Catalytic activity of a supported palladium-benzimidazole complex toward alkene hydrogenation

Article abstract of DOI:10.1007/s11243-012-9597-0

A polymer anchored palladium complex was synthesized by sequential attachment of benzimidazole and palladium chloride to chloromethylated polystyrene divinyl benzene co-polymer with 6.5 % cross-linking. The product was characterized by XPS, UV-vis. spectrophotometry, FTIR and TGA. Various physico-chemical properties such as bulk density, surface area and swelling behavior in different solvents were also measured. The polymer anchored complex was tested as a catalyst for reduction of olefins. The kinetics of hydrogenation of 1-hexene was studied by varying the temperature, catalyst concentration and substrate concentration. The energy and entropy of activation were evaluated from the kinetic data. The catalyst could be recycled a number of times and no leaching of metal from the catalyst surface was observed.

Full text of DOI:10.1007/s11243-012-9597-0

Transition Met Chem (2012) 37:367–372
DOI 10.1007/s11243-012-9597-0
Catalytic activity of a supported palladium–benzimidazole
complex toward alkene hydrogenation
•
S. Alexander V. Udayakumar V. Gayathri
•
Received: 25 October 2011 / Accepted: 12 March 2012 / Published online: 27 March 2012
Ó Springer Science+Business Media B.V. 2012
Abstract A polymer anchored palladium complex was
synthesized by sequential attachment of benzimidazole and
palladium chloride to chloromethylated polystyrene divinyl
benzene co-polymer with 6.5 % cross-linking. The product
was characterized by XPS, UV–vis. spectrophotometry,
FTIR and TGA. Various physico-chemical properties such
as bulk density, surface area and swelling behavior in
different solvents were also measured. The polymer
anchored complex was tested as a catalyst for reduction of
olefins. The kinetics of hydrogenation of 1-hexene was
studied by varying the temperature, catalyst concentration
and substrate concentration. The energy and entropy of
activation were evaluated from the kinetic data. The cata-
lyst could be recycled a number of times and no leaching of
metal from the catalyst surface was observed.
Abbreviations
PSDVB
Chloromethylated polystyrene
divinyl benzene
PSDVB-BzlH
Benzimidazole Functionalized
beads
PSDVB-BzlH-PdCl2 Functionalized beads with PdCl2
Introduction
The hydrogenation of organic functional groups is a common
application of catalysis in the synthesis of organic com-
pounds [1, 2]. Although transition metal complexes of plat-
inum, rhodium and ruthenium have long been used as
catalysts for the hydrogenation of organic nitro compounds
and alkenes, palladium is currently the metal of choice [3, 4].
It is an extremely active catalyst for alkene saturation, par-
ticularly in the presence of multiple unsaturation. However,
platinum group metal complexes are often expensive, so
their immobilization on a solid support makes for commer-
cial advantage.
Keywords Polymer-supported Pd(II) catalyst ꢀ Olefin ꢀ
Hydrogenation ꢀ Kinetics and rate equation
The heterogenization of homogeneous transition metal
catalysts can provide a number of useful advantages [5, 6].
Besides high activity and selectivity, easier separation of
catalyst from the product makes them attractive. Particu-
larly for industrial applications, recyclability of expensive
catalysts is a priority and so the immobilization of catalysts
that are normally used in homogenous phase is a topic
being investigated thoroughly [7]. We have earlier reported
the synthesis and characterization of palladium complexes
supported on polystyrene cross-linked with 6.5 % divi-
nylbenzene (PSDVB) and their use in the reduction of
Schiff bases and alkenes [8, 9]. In this paper, we report the
synthesis and characterization of palladium chloride sup-
ported on PSDVB functionalized with benzimidazole as a
Electronic supplementary material The online version of this
article (doi:10.1007/s11243-012-9597-0) contains supplementary
material, which is available to authorized users.
S. Alexander ꢀ V. Udayakumar ꢀ V. Gayathri (&)
Department of Chemistry, Bangalore University,
Central College Campus, Bangalore 560 001, India
e-mail: gayathritvr@yahoo.co.in
123

