Synthesis of mesoporous iridium nanosponge: A highly active, thermally stable and efficient olefin hydrogenation catalyst

Article abstract of DOI:10.1039/c7dt01358b

Three dimensional porous structures offer high specific surface areas and large pore volumes, which enhance substrate diffusion within the porous structures and provide a large number of surface active sites. Such types of structures find applications in catalysis. Herein, we report a simple synthetic strategy for the preparation of iridium nanosponges by the capping agent dissolution method. An Ir@BNH_x nanocomposite was prepared starting from different iridium precursors by a solid state reduction method using ammonia borane wherein iridium(0) nanoparticles are embedded in a BNH_x polymer. Capping agent (here, the BNH_x polymer) dissolution using water under ambient conditions resulted in the formation of a mesoporous iridium nanosponge. This iridium nanosponge exhibits a surface area of 33.5 m² g^-1. The iridium nanosponge was found to be catalytically active for hydrogenation of a variety of olefinic substrates including linear and cyclic alkenes and α,β-unsaturated esters under relatively mild conditions and exhibits high turnover frequencies. It was also found to exhibit much better catalytic activity as compared to other iridium based heterogeneous catalysts for olefin hydrogenation reactions. Additionally, catalyst recovery was achieved via simple filtration from the hydrogenation reaction mixture. The catalytically active surface area of iridium nanosponge was estimated using H₂-temperature programmed desorption (TPD) experiments. Moreover, the catalyst was found to be thermally quite robust. The catalyst was recyclable over seven cycles of styrene hydrogenation and was found to be capable of hydrogenating 99% of styrene to ethyl benzene after seven cycles.

Full text of DOI:10.1039/c7dt01358b

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Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
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Received 00th January 20xx,
Synthesis of mesoporous iridium nanosponge: A highly active,
thermally stable and efficient olefin hydrogenation catalyst
*
Sourav Ghosh, Balaji R. Jagirdar
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Three dimensional porous structures offer high specific surface area and large pore volumes, which enhance substrate
diffusion within the porous structures and provide a large number of surface active sites. Such type of structures find
applications in catalysis. Herein, we report a simple synthetic strategy for the preparation of iridium nanosponge by
capping agent dissolution method. Ir@BNH
solid state reduction method using ammonia borane wherein iridium(0) nanoparticles are embedded in a BNH
Capping agent (here BNH polymer) dissolution using water under ambient conditions resulted in the formation of
x
nanocomposite was prepared starting from different iridium precursors by a
x
polymer.
x
2
mesoporous iridium nanosponge. This iridium nanosponge exhibits surface area of 33.5 m /g. The iridium nanosponge was
found to be catalytically active for hydrogenation of a variety of olefinic substrates including linear and cyclic alkenes and
α,β-unsaturated esters under relatively mild conditions and exhibits high turnover frequencies . It was also found to
exhibit much better catalytic activity as compared to other iridium based heterogeneous catalysts for olefin hydrogenation
reaction. Additionally, catalyst recovery was achieved via simple filtration from the hydrogenation reaction mixture.
Catalytically active surface area of iridium nanosponge was estimated using H
TPD) experiments. Moreover, the catalyst was found to be thermally quite robust. Catalyst was recyclable over seven
cycles of styrene hydrogenation and was found to be capable of hydrogenating 99% of styrene to ethyl benzene after
2
-temperature programmed desorption
(
seven
cycles.
analogues of heterogeneous catalytic systems. There have
been a few reports of hydrogenation of alkenes brought about
1
. Introduction
1
2
by iridium nanoparticles having capping agents. Removal of
capping agent from the surface which is usually carried out at
high temperatures in order to achieve greater catalytically
active surface area causes particle agglomeration and
eventually reduces the catalytic activity. Incorporation of
metal clusters within porous materials or on polymeric
systems prevents agglomeration to a certain extent. Thus,
Catalytic hydrogenation is truly a very important chemical
reaction that is carried out on a massive scale in the industry.
Several transition metal based catalysts have been developed
1
-3
over the years for hydrogenation reaction.
Among the
several active catalysts based on platinum group metals, the
ones based on iridium have proven to be potential candidates
both academically and industrially for hydrogenation
1
3
4
incorporation of iridium nanoparticles in hydrous zirconia,
1
reactions. Homogeneous iridium complexes (Crabtree
4
15
polymeric membrane, zeolitic imidazole framework (ZIF-8),
and hyperbranched polystyrene bearing ammonium salts
complex and its analogues) have been reported in the past for
5
olefin hydrogenation catalysis. In these cases, upon substrate
16
have been carried out. Such systems were found to exhibit
hydrogenation catalytic activities. Low temperature synthesis
of metal nanoclusters could preserve the active sites on the
catalyst surface which often cannot be realized in
heterogeneous processes. A challenge that remains to be
realized is to design a thermally and chemically stable, highly
active, easy to separate out and a robust heterogeneous
catalyst operating at ambient conditions for hydrogenation
reactions.
consumption, in presence of hydrogen, the catalyst gets
transformed into deactivated iridium dimer or trimer
6
complexes. Hydrogenation carried out at high pressure and
temperature using some homogeneous iridium catalysts lead
to their decomposition into iridium nanoclusters which are
7
catalytically active species. Earlier this observation of
decomposition of iridium complexes was deliberately used to
obtain precious metal nanoclusters stabilized by ionic liquids
and used as heterogeneous catalysts for several catalytic
8
-11
Porous materials are of immense interest in the field of
heterogeneous catalysis due to their inherent spongy structure
and high surface area which facilitates an easy diffusion of
substrate into the pores and provides large number of surface
transformations.
