Selective C=C Hydrogenation of Unsaturated Hydrocarbons in Neat Water Over Stabilized Palladium Nanoparticles Via Supported 12-Tungstophosphoric Acid

Article abstract of DOI:10.1007/s10562-019-02763-1

Stabilized Pd(0) nanoparticles by supported 12-tungstophosphoric acid (Pd(0)-TPA/ZrO₂) was explored as a sustainable recyclable catalyst for selective C=C hydrogenation of cyclohexene and crotonaldehyde. The catalyst shows an outstanding performance [catalyst to substrate ratio (1:1.31 × 10⁴)] towards high conversion as well as 100% selectivity of the desired product with high turnover number (> 10,000) and turnover frequency (> 2600?h^-1) for both the systems. The use of neat water as a solvent and mild reaction conditions makes the present system environmentally benign and green. Moreover, the catalyst could be recovered and reused up to five cycles without any significant loss in their conversion as well as selectivity. The viability of the catalyst was evaluated towards different aromatic as well as aliphatic arenes and found to be excellent in all the cases. The obtained selectivity, especially butyraldehyde, was correlated with the nature of the catalyst as well as solvent and based on the study, a plausible mechanism for both the reactions was also proposed. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-019-02763-1

Catalysis Letters (2019) 149:1476–1485
https://doi.org/10.1007/s10562-019-02763-1
Selective C=C Hydrogenation of Unsaturated Hydrocarbons in Neat
Water Over Stabilized Palladium Nanoparticles Via Supported
12‑Tungstophosphoric Acid
Anish Patel1 · Anjali Patel1
Received: 8 January 2019 / Accepted: 24 March 2019 / Published online: 4 April 2019
©
Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract
Stabilized Pd(0) nanoparticles by supported 12-tungstophosphoric acid (Pd(0)-TPA/ZrO ) was explored as a sustainable
2
recyclable catalyst for selective C=C hydrogenation of cyclohexene and crotonaldehyde. The catalyst shows an outstanding
4
performance [catalyst to substrate ratio (1:1.31 × 10 )] towards high conversion as well as 100% selectivity of the desired
−
1
product with high turnover number (>10,000) and turnover frequency (>2600 h ) for both the systems. The use of neat
water as a solvent and mild reaction conditions makes the present system environmentally benign and green. Moreover, the
catalyst could be recovered and reused up to ꢀve cycles without any signiꢀcant loss in their conversion as well as selectivity.
The viability of the catalyst was evaluated towards diꢁerent aromatic as well as aliphatic arenes and found to be excellent
in all the cases. The obtained selectivity, especially butyraldehyde, was correlated with the nature of the catalyst as well as
solvent and based on the study, a plausible mechanism for both the reactions was also proposed.
Graphical Abstract
Keywords Selective C=C hydrogenation · Crotonaldehyde · Cyclohexene · Neat Water · Pd nanoparticles · Supported
1
2-tungstophosphoric acid
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10562-019-02763-1) contains
supplementary material, which is available to authorized users.
1
Introduction
Selective C=C catalytic hydrogenation is a diverse and ver-
satile acceptable route to synthesize precursors for various
intermediates [1] and still is fascinating and challenging
*
Anjali Patel
anjali.patel-chem@msubaroda.ac.in
Anish Patel
ꢀ
eld. Cyclohexane, a product of cyclohexene C=C hydro-
anish.patel-chemphd@msubaroda.ac.in
genation, has immense importance for its uses as solvent
and to synthesize mainly cyclohexanol and cyclohexanone,
which are the chemical commodities to produce adipic acid,
1
Polyoxometalates and Catalysis Laboratory, Department
of Chemistry, Faculty of Science, The Maharaja Sayajirao
University of Baroda, Vadodara, India
V
1
ol:.(1234567890)
3

Selective C=C Hydrogenation of Unsaturated Hydrocarbons in Neat Water Over Stabilized Palladium…
1477
caprolactam, nylon 6 and nylon 66. Similarly, n-butyralde-
hyde, one of the selective products of crotonaldehyde C=C
hydrogenation, is the chemical intermediate for n-butyl alco-
hol, 2-ethylhexanol, trimethylolpropane (TMP), polyvinyl
butyral (PVB), pharmaceuticals, pesticides, antioxidants,
ꢂavors, synthetic perfumes, vulcanization accelerators, crop
protecting agent and many more.
crotonaldehyde in harsh conditions, was carried out either
in organic solvents or in aqueous solvents. In addition, no
study has been carried out in which neat water was used
as a solvent. It would be interesting and environmentally
benign if hydrogenation reactions would be carried out in
neat water.
