Synthesis and catalytic activity of polymer-supported Rh(I) complex

Journal of Molecular Catalysis, 49 (1989)

SYNTHESIS AND CATALYTIC Rh(1) COMPLEX

285

285 - 298

ACTIVITY

OF POLYMER-SUPPORTED

D. T. GOKAK and R. N. RAM Department of Chemistry, Faculty of Science, M.S. University of Baroda, Baroda 390 002 (India) (Received February 25, 1988; accepted June 15,1988)

Summary The Wilkinson catalyst has been ‘heterogenized’ by anchoring to a copolymer (styrene-divinylbenzene) via ligand exchange reaction, The polymer support with a chelating ligand was prepared by sequential attachment of ethylenediamine to chloromethylated styrene-divinylbenzene (2% and 5%) copolymer. The activity of these catalysts is demonstrated in the hydrogenation of 1-octene and also in the decomposition of hydrogen peroxide. The influence on reaction rate of factors such as temperature, concentration of the substrate as well as of the catalyst, and the solvent has been studied.

Introduction

The development of catalysis over the last few years has been characterized by the wide application of metal complexes and organometallic compounds as catalysts. Immobilization of homogeneous catalysts on insoluble supports has received wide attention in recent years [ 1 -31. A number of rhodium complexes as catalysts both in homogeneous and heterogenized systems have been studied by many research workers, mainly due to their high catalytic activity under mild operating conditions [ 4 - 71. In most systems investigated, the metal was anchored to the macromolecular ligand through a phosphorus atom. Studies on such immobilized complexes have revealed that these catalysts are sensitive to air and may lead to metal leaching or the agglomeration of metal crystals [8,9]. Catalysts with sufficient stability and catalytic activity towards hydrogenation of olefins, in which the rhodium and palladium complexes are anchored through ligands containing nitrogen as the donor atom have been reported [ 10 - 121. In the present paper, an attempt is made to heterogenize the Wilkinson catalyst by reacting it with polymer supports (with two different crosslinkings) functionalized with chelating nitrogen-containing ligands. The activity of these two catalysts towards the hydrogenation of 1-octene and the decomposition of hydrogen peroxide has been investigated. 0304-5102/89/$3.50

@ Elsevier Sequoia/Printed

in The Netherlands

286

Experimental Materials and equipment Styrene, divinylbenzene, tetrahydrofuran, dioxan, methanol, 1-octene and n-heptane were purified according to the published method [ 131. Chloromethyl methyl ether and ethylenediamine were distilled before use. Benzoyl peroxide, triphenylphosphine and glycine were recrystallised. Aluminium chloride was purified by sublimation. RhCls- 3H,O (obtained from Sisco, Bombay) was used without purification. Ultraviolet spectra of solid samples were recorded using a Shimadzu UV-240 instrument. Infrared spectra of solids were obtained using a Beckman IR-12 instrument, and the magnetic moment was measured using a Guoy balance. Elemental analysis was carried out in our laboratory using a Coleman instrument. Surface area was determined using a Carlo Erba 1800 instrument. Synthesis of polymer support Macroporous styrene-divinylbenzene copolymers with 2% and 5% crosslinking were prepared by the suspension polymerization technique using benzoyl peroxide as initiator; the detailed procedure is described elsewhere [15,18]. The polymer beads were chloromethylated by a reported procedure [14] using chloromethyl methyl ether*. After the reaction, the chloromethylated beads were washed sequentially with 50% aqueous dioxan, aqueous dioxan containing 10% aqueous HCl (v/v), dry dioxan and finally with deionized water till the filtrate was free from chloride ions. Beads were dried at 60 - 65 “C for 24 h under vacuum. The elemental analysis found is as follows: Polymer crosslink 2% 5%

&t.%) 71.66 78.75

H (Wt.%) 5.55 6.39

Cl (Wt.%) 17.71 14.75

Ligand introduction onto the polymer matrix The reaction was carried out at room temperature for 48 h with constant stirring using THF as solvent. The detailed procedure is described elsewhere [15]. Elemental analysis found after ligand introduction is given below: Polymer crosslink 2% 5%

*CAUTION monochloroether

C (Wt.%) 68.60 79.91

H (Wt.%) 5.88 7.38

is advised during chloromethylation, is a carcinogen.