These could be treated as soluble
1
7,18
active catalytic sites.
But, synthesis of porous metals still
1
remains a challenge in the field of materials science.
9
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2
0-22
Template based synthesis of porous metals
which is scanning transmission electron microscopy (STEM) imaging
typically carried out at high temperatures results in sintering of were performed using a JEOL 2100F transmission electron
nanoparticles causing a drop in the catalytic activity. Also, microscope equipped with a field emission gun. The operating
template synthesis is quite selective towards a metal which voltage of the field emission gun (FEG) is 200 kV. The HAADF
resists oxidation when a high temperature template removal STEM elemental mapping was accomplished using Oxford EDS
process is carried out. Electrochemical and chemical dealloying spectrometer attached to TEM. Typically 2 mg of iridium
of multi-metallic alloys for synthesis of porous metals are also powder was dispersed in 2 mL of THF and a 5 µL colloid was
2
3,24
metal specific.
Synthesis of three dimensional porous drop casted on formvar coated 300 mesh copper grid. The
2
5
metals has been carried out using assembly of ex-situ and in- powder X-ray diffraction measurements were carried out using
2
situ (kinetic assembly) prepared nanoparticles. Our group a PANalytical EMPYREAN diffractometer using Cu Kα radiation.
6
and others reported the synthesis of transition metal The FT-IR spectra were acquired on a Bruker ALPHA FT-IR
2
7-29
nanoparticles by amine borane reduction both in solution
spectrometer. X-ray photoelectron spectroscopy (XPS) was
In the solid-state reduction, we carried out using a kratos AXIS Ultra XPS spectrometer. Surface
demonstrated that ammonia borane acts both as a reducing area and porosity were measured using Micromeritics ASAP
agent and also stabilizing agent (in the form of BNH polymer 2020 instrument. Nuclear magnetic resonance spectroscopy
which is formed in situ during the reduction) for the metal was carried out using Avance Bruker 400 MHz spectrometer.
3
0-33
and in the solid state.
x
3
1,32
nanoparticles. Demirci and his co-workers showed that this Agilent 7890A model instrument was used and flame
polymer could be hydrolyzed in presence of water to generate ionisation detector was used for the quantification of the
3
4
hydrogen gas. Thus combining these two approaches, we products of hydrogenation reactions. Thermo Trace GC
attempted to prepare mesoporous iridium nanosponge via instrument (GC-MS) coupled with Thermo DSQ II model mass
dissolution of the BNH
x
x
polymer in Ir@BNH in presence of spectrometer was used for the identification of the products of
water. The evolved H
2
5,36
gas acts as a dynamic template to form hydrogenation reactions. In some cases, Shimadzu GCMS-
3
iridium nanosponge.
The resulting iridium nanosponge was QP2010 SE was used for identification of the compounds.
found to be a highly active hydrogenation catalyst for a variety Optical rotations were measured using JASCO P-2000
of olefins. Herein, we describe the preparation and polarimeter.
characterization of iridium nanosponge and the results of
2
.3 Synthesis of iridium nanosponge from H
Iridium nanosponge was synthesized from the Ir@BNH
nanocomposite. Earlier the synthesis of Ir@BNH
2 6
IrCl
catalytic hydrogenation reactions.
x
x
3
1
2
. Experimental
nanocomposite was reported by our group. In a typical
experiment, 155 mg (5 mmol) of ammonia borane was taken in
o
a Schlenk tube immersed in an oil bath preheated to 60 C.
2.1 Materials and methods
Hexachloroiridic acid (H
2
IrCl
6
)
and iridium chloride Hexachloroiridic acid, H IrCl (407 mg, 1 mmol) was added to
2
6
(
IrCl
India. Sodium borohydride (NaBH
sulfate ((NH SO ), CH Cl , and THF were obtained from S. D. of the reaction although a color change was observed from
3 2
.xH O) were purchased from Arora Matthey Limited, the Schlenk tube and vigorously stirred for 12 h under inert
3
1
4
), n-heptane, ammonium atmosphere. No gas evolution was noted during the course
4
)
2
4
2
2
Fine Chemicals Limited, India. 1,5-Cyclooctadiene was black to greenish yellow. After the reaction mixture was
purchased from Sigma Aldrich. Ethanol was dried and distilled allowed to cool down to room temperature, 7 mL of water was
over Mg(OEt)
benzophenone and distilled. CH
used for hydrogenation reactions. The iridium dimer, out and washed thoroughly for 5 to 6 times with deionized
2
. THF and n-heptane were dried over Na- added. This resulted in vigorous bubbling for 1-3 min. After
2
Cl was distilled over CaH and that, the black precipitate of iridium nanosponge was filtered
2
2
[
Ir(COD)Cl]
2
and ammonia borane (H
3
N·BH
3
,
7,38
AB) were water and then finally with acetone and kept for overnight
Styrene, drying. Yield obtained was 144 mg (75%). The black precipitate
3
synthesized following literature reports.
cyclohexene, and 4-vinyl cyclohexene were distilled and stored of iridium nanosponge was characterized using XRD, FT-IR
in freezer. The remaining alkenes used in this work were used spectroscopy, TEM, and SEM. The filtrate was collected, dried
o
11
as received without further purification.
at 80 C for 12 h, and characterized by PXRD, B NMR and FT-
4 2 4 7
IR spectroscopy and found to be comprised of (NH ) B O ,
34
2.2 Characterization
4 5 8 4
NH B O and NH Cl.