Number of methods are available in literature for sta-
bilization of Pd(0) nanoparticles with diꢁerent stabilizing
agents [10–24], more recently Scarso et al. also reported
anionic surfactant stabilized Pd nanoparticles for selec-
tive hydrogenations and dechlorinations in neat water [25],
as well as polyoxometalates (POMs) [26–29]. Amongst
these, POMs are very fascinating and excellent as stabi-
lizing agents due to their number of convergent properties
such as robust oxoanionic nature, their negative charges,
large relative sizes, stability, reducing and encapsulating
agent [26–29]. However, only two reports are available
in which palladium and supported heteropolyacid were
used for carrying out hydrogenation reaction. First, in
2002, Newmann and co-workers synthesized palladium
substituted polyoxometalate (K PPdW O /C) as pre-
Due to the importance of these products, number of
groups have been working on the same and as a result over
the last few years, hydrogenation using Pd(0) nanoparticles
as catalysts, has gained lot of interest because of their great
eꢃciency towards the selective hydrogenation of C=C bond.
However, C=O selectively can be reduced by either design-
ing the suitable bimetallic catalyst in which one of the metals
having oxidic species (acidic sites) would attract the lone
pair of the carbonyl group, or using the alcohols as preferred
solvents which can easily activate the C=O bond. However,
use of alcohols can arise the undesirable secondary eꢁect
such as formation of acetals and hemiacetals [2].
Cyclohexene hydrogenation reaction on the basis of sup-
ported Pd(0) nanoparticles had been carried out by various
groups such as, in 2009, Liu et al. immobilized Pd nanopar-
ticles on sepiolite (SH-IL-1.0 wt% Pd) and carried out the
reaction in stainless steel autoclave with a magnetic stirrer
5
11 39
catalyst for hydrogenation of aromatic compounds [30]
and second, in 2013, Parida et al. reported palladium based
lacunary phosphotungstate supported mesoporous silica
(50LPdW/MCM-41) for hydrogenation of p-nitrophenol to
p-aminophenol at room temperature [31]. In both of these
reports the use of Pd(0) nanoparticles had not been men-
tioned. No report is available in the art, in which supported
Pd(0) nanoparticles stabilized polyoxometalate were used
for the hydrogenation of unsaturated hydrocarbon.
to obtain 99% conversion in 1 h at 60 °C under 2 Mpa H
2
pressure [3]. In 2014, Zhang group immobilized Pd nanopar-
ticles in a microporous/mesoporous composite (Pd/MSS@
ZIF-8) to obtain 5.6% conversion at 35 °C in 6 h using ethyl
acetate as a solvent [4]. In 2016, Sapi et al. synthesized
mesoporous carbon-supported Pd nanoparticles (Pd/CNT)
and performed hydrogenation in a continuous ꢂow reactor
system at 40 °C, where the ratio of cyclohexene: hydrogen:
Rrecently we have published, Suzuki-Miyaura cross
coupling reaction in aqueous medium as well as in neat
water using stabilized Pd(0) nanoparticles by zirconia sup-
−1
nitrogen was 1: 10: 90 with a total ꢂow rate of 101 mL min
at 1 bar [5]. In 2017, Leng et al. immobilized palladium nan-
oparticles on nitrogen-doped carbon (Pd@CN) to get 96%
conversion in 12 h at 90 °C using formic acid as solvent [6].
As per our knowledge, there are only three reports
on hydrogenation of crotonaldehyde to butyraldehyde
using supported Pd (0) nanoparticles. In 2003, Zhao et al.
reported carbon supported palladium nanoparticles (Pd/C)
to achieve 100% conversion with 100% selectivity using
ported 12-tungstophosphoric acid (Pd(0)-TPA/ZrO ) [32].
