Polymers for Advanced Technologies Volume 7, pp. 634-638

Immobilization of Polymer-protected Metal Colloid Catalysts by the Formation of Polymer Hydrogen Bond Complexes Yuan Wang, Hanfan Liu* and Yanxia Huang Institute of Chemist y, Chinese Academy of Sciences, Beijing 100080, China

ABSTRACT

poly(N-vinyl-2-pyrrolidone)-(PVP) and polyvinyl alco- hol-(PVA) protected nanoscopic noble metal colloidal catalysts were immobilized to prepare the corresponding heterogeneous catalysts by forming polymer hydrogen bond complexes of polyacrylic acid ( P . ) with PVP or PVA. A PVP-PAA-Pd catalyst was found to be very active and selective for the partial hydrogenation of cyclopentadiene to cyclopentene.

KEYWORDS: polymer-protected metal colloid; immobilization; catalyst; hydrogenation; cyclopentadiene

INTRODUCTION Much attention has been given to the catalytic properties of nanoscopic metal clusters or colloids [l-71. Metal particles, sized in one or several nm, are of importance for understanding quantum size effect and the relationship between homogeneous and heterogeneous catalysts. Stimulated by these scien- tific motives, great success has been achieved in research on preparation, characterization and cata- lytic properties of polymer-protected nanoscopic metal colloid with narrow particle size distribution. Recently, successful preparation of the polymer- protected bimetallic colloids, especially the alloy colloids composed of a noble metal and a light transition metal, threw light on designing new catalysts with narrow particle size distribution and controllable composition or structure, from which some special catalytic selectivity may be derived

* To whom correspondence should be sent.

[ 8-1 81. When polymer-protected metal colloids are used

as catalysts, a practical problem arises in that it is difficult to separate the substrates and products from the homogeneously dispersed catalytic system. In order to solve this problem, several strategies for the immobilization of polymer-protected metal colloids have been proposed [19-221. However, there are several problems about these known methods; for example, the limitation of the protective agent for metal colloid and the complicated process for synthe- sis of proper supports. Intensive studies have been conducted to find the best protective agent which should be able to produce metal colloids with small particle size, narrow size distribution and high catalytic activity. Poly(N-vinyl-2-pyrrolidone) (PVP) has been proved to be the most suitable polymer protective agent. Unfortunately, the known immobi- lization methods, for except one named coordination capture method 1221, are not suitable for immobiliz- ing the PVP-protected metal colloid, while the coordination capture method suffers from the com- plicated synthetic process of its supports anchored with strong coordination ligands and the influence of strong coordination ligands on catalytic properties for some reactions.

In this paper we report a new, convenient and effective method for the immobilization of polymer- protected metal colloids, i.e. using a process of forming polymer hydrogen bond complexes of poly- mer-protective agents of metal colloids with another polymer, such as polyacrylic acid (PAA). This immo- bilization method is suitable for PVP- or polyvinyl alcohol (PVA)-protected metal colloids. Palladium catalyst prepared by the present method is highly active and displays selective hydrogenation of cyclo- pentadiene to cyclopentene.

CCC 1042-7147/96/070634-05 0 1996 by John Wiley & Sons, Ltd.

Received 2 November 1995 Accepted 30 Ianua y 1996

Immobilization of Polymer-protected Metal Colloid Catalyst / 635

EXPERIMENTAL

Materials and Equipment Palladium chloride (PdC1,) and chloroplatinic acid (H,PtCl,. 6H,O) are of analytical grade supplied by Beijing Chemical Co., rhodium trichloride hydrate (RhC1, .3H,O) was prepared according to the lit- erature [221, PVP (M,40,000) was purchased from Aldrich Chemical Co. PVA was purchased from Kurashiki Co. (degree of polymerization: 2400, degree of hydrolysis: 99 mol%). Polyacrylic acid (M, 50,000) was prepared in water at 60°C by using benzoyl peroxide as initiator. Cyclopentadiene (CPD) was obtained by a thermal splitting decomposition of its dimer at 140°C prior to use. Other reagents had at least a level of guaranteed reagent (GR) and were used without further purification. Transmission elec- tron microscopy (TEM) photographs were taken by using an Hitachi-9000 NAR-electron microscope.

Preparation of Polymer-protected Metal Colloids

PVP-protected Pd, Pt, and Rh colloidal dispersions and PVA-protected Pd colloid were prepared sim- ilarly to the method in the literature [5,81. PVP (1.125 g, 10.13 mmol as monomeric residue) and PdCl, (44.50 mg, 0.251 mmol used as hydrate) were dissolved in 342 ml methanol. After refluxing for 30 min, 38 ml methanol solution of NaOH (0.034 M) was added to the mixture. A dark-brown PVP-protected Pd (PVP-Pd) colloidal dispersion was obtained after further refluxing for 10 min.

PVP-Rh, PVP-Pt and PVA-Pd were prepared by refluxing a methanol-water (1:l) solution of the corresponding polymer-protective agent with a noble metal compound for 4 hr without adding NaOH.

