Understanding the surface and structural characteristics of tungsten oxide supported on tin oxide catalysts for the conversion of glycerol

Article abstract of DOI:10.1007/s12039-015-0848-4

Abstract Catalysts with varying WO₃ content on SnO₂ were prepared and characterized by X-ray diffraction, in situ Raman spectroscopy, X-ray photoelectron spectroscopy and temperature programmed desorption of NH₃. In situ Raman analysis reveals the presence isolated monomers and polymeric species of WO₃. These catalysts were evaluated for the conversion of glycerol into value added chemicals. Etherification of glycerol with tertiary butanol and preparation of glycerol carbonate from glycerol and urea are studied over these catalysts. The catalytic activity results suggest that the glycerol conversion and selectivity depends on the morphology of WO₃ which in turn is related to its content in the catalyst. The catalysts with 5 wt.% of WO₃ on SnO₂ resulted in high dispersion with larger number of strong acidic sites. The selectivity in the glycerol etherification is related to the nature of the catalyst and reaction time. These catalysts also exhibited high activity for synthesis of glycerol carbonate. The effect of various reaction parameters was studied to optimize the reaction conditions. The catalysts also exhibited consistent activity upon reuse.

Full text of DOI:10.1007/s12039-015-0848-4

c
J. Chem. Sci. Vol. 127, No. 5, May 2015, pp. 897–908. ꢀ Indian Academy of Sciences.
DOI 10.1007/s12039-015-0848-4
Understanding the surface and structural characteristics of tungsten
oxide supported on tin oxide catalysts for the conversion of glycerol
M SRINIVASa, G RAVEENDRAa, G PARAMESWARAMa, P S SAI PRASADa,
S LORIDANTb and N LINGAIAHa,∗
aCatalysis Laboratory, I & PC Division, CSIR-Indian Institute of Chemical Technology,
Hyderabad 500 007, India
bInstitut de Recherchessur la Catalyse et l’Environnement de Lyon, IRCELYON, UMR 5256, CNRS,
Université Lyon I, 2 avenue Albert Einstein, 69626 Villeurbanne Cedex, France
e-mail: nakkalingaiah@iict.res.in
MS received 28 August 2014; revised 1 December 2014; accepted 28 December 2014
Abstract. Catalysts with varying WO3 content on SnO2 were prepared and characterized by X-ray diffrac-
tion, in situ Raman spectroscopy, X-ray photoelectron spectroscopy and temperature programmed desorption
of NH3. In situ Raman analysis reveals the presence isolated monomers and polymeric species of WO3. These
catalysts were evaluated for the conversion of glycerol into value added chemicals. Etherification of glycerol
with tertiary butanol and preparation of glycerol carbonate from glycerol and urea are studied over these cata-
lysts. The catalytic activity results suggest that the glycerol conversion and selectivity depends on the morphol-
ogy of WO3 which in turn is related to its content in the catalyst. The catalysts with 5 wt.% of WO3 on SnO2
resulted in high dispersion with larger number of strong acidic sites. The selectivity in the glycerol etherifica-
tion is related to the nature of the catalyst and reaction time. These catalysts also exhibited high activity for
synthesis of glycerol carbonate. The effect of various reaction parameters was studied to optimize the reaction
conditions. The catalysts also exhibited consistent activity upon reuse.
Keywords. Tungsten oxide; tin oxide; etherification; glycerol; tert-butanol; glycerol ethers
1. Introduction
is glycerol carbonate (GC), a highly value added product
with many potential applications.12–16 Etherification of
glycerol is generally facilitated by acid catalysts. Thus,
acid catalysts such as sulfuric acid, p-toluenesulfonic
acid, acidic ion exchange resins, wide pore zeolites, and
acid-functionalized mesostructured silicas have been
used for etherification of glycerol.17–27 Etherification
of glycerol with tert-butanol produce a mixture of mono-
tert-butyl glycerol ether (ME), di-tert-butyl glycerol
ether (DE), and tri-tert-butyl glycerol ether (TE). Selec-
tive etherification of glycerol into monoalkyl glycerol
ethers, which exhibit a wide spectrum of biological
activities such as anti-inflammatory, anti-bacterial, anti-
fungal, immunological stimulation and anti-tumour
properties.28 Another important derivative of ME, diox-
olanes, is also used as co-fuels for the diesel fraction.20
The main routes proposed for the synthesis of GC
are: (i) reaction of glycerol with phosgene,29 (ii) trans-
esterification of glycerol with dimethyl carbonate30–32
or ethylene carbonate,33 (iii) reaction of glycerol with
urea,33–36 and (iv) carbonation of glycerol with carbon
dioxide.37,38 Among the possible routes for the prepara-
tion of GC, the reaction of glycerol and carbon dioxide
in the presence of a catalyst is the best approach.