2
The obtained excellent catalytic activity encouraged us
to extend our work for another important and challeng-
ing transformation i.e. hydrogenation of cyclohexene and
crotonaldehye.
In the present paper, eꢃciency of the catalyst was evalu-
ated for hydrogenation of cyclohexene and crotonaldehyde
in neat water by varying diꢁerent parameters such as catalyst
amount, time, temperature, pressure and eꢁect of solvents
were studied for maximum conversion as well as selectivity
of the desired product. Under optimized condition, hetero-
geneity test, control experiment as well as catalytic activity
of recycled catalyst were performed. Regenerated catalyst
was characterized by EDS, FT-IR and XPS techniques to
conꢀrm its stability. Plausible reaction mechanism for both
the reactions was also proposed.
supercritical CO as a solvent at 100 °C. Here, the reac-
2
tion was carried out under very harsh condition i.e. 80 bar
pressure [7]. In 2005, Iwasa et al. reported the synthesis
of palladium nanoparticles supported on CeO (Pd/CeO )
2
2
and achieved 100% conversion with 40% selectivity of
butyraldehyde at 200 °C using ꢀx bed reactor [8]. After
8
years, in 2013, Harraz et al. reported the synthesis of
stabilized palladium nanoparticles by polyethylene glycol
(
Pd/PEG) to obtain 100% conversion and 100% selectivity
of butyraldehyde in 90 min at room temperature [9].
Thus, literature survey shows that hydrogenation
of the unsaturated compounds, especially in case of
1
3

1478
A. Patel, A. Patel
2
Experimental
2.4 Catalytic Activity
2
.1 Materials
The catalytic reaction was carried out using Parr reactor
instrument having three major components: The batch type
All chemicals used were of A.R. grade. 12-tungstophos-
phoric acid, zirconium oxychloride, ammonia, palladium
chloride, cyclohexene, crotonaldehyde and dichlorometh-
ane were obtained from Merck and used as received.
reactor of 100 mL capacity is made up of SS-316, H res-
2
ervoir with electronic temperature and pressure controller.
For example, in typical reaction, 9.87 mmol of cyclohex-
ene with 50 mL of water as solvent and 20 mg of catalyst
were charged to the reactor vessel. The reactor was ꢂushed
thrice with H gas to remove the air present in the empty
2
2.2 Catalyst Synthesis
part of the vessel. Finally, 10 bar H pressure was applied
2
for the reaction. The reaction was set at 80 °C with the stir-
ring rate of 1700 rpm for 4 h. The continuous decrease in
pressure inside the vessel was utilized for determination of
the reaction progress. After reaction completion, the reac-
Zirconia (ZrO ) [33] and Zirconia supported 12-tung-
2
stophosphoric acid (TPA/ZrO ) [34] was synthesized by fol-
2
lowing the same method reported by us. Pd(0) nanoparticles
were deposited on supported 12-tungstophosphoric acid via
tion mixture was cooled to room temperature and then H
2
exchanging the available protons of TPA/ZrO [32]. 1 g of
pressure was released from the vent valve. The organic layer
was extracted using dichloromethane, while catalyst was
collected from the junction of the liquid phases and ꢀnally
recovered by centrifugation. The organic phase was then
dried with anhydrous magnesium sulfate and analyzed by a
gas chromatograph (Shimadzu-2014) using a capillary col-
umn (RTX-5). The products were recognized by comparison
with the standard samples.
2
TPA/ZrO was soaked with 25 mL 0.05 M aqueous solution
2
of PdCl for 24 h with stirring. The solution was ꢀltered,
2
washed with distilled water in order to remove the excess of
palladium and dried in air at room temperature. The result-
ing brown catalyst was designated as Pd(II)-TPA/ZrO .
2
Then, synthesized catalyst was charged in a Parr reactor
under 1 bar H pressure, at 40 °C for 30 min to reduce Pd(II)
2
to Pd(0). After that the catalyst was removed from reactor
and kept in air to attain the room temperature. The obtained
black colored catalyst was designated as Pd(0)-TPA/ZrO .
