Synthesis and characterization of supported stabilized palladium nanoparticles for selective hydrogenation in water at low temperature

Article abstract of DOI:10.1039/d1ra00239b

Zirconia supported vacant phosphotungstate stabilized Pd nanoparticles (Pd-PW₁₁/ZrO₂) were prepared using a simple impregnation and post reduction method, characterized and their efficiency for selective C=C hydrogenation of unsaturated compounds explored. The establishment of a hydrogenation strategy at low temperature using water as solvent under mild conditions makes the present system environmentally benign and green. The catalyst shows outstanding activity (96% conversion) with just a small amount of Pd(0) (0.0034 mol%) with high substrate/catalyst ratio (29 177/1), TON (28 010) and TOF (14?005 h^?1) for cyclohexene (as a model substrate) hydrogenation. The catalyst was recovered by simple centrifugation and reused for up to five catalytic cycles without alteration in its activity. The present catalyst was found to be viable towards different substrates with excellent activity and TON (18 000 to 28 800). A study on the effect of addenda atom shows that the efficiency of the catalyst can be enhanced greatly by increasing the number of counter protons. This challenging strategy would greatly benefit sustainable development in chemistry as it diminishes the use of organic solvents and offers economic and environmental benefits as water is cheap and non-toxic.

Full text of DOI:10.1039/d1ra00239b

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Synthesis and characterization of supported
stabilized palladium nanoparticles for selective
Cite this: RSC Adv., 2021, 11, 8218
hydrogenation in water at low temperature†
*
Anish Patel
and Anjali Patel
Zirconia supported vacant phosphotungstate stabilized Pd nanoparticles (Pd–PW11/ZrO2) were prepared
using a simple impregnation and post reduction method, characterized and their efficiency for selective
C]C hydrogenation of unsaturated compounds explored. The establishment of a hydrogenation
strategy at low temperature using water as solvent under mild conditions makes the present system
environmentally benign and green. The catalyst shows outstanding activity (96% conversion) with just
a small amount of Pd(0) (0.0034 mol%) with high substrate/catalyst ratio (29 177/1), TON (28 010) and
TOF (14 005 hꢀ1) for cyclohexene (as a model substrate) hydrogenation. The catalyst was recovered by
simple centrifugation and reused for up to five catalytic cycles without alteration in its activity. The
present catalyst was found to be viable towards different substrates with excellent activity and TON
(18 000 to 28 800). A study on the effect of addenda atom shows that the efficiency of the catalyst can
be enhanced greatly by increasing the number of counter protons. This challenging strategy would
greatly benefit sustainable development in chemistry as it diminishes the use of organic solvents and
offers economic and environmental benefits as water is cheap and non-toxic.
Received 11th January 2021
Accepted 12th February 2021
DOI: 10.1039/d1ra00239b
rsc.li/rsc-advances
variation in both symmetry and size.10 In addition, they have
following advantages: (i) Robust oxoanionic nature which greatly
Introduction
enhance its stability power, (ii) reducing capacity, favours the
stability of Pd in to its most stable oxidation state zero,11 (iii) large
relative sizes which sterically hindered PdNPs and prevent it from
agglomeration during the synthesis as well as its catalytic reaction
and (iv) it avoids the use of external ligands for stabilization of
PdNPs, which are mostly organic-toxic materials.
Palladium nanoparticles (PdNPs) are one of the most studied
catalysts for organic transformations and historically dominant
over other metals, and they still serve as a central tool for
innumerable important organic transformations and total
synthesis.1 Nanosizing of Pd based bulk catalysts is an emerging and
promising eld owing to the allowance of Pd's maximum utilization,
and enhanced efficiency, activity and selectivity.2 Particle size is a key
factor of catalytic performance: by decreasing the metal size, the
specic activity can be increased greatly3 (Fig. S1†). However, due to
high surface energy, PdNPs are very much prone to form aggregates
during either their synthesis or use in catalytic reactions, and hence,
stabilization of PdNPs is necessary.4
Number of efforts have been made for the same using
different stabilizing agents such as dendrimers,5 phosphine base
ligands,6 surfactants,7 ionic liquids,8 etc. As the reported stabilizing
scaffolds are mostly organic, toxic and expensive, it is important to
replace the same by some alternatives. Heteropoly acids (HPAs) are
the excellent candidate for the same. HPAs are discrete early
transition metal–oxide cluster anions and comprise a class of
inorganic complexes9 having unrivalled versatility and structural
Liquid-phase selective hydrogenation is a diverse and versatile
acceptable route to synthesize precursors for various intermediates
and still it is fascinating and challenging eld.12 In this regards,
triethanolamine,13 isopropanol14 and oxalic acid15 etc., are generally
used as electron and proton donors for hydrogenation, which
makes the process less sustainable. Moreover, the processes which
involves the use of molecular hydrogen are usually suffer from
harsh reaction conditions, lower selectivity16 and use of high active
amount of the catalyst.12d In advancement, it is highly attractive to
design a green strategy for direct utilization of the molecular
hydrogen using water as solvent under mild reaction conditions.
