Hydrogenation of olefins catalyzed by polymer-supported palladium-imidazole

Article abstract of DOI:10.1016/S1872-2067(10)60176-3

A polymer-supported palladium-imidazole catalyst was used to catalyze the hydrogenation of various olefins under mild conditions. The rate of hydrogenation was studied. The effects of factors such as substrate concentration, catalyst concentration, partial pressure of hydrogen and temperature on initial rate of reaction of selected olefins were investigated. A mechanism for the reaction was proposed from the rate equation. The effects of the solvent and structure of the olefin on the rate of hydrogenation were investigated. The catalyst showed good reusability without any leaching of metal from the support. The homologous analog of the polymer-supported catalyst could not be used as catalyst for the hydrogenation of olefins in methanol because there was precipitation of the metal during reaction.

Full text of DOI:10.1016/S1872-2067(10)60176-3

CHINESE JOURNAL OF CATALYSIS
Volume 32, Issue 2, 2011
Online English edition of the Chinese language journal
RESEARCH PAPER
Cite this article as: Chin. J. Catal., 2011, 32: 280–285.
Hydrogenation of Olefins Catalyzed by Polymer-Supported
Palladium-Imidazole
Udayakumar VELU1, Alexander STANISLAUS1, Gayathri VIRUPAIAH1,*, Shivakumaraiah2,
Viswanathan BALASUBRAMANIAN3
1Department of Studies in Chemistry, Central College Campus, Bangalore University, Bangalore 560 001, India
2Siddaganga Institute of Technology, Tumkur, Karnataka 572 103, India
3National Centre for Catalysis Research, I.I.T. Madras., Chennai 600 036, India
Abstract: A polymer-supported palladium-imidazole catalyst was used to catalyze the hydrogenation of various olefins under mild condi-
tions. The rate of hydrogenation was studied. The effects of factors such as substrate concentration, catalyst concentration, partial pressure
of hydrogen and temperature on initial rate of reaction of selected olefins were investigated. A mechanism for the reaction was proposed
from the rate equation. The effects of the solvent and structure of the olefin on the rate of hydrogenation were investigated. The catalyst
showed good reusability without any leaching of metal from the support. The homologous analog of the polymer-supported catalyst could
not be used as catalyst for the hydrogenation of olefins in methanol because there was precipitation of the metal during reaction.
Key words: polymer-supported palladium-imidazole catalyst; cyclohexene; relative reactivity; sequential hydrogenation; recycling effi-
ciency
Many metal complexes that are homogeneous catalysts for
various chemical transformations can be bonded onto supports
and be used as heterogeneous catalysts [1,2]. The attachment of
metal complexes onto a support to make it insoluble prevents
the loss of expensive catalyst material and contamination of
reaction mixture and product [3]. Such complexes can catalyze
various reactions like hydrogenation, oxidation, and Suzuki
cross-coupling [4–6]. Environmental concerns continue to
encourage the development of these eco-friendly catalysts that
work under mild conditions, can be recovered easily, and re-
cycled without much loss of reactivity because they solve
many chemical, environmental, and economical problems.
Hydrogenation is a useful test reaction widely used in sup-
ported metal complex catalysis study [7].
under ambient conditions [8]. A polymer-supported palla-
dium-Schiff base complex has been prepared, characterized,
and used as catalyst for the hydrogenation of selected olefins [9]
and nitroarenes [10].
In the present work, a Pd(Imz)2Cl2 (Imz = imidazole) com-
plex was synthesized and used as homogeneous catalyst for the
hydrogenation of olefins in methanol under ambient conditions.
Precipitation of metal in the reaction mixture was observed
during the course of reaction. Hence, it was not a good catalyst
for these reactions. The Pd(Imz)2Cl2 complex was insoluble
and stable when bonded to a polymer support. The polymer
supported palladium-imidazole complex was used as a het-
erogeneous catalyst for the hydrogenation of various olefins
under mild conditions. Kinetic studies were performed on
selected olefins. On the basis of the kinetic study and rate
equation, a reaction mechanism was proposed. The catalyst
showed good reusability without any leaching of metal from
the support.
