Microwave assisted hydrogenation of olefins by Pd NPs@polystyrene resin using a gas addition kit: A robust and sustainable protocol

Article abstract of DOI:10.1039/c8nj03298j

Polystyrene (PS) resin bead supported palladium nanoparticles (Pd NPs@PS resin) were prepared and their catalytic activity for the hydrogenation of olefins was investigated under microwave heating. The hydrogenation of styrene was effectively carried out in EtOH/H₂O, in the presence of 0.00035 mmol of the catalyst to afford the corresponding ethylbenzene in high yield within 20 min under microwave heating. The catalyst efficiency measured in terms of turn over number (TON) and turn over frequency (TOF) was found to be 2829 and 8573 (h^-1), respectively. The encapsulated palladium nanoparticles were easily recovered by a simple filtration method and reused several times without significant loss in their catalytic activity. Further, the method showed a wide substrate scope under mild reaction conditions, making it a green versatile and highly sustainable protocol.

Full text of DOI:10.1039/c8nj03298j

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Microwave assisted hydrogenation of olefins by
Pd NPs@polystyrene resin using a gas addition kit:
a robust and sustainable protocol†
Cite this: New J. Chem., 2018,
42, 18935
Anuj S. Sharma and Harjinder Kaur
*
Polystyrene (PS) resin bead supported palladium nanoparticles (Pd NPs@PS resin) were prepared and
their catalytic activity for the hydrogenation of olefins was investigated under microwave heating. The
2
hydrogenation of styrene was effectively carried out in EtOH/H O, in the presence of 0.00035 mmol of
the catalyst to afford the corresponding ethylbenzene in high yield within 20 min under microwave
Received 2nd July 2018,
heating. The catalyst efficiency measured in terms of turn over number (TON) and turn over frequency
Accepted 12th October 2018
À1
(
TOF) was found to be 2829 and 8573 (h ), respectively. The encapsulated palladium nanoparticles
were easily recovered by a simple filtration method and reused several times without significant loss
in their catalytic activity. Further, the method showed a wide substrate scope under mild reaction
conditions, making it a green versatile and highly sustainable protocol.
DOI: 10.1039/c8nj03298j
rsc.li/njc
Filterability can be another problem with metal nanoparticles
supported on classic materials such as carbon, owing to their
Introduction
The hydrogenation of olefins is one of the oldest and most tendency of pulverisation to give nanometre-sized catalyst particles
important catalytic reactions in industry that can be effectively that turn out to be very difficult to recover and be successfully
13–16
catalyzed by transition metals. In most hydrogenation reactions, reused for the next cycle.
With the advancements in nano-
addition of hydrogen to unsaturated compounds such as alkenes, technology, new methods for the synthesis of nanoparticles with
alkynes, etc. is carried out in the presence of relevant catalysts and perfect size control and new porous supports with better accessi-
this protocol with 100% atom economy is arguably the cleanest bility have come into play that can be used to achieve high
possible way to reduce an organic compound. Furthermore, turnover numbers (TONs) and turn over frequencies (TOFs).
catalysts can be recycled with ease. This reaction using molecular
Over the last few decades, the area of microwave chemistry
hydrogen requires harsh reaction conditions such as high has become a powerful tool for performing challenging organic
pressures and temperatures and thus a lot of research is directed reactions in a short span of time with high yield and purity.
1–5
towards making these reactions energy efficient.
As with several other types of metal catalyzed reactions,
Palladium is one of the most useful metal catalysts in chemical hydrogenation reactions have also benefited from the use of
industry. Among the various applications of palladium catalysts, microwave irradiation to accelerate the reaction and improve
1
7–20
the most important are its ability to catalyze the formation of C–C yields.
As mentioned earlier, the utilization of molecular
bonds, C–N bonds, and the C–H activation and hydrogenation hydrogen as a reductant typically decreases the steps required
6–10
reactions.
Though expensive, its insensitivity towards water for the purification of products, however, getting the hydrogen
and oxygen makes palladium an attractive catalyst in industry as gas into the reaction vessel in a microwave oven can be
11,12
21–24
well as in academia.
The heterogenization of Pd nanocatalysts challenging.
To date, there have been only a few reports
(
heterogeneous catalysts) has shown great potential in organic of hydrogenation reactions carried out in special microwave
2
5–27
reactions as low Pd loadings and high reusability of the catalyst up reactors attached with a gas addition tool.
