Table Of ContentStudies in Surface Science and Catalysis
Advisory Editors: B. Delmon and J.T. Yates
Vol. 62
CHARACTERIZATION OF
POROUS SOLIDS II
Proceedingso f the IUPAC Symposium (COPS 11). Alicante, Spain,
May 6- 9,1990
Editors
F. Rodriguez-Reinoso
Departamento de Quimica lnorgdnica e Ingenieria Quimica, Universidad de
Alicante, Apartado 99, Alicante, Spain
J. Rouquerol
Centre de Thermodynamiquee t 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 -6500M ainz, F. R. G.
- - -
ELSEVIER Amsterdam Oxford New York Tokyo 1991
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Library of Congress Cataloging-in-Publication Data
IUPAC Symposium. COPS (2nd : 1990 : Alicante. Spain)
Characterization of porous solids I1 : proceedings of the IUPAC
Symposium, COPS 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., 1941-
11. International Union of PGre and Applied Chemistry. 111. Title.
IV. Title. Characterization of porous solids 2. V. Title:
Characterization of porous solids two. VI. Series.
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
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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. Rodriguez-
Reinoso 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
F. Rodriguez-Reinosoe t al. (Editors), Characterization of Porous Solids II
1991 Elsevier Science Publishers B.V., Amsterdam
CHARACTERIZATION OF POROUS SOLIDS: AN INTRODUCTORY SURVEY
Kenneth S.W. Sing
Department of Chemistry, Brunel University, Uxbridge,
Middlesex, UB8 3PH, United Kingdom.
BACKGROUND
The widespread interest in porous solids is well illustrated by the
multifarious nature of the contributions to this volume. Much of the work
reported was undertaken on materials of technological importance such as
adsorbents, catalysts and constructional materials and the solids studied
include carbons, oxides, cements, clays, polymers, zeolites and metal films.
In view of this wide diversity of interest, it is pertinent to ask whether
such a broadly based symposium is likely to be useful from a scientific
standpoint. The first IUPAC Symposium on the Characterization of Porous
Solids was held in 1987 (i.e. COPS I, Elsevier, 1988) and it was evident
then that there was a need for further systematic work. In his introductory
paper, Everett drew attention to some of the outstanding problems including
the important requirement of predicting technological performance from the
results of characterization measurements. This became all the more urgent
with the development of advanced materials and shape selective catalysts
which require the application of sophisticated characterization techniques.
The way was therefore prepared for the COPS II Symposium to be held
in 1990 and the opportunity was then taken to review the status of the more
traditional techniques such as adsorption and fluid penetration alongside the
newer experimental techniques and computational procedures (e.g. small angle
scattering, computer simulation and molecular modelling). This introductory
survey is not designed to provide a systematic appraisal of the work
described here, but rather to set the scene for these Proceedings of an
important symposium.
TERMINOLOGY AND MODEL SYSTEMS
The ubiquity of porous materials has led to confusion in the usage of
such terms as 'micropore', 'macropore', 'total pore volume' and 'internal area'.
In the IUPAC classification of pore size, the micropore width is taken to not
exceed about 2 nm (2ORJ. the mesopore width to be in the range 2-50 nm
and the macropore width to be above about 50 nm (0.05 pm). In recent years
these definitions have served us well, especially in the context of gas
'
adsorption and mercury porosimetry, but it is becoming increasingly clear
that some refinements are required and that account should be taken of pore
shape.
It is apparent that the pore structures of many systems of technological
importance (e.g. building materials) are made up of cracks, cavities and
channel networks of varying size, shape and connectivity. On the other
hand, pore structures can now be prepared which are remarkably uniform
and correspond fairly closely to model systems.
2
Zeolitic structures of high Si/AI ratio are generally quite difficult to
synthesise in the form of large crystals. It is therefore noteworthy that
Unger and his co-workers have been able to synthesise large crystals of
ZSM-5, Silicalite 1 and ZSM-48. This has enabled Reichert el al to gain a
much improved understanding of the intrinsic properties of these zeolites
than was formerly possible.
Molecular seive carbons can now be prepared from various polymeric
precursors. High-resolution electron microscopy has revealed that the pores
are predominanently slit-shaped. Other systems which exhibit slit-shaped
pores are the pillared clays and certain inorganic oxides produced by the
controlled thermal decomposition of parent hydroxides such as Ca(OHI2 and
Mg (OH) 2.
