Stabilized Rh0-nanoparticles-Montmorillonite clay composite: Synthesis and catalytic transfer hydrogenation reaction

Article abstract of DOI:10.1016/j.apcata.2013.10.049

Rh⁰-nanoparticles of around 5 nm size distributed homogeneously into the nanopores of acid activated Montmorillonite clay were generated by incipient wetness impregnation of RhCl₃, followed by reduction with ethylene glycol. Acid act

Full text of DOI:10.1016/j.apcata.2013.10.049

Applied Catalysis A: General 470 (2014) 355–360
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
0
Stabilized Rh -nanoparticles-Montmorillonite clay composite:
Synthesis and catalytic transfer hydrogenation reaction
∗
Podma Pollov Sarmah, Dipak Kumar Dutta
Materials Science Division, CSIR-North East Institute of Science and Technology, Jorhat 785006, Assam, India
a r t i c l e i n f o
a b s t r a c t
0
Article history:
Rh -nanoparticles of around 5 nm size distributed homogeneously into the nanopores of acid activated
Received 31 May 2013
Montmorillonite clay were generated by incipient wetness impregnation of RhCl3, followed by reduction
with ethylene glycol. Acid activation of the Montmorillonite clay was carried out by treating with H2SO4
under controlled condition to increase the surface area by generating nanopores upto about 10 nm sizes,
Received in revised form 24 October 2013
Accepted 26 October 2013
Available online 5 November 2013
0
which act as a host and stabilize nanoparticles into the pores. The synthesized Rh -nanoparticles-clay
0
composites characterized by PXRD, SEM-EDX, HRTEM, XPS and N2-adsorption confirm generation of Rh -
Keywords:
nanoparticle below 5 nm size and fully reduced to metallic state. The supported metal nanoparticles serve
as efficient heterogeneous catalyst for reduction of some important aromatic carbonyl compounds lead-
ing to corresponding alcohols through transfer hydrogenation up to 100% conversion (GC) and selectivity,
where isopropanol was used as both solvent and reductant. The catalyst remained active for several runs
without significant loss of its catalytic activities.
0
Rh -nanoparticles
Acid activated Montmorillonite
Nanopores
Transfer hydrogenation
Reduction
©
2013 Elsevier B.V. All rights reserved.
1
. Introduction
stability and wide spread applications as industrial catalyst. Stabi-
lized and dispersed Rh -nanoparticles are found good catalyst for
0
Metal nanoparticles show enormous applications in different
a wide range of organic reactions like hydrogenation, oxidation,
hydroformylation reactions, etc. [23–31].
fields for their intrinsic physical and chemical properties like high
reactivity, quantum size effect, low melting point, size dependent
colour, etc. that are lacked in the bulk metal [1–6]. These prop-
erties are observed due to the high surface to volume ratio, i.e.
most of the atoms in nanoparticles are exposed on the surface
Reduction of the carbonyl group of organic compounds to the
corresponding alcohols is an important organic reaction. These
compounds are very vital components in different synthesis of fine
chemicals, pharmaceuticals, dyes, biologically active compounds,
etc. [32]. Various techniques like catalytic hydrogenation, elec-
trolytic reduction, metal mediated reductions [33], etc. are utilized
and among them, transfer hydrogenation is one of the ‘Clean’ and
‘Green’ approaches. Transfer hydrogenation is advantageous over
the other hydrogenation technique in which an alcohol like iso-
propanol, considered as one of the green solvents, acts as solvent
as well as the source of hydrogen and the reaction takes place at
atmospheric pressure and relatively low temperature. Moreover,
the co-produced acetone in the reaction is also a useful chemical
and can easily be separated from the reaction mixture by sim-
ple distillation. Wide varieties of homogeneous complexes have
been reported as transfer hydrogenation catalyst with very high
turnover frequency and selectivity [34,35]. However, very few het-
erogeneous catalysts have been reported though it has several
advantages over homogeneous process in respect of catalyst sepa-
ration, recyclability, higher stability, economic viability etc. With
stringent and growing environmental norms, supported metal
nanoparticles may be advantageously used as heterogeneous cata-
lyst in different organic transformations. Recently, different metal
[
1–6], and for that the nanoparticles are very reactive and tend to
agglomerate to form bulk metal and thus reduces its activity [1–10].
Therefore, it is very challenging task for researchers to generate
metal nanoparticles of uniform size, shape and to stabilize them
in the nano domain. Various supports/stabilizers like mesoporous
solid, organic ligand, polymer, carbon materials, etc. are used for
stabilization of nanoparticles [1–3,11–19]. Montmorillonite clay
is one of the suitable supports where metal nanoparticles can be
stabilized inside the interlayer spacing or into the pores on the
surface, which can be generated by activating with mineral acids
under controlled condition [9,10,18–22]. Pores are generated due
to leaching out of aluminium ion by mineral acid from the clay
matrix. Thus, by controlling the acid concentration, activation time
or temperature, the pore size and pore volume can be customized
to desired dimension. Among the different metals, platinum group
metal nanoparticles have aroused much interest for their greater
∗ Corresponding author. Tel.: +91 376 2370081; fax: +91 376 2370011.
E-mail addresses: dipakkrdutta@yahoo.com, dutta dk@rrljorhat.res.in
D.K. Dutta).
and metal oxide nanoparticles like Ni, Co, Ru, Rh, MgO–ZrO , etc.
2
(
were reported [36–40], but there is still lot of scope to enhance the
0
926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2013.10.049

