Aromatic ring hydrogenation catalysed by nanoporous montmorillonite supported Ir(0)-nanoparticle composites under solvent free conditions

Article abstract of DOI:10.1039/c5nj03030g

Ir(0)-nanoparticles (Ir-NPs) were synthesized into the nanopores of modified montmorillonite clay by incipient wetness impregnation of IrCl₃ followed by reduction with ethylene glycol. The activation of the montmorillonite clay was carried out by treatment with HCl under controlled conditions to increase the surface area by generating nanopores which act as host for the metal nanoparticles. The synthesized Ir-NP-montmorillonite composites were characterized by N₂-sorption, powder XRD, SEM, EDS, TEM, XPS, etc. The composites exhibit high surface area of 327 m² g^-1 and the Ir-NPs with size around 4 nm are uniformly distributed on the support. The Ir-NPs show efficient catalytic activity in aromatic ring hydrogenation under solvent free conditions with maximum conversion up to 100% and Turn Over Frequency (TOF) up to 79 h^-1. The catalyst can be easily separated by simple filtration and remained active for several runs without significant loss of catalytic efficiency.

Full text of DOI:10.1039/c5nj03030g

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Aromatic ring hydrogenation catalysed by nanoporous
montmorillonite supported Ir(0)-nanoparticles composite under
solvent free condition
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Prabin Das, Podma Pollov Sarmah, Bibek Jyoti Borah, Lakshi Saikia and Dipak Kumar Dutta
www.rsc.org/
Ir(0)-nanoparticles (Ir-NPs) were synthesized into the nanopores of modified montmorillonite clay by incipient wetness
impregnation of the IrCl
3
followed by reduction with ethylene glycol. Activation of the montmorillonite clay was carried out
by treating with HCl under controlled condition to increase the surface area by generating nanopores which act as host for
2
the metal nanoparticles. The synthesized Ir-NPs-montmorillonite compositeswere characterized by N -sorption, powder
2
XRD, SEM, EDS, TEM, XPS etc. exhibiting high surface area of 327 m /g and size around 4 nm uniformly distributedon the
support. The Ir-NPs show efficient catalytic activity in aromatic ring hydrogenation under solvent free condition with
-1
maximum conversion up to 100% and Turn Over Frequency (TOF) upto 79h . The catalyst can be easily separated by
simple filtration and remained active for several runs without significant loss of catalytic efficiency.
reactions to produce the desired products in the fine chemicals
3
7
Introduction
and pharmaceutical industries . Moreover, aromatic ring
hydrogenation by efficient heterogeneous catalyst may pave
Over the past several decades, nanostructured materials are
the most intensive area of research due to their potential
3
8-40
the way for more efficient process for liquefaction of coal
.
1
-5
A large number of homogeneous and heterogeneous catalysts
prepared from various metals like Ni, Fe, Ru, Rh etc. have been
reported for hydrogenation reactions. However, applications
of Ir-metal for such reactions are less exploited and therefore,
applications in diverse fields . The high surface to volume
ratio and high density of active sites in nanomaterials lead to
6
the development of sustainable catalysis . Various industrially
important organic reactions like oxidation, hydrogenation,
hydroformylation, carbonylation, C-C bond formation
41-47
needs to be addressed
.
6
-11
In the present manuscript, we report the synthesis of Ir-NPs of
less than 4nm size into the nanopores of modified
montmorillonite clay. The catalyst can be easily separated by
simple filtration technique. The montmorillonite clay is
modified by acid activation to achieve high surface area and
nano-porosity which act as a support and restrict the size in
the nano-particle range. The synthesized Ir-NPs are
reactions are catalyzed by a wide variety of nanomaterials
.
However, the metal nanoparticles are very reactive and tend
to agglomerate during synthesis, if no stabilizing agents are
used. Therefore, a large numbers of stabilizing agents or
supports such as, natural and activated montmorillonite clay,
zeolites, activated carbon, polymers, organic ligands, metal
1
2-25
oxides etc.
are employed which prevent the agglomeration
1
1-21
2
characterized by powder XRD, TEM, XPS and N -sorption
of metal nanoparticles and controls the particle size
.
analysis etc. and evaluated as catalyst for the hydrogenation of
some aromatic compounds.
