Preparation, characterization and catalytic activity of polymer-supported trimethylenediamine-ruthenium complex

Journal of Molecular Catalysis, 60 (1990) 141-154

141

PREPARATION, CIIARACTERIZATION AND CATALYTIC ACTIVITY OF POLYMER-SUPPORTED TRIM.RTHYLENEDIAMIlWRUTIIENIUM COMPLEX JATIN N. SHAH, D. T. GOKAK and R. N. RAM Department of Chemistry, Faculty of Science, MS.

University of Bar&a, Baroda 390 002 (India)

(Received August 21,1989; accepted January 3,199O)

Summary Polymer-supported ruthenium( trimethylenediamine) complexes have been prepared with 3% and 10% divinylbenzene-styrene copolymer porous beads by sequential attachment of the bifunctional ligand, followed by treatment of metal salt with the functionalised polymer beads. Physiochemical properties such as surface area, moisture content, bulk density, swelling, crushing strength, morphology and thermal stability of support and supported catalysts have been studied. Probable structures based on spectroscopic studies have been proposed and catalytic activity evaluated for the model reaction of cyclohexene hydrogenation.

Introduction Homogeneous and heterogenized homogeneous catalytic hydrogenation of olefins by platinum group metal complexes have received much attention in recent years [l-5]. A number of ruthenium complexes have been studied for various reactions, mainly due to their high catalytic activity under mild operating conditions 161. Polymer-anchored ruthenium complexes are found to catalyse a number of reactions [7,81. It has also been reported that polymer-anchored non-chelated complexes are less stable and leach out in solution [9,101. Hence the present investigation was undertaken in order to anchor the chelated ruthenium complex on the polymer and also to study the effect of the polymer crosslink on the stability and activity of the catalyst in a model hydrogenation reaction. Experimental Materials and equipment Styrene, divinylbenzene (DVB), dioxan, methanol, cycle-hexene and n-heptane were purified according to the published method 1111. Chloromethyl methyl ether (CMME) and trimethylenediamine (TMDA) were distilled before use. Aluminium chloride was purified by sublimation. RuCl,=3H20 (obtained from Johnson Matthey Inc., New Jersey) was used without purification. iQ Elsevier Sequoia/Printed in The Netherlands

142

Ultraviolet spectra of the solid samples were recorded on a Shimadzu W-240 instrument, using BaS04 as standard and liquid sample in ethanol. Infrared spectra were recorded on a Beckmann IR-4220 instrument, while the surface area of the supports as well as the catalysts was measured using Carlo Erba Strumentzione 1800. Elemental analyses and TGA were carried out in our laboratory on a Coleman analyzer and a Shimadzu thermal analyser DT-30. X-ray analysis was done on a Perkin Elmer instrument, and Jeol scanning microscope JSM-T300 was used for scanning electron microscope; crushing strength was measured on a Baker Mercer Type C-12 machine. Swelling studies of the catalysts were carried out using polar and non-polar solvents at a constant temperature. The detailed procedure has been described earlier [ 121. Synthesis of polymer support Styrene-divinylbenzene copolymer with 3% and 10% cross-link were synthesised by the suspension polymerization technique, using benzoyl peroxide as initiator [ 121. After polymerization the beads were washed with distilled water, water-ethanol mixture (1:l) and finally with ethanol. Any adsorbed material was removed by extracting the polymer using soxhlet extractor with ethanol-benzene (1:l) mixture for a total period of 20 h. The polymer beads were chloro-methylated using chloromethyl methyl ether (CAUTION is advised in chloromethylation and in handling CMME because the related compound, dichlorodimethyl ether, is carcinogenic). The detailed procedure for the chloromethylation has been described earlier [12]. The elemental analysis after chloromethylation is found to be as follows: Polymer crosslink

C (wt.%)

H (wt.%)

