Synthesis of glycerol carbonate from glycerol and urea over tin-tungsten mixed oxide catalysts

Article abstract of DOI:10.1016/j.apcata.2013.09.041

Tin-tungsten mixed oxide catalysts with varying their mole ratio were prepared by co-precipitation method. The catalysts physico-chemical properties were derived from FT-infrared, Laser Raman, X-ray diffraction, UV-Vis DRS, BET surface area and temperatur

Full text of DOI:10.1016/j.apcata.2013.09.041

Applied Catalysis A: General 469 (2014) 165–172
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Synthesis of glycerol carbonate from glycerol and urea over
tin-tungsten mixed oxide catalysts
a
a
a
b
a,∗
K. Jagadeeswaraiah , Ch. Ramesh Kumar , P.S. Sai Prasad , S. Loridant , N. Lingaiah
a
Catalysis Laboratory, I & PC Division, CSIR—Indian Institute of Chemical Technology, Hyderabad 500007, India
Institut de Recherches sur la Catalyse et l’Environnement de Lyon, IRCELYON, UMR 5256, CNRS, Universite Lyon I, 2 avenue Albert Einstein, 69626
b
ꢀ
Villeurbanne Cedex, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 June 2013
Received in revised form
Tin-tungsten mixed oxide catalysts with varying their mole ratio were prepared by co-precipitation
method. The catalysts physico-chemical properties were derived from FT-infrared, Laser Raman, X-ray
diffraction, UV–Vis DRS, BET surface area and temperature-programmed desorption of NH3. The cata-
lysts activities were evaluated for the synthesis of glycerol carbonate from glycerol and urea. The activity
results showed that Sn-W mixed oxide catalysts are highly active for selective formation of glycerol
carbonate. Sn-W catalyst with 2:1 molar ratio exhibited about 52% of glycerol conversion with >95%
selectivity towards glycerol carbonate. The active catalyst was subjected to calcination at different tem-
peratures and evaluated for their activity in glycerol carbonate synthesis. The activity of the catalysts
depends on mole ratio of Sn/W and treatment temperature which are influencing the surface-structural
characteristics of the catalysts. Different reaction parameters such as glycerol to urea molar ratio, reaction
temperature and catalyst loading were studied and optimum conditions were established. The catalysts
showed consistent activity upon repeated use.
1
7 September 2013
Accepted 24 September 2013
Available online xxx
Keywords:
Glycerol
Glycerol carbonate
Tin oxide
Tungsten oxide
Urea
©
2013 Elsevier B.V. All rights reserved.
1
. Introduction
glycerolysis in which glycerol is reacted with urea in the presence
of a catalyst. This approach is an indirect route for utilization of
The development of biodiesel industry from biomass resource
CO as it is used in the production of urea. The synthesis of glycerol
2
led to over plus of glycerol, produced as a byproduct in the synthesis
of biodiesel. The conversion of glycerol to valuable chemicals has
attracted considerable attention in recent times not only due to its
availability but also for the presence of more number of functional
groups [1–10]. Among different chemicals possible from glycerol,
glycerol carbonate is a valuable glycerol derivative. This is being
widely used in cosmetics industry, gas separation, coatings, deter-
gents and polymers [11–15]. A number of synthetic processes are
known for the preparation of glycerol carbonate from glycerol [5].
The well known methods for preparation of glycerol carbonate are
the reaction from glycerol with phosgene [16] and transesterifica-
tion of glycerol with a carbonate source [17]. The most interesting
route to produce glycerol carbonate is from the reaction of glycerol
with CO2 in the presence of a catalyst [18]. This reaction requires
elevated temperature, pressure and the yield of glycerol carbonate
is too low to be used for practical purposes. The alternative route is
carbonate by glycerolysis is studied by different authors [17,19,20].