It was in 2007
to several cycles enhance its sustainability. Currently up to 75% when Vanier et al. first reported a microwave-assisted gas
of hydrogenation reactions in industry are carried out over Pd/C addition tool for the hydrogenation of several different types
(heterogeneous catalyst). However, aggregation and Ostwald of molecules at moderate temperature and pressure using Pd/C
ripening lead to larger particles and, thus, to catalyst deactivation. as the catalyst. The reaction was carried out at 80 1C in the
2
5
presence of 10 mol% catalyst. After this report, Taddei et al.
developed a protocol for the hydrogenation of pyridine rings
Department of Chemistry, School of Sciences, Gujarat University, Ahmedabad,
using PtO
2
(10 mol%) as the catalyst under microwave
India. E-mail: hk_ss_in@yahoo.com; Fax: +91 79 26308545; Tel: +91 79 26300969
2
6
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/c8nj03298j irradiation.
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In our lab, we have reported various C–C couplings under
NJC
microwave heating catalyzed by Pd NPs@PS resin and found
that the catalyst is robust under microwave conditions as no
deterioration of support or agglomeration of nanoparticles was
2
8–30
observed under microwave heating.
Inspired by this, we
evaluated Pd NPs@PS resin for the catalytic hydrogenation of
styrene in a microwave attached with a gas addition kit as it is
an important industrial reaction. A special gas addition kit was
used for passing hydrogen gas and the effect of various impor-
tant reaction parameters such as solvents, temperature, catalyst
quantity, reusability and reaction time was studied. In addition,
the usefulness of the protocol for the industrially important
reduction of cyclohexene, cyclooctene, indene, trans-stilbene,
etc. was explored.
Fig. 2 PXRD pattern of Pd NPs@PS resin.
Results and discussion
Amberlite XAD-4 (polystyrene divinylbenzene cross-linked
macroporous resin) is a commercial resin with low cost that
is widely employed as an ideal support material for the stabili-
of Pd 3d of Pd NPs@PS resin showed the first pair of Pd 3d peaks
at 335.6 and 340.9 eV due to Pd 3d5/2 and Pd 3d3/2, respectively,
which are characteristic of Pd(0), as shown in Fig. 3(D). The second
2
À1
zation of catalysts. The high surface area of resins (750 m g ),
excellent stability (chemical and thermal), inert behaviour
during the reaction, and easy separation from the reaction
medium make them a popular support especially for liquid
phase reactions. As they do not possess any functional group,
metal loading is generally low, which has rather become a
desirable feature for microwave enhanced reactions as a high
concentration of a metal can produce hot spots. Also, in the
absence of ligands, the capping of surface atoms is not very
strong, which enhances catalyst activity. Palladium nano-
particles were synthesized by a simple sorption followed by
chemical reduction, in a method developed in our lab earlier
pair found at 337.10 and 342.60 eV corresponded to the Pd 3d5/2
37–39
and Pd 3d3/2 electronic states characteristic of Pd(II).
So, as the
second peak is weaker, it can be concluded that the nanoparticles
comprise Pd in the zero oxidation state with a thin layer of oxide
over it.
Scanning electron microscopy (SEM) is used to investigate
the composition and morphology of the surface of a catalyst
material. The SEM images of whole beads and broken beads
were taken. They indicated that there are no big nanoparticles
or agglomeration present on the exterior surface of the resin
beads (Fig. 4). It was concluded that overall, palladium nano-
particles are encapsulated in the interior polymer matrix of the
resin. Even in the broken beads, the Pd NPs could not be seen
clearly probably due to the dense nature of the resin, however,
they were clearly visible in the TEM pictures. The total quantity
of Pd loaded onto the polystyrene resin was investigated by
ICP-AES (inductively coupled plasma-atomic emission spectro-
scopy) analysis. 500 mg of the catalyst was incinerated in a
crucible for 4 h at 550 1C. The resultant residue was dissolved in
3
1–34
(Fig. 1).
After impregnation of nanoparticles, the poly-
styrene resin beads became light brown (Fig. 1).
The X-ray diffraction pattern (XRD) pattern of the finely
powdered catalyst (Pd NPs@PS resin) showed the characteristic
diffraction peaks of palladium at 2y = 39.121, 40.461 and 64.911
(JCPDS card no. 46-1043) that corresponded to the (111), (200)
and (220) reflections of the face centred cubic (fcc) arrangement
3
5,36
of Pd(0) nanoparticles depicted in Fig. 2.
The XPS survey
spectrum for Pd NPs@PS resin is depicted in Fig. 3(A). The
spectrum showed a strong existence of C 1s (285 eV) core level
binding energy peaks along with O 1s (532 eV) and Pd3d (335 eV).
