Polymer International 34 (1994) 369-373

Synthesis, Characterisation and -

Catalytic Activity of Styrene- Divinyl benzene Copolymer

(XA-D-2) Bound Palladium(ll)-Complex

Jacob John & R. N. Ram*

Department of Chemistry, Faculty of Science, M.S. University of Baroda, Baroda-390 002, India

(Received 25 August 1993; revised version received 30 November 1993; accepted 8 February 1994)

Abstract: Polymer-anchored Pd(I1) complex catalyst was synthesised by sequen- tial attachment of 1,2-diaminopropane as a ligand to the chloromethylated com- mercially available styrene-divinylbenzene copolymer (XAD-2). The catalyst was characterised using techniques such as FTIR, SEM, EPR, and UV-VIS reflec- tance spectroscopy. Other physicochemical properties, such as bulk density, moisture content, swelling and surface area by the BET method at low tem- perature by nitrogen adsorption, were also studied. The catalyst was tested for the rate of hydrogenation of cyclohexene. The influence of various factors such as temperature, concentration of catalyst, substrate, and quantity of solvent have been determined. The results are compared with the homogeneous analogue. The recycling efficiency of the catalyst was also studied.

Key words: chloromethylation, styrene-divinylbenzene copolymer, 1,2-diamino- propane, cyclohexene, kinetics of hydrogenation.

I NTRO D U CTlO N

A large number of metal complexes and organometallic compounds have attracted much attention in the field of catalysis. Many research workers have studied the cata- lytic activity of Pd(I1) and Ru(II1) complexes in homo- geneous systems for various reactions, mainly due to their high activity under mild operating condition^.'-^ A variety of polymeric supports have been used to het- erogenise the homogeneous catalysts, which enhances the catalytic activity of the Most of the sup- ported catalysts studied were non-chelated. The main purpose of this study was to synthesise a chelated metal complex catalyst on a polymeric support and to measure its catalytic activity for a model hydrogenation reaction. We report a Pd(I1) ion heterogenised on a polymer support functionalised with a ligand, hence forming a stable chelated metal complex, attached to

the polymer matrix. Cyclohexene was used as a sub- strate to study the catalytic activity of the above cata- lyst.

EXPERIMENTAL

Materials and equipment

A styrene-divinylbenzene copolymer (XAD-2) obtained from Fluka A G was purified by Soxhlet extraction using a benzene/ethanol (1 : 1) mixture and dried at 70°C. THF, dioxane, methanol and cyclohexene were purified according to reported procedure^.^ 1,2- Dichloroethane and diaminopropane were distilled before use. Aluminium chloride was purified by subli- mation. PdCl, (Lobachemie, Bombay) was used as received. Elemental analysis was carried out in our laboratory using a Coleman analyser unit. UV-visible reflectance spectra of solid samples were recorded using a Shimadzu UV-240. FTIR of the catalyst was recorded * To whom correspondence should be addressed.

Polymer International 0959-8103/94/%09.00 0 1994 SCI. Printed in Great Britain 369

370 Jacob John, R. N. Ram

on a Perkin Elmer 172OX. Surface area was determined using a Carlo-Erba 1800, and TGA with a Shimadzu DT-30.

Synthesis of the catalyst

Beads of XAD-2 dried at -70°C were chloromethyl- ated in the presence of para-formaldehyde and 1,2- dichloroethane using AlCl, as catalyst at 80"C.8 They were washed with 50% aqueous dioxane containing 10% HCl (v/v), dry dioxane and deionised water till the removal of chloride ions was complete, and finally dried at 60-65°C for 24h under vacuum. The C, H and C1 contents in the above sample were found to be 62.86, 7.42 and 5.7 wt%, respectively.

Ligand introduction on to the polymer support

In order to attach 1,2-diaminopropane, a chelating ligand, chloromethylated beads were kept in contact with an appropriate quantity of ligand at 60°C using ethanol as solvent. The detailed procedure is described el~ewhere.~ The percentages of C, H and N after ligand introduction were found to be 61.26, 7.31 and 1.80, respectively.

Attachment of metal ion on to the liganded polymer

A 25 g sample of the functionalised polymer was kept in contact with 80ml of ethanol for 30min in a round- bottomed flask. PdCl, (0.5 g) dissolved in AR HCl(3 ml) and diluted with ethanol was placed in a pressure equal- king addition funnel, fitted to the reaction vessel by means of a standard joint, the end being closed with a stopcock. The solution was added to the reaction vessel over a period of 30min. The reaction mixture was stirred for 7 days using a magnetic stirrer. The colour of the supernatant solution changed from dark brown to light red and polymer beads became golden yellow. Beads were filtered and washed thoroughly with benzene, ethanol and dry ether. The anchored catalyst so obtained was dried in vacuum and stored. The metal content was determined by refluxing metal-containing polymer beads with conc. HCl (AR) for 24 h, and then estimating the metal concentration in the solution by a spectrophotometric method" after complexation with nitroso-R salt.