N (Wt.%) 7.80 7.82

because the related compound

Rh(PPhs)sCl was prepared as described by Osbom et al. [ 161. (Found C, 70.08%; H, 4.90%; m.p. 157 “C, diamagnetic.) Attachment of Rh(PPh&Cl onto the liganded polymer 25 g of the functionalized polymer was kept in contact with benzene (120 ml) for 30 min in a round-bottom flask. 0.5 g Rh(PPh&Cl was taken up in benzene (80 ml) in a pressure-equalizing addition funnel, fitted to the reaction vessel by means of standard tapered joint, the end being closed by a stopcock. The system was evacuated before addition of the Rh(PPhs),Cl solution. The complex solution was then added to the reaction vessel under vacuum over a period of 30 min. The reaction mixture was stirred using a magnetic stirrer at room temperature under vacuum for 7 days. A constant vacuum was maintained during the reaction by connecting the system to a McLeod gauge. The colour of the supematant solution changed from dark red to light red, and polymer beads became light brown in colour, indicating the attachment of the complex to the polymer matrix. The beads were filtered, washed thoroughly with benzene, ethanol and dry ether. The anchored catalyst so obtained was dried in vacuum and stored. The rhodium content was determined by refluxing metal containing polymer beads with cont. HCl (A.R.) for 24 h and then estimating the metal concentration in the diluted solution followed by calorimetry [ 171. Decomposition of Hz 0, A known weight of the catalyst was kept in contact with 10 ml (-5 ~01%) of H,Oz at 35 f 0.1 “C and the volume of oxygen evolved was measured at various time intervals as well as after complete decomposition, using the gasometric technique. The detailed procedure has been described earlier [ 151. Hydrogenation procedure The hydrogenation reaction was carried out at atmospheric pressure in a magnetically stirred glass reactor in a chemically controlled regime [18]. The rate of hydrogen consumption was measured using a glass manometric apparatus. The detailed experimental setup and hydrogenation procedure are described elsewhere [ 15,181. Results and discussion The surface area as well as the pore volume of the support was given as below: Polymer support 2% SDVB 5% SDVB

Surface area

Pore volume b”d8gs5I)NTP 0:212

288

It was found that the surface area of polymer anchored catalyst reduced to -1 m2 g-l. This may be due to the blocking of pores of the support by the sequential attachment of chloromethyl group, ligand and formation of metal complex. The change in the morphology of support and the supported catalyst was observed by the use of scanning electron micrographs [ 181. Elemental analysis and metal estimation of the catalysts indicate a low level of anchoring of the metal complex onto the liganded polymer. This is probably due to steric hindrance of the Rh(tpp)sCl complex and low dissociation in benzene. It has been reported by Eaton and Stuart [19], Lehman et al. [20] and Arai and Halpem [ 211 that the dissociation of the Wilkinson complex in benzene is below 5% at 10v2 M and the equilibrium constant is (1.4 fr 0.4) X 10m4M. However, we were successful in synthesizing Rh(tpp),enCl complex, as was evidenced by UV, far IR, IR and NMR studies

WI.

The anchoring of the Wilkinson catalyst to the polymer was clarified by a detailed comparative study of the IR spectra of the catalyst on crosslinked polystyrene, chloromethylated polymer, liganded polymer and also the Wilkinson catalyst alone in the far-IR region [ 181. Both the catalysts showed weak but distinct bands around 480 cm-’ (V(Rh-N)) and 275 cm-i (v(Rh-Cl)) which are exhibited by the Wilkinson catalyst also. In the near IR region, anchored catalysts showed a broad band at 3400 cm-’ due to N-H stretching (Fig. 1). The reflectance spectra of the anchored catalysts in BaS04 were recorded in the UV region, using crosslinked polystyrene in BaS04 as the blank. The polymer-bound catalysts showed three broad absorption bands between 200 and 600 nm which are assigned to the d-d transition of Rh(1) [23]. Based on the above discussion, the polymer-bound catalysts can be represented as given below: P2enRh(tpp)s _ .Cl for 2% crosslinked polymer P5enRh(tpp)3 _ $1 for 5% crosslinked polymer

100

Fig. 1. Infrared spectrum of P5enRh(tpp)s_,Cl.