3
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P.P. Sarmah, D.K. Dutta / Applied Catalysis A: General 470 (2014) 355–360
catalytic activity by improving the reaction time, catalyst quantity,
etc. As a part of our continuous research activity [9,10,18,19,41–43],
here we have reported the greener synthetic procedure for genera-
1600
1400
40.9
0
tion of Rh -nanoporticles using environmentally benign activated
Montmorillonite (AT-Mont) clay as support and ethylene glycol
as reductant. Naturally occurring Montmorillonite clay (Mont) is
processed and activated using mineral acid under controlled con-
dition to generate micro and mesoporous matrix with high surface
area and mean pore diameter of about 5 nm on the surface. These
1
1
200
000
4
7.7
8
6
4
2
00
00
00
00
0
pores act as a host for the Rh -nanoparticles and restrict their size to
3
5.4
grow further. Literature survey reveals that for the first time, we are
0
going to report the use of this supported Rh -nanoparticle as excel-
69.7
lent heterogeneous catalyst for transfer hydrogenation of some
important organic carbonyl compounds to corresponding alcohols.
The catalysts were recycled several times and found active without
significant loss of catalytic efficiency.
3
0
40
50
60
70
80
Two theta (Degree)
2
. Results and discussion
0
Fig. 2. The powder XRD pattern of Rh -nanoparticles supported on AT-Mont.
2.1. Characterization of the modified support
sheet and shows broad absorption bands at 3633 cm 1 due to
−
The powder XRD of the parent Mont shows an intense reflection
◦
stretching vibrations of OH groups of Al OH [47]. The bands at
O Al and Si O Si bending vibra-
tions mode, respectively. Upon acid activation, the Si O stretching
vibration band shifts from 1034 to 1083 cm indicating the change
in bonding environment surrounding the tetrahedral sheet.
at 2Â = 7.06 corresponding to a basal spacing (d 0 1) of 12.5 A˚ while
0
−
1
5
22 and 460 cm are due to Si
in AT-Mont, the reflection almost disappears indicating depletion
of the lamellar structure [44]. The cation exchange capacity (CEC) of
the purified natural Mont decreases from 127 to 40.8 milli equiva-
lent (meq)/100 g of clay upon acid activation for 1 h. The surface
area and pore size distribution of the AT-Mont determined by
−
1
0
N -adsorbtion study (Fig. 1) reveals that AT-Mont contain both
2.2. Characterization of supported Rh -nanoparticles
2
micro and mesoporous with average pore diameters of 5 nm and
2
0
exhibits high specific surface area of about 417 m /g with large
Evidence for the formation of Rh -nanoparticles was obtained
3
specific pore volumes of 0.57 cm /g. Acid activation modifies the
from powder XRD analysis (Fig. 2), wherein peak centred at
◦
layered structure of Mont by leaching out aluminium from octahe-
dral sites, thereby creating porous matrix with a high surface area.
The adsorption–desorption isotherms were of the type IV with a
H3 hysteresis loop at P/P0 ∼0.4–0.9, indicating mesoporous solids
2Â = 40.9 and 47.7 can be assigned to the (1 0 0) and (1 1 1) reflec-
tions from face centred cubic (fcc) Rh0 nanoclusters respectively,
which are consistent with the standard rhodium metal data [48].
The size of Rh nanoparticles calculated for the (1 1 1) peak using
Scherrer equation was found 5.9 nm. The binding energies of the
[
45,46]. The BJH pore size distribution plot indicates relatively a
0
narrow pore size distribution with a peak pore diameter centred
within 4–6 nm.
supported Rh -nanoparticles shown in the XPS spectrum peaks
(Fig. 3) at 306.7 and 311.4 eV, are assigned to Rh0 3d5/2 and d3/2
electronic state which are slightly higher by 0.3 and 0.4 respec-
tively than those of the free rhodium metal [49]. These higher values
3
IR study reveals that the parent Mont exhibits an intense absorp-
−1
tion band at 1034 cm for Si O stretching vibrations of tetrahedral
0
may be attributed to the interaction of Rh -nanoparticles with the
framework oxygen of the clay matrix, which is expected to induce
a positive charge on the metal surface and thereby increasing the
0
binding energies of Rh -nanoparticles [10,50].
3d5/2
3d3/2
70000
65000
60000
Rh(0)
305
310
315
320
Binding Energy (eV)
Fig. 1. N2 adsorption–desorption isotherm and BJH pore size distribution curve of
AT-Mont.
0
Fig. 3. XPS pattern of Rh -nanoparticles supported on AT-Mont.

