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REVIEW
Catalysis by Metal Nanoparticles Supported on Functionalized Polymers*
a M . KRÁLIK, b B . CORAIN, and b M . ZECCA
aDepartment of Organic Technology, Faculty of Chemical Technology, Slovak University of Technology, SK-812 31 Bratislava
e-mail: [email protected]
b Dipartimento di Chimica Inorganica, Metallorganica e Analitica, 1-35131, Padova, Italy, and Centro per lo Studio delia Stabilita' e Reattivita' dei Composti di Coordinazione, C.N.R., 1-35131 Padova, Italy
Received 14 June 1999
This contribution is a brief review dealing with metal catalysts prepared on functionalized polymers. The following topics will be touched: i) preparation and properties of metal catalysts based on functionalized polymers; ii) methods for the characterization of these catalysts; iii) effect of mass transport phenomena on the overall rate of a process and on the selectivity in complex reaction systems; iv) deactivation of the polymer-based catalysts; v) applications - selective hydrogenations, one-pot synthesis of methyl isobutyl ketone, removal of nitrates from water. A particular attention is paid to the comparison of polymer-based metal catalysts with those supported on inorganic materials. The mechanical and chemical stability vs. the activity and the cost of the catalysts are discussed. The emphasis will be put on the main advantages of polymer-based catalysts, stemming from easily controlled micro- or macroporous nature and hydrophobic-hydrophilic properties, from the possibility to attach various special functional groups, precise and well defined metal loading and metal distribution in the final catalyst. In the part describing the methods for the characterization of metal polymer-based catalysts, the techniques as evaluation of the mobility on the basis of electron spin resonance and pulse field gradient spin echo NMR will be discussed.
Activity of catalysts with dispersed metals depends mainly on the chemical nature of a metal, size of metal crystallites, organization of a crystal lattice (defects), and accessibility of these metal crystallites. Experience and literature information [1, 2] report tha t the highest specific activity is usually reached with metal crystallites of a nanometric size. Convenient preparation of this size of particles using a functionalized polymer [3], as well as other peculiar properties like e.g. an interaction between metal crystallites and the polymer, have been a motivation to consider catalysis by metal nanoparticles supported on functionalized polymers in more detail. Hereafter, the word "polymer" will be used in a general sense, the word "resin" will be used to stress a polymer insoluble (usually cross-linked) in any common solvent.
Catalysts based on functionalized polymers (mainly a resin type) are so far exploited mainly for acid
*Presented as a plenary lecture at the 26th International Jasná - Demänovská dolina, 24—28 May 1999.
catalysis [3, 4], for the immobilization of enzymes and for peptide syntheses [5]. The majority of acid catalysts is represented by sulfonated poly(styrene— divinylbenzene) resins [6, 7] and the production of methyl tert-hutyl ether is the most widespread application of this type of catalysts all over the world [5, 6]. Macroporous (or macroreticular) and microporous (or gel-type) functional resins are isotropic materials formed by chemically interconnected polymer chains, normally insoluble in any conceivable solvent [5, 8]. In general, however, strong hydrogen bonding and the mutual entanglement of polymer chains ("physical cross-linking") may bring about insolubility of these materials under conditions of practical interest, as in the case of polybenzimidazole resins [8—10]. Macroporous and microporous materials differ significantly in their chemical composition and largely in their micro- and nanostructure. Gel-type resins are usually
Conference of the Slovak Society of Chemical Engineering,
254 Chem. Papers 54 (4)254—264 (2000)
METAL CATALYSTS BASED ON FUNCTIONAUZED POLYMERS
2—8 % cross-linked, while the macroporous ones are 8—20 % cross-linked materials and are normally synthesized in the presence of porogenic components [8, 11]. In the dry state gel-type materials do not possess any porosity, but they develop an extensive nanometer scale "porosity" (hereafter referred to as nanoporos-ity) in the swollen state. On the contrary, macro-porous resins do possess a permanent micrometer scale porosity even in the dry state (hereafter referred to as macroporosity). Macroporous resins also undergo swelling, although to a much lower extent than the gel-type ones, so developing nanoporosity in addition to the permanent macroporosity. The latter is practically unaffected by the swelling process. In chemical applications resins are used as beads (0.2 to 1.25 mm) or powders, in fixed-bed reactors or suspension reactors (often operated batchwise) or more frequently in flow-through reactors. Working temperatures range from room temperature up to about 120 °C. The mechanical strength of these solids is relatively poor, but this drawback can be managed by means of various technical solutions [6].
The lower mechanical strength and chemical stability are probably the mean reasons why functionalized resins are not employed as supports for metal catalysis as often as acidic catalysts. However, under certain conditions these materials have sufficient lifetime and their utilization may be more efficient in comparison with inorganic materials. One of the best known industrial applications is the removal of oxygen from water to be used in heat exchangers [3].
The aim of this paper is to provide a reader with a brief summary of the main features of polymer catalysts and to tentatively advise for which type of processes a certain catalyst may be used and how to prepare it. In order to stress some technological aspects, problems of deactivation are discussed, too. In this paper, we will not review only literature information,
but we will try to generalize also our results and experience in this area [12—21]. A particular emphasis will be given to the behaviour of microporous catalysts swollen in a compatible solvent. Although the main area of interest is supported metal catalysis, as indicated by the title of this contribution, many of the presented topics are of more general interest, such as the assessment of the accessibility of the catalyst interior.
