Catalysis by metal nanoparticles

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Journal of Molecular Catalysis A: Chemical 177 (2001) 113–138
Catalysis by metal nanoparticles supported on
functional organic polymers
M. Kralik
a
,
1
, A. Biffis
b
,
2
a
Department of Organic Technology, Slovak University of Technology, Radlinskeho 9, SK-81237 Bratislava, Slovak Republic
b
Dipartimento di Chimica Inorganica, Metallorganica ed Analitica, via Marzolo 1, I-35131 Padova, Italy
Received 15 March 2001; received in revised form 20 May 2001; accepted 5 June 2001
Abstract
The preparation and catalytic applications of dispersed metal catalysts supported on organic functional polymers are
presented. The advantages of these catalysts, such as the easy tailoring with respect to the nature of the used support, the
“nanoscale” size control of metal crystallites by the polymer framework, the high accessibility and consequent catalytic
activity in a proper liquid or liquid–vapor reaction systems are stressed. Various proposed catalytic processes making use of
these materials are presented and evaluated, including multifunctional catalysis, e.g. redox-acid. Interesting peculiar aspects
such as the enhancement of the hydrogenation rate by nitrogen containing moieties anchored to the polymer backbone are
emphasised. When suitable, a comparison with catalysts based on inorganic supports is given. © 2001 Elsevier Science B.V.
All rights reserved.
Keywords:
Functional organic polymers; Functional resins; Ion-exchange; Dispersed metals; Hydrogenation; Oxidation; Multifunctional
catalysis
1. Introduction
find application in such diverse fields as photochem-
istry, nanoelectronics, optics, and catalysis [1–7]. In
fact, often enough these particles do possess physi-
cal as well as chemical properties, which are distinct
both from the bulk phase and from isolated atoms and
molecules. Moreover, such unique features of metal
nanoparticles appear to be significantly influenced by
parameters such as the metal nanoparticle size, the or-
ganisation of the nanoparticle crystal lattice (i.e. the
nature and amount of defects) and the chemical nature
of the microenvironment surrounding the nanoparti-
cle. Thus, there is a large potential for the development
and application of metal nanoparticles with tailored
physical and chemical properties in both catalysis and
material science.
In the frame of this review, we shall concentrate on
the utilisation of metal nanoparticles in catalysis. In
Metal nanoparticles are objects of great interest in
modern chemistry and materials research, where they
Abbreviations:
APSDVB, tetraalkylammonium PSDVB; CAL, cin-
namaldehyde; CAn, chloroaniline; CNB, chloronitrobenzene; COL,
cinnamyl alcohol; DAA, diacetone alcohol; Et, ethyl; 2-EtAQ,
2-ethylantraquinone; 2-EtHQ, 2-ethylantrahydroquinone; EXAFS,
extended X-ray absorption fine structure spectroscopy; MA, metha-
crylic acid; Me, methyl; MEFO, melamino-formaldehyde resin;
MIBK, methylisobutyl ketone; MSO, mesithyl oxide; MTBE,
methyl-
tert
-butyl ether; PS, poly-styrene; PSDVB, poly-styrene-
co-divinylbenzene; PVP, poly-
N
-vinyl-2-pyrolidone; SPSDVB,
sulphonated PSDVB; TOF, turnover frequency; XPS, X-ray photo-
electron spectroscopy; XRMA, X-ray microprobe analysis
1
Tel.:
+
421-7-52495242; fax:
+
421-7-52493198.
E-mail
address:
kralik@chtf.stuba.sk
2
E-mail address:
biffis@chin.uinpd.it
1381-1169/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S1381-1169(01)00313-2
 114
M. Kr alik, A. Biffis / Journal of Molecular Catalysis A: Chemical 177 (2001) 113–138
particular, we will focus on the application of metal
nanoparticles supported on organic functional poly-
mers. The polymer support can be a soluble linear
or branched macromolecule or a micellar aggregate
which “wraps” the metal nanoparticle in solution, thus,
preventing metal sintering and precipitation. On the
other hand, it can be a
resin
, i.e. an insoluble mate-
rial consisting in a bundle of physically and/or chem-
ically cross-linked polymer chains in which the metal
nanoparticles are embedded. There appears to be no
sharp boundary between these two typologies of poly-
mer supports. For example, it is possible to prepare sol-
uble cross-linked polymers (“microgels”), which have
been reported to effectively stabilise metal nanopar-
ticles [8–10]. Furthermore, metal colloids protected
by soluble linear polymers have been conveniently
grafted onto insoluble resin supports to yield insolu-
ble catalysts [11]. This review will be mainly devoted
to metal nanoparticles on insoluble resin supports,
since the area of soluble polymers as stabilisers for
metal colloids has already been the object of thorough
review [5]
3
. Hereafter, the word “polymer” will be
used in a general sense, whereas the word “resin” will
be employed to stress a polymer (usually cross-linked)
insoluble in any common solvent.