3 Results and Discussion
2
3.1 Characterization
2.3 Characterization
EDX values of W (16.87 wt%) and Pd (0.47 wt%) is in good
agreement with calculated one (16.91 wt% of W, 0.4 wt%
of Pd). Very low % of Pd indicates that only the protons of
TPA were exchanged [35]. EDX elemental mapping of the
catalyst is shown in Supplementary Fig. S1.
A detailed study on the characterization of Pd(0)-TPA/
ZrO can be found in our recent publication [32]. However,
2
in the present article EDS, XPS, TEM, BET and XRD are
given for reader’s convenience. In addition, we have char-
acterized fresh and regenerated catalyst by XPS for W also
in order to conꢀrm its reduction. Elemental analysis of the
solid catalyst was performed by JSM 5610 LV combined
with INCA instrument. Leaching of Pd in the reaction
mixture was checked by using atomic adsorption spec-
trometer AAS GBC-902 instrument. X-ray photoelectron
spectroscopy (XPS) measurements were performed with
Auger Electron Spectroscopy (AES) Module PHI 5000
Versa Prob II. TEM analysis was carried out on JEOL/
EO TEM instrument (model-JEM 1400) with accelerat-
ing voltage of 120 kV. Samples were prepared by drop-
ping the dispersed sample on 300 mesh carbon coated Cu
grid. Adsorption–desorption isotherms of samples were
recorded by Micromeritics ASAP 2010 surface area ana-
lyzer at − 196 °C. XRD pattern was performed by using
a Philips PW-1830 diꢁractometer. The conditions were:
Cu-Kα radiation (1.54 Å), scanning angle from 10° to 80°.
The XPS of Pd(0)-TPA/ZrO is displayed in Fig. 1. A
2
high intense peak at 532 eV (3p ) with broad peak at
3
/2
558 eV (3p ) and low intense peak at 335 eV (3d ) are
1
/2
5/2
in good agreement with available reports [29, 32, 36–38]
respectively, which conꢀrm the presence of Pd(0) nanopar-
ticles on the surface. However, there is still low amount of
Pd(II) present, as evident by corresponding peak at 345.5 eV
(3d ). This may be because of air oxidation of Pd(0) during
3/2
the drying process. This is further conꢀrmed by TEM. The
W4f peak is composed of a well resolved spin orbit doublet
(33.9 eV and 35.9 eV for W4f and W4f respectively),
7
/2
5/2
typical of W(VI) atoms in agreement with literature data on
Keggin-type POMs [29], conꢀrming no reduction of W(VI)
during the synthesis of Pd(II)-TPA/ZrO to Pd(0)-TPA/ZrO .
2
2
TEM images of fresh catalyst are shown in Fig. 2 at var-
ious magniꢀcations. Images (a) and (b) show the dark uni-
form suspension in the amorphous nature of the catalyst.
1
3

Selective C=C Hydrogenation of Unsaturated Hydrocarbons in Neat Water Over Stabilized Palladium…
1479
Fig. 1 XPS of Fresh Pd(0)-TPA/
ZrO for a Pd and b W
2
Fig. 2 TEM images of dispersed
Pd(0)-TPA/ZrO2
This indicates the high dispersion of Pd(0) nanoparticles
over the surface of the catalyst.
The increase in surface area of the Pd(II)-TPA/ZrO
2
2
−1
2
(
169 m g ) as compared to that of TPA/ZrO (146 m
2
−
1
g ) indicates high uniform dispersion of the Pd on the
surface of TPA/ZrO [35]. The observed drastic increase
2
2
in the value of surface area of Pd(0)-TPA/ZrO (203 m
2
−
1
2
−1
g ) as compared to that of Pd(II)-TPA/ZrO (169 m g ),
Scheme 1 Cyclohexene hydrogenation
2
conforms the reduction of Pd(II) to Pd(0) and is in good
agreement with the known fact that nanoparticles have
higher surface area than the parent one. It is very interest-
ing to note down that in spite of diꢁerent surface areas, the
nitrogen adsorption desorption isotherms (Supplementary
Fig. S2) are almost similar for all systems conꢀrming no
change in the basic structure.