Mouloungui et al. patented the synthesis of glycerol carbonate by
carbonylation of glycerol with urea over heterogeneous zinc cata-
lysts such as zinc sulfate, zinc organosulfate and zinc ion exchange
resins [19]. Calcined ZnSO4 gave 86% of glycerol carbonate in 2 h
◦
at a reaction temperature of 150 C and 40 mbar of pressure. The
major drawback of this catalyst is that, ZnSO salt is soluble in glyc-
4
erol. There is a strong need to develop heterogeneous catalyst for
strategic and environmental benefit. Heterogeneous catalysts are
particularly efficient for carbonylation of glycerol with urea [17,20]
affording good yield under moderate reaction conditions. Climent
et al. [17] have recently reported the carbonylation of glycerol with
◦
urea at 145 C over heterogeneous catalysts such as basic oxides
(MgO and CaO), Al/Mg and Al/Li mixed oxides derived from hydro-
talcites with adequate acid–base pairs. These catalysts showed
about 72% of glycerol carbonate yield in 5 h of reaction time. In
our previous study, we reported samarium exchanged heteropoly
acid catalyst for carbonylation of glycerol with urea. The Sm0.66TPA
catalyst showed good selectivity (85.4%) towards glycerol carbon-
ate with 49.5% glycerol conversion [21]. Recently Hutchings and
co-workers reported the synthesis of glycerol carbonate using gold
∗ Corresponding author. Tel.: +91 40 2719 1722; fax: +91 40 2716 0921.
E-mail addresses: nakkalingaiah@iict.res.in, lingaiah66@yahoo.co.in
(
N. Lingaiah).
0
926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2013.09.041

1
66
K. Jagadeeswaraiah et al. / Applied Catalysis A: General 469 (2014) 165–172
and Pd based supported catalysts [22]. These catalysts are expen-
sive and overall activity is not very high and requires long reaction
time. The catalysts with acidic and basic sites such as Zn-Al hydro-
talcite and ␥-zirconium phosphate were also reported [17,20]. The
catalysts containing basic sites were reported for the selective syn-
thesis of glycerol carbonate with high activity [23,24]. However,
these base catalysts exhibited low activity during recycling. There is
a need to develop highly active and stable catalysts for the synthesis
of glycerol carbonate from glycerol and urea.
*
M-monoclinic WO3
*
SnO2
*
*
*
(e)
*
*
M
M
(
d)
Solid acid catalysts are promising catalysts for the carbonylation
of glycerol with urea [21]. Tungsten oxide based mixed oxide cata-
lysts are known for their role in acid catalyzed reactions. Moreover
these mixed oxide catalysts are known for their stability.
In the present study, a series of tin-tungsten mixed oxide cata-
lysts were prepared with varying Sn to W molar ratio and studied
for carbonylation of glycerol with urea under reduced pressure.
The surface and structural properties of the catalysts were varied
by treating the samples at different temperatures. The proper-
ties of the catalysts were investigated from different spectroscopic
methods to establish the catalytic activities. A detailed study was
undertaken to optimize the reaction parameters to achieve best
results.
(
c)
(
(a)
b)
1
0
20
30
40
50
60
70
80
o
2
Theta ( )
◦
Fig. 1. XRD profiles of the SW mixed oxide catalysts calcined at 500 C. (a) SW11,
(
b) SW21 (c) SW31 (d) SW51 (e) SW12.
2
. Experimental
Temperature programmed desorption of ammonia (TPD-NH3)
was carried out on a laboratory-built apparatus equipped with a gas
chromatograph using TCD detector. In a typical experiment about
2.1. Preparation of Sn-W mixed oxide
0
.05 g of oven dried sample was taken in a quartz tube. Prior to TPD
◦
Sn-W mixed oxide catalysts were prepared by co-precipitation
run, the catalyst sample was treated at 300 C for 1 h by passing pure
method. The preparation of Sn-W oxide with a Sn/W molar ratio
He gas (50 ml/min). After pretreatment, the sample was saturated
◦
of 2 is given as an example. In a typical procedure, Na WO ·2H O
with anhydrous ammonia (10% NH balance He gas) at 100 C with
2
4
2
3
(
4.94 g, 15 mmol) was dissolved in deionized water, followed by
a flow rate of 50 ml/min for 1 h and was subsequently flushed with
He gas at the same temperature to remove physisorbed ammonia.
The process was continued until a stabilized base line was obtained
in the gas chromatograph. Then the TPD analysis was carried out
the addition of SnCl4 (7.815 g, 30 mmol) in a single portion. The
solution stirred for 1 h at room temperature and deionized water
was added to form white slurry. This slurry kept for stirring for
◦
◦
2
4 h at room temperature and the resulting white precipitate was
from ambient temperature to 700 C at a heating rate of 10 C/min.
