t should be noted, t h a t i n interferometry different dispersion of matrix and solution c a n lead t o i n c o r r e c t i d e n t i f i c a t i o n of t h e zero o r d e r f r i n g e i n w h i t e light. In this case, t h e colourless fringes a r e not necessarily identical with t h e position of t h e Is* order. Different d i s p e r s i o n i s however d e t e c t a b l e by t h e asymmetry of t h e interference colours in white light.
Fractal interpretation: The pore s i z e d i s t r i b u t i o n of g e l s c a n be described i n f r a c t a l terms. The existence of biggest and smallest details of a n y natural object leads to fractal properties within certain limits. Thus selfsimilarity can be reached only asymptotically in a confined range of resolution E (= M R , MW). F o r graph types like kna vs. E, w e use a log-logistic function (LLF) i n order t o expand t h e s t r i c t definition of fractality from t h e purely selfsimilar range (linear dependence of log kna vs. log E ) t o t h e entire range of non-Euclidian behaviour (Fig. 6a for cornea d a t a ) (Se88, Se891. Let y be t h e o r d i n a t e q u a n t i t y , h e r e y = knn. An LLF i s o b t a i n e d by extending t h e selfsimilar power function y = aeb t o a n expression regarding a n upper limit ymaw, a n d a lower limit Y m i n as a f r a c t a l o r topological scaling residue [Bi89. Bi9Ol. With X = In E, Q = parameter of position, R = ymin/Ymax. P = e-bQ, t h e following, equivalent forms of t h e LLF can be written: Y = ymax ( 1 -
1
-
ymin/ymax
1 +
) eb(X-Q)
= ymin
+
ymax - ymin 1
+
e-bW-9)
--
Y ~ ~ x *+Pymin E ~ PEb
+
(Eq. 3) 1
For fractal characterization, one can use either t h e logarithm of t h e ordinate, Y = log y. or t h e logarithm only of t h a t p a r t B = log (y-ymin) exceeding t h e scale invariant residue. For t h e present investigations, t h e scaling residue h a s a considerable e f f e c t on t h e LLF, s i n c e ymin= M r a n g e s from 15 % t o 29 96, compared t o approximately only 2 % t o 6 % for Sepharose gels ISe88l. The fractal embedding [Ma821 i s literally identical with t h e experimental embedding of t h e matrix with molecules as t e s t elements. Y = log kns describes t h e scaling of t h e matrix network, which i s t h e complement of t h e accessible volume (Fig. 6b). In
148 contrast, Y = log (kna-M) describes t h e scaling of t h e inaccessible pore volume, disregarding t h e matrix network (Fig. 6c). The LLF i s a n approach to solve t h e question: Which range in X = In E is necessary to allow h c t a l evaluation? The LLF i s n o t r e s t r i c t e d t o t h e ( a r b i t r a r i l y chosen) n e a r l y l i n e a r region, b u t covers t h e entire range. I t describes t h e continuous fading of fractal properties towards t h e limits. Here t h e fractal dimension DF(X) itself i s a function of the resolution and h a s t o be regarded as a differential fiactal dimension. DF(X) i s c a l c u l a t e d from t h e local s l o p e p e i t h e r of dY/dX v s . X or of dY/dX v s . X, depending on t h e specific requirement. In t h e case of measuring with E = MR then DF(X) = 3 - p, whereas DF(X) = 3 - 2.7.p in t h e case of E = MW P r o te in [BigO]. In terms of Y, t h e local slope p at t h e t a i l s approaches zero. This means, t h a t in both ranges of scale t h e matrix h a s t o be considered as a solid volume (DFZ 3) whereas a t t h e inflexion p o i n t Q' = Q + (log R)/2b of Y vs. X t h e slope p becomes b . ( l - n ) / ( l + a ) , and t h e dimension o f t h e matrix reaches a minimum. In terms of Y, t h e slope of t h e netto inaccessibility vanishes for t h e larger scales, as well (p-+O). For small scales, p approaches b, leading t o DF = 3-b. ( A t X=Q*. p = b/2). The distribution of fractal properties results from t h e derivative dy/dX (Fig. 6d). The exponent b represents t h e dimensional excess in t h e unlimited power function. Here it assumes t h e meaning of a measure of dispersion as well, e.g. as width of t h e pore volume distribution around X = Q [Se89, Bi9Ol. F u r t h e r f r a c t a l regions may exist o u t s i d e t h e one i n v e s t i g a t e d h e r e . For instance with granulated porous materials, t h e interparticle wedge volume might show similar c h a r a c t e r i s t i c s . On t h e o t h e r s i d e , f o r s m a l l e r s c a l e s , t h e intramolecular space could be reached [Seas].
Measuring f r a c t a l s by fractals: Special a t t e n t i o n was paid t o t h e problem of measuring f r a c t a l s (gels, cornea, cartilage) by f r a c t a l s ( p r o t e i n or PEG molecules) [Se89l. Polyethylene glycols (PEGS) as test molecules yield lower kne values. This increased accessibility for l a r g e r PEGS c o m p a r e d t o p r o t e i n s c a n b e e x p l a i n e d by t h e i r l o w e r f r a c t a l dimension. For measurement of a fractal s t r u c t u r e by embedding, it is necessary t h a t t h e embedding probe h a s a higher dimension t h a n t h e embedded sample. Thus t h e globular p r o t e i n s (DF z 2.6 t o 2.8 [FaSSl; DF sz 2.7 lSe88n are a p t f o r characterization of t h e s t r u c t u r e s investigated here, whereas t h e elongated PEGS (DF sz 1.9 [Se89/) cannot be used t o measure structures with fractal dimensions above 1.9. The use of PEGS is meaningful only in scales, where t h e differential dimension does not exceed 1.9, t h a t is i n our case, towards t h e limits of t h e pore size distribution.
149
a
b
-.a -
I
-1
'
"
'
I
1
"
'
-.5
1 0
,
,
,
,
,
,
,
.5
,
,
,
-1
-
1
7
---s
I
= lag m
a
-I
-.5
0
.5
C
x.lcgm
Fig. 6: Fractal analysis of the porosity from cornea data (Fig. 5d with Q = 0); abscissa X = loglo MR. (a) Fitting function y=kn. (b) scaling of inaccessability Y = loglo y (inflexion point Q') (c) scaling of "netto" inaccessability rY = log10 (y-M) (d) Distribution of the porosity dy/dX.
T a b l e I1
150 REFERENCES Bittner. P . Wlczek. M. Sernetz: Characterization of fractal biological objects by image analysis. Acta Stereologica 8 (1989) 31-40. [Big01 H.R. Bittner. M. Sernetz: Selfsimilarity within limits: Description with t h e log-logistic function. (1990) Proceedings of t h e FRACTAL ‘90 conference, Lisbon, June 1990; Elsevier (in press). lFa88l D. F a r i n . D. Avnir: The f r a c t a l n a t u r e of molecule-surface chemical activities and physical interactions i n porous materials &; Characterization of Porous Solids. K.K. llnger, J. Rouquerol, K.S.W. Sing and I%. Kral (eds.), Elsevier Amsterdam; (1988) 421-432. lG188l B. Glittenberg: Mikrointerferometrische Untersuchung der Porenradienverteilung a m hyalinen Knorpel. Thesis, Giesen (1988). IMa82/ B.B. Mandelbrot: The Fractal Geometry of Nature. Freeman, New York (1982). l S e 8 8 l M. Sernetz. H.R. Bittner. C. Baumhoer, S. Sehwarz, H. Willems: Interferometric Characterizadetermination and fractal characterization of gel porosity t i o n of Porous Solids. K.K. Unger, J . Rouquerol, K.S.W. Sing a n d H. Kral (eds.), Elsevier Amsterdam; (1988) 461-472. lSeB9l M. Sernetz. H.R. Bittner, H. Willems, C. Baumhoer: Chromatography. & The Fractal Approach to Heterogeneous Chemistry. D. Avnir (ed.) Chapter 4.2.3.. John Wiley & Sons Ltd., Chichester (1989) 361-379. lBiS9l H.R.
F. Rodriguez-hinoso et al. (Editors), Characterization of Porous Solids II 0 1991 Elsevier Science Publishers B.V., Amsterdam
151
DETERMINATION OF SURFACE PROPERTIES OF POROUS SOLIDS
K.S. Birdi', D.T. Vu', S.I. Andersenl, A. Winter', Christensen'.
H. Tops0e3 and S.V.
'Fysisk-Kemisk Institut, Technical University, Building 206, 2800Lyngby, Denmark *Geological Survey of Denmark (D G.U. ) , Thoravej-8, Copenhagen, Denmark 'Haldor Tops0e A/S, Lyngby, Denmark.
.
ABSTRACT The surface area of porous solids was determined from the heat of immersion method using calorimetry. The heat of immersion of different solid powders in n-hexadecane was measured. These data agreed with the BET method. The surface structure of solids (glass, teflon) and porous solids (glass spheres, catalysts) have also been analyzed by liquid evaporation method. The liquid in a cylinder which contained fine solid powders was found to evaporate at three distinct different rates. The first evaporation rate of liquid molecules is for the bulk liquid, the second rate is for liquid molecules which are present between particles and the third rate is from the liquid molecules which are present in the pore volume of particles. In addition, the effect of some detergents on evaporation rate of sessile drop have been studied. INTRODUCTION The porous materials are known to be of importance in many different industrial processes; e.g., catalysis, oil recovery, soil pollution, chromatography and separation. In all these systems, the pore structure is known to determine the physico-chemical characteristics. The pore shape and form is not easily determined. Microsporous material is not easily analyzed using electron microscope or diffraction methods, when the mean pore-radius is 2 50 pm. One generally uses mercury porosimetry for larger pores, which is based on a capillary phenomena. Other methods have also been used, which are based upon the effect of the curvature of a liquid on its solid - liquid phase transition equilibria, i.e. freezing point depression, vapor pressure or heat of evaporat on.
152
The purpose of this study is to report on the analyses of the surface structure of porous solids, as regards: [ A ] heat of immersion and surface area [B] rates of evaporation of liquids from porous solid surfaces. The heat of immersion, him, is known to be related to the magnitude of the surface area of solids, when using inert liquids (ref. 1). In recent studies, we have shown that the evaporation rate studies of liquid drops placed on solid surfaces can provide much useful information (ref. 2 ) . In this report we will describe the evaporation of liquids from porous media. The porous media consisted of different types, e.g. small glass spheres, solid powders as kieselguhr and catalysts. EXPERIMENTAL A . Heat of immersion of solids in fluids
When a solid exposed to air is immersed into a fluid, the change in enthalpy, hi,, is related to the different phases as (refs. 1, 3 ) : hi,
=
h,, - h,
where subscripts sl and s denote the enthalpy of the solid - liquid interface, and solid interface, respectively. The magnitudes of hi, for different solid powders in nhexadecane (n-C16) are given in Fig. 1. The data point for 10 m2/g corresponds to an average value for different solids (e.g. A l p O3 , Si02 ,Ti02). The data can be fit to the following linear equation (with a corr. coeff. ca.0.95): hi,
=
0.0375 Joule/m2
(2)
These data thus indicate that the surface area of porous materials can be determined by the hi, method, and it compares with the BET data. The him, however, can also be useful in the estimation of the pore size distribution. B. Rate of EvaDoration of liauid drops Dlaced on solids In continuation to previous investigations (ref. 2 ) ,
the
153
evaporation rates of drops of fluid placed on solid surfaces were investigated under varying conditions. B.l. Evaporation of sessile drop of liauids restina on different solid surfaces. The rate of evaporation of a sessile drop was investigated by weighing. The experimental procedure has been described in reference (2). Evaporation experiments have been performed with different liquid drops such as water, alkanes, or alcohols placed on different solids, e.g. glass, teflon, graphite, or porous solids.
- .2
M 1472-
E E 3
lo-
86-
/ '
-
1 SLOPE = 0.0375 I
42-
0
I' 4 , 0 2
~,
4
,
6
8
I
,
I
10
12
14
__
--.wI
16
18
20
Surface Area [m */,.I
Fig. 1. Heat of immersion (him) vs. surface area (m2/g: from BET data) of different powders (fluid= n-Hexadecane).(25-C). The evaporation behaviour was found to vary from system to system. The data in Fig. 2 shows the evaporation process of wateror n-octane (C8) drops when placed on glass or teflon surface, respectively. In contrast to the other systems, the evaporation of water drops placed on a glass surface, and n-octane drops placed on a teflon surface was found to be stationary process. (i.e., the rate of evaporation of the liquid drop is constant during evaporation process). This occurs when the size of evaporating drop decreases, and the contact angle decreases while the radius of the liquid-solid
154
interface remains constant during evaporation. Furthermore, the rate of evaporation, in these cases, was found to be linearly proportional to the radius of the liquid-solid interface (Fig. 3 ) . The data obtained from the evaporation of water drop placed on a glass surface and from the evaporation of an n-octane drop on a teflon surface can be explained by considering the evaporation as being essentially a gas diffusion process.
d
0.005
ho
Y
0.0°4j
.r(
P 8 I2
0.0°3
0.002
0.001
0
Time lsecl
Fig. 2 . Variation of drop weight vs. time for different liquidsolid systems. The rate of diffusion of the vapour of the droplet across a spherical interface with radius r is given as (ref.2): rate
=
I = -4nr2(dc/dr) D
g/s
(3)
where D is the diffusion constant of the vapour and c its concentration (g/cm3). At infinite distance from the liquid interface, the following boundary conditions exists: c = c- when r = m and c = co when r = rd
(4) (5)
155
where rd is the radius of the liquid drop. From the above equations one obtains:
I
=
4rrrdD(co - c,)
(6)
The rate of evaporation, I, under these conditions is therefor completely determined by the rate of diffusion of the vapour in the medium. Further, it is also observed that in the present case the rate of evaporation is proportional to the radius of the drop rather than the surface area (as is the case from flat fluid surfaces! ) As mentioned above, the evaporation of a sessile drop of water or noctane placed, respectively, on a glass or teflon surface is a stationary process. The rate of evaporation of these drops was found to be linearly proportional to the radius of the liquid-solid interface. Equation (6) thus can be used to describe the evaporation process for these cases.
.
3
0.0006
u
t
Q, 0.0005 0) 0.0005-
/
/
A
c,
I
id
I
:E L d
3
g
0.0003
/
/
1
0.0002
id
P
W
nnnni
Radius of Liquid-Solid Interface [mml
Fig. 3 . Variation of evaporation rate vs. radius of the liquidsolids interface. B.2. Effect of added deteraent on the evaporation rate of liauid drops. It is well known that the surface tension of an aqueous solution is reduced by the addition of a detergent. The
156
purpose of this study was to compare the evaporation rates of drops of aqueous solutions of anionic (Na-n-C12H25S04, SDS) or cationic (nhexadecyl trimethylammonium bromide, CTAB) detergents when placed on a surface of glass (negatively charge surface). It can thus be deduced from the above equations that a sessile drop of solution of detergent has therefor a smaller contact angle, and hence a larger radius of liquid-solid interface, than a water drop with the same volume. We have used 5 pL drop of aqueous solutions of SDS and CTAB (concentration of 5 g/L and much >CMC) in the evaporation experiments. The evaporation rates of SDS-drop [3.1 10-6g/s] and CTAB-drop [2.3 10-6g/s]and pure water [1.9 10-6g/s]were found to be different, due to the above described differences in contact angle and radii. However, the CTAB-drop was found to evaporate with a constant rate while the SDS-drop evaporated with a decreasing rate. This shows that the radius of the CTAB-glass interface remains also constant during evaporation. It is larger than the radius of the water-glass interface but smaller than the radius of the SDSglass interface. The less spreading of CTAB compared to SDS may be explained by a mutual attraction between the glass surface (due to a negative charge) and CTAB (which is a positively charged detergent). This is not the case for SDS which is a negatively charged detergent. The same charge for a glass surface and SDS-drop causes the radius of the SDS-Glass interface not to remain constant as was the case for CTAB-Glass. B.3. EvaDoration of liquids from porous media. Measurements of the rates of evaporations of liquid drops placed on a porous solid surface were conducted using a small cylinder of polystyrene (Height = 10.5 nun, Diameter = 6.6 mm) which contained the solid powder or small glass spheres, so as to fill up to ca. 4 the volume of a cylinder (Fig. 4 ) . The cylinder was placed in a chamber with controlled temperature and humidity. Thereafter a given volume of the liquid was poured into the cylinder. The weight of the liquid was measured with a sensitivity of k 5 pg. The rates of evaporation of n-hexane from glass spheres with diameter of 0.1 nun or 0.5 mm, respectively, were investigated. The data in Fig. 5 show that n-hexane evaporates with different rates in the two cases, and also that the evaporation shows clearly three different stages (marked as rate 1, rate 2 and rate 3 in Fig.
157
5). The first rate (rate 1) is approximately constant and corresponds to the evaporation of n-hexane molecules of the bulk liquid. The n-hexane molecules which are present in the pores between the glass spheres evaporate with a constant rate (rate 2) but the magnitude is lower than rate 1. The remaining n-hexane molecules which are adsorbed on the surface of the glass spheres evaporate with a decreasing rate (rate 3).
Fig. 4 .
The evaporation of liquid from porus medium.
By using linear regression, the value of the first rate is found to be ca. 5.6 g/sec and 5.0 g/sec for n-hexane/glass spheres with diameter of 0.1 mm and 0.5 mm, respectively. The value of the second rate for both sizes of glass sphere is found to be the same, i.e. 3.0 1 0 - ~g/sec. In order to determine the dependence of the evaporation rate On solid particles with different composition and pore volume, we have used fine solid powders, e.g. SVc45 (a-alumina standard, total pore volume Vp = 0.037 cm3/g), SVCSO (alumina carrier, Vp = 0.312 cm3/g), SVC52 (kieselguhr, Vp = 0 . 0 0 9 cm3/g) and SVC71 (graphite, Vp = 0.0608 cm3/g). These powders and their data have been provided by Haldor Topspre A / S . The data for the evaporation of n-hexane from these powders VS. time are given in Fig. 5 . These data show that n-hexane in these liquid/powder systems also evaporated at three different rates. The first rate (rate 1) and the second rate (rate 2) are constant and were estimated by using a linear regression:
158
rate 1: Y = A 1 + B1 * X rate 2 : Y = A2 + B 2 * X where X is time (sec), Y is weight of liquid, A 1 and A2 are constants and B1 and B2 are slopes of plot (=rate of evaporation (g/sec)) The magnitudes of A l l B 1 , A2, and B2 for different systems are given in Table 1.
.
0.1
0.08
E
Glass (0 .5 mm -
Glass (0.1 mrn
0.04
0.02
0 0
Fig. 5.
1000
2000
3000
4000
5000
6000
Evaporation of n-Hexane from porous media.
The results from Table 1 show that the values of rate 1 and rate 2 for all systems do not deviate very much from each other. It is seen that n-hexane molecules in bulk liquid evaporate three times faster than n-hexane molecules in the pores between particles. The most interesting observation was made for the evaporation rate which involves the adsorbed layers of liquid molecules which are in the pores of the solid material. It was found that n-hexane in the particles which have a high pore volume (SVC50) evaporated at a lower rate than from low pore volume particles (SVC45 or SVC52 or SVC71).
159
TABLE 1 The first and second rate for different n-Hexane/Powders system.
I n-Hexane/ powder svc45 SVC50 svc52 svc71
Rate 1 Y = A1 + Bl*X A1
B1
Rate 2 Y = A2 + B2*X A2
B2
R-Squared
(*I 0.11360 0.11186 0.11165 0.11059
-9.1E-5 0.09035
-2.6E-5
0.99988 0.99831
0.09920
-3.3E-5
0.99996 0.99852
0.09397
-2.7E-5
0.99989 0.99983
0.09742
-2.9E-5
0.99979 0.99968
-1.OE-4 -8.7E-5 -8.5E-05
CONCLUSION
The surface area of solids was determined from the heat of immersion method using calorimetry. These data agreed with the BET method. These data are of use for describing the evaporation of liquids from solid surfaces (both the rates and the heat of evaporation). The evaporation of sessile drops of water (resting on glass surface) and n-octane (on teflon) was found to be a stationary process. In both cases, when the size of evaporating drop decreases, only the contact angle decreases while the radius of liquid-solid interface remains constant during evaporation. Furthermore, the rate of evaporation, in these cases, was found to be linearly proportional to the radius of liquid-solid interface. Sessile drop of water, SDS-, and CTAB-solution when placed on a glass surface, were found to evaporate into the air with different rates. A CTAB-droplet will evaporate at a faster rate than a waterdroplet but it evaporates at a slower rate than a SDS-droplet. Both water- and CTAB-droplets evaporate with a constant rate, while the SDS-droplet evaporates with a decreasing rate. This suggests that there is a different effect between the two detergents: negative detergent (SDS) and positive detergent CTAB onto glass surface.
160
The liquid in a cylinder which contained small particles such as glass spheres or solid powders will evaporate at three different rates. The first rate is the evaporation rate of liquid molecules in the bulk liquid, the second one is the evaporation rate of liquid molecules which are present in the pores between particles and the third one is for the liquid molecules which are present in the pore volume of particles. In general, both the first and second rate are constant but the third rate is a decreasing rate. Although the first and second rate for most systems are about the same, the third evaporation rate of n-hexane will be slower in particles which have a high total pore volume. This may be due to the possibility that more volume of liquid penetrates into the pores and then it takes a longer time to evaporate. This suggests that one can estimate the pore volume of the porous solid from the latter. ACKNOWLEDGEMENTS This work was supported by BRITE contract [No. RI lB-0290-Cl. REFERENCES 1 Chattoraj, D.K. & Birdi, K.S., Adsorption & the Gibbs Surface Excess, Plenum Press, New York, 1984. 2 (a) Birdi, K.S., Vu, D.T. and Winter A., J. Phys. Chem. 1989, 93, 3702. (b) Birdi, K.S. & Vu, D.T., Mechanisms of Oil Recovery, Report, 1988, Danish Ministry of Energy, Copenhagen. 3 Birdi, K.S., Lipid & biopolymer Monolayers at Liquid Interfaces, 1989, Plenum Press, New York.
F. Rodriguez-Reinoso et al. (Editors), Characterization of Porous Solids II 0 1991 Elsevier Science Publishers B.V., Amsterdam
161
A NEW APPARATUS FOR CONTINUOUS ADSORPTION. APPLICATION TO THE CHARACTERIZATION OF MICROPOROUS SOLIDS H. AJOT, J.F. JOLY, F. RAATZ and C. RUSSMANN INSTITUTFRANCAIS DU PETROLE, 1et 4 avenue de Bois Preau, BP311,92506Rueil Malmaison (France).
SUMMARY A new apparatus for continuous adsorption is described. This apparatus is based on anew technique which uses proportional valves with an upstream pressure regulation. The decrease of upstream pressure is programmed at about 1 torr/min, leading to constant flow rates in the range of 0.17 to 0.2 ml(STP)/rnin independent of the downstream pressure. Complete nitrogen adsorption-desorption isotherms can be recorded with an automatic procedure. The main advantages of this new apparatus are: simplicity, accuracy and a wide field of applications, including the study of microporous solids. INTRODUCTION Gas adsorption is the standard technique to study the textural properties of porous solids (ref. 1). The classical technique is based on incremental additions of gas providing adiscret set of points along the isotherm. This technique can be referred as a “discret” technique. It appears to be limited and time consumming when high accuracy is required. This is especially true in the case of microporous solids. To overcome this intrinsec limitation, “continuous” adsorption techniques have been developed. In these techniques the adsorbate is admitted to the sample tube at a slow flow rate. As it is critical to maintain, at any given time, the adsorption equilibrium, very low adsorbate flow rates must be used. Volumetric and gravimemc apparatus have been developed in order to obtain a continuous adsorbate addition. Two main volumetric techniques can be distinguished, the use of i) narrow restrictions such as capillaries (refs. 2,3) and orifices in metal foils (ref. 4), and ii) sonic flow restrictions (refs. 5,6). Constant flow rates cannot be obtained using capillaries and orifices, even for downstream pressures lower than 100 mbars. Thus, complete adsorption-desorption isotherms cannot be easely obtained. In contrast, constant flow rates can be obtained with sonic flow restrictions, in the entire range of desired downstream pressures (up to 1 bar without experimental difficulties) by using high upstream pressures (up to 10 bars) (ref. 7). Gravimetric techniques as described in ref. 5, lead to complete adsorption-desorption isotherms without downstream pressure limitations (a needle valve is used). We have developed a new apparatus leading to the acquisition of complete adsorption-desorption isotherms. In this apparatus the adsorbate is admitted to the sample tube at a slow constant flow rate
162
using a proportional valve and an upstream pressure regulator (in the desorption mode the adsorbate is removed from the sample tube according to the same principle). The flow rate is thus independent
from the downstream pressure. This new concept has been protected by a patent. APPARATUS AND PROCEDURE ApyratuS A schematic of the apparatus is presented in figure 1. The adsorbate, usually nitrogen, is admitted to the sample tube at a slow flow rate (typical value: 0.17 ml(STP)/min) through a proportional valve. The decrease of the upstream pressure is typically programmed to 1 torr/min. Two proportional MKS valves are used, one for the adsorption, one for the desorption. During desorption, the difference between upstream and downstream pressures has to be in the range of 76 to 228 torrs to insure a constant flow rate. Both adsorption and desorption branches of the isotherms are depicted with theoritically an infinite number of points (practically loo0 points are recorded).
Fig. 1. Schematic of the apparatus. V10 and V11: proportional valves; C1, C2 and C3: pressure gauges; R1: adsorbate container; R2: calibration volume; S.T.: sample tube; V: vacuum line. V1 to V9: valves.
163
Procedure The sample is first ou.tgassed down to torr using a turbomolecular pump, with a specific temperature programme depending on the nature of the studied sample. The sample is then isolated from the vacuum system. A dewar flask containing liquid nitrogen is placed around the sample tube, the level of nitrogen is kept constant by periodically replenishing the liquid lost by evaporation. The dead volume is determined by helium, the measurment is automaticaly conducted by the use of a computer. Helium is withdrawn from the apparatus, the sample is outgassed under vacuum until the vacuum is close to lO.'torr. The initial upstream pressure is of about 1290 torrs, the pressure regulator is programmed so that the decrease of pressure is close to 1 torr/min. Nitrogen is thus slowly admitted at a slow constant flow rate in the range of 0.15-0.20ml(STP)/min through the proportional valve to the sample tube. The nitrogen admission is stopped when the partial pressure in the sample tube is close to 1, the adsorption branch is thus described with a high accuracy. To record the desorption branch, the upstream pressure is lowered to lO.'torr, and nitrogen is evacuated from the sample tube at a slow constant flow rate in the range of 0.15-0.20 ml(STP)/min. During the adsorption procedure, the sample tube can be isolated, it is thus possible to check that the equilibrium is reached at any given time. Other adsorbates than nitrogen can be used in the same way. Isotherm acauisition Volumes of nitrogen container and of the sample tube are known with a high accuracy. By recording simultaneously upstream and downstream pressures, the quantity of adsorbed nitrogen is calculated, the isotherms are obtained by plotting it as a function of partial pressure P/Po. RESULTS This new continuous adsorption technique has been used to determine the textural properties of a mesoporous solid ($alumina) and of microporous solids (zeolites). Results have been compared to those obtained with the "discrete" technique. For the experiments, nitrogen is used as adsorbate. 1.f-aIumina d-alumina (Rhone Poulenc product) has been outgassed at 723K under vacuum, lo6torr. The adsorption branch has been determined at 77K with nitrogen at constant flow rate of 0.17 ml(STP)/min, 967 points are recorded. T h e desorption branch is obtained by evacuation of nitrogen at a constant flow rate of 0.17 ml(STP)/min, 930 points are recorded. The complete nitrogen isotherm is reported in figure 2. The calculated B.E.T. surface area is 257 m'/g (252 m'/g using the "discrete" technique). The agreement between the two techniques is very satisfactory.
2. NaY zeolife torr). After Nay, provided by Union Carbide, has been outgassed at 773K under vacuum cooling the sample tube at 77K,the complete nitrogen isotherm is recorded and is reported in figure 3.
164
Fig. 2. Nitrogen isotherm at 77K of #alumina.
Fig. 3. Nitrogen isotherm at 77K of Nay.
165
The adsorption branch is obtained using a constant flow rate of 0.17 ml(STP)/min, 967 points are recorded. To record the desorption branch the same flow rate is used, 930 points are then recorded. The calculated B.E.T. surface area is 871 m2/g (900 m2/g indicated by Union Carbide). It should be noticed that the Union Carbide value has been obtained using a dynamic apparatus ("one point" B.E.T.). 3. ApDlication to H-zeolites with different structures H-Beta, H-mordenite and H-MFI zeolites have been prepared from as-synthesized zeolites using classical modification procedures: ionic exchanges in NH,NO, solutions followed by calcination under air at 823K. The main physicochemical characterists of the three solids are given in table I. TABLE 1 Physicochemical characteristics of H-zeolites
I
zeolites
I
Si/Al total
I
% DX
I
% Na
I
H-BETA
H-MFI
0.013
Dubinin volume, B.E.T. and t surface areas of these zeolites are determined using the continuous nitrogen adsorption at 77K with flow rate values close to 0.2 ml(STP)/min. The values obtained are summarized in the table 11.
TABLE 2 Dubinin volume, B.E.T. and t-surface areas of H-zeolites determined using continuous nitrogen adsorption
I
zeolites
I
H-MFI
I
I
0.204 0.193
S BETm2/g
1
692
0.280
H-BETA rHMORD.
V Dubinin cm3(liq)/g
I
361 441
St m2/g
I
37
I
10
I
164
Values found for Dubinin volume, B.E.T. and t surface areas reported in table I1 are in good agreement with that generally reported for such zeolites.
166
The isotherm of H-Beta is reported in figure 4 and exhibits an hyterisis loop indicating that mesopores are present, the closer point of the hyterisis loop is found to be close to P/Po=O.42. Catastrophic desorption of mesopores is seen as generally found for dealuminated HY zeolites (ref. 8), one can discuss about the origin of this phenomenon. It is certainly due to mesopores, evidenced in transmission electron microscopy, which are not directly connected to the exterior of the crystals. The formation of these mesopores is probahly related to the presence of faults in the stacking sequence of polytypes as mentioned in ref. 9.
Fig. 4. Nitrogen isotherm at 77K of H-Beta.
CONCLUSION We have developed a new continuous adsorption technique. This technique is different from those already described in the literature since it does not employ any mass flow controllers, capillaries, orifices in metal foils or sonic flow system. It is based on the use of proportional valves (one for the adsorption, one for the desorption) with upstream pressure regulation (or downstream regulation in the desorption mode). The programmed decrease of the upstream pressure is around 1 torr/min with typical flow rates of 0.17-0.20 ml(STP)/min. The main advantages of this new apparatus are:
167
1. It's simplicity, 2. Flow rates low enough as to ensure a thermodynamic equilibrium at any given time, 3. Controlled and constant flow rate, independent of the downstream pressure, 4. Complete adsorption-desorption isotherms can be recorded, 5. Wide field of applications including microporous solids, since high accuracy is obtained in the very low pressure range.
ACKNOWLEGMENTS We would like to sincerely acknowledge Mr GARNER and MAFWY REFERENCES 1. S.J. Gregg and K.S.W. Sing in "Adsorption, Surface area and Porosity", Academic Press Inc., second edition, 1982. 2. K.R.Lange, 1. Colloid. Sci., 18 (1963), pp. 65-72. 3. E.G. Schlosser, Chemie Ing. Techn, 31 (1959), 799. 4. P.S. Northrop, R.C Flagan and G.R. Gavalas, Langmuir, 3 (1987), pp. 300-302. 5. J. Rouquerol, F. Rouquerol, Y. Grillet and R.J. Ward, Proceeding of the IUPAC Symposium (COPS I) Bad Soden, April 26-29, 1987, Studies in Surface Science and Catalysis, Elsevier, Amsterdam, 1988, vol. 39, pp 67-76. 6. Y. Grillet, F. Rouquerol and 1. Rouquerol, J. Chim. Phys., 74 (1977), pp. 179-182. 7. J. Rouquerol, personal communication. 8. J. Lynch, F. Raatz and P. Dufresne, Zeolites, vol. 7 (1987), pp. 333-340. 9. H. Ajot, P. Caullet, J.F. Joly, J. Lynch andF. Raatz, Preprints of the COPS IIIUPAC Symposium, Alicante 6 9 May 1990, pp. 62-64.
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F. Rodriguez-Reinosoet al. (Editors), Characterizationof Porous Solids II 1991 Elsevier Science Publishers B.V., Amsterdam
169
A NEW MERCURY INTRUSION-RETRACTION SIMULATOR USED AS A MEANS FOR THE CHARACEREATION OF POROUS MATERIALS
CHRISTOS D. TSAKIROGLOU and ALKIVIADES C. PAYATAKES Department of Chemical Engineering, University of Patras, and ICE/HT-FORTH, GR 261 10 Patras, Greece
SUMMARY Information about the pore structure of porous solids is embedded in mercury intrusionretraction curves in highly convoluted form. Any attempt to derive a "pore-size distribution" must inevitably depend on postulates concerning the pore shapes and the pore network skeleton. For an important class of porous materials the pore space can be represented as a matrix of chambers interconnected through narrow throats. Information about the chamber size distribution and the network skeleton can be obtained from serial tomography. Information about the throat size distribution can, then, be obtained by deconvolving the intrusion-retraction curves. To this end, a reliable mercury intrusion-retraction simulator must be available. Such a simulator for three dimensional chamber-and-throat networks is developed here. This simulator takes into account the mechanisms with which mercury menisci move in pores and stop at entrances to throats or (in certain cases) chambers. It also takes into account the mechanism of snap-off, which leads to the disconnection and entrapment of mercury. The simulator is used to study the effects of the main geometrical, topological and statistical network parameters on the capillary pressure curves. INTRODUCTION Mercury porosimetry produces a set of capillary pressure curves which contain information about structural characteristics of porous media in highly convoluted form. The conventional method of analysis (refs. 1-3) is based on the tube-bundle model and ignores important characteristics of porous media, such as the existence of chambers and throats and the high interconnectivity of the pores. Several researchers have tried to interpret mercury porosimeay data of unconsolidated porous materials (such as sandpacks, soil etc) by assuming that the pore space is similar to that in a packing of uniform spheres (refs. 4-9). Pore network models have also been used to study the effects of geometrical, topological and statistical parameters of porous media on mercury capillary pressure curves. In these models the pore space is represented by a network of nodes and bonds in which shape and size are assigned either only to the bonds or both to the nodes and the bonds (ref. 10). In this way the porous medium can be considered as a network of interconnected capillaries of different sizes (refs. 11-13), or as a network of large pores (chambers) interconnected through narrow constrictions (throats), (refs. 14-19). Optical studies of certain sedimentary rocks (mostly sandstones) indicate that chamber-and-throat network models can be used to represent their pore space (refs. 20-23). Experimental studies in planar chamber-and-throat networks etched in glass plates have provided information about the mechanisms of mercury intrusion and retraction, and about the
170
effects of the wettability of the air/mercury/solid system and the geometrical, topological and statistical properties of the networks (refs. 24-29). In recent years it has been recognized that a more accurate method of pore analysis should consist of an appropriatecombination of techniques of which mercury porosimeay is but one of the components (refs. 18,30,31). First, serial sectioning analysis of pore casts (refs. 32-35) can be used to determine the chamber-size distribution, the correlation between the sizes of adjacent chambers, and information pertaining to the interconnectivity of the network (e.g. specific genus and coordination number). Then, the capillary pressure curves can be used to determine the throatsize distribution, and the correlation between the sizes of contiguous throats and chambers. In order to deconvolve these curves a reliable simulator of intrusion and retraction of mercury in evacuated chamber-and-throatnetworks must be developed. In (ref. 37) and in the present work a new simulator of mercury intrusion into and retraction from a three-dimensional chamber-and-throat network is developed. The capillary resistance encountered at entrances to chambers under certain conditions during mercury intrusion, and the snap-off in throats during mercury retraction are taken into account. The effects of geometrical, topological and statistical parameters and of the intrusion and retraction contact angles on the form of capillary pressure curves are studied. Comparisons between the actual throat and chamber size distributions and the measured "pore size distributions"(by the convetional method of analysis) are also made.
PORE NETWOFX MODEL Here the skeleton of the pore network is considered as a cubic lattice. Other networks, including random ones, can also be used. Large pores (chambers) are placed at the nodes and long, narrow pores (throats) at the branches, thus creating a 3-D chamber-and-throatnetwork. The diameters of chambers and throats are randomly chosen from preselected distributions (e.g. Gaussian, Lognormal, etc) and they are assigned at random to the nodes and the bonds of the network (a different procedure is followed for correlated networks, as described below). The distance between the centres of two adjacent chambers (length of periodicity) is adjusted so that the porosity of the network matches that ot the prototype. A porosity value of &=0.20 was considered for all the networks in the present work. A more detailed description of the chamber-and-throat network construction is given in (refs. 36,37).
SIMULATION OF MERCURY INTRUSION AND RETRACTION 1) Simulation of mercury intrusion Once the pore network has been constructed (see above) mercury menisci are placed at the entrances to all boundary throats and the external pressure is set at an initial value which is smaller than the capillary pressure of the smallest throat, so that no mercury enters into the network. Then, the pressure is increased by a small step AP and the effect of this change on the positions of the mercury menisci is examined. First, all menisci posed at entrances to throats are examined. If P,,sP,, where P , is the capillary pressure required for a throat to be filled by mercury (ref. 37),
171
the throat is filled and the meniscus is placed at the entrance of the downstream chamber. Each time two branches of mercury meet, they are assumed to coalesce instantly. Second, all menisci posed at entrances to chambers are examined. If the external pressure exceeds the minimum capillary resistance of entry into the chamber (ref. 37), the chamber is filled with mercury and new menisci are placed at the entrances to all contiguous throats. At the end of this scanning, all menisci newly placed at entrances to throats and chambers are scanned for stability and so on. The procedure is continued until no more unstable menisci can be found. Next, the pressure is increased by another step and the procedure is iterated until all the network becomes filled with mercury. 2) p n When the network is completely filled with mercury, the external pressure is decreased by a small step AP and the network is scanned in search of sites where flow events can take place under the existing conditions, The following types of flow events may occur. a) A throat full of mercury connects two chambers also occupied by mercury and has the required length for a collar to be developed. If PexlPts,where Pt, is the critical capillary pressure for snapoff in a throat (ref. 37), then snap-off takes place, the throat is emptied and two new mercury menisci are formed at its ends. b) A throat is occupied by mercury and at one of its ends there exists an empty chamber. If PexQti, where Pti is the capillary pressure for piston-type retraction (refs. 26,37), the throat is emptied and a meniscus is placed at the entrance to the other adjacent chamber. c) A chamber filled with mercury is connected with at least one empty throat. If P,
172
TSDs are given in Fig. la. As the TSD moves to smaller sizes the intrusion curve moves towards higher pressures, and also retraction of mercury from the pore network and snap-off in throats begin at higher pressures. As it can be seen, both the form of the retraction curve and the residual mercury saturation depend strongly on the ratio of the mean chamber diameter over the mean throat diameter, /. Retraction is governed by two competing mechanisms, namely, snap-off in throats and emptying of chambers. If the ratio &>/at> exceeds the critical aspect ratio E=Pti/Pt, (refs. 26-27,37) snap-off intensifies and a large amount of mercury progressively loses its continuity and it is trapped in the network. If the ratio / is comparable to the critical aspect ratio E, snap-off in throats and emptying of chambers occur in the same pressure range and this results in higher retraction efficiency. Comparison between the actual TSD and CSD,on one hand and the pore size distributions derived by the differentiation of the intrusion (PSDI) and retraction (PSD2) curves, on the other, is made in Fig.lb,c,d .
100
/ = 2.5
80 4-
-TSD CSD
5 6 0 % 40
i
f
20 /
50
100
CSD
............. PSDl
,: ;.
: :
I
-,
0
f
NETWORK S I Z E . 20XPOX20
b
.," _
150 P.#&
200
250
MO
::
Fig. 1. (a) Simulated capillary pressure curves for networks with the same lognormal CSD =l6.0 pm, (=40.0 pm, 0,=15.0) and three different lognormal TSDs : (I) (--) 0,=8.0, (11) (-.. -) =13.0 pm, 0,=5.0, (111) (- -) =lO.O pm, 0,=5.0. (b), (c), (d) Comparison between the actual TSD and CSD and the pore size distributions (by number) obtained by differentiating the intrusion (PSD1) and retraction (PSD2) curves, according to the tube bundle model. The relative frequencies of PSDl and PSD2 refer to number of pores, and they are obtained using the conventional method, which assumes that all pores are non-communicatingcylinders of equal length but of varying diameter. As it can be seen PSDl is comparable only to the TSD, being narrower than TSD in both directions of larger and smaller sizes. This is explained by the fact that
173
large throats are shadowed by smaller ones, and small throats of negligible volume are filled in the last stages of the process (see also refs. 16-17). PSD2 lies between the TSD and CSD curves, as mercury retraction is controlled by the emptying of chambers and snap-off in throats. PSD2 moves closer to the TSD, as the ratio / increases, because then it is mainly throats that are emptied by snapoff while an even larger amount of mercury is trapped in chambers. The mean coordination number is a measure of the connectivity of a pore network. Simulated capillary pressure curves for networks with the same Gaussian CSD and TSD, but with different mean coordination number (=6,5,4,3), are given in Fig. 2a. As the mean coordination number decreases the accessibility of the interior pores to the boundary of the network decreases. Hence, a given saturation value is obtained at higher pressures during intrusion and at lower pressures during retraction so that the hysteresis between capillary pressure curves widens and the residual mercury saturation increases. As the mean coordination number decreases, PSDl moves to smaller sizes because the shadowing of large throats increases (Fig.2b), whereas no important change occurs to PSD2 (Fig.2~).
037
I
TSD PSDl (c,=6.0)
025-
0
0
,=I
50
150 ZOO P. k h
!OO
250
300
350
,
-
i-\
0.1
.....
TSD CSD PSDZ (c,=6.0) PSDZ ( ~ ~ 4 . 0 ) PSD? (c,=3.0)
....... :
NETWDRK SIZE : 20 X 20 X 20
OD6 0134
O
B
2
J
J
) U 0.P
)
5
0
"0
50
100
!50 P.kFa
200
250
30
Fig. 2. (a) Effect of the mean coordination number, , on simulated capillary pressure curves for a network with Gauusian TSD (43>=10.0 m, 0,=3.0) and CSD (=30.0pm, cs =7.5). (b) Comparison between the actual kSD andlthe pore size distributions PSDl's o b t h e d by differentiating the above intrusion curves. (c) Comparison between the actual TSD, CSD and the pore size distributions PSD2's obtained by differentiating the above retraction curves. (d) Effect of the intrusion contact angle, 01, and of the retraction contact angle, OR, on simulated capillary pressure curves for a network with Gauusian TSD (=10.0 pm, 0,=3.0) and CSD (=30.0 pm. 0,=7.5).
174
Simulated capillary pressure curves for a network 20x20~20obtained by using various intrusion and retraction contact angles are given in Fig. 2d. It seems that the form and the degree of hysteresis of intrusion and retraction pressure curves are strongly affected by the values of the contact angles. Simulated capillary pressure curves for networks with the same CSD and two different bimodal TSD’s having the same and (T, values are shown in Fig. 3a. Since intrusion is controlled mainly by the large throats, the intrusion curve widens and extends to a higher pressure range as the fraction of large throats decreases. In these networks, which have high ratio /d),>, mercury retraction is controlled by snap-off events, and as the frequency of narrow throats increases, snap-off occurs over a wider pressure range with the result that the retraction curve widens and the residual mercury saturation increases. As it can be seen in Fig.3b,c, the large sizes of throats and chambers are not reflected in PSDl and PSD2 because of the shadowing effect during intrusion, and the entrapment of mercury in a large number of them during retraction (/=4.0).
20
0
6
TSD (c = 0.8) CSD
...........PSDl 4
f
50
b
I00
150 200 P.kFa
‘1 4
E
250
C
300
3 0
STSD D (c = 02)
...........PSDl PSDZ
C
....
2
0 t
Fig. 3. (a) Simulated capillary pressure curves for networks with the same lognormal CSD (=40.0 pm, 0,=15.0) and two different bimodal TSD’s (=10.0 pm, 0,=5.0) . Parameters of the component lognormal size distributionsused as input data for the TSD’s: (1) -( ) =12.0pm,0,1=3.0, =2.0 pm, ot2=3.0, c=O.8 (11) (.----) =l8.Opm, 0,,=6.O, =KO m, (T 1.5, c=0.2 (b), (c) Companson between the actual TS8 and C!$D and%: pore size distributions (by number) obtained by differentiating the above intrusion (PSD1) and retraction (PSD2) curves, according to the tube bundle model value can be seen in Fig. 4a. The effect of the width of bimodal TSD’s having the same a,>
175
The intrusion curve moves to lower pressure ranges as the frequency of wide throats increases. Since mercury retains its continuity to the external mercury sink through large throats (which become disconnected by snap-off at lower pressures) the possibility of mercury retraction from chambers increases, and the residual mercury saturation decreases as the frequency of wide throats increases. In these cases where the fractions of large and small sizes of throats are comparable, neither very large sizes nor very small ones are reflected in PSDl (Fig.4b,c,d). It must be noted that the shape of PSD2 is affected by the shape of TSD as snap-off in throats intensifies during mercury retraction ((/=4.0).
4-
20
1
b
= 05)
.j.
..
-z:
,
,
~
..i '
50
-
!OO
6- -TSD
150
200
P ILFa
(C
= 0.4)
:.
........... CSD PSDl 4-
(C
.........
),:/
0 0
r
TSD
CSD ........... PSDl
PSD2
250
300
EO
C
::
... ... .. ..
Fig. 4. (a) Simulated capillary pressure curves for networks with the same lognormal CSD (=40.0 pm, o =15.0) and three different bimodal TSD's. Parameters of the component lognormal size distributions used as input data for the TSD's: (I) (-) =18.0 pm, ot,=8.0, =2.0pm, oQ=2.83, c=OS (=10.0 pm, o,=10.0) (11) (-. -) =15.0pm, o,,=4.0, =6.67pm, oQ=1.8, c=0.4 (=10.0 pm, 0,=5.0) (111) (- -) =13.0pm,ot1=4.0,=5.5 pm, o,=2.18, c=0.6 (=lO.O pm, vt=5.0) (b), (c), (d) Comparison between the actual TSD and 6SD and the pore size distnbunons (by number) obtained by differentiating the above intrusion (PSD 1) and retraction (PSD2) curves, according to the tube bundle model
EFFECTS OF C-t AND
SIZE-CORRELATIONS Some characteristics of experimental capillary pressure curves, especially the width of the pressure range, have been interpreted by constructing pore networks where the sizes of the throats are not randomly arranged,but are correlated to the sizes of adjacent chambers (refs. 14,18,19,38). Actually, at least two types of correlated networks can be considered depending on the arrangement of the sizes of throats and chambers in the network. C-c
176
a) Networks with chamber-to-throat size correlation (c-t correlation) The sizes of the chambers themselves are arranged completely at random, but each chamber casts a "vote" on the size of its adjacent throats according to the relation
where s is a dimensionlessparameter. The vote value for each throat is defined as the average of the votes of the two adjacent chambers. The sizes of the throats are ranked in descending order and they are assigned to the bonds of the network according to the results of the voting (ref. 36). For s=O there is no c-t correlation.For s=l (used here) there is a significantc-t correlation. b) Networks with chamber-to chamber size and chamber-to-throat size correlation (c-c and c-t correlation) The sizes of chambers are ranked in ascending order and they are partitioned in classes of equal width. A few sizes are randomly chosen from the set of the available sizes as seeds, and they are randomly assigned to chambers of the network. For each of these seed chambers the following procedure is followed. If the seed chamber belongs to class I, the sizes of its immediate neighbors are also chosen from class I. If this class becomes empty, then sizes from classes 1-1 and 1+1 are chosen, and so on. Each chamber that is assigned a size in this way is treated from that point on as -~
10-
8f
6-
UNCORRELATED NETWORK
............. ____
TSD CSD PSDl PSD2
4-
::
ii ;;
... ...
4 6-
4f
C-t
C-t
CORRELATED
NETWORK
-.I-.
TSD CSD PSDl PSD2
C
:i ii
:: :
4 ' c-c
CORRELATED
3 NETWORK 1 2 1 0
Fig. 5. (a) Effect of the degree of correlation between chamber-sizes and throat-sizes on simulated capillary pressure curves for networks with lognormal CSD (=40.0pm, oc=15.0) and TSD (=13.0 pm, 0,=5.0, ). (b), (c), (d) Comparison between the actual TSD and CSD and the pore size distributions (by number) obtained by differentiatingthe above intrusion (PSD1) and retraction (PSDZ) curvesusing the tube bundle model.
177
a new seed chamber. The procedure is iterated until all the available sizes become assigned to the chambers of the network. The arrangement of the throats follows the rules described above in (a). Comparison between the capillary pressure curves of an uncorrelated and two correlated networks with the same TSD and CSD is made in Fig. 5a. As the degree of correlation becomes stronger the intrusion curve widens in both directions. The retraction curve is affected mainly in the portion near the end; the residual mercury saturation decreases as the correlation increases. As the degree of correlation increases, PSDl widens (Fig.Sb,c,d), following the increase of the width of the intrusion curve. Especially in the case of significant c-c&c-t correlation (FigSd), PSDl becomes multimodal, as mercury fills small subnetworks of different throat sizes at progressively higher pressures. As the degree of correlation increases, PSD2 approaches the CSD, and it widens (Fig.Sb,c,d) as the fraction of the chambers which are emptied increases. In the case of c-c&c-t correlation (FigSd) PSD2 becomes bimodal because chambers belonging to clusters of similar throat and chamber sizes are emptied at once in the last stages of the process. CONCLUSIONS - DISCUSSION A new theoretical simulator of mercury intrusion in and retraction from three-dimensional chamber-and-throat networks is developed. Stepwise porosimetry is modelled as a sequence of flow events occurring at each new external pressure value. The main conclusions resulting from the study of the effects of geometrical, topological and statistical parameters on the capillary pressure curves are listed below. * The intrusion curve is conmlled mainly by the TSD and moves to higher pressure ranges as the TSD moves to smaller sizes (unimodal distributions), or as the fraction of the wide throats decreases (bimodal distributions). * The form of the retraction curve is the result of two competing processes, namely snap-off in throats and emptying of chambers. As snap-off intensifies with increasing / ratio the quantity of trapped mercury also increases. * As the mean coordination number decreases the intrusion and retraction curves widen, the degree of hysteresis increases and the residual mercury saturation increases. * The capillary pressure curves are strongly affected by the values of the intrusion and retraction contact angles. Exact determination of these parameters is needed in order for adequate information about the pore structure to be extracted. * Correlations between the sizes of neighboring chambers and between the sizes of chambers and adjacent throats affect the form of capillary pressure curves strongly. As the degree of correlation increases, the intrusion curve widens and the residual mercury saturation decreases. * The pore size distribution obtained by differentiation of the intrusion curve (PSDI) is narrower than the TSD in both directions. It moves to smaller sizes as the mean coordination number decreases, and it widens as the degree of correlation increases (in the cases studied). * The pore size distribution obtained by differentiation of the retraction curve (PSD2) lies between the TSD and CSD and it moves closer to the TSD as the ratio / increases. For large ratios / the shape of PSD2 is affected by the shape of TSD as snap-off intensifies during mercury retraction. PSD2 becomes wider and moves closer to CSD, as the degree of
178
correlation increases. The simulator was developed as a part of a generalized method for the deconvolution of capillary pressure curves in order to obtain the equivalent capillary throat diameter distribution and to determine the throat-to-chamber size correlation. To this end information about the chamber-size distribution, the mean coordination number, and the chamber-to-chambersize correlation must be available from serial tomography. ACKNOWLEDGEMENTS This work was supported by EC, Contract No. TH 15.73/85, and by the Institute of Chemical Engineering and High Temperature Chemical Processes. REFERENCES 1 E.W. Washburn, Phys. Rev., 17 (1921) 273-283. 2 L.C. Ritterand H.L. Drake, Ind. Eng. Chem., 17 (1945) 782-786. 3 H.L. Drake and L.C. Ritter, Ind. Eng. Chem., 17 (1945) 787-791. 4 S. Kruyer, Trans. Faraday Soc., 54 (1958) 1758-1767. 5 L.K. Frevel and L.J. Kressley, Anal. Chem., 35 (1963) 1492-1502. 6 J.C. Melrose, Soc. Pet. Eng. J., 5 (1965) 259-271. 7 R.P. Mayer and R.A. Stowe, J. Colloid Sci., 20 (1965) 893-911. 8 R.P. Mayer and R.A. Stowe, J. Phys. Chem., 70 (1966) 3867-3873. 9 D.M. Smith and D.L. Sterner, J. Colloid Interface Sci., 111 (1986) 160-168. 10 F.A.L. Dullien, Porous Media; Fluid Transport and Pore Structure, Academic Press, New York, 1979. 11 I. Fatt, Trans. AIME, 207 (1956) 144-159. 12 I. Chatzis and F.A.L. Dullien, J. Can. Pet..Tech., 16 (1977) 97-108. 13 G.P. Androutsopoulos and R. Mann, Chem. Eng. Sci., 34 (1979) 1203-1212. 14 I. Chatzis and F.A.L. Dullien, Int. Chem. Eng., 25 (1985) 47-66. 15 G.R. Lapidus, A.M. Lane, K.M. Ng and W.C. Conner, Chem. Eng. Commun., 38 (1985) 33-56. 16 W.C. Conner, A.M. Lane, K.M. Ng and M. Goldblat, J. Catal., 83 (1983) 336-345. 17 W.C. Conner and A.M. Lane, J. Catal., 89 (1984) 217-225. 18 Y. Li, W.G. Laidlaw and N.C. Wardlaw, Adv. Colloid Interface Sci., 26 (1986) 1-68. 19 C.E. Diaz, I. Chatzis and F.A.L. Dullien, Transp. Porous Media, 2 (1987) 215-240. 20 F.A.L. Dullien and P.N. Mehta, Powder Technol., 5 (1971/72) 179-193. 21 F.A.L. Dullien and G.K. Dhawan, J. Colloid Interface Sci., 47 (1974) 337-349. 22 N.C. Wardlaw and J.P. Cassan, Bull. Can. Pet. Geol., 26 (1978) 572-585. 23 N.C. Wardlaw and J.P. Cassan, Bull. Can. Pet. Geol., 27 (1979) 117-138. 24 N.C. Wardlaw and M. McKellar, Powder Technol., 29 (1981) 127-143. 25 I. Chatzis and F.A.L. Dullien, Powder Technol., 29 (1981) 117-125. 26 Y. Li and N.C. Wardlaw, J. Colloid Interface Sci., 109 (1986) 461-472. 27 Y. Li and N.C. Wardlaw, J. Colloid Interface Sci., 109 (1986) 473-486. 28 N.C. Wardlaw and Y. Li, Transp. Porous Media, 3 (1988) 17-34. 29 C.D. Tsakiroglou and A.C. Payatakes, AIChE 1988 Annual Meeting, paper No 102L, Washington, D.C., Nov. 27-Dec. 2, 1988. 30 A.C. Payatakes and M.M. Dias, Rev. Chem. Eng., 2 (1984) 85-174. 31 M. Yanuka, F.A.L. Dullien and D.E. Elrick, J. Colloid Interface Sci., 112 (1986) 24-41. 32 R.T. DeHoff, E.H. Aigeltinger and K.R. Craig, J. Microsc., 95 (1972) 69-91. 33 M. Yanuka, F.A.L.Dullien and D.E. Elrick, J. Microsc., 135 (1984) 159-168. 34 I.F. MacDonald, P. Kaufman and F.A.L. Dullien, J. Microsc., 144 (1986) 277-296. 35 I.F. MacDonald, P. Kaufman and F.A.L. Dullien, J. Microsc., 144 (1986) 297-316. 36 G.N. Constantinides and A.C. Payatakes, Chem. Eng. Commun., 81 (1989) 55-81. 37 C.D. Tsakiroglou and A.C. Payatakes, J. Colloid Interface Sci., 137 (1990) 315-339. 38 N.C. Wardlaw, Y. Li and D. Forbes, Transp. Porous Media, 2 (1987) 597-614.
F. Rodriguez-Reinosoet al. (Editors),Characterization of Porous Solids II 0 1991 Elsevier Science Publishers B.V., Amsterdam
179
FILM SURFACE AREA MEASUREMENTS FOR MICROPOROSITY AND SURFACE ROUGHNESS ANALYSIS Gregory P. Johnstonl,Douglas M. Smithl, Alan J. Hurd2, Peter Heifer3 1 UNM/NSFCENTER FOR MICRO-ENGINEEREDCERAMICS,University of New Mexico Albuquerque, NM 87131 USA 2 Division 1153, Sandia National Laboratories, Albuquerque, NM 87185 USA 3 Department of Physics, University of Missouri, Columbia, MO 65211 USA ABSTRACT From measurements of the change in nitrogen surface area as a function of the quantity of vapor preadsorbed on a solid powder or porous solid, surface/pore structural parameters may be obtained. The approach is demonstrated for water vapor adsorbed on various fumed silica powders, Vycor phase-separated glass, and several silica gels for surface roughness and micropore size distribution. INTRODUCI'ION AND BACKGROUND In principle, measurements of the surface area with varying coverage of a
preadsorbed vapor (film surface areas) yields additional information concerning surface and pore structure. The basic principle is that one measures the surface area of a dry porous solid with nitrogen adsorption, equilibrates the sample with a vapor at higher temperature, rapidly cools the sample, and remeasures the surface area via nitrogen adsorption. The surface area decrease (or increase) with increasing film content when combined with an appropriate physical model should contain structure information. Properties which can be probed include coordination number in particle compacts, surface roughness, and micropore pore size distributions. Karasz and co-workers [l]first reported the use of nitrogen and argon adsorption on water preadsorbed on a solid surface as a structure probe. Wade [2,3] adsorbed water on silica and alumina and measured the N2 surface area as a function of water coverage. The reduction in surface area was used to extract the coordination number (i.e., average number of particle contacts per particle) for samples pelleted at different pressures. Coordination numbers obtained from these film surface area measurements and Wade's model of water adsorption/condensation resulted in coordination
180
numbers which were 30 to 60% larger than that expected from the porosity. Smith and co-workers [4,5]used a more complete model of the adsorption and toroidal condensation processes in conjunction with both Wade's data and film surface area measurements on packings of monodisperse silica spheres and obtained coordination numbers in reasonable agreement with porosity-derived values. In addition to coordination number, film surface area measurements have been
used to obtain qualitative information concerning surface roughness [6]. This process is illustrated in Figure 1 for which the surface available for surface area measurement of a rough surface is smoothed with increasing film thickness. This idea could also be extended as a pore size distribution probe. In other words, as the thickness of a film approachs the pore size, the surface area of the pore would rapidly decrease to zero. Although Figure 1 illustrates the film as a smooth layer of constant thickness, we do not mean to imply this is actually the case except for films of many monolayers. Instead, this is a conceptual model of the statistical film thickness.
Figure 1 Decrease of surface area on a rough surface as a function of increasing film thickness.
181
THEORETICAL BACKGROUND
For a fractal surface, the reduction in surface area is related to the volume (or
where V is the volume of film preadsorbed, Z is the N2 surface area, Zo is the N2 surface area on the dry solid, and Ds is the surface fractal dimension. Thus, a log-log plot of Z/Zo versus V should be linear and have a slope which is a function of Ds. For pore structure analysis, the pore size distribution ( E D ) may be obtained from Z(V) by following:
dv
1)
r(V) = l / Z ( V ) dV, assigns a value of r to every measured value of Z(V).
2)
Denote Z(V(r))as z(r), where V(r) is the inverse function of r(V)
3)
dV/dr (r) = - r dE/dr (r) where dV/dr (r) is the pore size distribution.
(2)
The complete proof for (2) is given elsewhere [7]. Several special cases of (2) may be considered. For a flat surface: Z(V) = S (a constant) which implies: dV/dr (r) = 0
(3)
For a fractal surface: Z(V) = C V
(2-Ds)’(3DS)
where C is a constant. This implies:
(4)
EXPERIMENTAL
For surface roughness studies, fumed silica of different roughness (Cab-0-Sil grades L90, MS7, HS5, EH5) was employed. These had been previously characterized with SAXS and molecular tiling experiments [6]and Ds was found to vary as L90 (2.02.1), MS7 (2.1-2.3),HS5 (2.2-2.3),and EH5(2.5). For pore structure analysis, two xerogels
were prepared using either a two-step acidtatalyzed tetraethylorthosilicate(TEOS) reaction system (designated as A2) which results in microporosity or a two-step basecatalyzed TEOS system which results in a broader PSD with pores ranging up to 10 nm [8]. In addition, a commercial sample of Vycor phase-separated glass was employed.
Surface area and pore size analysis was conducted using N2 at 77 K.
182
After sample outgassing at 383 K for three hours under vacuum, film surface areas were obtained by equilibrating the samples with water vapor at 293 K and the desired relative pressure and measuring uptake (volumetrically or gravimetrically). The samples were then rapidly cooled to 77 K and a conventional BET nitrogen surface area experiment (5 points, 0.05cP/Po
RESULTS Previously [6],film surface areas were measured for fumed silica materials for which roughness had also been characterized via scattering and molecular tiling. A plot of these film surface areas versus P/Po is shown in Figure 2. A greater reduction in surface area is observed as the nominal surface roughness increases. As suggested by (l),Figure 3 is a plot of the log of surface area versus the log of vapor P/Po (note: uptake was not measured but for similar samples, the BET "C' value was low and therefore, we assume as a first approximation that uptake is proportional to P/Po). For all four samples, linear log-log behavior is observed over the limited P/Po range studied. Analysis of the slopes yields Dsvalues ranging from 2.04 (L90) to 2.10 ( E M ) which with the exception of L90 are considerably lower than the
scattering/ tiling experiments. However, we attribute these differences to the self-affine character of these fumed silicas [61. The thickness of the roughness on these materials has been estimated to be 1-2nm and it appears that even one monolayer of water (our lower limit of uptake) serves to smooth out most surface roughness.
183
A
'
0
2
B
0.8
arn
0
A% U
A
L90
MS7
0.7
-
0.61 0.0
I
0.1
.
I
-
I
0.3
0.2
A
.
I
-
0.4
0.5
p1p0
Figure 2
Effect of water film relative pressure on the reduced
surface area.
0.1
0.2
0.3
0.4 - 3
- 2
L n(P/Po)
- 1
0
Figure 3 Surface roughness plots for water preadsorption on fumed silica.
184
Sample A2 B2 Vycor
Pore volume (cm3/d 0.41 1.41 0.22
Surface area (m2/g) 763 918 159
rhm) 1.1
3.1 2.8
Table 1 Summary of nitrogen adsorption/condensation results.
=. .
0 Vycor
A
H A2 silicagel
AA
v
1 500
0.0
0.1
A
82silicagel
A
0.2
A
0.3
0.4
0.5
g-H20/g-solid Figure 4 Variation of nitrogen surface area with water content for Vycor glass and A2 and B2 silica xerogels.
The Vycor and silica gel samples were selected for the pore size distribution portion of this study so as to provide a range of pore structures. N2 surface area, pore volume, and mean pore radius for the three samples are provided in Table 1. Film surface area measurements for the silica xerogels and Vycor glass are presented in Figure 4. Qualitative observations can be made from the surface area change such as the narrower pore size distribution of the A2 xerogel occurring at smaller pore size as compared to the B2 xerogel, the larger pore volume of the B2 material, and the broad pore size distribution of the Vycor. These are consistent with trends in Table 1. The discrepancy between the N2 pore volume and the water uptake for the A2 sample could be the result of a few large pores or the fact that for microporous materials, pores need
not to be completely filled to restrict the entrance of nitrogen.
Pore size distributions
185
have been calculated from the film surface area results presented above, from Kelvin equation analysis of nitrogen condensation, and from micropore analysis of nitrogen adsorption for P/Po between 0.1 and 0.4. This PSD comparison is presented in Figure 5 for the B2 xerogel, Figure 6 for the A2 xerogel and Figure 7 for the Vycor glass. Noting that the film surface area PSD and the nitrogen adsorption/condensation PSD's are completely independent, excellent agreement between the methods is obtained for the applicable pore size ranges of the different techniques (in general). However, for the A2 sample (and all samples), we expect the pore size obtained from film surface area to be several angstroms to small because of nitrogen accessibility questions. The Vycor glass has a narrow pore size distribution in the mesopore range, so micropore analysis using nitrogen adsorption was not carried out. The upper size pore size limit that we were able to probe with film surface areas (because of the high water
P/Po required ) was approximately 2 nm. Although the desorption N2 desorption branch indicates a large fraction of pores with this size, both the adsorption branch and the film surface area PSD's agree over the limited size range that they overlap. In general, one would expect the film surface area to agree with the adsorption rather than the desorption branch. If the same film results are plotted following (l),a linear relationship is obtained which yields a fractal dimension of 2.3 (see Fig. 8). This Ds value is in reasonable agreement with values of 2.2 for similar phase separated glasses obtained from scattering and molecular tiling [9,101.
186
5
\
-
1
h
Film SA
I . - Micropore-N2
-
0.8-
--I, Kelvin-NP
P m v
0.4-
a \ \ \
b
0.0
!
I
.1
..
I
r (nm)
10
100
Figure 5 Pore size distributions calculated for the B2 xerogel.
-
1
-- .O
.1
Film SA
""..~*"~
1
r(nm)
Micropore-NP Kelvin-N2
10
Figure 6 Pore size distributions calculated for the A2 xerogel.
100
187
,M m 3
ti V
I' II
A Y
:2 0
* &
$
a
1 0 .1
1
10
100
r (nm) Figure 7
Vycor pore size distributions calculated from film surface areas,
and nitrogen condensation (adsorption and desorption).
-4
-3
-2
Lno Figure 8
Film surface area surface roughness plot for Vycor.
188
CONCLUSIONS Film surface areas as a probe for of surface fractal dimension agree with previous SAXS and molecular tiling results for materials which are not self-affine. Pore size distributions calculated via film surface areas seem to provide reasonable results in the microporous region although the method should always underestimate pore size and pore volume but a correction could be developed for this systematic deviation. However, because of the sensitivity of vapor volume uptake to PIPo,this method should not be used for mesopores. A more rigorous test of the film surface area PSD method awaits results for solids with better described pore structure such as zeolites. REFERENCES Karasz, F.E., Champion, W.M., and Halsey, G.D., J.Phys.Chern., 60,376, (1956). Wade, W.H., J.Phys.Chern.,68, 1029, (1964). Wade, W.H., J.Phys.Chern., 69, 322, (1965). Smith, D.M., J.Phys.Chern., 90, 2725, (1986). Smith, D.M., and Olague, N.E., J.Phys.Chern., 91,4066, (1987). Ross, S.B., Smith, D.M., Hurd, A.J., and Schaefer, D.W., Langmuir, 4,977, (1988). Pfeifer, P., Smith, D.M., Hurd, A.J., and Johnston, G.P., in preparation. Brinker, C.J., Keefer, K.D., Schaefer, D.W., and Ashley, C.S., J.Non-Cryst.SoZids, 48,47, (1982). Hohr, A., Neumann, H.B., Schmidt, P.W., Pfeifer, P., and Avnir, D., Phys.Rev.B., 38,1462, (1988). Stermer, D.L., Smith, D.M., and Hurd, A.L., J.CoZloid Interface Sci., 131,592, (1989). ACKNOWLEDGEMENTS This work was supported by Sandia National Laboratories (#05-5795) under DOE
contract DE-AC04-76-DPO0789.
F. Rodriguez-Reinosoet al. (Editors),Chcrraeterization of Porous Solids I1
189
0 1991 Elsevier Science Publishers B.V., Amsterdam
SOME PROBLEMS ABOUT GAS ADSORPTION ISOTHERM MEASUREMENTS B Y AUTOMATED PROCEDURES IN MANOMETRIC DEVICES
J.L. GINOUX, L. BONNETAIN Laboratory S2MC (URA C N R S 413) - ENSEEG - INP Grenoble - Domaine Universitaire
- BP 75 - 38402 Saint-Martin d’H6res (France)
ABSTRACT Some r e m a r k s a r e m a d e a b o u t t h e conception a n d t h e a u t o m a t i o n of volumetric devices for t h e d e t e r m i n a t i o n of g a s adsorption isotherms. The calibration problems discussed concern : a procedure to c h e c k t h e m a n o m e t e r linearity ; t h e o p t i m a l pressure r a t i o for t h e calibration of volumes by g a s expansion ; m e a s u r e m e n t s of H e adsorption in z e o l i t e s j minimal measurable s u r f a c e a r e a . T h e possibility of readily setting e x p e r i m e n t a l points in a prescribed z o n e of t h e isotherm by using operating lines i s discussed, and rules for t h e extrapolation of t h e isotherms a r e suggested f o r s o m e cases : a modified BET equation f o r adsorption in micro/mesoporous solids, and a relation between t h e s a t u r a t i o n volume a n d t h e capillary condensation pressure.
INTRODUCTION Over t h e p a s t f i f t y years, t h e g a s adsorption has b e c o m e t h e main technique for t h e c h a r a c t e r i z a t i o n of porous or finely divided solids. This r i s e i s d u e to t h e success
of t h e BET and BJH t h e o r i e s (and r e l a t e d procedures), as well as t o t h e growing needs of c h a r a c t e r i z a t i o n d a t a for t h e inorganic, organic and p h a r m a c e u t i c a l industries or research centers. O t h e r needs c o n c e r n c r u d e m e a s u r e m e n t s of adsorbed a m o u n t s (chemisorption, g a s separation data). Consequently, many laboratories now buy c o m m e r cially available devices, or build t h e m , to p e r f o r m t h e s e measurements. The developm e n t of t h e c o m p u t e r s c i e n c e h a s also provided f a c i l i t i e s for t h e a u t o m a t i o n of t h e e n t i r e e x p e r i m e n t a l run and d a t a reduction. Among a l l t h e r e l e v a n t methods ( I , 21, t h e v o l u m e t r i c (or manometric) is t h e most frequently used, conceivably f o r i t s insensitivity t o t h e physical properties of t h e gas, contrarily to t h e c h r o m a t o g r a p h i c (sensitive t o t h e t h e r m a l conductivity) and t h e g r a v i m e t r i c (sensitive to t h e molecular weight) methods. Our a i m is to expose s o m e ideas w e have y e t t e s t e d , a b o u t t h e design of such devices a n d t h e i r automation.
190 0
1 - THE VOLUMES AND MANOMETER CALIBRATIONS
The volumetric devices basically include a m a n o m e t e r and some c a l i b r a t e d volumes (introduction volume(s) and t h e solid s a m p l e cell). T h e t e m p e r a t u r e in e v e r y p a r t is controlled. The a d s o r b a t e m a y b e introduced continuously (flow method) or as successive doses ( s t a t i c method) f r o m t h e introduction volume to t h e cell. Whatever t h e method, t h e number of moles adsorbed i s obtained by a m a t e r i a l balance and subsequent introduction of P-V-T d a t a in a suitable gas-phase equation of state. The crucial variable to b e measured i s t h e pressure, f r o m which t h i s m e t h o d m a y b e called "manometric". T h e main problems to b e considered a r e t h e assessment of t h e quality
of t h e pressure transducer, and t h e volume calibrations. W e will not consider h e r e t h e devices with varying volumes (3).
The linearity of the pressure transducer f o r t h e s t a t i c d e v i c e s m a y b e periodically c h e c k e d by a s e r i e s of g a s expansions f r o m a volume V,, t o a volume V2, t h e l a t t e r being e v a c u a t e d a f t e r e v e r y expansion, t h e f o r m e r being n o t restocked with gas. The pressure Pn a f t e r t h e n-th expansion i s obtained by a s e r i e s of m a t e r i a l balances :
PI (V,
Po
V1
=
t
k V,)
PI
V1
= P, (V, +
k V,)
............
(k a c c o u n t s for a n y t e m p e r a t u r e d i f f e r e n c e o r heterogeneity in t h e a p p a r a t u s ; k is 1 for a n isothermal process). A plot of In P
vs. n is e x p e c t e d to b e a s t r a i g h t line. So, t h e d e p a r t u r e f r o m
linearity, expressed as : In Pn (experimental) / Pn (linear regression) measures t h e r e l a t i v e e r r o r on Pn. Fig.
1 displays t h i s c u r v e for a c a p a c i t a n c e
manometer. If kVz/VI, o r o n e pressure of t h e s e r i e s is known (for i n s t a n c e with t h e help of t h e local meteorological office), t h e e n t i r e pressure c o r r e c t i o n c u r v e i s obtained. This procedure i s easily a u t o m a t e d , and t h e c o r r e c t i o n c u r v e implemented in t h e c o m p u t e r program. In t h e case of a continuous introduction, a c o n s t a n t mass flow m a y b e a d m i t t e d in a cell, and t h e pressure recorded. Here, t h e d e p a r t u r e f r o m linearity of t h e pressure vs. t i m e plot m a y b e ascribed e i t h e r to t h e pressure transducer, or to t h e flow
controller.
-
191
P/ k Pa
50
100
-5
-
20
0
0
0
0
1
1:
10
I
1
Deviation f r o m linearity of a model 1176 Datametrics pressure transducer
( see text). The volume calibration is generally obtained by g a s expansions f r o m a volume V I , calibrated before t h e building of t h e a p p a r a t u s a n d t a k e n as t h e primary s t a n d a r t , to t h e unknown volume V2, which is t h e introduction volume or t h e dead s p a c e of t h e
sample cell. The r a t i o V /V has to b e s e t t l e d a t a n o p t i m a l value, as will b e shown 1 2 here. T h e m a t e r i a l b a l a n c e for t h e gas expansion is as previously Po V1
=
w r i t t e n as :
PI (V, + k V,)
The u n c e r t a i n t y
AP a b o u t t h e pressure readings i s generally n o t pressure-
dependant in a wide r a n g e including Po and P I . So t h e u n c e r t a i n t y A V 2 a b o u t V2 d u e t o t h e pressure transducer is obtained as :
with : r = P o / P I = I + k
v 2/v 1
192
For a given m a n o m e t e r and a given minimal when P to Vl/k V2 =
AP, t h e right hand side of
AV2/V2
is
is at i t s maximal value, and P o / P I equal to I + F ( c o r r e s p o n d i n g
'GI. T h e
c a l i b r a t e d vs. V2. For
s a m e o p t i m a l expansion r a t i o is found if V I h a s to b e
AP/Po =
one calculates :
high value in this o p t i m a l case. Fortunately t h e
4V,/V,
dV2/V2
= 0.58%, a r a t h e r
vs k V2/Vl
c u r v e is r a t h e r
f l a t around t h e optimum, reaching t w i c e t h e previous value only with k V 2 / V l
= 8.4 or
Vl/k V2 = 4.2. The cell dead space volume is c a l i b r a t e d by helium expansion, assuming t h a t
helium i s n o t adsorbed. This is n o longer t r u e when t h e cell contains microporous adsorbents s u c h as z e o l i t e s at 77 K. This erroneous assumption c a u s e s t h e dead s p a c e to b e overvalued, and t h e isotherm slope undervalued, a t i r e s o m e effect when t h e t (or
Q!
S
) method h a s to b e used. I t also gives rise to t h e "apparent abnormal adsorption
of nitrogen" o n NaA z e o l i t e (4, 5). I t is possible to e v i d e n c e t h e adsorption of helium by a plot of (Po/P, - I ) vs.
1/T, T being t h e adsorbent t e m p e r a t u r e (4, 6). The dead s p a c e is y e t m o r e safely evaluated when t h e micropores a r e s a t u r a t e d by a low-pressure a d s o r b a t e (H 0, 2
n-nonane, ref. 71, o r when t h e i r o u t e r a p e r t u r e s a r e c o a t e d by a hard Gibbs s u r f a c e ( 2 ) , f o r i n s t a n c e C 0 2 introduced at low t e m p e r a t u r e (4). Conversely t h e adsorption isotherm of helium c a n b e d e t e r m i n e d when t h e dead s p a c e is known. Fig. 2 gives t h e mol e x a m p l e of z e o l i t e 4A at 77.3 K. From t h e slope of t h i s linear isotherm (0.85 1 Pa-' kg- 1, i n t e r e s t i n g comparisons a r i s e : t h e deadspace volume is overvalued by 3 a b o u t 0.55 c m per g. of zeolite, a value nearly equal to t h e z e o l i t e c r y s t a l l i t e volume 0.66 cm3.g-l
; t h e mean density of helium adsorbed in t h e
Q!
c a g e s of z e o l i t e 4A
m3 per kg. of zeolite), is t w i c e as l a r g e as in t h e g a s phase under s a m e
(0.274
pressure and t e m p e r a t u r e (3.1 mmol Pa-'
m-3 c o m p a r e d to 1.56 mmol Pa-'
K3).
The minimum value of the BET surface area c o r r e c t l y d e t e r m i n e d by e i t h e r
s t a t i c or flow method may b e derived as follows. The adsorbed a m o u n t is e v a l u a t e d as t h e d i f f e r e n c e b e t w e e n t h e a m o u n t introduced in t h e cell, and t h e g a s phase remaining in t h e d e a d space. The main uncertainty a b o u t t h e d e v i c e volumes c o n c e r n s t h e cell dead space. So, t h e following r a t i o : g a s phase a m o u n t F = ( g a s + adsorbed a m o u n t ranging b e t w e e n 0 and I , has to b e d e c r e a s e d a s low as possible to e n s u r e a good a c c u r a c y a b o u t t h e adsorbed amount, and is profitably e v a l u a t e d a l l along t h e ackorption run. W e recommend i t d o n o t e x c e e d 0.67 a t t h e monolayer completion
193
mA2 : A d s o r p t i o n i s o t h e r m
o f h e l i u m a t 7 7 . 3 K on z e o l i t e 4 A c o n t a i n i n g 20.6 H,O p e r ct c a g e ( o ), or c o a t e d by CO, ( 0 ).
(point B) i.e. a t r e l a t i v e pressure P/Po
N
:
outgassed (
x
t
0.10. Neglecting t h e p a r t of t h e volume cell
which is at t h e a m b i e n t t e m p e r a t u r e , o n e may d e r i v e t h e s u r f a c e a r e a S corresponding t o t h e previous conditions :
(
N = Avogadro number,
u and Po
: molecular a r e a
and vapor pressure of t h e
adsorbate, Vs : d e a d s p a c e volume, T : s a m p l e temperature). 2 2 For nitrogen ( U = 0.162 n m ) and krypton ( U = 0,20 m m at 77.3 K, o n e finds 2 2 S/Vs = 0.8 m per c m 3 and 21 c m respectively, which may practically be 2 3 modified i n t o 0.9 rn2 and 24 c m .cm- , accounting for t h e neglected volume. 11
-
SETTLING TARGETS T O THE COMPUTER PROGRAM Setting readily a limited number of e x p e r i m e n t a l points on t h e i n t e r e s t i n g
part(s) of a n isotherm i s a n i m p o r t a n t topic, and a d e l i c a t e operation t h a t may be c o m m i t t e d to a computer. P r e c i s e t a r g e t s a r e s o m e t i m e s prescribed : fixed pressures in t h e range of r e l a t i v e pressure of a d s o r b a t e : x = P/Po = 0.05 t o 0.35
for t h e BET
method, o r fixed a m o u n t s for t h e d e t e r m i n a t i o n of isosteric h e a t s of adsorption, etc... Sometimes, m o r e fuzzy rules may b e given, such as : see to i t t h a t enough points
1,
194 a r e l o c a t e d on a peculiar p a r t : knee (type I isotherm, chemisorption), plateau (type I, IV, V and stepwise isotherms), o r nearly v e r t i c a l p a r t s (capillary condensation, steps). The complexity of t h e managing program depends on t h e desired versatility. The s y s t e m s with a continuous introduction, o r with variable volume ( 3 ) , in principle o v e r c o m e this difficulty, b u t a r e n o t f i t for all gas-solid pairs, so w e will mainly discuss t h e case of a s t a t i c apparatus, restricting t h e m a t t e r t o s e t t i n g points at prescribed pressure o r adsorbed amount. A t first, t h e c o n v e r s e c o o r d i n a t e (amount q o r pressure P) is e v a l u a t e d by a n appropriate extrapolation of t h e isotherm (see below). Then, t h e introduction pressure Pintr. may b e e v a l u a t e d by a m a t e r i a l balance :
Here, qn and P
a r e t h e c o o r d i n a t e s of t h e previous equilibrium point ; A and B a r e in
t h e f o r m : volume/m RT, A is for t h e introduction space, B for t h e cell. m is t h e adsorbent weight. Ideal gas i s assumed for this c r u d e evaluation. So, in t h e q vs. P plot, t h e point indicating a t a n y t i m e t h e a c t u a l c o o r d i n a t e s
follows a s t r a i g h t line ( t h e operating line) (Fig. 3), and at infinite t i m e r e a c h e s t h e isotherm equilibrium curve. If t h e introduction volume c a n b e modulated, t h e slope :
- (A + B) will b e varied. S t e e p e r slopes (larger volumes) a r e convenient when t h e equilibrium p r e s s u r e is prescribed : a n ill-extrapolated i s o t h e r m will c r o s s t h e operating line a t a pressure not too f a r f r o m t h e anticipated. This is especially
7 Pn
mt3 : Known part
a
b
+ P
( ___ ) and extrapolated part ( - - - ) of an adsorption isotherm, and two possible operating lines ( a , b ) reaching the target point ( M ). q, and Pn are the coordinates o f the n-th experimental point.
195 i m p o r t a n t when a p l a t e a u is expected. Conversely, f l a t t e r slopes a n d smaller volumes a r e convenient when t h e adsorbed a m o u n t s a r e prescribed, o r when a s t e e p p a r t is e x p e c t e d f o r t h e isotherm. Concerning t h e extrapolation of isotherms f r o m t w o o r m o r e e x p e r i m e n t a l points, n o universal d e f i n i t e equation may b e given ; moreover, w h a t e v e r t h e c a r r i e d o u t method, t h e program must b e a b l e to d e t e c t miscalculations, such as n e g a t i v e slopes o n t h e e x t r a p o l a t e d isotherm ; t h i s case may o c c u r for i n s t a n c e with polynomial extrapolation. Concerning t y p e I isotherms, t h e Langmuir equation r e m a i n s probably t h e b e s t way for e x t r a p o l a t i o n s based on t w o known equilibrium points only. In particular, i t c a n n o t result in a negative slope. Concerning now t y p e I1 and I V isotherms, t h e well-known BET equation may p r e d i c t a n e g a t i v e slope in t h e extrapolat e d p a r t : this case o c c u r s when i t s t w o c o n s t a n t s a r e c a l c u l a t e d in a too s t e e p e x p e r i m e n t a l p a r t of
t h e isotherm, a case e n c o u n t e r e d with t y p e I V i s o t h e r m s
(capillary condensation). The BET equation also results in bad e x t r a p o l a t i o n s when t h e isotherm slope is too small, a case frequently m e t when micropores and mesopores (of f l a t surface) c o e x i s t in t h e s a m e solid, leading to a composite-type (1 + 1V o r I + 11) isotherm, o r when adsorption is m a d e on uniform solids (7). On a n ideal t y p e 11 isotherm, described by t h e BET equation (Fig. 4a), t h e t a n g e n t t o t h e inflection point crosses t h e P = 0 and P = Po v e r t i c a l a x e s at q A and qF, t h e r a t i o qF/qA ranging between 2.2 and 6-8 for usual values of t h e C c o n s t a n t (Fig. 4b). So, i f t h e local slope of t h e isotherm is lower t h a n t h e t h e o r e t i c a l minimum given by qF/qA = 2.2, or if a n y e v i d e n c e e x i s t s f o r t h e presence of micropores o r e n e r g e t i c u n i f o r m i t y , i t i s a d v i s a b l e t o m o d i f y t h e BET e q u a t i o n as f o l l o w s :
0
0
4 q,
:
0.2
( a )
/ q,
0.4
0.6
08 P/ Ps
1
10
20
50
100 200
500
103
C
I d e a l t y p e I1 i s o t h e r m , and t h e t a n g e n t t o t h e i n f l e c t i o n point I . ( b)
v s . the C constant value.
196 Here, q o and q1 may b e roughly i n t e r p r e t e d as t h e micropore (or uniform s u r f a c e ) capacity, and o u t e r s u r f a c e monolayer c a p a c i t y , respectively. To avoid using t h r e e e x p e r i m e n t a l points to d e t e r m i n e qo, q1 and C, a fixed value (say : 5 0 ) is recommended for t h e BET c o n s t a n t C. On t y p e 1V isotherms, i t i s i m p o r t a n t to precisely d e t e r m i n e t h e height of t h e s a t u r a t i o n plateau (vs, expressed a s a volume of liquid adsorbate) and i t s lower limit ( P / P o ) . So w e must f o r e s e e t h e s e values to properly place t h e corresponding experi1 m e n t a l points. In t h e cyclindrical pore model, v is r e l a t e d to t h e p o r e s u r f a c e a r e a , likened to t h e BET a r e a SBET, and to t h e pore radius r m :
A s SBET is d e t e r m i n e d b e f o r e vs, this relation gives a maximal limit for v
at
any prescribed abscissa P/Po on t h e adsorption branch, provided t h e r relation i s known. On t h e desorption branch v
vs.P/Po m i s y e t known, and o n e deduces t h e r m
value f r o m i t ; r
in t u r n gives (P/Po), a t t h e end of t h e plateau. m ~ was t e s t e d a s follows. The validity of four theories for t h e r m V S . ( P / P ~ )relation
r
m was e v a l u a t e d f r o m e x p e r i m e n t a l values of v and SBET, and plotted versus t h e P/Po value c h a r a c t e r i s t i c of t h e capillary condensation o r decondensation (Fig. 5 ) , for
m15
: Cylindrical pore radius r m v s . t h e r e l a t i v e p r e s s u r e of n i t r o g e n a t 7 7 . 3 K f o r t h e c a p i l l a r y condensation ( b ) , or decondensation ( a ), a c c o r d i n g t o v a r i o u s t h e o r i e s : ( 1 ) W h e e l e r ( o r K e l v i n and C o h a n ) ; ( 2 ) : K a r n a u k h o f and K i s e l e v : : Broekhoff and D e B o e r : ( 4 ) : C o l e and S a a m . E x p e r i m e n t a l points f o r porous s i l i c o n .
( 3 )
I
a75
a50
197
t h e adsorption of nitrogen a t 77.3 K on a s e r i e s of porous silicons exhibiting unimodal p o r e d i a m e t e r s (8, 9). The e x p e r i m e n t a l points w e r e c o m p a r e d to four t h e o r e t i c a l c u r v e s according to B J H (lo), KARNAUKHOV a n d KISELEV ( ] I ) , BROEKHOFF a n d DE BOER (121, a n d C O L E and SAAM (13). T h e physical p r o p e r t i e s of liquid nitrogen w e r e t a k e n a t t h e i r s t a n d a r t values, a n d t h e relation b e t w e e n t h e nitrogen m u l t i l a y e r thickness t and P / P o w a s t a k e n as :
A s may be observed in Fig. 5, t h e simpler t h e o r y ( t h e KELVIN equation f o r desorption, t h e COHAN equation f o r adsorption) gives t h e best, although not p e r f e c t , fit. 111
-
CONCLUSION Some r e m a r k s h a v e been m a d e a b o u t t h e design of a m a n o m e t r i c d e v i c e and i t s
c o m p u t e r program, to i m p r o v e t h e a c c u r a c y in d e t e r m i n i n g t h e adsorption isotherms. W e think t h e largest d e v e l o p m e n t t o b e e x p e c t e d in t h i s domain is a n increased use of
computers, perhaps a l s o of t h e a r t i f i c i a l intelligence, to recognize during t h e adsorption run t h e m o r e c r u c i a l p a r t s of t h e i s o t h e r m a n d then deliver i t s o p t i m a l a c c u r a c y :
- modulating t h e flow r a t e in flow systems, - modulating t h e e x p e r i m e n t a l points density in s t a t i c ones.
LITERATURE
9
10 I1 12 13
S.J. ROTHENBERG, P.B. DENEE, Y.S. CHENG, R.L. HANSON, H.C. YEH, A.F. (1982) 223. HEIDSON, Adv. in Colloid Interf. Sci. K.S.W. SING, in : F u n d a m e n t a l s of A d s o r p t i o n , p. 5 6 7 , A.L. M Y E R S , G. BELFORT, Eds., Engineering Foundation (1983). B. RASNEUR, Bull. SOC. Fr. C e r a m . 101 (1973) 21 ; B. RASNEUR, J. CHARPIN, in : Fine P a r t i c l e s , 2nd Int. Conf. Boston, p. 58, E l e c t r o c h e m i c a l SOC. (1973). J.L. GINOUX, These, Grenoble (1983). I. SUZUKI, H. SATO, K.1. SAITO, S. OKI, J. of C a t a l y s i s 113 (1988) 540. R.J. HARPER, G.R. STIFEL, R.B. ANDERSON, Can. J. C h e m . 47 (1969) 4661. S.J. GREGG, K.S.W. SING, Adsorption, S u r f a c e A r e a and Porosity, A c a d e m i c P r e s s (1982). L. BONNETAIN, J.L. GINOUX, M. CABEDO, in : C h a r a c t e r i z a t i o n of Porous Solids, p. 223, K.K. UNGER et al. Ed., Elsevier (1988). R . H E R I N O , G. B O M C H I L , K. B A R L A , C. B E R T R A N D , J.L. G I N O U X , J. Electrochem. SOC. 134 (1987) 1994. E.P. BARRETT, L.F. J O Y N E R , P.H. HALENDA, J. Am. C h e m . SOC., 7 3 (1951) 373. A.P. KARNAUKHOV, A.V. KISELEV, Russ. J. Phys. Chem., 34 (1960) 1019. J.C.P. BROEKHOFF, J.H. D e BOER, 3. C a t a l y s i s 9 (1967) 8. M.W. COLE, W.F. SAAM, Phys. Rev. L e t t e r s 32 (1974) 9 8 5 ; W.F. SAAM, M.W. COLE, Phys. Rev. B I 1 (1975) 1086.
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F. Rodriguez-Fbinosoet al. (Editors), Characterization of Porom Solids I1 1991 Elsevier Science Publishers B.V., Amsterdam
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MORPHOLOGICAL, INFLUENCES ON UNS'EADY GAS DIFFUSIVITIESIN POROUS SOLIDS
w. CURTIS CONNER~,STEVEN. w. WE B B ~P. , BUCKLEY~,S.V. CHRISTIANS EN^, G. PARTHUN2, J. A. HANSEN2, and H. TOPSldE2 IDept. of Chemical Engineering, University of Massachusetts, Amherst, MA, USA 01003 2Haldor Topsae Research Laboratories, DK-2800 Lyngby, Denmark ABSTRACT The objective of this study is to be able to estimate the effective transport resistance of a porous medium by characterizing its void morphology by mercury porosimetry. A series of porous catalyst solids were obtained differing only in void morphology, overall porosity and pore sizes. We calculated the tortuosity by a dynamic experiment employing solid-gas chromatography,SGC. Tortuosities of all solids were very similar; in the range of 5-25. Transport resistance is more easily related to overall volume porosity rather than specific network architectural features observable by porosimetry.
INTRODUCTION The estimation of transport properties of porous materials has long been a source of interest in both the applied and theoretical arenas. Many practical processes occur in porous media in which transport is particularly critical. the morphological properties of the void network are crucial in determiningthe performance of the reaction or separation processes in all these situations. There is currently no way to estimate the transport resistance of a porous medium without performing the transport experiment. While direct, this procedure provides no understanding of the relationship between morphology and transport resistance and therefore no intuition into how the void network could be manipulated to effect a desired result. In many practical situations reliable sampling is not possible or the transport experiment is not feasible; the transport resistance must be estimated from an empiricism. In these studies we have investigated the dependence of the transport ( as reflected by the tortuosity) on two variables derived from porosimem : (1) retained volume and (2) pore to throat size ratio (PS/TS). Retained volume is the amount of mercury which is not recovered after the 1st intrusiow'retraction cycle back to 1 am. Retention of mercury is caused by two factors: network effects and "snap-off'. As the intrusion and retraction are governed by different physical attributes of the void (constrictions for intrusion and openings for the retraction), network effects are able to isolate mercury from a continuous path to the solid exterior and therefore to the mercury bulk. The result is that a fraction of the initially intruded mercury can be isolated and retained during the retraction process. Snap-off occurs due to stress accumulation in the mercury fluid threads spanning the void network of the solid. Stress is accumulated from two sources: (1) dynamic imbalance between stress developed by the surface tension within the fibrils of fluid during depressurizationof the extraparticle mercury pool
200
and stress dissipation by fluid flow and (2) stress magnification along the fluid threads due to pore/throat cross-sectional area variation. A solid with very narrow throats, a large PSRS ratio, or large network complexity may retain considerable mercury. Stress induced mercury retention is related to the average length of mercury fluid threads within the void network. Long fluid fibrils increase the stress transport time and increase the probability of fracture and mercury isolation within the solid at a given depressurization rate. Thus, retained mercury should be related to solid tortuosity which is commonly defined as the average pore path length divided by the particle size. It is proposed that this concept of a connection between a morphological variable (readily accessible with common instruments) can be used (1) to provide a more tangible definition of tortuosity, which in transport models has a "fudge" factor reputation, (2) to provide a means of measuring a transport resistance without performing a more difficult transport experiment and (3) to contribute to the understanding of the relationship between void morphology and transport resistance. There are basically two problems with transport in porous media: 1) there is no accepted manner to characterize the phase morphology of a porous medium and 2) there is no universal, unambiguous way to relate void morphology of a porous to its transport resistance. Work done in this laboratory is aimed at 1) determining the set of meaningful, independent variables that can fully characterize the void morphology of a solid-gas porous medium using common analvtical techniques, 2) using these variables to estimate the transport resistance of the medium and 3) elucidating a relationship between properties that describe the void morphology of a porous solid and its transport resistance. Short of detailed simulation and modelling there are two approaches recently employed to enhance the prediction of transport properties in porous media by accounting for morphological variations: percolation theory and effective medium theory (EMT). In percolation one analyzes a network of assumed void connectivity. The model is as a network of sites which are interconnected by bonds. The transport through the network can then be analyzed by considering either progressive allowing random access (percolation) to the sites or to the bonds. Percolation theory applied to random porous solids is most useful in cases of high or low porosity (ref. 1). In these outer regions of porosity, transport resistance exhibits the highest sensitivity to morphology and solute transport is characterized by "threshold values" and "critical" exponents. However, there do not exist characterization tools that can provide the detailed and quite specific morphological information necessary to make percolation models useful at a practical level. Effective diffusivities and EMT are more often used to represent the resistance of a particular porous medium to molecular transport (refs. 2-4) . There are of course many simplifications inherent in EMT but many do not pose any particular problem for real porous catalysthorbent solids. It is necessary to assume that the porous medium is isotropic, spatially uniform, randomly structured and of "intermediate"(-50%) porosity.
An alternate approach is to use existing characterization instruments and improved analysis to provide specific morphological information about the medium and use these to correlate with transport resistances within the existing framework of EMT and the concept of tortuosity. This approach is more
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direct but is also heavily reliant on the interpretation of the techniques employed in the characterization. This paper is a first attempt to apply this approach to the study of transport in porous media. It is the purpose of this work is to improve EMT by introducing 1) an experimental analysis method to characterize the morphology of a porous solid using existing and common analytical instruments and 2) to apply the more detailed morphological information to the estimation of tortuosities. This approach would decrease the "fudge factor" basis of tortuosity and extend the utility and predictive ability of EMT to more varied morphologies and wider porosity region. This approach does not compete with other descriptions of porous media transport such as percolation theory. In fact, both approaches recognize that morphological influences can be severe and should be incorporated in any transport model. POROUS MEDIUM CHARACTERIZATION Characterization of the morphology of a porous medium is not an unambiguous problem. It generally involves some assumption regarding the architecture of the media before any data may be analyzed. For catalysts made from random agglomeration and fusion of non-porous microparticles, a useful and sufficiently general structure is the pore-throat model. In this visualization, it is assumed that internal void volume is distributed within channels which connect pores. Channels provide the connecting paths between pores and ultimately between transport boundaries. The properties that are sufficient to define this type of medium are; 1. total void volume (volume of voids to volume of solid) 2. network size of the void space (dimension of the system) 3. average void connectivity between pores (Le., average no. of throats that connect pores) 4. pore size distribution 5. throat size distribution It is necessary to assume that the medium is isotropic, uniform, randomlv constructed and represented by a pore-throat geometry between its boundaries. Connectivity of 1 implies a poorly connected or dead-ended pore; connectivity of zero defines an isolated pore. Parallel, non-intersecting pores would define a one dimension void space of connectivity 2. Time effects due to intraparticle back-mixing would create an extra dimension and lead to dynamic sensitivity of apparent transport resistance. Being able to identify the necessary properties to define a porous medium in no way implies that there are analytical methods to evaluate them. Using porosimetry and sorption, it is possible to measure 6 properties of the porous medium structure (refs. 5-7). It is not clear that these measures are independent. These properties are obtained by analyzing both mercury intrusion and retraction profiles (i.e., the nature of the hysteresis commonly found with most porous materials). These properties are enumerated below:
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1. pore size distribution 2. throat size distribution 3. pore size/throat size ratio versus void space filled ( i e , hysteresis) 4. retained void volume (volume mercury intruded minus that retracted) 5. total porosity 6. surface area/surface roughness
It is still speculative as to how these measures obtained from porosimetry relate to void connectivity and network size. It is even less certain how these properties influence transport resistances. It is also conceded that the evaluation of these properties is not without some ambiguity, particularly for samples with low porosity or extended networks, e.g., large particle sizes. Throat size distribution is given from the intrusion profiie. Pore body size distribution is given from the retraction profile (refs. 8-9). The ratio of the pore size to throat size at a given fraction of pore volume filled provides an indication of the shape or dimension of the void space. The amount of retained mercury (difference between intruded volume and extruded volume) gives some indication about the average connectivity and network size of the void space. Finally, total porosity in the range of 0.005-0.5microns in opening diameter is obtained from the total amount of mercury intruded and the solid densities from mercury displacement at 1Bar. The utility of these characteristics, as obtained from mercury porosimetry, have been tested with real systems. Ali (ref. 10) used porosimetry to study the influence of olefm polymerization on the morphology of chromium oxide polymerization catalyst as it changed during nascent polymerization. Conner, Weist and Pedersen (ref. 11) used this type of porosimetric analysis to study the influence of hydration and calcination of aluminas. Nevertheless, this analysis has yet to be commonly employed in the current literature. Microporosity (< 20 A) is best examined using nitrogen adldesorption. While different physics are at work in characterizing sorption data, the effects of network structure yielding hysteresis in the adsorption and desorption profiles is similar to that found from mercury porosimetry (ref. 12) for mesopores. In this work solid-gas chromatography is used to measure dynamic diffusion coefficients of argon in various porous solids. Mercury porosimetry is used to study the internal macroporosity and macro-morphology of these solids. Finally, an attempt is made to elucidate a relationship between the tortuosity measured from the transport experiment and the internal structure of the porous medium as characterizedby porosimetry. Moruholoeical Variables from Mercurv Porosimetry Mercury porosimetry intrusion-extrusion curves were obtained using Quantachrome High (20050,000 psi) and Low (1-500 psi) Pressure Autoscan Porosimeters. An Analog Devices Macsym 150 computer was used to collect and analyze the data. Typical run times for intrusion and retraction was 78 minutes. 480 dyne cm-l was used for the surface tension of the mercury with a contact angle of 140 degrees in the calculation pore dimensions. Total pore volume (maximum mercury intruded to 50,000
203
psi), pore s u e distribution mean (from the retraction), throat size distribution mean (from the intrusion curve) are reported for each sample. Also, morphological analysis based on the amount of the hysteresis was attempted. Morphology is determined by two variables; the shape of the retraction vs intrusion pore sizes with volume fraction pores fiiled (a " R 5 i vs I$'' plot) and the amount of retained mercury as compared to the pore volume. MorpholoizicalVariable #1: Retained Mercury Retained mercury is caused by network effects and by extension and fracture of the threads of fluid mercury in the pores of the solid during depressurization (retraction) in porosimetry. Stress is generated by the pressure difference between the mercury inside the pores and bulk external mercury pool. Since the stress in scanning porosimetry is applied dynamically it must be dissipated by the fluid (assuming the solid is incompressible). Dissipation occurs by flow and extension of the fluid thread. If dissipation by flow is too slow relative to its rate of application, stress will accumulate in the threads of mercury. If the stress on the mercury threads exceeds the tension on the fluid in the pore (approximated by the surface tension of mercury divided the pore diameter om),fluid thread fracture, residual gas expansion and isolation of mercury inside the porous solid will occur. Macroporous solids (pores > 10 microns) expel almost no mercury during retraction to one atmosphere because the restoring force is too small to preserve the fluid continuum at virtually any scanning rate. The time for dissipation of stress will depend on the length of the mercury thread and the kinematic viscosity of mercury. The length of the mercury thread will of course be distributed, but on the average will be equal to the product of tortuosity of the void structure and particle size. This time may be compared to the rate of depressurization to evaluate the probability of fracture of a mercury thread, shown in eq. 1. In eq. 1, P is the measured external pressure, t is time, p is the viscosity and p is the density of mercury. P(TR2) 1dP fracture probability = p Pdt
As pressure decreases (P --> 0) the probability of fracture will increase. However, while fracture is more likely during the end of depressurization (i.e., at 1 atm) it may occur during all stages of retraction. Fracture will cause the retraction profile to be shifted to higher pressure and produce altered estimates of mean pore size. Retained mercury due to this stress should be sensitive to the rate at which the pressure is decreased. We propose that this morphological variable would therefore related to a network variable (tortuosity) and perhaps may be directly applied to the evaluation of the transport resistance of the medium. It is the goal of this work to correlate tortuosity measured in dynamic transport experiments to retained mercury. This would be the first technique that may independently estimate the transport resistance of a porous medium by studying its morphology alone. If there is a pore-throat morphology with a large distribution in pore sizes, then the stress along the fluid thread will vary. The applied stress will be magnified according to the variation in liquid-
204
thread cross-sectional area. Stress on mercury in a throat will be much greater than stress in a pore. Fracture, if the stress is not dissipated fast enough by flow, will occur mainly in the narrowest throats. However, for sufficiently narrow throats, stress may exceed the tension regardless of how slow the stress is applied. For this reason solids with a large variation in sizes (characterized by the pore size to throat size ratio; "PS/TS")and small throats will show higher retained mercury independent of scanning time. Thus, retained mercury due to snap off has two components, one which depends on rate and is sensitive to tortuosity and the other which is rate independent and sensitive to PS/TS. The variation of stress along the thread due to diameter variation will confound the dynamic sensitivity above and complicate the correlation of tortuosity considerably. Unfortunately, in most real porous solids, a large PSmS ratio is the nonn rather than the exception. It has been observed in some porous solids (e.g., compressed Aerosils) that there is no scanning-rate influence on retained mercury. The dependence of porosimetry on scanning rate is shown in Table 1 for two selected porous solids. Rate is calculated as the time required to scan 1 atm to 50,000 psi at a constant rate. It is evident that properties calculated from intrusion of mercury (throat size and pore volume) are insensitive to scanning rate. Properties obtained from the retraction curve (retained mercury and pore size) are sensitive to scanning rate. This is due to mercury fracture during depressurization. During intrusion there is no possibility of fracture since the fluid source is a large pool and the mercury in the pores is being compressed together maintaining the continuum. The sensitivity to scanning rate is greater for solids with a larger PSmS ratio, or a wider distribution of pore-throat sizes. This is because of the stress magnification effect superimposed on the dynamics of extension of the mercury thread. TABLE 1 Porosimetric results for different scanning rates (samples: SVC46=1, SVC47=2, SVC50k3, Shell Silica D2.2=4) Average Rate Av.Pore Size (Kpsumin) (A) 1. 5 5 80 10 485 2. 10 624 25 546 3. 6 140 15 141 23 141 4. 6 171 15 177 23 191
Av.Throat Size
(A) 71 71 128 117 42 41 42 69 68 69
Retained Pore Volume Volume (cc) (cc/gram) 0.144 0.243 0.248 0.185 0.0 0.064 0.01 0.064 0.355 0.161 0.199 0.354 0.205 0.347 0.666 0.263 0.725 0.361 0.418 0.820
The retraction profile in mercury porosimetry must be used with care in order to interpret morphological variables properly. In particular the dynamics of depressurization must be. controlled in
205
order to effect meaningful comparisons of the hysteresis among several solids. The intrusion profie remains the most reproduceable part of the porosimetry experiment. Retained mercury may be plotted against the scanning rate in order to evaluate the influence of mercury retraction dynamics. These will depend on two parts: (1) the tortuosity of the void network which is scanning rate dependent and (2) the network effects whose influence is scanning rate independent. The network effects include pore size scale inhomogeneity (PSDS) and void connectivity both of which are scanning rate insensitive (ref. 8). Table 2 shows the slope and intercept of the retained mercury vs. porosimetry scanning time plot and compares them to measured values of tortuosity (from gas phase diffusion; Table 3) and PSDS. There may be some correlation between tortuosity and the slope of the retained mercury line. TABLE 2 Comparison of Dynamic Retained Mercury with Solid Tortuosity and PSnS Ratio PS/TS Solid Slope Intercept Tortuosity (min.) (dim.) (dim.) (dim.) svc46 1.3 0.44 13 7 11 2.3 1.3 0.39 shell silica 0.15 6 3.8 svc50b 0.08 Solid-Gas ChromatomaDhy Tortuosity of selected porous solids was measured by gas phase diffusion. Tortuosity is simply defined in the context of effective medium theory described above. The gas phase diffusion coefficient of dilute argon in nitrogen at 50°C was determined using solid-gas chromatography and the analysis and technique f i s t developed by Kubin and Kucera (refs. 13-14) and utilized by Smith and others (refs. 15-17). The technique involves sending a pulse of tracer in a carrier gas across a column packed with a porous solid. The variance of the elution peak is dependent on the intraparticle diffusion time and axial diffusion. It is possible to determine the intraparticle effective diffusion coefficient by extrapolatingthe variance to infinte gas velocity according to the following equation.
In this above equation, p is the calculated volume porosity of the solid from porosimetry, c1 is the measured column porosity, R is the average radius of the solid, s2 is the variance of the elution peak, L is the column length, v, the carrier gas velocity based on an empty column. The regression was performed with 10-14 points within L/v, from 45 to 15 seconds.
206
RESULTS AND DISCUSSION The transport experiments highlight some important differences between the solids, shown in Table 3. This table presents the results of measurements of very dilute, gas phase argon in nitrogen diffusivities for a set of porous solids, along with morphological properties obtained from mercury porosimetry. Axial diffusivities were in the range of 0.19-0.22cmz/sec. PSnS ratios for all solids were in the range of 2-8.Retained volume was in the range of 20-100%. Both these ranges in morphological variables of the selected porous solids are significant and demonstrate that the void network of each solid is different. However, calculated tortuosities are all in the range of 5-25with no apparent correlation to retained mercury, surface roughness or PSDS. Tortuosity is roughly correlated to volume porosity as shown in Figure 1. High porosity seems to lead to lower tortuosity independent of the PSDS and retained mercury. TABLE 3
Measured Dilute Gas Diffusivities in Nitrogen at SO"C/latm.Using SGC and Morphological Properties Measured by Mercury Porosimetry (all solids of mean diameter: 1.5 mm; regression emors in Dd were less than 5%. calculations based on Dm (Ar:N2) of 0.202 cm2/sec at 50°C and 1 a m ; argon injection volume was -90 pmol; argon concentration was 60 mmolfliterintraparticle volume)
p
a
Throat Size
Pore Pole Defi Retained1 PSmS ~2 Size V O ~ . (cmz/s); volume (throat) (dim.) (dim.) (A) (A) (cc/g) ( x ~ o - ~ ) (W (dim) (dim.) SVC57 0.35 0.51 218 4.0 874 0.15 2.0 26 32 SVC48 0.45 0.50 1090 -5000 0.24 3.1 100 5.0 23 a-alumina 0.45 0.49 3800 15200 0.19 3.7 20 4.0 24 SVC56 0.34 0.52 230 706 0.15 1.8 3.1 31 22 SVC46 0.49 0.55 7 2 546 0.26 2.3 73 7.5 14 SVC463 0.44 0.47 7 1 485 0.25 2.4 72 6.8 12 SVC45 0.43 0.50 428 2180 0.23 5.3 19 5.1 13 Shell (S980) Silica D2.2 0.43 0.51 6 8 156 0.52 1.3 2.3 21 11 SVC5Ob 0.51 0.56 4 1 156 0.37 2.0 51 3.8 6 y-alumina 0.65 0.55 85 198 0.68 3.0 33 2.3 6 SVC47 0.50 0.50 128 624 0.06 5.2 18 4.8 7 glass beads ----2.95 (contr01)~ 0.00 0.37 Solid
1: retained mercury from scanning time of roughly 5
SA
pJSA5
(m2/g)
(A)
11 5.8 <2 16 69 69 5.3
136 413 >loo0 94 37 37 434
75.5 155 147 11
69 23 46 55
minutes calculated from porosimetry intrusion profiles repeated experiment to test precision of SGC and porosimetry non-porous quartz glass beads of diameter 175 microns used to test SGC s: used as a measure of surface roughness
2: 3: 4:
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There are several possibilities. (1) SGC may not be sensitive enough to characterize the subtle differences in transport resistance between these solids. (2) The morphological variables that we employed were not a complete or appropriate description of void smcture. (3) Gas transport in porous solids of intermediate porosity do not possess void network sensitivity. The tormosity is well-correlatedto porosity.
Morphological sensitivity may not be observed until medium porosity becomes extreme, particularly near the percolation threshold (- 30% v/v for ideal amorphous solids).
0.3
0.5 Void Fraction (dim.)
0.4
0.6
0.7
Figure 1: Relationship between Tortuosity in a Gas Phase Diffusion Experiment and Solid Volume Porosity For samples of intermediateporosity, produced by random agglomerative processes such as the selected solids used in this work, tortuosity is best estimated in the range of 10-20. This is considerably higher than 1.5-2 which would be estimated assuming a void morphology of well packed microspheres.
CONCLUSIONS An attempt was made to correlate a measured tortuosity in a gas phase transport experiment to morphological characteristics derived from mercury porosimetry. Unfortunately, no such direct correlation is evident for the solid samples and techniques used. Tortuosity is not apparently sensitive to the characteristics derived directly from porosimetry using samples of intermediate porosity. Values of tortuosity in the range of 5-25 are measured for solids with pore sizes ranging from 200 to 150008, and porehhroat ratios of 2-8. Tortuosity appears more sensitive to porosity rather than to morphologicalparameters derived from the analyses of porosimetry for the set of solids tested.
208
Since all solids were in the intermediate range of porosity (4560%v/v), the measured a m p o r t might be less sensitivity to void morphology. Sensitivity to void architecture will be greater at extremes of porosity where percolation concepts might also be applicable. ACKNOWLEDGMENTS Partial support for this work (Haldor-Topee) was obtained through a BRITE contract No. RI
1B-0290-C.
REFERENCES 1. S. Reyes and K.F. Jensen, Estimation of Effective Transport Coefficients in Porous Solids Based on Percolation Concepts, Chem. Eng. Sci., 40(9) (1985) 1723-1734. S.C. Camiglia, Construction of the Tortuosity Factor from Porosimetry, Journal of Catalysis 2. 102 (1986) 408-418. 3. C.T. Wang and J.M. Smith, Tortuosity Factors for Diffusion in Catalyst Pellets, AICHE Journal (1983) 132-136. K. Wohlfahrt,The Design of Catalyst Pellets, Chem. Eng. Sci. 37 (1982) 283-290. 4. W.C.Comer, A.M. Lane, K.M. Ng, and M. Goldblatt, Measurement of the Morphology of 5. High Surface Area Solids: Porosimetry of Agglomerated Particles, Journal of Catalysis 83 (1983) 336-345. 6. W.C. Conner, A.M. Lane, and A.J. Hoffman, Measurement of the Morphology of High Surface Area Solids: Hysteresis in Mercury Porosimetry ,Journal of Coll. and Int. Sci., lOO(1) (1984) 185-193. W.C. Comer, C. Blanco, K. Coyne, J. Neil and J. Pajares, Analysis of the Morphology of 7 High Surface Area Solids: Studies of Agglomeration and the Determination of Shape, J. Cat. 106 (1987) 202-209. 8. A., Lane, lnterpretation of Mercury Porosimetry Data, PhD Thesis, University of Massachusetts, Amherst, 1984. 9. M. Ciftcioglu, D.M. Smith, and S.B. Ross, Mercury Porosimetry of Ordered Sphere Compacts: Investigation of Intrusion and Extrusion Pore Size Distributions, submitted to Powder Technology Journal, February, 1988 10. A Ali,, Kinetics and Morphology Study of Phillips Polymerization Catalyst, Masters Thesis, Dept. of Chemical Engineering, Univ. of Massachusetts,Amhexst, 1985. 1 1 . W.C. Conner, E.L. Weist, and L.A. Pedersen, Understanding the Morphological TransformationsThat Occur in the Preparation of Alumina Supports, in: B. Delmon et al. (Ed.) Preparation of Catalysts IV, Elsevier, 1987. 12. W.C. Conner and J.F. Cevallos-Candau,Characterization of Pore Structure: Porosimetry and Sorption, Langmuir 2 (1986) 151-154. 13. M. Kubin, Beitrag zur Theorie der Chromatographie,Coll.of Czech. Chem. Comm. 30 (1965) 1105-1116. 14. E. Kucera, Contribution to the Theory of Chromatography:Linear, Non-equilibrium Elution Chromatography, J.of Chrom. 19 (1965) 237-248. 15. P. Schneider and J.M Smith, Adsorption Rate Constants from Chromatography,AICHE Joumal, 14 (5) (1968) 763-771. 16. A. Baker, M. New and W. Richan, Determination of Intraparticle Diffusion Coefficients in Catalyst Pellets: A Comparative Study of Measuring Methods, Chem. Eng. Sci. 32 (1982) 643656. 17. S.W. Webb, W.C. Conner, and R.L. Laurence, Monomer Transuort Influences in the Nascent Polymerization of Ethylene by Silica-SupportedChromium Oxide Catalyst, Macromolecules 22(7) (1989) 2885-2895.
F. Rodriguez-Reinosoet al. (Editors),Characterization of Porous Solids I1 0 1991 Elsevier Science PublishersB.V., Amsterdam
TEXTURAL CHARACTERIZATION OF ULTRAFILTRATION MEMBRANES BY THERMOPOROMETRY AND LIQUID FLOW MEASUREMENT
J.F. QUINSON~,N. NAMERI~,B. BARIOU~ 1 Universite Claude Bernard - LYON I, Laboratoire de Chimie Appliquee et Genie Chimique (CNRS, URA 417), 43 Boulevard du 11 Novembre 1918, 69622 Villeurbanne cedex, France 2 Universite de RENNES, E.N.S.C.R., Avenue du General Leclerc, 35700 Rennes France
SUMMARY The porous texture and the permeability of an organic and symmetrical ultrafiltration membrane, aged in different solvents, are determined by water thermoporometry and water flow measurement, respectively. The permeability variation observed, when a solvent substitution is operated, is explained by a textural modification due to the solventhembrane interaction.
INTRODUCTION The flux, J, of a liquid through an ultrafiltration membrane under the influence of a pressure gradient, AP, is given by the relation : J = L A P (1) with AP: pressure difference above and below the membrane L : membrane permeability, inversely proportionnal to the liquid viscosity, q. Generally, for organic membranes, the viscosity correction cannot explain the permeability variation observed when a solvent substitution is operated. It is necessary to introduce a supplementary parameter ci (refs. 1-21: L2 - - u -91 L1 12 The present work aims at linking the parameter ci with a possible textural modification, due to the solventhembrane interaction.
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EXPERIMENTAL PART Material In this study, a Nuclepore polycarbonate symmetrical membrane has been used. Its characteristics, given by the manufacturer (Nuclepore Catalog), are summarized in table 1
.
TABLE 1 Membrane characteristics
7.5
Where:
6
0.6
I
6 x 10'
R is the membrane pore radius, directly measured by microscopy h, the thickness, which, in this case, is also the pore length W, the weight and I, the pore density
The membrane is produced by using an irradiation and etching process on polycarbonate film. In theory, this fabrication procedure allows to obtain uniform pores, approximatively cylindrical, as it is illustrated in Pig.1:
Fig. 1. SEI micrography of external surface of a Nuclepore symmetrical membrane, in which the pore radius is R = 50 nm (data of the manufacturer)
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Actually, thermoporometry shows a pore size distribution probably due to an overlap of irradiated tracks within the membrane. Permeability measurement As far
as the membrane structure is not modified under the pressure
effect, 1.e. for values of LIP not beyond a few bars, the experimental flow versus pressure gradient is a straight line. The curve slope divided by the membrane surface characterizes the permeability, L (Fig.2). Values of permeability are deduced from solvent flow measurements realized in an Amicon cell at three different pressures: 0 . 5 , l and 1.5 bars.
I PRESSURE GRADIENT
Fig. 2. Correlation between liquid flow and pressure gradient for low values of AP. Textural characterization by thermoporometry (refs. 3, 4 ) The pore size distribution and the pore shape of the membrane are determined by thermoporometry. Thermoporometry is based on the thermodynamic conditions of the liquidsolid transformation of a capillary condensate inside a porous material. The thermal analysis of this transformation leads to the pore size distribution through the solidification curve (Fig.3): - the size of pore is obtained by means of the measurement of the solidification temperature.
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the volume of these pores is determined from the amount of the energy involved in the phase transformation. The characterization of the pore shape is made possible by the existence of a hysteresis phenomenon between the solidification and melting curves: from a comparison of these two curves, a shape factor FT can be deduced; FT varies between 1 (for nearly spherical pores) and 2 (for nearly cylindrical pores).
r
PoRERmlUS
i
T
v
POROUS VOLUME
4i
TEMPERATURE
Fig. 3 . Thermoporometry method : schematic determination of the pore size distribution from the solidification curve. Sample Dreparation The studied membrane is aged in three solvents respectively: ethanol (1 hour), water ( 2 days), dodecane (11 days). After aging in these different solvents, the membrane samples are always characterized by using water as the liquid for flow and thermoporometry measurements.
213
When the solvent under study is the ethanol or the dodecane, the substitution by the water is quickly performed in the course of successive washing sessions : ethanol +water or dodecane -+ ethanol + water. RESULTS AND DISCUSSION The textural characteristics and the permeability of the membrane samples are summarized in table 2 :
Sample A (aged in water) L
lo'*
Sample C Sample B (aged in dodecane) (aged in ethanol)
38.3
8.3
35.8
2.2
7
7.6
9
11.5
12.5
3.6
6
6.3
2
1.9
1.8
(m s-' Pa-')
v (mm3 g-')
-
R (nm) AR
(nm) FT ~
~~~
Where: L is the permeability, referred to the surface unit of the membrane V, the total porous volume R, the mean pore radius corresponding to 50% of the total porous volume. AR, the spreading of the pore radius distribution curve, measured by the difference of the radii corresponding to 10% and 90% of the porous volume. FT, the shape factor corresponding to the mean pore radius, R. The pore size distribution, AV/AR = f(R), are given in Fig.4.
of samples A, B, and
C
214
Fig. 4. Pore size distribution curves of samples A, B, C. From these different results, it should be noted that the aging solvent induces not only a permeability variation but also a modification of the pore size distribution. The nearly cylindrical shape of the pores only appears to subsiste. This last observation can be the indication of an isotropic swelling whatever is the solvent in which aging is done. Taking into account the pore radii measured by thermoporometry, the permeability variation is explained from the application of the Poiseuille's law which leads to : (3)
where:
N is the number of pores per surface unit of the membrane R, the pore radius h, the pore length, assumed equal to the membrane thickness
215
In the case of a pore size distribution,
=
f(R) and
"
L =-
8 1 h I i 9 dR Rl dRi
(4)
dV if the calculation is achieved by step by step integration of the ; iii Q S . R curve. Indeed, for cylindrical pores with radius R, dN is linked to the volume variation , by the relation:
f
From equations (4) and ( 5 1 , the permeability has been calculated in the case of sample A for instance. The R and values deduced from thermoporo-
f
metry, and the N and h values given by the manufacturer are used for calcula t ion. The difference observed between the measured permeability, Lexp = 8.3x10-" m s-' Pa-', and the calculated permeability, = 4 ~ 1 0 - lm~ s-' Pa-', can be explained by the uncertainty on the esLcalc timated values N (accuracy: 15 %), and h (pore, assumed normal to surface within i 34O). It is possible to disregard these parameters by estimating the ratios, Li
c-. j
water
i = sample C j = sample A
i = sample B j = sample A
i = sample C j = sample B
experimental
4.6
4.3
1.1
4.1
3.6
1.2
L. =
(<)
L (&)water I
-
R. I
Ri
I: dN Ri4 dRi
a
I:
Ri dR
j
216
In table 3, the measured permeability ratios agree very well with the calculated values by assuming an isotropic variation of the membrane thick-
Thus, in these particular cases, the M parameter displays very well a pore size modification due to a swelling of the membrane which varies according to the solvent.
REFERENCES 1 Q.T. Nguyen, Thesis, INP Nancy, France, 1980. 2 B. Bariou, Communication presented at the Summer School on Membrane Science and Technology, Cadarache, September 3-7, 1984. 3 M. Brun, A. Lallemand, J.F. Quinson, C. Eyraud, Thermochimica Acta, 21 (1977) 59-88. 4 J.F. Quinson, M. Brun, in Characterization of Porous Solids, (Eds.), K.K. Unger and al, Elsevier,Amsterdam, 1988, pp307-315
F. Rodriguez-Reinoso et al. (Editors), Characterization of Porous Solids II 0 1991 Elsevier Science Publishers B.V., Amsterdam
217
CHARACTERIZATION O F THE SURFACE FRACTAL DIMENSION O F EVAPORATED SILVER AND GOLD FILMS THROUGH ADSORPTION ISOTHERM MEASUREMENTS
J. KRIM and V. PANELLA Physics Department, Northeastern University, Boston, MA 02115, USA ABSTRACT Thin solid films are receiving much current attention as model systems for nonequilibrium growth processes and also on account of their potential for use as protective overcoats. The technique of adsorption is particularly advantageous for the characterization of the roughness and porosity of such films because of the microscopic size of the probe and because the surface area probed by the adsorbed monolayer includes that contained in pores. Adsorption studies have been carried out for various silver and gold films which have been deposited under controlled conditions onto the surface electrodes of a quartz crystal microbalance. We have studied films deposited at rates between .5 and 75 A/sec and at temperatures ranging from 80-500 K. Within this range, there results a wide variety of film behavior, ranging from amorphous to crystalline to porous to potentially self-fine fractal structures. INTRODUCTION In deposition processes used to fabricate thin solid films, there is a very sensitive and complex dependence of film structure on growth conditions (ref. 1). Film structures grown under nonequilibrium conditions and/or via aggregation processes have been the topic of much recent theory and computer simulation, with certain growth conditions resulting in self affine or fractal surfaces (refs. 2,3). The technique of adsorption is an excellent choice for the characterization of film microstructure because it involves covering the surface with particles which probe microscopic lengths and yet samples the entire macroscopic extent of the surface. We report here an adsorption isotherm study of evaporated silver and gold films which have been deposited under controlled conditions onto the surface electrodes of a quartz crystal microbalance. Adsorption isotherms recorded by standard volumetric techniques are potentially quite difficult when carried out on solid film samples. This is primarily due to the small surface area and thinness of many of the films which are of interest. This difficulty can be eliminated however by depositing the film onto a quartz crystal microbalance. There are several major differences associated with the recording of an isotherm by this technique with respect to those recorded by a volumetric technique. (1) The surface area of the film is on the order of only 1 cm2. Equilibrium times are therefore quite short, and the entire isotherm is typically recorded in a period of 1-8 hours. (2) The microbalance records
218
adsorption information directly in terms of mass per unit (macroscopic electrode) area of the film. A standard isotherm corresponding to adsorption on a flat surface is quite readily available, and no assumption need be made concaning the area per particle of the adsorbate particle. (3) The microbalance registers only the mass of the film which actually adsorbs on its oscillating electrodes. No corrections need be made concerning the dead volume of the gas dosing system. (4) The surface of the film has an open geometry. This allows one to record adsorption information in a thick film region without the complications of capillary condensation associated with porous or high surface area materials generally utilized for volumetric isotherms. In the work which we present here, our analysis will focus on an analysis of adsorption for films thicker than three layers. THEORETICAL FORMS FOR AN ADSORPTION ISOTHERM The form of an adsorption isotherm on a geometrically flat substrate in the thick fJm regime can be described by the so called "Frenkel-Halsey-Hill" (FHH) theory, under the assumption of complete wetting of the surface by the a m (ref. 4). The FHH vapor pressure is of the form:
where a is a coefficient which reflects both the substrate-adsorbate and adsorbate-adsorbate van der W d s interactions (ref. 5), T is the temperature, 6 is the quantity of adsorbed material and n = 3. Numerous comparisons of experimental isotherms with the FHH theory show apparent exponents with n significantly less and 3 (ref. S), particularly for f ilm thicknesses in the range 1.5 to 3 layers (ref. 7). Steele and also Bakaev (ref. 8) have suggested refinements of the FHH theory associated with finite film-gas interfacial thickness. Steele obtained an exponent of n = 2.8 for an interface which is spread over a thickness of two atomic layers. More recently Finn and Monson (ref. 9) have carried out computer simulations which are consistent with n = 3 in the thick film regime. Halsey (ref. 10)has shown that films conformingto the FHH theory adsorbed on a heterogeneous substrate yield isotherms with apparent n significantly below 3. In light of these results, we carry out our data analysis only for films greater than 3 layers and also utilize only homogeneous substrates: evaporated silver or gold films which have not been exposed to air after the evaporation. Since we do not have precise knowledge of the width of the liquid-vapor interface, we do not explicitly treat this correction. This correction is incorporated into the error bar associated with the fractal dimension assigned to the surface in question. According to recent theory (ref. ll), the form of an adsorption isotherms corresponding to adsorption on a fractally rough surface is an adaptation of the FHH theory, with n = 3/(3 - D), where D is the fractal dimension of the surface. Adsorption isotherms carried out on commercially prepared electron-beam evaporated silver films (ref. 5) have exhibited n = 4.3(0 = 2.3)for adsorbed film thicknesses in the range 8-50A(3-17layers).
219
Various issues are raised by this result: Will the result change if the effects of surface tension are incorporated into the theory? Kardar and Indekeu (ref. 12) have suggested that the incorporation of surface tension effects might instead relate the experimentally observed exponent n = 4.3 to a self-&ne surface whose local fractal dimension is 2.6. Do other experimental techniques which measure fractal dimension yield the same result? Does the range 8-508 correspond to the upper and lower cutoffs for fractal scaling or merely to the range over which the measurement technique is applicable? What are the physical parameters which influence the observed behavior, and how do these relate to the computer simulations of solid film growth? We report here our efforts to understand the physical growth mechanisms underlying the experimentally observed fractal scaling in order to provide a link with theories of adsorption and film growth.
EXPERIMENTAL A schematic of our experimental setup is shown in Fig. 1. Isotherms are recorded by monitoring adsorption onto metal electrodes which we have evaporated onto the major surfaces of a quartz crystal oscillator which vibrates in a transverse-shear mode. The crystal is driven at its resonant frequency by means of a Pierce oscillator circuit (ref. 13). Changes in frequency are proportional to the quantity of gas adsorbed (ref. 14), so an isotherm is a plot of frequency shift versus pressure at fixed temperature. The microbalance crystals for these studies were polished single crystals of quartz which had quality factors near lo5 (ref. 15). We produced the electrodes by evaporation of 99.999% torr onto the faces of the quartz blanks. The temperature pure Ag or Au at 5 x of the substrate was variable between 80 and 500 K. The deposition rate was variable between .5 and 75 hils. After the metal electrodes were deposited, the crystal was transferred in vacuum to the gas adsorption chamber (held at 77.4 K), where the frequency shift was recorded as a function of the surrounding vapor pressure of adsorbate gas. The experiments were carried out in a regime satisfying the condition mfilm << Maare,where mfilm is the mass of the adsorbate (liquid nitrogen) and Mbarc is the mass of the oscillator (quartz electrodes) with no adsorbed fdm. In this regime, there is a linear response between the change in frequency, Sj and the mass of the adsorbed film':
+
Changes in resonant frequency due to adsorption are therefore proportional to the mass per (geometric) unit area of the adsorbed film. One monolayer of liquid nitrogen (34.5 ng/cm2) adsorbed on flat electrodes of an 5 MHz resonant resonant frequency crystal will produce a shift of 4 Hz. If the electrodes are rough, the monolayer frequency shift (as determined by standard BET analysis) will be increased by a roughness factor A / A f , where A is the actual surface area and A f is the geometrically flat area. This measurement allows separation of the flat and rough samples. The thick film regime is then analyzed
220
L
........
e 1
a
Ib
Figure 1 Schematic of experimental setup. (a) quartz microbalance; (b) vacuum chamber; (c) gas dosing system; (d) pressure measurement system; (e) vacuum jacket; (f) thermal link, including heater and thermometer; ( g ) oscillator circuit.
221
in order to further characterize the roughness. RESULTS We first examined films grown under conditions similar to that described by the manufacturer of the commercially prepared samples which exhibited adsorption isotherms characterized by n = 4.3 (1000 - 2000 h; thick films deposited at rates of 2-20 a/sec) Fig. 2a shows the data for one such isotherm. The isotherm was recorded on a silver film which was deposited at room temperature onto optically polished quartz at a rate of 5h;/s and grown to a thickness of 1500h;. The isotherm exhibits power law scaling, consistent with that of Eq.( 1). The exponent n=3, however, would indicate a two-dimensional substrate. Similar behavior was observed for all deposition rates which we studied in the range
.5.&/s to 75h;ls. Solid Kr adsorption adsorption on these same substrates indicated more crystalline surfaces at the lower deposition rates and more amorphous surfaces at the higher deposition rates. Only slight increases in geometric surface area were observed as the rate of deposition increased, but the thick film scaling always indicated a twodimensional substrate.
3
3.5 4 4.5 5 In(coverage) (ng/cm2)
5.5
6
Figure 2. Nitrogen adsorption data for silver films which have been evaporated under identical conditions onto an optically polished (dots) and mechanically polished (stars) quartz substrate. The slope of the plot is the exponent n in Eq.(l). This exponent indicates D = 2.0 for the optically polished substrate and D = 2.4 for the mechanically polished quartz. The solid line shows the theoretical prediction for adsorption on a smooth substrate.
222
We then removed the silver electrodes from the commercially prepared samples and reevaporated it in conditions identical to those under which we had observed two-dimensional scaling for the polished quartz samples. The adsorption isotherm recorded on one such sample is shown in Fig. 2b. The fractal scaling behavior is retrieved, with n=5.0 (D=2.4). We conclude that the suggested fractal scaling behavior reported in Ref. 11 originates in the roughness of the underlying quartz substrate and is not intrinsic to the room temperature silver depositions process. It is therefore inappropriate to attempt to link this particular result .with theoretical predictions of self-&ne scaling based on deposition models. In order to study roughness intrinsic to the deposition process, we evaporated Ag at a rate of .5A/s onto an optically polished crystal held at 80 K, (Figures 3a,4a), and Au at a rate of .5A/s onto an optically polished crystal held at approximately 500 K (Figs. 3b,4b). Both films were 1500A thick. Both are rough, as indicated by monolayer step heights which are significantly greater than that of the flat surface, (Figs. 3c,4c). 1
80
1
1
1
G
60
1
1
1
I
I
I
I
I
l
l
1
0
Au(5OOK)
0.2
0.4
I
I
I
I
4
I-
(b) n
1
L
v
w
a
40
I
20 0
0
0.6
0.8
1
p/p, Figure 3. Adsorption isotherms for nitrogen at 77 K on three surfaces deposited at different temperatures. (a) Ag deposited onto a substrate held at 80 K; (b) Au deposited onto a surface held near 500 K; and (c) Ag deposited onto a room temperature surface. One monolayer of nitrogen produces a 4 Hz shift on a flat surface.
223
The flat surface scales as a two dimensional surface while the silver film deposited at low temperature exhibits scaling consistent with D = 2.4 The film deposited at elevated temperature shows features which may be associated with the effects of pore filling and surface tension. Upon desorption, it exhibited a great deal of hysteresis, with much of the adsorbed material remaining trapped on the surface, presumably in pores. Films which were evaporated in similar conditions to those shown in Figs. 3 and 4 were also studied by means of x-ray reflectivity.[l5]The Ag film which was deposited onto the substrate held at low temperature was the only one of the three films to be identified by x-ray reflectivity to exhibit self-fine fractal scaling. A fractal dimension D = 2.5 was obtained from fits to the x-ray data for this latter sample. We note that adsorption on the films described above was substantially different after air exposure and, in the case of the low temperature deposited silver films, even after the first exposure to liquid nitrogen. Caution should therefore be exercised in extending our results to films which have been exposed to air.
0
#'%
L n n
e
-2
PI W
-4
0
F: v-4
(b)
W
d v-4
0
Au(500K)
(c) 0 Ag(300K)
'
-8' 3
I '
I 5
I
4 ln(coverage)(ng/cm2)
'
r % 0 ' " 6
7
Figure 4. Plot of the same data shown in Figure 3 on a logarithmic scale. (a) This silver sample scales as a potential fractal or self-fine surface with dimension D = 2.4 (n = 5.0); The gold sample shows feature consistent with porosity and surface tension effects; (c) This silver sample scales as a two dimensional surface ( n = 3.0).
224
CONCLUSIONS Our results show that the scaling behavior reported in Ref. 11 originates in the roughness of the underlying quartz substrate and is not intrinsic to the room temperature silver deposition process. Among the systems which we have studied, The films deposited onto optically polished substrates held at low temperature appear to provide the closest experimental realizations of theoretical deposition models. This is reasonable, since restricted growth can be expected at low temperature. Based on comparisons with x-ray reflectivity measurements, we conclude that the fractal dimension deduced from our liquid nitrogen isotherms is accurate to 60.1. ACKNOWLEDGEMENTS This work has been supported by the Petroleum Research Fund, grant number 22008AC5, and the National Science Foundation, grant number DMR8910315. REFERENCES 1 M. Sikkens, I.J. Hodgkinson, F. Horowitz, H.A. Macleod and J. Wharton, Optical Engineering, 25 (1986) 142. 2 P. Meakin, CRC Critical Reviews in Solid State and Materials Sciences 13, No.2 (1987) 143, and references therein. 3 S.H. Lu, Solid State Physics 39 (1986) 207. 4 J. Frenkel, Kinetic Theory of Liquids, Oxford Press, London, 1949; G.D. Halsey, J. Chem. Phys., 17 (1949) 520; T.C.J. Hill, J. Chem. Phys., 17 (1949) 590. 5 E. Cheng and M.W.C. Cole, Phys. Rev. B, 38 (1988) 987. 6 W.Y. Lee and L. Slutsky, Surf. Sci., 54 (1976) 169. 7 P.J.M. Carrott, A.I. McLeod and K.S.W. Sing, Adsorption at the Gas-Solid and Liquid-Solid Interface, Elsevier, Amsterdam, 1982. 8 W.A. Steele, J. Coll. Int. Sci., 75 (1980) 13; V.A. Bakaev, Izvestia Akademii Nauk SSSR, Ser. Khim., (1977) 8.
9 J.E. Finn and P.A. Monson, Phys. Rev. A, 39 (1989). 10 G.D. Halsey, J. Amer. Chem. SOC., 73 (1951) 2693. 11 P. Pfeifer, Y.J. Wu, M.W. Cole and J. Krim, Phys. Rev. Lett., 62 (1989) 1997. 12 M Kardar and J. Indekeu, Phys. Rev. Lett., in press 13 M.E. Fkerking, Crystal Oscillator Design and Temperature Compensation, Van Nostrand, New York, 1978, pp 67-68 14 C.D. Stockbridge, Vacuum Microbalance Tech. 5, Plenum, New York, 1966. 15 R. Chiarello, V. Panella, J. Krim and C. Thompson, submitted to Phys. Rev. Lett.
F. Rodriguez-Reinosoet al. (Editors), Characterizationof Porous Solids II 0 1991 Elsevier Science Publishers B.V., Amsterdam
225
INFLUENCE OF PORE STRUCTURE PARAMETERS ON THE INTRAPARTICLE PRESSURE CHANGE DURING ADSORPTION
Stephan E. Scholl' and Alfons B. Mersmann Department B for Chemical Engineering, Technical University of Munich POB 20 24 20, D - 8000 Munchen 2, FRG
ABSTRACT The influence of pore structure parameters on intraparticle total pressure change during gas phase adsorption is investigated. Dusty Gas theory is applyed to quantify pore mass transport for the case of non-isothermal, non-isobaric single component adsorption from an inert carrier medium. Comparing the results for activated carbon Chemviron BPL and a large pore adsorbent it is found that intraparticle total pressure reduction is less pronounced for the large pore solid. This is due to a larger intraparticle mole flux of the adsorbed component thus resulting in a faster equilibration of the gas phase mole defect caused by adsorption. INTRODUCTION Modeling the ad- and desorption kinetics of gas phase systems it is most commonly assumed that the total pressure within the pore system remains constant (refs. 1,2). However, this might not a priori be true in cases where the concentration of the adsorptives is high (which is the case in adsorptive separation in contrast to purification processes) or when the ambient pressure varies during operation (as in pressure swing adsorption). Then the intraparticle total pressure may change in the course of the process thus resulting in additional pore mass transport due to viscous flow. Besides operational parameters the quantitative contribution of this effect is determined by pore structure properties of the utilized adsorbent material. Therefore it is the purpose of this work to investigate the influence of structural parameters of the employed adsorbent material on the intraparticle total pressure change. This is studied for the case of single component gas phase adsorption from an inert carrier medium. A constant composition gas mixture comprising of an adsorptive and a noii-adsorbable carrier passes with constant rate a collective of spherical adsorbent particles at constant temperature and ambient total pressure. By use of a kinetic model concentration, total pressure, and temperature profiles inside a particle during adsorption may be calculated. The investigations are performed for two different adsorbents: Activated carbon Chemviron BPL 4x10 and a large pore adsorbent.
THEORY A quantitative answer to above questions may be given through the theoretical modeling of non-isobaric, non-isothermal single component gas phase adsorption. External heat and mass transfer, intraparticle mass transport through Knudsen diffusion, Fickian diffusion, sorbed phase diffusion and viscous flow as well as intraparticle heat conduction are accounted for. Fig. 1 presents the underlying assumption on the combination of the different mass transport mechanisms in the pore system. It is shown elsewhere that the assumption of instantaneous 'Author to whom correspondence should be addressed
226
establishment of adsorption equilibrium between pore fluid and adsorbed phase is justified (ref. 3).
A
t Knudsen diffusion sorbed phase
viscous
gos phase transport
1
diffusive transport
1
Fig. 1: Combination of the different mass transport mechanisms in the pore system of the adsorbent (ref. 4) Flux equations for pore mass transport The Dusty Gas Model (ref. 4) is employed to quantify intraparticle pore mass transport. The Dusty Gas theory considers the porous adsorbent material as a collective of large motionless molecules distributed isotropically in space and penetrated by the gas mixture. The dust molecules are (n+l)st component additional to the n-component true gas mixture. The transport expressions of the kinetic theory of gases are then applyed to the dust/gas-mixture (ref. 4). Remembering that the dust molecules have zero flux and interpreting Knudsen diffusion as intermolecular collisions of gas phase constituents with the dust matrix the appropriate transport expressions for the Dusty Gas Model may be derived (refs. 3, 4). For isobaric conditions the obtained formulations are identical to those from an analysis of intermolecular collisions of ideal gases in capillaries and porous solids (ref. 5). Due to the visualization of a porous medium as an ensemble of large dust molecules in the Dusty Gas Model pore structure properties such as porosity, tortuosity, and pore size distribution are not directly included. All information on pore structure characteristics is contained in the permeability constants Co, C1, and Cz. Heteroporosity as originating from a wide pore size distribution is not accounted for specifically. On the other hand the Dusty Gas Model has the advantage to allow a separation of the influence of pore structure characteristics on the different transport mechanisms. The influence of the adsorbent material pore structure on gas phase mass transport is incorporated through the parameters Co, C1, and Cz resp. They are determined by flux experiments for the specific adsorbent material (refs. 4, 6). The values for the different transport mechanisms may be correlated to structural parameters such as representative pore diameter d,, porosity E,, and tortuosity factor r, by the expressions:
d*,Kn Knudsen diffusion: C1 = 3 rpp,Kn
,
227 Ep,Fi
Fickian diffusion: Cz = -
,
Tp,Fi
(3)
Above definitions may be derived by formulating pore mass transport equations for the well known pore diffusion model (refs. 1,2). It assumes non-intersecting straight, cylindrical pores of constant circular cross-section with diameter d, lying along the pellet radius. Those equations are matched to the corresponding Dusty Gas expressions (refs. 3, 4). The parameters E, and d, for the different pore regimes may be determined from pore structure analysis by use of nitrogen adsorption or mercury intrusion methods (ref. 7), see also Fig. 2. Tortuosity factors r, for the different mechanisms should be a function of representative pore diameter (ref. 8). rp,visc, r p , K n , and T,,F~ resp. will thus not have identical values in above equations. Given the pore size distibution of an adsorbent material one may in principle estimate values of CO,C1, and Cz from above equations and thus predict the kinetic behaviour of an adsorption system. The flux equations for pore mass transport are formulated by use of the Dusty Gas Model. For the adsorptive the following expression is valid (refs. 3, 4):
ya is the mole fraction of the adsorbed component in the gas mixture and p is the total pressure. A l l and A12 are complex functions of the mole fractions of the gas components, total pressure, temperature, and the permeability constants Co, C1, and C2. They are derived from the complete formulation of the mass transport rates according to the Dusty Gas Model and the full definitions are given in the appendix.