3.2 Catalytic Activity
The catalytic activity of Pd(0)-TPA/ZrO2 was evaluated for
hydrogenation of cyclohexene and crotonaldehyde. Eꢁect of
various reaction parameters like solvent, substrate-catalyst
ratio, time and temperature and hydrogen pressure were
studied in order to obtain maximum conversion.
XRD patters of ZrO , TPA, TPA/ZrO and Pd(0)-TPA/
2
2
ZrO are presented in Supplementary Fig. S3. The absence
2
of any crystalline peak in TPA/ZrO corresponding to
3.2.1 Cyclohexene Hydrogenation
2
TPA, indicates the high dispersion of material over the
surface of the support. The XRD patterns of TPA/ZrO
To evaluate the eꢃciency of the catalyst for hydrogenation,
cyclohexene was selected as test substrate (Scheme 1). Eꢁect
of diꢁerent reaction parameters such as Pd concentration,
solvent, temperature, pressure and time were studied to opti-
mize the conditions for maximum conversion.
2
and Pd(0)-TPA/ZrO are almost identical, indicating the
2
retention of the TPA structure even after exchanging and
reduction of palladium. Absence of any crystalline peak
corresponds to Pd(0) in Pd(0)-TPA/ZrO spectrum, is due
2
to its very low concentration on the surface of the catalyst
as well as high dispersion of Pd(0) nanoparticles on the
3.2.1.1 Eꢀect of Palladium Concentration (Substrate/Cata‑
lyst Ratio) The eꢁect of Pd concentration was evaluated by
surface of TPA/ZrO .
2
1
3

1480
A. Patel, A. Patel
Fig. 5 Eꢁect of temperature. Reaction conditions: cyclohexene (9.87
Fig. 3 Eꢁect of Pd Concentration. Reaction conditions: cyclohexene
-4
mmol), conc. of Pd (7.48 × 10 mmol), H O (50) mL, H pressure
2
2
(
9.87 mmol), MeOH:H O (20:30) mL, Temp. (80 °C), H pressure
2 2
(10 bar), time (4 h)
(
10 bar), Time (4 h)
(
i.e. 96%). Hence, water was selected as a solvent for further
study.
3
.2.1.3 Eꢀect of H Pressure Hydrogen pressure inꢂuence
2
was evaluated and obtained results are presented in Supple-
mentary Fig. S4. Initially, with increase in pressure from 5
to 10 bar, the % conversion also increased linearly. Hence,
the reaction is ꢀrst order with respect to H pressure. With
2
further increase in pressure, no eꢁect on the conversion was
observed. Highest 96% conversion was obtained for 10 bar
H pressure.
2
3
.2.1.4 Eꢀect of Time The eꢁect of time was investigated
by varying the reaction time from 2 to 5 h (Supplemen-
tary Fig. S5). It is seen from the results that initially with
increase in time up to 4 h, the % conversion also increases,
which is in good agreement with the well-known fact that
as time increases, the formation of reactive intermediates
from the reactant increases, and ꢀnally converted into the
products. On further increasing the reaction time, the %
conversion decreases. This might be due to occurrence of
reversible process.
Fig. 4 Eꢁect of solvent. Reaction conditions: cyclohexene (9.87
-
4
mmol), conc. of Pd (7.48 × 10 mmol), solvent ratio (20–30) mL,
Temp. (80 °C), H pressure (10 bar), time (4 h).
2
varying the catalyst amount from 10 to 25 mg (Substrate/
Catalyst ratio from 26259/1 to 10504/1 respectively). Ini-
tially, with increase in concentration of Pd, % conversion
also increases i.e. 10 mg to 20 mg (Fig. 3). Higher concen-
tration of Pd would means a higher number of active sites
to tolerate the substrate and gives higher conversion. With
further increasing Pd concentration, the conversion remains
constant. The highest, 99% conversion was achieved by
3.2.1.5 Eꢀect of Temperature Eꢁect of temperature was
screened from 50 to 90 °C and obtained results are pre-
sented in Fig. 5. Results shows that % conversion increases
with increase in temperature from 50 to 80 °C, as expected.
