Molecular Sieves—II

723 Pages·1977·12.63 MB·English

by  James R. Katzer (Eds.)

Molecular Sieves—II
James R. Katzer, EDITOR
University of Delaware
The Fourth International
Conference co-sponsored by
the Divisions of Colloid and
Surface Chemistry,
Petroleum Chemistry, and
Physical Chemistry of the
American Chemical Society
and by the University of
Chicago at the University
of Chicago, Chicago, Ill.,
April 18-22, 1977
ACS SYMPOSIUM SERIES 40
In Molecular Sieves—II; Katzer, J.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
Library of Congress CIP Data
International Conference on Molecular Sieves, 4th, Uni
versity of Chicago, 1977.
Molecular sieves II.
(ACS symposium series; 40 ISSN 0097-6156)
Papers of the 4th of a series of meetings; papers of the
3rd are entered under the title: Molecular sieves.
Bibliography: p. Includes index.
1. Zeolites—Congresses. 2. Molecular sieves—Con
gresses.
I. Katzer, James R., 1941- . II. American Chemical
Society. Division of Colloid and Surface Chemistry. III.
Title. IV. Series: American Chemical Society. ACS sym
posium series; 40.
TP245.S5I59 661'.06'8324 77-720
ISBN 0-8412-0362-8 ACSMC 8 40 1-732
Copyright © 1977
American Chemical Society
All Rights Reserved. No part of this book may
be reproduced or transmitted in any form or by
any means—graphic, electronic, including photo
copying, recording, taping, or information storage
and retrieval systems—without written permission
from the American Chemical Society.
PRINTED IN THE UNITED STATES OF AMERICA
In Molecular Sieves—II; Katzer, J.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
ACS Symposium Series
Robert F. Gould, Editor
Advisory
Donald G. Crosby
Jeremiah P. Freeman
E. Desmond Goddard
Robert A. Hofstader
John L. Margrave
Nina I. McClelland
John B. Pfeiffer
Joseph V. Rodricks
Alan C. Sartorelli
Raymond B. Seymour
Roy L. Whistler
Aaron Wold
In Molecular Sieves—II; Katzer, J.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
FOREWORD
The ACS SYMPOSIU
a medium for publishing symposia quickly in book form. The
format of the SERIES parallels that of the continuing ADVANCES
IN CHEMISTRY SERIES except that in order to save time the
papers are not typeset but are reproduced as they are sub
mitted by the authors in camera-ready form. As a further
means of saving time, the papers are not edited or reviewed
except by the symposium chairman, who becomes editor of
the book. Papers published in the ACS SYMPOSIUM SERIES
are original contributions not published elsewhere in whole or
major part and include reports of research as well as reviews
since symposia may embrace both types of presentation.
In Molecular Sieves—II; Katzer, J.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
PREFACE
ith the" discovery of the catalytic properties of zeolites in the late
• • 1950s, the level of research activity into zeolite synthesis, structure,
and properties changed from one of slow continual progress to intense
pursuit. This led to three International Conferences on Molecular Sieves.
The first in London in 1967 was in recognition of the pioneering work
of R. M. Barrer, the second was at Worcester Polytechnic Institute in
1970, and the third was in Zurick, Switzerland in 1973. About 300 sci
entists and engineers fro
backgrounds attended the sessions. These meetings attempted to cover
the scope of molecular sieve science including mineralogy, structure,
synthesis, modification, adsorption, diffusion, catalytic properties, and
technological applications. This intermingling of disciplines has been a
critically important benefit of previous conferences and of the resulting
proceedings.
This volume of the ACS Symposium Series contains the program
papers of the Fourth International Conference on Molecular Sieves held
at the University of Chicago in April 1977. It includes papers on all the
aspects of molecular sieve science that were covered at the last three
conferences except mineralogy. Due to a Natural Zeolites meeting held
in Tuscon, Arizona in June 1976, no mineralogy papers were submitted
to this conference. It is unfortunate that this separation has occurred,
and a reuniting of these two areas would benefit the entire field of
zeolite science.
The program papers have been categorized into the topical sections:
Structure, Synthesis and Modification, Adsorption and Diffusion, Cataly
sis, and Technology. Such categorization was at times subjective because
many papers overlapped several topical areas. For example, papers con
cerning the structure and electronic properties of transition metal com
plexes in zeolites relate to the structural (location) properties (albeit not
framework structure) of zeolites, involve modification, and are of vital
interest to the catalytic properties. Similarly papers that characterize
metals, partially or completely reduced, are not truly catalytic and were
typically included as a modification.
A review paper by M. M. Dubinin is presented first. Reviews on
theoretical calculation of zeolite structure (Gibbs et al.), ion exchange
(Cremers), adsorption (Schirmer et al.), transition metal complexes in
zeolites (Lunsford), the acid catalytic properties of zeolites (Bartho-
ix
In Molecular Sieves—II; Katzer, J.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
meuf), adsorption applications (Anderson), and catalytic cracking tech­
nology ( Magee ) open each topical section.
