(temperature of 120 °C, molar ratio of butanol and acetic acid of
1 : 2 and the reaction time of 6 h), the 20 wt% WO3/β-SiC shows
better catalytic activity, having 88% conversion with 100%
selectivity to butyl acetate.
The bonding and structural information of the prepared β-SiC
and WO3 promoted β-SiC catalysts were recorded using Fourier
transform infrared spectroscopy on a Perkin Elmer spectrum Gx
in the range between 500–2000 nm.
The electronic states of Si, C, W, O were examined by X-ray
photoelectron spectroscopy (XPS, Kratos Axis 165 with a dual-
anode (Mg and Al) apparatus) using an Mg Kα source. All the
binding energy values were calibrated by using the contaminant
carbon (C 1s = 284.9 eV) as the reference. Charge neutralization
of 2 eV was used to balance the charge of the sample. Binding
energy values of the samples were reproducible within 0.1 eV.
UV-vis investigation in diffuse reflectance mode was recorded
in a UV-vis spectrophotometer (Varian, Australia). The spectrum
was recorded in the range of 200–800 nm using boric acid as the
reflectance standard.
The shape, size and impurities (if any other than Si, C, W and
O) of the β-SiC and WO3 promoted β-SiC catalysts were deter-
mined by Transmission electron microscopy (TEM) equipped
with energy dispersive spectroscopy (EDS) using FEI,
TECNAIG2 20, TWIN instrument and the images were recorded
by using a Gatan CCD camera.
The BET surface areas and pore volume distributions of
the catalysts were determined by N2 adsorption at 77 K
(ASAP2010). A known amount of catalyst sample was evacuated
for 2 h at 110 °C to remove physically adsorbed water prior to
surface area measurements.
The acid character of the catalysts was studied by TPD-NH3
with an AutoChem-II, (micromeritics) Chemisorption analyzer
equipped with a thermal conductivity detector (TCD). About 1 g
of powdered sample was contained in a quartz “U” tube and
degassed at 250 °C for 1 h with ultra pure nitrogen gas. After
cooling the sample to room temperature, NH3 (20% NH3
balanced with helium) gas was passed over the sample while it
was heated at a rate of 10 °C min−1 and the profile was recorded.
The morphological and structural properties of synthesized
samples were studied with a field-emission scanning electron
microscopy (FE-SEM, ZEISS 55) analyser.
Experimental section
Materials preparation
Preparation of β-SiC powder as precursor. Rice husk has
been considered as a potential raw material for the preparation of
silicon carbide.15 Hence, in the present study the locally avail-
able rice husk obtained from Khurda District of Odisha was pro-
cessed in plasma using an indigenously developed 50 kW
extended arc thermal plasma reactor with applying load voltages
of 50 V and an arc current of 300 A. The plasma forming argon
gas flow was kept fixed at 1 lpm. The thermal plasma reactor
provides a higher temperature above 2100 °C which is the main
advantage to convert rice husk to β-SiC within a very short time
period of 20 min.
The detailed synthesis procedure was documented else-
where.16 The plasma synthesized product obtained from the
plasma reactor contained a mixture β-SiC and a small percentage
of carbon and silica. The plasma produced sample was heated in
a Muffle furnace at 700 °C for 2 h for the carbon removal. The
particle sizes are found to be in the order of 10 mm. For further
reduction of particle sizes, the plasma produced carbon free
β-SiC powder sample was ground using a Retsch PM-100 plane-
tary ball mill with 3 mm stainless steel balls. The grinding was
carried out in a 500 ml stainless steel jar in an ethyl alcohol
medium at a fixed rpm of 350 for 4 h. Then this ground sample
was thoroughly washed with 1 : 1 HCl, 1 : 2 HNO3 and 40% HF
for the complete removal of silica and other metallic impurities
(if any), present in the sample. The XRF analysis shows that no
inorganic impurity other than silicon is present on SiC surface.
Preparation of tungstate promoted β-SiC. Tungstate promoted
β-SiC catalysts with different wt% were prepared by a wetness
impregnation method using water as a solvent. The requisite
amount of promoter source (WO3 as ammonium meta tungstate)
was dissolved in 40 ml of distilled water. The solution was
stirred on hot plate–magnetic stirrer for some minutes and then
the desired amount of β-SiC powder was added. Stirring was
continued on the magnetic stirrer at a low temperature until com-
plete evaporation of the solution took place. The residue was
completely dried in an oven at 120 °C for 8 h. These dried
samples were calcined at 700 °C for 2 h. In this way different
compositions of tungstate (5, 10, 15 and 20 wt% of WO3) pro-
moted β-SiC catalysts were prepared.
Catalytic activity test
The esterification reaction was carried out in a 100 ml double
necked round bottom flask equipped with a reflux condenser and
a magnetic stirrer by taking 0.05 g of the catalyst, 2.7 ml of
n-butanol (Merck, 98%) and 3.4 ml of acetic acid (Merck,
99.8%). The contents were then refluxed gently at 100 °C for
6 h. The reaction mixture was filtered and the products were ana-
lyzed by offline GC (Shimadzu, GC-17A) equipped with capil-
lary column (ZB-1, 30 m length, and 0.5 nm ID and 3.0 μm film
thickness) using flame ionization detector (FID).
Characterization
Results and discussion
β-SiC and WO3 promoted β-SiC catalysts were characterized by
X-ray powder diffraction (XRD, PANAlytical) using Mo-Kα
radiation of 0.7093 Å as the X-ray source.
Raman spectra of the precursor and promoted samples were
obtained on a Renishaw inVia Raman microscope for the deter-
mination of structural information. The spectra were recorded in
the range between 500–2000 nm.
Fig. 1 displays the XRD patterns of (a) β-SiC, (b) 5 wt%
WO3/β-SiC, (c) 10 wt% WO3/β-SiC, (d) 15 wt% WO3/β-SiC
and (e) 20 wt% of WO3/β-SiC catalysts. The highly intense
peaks of β-SiC at 2θ = 16.24°, 18.72°, 26.62°, 31.41°, 32.69°
and 37.97° have been found in the XRD patterns of all the
products and matched with the JCPDS data (# 02-1050) having
14300 | Dalton Trans., 2012, 41, 14299–14308
This journal is © The Royal Society of Chemistry 2012