Further increase in the temperature showed no eꢁect on the
% conversion.
2
0 mg of catalyst.
3
.2.1.2 Eꢀect of Solvent Eꢁect of various solvents was
studied and obtained results are presented in Fig. 4. Compar-
The optimized conditions for the maximum % conversion
(96) are: cyclohexene (9.87 mmol), conc. of Pd(0) (7.48 ×
atively, low % conversion was obtained in case of CH CN–
3
−
4
H O solvent system due to its oxidizing nature which resists
10 mmol), H O (50 mL), H pressure (10 bar), time (4 h)
2 2
2
the reduction of cyclohexene. While it was achieved maxi-
and temp. (80 °C), Substrate/Catalyst ratio (13130/1) and
TON (12,667).
mum for CH OH–H O, IPA–H O and CH OH–H O system
3
2
2
3
2
and this may be due to their reducing nature. However, it
was miracle that higher % conversion was obtained under
the identical reaction conditions for neat water as a solvent
1
3

Selective C=C Hydrogenation of Unsaturated Hydrocarbons in Neat Water Over Stabilized Palladium…
1481
Scheme 2 Crotonaldehyde hydrogenation
3
.3 Crotonaldehyde Hydrogenation
The optimized conditions for the maximum % conversion
89) are: crotonaldehyde (9.87 mmol), conc. of Pd(0) (7.48
(
−
4
To evaluate the eꢃciency of the catalyst for hydrogena-
tion, crotonaldehyde was also selected as test substrates
× 10 mmol), H O (50 mL), temp. (80 °C), H pressure
2 2
(8 bar) and time (4 h). Substrate/Catalyst ratio (13130/1),
TON (11744).
(
Scheme 2). Eꢁect of diꢁerent reaction parameters such
as Pd concentration, solvent, temperature, pressure and
time were studied to optimize the conditions for maximum
conversion.
3.4 Control Experiment
As described in section of cyclohexene hydrogenation,
in present case also, all parameters were varied and results
are shown in respective ꢀgures (Fig. 6). It should be noted
that the explanation will remain the same as given in the
section of cyclohexene hydrogenation. Here, in all the
cases, butyraldehyde was obtained with 100% selectively.
In both the cases, control experiments were carried out with
ZrO , PdCl , TPA/ZrO and Pd(0)-TPA/ZrO under opti-
2
2
2
2
mized conditions and results are shown in Supplementary
Table S1. It is seen from the data that ZrO and TPA/ZrO
2
2
were inactive towards the reactions. Almost same conver-
sions were found in the case of PdCl and Pd(0)-TPA/ZrO
2
2
Whereas in case of EtOH: H O (20: 30) mL solvent system,
in both the cases. This indicates that Pd is real active species
2
we achieved 91% conversion with 60% butyraldehyde and
responsible for the reaction.
4
0% crotylalcohol selectively. An explanation for the same
is provided in mechanism Sect. 3.7.3.
3.5 Leaching and Recycling Test
Finally, the eꢁect of time on reaction was also studied
and obtained results are presented in Supplementary Fig. S6.
From the data, it can be seen that reaction progress increases
with increase in time up to 4 h. Further increase in time
didn’t show any eꢁect on the conversion indicating the satu-
ration point of the reaction. Maximum 89% conversion was
obtained in 4 h.
The leaching of Pd from Pd(0)-TPA/ZrO was conꢀrmed
2
by carrying out an elemental analysis of the recovered cata-
lyst by EDS as well as of the reaction mixtures by Atomic
Adsorption Spectroscopy (AAS). The analysis of the recov-
ered catalyst and reaction mixture did not show appreci-
able loss in the Pd content. To evaluate the eꢃciency of
Fig. 6 Reaction conditions.
a Eꢁect of Pd concentration:
crotonaldehyde (9.87 mmol),
H O (50) mL, Temp. (80 °C),
2
H pressure (8 bar), Time (4 h).