Biodiesel, an eco-friendly biofuel is being chosen as an
alternative to fossil fuels. Biodiesel has been produced
in large quantities by transesterification of vegetable
oils or animal fats.1 During the production of biodiesel
approximately 10 wt.% of glycerol is obtained as by-
product.2 As the production of biodiesel is increasing
with time, large quantities of glycerol have been accu-
mulated in an already saturated market. This has led to
a dramatic reduction on the economic value of glycerol
as a raw material. This situation has fuelled a growing
interest to look for new glycerol uses.3 Glycerol can
be converted into several important chemicals4–8 such
as 1,2-propanediol, 1, 3- propanediol, acrolein, glyceric
acid, esters of glycerol and syngas, etc. The details
about the glycerol conversion are comprehensively dis-
cussed in recent reviews.9–11 Etherification of glycerol
with isobutylene or tert-butanol towards fuel oxygena-
tes has been considered recently as a promising and
economically viable method for the conversion of glyc-
erol. Another important chemical derived from glycerol
∗For correspondence
897

898
M Srinivas et al.
However, this method requires severe reaction condi- designated as x% WS, where x indicates the weight per-
tions and the yield of glycerol carbonate is too low centage of WO3 on SnO2 and W and S stands for WO3
to be used for practical purpose.37,38 An alternative and SnO2.
route for the GC synthesis is carbonation of glycerol
with dimethyl carbonate or urea. Several authors stud-
2.1 Characterization of the catalysts
ied the performance of solid catalysts including ZnO,39
zinc sulfate,40 ZnCl2,41 γ –zirconium phosphate,42
X-ray diffraction (XRD) patterns of the catalysts were
HTc–Zn derived from hydrotalcite,43 Co3O4/ZnO,44
recorded on a RigakuMiniflex diffractometer using
Sm exchanged heteropoly tungstate,45 gold supported
ZSM–5,46 manganese sulfate,46 etc., for the preparation
of GC from glycerol and urea. Although useful catalysts
are reported, it is still a challenging task to develop
new heterogeneous catalysts having a high performance
under milder conditions.
CuKα radiation (1.5406 A◦) at 40 kV and 30 mA. The
measurements were obtained in steps of 0.045◦ with an
account time of 0.5 s and in the 2θ range of 10–80◦.
Confocal micro-Raman spectra were recorded with a
LabRam HR Raman spectrometer (Horiba-JobinYvon)
equipped with BXFM confocal microscope, interfer-
ence and Notch filters and Charge-Coupled Device
detector. The exciting lines at 514.5 nm of a 2018 RM
Ar+–Kr+ laser (Spectra Physics) were focused using
×50 long working distance objective and the diffused
light was dispersed with grating of 1800 lines−1.mm−1
leading to spectral resolution of ca 0.5 cm−1. In situ
Raman spectra were recorded between RT and 873 K
under air using THMS600 cell (Linkam). The homo-
geneity of the catalysts at the micrometer scale was
controlled achieving spectra on different points.
Temperature programmed desorption (TPD) of
ammonia was carried out on a laboratory-built appara-
tus equipped with a gas chromatograph using a TCD
detector. In typical experiments about 0.1 g of the oven
dried sample was taken in a quartz tube. Prior to TPD
studies, the catalyst sample was treated at 573 K for 1
h by passing pure He gas (99.995%, 50 mL/min). After
pre-treatment, the sample was saturated with anhydrous
ammonia (10% NH3 balance He) at 373 K with a flow
rate of 50 mL/min for 1 h and was subsequently flushed
with He 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 from ambient temper-
Tin oxide (SnO2) is one of the most attractive func-
tional materials because of potential applications par-
ticularly as a catalyst and also as a carrier for supported
catalysts. Furuta et al.47 compared the activities of sul-
fated zirconia and sulfated tin oxide for etherification
reaction of methanol and reported that sulfated tin oxide
is active due its high acidity. However, in order to pro-
mote the thermal stability and catalytic property of the
pure tin oxide, using high valence cation of oxide was
previously reported.48,49 Thus, it is of interest to study
the novelties of WO3 supported SnO2 catalysts for the
conversion of glycerol.
In the present study, a series of WO3 supported on
SnO2 catalysts were prepared and studied for selective
etherification of glycerol with tert-butanol and synthe-
sis of glycerol carbonate from glycerol and urea. Under-
standing the surface and structural properties of the
catalysts towards glycerol etherification and optimiza-
tion of reaction parameters was also the aim of the
present study.
