Layered double hydroxides supported nano palladium: An efficient catalyst for the chemoselective hydrogenation of olefinic bonds

Article abstract of DOI:10.1016/j.molcata.2012.08.017

Chemoselective hydrogenation of olefinic double bonds in the presence of various functional groups using layered double hydroxides supported nanopalladium (LDH-Pd⁰) catalyst is described. LDH-Pd⁰ was recovered quantitatively by simple filtration and reused several times with consistent activity and selectivity.

Full text of DOI:10.1016/j.molcata.2012.08.017

Journal of Molecular Catalysis A: Chemical 365 (2012) 115–119
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Layered double hydroxides supported nano palladium: An efficient catalyst for
the chemoselective hydrogenation of olefinic bonds
∗
Lakshmi Kantam M , Parsharamulu T, Manorama S.V.
Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology, Hyderabad 500007, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 June 2012
Received in revised form 18 August 2012
Accepted 20 August 2012
Available online 29 August 2012
Chemoselective hydrogenation of olefinic double bonds in the presence of various functional groups
using layered double hydroxides supported nanopalladium (LDH-Pd ) catalyst is described. LDH-Pd
was recovered quantitatively by simple filtration and reused several times with consistent activity and
selectivity.
0
0
©
2012 Elsevier B.V. All rights reserved.
Keywords:
Hydrogenation
Chemoselectivity
Layered double hydroxides
Palladium
Recyclability
1
. Introduction
bead for the hydrogenation of olefins [17]. Similarly, Zecca et al.
used the ion-exchange resins-supported palladium catalyst for the
same reaction [18]. Manorama et al. disclosed a promising cat-
alytic protocol for hydrogenation of nitro arenes, azides and olefins
employing Pd immobilized on the surface of amine-terminated
Fe O and NiFe O4 nanoparticles [19]. Dendrimer-encapsulated
palladium nanoparticles were also explored for the chemoselec-
tive hydrogenation of olefins with modest success [20]. All these
reports reveal the important requirement for an improved catalytic
system for the chemoselective hydrogenation of olefin in presence
of various sensitive functional groups.
Chemoselective hydrogenation of olefinic double bonds is an
indispensable tool for the synthesis of natural products, pharma-
ceuticals, and functional materials [1–3]. The most promising
method for performing these transformations are by employing
transition metals particularly palladium under catalytic conditions
using molecular hydrogen. Many of these processes were realized
using Pd/C in combination with various additives like pyridines,
amines, ammonium acetate, ammonia, and diphenylsulfide that
results in the selective reduction of olefinic bond in presence of aro-
matic halogens [4,5], aromatic carbonyls [4,5], epoxides [6], benzyl
3
4
2
Recently, we have reported the chemoselective hydrogenation
of olefinic bond using a Pd/Mg-La mixed oxide catalyst [21]. In
continuation to these studies, herein we report the chemoselec-
tive hydrogenation of olefinic double bonds at room temperature
with good to excellent yields using heterogeneous LDH-Pd0 cata-
lyst (Scheme 1). This protocol tolerates a wide variety of functional
group with excellent selectivity. Earlier, our group has reported
the synthesis and characterization of layered double hydroxides
[
7–10], benzyl esters [11], and N-Cbz protective groups [11]. Selec-
tive hydrogenations of substituted ␥-amino ␣,␤-unsaturated esters
were accomplished without affecting the benzyl ester or benzyl
ether functionality using Pd/C in EtOAc in absence of additives [12].
Notably, Felpin and Fouquet has reported the selective hydrogena-
tion of olefins by in situ generated Pd /C catalyst from Pd(OAc)2
and charcoal in methanol [13].
0
0
A chemoselective hydrogenation method was demonstrated by
Hirota et al. using finely dispersed palladium catalyst in silk-fibroin
supported nanopalladium catalyst (LDH-Pd ) and its activity in
Suzuki, Heck, Sonogashira, and Stille type coupling reactions of
chloroarenes [22]. LDHs have received much attention in view of
their potential usefulness as materials [23–26], anion exchangers
and more importantly as catalysts [27–34]. The LDH consists of
[
14]. In addition, hydrogenation of olefins was accomplished using
Pd nanoparticles in water-in-CO2 and water-in-oil microemulsion
as catalysts [15,16]. Ihm et al. showed the potential application of
Pd(II) complexes supported on a modified macroporous polymer
x+
n−
alternating cationic M(II)1 M(III)x (OH)2 and anionic A ·zH O
−x
2
layers [35]. The positively charged layers contain edge-shared
metal M(II) and M(III) hydroxide octahedra, with charges neu-
n−
∗ Corresponding author. Tel.: +91 40 2719 3510; fax: +91 40 2716 0921.
E-mail address: mlakshmi@iict.res.in (L.K. M).
tralized by A
anions located in the interlayer spacing or at
the edges of the lamellae. Small hexagonal LDH crystals with
1
381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2012.08.017

