Hydrogenation of alkenes with rhodium nanoparticles supported on SBA-15

Article abstract of DOI:10.1007/s10562-012-0956-4

Rhodium nanoparticles were prepared by chemical reduction of RhCl ₃·3H₂O in presence of polyvinyl pyrrolidone and then immobilized on SBA-15 by impregnation. The catalyst was used for hydrogenation of unsaturated hydrocarbons at room temperature. The progress of the reaction was monitored by GC-MS and 100 % conversion was achieved in all cases.

Full text of DOI:10.1007/s10562-012-0956-4

Catal Lett (2013) 143:276–281
DOI 10.1007/s10562-012-0956-4
Hydrogenation of Alkenes with Rhodium Nanoparticles
Supported on SBA-15
•
Nayanmoni Bhorali Jatindra Nath Ganguli
Received: 10 November 2012 / Accepted: 17 December 2012 / Published online: 9 January 2013
Ó Springer Science+Business Media New York 2013
Abstract Rhodium nanoparticles were prepared by
chemical reduction of RhCl3Á3H2O in presence of polyvi-
nyl pyrrolidone and then immobilized on SBA-15 by
impregnation. The catalyst was used for hydrogenation of
unsaturated hydrocarbons at room temperature. The pro-
gress of the reaction was monitored by GC–MS and 100 %
conversion was achieved in all cases.
structure of the particle and the stabilization of the colloids.
Among various techniques, chemical reduction techniques
are important for the synthesis of nanoparticles, because
these methods can be applied under simple and mild con-
ditions and can also be used to prepare nanoparticles on a
large scale [4]. Though transition metal nanoparticles act as
good catalyst for many organic synthesis, the liquid sus-
pensions of metal nanoparticles in catalysis is difficult to
separate and recycle. Our work focuses on immobilization
of metal nanoparticles on mesoporous solid support mate-
rials. Mevellec et al. [5] reported a synthetic method for
preparation of rhodium nanoparticles by addition of silica
to a pre stabilized rhodium aqueous colloidal suspension.
Although Somorjai and coworkers [6] also reported syn-
thesis of Rh nano particles and ethylene hydrogenation,
their method is high temperature synthesis and the catalysis
is also at high temperature and pressure, in comparison our
method reported in this paper is simple, starting from
RhCl3, room temperature synthesis of Rh nano particles
and liquid phase heterogeneous hydrogenation of various
alkenes at low pressure and room temperature.
Keywords Mesoporous silicates Á Rh-SBA-15 Á
Rhodium-nanoparticles Á Hydrogenation
1 Introduction
The overall performance and activity of catalyst and
selectivity are ruled by size and morphology of active
metals [1–3]. Though metal nanoparticles with different
sizes and morphologies have been synthesized, yet their
successful application in catalysis is a big challenge for
chemists. The main disadvantage in this regard is the
possibility of agglomeration and sintering of the nanopar-
ticles under drastic catalytic reaction conditions. Hence it is
important to support the nanoparticles effectively on solid
and stable materials which would prevent agglomeration
and deactivation. Interest in the synthesis and properties of
colloidal metal nanoparticles has been increasing because
of their unique properties and applications as catalyst. Two
main aspects in the synthesis of the metal colloid nano-
particles are to control the growth of metal nanoparticles
in terms of particle size, particle size distribution and
2 Experimental: Materials and Methods
Rhodium chloride hydrate was obtained from Arora Mat-
they, sodium borohydride and the alkenes were purchased
from Merck and were used without further purification.
Tetraethylorthosilicate (TEOS) and block polymer—
EO20PO70EO20 were purchased from Aldrich.
Growth of the rhodium nanoparticles by sodium
borohydride reduction in PVP was monitored by absorp-
tion spectroscopy. UV–Visible spectroscopy was carried
out in a double beam Hitachi (U-4100) spectrophotome-
ter. Rh(0) sol was studied in 10 mm quartz cuvettes.
N. Bhorali Á J. N. Ganguli (&)
Department of Chemistry, Gauhati University, Gauhati,
Assam, India
e-mail: jatin_ganguli_gu@yahoo.co.in
123