P R E P A R A T I O N A N D P R O P E R T I E S O F M E T A L CATALYSTS B A S E D
O N F U N C T I O N A L I Z E D P O L Y M E R S
The preparation of metal catalysts as metal-functionalized polymer composites may be carried out via four basic routes (A, B, C, D) outlined in Table 1. For the purpose of this contribution, functionahzation means the introduction of an ionic group (anionic, e.g. sulfonic or cationic, e.g. tetraalkylammonium) or a group enabling to form coordination bonds, e.g. amino or amido). Resins with pendapt anionic groups can exchange cations, therefore they are known and used as cation exchangers. Similarly, resins with pendant cationic groups can serve as anion exchangers [7]. These functional groups can either be introduced after polymerization (the route A) or be already present in the monomers (the route B). Commercial catalysts are mostly prepared according to the route A [3— 9]. Usually styrene copolymers are formed at the first stage, then either sulfonated or chloromethylated and animated with a triamine compound, which results in the formation of tetralkylammonium groups [8].
In order to increase the mechanical strength of polymers, as well as to ensure their insolubility, cross-linking agents (monomers with two or more reactive groups) are used. Divinylbenzenes (DVB, 1,2-, 1,3-, 1,4-isomers), N, TV'-methylenebisiacrylamide), ethy-
Table 1. Routes for the Preparation of Metal Catalysts Supported on Functionalized Polymers
Step
Preparation of a metal colloid
Preparation of nonfunctionalized monomers
Polymerization
Functionalization
Preparation of functionalized monomers without metals Polymerization Charging with a metal compound
Preparation of functionalized monomers with metal atoms Polymerization
Activation (reduction; generation of metal clusters)
A
X
X
X
X
X
В
X X X
X
Route
С
X X X
D
X
X
X
(X)
Dashed line denotes the start of individual routes.
Chem. Papers 54 (4) 254—264 (2000) 255
M. KRÁLIK, B. CORAIN, M. ZECCA
lene glycol dimethylacrylate (EGDMA), trimethy-lolpropane trimethacrylate (TRIM, l,l,l-tris(metha-cryloyloxymethyl)propane) are among the most frequently used cross-linkers [8]. Palladium catalysts supported on sulfonated, or aminated poly (styrene— divinylbenzene) are typical examples [3, 4]. The route В is exploited to a smaller extent than the route A. For example, resins which contain carboxylic groups, resulting from methacrylic acids may be prepared. In a similar way, anionic resins with pendant sulfonic group can be prepared from a comonomers mixture containing 4-vinylbenzenesulfonic acid [12—14]. An advantage of the route В in comparison with the route A is the much more homogeneous distribution of the functional groups, especially when a low concentration of the functional groups is desired. To achieve this result, however, proper polymerization conditions need to be applied. The route С is exploited very seldom, for instance when catalysts with peculiar properties are desired. A nice example is a Pd-catalyst prepared from poly[AT,A^-dimethylacrylamide-co-bis(3-isocyanopropylacrylato) dichloropalladium-co-N, N'-methylenebis(acrylamide)] which proved to be very stable in the hydrogenation of aromatic nitro compounds [21]. The route D is represented by steps which may be in a reverse sequence in comparison with the previous routes, i.e. metal nanoclusters are formed firstly and then they are stabilized (immobilized) by a polymer. However, metal nanoclusters can be also formed in such a way that they are generated in the entanglement of polymer chains by techniques usually used for the preparation of metal colloids, whereas complexation of metal cations with functional groups of resin is exploited. An example of this procedure has been recently described by Akashi et al. [22] who prepared platinum nanoclusters by fixation of colloid particles in the swollen mass of poly(7V-isopropylacrylamide) grafted on polystyrene beads.
The routes А, Б, C, D differ in the number of steps and complexity of monomers. Higher complexity implies higher costs, and a compromise dictated by economical rules must be found. In addition, changes in solubility and reactivity [23] of functional-ized monomers need to be considered in the choice of a polymerization technique.
Each of the routes A, B, C, D involves the polymerization step. Theoretical aspects of polymerization and experimental techniques may be found in the literature [8, 11, 23]. In this paragraph we only discuss a few features dealing with textural (shape and size of resin particles; shape, size, and distribution of pores) and functional homogeneity (distribution of the functional groups throughout the volume of polymer particles). It is important to mention that due to the swelling, textural properties in a reaction system which contains a solvent compatible with a function-alized resin are strongly influenced by the homogeneity of functional groups. All polymerization techniques
start with an initiation step, which should be chemical, thermal or radiochemical [23]. Peroxy compounds, e.g. dibenzoyl peroxide (CGH 5COO—OOCC 6H 5), 2,2'-azobis(isobutyronitrile) (AIBN, (CH 3 ) 2 C(CN)N= NC(CN)(CH 3) 2), and potassium peroxodisulfate (K 2 S 2 0s) represent the most frequently used initiators for the radical polymerization [23, 24]. Generation of free radicals by heat and/or irradiation by using 7-rays or X-rays is not so widely used as the chemical way. However, irradiation techniques enable to achieve resins with better homogeneity when comonomers with a different reactivity are applied [25].