Scanning electron microscopy (SEM) imaging and energy
2
.4 Synthesis of iridium nanosponge from IrCl
3 2
.xH O
dispersive X-ray spectroscopy (EDS) were performed using FEI
Sirion XL30 FESEM. The material was deposited over double
Iridium nanosponge was synthesized from IrCl .xH O by
3
2
sided conductive carbon tape supported by aluminum stub, following the above mentioned procedure using 155 mg (5
and evacuated overnight before imaging. The SEM imaging mmol) of ammonia borane and 299 mg (1 mmol) of
3
1
was carried out at 10 kV source voltage. The bright field IrCl .xH O. In this case also, black to greenish yellow color
3
2
transmission electron microscopy (BF) imaging, high-resolution change was observed. Addition of 7 mL of water to the
TEM (HR-TEM) imaging, selected area electron diffraction reaction mixture upon cooling resulted in vigorous gas
(SAED) patterns and high-angle annular dark field (HAADF) evolution for 1-3 min followed by precipitation of a black
2
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substance of iridium nanosponge. The precipitate was isolated activity) using hydrogen temperature programmed desorption
by filtration and washed thoroughly with water and acetone (H -TPD) experiments and found it to be 13%. Further,
2
and dried overnight. Yield obtained was 137 mg (71%). The turnover frequency was calculated based on the surface
black precipitate of iridium nanosponge was characterized exposed iridium atoms which are available for catalysis
thoroughly using powder XRD, FT-IR, TEM, and SEM. The (Turnover frequency (TOF) = moles of substrate transformed
o
filtrate was collected, dried at 80 C for 12 h, and characterized per mole of exposed iridium per hour (available for substrate
1
by PXRD, B NMR and FT-IR spectroscopy and found to binding and catalytic activity)). The details of the calculation
1
3
Cl.
4
contain (NH
4
)
2
B
4
O
7
, NH
4
B
5
O
8
and NH
4
are given in the ESI.
Hydrogenation of 1-butene (99.5%, Bhuruka Gases Limited,
India) was performed in a stainless steel (T316) autoclave using
2.5 Synthesis of iridium nanosponge from [Ir(COD)Cl]
2
In this case as well, we followed a similar procedure as Parr 4590 series high pressure micro-reactor set up. In a typical
3
1
described above. Here, we used [Ir(COD)Cl]
mmol) as the iridium precursor and ammonia borane (77 mg, added to 5 mL of CH
.5 mmol) to obtain Ir@BNH nanocomposite powder which the autoclave and sealed. Subsequently, hydrogen was flushed
2
(336 mg, 0.5 reaction, 1.9 mg (0.01 mmol) of iridium nanosponge was
2
Cl in a quartz reactor and placed within
2
2
x
was characterized by TEM, PXRD, and FT-IR spectroscopy (ESI). for 4 times; then, 1-butene was introduced at a pressure of 2.5
Color change from orange to greenish yellow was noted. In bar. Then, hydrogen gas was introduced at 5 bar pressure
this case also upon addition of 7 mL of water to the reaction (total pressure 7.5 bar) and the pressure drop was monitored
mixture at room temperature resulted in gas evolution. Finally, until gas consumption ceased. The reaction vessel was
the black residue of iridium nanosponge was washed carefully depressurized and the solution was quickly cooled to
o
and –90 C using liquid nitrogen and acetone bath to minimize the
thoroughly with deionized water, acetone, and CH
2
Cl
2
o
dried overnight. Yield obtained was 128 mg (67%). The black product evaporation (butane b.p. –1 C). GC-MS experiment
sample was characterized using PXRD, SEM, TEM, and FT-IR was carried out with the neat sample which confirms the
o
spectroscopy. The filtrate was collected and dried at 80 C for presence of CH
2
Cl
2
, butane, and acetone (ESI).
1
2 h. Presence of (NH
4 2 4 7 4 5 8 4
) B O , NH B O and NH Cl in the filtrate 2.6.2 Effect of Pressure on Hydrogenation Catalysis. Styrene
1
was established using PXRD, B NMR and FT-IR spectroscopy hydrogenation reaction was carried out at fixed styrene (20
1
3
4
techniques.
mmol of substrate) to iridium catalyst (0.01 mmol, 0.05 mol%)
ratio (substrate/catalyst 2000) in CH Cl at room
temperature. Reactions were carried out at 2, 3, 4, 5, and 6 bar
Hydrogenation reactions were carried out using a Parr of hydrogen gas pressure separately. After gas consumption
hydrogenation apparatus. Ultra-high purity hydrogen ceased, the reaction mixture was centrifuged and the
=
2
2
2.6 Hydrogenation Catalysis
1
(
99.997%) gas obtained from Bhuruka Gases Limited, India was supernatant was used for GC-MS and H NMR spectroscopic
used for these experiments. Hydrogenation reactions were studies. A blank experiment of hydrogenation of styrene using
carried out in CH Cl solvent. iridium catalyst in the absence of hydrogen gas gave no ethyl
.6.1 General Procedure for Olefin Hydrogenation. In a typical benzene.