In this context, our previous report12d demonstrates the use
of zirconia supported 12-tungstophosphoric acid (Pd–TPA/ZrO2)
as stabilizing scaffold and its outstanding activity towards hydro-
genation of different aromatic and aliphatic compounds inspired
us to design another catalyst. Our main objective is to develop the
catalyst having upgraded activity for water mediated hydrogena-
tion with high substrate/catalyst ratio and low active amount of Pd
under mild reaction conditions. Hence, in present case we have
selected mono lacunary tungstophosphoric acid (PW11) supported
Polyoxometalates and Catalysis Laboratory, Department of Chemistry, Faculty of
Science, The Maharaja Sayajirao University of Baroda, Vadodara-390002, Gujarat,
India. E-mail: anjali.patel-chem@msubaroda.ac.in
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d1ra00239b
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on zirconia (ZrO2) as a stabilizing agent. The selection of PW11 was
the eye-catchy step as it has seven counter protons (higher than the
parent one, TPA ¼ PW12) which can accelerate the formation of
Pd–H to enhance the hydrogenation rate.
Step-1: Synthesis of supported mono lacunary tung-
stophosphoric acid (PW11/ZrO2) by incipient wet impregnation
method. Sodium salt of mono lacunary tungstophosphoric
acid17 (Na7PW11O39, later Na7PW11), zirconia18 (ZrO2) and PW11/
ZrO2 (ref. 19) were synthesized following the methods reported
by Brevard et al. and our group, respectively (for detail, see ESI†)
as shown in Fig. 1a.
Step-2: Synthesis of Pd–PW11/ZrO2 by soaking and post
reduction method. Palladium was deposited on PW11/ZrO2 via
exchanging the available protons of PW11 following the same
method as reported earlier by our group for Pd–PW12/ZrO2
synthesis,11 which can be used for any polyoxometalate based
material. In synthesis, 1 g of PW11/ZrO2 was soaked with 25 mL
of 0.05 M aqueous solution of PdCl2 for 24 h with stirring. The
solution was ltered, washed with distilled water in order to
remove the excess of PdCl2 and dried in air at room tempera-
ture. The resulting (brown colored) material was designated as
Pd(II)–PW11/ZrO2. Finally, the synthesized material was charged
into the Parr reactor under 1 bar H2 pressure, at 40 ꢁC for 30 min
to reduce Pd(II) to Pd(0). The obtained (black colored) material
was designated as Pd–PW11/ZrO2. The synthetic scheme is pre-
sented in Fig. 1b. The same procedure was followed for the
synthesis of Pd/ZrO2 (for detail, see ESI†).
Herein, rst time we present a simple engineering method
for PdNPs, based on supported mono lacunary tungstophos-
phoric acid (PW11/ZrO2) as effective stabilizing agent. Synthe-
sized catalyst (Pd–PW11/ZrO2) was characterized by different
spectroscopy techniques and sole presence of PdNPs was
exposed by HRTEM and dark/bright eld STEM. The efficiency
of the catalyst was evaluated for the hydrogenation of cyclo-
hexene as a model substrate using molecular hydrogen and
water as solvent. The inuence of various reaction parameters like
catalyst amount, reaction time, temperature, pressure and solvent
were studied deeply. Scope and limitations of the catalyst was also
evaluated for various aromatic and aliphatic. The stability of the
catalyst was conrmed by characterization of regenerated catalyst
by EDX, FT-IR, XRD, XPS and TEM. The competence of the catalyst
was compared with reported catalytic systems. Further, in order to
understand the role of addenda atom, the catalytic activity was
compared with Pd–PW12/ZrO2 (Pd–TPA/ZrO2) and explained on the
bases of available protons. Mechanistic investigation was also
studied by using D2O as solvent.