In the high resolution scans of C 1s, the spectrum possesses
different deconvolution components. The deconvolution spectrum
1
mL of aqua regia and diluted to 5 mL with DI water. The
average concentration of palladium (3 trials) as determined by
À1
À1
ICP-AES was found to be 0.00235 mmol g (0.249 mg g ).
TEM and HR-TEM were mainly used to analyse the shape
and size of Pd NPs@PS resin and the results are shown in
Fig. 5. It was observed that all of the encapsulated Pd NPs were
spherical in shape and well dispersed. The average particle size
of the Pd NPs was found to be between 1 and 6 nm. The high-
resolution TEM (HR-TEM) images showed the lattice fringes
with the crystal lattice spacing of Pd NPs@PS resin at 0.225 nm,
which could be ascribed to the Pd(111) plane and matched well
4
0
with the literature value.
The energy dispersive X-ray (EDX) analysis was used to verify
the presence of palladium in the matrix of the resin beads. The
EDX results showed a small peak of palladium (0.02 wt%) that
Fig. 1 Schematic preparation of Pd NPs@polystyrene resin (PS).
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Fig. 3 Survey spectrum of Pd NPs@PS resin (A) and deconvoluted spectra of C 1s (B), O 1s (C) and Pd 3d (D).
Fig. 4 SEM images of Pd NPs@PS resin (A), PS resin bead and (B) crushed
resin showing interior of a bead.
confirmed its presence in the matrix of the resin, as shown in
Fig. 6(A). In addition, EDS mapping also confirmed the homo-
genous distribution of the Pd NPs in the resin (Fig. 6(B)).
Scope of Pd NPs@PS resin for the hydrogenation of olefins
under microwave irradiation
Fig. 5 TEM and HR-TEM images of the Pd NPs@PS resin taken after
crushing the beads.
The catalytic activity of Pd NPs@PS resin was assessed for the
hydrogenation of olefins under microwave irradiation attached
with the gas addition kit. The gas addition kit attached to a CEM
microwave provides the ability to add a gaseous reagent to a
Optimization of the reaction conditions
10 mL reaction vessel prior to microwave irradiation. A fiber optic
probe attached to the gas addition kit was used to continuously Initially, to optimize the reaction conditions, styrene was chosen
monitor the temperature during the reaction. The reaction as a model substrate and a series of experiments was carried out
assembly for conducting the hydrogenation of olefins is shown with varying amounts of the catalyst in the presence of molecular
in Fig. S1 (ESI†).
hydrogen (Scheme 1) at 60 1C and 50 psi pressure. According to
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Table 1 Effect of solvents on Pd NPs@PS resin catalyzed hydrogenation
of styrene
b
c
Entry
Solvent
Conversion (%)
TON
TOF
a
1
—
Trace
58
65
45
05
82
87
99
—
—
2
3
4
5
6
7
8
Toluene
1,4-Dioxane
Acetonitrile
1657
1857
1286
143
2343
2486
2829
5021
5627
3897
433
7100
7533
8573
H
2
O
MeOH
EtOH
EtOH : H
2
O (4 : 1)
Reaction conditions: styrene (1 mmol), Pd NPs@PS resin (150 mg =
.00035 mmol of Pd), solvent (5 mL), 20 min, 60 1C, 50 psi, 80 W.
0
a
b
Conversion was determined by GC-MS. Absence of solvent. Moles of
product per mole of Pd. TON per hour (h).
c
2
solvents such as 1,4-dioxane, toluene, MeOH, EtOH and H O,
as depicted in Table 1. The results showed that conversion is
higher in polar protic solvents compared to non polar solvents.
In the case of hydrogenation reactions, this is attributed to
stabilization of the highly reactive catalytic intermediate by
4
1
these solvents. Though water is a desirable solvent according
to the green and sustainable chemistry laws, the reaction
Fig. 6 EDX spectrum of Pd NPs@PS resin (A). EDS mapping of C and Pd
present in the Pd NPs@PS resin (B).
conducted in H
low solubility of olefins in water. EtOH : H
2
O gave poor conversion. This was due to the
O mixed in a molar
2
ratio of 4 : 1 gave excellent results. We also tried the hydrogena-
tion of styrene under solvent free conditions and found only
trace conversion taking place. This was attributed to poor mass
transfer in the presence of styrene alone and the tendency of
styrene to polymerize.