The amount of NH3 evolved was calculated from the peak area of
the already calibrated TCD signal.
filtered off, washed with large amount of deionized water until it
◦
is free from chlorine. The precipitate was dried in oven at 120 C
to afford Sn-W hydroxide as a white powder. SW hydroxides with
different Sn/W molar ratios (Sn/W=0.5, 1, 2, 3, and 5) were success-
fully prepared by changing the molar ratios of the starting metal
2
.3. Reaction procedure
◦
solutions. The hydroxide precursors were calcined at 500 C for 4 h
The reactions were performed in a 25 mL two neck round-
under an air atmosphere to yield final catalyst. The catalysts are
denoted as SW11 where the alphabets represent the metal oxide
and the numerical number related to their molar ratio.
bottom (RB) flask under reduced pressures. In a typical experiment
glycerol (2 g), urea (1.306 g) and catalysts (0.2 g) were taken in
the round bottom flask and heated in an oil bath at 140 C with
◦
constant stirring. One neck of the RB flask was connected to vac-
uum line. Reaction was run under a reduced pressure in order to
remove the ammonia formed during the reaction. After completion
of reaction or stipulated time, methanol was added and the cata-
lyst was separated by filtration. The products were analyzed by a
gas chromatograph (Shimadzu, 2010) equipped with flame ioniza-
tion detector using inno wax capillary column (diameter: 0.25 mm,
length 30 m). Products were also identified by GC–MS (Shimadzu,
GCMS-QP2010S) analysis.
2
.2. Characterization of the catalysts
X-ray diffraction (XRD) patterns of the catalysts were recorded
a Rigaku Miniflex diffractometer using CuK␣ radiation
on
(
1.5406 A˚ ) at 40 kV and 30 mA. The measurements were obtained
◦
in steps of 0.045 C with account time of 0.5 s and in the 2ꢀ range
of 10 –80 .
◦
◦
Confocal Micro-Raman spectra were recorded at room temper-
−
1
ature in the range of 200–1200 cm
Lab Ram HR spectrometer with
using a Horiba Jobin-Yvon
17 mW internal He–Ne
a
3. Results and discussion
(
Helium–Neon) Laser source of excitation wavelength of 632.8 nm.
The catalyst samples in powder form (about 5–10 mg) were loosely
spread onto a glass slide below the confocal microscope for mea-
surements.
3.1. Catalyst characterization
◦
X-ray diffraction patterns of SW catalysts calcined at 500 C are
The UV–Vis diffuse reflectance spectra (UV–Vis DRS) were
recorded on a GBC UV–Visible Cintra 10e spectrometer in the range
of 200–800 nm. BET Surface area was measured on Quadrasorb—SI
instrument at relative pressure range of 0.05–0.3. Before analysis,
the samples were degasified at 150 C for 2 h to remove moisture
on the surface of the catalyst.
shown in Fig. 1. The XRD patterns of SW catalysts with the molar
ratio of 1:1 and 2:1 were intrinsically identical to that of amorphous
Sn-W hydroxide precursor. Whereas in the case of SW12, SW31
catalysts, intense diffraction peaks related to crystalline tin oxide
◦
◦ ◦
were observed. The peaks observed at 2ꢀ values of 26.5 , 33.9 ,
◦ ◦ ◦ ◦ ◦
37.9 , 43.5 , 51.8 , 54.7 and 61.9 are corresponding to tetragonal

K. Jagadeeswaraiah et al. / Applied Catalysis A: General 469 (2014) 165–172
167
(e)
(f)
(
e)
d)
(c)
b)
a)
(
d)
(
(
c)
(
b)
(
(
(
a)
300
400
500
600
700
800
2
00
400
600
800
1000
-
1
Wavelength (nm)
Raman shift (cm )
Fig. 3. UV–Vis diffused reflectance spectra of SW mixed oxide catalysts calcined at
◦
Fig. 2. Laser Raman spectra of SW mixed oxide catalysts calcined at 500 C. (a) SW11,
◦
500 C. (a) SW11, (b) SW21, (c) SW31, (d) SW12, (e) SW51 and (f) WO3.
(
b) SW21, (c) SW31, (d) SW51 and (e) SW12.
SW mixed oxide catalysts (except SW51) showed broad bands in
the range of 220–378 nm and band at 398 was absent indicates that
these broad bands can be assigned to isolated tungsten species or
low-condensed polymeric tungsten oxide species [33]. In the case
of SW51 catalyst, in addition to the broad band in the range of
rutile structure SnO crystal planes of (1 1 0), (1 0 1), (2 0 0), (2 1 0),
2
(
2 1 1), (2 2 0) and (0 0 2), respectively [25–27]. Catalyst with 5:1
molar ratio of Sn/W showed diffractions peaks related to both tin
◦
◦
oxide and tungsten oxide. The diffraction peaks at 2ꢀ = 23.3 , 24.3 ,
3
◦ ◦
3.29 and 34.1 can be indexed to (0 0 2), (2 0 0), (0 2 2) and (2 0 2)
2
20–360 nm, a shoulder peak is observed at 398 nm which corre-
of monoclinic WO phase, respectively (JCPDS No. 43-1035). These
3
sponds to the bulk WO . These results indicate that SW51 catalyst
3
peak positions and their relative intensities are in good agreement
with the literature [28]. The catalysts with Sn/W molar ratio of
exists in two phases. The results obtained from XRD and Raman
results are in good agreement with these results.