P.P. Sarmah, D.K. Dutta / Applied Catalysis A: General 470 (2014) 355–360
357
0
Fig. 4. EDX patterns of AT-Mont (a) and Rh -AT-Mont (b).
The energy dispersive X-ray spectroscopy (EDX) [corresponding
SEM images are given in the supplementary information] pat-
0
terns of AT-Mont and Rh -AT-Mont (Fig. 4) indicate the presence
of only alumino silicates in acid activated Montmorillonite clay
(
Fig. 4a) and Rh metal on the clay surface of the nanocomposite
0
(Fig. 4b). The morphological investigation of the supported Rh -
nanoparticles by transmission electron microscopy (TEM) analysis
0
(
Fig. 5) reveals that the average sizes of the Rh -nanoparticles were
below 5 nm, which is in good agreement with value calculated
from Scherrer equation. The particle size histogram (Fig. 5(inset))
shows a narrow size distribution range 2–5 nm of the synthesized
nanoparticles. High resolution transmission electron microscopy
0
(
HRTEM) image of a single Rh -nanoparticle (Fig. 6) shows the
reticular lattice planes inside the nanoparticles. The lattice planes
continuously extended throughout the whole particles without
stacking faults or twins, indicating the single crystalline nature.
The measured inter-planar lattice fringe spacing is about 0.21 nm,
0
which corresponds to the (1 1 1) plane of fcc Rh crystals. The
corresponding selected area electron diffraction (SAED) pattern of
Rh -nanoparticles was obtained by focusing the electron beam on
0
Fig. 6. HRTEM images of Rh -nanoparticles supported on activated clay.
0
the nanoparticle lying on the TEM grid (Fig. 6(inset)). The formation
of hexagonal symmetrical diffraction spot patterns indicates that
3
3
0
of the nanocomposite decreases to 0.43 cm /g from 0.57 cm /g (AT-
Mont) after metal nanoparticles generation and the BJH pore size
distribution shows shifting of pore diameter towards lower value
compared to the support, which indicate that the nanoparticles are
generated inside the pores resulting in decrease of pore volume and
the generated Rh -nanoparticles are mono-crystalline nature. The
0
specific surface area measurement of the Rh -nanoparticles sup-
ported on AT-Mont by N -adsorption (Fig. 7) reveals that the value
2
2
significantly decreases from 417 to 376 m /g, which indicates the
generation of the Rh -nanoparticles inside the pores on the clay
0
0
pore sizes. The Rh content in the Rh -AT-Mont was estimated using
ICP-AES, which shows the presence of 0.047 mol of Rh per 100 g of
the nanocomposite.
matrix, resulting in decrease of surface area. The total pore volume
0
Fig. 7. N2-adsorption isotherm and BJH pore size distribution curve of Rh0-
Fig. 5. TEM images of Rh -nanoparticles and particle size histograms with a Gauss-
ian curve fitting (inset) supported on AT-Mont.
nanoparticles supported on activated clay.

