Polymer-supported palladium-imidazole complex catalyst for hydrogenation of substituted benzylideneanilines

Article abstract of DOI:10.1016/j.molcata.2009.10.030

The polymer-supported palladium-imidazole complex catalyst was synthesized and characterized by various techniques such as elemental analysis, IR spectroscopy and TG analysis. The physico-chemical properties such as bulk density, surface studies by BET me

Full text of DOI:10.1016/j.molcata.2009.10.030

Journal of Molecular Catalysis A: Chemical 317 (2010) 111–117
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Polymer-supported palladium-imidazole complex catalyst for hydrogenation of
substituted benzylideneanilines
a
a
a,∗
b
c
d
V. Udayakumar , S. Alexander , V. Gayathri , Shivakumaraiah , K.R. Patil , B. Viswanathan
a
Department of Studies in Chemistry, Central College Campus, Bangalore University, Ambedkar Veedhi, Bangalore 560001, India
Siddaganga Institute of Technology, Tumkur 572103, Karnataka, India
National Chemical Laboratories, Pune 411008, India
National Centre for Catalysis Research, I.I.T. Madras, Chennai 600036, India
b
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 August 2009
Received in revised form 30 October 2009
Accepted 30 October 2009
Available online 11 November 2009
The polymer-supported palladium-imidazole complex catalyst was synthesized and characterized by
various techniques such as elemental analysis, IR spectroscopy and TG analysis. The physico-chemical
properties such as bulk density, surface studies by BET method and swelling studies of catalyst in different
solvents were investigated. XPS studies were carried out to identify the oxidation state of palladium
in the catalyst. The morphology of the support and the catalyst was studied using scanning electron
microscope. Using the synthesized catalyst, hydrogenation of benzylideneaniline and a few of its para
substituted derivatives was carried out at ambient conditions. The influence of variation in temperature,
concentration of the catalyst as well as the substrate on the rate of reaction was studied. The catalyst
showed an excellent recycling efficiency over six cycles without leaching of metal from the polymer
support.
Keywords:
Polymer-supported palladium-imidazole
complex catalyst
Hydrogenation
Benzylideneaniline
Kinetics
© 2009 Elsevier B.V. All rights reserved.
1
. Introduction
Catalytic reduction of organic compounds is an important
inert and thermally stable materials like silica, alumina, carbon or
polymers are used as supports [4–16]. Polymer supports have sev-
eral advantages as they can be functionalised with different types
of ligands and the functionalised polymer supports can form sta-
ble complexes with different metals suitable for various organic
transformations. Catalysts can be tailor made for reactions.
Earlier researchers have reported the hydrogenation of Schiff
bases like benzylideneaniline using transition metal complexes
as catalysts under high pressure and temperature [1,17]. In the
present work, chloromethylated polystyrene beads (PS-DVB) were
functionalised with imidazole to create an environment analogous
to those in active homogeneous catalysts. Palladium chloride was
anchored on to the functionalised beads and was used for hydro-
genation of benzylideneaniline and few of its para derivatives.
process in the laboratory and industry and continues to be a
subject of active research. Methods based on catalytic hydrogena-
tion in homogeneous and heterogeneous media are commonly
adopted for the hydrogenation of organic compounds. Transition
metal complexes are reported to be powerful catalysts for organic
transformations [1]. Studies have revealed that the complexes of
platinum group metals such as palladium, platinum and rhodium
are effective catalysts for hydrogenation of nitro compounds, alde-
hydes and Schiff bases [2,3].
In the recent past, there has been an increasing interest in devel-
oping environmental friendly greener processes which are also
economically viable. Heterogeneous catalysts can be used in flu-
idized and packed bed reactor where recycling is easy. This becomes
more significant when noble metals are used as catalysts. Highly
2. Experimental
2.1. Materials, methods and equipment
PS-DVB with 6.5% cross-linking was obtained as a gift by Ther-
Abbreviations: A, chloromethylated polystyrene beads cross-linked with 6.5%
divinylbenzene (PS-DVB); B, PS-DVB functionalised with imidazole; C, function-
alised beads treated with palladium chloride; D, C treated with methanolic solution
of sodium borohydride; E, D after recycling; F, Pd(Imz)2Cl2; Bza, benzylideneaniline;
p-ClBza, p-chlorobenzylideneaniline; p-MeOBza, p-methoxybenzylideneaniline; p-
NO2Bza, p-nitrobenzylideneaniline; p-OHBza, p-hydroxybenzylideneaniline.