In our previous work, a palladium-imidazole complex
bonded to chloromethylated polystyrene beads cross-linked
with 6.5% divinylbenzene was characterized by various tech-
niques and used as catalyst for the hydrogenation of ben-
zylideneaniline and some of its para substituted derivatives
Received 19 September 2010. Accepted 11 November 2010.
*Corresponding author. Tel: +91-080-2296 1348; E-mail: gayathritvr@yahoo.co.in
Foundation item: Supported by University Grants Commission, Government of India.
Copyright © 2011, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved.
DOI: 10.1016/S1872-2067(10)60176-3

Udayakumar VELU et al. / Chinese Journal of Catalysis, 2011, 32: 280–285
Table 1 Hydrogenation of cyclohexene in various solvents
S
U
P
CH2
CH2
Solvent
Initial rate × 10–4 (mol/(L·min))
N
N
N
Cl
Cl
Methanol
9.56
9.38
5.68
5.26
5.07
3.38
0.96
0.89
8.96
7.91
6.86
Pd
P
O
R
T
N
Ethanol
Toluene
Tertahydrofuran
Acetone
Scheme 1. Polymer-supported palladium-imidazole complex.
Benzene
1 Experimental
Dimethyl sulfoxide
Dimethylformamide
75% methanol-25% benzene
50% methanol-50% benzene
25% methanol-75% benzene
1.1 Synthesis of the catalyst
Chloromethylated polystyrene beads cross-linked with 6.5%
divinylbenzene was donated by Thermax India Ltd., Pune,
India. The solvents and all other reagents were purified before
use.
10–2 mol/L and [Catalyst] = 5.92 × 10–3 mol/L (calculated based
o
on Pd), at temperature = 30 C, 79.5 kPa hydrogen pressure
using 30 ml of different solvents (Table 1). The results revealed
that the polar solvents favored the hydrogenation reaction.
Methanol, which is polar and has good hydrogen solubility [12],
shows the highest rate of reaction. Better swelling of the sup-
ported catalyst in methanol can be another reason for the high
rate of reaction [8,13]. Dimethylformamide and dimethyl sul-
foxide, although polar, can coordinate strongly and irreversibly
with the catalyst and hindered the coordination of hydrogen
and olefin with the catalyst, resulting in very low rates of re-
action. Since a higher reaction rate was observed in methanol,
further studies were carried out with methanol. The effect of
increased benzene concentration in the reaction medium was
also studied. The results are included in Table 1 [14–16].
The polymer-supported palladium-imidazole complex
(Scheme 1) was prepared by the procedure described in our
earlier publication [8]. The Pd(Imz)2Cl2 complex was prepared
according to a literature report [11].
1.2 Hydrogenation of olefins
In a typical hydrogenation experiment, the catalyst was
added along with the solvent into a glass reactor, flushed with
hydrogen and evacuated. Water was circulated at a desired
temperature from a thermostat through the outer jacket of the
reactor and that of the hydrogen gas burette. A known quantity
of the substrate was injected and the system was opened to the
hydrogen gas burette. The reactions were monitored by the
hydrogen uptake at set intervals of time. At the end of the each
reaction, the catalyst was separated by filtration and the prod-
ucts were identified by GC and GC-MS techniques. A blank
reaction was also done in the absence of catalyst. GC data were
obtained with a Shimadzu GC with BP5 capillary column.
GC-MS data were obtained using a Shimadzu GC-MS
QP5050A with BP5 column.
2.2 Reaction kinetics and mechanism
Kinetic studies were carried out using cyclohexene (cyclic
olefin), diethylmaleate (DEM), and diethylfumarate (DEF)
(internal olefin, geometric isomers) as substrates.