2
2
2
catalytic reaction, 1.9 mg (0.01 mmol, 0.05 mol%) of iridium 2.6.3 Effect of Temperature on Hydrogenation Catalysis.
nanosponge was taken in a high pressure Parr Hydrogenation Hydrogenation reactions were carried out at 50 °C and 75 °C in
reaction bottle and dispersed in 5 mL of CH
n-heptane was used as a solvent instead of CH
mmol of substrate (olefin) was added. Before starting the solubility of both the substrate and hydrogen and moderate
2
Cl
2
. In some cases, n-heptane as a solvent. n-Heptane was used as a solvent for
2
2
Cl . To this, 20 high temperature hydrogenation experiments due to good
1
o
o
42
reaction, an aliquot was drawn for recording H NMR temperature window (30 C - 98.4 C). Reaction was carried
spectrum. The reaction bottle was attached to the out in a similar manner as described above. The hydrogenation
hydrogenation apparatus. The reaction mixture was saturated reaction bottle was heated using a heating tape wound around
with hydrogen gas over four cycles. Hydrogenation reactions it and the temperature was monitored using a thermocouple.
were carried out at 4 bar of hydrogen gas pressure at room The reaction was continued until hydrogen pressure drop in
temperature. The reaction was continued until gas the gauge ceased. The reaction vessel was cooled down to
consumption ceased which was evident by a drop in pressure room temperature, hydrogen gas was released and the
on the pressure gauge of the hydrogenation apparatus. Then solution was taken out for further analysis.
the reaction bottle was detached and the solution was 2.6.4 Catalyst Poisoning. Catalyst poisoning experiment was
centrifuged to separate the catalyst. A small amount of the done using CO gas. Two separate reactions were carried out. In
reaction mixture was used for GC-MS study and another the first experiment, 1.9 mg (0.01 mmol, 0.05 mol%) of iridium
1
aliquot was taken for recording H NMR spectrum. The NMR nanosponge was dispersed in CH
2
Cl
2
and purged with CO gas
o
for 45 min at 0 C. After that, nitrogen gas was purged to
3
sample was prepared in CDCl .
The turnover frequency was calculated based on the ensure that there is no free CO gas present in the solution.
fraction of total iridium metal that is exposed at the surface Subsequently, 20 mmol of styrene was added and the
and is available for catalytic activity. We quantified the fraction hydrogenation reaction was carried out at 4 bar of hydrogen
of total iridium metal that is exposed at the surface and is gas pressure and room temperature for 2 h. Then, the reaction
available for substrate binding (or available for catalytic
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Figure 1: Iridium nanosponge: a) SEM image, b) BF-TEM image, c) HRTEM image, d) SAED pattern, e) PXRD pattern, f) BET isotherm, g) BJH pore size distribution.
mixture was taken out from the hydrogenation apparatus and reaction, the reaction mixture was centrifuged and the
1
centrifuged to collect the supernatant for NMR and GC-MS supernatant was taken out for H NMR spectroscopy and GC-
analysis.
In another experiment, we poisoned the catalyst during the times via re-dispersion and centrifugation cycles. Again, the
course of hydrogenation reaction. At first, hydrogenation catalyst was re-dispersed in CH Cl and 20 mmol of substrate
reaction was carried out with 1.9 mg (0.01 mmol, 0.05 mol%) was added for the next catalytic cycle and reaction was
of catalyst dispersed in CH Cl and 20 mmol of styrene. After monitored for 4 h. After the 7th cycle, the catalyst was
continuing the reaction at 4 bar hydrogen pressure for 1 h 40 separated by centrifugation and washed repeatedly with
min, the reaction was stopped deliberately by depressurizing CH Cl and dried. TEM and powder XRD studies were
2 2
MS studies. Catalyst was washed thoroughly using CH Cl four
2
2
2
2
2
2
the vessel. An aliquot was taken for NMR and GC-MS analysis. performed to check the catalyst stability after 7 cycles.
The reaction mixture was then cooled down in ice bath and 2.6.7 Thermal Stability of the Catalyst. Two samples of iridium
o
o
subsequently purged with CO for 45 min. After that, nitrogen nanosponge were heated to 150 C and 300 C in argon filled
was purged to remove the remaining CO in the solution. Then, ampules for 3 h. Then the catalysts were used for
hydrogenation reaction was resumed at 4 bar hydrogen hydrogenation reactions. The reaction conditions were the
pressure and room temperature and was monitored for 2 h. same as employed in above experiments: (0.05 mol% iridium
After that, the catalyst was separated by centrifugation and catalyst, 5 mL CH
the supernatant was used for NMR and GC-MS analysis. pressure at room temperature); the reaction was monitored
.6.5 Catalyst Lifetime. The catalyst lifetime of iridium for 4 h. Upon completion of the reaction, the catalyst was
2 2
Cl , 20 mmol styrene, 4 bar hydrogen
2
nanosponge for styrene hydrogenation was determined by separated by centrifugation and supernatant was
measuring the total turnover number (TTN). At first, 1.9 mg characterized using NMR and GC-MS techniques. The samples
(
0.01 mmol, 0.05 mol%) of iridium nanosponge was added to 5 are abbreviated as Ir-RT (as-prepared, not heat treated), Ir-150
o
mL of CH Cl2 in hydrogenation reaction vessel and (annealed at 150 C, 3 h), and Ir-300 (annealed at 300 C, 3 h).
o
2
subsequently, 20 mmol of styrene was added to the reaction 2.6.8 H -Temperature Programmed Desorption experiment.