∗ Corresponding author. Tel.: +91 80 22961348.
E-mail address: gayathritvr@yahoo.co.in (V. Gayathri).
max India Ltd., Pune, India. Palladium chloride was procured from
Arora Matthey Ltd., India. Benzylideneaniline and its derivatives
were prepared as per the literature method [18]. All the solvents
were purified before use. The IR spectra were recorded using a
Shimadzu FTIR-8500S spectrometer and far IR spectra by using a
PerkinElmer, Spectrum 1000, FTIR spectrometer. Surface area was
1
381-1169/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2009.10.030

1
12
V. Udayakumar et al. / Journal of Molecular Catalysis A: Chemical 317 (2010) 111–117
Scheme 1. Preparation of the polymer-supported palladium-imidazole catalyst.
Table 2
measured using a Nova 1000 instrument in nitrogen atmosphere.
TGA studies were carried out using a PerkinElmer, Diamond instru-
ment under inert atmosphere. Scanning electron micrographs were
recorded on a Leica S440i. HPLC studies were recorded using a Shi-
madzu Prominence, 2A with C18 column and UV detector. GC–MS
were recorded with a Shimadzu GC–MS instrument. C, H and N
analyses were carried out using Finnegan Eager 300 and a Elemen-
tar Vario EL III, Carlo Erba 1108, at SAIF, Cochin, CDRI, Lucknow.
Palladium content was determined using a GBC, Avanta PM, atomic
absorption spectrophotometer and chloride content by Volhard’s
method. Photoelectron spectra were recorded on a ESCA-3000
electron spectrometer, equipped with a hemispherical electron
analyzer using the Al K␣ radiation (1486.6 eV) X-ray source. The
Elemental analysis of polymer support and polymer bound catalyst.
% C
% H
% N
% Pd
A
B
C
D
E
69.50
62.92
50.60
52.13
53.9
5.78
5.21
4.78
5.06
4.87
–
–
–
8.80
6.52
6.22
5.00
10.03
9.95
9.86
3. Results and discussion
3.1. Characterization of the catalyst
neat complex Pd(Imz) Cl2 (Imz = imidazole) was prepared accord-
ing to a literature method [19].
2
3.1.1. Surface area and porosity of the catalyst
Surface area and porosity of the catalyst influences the activity
of the catalyst. Bulk density is also an important pre-requisite to be
fulfilled by a catalyst. High bulk density facilitates the efficient use
of reactor volume and therefore is economical. Surface area, pore
volume and pore diameter determined by BET method is tabulated
along with apparent bulk density in Table 1. Decrease in surface
area, pore volume and pore diameter after functionalisation and
activation of the catalyst was observed which may be attributed
to the blocking in pores of the polymer-support. Increase in bulk
density is due to the functionalisation and anchoring of the metal
salt on the support.
2
.2. Preparation of the polymer-supported palladium-imidazole
catalyst
Functionalisation of PS-DVB with imidazole was carried out
by treating PS-DVB with imidazole, in a mixture of 1:1 toluene
and acetonitrile on a hot water bath maintained at 50 C for 60 h
◦
[
6,14]. The functionalised beads were filtered and washed by Soxh-
let extraction using ethanol as a solvent. The beads were treated
with ethanolic solution of palladium chloride for 10 h during which
the beads colour changed to buff [2]. Beads were finally washed by
Soxhlet extraction using ethanol and dried (Scheme 1). The catalyst
was activated by treating with methanolic solution of sodium boro-
hydride, washed several times with methanol and vacuum-dried.
