Catalysis by metal nanoparticles.pdf

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

Department of Organic Technology, Slovak University of Technology, Radlinskeho 9, SK-81237 Bratislava, Slovak Republic

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

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

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

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

homogeneous

distribution

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)

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

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 −

(

)

–N

(

R 3 ) + } 2 [PdCl 4 ] 2 − +

2Cl −

{ (

)

–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

(

)

–SO 3 H

(

OOCCH 3 ) 2

(

)

[

–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

inorganic

solids

may

employed.

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