Chemically Modified Surfaces in Catalysis and Electrocatalysis

298 Pages·1982·4.73 MB·English

by  Joel S. Miller (Eds.)

Chemically Modified Surfaces
in Catalysis and Electrocatalysis
In Chemically Modified Surfaces in Catalysis and Electrocatalysis; Miller, J.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
In Chemically Modified Surfaces in Catalysis and Electrocatalysis; Miller, J.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
Chemically Modified Surfaces
in Catalysis and Electrocatalysis
Joel S. Miller, EDITOR
Occidental Research Corporation
Based on a symposium jointly
sponsored by the Divisions of
Inorganic, Analytical, and
Petroleum Chemistry at the
182nd ACS National Meeting,
New York, New York,
August 23-25, 1981
192
ACS SYMPOSIUM SERIES
AMERICAN CHEMICAL SOCIETY
WASHINGTON, D. C. 1982
In Chemically Modified Surfaces in Catalysis and Electrocatalysis; Miller, J.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
Library of Congress Cataloging in Publication Data
Chemically modified surfaces in catalysis and electro-
catalysis.
(ACS symposium series, ISSN 0097-6156; 192)
Includes bibliographies and
1. Materials—Surfaces—Congresses. 2. Electrodes—
Congresses. 3. Catalysis—Congresses.
I. Miller, Joel S. II. Series.
TA418.7.C48 1982 600.2'9453 82-8731
ISBN 0-8412-0727-5 AACR2 ACSMC8 192 1-292
1982
Copyright © 1982
American Chemical Society
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In Chemically Modified Surfaces in Catalysis and Electrocatalysis; Miller, J.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
ACS Symposium Series
M. Joa Comstock Series Editor
Advisory Board
David L. Allara Marvin Margoshes
Robert Baker Robert Ory
Donald D. Dollberg Leon Petrakis
Robert E. Feeney Theodore Provder
Brian M. Harney Charles N. Satterfield
W. Jeffrey Howe Dennis Schuetzle
James D. Idol, Jr. Davis L. Temple, Jr.
Herbert D. Kaesz Gunter Zweig
In Chemically Modified Surfaces in Catalysis and Electrocatalysis; Miller, J.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
FOREWORD
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In Chemically Modified Surfaces in Catalysis and Electrocatalysis; Miller, J.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
PREFACE
ALT MANY ACADEMIC AND INDUSTRIAL RESEARCH LABORATORIES, surface
scientists as well as inorganic, organic, polymer, and analytical chemists
have focused on the chemical modification of surfaces to alter the bulk
properties of materials. Chemical bonding of substituent groups to surfaces
has been used to achieve catalytic, electron transfer, surface wetting, corro
sion resistance, photochemical, and polymer binding properties that bulk
materials inherently do no
these areas as synthetic an
developed and exploited. The types of materials that have been chemically
modified range from inorganic solids (such as semiconducting titanium
dioxide and tin dioxide) through alumina, silica, and silicates (such as
clays) as well as ordered material such as zirconium phosphates. In con
trast, organic materials ranging from the semimetal graphite to polymers,
such as insulating polystyrene and the unusual conductor polypyrrole,
have been of major concern in numerous research laboratories throughout
the world.
Major concerns are the availability and limitations of the analytical
techniques necessary to determine that surface modification has occurred,
and the extent to which k has occurred. Herein, the state-of-the-art of
the chemical modification of surfaces is presented by 17 chapters that also
discuss the nature of the binding of the pendant groups to the surface and
their frequency and spatial distributions. The principal focus in these
chapters is on modification of materials for catalytic purposes and the
modification of organic and inorganic electrode materials for electrocata-
lytic and photoelectrochemical applications.
JOEL S. MILLER
Occidental Research Corporation
Irvine, California
February 1982
In Chemically Modified Surfaces in Catalysis and Electrocatalysis; Miller, J.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
1
Chemically Modified Surfaces in Catalysis
DAVID E. BERGBREITER
Texas A&M University, Department of Chemistry, College Station, TX 77843
In recent years, considerabl
methods in which organi
modified in order to increase their usefulness in catalytic pro
cesses. The materials resulting from such chemical modifications
have considerable potential as alternatives to conventional homo
geneous and heterogeneous catalysts. For example, homogeneous
catalysts immobilized on an organic or inorganic support can in
principle possess both the experimental advantages of typical het
erogeneous catalysts and the reactivity and selectivity of their
homogeneous analogs. Immobilized metal clusters can be used as
catalysts themselves or as precursors to highly dispersed metal
crystallites or as precursors to metal oxide particles. Immobili
zation of transition metal complexes and metal clusters both pro
vides reliable routes to a type of heterogeneous catalyst whose
nature and mechanism may be more readily understood and, potenti
ally, offers ways to manipulate metal particle and crystallite
size to achieve new types of catalytic reactions. Surface modifi
cation of electrodes is another example of this type of chemistry.
Chemical modification of electrode surfaces either by adsorption
of specific molecules or by binding a polymer or molecule to the
electrode covalently provides opportunities to modify the stabil
ity and reactivity of electrodes to facilitate more useful elec
trocatalysis. This introduction discusses some of the general
problems encountered in this rapidly developing field of chemistry
and some of the advantages of these approaches to developing new
types of catalysts. More specific examples of ways in which chem
ically modified surfaces have been used in studying or developing
new catalysts are discussed in the accompanying papers in this
volume and in recent reviews (1-5).
Modification of organic surfaces or more generally of organic
polymers for the preparation of new types of catalysts illustrates
the potential and many of the problems common to this type of
chemistry. Modification of organic polymers may entail several
different approaches. Typically organic polymers are modified
either by chemical modification of an existing polymer or by poly-
0097-6156/82/0192-0001 $6.00/0
© 1982 American Chemical Society