Karnaukhov has classified porous solids as spongy and corpuscular.
Many corpuscular systems are unconsolidated or only weakly aggregated. If
the area of contact between an assemblage of globular particles is small the
system will behave in some ways as a non-porous powder (e.g. with respect
to gas adsorption). If the powder is subjected to compaction or heat
treatment it will tend to undergo an irreversible change. The weakly-bonded
aggregate is thus converted into a more compact agglomerate with a
well-defined pore structure. Systems of this type are discussed by
Karnaukhov, Mason, Ramsay and others.
Dubinin and his co-workers first suggested that micropores should be
sub-divided into two groups, which are now usually termed ultramicropores
<
and supermicropores. Ultramicroporous solids (of pore width ca 0.7 nm)
are likely to exhibit molecular sieve properties, whereas supermicroporous
solids generally have larger internal areas and pore volumes which are
accessible to a wider range of adsorptive molecules. If these somewhat
inelegant terms are to be retained it would be desirable to define the ranges
of size more precisely in relation to pore shape (e.g. slits and cylindrical
channels).
It is obvious that as the pore width is reduced and approaches
molecular dimensions so the absolute magnitude of the pore volume becomes
more difficult to evaluate. For this reason it has been recommended that the
term effective pore volume should be employed and the operational procedure
used for its evaluation clearly specified.
The COPS-I Symposium (Elsevier, 1988) provided the first opportunity
for an extensive discussion of the role of fractal analysis in the
characterization of the texture of solids. Although some aspects are open to
criticism there is little doubt that fractal geometry has been shown to be a
useful tool in the analysis of data obtained with porous solids or rough
surfaces. The studies by Krim and Panella, Johnston et al, Dore and North
and Sernetz and his co-workers illustrate the application of fractal geometry
for the analysis of various types of experimental data obtained with rough
surfaces and porous materials. At the very least, the proponents of fractal
analysis can justifiably claim that the approach provides a systematic basis
for the analysis of experimental data obtained with structurallv complex
nlHerials. Unfortunately, the results of the analysis are ofte; difficult
to interpret!
ADSORPTION
Experimental Techniques
The measurement of adsorption at the gas/solid interface continues to
be one of the most popular techniques for the study of microporous and
3
mesoporous solids. It is not surprising therefore that many papers in this
symposium are concerned with the determination and interpretation of gas
adsorption data.
Great advances have been made in the development of automated equipment for
adsorption isotherm measurements, but it is not always easy to obtain reliable
data. Robens and Krebs stress the desirability of calibrating new instruments
with the aid of reference materials and Ginoux and Bonnetain also draw
attention to some of the likely sources of error in isotherm measurements.
The papers by Conner, Kaneko, Rouquerol, Unger and their co-workers
underline the importance now attached to the determination of physisorption
isotherms at very low levels of surface coverage or fractional micropore filling,
i.e. in the region of very low p/po. Such high resolution adsorption (HRADS)
measurements have been shown to be especially useful for the characterization
of the adsorptive properties of zeolites, aluminophosphates and molecular
sieve carbons.
Another benefit of automated instrumentation is that the detailed course
of an isotherm can be established over any pre-selected range of p/po.
Equipment of this type operating in the mode of continuous flow was first
used by Rouquerol and his co-workers in conjunction with microcalorimetry
for studying changes in state of the adsorbed phase. The results of
continuous adsorption measurements are also reported here by Ajot et al.
Micropore Fillinq
It is now generally agreed that physisorption within the narrowest micropores
(i. e. the ultramicropores) does not involve monolayer formation, but instead
takes place preferentially at very low p/po (initially around p/po/v
This process is associated with enhanced adsorbent-adsorbate interactions and
results in an appreciable distortion of the adsorption isotherm. The mechanism
of physisorption in the wider micropores (i. e. the supermicropores) is much
less well understood, but appears to involve cooperative adsorbate-adsorbate
interactions so that adsorption takes place at somewhat higher p/po (-0.01-0.2)
by an assemblage of molecules, i.e. giving quasi-multilayer formation. In this
connection it is of interest to note the findings of Nicholson and Tan & Gubbins.