Hydrogenation of Alkenes
277
Powder XRD spectra were recorded on a Brucker D8
X-ray diffractometer from 5–75o 2h using Cu-Ka source
˚
(k = 1.54 A). Low angle XRD spectra were recorded on
a Rigaku X-ray diffractometer from 0.5–10o 2h using Cu-
˚
Ka source (k = 1.54 A). The nitrogen adsorption and
desorption isotherms were measured using Micromeritics
Tristar 3000 system. Before adsorption, the samples were
degassed at 250 °C for 3 h. Pore volume were derived
from the isotherms using the Barrett–Joyner–Halenda
(BJH) method. Scanning electron microscopy (SEM)
images and energy dispersive X-ray Spectroscopy (EDX)
were collected with JEOL, Model: JSM6390LV, operated
at 15 kV. High resolution TEM images were recorded in
JEOL JEM 2100 TEM instrument and operated at
200 kV. Aqueous suspension of the sample was prepared
in dry carbon coated Cu-grid. Hydrogenation reactions
were carried out under room temperature (27 °C) and
4 bar pressure in a Parr hydrogenation apparatus. The
reaction was monitored by the volume of gas consumed
(the pressure indicator in the Parr apparatus gradually
falls down with the consumption of the H2 gas) and using
a Perkin Elmer GCMS (Clarus 600) with an Elite 5MS
column (30 m, 250 lm i.d.).
Fig. 1 Low angle XRD of Rh-SBA-15
2.3 Synthesis of the SBA-15 Supported Rhodium
Nanoparticles
SBA-15 silica (1 g) was added under vigorous stirring to
50 mL of deionized water. After 2 h, the above prepared
Rh(0) colloidal suspension was added under vigorous
stirring to this suspension. The system was kept under
stirring for 4 h. Then the content was filtered and the brown
powder was washed several times with distilled water to
eliminate the PVP and evaporated to dryness at 353 K
(Fig. 1).
2.1 Synthesis of SBA-15 Silica
The synthesis of mesoporous silica SBA-15 was done
according to a previously reported procedure [7]. Pluronic
P123 (4 g) was dissolved in 144 mL 2 M HCl solution
with constant stirring at 308 K for 2 h. TEOS (9 mL)
(TEOS/P123 = 2) was added to the solution with constant
stirring at 308 K for 4 h. The mixture was aged for 12 h,
transferred to a high pressure Teflon lined stainless steel
autoclave and then heated at 373 K for 48 h. The white
powder was recovered through filtration, washed with
water and ethanol thoroughly and dried at 353 K for 12 h
and finally calcined at 813 K for 3 h.
3 Results and Discussions
3.1 X-ray Diffraction Analysis
The calcined SBA-15 had an ordered hexagonal structure,
as indicated by the intense (100), (110), and (200) reflec-
tion in the small angle region of powder X-ray diffraction
pattern shown in Fig. 2. The intense (100) peak corre-
2.2 Synthesis of Aqueous Rhodium (0) Suspension:
Polymer Stabilized Rh(0) Nanosols
˚
sponds to a d spacing of 97 A and this remained unchanged
To an aqueous solution of PVP (0.02 g/100 mL), sodium
borohydride (0.1 g) was added. Then this solution was
quickly added under vigorous stirring to an aqueous solu-
tion (100 mL) of precursor RhCl3Á3H2O (0.1 g, 0.4 9
10-3 mol) to obtain an aqueous Rh(0) colloidal suspension.
The reduction occurred instantaneously and was charac-
terized by color change from red to black. Since complete
reduction of Rh3? to Rh(0) occurs, amount of Rh(0) is
0.04 g.
after the incorporation of rhodium nanoparticles. Other
weak peaks found in 2h range of 1°–3.5° correspond to the
(110), (200) reflections indicating high degree of hexagonal
mesoscopic structure.
The X-ray diffraction (XRD) pattern of Rhodium
nanoparticle (Fig. 3) shows a diffuse ‘‘peak’’. The dif-
fraction angle of the peak is found nearly 2h = 42.8°,
which is consistent with the d-value of rhodium metal. A
very broad signal at 2h = 23o for amorphous SiO2.
123

