Silica supported palladium phosphine as a robust and recyclable catalyst for semi-hydrogenation of alkynes using syngas

Article abstract of DOI:10.1016/j.molcata.2016.01.004

This work reports a chemo-selective semi-hydrogenation of alkynes to alkenes using silica supported palladium phosphine catalyst with syngas (CO/H₂). This developed methodology is an alternative to classical Lindlar catalyst for chemo-selective semi-hydrogenation of alkynes to alkenes. Various alkynes were smoothly convert to alkenes in 60-97% conversion with 85-98% selectivity. The prepared catalyst was well characterized by Field Emmission Gun Scanning Electron Microscopy (FEG-SEM), Energy Dispersive X-ray Spectroscopy (EDS), X-ray Photoelectron Spectroscopy (XPS), Inductively Coupled Plasma- Atomic Emmission Spectroscopy (ICP-AES) analysis techniques. In addition, catalyst was effectively recycled up to four consecutive run without significant loss in its catalytic activity and selectivity.

Full text of DOI:10.1016/j.molcata.2016.01.004

Journal of Molecular Catalysis A: Chemical 414 (2016) 78–86
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Silica supported palladium phosphine as a robust and recyclable
catalyst for semi-hydrogenation of alkynes using syngas
Samadhan A. Jagtapa, Takehiko Sasakib, Bhalchandra M. Bhanagea,∗
a Department of Chemistry, Institute of Chemical Technology, Matunga, Mumbai 400019, India
b Department of Complexity Science and Engineering, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa, Chiba
277-8561, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
This work reports a chemo-selective semi-hydrogenation of alkynes to alkenes using silica supported
palladium phosphine catalyst with syngas (CO/H2). This developed methodology is an alternative to clas-
sical Lindlar catalyst for chemo-selective semi-hydrogenation of alkynes to alkenes. Various alkynes were
smoothly convert to alkenes in 60–97% conversion with 85–98% selectivity. The prepared catalyst was
well characterized by Field Emmission Gun Scanning Electron Microscopy (FEG-SEM), Energy Dispersive
X-ray Spectroscopy (EDS), X-ray Photoelectron Spectroscopy (XPS), Inductively Coupled Plasma- Atomic
Emmission Spectroscopy (ICP-AES) analysis techniques. In addition, catalyst was effectively recycled up
to four consecutive run without significant loss in its catalytic activity and selectivity.
Received 20 November 2015
Received in revised form
19 December 2015
Accepted 5 January 2016
Available online 8 January 2016
Keywords:
Heterogeneous catalysis
Semi-hydrogenation
Palladium catalyst
Syngas
© 2016 Elsevier B.V. All rights reserved.
Alkynes
Alkenes
1. Introduction
[19], Rh [20,21], Ir [22] and Ru [23]. Despite the potential utility
of Lindlar catalyst (Pd/CaCO3) treated with Pb(OAc)2) suffers from
The synthesis of alkenes via chemo-selective semi-
hydrogenation of alkynes is an important transformation in
the organic synthesis [1,2]. Alkenes are essential intermediates in
the pharmaceutical drug moieties, natural products and biological
active compounds [3–5]. The styrene derivatives obtained by
semi-hydrogenation of phenylacetylene plays an important role
in the synthesis of essential monomer of polystyrenes, synthetic
rubbers and the various organic commodities [6]. In addition,
olefins are also one of the most important starting material for
various reactions such as Markovnikoffs addition, epoxidation,
ozonolysis, hydroboration, hydrogenation, hydroformylation
reactions, etc. [7–9]. The literature survey showed that, various
homogeneous catalytic systems were reported for the semi-
hydrogenation of terminal alkynes [10]. The first chemical reaction
of semi-hydrogenation of alkynes was reported by Heck in 1978
by using Pd/C with tri-alkyl-ammonium formate as a hydrogen
source [11,12]. Moreover, the various catalysts has been reported
for the conversion of alkynes into alkenes such as, Pd [13–18], Ni
drawbacks such as use of poisonous lead (Pb) salt and excess use
of quinoline to control the over hydrogenation, which limits in
the substrate scope and selectivity [24]. Heterogeneous catalysis
based on Pd and Ni metals has been extensively studied for
reduction of alkynes to Z-alkenes using H2 as a reducing source,
but it suffers from the over reduction of alkenes [25–27]. In recent
times, heterogeneous Palladium (Pd), Platinum (Pt) and Iron (Fe)
catalyst are reported for selective hydrogenation/dehydrogenation
reactions [28–30].