Characterization
Experimental
The synthesized materials were characterized by different
physico-chemical techniques which are generally used for
nanomaterials such as EDX, TGA, FT-IR, BET, XRD, XPS, TEM,
HRTEM and BF/DF-STEM. The instrumental details are
included in ESI.†
Materials
Anhydrous disodium hydrogen phosphate, sodium tungstate
dihydrate, acetone, nitric acid, hydrochloric acid, zirconium oxy-
chloride, 25% (v/v) ammonia, palladium chloride, cyclohexene and
dichloromethane were used as received from Merck without further
purication. All chemicals used were of analytical reagent grade.
Hydrogenation
The catalytic hydrogenation was carried out using Parr reactor
as described in our recent article.20 In typical experiment,
Catalyst synthesis
Stabilized PdNPs via supported mono lacunary tungstophos- cyclohexene with water as solvent and catalyst were charged into
phoric acid (Pd–PW11/ZrO2) was synthesized by wet chemistry the reactor vessel. The reactor was ushed thrice with H2 gas to
method in two steps.
remove the air present in the empty part of the vessel. Finally,
Fig. 1 Synthesis of (a) PW11/ZrO2 and (b) Pd–PW11/ZrO2.
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H2 pressure was applied for the reaction. The reaction was set at PW11 exhibits bands at 1088, 1042, 964, 903 and 810 cmꢀ1
desired temperature with the stirring rate of 1700 rpm for 2 h. corresponding to P–O, W]O and W–O–W stretching, respec-
The continuous decrease in pressure inside the vessel was tively. Here, the splitting of P–O bond is due to the lowering of
utilized for determination of the reaction progress. Aer reac- symmetry around central hetero atom phosphorus, indicates
tion completion, the reaction mixture was cooled at room the formation of lacunary species.
temperature and H2 pressure was released from the vent valve.
Similarly, PW11/ZrO2 exhibits bands at 1090, 1047, 964 and
The organic layer was extracted by dichloromethane, whereas 812 cmꢀ1 corresponding to P–O, W]O, and W–O–W stretching
catalyst was collected from the junction of the liquid phases and vibration frequencies, respectively. No signicant change in the
nally recovered by centrifugation. The organic phases were bands indicate the retention of the Keggin unit in the synthe-
dried with anhydrous magnesium sulfate and analyzed by a gas sized material. The spectrum of Pd–PW11/ZrO2 shows bands at
chromatograph (Shimadzu-2014) using a capillary column 1092, 1049, 960 and 806 cmꢀ1 corresponding to P–O, W]O and
(RTX-5). The products were recognized by comparison with the W–O–W, respectively. The slight shi in the bands may be due
standard samples.
to the change in environment by Pd.
The BET surface area is the useful tool to probe the quality
and character of solid phase materials. Specic surface area of
ZrO2, PW11/ZrO2, Pd(II)–PW11/ZrO2 and Pd–PW11/ZrO2 were
measured to investigate the chemical interaction between
support and active species. The surface area of PW11/ZrO2 (224
m2 gꢀ1) is found to be higher than support ZrO2 (170 m2 gꢀ1).
Results and discussion
Characterization
The gravimetric analysis of standard solution and ltrate
showed 6.05 wt% and 0.72 wt% of Pd in Pd/ZrO2 and Pd(II)–
PW11/ZrO2, respectively.21 For Pd–PW11/ZrO2, EDX values of W
(15.57 wt%) and Pd (0.76 wt%) are in good agreement with
calculated one (15.19 wt% of W, 0.72 wt% of Pd). EDX elemental
mapping of Pd–PW11/ZrO2 is shown in Fig. 2a.
This is because of bulky anionic nature of PW11.
19 The decrease
in surface area of Pd(II)–PW11/ZrO2 (199 m2 gꢀ1) compared to
PW11/ZrO2 indicates the strong interaction of Pd with the
loaded material. The drastic rise in surface area of Pd–PW11/
ZrO2 (213 m2 gꢀ1) compared to Pd(II)–PW11/ZrO2 is the rst
evidence for the presence of Pd(0) nanoparticles (PdNPs), due to
the downsizing of Pd during the reduction of Pd(II) to Pd(0). In
spite of having different surface area, the unaltered nature of
the N2 sorption isotherms of PW11/ZrO2 and Pd–PW11/ZrO2
(Fig. 2c) indicate the identical basic structure, the same is also
reected by FT-IR analysis.