For the carrier gas the analogous equation for the molar transport rate applies:
again with Dusty Gas constants Azl and
A22
as defined in the appendix.
Diffusion in the adsorbed phase is described according to Fick’s law. The appropriate flux equation for the adsorbate is therefore given by
Ds is the transport coefficient for activated sorbed phase diffusion with
In contrast to gas phase diffusion coefficients the frequency factor for surface diffusion DO, may not be estimated quantitatively from theoretical considerations but has to be determined from experiment. It therefore includes also structural characteristics of the employed adsorbent material relevant for surface diffusion. Es is the activation energy for sorbed phase diffusion. It is proportional to the heat of adsorption with proportionality factor approximately ranging from 0.15 to 0.6 (ref. 9). Material and energy balances Assuming local adsorption equilibrium between pore fluid and adsorbed phase and eliminating the mole fraction of the carrier gas by yc = 1 - ya the following material and energy balances are obtained. For the material balance of the adsorbed component mass transport in
228
the gas phase and in the adsorbed phase are accounted for. The appropriate expression is
d X / d y a ,d X / d p , and a X / d T represent the partial derivatives of adsorption equilibrium loading with respect to mole fraction of the adsorbed component, total pressure, and temperature resp. For convenience adsorption equilibrium was represented in the calculations using the Langmuir expression (ref. 1):
Above expression is used solely as adsorption equilibrium formulation in adsorption kinetics modeling and not as true representation of exchange kinetics mechanism between pore fluid and adsorbed phase. The material balance for the carrier gas is given by
Accounting for intraparticle heat conduction the energy balance may be formulated as
cp is heat capacity, p p pellet density, and A, effective thermal conductivity of the porous solid as obtained from an Effective Medium Approach by Marcussen (ref. 10). Q a d is the isosteric heat of adsorption which gives rise to the energy source term in above equation.
(i) Initial and boundary conditions. The above presented material and energy balances were solved with respect to the following boundary conditions: The symmetry condition at the pellet center requires that
If adsorption at the external pellet surface is neglected (ref. 11) the boundary condition at the pellet surface for the adsorbed component material balance is:
N A G , r = d / 2 represents the molar flux density of the adsorbed component in the gas phase as given by eq. (4)and evaluated at r = d/2. The transfer coefficient for external mass transfer p is calculated from the well known dimensionless relations employing the Reynolds and Schmidt number (ref. 12).
Neglecting a pressure built-up in the surrounding boundary layer of the particle the total pressure at r = d/2 is given by: plr=d/2
= Pm
.
Accounting for external heat exchange through convection and radiation it follows:
(14)
229
As the investigated effects will be most pronounced if adsorption starts from a completely unloaded pellet the following initial conditions were used:
ya(r,t = 0s) = X ( r , t = 0s) = 0
,
T ( r , t = 0s) = T, p(., t = 0s) = p ,
, .
(16) (17) (18)
The obtained system of three coupled partial differential equations describing intraparticle absorbed component mole fraction, total pressure, and temperature profile was transformed into dimensionless form by use of appropriate quantities. It was solved numerically with orthogonal collocation method (ref. 13). RESULTS AND DISCUSSION The presented results were obtained for the parameters enlisted in Table 1. pellet diameter pellet porosity pellet density mole fraction of adsorptive ambient total pressure ambient temperature superficial gas velocity Dusty Gas parameter for Chemviron BPL 4x10
d EP PP
ya,, PCC T, W O
Co
c1 c2
Dusty Gas parameter for large pore adsorbent
Co c1
c2
heat of adsorption number of collocation points
Qad
NTOK
4 mm 0.4 850 kg/m3 9.872 x lo-' 1.013 x lo5 Pa 298.15 I< 0.1 m/s 1.045 x ni2 1.139 x lo-' m 5.743 x 10-2 6.53 x m2 1.139 x lo-' I n 2 8.615 x lo-' 53.42 kJ/mol 7
'ABLE 1: Main parameters used in the calculations
mrn)-kcmVg-
Transition Region 7Fickian
I
Diffusion
1
Pore Radius Fig. 2: Pore size distribution of activated carbon Chemviron BPL 4x10 with diffusion regimes for c-hexane In most cases of practical relevance overall adsorption kinetics of physical gas phase adsorption systems is governed by intraparticle pore mass transport (refs. 1, 3). In the present
230
study this is considered to be established by Knudsen diffusion, Fickian diffusion, viscous flow, and surface diffusion. The relevant pores are those with diameters in the order of approx. 1/10 of the adsorptive mean free path and larger. Fig. 2 therefore depicts the differential pore size distribution of activated carbon Chemviron BPL 4x10 (Lot D 80618) as determined by mercury intrusion method with diffusion regimes for c-hexane at operating conditions (ref. 3). The mean free path of c-hexane is about 39 nm. The corresponding values for the Dusty Gas parameters Co, C1, Cz as reported by Gloor et al. (ref. 6) are given in Table 1. It may be seen from Fig. 2 that most of the pores of the activated carbon are operating in the transition region between Knudsen and Fickian diffusion. It is therefore obvious that the use of sophisticated models for a proper quantification of pore mass transport in the transition region should be of primary interest for the succesful modeling of sorption kinetics.
For this study a second fictive large pore adsorbent was investigated. It was assumed that the two materials have identical values for total pellet porosity E~ and density pp. Porosity c p , ~ i of large pores where Fickian diffusion is the dominating diffusion mechanism was assumed to as well be 50% higher for the large pore adsorbent than for the carbon. Tortuosity factor T~,K,, as the Dusty Gas parameter for Knudsen diffusion C1 were set constant for both materials. For the viscous flow parameter Co it was assumed that the characteristic pore diameter dp+= was 2.5 times higher for the modified adsorbent.
P 3
% a
0)
&&-
0.
Lorge Pore Adsorbent
-20 0
~
0
-
0
-
0
-
/
o
1
.
.
0
-40 - 60
Chemviron BPL
-80
o
-100+
.
:
/ ~
i
o
g
#-60 =
~
.
-LO
o
:
-
:
o
-
:
-80 r
-
, i
- 100
Fig. 3: Calculated intraparticle total pressure profiles for the adsorption of c-hexane on activated carbon Chemviron BPL 4x10 and a large pore adsorbent Fig. 3 presents calculated profiles of the intraparticle total pressure in the t w o adsorbent pore systems for the adsorption conditions given in Table 1. Circles indicate the location of the collocation points. From the computations it may be seen that the maximum pressure drop is obtained for the initial operation period (ref. 14). Therefore the pressure profiles are given at t = 0.75 min, the moment of maximum pressure drop. The total pressure reduction is always higher for the activated carbon than €or the large pore adsorbent. Further investigations show that the total pressure reduction in the pore system for adsorbent material with small transport pores at high adsorptive concentrations may be up to 1% of the ambient total pressure (ref. 14). The reason for this effect is demonstrated in Fig. 4.It presents the total intraparticle mole flux of the adsorbed component at t = 0.75 min. Here the values obtained for the large pore adsorbent are higher than for the activated carbon. This results in a faster equilibration of the
231
mole defect in the gas phase caused by the adsorption of gas phase constituents.
mmol m's
t = 0.75 min
12 f
O
0
J
"
.
0.2
.
"
' 0.4
"
!
0.6
0.8
1
0
Reduced Rodius 2r/d Fig. 4: Calculated total intraparticle mole flux Nt at t = 0.75 min for activated carbon Chemviron BPL 4x10 and large pore adsorbent CONCLUSIONS During adsorption a reduction of total pressure in the pore system of the adsorbent takes place. Besides operational and system parameters such as adsorptive concentration and adsorption equilibrium the effect is influenced by pore structure characteristics. It was found that it is less pronounced for adsorbent material with large transport pores. This is due to a higher intraparticle mass transport rate and thus a faster equilibration of the intraparticle total pressure drop. The maximum reduction of intraparticle total pressure is obtained at the initial operation period. It is therefore most crucial for adsorption processes with small characteristic time scales. NOMENCLATURE parameter for the calculation of gas phase A n , Azi mol/m s A12, A 2 2 mol m/N s mass transport with the Dusty Gas model Langmuir constant in eq. (9) br m2/N pellet heat capacity CP kJ/kg K parameter for viscous flow co mz parameter for Knudsen diffusion C1 m parameter for Fickian diffusion cz pellet diameter d m representative pore diameter dP m Fickian diffusion coefficient V mz/s Knudsen diffusion coefficient D m2/s effective Knudsen diffusion coefficient D" m2/s sorbed phase diffusion coefficient Ds m2/s frequency factor for sorbed phase diffusion DD, m2/s activation energy for surface diffusion Es J/mol molar mass M kgfkniol number of gas phase components n molar flux n mol/s molar flux density N mol/m2 s
232
NTOK P Qad
r R t T wo X X,, Ya ff
P EP
17 PP U
TP
number of collocation points total pressure heat of adsorption coordinate along pellet radius with 0 5 r 5 d/2 gas constant time S temperature K superficial gas velocity m/s loading mollkg loading at monomolecular coverage mollkg mole fraction of adsorptive in the gas phase mol/mol W/m2 K heat transfer coefficient mass transfer coefficient m/s pellet porosity Pa s viscosity pellet density k/m3 W/m2 K4 radiation constant tortuosity factor
Pa J/mol m J/mol K
Subscripts and superscripts A Kn
Fi Fickian adsorptive C carrier gas S sorbed phase visc viscous flow Knudsen
G 03
gas phase bulk
LITERATURE 1 Ruthven, D. M.: Principles of Adsorption and Adsorption Processes. John Wiley & Sons, New York 1984. 2 Sun, L. M. and F. Meunier: Non-isothermal Adsorption in a Bidisperse Adsorbent Pellet. Chem. Eng. Sci. 42 (1987) 12, pp. 2899 - 2906. 3 Scholl, S. E.: On Sorption Kinetics of Physically Adsorbed Species on Porous Adsorbents. PhD-thesis TU Miinchen 1990. 4 Mason, E. A. and A. P. Malinauskas: Gas Transport in Porous Media: The Dusty Gas Model. Elsevier Sci. Publ. Amsterdam 1983. 5 Scott, D. S., and Dullien, F. A. L.: Diffusion of Ideal Gases in Capillaries and Porous Solids. AIChE J. 8 (1962) 1, pp. 113 - 117. 6 Gloor, P. J. et al.: Dusty-Gas Parameters of Activated Carbon Adsorbent Particles. Chem. Eng. Comm. 59 (1987), pp. 95 - 105. 7 Gregg, S. J. and K. S. W. Sing: Adsorption, Surface Area and Porosity. 2nd ed., Academic Press, London 1984. 8 Klavetter, E. A. et al.: Comparison of Mass Fluxes Predicted by the Dusty-Gas and a Modified Dusty-Gas Model. Chem. Eng. Sci. 37 (1982) 7, pp. 997 - 1005. 9 Gilliland, E. R. et al.: Diffusion on Surfaces. I. Effect of Concentration on the Diffusivity of Physically Adsorbed Gases. Ind. Eng. Chem. Fund. 13 (1974), pp. 95 - 100. 10 Marcussen, L.: Mathematical Models for Effective Thermal Conductivity. In: Thermal Conductivity 18. Eds.: T. Ashworth and D. R. Smith. Plenum Press New York 1985. 11 Do, D. D., and Rice, R. G.: A Mathematical Formulation of Diffusion for Nonequilibium Adsorption in a Single Particle. Chem. Eng. Sci. 43 (1988) 10, pp. 2897 - 2900. 12 Mersmann, A.: Stoffiibertragung. Springer Verlag, Berlin 1986. 13 Holland, C. D. and A. I. Liapis: Computer Methods for Solving Dynamic Separation Problems. McGraw-Hill Book Comp., New York 1983. 14 Angerhofer, M.: Calculation of Non-isothermal Sorption Kinetics with the Dusty Gas Model
233
(text in german). Diploma thesis T U Munchen 1988. 15 Fuller, E. N. et al.: A New Method for Prediction of Binary Gas-Phase Diffusion Coefficients. Ind. Eng. Chem. 58 (1966) 5 , pp. 19 - 27. APPENDIX The transport, coefficients for the evaluation of the gas phase molar flux of the adsorptive as used in eq. (4)are defined as
Ail = The constants g,l to g,
913 9 2 2
+ 912
911 Q22
-
923
912 921
,
A12
=
914922
-
912924
911 9 2 2 - 9 1 2 9 2 1
are given by
For the carrier gas the analogues formulations apply: A21
= -
911 9.23 911 9 2 2
+
913 921
- 912
921
,
A22
=
911 9 2 4 911 Y22
-
914 g21 912 921
with constants g21 to 924 defined as
DE and D: are effective Knudsen diffusion coefficient for adsorbed and inert component, resp. They are given by -
D" = C1
\/M, RT
where M represents the molar mass of the species considered. Correspondingly, 'Dz, and 'DEaare effective free molecular diffusion coefficient of the two gas mixture constituents, resp. with
'Dacand D ' , are theoretical values for free inolecular diffusion coefficients as obtained from the kinetic theory of gases or Fuller's semi-empirical relation (ref. 15).
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F. Rodriguez-Reinosoet al. (Editors),Characterization of Porous Solids II 0 1991 Elsevier Science PublishersB.V., Amsterdam
235
NEUTRON SCATTERING INVESTIGATION OF ADSORPTION PROCESSES IN MODEL POROUS SYSTEMS J.D.F.
RAMSAY and R.G. AVERY
AEA Technology, Harwell Laboratory, Oxfordshire, O X 1 1 ORA, United Kingdom. SUMMARY Model porous adsorbent gels (silica, ceria) composed of regular packings o f spherical oxide sol particles have been prepared. The pore size in the gels was controlled by the diameter of the sol particles. The mechanisms o f adsorption and capillary condensation of water in the gels was investigated by small angle neutron scattering using H,O/D,O mixtures which had a composition which gave a contrast match with the solid oxide. Behaviour o f mesoporous silicas is in accord with previous theoretical models based on multilayer and capillary condensation whereas for microporous ceria there is clear evidence of a volume filling mechanism where the sorbed water has a density significantly less than in the bulk. INTRODUCTION Neutron scattering techniques have recently become established as a versatile probe for the determination of the structure of porous solids (ref. 1). Thus small angle neutron scattering (SANS), in common with small angle X-ray scattering (SAXS), gives details of the size, shape, concentration and surface area of inhomogeneities, such as pores in materials (ref.,?). Both SANS and SAXS are non-destructive techniques which are applicable to systems containing closed or molecular sized pores. The principal advantages o f SANS arise from the lack of restriction on the sample size and environment and the unique possibilities afforded by contrast variation, which allows neutron scattering selectively to probe the structure of heterogeneous or multicomponent pore systems (ref . 3 ) . Here we describe a novel application where the contrast variation technique has been applied to investigate the structure and adsorption mechanisms which occur with porous oxide gel systems. Here the progressive uptake of water vapour in silica and cerium oxide gels with well defined pore structure has been investigated and compared with model predictions based on capillary condensation theory. This study, made with gels having different pore sizes, has provided new insight into the mechanisms of capillary condensation and micropore filling. In this investigation the contrast variation technique has been applied by adjusting the water (H20/D20) and solvent (protonated or deuterated) composition to give appropriate contrast match characteristics comparable with the solid matrix. The scattering theory involving contrast is outlined below.
236
THEORY Small angle neutron scattering arises from variations of scattering length density which occur over distances dSAS (dsAs % X/20) exceeding the normal interatomic spacings in solids (refs. 2 , 4 ) . Such an effect thus occurs with colloidal dispersions of particles and porous solids - which may be composed of assemblies of small particles - as in the oxide gels considered here. The theory of scattering has previously been described extensively and hence we will only outline the main features of importance here. Thus for a concentrated system of discrete monosized particles, such as the oxide gels studied here, the intensity of scattering, as a function of the scattering vector, Q, ( = 4n sin 8 ) is given by
Here V and n are the particle volume and number densities respectively and p P P P and pQ are the scattering length densities of the particles and that of the liquid filling the pores. (N.B. the difference, (p, - p a ) , defines the contrast. When p and pQ are identical, we have matching and zero scattering. P For an outgassed gel, pa is effectively zero. P(Q) is the particle form factor, which for spheres, of radius, R, as considered here, is given by:
p(Q)
= [slsin
(QR) - QR cos (QR)I Q3R3
The static structure factor, S(Q), defines the spatial ordering of the particles. This is described in terms of the particle pair distribution function, g(r) which is given by:
The gels studied here have a short range order which is typified by a maximum in g(r) corresponding to a mean interparticle separation, r*. This relates to a corresponding maximum in S(Q), as reflected in the scattering behaviour, where
In the limit o f high Q the scattering is dependent on the interfacial area, S , between the two phases of the system such that in general we have :
237
This asymptotic decrease in the high Q tail is described as the Porod law region and arises when QR 2 4, where R refers to the dimension of the scattering heterogeneity, such as a particle say. For an outgassed gel or where the pore space contains only air we have the condition, p2 = 0. In the present investigation we have investigated the effect on the SANS during progressive uptake of water with a composition (D20/H20) which has a scattering length density identical to the solid gel phase, viz. p1 = p 2 . (See values o f scattering length density in Table 1). Under these conditions it has been possible to assess the interfacial area o f the adsorbed water film and gain an insight into the location and form of the capillary condensate within the gel structure.
TABLE 1 Scattering length densities, Oxide
p,
for water and oxides
p/10lo
HO ,
-0.56
D,O
6.36
SiO,
3.5*
CeO,
4.5*
* Denotes experimentally determined value.
TABLE 2 Surface and porous properties o f gels Gel
D/nm*
SBET/mZ 9 - l
VD/cm3 g-'
rD/nm
Ceria, C 1
6-7
160
0.10
-
Silica, S2
12
258
0.24
42.0
0.34
Silica, S4
30
128
0.20
2.9
0.31
~
* Size
E
0.41
o f sol particles determined by electron microscopy; SBET, Vg and F correspond to specific surface area, total pore volume and me n pgre radius determined from N, adsorption isotherms at 77K; E - porosity.
238
EXPERIMENTAL Samples of silica and ceria gel flakes were prepared by slowly evaporating concentrated sols in air. Two different silica sols (S2 and S4) and a ceria sol ( C l ) , having primary particle sizes of 12, 30 and 6 nm respectively, were similar to those studied previously (ref. 5). Surface and porous properties o f the gels, as determined from nitrogen adsorption isotherms at 77K, are given in Table 2. Gel flakes were gently ground and sieved, and a powder size fraction between 106 and 212 um was selected for subsequent SANS studies: identical amounts of powder (0.25 and 1.0 g for silica and ceria respectively) were transferred to several quartz spectrometer cells (path length 2 mm) and outgassed in a vacuum oven at 423K. The cells containing the powder were reweighed and then transferred to small desiccators to equilibrate at different vapour pressures (p/po, 0.08, 0.43, 0.62, 0.75, 0.97) above saturated salt solutions. Equilibrium uptakes for the three gels are shown in Figure 1. The volume ratios of H20 and D20 in the water of these saturated solutions were selected to give contrast match conditions (61% v/v and 75% v/v 020 for silica and ceria respectively). Samples were also exposed (p/po = 0.97) above water of zero scattering length density (8% v/v DZO) to compare the SANS with that of the outgassed samples. Such a comparison was expected to reveal any possible structural change in the porous gels resulting from water sorption. SANS measurements were made a5 previously at a nelitron wavelength, 1, of 6 8, using the multidetector instrument installed in the Pluto reactor at the Harwell Laboratory. I ,
RESULTS AND DISCUSSION The effect of progressive water vapour adsorption on the SANS of silica gel S4 is shown in Figure 2. For the outgassed sample (a) the scattering i s characteristic of a structure formed by the packing of spherical sol particles as described previously. Thus the pronounced maximum at Q = 0.026 A-' arises from the interference in the scattering from a partially ordered structure, where the interparticle separation is given approximately by Zn/Qmx, viz. ?I 24 nm. The inflexion at Q 0,040A-' results from the form factor, P(Q), of the spherical particles. Thus for monodispersed spheres, of radius R , the decay o f P(Q) has a secondary maximum at QR ?I 5.9. Although this feature is smeared due to the slight polychromaticity of the incident beam and particle 29 nm polydispersity, the position corresponds to a particle diameter o f which is in good accord with that determined (I, 30 nm) in previous SANS and electron microscopy investigations on dilute sols. Beyond the inflexion, I(Q) decays linearly with Q-4, in accord with the Porod law. Changes in the SANS after exposing gel samples to water vapour having the same scattering length density as silica (viz. 61% v/v D20) are shown in I ,
I ,
239
12
e
I
4
L
ul c
I
E o
c
\
C 73 \
W 0,
0
4
7
,
u o 0
0
0.5
PI Po
0
C 1
0.0 5 Q I A-’
Fig. 1. (Left). Equilibrium adsorption of water at different p/po (T , silica S2; 0 , silica S4; x , ceria C1.
%
29810:
Fig. 2 . (Right). SANS of silica gel S4 after equilibrating with water (O,O, 61% v/v) at different relative vapour pressures, p/po: (a), 0; (b), 0.62; (c), 0.75; (d), 0.97. Figures 3b, c, d and e. Here the uptakes correspond to p/po of 0.08, 0.62, 0.75 and 0.97 respectively. The scattering remains almost unchanged up to p/po = 0.62 but on further uptake (p/po = 0.75) marked differences occur: the almost total elimination of the interference peak is the most striking. Less evident are the suppression of the inflexion due to P ( Q ) , and the reduction in the intensity in the Porod region by a factor of % 0.83. These features are discernible in Figure 3d. Such changes arise from the partial filling of the pore space by a process of capillary condensation and the development of the
?: -.... ..
10
1
10-
(fl‘ I
Illllll
I
1 1 1 1 1 1 1
I
I111111
I
I111111
I
I 1 1 1 1 1 1
I
I l l l l L
Fig. 3 . SANS o f silica ge1 S4 equilibrated with water at p/po: (a), 0; (b), 0.08; (c), 0.62; (d), 0.75; (e), 0.97; ( f ) , 0.97. Water compositions (D,O and 8% for ( f ) respectively. Broken line shows Q - 4 % /v) are 61% for (b)-(e) power law.
surface film of water. The extent of reduction in I(Q) in the Porod region implies that the effective surface area of the water film, SF, compared with that of the bare surface, So, is I, 0.8. This result is in satisfactory accord with model calculations as will be later described. At the highest p/po there is a dramatic reduction in I(Q) (Figure 3e) corresponding to the saturation o f the pore space with water, which results in almost complete contrast matching. Here the intensity has decreased by a factor of > 102 compared with that of the outgassed gel. This feature is vividly illustrated in Figure 3f, which shows the scattering from a gel sample saturated with water having zero scattering length density (8% v/v D20), after exposure at the same p/po. Here the scattering curve is virtually identical to that o f the outgassed gel (cf. Figure 3 a ) . Changes in the SANS of silica gel S2 after exposure to water vapour are shown in Figure 4. Here the primary particle size of the sol forming the gel is considerably smaller (,I 12 nm) and more polydispersed. Consequently for the outgassed gel (Figure 4a) the maximum in the interference peak occurs at a lower Q of 0.057 A-’, corresponding to an interparticle separation of 11 nm. The less pronounced maximum compared with the S4 gel, can be ascribed to partial coalescence of the contacting particles after the outgassing treatment at 423K.
241
10 -i L
-
In
I
. E v
’
G
:
W
lo-’
Fig. 4. SANS of silica gel S2 equilibrated with water at p/po: (a), 0; (b), 0.08; (c), 0.43; (d), 0.97; (e), 0.97. Water compositions (D,O % ‘/v) are 61% for (b)-(d) and 8% for ( e ) respectively. Broken line shows Q - 4 power law.
0.08) causes a significant Exposure to a low water vapour pressure (p/po change in the interference peak (4b) in contrast to the S4 gel. This difference can be ascribed to the smaller size of the sol particles forming the gel. Since the uptake will be close to a monolayer at this pressure, the thickness of the adsorbed film will result in a more pronounced ‘neck’ at the points o f particle contact in the S2 gel. The effect on the scattering will thus be somewhat akin to that which arises from particle coalescence already noted. Beyond the interference maximum there is no perceptible difference in I(Q) in the Porod region. At a p/p, of 0.43 (4c) the scattering is unchanged but at a p/po of 0,97 (4d), where saturation occurs there is a dramatic reduction in I(Q), as previously noted with the S4 gel. At the corresponding pressure where the gel is saturated with water of zero scattering length density ( 4 e ) the scattering is identical to that of the outgassed gel indicating that the surface and structure o f the gel is unchanged on saturation with water. The processes of adsorption and capillary condensation of vapours in regular packings of monodispersed spherical particles have been described theoretically
242
I
LL
In
0’7
t
I
I
II
0
Fig. 5. Dependence of relative surface area of adsorbed water film, SF/S,, on p/po, for a sphere packing with n = 8. Sphere diameters are (a) 100; (b) 200; and ( c ) 300 8, respectively.
101
I
I
I
I
1
\I
Fig. 6. SANS o f ceria gel C1 equilibrated with water at p/po: (a), 0; (b), 0.43 and (c), 0.97. Water compositions (0,O % ‘/v) are 75% for ( b ) and 8% for (c) respectively. Broken line shows Q-4 power law.
243
by several workers (refs. 6-9). In general three processes occur as the vapour pressure is increased. These include (a) multilayer adsorption on the sphere surfaces, (b) the gradual filling of the capillary condensate around the points of contact between spheres and (c) condensation in the cavities between spheres. It has been shown that the area of the surface film only becomes appreciably less than that of the solid when the sphere size is small (diameter < 15 nm) and n, the particle co-ordination number, is large. Such effects are 2, then only important when the p/po is approaching the point when spontaneous capillary condensation occurs. This feature is illustrated in Figure 5 which shows the ratio of adsorbed water film to solid area, SF/So, for n = 8 and for sphere diameters, 0, which are in a range relevant to the oxide gels here. The relative insensitivity of SF/So to p/po arises because the loss in interfacial area due to the growth of the meniscus at the points of contact, is offset by the contribution of the meniscus itself. Such behaviour is in accord with the SANS results described for the S4 and S2 gels. The foregoing model is somewhat idealised and the analysis, assuming w e l defined multilayer and capillary condensation processes, may become invalid when the pore size of the sphere packings approaches the micropore range. In this respect the SANS results for the ceria gel (Figure 6) provide important insight into the water sorption process. Here the particle size of the sol (% 7 nm) is sufficiently small to give a microporous gel showing type I isotherm behaviour. The outgassed gel (6a) shows a maximum in the interference between % 0.09 and 0.10 A - l corresponding to an interparticle separation of 2, 6 to 7 nm. This feature is less pronounced and broad indicating more extensive particle coalescence than with the silica gels. However on saturation of the gel with water (75% v/v 020) at p/po = 0.43 (6b) there is still, perhaps surprisingly, considerable scattering. Thus the interference feature is suppressed, as observed previously with the silica gels, and the intensity in the Porod region is reduced by a factor of 2, 2. The scattering from the gel saturated with water of zero scattering length density (6c) i s again virtually identical to the outgassed sample. The marked scattering observed when the gel i s saturated with 75% v/v D20 clearly shows that this is not a contrast match composition, despite having been previously established with the ceria sols. This suggests that the effective density of the sorbed water in the pores is less than the bulk density. This lower effective density may arise from differences in the ordering of water molecules when confined in the ceria micropores compared with that imposed by H-bonding in the bulk liquid. Such effects of pore geometry have indeed recently been discussed by Sing et a1 (ref. 10) to explain inhibition of water uptake in the molecular sieve silicalite, compared with that of l e s s ordered molecular liquids such as nitrogen. Indeed other evidence
244
based on incoherent inelastic and quasielastic neutron scattering indicates that the H-bond structure and diffusion of water in the microporous ceria gel differs markedly from that reported with mesoporous silicas (ref. 11). These general conclusions on the mechanisms of sorption in micropores are based on the limited SANS measurements with the ceria gel. Evidently there is scope for further investigations, particularly with microporous solids with different pore geometry, such a slits, which may be compared more readily with recent theoretical simulations of sorbate structure (ref. 12). ACKNOWLEDGEMENTS The work described was undertaken as part of the Underlying Research Programme of the UKAEA. The experimental assistance of Mr. B.O. Booth and Mr. M . Scanlon in the preparation of gel samples used in SANS measurements is gratefully acknowledged. REFERENCES 1 B . O . Booth and J.D.F. Ramsay, in: J.M. Haynes and P. Rossi-Doria (Eds.), Principles and Applications of Pore Structural Characterisation, J.W. Arrowsmith Ltd., Bristol, 1985, p. 97. 2 A. Guinier and G. Fournet. in: Small Anale - Scatterinq- of X-rays. - . Wiley. -. New York, 1955. 3. J.D.F Ramsay, in: K.K. Unger, J. Rouquerol and K.S.W. Sing (Eds.), Characterization o f Porous Solids. Elsevier. Amsterdam. 1988,. .P. 23. 4 J.D.F. Ramsay, Chem. SOC. Rev., 15 (1986) 335. 5 J.D.F. Ramsay and B.O. Booth, J. Chem. SOC., Faraday Trans. I, 79 (1983) 173. 6 B.G. Aristov, A.P. Karnaukhov and A . V . Kiselev, Russ. J. Phys. Chem., 36 (1962) 1159. 7 D. Dollimore and G.R. Heal, J. Colloid Interface Sci., 42 (1973) 233. 8 W.H. Wade, J. Phys. Chem., 69 (1965) 332. 9 D.M. Smith and N.E. Olague, J. Phys. Chem., 91 (1987) 4066. 10 M . B . Kenny and K.S.W. Sing, Chem. & Ind., 39 (1990). 11 J.D.F. Ramsay and C. Poinsignon, Langmuir, 3 (1987) 320. 12 B.K. Peterson, J.P.R.B. Walton and K.E. Gubbins, J. Chem. SOC., Faraday Trans. 11, 82 (1986) 1789.
F. Rodriguez-Reinosoet al. (Editors), Characterization of Porous Solids II 0 1991 Elsevier Science Publishers B.V., Amsterdam
245
SMALL ANGLE AND ULTRA-SMALL ANGLE SCATTERING TECHNIQUES FOR CHARACTERIZATION OF POROUS MATERIALS
J.C.Dore and A.N.North Physics Laboratory, University of Kent, Canterbury, CT2 7NR, UK
SUMMARY Small-angle X-ray (SAXS) and small angle neutron (SANS) scatterin can be used to study the properties of porous materials over a range of 10f100nm. Experimental methods using reactor neutrons and synchrotron radiation are described and current methods of analysis reviewed. New developments which extend the range of the measurements down to low scattering vectors (- 10-5A-1)are presented and the application to various systems critically examined. Spatial features covering a range of log, to 2~ can be investigated and are illustrated by reference to work on materials of varying pore size, pore shape, surface texture and composition. Recent work using the principle of "contrast-matching'' to study multi-phase systems such as liquids in pores (partial-filling) is briefly described and the basic features of neutron and X-ray methods are critically compared. The complementary nature of the two measurements is emphasised and linked to likely future developments of the technique.
INTRODUCTION The structural features of porous solids may be of many different forms depending on the size, shape, connectivity and surface texture of the materials from which they are formed. Furthermore, the basic characteristics may be ordered as in the case of zeolites, or partially disordered in the case of structures formed through sol-gel or spinodal decomposition processes. A full mathematical representation of this complex spatial distribution is rarely possible and it is necessary to make approximations in order to extract information on comparative properties for real materials from a limited set of experimental measurements. Conventional methods such as gas adsorption (surface area, pore size) and electron microscopy (direct imaging in two dimensions) are already well established. In recent years the techniques of small-angle scattering have played in increasing role as an alternative means of investigation. Recent improvements in beam intensities and instrumentation for neutron and X-ray techniques have greatly enhanced the value of these methods. Since the scattering profile results from coherent interference effects it represents a direct observation of the spatial distribution of scattering centres in the sample and is not subject to any approximations. The data is an effective
246
average over the irradiated area and is therefore complementary to the image techniques of electron microscopy. This short review provides an update of some recent developments, particularly in the area of ultra-low small angle scattering (USAS),and gives consideration to possible extensions into new methods of studying the structural features of porous materials using both X-ray and neutron techniques.
THEORETICAL FORMALISM The basic theory of small-angle scattering has been reviewed elsewhere [1,21 and only a short digest of fundamental principles will be reproduced here. The intensity of coherent scattering from an assembly of scatterers may be written as
where a(Q) is the scattering amplitude and Q is the elastic scattering vector which has a magnitude:-
4n 0 Q = -sin2
x
for a scattering angle 0 with incident radiation of wavelength, h. At high Q-values (2 1A-1)the pattern exhibits diffraction effects characteristic of the atomic arrangement but at low Q-values the intensity will depend on large-scale inhomogeneities in the sample. The mean coherent scattering amplitude p(r) represents an effective average over the scatterers for a region larger than the atomic dimensions and the intensity can then be formally written as:-
and for an isotropic material becomes:-
where p(r) represents the spatial distribution. Comparative values of p(r) for X-rays and neutrons are different as shown in Table 1.
241
TABLE 1:
Some typical values of scattering length densities, p.
Material
I
H20 D20 CCh, c7D16 c7D16 Si02 CBr4
I
Neutrons -0.56 6.34 6.30 -0.48 6.28 4.09 4.73
I
] } I
X-rays 9.3 13.0 7.9 26.0 26.0
For an idealised two component system of an amorphous nature with no preferred orientation the intensity simplifies to
where (Ap) is the contrast difference, F(Q) is a form-factor for the individual "particles" in the assembly and S(Q) is a structure factor representing the distribution of particle centres. For porous materials prepared by the sol-gel process, and conveniently modelled by an aggregation of hard spheres, the form-factor corresponds to the scattering by an isolated sphere and the first peak in the structure factor results from interparticle correlations that are usually linked to the mean separation of the centres. The geometrical structure of the pore network in real materials is clearly of a more complex nature but it is usual to make the assumption that the effective form-factor can be averaged over a distribution, N(R) of pore sizes such that:-
If assumptions are made about the pore shape, which is usually assumed to be spherical, it is possible to extract information on N(R). Alternative assumptions can be made about the spatial distribution of mass in the aggregate and one which has attracted much recent attention is that based on a fractal formalism. [3,4] I n this case the intensity has a very simple form:-
248
where DM is the fractal dimensionality. This power law cannot apply over all length scales in a real system and at larger Qrange will be sensitive to correla-tions in the surface texture. The relationship becomes:-
where Ds is a surface fractal and eventually, aT an asymptotic limit, reaches the Porod regime, where:-
All of these concepts play a role in the interpretation of SAS data as shown in the following examples.
INSTRUMENTATION Most conventional small angle scattering methods with X-ray or neutrons use an incident monochromatic beam and a multi-detector as shown schematically in Fig.1. The presence of a beam stop to prevent the primary beam entering the detector provides a lower limit to the Q-values accessible for study. Since the scattering formalism scales as a function of QR, this gives a cut-off in the size range that can be 2.10-3A-1 means that structural investigated; a typical value of Qmin inhomogeneities with a scale > 2000A cannot be studied. The SAXS and SANS method is therefore suitable for studies of microporous media but is less useful for the higher range of mesoporous systems.
-
Fig.1: Schematic layout for a conventional SAS measurement. All research reactors have S A N S facilities and typical examples are the D11 and D17 instruments at ILL, Grenoble [5] and the PACE, instrument on the Orph6e reactor at CEN, Saclay [6] which use area multidetectors. X-ray facilities can use a Kratky camera with a laboratory-based generator but the use of dedicated instruments on a
249
synchrotron gives a huge gain in intensity. Typical examples are the SAXS instruments on lines 2.1 and 8.2 of the Synchrotron Radiation Source at the Daresbury Lab. [A An extension of the range to lower Q-values for X-ray studies can now be achieved by using an alternative technique based on the Bonse-Hart camera. The basic layout is shown in Fig.2 where a single channel cut crystal is used to give a well oriented monochromatic X-ray beam from four Bragg reflections and a similar arrangement is used to determine the intensity of the scattered beam. The tight collimation of this arrangement enables measurements to be made at very small scattering such that Qmin is reduced to 2.10-5 A-1 corresponding to an effective length scale up to 2p. The full evaluation of the data requires a deconvolution analysis particularly when the scattering intensity is a rapidly varying function of Q. Although the principles of the method have been known for many years, it is only recently that the full advantages have become apparent [8] through the use of synchrotron radiation sources (e.g. USAXS on line 2.2 at the Daresbury Lab). Corresponding methods are under development for neutrons [9] using longer wavelengths (1lo& but the instrumentation is still at a relatively early stage.
-
-
Fig.2 Schematic layout for a conventional USAS measurement It is useful to comment briefly on the comparison of X-ray and neutron techniques since the measurements for a two-component system such as a porous material should give identical results using either radiation probe. For cases where the pores are void, the contrast Ap is high and a full SAS intensity profile can usually be determined to satisfactory precision within 10 minutes using modern facilities. One problem which can arise in neutron studies is the subtraction of the flat incoherent contribution which can be quite large in the case of hydrogenous materials This disadvantage is partially offset by the possibility of using isotopic substitution to vary the Ap-value in a systematic way and this technique is very powerful when the pores are filled with a liquid medium. The incoherent scattering and background poses a smaller problem in SAXS studies and the higher intrinsic intensity of the main beam means that the I(Q) profile can be determined over a more extended Q-
250
range and with better Qresolution. However, there is no means of varying the A p value unless anoma-lous scattering techniques are adopted and these have not yet been tried. It now seems clear that SAXS, S A N S and USAXS can be used in a complementary manner to give information that would not be obtained from a single method. [lo] This factor will be illustrated in the varied examples of the following section. LLUSTRATIVE EXAMPLES The relative merits of the SAS technique can be illustrated by reference to some of the current collaborative studies undertaken by the UKC Scattering Group.
Silica and Alumina svstems. Porous silica may be routinely manufactured with a high surface area and pore volume but the preparation and treatment leads to a wide range of different characteristics. The standard method uses a sol-gel technique leading to an aggregated structure which is usually dried and heat treated. This process leads to a material with a high surface area and reasonably monodisperse pore size according to gas adsorption measurements. A typical example of a SAXS measurement (Aldrich silica; nominal 60 A pore size) and a fitted pore distribution is shown in Fig.3.a. An alternative method based on a leaching process for a two component glass in which one water-soluble component may be removed to give a spinodal glass leads to a quite different pore structure as shown by the data given in Fig.3b for a specially prepared research material (Schott glass). Current analysis of the available data suggests that neither system can be readily described by either a fractal or a simple pore distribution function but further analysis is in progress.
o-vahle
a I A-l
Fig.3: S A N S studies of porous silica samples a) commercial gel system (Aldrich); b) leached glass (Schott) The heat treatment of materials can lead to significant changes in the pore structure. The data for a finely divided research glass of small pore size [ll]which has
251
been heated to different temperatures is shown in Fig.4. The main effect is to reduce the pore volume, as expected, but the data also show a second contribution to the scattered intensity at low Q-values which becomes the dominant feature for the high temperature material The presence of this scattering is not fully understood but could arise from particle size or surface texture effects. This example shows that some care will often be required to interpret unusual features in the measured profile but that this could also reveal previouslv-unknown factors, in th characterisation of the material. Sintered Silica A : 366 ' C 6 : 406 OC
C : 760 O C D : 1025 O C
i 0.1
0.2
Fig.4: Sintenng effects in porous silica arising from heat treatment at various temperatures In some speaalised cases the pore structures are known to be anisotropic. This occurs in porous fibres and can be studied by measurements in which the sample orientation is varied to give conditions in which the Q-vector is either parallel or perpendicular to the fibre axis. An example is given by Stacey [12] for S A N S studies of alumina and Fig5a shows a similar measurement made with X-rays; Fig.5b shows an example for a naturally-occurring silica fibre taken from the skeletal structure of a sea animal [ l l ] where the anisotropy is more pronounced. It can be seen that the scattering intensity is dramatically altered by changes in the sample orientation. Suitable analytic methods are now being developed to extract information on the alignment properties of the pores based on partially-oriented cylindrical voids. Geoloeical materials Sedimentary rocks are often porous and can be studied by SAS techniques. Early work by Mldner, Hall and co-workers 1131 has shown that these structures can be represented by a fractal formalism. We have extended some of their work by using a combination of SAXS and USAXS techniques. The results are given in Fig.6. The (U)SAXS intensity shows a power law scattering with an exponent of -3.49 suggesting a surface fractal dimensionality of Ds= 2.51 whereas the S A N S value is Ds= 2.61.
252
Alumina Fibres
(la)
Channel Number
Fig.5: Anisotropic scattering for a) synthetic alumina fibres [12]; b) natural bio-silica from a sea animal [ll] It would be expected that the extension to lower Q-values would reveal a decrease in the slope corresponding to the change over to a mass fractal on a larger length scale. Surprisingly, the USAXS data exhibit a continuation of the same relationship and the I(Q) plot is therefore dominated by surface effects. Mildner et a1 [13] show that a comparison of neutron and X-ray data, which give different slopes is probably linked to the occluded pockets of oil in the interfacial regions and the information could be of importance for oil-recovery. Further work on geological specimensisisplanned. planned. -,specimens 1 Bakken Shale
USAX$
:I
...\**A
LoQclta)l -
2 4--
0 ,
\ \ , I
I
I
I
1
I
I
I
I
Fig.6: SAXS, USAXS and S A N S measurement son Bakken shale (geological) showing a surface fractal characteristic over six decades of I(Q).
253
Liauids in m r e materials The pore material may be the host for other distributed matter in a solid or liquid form. The modified behaviour of liquids in constrained geometry is currently attracting much attention [141. If the pores are filled with the liquid the only change to the scattering is due to the changed magnitude of A p . The use of hydrogen/deuterium mixtures in SANS studies is particularly important since it is often possible to contrast match the liquid to the substrate. This phenomenon is illustrated in Fig.7 for H20/D20 water in porous silica. The match point is achieved for 64% D20 (mole fraction) and confirms that all pores are open to the general network. If the pore filling is restricted it is possible to investigate the effects of capillary condensation as shown by Ramsay [15]. Other work has been reported by Li et a1 [16] in which a quantitative analysis has been carried out, based on the assumption of a fractal distribution with a spinodal structure factor. This group also point out that the process of drying at high temperature can lead to surface cracking and a dramatic increase in the scattering intensity. Similar work by the UKC group I171 is not yet published. There is clearly much scope for more extended work in this field on differing absorbants and absorbents under varying conditions.
Fig.7: Contrast-matching of water in porous silica by variation of isotopic composition in (H20/D20) mixtures
SUMMARY AND PROJECTION OF FUTURE WORK The opportunities for the use of SAS and USAS techniques have become attractive in recent years but relatively little work has yet been done. The high intensities of modern X-ray and neutron facilities can be exploited in various ways,
254
making use of a Q-range suitable for the particular problem under investigation. Several specific developments deserve mention in this context:routine measurement of pore distribution functions and changes i) due to sintering or any other external variable; direct observation of capillary condensation and study of spatial ii) distribution of fluids; iii) anisotropic studies in oriented samples such as fibres and layered materials. iv) timeresolved I(Q) measurements to study pore-filling processes, liquid flow or displacement by viscous fingering; study of complex interfaces and surface texture by complementary v) measurements with both X-rays and neutrons; The development of the scattering method will, in some cases, require an extension of the existing theoretical framework for interpretation of the measurements. It will also enable checks to be made on the simplifying assumptions inherent in the interpretation of the data obtained from other less direct means of investigation. The next few years should see a dramatic increase in the use of this technique for a wide range of materials considered at this meeting and it is to be expected that substantial developments will have been achieved for presentation at
COPS m. ACKNOWLEDGEMENTS We wish to thank various people who have contributed to this work through provision of facilities, computational knowledge or specific samples; they include Josb Teixeira (LLB), Wim Bras (SRS), John Harries (SRS), Martyn Stacey (ICI), Peter Hall (Schlumberger), Peter Langer (Schott Glass), Carole Perry (Brunel University) and others. ANN would like to thank the SERC for financial support that made this work possible.
255
REFERENCES 1. 0.Glatter & O.Kratky, 'Small angle X-ray scattering', Academic (Pub) 1984. 2. A.Kostorz, p.227, in 'Treatise on Materials Science and Technoloy', Vo1.15, Neutron Scattering, Academic (pub), 1979. 3. H.Bale & P.W.Schmidt, Phys.Rev.Lett., 1984, 53,596 4. J.Teixeira, in H.E.Stanley & N.Ostrowski (eds) 'OnGrowth and Form' Martinus Nijhoff (pub) 1986, p145. 5. D11 and D17 instruments: 'Neutron Research Facilities at the ILL High Flux Reactor', B.Maier (ed). 6. PACE and PAXE instruments, Orphke Reactor,Lab.Leon Brillouin, C.E.N., Saclay A.N.North et al., Nuc.Inst. & Methods, 1988,834,188. 7. 8. A.N.North, J.C.Dore, A.RMackie, A.M.Howe and J.Harries, Nuc.Inst. & Methods, in press (1990). 9. C.Zeyen & E.Davis, private communication. 10. A.North, in M.C.Fairbanks, A.N.North and R.J.Newport (eds). 'Neutron and X-ray Scattering: Complementary Techniques', Adam Hilger (pub), 1989.~.181. 11. S a m s e provided by C.Perry, Brunel University. 12. M.Stacey, this meeting D.F.R.Mildner, R.Rezvani, P.L.Hal1 and R.L.Borst, Appl.Phys.Letts., 13. 1986,48,1314. 14. J.Ramsay, this meeting. 15. J.-C. Li, M.J.Benham, L.D.Howe and D.K.Ross, p.155, in M.C.Fairbanks, A.N.North and R.J.Newport (eds), 'Neutron and X-ray Scattering: Complementary Techniques [as ref.101. 16. J.C.Dore and A.N.North, unpublished data.
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F. Rodriguez-Reinosoet al. (Editors), Characterization ofPorous Solids I1 0 1991 Elsevier Science Publishers B.V., Amsterdam
257
GEL-PRECIPITATED OXIDE GELS WITH CONTROLLED POROSITY - DETERMINATION OF STRUCTURE BY SMALL ANGLE NEUTRON SCATTERING AND ADSORPTION
ISOTHERM MEASUREMENTS. J. D. F. Ramsay, P. J. Russell and S. W. Swanton Colloid Chemistry Section, AEA Industrial Technology, Harwell Laboratory, Didcot, Oxfordshire, OX1 1 ORA, United Kingdom. SUMMARY In the gel precipitation process hydrous oxide gels in a polymer matrix are formed by precipitation (eg. as spheres, fibres etc.) by controlled neutralisation of a metal salt solution (eg. Th(IV), AI(III), Zr(IV), Ti(1V) etc.) containing a water soluble polymer (eg. polyacrylamide). On immediate precipitation such gels are markedly porous (c-0.90).Surface and porous properties of the dried gel may be controlled and are determined by the method of dehydration. The displacement of water by a partially miscible solvent (butanol) results in little shrinkage whereas drying in air results in a marked contraction of the gel structure. The surface and porous properties of dry gels have been characterised by nitrogen adsorption isotherm measurements and the evolution of the structure from wet to dry gels has been studied by small angle neutron scattering (SANS). Using a model mixed oxide system containing zirconia and titania, which are similar chemically but have a large difference in scattering length density, the properties of the inorganic oxide phases (size, surface area, homogeneity) and the polymer have been studied by SANS by the contrast variation technique. INTRODUCTION The process known as gel precipitation was developed at Harwell as a route to mixed oxide ceramic nuclear fuel (ref. 1). The process, which is outlined in the flow-diagram of Fig.1, differs from conventional sol-gel routes to oxide ceramics by the incorporation of a high molecular weight water soluble polymer (eg. polyacrylamide, polyvinyl alcohol). The role of the polymer is very important in the precipitation process. Firstly it allows considerable control in the shape (spheres, fibres) of the gel which is produced on precipitation of the aqueous metal salt solution (eg. Th(IV), U(VI), AI(III), Zr(IV), Ti(1V) etc.) in the external basic medium. Secondly it has a marked effect on the porosity of the gel. Thus on immediate precipitation the gel formed is markedly porous (porosity, 00.90). The surface and porous properties of dried gels are determined and may be controlled by the method of dehydration. Thus displacement of water with a partially miscible solvent (such as as a short chain alcohol) results in little shrinkage of the gel whereas drying by evaporation in air leads to a marked contraction of the gel structure. Our technical interest in gel precipitation stems from the potential to produce porous oxide adsorbents with controlled pore structure. Furthermore mixed oxide gels can be produced with high homogeneity.
258
METAL SALT SOLUTION
POLYMER SOLUTION
Droplet formation
I
Precipitation in conc. NH,
I
Washing (water)
Solvent displacement
DRY GEL SPHERES
Dry in air
DRY GEL SPHERES low porosity
Debonding
I POROUS CERAMIC SPHERES 1 I S int er ing
I DENSE
CERAMICS
I
Fig.]. Chemical flow diagram for the gel-precipitation route to single- and mixed-metal oxide microspheres of controlled porosity. The process has seen considerable technical development but more limited study of the basic mechanisms involved. Little is known about the development of the structure during the early, wet stages of the process and in particular the role and interactions of the polymer. This problem is being addressed by the application of small angle neutron scattering (SANS). SANS is particularly suitable because the penetrating power of neutrons makes it possible to study gel microstructure in the wet state. In addition using contrast variation techniques (refs. 2-4) it is possible to study the structure of the individual components in the gel (eg. polymer and oxide phases) and to examine the homogeneity of mixed oxide gels. We report here SANS measurements made at Harwell on zirconia, titania and mixed zirconia/titania gel systems formed in the presence of polyacrylamide. Zr(1V) and Ti(IV) are similar chemically but their oxides have appreciable differences in scattering length density. Measurements have been made on wet (undried) gels and gels that have been dehydrated directly
259 in air and by organic solvent displacement. Contrast variation experiments have been made by exchanging wet gels and rewetted dried gels with appropriate H,O/D,O
mixtures. These
experiments have been complemented by measurements of gas adsorption isotherms from dehydrated gels. THEORY OF SMALL ANGLE SCATTTERING Small angle scattering arises from variations in scattering length density, p , which arise over distances d,,
(d,,,
- A/28) in the range 1-100 nm such as may arise in dispersions of colloidal
particles and porous solids for example. The fundamental equation relating the small angle scattering intensity as a function of the scattering vector, Q, (where Q
=
4 r sin $/A, and 28 is the
scattering angle) to the structure of the scattering inhomogeneities for a statistically isotropic system is (ref. 5)
where K is an experimental constant. q2 is the mean square fluctuation in scattering density and the function -y(r) contains all the information from the effects of the form (size, shape) of the heterogeneities and their mutual arrangement. Although separation of this information is difficult, precise interpretations can be obtained from -y(r), in particular for two-phase systems such as porous media. For a two-phase system with sharp interphase boundaries
where
v2 is given by
4, and 9, are the respective volume fractions of the two phases with scattering length
densities p, and p z , and (p,-p,) is the contrast. For the gel systems considered here certain generalisations can be made regarding the dependence of I(Q) on Q. Thus for aggregated systems which have fractal properties it can be shown that I(Q) scales with an exponent corresponding to the fractal dimension, D, (refs. 6-8) namely UQ)
- Q-D
(3)
The value of the exponent relates to the mechanism of formation of the aggregates and the range over which the power scaling occurs to the size or extent of the cluster. Thus for the process of diffusion limited aggregation (DLA), D has a value of 2.5. Such a scaling effect relates to the mass fractal dimension and arises for a dimension in reciprocal space (Q) between the size of the cluster, a,, and the primary particles, a2 (see Fig.2). At higher Q it can be shown that for a
260
/
Range of fractal self similarity
log
Region”
+:-
Q
Fig.2. Schematic representation of a particle aggregate (a) having a range of self-similarity between approximately al and a2. The form of the scattering is depicted in (b).
two-phase system with sharp interphase boundaries the scattering in the limit of high Q is dependent on the surface area, S, of the system and obeys a Q-4 power law:
This
Q-4
scaling of the intensity at high Q is described as the Porod law region and arises when
Q p 4 , where r refers to the half dimension of the scattering inhomogeneity eg. a pore. EXPERIMENTAL Materials Gel-precipitated spheres were produced by the method outlined schematically in Fig.1. Feed solutions were prepared by mixing equivalent volumes of 4% polyacrylamide solution in formamide/water with aqueous solutions of zirconium nitrate and or titanium chloride (total metal concentration 0.8 mol dm-’). Feed droplets (-1 mm diameter) were produced by pumping the feed solution through a vibrating jet as shown in the photograph, Fig.3a. The droplets gelled (retaining their integrity as individual spheres) and metal hydrous oxide precipitation occurred on immersion in concentrated ammonia solution. The gel spheres were washed repeatedly with water (or ammonia solution in the case of titania) to remove salt. Batches of wet gel were divided into three, one portion being retained and another dried by evaporation in air. The third was
261
Fig.3. Photographs showing (a) rapid drop formation from a vibrating jet, and (b) the resultant oxide gel spheres after dehydration. [Courtesy of Harwell Laboratory]
dehydrated by solvent displacement: the aqueous phase was exchanged repeatedly with butanol and the solvent was removed by subsequent evaporation in air. The uniform size and highly regular spherical shape of dried gel spheres are illustrated in Fig.3b. Samples of the gel spheres (either wet or dry) were transferred to silica cuvettes (path length 1 mm) for SANS measurements. For contrast variation studies the dry gels were rewetted with H,O/D,O mixtures of the required composition and the supernatant aqueous phases of the wet
and rewetted samples were exchanged with the appropriate H,O/D,O mixture repeatedly to attain the required isotopic composition. Small annle neutron scattering Measurements were made at a wavelength, A, of 6 A using the multidetector SANS spectrometer installed in the PLUTO reactor at Harwell Laboratory (ref. 9). Data were analysed using standard programs to normalise for detector efficiency, and correct for sample self-absorption and background contributions.
262
Adsorotion isotherm measurements Nitrogen adsorption isotherms at 77 K were measured volumetrically using a Digisorb 2600 (Micromeritics Instrument Corporation). Dried gel samples were outgassed at ambient temperature for approximately 16 hours. Specific surface areas, SBET,and pore volumes, Vp, were calculated in the standard manner. Mean pore radii, rp, were derived from the desorption branches of the isotherms from the maxima of pore size distributions computed on the basis of the Kelvin equation, using a cylindrical pore model as previously (refs. 10-12).
RESULTS AND DISCUSSION Adsorotion isotherms The method of dehydration has a marked effect on the surface and porous properties of the dried gels. This is illustrated by the nitrogen adsorption isotherms in Fig.4 for zirconia gels which have been dried in air (a) and by solvent displacement (b). The isotherm for the solvent displacement dried gel is almost Type I1 in character which is typical of structures composed of an assembly of particles with a very open packing (ref. 10). This feature is demonstrated by the very high uptake at saturation, which corresponds to a considerable porosi:y (00.90) in the gel and the large pore size (see Table 1). In contrast the isotherm for the air dried gel is Type IV in character, and has features which indicate a gel with a considerably reduced porosity where the size of the pores are approaching the micropore range (52 nm). This is illustrated by the marked reduction in the uptake at saturation and the shift of the hysterisis loop to much lower pressures. Indeed the restricted size of the hysterisis loop shows that the isotherm is almost reversible and therefore approaching Type I behaviour
-
which is typical of a volume filling process in a microporous
solid. Such behaviour is an indication that marked shrinkage has occurred leading to a highly compact assembly of very small particles The marked differences in specific surface area,,,,,S Vp and rp depending on the method of dehydration are listed in Table 1.
TABLE I Surface and porous properties of zirconia gels dehydrated by different routes from nitrogen adsorption isotherms. ~~
Drying method
SBE,/m2 g-'
Mean pore radius/nm
~
~
Pore volume/cms g-'
Air dried
130
52
0.09
Solvent displacement dried
320
22
1.76
~~
263
100~00
I
I
I
I
I 0.02
I
1
I
0.05
0.10
0.2
10.00
1.00
0.10
0.01
'0
m 0.25
0.5
0
I
1.0
0.75
Fig.4. (Above left) Nitrogen adsorption isotherms at 77 K for zirconia gels. (a) Gel dehydrated by evaporation in air. (b) Gel dried after water displacement with butanol. Open and closed symbols represent adsorption and desorption respectively. Fig.5. (Above right) SANS from zirconia gels. (a) 'Wet' gel in water, 0 , (b) butanol displacement dried gel reimmersed in water, 0 , and (c) gel dried in air then reimmersed in water, 0. N.B. Data normalised for equivalent zirconia concentration.
As an example of the scattering data obtained, Fig.5 compares the scattering from dried zirconia gels after evaporative and solvent displacement drying reimmersed in water with that measured from the original wet gel. The scattering curves have been normalised to take account of different sample transmission and metal concentrations. At low Q the scattering from the wet gel (0)
shows a power law scaling of I
Q
Q-"'';
such behaviour is typical of a fractal aggregate
system. Similar scattering behaviour is shown by the rewetted solvent displacement dried gel
(0)
indicating that there is little microstructural change occuring during solvent displacement drying.
In contrast the marked macroscopic contraction of the wet gel on drying in air
(0) is
accompanied
by a significant reduction in scattering intensity and a change in the shape of the scattering curve indicating a considerable microstructural change. Also at higher Q (>lo-'
A-') the scattering
264 5.0
.'
L.0
3.0
2.0
k
1.0 0-0 -1.0 -2.0 -3.c
Fig.6. SANS contrast variation results for solvent displacement dried gels of (a) titania, (b) an equimolar mixture of titania and zirconia and (c) zirconia. Intensity corresponds to Q/A-' of 2.5~10'~. eventually tends towards Q-4 behaviour
- suggesting
that the structure is composed of very small
primary units ( ~ 3 0A). The scattering in Fig.5 indicates a reduction in surface area to about 40% of that of the wet gel on air drying. This reduction in surface area is consistent with the changes in,,,S
measured by nitrogen adsorption listed in Table 1 .
Contrast variation studies Other investigations (eg. XRD, EXAFS) indicate that the structure of the oxide phase is amorphous. This is of interest in the context of mixed oxide systems for the preparation of homogeneous gels and the homogeneity of gels at the microscopic level is being investigated by SANS using the contrast variation technique. This is illustrated in Fig.6. for experiments using
solvent displacement dried titania and zirconia gels and an equimolar mixed titania-zirconia gel immersed in H20/D20 mixtures. Fig.6. shows the square root of the intensity measured at a fixed scattering angle plotted against the isotopic composition of the aqueous phase. For each gel system the plots are reasonably linear over the range of contrasts (except for the zirconia system close to
its contrast match point as discussed below) and we note that the mixed oxide gel has a scattering length which is intermediate between that of the single component gels which indicates that the gels are homogeneous and can be regarded as two-phase systems (ie. solid and pores) over the scale length, d,,
s 20 A,
measured. This can also be concluded from the SANS behaviour (not
shown here) of the mixed oxide which shows similar shape throughout the range of contrasts.
265
We note that the contribution to the scattering from the polymer component of the gels is weak compared to that from the oxide component. Only in the case of the zirconia gels close to the match point of zirconia (-95% D,O), which is also of greatest contrast to the scattering length density of the polymer, would we expect a significant contribution to the total scattering from the polymer. It is this which causes the anomalously high intensities close to the match point of the zirconia gels. ACKNOWLEDGEMENT This work was undertaken as part of the Underlying Research Programme of the UKAEA. We would like to thank Mr S. J. Wilkinson for experimental assistance with the SANS measurements. REFERENCES 1. 2. 3 4. 5. 6. 7. 8. 9. 10. 11. 12.
B. Stringer, P.J. Russell, B.W. Davies and K.A. Danso, Radiochimica Acta, 36 (1984) 31. J.D.F. Ramsay, Chem. SOC.Rev., 15 (1986) 335. J.D.F. Ramsay, R.G. Avery and L. Benest, Faraday Discuss. Chem. SOC., 76 (1983) 53. J.D.F. Ramsay and R. G. Avery, this meeting. A. Guinier and G. Fournet, Small angle scattering of X-rays, Wiley, New York, 1955. T.A. Witten and L.M. Sander, Phys. Rev. B, 27 (1983) 5686. P. Meakin, Phys. Rev. A, 27 (1985) 1495. S.R. Forrest and T. Witten, J. Phys. A, Math. Nucl. Gen., 12 (1979) 109. D.I. Page, Atomic Energy Res. Estab. Rep. AERE-R9878, 1980 R.G. Avery and J.D.F. Ramsay, J. Colloid Interface Sci., 42 (1973) 597. J.D.F. Ramsay and B.O.Booth, J. Chem. SOC.Faraday Trans. I., 79 (1983) 173. B.O. Booth and J.D.F. Ramsay in: J.M. Haynes and P. Rossi-Doria (Editors), Principles and applications of pore structural characterisation, J.W. Arrowsmith, Bristol, 1985, p.97.
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F. Rodriguez-Reinosoet al. (Editors),Characterization of Porous Solids I1 0 1991 Elsevier Science Publishers B.V., Amsterdam
267
SMALL-ANGLE NEUTRON SCATTERING STUDY OF FUMED SILICA POWDER COMPACTION ALAN J. HURD1v2, GREGORY P. JOHNSTONZ, and DOUGLAS M. SMITHZ lSandia National Laboratories, Albuquerque, New Mexico 87185-5800 (USA), 2Center for MicroEngineered Ceramics, University of New Mexico, Albuquerque, New Mexico 87131 (USA) ABSTRACT In a previous study of fumed silica by mercury porosimetry (ref. l), we established an inverse dependence of the powder compressibility n on applied pressure P, heralded by power-law differential volume vs pressure curves. Independent small-angle neutron scattering (SANS) measurements, undertaken to establish microstructure, indicated a decreasing zero-angle intensity, I(q+O), with increasing sample compression. Preliminary analysis suggested that I(q+O) is proportional to the compressibility n . Since it is well known that I(q-+O) is proportional to n for systems in thermodynamic equilibrium, our results implied an analogous relation for systems in mechanical equilibrium. However, the preliminary study was incomplete, lacking scattering data for a wide range of compactions. In this paper, we have tested this relationship, in the low pressure regime, by correlating the scattered neutron intensity from samples in various degrees of compaction. We find that, while the intensity does decrease with increasing compaction (after removing the effects due merely to greater sample density) as expected according to the compressibility analogy, the dependence is not the same as that found by mercury porosimetry. Lightly compressed fumed silica has been studied as a percolating system by several groups (refs. 2-4); our results pertain to states of greater compaction. INTRODUCTION Previous studies (refs. 1.2) of fumed silica compaction have established an empirical "equation of state" between powder pore volume and applied pressure,
The compressibility, which is defined as
&
P
-
1 -
dV ~
V
dP
1
dV
V
dP
= -A
(2)
is therefore inversely proportional to pressure. Here V, the total volume of the powder, is comprised of compressible pore volume V, silica volume V,:
V=V,,+V,.
and incompressible
The scattered intensity at zero angle I(q+O)
might be expected to be proportional to
n
by analogy to thermalized systems.
In fact, preliminary scattering studies on compacted fumed silica powders demonstrated that I(0) decreases with increasing compression. we attempt to test the relationship
In this paper
268
which, if true, would imply a straightforward statistical mechanics for powder behavior. Thus, it should be possible to understand the empirical equation of state in Eq. (1) on a statistical basis.
Our study was motivated by interest
in the pore space surrounding the fractal powders as they are forced together. To illustrate what we mean by a statistical understanding of the state of the powder, we refer the reader to Edwards and Oakeshott (ref. 5).
If the
particles are sufficiently numerous and the local construction rules welldefined, then the macroscopic properties of the powder should be predictable and interesting.
The essential idea is that all configurations consistent
with mechanical stability are equally probable, but that for the overwhelming majority of these states the measurable properties are essentially the same. Thus, we should be able to predict, for example, the volume of a heap of sand. Unfortunately, it is necessary to understand the role of energy in our powder system in order to develop a calculus for the pressure-volume equation of state and the suppression of density fluctuations; we hope to provide these insights with future experiments.
The present study is a first step toward
that goal. Fumed silica is "glass soot" made by burning silicates in flames.
It is
known to be mass-fractal particles, i.e. submicron-sizeaggregates composed of random, weakly branched strings of 100 A silica spheres, with a great deal of internal porosity. Since the limiting small-angle intensity can be related quite generally to the fluctuations in the sample, I(0)
a
2
4 N > = <(N
- UC-)2>
(4)
the relationship being tested here is the proportionality
I
v
'a
? ci
2
<6N >
(5)
ap
The probability W(N,V,A,. . . ) of having N particles in a given total volume V must be sharply peaked at for large N.
W depends on some number of
extensive parameters A , . . . , which remain unspecified in our discussion of powders; for now we assume that a single function A suffices to encapsulate
269 our ignorance.
By expanding W in the neighborhood of its peak at N 4 > , we
can express the width, or fluctuation in N, in terms of the curvature in A.
W(N,V,A,. . . ) = W()
+
1 2 a2A (N-) - I 2 aN2 N-+>+
' '
(By stability arguments, the linear term in the expansion must be zero and the sign of the A-curvature must be negative.)
Thus, the Gaussian width-squared
is 2/(a2A/aN2) and is equal to 2<6N2>. But adding particles to a constant volume is equivalent to compressing a sample of constant mass; hence, it is actually the variation in A with volume that matters. -(aA/aV),
In thermodynamic systems, A is the (Helmholtz) free energy and
is the pressure, so
a2A
2
----..- 2 aN2 <6N >
v2 N~
ap -
av
(thermodynamic systems). (7)
Thus, in thermodynamic systems, Eq. ( 5 )
follows directly from the key
relation between free energy and pressure.
For powder compaction, it must
again be an energy argument, but we have yet to identify the fate of the mechanical energy put into the system. EXPERIMENT Eight samples of Cab-0-Sil (grade EH-5, Cabot Corporation) fumed silica were prepared in closed aluminum cells with 3 mm path length.
The cell was an
aluminum cylinder, 1 . 2 6 cm inside diameter, with 1 mm thick aluminum windows pressed into each end.
Densities ranged from 0,038 g/cc to 1 . 2 g/cc; the
loose powder density (see below) is even less than that of sn aerogel. Small-angle neutron scattering was performed at the Manuel Lujan Los Alamos Neutron Scattering Center (LANSCE). 0.05
The useful wave vector range was
< q < 0.16 A-1 after correction for scattering from an empty cell. The
high sample densities (1.2 g/cc) were done at the Missouri University Research Reactor (MUM). ANALYSIS Figure 1 shows the measured sample density as a function of l/a, where a is a compression factor defined by a-V/V, with V the compressed sample volume and V, the volume o f loose powder that was compacted. By least-squares fitting to p=po/a, we found po=0.0296 g/cc (loose powder density).
270
Q
9 0
2.0
0.0
4.0
80
60
1 /.
Figure 1.- Densities of compressed fumed silica samples. In the very low pressure regime, we noticed that the scattering curves all had the same form but different amplitudes.
Each curve was divided by
p
and
by sample thickness to correct for the effects of scattering mass; this brought the very low pressure curves into coincidence, proving that very little structural difference existed between these samples; the higher density samples did, however, exhibit deviations at low q as seen in Figure 2 .
We
compared the total scattered intensity between curves by dividing each curve by a "reference curve" f(q)
in order to leave only amplitude information.
f(q) was formed by simply averaging the six lowest density curves and is shown in Figure 2 with two other representative curves. Finally, the lowest-q intensity datum from each normalized curve was taken as our approximate I(0)
.
(rather than attempt a dangerous extrapolation to
0) and plotted in Figure 3 . The abscissae are VP., where V, is the pore q volume (calculated from the density) and V, is the silica volume (calculated from the mass). For low densities (V,/V, > lo), the intensity was found to be constant when normalized in the above manner indicating no significant interparticle interference accessible to the
SANS.
At higher densities,
however, I ( 0 ) was found to drop as particles packed closer together and scattered more coherently.
271
0.001
0.01
0.1
9 Figure 2.- Scattering curves of compressed fumed silica. scattering curve f(q) p-0.306
for very low density
g/cc; (c) p=1.00 g/cc.
samples p C 0 . 0 9
(a) Average g/cc;
The denser samples scatter less intensity at
small angles.
0
1
(b)
10
100
VdVS
Figure 3.- Approximate zero-angle intensity vs. normalized pore volume.
272 [We note that the large-q data in the raw scattering curves do not quite approach a Porod asymptote (slope of -4), as noted previously (ref. 6), indicating a somewhat rough primary particle surface.
Since the curves
coincide at large q (after correcting for scattering mass),
the interfacial
area is unchanged with compaction except at the highest densities, when it decreases.
We infer that chains in the aggregate do not break to form new
surface area (or, if they do, an equal amount of surface area is annihilated simultaneously).] Using n
a
1/P from Eqs. (1) and (2). we would expect
I(O)
a
P-'
a
v3
P
Instead we observe (for V,/V,
a
vY P
v
=
0.75 f 0.25
(9)
We conclude that Eq. ( 3 ) does not hold for our powder samples. The failure of Eq. (3) does not mean that it has no ergodic analog for fumed silica powder, but merely that the large scale fluctuations measured by Eq. (4) do not control the compressibility in the same way as in thermal systems. ACKNOWLEDGMENTS The scattering experiments were done at LANSCE (Los Alamos Neutron Scattering Center) and at MURR (Missouri University Research Reactor).
We are
indebted to Phil Seeger (LANSCE) and David Mildner (MURR) for their expert help, to Peter Pfeifer for helpful discussions about pores, and to Don Stuart for technical assistance. This work was supported by Sandia National Laboratories under DOE Contract DE-AC04-76-DP00789and the UNM/NSF Center for MicroEngineered Ceramics. REFERENCES
1. 2. 3.
4. 5. 6.
D. M. Smith, G . P. Johnston, and A . J. Hurd, J . Colloid Interface Sci., in press. F. Ehrburger and J. Lahaye, J. Phys. France 5 0 , (1989) 1349. J. Forsman, J. P. Harrison, and A. Rutenberg, Can. J. Phys. 65, (1987) 767. J. M. Heintz, F. Weill, and J. C. Bernier, Mater. Sci. and Engin. m, (1989) 271. S. F. Edwards and R.B.S. Oakeshott, Physica D 3 8 , (1989) 8 8 . A. J. Hurd, D. W. Schaefer, D. M. Smith, S . B. Ross, A. Le Mehaute, and S . Spooner, Phys. Rev. B 2 (1989) 9742.
F. Rodriguez-Reinosoet al. (Editors), Characterization of Porous Solids II 0 1991 Elsevier Science Publishers B.V., Amsterdam
THE DETERMINATION OF PERMEABILITY AND BINARY COEFFICIENTS IN NOVEL FORMS OF POROUS CARBONS
273
GAS
DIFFUSION
S.B. BHOWMIK,' S.P. WALDRAM,' R. McMURRAY' AND S.R. TENNISONa 'Chemical and Biochemical Engineering, University College London, G t Britain. 'BP Research, Sunbury Research Centre, G t Britain. SUMMARY Permeabilities and binary diffusion coefficients for helium/nitrogen mixtures These are prepared from are reported for samples of carbon materials. primary particle sizes between 50 and 250 pm by a novel technique which allows the pore size distribution of the carbon to be tailor-made for particular duties. The reported diffusivities correspond to tortuosity factors between 1.7 and 3.0. Permeabilities fall in the range of 2 to 16 x lo-'' m ' . INTRODUCTION The bi-modal porous carbons were prepared using the route shown in Figure 1, (ref. 1). The commercial Novolak resin precursor was carefully pre-cured so that it was no longer thermoplastic but still retained significant capacity for further cross linking. This was characterised by a relatively low glass transition temperature. The partially cured resin was then ground and sieved to produce a series of particle cut sizes, < 50 pm, 50-100 pm, 150-200 p m and 200-250 pm. The powdered material was mixed with additional hexamethylene tetramine cross linking agent and placed in glass test tubes. Further cross linking within the particles and chemical bonding between the particles then took place a s the sample was heated to 150'C.
PHENOLIC R E S I N CARBON PRCIDUCTION ROUTE
I
NOVOLAK
RESIN
I
1
Figure 1. route
Carbon production
Figure 2. SEM of porous carbon (50 p m primary particle size)
274
The porous phenolic resin artefact was then removed from the tube and carbonieed in a nitrogen atmosphere to 9OO'C over a period of 18 hours. During this process the resin lost approximately 50% by weight and shrank axially and radially by approximately 30%. The resultant carbon had a uniform three dimensional macroporosity, characterised by the interconnected voids This gives rise to a between the sintered primary particles, Figure 2. macropore distribution that is directly related to the initial resin particle size: this is illustrated by the pore size distributions determined by mercury porosimetry, Figure 3. The total macroporosity remains approximately constant a t 50%, see column 4 of table 1. Total porosity is also relatively constant at between 60 and 64%. Samples A2 and D2 fail to follow this trend because of the presence of undesired large scale macroporous flaws. PHENOLIC R SIN CARBON MACROPOR STRUCTURE 7MERCURY Porn i METRYE) 7 6
5 .? -
n
-
0
4
Q
3
v
zp
2 1
0
.5
.75
1
1.5
l.25
1.75
2
Log (Mean Pore Diameter -pm) Particle Size -TF
Figure 3.
<50um
+ 50-100um
Q
150-200um
+ 200-250um
Pore size distribution of macropore a s determined by mercury porosimetr y
THEORY Stimulus-reswnse experiments. transfer functions and s y s t e m moments Consider the system illustrated in Figure 4.
SYSTEM
Figure 4.
Stimulus response test.
L e t the transfer function relating fout(s) to fin(s) be G ( s ) and consider the response of the s y s t e m to a dirac delta function. Then fin(s) = 1 and,
275
1.
2.
where an represents the nth unnormalised moment of fout(t) about the origin and is defined by equation 3. a, =
]
t” f o u t ( t ) dt.
3.
0
From equation 2 it is plain that G ( s ) may be considered in s, the coefficients of which are moments of t h e system Note that the eyetem mean M 1 is equal to a l / u o and that if in the form of equation 4 then expressions for a. and u 1 are
4.
1 G(s)
=
(a
+
bs
+
a s a power series impulse response. G ( s ) is expressed a s specified in 5.
cs’ + ds’ +
*.
)
These, and other, relationehipe are reviewed in more detail, (ref. 2). Gibilaro and Waldram discuse further details of stimulus-response experiments including the property of step response curves illustrated in Figure 5, (ref. 3).
1
SHADED AREA
=
dl
STEP RESPONSE TIME Figure 5.
The step response curve and its properties which enable system moments to be determined.
Permeation exDeriments When a pressure differential is imposed across a porous medium of constant cross-sectional area, the convective, or permeation, gas flux through the medium is given by equation 6. 6.
216
For small changes in total pressure this can be integrated directly to give:
7.
where L is the length of the sample and PL and pR are the pressures applied to its end faces. The permeability, %, of the sample can be evaluated by rearranging equations 6 or 7 and determining all other quantities in these equations by independent means. These are loosely referred to a s the "derivative" and "integral" methods for finding BO respectively. Steady- and unsteady-state diffusion exmriments. Effective diffusion coefficients can be measured using a Wicke-Kallenbach type cell, (ref. 4), Figure 6.
Figure 6.
Wicke-Kallenbach diffusion cell geometry.
Descriptions of alternative techniques which may be used to determine effective diffusion coefficients in porous particles have been presented together with an analysis of the relative merits of each, (refs 5,6,7). The s y s t e m illustrated in Figure 6 can be modelled with a system of partial and ordinary differential equations. Implicit in this model are the following assumptions: i) ii) iii) iv) v)
Volumes V L and V R are perfectly mixed
No tracer enters V R in the inlet stream of flow rate qR There is negligible mass transfer resistance adjacent to the flat end faces of the pellet The diffusion tests are isobaric Adsorption can be neglected.
( i ) Mas6 balance on tracer in volume VI 8.
277 jii) Mass balance on tracer in porous Dellet
5 dxz -
Jc with
C = CL at x = 0 C = CR at x = L
9.
C = 0, a l l x a t t = 0
(iii) Mass balance on tracer in volume VD 10
Note that diffusion fluxes through the pellet are strictly non-equimolar so that inlet and outlet flow rates to a given chamber are not identical. Experimental conditions are chosen such that this is a minor effect and the assumption of constant values for qL and qR introduces negligible error. Equations 8 to 10 inclusive may be solved by Laplace transformation or other methods, (refs 8, 9). When a step change in inlet concentration of trace species is made to the left hand chamber, responses of the form illustrated in The final steady values of C L and C R enable the Figure 7 are observed. effective diffusion coefficient to be determined from steady state data whilst the specific shape of the transient in c L or CR enables the effective diffusion coefficient to be found from transient data.
A
CL
Helium
conoentratlon In d l outlet flow streams
CR
Y
Tlme. t Figure 7. Typical step response curve from the Wicke-Kallenbach cell. When the step change in trace species is made to the left hand side of the cell the shape of the response on the opposite, i.e. right hand side, is more dependent on the pellet properties than that on the left hand side: the latter tends to be dominated by the mixing characteristics within volume VL. The transfer function relating C,(S) to Cln(s) can be shown to be of the form given in equation 11. cR(S)
9L
I
[ D+s
J
Csch@ a 1
where a =
11.
After considerable tedious algebraic manipulation this can be expressed in the form of equation 4 and the system moments may thus be determined from the relationship of 5.
278 a.
=
1
= Y (Fig. 7)
12.
For pellets in which the total porosity is not totally continuously interconnected, but rather has some dead ended porous structure, the effective diffusivities determined by these two means may be expected to differ. EXPERIMENTAL Each sample of carbon was pressed into a tightly fitting piece of soft, flexible silicone tubing. Purpose built gas distributors housing miniature pressure transducers were then inserted into t h e tubing to form a Wicke-Kallenbach type diffusion cell. The distributor and cell are shown in Figures 8 and 9 with the diffusion cell of Figure 9 being of t h e form illustrated in Figure 6 and described theoretically in t e r m s of equations 8, 9 and 10.
Figure 8. Gas d i s t r i b u t o r housing miniature pressure transducer. Outside diameter is 18 mu.
Figure 9. Assembled diffusion/permeation cell containing the porous cerbon sample t o be tested.
EXPERIMENTAL RESULTS Pellet Permeability The pressure transducers w e r e calibrated and t h e readouts adjusted so they had a matched sensitivity of 19.48 mv/cm of H,O. This is equivalent to a sensitivity of 1370.87 mv/psi. The first carbon sample was assembled in the test cell and flow rates qL and q R adjusted so that PL and PR w e r e exactly balanced. The flow rate q R was then set at a slightly higher, or lower, value and t h e steady value of qL measured a t t h e cell outlet using calibrated rotameters. This flow rate is augmented, or diminished, from that value a t exact pressure balance by the permeation flux through the pellet. Approximately ten measurements were taken for each pellet with imposed 20 N/mz. Typical results are shown in pressure differentials between 0 and Figure 10. A least squares f i t of a second order polynomial was made through the data. Curvature was mild, but most pronounced for the m a t e r i a l s composed of the largest particle sizes. Each data point associated with figure 10 is taken a t known values of P L and PR so that equation 7 can be applied directly to find B,, since all other parameters are known. This is referred to a s the integral method of finding Bo and gives a value for every data point: the mean value of Bo and the standard deviation of these results can then be found.
279 An alternative is to use the least squares fit to Figure 10 to evaluate dp/dx a t AP = 0 and to find & from equation 6. This is referred to a s the differential method and yields a single value for Bo with an averaging over all data points being implicit in the least squares fit. The results calculated by these two methods for all eight pellets are shown in table 1.
SIZE: 150-200 MICRONS SRMPLE 2
u
-
.9
0
:.a 5
2
.7
-
.6
LL
u
+ .5 0
' .3-28
-15
-10
-5
0
5
10
15
Dlfferentlal P r e s s u r e ( m v )
Figure 10. Volumetric flow rate, q L , a s a function of the differential pressure across the test cell. 150-200 p m material, sample 2. Effective binary gas diffusion coefficients in pellet samples Flow rates qL and qR to the test cell are adjusted 80 that there is no pressure differential across the pellet, i.e. PL = PR. A rapid step change in is then made whilst holding flow rates and inlet gas concentration C,, pressures constant. Concentration transients in CL and CR (as illustrated in Figure 7) are measured using microkatharometera. Both outputs of these are sampled at 50 Hz and the data recorded in the HP 9826 computer, see Figure
11. The microkatharometers have been previously calibrated a t the appropriate gas flow rates and applied bridge voltage so that the specific gas concentrations corresponding to the final steady values in C L and CR, see figure 7, are known. In all the diffusion tests reported the binary gas system was He/Na and the step change in concentration was from 0 to 5 mol percent helium. The relatively small change in composition ensures that non-equimolar, and hence non-isobaric, diffusion effects are minimised. The steady state effective diffusivity, De, can be found from equation 14 when dc/dx is written a s (cL-cR)/L
Diffusive flow through pellet =
- D,& &
14.
dx Unsteady state estimates of De are found from the step response in cR using the area relationship of Figure 5 and equation 13. All parameters in 13 are known BO that the unsteady state diffusivity may be determined directly. All diffusivity measurements are reported in table 1. CONCLUSIONS The total porosities of the carbon samples were in the range of 60 to 72%, the high values for samples A2 and D2 being due to undesired macro voids formed during sintering and carbonisation. Steady and unsteady state diffusivities both fell with increasing primary particle size. A t larger particle
280
HEWLETT PACKARD 9826 COMPUTER AND A TO D CONVERTER
r--------------
I
I
C2M)Hzl
t
I I
PRINTER
ROTAMETER,
ROTAMETER,
. 1 1 F'ROWCTION
I Figure 11.
Line diagram of diffusion and permeation apparatus
Density
3
1 2
B
50-1OOpm 1
2
C
150-200pm 1
]
1.890
]
1.859
]
1.904
200-250pm 1 2
]
Bulk
0.715
.62
0.523
.72
Steady
Unsteady
Integral
Slate
State
Method
4
-
.64
.25
.26
3.1 St. Dev
.25
.24
3.8
st. Dev
3.5 St. Dev
21
L
D
-
2
1
A<50pm
BO x 1012/m2
E
--Skeletal
Permeability
>orosity
p ig cm-3
Sample
Ne He/N2 BLEND
AND REOULATORS
-
0.23
3.2
0.56
3.8
0.30
35
0.16
2.4
0.719
.61
.52
.19
.18
2.3 SI. Dev
0.758
.60
.52
.I4
.I6
5.6 SI. Dev
=
0.40
5.7
0.736
.61
47
.I4
.I7
7.3 St. Dev
=
0.87
7.2
0.699
63
.M
.15
.I9
13.0 SI. Dev
2.7
11.1
0.679
.64
.69
.I7
.I8
16.2
st. Dev = 2.8
14.9
-
-
1.885
Tablc 1
-
Evperimcnlal Results
I
Determined from Helium F'ycnometry
2
Determined from Mercury immersion
3
Determined from skeletal and bulk densities
4
Determined from Mercury porosimetry
281
sizes there w a s evidence of some dead-ended pore structure in that the effective diffusivity determined by unsteady state testing techniques is greater than that found by steady state experiments. Under the conditions studied the binary gas diffusion coefficient for He/Na is 0.71 so if all diffusion w e r e in the bulk mode these results would correspond to tortuosity factors in the range of 1.7 to 3.0. The assumption of bulk diffusion is in fact poor as u p to 8% of the total pore volume is associated with pores less than 100 nm. Apart from sample D the permeabilities determined by integral and differential methods agree to within 4%. The differential method is preferred as this allows the permeability to be found by interpolation, at conditions where the imposed pressure imbalance is infinitesimally small. A s the primary particle size is increased by a factor of more than 8 the permeability rises by a factor of only 5. This is significantly less than the squared dependency on primary particle size which t h e Blake Kozeny equation predicts, (ref lo), but is in agreement with the variation of the volume average pore diameter revealed by mercury porosimetry. The high permeabilities of samples A2 and D2 a r e probably reflections of the macro-voids referred to in the paragraph above. These permeabilities a r e comparable with those found for packed sand and are a s much a s 104 t i m e s greater than in strong porous compacts produced by pelleting in tabletting machinery. ACKNOWLEDGEMENTS Permission to publish this paper has been given by the British Petroleum Company plc. The permeability and diffusivity measurements w e r e made a t University College London a s part of a British Petroleum Research contract. REFERENCES 1. European patent EP 254551 (pending). 2. S.P. Waldram Non-ideal flow in chemical reactors. Chapter 6, vol. 23, Comprehensive Chemical Kinetics Ed C.H. Bamford et al. Elsevier 1985. 3. L.G Gibilaro, S.P. Waldram, Chem. Eng. J., 4, 1972, 197. 4. E. Wicke, R. Kallenbach, Z. Kolloid, 97, 1941, 135. 5. L.G. Gibilaro, S.P. Waldram, J. of Catalysis, 67, 1981, 392. 6. D.L. Cresswell, N.H. O r r , in "Residence t i m e distribution theory in chemical engineering", Eds A. Petho, R.D. Noble, Verlag Chemie, 1982. 7. P.E. Bower, M.P.DudukoviE, P.L. Mills, S.P. Waldram, I. Chem. E Symposium series No. 87 (ISCRE 8 ) , 1984, 9. 8. A. Burghardt, J.M Smith, Chem. Engng Sci., 34, 1979, 267. 9. M.P DudukoviE, Chem. Engng Sci., 57, 1982, 153. 10. R.B. Bird, W.E. Stewart, E.N. Lightfoot, Transport Phenomena, Wiley, 1960. NOMENCLATURE ( D ~ / ~ ) K, see equation 11. a Coefficients of s, see equation 4 a,b,c,d A C r o s s sectional area Permeability B, Concentration of diffusing species C C(5) Laplace transform of c De Effective diffusion coefficient fin(t) Forcing function fout(t) Response Laplace transform of fout(t) fou t(s)
ms-x
-
ma
mz mol mol m-3 s ma s-'
282 G(s)
L P 9
R S
t T V X
Y
System transfer function Pellet length Pressure Volumetric gas flow rate Ideal gas constant Laplace variable Time Temperature Volume Length variable in pellet Steady state value of CR, see figure 4
Greek an c P P
r
SubscriDts in out L
R P
nth unnormalised moment about the origin Pellet porosity Viscosity Density Mean residence t i m e inlet outlet left chamber of cell right chamber of cell permeation (on q), pellet (on r)
F. Rodriguez-Reinosoet al. (Editors), Characterization of Porous Solids II 0 1991 Elsevier Science Publishers B.V., Amsterdam
283
PORE-SIZE ANALYSIS FOR PERMEABILITY ESTIMATION IN POROUS MATERIAL
.
T SAT0 Dept. of Civil Engineering, Gifu university, Yanagido, Gifu 501-11, JAPAN
SUMMARY Hydraulic conductivity, which is one of the important soil parameters for seepage analysis, is estimated from the pore-size distributions measured by a mercury intrusion porosimeter. Two different relations are indispensable for analysis of partial saturation soil because hydraulic conductivity depends on the capillary potential, which is usually characterized by soil-water retention. The network model is developed for estimating the hydraulic conductivity and soil-water retention taking into account the spatial differences of the pore size. The model is checked with published laboratory tests. It is clear that the model demonstrates the hysteresis between the wetting and drying, and shows good similarities of the relative hydraulic conductivity with the laboratory values.
INTRODUCTION Soil mechanics is the application of the laws of Mechanics and Hydraulics to problems dealing with soil(ref. 1). soil
is
an
the
important problem because the soil mass behavior
interactions between seepage
The seepage flow through
the soil grains and pore water.
unsaturated
depends
The parameters
analysis require moisture retention as well as hydraulic
on
the
for
the
conductivity,
since hydraulic conductivity depends on water saturation which also relates to capillary suction.
The moisture retention is usually described by the relation
between the water saturation and capillary potential. Several relations.
different types of The
models have been proposed for estimating
most common method is to carry out laboratory
soil, but some problems still exist. the as a
steady-state condition. a
tests
Testing is too long, if carried out
In addition, special equipment is
gamma beam for measuring the saturation under
the
those
using
required,
transient
a
under such
condition,
glass or ceramic filter to protect air intrusion into the water supply
units
and so on. This
study
forcuses on the application of the pore-size
measurements
for
284
predicting the soil properties of unsaturated soil. are used in the pore size measurement.
Two different types of soil
The measurement is performed by the
use
of the standard commercial item of the mercury intrusion porosimeter. The through
of
network model is a useful tool for computating the pore-water movement the soil while taking into account the pore-size measurement.
computating is the same for the wetting and drying
values
process.
The
The
of the relative hydraulic conductivity and water retention are
with the laboratory values obtained from
checked
the instantaneous profile(ref. 2 )
the controlled air pressure methods(ref. 3).
way
computed
More work stills needs to be
and done
for application to real problems, but it is clear at this stage that the network model
indicates the hysteresis of the water retention curve during wetting
and
drying, and shows a good similarity of the relative hydraulic conductivity with the laboratory values.
CONVENTIONAL WAYS FOR HYDRAULIC CONDUCTIVITY There
are various kinds of the conceptual models for estimating
conductivity, taking clustered
into
structure
of
the
soil
into
account the pore size
soil.
The models
three types from the mathematical treatment of as
shown in Table 1.
laboratory values on water retention. conductivity without conductivity
of
hydraulic
knowledge
All
these
are
models
soil-water
retention.
The
hydraulic hydraulic
of the partial saturation depends on water saturation, which
relates to the capillary potential.
It is nessesary to establish two
for the modelling of pore-water movement through partially saturated
S I N G L E CAPILLARY T U B E M 3 D E L BUIWLE OF CAPILLARY T U B E
M3DEL PROBABILITY M 3 D E L
i
Averjanov(ref.4) Irrnay(ref.5) Burdine( ref. 6 ) Sinclair(ref.7) Laliberte(ref.8)
Childs-coil is G e o r g e ( r e f . 9 ) Marshall(ref.10) Jackson(ref.11) G r e e n and C o r e y ( r e f . 1 2 ) Carnpbell(ref.13) Mualern(ref. 14) Mualern and D a g a n ( r e f . 1 5 )
also
relations soil.
TABLE 1. Models of hydraulic conductivity for partially saturated soil
I
are pore
requiring
Therefore, they cannot predict
of
the
285 TABLE 2. Models of soil-water retention characteristics
Three
G R A I N SIZE
S e n o ( r e f . 16)
PORE SIZE
N a k a n o ( r e f . 17)
CURVE FITTING BASED ON MEASURIEMENT
K i n g ( r e f . 18) Brooks and Corey(ref.19) Rogowski(ref.20) Kroszynski(ref.21) Van Genuchten(ref.22)
different
approaches have been proposed for
calculating the
water
retention
curve as shown in Table 2. The third approach is the fitting method,
in
the
which
pensable.
laboratory test values on the soil-water
overcome, since they
indisto
spatial
be
cannot indicate the hysteresis between the wetting and the
The soil-water retention usually shows a strong hysteresis because
drying. the
retention are
The first and second are unique, but there are still problems
differences of the pore size distribution, so
these
models
of
are
not capable of predicting the hysteresis.
NETWORK MODEL APPLICATION The
model
predicts Fig. 1
in this study takes into account the
both
is
the
the
spatial differences, and
hydraulic conductivity and water retention.
case
of
vertical one-dimensional flow.
The
The
model
model
has
in two
components. The branch plays the role of the water conduit and the node that of the water storage. from
the
assigned
usual to
It is hard to separate the two different types of pore PSD(Pore Size Distribution) curve. The same pore
size
radius
each component according to the PSD curve, which is given
is
by
pore-size measurement by the porosimeter. The computation is based on the
the Monte
Carlo method. The procedures are as follows: (1)Assignment of
the number of nodes and branches to the model. The
model
of
this study has 100 branches horizontally and 10 vertically. (2)Assignment of
the
pore radius to each branch and
node
according
to
the
(3)Determination of the potential domain by giving a capillary potential to
the
probability given from the PSD curve by the porosimeter. model. (4)Selection of the location for water intrusion according to random variables. (5)Search the
flow path in which the branch has smaller pore radius
than
the
286 preceding advances
during wetting, and acount of the
number of branches the pore
the number of the trial times, in which water
and
water
perfectlly
passes
through the model. (6)Selection of the location for the air intrusion according to the same manner as in step (4). (7)Investigation of the flow path in which the air perfectly passes through
the
model during drying. (8)Computation in the same manner as in Step
5).
(9)Repetition of the procedures from (2) to
8).
Each trial has the
same
curve, and the pore radius is assigned to the components according to the
PSD
Monte
Carlo method.
PORE-SIZE MEASUREMENTS
Two different types of soil are used for the pore-size grain
measurements.
size distributions are shown in Figure 2. The sand, which
sand, has a narrow grain size which varies from 74 to 420um.
is
The
The
Toyoura
silty
clay
shows a wider grain size than the sand. Figure 3 shows a histogram of the pore-size distribution of the sand. It hard to make a sand lump specimen. lump for
Thus, the sand is mixed with clay to make
the pore-size measurement. These effects
distribution
in
Fig. 3 .
There are two clusters.
appear
in
The cluster
the
of
a
pore-size
the
radius reflects on the pore size composed of the sand, and the smaller
is
larger
reflects
on the clay mixed with the sand.
INFLOW
Fig. 1. Network model for estimating pore water movement through soil.
287
The pore radius assigned to each component of the network model is generated random numbers from 0 to 1 according to the probability density from the
the pore-size measurement.
pore size is important in this model.
The water saturation and
conductivity computed depend on the spatial distribution of the pore The
pore
radii
larger than 3 0 h , which can not be
intrusion method,
given
The numerical assumption of the occurrence of
measured
by
hydraulic radius.
the mercury
are approximated to appear at the same probability
as
301m
given by the PSD curve of Toyoura sand.
- 100
-
././--
P
E
e
w
60 -
3
2
60
i
-
a GRAIN SIZE
Fig. 2.
(mm)
Grain size distribution of the soils for pore-size measurement by the mercury intrusion method.
0
APPARENT PORE RADIUS
Fig. 3.
(urn)
Pore size distribution of Toyoura sand.
288 NETWORK RESULTS The model
soil-water
retention of the sand is
The
shown in Figure 4.
computations, displayed as solid lines, indicate
network
curves for wetting and drying, and the effect shown by the laboratory values are displayed as circles. of
tests:
measuring the
obtained from two different
instantaneous profile method,
using
the
gamma
the saturation and (b)the controlled air pressure method
condition
dashed for
The laboratory values have been (a)the
different
of the constant capillary potential in the soil
for
for making
specimen.
The
line shows the Burdine model, based on the bundle of the capillary
tube
estimating
the
hydraulic conductivity.
The
water
retention curve
directly estimated from the cumulative pore-size distribution curve. was
types
beam
The
was
curve
divided into the same intervals with reference to the cumulative intrusion
volume.
The water saturation is given by adding the intervals, in order,
the minimum pore radius.
from
Using the average pore radiustr) of the interval, the
capillary potential(p) is computed as p=O.l5/r(cm). It
is
hard
predictions.
to
compare
the
laboratory measurements with
quantitative difference between
A
laboratory values can be seen.
the
network
the
model
model
and
The laboratory tests employed the sand specimen,
but the pore-size measurement was performed for the specimen of
sand mixed with
clay because the sand has no cohesion to form a lump. The network entry
critical
capillary head of the sand is estimated to be 97.7cm
model, and 25cm in the laboratory values.
value of the sand.
It is also clear that
in
the
This coincides with the
air
the differences in
the
grain
size distribution has an effect on it. The
other
important relation between the
saturation is displayed in Figure 5. of
the
hydraulic
conductivity and
The horizontal axis is the relative
hydraulic conductivity which is the ratio of the value of
saturation to
that of the fully saturation.
The value was
the
estimated
value
partial by
the
proportion of the trial times, in which water reaches the exit end, to the total trial number. The
network model
displayed as
circles.
shows a good similarity with
the
laboratory values
The Burdine model tends to give a smaller value
relative hydraulic conductivity than the network model due to a neglect of spatial difference of the pore size.
for the
289
5
0 0 Laboratory Values
Burdine Model Network Model -
4
3 LL
n
2
1
0
0
20
40
80
60
SATURATION
100
(%)
Fig. 4. Measured and computed curves of soil-water retention of Toyoura sand. (00:controlled air pressure method, ++:instantaneous
profile method)
-
-
0 0 L a b o r a t o r y Value
Burdine Model Network Model
20
0
40
60
SATUFWTION Measured
Fig. 5. (
and
0:controlled
computed
values
80
100
(Oh)
of
air pressure method,
-
relative 0
hydrau lic
conductivity.
:instantaneous profile method)
290 CONCLUSION The
soil
properties, affecting the pore-water movement
saturated domain, network
model.
hydraulic
are
In
the
measurement
No mathematical model exists that
can
partially
through
The model can estimate the soil-water retention as
conductivity.
properties.
estimated from the pore-size
in
well
compute both
addition, no model exists indicating the hysteresis
wetting and drying.
the as
between
The network model also predicts the hysteresis in the water
retention. The
network model
still has some problems
to
overcome.
The
numerical
assumption of the probability of pore-size occurrence has to be improved. difficult
It is
to compute the soil-water retention near the full saturation because
the limitation of the mercury intrusion measurements. It has been said
that
the mercury intrusion method can be applied to the measurement of the pore
size
of
less
than 1OOm.
An effort has to be made to obtain an
approximate pore-size
distribution more than 1 0 0 ~ . The
application
properties
to the soil mechanics was limited to estimating
relating to the pore-water movement through the
partial
the
soil
saturation
domain, and there are some other problems the model can be applied to.
REFERENCES 1 K. Terzaghi, Theoretical Soil Mechanics, John Wiley and Sons, 1943. 2 I. Kono, and M. Nishigaki, An Experimental Study on Chracteristics of Seepage through unsaturated Sandy Soil, JSCE, Vo1.307, pp.59-69, 1981(in Japanese). 3 T. Uno, T. Sato, T. Sugii and H. Tsuge, Method of Test for Permeability of Unsaturated Sandy Soil with Controlled Air Pressure, JSCE, Vo1.418, pp.115-124, 1990(in Japanese). 4 Y. Mualem, Hydraulic Conductivity of Unsaturated Porous Media, Water Resour. Res., Vo1.14, pp.325-334, 1978. 5 W. Brusaert, Some Methods of Calculating Unsaturated Permeability, Trans. ASAE, Vol.10, pp.400-404, 1967. 6 N.T. Burdine, Relative Permeability Calculations from Pore Size Distribution Data, Trans. AIME, Vo1.198, pp.71-78, 1953. 7 L.R. Sinclair, D.W. Fitzsimmons and G.L. Bloomsburg, Permeability of Unsaturated Field Soils Calculated from Laboratory Desaturation Data, Trans. ASAE, Vo1.17, pp.399-405, 1974. 8 G.E. Laliberte, R.H. Brooks and A.T. Corey, Permeability Calculated from Desaturation Data, ASCE, IR.94, pp.57-71, 1968. 9 E.C. Childs, An Introduction to The Physical Basis of Soil Water Phenomena, John Wiley and Sons, 1969. 10 T.J. Marshall, A Relation between Permeability and Size Distribution of Pores, J. Soil Sci., Vo1.9, pp.1-8, 1958. 11 R.D. Jackson, On The Calculation of Hydraulic Conductivity, Soil. Sci. SOC. Amer. Proc., Vo1.36, pp.380-382, 1972. 12 R.E. Green and J.C. Corey, Calculation of Hydraulic Coductivity, Soil Sci. SOC. Amer. Proc., Vo1.35, pp.3-8, 1971. 13 G.S. Campbell, A Simple Method for Determining Unsaturated Conductivity from Moisture Retention Data, Soil Sci., Vo1.117, pp.311-314, 1974. 14 Y. Mualem, A New Model for Predicting The Hydraulic Conductivity of
291 Unsaturated Porous Media, Water Resour. Res., Vo1.12, pp.513-522, 1976. 15 Y. Mualem and G. Dagan, Hydraulic Conductivity of Soils, Soil Sci. SOC. Amer. Proc., Vo1.42, pp.392-395, 1978. 16 M. Seno, A Study on The Soil Structure based on The Energy Concept of The Soil Water, Trans. AESJ, Vo1.14, pp.ll-15, 1965(in Japanese). 17 M. Nakano, The Relation between Soil Water and Suction, Trans. JSIDRE, Vo1.35, pp.1-9, 1971(in Japanese). 18 L.G. King, Description of Soil Characteristics for Partially Saturated Flow, Soil Sci. SOC. h er. Proc., pp.359-362, 1965. 19 R.H. Brooks and A.T. Corey, Properties of Porous Media Affecting Fluid Flow, ASCE, IR92, pp.61-88, 1966. 20 A.S. Rogowski, Watershed Physics; Model of The Soil Moisture Characteristic, Water Resour. Res., Vo1.7, pp.1575-1582, 1971. 21 U. Kroszynski, Flow in A Vertical Porous Column Drained at Its Bottom at Constant Flux, J. Hydrology, Vo1.24, pp.135-153, 1975. 22 M.T. vanGenuchten, A Closed-Form Equation for Predicting The Hydraulic Conductivity of Unsaturated Soils, Soil Sci. SOC. Amer. Proc., Vo1.44, pp.892-898, 1980.
This Page Intentionally Left Blank
F. Rodriguez-Reinoso et al. (Editors), Characterization of Porous Solids I1 0 1991 Elsevier Science Publishers B.V.. Amsterdam
THE EFFECTS OF PORE AND PARTICLE GEOMETRY ON NMR DIFFUSION MEASUREMENTS IN ADSORBED LIQUIDS
S. Bahceli: A.R.S. Al-Kaisi,K. Krynicki and J.H. Strange Physics Laboratory, University of Kent, Canterbury, Kent, CT2 7NR, UK.
SUMMARY Nuclear magnetic resonance is sensitive to molecular mobility and local magnetic fields. Motion is modified in liquids contained in confined geometry. Local magnetic field variation results from susceptibility effects at interfaces. Both phenomena are observed for liquids contained in porous solids. This paper critically examines these effects and their use in characterisation of porous materials. The principles are illustrated with porous silicas and preliminary results are given of diffusion measurements on n-butane in silica as a function of temperature and pore geometry.
INTRODUCTION Nuclear magnetic resonance (NMR) provides a powerful method for the study of molecular motion. The techniques can distinguish molecular reorientation and translation and have proved particularly valuable for the study of self-diffusion in bulk liquids. The molecular motion of liquids in the confined geometry provided by their containment in porous materials has been of considerable interest for many years. It is of importance both as a fundamental scientific problem and because of its technological importance in such diverse systems as oil recovery from rocks and catalytic agents. The purpose of this paper is to question the reliability of many previous investigations and the validity of their interpretation. Potential sources of error are demonstrated by measurements on mobile liquids adsorbed into porous silicas with different geometrical characteristics. The principles illustrated are equally valid for other porous systems. Preliminary measurements of the diffusion coefficient of nbutane in silica as a function of temperature and the effect of pore dimensions are presented. *Department of Physics, Faculty of Science, Ankara University, Turkey.
293
294
NMR LINEWIDTHS The NMR linewidth or its inverse, the transverse relaxation time T2, is often taken to reflect the extent of molecular motion. The extreme line narrowing observed in mobile liquids is a result of the rapid motion of the molecules. It has frequently been reported that the linewidth of liquids contained in porous media is greater, sometimes by orders of magnitude, (1) than that observed in the bulk liquid. This has often been taken as evidence of reduced molecular mobility in the adsorbed state. While mobility may, in fact, be reduced we believe that the line-broadening often observed is a characteristic of the porous material and depends on the pore size and shape and the grain size and shape. This effect can, in principle, be used to characterise the porous media. It is a result of the heterogeneity of the sample and the variation of the magnetic susceptibility between the confined liquid and its surroundings. The nuclear magnetic resonance frequency, wo, of a sample placed in a magnetic field Bo can be represented by the relation
where y is the gyromagnetic ratio of the resonant nuclei. Bloc is a local magnetic field seen by the nuclei and is due to its environment. In mobile liquids this field is usually very small, but in heterogeneous samples a variation in Bloc arises due to the fact that the internal field inside a magnetically polarizable material depends on the shape of the sample. The local field within a thin cylinder of material, for example, will depend on the orientation of the cylinder's axis to the polarising field. Thus, in fluid contained in pores which are randomly oriented, the local field will vary spatially according to the pore geometry, the pore filling factor, and the difference in magnetic susceptibility between the liquid, gas and porous medium. Similarly, in grains of porous material, the local field will depend on the shape and orientation of the grains and the difference in susceptibility between the grains and their surroundings. Of course, for isolated spherical grains the value of Bloc due to the grain approaches zero whereas for irregular shaped grains (and pores)
295
x
where a is a geometrical factor of order unity and is the magnetic susceptibility of the fluid. Nuclei in a sample will thus have a distribution of resonance frequencies and therefore an NMR linewidth given by a B,. The effect is clearly demonstrated for butane and for cyclohexane absorbed in various porous silicas with pore sizes of 60A, 90A and 140A and grain dimensions from lop to 1 5 0 ~ .The grain shapes varied from the highly irregular (David silica gel supplied by Aldrich Chemical Co. Ltd.)to a highly uniform spherical shape (Spherisorb Slow marketed by Phase Separations Ltd.). The latter produced a much narrower Nh4R line than the former. Linewidths were found to be proportional to the applied magnetic field Bo as expected for susceptibility broadening. An illustrative set of results is given in table 1.
x
TABLE 1 I
I
I
60A l50g Irregular
Pore size Particle size Particle character
9 0 i 10M Spherical
140i 1.50~
Irregular
I Cyclohexane
Wane
axBo
observed
axBo
n.Butane
n-Butane
observed
aXBo
observed
aXBo
1 at
45a
7.05
49.8a
3.5
45a
10.7
60 MHz
2.56
1Oa
1.64
Il.la
0.94
IOa
2.0
I Oa
18.2 MHZ'
0.82
3.la
0.48
0.24
3.la
0.72
3.111
270 MHz
Average a
12.2
a
0.26
.
3.4a
a = 0.14
a = 0.08
45a
a = 0.22
Equating observed linewidths with a X Bo gives coefficients a which depend on grain shapes and vary from 0.1 for spherical to 0.26 for irregular shapes. The pore dimensions were those quoted by the suppliers (Davisil for 60 A and 140A; Phase Sep for 90A) and had been determined by BET Nitrogen analysis and mercury porosimetry. Grain sizes were obtained by sieving. * Extrapolated from high frequency data. The pore dimensions used here are small, changing direction and shape in about lOOA. This distance is small and the frequency as,with which molecules sample the various local fields due to molecular diffusion is large compared to the spread in resonance frequencies (y B1oc) arising from the pore geometry. This source of local field variation is therefore averaged to zero for these experiments. The grain sizes (10 to 100pm) are, however, sufficiently large that molecules sample the various local fields due to this source at a rate that is much less than
296
y Bloc due to grain geometry and the grain characteristics provide the NMR line-
broadening mechanism in our samples. DIFFUSION MEASUREMENTS The nuclear magnetic dipoles precess at the Larmor frequency wo determined by the magnetic field that they experience. A sample placed in a magnetic field gradient, G, will have a distribution of frequencies. If nuclei move randomly in the field gradient, phase memory will be lost. It is the application of this principle that is now well established as an NMR method for the measurement of the self-diffusion coefficient, D, (2) and it manifests itself as a reduction in the amplitude of a spin echo occurring at time 22 after an initial 90" excitation pulse in the NMR 9002 18002 echo experiment. The echo amplitude is given (21, in the simplest experiment, by
e
E=E,exp- -+G*D23
(3)
where other relaxation is ignored. This method is simple to apply in bulk liquids. In the heterogeneous systems described here two basic problems arise and both tend to produce systematic errors giving D values that are too high. This may well account for some reports that diffusion in porous media is apparently enhanced with respect to bulk material (3). The first source of error is in the accurate knowledge of G, the actual magnetic field gradient as seen by nuclei. In addition to the external gradient there is the internal distribution of magnetic fields arising from the susceptibility effects (4) described in the previous section. In our measurements of D for cyclohexane and butane in porous silica, we use a Larmor frequency of 18.3 MHz. We estimate the variation of magnetic field within a silica grain to be of order XB, where X is the volume susceptibilityof the fluid and is approximately8 x 10 -6 for both cyclohexane and butane, giving a X B, 4 10 pT. Actual values for various values of wo are given in table 1. The variation over particles that are 100pm in size at 10 MHz produces a background magnetic field gradient of about 10 mT m-1. For lOpm particles the gradient could be of order O.1Tm-1 which is comparable to the values of G normally applied in spin echo measurements. An error in G used in equation (3) arising from this source can result in D being grossly overestimated. This probably accounts for the high value of D shown in figure 1 for butane in lop size particles of porous silica.
291 I
I
I
0 (0.17)
Fig. 1: The temperature dependence of the self-diffusion coefficient D of n-butane adsorbed on porous silica of various grain size (gs) and pore size (ps):o - gs 10pm, ps 9 nm; 0 - cgs> 150pm, ps 14 nm; A - 150 pm, ps 6nm. The bracketed numbers indicate weight coverage. The dotted line gives D values for pure butane.
If the diffusion coefficient and X of the absorbed liquid are known, the echo experiment can provide information on the grain and pore characteristics. The background field gradient calculated from the observed local magnetic field distribution and the known grain dimensions is compared with the effective gradient measured by the spin echo experiment (assuming the self-diffusion coefficient of the liquid is that of the bulk liquid) in table 2. The actual gradients with the grains for spherisorb are less than estimated but larger for the irregular shaped grains of the silica gel (Aldrich). This is probably due to the regular (spherical) shape of the spherisorb grains (figure 2). A second source of error is that, in the time of the experiment (1 to 10 ms) the molecules of a typical liquid whose D = 4 x 10-9m2s-1 diffuse a distance
298
a
b
Fig. 2. Silica grains photographed using an optical microscope (a) Spherisorb (b) silica gel. b
A
Bulk diffusion
0
a0
110
15c
Coverage Z
Fig. 3. Apparent self-diffusion coefficient of cyclohexane C6 H12 adsorbed on silica, 90 A, ~Op(gs)as a function of coverage.
299
TABLE 2 Table 2 Estimation of Background (susceptibility) field gradient Gradient due to grain a ax Bo
n-Butane
+ grain
Spherisorb (d = 1011) Gradient w T m - 1
in
dimension, d.
Silica Gel (d = 1 5 0 ~ ) Gradient pT m - * ~~
aXBo 5 d Experimenta! gradieni from linewidth due to Bloc at 18.3 MHz
24
Effective Field Gradient from Diffusion Experiment
4.4 x 103
I
s
103
17
103
103
I
d ex*> = d6Dt or 5 to 15pm. Molecules reaching the surface of the adsorbed liquid can enter the gas phase and diffuse rapidly. For small grains most molecules will spend a significant time in the gas phase during the experiment since the diffusion distance is longer than the grain dimension and again diffusion is apparently enhanced. This effect has been recognised previously (3) and can be greatly reduced by over-filling pores slightly to block the evaporation. When this is done D values obtained are significantly lower than obtained for partly filled pores, see figure 3. The effect is much more pronounced at high temperatures where diffusion is faster and for small grains as shown by the results in figure 1. Having identified these sources of error in D measurement we are now embarking on a study to obtain reliable diffusion data for liquids in porous silica and as shown in figure 1, our preliminary results indicate that self diffusion for liquid butane in porous silica is reduced by the confined geometry, by 20% in 140 A silica and 50% in 60A silica. This is in qualitative agreement with the conclusion of experiments on other liquids in porous glass (5). Activation energies are similar to the bulk liquid (1.6k cal mol-1) as shown by the parallel temperature dependence in figure 1.
300
REFERENCES 1.
2. 3. 4. 5.
R. T. Pearson and W. Derbyshire; J. Colloid and Interface Science64 232, (1974). J. Karger, H. Pfeifer and W. Heink, Adv.Mag.Res., 12,1, (1988). F. DOrazio, S. Bhattacharja and W.P. Halperin; Phys.Rev.Letter, 43, (1989). D. Zamir, R.C. Wayne and R.M. Cotts; Phys.Rev.Letter, 327, (1964). J. Karger, J. Lenzner, H. Pfeifer and H. Schwabe; J.Am.Cer.Soc.,& 69, (1983).
a
F. Rodriguez-Reinosoet al. (Editors), Characterization of Porous Solids 11 0 1991 Elsevier Science Publishers B.V., Amsterdam
301
PORE SIZE ANALYSIS OF WET MATERIALS VIA LOW-FIELD NMR
Douglas M. Smith and Pamela J. Davis UNM/NSF CENTER FOR MICRO-ENGINEERED CERAMICS, University of New Mexico, Albuquerque, NM 87131 USA ABSTRACT
Conventional pore size analysis techniques require the use of dried materials but many materials undergo drastic structural rearrangement during drying (i.e., before analysis). The use of low-field NMR spin-lattice relaxation measurements is demonstrated for pore structure analysis for a number of wet solids including silica gels and ion exchange resins. INTRODUCTION
Most pore structure analysis techniques (adsorption/condensation, mercury porosimetry, TEM/SEM, etc.) are not appropriate for "wet" materials since they require the removal of Dore fluid before analysis. Since drying the sample can induce significant, irreversible changes (and is often a topic of study in its own right), a "nonintrusive" technique is required. The ability to monitor pore structure changes during materials processing would be of great utility for many materials. In general, changes during processing have been inferred from the pore structure of the final dried material. However, chemistry and structure often continue to evolve during drying and the interpretation of how a parameter affects the final pore structure is not straightforward. The few studies of pore structure evolution during processing use either scattering (SAXS, SANS), thermoporometry, NMR relaxation, or magnetic resonance imaging (MRI). BACKGROUND
Scattering has primarily provided information on nucleation and growth mechanisms in solution and/or the structure of the final dried material. The use of scattering for in-situ pore structure analysis suffers from limited length scales (1-20 nm, SAXS only), contrast problems, relation of results to pore size, multiple scattering, and
302
errors resulting from desmearing. However, the approach is quick, allows extraction of all length scales at once, and accesses closed porosity. Thermoporometry provides pore size information from comparison of melting and solidification thermograms (ie., the freezing/melting temperature of pore fluid is a function of pore size) [1,2]. This approach is useful for determining pore size distribution with pores in the size range of 1.5 to 150 n m but suffers from several limitations regarding its use. These include the fact that the pore fluid must be very pure (requires multiple washing which can change structure], is nonisothermal, requires a pore shape assumption to relate temperature to freezing point, suffers from network/percolation effects, and the volume changes associated with phase change can significantly affect the structure of the sample. In addition, the nature of the thermoporometry experiment precludes the continuous study of change in a single sample as it undergoes processing. The use of low-field NMR has been suggested as a pore structure tool and offers advantages [3,4]including the use of the existing pore fluid as the probe, a large pore size range (4nm to > 10 pm), the lack of percolation effects, and a pore shape assumption is only required for pores smaller than several nm. This technique is well suited for in-situ studies of gel structure as it is non-intrusive in the sense that the pore fluid is used as the probe, high purity fluids are not required, and the temperature is held constant. Pore size and surface area information are obtained from the fact that fluid near a surface will undergo spin-lattice (Ti) and spin-spin relaxation (Ti) at a faster rate than for the bulk fluid. From the two-fraction, fast exchange model, the measured Ti or T2 is related to the pore size by [51:
where the pore size, rp, is the hydraulic radius (2PV/As). The physical model associated with Equation 1 is illustrated in Figure 1. When the pore volume is large as compared to the surface area, (i.e., for pore size larger than 3-5nm) the volume of the surfaceaffected phase is small and the pore volume to surface area ratio is obtained directly from Equation 1. For smaller pores, assumptions concerning pore geometry
303
and the thickness of the surfaceaffected phase are required [61. The thickness of this surface-affected phase is typically 0.3 +/- 0.1 nm.
4 1
Bulk Fluid
I
U Pore Wall
Schematic diagram of pore fluid during a NMR experiment.
Figure 1
From relaxation measurements of fluid in the pores and the bulk fluid, the pore size maybe obtained if the surface interaction parameter,
p, is known. p is found by either
performing a series of relaxation experiments on partially saturated samples with different moisture contents [7] or by performing relaxation experiments on samples with submonolayer fluid coverage to directly obtain the surface relaxation time [81. For a porous solid, a distribution of relaxation times exists which must be extracted from the measured magnetization relaxation data. This requires the solution of Tlmax M(d = MO
[1-2~p[-dT111f[T11 dT1
(2)
Tlmin where M(T) is the measured magnetization at different delay times, 2, MO is the equilibrium magnetization, and f[T1] is the desired distribution of relaxation times which is directly related to the pore size distribution via Equation 1. Equation 2 may be solved bj a number of approaches such as the method of regularization [9].
EXPERIMENTAL Silica gels were prepared from tetraethyl orthosilicate using a two-step, basecatalyzed scheme described by Brinker and co-workers [lo]. This system was selected since it yields a fairly broad pore size distribution in both the initial(wet) and final (dried) states. The pore size distribution and surface area were determined durine
304
processing using a 20 M H z NMR and a 1800-~-900 pulse sequence as described elsewhere [3]. Before drying, samples were aged under various conditions including washing the samples with ethanol to remove the mother liquor (a mixture of water, ethanol, and unreacted TEOS),aging with various pH fluids (water and KOH), and aging in mother liquor for extended time. Samples were dried at ambient conditions for 1 week and then at 383 K. In addition to silica gels, commercial materials such as phase separated Vycor glass (Corning Glass) and ion exchange resins (Dow Chemical) were studied. For NMR pore size analysis, the Vycor was saturated by placing samples in an evacuated chamber and partially filling the chamber with ethanol such that only ethanol vapor contacted the sample. Ion exchange resins were both analyzed "as is" as well as after washing in distilled water. In addition to NMR,some samples were analyzed by thermoporometry using water as the pore fluid and a heating rate of 0.5 K/min. To convert the thermograms to pore size distributions, the method described by Eyraud et al. 121 was employed. N2 adsorption/condensation (77K)was used to obtain surface area [Spoint BET analysis (0.05
RESULTS AND DISCUSSION A significant part of pore structure characterization via NMR relaxation measurements is the determination of the surface-interaction parameter, required to relate relaxation time to pore size (see Eq. 1).
p, which is
p is a function of temperature,
fluid, surface chemistry, and field strength. As the field strength (and proton frequency)
p increases leading to greater sensitivity to pore size. Assuming that temperature and field strength are fixed, p for a given fluid-porous solid decrease, the magnitude of
combination can be found via several approaches. For high surface area materials, one can dry the sample sufficiently such that the magnitude of the surface relaxation time and the thickness of the surface-affected phase may be measured directly. Alternatively, one can measure relaxation on an unsaturated sample as the fluid content changes (effectivelychanging the ratio of bulk to surface-affected phases). If one knows the total surface area of the sample (for example, from nitrogen adsorption), a plot of 1/T1 versus
305
the product of Mv (mass of solid per volume of fluid) and surface area should be linear with a slope proportional to p. This type of plot is illustrated in Figure 2 for Vycor glass. In general, we have found that that the value of
p does not vary greatly (i.e. greater than
50%) for a given fluid and a wide range of solids if the solids do not contain a significant
concentration of paramagnetic impurities. With
p known, the pore size distribution
(PSD) may be calculated via the solution of Equation 2. The NMR-derived PSD as well as nitrogen condensation results (adsorption and desorption) are included in Figure 3 for Vycor. As expected, the NMR PSD exhibits a slightly broader distribution and a mean pore radius which is approximately 50% larger than the adsorption branch. This is a result of two factors: the skewing of condensation results to smaller pore size as a result of networklpercolation effects and the fact that NMR obtains the hydraulic radius (i.e., twice the pore volume to surface area) which only agrees with the condensation pore size when the sample contains uniform, smooth cylindrical pores.
0
1000
M v As
2000
3000
Figure 2 Variation of relaxation time with moisture content for Vycor.
306
1
10
100
r (nm) Figure 4 Pore size distribution change during drying of a B2 silica gel. Also included is the PSD of the dried gel by nitrogen condensation and NMR. The synthesis of ceramics via sol-gel processing is an area for which the ability to study pore structure in-situ is of great practical importance. By changing processing conditions, the PSD of a wet gel--
be changed significantly to either change further
processing steps (e.g., drying) or the structure and properties of the final dried gel. How the pore sue distribution changes during drying is illustrated in Figure 4 for a base-
307
catalyzed silica gel as it dries over a one week period. After complete drying at 383 K, the sample was analyzed via N2 condensation and subsequently resaturated for NMR analysis. During the initial stages of drying, the large pores disappear as the wet gel shrinks and the entire gel remains saturated. As drying continues, the matrix eventually stiffens such that the vapor-liquid menisci penetrate the gel. From weight loss and volume measurements, we calculate this to occur at approximately 75% solvent loss. During this final stage of drying, a large decrease in pore size is noted as a result of the large capillary forces in pores less than 10 nm. The N2 and NMR results for the dried gel show good agreement. Often, the pore fluid in a wet gel is changed to modify the pore structure before drying. Larger pores in the wet gel imply lower capillary forces during drying and hence, faster drying rates without cracking and larger pore size in the final dried material [ll]. This increase in the pore size distribution is illustrated in Figure 5 for a gel washed repeatedly in water. Note that the smallest pore size is now 8 nm as compared to 3.5 nm for the gel washed in ethanol (see Figure 4). Water has the effect of increasing hydrolysis/condensation in the gel resulting in larger pores. Also shown in Figure 5 is the pore size distribution obtained from thermoporometry. Although the two techniques show reasonable agreement considering that they are completely independent, the thermoporometry is shifted to smaller pore size and also had a second peak at pore size greater than 1 pm. Pores this large are not contained in the wet gel since it is transparent. This is probably the result of structural rearrangement during freezing because of the fairly large cooling rates (0.5 C/min) associated with our instrument.
-
308
2o
,-I),
NMR Themopommetry
10
01
. m
1
1
10
100
1000
r (nm) 2500 A
unwashed EtOH washed 1 time A EtOH washed 5 times
W 0
2000-'i
m
2E
1500-
U J
lWO;,
a
500-
w
Y
0
0
I
O
0
I
A.
1
I
For the study of some problems, the specific surface area is a more useful then the pore size distribution. This is illustrated in Figure 6 for three samples (unwashed, washed once in EtOH, and washed five times in EtOH) during drying. Previously, we have observed 181 a maximum in the surface area-solvent content curve in the vicinity of 4040% solvent loss. Also, it is often observed that when transparent wet silica gels are immersed in water, they become opaque and this is attributed to phase separation of formally unreacted TEOS in the pore fluid [ll]. We attribute the surface area increase during drying to this same effect. That is, as drying proceeds, the concentration of TEOS
309
and water in the pore fluid increases and results in the precipitation of high surface area silica. Evidence for this is presented in Figure 6 since only with extensive washing (5 times) does the surface area increase disappear.
J
d :: * m
,M m
ti
U
.
2-
A
&
p i
I I
Y
M
1
I
: ;
Y
Themopormetry
....)..
S I
10
100
r (nm) Figure 7
Comparison of pore size distributions for Dow IX resin.
In addition to inorganic materials, NMR may also be used to study pore structure in organic materials such as ion exchange resins. A comparison between NMR and thermoporometry is presented in Figure 7. Good agreement is obtained between the techniques and the reason for the NMR peak at 70-80 nm is probably the result of "free" water on the outside of the small ion exchange particles. CONCLUSIONS
The use of low-field NMR appears to offer reasonable pore structure information (pore size distribution, surface area, etc.) for wet porous solids which could not be previously obtained by conventional methods. ACKNOWLEDGEMENTS
This work has been supported by Sandia National Laboratories (#05-5795) and
the UNM/NSF Center for Micro-Engineered Ceramics which is a collaborative effort of the NSF (CDR-8803152), Sandia and Los Alamos National Laboratories, the NMRDI, and the ceramics industry. The authors thank G. Johnston and K. Moore their help.
310
REFERENCES Brun, M., Lallemand, A., Quinson, J., and Eyraud, C., Thermochim. Acta, 1977, 1.
a.
21,59. Eyraud, C., Quinson, J.F., and Brun, M., in CHARACTERIZATION OF POROUS SOLIDS, Eds. Unger, Rouquerol, Sig, Elsevier, Amsterdam, (1988). Gallegos, D.P., Munn, K., Smith, D.M., and Stermer, D.L., J. Colloid Interface Sci., 1987,119,127. Bhattacharja, S., DOrazio, F., T a r m n , J.C., Halperin, W.P., and Gerhardt, R, J.Am.Ceram.Soc., 1989, 72, 2126. Brownstein, K.R.,and Tam, C.E.,1. Mag. Resonance, 1977, 26, 17. Gallegos, D.P., Smith, D.M., and Brinker, CJ., J. Colloid Interface Sci., 1988, 124, 186. Davis, PJ., Gallegos, D.P., and Smith, D.M., Pow. Tech., 1987, 53,39. Glaves, C.L., Brinker, C.J., Smith, D.M., and Davis, P.J., Chem. Materials, 1989,1,
9. 10.
34. Gallegos, D.P., Smith, D.M., J. Colloid Interface Sci., 1988,122,143. Brinker, C.J., Keefer, K.D., Schaefer, D.W., Ashley, C.S., J. Non-Cryst. Solids, 1982,
11.
48,47. Scherer, G.W., J. Non-Cryst. Solids, 1988, 100,77.
2. 3. 4.
5. 6.
7.
F. Rodriguez-Reinosoet al. (Editors), Characterization of Porous Solids ZZ 0 1991 Elsevier Science Publishers B.V., Amsterdam
311
CHARACTERIZATION OF MICROPOROSITY AND SURFACE HOMOGENEITY BY
TRE STUDY OF ARGON AND "ROGEN
ISOTHERM CROSSING AND
MEASUREMENT OF DIFFERlENTIAL ENTHALPIES OF ADSORPTION J.M.Mart in-Martinez', F.Rodriguez-Reinoso', Y.Grille$, F.Rouquero13 and J.Rouquero12 Departamentode Quimica Inorganica e Ingenieria Q u imica. Universidad de Alicante. Alicante. Spain. Centre de Thermodynamiqueet de Microcalorimttrie du C.N.R.S., 26 rue du 141tme R.I.A., 13003 Marseille, France. Universitt de Provence, Place V. Hugo, 13331 Marseille Cedex 3, France. ABSTRACT The analysis of crossing points in adsorption isotherms of microporous silicas and nonporous homogeneous lamellar halide has been carried out. The explanation for the crossing is different according to the type of solid. For a microporous silica a crossing appears when the proportion of microporous surface increases. However, in the case of lamqlar halides the crossing point is associated to a high surface homogeneity. On the other hand,' the microcalorimetric results measured for the different samples were more reliable than the isostenc heat of adsorption values calculated from the Clausius-Clapeyron equation.
INTRODUCTION The first part (say, up to P/Po = 0.4) of the adsorption isotherm of N2 or Ar on a solid is known to contain a great amount of information about the gas-solid interactions and the microporous structure of the adsorbent. Widely used methods for the analysis of this part of the isotherm are the BET (ref. l), Dubinin (ref. 2), "t" (ref. 3) and "a," (ref. 4) methods. We wish to examine here the additional information about the microporosityor homogeneity of the surface, which may be provided on the one hand by the crossing points of the adsorption isotherms of N2 or Ar and on the other hand by the enthalpies of adsorption.
The existence of crossing point -under given conditions- between two or more adsorption isotherms (for a given solid and adsorptive) obtained at neighbouring temperatures when the
amounts adsorbed are plotted as a function of the relative pressure has been previously reported (ref. 5). Actually, the crossing point is mainly observed in homogeneous adsorbents (such as graphite, boron nitride, etc.), i.e., with systems giving rise to stepwise adsorption isotherms (refs. 6,7).Nevertheless, since the reverse is not necessarily true we considered interesting to see
312
whether these crossovers could be associated or not with phase changes of the adsorbed phase. This would be done by taking advantage of the safety of isothermal low temperature adsorption microcalorimetric technique in debxting such two-dimensional phase. changes.
On the other hand, there is another kind of isotherm crossing found when plotting adsorption isotherms of two different adsorptives at a given temperature as a function of the relztive pressure (refs. 8,9). Of course, this crossing point has a different meaning and could be associated to the microporous texture of a given solid. Therefore, the particular aspects to be discussed are: i) the crossing point between adsorption isotherms of N, and Ar determined at 77 K, ii) the crossing point between adsorption isotherms of one gas (N, or Ar) determined at two neighbouring temperatures, iii) the enthalpies of adsorption determined by the isosteric method from the above isotherms and iv) the enthalpies of adsorption determined by direct microcalorimetric measurement. EXPERIMENTAL An amorphous microporous silica (previously studied in the scope of an SCI-WAC project (ref. 10)) is examined (fractal dimension ca 2.5 (ref. 11)). It is outgassed at increasing temperatures from 423 to 1173K by Controlled TransformationRate Thermal Analysis (ref. 12). The two lamellar halides, CdC12.2H,0 and CdBr,.4Hz0 (from Merck) are first dehydrated by room temperature outgassing and then heated at a rate of 0.5 K min" up to 423 K at which they are maintained under vacuum during times ranging from 20 to 310 h in order to increase
their surface homogeneity (ref. 13) .4dsorption volumetry of N2 or Ar is carried out at 77, 87 or 90 K, either with a conventional point-by-point procedure or with the quasi-equilibrium procedure allowing a continuous recording of the adsorption isotherm (ref. 14). When needed, the latter procedure is associated with adsorption micrdorimetry.(ref. 15). RESULTS AND DISCUSSION
A/ CrossinP mint between adsorption isotherms of N1and Ar. both determined at 77 K If one plots the adsorption isotherms of N2and Ar adsorbed on the same sample at a given
temperature (with PIPo on the abscissa), as is done in Fig. 1, one may or may not get a crossing of the isotherms. The presence of such crossing points in different silicas was also reported
previously in the literature by different research groups (refs. 16,17). The reason for the crossing may be derived from previous works (refs. 8,9) where a systematic comparison was carried out
313
between the BET monolayer capacity for N2and Ar on a variety of porous or non-porous solids. It was consistently observed that the greater the proportion of microporous surface the smaller the ratio of the N2 over Ar monolayer capacities. For a fully microporous sample (i.e. with comparatively negligible external surface arm) this ratio tends to 0.82 which is also the ratio of the molar volumes of these adsorptives in the three-dimensional liquid state. Nevertheless, because of a somewhat higher interaction of N2 with the surface (reflected into a much higher value of the BET parameter "C"; for instance 91 instead of 26 for Ar in the
N2 adsorption isotherm always begins steeper and higher than the Ar isotherm and then, after the "knee",always becomes more horizontal than the
case of the 1173 K sample in Fig. 1) the
Ar isotherm. In these conditions, the existence of a crossing point is directly related with a low v,(Nz)/v,(Ar)
ratio and therefore with a high proportion of microporous surface.
15OOC 40OoC 65OoC 0 900°C A
!03
0
P
Fig. 1. N2and Ar adsorption isotherms at 77 K on a microporous silica gel outgassed at various temperatures. BI Crossing mint between the 77 and 90 K adsorption isotherms of N, (or Ar)
If one now plots the adsorption isotherms of a given adsorbate on a given adsorbent but determined at two different temperatures, one may also find a crossing of the isotherms (if they are plotted versus P/P,) but with a quite different meaning. This was observed by Prenzlow and
314
Halsey (ref. 6 ) and by Lbpez-GonzaSez and Rodriguez-Reinoso (ref. 5) in the case of homogeneous crystalline surfaces. It seems that the crossing of the isotherms is here directly related with the existence of a stepwise (Type VI (ref. 18)) adsorption isotherm: as the temperature is increased over the 2D critical point of adsorbed N2or Ar, the portion of the isotherm between two successive steps becomes less horizontal and therefore crosses the correspiiding portion determined at a lower temperature, as shown in Fig. 2. Nevertheless, the
dative pressure at which the crossing occurs is certainly
CADMIUM CHLORIDE /ARGON
0
77.3 K
90.1 K
PIP0
-c
Fig. 2. Ar adsorption isotherms at 77 K and 90 K on CdC1, outgassed 310 hours at 423 K. a sensitive indication of the surface homogeneity, specially when the latter is limited. A good way
to assess the homogeneity is indeed that suggested by Larher (ref. 13), i.e to determine the ratio between the total height of the first step (up to A", see Fig. 2) and the height of the "foot" of that step (up to A', which is due to the heterogeneous part of the surface). Nevertheless, as can be Seen in Fig. 2, these heights are not easy to determine with accuracy when the shape of the isotherms departs from a clear-cut Type VI. On the contrary, the location of the crossing point and of the corresponding relative pressure may always be precisely determined: in the case of the
CdCl, sample with Ar, it occurs at P/Po= 0.28 after 10 h outgassing at 423 K and at P/Po= 0.18 after 310 h outgassing which is expected to increase the homogeneity, whereas the relative heights of the step and foot are not seen to change.
315
if adsorpb'on and m & en mints
-Dies
In the case of a microporous adsorbent leading to crossing points like those given in Fig.
1, the differential enthalpy of adsorption of N2or Ar may be expected to start from high values at low coverage. Quite often, this is not what is observed when applying the Clausius-Clapeyron equation to derive the "isosteric heat" of adsorption which is equal to the differential enthalpy. The problem with the isosteric method in the low m e r a g e region lies on the fact that small changes in the pressure measured (due to a slow, thermally activated, gas diffusion, to minor impurities in the adsorbate or to a lack of accuracy of the pressure transducer) may heavily distort the results. This problem is found when using a conventional adsorption equipment with only one pressure gauge (for instance 0-loo0mbars for N, adsorbed at 77 K). It may be overcome with multi-gauge modem equipment capable of a precise measurement in the low pressure range corresponding to the micropore filling. On the other hand, the direct microcalorimetric determination (cf. Fig. 3, the steadily decreasing plain curve) safely gives, in any case, the expected information on the micropore filling.
0
I
Fig. 3. Differential enthalpy of adsorption of N2 at 77 or 87 K on a microporous silica gel outgassed at 423 K, as determined by the isostenc (points) or microcalorimetric @lain CUrVe) methods.
In the case of adsorbents leading to crossing points of the type represented in Fig. 2, the isosteric method may give the expected variation of the net derivative enthalpy of adsorption in
316
the case of a homogeneous surface, as shown by the circles in Fig. 4: a decrease (due to a first adsorption on the heterogeneous portions of the surf=) is followed by an increase (due to the increasing part of the adsorbate-adsorbate interactions) and a maximum just before the
statistical completion of the monolayer. Nevertheless, here again a direct microdonmetric determination is somewhat safer, both in the region of relative pressures lower than 0.25 (for which we do not report the results from the isostenc method) and in the region above the monolayer (for which the isosteric method gives an endothermal enthalpy change which is likely to be an artefact).
CADMIUM BROMIDE /ARGON 77.3K
Fig. 4. Net differential enthalpy of adsorption of Ar at 77 K on CdBr, treated 20 hours (left) or 170 hours (right) at 423 K.
317
REFERENCES 1. S. Brunauer, P.H. Emmett and J. Teller. J. Am. Chem. Soc. 60 (1938) 309. 2. M.M. Dubinin and L.V. Radushkevich. Proc. Acad. Sci. USSR 55 (1947) 331. 3. B.C. Lippe-ns and J.H. de Boer. J. Catal. 4 (1965) 319. 4. K.S.W. Sing,in "Surface Area Determination", Roc. Int. Symp. Eds. D.H. Everett and R.H. Ottewill. Butterworths. London. (1970) 25. J.D. Lopez-Godez and F. Rodriguez-Reinoso, in "Adsorption at the gas-solid and 5. liquid-solid interface". Eds. J. Rouquerol and K.S.W. Sing. Studies in Surface Science and Catalysis vol. 10, p. 479. Elsevier Scientific Pub. Company. Amsterdam (1982). C.F. Prenzlow and G.D. Halsey. J. Phys. Chem. 61 (1957) 1158. 6. A. Linares-Solano, J.D. Lopez-Gonzalez, C. Moreno-Castilla and F. Rodriguez-Reinoso. 7. Carbon, 16 (1978) 397. F. Rouquerol. Ph D Thesis, Paris (1965) p. 91. 8. J. Rouquerol, F. Rouquerol, Y. Grillet and M.J. Torralvo, in "Fundamentals of 9. Adsorption".p. 501. Eds. A.L. Myers and G. Belfort. Engineering Foundation. New York (1984). D.H.Everett, G.D.Parfitt, K.S.W. Sing, and R. Wilson, J. Appl. Chem. Biotechnol., 10. 24 (1974) 199; D.C. Harvard and R. Wilson, J. Colloid Interface Sci., 57 (1976) 276. P. Levitz, H.van Damme and J.J. Fripiat. Langmuir, 4 (1988) 781. 11. I. Rouquerol. J. Therm. Anal. 2 (1970) 123. 12. Y. Larher, Ph D Thesis, Orsay (1970) p. 203. 13. Y. Grillet, F. Rouquerol and F. Rouquerol., J. Chim. Phys., 2 (1977) 179. 14. J. Rouquerol, in "Thermochimie", Marseille, 1971, C M S ed., Paris. 15. D.A. Payne, K.S.W. Sing and D.H. Turk., J. Colloid Interf. Sci. 43 (1973) 287. 16. J. Rouquerol, F. Rouquerol, C. Peres, Y. Grillet and M. Boudellal, in "Characterization 17. of Porous Solids". p. 107. Eds. S.J. Gregg, K.S.W. Sing and H.F. Stoeckli. London Soc. Chem. Industry (1979). 18. S.J. Gregg and K.S.W. Sing, in "Adsorption, Surface Area and Porosity", p. 303. Eds. S.J. Gregg and K.S.W. Sing, 2" ed.. Academic Press. London (1982).
This Page Intentionally Left Blank
F.Rodriguez-Reinosoet at. (Editors),Characterization of Porous Solids ZZ
319
1991 Elsevier Science Publishers B.V., Amsterdam
ADSORPTIVE PROPERTIES OF ACTIVATED CARBONS PREPARED FROM KEW.,AR@*
J.J.
FREE MAN^,
'Department
F.G.R. GIMBLETT~,R.A.
HA YES^, z. MOHD. AM IN^
AND K.S.W.
SING^
of Chemistry, Brunel University, Uxbridge, Middlesex UB8 3PH (U.K.)
2School of Chemical Technology, South Australian Institute of Technology, The Levels, SA 5095 (Australia) 3School of Chemical Sciences, Universiti Sains Malaysia, 11800 Penang (Malaysia)
SUMMARY Activated chars were prepared from Kevlar" and were characterised by Ng, neoC5H12, H20 and C02 sorption. They had a greater affinity for H20 and C02 than comparable rayon chars, and could separate C02 from air. Pore widening with burn-off was very limited, probably due to the crystallinity of the precursor. INTRODUCTION Fibrous activated carbons are currently manufactured from a number of precursors including viscose rayon, polyacrylonitrile, and coal tar pitch. Rayonbased materials are particularly versatile in terms of the range of pore sizes which can be formed during activation, after suitable pre-treatment (refs. 1-3). Regular viscose rayon has quite low crystallinity and it is possible that more highly ordered polymers may yield activated chars with alternative adsorptive properties. Carbon fibres have been produced (ref. 4) from the highperformance aramid fibre, Kevlar@, but no attempt has been made to develop porosity i n such chars by gaseous activation. We report here the preparation of activated carbons from Kevlare and an assessment of their adsorptive properties. Kevlar@ is the condensation product of 1,4-diaminobenzeneand isophthalic acid.
The fibres are highly crystalline and consist of a system of sheets regu-
larly pleated along their long axes and arranged radially (ref. 5). The relatively small amount of disorder in the structure is due to chain termination or defects in the packing of the sheets. The grade chosen for this study was Kevlar" 29, a low modulus form available in a variety of textile constructions. EXPERIMENTAL PreDaration of activated chars Samples of two Kevlar@ 29 textiles were obtained from P. & S. Textiles Ltd., Bury, Lancs., a non-woven felt, Arrowsafe@ K103, and a plain-weave cloth, Arrow*Kevlar@ is the registered trade mark of E. I. Du Pont de Nemours & Co. Inc.
320 safe@' K280.
Kevlar" 29 yarn was obtained from the Scottish College of Textiles,
Galashiels. A further woven cloth, Type D0235/001, was supplied by Fothergill Engineered Fabrics, Littleborough, Lancs. Pieces of Type D0235/001 cloth were washed by soaking for 4 8 h in aqueous solutions of hydrochloric acid (AnalaR, BDH Chemicals Ltd.) of various concentrations up to 3M, followed by washing with distilled water and drying overnight at 60'C.
Strips of the textiles were cut (dimensions, 6 x 1 . 5 cm) and
suspended in a gravimetric tube furnace (described in ref. 6).
The yarn was
used as a continuous length, bound together to make a bundle of ca. 2 g in weight and 20 cm in length. Each sample was subjected to: (1) pyrolysis in oxygen-free nitrogen (flow rate, 4 dm3 min-l) at a heating rate of 1 0 ' C min-l to 860^C; ( 2 ) activation in carbon dioxide gas (flow rate, 4 dm3 min-l) at 860'C for the time necessary to obtain the required burn-off; and ( 3 ) cooling to ambient temperature in flowing nitrogen.
The nitrogen employed for pyrolysis
and adsorption measurements was high purity grade ( > 9 9 . 9 9 % , Air Products Ltd.), and the carbon dioxide was of 9 9 . 7 5 % purity (Distillers M.G. Ltd.). Characterisation Neopentane was chosen in addition to nitrogen to assess the effect of molecular diameter on the nature and extent of adsorption on the chars studied (ref.
7).
Nitrogen adsorption isotherms were determined at 77K using a Carlo Erba
Sorptomatic 1800 and an Omnisorp 100, the latter being used for measurements at very low relative pressures. Neopentane isotherms were measured at 273K using a
CI Robal vacuum microbalance, while a quartz spring McBain-Bakr type vacuum microbalance was used to measure water sorption isotherms at 298K.