Recently, Wagh et al. reported the transfer semi-hydrogenation
of alkynes by using heterogeneous gold catalyst with HCOOH/NEt3.
In this method, HCOOH served as a hydrogen source and NEt3 as
an additive [31]. Alternatively, the number of lead-free catalytic
systems have been reported for semi-hydrogenation of alkynes in
literature [32–47]. Takahashi et al. also reported that lead free cat-
alytic system for the selective semi-hydrogenation of alkynes by
using silica supported Pd nanoparticles (PdNPs) with dimethyl-
sulfoxide (DMSO) as an additive [48]. Considering all above the
environmental issues, the development of the simple catalytic sys-
tems, additive free, co-catalyst free protocol is highly desirable.
The syngas is commercially produced by steam reforming or
partial oxidation technology of natural gas. It is extensively used for
the synthesis of ammonia, methanol, hydrogen and other impor-
∗
Corresponding author. Fax: +91 22 33611020.
E-mail addresses: bm.bhanage@ictmumbai.edu.in, bm.bhanage@gmail.com
(B.M. Bhanage).
http://dx.doi.org/10.1016/j.molcata.2016.01.004
1381-1169/© 2016 Elsevier B.V. All rights reserved.

S.A. Jagtap et al. / Journal of Molecular Catalysis A: Chemical 414 (2016) 78–86
79
R1
Pd(OAc)2Ph2PEt@SiO2
R2
R1
R2
R2
R1
2a-2u
Syngas (150 psi),
THF,70 oC,6 h
1a-1u
3a-3u
2%
R1 = phenyl or alkyl
R2 = H,alkyl, aryl,
heteroaryl etc.
Selectivity - 98%
Conversion - up to 97%
Additive Free !!
Co-Catalyst Free !!
Recyclable Catalyst !!
Wide substrate scope !!
Scheme 1. The semi-hydrogenation of terminal and internal alkynes.
Fig. 1. FEG-SEM images of Pd(OAc)2Ph2PEt@SiO2 fresh catalyst before the reaction.
as well as terminal alkynes which affords the respective alkenes
with 85–98% selectivity.
2. Experimental
2.1. Materials and methods
All the reactions were performed in inert conditions under the
nitrogen atmosphere by using 100 mL stainless steel high pres-
sure reactor. All the chemicals and reagents were purchased form
Sigma Aldrich, S.D. Fine and commercial suppliers. The solvents
were purchased from commercial suppliers and used without
further purification. The prepared Pd(OAc)2PPh2Et@SiO2 catalyst
was well characterized by using FEG-SEM, EDS, XPS and ICP-
AES spectroscopic analysis techniques. The loading of catalyst
was calculated by XRF measurements (SEA-2010, Seiko Electronic
Industrial Co., Japan). The XPS of Pd(OAc)2PPh2Et@SiO2 was mea-
sured using a PHI5000 Versa Probe with a monochromatic focused
(100 ␮m × 100 ␮m) Al K␣ X-ray radiation (15 kV, 30 mA) and dual
beam neutralization using a combination of Argon ion gun and
electron irradiation. Reaction monitor by using PerkinElmer Clarus
400 gas chromatography equipped with flame ionization detector
with a capillary column (Elite-1, 30 m × 0.32 mm × 0.25 ␮m). GC-
MS-QP 2010 instrument (Rtx-17, 30 m × F5 mm ID, film thickness
(df) = 0.25 ␮m) was used for the mass analysis of the prod-
ucts. Products were purified by column chromatography on silica
(120–200 mesh). Nuclear magnetic resonance spectra were taken
on Bruker in CDCl3 solvent (1H, 400 MHz; 13C, 100 MHz) spec-
trometer using tetramethylsilane (1H) as an internal standard. The
chemical shifts values are reported in parts per million (␦) relative
to tetramethylsilane as an internal standard. The J (coupling con-
Fig. 2. EDS image of fresh Pd(OAc)2Ph2PEt@SiO2 catalyst.
tant products in the industries [49]. Generally, the syngas is widely
used in the well known hydroformylation reaction for the conver-
sion of olefins to aldehydes [50–53]. Takahashi et al. disclosed the
use of syngas for the hydrogenation of aldehydes to alcohols by
using ruthenium as a catalyst [54]. However, the important class
of semi-hydrogenation reactions has not yet been reported using
syngas as hydrogen source.