2
mixture and then attached to the hydrogenation apparatus. Temperature programmed desorption (H -TPD) experiment
2
The reaction bottle was saturated with hydrogen gas in four was performed using multipurpose unit coupled to a
cycles and finally charged with 4 bar of hydrogen gas. The quadrupole mass spectrometer (BELCAT II catalyst analyser,
reaction was monitored until gas consumption ceased. Then Japan). For H -TPD experiment, the iridium sample was dried
2
o
the bottle was opened and more styrene was added to the at 100 C for 60 min under a flow of He (50 mL min ). Then,
-1
mixture and after saturating with hydrogen gas in a few cycles, the sample was pre-treated with 5% H /Ar gas flow (50 mL
2
-1
finally the bottle was charged with 4 bar of hydrogen gas and min ) for 60 min and the temperature was maintained at 100
o
the cycle was continued.
C. After H treatment, sample was purged with He flow (50
2
-
1
o
.6.6 Catalyst Recyclability Test. Catalyst recyclability was mL min ) for 15 min at the reduction temperature (100 C).
2
o
checked for 7 cycles. Reaction conditions were similar to those Then, the temperature was increased to 500 C at a heating
o
described above. 1.9 mg (0.01 mmol, 0.05 mol%) of iridium rate of 5 C min , under He flow, and H desorption was
-1
2
dispersed in 5 mL of CH Cl and 20 mmol of styrene were monitored.
2
2
taken in the hydrogenation reaction bottle. The reaction was
continued for 4 h at 4 bar of hydrogen gas pressure at room
temperature. Each time after performing the catalytic
3
. Results and discussion
4
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x
Figure 2: Iridium nanosponge: a) FT-IR spectral stack plot of iridium nanosponge and Ir@BNH , b) EDS spectrum, c) STEM-HAADF image, d) STEM EDS elemental (Ir) map, e)
4
f core level XPS spectrum.
3
.1 Synthesis and Characterization of Ir Nanosponge
that of Ir nanosponge is featureless. The EDS spectrum (Fig 2b)
also showed the presence of only iridium in the sample. The
sole presence of iridium was further established by STEM
HAADF and STEM EDS elemental mapping (Fig 2c, d). Further,
the XPS 4f core level spectrum of iridium consists of two peaks
with binding energies of 60.9 eV (4f7/2) and 63.8 eV (4f5/2)
We reported the synthesis of Ir nanoparticles encapsulated
in a BNHx polymer (Ir@BNHx nanocomposite) earlier via
reduction of an iridium precursor (H IrCl ) using ammonia
2
6
3
1
borane.
Further, addition of water to the Ir@BNHx
nanocomposite resulted in vigorous H2 evolution and
formation of an iridium nanosponge. The iridium nanosponge
was thoroughly characterized using SEM, TEM, BET surface
area analyzer, XRD, FT-IR spectroscopy, and XPS techniques.
Additionally, the composition of the filtrate was determined
4
0
which evidences the presence of Ir(0) (Fig 2e). The other two
peaks corresponding to IrOx (Ir(IV)) arise due to surface
oxidation of the metal under ambient conditions. Upon
hydrolysis of the Ir@BNH nanocomposite, in addition to the
x
1
1
iridium nanosponge and ammonium borate and ammonium
chloride was formed which stays in the filtrate.
using PXRD, B NMR and FT-IR spectroscopy and found to be
3
comprised of (NH ) B O , NH B O and NH Cl (ESI). Porous
4
4
2
4
7
4
5
8
4
Based on our experimental observations, we propose a
general mechanism for the synthesis of iridium nanosponge.
Upon addition of water, hydrolysis of the B-H bond takes place
and iridium nanoparticles that are present also catalyze the
nature of the sample is evident from the SEM image shown in
Fig 1a. The BF TEM image is shown in Fig 1b; the HRTEM image
(Fig 1c) shows lattice fringes of 0.22 nm which corresponds to
the (111) plane of FCC (face-centred cubic) lattice of iridium.
Additionally, the porous structure consists of 2 to 3 nm sized Ir
nanoparticles, which is apparent from the HRTEM image. The
SAED pattern (Fig 1d) matches exactly with the FCC lattice of
iridium. The powder XRD pattern (Fig 1e) which matches well
with the FCC lattice of Ir, corroborates with the electron
diffraction pattern. BET surface area was measured to be 33.5
3
4
hydrolysis process. Simultaneously, during the hydrolysis
process, the Ir nanoparticles formed aggregates since there
was no capping agent present to provide stability. The
hydrogen gas bubbles produced rapidly act as dynamic
templates to generate iridium nanosponge instead of bulk
iridium.