2
b Eꢁect of solvent: crotonal-
dehyde (9.87 mmol), conc. of
-
4
Pd (7.48 × 10 mmol), solvent
ratio (20–30) mL, Temp. (80
°
C), H pressure (8 bar), time
2
(
4 h). c Eꢁect of temperature:
crotonaldehyde (9.87 mmol),
-
4
conc. of Pd (7.48 × 10 mmol),
H O (50) mL, H pressure (8
2
2
bar), time (4 h). d Eꢁect of H
2
pressure: crotonaldehyde (9.87
-
4
mmol), conc. of Pd (7.48 × 10
mmol), H O (50) mL, temp. (80
2
°
C), time (4 h)
1
3

1482
A. Patel, A. Patel
Pd(0)-TPA/ZrO towards recyclability, after reaction com-
active species on the support during the reaction shows that
present catalyst is of category C [32]. In both the cases, the
same trend was found and hence we have reported here only
the results of cyclohexene.
2
pletion, the catalyst was recovered by simple centrifuga-
tion, washed with dichloromethane followed by water and
then dried in oven at 100 °C for an hour and ꢀnally used
for the next cycle. Obtained constant results (Fig. 7) shows
the stable activity up to number of cycles without leach-
ing of Pd(0) nanoparticles. Recycling test was carried out
for both reactions and almost same trend was found upto 5
cycles. However, we have reported here only the results for
cyclohexene substrate.
3.7 Characterization of Regenerated Catalyst
In order to check the stability, the regenerated catalyst was
characterized for EDS, FT-IR and XPS.
EDX values of W (16.90 wt%) and Pd (0.46 wt%) of
regenerated Pd(0)-TPA/ZrO is in good agreement with
2
3.6 Heterogeneity Test
values of fresh catalyst (16.87 wt% of W, 0.47 wt% of Pd)
conꢀrming no leaching of TPA from ZrO . This indicates
2
In both the cases, reaction was carried out for 2 h, catalyst
was removed and then ꢀltrate was allowed to react up for
that TPA plays an important role as a stabilizer to keep the
Pd(0) active and also prevent its leaching from the support.
4
h. After that, obtained reaction mixtures were extracted
The XPS of fresh and regenerated Pd(0)-TPA/ZrO is
2
by using dichloromethane and analyzed by Gas chromatog-
raphy. Obtained results (Supplementary Table S2) showed
that there was no signiꢀcant change in % conversion of the
reactants after removing the catalyst, indicating the absence
of active species in the reaction mixture which proves that
there was no leaching of Pd during the reactions. This study
displayed in Fig. 8. The obtained spectra are identical with
fresh one, which conꢀrms that Pd(0) nanoparticles are stabi-
lized by TPA and catalyst is sustainable during the reaction.
Here, the intensity of regenerated catalyst spectrum is low,
which may be due to the sticking of the substrates on the
surface, although this might not be signiꢀcant in the reutili-
zation of the catalyst.
indicates that TPA/ZrO stabilized Pd nanoparticles and
2
does not allow to leach it into reaction mixture, making it
a true heterogeneous catalyst to recycle and reuse. Here,
heterogeneous nature of the catalyst and withholding of the
3.7.1 Substrate Study
In order to see the viability of the present catalyst, hydro-
genation of diꢁerent substrates was carried out over Pd(0)-
TPA/ZrO and the results are presented in Table 1. From
2
the results it is clear that the catalyst was found to be active
for selective hydrogenation of C=C bond without tolerating
the C=O bond.
3
.7.2 Comparison with Reported Catalysts
Catalytic activity of the present catalyst is also compared
with reported catalysts in both the cases.
In case of cyclohexene hydrogenation (Table 2), Liu et al.
