2
A.A. Dabbawala et al. / Catalysis Communications 42 (2013) 1–5
Scheme 1. Dehydration of sorbitol to isosorbide.
with a heating rate of 5 °C min–1 in flowing air. According to the
calcination temperature, STO catalysts were denoted as STO-T (where
T = temperature). The dehydration of sorbitol was carried out in liquid
phase according to our previously reported procedure [12,13]. The com-
position of the dehydrated product was analyzed using HPLC (Younglin
Instrument, Acme 9000) equipped with RI detector and Asahipak
column (NH2P-50 4E, No. N712004). The products detected were
isosorbide and sorbitan (mostly 1,4- or 3,6-sorbitan with small
amount of 1,5- and 2,5-anyhydrides). The selectivity of isosorbide
(IS) and sorbitan (ST) were based on molar composition. The non-
indentified soluble and insoluble compounds were considered as “others”
(unknown). The reproducibility of the experimental results was con-
firmed by repeating the same experiment at least two times and was
found to be in a range of 3–5% due to process and analytical variations.
After dehydration reaction, the recovered catalyst was thoroughly
washed with water and ethanol. The catalyst was then dried and finally
calcined at 450 °C for 5 h to get the regenerated catalyst.
The BET surface area and S content (determined by element analysis)
of catalysts are listed in Table 1. The S content remain unchanged when
the calcination temperature was below 350 °C but decreased sharply
with an increase of the calcination temperature above 350 °C. BET surface
area of samples increased first and then decreased with an increase of the
calcination temperature. An abrupt change in surface area and S content
above temperature of 350 °C during the calcination confirmed the
transformation of SnSO4 to SnO2 by desulfation, which is necessary
to generate medium–strong acid sites on the surfaces. The above results
are in accordance with reported literatures indicating that the acidic
properties and catalytic activities of STO catalyst depend on BET surface
area and the sulfur content which are function of the calcination
temperature [15–17].
The FT-IR spectra of the STO catalysts are shown in Fig. 3. From FT-IR
spectra, it can be seen that the bands at 1165, 1038 and 975 cm–1
confirmed the presences of sulfate group and coordination of sulfate to
tin ions through a bidentate chelating mode [18]. Due to the existence
of sulfate groups on the surface, a strong acidity can be realized in the
STO catalysts.
3. Results and discussion
3.1. Characterization of the catalyst
3.2. Catalytic dehydration of sorbitol
Powder XRD of STO catalysts calcined at different temperature indi-
cates the change in the crystalline phase of stannous sulfate before and
after calcination at 350, 400, 450, 500 and 600 °C (Fig. 1). The diffraction
peaks at 24.95°, 27.02°, 29.08°, 32.48°, 41.69° and 42.99°, corresponding
to the characteristic diffraction of SnSO4, with additional peaks at 26.63°,
33.97°, 38.10° and 51.83°, corresponding to the characteristic diffractions
of SnO2 appeared in the sample calcined at 350 °C. This suggests that the
decomposition of SnSO4 to SO24−/SnO2 occurred at 350 °C. The char-
acteristic diffraction peaks of SnSO4 disappeared in all the samples
calcined above 350 °C, indicating complete conversion of SnSO4 to
SnO2 phase.
The SEM images of STO calcined at 450 °C are shown in Fig. 2 along
with SEM-EDX spectrum. From the SEM images, it is clear that the
catalyst has crystalline cubic morphology with particles sized in the
range of 30–35 nm, whereas SEM-EDX spectrum indicates the presence
of tin, oxygen and sulfur.
The catalytic activities of STO calcined at different temperature were
tested in dehydration of sorbitol. To understand the role and effect of
sulfate group in catalysts, the dehydration experiments were also
performed with SnO2 and SnSO4 for comparison. All STO catalysts
and SnSO4 showed high catalytic activities towards the dehydration
of sorbitol compare to unsulfated analogue, SnO2 (Table 2, entry 1).
The catalytic behavior of the STO changed with their calcination tem-
perature due to the variation in sulfur content at different calcination
temperature (Table 2, entries 3–7). These results indicated the role
of the sulfate group to enhance conversion and selectivity.
The dehydration activity of the catalysts was in the order of
SnSO4 N STO 350 N STO 450 N STO 400 N STO 500 N STO 600. The
SnSO4 exhibited highest activity due to its homogeneous nature. In
the case of STO catalysts, the catalyst activity was decreased with de-
crease in sulfur content as acidity of catalyst varied with sulfur content
[11–13]. The STO calcined at 350 °C having sulfur content 8.02% displayed
the complete conversion and high isosorbide selectivity (51%) in 1 h at
180 °C, although it was not fully converted to its oxide form. Therefore,
the STO calcined at 450 °C having sulfur content 3.71% showed 46%
isosorbide selectivity was selected for further optimization.
The effect of reaction temperature on conversion and selectivity of
sorbitol dehydration was studied in the temperature range from 170
to 220 °C for reaction time 1 h using 0.15 g of STO-450 catalyst. The
complete conversion was achieved in all cases when temperature was
greater than 170 °C, while isosorbide selectivity considerably influence
by variation in temperature (Fig. 4).
The low isosorbide selectivity was observed at 170 °C, which
increased by factor of 2 at 180 °C. The isosorbide selectivity in-
creased gradually with increasing temperature from 180 to 210 °C
then decreased at 220 °C. At 170 °C, isosorbide selectivity was 23%
which increased to 46% at 180 °C and enhance up 55% at 210 °C.
With an increase in temperature, formation of dark brown colored
side products were observed to increase which is difficult to character-
ize. These side products might be originated from self-polymerization
10000
8000
STO-600
STO-500
6000
STO-450
4000
O
O
O
STO-400
O
O
O
O
O
O
O
2000
0
STO-350
SnSO4
10
20
30
40
50
60
70
80
2theta/ degree
Fig. 1. XRD patterns of STO calcined at different temperatures (O–SnO2).
Dabbawala, Aasif Asharaf