2
3.2 Olefin Hydrogenation Catalysis Using Iridium Nanosponge
m /g. The BET isotherm plot (Fig 1f) evidences a type IV
isotherm (H3 type) and the hysteresis is a signature of
The potential of Ir nanosponge towards olefin
3
9
mesoporosity of the sample. BJH pore size distribution hydrogenation was tested using styrene as a model substrate.
calculated from desorption branch also confirms a wide peak Hydrogenation of styrene was carried out at room
centered at 27.7 nm which further validates the nature of the temperature using Ir nanosponge at a hydrogen pressure of 4
isotherm (Fig 1g).
bar. Ethyl benzene was obtained in quantitative yield in 4 h
-1
The dissolution of BNHx polymer was probed using (turnover frequency (TOF): 3846 h based on iridium atoms
different analytical techniques (FT-IR spectroscopy, STEM-EDS available for catalytic activity, ESI). Moreover, we found that
elemental mapping). The FT-IR spectrum (Fig 2a) of Ir@BNH is Ir@BNH nanocomposite is not an active catalyst for styrene
x
x
comprised of B–H, N–H, and B–N stretching vibrations whereas hydrogenation under similar reaction conditions. In this case,
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Table 1 Hydrogenation of styrene at different substrate to catalyst molar ratio, at
Table 2 Hydrogenation of alkenes by iridium nanosponge
different pressures and temperatures and in different solvents
-
1 a
Entry
Substrate
T (h)
Product
TOF (h )
Styrene/Ir
ratio
P
(bar)
4
T
Time
(h)
%
Conv
>99
>99
80
-1 e
b
Entry
o
d
TOF (h )
1
2
Styrene
4
4
-
-
( C)
30
30
30
30
30
30
30
30
30
50
75
Styrene
Ethylbenzne
500(3846)
a
1
2
3
4
5
6
7
8
9
1000
2.5
4
250 (3077)
500 (3846)
167 (1282)
208 (1602)
227 (1748)
308 (2366)
526 (4048)
571 (4395)
1143 (8791)
2667 (20512)
4762 (36630)
4
-Chloro
ethylbenzne
-Methyl
a
2000
4
3
4
4- Chlorostyrene
4
4
500(3846)
a
5000
4
24
4
a
10000
4
24
50
4- Methylstyrene
500(3846)
ethylbenzne
Hexane
b
2000
2
8.8
6.5
3.8
3.5
1.75
0.75
0.42
>99
>99
>99
>99
>99
>99
>99
5
6
7
8
9
1-Hexene
1-Octene
4
5
500(3846)
400(3077)
333(2564)
333(2564)
500(3846)
400(3077)
222(1709)
571(4395)
b
2000
3
Octane
b
2000
5
1-Hexadecene
1-Octadecene
Cyclohexene
Cyclooctene
6
Hexadecane
Octadecane
Cyclohexane
Cyclooctane
b
2000
6
6
c
2000
4
4
c
1
1
0
1
2000
4
1
1
0
1
5
c
2000
4
c
1,5-Cyclooctadiene
Ethyl acrylate
9
Cyclooctane
a
o
All reactions were carried out at a constant temperature of 30 C and 4 bar H
2
12
3.5
Ethyl propionate
t-Butyl
b
gas pressure in CH
2
c
Cl
2
; All reactions were carried out at a constant temperature
o
13
t-Butyl acrylate
4
500(3846)
of 30 C in CH
2
Cl
2
; All reactions were carried out at a constant pressure of 4 bar
propionate
d
in n-heptane as a solvent at different temperatures; conversion determined by
Methyl-2-methyl
propionate
e
14
Methylmethacrylate
Cyclohexenone
3.75
3.83
9
533(4102)
522(4016)
222(1709)
GC/MS analysis; turnover frequency = mol (styrene)/mol (Ir nanosponge)·h
based on total metal, in parenthesis based on available surface atoms of catalyst.
d
1
5
Cyclohexanone
Ethyl
BNH
hence blocks the catalytically active sites, which is not the case
for iridium nanosponge. Subsequently, H -TPD experiment was
x
polymer protects and stabilizes iridium nanoparticles and 16
4-Vinylcyclohexene
e
cyclohexane
f
1
1
7
8
R-Limonene
R-Limonene
4.5
0.5
p-Menth-2-ene
444(1709)
4000(15384)
Not
g
2
p-Menth-2-ene
performed to quantify the active surface area of Ir
nanosponge. It revealed that 13% of iridium metal exposed at
the surface of nanosponge is involved in hydrogen adsorption,
which takes part in catalysis (ESI). Subsequently, TOF values
h
1
9
1-butene
3
Butane
determined
All reactions were carried out with substrate/iridium catalyst ratio of 2000 (20
o
mmol substrate, 0.05mol% Ir catalyst) and at a constant temperature of 30 C and
were determined based on total iridium concentration and H2 pressure of 4 bar in CH Cl except for entry number 17,18; 100% conversion
2
2
a
surface exposed iridium metal (ESI). With a view to understand determined by GC/MS for all the reactions; turnover frequency
=
mol
(
substrate)/mol (Ir nanosponge)·h for total metal, in parenthesis based on
the hydrogenation catalytic activity of Ir nanosponge, we
carried out studies under different conditions using styrene as
a model substrate.
b
x
available surface atoms of catalyst; iridium catalyst is Ir@BNH and for rest of
c
d
the cases, iridium nanosponge; conversion in cyclooctane; 100% conversion to
e
cyclohexanone and cyclohexanol with 89:11 ratio; conversion of 4-
Various substrate (styrene) to catalyst concentration ratios vinylcyclohexene to ethylcyclohexane; (substrate to cat. ratio-1000, 30 C, 4 bar
f
o
g
were used to understand the effect on catalyst activity. These
hydrogenation reactions were carried out in CH Cl solvent.