Immobilization of Polymer-protected Metal Colloid After adding water (300ml), methanol in the PVP- protected I'd colloidal solution (342 ml) was removed by evaporating under vacuum at 50°C (for the PVA-protected metal colloids, this process is not necessary) to give an aqueous colloidal dispersion of PVP-Pd (200 ml). To the obtained metal sol (190 ml), silica support (391 mg, surface area 375 m2 / g) was added, followed by the addition of an aqueous solution of polyacrylic acid (0.39 g, 5.4 mmol as monomeric residue in 600 ml water) under vigor- ously stirring. The mixture was stirred at room temperature for 1 hr. Almost all palladium particles were immobilized onto the support to give a dark- brown supported catalyst, leaving a clear, colorless solution. The I'd concentration of the residue solu- tion was 0.24 ppm as measured by atomic absorption spectroscopy (AAS). The immobilized catalyst PVP- PAA-Pd was washed with water and alcohol and was dried under vacuum. Immobilized catalysts PVA-PAA-Pd, PVP-PAA-Pt and PVP-PAA-Rh were prepared similarly from the corresponding polymer-protected metal colloids.

Hydrogenation of Cyclopentadiene

A 30 ml flat-bottomed flask was charged with cata- lyst and 19 ml of solvent. Hydrogen was introduced to replace air several times. The catalyst was acti- vated under an atmosphere of hydrogen with magnetic stirring for 1 hr at 30°C. CPD (0.5 mmol in 1 ml solvent) was then aded and the hydrogenation reaction rate was measured by the uptake of hydro- gen. The products were analyzed by gas chromatography (GC) with a 2 m column of squa- lane.

RESULTS AND DISCUSSION

The Immobilization Process It is well known that a polymer hydrogen bond complex can usually be obtained as a precipitate from the reaction between two kinds of soluble polymer having groups for forming hydrogen bonds. Figure 1 shows the mechanism of the present immobilization method. The polymer hydrogen bond complexes containing colloidal metal particles formed by the interaction of the protective agent PVP or PVA with PAA were dispersed on the surface of the silica support in order to give a well-dispersed heterogeneous catalyst. The present process is very effective for immobilizing PVP or PVA-protected metal colloids; it can be deduced from the AAS measurement that higher than 99% of the metal particles is present in the heterogeneous catalyst.

Figure 2 shows the TEM picture of PVP-Pd and PVP-PAA-Pd catalyst with their size distribution of Pd particles. Pd particles in both PVP-Pd and PVP- PAA-Pd have an average diameter of 1.8 nm and a size distribution from 1.0 to 3.0 nm. Thus, the present method provides a widely available route to prepare heterogeneous catalyst, in which not only the particle size and size distribution of metal particles but also the metal composition and structure of bimetallic particles can be controlled by the synthesis process of the polymer-protected monometallic and bimetallic colloids, because the protective agent PVP is of universal significance for the preparation of different metal and alloy colloids.

FIGURE I. Schematic representation of the immobilization process of a olymer-protected metal colloid by forming a hydrogen bon 8 complex.

636 / Wang eta/.

60

5 0 s . 4 0 8 '3 3 0 P

8 2 0

10

5 s

3 2 a ;:i 1

(b) dav = 1.8 nm 7-

I

I

I I I

I

I 8

I I I

I

- -

Particle size / nm 50.0 nm

FIGURE 2. TEM picture and metal particle size distribution: (a) PVP-Pd; (b) PVP-Pd-PAA.

Hydrogenation of Cyclopentadiene The time courses of the hydrogenation of CPD catalyzed by different immobilized colloid catalysts are shown in Fig. 3. The initial rate of the hydro- genation of CPD (V,) and that of cyclopentene (VJ are listed in Table 1. The relative selectivity for the monoene formation (RS) is expressed by the ratio of V, to V,. From the catalytic results listed in Table 1, it can be seen that the catalytic selectivity for the formation of monoene is in the order PVA-PAA-

and the order of activity is PVP-PAA-Pd > PVP- Pd > PVP- PAA -Pd > PVP- PAA -Rh > PVP-PAA - Pt

0 200 400 600 800

Time I s

FIGURE 3. Time

20 ml; temperature 30°C; ressure, 1 atm. PVP-PAA- Pd; PVP-PAA-Rk; A PVF-PAA-Pt; + PVA-PAA-Pd.

catalyzed b immobilized Pd, 3.3 x 1 & 3 mmol;

PAA-Rh > PVP-PAA-Pt > PVA-PAA-Pd. When pal- ladium is used as the catalyst, it seems that larger metal particles provide a better selectivity for mono- ene although increasing in particle size is accompanied with the loss of activity.