368
Transition Met Chem (2012) 37:367–372
ligand. The catalytic activity of this anchored catalyst
toward the hydrogenation of alkenes has been investigated.
were calibrated with respect to the C1S component of the
binding energy set at 285 eV. GC was recorded using a
Shimadzu 14 B using BP5 capillary column.
Experimental
Preparation of the catalyst
Materials and equipment
The PSDVB polymer beads were first washed with a
mixture of tetrahydrofuran and water (4:1) using a Soxhlet
extractor for 10 h. The beads were then vacuum dried. This
polymer was functionalized with benzimidazole by treating
the beads (5 g) with benzimidazole (2.845 g) in acetonitrile
plus toluene mixture (1:1, 200 ml) on a water bath at 60 °C
for 30 h. The beads were then filtered off, Soxhlet
extracted with ethanol and dried.
Chloromethylated polystyrene divinyl benzene copolymer
with 6.5 % cross-linking (PSDVB) was obtained as a gift
from Thermax India Ltd., Pune, India. Palladium chloride
was obtained from Arora Matthey Ltd., Benzimidazole was
procured from S.D Fine Chemicals. 1,5-cyclooctadiene
(1,5-cod), 1-hexene, 1-heptene, 1-octene, norbornadiene
(nbd) and cyclohexene were purified and distilled before
use. Other substrates were used as such. Super dry meth-
anol was prepared according to the literature method [10].
C, H and N analyses were carried out using a Thermo
Finnegan Eager 300 analyzer at RRI, Bangalore and
Thermo Finnigan Flash EA1112 CHNS analyzer at the
Department of Organic Chemistry, I.I.Sc., Bangalore.
Palladium was estimated by AAS using a GBC Avanta PM
spectrophotometer at Siddaganga Institute of Technology,
Tumkur. IR spectra (in KBr) were recorded using a Shi-
madzu FTIR-8400S spectrometer. Far-IR spectra were
recorded on an optical spectrometer Bruker IFS-66 V/S.
Surface area was measured using a Nova 1000 instrument
under nitrogen atmosphere. TGA studies were carried out
using a Perkin-Elmer 7 series instrument. XPS was recor-
ded at 358 take off angle (relative to the surface normal)
with an SSX-100 spectrometer using monochromatized Al
Ka radiation (1,486.6 eV). The analyzed core level lines
Benzimidazole functionalized PSDVB beads (2 g with
6.3 % of nitrogen) were soaked in toluene and acetonitrile
mixture (1:1, 50 ml) for 1 h. A methanol solution con-
taining 500 mg of palladium chloride (0.3 mmol of Pd)
was added to the soaked beads and the mixture was heated
at 60 °C on a water bath for 48 h. The beads were filtered
off, washed several times with ethanol and Soxhlet
extracted with the same solvent (Scheme 1).
The supported palladium complex was activated prior to
use by treating with an ethanolic solution of sodium
borohydride (500 mg/100 ml) then washed several times
with ethanol and dried.
The supported catalyst was added to 30 ml of deaerated
methanol and the reaction system was allowed to saturate
in an atmosphere of hydrogen for 2 h. The system was
evacuated and again saturated with hydrogen gas. A known
quantity of the substrate in methanol was injected into the
system, followed by releasing the system to a gas burette
2
Scheme 1 Synthesis of the polymer-supported palladium—benzimidazole catalyst
123