Among these, montmorillonite clay, one of the
environmentally benign, cheap, easily available and robust
supports, is used for the synthesis of different metal
nanoparticles to make heterogeneous catalysts having several
advantages over homogeneous process, like easily separable
Experimental
2
6-35
Materials and methods
from reaction mixture
. Acid activated montmorillonite clay
is partially delaminated and exhibits higher surface area
containing both micro and mesopores, and can be utilized as a
Purification of the naturally occurring Bentonite clay (procured
from Gujarat, India) was carried out by sedimentation
technique by following Stokes’ law to remove impurities like
quartz, iron oxide etc. to produce pure montmorillonite clay
3
host for synthesis of metal nanoparticles .
6
Hydrogenation of aromatic compounds is one of the important
(mont.) of size less than 2 μm. The basal spacing (d001) of the
air dried samples, determined by powder XRD, was about 12.5
Å. The surface area (BET) of the mont. as determined by N
2
2
-1
sorption technique was 101 m g . The analytical oxide
composition of Bentonite determined was SiO : 48.62%; Al O :
2
2
3
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2
0.02%; Fe
2
O
3
: 7.49%; MgO: 2.82%; CaO: 0.69%; loss on and washed with methanol followed by distilled water. The
O, K O and TiO
): 2.95%. composite was dried under vacuum and stored in airtight
ignition (LOI): 17.51%; and others (Na
2
2
2
Mont was converted to the homoionic Na-exchanged form by bottle. The sample thus prepared was designated as Ir-NPs.
stirring in 2 M NaCl solution for about 48 h, washed and Aromatic hydrogenation reaction. All the reactions were
dialysed using deionized distilled water until the conductivity carried out in a high pressure reactor of 50 ml capacity (Parr
of the water approached that of distilled water. The cation
exchange capacity (CEC) was 126 milliequivalent (meq) per 100
g of clay (sample dried at 120 °C).
Instrument Co.) taking 10 mmol of substrate and50 mg of
o
2
catalyst (Ir-NPs) and pressurized with H gas to 5 bar at 25 C.
o
The reactions were performed at 75 C under stirring at 500
rpm for different reaction time (1, 2, 3, 4, 5, and 6h). The
products of the reactions were analyzed using Gas
Chromatography. The catalyst (Ir-NPs) was recovered by
simple filtration, washed and dried for further recycling. The
effects of catalyst amount on the substrate conversion were
also carried out by varying the catalyst amounts (10, 20, 30, 40
and 50mg).
3
IrCl , ethylene glycol, ethanol, methanol and all the substrates
were purchased from Sigma-Aldrich, USA and Acros Organics.
All reagents were used as supplied.IR spectra were recorded
on KBr discs in a Shimadzu IR Affinity-1 spectrophotometer.
Specific surface area, pore volume, average pore diameter
were measured with the Autosorb-1 (Quantachrome, USA).
