Iridium nanoparticles deposited on hypercrosslinked polystyrene: synthesis and application in the hydrogenation of aromatic compounds

Article abstract of DOI:10.1007/s13738-020-01854-w

A novel method for the incorporation of iridium nanoparticles into a hypercrosslinked polystyrene matrix is developed using supercritical CO₂ as reaction medium. The composite has regularly distributed iridium nanoparticles with monomodal size of ca 5?nm. The catalyst shows high activity in the hydrogenation of benzene and can be recycled ten times without any decrease in productivity. The catalyst gave a full conversion in the hydrogenation of toluene to methylcyclohexane and fluoro- and chlorobenzene to cyclohexane.

Full text of DOI:10.1007/s13738-020-01854-w

Journal of the Iranian Chemical Society
https://doi.org/10.1007/s13738-020-01854-w
SHORT COMMUNICATION
Iridium nanoparticles deposited on hypercrosslinked polystyrene:
synthesis and application in the hydrogenation of aromatic
compounds
Sergey E. Lyubimov1 · Marina V. Sokolovskaya1 · Alexander A. Korlyukov1 · Oleg P. Parenago2 · Vadim A. Davankov1
Received: 15 October 2019 / Accepted: 3 January 2020
© Iranian Chemical Society 2020
Abstract
A novel method for the incorporation of iridium nanoparticles into a hypercrosslinked polystyrene matrix is developed
using supercritical CO2 as reaction medium. The composite has regularly distributed iridium nanoparticles with monomodal
size of ca 5 nm. The catalyst shows high activity in the hydrogenation of benzene and can be recycled ten times without
any decrease in productivity. The catalyst gave a full conversion in the hydrogenation of toluene to methylcyclohexane and
fuoro- and chlorobenzene to cyclohexane.
Keywords Hypercrosslinked polystyrene · Iridium nanoparticles · Hydrogenation · Arenes
Introduction
protection [11–18]. It should be noted that the catalytic
activity of heterogeneous catalysts depends not only on its
stability but also on the accessibility of their surface area to
the reacting molecules. One advanced polymeric support
characterized by very high specifc surface area (typically
in the range of 1000 m2/g) is hypercrosslinked polysty-
rene (HPS) [19]. Moreover, HPS polymers can swell in a
wide range of solvents, even thermodynamically poor ones.
Accordingly, Pt, Pd or Ru nanoparticles incorporated into a
HPS matrix were successfully used in the oxidation, cross-
coupling, hydroformylation and hydrogenation reactions
[20–24].
New methods for the synthesis of metal nanoparticles have
attracted increased attention because of their inherent large
surface-to-volume ratio and quantum size efects [1–5]. The
high surface area and thereby large number of active cent-
ers make the metal nanoparticle dispersions very attractive
for catalysis application [6, 7]. Nevertheless, these ultrafne
particles often agglomerate to form either lumps or second-
ary larger particles in order to minimize the total surface or
the interfacial energy of the system [8–10]. Therefore, it is
very important to stabilize the nanoparticles against adverse
agglomeration at preparation, storage and usage stages. The
use of stabilizing agents such as surfactants, ionic liquids,
polyoxoanions, carbon nanotubes and polymers provides
metal nanoparticles stabilized by electronic and/or steric
Among the nanosized transition metals commonly used
in catalysis, synthesis of iridium nanoparticles is limited
to a few examples. Diferent supports, such as ionic liq-
uids, silica, carbon nanotubes, alumina and polyvinylpyr-
rolidone, have been studied to immobilize Ir nanoparticles
[11, 25–30].
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s13738-020-01854-w) contains
supplementary material, which is available to authorized users.
Here we report a new method for the formation of Ir nan-
oparticles in HPS matrix, which consist of decomposition-
reduction of [Ir(COD)Cl]2 in supercritical CO2/H2, as well
as a successful use of the composite in the hydrogenation of
benzene, toluene, fuorobenzene and chlorobenzene.
* Sergey E. Lyubimov
lssp452@mail.ru
1
A.N. Nesmeyanov Institute of Organoelement Compounds,
Russian Academy of Sciences, Vavilov Str. 28, Moscow,
Russia 119991
2
Institute of Petrochemical Synthesis, Russian Academy
of Sciences, Leninsky Prospekt 29, Moscow, Russia 119991
Vol.:(0123456789)
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Journal of the Iranian Chemical Society
General procedure for arene hydrogenation
Experimental part
76 mg of the polymer-Ir composite was placed into a stain-
less-steel autoclave, and 4.43 mmol of arene was added. The
autoclave was flled with H2 (45 atm) and maintained for 5 or
24 h (25 or 100 °C), cooled to 25 °C and then depressurized.
The liquid reaction mixture was separated from the solid
catalyst using a Pasteur pipette and analyzed by 1H NMR.
For reuse experiments, 0.4 mL of benzene and 45 atm H2
were added again and the autoclave was heated for 5 h.
General
Biporous (characteristic pore size 71 and 1.6 nm) support
(insoluble in organic and inorganic solvents 0.3–1.2 mm
beads), hypercrosslinked polystyrene Macronet™
MN-200 with the specific surface area of 1100 m2/g
(Purolite, UK) was washed with MeOH and dried in vacuo
before the use. [Ir(COD)Cl]2 (Sigma-Aldrich) was used as
received. Samples morphology was studied using Hitachi
HT7700 transmission electron microscope. Images were
acquired in bright-field TEM mode at 100 kV acceler-