The industrial application of catalysts based on
functional resins, has thus, far largely been confined to
acid catalysis [12], the production of methyl-
tert
-butyl
ether (MTBE) being the most renowned example. The
resins employed for this purpose are mainly SPSDVB
copolymers. Other applications of functional resins
in the field of catalysis include their use as supports
for enzymes in some biocatalytic processes, e.g. the
Nitto process for acrylamide synthesis [13]. In addi-
tion, there is a huge amount of literature on the use of
functional resins as supports for transition metal com-
plex catalysts (“hybrid” catalysts) [14,15]. In spite of
the fact that up to now no large-scale process based
on hybrid catalysts has reached commercialisation,
the academic and industrial research in this field is
still lively, particular attention being currently paid to
the immobilisation of costly asymmetric catalysts.
Resin-supported metal nanoparticles are currently
being employed as catalysts in some smaller scale in-
dustrial processes. Thus, strongly acidic ion-exchange
resins are used as active supports for metal palladium
in the preparation of bifunctional catalysts comprising
acid as well as hydrogenation-active centres. Such
catalysts are employed, e.g. in the industrial synthe-
sis of methylisobutyl ketone (MIBK) (Bayer catalyst
OC 1038) [16,17], where the acid centres catalyse
the dimerisation of acetone to diacetone alcohol
(DAA) and its dehydration to mesityl oxide, which
is then hydrogenated on the metal surface to the end
product. Similar catalysts based on anion exchange
resins (Bayer catalysts K 6333 and VP OC 1063)
[16] are employed in industrial heat-exchange units
for the reduction of dioxygen level in water from
ppm to ppb. Other applications include an alternative
route to MTBE (EC Erd olchemie process) [17,18]
and the etherification–hydrogenation of mixtures of
unsaturated hydrocarbons to give blends of alkanes
and branched ethers for the manufacture of unleaded
petrol (BP Etherol Process) [18].
In the above-mentioned applications, the resins are
generally used as beads (0.2–1.25 mm diameter) or
powders, in fixed-bed or suspension reactors (often op-
erated batchwise) or, more frequently, in flow-through
reactors. Working temperatures range from room tem-
perature up to about 120
◦
C. Most resin materials suffer
from relatively low mechanical, thermal and chemical
stability, which represents the main drawback of these
supports in comparison to more traditional inorganic
materials. For this reason, resin-based catalysts are
mainly applied as fixed-beds; alternatively, special
technical solutions are sometimes needed in order to
cope with this problem [19,20]. On the other hand,
resin supports do have other advantages in compari-
son to conventional supports. As we will see in more
detail below, this stems from the fact that in functional
resins the majority of the functional groups is embed-
ded
inside
the polymer matrix, and not simply on the
surface of the support particles, as it is commonly the
case with inorganic supports. Even when permanent
pores with high surface area are present in the resin,
only a negligible fraction of the functional groups is
truly positioned on the pore walls. Thus, when the
resin is in the dry state, most of the catalytically active
groups are located in the
glassy
polymer matrix and
are inaccessible to reactant molecules. They become
accessible when the resin is
swollen
by a suitable
liquid medium having a good compatibility with the
polymer, but even under these conditions they are
still surrounded by a medium having a relatively high
3
See the chapter written by J.S. Bradley in [1,46].
M. Kr alik, A. Biffis / Journal of Molecular Catalysis A: Chemical 177 (2001) 113–138
115
“concentration” of polymer chains from the support.
This particular situation can advantageously affect
the reactivity of the supported catalysts, e.g.
5. it allows the generation of metal nanoparticles with
a controlled size and size distribution;
6. it provides a mean to influence the chemical
behaviour of the metal nanoparticles through the
direct interaction of the metal surface with the
polymer-bound functional groups.
1. the concentration of reagents and products inside
the swollen resin can be significantly different in
respect to that in the bulk solvent, with potentially
beneficial effects on catalyst specificity and selec-
tivity;
2. equilibrium reactions taking place within the resin
can be conveniently shifted to the right if the prod-
ucts have a low compatibility with the resin, and
are therefore, expelled therefrom;
3. the kinetics of a given reaction can be substan-
tially influenced by the microenvironment inside
the swollen resin, thus, making it possible to change
the preferred reaction pathway in comparison to the
bulk solution;
4. size-selectivity effects are possible when reagents
with different solvated dynamic radii are used
simultaneously.
In connection with metal nanoparticles as the
catalytically active moieties, the use of functional
resins as supports offers some further convenient
features, namely
The aim of this paper is to provide the reader with
a thorough account of the state of the art in the field
of catalysis with polymer-supported metal nanoparti-
cles. Whenever possible, comparisons will be traced
between the performance in a given reaction of
polymer-supported catalysts and of catalysts based on
more conventional supports like carbon or inorganic
oxides.