Specific surface area of the samples was measured by
adsorption of nitrogen gas at 77 K and applying the Brunauer–
Emmett–Teller (BET) calculation. Prior to adsorption, the
samples were degassed at 250 °C for 3 h. Pore size
Results and discussion
distributions were derived from desorption isotherms using Ir-NPs were synthesized into the nanopores of modified mont. by
the Barrett-Joyner-Halenda (BJH) method. Powder XRD were incipient wetness impregnation of the IrCl followed by reduction
3
recorded on a Rigaku Ultima IV diffractometer using a CuKα with ethylene glycol and tested as catalyst in the hydrogenation of
source (λ = 1.54 Å). The products were analyzed by Gas aromatic ring. Modification of the mont. was carried out by treating
Chromatography (Thermo Scientific TRACE 1300, FID detector). with HCl under controlled condition. Powder XRD analysis of the
Scanning Electron Microscopy (SEM) images and energy mont. shows the intense 2θ reflection at 7.06°, with corresponding
dispersive X-ray spectroscopy (EDS) patterns were obtained basal spacing (d001) of 12.5 Å, but upon acid activation it diminishes
from the samples by using Carl Zeiss SIGMA FE-SEM operated indicating partial delamination of the clay layered structure. The
at 5 and 20 KV and Oxford X-Max 20 EDS detector. CEC of the AT-mont. was found 40.8 meq/100 g, which is much
Transmission electron microscopy (TEM) images were lower compared to mont. (126 meq/100 g), also indicating the
recorded on a JEOL JEM-2011 electron microscope; the delamination upon activation. IR spectrum of the mont. and AT-
specimens were prepared by dispersing powdered samples in mont. have shown a shifting of Si-O stretching vibration from 1034
-1
isopropyl alcohol and placing them on a carbon coated copper to 1083 cm , which is due to change in the clay framework during
2
grid followed by drying. X-ray Photoelectron Spectra were acid activation. AT-mont. shows surface area of about 418 m /g
3
recorded on Kratos ESCA model Axis 165 spectrophotometer.
with high pore volume (0.60 cm /g) compared to mont. (Surface
2
3
area: 101 m /g, pore volume 0.15 cm /g), which is due to the
3
+
Synthesis
leaching of Al from the clay structure. The presence of type-IV
isotherm with H3 type hysteresis loop at P/P
o
~ 0.4-0.9 (Figure 1)
Preparation of Support. The purified mont. (7.0 g) was dispersed
in 350 mL 4 M hydrochloric acid and refluxed for 1 h. After
cooling, the supernatant liquid was discarded and the acid
250
activated mont. was repeatedly redispersed in deionized water
−
until no Cl ions could be detected by the AgNO
3
test. The
200
modified mont. was recovered, dried in air at 50 ± 5 °C
overnight to obtain the solid product. The acid activated mont.
was designated as AT-mont. The analytical oxide composition
of Bentonite determined was SiO
Fe2O : 2.39%; MgO: 0.19%; CaO: 0.56% loss on ignition (LOI):
3.74%; and others (Na O, K O and TiO ): 2.40%.
Synthesis of Ir-NPs. 1 g AT-mont. was impregnated with 0.50
mmol IrCl in aqueous medium (15 mL) under vigorous stirring
2 2 3
: 72.59%; Al O : 8.13%;
1
50
3
2.5
2.0
1.5
1.0
1
2
2
2
100
3
0
.5
condition for about 24 h. and thereafter evaporated to dryness
in a rotavapour. 0.5 g of the composite was dispersed in 20 mL
o
ethylene glycol and refluxed at 197 C for 5-6 h to ensure
0.0
0
2
4
6
8
10
12
PoreDiameter(nm)
50
complete reduction of the Ir salt. The colour of the solid mass
changed from reddish purple to black. The solid was allowed
to settle and washed repeatedly with distilled water for
several times and finally filtered through a sintered crucible
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure P/P0
Figure 1. N -sorption isotherm and BJH pore size distribution curve
2
of AT-mont.
2
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clearly indicates the mesoporous nature of the activated clay. A
narrow pore size distribution with a peak pore diameter centered at
4
nm was observed from the BJH pore size distribution plot, which
can act as host for supporting and stabilizing metal nanoparticles.
The typical SEM images of mont. and AT-mont. (ESI) indicate the
formation of porous surface on mont. upon acid activation. The EDS
patterns (ESI) clearly show a decrease in Al content upon acid
3
+
activation substantiating the leaching of Al .
