Hydrogenation of 1-alkenes catalysed by anchored montmorillonite palladium(II)complexes: a kinetic study

B. Ramesh, D. Thomas Sadanand, S. Gopikanth Reddy, K. Venkata Swamy and P. K. Saiprakash*Department of Chemistry, University College of Science, Osmania University, Hyderabad - 500 007, India

Received 27 October 1999; accepted 26 January 2000

Abstract

Hydrogenation of 1-hexene, 1-heptene and 1-octene was carried out using anchored montmorillonitebipyridine-palladium(II) acetate (CI), montmorillonitebipyridinepalladium(II) chloride (CII) and montmorillonitediphenyl-phosphinopalladium(II) chloride (CIII) in THF. Under the reaction conditions 100% saturation of the carbon±carbon double bond was observed. The observed rates were ®rst order with respect to the partial pressure ofhydrogen and fractional order with respect to [substrate] and [catalyst]. The hydrogenation rates were found to be:1-hexene > 1-heptene > 1-octene for all three catalysts. The reactivity order of various catalysts is: CI>CII>CIII.Thermodynamic and activation parameters were evaluated. A rate law and a plausible mechanism has been proposed.

Introduction

Palladium and rhodium complexes have been used ascatalysts in many heterogeneous and homogeneousreactions [1±5]. The design of catalysts for heterogeneousreactions is not easy because the catalytically active sitesare not well-de®ned. Although this problem is solved inhomogeneous catalytic systems, inherent practical pro-blems like corrosion, deposition of the catalysts on thewalls of the reactor, and recovery of the catalysts from thereaction mixture remain, and the homogeneous catalystshave been applied in only limited number of chemicalprocesses [6, 7]. The ideal union of the chemicaladvantages of homogeneous catalysis with the practi-cality of a solid heterogeneous system gave rise toanchored catalysis [8]. These new generation catalysts inwhich metal complexes are bound to insoluble organic orinorganic supports have advantages for both homoge-neous and heterogeneous catalytic systems. Apart fromenhanced activity, these anchored catalysts o�er manyadvantages such as recovery and recycling of the activespecies as well as enhanced speci®city and activity [9].Platinum group metals have been widely used for

decades as hydrogenation catalysts. The most e�ectivemetal varies with each substrate. Though palladium,platinum, rhodium and ruthenium have been anchoredto a variety of supports, it appears that, except in somecases, palladium is the metal of choice. It is an extremelyactive catalyst for carbon±carbon double bond satura-tion, particularly in the presence of multiple unsatura-tion, as it is much more labile than other metals. The

activity and selectivity of a catalyst vary with themethod of preparation, the type of metal atom or ionand the nature of ligands attached to metal [10±12].Catalytic hydrogenation of organic functional

groups is one of the most common steps in thesynthesis of organic compounds. Unfortunately duringthe past few decades, there has been very littlefundamental research into kinetics [13±19]. Thereforein order to understand the kinetics of hydrogenationsof ole®ns using anchored catalysts, hydrogenation of1-hexene, 1-heptene and 1-octene have been carried outusing anchored montmorillonitepalladium(II) complexessuch as montmorillonitebipyridine palladium(II) acetate,montmorillonitebipyridinepalladium(II) chloride andmontmorillonitediphenylphosphinopalladium(II) chlor-ide. Montmorillonite, the solid support used to anchorthe palladium(II) complexes in the present study, is anaturally occurring clay [8]. It possesses large amountsof highly reactive surface consisting of two-dimensionalpolymeric silicate anions separated by layers ofhydrated cations, normally Na+ and Ca2+. The mostimportant property of montmorillonite from thestandpoint of catalyst design is its ability to swell indi�erent solvents to a di�erent extent and, exert cationexchange capacity and intercalation. This paper de-scribes the e�ect of increasing the chain length ofole®ns, the nature of ligands around the metal ion andthe e�ect of interlayer spacing of the clay on hydro-genation rates.

* Author for correspondence

Transition Metal Chemistry 25: 639±643, 2000. 639Ó 2000 Kluwer Academic Publishers. Printed in the Netherlands.

Experimental

Materials

Montmorillonite was obtained from Fluka Chemicals,Switzerland. Bipyridine and silicone rubber septa(14 joint) were obtained from Aldrich, USA. Pd(OAc)2was obtained from Alchem laboratories, India. n-BuLiwas obtained from E-Merck, Germany. All otherchemicals were purchased from SD Fine Chemicals,India. Air and moisture-sensitive reactions were carriedout in a N2 atmosphere. All the reagents were eitherrecrystallised or distilled, wherever necessary, bystandard literature methods [20].