I

289

Kinetics of H202 decomposition We have reported earlier that metal ions such as Cu2+, Co’+, Cr3+ and their complexes when supported on alumina or silica exhibit catalase-like activity in the decomposition of hydrogen peroxide [24 - 261. In continuation of our earlier study, the hydrogen peroxide decomposition reaction was chosen as a model reaction to study the catalytic activity of polymer-bound and unbound rhodium complex catalysts. In the present investigation, the kinetic analysis is based on the initial rate data, which was calculated from the slopes of the plot of oxygen evolved against time. The order of reaction with respect to hydrogen peroxide was determined from the slope of the linear plots of log(initia1 rate) us. log[H,O,] (Fig. 2) by using the same amount of catalyst. The order of reaction with respect to the catalyst concentration was obtained from the slope of log(initia1 rate) us. log[catalyst] (Fig. 3), keeping hydrogen peroxide concentration constant. The values of the reaction orders for different catalysts are given in Table 1. Based on the average orders of the reaction with respect to catalyst and hydrogen peroxide, the power-law expression for the catalytic decomposition process can be written as;

d[O,l

r= -

= K,[cat]“‘[H202]” dt where r = rate of reaction

(1)

K, = reaction rate constant

-11 -3.0

-2.9

-2.e

-27

-26

-2.5

-b9IH2021+

Fig. 2. Plot of log(rate) PSenRh(tpp)3_,Cl.

us. log[HzOl]

for (1) PBenRh(tpp)3_,Cl,

Fig. 3. Plot of log(rate) P5enRh(tpp)3_,Cl.

us. log [catalyst]

(1) PPenRh(tpp)3_,Cl,

(2) Rh(tpp)&l,

(3)

(2) Rh(tpp)&l,

(3)

TABLE 1 Reaction peroxide

orders calculated

for different

catalysts in the decomposition

Order of reaction with respect to [catalyst ]

Order of reaction with respect to

Catalyst

1

[Hz02

0.6 0.4 0.7

0.4 0.3 0.6

RWw IF1 PSenRh(tpp)s_,Cl P5enRh(tpp)s_,Cl

of hydrogen

016 -

1 --&at104

2

0

3 x lb’

HIO+--+

a5

1.0

ls x 16’

-lC0tp”1H202]~~

6x166 -I

Cat

lo.‘1 H202

]o.6

>

Fig. 4. Plot of rate of reaction us. [catlm[H2021n (mol Rh)m(moll-l)n (B) P2enRh(tpp)s_,Cl, (C) P5Rh(tpp)3_XC1.

of (A) Rh(tpp)&l,

The plots of -d[OJ/dt us. [cat] m[H,O,]” (Fig. 4) according to eqn. (1) for different catalytic processes are linear and pass through the origin, indicating good agreement of the rate data with the power model. The frac-

291

tional order might be due to steric hindrance and nonaccessibility of the active sites for the substrate molecules. For comparative purposes, the apparent reaction rate constant K, in the rate eqn. (l), which includes some other constants, can be taken as a direct measure of the catalytic activity. The values of K, were obtained from the slopes of the linear plots of the corresponding rate against [cat]“[HsOJ”. The units of K, for different catalysts are given in Table 5. A set of experiments was carried out to study the influence of various parameters, such as loading of catalyst, substrate concentration and temperature of the system, on .the rate of decomposition for different catalysts. The results are summarised in Tables 2 - 4. TABLE 2 Effect of [catalyst] Catalyst

PSenRh(tpp)s _,Clb P5enRh(tpp)s_,Cl

WW

I$1

in the decomposition of hydrogen peroxidea Specific reaction rate constant Krc as a function of the amount of catalyst used in reaction 0.2 g