ating voltage. X-ray powder diffraction (XRD) patterns
were taken by a Bruker D8 Advanced X-ray diffractome-
ter using CuKa radiation (λ= 1.5406 Å). 1H NMR spectra
were recorded on a Bruker 400 spectrometer in CDCl3.
X-ray fluorescence analysis was performed on an Innov-X
α-2000 X-Ray spectrometer.
Results and discussion
The Ir(0) nanoparticles immobilized in Purolite Macronet
MN-200 resin were obtained by adsorption of [Ir(COD)
Cl]2 dissolved in CH2Cl2, followed by vacuum draying and
reduction with hydrogen in supercritical CO2. According to
X-ray fuorescence analysis, the average iridium content in
the resin was found to be 2.8%. The powder X-ray difrac-
tion measurement showed difraction peaks 2θ at of 40.7°,
47.3° and 69.1° which correspond to face-centered-cubic
crystalline structure of the Ir nanoparticles (Fig. 1) [31].
Calculation of crystallite size from integral breadth of refec-
tions using Lvol-IB formalism showed that the average size
of the Ir nanoparticles amounts to 0.5 nm. Such a low value
of crystallite size can be explained by weak difraction of
iridium nanoparticles that faded by scattering from amor-
phous polystyrene.
Catalyst preparation
[Ir(COD)Cl]2 (64 mg) was dissolved in CH2Cl2 (5.4 mL)
and added to industrial Macronet MN-200 resin (0.7 g);
the solvent was removed in vacuo. The resin was placed
into a 10-mL stainless-steel autoclave and the vessel was
pressurized with H2 (45 atm) and then filed with CO2 by
means of a syringe press (High Pressure Equipment) to
the total pressure of 150 atm. The mixture was heated for
45 min (50 °C) and then slowly depressurized. A gray
composite was washed by acetone (3 × 5 mL) and dried
in vacuo.
The size and morphology of the Ir(0) nanoparticles
were also investigated by TEM analysis of the milled
MN-200 resin-iridium composite (Fig. 2). The nanopar-
ticles were enough regularly distributed and have mono-
modal size of ca 6 nm. The histogram of nanoparticle
Fig. 1 XRD patterns of the catalyst
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Journal of the Iranian Chemical Society
Fig. 2 TEM image of the milled
catalyst
1). Lowering of H2 pressure led to a decrease in the reac-
tion rate (Table 1, entry 2).
The use of a higher reaction temperature (100 °C) gave
full conversion of benzene in 5 h (Table 1, entry 3). To test
the reusability of the catalyst, a series of twelve consecutive
runs were carried out. A complete conversion was obtained in
ten catalytic cycles, but decreased starting with the eleventh
catalytic run (Table 1, entries 3–14).
We have also examined the hydrogenation of toluene, chlo-
robenzene, fuorobenzene and aniline. In the case of toluene
full conversion to methylcyclohexane was obtained in 5 h at
100 °C (Table 1, entry 15). Under the same conditions, chlo-
robenzene and fuorobenzene were exposed to hydrodehalo-
genation process forming cyclohexane as the only product
(Table 1, entry 16, 17). Accordingly, the process may be inter-
esting to utilize toxic, thermally stable and ozone depleting
aryl halides to the harmless cyclohexane after synthetic use
[32, 33]. In the case of aniline, the catalyst gave a moderate
conversion to aminocyclohexane, probably due to deactivation
of nanoparticles by the strongly coordination active substrate
(Table 1, entry 18).
Fig. 3 Nanoparticles distribution histogram
distribution is represented in Fig. 3. Interestingly that each
particle exists as a non-spherical shape-cluster.
The obtained iridium composite was tested as a hydro-
genation catalyst for some model aromatic hydrocarbons.
The HPS-based system was frst used for benzene hydro-
genation yielding cyclohexane which is commonly used as
a non-toxic solvent. At 45 atm H2 and 25 °C, conversion
was 99.8% using only 0.25 mol% of iridium (Table 1, entry
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Journal of the Iranian Chemical Society
Table 1 Hydrogenation of
Entry
Substrate
Cycle
PH2, atm
T, °C
t, h
Conversion, %
arenes
1
2
3
4
5
6
7
8
Benzene
Benzene
Benzene
Benzene
Benzene
Benzene
Benzene
Benzene
Benzene
Benzene
Benzene
Benzene
Benzene
Benzene
Toluene
–
–
45
10
45
45
45
45
45
45
45
45
45
45
45
45
45
45
45
45
25
25
24
24
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
99.8
72
1
2
3
4
5
6
7
8
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
70
9
10
11
12
13
14
15
16
17
18
9
10
11
12
–
–
–
30
100
100
100
20
Chlorobenzenea
Fluorobenzenea
Aniline
–
5
Ir—0.25% mol
aCyclohexane obtained as the only product
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Conclusions
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Organomet. Chem. 30, 823–830 (2016)
We report a novel rapid method for obtaining a reusable
Ir(0) catalyst by decomposition-reduction of [Ir(COD)Cl]2 in
supercritical CO2/H2 within the matrix of hypercrosslinked
polystyrene. The method allows obtaining composite with
regularly distributed iridium nanoparticles having mono-
modal size of ca 5 nm. The HPS/Pd composite showed high
activity in the hydrogenation of benzene and can be recycled
ten times without any decrease in productivity. The cata-
lyst gave a full conversion in the hydrogenation of toluene
to methylcyclohexane and fuoro- and chlorobenzene to
cyclohexane. Accordingly, the last process may be interest-
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halides to the harmless cyclohexane after synthetic use.
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