Our knowledge of zeolite properties and the emphasis in zeolite
research has changed markedly over the last two decades as illustrated
by the papers in this volume and those of the previous three conferences.
Classical framework structural papers are no longer present as they
were appropriate when the field was new and growing. They have been
succeeded by papers concerned with the locations and electronic prop­
erties of cations in the cages and detailed structural information on the
location and properties of transition metal complexes in zeolites. The
physical-chemical approach of investigating zeolite properties is being
replaced by application of sensitive electronic (X-ray emission, XPS),
nuclear (NMR, Szilard-Chalmers cation recoil), and other probes
(Raman, IR, uv). Catalyti
studies are concerned no
and to quantify previously stated concepts. In the second trend new,
different chemistry is being investigated. An example of the latter is
the studies of transition metal complexes in zeolites. Catalytic properties
remain the least well characterized properties of zeolites and are the
topic of the most papers.
Adsorption and catalytic applications papers were encouraged in
recognition of the technological impact of zeolites. Because of the pro­
prietary nature of commercial catalysts and catalytic processing, few
papers were obtained in this area.
Thanks are due to the organizing committee which consisted of
J. V. Smith (Conference Chairman), D. W. Breck (Technical Program
Chairman), J. W. Ward (Financial Committee Chairman), D. M. Ruth-
ven, G. T. Kerr, and J. B. Uytterhoeven (previous chairmen) for all their
assistance in getting the program together. Special thanks are due to all
of the reviewers who have contributed greatly to the quality of the papers
in this volume, to the authors for their excellent preparation of the final
manuscripts, to ACS for their cooperation, and particularly to J. B.
Uytterhoeven for his advice on paper handling grained from his work on
the Third International Conference on Molecular Sieves. Support was
obtained from several organizations which are listed in the proceedings
of the conference.
University of Delaware JAMES R. KATZER
Newark, Delaware
November 18, 1976
χ
In Molecular Sieves—II; Katzer, J.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
1
Investigations of Equilibria and Kinetics of Adsorption
of Gases on Zeolites
M. M. DUBININ
Institute of Physical Chemistry, Academy of Sciences of the U.S.S.R.,
Union of Soviet Socialists Republic
A thermodynamic theory of adsorption equilib
rium based on the vacancy solution model and the
analogy between osmotic and adsorption equilibria
has been developed to describe adsorption on zeolites
over wide ranges of pressure and temperature. New
methods for investigating the kinetics of vapor ad
sorption by microporous adsorbents and for theoreti
cal description of these processes on the basis of
a biporous adsorbent model have been proposed. The
experimental data agree with theory.
Introduction
i^or the subject of the opening lecture which I
have been invited to deliver, I haven chosen the most
important results obtained recently at the Sorption
Processes Department headed by me at the Institute of
Physical Chemistry of the USSR Academy of Sciences.
These are theoretical and experimental investigations
into equilibrium physical adsorption carried out by
B.P.Bering and V«V»Serpinskii with the participation
of T.S.Yakubov and A.A.Fomkin, and studies into the
kinetics of physical adsorption far biporous-structu-
re adsorbents conducted by P.P.Zolotarev, A.M.Volo-
shchuk with the participation of I.T#Erashko, V#A#GOD-
lov, G.Schon arji V.I.Ulin.
Based on the sizes of their pores, zeolites are
typical microporous adsorbents. The commensurability
of the sizes of micropores and the molecules adsorbed
leads to a sharply defined effect of increase in ad
sorption potentials due to dispersion forces.Cations
in the zeolite voids considerably enhance (owing to
electrostatic interactions) the energy inhomogeneity
1
In Molecular Sieves—II; Katzer, J.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
2 MOLECULAR SIEVES—II
of the adsorption space of the micropores as compa
red with adsorbents of a different chemical nature,
such as activated carbons*
Our investigations of equilibrium adsorption of
the vapors of various substances in micropores, have
lead to the concept that there was a qualitative
difference between adsorption in micropores and ad
sorption on the surface of nonr-parous and relatively
large-pore adsorbents of identical chemical nature*
As a reasonable approximation far describing adsorp
tion in micropores, we proposed the theory of volume
filling of micropores* A survey of these investiga
tions is given in (1,2)#
In development of this theory we meet with some
difficulties. The theory indicates only a decrease
in differential heat
in filling. At tenperature T>T the theory is
C
inapplicable.
It follows from, the foregoing that it is expe
dient to search for a more perfect model of equilib
rium adsorption of gases in micropores and develop
a theory free from the above drawbacks. B.P.Bering
and V.V.Serpinskii have made a successful attempt
to develop and substantiate experimentally a more ge
neral thermodynamic theory of equilibrium adsorption,
which was named the osmotic theory of adsorption.