Characterization of supported Ir-NPs
The formation of Ir-NPs was substantiated from the powder XRD
analysis (Figure 2) wherein, a broad peak centered at 2θ = 40.9° can
be assigned to the (111) reflections of Ir-NPs (JCPDS 01-1212) along
3
5
with other characteristic peaks of Ir metal . To confirm the
complete reduction of the impregnated iridium salt into the pores
of the AT-mont., XPS analysis was carried out and the binding
energies (Figure 3) of the supported Ir-NPs show spectrum bands at
Figure 4. (a) EDS (b) EDS dot mapping of Ir (c) EDS dot mapping of
Si (d) EDS dot mapping of Al of Ir-NPs surface
6
0.8 eV and 63.4 eV due to Ir(0) 4f7/2 and Ir(0) 4f5/2 respectively,
which are marginally higher than that of iridium metal and
4
6
therefore indicating the formation of metallic particles . These
NPs with the framework oxygen of the clay matrix which is
expected to induce a positive charge on the metal surface and
marginally higher values may be attributed to the interaction of Ir-
1
9
4
4
3
3
2
2
50
00
50
00
50
00
thereby increasing the binding energies of Ir-NPs . The EDS spectra
and EDS dot mapping (Figure 4) of Ir-NPs clearly indicates the
presence of homogeneously distributed Ir on the AT-mont. The
morphology of the supported Ir-NPs was investigated by TEM
analysis (Figure 5) and the sizes of Ir-NPs were found to be around 4
nm (with average size of 4.41 nm and standard deviation of less
than ±15% calculated from analysis of 100 particles in the TEM
image) which is almost equal to pore sizes of the acid activated
mont. and evenly distributed in the support matrix. The particle size
histogram shows the distribution of the sizes of Ir-NPs nanoparticles
4
0.9
in the range 2-8 nm. The N
2
-sorption analysis of the Ir-NPs
2
0
30
40
50
60
70
80
supported on AT-mont. reveals that the surface area value
Two theta (degree)
2
2
remarkably decreases to 327 m /g from 418 m /g, which indicates
the generation of the Ir-NPs inside the pores of the clay matrix
resulting in decrease of surface area. From ICP-AES, the actual Ir
content in AT-mont. was determined and found to be 0.42 mmol of
Ir per 1 g of the AT-mont.
Figure 2. Powder X-ray diffraction pattern of Ir-NPs supported
on AT-mont.
9
84000
82000
80000
78000
76000
74000
72000
70000
68000
4f5/2
9
9
9
9
9
9
9
9
4f7/2
Catalytic application
The synthesized Ir-NPs composite was evaluated as a catalyst
in the hydrogenation of some aromatic compounds under mild
condition (Scheme 1). All these reactions were carried out in a
X
X
Ir-NPs
H ] 5 bar / 75 C
o
[
2
Y
Y
X = -H, -OH, -COCH , -C H
2 3
3
54
56
58
60
62
64
66
68
70
72
Y = -H, -CH3
Binding energy (eV)
Figure 3. XPS spectra of Ir-NPs supported on AT-mont.
Scheme 1. Aromatic hydrogenation reaction catalyzed by Ir-NPs
supported on AT-mont.
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solvent free condition at 75 ºC under initial 5 bar H
2
pressure
(25 ºC), stirring at 500 rpm and the results are shown in Table-
1. The effect of catalyst amount on reaction was also studied
by varying the catalyst to substrate ratio in the hydrogenation
of toluene. The rate of conversion increases with the increase
of catalyst amount. The total time required for complete
reduction of toluene was also carried out by performing the
reaction with a fixed amount of catalyst. It was confirmed that
the total conversion of toluene takes place in 6 h of reaction
time (ESI). It has been observed that the conversion to
hydrogenated product for ortho, meta and para substituted
xylene was in the order of 90, 97 and 100% respectively. This
can be explained from the electronic effect on the aromatic
40
30
4
9,50
ring which hinders the hydrogenation process
.Additionally,
20
the steric effect also plays an important role for the low
conversion in the ortho position of the methyl group in
comparison to the meta and para position. The same trend
was also observed for ortho and meta cresolwith conversion of
1
0
0
2
4
6
8
Particle size (nm)
4
3% and 65% respectively. It has been found that the mono-
Figure 5. TEM image and particles size histograms with a
alkyl substituted aromatic rings are more easily hydrogenated
in comparison to multi substituted aromatic compounds,
Gaussian curve fitting of Ir-NPs on AT-mont.