3% 10%

82.85 79.84

6.77 6.72

Z.l%, 10.08 11.20

Ligand introduction onto the polymer matrix 25 g chloromethylated polymer beads were kept in contact with ethanol for 30 min in a three-necked 500 ml round-bottom flask fitted with a condenser, mechanical stirrer and adding funnel. TMDA (15 ml in 100 ml ethanol) was added dropwise into the reaction vessel over a period of 15 min by using the adding funnel. The reaction mixture was refluxed for 4 h with occasional stirring. After reaction the beads were cooled and filtered. The beads were washed with 0.5NNaOH and deionized water, then dried at 70 “C for 24 h under vacuum. The color of the functionalized polymer beads was yellow. The elemental analysis found after ligand introduction is given below: Polymer crosslink 3% 10%

C (wt.%)

H (wt.%>

79.01 79.02

7.55 7.36

(2%) 2.00 4.47

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Formation of metal complex on the polymer matrix 25 g of the functionalized polymer was kept in contact with 100 ml ethanol for 30 min in a round-bottom reaction vessel. A solution of RuC13.3Hz0 in ethanol (2.5 g in 100 ml ethanol), taken in an adding funnel fitted to the reaction vessel, was slowly added to the reaction mixture over a period of 30 min. The reaction mixture was initially refluxed for 7 h and then was allowed to continue to react at room temperature for 3 days. During this period the color of the supernatant solution changed from dark orange to light orange and the polymer turned light grey. The beads were filtered and washed thoroughly with ethanol. The anchored catalyst thus obtained was dried in vacuum at 70 “C for 24 h and stored. Ruthenium content was determined by refluxing the metal-containing polymer beads with concentrated hydrochloric acid (A.R.) for 24 h and then estimating the metal concentration in the diluted solution by colqrimetry [131. Nomenclature The catalysts are designated by the general formula: NPML where N = percent crosslink, P = copolymer of styrene-divinyl benzene (PSDVR), M = metal atom, L = type of ligand. Let us define Catalyst A = 3PRu”‘(TMDA), Catalyst B = lOPRu”‘(TMDA). Hydrogenation procedure The hydrogenation reaction was carried out at atmospheric pressure in a magnetically stirred glass reactor in a chemically controlled regime 1143. The rate of hydrogen consumption was measured using a glass manometric apparatus. The detailed experimental set-up and hydrogenation procedure has been described elsewhere 1151.

Results and discussion The surface area of the polymer support as well as of the catalysts is given in Table 1. A decrease in surface area was observed after loading the metal ions on the polymer support. This may be due to the blocking of the pores of the polymer support after introduction of the ligand as well as the metal ion. Similar results have been reported by us in earlier studies [12, X5-171. The nature of the solvent is an important factor that can be varied in order to control the activity and selectivity of the polymer-supported catalysts. A solvent can influence the reaction through its ability to swell the support. It is therefore essential to study the swelling of the polymeric support and the catalysts. Studies of the catalyst swelling have been carried out using various solvents. The results are summarised in Table 2. A

144 TABLE 1 Catalyst characterization Physical properties

3PRu”’ (TMDA)

1OPRu”’ (TMDA)

particle size (mm) surface area (m2 g-l) NTP Apparent bulk density (g cm-‘) moisture content (wt.%) crushing strength (kg)

2 -1

2 -1

0.4826

0.4744

1.685

2.510

0.15

0.20

decrease in swelling was observed on increasing the degree of crosslinking of the polymer support, which is indicative of the rigidity of the catalyst. The optimum solvent is one that combines good swelling ability and high polarity. Methanol was observed to be a good solvent by causing greater swelling, as well as having a larger capacity for the dissolution of the substrate. However, maximum swelling was found using water as a solvent, perhaps due to the hydrogen bonding of water molecules with amino groups. X-ray studies indicated that the metal complex does not crystallize on the polymer surface. The change in the morphology of the support and the supported catalysts was observed by the use of SEM photographs (Fig. 1). Elemental analysis and metal estimation of the catalyst indicate a low level of anchoring of the metal ion onto the aminated polymer. Similar observation was noticed by us while loading cobalt and rhodium on styrene-DVB crosslinked polymer [12,15-171. This may be due to the non-accessibility of ligands to the metal ion. However, anchoring of the metal complex onto the polymer was confirmed by comparative spectral studies of polymer-bound and unbound complexes (Fig. 2a, 2b). TABLE 2 Swelling studies Solvent