1
:1, 2:1 and 3:1 exhibited diffraction peaks related to only SnO2
The TPD profiles of the SW catalysts along with pure WO3
are shown in Fig. 4. The amount of ammonia desorbed from the
catalyst surface is calculated and presented in Table 1. All the
samples showed broad desorption peaks indicating that the sur-
face acid strength is widely distributed. SW11 catalyst showed
and patterns related to WO were absent. This might be due to (i)
3
strong interaction between tin oxide and tungsten oxide species,
(
ii) poly tungstate species are highly dispersed on SnO2 and for-
mation of homogeneous solid solution. However, SW51 catalyst
showed peaks related to WO . This indicates that, agglomeration
3
◦
two unresolved desorption peaks at 390 and 500 C correspond-
of tungsten oxide species took place at high Sn to W ratio. Thus,
homogeneity of the mixed oxide lost beyond the Sn/W molar ratio
of 3:1. These results suggest that catalysts with Sn/W molar ratio
of 1:1 to 3:1 form a homogeneous solid mixed oxide.
ing to moderate and strong acidic sites. Whereas the SW21 catalyst
showed two broad and intense desorption peaks in the range of
◦
◦
◦
1
00–460 C and 450–650 C. The peak centred at 250 C is related to
Fig. 2 shows the Raman spectra of the catalysts. The cata-
−
1
lyst SW11 showed an intense band at 953 cm
corresponding
(f)
to stretching mode for octahedrally coordinated poly tungstate
species [29]. When the SW molar ratio varied from 1:1 to 3:1
(e)
−
1
and 1:2, the band at 953 cm
related to W=Ot bond shifted to
−
1
higher wave number (953 to 979 cm ). This suggests that different
two-dimensional tungsten oxide species might be present in Sn/W
catalysts and no crystalline WO3 phase was present. The SW51
(d)
−
1
catalyst showed bands at 273, 717 and 808 cm
corresponding
to W–O–W deformation mode, W=O bending and W=O stretching
(c)
mode, respectively, of crystalline WO [30,31]. These results indi-
3
cate that, 1:1 to 3:1 ratio of S/W mixed oxide form a homogeneous
mixture. The XRD patterns also supports the results observed from
Raman spectra.
UV–Vis diffuse reflectance spectral technique is sensitive for
distinguishing the chemical nature and coordination states of tung-
sten oxide species. Fig. 3 shows the UV–Vis DRS of SW catalysts
(
b)
(a)
along with bulk WO . Catalyst with SW molar ratio of 1:1 clearly
3
showed a absorption band at 303 nm which corresponds to poly-
meric WO6 species [32]. This band is unresolved in the case of
catalysts SW21, SW31 and SW12 due to strong absorption in the
1
00
200
300
400
500
600
o
Temperature ( C)
range of 220–378 nm. In general bulk WO shows a broad band at
3
Fig. 4. Temperature programmed desorption of ammonia patterns of the SW mixed
oxide catalysts calcined at 500 C. (a) SW11, (b) SW21, (c) SW31, (d) SW12, (e) WO3
3
98 nm with a shoulder at 359 nm [33]. UV–Vis DRS of bulk WO3
◦
sample showed a broad band centred at 390 nm. On the other hand
and (f) blank run of SW21.

1
68
K. Jagadeeswaraiah et al. / Applied Catalysis A: General 469 (2014) 165–172
Table 1
Acid strength distribution and BET surface area of SW mixed oxide catalysts.