2. Experimental
All the chemicals used in this study were of analytical ature to 973 K at a heating rate of 10 K/min. The amount
grade. Glycerol (99%) was obtained from Fisher Sci- of NH3 evolved was calculated from the peak area of
entific. tert-butanol (99%), ammonium meta-tungstate the already calibrated TCD signal.
(99.5%) and tin oxide (99.9%) were obtained from
X-ray photo electron spectroscopy (XPS) measure-
Sigma-Aldrich. A series of WO3/SnO2 catalysts were ments were conducted on a KRATOS AXIS 165 with
prepared by impregnation method. In a typical method a DUAL anode (Mg and Al) apparatus using Mg Kα
required amount of ammonium meta-tungstate hydrate anode. The non-monochromatized AlKα X-ray source
[(NH4)6H2W12O40. nH2O] was dissolved in distilled (hν = 1486.6 eV) was operated at 12.5 kV and 16 mA.
water and this solution was added to calculated amount Before acquisition of the data, each sample was out-
of tin oxide with stirring. The solution was stirred for gassed for about 3 h at 373 K under vacuum of 1.0
a period of 2 h and the excess water was removed on a × 10−7 Torr to minimize surface contamination. The
hot plate. The dried sample was kept overnight for fur- XPS instrument was calibrated using Au as standard.
ther drying at 393 K. The final catalyst was obtained by For energy calibration, the carbon 1s photoelectron line
calcination at 773 K for 4 h. The amount of WO3 on was used. The carbon 1s binding energy was taken as
SnO2 is varied from 2 to 10 wt. %. The catalysts were 285 eV. Charge neutralization of 2 eV was used to

Catalytic conversion of glycerol to ethers
899
Scheme 1. Etherification of glycerol with tert-butanol.
balance the charge up of the sample. The spectra were in a 25 mL two neck round-bottom (RB) flask under
decomposed into individual components using Sun reduced pressures. In a typical experiment glycerol (2
Solaris Vision-2 curve resolver. The location and the g), urea (1.306 g) and catalysts (0.2 g) were taken in
full width at half maximum (FWHM) value for the the round bottom flask and heated in an oil bath at
species were first determined using the spectrum of a 413 K with constant stirring. One neck of the RB flask
pure sample. Symmetric Gaussian functions were used was connected to vacuum line. Reaction was run under
in all cases. Binding energies for identical samples a reduced pressure in order to remove the ammonia
were, in general, reproducible within 0.1 eV.
formed during the reaction. After completion of reac-
tion or stipulated time, methanol was added and the
catalyst was separated by filtration. The products were
identified by GC–MS (Shimadzu, GCMS-QP2010S)
analysis. The products were analyzed with a Shimadzu,
2010 gas chromatograph equipped with INNO-WAX
capillary column (diameter: 0.25 mm, length 30 m) and
flame ionization detector.
2.2 Reaction procedures
2.2a Etherification of glycerol with tert-butanol:
Etherification of glycerol with tert-butanol (scheme 1)
was carried out in a 100 mL haste alloy PARR auto-
clave. Required quantities of glycerol, tert-butanol and
catalyst were introduced in to the autoclave and purged
three times with N2. After the third purge, the reactor
was heated to desired reaction temperature. During
the course of reaction, pressure was developed autoge-
nously. After the reaction, the gas phase products were
collected in a gasbag and the liquid phase products were
separated from the catalyst by filtration. The liquid sam-
ples were analyzed by using a gas chromatograph (Shi-
madzu, 2010) equipped with flame ionization detector
(FID) using INNO-WAX capillary column (diameter:
0.25 mm, length 30 m). The products were confirmed
by GC–MS (Shimadzu, GCMS-QP2010) analysis. The
gas products were analyzed by using a gas chromato-
graph (Porapak Q column) equipped with a thermal
conductivity detector (TCD). The products identified
during etherification of glycerol were mono tert-butyl
glycerol ethers (ME), di tert-butyl glycerol ethers (DE),
and tri tert-butyl glycerol ethers (TE).
Conversion of the glycerol and selectivity was calcu-
lated on the basis of the following equation with respect
to glycerol:
Number of moles of glycerol reacted
Conversion (%) =
Selectivity (%) =
×100
Total number of moles of glycerol taken
Number of moles desired product
×100
Total number of moles of all products
3. Results and Discussion
3.1 X-ray diffraction of catalyst
XRD patterns of the catalysts are shown in figure 1. The
patterns of the SnO2 support and WO3 were included
for the sake of comparison. The XRD pattern were
quite consistent with the tetragonal cassiterite (rutile)
(JCPDS No. 41-1445) structure of SnO2,38 and no char-
2.2b Carbonylation of glycerol with urea: Carbony- acteristic diffraction peaks of the impurities and other
lation of glycerol with urea (scheme 2) was performed compounds were found with the lower loadings of
Scheme 2. Reaction scheme for the carbonylation of glycerol with urea.