3PRu”’ (TMDA) (mol%)

1OPRu”’ (TMDA) (mol%)

water methanol ethanol dioxane NJ-dimethylformamide acetone t&ahyd&llran benzene cyclohexane n-heptane

0.9389 0.6392

0.9170 0.6195 0.2722 0.1388 0.1245 0.1010 0.0988 0.0823 0.0493 0.0344

0.2886 0.1624 0.1523 0.1131 0.1125 0.1013 0.0634 0.0514

525

Fig. 1. *g electron mkrographs of polymer supports and catalysts: (A) 3P, (B) Catalyst A, (C) lOP, (D) Catalyst B.

ClOP

A3P

146

The various frequencies are assigned as given below [l&211:

IR (cm-’

)

v(N-H)

v(Ru-N)

v(Ru--Cl)

3400 (3400)”

385,355 (380,340)

275

260 (270)

330 (350)

(270)

d-d transition of Ru (III) 230 (230)”

uv (nm)

* Values given in parenthesis

refer to the unbound Ru”‘(TMDA).

Based on all the above physicochemical properties and spectral studies, a possible structure may be proposed as shown in Scheme 1. However, further investigation is needed to elucidate the exact structure. Thermal studies Thermal stability of polymer-supported catalysts is one of the most important criteria for higher temperature applications. Thermogravimetric

(a)

WAVE

WAVENUMBER

(b) Fig. 2. IR spectra (Ru”‘UMDA)Cl,).

of

(a)

1OPRu”’

NUMBER

(Cm?

Ct.?

(TMDA)

and

(b)

unbound

ruthenium

complex

147

Cl

0

NC-Nti*

1 P

ClRL,

A\ p NL/-NH2 0

Scheme 1.

analysis of the catalysts was carried out from room temperature to 500°C under Nz atmosphere with a heating rate of 10 “C mine1 (Fig. 3). It was found that in both cases polymer degradation starts above 150 “C; The -2% weight loss below this temperature may be due to moisture. In order to investigate the stability of the support, a separate experiment was carried out. The supports were isothermally heated at 175 “C for a period of 5 h (Fig. 4). It was noticed that in the case of 3% crosslinking, degradation occurs to lesser extent (i.e. -Swt.%) and in the case of 10% crosslinking degradation occurs up to 8% weight loss within 5 h time, after which the polymer is stable. This loss may be due to degradation of polymer to some extent, or due to the evaporation of organic matter entrapped inside the fine pores. Similar observations have been made by DTA. The changes in the morphology of supports after heating isothermally at 175 “C for 5 h were apparent in the SEM photographs (Fig. 5). Hence it was ensured that the polymer-anchored catalysts can be used in catalytic studies below 150 “C. Hydrogenation The kinetics of cyclohexene hydrogenation was investigated for catalysts A and B.The data for the hydrogenation reaction were obtained in a kinetic regime [14] using a stirring speed of 700 rpm in the temperature range

148

TlME

(min)

1

1 TEMPERATURE

1 OC 1

2

3 TlME

I

1

c

5

(hrs.)

Fig. 3. Thermogravimetric studies of polymer-supported Catalyst A (1) and catalyst B (2). Fig. 4. Thermogravimetric curves of polymer supports 3P (1) and 1OP (2) isothermally heated at 175 “C for 5 h.