2.2.1 Effect of catalyst concentration on the rate of
hydrogenation
2 Results and discussion
The order of reaction, calculated from the slope of the plot of
log(Initial rate) against log[Catalyst], revealed that the initial
rate of hydrogenation for all the substrates was first order in
catalyst concentration in the range 1.46 ×10–3 to 7.40 × 10–3
mol/L , at constant [Substrate] = 6.67 × 10–2 mol/L, 79.5 kPa
The initial rates of hydrogenation of the various olefins were
determined from the slope of the plot of volume of hydrogen
uptake as a function of time. The effects of various parameters
on the rate of reaction were investigated.
o
hydrogen pressure, temperature = 30 C, and with 30 ml
methanol (Fig. 1) [16].
2.1 Effect of solvent on the rate of hydrogenation of
cyclohexene
2.2.2 Effect of substrate concentration on the rate of
hydrogenation
Polarity, coordinating ability of the solvent with the catalyst,
capacity to swell the polymer support, and hydrogen solubility
in the solvent can influence the rate of hydrogenation. To study
the effect of solvents, hydrogenation of cyclohexene was car-
ried with constant concentrations of [Cyclohexene] = 6.67 ×
The effect of substrate concentration on the rate of reaction
was studied by varying the substrate concentration in the range
3.3 × 10–2 to 16.7 × 10–2 mol/L at constant [Catalyst] = 5.92 ×

Udayakumar VELU et al. / Chinese Journal of Catalysis, 2011, 32: 280–285
-3.00
-3.05
-3.10
-3.15
-3.20
-3.25
-3.30
-3.35
-3.40
-3.45
Cyclohexene
DEM
-3.0
-3.1
-3.2
-3.3
-3.4
-3.5
DEF
Cyclohexene
DEM
DEF
-2.9 -2.8 -2.7 -2.6 -2.5 -2.4 -2.3 -2.2 -2.1
log[Catalyst]
-2.85 -2.80 -2.75 -2.70 -2.65 -2.60 -2.55 -2.50
log[Hydrogen]
Fig. 3. Dependency of initial rate of hydrogenation on partial pressure
Fig. 1. Dependency of initial rate of hydrogenation on catalyst concen-
of hydrogen.
tration.
10–3 mol/L, temperature = 30 C, 79.5 kPa pressure of hydro-
total pressure of the mixture of gases as 1.0 × 105 Pa. The
concentration of dissolved hydrogen in the solvent was calcu-
lated using Henry’s law [14,18]. Hydrogenation was carried
out in the hydrogen concentration range 1.52 × 10–3 to 3.04 ×
10–3 mol/L with constant [Catalyst] = 5.92 × 10–3 mol/L, [Sub-
o
gen, and with 30 ml methanol. The slope obtained from the plot
of log(Initial rate) against log[Substrate] revealed that the
reaction followed first order kinetics in the substrate concen-
tration range 3.3 × 10–2 to 1.0 × 10–1 mol/L (Fig. 2). Above this
range, the initial rate of reaction followed fractional order
[16,17]. This can be attributed to the fact that as the catalyst
concentration was constant, all the catalyst would have been
used by the substrate and there was insufficient catalyst at
higher concentrations of the substrate.
o
strate] = 6.67 × 10–2 mol/L, temperature = 30 C, 30 ml
methanol. The order of reaction obtained from the slope of the
plot of log(Initial rate) against log[Hydrogen] was first order
(Fig. 3) [19].
On the basis of the kinetic studies, a reaction mechanism
(Scheme 2) and rate equation (Eq. (1)) were proposed [18]. In
this proposed mechanism, the alkene initially forms a
S-complex intermediate in a reversible manner with the
polymer-supported catalyst. This intermediate reacts with
hydrogen to form a dihydrido complex to subsequently form an
alkyl-palladium complex that then undergoes reductive elimi-
nation reaction to yield the alkane.
2.2.3 Effect of partial pressure of hydrogen on the rate of
hydrogenation
A range of partial pressures of hydrogen was obtained by
mixing pure nitrogen and hydrogen in a gas burette with the
-2.7
Kk1[Catalyst][Substrate][H2 ]
Cyclohexene
-2.8
Rate
(1)
1ꢀ K[Substrate]
DEM
-2.9
The deduced rate equation (Eq. (1)) described the experi-
mental data reasonably well. In Eq. (1), K is the equilibrium
constant for the reversible reaction, and k1 is the rate constant
for the rate determining step. Equation (1) is consistent with
that the reaction follows first order with catalyst concentration
and partial pressure of hydrogen, and follows first order with
respect to substrate concentration in its lower concentration
range and fractional order at higher substrate concentrations.