Transition Met Chem (2012) 37:367–372
369
filled with hydrogen. The reaction was monitored by the
volume of hydrogen absorbed at different intervals of time.
The reaction mixture was analyzed by GC, using a BP-5
capillary column. No hydrogen uptake was observed when
a blank reaction was carried without the catalyst and
1-hexene as the substrate.
and polymer-bound complex at various stages are listed in
Table 1. The palladium content in the functionalized
beads was found to be 13.5 %. On activation with sodium
borohydride, the palladium content was reduced to
12.7 %. The metal contents in the activated beads and
recycled beads were found to be the same, indicating that
there is no leaching of the palladium even after recycling
for six times.
Results and discussion
The IR and Far-IR spectra at various stages of catalyst
preparation have been recorded. The chloromethylated
beads exhibit characteristic bands at 1,265 and 825 cm-1
due to –CH2–Cl (Fig. 1). On functionalization with benz-
imidazole, the intensity of these two peaks decreases
considerably indicating that some of the chlorides are
displaced by benzimidazole. The far-IR spectrum of the
polymer-bound palladium chloride complex shows an
intense band around 330 cm-1 (Fig. 2 in supplementary
material) which is assigned to terminal mPd-Cl [13–17].
Characterization of the catalyst
Functionalization and loading of the metal was confirmed
by various physicochemical analyses. A decrease in surface
area of the beads on treatment with sodium borohydride
was observed, which is attributed to blockage of the
pores of the polymer support upon activation [11, 12].
Elemental analysis and physical properties of the polymer
Table 1 Elemental analysis
and physical properties of
polymer and polymer-bound
Pd
C
H
N
Pore volume Surface area Apparent bulk
(m2/g)
(%) (%) (%) (%) (cm3/g)
density (g/cm3)
complex
Chloromethylated polystyrene
Divinyl benzene (PSDVB)
–
–
74.5 6.3
–
0.05
36
0.42
Benzimidazole Functionalized
beads (PSDVB-BzlH)
68.0 6.0 6.3 0.04
35
39
0.45
0.43
Functionalized beads with PdCl2 13.5 67.0 6.0 6.0 0.05
(PSDVB-BzlH-PdCl2)
Activated PSDVB-BzlH-PdCl2
12.7 65.0 7.0 6.0 0.05
33
35
0.43
0.43
Recycled beads after six cycles 12.7 64.0 6.6 6.0 0.05
Fig. 2 Hydrogenation of 1-hexene; [1-hexene] = 0.2 mol/dm3. [Cat-
alyst] = 19.88 9 10-3 mol/dm3, 596 mm Hg pressure of hydrogen,
303 K in 30 ml methanol
Fig. 1 The Overlay of the IR spectra of: a PSDVB b PSDVB-BzlH
c PSDVB- BzlH-PdCl2 d Activated PSDVB-BzlH-PdCl2
123