30-50°C. The stoichiometry of the reaction was verified by carrying out several experiments at constant temperature and 1 atm pressure of hydrogen at different concentrations of cyclohexene. In all these experiments, the amount of hydrogen absorbed was stoichiometrically proportional to the cyclohexene converted (calculation based on CC analysis of the product). This suggests that no side products were formed. The initial rate data (based on H2 uptake measurements) were used to evaluate the kinetics of the reaction. In each run, the rate of hydrogenation was calculated from the slope of the plots of volume of Hz absorbed (STP) as a function of time, and a summary of the results is given in Table 3(a) and (b). The effect of various parameters on the rate was discussed on the basis of the experimental observations. Influence of cyddwxene concentration The influence of substrate concentration on the rate of hydrogenation was determined for both catalysts in the range of 3.5 x 10m3to 6.3 x 10Y3M at constant catalyst concentration of 5.44 X 10m6M Ru at 35 “C. It was found that the rate varies linearly with the substrate concentrations. The rate of hydrogenation was observed to be higher in case of catalyst B than catalyst A

149

A

525

B

BOO Fig. 5. SEM photographs of isothermally heated polymer supports (A) 3P and (B) 1OP after 5 h.

(Fig. 6). This may be due to coordinatively saturated inactive complexes formed in the case of catalyst A because of the ease of swelling and higher mobility of the polymer fragment. Influence of catalyst concentration The change in the rate of hydrogenation with catalyst concentration was investigated in the range of 0.54 x 10m5 to 1.63 x low6 M Ru at substrate concentration of 4.87 x 10m3M at 35 “C for both the polymer-bound catalysts. It was found that the rate followed a fractional order with respect to catalyst concentration (i.e. 0.25 for Catalyst A and 0.2 for Catalyst B (Fig. 711, perhaps because of the non-accessibility of catalytic sites due to the lack of swelling and the steric hindrance of chelating ligand molecules. Influence of temperature The effect of temperature on the rate of hydrogenation for both polymer-bound ruthenium complexes has been studied in the range of 30-55 “C and the results are given in Table 3. The energy of activation

150

TABLE 3 Summary of the kinetics of cyclohexene hydrogenation for lOPRu”‘(TMDA) catalysta in methanol at atmospheric pressure Catalyst (Ru mol 1-l x ld)

(a) lWRu”(TMDA) 0.544

Cyclohexene (mall-’ x 10’)

3.506 4.236 4.869 5.648 6.330

Temp.

Methanol

NJ)

(mu

and 3PRu”‘(TMDA)

Rate of reaction (ml min-’

35

25

0.897 0.94 1.02 1.23 1.40

0.544

12.17

30 35 40 45 50

10

0.93 1.021 1.32 1.59 2.13

0.544

4.67

35

10 20 25 40

0.885 0.914 1.05 1.21

0.544 0.82 1.08 1.36 1.63

4.87

35

25

1.02 1.04 1.12 1.14 1.31

5.51 4.24 4.87 5.59 6.33

35

25

4.31 4.65 5.00 5.62 6.16

0.544

12.17

30 35 40 45 50

10

4.5 5.0 6.7 8.4 11.2

0.544

4.87

35

10 20 25 40

5.0 5.3 5.6 6.0

0.54 0.81 1.08 1.36 1.63

4.87

35

25

5.0 5.2 5.5 5.9 6.0

(b) 3F’Ru”‘(TMDA) 0.544

X 102)

151

I

Cyclohexene

concentrdion

(moles/litre)

Fig. 6. Influence of cyclohexene concentration on the rate of hydrogenation for polymer-bound ruthenium complexes (1) catalyst A and (2) catalyst B.

1

0.2.

1a2x 0 0

0.2

04

0%

Concentration

0.8

1.0

1.2

1.4

1.6 x

3

of Catalyst (moles,,Litre)

Fig. 7. Influence of catalyst concentration on the rate of cyclohexene hydrogenation for polymer-bound catalyst A (1) and catalyst B (2).