1
16
L.K. M et al. / Journal of Molecular Catalysis A: Chemical 365 (2012) 115–119
0
2
.15 mmol) and AlCl ·6H O (12.07 g, 0.05 mmol) was dissolved in
3
2
00 mL of deionized water. To this aqueous solution, 100 mL of
◦
NaOH (2 M) solution was slowly added at 25 C and further 2 M
NaOH solution was added to maintain a pH of 10 under nitrogen
flow. The resulting suspension was stirred overnight at 70 C. The
◦
solid product was isolated by filtration, washed thoroughly with
◦
deionized water and dried overnight at 80 C.
Scheme 1. Chemoselective hydrogenation of olefinic bonds over LDH-Pd0 catalyst.
2.3. Preparation of LDH-PdCl4
The preparation of LDH-PdCl4 was based on the literature pro-
cedure [22]. LDH-Cl (Mg:Al = 3:1, 1.5 g) was suspended in 150 mL
of aqueous Na PdCl (0.441 g, 1.5 mmol) solution and stirred at
2
4
Mg1 Alx(OH) (Cl)x·zH O composition were synthesized following
−x
2
2
◦
2
5 C for 12 h under nitrogen atmosphere. The solid catalyst was fil-
the existing procedures (here x = 0.25) [22].
tered, washed thoroughly with 500 mL of water and vacuum dried
to obtain 1.70 g of LDH-PdCl4.
2
. Experimental
2.1. General
2
.4. Preparation of LDH-Pd0
All the chemicals were purchased from Sigma Aldrich or Alfa
Aesar Company and used without further purification. Thin-layer
chromatography was performed on Merck-precoated silica gel
The preparation of LDH-Pd0 was based on the literature pro-
cedure [36]. LDH-PdCl4 (1 g) was reduced with hydrazine hydrate
(
1 g, 20 mmol) in ethanol (10 mL) for 3 h at room temperature, fil-
6
0-F254 plates. All the other solvents were obtained from com-
0
tered and washed with ethanol to give LDH-Pd as an air stable
black powder (0.89 mmol of Pd/g).
mercial sources and purified using standard methods. The particle
size and external morphology of the samples were observed on a
Philips TECNAI F12 FEI transmission electron microscope (TEM).
NMR spectra were recorded on a Bruker Avance (300 MHz), Var-
ian (500 MHz) spectrometer using TMS as an internal standard in
.5. Characterization of LDH-Pd0
2
CDCl . XPS spectra were recorded on a KRATOS AXIS 165 equipped
3
with Mg K␣ radiation (1253.6 eV) at 75 W apparatus using Mg K␣
anode and a hemi spherical analyser. The C 1s line at 284.6 eV was
used as an internal standard for the correction of binding ener-
gies. The X-ray diffraction (XRD) patterns of the fresh and used
samples were obtained on a Rigaku Miniflex X-ray diffractometer
using Ni filtered Cu K␣ radiation (ꢀ = 0.15406 nm), at a scan rate
The catalyst was well characterized by XRD, TEM and XPS anal-
ysis (see supporting information (SI)). The XRD patterns of the
0
fresh and used LDH-Pd catalysts (Fig. 1(a) and (b), see SI) revealed
the presence of both LDH (Mg–Al–Cl) and metallic Pd phases. The
diffraction peaks at 2Â = 40.21, 46.62 [ICSD # 87-0639] are corre-
sponding to the metallic Pd species and those at 2Â = 11.20, 22.41,
◦
−1
of 2 min , with the beam voltage and beam current of 30 kV and
3
4.51, 38.30, 60.20, 61.60 are due to the LDH support. Studies of XRD
1
5 mA, respectively.
pattern indicate that the catalyst and support maintain the crystal
structure after the reaction. Similarly the XPS data of catalyst shows
the characteristic peaks at 335.60 eV and 340.76 eV corresponding
to Pd 3d5/2 and Pd 3d3/2. Moreover, the position of the Pd phases
of the catalyst after the reaction is unchanged at 335.65 eV and
341.02 eV.
2.2. Preparation of Mg–Al–Cl (LDH) support
The preparation of LDH (Mg–Al–Cl) was carried out following
the reported procedure [34]. A mixture of MgCl ·6H O (30.49 g,
2
2
0
Fig. 1. TEM images of the LDH-Pd : (a) fresh catalyst, Inset shows the corresponding SAED pattern of Pd nanoparticle and (b) used catalyst.