The
neopentane used was of 9 9 . 0 % purity (Argo International Ltd.) while the deionised water was subjected to repeated freeze/thaw cycles under vacuum to remove dissolved air, before use. 250*C to a residual pressure o f <
All samples were outgassed for 2 15 h at mbar prior to isotherm measurement.
Carbon dioxide sorption was investigated dynamically rather than via the static methods listed above.
The sorbent was prepared in the form of a column
by packing a number of discs cut with a cork borer as layers (2-50) in a short stainless steel tube of 4 . 6 mm i.d. This resulted in column lengths of 1-25 mm being produced.
The columns were installed in a Carlo Erba Vega GC 6000 gas
chromatograph fitted with a hot wire detector and were conditioned, typically, at 25OOC in helium (high purity grade, Air Products Ltd.).
After cooling, C02
was injected at various temperatures and the resultant peaks were integrated using a Jones Chromatography JCL 6000 system. The carrier gas employed was helium (flow rate, 1 3 cm3 min-l), while the column temperature was 4 0 * C .
The
chromatograms were transformed to breakthrough data using methods described elsewhere (ref. 8).
Scanning electron micrographs o f various samples were obtained using a Cambridge Stereoscan S250 instrument, and electron probe microanalyses performed using a Link Systems 860 EDXA attached to the microscope.
Thermal analyses
(DTA, TG and DTG) were performed on Kevlar" 29 cloth using a Stanton Redcroft STA780 instrument. An atmosphere of dry N2 (flow rate, 50 cm3 min-l) and a heating rate of 1 0 ' C min-l up to a maximum temperature of 950eC, were employed. Elemental analysis (for C , N and S ) was performed on the unwashed Kevlar" (Fothergill Type D0235/001) and on unwashed and washed chars derived from this precursor which had been activated to 50% burn-off.
RESULTS Thermal analvsis Thermal. analysis data obtained for the unwashed and washed cloths showed that the decomposition was endothermic in all cases, the major DTA peak being at 615'C with a shoulder at 575'C.
A single DTG peak also occurred at 615'C,
indicating that the major endotherm was associated with a loss in weight. the TG curve, the char yield at 950'C
From
was 36.5%. Similar data were also
obtained f'or all the washed samples, indicating that such treatment had no influence on the subsequent thermal behaviour of the polymer.
Elemental analysis The ratios of carbon to nitrogen to sulphur, with carbon adjusted to 100, were 1 0 0 : 1 6 . 6 : 0 . 8 for the unwashed Fothergill woven Kevlar", and 1 0 0 : 9 . 4 : 1 . 0 and
100:9.3:1.0,respectively, for the unwashed and washed activated chars derived from it. The nitrogen present is likely to be a residue of the amide groups in the polymer, whereas sulphur was probably introduced during fibre spinning.
Carbonisation and activation The average carbonisation yield obtained from the weight change during preparation of the chars was 36.5 k 0.6%. All the unwashed/washed samples gave similar traces of weight against time during activation, demonstrating that washing also had no influence on the activation process. A progressive increase was apparent in the rate of weight l o s s as burn-off proceeded. Scannine electron microscouv and EDXA Scanning electron micrographs of chars prepared from unwashed and washed precursors are depicted in Figs. l(a) and (b).
Particulate residues were found
on the fibre surface of the unwashed precursor and this may be linked to extensive pitting of the fibre surface during activation (Fig. l(a)).
EDXA
studies showed that the elements Ca, Fe, K , Si, S , P and A1 were present in the
322 surface residues, with S and K also being present on parts of some fibre surfaces free from visible residues.
Fig. 1 . Scanning electron micrographs of (a) a 40% burn-off char prepared from unwashed woven Kevlar@ 29 and (b) a 40% burn-off char prepared from 3M H C 1 washed woven Kevlar@ 29. Residues containing heavy metals were greatly diminished by washing the precursor with aqueous H C 1 of varying concentrations, and were virtually eliminated by the use of a 3M H C 1 solution.
In contrast, K, Ca and S residues
appeared to be unaffected by such treatment, S residues being always present in the fracture surfaces of activated chars. The removal of heavy metal residues by washing also reduced the extent of pitting, although some surface modification was still visible even after 3M acid washing (Fig. l(b)).
The residues
were probably introduced during the manufacture of Kevlar@ at the spinning and/or finishing stages, and cannot be the result of contamination during hand1ing . NitroZen adsorption Typical nitrogen isotherms for activated chars derived from unwashed Kevlar@ (in this case, yarn) are shown in Fig. 2(a).
Both isotherms, and those not
illustrated for woven and non-woven chars, are of Type I character and most alsc display a small hysteresis loop. The sample activated to the lowest burn-off ( 2 7 . 4 % ) of those studied exhibited low pressure hysteresis (Fig. 2(a)).
Corres-
ponding a, plots, constructed using the reference data of Carrott et al. (ref. 9),
are depicted in Fig. 2(b).
External surface areas and pore volumes have
323
0 0
MFKlOl
-I
0 I MFKl02
0
0.2
0.4
0.6
0.8
0 MFK/OZ
1.o
1.0 0
PIP"
2.0
3.0
0,
Fig. 2. (a) Isotherms and (b) a, plots for the adsorption of nitrogen at 77K on activated Kevlar@ 29 yarn chars. been derived from the slopes and intercepts of the linear regions of the plots (ref. 10). Not all of the plots attain a plateau at as
=
1, the value usually
considered to mark the completion of micropore filling, and there may, therefore, be some overlap of pore filling processes in this region of the isotherms. In these cases, the pore volume obtained by back extrapolation from the plateau may include a mesopore contribution in addition to the micropore volume. For this reason, the extrapolated values have been designated total pore volumes. BET surface areas were calculated in the usual way,
1
and the adsorption data derived from the isotherms are listed in Table 1. The nitrogen isotherms obtained for an unwashed sample and a 3M HC1-washed sample of the same burn-off were virtually identical, except for the maximum uptake at .-_--------
high p/pc for the washed sample which was marginally lower than that for the unwashed. The nitrogen isotherms (Fig. 3 ) measured over very low relative
0
0.001
P/PO
0 . o(12
Fig. 3 . Nitrogen adsorption isotherms at very low relative pressures for microporous carbons: 1, Carbosieve S ; 2, rayon char J F 5 1 6 I ; 3, woven Kevlar@ char.
324 pressure ranges (p/p^
=
0-0.002) show clearly that the initial uptake of
nitrogen by the activated Kevlar@ char is significantly greater than that exhibited by the other microporous carbons studied, including the molecular sieve carbon, Carbosieve S . TABLE 1 Nitrogen and neopentane adsorption on activated Kevlar@ 29 chars Sample
Burn-off
Nitrogen adsorption
ABET
(X)
2 , Non-woven ZFK/l031 ZFK/lO 3 2 ZFK/1033
Woven ZFK/ 2 801 ZFK/2802 ZFK/2803 ZFK/2804
g-l , 2
NeoDentane adsomtion
AS "P g-l 3,. g-l
Vp g-l 3,. g-l
As
ABET
, 2
g-l , 2
32 51.5 70.0
691 790 985
22 18 26
0.33 0.38 0.48
738 544 726
4
0.30 0.26 0.30
31.2 42.4 59.1 47.3
692 750 1077 803
29 6 11 8
0.32 0.34 0.50 0.36
602 685 1017 857
<1 4 6 8
0.28 0.27 0.42 0.36
27.4 53.0
537 789
16 12
0.24 0.36
7 6
Yarn MFK/Ol MFK/02
NeoDentane isotherms Neopentane isotherms for woven chars derived from unwashed precursors are shown in Fig. 4, together with the a s plots constructed using the reference data 0.5
I
0.4
0.3
0.2 0 0 0
0.1
A A
v
0
0.2
T
ZFK1.2801 ZFK12802 ZFK12803 ZFK/2804
0.6
0.4
PIP0
0.8
I
0 ZFK12801
ZFK12802 ZFK12803 v ZFK12804 0
A
1.0 0
1 .o
2.0
3.0
as
Fig. 4. (a) Isotherms and (b) a s plots for the adsorption o f neopentane at 273K on activated woven Kevlar@ 29 chars.
325 of Carrott et al. (ref. 7).
A s expected, the neopentane isotherms differ from
those of nitrogen in several ways.
Thus all the isotherms exhibit low-pressure
hysteresis, although that obtained for the 70% burn-off sample ZFK/1033 (not illustrated) was almost reversible at low p/po.
The interpretation of the a s
plots is, again, not straightforward, there being some evidence that pore filling processes overlap in the region around a s
=
1. Values of the BET
surface area and the total pore volume are again listed in Table 1.
Water sorution Fig. 5 depicts the isotherms for water sorption on 50% burnoff Kevlar@ chars derived from
l
P
unwashed and 3M HC1-washed precursors. Both samples exhibited a similar uptake of water vapour as the relative pressure was 0
increased, but the maximum uptake
A
A
GFK/035/unwashed GFK/035/washed
of the unwashed sample was greater than that attained by the washed sample.
The extent of the initial
uptake at low p/po, together with the early upswing isotherm at p/po =
0
0.2
0.6
0.4
0.8
1.0
PIP"
0.2-0.3,indicates that the
Kevlar@ chars have a substantially greater affinity for water vapour than comparable rayon chars.
Fig. 5. Water sorption isotherms at 298K on activated woven Kevlar@ 29 chars derived from unwashed and 3M hydrochloric acid-washed precursors.
Carbon dioxide breakthrough Activated Kevlar@ samples derived from unwashed woven precursors interact strongly with carbon dioxide. This interaction manifests itself in two interesting w a y s .
Firstly, after being conditioned prior to
CO2
injection, the
activated materials appeared capable of retaining a critical volume of the gas with no elution occurring unless the column temperature was increased significantly. Subsequent injections of C02 resulted in partial and finally total elution of the injected volume.
Secondly, even after saturation with CO2
gas, the activated materials were still capable of separating C02 from air.
The
chromatogram depicted in Fig. 6 shows a distinct separation between the peaks from the two gases effected on a column consisting of 30 layers of 59% burn-off material.
Such behaviour was not observed with unactivated samples, indicating
that the C02 activity was not simply due to the presence of metal residues on the unwashed precursor but associated with the porosity induced by activation.
326
01 20
10
0
Time ( n i i n )
Fig. 6 . Chromatogram for column containing 30 layers of 59.1% burn-off Kevlar@ char ZFK/2803 (presaturated with Cog) illustrating separation of Cog from air. The C02 breakthrough curves for a viscose rayon-based activated
p
120,
carbon cloth and a Kev1ar"-derived material are depicted in Fig. 7. There is a major difference between the C02 activities of the two materials, with that based on Kevlar" retaining 283-times more C02 than the rayon-based char. Furthermore, the latter showed little ability to separate air and C02 from gaseous mixtures, in marked contrast to the behaviour of the Kev1ar"-derived materials. DISCUSSION
Comparison of the nitrogen and
cL
0
50
100
150
200
250
300
Cumulative volume of C02 injected (PI) Fig. 7 . Carbon dioxide breakthrough curves f o r woven Kevlar" 29 and rayon chars (both activated to 60% burn-off)
neopentane adsorption data for the woven and non-woven chars derived from unwashed precursors (Table 1) reveals that, with the exception of sample ZFK/2804, the total amount of nitrogen adsorbed was always greater than that of neopentane. It also appears that the ratio of the total amounts of nitrogen and neopentane adsorbed tends to increase. with burn-off, suggesting that part of the pore structure is becoming less accessible to the larger neopentane molecules as burn-off proceeds.
This
pattern of pore volume development contrasts with that found in the case of viscose rayon chars, where the agreement between the two measured volumes is
327 poor at low burn-off but improves as the micropores are widened during activation (ref. 11).
In general, the pore volume obtained for a given
percentage burn-off is rather lower for Kev1ar"-derived chars than for those obtained from viscose rayon. A first step towards an explanation of these effects may be obtained from the
isotherms themselves. The persistence of low pressure hysteresis in the neopentane isotherms (Fig. 4(a)),
even after activation to 70% burn-off, suggests
that the pore dimensions have been increased by activation to a relatively small extent. Nitrogen isotherms obtained for microporous rayon chars are generally reversible after ca. 15% burn-off, while the corresponding neopentane isotherms are reversible after ca. 40% burn-off (ref. 12).
Clearly the scope for micro-
pore widening in Kevlar" chars is very limited, and it is likely that this originates from the highly ordered structure of the precursor which yields a denser, less defective char than rayon. This could also explain the considerable uptake of nitrogen exhibited by Kevlar@-derived materials at very low relative pressures.
The existence of narrow pores, not significantly widened by
activation, could result in enhanced adsorbate-adsorbent interactions and lead to the substantial primary micropore filling observed. Table 1 shows that the most highly activated non-woven char (70.0%burn-off) had a lower uptake of neopentane than the woven char activated to a lower burnoff (59.1%) and only a slightly greater uptake of nitrogen. One possible explanation of the reduced uptake at the higher burn-off could be that the effect of gasification-induced shrinkage during activation between 60 and 70% burn-off more than compensates for any pore widening (ref. 1 3 ) .
Such
densification may have involved pore closure, collapse or sintering, thus accounting for the reduced pore volume available to neopentane. KevlarB-derived chars are also in marked contrast to rayon chars of similar burn-off in their affinity to water vapour. Whereas the latter are relatively hydrophobic, with little uptake of water until ca. 0.7 p o , both Kevlar" char samples exhibited a 'knee' in the isotherm at low relative pressure and a main upswing at p/po
=
0.1-0.3(Fig. 5).
This behaviour was not influenced by acid
washing; washing appears to have a greater effect on the final uptake of water, with the washed sample exhibiting a lower value than unwashed sample. Recent investigations in these laboratories (ref. 1 4 ) of the effects of pore width and surface polarity on the nature of water sorption suggest that narrow pore widths alone are unlikely to produce displacements in the main upswing in the isotherm of the magnitude observed with the Kev1ar"-derived chars. Hence, such behaviour must be attributed largely to inorganic impurities remaining in the materials after washing, leading to the adsorption and nucleation of water molecules on polar sites. The presence of nitrogen and sulphur, distributed throughout the char structure, could explain the increased number of such sites,
328
whereas the removal of heavy metal residues could account for the reduced uptake of water near saturation by samples derived from acid-washed precursors. When expressed on a volume basis, the uptake of water at high p/pc for both washed and unwashed samples was some 30-35% less than that of nitrogen. A similar variation was noted by Kaneko et al. (ref. 15) for cellulose- and polyacrylonitrile-based activated carbon fibres. The latter chars exhibited hydrophilic character somewhat similar to that reported here for Kevlar@-based materials, which these authors attributed to residual nitrogen in the char. The presence of sulphur and nitrogen in the Kevlar@-based chars also provides an explanation for the greatly enhanced uptake of carbon dioxide exhibited by these materials. Polar sites derived from such residues are active towards the adsorption of both COq and water, and the distribution of sulphur and, presumably, nitrogen throughout the fibre cross-section would lead to such sites being located on the pore walls as well as on the external surface. This, in turn, would promote enhanced adsorption of C02 molecules through the action of polar forces in addition to the adsorbate-adsorbent interactions normally present ACKNOWLEDGEMENTS Financial support from the Ministry of Defence, and a travel grant (for ZMA) from Universiti Sains Malaysia, are gratefully acknowledged. Drs. D.L. Brydon and R.R. Mather of the Scottish College of Textiles are thanked for helpful discussions and for providing samples. P. h S . Textiles Ltd. and Fothergill Engineered Fabrics are thanked for supplying samples. Mr M.B. Kenny is thanked for isotherm measurements using the Omnisorp 100 Analyser kindly provided by Omicron Technology Corporation and Malvern Instruments Ltd. REFERENCES
1 J.J. Freeman, F.G.R. Gimblett, R.A. Roberts and K.S.W. Sing, Carbon, 25 (1987) 559; Carbon, 26 (1988) 7. 2 J.J. Freeman, F.G.R. Gimblett and K.S.W. Sing, Carbon, 27 (1989) 85. 3 J.J. Freeman, F.G.R. Gimblett, K.S.W. Sing and J.E. Wright, in: Proc. Carbon ' 8 8 , IOP Publishing, Bristol, 1988, p p . 510-512. 4 I. Tomizuka, Y. Isoda and Y. Amamiya, Tanso, No.106 (1981) 93. 5 M.G. Dobb, D.J. Johnson and B.P. Saville, J. Polym. Symp., 58 (1977) 237; J. Polym. Sci., Polym. Phys. Ed., 15 (1977) 2201. 6 J.J. Freeman and F.G.R. Gimblett, Carbon, 25 (1987) 565. 7 P.J.M. Carrott, R.A. Roberts and K.S.W. Sing, Langmuir, 4 (1988) 740. 8 R.A. Hayes, Ph.D. Thesis, Brunel University, 1988. 9 P.J.M. Carrott, R.A. Roberts and K.S.W. Sing, Carbon, 25 (1987) 769. 10 P.J.M. Carrott, R.A. Roberts and K.S.W. Sing, Carbon, 25 (1987) 59. 11 P.J.M. Carrott, R.A. Roberts and K.S.W. Sing, in: K.K. Unger, J. Rouquerol, K.S.W. Sing and H. Kral (Eds.), Characterisation of Porous Solids, Elsevier, Amsterdam, 1988, p.89. 12 R.A. Roberts, K.S.W. Sing and V.S. Tripathi, Langmuir, 3 (1987) 331. 13 R.H. Hurt, D.R. Dudek, J.P. Longwell and A.F. Sarofim, Carbon, 26 (1988) 433; P.J.M. Carrott and J.J. Freeman, Carbon, in the press. 14 P.J.M. Carrott, M.B. Kenny, R.A. Roberts, K.S.W. Sing and C.R. Theocharis, this conference. 15 K. Kaneko, Y. Fujiwara and K. Nishikawa, J. Colloid Interface Sci., 127 (1989) 298.
F. Rodriguez-Reinosoet al. (Editors),Characterization of Porous Solids I1
329
0 1991 Elsevier Science PublishersB.V.. Amsterdam
MODIFICATION I N POROUS TEXTURE ACTIVATED CARBONS BY OXIDATION
M.
MOLINA-SABIO,
AND
OXYGEN SURFACE
GROUPS
OF
M.A. MUNECAS-VIDAL and F. RODRIGUEZ-REINOSO.
Departamento de Quimica Inorghnica e Ingenieria Quimica. Universidad de Alicante. Alicante. Spain.
SUMMARY
Four a c t i v a t e d c a r b o n s h a v i n g d i f f e r e n t p o r e v o l u m e s a n d micropore size distribution were oxidized with air at 573K or w i t h aqueous s o l u t i o n s of H N 0 3 or H 2 0 2 u n d e r m i l d c o n d i t i o n s . T h e analysis of t h e a d s o r p t i o n i s o t h e r m s o f N 2 ( 7 7 K ) a n d C 0 2 ( 2 7 3 K ) indicates that the oxidation treatments used d o not substantially modify t h e m i c r o p o r o s i t y . T h e n a t u r e o f t h e o x y g e n s u r f a c e g r o u p s introduced is a f u n c t i o n of t h e o x i d a t i o n a g e n t , a s s h o w n b y Temperature Programmed Desorption (TPD). Carbons treated w i t h H N 0 3 and H 2 0 2 have a large amount of low-temperature C 0 2 groups whereas air oxidation introduces only high-temperature C 0 2 groups. A linear relationship h a s b e e n f o u n d b e t w e e n g r o u p s t h a t d e c o m p o s e to C 0 2 i n T P D and groups titrated by NaOH. INTRODUCTION The p e r f o m a n c e o f a n a c t i v a t e d c a r b o n in a g i v e n a p p l i c a t i o n is a f u n c t i o n o f b o t h t h e p o r e s t r u c t u r e a n d t h e c h e m i c a l n a t u r e o f its s u r f a c e structure,
[l]. A c t i v a t e d c a r b o n s b e s i d e s h a v i n g a g i v e n p o r e
are
invariably associated
with
heteroatoms
such as
oxygen and hydrogen, which will influence the adsorption behaviour. The oxygen surface groups are very c o m m o n and better k n o w n in activated c a r b o n s and t h e y m a y b e e a s i l y i n t r o d u c e d by o x i d a t i o n with n i t r i c a c i d , h y d r o g e n p e r o x i d e , p o t a s i u m p e r m a n g a n a t e , a i r , etc.
[ 2 ] ;
complex.
however, their characterization and evaluation is rather Boehm
I31 proposed
a method
based
in the
selective
titration o f the groups by bases of different strengh; in this w a y , the
groups may
be
classified
as
a
function
of
their
acidity
assuming t h a t t h e i r p K a is c o i n c i d e n t w i t h t h a t of s i m p l e o r g a n i c molecules containing these functional groups. This method has been widely used because of its simplicity [4]. Selective
titration
may
be
complemented
by
temperature-
programmed desorption (TPD), the origin of which is to a s s u m e that, upon heat treatment of the carbon, the oxygen surface groups evolve at
different
temperatures
either
as C O
( c a r b o n y l or
quinonic
structure), C 0 2 ( c a r b o x y l or l a c t o n i c s t r u c t u r e ) or C O p l u s C 0 2 (carboxyl anhydrides structure) [5].
330 A given oxidation treatment will modify the pore structure and the c h e m i c a l n a t u r e of t h e s u r f a c e o f a n a c t i v a t e d c a r b o n , t h e extent o f t h e m o d i f i c a t i o n b e i n g a f u n c t i o n o f b o t h t h e s t r u c t u r e of t h e c a r b o n a n d t h e o x i d a t i o n t r e a t m e n t . T h e a n a l y s i s o f t h e s e two a s p e c t s a n d
the comparison of selective titration and T P D
methods o f a n a l y s i s of o x y g e n s u r f a c e g r o u p s a r e t h e
t w o main
objectives o f t h i s w o r k .
EXPERIMENTAL Three a c t i v a t e d c a r b o n s w e r e p r e p a r e d by c a r b o n i z a t i o n ( N 2 1 1 2 3 K ) o f p e a c h s t o n e s f o l l o w e d by a c t i v a t i o n ( C 0 2 1098K): B O ( 1 1 %
burn-off),
MO
(24% burn-off) and
A0
( 5 3 % burn-off).
A
fourth
carbon, RO w a s prepared by carbonization ( N 2 , 1 1 2 3 K ) of plum s t o n e s followed
by
activacion
(N2/steam,
1 0 9 3 K ) to 4 8 % b u r n - o f f .
All
carbons w e r e heat treated in H 2 for 90 min. at 1 2 2 3 K to reduce to a minimum the number of oxygen surface groups. T h e reduced carbons were oxidized (see Table 1 for details and nomenclature)
in air
( 5 7 3 K ) , or a q u e o u s s o l u t i o n s
volume/carbon w e i g h t = 6 / 1 ) of H N 0 3 and H 2 0 2 .
(solution
S o m e of the oxidized
carbons - M P 1 , M 4 N , M 7 N and M15N- w e r e h e a t t r e a t e d in N 2 a t 9 2 3 K i n order to r e d u c e s e l e c t i v e l y t h e n u m b e r o f o x y g e n s u r f a c e g r o u p s introduced (T is added in the nomenclature). TABLE 1 Preparation conditions of the oxidized carbons Reduced Carbon
BO
MO
A0
RO
Oxidation conditions
Yield ( % )
Designation
4N H N 0 3 , lh. 15N HN03. lh.
106 107
B4N B15N
Air 573K, 8h. Air 573K, 8h.
100 100
M8A M24A
4N H N 0 3 , lh. 7N HN03, lh. 15N H N O 3 , lh.
107 108 109
M4N M7N M15N
5M H 2 0 2 , lh. 5M H 2 0 2 , 2h. 5M H 2 0 2 , 3h. 4N HN03, lh. 15N FINO3, lh.
100
MP1 MP2 MP3
110 113
A4N A15N
Air 573K, 8h. Air 573K, 24h.
100 99
R8A R 2 4A
4N HNO3, lh. 7N HN03, lh. 15N HN03, lh.
109
110
R4N R7N R15N
97 94
112
33 1 The pore structure o f the carbons w a s evaluated by adsorption of N2 (77K) and C 0 2 (273K) in a conventional gravimetric adsorption system. The micropore volume w a s deduced
from the adsorption data
by m e a n s of the Dubinin-Radushkevich (DR) equation [ 6 1 in the range
o f r e l a t i v e p r e s s u r e of 0.05-0.3 f o r N 2 a n d 0.003-0.03 f o r C02. Oxygen surface groups were determinated by titration w i t h NaOH and NaHC03 solutions " 2 1 .
TPD was used to determine the desorption
profiles o f C O a n d C02: 100 m g of c a r b o n w a s h e a t e d u n d e r a h e l i u m flow at
5K/min
from
300 t o
1325K and
the gases evolved w e r e
analyzed by gas chromatography using a TCD detector (Na2C204
.
H20
was u s e d a s r e f e r e n c e m a t e r i a l t o c a l i b r a t e t h e a m o u n t s of C O a n d C02 desorbed).
RESULTS AND DISCUSSION Figure 1 i n c l u d e s t h e N 2 ( 7 7 K ) a d s o r p t i o n i s o t h e r m s o f t h e reduced o r i g i n a l c a r b o n s . A l l corresponds
to
essentially
activation in C 0 2 produces
isotherms are of
microporous
type I a s
carbons.
it
Increasing
a gradual development of microporosity
of i n c r e a s i n g w i d t h , a s d e s c r i b e d i n p r e v i o u s w o r k [7,81. C a r b o n RO, p r e p a r e d by s t e a m a c t i v a t i o n , e x h i b i t s a w i d e r m i c r o p o r o s i t y and a more important mesoporosity contribution than carbon A O , similar burn-off i n C02.
20
-
15
M
\
a, rl
0 E E
10
v
c
5
0
0
0.2
0.4
0.6
0.8
1.0
P/P, Fig. 1. N2 (77K) adsorption isotherms for reduced carbons
with
332 The a p p l i c a t i o n of t h e D R e q u a t i o n t o t h e a d s o r p t i o n d a t a isotherms o f Fig.
1 leads to the
micropore
volume,
Vo,
of
the
carbons p l o t t e d i n Fig. 2 . T h e r e is a n i n c r e a s e in Vo f r o m BO to M O t h e Vo o f R O b e i n g s l i g h t l y l o w e r t h a n t h a t o f A O .
and A O ,
The
evolution o f V o m e a s u r e d by C 0 2 is s i m i l a r b u t t h e a c t u a l v a l u e s
, ,#
are s i m i l a r o n l y f o r c a r b o n s BO a n d MO. T h i s m e a n s t h a t i n t h e s e two carbons, the microporosity is narrow and relatively homogeneous [91; t h e l a r g e d i f f e r e n c e b e t w e e n the v a l u e s d e d u c e d f r o m N 2 C02
for carbons A 0
and
RO
is d u e
to the wider
micropore
and size
distribution, especially in carbon RO.
,
0.6
‘ jc;;; I ;,; , ;, 0.4
h
T i
. . . . . . . . . . .
M
. . . . . .. .. .. .. ..
m
E
. . . . . . . . .. .. . .. . .. . .. . ... ... ... ... ... . . .. . . . .. . . . . .. .. .. .. . . . . . . . . . . . .. .. .. ..
0.2
0
v
0
>
.. . . . . . .
o.o
.. ..
. ...
.. ..
.. ..
.. ..
.. ..
.. .. .. . . . . . . . . . . . . . . . . . ..
o N2 77K
Fig 2 .
. . . .
.. . .
c,
. . . ... .. .. ... ... . .
. ... ..
. . . . ..
... ... ... . . . . .. ... ... ... .. .. ..
...
. .
C 0 2 273K
Micropore Volume (cm3/g) of oxidized carbons
There a r e n o s i g n i f i c a n t c h a n g e s in Vo w h e n t h e c a r b o n s a r e oxidized u n d e r m i l d c o n d i t i o n s . T h u s , f o r c a r b o n M O a n d d i f f e r e n t oxidation t r e a t m e n t s ( s e e Fig. 2 ) t h e r e i s a s l i g h t m i c r o p o r o s i t y widening
only
for s a m p l e s M P 2 and
MP3.
This means
that
the
oxidation treatment may introduce changes in the chemical nature of the carbon surface without affecting much the porosity. Vo
(C02)
treated in N 2
keeps a l m o s t unchanged for carbons that w e r e heat (92310
increase i n V o ( N 2 ) ,
a f t e r o x i d a t i o n , b u t t h e r e is a n o t i c e a b l e the difference increasing w i t h the d e g r e e of
oxidation of the carbon. However, the adsorption isotherms of these carbons a r e p a r a l l e l t o that of t h e o r i g i n a l ( u n o x i d i z e d ) c a r b o n MO in the
0.3-1.0
relative pressure
range.
These results indicate
333 that t h e N 2 t r e a t m e n t at 9 2 3 K p r o d u c e s a s e l e c t i v e g a s i f i c a t i o n through t h e e l i m i n a t i o n o f a f r a c t i o n o f o x y g e n s u r f a c e g r o u p s ; this g a s i f i c a t i o n , in t u r n , p r o d u c e s a n i m p o r t a n t c h a n g e i n t h e micropore s i z e d i s t r i b u t i o n a s d e n o t e d by between V o ( N 2 ) and V o
the large d i f f e r e n c e
i n c a r b o n s s u c h a s M15NT.
(C02)
Since the
n a r r o w microporosity, measured by C 0 2 adsorption [91 r e m a i n s a l m o s t constant, i t is deduced that the oxygen surface groups that evolve
at temperatures up to 923K (almost exclusively as
C02)
w e r e located
on w i d e micropores. The m i c r o p o r e v o l u m e s V o ( N 2 ) o f Fig. 2 a l l o w a n a n a l y s i s o f the effect of HN03 oxidation on carbons with different porosity and
pore s i z e d i s t r i b u t i o n . T h e r e is, in g e n e r a l , ( e x c e p t i n t h e c a s e of carbon BO) a slight decrease in micropore volume with increasing oxidation, b u t t h e d e c r e a s e is m o r e n o t i c e a b l e f o r c a r b o n s A 0 a n d R O , w i t h wider microporosity (larger difference between V o
(N2) and
V o ( C 0 2 ) values). H o w e v e r , t h e e v o l u t i o n o f V o ( N 2 ) a n d V o ( C 0 2 ) i s very s i m i l a r , e x c e p t microporosity.
for R15N,
with
slight gasification of
to t h e
This general decrease may be adscribed
percentage of oxygen a t o m s occupying the surface of carbon. The o x i d a t i o n o f c a r b o n BO w i t h H N 0 3 a p p a r e n t l y p r o d u c e s a different
effect.
oxidation
with
Thus V o
4N
and
remains almost
(C02)
15N
nitric
acid
constant
solutions
after
since
microporosity w a s v e r y n a r r o w and u n i f o r m . H o w e v e r , t h e V o
the (N2)
values are very l o w but this is only due to an activated diffusion effect
[lo] caused
by
the
oxygen
surface
groups
blocking
the
entrance of N2 molecules to the micropores at the low temperatures of
adsorption Although
(77K). the p o r o s i t y o f t h e c a r b o n s is n o t s i g n i f i c a n t l y
modified by t h e o x i d a t i o n t r e a t m e n t , t h e T P D p r o f i l e s o f Fig. 3 show
that
the
chemical
changed by oxidat ion. to
C02
in
the
nature
of
the surface is considerably
Most carboxyl and
475-875K
range
lactonic groups d e c o m p o s e
whereas
carbonyl,
quinonic
phenolic g r o u p s e v o l v e a s C O in t h e 8 7 5 - 1 3 0 0 K t e m p e r a t u r e
and
range
[5]. At about 1325K, the evolution s e e m s to be almost finished. Fig. 3a s h o w s the effect of different oxidation t r e a t m e n t s o n a c o m m o n carbon, MO. Thus, oxidation in air does not introduce l o w temperature
C02
groups
because
their
inestability
at
the
temperature used for oxidation (573K). Oxidation w i t h H202 and H N 0 3 introduces groups desorbed as CO and C 0 2 although the stability
of
the latter is lower for the H202-treated carbon. On the other hand, the e x t e n t o f o x i d a t i o n d e c r e a s e s
in
the
order:
H N 0 3 > H202 >
334
,--.
m
0 rl
1.2
4
I
?c
6
I
1.0
M a, d
0
0.8
E E
v
C
0.6
.rl
Y
5
4 0
>
0.4
3M 0.2
k 0
a, Y (d
0.0
a:
1100
700
300
1100
700
300
Temperature ( K ) a) b)
Fig. 3 .
- M15N _ _ M15NT - B15N ---A15N
- MP3 ........
....... M24A
R15N
T P D profiles for oxidized carbons
air, i t
is r e m a r k a b l e t h a t a l a r g e a m o u n t o f o x y g e n c a n b e
introduced b y H N 0 3 i n c a r b o n M 1 5 N - n e a r 10 m m o l e of 0 p e r g r a m m e o f carbon-
without
modifying
the porosity.
This amount of oxygen
groups i s c o n s i d e r a b l y r e d u c e d a f t e r a h e a t - t r e a t m e n t 9233:
in N2 at
the low-temperature C 0 2 groups and the fraction of thermally
unstable CO g r o u p s a r e missing. T h e i n c r e a s e in h i g h - t e m p e r a t u r e C02
groups after this treatment indicates a
transformation
of
carboxyl to anhydride groups. Fig. 3 i n c l u d e s s o m e TPD p r o f i l e s of c a r b o n s w i t h d i f f e r e n t porosity oxidized by 15N HN03 solutions. Although the profiles are very similar,
the
order
treatment ( B < R < M N
A)
found does
for
the oxygen introduced by the
not
s e e m to be r e l a t e d to t h e
porosity o f t h e c a r b o n s . T h u s , i n R 1 5 N t h e g a s e v o l u t i o n is l o w e r than expected from the large porosity and extent of oxidation, and the large variation in porosity of the
carbons
MO
and
A0
(see
Fig 2 ) i s n o t p a r a l l e l e d by t h e s m a l l i n c r e a s e in o x y g e n s u r f a c e groups of the corresponding oxidized carbons.
This means that the
335 porosity i s n o t p l a y i n g a n i m p o r t a n t r o l e in t h e f i x a t i o n o f o x y g e n during oxidation by HN03.
O B O M O R 8 A
0
U
0
0.2
CO
0.4
0.6
+ C 0 2 surface groups reduced carbons
Fig. 4 . Relation between surface groups on the reduced carbons surface groups o n HN03-oxidized carbons (in meq/g)
Fig.
4
includes the plots of
total CO + C 0 2
evolved
and
from
carbons o x i d i z e d w i t h 4 N , 7 N and 1 5 N s o l u t i o n s a s c o m p a r e d t o t h e amounts e v o l v e d f r o m t h e r e d u c e d o r i g i n a l c a r b o n s . T h e d a t a f i t s l o p e of
straight l i n e s ,
the
strength
HN03
of
the
which
solution.
increases
This
means
with
that
the
increasing extent
of
oxidation - w h i c h a s s h o w n a b o v e is n o t g o v e r n e d by t h e p o r o s i t y - , will
be
r e l a t e d to t h e a m o u n t o f o x y g e n s u r f a c e g r o u p s o f
the
original carbons which, in turn, is a function of the experimental conditions u s e d burn-off),
in the a c t i v a t i o n p r o c e s s
(especially, gas and
although the groups seem to be located preferentially in
wider m i c r o p o r e s . I f t h i s w e r e n o t t h e c a s e , b e s i d e s a c c e s i b i l i t y problems o f t h e o x i d a t i o n a g e n t to t h e n a r r o w (C02)
micropores,the V o
v a l u e w o u l d h a v e d e c r e a s e d c o n s i d e r a b l y : t h u s , in a c a r b o n
with narrow microporosity such as B15N, the area occupied by oxygen surface g r o u p s ( a s s u m i n g a n a r e a of 0.083 n m 2 f o r a n o x y g e n a t o m [ll]) would be 2 0 8 m2/g
and this would decrease V o ( C 0 2 )
f r o m 0.242
cm3/g f o r BO to 0.164 c m 3 / g f o r B 1 5 N , w h e n t h e e x p e r i m e n t a l v a l u e is 0.234 cm3/g. A similar situation is found for carbons M15N, A15N and to a lesser extent, R15N, with wider microporosity. the d a t a f o r c a r b o n s h e a t - t r e a t e d
in N 2 at
Above all,
923K indicate that
336 porosity changes take place mainly in the wider microporosity since
Vo
(N2) increases considerably V o (C02) remains a l m o s t Constant. As
mentioned above, selective titration is also considered to
be a useful method to differenciate the surface groups introduced by t h e o x i d a t i o n t r e a t m e n t s . T h u s , a c c o r d i n g t o B o e h m
[ 3 1 r NaOH
titration w o u l d m e a s u r e p h e n o l i c , l a c t o n i c and c a r b o x y l g r o u p s , whereas N a H C 0 3 , a w e a k e r b a s e , o n l y c a r b o x y l g r o u p s . I n f a c t , t h e amount of NaHC03 consumed by all oxidized carbons is m u c h s m a l l e r than t h e a m o u n t o f NaOH. A s s h o w n i n Fig. 5 , e x p e r i m e n t a l d a t a f i t a straight line but there are significant deviations for c a r b o n s h e a t - t r e a t e d a t 9 2 3 K i n N 2 a f t e r o x i d a t i o n ( s e r i e s T). T h i s m e a n s that i n d e p e n d e n t l y of t h e t y p e of o x y g e n s u r f a c e g r o u p s o f
the
carbon, t h e r e is a r e l a t i o n s h i p b e t w e e n t h e a m o u n t s c o n s u m e d o f NaOH a n d N a H C 0 3 . O t h e r a u t h o r s [ 1 2 , 1 3 1 h a v e a l s o d e s c r i b e d l i n e a r relationships between the amounts consumed of NaOH, NaHC03, NaZC03 and N H 4 0 H w i t h c a r b o n s v e r y d i f f e r e n t to t h o s e d e s c r i b e d here. I t is to b e n o t e d t h a t t h e a c i d i t y o f t h e d i f f e r e n t o x y g e n s u r f a c e groups w i l l b e a f u n c t i o n o f n o t o n l y t h e f u n c i o n a l g r o u p i t s e l f (with a
c o r r e s p o n d i n g c o n s t a n t v a l u e o f pKa),
but
also of
its
position o n the carbon structure. I n any case, these relationships are a c o n f i r m a t i o n of the difficulty of differenciating o x y g e n surface g r o u p s by s e l e c t i v e titration. T h e d e v i a t i o n s f o u n d f o r
I
I
I
I
0
1
2
3
NaOH consumed (meq/g) MA
AMP
A MN
v AN
eRA
ORN
0
Fig. 5 . Relation between NaHC03 consumption by selective titration
oMT
(meq/g)
and
NaOH
(meq/g)
337 carbons of series T may be explained considering that these carbons d o not have low-temperature C02 groups and that the acidity of the high temperature C02 groups remaining in the carbons after the heat treatment i n N 2 is w e a k e r , l e a d i n g to a s m a l l e r a m o u n t o f N a H C 0 3 consumed in titration with respect to NaOH. It i s a l s o i m p o r t a n t to n o t e that
the
large reduction
in
acidity t a k i n g p l a c e a f t e r t h e h e a t t r e a t m e n t o f o x i d i z e d c a r b o n s (e.g., carbon M 1 5 N consumes 2.3 meq/g
NaOH, but only 0.72 meq/g
are
consumed for carbon M15NT) is indicative of a relationship f o r the oxygen s u r f a c e g r o u p s t i t r a t e d by N a O H a n d t h o s e d e c o m p o s i n g to C02.
Figure 6 includes the data for the amounts of NaOH consumed in
the t i t r a t i o n v e r s u s t h e a m o u n t o f C02 g r o u p s m e a s u r e d by TPD. There is a linear relationship f o r all carbons the slope being near unity ( e x c e p t t h o s e of s e r i e s T and B). T h i s m e a n s t h a t t h e C 0 2 evolving groups will undergo hydrolysis to weak acids w h i c h may be stoichiometrically
titrated
by
NaOH.
Carbons of
series T
fit a
different s t r a i g h t l i n e , w i t h a s l o p e h a l f of t h e o n e d e f i n e d b y the other carbons, because structures) b e h a v e
in a
high-temperature
different
way.
C 0 2 groups
Upon
(anhydride
hydrolysis,
these
groups w o u l d g i v e t w o a c i d s w h i c h , a l t h o u g h t h e y c a n b e t i t r a t e d by NaOH, would decompose in TPD to CO and C 0 2 molecules.
I n the other
oxidized c a r b o n s , t h e p r e d o m i n a n t C 0 2 g r o u p s a r e l o w - t e m p e r a t u r e groups
(carboxyl o r
lactonic
structures);
each
group
will
Do
\
aJ
il
0
E E
v
Q
m
>
4
0
> m
N
0 U
0
1
2
3
NaOH consumed ( m e q / g ) o p4A V AN
A MP
RA
AMN
0 RN
-MT OBN
Fig. 6. Relation between C02-evolving groups (mmole/g) and measured by NaOH titration (meq/g)
acidity
338 correspond to a n a c i d t h a t c a n be t i t r a t e d a n d w o u l d d e c o m p o s e to one C02 molecule. The data for the oxidized carbons of series B s h o w a large discrepancy between the t w o techniques, the l o w values obtained by titration are d u e to the narrow microporosity of the samples w h i c h makes difficult the access of the alkali to the surface groups (as shown in Fig. 2, N2 adsorption is restricted in this s a m p l e s at the l o w t e m p e r a t u r e o f 7 7 K ) . T h e s e r e s u l t s m a y b e t a k e n to i n d i c a t e that s e l e c t i v e t i t r a t i o n o f o x y g e n s u r f a c e g r o u p s c a n b e a p p l i e d only in carbons with relatively wide porosity a s those 0 s series M , A
a n d R. Finally,
it
to
is
be
noted
that
there
is
not
a
clear
relationship b e t w e e n t h e a m o u n t o f C O g r o u p s m e a s u r e d b y TPD a n d N a O H t i t r a t i o n . i n d i c a t i n g t h a t a r e l a t i v e l y l a r g e p r o p o r t i o n of the groups are very weakly acidic and cannot be titrated by NaOH.
CONCLUSIONS The
oxidation
treatments
substantially m o d i f y
described
the microporosity
in
this
study
do
not
( m e a s u r e d by b o t h N 2 or
C02), e x c e p t if t h e o x i d i z e d c a r b o n s a r e heat-treated ( 9 2 3 K ) i n N 2 ( t h e r e is t h e n a n i n c r e a s e in w i d e m i c r o p o r o s i t y ) .
For a given
carbon, t h e o r d e r of i n c r e a s i n g e x t e n t o f o x i d a t i o n p r o d u c e d i s HNO) > H 2 0 2 > a i r a n d i t is a l m o s t i n d e p e n d e n t o f t h e p o r o s i t y o f the c a r b o n s , b e i n g a f u n c t i o n of t h e c o n t e n t in o x y g e n s u r f a c e groups of the original carbon. The surface groups introduced s e e m s to be preferentially located in the wide micropores. The function
o f t h e o x y g e n s u r f a c e g r o u p s i n t r o d u c e d is a t h e n a t u r e of t h e o x i d a t i o n agent. T h u s , c a r b o n s
nature
of
treated w i t h H N 0 3 a n d H 2 0 2 h a v e a l a r g e a m o u n t o f l o w - t e m p e r a t u r e C 0 2 groups whereas air introduces only high-temperature C 0 2 groups. When the oxidized carbons are heat treated in N2, they have a large proportion o f C O g r o u p s . There
is a
linear relationship between the groups that
decompose to C 0 2 in T P D a n d t h e g r o u p s t i t r a t e d by NaOH. b u t s u c h relationship h a s results
not
described
in
been
found
this
with
study
CO-evolving
show
that
TPD
groups. is
a
The more
comprehensive t e c h n i q u e t h a n s e l e c t i v e t i t r a t i o n to s t u d y o x y g e n surface g r o u p s i n carbon: o n t h e o t h e r h a n d , t i t r a t i o n c a n n o t b e used in carbons with narrow micropores.
339 REFERENCES 1.
2. 3. 4.
5. 6.
7. 8.
9.
10.
11. 12. 13.
J. V a n D r i e l , " A c t i v e carbon... a f a s c i n a t i n g m a t e r i a l " . Ed. Norit. N.V. A m e r s f o o r t . H o l l a n d (1983). B.R. P u r i , " C h e m i s t r y a n d P h y s i c s o f C a r b o n " , vol. 6. Ed. P.L. Walker, j r . M a r c e l D e k k e r . N e w Y o r k (1970). H.P. B o e h m , Adv. in C a t a l y s i s , 16, ( 1 9 6 4 ) 179. J. B. M a t t s o n and H. B. M a r k , j r , " A c t i v a t e d c a r b o n s u r f a c e Chemistry a n d A d s o r p t i o n f r o m Solution". M a r c e l D e k k e r . N e w York (1971). G. T r e m b l a y , F.J. V a s t o l a and P.L. W a l k e r , j r , C a r b o n l a , (1978) 3 5 M. M. D u b i n i n , " C h e m i s t r y and P h y s i c s o f C a r b o n " , vol. 2 . Ed. P. L. W a l k e r , j r . M a r c e l D e k k e r . N e w Y o r k (1966). J.D. L o p e z - G o n z a l e z , F. M a r t i n e z V i l c h e z a n d F. R o d r i g u e z Reinoso, Carbon 18 ( 1 9 8 0 ) 413. J. G a r r i d o S e g o v i a , A. L i n a r e s S o l a n o , J.M. M a r t i n M a r t i n e z , M. Molina Sabio, F. Rodriguez Reinoso and R. Torregrosa M a c i a , J. C h e m . SOC. F a r a d a y T r a n s , I., s2 ( 1 9 8 7 ) 1081. J. G a r r i d o S e g o v i a , A. L i n a r e s S o l a n o , J.M. M a r t i n M a r t i n e z , M. Molina Sabio, F. Rodriguez Reinoso and R. Torregrosa Macia, Langmuir, 3 ( 1 9 8 7 ) 76. S.J. G r e g g a n d K.S.W. S i n g , " A d s o r p t i o n , S u r f a c e A r e a a n d Porosity Acad. P r e s s . L o n d o n (1982). N.R. L a i n e , F.J. V a s t o l a and P.L. W a l k e r , jr., J. o f P h y s . Chem. 61 ( 1 9 6 3 ) 2030. H.P. B o e h m , E. D i e h l , W. H e c k a n d R. S a p p o k , A n g e w C h e m . Internet. Edit. 2 ( 1 9 6 4 ) 664. O.P. M a h a j a n , A. Y o u s s e f f and P.L. W a l k e r , jr., Sep. SCi. 6 Technol. 13 ( 1 9 7 8 ) 487.
".
This Page Intentionally Left Blank
F. Rodriguez-Reinosoet al. (Editors), Characterization of Porous Solids II 0 1991 Elsevier SciencePublishersB.V.. Amsterdam
341
ADSORPTION OF METHANOL AND WATER BY CHARCOAL CLOTH
A.M. GonFalves da Silva Centro de Quimica Estrutural, Complexo I, Instituto Superior TBcnico, 1096 Lisboa Codex, Portugal. M.M.L. Ribeiro Carrott, P.J.M. Carrott. M.M. Brotas de Carvalho Departamento de Quimica, Faculdade de Ciencias, Universidade de Lisboa, Rua da Escola Politecnica, 5 8 , 1294 Lisboa Codex, Portugal.
ABSTRACT Adsorption isotherms of the pure vapours and of liquid mixtures of water and methanol on charcoal cloth at 293K have been determined. The surface excess isotherms show that methanol is always the preferentially adsorbed component over the whole composition range. The isotherms exhibit an unusual feature with a secondary maximum at high mole fractions of methanol. In order to explain this behaviour charcoal cloth was studied before and after modification of the surface chemical structure. Evidence was found that as the polarity decreases the surface excess increases and the secondary maximum becomes less pronounced.
INTRODUCTION
It is well known that the adsorption of water by carbon adsorbents is very sensitive to the concentration and nature of the active sites present on the surface I l l . The adsorptive behaviour of other polar molecules, such as n-alcohols, may also be complex. In the case of methanol, for instance, gas chromatographic retention volumes and limiting isosteric heats of adsorption change significantly when the carbon surface is modified [21. The shape of the adsorption isotherm at low p/po is also dependent on the polarity of the surface [31. Surface excess isotherms from solution determined on acid-washed (and therefore probably slightly polar) graphitised carbon black for the systems water + methanol and water + ethanol exhibit an unusual feature with a "step" occurring at high mole fractions of alcohol 141. One must take into account
that
the
chromatographic results were
determined
at
very
low
concentrations of methanol, whereas the step in the solution isotherms occurs at high concentrations of methanol. It is not clear, therefore, if the two effects are related or not. In order to explain this behaviour measurements on the
pure vapour/solid
and
liquid mixture/solid
interfaces of
water
and
methanol on a well characterised microporous adsorbent (charcoal cloth) were
342
carried out. A first attempt at comparing the isotherms was made. Furthermore we have investigated how the magnitude and the position of the step in the surface excess isotherms of water + methanol is affected by modification of the polarity of the surface.
EXPERIMENTAL Samples of charcoal cloth were prepared from viscose rayon by carbonisation in nitrogen at 1123K followed by activation in C02 at the same temperature. Modification of the surface was carried out by heating in flowing hydrogen at 673K for 4h (sample B ) . Nitrogen isotherms at 77K were determined using a Carlo-Erba Sorptomatic. Methanol isotherms at 293K were obtained using a conventional volumetric apparatus with greaseless taps and a Datametrics Barocel pressure gauge. The results were checked in another volumetric apparatus with a Schaevitz pressure transducer (Type P-274). Water vapour isotherms at 293K were determined gravimetrically in a CI microbalance, with a Robal control unit and a Bell & Howell (Type BHL-4105) pressure transducer. Adsorption from solution, at 293K. was measured by the conventional immersion method, which involves agitation of the adsorbent within the solution, using a Waters Associates R401
differential
refractometer. Prior
to
adsorption
measurements samples were outgassed at 523K to a residual pressure < 10-'Pa.
RESULTS
The vapour phase isotherms obtained on sample A are shown in Fig.1, the adsorbed amounts being referred to the outgassed weight
in each case.
Application of the Gurvitsch rule leads to the values of pore volume, v , P
given in Table 1. There is excellent agreement between water and methanol, although the values are only about 70% of that obtained with nitrogen. The "monolayer equivalent capacities" and corresponding surface areas estimated from analysis of the isotherms by the BET method, over the range of 0.01 < p/po < 0.1 are given
in Table 1.
Also
given
in Table 1
is the
concentration of polar groups of the surface, estimated from the first point of inflexion of the water isotherm at p/po = 0.08. In Fig.2 are presented the specific surface excess isotherms for the systems (water ( 1 ) + methanol (2)) / sample A and (water ( 1 ) + methanol (2)) / sample B. The specific surface excess of component (2) is expressed as
e
noAx2/m, where no is the total amount of 2 and
e Ax
2
= xo 2
-
1
in the system and
xe is the variation of the bulk mole fraction when the solution, of 2
initial concentration xo is equilibrated with a mass m of the solid. 2'
343
0.15
I
10
0.8 PIP0 Fig.1 Adsorption isotherms of nitrogen at 77K ( n ) and c)f methanol (V) and
water
TABLE
(0)at
293K on sample A.
1
Amounts adsorbed, ns, monolayer equivalent areas, as, and pore volumes, v P'
evaluated from the gadsolid and liquidlsolid data for sample A. Cross-sectional areas
-
a (N 1 = 0.162 nm', m
2
am(CH OH) = 0.219 nm2.
v from Gurvitsch rule at 0.95~'.
CH30H
Hzo
cn30n+H20
( G/S 1
( G/S 1
( G/S
(L/S)
9.4
5.2
(BET)
(BET
ar/m2g-'
914
636
v /cm3g-'
0.39
0.27
ADSORPTIVE (INTERFACE) ns/mmol
g-'
1
(Point B)
0.26
5.1
(Eq.1)
344 L
3
r
-oI 0
E 2 \
E \ urn
a C
1
Fig.2 Specific surface excess isotherms at 293K for water (1) + methanol ( 2 ) on samples A (0)and B ( 0 1 .
DISCUSSION
The shape of the nitrogen isotherm, with a well defined plateau, shows that the charcoal cloth is a microporous material with a very low external area. Furthermore it is evident from the values of "monolayer equivalent area" and pore volume (Table 1) that the surface is less accessible to methanol and water than to nitrogen. Although the total pore volume evaluated with methanol and water are similar. the shapes of the isotherms indicate that the adsorption mechanisms are different. In the case of water, adsorption involves strong specific interactions between the water molecules and polar groups on the surface which determine the isotherm shape at low p/po. In comparison with other results [51 the upswing in the water isotherm occurs at quite a low p/po indicating that the surface is relatively polar. The value of 1 mmol g-l for the concentration of polar groups is consistent with this. In the adsorption of methanol there
345
I
0.2
I
t
0.L
0 5
x2
ee
e
Fig.3 x x /(noAx /m) as a function of xe for sample A.
might also be specific interactions with the surface although in this case the dispersion forces, enhanced in the micropores, are certainly stronger and are those which determine the shape of the isotherm. Adsorption results from solution (Fig.2) show that methanol
is the
component preferentially adsorbed over the whole composition range, this behaviour being accentuated in the reduced sample. An unusual feature of the isotherms is the presence of a step at high mole fractions of methanol. For sample A the step appears as a well defined maximum, while for sample B the step is less well pronounced. The analysis of adsorption data from solution obtained on sample A was carried out using the Everett equation I61 written in the form
c e e = (l/n”)(xz+(l/K-l)) c xlx2/(noAxz/m)
(Eq.1 )
where ns is the amount adsorbed per unit mass of solid and K is the adsorption
346
equilibrium constant. This equation assumes both ideal bulk and adsorbed phases of equally sized molecules. It is noteworthy, however, that the Everett equation doesn't require any assumption about the thickness of the adsorbed
ee
e
phase. The representation of xIx2/(noAx2/ml versus xt for sample A is linear over the range 0 <
e x2
< 0.5 (Fig.3).
The ns value estimated over this range is very similar to the "methanol monolayer equivalent" of the gas-solid interface (Table 1). This agreement
x = 0.5 can be described by a suggests that the liquid-solid interface up to : monolayer model. This model doesn't explain the step occurring at higher methanol mole fractions which can be associated with the formation of a second layer [71 or a different interface structure. As far as we know, such an isotherm conformation, with a secondary maximum at higher mole fractions of the preferentially adsorbed component has not yet been readily explained
It is interesting to notice that as the polarity decreases the second maximum becomes less pronounced and the isotherm shape resembles more that obtained on Graphon I41. The similarity of the isotherm shape in two adsorbents with such a different surface texture (microporous and non-porous) suggests that the step in the isotherm is related to the nature of the surface rather than the porosity.
ACKNOWLEDGEMENTS The authors are grateful to Dr. J.J. Freeman of Brunel University for providing the sample of charcoal cloth and to Junta Nacional de InvestigaqZo Cientifica e Tecnol6gica (Portugal) for financial support.
REFERENCES 1 R.C. Bansal, J.B. Donnet & F. Stoeckli, Active Carbon, Marcel Dekker, New York, 1988. 2 P.J.M. Carrott, M. Brotas de Carvalho & K.S.W. Sing, Adsorp.Sci. Technol., 6 (1989) 93. 3 A.V. Kiselev, Adv.Chromatography, 4 (1967) 113. 4 D.H. Everett & A.J.P. Fletcher, J.Chem.Soc., Faraday Trans.1, 82 (1986) 2605. 5 P.J.M. Carrott, R.A. Roberts, M.B. Kenny, K.S.W. Sing & C.R. Theocharis, The Adsorption of Water Vapour by Microporous Solids, this volume. 6 D.H. Everett, in R.H. Ottewill, C.H. Rochester & A . L . Smith (Eds. 1, Adsorption from Solution, Academic Press, London, 1983. 7 R.C. Bansal, J . B . Donnet & F. Stoeckli, Active Carbon, Marcel Dekker, New York, 1988, p.237.
F. Rodriguez-Reinosoet al. (Editors),Characterization of Porous Solids II 0 1991 Elsevier Science Publishers B.V., Amsterdam
INFLLENCE OF COAL
PREOXIDATION AND
REACTIVE GAS
347
FLOW
RATE ON
TEXTURAL PROPERTIES O F ACTIVE CARBONS J . A , Pajares, J.J. Pis, A.B. Fuertes, A.J. P&rez, M. Mahamud and J.B. Parra
Instituto Nacional del Carbbn, CSIC, Aptdo. 73, 33080 Oviedo, SPAIN
ABSTRACT The effects of air preoxidation of coal and flow rate of activating gas on the preparation of activated carbon were studied. A high-volatile bituminous coal was used as starting material. Coal preoxidation has a beneficial influence on the textural development of chars and activated materials. The flow rate of the activating gas (CO,) has a big influence on the development of the porous network of the activated carbons. The best results were obtained with a high degree of coal preoxidation and a low flow rate of CO, in the activation step. INTRODUCTION Activated carbon is a material with an increasing number of applications in processes related to environmental protection, especially in the treatment of waste water and gas emissions. Its traditional use as an adsorbent and catalyst is well known (refs. 1,2).
Continuous increase in demand makes it necessary to make use
of different raw materials for activated carbon production, e.g.
coals, coconout shell and peat. Coals
are
the
main
source
employed
in the
production
of
activated carbons (ref. 3 ) . Previous pyrolysis is used in order to produce a porous coke or char. These are then activated by heating in a suitable oxidizing agent, e.g. steam (ref. 4 ) , carbon dioxide (refs. 5 , 6 ) or air (ref. 7). During gasification, in the activation step, loss of carbon takes place and as a result the texture of the material undergoes important modifications. The pore size distribution changes, with an increase in the specific surface area and of the meso and micropores network. As a result the accessibility of molecules to the whole solid bulk is improved. The properties of activated carbons are strongly conditioned by the features of their precursors: coals and chars. Air oxidation of
348
bituminous
coals
causes
an
important
modification
in
their
characteristics, so caking properties can be completely destroyed (refs. 8 , 9 ) . Gasification conditions have a big effect on the development of porosity. In this work coal preoxidation and flow rate of the activating agent, CO,, were studied in order to find out the effects on the textural properties of the activated carbons obtained. EXPERIMENTAL
A high-volatile A bituminous coal from the Ma Luisa mine, from the Central Asturian Basin, in the North of Spain, was used. The most important characteristics of the starting material are given in Table 1. The coal was ground and the size fraction + 0 . 1 2 5 - 0 . 4 2 5 mm was used. TABLE 1 Characteristics of the coal used Proximate analysis (%wt)
Ultimate Analysis (%wt, daf)
Moisture
V.M. Ash (dry) (daf)
C
H
N
S
0 (diff.)
1.35
3.80
86.70
5.04
1.27
0.52
6.47
37.16
Arnu test
Maceral Composition
Tr,K
Ts,K
Tc,K
b,%
Vit.
Exi.
622
692
738
179
65.2
10.6
Semif. 7.4
F.S.I.
C.V.
--
Fus.
8
13.6
kcal/kg 8482
F.S.I. : Free Swelling Index; C.V.: Calorific value. The oxidation of coal was carried out in an oven with forced circulation, at 4 7 3 K for different periods of time: 0, 6 , 1 2 , 1 8 and 2 4 hours. Change in chemical composition and weight gain are given in Table 2 . An increase up to 7.1% of weight was measured. The pyrolysis of fresh and oxidized coal samples was performed under nitrogen at 1 1 2 3 K with a heating rate of about 6 0 K min-' and 5 min of soaking time. The activation was carried out with CO, in a vertical quartz reactor (I.D. 2 0 mm) (ref.lO), at 1 1 2 3 K and two different CO, flow rates: 7 and 5 0 0 cm3 min-l. Gasification was performed under isobaric conditions, at 1 0 2 . 7 kPa ( 7 7 0 mm Hg) until 5 2 k 2 % burn-off. The process evolution was followed by GC of exhaust gases and the yield evaluated by weight. Textural properties were
obtained
from measurement
of
true
349
(helium) and apparent (mercury) densities, total open pore volumes and pore volume distributions. For determination of the helium densities, a Micromeritics Autopycnometer 1 3 2 0 was used. Apparent densities were determined in a Carlo Erba Macropore Unit 1 2 0 . The pore volume distributions were evaluated with a mercury porosimeter, Carlo Erba 2 0 0 0 .
The volume of pores with radius smaller than 3 . 7
nm was calculated by difference between total pore volume (from helium and mercury densities) and the pore volume as determinated by mercury porosimetry. Specific surface areas were determined by physical. adsorption in a Omnisorb 3 6 0 and a Sorptomatic Carlo Erba 1900.
N, at 77 K and
CO,
at 2 7 3 K were used. We
assumed one
cross-molecular area for a molecule of N, of 0 . 1 6 2 nm2 and 0 . 1 8 7 nm2 for a molecule of CO,.
All textural properties are expressed on a
dry ash free basis (daf). RESULTS AND DISCUSSION Coal preoxidation is a crucial step in the preparation of activated carbons. Air oxidation produces a decrease in the plastic properties of bituminous coals, that can be totally destroyed (refs. 8 , 9 ) . An important transformation in the chemical composition and in the porous structure of the coals is produced (ref. 1 1 ) . Some chemical and textural data of oxidized coals are given in Table 2 . The caking properties of samples decrease as a result of air oxidation, so a drastic reduction in free swelling index (FSI) occurs, from 8
in fresh coal to 1 and 0 in oxidized samples.
Likewise, an important decrease in carbon content, and a parallel increase in volatile matter and oxygen content, mainly in this latter element, is observed. CO, surface areas for the fresh and oxidized coal samples are given in Table 2 . Preoxidation have a clear effect on the CO, surface areas of coal samples. The enhancement of surface area due to preoxidation is of the same order as those obtained by other authors (ref. 1 2 ) working with a hvA ( 3 0 . 4 %V.M.) and lv ( 1 9 . 5 %V.M) bituminous coals and using 4 4 4 K ,as temperature of oxidation. Pyrolysis Coal preoxidation affects the textural properties of the chars subsequently obtained by pyrolysis as shown in Table 3 . The drastic reduction in plastic properties of bituminous coals which occurs as a result of oxidative treatment, seems to be the principal cause of this variation (ref. 1 3 ) .
350
TABLE 2 Characteristics of the oxidized coals.
0 2.4 4.3 6.0 7.1
0 6 12 18 24
8
1 0 0 0
37.2 32.6 32.5 33.1 33.0
86.7 81.7 80.0 78.9 78.1
1.3 1.7 1.7 1.7 1.7
5.0 4.1 3.8 3.6 3.4
0.5 0.4 0.4 0.4 0.4
6.5 11.9 14.1 15.2 16.4
146 159 180 180 197
All results are expressed on a d.a.f. basis. The pore volume distribution of the chars is given in Table 3 . An enhancement of the volume of pores with a radius smaller than 3 . 7 nm was observed, as a result of the increase in coal preoxidation. A relative decrease in those pores with a radius greater than 2 5 nm was also produced. No significant variation was observed in mesopores, which in all cases present a percentage less than 5% of the total pore volume. TABLE 3 Textural properties o f the chars obtained Coal oxidation
Density ( g/cn? )
CO, surface area (dl3-l)
Porosity
POIW
volume (nm3g-l)
(%I radius (nm) Total >25
3.7/25
(3.7 ______~ ~
0 6 12 18 24
1.767 1.857 1.838 1.840 1.856
1.476 1.328 1.313 1.295 1.338
196 547 594 616 605
16.5 28.5 28.6 29.6 27.9
111 215 218 229 209
48 79 89 78
4 9 9 11
73
6
59 127 120 140 130
An important increase in the CO, surface area of the chars was observed, especially in the first steps of coal oxidation. This increase, up to 3 0 0 % , is bigger than that obtained by other authors (ref. 1 4 ) . Values of N, surface areas are smaller than those determined from CO, adsorption, showing the importance of the micropore network in this stage. Activation During the pyrolysis step, a primary pore structure is developed. The increase in porosity and initial pore structure during gasification is strongly influenced by previous treatment of
351
the caking coals, e.g. air oxidation. The evolution of the surface area (CO,,
2 7 3 K ) of coals, chars
and activated chars with the time o f oxidation of the coal is shown in Figure 1 . Coal preoxidation determines a big increase in the surface area of chars. A large and continuous increase in the CO, surface
area
of
activated
carbons
is
also
produced
during
gasification, as a consequence of a more intense air oxidation of the raw coal samples.
S8O0 E
a
& 600a
W 0
2
400-
3 VJ
N
s
200 -
/
COAL
0 0 -
0-0-0-
I
I 6
0
0
I 12
I
I 18
24
TIME OF COAL OXIDATION,h
Figure 1. Variation of surface area (CO,, 2 7 3 K ) of coal, chars and activated carbons, with the time of coal oxidation. The control of
operational parameters in the preparation of
activated carbons is of great importance for tailoring their texture for specific applications, Flow rate of activating gas in the activation step is an important parameter that can be used in this way. In order to
study the
influence of
CO,
flow rate, a series
of experiments were performed, in which two different flows, 7 and 500 cm3/min, were used. A s a consequence of the modification of the
experimental conditions, char reactivity and CO concentration change significantly. In fact, the semi-reaction time varies from 8 . 4 h to 42.6
h when 5 0 0 and 7 cm3/min were used, respectively. A l s o CO
concentration moves from values of about 3 - 1 2 % at a flow rate of 500 cm3/min, to 4 5 - 8 0 % when a flow rate of about 7 cm3/min was used. The evolution of pore volume distributions, calculated from mercury porosimetry and helium densities, of activated carbons
352
obtained from the subsequent chars of oxidized coal samples, is shown in Figure 2. The total pore volume of activated carbons increases sharply in the first steps of coal preoxidation and then it remains practically constant. This increase is more accused when low flow rates of the activating agent are employed. During gasification a progressive enlargement of
the pores
previously formed in the pyrolysis step is produced. This fact determines the change of porosity in a micro-meso-macro sequence (ref. 15). Consequently it seems that the pore volume distribution of chars has a strong influence on the textural properties of activated materials. When low flow rates of CO, were used, the increase of pore volume is especially noticeable for the pore volume contribution of pores with a radius smaller than 3 . 7 nm. These results are in agreement with our previous results (ref. 16) and with the studies of Rand and Marsh (ref. 171, who observed that a greater micropore volume is developed by gasification when a lower flow rate is used. A s can be seen in Figure 2, low flow rates of CO, give activated
carbons with a bigger development of pores in the range 3 . 7 - 2 5
nm.
This is of great importance in view of the eventual use of these materials (ref. 18). In fact, a well-balanced pore size distribution is essential in processes in which both surface area and
the
penetration of reactive gases
the
into the inner porosity of
particle are important.
25 n m “Rp -25 3.75 nm
600
400
200
0
0
200
400
600
PORE VOLUME, rnrn3/g Figure 2. Pore volume distributions of activated carbons from oxidized coal samples.
353
Figure 3 shows the influence of parent coal preoxidation on the porosity of activated carbons obtained when two different flow rates of CO, are used in the activation step. In both cases an increase in porosity is observed with coal preoxidation, which is more important for lower flow rate of
CO,.
-
0
0
0
7cm3 ~ 0 2 / m i n
0
o
0
500cm3~0~/min
12
6
24
18
T I M E OF COAL OXIDATION, h
Figure 3 . Evolution of activated carbon porosity with the extent of parent coal preoxidation, when two different flow rates of activating gas were used. As can be seen in Figure 4 the surface area of the activated materials increases with the extent of coal preoxidation. This increase is more important in the N,-BET specific surface area ( S B E T than ) in the C0,-DR
Activated
carbons
corresponding equivalent surface area ( S D R ) .
prepared
from
the
most
oxidized
coals
and
activated at the lowest flow rates, exhibit the highest adsorption capacity. are twice When low flow rates of CO, are used, the values of ,,S, as big as those obtained at high flow rates. This suggest an enlargement in the diameter of micropores when low flow rates of
CO, are used. These results agree with those of Marsh (ref. 19) who maintains that the increase in (S,,,-S,,) is due to a growth in the diameter of micropores.
7 6
0
I2
18
T I M E OF C O A L OXIDATION,h
0
6
I2
i8
24
24
TIME OF COAL OXIDATION, h
Figure 4 . Evolution of activated carbon surface area with the extent of parent coal preoxidation. CONCLUSIONS Coal preoxidation greatly affects the textural properties of chars obtained by pyrolysis. A very important increase in the CO, surface area of chars was reached as a result of a convenient oxidative pretreatment of bituminous coal. Raw coal preoxidation always produces an important increase in the surface area of the activated carbons obtained. The flow rate of the activating agent has a big influence on the development of the porous network of the activated carbons. Low CO, flow rates in the activation step gives materials with a better balanced pore distribution and a more highly developed microporosity than those obtained at high flow rates. A right choice of the degree of coal preoxidation and activating gas flow rate, can produce activated carbons with suitable textural development. ACKNOWLEDGEMENTS The authors thank Fundaci6n para el Foment0 de la Investigacion en Asturias (FICYT) for financing this work. A.J.P. and M.M. wish to express their thanks to M.E.C. for F.P.I. grants.
REFERENCES
1
R.C. Bansal, J.B. Donnet and F. Stoeckli, Active Carbon, Marcel Dekker, New York, 1988.
355
10 11 12 13
14 15 16 17 18 19
D.L. Trimm, in Catalysis, vol. 4, p. 210, The Chemical Society, C. Kemball and D.A. Dowden (Eds.), London, 1981. J. Wilson, Fuel, 60 (1981) 823-831. E. Klose and M. Born, Fuel, 64 (1985) 1313-1316. M. Kawahata and P.L. Walker, Jr., in Proc. 5thCarbon Conf., Vol. 2, Pergamon Press, New York, 1963, pp. 251-263. K.E. Makhorin and V.A. Borodulya, in Proc. 5th Eng. Found. Conf. on Fluidization, Engineering Foundation, New York, 1986, pp. 595-602. T. Boudinova, N. Perov and G. Angelova, High Temp. Technol., 4 (1986) 97-100. J.J. Pis, A. Cagigas, P. SimBn and J.J. Lorenzana, Fuel Processing Technol., 20 (1988) 307-316. D.J. Maloney, R.G. Jenkins and P.L. Walker, Jr., Fuel, 61(2) (1982) 175-181. A.B. Fuertes, J.J. Pis, A.J. Pkrez, J.J: Lorenzana, J.A. Pajares and J.M. Palacios, Vacuum, 39 (1989) 677-681. M.M. Ludvig, G.L. Gard and P.H. Emmett, Fuel, 62 (1983) 1393-1396. D.J. Maloney and R.G. Jenkins, Fuel, 64 (1985) 1415-1422. J.J. Pis, J.A. Pajares, A.B. Fuertes, M. Mahamud, J. B. Parra, A.J. Pkrez and B. Ruiz, Carbone 90, July 16-20, Paris, 1990, pp. 114-115. O.P. Mahajan, M. Komatsu and P.L. Walker, Jr., Fuel, 59 (1980) 3-10. F . Rodriguez-Reinoso, A. Linares-Solano, J.M. Martin-Martinez and J.D. L6pez-Gonzblez, Carbon, 22 (1984) 123-130. J.J. Pis, A.B. Fuertes, A.J. Pkrez, J.J. Lorenzana, S . Mendioroz and J . A . Pajares, Fuel Processing Technol., 24 (1990) 305-310. B. Rand and H. Marsh, Carbon, 9 (1971) 79-85. T. Wigmans, Carbon, 27 (1989) 13-22. H. Marsh, Carbon, 25 (1987) 49-58.
This Page Intentionally Left Blank
F.Rodriguez-Reinosoet al. (Editors),Characterization of Porous Solids II
357
0 1991 Elsevier Science PublishersB.V., Amsterdam
EVALUATION OF MICROPOROSITY IN STEAM ACTIVATED BROWN COAL HUMIC ACIDS CHARS T. Siemieniewskal ,
K. Tomkow’ ,
J . Kaczmarczyk’ ,
A. Albiniak’ , Y. Grillet‘ and
M. F r a n ~ o i s ~
’ Institute
of
Chemistry
and
Technology
of
Petroleum
and
Coal, Technical
University of Wroclaw, Wroclaw (Poland)
‘ Centre de Thermodynamique et de Microcalorimetrie du CNRS, Centre de
Recherches
sur
la Valorisation
des
Marseille (France)
Minerais, Ecole
Nationale
Superieure de Geologie, Vandoeuvre (France)
ABSTRACT Different steam
approaches were
activated humic
tried to
evaluate nitrogen sorption data on
acids chars in terms of microporosity. Special attention
was payed to C02 adsorption at temperatures from 195 K to 298 K.
INTRODUCTION The
as-method (1,2) provides a
useful means to detect the presence of
mesoporosity in a porous solid ; it can also be successfully applied to evaluate the volume of micropores, especially in solids devoid of mesopores. However, the capillary structure of activated carbons is frequently composed of both kinds of porosity by
and so
adapt
them to
adsorption of
the evaluation might become more difficult. Attempts were made
various authors
to transform
a single kind o f
on the
volume filling
the
the aim present
attention
purpose to
can be done by elimination of
surface of mesopores, as is the case when the Dubinin theory of micropores
-the carbon dioxide with
experimental isotherms, with the porosity. This
( 3 ) . Another interesting approach
is applied
subtraction method
(CDS)- has recently been proposed ( 4 ) ,
to obtain an isotherm for porous systems devoid of micropores. In paper
some
related problems
are
considered, with
particular
to the evaluation of the carbon dioxide adsorption data. The research
was carrj.ed out on a suite of progressively activated chars from humic acids.
EXPERIMENTAL Humic
acids
(HA)
were
obtained
by
alkaline
extraction
of
a
humodetrinitic brown coal, followed by precipitation with hydrochloric acid. The pyrolysis of HA (grain size 0.5 - 1.0 mm) was carried out in an atmosphere of Ar up
to 8S0°C, at a
chars
(HA 850) were
heating rate of 5 K/min. After a holding time of 30 min the cooled to
room temperature
in Ar. From HA 8 5 0 , activated
358 chars were
obtained by
activation in steam at 800°C in a thermogravimetric
apparatus. Sorption measurements were carried out using N,
at 77 K (volumetric
apparatus) and CO, at 195-298 K (gravimetric apparatus - McBain quartz springs). The
density of N, in the adsorbed state was taken as 0 . 8 0 8 g/cm3. The values of
the
cross-sectional areas were taken :
and
for the
for the N, molecules as 0.162 nm2 (2),
at 195 and 273 K as 0 . 1 7 run2 (5) and 0.187 nm2 ( 6 ) ,
CO, molecules
respectively. For standard adsorption isotherms Spheron 6-2700 with a BET (7) surface area
81 m2/g
of
,)
,,,S(,,
distribution of mesopores were based
),
(Sws
(8),
on the
was
used. Calculations of the pore size
including the
desorption branches
surface area of the mesopores of the nitrogen isotherms. The
volumes of macropores were determined by mercury porosimetry. RESULTS AND DISCUSSION Carbon dioxide adsorption at various temDeratures CO, adsorption data can be fitted, without significant deviations, into one
characteristic curve, when
calculated on
the basis
of
the
the
CO,
densities in the adsorbed state are
"b" constant in the Van der Waals equation,
instead of taking them as densities of CO, as bulk liquid (Fig. 1 and 2). The
CO, adsorption data (corrected for adsorption on the surface of the
mesopores, assuming a
slit-like shape) were
found to
follow the two-term
Dubinin-Radushkevichequation ( 3 ) :
where :
A
=
RTln(po/p)
- differential molar work of adsorption ; V-current
adsorbed volume ; Vo, , Vo2 and EO1 , EO, micropores
and
-
limiting volumes of narrower and wider
respective characteristic energies ; p - affinity coefficient
(standard vapour : benzene). Vol and EO1 were calculated from the characteristic curves in Dubinin coordinates (Fig. 3 ) . assuming that at sufficiently low relative pressures the amounts the
of CO,
adsorbed in Vo2 are negligible. The calculations were based on
low pressure
parts
(p/po
=
0.0002 - 0.002) of
the
273 K
plots ;
the
corresponding results for 253 and 298 K did not differ significantly. Adsorption data
at
195 K
could not be
relative pressures
used
the molecular
to
calculate Vol
and EO1, because at low
sieve effect, probably
due
to activated
diffusion, was -in all cases- significant. The values of Vo1(273) of
the
increase
chars. As in the
(Table I) are not much influenced by the burn-off
it can be
expected
that with progressing activation the
geometrical surface area of the micropores is less pronounced
than the increase in their corresponding volumes, the observed similarity of the Vo,(273)
values
for all burn-offs (including even, to
some extent, the
359 1.3
co,
1 -60
-40
llqu
-20 0 20 Temperature 1 "C1
u
Fig.1 Densities of liquid carbon dioxide : A at tripple point ; B at critical temperature ; C from constant "b" of the Van der Waals equation.
a Density fmm A 8 In Fig.2
Fig.2 Characteristic O a t 273 K ;
0
curves
for
C02
adsorption
at 298 K) on HA 850, burn-off 50%.
(a at
195 K ;
at 253 K ;
360
P’P. 0 501 0 02 0005 0 00100005-195K 00010 00005-273K 0025 0010 0005 00025 Adsorotion a t .
-5 -6 -7
f r o m A C in Fig.2
Fig.3. Characteristic curves in the DR coordinates for carbon dioxide adsorption
non-activated char), might be connected with adsorption of C02 occurring on the walls
of the micropores only, without the effect of volume filling. High values
of E0,(273) support this supposition. These results seem to confirm the view of several authors ( 9 , 10) that carbon dioxide, because of the specific structure of
its molecule, will form
materials
only a
monolayer on
the surface
o f carbonaceous
(provided the adsorption temperature is not too low), and
so
will not
be adsorbed by volume filling of the micropores. The
observed lack
of significant influence of burn-off on the value of
Vo,(273) (Table I), can be also regarded as being indicative o f the mechanism of activation of the HA chars : the development of microporosity in this process seems to be based not so much on creation of new porosity, as on the widening of a porous system, pre-existing in the non-activated char. Voz
and
EOZ were
calculated basing
on the
extrapolated low pressure
parts of the 273 K plots in Fig. 3 , and the successive points of the upper parts
of
the characteristic
curves corresponding
0.03) and b) 195 K (p/po from 0.02 to 0.20) :
to : a) 273 K
(p/po from
0.01 to
361
Vo2(z73) , (Table I),
but for
Vo2(z73) . wider
as
well
However, it
can be
to assume
small values
increase with
hardly expected
burn-off
is always smaller than
that it is the accessibility of diffusion ; it
seems more
do
not
represent realistic volumes of their wider
It is rather the volume of the narrower micropores (Volc273, ) which
micropores. might
strongly
that for chars with low burn-offs the formally calculated
Vo2(195)
of
VOz(195) ,
lower burn-offs Vo2(195)
is influenced by activated
micropores which
reasonable
as
chars with
be entirely
for C 0 2 at 195 K. Therefore, the
or partly, not accessible
differences in the formally calculated values of Voz at 195 and 273 K , permit to discern, within the narrower micropores (VolCzn) ) of these chars, two types of pores :
a) extremely fine
micropores, accessible
195 K : V,,
'
C 0 2 at 273
K and also at 195 K : Vo, "
=
While
at 273 K but not at
for C 0 2
Vo2(195) , and b) narrow micropores, accessible for
-
Voz(273)
the adsorption
=
- Vol
Vo,(273)
I .
of C 0 2 at 298 K and 273 K, and even 253 K, is not
much affected by the burn-off of the chars, the influence of the degree of
very
activation
becomes
very
pronounced
at
195 K.
While
at
253
-
298 K only a
monolayer seems to be formed on the walls of the micropores, at 195 K adsorption of COz leads to volume filling. Adsorption of nitrogen Nitrogen chars
adsorption indicates
the development
that in
the course of activation of the
of their microporous system is accompanied by successive
creation of mesoporosity. This but
only
medium
mesoporosity is
if
desorption
relative pressures
capillary
easily detected
data
are
up to
condensation, are
(1,2) in Fig. 4 ,
on the %-plots
considered. On the adsorption branches, at
about 0.75, no upward deviations, pointing to
observed, but
rectilinear
sections are formed,
indicating multilayer formation. Similar of
capillary
observations, concerning
condensation, were
made
the formation of multilayers, instead recently
by
Dubinin (11) for benzene
adsorption on carbonaceous mesoporous materials at p/po up to 0 . 8 . The values of SEeS
and Vt
calculated from
rectilinear sections in Fig. 4 , are approximately
consistent with respective values of mesopore surface areas (S,,,
(V,) resulting
volumes
) and micropore
from calculations based on the pore size distributions
(8), assuming cylindrical or slit-like shapes of the mesopores. The activated Fig. 5),
application of
chars
permits,
to transform
the CDS in
case
method ( 4 ) of
chars
to nitrogen
adsorption on the
with high burn-offs (example in
the experimental nitrogen isotherm (isotherm I) into an
362 isotherm for predominantly mesoporous systems. For this purpose, however, it sufficient co
appeared not adsorption residual
in Vo1(z73)
out
the
substraction referring
to
COz
(reconstructed isotherm 11-1). The so obtained
isotherm (111-1) still indicated the presence of a significant volume
Calculated fmn pore size distrlbutw-1
- 1 VOLVlf
carry
only
PRFACL AREA
OF
01 0.3
(15'
0.7
0.8
[mz. g'I
5,
L,
'i'
400 251
244
uc
56 39
19
o
a5
24
E ~r 2!
1.0
Fig.4 as plots for nitrogen adsorption (77 K ) on steam activated HA 850 chars ;
0-
adsorption,
- desorption.
of micropores, as can be seen in the respective aS-plots ; also, the high values of
(Table 11)
S,,
indicate that here
this parameter signifies rather the
effective surface area (including the effect of volume filling of the micropores with
nitrogen)
adsorption
in
than (Vol
consideration, that
the
+
Voz
in
surface area
)za
the
of
the mesopores.
It
(reconstructed isotherm 11-2) is
is only when taken
into
residual nitrogen isotherm (111-2) microporosity
becomes almost eliminated. At were
low burn-offs, for the residual isotherms negative adsorption values
obtained (Table 11).
This is
caused by
the molecular sieve effect, when
adsorption of N2 at 77 K is compared with adsorption of C 0 2 at 273 K (or even at 195 K).
363 TABLE I Dubinin-Ra u s h k e v i c h ( D R ) p a r a m e t e r s c a l c u l a t e d on t h e b a s i s o f correcteda' C O z a d s o r p t i o n i s o t h e r m s ( A - based on t h e two-term and B - on t h e one-term e q u a t i o n C31
195
Vor‘ Vor’
195
-
VO2
E 02
’
0.054 0.050
-
0.284
0.335
0.117 0.096 0.010 0.039 .0.099 0.173
-
-
0.180
0.181
12.1
12.2
11.2
0.010
0.085
14.9
13.3
0.266
Residual isotherms calculated eccording to the method. S u b t r a c t e d volumes correspond to a) Vi(z79). ( V ~ + V z ) w s , and c) (V%+Vz)z73, at respective values of 62'; negative values were obtained.The values of (3 (ratio p a r a c h o r e s o f t h e a d s o r p t i v e s and b e n z e n e ) w e r e for Nz and e q u a l 0 . 3 4 and 0 . 5 7 , r e s p e c t i v e l y . a)-&:
CDS t: di
of COz
364
I
0
05
05
0
11
Fig.5 Application
75% :
I
=
15
10
2:
2.0
4
P/R
of the
experimental
CDS method to nitrogen adsorption on HA 850, burn-off
nitrogen
isotherm
at
77 K ; 11-1 and 11-2 = nitrogen
isotherms constructed from Cog adsorption data (characteristic curves in Fig. 3 ) for
V,(273)
and
for (V,+V2)273 , respectively ; 111-1 and 111-2 = corresponding
residual nitrogen isotherms.
Develoument of uorositv during activation Steam macro-,
contribution
the HA
activation of
meso-
and
microporosity
of wider
pores
chars leads to a systematic development of (Fig. 6).
Cog-accessible, micropores (Vo, ' - Table I) pore
volume
present, and
is
developed
about
half
at of
With
increases, while
a the
the
increasing burn-off the extremely fine, and only
disappear completely. The greatest
burn-off of 85 % , with almost no macropores total
porosity corresponding to mesopores.
Nevertheless, the effective surface area of this char is contained mainly in the primary (2) micropores.
365 The values of the parameters : a)
Vm,BET ,
b)
Vm,BET
c)
(Vo,
+
- for nitrogen at 77 K,
V t , VO,DR and Vo(slit)
and VO,DR - for carbon dioxide at 1 9 5 K,
- for carbon dioxide at 273 K ,
V o p )DR
are -for each of the considered burn-offs- not too widely dispersed. They could, therefore, be all considered as parameters reflecting the micropores volumes o f these
chars. A
particularly good
correlation was obtained between VmCBET) and
chosen parameters for nitrogen (77 K) and for
14
GOz
at 195 K (Fig. 7).
0.
--2oa -E 'm
n'
a
aJ
L
1501 aJ u
d L L
Ln 3
aJ
F " 100: aJ
L
W
50
Micropom(N,
C Q only
-... 0
10
25
7
50
10
€!urn- off I
Fig.6 Influence of burn-off in the
primry
-...-__
/-----__
*.
25
-.--. ---.___ 50
7'
I
steam activated humic acids
chars on the
development of macro-, meso- and microporosity.
Brown
coal hiimic
organic matter of
acids can be regarded as a
low rank
model substance of the
coals (12). Therefore, the results obtained in the
present study migh be relevant to similar carbonaceous adsorbents based on solid fuels. ACKNOWLEDGEMENTS
Part
of
this work
was
(Scientifi.c Program CPBP 01.16).
sponsored by
the Polish Academy of Sciences
366
0.8
, , ,
N, at 77 K
/
/ /
sr.’
- 0.E
/Q /
-7
/
(51
/
”‘
-
/
CO, at 195K
E
0
a,
06
E
-
*
vo,DR /
3
* 3%’/
2
*;”
/
/
0.2
/
/&I
*’
/ /
0.2
Fig.7 Correlation
0.4
between VrncBET)
0.6
and
(
3
other parameters
representing the
micropore volumes of steam activated humic acids chars. REFERENCES
1 K.S.W. Sing, Chem. Ind., 20 (1967) 829-830. 2
S.J. Gregg
and K.S.W. Sing, “Adsorption, Surface Area
and
Porosity”,
Academic Press, London 1982. 3 M.M. Dubinin, Carbon, 27 (1989) 457-467.
4 C. Salinas-Martinez de
Lecea, A. Linares-Solano, F. Fodriguez-Reinoso and
A. Sepulveda-Escribano, in “Characterization of Porous Solids“, pp.173-182, Academic Press, Elsevier, Amsterdam 1988. 5
P.H. Emmett, “Catalysis”, v o l . I , p.38, Reinhold, New-York 1954.
6 M. Iley, H. Marsh and F. Rodriguez-Reinoso, Carbon, 11 (1973) 633-638.
7
S . Brunauer, P.H. Emmett and E. Teller, J.Am.Chem.Soc.,60 (1938) 309-319.
8
C. Pierce, J . Phys. Chem., 57 (1953) 149-152.
9
C.H. Amberg,
D.H. Everett, L.H. Ruiter
and
F.W. Smith, in
“Solid/Gas
Interface“, vol. 11, pp.3-16, Butterworths Sci. Publ., London 1957. 10 H. Marsh, Fuel, 44 (1965) 253-268.
11 M.M. Dubinin, in “Characterization of Porous Solids“, pp.127-137, Academic Press, Elsevier, Amsterdam 1988. 12 K. Tomkow, T. Siemieniewska, A. Jankowska, E. Broniek and M. Jasienko, Fuel, 65 (1986) 1423-1428.
F. Rodriguez-Reinosoet al. (Editors), Characterization of Porous Solids ZI 0 1991 Elsevier Science Publishers B.V., Amsterdam
367
INDUCED POROSITY IN ACTIVATED CARBONS BY CATALYTIC ACTIVATION A. Linares-Solano, M. Almela-Alarcon, C. Salinas-Martinez de Lecea, MaJ. Munoz-Guillenaand M*J. Illan-Gomez
Departamento de Quimica lnorganica e Ingenieria Quimica. Facultad de Ciencias. Universidad de Alicante. Apartado 99. Alicante. SPAIN ABSTRACT Two carbon precursors with very different initial pore size distributions (from almond shells and phenolformaldehyde polymer resin) have been activated in CO, and steam. Catalyzed and uncatalyzed activations have been compared using calcium as catalyst. The addition of a catalyst to the carbon activation process influences both the gasification rate and the adsorption capacity of the activated carbon allowing to reduce reaction temperature and to select tailoring of the activated carbon pore size distribution. Total pore volume increases in both carbon series with the extent of burnoff. Catalytic activation in CO, produces, in respect to the uncatalyzed process, a remarkable development of the mesoporosity and, as a result, a much wider pore size distribution is obtained. The effect of calcium in the steam activation is much less noticeable. INTRODUCTION The properties of activated carbons are a function of the carbonaceous precursor material and of the preparation conditions used (ref. l ) , one of which could be the rate of the activation process. Changes in the activation process rates may be obtained by varying the activation temperature, the partial pressure of the activating agent or by use of a catalyst. Although catalytic carbon gasification has been subject of many investigations (ref. 2), its application to activated carbon preparation has not been widely analyzed. Recently (ref. 3) the use of calcium as a catalyst of the carbon-CO, reaction, was investigated in the preparation of activated carbons. Two different porous carbon precursors were used and the porosity of the activated carbons obtained with and without calcium were compared. It was found that the addition of calcium to the C0,-carbon activation influencesthe gasification rate and the adsorption capacity of the resulting activated carbons. It was proposed that catalytic activation may be used to tailor the pore size distribution in a way which is not possible by the usual uncatalyzed activation process.
368
Using the same carbon precursors as mentioned above (ref. 3) this paper analyzes in detail the effect of calcium in the preparation of activated carbons by CO, and steam carbon reactions. The calcium catalytic activity in both gas atmospheres is analyzed to interpret the porosity of the activated carbons. EXPERIMENTAL Two carbons (A and B) have been used. Carbon A prepared by carbonization of phenolformaldehydepolymer resin and carbon B prepared by carbonization of almond shells as described elsewhere (ref. 3). Carbons A and B have been treated with 15N HNO, acid solution at 353K to dryness and then washed with distilled water until free of NO,- ions (carbons A2 and 82). The catalyst precursor, calcium acetate, has been ion-exchanged with the carbons for 4h. after that, samples were washed, dried and heated in flowing N, up to 1123K and held at this temperature for 1h. (carbons ACa and BCa). Calcium weight percentage has been determined by atomic absorption spectroscopy. The four carbons have been activated in a horizontal furnace, in CO, (0.1 MPa) at 1073K and steam (19.7 KPa) at 1173K during different periods of time. The nomenclature used includes, parent carbon, activating agent (C or S) and degree of burn-off (e.g. a carbon from polymer, with catalyst, activated in CO, at 23 % of burn-off will be ACaC-23). Porous texture has been studied by adsorption of N, (77K) and CO, (273K) and by mercury porosimetry (Carlo Erba 2000). The different pore volumes have been estimated from CO, and N, adsorption and from mercury porosimetry in the following manner: 1) the micropore volumes V(micro) from CO, adsorption data (DR equation); 2) the supermicropores volumes (Vsuper) by difference of Vmicro of N, (DR equation) and Vmicro of CO,; 3) the mesopore volumes (Vmeso) by adding the mesopore volumes deduced from N, and mercury porosimetry; from N, adsorption the mesopore volume is obtained by difference in the volume adsorbed at P/P,= 0.7 and P/P,= 0.2 and from mercury porosimetry using the pore size range from 7.5 to 50 nm in diameter; 4) the macropore volumes (Vmacro) from mercury porosimetry, pore size > 50 nm in diameter. Temperature programmed desorption experiments (TPD) were used to quantify the amount of CO, and CO evolved upon heat treatment in He flow to study the effectiveness of the carbon oxidizing treatment. RESULTS AND DISCUSSION Carbons A and 6 have been selected, considering their different origins and porous texture, to test the influence of the starting porosity in the catalytic
369
activation process. Table 1 summarizes some properties of these two carbons. Carbon A has a well developed mesoporosity whereas carbon B has a narrow microporosity which gives rise to a strong activated diffusion effect with N, at 77K; even after waiting 10h for each experimental point the real equilibrium is not reached (ref. 4). The oxidizing treatment does not change significantly either the surface area or the pore volume. However, a slight activation and opening of the micropore entrances is observed mainly for carbon B which does not show, after oxidation, any activation diffusion effect with N, adsorption at 77K. Thermal evacuation of both oxidized carbons, in the gravimetric system, leads to a carbon loss (as CO,) which has been estimated as 3 to 4 wt%. This small degree of activation is the reason for the different adsorption behaviour of carbon B and 82 in N, at 77K. TABLE 1 Some properties of carbon precursors. Carbon Surface area (m’.g-’) Pore volume (cm3.g-’) TPD ton-exchange N,(DR) CO, (DR) Vmicro Vmeso GO CO, (wt%) A
A-2 B 8-2
723 878 457 547
696 712 567 527
0.266 0.272 0.201 0.187
0.387 0.355 0.032 0.020
300 2797 237 1174
178 1968 130 2706
0.4 3.2 0.2 0.6
The oxidizing treatment increases the amount of oxygen surface groups and therefore the ion-exchange capacity (see Table 1). The amount of calcium exchanged by carbons A and A2 agrees very well with the amount of CO, complexes, assuming that all evolved CO, comes from carboxyl groups and that two H’ are exchanged by one Ca” (ref. 5). This is not true for carbon B2 where, although higher amounts of CO, are evolved than for carbon A2, its calcium ion-exchange capacity is only 0.6 wt%. The effect of their different initial porosity is evident; the pores in carbon A2 are accessible to Ca”, whereas this is not the case for carbon 82 because of its narrow porosity. Our reactivity results show that calcium is a very active catalyst for the carbon-CO, reaction, whereas calcium effectivity is quite low for the carbonsteam reaction, probably due to the different experimental conditions used (19.7KPa in steam versus 0.1 MPa in CO,) in agreement with isothermal reactivity data obtained in a TG system (ref. 7). It should be noted that carbon B is more reactive than carbon A in CO, probably due to its inherent calcium content (0.15%) compared to carbon A. On the other hand, because the calcium content of carbon A2 is higher than
370
that of B2 the CO, activation rates of the ACa series are higher than those of BCa. In both carbons the catalytic CO, activation allows the time needed to reach a given burn-off to be reduced significantly compared to the uncatalyzed process, as can be observed on Table 2 where some results of burn-off versus time of activation are included. TABLE 2 Time of activation and burn-off for the uncatalyzed and catalyzed processes. Carbon
Time (h)
A ACa
24.00 0.33 8.00 0.50
B BCa
Burn-off
(W 14 15 19 15
Figure 1 and 2 include the adsorption isotherms of N, for series A and B respectively. The isotherm shapes are characteristic of microporous solids being mostly type I, but with activation they show some tendency to become type II isotherms, depending on the starting porosity of the carbon, the activating agent and the use of catalyst. Uncatalvzed activation Figure 1 (a and c) for carbons A and Figure 2 (a and c) for carbons B show a gradual development of adsorption capacity with the degree of activation during the uncatalyzed CO, and steam activation. The different initial porosity of carbons A and B is important in determining the properties of the resulting activated carbons. Thus as the burn-off increases the isotherms shapes are parallel to those of the initial carbons. Carbon A, although microporous has an important contribution of meso and macropores and this characteristic remains after activation. However carbon B is essentially microporous and consequently series B activated carbons are more microporous than series A. The effect of activating agent can be observed by comparing Figures 1a with l c and 2a with 2c. It is found that the adsorption isotherms of activated carbons prepared with steam have higher slope of the plateau than those prepared with CO,.
25
n (mmole/g)
20 AC-19 AC-32
15 -
..........
5 -
OL
0.2
0.6
0.4
0.8
1
0
0.2
0.4
P/PO
20
0.2
0.6
0.8
1
ACaS-I3 ACaS-23
AS-24
0
1
I
- AS-13 ....
0.8
n (mmole/g)
n (mmole/g)
25,
20
0.6
P/PO
0.4
0.6
P/PO
0.8
1
0 0
0.2
0.4
P/P
O
Fig. 1. Nitrogen adsorption isotherms at 77K for carbon series A: a)uncatalyzed CO, activation,b)catalyzed CO, activation, c)uncatalyzedsteam activation and d)catalyzed steam activation.
372
15
b) .
/
lo-
.
_ _ - -..---_._.--
_ _ - _- _ - - -
* - - -
5-
5
...
-
- BC-10
_-
BC- 19
0.6
0.4
- BCaC-15
BCaC-23
I 0.2
8-2
. ~ ~ .
BC-32
0‘ 0
T
0.8
1
65-14
65-30
,
1
0
0
0.2
0.4
0.6
P/PO
0.8
1
.
P/P
O
BCaC-43
0
0
0.2
0.4
P/PO
5 1 ~ L72, ,
’
I0
0.8
1
- BCaS-10
8-2
BCaS-23
0.2
0.6
-
0.4
BCaS-40
0.6
0.8
1
P/PO
Fig. 2. Nitrogen adsorption isotherms at 77K for carbon series B: a)uncatalyzed CO,activation, b)catalyzed CO, activation,c)uncatalyzedsteam activation and d)catalyzed steam activation.
373
Figure 3 includes plots of cumulative pore volume deduced from mercury porosimetry for carbon of series B, activated in CO, and steam by the uncatalyzed and catalyzed activation process. Uncatalyzed activation with CO, only develops the macroporosity especially in the pore range 70-1500 nm. In the case of steam activation the porosity development occurs in pore sizes below 300 nm, in agreement with the results obtained from N, adsorption. Similar trends are found with carbons of series A. These finding are in agreement with data published previously where the porosity of activated carbons prepared by CO, and steam activation of carbonized plum and olive stones were compared (ref. 7). Catalvzed activation When calcium is used in the preparation of activated carbons it is found that the adsorption capacity increases in both carbon series with the extent of burn-off. However changes in the porosity of the activated carbons with burnoff differ considerably in the presence of calcium, mainly for CO, activation. Figure 1 (a and b) for carbon A and Figure 2 (a and b) for carbon B show the remarkable effect of the catalyzed carbon-CO, activation. The adsorption isotherms shapes are very different from those found for the uncatalyzed activation. Isotherms are a combination of type I and II in contrast to the well defined type I isotherms obtained for the uncatalyzed CO, activation. Carbon A2 and 62 behave differently (Figure 1b and 2b) probably due to their different initial porosity and calcium contents. In any case, catalytic activation in CO, gives rise to a noticeable development of mesoporosity and, as a result, a much wider pore size distribution. Mercury porosimetry, Figure 3 (a and b), show the very different pore size distributions obtained by catalytic activation with calcium; mesoporosity development is very noticeable in agreement with the N, adsorption data. Catalytic activation in steam does not produce significant differences compared to uncatalyzed activation as can be deduced from N, isotherms (Figure 1 and 2) and from mercury porosimetry (Figure 3). Furthermore, the adsorption capacity for a given burn-off has been found, in some cases, to be slightly lower for the catalyzed steam activation, probably due to partial pore blocking by the catalyst. The low influence of calcium in the steam activation may be related to its low catalytic activity. Activation in higher partial pressures of steam needs to be studied to check this unusual low activity of calcium in the carbon-steam reaction. Also lower activation temperatures should be used to differentiate the catalyzed and the uncatalyzed activation process. In any case, with 19.7 KPa partial pressure of steam calcium is not useful in the activation process.
374 Volume (crn’/g)
0.5
b) BC-10
0.4
-
‘
04-
..
I
BC-32 0.3
-
I
1000
10000
.
\ .
1000
100
10
1
10000
R (nm)
R (nm)
0.5
0.5
0.4 -
BS- 14
.... 83-22
..
-
8-2
- BCaS- 10
0.4 -
~ ~ . .
BCaS-23
..
BS-30
BCaS-40
0.3 -
0.3 -
~
4
-
c)
0.2
BCaC-27
. I
100
.
.
I
10
BCaC- 15 BCaC-23
._
1
8-2
.
0.2 -
,
\
.
.
.. \ . .
Fig. 3. Cumulative pore volume from mercury porosimetry for carbon series B: a)uncatalyzed CO, activation, b)catalyzedCO, activation, c)uncatalyzed steam activation and d)catalyzed steam activation.
375
Figure 4 represents, in histogram form, the evolution of the different pore volumes for activated carbons of series A and 6 as a function of burn-off degree obtained in CO,. Uncatalyzed and catalyzed process have been included for comparative purposes. The histograms clearly show both the important effect that calcium has in the preparation of activated carbons by CO, and the marked influence of carbon initial porosity especially in the catalyzed process. Important general features that can be extracted from these distributions are: 1) catalytic activation does not develop the narrow microporosity (Vmicro); 2) supermicropores and mesopores are significantly more developed in the catalyzed activation, the former especially in carbons A and the latter in carbons B; 3) macroporosity (Vmacro) in carbon A is not noticeably influenced by the use of calcium as a catalyst, however, for carbon 6 the use of calcium as a catalyst reduces the macropore volume of the resulting activated carbons. It is concluded that the initial porosity of the starting carbon controls the amount and the distribution of Caz*ions during the preparation step. Thus for carbon A which has an open porosity 3.2 wt% of calcium is exchanged, whereas for carbon B having a narrow microporosity only 0.6 wt% of calcium is exchanged. Evolution of porosity with burn-off for the catalyzed activation process indicates that once the catalyst has reached a given porosity during the preparation step, its catalytic effect during the activation process will be focussed in that region. The rate of gasification increases significantly in the area where the catalyst is confined (as observed in Table 2) and hence the porous texture in contact with the catalyst will be developed. Thus, the use of calcium during activation in CO, is very useful for preparing activated carbons with a well developed mesoporous provided that a carbon with a suitable starting porosity is used, e.g. carbon A. For carbon 6 a well developed mesoporosity cannot be attained unless a high degree of activation is used (ref. 8 ) . CONCLUSIONS The addition of calcium to the C0,-carbon activation process influences noticeably the gasification rate reducing considerably the time needed to prepare activated carbons (see Table 2). The pore size distribution of activated carbons prepared by catalytic CO, activation is different from that obtained by the usual non-catalytic activation process. In both cases the initial porosity of the starting material is important, but in the catalytic activation the distribution of the catalyst and its amount play an additional important role.
376
1.5
Pore Volume (cm3/g)
Unc a t a l y z e d
a)
Catalyzed
1
0 5
0 AC-32
AC-19
Vmicro
1.5
,P o r e
AC-14
A
RB Vsuper
A-2
ACaC-15ACaC-23ACaC-45
0Vmeso
0V m a c r o
Volume (cm3/g)
Unc a t a lyzed
b)
Catalyzed
05
0 BC-32
BC-19
I EBB
Vrnicro
BC-10
B
Vsuper
B-2
~3
BCaC-15 BCaC-23 BCaC-43
Vrneso
o Vmocro
I
Fig. 4. Pore size distribution for CO, activated carbons: a)series A and b)series B.
377
Catalytic CO, activation may be used to tailor the pore size distribution in a way which is not possible using the uncatalyzed activation. Uncatalyzed steam activation develops the mesoporosity of the activated carbons to a higher extent than does the uncatalyzed CO, activation but to a lesser extent than the catalyzed CO, activation. ACKNOWLEDGEMENTS Thanks to the DGICYT for financial support (project nQ86-0286). REFERENCES 1 R.C. Bansal, J.B. Donnet and F. Stoeckli, "Active carbon", Marcel Dekker, Inc., New York, 1988. 2 "Carbon and coal gasification", J.L. Figueiredo and J.A. Moulijn Ed., Martinus Nijhoff Publishers, Dordrecht, 1986. 3 M. Almela-Alarcon, A. Linares-Solano and C. Salinas-Martinez de Lecea, Proc. 18th Biennial Conference on Carbon, Worcester, USA, 338, 1987. 4 F. Rodriguez-Reinoso and A. Linares-Solano, "Chemistry and Physics of Carbon", Vol. 21, P.A. Thrower and Marcel Dekker, Inc., New York, 1, 1989. 5 D. Cazorla-Amoros, C. Salinas-Martinez de Lecea, A. Linares-Solano and J.P. Joly, Proc. 19th Biennial Conference on Carbon, University Park, U.S.A., 598, 1989. 6 C. Salinas-Martinez de Lecea, M. Almela-Alarcon and A. Linares-Solano, Fuel 69, 21, 1990. 7 F. Rodriguez-Reinoso, A. Linares-Solano, M. Molina-Sabio and 1. PerezLledo, Proc. 17th Biennial Conference on Carbon, Lexington, U.S.A., 239, 1985. 8 F. Rodriguez-Reinoso,J.D. Lopez-Gonzalez and C. Berenguer, Carbon 20, 513, 1982.
This Page Intentionally Left Blank
F. Rodriguez-Reinosoet al. (Editors), Characterization of Porow Solids II 0 1991 Elsevier Science Publishers B.V., Amsterdam
379
CHARACTERIZATION OF ACTIVATED CARBON: AN APPROACH TO THE ACTIVATION PROCESS BY SAXS AND OPTICAL MICROSCOPY J.M. GUET, 0. LIN, A. LINARES-SOLANO’ and CSALINAS-MARTINEZ de LECEA’ Laboratoire de Cristallographie, Universite d’Orleans 45067 Orleans Cedex 2, France * Departamento de Quimica lnorghnica e Ingenieria Quimica Universidad de Alicante, EspaAa
SUMMARY Characterization of carbons (from different origins) and their corresponding activated carbons, prepared in a steam or carbon dioxide flow, have been performed by adsorption measurement of gases, mercury porosimetry, small angle X-ray scattering and optical microscopy. The combination of all these techniques allows to examime the influence that the starting material and t h e activation burn-off have on the porous texture of the activated carbons. SAXS data, in agreement with gas adsorption data, show that the lower is the mesoporosity of the starting material t h e higher is the mesoporosity range developed after activation. Optical microscopy observation allows to better understanding the origin of macroporosity and its development in the carbonization and the activation processes. INTRODUCTION Activated carbons are high surface area materials prepared from various amorphous carbon-based materials and exhibiting a high degree of porosity (ref. 1). Depending on the origin of the carbonaceous precursor and its preparation method t h e adsorptive properties of activated carbons may vary considerably (refs. 1,2). An activated carbon needs to have an appropiate pore size distribution, to fit a given application. The direct relationship existing between the final properties of a given activated carbon and its precursor needs to be investigated in each case. On account of the difficulty that active carbon pore size distribution determination has, the combination of different techniques is justified. The present paper deals with activated carbons characterization which has been performed with different techniques: adsorption measurement of gases, mercury porosimetry, Small Angle X-ray Scattering (SAXS) and optical microscopy. The combination of all these techniques allows to examine the influence
380
that t h e starting material and the activation burn-off have on the porous texture of t h e activated carbon. Olive stone, almond shell and a polymer resin have been used as raw material to prepare carbon materials and activated carbons.
EXPERl M ENTAL Sample meparation Olive stones and almond shells were crushed and sieved to a particle size of around 2 m m before been treated with a 10 % solution of H,SO, for 6 hours. After treatment t h e samples were washed with distillated water to zero acid removal. A two-stage physical activation process was used. During the first stage, the clean raw material was carbonized for 2 hours in a continuous N, flow at 850°C for olive stone and 900°C for almond shell, using a constant heating rate of 5 K/min. Carbonized olive stone (carbon C-I) was activated in H,O/N, mixture at 800°C during 46 hours to give carbon AC-I with a 63% burn-off. Carbonized almond shell (carbon C-11) was activated in CO, at 875°C during 12 hours to give carbon AC-II with 35% burn-off. A phenol-formaldehyde polymer resin was carbonized in N, flow at 1000°C during 1 hour, heating rate of S"C/min (carbon C-Ill) and then activated in CO, at 800°C: for 60 hours to reach 32% burn-off (carbon AC-Ill). Further details of sample preparation and characterization is given elsewhere: (ref. 3 ) for olive stone and (ref. 4) for almond shell and the polymer. A
m y porosimetry Porous texture has been characterized by N, (77 K) and CO, (273 K) adsorption isotherms carried out in a conventional McBain silica spring balance and mercury porosimetry (Carlo Erba, series 200). Small Angle X-ray Scatterinp_ (SAXS) . SAXS measurements were carried out in a 12 kW rotating anode X-ray RIGAKU generator, equipped with a copper target. The K a , reflection is selected by a germanium incident-beam monochromator. The intensity of the scattered beam is recorded during 4000 seconds by a linear localization detector connected with a multichannel analyzer for data acquisition and analysis. All the experiments are performed with the tube operating at a medium power (50 KV and 60 mA), under pinhole collimation conditions combining two focusing distances in order to obtain a complete scattering curve from S = 0.8*10" A-' to 2.0*10-' A-' using s = 2sino/aas scattering vector. Optical microscopy The activation process have been specially studied on olive stones by optical
381
microscopy using a Leitz Orthoplan microscope with a wavelength radiation of 546 nm.
RESULTS AND DISCUSION Adsorption and mercury Dorosimetry The different pore volumes of the carbons and their corresponding activated carbons are compiled in Table I ; the micropore volume has been deduced from N, (77 K) and CO, (273 K) adsorption isotherms by use of DK equation (ref. 5 ) and the meso and macropore volume from mercury porosimetry. Fable 1 also shows the N, surface area deduced from BET equation and t h e CO, surface area from D R K equation (ref. 5). Important features related with these data are: (i) Raw carbons. The starting carbons have very different pore size distribution; their meso and macropore volume differ considerably, as it is shown in Figure I , where the cumulative pore volume is plotted versus t h e mean pore size. Carbon C-I (from olive stone) presents the most developed macroporous network system whereas the highest mesopore volume corresponds lo C-I I I (polymer carbon). The observed mesopore volume sequence is : C-II< C-I< C111.
The micropore volume of carbons C-I and C-II are quite similar and, in both cases, the much lower N, values in respect to CO, values indicate that t h e carbons have narrow micro-porosity which gives rise to a strong activated diffusion effect with N, because the low adsorption temperature used. Very long equilibrium time is needed to reach a "real" equilibrium (refs. 2,6); t h e N, values of Table I are not under equilibrium. Carbon C-Ill has a slight higher micropore volume with a wider pore entrances than C-I and C-II which is equally accessible to both N, and CO,. Its N, adsorption isotherm shape (see Figure 2) indicates that in addition to micropores there is a well developed mesoporosity which gives rise to an important slope of the isotherm plateau. TABLE I Pore volume (cm3.g-') and surface area (rn'g'). Pore volume Sample Precursor microd meso" m;icroc C-I Olive stone 0.227 0.022 0.225 AC- I c-I 0.332 0.234 0.593 c-II Almond shell 0.201 0.01 1 0.038 AC-II C-ll 0.471 0.120 0.100 c-Ill 0.266 0.339 0.044 AC-Ill ?I%ller 0.459 0.445 0.054
Surface area N,(77K)" C0,(273K)' 38 593 1450 883 141 567 1008 1233 585 696 1195 1201
from CO, (DR); from mercury Forosirl)etrl: 75 A < CDd, 500 A. from mercury porosimetry: 500 A < CD < 1.0000 A ; from BET; trom DkK.
a
382
(ii) Activated carbons. Carbons C-Il and C-Ill have been activated in CO, to comparable burn-off levels; in both cases the adsorption capacity of t h e activated carbons increase noticeably. Figure 2 shows, as an example of the activation process effect o n adsorption capacity development, the N, adsorption isotherms of carbon C-Ill and AC-Ill. The activation process develops the micro and supermicropore volumes but does not change significantly the wider mesopore volume as it can be deduced from the isotherm shape of AC-Ill which is almost parallel to that of C-Ill. Activation of C-I has been carried out in a N,/H,O mixture to a much higher burn-off level as a result of which the porosity of this activated carbons has been noticeably developed (see Table 1). 25 n (mmoI/g) c-111 AC-I11
1
10
100
1000
10000
0' 0
0.2
0.4
0.6
0.8
1
R (nm)
P/PO
Fig. 1. Cumulative pore volumes for carbonized samples (mercury porosimet ry ) .
Fig. 2. Nitrogen adsorption isotherms at 77K for samples C-Ill and AC-Ill.
Small Angle X-ray Scattering (SAXS) SAXS has been extensively developed to characterize the pore structure of coals, carbonaceous materials and activated carbons, on ii scale from 10 to 1000 A (refs. 7-1 1). It is a non intrusive technique which does not interact chemically with the sample and is able to probe both open and closed pores. All polydisperses systems such as porous carbonaceous materials produce small angle X-ray scattering which origin-ates from the electron density difference between voids and solid carbonaceous matrix. The expression of the intensity of t h e scattered beam is: sin 2mr z(r)-_________ 4 rZdr 2nsr
-
383
where: s : scattering vector I, : incident beam intensity I, : intensity scattered by ;in electron V : irradiated sample volume p, : electron density of carbon p2 : electron density of pore $, : volumic fraction of matter in carbon i,h2 : volumic fraction of pores in carbon z(r): correlation function, which is related to the probability that a line of length r will have both ends situated in pores. This equation can be applied to our carbons because we are typically in t h e case of a two-phases sample consisting only of voids and density packed carbonaceous materials. A statistical isotrope repartition of pores has to be admi tted. Typical SAXS curves, expressing the experimental intensity (logarithmic scale) versus the scattering vector S (logarithmic scale), are represented in Figure 3, for all the samples. On account of the absence of a linear range in the Guinier plot (suggesting a broad distribution in pores size) and a deviation from Porod’law, at the large angle, for all the samples, we have not been able to compute t h e conventional Fourier transformation requiring two extrapolations of each curve. The scattering curves for these samples, have been calibrated in connecting them to high angle d raction curves at the absolute scale, so the highest intensity curve indicates, more or less, the largest porosity of a given sample. Thus,the scattering curves of Figure 3a, corresponding to the carbonized samples, show the important influence of the starting material; the porosity range analyzed which goes from 50 to 800 indicates that there is significant porosity difference in these three samples. The porosity analyzed by SAXS follows the sequence C - l l i C-I< C-Ill. It should be pointed out that mercury porosimetry also shows, in the same porosity range, a similar sequence. Intensities scattered by these three carbons at higher scattering angles are quite similar. This is expected considering that this technique is able to probe both open and closed pores and considering the micropore volume deduced from CO, adsorption. Figures 3 (b,c and d) represents, for each precursor, scattering curves comparing the carbonized state ( C ) with t h e activated one (AC). In the three cases, as it was expected ,the activation process develops t h e porosity of t h e carbonized materials. N,adsorption isotherms for samples C-Ill and AC-Ill (see Figure 2) and SAXS results (see Figure 3d) allow to reach t h e same conclusion; the activation of this carbon is mainly restricted to the micropore and very small
A
A
384
9.5
7.5
5.5
3.5
-7
-6.5
-6
-5.5 -5
LN
-4.5
-4
-3.5
-3
1.5 -4.8-4.6-4.4-4.2
(S)
Fig. 3. Small Angle X-Ray Scattering curves.
-4 -3.8-3.6-3.4-3.2
LN (S)
-
385
mesopores. Finally, SAXS data show clearly that the porosity development upon activation is ;I function of the carbonized material porosity; the lower is the original mesoporosity the higher is the range of mesoporosity developed. Thus C-I1 develops porosity from 30 to 1000 A, C-I from 20 to 250 A and C-Ill from 20 to 50 A. ODtical microscouy Optical microscopy was used to study the activation process of olive stone. A crude olive stone, a weak activated carbon (9% burn-off) and the AC-I were studied. The observation of such samples allows to have a better understanding of the origin of macroporosity and its development during t h e carbonization stage (or weak activation) and during a high activation process. The crude olive stones, as shows t h e photography of Figure 421, presents anisotropic vegetal cells of size 10 to 50 m. The carbonization stage and the weak activation are similar stages and correspond to the removal of volatile matter and the large opening of the cells (see Figure 4b). In a second stage, the activation process develops porosity at the frontier of cells connecting all the macropores (see Figure 4c). During this stage an important damage and disappearance of the cells are observed.
Fig. 4. Pore images of crude olive stone (a), 9% activated olive stone (b) and 63% activated olive stone (c).
386
Fig. 4. Cont.
387
ACKNOWLEDGEMENTS Thanks to the DGICYT (Project n. 88-0295) for financial support. REFERENCES 1 R.C. Bansal, J.B. Donnet and F. Stoeckli; "Active Carbon", M. Dekker; New York (1988). 2 F. Rodriguez-Reinoso and A. Linares-Solano; "Chemistry and Physics of Carbon"; vo1.21; P.A. Thrower, M. Dekker, New York p.1 (1989). 3 I. Perez-Lled6, M. Almela-Alarcon, A. Linares-Solano, C. Prado-Burguete and F. Rodriguez-Reinoso. XIX Carbon Conference. p. 608 (1989). 4 M. Almela-Alarcon, A. Linares-Solano and C. Salinas-Martinez de Lecea, XVlll Carbon Conference, p. 338 (1987). 5 A. Linares-Solano, in Carbon and Coal Gasification (J.L. Figuereido and J.A. Moulijn, eds.), Martinus Nijhoff, Dordrecht (The Netherland), p. 137 (1986). 6 F. Rodriguez-Reinoso, J. D. Lopez-Gonzalez and C. Berenguer, Carbon 20, 513 (1982). 7 M. Kalliat, C.Y. Kwak and P.W. Schmidt; New approaches in Coal Chemistry" A.C.S. Symposium Series N"169, Washington D.C. p. 1 (1981). 8 A. Renouprez and J. Avom; Characterization of Porous Solids; K.K. Unger, J. Rouquerol, K.S.W. Sing and H. Kral; Elsevier p. 49 (1989). 9 J.M. Guet; "Advanced Methodologies for Coal Characterization" H . Charcosset; Elsevier p. 97 (1990). 10 P.W. Schmidt, Characterization of Porous Solids; K.K. Unger, J. Rouquerol, K.S.W. and Kral; Elsevier p. 35 (1989). 11 A. J5nosi and t-1. F. Stoeckli, Carbon 17, 465 (1979).
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F.Rodriguez-Reinosoet al. (Editors), Characterization of Porous Solids 11 0 1991 Elsevier Science Publishers B.V., Amsterdam
389
DYNAMIC MICROPORE STRUCTURES OF MICROGRAPHITIC CARBONS DURING ADSORPTION
K. Kaneko, T. Suzuki, Y. Fujiwara" and K. Nishikawa" Department of Chemistry, Faculty of Science, Chiba University 1-33 Yayoi, Chiba 2 6 0 , Japan ( * ) Department o f Chemistry, Faculty o f Science, Gakushuin University, Toshima-ku, Tokyo 171, Japan
ABSTRACT The microporosity o f two kinds of activated carbon fibers (ACF'S) and coconut-shell based activated carbons (AC) was determined by N2 adsorption isotherms at 77 K and their water adsorption isotherms were measured at 298 K. The changes in the in situ X-ray diffraction(XRD) patterns of the water-adsorbed ACF and AC upon evacuation were determined at 2 9 8 K. The interlayer distances of the graphitic layers of cellulose-based ACF and AC increased remarkably with desorption of water. The change in the small angle X-ray scattering (SAXS) with water adsorption was measured at 298 K; the SAXS changes sensitively with adsorption. The gyration radius obtained from the Guinier plot increased with the amount of water adsorbed. The slit-shaped micropores swelled adn developed with water adsorption. The geometrical changes of micropores were associated with their micrographitic structures.
INTRODUCTION Activated carbon fibers (ACF'S) are highly microporous with small external surface areas and very little mesoporosity[l-31. The microporosity of ACF'S has been examined by molecular adsorption. The microporosity of ACF should be associated with the structure of the micropore-wall. McBain et a1[4] reported the linear expansion of an activated carbon block upon adsorption of water by about 0.1 %. Dacey and Evans[5] found that activated carbons show a contraction in volume of 0.1 % followed by a larger expansion near saturation. Franklin[6] showed that activated carbons have micrographites whose interlayer spacing is greater than that of graphite. Also Dubinin[7] suggested that the microporosity o f activated carbons is closely related with the micrographite-
390
structure. Recently Huttepain and Oberlin[8] showed clearly by a high resolution transmission electron microscopy that the micropores and ultramicropores of activated carbons are slit-like pores. Dubinin and Stoeckli[9] combined two separate experimental results of N2 adsorption and small angle X-ray scattering (SAXS) on activated carbons and proposed a useful empirical relationship between the micropore size and the characteristic adsorption energy; they suggested an importance of a simultaneous experiment with structural techniques and molecular adsorption on microporous carbons. These authors reported the micrographitic structural changes of ACF'S by in situ XRD[10] and the swelling of the micropores by in situ SAXS[11] during adsorption of water in the preceding letters. The iron oxide-dispersed ACF exhibits marked micropore filling of supercritical N0[12], which is presumed to be caused by a magnetic interaction. It was found that the micropore filling of NO on ACF'S is enhanced by application of the external magnetic field[l3], which is postulated to be associated with the structural change of the micropore-walls due to the instantaneous induction current. Therefore, the structural dynamics of ACF'S upon molecular adsorption should be carefully studied. In this article, in situ XRD and in situ SAXS studies on two kinds of ACF'S upon water adsorption have been described and the structural changes have been discussed with relevance to molecular adsorption data.
EXPERIMENTAL Cellulose(CEL)- and polyacrylonitrile(PAN)-based ACF'S and coconut-shell granular activated carbons (AC) were used in this study. Also nonporous carbon black(NPC) whose surface area is 81 m2g-l was examined by in situ XRD for comparison. The adsorption isotherms of water at 298 K and N2 at 77 K on samples were measured gravimetrically. The ACF sample was pre-evacuated at 383 K and 1 mPa for 15 h prior to the adsorption. The evolved gas analyses (EGA) of ACF samples preheated at 373 K under 1 mPa for 15 h were carried out at a heating rate of 10 K min" with the aid of a mass filter(ULVAC, MSQ-150A). The elemental analysis for the nitrogen atom of the ACF samples was done; the ratios of nitrogen to carbon in wt. % of CEL and PAN were 2 and 6-7, respectively. The in situ XRD patterns of ground ACF samples were measured with an automatic powder diffratometer (Rigaku 2028) of the diffraction cell with Kapton windows[lO]. The radiation was Nickel filtered CuKo! operated at 35 kV and 10 mA. The accuracy in the reflection angle was examined by use of a Grafoil. The ground ACF sample after drying at 3 8 3 K for 2h was wetted in water, then it was compacted to a pellet, which was mounted in the diffraction cell.
391
The changes in XRD patterns of the wet ACF pellet with evacuation by a rotary pump were measured at 298 K. The SAXS spectra of ACF previously dried at 403 K were measured under varying relative humidities (RH) at 298 K. The Cu Ka! X-ray beam operated at 30 kV and 30 mA was used after monochromatization[l4].
RESULTS AND DISCUSSION Microporosity and water adsorption
The adsorption is0therms of N2 on both ACF'S at 77 K were of T y p e I. T h e detailed description on the
TABLE I Numerical adsorption data on N2 and water N2 Adsorption
/m'g-'
at /m'g-' GEL
1410
PAN
850
AC
910
at,ext 22
W,
,
/mlg-'
w0 ,a/ m l g -
0.59
0.61
7.8
0.35
0.34
9.0
0.36
0.35
1
Water Adsorption
already PAN 0.20 0.32 0.94 reported AC 0.07 0.40 1.11 in other papers 600 [15,16]. Also the adsorption isotherm of N2 'w e" o n AC w a s of T y p e I. T h e s e 400 samples have considerably uniform micropores. The surface j area at, external surface area 200 ae,t,t and the micropore volume f f r o m the t - p l o t and the micropore volume W from the ds0 d 0 0.2 0.4 0.6 0.8 1.0 plot, are shown ino ,Table 1. Here w e used the s t a n d a r d d a t a o f P/P, ungraphi-tized nonporous carbon FIGURE 1 Adsorption isotherms of water black for construction of these molecules on microporous carbons. plots [15]. Figure 1 shows adsorption isotherms of water on ACF'S and AC at 298 K. The isotherm of water on CEL is of Type V, having a hysteresis. PAN adsorbs considerable amounts of water even below 0.4 of P/Po, showing a little r(
West
392
hysteresis. Thus, P A N is more hydrophilic than CEL. The adsorption isotherm of water on AC is close to that of CEL. The nitrogen atom in the carbon structure o f PAN must work as a polar site, as suggested by Mochida et a1[17]. The adsorption isotherms of water were analyzed by the Dubinin-Serpinsky(DS) equation[l8201. The DS equation is expressed in quadratic form,
that a graph of the left-hand side of eq. ( 1 ) against the amount of adsorbed W should be an inverted parabola, if the equation is obeyed. Here . a is the adsorption on the polar sites and W0(H20) and C are the micropore volume for water and a constant, respectively. We can determine the Wo(H20) value from the extrapolation to P/Po = 1. T h e p l o t o f W(Po/P) vs. W provides the a, value, that is, so
900 rl
M
E
h r?
L
2
0
\
P
~, PAN
500-
2
I
2
300
100
0
200
t h e number o f - t h e p o l a r s i t e s from t h e c o o r d i n a t e s of t h e t o p of t h e parabola. F i g u r e 2 shows
~,,,
700-
v
;
P, CEL
\
400
600
the DS plots. The a, value was uater adsorption is0therms. estimated from the approximated parabola. Wo(H20), a . and the ratio of W0(H20) to Wo,,&N2) are CEL also listed in Table 1. PAN has about two times greater a, value than CEL; also the EGA spectra are indicative of the difference between CEL and PAN surfaces. Figure 3 shows temperature profiles of the evolved CO (and/or N2 for PAN) and C02 from ACF'S. C02 is derived from COOH and lactone groups on the carbon FIGURE 3 Temperature profiles of surface, whereas CO comes from phenolic OH and quinone-type evolved CO and C02. oxygens[21,22]. CEL has similar amounts of COOH and lactone groups and phenolic OH and quinone-type oxygen groups. On the other hand, the COOH and lactone groups are greater than phenolic OH and quinone-type oxygen groups in the case of PAN.
c
0
393
In s i t u X-ray d i f f r a c t i o n
Figure 4 shows the changes in XRD pat terns microporous desorption
Evacuation for
of
The water-adsorbed
20
f r o m (002) a n d overlapped __ from
30
40
50
20
30
LO
50
1
Diffraction angle/ d e g .
FIGURE 4 Changes in X-ray diffraction patterns of
(100) a n d (101) water-adsorbed carbons with evacuation. [23]. PAN has a sharp peak on the broad 002 peak at 25.0°, s o lZ . ~-+considerably developed micrographi tes coexist in the micrographites of PAN samples. T h e p e a k p o s i t i o n is lower compared with that o f CEL grafoil(26.2'). The diffraction angle of the (002) plane was a40 determined after substraction o f the increment of the background g 038 d u e to S A X S ; the peak thus 0.36 determined is shown by a dot 0.34 line for the water-adsorbed 0 . 3 2 1 , , , 1 , , sample. The evacuation of the 1 2 3 ' 20 25 water-adsorbed CEL and AC for 30 Evacuation Time/ h min leads to the remarkable broadening and significant shift FIGURE 5 Changes in the crystallite si of the 002 peak toward lower ~ O and Z the interlayer spacing do angle and to the tail of marked with evacuation. SAXS. The diffraction angle of the (002) plane decreases and the peak becomes broader with desorption time; the XRD patterns after evacuation for 18 h resembles those of CEL dried at 383 K for 17 h in air. In contrast to GEL, PAN shows less noticeable change in the XRD patterns upon desorption than GEL. The diffraction angle of the broad 002 peak slightly decreases with desorption, while the sharp 002 peak does not change. Figure 5 shows the changes in the
. 0
394
interlayer spacing do02 and the crystallite size Lo02 from the 002 peak as a function of evacuation time. The increment of the do02 of CEL upon desorption is the greatest, reaching 0.06 nm. The do02 of CEL after desorption for long time coincides with that of CEL dried at 373 K. On the contrary, the dO02 of the less-crystalline texture of PAN increases by 0.01 nm at best. With AC, the do02 rises steeply within 30 min and become constant. The change in the No change Lo02 steeply decreases with desorption for all samples. in the XRD patterns of the water-adsorbed NPC was observed.
In situ small angle X-ray scattering
Figure 6 shows the relationships between the scattering intensity I(s) and the square of the scattering parameter s(=4n sine/A) as a function of RH. Here 20 and % are the scattering angle and X-ray wave length, respectively. Generally speaking, the absolute I(s) value decreases with the increase of RH for the whole s2 range. With the scattering curves of both samples under RH = 0 %, we cannot observe a marked increase of the I(s) value in the lower angle region. The decrease in the I(s) value with an increase of RH may arise from the disappearance of the heterogeneity of the electron density between the carbon-wall and the micropore space. The ACF texture can be described by a "two density model" in which Ape is the difference between the electronic density of the carbon matrix and that of the pores. The intensity I(s) scattered by a diluted system of N "microporeparticles" of volume V is given by the Guinier's law [ 2 4 , 2 5 ] .
where RG is the radius of gyration o f the "microporeparticle". This approximation i s valid f o r s R G < < 1. Figure 7 shows the log I(s) $ vs. s 2 plots at the lowest f angle region. The Gunier plots are briefly linear in the s 2 region of 1.6 x to The slope of the
CEL a:
i .:
Plus
YC~EYD
':\.
-
d : 53 1
e: 85 1
f: 100
______
*.>..? lo4-
b: 0 7. s: 22 1
'\\
"'1 0
I
I
I
I
0.01
0.02
0.03 .2/
0.01
0
I 0.02
I
0.03
*"-2
Gunier plot for in FIGURE 6 Changes in the intensity of the small angle Xvacuo is almost identical ray scattering with the square of the scattering to that for samples under parameter as a function of relative humidity. R H = 100 % ; t h e s l o p e increases with RH. The slope of the Gunier plot provides the RG value. It is not definitive whether the micropores in the carbon texture can be simply described as a dilute system of "micropore-
395
particles" or not. As the I CEL I(s) vs. s2 relationships in Fig. 7 do not have a maximum because of the interparticle interference effect, probably geometrical size o f the micropores of ACF samples can be described by the RG value [26]. The slope of the 0.002 Gunier plot provides the RG 0.002 0.004 value; the RG increases with RH. The changes of RG with the fractional filling of FIGURE 7 Guinier plots for the small angle water molecules are shown in X-ray scattering spectra. Fig. 8. Here the fractional filling was the ratio of the amount o f water adsorption t o Wo(H20). The increase of the RG value is proportional to the fractional filling of water molecules for both samples, which is assumed to be associated with swelling and development of the micropores. I
I
4
Swelling and development of micropores due to adsorption
The RG value does not directly express the geometrical micropore size, but the radius 3 of gyration of the "microporeparticle". The shape of the micropore o f ACF can be p approximated as a slit from the high resolution electron 2 2 microscopic observation [27]. The width o f the slit-shaped pore was determined by the N2 adsorption. Therefore, we I I I I I presume that the "micropore1 1.0 0 0.5 particle" is rectangular. W e Fractional filling can estimate the rectangular size of the "micropore-particle" FIGURE 8 The Gunier radius as a function from the R G value. When the s h a p e of t h e p o r e - w a l l is of the fractional filling for water. assumed to be a square of x in length and the slit width is w, the Rg can be given by eq. (3)[28].
[2(x2 + w ~ +) 4 ~x4]'/*/3 (3) We can assume that the micropore-wall consists of about three
RG
=
396 0.39
nm
A-
FIGURE 9
0.33 n m
-1.-
Without water
F i l l e d with water
2.0 nm
3.1 nm
Without water
F i l l e d with v a t e r
Without v a t e r
F i l l e d v i t h vater
(A) Schematic model of the compression of
the micrographitic
spacing and swelling of
the
micropore- due to water adsorption f o r CEL.
(B) T h e e s t i m a t e d m i c r o p o r e s of CEL w i t h o u t adsorbed water and filled with adsorbed water.
(C) Change f r o m the wedge-shaped pore without adsorbed water to the parallel slit-shaped pore filled with adsorbed water.
397
graphitic layers from the Lo02 value and the electron microscopic observation[29]. The do02 value of CEL decreases by 0.06 nm upon saturated adsorption of water and the slit width w of a micropore increases by 1.2 nm from the in situ XRD study. Table 1 indicates that the micropore volume for water almost agrees with that €or N2; the micropore width of CEL filled with water should be equal to that with N2(0.92 nm [16]). Hence the pore width of CEL without adsorbed water may be (0.92 0.12) nm, as shown in Fig. 9A. Figure 9B shows the micropore-models of CEL with saturated adsorbed water and without adsorbed water, which are consistent with the RG value from eq.(3). Why does the slit-shaped micropore develop upon adsorption ? In the CEL texture, the graphitic layers should be randomly oriented and a part of opposite pore-walls are too close each other to induce the S A X . Probably CEL has wedge-shaped micropores (Fig. 9C). Here the graphitic layer is illustrated by a The edge carbons of the bold line in Figs. 9(A) and (C). micrographite of the CEL are oxidized to become the surface functional groups. The adsorbed water layers on the oxidized sites on both pore-walls connect each other and swell the narrow entrance of the micropore. Under the conditions of saturated adsorption water molecules near the entrance should migrate into the inner part of the micropore to swell the whole micropore to develop the effective micropore. The micropores of AC should change upon adsorption in a similar way to CEL, although we do not have the in situ data on AC. In the case of water adsorption on PAN, PAN is more resistive to distortion upon water adsorption, because PAN has a mixed texture of well- and less-developed graphitic structures, as shown by the XRD examination. Although we do not measure the macroscopic extension of these carbon samples yet, the swelling of the micropore which does not originate from the compression of the interlayer spacing may be neglected considering the data by McBain et al[4](0.001 nm per the slit width at best). Hence we conclude that water adsorption on microporous carbons leads to the compression and reorientation of the micrographitic layers, accompanying the marked swelling and development of the micropores. However, the micropore fully occupied with water molecules is not a simple "void particle" of free of electrons, so further study on in situ SAXS results near the saturated conditions is necessary.
-
Acknowledgment is made to Professor H.P. comment.
Boehm for an important
398
REFERENCES 1 J.J.Freeman, F.G.R.Gimblett & K.S.W. Sing, Carbon,27 (1989) 85. 2 K.Kaneko, Langmuir, 3 (1987) 357. 3 K.Kaneko & Shindo, Carbon,27 (1989) 815. 4 J.W.McBain, J.L. Porter & R.F. Sessions, J. Amer. Chem. SOC., 55 (1933) 2294. 5 J.R.Dacey & M.J.B. Evans, Carbon, 9 (1971) 579. 6 R.E. Franklin, Acta Cryst., 4 (1951) 253. 7 8 9 10 11 12 13 14
15 16 17 18 19 20 21 22 23 24 25 26 27 28
M.M. Dubinin, in S.J. Gregg, K.S.W. Sing & H.F. Stoeckli (Editors), Characterisation of Porous Solids, SOC. of Chem. Ind., London (1978) pp.1. M.Huttepain & A. Oberlin, Carbon, 28 (1990) 103. M.M. Dubinin & H.F. Stoeckli, J.Colloid Interface Sci., 75 (1980) 34. T. Suzuki & K.Kaneko, Carbon, 26 (1988) 743. K.Kaneko, Y. Fujiwara & K. Nishikawa, J. Colloid Interface Sci., 127 (1989) 298. K. Kaneko, Colloid Surf., 37 (1989) 115. H.Uchiyama, S.Ozeki & K.Kaneko, Chem. Phys. Lett., 166 (1990) 531. K.Nishikawa, Y. Kodera & T. Iijima, J. Phys. Chem., 91 (1987) 3694. K. Kakei, S. Ozeki, T. Suzuki & K.Kaneko, J. Chem.Soc. Faraday, 86 (1990) 371. K.Kaneko, T. Suzuki & K. Kakei, Tanso, (1989) 288. Y. Komatsubara, S.Ida, H. Fujitsu & I. Mochida, Fuel, 63 (1984) 1738. M.M. Dubinin & V.V. Serpinsky, Carbon, 19 (1981) 402. M.J.B. Evans, Carbon, 25 (1987) 81. K.Kaneko, N. Kosugi & H. Kuroda, J.Chem.Soc. Faraday Trans.I,85 (1989) 869. H.P. Boehm, Adv. Catal., 16,179 (1966). D.Rivin, Rubber Chem. Technol. 44 (1971) 307. K.J. Masters & B. McEnaney, in S.J. Gregg, K.S.W. Sing & H.F. Stoeckli (Editors), Characterization of Porous Solids, SOC. Chem. Ind. London (1979) pp.79. A. Guiner & G. Fournet, Small angle scattering of X-rays, Wiley, New York (1955) Chap.4. J.D.F. Ramsay, Chem. SOC. Rev., 15 (1986) 335. A. Craievich, M.A. Aegerter, D.I. dos Santos, T. Woignier, & J.Zarzycki, J. Non-Cryst. Solids, 86 (1986) 394. K. Kakei, S . Ozeki, T. Suzuki & K. Kaneko, IUPAC symposium on Characterization of Porous Solids 11, Alikante, Spain (1990)) in press. G.W.Housner & D.E. Hudson, Applied Mechanics Dynamics, Van Nostrand, Princeton (1959) pp.369.
F. Rodriguez-Reinosoet al. (Editors),Characterization of Porous Solids II 0 1991 Elsevier Science PublishersB.V., Amsterdam
CHARACTERIZATION
OF THE
POROSITY OF
ACTIVATED
399
CHARCOALS
BY ADSORPTION
FROM
SOLUTION J . Fernandez-Colinas’, R. Denoyel and J . Rouquerol Centre
de
Thermodynamique
et
de
du C.N.R.S.,26 rue du
Microcalorimetrie
141eme R.I.A., 13003 Marseille, France
’
On
leave from
Depto. de Quimica Inorganica, Facultad de Quimica, Universidad
de Oviedo, 33071 Oviedo Spain
ABSTRACT char:,2terization of microporosity in two sets of activated charcoal
The is
carried out
a,
method to
acid,
here along two main lines. The first is the extension of Sing’s
the case
of adsorption of various
ter-butanol and
molecules (iodine, salicylic
3-methyl-3-pentanol). Only
primary
filling
of
the
micropores is then detected. The second approach uses two sets of flat (benzene, naphthalene, ter-butanol
pyrene and
and
perylene)
or
more
bulky
3-methyl-3-pentanol) molecular
(methanol, isopropanol,
probes
which
evidence
the
existence of slit-shaped and bottle-necked micropores.
INTRODUCTION Adsorption capacity dyes
from
lead to
sometimes
used to assess the adsorptive
mixtures (5)
a method
as wide
or surfactants and generally
( 6 ) have been used, but this
accepted as those based on gas
like the BET, BJH, cis or Dubinin-Radushkevitch methods (7). The two
adsorption, main
is
the surface area of adsorbents. For instance, iodine ( 1 , 2 ) ,
and hence
(3,4), organic
never
solution
reisons
are
certainly
that
adsorption from
solution is a competitive
phenomenon between solvent and solute which is more difficult to interpret than the
a single
adsorption o f
subject
to more
also that
solute molecules are generally
. . .) .
conventionaly used ( N 2 , Ar In
gas and
specific interactions with the surface than the gas molecules
a preceding
solution, followed
by
paper (2)
we saw
determining
both
that iodine the
adsorption from aqueous
adsorption
isotherms
and
the
corresponding enthalpies of displacement, was able to bring a useful information about
the
auproach
moleculai
porosity
to
the
of
use
size and
activated of
other
shape) and
charcoals. Our
aim
is now to extend this
adsorutives in aqueous solution (with varying to compare the results with those obtained from
gas adsorption ( 8 ) and immersion microcalorimetry ( 9 ) .
400 EXPERIMENTAL
Two
charcoals were prepared by CECA S.A. R and D Laboratories.
sets of
The first set is made up of four charcoals named C1, C2, C3 and C4 respectively. The original sample was cokefied at 900°C to give C1 and then activated by water vapour
at 900-1000°C during increasing
second
set (Bl, B2, B3)
carbon black, from isotherms were preliminary the
times, leading
to C2, C 3 and C4. The
was activated by phosphoric acid. Untreated "vulcan"
Cabot, was
determined by
used the
as
a
non porous reference. Adsorption
conventional
"immersion" method
(10). A
kinetic study was carried out for each system in order to determine
equilibrium time. Equilibrium concentrations were monitored by either an UV
spectrometer
or
a
differential
refractometer. All solutes were of purissima
grade from Fluka. The enthalpies of displacement were measured with a batch cell in
a
(11) :
Tian-Calvet microcalorimeter
suspension
the
charcoal was
maintained
in
(initially in pure solvent) by stirring and the mother solution was
introduced in small successive increments. After taking into account the thermal effect due
to
the
dilution of
the mother solution, a "pseudo-differential''
enthalpy of displacement could be evaluated. A l l experiments were carried out at 25°C. RESULTS AND DISCUSSION
The
two
sets
characterized by gas liquids
of
activated
adsorption (8)
charcoals
studied
here, were
already
and immersion microcalorimetry into pure
(methanol, benzene, cyclohexane, n-hexane and a-pinene) (9). The main
results from nitrogen adsorption are summarized in Table I.
TABLE I BET surface area and microporous volumes obtained by the standard as method with nitrogen. Vas,l
corresponds
to
primary
micropores
and V, s
microporous volume
(8).
cm3g" 0.09 0.13 0.13 0.13
cm3g'l 0.19 0.25 0.27 0.31 0.29 0.37 0.54
,2
is the overall
401 Generalization
of the
C L ~ method
to the case of adsorption at the liquid/solid
interface After
making with
adsorption from a instance, the provides
adsorption of
the data
first trial
to extend
the as method to
(2) we are trying here other molecules. For
salicylic
reported in
isotherms, the enthalpies interesting
iodine our
liquid solution
acid
Fig. 1, 2
of displacement
the
on
and 3 ,
first set of charcoals
which include the adsorption
and the
as plots, respectively. An
be shown between the coverage corresponding to the
correlation may
filling of the micropores (as obtained from this generalized
C L ~ method)
and that
corresponding to the final decrease of the enthalpy of displacement, as shown by the
arrows
in
Fig. 2.