which is due to the ease of adsorption of mono-alkyl substrate change in the catalyst (ESI). The N sorption isotherm shows
2
4
9
aromatic ring on the catalyst surface . The recovery of the that the pore diameter of recovered (third run) Ir-NPs is
catalyst was carried out by simple filtration followed by increased marginally compared to that of fresh catalyst (ESI).
vacuum dry at 150 ºC and was used in the hydrogenation of The marginal change is may be due to the rupture of the pore
toluene up to three runs by maintaining the stoichiometric walls of some pores leading to larger pores. Ir-NPs have been
ratio of the substrates and catalysts. The powder XRD pattern seen in good morphology with minimum aggregation of few
of the recovered catalyst shows that there is no significant nanoparticles from the TEM images (ESI) of the recovered
[
a]
Table 1. Tablehydrogenation of arenes by Ir-NPs under solvent free condition.
[b]
[c]
Conversion
%)
TOF
Entry
Arene
Product
-
1
(
[h ]
CH
3
[1st]
CH
3
1
96
9
00[
79
76
75
2nd]
1
[
3rd]
5
CH
3
CH
3
CH
3
CH
3
2
3
90
71
77
CH
3
CH
3
97
CH
3
CH
3
CH
3
CH
3
4
5
100
43
79
34
CH
3
CH
OH
3
OH
CH
3
CH
3
OH
OH
6
7
65
45
73
51
35
57
CH
3
CH
3
O
CH3
HO CH3
8
[
a] Conditions: catalyst 50 mg, H
2
pressure :5 bars, temperature: 75 ºC, time: 6 hours, stirring: 500 rpm.[b] Determined by GC analysis. [c] TOF with
This journal is © The Royal Society of Chemistry 20xx
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catalyst (third run) and the particles are supported into the
pores of the mont. having average particle size of 4.67 nm with
a standard deviation of 1.27 nm based on the measurement of
3
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2
1
4
5
100 particles. XPS analysis shows two peaks centered at 60.6
eV and 63.3 eV (ESI) corresponding to Ir(0) 4f7/2 and Ir(0) 4f5/2
respectively for the recovered catalyst indicating no change in
the oxidation state of the Ir metal after the third run of
reaction. This indicates that the catalyst is very robust, active
with long life time and recyclable for many catalytic runs. To
check the heterogeneity of the catalyst, hot filtration test was
performed. During the test, a hydrogenation reaction of
toluene was carried out and the catalyst was separated from
the reaction mixture by filtration after 3 h of reaction and the
products were analyzed using GC. The filtrate was further
allowed for 3 h reaction time under the same reaction
condition, but no increase in the reaction products were
observed which indicates that the Ir nanoparticles were intact
in the porous clay matrix without any leaching. A number of
catalysts with different stabilizing agents/supports have
already been reported for hydrogenation of aromatic
6
7
8
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1
1
1
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3
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1
2
In conclusion, highly stable Ir-nanoparticles can be synthesized
by simple incipient wetness impregnation of IrCl into the
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4
3
2
2
2
2
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4
2
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-sorption analysis. Ir-
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a
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0
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-1
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easily separated by simple filtration. The recyclability test with
2
2
recovered catalyst was carried and found almost active as 27 D. K. Dutta, B. J. Borah and P. P. Sarmah, Catal. Rev., 2015,
5
7
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and found very robust without any remarkable change.
2
2
3
3
,
2
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The authors are grateful to Dr D. Ramaiah, Director, CSIR-
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,
,
1
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are also thankful to Dr. P. Sengupta, Head, Materials Science
Division, CSIR-NEIST, Jorhat, for his constant encouragement.
Thanks are also due to CSIR, New Delhi for a financial support
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Notes and references
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1
2
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DOI: 10.1039/C5NJ03030G
Graphical Abstract
Ir nanoparticles supported on nanoporous montmorillonite clay showing efficient catalytic activity
for hydrogenation of aromatic compounds.