370
Transition Met Chem (2012) 37:367–372
The diffuse reflectance spectra of the functionalized
Table 2 Initial rate of hydrogenation of various substrates, [sub-
strate] = 0.2 mol/dm3
beads and beads after reaction with palladium chloride are
given in Fig. 2 in the supplementary material. The beads
obtained after reaction with palladium chloride exhibit a
broad peak around 460 nm which is assigned to the d–d
transition of the metal in d8 configuration.
Substrate
Initial rate 9 10-5
(mol/dm3/s)
1-hexene
7.73
7.13
4.96
8.61
6.66
1.92
2.95
5.21
10.17
5.81
1-heptene
XPS studies were carried out to find the oxidation state
of the metal in the catalyst, before and after activation
(Figs. 4 and 5 in supplementary material). The data col-
lection was based on the binding energy values of C 1s
fixed at 285 eV [18]. From the spectra it can be seen that
there is no significant change in the oxidation state of the
palladium species in either the activated catalyst or the
recycled catalyst. Prior to activation, the binding energy
values for the palladium 3d5/2 and 3d3/2 levels are 338.9
and 344 eV respectively. Upon activation with sodium
borohydride these peaks do not shift significantly, but
develop a shoulder toward lower binding energy. This
indicates that the palladium has been partially reduced to
the zero state, since the palladium 3d5/2 level with 336 eV
corresponds to the binding energy of the inner electrons of
Pd(0) [19].
1-octene
Diethyl maleate
Diethyl fumarate
Cinnalmaldehyde
Ethyloleate
1,5-cyclooctadiene
Norbornadiene
Cyclohexene
[Catalyst] = 19.88 9 10-3 mol/dm3, 596 mm of Hg pressure of
hydrogen, 303 K in 30 ml methanol
length of the carbon chain. This order of reactivity may be
explained based on steric and electronic factors. Thus, the
decrease in activity from 1-hexene to 1-octene may be due
to crowding in the transition state with increase in length of
the carbon chain. Similar observations have been reported
by Ramesh et al. for the hydrogenation of 1-alkenes using
montmorillonite-bipyridine palladium(II)acetate and pal-
ladium chloride complexes [3]. Increasing carbon chain
length enhances electronic transmission through inductive
effects, but this effect is generally insignificant beyond the
third carbon.
From the XPS and far-IR spectra it can be suggested that
the major oxidation state of palladium in both the activated
and recycled catalysts is ?2 (Fig. 6 in supplementary
material). Bruner and Bailar [20] have previously synthe-
sized a palladium complex of a polymeric diphenylben-
zylphosphine ligand with good selectivity for the reduction
of dienes to monoenes as well as greater reactivity toward
conjugated dienes in which the active species was identi-
fied as palladium(II) and not palladium(0).
Comparing the geometrical isomers diethyl maleate and
diethyl fumarate, the cis isomer is reduced more rapidly
than the trans isomer, as it can form an intermediate
complex more easily than the trans isomer [23–26]. Due to
the highly strained nature of nbd, it is reduced much faster
than the non-conjugated diene 1,5-cyclooctadiene [27].
TGA analysis of the activated palladium chloride
anchored catalyst shows that it is stable up to 200 °C
(Fig. 7 in supplementary material). Upon activation and
recycling the thermal stability of the beads does not alter.
Swelling studies show that the swelling is greater in polar
solvents than in non-polar solvents. Though the maximum
swelling of the beads is in water which may be attributed to
strong hydrogen bonding, methanol appears to be a better
choice for hydrogenation reactions since the miscibility of
substrates and solubility of hydrogen is greater in this solvent
[21, 22]. The results of these swelling studies are tabulated in
Table 2 (supplementary material).
Hydrogenation of 1-hexene
In experiments using 6 mmol of 1-hexene, in the first
20 min 3 mmol undergo hydrogenation while the remain-
der undergoes isomerization to cis-2-hexene and trans-2-
hexene (Fig. 8). These two isomers are subsequently
hydrogenated into n-hexane at a point when most of the
starting alkene is consumed. The cis isomer is formed in
smaller quantities than the trans. Efraty and Feinstein [28]
reported that hydrogenation of 1-hexene in the presence of
a rhodium catalyst at relatively high temperature and
pressure predominantly gave n-hexane, with small amounts
of cis- and trans-2-hexene. In our study, though the reac-
tions were carried out under ambient conditions, very little
isomerization has taken place.
Catalytic hydrogenation of olefins
To screen for hydrogenation of olefins, studies on 1-hex-
ene, 1-heptene, 1-octene, diethyl maleate, diethyl fumarate,
cinnamaldehyde, ethyl oleate, 1,5-cyclooctadiene (cod) and
norbornadiene (nbd) were carried out using the catalyst.
The results of these experiments are presented in Table 2.
Under our experimental conditions, the rate of hydro-
genation of terminal alkenes decreases with increasing
The kinetics of 1-hexene hydrogenation using our sup-
ported catalyst was studied in methanol by following the
123