The
adsorption isotherms
of
salicylic acid on the
charcoals of the second set (figure ( 4 ) ) show a lower affinity of salicylic acid for this set of charcoals. This is confirmed by the generalized as plot (fig. 5) which
leads to
straight lines
could not detect absence
here any
of primary
intercepting the
microporosity. This
micropores, as
origin : as if salicylic acid fact may
be understood by the
shown by gas adsorption (8) : in micropores
whose width is larger than 7 - 8 b . , salicylic acid behaves like on an open surface.
If a flat conformation is assumed for the adsorbed molecule (thickness around 4 A ) , a micropore size over c.a. 8A is enough to avoid any measurable enhancement of
the adsorption potential, as shown by calculation (11) and experience (12).
Contrary
to what is seen by gas adsorption (13), no secondary micropore filling
is detected from these adsorption isotherms in solution. Kai
/rmo
I . m-2
L3
c2 c3
I
4.5 x 10-3
2~ 0 - 3 Fig. 1. Adsorption
isotherms of
salicylic acid
on charcoals of the first set.
Amounts adsorbed are refered to the BET surface area.
402
90
60
30
4
0
0.4
0.2
Fig. 2. Derivative charcoa
i
enthalpy of
0.8
0.6 displacement of
water by
1 salicylic acid
on 4
(increasing activation from C, to C 4 ) v s surface coverage
3
c3
e4 \ 83-
c1
82/
ei
5 7
0
0
1
Fig. 3. a, plots adsorbed as
=
o
:
(generalized
on charcoals
e,
C1 to
to adsorption from solution) for C 4 . Coverages
- 0.75, e, - 0.64, e, -
0.55,
e,
corresponding to
- 0.4.
salicylic acid the intercept at
403
o
B1
B2
A
B3
2 x10-3 F i g . 4 . Adsorption
i s o t h e r m s of
4.5 x 10-3 s a l i c y l i c a c i d on c h a r c o a l s of t h e second s e t .
Amounts adsorbed a r e r e f e r e d t o t h e BET s u r f a c e a r e a .
c c
F i g . 5. as plots (generalized a d s o r b e d on c h a r c o a l s B1 t o B 3 .
to adsorption
from solution) f o r
salicylic acid
404
the a, method to the adsorption of 3-methyl 3-pentanol
also applied
We (3M-3P)
and
calculated
ter-butanol for
the
(tBuOH).
four
Microporous
considered
volumes
molecules
and
external
areas
(iodine, salicylic
acid,
ter-butanol and 3-methyl 3-pentanol) are reported in table II together with the minimum
size
other,
of
each
although we
microporous
molecule. Results
must
admit
volumes obtained
correspond to to
that
there
with the
different parts
are reasonably consistent with each is no clear relation between the
various molecules. This is because they
of the
microporous distribution whereas the
external surface area is available to all molecules. In the case of charcoal C 1 , we
can
remark
the
much
higher
microporous
volumes obtained for iodine and
salicylic acid, is probably to a large proportion of micropores smaller than 5A, i.e.
primary micropores as defined by nitrogen adsorption : a
corresponding to
small
change in
the shape or size of the probe molecule can drastically change
its access to the primary micropores. Nevertheless,
this
extension of
the
a,
method
to
adsorption from
solution is not alunys possible. The systems for which we found difficulties can be divided into two types :
- - those
for
sample
which
or the
the
adsorption isotherm
reference s o
that the
is H-shaped (14) either for the
amount adsorbed at the plateau is the
only known value ( f o r example with methylene blue)
- - those for the adsorption isotherm of the non-porous reference is S-shaped. We got
such a
result for
the adsorption, from water,
of small alcohols like
isopropanol. The following conclusions may finally be drawn from this trial to extend the CC,method to adsorption from -,~i?ution : --
the a, method provides the
v<,.i.-F
1 to 3 times the minimum siz,
- - with
the systems
studied we
(i.e. which would
-
of those micropores whose width ranges from the adsorbed molecule
do not
mainly involve
detect any secondary micropore filling
adsorbate-adsorbate interactions).
It is
possibly not a general result, since it should depend on the solute used.
_ _ to
be
meaningful,
precision
this
as method
is
highly
demanding
concerning
the
of the adsorption isotherms in the range of low coverage, for both
the sample studied and the reference adsorbent.
Use of probe-molecules of different shapes An of
priori, of adsorption from solution is the large choice
interest, a
possible
solutes
as
showed
in
the
introduction. Nevertheless, a probe
molecule must be simple in order to make easier the interpretation of adsorption isotherms. A s conformations of
a
consequence, molecules
on the
with
a
large
number
of
possible
surface (because of their length non-rigidness or variety
chemical functions)
must be eliminated (for example surfactants). If we add
405 TABLE II Microporous
volumes and
external surface
area calculated by the as method for
iodine, salicylic acid, 3-methyl 3 pentanol and ter-butanol. AC . Sal.(4.4)
3M.3P(6A) V A m2 g-' cm3g-' m2g.' 0.02 a5 0.06 220 0.11 342 0.12 438
Charcoals cm3g' el c2 c3
0.07 0.07
c4
to
601
these limitations
0.25 0.25
the problems
of low
solubility or
tBuOH ( 6.k) m2 g'l
0.09 0.15
627 954
high volatility, the
choice is finally reduced. Taking into account these facts, we chose two sets of molecules :
__
a
first set of rather
: benzene, naphthalene, pyrene and
"flat" molecules
perylene. Their interest is to be made of the repetition of the same pattern. Their
shortcoming is
(for the largest ones) their insolubility in water. We
therefore used ethanol as the solvent. a
._
second
set
of
"spherical" molecules :
rather
methanol,
isopropanol,
ter-butanol and 3-methyl 3-pentanol, with water as the solvent. For
the
flat molecules, the
anchoring points H-shape (14). The
increases)
the
apparent areas
larger
closer
is
they
are
(i.e. the number of
their adsorption isotherm to the
per molecule
calculated at
the plateau are
in table IU. In the case of benzene, for which the adsorption isotherm
reported
covers the whole range of molar fractions, the relative excess amounts have been transformed to
in adsorbed
4A (15).
TABLE JE Average
area
It seems
per
amount by assuming a thickness o f adsorbed layer equal
that nitrogen and benzene have essentially access to the
molecule
(
A2
),
calculated
at
the plateau of adsorption
isotherms.
Charcoal
benzene
naphthalene
pyrene
perylene
c1 c2 c3 c4 Vul can
39 34 34 34
185 160 215 215 160
307 170 170 170 166
494 283 283 255 307
34
406 same
porosity
observe
as
already shown by iinmersion enthalpies measurements (9). We can
that, despite
the increase
in molecular
size (in the order benzene,
naphthalene, pyrene and perylene, respectively) apparent areas per molecule are very
similar for
a given
solute (for charcoals 2 to 4 ) , which is in favour of
slit-shaped micropores. Charcoal adsorption isotherm of
shows
C1
a
different behaviour, since the
pyrene and perylene are very different from each other.
This difference seems to show the existence, in this sample, of micropores whose appertures
are between one and two benzene ring widths (benzene and naphthalene
can
penetrate but
not pyrene and perylene). This fact has to be related to the
low
enthalpies of
displacement measured
the
case
of
microporous
C1 :
an
sample and
explanation may be
amazing result should lead
for salicylic acid at low coverage in if we
to the
consider that
C1 is the most
highest adsorption potential, The
that the smallest micropores are bottle-necked and are made
accessible by activation. For are
the spherical
L-shaped, and
molecules (except methanol) the adsorption isotherms
the initial
slope increases with the size of the molecule,
which
is due to the higher number of anchorage points per molecule. We can a l s o
point
out, as it has often been outlined (16,17), that the lower the solubility
m, the
the higher is the affinity for the surface. In table
amounts adsorbed at
the plateau are reported for three alcohols. For a given charcoal
(except Cl),
TABLE Amounts
of
(pmo1.m.'
of BET surface area)
alcohol
adsorbed
at
the
plateau
Charcoal
isop.
tBuOH .
3M.3P.
c4
2.5 3.5
2.6
2.35 2.8
they
decrease from
isopropanol to
3-pentanol. These values given
molecule to
are a
terbutanol and
result of
of
adsorption
then increase
isotherms
for 3-methyl
the balance between the access o f a
the microporosity and its ability to displace water from the
surface. This is why, for the non-porous reference sample (vulcan), the average area per molecule
calculated at
the plateau decreases when the molecular size
increases, which shows that water is not fully displaced from the surface by the smallest alcohols
(
cf. table P, where
are also
reported the
surface areas
calculated from the immersion enthalpy measurements ( 9 ) in the pure liquid).
407
Nevertheless, these values
stay relatively
constant from
charcoal to
charcoal, indicating that the surface available to the pure liquid is nearly the same
as that
available to
immersion and
from
the molecule solubilized in water. The results from
adsorption from
solution therefore lead to a consistent
interpretation.
TABLE !J Surface areas (A/ m'g-'
)
calculated by the immersion method (9) and average area
per molecule ( a / A 2 )
isop.
Charcoal A c1 c2 c3 c4 Vulcan
The
use
3M-3P
tBuOH U
A
a
A 420
127 85 70
U
482
ao
274
70
826 986 1172
72 65 56 68
586 867
72
765
71
1108
60 60
993 1193
of
these
various
probe-molecules finally
70 54
leads
us to the
following conclusions :
- - The primary micropores assessed by adsorption from solution are comparable to those detectable by nitrogen
--
The
set of
flat molecules
allows to evidence slit-shaped and bottle-necked
pores
- - The
data obtained
for adsorption from solution are also
a b l e to show the
enlargement of the micropore openings on activation.
- - The
results
immersion
of
adsorption from
enthalpy
measurements
solution are when
they
may
comparable with be
carried
those of
out with the
pure liquid solute. ACKNOWLEDGEMENT
This the
work was made possible thanks to the grant received by J.F.C. from
cooperation program between the Spanish "Ministerio de Educacion y Ciencia"
and the French "Mipistere de la Recherche et de 1'Enseignement Superieur" during his
stay
in Marr.?ille. The
CECA S.A. Finally, we Research
laboratory
also received financial support from
thank J . Vandermersch
and
J.L. Reymonet
(from CECA
and Development Laboratories, Levallois-Perret, France) for fruitful
discussions.
408
REFERENCES
1 M. Molina-Sabio,
C. Salinas-Martinez
de
Lecea,
F. Rodriguez-Reinoso,
C. Peunte-Ruiz and A. Linares-Solano,Carbon, 23 (1985) 91.
2 J . Fernandez-Colinas, R. Denoyel and
J . Rouquerol, Adsorption Science and
Technology, 6 (1989) 18. 3 C.H. Giles, A.P. D'Silva
and A.S. Trivedi, in Surface Area Determination,
D.H. Everett, R.H. Ottewill Eds., Butterworths, London, (1970) p.317. 4 G. McKay, J. Chem. Technol. Biotechnol., 32 (1982) 759. 5 G. Schay, in Surface
Area Determination, D.H. Everett, R.H. Ottewill Eds.,
Butterworths, London, (1970) p.273. 6 P. Somasundaran and D.W. Fuerstenau, J. of Phys. Chem., 70 (1966) 90.
7 S.J. Gregg
and K.S.W. Sing, in Adsorption Surface Area
and Porosity, 2nd.
Ed., Academic Press, London, (1982). 8 J . Fernandez-Colinas, R. Denoyel, Y . Grillet, F. Rouquerol and J . Rouquerol,
Langmuir, 5 (1989) 1205. 9 J . Fernandez-Colinas, R. Denoyel, Y . Grillet, J . Vandermersch, J.L. Reymonet, F. Rouquerol
and
J. Rouquerol, in
Proceedings of the Third Conference on
"Fundamentals of Adsorption", Sonthofen (1989). 10 D.H. Everett, Pure Appl. Chem.
58, (1986) 967.
11 D.H. Everett ar,d J.C. Powl, J. Chem. SOC., Faraday Trans. I , 72, (1976) 619. 12 S .
Partyka, F .
Rouquerol
and
J. Rouquerol, J. Colloid Interf. Sci., 68,
(1979) 21. 13 M.M. Dubinin, in Progress J.F. Danieli and
in Surface
M.D. Rosenberg
and Membrane Science, D.A. Cadenhead,
(eds.), Academic
Press, New
Y o r k (1975),
Volume 9 , p. 1. 14 C.H. Giles, D.
Smith and
A. Huitson, J . Colloid and Interface Sci., g ,
(1974) 755. 15 I. Johnson, R. Denoyel, J. Rouquerol and D.H. Everett, accepted in Journal of Colloid and Interface Science. 16 K. Urano, Y. Koichi
and Y. Nakayama, J . Colloid
and Interface Sci., 81,
2 ( 1 9 8 1 ) 477.
1 7 J . Kipling,
in
"Adsorption from
Press, London ( 1 9 6 5 ) .
Solutions
of Non-Electrolytes", Academic
F. Rodriguez-Reinosoet al. (Editors), Characterization of Porous Solids II 0 1991 Elsevier SciencePublishers B.V., Amsterdam
409
THE POROSITY OF TEXTILE FIBRE SURFACES A. McINALLY1, R.R.
MATHER1 and K.S.W.
SING2
IScottish College of Textiles, Netherdale, Galashiels, Scotland, TD1 3Hp. %epartment of Chemistry, Brunel University, Uxbridge, Middlesex, England, uB8 3PH.
SUMMARY The effects of dyeing on the porosity of samples of charcoal cloth and cotton fabric have been examined from nitrogen adsorption isotherms. So too has the effect on the cotton fabric of mercerisation, a treatment which causes dimensional changes to the constituent fibres. On application from 1% sodium chloride solution, increased contents of dye in the charcoal cloth progressively block the micropores and reduce mesopore area; but treatment with sodium chloride solution alone increases microporosity. Mercerisation of the cotton fabric enhances the mesoporous nature of the surface. The effect on porosity of treatment with a reactive dye is less clear-cut; it is tentatively suggested that application of the dye narrows the mesopores.
INTRODUCTION
The surfaces of textile fibres play a key role in the manufacture of textile materials. the
Substances, such as dyes, which are incorporated into the body of
fibres, must
first be
adsorbed by
the fibre surfaces before
subsequently diffuse into the internal structure.
they
Moreover, processes such as
shrinkproofing generally rely on modificaton of the fibre surfaces. The porous properties of textiles vary widely.
Textile fibres of natural
origin generally have a higher degree of porosity than synthetic fibres, and this difference extends to the fibre surfaces.
Wool fibres, for example, have
scaly surfaces and cotton fibres have ridged surfaces, whereas those of synthetic fibres are generally much smoother. Despite
its wide
application overall, the use
of
gas
adsorption to
investigate textile surfaces has been very limited even though the first report, known to us, appeared over forty years ago (I).
Values obtained for
the BET-nitrogen surface areas were all less than 1 mLg-’: adsorbed nitrogen inolecules were
apparently confined
to the
fibre surfaces and
were
not
410 perletrating into the extensive internal porous network. In view of the low BET-surface areas, few attempts have been made to characterise the porosity of fibre surfaces, although an investigation on the porosity of wool samples has recently been presented (2).
In this paper, we
present some preliminary results on differences in the porosity of the surfaces of some cotton fabrics, as a result of mercerisation and treatment with a reactive dye.
Mercerisation is a treatment by strong solutions of sodium
hydroxide, which causes lateral swelling of the fibres and maybe longitudinal shrinkage.
A reactive dye forms covalent bonds with the cellulosic chains of
the cotton by reaction with its hydroxyl groups.
To check the capability of
the technique, studies have also been made of the application of a direct dye to a charcoal cloth of well-characterised mesoporous structure.
HATERIALS AND HEThe charcoal cloth has been described elsewhere ( 3 ) .
It had been obtained
from a woven Moygashel viscose rayon precursor, impregnated with aqueous phosphate-containing solutions. The precursor was first carbonized in nitrogen and then activated to 50% burn-off in carbon dioxide at 850°C.
A sample of an
ungraphitised carbon black, Elftex 120 (4), was also included for reference purposes. Samples of the cloth were dyed in sealed tubes in a Rotadyer at 95°C with the direct dye,
C.I.
Direct Blue 71, for a period of 22 h.
The uptake of dye
by the cloth was determined by measuring spectrophotometrically at a wavelength
of 580 nm the initial and final concentrations of dye in the liquor.
The
structural formula of the dye is:
The dye liquor also contained 1% (w/v) sodium chloride, normally used to assist
411 the application of direct dyes. A series of dyed cloths was thus simultaneously prepared with progressively increasing dye content (Table 1).
In addition, one
sample of cloth was exposed to a bath containing neither salt nor dye, and a second sample to a bath containing salt but no dye.
TABLE 1 Properties of charcoal cloth samples Treatment
Micropore volume V p (cm”g-’)
Mesopore area S, (m2g-=) ~~
None Blank Salt 5.4% dye* 7.3% dye* 10.0% dye*
~
0.130 0.113 0.159 0.068 0.036 0.008
~
~
~~
824 908
1003 885 783 705
* The percentages of dye are expressed with respect to the mass of charcoal cloth. In these cases, salt was present in the dyebath.
The cotton samples were all supplied by ICI Colours and Fine Chemicals, Manchester, England.
Sample 1 had received no special treatment. Sample 2 had
been dyed with the reactive dye, C.I. Reactive Blue 168, and Sample 3 had been mercerised.
Sample 4 had been initially mercerised and then dyed with the
reactive dye (Table 2).
The structural formula of the dye has not been
disclosed.
TABLE 2 Properties of cotton samples Sample
Treatment
BET - plot S,
(m”g--’) 1 2 3 4
Untreated Dyed Mercerised Mercerised, then dyed
0.88 0.88 0.72 0.79
c 14 17 19 14
r 3.23 3.13 2.73 2.66
FHH - plot Range of linearity (p/p,) 0.65-0.88 0.75-0.88 0.65-0.88 0.65-0.78
Nitrogen isotherms were determined at 77K by a volumetric method, with a
412 semi-micro appartus of the type designed by Harris and Sing (5). of charcoal cloth had been outgassed at 250°C. the dyed
samples revealed
outgassing.
no
The samples
Thermogravimetric studies of
decomposition of
the adsorbed dye during
To determine the nitrogen isotherms for the cotton samples, the
apparatus was modified to accommodate a bigger sample vessel, which could hold up to 20-23 g of fabric.
Before determination of an isotherm, each sample was
outgassed at room temperature for 68 h. The cotton samples were also examined by scanning electron microscopy, using a Hitachi S530 instrument.
RESULTS AND DISCUSSION Charcoal Cloth
Representative adsorption isotherms are shown in Figure la.
All the cloths
give broadly similar types of isotherm of combined Type I and Type IV character and hence contain both micropores and mesopores. differences in the detailed nature of the isotherms.
However, there are clear For example, the effect
of treatment with liquor containing salt but no dye is an upward displacement o € the isotherm, such that an enhanced interaction with nitrogen is suggested.
However, the breadth of the hysteresis loop is only slightly increased.
The
effect of treatment with liquor containing neither salt nor dye (not shown in Figure l a ) is much less marked: a slight upward displacement is apparent. Dyed samples of cloth show an opposite effect:
as the content of dye is increased,
the isotherm is displaced downwards, but the nature of the hysteresis loops is not appreciably affected. Figure lb shows the &,-plots
(6) corresponding to the isotherms in Figure
la. Isotherm data determined for the non-porous carbon, Elftex 120, on the same apparatus were taken as the standard. 0.5<&
and extrapolation of
positive intercepts. mesopore areas, S , ,
The plots are linear in the range of
the linear plots to the ordinate gives
From the gradients of the linear portions of the plots, for the cloths can he calculated;
these areas strictly
413
Relative pressure
d S
Fig. 1. :a) Nitrogen adsorption isotherms and (b) corresponding d e plots for samples of charcoal cloths. Open points - adsorption; Filled pointsdesorption. A, cloth exposed to 1% sodium chloride solution; B, untreated cloth; C, cloth containing 10.0% dye.
Percentage of dye in cloth
Mesopore area, Ss
(mag1)
Fig. 2 . Plots of micropore volume, Vr, (a) against dye content and (b) against mesopore area, S,. Point 1 corresponds to the untreated charcoal cloth.
414 also include a small contribution from the truly external surface area.
The
irltercepts of the plots on the ordinate provide values for the micropore volumes,
vr
(Table 1).
In the range d,,>1.1, the plots clearly deviate from
linearity, a trend expected from the mesoporous nature of the cloths. Table 1 lists the values of S, and Vr Figure
2a
illustrates the
increasing content of dye.
found for the different cloths, and
progressive blocking
of
the micropores
with
It is noteworthy too that blocking of the
micropores accompanies a reduction in mesopore area,
S,.
In an analogous
manner, the increased microporosity induced by salt treatment accompanies an increase in S.,
The relation between Vr
and S, is shown in Figure 2b.
Cotton Fabric
Some of the nitrogen adsorption isotherms on the cotton samples are shown in Figure 3 .
In agreement with the results of previous workers
adsorption of nitrogen was found to be low.
(l),
the
The isotherms are of Type I1
character and appear reversible, or nearly so. BET plots for the isotherms are linear between relative pressures of 0.05 and 0.35.
derived from the plots, together with values of
nitrogen surface areas, S,,,,
the BET parameter, c, are given in Table 2. match
the
The values of the BET-
The low values of S,
broadly
corresponding geometric surface areas, assessed from scanning
electron micrographs for the component fibres of each fabric.
Thus, nitrogen
is significantly adsorbed only onto the external surfaces of the fibres.
The
small values of c indicate a weak interaction between nitrogen and the fibre surfaces. The Frenkel-Halsey-Hill (FHH) equation has been used
to analyse the
behaviour of the isotherms at higher relative pressures (7).
It has been
applied in the form: log,
(PJP) = k /V'.
V is the volume (reduced to stp) of nitrogen adsorbed at the equilibrium relative pressure, p/p-;
k and r are constants.
FHH plots of log,V
against
415
Relative pressure Fig. 3. untreated;
Nitrogen adsorption isotherms on samples of cotton fabric. (b) mercerised, then dyed. 0, adsorption; 0, desorption.
(a)
Amount adsorbed by Sample 1 Fig. 4. Nj.trogen comparison plots for samples of the cotton fabric. Axes are in units of cm3(stp)g-l. (a) Sample 3 vs. Sample 1. (b) Sample 4 vs. Sample 1.
416 log,[log,(p,,/p)]
were accordingly constructed for the isotherms. In all cases,
linear portions of the plots could be identified, although for the dyed fabrics the range of linearity was limited.
Moreover, linearity begins only at or
above a relative pressure as high as 0.65,
thus providing further evidence for
the low adsorbent-adsorbate interaction suggested by the small values of c. Such behaviour has also been noted for commercial samples of the disperse dye, C.I.
Disperse Blue 79 (8).
Values of the FHH-constant, r, are included in Table 2; that the values for Samples 1 and 2 are greater than 3 .
it is noteworthy
Moreover, comparisons
between Samples 1 and 3 and between Samples 2 and 4 reveal that mercerisation significantly reduces the value of r. At this stage, no unique solution can be offered to these differences in r-values.
The BET c-values reveal that all
four samples have surfaces of low energy.
The high r-values, especially for
Samples 1 and 2, may suggest then that the multilayer structure of the adsorbed nitrogen molecules is different from their structure on surfaces of higher energy.
Indeed, r-values of 2.8-2.9 have previously been determined on some
dye samples with low c-values (8).
The reduction in r-value on mercerisation
suggests that mesoporosity is induced or extended at the cotton fibre surfaces. Values of r greater than 3 have also been noted in microporous samples (7). On this basis, the reduction in r could be interpreted by a reduction in micropore content. However, microporous samples would be expected to have much higher c-values than those listed in Table 2, so this interpretation seems more doubtful. The effect of dye on porosity is intriguing.
From comparisons between
Samples 1 and 2 and between Samples 3 and 4 , it appears that application of the reactive dye lowers the value of r only slightly, if at all. Nevertheless, the range of linearity of the FHH plot is considerably reduced.
This effect could
perhaps be explained by a narrowing of the mesopores as a consequence of applying the dye, but such a conclusion can as yet be only tentative. The effect of mercerisation has been examined further:
the adsorption of
417
nitrogen by Sample 3 has been compared with that by sample 1, using the comparison plot method ( 9 ) .
Figure 4a illustrates the plot.
relative pressures, 0.10-0.65, the plot
is linear.
In the range of
At higher relative
pressures the deviation away from linearity reflects the greater surface porosity in the mercerised fabric.
It is noteworthy too that extrapolation of
the linear region at. very low relative pressures leads to a clear positive intercept on the ordinate. The effect of dyeing the mercerised fabric can also be similarly examined. In Figure 4b the adsorption of nitrogen by Samples 1 and 4 is compared. linear region can now be extrapolated back to the origin.
The
Moreover, when the
adsorption of nitrogen by Samples 1 and 2 is compared, to examine the effect of dyeing without mercerisation, a similar result is obtained. suggest that mercerisation not only induces mesoporosity; give rise to greater interaction with nitrogen.
These results
it appears also to
Such increased interaction
could be explained by the exposure of more hydroxyl groups in the cellulose chains comprising the cotton.
These groups are masked by subsequent reaction
with the dye
ACKNS -
The authors would like to thank ICI Colours and Fine Chemicals, Manchester, England for providing the cotton samples and the technical staff at both the Scottish College of Textiles and Brunel University for their assistance.
REFZRENC!ZS
1 2. 3.
J.W.
Rowen and R.L. Blaine, Ind. Eng. Chem., 39 ( 1 9 4 7 ) 1659-1663.
U. Schumacher-Hamedat, C. Laurini and V. Schneider, Proc. 8th. Int. Wool Text. Res. Conf., Christchurch, 1990, in press. J.J. Freeman, F.G.R. Gimblett, R.A. Roberts and K.S.W. Sing, Carbon, 26 ( 1 9 8 8 ) 7-11.
4.
5. 6. 7.
P.J.M. Carrott, R.A. Roberts and K.S.W. Sing, Carbon, 25 ( 1 9 8 7 ) 59-68. M.R. Harris and K.S.W. Sing, J. Appl. Chem., 5 ( 1 9 5 5 ) 223-227. K.S.W. Sing, in D.H. Everett and R.H. Ottewill (Editors), Surface Area Determination, Proc. Int. Symp. 1969, Butterworths, London, 1970, pp25-42. P.J.M. Carrott, A.I. Mcleod and K.S.W. Sing, in J. Rouquerol and K.S.W. Sing (Editors), Adsorption at the Gas-Solid and Liquid-Solid Interface, Elsevier, Amsterdam, 1982, pp403-410.
418 8.
9.
R.R. Mather, in K.K. Unger, J. Rouquerol, K.S.W. Sing and H. Kral (Editors), Characterisation of Porous Solids, Elsevier, Amsterdam, 1988, pp263-271. C.E. Brown and P.G. Hall, Trans. Faraday SOC., 67 (1971) 3558-3564.
F. Rodriguez-hinoso et al. (Editors),Characterization of Porous Solids ZZ 0 1991 Elsevier Science Publishers B.V., Amsterdam
419
FURTHER COMMENTS ON LOW PRESSURE HYSTERESIS IN ACTIVATED CAREONS: EFFECT OF PREPARATION METHOD
F. Rodriguez-Reinoso, J.M. Martin-Martinez, A. Linares-Solano and R. Torregrosa Departamento de Quimica Inorginica e Ingenieria Quimica. Universidad de Alicante. Apartado 9 9 . 03080 Alicante. Spain
SUMMARY
Low pressure hysteresis (LPH) in activated carbons is not very common and the few reports found in the literature do not include the role played by the preparation method used. This work compares the results found in the evolution of LPH in two series of carbons prepared by reaction with CO, or air of activated carbons obtained from olive stones. Reaction with CO? leads to LPH only in carbons with low burn-off whereas reaction with air gives LPH in all carbons and increases with burn-off. Some new examples are presented to show that LPH independently of the hydrocarbon used as adsorptive - is always present when the carbons have been reacted with air at relatively low temperature (623K).
-
INTRODUCTION
Adsorption-desorption isotherms of hydrocarbons on activated carbons usually exhibit hysteresis loops which may be limited with a closing point at a given relative pressure (High Pressure Hysteresis, HPH) or may persist down to very low pressure (LPH). LPH is observed less frequently than HPH and has not been so extensively studied. The reported reasons for the existence of LPH are usually related to porous texture distortions of the solid caused by swelling during the adsorption process which opens up cavities previously inaccessible to the adsorbate molecules (refs. 1,2). Furthermore, the stress induced by adsorption swelling could be enough to fracture the carbon adsorbent structure, as has been argued by McEnaney (ref.3). The trapped adsorbed molecules can only be removed if outgassing is carried out at temperatures higher than that of adsorption. Important features related to LPH, extracted from the limited data available, are: i) it is usually found that LPH does not
420
depends on both the adsorbent porosity and the adsorbent-adsorbate system (ref. 4) although it has been usually argued that the adsorbent pore size should be comparable to that of the adsorbate molecule (refs. 2,3). It is clear that the carbon burn-off may or may not control the existence of LPH. However, despite its importance, little attention has been paid to the analysis of LPH loops and their evolution with burn-off. Furthermore, contradictory results have been published as commented elsewhere (ref. 5). The adsorption-desorption isotherms of several hydrocarbons have been compared for two series of activated carbons prepared from almond shells (ref. 5 ) . Both series of activated carbons were prepared with similar percentage burn-offs but one of them was activated in CO1 (ref. 6) and the other by reaction with air (ref. 4). In the case of air-reacted samples LPH was present and, contrary to what could be expected, its size increased significantly with burn-off. This behaviour was not found for CO?activated carbons even if they had comparable burn-off levels (ref. 5).
In order to investigate the 'anomalous" behaviour of LPH in airreacted carbons, this paper presents further adsorption-desorption isotherms of hydrocarbons on a series of air-reacted activated carbons prepared from olive stones. The results, focussing on the desorption paths, will be compared with those previously published, mainly with carbons prepared from the same precursor, but activated in CO, (ref. 7). EXPW IHENTAL
An activated carbon, prepared by CO, activation of carbonized olive stones, to a 32 wt% burn-off was reacted with dry air at 623K for different periods of time, as indicated in (ref. 8 ) . The nomenclature of this series B of carbons includes the degree of activation in air. The adsorption-desorption isotherms of benzene, cyclohexane, 2,2-dimethylbutane (2.2-DMB) and isooctane were determined at 298K in a grease-free gravimetric system using silica spring balances (ref. 4 ) . For each experimental point an "equilibrium" time of lh was allowed for all hydrocarbons (4h for the first point) except for isooctane (4h for each point and 24h for the first). Before proceeding to desorption the system was held under saturation conditions for 60h and an 'equilibrium" time of 3h was then used for each desorption point.
42 1 RESULTS AND DISCUSSION
The adsorption-desorption isotherms of benzene, cyclohexane, 2,2-dimethylbutane and isooctane are presented in Figs. 1 to 4 respectively, in order to show the effect of burn-off on both the adsorption capacity and shapes of the corresponding adsorptiondesorption HPH and LPH loops.
12 10
a 6
4
2 0 0 0.2 0.4 0.6 0.8 1
P/P
0 0.2 0.4 0.6 0.8 1
P/P
O
O
Fig. 1. Adsorption (open symbols)-desorption (closed symbols) isotherms of benzene at 298K on activated carbons of series B.
3
B-0
2
I 0
0.2 0.4 0.6 0.8 P/PO
1
I
0
I
0.2 0.4 0.6 0.8 1
P/PO
Fig. 2 . Adsorption (closed symbols)-desorption (open symbols) isotherms of cyclohexane at 298K on activated carbons of series B. The adsorption capacity for all adsorptives increases as a consequence of the reaction with air up to a maximum value for sample B-31, significantly decreasing thereafter to sample B-71.
422
This adsorption capacity evolution associated to air-reacted samples confirms previous findings (refs. 7-9). The micropore volumes deduced from the Dubinin-Radushkevich (DR) equation are compiled in Table 1. From both the data in this Table 1 and the isotherm shape evolution (Figs. 1-4) it can be deduced that the reaction with air produces an important change
L
n mmol
(mmol/g)
B-5
0 0.2 0.4 0.6 0.8 P/PO
0
1
0.2 0.4 0.6 0.8 1 P/PO
Fig. 3 . Adsorption (open symbols)-desorption (closed symbols) isotherms of 2,2-dimethylbutane at 298K on activated carbons of series B.
1 4
0' 0
B-31
I
0.2 0.4 0.6 0.8 1 P/P O
Fig. 4 . Adsorption (open symbols)-desorption (closed symbols) isotherms of isooctane at 298K on activated carbons of series B.
423
in the porous texture of the starting activated carbon. Thus, Fig. 1 - as an example of all other - clearly shows that sample B-52 and B- 71 have undergone a considerable porosity erosion as it can be deduced from their much lower adsorption capacity their more rounded knees and larger slope of the plateau region of the isotherms. If the amounts adsorbed in Table 1 (expressed as mmol.g-') for the different hydrocarbons are compared for a given adsorbent the following sequence is generally found: benzene> cyclohexane> 2,2dimethylbutane> isooctane, corresponding to the same order of increasing minimum dimension of the molecules.
TABLE 1 Micropore volumes (cm3/g) deduced from the DR equation. Cyclohexane
B-52 B-71
0.28 0.20
I
Isooctane
I
0.13
The hysteresis loop (HPH) gradually increases in size upon gasification in air. To quantify this feature the relative height (h) of the HPH loops has been calculated as the difference between the amounts adsorbed on the desorption and adsorption branches at P/P" = 0.6 divided by the desorption value. Such calculations which are compiled in Table 2 - confirm that there is a noticeable increase of the HPH with air activation in contrast with the results found with the same precursor but CO, activation (ref. 5) where the HPH loops did not change with burn-off (Series D in Table 2). TABLE 2
h values for different adsorptives.
B-31 B-52 B-71
7.08 16.18 23.50
12.30 27.27
D-52 D-70 D-80
Benzene
2,2-DMB
0.10
0.08
0.13
0.08 0.08
0.12 0.11
0.10
Figs. 1 to 4 show the presence of LPH, its magnitude being dependent on the carbon burn-off; in spite of the fact that the
424
original activated carbon (B-0), with a 32% burn-off in CO, does not have LPH (except for isooctane, Fig. 4 ) , all the other air-reacted carbons exhibit LPH loops increasing with burn-off so that, as a result of this feature, the above mentioned HPH increase with activation should be the consequence of an increasing contribution from LPH as has been previously pointed out (refs. 2 , 4 ) . Similar evolution of LPH with activation was found in another series of air-reacted carbons prepared from almond shells (ref. 4 ) . The results found with these two series of air-reacted carbons olive stones and almond shells precursors - differ greatly however from those obtained when these two precursors are activated in CO, to comparable burn-off levels (ref. 5 , 6 ) . Fig. 5 - redrawn from (ref. 5 ) - shows the benzene adsorptiondesorption isotherms of benzene on two series of activated carbons prepared from almond shells but activated in CO, (Fig. 5A) and reacted with air (Fig. 5B), the burn-off of these series being very similar. Fig. 6 - redrawn from (ref. 6) - shows the corresponding isotherms on the series prepared from olive stones in CO, for which the burn-off levels are also comparable. A detailed comparative analysis of the different porous texture of carbons prepared by Co, activation and by air-reaction has been published elsewhere (refs. 5r7).
::p, n (mmol/g)
I
12
I
. B
10 c-38
6 4
I
I
A-33 A
I
c-9
2 1
0 0.2 0.4 0.6 0.8 1 P/PO
0 0.2 0.4 0.6 0.8 1 P/P O
Fig. 5. Adsorption (open symbols)-desorption (closed symbols) isotherms of benzene at 298K on A)activated carbons of series C (almond shells activated in CO?); B) of series A (almond shells reacted with air).
425
From Fig. 5 and by comparing Figs. 1 and 6 it is clearly observed that air and CO, give rise to different porous texture evolution. Important features extracted from the figures are: i) the evolution of the amount adsorbed with burn-off are very different in CO, and in air, increasing continuously with burn-off
101 -70
8
-52
“i OL
I
0 0.2 0.4 0.6 0.8 1
P/P
O
Fig. 6. Adsorption (open symbols)-desorption (closed symbols) isotherms of benzene at 298K on activated carbons of series D (olive stones activated in CO,). in CO, whereas in air there is an increase up to 31% burn-off decreasing thereafter. ii) the HPH loops in air reacted samples independently of the precursor (almond shells or olive stones) increase with burn-off; this behaviour being unobserved for the CO, activated series. iii) both air-reacted series show increasing LPH loops with burn-off; however, no LPH is found in the benzene isotherms of the two C0,-activated series, including the original activated carbon (Figs. 1 and 5, respectively). As mentioned above LPH loops and their evolution with increasing burn-off have been related to microcracks (ref. 3) or to more or less elastic distortion (or deformation) produced in the carbon texture during the adsorption process by induced pressure swelling (refs. 1,2). From a series of repeated adsorption-desorption cycles in a given air-reacted sample it was concluded that the induced pressure swelling did not produce any appreciable fracture or the adsorption isotherm being reversible microcracks (ref. 4 ) after heat evacuation so that the porous texture deformation seems to be a more reliable explanation for the occurrence of LPH.
426
The observation that steam activation of carbons removed LPH or decreased its extent (ref. 3) seems to confirm the idea that a pore size comparable to the adsorbate molecules will favour the existence of LPH. Since the reaction with air produces a considerable widening of the micropores (refs. 8,9) one could expect a decrease (or even absence) of LPH and not an increase with burn-off Our "anomalous" findings agree better with the results observed by Everett and Whitton (ref. 10) for a coconut shell activated carbon with a large burn-off (81%). The weak porous texture due to the large burn-off was the argument used to explain its LPH (ref. 2); high burn-off levels will cause a less rigid carbon structure which could be more easily deformed. The burn-offs used in the C02-activated carbons series are comparable to those of the air-reacted samples and in some cases are very large. Nevertheless, LPH in air-reacted samples differ from those prepared by CO,, consequently, the burn-off cannot be used by itself to interpret our results. It is evident that, besides the important role played by the porous texture of the solid, there are probably other unknown factors affecting the LPH. The reasons for the "anomalous" behaviour observed in airreacted carbons from both precursors (almond shells and olive stones) are not yet well understood. However, they must be related more to the activating agent used (air) for the reaction than to the burn-off levels reached. It is possible that different oxygen surface groups formed during reaction with air compared to the C02 reaction, give rise to a more flexible and deformable carbon structure. This paper confirms previous work (refs. 4 , s ) where the reactant gas was introduced as a new parameter affecting LPH in addition to other well known factors such as porosity, maximum pressure reached in the adsorption and adsorptive molecular size.
.
REFERENCES 1 J.C. Arne11 and H.L. Mc Dermott, Proc. Second Int. Cong. Surface Activity, Vol. 11, p. 113. Butterworths. London (1957). 2 A. Bailey, D.A. Cadenhead, D.H. Davies, D.H. Everett and A.J. Miles, J. Chem. SOC., Faraday Trans. I, 67, 231 (1971). 3 B. McEnaney, J. Chem SOC., Faraday Trans. I, 70, 84 (1974). 4 J.M. Martin-Martinez, A. Linares, F. Rodriguez-Reinoso and J.D. L6pez-Gonz&lez, Ads. Sci. Technol., I, 317 (1984). 5 F. Rodriguez-Reinoso and A . Linares-Solano in "Chemistry and Physics of Carbon"; Vol. 21, p.1. Ed. P. A. Thrower. Marcel Dekker Inc. New York (1989). 6 J. Garrido, J.M. Martin-Martinez, M. Molina-Sabio, F. RodriguezReinoso and R. Torregrosa, Carbon, 24, 469 (1986).
421
7
J. Garrido, A. Linares-Solano, J.M. Martin-Martinez, M. MolinaSabio, F. Rodriguez-Reinoso and R. Torregrosa, J. Chem. SOC. Faraday Trans. I, 83, 1081 (1987). 8 P. Gonz61ez-VilchezI A. Linares-Solano, J.D. Lbpez-Gonzhlez and F. Rodriguez-Reinoso, Carbon, 17, 441 (1979). 9 J.M. Martin-Martinez, A. Linares-Solano, F. Rodriguez-Reinoso and J.D. Lbpez-Gonzilez, Ads. Sci. Technol., I, 195 (1984). 10 D.H. Everett and W.I. Whitton; Proc. Roy. Soc. London, m, 91 (1955).
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F. Rodriguez-Reinosoet al. (Editors),Characterization of Porous Solids II ElsevierScience Publishers B.V., Amsterdam
429
0 1991
MULTI-STAGE
MICROPORE FILLING OF N2 AND Ar
BY MICROPOROUS
CARBON FIBERS
K. Kakei, S. Ozeki, T. Suzuki, and K.Kaneko Department of Chemistry, Faculty of Science, Chiba University Yayoi, Chiba 260 Japan
SUMMARY The detailed adsorption isotherms of Ar on activated carbon fibers(ACF's) at 77 K were measured with the aid of a computercontrolled apparatus and were compared with the detailed N2 adsorption isotherms. The DR plots of the Ar adsorption isotherms on ACF'S are concave against the abscissa, while the DubininAstakhov plots(n=3) of the Ar isotherms have three linear sections. Three linear sections could be explained with the multi-stage micropore filling (MSMF) mechanism which covers two-stage mechanism proposed by Sing et al. The presence o f the monolayer adsorption process on the micropore wall, which was not considered in the twostage mechanism, was shown even in the case of micropore filling of Ar without the quadrupole.
INTRODUCTION Micropore filling by activated carbons has been extensively investigated[l,2]. Usually the micropore filling process is described by the Dubinin-Radushkevich (DR) equation based on the potential theory. Dubinin and Astakhov proposed a general equation known as the DA equation [3]. W = Wo exp[-(A/
pEo>nl
(1)
Where W is the volume o f adsorbates adsorbed in micropores at temperature T and relative pressure P/Po; Wo is the limiting volume of the adsorption space, A(=RT ln(Po/P)) is the adsorption potential, n, p , and Eo are specific parameters of the system under investigation. The DR equation is one form of the DA equation with n = 2 . Analyses of numerous adsorption experiments have shown that the DR equation is useful to describe phenomenologically vapor adsorption on activated carbons. However, deviations from linear DR plots are frequently encountered. The deviation is mainly ascribed to some heterogeneity in the micropore structure. Dubinin and Stoeckli [ 4 ] proposed the DR equation having two terms which
430
originate from two independent structures. Marsh [5] and Master and McEnaney[6] discussed the relationship between the deviation from the ideal DR equation and the micropore structure of carbonous materials. McEnaney[7] has tried to describe the micropore filling in terms of general formula. Jaroniec and Choma[8] have proposed new mathematical expression for the micropore filling in micropores of energetic heterogeneity. On the other hand, Sing et a1[9-11] have proposed two-stage mechanism o f the micropore filling on the basis of ds-and calorimetric analyses for abundant adsorption data; two elementary processes are a significantly enhanced 'primary' process that occurs at lower P/Po and a 'secondary' or 'cooperative' process at higher P/Po. The micropore filling mechanisms by carbonous materials, however, are not sufficiently established yet. Activated carbon fibers (ACF'S) are highly microporous with small external surface areas and very little mesoporosity[l2-141. ACF's have been extensively investigated from both fundamental and practical aspects. The detailed adsorption isotherm of N2 on ACF should provide an important key to the micropore filling mechanism. In the preceding papers[l5,16], the adsorption isotherms of N2 on ACF'S were statically measured with the aid o f a computercontrolled gravimetric apparatus; the detailed DR plot indicated a multi-stage micropore filling (MSMF) mechanism including monolayer adsorption on the pore wall of supermicropores. The quadrupole of N2 interacts with the carbon surface; the quadrupole produces the herringbone pattern o f the N2 molecules adsorbed on the graphitized carbon black in the submonolayer[l7,18]. Adsorption isotherms of N2 on ACF's have two steps below 0.01 of P/Po which should be associated with the submonolayer phase transition. As Ar without the quadrupole has the molecular size and polarizability similar to N2, N2 and Ar adsorption isotherms are frequently compared with each other to assess the true microporosity[7,10,20]. Hence the analysis of Ar adsorption isotherms on ACF'S should give a further evidence for the MSMF mechanism. In this paper the adsorption isotherms of N2 and Ar on ACF'S will be compared and the micropore filling mechanism in the ACF system will be discussed.
EXPERIHENTAL
Cellulose(CEL, Toyobo KF1500)-, pitch(PIT,Osaka gas A10)-, and polyacrylonitrile(PAN, Tohorayon FE200)-based ACF'S have been used in this study. Nonporous carbon black(NPC, Mitsubishi Kasei 32 [21]) was also used for comparison. The adsorption isotherms of N2 and Ar were statically measured by an automatic gravimetric apparatus at 77 K. The detailed description of this apparatus has been reported[15]. We obtained adsorption isotherms of more than
431
70 measuring points over about 20 h by this apparatus. The samples were pre-evacuated at 383 K and 1 mPa for 2 h. Gases of more than 99.99 % purity were dried by slow passage through a cold trap. X-ray diffraction of the ground sample was measured by an automatic X-ray diffractometer (Rigaku Denki 2028). High resolution electron micrographs were taken on a JEM-200FX instrument operated at 200 kV. The ACF sliced at a right angle to the fibrous direction in 50-60 nm thickness was observed.
RESULTS AND DISCUSSION Characterization of ACF X-ray powder diffraction patterns of ACF samples have two broad peaks at 28= 25' and 28= 43 O . which are reflections from the (002) planes and from the (001) and (101) planes, respectively. The interlayer distance from the 002 planes was 0.35-0.36 nm for all nm, corresponding to ACF's. The 002 crystallite size was 0.7-0.9 ca. 2-3 times of the interlayer distance. The details on X-ray diffraction study were already reported in the preceding papers[15,17]. Also the detailed description of the microporosity from the N2 adsorption isotherms previously appeared. The micropore volumes Wt and WOLfrom the t- and ds-plots, respectively are described here[ Wt in mlg": CEL; 0.590, PAN; 0.354, and PIT; CEL; 0.606, PAN; 0.344, and PIT; 0.3241. 0.330. WOrin mlg": Figure 1 shows electron micrographs of ACF samples. It is not to easy to conclude the microporosity from the micrographs[23,24]. The micrographs of these ACF samples with greater magnification are relatively homogeneous with microporosity due to distorted slits, ca. 1 nm in width; these pores are separated by walls of about three graphite-like layers in thickness, coinciding with the X-ray diffraction data. Meanwhile the low magnification micrograph of CEL shows greater heterogeneity, which probably originates from the mesoporosity. Other PIT and PAN do not have such images on the low magnification micrograph due to the mesoporosity.
DR plots €or N2 adsorption isotherms micropore
filling
and
multi-stage
mechanism
The DR plots of N2 adsorption isotherms are composed of three or four lines with different slopes, as shown in Fig. 2.15 The inflection points appear near 0.004, 0.05, and 0.3 of P/Po. These linear sections are denoted L-, M-, H-, and S-regions in the order of the P/Po increase. The broad X-ray diffraction patterns and the high resolution electron microscopic observation show that ACF's consist of micrographites. The micropores of ACF'S may be assumed to stem from defaults of the graphitic layers upon activation; the
432
F i g . 1. E l e c t r o n m i c r o g r a p h s of ACF'S. (c): P I T
and ( d ) : PAN.
( a ) and (b) : CEL,
433
micropore size is approximated by integral multiples of the graphitic layer's thickness (0.34-0.35 nm). We can express the micropore size in terms of 0.3 0.05 0.004 P/P, the width of an adsorbed N2 molecule, since the width of an N2 molecule (0.34 nm) is nearly equal to the interlayer distance of the graphitic structure. The micropore(MP) analysis[25] for the t-plot indicated qualitatively the presence of the two-four N2 l"* ( P , / P )
X]g--fg
layer-sized micropores which come from defaults of two-four :$. graphitic layers; we can associate the micropore size estimated by the MP analysis w i t h each region of the (4 (b) detailed DR plot according to the multi-stage micropore filling(MSMF) mechanism. In A detailed DR plot and the MSMF mechanism[l51, N2 Fig. 2 molecules are filled in the schematic model for the multibilayer-sized micropores at stage micropore filling for N2 t h e L r e g i o n , t h e y a r e adsorption on ACF. (a) cooperative monolayerly adsorbed on the filling on the monolayerly covered micropore-walls at the M- supermicropores, (b) monolayer region, they are adsorbed in adsorption on supermicropores and t h e m o n o l a y e r covered(c)pore f i l l i n g i n the N2 micro por es a t the H- region bilayer-sized ultramicropores (this process corresponds to the cooperative micropore filling proposed by Sing et al[9-111, and then they are adsorbed on the external surface in the S-region, as illustrated in Fig. 2. The MSMF mechanism includes the primary and cooperative pore filling mechanism by Sing et a1[9-111 and each stage of the MSMF mechanism can be explicitly determined, while the two-stage mechanism by Sing et a1 does not show a clear boundary between two elementary processes. The analysis of the detailed DR plot can distinguish each elementary process in the micropore filling by different o E o value. Here B E o value is associated with the isosteric heat of adsorption, qst, B=l/e at the fractional filling of l/e, as expressed by eq. 2[25].
IF
(C)
-
.
qst,e=l/e
=
PEo
+ A% (&:
heat o f vaporization)
(2)
In the preceding study[l5], it was shown that the qst value corresponding to each elementary process agrees with the literature
434
value. In later section, it will be examined whether the M S M F mechanism can be applied to the Ar adsorption data. DA and DR plots for Ar adsorption isotherms Figure 3 shows the adsorption I isotherms of Ar. Here we use E the solid phase vapor pressure of 27.4 kPa at 77 K as the saturated vapor pressure of Ar. All isotherms are of Type I, 6 being almost identical to those of N2. The uptake of Ar at the low pressure region is more 400 gradual than that of N2, which should be attributed to the absence of the quadrupole in the A r - A C F s y s t e m . T h e s e Ar Y adsorption isotherms were a n a l y z e d by the D R plots. Figure 4 shows the DR plot of the Ar adsorption on CEL. The 0 DR plot is not linear but there 0 0.2 0.4 0.6 0.8 1.0 are three concave regions P/P, against the abscissa; the DR equation does not express the Ar adsorption. Ar does not form Fig. 3. Adsorption isotherms Of the liquid-like adsorbed layer Ar on ACF's at 77 K * upon adsorption of Ar at 77 K, but should form the solid-like phase on the surface. The DA e q u a t i o n of n = 3 may be applicable to such a 6.8 system[3,27]. Figure 5 shows the DA(n=3) plots for the Ar adsorption isotherms on ACF'S at 77 K. The DA plot has three sections as well as the DR plot for the N2 adsorption isotherm. It bends upwards at two points w i t h i n c r e a s i n g P / P o ; the inflection point is different from each other. The DA plots for PIT and PAN have steep rises Fig.4. DR plot f o r the just near the ordinate. We may designate these linear sections Ar adsorption isotherm on in the DA plots L-, M-, and HCEL. regions in the order of the P/Po
1
2
7
435
increase as well as the case of the DR plots for N2. Probably the steep r i s e near the ordinate in the DA plot for Ar corresponds to the S region(multi1ayer adsorption on the external surface) in 7 the DR plot for N2; it is 2 difficult to determine the 3 bending point of the S-region from the H-region in the Ar adsorption.
7
.
0
1
7
7.0
h
.
6.5
6.5
v
Comparison analyses
of and
Ar and
N2
6.0 6.0
5.5
I
I
I
0
I
50
100
150
5.5
ln3(p0/p)
multi-stage
micropore filling mechanism. Fig. 5. DA(n=3) plots for the T h e coordinates of the Ar adsorption isotherms inflection points of the DA plot for Ar and DR plot for on ACF'S. N2 and oEo values from the slopes are compared in Table 1. H e r e t h e a m o u n t o f adsorption,W, is more important than the P/Po value in the inflection point for comparison of the Ar and N2 a n a l y s e s ; t h e a m o u n t of adsorption W for Ar in mlg-' 5 % 4 is calculated with the liquid density value(l.40 gml-l) at ; 4.0 8 7 K. The W value of the 'a inflection point in the DA 3.5 plot for Ar agrees with that ; * in the DR plot for N2 within 3.0 20 % d e v i a t i o n . If we compare the W value o f the inflection point expressed in 0 50 100 150 the ratio against each W o value, the agreement is ln3(po/p) improved to 10 % deviation at maximum. Therefore, the Ar adsorption can be explained by Fig.6. DA(n=3) plot for the Ar the MSMF mechanism. The adsorption isotherm of adsorption isotherm on NPC. Ar on NPC is also expressed by the DA(n=3) equation, as shown
5
436
$4
$4
4 0
w rn Y
a
4
0
e
II
m
W
d a, 5 52
V Q N
$4
z 0 rn
rcl Y
a
0 4
a,
a
rcl
5 0
cn
c 0
M
a, LI
. o. m.
m r . m
m
. om.o m-.
m
0
0
0
0
-
0
N
o
0
u
a
a
0
0
0
m
-
”9‘s.
0
*.-lo
*
. . . O
0
m
. m. - .
a c u m
o
p
0
N
. .
0
-3
i t m
N
m i t
a
? ? ” m
0
l
u
. m. o.
u -
r
. .
m a r -
a
00 I n -
0
m . 3
0
i t 4 0 0
m
e u
rl
0
0
0
a
0
0
. .
m
“.?1 0
0
. . 0
a
e
0
o
0
0
. . .
m
4
0
437
in Fig. 6. The DA plot is composed of a long line and a steep rise, which corresponds to the monolayer adsorption and the multilayer adsorption, respectively. As the DA equation is based on the potential theory, the monolayer adsorption may be expressed by the DA equation even in the nonporous system[28]. The slope of this linear region gives .BEo of 5.4 kJmol-l. The BEo value for Ar on ACF in the same region may be compared with that on NPC. Table 1 lists also the BEo value for each linear sections. The B E o values of Ar on ACF'S for the L- and M-regions are greater than the BEo value for the monolayer formation on NPC by more than 1.8 kJmol-l and 0.1-0.6 kJmol-', respectively. These differences should be caused by the enhancement by the micropore field. Consequently, Ar is adsorbed by the micropore filling in the Ar bilayer-sized pores (L-region) at first and then Ar forms the monolayer on the pore-walls of the three and/or four Ar layer-sized pores (M-region), accompanying the cooperative filling of Ar in the H-region. Thus, analyses of adsorption data o f Ar without quadrupole support the MSMF mechanism. That is, the importance of the introduction of elementary processes into the micropore filling proposed by Sing et a1[9-111 has been shown and their two-stage mechanism has been extended to three-stage mechanism with the monolayer adsorption on the micropore walls by analyses of the d e t a i l e d N 2 a n d Ar a d s o r p t i o n i s o t h e r m s . A l t h o u g h the micrographitic structures and the micropores change with gas adsorption by in situ X-ray diffraction and in situ small angle Xray scattering[21,29], this study neglects such a dynamic effect of microporous carbons. In future, the dynamic micropore structure should be taken into account in the micropore filling mechanism and also calorimetric study will be needed.
ACKNOWLEDGEMENTS This work was partly supported by a Grant in Aid for Fundamental Scientific Research from the Ministry o f Education of Japan. Special thanks are due to Dr. T. Okada for observation of ACF samples with a high resolution transmission electron microscopy.
REFERENCES 1 2 3
4
K.S.W.Sing, Carbon, 27 (1989) 5. H.F. Stoeckli, Carbon, 28 (1990) 1. M.M. Dubinin, in P.L. Walker Jr. (Editor), Chemistry and Physics of Carbon, Marcel Dekker, New York (1966) pp.51. M.M.Dubinin & H.F. Stoeckli, J. Colloid Interface Sci., 75 (1980) 34.
438
5 6
H. Marsh, Carbon, 25 (1987) 49. K.J.Master & B. McEnaney, J . Colloid Interface Sci., 95 (1983) 340. 7 B. McEnaney, Carbon, 26 (1988) 267. 8 M.Jaroniec & J. Choma, in P.L. Walker Jr. (Editor), Chemistry and Physics of Carbon, Marcel Dekker, New York (1989) pp.197. 9 D. Atkinson, A.I. McLeod & K.S.W. Sing, J. Chim. Phys., 81(1984) 791. 10 D. Atkinson, P.J.Carrott, Y.Grillet, J. Rouquerol & K.S.W. Sing, in A.I. Liapis(Editor), Fundamentals of Adsorption, Engineering Foundation, New York (1987) pp.89. 11 P.J.Carrott & K.S.W.Sing, in K.K. Unger, J.Rouquero1, K.S.G. Sing & H . Kral (Editors), Characterization of Porous Solids, Elsevier,Ansterdam (1988) pp.77. 12 Y. Komatsubara, S.Ida, H. Fujitsu & I. Mochida, Fuel, 63 (1984) 1738. 13 J.J.Freeman, F.G.R.Gimblett, R.A.Roberts & K.S.W. Sing, Carbon, 25 (1987) 559. 14 K.Kaneko, N. Kosugi & H. Kuroda, J.Chem.Soc. Faraday Trans., 85 (1989) 869. 15 K.Kakei, S.Ozeki, T.Suzuki & K.Kaneko, J.Chem.Soc. Faraday Trans., 86 (1990) 371. 16 K.Kaneko, T. Suzuki & K.Kakei, Tanso, (1989) 288. 17 J. Rouquerol, S. Partyka & F. Rouquerol, J.Chem.Soc. Faraday Trans. I, 73 (1977) 306. 18 W.A. Steele, A.V.Vernov & D.J. Tildesley, Carbon, 25 (1987) 7. 19 K.Kaneko, T.Suzuki & K.Kakei, Langmuir, 5 (1989) 879. 20 D.A. Wickens, Carbon, 28 (1990) 97. 21 S. Hagiwara, K.Tsutsumi & H . Takahashi, Carbon, 16 (1978) 89. 22 T. Suzuki & K.Kaneko, Carbon, 26 (1988) 745. 23 R.W.Inness, J.R. Fryer & H.F. Stoeckli, Carbon, 28 (1989) 71. 24 M.Huttepain & A. Oberlin, Carbon, 28 (1990) 103. 25 R.S.Mikhai.1, S. Brunauer & E. E. Boder, J. Colloid Interface Sci., 26, 45 (1968). 26 K.Kawazoe, V.A. Astakhov & Y. Eguchi, Kagaku Kogaku, 35 (1971) 1006. 27 T.Kawai, Rep. of Eng. Inst. of Kanagawa Univ., No.1 (1971) 38. 28 S.J. Gregg & K.S.W.Sing, Adsorption, Surface Area and Porosity, Academic press, London (1982) pp.222. 29 K.Kaneko, Y. Fujiwara & K. Nishikawa, J.Colloid Interface Sci. 127 (1989) 298.
F. Rodriguez-Reinoso et al. (Editors), Characterization of Porous Solids II
439
0 1991 Elsevier Science Publishers B.V., Amsterdam
POROUS
N.T.
STRUCTURE
A.M.
KARTEL,
Division
of
Ukrainian
OF
SYNTHETIC
ACTIVE
and V.V.
STRELKO
WZY
sorption.
institute
of
Academy
of
Sciences.
CARBONS
General
Kiev
and
inorganic
Chemistry.
(USSR),
ABSTRACT Synthetic active carbons produced f r o m some types o f porous polymers or resins belong t o a new class o f carbon adsorbents which exhibit a unique combination o f physicochemical properties. Spherical-granule nitrogen-containing carbon SCN and pure carbon scs have been developed, which possess (according t o benzene vapour adsorption d a t a ) easily c o n t r o l l a b l e porous structure. The methods o f synthesis developed make it possible t o develop t h e micropore volume o f such carbons t o 0.6-0.9 cm3/g. The medium micropore-size can b e varied over a wide range f r o m narrow micropores nm). Adsorption-Weight analysis (halfwidth 0.3 nm) t o supermicropores (0.9-t.4 d a t a indicate a developed mesoporous s t r u c t u r e (volume 0.5-1.4 Cm3/g, specific mesopore surface area 100-500 m2/g). It has been shown by mercury porosimetry t h a t SCN and SCS carbons are characterized by a narrow mesopore volume distribution ( t h e main peak lies in t h e region 12-54 nm) and a small macropore volume development (0.1-0.2 cm3/g). The pecularities o f t h e porous s t r u c t u r e o f synthetic carbons are controlled by t h e prolonged and, in some cases, molecular-sieve nature o f their sorption f r o m solutions, which is o f prime importance f o r solving a number o f problems for s o r p t i o n technologies and medicine.
INTRODUCTION Synthetic porous
active
copolymers
subsequent
and
vapour-gas
properties pore
carbons
including
are
a
new
spherical activation.
increased
type
of
granulated The
carbon
resins
sorbents
mechanic81 s t r e n g t h
as
sorbent
by
prepared
their
DYrolysis
obtained well
have
as
a
from and
improved
highly
specific
structure.
The
structure-sorption
generally
determine
important carbons
its
characteristics and
SCN
VIZ
divinilbenzene
characteristics
of
practical
application,
of
porous
the
SCS
co-polymers,
any
adsorptive
material
will
this
paper
present
the
so
structure
from
obtained respectively
of
two
WIII
types
vinylpyridine
(ref.
and
of
synthetic
Styrene
with
t,?).
METHODS
For
the
degree o f studied were
by
present burn-off
study,
conventional
determined
by
SCN
and
SCS
have been selected. methods.
mercury
carbon
speciments
with
The porous structure o f
Macro-
porosimetry
and (Pore
mesopore Sizer
a
distribution
R9300,
progressive
t h e carbons was by
Micromeritics
radii Inc.,
The threshold radius was 3 nm (limiting pressure - 2500 kg/cmz). Apparent ( 8 ) and t r u e (d) densities were determined pycnometrically
USA).
mercury and benzene respectively. have
been
and water
obtained vapour
quartz
wlth
Specific
from
Data on t h e carbon micro-
adYorption-desorption
isotherm3
using
and mesostructures
of
benzene,
methanol
a t 20% using a t h e r m o s t a t t e d vacuum adsorption unit supplied
spring
balance.
Mesopore
volume
distributions
radii
by
surface values were computed f r o m t h e desorption isotherm
and
bT617ChW.
The microstructure o f t h e carbons was also determined by application o f microp@rous zone model developed the
sorption
isotherms
2,4-dibromphenol) evaluation
from
(ref.
of
by
Dubinin ( r e f .
ti.
low-soluble
water
SOIUtiOnS
organic
also
was
3).
the
method based on
A
substances
(pW6-Chlorantlrne,
used
a
for
microstructure
4).
RESULTS TYprC.31 porograms o f are shown in Figure volume
distribution
the
follows
As
1.
by
synthetic
radii,
the
carbons
from
the
obtained
mercury
by
porosimetry
respective d i f f e r e n t i a l
macropores (r>100 nm)
are
curves o f
actually
completely
absent in t h e SCN and SCS carbons and t h e Predominant mesoporous sizes are 35 and
nm,
12
respectively.
Location o f factors: content
of
obtained in
t h e most
degree
of
probable peak f o r
polymer
pore-former.
cross-linking
The
pore
volume
f r o m styrene-divinylbenzene
Figure
t h e mesopores depends on numerous
(divinylbenzene distribution
matrices
content), radii o f
by
defferent
Of
nature
types
the
isotherms
for
benzene,
methanol
and
water
vapour
t.he SCN and SCS carbons specimens w i t h d i f f e r e n t degrees o f burn-off Figure
The
3.
results
mesoporous
content
substantial
mesoporosity
6re
reacting those
are
considerably
pycnometric
at of
lower
calculate
the
the
structural
width
and
-
occurring.
than
total
the
their
(V,=1/6 -1/d). i.e.
adsorption variance
the
t h e theory
presented in
giving
information
on
its
the
(ED),
micropores
pores
adsorption
specific
adsorption
of
Table
halfwidth
In
Table
2
mesopores
obtained
from
isotherms,
one
of
as
of
well
as
micropores
the
distribution
micropore volume 1.
activation
of
surface
volume
Volume
are shown
(Vs=Vn,+Vnc), however,
Dubinin-Radushkevich
parameters constituting t h e s t r u c t u r e o f and
mesopore
limiting
the
of
on
insignificant
futher
volumes
values
Using benzene and
under
The
total
values
energy
of
constituting
obtained are
is
but
rel6tively
a
60% whereas
to
development
fundamental equations f o r values
demonstrate
micropores
parameters.
(X),
( W)
up
micropore volumes
characterictic
micropores
obt6ined
burn-offs
measurements
may (Wg),
carbons
are presented
2.
AdSorption-desorpt1on in
6nd
slit-like over
their
and
Dubinin-StoecKli
filling
(ref.
the
slrt-like
3).
The
micropore
t h e carbon adsorbent microporous zones
re-structure
during
progressive
activation
are
441
ddV zr
2
35 nm
1
Fig.
Pore
1.
r,nm
1000
100
s i z e d i ~ t r i b u t i o nf o r s y n t h e t i c carbons SCN ( 1 , 2 )
and scs ( 3 , 4 )
w ~ t hburn-offs: 40% 11.3) and 75% 12.4).
56 nm
10 F i g . 2.
100 I 0
Pore s i z e d i s t r i b u t i o n f o r
100
100
I0
synthetic
carbons
100 r,nm
10
prepared
s t y r e n e - d i v i n y l b e n z e n e copolymers w l t h v a r i o u s c r o s s - i inkage c o n t e n t : ( a ) 10/80%; ( 5 ) 30/130%; ( c ) 30/160%; (d) 40/180%.
from
and
p o r e -former
442 Q.,
C
40%
SCN: 20
r
L
1c
0.5
0.5
I
0.5
SCS: 40%
I(
0.5 Fig.
3.
0.5
0.5
A d s o r p t i o n i s o t h e r m o f benzene ( a ) , methanol (b) and water vapour
on s y n t h e t i c a c t i v e carbons SCN and SCS w i t h v a r i o u s b u r n - o f f s .
(c)
443 TABLE 1 Porous s t r u c t u r e o f s y n t h e t i c a c t i v e carbons ~~
Parameter
Burn-off,
X
SCN-1M
SCN-2H
SCN-1K
SCS-1
SCS-2
SCS-3
40
60
75
40
60
75
Pore vo~ume, c d / g
- total
0.75
0.93
1.59
0.53
0.86
1-15
- i n t r u d e d by Hg
0.41
0.46
0.95
0.34
0.52
0.71
- macropore
0.02
0.03
0.09
0.08
0.11
0.12
- s o r p t i o n (by benzene)
0.45
0.65
0.97
0.44
0.10
0.93
- micropore
0.36
0.48
0.61
0.19
0.34
0.44
t o t a l (by Ar)
420
560
1320
540
880
1380
mesopore (by benzene)
36
74
199
66
88
200
0.485
S p e c i f i c s u r f a c e area,
-
d/g
D u b i n i n - S t o e c K I i e q u a t i o n parameters
- w,,
cd/g
0.375
0.520
0.650
0.189
0.356
-
KJ/mole
27.3
18.5
14.9
33.1
18.5
14.2
0.418
0.681
0.859
0.298
0.559
0.706
Eg,
- m i c r o p o r e h a l f w i d t h , nm
TABLE 2 Structural
parameters
o f S y n t h e t i c a c t i v e carbons a c c o r d i n g t o
m i c r o p o r e zone (MZ)model
Parameter
SCN-1M
SCN-2M
T o t a l MZ volume, cn?/g
0.860
MZ f a c e s i z e , nm
96
Mesopore h a l f w i d t h , nm
SCN-1K
SCS-1
SCS-2
SCS-3
0.978
1.090
0.766
0.866
0.963
53
22
47
39
10
2.2
1.8
2.1
3.4
3.1
2.8
T o t a l MZ q u a n t i t y , Nw10-'6
0.10
0.66
10.4
0.76
1.4
13.5
l 5 0 l a t e d m i c r o p o r e volume, n d
1.24
4.15
8.0
0.64
2.46
4.58
S i z e o f c r y s t a l l i t e face, nm
1.61
2.33
2.81
1.48
2.07
2.45
Crystallite Quantity i n i s o l a t e d MZ
171070
8920
320
31500
5980
385
Micropore Q u a n t i t y in i s o l a t e d MZ
294440
17530
730
38960
9740
710
Geometrical s u r f a c e area o f micropore, d/g
861
705
710
634
637
623
444 shown.
Isotherms o f acetylene black in
Figure
tsalswbed ?TWn Waxer
p-&XlltKMki%ng
5CM EWkWS
b+'
$OWt?QflS
t h e AeltbOll - Abhck coordinates are presented
plotted f o r
4.
DISC 11S S l O N According
to
mercury
mesoporous b u t sorbents. by
These
conflicting
independent
methods,
macropores
that
volume o f
the
classified
be
higher The
above
as
t h e radius o f polymer
are
that
a
evaluating by
volume,
structure
since
is
described
cOpOlymW
mesoporous
the
porous
specific
structure Shape
of
Then, t h e whole
their
filling
up
would
occur
at
typical
investigations we
content
benzene
t h e most
have
copolymers
(divinylbenzene (alkyl
of
characteristic
the
a method o f
typical
the
synthetic
the
most
resins, Locat.ion o f
CWbOtiS
probable size
channels would be determined by t h e properties o f
a
divinylbenzene
%3ttlfiy o f
as
characterized
they are macroporous
macropores by t h e mercury porosimetry method would
Thus
gore-formers
by
interpreted
be
cross-linking
and
be
and
and
demonstrated
obtained
matrix
type.
carbons
5).
macroporous
the
carbone
should
mesopores
(ref.
for
Styrene
results,
"latent"
the
mesopore from
synthetic
isotherms would Suggest
could be reached via t h e more narrow mesopores.
pressures
obtained
porosimetry,
benzene vapour
of
of
been
for
the
conducting With
dif fering
by
5-60%),
amount
their
paraffin,
60-160%,
varying b o t h t h e
value
and
process
definitive commercial
degree
of
nature
of
oils,
etc.)
t o t a l and sorption volumes o f
Drobable radius f o r
pores
t h e t r a n s p o r t pores f r o m 3 up t o
nm as shown in Figure 2.
210
CharacteriStiCS
of
the
synthetic
progressive increase in macroincreases. Water well
Shifts
Vapour 85
When
the
position
associated with
the
SCN
desorption
linear
the
of
water
carbons
chemically-bound
t h e degree o f conforms burn-out
to of
Intercrystallite burning their
widening o f
burn-off a
micropores share
of
section
at
vapour
the
low
are
inflection
burn-off of
relative
pressure
of
benzene
adsorption P/Po
the
nitrogen atoms the
radii.
present
micropore
Crystallite
resulting
in
contributing
is approaching
1,
but
an more
the
one
values.
is above 4 0 % and, If s t i l l hlgher carbon
degree o f
branch
a
demonstrate
1
the
in
the
the as and
is
note
the
probably
sorbent.
model (Table
2)
the crystallites when
a half-width o f a slit
enlargement
improved
may
arrangement
significantly,
volume,
may
This
zone
t h e s l i t s and enlargement o f
supermicropore
amorphous
values
isotherms
Analysing t h e carbon characteristics f r o m one may n o t e t h e
Table
also confirmation o f t h e above f a c t .
isotherms
for
convexity
the
towards
initial
adsorption are
studying t h e
of
isotherm
in an
decrease
8
methanol v m o u r
isotherm
in
adsorption
in
carbons
and mesopore volumes as
however,
viz
at
indicate of
the
60-75%
makes up only
a
445
0.4
0.2 /
/ I
/
/ I
I I
I
I I
Fig.
4.
SYnthetlc
AdsOrptlOn actlve
lsotherm
carbons
of wlth
SCN
p-chloranlllne varlous
from
aqueous
burn-offs
and
actlve
carbons
solutlon
carbon
black.
TABLE 3 Mlcropore by
structure
adsorption
of
parameters dlssolved
Parameter
synthetic
2,4-d1bromphenol
Symbol
SCN-1M
SCN-2M
SCN-1K
SCN-2K
Vtn;
0.27
0.32
0.31
0.26
V,
0,27
0.43
0.53
0.4b
v,,~
0.09
0.16
0.30
0.36
superml c r o p o r e
Vsml.1
0
0.11
0.22
0.20
mnolayer f I I led supermlcropore
V s m ~ 2 0.09
0.05
0.06
0.la
120
340
1160
Pore volume,
cm3/g
- micropore - volume f l i i e d m l c r o p o r e - t o t a 1 superml c r o p o r e - volume f i l l e d
-
of
parachloran~lme and
S D e c if ic surf ace area o f monolayer f I I l e d pore, mz/g
s1
55
on
446 half
the
total
Vaiuco,
of
conflicting
processes,
shrinkage within
micropores micropore
and
the
their
distributions adSOrptiOn their
adsorption
on
are filled
of
by
the
VOlUmeS of
determined
by
the
is
the
to
a
due
black.
microporous
true
micro-
While from
According
volume
mechanism
monolayer
to
to
structure
area
solutions
the
alters
supermicropore
in
of
studies
wider
the
comparison (ref.
micropores and a
and
the
Therefore
isotherms
earlier
true
a
mass.
and
the
of
4,6)
part
of
supermicropores
formation.
t h e Pores being filled as
effect
surface
studying
up t h e
the
equation
micropores
are filling
up by t h e mechanism o f
Specific
sum
geometric
substances
carbon
a
loss in carbon
a
detect
organic
express
the
of
the
to
substances
supermicropores
of
because
possible
low-Soluble
organic
surfaces
narrowing
Pecularities were
of
low-soluble narrow
i.e.
extension
40-75% burning range
insignificantly.
with
volume.
geometric
up by a volume
mechanism may be
follows:
8
a,
where
conforming pores which Avogadro When
a
sorption
may
volumes
where
of
whose
and
the
under
two
precipitation
substances
molecules
micropores
are
with Diane,
oriented
a
following
may
calculated
the
monolayer
*- s o r p t i v e e.g.
a
concentration
of
J
over
in t h e
be
surface
determined
IS
balanced
specific
dissolved substance
be calculated
of
value
soiution,S,is
saturated
number, W -monolayer
value
The
adsorption
are adsorbing a
2,4-d1bromphenol
sq
limiting
sorption
molar
volume.
para-chloraniline
surface
N-
formation,
and
nonuniformly
the
way:
the
by
reduction
method,
e.g.:
(rcb -values o f substances adsorbed a t t h e carbon and t h e black a,,#= vmL - volume o f the true micropores; s,b -
a t similar concentrations; specific
surface
of
between monolayer
vs,,,il
carbon
black.
S, , VmL and Vsmi values
The computed
and
(being filled up in a volume way) and
manner) are represented
reaching values comparable with
in Table
Vmi,
3.
the
studies on SCN and
distribution
vsmiz
It follows
; the
supermicropores a t 60-75% burn-off makes up over 2/3 Results o f
the
proportion
of
the
(filling
up
latter in
a
f r o m t h e data obtained of
the
volume-filling
Vsmi
scs synthetic carbons porous structure by
447 independent methods a r e r e p o r t e d i n our Papers ( r e f . 1, 2 , 7) total
data
allows
the
formation
of
in
detail.
The
t h e p o r e s o f d i f f e r e n t t y p e s and t h e i r
S u r f a c e deVelOPment, dependent on a c t i v a t i o n degree t o be o b s e r v e d
and
are
a
b a s i s t o conclude t h e f o l l o w i n g :
1.
SCN
and
Synthetic
SCS
a r e distinctive f o r t h e c o n s i d e r d b l e
carbons
development o f s u p e r m i c r o - and mesopores. T h e i r volume c o u l d be even
higher
carbons.
than
the
SorDtion
volume
o f the maJority o f comercia1 active
The g i v e n s o r b e n t s possess " l a t e n t "
mesoporous necessary,
comp~lrable or
macropores
accessible
via
the
whose e f f e c t i v e r a d i a a r e 35 nm (SCN) and 12 nm (SCS). i f
channels
t h e s i z e c o u l d be v a r i e d f r o m 3
up
to
210
nm
depending
on
the
i n i t i a l polymer s o u r c e s . 2.
Supermicropores
ones t h a t
filling
supermicropores
up
whose
For o r g a n i c substances, capacity
not
oniy
for
of by
the the
f i 1 1 ing
above s o r b e n t s a r e s u b d i v i d e d i n t o t h e n a r r o w volume
filling
mechanism,
and
t h e s y n t h e t i c carbons t h e r e f o r e have
a
biological
Widened
high
sorpt!on
low b u t a l s o f o r t h e m i d d l e and h i g h m i e c u i a r w e i g h t
m a t e r i a l s w h l c n 1 5 o f g r e a t s i g n i f i c a n c e f o r p u r i f y i n g a number specifically
the
i s p r imar i l y o c c u r r i n g by monoiayer f o r m a t i o n .
iiquids.
The
of
solutions,
w e i i developed s t r u c t u r e o f t h e l a r g e
s u p e r m i c r o - and mesopores Should l e a d t o h i g h k i n e t i c r a t e s o f s o r p t i o n on
s yn t h e t i c car bons
the
.
RE F E RE NCE S 1 V . V . S t r e l k o , T.G. Plachenov, N.T. K a r t e l e t a l . , P e c u l i a r i t i e s o f 5trUCtUt-e o f s p h e r i c g r a n u l e n i t r o g e n - c o n t a i n i n g s y n t h e t i c carbons p r e p a r e d f r o m r e s i n s , i n : Carbon a d s o r b e n t and t h e i r i n d u s t r i a l a p p l i c a t i o n s (Russ.), Nauka, Moscow, 1983, pp. 172-185. 2 V . V . S t r e l k o , Y . F , K o r o v i n , N.T. K a r t e l and A . M . P U Z Y , S t r u c t u r e - s o r p t i o n c h a r a c t e r i s t i c s o f a new s y n t h e t i c CarbOIlS SCS t y p e , U k r a i n i a n Chem. J. (Russ.), 50 (11) (1984) 1157-1162. 3 M.M. Dubinin, Micropore s t r u c t u r e o f carbon adsorbents. Report 1 . C 6 m n c h a r a c t e r i s t i c o f m i c r o - and s u p e r m i c r o p o r e s f o r s l i t - l i K e mOdel, P r o c e e d i n g o f USSR Acad. SCI., ser. chem. ( R u s s . ) , 8 (1979) 1691-1696. 4 A . M . Koganovsky, T . M . Levchenko, V . A . ~ iichenko, r ~ d s o r p t i o n o f resolved substances ( R u s s . ) , Naukova Dumka, K l e v , 1977. 5 0. K a d l e c , A . Varhanikova, A . Z u k a l , S t r u c t u r e o f p o r e s o f a c t i v e carbons p r e p a r e d by water-vapour and z i n c - d i c h l o r i d e a c t i v a t i o n , Carbon, 6 (4) (1970) 321-331. A . V . Mamchenko and A . M . K6qanOVSkY, A d s o r p t i o n o f r ? w l v e d 6 T. I . Yakimlva, substances i n super- and n a r r o w m i c r o p o r e s o f a c t i v e cartjons, J . Phys. Chem. (RUSS.), 5 4 (3) (1980) 741-743. 7 S . L . Medvedev, A . V . Mamchenko, N.T. K a r t e l and T . I . Yakimova, E s t i m a t i o n o f m i c r o p o r e s t r u c t u r e o f a c t i v e c a r b o n SCN t y p e aCC6rding w i t h a d s o r p t i o n o f r e s o l v e d substances d a t a , U k r a i n i a n Chem. J . ( R u s s . ) , 53 (6) (1967) 581-584.
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F. Rodriguez-Reinosoet al. (Editors),Characterization of Porous Solids IZ 0 1991 Elsevier Science Publishers B.V.,Amsterdam
EVALUATION OF MICROPOROSITY IN ACTIVATED CARBONS WITH HIGH ASH (Cr203)CONTENT M.A. Martinez-SAnchez', J.M. Martin-Martinez', A.C. Orgiles- Barcelo', F. Rodriguez-Keinoso2 and M.J. Selles-Perez2. 'INESCOP. Asociacion de Investigacion de las lndustrias del Calzado y Conexas. Elda. Alicante. Spain. 'Departamento de Quimica InorgAnica e Ingenieria Quimica. Universidad d e Alicmte. Alicante. Spain.
INTRODUCTION Activated carbons are adsorbents with a wide pore size distribution and consequently the precise determination of their porous structure is a rather difficult task. Since activated carbons are essentially microporous, most work devoted to their characterization is centred around the determination of t h e microporosity. Several methods have been extensively used to analyze the adsorption isotherms of nitrogen and other adsorptives. The micropore volume filling theory of Dubinin has been successfully used but there are well-known problems when the micropore size distribution is heterogeneous (refs. 1,2). The n-nonane preadsorption technique has also been used in the last few years but it provides information only on narrow microporosity and t h e results are conditioned by interconnectivity network of t h e porosity (refs. 3,4). The t- and &-plot methods have also been widely applied using
different non-porous reference materials, the selection of which may be critical (refs. 5,6). The main objetive of the work described here is to evaluate the microporosity in
;I
series of activated carbons with increasing burn-off and
ash (Cr203) content using the isotherms on non-porous carbon and Cr'O, samples ;IS references in the a-plot method and the comparison with results obtained from the Dubinin theory and n-nonane preadsorption.
44Y
450
EXPERIMENTAL
Series P of activated carbons was prepared by carbonization in N, ( 1 123K) of chromium-tanned leather waste followed by activation in CO,
(1098K) for different periods of time to cover the 7-70% burn-off range (burn-off is included in the nomenclature of the samples). Adsorption of N,
(77K), CO, (27310 and n-butane (273K) and preadsorption of n-nonane were determined in conventional gravimetric systems in order to characterize the carbons.
The non-porous reference carbon was prepared by heat treating in Ar (2100K, 30 min.) an activated carbon prepared from olive stones; BET
surface area was 7.3 m2/g and the value for the C constant was 158. The N, adsorption isotherm for this carbon is shown in Fig. l(a). ‘The isotherm is in excellent agreement with the one proposed by Rodriguez-Keinoso et al
(ref. 7) and in good concordance for the relative pressure range 0.1-0.8 with
one published by Sing et al (ref. 8). Three different types of non-porous reference material for Cr,O, were selected: two from the literature, B2(280)110 and B3(880)2 (ref. 9) and the ashes obtained from sample P-70 (more of 90% of which is Cr,O,). The N, adsorption isotherms for the three samples are included in Fig. l(b), together with some relevant data. There is an acceptable agreement only in isotherms for t h e ashes and B3(880)2), specially up to a relative pressure of
0.8. RESULTS AND DISCUSSION
The N2 adsorption isotherms for carbons of series P given in Fig. 2 show the development of micro- and mesoporosity from P-7 to P-58 and a
decrease in adsorptive capacity thereafter. Table 1 shows that there is an important increase in ash content with increasing burn-off so that only a 31%
of
sample
P-70
is
carbon.
The
application
of
the
Dubinin-Radushkevich (DK) equation to the adsorption data of the N,
(77K), CO, (27310 and n-C,H,, (27310 for all carbons leads to the micropore volume (V,) values listed in Table 1. There is an increase in V,
451
up to 58% burn-off decreasing thereafter. On the other hand, increasing burn-off modifies the pore structure of carbons; thus, carbon P-7 has a narrow and uniform microporosity since V, (N,)
=
V, (CO,) and such a
Fig. 1. N, (77K) adsorption isotherms on a) Carbon Ap; b) different Cr,O, samples.
hD
10
Z
8
\ 4
E
v
F I 4
0
0
0.4
0.8
Fig. 2. N, (77K) adsorption isotherms of series P. b)P7 (X)P25 @P39 @P58 (o)P70.
(0)P64
452
porosity is not enterely accessible to n-butane. Increasing burn-off produces a widening of t h e microporosity - V, (n-butane)
- V,
(N2) > V, (CO,) - the
limiting case being P-64 since the difference decreases for P-70. TABLE 1
Micropore volumes (V,,, cm3/g) from DK equation CARBON
ash (%)
N,(77K)
C0,(273K)
n-C4H,,,(273K)
0.24 0.27 0.27 0.23 0.20 0.14
0.25
P7 P25 P39 P58 P64 P70
I
0.35 0.33 0.22
0.21 0.31 0.34 0.37 0.33 0.2s
Fig. 3 includes the a-plots for the adsorption of N, (77K) using the Ap non-porous carbon as reference material. The shapes of the plots indicate the widening of microporosity with burn-off and t h e plots for carbons with medium burn-off exhibit a clear deviation at large values of
the meaning
a 14 12 10 h
3
8
i f 5
v
c 4 2
u
I I
I
I! 1/
k 2 3
Fig. 3. N, (77K) a plots for carbons of series P. [Reference: Carbon Ap.] (O)P7 @P25 @)P39 @PSS @P64 (.)P70.
453
of which (i.e, the meaning of the pore volume deduced from its extrapolation) is not well established. The V, values (Table 2) deduced by extrapolation of t h e straight portion of t h e plots to
a =
0, follows the same
evolution deduced from the DR equation but they are slightly lower (up to 10%) in carbons of medium burn-off. The external surface areas deduced
from the corresponding slopes increases with burn- off up to P-58 remaining almost constant thereafter. Since the ash content of t h e samples increases
with burn-off (see Table 1 ) one could question the validity of using a reference material entirely made of carbon as stated in t h e IUPAC recornendations (ref. 10). TABLE 2 CY
method applied to carbons of series P
non-porous Ap I
non-porous B2(280)110
CARBON
V,,(cm”g)
0.25 0.28 0.32 P5 8 P64 P70
0.32 0.30 0.21
S,(rn2/g)
V,,(cm3/g)
4 71 91 I42 I44 134
S,(m2/g)
0.25
3
0.32
80
0.22
108
The reduced isotherms of the different reference materials (Fig. l(b)) show that carbon Ap is relatively coincident with the other three materials only up to a relative pressure of 0.4; sample B3(880)2 and the ashes are
rather coincident up to a relative pressure of 0.75 and both differ from the isotherm for B2(280)110. Fig. 4 includes the
CY
plots for carbons P-7, P-39
and P-70, using the three Cr,O, reference materials. For carbon P-7 the a-plots are similar and lead to the same value of V, (0.25 cm3/g) and S, (2-3 m2/g). The a plots for samples P-39 and P-70 are rather curved, especially
if t h e ashes o r B3(880)2 are used as reference material, t h u s making difficult the evaluation of V, and S,. One could expect the ashes to be the
454
2L OO
2
1
3
0
1
2
3
a
U
12
10 h
M
2 % &
-
E C
6 ' a A
4
2
0
2
1
3
U
Fig. 4. N, (77K) cx plots for carbons of series P. a) Reference: Cr,O, (ashes). b) Reference: B2(280)110. c) Reference: B3(880)2. @)P7 0) P39 QP70.
455
most adequate material if the chemical nature of the samples were the main factor in using the a-plot method (ref. 10) but the results of Fig. 4 show that this is not the case for the test samples (series P) used in this work. This means that the similarity in chemical nature of reference and test samples
is not the only factor to be considered. On the other hand, the reference
B2(280)110 seems to define less curved a-plots although there are two possible straight portions that can be drawn as in the a-plots of Fig. 3. The results given in Table 2 indicate that t h e results are, surprisingly, in very good agreement with those obtained using the carbon Ap as reference material. Preadsorption of n-nonane may help to evaluate t h e applicability of t h e a-plot method. Table 3 includes the data for samples P-39, P-58 and P-70, selected because their wide micropore size distribution. The V,' values (given by the difference at P/P,=0.80 between the isotherms without and with n-nonane adsorbed) are very similar to those given in Table 2 for CO, (27310, these values giving then the volume of narrow micropores. T h e relatively lower values of V, (volume of n-nonane retained by the samples) for P39 and P53 are indicative of porosity interconectivity typical of carbons with medium burn-off as shown elsewhere (ref. 1 I). The application of the a-plot method to the N, (77K) isotherms after n-nonane preadsorption - only Ap and B2(289)110 reference materials give non-curved plots - is shown in Fig. 5 and the corresponding values of V, and S, are included in Table 3. The plots are almost parallel to these for the
original carbons (Fig. 3 ) , indicating that the n-nonane preadsorption only affects to t h e narrow microporosity. Again, it is surprising that t h e two reference materials lead to similar values of micropore volume outside t h e narrow microporosity. The results of Table 3 clearly differentiate the
narrow (V,') and wide (V,") microporosity of the carbons. It is importan to note the similarity of V,'and V, (CO,) confirming the validity of adsorption of CO, at 273K to evaluate the narrow microporosity of activated carbons
(ref. 11). Series P, however, show a general behaviour which is not exactly coincident with other series of carbons previously studied (refs. 12-14) in the
456
TABLE 3 Results from n-nonane peadsorption. Volumes, (cm'/g); Surfaces, (m2/g) non-porous Ap 'ARBON
P39 P58 P70
0.24
0.29
0.23 0.13
0.25
0.03 0.08
0.14
0.08
a
85 120 109
1
non-porous B2(280)110
0.04 0.09 0.09
64 98 91
a
Fig. 5. N, (77K) a plots of some carbons of series P with preadsorbed nonane. a) Reference: Ap. b) Reference: B2(280)110. @)Pp39 QP58 QP70.
457
sense that the -method
usually yields somewhat larger values of micropore
volume than the DR equation for carbons with medium - to - high burn-off;
the results given here show a 10% larger values for t h e DR equation. Whether this is due to the uncertainty in the selection of the reference material is still a problem to be solved. I t is clear however that these results for series P show that the role of the reference material in the a-method is not as clear iis expected. Further work on samples of mixed chemical nature is needed.
REFERENCES 1
2 3
4
5 6 7 8 9 10
II 12 13
14
F. Rodriguez-Reinoso and A. I-inares-Solano. "Chemistry and Physic of Carbon". 1 (1989). Marcel Dekker. New York. J. Garrido, A. Linares-Solano, J.M. Martin-Martinez, M. Molina-Sabio, F. Rodriguez-Reinoso and K. Torregrosa; Langmuir 3, 76 (1987). A. Linares-Solano, J.D. Lopez-Gonzblez, J.M. Martin-Martinez, and F. Rodriguez-Reinoso; Ads. Sci. 'I'echnol. 1,123 (1984). F. Rodriguez-Reinoso, J.M. Martin-Martinez, M. Molina-Sabio, R. Torregrosa and J. Garrido; J . Colloid. Interf. Sci. 106,305 (19%). (1. Pierce; J. Phys. Chem. 72, 3673 (1968). S.J. Gregg and K.W.S. Sing. "Adsorption, Surface Area and Porosity". 2nd ed. Academic Press. London (1982). F. Rodriguez-Reinoso, J.M. Martin-Martinez, C. Prado- Burguete and B. Mc Enaney; J. Phys. Chem. 91,515 (1987). P.J.M. Carrott, R.A. Roberts and K.S.W. Sing; Carbon 25, 769 (1987). F.S. Baker, J.D. Carruthers, R.E. Day, K.S.W. Sing and L.J. Stryker. Disscusion Faraday Society 52, 173 (1971). Reporting Physisorption Data for Gas/Solid Systems. Pure Appl. Chem. 57, 603 (1985). F. Rodriguez-Reinoso. Pure Appl. Chem. 61,1859 (1989). J. Garrido, A. Linares-Solano, J.M. Martin-Martinez, M . Molina-Sabio, F. Rodriguez-Reinoso and K. Torregrosa; J. Chem. Soc. Faraday 108 (1987). Transactions I, €3, P. Gonzlilez-Vilchez, A. L,inares-Solano, J.D. Ldpez-Gonzlilez and F. Rodriguez-Reinoso; Carbon l7,44 (1979). J. Garrido, J.M. Martin-Martinez, M. Molina-Sabio, F. Rodriguez-Reinoso and R. Torregrosa; Carbon 24, 469 (1986).
a,
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F. Rodriguez-Reinosoet al. (Editors), Characterization of Porow Solids ZI 1991 Elsevier Science Publishers B.V.. Amsterdam
459
INFLUENCE OF COAL OXIDATION ON COKE POROSITY
J.J. Pis, R. Menendez, J.J. Lorenzana, A.J. Perez, H. Marsh' and E. Romerol lnstituto Nacional del Carbbn, CSIC, Aptdo 73, Oviedo 33080, Spain. Northern Carbon Research Laboratories, Dept. of Chemistry, University of Newcastle upon Tyne, Newcastle upon Tyne, NEI 7RU, U.K.
SUMMARY This paper studies the effects of low temperature pre-oxidation on cokes from three bituminous coals of different, rank. The development of porosity was quantified by microscopy analysis and mercury porosimetry and results compared and evaluated in terms of coal rheological behaviour during heating and the mechanical properties of resultant cokes.
INTRODUCTION Several studies, using different approaches (refs. 1-7) are reported of coal oxidation. As yet, no unified approach exists to study coal oxidation and its effects on subsequent coal processing. The mechanism of oxidation is complex and appears to differ for temperatures below and above 70°/800C (refs. 8-11). Martin (ref. 10) suggested the formation of peroxides at the lower temperatures, whereas at the higher temperatures the initial formation of peroxides is followed by their decomposition and subsequent formation of carboxylic acids. The majority of studies of coal oxidation relate to coal carbonization. Coal oxidation is known to be detrimental to the coking properties of coals (ref. 12). The introduction of oxygen-functional groups into coal produces loss of mobile hydrogen and formation of cross-linkages within the coal, either during oxidation or during the pyrolysis of coal. These changes are responsible for loss of fluidity and consequently for changes in resultant coke structure and properties. Crelling (ref. 13) quantified effects of additions of weathered coals during co-carbonizations with fresh coals. A decrease in coke stability and an increase in coke reactivity and breeze content were observed as the proportion of weathered coal in the blend was increased. An increase of reactivity and a decrease of strength was also observed by Pis U .(ref. 14) in cokes from low temperature oxidised coals. Pre-
u.
u.
460
oxidation inhibits the development of anisotropy via a fluid phase, so increasing amounts of isotropic carbon (coke) (ref. 15, 16) or decreasing the size of the optical texture of the coke (ref. 17). Metallurgical coke strength is mainly controlled by the porosity contained within its structure (ref. 18). Decreases in coke strength caused by coal oxidation could be due to modifications of the porosity of coke. Optical microscopy linked to an image analysis system describes macroporosity in cokes (ref. 19). A limitation of the image analysis method is that the information obtained usually relates to a twodimensional image of the object. However, with controlled progressive polishing to remove known depths of the specimen, image analysis can give a three-dimensional assessment of porosity (20). Mercury porosimetry is a convenient method to characterise macropores in cokes, covering a wide range of mesoporosity and approaching the microporosity, 7.5 pm to 3.75 nm (at a pressure of 200 MPa). There are, however, several serious limitations (ref. 21), to mercury porosimetry. There is mercury contamination, advancing or retreating of mercury over the solid surface, and a measurement of pore entrance radii which may be smaller than the main body of the pore. This work describes the development of porosity in metallurgical cokes obtained from three series of coals with different extents of preoxidation. Mercury porosimetry and an optical microscope allied to an image analyser were used to measure porosity of cokes and results are compared. EXPERIMENTAL Three bituminous coals, of decreasing rank, with volatile matter content 17.8 to 32.2 wt.% were used. The most important characteristics of the parent coals are given in Table 1 and indicate that the Turon coal has the maximum fluidity. The three coals were ground to c 1 mm. The ground coals were placed in trays and oxidised in an oven, in air, at 140°C up to 24 h (Gregory 8 h). To carbonize the coals, 400 g of fresh and oxidised coal was placed in stainless steel cylinders (1 1.5 cm high, 9 cm internal diameter) within an electrical furnace and heated at 5 K min-l to a final heat treatment temperature of 1000°C. Strength and reactivity data of resultant cokes are published (ref. 14).
461
TABLE 1 Petrographic and chemical analysis of coals used Coals Chemical analvsis. wt.%. fdry) Volatile matter Ash Carbon Hydrogen Sulphur (N+O) (diff.) Plastic D roDert ies Arnu dilatation (Yo) Gieseler fluidity (ddpm) Petroaraohic Analvsis. % vol Vitrinite Exinite Semi-fusinite Fusinite
Alpheus
17.8 7.1 84.3 4.2 0.7 3.7 81 71 86.0 0.0 5.7 8.3
Turon
Gregory
26.7 9.5 79.4 4.8 0.9 5.4
32.2 8.2 77.3 5.0 0.6 8.9
161 2754 89.2 2.7 1.9 6.2
62 178 76.3
5.0 5.9 12.8
For image analysis, cokes from the three coals each with different extents of oxidation were mounted in blocks, polished and surfaces examined using a Vickers M41 microscope. The extents of porosity were determined by using an Optomax V image-analysis (I.A.) system. Using the "feature-analysis'' method the computer software of the I.A. system recognised differences in grey levels of a screen-image of porosity of the specimen. The computer is programmed to give several porosity features, such as total porosity, size distributions and shape, the number of pores examined, their mean area, perimeter, diameter, shape (form factor) and percentage of porosity, as a percentage of the total area, were obtained. Approximately 20 fields in each of the two blocks were examined providing a data base from about 40 fields of view. Resolution is limited to about 5 pm diameter. Coke porosity was also studied by mercury porosimetry; true For the (helium) and apparent (mercury) densities were measured. determination of the helium density a Micromeritics Autopicnometer 1320 was used. Apparent density to mercury was determined in a Carlo Erba Macropores Unit 120.
462
RESULTS AND DISCUSSION Figure 1 shows .the variation of coke porosity obtained by microscopic image analysis with oxidation time for the three series of cokes.
60
-
. 50 -
~I’-o
i$
Alpheus
.= 40%
v)
20
a
v /*
y
30 -
I ! 0
Turon
1
I
I
5
10
15
Oxidation time, h
Fig. 1. Variation of coke porosity determined by image analysis with oxidation time of parent coal. Cokes from Alpheus and Turon, of highest rank and highest vitrinite content, do not show a significant change; there is a slight decrease in porosity (38 to 32%) in the intermediate stages of oxidation (between 1 and 9 h) for Turon cokes, in agreement with results from mercury porosimetry (Figure 5). Cokes from Gregory coal, of lowest rank and minimum vitrinite content give a pronounced increase in porosity, 41% (fresh coal) to 63% (8 h oxidised coal). Figure 2 shows the variation of the mean perimeter of pores in the cokes with pre-oxidation time. Cokes from Alpheus coal develop a slightly smaller sized porosity. The size of the pores in Turon cokes does not change appreciably. For Gregory cokes the mean perimeter of pores increases from 540 pm (fresh coal) up to 660 pm (6 h oxidation).
463
TABLE 2 Abrasion indices of the cokes from oxidised coals (ref. 14). time oxidation (h) 0 1 2 3 6 8 12 18 24
.
Alpheus 5.8 5.8
Turon 5.3 6.0 5.9 6.1 6.4 7.1 12.3 42.3 70.3
6.1 7.0 7.5 9.9 28.7
/
700-
Gregory 6.7 6.3 6.7 6.6 26.4 38.6
Gregory
L
0) L
0)
E 'i 60 0 W
Q
A
c
0 W
4/
/ A
0
500-
0 Turdn
o - - * I
Alpheus 1
1
1
Fig. 2 Variation of pore size with oxidation time of parent coal from image analysis. A coke quality criterion is strength. Comparison of porosity determined by image analysis (Figure 1) with the coke strength (Table 2) shows increasing an abrasion index (decreasing strength) with increasing percentage porosity and pore size. Alpheus and Turon are the more resistant to oxidation; after 10 h strength has not significantly changed, being coincident with the evolution of total porosity (I.A.). Gregory cokes
464
undergo a dramatic increase in abrasion index at the same point.
This is in
agreement with the concept that large pores mainly control coke strength (ref. 19). Variation of porosity, studied by mercury porosimetry, of the three series of cokes, is shown in Figure 3. An increase in coke porosity is observed in the cokes from oxidised samples of Alpheus, Tur6n and Gregory coals, the largest being for Alpheus and Gregory coals. For Gregory coal the increase in porosity is very significant. In fact, for Turon coal a decrease in porosity is observed in the first stages of coal preoxidation, and after this a slight enhancement in porosity is produced.
0
5
10
15
Oxidation time, h
Fig. 3. Variation of coke porosity with oxidation time of parent coal, from mercury po rosimetry data. Figure 4 shows the cumulative pore volume distribution with pore diameter of Alpheus cokes, produced from pre-oxidised coal samples. Cumulative pore volumes of cokes from oxidised coal are larger than those of cokes from the fresh coal. A very similar evolution is observed for cokes from oxidised samples of Gregory coal (data not reproduced). The drastic reduction in plastic properties of coals, which occurs as a result of oxidation, seems to be the principal cause of this increase (ref. 22).
465
However, for cokes from preoxidised Turon coal, the situation is different (Figure 5). The sample preoxidised for 2 hours has a smaller cumulative pore volume than that from fresh coal. With 24 h of coal preoxidation the trend is reversed and cumulative pore volumes of cokes are now larger than those of cokes from the fresh coal. The Tur6n coal has the highest dilation when fresh, (Table l ) , the other two coals exhibiting lower dilations 8 and 62% respectively. The decrease in porosity of cokes from pre-oxidised Turon coal is similar to that observed by BCRA (ref. 23). The high volatile bituminous coals with a total dilatation between 115 and 280% were oxidised at 100°C until a dilatation of about 65% was reached. The largest decrease in porosity (determined by microscopy) was observed in coal with the highest value of dilatation. These conditions are close to those given in the initial stages of Tur6n. It could be inferred that in coals with high dilatation values (high values of plastic properties), a slight oxidation involves a reduction in macroporosity, perhaps due to a partial collapse of pores as a consequence of both swelling in the plastic stage and a decrease in the permeability of the plastic layers. The two techniques for pore analysis in cokes, Le. image analysis based on optical microscopy, and mercury porosimetry are complementary to each other. The limiting resolution of the optical microscope, in terms of the pixel density of the computer screen is about 5 pm. Pores with diameters of -5-200 pn are identified. Mercury porosirnetry provides information in the range of - 4 to 7500 nm (7.5 pm). It is reported by Patrick U .(ref. 19) that coke strength correlates well with porosity (5200 pm diameter). This study confirms the results of Patrick &A. The mercury porosimetry data indicate that significant changes also occur in porosities of diameter >7.5 pm, and this aspect has not been discussed significantly, before. Whether or not these changes simply parallel the changes in larger porosities, or possibly have an important role,within themselves, in crack generation and propagation when coke is stressed in the on-going study.
466
-9 9
0.1
0.09
7
, "
0.08
5
v
0.07
w
3
0.06
2
0.05
2
0.04
2w
0.0s
1 +
0.02
22
0.01
3
0
0
0.4
0,8
1.2
1.6
2
2.4
2.8
3.2
3.6
4
log R (nm) Fig. 4. Cumulative pore volumes of cokes from preoxidised samples of Alpheus coal using mercury porosimetry.
7-
0.09
j
24 h
0
w [r
g ,,,
0.04
0.03
> F
0.02
3
0.01
a 1
I
s o
0.4
0.8
1.2
1.6
2
2.4
2.8
3.2
3.6
4
log R (nm) Fig. 5. Changes of cumulative pore volume of cokes from preoxidised samples of Turon coals using mercury.
467
CONCLUSIONS Coal oxidation produces an increase in porosity in resultant cokes for i) all three coals, when studied by mercury porosimeter, pores <5 pm diameter. Porosity increases significantly only for Gregory coal when studied ii ) by image analysis, pores 5 to 200 prn diameter. i i i ) Abrasion indices correlate with porosity data of image analysis. Pore volumes, <5 prn dia., can double in cokes as a result of oxidation iv) of parent coals. The influence of this porosity on abrasion resistance (crack propagation) has to be ascertained. v) The coal of highest dilation and fluidity (Turon) shows small decreases in all porosities after 2 h of pre-oxidation. REFERENCES 1. 2. 3.
4. 5. 6.
7. 8. 9. 10. 11. 12. 13. 14. 15.
F. Goodarzi and D.G. Murchison, Fuel, 52, (1973) 90-92, J.G. Prado, J. Microsc., 109(1), (1977) 85-92. B.M. Nandi, L.A. Ciavaglia and D.S. Montgomery, J. Microsc., 109(1), (1977) 93-103. S.R. Kelemen and H. Freund, Energy & Fuels, 3, (1989) 498-505. A.H. Clemens, T.W. Matheson, L.J. Lynch and R. Sakorvos, Fuel, 68, (1989) 1162-1167. J.C. Quick, A. Davis and D.C. Glick, Ext. Abs. 19th Biennial Conf. on Carbon, Pennsylvania, U.S.A., American Carbon Society, (1989), pp. 232-233. S.L. Bend, Ph.D. Thesis, University of Newcastle upon Tyne, Uk, pp. 153-155 (1989). J.W. Larsen, D. Lee, T. Schmidt and A. Grint, Fuel, 65, (1986) 595-596. E. Jakab, B. Hoesterey, W. Windig, G.R. Hill and M.L.C. Meutelaar, Fuel, 67, (1988) 73-79. R.R. Martin, J.A. Macphee, M. Workinton and E. Lindsay, Fuel, 68, (1989) 1077-1 079. R. Rausa, V. Calemma, S. Ghelly and E. Girardi, Fuel, 68, (1989) 11681173. L.D. Schmidt in "Chemistry of Coal Utilisation", Ed. H.H. Lowry, Volume 1, John Wiley ?L Sons, New York, USA, (1945) p. 627. J.C. Crelling, R.H. Schraeder and L.B. Benedict, Fuel, 58, (1979) 542546. J.J. Pis, A. Cagigas, P. Simon and J.J. Lorenzana, Fuel Proc. Technol., 20, (1988) 307-316. A. Grint, H. Marsh and D.E. Clarke, Fuel, 62, (1983) 1355-1358.
468
16. 17.
18. 19. 20. 21.
22.
23.
R. Menendez, H. Marsh, J.J. Pis, R . Alvarez and J.J. Lorenzana, 1989 Intern. Conf. on Coal Science, Tokyo, Japan, Vol. I, pp. 591-594. S. Ragan, W. Hibbard, A.M. Squires and H. Marsh, Ext. Abs. 15th Biennial Conf. on Carbon, Philadelphia, USA, American Carbon Society, June (1981) p. 204-205. J.W. Patrick, J. Microscopy, 132 (1983) 333-343. J.W. Patrick and A. Walker, Carbon 27(1), (1989) 117-123. M.J. Kwiecien, I.F. Macdonald and F.A.L. Dullien, J. Microscopy, 159 (1990) 343-359. B. McEnaney and T.J. Mays, "Porosity in Carbons and Graphites" in 'Introduction to Carbon Science', Ed. Harry Marsh, Butterworths, London, UK (1989) pp. 152-196. J.J. Pis, J.A. Pajares, A.B. Fuertes, M. Mahamud, J.B. Parra, A.J. Perez and B. Ruiz, Ext. Abst. of Carbone 90 Conference, lnternationale sur le Carbone, Paris, France 1990, p. 114-115. British Coke Research Association, Carbonization Research Report No. 57, Aspects of pore-structure development during carbonization of preoxidised and preheated coals, (1978), B.C.R.A., Chesterfield, Derbyshire, U.K.
F. Rodriguez-Reinosoet al. (Editors), Characterization of Porous Solids I1 0 1991 Elsevier Science Publishers B.V., Amsterdam
469
COMPARATIVE STUDIES OF THE MICROPOROUS STRUCTURE PARAWETERS EVALUATED FROM THE ADSORPTION ISOTHERnS OF VARIOUS ADSORBATES ON ACTIVATED CARBONS
M. Jaroniec', J. Choma'. Martinez'
F. Rodriguez-Rein~so~and S.M. Martin-
'Department of Physics, Kent State University, Kent, Ohio 4 4 2 4 2 , U.S.A. 'Institute of Chemistry, WAT, 00908 Warsaw, Poland bepartamento de Quimica Inorginica e Ingenieria Quimica, Universidad de Alicante, Apartado 99, 03080 Alicante, Spain SUMMARY
Microporous Structure parameters such as micropore volume and average micropore size are evaluated by means of the DubininRadushkevich (DR) equation applied to adsorption of carbon dioxide, nitrogen, n-butane and iso-butane on two microporous activated carbons. A comparison of these parameters provides information about accessibility of the micropores of a solid adsorbent for adsorbates of different molecular sizes and explains why the DR plots for some adsorbates do not coincide. INTRODUCTION Although the Dubinin-Radushkevich (DR) equation (ref. 1) was proposed over 4 0 years ago, it is still one of the most popular equations used for describing physical adsorption of gases and vapours on microporous activated carbons (refs. 2 - 4 ) . This equation is also used to describe gas adsorption on energetically heterogeneous surfaces (refs. 5,6). Starting with this equation, it was possible to formulate a theory of gas adsorption on structurally heterogeneous microporous solids (refs. 6,7). In the majority of papers [cf., refs. 2 - 4 , 6 , 7 and references therein], the DR equation has been used to characterize different activated carbons on the basis of adsorption isotherms for one adsorbate (usually benzene). There are no extensive studies dealing with the use of DR equation to describe adsorption of various adsorbates on selected samples of microporous activated carbons. This problem is addressed in this paper. A brief description of adsorption in microwres The total adsorbed amount a, is a simple sum of the amount ami
470
adsorbed in the micropores and the amount am adsorbed on the mesopore surface (ref. B ) , i.e., a, = a,, + am. Using the standard adsorption isotherm 8, on a reference nonporous carbonaceous adsorbent, the adsorbed amount a,, can be extracted from the total adsorbed amount a, (refs. 8 , 9 ) : ami= a, - a",,,,
(1)
some
is the monolayer capacity of the mesopore surface, and where e r is the relative coverage of the reference adsorbent surface of the solid; it is assumed that physicochemical nature of the mesopore surface of the solid studied is identical with that of the reference adsorbent, and that there is no capillary condensation. The monolayer capacity a", can be evaluated by the a,-method (refs. 9,lO). Note that the equation (1) defines the relationship between the total adsorbed amount a, and the standard adsorption isotherm er for each value of the relative pressure P/P" at a constant absolute temperature T. To obtain the dependence of the adsorbed amount amion the relative pressure P/P", the value of a, and er should be compared via equation (1) for different values of P/P" over the micropore filling region. For microporous activated carbons with narrow ranges of pore size, the DR equation gives a good representation of the pressuredependence of a,, (refs. 1, 2, 11, 12). This equation may be written as follows:
where A
=
RT In (P"/P)
(3)
Here, aomiis the maximum adsorbed amount in the micropores (if density of the fluid in filled micropores does not differ significantly from the bulk density then the amount ao,,,,multiplied by the adsorbate molar volume y, is equal to the micropore volume V,, in other case the molar volume of the fluid in filled micropores should be used to calculate V"); A is the adsorption potential; E
471
is the characteristic energy for a given adsorbate-adsorbent system and R is the universal gas constant. To extract information about microporosity of a solid from the characteristic energy E , Dubinin (ref. 1) proposed the use of a reference adsorbate (benzene). His experimental studies (ref. 1) showed that for different activated carbons the ratio of the characteristic energy E of a given adsorbate to the characteristic energy E, of the reference adsorbate is nearly constant, i.e., l3 = E/E,, where D is the so-called affinity (similarity) coefficient. Also, Dubinin (ref. 8 ) proposed the following empirical relationship between the half-width of slit-like micropores, x, and the characteristic energy E, (ref. 8 ) : x = k/E,
(4)
Dubinin (ref. 8 ) treated the parameter k in equation (4) as a constant, which is independent on the micropore size x in the whole micropore region. Extensive experimental studies of adsorption on microporous carbonaceous adsorbents including molecular carbon sieves (refs. 11, 13, 14) showed that the parameter k in equation ( 4 ) depends on the micropore size. Stoeckli et al. (refs. 11, 13) found a relationship between x and E, which is more complex than that expressed by equation ( 4 ) . Because the current study is limited for evaluating the characteristic energy E, by means of the DR equation ( 2 ) for various adsorbates, the simple relationship ( 4 ) can be used to estimate the average micropore size x (ref. 15); however, it is noteworthy that knowledge of an accurate relationship x(E,,) is of great importance for evaluating the micropore-size distribution from experimental adsorption isotherms (ref. 12). It was mentioned above that micropore capacity, aomi,can be converted to micropore volume, V,. For the same adsorbent it might be expected that access to small micropores is limited for large adsorbate molecules (ref. 16). Accordingly, similar values of V, suggest that molecules of different adsorbates are accessible to the microporous structure of the adsorbent to the same degree. In this context, comparisons of DR parameters and micropore volumes estimated from adsorption of carbon dioxide, nitrogen, n-butane
472
and iso-butane on two activated carbons are discussed below. RESULTS AND DISCUSSION
Two activated carbons were obtained from carbonized olive stones by CO, gasification at 1098K to 8% (D-8) and 19% burn-off (D-19), respectively (ref. 17). The adsorption isotherms of carbon dioxide, nitrogen, n-butane and iso-butane on the D-8 and D-19 microporous activated carbons were analysed by means of the DR equation (2). Table 1 contains the values of the affinity coefficient 13 and the molar volume y,,, for the adsorbates studied as well as the temperatures of the adsorption measurements. TABLE 1 Selected physicochemical adsorption studies
i-C,H,,
273 273
parameters
34.7 96.7 100.0
for
adsorbates
used
in
0.87 0.92
We selected these two carbons because their mesopore surface areas are very small (about 20 m2/g) (ref. 18); accordingly, the total adsorbed amount a, may be identified with the amount a,,,, adsorbed in the micropores. For microporous activated carbons with large specific surface areas of the mesopores equation (1) should be used to extract the adsorbed amount a,,,,from the total adsorbed amount a,. Analysis of the adsorption isotherms for carbon dioxide and nitrogen by means of the DR equation (2) shows that both adsorbates provide almost the same values for the micropore volume; the average value of V, obtained for these adsorbates is equal to 0.25 cm3/g for D-8 carbon, and 0.31 cm3/g for D-19 carbon (cf., Table 2). This result suggests that all micropores of these carbons, which are accessible to carbon dioxide molecules, are also accessible to nitrogen molecules. If the values of V, obtained from adsorption data for some adsorbates are identical and the characteristic energy of the reference compound, E,, is known, then
473
the affinity coefficient D for these adsorbates can be estimated by comparing the DR linear plots; in this paper benzene was used as the reference compound (ref. 18). A proper selection of the values of I3 leads to one characteristic line In 8 ( = a,/aomi) vs. ( A / I ~ ) ~ , which contains experimental points for different adsorbates. This situation is illustrated in Fig. 1, which presents the DR plots In e v s . (A/I3l2 for carbon dioxide and nitrogen on the D-8 and D-19 activated carbons. It follows from this figure that the experimental points of both adsorbates form a single straight line for each activated carbon. This single straight line is obtained by assuming A = 0 . 3 3 for nitrogen and D=0.42 for carbon dioxide.
I
I
I
I
1
(a)
(b)
2, -1.0
-
\x
I
k \X
\
-
\
\
- 1.5 Fig. 1. DR plots for carbon dioxide (x) and D-8 (a) and D-19 (b) activated carbons
x,
nitrogen
~
( 0 ) on
the
While the value of D for nitrogen agrees with the literature data (ref. 3 ) , the value of D=0.42 obtained by us for carbon dioxide is higher than D=0.35 used earlier in the literature for this adsorbate. Note that the experimental values of D may differ slightly for various microporous carbons; this difference can be due to the fact that the molecular structure and density of liquid phases formed by various adsorbates in the micropores with different structural surface heterogeneities may be different; consequently, the adsorbate parameter A can slightly depend on the nature of activated carbon. The slope of the straight line In0 vs. ( A / D ) 2 is equal to E i 2
414
and permits evaluation of x from E, if the relationship x(E,) is known. Table 2 contains the value of x for each carbon calculated
Carbon D-8
Adsorbate
Micropore size x (nm)
i-C,Hlo
0.18
0.48 0.48 0.49 0.60
co2
0.31 0.32 0.28 0.26
0.50 0.50 0.53 0.55
co2 N2 n-c4H10
D-19
Micropore volume v,, (cm3/9)
N2 n-c4H10
i-C4Hlo
0.25 0.26
0.21
from E, by means of equation ( 4 ) with k=l2kJ/mol (ref. 8). Both carbon dioxide and nitrogen provide the same value of x for a given carbon, because these adsorbates provide identical values of V,. The micropore volumes for D-8 and D-19 carbons obtained from the adsorption isotherms of n-butane and iso-butane are smaller than that evaluated by nitrogen and carbon dioxide adsorption (cf., Table 2); this means that the smallest micropores, which are
Fig. 2. DR plots for n-butane ( 8 ) and iso-butane (A) on the D-8 (a) and D-19 (b) activated carbons.
475
accessible for carbon d oxide and nitrogen molecules, are not accessible for molecules of n-butane and iso-butane. It follows from Fig. 2 that the DR plots for n-butane and iso-butane do not coincide and consequ ntly, they provide different values of E,. Table 2 contains the average values of x calculated according to equation ( 4 ) from the values of E, for n-butane and iso-butane. It follows from this table that the average values of x are slightly higher than that obtained by nitrogen and carbon dioxide adsorption. Fig. 3 presents a comparison of the values of the micropore volume as a function of the average micropore size x for all adsorption systems studied. This figure shows that for n-butane and iso-butane the DR equation ( 2 ) predicts the smaller values of V, and the higher values of x than those obtained for carbon dioxide and nitrogen. This result is expected for activated carbons possessing small micropores, which are not accessible for
I
04
05
06 x (nm)
II
04
05
06
x (nm)
Fig. 3 . Dependence of the micropore volume V, on the average micropore size x for the D-8 (a) and D-19 (b) activated carbons; solid line denotes data for carbon dioxide, dotted line for nitrogen, dashed line for n-butane and dashed-dotted line for isobutane adsorption of larger molecules. The above discussion indicates that analysis of the micropore adsorption data for various adsorbates of different molecular size by means of DR equation (2) or its extended forms provides a valuable information about microporous adsorption systems in addition to that obtained by the a,-method.
476
REFWENCES
1 M.M. Dubinin and L . V . Radushkevich, Dokl. Akad. Nauk SSSR 55, 331, 1947. 2 M.M. Dubinin, Progress Surf. Membrane Sci. 9 , 1, 1955. 3 M.M. Dubinin, Chemistry and Physics of Carbon 2 , 55, 1966. 4 F. Rodriguez-Reinoso and A. Linares-Solano, Chemistry and Physics of Carbon 21, 1, 1988. 5 G.F. Cerofolini, Colloid Science 4 , 59, 1982. 6 M. Jaroniec and R. Madey, Physical Adsorption on Heterogeneous Solids, Elsevier, Amsterdam, 1988. 7 M. Jaroniec and J. Choma, Chemistry and Physics of Carbon 2 2 , 197, 1989. 8 M.M. Dubinin, Carbon 2 3 , 373, 1985; 2 5 , 593, 1987; 2 7 1 457, 1989. 9 M. Jaroniec, R. Madey, J. Choma, B. McEnaney and T. Mays, Carbon 2 7 , 77, 1989. 10 K.S.W. Sing, Colloids and Surfaces 38, 113, 1989. 11 H.F. Stoeckli, F. Kraehenbuehl, L. Ballerini and S . DeBernardini, Carbon 2 7 , 125, 1989. 12 H.F. Stoeckli, Carbon 2 7 , 962, 1989. 13 H.F. Stoeckli, L. Ballerini and S . DeBernardini, Carbon 2 7 , 501, 1989. 14 B. McEnaney, Carbon 2 5 , 69, 1987. 15 M.M. Dubinin, Carbon 2 6 , 97, 1988. 16 P.J.M. Carrott, R.A. Roberts and K.S.W. Sing, in Characterization of Porous Solids; K.K. Unger, J. Rouquerol, K.S.W. Sing and H. Kral eds., Elsevier, Amsterdam, 1988. 17 J. Garrido, A. Linares-Solano, J.M. Martin-Martinez, M. MolinaSabio, F. Rodriguez-Reinoso and R. Torregrosa, J. Chem. SOC. Faraday Trans. I 83, 1081, 1987. 18 M. Jaroniec, J. Choma, F. Rodriquez-Reinoso, J.M. MartinSOC. Faraday Trans. I Martinez and M. Molina-Sabio, J. &em. 85, 3125, 1989.
F. Rodriguez-Reinosoet al. (Editors), Characterization of Porous Solids II 0 1991 Elsevier Science Publishers B.V., Amsterdam
ESTIMATING MICROPORE SIZES IN ACTIVATED CARBONS FROM ADSORPTION ISOTHERMS
B. McENANEY and T. J. MAYS School of Materials Science, University of Bath, BATH, Avon BA2 7AY, U. K.
SUMMARY Various methods for estimating micropore sizes in activated carbons from a single adsorption isotherm are reviewed. The methods include: (i) single parameter estimates of micropore size based upon Dubinin's theory of the volume filling of micropores; (ii) estimates of micropore size distributions based upon a generalised Dubinin-Radushkevich equation, and (iii) the use of intermolecular potentials in model micropores. INTRODUCTION The International Union of Pure and Applied Chemistry (ref. 1) has adopted a classification of pores according to their average width, w: for micropores w < 2 nm; for mesopores
2 5 w I 5 0 nm, and for macropores w > 50 nm. This classification is based upon adsorption criteria. In micropores adsorption is enhanced as a result of the overlap of dispersion forces from proximate pore walls. In mesopores adsorption occurs as on free surfaces until capillary condensation takes place as a result of interactions between adsorbate molecules on opposite pore walls. In macropores condensation takes place as in mesopores, but at relative pressures SO
close to unity that effects on isotherms are virtually impossible to detect. Thus, while
macropores are important in providing routes for adsorptives to gain access to mesopores and micropores, they effectively do not influence isotherms so that their structure cannot directly be investigated from adsorption measurements. Techniques for studying macropore structure in carbons such as porosimetry, gas transport and microscopy have been reviewed in ref. 2. There are many well-established methods available for estimating mesopore sizes in activated carbons, and other porous adsorbents, from adsorption isotherms using models for capillary condensation (ref. 3). Such methods are used routinely, both in the laboratory and in industry, and, in recent years, they have been incorporated into the software of commercial, automated, adsorption instruments, for example Omicron Technology Corporation's Omnisorp
477
478
systems and the Sorptomatic series instruments manufactured by Car10 Erba. It would be a highly desirable development in the characterisation of porous solids if reliable methods could also be devised for routine use in estimating micropore sizes. This paper is a review of methods for estimating micropore sizes from a single experimental adsorption isotherm. Laborious ways for probing micropore structure involving analyses of isotherms of adsorptives of different molecular size and shape are not directly considered. Standard analyses of adsorption isotherms for non-microporous solids usually involve models for adsorption on free surfaces, such as the Brunauer-Emmett-Teller (BET) equation (ref. 4), and models for capillary condensation such as the Kelvin equation (ref. 5). However these analyses are limited when applied to microporous adsorbents such as activated carbons. Adsorption on these solids at low relative pressures (p/ po = x less than about 0.4) predominantly involves the filling of micropores in which there is enhanced adsorption, rather than the formation of multilayers and subsequent capillary condensation. It is for this reason that the MP method (ref. 6) for determining micropore size distributions using a development of the BET model has failed to gain wide acceptance. Methods for determining micropore structure based on adsorption isotherm data, which offer the potential to be developed into programmes for routine characterisations, are reviewed in this paper. These methods involve analyses of a single isotherm based on the Dubinin-Radushkevich equation (ref. 7), a development of the potential theory of adsorption (ref. 8). Techniques for obtaining both single parameter estimates of micropore size and micropore size distributions are described. The estimation of micropore size distributions using intermolecular potentials in model micropores is also considered.
SEPARATION OF EQUILIBRIUM MICROPORE ISOTHERMS In the first stage of determination of micropore structure, adsorption measurements should be tested to ensure that the adsorption system is in equilibrium. This prevents including in analyses non-equilibrium adsorption data which in activated carbons result mainly from the effects of thermally activated entry into micropores via narrow entrances (refs. 9, 10). Once the system is in equilibrium, micropore isotherms may be separated from total isotherms, which may include adsorption on non-microporous surfaces, using techniques such as preadsorption (ref. 1 l), isotherm subtraction (ref. 12), t-plots (ref. 13) and as-plots (ref. 14). A comparative study of these methods for activated carbons with different extents of activation was made in ref. 15.
479
SINGLE PARAMETER ESTIMATION OF MICROPORE SIZE The Dubinin-Radushkevich @R) equation may be written as
v = V,
A 2 exp ( - -) PEO
where V is the volume of micropores filled at relative pressure x, V, is the total micropore volume, A = RT ln(l/x) is the adsorption potential or differential molar work of adsorption (R is the gas constant, T is absolute temperature),
p is the affinity or similarity coefficient
which depends on the adsorptive and E, is a characteristic energy which depends on the microporous structure of the adsorbent. It has been suggested (ref. 16) that the DR equation represents adsorption in homogeneous microporous solids, that is in these solids E, is a constant for all micropores. For microporous carbons an empirical, inverse correlation has been found (ref. 17) between E, estimated from adsorption data using the DR equation, and R,, the average Guinier radius of gyration, measured using small-angle x-ray scattering (SAXS) methods, which may be written as
E, =
14.68 (+-0.16)
I kJ moil
(2)
Rg
A single micropore size may be estimated from Rg assuming a simple pore shape, for example
R
g
b2
w2
2
12
=(-+-)
0.5
(3)
for a disc-shaped pore of width w and with two circular walls of radius b. Similar inverse correlations have been observed between E, and average micropore size less than about 1 nm estimated from gas-solid chromatography experiments (ref. 18) and using molecular probes and immersion calorimetry (ref. 19), for example
E ,= -
K W
(4)
480
where w is the width of the micropore determined from molecular probe studies and K is a weak function of E,.
In ref. 20 SAXS and molecular probe data were correlated with the
empirical expression
w = 6.64 - 1.79 lnEo
(5)
as shown in Fig. 1. In ref. 21 eqns. (2-5) were further analysed in terms of a disc-shaped model pore.
3.2 2.8
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2.0 1.6 1.2
-
0.4 0.8
-
0.0
,
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-
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b. 0 Molecular probe data SAXS data I
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Fig. 1. Correlation of micropore width, w, in activated carbons with Dubinin's characteristic energy, E,, (eqn. 5 ) using data from small-angle x-ray scattering (SAXS) and molecular probes (after ref. 20). It is clear that eqns. (2-5) provide simple means of obtaining single parameter estimates of micropore size from a DR isotherm. However a number of criticisms can be made. First, concerning SAXS measurements, the poor grouping of some SAXS data (refs. 22,23) and the lack of a Guinier limit for some carbons (ref. 24) limit the statistical reliability and the extent of applicability of eqn. (2), as was acknowledged by its authors. Second, concerning molecular probe data, correlations such as eqns. (4) and ( 5 ) relate E, to micropore size using dimensions of the probe molecule. Such correlations do not take into account the compressibility of the
481
adsorptive molecule in the force fields between micropore walls. For example, it was estimated in ref. 25 that a correction of about 0.1 nm must be made for the compressibility of argon and xenon in micropores; thus, failure to correct for compressibility of the adsorptive molecule may result in a substantial overestimate of micropore size. Another criticism stems from the assumption that the DR equation is homogeneous and that therefore all of the carbons used to establish the correlations in eqns. (2-5) are also homogeneous as regards micropore sizes. This is certainly not the case; evidence for structural heterogeneity in activated carbons, that is micropores of different sizes, has been obtained from electron microscopy and molecular probe studies (refs. 26-28). Recently, in ref. 29, it has been suggested that for some activated carbons the DR equation results in a good fit to micropore adsorption data, even though molecular probe experiments indicate that the carbons are structurally heterogeneous. A further criticism of the methods for single parameter estimates of micropore size is that the DR equation frequently does not result in a good fit to equilibrium, microporous adsorption data. To represent such data the Dubinin-Astakhov @A) equation (ref. 30) has been proposed. This may be written as
v = V,
>"
A exp ( - PEO
where the DA exponent n 2 1 is an adjustable parameter which may be estimated from adsorption data; thus, the DR equation, eqn. (l), is the special case of the DA equation, eqn. ( 6 ) ,for n = 2. While fits to data are frequently better using the DA equation - which to some extent is statistically inevitable, since the DA equation has an extra adjustable parameter no physical significance for the exponent n has yet been determined, although values of n > 2
are found for adsorption in carbons with narrow micropores and n < 2 for wide-pore carbons. An alternative generalisation of the DR equation which gives improved fits to data, (see ref. 16), accounts for non-identical micropores (heterogeneity) by including a distribution function for Eo. Thus, if it is assumed that E, is correlated with pore size, as discussed above, then Stoeckli's generalised DR equation has some physical significance, unlike the DA equation. The generalised DR (GDR) equation is discussed further in the next section as a basis of estimating micropore size distributions.
482
ESTIMATION OF MICROPORE SIZE DISTRIBUTIONS The generalised Dubinin-Radushkevich eauation The GDR equation may be written as
V =
J
V, exp ( -
0
A >z f(Eo) dE,
PEO
where f(E,) is the probability density function (pdf) of E,, such that
f(E,) 2 0
non-negativity
(84
-
I 0
f(E,) dE, = 1
normalisation to unity
A simple treatment which anticipated the GDR (ref. 31) effectively assumed that f(EJ is the
sum of two Dirac &functions, that is there are two classes of micropore each with a different total volume and E,. The isotherm equation in ref. 3 1 may be written as
where the subscripts 1 and 2 refer to the two classes of micropore. Later, in ref. 32, f(Eo) was related to the normal distribution which gives the following isotherm equation
V = Voexp ( - B, y ) exp ( -
y2
T)
1 - erf(u) 1 2
where B, is proportional to the mean squared value of E,, y and u are functions of B,, A, A and P, A is the standard deviation of the distribution of B, and erf(.) is the error function. The application of these two approaches to activated carbons was considered in ref. 33. Recently (ref. 34) different model functions for f(E,) were considered, and applied to a
483
number of different carbon adsorbents. For example one isotherm equation for heterogeneous microporous adsorption (ref. 34) may be written as
where q and m are parameters of a gamma-type distribution for B, [the same B, as in eqn. (lo)]. There are severe statistical difficulties in estimating f(Eo) from the GDR because it is a linear, one-dimensional Fredholm integral equation of the first kind (ref. 3 3 , in which the total micropore isotherm is the driving term, the DR equation, eqn. (l), is the kernel and the energy distribution, f(E,), is the unknown function which is sought. Equations of this kind are illposed or improperly- or incorrectly- posed which means here that many different energy distributions will, on substitution in the GDR, give similar total isotherms. Therefore fidelity of the model to the data does not in itself validate an estimate of the energy function. The problems of ill-posedness for equations of the same form as the GDR were discussed in ref. 36. The simplest and most widely used way to 'solve' equations similar to the GDR is to assume that the unknown function is defined by a mathematical formula, which is selected to allow direct integration to give an analytic function for the total isotherm. The parameters of the isotherm are then estimated, for example by regression analysis, and substituted back into the formula for the unknown function to define a 'solution'. Eqn. (10) is used in this way; the method in ref. 31 and many of those discussed in ref. 34 also use this approach. However, it should be noted that, almost exclusively, the assumed form of the energy function is selected for the mathematical convenience of being able directly to integrate the GDR. Ill-posedness still remains in that different functions f(E,) will give similar isotherms. It is a matter of discretion as to which of the range of possible parametric energy distributions is finally chosen to represent heterogeneity, since any pdf on the interval [0, -) may be given as an estimate of f(Eo), provided that the model total isotherm fits the data, say to within experimental error. Estimation of micropore size distributions from the eeneralised Dubinin-Radushkevich eauation If a monotonically decreasing function, E, = h(z), between E, and micropore size z is known or assumed then from f(E,) the pdf of z, g(z), is given by
484
g(z) =
I
f[h(z)]
This function, which satisfies constraints equivalent to eqns. (8a, 8b) for f(E,), characterises the structural heterogeneity of the microporous adsorbent (at absolute temperature T, with respect to the adsorptive X and adsorbent Y). The domain of g(z) will be limited by the size of the smallest micropores in Y which molecules of X can enter, zmin,and the largest pores in which micropore filling occurs, zma. This imples that in addition to the constraints on f(E0) in eqns. (8a, 8b), the domain of f(E,) will also be constrained to some finite range [h(z,,,),
h(zmin)]. This is both a useful additional constraint on the choice of functions for
f(E& selected for analytical solutions of eqn. (7) and a useful test for numerical solutions. Thus for a given estimate of f(E,) the method for obtaining micropore size distributions involves: (i) the determination of a relationship between E, and pore size z, E, = h(z), and (ii) the calculation of the micropore size distribution g(z) from f(E,,) using eqn. (12). The first of these steps is the more important; the second is a simple mathematical wnsformation. A relationship equivalent to eqn. (2) has been used recently to estimate pore size
distributions in activated carbons (ref. 37), see Fig. 2.
0
1
2
3 w/nm
4
5
6
Fig. 2. Some examples of distributions of the width, w, of slit-shaped micropores in activated carbons for benzene adsorption at 293 K (after ref. 37).
485
Clearly the criticisms above of the use of eqn. (2) for obtaining single parameter estimates are also applicable to its use in obtaining pore size distributions. In particular, if the DR equation is not homogeneous, then it should not be used as the kernel of the GDR equation, eqn. (7), since E, does not correspond to a single-valued adsorption energy in pores of uniform size and therefore the relationship E, = h(z) is not valid. The probable bias in micropore size distributions estimated using methods which involve this inconsistency needs to be explored.
GENERAL DISCUSSION Here wider consideration is given to models of adsorption and structure in activated, microporous carbons. This general discussion leads to suggestions for future work in this area. Methods based upon the Dubinin-Radushkevich equation
An assumption which is made in both single parameter and distribution estimates of micropore size is that there is a single energy factor, E, from the DR equation, which is associated with each micropore. This much simplifies the probable physical nature of micropores in activated carbons, which involves: (i) spacial variations in adsorption energy, due to different degrees of adsorption energy enhancement across the pore width (important in wide pores), and to different pore shapes [for example in wedge-shaped pores (ref. 38)l and (ii) the inherent energetic heterogeneity of carbon surfaces, due to surface defects, heteroatoms, etc. Thus for a single micropore E,, or more precisely E = PE,, represents some measure of the energy of interaction between the adsorptive and the adsorbate. These simplifications are compounded when relationships, which have been criticised here, are estimated between E, and pore size, z. The generalised adsorption isotherm (GAI) for heterogeneous, microporous solids (ref. 36) may be written as
c
where N(p), the total isotherm (the driving term), is the total amount adsorbed at p, n(p, E), the local isotherm (the kernel), is the amount adsorbed at p in micropores charactensed by an
486
energy E, and F(E) (the unknown function which is sought) is the pdf of E. Thus the GDR is a special case of the GAI where amounts adsorbed are expressed by volume and the local isotherm is the DR equation (so that
E=
E,).
The GAI gives the DR equation when a
Langmuir kernel is approximated by a step-function (the condensation approximation) and it is assumed that the energy function is a Rayleigh distribution of the molar isosterk heat of adsorption q. This interpretation, in which the DR equation is heterogeneous, may explain its success in representing adsorption on a wide range of solids (ref. 39). Deviations from the DR equation represented by the DA equation, may also be accounted for by the condensation approximation, but with different forms of F(E). As for the DR methods reviewed here, in principle pore size distributions may be estimated from the GAI for a selected local isotherm if a function
E = G(z)
relating
E
to pore size z is
known or assumed. The present authors have used the Langmuir isotherm (ref. 40)and the n-layers BET equation (ref. 41). together with suitable distribution functions for the heat of adsorption q, to represent adsorption in activated carbons. In a wider context many different combinations of local isotherm and energy distribution functions in the GAI have been applied to carbons and other microporous adsorbents (ref. 34). However, no analyses have yet been
published to relate heats of adsorption, or other characteristic adsorption energies, to pore size, other than those reviewed here which are based on the DR equation and methods based upon intermolecular potentials discussed below. Further work in this area is much needed.
A different and promising approach to estimating micropore sizes is based on intermolecular potentials. Using Lennard-Jones intermolecular potential functions, relations between the potential Q in model micropores of width w or radius r and the location of a single molecule in the pore were derived (ref. 25). The minimum potential Qo(z) was noted to decrease with increasing pore size z = w or r, eventually reaching the value of the minimum for a free surface, $,(-).
as expected from qualitative considerations of the superposition of
dispersion forces from proximate pore walls. It was shown in ref. 42 for many different activated carbons that E = PE, from the DR equation was proportional to the difference between the heat of adsorption in micropores at low surface coverage, qmi,and the heat of adsorption on a (nonporous) graphitised carbon black, 9,. From this correlation, assuming that qmi/qg= Qo(z)/Qo(-), a model, inverse correlation between E, and z for the adsorption of argon in micropores was derived (ref. 20) based upon the 10:4 intermolecular potential
487
function for slit-shaped pores obtained in ref. 25, see Fig. 3. In principle it would be possible to calculate similar correlations for different adsorptives, and for different pore shapes, which could be used to transform distributions of E, from the GDR into pore size distributions. While this has not yet been done, the advantage of this approach compared with that involving correlations between E, and pore size from SAXS or molecular probe data is that a value of E, is related to a single pore size. A further extension of this approach would be to explore correlations between energy parameters of different local isotherms, for example the heat of adsorption in the Langmuir equation, with model potential functions.
36 .
-
32
-
28
-
24
-
8
20-
9
-
I
I
I
I
I
I
&
' 16 mo 12
-
8 4 -
0
Fig. 3. Variation of Dubinin's characteristic energy, E,, for argon adsorption in carbons with the width, w, of model, slit-shaped micropores (after ref. 20). The calculations in ref. 25 for model micropores only consider interactions between a single adsorptive molecule and the walls of the model micropore. They do not account for interactions between adsorptive molecules and so cannot model the process of micropore filling. Recently (ref. 43) results from molecular modelling studies were reported for the adsorption of nitrogen on porous carbons in which both adsorptive-adsorbent and interadsorptive interactions were considered. Using an approximate theory of inhomogeneous fluids known as mean-field theory, a function p(p, w) was derived (ref. 43) which relates the
488
density p of nitrogen in pores to pressure and pore width. This function was subsequently used as the kernel in the GAI, eqn. (13), and parametric estimates of pore width dismbutions were obtained. A significant aspect of this work is that it applies to adsorption both in micropores and in mesopores. Another important observation is that adsorption in a single micropore is not a smoothly increasing function of pressure; rather, for a pore of width W, a steep rise in amount adsorbed occurs at a critical pressure which is related to w. Although this recent molecular modelling approach in the estimation of pore sizes (ref. 43) is an improvement on any of the other pore size estimation methods considered in this paper, a notable shortcoming is the extensive computations required to derive the density function p(p, w), which are beyond the capability of current microcomputers attached to commercial adsorption equipment. Also, the micropore size distributions presented in ref. 43 have a lower limit of 1.3 nm which is determined by the lowest relative pressure, x =
at which
experimental measurements were made. To obtain meaningful size distributions for smaller micropores using this method will require precise measurements of adsorption isotherms at very low relative pressures (in principle this is a general requirement of any method for obtaining micropore size distributions from adsorption data). Also, for general applicability, density functions for different adsorptive-adsorbent-temperature systems would need to be determined and, in addition, the sensitivity of the method to the form of the integrated adsorptive-adsorbent potential (which involves factors such as pore shape and pore wall thickness) needs to be explored.
CONCLUSIONS The estimation of mesopore size distributions from a single adsorption isotherm is a widely accepted technique. Because understanding of adsorption in micropores is much poorer than adsorption in wider pores, methods for estimating micropore sizes from a single adsorption isotherm have not been widely accepted. Single parameter estimates of micropore size based upon correlations between the Dubinin's characteristic energy, E,, and measures of pore size using small angle x-ray scattering and molecular probe studies have been reviewed. Although these methods are easy to use, they are subject to a number of criticisms, central among these being the assumption that the DR equation is homogeneous, that is it applies to adsorption in pores of uniform size. There is much evidence that activated carbons are heterogeneous microporous solids and this can be accounted for by using the Ceneralised DubininRadushkevich (GDR) equation to obtain a micropore size distribution. A problem with this
489
approach is that the GDR equation is ill-posed, which means that many different pore size distributions can give similar fits to adsorption data. A different and promising method for estimating micropore sizes is based on intermolecular potentials. This approach has the advantage of being based upon sound physicochemical principles and can avoid the empiricism of methods based on the DR equation. However, at present considerable computing power is required to obtain pore size distributions using this method.
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A Comparative Using Benzene
491
Study of t h e Porous S t r u c t u r e of A c t i v e Carbons and Water Adsorption, Immersion Calorimetry and Liquid Chromatography
R.H.Raaeke; P.Bruckner C 8 n t r 3.1 I fi s t i t u t e o f F 11 y si c a 1 C e m i s t r y U 1 1 9 9 B e r l i n . Germany
ABSTRACT Some m e t h o d s f o r p o r e s t r u c t u r e a n a l y s i s h a v e b e e n p r e s e n t e d : T h e a d s o r p t i o n o f b e n z e n e and t h e e v a l u a t i o n of isotherms through t . h s Dubiinin - R a d u s h k e v i c h e q u a t i o n , t h e e s t i m a t i o n o f immersiijin h e a t s i n benzene, t h e a d s o r p t i o n of w a t e r a t r e l a t i v e p r e s s u r e s of h=C).6 a n d 1 . O , t h e s i z e e x c l u s i o n l i q u i d c h r o m a t o g r a p h y w i t h t r a c e r s o f ( d i f f e r e n t m o l e c u l a r d i a m e t e r s and t h e one - p c i n t . adsorption o f n i t r o g e n . S i x a c t i v e c a r t o n s are i n c l u d e d i n t h e i n v e s t i g a t i o n s . I t is n o t p o s s i b l e t o o h t a i n r e l i a b l e v a l u e s w i t h t h e s i m p l e w a t e r a d s o r p t i o n m e t h o d . The r e s u l t s o b t a i n e d w i t h s ~ t h e r m e t h o d s a r e compared w i t h p e r f o r m a n c e s o f adsorpt.ion of p h e n o l from aqueous s o l u t i o n s as o b t a i n e d from m e a s u r i n g e q c i i i b r i a and column d y n a m i c s . I t is shown, t h a t t h e r a n k o f the r e s u l t s of p u r e s t r u c t u r e a n a l y s i s i s t h e same as f r o m t h y dynamic e x p e r i m e n t s .