In order to investigate the effect of temperature on styrene
conversion, the hydrogenation of styrene with molecular hydrogen
was conducted at temperatures ranging from RT to 70 1C
employing Pd NPs@PS resin as the catalyst (Table 2). It was noted
that the conversion increased from 25 to 99% with an increase in
temperature from RT to 60 1C. The best conversion of styrene
Scheme 1 Hydrogenation of styrene catalyzed by Pd NPs@PS resin.
(
99%) was achieved at 60 1C for which the calculated values of TON
À1
and TOF were 2879 and 8573 h , respectively. Therefore, 60 1C
the ideal gas law, the amount of hydrogen necessary for complete
conversion to the product is about 45 psi. The reaction was
performed by charging the reaction vessel up to 50 psi to give
only a slight excess of hydrogen gas resulting in minimal waste.
The progress of the reaction was monitored by GC-MS. The peaks
for styrene and ethylbenzene were observed at retention times of
was selected as the optimum temperature for the microwave
assisted hydrogenation of styrene and the effects of catalyst
concentration and reaction time were studied at this temperature.
Effect of catalyst concentration and reaction time
4.50 and 3.80 min, respectively. Ethylbenzene (EB) was detected A blank reaction was carried out in the absence of the catalyst;
as the main product.
no conversion of styrene was detected, which indicated the
Effect of solvents and temperature
Table 2 Effect of temperature on the hydrogenation of styrene catalyzed
In the case of resin based catalysts, the choice of solvents
strongly affects the performance of the catalyst. Generally,
solvents act as a medium for dissolving the particular reactant Entry
by Pd NPs@PS resin
a
b
Temperature
Conversion (%)
TON
TOF
molecules and their interaction with the catalyst surface. How-
ever, in the case of resin encapsulated catalysts, they also
enhance activity by swelling the resin and thus enhance acces-
sibility to the available catalytic sites during the organic reac-
tions. To investigate the effect of solvents on the performance
of the reaction, the model reaction was conducted in the
1
2
3
4
RT
40
50
60
25
55
78
99
714
1572
2229
2829
2164
4764
6755
8573
Reaction conditions: styrene (1 mmol), Pd NPs@PS resin (150 mg =
.00035 mmol of Pd), solvent (5 mL), 20 min, 50 psi, 80 W. Conversion
0
a
b
was determined by GC-MS. Moles of product per mole of Pd. TON
presence of the Pd NPs@PS resin catalyst with different per hour (h).
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necessity of the catalyst in the reaction. Additionally, the Table 3 Effect of time on the hydrogenation of styrene catalyzed by Pd
NPs@PS resin
reaction was also conducted by employing polystyrene resin
alone as the catalyst and trace conversion was observed.
To study the effect of catalyst concentration on the reaction
rate, we changed the quantity of the catalyst from 70 to 150 mg
while keeping the other parameters constant. It was observed
that as the catalyst quantity increased from 70 to 150 mg,
a
b
Entry
Time (min)
Conversion (%)
TON
TOF
1
2
3
4
05
10
15
20
34
58
85
99
971
1657
2429
2829
11 699
9982
9716
8573
styrene conversion increased up to 99% (Fig. 7). 150 mg of the Reaction conditions: styrene (1 mmol), Pd NPs@PS resin (150 mg =
.00035 mmol of Pd), solvent (5 mL), 60 1C, 50 psi, 80 W. Conversion
0
catalyst contains 0.00035 mmol of Pd giving a TON and TOF of
a
b
was determined by GC-MS. Moles of product per mole of Pd. TON
per hour (h).
2829 and 8573, respectively.
To ascertain the conversion of styrene, reaction mixtures
were analyzed at intervals of 5 min by GC-MS. It was observed
that after 5 min, the conversion of styrene was 34% with a TON Table 4 Hydrogenation of olefins catalyzed by Pd NPs@PS resin
À1
and TOF up to 971 and 11 699 h (Table 3, entry 1). It was
Time
(min)
Conversion
(%)
observed that as the conversion of styrene increased from 34 to
9%, the reaction time increased from 5 to 20 min (Table 3,
a
b
Reactant
Product
TON TOF
9
entry 4). This observation indicates that the TOF values
decrease as the reaction proceeds. This is mainly attributed
to a decrease in the accessibility of active sites due to the
passivation of the metal surface by product accumulation as
2
2
0
0
99
87
2829 8573
2486 7533
4
1,42
the reaction proceeds.
In addition, it was reported that the
styrene hydrogenation reaction intrinsic kinetics coincides with
the zeroth-order reaction law. Thus, the rate of conversion of
4
3–46
styrene remains constant throughout the reaction.