Thermal stability of Pd–PW11/ZrO2 was evaluated by TGA and
the obtained curve is plotted in Fig. S2.† Curve indicates 8.13%
weight loss in the temperature range of 50 to 110 ꢁC due
adsorbed water molecule. Alongside, there was no signicant
ꢁ
weight loss observed up to 500 C indicating the high thermal
stability of the material.
The FT-IR spectra of ZrO2, PW11, PW11/ZrO2, Pd–PW11/ZrO2
are shown in Fig. 2b. ZrO2 shows broad bands in the region of
1600, 1370, and 600 cmꢀ1 attributed to H–O–H and O–H–O
bending and Zr–OH bending, respectively. FT-IR spectrum of
To study the surface morphology and rate of dispersion in
synthesized materials, the XRD patterns of PW11, ZrO2, PW11/
ZrO2 and Pd–PW11/ZrO2 were recorded (Fig. 2d). XRD patterns
Fig. 2 (a) EDX elemental mapping of Pd–PW11/ZrO2; (b) FT-IR spectra; (c) N2 sorption isotherms and (d) XRD spectra.
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Fig. 3 XPS spectra of PW11/ZrO2 (a–c) and Pd–PW11/ZrO2 (d–f).
of PW11 shows the characteristic peaks between 2q range of 20ꢁ presence of W(VI). Pd–PW11/ZrO2 also shows (Fig. 3f) a single
to 35ꢁ. Whereas, the absence of all patterns corresponds to spin–orbit pair at binding energy 35.6 and 37.5 eV (spin–orbit
PW11 in PW11/ZrO2 indicates the uniform dispersion over the splitting, 1.9 eV) conrming no reduction of W(VI) during the
surface of the support. XRD patterns of Pd–PW11/ZrO2 did not synthesis.23b,25
reected any diffraction corresponds to PW11 as well as Pd,
indicates the high degree of dispersion over the surface as well (Fig. 4b and c) were recorded at various resolution. TEM
as no sintering of Pd was form during the synthesis. micrographs of PW11/ZrO2 show the homogeneous dispersion
TEM images of PW11/ZrO2 (Fig. 4a) and Pd–PW11/ZrO2
To conrm the electronic state of the Pd, W and O high of PW11 over the surface of the ZrO2. Selected area electron
resolution XPS of PW11/ZrO2 and Pd–PW11/ZrO2 were recorded diffraction (SAED) image of Pd–PW11/ZrO2 shows the non-
(Fig. 3). PW11/ZrO2 shows (Fig. 3 and b) a very intense peak at crystalline nature of highly dispersed Pd in the synthesized
binding energy 532 eV corresponds to O 1s as it contains material (Fig. S3†). Whereas, Fig. 5b and c show the high degree
number of O atoms of PW11 and ZrO2 support, whereas Pd– of homogeneously dispersed very small isolated PdNPs. To
PW11/ZrO2 shows (Fig. 3d) the direct overlap peak between Pd further conrm, HRTEM micrographs were also recorded as
3p3/2 and O 1s peaks at binding energy 532 eV, which is in good shown in Fig. 4d and e, which clearly show the uniform
agreement with the reported one,22 and cannot be assigned to dispersion of PdNPs (particle size, ꢂ2 nm) throughout the
conrm the presence of Pd(0). Hence, we have presented morphology, without aggregates formation, conrming the
instrument generated full spectra (Fig. 3a and d) images, sup- stabilization of PdNPs by PW11. TEM images of Pd/ZrO2 are
porting the presence of Pd(0). This is further conrmed by presented in Fig. S3† which show the aggregates formation of
recording the high resolution Pd 3d spectrum which (Fig. 3e) Pd(0), indicating the non-stabilized nature of the Pd.