Appearance of a polymer depends on the used polymerization procedure [24]: i) bulk - a block of the polymer is formed; ii) solution - the polymer remains dissolved in a solvent, unless cross-linked; iii) suspension - bead particles are formed: iv) emulsion -the suspension of polymer particles with a lower size (0.05—0.2 /mi) than in the suspension polymerization are formed, the initiator must be soluble in water. If a regular form of polymeric bead is required, the suspension polymerization is the best one. This process is typically carried out in the presence of two liquid phases (water and "oil"); if a macroporous material is desired, a porogenic agent (e.g. toluene in the case of copolymerization of styrene with divinylbenzene) is added [8]. In the case of the block polymerization resulting in cross-linked insoluble polymers (resins), the resin after the polymerization needs to be cut and a proper size fraction is obtained by sieving [12—18]. We successfully used the bulk polymerization initiated by 7-rays to prepare various resins [12—19, 21] differing in composition, lipophilicity, and mechanical properties.
Metal compounds can be introduced by ion exchange into resins prepared according to either route A or B. If pendant anionic groups are desired or do not disturb the catalytic process, the "forced" ion-exchange technique with metal acetates is very efficient (the term "forced" does not imply that the process is not spontaneous, but only stresses the thermodynamic more favourable conditions for the process; see below). Metal cations are quantitatively or almost quantitatively incorporated into the acid form of a resin (most frequently —SO3H or —COOH groups, which need to be in excess with respect to the metal cations desired to be confined in the polymer) in a single step due to the weakness of the formed acetic acid ((P) and M denote the polymer backbone and divalent metal, respectively)
2(P)—SO3H + M(OOCCH 3) 2 <* & [(P)—S0 3] 2M + 2CH3COOH {A)
A drawback of this method, albeit a minor one, is the low solubility of some metal acetates in water, e.g. [Pd(OOCCH3)2]. On the other hand, this compound is readily soluble in acetone. Anionic resins are usually
256 Chem. Papers 54 (4)254—264 (2000)
METAL CATALYSTS BASED ON FUNCTIONALIZED POLYMERS
very hydrophilic, i.e. they swell and open their interior in water, but not in organic solvents of low polarity, e.g. acetone. This problem is solved either by the dissolution of palladium acetate in the mixture of acetone and water [13, 14], or in the mixture of acetone and methanol [16, 17]. For example, more than 95 % of both palladium and copper available in solution were introduced into the support by ion exchange during the preparation of bimetallic catalysts [20]. The described technique is used both in the laboratory and industrial scale.
The final step in the preparation of metal-supported catalysts according to the routes А, В, С is their activation, usually the reduction of the metals. To this purpose, similar techniques as in the preparation of metal catalysts supported on inorganic solids may be used. However, two differences need to be taken into account: i) the lower thermal stability of resin-based catalysts, and ii) the necessity to "open" the micro-porous interior of gel-type resins. The latter factor requires that the reduction is carried out in a liquid phase, predominantly formed by a solvent with the proper compatibility. The most frequent reduction procedures are as follows: i) hydrogen in methanol, ii) sodium borohydride in ethanol, iii) formaldehyde in water. The size of metal crystallites, as well as the distribution of a metal in the resulting catalyst depend on the used procedure and the metal concentration. We observed increasing homogeneity of the metal distribution with decreasing metal concentration and increasing of the concentration of the reductant [12— 14]. Industrial catalysts are often activated in such a way to force nonuniform distribution of metals, e.g. in the catalysts for the removal of oxygen from water [3].
In comparison with inorganic catalysts, the greatest difference of metal catalysts dispersed onto func-tionalized resins is a significantly higher interaction between the catalyst and components of a reaction mixture. Probably, the most striking different property is the swellability, which may result in an expansion of the resin volume by a few hundred of percent with respect to the volume in dry state of a micro-porous resin. Swellability also occurs on the surface of the macroporous resins. For example, recent measurements by D'Archivio et al. [10] have shown an increase of the surface of PBI particles from approximately 20 m 2 g - 1 in the dry state (nitrogen porosime-try) to more than 600 m 2 g _ 1 and 220 m2 g _ 1 when the resin was swollen in tetrahydrofuran and water, respectively. This example shows that i) the derivation of catalytic properties of resin-based catalysts from their properties in dry state is not straightforward, and ii) these catalysts possess much higher molecular accessibility of catalytic sites and consequently higher specific catalytic activity when they are swollen in a proper solvent. Of course, information about the catalyst in dry state is important for the overall description of a catalyst. Similarly to inorganic materials, evalu
ation of the following properties is desired: i) shape and size, or size distribution of catalyst particles; ii) shape, size, or size distribution of pores in the case of macro- and meso- (above 2 nm) materials; iii) elemental analysis; iv) average concentration and distribution of functional groups; v) content of metals and its distribution; vi) type of metal crystallites and their size, or size distribution. In addition to this list, evaluation of properties in the swollen state is necessary. The main quantities describing the swollen state are as follows: i) swellability; ii) distribution of polymer chain concentration; iii) diffusivity in the swollen state. These general properties need to be completed with results from catalytic tests, and, in this connection, we again stress the different behaviour in various solvents due to the swelling and interactions with functional groups [3, 8].