Herein, we first time report the use of syngas as hydrogen source
for the selective semi-hydrogenation of alkynes to alkenes using
silica-supported palladium-phosphine as a heterogeneous catalyst
(Scheme 1). The developed method tolerates wide range of internal

80
S.A. Jagtap et al. / Journal of Molecular Catalysis A: Chemical 414 (2016) 78–86
Fig. 3. Recyclability study of the Pd(OAc)2PPh2Et@SiO2 catalyst.
Fig. 4. XPS images of the Pd(OAc)2PPh2Et@SiO2 catalyst (a) wide scan, (b) Pd 3d region, (c) P2p region, (d) P2s region.
stant) values were described in Hz. Splitting patterns of proton are
depicted as s (singlet), d (doublet), t (triplet) and m (multiplet). The
3.0 g of PPh2Et/SiO2 (2-diphenylphosphinoethyl functionalized sil-
ica, 0.7 mmol/g) acquired from Aldrich were mixed properly, stirred
and refluxed with acetonitrile under nitrogen atmosphere for 48 h.
The material showing black was filtered with 0.45 ␮m PTFE filter
and washed several times with acetonitrile then dried at room tem-
perature. The metal loading of Pd(OAc)2PPh2Et@SiO2 was 6.4 wt%
as determined by XRF measurements (SEA-2010, Seiko Electronic
Industrial Co., Japan).
products were confirmed by the comparison of their GC, GC-MS, 1
and 13C NMR spectra with those of authentic data.
H
2.2. Synthesis and characterization of the catalyst
2.2.1. Synthesis of silica supported Pd(OAc)2PPh2Et catalyst
The procedure for the silica supported Pd(OAc)2PPh2Et cata-
lyst is: in a 100 mL round bottom flask 0.472 g of Pd(OAc)2 and

S.A. Jagtap et al. / Journal of Molecular Catalysis A: Chemical 414 (2016) 78–86
81
Table 1
The study of catalyst screening and loading for semi-hydrogenation of Phenyl acetylene (1a) to styrene (2a).a
Entry
Catalyst
Catalyst loading (mol%)
Conversion of 1a (%)a
Selectivity of 2a/3a (%)b
1
2
3
4
5
6
7
8
Pd(OAc)2
PdCl2(PPh3)2
Pd(OAc)2Ph2PEt@SiO2
Pd/C (10%)
PdBr2
Pd(OAc)2Ph2PEt@SiO2
Pd(OAc)2Ph2PEt@SiO2
Pd(OAc)2Ph2PEt@SiO2
Pd(OAc)2Ph2PEt@SiO2
Pd(OAc)2Ph2PEt@SiO2
No catalyst
0.50
0.50
1.28
3.00
1.20
0.50
0.64
0.96
1.60
1.92
–
80
40
98
90
67
90
95
97
99
100
–
85/15
90/10
92/08
88/12
76/24
92/08
95/05
98/02
90/10
82/18
−/−
9
10
11
a
Reaction Conditions: 1a (1 mmol), syngas (150 psi), THF (10 mL), temp. (80 ◦C), time (8 h).
GC Yield.