shows a low intense spin orbit doublet peak at binding energy
For more insight of PdNPs, an advance technique, STEM was
335.9 eV and 340.5 eV correspond to Pd 3d5/2 and Pd 3d3/2
,
utilized to probe the behaviour of PdNPs. Bright/dark eld
conrming the presence of Pd(0).23 Two additional high intense STEM (BF/DF-STEM) images (Fig. 4f and g, respectively) show
peaks at binding energy 331 eV and 345 eV attributed to Zr 3p3/2 the highly dispersed PdNPs all over the morphology of the
and Zr 3p1/2, respectively24
material. Whereas, overlapping image (Fig. 4h) as well as
PW11/ZrO2 shows (Fig. 3c) a well resolved spin–orbit doublet elemental image of Pd (Fig. 4i) clearly indicates the presence of
of W 4f7/2 and W 4f5/2 at binding energy 35.6 and 37.6 eV (spin– isolated PdNPs homogeneously dispersed without any cross
orbit splitting, 2.0 eV), characteristic of W(VI), conrming the talks between them. The absence Pd of aggregates conclude that
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Fig. 4 TEM micrographs of PW11/ZrO2 (a) and Pd–PW11/ZrO2 (b and c), HRTEM micrographs of Pd–PW11/ZrO2 (d and e), STEM images bright field
(f) & dark field (g) of Pd–PW11/ZrO2, overlapping image (h) and Pd elemental image (i) of Pd–PW11/ZrO2 at various magnifications.
PW11/ZrO2 is very much capable to decrease the high surface
The effect of substrate to catalyst ratio was evaluated by
free energy of PdNPs, by providing combinedly the facility of varying the catalyst amount from 5 to 20 mg. Obtained results
high surface area for dispersion as well as stabilizing nature. (Fig. 5a) show that with increasing the catalyst amount from 5 to
More representative images can be found in Fig. S4.†
20 mg there was no effect on the reaction conversion. Here, very
FT-IR shows the retention of Keggin structure even aer small amount of catalyst i.e. 5 mg (0.0034 mol% of Pd) has
impregnation, soaking and post-reduction of the catalyst. XPS sufficient active sites and capable to tolerate very high amount
conrms the presence of Pd(0) and W(VI). TEM, HRTEM and of substrate (substrate to catalyst ratio, 29 177/1), clearly indi-
STEM indicate the sole presence of highly dispersed PdNPs over cates the high efficiency of PdNPs.
the surface of the catalyst.
The inuence of time was assessed between 1 to 4 h (Fig. 5b).
Initially, up to 2 h, with increasing time, z1.14-fold %
conversion also increases. Hence, 2 h is the sufficient time for
the maximum productive collision of the substrates to yield
cyclohexane. Further, increase in time (up to 4 h) has no
inuence on the % conversion, maximum 96% conversion was
achieved in 2 h.
Catalytic activity
The efficiency of Pd–PW11/ZrO2 was evaluated towards the
hydrogenation. Cyclohexene was selected as the ideal substrate
for the reaction to optimize the different inuential reaction
parameters such as substrate to catalyst ratio (catalyst amount),
time, temperature, pressure and solvent for highest product
conversion. Reaction scheme is shown in Scheme S1.†
The effect of temperature on reaction was screened in the
ꢁ
region of 30–60 C (Fig. 5c). Obtained results show z1.71-fold
increase in % conversion with increasing temperature from 30
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Fig. 5 Optimization of reaction parameters: (a) effect of catalyst amount. Cyclohexene (9.87 mmol), H2O (50) mL, temp. (80 ꢁC), H2 pressure (10
bar), time (4 h); (b) effect of time. Cyclohexene (9.87 mmol), conc. of Pd (0.0034 mol%), H2O (50) mL, temp. (80 ꢁC), H2 pressure (10 bar); (c) effect
of temperature. Cyclohexene (9.87 mmol), conc. of Pd (0.0034 mol%), H2O (50) mL, H2 pressure (10 bar), time (2 h) and (d) effect of pressure.
Cyclohexene (9.87 mmol), conc. of Pd (0.0034 mol%), H2O (50) mL, temp. (50 ꢁC), time (2 h). (e) Leaching test: cyclohexene (9.87 mmol), conc. of
Pd (0.0034 mol%), substrate/catalyst ratio (29 177/1), H2O (50) mL, temp. (50 ꢁC), H2 pressure (10 bar), time (2 h). (f) Recyclability test: cyclo-
hexene (9.87 mmol), conc. of Pd (0.0034 mol%), substrate/catalyst ratio (29 177/1), H2O (50 mL), temp. (50 ꢁC), H2 pressure (10 bar), time (2 h).
to 50 ꢁC. Further, rise in temperature shows no appreciable conversion, respectively). This indicates the Pd is only the
increase in % conversion. At higher temperature there are two responsible real active species for the reaction conversion.
facts which resist the reaction conversion: (i) desorption of
cyclohexene from the surface of the catalyst and; (ii) lower
adsorption rate of hydrogen over the surface of catalyst.26
Hence, required activation energy was obtained in just 50 ꢁC for
96% conversion.