80
100
90
Catalyst
Acidity (mmol/g)
Weak/moderate
BET surface
area (m /g)
2
70
60
50
Strong
Total
8
0
0
SW11
SW12
SW21
SW31
SW400
SW600
SW700
0.085
0.195
0.404
0.231
0.021
0.177
0.136
0.020
0.015
0.068
0.035
0.018
0.045
0.016
0.105
0.210
0.472
0.266
0.039
0.222
0.142
12.6
27.7
37.4
47.6
14.8
39.5
51.1
7
60
50
◦
moderate acidic sites and other one centred at 530 C corresponds
to strong acidic sites. Catalyst with a Sn/W molar ratio of 3:1 (SW31)
40
30
40
30
◦
showed a strong desorption peak at 200 C, along with small peaks
centred at 370 and 520 C. The first peak is related to weak acidic
sites and two small desorption peaks corresponding to moderate
and strong acidic sites. SW12 catalyst showed two desorption peaks
at 380, 515 C related to moderate and strong acidic sites, respec-
◦
400
500
600
700
o
Catalyst calcination temperature ( C)
◦
◦
Fig. 5. Effect of catalyst calcinations temperature on the glycerol conversion and
glycerol carbonate selectivity. Reaction conditions: glycerol: 2 g, urea: 1.306 g, cat-
alyst weight: 0.2 g, Reaction temperature: 140 C, reaction time: 4 h.
tively, along with a small hump at 180 C. The intensity of the
desorption peaks related to moderate to strong acid sites increased
with increase in Sn/W ratio up to 2:1. Further increase in Sn/W
ratio to 3:1 intensity of the desorption peaks decreased and shifted
to lower temperatures. In the case of bulk WO3 sample, there is
no considerable ammonia desorption peak and desorbed ammonia
was found to be negligible. Compared to all other catalysts, SW21
showed broad and strong desorption peaks. The acidity values of
SW21 catalysts (Table 1) indicating that this catalyst contains high
acidity compared to all other catalysts. The high acidity of this cat-
alyst might be related to the presence of highly dispersed WO3
◦
of glycerol was about 10%. Pure WO3 and SnO2 showed 26% and
4% of glycerol conversion with 42% and 35% selectivity towards
glycerol carbonate, respectively. The main by-products formed in
the glycerol carbonylation are 5-(hydroxymethyl) oxazolidin-2-
one and (2-oxo-1,3-dioxolan-4-yl)methyl carbamate. In the case
of SW12 catalyst, 30% of glycerol conversion was observed with
1
4
8% selectivity. With increase in Sn/W molar ratio from 1:1 to 3:1,
the conversion of glycerol increased up to 52% and at the same
time the selectivity towards glycerol carbonate is varied signifi-
cantly. The catalyst with Sn/W molar ratio of 1:1 exhibited about
domains on SnO as evidenced from XRD and Raman analysis.
2
In order to know the nature of the gas released, a TPD run of
SW21 catalyst was performed without pre-treatment of ammonia
gas and the TPD profile is shown in Fig. 4. From the TPD profile it
can be noted that no considerable peak is observed above 500 C.
This result indicates that the desorbed gas during TPD run is related
to ammonia adsorbed on the acidic sites of the catalyst.
BET surface areas of the catalysts are presented in Table 1. From
the table it can be observed that as the increase in SW molar ratio
from 1:1 to 3:1 the surface area of the catalysts also increased. Cat-
alyst with SW molar ratio of 1:2 showed a surface area of 27.7 m /g.
In the case of SW21 catalyst calcined at in the range of 400–700 C
6
7% selectivity. Although SW21 and SW31 catalysts showed same
◦
glycerol conversion, but high selectivity about 95% towards GC
was observed for SW21 catalyst. SW31 catalyst showed about 66%
selectivity. The high activity of SW21 catalyst can be explained
based on the catalysts characteristics. This catalyst exhibited high
acidity compared to other catalysts. The acidity profiles of the cat-
alysts derived from ammonia TPD are in good agreement with the
observed catalytic activity, as SW21 catalyst showed strong and
intense desorption peaks of ammonia. The presence of high acidity
is mainly due to the presence of nano sized WO domains on SnO .
The existence of this type of WO3 species are supported by XRD
and Raman analysis. These results indicate that catalyst Sn/W with
:1 molar ratio is the most active catalyst compared to its counter
parts.
In order to know the thermal stability, surface and structural
changes of the catalyst, the most active catalyst was subjected
to calcination at different temperatures and the catalytic activity
was investigated for carbonylation of glycerol. Fig. 5 shows the
catalytic performance of SW21 catalyst calcined at different tem-
peratures. With increase in catalyst calcination temperature from
00 to 500 C conversion of glycerol increased from 37% to 52%
and selectivity towards glycerol carbonate also increased from 71%
to 95%. Increase in calcination temperature to 600 C, conversion
of glycerol was decreased to 37% and glycerol carbonate selec-
tivity is found to be 71%. Further increase in catalyst calcination
temperature to 700 C, there is no variation in the glycerol con-
version compared to catalyst calcined at 600 C, however, glycerol
carbonate selectivity was decreased to 62%.