[
3] reported very high conversion at lower temperature with
very small amount of catalyst but they had utilized very high
Fig. 7 Recycling test. Reaction conditions. Cyclohexene (9.87
-
4
H pressure (20 bar) for the reaction compared to present
mmol), Conc. of Pd (7.48 × 10 mmol), H O (50) mL, Temp. (80
2
2
°
C), H pressure (10 bar), time (4 h)
work. Zhang et al. [4] reported the same reaction at 35 °C
2
Fig. 8 XPS of fresh and regen-
erated Pd(0)-TPA/ZrO for a Pd
2
and b W
1
3

Selective C=C Hydrogenation of Unsaturated Hydrocarbons in Neat Water Over Stabilized Palladium…
1483
Table 1 Substrate study
the desired product butyraldehyde was only 31%. Here, rea-
son for the low selectivity was weaker adsorption of formed
product butyraldehyde over the surface of the catalyst, which
easily gets disorb and hydrogenated to form another products
alcohol. Zhao et al. [7] achieved 100% conversion at 100 °C
with very low amount of Pd, but they had used super critical
Substrate
Product
% Con-
version/
selectiv-
ity
a96/100
CO as solvent under 80 bar pressure (very harsh condition).
2
Here, the use of CO favours to obtain 100% selectivity of
2
butyraldehyde.
a72/100
Here in both the cases, high activity as compared to
reported systems, may be due to its single atom catalytic
(
SAC) properties. Recently, in SAC, it is reported that the
a86/100
a79/100
activity of the catalyst as well as high density of the active
sites can be increased by either increasing the dispersion of
the metal atoms to the support or reducing the size of metal
by converting it into nanoparticles [39, 40].
3
.7.3 Plausible Mechanism
b89/100
b76/100
Hydrogenation of crotonaldehyde gives three products
selectively (i) Butyraldehyde (ii) Crotyl alcohol and (iii)
n-butanol. It is well known that thermodynamic parameters
favor the hydrogenation of C=C over the C=O bond [41]
due to high activation energy of C=O tolerance to yield
selectively butyraldehyde. However, C=O selectively can be
reduced by either designing the suitable bimetallic catalyst
or using organic solvent as discussed in introduction part.
In the present work, we achieved single selective prod-
uct that is butyraldehhyde instead of product mixture.
There might be two reasons for the selective reduction
of C=C bond. (i) The presence of only water as a solvent
instead of any organic solvent as discussed above. Here,
the absence of organic solvent would not activate C=O
bond to selectively produce butyraldehyde only. To check
the same, we have carried out the same reaction under
Reaction Condition. Substrate (9.87 mmol), Conc. of Pd (7.48 ×
−
4
a
1
0
mmol), H O (50) mL, Temp. (80 °C), H Pressure ( 10 bar and
2
2
b
8
bar), Time (4 h)
with very low conversion, moreover use of catalyst amount
was too high. Leng et al. [6] achieved high conversion using
formic acid as a sovent with very high amount of the catalyst
compared to the present catalytic system.
In case of crotonaldehyde hydrogenation (Table 3), Iwasa
et al. [8] reported 100% conversion at 200 °C temperature
with very high amount of catalyst, moreover selectivity of
the optimized condition using EtOH: H O (20:30) mL as
2
Table 2 Comparison with
Catalyst
Pd (mol%)
Solvent
Temp. (°C)
% Conversion
TON
5000
560
44
reported catalyst in case of
cyclohexene hydrogenation
SH-IL-1.0 wt%Pd [3]
Pd/MSS@ZIF-8 [4]
Pd@CN [6]
0.02
Auto-clave
Ethyl acetate
Formic acid
Water
60
35
90
80
99
5.6
96
96
0.1738
2.208
0.0384
Present work
12,667
Table 3 Comparison with
reported catalyst in case of
crotonaldehyde hydrogenation
Catalyst
Pd (mol%) Solvent
Temp. (°C) % Conversion % Selectivity of TON
butyraldehyde
1
0% Pd/C [7] 0.0094
CO (80 bar)
Fix bed reactor 200
100
100
100
100
89
100
31
2000
–
2
Pd/CeO [8]
10
2
Pd/PEG [9]
Present work
0.5
0.0384
Ethanol
Water
RT
80
100
100
1277
11,744
1
3

1484
A. Patel, A. Patel
Compliance with Ethical Standards
Conflict of interest There are no conꢂicts to declare.
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