900
M Srinivas et al.
(a)
873 K
773
K
723 K
673 K
573 K
473 K
RT
200
400
600
800
-1
1000
1200
Raman Shift (cm
)
(b)
873 K
823 K
773 K
748 K
723 K
698 K
673
573
K
K
473 K
RT
200
400
600
800
1000
1200
-1
Raman Shift (cm
)
Figure 1. XRD patterns of x% WO3/SnO2 catalysts (a)
2WS (b) 5WS, (c) 7.5WS, (d) 10WS and (e) WO3.
(c)
873 K
773 K
WO3, this implies that the WO3 (≤5 wt.%) might exist
as an amorphous oxide on the SnO2, and is considered
to act as a barrier against development of grain bound-
aries of SnO2 and effectively prevent the grain growth.50
When the composition of the WO3 in the catalyst was
higher than 5 wt.%, the diffraction peaks of monoclinic
WO3 crystals were observed in the XRD patterns.51,52
The crystallization of WO3 was accompanied by a loss
of specific surface area.
723 K
673 K
573 K
473 K
RT
3.2 In situ Raman spectra
200
400
600
800
1000
1200
In situ Raman spectra were achieved during calcina-
tion up to 873 K under air of freshly prepared 2, 5
and 7.5WS catalysts. Raman spectra of these catalysts
were measured with stepwise increase in temperature.
The most relevant spectra of 2, 5 and 7.5WS catalysts
are shown in figures 2(a), 2(b) and 2(c) respectively.
-1
Raman Shift (cm
)
Figure 2. In situ Raman spectra of x% WS catalysts (a)
2WS, (b) 5WS and (c) 7.5WS

Catalytic conversion of glycerol to ethers
901
The 2WS catalyst showed predominantly the bands
(e)
at 475, 629 and 768 cm−1 related to SnO2 cassi-
terite crystalline phase. The band at 685 cm−1 was
assigned to an A2u (LO) mode that becomes weakly
Raman-active because of structural distortions induced
by disorder.53,54 Additionally, the broad bands at 850
and 985 cm−1 were respectively attributed to stretch-
ing vibrations of W-O-W bridging bonds (ν(W-O-W))
and to stretching vibrations of terminal W=O bonds
(ν(W=O)) of hydrated polymeric by analogy with the
spectra of such species supported over other oxides.55,56
(d)
(c)
(b)
(a)
The bands near 270, 715 and 805 cm−1 related to crys-
57,58
talline α-WO3
were absent even after the catalyst
373
473
573
673
773
873
973
exposed to 873K. This suggests that the WO3 was well
dispersed on SnO2. Upon raising the temperature up
to 873 K, the ν(W=O) vibrations were shifted to 998
cm−1 and became thinner because of dehydration55–57
whereas the ν(W-O-W) vibrations around 850 cm−1
remained almost unchanged. The Raman spectra of
5WS catalysts showed similar spectra as that of 2WS
at low calcination temperatures. Above 673 K, a thin
(W=O) band at 999 cm−1 and a broad one at 1017 cm−1
were distinguished and ascribed to isolated monomers
and polymeric species respectively.56,59 Broad bands
near 715 and 805 cm−1 that remained weak from 673 to
773 K were additionally observed and were ascribed to
WO3 nano particles. The micro-Raman spectra obtained
on different areas of the sample revealed good homo-
geneity at the micrometre scale which suggested that
these nano particles were well dispersed on SnO2.
Above 773 K, the bands of crystalline α-WO3 became
intense revealing crystalline growth and the micro-
Raman spectra achieved on different areas showed the
heterogeneities at the micro meter scale. The Raman
spectrum of uncalcined 7.5WS catalyst was differ-
ent from the spectra of 2 and 5WS catalysts. Indeed,
in addition to the bands of SnO2, some bands were
observed at 975, 960 and 935 cm−1 and attributed to
(NH4)6H2W12O40 salt54 that re-precipitated during dry-
ing because of the high concentration of the impreg-
nation solution. Additionally, formation of crystalline
WO3 was evidenced above only at 673 K. This fea-
ture was related to the higher WO3 surface density
at which the monolayer coverage was reached. These
results indicated that the maximal coverage of WO3
clusters on SnO2 was obtained for the 5WS catalyst
after calcination at 773 K which is in agreement with
XRD observations.