These two papers deal with adsorption in model slit-shaped pores within a
graphitic structure; the former by the application of grand canonical emsemble
simulation to follow the adsorption of argon and the latter by the use of mean-
field density-functional theory to model the behaviour of methane and ethane.
These studies appear to support the view that favourable circumstances exist
for the filling of pores of particular dimensions (in relation to the molecular
diameter) and point the way for further work.
The question of the validity of the Dubinin-Radushkevich (DR) equation
continues to attract a good deal of attention. Many authors still use the DR
plot for the assessment of the micropore volume whilst others are more
cautious in their interpretation of the derived values of micropore volume
and pore width. Confirmation is provided in a paper by Rodriguez-Reinoso
and his co-workers that excellent agreement can be obtained between the
values of micropore volume obtained by extrapolation of DR plots and the
corresponding a -plots provided that certain conditions are fulfilled - namely
that the microposre size distribution is not too broad. The Alicante scientists
also draw attention to the difficulty of obtaining suitable non-porous reference
materials when dealing with microporous carbons having high ash contents.
Kaneko and his co-workers have noted that some DR plots appear to
exhibit a succession of linear regions. These features are interpreted in
terms of a multistage mechanism of micropore filling, i.e. an extension of
the principles of primary and cooperative micropore filling.
4
As McEnaney and Mays point out, the simple DR equation is based on
the assumption that the micropore structure is homogeneous, i.e. that all the
micropores in the adsorbent give the same characteristic adsorption
.
potential, E Since the equation has a very general mathematical form, this
requirement'cannot be tested by simple inspection of the DR plot and there
is little doubt that most microporous solids are structurally heterogeneous.
To overcome this problem Dubinin, McEnaney, Stoeckli and Kadlec have
proposed generalised forms of the DR equation which in principle should be
applicable to heterogeneous microporous solids. In practice, the main
problem in adopting this approach is to arrive at a unique solution for the
probability density function of E and hence the micropore size distribution.
These aspects are discussed in &me detail by McEnaney and Mays.
As mentioned earlier, adsorption microcalorimetry is an invaluable
technique for studying the thermodynamic properties of adsorption systems.
The paper by Martin-Martinez et al provides a good example of how
adsorption enthalpy measurements can yield a clearer understanding of the
mechanisms of micropore filling and surface coverage. An improved isosteric
method has been developed by Rees and his co-workers. Their measurements
have revealed energetic heterogeneity in the adsorption of ethane and propane
by Silicalite I.
Jessop et al have developed a novel procedure for computing the pore
size distribution from nitrogen isotherm data. The method is based on the
application of mean-field theory for the calculation of a set of isotherms
corresponding to pores of given width and it is claimed that the molecular
model provides a realistic representation of the adsorbed fluid in pores of all
sizes and that the method can therefore be used for both micropore and
mesopore analysis. A limited number of comparisons have been made with
more conventional methods of pore size analysis, but it is probably too early
to judge the success of this interesting approach.
Adsorption Hysteresis
It is well known that capillary condensation in mesopores is generally
associated with hysteresis. Progress has been made in linking the
characteristic shapes of certain hysteresis loops with the nature of the pore
structure, but much remains to be done to explain the mechanisms of mesopore
filling and emptying. The papers by Mayagoitia, Neimark and Efremov and
Fenelonov on the role of porous networks show how further progress can be
made by the systematic computer-assisted analysis of a number of carefully
selected model systems. The fundamental question of whether to adopt the
adsorption or desorption branch of the hysteresis loop for mesopore analysis
remains unresolved. Indeed, it seems likely that there is no simple answer,
but that the computational procedure should be governed by the pore geometry
and network configuration.
For many years it was thought that any hysteresis appearing before the
onset of capillary condensation was the result of slow equilibration or
inaccurate measurements. It is now known, however, that there are two types
of low-pressure hysteresis which are associated with particular systems. The
first is a well-defined hysteresis loop appearing at p/po/v 0.1 and given for
example by N2 isotherms on HZSM-5 at 77K. 'This loop has been studied
in some detail by MUller and Unger, who attribute it to a phase transformation
(liquid-like to solid-like structures). The present paper by Pan and Mersmann
provides a somewhat different explanation based on a combination of localized
adsorption on a range of surface sites and interaction between adsorbed
molecules.