b
Table 2
Optimization study of semi-hydrogenation of phenyl acetylene (1a).a
Entry
CO/H2 (psi)
Time (h)
Temperature (◦C)
Solvent
Conversion of 1a (%)a
Selectivity of 2a/3a (%)b
Effect of syngas pressure
1
2
3
4
1/1(300)
1/1(200)
1/1(150)
1/1(100)
2/1(150)
1/2(150)
1/0(150)
0/1(150)
10
10
10
10
10
10
10
10
80
80
80
80
80
80
80
80
THF
THF
THF
THF
THF
THF
THF
THF
82
85
98
94
60
91
–
89/11
95/05
97/03
92/08
71/29
79/21
−/−
5
6
7c
8d
100
−/100
Effect of time
9
10
11
1/1(150)
1/1(150)
1/1(150)
8
6
4
80
80
80
THF
THF
THF
97
97
94
97/03
98/02
97/03
Effect of temperature
12
13
14
15
1/1(150)
6
6
6
6
60
70
90
100
THF
THF
THF
THF
92
97
98
100
95/05
98/02
90/10
85/15
1/1(150)
1/1(150)
1/1(150)
Effect of solvent
16
17
18
19
20
1/1(150)
1/1(150)
1/1(150)
1/1(150)
1/1(150)
6
6
6
6
6
70
70
70
70
70
1,4 Dioxane
EtOH
MeOH
Toluene
DMF
99
89
86
79
92
85/15
92/08
89/11
90/10
94/06
a
Reaction Conditions: 1a (1 mmol), solvent (10 mL), Pd catalyst (0.96 mol%), 400 rpm.
GC Yield.
Reaction under CO atm.
Reaction under H2 atm.
b
c
d
2.2.2. Characterization of prepared silica supported
Pd(OAc)2PPh2Et catalyst
The energy dispersive X-ray spectrum (EDS) was recorded by
using an INCA x-act Oxford instrument (Model 51-ADD0007). The
EDS analysis shows the elements present in the material. EDS anal-
ysis of fresh Pd(OAc)2Ph2PEt@SiO2 catalyst shows the presence of
Si, P, O, Pd elements (Fig. 2).
The silica supported Pd(OAc)2PPh2Et catalyst was character-
ized by FEG-SEM, EDS, XPS, ICP-AES analysis techniques. The
morphology of the samples was examined using field emission gun-
scanning electron microscopy (FEG-SEM) analysis (Tescan MIRA 3
model). The FEG-SEM analysis shows the surface morphology of
the material. The as synthesized fresh Pd(OAc)2Ph2PEt@SiO2 cata-
lyst shows the flakes like surface morphology. The FEG-SEM images
(Fig. 1) display the presence of rough surface of catalyst indicating
availability of high surface area to the reactants to react on the
surface of catalyst.
2.3. General experimental procedure for synthesis of alkenes via
semi-hydrogenation of alkynes
To a 100 mL high-pressure reactor, phenylacetylene (1 mmol),
Pd (0.96 mol%) in a 10 mL THF were transferred under an inert
atmosphere. The reactor was flushed three times with nitrogen

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S.A. Jagtap et al. / Journal of Molecular Catalysis A: Chemical 414 (2016) 78–86
Table 3
Effect of catalyst loading study for low conversion (ideally around 20%).
Entry
Catalyst in mg (mol%)
Conversion of 1a (%)a
Selectivity 2a/3a (%)b
1
2
3
4
5
6c
10 (0.6 mol%)
8 (0.48 mol%)
6 (0.36 mol%)
4 (0.24 mol%)
2 (0.12 mol%)
2 (0.12 mol%)
95
89
80
70
60
19
95/5
92/8
91/9
93/7
95/5
94/6
a
Reaction Conditions: 1a (1 mmol), syngas (150 psi), THF (10 mL), temp. (70 ◦C), time 6 h.
GC Yield.
time 2 h.
b
c
Table 4
ous homogeneous and heterogeneous Pd catalysts (Table 1, entries
1–5). It was observed that Pd(OAc)2Ph2PEt@SiO2 provided high
conversion of 1a with excellent selectivity toward the formation
of styrene (2a) (Table 1, entry 3). The Pd/C (10%) catalyst provides
good conversion for semi-hydrogenation reaction of 1a but selec-
tivity of 2a was less than prepared Pd(OAc)2Ph2PEt@SiO2 catalyst
(Table 1, entry 4). With increase in the catalyst loading, a significant
increase in conversion, but selectivity of 2a drastically decreases
(Table 1, entries 8, 9 and 10). The formation of products (2a and 3a)
was not observed in absence of the catalyst (Table 1, entry 11) it
indicates that catalyst is solely responsible for the reaction.