278
N. Bhorali, J. N. Ganguli
essentially unchanged. This suggests that the majority of
the nanometer scaled void space of the host SBA-15 was
still open, although a small portion of the mesoporous
channels may be occupied by the Rh-nanoparticles which
indicate the reduction in the surface area and the pore
volume [9, 10].
3.3 UV–Visible Analysis
The amount of reducing agent required for complete con-
version was determined with the help of UV–Visible ana-
lyser. To a 0.4 mM rhodium chloride solution containing
0.02 g/100 mL PVP, a 26 mM (0.1 g/100 mL) solution of
reducing agent was added with constant stirring. The pro-
cess of reduction at different NaBH4 concentration was
followed on the UV–Vis spectra as shown in Fig. 6. The
peaks which are characteristic of the chloro-complexes of
rhodium (225 and 198 nm) gradually disappeared and was
accompanied by a gradual rise of the descending branch of
the spectrum (in the range of 260–300 nm). In presence of
excess of reducing agent the new peak, characteristic of Rh
(0) nanoparticles appears at 194 nm. The curve shows that
1:1 Rh3?/NaBH4 is necessary for the complete reduction.
Fig. 2 Low angle XRD of SBA-15(Calcined)
3.4 SEM Analysis
From the figure (Fig. 7) it can be concluded that the SBA-
15 particles were cylindrical and freely distributed with
some aggregation. The EDX shows the atomic percentage
of Rh-nanoparticle as 2.3 %.
3.5 TEM Analysis
The morphology distributions of Rh nanoparticles on the
surface of the SBA-15 were shown in the TEM image
(Fig. 8). The image shows that the pore structure is regular
with well-ordered arrays, and the metallic nanoparticles are
well dispersed. The shape of the rhodium nanoparticles are
spherical or quasi-spherical and are homogeneously dis-
persed. The Rh nanoparticles have a narrow size distribu-
tion in the range of 3–4 nm. The Rh particles are well
separated with no agglomeration tendency. Histrogram in
Fig. 9 shows the particle size distribution of the Rh-nano-
particles obtained from 209 particles from TEM image.
Fig. 3 High angle XRD of Rh nanoparticles
3.2 Nitrogen Absorption–Desorption
Figures 4 and 5 shows nitrogen adsorption desorption
isotherm for SBA-15 and Rh-SBA-15 samples. The esti-
mated textural parameters are compiled in Table 1.
All of the nitrogen adsorption/desorption isotherms are
found to be of Type IV in nature as per the IUPAC
nomenclature and exhibited a H1 hysteresis loop which is
typical of mesoporous solid [8]. The adsorption branches of
each isotherm showed a sharp inflection in the relative
pressure range of about 0.60–0.68. This is a characteristic
of capillary condensation [7]. The position of the inflection
point is clearly related to a diameter in the mesopore range.
The sharpness of these steps indicates the uniformity of the
mesopore size distribution [8]. After Rh incorporation, the
total pore volume decreases, while the average pore size is
4 Catalytic Study
The SBA-15 supported Rh nanocatalyst previously
described has been tested in the hydrogenation of various
unsaturated compounds under mild conditions. In all cases,
a total conversion of substrate has been observed. The
amount of substrate was 1 mmol in all the cases; we have
123