3
58
P.P. Sarmah, D.K. Dutta / Applied Catalysis A: General 470 (2014) 355–360
0
surface of the Rh -nanoparticles in close proximity which, facilitate
the hydrogen transfer to the carbonyl group along with genera-
tion of one molecule of acetone [39]. The presence of base further
facilitates the adsorption of isopropoxide ion on the nanoparticles
surface [33].
A large number of homogeneous catalysts have been reported
in transfer hydrogenation reactions with very high turnover fre-
quency and selectivity [34,35]. However, only a few heterogeneous
catalysts are found for such reaction. Kantam et al. [36] have
reported an efficient Ru based heterogeneous catalytic system with
large number of substrates, however, a long reaction time from 20
to 48 h are required. Subramanian et al. [38] have reported a Cu
based catalyst supported on zeolite framework with high conver-
sions rate, however, the high conversions were found only when
the reaction was carried out in closed vessel. Literature survey
reveals that, for the first time, we have reported Rh metal based het-
erogeneous catalyst in transfer hydrogenation reaction with high
conversion rate and high stability compared to the existing reports.
The reduction of carbonyl group is greatly influenced by elec-
tronic property of the substituent’s as well as the steric effect. The
increase in electrophilicity of the carbonyl group facilitates the
reaction, as transfer hydrogenation proceeds through hydride ion
transfer. But, a reverse trend is observed in entry 1, 2 and 3, which
may be due to the steric hindrance of the bulkier group present
near the carbonyl group in entry 2 and 3. In case of para substituted
acetophenone, presence of electron withdrawing group increases
the electrophilicity and hence shows better conversion than the
Scheme 1. Transfer hydrogenation of aromatic carbonyl compounds to the corre-
sponding alcohols catalyzed by Rh -nanoparticles supported on AT-Mont.
0
2.3. Catalytic transfer hydrogenation
0
The synthesized Rh -nanoparticles were evaluated as
a
heterogeneous catalyst in transfer hydrogenation of some sub-
stituted aromatic carbonyl compounds to corresponding alcohols
(
Scheme 1). The yields (Table 1) of conversion of the carbonyl com-
0
pounds catalyzed by Rh -nanoparticles are found up to 100% with
1
00% selectivity. However, a control reaction with AT-Mont only (in
the absence of supported Rh) did not form any product, which indi-
cates that acid activated clay has no catalytic effect. To investigate
the effect of base on catalysis, reactions were carried out without
addition of any base. Although reactions were progressing, very low
rate of conversion was observed. Thus, the reaction rate increases
upon addition of a base. However, no products were observed in
presence of NaOH without addition of catalyst. Probably, the reac-
tion proceeds by adsorption of H-donor and H-acceptor on the
Table 1
0
Results of the transfer hydrogenation reaction of aromatic carbonyl compounds to the corresponding alcohols catalyzed by Rh -nanoparticles supported on AT-Mont.
Substratea
Product
Reaction time (h)
Yield (%)
Selectivity (%)
TOF (h
−1
)
Entry
O
OH
1
100 (1st run)b
53.2
H
H
H
2
100
b
9
9
9
8 (2nd run)
4 (3rd run)
4 (4th run)
52.1
50.0
50.0
b
b
O
O
OH
2
3
2
6
99b
94b
100
100
52.7
16.7
CH3
CH3
OH
O
OH
CH3
4
5
CH3
CH3
6
91c
100
100
16.1
7.4
HO
HO
O
OH
CH3
12
83c
H C
H C
3
3
O
OH
CH
6
CH
3
3
12
56c
100
4.9
H CO
H CO
3
3
a
Initial substrate concentration: 1 mmol.
GC yield.
Isolated yield.
b
c