Furthermore, in order to increase the conversion of 1a and selec-
tivity of 2a, we investigated the effect of syngas (CO/H2) pressure on
the model reaction at 80 ◦C for 10 h in THF as solvent. At 300 psi of
syngas pressure reaction provides conversion (82%) and selectivity
(89%) of desired product 2a (Table 2, entry 1). It was observed that,
decrease in the syngas pressure at 200 psi, the conversion (85%) and
selectivity (95%) of 2a increases (Table 2 entry 2). It was noted that
both the conversion as well as selectivity was better at 150 psi pres-
sure of syngas (Table 2, entry 3). The decrease in syngas pressure
below 150 psi the conversion of starting material and selectivity
was less (Table 2, entry 4).
Moreover, we studied the effect of relative stoichiometric ratio
of carbon monoxide (CO) and hydrogen (H2) gas (Table 2, entries
5–8). As the relative concentration of CO with respect to H2
increases, the conversion of 1a and selectivity of corresponding
product 2a decreases (Table 2, entry 5). On the other hand, with
increase in the concentration of H2 with respect to CO, the con-
version of 1a increases but considerably decreases the selectivity
toward 2a (Table 2, entry 6). Therefore, we kept equal ratio (1:1) of
CO and H2 gas as it provides the high conversion of 1a with excellent
selectivity toward 2a (Table 2, entry 3). These results clearly indi-
cate that the syngas (CO/H2) favors this transformation. No reaction
occurs only in presence of CO atmosphere (Table 2, entry 7). At
150 psi pressure of H2 gas, 100% conversion of 1a takes place but
instead of 2a complete hydrogenation product 3a was observed
(Table 2, entry 8).
Effect of temperature study for low conversion (ideally around 20%).
Entry
Temperature ( ◦C)
Conversion of 1a (%)a
Selectivity 2a/3a (%)b
1
2
3
4
70
60
50
40
30
30
30
97
92
23
21
NR
NR
NR
98/2
95/5
94/6
93/7
–
5
6c
7d
–
–
a
Reaction Conditions: 1a (1 mmol), catalyst (15 mg), syngas (150 psi), THF
(10 mL), time (6 h).
b
GC Yield.
Reaction time 4 h.
Reaction time 2 h.
c
d
then pressurized with desired 150 psi of syngas, then heated at
70 ◦C with constant stirring (400 rpm) for 6 h. After the comple-
tion of reaction, the reactor cooled down to room temperature and
the remaining syngas was carefully depressurized. The resultant
reaction mixture filtered off by simple filtration. The filtrate was
then collected in sample vial and the product was extracted for fur-
ther analysis such as GC, GC–MS, 1H & 13C NMR and matched with
those of authentic data. Selective experiments were performed in
triplicate and it was observed that results showed variation of 2%.
2.4. General experimental procedure for recycling of
Pd(OAc)2Ph2PEt@SiO2 catalyst
After the completion of reaction, the reaction mixture was
cooled to room temperature and the catalyst was recovered by
simple filtration. The filtered catalyst was washed with methanol
(3 × 5 mL) to remove all traces of product or reactant present. The
filtered catalyst was then dried under reduced pressure for 5 h. The
dried catalyst was then used for the next run for the recyclability
experiment of semi-hydrogenation of alkynes.
3. Results and discussion
Next, we investigated that the effect of time, temperature and
solvent for the effective progress of the reaction. The optimum reac-
tion time could be reduced to 6 h (Table 2, entry 10). Decreasing the
reaction temperature beyond 6 h lead to a significant decrease in
the conversion and selectivity of product 2a (Table 2, entry 11). Fur-
ther, we checked the effect of temperature on the reaction and it
was found that the conversion of 1a increases with increase in the
reaction temperature from 60 ◦C to 90 ◦C, but a gradual decrease in
the selectivity of 2a was also noted (Table 2, entries 3 and 12–14).
Increasing the reaction temperature beyond 90 ◦C did not show
any significant effect on the conversion, but drastically decreases
the selectivity of 2a (Table 2, entry 15). The temperature of 70 ◦C
was the optimized reaction temperature for the reaction to get the
excellent selectivity of product 2a (Table 2, entry 13). Moreover,
the effects of different solvents were investigated for the reaction.