Figure 4 shows the influence of solvent on the catalytic properties of the PVP-PAA-Pd catalyst, and the initial rate and RS are listed in Table 2. From Table 2, it can be seen that the selectivity for the monoene formation in a different solvent is in the order toluene > benzene > cyclohexane > methanol and the activity is in the order of methanol>cyclo- hexane > toluene > benzene. For the hydrogenation of olefin, alcohol is usually the best solvent for obtain- ing good activity. The fact that aromatic solvents increase the selectivity may be due to the coordina- tion of the benzene ring to palladium. The depressing effect of this coordination on the hydro- genation of monoene is expected to be much higher than that on the hydrogenation of conjugated diene because the coordination action of monoene with Pd is much weaker, thus resulting in a better selectivity in aromatic solvents. Hydrogenation products was analyzed by GC; when the PVP-PAA-Pd is used as catalyst in toluene, a selectivity for monoene forma- tion of higher than 98% was obtained at 100% conversion of cyclopentadiene.

Catalytic results of a Pd/C (Pd: 1%) catalyst are listed in Table 3. From Table 3, it can be seen that the catalytic activity of PVP-PAA-Pd is about 11 times higher than that of the Pd/C catalyst and the selectivity of PVP-PAA-Pd is also better than that of the Pd/C as shown by their RS values. Nakamura and Hirai [191 reported the catalytic properties for

TABLE 1. Catalytic Properties of Hydrogenation of CPD Catalyzed by Immobilized Catalysts

Particle size v d v, Metal content Catalyst (nm) (mo/ H2/s . mol MJ (mol H2/s . mo/ M) RS f%J PVP- PAA-Pd 1.8 2.25 0.68 3.31 0.43 PVA-PAA-Pd 25.3 0.29 0.01 8 16.1 1 0.6 1 PVP-PAA- Rh 2.0 1.93 1.35 1.43 0.44 PVP-PAA- Pt 1.6 1.35 1.35 1 .oo 1.57

Reaction conditions: the same as those in Fig. 3.

Immobilization of Polymer-protected Metal Colloid Catalyst / 637

0 500 lo00 1500

T i m e J S

FIGURE 4. Influence of solvent on the catalytic properties of PVP-PAA-Pd for the h drogenation of CPD.

28mM; solvent, 20 ml; temperature, 30°C; ressure, 1 atm. Ex erimental conditions: F; d, 3.3 x 1 0 - 3 mmol; [CPD],

methanol; 0 cyclohexane; A toluene; 4 E enzene.

hydrogenation of CPD of a catalyst PAA-Pd-PEI synthesized by immobilizing a PAA-protected Pd colloid by forming a polyion complex of PAA with polyethylene imine (see Table 3). Although its selec- tivity for CPE can also reach 98.5%, its activity in alcohol is about 11 times lower than that of the present PVP-PAA-Pd catalyst in toluene. This dif- ference may be because of the following reasons. PAA is not a good protective agent for colloid particles of palladium. Its protective value (0.07) is much lower than that of PVP (50) [231. Thus, a trace of black precipitate formed during the preparation of PAA-protected I'd colloid [191. According to our experience, the palladium particle size in PAA-Pd is

TABLE 2. PVP-PAA-Pd for the Hydrogenation of CPD

Influence of Solvent on the Catalytic Properties of

v d v e

(mol H2/s . mol Pd} (mol H 2/s . mol Pd} RS Solvent

Methanol 2.25 Toluene 1.04

Benzene 0.68 Cyclohexane 1.35

0.68 3.3 0.04 26.0 0.21 6.4 0.06 11.3

Reaction conditions: the same as those in Fig. 4.

TABLE 3. Selective Hydrogenation of CPD

Catalytic Properties of Different Catalysts for

v d Catalyst Solvent (mol H2/s . mol Pd} RS

PVP-PAA-Pd Toluene 1.04 26 PAA-Pd-PEI Methanol 0.09 32 Pd/C Toluene 0.09 21

Reaction conditions: the same as those in Fig. 4.

about 20 nm, whereas that in PVP-Pd is only 1.8 nm. Another reason for the poor activity of PVP-Pd-PEI may be because of the strong coordination ability of imine groups in PEI, by which most of the active sites would be blocked.

CONCLUSION This paper describes the method for the immobiliza- tion of polymer-protected nanoscopic metal colloids to prepare heterogeneous catalysts, which makes it possible to control the metal particle size, size distribution, composition and structure of metallic or bimetallic particles in heterogeneous catalyst by the synthesis of polymer-protected metal colloids. The present method is very effective for immobilizing the PVP- or PVA-protected metal colloids: more than 99% of the metal particles in the colloidal dispersions can be easily immobilized. Catalytic properties of the immobilized nanoscopic metal catalysts have been studied for the selective partial hydrogenation of CPD to CPE, and PVP-PAA-Pd has been found to be of high activity and selectivity in toluene. The activity is about 11 times higher than that of Pd/C and the published results of PAA-Pd-PEI catalyst.

ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (Contract No. 29474181) and the Fund of the Chinese Academy of Sciences for financial support (Contract No. Kh485-11-41. This work was also supported by the Fund of the Director of the Institute of Chemistry, the Chinese Academy of Sciences.

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