2. Preparation of metal nanoparticles
supported on functional polymers
The preparation of polymer-supported metal
nanoparticles can be carried out along different routes,
which are briefly outlined in Fig. 1.
Basically, the synthetic route involves three steps,
namely
(1)
synthesis
of
a
suitably
functionalised
Fig. 1. Routes for the preparation of metal nanoparticles supported on functional polymers.
116
M. Kr alik, A. Biffis / Journal of Molecular Catalysis A: Chemical 177 (2001) 113–138
polymer; (2) loading of the polymer with convenient
metal nanoparticle precursors; (3) generation within
the polymer of the metal nanoparticles. The first two
steps can be condensed in one upon utilisation of
metal-containing monomers in the polymer synthesis.
Furthermore, the third step can be omitted by directly
loading the polymer support with pre-formed metal
nanoparticles. The different strategies will be outlined
in more detail in the following paragraphs.
Suitable polymer supports can be prepared either by
copolymerisation of unfunctionalised monomers fol-
lowed by functionalisation of the polymer backbone
or, more directly, by copolymerisation of functional
monomers. The choice of the nature and amount of
functional groups to be built in the polymer is made
on the basis of the role that they have to play. Their
primary function is to bind metal ions or complexes,
which are the most common precursors of the metal
nanoparticles. Therefore, the kind of functionality
which is most usually built in the polymer support is
either an ionic moiety (anionic, e.g. sulphonate or car-
boxylate or cationic, e.g. tetraalkylammonium) whose
counter-ion can be readily exchanged, or a group
able to co-ordinate to metal centres (e.g. amino or
phosphino). Additionally, since the functional groups
determine the compatibility of the polymer support
with different reagents and solvents (a parameter of
chief importance for catalyst performance, as it was
discussed in Section 1), they have to be chosen ac-
cording to the requirements of the particular reaction
under study [19,21]. Finally, the functional groups
can be also selected in order to influence the catalytic
performance of the embedded metal nanoparticles
by directly interacting with the metal surface, a phe-
nomenon which was already observed, but which
still awaits thorough investigation and rationalisation
[11,22].
In order to prepare insoluble resin supports, a cer-
tain amount of a suitable cross-linking agent, i.e. a
molecule with more than one polymerisable group
such as divinylbenzene (DVB), ethylene dimethacry-
late, or
N,N
-methylene-bis(acrylamide), is usually
added to the monomer mixture. Thus, in the course of
the polymerisation, the different polymerisable groups
of the cross-linker are incorporated in different poly-
mer chains, yielding an insoluble polymer network
as the reaction product. The amount of cross-linking
agent needs to be carefully controlled, since it has a
profound influence on the morphology of the result-
ing resin. Depending on the cross-linking degree (but
not exclusively on this parameter), macroporous (or
macroreticular) or microporous (or gel-type) func-
tional resins can be prepared [23]. In the dry state,
gel-type resins do not possess any porosity, but they
develop an extensive nanometer scale “porosity”
(hereafter referred to as nanoporosity) in the swollen
state. On the contrary, macroporous resins do possess
a permanent micrometer scale porosity even in the
dry state (hereafter referred to as macroporosity).
Macroporous resins also undergo swelling, albeit
to a much lower extent than gel-type ones, and in
doing so they develop nanoporosity in addition to the
permanent macroporosity. The latter remains largely
unaffected by the swelling process. A deeper discus-
sion of this topic and, more generally, of the different
experimental techniques which can be employed to
prepare resin supports is beyond the scope of this
review. The interested reader is referred to other ex-
cellent articles and books more fully dedicated to the
subject [19,21,23,24].
Commercial catalysts are mostly prepared starting
from unfunctionalised monomers. Usually, PSDVB
resins are formed in the first stage, which are sub-
sequently either sulfonated or chloromethylated and
aminated with a tertiary amine resulting in the forma-
tion of tetraalkylammonium groups [19]. The route
starting from functionalised monomers is exploited
to a smaller extent, due to the higher costs of func-
tional monomers. For example, resins which contain
carboxylic groups, resulting from the copolymerisa-
tion of methacrylic acid (MA) can be conveniently
prepared. An advantage of this approach is the much
more precise control of the degree of polymer func-
tionalisation, which is especially valuable when a
relatively low concentration of functional groups is
desired. To achieve this, however, proper polymerisa-
tion conditions need to be applied in order to ensure
a
homogeneous
distribution
of
functional
groups
throughout the polymer mass [24].