Various polar and non polar solvents such as THF, 1,4-dioxane,
Initially, our aim was to develop a heterogeneous catalytic sys-
tem for the hydroformylation of terminal alkynes to the synthesis
of ␣,␤-unsaturated aldehydes. Under the hydroformylation reac-
tion conditions phenylacetylene provided an unexpected results
by using as prepared Pd(OAc)2Ph2PEt@SiO2 catalyst i.e., instead
of hydroformylation the semi-hydrogenated styrene obtained as
major product with trace amount of complete hydrogenation prod-
uct. This unexpected result inspired us to focus on the selective
semi-hydrogenation of alkynes for the synthesis of alkenes. The
semi-hydrogenation of phenylacetylene to styrene was chosen
as model reaction for optimization study. The various reaction
parameters such as catalyst screening, loading, the effect of syngas
(CO/H2) pressure, time, temperature and solvents were studied for
the selective semi-hydrogenation of phenylacetylene (1a) and the
results are summarized in Tables 1 and 2. Firstly, we screened vari-

S.A. Jagtap et al. / Journal of Molecular Catalysis A: Chemical 414 (2016) 78–86
83
Table 5
Substrate study for semi-hydrogenation of alkynesa.
Entry
Products 2a–2u
Conversion of 1a–1u (%)a
Selectivity 2a–2u/3a–3u (%)b
Terminal alkynes
1
2
97
94
98/2
96/4
3
92
97/3
4
5
93
94
94/6
92/8
6
7
90
92
92/8
94/6
8
9
90
85
92/8
94/6
10
89
85/15
11
12
97
95
98/2
98/2
13
14
84
82
85
91/9
90/10
92/8
15
Internal alkynes
16c
60
98/2

84
S.A. Jagtap et al. / Journal of Molecular Catalysis A: Chemical 414 (2016) 78–86
Table 5 (Continued)
Entry
17
Products 2a–2u
Conversion of 1a–1u (%)a
Selectivity 2a–2u/3a–3u (%)b
89
96/4
18
92
94
87/13
96/4
19
20
92
90
95/5
95/5
21
a
Reaction Conditions: alkyne (1 mmol), THF (10 mL), Pd catalyst (0.96 mol%), 400 rpm, CO/H2 (150 psi), Time (6 h).
b
c
GC Yield.
Time (12 h), Temp. (100 ◦C).
ethanol, methanol, toluene and DMF were screened (Table 2, entries
4. Catalyst reusability
3, 16–20). Among them, THF was found to be a best solvent which
provided high conversion and an excellent selectivity toward 2a
(Table 2, entry 13).
We studied the effect of catalyst loading for low conversion (ide-
ally around 20%), the loading of 0.6 mol% Pd(OAc)2Ph2PEt@SiO2
catalyst provides high conversion up to 95% (Table 3, entry 1). We
decrease the catalyst loading gradually from 0.6 mol% to 0.12 mol%
to check the reaction performance for low conversion (Table 3,
entries 1–6), the catalyst loading of 0.12 mol% provides low con-
version of 19% at 2 h (Table 3, entry 6).
We also studied the effect of reaction temperature for the little
conversion about 20%. The temperature of 70 ◦C and 60 ◦C affords
high conversion of 1a about 97% and 92% respectively (Table 4, entry
1 and 2). But a sudden change from high to low conversions of 1a
was observed 23% and 21% when temperature decreased at 50 ◦C
and 40 ◦C respectively (Table 4, entries 3 and 4).
These optimized reaction conditions were further applied to
investigate the wide substrate applicability of this protocol. Pheny-
lacetylene bearing electron-donating group such as Me, OMe,
The recyclability study of Pd-catalyst was examined for the syn-
thesis of styrene by semi-hydrogenation of 1a with synthesis under
the optimized reaction conditions. After completion of reaction,
the reactor was cooled to room temperature and remaining syngas
vented out carefully. The catalyst was recovered by simple filtration
and then washed three times with THF and finally by methanol to
remove traces of organic contents. The resulting catalyst was dried
under vacuum for 5 h and could be recycled up to four consecutive
cycles (Fig. 3). In order to prove that there was no catalyst leaching,
the mother liquor were examined for each run by ICP-AES tech-
nique. The results indicates that the palladium in the mother liquor
were below to the detectable level.
