Incorporation of Zn2+ ions into the secondary structure of heteropoly tungstate: Catalytic efficiency for synthesis of glycerol carbonate from glycerol and urea

Article abstract of DOI:10.1039/c4cy00253a

Zinc exchanged heteropoly tungstate (Zn_xTPA) catalysts were prepared and characterized by FT-IR, X-ray diffraction, Laser Raman spectroscopy, temperature programmed desorption of ammonia and pyridine adsorbed FT-IR spectroscopy. The activity of the catalysts was evaluated for the carbonylation of glycerol using urea as a carbonylating agent. Zn_xTPA catalysts showed high activity for glycerol carbonate synthesis compared to the parent TPA. The activity of Zn_xTPA catalysts depended on the number of Zn²⁺ ions in the secondary structure of heteropoly tungstate. Catalysts with partial exchange of Zn with the protons of TPA (Zn ₁TPA) exhibited high activity towards glycerol carbonate synthesis. Exchange of protons of TPA with Zn²⁺ ions resulted in generation of Lewis acidic sites. The changes in surface and structural properties of Zn ₁TPA catalysts with change in calcination temperature were also evaluated. The catalytic activities of Zn_xTPA catalysts were explained based on the variation in their properties. Reaction conditions such as reaction temperature, catalyst weight and glycerol to urea ratio were also optimized. the Partner Organisations 2014.