M e t h o d s for Character izat ion
The methods for characterization of resin-based catalysts in dry state are the same, or very similar to those employed for inorganic materials [26]. Some techniques, like mercury porosimetry, are not of general use for resin-based catalysts, due to their low mechanical stability. Other techniques, like X-ray powder diffraction analysis (XRD), are simpler because of amorphous features of resins. By contrast, poor electric conductivity of resins makes it more complicated to perforin Electron Spectroscopy Chemical Analysis (ESCA), and X-Ray Microprobe Analysis (XRMA).
In order to show how a resin catalyst can be characterized, a sequence of figures concerning Pd—Cu catalysts is presented. In this connection, we will also discuss the changes in the texture and nanomorphol-ogy of particles during the preparation of a catalyst containing 4 mass % and 1 mass % of Pd and Cu, respectively. Figs. 1—3 show electron microscopy images
Fig . 1. Electron microscopy image (150 x) of Pd4Cul/Dowex
particles after ion exchange.
Chem. Papers 54 (4) 254—264 (2000) 257
M. KRÁLIK, B. CORAIN, M. ZECCA
Fig . 2. Electron microscopy image (150 x) of Pd4Cul/Dowex particles after the reduction with H2 (50 °C, 1 MPa, 1 h).
Екщ^^^^^ШШ
LPd X-ray map j
Pd4Cul/Dowex
Cu X-ray map К К П
s • '
SEM x 100 / Ш и Й а Ш а
Fig . 4. XRMA image of the Pd4Cul/Dowex catalyst particle after the reduction with hydrogen in methanol (50°C, 1 MPa, 1 h).
Fig . 3. The egg-yolk shape of the catalyst particle after the reduction.
of particles after charging with metals (Fig. 1) and after the reduction with hydrogen in methanol (Figs. 2 and 3). The comparison of the particles shown in Figs. 1 and 2 gives evidence of smaller particles before the reduction. This behaviour should be prescribed to cross-linking with P d 2 + and Cu24", which affects the
size of not fully dry beads (hydrophilic resins can take more than 100 mass % of water if they are exposed to moist air [7]). A similar behaviour was also observed in 8 mass % of palladium catalysts supported on poly[A^,A^-dimethylacrylamide-co-(styryl-4-sulfonic acid)-co-[A^,iV/-methylenebis(acrylamide)] [12]. It is possible to observe breaking in structure of beads, which may result in the formation of the egg-yolk shape as nicely shown in Fig. 3. A possible explanation for breaking follows from mass transfer of P d 2 +
and C u 2 + cations and effects of Pd—Cu metal crystallites on polymer chains in the presence of hydrogen. Under certain conditions, metal crystallites are accumulated beneath the surface approximately one third of the particle radius measured, which is evident from Figs. 4 and 5. The higher concentration of metal crystallites causes more intensive destruction of the polymer network, which can even turn to breaking of the catalyst particle. When lower loadings of metals (less than 2 mass % of Pd) were applied, such a peculiar behaviour was not observed. The catalyst particles had a uniform distribution of metals after the reduction. The average size of metal crystallites was measured by
Fig . 5. The line profile for Pd from XRMA of the Pd4Cul/Dowex catalyst particle after the reduction with hydrogen in methanol (50 °C, 1 MPa, 1 h).
258 Chem. Papers 54 (4) 254—264 (2000)
METAL CATALYSTS BASED ON FUNCTIONALIZED POLYMERS
•a
<Pd>
/ ̂ л « У v
1 <CuO> ^
-
Pd4Cul
/ /
<Cu>
Pd4
Cul
35 40 45 20 /°
50 55
Fig . 6. X-Ray powder diffraction patterns of palladium (Pd4), copper (Cul), and palladium-copper catalysts (Pd4-Cul) supported on Dowexes (reduction with H2).
20 n m
Pd4Cul/Dowex
Fig . 7. ТЕМ image of Pd-Cu clusters in the Pd4Cul/Dowex catalyst after the reduction.
means of X-ray powder diffraction (XRD) and Transmission Electron Microscopy (ТЕМ) methods. From XRD patterns (Fig. 6), using the Sherrer's equation [13], an approximate size of 7 nm was estimated. As shown by Fig. 7 metal crystallites are organized in agglomerates of about 100 nm.