Measurements

The catalysts were prepared according to procedurescited in the literature [21]. X-ray power di�raction (thin®lm) showed a basal region expansion (d001) of 15.0,14.54 and 14.70 AÊ for montmorillonitebipyridinepalla-dium(II) acetate (CI), montmorillonitebipyridinepalla-dium(II) chloride (CII) and montmorillonitediphenyl-phosphinopalladium(II) chloride (CIII) respectively.The hydrogenation kinetics were carried out using a

specially fabricated system [22]. Hydrogen was puri®edby passage over a deoxo catalyst, through molecularsieves and drying tubes before admission to thevacuum system. Hydrogenations were carried out ina 100 cm3 two-necked round-bottomed ¯ask. The sidearm was packed with silicone rubber septa and theother arm was attached to glass vacuum manifoldequipment with a manometer, gas burette and gas-inlet. A weighed amount of the catalyst was placed inthe reaction vessel, which was then attached to thereaction system. To this, dry, THF (10 cm3) wasadded and the whole system was evacuated and¯ushed three times with pure hydrogen. The mixturewas then shaken for 30 min in the presence ofhydrogen. A weighed amount of substrate (alkene)in a measured quantity of solvent was then injectedinto the pre-equilibrated reaction system. Shaking wascarried out at such a speed as to ensure that thereaction rate does not depend upon shaking. Hydro-gen uptake commenced without any induction period.The rate of hydrogen uptake was monitored at regulartime intervals. At the end of each run, the catalystwas separated by ®ltration and thoroughly washedwith dry THF. To carry out the reactions at di�erenttemperatures, water was circulated through the outerjacket of the reaction system, using a thermostat orcryostat.

Stoichiometry and product analysis

The stoichiometry of the reaction was determined bymaintaining an excess of hydrogen with an ole®n(substrate) for several hours and the amount ofhydrogen consumed was measured. The total hydro-

gen consumption was found to correspond to com-plete hydrogenation of the carbon±carbon doublebond present in the substrate. The stoichiometry wasfound to be 1:1 (substrate:hydrogen). The hydroge-nated products were con®rmed by GC.

Kinetic results

The hydrogenation rates for ole®ns were found toincrease with increase in substrate concentration in the288±306 K (accuracy �0.5 K) range. The plot of log Vversus log[substrate] was linear with positive slope,indicating fractional order with respect to substrate(Figure 1a, b, c). The same trend has been observed for1-hexene, 1-heptene and 1-octene with all three catalysts.The rates were found to increase with the increase incatalyst concentration (Figure 1d, e, f). Plot of log Vversus log[catalyst] revealed fractional order dependenceon the catalyst concentration in the concentration rangestudied. However, the plots of log V versus log pH2

indicated unit order dependence on the hydrogen partialpressure (Figure 2a, b, c). Plots of reciprocal rate versusthe reciprocal hydrogen partial pressure were linear withintercepts on the rate axis (Figure 2d, e, f). Such linearplots were explained by Nayak et al. [17] on the basis offormation of intermediate hydride species in the reac-tion. Thus providing support for the hydride ion shiftfrom the initial intermediate complex to the substrate.Based on the above data the following mechanism is

proposed, in accord with the mechanism advanced byTurkervich [23] for hydrogenation of ethene usingheterogeneous catalysis involving palladium, repre-sented by Equations 1±3 and Scheme 1. This is also inparallel with the mechanism proposed by Terasawa

Fig. 1. (a,b,c) Plot of log V versus log[S]; (d,e,f) plot of log V versus

log[C].

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et al. [16] and Nayak et al. [17] for the hydrogenation ofvarious ole®ns catalysed by polymer-bound palladium-(II) complexes.

C�H2 ) *K1

C1 �1�

C1 � S ) *K2

C2 �2�

C2 ����!k

slowP� C �3�

Here `C' is the catalyst, `C1' is the complex-I which isformed between the catalyst and hydrogen, involvinghydride ion transfer to the palladium, replacingacetate ion. In the second equilibrium step alkene(S) forms a p-complex i.e., Complex-II (C2) with thepalladium and simultaneous hydride ion transfertaking place from palladium to alkene. The last stepis rate-determining, and involves transfer of a protonto the substrate, leading to the separation of thehydrogenated product from the catalyst. Thus thecatalyst is regenerated.The rate law derived is as follows:

dPdt� V � kK1K2�C��H2��S�

1� K1�C� � K2�S� � K1K2�C��S� �4�

This equation accounts for the ®rst order dependence onpartial pressure of hydrogen and fractional order withrespect to catalyst concentration and substrate concen-tration. In order to evaluate the individual equilibriumand rate constants K1, K2 and k Equation (4) is modi®edin terms of k¢, the pseudo-®rst order rate constant atconstant hydrogen partial pressure:

k0 � V�H2� �

kK1K2�C��S�1� K1�C� � K2�S� � K1K2�C��S� �5�

Taking reciprocals of the Equation (5)