0.5 g

0.7 g

1.0 g

0.61 x lo2 1.42x lo4

0.73 x 102 -

0.92 x 1.02 1.52 x lo4

0.93 x 102 1.75 x 104

0.01 (9)

0.02 (g)

0.025 (g)

2.08 x lo2

2.10 x 102

2.12 x 102

aReaction temperature 35 “C, concentration of Hz02 used in the reaction = 2.88 x 10” mol 1-r. b[HzOa] = 1.73 X 10” mall-r. CUnits of Kr for different catalysts are given in Table 5. TABLE 3 Influence of [ Hz021 in the decomposition of hydrogen peroxide Catalyst

Rh(tpp)sCl P2enRh(tpp)3_,Clb P5enRh(tpp)s_,Cl

Amount of catalyst used (g)

Specific reaction rate constant K, c as a function of [H20z] (mol 1-r) 1.23 x 10”

1.73 x 10-s

2.88 x 10”

0.01 0.50 0.50

1.98 x 10” 0.98 x lo2 2.59 x 104

2.15 x 1O-2 1.10 x 102 2.44 x lo4

2.09 x 10” 1.09 x 102 2.34 x lo4

aTemperature of reaction 35 “C. bTemperature of reaction 45 “C. CUnits of K, are given in Table 5.

Increasing the amount of catalyst increases the rate of reaction (Table 2), indicating that there is no dimerization of metal complexes in either the homogeneous or the heterogenized system in the range studied. However, it

TABLE 4 Effect of temperature on the rate of decomposition of HaOaa Activation energy (kcal mol-l)

Catalyst (amount used in 6)

Specific reaction rate constant Krb as a function of reaction temperature (“C) 35

45

55

Rh(tppN (0.01) PBenRh(tpp)a-&l

2.99 x 102

5.72 x lo2

8.09 x 102

5.1

0.62 x 102

0.89 x 102

1.44 x 102

5.4

2.32 x lo4

5.07 x 104

7.70 x lo4

6.9

(0.5) P5enRh( tpp)a _$I (0.5)

a[H202] = 2.88 x 1O-3 mall-‘. bUnits of Kr are given in Table 5.

was observed that the rate does not increase linearly with increasing amounts of catalyst; probably the rate is governed by fractional order with respect to the catalyst concentration. It was found that the rate constant was independent of initial concentration of H,O, in both the homogeneous and heterogenized catalytic processes (Table 3). It is seen from Tables 2 - 4 that the catalytic activity of polymer-bound catalysts is higher than unbound catalyst. However, no considerable change was observed in the values of the energy of activation calculated from the Arrhenius plots (Fig. 4, Table 4). P5enRh(tpp)s-~Cl catalyst was found to be more active than P2enRh(tpp)s_,Cl. This may be explained by the swelling ability of the support, which is lower in aqueous media [ 181, and also from the relationship between kinetics and crosslink density, The region involved in the former case may be the surface phase, which can improve the kinetics and catalytic activity [ 271. In the case of polymer-bound catalysts, the light yellow colour of the catalyst changed to light brown when brought in contact with H,O,. The brown colour persisted so long as any residual amount of hydrogen peroxide was present. After complete decomposition of the substrate, the catalysts TABLE 5 Units of specific reaction rate constant IQ for different catalysts where

Catalyst

Units of Kra

Rh(tpp)3Ci

M-i.0

PBenRh(tpp)a _,Cl PSenRh(tpp)s-$1 aM = moles of metal.