Since the main difference between resin- and inorganic-based catalysts rests in their different behaviour in the presence of compatible (with the resin) solvent, we focus on the methods which allow to describe properties of the resin-based catalyst in the swollen state. The best method for the characteri
zation of swollen resins is the Inverse Steric Exclusion Chromatography (ISEC) [27] which is a version of Steric Exclusion Chromatography (SEC) [28]. The latter is mainly used for the characterization of very swollen materials. In the ISEC method solutes of different sizes are let to flow through a column packed with a swollen resin, and from the elution curves the volume accessible for a solute with a certain size is calculated. Having a model for the distribution of polymer chains [29] or a model describing size distribution of pores [29, 30], it is possible to assess either the volume fractions of swollen polymer domains with a certain concentration of polymer chains, or to calculate size of pores. The higher is the proportion of very dense polymer domains (about 2 nm n m - 3 ) , the lower is accessibility of the interior of a swollen resin. This method is not only generally useful to characterize polymers, but it is also a tool for the evaluation of changes in the catalyst morphology after catalytic tests. We have shown [16, 17] that the swellability is directly connected with the affinity of a solvent towards the polymer backbone. This affinity may be assessed on the basis of the average" length of polymer chains exhibited by the unit mass of the polymer. For example, the length of polymer rods in a 2 mass % of palladium catalyst based on the amphiphilic resin poly[styrene-co- (methacryloyl—ethylenesulfonic acid)-co-A^,A^/-methylenebis(acrylamide)] (cat. SS2-MPd in Ref. [16]) was equal to 5.7 x 108 km g _ 1
and 16.4 x 108 km g _ 1 in water and tetrahydro-furan, respectively. Other useful techniques for the evaluation of accessibility and mobility of reaction species are: i) Electron Spin Resonance (ESR) [31], and ii) Pulse-Field-Gradient-Spin-Echo Nuclear Magnetic Resonance (PFGSE-NMR) [32]. The former allows to evaluate the rotational mobility of paramagnetic probes, connected to revolution of the molecule around its own axis continuously changing direction in the space. If ESR measurements are carried out on a sample of polymer (or polymer-based catalysts) swollen with a solution of a paramagnetic probe, then the signal intensity will be proportional to the amount of the probe confined in the swollen polymer. PFGSE-NMR is suitable for the evaluation of the translational mobility, connected to the movement of a molecule wandering in the space. Although these techniques were mainly developed and tested for swollen mi-croporous (gel-type) resins, recent investigations have shown their validity also for macro-microporous resins [10]. The coherence among the results of the individual techniques is demonstrated by the data in Table 2 (adapted from [17]). The catalyst with lower cross-linking (PlPd2) exhibited a higher swellability, as well as better accessibility in comparison with the more cross-linked catalyst (P3Pd2). A higher diffusional resistance of the interior of catalyst P3Pd2 was also evident in catalytic tests [17].
The full description of a catalyst must include cat-
Chem. Papers 54 (4)254—264 (2000) 259
M. KRÁLIK, B. CORAIN, M. ZECCA
Table 2. Swellability (S), Bulk Expanded Volume (BEV), Average Polymer Chain Concentration (av.pcc), Rotational Mobility (r) [13], Translations Mobility (D) [17], and Derived Quantities for Catalysts P lPd2 and P3Pd2 (details see in [16, 17jj
Property Catalyst
P lPd2 P3Pd2
S - I S E C - w a t e ^ / i c m ^ " 1 ) BEV-water/(cm3 g _ 1 ) BEV-THF/(cm3 g" 1 ) BEV-MeOH/(cm3 g" 1 ) av.pcc-watera/(nm n m - 3 ) r-MeOH6 /ps Dc x 10+5 / (cm2
s - i ) r / r o (TO = 37 ± 17 ps)d
D0/D (Do = (2.5 ± 0.3) x 10" l)d
2.72 4.1 6.3
20.5 0.16
60 ± 17 1.8 ± 0.2
1.6 1.4
1.89 2.2 3.9 9.5 0.24
115 ± 18 1.1 ± 0.15
3.1 2.3
a) Swellability and average polymer chain concentration determined in water [16]; b) Rotational correlation time of TEMPONE dissolved in methanol confined inside the microporous network of the catalysts; c) Diffusion coefficient of methanol inside the microporous network of the catalysts; d) Values for bulk methanol, units are the same as for quantities in the swollen catalyst. Errors in measured values were calculated on the basis of repeated measurements (5 or 6) using the standard procedure for the 95 % probability level.
alytic tests and investigation of its stability under catalysis conditions. Since transport phenomena often govern the productivity of the catalyst, their effect on the overall rate of the process must be taken into account in the treatment of kinetic data. We were successful [15] in the estimation of the parameters dealing with both mass transport phenomena and the intrinsic reaction kinetics. Highly reliable values of the parameters were achieved by simultaneous treatment of a few sets of kinetic data obtained under different conditions (different pressure of hydrogen, different well-defined size distribution of the used catalyst) in order to modify mass transport phenomena.
Effects of Mass Transpor t Phenomena on the Overall R a t e of a Process and on the Selectivity to React ions in Complex React ion Systems
The reduction of metal ions introduced into polymeric supports through ion exchange enables the formation of very active metal crystallites. In comparison with inorganic catalysts, the great advantage of metal catalysts supported on low-cross-linked functionalized resins is the full accessibility from all sides of the crystallites. However, the transport of the reaction species can be limited by the swollen polymer network, and the final activity is usually affected by diffusion phenomena. Fig. 8 shows the situation in the hydrogena-tion of cyclohexene over catalysts of different particle sizes. The expected decrease in efficiency at the increasing size of catalyst particles is evident.