3.1.2. Swelling studies
Swelling studies on the polymer support and the catalyst in var-
ious solvents were carried out at room temperature. Though the
swelling was found to be maximum in DMF, the reactions were
carried out in ethanol as the solubility of hydrogen is maximum in
ethanol [20–23].
2
.3. Hydrogenation procedure
All the reactions were carried out using ethanol as a solvent
3.1.3. Elemental analysis
at 596 mmHg hydrogen pressure in a glass reactor. In a typical
experiment, the catalyst was added along with the solvent into
the reactor, flushed with hydrogen and evacuated. A known quan-
tity of the substrate was injected and the system was opened to
hydrogen gas burette. The reaction was monitored by hydrogen
uptake at different intervals of time. The reaction mixture was then
filtered, evaporated to dryness and the resulting solid was recrys-
tallized. Products obtained were identified using HPLC, IR spectra
and GC–MS. A blank reaction was also carried out for all the sub-
strates without using the catalyst.
Elemental analysis for A, B, C, D and E were carried out and
the data are tabulated in Table 2. Decrease in chloride content
and the presence of nitrogen in B determined the extent of func-
tionalisation of A. Changes were also observed after treatment
of the functionalised beads with palladium chloride and treating
it with sodium borohydride. Amount of metal was determined
by stripping the bound metal from the support and analyzed
using atomic absorption spectrophotometer. Palladium content
remained almost unchanged even after recycling the catalyst for
six cycles.
Table 1
Physical properties of polymer-support and polymer bound catalyst.
Pore volume (cm3
g
−1
)
Surface area (m2
g
−1
)
Pore diameter (Å)
−3
Apparent bulk density (g cm )
A
B
C
D
E
0.0421
0.0299
0.0335
0.0344
0.0397
28.24
20.16
21.81
23.39
25.19
59.67
59.48
61.41
58.91
62.66
0.323
0.439
0.505
0.489
0.475

V. Udayakumar et al. / Journal of Molecular Catalysis A: Chemical 317 (2010) 111–117
113
Fig. 1. IR-spectra of A, B, C, D, E and F.
3
.1.4. Infrared spectral studies
The IR spectra of A, B, C, D, E and F were recorded in nujol and
a comparative study was made (Fig. 1). PS-DVB exhibits a peak
−1
at 1263 cm
attributed to –CH Cl (Spectrum A) and imidazole
2
−1
exhibited ꢀN–H around 3124 cm . The later peak was not found
after functionalisation of the polymer indicating that hydrogen of
(
N–H) imidazole is lost consequent to functionalisation and it is
bonded to the polymer support through nitrogen [11]. Decrease in
−1
intensity of band at 825 cm corresponding to ꢀC–Cl of A further
support the bonding of imidazole with A (Spectrum B) [24]. The
sharp absorbance due to ꢀN–H of palladium-imidazole complex at
−
1
3
256 cm in the spectrum F has not been observed after attach-
ment of the complex with the polymer support [25]. This further
confirms the bonding of complex with the polymer support. No
change in the spectral features was observed after activation of cat-
alyst with sodium borohydride or after recycling the catalyst over
six cycles (Spectra D and E, respectively). A high intense absorption
Fig. 2. (a) Scanning electron micrograph of A. (b) Scanning electron micrograph of
D.
−1
band at 312 cm in the far-IR spectrum recorded for D has been
assigned to Pd-Cl stretching.
3.2. Hydrogenation reaction
3.1.5. SEM and TGA studies
Scanning electron micrographs of A and D were taken to study
Palladium-imidazole complex, Pd(Imz) Cl , in ethanol was
2
2
the surface of the polymer support and the catalyst. Change in
morphology of the activated catalyst surface was observed when
compared with the support. The scanning electron micrographs are
depicted in Fig. 2a and b.
used as catalyst for hydrogenation of benzylideneaniline to N-
benzylaniline but the attempt was unsuccessful since it resulted
in the separation of metal from the solution during the course of
the reaction. Hence the polymer-supported palladium-imidazole
catalyst was synthesized and used for the reduction of benzylide-
neaniline and its derivatives. It was found that during the reaction
Thermal stability of the support and the catalyst was studied
by TGA–DTA analyses in an inert atmosphere with heating rate of
◦
◦
1
0 C/min up to 500 C for A, B, C, D and E. The TGA–DTA curve for
◦
D is shown in Fig. 3. The weight loss observed around 100 C could
be due to the moisture content. No significant weight loss has been
observed till 254 C.