Hydrogenation of Alkenes
279
Fig. 4 N2 adsorption–
desorption isotherm of SBA-15
Fig. 5 N2 adsorption–
desorption isotherm of Rh-SBA-
15
123

280
N. Bhorali, J. N. Ganguli
Table 1 Surface parameters of SBA-15 and Rh-SBA-15
Entry
BET surface area
(m2/g)
Pore volume (cm3/g)
BJH
adsorption
BJH
desorption
SBA-15
720
320
0.85
0.81
0.86
0.80
Rh-SBA-15
Fig. 8 TEM image of Rh-SBA-15
Fig. 6 Determination of amount of NaBH4 for total reduction of
Rh3? by adsorption spectrophotometry
Fig. 9 Particle size distribution of Rh-nanoparticles
conditions. But it takes more time for complete conversion
(Table 2).
Recycling experiments have been investigated on cata-
lytic systems. After each run, the catalyst was recovered by
simple filtration and dried at 333 K before another run with
new addition of substrate. Five run could be performed at
least with no loss of activity. These results show no
leaching of Rh nanoparticles during the catalytic reactions.
Fig. 7 SEM pictures of Rh-SBA-15
also observed by taking 10 mmol substrates keeping all
other conditions same. But we have not observed notice-
able change, except small change in time taken for com-
plete conversion. Active sites were calculated as function
of Rhodium-crystallite size from TEM. On the basis of
active site TOF for hydrogenation of the alkenes were
evaluated [11]
5 Conclusion
We have shown that an aqueous solution containing pre-
stabilized Rh nanoparticles can be used easily for particle
deposition onto SBA-15 through simple impregnation. The
For comparison, we have tested 1-Hexene hydrogena-
tion with Wilkinson catalyst RhCl(PPh3)3 under identical
123

Hydrogenation of Alkenes
281
Table 2 Catalytic activity in the hydrogenation of unsaturated hydrocarbons
Catalyst
Amount (g)
Reactant
Product
Time (h)
TOF
Rh-SBA-15
Rh-SBA-15
SBA-15
0.05
0.02
0.05
0.05
0.05
0.02
0.05
0.05
0.02
0.05
0.05
0.05
0.02
0.05
0.05
0.02
0.05
0.02
Cyclohexene
Cyclohexene
Cyclohexene
10 mmol of Cyclohexene
Geraniol
Cyclohexanea
Cyclohexanea
Cyclohexanea
Cyclohexanea
Citronellola
Citronellola
Citronellola
Haxanea
0.05
0.13
0.33
0.35
0.116
0.2
11,582
11,110
Rh-SBA-15
Rh-SBA-15
Rh-SBA-15
Rh-SBA-15
Rh-SBA-15
Rh-SBA-15
SBA-15
16,546
4,992
7,221
21,771
5,791
7,221
Geraniol
10 mmol of Geraniol
1-Hexene
0.266
0.10
0.20
24
1-Hexene
Haxanea
1-Hexene
1-Hexene
Haxanea
Rh-SBA-15
Rh-SBA-15
Rh-SBA-15
Rh-SBA-15
Rh-SBA-15
Rh-SBA-15
*Rh-SBA-15
*Rh-SBA-15
10 mmol of 1-Hexene
Styrene
0.166
0.083
0.166
0.166
0.116
2
34,887
6,977
8,700
34,887
4,992
722
Ethylbenzenea
Ethylbenzenea
Ethylbenzenea
Cyclohexanea
Cyclohexanea
Cyclohexanola
Cyclohexanolb
Cyclohexanoneb
Hexanea
Styrene
10 mmol of styrene
Benzene
Benzene
Phenol
2
289
Phenol
4
361
RhCl(PPh3)3
0.10
1-Hexene
19
a
*water as solvent, others methanol as solvent, 100 % conversion, 50 % conversion
b
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Acknowledgments The authors are thankful to Department of
Chemistry, Gauhati University, IISc, Bangalore, NEHU, Shillong for
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