The unbound complex in solution was obtained by mixing a 1 : 1 mixture of PdCl, and 1,2- diaminopropane in methanol. This metal complex cata- lyst was used in solution for kinetic studies of the hydrogenation of cyclohexene.

Kinetics of h ydrogenation

The kinetics of hydrogenation of cyclohexene were studied at atmospheric pressure by measuring hydrogen

uptake using a glass manometric apparatus. The detailed procedure and experimental set-up are described e l s e ~ h e r e . ~ ~ " The data for the hydrogenation reaction were obtained using a stirring speed of 650 rpm at 35 k 0.1"C. Several experiments were carried out in order to determine the stoichiometry of the reaction at constant temperature and at 1 atm pressure using differ- ent concentrations of cyclohexene. It was found that the amount of hydrogen absorbed was directly proport- ional to the cyclohexene converted (based on GC analysis). This indicates that no side product was formed. The initial rate was calculated from the slope of the tangent of the plot of the hydrogen uptake versus time.

RESULTS AND DISCUSSION

Physical properties of the supported catalyst are given in Table 1.

Thermal stability of the catalyst

In the case of polymer-bound metal complex catalysts, weight loss or enthalpy change may reveal stability and phase change as a function of environment. The support used for the preparation of the catalyst was found to be stable up to 150"C, whereas the support-plus-catalyst was less stable (Fig. 1). The initial weight loss may be due to moisture content as well as degradation of the polymer above 100°C. Therefore the catalyst could be used safely only below 100°C.

The morphology of the polymer-supported catalyst was studied by scanning electron microscopy (Fig. 2). The UV-visible reflectance spectra of the polymer- bound palladium complex gave broad peaks at 220nm and 400nm, which may be due to d-d transitions of Pd(I1). In the case of the homogeneous complex, the peaks appeared at 214nm and 400nm, confirming the presence of Pd ion in the same oxidation states both in homogeneous and heterogeneous systems. The EPR spectrum of the palladium complex was found to be inactive, which shows that Pd is present in the + 2 oxi- dation state.

The mode of anchoring of metal ion on the polymer matrix was confirmed by FTIR spectral studies. A band at 348cm-' for Pd-Cl and at 478cm-' for Pd-N indi- cated the attachment of metal ions to the liganded

TABLE 1. Physical properties of the supported catalyst

Particle size (nm) -0.5 Surface area (mZ g - ' ) NTP 326.00 Apparent bulk density (g cm-') 0.36 Moisture content (wt%) 0.38

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Styrene divinylbenzene copolymer (XAD-2) bound palladium(l1) complex 371

D Temperature eC )

Fig. 1. TG curves for support and catalyst. (A) XAD-2 polymer; (B) XAD-2 bound Pd(I1) catalyst.

polymer. Based on the spectroscopic evidence, a prob- able structure of the catalyst may be as shown in Scheme 1.

Fig. 2. Scanning electron micrograph of (top) XAD-2 support, and (bottom) XAD-2 bound palladium catalyst.

@ -CHz-NH-CHz-CH-CH, \ I ' P /NHz cl/ %I

Scheme 1

Hydrogenation reaction

The kinetics of hydrogenation of cyclohexene were chosen as a model reaction. The influence of various parameters such as temperature and concentration of the substrate and catalyst were studied. The recycling efficiency of the catalyst was measured for used and fresh catalysts. The experiment was carried out at 35°C for 16h. In each experiment lop1 of the substrate was injected. The rate of hydrogenation was measured as a function of time for both used and fresh catalyst. It was observed that the maximum rate of reaction was main- tained for about 10h, after which the rate decreased slowly.

Effects of cyclohexane concentration. The effect of con- centration of substrate on the rate of the hydrogenation reaction was studied at 35°C under a pressure of 1 atm keeping the catalyst concentration at 2.06 x 10-6mollitre-' of Pd. The cyclohexene concen- tration was varied from 4.71 x to 18.84 x 10-3mollitre-' (Table 2). A linear plot of l/rate versus l/[substrate] (Fig. 3) indicates that the rate of hydrogenation of cyclohexene, R, is related to [S], the concentration of cyclohexene by the relation- ship