◦
3.1.6. XPS studies
The oxidation state of the metal in the catalyst was ascer-
tained by carrying out XPS studies (Fig. 4). Spectrum of C revealed
the presence of palladium in a +2 oxidation state with binding
energies 338.0 eV (3d5/2) and 343.3 eV (3d3/2). After treating the
anchored catalyst with sodium borohydride (Spectrum D) it exhib-
ited one minor peak at 335.3 eV (3d5/2) along with two major peaks
at 338 eV (3d5/2) and 343.3 eV (3d3/2) indicating the presence of
small amount of palladium in zero oxidation state [5,26,27]. After
recycling the catalyst for six times its XPS was taken in which broad-
ening of 3d5/2 peak was observed (Spectrum E) which could be due
to increase in the concentration of palladium in zero oxidation state
on recycling.
Fig. 3. TGA-DTA curves of D.

1
14
V. Udayakumar et al. / Journal of Molecular Catalysis A: Chemical 317 (2010) 111–117
Fig. 5. Dependency of rate of hydrogenation on [catalyst].
3.3. Kinetic studies
The kinetics of hydrogenation of the benzylideneanilines were
studied by following the hydrogen uptake at 596 mmHg hydrogen
pressure. The rate of hydrogenation was calculated from the slope
after plotting the volume of hydrogen uptake by the substrate as
a function of time. The dependency of the rate on variables like
catalyst, substrate concentration and temperature was studied in
detail for all the substrates.
3.3.1. Effect of catalyst concentration
Fig. 4. XPS spectra of C, D and E.
The influence of catalyst concentration on the rate of hydro-
−
4
genation was carried out over
a
range of 14.8 × 10
to
−
3
−3
2
3
0.72 × 10 mol dm Pd at constant substrate concentration of
−
3
−3
◦
3.3 × 10 mol dm
at a temperature of 30 C with 596 mmHg
hydrogen pressure using ethanol as solvent [18]. The plot of
−
4
log(initial rate) against log[Catalyst] in the range of 14.8 × 10 to
metal does not leach out from the polymer support and was found
to be intact even after recycling for six times. All reactions were
conducted in ethanol at 596 mmHg of hydrogen pressure. Since the
activity of the activated catalyst (D) was more compared to palla-
dium chloride anchored catalyst (C), the activated catalyst was used
for hydrogenation reactions. [5,17]. The products were identified by
HPLC, IR and GC–MS. To determine the effect of substitution at the
para position of benzylideneaniline on hydrogenation, few deriva-
−3
−3
7
.4 × 10 mol dm Pd is depicted in Fig. 5. Order of the reaction
determined from the slope of the plot showed that all the substrates
followed fractional order kinetics. However, the range of concen-
tration in which the catalyst follows the fractional order on the rate
is not the same for all the substrates. This may be due to change in
structure of the substrate molecules. As the rate of hydrogenation
of hydroxyl derivative is close to that of chloro derivative the same
has not been included in the plot.
tives with –NO , –Cl, –OH and –MeO groups were chosen. Earlier
−3
−3
2
Above 7.4 × 10 mol dm
Pd concentration, initial rate was
found to be independent of catalyst concentration and it fol-
lowed zero order kinetics indicating that for the substrate
studies have shown that the introduction of substituents at para
position of the amine ring of the benzylideneaniline has very little
effect on the hydrogenation rate. In our studies it has been observed
that the insertion of substituents on to the para position of aldehyde
ring of benzylideneaniline reduced the activation energy markedly
−3
−3
4
× 10 mol dm Pd is sufficient to hydrogenate and higher con-
centration of catalyst may not be required. Hence the rate becomes
independent of catalyst concentration.