Temperature (K)
Figure 3. Temperature programmed desorption of ammo-
nia profiles of WS catalysts (a) SnO2, (b) 2WS, (c) 5WS, (d)
7.5WS, and (e) 10WS
surface, together with total acidity. The TPD profiles
of the catalysts are shown in figure 3. The TPD pro-
file of SnO2 sample was also included for sake of com-
parison. The catalysts with low WO3 (2 WS) content
did not show any appreciable acidity generated by the
presence of molecular tungsten oxide. However, with
further increase in WO3 content, a drastic enhancement
in the acidity was noticed. This catalyst showed NH3
desorption peaks centred between 743 K and 973 K.
The high temperature desorption of NH3 corresponds
to moderate to strong acidic sites of the catalysts orig-
inating due to well-dispersed WO3 clusters. After fur-
ther increasing the WO3 content, the high temperature
desorption peak shifted to low temperature. The cat-
alysts with high WO3 amount showed only one des-
orption peak at high temperature that was associated
to the crystalline WO3 present on SnO2 as revealed by
XRD patterns and Raman spectra. The acidity values
of these catalysts are presented in table 1. As a whole
the TPD results suggests the presence of well-dispersed
WO3 clusters on SnO2 leads to generation of maximum
number moderate to strong acidic sites.
Table 1. Acidity and surface area of the catalysts.
S. No. Reaction time (min) SBET(m2/g) Acidity (mmol/g)
1
SnO2
2WS
5WS
7.5WS
10WS
WO3
11.6
10.5
9.8
8.4
7.1
0.064
0.210
0.271
0.158
0.126
0.135
2.
3.
4.
5.
6.
3.3 TPD of ammonia
Ammonia adsorption–desorption technique permits to
determine the strength of acid sites present on catalyst
6.0

902
M Srinivas et al.
Table 2. Comparison of glycerol conversion and product
3.4 Activity measurements of the catalysts
selectivity with reaction time.
3.4a Etherification of glycerol: The etherification of
glycerol was carried over WO3/SnO2 catalysts and the
results are provided in figure 4. The etherification of
Reaction Time (Min) Conversion (%)
Selectivity (%)
ME DE
TE
glycerol over these catalysts leads to the formation of 30
68.6
88.8
90
94.5 3.8
87.8 7.5
87.1 6.6
85.8 7.7
1.7
4.7
6.3
6.5
60
mainly mono tertiary butyl glycerol ether with selectiv-
ities reaching ca 88%. A small amount of both di and tri
tertiary butyl glycerol ethers are also formed. There was
no much variation in the selectivities of the catalysts
with change in WO3 loading. Additionally, the figure
clearly show that the rate of the reaction increased with
increase in the WO3 loading up to 5 wt.% and thereafter
decreased. The optimum activity obtained for the cata-
lyst with 5% WO3 on SnO2. The catalytic reaction rate
Vt can be calculated by the following equation:
90
120
82.2
Reaction conditions: glycerol (1.84 g), tert-butanol (13.34 g),
catalyst weight (0.5 g), reaction temperature (393 K).
as the reaction prolonged, the selectivity towards di and
tri ethers increased marginally at the expense of mono
ether. Overall the catalyst is highly active and selective
towards ME within short reaction time.
ms
Rate of the reaction, Vt =
t Mcat
The activity of the catalysts can be explained based
on their surface and structural characteristics. The XRD
patterns suggest the presence of amorphous WO3 phase
for the catalysts with low WO3 content. The formation
of WO3 crystallites were observed for the catalysts with
>7.5WO3. The high activity of 5WS catalysts might
be due to the presence of well-dispersed WO3 clusters
on SnO2 surface. This catalyst also showed maximum
acidity with high number of moderate and strong acidic
sites. Similar behaviour is reported for WO3 based cat-
alysts where the presence of amorphous WO3 on ZrO2
resulted in generation of strong acidic sites and thereby
high esterification activity.60 The acidity of the catalysts
decreased with the presence of crystallites of WO3.
The 5WS catalyst showed the presence of more
number of acidic sites (table 1) which are responsi-
ble for high activity figure 5. Ball et al.61 reported that
the reaction between alcohols with urea to form alkyl
Where vt is the catalytic reaction rate, ms is the moles
of the glycerol converted within the reaction time of t,
and Mcat is the mass of a catalyst.