Due to the hydrodynamic resistance, relatively large particles (larger than 1 mm) are used in industrial processes with fixed-bed catalytic reactors. The proper ratio between the "chemical" and mass transport resistance is ensured by the use of catalysts with a low content of metals (less than 0.3 mass %), mainly located close to the surface [3, 7].
x/%
TJ/%
Fig . 8. Calculated conversion (a;, solid line) and the effectiveness factor (7/, clashed line) vs. time (t) in the batch hydrogenation of cyclohexene at 25 °C and 1 iMPa, over the Pd/polymer catalyst (2.3 mass % Pd, dry state) carried out on particles with different radius (1, 2, 3 - 0.01, 0.1, and 0.15 mm, resp.). Other details [15]: 0.00244 kg m - 3 swellability (5) of the catalyst, c(Pd) = 0.4 mmol d m - 3 in the reaction mixture, c(cyclohexene in methanol) = 1 mol d m - 3 ; the reaction rate: £v
k m o l - " § m4.5
- 1 ) = 0.61 k g - 1 s " 1 ; с н 2 , c-cen (kmol m - 3 ) , wPd
(kg m - 3 ) in the swollen polymer; diffusion coefficients: D\ = D ito exp (-0.0045 m 3 k g _ 1 / S ) , Д 0 - the value in the bulk liquid; Dii0: 2 x Ю - 9 m 2 s _ 1 and 1.7 x Ю - 8
m 2 s _ 1 for cyclohexene and hydrogen, respectively.
Mass transport usually affects the selectivity in a complex reaction system where the desired products may undergo further reactions, like in the removal of nitrates from drinking water through catalytic reduction over Pd—Cu catalysts. Nitrogen is the desired product, but if not readily released from the metal surface, the undesired "overreduction" to ammonia occurs [20, 32]. Mass transport hindrance can some-
260 Chem. Papers 54 (4) 254—264 (2000)
METAL CATALYSTS BASED ON FUNCTIONALIZED POLYMERS
times play a positive role, like in deactivation processes where leaching of metals is aided by the formation of metal complexes. In this case, the steric obstacle represented by a rigid polymer network limits the leaching process, thus protecting the catalyst from deactivation. Such a situation was observed in the hydrogenation of aromatic nitro compounds over a palladium catalyst supported on the very rigid resin [21].
DEACTIVATION OF P O L Y M E R - B A S E D CATALYSTS
Supported metal catalysts are generally deactivated in one of the following ways: i) sintering of metal crystallites; ii) formation of side products which stick on the surface of metal crystallites (fouling); iii) chemical changes in the nature of metal crystallites, e.g. oxidation, complexation, and leaching; iv) changes in the properties of the support. When the support is an organic polymer, its morphology is one of the properties which can change. It is surprising that very little information on the deactivation of metal catalysts supported on functionalized polymers can be found in the literature. Polymer-based catalysts are exploited rather in research laboratories than in practice, with some outstanding exception (see below). Laboratory tests usually last for relatively short times hence the lifetime of the catalyst is not of primary importance under this circumstance and so it is often neglected. However, we have faced strong deactivation of resin-based palladium catalysts in the hydrogenation of ni-troaromatics [13, 18, 19], and so we ha.ve started to investigate this problem in deeper detail.
Hereafter, we will focus on the deactivation due to the changes in the support. In the case of resin catalysts, this may be the result of splitting of the polymer chains, which continues in the degradation of the polymer backbone. Catalysts can be used in three types of reaction systems: i) gas phase; ii) liquid phase: iii) gas-liquid phase. Whereas in gas-phase reactions the situation is similar for inorganic and resin-based catalysts, it is quite different when a liquid is present (situations ii) and iii)), due to swelling of the polymeric support. We have proposed the hypothesis that a metal crystallite in an oxidative or reductive environment acts as a "hot stone" which helps the breaking of chemical bonds in the polymer network. According to this hypothesis, the following factors can contribute to the degradation of a polymer network in the presence of metal crystallites: i) high activity of catalytic sites; ii) high loading of a metal; iii) high concentration (pressure) of an oxidizing or reducing agent; iv) high temperature; v) functional groups hanging from the polymer chains, which have affinity to metal crystallites; vi) low cross-linking degree; vii) swellability; viii) chemical stability of the polymer chains. The interplay of these factors is also
reflected by the changes in accessibility and catalytic activity [19], but not to such an extent as it could be expected. It is even possible to predict that optimum performance of metal resin-based catalysts is the result of balance between high accessibility of the metal crystallites and a sufficient protection against their sintering. The egg-yolk particles in Figs. 2 and 3 represent an extreme case, in which the particles were broken locally during the reduction of metals. Fig. 2 may raise doubts about the lifetime of resin-based catalysts. In this connection, it is necessary to stress that the catalyst illustrated in this figure was subjected to the most favourable conditions for the polymer degradation: i) the content of palladium was relatively high; ii) the resin contained high concentration of sulfonic groups; iii) the solubility of hydrogen in methanol, the solvent employed in the reaction, is high; iv) cross-linking and swellability were of a middle value, i.e. metal crystallites were very close to the polymer chains. By contrast, it is possible to find examples in which the lifetime of the catalyst spans over several months, or even years. This is the case of the removal of oxygen from water [3] (low concentration of oxygen, stoichiometric amount of hydrogen), of the one-pot synthesis of methyl isobutyl ketone [3, 4, 33] (high cross-linked resin, very low concentration of palladium, crystallites very close to the surface accessible mainly from the site close to the catalyst surface, and in that, a very low interaction between the metal with chemisorbed hydrogen and a polymer chain), and of the reactions promoted by metal colloids stabilized by linear polymers (long distance between metal crystallites and polymer chains). Lifetime of the catalysts may be also increased by lowering the activity of metal crystallites, e.g. Lewis acids in the hydrogenation reaction systems.