The SW21 catalyst calcined at different temperatures was fur-
ther characterized by XRD and Raman spectroscopy to understand
the difference in activity with variation in treatment temperature.
XRD patterns of SW21 catalysts calcined at different tempera-
2
◦
3
2
exhibited an increase in BET surface area with increased calcination
temperature. This might be due to removal of hydroxyl groups on
the catalysts surface in the form of water molecules with increase
in calcination temperature.
2
3.2. Catalytic activity
Catalysts performance was investigated for carbonylation of
glycerol with urea and the results are presented in Table 2. The
reaction was also performed without catalyst and the conversion
◦
4
Table 2
Glycerol conversion and glycerol carbonate selectivity over SW mixed oxide
catalysts.
◦
Catalyst
Conversion of
glycerol (%)
Glycerol carbonate
selectivity (%)
◦
◦
Blank
SnO2
10.5
13.6
30.3
43.6
52.1
52.6
36.0
26.2
19.3
35.1
47.7
67.5
95.3
66.4
54.1
42.0
SW12
SW11
SW21
SW31
SW51
WO3
◦
tures are shown in Fig. 6. The catalyst calcined at 400 and 500 C

K. Jagadeeswaraiah et al. / Applied Catalysis A: General 469 (2014) 165–172
169
M-monoclinic WO3
*
*
SnO2
(d)
*
*
M
M
(c)
*
*
*
(d)
(
c)
b)
a)
80
(b)
(
(a)
(
1
00
200
300
400
500
600
o
1
0
20
30
40
2
50
Theta
60
70
Temperature ( C)
Fig. 8. Temperature programmed desorption of ammonia patterns of SW21 catalyst
calcined. at (a) 400 C, (b) 500 C, (c) 600 C and (d) 700 C.
◦
◦
◦
◦
◦
◦
◦
Fig. 6. XRD profiles of the SW21 catalyst calcined at (a) 400 C, (b) 500 C, (c) 600
and. (d) 700 C.
C
◦
◦
temperatures (≥500 C) this band is slightly shifted to lower wave
showed similar pattern as that of amorphous Sn-W hydroxide
precursor. Upon increasing the calcination temperature to 600 C,
numbers. Intensity of this band increased for the sample calcined
◦
◦
at 500 C and further increase in calcination temperatures to 600
◦
crystalline phase of tin oxide was formed. Further increase in calci-
and 700 C, intensity of the band is decreased. This might be due to
◦
nation temperature to 700 C, peaks associated to monoclinic phase
loss in homogeneity of solid mixture. XRD results are in support for
observations made from Laser Raman spectra.
◦
◦
of WO3 phase were observed at 2ꢀ values of 23.2 and 24.41
28]. The high temperature calcination resulted in the formation of
[
Fig. 8 shows the TPD of ammonia patterns of SW21 catalyst cal-
◦
poly tungstate aggregates. XRD results suggest that the crystallinity
cined at different temperatures. Catalyst calcined at 400 C showed
◦
increased with calcination temperature. The presence of well dis-
a small desorption peak at 320 C. A drastic change in the acidity
profile was observed for the catalyst calcined at 500 C. It showed a
strong desorption peaks at 250 and 530 C corresponding to moder-
◦
◦
persed amorphous phase of WO was maintained up to 600 C and
3
◦
◦
formation of WO3 crystallites were noticed at 700 C. XRD results
suggest that, at higher calcination temperatures (above 500 C),
◦
ate and strong acidic sites as discussed earlier. Catalyst calcined at
◦
homogeneity of the mixed oxide was lost and leads to formation of
polytungstate aggregates.
600 C showed three desorption peaks with low intensity. Desorp-
◦
tion peaks centred at 150 and 270 C corresponds to weak and
Laser Raman spectra of SW21 catalyst calcined at different tem-
peratures are shown in Fig. 7. All the catalysts showed an intense
moderate acidic sites, respectively. Another small desorption peak
observed at 580 C related to strong acidic sites. These desorption
◦
◦
band related to W=Ot bond. Catalyst calcined at 400 C showed
peaks intensity was further decreased with increase in calcination
−1
◦
a band at 981 cm , whereas for the catalyst calcined at higher
temperature to 700 C. The observation made from the XRD, Raman
◦
and TPD of NH suggests that the catalyst calcined at 500 C has high
3
acidity due to the presence of well dispersed WO domains on SnO2
3
surface. The high activity of the catalyst is related to its surface and
structural characteristics, which are directing the acidity of the cat-
alysts. Low conversion of glycerol for the catalyst calcined at higher
temperature related to decrease in acidity due to the formation of
large ensembles of WO3.