[
28] (Scheme 2).
When p-nitrobenzylideneniline was used as the substrate, both
N and nitro group were hydrogenated simultaneously and the
catalyst does not show selectivity towards C N. Hence the effect
of nitro group substitution on the rate could not be determined.
Similar observation was made by Arthur Roe and Montgonery [18].
3
.3.2. Effect of substrate concentration
The influence of substrate concentration on the rate of
C
−4
the reaction was studied in the range of 33.0 × 10
to
−2
−3 ◦
1
3.3 × 10 mol dm
of substrate at 30 C and at 596 mmHg
hydrogen pressure with a constant catalyst concentration of
Scheme 2. Hydrogenation of substituted benzylideneanilines.

V. Udayakumar et al. / Journal of Molecular Catalysis A: Chemical 317 (2010) 111–117
115
Fig. 6. Dependency of rate of hydrogenation on [substrate].
−4
−3
5
9.2 × 10 mol dm Pd for all the substrates in ethanol [18]. The
plot of log(initial rate) against log[substrate] is given in Fig. 6. Order
of the reaction in this concentration range for all the substrates
was found to follow fractional order. However, the range of con-
centration where the substrate follows a fractional order on the
rate is not the same for all the substrates. This may be due to struc-
tural changes in substrate molecules and coordinating ability of the
catalyst towards different derivatives.
−
3
−3
−3
In the range 33.3 × 10 to 66.6 × 10 mol dm substrate con-
centration, the initial rate was independent and followed zero order
[
9]. This could be attributed to the fact that as the catalyst concen-
Scheme 3. Probable mechanism of hydrogenation.
tration is constant, all the catalyst would have been used and no
catalyst is available for higher concentration of the substrate.
Based on the results obtained, a probable mechanism is pro-
posed for the hydrogenation reaction (Scheme 3). The polymer
bound catalyst binds reversibly with the substrate. In the slow step,
the ␲-complex reacts with hydrogen to form a dihydrido complex
which finally gives the amine.
A rate law derived for the above mechanism is given in Eq. (1):
[
C]total
[S]total
1 + K[C]unreacted
rate = Kk1 1
×
[H2]
(1)
+ K[S]unreacted
where [C]total is the total concentration of catalyst initially taken,
C]unreacted is the concentration of unreacted catalyst at equilib-
[
rium, [S]total is the total concentration of substrate, [S]unreacted is
the concentration of unreacted substrate at equilibrium. K is the
equilibrium constant for reversible reaction and k1 is the veloc-
ity constant for the rate determining step. All the reactions were
carried out at a constant hydrogen pressure of 596 mm of Hg. There-
fore, Eq. (1) reduces to
[
C]total
[S]total
Fig. 7. Effect of temperature: Arrhenius plot.
rate = Kk1 1
×
+ K [S]unreacted
1 + K[C]unreacted
This equation explains that at low concentration, the reaction
follows fractional order with respect to catalyst and substrate con-
centration, but at higher concentration the rate is independent of
both.
of log(initial rate) against 1/T (Fig. 7). The results are tabulated
in Table 3. Results revealed that activation energy decreased after
substituting –NO , –OH, –Cl and –MeO groups on to the para posi-
2
tion of aldehydes ring of benzylideneaniline [28]. Lower activation
3
.3.3. Dependency of the rate on temperature
Table 3
Rate of hydrogenation reactions for all the substrates were
Activation energy, activation entropy values for hydrogenation.