152

calculated from the slopes of the Arrhenius plot (Fig. 8) was found to be 5.39 and 4.87 kcal mol-’ for Catalyst A and Catalyst B respectively. Even though the bond dissociation energy for hydrogen molecule is quite high 123, the observed activation energy for the reaction is low. This may be due to the formation of intermediate complexes with the ruthenium metal, which provide an alternate low energy path for the reaction. Influence of hydrogen concentration in solution Figure 9 illustrates the influence of hydrogen concentration in methanol for both catalysts at fixed concentrations of catalyst and substrate at 35 “C. It can be seen that the rate increases with fractional order of hydrogen concentration. The effect is greater in Catalyst B than Catalyst A. Life cycle of catalysts One of the ways in which the polymer-bound catalyst can lose its activity is by loss of RuUII), which is brought about by leaching of the metal complex or reduction to free metal. In most cases the polymer-anchored complex detaches and the metal is leached out in solution [9,101. The effect is enhanced when the complex is anchored through monodentate ligands. It has however been reported by Drago et al. that polymer complexes anchored through chelating ligands are more stable under reaction conditions [91. In order to study the activity of the catalyst, the recycling efficiency of the catalyst was tested.

Fig. 8. Arrhenius

plot for the polymer-bound

ruthenium

catalyst A (1) and catalyst B (2).

153

’

Concentration

of Hydrogen in Methanol (ml/mlJ

Fig. 9. Influence of hydrogen concentration on the rate of hydrogenation with catalyst A ( 1) and catalyst B (2).

The experiment was carried out at 35 “C for about 16 h by injecting a known amount of substrate (i.e. 10 ~1) at 60min intervals. The rate of hydrogenation was measured us. time for both used and fresh catalysts. The results are summarized in Table 4. It can be seen that the maximum rate of reaction was maintained for about 10 h after which the rate decreases slowly. A 50% loss of metal complex from the polymer was found after the metal estimation of catalyst at the end of the reaction ( -16 h). The loss in activity and in metal complex may be due to the low mechanical strength of the TABLE 4 Life cycle study for 1OPRu*n(ThIDA) and 3PRu”‘(TMDA) Catalystsa Time (min)

Rate of reaction (ml mm-‘)

~OPRU”(TMDA)~ 90 (90) 240 (240) 390 (390) 540 (540) 690 (690) 840 (840) 970 (--_)

1.88 x lo-awJ8 x 10-z) 1.88 x 10-a (1.88 x 10-z 1.88 x 10-a (1.88 x 10-Y 1.88 x 10-z (1.19 x 10-2) 1.5 x 10-a (1.0 x 10-2) 1.1 x 1O-2 (0.96 x 10-2) 0.81 x 1O-2 (-_)

3PRu”(TMDA)’ 140 (140) 340 (340) 535 (555 ) 720 (775) 940 (-_)

7.06 x 1O-3 (7.02 x 10-3) 7.06 x 1O-3 (7.02 x lo-? 7.06 x 1O-3 (6.48 x 10-3) 6.4 x 1O-3 (3.82 x 10-3) 5.6 x 1O-3 (-_)

e Temperature of reaction 35 “C, amount of methanol 25 ml. b Amount of catalyst 0.7 g (7.62 x 10Y5 Ru mol-‘); total time on stream 16.25 h (14.40 h). “Amount of catalyst 0.7 g (7.75 x 10F5Ru mol-i); total time on stream 15.45 h (14.45 h). Values given in parentheses indicate data for used catalyst.

154

polymer support, and also to the polymer-bound complexes and polymerbound non-coordinated ligand molecules leaching out in solution and forming stable complexes which are inactive in the reaction.

Conclusions Polymer-bound ruthenium (TMDA) complexes were found to be active for the hydrogenation of cyclohexene reaction. The polymer-bound catalysts are stable for long periods and Catalyst B was found to be more active and stable than Catalyst A. The reaction mechanism appears complex and requires detailed investigation.

Acknowledgements The authors would like to acknowledge support for this work from Alembic Chemicals, Baroda (JNS). We would also like to thank Prof. P. K. Bhattacharya, Head, Chemistry Department and Catalysis Group R and D, IPCL, Baroda for providing the necessary facilities.

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