The effect of H2 pressure was studied in the region of 7 to 10
bar (Fig. 5d). Obtained results show that % conversion increases
from 80 to 96% linearly. Hence, the reaction is rst order with
respect to H2 pressure, which is in good agreement with the
reported one27 stating that hydrogenation is always rst order
with respect to H2 pressure. Further study for effect of high
pressure was not carried out as our main focus is to establish
environmentally green process. Highest 96% conversion was
obtained by applying 10 bar H2 pressure.
Leaching and heterogeneity test
In order to investigate the leaching of PdNPs as well as role of
PW11, the reaction was performed with Pd/ZrO2 and Pd–PW11/
ZrO2 up to 1 h, centrifuged to remove the catalyst, and nally
the reaction was continued for further 1 h (total 2 h). Organic
layer was separated by dicholoromethane, anhydrated by
magnesium sulphate and analysed by GC. In case of Pd/ZrO2,
obtained results (Fig. 5e) indicate the leaching of Pd from the
support (conversion, 61% aer 1 h and 68% aer 2 h). Whereas,
Pd–PW11/ZrO2 shows no rise in % conversion aer removal of
catalyst [conversion, 84% aer 1 h and 85% aer 2 h], conrm
no leaching of PdNPs during the reaction. To further conrm,
the heterogeneity of the Pd–PW11/ZrO2, the regenerated catalyst
was characterized by EDX (Fig. S4†) and reaction mixture by
atomic adsorption spectroscopy (AAS). No loss in Pd content
(0.71 wt%) indicates the stabilization of PdNPs by PW11 over the
surface of ZrO2. These obtained results conrm the true
heterogeneous nature of the catalyst, which is also reected in
recycling study.
The optimized conditions for the maximum % conversion
(96) with TON (28 010) and TOF (14 005 hꢀ1) are: cyclohexene
(9.87 mmol), conc. of Pd (0.0034 mol%), H2O (50 mL), H2
ꢁ
pressure (10 bar), time (2 h) and temp. (50 C) and substrate/
catalyst ratio (29 177/1).
Control experiments
Recyclability and sustainability of the catalyst
In order to investigate the role of individual components,
control experiments were carried out with ZrO2, PW11, PW11/ZrO2, Sustainability of the catalyst is the key challenge for any process,
PdCl2, Pd/ZrO2 and Pd–PW11/ZrO2 under optimized conditions. and for the same recycling test was studied for Pd/ZrO2 and Pd–
Obtained results show that ZrO2, PW11, PW11/ZrO2 are totally PW11/ZrO2 (Fig. 5f). In order to regenerate the catalyst, aer
inactive toward the reaction whereas almost same conversion is reaction completion, the catalyst was centrifuged, washed with
achieved by PdCl2, Pd/ZrO2 and Pd–PW11/ZrO2 (91, 95 and 96% dicholoromethane followed by consequent washes with
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Fig. 6 (a) FT-IR spectra of fresh and regenerated catalysts; (b and c) XPS spectra and (d and e) HRTEM images of regenerated catalyst.
distilled water and nally dried at 100 ꢁC for an hour to reuse it catalyst as well as no reduction of W(VI) during the catalytic
for next catalytic run. Obtained results show the gradual hydrogenation, assuring the sustainability of the catalyst.
decrease in the % conversion in case of Pd/ZrO2, conrm the
HRTEM images of regenerated catalyst are shown in Fig. 6d
leaching of active species Pd from the support. Whereas, Pd– and e. Images at various magnication clearly indicates the
PW11/ZrO2 shows consistent activity up to ve catalytic runs. retention of high dispersion of the isolated PdNPs over the
This indicates that there was no emission of PdNPs during the surface of the catalyst. No aggregation of Pd conrms the
reaction from the surface of the catalyst.