1

k0� 1

kK1K2�C��S� �1

kK2�S� �1

kK1�C� �1

k�6�

According to Equation (6) the least squares plot of 1/k¢versus 1/[S] should be linear at constant catalystconcentration with a positive slope and intercept. Thishas been realised, supporting the proposed reactionscheme. K2 is evaluated from the ratio of intercept andslope. Similarly the least squares plots of 1/k¢ versus1/[C] at constant substrate concentrations were linearwith positive slope and intercept. K1 was evaluated fromthe ratio of intercept and slope. The correspondingthermodynamic parameters were evaluated and pre-sented in Table 1. From the values of K1 and K2, and theintercept of plot of 1/k¢ versus 1/[S], the rate constant khas been calculated and details are presented in Table 1.The formation of complexes C1 and C2 seem to behighly favourable as indicated by the negative DG andDH values. DS is also negative indicating the compact-ness of the complexes.The ole®n reactivity order was: 1-hexene > 1-hep-

tene > 1-octene. A similar trend has been observed withall the three catalysts used. In each case the order withrespect to [substrate] was fractional, in contrast to

Scheme 1. Plausible mechanism for 1-hexene hydrogenation catalysed

by anchored montmorillonitebipyridinepalladium(II) acetate.

Fig. 2. (a,b,c) Plot of log V versus log pH2; (d,e,f) plot of 1/V versus

1/pH2.

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the liquid phase hydrogenation of similar substrates byRh/AlPO4 and Rh/Sepiolite carried out by Cabello et al.[24], where the order with respect to [substrate] wasfound to be negative and was attributed to the strongadsorption of alkene when compared to hydrogen in thecompetitive adsorption process. In the present study,the order of reactivity may be explained on the basis ofthe steric factors. The enthalpy of activation andentropy of activation are useful in this context. With aconsiderable increase in steric hindrance in the transi-tion state, a combined increase in enthalpy of activationand decrease in entropy of activation is to be expected[25]. The decrease in reactivity from 1-hexene to1-octene may be due to crowding in the transitionstate with an increase in ole®n chain length. Theactivation parameters also support this view, as thereis an increase in enthalpy of activation and decrease inentropy of activation from 1-hexene to 1-octene(Table 1). Normally transmission of electronic e�ectsby means of induction is not signi®cant beyond threer-bonds. Therefore the increase in chain length is notactivating the ole®nic double bond, but causing sterichindrance in entering the interlayers of clay catalysts,thereby causing a decrease in reactivity from 1-hexene to1-octene.All the three catalysts are active in ole®ns hydrogena-

tion. The di�erence in activity seems to be due to thechange in electronic environment associated with thepalladium complex in the interlayers of the montmor-illonite. This is also supported by the greater activity ofthese clay anchored catalysts when compared topolymer-anchored and homogeneous counterparts dueto the change in electronic environment around thepalladium metal [16, 26, 27].

The reactivity order for the di�erent catalysts inhydrogenation was found to be: montmorillonitebipyr-idinepalladium(II) acetate (CI) > montmorillonitebi-pyridinepalladium(II) chloride (CII) > montmorill-onitediphenylphosphinopalladium(II) chloride (CIII).The di�erence in the ®rst two catalysts is the change inthe secondary ligands from acetate to chloride which arebeing replaced by hydride ion in the ®rst equilibriumstep. Binding energies for CI and CII are 338.8 and339.4 eV respectively. The change in binding energy isdue to the di�erence in ligands associated with the metalions. The later showed a higher binding energy than theformer. This may be one of the reasons for the lowerreactivity of the CII. Similar observations were made byMathew and Mahadevan [26] with analogous polymer-anchored palladium(II) complexes. They found adecrease in activity with increase in binding energy ofthe complex. The favourable formation of the inter-mediate complexes is indicated by the higher values ofequilibrium constants with the catalyst CI resulting inhigher rates of hydrogenation than CII and CIII