The chemical stability of the polymer backbone was not discussed in this paragraph, because chemical nature of the support in the majority of the industrial catalysts is similar - sulfonated poly (styrene— divinylbenzene). More stable catalysts may be prepared from aromatic and heterocyclic polymers, e.g. polybenzimidazoles, poly(^-phenyleneterephthalami-de) [23, 24], etc.
A P P L I C A T I O N OF P O L Y M E R - B A S E D CATALYSTS
In the sixties metal resin-based catalysts were considered as very promising catalysts, even with better parameters than their inorganic counterparts [34]. Easy tailoring made them very good candidates for many chemical reactions. However, deactivation processes and major other drawbacks (in the case of the degradation of a polymer network - impossibility to regenerate these catalysts), shifted them out of the nub of the catalytic research. A renewed interest for metal catalysts dispersed on functionalized resins has
Chem. Papers 54 (4) 254—264 (2000) 261
M. KRÁLIK, B. CORAIN, M. ZECCA
started about 15 years ago, especially as multifunctional catalysts.
Probably, the most successful example of mono-functional metal catalyst is represented by removal of oxygen from water [3].
2H2(aq.) + 02(aq.) Pd * H 2 0 (B)
The catalyst is an ion-exchange resin, both macro-porous and gel-type, in beaded form (0.5—1.3 mm in diameter), in which palladium crystallites are deposited onto the outer shell of the beads (Bayer catalysts К 6333 and VP ОС 1063) [3]. Under typical conditions (0 = 70 °C; P = 0.15—0.2 МРа), the stoichio^ metric amount of dihydrogen is employed and residual O2 is less than 10 ppb. The amount of treated water is approximately 80 bed volumes per hour.
In the mid-seventies Deutsche Texaco commercialized a convenient process for the industrial synthesis of methyl isobutyl ketone (MIBK, Scheme 1) [33], a semicommodity produced on the tens of thousands tones scale worldwide [4] and currently utilized
as industrial solvent and a reactant [4]. The three-step synthesis is catalyzed by a strongly acidic synthetic macroporous resin "doped'1 [33] with metal palladium (about 0.1 mass %) dispersed as crystallites inside the polymer network (Bayer catalyst ОС 1038, 3 MPa?"l30—U0°C). The first two synthetic steps would lead to an equilibrium mixture in which only 17 % of MIBK would be produced. However, the copresence of the metal poly-dispersed phase promotes the regioselective hydro-genation of mesityl oxide to give the desired product.
Catalysts of this type are also employed in modern plants [4, 33] for the synthesis of MTBE (Scheme 2), the well-known important additive in the production of lead-free gasoline. In fact, the C4 fraction coming from the traditional MTBE reactor (primarily but-1-ene, but-2-ene, and saturated C4 hydrocarbons, "raffinate II") turns out to be contaminated by appreciable amounts of diolefins, acetylenes, and carbonyl compounds. These compounds can cause considerable troubles during the subsequent
H3cr ^CH3
Condensation
H+
Elimination of water
O CH, II I 3
/ C ^ --Q\ H,C CH2\ CH,
3 2 OH 3
DAA
o CH. I 3 Hydrogenation O CH,
н-,-н2о нзс^-сн^снз P d - + H* н 3 с - С - с н 2
С %
MSO MIBK
Scheme J
с н з ^ н+ 1 g
^ : C = C H 2 + CHoOH *- С Н о - С - О - С Н з CHo-^ 2 3 « 3 , 3
CH3
diolefines, acetylenes, H 2 / P d monoolefines, >C=0 compounds hydrogenation products
Scheme 2
Pd-Cu Pd-Cu
NO3" > N 0 2 " • N O *
-» NjO •+ N 2
•* N 2
-> NH/
Scheme 3
262 Chem. Papers 54(4)254—264 (2000)
METAL CATALYSTS BASED ON FUNCTIONALIZED POLYMERS
transformations of raffinate II and they can be readily eliminated upon hydrogenation in the same MTBE reactor, if these remarkable catalysts are used. Moreover, they do not catalyze the hydrogenation of either isobutene or but-1-ene and but-2-ene.