In order to investigate the variation in activity and
selectivity of the 5WS catalyst with reaction time, data
were collected at different reaction times and the results
are shown in table 2. The glycerol conversion increased
with reaction time from 30 to 60 min and thereafter no
much variation was noticed. The conversion of glyc-
erol was high even at 30 min suggesting the high activ-
ity of the catalyst. The overall selectivity to ME is
more. The formation of higher tertiary butyl glycerol
ether (HE) was noticed at longer reaction times. Ini-
tially, the selectivity towards mono ether was high and
Conversion of glycerol
ME
DE
TE
4.0
3.5
3.0
2.5
2.0
1.5
100
80
60
40
20
0
Rate of reaction
Acidity of the catalysts
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0.24
0.22
0.20
0.18
0.16
0.14
0.12
0.10
0.08
0.06
2.5
5.0
7.5
10.0
WO3 content (%)
673
773
Temperature
873
973
Figure 4. Catalytic performance of WS catalysts for the
synthesis of mono tert-butyl glycerol ether. Reaction condi-
tions: glycerol (1.84 g),tert-butanol (13.34 g), catalyst weight
(0.5g), reaction temperature (393 K), reaction time (1 h).
(K)
Figure 5. Comparison for acidity of the catalysts with the
rate of reaction.

Catalytic conversion of glycerol to ethers
carbonates can be improved using an adequate combi-
903
ME
DE
TE
100
80
60
40
20
100
nation of a weak Lewis acid and Lewis base. In the
present case also, moderate Lewis acidic sites are gene-
rated with the impregnation of WO3 over SnO2. The
catalysts which possess most of these sites showed
maximum activity. The high acidity for the catalyst was
generated from Lewis acid sites which can be observed
from TPD of NH3 (figure 3). In order to understand
further how the surface and structural characteristics
are playing a role in glycerol etherification, the most
active catalysts was subjected to calcination at different
temperatures.
80
60
40
20
353
373
393
413
433
Reaction temperature (
)
3.4b Effect of catalyst calcination temperature: The
catalyst 5WS was subjected to different calcination
temperatures in the range of 673 to 973 K and tested for
their etherification activity. The corresponding results
are shown in table 3. The conversion of glycerol
increased with increase in calcination temperature from
673 to 773 K and decreased thereafter. In order to find
the variation in activity with change in calcination tem-
perature, these catalysts were further characterized. The
acidity of the catalysts was measured from TPD of
ammonia. The acidity of the catalysts decreased gradu-
ally after calcination of the catalyst above 773 K. The
acidity of these catalysts and their etherification of glyc-
erol activity were correlated and the relevant plot is
shown in figure 5. The results indicate that there exists a
linear relation between acidity of the catalysts with that
of its glycerol etherification activity. Further, the in situ
Raman analysis supports the activity profiles of the cat-
alysts calcined at different temperatures. The catalysts
when exposed to temperature above 773 K showed the
bands related to crystalline α-WO3 phase. The growth
in the crystallites of WO3 resulted in the heterogeneity
on the catalysts surface at micrometer scale. This might
be a reason for low activity of the catalyst calcined
above 773 K. The conversion and selectivity during
glycerol etherification not only depends on the nature
Figure 6. Comparison of glycerol conversion and product
selectivity with reaction temperature. Reaction conditions:
glycerol (1.84 g), tert-butanol (13.34 g), catalyst weight
(0.5 g), reaction time (1 h).
of the catalyst but also on reaction parameters. The
influence of different reaction parameters such as reac-
tion temperature, catalyst weight and glycerol to tert-
butanol molar ratio were also studied.
3.4c Effect of reaction temperature: The etherifica-
tion of glycerol was carried out at different tempera-
tures ranging from 353 to 433 K and the results are
presented in figure 6. The percentage conversion of
glycerol increased with increase in reaction temperature
and attained an equilibrium at a reaction temperature
of 413 K. The selectivity also varied with increase in
reaction temperature. The formation of di and tri-tert-
butyl glycerol ethers was increased gradually with fur-
ther increase in reaction temperature. This is mainly due
to the attainment of maximum conversion of glycerol.
The successive etherification of ME is taking place at
high reaction temperature, as there was less availability
of glycerol. The optimum reaction temperature is 393K
as at this temperature maximum conversion and high
selectivity towards ME was obtained.
Table 3. Comparison of glycerol conversion and product se-
lectivity with 5WS catalyst calcined at different temperature.