Il.0

ml

tin-l

M-0.7 le.7 ml min-r M-1*3 11.3 ml mm-1

293

regained their original colour after a lapse of approximately 2 h. Similar results have been noted by Ram et al., [24 - 261 in the decomposition of H,O, using alumina- or silica-supported metal complexes as catalysts. On the other hand, no visible colour change was observed when unbound complexes were used in homogeneous media. It was also noticed that when a polymer support or functionahsed polymer alone was used in the reaction, neither change in the colour nor decomposition of H,O, occurred. It can therefore be concluded that the enhanced decomposition of HzOz may be due to the formation of an unstable brown-coloured intermediate complex on the polymer surface. Isolation of this brown intermediate was not possible because the metal complexes are chemically bound to the polymer. The polymer-anchored surface active intermediate complex, after washing with distilled water, alcohol and ether to remove the adhering hydrogen peroxide, was found to decolourise dilute KMn04 solution. This indicates the formation of a peroxo species in the presence of metal ions and hydrogen peroxide on the surface of polymer. This phenomenon has also been reported by other workers [28,29]. Attempts made to obtain spectral evidence (Le. IR and UV) were not successful, because the absorption bands were not sufficiently prominent due to the low content of rhodium metal. However, the formation of peroxo complexes has been supported by IR and UV spectral evidences in polymer-bound cobalt complexes [ 15,181. Based on the above observations, the following mechanism and rate equation may be proposed. It is known [ 28,301 that HzOz decomposes as: K, H202

-

H02- + H+

(2)

In case of polymer-bound metal complexes, all the active sites do not participate in the reaction, as evidenced from the fractional order values with respect to catalyst concentration. Therefore this may be expressed as: (MLn)totai = (MLn)accerribie + (ML,)i**ccessibie (where M = metal, L = ligand, n = integer). Thus the (ML,), may react with HO1 complex : (ML,),

+ H02- 2

(3)

ions to form an intermediate

[(ML&HO,-]

(4)

A second molecule of H202 may then interact complex in eqn. (4) to form the products: [W+d,HOll

+

with the intermediate

k3 H202

310~1

W+A,

+ 02

+ H2O

+ (X-I-

(5)

The above mechanism leads to the following rate equation:

(6)

294 KG

= constant where Kr = H+ Equation (6) is in agreement with the experimental results. Hydrogenation rem tion

The kinetics of l-octene hydrogenation was investigated for the P2enRh(tpp)a_XCl catalyst. The data for the hydrogenation reaction were obtained in a kinetic regime [ 191 using a stirring speed of 650 rpm in the temperature range of 30 - 35 “C. The stoichiometry of the reaction was verified by carrying out several experiments at a constant temperature and one atm pressure of hydrogen at different concentrations of I-octene for longer time intervals (i.e. 120 min). In all these experiments, the amount of hydrogen absorbed was stoichiometrically proportional to the 1-octene converted (calculation based on GC analysis of the product}. This suggests that no side products are formed, The initial rate data (based on H, uptake measurements) were used to evaluate the kinetics of the reaction. In each kinetic run, the rate of hydrogenation was calculated from the slope of the plots of volume of H, absorbed (STP) as a function of time, and the results are summarised in Table 6. The influence of various parameters on the rate is discussed on the basis of the experimental observations.

TABLE 6 Summary of the kinetics of l-octene THF at atmospheric pressure Temperature (“C) 30

Rh present (M ml-r x 109) 8.76

35

30

35

46 50

2.18 4.37 8.75 13.10 2.18 4.37 8.75 13.10 8.75

hydrogenation

for PSenRh(tpp)~_&l

catalyst in

l-octene (M ml-r x 106)

Rate of reaction (M min-’ X 1Oe)

6.38 12.76 19.15 25.53 6.38 12.76 19.16 25.53 6.38

0.667 0.730 0.940 1.120 0.870 1.070 1.857 1.920 0.475 0.547 0.657 0.820 0.66 0.94 1.21 2.00 4.50 7.27

12.76

295

Effect of I-octene concentrations The influence of l-octene concentration on the rate of reaction was studied at 30 and 35 “C under 1 atm of hydrogen, using a catalyst concentration of 8.75 X 10Vg M ml-” Rh and l-octene concentration in the range of 6.38 - 25.53 X low6 M ml-‘. The order of reaction calculated from the plot of log(rate of reaction) against lo&l-octenef (Fig. 5) was found to be 0.62.

-6.2

,

_ I.2

-4.1

(

, -LO

.

, -3r)

,

)

,

-3.0

, -31

, -26

-lopIl-octcnr1-

Fig. 5. Plot of log(rate) us. fog [l-octene]

for PZenRh(tpp)~_~~l

catalyst.