P.P. Sarmah, D.K. Dutta / Applied Catalysis A: General 470 (2014) 355–360
359
catalyst after first run may be due to the destruction of some of
the nanopores of the suppor t¸ but during subsequent run, it shows
almost steady value indicating high stability of the catalyst. The BJH
pore size distribution curve (presented in the supplementary infor-
mation) of the recovered catalyst shows a slight broadening of the
distribution pattern compared to fresh catalyst indicating break-
down of some pore walls and forming larger pores. The Rh content
in the recovered catalyst after the fourth run was estimated using
ICP-AES, which showed the presence of 0.046 mol per 100 g of the
catalyst, which indicates that only very small amounts of Rh metal
(∼2%) are leaching out from the clay matrix after fourth run.
3. Experimental
3.1. Materials and methods
Fig. 8. Conversion of acetophenone to 1-phenylethanol versus time catalyzed by
fresh and recovered catalyst.
Bentonite (procured from Gujarat, India) is clay, rich in Mont-
morillonite clay mineral along with other accessory materials like
quartz, silt, etc. and was purified by sedimentation technique to col-
1000
lect the <2 ␮m fraction of pure Montmorillonite clay before use. The
basal spacing (d0 0 1) of the air dried samples was about 12.5 A˚ . The
specific surface area determined by N2 adsorption was 101 m /g.
40.94
2
750
500
250
The analytical oxide composition of the Bentonite determined
was SiO : 49.42%; Al O : 20.02%; Fe O : 7.49%; MgO: 2.82%; CaO:
2
2
3
2
3
0
.69%; loss on ignition (LOI): 17.51%; and others (Na O, K O and
2 2
TiO ): 2.05%. Mont was converted to the homoionic Na-exchanged
2
47.8
form by stirring in 2 M NaCl solution for about 48 h, washed and
dialyzed using deionized distilled water until the conductivity of
69.8
the water approached that of distilled water. CEC was 127 meq per
◦
1
00 g of clay (sample dried at 120 C). RhCl , ethylene glycol, iso-
3
propanol and substrates were purchased from Sigma–Aldrich, USA.
All reagents were used as supplied.
2
0
30
40
50
60
70
80
IR spectra (4000–400 cm−1) were recorded on KBr discs in a Shi-
madzu IR Affinity-1 spectrophotometer. Powder XRD spectra were
Two theta (Degree)
◦
0
recorded on a Rigaku Ultima-IV from 2 to 80 2Â using CuK␣ source
A˚ ). Specific surface area, pore volume, average pore diam-
Fig. 9. The powder XRD pattern of recovered catalyst (Rh -nanoparticles supported
on activated clay).
(ꢀ = 1.54
eter 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 experiment, the samples were degassed
electron donation group. The electron withdrawing group at the
para position enhanced the reaction up to 91% (TOF: 16.1, entry 4,
Table 1). On the other hand, electron donors at the para position
showed as low as 56% (TOF: 4.9, entry 6, Table 1). In order to inves-
tigate the recyclability, the catalyst was reused up to fourth run in
the conversion of benzaldehyde to benzyl alcohol (Table 1, entry
◦
at 250 C for 3 h. Pore size distributions were derived from desorp-
tion isotherms using the Barrett–Joyner–Halenda (BJH) method
[39,41]. The 1H NMR spectra were recorded at room temperature
in CDCl3 solution on a Bruker DPX-300 spectrometer and chemical
1
). The used catalyst was recovered by simple filtration and dried
shifts were reported relative to SiMe . Mass spectra were recorded
4
under vacuum before using in the next run. The reactions were
carried out by maintaining the stoichiometry of the reactants and
recovered catalyst. Results show that the catalyst remains active for
several runs without significant loss in efficiency. The conversion
of acetophenone to 1-phenylethanol catalyzed by fresh catalyst as
well as recovered catalyst after first set of reactions are shown in
Fig. 8. The recovered catalyst shows almost the same trend of con-
version compared to that of the fresh catalyst, indicating that the
recovered catalyst remains active after reaction.
on ESQUIRE 3000 Mass spectrometer. SEM images and EDX pat-
terns were obtained by JEOL JSM-6390 LV operated at 15 kV on a
gold coated sample. TEM and HR-TEM images were recorded on a
JEOL JEM-2011 electron microscope and the specimens were pre-
pared by dispersing powdered samples in isopropyl alcohol, placing
them on a carbon coated copper grid and allowing to dry. X-ray
Photoelectron Spectra were recorded on Kratos ESCA model Axis
165 spectrophotometer having a position sensitive detector and
hemispherical energy analyzer in an ion pumped chamber.
The morphology of the recovered catalyst was further inves-
tigated through TEM, powder XRD and N -adsorption analysis.
2
0
TEM image shows that the Rh -nanoparticles were still inside the
3.2. Support preparation
clay matrix retaining its sizes as that of the fresh catalyst, which
indicates no significant morphological changes occurred after the
reaction (TEM images are presented in the supplementary infor-
mation). Further, the recovered catalyst showed almost the same
powder XRD pattern like the fresh catalyst (Fig. 9). The specific
surface area of the recovered catalysts decrease from the fresh cat-
Purified Mont (10 g) was dispersed in 200 mL 4 M sulfuric acid
and refluxed (around 100 C) for 1 h. After cooling, the supernatant
◦
liquid was discarded and the acid activated Mont was repeatedly
redispersed in deionized water until no SO42 ion could be detected
−
by the BaCl2 test. The modified Mont was recovered, dried in air at
2
◦
alyst 417 m /g to 356, 341 and 339 for second, third and fourth
run, respectively. The initial decrease of the surface area of the
50 ± 5 C overnight to obtain the solid product. The acid activated
Mont was designated as AT-Mont.

3
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P.P. Sarmah, D.K. Dutta / Applied Catalysis A: General 470 (2014) 355–360
0
3
.3. Rh -nanoparticles preparation
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◦
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[
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3
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