5
The second type of low-pressure hysteresis extends down to much
lower pressure and is due to either an irreversible change in the adsorbent
(e.g. swelling or surface chemical change) or to the slow passage of
molecules through very narrow pore entrances or between small aggregated
particles. Rodriguez-Reinoso and his co-workers now report new results
with microporous carbons, which reveal that the development of low-pressure
hysteresis is dependent on the atmosphere (COz or air) in which the carbons
are activated. The authors offer the tentative explanation that the
appearance of this type of hysteresis is associated with the development of
different surface structures.
Adsorption of Water Vapour
A number of papers in this volume are concerned with the adsorption of
water vapour, which is of great importance in the context of gas separation
or respiratory protection. Since activated carbon filters have a low affinity
for water vapour, very little pore blocking occurs at low relative humidity.
However, if the respirator is used in a humid atmosphere or the carbon
previously exposed to water vapour, the adsorption efficiency is seriously
impaired.
The work of Carrott et al has revealed that Silicalite is in effect more
hydrophobic than any microporous carbon studied so far, since it has both
low affinity and low capacity for water vapour. It is suggested that the low
water capacity is directly related to the tubular nature of the intracrystalline
channels in Silicalite and that a thin layer of hydrogen-bonded water molecules
can more easily form within the slit-shaped pores of activated carbons.
Adsorption from Solution
Adsorption from solution measurements have been employed for many
years to characterize industrial adsorbents, but the data obtained are often
difficult to interpret. Rouquerol and his co-workers have now made a
systematic study of a series of activated charcoals in which the results of
adsorption from solution are compared with data obtained by gas adsorption
and immersion calorimetry. By adapting the@- method, they have shown that
the adsorption of benzene from ethanol solutio8 is comparable with that of
nitrogen from the gas phase and that the adsorption from solution data
obtained with probe molecules of different shape provide a useful means of
studying the enlargement of mciropore entrances.
Another interesting study of solution adsorption reported here is that
of Eltekova and Eltekov on the adsorption of macromolecules by mesoporous
carbons and silicas. It is evident from this work that the adsorption of these
large solute molecules can be optimised by control of the pore structure and
it is tempting to suggest that 'micropore filling' effects should be taken into
account.
FLUID PENETRATION AND FLOW
As Everett has pointed out (see COPS I, Elsevier, 1988, p.7). the density of
porous solids is not a straightforward concept. A problem of interpretation
arises when the volume occupied by a given mass of solid appears to be
dependent on the fluid (gas or liquid) displaced. This disparity is indicative
of differences in the degree of penetrationof the fluids into the particular
pore structure and may be the result of either molecular sieving or the effects
of capillarity.
Wetting behaviour is often discussed in terms of contact angle
measurements, but the paper by Demlehher draws attention to the difficulty of
6
obtaining agreement between contact angles determined by different methods.
A way of avoiding the contact angle problem is discussed in the paper by
Winter, which deals with wetting and displacement of liquid in single pores
and capillary networks. Another problem is the swelling which occurs when
porous polymers are immersed in organic liquids or even subjected to vapour
sorption. However, Belyakova has found that this may be minimised by the
choice of adsorptive and control of p/po.
Mercury Porosimetry
Mercury porosimetry is featured in many of the contributions to this
volume. Indeed, it is now one of the most popular methods available for the
characterization of a wide range of porous materials and the derived pore
sizes are often quoted in the patent and technical literature. The method is
based on the non-wetting nature of mercury and the application of the
Washburn equation. The volume of mercury penetrating into a porous solid
is determined as a function of the applied pressure, which is assumed to be
directly related to the pore width.
In spite of the growing popularity of mercury porosimetry and the ready
availability of excellent automated equipment, the interpretation of the mercury
intrusion-extrusion data is still far from clear. The values of surface tension
and contact angle which must be inserted in the Washburn equation are still
uncertain - as are the limits of applicability of the equation itself. Other
problems include the reversible or irreversible deformation of the pore
structure, which undoubtedly occurs with some corpuscular or weakly
agglomerated systems.