The lines in Fig. 1 and Fig. 2 are parallel, which revealed that
cyclohexene (cyclic olefin), DEM, and DEF (linear internal
olefins, geometric isomers) follow the same reaction mecha-
-3.0
DEF
-3.1
-3.2
-3.3
-3.4
-3.5
-1.5
-1.4
-1.3
log[Substrate]
-1.2
-1.1
-1.0
Fig. 2. Dependence of initial rate of hydrogenation on substrate con-
centration.
H
Fast
K
C
C
k1
Pd4+
H
+
C
C
Pd2+
C
C
Pd2+
Fast
Pd4+
H
C
C
Pd2+
C
C
+
H2
Slow
H
H
H
Catalyst
Substrate
ʌ
-Complex (intermediate)
Dihydrido complex
ı
-Complex
Catalyst
Product
Scheme 2. Mechanism for the hydrogenation.

Udayakumar VELU et al. / Chinese Journal of Catalysis, 2011, 32: 280–285
Table 3 Relative reactivities of the olefins for the hydrogenation reac-
tion
-2.5
-2.6
-2.7
-2.8
-2.9
-3.0
-3.1
-3.2
Initial rate × 10–4
Relative
Substrate
(mol/(L·min))
reactivity*
100
84
1-Hexene
11.6
9.7
1-Heptene
Cyclohexene
1-Octene
7.5
65
DEM
Cyclohexene
Cyclooctene
9.5
82
5.6
49
DEF
Ethyloleate
9.1
79
Diethylmaleate
Diethylfumarate
2,5-Norbornadiene
1,5-Cyclooctadiene
1,3- Cyclooctadiene
8.9
77
7.4
64
3.10
3.15
3.20
(1/T)˜10ꢁ3 Kꢁ1
3.25
3.30
12.5
8.7
108
75
10.1
87
Fig. 4. Arrhenius plot for the hydrogenation reaction.
*Using initia1 rate of hydrogenation of 1-hexene = 100.
nism for hydrogenation (Scheme 2) under the studied condi-
tions.
long chain), cyclohexene, cyclooctene (cyclic olefins),
1,5-cyclooctadiene, 2,5-norbornadiene (non-conjugated cyclic
dienes), 1,3-cyclooctadiene (conjugated cyclic diene) were
chosen as substrates. The relative activities of the various
olefins and their initial rates of hydrogenation were studied
under identical conditions: [Substrate] = 6.67 × 10–2 mol/L,
[Catalyst] = 5.92 × 10–3 mol/L, 79.5 kPa hydrogen pressure, at
30 oC temperature and with 30 ml methanol. The data are listed
in Table 3.
2.2.4 Effect of temperature on the rate of hydrogenation
Hydrogenation reactions were carried out in the temperature
range 30 to 50 oC with constant [Catalyst] = 5.92 × 10–3 mol/L,
[Substrate] = 6.67 × 10–2 mol/L, 79.5 kPa pressure of hydrogen,
and 30 ml methanol. An increase in the rate of reaction with
temperature was observed. The activation energies found from
the Arrhenius plot (Fig. 4) and activation entropy are tabulated
in Table 2 [16]. The negative activation entropy values indi-
cated that the adsorption of the substrate on the catalyst surface
resulted in the loss of three translational degrees of freedom
and the adsorbed complex probably had only three vibrational
degrees of freedom. This type of interaction may facilitate
hydrogenation.
The reactivity of the olefins (Table 3) can be interpreted on
the basis of the complex formation ability of the olefin with the
catalyst (Scheme 2) [20]. The structure, position of the double
bond, and substituents around the double bond of the olefin can
influence its ability to form a complex with the catalyst, which
determines the reactivity.