Table 1 Substrate studya
Characterization of regenerated catalyst
Conv (%)/sel
(%)
The stability of the regenerated catalyst was studied by its
characterization also, such as elemental analysis (EDX), FT-IR,
XRD, XPS and HRTEM.
For regenerated Pd–PW11/ZrO2, the EDX values of Pd
(0.71 wt%) and W (15.08 wt%) are in good agreement with the
fresh one (0.72 wt% Pd and 15.19 wt% W) conrming no
emission of Pd as well as W from the catalyst during the reac-
tion. Elemental mapping is shown in Fig. S5.†
FT-IR spectra of fresh and regenerated catalysts are displayed
in Fig. 6a. The spectrum of regenerated catalyst was found to be
almost identical to fresh one, without any signicant shi in the
bands. However, the bands intensity for regenerated catalyst was
slightly low compared to fresh one, may be due to the sticking of
the substrates, which had no effect on the efficiency of the catalyst.
XRD spectra of fresh and regenerated catalyst are shown in
Fig. S6.† Obtained results reveal the retention of highly
dispersed nature of the catalyst. Absence of any characteristic
peaks regarding Pd aggregates as well as PW11 clearly indicates
the sustainability of the catalyst during the reaction.
Substrate
Product
TON/TOF (h–1
28 010/14 005
)
96
62
74
18 090/9045
21 591/10 780
99
28 885/14 443
26 843/13 422
92/100
71/100
20 716/10 358
a
Reaction conditions: cyclohexene (9.87 mmol), conc. of Pd
(0.0034 mol%), H2O (50) mL, temp. (50 ꢁC), H2 pressure (10 bar), time
(2 h).
XPS spectra of regenerated catalyst is presented in Fig. 6b
and c. Almost identical spectra of both fresh as well as regenerated
catalysts conrm the retention of Pd atoms over the surface of the
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Table 2 Comparison with reported systems in terms of cyclohexene substrate
H2 pressure
(bar)
Conversion
(%)
Catalyst
Pd (mol%)
Solvent
Temp. (ꢁC)
TON/TOF (hꢀ1
)
SH-IL-1.0wt%Pd16a
Pd/MSS@ZIF-8 (ref. 28)
Pd@CN29
Pd/SiO2 (ref. 16b)
Pd(0)–TPA/ZrO2 (ref. 12d)
Pd–PW11/ZrO2 (present work)
0.02
Auto-clave
60
35
90
25
80
50
20
1
—
10
10
10
99
5.6
96
96
96
96
5000/5000
560/93
44/4
1097/6582
12 604/3151
28 010/14 005
0.1738
2.208
0.091
0.0076
0.0034
Ethyl acetate
Formic acid (proton transfer)
CO2 (60 bar)
Water
Water
stabilization of the PdNPs during the reaction via PW11. More required more active amount of Pd as well as higher reaction
TEM images can be found in Fig. S7.† temperature compared to the present system. Obtain data
For regenerated catalyst, FT-IR indicates the retention of shows that the present catalyst is best one amongst all reported
PW11 unit although using it for number of consequent catalytic systems to till date in terms of used catalyst amount, TON as
runs. XRD shows the highly dispersed amorphous nature of the well as TOF under mild reaction conditions.
catalyst. XPS and HRTEM conrm the presence and homoge-
neous dispersion of the isolated PdNPs over the surface of the
catalyst.
Effect of addenda atom
In order to check the effect of addenda atom, the reactions were
performed using Pd–PW11/ZrO2 and Pd–PW12/ZrO2.
Scope of catalysis
(i) Comparison of TON (% conversion): hydrogenation was
carried out under optimized conditions using both the catalysts and
obtained results are enumerated in Table 3. It shows that Pd–PW11/
ZrO2 is highly active (z6.4-fold %) compared to Pd–PW12/ZrO2. (ii)
Comparison of TOF: for more insight, activity of the catalysts was also
compared in terms of TOF. For the same, the prolong reactions were
carried out under identical conditions to achieve possible maximum
% conversion, and the obtained results (Table 3) show that Pd–PW11/
ZrO2 requires only 2 h for fruitful collision of substrates to achieve
maximum 96% conversion with TOF 14 005 hꢀ1 whereas, Pd–PW12/
The scope for hydrogenation of different substrates was also
evaluated and the obtained results are enumerated in Table 1.