(d)
Conversion of glycerol and glycerol carbonate selectivity not
only depends on the nature of the catalyst but also on reaction
parameters. In order to optimize the reaction conditions, different
reaction parameters such as reaction time, reaction temperature,
glycerol to urea molar ratio and catalyst weight were studied by
(
c)
◦
using the most active SW21 catalyst calcined at 500 C.
(b)
3.3. Influence of the reaction time
(
a)
Reaction time is an important parameter and long reaction times
were adopted usually for the carbonylation of glycerol to obtain
reasonable conversion. The effect of reaction time was studied over
◦
2
00
400
600
800
1000
SW21 catalyst calcined at 500 C and the results are shown in Fig. 9.
-
1
The conversion of glycerol consistently increased with increase in
reaction time from 1 to 4 h. The selectivity towards glycerol car-
bonate is varied marginally with time. The overall selectivity of
the catalyst with reaction time is high suggesting that the catalyst
Raman shift (cm )
◦
◦
Fig. 7. Laser Raman spectra of SW21 catalyst calcined at (a) 400 C, (b) 500 C, (c)
◦ ◦
00 C. and (d) 700 C.
6

1
70
K. Jagadeeswaraiah et al. / Applied Catalysis A: General 469 (2014) 165–172
1
00
100
1
00
0
100
8
6
4
2
0
0
0
0
0
8
80
8
6
0
0
60
40
20
0
6
4
2
0
0
0
40
20
1
:0.5
1:1
1:2
1:3
1
2
3
4
Reaction time (h)
Glycerol to urea molar ratio
Fig. 9. Effect of reaction time on glycerol conversion and glycerol carbonates
Fig. 11. Influence of glycerol to urea molar ratio on glycerol conversion and glycerol
selectivity. Reaction conditions: glycerol: 2 g, urea: 1.306 g, catalyst weight: 0.2 g,
reaction temperature: 140 C.
carbonate selectivity. Reaction conditions: catalyst weight: 0.2 g, reaction temper-
ature: 140 C, Reaction time: 4 h.
◦
◦
(
55%). These results suggest that the optimum reaction tempera-
is selective in formation of glycerol carbonate. A decrease in the
yield of glycerol carbonate was reported with reaction time over
◦
ture is 140 C.
␥
-ZrP catalysts during glycerolysis of urea [20]. The present cata-
3
.5. Effect of glycerol to urea molar ratio
lyst is selective in formation of glycerol carbonate and there was
no increase in formation of byproducts with reaction time.
In order to study the influence of molar ratio of reactants the
reaction carried with variation in glycerol to urea molar ratio and
the results are shown in Fig. 11. From the figure it can be noted
that conversion of glycerol increased from 31% to 59% with increase
in the concentration of urea, at the same time selectivity towards
glycerol carbonate decreased from 100 to 61%. The decrease in
selectivity with increase in urea concentration related to formation
of a by-product which forms due to the reaction of free hydroxyl
group present in the glycerol carbonate with available urea.
When the glycerol to urea molar ratio is 1:1, the conversion
of glycerol is 52% with 95% selectivity towards glycerol carbonate.
Further increase in urea concentration (1:2 and 1:3 molar ratio of
glycerol to urea) the conversion of glycerol is increased. However
selectivity towards glycerol carbonate is decreased. These results
suggest that 1:1 molar ratio of glycerol to urea is optimum for high
glycerol carbonate selectivity.
3
.4. Effect of reaction temperature
In order to evaluate the influence of reaction temperature reac-
tion was carried out at different reaction temperatures ranging
from 100 to 160 C and the results are shown in Fig. 10. As expected,
the conversion of glycerol increased with reaction temperature.
Conversion of glycerol is only 13% at a reaction temperature of
◦
◦
1
00 C with 62% selectivity towards glycerol carbonate. As the tem-
◦
perature increased from 120 to 140 C the conversion of glycerol
also increases from 29% to 52%. At the same time selectivity to glyc-
erol carbonate is also increased from 78% to 95%. Further increase
in reaction temperature selectivity towards glycerol carbonate
decreased to 80% with marginal increase in glycerol conversion
1
00
100
80
60
40
20
3.6. Effect of catalyst weight
Fig. 12 shows the effect of catalyst weight on the glycerol
8
6
4
2
0
0
0
0
conversion. Increase in catalyst loading from 100 to 200 mg the
conversion of glycerol increased from 26% to 52%. Selectivity also
increased from 63% to 95%. Further increase in catalyst loading
to 200–400 mg, only a marginal increase in glycerol conversion
was observed with a decrease in selectivity towards glycerol car-
bonate. The catalyst present in excess might be favouring the
reaction between the product glycerol carbonate and urea to yield
5
-hydroxymethyloxazolidine-2-one.