2.3.1 Effect of olefin chain length
2.3 Effect of olefin structure on its reactivity for
hydrogenation
The increase in the chain length of the olefin did not facili-
tate activating the olefin double bond, but caused steric hin-
drance in forming the complex with the catalyst and thereby
caused a decrease in reactivity. The hydrogenation reactivity
followed the order: 1-hexene > 1-heptene > 1-octene [17]. This
may be due to that the electronic inductive effect is not sig-
nificant beyond three V-bonds in the olefin chain.
The structure, position of the double bond, and substituents
around the double bond of the olefin have a remarkable influ-
ence on the hydrogenation reactivity. To study the influence of
the structure of olefin on the rate of hydrogenation, some
open-chain and cyclic olefins were used as substrate. 1-Hexene,
1-heptene, 1-octene (open-chain terminal olefins), diethyl-
maleate, diethylfumarate (open-chain internal olefins with
carbon-carbon double bond in conjugation with two ester
groups, geometric isomers), ethyloleate (internal olefin with
2.3.2 Effect of olefin ring size
Hydrogenation of cyclohexene and cyclooctene were carried
out to establish the influence of ring size on reactivity. The
increase in ring size of a cyclic olefin decreased the rate of
hydrogenation, as reflected in the relative reactivity. The lower
activity of cyclooctene was attributed to the larger ring size, as
a larger ring size decreases the complexing ability of the
cyclooctene with the catalyst due to steric hindrance. Different
authors using various catalysts have reported different reactiv-
ity [21,22]. The order of reactivity 1-hexene > cyclohexene >
Table 2 Activation energy and activation entropy for the hydrogenation
reaction
Activation
Activation entropy
(J/(K·mol))
ꢁ170
Substrate
energy (kJ/mol)
Cyclohexene
DEM
42
48
49
ꢁ150
DEF
ꢁ152

Udayakumar VELU et al. / Chinese Journal of Catalysis, 2011, 32: 280–285
1-octene > cyclooctene revealed that the reactivity of olefins
Table 4 Percentage conversion of various olefins for the hydrogenation
decreases on moving from linear terminal alkenes to cyclic
olefins.
reaction
Substrate
Product
n-hexane
Conversion* (%)
1-Hexene
97
82
96
91
97
90
82
95
96
97
96
2.3.3 Effect of position of the carbon-carbon double bond
in the alkene chain
1-Heptene
heptane
1-Octene
octane
Cyclohexene
Cyclooctene
cyclohexane
cyclooctane
ethyl octadecanoate
diethyl succinate
diethyl succinate
norbornane
cyclooctane
cyclooctane
The position of the carbon-carbon double bond in the alkene
chain can affect its reactivity. An internal double bond in the
olefin has a larger steric hindrance for complex formation with
the catalyst. Hence the hydrogenation of an internal olefin like
ethyloleate is more difficult than that of a terminal olefin like
1-hexene.
Ethyloleate
Diethylmaleate
Diethylfumarate
2,5-Norbornadiene
1,5-Cyclooctadiene
1,3-Cyclooctadiene
*As per GC analysis.
2.3.4 Reaction with cis-trans isomers
initial stage of reaction was more selective to norbornene (NBE)
and very small amount of norbornane (NBA) was observed in
the reaction mixture. Subsequently, NBE was hydrogenated to
NBA only when most of the NBD was converted to NBE. This
high selectivity for NBD may be due to selective adsorption
and/or easy complex formation of NBD with the catalyst. The
conjugated diene 1,3-cyclooctadiene was hydrogenated at a
faster rate relative to non-conjugated 1,5-cyclooctadiene.
The conversions of various olefins obtained by GC under the
reaction conditions of [Substrate] = 6.67 × 10–2 mol/L, [Cata-
lyst] = 5.92 × 10–3 mol/L, 30 ml methanol, 30 oC temperature,
79.5 kPa hydrogen pressure are tabulated in Table 4.
On the basis of symmetry, one would expect cis alkenes to
possess more basic pi character than their trans isomer.
Cis-isomer diethylmaleate was hydrogenated faster than
trans-isomer diethylfumarate [23].