3.4d Effect of glycerol to tert-butanol molar ratio:
Figure 7 shows the effect of glycerol to tert-butanol
molar ratio on the etherification of glycerol. The con-
version of glycerol increased from 53.5 to 84% with the
increase in the molar ratio of glycerol to tert-butanol
from 1:3 and 1:6. Further increase in the molar does not
lead to appreciable increase in glycerol conversion. A
marginal decrease in glycerol conversion was observed
at a very high molar ratio of glycerol to tert-butanol of
1:12. The observed decrease in catalytic activity at high
molar ratio might be due to the saturation of catalyst
Temperature (K) Conversion (%)
Selectivity (%)
ME DE
TE
673
773
873
973
70.1
88.8
81.5
77.1
93.3 4.3
87.8 7.5
92.6 4.3
90.2 6.5
2.3
4.7
2.8
3.3
Reaction conditions: glycerol (1.84 g), tert-butanol (13.34 g),
catalyst weight (0.5 g), reaction temperature (393 K), reac-
tion time (1 h).

904
M Srinivas et al.
Conversion of glycerol
ME
DE
TE
100
80
60
40
20
0
120
100
80
60
40
20
0
1:3
1:12
1:6
1:9
Glycerol to tert-butanol mole ratio
Figure 7. Comparison of glycerol conversion and product selectivity with
mole ratio. Reaction conditions: catalyst weight (0.5 g), reaction temperature
(393 K), reaction time (1 h).
surface with tert-butanol by blocking the acidic centres. 3.4e Effect of catalyst loading: Catalyst loading is
Thus, one can say that there is a competitive adsorption an important parameter that needs to be optimized to
by tert-butanol on the acid sites with glycerol, which increase the glycerol conversion. Figure 8 shows the
reduces the efficiency of the catalyst. A glycerol to tert- effect of catalyst amount on glycerol conversion. From
butanol molar ratio of about 1:6 to 1:9 was found to be the figure, it can be seen that the glycerol conver-
optimum for the selective synthesis of mono ethers with sion increased with increase in catalyst amount and
high glycerol conversion.
reached maximum at a catalysts amount of 21.4%.
Conversion of glycerol
ME
DE
TE
4.0
100
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
80
60
40
20
0
5
10
15
20
25
30
Catalyst weight (%)
Figure 8. Comparison of glycerol conversion and product selectivity with
catalyst loading. Reaction conditions: glycerol (1.84 g), tert-butanol (13.34 g),
reaction temperature (393 K), reaction time (1 h).

Catalytic conversion of glycerol to ethers
905
Thereafter, further increase in catalyst loading no sub- completely converted into ME, the available acid sites
stantial increase in glycerol conversion was observed. favours the secondary etherification of mono tertiary
However, as more catalyst is introduced into the reac- butyl glycerol ethers to HE.
tion mixture, it provides an external contact surface area
that facilitates further etherification of mono-tert-butyl
3.4f Reusability of the catalyst: The catalyst was
used repeatedly for etherification of glycerol to inves-
tigate its reusability. The used catalyst was recovered
from the reaction mixture and washed with methanol,
dried and calcined at 773K for 2 h. This catalyst is
reused for etherification of glycerol to study its perfor-
mance and the results are shown in figure 9. The cata-
lyst was repeatedly used for 4 reaction cycles. The cat-
alyst activity did not change appreciably with cycles
of reuse. The conversion of glycerol and selectivity
towards mono- tert-butyl glycerol ether were same for
the used catalysts as that of the fresh catalyst. XPS
measurements were made on the 5WS catalyst before
and after reaction and relevant spectra are shown in
figure 10. The XPS spectra of Sn 3d showed two peaks
at 486.8 and 495.2 eV corresponding to Sn 3d5/2 and
Sn 3d3/2, respectively. These values correspond to that
of Sn (IV) oxide.52 The photoelectron peaks of W 4f7/2
and W 4f5/2 appeared at 35.9 and 37.9 eV which are
identical to that of W (VI) oxide.52 The XPS results
revealed the existence of both WO3 and SnO2 in their
simple oxide forms and that there was no formation of
any mixed oxides.
glycerol ether. As more ME is produced, it eventually
acted as co-solvent and subsequently, the reaction rate is
being controlled by the diffusion of the reactants to the
active sites, instead of catalyst loading. Once, increas-
ing the dosage of catalyst in the reaction mixture had
no significant effect on the glycerol conversion. How-
ever, with increase in the catalyst amount the selectivity
towards DE and TE have been increased. This is mainly
because there was less availability of glycerol, as it is
Conversion of glycerol
Selectivity
120
100
80
60
40
20
0
100
80
60
40
20
0
1
2
3
4
Number of cycles
3.4g Synthesis of glycerol carbonate: The high
active catalyst of the etherification of glycerol prompted
to study for the carbonylation of glycerol to yield glyc-
erol carbonate. The carbonylation of glycerol with urea
was performed with our previously repeated reaction
Figure 9. Reusability of 5 wt% WO3/SnO2 catalyst for
etherification of glycerol. Reaction conditions: glycerol (1.84
g), tert-butanol (13.34 g), catalyst weight (0.5 g), reaction
temperature (393 K), reaction time (1 h).