From the results it is clear that the catalyst is dominantly
viable for the selective hydrogenation of C]C including
aliphatic and aromatic compounds.
Comparison with reported systems
The efficiency of the catalyst with reported system is also
enumerated in Table 2 with respect to cyclohexene hydrogena-
tion in terms of % conversion, TON as well as TOF. Liu et al.16a
achieved 99% conversion at moderate temperature and very
high H2 pressure (20 bar) compared to present system. Zhang
et al.28 performed the reaction at low temperature with very poor
yield and with high catalyst concentration. Leng et al.29 reported
the use of formic acid as proton transferring agent to achieve
96% conversion using high temperature and catalyst amount
compared to the present system. Panpranot et al.16b obtained
96% conversion utilizing supercritical CO2 as a solvent at 60 bar
pressure (harsh conditions). Our previously reported system
Pd–TPA/ZrO2 (ref. 12d) was also found to be highly active but
ZrO2 requires 8 h for 97% conversion with TOF 3538 hꢀ1
.
Here, high activity of Pd–PW11/ZrO2 is attributed to greater
number of counter protons compared to Pd–PW12/ZrO2. It is
well known that for surface phenomenon type catalytic hydro-
genation, the formation of Pd–H is necessary, higher the
formation of Pd–H more the % conversion. As Pd–PW11/ZrO2
consists seven hydrogen (in the form of counter protons), which
accelerate the formation of Pd–H to enhance the hydrogenation
rate, whereas Pd–PW12/ZrO2 has only three counter protons and
as result it activates the reaction moderately.
Mechanistic study
The mechanistic study was investigated by carrying out, (i)
control experiments, (ii) reaction in inert atmosphere and (iii)
reaction using D2O as solvent.
Table 3 Effect of addenda atom on conversion/TON/TOFa
Time (h)/Conv%/
(i) Control experiments: to study the roll of each component as
well as the responsible active species, the reaction was per-
formed using ZrO2, PW11, PW11/ZrO2, PdCl2 and Pd–PW11/ZrO2
under optimized conditions. Achieved results (as shown in
control experiment study) indicate that Pd is only the active
sites for the reaction. (ii) Inert atmosphere: to ensure the
hydrogenation via molecular H2, the reaction was performed
under N2 pressure instead of H2 and no conversion assures the
necessity of H2 for reduction. (iii) D2O as solvent: to investigate
the role of water, reaction was performed using D2O as solvent,
Conversion
(%)/TONb
TOF
Number of
protons
Catalyst
(hꢀ1
)
Pd–PW12
ZrO2
Pd–PW11
ZrO2
/
/
15/4377
8/97/3538
3
7
96/28 010
2/96/14 005
a
Reaction conditions: cyclohexene (9.87 mmol), conc. of Pd
(0.0034 mol%), substrate/catalyst ratio (29 177/1), H2O (50 mL), temp.
(50 ꢁC), H2 pressure (10 bar). b Time (2 h).
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can be extended for the designing of other precious metal-based
NPs for various organic transformations.
Author contributions
Anish Patel: Conceptualization, formal analysis, investigation,
methodology, supervision, visualization, writing–review & edit-
ing. Anjali Patel: Conceptualization, supervision, writing–review
& editing. The manuscript was written through contributions of
all authors. All authors have given approval to the nal version
of the manuscript.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
We are thankful to Department of Atomic Energy (DAE) and
Board of Research in Nuclear Science (BRNS), project no. 37(2)/
14/34/2014-BRNS, Mumbai, for the nancial support. One of the
authors Mr Anish Patel is thankful to the same for the grant of
SRF. We are also thankful to Department of Chemistry, The
Maharaja Sayajirao University of Baroda for BET surface area
analysis.
Fig. 7 Proposed reaction mechanism.
no formation of deuterated product (cyclohexane, conrmed by
1H NMR) exposed that the hydrogen is directly transferred from
molecular H2, not from water. These results are in good agree-
ment with conventional mechanism for hydrogenation of
unsaturated hydrocarbon. Hence, we are proposing the same
mechanism in which mechanism runs by formation of palla-
dium hydride formation via homolytic cleavage of H2 molecule,
followed by hydrogen transfer to the unsaturated bond. Based on
this data, Fig. 7 shows the proposed mechanism.
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