2.3.5 Reactivity of dienes
Dienes may form chelate complexes with the catalyst and
the reactivity of these dienes depends on the ability of the diene
to form a complex with the catalyst. In case of the
non-conjugated dienes 1,5-cyclooctadiene and 2,5-norbor-
nadiene (NBD), the relative reactivity was a function of ring
strain. The higher double bond strain in NBD facilitates easy
complex formation with the catalyst and promotes hydrogena-
tion. GC and GC-MS analysis of hydrogenation of NBD re-
vealed the sequential hydrogenation of its double bonds (Fig. 5)
[24,25] and the reaction was very fast at the beginning. The
2.4 Recycling efficiency of the catalyst
The catalyst was recycled six times using cyclohexene as
substrate with constant [Cyclohexene] = 6.67 × 10–2 mol/L,
o
[Catalyst] = 5.92 × 10–3 mol/L, 30 ml methanol, 30 C tem-
H
2
perature, and 79.5 kPa hydrogen pressure. The initial rate
remained almost constant over six cycles without significant
loss of catalytic activity (Table 5). The palladium content of the
catalyst estimated using an atomic absorption spectropho-
tometer after six cycles was found to be almost the same as that
of the fresh catalyst. When an estimate of metal content was
carried out for the reaction mixture at the end of each cycle, no
metal was present in the solution, revealing no metal leaching
from the support.
H
2
NBE
NBD
NBA
100
80
60
40
20
0
NBA
NBD
Table 5 Recycling efficiency of the catalyst
Cycle
Initial rate × 10–4 (mol/(L·min))
Fresh
9.56
9.55
9.55
9.54
9.54
9.53
9.52
NBE
100 120
1
2
3
4
5
6
0
20
40
60
80
Time (min)
Fig. 5. Sequential hydrogenation of NBD. [Catalyst] = 5.92 × 10ꢁ3
mol/L, [NBD] = 6.67 × 10ꢁ2 mol/L, 79.5 kPa hydrogen pressure, tem-
perature = 30 oC, 30 ml methanol.

Udayakumar VELU et al. / Chinese Journal of Catalysis, 2011, 32: 280–285
13: 1
3 Conclusions
5
6
7
8
9
Islam M, Mondal P, Mondal S, Mukherjee S, Roy A S,
Mubarak M, Paul M. J Inorg Organomet Polym, 2010, 20: 87
Islam S M, Mondal P, Tuhina K, Roy A S, Mondal S, Hossain
D. J Inorg Organomet Polym, 2010, 20: 264
Islam M, Mondal P, Roy A S, Tuhina K. Transition Met Chem,
2010, 35: 427
A polymer-supported palladium-imidazole complex is a
promising catalyst for the hydrogenation of various olefins
under mild conditions. The solvent used and structure of the
olefin played a dominant role on the reactivity. The ability of
the olefin to form a complex with the catalyst during hydro-
genation provided the basis for interpreting the relative reac-
tivity of the olefins. Sequential hydrogenation of the double
bonds was observed in the case of 2,5-norbornadiene. The
catalyst was easily recovered and reusable, thus it is environ-
ment friendly.
Udayakumar V, Alexander S, Gayathri V, Shivakumaraiah,
Patil K R, Viswanathan B. J Mol Catal A, 2010, 317: 111
Alexander S, Udayakumar V, Gayathri V. J Mol Catal A, 2009,
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10 Alexander S, Udayakumar V, Nagaraj N, Gayathri V. Transi-
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11 Van Karalingen C G, De Ridder J K, Reeduk J. Inorg Chim
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Acknowledgements
12 Welton T. Chem Rev, 1999, 99: 2071
Authors wish to thank University Grants Commission for
Departmental research support and Department of science and
technology for Fund for improvement of science technology
programme, Thermax India Ltd., Pune, India, Sophisticated
testing instrumentation centre, Cochin and Central dug re-
search institute, Lucknow. Udayakumar VELU thanks Sree
Siddaganga Education Society, Sree Siddaganga Math, Tumkur,
Karnataka, India, for extending support for doctoral studies.
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