L.K. M et al. / Journal of Molecular Catalysis A: Chemical 365 (2012) 115–119
117
Scheme 2. Chemoselective hydrogenation of (E)-chalcone over LDH-Pd0 catalyst.
Table 1
to 98% (Table 2, entries 1–21). The catalyst exhibited high activ-
ity in the case of non-functionalized aromatic olefins (Table 2,
entries 2–4, and 17). Thus styrene, 4-methyl styrene, trans-
stilbene and 2-vinylnaphthalene were successfully converted to
ethyl benzene, 1-ethyl-4-methylbenzene, 1,2-diphenylethane, and
2-ethylnaphthalene, respectively, with 92–95% yield. In case of 4-
chlorostyrene, only the olefinic double bond was reduced and the
formation of dehalogenated product was not observed (Table 2,
entry 5). Importantly, carbonyl moiety was not affected and
only olefinic double bond was reduced selectively in the hydro-
genation of cyclic ␣,␤-unsaturated ketones (Table 2, entries 7).
Moreover, sterically hindered tri substituted olefinic ketones such
as ionone (Table 2, entry 6), pulegone (Table 2, entry 8), and
isophorone (Table 2, entry 9) reduced selectively at the olefinic
bond to give corresponding alkanes with high yields. Hydrogena-
tion of ␣,␤-unsaturated aldehydes such as cinnamaldehyde and
Screening of reaction parameters for the hydrogenation of chalcone.a
Entry
Catalyst
Solvent
Time (h)
Yield (%)b
LDH-Pd0
1
2
3
4
5
6
7
8
Ethanol
Ethanol
Toluene
Methanol
Ethyl acetate
Water
2.5
10
4
4
4
10
5
2.5
96
96
96
96
96
82
94
0
LDH-Pd
LDH-Pd0
0
LDH-Pd
LDH-Pd0
0
LDH-Pd
LDH-Pdn
Ethanol
Ethanol
c
Pd/C
60
a
Reaction conditions: (E)-chalcone (2 mmol), 15 mg of LDH-Pd0 catalyst
(
0.67 mol% of palladium), solvent (10 mL), stirring under one atmosphere of hydro-
gen pressure at room temperature.
b
Isolated yields.
Commercial 5% Pd/C catalyst was used.
c
␣
-methyl-cinnamaldehyde proceeded chemoselectively to furnish
2
.6. General procedure for the hydrogenation of different
the desired products (Table 2, entries 10 and 11). In a similar way,
␣,␤-unsaturated carboxylic acids and esters underwent chemose-
lective hydrogenation of olefinic bond to afford the corresponding
acids and esters in good yields (Table 2, entries 13, 14 and 16).
Interestingly, 1-ethyl-4-methoxybenzene also hydrogenated selec-
tively without affecting the methoxy group (Table 2, entry 18).
Hydrogenation of diphenylacetylene gave dibenzyl product with
96% yield (Table 2, entry 19). For terminal and disubstituted alkenes
this catalytic system afforded good yields and selectivity. Finally,
we observed hydrogenation of olefinic double bond substrates with
nitro and nitrile functionalities. Mixture of products was obtained
in the case of 3-nitro styrene (Table 2, entry 21). Nitro group was
reduced to give 3-ethylamine with 75% yield. Under Pd/C-catalyzed
hydrogenation condition nitrile functionality is known as reducible
functionality. In this case only olefinic bond was reduced selec-
tively using LDH-Pd0 catalyst with E-cinnamonitrile as a substrate
unsaturated compounds
In a typical reaction, 0.015 g of catalyst and 2 mmol of the reac-
tant were taken in 10 mL of ethanol under hydrogen atmosphere.
The reaction was monitored by thin-layer chromatography (TLC).
After complete disappearance of the starting material, the catalyst
was separated by simple filtration and the solvent was removed
under reduced pressure to obtain the pure product.
3
. Results and discussion
3
.1. Screening of catalysts and solvents
0
To know the catalytic activity of LDH-Pd catalyst for the hydro-