H
2
in CH
2 2
Cl ) conversion to p-menthene and p-menthane; (substrate to cat.
o
ratio-1000, 75 C, 4 bar H , in n-heptane) conversion to p-menthene and p-
menthane was noted; reaction was carried out with 0.01 mmol of iridium
2
2
2
h
Hydrogen pressure was maintained constant at 4 bar and the
temperature at 30 °C. The results of these studies are
o
catalyst in 5 mL of CH
2 2 2
Cl at a constant temperature of 30 C and H and 1-butene
pressure are fixed at 5 and 2.5 bar respectively; 100% conversion was determined
summarized in Table 1 (Entries 1-4). It is evident from the data by GC-MS.
that at a substrate to catalyst ratio of 2000, catalyst saturation
sets in, which is reflected in the lowering of activity at a ratio the results of hydrogenation of styrene carried out at different
of 1:5000 (Ir:substrate) or more. In another set of temperatures. Additionally, hydrogenation of styrene using Ir
experiments, we varied hydrogen gas pressure maintaining the nanosponge at room temperature in different solvents, e.g.,
other parameters constant (0.05 mol% catalyst, 20 mmol CH
substrate in 5 mL of CH Cl
at room temperature). The results ascribed to greater solubility of hydrogen in n-heptane
obtained in these experiments have been summarized in Table compared to CH
(Entries 2, 5-8). In these cases, we noted that the catalyst activity at 75 °C for terminal alkene, e.g., 1-hexadecene (TOF:
2 2
Cl and n-heptane gave different TOF values. This could be
2
2
4
.
2
2
Cl
2
We also noted very high catalytic
1
–1
activity does not change significantly upon raising the pressure 51282 h ) which is comparable to that of 1-decene
above 4 bar. This is evident from the TOF which did not hydrogenation using Ir nanoclusters. More importantly, we
9
increase substantially above a pressure of 4 bar. This found that catalyst activity using our Ir nanosponge catalyst for
observation could be rationalized as that above a pressure of 4 cyclohexene hydrogenation at 75 °C is far superior to other
bar, saturation of catalyst surface with hydrogen atoms takes heterogeneous iridium systems reported in the literature to
9
,14,46
place, and therefore, increase in hydrogen pressure does not date.
4
affect the TOF. Temperature variation of hydrogenation of
1
These results prompted us to carry out hydrogenation of
styrene on catalytic activity of Ir nanosponge was also studied: several other olefins under mild conditions (room
catalyst activity increased at high temperatures (75 °C) temperature, 4 bar pressure). In all the cases hydrogenation
proceeded to >99% yield of hydrogenated products and the
compared to that at room temperature. Table 1 summarizes
6
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ARTICLE
completely hydrogenated to give 90% of 1-
p-menthene (only R
isomer). Additionally, small amount of p-menthane (10 % of
fully hydrogenated product) was also noted which comprised
of equal amounts of cis and trans diastereomers (~5% each)
4
3
ESI). Further, simple gaseous alkene such as 1-butene can be
(
hydrogenated completely using the iridium nanosponge (ESI).
Catalyst recyclability was also investigated for styrene as a
substrate under similar conditions as detailed above. We
noted that the catalyst retained its high activity over seven
cycles and capable of hydrogenating 99% of styrene to ethyl
benzene even after seven cycles (Fig3a). In this context, it is to
Scheme 1: Hydrogenation of R-limonene to 1-p-menthene and diastereomers of
p-menthane.
results are summarized in Table 2. Separation of the catalyst be noted that our catalyst is quite stable and active than the
5
could be achieved by centrifugation followed by filtration. Crabtree’s catalyst reported in the literature. Catalyst
From the results tabulated in Table 2, it is apparent that recovery was achieved with ease via simple centrifugation. The
substitution at the para-position of the phenyl group of HRTEM image (Fig 3c) confirmed the presence of lattice fringes
styrene does not affect the catalytic activity as we found that with d spacing of 0.22 nm which corresponds to the (111)
the turnover frequency remained constant with or without the lattice plane of fcc phase of Ir(0). From the TEM data, it is quite
para-substituent. In case of linear chain terminal olefins, evident that no change in the morphology of the catalyst took
activity drops from 1-hexene to 1-octadecene. This could be place. The mesoporous nature of the catalyst is self-
ascribed to the steric encumbrance of the octadecyl moiety. sustainable for high catalytic activity and stability for a number
Hydrogenation of cycloalkenes, e.g., cyclohexene, cyclooctene, of cycles. The catalyst lifetime was determined separately by
and 1,5-cyclooctadiene was also carried out using iridium measuring the total turnover number (TTN) for styrene
nanosponge. The facility of hydrogenation in case of hydrogenation reaction. It was found that, iridium nanosponge
cyclohexene could be ascribed to the high solubility of retains its activity for up to 10 cycles with a turnover number:
cyclohexene in CH Cl and also the high hydrogen miscibility in 19,700 after 40 h, without deactivation of the catalyst. It
2
2
4
2
-1
-
cyclohexene. Activity of our Ir nanosponge is much better corresponds to an average TOF value of 492.5 h (TOF- 3788 h
1
than that of Ir nanoclusters reported in the literature.