The route starting from metal-containing monomers
[25] (the right hand part of Fig. 1) is seldom exploited,
for instance when catalysts with peculiar properties
are desired. A nice example is a Pd-catalyst prepared
from a copolymer of
N,N
-dimethylacrylamide with
N,N
-methylene-bis(acrylamide) and bis(3-isocyano-
propylacrylato)-dichloropalladium(II)
by
reduction
M. Kr alik, A. Biffis / Journal of Molecular Catalysis A: Chemical 177 (2001) 113–138
117
of the metal with sodium borohydride. This catalyst
proved to be particularly stable in the hydrogenation
of aromatic nitrocompounds [26].
Another possibility deals with the utilisation of
metal salts of polymerisable acids such as acrylates
or fumarates [27]. The metal-containing monomer
can be also formed in situ in the polymerisation
mixture. This strategy has been coupled with metal
reduction in the course of the polymerisation to yield
resin-supported metal nanoparticles from functional
monomers, a cross-linker and a metal precursor in
one step [28]. It is also possible to generate a layer
of a reactive monomer with simultaneous deposition
of metal nanoparticles and subsequent fixation of
these nanoparticles by polymerisation, as reported by
Zavjalov et al. [29], who used [2,2]paracyclophan and
palladium nanoparticles generated by an electric arc.
At low pressure (10
−
7
Torr) and temperature (77 K),
they deposited this mixture on a silica layer, and after
heating to room temperature, a polymerisation result-
ing in the formation of a poly(
p
-xylene) film occurred,
in which the Pd nanoparticles were embedded.
In most cases, the metal is introduced in the pre-
formed polymer support by reaction of the polymer-
bound functionalities with suitable metal precursors.
The metal precursors are easily accessible metal ions
or complexes which can be subsequently and con-
veniently reduced to the form of polymer-supported
metal nanoparticles. For example, metal cations can be
introduced by simple ion-exchange if pendant anionic
groups are present. In this connection, the “forced”
ion-exchange technique with metal acetates appears to
be a very efficient tool [30]. Here, the metal cations
are incorporated in high yields into a resin bearing
an excess of strongly acidic groups (most frequently
–SO
3
H groups). The lower acidity of the acetic acid
by-product as well as its volatility enable the reaction
to be rapidly driven to completion (Eq. (1); (
P
) and
M denote the polymer backbone and a divalent metal,
respectively):
bimetallic catalysts [31]. A shortcoming of this tech-
nique is the possible reduction of a portion of metal
if some reducing solvent, e.g. methanol or generally
alcohols, is used, according to the following reaction
scheme proposed by Yen and Chou [32]:
3CH
3
OH
+
(
CH
3
COO
)
2
Pd
(
CH
3
O
)
2
CH
2
+
H
2
O
+
Pd
+
2CH
3
COOH
(2)
CH
3
OH
+
CH
3
COOH
CH
3
COOCH
3
+
H
2
O
(3)
On the other hand, this phenomenon can be exploi-
ted for the direct generation of metal nanoparticles.
The reduction of Pd(II) to Pd(0) is easily monitored
by a change in colour of a resin from white (yel-
low, yellowish-brown) to dark brown, or even to black
depending of loading of metals [33].
In the case of cationic resins, metallation with
cationic species is possible only to a very little
extent due to the electrostatic field developed by the
pendant cationic groups. Utilisation of proper anionic
complexes, like, e.g. chlorocomplexes represents a
convenient solution [34].
R
3
)
+
Cl
−
+
[PdCl
4
]
2
−
2
(
P
)
–N
(
R
3
)
+
}
2
[PdCl
4
]
2
−
+
2Cl
−
{
(
P
)
–N
(
(4)
To ensure the stability of the chlorocomplexes,
ion-exchange is carried out in chloride solution; the
extent of metal incorporation is about 60–70%.
It is important to remark that a proper choice of
the reaction medium for the metal loading reaction is
fundamental, especially when resins are used as poly-
mer supports. Thus, a solvent must be chosen which
is able to solubilise the metal precursor, but which is
also capable of swelling the resin to an appreciable
extent, since swelling is needed in order to guarantee
the accessibility to the reactants of the majority of the
functional groups. The reactivity of the solvent, as in
the case mentioned above, needs also to be taken into
account.
The final step in the preparation of polymer-suppor-
ted metal nanoparticles is the generation of the
nanoparticles within the polymer, which is usually
accomplished by reduction of the polymer-bound
metal precursors. To this purpose, similar techniques
as in the preparation of conventional metal catalysts
supported
2
(
P
)
–SO
3
H
+
M
(
OOCCH
3
)
2
(
)
+
[
P
–SO
3
]
2
M
2CH
3
COOH
(1)
For example, almost quantitative incorporation
into an acidic support of both palladium and copper
available as acetates in solution was accomplished
by “forced” ion-exchange during the preparation of
on
inorganic
solids
may
be
employed.
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