In the last two years, we have started to investigate the removal of nitrates by the hydrogenation over Pd—Cu catalysts (Scheme 3). This scheme was also proved for water solutions containing SO^ - , COg", CI", HPC>4~, Ca2+, Na+, Mg2+ . Because of enhancement of the reaction rate in the presence of protons, the anionic resins were used. Various loading of metals and ways for the reduction were tested. The obtained results are promising [20], however problems with leaching of metals occurred [35]. In the case of the catalyst containing 4 and 1 mass % of Pd and Cu, respectively, the problem with changes in the nanomorphology during the reduction of metals occurred (see Figs. 1—3). During utilization of these catalysts in the water-phase reduction under mild conditions (about 50 kPa partial pressure of hydrogen), the texture of the catalyst did not change more.
As far as highly selective hydrogenation catalysts are concerned, the work of Michalská et al. [36] should be of a potential industrial interest. On the Pd/poly(heterocyclic polyamides) the reduction of oc-tadiene to octene was very selective (more than 80 % to octene at the virtual 100 % conversion of octadi-ene) and the stability of catalyst was proved by 11 recyclings without any evident loss of the catalytic activity. High stability of nanoclusters has been also described by Akashi et al. [22] who hydrogenated allylic alcohol over Pt/poly(7V-isopropylacrylamide) grafted on polystyrene beads (about 600 nm). After 6 recyclings of this catalyst there was no significant loss in activity and ТЕМ images revealed practically the same nanomorphology like at the start.
Other selective catalysts are represented by metal colloids stabilized by linear polymers [37—40], which should be considered as a dispersed metal catalyst supported on a polymer swollen to such an extent that it is closer to the solution state than to the solid one. Sulman et al. [37] have described the selective hydrogenation of dehydrolinalool (3,7-dimethylocta-6-en-l-yn-3-ol) to linalool (3,7-dimethylocta-l,6-dien-3-ol) at atmospheric pressure on Pd-colloids stabilized in micelle cores of polystyrene—poly(4-vinylpyridine) block copolymers. High activity and selectivity (more than 95 %) to the hydrogenation of triple bond to double bond were found and immobilization of the palladium-micelle cores on alumina allowed to recycle the catalyst 20 times without the loss in the catalytic activity.
Another interesting area deals with regioselec-tive hydrogenations [38—40]. Yu et al. [38] showed that the selectivity to p-chloroaniline in the atmospheric hydrogenation of p-nitrochlorobenzene over
monometallic palladium nanoclusters stabilized by poly(N-vinyl-2-pyrrolidone) was very poor. However, when bimetallic Pd-Pt colloid dispersion stabilized by the same polymer was used, the selectivity of 94 % was observed. More than 99 % of selectivity to o-chloroaniline [39] was obtained in the hydrogenation of o-chloronitrobenzene over Ru metal nanoparticles stabilized by poly(N-vinyl-2-pyrrolidone). Due to the low interactions between metal crystallites and polymer chains, in spite of a relative high pressure of hydrogen (4 MPa) the catalytic system was sufficiently stable. A similar system but with platinum nanoclusters was used [40] for the selective hydrogenation of a,/3-unsaturated aldehyde to a,/3-unsaturated alcohol at 4 MPa of hydrogen and 333 K. The highest selectivity was achieved in the presence of Lewis acids {e.g. ZnCl2, 99.8 %).
C O N C L U S I O N
We hope that this short overview on resin-based catalysts with dispersed metals would attract the attention of researchers committed to the development of new catalysts. Functionalized resins may be prepared as materials with well-defined macro- and mi-croporosity and hydrophobic-hydrophilic properties. In addition, other functional groups may be incorporated easily. It is evident that for mild reaction conditions functionalized resins are very suitable supports, and they are usually cheaper than e.g. zeolites. The wise combination of the intrinsic fine physico-chemical features of the swollen polymer gels with the catalytic potency of metal nanocrystallites is suitable for producing innovative catalysts in the commodities and fine chemicals industry. Actually, metal catalysis based on synthetic functional resins is already producing profits in three industrial applications, but other potentialities are feasible. Potentialities of these catalysts may be relevant to selective oxidation and hydrogenation reactions, to reactions in which metal— support—substrate interactions can be exploited, to multimetal and multifunctional catalysis, to the exploitation of polymer chains-tethered enantioselective controllers, etc.
Acknowledgements. This work was partly supported by
funds of the project SK-l/6049/99: New catalysts for industrial
applications. We acknowledge Mr. C. Furlan from the Univer
sity of Padova for electron-microscopy pictures and XRMA;
Dr. A. A. D'Archivio and Dr. L. Galantini from the University
di L'Aquila in Italy for PFGSE-NMR and ESR measurements;
Dr. K. Jeřábek from the Academy of Sciences of the Czech Republic for ISEC; Dr. W. Meyer-Zayka, the University of Essen for ТЕМ; Dr. Andrea Biffis from the University of Padova for
consulting results and organization of this paper; Dr. V. Jorik
from the Department of Inorganic Chemistry, Slovak Univer
sity of Technology, for X-ray powder diffraction analysis; Dr.
D. Gašparovičová and. V. Krátky from the Department of Organic Technology, Slovak University of Technology, for carrying out catalytic tests.
Chem. Papers 54 (4) 254—264 (2000) 263
M. KRÁLIK, B. CORAIN, M. ZECCA
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