Figure 10. XPS spectrum of the elements present in the fresh and used catalysts (a) O (b) Sn and (c) W.

906
M Srinivas et al.
with tert-butanol found in the literature compared with
by-product
glycerol carbonate
results obtained in this work. By analyzing the table,
it was possible to confirm that the results obtained
using 5WS as a catalyst for etherification of glyc-
erol with tert-butanol were quite satisfactory because
the conversion were similar or higher than other cat-
alysts presented in the literature.62,63 The use of cata-
lysts such as acid ion exchange resins (A-15 and A-
35) and sulfated carbon catalysts (SCC-S) showed the
reasonable conversion of glycerol and also showed the
formation of tert-butyl ethers as by-product at higher
temperatures. The best results were obtained with the
pure isobutylene but it is very expensive. Additionally,
these other catalysts had production costs much higher
than present 5WS catalyst prepared from low-cost raw
materials.
The present 5WS catalysts was also compared for its
carbonylation of glycerol to glycerol carbonate activ-
ity with our previous reported tin-tungsten mixed oxide
(SnW21) catalyst.64 SnW21 catalyst showed about
52.1% conversion of glycerol whereas the 5WS catalyst
showed 40.4% under similar conditions. The impor-
tant thing about present 5WS catalyst is that it showed
activity in both carbonylation and etherification of glyc-
erol, whereas SnW21 showed better activity only in
carbonylation reaction.
100
50
40
30
20
10
0
80
60
40
20
0
2.5
5.0
7.5
10.0
WO3 loadings (%)
Figure 11. Catalytic performance of x WS catalysts for the
synthesis of glycerol carbonate. Reaction conditions: glyc-
erol (2 g), urea (1.3 g), catalyst weight (0.2 g), reaction
temperature (413 K), reaction time (4 h).
procedure. The activity profiles for the synthesis of
glycerol carbonate from glycerol and urea are presented
in figure 11. In the glycerol carbonylation with urea,
the main product was glycerol carbonate. The other by-
products are 4-(hydroxymethyl) oxazolidine-2-one and
(2-oxo-1, 3-dioxolan-4-yl) methyl carbamate. In the
present study, only (2-oxo-1, 3-dioxolan-4-yl) methyl
carbamate was observed as by-product. In the synthe-
sis of glycerol carbonate also the 5WS catalysts showed
better activity. About 40.4% of glycerol conversion
with 85.4% selectivity towards glycerol carbonate was
achieved over this catalyst. These results indicate that
the WO3 supported on SnO2 are highly active solid
acid catalysts for the conversion of glycerol into value
added chemicals such as glycerol ethers and glycerol
carbonate.
4. Conclusions
Tungsten oxide supported on tin oxide catalysts are
highly active for the conversion of glycerol into its
ethers and glycerol carbonate. The catalytic activity
depended on the acidity of the catalysts which in turn
is related to the presence of well dispersed amorphous
WO3. The nature of WO3 species depended on the
amount of WO3 and the catalyst calcination tempera-
ture. The catalyst with 5 wt.% WO3 on SnO2 calcined
at 773 K for 4 h exhibited maximum activity. The con-
version and selectivity during the glycerol etherifica-
tion also depended on the reaction temperature, cat-
alyst, concentration and molar ratio of reactants and
these parameters have been optimized.
3.4h Comparison of the catalyst with other reported
catalysts: Table 4 presents etherification of glycerol
Table 4. Comparison of glycerol etherification activity of
present catalysts with other reported catalysts.
S. No Catalysts Time (min) Conversion (%)
Ref
Acknowledgements
MS and GR thank Council of Scientific and Industrial
Research (CSIR), India for the financial support in the
form of a Senior Research Fellowship. We also thank
Ms. Marline Daniel for Raman spectral analysis. NL
thanks CSIR for the award of Raman Research Fellow-
ship to carry part of the present work at IRCELYON,
France.
1
2
3
4
5
6
5WS
SCC-S
A-35
60
84
81.6
86
79
40.4
[Present work]
62
240
480
480
240
240
63
63
A-15
5WSa
SnW21a
[present work]
64
52.1
a These catalysts were used for synthesis of glycerol carbonate

Catalytic conversion of glycerol to ethers
907
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