3.7. Catalyst reusability
The most active SW21 catalyst was studied for its reusability
and results are presented in Fig. 13. Fresh catalyst showed about
2% of glycerol conversion with 95% selectivity towards glycerol
carbonate. After completion of the reaction, methanol was added
to reaction mixture and the catalyst was separated by filtration. The
separated catalyst was washed with methanol, dried in oven and
reused without any pre-treatment. In the first recycle the catalyst
1
00
120
140
160
5
o
Reaction temperature ( c)
Fig. 10. Effect of reaction temperature on glycerol conversion and glycerol carbo-
nates selectivity. Reaction conditions: glycerol: 2 g, urea: 1.306 g, catalyst weight:
.2 g, reaction time: 4 h.
0

K. Jagadeeswaraiah et al. / Applied Catalysis A: General 469 (2014) 165–172
171
Table 3
Comparison of catalytic activity of the present catalyst with other reported solid acid catalysts.
◦
Catalyst
Reaction temperature ( C)
Reaction time (h)
Conversion of glycerol (%)
Glycerol carbonate (%)
Ref.
Yield
Selectivity
SW21
140
145
140
150
150
140
4
5
3
4
4
4
52.1
70
80
80
66
49.7
26
76
39
21
95.3
37
100
48
32
85.4
Present work
[15]
[18]
[28]
[28]
Sn-beta zeolite
*
Zr-P
Au/Fe2O3
2
.5% Au/Nb2O5
Sm0.66TPA
49.5
42.3
[19]
*
This catalysts contains both acidic and basic sites.
8
7
6
5
4
3
2
0
0
0
0
0
0
0
100
3
.8. Comparison of SW21catalyst activity with reported solid acid
catalysts
Catalytic activity of SW21 catalyst is compared with other
80
reported solid acid catalysts and comparative results are presented
in Table 3. Among the different catalysts zirconium phosphate cata-
lysts exhibited about 80% glycerol conversion with 100% selectivity
towards GC [20]. This catalyst contains both acidic and basic sites.
However, its activity decreased with reaction time and showed
decline in activity upon reuse. Compared to acidic catalysts base
60
catalysts such as La CO3 [23] and gold supported on MgO [24]
2
relatively showed high activity in the synthesis of glycerol carbon-
ate. Even though these catalysts are highly active their reusability
is limited. The present solid acid catalysts showed better activ-
ity and selectivity compared to other reported acid catalysts such
as Sn-beta zeolite, samarium exchanged tungstophosphoric acid
40
20
(Sm0.66TPA) and gold supported on acidic supports. Moreover, the
100
200
300
400
present catalysts is reusable compared other basic and acidic cata-
lysts.
Catalyst weight (mg)
Fig. 12. Effect of catalyst weight on glycerol conversion and glycerol carbonate
selectivity.
4. Conclusions
◦
Reaction conditions: glycerol: 2 g, urea: 1.306 g, reaction temperature: 140 C, reac-
tion time: 4 h.
Tin-tungsten mixed oxide catalysts with varied Sn/W molar
ratio were synthesized and characterized by various spectroscopic
techniques. These catalysts showed high activity towards the syn-
thesis of glycerol carbonate from glycerol and urea. The activity
of the catalysts depends on the Sn/W molar ratio and catalyst
calcination temperature. The catalyst with Sn/W ratio 2:1 cal-
showed similar selectivity (95%) towards glycerol carbonate, with
about 2% decrease in glycerol conversion. Further recycling of the
catalyst there was no change in conversion. Interestingly as the
catalyst is recycled, the selectivity towards glycerol carbonate is
increased marginally. These results suggest that the SW21 catalyst
is stable and reusable without any appreciable loss in activity.
◦
cined at 500 C exhibited optimum activity. The glycerol conversion
also depends on the reaction parameters and optimum reaction
conditions were established. The catalyst is reusable without any
pre-treatment and exhibited consistent activity.
6
5
4
3
2
0
0
0
0
0
100
Acknowledgements
9
0
0
The authors KJ and ChRK thank CSIR, India for the award of senior
research fellowship. NL also thanks CSIR for the award of Raman
Research Fellowship.
8
70
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