9
, with respect to exposed metal atoms available for catalysis).
th
We also carried out selectivity studies of hydrogenation of After the 10 cycle, catalyst was still active and was able to
-vinylcyclohexene which has both primary and secondary hydrogenate 95% of styrene to ethyl benzene.
Thermal stability of Ir nanosponge was evaluated by
4
olefinic moieties. As expected, the primary olefinic double
bond gets hydrogenated first, followed by the secondary, both annealing it at 150 °C (Ir-150) and 300 °C (Ir-300). TEM images
at room temperature (ESI). Excellent hydrogenation activity (ESI) of the annealed samples showed no sign of sintering
was also noted for α,β-unsaturated esters like acrylates. indicating that mesoporous iridium is thermally stable. In
Hydrogenation of cyclohexenone resulted in 89:11 distribution addition, we recorded the powder XRD patterns (ESI) of these
of cyclohexanone and cyclohexanol. On the other hand, samples which revealed a very small increment in the
hydrogenation of 1-methylcyclohexene was unsuccessful. crystallite size, in particular in case of sample annealed at 300
Once again, in this case the lack of hydrogenation could be °C. Catalytic studies using these heat treated samples
ascribed to the steric encumbrance of the substituents. We evidenced that the sample annealed at 150 °C did not lose its
took advantage of the selectivity (primary > secondary >> catalytic activity, whereas the one annealed at 300 °C is only
tertiary olefin) of our catalyst and examined the hydrogenation marginally less active compared to the un-annealed Ir catalyst
reaction of the natural product, R-limonene (Scheme 1). (ESI). With a view to have an understanding of the nature of
Selectivity was noted for R
-limonene hydrogenation, as it was the active catalyst, we carried out catalyst poisoning
found that the methyne group (external double bond) got experiment with carbon monoxide (CO) which is an active
Figure 3: a) Catalyst recycling plot for styrene hydrogenation by iridium nanosponge in CH
2
Cl
2
, b) BF TEM image and c) HRTEM image of the iridium catalyst after 7 cycles
of hydrogenation.
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4
4
binding agent for iridium. Two different experiments were which could be utilized as potential catalysts for different
carried out to show that upon CO gas exposure, the catalyst chemical transformations.
lost its activity. When iridium nanosponge pretreated with CO
gas was used for hydrogenation of styrene, negligible (<2%)
Acknowledgements
conversion to ethyl benzene was noted. On the other hand,
after 1.66 h of hydrogenation, when the active iridium catalyst
was treated with CO gas and then hydrogenation was
resumed, no further conversion was observed. Treatment of
iridium nanosponge with CO gas renders it catalytically
inactive. Strong binding affinity of CO on iridium surface
results in unavailability of catalytically active sites leading to
catalyst deactivation. In addition to that, FT-IR spectroscopy
measurement was carried out to verify the adsorption of CO
on to iridium surface. FT-IR spectra exhibit three bands in the
The authors gratefully acknowledge the financial support
from the Council of Scientific & Industrial Research (CSIR),
India and the Indian Institute of Science for funding the
procurement of a 200 kV FETEM. We thank Mr. Vadivel for
TEM. S.G. thanks IISc for a fellowship. Authors thank SID
(Spectroscopy and Analytical test facility), IISc for providing
GC-MS facility and Dr. Deepalakshmi for her help with the
measurements. Author thanks Dr. Ashish Kumar Singh (BHU,
Varanasi) for valuable discussion. Author thanks Dr. Suresh B.
-1
range of 2000-2200 cm which confirms the CO adsorption
2
Kalidindi (PPISR, Bangalore) for his help with the H -TPD
(
ESI). We also prepared Ir@BNH
x
nanocomposite from other
O, [Ir(1,5-COD)Cl] . The Ir@BNH
was
characterized using various techniques (FT-IR, PXRD and TEM)
ESI). The data revealed that the properties of this material are
the same as those of the samples obtained starting from the
precursors, H IrCl and IrCl .xH O. Hydrolysis of Ir@BNH
nanocomposite obtained starting from IrCl .xH
COD)Cl] afforded iridium nanosponges independently and the
experiment. Author thanks Mr. Abhijnan Roy Chowdhury (OC,
IISc) for his help with the optical rotation measurements.
Author thanks Prof. S. Ramakrishnan (IPC, IISc) for giving
access to the GC-MS facility and Mr. Sujoy Bej for his
assistance with the measurements.
4
5
precursors such as, IrCl
3
.xH
2
2
x
nanocomposite obtained from [Ir(1,5-COD)Cl]
2
(
2
6
3
2
x
3
2
O and [Ir(1,5- Notes and references
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54x190mm (300 x 300 DPI)