THE PROBLEH The p o r o u s s t r u c t u r e o f a c t i v e c a r b c l n s i s t.he d e f i n i n g f a c t o r of t h e i r a d s o r p t i o n p e r f o r m a n c e : The p o r e d i a m e t e r d i s t r i h u t i o n d e t e r m i n e s t h e a d s o r p t i o n elnergy a n d t h e r e f o r e , t h e slope of adsorpt.iiln i s o t h e r m , whereas i n mainly microporous c a r b o n s t h e m i c r o p o r e volume l i m i t s t h e a d s o r p t i o n c a p a c i t y a t t h e h i g h e r end of the a d s o r p t i v e cconcentration. Furthermore, the chemical ~::ijmpositori o f t h e c a r b o n s u r f a c e i n f iuences ttie s e l e ~ t i v i t y ot adsorption, e.$. t h e c o m p e t i t i o n of t h e w a t e r a d s o r p t i o n w!lrn w o r k i n g i n s q u e o u s s o l u t i o n . However, h e r e o n l y t h e s t r u c t u r - a 1 cRrGoris a r e t a k e n i n t o a c c o u n t . p r o p e r t i e s of F o r s j t u d y i n g t h e s t r u c t u r e o f c a r b i ~ n ss e v e r a l c o n m e r c i a l equipn l z n t s h a v e treer: d e v e l o p e d , e . g . f o r t h e BET measuren1ent.s w i t ; - I n i t r o g e n 2 t 77 K , t h e m e r c u r y p o r o s i m e t . r y e t c . However, in the f o l l c i w i n p w e s h a l l p r e s e n t some m e t h o d s b a s e d o n t h e o r e t . i c a l snrl e x p e r i m e n t r l f i n d i n g s which are convenient, f o r i n v r s t i g a t i n g t h e c o n n e c t i o n between t h e c a r b o n s t r u c t u r e and the adsorption p e r f o r m a n c e : A d s o r p t i o n o f b e n z e n e and e v a l u a t i o n of isotherms t h r o u g h t h e D u b i n i n - K a d u s h k e v i c l h e q u a t i o n /l,,’! t h e e s t i m a . t i o n o f immersion h e a t s i n benzene / Z / , t h e o r p t i o n of water at r e l a t i v e p r e s s i i r e s o f h = 13.6 a n d 1 . 0 the zize exclusion 1i q u i d chromatography w i t h t r a c e r s of d i f f e r e n t molecular diameters / 4 / and t h e o n e - p o i n t a d s o r p t . i c n o f n i t r o g e n / s / . ~
492
THE THEORETICAL BACKGROUND OF HETHODS
1. : T h e p h y s i c a l a d s o r p t i o n o f b e n z e n e O n 9 t i v e carbon f r o n l t h e g a s p h a s e i s a s s u m e d t o o c c u r a s VOlume f i l l ng o f t h e m i c r o p o r e s and a l a y e r - b y - l a y e r c o v e r a g e of t h e masopare s u r f a c e .rklePefcr!re t,fIe ~ . ~ l l : . ~3.ii,l ~ ~ - 8.dst:~ lt. i I-! t.ti r 81 i :c Y I I p ~ I:! e x t r a c t e d f r o m t h e t o t a l a d s o r b e d amount a < ! ~ j :
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D u b i n i n e t a l . e v a l u a t e d t h e amount Q a a d s o r b e d p e r u n i t s u r f a c e area from t h e benzene i s o t h e r m measured frJr t h e n o n p o r o u s reference adsorbent / 7 / . we, however e s t i m a t e d t h e s p e c i f i c s u r f a c e 3 r e a Sme o f t h e m e s o p o r e s f r o m t h e a d s c r p t i o n i s o t h e r m s t u d i e d / 8 / . I n c a l c u l a t i o n s of t h e meso p o re s i z e d i s t r i b u t i o n a n d t h e s p e c i f i c s u r f a c e a r e a Sme i t h a s b e e n a s s u m e d t h a t t h e p a r a l l e l - s i d e d s l i t s 3 r e r i g i d and t h e s i z e d i s t r i b u t i o n d o e s not e x t e n d c o n t i n u o u s l y f r o m t h e m e s o p o r e i n t o b o t h t h e m a c r o p o r e the a n d m i c r o p o r e r a n g e . We h a v e u s e d t h e d e s o r p t i o n b r a i n c h of h y s t e r e s i s l o o p o f t h e i s o t h e r m f o r t h e c o m p u t a t i o n . The p r o c e d u re of B . F . R o b e r t s /9/ h a s been a p p l i e d . I n t h i s c o m p u t a t i o n , is a r i g o r o u s a p p l i c a t i o n of t h e c o n c e p t o f s i m u l t a n e o u s which c a p i l l a r y c c n d e n s a t i o n and m u l t i l a y e r a d s o r p t i o n , t h e adsorbed v o l u m e i s f i r s t e x p r e s s e d a s a f u n c t i o n o f p o r e s i z e ; t h e n it. i s c o n v e r t e d t o p o r e v o l u m e . A s t a n d a r d t - c u r v e /lo;/, which r e p r e s e n t s t h e b e n z e n e a d s o r p t i o n or1t.o n o n p o r o u s c a r b o n b l a c k s , has been used f o r c o r r e c t i o n f o r m u l t i l a y e r t h i c k n e s s .
We h a v e f i t t e d t h e a d s o r p t i o n d a t a t o t h e D u b i n i n - R a d u s h k e v i c h - e q u a t i o n /'ll/', u s i n g t h e n o n l i n e a r L e v e n b e r g - M a r q u a r d t method ,/12/: ~ N I I
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However, i f n o t having benzene adsorption d a t a , i t s e e m t o b e p o s s i b l e t o calculate a n e f f e c t i v e s u r f a c e a r e a Qlrr u s i n g the equation: Serr
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x w ( s e e F i g . 1;) msy h e e s t i m a t e d f r o m wo a n d S W : X U = wo/Su. w e found l a r g e d e v i a t i o n s from t h i s r u l e f o r s t r o n g l y However, itxidised s a m p l e s , s i n c e t h e y still y i e l d t h e t y p i c a l T y ~ e V isotherms b u t t h e isotherms are markedly d i s p l a c e d towards lower i e l a t i v e p r e s s u r e s . T k i e r e f o r e i n t h i s work o n i y c a r b o n s w i t h n o t t01; e x t r e m e s u r f a c e o x i d a t i o n s t a t e a r e i n c l u d e d . A s i n d i c a t e d i n Fig. 1, t h e e s t i m a t e d m i c r o p o r e v o l u m e s a n d specific surface areas are i n most c a s e s h i g h e r t h a n t h o s e from benzene a d s o r p t i o n and d o n o t h a v e t h e same t r e n d . A m a j o r prciblem i n t h e a n a l y s i s information concerning o f w a t e r a d s o r p t i o n d a t a is t h e l a c k o f t h e e f f e c t o f s u r f a c e h e t e r o g e n i t y on t h e c a r b o n - a d s o r b a t e i t is t h e r e f o r e no p o s s i b l e t o o b t a i n i n t e r a c t i o n s /'16/ a n d Another r e l i a b l e v a l u e s w i t h thf: above d e s c r i b e d s i m p l e method. p r o b l e m is, t h a t t h e f i r s t a s s u m p t i o n i n v o l v e s t h e a p p l i c a t i o n of t h a t t h e p o r e s a r e f i l l e d by the Gurvitsch rule /17/> i . e . [condensed a d s o r p t i v e o f n o r m a l l i q u i d d e n s i t y . T h i s c a n n o t kie t r u e when t h e p o r e s a r e o f m o l e c u l a r d i m e n s i o n s . F u r t h e r m o r e , i.t is n e c e s s a r y t o i n t r o d u c e t h e c o r r e c t i o n f o r a d s o r p t i o n in mesopores f o r o b t a i n i n g t h e real v a l u e s of t h e micropore parameters . T h a t ' s why t h i s p r o b l e m n e e d s f u r t h e r s t u d y . : The s i z e e x c l u s i o n c h r o m a t o g r a p h i c m e a s u r e m e n t s w i t h t r a c e r s of d i f f e r e n t m o l e c u l a r d i a m e t e r s g i v e i n f o r m a t i o n on t h e d i f f e r e n t i a l p o r o s i t y , i . e . on t h e p o r e d i a m e t e r d i s t r i b u t i o n / 4 / . T h e e v a l u a t i o n of t h e r e t e n t i o n times t R , i y i e l d s t h e p o r e volume , a v a i l a b l e t o t h e r e s p e c t i v e tracer i:
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Wa u s e d a c e t o n e a s c a r r i e r a n d t h e t r a c e r s b e n z e n e , e t h y l b e n z e n e , ! i r x y ? b e n z e n e a n d d e c y l b e n z e n e r t h e i r d i a m e t e r s were t a k e n f rom H a l a s z a n d V o B t e l ./18,/.A f t e r c a l i b r a t i o n w i t h t e x t u r e d a t a o b t a i n e d by b e n z e n e a d s o r p t i o n o n t o c a r b o n TVAX a n a v e r a g e d e q u i l i b r i u m c o n s t a n t Ki is e s t i m a t e d f o r e a c h of t h e t r a c e r s i . TVAX h a s b e e n u s e d a s a t y p i c a l c a r b o n w i t h a n a v e r a g e d e v e l o p e d m i c r o p o r o u s a n d m e s o p o r o u s s t r u c t u r e . The r e s u l t s f o r 5 c a r b o n s are shown i n F i g . 1 . 5. : A very convenient t o o l f o r rapidly characterizing adsorbents i s t h e one - p o i n t a-dsorption of n i t r o g e n / 5 / . However, in m i c r o p o r e s t h e a d s o r p t i o n d o e s n o t occur by monolayer completing b u t t h r o u g h voliume f i l l i n g . T h e r e f o r e , n o a b s o l u t e v a l u e s b u t a r a n k of a d s o r p t i o n c a p a c i t y may b e o b t . a i n e d .
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f i . v s c a r b o n s a r e i n c l u d e d c o n s i s t e n t l y i n al: investigatian-. I n 'l'ahle 3 , t i 5 0 j i s t h e h a l f t i m e o f t h e p h e n o l tlre3iitiirclugr! c u r v e , b i s t h e bed d e n s i t y and t < 5 U j / G are t h e r r e c t e d f o r differences i n d e n s i t y b r e a k t l i r o i ~ g hh a l f t i m e s , The b r e a k t h r o u g h h a l f times a r e taken a s a measure f o r t h e adsorptioi-t equilibrium constants.
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fron! p h e n o l i s a t h e r m m e a s u r e m e n t s i F i g . 3 ) i s t h e s3n:e as from d y n a m i c e x p e r i m e n t s . WRK i s a w a t e r g u r i f i c a t i o r ! c a r b o c made from h i g h t e m p e r a t u r e l i g n i t e coke,, w h i c h !]as t h e l o w e s t ad-,.,4rpLion c a p a c i t y i n a c c o r d m c e w i t h i t s h i g h pheriol n u m b e r . Dynamic measurenien?s h a v e r i o t h e r r . p e r f o r m e d w i t ! i W2E. A?l
methods of c h a r a c t e r i z i n g t h e psrouz s t r u c t u r e mierep-re vg3lumes and c i f i c s u r f a c e areas), w i t h tire e x c e p t ior? o f the w a t e r a d s c r p t i o n a t r e l a t . i v e p r e s s u r e s o f h = O . S a n d 1 .U, g i v e t h e same t r e n d s b u t . n o t e q u a l r e s u l t s . T!je simplest methods f o r , w i t h s m a l l e s t . expense i n t i m e aind s u r f a c e area e s t i m a t i o n s q u i p m e r t t , a r e the i m m e r s i o n c a l o r i m e t r y w i t h b e n z e n e .r!d t h e one -. p o i n t a d s o r p t i o n o f n i t r o g e n a t 77 K . 2 0 t h m e t h o d s g i v e t k e same t e n d e i n c i e s f o r all c a r b o n s , In c o c p a r i s o n t u t h e s e m e t h o d s , t h e time e f f o r t f o r b e n z e n e adscrl:t i o n measureme:-:t and 1i q u i d c h r o m a t o g r a p h y 1s m u c h h i g h e r , H o w e v e r . t h e r e s u l t s o f t h e I s t % c ? : met.hods h s v r a l s o t h e t e n d e n c i e s b u t f a i l tc. z g r e r a b s o l u t e ly. We t h i n k t h a t d i f f e r e n t a s s n m p t i o n s u n d e r l y i n g the several methods l e s d t o t h e s e d e v i a t . i o n s , e . g . t h e cummorily u s e 1 1 r i W i d : s l i t - p o r e iiiodel may n o t b e f u l f i l l e d i n e v e r y c a s e . 8~
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b e s t cavbon i n phenol adsorpt.iun performsnce is F i l t : a s o r h f o i i . i w e d b y BIiT a n d AG 3 , w h e i - e a s Hy 71 a n d WPK ( f r o n ; t h e i i i l i b r i u n i i s o t h e r m f o r t h e p h e n o l f r o m a q u e o c s s o l u t . i c ~ n , 5:ee e 3.t. t h e l o w e r e n d range . F i l t r a s o r b !;as t h e ! a ~ . g e , s t vc:lume wo and 3 ~!i.c-diump o r e w i d t h d . OI-~ t ] i e oc,i)er hand, WKK a n d Hy 71 h a v e s m a l l e r p o r e v ~ l u r n e . A d d i t i o n a i ? y , . WfiK h a s a very large r n i c : r o p o r e w i d t h #:see F i g . 1:. 11-1 s j y n a l ~ l i c m e a s u r e m e n t s T V A X h a s a p p a r e n t l y a s m a 1 . l r ~ p e r f c ; r m a r I c e a:: My 1'' ? - ) L i t . t h i s i s d u e t u its l o w b e d d e n s i t y . w h e r e a s t.he cc!rrec.ted for d e 1 i s i t . y p e r f o r m a n c e ( T a b l e 3 ) i s c o m p a r a b l e w i t h t h e HHT sam;.le. 'l'tie
400
I t is e v i - l e n t , t h a t t h e r a n k c f t h e r e s u l t s c;f p o r e s t ~ - ~ ~ : t u r - e a n a l y s i s , e x c e p t t h o s e o b t a i n e d from t h e w a t e r a d s o r p t i o n method, is t h e same 8 9 f r o m . d y n a m i c e x p e r i m e n t s . 'I'iie p r e s e n t s t u d y s h 0 k . r ~ t h e u s e f u l n e s s o f t h e d e s c r i b e d m e t h o d s for i n v e s t i g a t . i n g t h e c o n n e c t i o n b e t w e e n p o r e s t r u c t u r e a n d a d s o r p t i o n perft2rnlarlg;..j.
498
ACKNOWLEDGEMENT i w i s h t o t h a n k my c o w o r k e r s D r . G . B u n k c , i ~ h e X i . ~ l n gC. h . ( - ; h e n , i n g , E , ~ h i ~ -a nj d ~ ~ r s f. i . J u n g f o r t h e i r c o n t r i b u t i o n s .
REFERENCES 1 2 3
4
5 6 7
8 9
10
11
M.M. D u b i n i n . H.F. S t o e c k l i , J _ . C o l l . I n t e r f a c e b- c!1., 7.5 (l!380) 34. K . H . R a d e k e , Carbon, 22 i 1 9 8 4 j 473 G . A . A n d r e e v a , N . S . P o l y a k o v , M.M. D u b i n i n , K.M. N i k o l a e v , E . A . U s t i n o v , I z v . A . N . USSR. s e r . c h h. (1981) 2188 G. B u n k e , D . G e l b i n , O e m . E m . S c i . 40 ( i 9 8 5 j 2079 R . H a u l , G . Duembgen, chat^ Ina. T e c.b. 3% ( 1 3 6 0 ) 343 S . J . G r e g g , K . S . W . S i n g , B d s o r w t i o n . S u r f a c e Area a n d Forositv. 2nd A c a d e m i c P r e s s , L o n d o n . 1982 M . M . Q u b i n i n , Carbon, 23 ( 1 9 8 5 ) 373 F . B i l l i g , F . B r i i c k n e r , GroRmann. 8.. L e p p i n . M . , S c h m i d t , D . , Seltmann, U . , Thiede, E . , Tern-. i n p r e s s B . F . R o b e r t s , J . C o l l . I n t e r f a c e S c i , , 23 ( 1 9 6 7 ) 2G6 V. F o n e c , Z . K n o r , S . C e r n y , &&-o.n on cv ' R u t t e r w o r t h s , p . 5 5 8 , London ( 1 9 7 4 ) M . M . D u b i n i n , i n D . A . C a d e n h e a d ( E d i t o r j , -5s in d Membrane--SciencE, V o l . 5 , p p . 1 - 7 0 . Academic Fress, New Y o r k , ( 1975) W. H . P r e s s , B . F . F l a n n e r y . S . A . T e u k o l s k y , W. T . Vetterling, cipes. Cambridge U n i v e r s i t y F r e s s , C a m b r i d g e ( 1 9 8 8 ) p p . 5 2 5 -528 H . F . S t o e c k l i , L z v . A . N . USSR. s e r . c ( 1 9 8 1 ) 62 F . E . B a r t e l l , R . M . S u g g i t t . J . Phy.s. Chela_,, 58 ( 1 9 5 4 ) 3 6 L . R o b e r t , U.S o c . U.F r w., ( 1 9 6 7 ) 1 4 7 F . B r i i c k n e r , R . S . V a r t a p e t j a n , Chem. T e c h n , , i n p r e s s L . G u r v i t s c h . J . PhS O C .fiu.s.s-> 47 ( 1 9 1 5 ) 805 J . Halasz, P . V o g t e l , A n g c w - L - E S L l , 19 ( 1 9 8 0 ) 24 A. S e i d e l , E . T z s c h e u t s c h l e r , K . H. Radeke, D . G e l b i n , C h e 0 . E n s 3LLL, 40 119851) 215 G . R e s c h k e , K . H. f i a d e k e , E . G e l b i n , Ckiem. En ,b, S c i . , 4 0 (1986) 549
a.
u.
-
12 13 14 15 16 I7
18 19 20
3
m.
.?
F. Rodriguez-Reinoso et al. (Editors),Characterization of Pororrs Solids ZI 0 1991 Elsevier Science Publishers B.V., Amsterdam
499
MERCURY POROSIMETRY OF POROUS GLASS AND ACTIVE CARBON PRELOAUED
WITH N-DECANE OR WATER
H. Lentz and Y. Zhou*) Universitat-GH Siegen. Fachbereich 8. Postfach 101240. D-5900 Siegen (FRG)
ABSTRACT The possibilities of high-pressure mercury porosimetry for the investigation of preloaded porous solids a r e demonstrated using a mesoporous glass and a micro-porous active carbon preloaded with n-decane or with water. The volume of pores partially loaded with a non-interacting liquid decreases linearily with t h e increasing preload. Special interactions e.g. in t h e system porous glass and water a r e indicated a s a deviation of such regular behavior. If t h e pore radius i s calculated at a constant contact angle, t h e radius will formally increase with increasing preload. Hence a smaller contact angle has to be assumed for t h e solid preloaded with liquid in o r d e r to explain t h i s paradoxical result.
INTRODUCTION
Porous solids have been investigated with completely empty pores or - in o r d e r to s t u d y t h e s t a t e of t h e filling liquids
-
with completely filled pores.
However, in practice t h e r e a r e numerous examples of partly-filled
porous solids.
An investigation of these systems may also contribute t o a n understanding of t h e properties of adsorbed phases. The facilities of high-pressure mercury porosimetry for t h e investigation of preloaded porous solids will be demonstrated using mesoporous glass and rnicroporous activated carbon preloaded with n-decane o r with water (ref. 1). APPARATUS The a p p a r a t u s used w a s a non-commercial porosimeter which enabled u s to make accurate measurements between 0.4 and 2000 bar corresponding to a pore radius between 2
.
10'
and 3.6 nm. The porosimeter consists of a steel cylinder
and a piston forced into t h e cylinder by a r a m (ref. 2 ) . The p r e s s u r e and ttie volume change were measured accurately by a s t r a i n gauge and b y the displacement of t h e piston respectively. Up to a p r e s s u r e of 5 bar t h e mercury was forced into t h e porosimeter by a n air pump and t h e amount of mercury was determined accurately by a balance (ref. 3). Fig. 1 shows schematically t h e
*)
present address: Dr. Zhou, Yaping Si-Ji-Zun / 13-3-201 Tianjin University Tianjin / China
500
--------- 1 Vacuum
,
r--I
L_ _ _ _ _ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _J
Fiq. 1. Mercury porosimeter (Schematic), u p p e r part: p r e s s u r e from 0.4 to 5 bar, bottom part: p r e s s u r e u p t o 2000 bar 1 ) High-pressure vessel, 2 ) Piston, 3 ) Ram, 4 ) Support of ram, 5 ) Displacement indicator, 6 ) Strain gauge, 7 ) Electronic for strain gauge, 8) X-T-recorder, 9 ) Valve, 10) Balance, 11) Mercury storage vessel, 12) PVC-tubes, 13) Bourdongauge, 14) Vacuum meter, 15) Valve, 16) Needle valve, 1 7 ) Safety valve.
a p p a r a t u s in some detail. The measured p r e s s u r e P w a s used to calculate the pore radius r by t h e Washburn equation (ref. 4 )
The values taken for t h e surface tension and t h e contact angle were 0.48 Nm-I and 1400 respectively. The reproducibility of t h e measurements i s f 0.5 %. An estimation of the accuracy is difficult. However, t h e comparison with o t h e r methods indicates
2 5 % for t h e total pore volume and f 12 % f o r t h e pore radius.
For sorption measurements a volumetric method w a s used. In a thermostated constant volume of nearly 300 cm3 the mole numbers of different gas fillings have been determined by a n accurate piezo resistive p r e s s u r e gauge. POROUS MATERI.4LS The porous materials used were characterized b y adsorption and desorption measurements with nitrogen in a constant-volume-apparatus
and b y titrat.ion
(MotLlau-Fisher (ref. 5 ) ) with t h e same liquids as used later f o r the preloading u p to the complete filling of the pores.
501
0.8
1
1.2
1.6
1,4
13 [g r
Fig. 2. Pore size distribution of mesoporous glass 1: Sorption 2: Porosinietrv The mesoporous glass (CPG-10 240 A from Fluka) should have a pore volume of 960 mm"g-1
(Fluka); we measured 986 mm3g-l
(porosimeter u p t o 2000 b a r ) ,
990 rnr11~g-l (final sorption at P/Po = 0,98), 1040 mm3g-l
(titration with
n-decane). The pore radius should be 12.1 nm a n d w a s measured t o be 17.1 nm (porosimeter, 0 = 140°) and 15.5 nm (desorption 6,7) as demonstrated in Fig, 2. The surface a r e s should be 88.1 m2g-I
( B E T ) , 121 m2g-I
and w a s measured t o be 98.3 m2g-1
(Dubinin-Kaganer) and 116 m2g-l (porosimeter (ref. 8 ) ) .
The active carbon (Chevron) h a s a total pore volume (ref. 4 ) of 1150 mmsg-1 (n-decane or benzene) or 940 mm3gg-l (water). The porosimeter can measure 690 mm3g-l and the sorption of nitrogen (refs. 9, 10) results in 710 mm3g-1. The pore radii rarige between 0.38 and 8 nm with a peak a t 0.4 rim as calculated
from nitrogen adsorption (Y).The surface a r e a b y g s s adsorption measurements i s
502
1750
37
n1Lq-I
(Dublnin-Kaganer) a n d t h e s u r f a c e wetted by mercury (ref. 8 )
IS
mLg-1.
4000
-
n -..
m
G
5
3000
L
01
? 9 a 2000
1000
-I
10
10'
102
103
105
10' rlnm)
Fig. 3. Pore size distribution of activated c a r b o n 1: Sorption 2: Porosimetry PRELOADING The porous solids have been preloaded with liquid b y esposing t h e material t o t h e v a p o r of t h e boiling liquid o r by wetting t h e material i n t h e liquid a n d removing t h e liquid partially by heating i n a d r y chamber. Both methods dive t h e same results. The f i r s t one was mainly used for small amounts of preload a n d t h e partly-drying-method
w a s used f o r high preloads.
R E S U L T S AND DISCUSSION a ) Mesoporous glass
Fig. 4 s h o w s a plot of t h e experimental points of t h e pore volume as function of t h e n-decane load f o r mercury intrusion in the mesoporous glass. The e s t r u s i o n ( n o t shown i n Fig. 4 ) shows a h y s t e r e s i s in p r e s s u r e b u t releases t h e i n t r u d e d mercury almost CGmpktely.
503
160[
I40C 1701 1 ooc
BOO
600
400
/ 200
0 10‘
1O2
10’
JOY
los r (nml
Fig. 4. Pore size distribution of mesoporous glass with different contents of n-decane 0: 0; 1: 0.083; 2: 0.200; 3: 0.308; 4: 0.520; 5: 0.734 g n-decane/g glass. The pore volume as determined from t h e dashed line in Fig. 4 is indicated by open circles resulting in t h e s t r a i g h t line 1 in Fig. 5. The points measured by extrusion (indicated by c r o s s e s ) deviate only a little from t h e intrusion points. The volume of pores partially loaded with n-decane decreases linearily with the increasing preload and can be calculated from t h e m a s s and t h e density of the liquid a s demonstrated by the dashed line 2 in Fig. 5. The shift in the s t e p s in Fig. 4 corresponds t o a n increasing
pore r a d i u s
calculated a t constant contact angle with increasing preload ( s . Fig. 6). To explain this unrealistic result a change in t h e contact angle has to be assumed. The s t e p of c u r v e 1 in Fig. 4 will be in congruence with t h e s t e p of c u r v e 0 if the contact angle for t h e preloaded glass i s 135O instead of 140°. All f u r t h e r curves of Fig. 4 can then be interpreted as a successive filling of t h e pores. A detailed interpretation of t h e results i s only possible, if better information of the contact angle o r at least i t s change is available.
504 120 0 1100
500
-
~
LOO 300
~
-200
0
0,l
0.2
0.3
0,L
0,s
0.6
0.7
0,8 0,Y
1,0
1J
(g n-decane/gCPGl
Fig'. 5. Pore volume of mesoporous glass as function of t h e c o n t e n t of n-decarle 1: I n t r u s i o n ( 0 ) 2: Calculated from P,V,T-data, x extrusion
(g n-decane/gCPG)
Fig. 6 . Pore r a d i u s of mesoporous glass as function of t h e c o n t e n t of n-decane ( 0 = 140 OC)
505 If water is used as a preload of t h e nresoporous glass, t h e main features of t h e r e s u l t s of t h e non-interacting liquid n-decane remain. However, t h e pore volume d e c r e a s e s u p t o 0.1 g water p e r g mesoporous glass only a little (Fig. 7). This behaviour c a n probably be explained by a t i g h t e r packing of the f i r s t 2 or
3 molecular layers. Also t h e extrusion c u r v e ( 3 in Fig. 71 d i f f e r s widely from the intrusion c u r v e ( 1 in Fig. 7 ) f o r t h e porous glass p a r t l y loaded with water, t h u s
a relatively high amount of mercury is not released from t h e glass - wat,ei. system. b ) Microporous Active Carbon The r e s u l t s f o r preloaded activated carbon will b e described in sonre detail elsewhere (ref. 11) a n d c a n h e r e be summarised only shortly. The measured r e s u l t s of t h e pore volume occupied by m e r c u r j from 0.4 t o
2000 b a r as a function of t h e amount of loaded liquid
c a n naively be compared
with t h e difference between t h e pore volume a n d t h e volume occupied b y t h e
0
0.1
0.2
02
0.4
5.5
0.6
0,7
0.8
0.9
1.0
1.1
1.2
( g water / g CPG
I
Fig. 7. Pore volume of mesoporous glass as function of t h e water content. 1: Intrusion 2: Calculation from P,V,T-data 3: Extrusion
506
:oo
\
'
\ \ \ \
n
n7
V,L
n~ ",-
nh ",V
n R ",-
in t,"
1.,L.
I.,?
1. 6 I-
(g waterlg carbon I
0
0.2
0,4
0,6
0,8
1to
12
(gn-decane/g carbon1 Fig. 8. Pore volume (0.4-2000 b a r ) of microporous activated c a r b o n as function of t h e water ( 0 ) or n-decane (x) content. Total pore volume: water; n-decane 1: I n t r u s i o n 2: Calculated from P,V,T-data
507
liquid a t this temperature and a p r e s s u r e of 2000 bar (Fig. 8). There is a large deviation due to the fact t h a t t h e total pore x*olume of the microporous actix7ated carbon i s larger than t h e pore volume determined by mercury u p to 2000 bar.
A preload with n-decane and with water leads to similar results. Thus t h e r e is no indication of special interaction between liquid and solid phase.
Obviously the liquid occupies f i r s t t h e small pores outside the measuring range of the porosimeter. I n the pore range covered by t h e instrument t h e hehaviour is regular and can be predicted. ACKNOWLEDGEMENT
We thank the Deutsche Forschungsgemeinschaft and t h e Fonds d e r Chemischeri Industrie for financial support. REFERENCES 1 1'. Zhou, Thesis, Siegen, 1989. 2 G. Holzel and H. Lentz, High-Temp.-High Pres., 12 (1980) 113-116. 3 K. Becker, H. Lentz, E. Hinze, G. Nover and G. Will, Ber. Bunsenges. Phys. Chem., 90 (1986) 833-838. 4 E. W. Washburn, Phys. Rev., 17 (1921) 273-283. 5 A.Y. Mottlau and N.E. Fisher, Anal. Chem., 34 (1962) 714-715. 6 A. Wheeler, in: Catalysis 2: Fundamental Priciples, Reinhold, New York, 1959. 105-165. 7 S.J. Gregg and K.S.W. Sing, @sorption, Surface Area and Porosity, 2nd. Ed., Academic Press, London, 1982. 8 H.M. Rootare and C.F. Prenzlow, J. Phys. Chem., 7 1 (1967) 2733-2736. 9 S. Brunauer, R . S L Mikhail and E.E. Bodor, J. Colloid Interface Sci, 24 (1967) 451-463. 10 S. Brunauer, Z. Phys. Chem. N.F., 64 (1969) 54-63. 11 Y. Zhou and H. Lentz, in preparation for "Carhon".
This Page Intentionally Left Blank
F. Rodriguez-Reinosoet al. (Editors), Characterization of Porous Solids I1 0 1991 Elsevier Science Publishers B.V., Amsterdam
SORPTION OF HYDROCARBONS IN SILICALITE-1 AND NaY ZEOLITES
J.A. Hampson, R.V. Jasra and L.V.C. Rees Physical Chemistry Laboratories Imperial College of Science and Technology and Medicine London SW2 2AY INTRODUCTION The separation of binary gas mixtures by pressure swing adsorption (PSA) is becoming more widely used as a clean, efficient method in its own right, but as energy becomes more expensive it will become even more widely used for economic reasons. In the design of a PSA system it is essential to use an adsorbent which has optimum performance, both equilibrium and kinetic, for the specific binary mixtures to be separated. Zeolites could be excellent adsorbents for many PSA systems as they are so easily modified to produce the required performance characteristics. The cations in the zeolite channels, for example, can be easily exchanged to increase/decrease the electric fields present in the channels: the Si/Al ratio of the zeolite can be readily changed to give increased/decreased cation densities in the channels: the zeolite framework can be chosen to give the optimum channel dimensions to provide the required adsorbent/adsorbate intraction energies. However, the literature contains little information on the effects of such modifications on the adsorption of binary mixtures. The results to be reported in this contribution are part of a large programme designed to establish the preferred zeolite surfaces for the separation of n-hydrocarbons from branched hydrocarbons and their unsaturated counterparts. These studies are, of course, also of fundamental significance in the study of adsorbent/adsorbate interactions. EXPERIMENTAL The adsorbents used in this study were silicalite-l (l), the pure silica analogue of ZSM-5 (2), containing only trace quantities of aluminium and NaY zeolite with the unit cell formula of Na,,[ (A10,)56(SiOz),36] 260H,O The adsorbates usedwere ethane, propane, ethene and propene supplied by ARGO International with purities of at least 9 9 % . The sub-atmospheric sorption data have been obtained using an isosteric method originally developed by Bulow et a1 ( 3 ) . The apparatus used in the present studies is fully described by Graham et a1 (4). The high pressure sorption data were collected using a Sartorius electronic high-pressure ultra-microbalance, model S3D-P. RESULTS AND DISCUSSION The isosteres for ethane, ethene and propane sorbed in NaY are shown in Figures 1, 2 and 3 respectively. The linearity of The isosteres is excellent over the temperature range covered, 1.e. 15-5OoC, and they could be extrapolated over a wider temperature range with confidence. The isotherms calculated from these isosteres at 25OC are given in Figure 4 and the heats of
509
510 12
Fig. 1. Ethane/Na-Y isosteres. Sorbate loadings (in mmol/g): (1) 0.0947; (2) 0.1639 (3) 0.3038; (4) 0.4238; (5) 0.5239; (6) 0.6062 (7) 0.6984; (8) 0.8867; (9) 1.0500; (10) 1.2980 Po = 1 Pa
11
10
9
\ a
-
8
z
7
6
5
311
3 3
3:2 10311
3 4
K-I
8
Fig.2. Ethene/Na-Y isosteres. Sorbate loadings (in mrnol/g): (1) 0.0256; (2) 0.0612; (3) 0.0972; (4)0.1366; (5) 0.1967; (6) 0.2701; (7) 0.3697; (8) 0.4580; (9) 0.5448 Po = 1 Pa
7
6
-a
, a
z
5
4
3 1
3.3
3 2
103/T
3 4
K-'
Fig.3 Propane/Na-Y isosteres. Sorbate loadings (in rnmol/g): (1) 0.3106; (2) 0.3524; (3) 0.3939; (4) 0.4362 (5) 0.5025; (6) 0.5865; (7) 0.7186; (8) 0.7878; (9) 0.9098: (10) 0.9842 Po = 1 Pa
8
I
as a -
z
7
6
3:1
3.2 I03/T
3.3 K-'
3.4
511 3
JI 2
+.
Propane
#.
Ethene
*.
Ethane
rn
n
0 E \
m 01
:
1
U
0 10
0
20
36
50
40
P r e s s u r e / kPa
Fig. 4. Sorption isotherms in Na-Y at 25°C
40
38
34
--
32
3
30
E 0
\
=.
I
28
s,
Propane
I.
Ethene
if.
Ethane
26
24
22
20 0
I
2
c o v e r a g e / mmolg-'
Fig.5. Isosteric heats of sorption in Na-Y
6
512
adsorption obtained from the slopes of the isosteres are shown in Figure 5. Because of the high quality of the isosteric data the isotherms in Figure 4 are accurately defined. As expected the sorption of propane is much greater than that of ethane at 25°C. The initial slopes of these isotherms are linear within the experimental error of the data. It is interesting to note that the ethene isotherm at 25°C is almost coincident with the propane isotherm at lower equilibrium pressures indicating a balance in the sorption potential of a double-bond and a CH, group. However, the heats of adsorption in Figure 5 show significant differences between the sorption energies of ethene and propane. Both ethane and propane show heats of adsorption in Figure 5 which increase with increasing loadings due to sorbate-sorbate interactions over the range of 0-5 molecules per supercage covered in these measurements. This increase is only -2 kJ mol-’
for ethane but is -9 kJ mol-‘ for propane. The smaller ethane molecule seems to be able to detect some heterogeneity in the sorption sites of the NaY supercages at very low loadings (<0.3 molecules per supercage) which is not seen by the larger propane molecules. This heterogeneity was also detected when nitrogen was used as the sorbate. There is a suggestion of a decrease in the heat of sorption of propane at loadings in excess of 4 molecules per supercage. The heat of sorption of ethene with increasing coverage in Figure 5 shows a somewhat different behaviour from the corresponding heats of sorption of ethane and pro ane. Firstly the initial heat of sorption is some 14 kJ molPPgreater than that of ethane and some 7 kJ mol” greater than that of propane. Thus the specific interaction energy of the double bond in the high electric fields which exist in the NaY supercages is -15 kJ mol-’ (N.B. making a small allowance for the loss of two H atoms). The increase in the heat of sorption per CH, quoted in the literature for NaY is usually -12 kJ mol”. This figure is only observed at higher loadings in our measurements. The heat of sorption of ethene shows a sudden drop of -1.5 kJ mol-’ at a loading in excess of -1 molecule of ethene per supercage. The NaY used in these studies will have one Na+ ion per supercage sited on 4 ring (111) sites. This cationic site will have a much larger electric field gradient than the four other cationic sites which exist in the supercages of NaY [i.e.(II) sites; where Na+ ions are sited on 6-ring sites], Thus the specific interaction of the double bond seems capable of picking out these site 111 Na+ cations. Isosteres, of a quality similar to those given in Figures 1-3, have been determined for ethane and propane sorbed in
silicalite-1. From these isosteres the isotherms for these sorbates were calculated and these are shown in Figure 6 which includes the isotherms already presented in Figure 4 for comparison. The isotherms are more curved than for NaY demonstrating stronger sorbate/sorbent interactions because of the closer fit of the sorbate molecules in the smaller channels of silicalite-1 (-0.55nm diameter) compared with the larger supercages of NaY (-0.80nm diameter) These stronger intractions lead, obviously, to enhanced heats of sorption which is clearly
.
513
.. d 0
E 0 \ Iu
rn
m
L
m
>
U 0
0.2 0.1
J 0
0.0
C
1
2
3
4
5
6
7
8
1
0
10
20
30
40
50
Pressure / kPa
Fig.6. Sorption of hydrocarbons in Na-Y and Silicalite-1 at 25°C
40
36
Ethane/Propane !i0/50
34
Coverage / mmolg-'
Fig.7. Isosteric heats of sorption in Silicalite-I
6
514
shown in Figure 7. The heat of sorption of ethane and propane in silicalite-1 is -7 kJ mol-' greater than in NaY at low loadings. The heat of sorption of ethane in silicalite-1 stays sensibly constant with coverage up to 1 mmolg-' (i.e. 1.5 molecules per intersection) and thus differs from the small, gradual increase found with the sorption of ethane in NaY (see Figure 5). It is easier for sorbate-sorbate interactions to occur in the large supercages of NaY compared with the much smaller cavities at the intersections in the channel network of silicalite-1. In silicalite-1 the heat of sorption seems to decrease slightly on increasing the loading from 1.5 to 2.5 molecules per intersection. The sorption of ethane, ethene, propane and propene has been determined in silicalite-1 at pressures up to 25 atmospheres and temperatures between 0% and 70'C. The differences in the sorption behaviour of these sorbates can be seen in the 25% isotherms presented in Figures 8(a-d) . All of these isotherms are quite rectangular in shape with maximum loadings of -2 mmolg-'at 25OC (i.e. -3 molecules per intersection). Figures 8a and 8b show the enhanced sorption potential of propane and propene over ethane and ethene respectively at lower coverages but at higher loadings the silicalite-1 channels and intersections can accommodate a slightly larger number of the smaller sorbate species. Figure 8c shows that the sorption of ethane and ethene is v'ery similar at lower coverages but at higher coverages, higher equilibrium pressures there is a small enhancement in the sorption of the smaller, unsaturated ethene molecules. A similar behaviour is shown in Figure 8d in the sorption of propane and propene but there is a much smaller enhancement in the amount of the smaller unsaturated propene sorbed over the saturated, larger propane at higher equilibrium pressures. Finally, the sorption of an ethane/propane mixture in silicalite-1 was studied in the isosteric system. From the resulting isosteres the isotherm at 25'C was calculated for a constant sorbed phase composition of 49.35 mole % ethane and 50.65 mole % propane. This isotherm may be compared with the corresponding pure ethane and propane isotherms in silicalite-1 in Figure 9 and can be readily seen to be intermediate in behaviour to these two pure component isotherms. While determining these mixture isosteres the composition of the gas phase was determined with the on-line mass-spectrometer at temperatures between 25 and 50% and at four different loadings of the sorbed phase of the same composition as given above. The gas phase compositions are given in Figure 10. The separation = X,Y,/X,Y,, where X,, X, and Y,, YE are the mole factor, Q fractions of propane and ethane in the sorbed and gas phases respectively can be calculated from the data given in Figure 10. For a mole fraction YE of 0.895 for ethane in the gas phase a separation factor a, of 8.75 is obtained while for Y of 0.87 Q is 6.87. These experiments indicate that silicalike-1 is an excellent adsorbent for the separation of ethane/propane mixtures over the temperature range 25 to 5OoC.
515
+
* 0
Ethane Propane
1 ,
-
m
. %
2
W
m m L
W 0 >
o Ethene
U
I
# Propene
'ressure/kPa
+ Ethane
o Ethene Pressure/ kPa
-
-
m
P . W
m m
,
L W
0 >
*
Propane # Propene 0
C.T--. $00 200
300
do
-500
Pressure/kPa
Fig.8 High pressure sorption isotherms in Silicalite-l at 25°C
516 2
+
Ethane
o
Propane/Ethane
x
Propane
1
50/50
-
0
10
I
20
30
Pressure / CPa
Fig.9 Sorption isotherms in Silicalite-1 at 25°C
0.30 m D
c a m O1
0.63
c 4
=, r 0
2
0.88
u
c)
LL
r
0.87
0.86,
30 Temperature
40 OC
Fig.10. Mole fraction of ethane in gas phase as a function of temperature. Sorbed phase 50 mol% ethanelpropane sorbed in Silicalite- 1
517
At 25OC Figure 10 shows that the separation factor is nearly independent of loading in the range covered by the experiments varying only between 8.30 and 8.75. The isotherm at 25OC in Figure 9 for the mixture is only slightly curved up to loadings of -50% of that found at high pressures (see Figure 8a). In a PSA separation of such a mixture operating, say, between 1 atmosphere and vacuum Figure 8a shows that the propane isotherm is too rectangular at 25OC for an ideal PSA separation process. However, Figure 10 shows that the separation factors decrease only slightly with increasing coverage from 7.5 to 6.9 at 5OoC. These separation factors are still perfectly adequate for PSA separations and at this higher temperature the propane isotherm is now less rectangular and more suitable for the PSA method. CONCLUSIONS The preliminary results presented in this paper indicate that silicalite-1 could be used for the separation of ethane/propane mixtures, although the temperature may need to be raised to 5OoC to give more ideal performance. The results obtained also suggest that ethene/propene mixtures could also be separated in a similar manner with silicalite-1. NaY zeolite seems, also, to be capable of giving excellent separations of the above mixtures and would seem to be the better sorbent for the separation of ethane/ethene and propane/propene mixtures. REFERENCES 1.
E.M. Flanigen, J.M. Bennett, R.W. Grose, J.P. Cohen, R.L. Patton, R.M. Kirchner and J.V. Smith, Nature 271, 512 (1978)
2.
G.T. Kokotailo, S.L. Lawton, D.H. Olson and W.M. Meier, Nature, 272, 437 (1978).
3.
M. Bulow and P. Lorenz, "Fundamentals of Adsorption 11" (Ed. by A. Liapus) Engineering Foundation, New York, USA 1987, p.119.
4.
P. Graham, A.D. Hughes and L.V.C. Rees, Gas Sep. Purif. 3 , 56 (1989).
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F. Rodriguez-Reinosoet al. (Editors),Characterization of Porous Solids II 0 1991 Elsevier Science Publishers B.V., Amsterdam
519
HOW CAN AN ADSORPTION SYSTEM SHOW PHASE TRANSITION A case study on the adsorption of p-xylene in ZSM-5 Dongfeng Pan' , Alfons B. Mersmann Departmenl B of Chemical Engineering, Technical University of Munich POB 20 24 20, D-8000 Miinchen, FRG.
ABSTRACTS The intermediate plateau and the hysteresis loop of p-xylene/ZSM-5 isotherm are explained by using recent crystallographical results of adsorbate-loaded crystals. Two adsorption mechanisms - the occupation of low energy adsorption sites and the crystal lattice mediated interaction between the adsorbate molecules - are found to be responsible for the phenomena. Based on these considerations, qualitative model calculations are carried out and the results agree with their experimental counterparts. INTRODUCTION For physical adsorption systems which are concerned in this study, there are two classical answers to the title question: Capillary condensation and strong adsorbate-adsorbate attraction. The first case is often encountered in macroporous materials. The transition pressure lies near the saturation region; the second case is well known from low temperature adsorption experiments, where -wlkT is large, w, k , T being adsorbate-adsorbate interaction energy, Boltzmann constant and temperature, respectively. Their isotherms have the characteristical S-shape. The common background of these systems is that the adsorbent does not change its structure. Recent crystallographical investigations on adsorbate-loaded zeolite crystals [I ,2,3] indicate that a third answer is possible:
The adsorbate induced structural changes of the adsorbents induces in turn a phase transition of the adsorbate phase. This implies that the most basic postulate of the physical adsorption adsorbents
-
-
the inertness of
is not valid in some cases, especially in connection with high silicious ZSM-5 or
silicalite, where the electrostatic effects are small because few exchangable cations are present. Author to w h o m correspondence should be addressed.
520
Two most extensively investigated adsorption systems with a phase transition are N*/ZSM5 [4,5] and p-xylene/ZSM-5 [3,6,7]. Because no crystallographical details concerning the adsorbate loaded ZShI-5 crystals are available for the first case, this study is concentrated on the second case. The Ar/ZSM-5 [5] and benzene/ZSM-5 [9,8] also show the similar isotherm behavior.
STRUCTURE CHANGES OF ZSM-5 CRYSTALS The as-synthesized ZSM-5 crystal lattice consists of staggered layers [lo], possesing an orthorhombic structure. After calcination and at temperature below critical point of about
340K, the crystals assume a monoclinic phase [ l l ] . At the beginning of the adsorption, the p-xylene molecules first occupy the intersections between straight and sinusoidal channels [ 121. The reason is purely geometrical: Comparing to other places in the channel system, the intersection is just large enough to contain one p-xylene molecule (Kinetic diameter of p-xylene: 5.S5w [13]), providing a strong attractive interaction between adsorbent and adsorbate. At a p-xylene loading between 3 and 4 molecules per unit cell, the crystal lattice again undergoes a structure change, accompanied by deforming the rather cyclic pore shape of the sinusoidal channels to a more elliptical one [3]: from 5.891"ix 5.35A to 6.37Ax 4.76A. This transition is induced by the occupation of some sinusoidal channels. These sites have a higher potential energy for they are so close to the adsorbent atoms that the repulsive interaction takes place. Therefore the occupation probability is small according to the Boltzmann exponential factor. Because of geometrical reasons - the p-xylene molecule is flat, preferring a slit like pore - the occupied channel sections will be stretched to form a flat shape. Through the layer structure the stretching is propagated to the 4 neighboring channel sections [3]. From the energetical point of view the ZSM-5 crystal has some metastable phase which can be reached by changing the temperature or the adsorbate loading. The prerequisite of the deformation is that the two phase are energetically close enough to each other. The new pore structure provides now a better accommodation for the adsorbate molecules. In other words, the potential energy of these adsorption sites is lowered after a neighboring site of the same type is occupied. The more the neighboring sites are occupied, the larger the deformation will be, reducing more and more the original repulsion between atoms of the crystal and the adsorbate molecules. The lowering of the potential energy is assumed to be approximately proportional to the number of sites occupied. In an effective sense the energy difference before and after the occupation of the neighbors can be interpreted as a result of a long range inleraction between the adsorbate molecules. Therefore it can be called as a crystal lattice mediated interaction.
MODEL The crystallographical findings described above indicate two mechanisms which play the most importante role in the p-xylene adsorption on ZSM-5: The adsorption of p-xylene in the channel intersections and
521 0
the adsorbate induced structural changes, which in turn affect the adsorption.
For simplicity the first mechanism is modelled by the partition function of Langmuir lattice gas
where NI is the number of molecules adsorbed, q1 the partition function of an isolated admolecule, M1 the number of sites of first kind. The second mechanism is modelled by the quasi-chemical approximation for a twodimensional lattice gas with the nearest neighbor interaction energy w. As mentioned in the foregoing section,
u)
is used in an effective sense. The partition function Q2(Nz)is well
known in the literatures [14].
If the two mechanisms are not coupled, which means no interaction between the adsorbate molecules occupying different types of adsorption sites, the partition function of the entire system is given by:
C
Q(N) =
Qi(Ni)Qz(Nz)
(2)
NI +Nz =N
N being the total number of adsorbed molecules. Using standard procedures in statistical thermodynamics [14], one obtains
where p is the chemical potential of the lattice gas. It is related to gas pressure by p = po
+ kT l n p
(4)
Since the numbers of sites of type 1 and 2 are equal according t o the structure analysis, there is
N Ni -=(-+-)/2 M All
N2 M2
or
O=-
Qi
+ 02 2
(5)
From equation (3) it follows
K1 P = 1-82
K a p=
81
p-1+202
02( p + 1 - 202)
(7)
with
p
=
41 - 402(1 -
- exp(-w/kT))
This is the model isotherm. p, K1,IC, are the gas pressure, the Henry-constant of sites 1 and 2, respectively. The similarity of this model with two-patch models is evident. Only the physical background is different. Also the derivation of this model can be easily extended t o cases where
522
the coupling term C(Nl, N,) of the two adsorption mechanisms is known. In this case the partition function of the whole system can be written as
Q(W= C
Qi(Ni)Qz(Nz)C(N1,Nz)
N I +Nz =N
Therefore the isotherm is given by the following equations: PO
+
ln p = - a l n ~ ~ -( ~ ~ )
aNi
kT
(9)
aN1
P O + l n p = -alnQz(Nz) - alnC(Nl,Nz)
kT
aNz N = NI
aN2
+ Nz
Using Bragg-Williams approximation [14], the coupling term takes the form
(12)
InC(N1, N,) = -wABNlN2
Such corrections are of interests for quantitative fittings. But it is not importante for this qualitative study. RESULTS AND DISCUSSION Figure 1 shows some calculation results with different values of the model parameters. The model calculation with the parameter value w/kT = -2,
K1
= 100, I(Z = 1 reproduces
the most importante characteristics of the experimental p-xylene isotherm in figure 2 [6]:The intermediate plateau and the hysteresis loop. The finite slope at phase transition point is due to non-idealities of the crystals used in the experiments, which can be treated by the method of Dash and Puff [15]. Since phase transitions are cooperative phenomena, the “cooperation information
”
(In
this case, the information is the existence of neighboring adsorbate molecules) is transported by the interaction energy w. For small eu, entropy effects dominate, distorting the information. Therefore no phase transition occurs for w = -1. The difference of potential energies between the two types of sites Awl2 is reflected in the quotient I(l/I(zl which is given by K1
- = exp(-AwlZ/kT
I<,
-
2w/kT)
(13)
provided the sites have the same physical properties besides the different potential levels. For quantitative fittings the model should be modified to include the non-idealities such as energetic heterogenity, the dependency of the different adsorption mechanisms, the electrostatic effects and the crystal defects. In general the lower the Si/Al ratio of the crystal
is, the less the hysteresis loop and the plateau are pronounced [4,S]. Mostly the effects of these non-idealities could be lumped into an effective “heterogenity function”, therefore most isotherins could be fitted by this procedure.
523
1
Figure 1: Calculated isotherms using the model equations ( 5 ) , (6) and (7) with different sets of parameter.
10 Pressure (Scaled)
Figure 2: Experimental isotherm of p-xylene in ZSM-5/silicalite-l at 313.2K [6]. Open symbols indicate sorption, full symbols desorption. 1.0
M
: 0
E
0-5
D -1
0
1
log(Pressure [Pascal])
2
524
Another aspect of systems with phase transition is the “heterogenity” of the adsorbateadsorbate interaction energy. Because of the complexity of the ZSM-5 crystal lattice the interaction mediated by its deformation can be very complex. The model concept could also be used to explain the isotherm behavior of the system Nz/ZSM-5 and benzene/ZSM-5. In the first case the height of the hysteresis loop is smaller, indicating fewer sites of type 2. From the physical point of view it must be clarified if the crystal lattice plays the same role as for p-xylene/ZSM-5. Since the adsorption temperature of 77K is quite low, it is not sure if the crystal lattice also undergoes a deformation. CONCLUSION Based on crystallographical analysis a new mechanism of adsorbate phase transition is discussed and theoretically modeled. Qualitative comparisons between the model isotherm and its experimental counterpart are carried out. It can be concluded that the hysteresis loop in the p-xylene/ZSM-5 isotherms is caused by a strong crystal lattice mediated interaction between adsorbate molecules, while the intermediate plateau is the result of this interaction and the large energetic difference between two types of adsorption sites.
References [l] C.A. Fyfe, G.J. Kennedy, C.T. De Schutter, and G.T. Kokotailo, J . Chem. SOC.,Chem. Commun., (1984) pp541-542.
[2] G.T. Kokotailo, L. Riekert and A. Tissler, Stud. Surf. Sci. Catal. (1989). [3] H. van Koningsveld, F. Tuinstra, H. van Bekkum, J.C. Jansen, Acta Cryst. B45 (1989) ~~423-431. [4] U. Muller, K.K. Unger, Characterization of Porous Solids, K.K. Unger et aL(Editors), (1988) pp101-108. [5] U. Miiller, K.K. Unger, D. Pan, A . Mersmann, Y . Grillet, F. Rouquerol, J. Rouquerol. Zeolites us Catalysts, Sorbents and Detergent Builders, H.G. I<arge, J. Weitkamp (Editors). (1989) pp625-634. [6] R.E. Richard, L.V.C. Rees, Zeolites, 8( 1988) pp35-39. [7] B.F. Mentzen, F. Bosselet, C.R. Acad. Sci. Paris, (1988) S&ie 11, pp1533-153s. [S] C.-J. Guo, 0. Talu and D.T. Hayhurst, AZChE Journal. 35(1989) pp573-578. [9] H. Thamm, Zeolites, 7(1987) pp341-346 [lo] D.H. Olson, G.T. Kokotailo and S.L. Lawton, J. Phys. Chem., 85 (1981) 13132238-2243. [ll] H. van Koningsveld, J.C. Jansen, H. van Bekkuni, Zeolites, 7(1987) 1313564-568.
[12] B.F. Mentzen. Zeolites as Catalysts, Sorbents and Detergent Builders, H.G. Karge, J. Weitkamp (Editors). (1989) pp477-484. [13] D.W. Breck, Zeolite Molecular Sieves, New York, Wiley. (1974) p636. [14] T.L. Hill: A n Introduction to Statistical Thermodynamics. London: Addison-Wesley, 1962. [15] J.G. Dash and R.D. Puff, Phys. Rev. B. 24(1981) pp295-309.
F. Rodriguez-Reinosoet al. (Editors), Characterization of Porous Solids I1 0 1991 Elsevier Science Publishers B.V., Amsterdam
525
2
dakkmatkariev G.Uel , I s i r i k j a n A.B. 1. I m ; . r ; i t u t e of Chemistry o f U,bek SSii scauemy o f S c i e n c e s , Tashkent. 2. I n s t i t u t e of P h y s i c a l Chemistry of xcauemy o f S c i e n c e s of t h e USSR, iiloscow.
W3srHAcIL' D i f f e r e n t i a l h e a t s o f a d s o r p t i o n p o l a r and a p o l a r s m a l l mol e c u l e s K O , CE OH, GO C , H ( o n powderea z e o l i t e s ) N a X , MaA, MaY , L i Y ,'IVaZSIVi~5 and gili6aPit.e have been s t u d i e d . C o r r e l a t i o n w i t h b o t h a d s o r p t i o n heats on z e o l i t e s i n v e s t i g a t e d and t h e i r c r y s t a l l o c n e m i c a l s t r u c t u r e , m i c r o p o r e s shape arid s i z e have been e s t a b l i s h e d . x d s o r p t i o n of a p o l a r m o l e c u l e s o f hydrocarbons i s t h e most s e n s i t i v e t o c a v i t i e s s i z e , c h a n n e l s s i z e a m winaows i n z e o l i t e . The narrower i s t h e s o r p t i o n s p a c e o f m i c r o p o r e s t h e ,reatel- i s t h e energy of a d s o r p t i o n . I W d O D U C TIOlJ The forms mu s i z e s o f z e o l i t e s p o r e s are s t u d i e d well anu w i t n h i g h p r e c i s i o n (0.01 ~ x n )u s i n g X-ray s t r u c t u r a l methods. iias o r p t i o n o f iloi1poiar m o l e c u l e s (hyarocarbons) i s s e n s i t i v e t o por o s i t y geometric f a c t o r , and t h e narrower a r e a d s o r b i n g p o r e s t h e p e t i t e r i s t h e i r i n t e r a c t i o i i w i t h a d s o r b e n t . 8xchani;e c a t i o n s cail a c t i v e l y i i i t e r t l c t w i t h adsorbeci p o l a r s m a l l molecules. Here t h e chemical n a t u r e of acisorbind p o r e s p l a y t h e main r o l e w h i l e t o Geometric f a c t o r p o l a r uiolecules a r e riot s e n s i t i v e . Owiug t o t h e se f e a t u r e s we call s e p a r a x e l y s t u d y t h e i n f l u e n c e of g e o m e t r i c and chemical f a c t o r s on a u s o r p t i o n - e n e r ~ e t i c c h a r a c t e r i s t i c s o f t n y o t h e r adsorbelits. Z e o l i t e m i c r o p o r e s i n teriiis of a a s o r p t i o n - e n e r g e t i c f a c t o r a r e discrete-holnogeneous a a s o r b e r i t s (1 ); i . e . i n a d s o r b i n g z e o l i t e c a v i t i e s arid chaririels t h e r e a r e few g r o u p s o f hornojeiieous I n terms o f e n e r G e t i c e s c e i i t r e s . Xxchange c a t i o n s a r e such c e n t r e s r e l a t i v e t o p o l a r s i n a l l i;iolecules. More narrow f r a d i i e u t s
526
of porous s t r u c t u r e a r e s u c h c e n t r e s r e l a t i v e t o n o n p o l a r molec u l e s . 'I'hermodyntunic a d s o r p t i o n f u n c t i o n ( e n t h a l p y , f r e e e n c r gy, e n t r o p y , h e a t c a p a c i t y ) v e r s u s degree of z e o l i t e micropores volwne f i l l i n g c u r v e s d i s t i n c t p o i n t s i n which t h e y have s t e p wise d r o p s and f r a c t u r e s which c o r r e l a t e w i t h c r y s t a l l o c h e m i c a l s t r u c t u r e o f t h e s u r f a c e of aGsorbing s p a c e s of z e o l i t e s .
RdSULYS A.W DISCUSSIOIJ Powder z e o l i t e s of Linde firm N a A , liaX, Nay, i o n exchanged forms C a b , LgNak and L i Y , L I N a A , d e a l w n i n i z e a samples of f a u j a s i t e t y p e , NaZSivi-5 anu p u r e s i l i c a s i l i c a l i t e have been s t u d i e d . H20, liii3' CH 3 OH, C 2H5 OH, C O Z Y C 2H 2, , C21ib, C3Ho, n.C7H,4 and C6H6 were used as a a s o r b a t e s . To measure a d s o r p t i o n d i f f e r e n t i a l h e a t c a l o r i m e t e r s similar t o Ilian-Kalve o f our own d e s i g n ( 2 ) and plant-manufactured ones ( 3 ) as w e l l as t h o s e of Yrench f i r m Setarvn (model biS-70) were used. A11 e x p e r i n e n t s were performec. a t 258 anu ~ 0 1A and i n some c a s e s a t 450 K. Owing t o c a l o r i m e t r i c method we were t h e f i r s t who d i s c o v e r e u s t e p w i a e c h a r a c t e r of c u r v e s of d i f f e r e n t i a l h e a t s of water. v a p o r s a d s o r p t i o n on z e o l i t e s IJaA ( 2 ) and NaX ( 4 ) w h i l e a c c o r uint; t o numerous u a t a of s t u d y of t h e same a d s o r p t i o n systems d i f f e r e u t i a l h e a t s of a d s o r p t i o n ( c a l o r i r n e t r i c and i s b s t e r i c ) i s o f monotonous e x p o n e n t i a l l y d e c r e a s i n g c h a r a c t e r i n t h e main f i l l i n g r e g i o n . The l a t e r w a s t h e basis t o c o n s i d e r z e o l i t e s as e n e r g e t i c a l l y nonhomogeneous a d s o r b e n t s . Stepwise c h a r a c t e r of a d s o r p t i o n h e a t c u r v e s i s s p e c i f i e d by s t o i c h i o i n e t r i c i n t e r a c t i o n o f p o l a r molecules o f h20 w i t h c o o r u i n a t i v e l y u n s a t u r a t e d cat i o n s of sodium i n S l l , and S I 1 p o s i t i o n s of t h e s e z e o l i t e s . Discovery o f t h i s e v e n t made i t p o s s i b l e t o d i s p r o v e e x i s t e d concept about z e o l i t e nonhomogeneity and p u t Porwaru new concept about " d i s c r e t e homot;eneity" of t h e s u r f a c e of a d s o r b i n g c a v i t i e s of z e o l i t e s ( 1 ) wliich became t h e b a s i s f o r new d i r e c t i o n i n a d s o r p t i o n - e n e r g e t i c s t u d i e s of s y n t h e t i c and n a t u r a l z e o l i t e s ( 5 ) . The r e s u l t s o f o u r measurements o f d i f f e r e n t i a l h e a t s of w a t e r vapor a d s o r p t i o n on z e o l i t e s lLaA and IlaX a r e g i v e n i n F i t . l ( a a m b ) t o g e t h e i . w i t h t h e d a t a o b t a i n e d by t h e o t h e r s c i e n t i s t s ( 2 , 4 ) which r e p r e s e n t a " f a n r t of monotonous c u r v e s rounG o u r d i s t i n c t l y e x h i b i t e d s t e p curves. For many y e a r s , s t a r t i n l ; from (2,4), c a l o r i m e t r i c a n a l y s i s of s o r b t i o n systems w i t h z e o l i t e s a r e perforineci by u s i n t h r e e d i r e c t i o n s : ( 1 ) - i i i t e r m
521
\
Fib.l.
b
'.
D i f f e r e n t i a l h e a t s of w a t e r vapor a d s o r p t i o n on i J a A ( a ) and IiaX ( b ) z e o l i t e s a t 300 K : 1 and 2-5 ( b ) from (2,4)
.
-
our a a t e s ; 2-6 ( a )
o f m a c r o s c o p i c - t h e r m o u y n m i c a l i n o r d e r t o o b t a i n complete t h e r modynariiic f u n c t i o n s o i a a s o r p t i o n , ( 2 ) - i n terms o f 11ioleculars t r u c t u r a l i n o r d e r t o uetermine c o r r e l a t i o n between therinouynamic c h a r a c t e r i s t i c s m a c r y s t a l l o c h e m i c a l s t r u c t u r e of a d s o r b i i i G s u r f a c e s and ( 3 ) - i n terms of m o l e c u l a r - t e x t u r a l i n o r d e r t o d e t cn.iirie c o r r e l a t i o n be tween a u s o r p t i o n t herniodynaini c char a c t e r i s t i c s arid a d s o r b a t e molecules s i d e ( € o m ) frorn t h e one hand aria aasorbinr; s p a c e s s i z e a n a form ( p o r o c i t y ) on t h e o t h e r hand. I n t h e p r e s e n t r e p o r t t h e s e d i r e c t i o n s w i l l be s u p p o r t e d by r e p r e s e n t a t i v e examples from o u r i n v e s t i ; a t i o n s which a r e o r w i l l be p u b l i s h e d . 1,;easurinl; h e a t s of a d s o r p t i o n a t two t e m p e r a t u r e s ( s e e f o r example ( 6 ) ) we can c a l c u l a t e a d s o r p t i o n ilea t c a p a c i t y depend i n g on f i l l i n & . Out o f a l l f o u r thermodynamic a d s o r p t i o n funct i o n s o n l y d i f f e r e n t i a l values of a d s o r p t i o n f r e e energy can be p r e s e n t i n t h e f o r m of mathematical f u n c t i o n a l aepenuence owing t o o u r method ( 7 ) . Other thermodynaaic f u n c t i o n s a r e p r e s e n t e d as a r u l e e i t h e r i n t h e f o r m of t h e c u r v e s a g a i n s t f i l l i n g o r i n t h e fonn of' a t a b l e d a t a . A l l z e o l i t e s w i t h h i g h s i l i c o n c o n t e n t and uealuminized ones in p a r t i c u l a r ( 8 ) have s t r u c t u r a l d e f e c t s i n t h e form of hydroxyl groups (from 0.15 t o 0.4 mrnol/g arid h i g h e r ) . Hydroxyl kroups a r e r e s p o n s i b l e f o r h i G h i n i t i a l . a d s o r p t i o n h e a t of pol a r m o l e c u l e s , w h i l e w i t h a l c o h o l s t h e y i n t e r a c t cheriiically
528
w i t h fori,iation of t h e s u r f a c e methoxy-groups. Sodium exchaii,e c a t i o n s i n LSiJ-5 z e o l i t e a l s o cause i n c r e a s e d h e a t s of a d s o r p t i -
on i n i n i t i a l r e g i o n of f i l l i n g . A f t e r s t o i c h i o m e t r i c i n t e r a c t i m w i t h Oli-groups or/ana exchange c a t i o n s , water m o l e c u l e s a r e adsorbed on a l l v a r i e t i e s o f h i g h - s i l i c o u s z e o l i t e s w i t h t h e energy c l o s e t o t h e h e a t of vapor c o n d e n s a t i o n , w h i l e e n t r o p y in t h e s e r e g i o n s of f i l l i n g does n o t d i f f e r from e n t r o p y of normal f l u i d . Discovery of energy m a n i f i s t a t i o n of z e o l i t e d i s c r e t e horno&,etleit y which i s e x p r e s s e d i n s t e p w i s e c h a r a c t e r of a d s o r p t i o n h e a t curve s t i m u l a t e u f u r t h e r i n v e s t i g a t i o n s o f t h i s k i n d arid beginn i n g from (2.4) t h e y have become t r a d i t i o n a l f o r us. These i n v e s t i g a t i o n s a l s o g i v e v a l u a b l e i n f o r m a t i o n as i n terms o f macr o s c o p i c s o i n t h e terms of m o l e c u l a r - s t r u c t u r a l and makes i t p o s s i b l e t o determine c o r r e l a t i o n between h e a t s o f a u s o r p t i o n aria number o f homogeneous a c t i v e c e n t r e s .
2:piQ*
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Fig.2.
D i f f e r e n t i a l h e a t s o f w a t e r vapor a d s o r p t i o n a t 300 ( 1 ) and 450 ( 2 ) K on NaA ( a ) , BaX (b) z e o l i t e s . Above-entropy; below-heat c a p a c i t y b o t h i n J/mol*IC.
The most i n t e r e s t i n g examples a r e g i v e n i l l Pig.2-3. Differ e n t i a l h e a t s of w a t e r vapor a d s o r p t i o n on z e o l i t e s N a A (Yig.2, a), NaX (Fig. 2 , b ) and L i Y (Fig. 3 , a ) are c h a r a c t e r i s e u by s h a r p f a l l a f t e r f o r m e t i o n of h i g h e n e r g e t i c c0inplei:es: f o r N a A t h e end o f curve s t e p c o r r e s p o n d s t o t h e f i l l i n g of 3-4 molecules p e r one l a r g e c a v i t y , w h i l e f o r NaX - t o t h a t of 5 molecules p e r c a v i t y . T h i s number o f molecules of H20 p e r c a v i t y i s i n
529
agreement w i t h c o n c e n t r a t i o n s (number) o f sodium c a t i o n s i n pec u l i a r c r y s t a l l o g r a p h i c p o s i t i o n s of a l a r g e c a v i t y . Adsorption h e a t e x t r a p o l a t i o n t o z e r o f i l l i i i g gave similar f o r b o t h z e o l i t e s v a l u e s o f 110 kJ/mol which c o r r e s p o n d s t o the energy of i o n d i p o l e i n t e r a c t i o n of w a t e r molecule w i t h sodium c a t i o n . C a l c u l a t e d d i f f e r e n t i a l v a l u e s of e n t r o p y and h e a t c a p a c i t y on ItaA, TiaX z e o l i t e s a l s o a r e s u b j e c t e d t o s h a r p changes a t t h e above mentioned f i l l i n g s and i i i t h i s c a s e t h e s e changes a r e Itlore e x p r e s s i v e t h a n h e a t v a r i a t i o n , i.e. i f h e a t curve i s of s t e p w i s e c h a r a c t e r t h a n e n t r o p y c u r v e i s polyextreme w i t h minimums itna maximums which correspond t o t h e s t e p s on h e a t c u r v e s , however, r e l a t i v e v a r i a t i o n of ACa a t t h e same I'i1lint;s can be c o n s i d e r e d as phase t r a n s f o r m a t i o n s .
60 c
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Eig.3.
a
-
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-
40
D i f f e r e n t i a l h e a t s of water vapor a u s o r p t i o n a t 2100 ( 1 ) and 450 ( 2 ) K on L i Y z e o l i t e . Above - e n t r o p y , below - h e a t c a p a c i t y b o t h i n J/mol K. d i f f e r e n t i a l h e a t s of water vapor a d s o r p t i o n o n sil i c a l i t e (1 ) and NaSSb;-5 ( 2 ) a t 300 K. Above - e n t ropy o f a d s o r p t i o n i n J/mol K.
P e c u l i a r a t t e n t i o n s h o u l d be p a i d t o LiY-I-120 a d s o r p t i o n system ( F i g . j , a ) . h t t h e f i l l i n g of 2.3 m o l e c u l e s p e r c a v i t y t h e h e a t curve a l s o has g r e a t changes which a r e observed at ads o r p t i o n o f 3 molecules f o r Nab or 5 m o l e c u l e s of H 2 0 per c a v i t y f o r N a X , i . e . L i Y c o n t a i n s 2.8 c o o r d i n a t i o n a l l y g r e a t l y un-
530
s a t u r a t e u lithium c a t i o t i s . ivleanwhile accorciiiig t o t h e d a t a o f Lintie firm i n i t i a l sample o f N a Y c o n t a i n s o n l y 0.8 of a c t i v e l\ia c a t i o n s l o c a l i s e d i n S1 p o s i t i o i i s . T o t a l amount o f exchange c a t i o n s i n M Y i s 6.8 arid 2 i n S , p o s i - t i o n . Thus we can s u g g e s t t h a t i t i s t h e s e 2 c a t i o n s f r o m S1 p o s i t i o n move t o Sill p o s i t i on and a c t i v e l y i n t e r a c t w i t h a d s o r b a t e molecules. S i m i l a r l y t o p r e v i o u s c a s e s e n t r o p y and t h e r m a l c a p a c i t y v e r y g r e a t l y p r e c i s e l y during completion of f o r m a t i o n o f 2.8 of h i g h - e n e r g e t i c acis o r p t i o n complexes H20-Li+ (E'ig. 3 , a ) . A d s o r p t i o n volurries of t h r e e t y p e s of z e o l i t e c o n s i d e r e d above ( t y p e A , X , Y ) a r e c h a r a c t e r i z 2 d by r e l a t i v e l y l a r g e c a v i t i e s (1.2 nm) which a r e connected by oxigen windows (0.4-0.5) f o r t y p e k arid 0.74 run f o r X and Y ones. P e i i t a s i l z e o l i t e s do n o t have s o large m i c r o p o r e s a n d t h e i r a u s o r p t i o n s p a c e s a r e s p e c i i'ieu by i n t e r a c t i n g c h a n n e l s of 0.5-0.6 run. Z e o l i t e NaLSi-5 corit a i n s small number o f exchange c a t i o r i s o f sodium (0.5. mmol/g). These c a t i o n s a r e a c c e s s i b l e € o r i r i t e r a c t i o n w i t h p o l a r inolecul e s o f H 2 0 o r normal a l c o h o l s . Good c o r r e l a t i o n w a s observed between d i f f e r e n t i a l h e a t of v a p o r s a d s o r p t i o n o f H 2 0 , CH,OH and C 0 2 on NaZSM-5 and i t s c r y s t a l l o c h e m i c a l s t r u c t u r e . Heat c u r v e s a r e d i s t i n c t l y s t e p w i s e . However, d i f f e r e n t i a l h e a t s o f w a t e r vapor a d s o r p t i o n or1 NaZSM-5 (Pig. 3 , b ) a r e d i f f e r e n t as i n v a l u e s s o i n t h e c h a r a c t e r . o f cleperiderice 011 f i l l i n g d e g r e e f r o m h e a t s of w a t e r vapor a d s o r p t i o n 011 tile z e o l i t e s w i t h iiigh s i l i cot1 conteiit. i n i t i a l h e a t i s e q u a l Lo 100 kJ/mol and i s airnos-t; s i z i l a r t o t n e energy of i i i t e r a c i i o i i w i t h n o n l o c a l i z e d c a t i o i i s o f ?la i r i z e o l i t e s MaA aiid NaX. Xwiiber of h i g h l y a d s o r b a b l e w a t e r m o l e c u l e s b e i n g i n c o r r e l a t i o n w i t h t o t a l coriteiit o f N a c a t i o r i s (about 0.5 inmol/g ) i n NaZSIJ-5. L i i i e a r d r o p o f h e a t o v e r t h e eilt i r e s e c t i o r i o f forinatiori of h i g n l y e n e r g e t i c i o n - u i p o l e coiripl e x e s i s s p e c i f i e d by g r e a t r e p u l s i v e f o r c e ciisplaceineiit of ida c a t i o i i s from t h e i r e q u i l i b r i u m p o s i t i o n s . From 0.5 t o 1 .S nmol/g a d s o r p t i o n h e a t i s c o n s t a n t l y a t t h e l e v e l of 5'1 kJ/Itiol, b u t wheri t h e f i l l i n g i s 2.2 iiunol/g i t s h a r p l y d r o p s t o 46-47 kJ/mol and becomes c l o s e t o c o n d e n s a t i o n h e a t . Tnus, 3 iriore H20 rnolecul e s w i t h c o n s t a n t e n e r g y a r e added t o i o n - d i p o l e complex and t h i s p r o c e s s i s n o t accornponied wi.th complex phenomena. T e t r a aquacomplexes formed round Na c a t i o n a r e l i k e l y t o be p l a c e d i l l t h e i n t e r s e c t i o n s of s t r a i g h t and z i g z a g c h a n n e l s . F u r t h e r a a s o r p t i o n up t o f i l l i n g o f I.> mmol/g o c c u r s as i t was s t a t e d
531
above, with n e a t r e l e a s e equal t o conuensation n e a t b u t e x c e e a i n g i t by 2-3 kJ/mol.
(44 kJ/rnol)
E'urtner a a s o r p t i o n a o e s n o t a t
a l l d i f f e r from a a s o r p t i o n h e a t on c a t i o n - f r e e s i l i c a - s i m i l a r s t r u c t u r e o f z e o l i t e NaZSid-5. D i f f e r e n t i a l h e a t s of a d s o r p t i o n of o t h e r p o l a r molecules ( G O 2 , CH OH, C H OH) a l s o h a v e s t e p form.
3
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2
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F i g . 4. D i P f e r e n t i a l h e a t s of m e t h a n o l v a p o r a a s o r p t i o n oti s i l i c a l i t e (a) arid 1uaZSai-5 (b) a t 300 K. Above e n t r o p y o f a a s o r p t i o n i n J/mol K. I n i t i a l h i g h v a l u e s o f a l c o i l o l a d s o r p t i o n h e a t ( F i g . 4) c o r r e s p o n d t o i n t e r a c t i o n w i t h 0.5 mmol/c of ITa c a t i o i i s . Adsorpt i o n c a p a c i t y o f t h e second s e c t i o n w i t h t h e h e a t p l a t e a u b e i n g
a t t h e l e v e l of '/) kJ/mol, i s a l s o 0.5 inmol/g, i.e. t h e second m o l e c u l e o f a l c o h o l i s aadcd t o i n i t i a l coniplex. Exterit of' t h e t h i r d and t h e f o u r t h s e c t i o n s corresponcl Lo a u a i t i o r i oi' t h e t h i r d a n a t h e S o u r t h a l c o h o l m o l e c u l e s t o t h e p r e v i o u s colnplex; anu i r i t h i s case they ciirectly i n t e r a c t with c e n t r a l cation. Such t e t r a t h e a r a l s o r b t i o n complex c a n be formed o n l y i n t h e i n t e r s e c t i o n s of two t y p e s o f c h a n n e l s i n t h e s t r u c t u r e 01% pentas i l e z e o l i t e s . The Iirst alcohol m o l e c u l e d r s p l a c e s Iu'a c a i i o r i from e q u i l i b r i u m p o s i t i o n 111a l a t t i c e ( t h i s i s s u p p o r t e u by l i n e a r d r o p o f h e a t on t h e whole s e c t i o r i OP f i l l i t i g from 0 t o 0. I, mmol/e) and l o c a l i z e s i t O I I f a v o u r a b l e p o s i t i o n e d o x i g e n atoms o f a l a t t i c e 111 t h e p o i n t s o f chanriels i a t e r s e c t i o n . 111 t h i s c a s e m e t h y l and e s p e c i a l l y e t h y l g r o u p s p a r t i a l l y e n t e r s t r a i g h t chatinel o€ z e o l i t e . The second a l c o h o l m o l e c u l e i s a d ded t o l o c a l i z e d complex Srorn t h e o p p o s i t e s i d e as w e l l as w i t h a l k y l r a d i c a l p a r t i a l l y i n a s t r a i g h t charinel. To form s u c h a
532
l i n e a r complex ( w i t h o u t displacement from t h e p l a c e oi’ l o c a l i x a t i o n ) a l c o h o l m o l e c u l e b e s n o t have any s t e r i c d i f f i c u l t i e s and n e a t curve at t h e s e c t i o n o f 0.5-1.0 mmol/g i s c o n s t a n t . To add t h e t h i r d molecule of a l c o h o l from t h e s i d e of s t i l l f r e e z i g zag channel new displacement of l i n e a r complex t a k e s p l a c e and a d s o r p t i o n h e a t a t t h i s s e c t i o n of 1.0 t o 1.5 mmol/g drops l i n e a r l y . And a t l a s t , t h e f o u r t h molecule i s connected t o complex w i t h o u t any d i f f i c u l t y and w i t h c o n s t a n t h e a t a t t h e s e c t i o r i o f 1.5-2.0 rmnol/g. When a l l c a t i o n s of N a a r e i n v o l v e d i n t o complex f o r m a t i o n , f u r t h e r a d s o r p t i o n of a l c o h o l s similar t o t h a t 011 s i l i c a l i t e (Fig.4) i s accomponied w i t h s l i g h t i n c r e a s e i n a d s o r p t i o n h e a t , aciiievirig r a t h e r low iiiaxirflwu w i t h t h e f o l l o w i n g s h a r p drop t o c o n d e n s a t i o n h e a t i n t h e r e g i o n o f s o r p t i o n completion. Entropy aiagrams i n accordance w i t h f o u r - s t e p c h a r a c t e r o f ads o r p t i o n h e a t c u r v e s have polyextremal form and minimum o f ent ropy curve c o r r e s p o n d s t o each s t e p on a h e a t curve. Adsorption of C02 on IUaZSlii-5 (Fig. 5) i s s u b s t a n t i a l l y g r e a t e r t h a n on s i l i c a l i t e , b u t i n the i n i t i a l r e g i o n of f i l l i i i g of 0.0-0.1 mmol/g a d s o r p t i o n h e a t s on NaZShi-5 and on s i l i c a l i t e a r e similar and r a t h e r h i g h , i . e . a t t h i s s e c t i o n of f i l l i n g energy of i n t e r a c t i o n w i t h 013-groups which i s g r e a t e r t h a n t h a t o f i n t e r a c t i o n w i t h Na c a t i o n i s m a n i f i s t e d (while i n t h e c a s e w i t h alcohol the s i t u a t i o n i s contrary).
40 30
Pig.5.
D i f f e r e n t i a l h e a t s o f carbon d i o x i d e a d s o r p t i o n on s i l i c a l i t e ( a ) and IVaZSivi-5 ( b ) a t j00 K. Above - e n t r o p y o f a d s o r p t i o n i n J / m o l K.
533
uveragy energy of f o r m a t i o n o f lia+-CO2 a a s o r p t i o n complex i n NaZSl4-5 z e o l i t e ( e x t r a p o l a t i o n t o z e r o f i l l i n g ) makes up 50.0 k J / mol. I n t h i s c a s e l i n e a r drop of t h e h e a t evideiices, as i t w a s a u r i n g a l c o h o l a d s o r p t i o r i , about displaceilient of Na c a t i o n froiil i t s e q u i l i b r i u m p o s i t i o n i n a l a t t i c e . A f t e r s t e p w i s e berid a t f i l l i n g o f 0.5 miol/i;: tile h e a t curve a g a i n drop l i n e a r l y from 40 t o 30 kJ/mol t o f i l l i n g of 1 .O mniol/~;, i.e. s t o i c h i o m e t r i c mechanism of complex forrilation w i t h exchange c a t i o n s i s observed i n cormon w i t h t h e c a s e w i t h a l c o h o l . fiowever, h e r e t h e energy of complex f o r m a t i o n i s s u c h t h a t r e l a t i v e weak i n t e r a c t i o n v i i t k i cat i o i i a n u r e l a t i v e l y s t r o n g i n t e r a c t i o n w i t h l a t t i c e does n o t a l low l a r g e l i n e a r complex OcO-iva+-OCO t o move i i i a s t r u i d i i t chaiiiiel o f b e o l i t e . “iiat i s why t h e l a s t s t a g e of f i l l i n g o f 1.0-1.5 i u i i o ~ / g corresporius t o a u s o r p t i o r i oil c a t i o i i - f r e e s e c t i o i i o ~ ? z s s c t e s t r u c t u r e , w h i l e a d s o r p t i o t i iieat v a l u e s i n t h i s r e g i o i i c o i n s i a e w i t h a u s o r p t i o n iieat l e v e l oil s i l i c a l i t e . Tile d e u s i t y of inolecules arrailgemeiit of v a r i o u s n a t u r e an6 geometry i n p e n t a s i l z e o i i t e s v a r i e s . For example, n.alkaries i n s i l i c a l i t e a r e a r r a n g e u v e r y dense (erici t o e n d ) and occupy all acisor*ptiori s p a c e ( 9 ) . 1G.alcohols f i l l 0.U of s o r p t i o n volume o f t h e i r c h a n n e l s w h i l e beiizerie o n l y 0.b of t h e volume. Thus, i n one c a s e i n t e r m o l e c u l a r i n t e r a c t i o n o f a l c o h o l s w i t h each o t h e r through hycirogen bonds p r e v e n t t h e i r c l o s e packing i n t h e chaiin e l s w h i l e i n t h e o t h e r c a s e (when benzene i s a d s o r b e d ) rnolecul a r conformation p r e v e n t i t from p e n e t r a t i o n i n t o inore narrow zig-zag channels. Adsorption o f a p o l a r m o l e c u l e s of hydrocarbons i s t h e most s e i i s i t i v e t o c a v i t i e s s i z e , t h e s i z e o f c h a n n e l s and. v a r i o u s tlwinuowslli n z e o l i t e s . For example, u i f f e r e n t i a l h e a t s o f n.alkanes a d s o r p t i o n on z e o l i t e s a t z e r o f i l l i n g ( o b t a i n e d by e x t r a p o l a t i o n o f l i n e a r l y i n c r e a s i n g s e c t i o n of t h e h e a t curve t o z e r o f i l l i n g ) i s i n c r e a s i n g l i n e a r l y w i t h t h e Lrowth o f t h e number of carbon atoms i n n.alkane m o l e c u l e , aria f o r s i l i c a l i t e t h i s dependence i s e x p r e s s e d by r e g r e s s i o n 11.6+10.0 n kJ/mol, where 111111 i s t h e l i m b e r of carbon atoms i n molecule. For n . a l c o h o l s a d s o r p t i o n h e a t on s i l i c a l i t e l i n e a r dependence i s d i f f e r e u t : 34.0 + 9.0 n. For t h e a l c o h o l s f r e e menber of r e g r e s s i o n which r e p r e s e n t s w a t e r ‘molecule a d s o r p t i o n energy i s much Less t h a n w a t e r vapor c o n d e n s a t i o n h e a t . T h i s f a c t e x p l a i n s h y d r o p h o b i c i t y o f s i l i c a l i t e (9).
534
From s i m p l e t h e o r e t i c a l p r e m i s e s i t i s c l e a r t h a t t h e n a r rower i s t h e s o r p t i o n s p a c e t h e g r e a t e r i s t h e e n e r g y of d i s p e r s i o n i n t e r a c t i o n of a p o l a r m o l e c u l e arid i t i s a peak one f o r s u c h p o r e s i n which m o l e c u l e i s c l o s e l y a t t a c h e d t o t h e w a l l s . The c h a n n e l s i n hydrogen form of n a t u r a l s e o l i t e - c l i n o p t i l o l i t e (H-CL) i s l i k e l y t o be c l o s e l y a t t a c h e d t o n.alkane m o l e c u l e s (10). H-CL c h a n n e l s a r e s o narrow t h a t n.alkane m o l e c u l e s w i t h n = 2 do n o t p e n e t r a t e t h e r e o r p e n e t r a t e w i t h g r e a t d i f f i c u l t y ( w i t h h i g h a c t i v a t i o n e n e r g y a n d v e r y slow). Ethane a d s o r p t i o n h e a t on H-CL a t zeyo f i l l i n g made up 40.0 kJ/mol (10). Taking i n t o a c c o u n t t h e f a c t t h a t for most o f z e o l i t e s w i t h open porous s t r u c t u r e s r e g r e s s i o n f r e e members of n . a l k a n e s a d s o r p t i o n h e a t iilakes up 7.0 kJ/mol ( 1 ) arid f o r t h e most narrow o n e s t h i s v a l u e can n o t exceed 12 kJ/mol ( f o r s i l i c a l i t e , as i t w a s g i v e n above, qo= 11.6 kJ/moi) we o b t a i n h e a t i n c r e m e n t p e r CH2-group f o r H-
KL which i s 14.0 kJ/mol.
On t h e same b a s i s , i f we t a k e qo= 10 kJ/mol for KL z e o l i t e ( c h a n n e l d i a m e t e r b e i n g 0.71 nm) t h e n i n crement p e r CI12-group w i i l make up -9.5 kJ/mol. The o b t a i n app r o x i m a t e v a l u e s o f h e a t i n c r e m e n t p e r CH2-group - 14.0, 10.0
and 9.5 kJ/mol f o r t h r e e dimensions ( d i a m e t e r s ) o f c h a n n e l s aver a g e c r o s s - s e c t i o n s (0.4,o.G and 0 . 7 ) c o r r e c t l y r e f l e c t a p r i o r i r e g u l a r i t y i n t h e growth of a d s o r p t i o n i n t e r a c t i o n e n e r g y w i t h d e c r e a s i n g i n p o r e s s i z e . However, t h i s r e g u l a r i t y i s q u a l i t a t i v e o f h a l f q u a n t i t a t i v e as a r e t h e r e s u l t s of c a l c u l a t i o n s o f t h i s k i n d f o r c o r r e s p o n d i n g models.
H&yARj&cj!s I . A.A. I s i r i k j a n . I n : A d s o r p t i o n arid a d s o r b e n t s . Nauka. Moscow. 198'7, 41-53. 2. lLM.Dubiriin, A . A . I s i r i k j a n , A.I.Sarakhov, V.V. S e r p i n s k i . Izv. &I USSH. Chem.(1968), No 8 , 1690-1699; (1969) No 1 1 , 2 3 5 5 - 6 0 . 3 . K.S.&hmedov, G.U.Kakhniatkariev, Ivi.Ivl.Dubinin and A . s , I s i r i k j a n . Izv. A& USSR. Chem. (1987), N o 8 , 1717-1721. 4. Ibi.Ui.Iktbinin,A.A.Isirikjan.G.U.Kalchmatkariev a n a V.V.Serpinski. I s v . Ai\i USSR. Cliem.(IY'/2), No 6 , 1269-1276 ; (1973), IT0 4 , 934y j 6 . 5. u . a . I s i r i k j a n . I n : C a l o r i m e t r y i n a u s o r p t i o n and c a t a l y sis. S O AN USSR. N o v o s i b i r s k (1 389), 21 7-214. 6. M.I.Dubinin, G.U.Hakhmatkariev and i i . A . I s i r i k j a n . I z v . f i l i USSH. Chern. (1984 ), iVo 1 2 , 207r[-2ij7tj. 7 . G.U.fiakhmatkariev,B.A.A.IsirikJan. Izv. AN USSK. Ciiern. ( 1 9 8 8 ) , No 1 1 . 224442245. 8. ~ ~ . ~ . D u b i r i i n , A . 8 . I s i r i k j a n ,I$. 1.Hegent , W. Sciirirner., H. S t a c h , H. T m r i , U. Lohse. I z v . Al! USSIi.Chern. ( 1 9 8 4 ) , 80 9. 1931-1938. 9. E.X.Flaiiigen, J,id.Bennet e t al. N a t u r e 271 (19'(8).512. 10, M.M.Dubiriin,E.Ch.Anaktschiari, A.A.Isirikjan. I z v . AN USSR. Chern. (1 9 8 7 ) , No IT .2631-2632. 1 1 . W.Ychrinier, ii.Stach e t al. Z.Phys.Chemie ( L e i p z i g ) 261 ( 6 ) ( 1 9801. 1 129-1 1 38.
F. Rodriguez-Reinosoet al. (Editors), Characterization of Porous Solids I1 0 1991 Elsevier Science Publishers B.V., Amsterdam
535
Sorption of argon and nitrogen on network types of zeolites and aluminophosphates
H. Reichert', U. Mullerl, K.K. Ungerl, Y. Grillet2, F. Rouquero12, J. Rouquerol2, J.P. Coulomb3 1Institut fur Anorganische Chemie und Analytische Chemie, Johannes Gutenberg-Universit, Joh. -Joachim-Becher-Weg 24, D-6500 Mainz, F.R.G. 2Centre de Thermodynamique et de Microcalorimetrie du C.N.R.S., 26 Rue du 141e R.I.A., F-13003 Marseille, France 3Departement de Pysique du C.R.N.S. F- 13003 Marseille-Luminy , France
Abstract Synthetic zeolites and aluminophosphates comprising 10- and 12- membered ring openings, unidimensional and network type of pore systems (MFI, MEL, E N , LTA, AEL, AFI and FAU) were used as model adsorbents to examine the impact of micropore structure on the sorption properties. Argon and nitrogen were employed as adsorptives. Adsorption measurements were carried out on gravimetric and volumetric sorption devices and also monitored by microcalorimetry. From the low coverage regime of the isotherm Henry's constants and isosteric heats of adsorption were derived. Both quantities allowed the discrimination between 10- and 12- membered ring systems. Unidimensional 10- and 12membered ring zeolites and aluminophosphates gave Type I isotherms for argon and nitrogen. Stepped isotherms were observed for argon and nitrogen on network types of molecular sieves. On MFI type zeolites with nitrogen a distinct hysteresis was observed between p/po = 0.1 and 0.15, as reported earlier. In-situ measurements of the system Silicalite I / nitrogen at 77 K by neutron diffraction experiments indicated discontinous changes in the diffraction pattern of both MFI and nitrogen upon increasing adsorbate coverage. Introduction Crystalline microporous solids such as synthetic zeolites and aluminophosphates have gained remarkable interest as adsorbents to study adsorption phenomena in microporous systems. These types of molecular sieves possess a well-defined, regular pore structure of molecular dimensions, comprising from unidimensional channels to three-dimensional networks [1-51. In our laboratory we have focussed on the synthesis of large and uniform crystals of ZSM-5, Silicalite-I, ZSM-48, A1P04-5 and AlPO,-ll was thoroughly investigated. Large crystals of microporous solids with negligible external surface area are essential to assess the intrinsic adsorption properties. Isotherms of nitrogen and argon were measured at 77-87 K at two distinct ranges: (i) The high coverage range at a reduced adsorption temperature of 0.5 < T/T, < 0.7 (T, is the critical temperature of the bulk adsorptive) is where the final micropore filling occurs and adsorbate-adsorbate interactions dominate.
536
(ii) The low coverage range at a reduced adsorption temperature of 1.8 < T/T, < 3.1 is where only adsorbent-adsorbate interactions take place. The objective of the study was to manifest the phenomena which gives rise to the deviations from Type-I-isotherms usually observed on microporous solids. Experimental The samples were prepared by optimization of synthesis procedures described in the Literature (see Table 1). The samples were calcined at 823 K. Prior to all adsorption measurements the samples were outgassed for 12 hours at 473 K and with a vacuum of mbar. The products were characterised by scanning electron microscopy, X-ray diffraction, thermal analysis and electron microprobe analysis. Additionally, the HZSM-5-crystals were analysed by IR and 2%-MAS-NMR spectroscopy. High resolution adsorption isotherms were recorded by a dynamic volumetric device (Omnisorp 360, Omicron Corp. U.S.A.). Investigations into the low coverage range were determined gravimetrically with a relative sensitivity of 106 (ultramicrobalance 4433, Sartorius, F.R.G.). The continous calorimetric measurements were performed on a reversible isothermal microcalorimeter of Tian-Calvet type (C.N.R.S. Thermodynamique et Microcalorimetrie, Marseille, France) [7]. Neutron powder diffraction data were collected at 77.4 K on a 2-axis diffractometer (D1B at ILL, Grenoble, France) at a constant wavelength of 2.525 A.
TABLE 1.: Molecular sieves used for this study Sample
Structure
Silicalite-I HZSM-5 ZSM-48 AIP04- 11 A1P04-5 A1PO4-17
MFI MFI (FER) AEL AFI ERI
Pore opening (-membered ring) 10 10 10 10 12 8
Literature 1, 2, 5, 6 1, 2, 5, 6 4 3 3, 5 3, 8
Results and Discussion In the low coverage range, at temperatures of 303 K to 373 K, coverages of less than one molecule of adsorbate per unit cell were observed (see Fig. 1). Which thus reflects pure adsorbate-adsorbent interactions. The slopes of these linear isotherms yielded the Henry constants KH. The heats of adsorption values were calculated according to the equation KH =KH ' .exp(-AH/RT) from the linear regression of the experimental data at different temperatures (see Fig. 2). In Table 2, the results from these measurements are shown.
537
Figure 1 Adsorption of nitrogen at low coverages on MFI type zeolites
PRESSURE
I
[mbar of Nitrogen]
1 7
Figure 2 Van't Hoff-Plots for MFI and AFI type structures for nitrogen
Nitrogen Van't Hoff-Plots
r.llM I
I
" "
3.0 -
-4
.73
"'
3.4
'
"
'
" "
I'
"
3.6
"
3.8
40-3
1/T
:
"
TABLE 2.: Isosteric Heats of adsorption Adsorbent
Structure
H-ZSM-5 MFI Silicalite-I MFI Silicalite-I MFI ZSM-48 PER) ~ 1 ~ 11 0 ~ - AEL AlP04-5 AFI
Adsorbate
AH[kJ/mol]
Nitrogen Nitrogen Argon Nitrogen Nitrogen Nitrogen
14.9k0.9 15.0k1.3 16.0k0.9
13.0k0.6 12.6+ 1.3 7.7k1.3
No difference was observed in the heats of adsorption between HZSM-5 (Si/Al= 1OOO) and the corresponding aluminium free structure Silicalite-I. Experiments with argon as an adsorptive also showed no significant difference in the heats of adsorption on the MFIstructures. On comparing the network-type MFI-structure with the unidimensional 10membered ring channel structure ZSM-48 no significant differences were either seen. However, significant diffierences in the heats of adsorption were found between the 12-membered ring channel structure A1P04-5 and the 10 membered ring channel structure AlP0,-1 1. Thus,
538
adsorption measurements in the Henry's-law region provide a useful tool to identify the size of micropores.
I 7
6-1 0
Figure 3 microcalorimetric measurements on EN-type of molecular sieve
Adsorption on AIP04-17
, , , ,
,
,
, ,
, , . ,
5
, ,
, , , .
,,
, ,,.
, , , ,
a
15
, , , , I
20
UPTAKE [rnolecules/unitcell]
The final micropore filling on network types of molecular sieves was studied in the high coverage range at 77 K. As previously observed on MFI-,MEL- and LTA-type network zeolites, nitrogen on A1P04-17 was found to exhibit a non-uniform behaviour. Microcalorimetric studies clearly indicated the onset of additional exothermic interactions at an uptake of approximately six molecules per unit cell (see Fig 3). This corresponds to a cooperative filling process after adsorption on the six adsorption potential minima in the structure [8]. The nonuniform behaviour of the heat curves cannot be seen on the adsorption isotherms which show pure Langmuir type behaviour (see Fig. 4). 20
Nitrogen
Figure 4 adsorption isotherms on ERI-type of molecular sieve
oa 0
E
Y
0
010
I " " l ~ " ' I ~ " I " " I ' " ~ I ' " ' I " ' ' l
0.2
04
0.6
RELATIVE PRESSURE [p/po]
As demonstrated on large and uniform crystals of MFI-Type zeolites the steps in the heat curves perfectly correspond to steps in the adsorption isotherm (see Fig. 5). As the large crystals were crushed with a pestle and mortar the steps in the isotherm were consequently flattened and the hyteresis (at p/pO = 0.1 to 0.15) was depressed. (see Fig. 6).
539
Figure 5 nitrogen isotherm and isosteric heat of adsorption on large MFI crystals at 77 K
Adswption of Nitrcpn on Slicdite-l
,I,, ~
, ,,
,,,, l
, , , , r
,, , , n
,
;"
,?, , , . ,
20
"
,,,,
~Y)
, , ,,
, 40 I
I
UPTAKE [molecukr/uritcdl]
Nitrogen Isotherm MFl/77 K
32 7
monodisperse
crushed crystals
20
o.w
OM
0.10
0.1s
Figure 6 isotherms on large, uniform and on crushed crystals of SiliditeI
'1'"'I"~'I'"'T""T""I
om
0.25
03
Rebtlvo Presswe p/pO
One can conclude that stepped isotherms and low pressure hysteresis loops may only be observed on monodispersed and large crystals of network types of molecular sieves. The hysteresis loop was examined in detail by scanning the region between the adsorption and the desorption branch of the hysteresis. The points between the two branches were stable for many hours, thus, the low pressure hysteresis is controlled by equilibrium properties and not by kinetical effects. Previous studies provided clear evidence that the stepped isotherms of argon and nitrogen on MFI-crystals can be rationally explained by lccalised adsorptive molecules at the channel walls and intersections [lo]. At higher loadings the step in the region of the hysteresis was proposed to be a result of solidification of the adsorbate. To obtain more evidence for this hypothesis in-situ neutron scattering experiments were performed on the system Silidite/Nz at 77 K at different pressures following the hysteresis loop (see Fig. 8-10 and Table 3).
540
Figure 7 scanning between adsorption and desorption branch of the hysteresis loop on Silicalite-I
.... ..... . &. ........ I+
r
;-
. ........:
I
:-
(i.
....................................... an
0.U
0.U
am
am
020
PfLATM PRESUlRE [p/po]
TABLE 3: Points on the isotherm used for neutron scattering Point
A B C D
relative pressure P/P0 0.07-10-3 0.081 0.321 0.127 11,
coverage N2/unit cell 18.4 23.5 30.1 29.6
branch adsorption adsorption adsorption desorption Figure 8 neutron diffraction patterns of N2/Silicalite-I at 77K at theangles 18"s20 5 97" uncovered MFI to point A
Although the data cannot yet be totally interpreted, the following explanation can be given. The diffractogram of the unloaded zeolite that shows some doublets change to singlets when the crystals have adsorbed nitrogen (see Fig 8). This indicates a transition from the monoclinic phase to the orthorhombic phase of the MFI-structure. This change was reported earlier for benzene and xylene and for higher temperatures 113-153. With further uptake of nitrogen the diffraction pattern changes distinctly (at 20=1.8 to 2.2) with peaks emerging (see Fig. 9).
541
Figure 9 neutron diffraction patterns of N*/Silicalite-I at 77K at the angles 18" 5 2 8 I 97" point A to C
By comparing the difference in the spectra at points B-C to the spectra of the solid hexagonal 5-Nitrogen phase, analogous peaks are seen (see Fig. 10). Differences at desorption to point D were not discovered. Thus evidence for a transition of the adsorbed nitrogen to a solid like phase was found. 1,5 1,25 -
3-N2(3D Solid)
Figure 10 diffraction pattern of a) solid 5-nitrogen and
(101)
1-
0,75
(002)
03 -
0,25. 0
j \
b) difference spectra of point B-C on the system Silicalite-I/N;! at 77 K
1
1.2
1,4
1,6
1,8
2
2,2
2,4
542
Conclusion To conclude, one can first point out the usefulness of the low-coverage isosteric heats of adsorption of nitrogen: they indicate a clear difference in behaviour between the 10-membered ring systems (MEL) and the 12-membered ring molecular sieves (AFI, MFI) with a 2-fold decrease in isosteric heat values. Finally, neutron diffraction experiments clearly confirm our previous conclusions drawn from the shape of the adsorption isotherms and from the microcalorimetric studies [16]: the step at at about p/po=O. 12 on the N2 adsorption isotherms is due to a fluid solid transition of the adsorbate. Also, as m n as nitrogen is adsorbed, the zeolite structure progressively undergoes a change from monoclinic to orthorhombic structure. +
Acknowledgements We would like to thank the Deutsche Forschungsgemeinschaft for their financial support. References U. Miiller, K.K. Unger, Z. Kristallogr., 182 (1988) 190. 1 U. Miiller, K.K. Unger, Zeolites, 8 (1988), 154. 2 S.T. Wilson, B.M. Lok, E.M. Flanigan, US Pat. 4,385,994 (1983). 3 E.W. Valyosik, US Pat. 4,585,747 (1986). 4 D.M. Bibby, N.B. Milestone, L.P. Aldridge, Nature, 285 (1980), 30-31. 5 6 U. Miiller et al., A.C.S. Symp. Series, 398 (1989), 346-359. 7 J. Rouquerol et al., J. Chem. S o c . , Faraday Trans. I, 73 (1977), 306 8 U. Miiller et al, in E.F. Vansant and R. Dewolfs (Us.), Gas Separation Technology, (1989), Elsevier, Amsterdam, 255 9 U. Miiller, Ph.D. thesis, Joh.-Gutenberg Universitiit Maim (1990). U. Miiller et al., Studies in Surface Science and Catalysis, Elsevier, Amsterdam, 46, 10 (1989),477 11 U. Miiller et al.,Proc. 3rd Int. Conf. "Fundamentals of Adsorption" Sonthofen, F.R.G. May 7-12, 1989, in press. 12 D. Pan et al.,"How can adsorption system show hysteresis", this procedmgs elsewere. C.A. Fyfe et al., Studies in Surface Science and Catalysis, Elsevier, Amsterdam, 46 13 (1989), 827-842. 14 B. F. Mentzen, Studies in Surface Science and Catalysis, Elsevier, Amsterdam, 46 (1989), 477-494. R.E. Richards, L.V.C. Rees, Zeolites, 8 (1988), 35-39. 15 U. Miiller et al., Fresenius Z. Anal. Chem. (1989) 333, 433 16
F. Rodriguez-Reinosoet al. (Editors), Characterization of Porous Solids II
543
0 1991 Elsevier Science Publishers B.V., Amsterdam
POROSITY OF SILICAS: COMPARISON OF NITROGEN ADSORPTION AND MERCURY PENETRATION D.R. Milburn, B.D. Adkins, and B.H. Davis Center for Applied Energy Research, 3572 Iron Works Pike, Lexington, KY 4051 1
INTRODUCTION Nitrogen adsorption and mercury penetration are two of the most common methods for obtaining information about the porosity of solids. Unfortunately, experimental realities limit the region where the two methods are both applicable to pore sizes of
m.10 - 30 nm.
To date there have been few studies that compare data
generated by the two techniques in this pore size range. Both nitrogen adsorption and mercury penetration provide a direct measure of pore size distribution and pore volume. However, these direct measures do not permit a determination of the morphology of the materials from pressure-volume relationships. DeBoer (ref. 1) was among the first to relate the shape of the nitrogen adsorption/desorption isotherms and the location and shape of the hysteresis loop to the type of pores associated with the material. Much attention has been paid to better define the relation between morphology of the pores of a material and the adsorption/desorption isotherms (ref. 2). Much less attention has been paid to the relationship of morphology to mercury penetration data (ref. 3). Most investigators, including Lowell and Shields (4-6) interpret the mercury penetration data assuming that all pores are nonintersecting and attribute the hysteresis to a change in the contact angle, 8, of mercury and the solid. Conner and coworkers (refs. 7,8)have recently utilized a porehhroat network model to obtain information about the morphology of materials from mercury penetration data. The void/solid structure is viewed as an interconnected network so that adsorption/desorption and retraction/intrusion can be associated with the openings and constrictions within the void network. These latter investigators analyzed the data as if the materials consisted of agglomerated microspheres. The measured ratio of the most probable radii of intrusion to those of retraction seemed to be characteristic of the void structure and pore shape. Conner et at. (ref. 8) developed a heuristic diagram for the classification of void/solid morphologies from a
544
plot of void fraction versus the radius ratio (Rextrusion/Rintrusion). These authors also made the interesting observation that intrusion and retraction porosimetry curves obtained for silica spheres essentially coincide so that a hysteresis loop is not obtained; this is probably the first time this observation has been reported except for nonporous macrosphere (G. 2 mm) silicas. The model developed by Conner et al. (ref. 7) that is based upon packing of spheres is of interest. We have found that a model based upon a simple cubic packing of nonporous spheres returns the best agreement among surface area, pore volume and average pore size data for nitrogen adsorption/desorption measurements with numerous metal oxide samples (ref. 9). It is of special interest to compare the data obtained from the two techniques with common materials to learn whether a model based on packings of spheres applies equally well. The Shell 980 series of silicas provides materials with three pore diameters
(ca.
15,30 and 60 nm) and for each of these, three pore volumes (ca. 1.I, 1.3,and 1.5 cm3g-’) are available. These materials would appear to provide a unique opportunity to compare porosity data obtained with two common experimental techniques:
mercury porosimetry and nitrogen adsorption. At the same time, the study would afford data to compare nitrogen adsorption to mercury depressurization and nitrogen desorption to mercury penetration.
EXPERlMENTAL Nitrogen adsorption-desorption isotherms were obtained with a Quantachrome Autosorb 6 instrument. Prior to analysis, samples were outgassed for several hours at about 10” torr and 200°C. Surface areas are calculated from the linear form of the BET equation. The model utilized for pore size distribution calculations is a packed particle model, assuming each primary spherical particle is in contact with six neighboring spheres. The primary particles agglomerate into larger, nearly spherical clusters ranging from B. 1.5 to 3.0 mrn diameter. This model was used to relate the neck opening in a face of the unit cell found by particle packing; corrections were made for an absorbed layer and for condensation at contact points (for more detail refer to reference 9). Mercury penetration curves were generated from pressure-volume measurements from 0 to 60,000 psia using a Quantachrome Autoscan 60 instrument. The surface areas are calculated using the Rootare-Prenzlow equation (ref. 10).
545
RESULTS AND DISCUSSION The absence of hysteresis for mercury penetration and depressurization curves was reported earlier for Shell S980 series silica samples, and this has been used as evidence for a network void structure theory (ref. 8). More recently these measurements have been repeated with portions of the same Shell S980 samples. These workers observed hysteresis in the second measurements but do not know what caused the incorrect data to be plotted in their earlier work (ref. 11).
............................................................ - ___---Extrusion
/
p
.-
..
I
I
.........................................
I I
I Intrusion I I I I I I I I
Shell Silica Spheres
S980 D2.2
Figure 1.
_-
1
ibo
1000
ioboo
Pressure. (psia)
-
............... ........ *' 0.80--
i
.
<
I
Extrusionj
; : ;
o.eo--
I I I
0 40--
: ; <
S980 02.3
I
I Intrusion
i
Shell Silica Spheres
(Crn3kl) Volume'
ll...YCYYY..'**'-.
I I I
,.."
........................................ .........
0.20--
I
Figure 2.
I I I
__-I
0.00100
Figures 1 & 2.
1000
10000
Sample Hg penetration curves for the Shell 980 series exhibit a stepwise volume increase and hystereses as well as some retention of Hg. Volumes retained varied from less than 0.2 cm3/g-' (Figure 2) to almost 0.8cm3g-' (Figure 1).
546
Figures 1 and 2 show typical mercury intrusion-extrusioncurves obtained in our study. The step volume increase and hysteresis are seen for all seven of the silica samples. The mercury extrusion volume is small for the data shown in Figure I,and is much larger for the curve in Figure 2. At least a portion of the volume of mercury remaining in the sample is most likely due to initial compaction of the loosely packed structure and is also seen in varying degrees for all samples. Agreement in surface areas as measured by the mercury penetration and nitrogen adsorption is shown in Figure 3. The surface areas calculated using the Rootare-Prenzlow equation for mercury penetration data are higher in every case than the areas calculated from nitrogen adsorption data using the BET equation; the greatest deviation is seen for the higher surface area materials. This is in agreement with results obtained for other silicas, carbons and aluminas (12-14).
40
Shell Silica Spheres
S980 Series
RootarePrenzlow Surface Area
(ma/o) 10
I
D
I
H
/ BET Surface Area. (mZ/O)
Figure 3.
A comparison of surface areas calculated from N, adsorption (BET equation) and Hg penetration (Rootare-Prenzlow equation).
547
49 I
Shell Silica S980 81.5
,o
OAdsorption ADesorption
............ *.---@-.
p.,,,p....*
0 0
0 2
0 4
*.+--
0 0
0 8
3
Relative Pressure. (PIP, )
Figure 4.
The N, adsorption/desorption isotherm for sample B illustrates type IV character and type " A hysteresis (Reference 1).
The adsorption and desorption isotherms are clearly of type IV and the hysteresis is type "A' (Figure 4). The total pore volumes obtained from mercury penetration show good agreement with the ones obtained from nitrogen desorption (Figure 5).
Total Pore Vol.. N p(cclo)
Figure 5.
A comparison of total pore volumes calculated from N, adsorption and Hg penetration.
548
The data in Figure 6 show that both mercury penetration and nitrogen adsorption rank the seven samples as indicated in the introduction. Thus, samples A, B, and C have a pore diameter of approximately 15 nm; D and F of approximately 30-40 nm and H in the range of 44-50 nm. The total pore volumes of the A,
G and
B and C set appear to
be more similar than the other two sets of data.
16
1
i H
H
P
l]
0.6
JP,oo
~
150
i i : ii
200
G
A
250
Pore Radius A )
Figure 6.
The samples may be grouped roughly into three pore size groups: A, B, and C with diameters of approximately 15 nm, D and F, approximately 30-40 nm and G,H, approximately 44-50 nm as measured by both Hg penetration and N, adsorption.
The ratio of the average pore size obtained by mercury penetration and nitrogen adsorption are compared to the BET surface area in Figure 7. This comparison is made on two bases. In one, the ratio of the maximum in the dV/dR of the nitrogen desorption isotherm to the maximum in the dV/dR of the mercury intrusion curve is plotted versus the BET surface area. The ratios show some scatter about the line defined for a ratio of 1.O. Alternatively, the data may be viewed to deviate from the line for low areas and to approach one for higher areas. Thus, the dV/dR maximum occurs at about the same pore diameter for the nitrogen desorption isotherm and mercury penetration. However, when the ratio is calculated from the maximum corresponding to the nitrogen adsorption isotherm and mercury extrusion curve the data are displaced downward from the line defined by the ratio of 1.O.
549
0
o.o--
Ratio Of
Radii
0 0
A
e--
a P
A
4 BET Surface Area, (m2/0)
Figure 7.
The ratio of the pore radii calculated using N, adsorption/desorption and mercury penetration (Reference 9) versus BET surface area.
The data in Table 1 shows that for the four pore size groups, there is reasonable agreement between three of the four columns of data: mercury intrusion, nitrogen desorption and nitrogen adsorption. The pore size maximum calculated for the mercury extrusion curve is considerably larger than the other three sets of data. It therefore appears that the pore size distribution calculated from the extrusion curve should be viewed with caution. The repeated mercury penetration - extrusion curves fall, apart from the difference due to irreversible retention of mercury, on the first mercury penetration extrusion curves. Others, including Lowell and Shields (3-6), have observed this also. This indicates that the hysteresis between penetration and extrusion should not be due to physical changes of the silicas. The reasons for observation of hysteresis for the seven silicas used in this study differ from those of reference 8 where hysteresis was not obtained. In this study, the Shell silicas exhibit hysteresis for mercury penetration - extrusion as has been observed in earlier studies with a variety of silicas. The value for the "correct" contact angle used to calculate area or porosity using mercury penetration/extrusiondata is not certain. Likewise, it is not certain whether the same contact angle should be used in making calculations using both extrusion and intrusion data. Davis (15) made a comparison of the surface area
550 TABLE 1 Shell Silica Spheres
Sample
Radius at dVd /R, Ha Intrusion
Radius at dVd /R,
N, DeSorDtion
a
Radius at dVd /R., Hq Extrusion
Radius at dV/dR,
N, Adsorption
A
74
80
182
103
B
70
85
202
103
C
68
76
184
103
D
217
178
817
250 166
F
145
150
254
F
177
150
568
166
G
220
145
562
150
H
257
232
511
250
calculated from nitrogen adsorption and mercury penetration data for a large number of materials, including silica. He concluded that the contact angle had to vary by more than 50' from the value of 130 he utilized to cause the two surface area values to agree in some instances. Thus, adjustment of the contact angle to effect agreement among the present data does not appear merited. As the data in Figures 1 and 2 indicate, the amount of mercury retained by the
sample during extrusion varied widely. The volume of retained mercury, defined as the difference in two curves in the 100-500 psia pressure region, does not appear to depend simply upon the pore volume surface area or pore radius. The limited data set suggest that a slightly skewed bell-shaped curve results when the total pore value is plotted versus the volume of mercury retained (Figure 8). A plot of the surface area (either nitrogen BET or mercury penetration) versus the value of mercury retained resembles that shown in Figure 8 except the maximum occurs slightly below 0.4 cc/g.
A similar curve, except inverted, results from a plot of the most probable pore radius versus the volume of mercury retained; the minimum in this case occurs at g.0.3 cc/g. Thus, the volume of mercury retained does not appear to be uniquely determined by the total pore volume, the pore radius or the surface area. In summary, similar values are obtained for surface area and pore size distribution when nitrogen adsorption and mercury intrusion data are compared.
551
i.6.
Total Pore Volume
(cm3/a) 10.
0.6
0:2
0:4
0:e
018
VOI. Hg Retained After Extrusion (crrP/g)
Figure 8.
The volume of Hg retained following extrusion varies with total pore volume as measured by N2 adsorption and Hg penetration.
REFERENCES
1. 2. 3. 4.
5. 6. 7. 8.
J. H. deBoer, The Structure and Properties of Porous Materials, Butterworths, London (1 958). K. S.W. Sing, D. H. Everett, R. A. W. Haul, L. Moscou, R. A. Pierotti, J. Rouqubrol, and T. Siemieniewska, Pure and Appl. Chem., 57 (4).(1985)603. L. K. Freuel and L. J. Kressley, Anal. Chem., 35, (I 963) 1492. S.Lowell and J. E. Shields, J. Colloid. Interfac. Sci., 80,(1980)192. S.Lowell and J. E. Shields, J. Colloid. Interfac. Sci., 90,(1982)203. S.Lowell and J. E. Shields, Powder Technol., 29,(1980)37. W. C. Conner and A. M. Lane, J. Catal., 89,(1984)217. W. C. Conner, Jr., C. Blanco, K. Coyne, J. Neil, and J. Pajares, J. Catal., 106,
(1987) 202. 9. 10. 11. 12. 13.
B. D. Adkins and B. H. Davis, J. Phys. Chem., 90,(1986)4866. H. M. Rootare and C. F. Prenzlow, J. Phys. Chem., 71,(1967)2734. W. C. Conner, personal communication. D. R. Milburn, B. D. Adkins and 6.H. Davis, Fundamentals of Adsorption, Engineering Foundation Inc., New York, NY, (1987)401. B. D. Adkins and B. H. Davis, Ads. Sci. & Tech., 5 (I),Multi-Science Pub.,
(1988). 14. 15.
D. R. Milburn, B. D. Adkins and B. H. Davis, Characterization of Porous Solids, K. K. Unger, (Ed.), (1988).501 B. H. Davis, Appl. Catal., 10,(1984)185.
This Page Intentionally Left Blank
F. Rodriguez-Reinoso et al. (Editors), Characterization of Porous Solids II
553
0 1991 Elsevier Science Publishers B.V., Amsterdam
CHARACTERI ZATI ON AND STAB1L I TY OF POROUS STRUCTURE OF T I TAN1 UM-
-SI L I CALI TE BY SORPTION METHODS
G. A.
1 Genoni , M.
1 L e o f a n t i 3 , F. Zecchina .
1 Padovan ,
G.
Petrini
'Montedipe S . r . l . Units d i Ricerca d i Bollate, 20021 B o l l a t e ( M i l a n o ) , I t a l y .
1
,
G.
Trezza
2
,
Via San P i e t r o 50
'Montedipe S. r. 1. C e n t r o R i c e r c h e d i Marghera, Via d e l l a Chimica 5 30175 P o r t o Marghera ( V e n e z i a ) , I t a l y . 3 D i p a r t i m e n t o d i Chimica I n o r g a n i c a Chimica F i s i c a e d e i M a t e r i a l i Via P. G i u r i a 7, 10125 Torino, I t a l y .
SUMMARY S o r p t i o n measurements of s m a l l ( N 2 ) and l a r g e ( i .e. xylenes) p r o b e molecules t o g e t h e r w i t h s p e c t r o s c o p i c and d i f f r a c t o m e t r i c t e c h n i q u e s have been used t o c h a r a c t e r i z e t h e porous s t r u c t u r e of T i - s i l i c a l i t e b e f o r e and a f t e r t h e r m a l t r e a t m e n t s i n t h e r a n g e 823-1573K. I t i s n o t i c e a b l e t h a t N2 and p-xylene a d s o r p t i o n a r e c o r r e l a t e d t o t h e z e o l i t e framework w h i l e m-xylene u p t a k e depends on t h e c o n t e n t of T i i n t h e framework. The r e s u l t s a l s o i n d i c a t e t h a t t h e T i e x t r a c t i o n from t h e l a t t i c e d u r i n g t h e t h e r m a l t r e a t m e n t s b e g i n s b e f o r e t h e framework c o l l a p s e . INTRODUCTION During t h e l a s t p e r i o d c o n s i d e r a b l e a t t e n t i o n has been g i v e n t o Ti-silicalite,
a
zeolite
derived
s u b s t i t u t i o n o f framework
from
S i with T i
silicalite
(refs.
1-3).
by
partial
This
zeolite
e x h i b i t s v e r y v a \ u a b l e c a t a l y t i c p r o p e r t i e s f o r a v a r i e t y of r e a c t i o n s of
industrial
interest, i n
particular
for
cyclohexanone
ammoximation, phenol h y d r o x i l a t i o n and o l e f i n s e p o x i d a t i o n 4,
5).
Despite t h a t , t h e s t a b i l i t y
fundamental a s p e c t s
of i t s
of T i - s i l i c a l i t e ,
c h a r a c t e r i s t i c s and
(refs.
as w e l l
catalytic
as
beha-
v i o u r , has n o t been s t u d i e d . I n t h i s work s o r p t i o n measurements of and m-xylene) probe molecules w e r e
per-
formed t o c h a r a c t e r i z e t h e porous s t r u c t u r e of T i - s i l i c a l i t e ,
be-
s m a l l ( N 2 ) and l a r g e ( p -
f o r e and a f t e r v a r i o u s t h e r m a l t r e a t m e n t s .
The r e s u l t s ,
compared
t o t h o s e o b t a i n e d by s p e c t r o s c o p i c and d i f f r a c t o m e t r i c t e c h n i q u e s , provided information
on s t a b i l i t y
t i t a n i u m i n s e r t e d i n t h e framework.
of
both z e o l i t e
lattice
and
554 EXPERIMENTAL
Sample p r e p a r a t i o n The samples have been obtained following t h e p a t e n t l i t e r a t u r e ( r e f . 6 ) s t a r t i n g from a l k y l s i l i c a t e s and t i t a n a t e s and an aqueous s o l u t i o n of tetrapropylammonium hydroxide. A f t e r removal of t h e alcohols formed by t h e hydrolysis of S i and T i compounds w i t h t h e organic base t h e s o l u t i o n has been charged i n t o a vessel equipped with s t i r r e r and kept a t 443R f o r t h e required time. The so obtained s l u r r y has been centrifuged, t h e s o l i d c a r e f u l l y washed with water, d r i e d a t 3 9 3 R f o r 16 hours and f i n a l l y c a l c i n e d a t 023K f o r 4 hours. The r e s u l t i n g z e o l i t e sample has been t h e n part i t i o n e d i n t o equal p a r t s which were submitted to a f u r t h e r t r e a t ment i n t h e 973-1573K range f o r 4 hours. When needed, t h e samples
examined i n t h e p r e s e n t paper a r e i n d i c a t e d by TS followed by a number corresponding to t h e c a l c i n a t i o n temperature ( f o r i n s t a n c e TS1173 r e p r e s e n t s a sample calcined a t 1173K). Besides T i - s i l i c a l i t e , s i l i c a l i t e and s i l i c a samples have been used i n t h e preS i l i c a l i t e has been obtained s e n t work as r e f e r e n c e materials. following t h e above procedure, obviously a p a r t from a l k y l t i t a n a t e a d d i t i o n . Amorphous s i l i c a i s a commercial sample (Grace SG360).
-N2
adsorption N2 a d s o r p t i o n isotherms a t
77K have been
measured using a
C.
Erba SORPTOMATIC 1900 on samples outgassed a t 57313 ( 1 0 hours, final vacuum mbar). The z e o l i t i c channels volume was determined by t h e as method ( r e f . 7 ) by e x t r a p o l a t i n g t o as=O t h e l i n e a r The method has been checked by m u l t i l a y e r r e g i o n of t h e as p l o t . comparing t h e higher as region of t h e TS823 sample with t h e corresponding as region of TS393 sample s t i l l containing tetrapropylammonium hydroxide. These samples d i f f e r e x c l u s i v e l y i n t h e a v a i l a b i l i t y of z e o l i t i c channels which a r e made f r e e only under high corresponding temperature treatment. According t o t h a t , t h e as-plots show t h e same s l o p e i n t h e high as region and d i f f e r e n t o r - d i n a t e values when e x t r a p o l a t e d to a s = O ; i n p a r t i c u l a r the s t r a i g h t l i n e corresponding t o t h e sample containing t h e organic base passes through t h e o r i g i n of t h e axes. The e x t e r n a l s u r f a c e a r e a of t h e c r y s t a l s or, more exactly, of t h e primary p a r t i c l e s has been determined on t h e b a s i s of above considerations from t h e s l o p e of l i n e a r high as region, using as a r e f e r e n c e t h e d a t a of nonporous hydroxylated s i l i c a of r e f . 7.
555
Xyl enes a d s o r p t i o n P a r a and meta-Xylene a d s o r p t i o n has been performed i n a m i c r o b a l a n c e connected t o a vacuum manifold ( u l t i m a t e vacuum
Cahn lo-'
mbar). Before t h e a d s o r p t i o n measurement e a c h sample has been The t e m p e r a t u r e t r e a t e d i n s i t u a t 573K up t o c o n s t a n t weight. has been t h e n lowered t o 303K, t h e x y l e n e i s o m e r vapour dosed on t h e sample and t h e weight i n c r e a s e due t o t h e a d s o r p t i o n recorded. Xylenes TPD (Temperature programmed d e s o r p t i o n ) Before t h e measurement, each sample has been i n s e r t e d i n t o a h o l d e r connected t o a vacuum manifold ( u l t i m a t e vacuum mbar) and h e a t e d a t 573K f o r 2 hours. The h o l d e r has been t h e n c o o l e d t o r . t . , i s o l a t e d from vacuum, and a s l i g h t e x c e s s o f l i q u i d x y l e n e i n j e c t e d i n t o it. A f t e r 10 minutes t h e w e t sample has been removed, p u t i n t o a Mettler TA 3000 thermobalance and k e p t a t r . t . i n a H e f l o w ( 1 0 0 cm3 m i n - l ) f o r a w h i l e u n t i l removing t h e most x y l e n e e x c e s s . F i n a l l y t h e sample has been h e a t e d t o 673K a t 4 K min-l i n a H e f l o w and t h e weight loss r e c o r d e d . R e s u l t s have been e x p r e s s e d a s c o n v e n t i o n a l TG c u r v e s . XRD -
The X-ray
diffraction
patterns
have
been
recorded
with
a
P h i l i p s d i f f r a c t o m e t e r equipped w i t h a p r o p o r t i o n a l c o u n t e r by u s i n g a N i - f i l t e r e d CuKa r a d i a t i o n . The samples have been examined w i t h o u t any p r e v i o u s p r e t r e a t m e n t . The c r y s t a l l i n i t y d e g r e e has been d e t e r m i n e d by a p r o c e d u r e developed i n o u r l a b o r a t o r y ( r e f . 8 ) . The method i s based on a comparison of t h e i n t e g r a t e d i n t e n s i t i e s of two d i f f e r e n t s p e c t r a l ranges a f f e c t e d r e s p e c t i v e l y by t h e c r y s t a l l i n e and amorphous f r a c t i o n s of t h e s o l i d . I n t h i s mann e r t h e u s e of s t a n d a r d s w i t h a known c r y s t a l l i n i t y c a n be a v o i d The c r y s t a l l i t e s i z e has been determined from t h e h a l f h e i g h t ed. width using the Scherrer equation (ref. 8 ) a f t e r t h e c o r r e c t i o n s f o r t h e K a l , a 2 d o u b l e t and t h e i n s t r u m e n t a l broadening. I R-DRS
D i f f u s e r e f l e c t a n c e s p e c t r a have been measured between 400 and 4000 cm-' u s i n g a P e r k i n E l m e r F T - I R 1640 s p e c t r o p h o t o m e t e r equipped w i t h a d i f f u s e r e f l e c t a n c e attachment. Before e a c h measure t h e samples have been f i n e l y ground w i t h KBr. The r e s u l t s have been e x p r e s s e d i n terms of Kubelka-Munk f u n c t i o n ( r e f . 9 ) .
556 UV-Vis
DRS
D i f f u s e r e f l e c t a n c e s p e c t r a i n t h e 12500-50000 cm-' been o b t a i n e d
with a
Perkin
E l m e r LAMBDA
15
range
have
spectrophotometer
equipped w i t h a d i f f u s e r e f l e c t a n c e a t t a c h m e n t u s i n g MgO a s a r e f e r e n c e . A s p e c i a l l y d e s i g n e d quartz c e l l a l l o w e d t h e c o n n e c t i o n electric
furnace
(maximum
e l i m i n a t e t h e adsorbed 393R b e f o r e
temperature
1100K).
w a t e r , each sample
Rubelka-Munk f u n c t i o n has
In
to
order
has been o u t g a s s e d
i n t h e c a s e of been used t o e x p r e s s t h e
t h e measurement.
to
mbar) and i t s i n s e r t i o n i n a
a vacuum s y s t e m ( u l t i m a t e vacuum
Like
IR-DRS,
at the
experimental
data. RESULTS AND DISCUSSION
-N2
adsorption The channel volume of TS823 sample
was 0. 195 cm
3
g
-1
.
By
c r e a s i n g t h e t r e a t m e n t t e m p e r a t u r e up t o 1273K, o n l y a v e r y change i n N2 a d s o r p t i o n i s o t h e r m s
i s observed ( F i g . l a ,
insmall
lb).
For
higher treatment temperatures t h e z e o l i t e progressively l o s s e s i t s porosity;
at
1573K
i n i t i a l value.
A
t h e microporosity
similar
behaviour i s
is
less than shown
by
5% of the
the
external
s u r f a c e a r e a (Fig. l c ) .
y
mE 0 1 l00
.. '., .
5
O
V
z
I d, 1.0
2.0
973
1173
1373
K
Fig. 1. N2 a d s o r p t i o n a t 77K: a ) a d s o r p t i o n i s o t h e r m s (from t h e t o p t o t h e bottom: TS823, TS973, TS1173, TS1273, TS1373, TS1473, TS1573); b ) v a r i a t i o n of micropore volume w i t h t h e t r e a t m e n t t e m p e r a t u r e ; c ) v a r i a t i o n of e x t e r n a l s u r f a c e a r e a w i t h t h e treatment temperature. para-Xylene A t r. t . ,
a d s o r p t i o n and TPD t h e TS823
sample r e a d i l y
a d s o r b s p-xylene
vapours,
showing a n i s o t h e r m w e l l comparable t o t h o s e r e p o r t e d i n r e f s .
10,
11 f o r s i l i c a l i t e .
the
The adsorbed v a l u e
a t 1 mbar ( t h a t i s i n
557
l o w e r p r e s s u r e p a r t of t h e p l a t e a u r e g i o n i n o r d e r t o minimize t h e a d s o r p t i o n on t h e e x t e r n a l s u r f a c e of t h e c r y s t a l s ) c o r r e s p o n d s t o 8. 5 molecules/u. c.
As observed
for
s i l i c a l i t e (ref.
ll),
the
s o r b a t e c a n be e a s i l y removed under vacuum. The same sample p r e v i o u s l y impregnated under vacuum w i t h l i q u i d p-xylene,
TG c u r v e s r e p o r t e d i n Fig.
g i v e s under h e a t i n g t h e
The c u r v e s show two
weight l o s s e s : one below
(complex) between 330 and 500K. l i q u i d p-xylene
wetting t h e
f i r s t loss i s due t o
The
2a.
310R and t h e
external surface
of t h e
other excess
particles,
w h i l e t h e 330-500K weight loss corresponds t o p-xylene adsorbed i n the zeolite
channels.
following results:
These
i) in
attributions are
a TG
confirmed by
the
a n amorphous,
non
l i q u i d p-xylene o n l y t h e
low
experiment on
microporous s i l i c a impregnated w i t h
t e m p e r a t u r e weight l o s s i s observed ( t h e TG c u r v e i s c l o s e t o t h a t of t h e TS1573 sample shown i n Fig.
2 a ) ; i i ) t h e weight loss i n t h e
330-350K r a n g e (TS823 sample) corresponds t o 8. 2 molecules/u. c. of adsorbed p-xylene,
i n good agreement
d i r e c t a d s o r p t i o n measurement. techniques t h e
d a t a of
with t h e value obtained
Due t o t h e e q u i v a l e n c e of t h e
t h e adsorption
c a p a c i t y vs
the
thermal
t r e a t m e n t s have been o b t a i n e d by u s i n g t h e f a s t e r TG method 2b).
The t o t a l
amount of
so showing t h a t
c o r r e l a t e d each other, adsorption capacity capacity i s
adsorbed p-xylene and
up
found o n l y
t o 1273K. for
A
N2 a r e
(Fig. closely
t h e samples r e t a i n
their
of
this
higher
than
progressive loss
treatment temperatures
by two
1273K and goes t o z e r o a t 1 ' 3 K .
973
\
1173
1373
K
Fig. 2. p-xylene a d s o r p t i o n by TG a n a l y s i s : a ) s e l e c t e d TG c u r v e s ; - b ) v a r i a t i o n of 373 473 573 K adsorption capacity (calculated from t h e weight loss i n t h e 310-500R r a n g e and e x p r e s s e d a s volume of l i q u i d p - x y l e n e ) w i t h t h e treatment temperature. TS 1573
meta-Xylene a d s o r p t i o n and TG The b e h a v i o u r of m-xylene i s remarkably d i f f e r e n t from t h a t
of
558
t h e p-isomer. The a d s o r p t i o n i s v e r y slow ( t h e p r o c e s s i s n o t complete even a f t e r some t e n s of h o u r s ) so t h a t t h e e q u i l i b r i u m v a l u e s a r e d i f f i c u l t t o d e t e r m i n e (see a l s o r e f . 1 2 ) . D e s p i t e t h e low a c c u r a c y , t h e e q u i l i b r i u m v a l u e s seem t o be l o w e r t h a n t h o s e o b t a i n e d f o r t h e p-isomer i n agreement w i t h l i t e r a t u r e d a t a on ZSM-5 t y p e z e o l i t e s ( r e f . 13, 1 4 ) . Thermogravimetric a n a l y s i s on samples impregnated w i t h l i q u i d m-xylene g i v e s t h e TG c u r v e s shown i n Fig.
3a.
The TG c u r v e s
are
c h a r a c t e r i z e d , by a w e l l d e f i n e d weight l o s s i n t h e 370-500K i n t e r v a l ( b e s i d e s by a low t e m p e r a t u r e l o s s due t o l i q u i d l i k e m-xylene). According t o t h e p-xylene r e s u l t s , w e a t t r i b u t e t h e 370-500K w e i g h t loss t o m-xylene e n t r a p p e d i n t o T i - s i l i c a l i t e c h a n n e l s . On t h e examined samples, t h e 370-500K weight l o s s corresponds t o a value not exceeding 2 m o l e c u l e s / u . c . I f the above e x p e r i m e n t i s performed on T i f r e e s i l i c a l i t e and amorphous s i l i c a t h e r e s u l t s a r e c o m p l e t e l y d i f f e r e n t : t h e w e i g h t l o s s due t o d e s o r p t i o n of m-xylene o c c u r s c o m p l e t e l y below 320K ( t h e TG c u r v e s a r e c l o s e t o t h e c o r r e s p o n d i n g c u r v e of TS1573 sample r e p o r t e d i n Fig. 3 a ) . I n c o n c l u s i o n , under t h e adopted e x p e r i m e n t a l c o n d i t i o n s , T i - s i l i c a l i t e a d s o r b s m-xylene ( t h e adsorbed amount b e i n g however l o w e r t h a n t h a t found f o r t h e p-isomer) w h i l e s i l i c a l i t e does not. A second i m p o r t a n t o b s e r v a t i o n i s t h a t T i - s i l i c a l i t e shows, upon t h e r m a l t r e a t m e n t s , a loss of a d s o r p t i o n c a p a c i t y ( a s measured by ( F i g . 3b). The TS1373 sample does n o t a d s o r b any more m-xylene, w h i l e it s t i l l r e t a i n s a b o u t 70% o f t h e a d s o r p t i o n c a p a c i t y towards N2 and p-xylene.
T G ) s t a r t i n g below 1170K
I 2ht
\
1
\ TS1573 & 373
473
573
n
973
1173
1373
K
Fig. 3. m-xylene a d s o r p t i o n by TG a n a l y s i s : a ) s e l e c t e d TG curves; b ) v a r i a t i o n of adsorption capacity (expressed a s volume of l i q u i d m-xylene) with t h e treatment temperature.
559 XRD -
The XRD p a t t e r n of t h e TS823 sample shows ( F i g . 4a) a c r i s t a l l i n i t y degree (99%). The s u r f a c e a r e a c a l c u l a t e d
high from
c r y s t a l l i t e s i z e a g r e e s w i t h t h e e x t e r n a l s u r f a c e a r e a a s measured by N2 a d s o r p t i o n ( 4 7 and 39 m2 g - l r e s p e c t i v e l y ) . Upon h e a t i n g up t o 1273K, t h e c r y s t a l l i n i t y does n o t change.
A
f u r t h e r r i s e of c a l c i n a t i o n t e m p e r a t u r e c a u s e s p r o g r e s s i v e loss i n z e o l i t e c o n t e n t ( w i t h o u t s i g n i f i c a n t change i n c r y s t a l s i z e ) a t 15733 no more r e s i d u a l z e o l i t e i s d e t e c t e d ( F i g . 4b, 4 c ) .
and The
spectrum of TS1473 sample i s a m i x t u r e of T i - s i l i c a l i t e and c r y s t o b a l i t e p a t t e r n s w h i l e i n t h e spectrum of TS1573K sample o n l y t h e c r y s t o b a l i t e i s detected. a
I
b 75
Fig. 4. XRD a n a l y s i s : a ) s e l e c ted patterns; b ) v a r i a t i o n of c r y s t a l l i n i t y degree with the treatment temperature; c ) v a r i a t i o n of c r y s t a l l i t e s i z e ( c a l c u l a t e d from two d i f f e r e n t p e a k s ) w i t h t h e t r e a t m e n t t e m p e r a t u r e . 10
IR
-
30
50
DRS
The I R s p e c t r a of T i - s i l i c a l i t e shows t w o s i g n i f i c a n t bands, t h e f i r s t a t a b o u t 550 cm-l and t h e l a t t e r a t a b o u t 970 cm-' ( F i g . 5 a ) . The 550 cm-' band i s t y p i c a l of a l l t h e ZSM-5 f a m i l y z e o l i t e s and can be r e l a t e d t o t h e five-membered r i n g system c h a r a c t e r i s t i c of t h e z e o l i t e framework ( r e f . 1 3 ) . I t s i n t e n s i t y depends on b o t h c r y s t a l l i n i t y and c r y s t a l l i t e s i z e and c a n be used t o e v a l u a t e t h e t h e r m a l s t a b i l i t y of t h e samples. I n Fig. 5b t h e r a t i o between t h e i n t e n s i t i e s of t h e band a t 550 -1 -1 cm and of t h a t a t 800 cm (common t o b o t h c r y s t a l l i n e and amorphous s a m p l e s ) i s r e p o r t e d a s a f u n c t i o n of t h e t r e a t m e n t temperature. I t c a n be n o t i c e d t h a t t h e dependance of 1550/1800
560
c
b
.-u) Y
c
U
3
1000
800
600
cr
I
913
1173
1373
K
Fig. 5. IR-DRS: a ) selected I R spectra; b) v a r i a t i o n I w i t h t h e t r e a t m e n t t e m p e r a t u r e ; c ) v a r i a t i o n of I 9 7 0 ” 800 treatment temperature. upon t e m p e r a t u r e a g r e e s w e l l w i t h t h e XRD r e s u l t s . The 9 7 0 cm-l band observed on T i - s i l i c a l i t e i s a s s o c i a t e d a s t r e t c h i n g mode of a [ T i 0 4 ] u n i t i n v o l v i n g a framework T i
with (ref.
3, 1 7 ) . This band, which i s p r e s e n t on amorphous S i 0 2 - T i 0 2 g l a s s e s 14, 15) g i v e s i n f o r m a t i o n on t h e and a b s e n t i n T i 0 2 ( r e f .
concentration
of
substitutional
Ti
(either
in
the
zeolite
framework o r i n a n amorphous p h a s e ) and t h e r e f o r e on d e t i t a n a t i o n .
In Fig. 5c t h e dependence of t h e 1970/1800 r a t i o upon t u r e treatment i s i l l u s t r a t e d .
tempera-
The
f o l l o w i n g can be n o t i c e d : i) i s n o t going t o z e r o a s t e m p e r a t u r e approaches 1573K: t h i s c a n be e x p l a i n e d w i t h t h e format i o n of a n amorphous p h a s e s t i l l c o n t a i n i n g s u b s t i t u t i o n a l t i t a nium. i i ) l i k e t h e m-xylene a d s o r p t i o n c a p a c i t y , t h e ‘970”550 r a t i o d e c r e a s e s a l r e a d y a t t e m p e r a t u r e lower t h a n 1 1 7 3 K ; this means t h a t s u b s t i t u t i o n a l T i is going i n t o extraframework p o s i t i o n b e f o r e t h e o c c u r r e n c e of d r a m a t i c changes i n sample c r y s t a l l i n i t y ratio). ( a s r e v e a l e d by XRD and 1550/1800 unlike t h e previous case, t h e r a t i o
UV-Vis
DRS
The framework t e t r a h e d r a l T i
i n pure T i - s i l i c a l i t e i s
a t e d w i t h a v e r y s t r o n g band a t 48000 cm-’
associ-
having a l i g a n d t o m e t -
a l c.t. character (ref. 3 ) . Bands a t l o w e r f r e q u e n c y ( a s found on Si02-TiOZ amorphous c o p r e c i p i t a t e ( r e f . 16, 1 7 ) ) a r e i n d i c a t i v e of non t e t r a h e d r a l T i .
As a consequence t h e DRS i s a s e n s i t i v e
tool
f o r b o t h framework and extraframework T i . However, due t o i t s high i n t e n s i t y , t h e 48000 cm-‘ band c a n n o t be used f o r q u a n t i t a t i v e de-
561
t e r m i n a t i o n of t h e framework T i . can n o t be
I n f a c t t h e Kubelka-Munk values as l o w
u t i l i s e d f o r reflectance
theory
as those
ob-
s e r v e d a t 48000 cm-'. The e v o l u t i o n of DRS spectrum of t r e a t m e n t t e m p e r a t u r e i s shown
a TS sample a s a f u n c t i o n
i n Fig.
6a.
, I n the starting
of ma-
most of t h e t i t a n i u m i s i n framework p o s i t i o n (band a t 46000 cm-') w h i l e a minor p a r t is p r e s e n t a s an a n a t a s e l i k e phase -1 (band a t 30500 cm ). By i n c r e a s i n g t h e c a l c i n a t i o n t e m p e r a t u r e
terial,
t w o new bands show up: one a t f r e q u e n c y lower t h a n t h e 30500
cm
-1
band and s t r o n g l y o v e r l a p p e d w i t h t h e a n a t a s e band and t h e o t h e r -1 a t a b o u t 40000 cm The f i r s t band c a n be a s s o c i a t e d w i t h a
.
r u t i l e l i k e phase
with i t s appearance
i n agreement
highest calcination
temperatures
(1473, 1573K)
only a t
(Fig.
the
6b).
The
second peak i s of more u n c e r t a i n a t t r i b u t i o n ; however t h e p r e s e n c e of
bands
at
similar
frequency
c o p r e c i p i t a t e s w i t h v e r y low t h a t we a r e dealing with
in
Si02-Ti02
T i concentration ( r e f .
amorphous
17)
suggests
amorphous domains c o n t a i n i n g v e r y
small
T i c l u s t e r s o r i s o l a t e d T i atoms i n o c t a h e d r a l c o o r d i n a t i o n .
a t t r i b u t i o n of
the
40000 cm-'
band
is also
appearance a t moderately l o w t e m p e r a t u r e s ,
confirmed
by
before t h e collapse
t h e c r y s t a l l i n e s t r u c t u r e (1537K) ( F i g . 6 d ) .