The XPS analysis was performed for the fresh, first and fourth
recycled catalyst as shown in Fig. 4. The wide scan of the fresh and
recycled Pd(OAc)2Ph2PEt@SiO2 catalyst [Fig. 4(a)] which indicates
the presence of P, C, Pd, O, and Si elements in the catalyst. The XPS
spectra of Pd(OAc)2Ph2PEt@SiO2 catalyst shows that two peaks at
334.6 and 340.5 eV for fresh Pd(OAc)2Ph2PEt@SiO2 are assigned as
3d5/2 and 3d3/2 for Pd2+ species, respectively [Fig. 4(b)]. For the first
recycle sample, no change in peaks appear at 334.5 and 340.5 eV,
which are assigned as 3d5/2 and 3d3/2 for Pd2+ species, respectively,
indicating that the no change in oxidation state of Pd during the
catalytic reaction. From the spectrum of the fourth recycle sample,
it is obvious that the component of Pd2+ is slightly decreasing upon
recycles, although the component of Pd2+ remains constant. The
peak at 129.8 eV corresponds to the P2p recognized to the presence
of phosphine in the catalyst shown in Fig. 4(c). The peak at 192.5 eV
corresponds to the P2s recognized to the presence of phosphine
in the catalyst shown in Fig. 4(d). These results indicate that the
coordination bond between Pd and phosphine are formed and they
are still remains on silica support even after 4th recycle.
NMe2, NH2, OH (2b–2 g) and electron withdrawing group
F
(2 h) resulted into an excellent selectivity toward corresponding
alkenes (Table 5, entries 2–8). The alkyne containing heteroatom
(1i) could also be reduced successfully providing 94% selectively of
the desire product 2i (Table 5, entry 9). The terminal alkynes (2j–2o)
were easily reduced to corresponding products with 85–98% selec-
tivity (Table 5, entries 10–15). To our delight, aromatic and aliphatic
internal alkynes can be smoothly converted in to the respec-
tive alkenes providing 87–98% selectivity. The alkyne 1p provided
moderate conversion but selectivity of 2p is excellent at high tem-
perature (100 ◦C) for longer reaction time of 12 h (Table 5, entry
16). The various aromatic and aliphatic internal alkynes (1q–1u)
were reduced to corresponding alkenes providing moderate to high
conversion as well as selectivity (Table 5, entry 17–21).
The SEM images of fresh, 1st recycled and 4th recycled
Pd(OAc)2Ph2PEt@SiO2 catalysts are shown in (Fig. 5), which shows
the surface morphology of the catalyst. The FEG-SEM analy-

S.A. Jagtap et al. / Journal of Molecular Catalysis A: Chemical 414 (2016) 78–86
85
Fig. 5. FEG-SEM images of (a) fresh, (b) 1st recycled and (c) 4th recycled Pd(OAc)2PPh2Et@SiO2 catalyst.
sis of fresh and recycled catalyst points the Pd species present
on the surface of catalyst. After the 4th recycled the palla-
dium metal slightly agglomerized on the surface of the catalyst
[Fig. 5(c)]. The SEM images of fresh, 1st recycled and 4th recy-
cled Pd(OAc)2Ph2PEt@SiO2 catalysts with metal mapping acquired
using EDX analysis are shown in Fig. S2, 4 and 6 respectively (please
see in the supporting information). It shows that the Pd species is
well dispersed on the surface of catalyst.
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In conclusion, we have developed a simple, efficient, heteroge-
neous Pd(OAc)2Ph2PEt@SiO2 catalyst and used for the synthesis of
alkenes via semi hydrogenation reaction of alkynes using syngas
as a hydrogen source. In the present protocol, the syngas contains
an equal ratio of carbon monoxide and hydrogen gas is used as a
controlling system to prevent the rate of over hydrogenation. The
present catalytic system eliminates the use of poisonous metals,
additives, co-catalysts and excess amount of chemicals. This devel-
oped method tolerates wide range of internal as well as terminal
alkynes and affords the respective alkenes from moderate to high
conversion (60–97%) with good to excellent selectivity (85–98%).
The catalyst could be easily recovered and shows significant recy-
clability up to the fourth run without loss of its catalytic activity.
Acknowledgments
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The Author S.A. Jagtap thanks to the Council of Scientific and
Industrial Research (CSIR) India, for providing Junior Research
Fellowship (JRF). The XPS measurements were conducted at the
Research Hub for Advanced Nano Characterization, the University
of Tokyo, supported by the Ministry of Education, Culture, Sports,
Science and Technology (MEXT), Japan.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.molcata.2016.01.
004.
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