(Table 2). The reason for the lowest reactivity of CIII

may be due to the diminished electron density overpalladium due to the backbonding M ® P in themontmorillonite diphenylphosphine complex, thusdeactivating the catalyst.In the catalytic activity of the hydrogenation of

ole®ns, the interlamella spacing of the catalysts alsoplays a dominant role. Large basal spacing contributesto the development of a larger surface area and porousvolumes, which are the main requirements for anacceptable catalyst. The larger spacing allows greater¯exibility for the molecules during the process ofhydrogenation. The interlayer spacings of the catalysts

Table 1. Hydrogenation of 1-alkenes catalysed by anchored montmorillonitepalladium(II) complexes ± e�ect of temperature and activation

parameters

104V (mmol s)1) 10)5k

(s)1)

Eaà DHà DGà )DSà

288 K 294 K 300 K 306 K (kJ mol)1) (J mol)1 K)1)

1-Hexene

CI 6.25 6.90 7.60 8.10 3.3 40.2 37.8 59.0 70.9

CII 4.61 5.05 5.46 6.02 3.1 40.9 38.4 59.7 69.0

CIII 3.35 3.91 4.40 5.16 3.4 44.9 42.4 58.8 54.9

1-Heptene

CI 5.36 5.85 6.30 6.89 2.9 44.8 42.3 59.3 56.7

CII 3.97 4.46 4.92 5.53 2.8 41.9 39.4 59.4 66.6

CIII 3.27 3.71 4.65 4.80 2.7 52.3 49.8 59.4 32.1

1-Octene

CI 4.75 5.24 5.73 6.24 3.3 53.6 51.2 58.8 25.4

CII 3.99 4.37 4.72 5.16 2.8 42.6 40.1 59.4 64.4

CIII 2.99 3.37 3.70 4.20 3.2 53.4 50.9 59.1 27.3

Reaction conditions:

[Substrate]: 1.00 ´ 10)2 mmol, [Catalyst]: 0.20 ´ 10)2 mmol.pH2: 1.063 ´ 102 kN m)2, Solvent: THF.

Temp. range: 288±306 K.

CI: Montmorillonitebipyridinepalladium(II) acetate.

CII: Montmorillonitebipyridinepalladium(II) chloride.

CIII: Montmorillonitediphenylphosphinopalladium(II) chloride.

Values of k are presented at 300 K (accuracy �4%).

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were found to be CI > CII > CIII. The decrease inactivity from CI to CIII is in parallel with decrease inthe interlayer spacing. Thus the di�erence in activityseems to be a combined e�ect of electronic environmentover the metal complex and the interlayer spacing of theclay.

Acknowledgements

BR and SGKR thank the CSIR and UGC, New Delhi,respectively, for providing Fellowships.

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TMCH 4672

Table 2. Hydrogenation of 1-alkenes catalysed by anchored montmorillonitepalladium(II) complexes ± thermodynamic parameters involving K1

and K2

10)2K1 )DHà )DGà )DSà 10)2K2 )DHà )DGà )DSà

(kN)1 m2) (kJ mol)1) (J mol)1 K)1) (mmol)1) (kJ mol)1) (J mol)1 K)1)

1-Hexene

CI 2.58 25.26 13.85 38.02 1.88 41.41 13.07 94.48

CII 1.81 27.52 12.97 48.50 1.74 25.82 12.87 43.16

CIII 1.37 21.00 12.28 29.08 1.34 24.65 12.22 41.44

1-Heptene

CI 2.29 22.40 13.55 29.48 1.86 41.71 12.99 95.56

CII 1.69 23.07 12.80 34.23 2.11 27.77 13.34 48.09

CIII 1.55 18.72 12.58 20.47 1.39 27.82 12.36 51.72

1-Octene

CI 2.08 33.46 13.31 67.12 1.29 35.70 12.12 95.97

CII 2.03 27.01 13.26 45.86 1.26 23.22 12.06 34.83

CIII 1.71 22.52 12.90 32.29 0.76 34.57 10.80 79.26

Reaction conditions:

[Substrate]: 1.00 ´ 10)2 mmol, [Catalyst]: 0.20 ´ 10)2 mmol.pH2: 1.063 ´ 102 kN m)2, Solvent: THF.

Temp. range: 288±306 K.

CI: Montmorillonitebipyridinepalladium(II) acetate.

CII: Montmorillonitebipyridinepalladium(II) chloride.

CIII: Montmorillonitediphenylphosphinopalladium(II) chloride.

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