In Chemically Modified Surfaces in Catalysis and Electrocatalysis; Miller, J.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
2 CHEMICALLY MODIFIED SURFACES
merization of appropriately derivatized monomers* In many cases,
the resulting chemically modified polymers contain a ligand suit
able for immobilization or "heterogenization" of a catalyst deri
ved from a transition metal complex or metal cluster (1,2). Al
ternatively, the polymer may contain a non-metallic catalyst (3-
5)• Although various types of organic polymers can or have been
chemically modified for use as catalysts or catalyst precursors,
divinylbenzene (DVB) crosslinked polystyrene has been the most
widely used polymer. This particular polymer is easily derivati
zed either before or after polymerization by unexceptional elec-
trophilic aromatic substitution reactions. Furthermore, the phys
ical restraints imposed by the polymer backbone on chemical reac
tivity of the attached catalysts are also readily controllable by
varying the percentage of divinylbenzene crosslinking reagent and
the polymerization method. For example, rigid macroporous DVB-
cross linked polystyrene possessing relatively large pores whose
size is not very solven
crosslinked polystyren
sites is very solvent dependent are both readily available or can
be readily prepared.
One of the earliest examples of the use of chemically modif
ied polymers as catalysts is the use as a catalyst of sulfonated
DVB-crosslinked polystyrene which is commonly available in the
form of ion exchange resins (4). This strongly acidic polymer has
found application in a number of industrially important processes
catalyzed by acids including dehydration reactions, esterifica-
tions, and olefin isomerizations• This heterogeneous polymeric
acid is useful as a general substitute for mineral acids such as
sulfuric acid in these processes. The primary advantage of this
modified polymer as a catalyst in these reactions is its hetero
geneity and the resulting ease with which this strong acid can be
handled and reused.
More recent work with chemically modified organic polymers
containing non-metallic catalysts has tended to emphasize the use
of polymers to bind more exotic catalysts or ligands to facilitate
the separation of catalysts and products after reaction. Phase
transfer catalytic reactions are one example of this more recent
chemistry. Phase transfer catalysts such as tetraalkylammonium
or tetraalkylphosphonium salts or macrocyclic ethers have been
successfully attached to polymers such as DVB-crosslinked poly
styrene to produce heterogeneous organic catalysts or so called
"triphase" catalysts (3,5). These catalysts are used to facili
tate common organic reactions such as nucleophilic substitution
reactions in which a polar reagent more soluble in water than the
usual nonpolar organic solvents is required. Again, the primary
advantage of immobilization of these catalysts is their facile
recovery and the ease with which the reaction products can be sep
arated from the catalysts.
Immobilization of otherwise homogeneous transition metal cat
alysts represents the second broad area in which chemically modi-
In Chemically Modified Surfaces in Catalysis and Electrocatalysis; Miller, J.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
1. BERGBREITER Chemically Modified Surfaces in Catalysis 3
fied organic polymeric surfaces are of increased utility in catal
ysis Q,2). In this case, the objective is to transfer the exper
imental advantages usually associated with heterogeneous catalysts
to homogeneous catalysts while retaining the reactivity, selectiv
ity and mechanistic understanding usually associated with homo
geneous systems. The immobilization procedures by which common
homogeneous catalysts are attached to organic polymers typically
involve chemically modifying the organic polymer so that it con
tains a suitable ligand that can bind the transition metal. In
many cases, this ligand is a relatively simple triaryl- or alkyl-
diarylphosphine. However, inclusion of more complex phosphine
ligands including chelating and optically active phosphine ligands
has also been successfully accomplished (6). In fact, virtually
every imaginable type of ligand has been incorporated into organic
polymers or could be incorporated into an organic polymer to bind
a catalyst. Some common examples include cyclopentadienyl ligands,
bipyridyl ligands, ioni
ates and diketonates, an
groups of a polymer like polystyrene serve as the liganding group
(2). The diversity of types of ligands attached to organic poly
meric surfaces has led to a corresponding diversity in terms of
the types of catalysts which can be bound to polymers. The only
limitations to binding a catalyst to a polymer such as polystyrene
are the possible reactivity of the polymer backbone to a particul
arly reactive catalyst, the physical diffusional restraints which
complicate catalyst immobilization, or the hydrophobicity of the
interior of organic polymers like polystyrene which could impose
chemical restraints on the type of species which could exist with
in the interior of a polystyrene matrix. All types of catalysts
including catalysts for typical homogeneously catalyzed hydrogen-
ations, asymmetric hydrogenations and hydroformylations, olefin
dimerizations, olefin isomerizations, hydroformylation, and hydro-
metallation have been successfully attached to organic polymers.
Polymer bound versions of homogeneous catalysts usually function
as catalysts for the same reactions as their homogeneous counter
parts although some recent reactions have shown that this condi
tion is not always true (8,_9).
Chemically modified polymers may also be used to support
transition metal carbonyl clusters which may in turn either be
used as "heterogenized" analogs of homogeneous clusters in catal
ysis in subsequent reactions or which may be decomposed to form
small metal crystallites. Transition metal carbonyls are most
commonly bound to organic polymers such as polystyrene using
phosphine ligands although other ligands such as bipyridyl ligands
have also been used. Mixed metal clusters can also be immobilized
(10).
Immobilization of homogeneous catalysts on organic polymers
has many real advantages and considerable potential for catalysis.
The most obvious advantage is the experimental advantage of heter
ogeneity. This permits recovery of both catalyst and ligand along
In Chemically Modified Surfaces in Catalysis and Electrocatalysis; Miller, J.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1982.