Many different explanations have been proposed for the appearance of
intrusion-extrusion hysteresis which appears to be a universal feature of
mercury porosimetry. The paper by Day et al helps to provide a better
understanding of this phenomenon and also the related irreversible
entrapment of mercury. The ICI scientists have extended and improved the
network model approach originally used by Haynes, Mann and Conner. By
computer simulation of a three-dimensional network it is possible to model the
pathways of advancing and receding mercury threads and explore the effects
of blocking and knocking out pores. The work is still in progress, but the
comparisons with real systems made so far indicate that a mechanism
involving the spontaneous nucleation of the mercury meniscus at the start of
extrusion is untenable and that some form of air seeding is probably
essential. Careful experimental work in the IC I laboratories has confirmed
that a high level of reproducibility can be achieved in partial intrusion,
scanning and recycling experiments.
Lentz and Zhou have carried out an interesting investigation of the effect on
mercury intrusion of partially filling the pores with another liquid. They
explain their results by postulating a change in the contact angle, but this
explanation is open to question in view of the complexity of the pore
structures studied so far. However, it should be rewarding to carry out
more work of this type with carefully selected systems.
Davis and his co-workers have extended their investigations of well-defined
porous silicas. They report fairly good agreement between the pore volumes
and pore size distributions determined by mercury porosimetry and nitrogen
adsorption, but lack of agreement between the corresponding surface areas.
(The latter values calculated from the mercury intrusion curves are
.
appreciably higher than the corresponding BET-areas) These and other
results underline the urgent need for more fundamental work to provide a
more rigorous basis for the interpretation of mercury porosimetry data.
Fluid Flow
The rate of movement of fluids into and through porous media is of
great importance in agriculture, civil engineering, catalysis and separation
technology. As Conner et al point out, many attempts have been made to
correlate permeability (or transport resistance) with the morphology of a
porous solid. However, it is not surprising to find that no simple correlation
can be found between the transport properties and the porosity as studied by
gas adsorption or mercury porosimetry. Another complication is that
adsorption kinetics are notoriously difficult to model at the molecular level.
Thus, although gaseous diffusion in zeolites and molecular sieve carbons has
been widely studied, the data in the literature show many anomalies and
inconsistencies.
The problems encountered in experimental permeability studies are
discussed in a number of papers (e.g. Sato; Bhewmik et al; Quinson et al;
Sing and Yates). An unexpected development of high permeability in
porous plugs or membranes is often the result of uneven macropore or crack
formation during manufacture, storage or operation (e.g. dimensional changes
of membranes). In their study of model systems, Kanellopoulos and his
co-workers discuss the effects on gas permeability of different forms of
network heterogeneity. It appears from this and other studies that similar
changes in permeability and percolation thresholds may originate in quite
different ways and highlights the need for caution in the interpretation of
permeability data. An alternative approach is presented in the paper by
Mason and Mellor, which follows the earlier work by Mason (see COPS I) on
percolation and network theory. In their present paper, attention is given to
beds of packed spheres and it isconcluded that such systems can be treated
as networks arranged in the form of the 3-D diamond lattice. However,
simulation of drainage and imbibition appears to indicate that the bond and
cavity sizes are not randomly distributed throughout the network.
Mass transport
The role of the pore structure in mass transport in adsorbents and
catalysts is discussed in the papers by Scholl and Mersmann and Boon et al.
In the former study, which involves modelling the adsorption kinetics,
allowance is made for the effect of variation of total pressure on concentration
and temperature profiles within a spherical particle and thus simulate the
conditions of pressure swing adsorption. The other study by Boon et al is
concerned with the behaviour of porous oxide-based catalyst spheres prepared
by the sol-gel method. Although they deal with very different systems and
circumstances, these two papers both draw attention to the importance of
macroporosity in diffusion control and mass transport.
M I SC ELLANE OUS TECH N IQ UE S
Microscopy
Although they do not appear to occupy a prominent place in the present
volume, microscopic techniques continue to play a vital role in the
characterization of many porous materials. Thus, confidence can be gained in
the interpretation of adsorption or flow data if independent evidence can be
obtained of pore shape or texture uniformity.
The paper by Pis et al provides a good example of the application of
optical microscopy. In this case,image-analysis has been used to provide a
quantitative evaluation of the number, size and shape of pores in cokes
produced by progressive oxidation. The results are compared with the
mercury intrusion data and the two techniques shown to be complementary.