Its basic principles have been published only in
Russian Q ) . We will consider the main version of
the theory as applied to microporous adsorbents,using
zeolites as examples*
ffundamsntal of Osmotic Theory of Adsorption
in Micropores
As demonstrated by Hill (4), in describing equi
librium between an adsorbent and the gas phase in
physical adsorption, methods of adsorption thermody
namics and of solution thermodynamics can be applied
with equal success. Usually, the adsorbent is as
sumed to be thermodynamic ally inert in adsorption,
and methods of adsorption thermodynamics are used.
However the chemical potentials of the microporous
adsorbents and particularly of zeolites change in
the course of adsorption (|). Therefore the methods
of solution thermodynamics are more expedient in
this case. In connection with this it was also shown
(6) that the linear dimensions of the zeolite crys
tals change in the course of adsorption.
This approach to the description of adsorption
In Molecular Sieves—II; Katzer, J.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
1. DUBININ Adsorption of Gases on Zeolites 3
equilibria has proved to be the most fruitful, when
the theory was based on the solution model, which was
named ,9the vacancy solution1.'. In this model, one of
the solution components is not a solid adsorbent, as
in (5), but so-called "adsorption vacancies11* This
term means the free elementary volume of the adsorp
tion space of the micropores which is filled by one
adsorbate molecule in adsorption* Since the adsorp
tion space of the micropores is limited by their
volume, for a unit mass of the adsorbent there is a
maximum number of vacancies, which is equal to the
limiting number of the molecules adsorbed. The adsorp
tion value tends to this quantity asymptotically with
an unlimited increase of the equilibrium pressure in
the gas phase.
At any equilibrium pressure of the adsorbate the
adsorption space of the micropores contains adsorbed
molecules and adsorption vacancies which form a bina
ry vacancy solution* At a constant temperature, the
dependence of the equilibrium pressure in the gas
phase on the molar fraction of the dissolved substan
ce (adsorbate), i.e. the adsorption value, represents
a curve of the partial vapor pressure over the vacan
cy solution, or the adsorption isotherm. Thus, the
adsorption eequilibrium of the adsorbent with the gas
phase is equivalent to the equilibrium of the vacancy
solution with the same gas phase. It is natural to
analyse this equilibrium by methods of solution ther
modynamics.
Note that these concepts have proved to be parti
cularly fruitful because there is a deep, formally
thermodynamic, as well as physical, analogy between
adsorption equilibrium and osmotic equilibrium. The
idea of the existence of this similarity in the sim
plest case of adsorption on the surface of a liquid
was first suggested by Erumkin as far back as 1925
(7)i and then was absolutely clearly formulated by
Adam (8). In subsequent years, however, it was not
developped on a sufficient scale.
Consider now the physical background of this anar-
logy. Denote the number of moles of the substance
adsorbed and the number of vacancies per unit mass of
the adsorbent by a and d* , respectively. At any
equilibrium conditions
a + a*= a
m ( 1 )
where a is limiting adsorption, which is assumed
m
to be temperature invariant. Introduce the concept
In Molecular Sieves—II; Katzer, J.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
4 MOLECULAR SIEVES—H
of molar fractions of the adsorbate x and of the
vacancies x* » in the vacancy solution:
x = a / a ^ , x * - a V c i m (2)
We will regard the equilibrium of the micropo
rous adsorbent with the gas phase as equilibrium of
two vacancy solutions of different concentration.
One of these solutions is the vacancy solution i n
the micropores, and the other, the solution formed
by the gas molecules in a vacuum, in which the role
of the "solvent" is played by the vacancies in the
gas phase. Two solutions of different concentration
formed by unlimited soluble components can be at
equilibrium only when one of them is in the external
potential field. I
of this field is formally equivalent to the diffe
rence of the hydrostatic pressures existing in these
solutions. In the solution theory the difference of
these pressures is called the osmotic pressure.
We will now write down the expressions for the
chemical potentials of the vacancies in these two
solutions, denoting the values referring to the gas
phase by the subscript oC . Note that a vacancy so
lution corresponding to the gas phase is always high
ly diluted ( X^c^1 ; X « 1 ) and therefore can be
o C
regarded as ideal
R T ^ < P*V* )
+
(3
M-*- M* + gRT In. X * + pV*
( 4 )
where V is the molar volume of the vacancies, and
g is the osmotic coefficient, which, like the acti
vity coefficient, characterizes the non-ideality of
the vacancy solution in the micropores (9)»
At equilibrium, JUl* = U * , and, according to (3)
and (4), ' ' *
It is obvious that at 3C*.= 1, i.e. at zero concen
tration of the substance in the gas phase X * ^ , g=1
and Pot = P • Therefore, in (5) U * = • Introducing
the notation / 0
n « P-Poc (6)
for the osmotic pressure, we obtain
TTV*= -gRTtVtx* + RT &rt X * (7)
In Molecular Sieves—II; Katzer, J.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1977.