The
initial reaction rate constant was calculated from the straight
À1
line and was estimated to be 0.048 mmol min (Fig. S2, ESI†).
20
99
98
2829 8573
Scope of the microwave assisted hydrogenation catalyzed by Pd
NPs@PS resin
Encouraged by the excellent performance of Pd NPs@PS resin,
a variety of functionalized aromatic compounds containing
electron withdrawing and electron donating groups such as
-nitro styrene and 4-bromo styrene, and 4-methyl styrene and
-chloro styrene were evaluated for the hydrogenation reaction
20
2800 8485
2771 8397
4
2
20
97
10
under the optimized reaction conditions (Table 4). We observed
that the substituted styrene containing electron donating groups
30
286
572
1
1
1
5
5
5
98
96
99
2800 11 200
2743 10 971
2829 11 316
Reaction conditions: substrate (1 mmol), Pd NPs@PS resin (150 mg =
0
.00035 mmol of Pd), EtOH : water (4 : 1) (5 mL), H
2
(50 psi), 60 1C,
a
b
80 W. Moles of product per mole of Pd. TON per hour (h).
(
–CH , –Br and –Cl) afforded excellent conversion. In the case of
3
4-nitro styrene, it was assumed that the hydrogenation will take
place at only one of the reducible functionalities (CH QCH ) to
2
2
afford 4-nitro ethylbenzene. However, we observed that the
catalyst was not selective and the nitro group was also reduced.
On employing three equivalents of molecular hydrogen, 3-ethyl
aniline was obtained as the exclusive product under the
Fig. 7 Effect of catalyst quantity on the conversion of styrene (reaction
conditions: styrene (1 mmol), EtOH : H
0 W.)
2
O (5 mL), 20 min, 60 1C, 50 psi,
8
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optimized reaction conditions. In addition, we also tried the Reusability and stability of the catalyst
hydrogenation of cyclohexene, cyclooctene, indene and trans-
As far as heterogeneous catalysts are concerned, recyclability is
the most important and essential parameter in small as well as
large scale catalytic reactions. After each single catalytic run,
the catalyst was separated from the reaction mixture by a
simple filtration method and frequently washed with hot DI
water and acetone to remove any sorbed products. After drying,
it was reused for the next cycle. The experimental results of
each catalytic run indicated that the Pd NPs@PS resin catalyst
did not exhibit any significant loss in activity (Fig. 8). The
TEM images of the catalyst after the fifth run revealed that no
big aggregation of particles occurred and the average size of
particles was still found to be 1–8 nm, as depicted in Fig. 9.
Furthermore, it was also noted from ICP-AES that the loaded
quantity of Pd (150 mg = 0.00035 mmol) present in the Pd
NPs@PS resin catalyst after five catalytic runs was almost to
same as that of the fresh catalyst.
stilbene. The results indicated high conversions in the case of
cyclohexene, cyclooctene and indene in a short period of time
(15 min). Poor conversion was observed in the case of trans-
stilbene even after increasing the reaction time (30 min). This
may be due to the poor interaction of trans stilbene with the
available catalytic sites on the palladium nanoparticles. All the
compounds were identified by their mass spectra and some were
1
13
also confirmed by H-NMR and C-NMR spectroscopy. Thus,
it can be stated that the protocol has high applicability and
we achieved good turnover numbers (TONs) and turnover
frequencies (TOFs) for almost all the reactants.
Conclusions
In summary, we have developed a protocol for the reduction of
styrenes with a significant increase in reactivity, reduction in
the reaction time and substantially high yields (offering 4 99%
conversion of styrene to ethylbenzene at 50 psi pressure in
20 min) under microwave irradiation conditions. A high TON
and TOF confirmed the efficiency of the catalyst. The catalyst
can be easily separated (filtration) from the reaction medium
and used for more than five times without significant loss in its
activity. The catalyst (Pd NPs@PS resin) is easy to synthesize
and exhibits good activity, stability and reusability. The leaching
of palladium is also negligible. The study concluded that there is
great potential for the development of the microwave assisted
hydrogenation of C–C unsaturated compounds under mild and
sustainable reaction conditions. Also, Pd NPs@PS resin may be a
useful alternative to Pd/C for microwave assisted hydrogenation
reactions.
Fig. 8 Reusability of Pd NPs@PS resin (reaction conditions: styrene
(
(
1 mmol), Pd NPs@PS resin (150 mg = 0.00035 mmol of Pd), EtOH : water
4 : 1) (5 mL), H (50 psi), 60 1C, 80 W.)
2
Conflicts of interest
There are no conflicts to declare.
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