The
its of
The p e r s i s t e n t h i g h
.15 -
.05-.
.
I
./
2.2
Y
Y
0 1 20000
40000
cm
1
U V- V i s DRS: a ) U V - V i s 0 Fig. 6. 973 1173 1373 K s p e c t r a (from t h e t o p t o t h e bottom t h e d a t a of TS823, TS973, TS1173, TS1273, TS1373, TS1473, TS1573 a r e r e p o r t e d ( t h e c a p i t a l l e t t e r s i n d i c a t e t h e p o s i t i o n of t h e bands a t t r i b u t e d t o r u t i l e l i k e phase (A), a n a t a s e l i k e phase (B), isolated o r nearly isoleted c1 o c t a h e d r a l T i (C), framework i s o l a t e d t e t r a h e d r a l T i (D)). b, d ) v a r i a t i o n of i n t e n s i t i e s a t 25500, 30500 and 40000 cm r e s p e c t i v e l y with t h e treatment temperature.
562
i n t e n s i t y of t h e peak a t 48000 c m - l i n TS1573 s u g g e s t s t h a t a h i g h p e r c e n t a g e o f Ti remains i s o l a t e d and t e t r a h e d r a l l y c o o r d i n a t e d i n a S i 0 2 - T i 0 2 s o l i d s o l u t i o n even a f t e r t h e t o t a l c o l l a p s e of t h e zeolite structure. FINAL CONS1 DERATIONS
The c o l l e c t i o n of numerical t e c n i q u e s i s shown i n Tab. 1. TABLE 1
-
values
obtained
with
various
C h a r a c t e r i s t i c s of t h e s a n p l e s c a l c i n e d at various t e n p e r a t u r e s
U2 AOSOBPTIOU EXTERUAL SURFACE AREA
SAIPLE
XYLEM ADSORPTIOU PBOH FC B-XIL
XBD AUALYSIS CRYSTALLIUITY CBISTALLITB DEGBKE
n-XyL
64
IB-DRS
08-VIS DRS
Ig60
I 25500 I 30500 I 40000
'800
'800
1.45
4.20
l'S823
,195
39
,168
,042
99
TS973
,192
37
1.354.05
34
,036 ,032
71
,188
-
97
TS1173
96
59
1.15
4.00
TS1273
,180
37
.156
,015
.953.85
TS1373
,156
34
,141
,002
81
60
TS1173
,078
16
,057
29
56
TS1573
,009
2
(.005
0
.04
.17
-23
-04
.19
.26
.04
,20
.27
-04
.I7
,61
,631.60 -48 ,95
.04
.20
,08
-31
.74 .74
-
.16
-38
1.15
I n Fig. 7 t h e r e s u l t s o b t a i n e d by n o r m a l i z i n g t h e above d a t a a r e r e p o r t e d . On t h e o r d i n a t e w e have t h e RT f u n c t i o n e x p r e s s e d as: RT
=
T' v053
-
'1573 v1573
-
Fig. 7. Normalized d a t a from v a r i o u s t e c h n i q u e s vs t r e a t m e n t temperature:
RT .75
A c r y s t a l l i n i t y d e g r e e by XRD p-xylene adsorbed by TG 0 micropore volume by N2
0
adsorption
.50
/I by I R s p e c t r p s c o p y w ia?8ns!@ a t 40000 cm- by UV-Vis s p e c t r o s c o p y A I /I by I R s p e c t r o s c o p y 0 m9@le@0adsorbed by TG 0 I
.25
0
973
1173
1373
K
563
d a t a from a
where V i n d i c a t e s t h e
volume o r
t h e micropore
the
given technique ( f o r
XRD c r y s t a l l i n i t y ) ,
i n d i c a t e s t h e t e m p e r a t u r e t o which and V1573
are
the
reference
the
band
On t h e c o n t r a r y
with
latters
t h e m-xylene a d s o r p t i o n c o r r e l a t e s w e l l w i t h c m - 1 and t h e U V - V i s peak a t 40000 cm-' which T i e x t r a c t i o n from t h e framework i s
a r e s t r u c t u r a l T i parameters.
1173K and i s accompanied by
o c c u r r i n g a t t e m p e r a t u r e a s low a s small relaxation
is
data,
are s t r i c t l y correlated
This f a c t i n d i c a t e s t h a t t h e
2 a r e mainly dependent upon t h e o v e r a l l c r y s t a l l i n i t y .
t h e I R band a t 910
v~~~ it
figure
XRD and I R 550 cm-'
which a r e c r y s t a l l i n i t y p a r a m e t e r s , and p-xylene a d s o r p t i o n .
From
can be c o l l e c t e d i n t w o groups.
I t i s most remarkable t h a t t h e N
subscript
t h e d a t a i s r e f e r r e d and
values.
apparent t h a t a l l t h e parameters
instance
the
of t h e
framework
whith subsequent
change
a
the
c h a n n e l s dimensions ( w i t h o u t s i g n i f i c a n t m o d i f i c a t i o n of t h e t o t a l p o r e volume).
m-Xylene,
having
r e s p e c t t o p-xylene and N2 transformation:
a larger
k i n e t i c diameter
with
( r e f . 1 8 ) , i s a s e n s i t i v e p r o b e of t h i s
consequently
it
correlates very
well
with
the
s t r u c t u r a l T i parameters.
As a f i n a l remark w e p o i n t o u t t h a t some a s p e c t s r e l a t e d t o t h e framework/extraframework T i ( i . e . , t h e n a t u r e of t h e d e t i t a n a t i o n process
at
temperature
s t r u c t u r e of o c t a h e d r a l treatment), a r e
below
1513K
s p e c i e s of T i
not s u f f i c i e n t l y
(Fig.
7)
and
the
formed d u r i n g t h e
c l a r i f i e d and
require
exact thermal further
i n v e s t i g a t i o n s (work s t i l l i n p r o g r e s s ).
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9
10
11 12
13 14 15 16
17
18
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F. Rodriguez-Reinoso et al. (Editors),Characterization of Porous Solids II 0 1991 Elsevier Science Publishers B.V., Amsterdam
565
STUDY OF THE PORE NETWORK OF DEALUMINATED FAUJASITES BY WATER VAPOR ADSORPTION
M.H.
SIMONOT-GRANGE1,
A.
ELM'CHAOURI1,
M. NAFISl, G. WEBERl and P.
DUFRESNE2,
F. R A A T Z ~ , J.F. JOLY* l U n i v e r s i t 6 de Bourgogne, L a b o r a t o i r e de Recherches s u r l a R B a c t i v i t B des S o l i des, CNRS UA 23, B.P. 138, 21004 D i j o n cedex, (France) 2 I n s t i t u t FranGais du P B t r o l e , B.P.
311, 92506 R u e i l Malmaison cedex, (France)
SUMMARY The a d s o r p t i o n and d e s o r p t i o n i s o t h e r m s o f w a t e r vapor a r e drawn a t 25OC f o r dealuminated HY z e o l i t e s upon framework Si/A1 r a t i o . The i s o t h e r m s a r e compared t o t h a t o f t h e p a r e n t NaY z e o l i t e . The i s o t h e r m changes i n shape f r o m t h e t y p e I t o t h e t y p e I V w i t h an h y s t e r e s i s l o o p changing f r o m t h e t y p e H4 t o t h e t y p e H2, as i n c r e a s e s t h e S i / A l r a t i o . The POLANYI-DUBININ t h e o r y i s used t o d e t e r m i n e t h e m i c r o p o r e volume a c c e s s i b l e t o water. I t decreases w i t h i n c r e a s i n g S i / A l r a t i o s , down t o z e r o a t a Si/A1 r a t i o o f 35. Such a r e s u l t i s accounted b y t h e a d s o r p t i o n on t h e h y d r o p h i l i c c e n t e r s which a r e t h e c a t i o n s (H') a s s o c i a t e d w i t h t h e s t r u c t u r a l aluminium ions, each c a t i o n b e i n g c o o r d i n a t e d b y 8H2O. INTRODUCTION The a d s o r p t i o n
and d e s o r p t i o n
dealuminated HY z e o l i t e s shape
of
procedure. the
the
i s o t h e r m s o f w a t e r vapor on a s e r i e s o f
a r e e x p e r i m e n t a l l y drawn t o s t u d y t h e
i s o t h e r m upon
both
the
Si/A1
ratio
and
the
dealumination
T h i s s t u d y i s r e a l i z e d on t h e one hand, t o a s c e r t a i n whether o n l y
shape o f t h e c u r v e foreshadows t h e v a l u e o f t h e Si/A1
dealumination
procedure and on t h e o t h e r hand,
To
allow
for
a
satisfactory
r a t i o and t h e
t o t e s t the efficiency o f
t h e m o l e c u l a r H20 probe f o r d e t e r m i n i n g t h e s t r u c t u r a l volumes.
change i n
interpretation
and secondary p o r e the
adsorption
and
d e s o r p t i o n i s o t h e r m o f w a t e r vapor on t h e p a r e n t z e o l i t e i s s t u d i e d i n wide ranges o f temperature and f i l l i n g c o e f f i c i e n t . LITERATURE SURVEY Zeol i t e s Only a few works deal w i t h t h e a d s o r p t i o n and d e s o r p t i o n e q u i l i b r i u m o f w a t e r vapor on Nay.
The s t u d i e s a r e r e s t r i c t e d t o an i s o t h e r m a t 25°C
in
a small range o f f i l l i n g c o e f f i c i e n t s ( r e f . 1 ) . On t h e o t h e r
hand,
the
study o f water
vapor a d s o r p t i o n on Y z e o l i t e s
566
dealuminated b y steaming,
EDTA o r S i c 1 4 s e t t h e f o l l o w i n g i n f o r m a t i o n s
( r e f . 2) : *for
a
Si/A1
ratio
smaller
than
5.5,
the
water
adsorption
decreases weakly w i t h d e c r e a s i n g t h e aluminium c o n t e n t .
capacity
T h i s i s due t o t h e
d e c r e a s i n g m i c r o p o r e volume and t o t h e d e c r e a s i n g i n t e r a c t i o n energy between w a t e r and z e o l i t i c framework.
Such a decrease i s a l i n e a r f u n c t i o n o f t h e
Si/A1 r a t i o f o r v a r i o u s c a t i o n i c dealuminated f a u j a s i t e s ( r e f . 3). *for a Si/Al adsorption
r a t i o h i g h e r t h a n 5.5,
results
t h e s t r o n g r e d u c t i o n o f w a t e r vapor
f r o m t h e v e r y h i g h decrease o f
t h e i n t e r a c t i o n energy
between w a t e r and t h e z e o l i t i c framework. Water does n o t more f i l l c o m p l e t e l y m i c r o p o r e s and t h e z e o l i t e becomes l e s s and l e s s h y d r o p h i l i c ( r e f . 2). *for
r a t i o h i g h e r t h a n 30,
a Si/A1
z e o l i t e s a r e n e a r l y hydrophobic and
t h e w a t e r vapor s t r o n g a d s o r p t i o n i n t h e range o f h i g h p r e s s u r e s i s due t o c a p i l l a r y condensation i n t o mesopores c r e a t e d d u r i n g steaming. Adsorption o f water
hydrocarbons
adsorption,
with
o r nitrogen r e f l e c t s the
respect
to
t h e biporous
same t r e n d as d i d
c h a r a c t e r o f dealuminated
2,
samples ( s t r u c t u r a l m i c r o p o r e s and secondary mesopores) ( r e f s .
4-6).
The
non p o l a r c h a r a c t e r o f n-hexane l e a d s t o use such a m o l e c u l e f o r d e t e r m i n i n g t h e m i c r o p o r e volume b y t h e POLANYI-DUBININ t h e o r y . size
distributions
from
adsorption
isotherms
The s t u d y o f t h e p o r e
and
mercury
porosimetric
measurements c h a r a c t e r i z e s two p o r e d i a m e t e r s o f t h e secondary p o r e network, t h e one between 3 nm and 4 nm and t h e second t o about 20 nm ( r e f s . 7, 8). The decrease o f t h e t o t a l a d s o r p t i o n c a p a c i t y o f steamed HY z e o l i t e s w i t h respect
to
that o f
the parent z e o l i t e
i s due t o t h e o b s t r u c t i o n o f t h e
secondary network by t h e a l u m i n i c s p e c i e s c r e a t e d d u r i n g t h e e x t r a c t i o n o f t h e framework aluminium atoms. An a c i d l e a c h i n g s o l u b i l i z e s t h e extraframework species
and
so
m i c r o s c o p y and
on
increases
widely
n i t r o g e n adsorption
the
are
adsorption
used
to
capacity.
study
Electronic
quantitatively
the
f o r m a t i o n o f t h e secondary network and t o c h a r a c t e r i z e t h e r e l a t i v e e x t e n t o f t h e s t e p s o f c a l c i n a t i o n and a c i d l e a c h i n g ( r e f . 9). Lastly, the
a s t u d y on dealuminated s y n t h e t i c m o r d e n i t e s ( r e f .
adsorption
of
water
w a t e r m o l e c u l e s and
molecules
hydrophilic
involves
centers
specific
which
are
o r c a t i o n s a s s o c i a t e d w i t h s t r u c t u r a l aluminium atoms. dealuminated z e o l i t e s ,
10) shows t h a t
i n t e r a c t i o n s between
silanol
groups
(-SiOH)
For h i g h l y c r y s t a l l i n e
t h e pore b u l k s t r u c t u r e i s n o t c o n s t i t u t e d o f s i l a n o l
groups b u t s o l e l y o f - S i - 0 - S i -
w h i c h a r e hydrophobic. T h e r e f o r e , t h e a d s o r p t i o n
o f w a t e r m o l e c u l e s t a k e s p l a c e on a l u m i n i c s i t e s . The decrease o f t h e number o f such s i t e s i n v o l v e s a decrease o f t h e adsorbed w a t e r c a p a c i t y . a l u m i n i c s i t e i s a s s o c i a t e d w i t h f o u r w a t e r molecules.
Every one
567
The POLANYI-DUBININ t h e o r y The POLANYI-DUBININ
adsorption p o t e n t i a l
t h e o r y i s used t o c h a r a c t e r i z e
10). An i s o t h e r m a t a g i v e n temperature
t h e m i c r o p o r e network o f z e o l i t e s ( r e f .
W,
T (expressed i n volume adsorbed p e r a c t i v a t e d z e o l i t e mass u n i t ,
as a
f u n c t i o n o f t h e r e l a t i v e p r e s s u r e p/po) i s t r e a t e d i n t h e DUBININ-RADUSHKEVICH model ( r e f . 11) (denoted D-R) i n t h e l i n e a r f o r m l o g l o g W = l o g Wo
-
W
= f[(Tlog
po/p)2]:
D(T1og po/p)2.
The o r d i n a t e a t t h e o r i g i n o f t h e l i n e d e f i n e s t h e v a l u e o f t h e maximal volume,
Wo, a c c e s s i b l e t o water. The D-R model b e i n g v e r i f i e d o n l y i n t h e range o f t h e more g e n e r a l l i n e a r model o f DUBININ-ASTAKHOV
high f i l l i n g c o e f f i c i e n t s , (ref.
12) (denoted D-A) : l o g W
= f[(Tlog
~ ~ / p )i s~ t] h e n used. Two domains
a r e commonly d e f i n e d w i t h t h e w a t e r adsorbate : t h e one i s t h e t y p e D-R domain w i t h n=2 i n t h e h i g h p/po
t h e second i s t h e t y p e 0-A domain w i t h n = I
13) and even n=2 ( r e f .
(ref.
characterizes specific
D-R
and,
domain
[(Tlog
non =
14) i n t h e l o w p/po.
The t y p e
D-A
domain
i n t e r a c t i o n s between w a t e r and c a t i o n s and t h e t y p e
specific
0 of
interactions
the
D-R
(ref.
15).
domain l e a d s t o
The
extrapolation
the determination
of
to the
Wo. The volume a t t h e t r a n s i t i o n
m i c r o p o r e maximal volume a c c e s s i b l e t o water,
p o i n t between t h e two D-R and D-A domains i s Wol i n w h i c h s p e c i f i c i n t e r a c t i o n s predominate. EXPERIMENTAL I s o t h e r m s and using
a
Mc
isobars
Bain
are
balance
drawn
well
from
suitable
thermal to
c o n t r o l l e d by means o f a " c o l d p o i n t " ( r e f . in
graduated
in
small
steps
by
successive
increasing
increments.
(or
a c t i v a t e d i n s i t u a t 400°C a t 10-1 Pa.
impose w a t e r
measurements
vapor
pressures
16). The curves a r e c o n s t r u c t e d
decreasing)
Before
gravimetric
each
p r e s s u r e o r temperature
experiment,
the
zeolite
is
For adsorption t h e i n i t i a l s t a t e i s
t h e a c t i v a t e d s t a t e a t 350"C, e i t h e r a t a p r e s s u r e p under i s o b a r i c c o n d i t i o n s , o r a t a p r e s s u r e o f 101 Pa under i s o t h e r m a l c o n d i t i o n s . a s t a t e close t o saturation.
The f i n a l s t a t e i s
F o r d e s o r p t i o n measurements t h e p r e v i o u s d e f i n e d
i n i t i a l and f i n a l s t a t e s a r e r e v e r s e . The dealuminated z e o l i t e s a r e p r e p a r e d e i t h e r b y steaming (denoted s), by a c i d l e a c h i n g (denoted a )
o r b y a c o m b i n a t i o n o f t h e s e two procedures
(denoted sa o r sas). RESULTS AND DISCUSSION Shape of w a t e r vapor a d s o r p t i o n i s o t h e r m s and p o r e network
568
( i ) NaY z e o l i t e .
The w a t e r vapor a d s o r p t i o n i s o t h e r m a t 25°C o f t h e p a r e n t
z e o l i t e i s t y p i c a l o f a microporous a d s o r b e n t ( F i g . the type I ( r e f .
17).
The m i c r o p o r e t o t a l volume,
0-R p l o t drawn f r o m t h e i s o t h e r m ( F i g . geometrical
volume
of
micropores
since
2).
1 ) . The i s o t h e r m i s o f
Wo,
i s estimated from the
T h i s volume r e p r e s e n t s t h e t o t a l
water molecules f i l l
too sodalite
cavities.
0.3
7
c? (=1
\
E 0.2
0 .I
-
r 0.2
0
0.4
0.6
0.9 P f Po
F i g . 1. A d s o r p t i o n b l a c k key symbols] and d e s o r p t i o n [white key symbols3 i s o t h e r m s o f H20 on NaY ( 0 , o ) and H Y s l ( S i / A l = 7.2 ; B , o ) , HYs2 ( S i / A l = 9.9 ; A , A ), HYS3 ( S i / A l = 12.4 ; v , v ) a t 25°C. However,
e
the
i s o t h e r m d e s c r i b e s a narrow range o f f i l l i n g c o e f f i c i e n t s
f r o m 0.3 t o 1. F o r e x t e n d i n g t h i s range down t o 0.03,
isobars are t h e r e a f t e r
drawn a t p r e s s u r e s o f 1712 Pa and 2209 Pa f o r temperatures r a n g i n g f r o m 350°C t o 25°C. D-A 8 )
From t h e model D-A
t h e two a d s o r p t i o n domains appear : t h e domain
e < 0.23, and t h e domain D-R and Wo a r e 0.076 cm3.g-1 and 0.324
(n=2) i s l o c a t e d a t 0.23 ( F i g . 2). W o l
(ii)Dealuminated HY z e o T i t e s . zeolites
(denoted H Y s l ,
HYs2,
HYs3) w i t h S i / A l
HY,
29 and 29 r e s p e c t i v e l y ,
respectively.
r a t i o s o f 7.2,
9.9
and 12.4
HYsa2, HYsa3) w i t h Si/A1 r a t i o s
prepared b y a c i d l e a c h i n g o f t h e l a s t t h r e e
samples ; a n o t h e r HYsa4 sample w i t h a S i / A l
homologue i n t h e HY,
cm3.g-l
The s t u d i e d samples a r e : t h r e e steamed
r e s p e c t i v e l y ; t h r e e z e o l i t e s (denoted H Y s a l , o f 24.5,
(n=2) i s located a t
r a t i o o f 29 h a v i n g n o t any
series.
The i s o t h e r m s a t 25°C a r e drawn i n F i g s . 1,3 and f i t t h e f o l l o w i n g f a c t s : *isotherms
of
steamed HY,
z e o l i t e s are o f the type
I form although a
d e v i a t i o n i n shape i s more and more observed w i t h i n c r e a s i n g d e a l u m i n a t i o n ; however, two o f these i s o t h e r m s e x h i b i t a n e a r l y p a r a l l e l branches h y s t e r e s i s c l o s e d a t v e r y l o w p r e s s u r e ; such a shape o f an h y s t e r e s i s l o o p o f t h e t y p e H4 c h a r a c t e r i z e s narrow s l it - 1 ike p o r e s ( r e f . 17). W i t h r e s p e c t t o Nay,
d e a l u m i n a t i o n i n v o l v e s a decrease o f t h e a d s o r p t i o n
569
2
-
5-0.82
5
\
-
3 -1.10
- -1.38 0 0
-1.66
30
20
10
-0.54
*
Isotherm 2 S ' C * Isobar 16.9mbar A
Nay
"Isobar 21.8mbar
-
I
F i g . 2. Dubinin-Astakhov t r a n s f o r m a t i o n f o r a d s o r p t i o n o f H20 on Nay. c a p a c i t y i n t h e t o t a l range o f p r e s s u r e ; t h e h i g h i s t h e decrease, i s the dealumination.
Indeed,
the total
volume,
Wo,
decreases
the high
t o 35 X as
t h e Si/A1 r a t i o i s i n c r e a s e d f r o m 2.5 t o 12.4 ( T a b l e 1). F o r t h e HYsl sample,
t h e i s o t h e r m does n o t r e a c h s u f f i c i e n t l y l o w f i l l i n g
c o e f f i c i e n t s t o c h a r a c t e r i z e t h e two a d s o r p t i o n domains.
On t h e o t h e r hand,
t h e volume i n which s p e c i f i c i n t e r a c t i o n s between c a t i o n and w a t e r predominate i s much h i g h e r f o r HYs2 and HYs3 t h a n f o r NaY and i t i s l o c a t e d a t a v a l u e o f t h e r e l a t i v e p r e s s u r e p/po much h i g h e r t o o ( T a b l e 1). T h a t r e s u l t s from t h e decrease o f t h e r e s t r a i n t due t o d e a l u m i n a t i o n . *isotherms o f HYsal, dealumination,
by the
HYsa2. HYsa3 come c l o s e t o t h e t y p e I V as i n c r e a s e s progressive
i n t h e range o f h i g h pressures,
formation
of
a type
H2 h y s t e r e s i s
loop
showing up t h e b i p o r o u s c h a r a c t e r o f samples.
The comparison o f e v e r y i s o t h e r m o f HYsa w i t h t h a t o f HYs shows t h a t t h e a d s o r p t i o n c a p a c i t y o f w a t e r decreases a t s m a l l e s t f i l l i n g c o e f f i c i e n t s whereas
P/Po
F i g . 3. A d s o r p t i o n [black key symbols] and d e s o r p t i o n [ w h i t e key symbols] HYsa2 (Si/A1 = 29 ; A ,A), i s o t h e r m s o f H20 on HY, 1 ( S i / A l = 24.5 ;.,a), HYs,3 ( S i / A l = 29 ; V , V ! , HYsa4 ( S i / A l = 29 ; O , 0 ) .
570
TABLE 1 C h a r a c t e r i s t i c values o f t h e sample f i r s t s e r i e s ( w i n cm3.g-1) a : a c i d leaching)
( s : steaming;
0
Y
Si/Al
ao/A
2.5 7.2 9.9 12.4 24.5 29 29 29
24.44 24.39 24.36 24.30 24.29 24.29 24.29
Wol
wo
PIP0
0.076 lom4 n o t accessible 0.126 0.33 0.158 0.35 0.151 0.16 0 0 0
P/Po
0.324 0.275 0.252 0.209 0.327 0.095 0.095 0.085
0.63 0.73 0.68 0.71 0.77 0.29 0.34 0.41
pip0
Wf
0.314 0.262 0.240 0.348 0.340 0.330 0.314
0.90 0.85 0.86 0.90 0.99 0.99 0.99
TABLE 2 C h a r a c t e r i s t i c values o f t h e HY second s e r i e s (w i n crn3.g-l) a : acid leaching)
( s : steaming ;
0
Y
Si/A1
HYs5 4.4 HYsa6 24 HY (sas6) 1 35 HY(sas6)1 35
ao/A 24.55 24.30 24.28 24.28
p/p0
wol
n o t accessible 0 0 0
wo
PiPo
0.302 0.083 0 0
0.83 0.25
W f
0.275 0.09 0.03
p1p0
--
0.99 0.99 0.99
TABLE 3 S p e c i f i c adsorption energy o f t h e cation-(HzO)N c l u s t e r ( s : steaming ; a : a c i d leaching)
Y
N1
El/kJ.mol-l
NI*El/kJ
NaY NYs5 NYS 1 NYs2 NYs3 NYsa6 NYsa2 NYsa3 NYsa4
0.9
28.0
25
5.7 9.0
7.2 5.3
41 48
N 4.2 5.9 9.0 11.4 11.9 6.9 9.8 9.8 8.7
E/kJ.mol-l 15.8 10.9 7.7 4.3 3.6 9.3 6.2 7.1 6.3
N*E/kJ 66 65 69 49 43 64 60 69 55
571
ao/A
t
24.2
Fig. 4. Dependence o f m i c r o p o r e volume a c c e s s i b l e t o water, r a t i o and t h e u n i t c e l l parameter, a.,
Wo, on t h e Si/A1
i t increases
t o dealumination
at
highest f i l l i n g
coefficients.
I t i s due
and t o d i s s o l u t i o n o f t h e extraframework a l u m i n i c s p e c i e s which o b s t r u c t e d cavities.
The t o t a l
p o r e volume,
l o o p c l o s u r e a t h i g h pressures,
estimated a t the p o i n t o f the hysteresis Wf,
i s q u i t e t h e same as t h a t o f NaY f o r
a l l HYsa ( T a b l e 1). The i s o t h e r m o f HYSa4 does n o t e x h i b i t p a r a l l e l branches o f a d s o r p t i o n and d e s o r p t i o n
i n t h e l o w p/po c o n t r a r y t o what i s observed
f o r t h e o t h e r compounds.
For
the
three
zeolites
of
29
Si/A1
ratio,
only
one
volume,
Wo,
is
d e t e r m i n a t e d f r o m t h e D-R p l o t and i t s v a l u e i s o f t h e same o r d e r o f magnitude f o r t h e t h r e e samples
( T a b l e 1). The absence o f t h e volume W o l
indicates
so, a weakening o f t h e e l e c t r i c a l f i e l d due t o t h e e x t r a c t i o n o f t h e s t r u c t u r a l aluminium i o n s and w h o l l y ,
t h e s t r o n g s p e c i f i c i n t e r a c t i o n s H20-H+ f a d e o u t
i n f a v o r o f the non-specific dispersion forces. adsorption
characterized
c a p i l l a r y condensation
by
the
hysteresis
i n t o mesopores.
A t high pressures the strong
loop
results
The volume
thereafter,
from
o f w a t e r condensed i n t o
t h e s e mesopores i s 74 % o f t h e t o t a l volume o f adsorbed water.
On t h e o t h e r
Wo a r e d e f i n e d f o r t h e sample w i t h a Si/A1 r a t i o o f 24.5. *as show i n F i g . 4, Wo i s a l i n e a r f u n c t i o n o f t h e S i / A l r a t i o , e x c e p t
hand, Wol and for
HYsa4 ( S i / A l
=
24.5).
i s r e l a t e d t o the Si/Al
Since t h e u n i t c e l l ratio,
these two q u a n t i t i e s ( F i g .
4).
parameter o f such z e o l i t e s
a s i m p l e c o r r e l a t i o n i s e s t a b l i s h e d between It follows that,
t h e z e o l i t e i s hydrophobic 0
when t h e Si/A1 r a t i o i s h i g h e r t h a n 35 ( a o = 24.77 A). of
samples
allows
to
assume
that
the
only
The active
high c r y s t a l l i n i t y centers
of
the
i n t r a c r y s t a l l i n e network a r e H+ c a t i o n s a s s o c i a t e d w i t h framework aluminium ions.
Therefore,
d e a l u m i n a t i o n r e s u l t s i n t h e h y d r o p h o b i c i t y o f micropores,
512
unless they have been f u l l y destroyed during the procedure, which i s a l s o an assumption t o be taken i n t o account. Another sample s e r i e s i s prepared t o a s c e r t a i n t h i s r e s u l t : HYs5 (steamed sample with a Si/A1
r a t i o of
4.4),
HYsa6 (prepared by steaming and acid
leaching successively), with a Si/A1 r a t i o of 24,
HY(sas6)1 and HY(sas6)~
(prepared by an additional steaming of HYsa6 a t two d i f f e r e n t temperatures) with a Si/Al r a t i o of 35 (Table 2).
As a n t i c i p a t e d , only Wo i s obtained f o r HYs5 since the range of f i l l i n g c o e f f i c i e n t s i s too narrow a t 25°C. Not any micropore volume i s characterized and HY(sas6)2. On the o t h e r hand, HYsa6 i s characterized by
f o r HY(,,,6)1
only one volume Wo,
HYSa1
which value i s much smaller than t h a t of i t s homologue
of the f i r s t s e r i e s . The values o f the volume Wo of this new s e r i e s
agree well w i t h the previous established c o r r e l a t i o n s (Table 2). The isotherms of
HYSa1
and HYsa6 with the same Si/A1
r a t i o a r e very
d i f f e r e n t in shape. The HYSa1's i s close t o the type I whereas the HYsa6'S i s of t h e type IV. Moreover the mesopore network i s characterized f o r the l a s t sample while the micropore network i s retained f o r the f i r s t sample (Tables 1, 2). Aluminic s i t e energy ( i ) Water molecules associated with an aluminic s i t e . The number ( N ) of water molecules associated with an aluminic s i t e i s : N = n(HZO)/n(Al) ;
n(H20)
is
the
number of
calculated from W o l
and Wo,
water molecules per
gram of
anhydrous z e o l i t e
and n(A1) the number of the framework aluminium
atoms per gram of anhydrous z e o l i t e calculated from t h e framework and global Si/A1 r a t i o s ( r e f . 18). For steamed HY, z e o l i t e s an aluminium atom i s associated with 6-9H20 (N1) in the domain of s p e c i f i c i n t e r a c t i o n s whereas an aluminium i s associated with 6-12H20 (N) i n the t o t a l micropore domain accessible t o water (Table 3 ) . For HYsa samples, an aluminium atom i s associated with 9H20 on an average in the micropore domain accessible to water (non-specific i n t e r a c t i o n s ) (Table 3 ) . Thereafter, a cation Ht associated with a s t r u c t u r a l aluminic s i t e i s surrounded by 8H2O on an average, the adsorption on these s i t e s being therefore characterized by the determination of e i t h e r , W o l f o r steamed samples o r , Wo f o r z e o l i t e s undergoing an additional acid leaching. The increase of the t o t a l number of water molecules associated w i t h an aluminic s i t e , a s a function
573
of
dealumination r a t i o i n t h e HY,
series,
i s a t t r i b u t a b l e t o an a d d i t i o n a l
a d s o r p t i o n on t h e aluminium i o n s o f extraframework species ( g l o b a l a l u m i n i c site).
However,
f o r Nay,
a framework aluminium atom i s surrounded by o n l y
1H20 i n t h e domain o f s p e c i f i c i n t e r a c t i o n s and t o 4H20 i n t h e t o t a l micropore domain.
This arises
from r e s t r a i n t s
which disappear as f a r
as micropores
a r e destroyed d u r i n g dealumination. (ii)
Adsorption
energy.
The a d s o r p t i o n energy,
i s c a l c u l a t e d from t h e
f o l l o w i n g equation :
E = 2.3(n--l)/n
.R
Dl/n ;
n=2 and 0 i s t h e constant c h a r a c t e r i z i n g t h e 0-A and D-R equations. The molar energies E l (D-A)
and E (D-R)
o f water a d s o r p t i o n on HY decrease
s t r o n g l y (three-times s m a l l e r ) w i t h r e s p e c t t o NaY (Table 3). The
ratio
characteristic
E1/E
which
is
about
two,
is
the
same as
that
of
Nay.
A
energy o f an a l u m i n i c s i t e may be c a l c u l a t e d by m u l t i p l y i n g
t h e molar energies E l and E by t h e number o f water molecules associated w i t h such a s i t e (framework aluminium,
o r g l o b a l aluminium).
Thereafter,
the t o t a l
energy associated w i t h a framework aluminium o f t h e t o t a l micropore domain a c c e s s i b l e t o water i s a s c e r t a i n e d t o be independent o f t h e Si/A1 r a t i o and o f t h e same o r d e r as t h a t o f t h e t o t a l volume o f NaY (Table 3). CONCLUSION Adsorption and d e s o r p t i o n isotherms o f water vapor on HY z e o l i t e s change i n shape w i t h i n c r e a s i n g dealumination.
Thus,
f o r o n l y steamed samples t h e
isotherm i s c l o s e t o t h e type I whereas f o r z e o l i t e s submitted t o an a d d i t i o n a l a c i d leaching,
i t i s c l o s e t o t h e type I V w i t h a p r o g r e s s i v e l o w e r i n g o f
p o i n t B towards
the
the
pressure
hydrophobic c h a r a c t e r o f adsorption.
axis,
what
emphasizes
a more and more
The micropore volume a c c e s s i b l e t o water
and t h e u n i t c e l l parameter a r e s i m p l y c o r r e l a t e d w i t h t h e framework S i / A l r a t i o . The water molecule does n o t a l l o w t o determine q u a n t i t a t i v e l y s t r u c t u r a l and secondary pore volumes,
b u t appears t o be a s e l e c t i v e molecular probe
o f t h e s t r u c t u r a l aluminium ions.
For HY z e o l i t e s and w i t h o u t any r e s t r a i n t ,
8H2O a r e associated w i t h such a framework a l u m i n i c s i t e . REFERENCES 1 D.W. Breck, Z e o l i t e Molecular Sieves, 2 U. Lohse, G. Engelhardt, E. A l d o r f ,
3 4
J. Wiley and Sons, New-York, 1974. P. Kolsch and M. F e i s t , Adsorption
Science and Technology, 3 (1986) 149-158. Y. L i and L.V.C. Rees, Z e o l i t e s , 6 (1986) 217-220. U. Lohse, H. Stach, H. Thamm, W. Schirmer and A.A. A l l g . Chem., 460 (1980) 179-190. C.
Isirikjan,
Z.
Anorg.
514
M.W. Anderson, J.J. K l i n o w s k i , Chem. SOC. Faraday SOC. 1, 82 (1986) 35693586. 6 U. Lohse, G. E n g e l h a r d t and V. Patzelova, Z e o l i t e s , 4 (1984) 163-167. 7 U. Lohse, M. M i l d e b r a t h , Z. Anorg. A l l g . Chem., 476 (1981) 126-135. Materials: 8 T.E. Whyte, R.A.D. B e t t a , E.G. Derouane and R.T.K.Baker,Catalytic r e l a t i o n s h i p between s t r u c t u r e and r e a c t i v i t y , i n : American Chemical S o c i e t y (Ed.), ACS Symposium S e r i e s 248, San Francisco, C a l i f o r n i a , June 13-16, 1983, E l s e v i e r , Washington D.C. , 1984, pp. 157-200. 9 J. Lynch, F. Raatz and P. Dufresne, Z e o l i t e s , 7 (1987) 333-340. 10 N.Y. Chen, J. Phys. Chem., 80 (1976) 60-64. 11 M.M. Dubinin, E.D. Z a v e r i n a and L.V. Radushkevich, Zh. F i z . Khim., 21 (1947) 1351-1355. 12 M.M. D u b i n i n and V. A. Astakhov, B u l l . Acad. S c i . USSR, 20 (1971) 3-7. 13 M.H. Simonot-Grange, F. Be1 hamidi-El Hannouni , Thermochimica Acta, 77( 1984) 311-326. 14 A. Elm'Chaouri, M.H. Simonot-Grange, H y d r a t i o n thermodynamic f u n c t i o n s o f h e c t o r i t e and m o n t r o n i t e f r o m e x p e r i m e n t a l i s o t h e r m s and t h e P o l a n y i D u b i n i n t h e o r y , 9 t h I n t e r n a t i o n a l C l a y Conference, Strasbourg, France, August 28 September 1, 1989. 15 A.V. K i s e l e v , D i s c Faraday SOC., 40 (1965) 205-231. 16 M.H. Simonot-Grange, C l a y s and C l a y M i n e r a l s , 27 (1979) 423-428. 17 IUPAC " R e p o r t i n g P h y s i s o r p t i o n d a t a f o r g a s - s o l i d systems w i t h s p e c i a l Reference t o t h e D e t e r m i n a t i o n o f S u r f a c e Area and P o r o s i t y " (K.S.W. Sing, D.H. E v e r e t t , R.A.W. Haul , L. Moscou, R.A. P i e r o t t i , J. Rouquerol, T. Siemieniewska) Pure Appl. Chem., 57 (1985) 603-619. 18 P r i v a t e communication, I n s t i t u t F r a n c a i s du P b t r o l e , France.
5
F. Rodriguez-Reinosoet al. (Editors), Characterization of Porous Solids II 0 1991 Elsevier Science Publishers B.V., Amsterdam
515
and Yu.A.ELT5KOV I n s t i t u t e o f P h y s i c a l Chemistry, USSR Academy o f S c i e n c e s , Lenin p r . 31, Moscow 117071 ( USSR ) N.A.ELLTdKOVd
ABSTIiACT The a d s o r p t i o n i s o t h e r m s o f d e x t r a n e s , p o l y e t h y l e n e g l y c o l s and p o l y s t y r e n e s from d i l u t e s o l u t i o n have been s t u d i e d f o r n o m u s s i l i c a and carbon s o r b e n t s . The dependences o f maximum a d s o r p t i o n v a l u e s on t h e m o l e c u l a r weight o f t h e s e polymers have been analysed. These dependences have been used f o r t h e c a l c u l a t i o n o f s u r f a c e areas and p o r e volume d i s t r i b u t i o n s . The comparison o f r e s u l t s o b t a i n e d by macromolecularand Hg-porosimetric methods h a s shown s a t i s f a c t o r y agreement. INTRODUCTION
Porous s t r u c t u r e o f s o r b e n t s and c a t a l y s t s d e t e r m i n e s b a s i c a l l y t h e a d s o r p t i o n , d i f f u s i o n and dynamical c h a r a c t e r i s t i c s o f many s o r p t i o n and c a t a l y t i c p r o c e s s e s . T h e r e f o r e t h e d e t e r m i n a t i o n o f pore s t r u c t u r e p a r a m e t e r s ( s u c h as p o r e volume arid s u r f a c e a r e a ) i s a t t r a c t e d t h e a t t e n t i o n o f many s c i e n t i s t s . T r a d i t i o n a l methods f o r t h e s t u d y o f porous s t r u c t u r e ( t h e He;p o r o s i m e t r y and c a p i l l a r y method) r e q u i r e d u s u a l l y t h e complex s e t up, which has many d i s a d v a n t a g e s and l i m i t a t i o n s ( r e f . 1, 2). P r e s e n t l y t h e polymer s t a n d a r d s f o r s i z e e x c l u s i o n chromatography are commercial chemicals, which have d i f i n i t e c h a r a c t e r r c t i c s ( w i t h t h e p o l y d i s p e r s i b i l i t y r4,/Gn l e s s 1.5). The fundarlierit a l s t u d i e s o f polymer a d s o r p t i o n by porous s o l i d s have shown t h a t t h e a c c e s i b l e i n n e r s u r f a c e a r e a o f p o r e s depends on t h e r e l a t i o n s h i p between t h e p o r e s i z e and t h e polymer m o l e c u l a r weight ( r e f . 3 , 4). F l e x i b l e macromolecules o f l i n e a r polymers a r e c o i l e d i n d i l u t e s o l u t i o n . Such macromolecule c o i l i s formed as i n a r e s u l t o f i n t r a - and i n t e r m o l e c u l a r i n t e r a c t i o n s between c h a i n segments and s o l v e n t molecules. The macromolecular c o i l s p r e a d s when it i s adsorbed on t h e s o l i d s u r f a c e . I n t h i s work we d e s c r i b e t h e polymer a d s o r p t i o n on some POr o u s s i l i c a and carbon a d s o r b e n t s w i t h t h e aim o f e v a l u a t i n g
576
t h e porous s t r u c t u r e p a r a m e t e r s . EY2KRIrGNTAL Porous a d s o r b e n t s New data have been o b t a i n e d f o r c h a r a c t e r i z i n g s i l i c a and carbon a d s o r b e n t s e a r l y used i n p r e v i o u s a d s o r p t i o n o r chromatigraphy s t u d i e s (ref. 3 , 5 9). The porous s t r u c t u r e charact e r i s t i c s o f s o r b e n t samples c o n s i d e r e d are given i n Table 1, where AgdT and d a r e s u r f a c e a r e a detemined by t h e BKT method, and predominant pore d i a m e t e r , r e s p e c t i v e l y . TAELE 'I S u r f a c e Areas and pore diameters of samples Silica d ref. d ref. mL/g nm nm
-
tBET
KCK-2 C-80
cx-2
340
85
50
14
55
110
3, 5
CS-1
6
cs-3
cs-2
550
20
870 2 00
14
5
7
8
9
It can be s e e n t h a t i n g e n e r a l s e l e c t e d s i l i c a and carbon a d s o r b e n t s commonly choosen a r e t h e most s u i t a b l e s o l i d s f o r o u r study. Before t h e a d s o r p t i o n experiments a l l samples were d r i e d i n vacuum a t 1 5 O o C for about 4 hours.
Polymer s t a n d a r d s The polymers d e x t r a n s (DX), p o l y e t h y l e n e g l y c o l s (PEG) and p o l y s t y r e n e s (PS) were used i n o u r experiments. I n p a r t i c u l a r y we used PEG (Schuchardt, YRG) 300, PEG 600, PEG 1000, PEG 6000, PZG 15000, PEG 20000 and P d G 40000, d e x t r n n s (Pharmacia, Sweden) T 20, T 40, T 70, T 100, T 160, T 500 and T 2000, p o l y s t y r e n e s (Waters, USA) w i t h m o l e c u l a r weight from 5000 t o 2000000. The average weight-molecular m a s s Mw o f t h e s e polymers v a r i e s i n range from 300 t o 2 GO0 000. The p o l y d i s p e r s i b i l i t y ( a s t h e r a t i o o f a v e r a g e weight-molecular mass t o average number-molec u l a r mass) o f t h e s e polymers w a s less 1.5. Twice d i s t i l l e d wat e r and f r e s h d i s t i l l e d t e t r a c h l o r e m e t h a n e were used as t h e solvents. Polymer a d s o r p t i o n measurements have been c a r r e i d o u t i n d i l u t e scli.~tio::s I>; t r a d i t i o n a l method as d e s c r i b e d elsewhere ( r e f . 9, 10). Adsorption e x c e s s v a l u e s r l f o r polymers were c a l c u l a t e d by t h e d i f f e r e n c e ( 0 C ) o f t h e s o l u t i o n concentrat i o n s b e f o r e and a f t e r a d s o r p t i o n measurements.
-
where m ti v e l y
.
and
ma
- masses
o f s o l u t i o n and a d s o r b e n t , respec-
RESULTS AND DISCUSSION Ad s o r p t i o n is0 therms
f-A
Y
I
I
8-A
mg g-'
F i g u r e 1 shows t h e a d s o r p t i o n i s o t h e r m s o f PEG and DX i n aqueous s o l u t i o n s on mesoporouse carbon s o r b e n t CS-2 (Carboraff i n e ) . The c o i n c i d e n c e o f a d s o r p t i o n i s o t h e r m s f o r PEG 20000 and DX T 20 can be noted. Bone a d s o r p t i o n i s o t h e r m s a r e char a c t e r i z e d by s h a r p maximum. It can bee s e e n from Fig. 1 t h a t increase with t h e a d s o r p t i o n v a l u e s € o r PEG on t h i s sample growth o f m o l e c u l a r weight (from ?EG 300 till PEG 60001, b u t t h e n t h e a d s o r p t i o n amounts d e c r e a s e w i t h t h e f u r t h e r growth i n PIIq v a l u e s . The r e s u l t s i n d i c a t e t h e d i f f e r e n t a c c e s s i b i l i t y o f mesopore s u r f a c e f o r t h e polymer macromolecules a d s o r p t i o n .
578
Iwlolecular weight dependences f o r polymer a d s o r p t i o n The r e l a t i o n s h i p between t h e maximum a d s o r p t i o n v a l u e and t h e a v e r a g e m o l e c u l a r weight Mw a l l o w s i n some c a s e s t o e s t i m a t e t h e mean s t a t i s t i c a l t h i c k n e s s of adsorbed polymer lgye r . If adsorbed macromolecular c o i l s s t r a i g h t e n e d o u t i n t h e f i e l d o f a d s o r p t i o n f o r c e s , it c o u l d be expected t h a t t h e deU pendence nmax v s M, is v e r y s l i g h t . Such dependence w a s f o und for PEG a d s o r p t i o n i n w a t e r s o l u t i o n on g r a p h i t i z e d carbon b l a c k ( r e f . 4.). U The dependences nmax v s PIw were o b t a i n e d f o r t h e p o l y s t y r e n e a d s o r p t i o n i n d i l u t e s o l u t i o n s o n porous s i l i c a and carbon s o r b e n t s ( F i g . 2). It i s e v i d e n t t h a t t h e p o s i t i o n s o f curve ma-
Fig. 2. P l a t e a u a d s o r p t i o n o f o l y s t y r e n e as f u n c t i o n o f moleCS-1 (27, ‘2-80 ( 3 ) , CX-2 (4). c u l a r m a n s . KCK-2 (I), xima and t h e i r s h a p e s depend b a s i c a l l y on t h e p o r e s t r u c t u r e par a m e t e r (i. e. p o r e volume d i s t r i b u t i o n ) . The low Lroping b r a n c h s nf c’ii’ves a r e r e s p o n s i b l e for t h e macromolecule p e n e t r a t i o n i n t o a d s o r b e n t p o r e s and a l l o w s t o e v a l u a t e t h e f r a c t i o n o f i n t e r n a l p o r e s u r f a c e accessible f o r t h e macromolecular c o i l s . The maxima p o s i t i o n s a r e c h a r a c t e r i s t i c s f o r t h e mesopores i n a d s o r b e n t sample, which a r e e n t i r e l y a c c e s s i b l e f o r macromolecular c o i l s w i t h t h e g i v e n diameter o r less.
579
Hydrodynamic diameter D of f l e x i b l e c h a i n macromolecule i n d i l u t e s o l u t i o n can be c a l c u l a t e d by u s i n g Flory-Fox e q u a t i o n (ref. 12)
-
Flory parameter, equal where [ q 1 - i n t r i n s i c v i s c o s i t y and @ t o 2.6 lo2' l/mole ( r e f . 12). The D-values f o r p o l y s t y r e n e macr o m o l e c u l a r c o i l s i n CC14 s o l u t i o n s were c a l c u l a t e d by t a k i n g i n t o account t h e Mark-Howink e q u a t i o n p a r a m e t e r s k = 2.75 10-4 and a = 0.69 r e s p e c t i v e l y ( r e f . 13). For PEG i n w a t e r s o l u t i ons k = 1.25 lo-' and a = 0.78; for DX i n water s o l u t i o n s k = 9.78 and a = 0.5 ( r e f . 14). We presume t h a t macromol e c u l a r c o i l s w i t h hydrodynamic d i a m e t e r D can p e n e t r a t e i n t o p o r e s which have openings i n t o c a v i t y more o r same d i a m e t e r ( d >, D). The dependences o f s u r f a c e coverage f o r d i f f e r e n t macromolecules on t h e i r c o i l s i z e s can be c a l c u l a t e d f o r t h e adsorbents with various pore s t r u c t u r e s .
Pore d i s t r i b u t i o n s A s t h e r e i s a r e l a t i o n s h i p between t h e macromolecular c o i l s s i z e and t h e p o r e opening diameters, it i s p o s s i b l e t o c a l c u l a t e t h e dependence o f a c c e s s i b l e p o r e volume V on p o r e diame t e r s d. For t h e model o f c y l i n d e r p o r e shape we used t h e f o l l o wing e q u a t i o n
and d are e x p r e s s e d i n m2 g-' and nm, r e s p e c t i v e l y . Fig. 3 shows t h e com2arison o f t h e p o r e volume d i s t r i b u t i o n s c a l c u l a t e d by macromolecular and mercury p o r o s i m e t r y methods for two macroporous s i l i c a samples C-80 and CX-2. Porograms found by irercury p o r o s i m e t r y were o b t a i n e d i n Karnaukhov l a b a t I n s t i t u t e o f C a t a l y s i s ( N o v o s i b i r s k , USSR). T h i s f i g u r e d e m o n s t r a t e s satisf a c t o r y agreement between t h e maxima p o s i t i o n s o f t h e s e d i s t r i butions. Table 2 g i v e s t h e numerical v a l u e s o f p o r e s t r u c t u r e 1 charact e r i s t i c s f o r s i l i c a samples and carbon s o r b e n t s . Values o f d and V were c a l c u l a t e d from t h e c u r v e s o f d i f f e r e n t i a l p o r e volume d i s t r i b u t i o n by t h e g r a p h i c i n t e g r a t i o n . The comparison if
A
580
o f c o r r e s p o n d i n g v a l u e s shows a l s o good agreements i n some cases.
n 0.4
I
I
1
I
d, nm
120
80
40
Pig. 3 . Comparison o f p o r e d i s t r i b u t i o n i n C-80 (1, 2) and CX-2 (3, 4) measured by polymer- (1, 3) o r Hg-porosimetry (2, 4)
TABLE 2 V and d
-
-
v a l u e s for s i l i c a and carbon s o r b e n t s . 1 polymer porosimetry, 2 c a p i l l a r y condensation, 3 mercury porosimetry
-
-
--__-
I .
Kch-2
1
CS-2
C-&3
’>
‘_
i
3
-
1
Cb-3
2
‘1
3
CONCLUSION Macromo l e c u l a r po ros imet ry shows t h e new p o s s i b i l i t i e s for t h e s t u d y o f meso- and macroporous s o l i d s t r u c t u r e (ref. 15). Tfowever, t h i s method r e q u i r e s many polymer s t a n d a r d s w i t h nar-
581
row m o l e c u l a r weight d i s t r i b u t i o n s and t h e o p t i m i z e d c o n d i t i o n s
for t h e predominant a d s o r p t i o n o f f l e x i b l e macromolecules i n s o l v e n t , molecules o f which a r e adsorbed weakly. Accoding t o u s f u r t h e r experiments are need f o r t h e development o f macromolec u l a r p o r o s i m e t r y method. ACKNOWLEDGEMENT I n t h i s work we used many R i s e l e v ’ s i d e a s . F33FEHEn’CES 1 2
3 4
5
6
7 8
S.J. Gregg and K.S.W. S i n g , Adsorption, S u r f a c e Area and P o r o s i t y , 2 nd. Ed., Academic ?ress, London, 1982. A.V. Kiselev, I n t e r m o l e c u l a r I n t e r a c t i o n s i n Adsorption and Chromatography, Vysshaya Shicola, Noscow, 1986. E.K. Bogacheva, A.V. K i s e l e v , Yu.S. N i k i t i n and Yu.A. E l t e kov, Zh. Fiz. Khim., 39 (1965) 1777. Yu.A. Eltekov, Pure and Appl. Chem., 61 (1989) 1987. Yu.S. N i k i t i n , i n I . V . B e r e z i n ( E d i t o r ) , Immobilized Enzimes, p. 68, DIGU, i~~oscow (1976). S.P. Ztdanov, A.V. K i s e l e v , A.S. N a s a n s k i i and Yu.A. E l t e kov, Koll. Zh., 39 (1977) 354. N.A. E l t e k o v a , D. Berek and I. Novak, Zh. 7 i z . Khim., 63
(1989) 2675.
M.M. Dubinin, L.I. Kataeva and h.S. Polyakov, Izv. Acad. Nauk SSSR, S e r . Khim., (1986) 719. 9 M.M. Dubinin, L.I. Kataeva, N.S. Polyakov and V.F. Surovik i n , Izv. Acad. Nauk SSSR, S e r . Khim., (1987) 1453. I 0 N.A.Eltekova a n d Yu.A. E l t e k o v , Zh. Fiz. Khim., 60 (1986)
2272.
H r n e i r , and E.J. Xornmelzvaal, Bcta. H y d r o c h h Hydrobiol., 6 (1978) 153. 1%P. F l o r y , S t a t i s t i c a l Mechanics of Chain Molecules, C o r n e l l , I t h a c a , N. Y., (1969). 13 A.V. K i s e l e v , A.S. N a s a n s k i i and Yu.k. Eltekov, Koll. Zh., 7 1 J. Chudoba, R.
37 (1975) 556.
1 4 Yu.A. E l t e k o v , N.M. S t r a k h o v a , I. Kalal, I. Peska and I. Stemberg, J. Polym. S c i . , Polym. Symp,, 68 (1980) 247. I 5 N.A. E l t e k o v a and Yu.A. E l t e k o v , USSR. P a t . , 1500914 (1980).
This Page Intentionally Left Blank
F. Rodriguez-Reinosoet al. (Editors), Characterization of Porous Solids ZI 0 1991 Elsevier Science Publishers B.V., Amsterdam
583
FORMATION OF SECONDARY PORES IN ZEOLITES DURING DEALUMINATION: INFLUENCE OF THE CRYSTALLOGRAPHIC STRUCTURE AND OF THE SVAI RATIO H. AJOT', J.F. JOLY', J. LYNCH', F. RAATZ' and P. CAULLET* 'INSTITUT FRANCAIS DU PETROLE, 1 et 4 avenue de Bois M a u , 92506 Rueil Malmaison Cedex, France 2ENSCM~,3 rue A. Werner, 68093 Mulhouse Cedex, France SUMMARY The parameters affecting the genesis of mesopores in zeolites during dealumination have been investigated The formation of mesopores is essentially controlled by the structural defects density. Structural defects correspond to: i) framework vacancies, ii) crystalllographic defects andiii) trivalent elements incorporatedin the framework. At least two parameters control the structural defects density: the initial Si/Al ratio and the synthesis conditions. INTRODUCTION Dealumination of zeolites with low initial SVA1ratios leads to materials with new textural properties. The creation of extensive secondary porous network has been observed in dealuminated zeolites with low initial SVAl ratios, such as Y (refs.1-4), omega (ref. 5). offretite (ref. 6), mordenite (ref. 7) and CSZ-1 (ref. 8) structures. Two main parameters seem to affect the formation of mesopores in zeolites during dealumination: the crystallographic structure and the SUA1ratio, in addition the synthesis conditions have certainly to be considered as well. In order to determine the effect of each of these parameters, different zeolites with different initial Si/Al ratios were submitted to classical dealumination treatments. The effect of the initial SVAl ratio was studied with mordenite (Si/A1=5 and 10) and Y zeolite (Si/A1=2.7 and 7). The effect of the synthesis conditions was studied in the case of Beta zeolite synthesized in alkaline and fluoride medium. The specific effect of the crystallographic structure will be deduced from the two first approaches and from literature data. EXPERIMENTAL Starting from two Toyo Soda mordenites NaMI(Si/Al=5) and NaM2 (Si/AI=IO), dealuminated mordenites DeHMl and DeHM2 were prepared by steaming of low sodium forms NH.,Ml and NH.,M2 respectively, at 1073K (4 hours) followed by acid leaching in 3N HNO, at 373K (2 hours). A low sodium NH,Y form, obtained by repetitive ammonium exchanges of NaY (LZY-52 Union Carbide) was submitted to (NI-I,,)+3F6treatment leading to DeHY 1 with a framework Si/A1 ratio close to 7 (total SVAl ratio: 5.4). In a second step, DeHYl was submitted to steaming treatment at 1073K leading to DeHY2. DeHY2 has been acid leached in 1.5N HNO, solution leading to DeHY3.
584
Beta1 zeolite was synthesized using the classical alkaline medium synthesis method. The HBeta form was then obtained by calcination under air at 823K followed by two successive NH,NO, exchanges leading to the low sodium form HBetal. HBetal was submitted to a direct acid leaching treatment in 0.5N HNO, solution at 373K during 2 hours, leading to DeHBetal. Beta2 zeolite has been synthesized in the fluoride medium by J.L. GUTH and his team (ref. 10). A 823K calcination leads directly to the H form HBeta2. DeHBeta2 was then obtained by steaming at 1023K, 4 hours, followed by acid leaching in 3N HNO, solution, 2 hours at 373K. The main physicochemical charecteristics of dealuminated samples are summarized in table 1. TABLE 1 Physicochemical characteristics of dealuminated zeolites.
I
reference
aonm
Si/Al,
DeHMl
---
> 110
DeHYl DeHY2 DeHY3
2.445 2.427 2.426
7
DeHBetal DeHBeta2
---
Si/Al total 110
DX% 98
BETS m2/g 468
5 5 12 29 1lo
87 107 111
628 470 559
---
573 338
I
I I
---
41
62
-----
---
Nitrogen adsorption The nitrogen adsorption-desorptionisotherms were recorded at 77K with a#SORB apparatus (licence IFP). The samples were pretreated at 723K during 12 hours under vacuum (lo-’ torr) before isotherm acquisition. Electron microscopy The internal porosity of zeolites was investigated by observation of ultrathin sections of grains embedded in resin, using a Jeol. 120CX transmission microscope (TEM). To avoid artefacts many grains in different orientations were observed enabeling a global analysis. RESULTS . .. . 1. p (i) Nitrogen adsorption The complete nitrogen isotherms of DeHMl and DeHM2 are reported in figure 1 and are characterized by an hysterisis loop with a lower closer point at about P/po=O.42. This indicates that, as
585
0.4 cn
.6 0.3 v
%
0.2
E 3
-6 0.1
>
O.P 1 0.2 0.4 0.6 0.8 . 1 Partial pressure
"8!0
012 0:4 016 0:8 Partial pressure
1 0
Fig. 1. Complete nitrogen isotherms at 77K of A) DeHM1, B) DeHM2, C) DeHBetal and D) DeHBeta2.
Fig. 2. Microtome sections of dealuminated mordenites: a) NH4M1, b) DeHM1, c) NH,M2 and d) DeHM2.
586
in the case of dealuminated HY (ref. 3), the mesopores are not directly connected to the exterior of the crystals leading to catastrophic desorption of mesopores. The hysterisis is more developed in DeHMl indicating a more developed secondary porous network than in DeHM2. (ii) Electron microscopy Figure 2 depicts characteristic mipgraphs of m M 1 , DeHM1, NHJ42 and DeHM2 samples. As expected the starting materials show no evidence of mesopores, individual grains exhibiting uniform contrast. Mesoporescan be directly seen in DeHM 1 as low density regions. These mesopores are approximately equiaxed with average diameter about 5 nm and appear randomly distributed throughoutthe grain sections.No evidence for direct connection of the mesopores to the grain surface is observed. In D e w sample, mesopores are only rarely observed and on a small number of grains.
Kx Electron microscopy Figure 3 depicts micrographs of DeHY 1, DeHY2 andDeHY3samples. As previouslymentionned in the literature (ref. 9), the (NH.,),SiF6 treatment leads to mesopores close to the exterior of the crystals due to the fact that dealuminationof the bulk crystal is limited using this technique. These mesopores have diameters ranging from 10 to 25 nm. Steaming at 1073K leads to the apperance of mesopores randomly distributed throughout the crystals sections. Their average diameter, about 30 nm, is much larger than that previously described for steamed W Y (refs. 1-4). Nodules of 4 nm to 6 nm diameter are present in steamed samples. In addition, when the zeolite grains of DeHY2 are oriented so that (111) type lattice fringes are visible, the pores are seen to be straight-edged with faces perpendicular to the (111) direction.
..
2. (Beta z&& .) (i) Nitrogen adsorption Completenitrogen isotherms of DeHBetal and DeHBeta2 are reported in figure 1. The isotherm of DeHBetal exhibitsan hysterisisloop indicatingthat despite high initial Si/Al ratio (10)mesopores have been created during dealumination. Their formationis certainlydue to the presenceof a high density of defects in the structure,clearly visible in high resolution micrographs as an apparent distorsion of the lattice planes (figure 4). These mesopores seem not to be connected to the exterior of the crystals as indicated by the catastrophic desorption at PPo about 0.42. In contrast, the isotherm of DeHBeta2 exhibits a slight hysterisis loop. (ii) Electron microscopy Migrographs of ultra-microtome sections of DeHBetal and DeHBeta2 are shown in figure 4. DeHBetal exhibits a very dense network of small (< 4 nm diameter) pores fquently superposedin the micrograph due to the relatively large (> 50 nm) section thickness. In the case of DeHBeta2 the mesopores are of particular type. They are cylinders running along directions perpendicular to the c-axis. Viewed end on these pores are seen to traverse the solid (at least over a length equivalent to the section thickness), providing a direct connection to the exterior of the crystals.
587
Fig. 3. Microtome sections of dealuminatedHY zeolites: a) DeHY 1, b) DeHY2, c) DeHY3 and d) DeHY3 (high magnification).
Fig. 4. Microtome sections of Beta zeolites: a) metal (high magnification), b) DeHBetal, c) DeHBeta2 section perpendicularto c-axis and d) DeHBeta2 in a plane containing c axis.
588
DISCUSSION The combined use of nitrogen adsorption and CI'EM analysis leads to a coherent description of the formation of secondary pores in zeolites duringdealumination by classical techniques (steam and acid leaching treatments). Effect of the- i In the case of mordenite, the effect of the initial Si/Al ratio on mesopores formation during
dealurnination is clear. Dealurnination of mordenite with WA1ratioof 5 leads to mesopores formation. When the initial Si/Al ratio is greater than 10 very few mesopom are created even during severe dealumination. In the case of HY with initial framework Si/Al ratio close to 7, mesopores are created during dealurnination. These mesopores are larger (diameters up to 30 nm) than expected and exhibit a particular shape. This could be related to a particular distribution of framework aluminium atoms after the (NH&SiF6 treatment during which only 60% of the initial aluminium have been removed (specific aluminium atoms may have been extracted). It thus appears that if the initial Si/Al ratio is a parameter controlling the genesis of mesopores in zeolites during dealurnination, the initial crystallographic distribution of aluminium has also to be considered. .. Effect of the 7 The specific effect of the synthesis conditions (alkaline or fluoride medium) has been studied in the case of Beta zeolite which present the particularity to be composed of at least two polytypes (refs. 11,12).
The case of Beta zeolite appears particularly since mesopores have been created during dealumination despite relatively high Si/A1 ratios (10 and 17). Considering alkaline medium synthesized Beta zeolite, mesopores are seen as cavities with average diameter of 4 nm. Mesopore generation could be related to the density of structural defects present in this zeolite. The case of Beta zeolite synthesized in fluoride medium is similar since the genesis of mesopores could also be explained by the presence of structural defects. But as the mesopores are seen to be cylinders running along particular directions, we can suppose that these structural defects are oriented in specific directions too. The exact nature of these defects is uncertain but they are most likely related to the presence of polytype stacking since they exhibit specific crystallographic orientations. The density and perhaps the nature of such structural defects can be strongly influenced by the synthesis conditions. One may suspect the defects to be either locally high concentrations of aluminium or faults in polytype stacking sequences leading to a high disorder.
To rationalize the influence of each of the parameters (initial Si/Al ratio, synthesis conditions) on the formation of mesopoms during dealumination, we propose the following scheme: During dealumination, apart from framework dealumination, formation of aluminium rich nodules occurs. These nodules will lead to mesopores after acid leaching treatment providing that the framework Si/Al ratio is high enough so as not to be damaged by the acid leaching. The aluminium rich nodules arise from local framework destructions due to high local density of vacancies created
589
by aluminium extraction or already present in the as-synthesized zeolite. The initial Si/Al ratio appears logically to be one of the most important parameters for mesopores formation since it controls the framework aluminium density, thus the density of potential vacancies (A1 atoms). Post synthesis modifications ("secondary synthesis") can also lead to particular framework aluminium distributions and thus to particular vacancies distributions. In addition to existing or potential framework vacancies, slructural defects may also be present in as-synthesized zeolites (ex. Beta zeolite). Structural defects also lead to mesopores formation during dealumination. If such defects are characterized by specific orientations, the mesopores created can present similar orientations (this is the case for dealuminated fluoride medium synthesized Beta). The term "structural defects" could also be generalized by considering that it could refer to: i) framework vacancies, ii) crystallographic defects and iii) potential framework vacancies (Al, Ga, Fe etc..). Trivalent elements are thusconsideredas structural defects with respect to mesopore formation. A high density of structural defects will lead in most cases to mesopores formation in the course of dealumination treatments. CONCLUSION In this study, starting from different zeolitic structures (Y, mordenite and Beta) with different Si/A1 ratios, the parameters affecting the genesis of mesopores during dealumination have been investigated Taking into account our results and literature data, we can propose the following scheme for mesopores formation: a single structural factor can be identified which plays the major role in the formation of mesopores during classical dealumination treatments. This parameter is the structural defects density and distribution, and is related to: i) the density of existing vancies in as-synthesized zeolites, ii)the presence of crystallographic defects, and iii) the density of trivalent elements incorporated in the framework (Al, Ga, Fe..). Initial Si/Al ratio and synthesis conditions are thus indirectly two factors controlling the genesis of mesopores in zeolites. This general scheme has the advantage of predicting the behaviour of other solids not studied here. It thus appears for instance that it will be very difficult to improve the porosity of high silica zeolites by conventional dealumination treatments if the as-synthesized zeolites have a low density of structural defects. ACKNOWLEDGMENTS We would like to sincerely acknowledge Mrs BURNICHON, DUPONT, LEVEQUE,RUSSMANN and TOROSSI for the preparation of dealuminated zeolites and adsorption measurments. REFERENCES V. Bosacek, V. Patzelova, D. Tvaruzkova, D. Freude, U. Lohse, W. Schimer, H. Stach and 1. H. Thamm. J. of Catal., 61 (1980) 435-442. A. Zukal, V. Patzelova and U. Lohse, Zeolites, Vo16 (1987) 133-136. 2.
3. 4.
J. Lynch, F. Raatz and P. Dufresne, Zeolites, Vol7 (1987) 333-340. W. Schirmer and H. Tham, Izv Akad Nauk Gruz SSR Ser Khim, 5 (1979) 217.
590
5. 6. 7. 8. 9.
B. Chauvin, P. Massiani, R. Dutame, F. Figueras, F. Fajula and T. Des Courieres, Zeolites, V0110 (1990) 174-182. C. Fernandez, J. Vedrine, J. Grosmangin and G. Szabo, Zeolites, Vo16 (1986) 484-490. B.L. Meyers, T.H. Fleish, G.J. Ray, J.T. Miller and J.B. Hall, J. of Catal., 110 (1988) 82-95.
S. Cartlidge, H.U. Nissen and R. Wessicken, Zeolites, Vol9 (1988) 346-349 J. Lynch, F. Raatz and Ch. Delalande,Studies in Surface Science and Catal, Elsevier, 39 (1988) 547-557. 10. non published results 11. J.M. Newsam, M.M.J. Treacy, W.T. Koetsier and C.B.De Gruyter, proc. R. Soc.London, A, 1988,420 (1859), 375. 12. J.B. Higgins, R.B. Lapierre, J.L. Schlenker, A.C Rohrman, J.D. Wood, G.. Kerr and W.J. Rohrbaugh. Zeolites. Vol8 (1988), 446-452.
F. Rodriguez-Reinoso et al. (Editors), Characterization of Porous Solids II 0 1991 Elsevier Science Publishers B.V., Amsterdam
591
VACUUM THERMAL STABILITY AND TEXTURAL PROPERTIES OF ATI'APULGITE
J.M. CASES(~),Y. GRILLET(~),M. FRANCOIS(~),L. MICHOT(~),F.VILLIERAS(~) and J. YVON(l) (1) Centre de Recherche sur la Valorisation des Minerais et U.A. 235 - B.P. 40 54501Vandoeuvre Cedex (France). (2) Centre de Thermochimie et de Microcalorim6trie - 26, rue du 1412me R.I.A. 13003Marseille (France). SUMMARY Evolution of the external surface area and the two types of microporosity of attapulgite (structural and inter-fiber) were examined as a function of a vacuum thermal treatment upt to 500OC. The methods used include: controlled transformation rate thermal analysis, N2 and Ar low temperature adsorption calorimetry, water vapor adsorption gravimetry and quasi equilibrium gas adsorption procedure of N2 at 77K and C02 at 273 and 293K. Depending on the outgassing conditions,i.e. the residual pressure, the structure folds 150 to 7OOC. For lower temperature, only a part (18%) of the structural microporosity is available to N2, 13% to argon and 100% to C02.With water, the structure can rehydrate after the structure is folded up to an outgassing temperature of 225°C. INTRODUCI'ION Attapulgite or palygorskite is a fibrous aluminum-magnesianclay rare used by man for a long time because of their sorptive properties (refs. 1-2).Studies of the structure have shown that attapulgite is made up of talc-like layers arranged in long ribbons stuck together to form the fibers and in staggered rows separated by channels parallel to the fiber axis. These channels are referred to as structural micropores or intramicropores. The unit cell parameters are: a sin p = 12.7 A; b = 17.9 A; c = 5.2 A (ref. 1). In a half unit cell, four H20 molecules are present in the channels (zeolitic water) and four others are bound to octahedral cations. On heating, these latter water molecules are lost in two stages; when the first two water molecules are lost, the structure collapses by alternate rotation of ribbons folding, (refs. 3-4). The porous structure is complicated by the sticking of fibers with each other which creates an intermicroporosity (ref. 5). As many authors suggested that the availability of the structural channels to different adsorbents as nitrogen molecules is limited (ref. 2), the aim of this study was to use different adsorbents (N2, Ar,C02, H20 vapor) and methods able to reveal information about relative pressures < 0.07 in order to distinguish the filling of the two kinds of microporosity and the adsorption on external surface mainly consisting of (011 ) crystal corrugated faces. These studies were made as a fonction of both surface coverage and thermal treatment, i.e. as the porous structure progressively folds and fibers sinter.
592
SAMPLE AND METHODS The attapulgite studied here was from the Montagne de Reims (France) and was supplied by BRGM (OrlCans, France). The approximate structural formula is si8 (A11.38 Fe0.22~' Fe().312+ 0 0 . 8 9 ) 0 2 0 (OH)2 (H20)4 (H20)4 Kg.13+ Nq.01+ C q 0 2 ~ + The . major impurities are quartz (4%), anorthite (0.8%), calcite (0.6%), anatase (1%) and mica (1.0%). Seen under transmission microscope, the size of fibers is variable ranging from 0.5 to 2.0 p m in length and 250 to 360 A wide. Outgassing for adsorption microcalorimetry and thermal analysis was carried out by controlled transformation rate thermal analysis (CTRTA) (ref. 6). Its interest is both the rather high resolution achieved on the thermal analysis curves (ref. 7) and the possibility of carrying out the experiment directly with the sample bulbs needed for adsorption microcalorimetry. The experimental conditions selected were a sample mass of about 0.260 g, a residual pressure of 2 Pa over the sample and a dehydration rate of 2.77 mglh. Adsorption microcalorimetry of N2 and Ar at 77K was carried out with an equipment described by Rouquerol (ref. 8) and which associates quasi equilibrium adsorption volumetry with isothermal low temperature microcalorimetry (using Tian Calvet heat flow-meters) so that two curves are continuously recorded (heat flow and quasi equilibrium pressure) as a function of the amount of gas introduced into the systems. Continuous plots of the adsorption isotherm and of the derivative enthalpy of adsorption Aads h vs surface coverage may easily be derived (refs. 4,7).
A quasi equilibrium gas adsorption procedure recently presented (ref. 9) was used to examine surface heterogeneity and microporosity of attapulgite in more details. With this method a slow, constant and continuous flow of adsorbate (C02 at 273 and 293K, nitrogen at 77K) was introduced into the adsorption cell. From the recording of the quasi equilibrium pressure (in the range of 0.01 to 5 . lo4 Pa) vs time, the adsorption isotherms were derived. The experimental conditions were a sample mass of about 0.400 g with outgassing under 0.1 Pa up to a final temperature of 25,70, 100, 130 and 15OOC for C 0 2 and 25,70 and 380°C for nitrogen. Adsorption gravimetry of water vapor was carried out with the experimental apparatus described in ref. 10. Prior to each experiment 100 mg samples were outgassed with a residual pressure of 0.1 Pa during 18 h and a temperature of 25,70, 100, 130,225,300,380 and 500OC. RESULTS AND DISCUSSION The dehydration curve can be divided in three steps that successively correspond to the evolution of 1) the zeolitic and adsorbed water on external surface (T < 75"C), 2) coordination water linked at the edge magnesium atoms inside of the channels (in two times, domains 75-150OC and 15O-37O0C,two molecules each and weight losses 10.48% of the final mass), 3) structural(2.61%), one molecule due to two hydroxyls from the octahedral layer of the talc ribbon) and decomposition of calcite. According to ref. 3, the structure folds when approximately half of the coordination water is removed, i.e. here under 2 Pa residual pressure between 100 and 130OC. Regarding now the enthalpy curves (Fig.1 and 2) notice that the curves obtained for outgassing temperatures lower than that corresponding to the folding may be separated into three parts:
593
lbads
'hl A
25 -0-
100°C
-'-130°C
20
150°C +
225°C
+
380°C
-x-
500°C
15
10
5 i 0
I
0,2
0,4
0,6 0.8 surface coverage
1
1.2
Fig. 1. Derivative enthalpy of adsorption versus coverage for attapulgite-nitrogen systems at 77K and various outgassing temperatures - Part a,where the derivative enthalpy of adsorption is constant as it has been observed either
on homogeneous surface or homogeneous porous solid (molecular sieves). It is therefore reasonable to assume this part corresponds to the filling of the structural or intramicroporosity. The value indicated at point A is no longer detectable on samples obtained for outgassing temperature higher than that corresponding to the folding of the crystal, suggesting that the structural microporosity is not available to nitrogen or argon molecules. Point A corresponds to a nitrogen liquid volume adsorbed of about 38.8 mm3 . g-l, and 26.5 mm3 . g-l for argon and final outgassing temperature of 25°C (Table 1). - Part 0,where the derivative enthalpy of adsorption decreases (down to inflexion point C) and which is likely due to the filling of the inter-fiber microporosity or to defect in the arrangement of the structural units (ref. 5). The inter-fiber micropore volume, as measured from the width of region 0, is not influence; by final outgassing temperature for nitrogen (- 22.2 mm3. g-l - Table 1). The values of IAadS hl measured with nitrogen increase up to 380°C (more than 26 KJ . mole-l). This phenomenon is more likely due to the increased energy of the adsorption sites for the quadrupolar nitrogen molecules than to a smaller size of the micropores (only a slight increase is observed with argon). - Part y, where the monolayer capacity is reached on the external surface of the fibers. This part goes up to 6 = 1 (which corresponds to Emmett and Brunauer's point B). The width of y allows determination of an external surface area which is kept constant with outgassing temperature up to 500"C.The arithmetical mean value obtained with nitrogen (64m2/g) is higher than for argon (54 m2/g). The difference could be attributed to the cross sectional area taking into account the
594
calculation of the specific surface area (nitrogen 16.2 A2, argon 13.8 Az).The molecules do not cover the same area on the corrugated attapulgite surface as on a flat surface. In contrast to sepiolite (ref. 4), the constancy of domains 13 or y with outgassing temperature suggests that there is no important change in shape or size for inter-micropores or fibers due to the folding of the crystal and to structural modification at higher temperature. The value of external surface area, thus calculated, corresponds to that obtained from statistical measurements by transmission electron microscopy (58 m2 g-1) (ref. 7).
IAads
19
'hl
Jr(kJ'mo'e'
A
-0-
100°C
-I-
130°C
-
-O-
-
150°C 225°C 380°C
-"- 500°C
5
I
0
0,2
0,4
0.6
0,8
1
1,2
surface coverage
Fig. 2. Derivative enthalpy of adsorption versus coverage for attapulgite-argon system at 77K and various outgassing temperatures The liquid volume VB corresponding to the amount adsorbed at "point B" may also be used to calculate an "equivalent specific surface area" (Table 1, column 3). Here the word equivalent is, of course, used to point out the partial inadequacy of the above calculation for a microporous solid in which the molecule does not cover all the same area as on a flat surface (ref. 11) Table 1 shows that area for attapulgite is maximum at 100°C when the zeolithic water has been lost and then decreases up to about 130 m2. g-l after the structure is tilted when about half of the bound water is driven off, and alternate ribbons rotate positively or negatively to close the channels (intramicroporosity)forming what is known as folded structure. Tables 2 and 3 give the main results obtained from C 0 2 and water vapor adsorption respectively. The folding of the structure under different outgassing condition (0.1 Pa) is associated with a decrease of the micropore volume accessible to C02 observed between 70 and 130OC. The volume of gas, which once adsorbed is able to completely fill the micropores, was calculated using Dubinin's equation (ref. 12) and convertied into liquid volume using 1.08 and 1.05 g/cm3 for liquid C 0 2 at 273K and 293K respectively. The values obtained for temperature lower than that
595
corresponding to the folding of the structure are higher than the values observed for nitrogen and argon.
TABLE 1. Low temperature adsorption calorimetry results for palygorskite
(1) Total specific surface area ; (2)monolayer capacity obtained from the B point per unit mass of adsorbent; (3) micropore volume per unit mass of adsorbent as calculated with density of the liquid adsorptive; (4) external specific surface area. TABLE 2. Micropore volumes Vo (liq) of paligorskite obtained from C@ adsorption
25
70
100
130
273
0.2279
0.2556
0.1438
0.0241
293
0.2274
0.0719
150
0.0373
596
For water, the monolayer capacity calculated by the B.E.T. method was converted in equivalent surface area using a cross sectional area of 14.8 A2 for the water molecule. Results plotted in table 3 (column 4) show that regarding rehydration after heating at different temperatures, the attapulgite can rehydrate after the structure is folded up to a final outgassing temperature of 225OC. This value is the same that observed for sepiolite (ref. 4). Beyond this temperature, the new bonds originating in the anhydrous structure resist rehydration. From the structural parameter and formula it is possible to calculate the theoretical microporosity. The value obtained is 0.2096 0 3 g-1. If the value of intermicroporosity obtained from nitrogen and argon are taken into account (arithmetical mean 0.0241 cm3/g), the sum (0.2337 cm3/g) is in good agreement with the value derived from C02 adsorption. It is possible to conclude, as observed with sepiolite (ref. 9), that C02 fills all the microporosity, nitrogen no more than 18% and argon about 13%. Classical parameters given for the dimension of channels (3.7 x 6.4 A2) (ref. 1) and that derived from the zeolitic water content give a value for intramicroporosity of about 0.085 cm3 g-l. This is, incidently, near of the total volume obtained either for nitrogen and argon at point TABLE 3. Equivalent specific surface area and energetic constant as calculated from the BET theory and obtained from water adsorption
s total
C
m2/g
0.0580 0.0819 0.0805 0.0774
381
0.0819
399
0.0198
98
~~
I I I
22.5 25
I
0.0198
98
0.0270
135
I
50 13
597
B or for water from the monolayer, capacity calculated from the BET therory. But these values contain the inter-fiber microporosity and the external surface area. Thus, the dimension usually given for the channels are too low to account of all the adsorption data. In order to check the influence of vacuum condition on the folding temperature, an another run was conduced with the quasi-equilibrium gas adsorption procedure on a sample outgassed at 7OoC during 4 hours with a residual pressure of 10-5 Pa. The isotherm obtained with N2 at 77K is plotted in Fig. 3 in the form 8 (where 8 = Va/Vm) vs In (P/Po) where Va represents the adsorbed volume, Vm the monolayer capacity, and P/Po the relative equilibrium pressure. This plot can be used to study surface heterogeneity (ref. 13). The BET treatment leads to a liquid V, value of 0.0415 cm3 g-1 that corresponds well to the values given in Table 1 indicating that the structure is already folded. Then it is possible to plot (AWA In P/Po) against In (P/Po). That plane gives access to the different
homogeneous domains of the surface (ref. 13). Using a special procedure and BET treatment (ref. 14) for each homogeneous domain, three different domains are observed : 1) high energetic domain A which represent 27% of the total liquid volume V, 2) moderate energetic domain B (16%), 3) low energetic domain C (56%). In these conditions, the cumulative BET isotherms fit completely the experimental curve. The domain C corresponds to a surface of 65 m2 g-1 in good agreement with the value for the external surface area given in Table 1 and the general interpretation of the adsorption enthalpy curves. This complementary run show that the inter-fiber microporosity could be divided in two domains (A + B) corresponding to a liquid volume of 0.0178 cm3/g, a value slightly lower than those presented in Table 1.
I
IFTA 0.5
0
-20
-13.6
-10
-6.2
-3.5
I.@G ( P / h J
Fig. 3. The heterogeneity of attapulgite outgassed at 7OoCand 10-5Pa observed by quasi equilibrium gas adsorption procedure and calculated after special Ueatment (I) : isotherm, (11) derived isotherm.
0
598
ACKNOWLEDGEMENT This research was supported by the Phygis program of the Ministkre de la Recherche. REFERENCES 1 2 3 4 5 6 7 8 9 10 11
12 13 14
Jones, B.F., Galan, E. in Review in Mineralogy: S.W. Bailey, ed., Hydrous Phyllosilicates (exclusive of micas), 19, Mineral. Soc. of America, Washington, 628-674. 1988. Barer, R.M., Mackenzie, N., and MacLeod, D.M., J. Phys. Chem., 58 (1959) 568-573. Van Scoyoc, G.E., Serna, C., Ahlrichs, J.L., Am. Mineral., 64 (1979) 216-223. Grillet, Y., Cases, J.M., Franqois, M., Rouquerol, J., Poirier, J.E., Clays and Clay Minerals, 36 (1988) 233-242. Rautureau, M. and Tchoubar, C., Clays and Clay Minerals, 24 (1976) 43-49. Rouquerol, J., Thermochimica Acta, 144 (1989) 209-224. Cases, J.M., Grillet, Y., Franqois, M., Michot, L. VilliCras, F., Yvon, J,to be published in Clays and Clay Minerals. Rouquerol, J., J. Thermal Analysis, 2 (1970) 123-140. Michot, L., Franqois, M., Cases, J.M., Langmuir., 6 (1990) 677-681. Poirier, J.E., Franqois, M., Cases, J.M., Rouquerol, F. in Fundamentals of Adsorption, T. Athanasios, T. Laiapis eds., A.I.C.H.E., New York, 472-782, 1987. Sing, K.S.W., Everett, D.H., Haul, R.A.W., Moscou, L., Pierotti, R.A., Rouquerol, J. and Siemieniewska, T., Pure Appl. Chem., 57 (1985) 603-619. Dubinin, M.M., Pure Applied Chem., 10 (1966) 309-321. Cases, J.M, Bull. Minkral., 102 (1979) 684-707. VilliCras, F. Internal communication. (1990)
F.Rodriguez-Reinosoet al. (Editors),Characterization of Porous Solids II
599
1991 Elsevier Science PublishersB.V., Amsterdam
CHARACTERISATION OF POROUS Si02-A1203 SOL-GELS: MODEL HETEROGENEOUS CATALYSTS
* , T.J.
P.A. SERMON
WALTON, M.A. MARTIN LUENGO (YATES) and M. YATES
Department of Chemistry, Brunel University, Uxbridge, Middlesex UB8 3PH, UK
SUMMARY Silica-alumina jql-gels have been prepared with a variety of compositions; the addition of A1 reduces the adsorption capacity, the total surface area and the microporosity os+the gels. This can be explained if the adsorption or reduces the charge on the silica sol-gel particle? and incorporation of A1 + and enhances their adhesion. The gels readily allowed ion-exchange with Cu temperature-programmed reduction and X-ray photoelectron spectroscopy has been used to probe the environment of these cations in model precursors of heterogeneous catalysts. The potential of this approach for the analysis of solutions is considered. INTRODUCTION It has been argued [ l ] that many of the intricacies ofheterogeneous catalysts arise prior to producing the supported catalyst surface itself, i.e. at a precursor state when species are adsorbed from solution at the solid/solution interface during catalyst preparation.
Such species probably template and
define irreversibly the selectivity and reactivity of the catalyst surface produced after reduction, calcination or sulphidation. Multinuclear NMR has been used to probe the prevailing precursor chemistry at the solution/solid interface in the pores of such catalysts [ l ] . attempted to use X-ray photoelectron spectroscopy (XPS).
Here we have
Traditionally XPS of
solutions has only been possible in thin films [2] which are constantly replenished to compensate for rapid evaporation under high vacuum conditions. Porous oxide gels provide an alternative framework for a new mode of analysis of liquid phases contained therein. These are readily prepared via the sol-gel route [3] involving alkoxide hydrolysis M(OR),
+
X H ~ O= M(oH)~(oR)~-~ + XROH
and subsequent condensation -M-OH
+ OH-M-
c
-M-0-M-
+
H 0 2
In such preparations the prevailing acidity-alkalinity and the H20:M(OR)n alkoxide ratio are critical in defining the properties of the gel produced.
The
introduction of a second (or third) metal cation is possible and in the case of silica-alumina gels the Si04 and A104
-
tetrahedral building blocks have ready
600
similarities which allow
incorporatian a f the two producing:
Si-OH 0 1
-0-Si-0-A1-OH 1
0 1
Si-OH Naturally then the Si:A1 ratio will define the surface area, acid site density, etc. of the gel.
The latter property may well modify the ion-exchange
capacity of the gels.
This in turn will affect their ability to incorporate other say transition metal or IB metal cations. Such as ion-exchange
mechanisms have been postulated [4] to produce: S i-OH
0 1
-0-Si-0-Al-0-Cu(0H ) + 2 5 1
+ H+
0 Si-OH when the solvated Cu2+ cation is used and similar process are thought to occur with Pt, etc. [5]. These porous sol-gel derived matrices are therefore model of heterogeneous catalysts.
Since the surface area exhibited by the internal surface of the
pore volume (V ) (if this is composed of non-intersecting uniform cylindrical P pores of radius r) is given by S int where S int=2V /r and s o their high P surface-are a will enable a high fraction of surface-held catalyst precursors to be investigated per unit volume.
In addition their small pore size may reduce
the rate of solution l o s s under vacuum conditions and allow in-situ X-ray pnotoelectron spectroscopy of these species at the solution-solid interface of this precursor to heterogeneous catalysts. This approach is illustrated here in terms of the study of model porous Si02-A1203 sol-gels with and without the addition of Cu2+.
EXPERIMENTAL Tetraethoxysilane (Si(OC2H5)4
TEOS), absolute ethanol, water and 3M HC1 were
used in the molar ratio 1:4:4:0.07
in the sol-gel preparations.
TEOS in
ethanol at 298K was mixed with constant stirring with HC1-water at 298K for 60min before allowing gelation in a polyethylene beaker. aluminium nitrate (A1(N03)3.9H20)
was added.
In some samples
An attempt was made to introduce
Cu2+ to each of the gels by immersing the gel in an aqueous solution of cupric
601 nitrate. Total surface areas were investigated by application of BET theory to the extent of N
2 adsorption at 77K measured in a Carlo-Erba 1800 Sorptomatic instrument after outgassing samples (0.3-0.5g) at 393K for 16h, assuming that the cross-sectional area of N 2 in the monolayer was 0.162nm2 Thermogravim-
.
metric analysis was carried out in a Stanton Redcroft 780 and X-ray photoelectron spectroscopy in a Kratos ES300.
Temperature-programed reduction was
carried out in 4%H / N during heating at 5K/min. 2 2 RESULTS AND DISCUSSION The adsorption isotherms for N2 at 77K on the silica-alumina sol-gel derived samples are shown in Figures 1-2 and the surface area data derived from BET analysis of these isotherms are shown in Table 1.
TABLE 1 Textural Information in SiO -A1 0 Samples Derived from the Sol-Gel Route 2 2 3 %Si02%A1203
'total
100 0 75 25 50 50 25 75
653 410 261 23
C
387 252 191 15
'ext
53 377 234 16
int
600 33 27 7
t'
0.31 0.33 0.24 0.06
'mic
0.24 0.04
0.01 0.0
In Figure 1 and Table 1 it can be seen that Si02 with a surface area of 2 653m /g showed substantial micropososity (r<2nm) with a largely reversible type
I adsorption isotherm, a high C value and no real hysteresis. surface area S
Its external
was calculated by the a method to be only 53mL/g and so the
ext majority of its surface area and pore volume is within the micropores predominating in this sample. However, silica(75%)-alumina(25%)
2 (410m /g) showed an adsorption capacity
and a surface area which had decreased by almost a third from that seen for the silica-only gel.
Its isotherm (see Figure 2) was of type IV (H2) which is
typical of capillary condensation within a mesoporous matrix (2<rp<50nm); the gel appears to maintain only about 10% of its pore volume within micropores and the C value is correspondingly low.
Figure 2 and Table 1 also show that with
the incorporation of 50% A1 the adsorption capacity, surface area and pore volume have all decreased markedly still further and the isotherm has a type I1 form; the incorporation of this concentration of alumina has clearly left the
602
- '
0
I
I
I
8 0
I
0
O
e
r\
0
N
%
ua
3
z
C
O
u
a
.A U
a
m
U
m M a r . . mr.m
0
a c u o
U
u
P
O
n
I
I
I I
I
0
'0
O
cd
v
0
\a Q
l!! I I
b
i: 40
a
m
U
Figure 2:
adsorption isotherms at
77 K . m w 0
604
samples with a wide range of micro-, meso and macro pores. alumina-containing samples the
01
With both of these
plot was most effectively refered to a
standard alumina sample (unlike the silica only sample which appeared best referred to a standard silica) and this may reflect the fact tht the surfacesof the sol-gel particles are enriched in A 1 ions. Finally, by the time threequarters of the cations are A 1 the adsorption capacity, surface area and pore volume have been suppressed to a very low level (see Figure 2 and Table 1) In summary the introduction of A 1 0 4 has progressively converted a high area essentially microporous silica gel into a low area gel containing a wide range Yet caution needs to be exercised since thermal analysis of the
of pore sizes.
gels in flowing N2 at 4 7 3 K revealed weight losses of %Si02 % A 1 0 2 3 100 0 7 5 25 50 5 0 25 7 5
19% wtloss 74% 75% 71%
These are in line with expectation bearing in mind the pore sizes in these samples and hence f o r the silica-only sample the outgassing prior to N 2 adsorption might not have removed all water contained therein, and hence its surface area may have been underestimated. Samples (0.1-0.2g) of the gels were immersed in NaOH solution of known volum and strength, which after filtering was titrated with H C 1 using phenolphthalein indicator, the consumption of NaOH increased in line with A 1 concentration in the samples but this may have been related to surface acidity or dissolution of the alumina component. However, the ion-exchange capacity of the gels for Cu2+ was greatly enhanced by A 1 incorporation in gels and attention was given to 2+ 2+ Cu /silica and Cu /silica(75%)-alumina(25%). Figure 3 shows that,while no hydrogen consumption was seen for the gels alone,reduction of Cu2+ in the microporous silica sol-gels was readily seen to be a two-stage process (possibly + + related to Cu2+ to Cu and then Cu to Cuo or to reduction of Cu2+ in two different states); which in turn throws further light of the ion-exchangeable sites on the surface of the sol-gels, but further work is required on this point.
Figure 3 also shows the XPS results obtained Tor Cu2+/silica (75%)-alumina
(25%); since the Cu shake-up satellites are seen it is clear that the copper within the sol-gel is partly in the 2+ oxidation state. It also follows conversely that a small fraction _may be exchanged onto the surface and in a 1+ oxidation state.
Again further work is required to unravel this point.
Nevertheless, the binding energy for the Cu 2 p ( 3 / 2 ) in the sol-gel environment was about lev higher than that f o r Cu2+ in curpic nitrate itself and this may result from Cu2+ stabilisation by the solution or the oxide surface sites.
1
0960
970
950
940
930
(b)
I
I
273
473
I 673
I I 873 T(K)
I
Figure 3: X-ray photoelectron spectroscopy of Cu2+/silica( 75%) -alumina (25%) (a) and temperature-programmed reduction of Cu*+/silica ( b ) .
606
Further investigation of this point is required.
CONCLUSIONS Silica-alumina sol-gels can readily be prepared with a range o f compositions. The composition affects the total surface area, pore size distribution, acidity and ion-exchangeability of the samples. Other binary and multiple oxides can no doubt be formed in the future with controlled properties.
The effects of
adding A13+ to the silica gels clearly reduces the extent of microporosity (and total surface area) possibly by binding the silica particles together by reducing the overall charge of the silica primary particles and increasing their adhesion.
In future MAFS will possibly reveal the intimacy of contact between
Si04 and A104
-
tetrahedra and the local order of such samples.
The introduction of transition metals can reveal much about the surface chemistry of the sol-gels (i.e. in TPR profiles and X P S ) , but the metalcontaining sol gels can act as model precursors of heterogeneous catalysts. This is true at the stage of the solution-solid interaction, since analysis by XPS can be applied while the samples contain a significant fraction of the
exchanging or impregnating solution. However, this chemistry is naturally complex and requires the solution chemistry to be differentiated from the chemistry atthe solution-solid interface.
If this can be achieved then this
approach may even allow microporous sol-gels to act as micro-containers of solutions for analysis in XPS in a far simpler manner than is possible at the present time.
REFERENCES 1 S.A. Lawrence and P.A.
Sermon Spectrochim Acta
w,
1461, (1987)
2 H. Siegbahn J. Phys. Chem. 89, 897, (1985) 3
J. Non-Cryst. Solids
2, (1986)
4 K. Hachiya, M. Sasaki, Y. Saruta, N. Mikami and T. Yasunaga
Phys. Chem. (1984) Santacessaria, S . Carra and I. Adami Ind. Eng. Chem. Prod. Res. Dev. 16, 41, (1977)
88, 23 (1984); K. Hachiya, M. Sasaki J. Phys. Chem. 5
e, 27,
3.
F. Rodriguez-Reinosoet al. (Editors), Characterization of Porous Solids II 0 1991 Elsevier Science Publishers B.V., Amsterdam
607
EFFECT OF La(II1) ON THE THERMAL STABILITY OF A1-PILLARED MONTMORILLONITE J.M.Trillo,M.D.Alba,R.Alvero,M.A.Castro,J.Poyato and M.M.Tobias Department of Inorganic Chemistry. Institute of Materials Science. University of Seville- C.S.I.C.. P.O.Box 8 7 4 . Seville. Spain ABSTRACT The promotion effect of La(II1) addition upon the thermal stability of an alumina-pillared montmorillonite has been studied. The accessibility of molecular water to the interlamellar spacing has been measured as a test for the adsorbance capacity of the active solids. Nitrogen preheating at 300°C exert a negative influence on the samples containing lanthanum. Thermal treatments at temperatures higher than 300°C lead to a reduction in the uptake of H20, which is not accompanied by a parallel change in the specific surface area or in the basal spacing of the smectite. INTRODUCTION
Pillared clays represent a new class of porous, high surface area materials of potential interest as catalysts and adsorbents. The size of the pillaring species determines the porosity of the material (1). Characterization measurements of pillared clays consist, usually, of nitrogen BET specific surface area, SBET, and X-ray diffraction powder patterns, as well as of the adsorption capacities for a series of adsorbates, although a proper differentiation is not usual between the amount taken up into the interlamellar spacing and that on the external surface, as multilayer and condensed in mesopores. Upon calcination, there is a contraction of the d(001) spacing. Diverse trivalent lanthanide cations are added by ion exchange in order to improve the thermal stability of the pillared clays. From SBET measurements, the existence of a positive effect by the lanthanides has been suggested ( 2 , 3 ) . The aim of this work is to determine the effect of La(II1) addition upon the thermal stability of an alumina-pillared montmorillonite. The accessibility of molecular water to the smectite interlamellar spacing has been measured.
608
EXPERIMENTAL A Trancos smectite mineral from Gador, Almerla (Spain), was used (4). The <2 pm fraction was collected, corresponding to montmorillonite with a charge deficit per unit cell of 0.87 (0.36
+ 0.51) (5). XPS spectra of lanthanide saturated montmorillonite, La-M8 showed no migration from interlamellar positions to octahedral ones upon heating at 3OO0C(6). The montmorillonite pillared with alumina, A1-CLM, as well as that containing alumina and lanthanum, Al(La)-CLM, were prepared according to the method described by Yamanaka and Brindley (8). Hydroxyaluminum solutions of composition ratio OH/A1= 1.84 were prepared by adding a 0.01M NaOH solution to another one 0.1M A1(N03)3. In the case of Al(La)-CLM, the precursor solution of Al(II1) contained 4 % of La(II1). The final pH values were 4.32 and 4.778 respectively. These solutions were aged at reflux temperature (85"-95"C) for 10 h, which appears to have a beneficial effect on the subsequent pillaring process in accordance with data reported by Tokarz et a1.(3). Figure 1 shows the evolution of the experimental potentiometric A1(III)-4.10-3M La(II1) titration curves of the 0 . 1 ~A~(III); 0 . 1 ~ and 0.1M Al(111)-2~10~2M La(II1) solutions as a function of the
A
5.0.
4- 0
I
a
4- 0
-
5-0 ;!rq u
4- 0
3. 0
0.0
0.5
1.0
1.5
2.0
2.5
0.0
0.5
1.0
1.5
2.0
2.5
'OH/A~
Fig. 1.-Comparison of the experimental and calculated -Bottero et a1.(8)- pH vs. R O W A ~titration curves of the A1C13 solutions: the calculated;(A); the experimental ones for [Al(III)]= 0.1M plus ILa(III)l=OM (B), [La(III)]=4-10-3M (C) and [La(III)]=2.10-2M(D).
609
OH/A1 neutralization ratio (rOHIA1), after ageing times of 24 h. NO differences were observed among the three resulting curves. Likewise, the theoretical potentiometric titration curve assuming the existence in solution of Al(OH)+', Al(OH)$, Alz(OH)H4, A11304(OH)il and A1(OH)3 species is shown, according to the calculation by Bottero et a1.(8). The agreement between the theoretical curve and the experimental ones is notable. Herewith the rOH/A1=1.84 was selected, for which the most concentrated species is AlI3. Separate 300 mg samples of the Na-saturated montmorillonite were weighed into 50 ml-centrifuge tubes and 25 ml-aliquots of the aged Al(II1)Al(III)/La(III) solutions were added, these dispersions being favoured by ultrasonic radiation. The suspensions were stirred and allowed to react for 3 h, after which they were centrifuged and another 25 ml of solution added. The above experiment was repeated four times. The A1-CLM and Al(La)-CLM thus synthesized were air-dried at 100°C. All the samples were heated in Nz at 300°C for 5 h to transform the hydroxycations into oxide pillars. This treatment decreased the basal spacing value from 19.2 A to 18.2 A (Al-CLM) and from 18.8 A to 18.4 A (Al(La)-CLM). X-rays powder diffraction diagrams were obtained with a Siemens Kristaloflex D-500 instrument, using Cu Ka radiation and a Ni filter, 36 kV and 26 mA. The Nz adsorption isotherms were measured with a Micromeritics system, model 2200 A, at 77.35 K. A conventional diffusion pumped glass system was utilized to obtain the water sorption isotherms at 2O"C, the HzO pressure being measured by means of silicone McLeod
.
RESULTS AND DISCUSSION
Molecular water taken up by swelling clay minerals stands up as multilayer on the external free surface, filling micropores, condensed in pores and bonded in the interlamellar spacing. Up to now, very over-simplified models have been used to differentiate one form of molecular water from another. For example, the amount of interlamellar water has often been calculated by assuming densities close to that of the liquid water, 0.95 Kg-d~n-~ by Ormerod and Newman (9), from earlier estimations where the other forms were under-estimated. Push (10) has recently observed, by humidic cell high voltage microscopy, that during the expansion of
610
the aggregate a stable condition is reached in which a small amount of the water was contained within the aggregates, while a major portion occupies larger inter-aggregate voids. In a previous work (6), it has been observed that the present montmorillonite saturated with L~(III) shows, over a wide range of humidities, a basal spacing of about 15.6 and a constant interlamellar water content of 1 0 4 cm3(NPT) .g-', which corresponds to a calculated apparent density of 0.36 kg-dm-3 Brunauer et al. (11) have obtained adsorption isotherms of H20 on a series of non-porous solids, grouping the results into five "t" reference curves. The "t" curves have been plotted from the appropriate Brunauer reference curve and H O sorption isotherms on Li-M, A1-CLM and Al(La)-CLM samples,, Figures 2 to 4 . The curve for the Li-M sample, previously heated at 300°C in air or under vacuum, consists of a straight line, from which the absence of both condensation in mesopores and swelling of the clay is inferred. The "t" specific surface area agrees with the BET nitrogen one, assuming a cross section area for an adsorbed H20 molecule of 1 0 . 6 i 2 ( 1 1 ) . The intercept of the "t" straight line indicates the absence of microporosity in the montmorillonite. The "t" plots for the initial and 300°C preheated pillared samples consist of straight lines at relative pressures above ca. 0.5, Figures 2 to 4, with slopes similar to the one for Li-M (30OOC). This means that the contribution of the H20 adsorbed as multilayer to the total taken up by the solids is the same for the pillared and non-pillared montmorillonites. The negligible mesopore volume in the Li-M samples may be extended to the pillared montmorillonites, according to both present and previous results. Van D a m e and Fripiat (13) have attempted to determine the fractal dimension D of pillared clays. The D value obtained, about 2 , shows that the surface is molecularly rather smooth, as found earlier with Y-zeolites. The intercepts of the above mentioned linear traces extrapolated back to the Y-axis have been interpreted to yield the interlamellar water content. Their values are shown in Table I. Preheating at 300°C does not exert practically any influence on the interlamellar water content of the A1-CLM sample. However, the addition of lanthanum (4%) leads to its strong decrease. The effect of La(II1) is difficult to understand, as it is in heterogeneous catalysis. Figure 1 suggests that lanthanide cations are incorporated into the montmorillonite as exchangeable ions.
611 -
m
E
i
n
G
.t
d
(D
200
200
100
100
m
m
6
m
i a i
8'
ai
ai
2
4
-
P/Po
ai
ai
..
4. 0
I
2 5
0
0 E
2
4
6
0
€4
6
e
6
B
m
0 "
>
B
2
4
6
B
t tx,
E
2
4
Fig. 2.-"t" plots for water vapour sorption at 20°C upon Li-M: initial (A), vacuum and air preheated at 300°C (B); upon A1-CLM nitrogen preheated at 300°C during 10 h (C) and 30 h (D). The lower straight line corresponds to only multilayer adsorption.
P/ P o
__ -
0w
0
Fig. 3.- "t" plots for water vapour sorption at 20°C upon Al(La)-CLM nitrogen preheated at 300°C during 10 h (A), 2 0 h (B) and 30 h (C). The lower straight line corresponds to only multilayer adsorption.
2
4
c 6
8
Fig. 4 . - "t" plots for water vapour sorption at 20°C upon A1-CLM nitrogen preheated during 10 h at 300°C (A), 500°C (B) and 700°C (C). The lower straight line corresponds to only multilayer adsorption on the sample preheated at 300°C.
612
Furthermore, the previous XPS data already mentioned demonstrate that they remain in the interlamellar spacing after heating. The nature of the interaction of the interlamellar pillars with the tetrahedral and/or octahedral sheets of the montmorillonite could afford an explanation. New experiments are being carried out in order to interpret these results. It is noticeable that the SBETand d(001) parameters are not appreciably influenced by preheating Al(La)-CLM, :Table I. In spite of this, spacing is generally used to follow the thermal evolution of these materials. Apart from the present results, examples from the literature show the non-existence of relationship between both parameters and the pore (V,) and micropore volumes. In (14) the values: 18.4 A (d(001)), 242 rn2.g-' (SBET) and 0.20 cm3 .g-1 (V,) were obtained for an alumina-montmorillonite; while for another one, prepared under different experimental conditions, the values: 15.5 A (d(001)), 249 m2.9-' (SBET)and 0.23 cm3.g-' were found. TABLE I : INTERLAMELLAR MOLECULAR WATER ( v cm3
Sample
g-'
d(001)
V
(A) Li-M A1-CLM A1-CLM A1-CLM A1-CLM Al(La)-CLM Al(La)-CLM Al(La)-CLM
300 300 300 500 700 300 300 300
10 10 30 10 10 10 20 30
95 225 225 150 50 215 215 215
9.6 18.2 18.2 17.2 v.b. 18.4 18.4 18.4
0 86 77 31 15 91 70 41
Pretreatment by heating at temperatures higher than 300°C determines a decrease of the interlamellar water content for the A1-CLM, which is not accompanied by a parallel change in the basal spacing, Table I. Strong reductions in the catalytic activity of montmorillonite pillared with alumina, which were not accompanied by appreciable changes in the basal spacing have been observed in the literature. The diminution of activity has been related exclusively to poisoning effects or to changes in the Lewis-Bronsted acid sites ratio. For example, Matsuda and Kikuchi (15) have observed the activities: 41.8% (300"C), 19.4% (400"C), 7.0% (550°C) in the reaction of conversion of 1,2,4-trimethylbenzene, at the indicated temperatures.
613
The interlamellar water content calculated on the basis of a 8.6 A interlamellar thickness and the above mentioned apparent density of 0.36 kg.dm-3 is 120 cm3(NPT). The experimental values, Table I, are expected to be decreased by: i) interstratification between collapsed and open lamellar; ii) variable heights of the pillars, sometimes too small to allow the efficient packing of HzO molecules; and iii) different types of ordering of the pillars within the interlamellar space. Some of these effects must influence the time of preheating at 300°C, in Al(La)-CLM and the temperature of preheating of A1-CLM. From these data it is concluded that the basal spacing and BET specific surface area are not sufficient by themselves to represent the potential activity as adsorbents and/or catalysts of the pillared-smectites. ACKNOWLEDGEMENT
We thank CICYT for financial support. REFERENCES
1.-F.Figueras; Cat.Rev. -Sci.Eng. 30(3), (1988) 457-499. 2.-J.ShabtaiIM.Rosel1 and M.Tokarz; Clays and Clay Miner. z ( 2 ) I (1984) 99-107. 3.-M.Tokarz and J.Shabtai; Clays and Clay Miner. 33(2), (1985) 89-98 4.-E.ReyesIF.Huertas and J.Linares, in A.Pietracaprina (Ed.), Proc. 1st Congr.Bentonites, Sassari-Cagliari (Cerdefia), (1978) 149-157. 5.-J.PoyatoIM.M.Tobias and J.M.Trillo; Applied Clay SCi., 4,(1989) 499-508. 6 .-~.M.Trillo,J.Poyato,M.M.Tobias and M.A.Castro;Clay Miner. In press. 7.-G.W.Brindley and S.Yamanaka; Am.Miner. 64, (1979) 830-835. 8.-J.Y.BotteroI J.M.Cases, F.Flessinger and J.E.Poirier; J.Phys.Chem. 84, (1980) 2933-2939. g.-E.C.Ormerod and A.C.D.Newman; Clay Miner. Is, (1983) 289-299. lO.-R.Pusch; Applied Clay S c i . 2, (1987) 343-352. 11.-J.Jr.Hagymassy,S.Brunauer and R.Sh.Mikhai1; J.Colloid Interface Sci. 29, (1969) 485-491. 12.-T.M.El-AkkadI N.S.Flex, N.M.Guindy, S.R.El-Massry and S.Nashed; Surface Techn. J J , (1982) 69-77. 82, (1985) 13.-H.Van Damme and J.J.Fripiat; J. Chem.Phys. 2785-2790. 14.-J.Sterte,J.E.Otterstedt H.Thulin and F.E.Massoth; Applied .~ Catal. (1988) 119-129. 15.-T.MatsudaIM.Asanuma and E.Kikuchi; Applied Catal. 38, (1988) 289-299.
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F. Rodriguez-hinoso et al. (Editors),Characterization of Porozu Solids ZI
615
0 1991 Elsevier Science Publishers B.V., Amsterdam
EYOLUTION OF WROSIlY DUFUNG CONVERSION OF ~-ATJMlJUTo A NOVEL FOROUS a-ALWEN?. FIBRE
By
M.H.STACEY
ICI manced Materials, P . 0 . W 11, The Heath, Runcorn, England
ABSTRACT AI-I exprimental sol-gel 1.9% silica-doped almina fibre has been studied before and after the n t o a-almina phase change. The initial fibres contained roughly equal munts of randm pores and pores aligned parallel to the fibre axes, but only the f o m r were eliminated during the phase change. The product a-alumina fibre has therefore 90% axially-aligned pores and should have an excellent s p x i f i c modulus. This behaviour is contrasted with that of 5% silica-doped alumina fibres previously studied.
IN'IRODUCTION
Generally alumina fibres made by sol-gel mthcds and calcined below On heating t o higher 1000°C consist of nesoporous 0-almina. temperatures eventually the thermodynamically stable a-almina i s formed (ref 1). This phase transition results i n a significant voluw shrinkage (10-20%) of the alms and so a reorganisation of the pore structure i s t o be expected. It is also generally knm that the
details of this phase transition are strongly affected by s i l i c a doping. In particular the transition temperatures are &creased markedly and the porosity elimination is delayed t o higher terqxratues.
For example, a t
about 4% s i l i c a the initial msoporosity can be almost ccanpletely eliminated, whereas without s i l i c a considerable porosity remains. The fibres studied here were experimental fibres made by a proprietory sol-gel prccess incorporating 1.9% s i l i c a additive.
This additive i s in a lower m u n t than i n previously reported materials (refs 2.3) but i s s t i l l expected t o influence the phase change m r a t u r e significantly. A particular feature of these fibres i s that about 50% of the i n i t i a l porosity was preferentially aligned w i t h the fibre axis: this i s again different frm the fibres previously reported (ref 2) which contained only randcanly orientated cylindrical pores w i t h a narrow pore size distribution. The effects of the r- to a phase change i n these fibres has m been studied using X-ray m
r diffraction (XRD),
616
nitrogen adsorption isotherms, true density determinations, the optical properties, -1-Angle Neutron Scattering (SANS), and TEN t o derive a canplete description of the microstructural changes. The resulting a-almina pssesses an unusual type of porosity due t o the nature of the precursor. WERIMENTFL
Silica-doped sol-gel aligned fibre t a w were ma& by a proprietory process w i t h a man fibre diameter of 3 p . They were finished a t 900°C in order to produce the mesoprous q-alumina phase. They were then further calcined a t 1300°C in a muffle furnace i n order t o convert the fibres t o the a-alumina phase. P o k i e r XRD patterns here determined t o confirm the phases. characterisation of the two typs of fibres was by nitrogen adsorption a t 77K using a Micrcmxitics Inc Digisorb 2500 equi-t. Samples were degassed beforehand to 2 5 0 T and 0.01 mbar for 16 hrs. BJTC surface areas were calculated frm results below a relative pressure of 0.3 and the Gurvitsch pore volurne frm the plateau uptake a t relative pressure approaching saturation using a liquid nitrogen density of 0.8081g/ml. Pore-size distributions w r e calculated frm the adsorption branches using the &sH mthcd assuming cylindrical p r e s . The optical properties were evaluated using a Nikon transmitted light microscope equipped w i t h polarisor and analyzer and using a S m n t canpensator to determine the amount of double refraction.
The difference
between refractive indices in the axial and radial directions for a fibre
was calculated frm the equation
na-nr
=
e*A/l80*d
where e i s the angle of rotation of the Senannont ccanpensator w h i c h causes maximum darkening of the fibre image, h is the wavelength of the light used ( 0 . 5 5 ~ ) and d= fibre d i a m t e r ( p ) . Smdll-Angle Neutron Scattering expriments were performed either a t Institute Laue-Langevin. Grenoble using spectrmter D17, or a t Rutherford-Appleton Laboratory, Harwell, using the LCQ spxtrcneter. The centre of the detector for D17 was offset frm the main neutron beam axis so as t o obtain a greater Q-range frm the results. A t IJL 12.0A neutrons were used, whereas a t RAL the pulsed neutron source has a range of wavelengths (4-1OA). In both cases standard data reduction p r o g r m s enabled the scattering intensity t o be calculated as a function of the
617
scattering vector Q defined by
where 28 is the scattering angle and h ( A ) is the wavelength of the neutrons. The Q-range accessible w a s 0.006-0.3A-1on D17 and 0.0056-0.228-1 on
a. The fibre sarrg?les (ca 0.5g), consisting of tows of aligned fibres with a volume fraction of ca 10% were loaded into l h id fused silica tubes, heated under vacuum (0.01mbar) at 200°C and the tubes then sealed under vacuum. In this way absorbed water was eliminated f r m the fibres. me neutron transmission was ca 70-80% for these m u n t s of fibres. The scattering data from the two-dimensional area detector were found to be anisometric and were therefore converted to one-dimensional data by using sector masks. Scattering in the fibre direction was derived from a 75" wide mask parallel to the fibres and the scattering normal to the fibres was derived from a 15" wide mask perpndicular to the fibre direction. Samples were prepared for TEM by embedding in epoxy resin, and thinning in an Ion Tech A t m M i l l operated at 5kV and
618
precursor 900°C f i b r e s proved t o be unusual; i n this case the &sorption branch of the hysteresis loop was convex towards the relative pressure
axis giving the hysteresis loop i t s e l f a "banana"shape ( f i g . 1). The maximum uptake o f the 900°C f i b r e s was wll d e f i n e d , but uncertain for
the 1300°C f i b r e s as the adsorption branch rises steeply very close to
P/Po=l.
Calculations o f the man p o r o s i t y , pore diamter and surface
area parareters (Table 1) shoved that d t h o u t doubt there has been
substantial growth i n pore s i z e (4x)during heating frm 900°C t o 1 3 0 0 T even though the loss of pore volume i s modest (<50%); the p r o s i t y was even less affected ( r a c e d frcan 30 t o 24%) as there has been a s u b s t a n t i a l increase i n true density (3.08 t o 3.80 g/ml).
Consequently
the f i b r e s shrunk macroscopically ca 10%. TABLE 1
P r o w r t i e s of A 1 b Fibres Analysis
PHASE rl
U
N i t r o g e n Adsorption
Pore V o l cc/g
0.142
>0.082
Porosity %
29.4
> 24
BET Area m 2 / g
83.7
10.5
6.8
(>31)
Pore Diam/m D e n s i t y g/m3
Apparent Density g/m3
3.08
3.80
2.06
2.90
0.163
0.069
112
12.1
Experinental
0.0178
0.0390
Theory (100% a x i a l )
0.0356
0.0442
50
88
Cumulative Pore V o l m3/g Cmulative Area m * / g Double refraction
Actual/theory %
619
80 -
0 PIP0
Fig. 1 Nitrogen Adsorption Isotherms a t IIK
2
5 10 20 50 Mean Pore Diameterlnm
Fig. 2 Pore Size Distributions (AdsOrptiOIl)
The unusual hysteresis loop shape of the 900°C fibres mans that the a -plot of the adsorption branch was of the n-esoprous type but was also S
unusual in having two inflection p i n t s before the maxirmrm adsorption is reached. These occurred a t as values of ca 1 . 0 and 1.5. consequently the p r e size distribution calculated frm the adsorption branch clearly suggests a bimoddL characteristic With the most frequent pore dianeters a t 4 and 10 nm ( F i g 2). Fig. 2 also sham that the psd of the 1300°C sample had becarte moncmxkl where the mst c m n pore size (30nm) agrees well with that calculated frm the BET area and total pore volume. The cmulative figures calculated frm the adsorption branches in both cases also agree f a i r l y w e l l with the BET areas and Gurvitsch v o l m s (Table l), but the same calculation on the &sorption branches gave a much greater disagreement w i t h the BET areas and Gurvitsch v o l m s . (ii)Optical masurerwnts Both these fibre samples were strongly and uniformly positive
double
refracting, the 1300°C fibres having more than twice the value of the 900°C fibres (Table l)! This i s surprising since the q-alumina phase is dlmost cubic and so cannot be double refracting while, although a-alumina is a biaxial crystal, its' refractive index difference is only 0 . 0 1 (cf 0.039 found). It is m r t a n t to note that neither the polycrystalline a-almina fibres nor the q almina fibres showed any sign of variation of double refraction w i t h i n a single fibre over long lengths. This mans mat if the individual crystals do contribute t o the double refraction
620
they all do so in the same sense or they are t w small to be resolved in the optical microscope. The optical path difference (masured by 8) did not vary much bebeen fibres of different diameter within each sample. However as the refractive index difference iS inversely proportional to fibre diameter, the experimentdl value quoted in Table 1 is a mean value for these 3 p fibres and does not hold for fibres of a different diameter within the sample. The standard deviations were only about 3% of the man values. The fibres are double-refracting because of their --phase nature. This is knm as form birefringence and arises because of the differing refractive indices of pres (1.00)and alms (q 1.56; a 1.75) and because of the nonspherical shaE of saw of the pores (ref. 4). Provided that the pores are of a dimter below the wavelength of light (550m) then positive birefringence indicates porosity which is fibrillar and parallel to the fibre axis. The quantitative theory of form birefringence was derived by Wiener in 1908 so that knowing the total porosity fran nitrogen adsorption and the refractive index of alumina we may then calculate the form birefringence if fibres contained axial fibrillar pores only (Table 1). The values so calculated were larger than those found experirrWtdlly indicating that only a fraction of the pores were likely to be fibrillar and axially aligned. In fact part of the increase on calcining at 1300°C is due to the q to a transition, the t m phases having different refractive indices. Even so the birefringences found show that there was an increase in proportion of axially aligned pores on calcining the 900°C fibres at 13OO0C (frm 50% to almst 90% of that expected for perfectly axial porosity). (iii) SANS and TEM The characterisation of the fibre porosities w a s pursued further by TEM and by Small-Angle Neutron Scattering. A TEM of the q-alwCina sample (Fig 3) showed that unlike a previous sample (ref 3) a coarse axial texture was present. The c m s e texture has a scale of ca lOOnm in the fibre radial direction but the origin of the observed contrast between zones is so far unknm. The porosity visible within both light and dark zones is generally in the 5-1Onm diameter range but it is clearly only partly aligned with the fibre axis (which is vertical in the "s).In the 13OOOC fired sample wide axial pores (ca 30 nm wide) are dcaninant with substantial blocks of solid phase ca lOOnm wide (fig 4). Randcpn pores make up only a small fraction of the visible porosity.
621
Fig. 3 T€N of thinned 1.9% Silica/ A l m a Fibres. q-phase and
Fig. 4 Ditto a-phase
AXIS vertical The SANS data also lend support t o the view that wst residual porosity after 1300°C calcination is now oriented along the fibre axis while in the precursor this is much less marked. It i s not possible t o give a f u l l quantitative explanation of the SANS data here as the necessary theory i s extensive and w i l l be presented later in another paper (ref 5 ) . The data w i l l therefore be discussed only qualitatively in order t o bring a b u t the mst relevant features under discussion. SANS
The
scattering fram both sanples was anisotropic i n the detector plane
(ref 6 ) , the intensity being greater normal t o the fibre axes than parallel. This azimutkal variation is proof that both axially-aligned and randm pores are present in the fibres as inferred frm the form birefringence and seen by "I. By examining the variation of intensity w i t h Q in the two sectors parallel and normal t o the fibre axes, the relative pore sizes of the two types of pore can be deduced. The randan pores scatter uniformly over all azimuthal angles m l e the axial pores only scatter i n directions normal t o the fibre axes. The axial pore scattering can therefore be measured by the difference bethe two sectors and that frm the randan pores by the 7.5" sector parallel t o the fibres. (see fig. 5 ) . (Data below Q=O.OZ-l for the normal sector i n the rl a l m a case only have been eliminated frm the plots since this feature has been proved to be due to the external surface of the fibres. The data sham here are wholly due t o the internal porosity).
622
10000000
randompores 10 0.005
.hA
0.010
0.020
0.050
Fig. 5 SANS from Porous Alumina Fibres.
0.100
%\
0.200
Contributions from axial and
randm pores Fig 5 shows that the SANS intensity from axi,al porosity i n the 1300'C fibres was lox greater than that from randcan pores a t all Q-values whereas in the precursor fibres' case the intensities were similar below
This indicates that a x i a l pores dcarcinate i n the 1300°C fibres but that the two kinds are present in similar munts in the 900°C precursor fibres. In the l a t t e r case the axial wre scattering falls off Q=O.07k1.
much m r e rapidly with Q than that of the randm pore scattering shcrWing that the axial pores are also larger than the randan pores.
In fact an
estimate of the average pore d i e t e r can be deduced for the randan pores from a Guinier plot of the parallel sector data. This yields a radius of gyration of 2.4nm and hence a mean diameter of 5nm.
These pores can
therefore be identified as those below 6m in Fig 2 whereas the axially-aligned pores are the larger lorn diameter pores.
Increased
a x i a l pore size i n the a-almina is proven by the fact that scattering falls off a t high values of Q more rapidly than for those present i n rl-alumina. Since the tho scattering curves are now of the same form mans that the sizes of the randm and a x i a l pores are now similar. Consequenfly it apwars that it is the randm pores (originaLly less than 6m diameter) i n the q-alumina a c h have been preferentially eliminated during the a-almina formation. Only a very few remain now of dian-eter ca 3 0 m a 10-fold increase i n size. On the other hand the original larger diameter (1Om) axially aligned Wres have not been eliminatedbut
623
have grown i n diameter by about a factor of 3 t o 3 0 m . Crystallinity The 900°C fibres are q-dlmina and the line-broadening suggests a crystallite size of ca 6m. size.
This is of the saw order as the mean pore
Electron diffraction on a section of such fibres gives a uniform
ring pattern indicating totally randcan crystallinity. The 1300°C fibres are mre structured.
Electron diffraction on a
single fibre indicates that the crystdllite size is a t least of the s m order as the beam size ( l O O r n n ) , but using XRD on a fibre bundle s t i l l gives a uniform ring pattern. As an illustration a bright field/dark field pair of micrographs is sham in Fig 6.
Fig. 6
TEM a-alumina F i b r e s (a) Bright field
This illustrates that
(b) Dark field
within a single fibre there are many crystallites and that their orientations are different. The c m n orientation highlighted i n the dark field extends over ca 0.1-1pn both across and dong the fibre
direction
(including a t least one 30 m pore). There i s therefore no
long range preferred crystallite orientation as was deduced frcm the uniformity of birefringence i n the optical observations. CONCLUSIONS
These 900°C fibres have been shown t o contain birnoddl pore diameter distributions i n the msopxe range, w i t h the smaller pores being randcmly oriented and large pores being preferentially oriented parallel t o the fibre axes (previously examined 5% s i l i c a doped alumina did not possess any of these 1OOnm a x i a l pores). On
624 conversion t o a-alumina by heating to 1300°C, the smdll random pores were eliminated by sintering and the axial pores remained and
g r e w ca 3x i n diameter. have ca
The resulting porous a-alumina fibres
90% of t h e i r pores along the fibre axes (25%porosity) and
are expected t o have an attractive specific mdulus. The initial crystallinity of 900°C fibres is p l y c r y s t a l l i n e q-alumina of 5nm c r y s t a l l i t e s i z e and this converts a t 1300°C t o randm p l y c r y s t a l l i n e a-alumina w i t h c r y s t a l l i t e size between 0.1-1p. This work has shown that the evolution of crystallinity and porosity i n a fibrous 1.9% silica-doped alumina can prcceed by a different route fram that of 4% s i l i c a dowd fibres previously examined (ref 2 ) . In the l a t t e r case porosity was more cmpletely eliminated before
initial a-almina formation i n contrast t o this case where only the random pores could be eliminated while massive a-alumina i s f o m . O t h e r canpsitions need t o be examined t o ascertain whether these contrasting behaviours persist for other campositions and t o determine the controlling factors.
REFFBENCES 1. M.H.
Stacey, B r . Ceram. T r a n s . J. 87, (1988), 168-172.
2. M.H. Stacey, i n D. Taylor, R.W. Davidge, R. Freer and D.T. Livey (Eds.), Science of Ceramics 14, Sept 1987, I n s t i t u t e of Ceramics, Stoke-on-Trent, 1988, pp291-297.
3. M.H. Stacey, i n A.R. Bunsell, P. Lamicq and A. Massiah (Eds.), Developnents i n the Science and Technology of C c m p s i t e Materials, P r c c of EcCM3, 20-23rd March 1989, Bordeaux, Elsevier, London and New York 1989, pp65-70. 4 . J . R . Partington, An Advanced Treatise on Physical Chemistry IV, Iongram, London 1953, p275.
5. A.F. Jones, I.B. Parker and M.H. Stacey, J. Appl. Cryst. S u b n i t t e d for publication.
6. M.H. Stacey i n Inst. Phys. Conf. Ser. No 101. Neutron and X-Ray scattering: Ccxnplenxmtary Techniques, 29-31 March 1989, University of Kent, IOP Publishing 1989, pp197-211.
F. Rodriguez-Reinosoet al. (Editors), Characterization of Porous Solids II 0 1991 Elsevier Science Publishers B.V., Amsterdam
EVOLUTION OF THE TEXTURE AND THE THERMIC STABILITY WITH VARYING DIALYSIS TIME
625
OF A PILC-AI
C. PESQUERAI, F.GONZALEZI, 1. BENITO' and S. MENDIOROZ2 1 Departamento de Quimica, Facultad de Ciencias, Universidad de Cantabria. 39005 Santander (Spain) 2lnstituto de Catalisis y Petroleoquimica C.S.I.C. 28006 Madrid (Spain) ABSTRACT The effect of varying dialysis time on the texture of a Spanish montmorillonite pillared wirth polyoxycations of Al has been studied. The samples that had been dialysed for three days or more were thermically stable up to 700°C. The specific surface area of the original sample (61 m2/g) was increased by pillaring up to 362 m2/g. The high surface area is attributed to the porosity produced by stable aluminum oxide "pillars" formed by dehydroxylation of the Keggin ion [A11304(0H)24(H20)12]+7. The nitrogen adsorption isotherms are of Langmuir type and are consistent with adsorption in interlayer micropores. INTRODUCTION Pillared clays constitute a new family of molecular sieve materials obtained by the introduction of large polyoxycations between the silicate layers of clays. When they are heated, inorganic oxide clusters are formed which prop open the clays layers permanently. The first objetive in the pillaring process is to achieve as large a basal spacings as possible. Large basal spacings contribute to the development of high surface areas and porous volumes. The pore size of pillared clays can be controlled by the choice of conditions and method of preparation or by the kind of intercalating materials (refs. 1-7). Consequently, the application of pillared clays as shape-selective catalysts has become increasingly important, since reaction selectivity is a key ingredient in catalytic processes. The preparation procedures greatly affect the molecular sieve properties. Therefore in a previous study (ref. 8) we investigated the influence of the OH/AI and Al/clay ratios on the preparation of hydroxyaluminum montmorillonite using direct cation exchange. In order to determine the most appropiate washing conditions, in the present work we have studied the influence of different dialysis times on the stability
626
of the aluminum polyoxycations in the interlayer spacing with respect to possible hydrolysis reactions. The textural changes of the samples in the course of the dialysis and after thermal treatment were traced by the use of XRD, N2 adsorption and Hg-penetration. EXPERIMENTAL PART Materials The starting material was a bentonite, a yellowish-green material from La Serrata de Nijar, Almeria in SE Spain (ref. 9), supplied by Minas de Gador S.A. The previous chemical analysis of the sample (ref. 8), gave the following results: (si8.00)T (A12.69Fe0.32 Mg0.97)0 0 2 0 (OH),. (Ca0.13 Mg0.26 Na0.23 KO.02)EC The bentonite was purified by means of the conventional sedimentation, collecting the <2pm fraction. After that, it was washed two times with a 1M NaCl solution at 25°C to obtain Na-homoionic bentonite. The exchanged clay was washed with distilled water until chloride free. The suspension of the sample (S-Na) was stored as 2.5% stock at room temperature. PreDaration of pillaae nt An aluminum hydroxy oligomeric solution was prepared according to a reported method (ref. 1). A 0.5M NaOH solution was slowly added to a 0.2M AICI3.6H20 solution to obtain a OH/AI molar ratio of 2.0. Next distilled water was added until 0.1 mol solution of A P + was obtained. The resulting oligomeric solution was then aged for 2 hr before being used in the pillaring process. Pillarina process A clay slurry containing 25 g of solid/liter was used. To a specific volume of this solution we added dropwise the Al-hydroxyde solution, with vigorous stirring to obtain a 20 meq AP+/g clay ratio. The resulting system was left for 24 hr with constant stirring at room temperature. After that, the suspension was divided into nine portions. Seven of these were put in dialysis bags and immersed in distilled water which was changed every day. Thus, seven samples of pillared montmorillonite with different dialysis times were obtained (denoted P-AI-1, P-AI-2, P-AI-3 .... P-AI-7). The number in the denomination indicates the dialysis time in days. Finally, these samples were freeze-dried. One of the remaining two portions was only freeze-dried (P-AI-0); the other one was also taken as
627
a reference but the suspension was centrifuged and washed until chloride free (P-AI-Ow) before freeze-drying. Thermic treat ment Each sample was calcined in air in a horizontal open reactor for 2 hr at different temperatures from room temperature up to 700°C. The speed of heating (1O"C/min) was controlled by Stanton Redcroft equipment. Characterimtion of the Droducts The basal spacings of the products were followed by X-ray powder diffraction employing oriented clay films spread on glass slides. The Xray diffractograms were recorded with a Philips PW-1710 instrument using Cu-filtered, fine focus, CuKa radiation. Elemental analysis was carried out by atomic absorption spectrometry using a Perkin-Elmer mod. 560. TGA curves were obtained with a PerkinElmer TGS-2 connected to a temperature controller, System 7/4, and a data station 3600. Scanning electron microscopy was preformed using a ISI-130 microscope. Textural analysis was accomplished by N2 adsorption-desorption isotherms at 77K employing a conventional volumetric system and by mercury penetration porosimetry using a Micromeritics Poresizer 931 0. Surface areas were calculated according to the BET or to the Langmuir equation, depending on the type of the adsorption isotherm. The pore volumes were estimated as the volume adsorbed at a relative pressure of 0.98. In the isotherm analysis the Broekhoff criteria were followed (ref. 10) and the Pierce method (ref. 11) was used for the calculation of the <20A from the accumulation surface areas and volumes of pores desorption data. RESULTS AND DISCUSSION Structural characterization Table 1 shows the values of the d(001) peaks and their respective intensities of the montmorillonite pillared with Al for the dialysis times studied. The d(001) data shown in this table represent the distance between two montmorillonite layers, including the thickness of the clay sheet. By substracting this thickness, 9.6A (ref. 12), from the recorded d(001) value, the interlayer distance of the product can be estimated. As can be seen in this table, from the sample P-AI-2 onward there was no difference in the d(001) spacings. However, the d(001) peak of this sample was broad and less intense than those of the other samples
628
TABLE 1 Evolution of the basal spacing and peak intensity with the dialysis time at room temperature Sample
44
I (a. u.)
S-Na P-AI-OW P-AI-0 P-AI-1 P-AI-2 P-AI-3 P-AI-4 P-AI-5 P-AI-6 P-AI-7
15.41 19.06 15.55 15.55 18.86 19.07 19.22 18.92 19.03 19.1 3
1620 1220 300 480 81 0 780 1290 1440 1450 1590
(P-AI-3,P-AI-4 ... P-AI-7) although its chemical analysis showed that it had already incorporated 58.9 mg A1+3/g clay (the samples P-AI-4 and PAI-6 had incorporated 66.5 and 71.6 mg A1+3/g clay respectively). This may be due to the inadequate washing of this sample, because the conductivity of the last washing was 288 pS, whereas for the samples PAI-4 and P-AI-6 it was 22.0 and 2.9 pS respectively. Thus, the excess of C I - anions may impede the homogeneous diffusion of the [A11304(OH)24(H20)12]+7cations toward the interlayer space. The samples which were dialysed for more than two days show a very sharp and intense d(001) peak, suggesting regular intercalation of the Keggin ion between the clay layers. Fig. 1 shows the thermograms of the samples S-Na and P-AI-6 and their difference. The pillared sample showed lower mass loss than the S-Na up to 110°C. This step corresponds to the loss of water adsorbed on the external surface. Between 110-180°C there is higher mass loss in the pillared sample than in the S-Na. This may be due to the water adsorbed on the interior of the sheets or internal surface. Subsequently, between 180600°C in the pillared sample there is a new increase in the mass loss associated with the dehydration and dehydroxylation of the [Al1304(0H)24(H20)12]+7 cation incorporated. Finally, between 600-700°C, there is a decrease in the mass loss of the pillared sample with respect to the S-Na sample, due to the dehydroxylation of the OH- groups of the clay structure.
629
30
230
wo
630
830
1030
TEMPERATURE ('C)
Fig. 1. Thermograms: a) TG and DTG of S-Na; b) TG and DTG of P-AI-6; c) substracting between (a) and (b) TG.
Thermic treatment The effect of heat treatment on the basal spacings of the pillared samples can be deduced from the values summarized in Table 2. From the sample P-AI-3 onward, the basal spacings usually decreased with heating until they reached about 17 A at 500°C. This is thought to be due to the conversion of the aluminum hydroxy cluster cation into A1203 oligomeric. Samples dialysed for more than three days maintained a heat-stable pillared structure up to about 600°C. Textural characterization Table 3 shows the evolution of the textural parameters of pillared samples with varying dialysis time.
630
TABLE 2 Evolution of basal spacings
(A)
with thermic treatment
_---__I__I_______________
Sample
25
S-Na P-AI-OW P-AI-0
15.41 19.06 15.55
P-AI-1 P-AI-2 P-AI-3 P-AI-4 P-AI-5 P-AI-6 P-AI-7
15.55 18.86 19.07 19.22 18.92 19.03 19.13
100
200
13.07 18.62
13.46 18.09 12.67
16.76 18.99 18.40 18.47 18.23 18.64
18.66 18.74 18.33 18.46 18.11
Temperature (“C) 300 400 500 13.71 18.06
12.41 18.10 9.94
9.89 17.80
10.07
9.72
9.72
18.57 18.76 18.45 18.45 18.32
18.19 18.48 18.38 18.03 18.03
9.72 17.38 17.26 17.56 17.56 17.68
600
700
9.71
9.69
9.78
-
17.68 17.33 16.37 16.81 16.36
15.92
15.75
-
The adsorption isotherms of the samples (fig. 2) for the range of pressures P/Po 0-0.30 obeyed the Langmuir equation for monolayers rather than BET for multilayers. The N2 (77K) adsorption-desorption isotherms of some samples are represented in fig. 3. All of them have hysteresis loops of the H3 type corresponding to adsorbents having plate-like particles. Neither the pillaring process nor varying dialysis time produces important changes in the type of isotherm although the quantity of N2 adsorbed is affected and the hysteresis loops become narrower compared to that of the original sample. Nevertheless, the presence of microporosity can clearly be observed in the first part of the isotherm, P/Po< 0.30, of the stable pillared samples. The sample which was washed two days had excess chloride ions so, it had a small surface area. This could be because the CI- ions impede the adsorption of N2 and the diffusion of the pillaring agent toward the interior of the sheets. and The results shown in Table 3 exhibit a parallel evolution in S ~ E T microporous volumes throughout the experiment. The values of both series increase with dialysis time, reaching their maxima on the fifth day (SBET=362 mVg; V, = 0.193 cc/g). New information about the specific surface area can be deduced by the application of the Pierce method (ref. 11). The value of the specific area of the natural clay is due to mesopores (20-200 A), but in the stable pillared samples , only about 15% of the
631
specific area corresponds to pores > 20 A. This means that the increase of the surface area up to 362 m2lg is essentially related to the creation of micropores by the pillaring process.
TABLE 3 Textural parameters Sample ___-
S-Na P-AI-OW P-AI-0 P-AI-1 P-AI-2 P-AI-3 P-AI-4 P-AI-5 P-AI-6 P-AI-7
61 334 4 15 8 239 317 362 317 304
-
354 -
256 363 399 345 331
0.084 0.166 0.005 0.034 0.021 0.131 0.161 0.1 93 0.165 0.1 56
0.1 04
293
64 43
0.072 0.051
105 13
0.067 0.101 0.1 16 0.097 0.092
189 283 327 273 259
40 48 47 37 38
0.049 0.054 0.061 0.054 0.049
17 15 13 12 13
0.006 T
0.005
0.004
Y 0.003
0.002
0.001
0.000 0.00
0.05
0.10
0.15
0.20 P/Po
0.25
0.30
0.35
Fig. 2. Nitrogen adsorption data according to the Langrnuir [y=(P/Po).l/V] and BET equations [y=P/V(Po-P)] for a rnontmorillonite pillared with Al, dialysed for four days.
632
100
80
‘.-
4-
60
S-Na
-A- P - A I - 2
P- A l - 4
40
20
0 0.0
0.1
0.2
0.3
0.4
0.5 PIP0
0.6
0.7
0.8
0.9
1.0
Fig. 3. N2 adsorption-desorption isotherms of rnontmorillonite pillared with Al, for different dialysis times.
Macroporosity data obtained by mercury penetration for pore radii from 200 to 100,000 A are listed in Table 4. These data indicate that there was fewer macropores larger than 200 8, in the pillared samples than in the original one; this is in agreement with the SEM photographs where it can be seen that the aggregates of S-Na are much larger than those of the PAI-6. The smaller aggregates of P-AI-6 enclose much smaller interstitial voids, which results in a reduction of the pore volume with radius smaller than 200A (0.04 cclg), in comparasion with the pore volume in the same range of the S-Na (0.20 cclg).
633
CONCLUSIONS The results shown and discussed in the previous section allow us to draw the following conclusions: First, it is necessary to eliminate completely the excess ions resulting from the preparation of the sample by dialysis or centrifugation. Otherwise, they impede the diffusion of the oligomeric cations toward the interior of the clay layers and their subsequent intercalation. The samples were stable up to 600°C only when they had been dialysed for more than three days. The slight decrease of the basal spacing observed with progressive heating may be due to pillar dehydroxylation. Pillaring increased the specific surface area of the S-Na from 61 m2/g to 362 d / g , essentially by the creation of micropores > 20 A in diameter. Samples dialysed for four days or more resulted in stable values of S B E Tand Vmicropores; this means that it is not necessary to prolong the dialysis time to seven days. Apparently, there are no hydrolysis reactions in the mixture. ACKNOWLEDGEMENT The authors are gratefully indebted to the Comision Mixta Diputacion Regional de Cantabria-Universidad de Cantabria for financial support of this work through Project NQ 347. REFERENCES 1 S. Yamanaka and G.W. Brindley, Clays & Clay Miner.,27(1979) 119-124 . 2 N. Lahav, U.Shani and J. Shabtai,Clays & Clay Miner.,26,(1978) 107-115. 3 J. Shabtai and N. Lahav , U.S. Pat., 4 216 188, (1980). 4 G.W. Brindley and R.E. Sempels, Clay Miner., 12, (1977) 229-237 . 5 D.E.W. Vaughan , R. Lussier and J. Magee, U S . Pat. , 4 176 090 (1979). 6 T.J. Pinnavaia, M.S.Tzou, S.D. Landau and H.R. Raythatha, J. Mol. Catal., 27 (1984) 195-212. 7 D.E.W. Vaughan, R. Lussier and J. Magee, U.S.Pat., 4 271 043 (1981). 8 C. Pesquera, Ph. D. Thesis, University of Cantabria. (1989). 9 J.L. Martin-Vivaldi, J. Linares Gonzalez and L.J. Alias Perez , First Int. Clay Vol. 2 (1963) 229-231. 10 J.C.P. Broekhoff and J.H. De Boer, J.Catal., 9, (1967) 8-14. 11 C. Pierce, J.Phys. Chem., 6 (1953) 149-152. 12 M.B. McBride, T.J. Pinnavaia and M.M. Mortland, J. Phys. Chem., 79 (1975) 2430-2435.
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F. Rodriguez-Reinosoet al. (Editors), Churacterizatwn of Porous Solids 11 0 1991 Elsevier SciencePublishers B.V., Amsterdam
635
MICROSTRUCTURE OF EX-HYDROXIDE MAGNESIUM OXIDE & PRODUCTS OF REHMRATION
M.M.L. Ribeiro Carrott, P.J.M.
Carrott, M.M. Brotas de Carvalho
Departamento de Quimica, Faculdade de Cibncias, Universidade de Lisboa, Rua da Escola PolitBcnica, 58, 1294 Lisboa Codex, Portugal. K.S.W.
Sing
Department of Chemistry, Brunel University, Uxbridge, UBS 3PH, UK.
ABSTRACT The thermal decomposition of magnesium hydroxide under carefully controlled conditions, followed by subsequent rehydration, has been studied by means of nitrogen and neopentane adsorption isotherm measurements. Up to about 85% decomposition a uniform layered particle structure, consisting of plate-like microcrystallites intercalated by slit-shaped pores of width ca. 0 . 9 3 run, gradually spreads from the outside towards the centre of each crystal. At this stage each particle consists of MgO of normal crystallographic structure, with exposed faces partially covered by chemisorbed water. The presence of this water stabilizes the structure and, when it is removed at higher temperatures, a restructuring of the microcrystallites occurs leading to an increase in the mean pore width. Below 85% decomposition, narrow constrictions, near points of contact of the microcrystallites, are present. As a result of this, approximately half of the total micropore volume is inaccessible to nitrogen. On the other hand, the smaller water molecule appears to be able to penetrate the entire micropore structure. The restructuring of the microcrystallites at higher temperatures appears to lead to complete closure of some micropores, with the result that rehydration of the closed porosity now involves a bulk reaction and therefore occurs at a considerably slower rate.