genation reaction, we performed a reaction with (E)-chalcone using
ethanol as a solvent at room temperature under one atmosphere
hydrogen pressure (Scheme 2). Interestingly, the reaction pro-
ceeded with complete chemoselectivity and only the olefinic bond
was reduced. Further we studied various reaction parameters for
(Table 2, entry 15). Thus, this protocol tolerates a wide variety of
functional groups like keto, aldehyde, acids, esters, alcohol, halo-
gen, ether and cyano in the substrates.
0
hydrogenation of (E)-chalcone using LDH-Pd as a catalyst and
3.3. Recycling of the catalyst
molecular hydrogen as the reducing agent at room temperature to
develop a better catalytic system and the results are summarized
in Table 1.
The reusability of LDH-Pd0 catalyst was examined for the
chemoselective hydrogenation of methyl cinnamate. It is note-
worthy to mention that the LDH-Pd0 catalyst showed consistent
Next, we screened different solvents for the hydrogenation of
0
(
E)-chalcone (Table 1, entries 2–6). However, all the reactions pro-
activity and selectivity up to 5 cycles. After each cycle, LDH-Pd was
ceeded smoothly and gave good yields at longer reaction times.
Notably, similar result was obtained when LDH-Pd catalyst was
recovered by simple filtration, washed, air-dried and used directly
for the next cycle without further purification. All the cycles showed
consistent yield and selectivity for the hydrogenation reaction. The
Pd metal content of the fresh and used catalyst was found to be
9.48% and 9.32%, respectively, as measured by ICP AES, which sug-
gested that leaching was minimal.
The TEM images of the catalyst before and after the catalytic
reaction is shown in Fig. 1(a) and (b). The Pd nanoparticles seen as
dark particles are dispersed on the LDH support. Pd is confirmed
by the SAED (selected area electron diffraction) pattern as shown
in the inset of Fig. 1(a) and its corresponding d values agree with
the JCPDS data. Further as seen from the micrographs there is no
significant difference observed in the morphologies of fresh and
used catalysts.
II
used for the reaction (Table 1, entry 7). On the other hand only
moderate yields of the desired product was observed along with
the concomitant undesired side product (1,3-diphenylpropan-1-ol)
when commercially available 5% Pd/C was used as catalyst (Table 1,
entry 8).
3.2. Scope of the reaction
With the optimized reaction conditions in hand, the scope of
the reaction was extended to different unsaturated compounds
as summarized in Table 2. As shown, various unsaturated com-
pounds were reduced selectively with yields ranging from 85%

1
18
L.K. M et al. / Journal of Molecular Catalysis A: Chemical 365 (2012) 115–119
Table 2
Selective hydrogenation of different unsaturated compounds.
a
Entry
Substrate
Product
Time (h)
2.5
Yield (%)
96
Entry
12
Substrate
Product
Time (h)
2.5
Yield (%)
96
O
O
OH
OH
1
O
O
O
2
1.5
95
13
O
O
2
96, 92b
O
3
4
1
1
92
92
14
15
OH
OH
4
2
92
96
CN
CN
O
O
OH
OH
OH
OH
5
2
96
16
2
98
Cl
O
Cl
O
O
O
6
7
2.5
1.5
94
92
17
18
2.5
2.5
92
94
O
O
O
O
O
O
8
9
2
2
95
96
19
20
2
94
90
O
O
2.5
H
2
N
2
H N
O
O
1
0
1
H
H
2
94,
21
3
A = 75
B = 25
NH
2
NO
2
NO
2
A
B
O
O
1
H
H
2
94
a
Reaction conditions: substrate (2 mmol), 15 mg of LDH-Pd0 catalyst, ethanol (10 mL), stirring under one atmosphere hydrogen pressure at room temperature.
Yield at 5th cycle.
b
4
. Conclusion
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