Effect of catalyst concentration The effect of the P2enRh(tpp)s_,Cl catalyst concentration on the rate of reaction was studied over the range of 2.18 - 13.1 X lo-’ M ml-r Rh at 30 and 35 “C and 1 atm pressure of hydrogen. The results are given in Table 6. It was found from Fig. 6 that the order of reaction with respect to concentration of catalyst is 0.40.

T -56

p-w 2 . = T-6.0

I

-6.2

:

Fig. 6. Plot of log(rate) us. log [catalyst]

for P2enRh(tpp)3_&l

catalyst.

_)

Fig. 7. Arrhenius plot for P2enRh(tpp)a_,C1

catalytic hydrogenation reaction.

Effect of temperature The effect of temperature on the rate of hydrogenation for PZenRh(tpp)s_,Cl catalyst has been studied in the range 30 - 50 “C and the results are given in Table 6. The energy of activation calculated from the slopes of the Arrhenius plot (Fig. 7) was found to be 23.0 Kcal mol-“. Because of its closed electron cloud, the H, molecule is fairly inert, and its dissociation energy is 104 Kcal mol:’ [ 311. The observed low activation energy in the present ~vestigation could be due to the formation of an intermediate complex involving a metal atom and a hydrogen molecule. Effect of solvent The nature of the solvent can be varied in order to control the activity and selectivity of polymer-supported hydrogenation catalysts. The most suitable solvent is one that combines good swelling ability with high polarity. The influence of four solvents on the rate of reaction has been studied, the results are shown in Table 7. It is observed that all three solvents (i.e. THF, dioxan and methanol) were found to be fairly good, whereas benzene was a poor solvent for the hydrogenation reaction. This could be due to the poor swelling nature of the catalyst in benzene [ 181. Rate equation In the present investigation, the partial pressure as well as the concentration of hydrogen was kept constant by carrying out the reaction at constant hydrogen pressure and maintaining a fixed amount of solvent. In all the experiments, the solvent along with the catalyst was initially saturated with hydrogen, followed by the injection of olefin, and hence the actual consumption of hydrogen was measured.

297 TABLE 7 Effect of solvent on the kinetics of hydrogenation of l-octene at 1 atm Hz and 35 “C a Solvent

Rate of reaction (M min-‘) x lo6

THF dioxan methanol benzene

1.07 1.91 0.58 no absorption of hydrogen at < 120 min

a[Rhodium] = 8.75 P2enRh(tpp)s_,Cl.

x

lo4

M ml-l,

[l -octene]

= 12.76

X

10”

M ml-l,

[catalyst]

=

The mechanism of olefin hydrogenation using the Wilkinson catalyst or its derivatives has been extensively studied [4, 16, 311. The dissociation of Rh(tpp)sCl and formation of hydrido species, and the preferential attack of olefin on the hydrido complex of rhodium have been well established by various experimental studies and from the thermodynamic properties [16, 32 - 361. On the basis of the above literature evidence and present experimental observations, the following mechanism and thereby the rate equation was proposed for P2enRh(tpp), _ .Cl: Assuming x = 1, we know [ 16,341 that Rh(tpp)&l dissociates in the solvent as: solvent

PBenRh(tpp),Cl e PaenRh(tpp)Cl

PBenRh(tpp)Cl

+ tpp

K1

+ H, K_1 & P2enRh(tpp)(H)&l

PBenRh(tpp)(H),Cl

+ olefin sP2[enRh(tpp)(H)2-olefin]+

z PaenRh(tpp),Cl P2[enRh(tpp)(H)2(olefin)l'+tpp

+ Cl-

+ alkane

The above mechanism leads to the following rate equation Rate =

KK,[cat] [H,] [Olefin] K-i + K[Olefin]

where, K,, K-, and K are rate constants. Acknowledgements The authors would like to acknowledge support for this work from the Council of Scientific and Industrial Research, New Delhi. We would

298 also like to thank Prof. P. K. Bhattacharya, Head, Chemistry Department and Catalysis group R & D, IPCL, Baroda, for providing necessary facilities and for useful discussions.

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