INTRODUCTION
Although the thermal decomposition of magnesium hydroxide has been extensively studied in the literature the precise mechanism of decomposition is still not fully understood. Furthermore, there remains considerable controversy over the structure of the decomposition products. The bulk of the published work has involved either crystallographic studies of the structural transformations involved (reviewed in [1,21) or spectroscopic investigations of the nature of the active sites generated (reviewed in [ 3 , 4 1 ) . Rather less use has been made of detailed adsorption isotherm measurements to elucidate the textural and surface chemical properties of either Mg(0H) itself, the products of its thermal decomposition or their rehydration products (a guide to recent work can be found in 15-91).
636 At the outset of our investigation the state of knowledge ( i . e . the features which appeared to be generally accepted at the time, and which our results in no way subsequently contradicted) was as follows. The decomposition appears to proceed by the advance of a two-dimensional reaction interface from the edges of the crystal towards the centre [10,111. A definite orientation relationship is maintained between the crystal planes of the parent hydroxide and those of the product oxide, the major relationship deriving from the conversion of the hydroxide (0001) plane to a (111) plane of the oxide [2,101. A theoretical 54% contraction of the lattice is involved which, in the case of large crystals or when the decomposition is carried out too rapidly, leads t o the development of cracks and, ultimately, to the particle morphology being destroyed [ l l . Under conditions of slow thermal decomposition, on the other hand, the morphology of the parent hydroxide is preserved and a well defined micropore structure is generated [8,9l.Recent work from three independent laboratories has confirmed that approximately half of the total theoretical microporosity is inaccessible to nitrogen [6,8,91(although not necessarily to the smaller H20 molecule). Disagreement in
the
current
literature
centres mainly
around
the
structure o f the decomposition products, for which essentially two models have been proposed. In one [2,6,101it is argued that the product oxide consists of an array of MgO microcrystallites which are cubic in shape. The cubes are thought to be regular in size, with edge length ca. 2nm, and possibly in spatial distribution. The alternative model is based on an exfoliation of oxide into layers parallel to the basal faces of the crystals [8,9,121. One attempt has been made to try to combine both models by proposing that exfoliation into buckled layers of interpenetrating cubic microcrystallites occurs [lo]. Some of our experimental adsorption data has already been analysed in detail, and from a rather different point of view, in a preceeding paper [91. As
the reversibility of the decomposition might be expected to throw some
additional light on the structure of the decomposition products, additional results concerning the effect of rehydration were also obtained and are presented here. The structure of the decomposition products and the mechanism of decomposition are discussed in the light of these new results.
EXPERIMENTAL Details of the preparation of the two magnesium hydroxide samples, HID1 and HIDZ, and the controlled conditions used to carry out the decomposition have been given previously [91. Rehydration at room temperature was carried
637
out using freshly prepared samples, which had not been exposed to air, by equilibrating with water vapour at its saturated vapour pressure for 2 days. The
decomposed
“precursor/T1/Rh/T
‘I,
and
rehydrated
samples
are
designated
by
where the precursor is HIDl or HID2 and Rh indicates
that the sample was rehydrated after decomposition. TI and T are the final outgassing temperatures (15OoC if not stated) before and after rehydration respectively. Thus HID2/850/Rh refers to a sample of HID2 which was outgassed while gradually increasing the temperature in the manner previously described [91 over a period of about 2 weeks until a final outgassing temperature of
85OoC, then rehydrated, followed by outgassing firstly at room temperature for 15 hours and then at 150’C. Nitrogen adsorption isotherms at 77 K were determined using a Carlo-Erba Sorptomatic. Neopentane (ca. 273
adsorption isotherms at
ice melting
temperature
K) were determined using a CI Robal microbalance with pressure
measurement by means of a calibrated CEC strain gauge pressure transducer.
RESULTS Nitrogen isotherms were determined on samples of HIDl and HID2 at successive stages of decomposition and analysed by means of comparison plots using the corresponding undecomposed material as the reference adsorbent in each case [91. Each comparison plot had two linear regions. One at low pressures passed through the origin thereby enabling the total surface area, As,
of each sample to be estimated from the slope
-
excellent agreement with
the BET area was obtained in all cases. The second linear region, in the multilayer, was of slope equal to unity, thereby indicating that the external surface area,
*ext
of the samples was constant, and that the hexagonal
particle morphology and aggregate structure of both materials were not significantly altered during decomposition. From the intercept of the linear multilayer region micropore volumes, vs, were estimated. Assuming slit-shaped pores [8,91,hydraulic pore widths were estimated from d = 2 vS / (As - Aext)
(Eq. 1)
P
For both starting materials, HIDl and HID2, the variation of vs and dP with %
decomposition are shown in Figure 1 (a) and (b), respectively.
It is evident from Figure 1 that, up to 85% decomposition, the micropore volume gradually increases, while the micropore width remains almost constant and the same for both materials. At decomposition levels greater than 90%. on the other hand, the micropore volume tends to decrease, while the pore width increases significantly. This alteration
in the micropore
structure is
638 1.8-
t
C
1.6-
41
E, \
mal.L-
i
'0
'5
.
1.2-
: a
$
._
1.0-
OB
rehydrated
0.8-
I
20
60
LO
I
1
I
1
80
Yo decomposition
Fig.1
-
0
E
% decomposition
Evolution of micropore volume and pore width for HID1 (on) and HID2 (0.) during decomposition (0.1 and after rehydration (om).
0.2
0.6
0.L
0.8
P/PO
Fig.2
Nitrogen (a) and neopentane ( 0 ) isotherms corresponding to the two stages of decomposition.
accompanied by a characteristic change in the shape of the isotherms of nitrogen, but not those of neopentane, as can be seen in Figure 2, where representative isotherms for both stages are shown. It should be noted that this figure is a particularly good example of how the mechanism of adsorption, and hence the corresponding isotherm shape, depends on the ratio of pore width to molecular diameter and not just on pore width [131.
639
0.20
0.20
H I D1/300
--?
c
cn 0.15. 0
015
0
-
U 3
._ i+
-
._ I
i=i E u 0.10
m
E
u 0.10
\
\ U ul
U ul
P
p”
I
>
I1 1 original0 (3)rehydrated. 15%1
0.05
I
I
1
1
0.2
0.L
06
0.8
0.05
1
005
P/PO
Fig. 3
I
0.10
v,,.~,~HID1/150) / cd(liquid)g-’
Nitrogen isotherms and corresponding comparison plots for HIDl (01 HID1/300 (01 HID1/300/Rh (01 and HID1/300/Rh/250 (m).
r
I
I
I
0.2
04
0.6
I
I
08
P/P” Fig.4
Nitrogen isotherms and corresponding comparison plots for HID2 (01 HID2/850 (01 and HID2/850/Rh (01.
Representative nitrogen isotherms and corresponding comparison plots for the decomposed/rehydrated samples, below 85% and above 90% decomposition, are shown in Figure 3 for HIDl and Figure 4 for HID2. As with the decomposed samples, the comparison plots are linear in the multilayer region. For each sample, the micropore volume, v , was estimated from the intercept of the linear multilayer region. The results, given in Table 1 and Fi.gure 1 (a),
640
TABLE 1 Characterisation of HIDl and HID2 samples. %
decomposition, total surface area, As and micropore volume, vs.
Values refer to unit weight of stoichiometric Mg(OHI2.
sample
HIDl HID1/270 HID1/300 HID1/300/Rh HID1/300/Rh/250
HID2 HID2/300 HID2/300/Rh HID2/850 HID2/850/Rh
%
*s
0
78.6 85.6 4.5 82.2
0 86.6 8.3 100.0 40.8
V
8
m'8-l
cm3g-'
99 239 268
0 0.064 0.080 0
96 256
37 272 42 140 57
0.079 0 0.105 0.002 0.081 0.008
indicate that, for the same % decomposition, the micropore volumes of the rehydrated samples are somewhat less than those of the decomposed samples. In contrast to the decomposed samples, the low pressure region of each comparison plot gives rise to a negative intercept, indicating that the nature of the surface of the samples is modified after rehydration. Furthermore, a change in the shape of the capillary condensation hysteresis loop occurs after rehydration, thereby indicating that the aggregate structure is slightly modified (but without a modification of the external surface area or hexagonal particle morphology). For this reason the values of total surface area given in Table 1 were calculated from the BET equation rather than from the comparison plots. The hydraulic pore widths, indicated on Figure 1 fb), tend to be somewhat lower than those of the decomposed samples. Transmission electron microscopy confirmed that the hexagonal particle morphology is retained during decomposition and rehydration. Powder X-ray diffraction demonstrated
that during decomposition the lines of Mg(OHI2
gradually decrease, disappearing completely at 85% decomposition, while those of MgO gradually increase in intensity. During rehydration the MgO lines decrease in intensity while the Mg(0Hl2 lines reappear. Under the conditions used, the MgO lines never completely disappear indicating that rehydration is not complete.
641 DISCUSSION
A detailed analysis [91 of the nitrogen adsorption data on the decomposed samples has confirmed that decomposition starts at the periphery of each crystal. The maximum observed in the micropore volumes of
the samples
(Figure 1 (a)), taken together with TEM, XRD and thermogravimetric results, indicates that the reaction reaches the centre at about 85% decomposition. At this stage the particles consist entirely of MgO of normal crystal structure, with the residual 15% of H 0 being present, not as Mg(OHI2, but as chemisorbed water [91. As neither the aggregate structure nor the morphology of the particles changes during decomposition, the theoretical micropore volume, based on the crystallographic densities of Mg(OHI2 and MgO, is 0.23 cm3g-' (with respect to
It is evident therefore that less than
unit weight of stoichiometric Mg(OH)2).
50% of the total micropore volume is accessible to nitrogen molecules and, hence, that the difference in the maximum micropore volumes for HIDl and HID2 arises due to a difference in the ratio of accessible to
inaccessible
porosity. The constancy of the hydraulic pore width (Figure 1 (b)), and the fact that it is the same for both samples, is consistent with a uniform particle structure which probably consists of parallel layers of magnesium oxide rnicrocrystallites intercalated with micropores of mean width 0.93nm,
which
can be formally considered as 4 missing (111) MgO planes [ 9 1 . At higher levels of decomposition there is an increase in the micropore width. Although this can be seen from the neopentane isotherms (by a gradual roundening of the knee of the isotherm), it has a much more dramatic effect on the nitrogen isotherms. In this case, the micropore width is clearly in a critical region where the mechanism of adsorption of nitrogen has just changed from
primary
micropore
filling
to
secondary
micropore
filling
[141.
Furthermore, it is apparent that the two stages of the secondary process only become distinguishable when the pore width is greater than about three molecular diameters. The absence of a step in the neopentane isotherms is thus readily explained, as the pore width never exceeds three neopentane molecular diameters. In the case of HID1, the enlargement of micropore width is accompanied by a significant decrease in micropore volume, whereas, with HID2, the micropore volume only decreases to a relatively small degree. These results suggest that, after the decomposition reaction has reached the centre of the crystals, a
restructuring
of
microcrystallites
join
the
MgO
microcrystallites
together with
the
loss
of
occurs.
Adjacent
one micropore
and a
concomitant increase in the width of another. In the case of HIDl it is
642
evident that this process leads to a large increase in the amount of inaccessible porosity. Turning now to the results of rehydration, it is evident that the processes of decomposition and rehydration are not completely reversible. Thus, the aggregate structure and the nature of the surface are slightly modified (Figures 3 and 4) and, for the same % decomposition, the hydraulic pore width and micropore volume are decreased (Figure 1). In addition, the results in the Table, and the XFlD results, indicate that none of the samples was completely rehydrated. Actually, this conclusion is to be expected. The decomposition was carried out in a very controlled manner, allowing the reaction to proceed from the outside towards the centre of each crystal. Upon exposure of the samples to water vapour, the accessible micropores would initially have been completely filled by physisorbed water in a liquid-like state, and subsequent rehydration would therefore have occured simultaneously at all points throughout the crystal. The fact that the samples decomposed at 3OO0C are almost completely rehydrated indicates that most of the micropores, including those inaccessible to nitrogen, are accessible to the smaller water molecule. The molecular sieving of nitrogen is therefore probably associated with constrictions occurring near points of contact of adjacent microcrystallites, rather than with very narrow micropores p e r se. The presence of constrictions is supported by gas chromatographic measurements of isosteric heats of adsorption of n-alkanes at infinite dilution [151.
At
higher
levels
of
decomposition,
on
the
other
hand,
complete
rehydration is significantly more difficult to achieve, which suggests that, after restructuring of the microcrystallites has occurred, some of the micropore entrances become genuinely closed. Rehydration of the closed pores will therefore involve a bulk reaction which occurs at a significantly slower rate than rehydration of exposed surfaces [71. Because the micropore volume
of
the rehydrated samples is spread
throughout the particle, rather than concentrated at the peripheries as with the partially decomposed samples, it follows that, for a given pore volume, the mean pore width will be less (Figure 1). In addition, the pore walls will be fully, rather than partially, hydroxylated. As a result of these two factors, strong physisorption of molecular water can occur in the micropores, and an outgassing temperature of 15OoC may not be sufficient to desorb this water
[131. The
presence of
this molecularly adsorbed water, and
the
difference in the degree of hydroxylation of the exposed surfaces are consistent with the negative intercepts of the low pressure regions of the comparison plots in Figures 3 and 4.
643
CONCLUSIONS The results indicate that thermal decomposition of magnesium hydroxide occurs in two stages. The first stage, which involves conversion of Mg(0H)
to
microcrystallites of MgO, is complete at 85% decomposition. It appears that the residual water, which is chemisorbed on the exposed MgO faces, stabilizes the structure. The second stage of decomposition involves removal of the residual water, and leads to a restructuring of the microcrystallites. During this process the mean pore width is increased and some of the micropore entrances become closed. During the first stage the decomposition can (almost) be reversed by exposing the sample to water vapour. This reversibility, taken in conjunction with the constancy of the external surface area and the high degree of topotaxy of the reaction, provide very strong evidence in favour of the exfoliative mechanism of decomposition and indicate that the existence of cube-shaped MgO microcrystallites is extremely unlikely.
ACKNOWLEDGEMENTS The authors are grateful to Junta Nacional de Investiga@o Tecnol6gica
(Portugal) and
the
Commission of
the
Cientifica e
European Communities
(Belgium) for financial support and to The British Council for the award of travel grants under the Anglo-Portuguese Treaty of Windsor accord.
REFERENCES J. Green, J.Mat.Sci., 18 (1983)637. 1 2 M.G. Kim, U. Dahmen & A. W. Searcy, J. Am. Ceram.Soc. , 70 (1987)146. 3 A. Zecchina, S. Coluccia & C. Morterra, Appl.Spectr.Rev., 21 (1985) 259. 4 S. Coluccia, S. Lavagnino & L. Marchese, Mater.Chem.Phys., 18 (1988)445. M.M. Brotas de Carvalho, M.M. Lopes Ribeiro, M.R. Sales Grade & 5 A. Ruiz Paniego, Anal.Quim., 84 (1988)300. 6 D. Beruto, R. Botter & A.W. Searcy, J.Am.Ceram.Soc., 70 (1987) 155. 7 Y. Kuroda, E. Yasugi, H. Aoi, K. Miura and T.Morimoto, J.Chem.Soc., Faraday Trans. I , 84 (1988)2421. 8 H. Naono, Colloids G Surfaces, 37 (1989) 55. 9 M.M.L. Ribeiro Carrott, P.J.M. Carrott, M. Brotas de Carvalho & K. S.W. Sing, J. Chem.Soc., Faraday Trans., in press. A.F. Moodie & C.E. Warble, J.Crysta1 Growth, 74 (1986) 89. 10 11 P. J. Anderson & R.F. Horlock, Trans.Faraday Soc., 58 (1962) 1993. 12 J.C. Niepce, G. Watelle & N.H. Brett, J.Chem.Soc.,Faraday Trans. I , 74 (1978) 1530. 13 K.S.W. Sing, D.H. Everett, R.A.W. Haul, L.Moscou, R.A. Pierotti, J. Rouquerol & T. Siemieniewska, Pure 6 Appl.Chem., 57 (1985) 603. 14 P.J.M. Carrott & K.S.W. Sing, in K.K. Unger, J. Rouquerol, K.S.W. Sing & H. Kral (Eds.1, Characterization of Porous Solids, Elsevier, Amsterdam, (19881,pp. 77-87. 15 M.M.L. Ribeiro Carrott, Ph. D. Thesis, University of Lisbon, 1990.
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F. Rodriguez-Reinosoet al. (Editors), Characterization of Porous Solids I1 0 1991 Elsevier Science Publishers B.V., Amsterdam
TEXTURE AND SURFACE PROPERTIES OF SUPPORTED METALLIC OXIDE CATALYSTS: Na-DOPED, TITANIA AND ALUMINA-SUPPORTED VANADIA
M. DEL ARCO, E. HERNANDEZ, C. MARTIN, J. MATEOS and V. RIVES* Departamento de Quimica Inorginica. Universidad de Salamanca. Facultad de Farmacia. 37007Salamanca (Spain)
SUMMARY Adsorption of nitrogen at 77K has been studied on V205/MOx (M=Al, Ti) systems, used to oxidize hydrocarbons, obtained by impregnation of the supports with aqueous solutions of ammonium vanadate or by mechanical mixture of the oxides. The samples were doped with Na (0,1,3, or 6% weight) and calcined at 773K (to decompose the vanadium precursor) or 973K (to melt formed vanadia); characterization was performed by XRD and IR and V-UV spectroscopies. Incorporation of vanadium leads to a sharp decrease in the specific surface area, specially if sodium is simultaneously present. The specific surface area also decreases as the calcination temperature is increased in all samples, but not in Na-free alumina samples; in Na-rich alumina samples no effect of the calcination temperature on the specific surface area is observed. In the titania-supported samples the reaction is accompanied by a rutilisation of the support if Na and V are simultaneously present. The behaviour observed is discussed in terms of formation of stoichiometric and non stoichiometric Na-V-0, Na-Al-0, and AI-V-0 compounds.
INTRODUCTION Vanadia-based catalysts are widely used in industrial processes for partial oxidation of hydrocarbons (ref. 1-3), as well as for reduction of nitrogen oxides with ammonia (ref. 4-5). Its activity and selectivity in these reactions is largely modified depending on the nature of the support (generally another oxide) used to achieve a good dispersion of vanadia crystallites, and it has been recognized that the role played by the support is not as a mere dispersing agent, but that it also modifies the acid/base and electronic properties of the active phase (ref. 1). In this same sense, alkaline metal ions, sulphate, fluoride, phosphate, etc., species, are generally added as dopant agents, and sometimes they exist in real catalysts as impurities. We have previously studied the role played by the support (silica, alumina, magnesia and titania, both anatase and rutile) on the surface properties of vanadia (ref. 6-9), concluding that the interaction between the support and the supported phase, and hence their activity in the above mentioned processes, greatly depends on the difference in their basicities. The aim of the present paper is to insight in this study, analyzing the role played by a usual dopant (sodium) on the properties of alumina- and titania- supported vanadia, using two different methods to incorporate vanadia on the surface of the support, i.e., standard impregnation methods and mechanical mixture of the oxides, as we have observed (ref. 7 and 9) that some of the properties of the final
645
646
solids depend, in some sort of way, on the preparation method. EXPERIMENTAL The titania (P-25, ca. 50% anatase) and alumina (Al203-C), both from Degussa (Frankfurt), were submitted to calcination overnight at 773K prior to incorporate the active phase. Doping with sodium was achieved by impregnation with aqueous solutions of NaOH in a rotary vacuum evaporator (Heidolph VV-60), drying and calcination at 773K in oxygen, to yield final solids with 0, 1,3, or 6% (w/w) of sodium. Vanadium was incorporated by dry impregnation (series I) with aqueous oxalic solutions of NH4V03 (from Carlo Erba, p.a.) and calcination at 773 K or
973 K in oxygen. Samples belonging to series M were prepared by mixing vanadia (obtained by calcination in air of NH4V03 at 773 for 5 h) with the support (alumina or titania) and heating in
0, at 773 or 973 K. Calcination temperatures were chosen to yield decomposition of ammonium metavanadate and melting of vanadia (773 and 973 K, respectively). In all cases, the final content of vanadium amounts to yield a monolayer of vanadia on the support (equivalent to 4.1 % for the titania-supported samples, and 8.2 % for alumina-supported samples), as calculated from the specific surface area (SSA) of the undoped supports calcined at 773 K and the surface covered by a "molecule" of VO,,. Calcination treatments were performed at a heating rate of 10 Wmin (using a RAX PC-8601 temperature programmer/controller) up to the final temperature (773 or
973 K), that was maintained for 2 h, while 0 2 was continously flowed on the sample. Characterization of the samples was canied out by (i) X-ray diffraction (XRD), using a Philips 1730 instrument, with Cu K a i (h=154.05 pm) radiation and Ni filters; (ii) VisibleUltravioletDiffuse Reflectance (V-UVDR) spectroscopy, in a Shimadzu UV-240 spectrometer provided with a diffuse reflectance accessory and coupled to a Shidmazu PR-1 graphic printer, using a slit of 5 nm and parent support as a reference; (iii) infrared spectroscopy OR) in a PerkinElmer FTlR-1730 spectrometer using KBr pellets; and (iv) nitrogen adsorption measurements at
77K in a conventional high vacuum Pyrex system (residual pressure = lo4 N m-,) equipped with a silicon oil vapour diffusion pump, McLeod gauge and grease-free stopcocks, pressure changes being monitored with a MKS pressure transducer; the samples were outgassed in siru at 420 K for 2 h before performing the adsorption experiments.
RESULTS AND DISCUSSION The main features of the samples have been summarized in Table 1. The XRD diagrams of the Na-doped supports are very similar to those of the bare supports. Na4Ti04 and NaAlO, species are detected for the Na-rich samples calcined a 973 K, but NaA102 is already detected with the Na-rich support calcined at 773 K. The titania support calcined at 773 K shows a constant percentage of anatase (52 k 2%), but upon calcination at 973 K the Na-richest support still contains a small, appreciable, percentage of anatase, and the supports containing 0 or 1%Na are completely rutilised. On supporting vanadium on the Na-free samples, V2OS crystallites are detected on alumina
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upon calcination at 773, and A1VO4 is also detected upon calcination at 973 K, presumibly better crystallized in series I samples. The titania support behaves in a different way, and although no peak due to vanadium-containing species is recorded in the XRD profile of series I samples calcined at 773 or 973 K (indicating that vanadia should be very well dispersed on the surface of the support), vanadia peaks are recorded in the diagrams of series M samples. These results show that, probably due to their similar basicities, reaction between vanadia and titania is hindered, while reaction of vanadia with alumina (less acidic than titania) takes place. Taking into account the data on SSA sumarized in Table 1, the sharp sintering of the titania support upon calcination a 973 K should be responsible for the formation of fairly large crystallites of vanadia, making them detectable by XRD, even after melting upon calcination at 973 K. Moreover, this reaction should take place simultaneously to the topotactic anatase/rutile phase change upon calcination at 973 K, as both sets of samples obtained on titania calcined at 773 K contain ca.
50% anatase, this content sharply decreasing when calcination is performed at 973 K (0% anatase for sample belonging to series I and 16 % to sample belonging to series M).
TABLE 1 Specific Surface Area (BET, m2g-') for the Supports and the Vanadia-containing Samples support
Ti02
A'2°3
%Na
cr
0
1
3
0
1
6
support
773 973
53 9
48 c1
38 17
102 nm
108 nm
75 nm
Series1
773 973
46 6
24 5
32 <1
92 43
100 42
57 51
SeriesM
773 973
50 16
38 3
34 <1
84 73
89 51
59 54
CT=calcination temperature (K); nm=not measured. When vanadia is incorporated on the Na-rich, alumina support, XRD peaks due to stoichiometric and non-stoichiometric V-Na-0 and Al-V-0 species are recorded, NaA102 being absent; vanadia diffraction peaks are recorded upon calcination at 773 K. Increasing of the calcination temperature up to 973 K leads to more intense diffraction peaks, probably due to a better crystallization of the crystalline species, the peaks due to vanadia vanishing because of melting of the vanadia crystallites, their reaction with the alumina surface, and further formation of V-A1-0 species.
Similarly, stoichiometric (NaqV207, Na3V04, NaV03) and non-stoichiometric (bronzes, Na1.33V205) species are found in the Na-rich titania series I sample, and also for series I and series M samples calcined at 973 K. Stoichiometric species predominate when the percentage of
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Na is 3%, probably due to the fact that in these circumstances the Na:V ratio is closer to that of the stoichiometric compounds than in the other samples. It should be noted that formation of these species in sample M with 3 % Na calcined at 973 K seems to cancel the rutilisation observed with samples poorer in Na calcined at this same temperature. The V-UV/DR spectra of both series (titania-based and alumina-based) of samples are quite different, but agree with the results depicted above from the XRD studies. A band at 500-400 nm due to a O=-+VS+charge transfer process is recorded in those samples which XRD diagrams indicated the existence of vanadia, and is ascribed to the presence of distorted [VO6] octahedra. In addition, formation of NaV03 and Na3V04 in Na-rich samples containing vanadium is shown
by the presence of two absorption bands at 260 and 300 nm due to [VO4] species. In the case of the titania samples, the spectrum is less resolved, because of the presence of a band at 410-420 nm that has been previously ascribed (ref. 10 and 11) to formation of peroxide-like species on the surface of titania stabilized by the presence of vanadium ions. Similarly, the shift observed in the IR absorption band originated by crystalline V2O5 (ref. 12) due to
species from 1000
cm-1 in samples Ti@-I, to 1025 cm-1 (because of the improved crystallinity) in samples Ti02-M, together with its shift to 960 cm-1 because of bronzes formation (ref. 12) is in good agreement with the XRD results. As shown elsewhere (ref. 13), interaction with gaseous C02 leads to removal of this band in samples Al2O3-I, indicating that surface carbonates are formed through reaction with surface exposed V=O species, this reaction being enhanced in the presence of sodium. The nitrogen adsorption isotherms were fully reversible in the pressure range here studied (0 <
PPo I0.95), and belong to type I1 in the IUPAC's classification (ref. 14). No development of microporosity was found in any case, and the SSA values calculated following the BET method are shown in Table 1. Changes shown in this Table can be related to two different processes: (i) the topotactic anatasehutile phase change taking place in the titania-based samples, and (ii) formation of surface vanadium-containing species in both sets of samples. First of all, and paying attention to the behaviour shown by the supports, the increase in the SSA of A1203 upon incorporation of small amounts of sodium has been ascribed (ref. 15 and 16) to the creation of new pores. A further increase in the Na content, however, leads to a decrease of the SSA, as observed with the titania-supported samples. On the other hand, the simultaneous presence of Na and V on alumina leads to a further decrease in the SSA, but the final solids display the same SSA values (55k4 m2 g-I), whichever the way vanadium has been incorporated or the calcination temperature. This behaviour indicates that the formation of surface Al-V-0 compounds has in some sort of way stabilized the surface of alumina. With TiO2, however, the rutilisation taking place as the samples are calcined at 973 K favours its simultaneous sintering, this effect being more marked when Na is added. However, for samples with the same content in sodium (0 or 3%), the addition of vanadium does not modify the SSA values after calcination at 773 K (5Oi3 m2 g-1 for Na-free samples, and 35+3 m2 g-1 for samples with 3% sodium).
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f
0,o
0,2
0,4
p,po
0,6
0,8
1 .o
Fig.1 .-f-plots for the alumina supports (ref.: Na-free alumina)
Analysis of the isotherms have been carried out following the BET and Cranston and Inkley methods, as well as the t-method developed by de Boer (ref. 17 and 18). In addition, thefmethod developed by Gregg (ref. 19) has been also applied, as it allows a comparison of the adsorption capacities of the samples in the full relative pressure range studied. For the titaniasupported solids a steady decrease of the adsorption ability is observed, leading to horizontalfplots without any outstanding features, thus indicating that incorporation of vanadia on the surface of titania takes place with an homogeneous dispersion of the former on the surface of the later, blocking all mesopores in a similar way and thus decreasing the SSA. However, with the alumina-based samples a very different behaviour could be observed. First of all, and with regards to the support itself, incorporation of sodium leads to an appreciable decrease in the adsorption capacity, similarly to that above described for the titania-based samples with vanadium, Fig. 1. As shown above, the simultaneous presence of vanadium and large amounts of sodium (6% samples) leads to formation of V-A1-0 and Na-Al-0 species even after calcination at 773K. Formation of these species has a remarkable effect on the texture of these samples, as shown by the data reported in Fig. 2. Bothf-plots have been obtained using as a reference the nitrogen adsorption isotherm corresponding to the support doped with the same amount of sodium as the sample being studied, and thus deviation from a straight horizontal line at an ordinate value equal to one should be ascribed to changes taking place in the samples because of vanadium incorporation. Both samples show a similar behaviour, giving rise to parallel f-plots and their SSA are very close (58+1 m2g-1). However a sort of discontinuity is observed in the f-plot at P/Po=0.75, a relative pressure corresponding to pores with an average diameter close to
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8 nm. From the SSA of the solid, its specific gravity and assuming spherical or cube-like particles, it can be concluded that this size corresponds to that of the intercrystallite pores. So, it can be tentatively concluded that the ternary species formed upon reaction of the vanadium compounds (vanadate or vanadia) with the surface of alumina in the presence of sodium takes probably place with the most energetic sites of the surface, leading to soldering of the primary alumina particles, thus decreasing the SSA, mainly because of cancelation of such pores.
0’9
1
rn
V205Na6%M
1
0.0
0.2
I
OY4
I
PIP0
0,6
0,8
1 .o
Fig.2.-f-plots for the vanadia-alumina samples with 6% Na (ref.: 6%Na-doped alumina
CONCLUSIONS From the results above, it can be concluded that the surface properties of sodium-doped, vanadia-containing alumina and titania depend on the parameters here studied, i.e., the precursor salt of vanadium (although its effect is cancelled upon calcination at 973K). the calcination temperature and the presence of sodium. Formation of ternary V-AI-0compounds in the Na-rich samples leads to removal of pores with an average diameter of 8 nm, probably because of the soldering of the alumina primary particles. ACKNOWLEDGEMENTS Authors thank finantial support from C.I.C.Y.T. (MAT88-0556) and Consejeria de Cultura y Bienestar Social (Junta de Castilla y Le6n). REFERENCES 1
P.J. Gellings, in G.C. Bond and G. Webb (Editors), Catalysis, Specialist Periodical Reports, vo1.7, The Royal Society of Chemistry, London, 1985, Ch. 4, p. 105.
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2 3 4
5 6 7 8 9 10 11
12 13 14 15 16 17 18 19
K. Mori, M. Miura, A. Miyamoto and Y. Murakami, J. Phys. Chem., 88 (1984) 52325235. C. Martin and V. Rives, React. Kinet. Catal. Lett., 36, 401-406 (1988), J. Molec. Catal., 48 (1988) 381-391. F.J.J.G. Jansen, F.M.G. van der Kerkhoff, H. Bosch and J.R.H. Ross, J. Phys. Chem., 91 (1987) 5921-5927. Z. Sobalik, V. Pour, L.A. Sokolova, O.V. Nevskaya and N.M. Popova, React. Kinet. Catal. Lett., 30 (1986) 179-184. M. del Arco, M. J. Holgado, C. Martin and V. Rives, J. Catal., 99 (1986) 19-27. M. del Arco, M. J. Holgado, C. M a d n and V. Rives, Langmuir, 6 (1990) 801-806. M. del Arco, M. J. Holgado, C. Martin and V. Rives, J. Mat. Sci. Lett., 6 (1988) 616619. C. Martin; Ph. D. Thesis, Universidad de Salamanca, Spain (1987). M. del Arco, M. J. Holgado, C. Martin and V. Rives, Spectroscopy Lett., 20 (1987) 201211. J. P. Espinh, A. R. Gonzilez-Elipe, G. Munuera, J. Garcia, J. C. Conesa and E. Burattini, Physica B, 158 (1989) 174-175. J. Nakagawa, T. Ono, H. Miyata and Y. Kubokawa, J. Chem. Soc. Faraday Trans. I, 79 (1983) 2929-2936. M. del Arco, E. Hernindez, C. Martin and V. Rives, Spectroscopy Lett., 22 (1989) 11831191. K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol and T. Sieminiewska, Pure & Appl. Chem., 57 (1985) 603-617. R. Hombek, J. Kijenski and S . Malinowski, in B. Delmon, P. Grange, P. Jacobs and G. Poncelet (Editors), Preparation of Catalysts 11, Elsevier Scientific Publishers, Amsterdam, 1979, pp. 595-603 G. A. El-Shobaky, T. El-Nabarawy and G. A. Fagal, Appl. Catal., 52 (1989) 33-41. R.W. Cranston and F.A. Inkley, in A. Farkas (Editor), Advances in Catalysis, Vol. 9, Academic Press, New York, 1957, p. 143. B. C. Lippens and J. H. de Boer, J. Catal., 4 (1965) 319-327. S . G. Gregg, J. Chem. SOC.,Chem. Comm., (1975) 699-700.
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F. Rodriguez-Reinosoet al. (Editors),Characterization of Porous Solids ZI 0 1991 Elsevier Science Publishers B.V., Amsterdam
653
SORPTION OF WATER VAPOUR BY PARTIALLY DECOMPOSED CALCIUM HYDROXIDE Kenneth S.W. Sing, Charis R. Theocharis, and David Yeates Department of Chemistry, Brunel University, Uxbridge, Middlesex, UB8 3PH, UK.
ABSTRACT The surface chemistry of commercial grade calcium hydroxide was studied by sorption methods, thermal analysis and spectroscopy. Part of this investigation was the study of the sorption of water vapour by the partially thermally decomposed calcium hydroxide. It was found that when the decomposition takes place in the absence of air,the result is a high surfacearea, porous solid. The pores are slit-shaped. Complete rehydroxylation takes place upon exposure to water vapour. The whole process is reversible, and reproducible.
INTRODUCTION The long-standing industrial importance of calcium compounds has led to considerable interest being shown in the solution chemistry of calcium. In contrast, very little attention has been given to the surface or solid-state chemistry of calcium hydroxide, although it is well known that it is a very reactive solid. For example, the interaction of lime with moisture and carbon dioxide leads to a decrease in its reactivity and hence less of industrial performance. Furthermore, it seems likely that the serious effects of acidic airborne pollutants such as SO, and NO, on cement is at least partly due to their interaction with the calcium hydroxide element in cement. The work reported here was undertaken as part of an investigation of the surface properties of calcium
and of its thermal decomposition products. In this part of
the work we have studied the adsorption of nitrogen and the sorption of water vapour after outgassing samples taken from a commercial grade of Ca(OH)2 at different temperatures. We have carried out other studies using thermal analysis (TG and DT analysis), electron microscopy and Fourier-transform infrared spectroscopy'+2.Those results will be presented at subsequent publications.
654
EXPERIMENTAL The samples of calcium hydroxide used in these studies were commercially available and while every effort was made to protect them from exposure to the atmosphere, no purification was undertaken. Commercial calcium hydroxide is obtained by the hydration of calcium oxide, which in its turn had been obtained from the thermal decomposition of limestone -calcium carbonate-. Small amounts of impurities are known to be present, and include magnesium hydroxide (magnesium is always present in small amounts in calcium ores), undecomposed calcium carbonate and traces of quartz. This procedure yields a calcium hydroxide with a CdIz-likelayer hexagonal structure, of the brucite [Mg(OH)z] type. This was confirmed by powder X-ray diffractometry. Water isotherms were recorded at 298K by the gravimehic method by means of a McBain balance, employing a thermostated fused silica spring. The water used for adsorption was thoroughly degassed by several cycles of the freeze-evacuate-thawmethod. Pressures were measured by means of a transducer with a long linear range, which was calibrated against a mercury manometer. Weight changes upon adsorption were measured by the extension of the silica spring. The sample was outgassed before each isotherm in siru, overnight at 298,423,523 and 623K. On completion of these experiments an isotherm was recorded after outgassing at 298K, to check for reversibility. The same sample was used for all the sorption experiments and was continuously kept in the apparatus without exposure to the atmosphere. These experiments, took place over several weeks. Sorption measurements were obtained at a temperature of 298K.
RESULTS AND DISCUSSION Thermal analysis shows that for a fresh calcium hydroxide sample, there are two major weight loss events: the main one happens in the temperature range 613 to 743K, and the second, much smaller one from 373 to 523K. The latter event which represents a weight loss of ca 1%, corresponds to the removal of physisorbed water and other vapours from the surface. The former step, which represents a weight loss of 23% is the dehydroxylationof calcium hydroxide to the oxide. Some samples also show a smaller step at ca 900 to 950K which corresponds to the decomposition of small amounts of calcium carbonate. Fourier transform infrared spectroscopy was used to monitor samples of Ca(0H)Z before and after the measurement of water sorption isotherms after outgassing at 298K. Comparison of
655
these spectra indicated that water sorption resulted in additional water molecules being present on the surface compared with the sample before sorption, even after completion of the desorption branch, but not to the generation of extra OH groups. This means that the adsorption of water molecules occurs without dissociation. From the water sorption isotherms, the following data can be obtained. The two isotherms after outgassing at temperatures below 423K were very similar. However, for outgassing temperatures of 523K upwards, significant changes occured. These differences arise from a substantial increase in the amount of water vapour sorbed per unit weight. A significant change in the shape of the isotherm was also observed. The total amount
of water sorbed showed considerable increase with increasing outgassing temperature, signifying an enhancement of the hydrophilic character of the sample. It also reflects a genuine increase in the apparent surface area as shown by the results of nitrogen adsorption isotherms. Examination of the two isotherms presented in Figure 1 reveals that the sample which had been outgassed at the higher of the two temperatures (623K) sorbes a relatively larger amount of water at very low partial pressures, and continues to sorb substantially more than when the sample was outgassed at 298K, as shown by the difference in the slope of the two isotherms.
80
1 0.2
0.6
0.4
0.8
I
1.0
P/PO
Figure 1: HzO sorption isotherm after outgassing at 298K (circles), and 623K (squares).The empty symbols refer to adsorption, and the filled to desorption.
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The nitrogen and water sorption data can be understood in terms of the thermal analysis results. Outgassing at low temperatures causes the desorption of physisorbed layer water, but the sample continues to be stoichiometriccalcium hydroxide. Outgassing at the higher temperatures results in the partial decomposition of the hydroxide to the oxide. This, apparently results in a porous, very hydrophilic solid. The very high uptake of water at low partial pressures as shown in Figure 1, corresponds to the rehydroxylation of the CaO to the hydroxide. However, even after the rehydroxylation is complete, this sample remains significantly more hydrophilic than when the sample is outgassed at 298K. This can be shown by comparing the difference in amount adsorbed by the two samples, at two partial pressures. Thus, from Figure 1, at p/po= 0.1, the sample outgassed at the higher temperature sorbed
180 mg g-' more water than that outgassed at 298K3,whereas at p/po= 0.95 the excess has risen to 220 mg gl. This, it is suggested, is due to the generation of porosity upon partial decomposition. In order to highlight these features, the sorption isotherms have been drawn with the sample weights normalised to CaO as in Figure 2. On this scale, the weight for stoichiometric Ca(OH)2 corresponds to that for fully hydroxylated CaO. Taking the isotherm of the sample outgassed at 623K (diamond shapes in the Figure), it can be seen that by the time p/p,=O.l has been achieved, the sample has reached a weight above that of fully hydroxylated CaO (indicated by the start of the circles, and by the weight scale on the right of the diagram). However, even after this regeneration of stoichiometric Ca(OH)2the level of water uptake for this sample, continues to be significantly higher than for pristine Ca(OH)2. Thus, the sample outgassed at 623K, must have additional sites of adsorption for water, compared with stoichiometric Ca(OH)2. The existence of hysteresis in both the water sorption and nitrogen isotherms, signifies that this sample contains slit-shaped pores, which are not present in undecomposed Ca(OH)2. These have been generated by the removal of OH groups from the bulk. Saturation with water presumably allows a restructuring of the sample to occur which eliminates these extra pores. After desorption, all isotherms return to the same point on the weight scale, corresponding to stoichiometric hydroxide, indicating return to the initial state of the sample. This, apparently, occurs without an attendant trapping of water molecules
in the bulk. Table I shows the starting and finishing weights for each of these experiments: note that the final weights are the same, irrespective of the outgassing temperature. The
657
reversibility of the process is exhibited by the fact that the final isotherm for which outgassing was carried out at 298K was the same as the first one.
;= m 0
-1
,='
a3
0
w3
Figure 2: Series of water sorption isotherms carried out on a single Ca(OH)2sample, after outgassing at the following temperatures: 298K (circles), 523K (squares) and 623K (diamonds). Filled symbols refer to desorption.
It is noteworthy that CaO has a N2 surface area of 3 m2g-', whereas Ca(0H)z heated at 523K and therefore partially converted to CaO had a significantly increased area (85 m2 g-I). Furthermore, whereas neither CaO nor Ca(OH)2 are porous, these partially decomposed solids have slit-shaped pores. The crystal structure of CaO is very different to that of Ca(OH)2. It is suggested that porosity arises because when OH groups are eliminated to yield CaO, reorganisation of the Ca(OH)2 structure to that of CaO is incomplete, and therefore a very open structure results, which allows free access for water molecules to re-enter the structure and react. When decomposition takes place in air, this reorganisation can take place, probably
658
during the cooling down period, because of reaction with ambient moisture. The mechanism for the elimination of the pores upon rehydration under the conditions used here is not clear, but it is suggested that the water molecules in the pores assist the rorganisation of the lattice
during desorption. TABLE I Sample Weights for a Ca(OH)2 Sample Before and After Measuring Water Sorption.
Outgassing TemdK
298 423 523 623 298*
Outgassed Wt/g
0.1146 0.1148 0.0964 0.0907 0.1152
Wt after Isotherdg
0.1152 0.1151 0.1151 0.1151 0.1151
NB * Denotes last isotherm.
We acknowledge useful discussions with Dr J.F. Marsh. This work is sponsored by
EXXON Chemical Ltd.
REFERENCES 1. D. Yeates, PhD Thesis, Brunel University, (1989) 2. A.C. Gray, PhD Thesis, Brunel University, (1990)
F. Rodriguez-Reinoso et al. (Editors), Characterization of Porous Solids II 0 1991 Elsevier Science Publishers B.V., Amsterdam TEXTURE AND SINTERING OF ZIRCONIUM DIOXIDE-YTTRIUM OXIDE CERAMICS LECLOUX1,Silvia BLACHER', Pierre-Yves KESSELS 2, Pierre MARCHOT2, 2 2 3ean-Luc MERLO , Francis NOVILLE and Jean-Paul PIRARD2
And&
3.
'Solvay et Cie, Laboratoire Central de Recherche, rue de Ransbeek, 310, 6-1120 Bruxelles (Belgium) 'Laboratoire de Chimie Industrielle et de Gdiiie Chimique, Institut de Chimie au Sart Tilman (B6), Universitk de Ligge, 8-4000
Likge (Belgium).
ABSTRACT The modifications of porous texture and the sintering, as a function of temperature, of a zirconium dioxide-yttrium oxide ceramic were investigated by nitrogen adsorption-desorption isotherm analysis, by mercury porosimetry and by image analysis of scanning electron micrographs. The samples were prepared following a technique developed by Solvay (ref.1). This technique produces porous spherical particles which are rather monodisperse and exhibit few agglomerates. Densification of slabs prepared from this product seems nearly complete at 1450OC. This study shows that the results - at least for the investigated case - obtained using the different methods are consistent though the measurements are based upon completely different principles. The specific surface areas as well as the porosities determined by mercury porosimetry which is a global measurement agree with the corresponding quantities determined by micrograph image analysis which is a local measurement. INTRODUCTION Zirconia ceramics show interesting properties such as high melting point, low reactivity to molten metals, high ionic conductivity and good stability towards large temperature variations. Pure zirconia exists as four solid phases with monoclinic tetragonal, orthorhombic and cubic fluorite structures. Themonoclinic phase is stable up to ca.
llOO°C and transforms into the tetragonal phase when the temperature is
increased to 1200°C; this transformation is accompanied by a sudden volume contraction of around 9%. The transformation temperature exhibits a large hysteresis as the tetragonal to monoclinic transition occurs between 1000 and 8 5 O O C (refs. 2-3). In order to maintain a stable tetragonal phase at room temperature, small particles and stabilizers (Y203, MgO, CaO, CaF2) are used (refs. 2). These partially stabilized zirconias (PSZs) show enhanced mechanical strength, toughness and thermal shock resistance. Consequently, PSZs are extensively used as solid electrolytes in oxygen sensors, in high temperature heating elements as thermal barrier coatings and in thermomechanical applications. One o f t h e objectives of ceramic processing is to increase the density to its theoretical value in order to achieve definite properties. This densification will proceed at a reasonable rate only if the pore and the grain boundaries are
659
660
kept close together. A knowledge of the pore texture of the raw powder and the slabs during sintering is therefore important as it gives pertinent information about the capabilities of the material. Indeed, previous works evidenced that sintering is easier when samples of non agglomerated monodisperse spherical particles are used. (refs.4-5) EXPERIMENTAL Sample preparation Powders of partially stabilized zirconium dioxide-yttrium oxide ceramics (PSZs) were prepared at room temperature and under nitrogen at atmospheric
pressure by hydrolysis of zirconium and yttrium alcoholates solutions containing a carboxylic acid. If a careful selection of the acid type as well as the amount o f water is made, it is possible using this technique to obtain powders composed of nearly monodisperse spherical particles exhibiting few o r no agglomerates (refs. 1-6). After separation by filtration, the powder was dried at room temperature, then calcined at 45OOC under flowing nitrogen to remove the organic acid, the water and the organic solvents. Spherical particles (mean diameter d = 0.6 pm P (s.d = 0.1 pm); 4.1 wt.% Y ) were obtained. X-ray diffraction measurements show that the product is nearly amorphous o r formed o f very small crystallites, structure of which recall Zr02 tetragonal structure (41%) and Zr02 cubic structure ( 5 9 % ) . To study the powder porous texture modifications between 45OOC and 1000°C, the samples were brought to the temperature of the thermal treatment with a programmed temperature increase of 180°C/h and then kept at this temperature for 12h; the treatment was performed under flowing nitrogen between 45OOC and 85OoC, and under dry air above 85OOC to avoid partial reduction of zirconia. study the sintering process, two types of slabs were prepared with the powder calcined at 45OOC : - slabs of cylindrical shape (diameter 10 mm, thickness 4 mm, approximative weight 0.8 g) were obtained by uniaxially cold pressing a powder-water paste mixed with a binder ( 2 wt.-% of MOVIOL 8-88 a poly(viny1 alcohol) supplied by To
Hoechst) at 300 MPA. In order to remove the binder, the samples were heated to 800OC at a rate of 50°C/h and kept at this temperature for 2 h; this process was carried out under a flow of dry air (120 ml/min); - slabs of plate shape (length 60 mm, width 50 mm, thickness 2 mm) were obtained by slip casting of a powder-water paste without binder. The samples were then cooled to room temperature and separately sintered in an electric furnace in air up to a temperature between 800 and 1450OC. The heating conditions are shown in Fig. 1. Two temperature programs (TPI and TP2) were used.
661
Methods The specific densities of the sintered samples were calculated from the weight and the geometrical volume of the slabs. Specific surface areas were determined at liquid nitrogen temperature according to the BET theory (ref. 7) using a Carlo Erba Series 1800 sorptomatic apparatus controlled by an Apple I1 microcomputer. Nitrogen of high purity (99.98%) was used. Mercury penetration curves were carried out on a Carlo Erba 2000 automatic porosimeter working in the pressure range 0.1 - 200 MPa (ref. 7). Triply distillated mercury of high purity was used. After sintering, a specimen of each sample was mounted in a resin and polished for image analysis. A set of micrographs was produced on a 3EOL scanning electron microscope. The image analysis were realized with the Visilog software (Noesis - France) implemented on a Microvax 11, the image acquisition was obtained by a Burle FK/6990 Solid State CCD Camera. RESULTS AND DISCUSSION Powder porous texture analysis The evolution of the PSZs porous texture during calcination between 45OOC and 1000°C was investigated by the analysis of nitrogen adsorption-desorption isotherm following the method proposed by Lecloux (ref. 7-8). The powder calcined at 45OOC under flowing nitrogen has a high specific surface area as well as high pore volume (SBET = 114 m2/g, V = 0.141 cm’lg). P According to the BDDT classification, the adsorption-desorption isotherm shape is of type IV with a hysteresis loop of type E, characteristic of ink-bottle mesopores (2 nm < diameter < 3 0 nm) (Fig. 2). The t-plot o f Lippens and de Boer was calculated by the n-method of Lecloux (ref. 7-9). The upward deviation from the straight line passing through the origin indicates a capillary condensation in the material mesoporosity (Fig. 3). During the powder calcination between 45OOC and 1000°C, the porous texture remains virtually unmodified up to 65OoC, then progressively collapses to disappear completely at 1000°C. The largest modifications occur between 75OOC and 850OC. This description results from the analysis of the evolution of the adsorption-desorption isotherm shape as well as of the corresponding t-plot shape. Indeed, the shape of the adsorption-desorption isotherm is unmodified during the calcination (Fig. 2). It remains of type IV with a hysteresis loop of type E but the hysteresis loop progressively moves to higher relative pressures which indicates a pore mean readius increase. The corresponding t-plot evolution confirmsthis result (Fig.3). For all the samples, an upward deviation from the straight line is observed. The t-plots and consequently the porous texture remain nearly unmodified up to 65O0C. The largest modification occurs
662
'7 [ -
I 7 -
b
119.
in
5
1.
15 IIho
Heating rates of the slabs during sinterinq ( 1 ) = Temperature program 1 (TPI),
(2) = Temperature program 2 (TP2)
0
05
Fig.
2.
P./Po
Nitrogen adsorption-desorption
1.0
isotherms
obtained on powders calcined between
650 and 1000°C. ~~
. ~
~
__..____-
050.
025-
on 0
Fig.
3.
The v - t plots obtained for powders calcined between 450 and 1000°C.
~
n
1
Fig.
4.
Specific surface area dlstrlbution curve determined by isotherm analysis for powders calclned between 450 and 1000°C.
between 75OOC and 8 5 0 O C . For t-values higher than 1 nm, the t-plot is linear again with the same slope for all the samples. This slope allows to calculate a mean value of the particle external specific surface area S which ranges about 3.6 mZ/g. This last value ext agrees with the particle geometrical surface. Indeed an external specific surface area S of 3.5 m2/g i s estimated from geometrical data (particle diameter G d = 0.6 pm, porosity E = 0.47 for a 800°C calcined slab of cylindrical shape, a P theoretical density p = 6 . 0 8 g/cm' (ref. 1 0 ) ) . Table 1 shows, as a function of temperature, the evolution of the specific surface area SBET,o f the specific surface area S computed from the slope of the t t-plot straight line passing through the origin and the specific pore volume V P
663 TABLE 1
Effect o f calcination temperature on powder specific surface area and porosity Calcination temperature
Isotherm Hysteresis CBET type
type
IV
E E E
’BET St Scum V ’cum (m’/g) (m’/g) ( m * / g ) (cm’/g) (cm’/g)
(“C)
450 500 550 600 650 700 750 800 850 900 1000
IV IV IV IV
51 74 50 74 41 58 68 475 580 675
E E
E
IV IV IV IV
E
IV
E
E E
114 106 107 93 91 75 58 23 15 6 (1
113 106 105 97 95 72 58 26 16 5
95 89 100 77 79 68 53 22 13 5
0.141 0.125 0.133 0.108 0.110 0.098 0.085 0.051 0.039 0.031
0.154 0.139 0.194 0.103 0.120 0.128 0.101 0.059 0.044 0.030
TABLE 2
Effect of sintering temperature on porous texture of slabs. Cylindrical shape slabs ( T P l )
Sintering
Sintering temoerature
1100 1200 1300
0.143 0.137 0.123 0.111 0.095 0.090 0.071
2.68 2.78 2.98 3.33 3.87 4.57 5.30 6.04
800 900 1000 1100 1150 1200 1300 1450
1.99 1.81 1.67 1.48 1.32 1.28 0.93 (0.1
Plate shape slabs
0.102 0.075 0.036
2.9 2.1 1.1
2.4 2.2 1.3
0.465 0.454 0.428 0.403 0.366 0.354 0.302
Plate shape slabs (TPI)
b 0.144b 0.100 0.075 0.064 0.038
-
b
2.50b 1.83 1.78 1.44 0.92
b 0.467b 0.378 0.313 0.280 0.188
(TP2)
0.382 0.313 0.180
0.368 0.317 0.175
’determined in the same radius range (r > 7 . 5 nm) P bvalues difficult to correctly determine and likely overestimated because of the slab weak mechanical resistance
664
defined as being the volume of the liquid equivalent tothe gas quantity adsorbed per sample mass unit at the saturation pressure. The specific surface area distribution as a function of the pore radius, the and the cumulative specific pore volume cumulative specific surface area S cum Vcum were determined using the generalized Eroeckhoff-de Boer analysis on the adsorption in and desorption from capillaries of various shape (ref. 11). Excellent agreements between S and SBET as well as between V and V are cum cum P obtained if ink-bottle cylindrical pores closed at one end are considered (Table 1 ) . The mean pore radius slowly moves from 2 to 3 nm between 45OOC and 75OoC, then rapidly increases over 800OC. The specific surface area distribution is progressively widening.
(Fig. 4).
The pore volume distribution of the powder calcined at 45OOC determined by mercury porosimetry exhibits two peaks at pore radii o f 125 and 300 nm (Fig. 5). This result might be explained by the difficulty to obtain a compact packing of particles even when they are spherical and monodisperse. Milne (ref. 12) observed that a sample of monodisperse spherical particles could exhibit parts constituted'bya compact hexagonal lattice and other parts constituted by a cubic lattice. For the powder calcined at 45OoC, a pore volume V - 0.238 cm'/g and a Hg - 2.7 m'/g obtained. This last figure is to be compared
specific surface area S
Hg
-
with S6 = 3.5 m2/g and Sext = 3 . 6 m2/y. Sinterinq analysis The sintering process between 800 and 145OOC was followed by mercury porosimetry (ref. 13). The evolution of the pore volume as a function of pore radius is given on Fig. 6 for slabs of cylindrical shape ( T P I ) and on Fig. 7 for
.
slabs of plate shape (TPI)
The porous texture of each type of slabs and its evolution during sintering seems to be different. The slabs of cylindrical shape calcined at 800°C exhibit a bimodal pore volume distribution as the powder calcined at 45OOC : a predominant peak at 200 nm and a smaller peak at 135 nm. Duriny sintering process, the smaller peak at 135 nm disappears the first one. It moves to smaller and smaller pore radii whereas the position of the predominant peak remains unmodified at 200 nm. The slabs of plate shape exhibit a unimodal pore volume distribution. The pore volume collapse during sintering, simultaneously occurs with the mean pore radius decrease. The peak position moves from 135 nm at 9OOOC to 90 nm at 13OO0C. Let us recall that the preparation of the two types of slabs differs. For slabs of cylindrical shape, the binder is removed by calcination after compression whereas for slabs of plate shape, no binder is used. Referring to the results obtained on the powder, one is lead to assume that the sample preparation method produces different packings. The slabs of cylindrical shape
665
0'7r-----0.64
.
Sample 2
G, 0 5 -
0
E
1 4 ,
- 0.L0
0 m
-
," 0.3-J >
0.2-
Fig.
6.
Development of pore size distribution of cylindrical shape slabs during sintering ( T P I ) determined by mercury porosimetry.
X
0.7 5
9000
c
1
- - -O
Fig.
8.
X C X
Tensile strength of cylindrical shape slabs as a function of sintering temperature (TPl).
Il'CI IJ~OLC
w d p e siaos ourinq sintering ( T P I )
determined by mercury porosimetry.
tig.
7.
Length variations as a function o f sintering temperature.
666
characterized by a bimodal pore volume distribution would possess two types of packings : a compact one and a less dense one. The slabs of plate shape characterized by a unimodal pore volume distribution would only possess the compact packing.
Obviously, this difference could simply result from the
dissappear of the binder which would let largest voids. The evolution of the pore volume V and of the specific surface area S as a Hg Hg function of the sintering temperature is given in Table 2. This two quantities monotonically decrease and finally vanish. For the slabs of plate shape sintered below l l O O ° C , the S and V values Hg Hg are difficult to correctly determine and are likely overestimated because of the slab weak mechanical resistance when the pressure applied on the mercury is over 800 t o 1200 bar. In these conditions, the slabs broke up. An "apparent" pore volume is measured which could lead to an important overestimation of the
specific surface area. For the two types of slabs, the sintering seems complete at 14OO0C. porosity
The
corresponding to the interstices between particles disappears
completely.
Moreover, no residual mesoporosity is observed from scanning
electron micrographs nor from adsorption-desorption isotherm analysis. At very high mercury pressures about 2000 bar, we observed a non reproducible slight variation of the mercury column corresponding to pore volumes about cm3/g. We have not yet elucidated this phenomenon. Due to the high pressure applied, it might result from the occurence of cracks o r from the existence of closed cavities in slabs. The specific surface area S o f the weakly sintered slabs (Table 2 ) is Hg The slightly inferior to the particle external specific surface area SG. Karnaukhov model (ref. 14) relates, for spherical particles, the sire ofthe pore throat (pore diameter'), the particule diameter and the void fraction (ref. 14). In the case of the slabs of plate shape (unimodal pore volume distribution), this model predicts a porosity E = 0.465 for spherical particles with d = 0.6 pm and a P 0.27 pm pore throat. Tensile strength of the sintered slabs progressively increases when the temperature i s increased up to 14OO0C (Fig. 8). The disappearance o f the powder mesoporosity between 800°C and 1 0 0 0 ° C and the occurence of sintering between l l O O ° C and 14OO0C are also confirmed by dilatometric measurements in which the relative length variation of a slabs of cylindrical shape i s recorded as a function of temperature. Let u s note that the binder was preliminary removed at 8 0 0 ° C (Fig. 9 ) . The sintering process was investigated at 1100-1200-1300 and 14OO0C on scanning electron micrographs of polished sections of plate shape slabs ( T P 2 ) . Up to 12OO0C, the morphology of the powder remains nearly unmodified : the formation of necks occurs at the contact points between particles but the par-
667
ticle spherical shape remains perfectly apparent. At 13OO0C, the mass transfer is important, the powder morphology disappears but the porosity remains quite high.
Whereas at 14OO0C, the material seems completely sintered.
From scanning electron micrographs, we determined the porosity specific surface area S
and the la of the plate shape slabs (TP2) using the following E.
ia stereological relations (ref. 15) :
with N = total number o f pixels of the digitized image, No = number o f pixels o f the porous phase, N1 = number of exits from the porous phase along the line of analysis, L
=
image length, p = density.
For instance, the digital image of plate shape slab calcined at 13OO0C is reported on Fig. 10a. Original images were digitized in 256 grey levels on matrix of 256 x 256 pixels. In the case of a porous texture analysis, the contrast between void and solid phase is generally well marked. This allows to easily define threshold grey level to get a binary image. To improve the resolution of grain boundaries linear and morphological filtering techniques are used which enhances the bimodal character o f the grey levels intensity histogram (refs. 16-17). The result of the numerical image treatment of the digital image
Fig. 1Da. Digital image of p l a t e shape slab calcined a t 1300°C ( T P 2 ) (Detail).
Fig. lob. Binary image of 10a (Detail).
668
(Fig. 10a) leads to the binary image reported on Fig. 10b where the voids are represented in light colour. In Table 2, results obtained by image analysis and by mercury porosimetry are reported for plate shape slabs calcined at 1100-1200 and 13OO0C (TP2). The agreement between the results obtained by the two methods is encouraging. These results were determined in the same pore radius range (r > 7.5 nm). In our P experimental conditions, it may be computed that one pixel is equivalent to a pore diameter of 15 nm. CONCLUSIONS The technique developed by Solvay (ref. 1) to prepare PSZs ceramics using a SOLGEL method allows to produce powders o f monodisperse spherical porous particles, the mesoporosity of which disappears around 1000°C. The sintering process of slabs obtained by pressing and by slip casting o f a powder-water paste was followed by different methods and seems to be complete at 1450OC. The description o f the PSZs porous-texture resulting from the different methods is coherent. In particular, similar results were obtained by scanning electron micrograph image analysis and mercury porosimetry. REFERENCES 1. F. Legrand, P. De Bruycker, L. Lerot, European patterns EP 0 238 103, EP 0 263 544, EP 0 286 155. 2. 1.3. Mc Colm, Ceramic Science for Materials Technologists, Chapman and Hall, New York, 1983, pp. 272-284. 3. Kirk-Othmer Encyclopedia o f Chemical Technology, Vol. 24, Wiley, New-York, 3rd ed., 1984, pp. 863-902. 4. A.3. Lecloux, P. Verleye, 3. Bronckart, F. Noville, P. Marchot, 3.P. Pirard, React. Solids, 4 (1988) 309. 5. B. Fegley, P. White, H.K. Bowen, Am. Ceram. SOC., 64 (1985) 1115. 6 . L. Lerot, F . Legrand, P. De Bruycker, submitted to 3. Mater. Sci. 7. A.3. Lecloux, in 3.R. Anderson and M. Boudart (Eds), Catalysis, Science and Technology, Vol. 11, Springer-Verlag, Berlin, 1981, pp. 171-230. 8. 3.L. Merlo, Etude de la texture poreuse de poudres d'oxydes de zirconium et d'yttrium, Ms.D., Liige University, 1989. 9. A.3. Lecloux, 3.P. Pirard, 3. Colloid Interface Sci., 70 (1979) 265. 10. R.P. Ingel, D. Lewis 111, 3. Am. Ceram. SOC., 69 (1985) 325. 11. A.3. Lecloux, 3. Bronckart, F. Noville, 3.P. Pirard, in K K . Unger et al. (Eds), Characterization of Porous Solids, Elsevier, Amsterdam, 1988, pp. 233-242. 12. S.3. Milne, Br. Ceram. Proc., 38 (1986) 81. 13. P.Y. Kessels, Etude du frittage de cdramiques d'oxyde de zirconium stabilisdes 2 l'yttrium, Ms.D., Likge University, 1989. 14. A.P. Karnaukhov, in S . 3 . Gregg et al. (Eds), Characterization of Porous Solids, Society of Chemical Industry, 1979, pp. 301-311. 15. 3.P. 3ernot, C. Lantwejoul, in E . Guyon et al. (Eds), Disorder and Mixing, Kluwer Academic Publishers, Dordrecht, The Netherlands, 1988, pp. 327-337. 16. A. Rosenfeld, A.C. Kak, Digital picture processing, Academic Press, London, 1976. 17. M. Coster, 3.L. Chermant, Prdcis d'analyse d'images, Editions du Centre National de la Recherche Scientifique, Paris, 1985.
F. Rodriguez-Reinoso et al. (Editors), Characterization of Porous Solids ZZ 0 1991 Elsevier Science Publishers B.V., Amsterdam
669
THE POROSITY AND PERMEABILITY OF MACRODEFECT FREE CEMENTS
K.S.W.
S i n g and M.
Yat.es
Department o f Chemistry, Brunel U n i v e r s i t y , Uxbridge, Middlesex, England
ABSTRACT Very h i g h s t r e n g t h c e m e n t s o f l o w p o r o s i t y h a v e b e e n p r o d u c e d from b o t h O r d i n a r y P o r t l a n d Cement (OPC) a n d High Alumina C e m e n t (HAC) b y t.he i n c o r p o r a t i o n of a w a t e r s o l u b l e p o l y m e r a t t h e e a r l y m i x i n g st-age. T h e s e m a t e r i a l s h a v e b e e n t e r m e d m a c r o d e f e c t free (MDF) c e m e n t s d u e t o t h e e x c l u s i o n o f the w i d e r pores b r o u g h t a b o u t by t h i s m a n u f a c t u r i n g process. I n t.he p r e s e n t s t u d y t h e e f f e c t o f h e a t t r e a t m e n t and r e h y d r a t i o n on t h e s e t w o s y s t e m s h a s been i . n v e s t 3 q a t e d by t h e u s e o f mercury p o r o s i m e t r y and methane gas permeability. T h e s e s t u d i e s were complemented by t h e r m o g r a v i m e t r y a n d g a s a d s o r p t i o n m e a s u r e m e n t s . The r e s u l t s o b t a i n e d show t h a t . d e c o m p o s i t i o n o f t h e p o l y m e r was t h e c o n t r o l l i n g f a c t o r i n t h e f o r m a t i o n o f t h e p o r e volume. However, t h e p o r e s i z e d i s t r i b u t i o n s of t h e t w o s y s t e m s were v e r y d i f f e r e n t b0t.h b e f o r e a n d a f t e r r e h y d r a t i o n a n d t h i s h a s a d i r e c t b e a r i n g on t-he measured p e r m e a b i l i t y .
INTRODUCTION
U n t i l r e c e n t l y it was w i d e l y a c c e p t e d t - h a t t h e s t r e n g t h of OPC was g o v e r n e d by t h e i n i t i a l w a t e r / c e m e n t r a t i o w h i c h d i r e c t l y l e d t o a p a r t i c u l a r 1 However, t h e work Of volume f r a c t i o n o f pores i n t h e h a r d e n e d c e m e n t p a s t e
.
Birchall e t
d e m o n s t r a t e d t h e e f f e c t . o f t.he wider p o r e s on t h e s t r e n g t h
and r e v e a l e d t h a t c o n s i d e r a b l e i m p r o v e m e n t c o u l d b e made e s p e c i a l l y i n t h e f l e x u r a l s t r e n g t h by t h e i r removal. To p r o d u c e t h e t w o c e m e n t s s t - u d i e d a w a t e r s o l u b l e p o l y m e r
(polyvinyl
a l c o h o l ) i s added t o t h e c e m e n t / w a t e r s y s t e m t o a c t a s a r h e o l o g i c a l a i d d u r i n g t h e h i g h s h e a r m i x i n g , t h u s l e a d i n g t.o closer p a c k i n g of t.he i n d i v i d u a l cement g r a i n s p r i o r t o h y d r a t i o n and f i n a l s e t .
This s p e c i a l processing
t e c h n i q u e is f u n d a m e n t a l t o t h e l o w p o r o s i t y / h i g h s t r e n g t h n a t u r e o f t h e MDF m a t e r i a l s . The main o b j e c t i v e o f t h e work d e s c r i b e d h e r e was t o s t - u d y t h e e f f e c t .
of removing t h e p o l y m e r on t h e p o r o s i t y a n d p e r m e a b i l i t y o f t h e MDF c e m e n t s . T h i s was a s s e s s e d u s i n g t h e t e c h n i q u e s o f m e r c u r y p o r o s i m e t - r y a n d m e t h a n e g a s permeability.
I n a d d i t i o n t o t.hese t e c h n i q u e s thermogravimet.ric a n a l y s i s w a s
u s e d t o f o l l o w + h e d e c o m p o s i t i o n o f t h e p o l y m e r a n d l o s s o f w a t e r from t h e materials during heat-treatment.
Gas a d s o r p t i o n m e a s u r e m e n t s were a l s o u s e d
t o st.udy t h e c h a n g e s i n t h e m i c r o s t - r u c t u r e c a u s e d b y t h e d e c o m p o s i t i o n o f t h e polymer.
EXPERIMENTAL The samples o f MDF cement s h e e t s were p r e p a r e d a t t h e Runcorn Heath I C I laboratories.
The GPC u s e d w a s a s t a n d a r d g r a d e Type I cement powder
produced by Blue C i r c l e and t.he HAC was S e c a r 71 produced by L a f a r g e . The t h r e e components
(cement, polymer and water) were premixed i n a n
o r b i t a l mixer t o g i v e a d r y g r a n u l a r "crumble".
T h i s g r a n u l a r dough was
c h i l l e d ( t o r e t a r d h y d r a t i o n ) and f e d i n t o a t w i n r o l l - m i l l , where it was converted i n t o a t h i n s h eet.
Any remaining o c c l u d e d a i r was removed by
p r e s s i n g t h e s h e e t ( a t 4MPa) f o r 16h a t ambient t e m p e r a t u r e .
The s h e e t w a s
t h e n l e f t t o c u r e f o r 5 d a y s a t a m b i e n t t e m p e r a t u r e and f i n a l l y d r i e d a t 8OoC overnight. The samples were examined b e f o r e and a f t e r h e a t - t r e a t m e n t rehydration.
and a f t e r
H e a t - t r e a t m e n t c o n s i s t e d of h e a t i n g t h e samples a t 5OC min-'
to
t h e s t a t e d t e m p e r a t u r e and h o l d i n g f o r 4 h o u r s b e f o r e c o o l i n g i n a d e s i c c a t o r . R e h y d r a t i o n was accomplished by immersing t h e s a m p l e s i n w a t e r f o r 1 week t h e n d r y i n g a t 8OoC o v e r n i g h t . Kercury p o r o s i m e t r y measurements were made u s i n g a C a r l o Erba 2000 porosimeter.
The c o n t a c t a n g l e of t h e mercury was t a k e n a s 141'
s u r f a c e t e n s i o n a s 480mN in-'. C a r l o Erba S o r p t o m a t i c 1800.
and t h e
Gas a d s o r p t i o n s t u d i e s were c a r r i e d o u t on a G a s p e r m e a b i l i t y measurements were made on a
home b u i l t a p p a r a t u s f o l l o w i n g a d e s i g n used by t h e B r i t i s h Cement Association4.
Thermogravimetric s t u d i e s were c a r r i e d o u t on a S t a n t o n
R e d c r o f t STA 780. a t 50cm3 min-' respectively,
A l l measurements were made i n a n atmosphere o f a i r f l o w i n g
w i t h a h e a t i n g r a t e o f S°C min-'
t o 150°,
300°
and 4.5OoC
i n o r d e r t o d u p l i c a t e t h e h e a t t r e a t m e n t regimes.
RESULTS The p r o p e r t i e s of t h e v a r i o u s t r e a t e d and u n t r e a t e d s a m p l e s o f MDF cements a r e summarised i n T a b l e 1 and r e p r e s e n t a t i v e i n t r u s i o n - e x t r u s i o n mercury p o r o s i m e t r y c u r v e s a r e shown i n F i g u r e s 1-4.
I t w i l l be noted t h a t
t h e o r i g i n a l GPC-MDF had a s m a l l b u t m e a s u r a b l e p o r o s i t y whereas t h e HAC-MDF
was a l m o s t non-porous.
I n t h e i r o r i g i n a l s t a t e b o t h cements e x h i b i t e d no
d e t e c t a b l e g a s p e r m e a b i l i t y , which i n d i c a t e s t h a t t h e pores i n t h e GPC-MDF d i d n o t e x t e n d a s a c o n t i n u o u s network t h r o u g h t h e sample.
I n b o t h cases h e a t
t r e a t m e n t b r o u g h t a b o u t a p r o g r e s s i v e i n c r e a s e i n p o r e volume, p e r m e a b i l i t y and s u r f a c e a r e a .
671 TABLE 1 Properties of MDP cements. Cement
Treatment
OPC-MDF
NONE
15OoC* 15OoC R+ 3OO0C* 3OO0C R+ 4 50oc* 45OoC R+ HAC- MDF
NONE
,I
15OoC* 15OoC R+ 3OO0C* 3OO0C R+ 45OoC* 45OoC R+
,I
*
P o r e Volume cm3g-1 0.020 0.046 0.030 0.055 0.032 0.082 0.034 0.002 0.011 0.021 0.066 0.044 0.086 0.015
P e r m e a b i 1i t y Barrer x
0 1.2-1.6 0.2-0.3 5.6-7.1 0.3-0.4 9.7-17.3 0.4-0.5
Flex strength MPa 82.2 15.7 23.4 14.0 47.3 11.2 33.4
0 0.1-0.2 07.6-346.8 40.7-182.9 0.6- 1 5 . 4 125.4-303.8 0.8-7.4
S u r f a c e Area m'9-l 3.9 16.4
-
21.5
-
24.5
-
0.1 24.1
-
32.6
-
30.1
-
Heated t o s p e c i f i e d t-emperature f o r 4 h o u r s.
+ Immersed i n w a t e r f o r 1 week.
Comparison o f F i g u r e s 1 a n d 3 r e v e a l s t h a t t h e pore s i z e d i s t r i b u t i o n of t h e h e a t e d s a m p l e s o f HAC-MDF was much b r o a d e r t h a n t h a t of t h e
c o r r e s p o n d i n g h e a t e d GPC-MDF.
Although t h e r e s u l t a n t p o r e volumes ( a s
d e t e r m i n e d by m e r c u r y p o r o s i m e t r y ) were n o t d i s i m i l a r , l a r g e r i n t e r n a l s u r f a c e a r e a s were g e n e r a t e d by h e a t t r e a t m e n t o f t h e HAC-MDF.
These heated samples
w e r e also f o u n d t o b e h i g h l y p e r m e a b l e , a l t h o u g h t h e r a t - e o f f l o w o f m e t h a n e was n o t e a s y t o r e p r o d u c e .
A p r o n o u n c e d d e c r e a s e i n p e r m e a b i l i t y was f o u n d
when 300° and 45OoC samples o f HAC-MDF were immersed i n w a t e r . A p a r t f r o m t h e u n e x p e c t e d b e h a v i o u r o f t h e 15OoC s a m p l e o f HAC-TDF. immersion i n water g e n e r a l l y p r o d u c e d a s u b s t a n t i a l r e d u c t i o n i n p o r o s i t y and permeability.
T h e s e e f f e c t s were e s p e c i a l l y s t r i k i n g i n t h e c a s e o f t h e 45OoC
s a m p l e Of HAC-MDF. H e a t t - r e a t m e n t of t h e OPC-MDF - e v e n a t 15OoC loss of f l e x u r a l s t r e n g t h .
-
resulted i n a drastic
T h i s was t o some e x t e n t r e s t o r e d by r e h y d r a t i o n ,
b u t t h e maximum v a l u e a t t a i n e d was s t i l l c o n s i d e r a b l y lower t h a n t h e o r i g i n a l strength.
612
F i g u r e 1. a t 15O0C
(-
Mercury p o r o s i m e t r y c u r v e s f o r OPC-MDF: O r i g i n a l (-) - ) , 3OO0C ( - . - ) a n d 45OoC ( - . - - ) .
and heated
F i g u r e 2. Mercury p o r o s i m e t r y c u r v e s f o r OPC-MDF: O r i g i n a l (-) and r e h y d r a t e d a f t e r h e a t t r e a t m e n t a t 15OoC ( - - ) , 3OO0C ( - . - ) a n d 45OoC ( - . ' - )
673
F i g u r e 3. Mercury p o r o s i m e t r y c u r v e s f o r HAC-MDF: a t 150°C (- - ) , 3OO0C ( - - - ) a n d 45OoC ( . . . . )
.
O r i g i n a l (-)
and h e a t e d
F i g u r e 4. Mercury p o r o s i m e t r y c u r v e s f o r HAC-MDF: O r i g i n a l (--) and r e h y d r a t e d a f t e r h e a t t r e a t m e n t a t 15OoC ( - - ) , 3OO0C ( - . - I a n d 45OoC ( . - . . ) .
674 DISCUSSION It is e v i d e n t t h a t t h e p e r m e a b i l i t y o f t h e h e a t e d and r e h y d r a t e d
c e m e n t s was n o t s i m p l y c o n t . r o l 1 e d by t h e p o r e volume a s d e t e r m i n e d by m e r c u r y porosimetry.
However, t h e s h a p e o f some o f t h e m e r c u r y i n t . r u s i o n c u r v e s i n
F i g u r e s 1-4 i n d i c a t e s t h a t a t t h e maximum a t t a i n a b l e p r e s s u r e m e r c u r y d i d n o t p e n e t - r a t e i n t o t h e n a r r o w e s t p o r e s of t h e h e a t e d or r e h y d r a t e d s a m p l e s . Furthermore,
t h e e x t e n s i v e i n t r u s i o n - e x t r u s i o n h y s t e r e s i s and t h e l a r g e
e n t r a p m e n t o f m e r c u r y a r e f e a t u r e s g e n e r a l l y a s s o c i a t e d w i t h complex p o r e n e t w o r k s made u p o f i n t e r c o n n e c t i n g c h a n n e l s a n d c a v i t i e s o f d i f f e r e n t dimensions
.
Alt-hough i t is n o t p o s s i b l e t o a r r i v e a t . a n u n a m b i g i o u s i n t e r p r e t a t i o n of t h e m e r c u r y p o r o s i m e t r y d a t . a , t h e r e i s l i t t l e d o u b t t h a t . on h e a t t r e a t m e n t t.he p o r e s i z e d i s t r i b u t i o n o f t h e HAC-MDF became much b r o a d e r t h a n t h a t o f t h e CPC-MDF.
I t seems t h a t t h e d e v e l o p m e n t o f t h e w i d e r pores i n the HAC s a m p l e s
was r e s w n s i b l e f o r t h e a p p e a r a n c e o f t h e i r v e r y h i g h p e r m e a b i l i t i e s .
On t.he
o t h e r h a n d , t h e c o m p l e t e removal of t h e p o l y m e r by h e a t t r e a t m e n t of t h e AAC-MDF a t 45OoC e v i d e n t l y l e d t o t h e f o r m a t i o n of a h i g h l y r e a c t i v e m a t e r i a l and an u n s t a b l e p o r e s t r u c t u r e .
The c u r i o u s r e s u l t o b t a i n e d on r e h y d r a t i o n o f
t h e 15OoC s a m p l e o f HAC-MDF was p r o b a b l y d u e t o t h e l e a c h i n g o u t o f r e s i d u a l polymer which i n t u r n l e d t o p o r e wid e n in g and i n c r e a s e d p e r m e a b i l i t y . I n T a b l e 2 t h e v a l u e s of p o r e volume a s d e t e r m i n e d by m e r c u r y p r o s i m e t r y a r e compared w i t h t-hose c a l c u l a t e d f r o m t h e l o s s o f p o l y m e r d u r i n g lieat treatment.
The 1att.er a r e c a l c u l a t e d from t h e t - h e r m o g r a v i m e t r i c c u r v e s .
The a g r e e m e n t b e t w e e n t h e two s e t s o f v a l u e s i s o n t h e w h o l e f a i r l y good a n d a p p e a r s t o confirm t h a t t h e p o r o s i t y g e n e r a t e d o v e r t h e t e m p e r a t u r e range of 150-450°C
i s m a i n l y d u e t o t h e t h e r m a l removal o f p o l y m e r .
TABLE 2
C o m p a r i s o n of P o r e V o l u m e s D e t e r m i n e d by M e r c u r y P o r o s i m e t r y a n d l o s s of P o l y m e r . Cement
CPC-MDF
Treatment
NONE
15OoC* 3OO0C* 4 5OoC* EiAC- MDF
NONE
15OoC* 3OO0C* 45OoC*
*
P e r c e n t a g e Polymer Decomposition
Calculated Pore Volume cm3g-l
0.0% 47.3% 81.3% 100.0%
0 0.031
0.0% 0.0% 31.9%
0 0.024 0.077 0.102
100.0%
Beated t o s p e c i f i e d temperature f o r 4 hours.
0.052
0.058
Measured P o r e Volume cm3g-l 0.020 0.046 0.055 0.082 0.002
0.011 0.066 0.086
675 W e s h o u l d l i k e t o a c k n o w l e d g e t h e s u p p o r t of B r i t i s h Gas a n d I C I i n
f u n d i n g t h e r e s e a r c h a n d i n p a r t i c u l a r t h e h e l p o f Norman P a r k y n s , Anthony Howard a n d P e t e r Cardew.
W e a l s o t h a n k David Lawrence of t h e B r i t i s h Cement
A s s o c i a t i o n f o r h i s h e l p and a d v i c e c o n c e r n i n g t h e p e r m e a b i l i t y measurements.
REFERENCES 1 2 3
H . J . G i l k e y ; J. Amer. Concr. I n s t . 5 8 , 1851-1878 ( 1 9 6 1 ) . J . D . B i r c h a l l ; I n d . Chem. B u l l . 167-170 ( 1 9 8 3 ) . K. K e n d a l l , A . J . Howard a n d J . D . B i r c h a l l ; P h i l . T r a n s . R.
4
139-153 ( 1 9 8 3 ) . C.D.Lawrence; 8 t h C o n g r e s s of Cement, B r a z i l
(1986).
SOC. Lond. A310,
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F. Rodriguez-Reinosoet al. (Editors), Characterization of Porous Solids I1 0 1991 Elsevier Science Publishers B.V., Amsterdam
677
AN APPRAISAL BY M.1.P OF THE CHANGES INDUCED IN THE MICROSTRUCTURE OF COMPLEX SULFIDE ORES BY REACTIVE THERMAL TREATMENTS IN H2 AND N2
M. FATEMI-SADR and P. BRACCONI Laboratoire de Recherches sur la R6activit6 des Solides (URA CNRS 23) Universite de Bourgogne. Dijon (France)
SUMMARY Changes in the microstructure of complex sulfide ores resulting from reactive thermal treatments in either H2 or N2 were investigated qualitatively by combined scanning electron microscopy and X ray microanalysis, and quantitatively by mercury intrusion porosimetry (MIP) and mercury pycnometry. With increasing treatment temperature, we successively observed : 1) the development of fractures, preferentially at interfaces between the various mineral components, 2) the development of meso and macropores and the simultaneous swelling of the solids and 3) the crystallization or recrystallization of the metal or sufide phase products. In all cases the specific pore volume and mean pore diameter were observed to increase with the temperature of treatment. Swelling is the major source of porosity whereas fracturing contributes very little. INTRODUCTION Complex sulfide Ores (CSO) constitute a major potential source of the base metals Cu, Zn and Pb, but due to their finely disseminated mineralization, they are not economically exploitable. We conjectured that thermal treatments under controlled atmosphere (not considering conventional oxydizing roasting) might be capable to bring about sufficient changes in the bulk and surface properties of such ores to make them amenable to conventional concentration techniques. With that idea in mind, we investigated the modifications of the microstructure of various ores as a function of treatment temperatures up to about 1000°C, in reducing and neutral atmospheres, hydrogen and nitrogen respectively. Microstructure is taken here to mean all the particle, grain or inclusion shape and size characteristics, aggregation state, porosity of all type, etc... The microstructure of the reactants and products have been investigated by scanning electron microscopy (SEMI, coupled with energy dispersive X ray microanalysis (XMA), on initially polished surfaces, and by mercury intrusion porosimetry (MIP) and mercury pycnometry on small lumps.
618
RESULTS The qualitative and quantitative interpretations of the modifications of the microstructure require that the chemical reactions brought about by various treatments be well understood. First, the mineralogic composition of the raw ores, i.e. the relative proportion and composition of the various mineral components had to be established. We restrict the scope of the present paper to two ores. One is a typical low grade ore referred to as LGO in the following, in which the base metal sulfides, chalcopyrite (CuFeS2) sphalerite (ZnS) and galena (PbS) are disseminated in a large excess ( = 72.6 w% ) of pyrite FeS2. The second is high grade ore (HGO) which may be used as a reference. Its concentrations in sphalerite (47.6w%) and galena (low%) are about 10 times larger than those of the LGO, and chalcopyrite amounts to 7 w%. Inclusions of these phases and of their reaction products are thus much more easily recognized and analysed by SEM-XMA and many of their characteristics can be extrapolated to low grade ores. Both ores contain As in the form of mispickel, and HGO contains chlorite in significant amount, which proves to have important consequences on the evolution of the microstructure. The chemical changes of these major mineral components resulting from the thermal treatments were then established by X Ray diffractometry (XRD) and thermogravimetric analysis (TGA). They are summarized in table 1 in parallel with the SEM results. Samples of the XRD and TGA results are presented in fig. 1 & 2 without further comments regarding these conventionnal techniques. MIP experiments were systematically performed as a function of treatment temperature and gas phase composition. The 9300 Pore Sizer from MICROMERITICS was used. Small lumps (a few tens to a few hundreds of mm3) were first treated in a gas flow of H2 or N2 at atmospheric pressure. In all cases the porosity (i.e. specific overall pore volume) increases with temperature. The results obtained with the LGO are shown in fig.3. The major increase around 600°C is most likely related to the decomposition of the major phase, FeS2, into pyrrhotite Fe 1-x S sponge. From that temperature on, the porosity still increases up to high values ranging from 30 to 50%. Now, if one compares such figures with the volume change resulting from the decomposition FeS2 -> Fel-x S and from the vaporization of galena, they appear to be approximately 3 to 5 times larger . Similar conclusions are reached for treatments in HZatmosphere, and to a lesser extent with the HGO. Mercury pycnometry experiments allowed to explain these discrepancies by revealing a strong swelling of the CSOs especially in nitrogen. The phenomenon was first strictly established for both ores by comparing the mercury intrusion curves before and after treatment at 7OOOC in nitrogen; the results are given in table 2. Next, a simplified procedure allowed us to obtain a good estimate for the swelling of every treated sample: we calculated the initial volume of the (unreacted)
679
Table 1 Chemical and microstructural transformations resulting from the thermal treatments
Chemical transformations
Miaostructure changes observed by SEM
Low temperature range : RT -> 35OOC treatment in nitrogen and hydrogen *No phase change in major *Fracturing statistically more frequent at solid / solid interfaces minerals; small weight losses *Recrystallizationof sphalerite and pyrrhotite in LGO, of pyrite in HGO Medium temperature range : 350 -> 55OOC 1 ) treatment in nitrogen : Thacopyrite ---> cubanite (CuFeZSg) *Cubanite forms sponge nnd large crystals at Chacopyrite/sphalerite interfaces Tyrrhotite forms sponge. *Pyrite ---> pyrrhotite *Chlorite inclusions contract and separate *Decomposition of chlorite from other phases. Small particles of sulfides crystallize on top 2) treatment in hydrogen : *Galena ---> Pb metal Thalcopyrite ---> cubanite --> ... ... Cu and Fe metals *Pyrite and pyrrhotite ---> Fe metal
*Liquid lead metal forms beads *Cu metal grows as whiskers and dendrites. *Fe metal grows as sponge.
High temperature ranve - : above 6OOOC 1 ) treatment in nitrogen : *Pyrrhotite (Fel-xS) ---> troilite (FeS) *Formation of meso and macropores and recrystallization. and cubanite structure destroyed *Formation of large voids *Galena vaporized 2 ) treatment in hydrogen : *Swelling is visible . *Above 1000°C : zinc vaporized; HGO weight loss = 77% and residue consists of Fe+Cu + (Mg, Al, Si oxides)
lumps from their respective weight and from an average experimental value for the specific gravity of the (unreacted) ore under concern. In all cases, positive and
680
physically consistent swelling values were obtained despite of the rather poor accuracy of the procedure. With HGO samples treated in nitrogen, swelling increases with treatment temperature as shown by fig. 4. Despite the large dispersion of the data, a steplike rise may be seen in the figure, between 600 and 7OO0C, separating the data into two ensembles of "low" and "high" values respectively. In the same temperature range, a large weight loss (= 10%) is measured and the structures of pyrrhotite, galena and cubanite disappear from the X ray diffractogrammes. Values as high as 25 % are finally measured. In hydrogen swelling remains more or less constant around 10 (+ 5) % between 200 and 800' C . This emphasizes that the phenomenon (swelling) is fundamentally dependent on the chemistry of the system, but that the relations between the two may not be simple.
I
I
I
CuFe2S3
I
FeS2 Fe7S8
I
I
RT
200
I
40 0
I
600
I
800
I
1000
Temperature (C)
Figure 1 : XRD phase identification in the reaction products of the LGO reacted in N2. Shaded bars represent the temperature ranges over which the solid phases are identified by three diffraction lines, at least, in the diffractogrammes. Shaded areas represent ranges of uncertainty : in the cases of ZnS and Fe7S8, between RT and 300°C or 200°C respectively, they are used to indicate that both phases are amorphous to X rays. The differential results of MIP experiments are shown in 3-Dimensional
681
histograms: specific pore volumes (in cm3g-1) versus pore diameters versus treatment temperature. The pore diameters were calculated through Washburn's formula with constant values for Hg surface tension (0,484 N.m-') and wetting angle (130 degrees). Though this is fundamentally incorrect there is no means to take into account the inhomogeneous and changing chemical nature of the pore walls. The difference between slit-like. The difference between slit-like and cylindrical pores also is neglected. The following is observed :
2 : Results of equilibrium thermogravimetric analysis of LGO treated in N2. Figure The different point markers refer to different runs. 1) a peak at low pore radius (high Hg pressures) and low temperature would be expected to correspond to intrusion in the fractures known to form at low temperatures from SEM. It is hardly detected in the LGO but in the case of the HGO one may consider the peak around = 0,l pm at 220°C in H2 and at 298 and 4OOOC in N2 corresponds essentially to mercury penetration in such fractures. The width of such slit like pores would be half the diameter for cylindrical pores i.e. about 0,05 pm, which is physically consistent with estimation by SEM. The corresponding porosity is of course very weak, typically 1 to 3 %. 2) the peak corresponding to intrusion in meso- and macropores grows and shifts towards larger pore openings with increasing treatment temperature and overall pore volume.
682
CONCLUSION Our observations support the conclusion that the high measured final porosity values result essentially from the swelling of the intermediate and final sulfide prod-
50
0
200
0
400
600
800
Temperature of treatment ("C) Figure 3 : Dependence of porosity of LGO on treatment temperature in N2.
30
I
20
"low values"
10
-
I
O!
200
=.
= I
I
I
400
600
800
Temperature of treatment
1000
("c)
Figure 4 : Increasing swelling of the HGO with treatment temperature in N2.
683
Table 2 Results of exact measurement of swelling by mercury pycnometry
Ore / Treatment
Initial volume (cc)
Final volume (cc)
Swelling; (%)
HGO / 700 C in N2 LGO / 700 C in N2
0,279
0,325
16,5
0,236
0,306
29.7
H pore diameter (run)
0.01
0.1
I
10
100
Figure 5 : 3D histograms relating the differential specific pore volume to pore diameter and temperature of treatment.
684
duct phases (especially pyrrhotite) and not from the volume deficit associated with the various decompositions or reductions. Obviously, the reactivity of the CSOs treated around 600-700 C (enriched in valuable components Cu, Zn and Pb) must be very high but whether the selectivity and/or yield of such concentration techniques as flotation may thus be enhanced is still to be investigated.
F. Rodriguez-Reinosoet al. (Editors), Characterization of Porous Solids I1 0 1991 Elsevier Science Publishers B.V., Amsterdam
685
THE ADSORPTION OF WATER VAPOUR BY MICROPOROUS SOLIDS
P.J.M. CARROTT~, M.B. KENNY, and C.R. THEOCHARIS
R.A.
ROBERTS^,
K.S.W. SING
Dept. of Chemistry, Brunel University, Uxbridge, Middlesex, UB8 3PH, U.K. 1 Faculdade de Cibncias, Universidade de Lisboa, Rua Escola Politecnica, 58, 1200 Lisboa, Portugal 2 Dowty Environmental and Safety Products, Adderbury, Banbury, Oxfordshire, OX17 lHJ, U.K. SUMMARY The adsorption of water vapour has been studied with a range of microporous carbons, zeolites and aluminophosphates in order to elucidate the relative influence of surface chemistry, pore size and pore shape upon the form of the water isotherm. It was possible to separate the adsorbents into three groups on the basis of their affinity and capacity for water vapour. The porous carbons were further examined using the BET and Dubinin-Serpinsky equations. The results show that the adsorption of water vapour at low p/po is largely dependent upon specific adsorbent-adsorbate interactions whilst at higher relative pressures the micropore size and shape control the extent of adsorption. It is proposed that hydrogen-bonded layers of water can be more readily accommodated in the narrow slit shaped pores (-0.5nm) of molecular sieve carbons than in tubular pores of similar width (e.g. Silicalite/ZSM-5). INTRODUCTION The many investigations of the adsorption of water vapour carried out over the last 20 years (refs. 1-5) have brought to
light several unusual features. Thus, large differences have been found in the shape of water isotherms determined with various porous solids such as carbons, oxides and zeolites. It is well known that the low polarisability, resulting from the small size of the water molecule, gives rise to weak non-specific interactions with graphitic carbon and dehydroxylated silica whilst the presence of a permanent dipole enables water molecules to undergo enhanced adsorbent-adsorbate interactions with surfaces where polar or cationic sites are exposed (refs. 7-8). The role played by the porosity of the adsorbent, particularly the microporosity, is much less clear. Conflicting reports have been
686
published regarding the mechanism of water adsorption in terms of the relative influence of the concentration of specific adsorption centres and the pore size distribution (refs 3 - 6 ) . The work reported here was designed to provide a systematic investigation of the adsorption of water by a number of well characterised microporous carbons, zeolites and aluminophosphates. In this way a more complete picture of the adsorption of water by microporous solids could be obtained and thereby allow a basis for the analysis of water isotherms in terms of texture and surface chemistry. EXPERIMENTAL Water vapour adsorption and desorption isotherms were determined gravimetrically at 298 K with the aid of quartz spring balances of the McBain-Bakr type. Prior to measurement each of the microporous carbons was outgassed at 573 K, the other samples at 673 K, for 16 hours to a vacuum of < torr. The adsorbents
used in this study are listed in Table I; they have all been employed in other related studies and their properties are described elsewhere (ref. 9). RESULTS AND DISCUSSION To provide a means of comparison between the different water isotherms the amounts of water vapour adsorbed, expressed as liquid volumes, at p/po = 0.01 and 0.95, are given in Table I as Vw(~.ol) and Vw(o.95) respectively. The apparent pore volume of each adsorbent was assessed from other adsorption measurements
(e-g. nitrogen at 77K) which allows a comparison of the different levels of fractional pore filling by water vapour, 8(o.01) and t3(o.g5), at p/po = 0.01 and 0.95. On the basis of these results the adsorbents in Table I are separated into three groups. The zeolites CaA, NaX and NaY are placed in Group I as each gives rise to a Type I isotherm in the IUPAC classification. These isotherms exhibit a pronounced rectangularity as indicated by the fact that both 8(o.01)and 8(o.95)are close to unity. An example of the Group I isotherms, on CaA, is given in Figure 1. The considerable affinity for water vapour shown by these zeolites results from the high concentration of specific cationic sites arising from their large Al contents (ref. 10). The increase in uptake at higher p/po and the
687 TABLE I
Adsorption 01
ADSORBENT
W d t r r Vapour
at Relative I’rcssures of 0.01 and 0.93
vw ( 0.01)
vw ( 0.95 ) ( ~ r n ~ q - ~ ) (cm39-1)
e(0.01)
e ( 0.95)
GROUP I ZEOLITES CaA NaX NaY
-
0.192 0.258 0.209
0 270 0.356 0.360
0.67 0.70 0.60
0.97 1-01 1.04
0 002 0.001 0.001 0.005 0.008 0.014 0.003 0.004 0.006
0.175 0.810 1.458 0 272 0.922 0.485 0.360 0.195
0.01 0.00 0.00 0.01 0.04 0.01 0.01 0.01 0.03
0.89 0.99 0.62 0.82 1.05 0.97 1.00 0.79 0.62
0.019
0.320 0.361
0.10 0.08
1.33 1.16
0.019
0.01 0.01
0.11 0.20
GROUP TJ. MICXOWROUS CARBONS
Takeda C 5%, Carbosieve Ax21
KCCl JF005 JF518 JF144 JFOlO JF025
charcoal cloth
f
0.338
-
ALUMINOPHOSPHATES ALPO-5 VPI-5
0.021
GROUP I11 ZSH-5 ZEOLITES SILICALITE I HZSM-5
0.001 0 004
-
0.038
characteristically small hysteresis loops are largely associated with intercrystalline effects. The microporous carbons and aluminophosphates are placed in Group I1 as they show a much lower affinity for water vapour (ref. This is mainly 11), as illustrated by the low values of @ ( o .ol). a consequence of the comparatively small number of specific sites present in these materials. The uptakes at p/po = 0.95, however, are generally equivalent to the amount required to give complete pore filling. In fact for the aluminophosphates the values of @ ( o . 9 5 ) are greater than unity as water is able to fit into the narrow six-rings which are unavailable to larger molecules (ref. 12). Each of the isotherms in Group I1 may be regarded as essentially Type V but in most cases an ill-defined Point B is discernible in the region of low uptake (Figures 1 and 2). The magnitude of this low pressure knee and the location of the point of inflexion varies widely between the adsorbents (Table I1 and Figure 2). It is also interesting that the aluminophosphates,
688
0.3
-I
W
c
E
= 0
0.2
5
I
v
>-
0.1
0
0.2
0.6
0.4
0.8
0
1.0
PIP'
PIP' Fig. 2.
Water vapour isotherms for the Group II
Fig. 1. Reduced water vapour isotherms for Group I ( 0 0 ) CaA and Group II (Om) JF025; [AA] adsorbents. Takeda 5A and lo*) KCCl adsorbents.
10.)
VPI-5 and (0.1 ALPO-5
which theoretically have neutral frameworks, show more rapid upswings at lower p/po than the carbons, possibly due in part to the presence of -OH groups at defect sites. Furthermore, VPI-5 is unusual in that it has a clear step at p/po = 0.02-0.06. Low pressure hysteresis was observed for each of these adsorbents, a small hysteresis loop was found even with the narrowest molecular sieve carbons. For the microporous carbons, it was noted that the hysteresis loop broadens as the pore width increased indicating that the high-pressure hysteresis is related to the process of capillary condensation. Again VPI-5 is unusual as it has a double hysteresis loop which closes at p/po - 0.07: the loop at lower p/po is associated with the step and cannot be explained by capillary condensation. Finally, the zeolite HZSM-5 and its aluminium-free analogue Silicalite I are placed into Group 111. These adsorbents may be termed truly hydrophobic since their affinity for water vapour remains low over the entire range of p/po as shown by the small values of 8(o.01)and 0(0.95)in Table I. The low uptake of water vapour exhibited by these zeolites is exemplified in Figure 3
689
0
0.2
0.A
0.6
0.8
PIP’ Fig- 3. Reduced water vapour and nitrogen isotherms for the Group 111 adsorbents. Silicalite I).Oi Water, (AA) nitrogen: . HZSM-5
1.0
Fig. 4. A sketch o f the hydrogen-bonded structure of water in a slit-shaped pore.
The circles
represent the positions of the oxygen atoms.
where the fractional pore filling by water and nitrogen are compared. The intracrystalline pores of the ZSM-S/Silicalite structure are tubular and of - 0.55 nm diameter. It is not surprising then that a three-dimensional array of hydrogen-bonded water molecules cannot easily be accommodated in such pores without considerable distortion of the directional hydrogen-bonds (ref. 13). On the other hand, as shown in Figure 4 , a thin slab of water can more easily develop in the slit shaped pores of molecular sieve carbons of similar width (e.g. Takeda 5A) and also the water structure is clearly able to form in the wider tubular However, if pores of ALPO-5 ( - 0.8 nm) and VPI-5 ( - 1.2 nm). favourable cationic or -OH groups are present on the internal surface of the Group I11 adsorbents, as in HZSM-5 (Si/Al = g o ) , then a limited number of water molecules may be adsorbed at low p/po through enhanced adsorbent-adsorbate interactions. A s shown in Figure 3 , the nitrogen and water isotherms are similar in the multilayer region and therefore, the hysteresis loops and much of the water adsorbed by Silicalite I is likely to be associated with secondary mesoporosity and intercrystalline effects.
690
In order to further elucidate the complex pore filling process in microporous carbons, the empirical Dubinin-Serpinsky (DS) equation (refs. 14-15) was used to assess the influence of polar sites on the shape of the isotherm. This equation was developed from the concept of adsorption of water molecules at uniform high energy primary adsorption centres. Molecules adsorbed on these sites act as secondary adsorption centres via a hydrogen-bonding mechanism. Thus, this model does not refer explicitly to the role played by pore size. The DS equation may be written in its modified form (ref. 15) as: P - - -
n
PO
D(no+n)(l-kn)
I
where n is the amount of water adsorbed at p/poI no is the concentration of primary adsorption centres, D is the ratio of the rates of adsorption and desorption, whilst k is a constant dependent on the uptake at p/po = 1. A best fit was achieved when the equation was applied in its quadratic form (ref. 16) but a reasonable fit was still only obtained for the Type I11 part of the isotherm, so that the range of fit was found to be dependent on the pore width of the adsorbent (Figures 5 and 6). Deviations at high pressures coincide with the plateau regions and at low pressures arise from the assumption that the primary sites are of one strength. This brings the validity of no into question, making it only significant from a comparative stand-point. The values of no and D calculated from these fits are given in Table 11. Also given are the BET monolayer capacities, nm, and the values of nm/ABET, where ABET is the BET area given by nitrogen adsorption. Of course, nm only gives an approximate indication of the number of primary centres as more than one water molecule may be associated with a particular site. Examination of Table I1 reveals no close agreement between nm and no and no obvious correlation between these values, or nm/ABET, and the location of the point of inflexion. However, there does appear to be some degree of correlation between the location of the point of inflexion and the values of D, which are in fact related to the pore widths (ref. 17). As can be seen for the charcoal cloths, the inflexion point is more strongly dependent on the percentage burn-off which directly controls the pore width. Therefore, the
691
80
60
40
20
0
8
0
24
16
I
/
Wide Pore Width
c p/p'=0.446
0
32
n [mrnol g-'l Fig. 5.
v 1
20
40
60
00
100
n Immol g-'l
Quadratic fit of the Dubinin-Serpinrky
equation to JF144 experimental data.
~ i6. ~ ~ . ~ f i t of the ~ Dubinin-Serpinsky d ~ equation to KCC1 experimental data.
number of specific sites appears to govern the uptake at p/po < 0.2 whilst the pore width controls the uptake at higher relative pressures. Furthermore, as noted above, the increase in width of the hysteresis loop with pore size also demonstrates the dominant It role played by micropore size distribution at higher p/po. TABIS 11 ADSOXBENT
Adsorption Of Water Vapour By Microporous carbons Pore Width
BET "m 4-l)
(-1
"mIABET ( m o l m-2) ~10-4
Inflexion Point (p/pO)
DS
D
"0
(-1
¶-I)
KCCL
W
0.66
2.22
0.78
1.03
1.42
Ax21
W
0.31
0.91
0.68
0.47
1.63
JFSl8 (7lrBO)
W
1-90
10.58
0.68
4.09
1.31
Takeda c 5A
0.35
9.02
0.50
0.99
2.28
0.81
7.26
0.46
0.94
2.08
0.32
2.72
0.43
0.31
2.52
JF14C (40tBO)
N N N N
1.41
11.41
0.40
1.16
N
1.46
21.64
0.37
--
2.62
JF025 (15rBO) JF005 (lOtB0)
N
1.32
14.98
0.32
1.50
2.76
JFOlO ( 5 0 t B O )
Carbosieve
Samples with the prefix Jf are charcoal cloths.
BO = Burn-off
--
~
~
692
would seem from the above that the calculated values of no and D have no real theoretical significance but do help to define the mathematical form of the water isotherm over a limited range. Lastly, the fact that VPI-5 has an inflexion point at lower p/po but contains uniformly wider pores than ALPO-5 suggests that a three-dimensional water array can form more easily in wider channels, assuming that -OH groups play a relatively minor role. Hence, within limits, somewhat wider tubular pores and narrower slit shaped pores result in micropore filling at low p/po. ACKNOWLEDGEMENTS
The authors wish to thank Dr. J.J. Freeman, Professor K.K. Unger and Dr. A. Venero for provision of samples, Dr. E.L. Short for assistance with curve fitting routines and the Ministry of Defence and the SERC for financial assistance. REFERENCES 1.
H.F. Stoeckli, F. Kraehenbuehl and D. Morel, Carbon, 21
2.
R.C. Bansal, T.L. Dhani and
(1983) 589. S.
Parkash, Carbon, 16 (1978)
389.
5.
M.M. Dubinin, Carbon, 18 (1980) 355. S . S . Barton, M.J.B. Evans and B.H. Harrison, J. Colloid Interface Sci., 45 (1973) 542. S . S . Barton and J.E. Koresh, J. Chem. SOC., Faraday Trans. 1,
6.
J.R. Dacey, J.C. Clunie and D.G. Thomas, Trans. Faraday SOC.,
7.
A.M. Youssef, T.M. Ghazy and Th. El-Nabarawy, Carbon, 20
8.
F.S. Baker and K.S.W. Sing, J. Colloid Interface Sci., 55
9.
R.A. Roberts, Ph.D. Thesis, Brunel University, Uxbridge, Middlesex, UK, 1988. D.W. Breck, "Zeolite Molecular Sieves", Wiley, 1973. C.R. Theocharis, M.R. Gelsthorpe and D. Yeates, J. Chem. SOC., Faraday Trans. 1, 85 (1989) 2641. M.E. Davis, C. Montes, P . E . Hathaway, J.P. Arhancet, D.L. Hasha and J.M. Garces, J. Am. Chem. SOC., 111 (1989) 3919. M.B. Kenny and K.S.W. Sing, Chem. Ind. (London), 2 (1990) 39. M.M. Dubinin, E.D. Zaverina and V.V. Serpinsky, J. Chem. SOC., (1955) 1760. M.M. Dubinin and V.V. Serpinsky, Carbon, 19 (1981) 402. S.S. Barton, M.J.B. Evans, J. Holland and J.E. Koresh, Carbon, 22 (1984) 265. M.M. Dubinin, K.M. Nikolaev, G.A. Petukhova and N.S. Polyakov, Izv. Akad. Nauk SSSR (Ser. Khim.), (1984) 743.
3.
4.
79 (1983) 1147. 54 (1958) 250. (1982) 113. (1976) 605. 10. 11.
12. 13. 14. 15. 16. 17.
F. Rodriguez-Reinoso et al. (Editors), Characterization of Porous Solids II 0 1991 Elsevier Science Publishers B.V., Amsterdam
693
POROSITY OF ANCIENT EGYPTIAN MORTARS
J . R A G A I l , K.S.W.
SING',
and
M. YATES2
'The American U n i v e r s i t y i n C a i r o , Egypt 2Brunel, The U n i v e r s i t y o f West London, Uxbridge, England
SUMMARY Mercury p o r o s i m e t r i c s t u d i e s were c a r r i e d o u t on A n c i e n t E g y p t i a n m o r t a r s e x t r a c t e d f r o m t h e Sphinx body c o r e and N o r t h s i d e c h e s t as w e l l as f r o m t h e K h a f r a V a l l e y temple. Such s t u d i e s were complemented b y t h e a p p l i c a t i o n o f Xr a y d i f f r a c t i o n and simultaneous t h e r m o g r a v i m e t r i c and d i f f e r e n t i a l thermal a n a l y s i s , The r e s u l t s o b t a i n e d suggest t h a t A n c i e n t E g y p t i a n m o r t a r s a r e c h a r a c t e r i z e d b y two s e t s o f pores w i t h i n t h e gypsum m a t r i x . The l a r g e r nores ( o f w i d t h '~1-15,um) appear t o be l o c a t e d between t h e CaS04 g r a n u l e s , whereas t h e s m a l l e r pores ( o f w i d t h < l o 0 nm) a r e p r o b a b l y formed as a r e s u l t o f t h e removal o f w a t e r f r o m w i t h i n t h e g r a n u l e s ( i . e . f r o m between and w i t h i n t h e i n d i v i d u a l crystals) ! INTRODUCTION I n h i s p i o n e e r i n g work on A n c i e n t E g y p t i a n M o r t a r s , A l f r e d Lucas ( r e f . 1 ) concludes t h a t t h e m o r t a r used i n A n c i e n t Egypt, b e f o r e Graeco-Roman times, was m a i n l y gypsum.
Lucas d i f f e r e n t i a t e s between two t y p e s o f gypsum i n Egypt.
One
poor q u a l i t y t y p e u s u a l l y a s s o c i a t e d w i t h v a r i o u s p r o p o r t i o n s o f CaC03 and sand Lucas
and a b e t t e r q u a l i t y gypsum i n which t h e CaS04.2H20 phase predominates.
c o n f i r m s , t h a t no i n s t a n c e of l i m e m o r t a r i s known t o have been used i n A n c i e n t Egypt b e f o r e t h e t i m e of Ptolemy I (323-283 B.C.).
Furthermore, gypsum was p r e -
f e r r e d r a t h e r t h a n l i m e because o f t h e s c a r c i t y o f f u e l . It i s w e l l known t h a t c h e m i c a l l y , gypsum i s c a l c i u m s u l f a t e d i h y d r a t e
"CaS04.2H20" which on h e a t i n g t o a t e m p e r a t u r e o f about 13OoC l o s e s t h r e e f o u r t h s o f i t s w a t e r and becomes t h e hemihydrate CaS04.3H20 ( P l a s t e r o f P a r i s ) . 13OoC Thus 2CaS04.2H20 --------+ (CaS04)2.H20 + 3H20. When p l a s t e r o f P a r i s i s made i n t o a p a s t e w i t h w a t e r i t s e t s t o a s o l i d mass i n a few m i n u t e s . The s e t t i n g i s due t o t h e r e f o r m a t i o n of t h e d i h y d r a t e i n t h e f o r m o f s m a l l c r y s t a l s which f i t c l o s e l y t o g e t h e r and produce a s o l i d mass. It i s g e n e r a l l y b e l i e v e d (refs.2,3)
t h a t the strength o f a given mortar o r
cement i s r e l a t e d t o t h e p o r e s i z e d i s t r i b u t i o n as opposed t o t h e volume f r a c t i o n s o f t h e pores.
The p o r e s i z e d i s t r i b u t i o n i s governed b y t h e n a t u r e o f t h e
s t a r t i n g m a t e r i a l s and b y t h e mode o f m i x i n g and compacting ( r e f . 4 ) . M i k h a i l and Malek ( r e f . 5 ) , u s i n g n i t r o g e n a d s o r p t i o n , have s t u d i e d modern E g y p t i a n gypsum m o r t a r s and have i d e n t i f i e d i n t h e s e systems two p o r e s i z e s
694
w.
8 and 70 Mercury porosimetric studies were n o t carried o u t on such systems, b u t Mikhail and Malek drew attention t o the similarity of corresponding to 10
the microstructures of hardened gypsum and Portland cement pastes. Since mercury porosimetry has been used t o a considerable extent i n the study of hardened Portland cement, i t seemed appropriate t o apply i t in the assessment of porosity of Ancient Egyptian Mortars. X-ray diffraction and simultaneous thermogravimetry and differential thermal analysis were used as additional techniques in the present study. EXPERIMENTAL Materials The SB1-SB5 samples refer t o mortars extracted from the Sphinx body core. The SN sample refers to a mortar from the Sphinx North Side Chest and the KV1KV3 samples r e f e r t o mortars extracted from the Khafra Valley Temple. A detailed account of the extraction s i t e s of these nine mortars suggests that sample KV1 i s l i k e l y to be the oldest one. The compositions of the mortars as determined by X-ray diffraction are reported in Table I . The thermal studies revealed t h a t the gypsum samples containing c a l c i t e exhibited a number of endotherms in the ranges 2O-12O0C, 120-170°C, 390-585'C and 637-850°C. These endotherms are attributed respectively t o the loss of i n t e r s t i t i a l H20, t o the transformation of the gypsum t o the hemihydrate and t o y-CaS04, followed by dehydration and the loss of structural water, and f i n a l l y t o the decomposition of CaC03 t o CaO. Thermogravimetric r e s u l t s indicated a f i r s t weight loss ranging from 22% t o 28% f o r most of the samples. Such a loss encompassed the two i n i t i a l DTA endotherms. Sample KV1 exhibited only 11% i n i t i a l weight loss followed by two very sharp additional losses of %2%around 350°C and 40OoC. We a t t r i b u t e the l a t t e r t o the formation of the so-called "soluble anhydrites" ( r e f . 6 ) . The present r e s u l t s accord very well with previously reported X-ray and thermal studies carried o u t on different s e r i e s of mortars (refs.6-9). Techniques Mercury intrusion-extrusion measurements were made w i t h a Carlo-Erba 225 pressure porosimeter. The X-ray diffraction patterns were obtained by means of a General Electric X-ray diffraction u n i t , model XRD-6, using Ni f i l t e r e d CuKa radiation. The thermal studies were carried out on a Stanton Redcroft STA-780, simultaneous thermal analyzer s e r i e s designed t o give simultaneous differential thermal analysis (DTA) and differential thermogravimetry ( D T G ) .
695
RESULTS Unheated Mortars Fig. 1 gives Hg intrusion-extrusion plots for six representative samples.
Fig. 1 . Mercury intrusion-extrusion plots for some representative mortars.
It
696
will be noted t h a t in a l l cases the intrusion-extrusion curves e x h i b i t considerable h y s t e r e s i s . With the exception of sample KV1 t h i s hysteresis is largel y confined t o the region of pressure below about 20-30 bars. Furthermore, a l l o f these intrusion curves e x h i b i t points of i n f l e c t i o n a t around 100-200 bars.
The corresponding extrusion curves t h e n remain almost horizontal over a very wide range of pressure leading t o l a r g e q u a n t i t i e s of mercury entrapment a t a pressure of 1 bar. A s t r i k i n g f e a t u r e i s t h a t the amount of entrapped mercury corresponds q u i t e closely t o t h e i n f l e c t i o n point a t 100-200 bars. In view of these findings we have subdivided the amount of mercury intruded i n t o two stages ' I ' and '11'. Stage ' I ' i s characterized by l a r g e r pores ( o f width -1-15 urn) whereas stage I 1 corresponds t o smaller pores ( o f width < 100 nm) . As indicated in Fig. 1 , i t i s evident t h a t sample KV1 behaves q u i t e d i f f e r e n t l y . Fig. 2 gives t h e mercury intrusion-extrusion p l o t s f o r mortars KV1, KV2 and
697
SB2 as w e l l as f o r t h e i r h e a t - t r e a t e d p r o d u c t s ( h e a t i n g i n a i r r e s p e c t i v e l y a t 2OO0C and 4OO0C f o r 3 h o u r s ) .
I t i s seen t h a t f o r samples KV2 and SB2, h e a t t r e a t m e n t a t 200°C and 4OO0C l e a d s t o a g r a d u a l decrease i n t h e p o r e volume
c o r r e s p o n d i n g t o s t a g e 'I'o f t h e mercury p e n e t r a t i o n , whereas t h e p o r e volume i n stage '11' i n i t i a l l y i n c r e a s e s a t 200°C ( t o g e t h e r w i t h an enlargement o f t h e pore r a d i i ) , f o l l o w e d b y a s m a l l decrease a t 40OoC.
As shown i n F i g . 2, t h e shape o f t h e mercury i n t r u s i o n - e x t r u s i o n p l o t s f o r t h e heated m o r t a r s KV2 and SB2 t o some e x t e n t resemble t h a t o f t h e o r i g i n a l m o r t a r K V l . GENERAL DISCUSSION The r e s u l t s r e p o r t e d h e r e suggest t h a t A n c i e n t E g y p t i a n m o r t a r s may be c h a r a c t e r i z e d by two s e t s o f p o r e s ' I ' and ' 1 1 ' w i t h i n t h e gypsum m a t r i x . Indeed t h e shape and e x t e n t o f t h e mercury i n t r u s i o n - e x t r u s i o n p l o t s F i g . 1, determined on t h e unheated m o r t a r s i n d i c a t e a f i r s t s t a g e 'I'o f t h e mercury p e n e t r a t i o n which corresponds t o t h e f i l l i n g o f l a r g e t h r e e dimensional c a v i t i e s connected by narrow t h r o a t s o r channels. ween t h e CaS04 g r a n u l e s .
Such pores appear t o be l o c a t e d b e t -
The p l o t s a l s o i n d i c a t e a second s t a g e ' 1 1 ' o f t h e
mercury p e n e t r a t i o n which seems t o correspond t o t h e f i l l i n g up o f s m a l l e r pores p r o b a b l y a r i s i n g f r o m t h e removal o f w a t e r f r o m w i t h i n t h e CaS04 g r a n u l e s . Hammond and Withrow ( r e f . 10) have shown t h a t l a r g e lumps o f CaS04.2H20 r e t a i n t h e i r o r i g i n a l shape and c r y s t a l s t r u c t u r e even a f t e r complete d e h y d r a t i o n . Such r e s u l t s suggest t h a t i n t h e p r e s e n t s t u d y pores r e l a t i n g t o s t a g e ' I ' o f t h e p e n e t r a t i o n r e s u l t from t h e s t a b i l i t y o f t h e gypsum phase, i . e . a r e i n t e r granular i n nature,
Outgassing t h e m o r t a r samples p r i o r t o c a r r y i n g o u t t h e
mercury i n t r u s i o n measurements seems, on t h e o t h e r hand t o be r e s p o n s i b l e f o r t h e s m a l l e r pores ( s t a g e '11' o f t h e p e n e t r a t i o n ) t h r o u g h t h e s t e a d y removal o f i n t e r s t i t i a l w a t e r . An enlargement o f t h e l a t t e r pores t o g e t h e r w i t h an i n creased p o r e volume would f o l l o w t h r o u g h t h e subsequent removal o f t h e w a t e r of c r y s t a l 1 iz a t i o n . An i n t e r e s t i n g comparison w i t h t h e work o f M i k h a i l and Malek ( r e f . 5 ) c o u l d be drawn a t t h i s p o i n t . As a l r e a d y mentioned e a r l i e r these a u t h o r s have i d e n t i f i e d two s e t s o f p o r e s i z e s i n modern E g y p t i a n hardened gypsum pastes. s m a l l e r s e t ' S ' w i t h an average h y d r a u l i c r a d i u s o f 10
8
One
was assumed t o c o n s t i -
t u t e t h e i n t e r s p a c e s between t h e c r y s t a l l i n e p a r t i c l e s whereas a l a r g e r s e t of pores ' L ' w i t h an average r a d i u s o f 70
fl -
80
8
was a t t r i b u t e d t o t h e w a t e r It i s there-
f i l l e d spaces i n t h e pastes which were i n t r a p a r t i c u l a t e i n n a t u r e .
f o r e q u i t e obvious t h a t t h e o r i g i n o f m i c r o p o r o s i t y as s t u d i e d i n t h e work o f M i k h a i l and Malek f o l l o w s q u i t e a d i f f e r e n t t r e n d t h a n t h a t observed i n o u r p r e s e n t s t u d y i n which much l a r g e r pores a r e i n v o l v e d . The d i f f e r e n c e observed i n t h e h e a t - t r e a t e d samples SB2 and KV2 a r e c o n s i s H e a t - t r e a t m e n t of t h e s e m o r t a r s a t 200°C
t e n t w i t h our present i n t e r p r e t a t i o n .
698
seems t o lead, as i n d i c a t e d i n F i g . 2, t o a general c o n t r a c t i o n o f t h e CaS04 s t r u c t u r e t o g e t h e r w i t h an enlargement of the s m a l l e r pore r a d i i . The former effect may be a t t r i b u t e d t o a c e r t a i n degree o f s i n t e r i n g , whereas t h e l a t t e r e f f e c t r e s u l t s from t h e removal o f t h e water o f C r y s t a l l i z a t i o n . The thermal studies have confirmed t h a t b o t h the i n t e r s t i t i a l and a good p r o p o r t i o n of the water o f c r y s t a l l i z a t i o n are removed a t 200°C. It i s noteworthy t h a t samples SB4 and SB5 as w e l l as KV1 and SN1 although
seemingly i d e n t i c a l i n terms o f component minerals and composition (Table I ) vary considerably i n terms o f ranges o f i n t r u s i o n pore r a d i i and volumes.
,
A
probable f a c t o r c o n t r o l l i n g pore s i z e i n these mortars i s the change i n t h e water content o f CaS04 gH20 when the l a t t e r i s made i n t o a paste. TABLE I Composition o f t h e mortar samples as determined by X-ray d i f f r a c t i o n . ~
~~
Sample SB1
~
~
Components
SB2
Gypsum (A), CaC03 (D), Q u a r t z ( D ) Gypsum (A), Q u a r t z (D)
SB3
Gypsum (A),
CaC03 (B), Q u a r t z (D)
SB4
Gypsum ( A ) ,
CaC03 (C), Q u a r t z (D)
SB5
Gypsum ( A ) ,
CaC03 (C), Q u a r t z (D)
SN1
CaC03(A), Gypsum (B) , Quartz (C)
KV 1
CaC03(A), Gypsum (B),
KV2
CaC03(A), Gypsum ( C ) Gypsum (A), CaC03 (B), Quartz ( C )
KV3
Quartz ( C )
A = Major component, B,C and D r e f e r t o components i n o r d e r o f decreasing amounts as i n d i c a t e d by the r e l a t i v e i n t e n s i t i e s o f the X-ray d i f f r a c t i o n peaks. X-ray d i f f r a c t i o n s t u d i e s on KV1 and on o t h e r s i m i l a r systems ( r e f s . 5,8) have i n d i c a t e d the unusual prevalence o f t h e y-CaS04 phase u n t i l temperatures as h i g h as 40OoC. The thermal s t u d i e s i n d i c a t e t h a t t h i s m o r t a r i n p a r t i c u l a r behaved q u i t e d i f f e r e n t l y e x h i b i t i n g a very small amount o f water o f c r y s t a l l i z a t i o n and very d i s t i n c t i v e sharp endothems corresponding t o the s o - c a l l e d " s o l u b l e anhydri tes". I t i s t h e r e f o r e l i k e l y t h a t ageing e f f e c t s may have l e d t o t h e removal o f t h e i n t e r s t i t i a l water f o l l o w e d by t h e slow d i f f u s i o n and e l i m i n a t i o n o f p a r t o f t h e
water o f c r y s t a l l i z a t i o n .
This would account f o r t h e observation o f a marked
h y s t e r e s i s i n stage '11' o f t h e p e n e t r a t i o n i n t h e unheated mortar KV1. Thermog r a v i m e t r i c r e s u l t s corroborate such an i n t e r p r e t a t i o n since a l l the h e a t - t r e a t ed mortars a t 200°C e x h i b i t e d a g r e a t e r i n i t i a l weight l o s s ( 2 2 %
-
28%) as w e l l
as an increase i n t h e I 1 pore volumes (Table 11) as compared w i t h m o r t a r KV1
699
where o n l y 11% weight l o s s was observed w i t h no increase i n pore volume.
In
keeping w i t h such an i n t e r p r e t a t i o n Okhotonikov e t a1 ( r e f . 11) have shown t h a t t h e k i n e t i c s o f dehydration o f CaS04.
nH20 i n t h e (010) d i r e c t i o n a r e d i f f u s i o n
c o n t r o l l e d i n which water molecules d i f f u s e through the s o l i d p r o d u c t CaS04. TABLE I1 Representative ranges o f i n t r u s i o n r a d i i and pore volumes corresponding t o stages 'I'and ' 1 1 ' o f mercury p e n e t r a t i o n f o r some unheated and heated mortars. Sample
r p ranges i n nm I I1
unheated SB1 SB4 SB5 KV1 SB2 heated KV1/200°C, KV1/400°C, SB2/200°C, SB2/400°C,
3 3 3 3
3 Ranges o f pore volumes A V (mm ) P I 11
8000-40 4000-40 15000-40 30000-200 8000-40
40-5 40-5 40-5 200-5 40-5
200 200 100 120 200
30 30 30 110 30
h r s . 30000-700 h r s . 15000-200 h r s . 8000-700 h r s . 8000-700
700-5 200-5 700-5 700-5
80 40 160 55
50 40 55 45
REFERENCES 1
A. Lucas and J.R. H a r r i s , Ancient Eqyptian M a t e r i a l s and I n d u s t r i e s , -- .
Edward A r n o l d Ltd., 1962: 2 J.D. B i r s h a l l , A.J. Howard and K. Kendall, Nature, 289 (1981) 388. 3 K. Kendall. A.J. Howard and J.D. B i r s h a l l , P h i l . Trans. Roy. SOC. Lond. , 310 (1983)-139. 4 R. Malinowski, Cem. Conc. Res., 1 (1971) 531. 5 R.Sh. M i k h a i l and R . I . A . Malek, J . Appl. Chem. Biotech., 21 (1971) 277. 6 H.Y. Ghorab, J . Ragai and A. Antar, Cem. Concr. Res., 16 (1986) 813. 7 J. Ragai, Cem. Concr. Res., 18 (1989) 9. 8 J. Ragai, Cem. Concr. Res., 19 (1988) 179. 10 W.A. Hamnond and J.R. Withrow, Ind. Eng. Chem., 25 (1933) 1112. 11 V.B. Okhotnikov, B . I . Yakobson and N.Z. Lyakhov, Ser. Khim. Nauk, 1 (1985) 23.
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F. Rodriguez-Reinoso et al. (Editors), Characterization of Porous Solids II 0 1991 Elsevier Science Publishers B.V., Amsterdam
70 1
THE POROUS STRUCTURE OF POLYMERIC SORBENTS OF DIFFERENT NATURE L. D. BELYAKOVA Institute of Physical Chemistry of the USSR Academy of Sciences, Leninsky Prospect 31, Moscow 117915 (USSR) SUMMARY Porous polymers relating to different structural types have now been synthesized. The paper generalized results on the investigation (using various physico-chemical methods) of the geometrical structure of copolymers of 2,3epoxypropylmethacrylate and ethylenedimethacrylate, styrene and divinylbenzene, hypercrosslinked polystyrenes ("Styrosorbs") initial and modified with amines and aminoalcohols. The effect of the porous structure of aminated polymers on the adsorption properties was dealt with mostly in the example which illustrated adsorption of carbon and sulfur dioxides. The paper showed the importance of optimization of the structure of polymeric adsorbents when they are used for practical purposes. INTRODUCTION Big successes have become possible in the recent years in surface chemistry, adsorption and chromatography due to the creation of porous polymeric adsorbents (ref.1). Porous polymers have some important advantages. They can be obtained not only as spherical granules of narrow fractions of different sizes but also as fabrics, films and membranes. The choice of monomers during the synthesis makes it possible to regulate their thermostability and acid or alkali resistance. The porous polymers adsorb substances both from dry and moist atmosphere. A variation in the conditions for synthesizing porous polymers makes it possible to regulate the structure of the resultant three-dimensional polymeric network and to obtain adsorbents having different of porosities. Polymeric sorbents with different surface chemistry can be readily obtained. CLASSIFICATION OF POLYMERIC ADSORBENTS ACCORDING TO THE GEOMETRICAL STRUCTURE A porous structure of most organic polymerizable-type polymers as well as inorganic xerogels is formed by drying gel; therefore they have an identical type of pore structures. The electronmicroscopy studies of some organic porous polymers showed that
702
their structure is globular (ref.2). The suspension copolymerization process results in creation of gel microsphere, their agglomeration and formation of globules. Pores are formed by channels between the globules. Classification of polymeric adsorbents according to the geometrical structure (Table 1) is based on concepts acceptable for traditional adsorbents. The structure is regarded as macroporous if the pore diameter d>100 nm, mesoporous if d=3-100 nm and microporous if d<3 nm (ref.3). As evident, methacrylatetype copolymers are mesoporous adsorbents, but under certain synthesis conditions macroporous samples can also be obtained. ST-DVB copolymers have a mesoporous structure, but they have generally few microporous due to parallel DVB-DVB copolymerization. Crosslinked isoporous copolymers of polystyrene may have a meso- and microporous structures. TABLE 1 Classification of polymeric adsorbents according to the porous structure.
Polymeric chain
Symbols of initial and modified polymers
2,3-epoxypropylmethacrylateethylenedimethacrylate (refs.4-6)
EPMA-EDMA EPMA-EDMA modified agent
d>10Qnm d=3-100nm macro meso
~~
-
ST-DVB Styrene-divinylbenzene (refs.7-12) ST-DVBmodified agent Polystyrenexylylenedichloride Polystyrenemonochlorodimethyl ether (refs.13-15)
PS-XDC Styrosorb PS-MCDE Styrosorbmodified agent
d<3nm micro
~
+
+
-
+
-
+
+
Thus, polymeric adsorbents of different structural types have been now synthesized (Fig.1). Polymeric adsorbents swell appreciably in many solvents. Complete and reliable information about the porous structure of such %on-rigidgf adsorbents can be obtained by using a number of
703
Fig.1. Adsorption isotherms of 1 n-C8F18 vapours at 25'C a) on 2 ,3-EPMA-EDMA copolymer (40% EDMA) modified with DETA b) on ST-DVB copolymer ( 8 % DVB, 100% n-decane as porogenic agent) modified with DETA c) on Styrosorb (2% DVB, crosslinking agent is MCDE, 2 porogenic agent is dichloroethane, mol.mass of PS is 3.105). on Styrosorb d) 1 (cross1inking agent is MCDE , mol. mass of PS is 3.105, 1 inert solvent is cyclohexane) 2 - on Styrosorb 1, modified with DETA. adsorption, Light points black - desorption. Crossed points is for second measurements. The arrow shows the adsorption change with time.
-
00
0.5
1.0
005
P/Po different methods. The investigation of adsorption isotherms of various vapours on polymers showed that these adsorbents don't swell only in the case of perfluoroalkane (which is capable of only weak non-specific interaction with the polymeric surface). The isotherms are fully reversible (Fig.1). At high p/po values capillary-condensation hysteresis is quite obvious as in the case with rigid skeleton inorganic adsorbents (ref.1). THE POROUS STRUCTURE OF METHACRYLATE-TYPE POLYMERS Methacrylate polymers were obtained in the process of suspension free-radical copolymerization with a different amount of an ethylenedimethacrylate (EDMA) crosslinking agent and a
704
different ratio between the cyclohexanol solvent and the dodecanol precipitator in an inert phase (refs.4-6). Depending on the copolymerization conditions, samples of different structure can be obtained. The surface area values of methacrylate polymers increase as the amount of EDMA increases. Various methods were used to investigate the structure of some aminated EPMA-EDMA samples (Table 2, samples 1-4). The surface area of adsorption film S' and the S values determined from n-C8F18 isotherms are substantially identical and the S values calculated from the adsorption of small nitrogen molecule and a rather large n-C8F18 molecule are similar. This is an indication that the investigated samples have no micropores. The VHg are approximate to va&... This fact indicates that the investigated samples do not contain any significant number of macropores with d>200 nm. TABLE 2 Characteristics of the porous structure of modified polymers: EPMA-EDMA (initial copolymer containes 40% EDMA, ratio cyclohexanol/dodecanol=91/9 (ref.12)); ST-DVB (initial copolymer contains 8 % DVB, porogenic agent-n-nonane=lOO% (refs.l1,12)); Styrosorb (inert solvent-cyclohexane, mol.mass of PS=3.105 (ref.15).
EPMA-EDMA 1 EPMA-EDMA 2 EPMA-EDMA 3 EPMA-EDMA 4 EPMA-EDMA ST-DVB 5 ST-DVB 6 ST-DVB 7 ST-DVB Styrosorb 8 Styrosorb
*
A MEA EDA DETA
-
MEA EDA DETA
-
DETA
69 100
-
-
106
100
1.03
-
60
1.08
-
1.13
70
70
0.93 1.00 0.97
66
60 60 70
GO
-
-
70 69
-
-
69
70
1.03
GO
-
62
55 53 49 545
58 55 50 400
53
0.84 0.77
-
0.74 0.79 0.40
0.79 0.89
-
510
53
55 280
-
-
0.60
-
-
49
-
57 57 57
60 60
4 4
-55
A - ammonia: MEA - monoethanolamine (2-hydroxyethylamine) ; EDA - ethylenediamine; DETA - diethylenetriamine.
As can be seen from Table 2, when polymers are modified, the porous structure parameters do not substantially change. Only if aminated with ammonia, S value increases due, perhaps, to additional crosslinking of the skeleton.
705
THE POROUS STRUCTURE OF ST-DVB POLYMERS Porous ST-DVB copolymers are obtained by suspension copolymerization of styrene with divinylbenzene. Their porous structure is determined by the amount of the DVB crosslinking agent, the type of porogenic agent and its concentration in admixture with polymerizable monomers (refs. 7-12). A number of different methods was used to investigate the porous structure of some modified polymers (Table 2, samples 5-7). The effect of the amount of the crosslinking agent (from 8 to 30% DVB) on the structure of polymers containing EDA groups was then investigated. As the amount of DVB in the polymeric matrix increases , the S , V and d values increase, since a more crosslinked skeleton more easily withstands capillary contraction forces as the porogenic agent is removed. The highly crosslinked sample contains a great number of small pores which are responsible for the high S value of this sample. The dependence of the structure of polymers containing EDA and DETA groups on the size of the molecule of hydrocarbon (inert solvent, porogenic agent) was also investigated. The largest pores contain polymers obtained with the n-decane solvent (refs.9,ll). Thus, the application of different methods for investigating the porous polymeric structure makes it possible to define in detail the structure of these complex objects. THE POROUS STRUCTURE OF STYROSORBS When in dry state hypercrosslinked styrene polymers (degree of crosslinking is 40-100%) display a developed porosity. Styrosorbs are obtained by crosslinking of linear polystyrene chains dissolved in organic solvents (dichloroethane or cyclohexane) with bifunctional compounds (ref.13,14). The investigation of the structure of Styrosorbs obtained in dichloroethane by different methods showed that they are microporous sorbents which contain predominantly pores with d of about 1 nm (refs.13,14). Fig-lc shows the sorption isotherms of n-C8F18 on this Styrosorb (ref.15) . The isotherm shape is typical of microporous sorbents, but sorption of perfluoro-n-octane is distinguished by slow kinetics. Fig.ld shows the typical sorption isotherm on Styrosorb sample obtained with cyclohexane. The location and form of the hysteresis loop depend on the molecular mass of PS (ref.15). It
706
should be pointed out to relatively high values of the sorption of n-C8F18 on Styrosorbs at low p/po values and irreversibility of the isotherms in the initial region. The latter may be attributed both to slight swelling of polymers in this sorbate and to retarded kinetics of the sorption of n-C8F18 in the micropores present in these samples. Thus, the type of porous Styrosorb structures can be varied within wide ranges and depends on the nature of solvent and the molecular mass of PS. when polymers are chloromethylated and aminated with DETA, the S and V values obtained from C8F18 isotherm increase and the d value remains constant (Fig.ld and Table 2). In case of polymer-
analogous transformation, small pores are perhaps formed still accessible for C8F18 molecules. THE EFFECT OF THE POROUS STRUCTURE OF POLYMERS ON THEIR ADSORPTION PROPERTIES. It has been shown above that the geometrical structure of polymeric sorbents is determined by the amount and the nature of components involved in their synthesis. Therefore, the paper examined the effect of these factors on the adsorption of some adsorbates, more particularly carbon and sulfur dioxides. Acid gases are selected because, firstly, they present interest from the practical point of view and, secondly, they are readily adsorbed by polymers containing amino groups. Adsorption isotherms of C02 and SO2 were determined by a static volumetric method as well as elution curves of C02 and SO2 on ST-DVB and EPMA-EDMA polymers containing various functional groups and with a different amount of the crosslinking agent (refs.l,16). Fig.2 shows the dependencies of acid gases capacities and the amount of amino groups of polymers on the content of the crosslinking agent for ST-DVB and EPMA-EDMA polymers. The C02 and SO2 capacities for modified ST-DVB polymers decrease as the amount of the crosslinking agent increases therein (Fig.2a). As the amount of DVB increases in the polymerizable system, the number of monomer links bearing functional groups decreases. The smaller number of functional groups causes the lower adsorption values of C02 and SO2 which on ST-DVB-EDA polymers due to are adsorbed
707
150
L
a
I0
E 20
,
30
, 40
Fig.2. Dependencies of specific surface area S, pore volume V, concentration of amino groups cN and C02 (light points) and SO2 (black) breakthrough capacity Eo on the amount of crosslinking in a) ST-DVB polymer containing EDA groups b) EPMA-EDMA polymer modified with EDA. dispersion interaction with the polymeric skeleton and specific interactions with functional groups, i.e. the interaction with functional groups is regarded as prevailing factor. The amount of SO2 adsorbed by a low crosslinked EPMA-EDMA polymers is far smaller than the number of amino groups (Fig.2b). This seems to occur because the S and V values for such polymers are low and the great number of amino groups is not accessible for the adsorbed SO2 molecules. As the EDMA amount increases and their accessibility increases and the SO2 capacity becomes higher. This is confirmed by the fact that as the amount of EDMA increases, the number of surface epoxy groups also increases (ref.4). It will be noted that despite different dependencies of acid gases capacities on the amount of crosslinking agents for ST-DVB and EPMA-EDMA polymers containing identical functional EDA groups, the optimum structure of the samples is identical ( S = 5 0 m2.g-l and d=10 nm) , although in the first case the copolymer is
708
obtained at 8% DVB, and in the second case, at 40% EDMA. Thus, this structure assures maximum accessibility for amino groups. CONCLUSION Thus, the investigation of the porous structure of polymeric sorbents and the determination of dependencies of the porous structure parameters on the synthesis condition (the amount and the nature of the crosslinking agent, the amount and the nature of the porogenic agent and the molecular mass of polystyrene) make it possible to synthesize the porous polymers of certain structure. The established laws of the adsorption of acid gases by modified polymers of different structure make it possible to optimize their porosity at which the maximum accessibility of functional groups for the adsorbed molecules is attained. REFERENCES 1.
2. 3. 4. 5. 6. 7. 8.
9. 10.
11. 12. 13.
14. 15. 16.
L.D.Belyakova, A.V.Kiselev, N.P.Platonova and T.I.Shevchenko, Ady.Colloid and Interface Sci., 21(1-2)(1984)55-118. J.Coupek and M.Pop1, Analysis, 7(1979)28-32. S.G.Gregg and K.S.W.Sing, Adsorption, Surface Area and Pocosity, Academic Pzess, London, New York, 1982. F.Svec, J.Hradi1, J.Coupek and J.K&lal, Angew.Makromol.Chem.L 48(1975)139-143. J. Lukai I M. Bleha, F.Svec and J.Kalal , Angew .Makromol Chem. , 9551981)129-137. F-Svec, Angew.Makromol.Chem., 144(1986)39-49. E.I.Lustgarten, T.K.Brutskus, A.B.Pashkov, G.A.Artyushin and V.A.Grigoryev, Plastmassy, 12(1980)216-222. A.A.Tager, M.V.Tsilipotkina, E.B.Makovskaya, E.I.Lyustgarten, A.B.Pashkov and M.A.Lagunova, Vysokomol. Soed., A13(1971)2370-2379. T.K.Brutskus, K.M.Saldadze, E.M.Gutkina, R.N.Kalinina, A.A.Davidchuk, G.A.Artyushin, E.A.Uvarova, A.K.Val'kova and A.V.Chistyakova, Zhurn.Prikladnoi Khim., 59(2)(1986)303-306. T.K.Brutskus, E.A.Uvarova, A.S.Zavadovskaya and G.K.Saldadze, Plastmassy, 8(1989)68-70. L.D.Belyakova, A.V.Kiselev, N.P.Platonova and T.I.Shevchenko, Kolloidn.Zhurn., 42(1980)216-222. L.D.Belyakova, T.I.Shevchenko, M.Bleha, E.Votdvova and J.K6lal, Kolloid.Zhurn., 49(1987)847-851. V.A.Davankov, S.V.Rogozhin and M.P.Tsyurupa, J.Polym.Sci. Symposium, 47(1974)95-101. V.A.Davankov and M.P.Tsyurupa, Pure and Appl.Chem, 61 (11)(1989)1881-1888. L.D.Belyakova, T.I.Shevchenko, V.A.Davankov and M.P.Tsyurupa, Adv.Colloid and Interfase Sci., 25 (1986)249-266. L.D.Belyakova, N.P.Platonova, T.I.Shevchenko, F.&ec and J.Hradi1, Pure and Appl.Chem., 61(11)(1989)1889-1896.
.
F. Rodriguez-Reinosoet al. (Editors), Characterization of Porous Solids I I 0 1991 Elsevier Science Publishers B.V., Amsterdam
709
DETERMINATION OF SPATIALLY RESOLVED PORE SIZE INFORMATION
Brent Ewingl.3, Pamela J. Davis2, Paul D. Major$, Gary P. Drobny3, Douglas M. Smith2, and William L. ~ ~ 1 1 khemistry and Laser Sciences Division, Mail Stop (3740, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (U.S.A.) 2NSF/UNM Center for MicroEngineered Ceramics, University of New Mexico, Albuquerque, New Mexico 87131 (U.S.A.) 3Department of Chemistry, University of Washington, Seattle, Washington 98 195 (U.S.A.) SUMMARY
Nuclear Magnetic Resonance is a useful technique for measuring pore structures of samples which are saturated with a pore fluid. These NMR methods can be combined with NMR imaging techniques to give pore structure information as a function of position in the sample. We demonstrate the principle of such applications with an example using a porous glass. We are able to measure porosity in the sample with spatial resolution of approximately 50 pm. We have also demonstrated image discrimination based upon pore size.
INTRODUCTION In the early days of Nuclear Magnetic Resonance (NMR) it was realized that the intrinsic nuclear magnetic relaxation times are affected by chemical exchange of molecules between environments (ref. 1). This was soon generalized to the measurement of pore sizes of saturated materials through the relaxation times of the solvent nuclei (ref. 2). The fundamental concept is that molecules adjacent to the surface of a pore have hindered molecular motion and thus have shorter relaxation timcs. Molecules in the center of the pore behave nearly like "bulk" solvent. There is chemical exchange between the two environments which results in an averaged relaxation time, weighted by the numbers of molecules in each site. It can be seen that, conceptually, this results in a measure of surface to volume if one knows the relaxation times in the two environments. The mathematical treatment of such data is more complex and it is not until recently that several groups have truly started to exploit these concepts (refs. 3-5). With some care it is possible to obtain accurate pore size distributions in saturated materials through NMR
710
longitudinal relaxation times, TI'S. These measurements work best at low applied magnetic field strengths. At high applied fields, the relaxation times of the surface and bulk molecules approach one another and the discrimination is not as p a t . However, the transverse relaxation h e , T2, is not affected by increased field and since the same surface vs. bulk arguments apply it should be possible to obtain pore size distributions from T2's. We have preliminary data that indicates the potential of this technique. Nuclear Magnetic Resonance Imaging, NMRI, has become a powerful tool in medical diagnostics but has found far less utility in physical measurements of "materials". Part of the reason for this is the cost and the fact that other imaging techniques such as x-rays can be applied with reasonable success. Ackerman and co-workers have produced a significant amount of information in which they have applied NMRI techniques to a variety of ceramics problems, including the imaging of liquids in pores (refs. 6-8). Hayashi, et al, have used NMRI to follow fluids during slip casting of advanced ceramics (ref. 9). Komoroski (ref.10) and Listerud and coworkers (ref. 11) have both used NMRI techniques to image elastomeric polymers with the intent of determining sample homogeneity. One useful pmperty of NMRI is that, in principle, any NMR parameter can be measured as a function of position through the proper selection of imaging modality. This implies that chemical shifts, relaxation times, etc can be spatially resolved. There are two primary NMRI techniques, Fourier Zeugmatography (ref. 12) and Filtered Back-Projection (FEiP) (ref. 13). In the work described below we have chosen to use FBP for several reasons that are of interest primarily to practitioners of the art. Essentially, the technique involves applying a magnetic field gradient during acquisition of the NMR "spectrum". This labels the position of nuclei in the sample. Nuclei at the high end of the field gradient appear at the high end of the spectrum and conversely for those at the low end of the gradient. It may be seen that this produces a projection of the sample in NMR spectral space. This is equivalent to the projection obtained in a n o d x-ray photograph. Projections are obtained as a function of the angle of the applied gradient to the sample. A number of projections are obtained through a total of 180° of arc. The generation of a true image from these projections is well known in the field of Computer Aided Tomography (CAT) scanning. The projections are multiplied by a suitable filter function and then back-projected into real space giving a three dimensional image. As noted above, it is possible to measure relaxation times in an equivalent fashion and thereby obtain spatially resolved pore size distributions. In the work below, we demonstrate the use of image intensity in a solid with saturated pores to obtain porosity. We also demonstrate NMR images which discriminate against fluid in small pores. It is now a question of data handling and NMR sensitivity to reach our desired goal of spatially resolved pore size
711
distributions.