◦
studied in the range of 30–45 C with a constant catalyst con-
−4
−3
centration of 59.2 × 10 mol dm Pd, substrate concentration of
Bza
34
p-NO2Bza
p-ClBza
p-OHBza
p-MeOBza
−
3
−3
3
3.3 × 10 mol dm in ethanol at 596 mmHg pressure of hydro-
Activation energy
(kJ/mol)
Activation entropy
31
9
24
12
gen. The rate of the reaction was found to be dependent on
temperature (T) of the system. Values of activation energy, acti-
vation entropy were calculated from the slope of Arrhenius plot
−200
−212
−286
−233
−276
(
J/K mol)

1
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V. Udayakumar et al. / Journal of Molecular Catalysis A: Chemical 317 (2010) 111–117
Table 4
Percentage conversion of various benzylideneanilines with supported palladium-imidazole complex catalyst.
Substrates
Products
Time (min)
Percentage conversiona
Fresh catalyst
After six recycles
Bza
N-Benzylaniline
60
100
80
100
60
72
88
78
97
97
67
84
71
83
97
p-ClBza
p-MeOBza
p-NO2Bza
p-OHBza
N-(4-Chlorobenzyl)aniline
N-(4-Methoxybenzyl)aniline
4-Aminobenzylaniline
4-(Anilinomethyl)phenol
a
Conversion based on the HPLC analysis.
−
3
−3 ◦
and temperature 30 C with 596 mmHg of
energy also indicates higher activity of the catalyst. The lower acti-
vation entropy indicates that the substrate molecules are bonded
to the catalyst surface and they have lost all translational degrees
of freedom and probably gained only three vibrational degrees of
freedom. Further, activation of the substrate with the catalyst may
be due to localization of the substrate on the catalyst surface and
this localized interaction may be necessary for the hydrogenation
reaction since a double bond (C N) involved is in the middle of the
substrate molecule.
33.3 × 10 mol dm
hydrogen pressure. The initial rate remained almost constant for
over six cycles without any loss in efficiency of the catalyst.
Percentage conversion of the substrates for fresh and recycled
catalyst is presented in Table 4. In the case of p-NO Bza, catalyst
did not show selectivity towards C N as both C N and nitro group
were hydrogenated simultaneously.
2
5. Conclusion
Polymer-supported palladium-imidazole complex could be
used for hydrogenation of C N group at ambient temperature and
pressure to synthesize corresponding amines. The catalyst was
3
.3.4. Effect of substituents
Effect of substituents on the rate of hydrogenation of benzyli-
deneaniline was studied by varying the para substituents with
different groups like –OH, –MeO, and –Cl [29]. It was observed that
the rate of hydrogenation was slower upon substitution with these
groups. When log(initial rate) was plotted against Hammett sub-
stituent constant (ꢁpara) it was a straight line with the slope of −0.99
◦
found to be stable up to 250 C as determined by TGA–DTA analysis.
It can be concluded from the Hammett plot, that the substituents
in para position of benzaldehyde ring of benzylideneaniline do not
have significant effect on rate of hydrogenation. However, the sub-
stitution at the para position reduces the activation energy. The
catalyst has an excellent recycling efficiency over six cycles without
leaching of metal from the polymer-support.
(
Fig. 8) indicates that the substituents do not have significant effect
on the rate of the reaction.
4
. Recycling efficiency of the catalyst
Acknowledgment
In order to test metal leaching from the catalyst, metal estima-
Authors wish to thank UGC for the DRS Programme, Ther-
max India Ltd., Pune, India, for providing the PS-DVB, Prof. B.K.
Sadashiva, Liquid Crystals Lab, RRI, Bangalore and CDRI, Lucknow
for C, H and N analyses and STIC, Cochin, for TGA.
V. Udayakumar thanks Sree Siddaganga Education Society, Sree
Siddaganga Math, Tumkur, Karnataka, India, for extending support
for doctoral studies.
tion was carried out at the end of first cycle and at the end of sixth
cycle of the reaction. Percentage of metal in the catalyst remained
constant in both the cases. Metal estimation for the reaction mix-
ture was also carried out to make sure that there is no loss of metal
from the catalyst. From this it is evident that there is no leaching of
metal from the catalyst.
Recycling ability of the catalyst was studied by carrying
out reactions over six cycles at a constant catalyst concen-
tration of 59.2 × 10−4 mol dm
−3
Pd, substrate concentration of
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