1 1 - = a - + b R CSl

POLYMER INTERNATIONAL VOL. 34, NO. 4, 1994

372 Jacob John, R. N . Ram

TABLE 2. Summary of the kinetics of hydrogenation of cyclohexene for palladium( II) catalyst (a) supported on XAD-2 and (6) in homogeneous

system

Temperature [Pd( 1113 [ Cyclohexene] Rate of reaction ("C) (mol litre-') (mol litre-' x lo3) (ml min-' x lo2)

a( 1 06) b( 1 03) a b

30 2.06 1.80 35 2.06 1.80

35 1.03 0.79 2.06 0.98 2.58 1.18 3.1 0 1.58 4.1 3 1.80

40 2.06 1.80 45 2.06 1.80

9.42 4.71 9.42 11.77 14.1 3 18.84 9.42

9.42 9.42

2.68 4.80 1.60 2.95 2.56 4.07 2.89 4.73 3.24 5.1 2 3.61 6.01 2.20 2.69 2.56 3.38 2.95 3.98 3.1 2 5.01 5.30 5.49 3.1 0 7.29 4.50 8.91

where a and b are the slope and intercept of the linear plot. The order of reaction calculated from the linear plot of log (initial rate) versus log [cyclohexene] was found to be fractional.

Efect of catalyst concentration. The effect of concentra- tion of catalyst on the hydrogenation reaction was investigated over a range of 1.03 x to 4.13 x 10-6mollitre-' of Pd at 35"C, and at a pressure of latm, with a substrate concentration of 9.42 x 10-3mollitre-1 (Table 2). The order of reaction calculated from the linear plot of log (reaction rate) versus log [catalyst] was found to be fractional with respect to catalyst concentration, which may be attrib-

uted to the non-availability of catalytic sites. The lack of swelling may be due to steric hindrance.

Efect of temperature. The effect of temperature on the rate of hydrogenation was studied over a range of 30-45°C at a catalyst concentration of 2.06 x 10-6mol litre-' of Pd at 1 atm pressure. The rate of hydrogenation was found to be dependent on temperature. The activation energy calculated from the slope of the plot of log (initial rate) against 1/T (Fig. 4) was found to be 5.85 kcal mol-'.

Hydrogenation with unbound complex [ P I DAPICI, . In order to compare results, the same quantity of catalyst

0

Fig. 3. Plot of I/rate versus l/[cyclohexene] for the supported catalyst.

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Styrene divinylbenzene copolymer (XAD-2) bound palladium(I I ) complex 373

-1.1

-1.2 ’

- c. 9 -1.3 n

I N

U

‘p -1. 4

3 -

-1.5

-1.6’

I 3. I

-1. 7 3.1 3.2 3.3

Fig. 4. Arrhenius plots for the catalytic hydrogenation of cyclohexene. (H) Homogeneous system, E , = 8.13 kcalmol- ’ ;

(P) polymer-supported system, E , = 5.85 kcalmol- ’.

was introduced into a homogeneous system as used for the polymer-supported catalyst but immeasurable hydrogen uptake was found. Therefore a large amount of catalyst was taken for the study.

The effects of various parameters such as concentra- tion of substrate and catalyst, temperature and solvent

TABLE 3. Effect of solvent on the kinetics of hydrogenation of cyclohexene at 1 atm pressure

Solvent Rate of reaction’ Rate of reactionb (mlmin-’ x lo2 ) (mi min-’ x 1 Oz)

Methanol 2.26 2.75 Ethanol 2.21 2.69 Dioxane 1.83 1.92 TH F 1.17 1.44 Benzene 0.72 0.90

a Data for supported catalyst. [Palladium] = 2.06 x 1 0-6 mot litre-’ ; [cyclohexane] = 9.42 x 1 0-3 mol litre-’.

Data for homogeneous complex. [Palladium] = 1.8 x 10-3mollitre-’; [cyclohexane] =9.42 x 10-3mol litre-’. Reaction temperature = 35°C. Amount of solvent used = 20 ml.

were investigated for the homogeneous complex under the same reaction conditions as the polymer-bound complex. The results are summarised in Table 2. The activation energy was found to be 8.13 kcal/mol (Fig. 4), which indicates that polymer-supported Pd(I1) complex catalyst has a higher activity than its homogeneous counterpart.

Efect of solvent. The effect of five different solvents on the rate of hydrogenation of cyclohexene was measured. It was found that the rate decreased as the nature of the solvent changed from polar to non-polar. The results are given in Table 3. The enhancement in the reaction rate may be due to the swelling of the polymer support and hence the availability of catalytic sites.

CONCLUSION

The results show that the polymer-bound palladium complex is more effective than the homogeneous complex for the hydrogenation of cyclohexene. The reaction mechanism is under study.

ACKNOWLEDGEMENTS

The authors would like to thank Prof. A. C. Shah, Head of Chemistry Department, for providing the necessary facilities ; UGC, New Delhi, for providing financial assistance to one of us (Jacob John); and R & D, IPCL, Baroda, for scanning electron micrographs.

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