EXPERIMENTAL
-
NMR imaging data were obtained on a modified commercial NMR spectrometer (Bruker
CXP-200) operating at an applied field strength of 4.7 Tesla. The NMR probe is nearly equivalent to one described by Listerud, et. al. (ref. 11). The NMR data were obtained as spin echoes according to the pulse sequence shown in figure 1. The delay between the pulses and the data acquisition was varied to obtain images with different T2 weights.
nl2 Proton rf
Magnetic field gradient Data acquisition
FID
n
Echo
Fig. 1. The pulse sequence used to acquire echoes for imaging.
This provides discrimination according to pore size. The magnetic field gradient was applied along the direction of the static field. Gradients of 40 or 50 gauss/cm were used for different experiments. The sample was rotated after obtaining each echo. Either 94 or 188 echoes were obtained through a total of 180° of rotation. The set of echoes was transferred to a microVAX I1 computer where they were Fourier transformed and corrected to yield projections. The projections were then filtered with a Hanning filter (ref. 14) and back-projected to give the images. Data handling in the microVAX was accomplished using a modified version of commercially available NMR software (FTNMR). The digital resolution in the images shown is about 40 p m per pixel. No slice selection was used in the generation of these images. This means that the data shown are projections of the number of solvent molecules down the axis about which the sample is rotated. The samples imaged consist of a microporous glass, vycor, obtained from Coming Glass Company. The vycor was in the form of annular cylinders with outer diameter of 3.4 mm and
712
inner diameter of 1.7 mm which were cut to 15.5 mm in length. The properties of the vycor, measured by N2 condensation techniques, are: specific surface = 172 m2/g, pore volume = 0.24 cm2/g, porosity = 34.5%, and average pore radius = 2.8 nm. Usually, the vycor was saturated by boiling in H 2 0 and images were obtained of the water in the pores. In one case, the vycor was saturated with ethanol and a small cylinder of sol-gel prepared silica gel was in the center of the vycor cylinder. The sol-gel silica was prepared from tetraethylorthosilicate (TEOS) using a method described in the literature (ref. 15). The gel was allowed to dry and was resaturated with ethanol. This resulted in a gel cylinder which was slightly smaller in outer diameter than the inner diameter of the vycor and the silica gel was also deformed from exact cylindrical geometry. Although the pore properties of this silica gel were not measured, in similar preparations we have found average porosities of about 90% and average pore radii of about 50 nm.
RESULTS AND DISCUSSION There are a variety of ways of presenting the information obtained in NMRI. The display used in medicine is a gray-scale which results in a picture similar to an x-ray photograph. In the images shown below, the contrast is extremely high so gray-scales look solid black or white. Figure 2 contains three images of a combined sample.
Fig. 2. Three presentations of the image of a combined sample which includes: an annular vycor cylinder saturated with ethanol, a cylinder of porous silica also saturated with ethanol, and a cylinder of pure ethanol. The presentations are: a stack plot, A; a contour plot, B; and a longitudinal cross section through B to demonstrate the amount of solvent in each region, C.
713
This is an annular vycor cylinder, saturated with ethanol, a distorted cylinder of extremely porous silica, also saturated with ethanol, in the center of the vycor, and a very small tube of pure ethanol. The "stack plot" of figure 2A is drawn in a manner that resembles a three dimensional picture. Each line in the figure represents the intensity of the NMR signal at that position. Figure 2B is a contour plot of the same data in which each contour level indicates a greater amount of ethanol. Figure 2C is a longitudinal slice through the contour plot, 2B. We note that all images presented are two dimensional images which represent the projection of the total number of solvent molecules (ethanol, in this case) down the axis of the cylinder. In the parlance of NMRI, no slice selection is used in the images. The data in figure 2 has very minor intensity distortions because the delay times in the pulse sequence were selected to get a good image in a short time rather than a faithful representation of solvent intensities. In spite of this it is clear that the silica gel is much more porous than the vycor. From longitudinal slices such as figure 2C we can obtain the porosity of a sample. The intensity is a measure of the number of solvent molecules at that point in the sample (projected down the length of the sample). In this particular image, the intensity is weighted by the effective relaxation times. Since the cylinder of pure ethanol has the same geometrical height as the vycor and nearly the same height as the gel, a simple comparison of height of signal yields porosity. In other words, the pure ethanol in the tube is 100% porous. An average of several measurements yields a porosity of 32.3 _+ 4.2%for the vycor and 89.5 f 1.0% for the gel. In order to accurately obtain porosities it is necessary to have a standard sample of known dimensions to calibrate the NMR data. In cases where the sample does not have the simple symmetry of the vycor cylinder, it will be necessary to select slices of the sample and measure the amount of solvent within each volume element of interest. Slice selection is well established in NMRI but it has the disadvantage of reducing the overall signal to noise and thus introducing uncertainty in the measurements. Figure 3 demonstrates the principles of pore size discrimination in NMR imaging. It relies on the difference in bulk T2 to distinguish between solvent in large pores and solvent in small pores. The latter has a significantly reduced relaxation time. Figure 3A contains a contour plot of an annular cylinder of vycor, saturated with water containing water filling the center tube. Figure 3B is another plot of the same sample taken at the same time but with a different set of delays in the pulse sequence of figure 1. In figure 3B the time between the first pulse and the echo was 20.4 ms. In this sample that is sufficient to allow NMR relaxation of the signal in the vycor through T2 processes.
714
'I
I
I
I
B
I
Fig. 3. Three images of a vycor annular cylinder saturated with water and filled with pure water in the central cylinder. The upper contour plot, A, is taken with a short delay in the echo sequence to image all of the water in the system. The lower contour plot, B, is taken with a long delay to image only pure water (not water in small pores). Figure 3C includes cross sections through 3A and 3B and the difference between them. The difference represents water in small pores.
This relaxation is due to the fact that the water in the vycor is contained in very small pores. In other words, 3B selects for water in large pores and since the water in the center cylinder is essentially in a single large pore, its signal is not very attenuated. Figure 3C contains radial cross sections through the two contour plots and the difference between them. The bottom plot is the difference and is a representation of the spatial variation of s m a l l pores in the sample. It demonstrates the position of the vycor annulus. This type of experiment can be extended to obtain spatial resolution of pore sizes and even pore size distributions. This involves obtaining a series of 10 or more images as a function of delay in the echo sequence. The mathematical analysis of the intensity as a function of echo delay yields the pore size. This analysis can be done
715
for each pixel in the image to give spatially resolved pore sizes. Since there are a total of 128 by 128 or 16384 pixels in the images presented, this is a rather formidable task for a complete image but it is not beyond the realm of straightforward modem computation. In cylindrically symmetrical samples such as those in this work, all of the information needed is contained in radial cross sections which reduces the problem to the order of 50 calculations which can be done with little problem.
CONCLUSIONS Nuclear Magnetic Resonance imaging techniques have potential for giving several types of information regarding pore characterization in saturated materials. The simplest information obtained is the spatial variation of porosity which can be obtained by careful selection of the pulse sequence and delays in the imaging experiment. It is possible to selectively image areas of a sample which have very large pores by inserting long delays in the echo sequence used to generate the data. A logical extension of this is to obtain a series of images as a function of delay time and by mathematical analysis of those to obtain spatial discrimination of pore size and even pore size distributions. We are working towards that goal. The primary disadvantage of this technique is the necessarily complicated hardware. This implies large costs and significant expertise to obtain reliable data. The consequence of that is that spatially resolved pore structure by NMR has primary application in studies of particular scientific interest rather than in surveys of large numbers of similar samples. The use of NMRI for detection of structural defects in materials is not likely to be of great utility with the present state of the art.
REFERENCES 1 H.S. Gutowsky and A. Saika, Dissociation, Chemical Exchange, and the Proton Magnetic Resonance in Some Aqueous Electrolytes, J. Chem. Phys. 21, (1953) 1688-1694. 2 R.J.S. Brown, Measurements of Nuclear Spin Relaxation of Fluids in Bulk and for Large Surface-to-Volume Ratios, Bul. Am. Phys. SOC.,Ser. 11, 1, (1956) 216; S.D. Senturia and J.D. Robinson, Nuclear Spin-Lattice Relaxation of Liquids Confined in Porous Solids, SOC. Pet. Eng. J., lo,( 1970) 237-244. 3 F. D'Orazio, J.C. Tarczon, W.P. Halperin, K. Eguchi, and T. Mizusaki, Application of Nuclear Magnetic Resonance Pore Structure Analysis to Porous Silica Glass, J. Appl. Phys., 65, (1989) 742-751.
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6
7
8 9 10 11 12 13 14
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D.M. Smith and D.P. Gallegos, Analysis of Pore Structure via Spin Lattice Relaxation Measurements, in K.K. Unger, J. Rouquerol, K.S.W. Sing, H. &al,@ds.), Characterization of Porous Solids, Elsevier Press, Amsterdam, (1988) pp. 391-400; D.P. Gallegos, K. Munn, D.M. Smith, and D.L. Stermer, A NMR Technique for the Analysis of Pore Structure: Application to Materials with Well-Defined Pore Structure, J. Colloid Interface Sci., 119, (1987) 127-139. S. Davies, M.Z. Kalam, K.J. Packer, and F.O. Zelaya, Pore-Size Distributions from NMR Spin Lattice Relaxation Data, Poster presented at the 31st Experimental NMR Conference, Asilomar, Calif. April (1990). W.A. Ellingson, J.L. Ackerman, L. Ganido, J.D. Weyand, and R.A. DiMilia, Characterization of Porosity in Green-State and Partially Densified A1203 by Nuclear Magnetic Resonance Imaging, Ceram. Eng. Sci Proc., 8, (1987) 503-512. L. Garrido, J.L. Ackerman, W.A. Ellingson, and J.D. Weyand, Determination of Porosity and Polymeric Binder Maps in Ceramics by NMR Imaging, Polymer Preprints, 28, (1988) 97. W.A. Ellingson, P.S. Wong, S.L. Dieckman, J.L. Ackerman, and L.Gamdo, Magnetic Resonance Imaging: A New Characterization Technique for Advanced Ceramics, Ceram. Bull., 68, (1989) 1180-1186. K. Hayashi, K. Kawashima, K. Kose, and T. Inouye, NMR Imaging of Advanced Ceramics During the Slip Casting Process, J. Phys. D: Appl. Phys., 21 (1988) 1037-1039. C. Chang and R.A. Komoroski, NMR Imaging of Elastomeric Materials, Macromolecules, 22, (1989) 600-607. J.M. Listerud, S.W. Sinton, and G.P. Drobny, NMR Imaging of Materials, Anal. Chem., 61, (1989) 23A-41A. A. Kumar, D. Welti, and R.R. Ernst, NMR Fourier Zeugmatography, J. Magn. Reson. 18, (1975) 69-83. P.C. Lauterbur, Image Formation by Induced Local Interactions: Examples Employing Nuclear Magnetic Resonance, Nature, London, 242, (1973) 190-191. D.A. Chesler and S.J. Riederer, Ripple Suppression During Reconstruction in Transverse Tomography, Phys. Med. Biol., 20, (1975) 632-636. C.J. Brinker, K.D. Keefer, D.W. Schaefer, and C.S. Ashley, Sol-Gel Transition in Simple Silicates, J. Non-Cryst. Solids, 48,(1982) 47-64.
F. Rodriguez-Reinoso et al. (Editors), Characterization of Porous Solids II 0 1991 Elsevier Science Publishers B.V., Amsterdam
717
THE INFLUENCE OF POROUS STRUCTURE AND EXTERNAL MORPHOLOGY ON THE ACTIVITY OF CATALYST SPHERES PREPARED BY THE SOL-GEL METHOD
A.Q.M. BOON1, C.J.G. van der GRTFT1, A.J.W. van VELDHUIZENZ and J.W. GEUS2 1 present address: Koninklijke/Shell Laboratorium, P.O. Box 3003, 1003 AA Amsterdam (The Netherlands) 2 Department of Inorganic Chemistry, University of Utrecht, Croesestraat 77a, 3522 AD Utrecht (The Netherlands)
SUMMARY In order to assess the effect of orous structure on the mass trarlsport within catalytically active bodies, the activit oFcata1yst spheres repared by the sol-gel method was measured for the oxidation olcarbon monoxide. &e pore size of the spheres was increased b means of a hydrothermal treatment. The treatment did not affect the intrinsic activity orthe catalyst. At low temperatures, the catalytic activity did not vary with the pore size. At increasing temperatures, diffusion in the narrow res of the spheres could not keep up with the catalytic reaction. Consequently, at hig er temperatures the reaction was confined to the outermost layer of the spheres, where the roughness of the external surface of the catalyst bodies controlled the rate of reaction. The effectiveness of the catalysts is thus controlled not on1 by the pore system, but also by the morphology of the external surface of the catalyst d e s .
r
INTRODUCTION The performance of heterogeneous catalysts not only depends on the reactivity of the active surface, but also on the heat and mass transport to and from this surface. Transport limitations are present within the stationary boundary layer surrounding the catalyst bodies and within the pores of the bodies. Usually, the porous solid consists mainly of a thermostable and catalytically inert support material, such as silica or alumina. The reaction proceeds on the surface of a catalytically active compound that has been deposited on the pore walls of the support. The active material usually is a metal, a metal oxide, or a metal sulfide, and is applied, for example, by impregnation of a solution of the metal, or by ion exchange of metal ions with the hydroxyl groups present at the surface of the pore walls. By calcination or reduction the active component is obtained. The Arrhenius law describes the dependence of the rate of the catalytic reaction on the temperature: rate = ko e-Ea/RT For most catalytic reactions, the activation energy is 50-150 kJ/mol. The temperature dependence of gas-phase diffusion is given by:
718
Deff=aTP with p= 0.5-1.5 Both continuum and Knudsen diffusion are not activated. However, a virtual activation energy of diffusion can be estimated by fitting the temperature dependence of diffusion to the Arrhenius law. This procedure leads to a virtual activation energy of 5-17 kJ/mol. At low temperatures, the rate of reaction is smaller than the rate of transport. The activity hence is controlled by the rate of reaction. With increasing temperatures, the rate of r e action increases more rapidly than the rate of transport. At a certain temperature, the rate of transport will start to limit the activity. The first transport process that adversely affects the activity is usually diffusion within the narrow pores of the catalyst. Due to this transport limitation, the catalyst will exhibit at the same temperature a lower activity than a catalyst the activity of which is not influencedby pore diffusion.
In the past, many models describing the influence of the porous structure of catalyst bodies on the activity have been presented. Pioneering in this field were Thiele ill, 2 1 dowitsch [2] and Jiittner 131, and some important extensions are by Wheeler [41 and A r i s 151.The models are based on the effective diffusivityof the reacting gases in the pore network. The effective ddfusivity is derived from the porous structure. Several approaches are possible, such as the dusty gas model, mixed Knudsen-continuum models, and others h7.l. In the present study, the influence of the pore size on the rate of mass transfer is investigated. Catalyst bodies were prepared with a bimodal pore size distribution, and subsequently the size of the wider pores was increased systematically 191. This was accomplished by incorporation of small particles (1040mm) of a highly active catalyst with very narrow pores (1-4 nm) into a silica gel matrix. A hydrothermal treatment raised the size of the pores in the gel matrix. The radii of the narrow pores of the catalyst, the pore volume, and the particle diameter were not affected by the hydrothermal treatment. Thus, all catalysts exhibited the same intrinsic activity. The porous structure of the catalysts was characterized by mercury penetration and nitrogen physisorption, and the morphology of the surface of the catalyst spheres was assessed by scanning electron micrcscopy (SEM). The oxidation of carbon monoxide [10,111 was used to investigatethe effects of transport limitations on the activity. The oxidation activity of the catalysts was correlated with the textural data. METHODS A 20 wt.% coppersn-silica catalyst with pores of 1-4 nm was prepared by homogeneous deposition-precipitation [12]. The hydrolysis of urea was used to raise the pH of a vigorously stirred suspension of the silica support (200VAerosil, Degussa) in an aqueous
719
solution of copper nitrate (Merck p.a.1. After complete precipitation, the catalyst precursor was washed extensively and concentrated to 0.11 g catalyst per g of suspension. The silica sol was prepared by addition of a 3.5 M sodium silicate solution to 40 ml of 2.5 M nitric acid at 273 K until a pH of 1 was reached. At this pH level, 30 g of the catalyst suspension was mixed with the silica sol. After homogenization the pH was carefully increased to 5 by further addition of the sodium silicate solution. Gelation of the mixture was accomplished by spraying the suspension as small droplets, 1-4 mm in diameter, into hot paraffin (353 K).After washing, the spheres were suspended in a 0.5 M sodium chloride solution and hydrothermally treated for 4,7, and 24 hours at 473 K in an autoclave 191. After extensive washing the spheres were dried at 393 K in air. The catalysts were denoted H4, H7 and H24, correspondingto the time of hydrothermal treatment. For comparison purposes, two catalysts obtained from the Engelhard Corporation were used. Both catalysts were prepared using ion exchange of copper tetramine nitrate at pH=lO.Carrier materials used were preshaped silica spheres with nmow and wide pores (Shell NP-S980-A1.5, WP-S980-G1.7). After the preparation the samples were characterized by scanning electron microscopy (Cambridge 150S), nitrogen adsorption (Omnisorb 100, Omicron), and mercury penetration (Porosimeter 2000, Carlo Erba Strumentazione). With each catalyst sample, COoxidation experiments were carried out on approx. 50 mg of smooth, unbroken spheres with a diameter of 1.40-1.50mm. Prior to the activity measurement the catalyst was calcined for 2 h at 723 K in a flow of 10 vol.% 02/N2, reduced for 2 h at 723 K in a flow of 10 vol.% H2/N2, and reoxidized for 2 h at 723 K in a flow of 10 vol.% 02/N2. At room temperature, a gas flow of 400 ml/min (0.25 vol.% CO, 1 vol.% 02, 98.75 vol.% N2) was passed through the reoxidized catalyst. Subsequently, the catalytic oxidation was measured as a function of the temperature (350-850 K). Details of the equipment used for these measurements are given elsewhere [111. The conversion-vs.-temperatureplots were linearized into Arrhenius plots. The slope of the Arrhenius curve gives the activation energy. RESULTS A scanning electron micrograph of a silica sphere is shown in Fig. 1. The sphere had an extensive surface roughness that ranges over several tens of micrometers. Also, some fissures and pits were present. The porous structure of the spheres is summarized in Table 1. Hydrothermal treatment resulted in an increase of the pore size, whereas the pore volume remained fairly constant. Mercury penetration of intact and crushed spheres gave identical plots, indicating that the pore size distribution over the cross-section of the spheres was uniform. In Fig. 2a and 2b conversion curves and Arrhenius plots are given for the hydrothermally treated samples. All curves were measured two to four times, and, after an initial small deactivation, could be reproduced excellently. The results presented,here are those of the second experiment.
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TABLE 1 porous structure of catalysts code hydrothermal treatment (h)
H4
4
w
7
H24 NP WP
24
pore radius (nm) N2-sorption Hg-penetration 12 9 10-20 15 10-30 20-25 7 8 22 25
pore volume (ml/ g) 1.1 1.2 1.0 1.0 0.9
At low temperatures, where mass transport limitation is absent, all samples showed the exponential rise of the activity with temperature associated with the Arrhenius behavior of the catalytic reaction. The activities of the hydrothermally treated samples at low temperatures were equal, indicating that the hydrothermal treatment had not affected the active surface of the copper-on-silica catalyst incorporated in the silica matrix. The activation energy was about 95 kJ/mol. Above 400 K mass transport limited the activity. The activation energy in the straight part of the curve (500-800 K)was about 6.0 kJ/mol.
Fig. 1. Scanning electron micrograph of the external surface morphology of a hydrothermally treated catalyst sphere.
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1.o
0.8
.-
0)
f
0.6
w
c
g
0.4 0.2
0.0 350
Fig. 2. Activity of hydrothermally treated catalyst spheres in the oxidation of CO. a (upper): conversion plot, b (lower): Arrhenius plot H4-H24 refer to duration of hydrothermal treatment Measured Arrhenius Plot
1.00e-3
2.00e-3
3.00e-3
l/T (1/K)
The activities and Arrhenius plots of the ionexchanged spheres are represented in Fig.3a and 3b. For both catalysts, the intrinsic activity at low temperatures was equal. The activation energy was 74 kJ/mol. At higher temperatures (400 K) a limitation in activity also was present. The activation energy between 450 and 550 K was about 8.5 kJ/mol. However, in contrast to the hydrothermally treated samples, the activation energy increased again at temperatures above 550 K. At these temperatures, the activity was not stable, as the curves measured at decreasing temperatures were considerably below the curves measured at increasing temperatures. Due to this deactivation, it was not possible
122
to determine an activation energy above 550 K. With higher carbon monoxide concentrations and higher gas velocities, keeping the contact time of the gas with the catalyst constant, exactly identical curves to those shown were measured. As a higher linear gas flow rate did not result in an increase of the activity, the mass transfer limitation should be present only within the catalyst body. This was confirmed by an evaluation of the criteria of Mears [13] and Anderson [14]for mass and heat transfer limitation both within the catalyst body and from the bulk of the gas flow to the external surface of the body. CO-oxidation 1.0-
0.8-
NP
A
wp
Fig. 3. Activity of ionexchanged catalyst spheres in the catalytic oxidation of CO. a (upper):conversion plot, b (lower):Arrhenius plot NI? narrow-pore spheres, WP: widepore spheres
CO-oxidation
1
!
I
0.001
0.002
-5
1lT (1/K)
0.003
723
However, Schliinder 1151 and others [16,171 have shown that at low Reynolds numbers the apparent stagnant layer around the catalyst particles is much thicker than predicted by the criteria of Mears and Anderson. They attributed the experimentally observed lower rate of mass transport from the gas phase to the catalyst body to the imperfect stacking of particles in the catalyst bed, so that most of the gas flows through the larger voids. This results in an effectively smaller contact time of the gas with the catalyst, and hence a lower activity. With the low Reynolds numbers of our experiments (approx. 6) we can expect external mass transport limitation by this mechanism. DISCUSSION In the case of internal mass transfer limitation, Wheeler has derived that the apparent activation energy of the catalytic reaction should become the mean of the activation energies of the reaction and diffusion 131. The reaction has an activation energy of 94 kJ/mol, and the temperature dependence of diffusion can be described by a virtual activation energy of 5-17 kJ/mol. In the case of internal mass transfer limitation, an activation energy of about 50 kJ/mol can thus be expected. However, the behavior of the catalysts measured in this work is different. At temperatures above 400 K the catalytic activity becomes limited, in agreement with the Thiele theory. However, the apparent activation energy gradually decreases from 94 to 6 kJ/mol, rather than to 50 kJ/mol, which implies that the apparent activation energy of diffusion is exhibited. Nevertheless, the size of the wider pores in the pellet does appear to affect strongly the activity. Therefore, it is impossible that merely external diffusion limitation, that is, diffusion from the bulk of the gas flow to the external surface of the catalyst body, is rate-determining. Since the catalyst spheres had the same diameter, the activity of all catalysts should be equal if external transport is determining the activity. As the concentration of reactants inside the particle is nearly zero, the pore size should be of no importance. However, this is in contradiction with the measurements. The cause for the experimentally displayed behavior of the catalysts becomes clear if one calculates the concentration profile in a catalyst sphere limited by internal diffusion [8]. In Fig. 4 the concentration of carbon monoxide is plotted over a H4 catalyst particle. The curves of Fig. 4 correspond to the points A-D in Fig. 2b. At point A, the concentration profile is rather shallow, and the activation energy (the slope of the Arrhenius curve) is high. In this case, the Thiele-Zeldowitsch theory still holds. At inaeasing temperatures, the concentration profiles become steeper, and the reaction only proceeds in the outermost layer of the catalyst body. At the points C and D, the activation energy is much smaller than predicted by the Thiele-Zeldowitschtheory. In this case only a layer at the external edge contributes to the activity. However, the layer is not smooth but rough, as can be seen in fig. 1. The voids caused by the surface roughness and the macropores are responsible for most of the transport of the reactants. It can be envisioned that the geometrical surface area of the particle has been enlarged. Over the enlarged surface a stagnant layer of gas is present, through which diffusion takes place. The stagnant layer will
124
hardly be influenced by the low gas velocity, as the surface roughness will retain the gas from flowing. Thus, the rough external surface of the catalyst sphere has two effects: firstly the interior of the sphere is made better accessible, and secondly the stagnant layer around the sphere is extended. Schliinder attributed the extension of the stagnant layer to an imperfect flow through the bed of catalyst particles at low gas velocities [15]. However, the thickness of the stagnant layer is dependent on the hydrothermal treatment. This can only be explained by the surface roughness, as the size and shape for all samples and, hence, their stacking in a catalyst bed is the same.
1.oo
0.75
P I Po 0.50 0.25 0.00 0.00
0.25
0.50
0.75
1 .oo
RIRo Fig. 4. CO concentration profiles in a mass-transport limited catalyst sphere. A-D refer to points in plot 2b. The increase in activity of the ion-exchanged spheres at high temperatures is attributed to another effect. As the temperature rises further, the reaction front will withdraw to the extreme edge of the catalyst. At very high temperatures, only some isolated peaks at the external surface will be active, provided catalytically active material is exposed directly to the gas stream. With the catalyst incorporated into a silica gel matrix, this increase of activity has not been observed, as the copper catalyst particles will always be covered by silica. With ion exchange, however, the silica support will be covered with small copper oxide particles. If the temperature becomes excessively high, the activity of the very small amount of copper oxide present at isolated peaks will be seen. The small amount of catalyst will be exposed directly to the flowing gas, so diffusion is not limiting the rate of reaction. Effectively, the thickness of the stagnant layer is zero. Normally, the activity of the catalyticallyactive surfacepresent at the peaks will not be meamred, as the contribution of these peaks to the activity of the whole body is negligibly small. However, at very high temperatures the activity of these peaks will be much higher than the activity of the re-
725
maining part of the catalyst body, that is hindered by diffusion. Therefore, the activation energy of the catalytic reaction itself is exhibited again. The activity will not be stable, as the heat liberated by the very high rate of reaction will cause the catalyst at the isolated peaks to sinter rapidly due to the local rise in temperature. CONCLUSIONS The Thiele-Zeldowitsch model presented in the literature to describe the limitation of the catalytic activity by internal mass transport does not take into account the surface roughness. At temperatures higher than those where the reaction is starting to be transport-limited, the reaction front withdraws to the outermost layer of the catalyst body, viz., to a layer of a thickness of 10-100 mm. This layer is rough, and therefore very well accessible from the gas phase. The activity of the catalyst is consequently best described as controlled by external diffusion. Thus, at low gas velocities and high reaction rates, the catalytic activity is controlled by the microscopic surface roughness of the catalyst particles. ACKNOWLEDGEMENTS The authors would like to express there sincere thanks to Mr. van Wingerden and Mr. van der Wal of VEG-Gasinstituut for helpful comments, and to Mr. Elberse for the porous structure determination. This work was supported by VEG-GASINSTITULJT, NOVEM and the European Community N N E project.
REFERENCES 1 E.W. Thiele, Ind. Eng. Chem.,31(1939)916 2 T.B. Zeldowitsch, Acta. Phvsicochim. USSR 10(1938)583 3 F. Jiittner, Z.ph .s. Chem,&5(1909)595 4 A. Wheeler, in J . G . Frankenburg, V.I. Komarewsky, and E.K Rideal (Eds.), Advances in Catal sis, Vo1.3, p 249,1951 5 R his, Mathematical Theory of Diffusion and Reaction in Permeable Catalysts, Clarendon Press, Oxford, 1975 6 C.N. Satterfield, Mass Transfer in HeterogeneousCatal sis, M.I.T. Press, 1970 7 R. Jackson, Trans ort in Porous Catalysts, in Chemical ngineering Monographs, Vo1.4, Elsevier, If77 8 P.B. Weisz and J.S. Hicks, Chem. Eng. Sa., 17(1%2)265 9 C.J.G. van der Grift, P.A. Elberse, J.W. Geus, J.F Quinson and M. Brun, 3rd International Conference on Fundamentals of Ads0 tion, Sonthofen FRG,1989, in press 10 J. van der Meijden, dissertation, University of gecht, 1981 11 I.I.M. Tijburg, dissertation, University of Utrecht, 1989, 12 J.W. Geus, in G. Poncelet, P. Gran e and P.A. Jacobs (E s.), Preparation of Catalysts III, Elsevier, Amsterdam, 1983, p.? 13 D.E. Mears, Ind. Eng. Chem. Process. Res. Develop., 10(1971)541 14 J.B. Anderson, Chem. Eng. Sci., 18(1961)147 15 E.U. Schliinder, Chem. Eng. Sci. 32(1977)845 16 P.A. Nelson and T.R. Galloway, Chem. Eng. Sci. 30(1975)1 17 P.N. Rowe, Chem. Eng. Sa. 30(1975)7
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F. Rodriguez-Reinosoet al. (Editors), Characterization of Porous Solids ZZ 0 1991 Elsevier Science Publishers B.V., Amsterdam
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CHARACTERIZATION OF POROSITY AND PORE QUALITY IN SEDIMENTARY ROCKS
M. E. Cather, N. R. Morrow, and I. Klich Petroleum Recovery Research Center, New Mexico Institute of Mining and Technology Socorro, New Mexico 87801 ABSTRACT Porosity in sedimentary rocks is commonly measured by point counting petrographic thin sections. Pore space is often formed by pores of much smaller dimensions than the 30 pm thickness of the thin section. This can lead to severe inconsistencies in identification of pore space because of a wide range of overlap between pore space and the mineral framework, and because of partial pore occlusion by very fine-grained material. Much improved images of pore space are provided by thin sections that are surface-stained with fluorescent dye and observed under reflected fluorescent light. The observed image more closely approaches the ideal of a two-dimensional mathematical slice, and fine details of pore structure that are not visible using standard transmitted light techniques are revealed. Allowance for partially occluded pores gives much closer agreement between laboratory measured porosities and those determined from thin section. The modified point count permits an index of pore quality, a measure of the overall degree of occlusion of pore space by fine material to be defined.
INTRODUCTION Petrographic thin-section analysis is a commonly used method for evaluating porosity in hydrocarbon-bearing rocks. Thin sections are prepared by first impregnating the rock with a bluedyed epoxy resin which fills pore spaces, and then grinding and polishing a slice of this rock to a standard thickness of
- 30 pm.
Under the petrographic microscope, porosity is identified by its blue
coloration. For porosity estimation, the thin section is examined and a certain number of points are counted on a grid pattern to provide a statistical sampling. Each point that falls under the microscope crosshairs is identified as either a pore space or solid.
For a set of low-permeability sandstones from the Mesaverde Group of Colorado, porosities determined by petrographic methods (dp) were sometimes quite different from those determined by laboratory core analysis techniques. Laboratory porosities (4,) were measured by comparing the volume and weight of a sample at complete saturation with that of the sample when dried to a constant weight at either 110°C and or 25°C and 45% relative humidity. The volume of water removed (void volume) was determined by the sample weight loss, and porosity was determined by the ratio of the void volume to the total volume. Two sets of point count measurements made for very similar sample sets were found to differ significantly, both from each other and from the more reliable 4, values (Fig. 1). Porosities determined by point counts of low-permeability gas sandstones were thus found to be highly dependent on the individual petrographer and the quality of the thin sections. The sometimes large discrepancies demonstrated the need for improved means of estimating porosity from thin sections. Two changes in our porosity analysis methods helped to lessen the differences between
dp and dg.
The first approach used epifluorescent microscopy to improve
visualization of microporosity and partially occluded pore space, and the second involved a change in counting technique to account for the presence of such pore spaces.
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8
20
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.-
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2 0 a .-.a5 -
IS
10
0
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.C
S
ir
BFEC 1 8 5
10
15
0
0
q,Gravimetric Porosity (%)
5
10
1s
20
Q, Gravimetric Porosity (%)
Fig. 1. Comparison of porosities from thin section (#p) and core analysis (dg) for similar sample sets. dp determined by a different petrographer for each set.
For the current study, thin sections from several different sources were examined. Multiwell
(MWX) samples are rocks representative of various depositional environments within the Mesaverde Group, and were obtained from the U.S.Department of Energy's Multiwell Experimental Site near Rangely, Colorado. These are all very low permeability sandstones, with Klinkenberg permeabilities ranging from 0.07
- 46 ~KI at 500 psi nominal confining pressure.
The second sample set, acquired
from the U.S.Geological Survey through the courtesy of W. Keighen, included fluvial and marine sandstones from the Almond Formation in Wyoming and the Dakota Formation in Colorado. Permeabilities of these samples ranged from 0.025
- 55
md. Thin sections of Berea (Illinois) and
Fontainebleau (France) sandstones, San Andres dolomite (Texas), and a bead pack were also examined. Available porosity and permeability data for some of the samples are included in Table 1; details of other data (35 MWX samples) are reported by Morrow et a1.l PROBLEMS OF THIN-SECTION POROSITY ESTIMATION Several factors were identified in the standard point-counting method that may reduce the accuracy of thin-section porosity estimates. 1. SamDle meoaration Some sandstones have such low permeabilities that achieving good impregnation of rocks is
729
difficult; as impregnation limits are approached, the dyes in the epoxy may be filtered out or adsorbed and dye colors are less intense. identification.
Coloration in smaller pores may be insufficient for porosity
TABLE 1 Porosity and permeability data for selected samples Sample ID Berea A Berea C C087WK44 CO87WK46 San Andres 5184 San Andres 5347 T-I1 9495 T-I1 9884.5 FB1
FB8 WY85WK16 WY85WK8 WY85WK9 wY86wK35 WY86WK38 wY86wK48 WY86WK49 WY86WK51 WY86wK.52 wY86wK9
19.13 21.17 6.89 4.78 24.29 13.60 2.89 9.41 5.7 15.7 10.99 6.97 8.01 2.10 16.59 23.09 14.14 10.90 11.60 5.64
19.05 20.9 8.1 1.9
0.77 0.80 0.49 0.33 0.89 0.96 0.33 0.58 .98
2.3 10.1
.%
0.41 0.39 0.39 0.35 0.62 0.63 0.50 0.39 0.49 0.34
12.4 9.6 6.2 1.1 21.2 22.3 20.1 21.2 17.2 6.2
100 1330 0.0339 0.0084 0.0617 0.0742 1.12 442.9 0.354 0.118 0.073 0.026 42.4 55.3 11.7 5.05 18.1 0.054
2. Grain overlao
In rocks where pore diameter is similar to or larger than the 30-pm thickness of the standard thin section, pore spaces are easily recognized and their boundaries appear well-defined. As grain and pore size diminishes relative to thin-section thickness, however, discrimination between pores and grains becomes increasingly difficult. Smaller pores will be overlain or underlain by grains (Fig. 2) to the extent that it becomes difficult to differentiate pores from grains.
glass beads; 100pm diameter
/
Fig. 2. Schematic cross section through a thin section of a bead pack. The thin section is 30 pm thick and the bead diameter is about 100 pm (equivalent to very fine sand). 3. Microoorosity Limitations in microscope resolving power also cause variations between laboratory and optical porosity measurements. Figure 3 shows water desorption isotherms for several M W X cores, along with relationships between relative humidity, capillary pressure, and pore size.’ Twenty to thirty
730
percent of the total pore volume in these samples is meso- and micropores (
The resolution limit for
a Nikon Optiphot microscope with a 40X objective is about 0.36 pm, so much of the microporosity in some samples is not visible or may be visible but difficult to resolve by optical microscopy. Even in coarser sandstones with large pores, there are many grains such as chert and partially dissolved lithic fragments that may contain significant amounts of microporosity not resolvable in transmitted light. 4. Observer variation
Subjective judgement is involved in assigning pore volume to areas where blue coloration is identified, but there is obviously some degree of pore occlusion. Some petrographers will ignore porosity in areas that are largely clay-filled, even though blue dye penetrates the region, while others may assign porosity to that same region.
I
lCENTRlFUGE LIMIT~J CRACK THICKNESS lnml
01
PORE RADIUS
!
Fig. 3. Water absorption-desorption isotherms for several MWX samples, showing the relationships between relative humidity, pore size, and capillary pressure.”
0 z
t
a
I-
Y)
U ” Y
Y)
Y LL 0
R E L A T I V E HUMIDITY
(-6)
Although some of these problems will never be completely solved, two alterations in the standard point-counting method gave improved results for dP and also provided additional information about rock pore structure not available from standard microscopic or laboratory measurements of porosity.
731
IMPROVEMENT IN POROSITY MEASUREMENT BY PETROGRAPHY Fluorescent Lieht Microscooy One of the major problems in estimating b,, in low-permeability sandstones is the difficulty in porosity identification. Most porosity in these sandstones is not apparent in transmitted light, so epifluorescent microscopy was used to improve pore space recognition. Gies:
and Yanguas and Dravis7 reported methods of porosity examination utilizing fluorescent
epoxies injected into the rock pore space. Although these methods proved to be extremely useful in illuminating pore structure in sandstones, grain/pore overlap and internal reflections from within the thin section still can cause difficulties in delineation of pore boundaries. Ruyzla and Jezek' described a simple method for surface-staining thin sections with a fluorescent dye. A film of dye is spread onto the surface of a clean, polished thin section. The dye adheres to epoxy-filled pores and, after an appropriate amount of time (determined by the type of epoxy and the desired brightness), the excess dye is removed by washing the thin section with alcohol. The stain is absorbed by most epoxies and, unlike previous fluorescence techniques, may be applied to preexisting thin sections. Porosity images produced by epifluorescent microscopy of surface-stained thin sections are superior in several respects to those obtained by dying the epoxy in bulk with visible or fluorescent pigments. Penetration of epoxy-filled pores is shallow, ranging from about I pm for a five-second exposure time to more than 15 pm for times over two minutes; therefore, problems of grain/pore overlap are minimized (Fig. 4). The strong contrast between fluorescing epoxy and nonfluorescing mineral grains makes porosity highly visible. The technique can be applied to impregnated thick sections and core plugs as well as standard thin sections. The process utilizes reflected, rather than transmitted, light, so intragranular porosity may be identified in opaque grains and organic matter.
Fig. 4. Photomicrographs of a thin section of a bead pack impregnated with red (lighter gray) and blue (darker gray) epoxy to simulate two fluid phases9 (a) Transmitted light. Arrows point to ambiguous beads which may be thin slices on the surface of the thin section underlain by pore space or else beads which do not quite intersect the surface plane of the thin section. (b) Epifluorescent light, same view as (a). The brightest areas are beads plucked from the thin section during its preparation. Blue epoxy fluoresces more brightly than red, and beads are black. Arrows point to same beads as in (a), showing the resolution of the ambiguous cases. Field of view is 2.6 mm.
732
Using epifluorescent microscopy, fine pore structures, such as pore-filling and pore-lining clay minerals, are seen in great detail. The presence of submicron-sized pores is revealed in areas that fluoresce less brightly than open pores. Sheet-like pores at grain boundaries are clearly visible. The average thickness of sheet pores in the MWX samples is about 0.2 pm at 500 psi confining pressure.' Although the limit of resolution for the combination of lenses and filters in the epifluorescent microscope is approximately .34 fim, structures smaller than this are visible, probably because they are fluorescent and emit visible light with wavelengths greater than the size of the pore. This can cause fine structures such as very narrow sheet pores" to appear larger than their true size. The Weiehted Point Countinn Method The amount of pore space in partially occluded pores and regions of microporosity revealed by fluorescence microscopy demonstrated the need to include these types of porosity in determinations of
dp. A method for estimating porosity was therefore tested wherein the degree of occlusion was
estimated for ambiguous points of partial porosity which, in the traditional method of point counting, are considered to be either pores or grains. In the new method, the degree of porosity occlusion for each point observed under the crosshairs is estimated, then the point is assigned to one of five categories based on the estimated degree of occlusion. Nonporous mineral grains are assigned a value of zero; discrete open pores are assigned a value of 100%. Areas of partly occluded porosity are assigned intermediate values in 25% increments. At the conclusion of the point count, points in each category are summed and multiplied by the value of the category. A sample calculation is given below
SAMPLE CALCULATION FOR MWXl 14-24B % Pore Space
#
of Counts 351 31 10
3 100%
1 400
If only the most obviously visible porosity
3511400 = .8775xO 311400 = .0775 x 25 101400 = .025 x 50 31400 = .0075 x 75 5/400 = 0.0125 x 100 Total Porosity
- the 75% to
100% open pores
=
0
= 1.94 = 1.25
= 0.51 = 5.0%
- were counted as pore
space, then estimated porosity could be as low as 1.8%. On the other hand, if all visible pore space was counted as porosity with no weighting, the porosity estimate for the same sample would be as high as 12.25%. Although still subjective, this method of point counting improved the accuracy of measurements of 4, (Fig. 5). Reasonable agreement was seen among counts made by a number of observers given only minimal instruction in point-counting techniques.
The agreement is quite good. importantly, it is now fairly evenly distributed above and below the 45" line.
More
733
Fig. 5 . #p vs. q5F for all samples. Porosity was identified using epifluorescent microscopy.
0
5
10
15
20
25
Iz$, Gravimetric Porosity (%) PORE QUALITY EVALUATION Rock porosity is made up of pore spaces that vary greatly in their degree of occlusion. In a mature sandstone containing little detrital or authigenic clay, such as the Berea sandstone, or in vuggy porosity in limestone, completely open pores may comprise as much as 85-100% of overall porosity.
In less mature sandstones such as the Mesaverde, open pores might contribute only 15 to 25% of overall porosity, and most porosity occurs in the highly occluded pore spaces. Because pore-space occlusion is an important factor governing reservoir quality, a quantitative method of characterization of this property should be of value. Estimates of the degree of pore occlusion made during point counting provide a useful approach to this problem. After point counting and apportioning each point to one of four categories (25, 50, 75, or 100% open pore space), the total number of points in each category is then multiplied by the
percent occlusion to determine "pore equivalents" for each category. Pore equivalents are summed and then divided by the total number of points for which some degree of porosity was observed. A demonstration of this technique is provided for MWXl 34-24B.
I
PORE QUALITY CALCULATION Space Counts
31 x.25= 10 x . 5 =
3 x .I5 = 100% Total
5x1
=
i 20.00
49
Pore Quality (Q6) = 20/49
2.25
=
.41
734
In this example, the 100% open porosity contributes only about 25% to the total porosity of the rock; 25% and 50% open pore spaces comprise over half of the total rock porosity. For the proposed scale, the pore quality (Qb) of a porous rock can vary from 0.25 (all pores being highly occluded) to 1.0 (a rock with no occluded pore spaces). Pore qualities for a variety of sandstones were determined by this method. The MWX samples examined in this study have extremely low QQ values ranging from 0.26 to 0.46. Sandstones from the Dakota and Almond Formations are of better quality, ranging from
0.3to 0.6. At the upper end of the spectrum, thin sections of Berea Sandstone had QQ of 0.77- 0.8, and samples from the San Andres dolomite had Qb of 0.89 and 0.96 (Table 1). Although the limestone contained very little clay, some of the intergranular pore spaces were so small that it could not be determined whether the crosshair was on a solid or a pore, so the overall porosity of the immediately surrounding area was estimated, much as estimates were made for microporous areas in sandstones. Figure 6 shows the distribution of points counted and estimated porosity for the MWX sample discussed above and also a sample of Berea sandstone.
MESAVERDE SANDSTONE Mwx114-24B
m-
#=S% 0, = .41
W W A
4 = 19.1% Q.
=.77
w-
a-
m -
polNTsoBsERveD
MsTRltwTloNoF PORE SPACE
O L
DlSTRlBLmONC PORE SPACE
Fig. 6. Distribution of porosity for a) MWXl 14-24b and b) Berea A. Porosity is shown in the lefthand histogram as the distribution of porosity points observed and on the right as the distribution of porosity after weighting by degree of occlusion. Weighting causes unoccluded pores to take on greater importance in overall sample porosity. It is clear from these two diagrams that the Berea sample has much better pore quality than the MWX sample. In the MWX samples, pore quality is closely related to the amount of material containing abundant microporosity, especially dissolved lithic fragments, partially altered feldspars, and clays. Pore quality also correlates fairly well with surface area from BET measurements (Fig. 7a). For all samples examined, pore quality correlated rather well with porosity and with permeability (Figs. 7b and c). It is interesting to note that permeabilities actually correlated better with bp than they do with These results suggest that 0, may provide a somewhat more useful indicator of sample permeability than dr
br
735
0.80
1
o.2 r = .448
0.00 0
100
50
150
200
:
i0
Surface Area (m2/cc) 1.oo
.EOBO
9 0.60 0 E
0.40
0 0.20 r = 0.847
0.00
5
10
IS
20
:
0gGravimetric Porosity (%)
100 101
1 0 - ~ 1 0 ~ 1 10-210-1 0-~
102
id 104
Permeability (md)
Fig. 7. Pore quality plotted as a function of various parameters. a) Pore quality vs. surface area/cc of pore (measured by single point BET technique) for M W X samples. b) Pore quality vs. porosity (4,) for all samples. c) Pore quality vs. permeability for all samples. Porosity, permeability, and pore quality are all interrelated parameters, but the relationships are complex and also influenced by other rock properties. Porosity in uncemented sands is primarily a function of grain shape, sorting, and compaction; diagenetic processes such as compaction, cementation, and dissolution can drastically change initial post-depositional porosity. Permeability is related to grain size, sorting, porosity, grain orientation, compaction, cementation, distribution of pore space, and general pore structure:'
In general, sandstones with the same porosities but coarser
grain size tend to have higher permeabilities.'2*'s point. They have very similar values of
The two samples from the Berea illustrate this
4 and Qb;however, their permeabilities differ by an order
of magnitude. This is quantitatively consistent with the smaller average grain size of the low-
permeability sample. The pore quality concept provides a method of quantifying the degree of pore occlusion in a rock, but in rocks where most porosity is not occluded, its usefulness as a predictor of permeability
736
is obviously limited. This is demonstrated by the Fontainebleau and San Andres samples. All of these samples have very high pore qualities (0.89- 0.98); however, only one, FB-8, has significant permeabiilty. For these rocks, pore quality determined from microscope analysis does not correlate well with either
4, or permeability, and permeability appears to be more dependent on the amount
and interconnectivity of porosity, rather than its quality. CONCLUDING REMARKS Improved estimates of thin-section porosity and a quantitative measure of pore space quality can be obtained by the methods described in this paper.
Accurate measurement of porosity and
interrelated parameters such as pore size, geometry, distribution, quality, and interconnectivity will be useful aids in assessing the production potential of hydrocarbon-bearing formations. Pore quality and identification of microporosity and its distribution by epifluorescence microscopy are also likely to be of value in interpretation of formation resistivity measurements, connate water retention in the reservoir, and capillary pressure behavior. REFERENCES 1.
2. 3. 4.
5.
6.
I. 8. 9. 10. 11.
12. 13.
Morrow, N.R., Buckley, J.S., Cather, S.M., and Brower, K.R.: "Rock Matrix and Fracture Analysis of Flow in Western Tight Gas Sands," final report (Feb. 1990), New Mexico Research and Development Institute Project No. 2-73-4313. Ward, J. S. and Morrow, N. R.: "Capillary Pressure and Gas Relative Permeabilities of Low Permeability Sandstone," SPEJ Form. Eval., 2 (Sept. 1987) 345-56. Everett, D. H.: "Definition, Terminology, and Symbols in Colloid and Surface Chemistry," J . Pure & Applied Chem. (1972) 518. Moshier, S. 0.:"Microporosity in Micritic Limestones: a Review," Sedimentary Geology, 69 (1989) 191-213. Pittman, E. D.: "Porosity, Diagenesis, and Productive Capability of Sandstone Reservoirs," SEPM, Special Pub. No. 26 (Mar. 1979) 159-73. Gies, R.M.: "An Improved Method for Viewing Micropore Systems in Rocks With the Polarizing Microscope," SPE Form. Eval. (June 1987) 209. Yanguas, J.E. and Dravis, J.J.: "Blue Fluorescent Dye Technique for Recognition of Microporosity in Sedimentary Rocks," J. Sed. Pefrology, 55, No. 4 (1985) 600-602. Ruzyla, K. and Jezek, D.: "Staining Method for Recognition of Pore Space in Thin and Polished Sections," J. Sed. Pet., 51, No 4 (1987) 777-778. Morrow, N.R.: "Physics and Thermodynamics of Capillary Action in Porous Media," Ind. & Eng. Chem. (June 1970) 32-56. Brower, K. R. and Morrow, N. R.: "Fluid Flow in Cracks as Related to Low Permeability Gas Sands," SPEJ (April 1985) 191-201. Pettijohn, F.J., Potter, P.E., and Siever, R.: "Sand and Sandstone," Springer-Verlag, Berlin (1972). Marzano, M.S.: "Controls on Permeability for Unconsolidated Sands From Conventional Core Data Offshore Gulf of Mexico," Gulf Coast Ass. Trans. 38 (1988) 113-120. Kodai, K.: "Graphic Representation of Rock Permeability," Bull. Geol. Surv. Japan, 35, No. 9 (1984) 419-434.
F. Rodriguez-hinoso et al. (Editors), Characterization of Porous Solids IZ 1991 Elsevier Science Publishers B.V., Amsterdam
I37
SURFACE CHARACTERIZATION OF AN UPPER-PERMIAN CARBONATE ROCK BY Nz ADSORPTION
Preben J. Mgller Laboratory of Physical Chemistry, H.C. Brsted Institute, University of Copenhagen, 5 Universitetsparken, DK-2100 Copenhagen 0 (Denmark) and Peter Frykman, Niels Stentoft and Chr. Bender Koch* Geological Survey of Denmark, Thoravej 8, DK-2400 Copenhagen NV (Denmark)
ABSTRACT From the oil-reservoir carbonates of the Danish Upper Permian two bulky samples have been investigated by low-temperature nitrogen adsorption as function of initial high-vacuum degassing temperature. Compositionally, the samples differ in the relative content of dolomite and anhydrite. The adsorption-desorption isotherms were of multilayer type with a marked hysteresis. BET analysis shows two groups of adsorption sites each of which are characterized by their specific surface area (approx. 0.1 m2g-1) and C-constant (20-180). The adsorption sites are not mineral specific. The areas were only slightly influenced by vacuum-heat-treatment, while the pore volume was significantly increased for one of the samples.
INTRO DUCT10 N The characteristics of the pore system in reservoir rocks is important for the understanding of the flow of liquids through the rocks, particularly in relation to the recovery of hydrocarbons. Commonly used methods in the assesment of these characteristics of reservoir rocks are optical microscopy of thin sections and measurements of total porosity by mercury dilatometry and of air or liquid permeabilities. The use of inert gas adsorption is a well established method for characterization of porous systems and their surfaces. From the literature it seems that this method has not been applied in the characterization of carbonate rocks, presumably due to the small specific surface area and porosity of the natural coherent rock. *Present address: Laboratory of Applied Physics, Technical University of Denmark, DK-2800 Lyngby.
738
Carbonate rocks, e.g. the Upper Permian (Zechstein) of Northern Europe, constitutes an important group of oil reservoir rocks. In this investigation we have studied the surface characteristics of two samples using volumetric adsorption at 77.6 I<. In order to overcome the problem of accuracy due to small volumes of gas adsorbed per g of sample we used sets of two large plugs with a total weight of 200-250 g and volume of about 87 ml for the analysis. The optimal outgassing condition for the samples is limited by a possible thermal decomposition of the mineral surface layers. In order to evaluate this we have carried out a systematic study of nitrogen adsorption as function of increasing vacuum-heat-treatment temperatures.
2. EXPERIMENTAL 2.1 Samples Two samples have been taken from a cored carbonate in the Lclgumkloster-1 well (Fig. 1). The carbonate is from Upper Permian (Zechstein), and stratigraphically it belongs to the Ca-2 formation of the Zechstein-2 groups (ref. 1).
V
Fig. 1. Map of Denmark, showing the main geological structures around the Lsgumkloster-1 well.
739
Each sample consist of two subsamples (vertical subplugs), each I+" x l + " ,see Fig. 2. The plugs were treated with toluene and methanol in a Soxleth extractor to facilitate r e moval of organic matter and soluble salts, and dried in an oven at 60 C.
Fig. 2. Sampling scheme indicating the relative positions of the samples in the core. Subplugs A and B were used for measurements of nitrogen gas adsorption isotherms. Subplug C was used for scanning electron microscopy (SEM) and prepations of thin sections for optical and electron back-scattering image analysis and for bulk chemical analysis. Prior to the gas adsorption subplug B was also used for the measurements of sonic velocities which involved saturation with deionized water and subsequent drying at 60 ' C. The amount of adsorbed gas is referred to the as-received weight of sample after oven drying. Prior to outgassing the samples were soaked in methanol to remove organic contaniination from the outer surface due to handling. The depth of the samples are referred to the top of subplug A measured from top of the corebox: sample
depth, in.
weight, g.
25 AB
2435.95
203.55
62 AB
2454.65
241.67
740
2.2 PetropaDhv The Ca-2 carbonate sequence, which is ca. 41 m thick at L0gumkloster-1, includes former oncoidal/algal muds and ooidal carbonate sands (the subdivision of the coated grains into ooids (= eggs) and oncoids (= nodules) is solely descriptive here) (ref. 2). These sediments have been subjected to a rather complex sequence of diagenetic events. However, the present porosity and permeability is primarily limited to four events: two leaching phases, a phase of chemical compaction and a late anhydritization. The present pore geometry of the rocks is complex. In the oolitic intervals the intra- and interooidal porosity types are the most conspicuous. In the oncolitic/algal intervals, intercrystalline porosity is often combined with inter/intraoncoidal porosity or vuggy porosity (ref. 3). Sample 25: Sample 25 represents an oolitic dolomite, CaMg(CO& (chemical analysis: 70.2 vol.% dolomite, 11.0 vol.% anhydrite). The single ooids are more or less obliterated during the diagenesis. However, the algal-like concentric structure is still discernible in some of them. The inner part of the ooids are dissolved (intraooidal porosity). The cavities around the ooids are partly filled with a dolomite rim cement consisting of 30-40 pm big subhedrals. At few scattered laths anhydrite, CaS04, is seen in the cavities, mostly in the intraooidal ones. Both the dolomite crystals of the ooids and the cements and the anhydrite crystals are attacked by leaching. Besides the intra/inter ooidal vugs, SEM reveals innumerable intercrystalline pores. Actually, the scattered inter/intra ooidal cavities are not quite isolated; they are connected by a complex network of micropores and associated pore throats. A circumstance confirmed by the fact that all the pores - both the vugs and the tiny intercrystalline pores - are epoxyfilled in the thin-section analyzed samples. Sample 62: The rock of sample 62 is a strongly anhvdritized dolomite (chemical analysis: 52.5 vol.% anhydrite, 43.6 vol.% dolomite), probably a former oncolite. It consists mostly of a rather massive, more or less felted mass of anhydrite laths with subordinate residuals of dolomite. The anhydrite has clearly grown at the expence of the dolomite. Only in places some indistinct more or less circular structures reveal the presence of the former oncoids. Only a few scattered areas or spots with intercrystallline pores and small vugs are connected with the dolomite. These porous areas are surrounded by oncoidal or algal structures of very finely crystalline to aphanocrystalline dolomite (< 16 pm) which probably has acted as a screening for the sulphate-rich, rising brines (probably from the underlying 2-1 Anhydrite Formation). The pores and small vugs are often surrounded by subhedral (subidiomorphic) dolomite crystals which are tightiy packed. However, as ail the tiny pores are epoxy-filled in the thin-section samples they must be interconnected by some narrow pore throats. Narrow passages must also be present between the tightly packed anhydrite crystals in the areas around the porous spots of dolomite.
741
20
-
15
.
V
10.
500
5e I
1
250
00
01 0
025
0.05
050
0.10
0 75 o h .
0.15
0.20
01 0
0.25
P/P.
vt
0
200 150.
100 50 01 0
0.05
0.10
0.15
020
025
n/o.
.
-
C
0 05
0 10
0 15
0 20
025
p/pn
.yo * 0.01
0.02
0 03
0.04 P/Po
Fig. 3. Nitrogen adsorption at 77.60 K onto sample 25 AB upon an overnight vacuum heattreatment at 170 ' C. ng:pmoles of N2 adsorbed per g sample, p: equilibrium pressure of N2, PO: saturation vapour pressure of Nz, y = p/n,(po-p). Only the absorption branch is shown here. (a) isotherm for 0 5 p/po 5 I , (b) detail of fig. 3a. The arrow marks the breakpoint of the isotherm (see text). (c) BET plot for 0.006 < p/po < 0.32, (d) BET plot of low (0.006 < p/po < 0.05) pressure range, (e) BET plot of high (0.05 < p/po < 0.32) pressure range.
742
2.3 Nitrogen vapour isotherms Vapour isotherms were obtained using an all-metal volumetric ultrahigh-vacuum apparatus with a graded seal Pyrex glass sample vessel. Due to the large volume of the samples, and to minimize dead-space corrections, the Pyrex glass containers were formed around the plugs that were precooled to -20 'C (the precooling assured a temperature below 0 " C during the formation of the glass container). A 3-mm internal-diameter constriction was formed in the container just above the samples, at the cryostat-coolant's surface. The residual pressure of the system was approximately 10-7 Pa. Equilibrium pressures were measured by capacitance manometry. Desorption branches were obtained down to the 0.05 to 0.2 p/po range only, due to experimental limitation (fixed volume of desorption reservoir in apparatus). The cold dead space (near 18 ml) was determined using helium gas at 77.6 K and 273.15 K. Nitrogen and helium were of 99.9992% and 99.9996% purity, respectively. Prior to adsorption the samples were outgassed at 5 x 10-4 Pa by pumping overnight at vacuum-heat-treatment temperatures (VHT) of 20, 80, 105 and 170 ' C, respectively.
RESULTS AND DISCUSSION The nitrogen adsorption isotherms of the two samples following the VHT are characterized by a multilayer adsorption (BET-type 11). Detailed analysis of the low-pressure range of the isotherms (e.g. see Fig. 3a and b) reveals a characteristic shape. In addition to the normal type I1 behaviour the isotherms show a relative rapid increase in the amount of adsorbed gas for a small increase in p/po at approx. p/po 0.14 (marked by arrow in Fig. 3b). This indicates the presence of two types of surface sites. In Table I is given the results of applying the BET equation in the low (0.006 < p/po < 0.05) and the high (0.05 < p/po < 0.32) relative pressure ranges. This method of analysis improved the correlation coefficients as compared to using one BET-equation in the entire range, 0.006 < p/po < 0.32, Fig. 3c. When desorption measurements were also taken, the isotherms show hysteresis (Fig. 4a) indicating the presence of a porous system. Pore volume VD-E as derived from a DubininRadushkevich plot (Fig. 4b), and amount adsorbed at p/po = 0.95 (Gurvitch volume, VG) were also determined. The results are included in Table I. Porosities were analyzed by use of the Kelvin equation on the adsorption branch. Assuming a cylindrical pore model the analysis yields an average pore radius distributed around a peak at 2 nm, i.e. at the very low end of the mesopore [ref. 41 range. From literature there has been much evidence that the lower closure point of the hysteresis loop occurs above a certain critical relative pressure characteristic of the adsorptive at a particular temperature; for nitrogen at 77 I< this critical relative pressure is about 0.45 [ref. 51. In the present experiments the related hysteresis loop is therefore rather unusual. It may result either from a distortion of the structure of the adsorbent which is difficult to reverse or from an activated passage of the nitrogen molecules. This may have some bearing on the type of 3.
N
743
porosity in these samples, but further experiments are needed in order to further elucidate this point. It might be expected that the process of anhydritization, which markedly reduces the total porosity of the dolomite (see description above), also may affect the mesopore radii considerably. However, analysis of the hysteresis indicates that the Kelvin mesopore-radii is essentially unaffected by the VHT and identical for the two samples.
Fig. 4(a) Nitrogen adsorption and desorption at 77.60 K onto sample 62 AB upon an overnight vacuum heat-treatment at 20 'C (b) Dubinin-Radushkevich plot from data of Fig. 4a. Specific surface areas as determined from the BET equation, using a nitrogen crosssectional area of 0.162 nm2, yield the areas asl and as2 of the sites in the high p/po range and in the low p/po range, respectively. They are both very small: around 0.15 and 0.10 m*/g, respectively. The corresponding adsorption-energy related C-constants C1 and Cz are also small: around 27 and 150, respectively. The isotherm (Fig. 3a) is composed of two superimposed Langmuir isotherms. This means that two different areas may be derived. Since asl includes as2 we have included in Table 1 also the values of the difference as1-as2, because the consequence would be two surface types, one with area asl and another with area as1-as2. Considering that the two samples consist mainly of two minerals: dolomite and anhydrite, the presence of two different specific surface areas might be associated with the two different minerals and two different ranges of C-constants. In an attempt to allocate one of the determined specific surface areas to a specific mineral, we have also carried out measure ments on a pure anhydrite sample. The isotherm for this sample was of BET pure type 11, and the C-constant obtained from this sample is approximately 50 and thus differs substantially from the C constants determined from samples 25 AB and 62 AB. Hence, an allocation of the sites to specific minerals does not seem justified. The effect of VHT on the as and C over the 20 'C < VHT < 170 'C range is shown in Table I. For sample 25 AB we note that asl increases (approximately 15%) with VHT, while as?, CI, VD-k and V, are practically unchanged. C2 is unchanged at the lower VHT
wd.
d.0
a m
m o
i i
+I
30
19 19
30
(3*
0 0
tc? (30
19 30 +I
30
-H
7 0
c?cn cn3
+I
zz
0t-0
+I
93 99
but has increased when VHT has reached 170 ' C. For sample 62 AB, we note that the areas asl and as2 are practically unchanged, while GI, C Z and VD-R increases with a factor of about two. At the same time V, drops a factor of about two. Thus, for sample 25 AB the effect of VHT is only moderate (except for the Cz value following VHT at 170 'C). In comparison the effect of VHT is rather strong for sample 62 AB on the pore volumes and on the adsorptionenergy related C -values. It is possible that the thermal decomposition process during the VHT treatment removes some residual organic hydrocarbon material that had not been removed in the p r e ceeding Soxleth-extraction treatment. In that case, this could explain the rather strong increase in the Dubinin-Radushkevich mesopore volume. It is not obvious, however, why this process then should cause a decrease in the total pore volume of the sample. Experimentally, the Gurvitch-volume measurements are not very reliable here because of the steep inclination of the isotherm near p/po = 0.95, and perhaps cannot, therefore, be taken as the uptake at saturation. In conclusion, the nitrogen adsorption isotherms indicate the presence of two types of surface sites in each of the two samples. Vacuum heat-treatment has no marked influence on t,he values of the specific surface areas. Sample 25 AB shows no major change in the pore volume upon VHT, while for sample 62 AB a major increase is observed.
ACKNOWLEDGEMENTS Assistance by Lene I. Jacobsen in obtaining volumetric data and by S0ren J. M ~ l l e rin computer programming is gratefully acknowledged.
REFERENCES 1 D.N. Clark and L. Tallbacka, Contr. Sedimentology, 9 (1980) 205-231. 2 D.K. Richter, in: T.N.Peryt (Editor), Coated grains, pp. 7-8, Springer-Verlag, Berlin, 1983. 3 N. Stentoft, Danmarks Geologiske Unders0gelse DGU Series B, no. 12 (1990) 1-41. 4 K.S.W. Sing (IUPAC Subcommittee), Pure & Appl. Chem., 54 (1982) 2201-2218. 5 S.J. Gregg, in: J. Rouquerol and K.S.W. Sing (Editors), Adsorption at the gas-solid and liquid-solid interface, p. 58, Elsevier, Amsterdam, 1982.
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F. Rodriguez-Reinoso et al. (Editors), Characterization of Porous Solids II 0 1991 Elsevier Science Publishers B.V., Amsterdam
I47
THE ADSORPTION OF SULPHUR BY MACROPOROUS MATERIALS L. DAZA, S. MENDIOROZ I n s t i t u t o de C a t a l i s i s y Petroleoquimica d e l C S I C , Madrid, Spain J. A. PAJARES I n s t i t u t o d e l Carbon d e l C S I C , Oviedo, Spain
SUMMARY The a c t i v i t y of
various
natural
concentrations (< 10% v o l ) of HS,
silicates
for
the
e l i m i n a t i o n of
low
from gaseous streams i s r e l a t e d t o t h e i r pore
s i z e d i s t r i b u t i o n . I n order t o f o l l o w more e a s i l y t h e changes i n macroporosity produced by t h e r e s u l t i n g sulphur, t h e authors propose t h e use o f 1 9 - p r o b a b i l i t y paper, from which t h e usual parameters, mean, mode and median values and standard d e v i a t i o n o f t h e pore r a d i u s can be estimated. The d e p o s i t i o n o f sulphur i s r e l a t e d t o t h e number o f c o n t a c t p o i n t s among p a r t i c l e s and r e l a t e d v o i d space, t h e exposed surface and t h e shape and s i z e o f t h e primary p a r t i c l e s involved. INTRODUCTION
(10% by volume) from gaseous The e l i m i n a t i o n o f low concentrations o f HS, streams has been c a r r i e d o u t by c a t a l y t i c o x i d a t i o n using porous m a t e r i a l s as c a t a l y s t s . When t h e r e a c t i o n takes p l a c e a t temperatures below t h e c r i t i c a l temperature
o f sulphur
(1040'C),
t h e elementary
sulphur produced,
may be
deposited on t h e c a t a l y s t reducing i t s a c t i v i t y as i t s porous system i s f i l l e d up w i t h t h e sulphur. I r o n oxides ( I ) , b a u x i t e ( 2 ) , alumina (3,4),
charcoal (5),
z e o l i t e s (6), anatase (4), e t c . have been used as c a t a l y s t s . I n previous papers we have shown (7, 8) t h a t n a t u r a l s i l i c a t e s can a l s o be used as c a t a l y s t s w i t h r e s u l t s comparable w i t h those o f t h e f o r e g o i n g m a t e r i a l s and t h e added advantages o f t h e i r low c o s t and t h e ease w i t h which t h e deposited sulphur can be recovered. The a c t i v e centers f o r t h i s r e a c t i o n , according t o
(6), are t h e a c i d centers o f t h e s o l i d on which t h e d i s s o c i a t i v e adsorption o f t h e hydrogen sulphide takes place. According t o Hedden e t a l . ( 9 ) , who worked w i t h a c t i v a t e d carbon, t h e f i r s t step i s t h e d i s s o c i a t i v e adsorption o f 0, which r e a c t s w i t h t h e hydrosulphide i o n s proceeding from t h e p a r t i a l i o n i z a t i o n o f
H,S i n t h e water f i l m adsorbed on t h e carbon surface. According t o S t e i j n s e t a l . (10) t h e H,S
o x i d a t i o n i s based on an o x i d a t i o n / r e d u c t i o n mechanism, w i t h
sulphur having on a u t o - c a t a l i t y c a c t i v i t y . Hepburn and Pate1 (11) s t a t e d t h a t t h e a d s o r p t i o n - d e p o s i t i o n o f sulphur on s i l i c a takes p l a c e i n successive stages.
I48
They consider that surface OH- groups of silica combine with Sz- ions which subsequently diffuse through the pores being physically and/or chemically adsorbed on the walls filling up the voids among the silica particles. Whatever the first step in the reaction is, the sulphur continues to be deposited on the surface of the solid until eventually forming a monolayer. Thus, the rate o f reaction in this first "induction" phase will depend on the density o f the active centers on the surface o f the solid and their accessibility, which means that activity in materials of a similar nature must be strictly related to their textural features, i.e. particle size, surface area and pore distribution. Once the sulphur has been produced, it acts as an autocatalyst (3-10) in the subsequent oxidation reaction, increasing the sulphur load until the loss in surface area and accessible sulphur makes the yield uneconomic. - - - - > S0tH,0t220 kJ/mol, is highly The oxidation reaction, H,St1/20, exothermic, therefore depending on the density of active centers, the rise in temperature can change the rate of the reaction, and thus, the sequence of the deposit. Once the autocatalysis by sulphur begins, depending on the rate o f reaction, it is possible that the sulphur produced is either orderly adsorbed in a mono-multilayer mechanism or, if the sulphur pressure is high enough, it is capillary condensed inside the pores and in the points of contact between particles, following the Kelvin equation. As the rate of reaction in this second phase will be linked to the accessible sulphur, the morphology and the packing density of the particles in the samples will also determine the final capacity for sulphur retention and, therefore, for the elimination of hydrogen sulphide from the gases. In this work we have tried to link the capacity to retain sulphur of different natural silicates, viz. diatomite, sepiolite, palygorskite and bentonite, all with differing morphologies and packing density, with their content of macropores (r>25nm). In order to facilitate the comparison, log-probability plots are used. With this device the distribution curves, Vp vs. radius, become straight lines, in which experimental and operator errors are smoothed out and statistical parameters (mean, mode, median, and standard deviation of the radius) can be easily read off the graph. FUNDAMENTALS
In a previous work ( 1 2 ) , we pointed to the possibility o f obtaining additional information on the textural features of the solids by the study o f the shape o f the pi/pe curves (intrusion pressure/extrusion pressure at a constant volume) vs. the fraction of total pore volume intruded by mercury (phi). Notwithstanding the validity of the obtained results the application of such a device to
749
"unconsolidated" systems (13) is not very straightforward. Mercury at high pressure can disaggregate the particles, invalidating the extrusion data and therefore the pi/pe ratio. The application of current statistical methods to the intrusion data and the conclusions inferred from them, seems to be a promising, more reliable and more general method of obtaining additional information on the porosity of the solids, the only condition being that the pore sizes fit a normal distribution. As the silicates used are natural products, some of their property are influenced by a large number of small effects that give rise to a frequency distribution of the property under study that fits a Gauss ian curve, provided the population sampled is large and that the results are presented as a frequency distribution. If the frequency distribution (y) is plotted on a normal distribution paper, y vs x, in which:
1
y=-
(X-xy
exp uJ2r
202
where x is the arithmetic mean value of x, the pore radius, and u is the standard deviation, a straight line will be obtained in which the value corresponding to the 50% frequency will be the mean value of the property studied (in this case the pore radius). The standard deviation (or its log, if log-probability paper is used instead) will be half the difference between the frequency values (or their log) of 84 and 16% ( 1 4 ) . MATERIALS Four natural silicates were used: diatomite or kieselguhr, sepiol ite, palygorskite and bentonite. All o f them were from Spanish deposits of great abundance and purity. The DIATOMITE sample used was from Elche de la Sierra: it is a very pure material, with an amorphous silica content basically opal of over 80%, and a low carbonate content. It is a globular material, given the high degree o f preservation of the original frustules, with a pore volume measured by mercury intrusion of 1.40 cm3/g and a surface area measured by nitrogen adsorption of 15.6 m2/g. The rest of the textural parameters are listed in Table 1. SEPIOLITE is an acicular mineral of great purity, over 95%, from Vicalvaro (Madrid). It was supplied by TOLSA S . A . , and has an aggregate size of between 0 . 4 and 0 . 6 mm, a total pore volume of 1.44 cm3/g and a surface area of 337 mz/g.
750
The PALYGORSKITE sample was also supplied by TOLSA S.A. as a very fine offwhite powder ( 2 pm). It is an acicular mineral, like sepiolite, but with a much TABLE 1 Textural parameters of raw materials Diatomite
Sepiolite
Palygorskite
Bentonite
15.6 166 0.042 62 0.7 9 1.40 0.18
337 223 0.546 32 70 5 0.894 0.0053
117
83 54 0.108 26 20 6 0.273 0.0063
115 0.268 46 11 6 2.245 0.037
r, rPp and R are average meso, micro and macropore radius from 20000/SB,, times Vo,98, ,V and V, respectively.
smaller fibre size, forming practically equidimensional aggregates along its longitudinal axis which reduces its packing density, in comparison with sepiolites. Its pore volume is 2.51 cm3/g and its surface area is 117 mz/g. Finally, the BENTONITE sample came from La Serrata de Nijar and was supplied by Minas del Gador. It is a lamellar silicate with a 2:l structure, a surface area of 83 m2/g and a pore volume of 0.381 cm3/g. Its montmorillonite content is over 90%. EQUIPMENT AND METHODS The surface area of the samples was measured by nitrogen adsorption at 77K in a Micromeritics Digisorb 2500 apparatus, using 0.162 nm2 as cross-sectional area of the adsorbed nitrogen molecule. Macroporosity was analyzed by mercury penetration porosimetry in a Micromeritics Poresizer 9310 apparatus which worked up to 30,000 psia ( 2100 Kg/cm2) corresponding to a Washburn pore radius of 30 A (contact angle 130", surface tension 484 dynes cm-'). The size of the particle and the morphology of the samples was studied by scanning electronic microscopy (SEM) in an IS1 DS-130.
75 1
The sulphur content of the samples was determined in a LECO-SC32, no. 780600 apparatus and occasionally by the'rmogravimetry in a Perkin-Elmer TGS-11. SULPHURATION The oxidation of H,S by 0, was carried out in a fixed bed glass reactor of 5 cm O.D., at atmospheric pressure and 12O'C, using nitrogen as a diluent, The molar ratio H,S/OJN, was 1:1:2, and the total gas flow 100-200 cm3 per minute. The materials to be used as catalysts were previously dried "in situ" at the reaction temperature for two hours, in a stream of nitrogen, so as to avoid the possibility of secondary reactions (15). The whole process and the specific conditions under which an even distribution of the sulphur deposited can be achieved are covered by Spanish Patent no. 551.862 (16). If, as already pointed out (8), the sulphur is produced through a surface reaction in the active centers of the solid, the amount o f sulphur deposited in each pore would be proportional to its remaining surface area, and the sulphurization rate would decrease with time. Eventually, the activity of the solid should disappear once a monolayer o f sulphur has been completed, at surface loads as high as 0.44, 9.53, 3.31 and 2.3% respectively, obtained for Kieselguhr, sepiol ite, palygorskite and bentonite, assuming a cross-sectional area of 150 A2 (17) for the molecule of S, sulphur produced. However, since the reaction continued at well over those values, it must be concluded with Prettre and Sion (3) that the sulphur generated in the reaction (10) is the actual oxidation catalyst. Thus, the activity of a given solid for hydrogen sulphide oxidation, where no reaction products other than sulphur are produced, will depend on two factors: the decreasing number o f active centers as the sulphur is deposited, and the increasing number of terminal sulphurs that, over and above the value of the monolayer, will be responsible for the continuity of the reaction. Likewise, the capacity for taking up sulphur will be related to the specific activity and to the porous system of the solids, the latter collecting the sulphur formed in the reaction, and the former determining the pore filling mechanism. For a given texture, the changes in porosity of the resulting samples will give an idea, not only of the progress of the reaction but also of the volume still available. The type of pores involved will scarcely affect the total quantity of sulphur, but their total volume and distribution, together with the inter-particule voids resulting from the density and regularity of the packing of the starting solid (18), would control the sulphur load.
752
RESULTS AND DISCUSSION Figure 1 shows the % sulphur deposited vs. time for each one of the samples, which can be related to their catalytic activity for the oxidation o f H,S. No reaction product other than sulphur was produced in the H,S oxidation, as previously shown by XRD and XPS (19). 965 35
30 -
/
25 20 -
15
-
+
10 5.-
J
DIATOMITE
SEPlOLlTE PALYGOASKITE
8 BENTONITE
-
n 0
5
10
15
20
1, h
Fig. 1. Deposited sulphur (X) vs. time (h). Table I1 summarizes the textural data of each series. In general terms, both the surface area and the total pore volume decrease and the average pore radius increases as the sulphur content grows. The radius of the most common pore (mode) stays almost constant in sepiol ites and palygorskites throughout the experiments; whereas in diatomites, and especially in bentonites, it first decreases and finally increases with the sulphur load. The BET constant reaches an almost constant value in every series, and nearly the same for all of them, once the value o f the monolayer o f sulphur deposited has been exceeded, in agreement with the same type o f adsorbate/adsorbent interaction. A pore radius of 25 nm (corresponding to an intrusion pressure of 7 x lo3 psia), in accordance with IUPAC recommendations (20) has been considered as the lower end of macroporosity. The total pore volume has been mentioned in the Table 1 and is the sum of the meso- and microporosity values resulting from N, adsorption isotherms at a relative pressure of 0.98 and macroporosity from mercury intrusion up to 25 nm pore radius, as previously mentioned.
753
TABLE I 1 Changes in textural parameters produced by sulphurization Sample
v,
XS
m'9-l
DTOO
-_
DTlO DT20 DT30
12.3 19.7 31.2
__
so0 s10 s20 S30 S40
cm3g-l
Radius*. A
cm3g-l Average
15.6 10.0 9.5 9.0
165 111 128 139
1.45 1.352 1.321 1.221
1.40 1.325 1.293 1.196
337 127 81 73 60 53
233 65 70 90 96 92
1.44 0.72 0.713 0.749 0.542 0.530
0.894 0.271 0.405 0.392 0.243 0.254
53 43 64 97 81 96
374 374 278 374 278 374 278 278 278 278 374
1794 1718 2649 1091 2722 926 2657 8867
S50
5.7 11.5 16.4 20.7 23.3
PO0 P10 P20 P30 P40
12.1 15.2 21.9 25.0
117 54 49 45 42
115 96 86 73 68
2.513 1.827 1.760 1.604 1.460
2.245 1.623 1.560 1.413 1.304
372 601 636 628 621
9.2 11.4 14.7 14.8 18.6
83 10 8 7 7 13
54 28 33 35 42 45
0.381 0.627 0.573 0.635 0.666 0.696
0.273 0.586 0.539 0.605 0.635 0.642
66 1172 1348 1729 1814 987
__
BOO"* B10 820
B30 B40
B50 * **
Mode
7x104 11x10~ 7x104 9x104 17x104 25x104
Exclusively related to macroporosity r > 25 nm. Bentonite is a macroporous sample. Only r > 1 pm have been considered.
The variations in accumulated macropore volume vs. pore radius with sulphur load are depicted in Fig. 2 a,b,c and d. A continuous decrease in all series but bentonite is patent, although the changes in every series do not affect in the same manner all pore radii. Thus, deposition is mostly produced on diatomite in pores up to 4 pm size, giving rise to porosity in the 0.3-1 pm range. In sepiolite, pores in the 10-100 ,urn range are specially affected, whereas they are in the 1-10 pm range in palygorskite, although minor differences among samples in the complete range are also patent. On bentonite the situation is completely different because an increase in all sizes is produced by sulphur deposition, When a representation of lg r vs. frecuency probability is computed, Fig. 3, the aforementioned changes are more distinct, and additional conclusions can be inferred from the plots.
754
'B
d 00
I
d 10
r
c .
d 30
,
i
+so01
-
0.8-
10
?F
+ s 30
*
c
0.8; 0.4-
s
' IS
P
-_
+-+% ,
-*.-. .
.
S 40 ~ $ 5 0 1 I
--_
.--*
p
-0
&pa0 Cp10 p 20
-1
8
O
+.*
L r ,"A
~
I
f
F
p 30
*p
40
Fig. 2 . Accumulated pore volume (cm3/g) vs lg r (pm): (a) diatomite, (b) sepiolite, (c) palygorskite, (d) bentonite. Thus, figure 3a, which corresponds to the diatomite series, shows a bimodal distribution of pores in all the samples, with parents overlapping between sizes o f 1 to 5 pm with many pores above and below these values. They could correspond to inter-particule voids that have resulted from the aggregation of decreasing size particles which in turn contain normally distributed decreasing pores. There is a very important gap between the medians of both distributions from 0.2 to 30 pm. This distribution matches the nature of the samples, amorphous silica from diatoms at different growth stages. Sulphur deposition takes place in the inter-particule voids above 4 pm ion size, giving rise to a number o f pores between 0.1 and 1 /MI. A bimodal distribution holds, but a shift of the medians to around 0.3 and 15 fim respectively occurs. A slight loss in surface area through sulphurization must be assigned to micropore occupation by the first produced sulphur. In its natural form sepiolite (Fig. 3b) shows a bimodal distribution with a total absence of porosity between 0.200 and 5 . 6 pm radius as a result of the morphology of the particles: very long needles along their longitudinal axis. The medians of the parent distributions are considerably separated from one another, probably corresponding to the voids between primary and secondary aggregates.
755
2.0
z VP
-
1.1-
lsq
....,..
0-
-1
1
I.
% VP
% 'JP
Fig. 3. Lg r (pm) vs. cumulative percentage pore volume on l g p r o b a b i l i t y paper: ( a ) d i a t o m i t e , (b) s e p i o l i t e , ( c ) p a l y g o r s k i t e , (d) b e n t o n i t e .
Pore
distribution
sulphurization,
i n s e p i o l i t e remains almost u n a l t e r e d through i n s p i t e o f t h e v a r i a t i o n s shown i n F i g . 2b and Table 2. Only
a decrease i n t h e 10-100 pm range i n favour o f t h a t o f 0.025-0.25 i s appreciated. This d i s t r i b u t i o n can be r e l a t e d t o a gradual f i l l i n g o f a l l pore sizes, r e s u l t i n g i n a l a r g e decrease i n surface area and pore volume t h a t leaves n e a r l y constant t h e values o f mean, median and mode pore r a d i u s . The r a t e o f sulphur d e p o s i t i o n should decrease i n l i n e w i t h t h e f a l l i n a c c e s s i b l e sulphur (Fig. 1). The macroporosity o f p a l y g o r s k i t e (Fig. 3c) shows a normal d i s t r i b u t i o n which s t r a y s s l i g h t l y a t t h e ends. The type
OF
sample, s h o r t f i b r e s attached a l l along
t h e l o n g i t u d i n a l axis, suggests a continuous v a r i a t i o n o f t h e i n t e r - p a r t i c u l e voids, t h e s i z e o f which i s r e l a t e d t o t h e number o f f i b r e s t h a t make up t h e aggregate. S u l p h u r i z a t i o n e s s e n t i a l l y a f f e c t s t h e l a r g e s t pores, s p e c i a l l y those i n t h e range 1-10 pm, as a r e s u l t o f t h e d e p o s i t i o n o f sulphur between t h e p a r t i c l e s i n c o n t a c t w i t h each o t h e r ( t h e same as i n t h e d i a t o m i t e s ) , and leaves almost unchanged t h e d i s t r i b u t i o n o f t h e smallest ones. The meso- and micropore volume and surface area data c o r r o b o r a t e these r e s u l t s . The p o r o s i t y o f b e n t o n i t e i s due mainly t o voids among p a r t i c l e s made up by aggregation o f l a m e l l a e by van der Waals forces. I t i s e s s e n t i a l l y macroporosity, b u t some micropores, probably from surface d e f e c t s , are present, which confer t o t h e sample i t s surface area and pore volume. The d i s t r i b u t i o n o f sizes i s
756
bimodal without overlapping between the groups, and a small difference between medians, 10 and 30 pm respectively. Sulphurization introduces a significant change in this distribution due to the appearance of macropores; the second part of the line practically disappears, giving way to a monomodal distribution which gradually moves towards higher sizes with its maximum exponent in B50. Microand mesoporosity remain practically constant, as does the surface area once the first deposition has been completed (B10). It is probable that here, condensation around intersecting or contacting edge-to-edge lamellae may explain this macropore volume "creation" giving rise to a more or less stable macropore system, in which mercury will penetrate following the laws of capillary intrusion. As a result of this macroporosity, there i s more accessible terminal sulphur in contact with the reactant gases, so increasing the rate of deposit (Fig. 1). CONCLUSIONS The oxidation of hydrogen sulphide at around 120°C using natural silicates as catalysts results in sulphur production and condensation inside the porous system of the solids, lowering their surface area and porosity. The deposited sulphur acts as an autocatalyst, taking over from the silicate active centres the catalytic action once the monolayer has been completed. The catalyst total performance is thus deeply related with the morfology and texture of the samples, and with the changes they undergo through sulphurization. The sulphur deposition is effected in a different manner in every sample, depending on its specific activity and texture. Thus, on fibrous silicates (sepiolite and paligorskite), a gradual deposition by a mono-multilayer mechanism in all pore sizes is produced, leaving unchanged the statistical values of pore radius. On the contrary, on lamellar and globular samples sulphur deposition in large macropores is more apparent, which in the former, results in sulphur condensation around intersecting 1 amellae, thereby increasing the total pore volume. The use of probability paper for presenting porosimetry data has been shown to be quite useful. Firstly, it enables a deeper insight into the texture, by showing the statistical parameters, mode, median, average, standard deviation and distribution of pore size. Secondly, because the shape of the curves involved seems to be closely related to the morphology of the samples, thus giving additional information on the system.
REFERENCES 1 A.R. Chaterjee, N.B. Bhatthacharyya, S.P. Sen., Technology 8 (1971) 48-55. 2 B.W. Gamson and R.H. Elkins, Chem. Eng. Progr., 4 (1953) 203-215. 3 M. Prettre and R. Sion, Z. Electrochem., 63 (1959) 100-105. 4 A. Datta, R.C. Cavell, J. Phys. Chem., 89 (1985) 450-454. 5 A. Swinarski and J. Siedlweski, Actes Congr. Int. Catal. 2nd, Paris 1960, 2 , (1961) 2345-2656. 6 7
A.B. Verver and W.P. van Swaaij, Appl. Catal., 14 (1985) 185-206. L. Daza, S. Mendioroz, J.A. Pajares and J.M. Palacios, React. of solids, 3 (1987) 351-364.
8 L. Daza, S. Mendioroz and J.A. Pajares, Appl. Clay Sci., 4 (1989) 389-402. 9 K. Hedden, Huber L and B.R. Rao, "Adsorptive reinigung von s c h w e f e l w a s s e r s t o f f h a l t i g e n Abgasen" VDI-Bericht N. 253, S 37/42, Dusseldorf, VDI-Verlag 1976. 10 M. Steijns F. Derks, A. Verloop and P. Mars, J. Catal., 42 (1976) 87-95. 11 C. Hepburn and P.I. Patel, Internt. Ruber Conf., Paris 1982. 12 Wm.C. Conner Jr., C. Blanco, K. Coyne, J. Neil, S. Mendioroz and J. Pajares Characterization of porous solids (COPSI) K.K. Unger Ed. Elsevier Sci. Pub. Amsterdam, 1988 273-281. 13 K.S.W. Sing, Chem. Ind. (London) July 1982, 475-480. 14 T. Allen, Particle Size measurement J.C. Williams Ed., Chapman and Hall London, 1974. p. 93. 15 M. Steijns and P. Mars, Ind. Eng. Chem. Prod. Res. Dev, 16, 4, (1977) 35-41. 16 S. Mendioroz, L. Daza and J.A. Pajares, Spanish Pat. NQ 551.862, Procedimiento de fabricaci6n de un adsorbente azufrado util para retener vapores de mercurio. 1986. 17 J. Klein and K.D. Henning, Fuel, 63 (1984) 1064-1067. 18 Y. Chang and D.D. Perlmutter, AIChE Journal, 33, 6, June 1987, 940-951. 19 L.Daza, S. Mendioroz and J.A. Pajares, React. of solids, in press. 20 K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. MOSCOU, R.A. Pierotti, J. Rouquerol and T. Siemieniewska., Pure & Applied Chem., 57, 4, (1985) 603-619.
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F. Rodriguez-Reinosoet al. (Editors), Characterization of Porous Solids I1 0 1991 Elsevier Science Publishers B.V., Amsterdam
759
THE DIFFERENCES IN THE ADSORPTION P R O C E S S E S IN MICRO A N D
S U P ERMICROPOR ES 0. Kadlec The Institute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of S c i e n c e s , 182 23 P r a g u e 8, Dolejskova 3, Czechoslovakia
SUMMARY A new approach t o micropore adsorption h a s been developed, which makes use of Amagate equation of s t a t e f o r the adsorbed fluids. It h a s been foundthat this s t a t e equation, o r i t s two-dimensional (2D) form, d e s c r i b e s well the s t a t e p r o p e r t i e s of typical a d s o r b a t e s confined in micropores as well as the s t a t e p r o p e r t i e s of 2D fluids. The r e a s o n is that the c r i t i c a l temperatures of both fluids are substantially lower than the c r i t i c a l temperature of the bulk fluid. Another situation h a s been found f o r fluids confined in s u p e r m i c r o p o r e s , In spite of the increasing number of the a d s o r b a t e molecules in the p o r e , the adsorbed fluid p r o p e r t i e s a r e h e r e the more s i m i l a r t o those of the bulk fluid. The thermodynamic of s m a l l sytems i n the field of adsorption f o r c e s enables to d e r i v e the equation of adsorption i s o t h e r m s . T h i s equation d e s c r i b e s well the adsorption isotherms of vapours and g a s e s on microporous a d s o r b e n t s at d i f f e r e n t temperatures a l s o in the cases, when the Dubinin’s theory cannot be applied (zeolites), as well as the isotherms on nonporous s o l i d s . The adsorption isotherms of methane, propane, n-hexane. n-heptane and benzene on zeolite N a X , measured a t different t e m p e r a t u r e s , as well as the adsorption isotherms of benzene on active c a r b o n s have been c h a r a c t e r i z e d by the d e r i v e d equation. The p a r a m e t e r s of this equation, the mean value of the potential e n e r g y 8 of the adsorbed molecule and the perturbation change of entropy AS* related t o +’ the elimination of the t r a n s l a t i o n d e g r e e s .of freedom of the s m a l l systems localized in micropores, are in good agreement with the theoretically expected and evaluated v a l u e s . F r o m the approach d e s c r i b e d it follows, that adsorption equilibrium of vapours and g a s e s on solids m a y be d e s c r i b e d by one temperat u r e independent c h a r a c t e r i s t i c isotherm a=cp[ln(p! /pe,o 3 g 5), where a is the a r e the equilibrium pres’sure and the same amount adsorbed, pg and pg 8-0 adsorptive p r e s s u r e a t the fiak?ion of saturation of sorbent with s o r b a t e and i t s dependence 8 - 0 . 5 , respectively. The values of the p r e s s u r e s pg on the temperature T , a r e controlled by the potenti%&igrgy @ and the change of the entropy AS*
+’
IN T R ODUC T ION It is w e l l known, that the knowledge of the s t a t e p r o p e r t i e s of 2D fluids enables t o evaluate the adsorption isotherms on nonporous s o l i d s . Now we t r y to evaluate s i m i l a r l y the adsorption isotherms on microporous solids on the
b a s i s of the 3D state equation of the adsorbed fluids. We have shown ”(ref.1)”
760
a that the three-dimensional p r e s s u r e p of the adsorbed fluid may be evaluated from the adsorption isotherm a(pg) by means of the equation lnpg Pa
=RTv J a(pg)dlnpg ,
@=const.
, Tzconst.
(1)
0
-05
where V is the volume of the adsorption s p a c e , The conditions of the validity of this equation a r e : i) the constant value of the potential energy
8
of the
adsorbed molecules and ii) that the fluid "phase transitions" connected with a the existence of the negative p p r e s s u r e no occure in the adsorption p r o c e s s . T h i s condition is evidently fulfilled, when only the gas-like fluid e x i s t s in the adsorption s p a c e . The question whether a l s o the liquid-like fluid may exist in micropores h a s been solved theoretically. Using mean field and density gradient methods, Evans and h i s co-workers "(ref. 2)" have shown that adsorbed fluid "condensation" should d i s a p p e a r , when the diameter R of the cylindrical micropores approaches to d the molecular diameter. According to the simplest form of t h e i r theory the c r i t i c a l temperature T (m) of the fluid confined in cylindrical micropores is determined by the approximate relationship (T -T (m))/T < d / R , c c where T is the bulk c r i t i c a l temperature. From this relation i t follows that e . g . benzene (d-0.4-0.68
nm, cf."(ref.3)")
adsorbed at room and higher
temperatures on zeolites and microporous active carbons behaves a s a superc r i t i c a l fluid. But the co-operative condensation of benzene a t room temperatures is not excluded in l a r g e supermicropores of r a d i i R > 1 . 2 nm
.
Because the i s o s t e r i c heat of adsorption of benzene on the zeolite NaX is practically independent on the amount adsorbed "(ref -4)" and the volume V
3
(=0.295 cm / g ) of supercages of this zeolite is known "(refs.5-6)", i t s a a state p , v , T relations on NaX may be evaluated by the equation (1) and by the relation v a =Vo/a(p8) from the adsorption isotherms. Similarly by the use of the adsorption isotherms of benzene, krypton, ethane e t c . on zeolites NaX and mordenites, w e have found that the s t a t e p r o p e r t i e s of this fluids adsorbed in micropores may be described by the Amagate state equation a a p (V -vb) where v
b
=
iRT
,
(2)
is the covolume of adsorbed molecules and i the parameter which cha-
r a c t e r i z e s the interaction between the adsorbed molecules. T h i s 3D analogy
761
of the Schofield-Rideal s t a t e equation h a s been f u r t h e r accepted as a useful a a approach t o a r e a l p , v , T r e l a t i o n s of fluids confined in m i c r o p o r e s .
THE ADSORPTION ISOTHERM OF FLUIDS ON ENERGETICALLY HOMOGENEOUS MICROPOROUS SOLIDS The derivation of the adsorption isotherm equation is based on the thermodynamics of small systems in the field of adsorption f o r c e s . The adsorbed fluid in the uniform microporous solid c o n s i s t s from a l a r g e number of equivalent, distinguishable, independent systems of fluid, each with fixed c e n t e r of m a s s , what eliminates the t r a n s l a t i o n d e g r e e s of freedom of individual systems. The s t a t e equation of the adsorbed fluid d e s c r i b e s the relation a a between the mean values of the p r e s s u r e p and the molar volume v , a t given temperature T , taken o v e r a l l ensemble of individual systems of the fluid, localized in individual m i c r o p o r e s . Since a l l systems of the ensemble a r e a equivalent, p and va a r e a l s o time a v e r a g e s f o r a single system. The quantit i e s r e f e r r i n g t o the adsorbed fluid are denoted by the s u p e r s c r i p t a and that concerning of the bulk g a s by s u p e r s c r i p t g . The derivation s t a r t s from the known Gibbsian condition of the diffusional equilibrium of the fluid in the field of e x t e r n a l f o r c e s pa
+ Nr)
=
pg
,
T=const.
,
(3)
where pa and pg a r e the chemical potentials of adsorbed and equilibrium bulk g a s , respectively and @ is the potential e n e r g y of the adsorbed molecules. From the statistico-thermodynamicalderivation "(ref. 7)" follows, that the equation (1) is quite c o r r e c t only in the c a s e , when the potential @ does not depend on the position r . Generally, condition (1) contains additional t e r m , which accounts the influence of the potential e n e r g y gradient d @ / d r . The chemical potential pg of the bulk g a s c a n be e x p r e s s e d by the known relation
pg
=
p+o
+ R T M p g /P+)g + B(P!
-
P
g
1 ,
(4)
where po is the chemical potential of the bulk fluid in the standard s t a t e ,
+ p+ the standard p r e s s u r e of this fluid and B the second v i r i a l coefficient of the
B e r l i n e r form of the v i r i a l g a s s t a t e equation. F o r convenience, we chose f u r t h e r the bulk fluid s t a t e as s t a n d a r d , w h e n i t s
762
pressure pg i s equal to the adsorbed fluid p r e s s u r e at half filling of the ada The fraction of saturation of sorbent with sorbate 8 sorption space p 8-0.5’ a i s here defined a s the ratio of the amount adsorbed a(-V /v 1 to the hypotheti0
c a l limiting amount adsorbed a (=V /v 1 i . e . o o b 8 = a / a o = vb/v
a
.
According to the above definition, a s follows from eqns. (2) and
’+
‘:=0.5
= iRT/vb
(5)
.
The evaluation of the chemical potential pa i s based on the thermodynamics of small systems. The change of the chemical potential connected with the transport of the fluid from the standard to the adsorbed state may be divided into two p a r t s . The f i r s t , further denoted a s perturbation change of the chemical potential Ap* is the change of p
t
, which
corresponds to the separa-
+
tion of the bulk fluid a t the standard p r e s s u r e pg into small systems localized in micropores. The second part is the change of chemical potential connected
+
with the compression of the adsorbed fluid from the standard pressure pg to a the pressure p Hence
.
where p
0
+
i s the chemical potential of the bulk fluid in the standard state.
w
According to the definition of free energy, Ap
+
X
Ap+ = RTlnv
=
AH:
-
may be written a s
,
TAS:
where v i s the perturbation activity, AH
+
(8) X
+
the molar perturbation enthalpy
and A S x the molar perturbation entropy, which correspond to the separation of the fluid into small systems and its localization i n the micropores. The effect of this separation may be illustrated by the hypothetical adsorption of the fluid on idealized hard wall noninteracting microporous solid of the same geometric structure a s the r e a l sorbent. For this idealized c a s e , when @SO, it can be shown on the basis of equa’tions
V=P,fPf where po
(3),(4),(7) and ( 8 ) that (9)
3
is the hypothetical equilibrious gas pressure above the idealized
microporous solid with @=O
,
when pa = P +
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The perturbation change of enthalpy of the bulk gas a t adsorption, when w e assume that behaves a s the van d e r Waals fluid, may be evaluated by the relation
A =2iay/v BH=ibRy/v and where y depends on the degree of sepab ’ b H ration. When the molecules a r e completely s e p a r a t e d , y = l . a and b a r e the
where
constants of v . d . W equation. The perturbation change of the entropy A S
x
+ e. g.
f o r a known microporous s t r u c t u r e of the zeolites, may be evaluated theoretiY
cally on the b a s i s of s t a t i s t i c a l thermodynamics. It is always negative ( A S < O )
t
a s r e s u l t of the loss of the translation d e g r e e s of freedom of small systems of fluids confined in micropores. When the p r e s s u r e pg may be neglected in the l a s t term of eqn.(4) in compar i s o n with pg
+’
then equations (2)
-
(10) yield the following equation of ad-
sorption isotherm of fluids on energetically homogeneous solids
determines the temperature dependence of adsorption isotherms and a l s o the form of adsorption i s o s t e r e at the filling
Qd.5.
The connection between the parameters of equation (11) and the i s o s t e r i c heat and entropy of adsorption i s obvious. It c a n be obtained from the widely used (but not quite c o r r e c t ) e x p r e s s i o n f o r the i s o s t e r i c heat of adsorption Q (blnpg/bTla
=
Q/(RT2,
.
(13)
When we assume, that the perturbation entropy does not depend on the temperat u r e , then from equations (12) and (13) follows the relation between the potential energy
8+
AH
=
8 and the
RT
heat of adsorption Q
- QQs0.
@PO. 5
a t half filling
(14)
The integration of the equation (13) and of Clausius-Clapeyron equation yields
764
the known r e l a t i o n s lnpg
=
+c
-Q/(RT)
,
a=const.
and
where C and C
0
a r e the integration c o n s t a n t s , A i s the heat of condensation
and pg the normal vapour saturation p r e s s u r e . The physical meaning of integr0
ation constants follows from a l s o widely used relation f o r the change of the f r e e e n e r g y AG=RTln(pg/pg) = AH 0
+ RT(C-Co),
where AH= - ( Q - A ) is the change
of the enthalpy and A S = -R(C -C ) is the change of the entropy of the t r a n s f e r 0
of the bulk fluid from the s t a t e of normal liquid t o the adsorbed s t a t e .
By means of the above r e l a t i o n s , the equation of adsorption isotherm (11) may be r e w r i t t e n in the following alternative form
lnpg
=
5 c0 - "Q=O. R ~
-
'Q=O.
5
RT
where the i s o s t e r i c heat Q
Q=O.5
AsQ=O. 5
t
iPn(i-$
0
0
t 1-8 -
11
1
and the change of the entropy a t adsorption
c o r r e s p o n d t o the half filling of adsorption space i . e . 8=0.5.
The
relation between the entropy changes used i n equations of isotherm (11) and
(17)
is
where
AS. is the change of entropy A S id €Lo.5 f o r the idealized c a s e , when is z e r o . F r o m equations (11),(14) and (17) the perturbation entropy AS:
then follows the e x p r e s s i o n f o r A S . id ASid
=
-R[ln(ieRT/vb)
-
Co]
+
BH
+
,
iBR/v,,
(19)
where e is the b a s i s of the n a t u r a l logarithm.
All p a r a m e t e r s of the equation (11) of the adsorption isotherm have a c l e a r physical meaning. On energetically homogeneous solids the d e c r e a s e of the parameter i indicates the i n c r e a s i n g r o l e of the interactions between the ads o r b e d molecules. The influence of this parameter on the c h a r a c t e r i s t i c isotherms r e p r e s e n t i n g the dependence of 8 on ln(pg/pg
)
8=0.5
from the equation (11), is illustrated on the F i g u r e 1.
as evaluated
765
Fiq. 1. The c h a r a c t e r i s t i c adsorption isotherms of fluids on energetically homogeneous solids, evaluated bv means of the eauation (11). The parameter i is the measure of the a t t r a c t i v e interactions between the adsorbed molecules,
THE ADSORPTION E O T H E R M S ON ENERGETICALLY HETEROGENEOUS
SOLIDS Energetically homogeneous microporous solid is more o r less the idealization only.
P r a c t i c a l l y in e v e r y microporous solid t h e r e e x i s t s any distribution
of the potential energy
0 i n the
adsorption s p a c e . Evidently the experimentally
measured isotherms must be related t o the f r a c t i o n of saturation Q averaged t over a l l possible adsorption potential energy values by the integral
'min where X ( @ > is the differential distribution of the adsorption space volumes according to the potential energy
0
of adsorbed molecules. This function con-
tains a l s o the additive term a r i s i n g from the influence of potential energy gradients, when the condition ( 3 ) is used in the more c o r r e c t form. A s evident from eqns. (11) and ( 2 0 ) and the F i g . 1, the heterogeneity "draw out" the isotherms in coordinates 8 v s . lnpg and effectively i n c r e a s e s the parameter i . The attractive f o r c e s between molecules and energetic heterogeneity effect
thus i n a somewhat opposite manner, when manifest himself on the shape of the adsorption isotherm "(ref. 8)". By analysing the experimental adsorption
766
isotherms of benzene on zeolite NaX, using
X(0)
function evaluated from the
benzene molecule potential e n e r g y profiles "(ref.9)" and eqn. (ZO), found that the r e a l value of the parameter i f o r this system is 0.70 r i s o n t o the effective value i=1.0
.
The effective values of
0
we have in compa-
+
and A S w
are n e a r to the mean values o v e r a l l ensembles of small systems of the adsorbed fluid. GENERAL PROPERTIES OF THE ADSORPTION ISOTHERMS A s h a s been shown e a r l y "(ref.
lY', the adsorption isotherms on energetical-
l y homoqeneous solids, when expressed a s the function of amount adsorbed on lnpg may be separated in two p a r t s , one of which depends on the potential enerqy
8
and the perturbation entropy AS
x
+
and the other f(aJ, which depends
on the siate p r o p e r t i e s of adsorbed fluids only. A s shown, the general equation can be written
where z=exp[(@+Ap*)/RT] = exp{((@+ AH)/RT]
- [(AS:
+ B H l / R]
-
iB/vb)
.
For adsorbed s u p e r c r i t i c a l fluids with state relations given by the equation(2J
0
+
0
-
- 11 and pg =iRT/vb (cf. eqn.(6)). Similar f(a) functif(a) =i[ln(-) 1-Q 1-0 o n s , corresponding to the v i r i a l , v.d. W . e t c . state equations, are a l s o independent on the temperature T . Thus the functions a = q[ln(p g / p g z)] , which -Ia r e inversional to f(a) , d e s c r i b e the "new c h a r a c t e r i s t i c curve" universal
+
for a l l temperatures. It is interesting that although the condition ( b q / b T ) -0 a a s h a s been
h a s been derived f o r energetically homogeneous adsorbents
proved experimentally, well c h a r a c t e r i z e s the adsorption equilib,rium on the r e a l a d s o r b e n t s . The above condition is not in contradiction with the known Polanyi postulate, but in many c a s e s may be used even f o r the systems, where the Polanyi postulate is failed ( e . g . benzene on the zeolite NaX).
EXPERIMENTAL PART The described theory h a s been verified on the adsorption isotherms of benzene measured mainly qravimetrically on t h r e e typical samples of microporous solids. The zeolite NaX and the microporous active carbon (a. c . ) (industrially steam activated beech wood carbonaceous products) r e p r e s e n t s the samples No 36 and 29 of o u r found of p r o b e s , respectively. The isotherms
.
on a . c 29 have been overtaken from "(ref. 10)'' and a l s o analysed e a r l y
767
"(ref. 11)". A s an example of supermicroporous solid the new type of active c a r b o n , prepared by L . Kavan by the chemical reaction of perfluorobenzene with the lithium amalgame, f u r t h e r denoted as a . c . from C6F6, h a s been used. The parameters of the equation (11) have been determined as follows. The ex0 0 perimental isotherms have been plotted in the coordinates h(-) 1 1-63 1-Q v s . Inpg, of the linearized form of equation (11), where the parameter a is
+
-
0
optimalized. The r e c i p r o c a l value of the sloDe determines i and the intercept
of the linearized isotherm with the lnpg a x i s the value of lnpg F r o m the 8-0.5' and A S on 1 / T , the parameters Q Q=O. 5 8-0.5 have Q=0.5 been determined according the equations (16) and (17). The parameters 8 and x AS have been determined by means of the following q(T) function obtained dependence of lnpg
t
from equations (2)
- (12).
+
q=-(@+Ap*)=-@ -(Rlnv)T=qo*SY+B +
{
H )T=RT ln[(iRT/vb)/pg8-0.5
1 -iB/vb) ,
(22)
where q =-(@+A ) and Rlnv = ( A / T ) - ( A S x S B ). When the coefficients A H 0 H H + H and B a r e small and the dependence of q on T l i n e a r , the tangens (=-Rlnv)
H
is equal t o A S
w
+
x
and the intercept with q a x i s is q =-(@+AH). When AS+ 0
depends on T , ASX(T)-(q-q )IT and the value -(@+AH) is equal to ?(T+O).
+
0
The covolumes v have been determined exactly on the zeolites only, where b the volumes V are known "(refs.5-6)". On active c a r b o n s i t h a s been assumed that the adsorption space volumes V a r e equal t o the volume VMI of micro0
p o r e s , determined e . g . by the t / F method "(ref. 12)". On microporous solids, the derived equation (11) c h a r a c t e r i z e s well all the
experimental adsorption isotherms of benzene and i t s dependence on the temperature, a s shown on the F i g . 2 . The small e x c e s s of the experimental amounts in comparison with the amounts evaluated a t high fillings, is probably caused by the changes of packing of the highly compressed fluid at high p
a
p r e s s u r e s o r eventually partially by the c a p i l l a r y condensation in contacts between the p a r t i c l e s of the zeolite N a X or in the mesopores of the active carbon. The another situation h a s been observed on typically supermicroporous active carbon p r e p a r e d from perfluorobenzene. Here in spite of co-operative condensation of benzene, the isotherms may be c h a r a c t e r i z e d by the equation (11) in n a r r o w e r region of 8 (0.4<0<1) only. But nevertheless the described g e n e r a l p r o p e r t i e s of adsorption isotherms may be demonstrated a l s o on this system. In agreement with the equation (21), they may be
768
transformed into one temperature independent c h a r a c t e r i s t i c isotherm a [ln(pg/p:=o.
5)] ,
a s shown on the Fig.3. The alternative form of the
equation (11) is r e p r e s e n t e d by the eqn. (17). The method of the parameters
'€LO.
5 and AsQ=O. 5
evaluation i s illustrated on the F i g . 4 . a imol / g
a mmol / g
5
3
4 2
3 2
1
1
- 10
-5
5
0
lnpg (p", €'a) Fig.
2.
The comparison of experimental (all points) and evaluated by eqn.
(11) (cIlrves) adsorption isotherms of benzene a) on the zeolite NaX (36) a t the temperature: 30,40,50,60,80,100,120,140,160,180,200,220,250,270,300, 320 and 350 C , denoted a s c u r v e s 1-17,respectively, b) on the microporoi>s active carbon (29) a t the temperatures 30,39.5,49.5,60,84,111.5,143.5, 181.4,226.8and 288.5OC,denoted a s c u r v e s 1-10,respectively. The c o r r e s ponding parameters of eqn. (11) a r e given in the Table 1.
400
200
F i g . 3. a ) the adsorption isotherms and b) the c h a r a c t e r i s t i c adsorption isotherms of benzene on supermicroporous active carbon p r e p a r e d by the chemical reaction of perfluorobenzene with lithium amalgame, a t 2OoC (o), 4OoC (A) and 6OoC (0). The full points desorption.
-
769
C
g
lnPQ=O. 5
1) lnff
S
(pg, torr)
2) lnpz
3) a . c . from C F 6 6
10
4) a . c . 29 5) NaX
5 0
-5 1
Fig.
2
3
4
4.
The illustration of the evaluation of the i s o s t e r i c differential h e a t s of adsorption from the dependence and the entropy changes A S 8-0 5 Q=o'50f lnpg on 1I T , for micropokous solids studied. The value C (pg 0 .o' P a L 2 2 . 9 3 hkk%%n used in the evaluations of the entropy changes according --R(C-C ). The value CO(fg Pa) is 21.89, where fg is the relation A S 0 0 0' the fugacity of &%&-saturated vapours. The condensation heat of benzgne h 133.45 kJ/mole. QQ=0.5=-@+(RT-AH) a s follows from the equation (Id).
Q
TABLE I Pa.rameters of the equation (11) of benzene on the zeolite NaX, microporous active carbon (29) and supermicroporous active carbon p r e p a r e d by the chemical reaction of C F with L i amalgame. 6 6 O+AH
As:
"Q=O.
5
i
Probe
V n
kJ/mole NaX(36) -76.7K -54.5 a.c.(29) a . c . from -48.5
J/(mole K ) J/(mole K)
-67.0* -63.0 -67.5
b
3
a
vO n
3
cm /mole cm / g -
-32.0*
1.00
92.3*
-29.2
1.93
-11.6
0.42
61.5+ 88.8
0.295 0.400
0.876
0 ______
mmole/g 3.20K
6.50t 9.87
'gF6
_ - _ _ _ _ _ _
K
0
sliqhtly depends on T , given data c o r r e s p o n d s to T between 120-200 C + t h e effective values in spite of the energetical heterogeneity (cf. i > l ) .
D I S C U S SION It h a s been shown that benzene confined in the cavities of the zeolite NaX behaves a s a s u p e r c r i t i c a l fluid, the s t a t e p r o p e r t i e s of which may be evaluated by the Amagate s t a t e equation, with the r e a l i s t i c value of the parameter i = l . The e x c e s s of this parameter o v e r one observed on the microporous
active carbon N o 29 may be explained by the i n c r e a s e of the energetical heterogeneity of this adsorbent. In agreement with the theory of Evans and h i s
770
co-workers, the co-operative condensation of benzene confined in the supermicropores of a . c . prepared from C6F6, seems to be presented. This follows from the low value i=O.42 of the Amagate equation. But this equation i s not the most appropriate for the description of properties of fluids condensable in supermicropores
. In the future the energetical heterogeneity of
supermicro-
porous active carbons should be accounted in the attempts to found the r e a l state properties of fluids confined in supermicropores. CON C LU S IONS On the b a s i s of the thermodynamics of small systems in the field of adsorpti-
on forces the adsorption isotherm equation has been derived, which i s able to characterize well the isotherms of fluids on zeolites a s well a s on microporous active carbons. This has been demonstrated on the isotherms of benzene, Quite analogous results have been obtained by above mentioned hydrocarbons on the zeolite NaX. The parameters of the equation,such a s (3, agree well with the values evaluated independently theoretically. E . g . the value of @ + A H -76.7 kJ/mole found f o r benzene on the zeolite NaX(cf.Tab. 1) agrees well with the mean value of (3-81.6
kJ/mole, evaluated f o r the same system by
A . V . Kiselev and his co-workers "(ref. 9)" on the basis of the theory of
intermolecular forces (the value of A
i s not probably higher than 10 kJ/mole). H Similar agreement has been a l s o found for methane on the zeolite NaX, where
both experimental and theoretical mean values of the potential energy n e a r to
0
are
- 17 kJ/mole.
REFERENCES O.Kadlec, Pure and Appl.Chem. , 6 1 (1989) 1867. P . C .Ball and R . Evans, Langmuir, 5 (1989) 714. D . W.Breck, W. J.Eversole,R.M.Milton,T .D.Read and T . L. Thomas 3. J . A . C . S . , 78(1956) 5963. 0. M.Dzigit, A . V . Kiselev, T . A.Rakhmakova, Zeolites, 4 (1984) 389. 4. 5. R . M . B a r r e r and W.M.Meier, T r a n s . Faraday S O C . , 54 (1958) 1074. 6. M. M. Dubinin, E . G . Zhukovskaya and K . 0. Murdmaa, Izv. Akad. Nauk USSR, Otd. Khim. Nauk, (1962) 760. J. S Rowlinson and B. Widom, Molecular Theory of Capillarity, 7. Clarendon P r e s s , Oxford, 1982. 8. W.Rudzinski and J. Jagiello, Ads. Sci.and Technology, 6 (1989) 35. A . G . Bezus, M . KoCiFik, A . V . Kiselev, A . A . Lopatkin and E . A.Vasilyeva. 9. Zeolites, 6(1986) 101. 10. A.Zukal, Disertation , Inst. Phys.Chem.Acad. of S c i . Prague, 1967. 11. O.Kadlec, Chemical P a p e r s (J.of Slovak Acad.of Sci.),29 (1975) 653. 12. 0. Kadlec, Collection of Czechoslovak Chem.Commun., 36 (1970) 2415. 1.
2.
.
771
AUTHOR INDEX Adkins, B.D.; 543 Ajot, H.; 161, 583 Al-Kaisi, A.R.S.; 293 Alba, M.D.; 607 Albiniak, A. 357 Almela-AlarcQ, M.; 367 Alvero, R.; 607 Andersen, S.I.; 151 Avery, R.G.; 235 Bach, P.; 141 Bahceli, S.; 293 Bariou, B.; 209 Bell, J.; 75 Belyakova, L.D.; 701 Benito, F.; 625 Bhowmik, S.B.; 273 Birdi, K.S.; 151 Bittner, H.R., 141 Blancher, S.; 659 Bonnetain, L.; 189 Boon, A.Q.M.; 717 Bracconi, P.; 677 Brotas de Carvdho, M.M.; 341, 635 Briickner, P.; 491 Buckley, P.; 199 Carrott, P.J.M.; 341, 635, 685 Cases, J.M.; 591 Castro, M.A.; 607 Cather, M.E.; 727 Caullet, P.; 583 Christensen, S.V.; 151, 199 Comer, W.C.; 31, 199 Coulom, J.P.; 535 Davis, B.H.; 543 Davis, P.J.; 301, 709 Day, M.; 75 Daza, L.; 747 Del Arco, M.; 645 Demlehner, U.; 97 Denoyel, R.; 399 Dore, J. C. ; 245 Drobny, G.P.; 709 Duffie, J.; 75 Dufresne, P.; 565
Earl, W.L.; 709 Efremov, D.K.; 115 Elm’Cjapiro. A.; 565 Eltekov, Yu. A.; 575 Eltekova, N.A.; 575 Ewing, B.; 709 Fatemi-Sadr, M.; 677 Fenelonov, V.B.; 115 Fernindez-Colinas, J. 399 Fletcher, R.; 75 FranCois, M.; 357, 591 Freeman, J.J.; 319 Frykman, P.; 737 Fuertes, A.B.; 347 Fujiwara, Y.; 389 Genoni, F.; 553 Gem, J.W.; 717 Gimblett, F.R.G.; 319 Ginoux, J.L.; 189 Glittenberg, B.; 141 Gonplves da Silva, A.M. 341 Gonzaez, F.; 625 Grillet, Y.; 311, 357, 535, 591 Gubbins, K.E.; 21 Guet, J.M.; 379 Hampson, J.A.; 509 Hansen, J.A.; 199 Hayes, R.A.; 319 Hernindez. E.; 645 Hurd, A.J.; 179, 267 111in-G6mez, M.J.; 367 Isirikjan, A.A.; 525 Jaroniec, M.; 469 Jasra, R.V.ii 509 Jessop, C.A.; 123 Johnston, G.P.; 179 Joly, J.F.; 161, 565, 583 Kaczmarczyk, J.; 357 Kadlec, 0.,759 Kakei, K.; 429 Kaneko, K.; 389, 429
112
Kanellopoulos, N.; 61 Karnaukhov, A.P.; 105 Kartel, N.T., 439 Kenny, M.B.; 685 Kessels, P.Y.; 659 Klich, I.; 727 Koch, Chr.E.; 737 Krebs, K.F.; 133 Krim, J.; 217 Krynicki, K.; 293 Lentz, H.; 499 Leofanti, G.; 553 Lecloux, A.J.; 659 Lin, Q.; 379 Linares-Solano, A.; 367, 379, 419 Lorenzana, J.J., 459 Lynch, J.; 583 Mahamud, M.; 347 Majors, P.D.; 709 Marchot, P.; 659 Marsh, H.; 459 Martin, C.; 645 Martin-Martinez, J.M.; 311, 419, 449, 469 Martinez-Sknchez, M.A.; 449 Martin-Luengo, M.A.; 599 Mason, G.; 41 Mate-os, J.; 645 Mather, R.R.; 409 Mayagoitia, V.; 51 Mays, T.J.; 477 McEnaney, B.; 477 McInally, A.; 409 McMurray, R.; 273 Mellor, D.W.; 41 Mendioroz, S.; 625, 747 MenCndez, R.; 459 Merlo, J.L.; 659 Mersmann, A.B.; 225, 519 Michot, L.; 591 Milburn, D.R.; 543 Mohd. Amin, Z.; 319 Molina-Sabio, M.; 329 Mdler, P.J.; 737 Morrow, N.R.; 727 Miiller, U.; 535 Muiiecas-Vidal, M.A.; 329 Muiioz-Guillena, M. J. ; 367 Nafis, M.; 565
Nameri, N.; 209 Neimark, A.V.; 67 Nicholson, D.; 11 Nishikawa, K.; 389 North, A.N., 245 Noville, F.; 659 OrgilCs-Barcel6, A.C.; 449 Ozeki, S.; 429 Padovan, M.; 553 Pajares, J.A.; 347, 747 Pan, D.; 519 Panella, V.; 217 Parker, I.B.; 75 Parra, J.B.; 347 Parthun, G.; 199 Payatakes, A.C.; 169, 267 Pkrez, A.J.; 347, 459 Pesquera, C.; 625 Petrini, G.; 553 Petropoulos, J.H.; 61 Petrou, J.K.; 61 Pfeifer, P.; 179 Pirard, J.P.; 659 Pis, J.J.; 347, 459 Poyato, J.; 607 Puzy, A.M.; 439 Quinson, J.F.; 209 Quirke, N.; 123 Raatz, F.; 161, 565, 583 Radeke, K.H.; 491 Ragai, J.; 693 Rakhmatkariev, G.U.525 Ramsay, J.D.F.; 235, 257 Rees, L.V.C.; 509 Reichert, H.; 535 Ribeiro Carrott, M.M.L.; 341, 635 Riddiford, S.M., 123 Rives, V.; 645 Robens, E.; 133 Roberts, R.A.; 685 Rodriguez-Reinoso, F.; 311, 329, 419, 449, 469 Romero, E.; 459 Rouquerol, I.; 311, 535, 399 Rouquerol, F.; 311, 535 Russell, P.J.; 257 Russmann, C.; 161
773
Salinas-Martinez de Lecea, C.; 367, 379 Sato, T.; 283 Scholl, S.E.; 225 Seaton, N.A.; 123 SellCs-PCrez, M.J.; 449 Sermon, P.A.; 599 Sernetz, M.; 141 Siemieniewska, T.; 357 Sitnonot-Grange, M.H.; 565 Sing, K.S.W.; 1, 319, 409, 635, 653, 669, 685, 693 Smith, D.M.; 179, 267, 301, 709 Stacey, M.H.; 615 Stentoft, N.; 737 Strange, J.H.; 293 Strelko, V.V., 439 Suzuki, T.; 389, 429 Swanton, S.W., 257 Tan, Z.; 21 Tennison, S.R.; 273 Theocharis, C.R.; 653, 685 Thomas, M.; 75 Tobias, M.M.; 607 Tomkow, K.; 357 Topsere, H.; 151, 199
Torregrosa, R.; 419 Trezza, G. 553 Trillo, J.M.; 607 Tsakiroglou, C.D.; 169 Unger, K.K.; 535 Van Veldhuizen, A.J.W.; 717 Van der Grift, C.J.G.; 717 Villieras, F.; 591 Vu, D.T.; 151 Waldram, S.P., 273 Walton, J.P.R.B.; 123 Walton, T.J.; 599 Webb, S.W.; 31, 199 Weber, G.; 565 Winter, A.; 85, 151 Yates, M.; 599, 669, 693 Yeates, D.; 653 Yvon, J.; 591 Zecchina, A.; 553 Zhou, Y.; 499
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115
KEYWORD INDEX Activated carbon, 319, 329, 367, 379, 419, 429, 449, 469, 477, 491 Activated charcoal, 399 Activation, 367 Activation by CO,, 347 Active carbon, 347, 499 Adsorbents, 235 Adsorption, 11, 21, 31, 51, 115, 123, 179, 189, 225, 329, 357, 369, 379, 509, 525, 565, 591, 685, 73 continuous, 161 enthalpies of, 3 11 from solution, 341, 399, 439 heats of, 535 hysteresis, 115 isotherm, 217, 341, 399, 477, 519, 701 of acid gases, 701 of Argon, 429 of Benzene, 491 of Methanol, 341 of Neopentane, 635 of Nitrogen, 635 of Water, 179, 319, 341, 389 Adsorption apparatus, 189 Adsorption processes, 235 Affinity coefficient, 469 Air gasification, 419 Al-CLM stability, 607 Al-pillared montmorillonite, 607 Alumina, 161, 615 Alumina-supported vanadia, 645 Aluminophosphates, 535 Argon, 591 Attapulgite, 591 BET method, 133, 737 Binary mixtures, 509 Bituminous coal, 459
Cadmium halides, 311 Calcium hydroxide, 653 Capillary, 41, 51, 97 Capillary condensation, 115 Capillary hysteresis, 67 Capillary network, 61 Carbon, 273, 439, 575 gasification CO,, 419
sorption, 319, 591 Carbones dioxide, 357 Carbonate rock, 737 Catalysis/catalysts (heterogeneous), 7 17 Catalytic activation, 367 Catastrophic desorption, 161 Cations, 525 Cement, macrodefect free, 669 Cement microstructure, 669 Ceramics, 659 Charcoal cloth, 409, 341 Charcoals, 357 Chromatography, 141 Chromium oxide, 449 Classification, 701 Coal oxidation, 347, 459 Coke porosity, 459 Computer simulation, 21 Condensation, 5 1 Contact angle, 97 Continuous adsorption, 161 Contrast variation technique, 235 Controlled porosity gels, 257 Cotton, 409 Dealumination, 565, 583 Densification, 319 Density functional theory, 21 Desorption , 161 Diffusion, 273, 293 Diffusion limitation, 717 Diffusion, relation to pore structure, by SGC, 199 Disordered media, 85 Drainage-imbibition, 41 Dubinin-Radushkevich equation, 469 Dusty gas model, 225 Dye, 409 Enthalpies of adsorption, 311 Epifluorescent microscopy, 727 Evaporation of liquid, 151 Faujasite, 565, 583 Fibres, 319, 615 Filling, 357 Film surface area, 179 Fractal, 217
116
porosity, 141 Fumed silica, 267 Gas adsorption, 369, 379 Gaseous, 273 Gasification by air, 419 by C 0 2 , 419 Gel precipitation, 257 Gels, 257 Glass, porous, 499 Gold, 217 Graphite, 11 Gypsum, 693 Heats, 525 of adsorption, 535 of inmersion, 151 Henry’s constants, 535 Heterogeneity, 61 Heterogeneous catalysts, 599 Heteroporosity, 61 High pressure hysteresis, 419, 535 HRADS, 31 Hydraulic conductivity, 283 Hydrocarbons, 509 Hydroxy-Al, 625 Hysteresis, 51, 67, 115 high pressure, 419, 535 low pressure, 419, 535 of water-retention, 283 Image, 709 Imbibition, 97 Immersion calorimetry, 491 Immersion, heat of, 151 Interferometry, 141 Isotherm crossing, 31 1 Kelvin Equation, 21 Kevlar, 319 Kinetics, 225 Lanthanum, 607 Leather waste, 449 Limited selfsimilarity, 141 Low pressure hysteresis, 419, 535 Macrodefect free cement, 669 Macromolecular porosimetry, 575 Macroporosity, 747 Magnesium hydroxide, 635
Magnesium oxide, 635 Magnetic susceptibility, 293 Mean-field theory, 123 Membrane, 209 Meniscus, 41 Mercury, 75 Mercury penetration, 379, 439, 543, 693 Mercury porosimetry, 459, 499 Mercury porosimetry, simulation of, 169 Mesopores, 161, 583 Mesoporosity, 67 Metallic oxides, 645 Microcalorimetry, 399 Micrographitic structure, 389 Micropore filling, 429 Micropore size distribution, 469 Micropore sizes, 477 Micropores, 11, 179, 525 Microporosity, 319, 357, 399, 449, 599, 607, 635, 653, 685 Microporous carbon, 389 Microporous solids, 685 Microscopy, 727 Model, 75 Model porous adsorbents, 235 Modelling, 105 Molecular probes, 469 Monolayer filling, 429 Montmonillonite, Al-pillared, 607 Mordenite, 583 Mortars, 693 Nay, 509 Needl-like materials, 105, 519 Nwpentane adsorption, 319 Network, 75 model, 283 Neutron diffraction, 535 Neutron scattering (small-angle), 235 Nitrogen, 123, 591 Nitrogen adsorption, 257, 319, 409, 429, 543 NMR, 293, 301, 709 Non-inert adsorbent, 519 Non-isobaric, 225 Optical microscopy, 379 Ores, 677 Oxidation treatments, 329 Oxide gels, 257 Oxygen surface groups, 329
Partially saturated soil, 283 Particles, 133 Percolation, 41 Percolation theory, 67 Permeability, 61, 209, 273, 669 Petrography, 727 Phase-change, 615 Pillaring, 625 Polanyi-Dubinin, 565 Polymer adsorption, 575 Polymeric sorbents, 701 Pore, 75, 615 dimension, 293 networks, 169 quality, 727 size, 189, 709 size distribution, 31, 123, 169, 245, 575 Pore structure, 199, 225, 439, 669, 685 volume, 543 size measurement, 283 Porosimetry, 75, 575, 677 Porosimetry, relation to diffusion, 199 Porosity, 141, 329, 669, 693, 709, 747 determination, 189 development, 367 measurement, 727 of organic tissue, 141 Porous, 51, 273 glass, 709 materials, characterization of, 169 media, analysis of, 169 medium, 97 networks, 115 silica, 293 solids, 85, 105, 115, 151 structure, 701, 717 materials (Synthetic and natural), 245 Power compaction, 267 Preadsorption, 449 Pycnometry, 677 Pyrolysis, 347 Reference material, 133 SAXS (small angle X-ray scattering), 379 Selectivity, 21 Silica, 311, 543, 575 Silicalite-1, 509 Silicates, 747 Silver, 217 Simulation, 11
Sintering, 659 Small angle X-ray scattering, 245, 389 Small-angle neutron scattering, 257, 267 Small-angle scattering (SANS and SAXS), 245 Smectite, 625 Soil, 283 Sol-gel, 615 SiO,-AI,O,, 599 Solvent, 209 Sorption of carbon dioxide, 319 Specific surface area, 625 Sphere packing, 41 Standardisation, 133 Statistical mechanics, 123 Steam and carbon dioxide activation, 367 Sulfides, 677 Sulphyrization, 747 Supported metallic oxides, 645 Surface area, 151, 179, 189, 543, 625 excess, 341 groups, 329 homogeneity, 31 1 roughness, 179 texture, 245 Swelling, 209, 677 Synthetic carbons, 439 Textile, 409 Textural properties, 347 Texture (porous), 659 Thermal stability, 553, 625 Thermoporometry, 209 Thiele theory, 717 Thin film, 85 Thin section, 727 Titania-supported vanadia, 645 Titanium-silicalite, 553 Tortuosity and pore structure, 199 Ultra-low small angle X-ray scattering (USAXS), 245 Vanadia, 645 Vanadia, Al,O,-supported, 645 Vapour adsorption, 439 Washburn equation, 97 Water, 565 Water adsorption, 179, 319, 341, 389 isotherm, 607 vapour, 685
778
vapour sorption, 653 Wet materials, 301 Wetting, 85 transition, 85 X-ray diffaction, 389 Xylene adsorption, 553
Yttrium oxide, 659 Zeolite and aluminophosphates, 535 Zeolite channels, 553 Zeolites, 31, 161, 525 Zirconium dioxide, 659
779
STUDIES IN SURFACE SCIENCE AND CATALYSIS Advisory Editors: B. Delmon, Universith Catholique de Louvain, Louvain-la-Neuve, Belgium J.T. Yates, University of Pittsburgh, Pittsburgh, PA, U S A .
Volume 1 Preparation of Catalysts I.Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the First International Symposium, Brussels, October 1417,1975 edited by B. Delmon, P.A. Jacobs and G. Poncelet Volume 2 The Control of the Reactivity of Solids. A Critical Survey of the Factors that Influence the Reactivity of Solids, with Special Emphasison the Control of the Chemical Processes in Relation to Practical Applications by V.V. Boldyrev, M. Bulens and B. Delmon Volume 3 Preparation of Catalysts II. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Second International Symposium, Louvain-la-Neuve, September 4-7, 1978 edited by B. Delmon, P. Grange, P. Jacobs and G. Poncelet Volume 4 Growth and Properties of Metal Clusters. Applications to Catalysis and the Photographic Process. Proceedings of the 32nd International Meeting of the Socibte de Chimie Physique, Villeurbanne, September 24-28, 1979 edited by J. Bourdon Volume 5 Catalysis by Zeolites. Proceedings of an InternationalSymposium, Ecully (Lyon), September 9- 1 1, 1980 edited by B. Imelik, C. Naccache, Y. Ben Taarit, J.C. Vedrine, G. Coudurier and H. Praliaud Volume 6 Catalyst Deactivation. Proceedings of an International Symposium, Antwerp, October 13- 15,1980 edited by B. Delmon and G.F. Froment Volume 7 New Horizons in Catalysis. Proceedings of the 7th InternationalCongress on Catalysis, Tokyo, June 30-July 4, 1980. Parts A and B edited by T. Seiyama and K. Tanabe Volume 8 Catalysis by Supported Complexes by Yu.1. Yermakov, B.N. Kuznetsovand V.A. Zakharov Volume 9 Physics of Solid Surfaces. Proceedings of a Symposium, Bechyiie, September 29October 3, 1980 edited by M. Liznieka Volume 10 Adsorption at the Gas-Solid and Liquid-Solid Interface. Proceedings of an InternationalSymposium, Aix-en-Provence, September 2 1-23, 198 1 edited by J. Rouqueroland K.S.W. Sing Volume 1 1 Metal-Support and Metal-Additive Effects in Catalysis. Proceedings of an InternationalSymposium, Ecully (Lyon), September 14-16. 1982 edited by B. Imelik, C. Naccache, G. Coudurier, H. Praliaud, P. Meriaudeau, P. Gallezot, G.A. Martin and J.C. Vedrine Volume 12 Metal Microstructures in Zeolites. Preparation - Properties - Applications. Proceedings of a Workshop, Bremen, September 22-24, 1982 edited by P.A. Jacobs, N.I. Jaeger, P. JirQand G. Schulz-Ekloff Volume 13 Adsorption on Metal Surfaces. An Integrated Approach edited by J. BBnard Volume 14 Vibrations at Surfaces. Proceedings of the Third International Conference, Asilomar, CA, September 1-4, 1982 edited by C.R. Brundle and H. Morawitz
780 Volume 15 HeterogeneousCatalytic Reactions Involving Molecular Oxygen by G. I.Golodets Volume 16 Preparation of Catalysts 111. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Third International Symposium, Louvain-la-Neuve, September 6-9,1982 edited by G. Poncelet, P. Grange and P.A. Jacobs Volume 17 Spillover of Adsorbed Species. Proceedings of an International Symposium, LyonVilleurbanne, September 12-1 6, 1983 edited by G.M. Pajonk, S.J. Teichner and J.E. Germain Volume 18 Structure and Reactivity of Modified Zeolites. Proceedings of an International Conference, Prague, July 9-13, 1984 edited by P.A. Jacobs, N.I. Jaeger, P. Jire, V.B. Kazansky and G. Schulz-Ekloff Volume 19 Catalysis on the Energy Scene. Proceedings of the 9th Canadian Symposium on Catalysis, Quebec, P.Q., September 30-October 3, 1984 edited by S. Kaliaguine and A. Mahay Volume 20 Catalysis by Acids and Bases. Proceedings of an InternationalSymposium, Villeurbanne (Lyon), September 25-27, 1984 edited by B. Imelik. C. Naccache, G. Coudurier, Y. Ben Taarit and J.C. Vedrine Volume 2 1 Adsorption and Catalysis on Oxide Surfaces. Proceedings of a Symposium, Uxbridge, June 28-29, 1984 edited by M. Che and G.C. Bond Volume 22 Unsteady Processes in Catalytic Reactors by Yu.Sh. Matros Volume 23 Physics of Solid Surfaces 1984 edited by J. Koukal Volume 24 Zeolites: Synthesis, Structure, Technology and Application. Proceedings of an InternationalSymposium, Portoroi-Portorose, September 3-8, 1984 edited by B. Deaj, S. HoEevar and S. Pejovnik Volume 25 Catalytic Polymerization of Olefins. Proceedings of the InternationalSymposium on Future Aspects of Olefin Polymerization, Tokyo, July 4-6, 1985 edited by T. Keii and K. Soga Volume 26 Vibrations at Surfaces 1985. Proceedings of the Fourth InternationalConference, Bowness-on-Windermere, September 15-1 9, 1985 edited by D.A. King, N.V. Richardsonand S. Holloway Volume 27 Catalytic Hydrogenation edited by L. Cervenq Volume 28 New Developments in Zeolite Science and Technology. Proceedings of the 7th InternationalZeolite Conference, Tokyo, August 17-22, 1986 edited by Y. Murakami, A. lijima and J.W. Ward Volume 29 Metal Clusters in Catalysis edited by B.C. Gates, L. Guczi and H. Knozinger Volume 3 0 Catalysis and Automotive Pollution Control. Proceedings of the First International Symposium, Brussels, September 8-1 1, 1986 edited by A. Crucq and A. Frennet Volume 3 1 Preparation of Catalysts IV. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Fourth InternationalSymposium, Louvain-la-Neuve, September 1-4, 1986 edited by 6. Delmon, P. Grange, P.A. Jacobs and G. Poncelet Volume 32 Thin Metal Films and Gas Chemisorption edited by P. Wissmann Volume 33 Synthesis of High-silica Aluminosilicate Zeolites by P.A. Jacobs and J.A. Martens Volume 3 4 Catalyst Deactivation 1987. Proceedings of the 4th InternationalSymposium, Antwerp, September 29-October 1, 1987 edited by B. Delmon and G.F. Froment
781 Volume 35 Keynotes in Energy-RelatedCatalysis edited by S. Kaliaguine Volume 36 Methane Conversion. Proceedings of a Symposium on the Production of Fuels and Chemicals from Natural Gas, Auckland, April 27-30, 1987 edited by D.M. Bibby, C.D. Chaney, R.F. Howe and S. Yurchak Volume 37 Innovation in Zeolite Materials Science. Proceedings of an International Symposium, Nieuwpoort, September 13-17, 1987 edited by P.J. Grobet, W.J. Mortier, E.F. Vansant and G. Schulz-Ekloff Volume 38 Catalysis 1987. Proceedings of the 10th North American Meeting of the Catalysis Society, San Diego, CA, May 17-22, 1987 edited by J.W. Ward Volume 39 Characterization of Porous Solids. Proceedings of the IUPAC Symposium (COPS I), Bad Soden a. Ts., April 26-29, 1987 edited by K.K. Unger, J. Rouquerol, K.S.W. Sing and H. Kral Volume 40 Physics of Solid Surfaces 1987. Proceedings of the Fourth Symposium on Surface Physics, Bechyne Castle, September 7-1 1, 1987 edited by J. Koukal Volume 4 1 HeterogeneousCatalysis and Fine Chemicals. Proceedings of an International Symposium, Poitiers, March 15-17, 1988 edited by M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, C. Montassier and G. Pdrot Volume 42 Laboratory Studies of Heterogeneous Catalytic Processes by E.G. Christoffel, revised and edited by Z.Pa61 Volume 43 Catalytic Processes under Unsteady-State Conditions by Yu. Sh. Matros Volume 44 Successful Design of Catalysts. Future Requirementsand Development. Proceedings of the Worldwide Catalysis Seminars, July, 1988, on the Occasion of the 30th Anniversary of the Catalysis Society of Japan edited by T. lnui Volume 45 Transition Metal Oxides: Surface Chemistry and Catalysts by H.H. Kung Volume 46 Zeolites as Catalysts. Sorbents and Detergent Builders. Applications and Innovations. Proceedings of an InternationalSymposium, Wirrzburg, F.R.G., September 4-8, 1988 edited by H.G. Karge and J. Weitkamp Volume 47 Photochemistry on Solid Surfaces edited by M. Anpo and T. Matsuura Volume 48 Structure and Reactivity of Surfaces. Proceedings of a European Conference, Trieste, Italy, September 13-16, 1988 edited by C. Morterra, A. Zecchina and G. Coste Volume 49 Zeolites: Facts, Figures, Future. Proceedings of the 8th InternationalZeolite Conference, Amsterdam, The Netherlands, July 10-1 4, 1989 edited by P.A. Jacobs and R.A. van Santen Volume 5 0 Hydrotreating Catalysts. Preparation, Characterizationand Performance. Proceedings of the Annual International AlChE Meeting, Washington, DC, November 27-December 2, 1988 edited by M.L. Occelli and R.G. Anthony Volume 5 1 New Solid Acids and Bases. Their Catalytic Properties by K. Tanabe, M. Misono, Y. Ono and H. Hattori Volume 52 Recent Advances in Zeolite Science. Proceedings of the 1989 Meeting of the British Zeolite Association, Cambridge, April 17-1 9, 1989 edited by J. Klinowski and P.J. Barrie Volume 53 Catalyst in Petroleum Refining 1989. Proceedings of the First International Conference on Catalysts in Petroleum Refining, Kuwait, March 5-8, 1989 edited by D.L. Trimm, S. Akashah, M. Absi-Halabi and A. Bishara
782 Volume 54 Future Opportunities in Catalytic and Separation Technology edited by M. Misono, Y.Moro-oka and S. Kimura Volume 55 New Developments in Selective Oxidation. Proceedings of an International Symposium, Rimini, Italy, September 18-22, 1989 edited by G. Centi and F. Trifiro Volume 56 Olefin Polymerization Catalysts. Proceedings of the International Symposium on Recent Developments in Olefin PolymerizationCatalysts, Tokyo, October 23-25, 1989 edited by T. Kelli and K. Soga Volume 57A Spectroscopic Analysis of Heterogeneous Catalysts. Part A: Methods of Surface Analysis edited by J.L.G. Fierro Volume 578 Spectroscopic Analysis of Heterogeneous Catalysts. Part 6: Chemisorption of Probe Molecules edited by J.L.G. Fierro Volume 58 Introduction t o Zeolite Science and Practice edited by H. van Bekkum, E.M. Flanigen and J.C. Jansen Volume 59 Heterogeneous Catalysis and Fine Chemicals II. Proceedings of the 2nd International Symposium, Poitiers, October 2-5, 1990 edited by M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, G. Perot, R. Maurel and C. Montassier Volume 6 0 Chemistry of Microporous Crystals. Proceedings of the InternationalSymposium on Chemistry of Microporous Crystals, Tokyo, June 26-29, 1990 edited by '6. Inui, S. Namba and T. Tatsumi Volume 6 1 Natural Gas Conversion. Proceedings of the Natural Gas Conversion Symposium, Oslo, August 12- 17, 1990 edited by A. Holmen, K.-J. Jens and S. Kolboe Volume 62 Characterization of Porous Solids II. Proceedings of the IUPAC Symposium (COPS 11). Alicante, May 6-9, 1990 edited by F. Rodriguez-Reinoso,J. Rouquerol, K.S.W. Sing and K.K. Unger Volume 63 Preparation of Catalysts V. Proceedings of the Fifth InternationalSymposium on the Scientific Bases for the Preparation of Heterogeneous Catalysts, Louvain-laNeuve, September 3-6, 1990 edited by G. Poncelet, P.A. Jacobs, P. Grange, and 6. Delmon
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