Transition Met Chem (2012) 37:367–372
371
hydrogen uptake at constant pressure. The effect of sub-
strate concentration [S] on the rate of hydrogenation was
studied over a range of substrate concentrations from 0.1 to
0.5 mol/dm3 at 30 °C and hydrogen pressure at 596 mm
Rewriting the rate equation, we have
1
1
1
¼
þ
Rate k1K½catalystꢁ½1 ꢂ hexeneꢁ½H2ꢁ k1½catalystꢁ½H2ꢁ
A plot of 1/rate versus 1/[1-hexene] at a constant catalyst
concentration of 19.88 9 10-3 mol/dm3 and 596 mm Hg
pressure of hydrogen gave a straight line with an intercept
on the y axis (Fig. 11 in supplementary material). From the
intercept and slope of these plots, the values of K and k1
were evaluated (Table 4 in supplementary material).
The kinetics of hydrogenation were also studied in the
range of 25–40 °C to determine the influence of tempera-
ture at a fixed catalyst concentration of 19.88 9 10-3 mol/
dm3, hydrogen pressure of 596 mm Hg and a substrate
concentration of 0.2 mol/dm3. An increase in the rate of
hydrogenation was observed with increasing temperature.
The apparent energy of activation was calculated from a
plot of log(initial rate) versus reciprocal of temperature
(Fig. 12 in supplementary material) as 41 kJ/mol. This low
value of Ea indicates high activity of the catalyst. The
activation entropy DàS indicates that there is significant
loss in freedom due to fixation of the catalyst molecule on
the polymer support. Although DàS is negative, the values
of DG and DH indicate favorable formation of the
p-complex in the equilibrium step.
Hg and at
a
constant catalyst concentration of
19.88 9 10-3 mol/dm3 (Fig. 9 in supplementary material).
The rate of hydrogenation (R) increased linearly with
respect to substrate concentration. The order of the reaction
calculated from the linear plot of log(initial rate) versus
log[1-hexene] was found to be fractional.
The catalyst concentration varied in the range of
(3.97–19.88) 9 10-3 mol/dm3 of palladium at 30 °C,
hydrogen pressure at 596 mm Hg and a constant substrate
concentration of 0.2 mol/dm3. The initial rate shows a
linear dependence with catalyst concentration (Fig. 10 in
supplementary material). A plot of log(initial rate) versus
log(catalyst) was used to calculate the order which is unity
for this substrate. This indicates that mass-transfer effects
can be neglected under the reaction conditions [29]. It also
suggests that all the catalytic sites are available for reaction
with the substrates [30, 31].
Based on the results obtained and evidence from the
literature [18], we propose a mechanism for the hydroge-
nation of 1-hexene as shown in Scheme 2.
In this mechanism, the supported catalyst binds revers-
ibly to the substrate to form a p-complex. In a slow step,
the p-complex reacts with hydrogen to form a dihydrido
alkyl palladium species which finally undergoes reductive
elimination to release the alkane product.
Recycling efficiency of the supported catalyst
Polymer-supported catalysts often suffer from an inherent
drawback of gradual metal depletion and hence loss in
activity under catalytic conditions. To evaluate the stability
and recycling efficiency of this polymer-bound catalyst, a
known quantity of 1-hexene was hydrogenated and the
initial rate was determined. The polymer beads were then
filtered off, washed, dried and reused. The catalyst was
analyzed for its metal content by AAS. The percentage of
metal remained the same. Aliquots of the reaction mixture
were also analyzed for palladium content at regular inter-
vals (every 10 min). No metal was found in these solutions,
suggesting that there is no leaching and readsorbtion of
metal during the reaction. The catalyst retained 87.4 %
activity on the sixth cycle (Table 5 in supplementary
material). The catalytic activity of its homogeneous analog
PdCl2(BzlH)2 was tested under similar reaction conditions;
in these experiments, palladium precipitated from the
solution, making it unsuitable for further reaction.
The rate law derived for this mechanism is as follows;
Kk1½catalystꢁ½1 ꢂ hexeneꢁ½H2ꢁ
Rate ¼
1 þ K½1 ꢂ hexeneꢁ
where K is the equilibrium constant for the reversible
reaction and k1 is the velocity constant for the rate deter-
mining step. This expression explains the fractional order
of the reaction with respect to substrate concentration and
first order dependence on both catalyst concentration and
hydrogen pressure.
C4H9
C4H9
K
Pd2+
+
P
Pd2+
P
C4H9
+ H2
C4H9
k1
P
P
Pd2+
P
Pd4+
H
slow
H
C4H9
H
C
H
fast
Pd4+
H
P
Pd4+
H
CH2-C4H9
H
Conclusion
H
C
H
fast
P
Pd4+
H
Pd2+
+
CH2-C4H9
H3C CH2-C4H9
P
The polymer-bound palladium–benzimidazole complex
described in this work was found to reduce various alkenes
under ambient conditions of temperature and pressure.
Scheme 2 Plausible mechanism for 1-hexene hydrogenation
123

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Transition Met Chem (2012) 37:367–372
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Recycling studies indicate that the catalyst could be used
repeatedly over several cycles without suffering apprecia-
ble loss in activity. Further, no loss of metal from the
polymer support was observed.
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Acknowledgments The authors would like to thank UGC for DRS
programme, Prof. Sadashiva, Liquid Crystal Lab. RRI for CHN
analysis and DST, Central Facility, Physics Department, I.I.Sc.
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