Full text of DOI:10.1039/c4cy00253a

Catalysis
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PAPER
2
+
Incorporation of Zn ions into the secondary
structure of heteropoly tungstate: catalytic
efficiency for synthesis of glycerol carbonate
from glycerol and urea
Cite this: Catal. Sci. Technol., 2014,
4, 2969
K. Jagadeeswaraiah, Ch. Ramesh Kumar, P. S. Sai Prasad and N. Lingaiah*
x
Zinc exchanged heteropoly tungstate (Zn TPA) catalysts were prepared and characterized by FT-IR, X-ray
diffraction, Laser Raman spectroscopy, temperature programmed desorption of ammonia and pyridine
adsorbed FT-IR spectroscopy. The activity of the catalysts was evaluated for the carbonylation of glycerol
using urea as a carbonylating agent. Zn
x
TPA catalysts showed high activity for glycerol carbonate synthesis
2
+
compared to the parent TPA. The activity of Zn
x
TPA catalysts depended on the number of Zn ions in the
secondary structure of heteropoly tungstate. Catalysts with partial exchange of Zn with the protons of
Received 26th February 2014,
Accepted 14th April 2014
TPA (Zn TPA) exhibited high activity towards glycerol carbonate synthesis. Exchange of protons of
1
2+
TPA with Zn ions resulted in generation of Lewis acidic sites. The changes in surface and structural
properties of Zn
activities of Zn TPA catalysts were explained based on the variation in their properties. Reaction conditions
such as reaction temperature, catalyst weight and glycerol to urea ratio were also optimized.
1
TPA catalysts with change in calcination temperature were also evaluated. The catalytic
DOI: 10.1039/c4cy00253a
x
www.rsc.org/catalysis
1
6,18–22
sources.
Glycerol carbonate can be prepared i) by
Introduction
1
0
transesterification of glycerol with dialkyl carbonates, ii) by
reaction of glycerol with carbon monoxide and oxygen,
iii) by reaction of glycerol with carbon dioxide, iv) from
1
2,13,19
Glycerol is a major by-product in the production of biodiesel
by the transesterification of oils and fats. High production of
biodiesel in recent times has resulted in a surplus of glycerol
as its traditional requirements in the chemical, food, phar-
1
8
2
3
glycerol and phosgene, and v) by reaction of glycerol with
1
0,14,24–26
urea.
1
–3
maceutical and cosmetics industries is limited. Glycerol is
used as a raw material in several fields like pharmaceutics,
cosmetics, food industry, and explosives production and
The use of carbon monoxide and phosgene as a
carbonylating agent is limited due to the toxicity and han-
1
7,21
dling difficulty in industries.
with CO is the best approach.
Carbonylation of glycerol
The main drawback of
4
–6
18,27
as a green solvent.
The transformation of glycerol to
2
value-added chemicals is one approach to address the surplus
of glycerol. Glycerol can be converted into different chemi-
cals by selective oxidation, hydrogenolysis, dehydration,
this reaction is that the overall yield of glycerol carbonate is
very low.
7
8
9
The use of alkyl carbonates like ethylene carbonate as a
carbonylating agent is also a useful approach. However, glyc-
erol carbonate is obtained along with ethylene glycol. The
1
0,11
12–14
transesterification, esterification,
carbonylation,
and
1
5
etherification. Among the different glycerol derivatives,
glycerol carbonate (4-hydroxymethyl-1,3-dioxolan-2-one) is
one of the most promising chemicals, due to its ideal
physico-chemical properties. Glycerol carbonate has many
applications in the synthesis of polycarbonates, polyure-
1
0
separation of the by-products is difficult. The use of
dimethyl carbonate as a carbonate source requires the use of
1
1
2
0,28
base catalysts such as K
catalysts often lose their activity after first use.
One of the practical routes for carbonylation of glycerol is
2
CO
3
and CaO.
However, these
1
6,17
thanes, surfactants, pharmaceuticals and cosmetics.
2
6
Several routes have been reported for the synthesis of
glycerol carbonate from glycerol using different carbonyl
the use of urea as a carbonate source. The major advantage
of this method over other processes is that urea is readily
available and cheap, and the by-product produced in this
reaction (ammonia) can be utilized for the synthesis of urea.
From the literature it is noted that catalysts with Lewis acid
Catalysis Laboratory, Inorganic & Physical Chemistry Division, CSIR-Indian
Institute of Chemical Technology, Hyderabad-500 007, India.
2
2,29,30
29
31
E-mail: nakkalingaiah@iict.res.in; Fax: +91 40 27160921; Tel: +91 40 27191722
sites such as ZnO,
zinc sulfate, ZnCl2, γ-zirconium
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2
5
14
phosphate, mixed metal oxides, HTc–Zn derived from
cooling the sample to room temperature, the FT-IR spectra of
the pyridine-adsorbed samples were recorded.
1
0
32
hydrotalcite,
Co O /ZnO,
Sm exchanged heteropoly
3
4
2
4
26
26
tungstate, gold supported ZSM-5, manganese sulfate,
X-ray powder diffraction patterns were recorded on a
Rigaku Miniflex diffractometer using Cu Kα radiation
(1.5406° A) at 40 kV and 30 mA and a secondary graphite
monochromator. The measurements were obtained in steps
of 0.045° with count times of 0.5 S, in the 2θ range of 5–80°.
Confocal Micro-Raman spectra were recorded at room
etc. produce high glycerol carbonate yields.
Recently, use of metal exchanged 12-tungstophosphoric
acid as solid acid catalysts has been developed for acid cata-
2
4,33,34
lyzed reactions due to their unique acidic properties.
Exchange of protons with certain metal ions generates Lewis
3
5
2+
−1
acid sites. Since Zn is a Lewis acid and can be used to
exchange the protons of tungstophosphoric acid (TPA) to gen-
erate Lewis acidity, zinc oxide based catalysts have been
temperature in the range of 200–1200 cm using a Horiba
Jobin-Yvon Lab Ram HR spectrometer with a 17 mW internal
He–Ne (helium–neon) laser source of excitation wavelength
of 632.8 nm. The catalyst samples in powder form (about
5–10 mg) were usually loosely spread onto a glass slide below
the confocal microscope for measurements.
2
2,29,30
proven to be useful
Li et al. reported the synthesis of Zn1.2H0.6PW12O40 nano-
tubes, and the catalytic activity was evaluated for the produc-
tion of biodiesel. However, variation in acidity with increase
in Zn content in the secondary structure of tungstophosphoric
acid and its detailed characterization was not studied.
for the carbonylation of glycerol.
3
3
The acidity of the catalysts was measured by temperature
programmed desorption of ammonia (TPD-NH ). In a typical
3
experiment, 0.1 g of catalyst was loaded and pre-treated in
In the present work, a series of zinc exchanged tung-
He gas at 300 °C for 2 h. After pre-treatment the temperature
stophosphoric acid (Zn TPA) catalysts was prepared by
was brought to 100 °C and the adsorption of NH was carried
out by passing a mixture of 10% NH balanced He gas over
3
the catalyst for 1 h. The catalyst surface was flushed with He
gas at the same temperature for 2 h to flush off physisorbed
x
3
2
+
exchanging the protons of TPA with Zn ions. The catalytic
activities of these catalysts were tested for the synthesis of
glycerol carbonate from glycerol and urea. The physico-
chemical properties of the Zn
content and treatment temperature were derived and corre-
lated with the catalytic activity.
x
TPA catalysts with varying Zn
3 3
NH . TPD of NH was carried out with a temperature ramp of
10 °C min and the desorbed ammonia was monitored using
a thermal conductivity detector (TCD) of a gas chromatograph.
−
1
Synthesis of glycerol carbonate
Experimental
Preparation of zinc exchanged TPA catalysts
The reactions were performed in a 25 mL two neck round-
bottom (RB) flask under reduced pressures. In a typical
experiment, glycerol (2 g), urea (1.306 g) and the catalyst
Zn exchanged TPA catalysts were prepared by a titration
method as reported in the literature. The required quantity
of TPA dissolved in a minimum amount of distilled water
2
4
(
0.2 g) were placed in the round bottom flask and the flask
was heated in an oil bath at 140 °C with constant stirring.
One neck of the RB flask was connected to a vacuum line.
The reaction was run under a reduced pressure in order to
remove ammonia formed during the reaction. After comple-
tion of the reaction or stipulated time, methanol was added
and the catalyst was separated by filtration. The products
were analyzed by a gas chromatograph (Shimadzu 2010)
equipped with a flame ionization detector using an inno
wax capillary column (diameter: 0.25 mm, length: 30 m).
The products were also identified by GC-MS (Shimadzu,
GCMS-QP2010S) analysis.
3 2
and to a calculated amount of aqueous Zn(NO ) solution
was added dropwise. The resulting mixture was aged for 1 h
at 80 °C. The excess water was evaporated to dryness on a
water bath. The obtained catalyst mass was oven dried at
1
20 °C overnight and calcined at 300 °C for 2 h. The catalysts
were designed as Zn TPA, where x = 0.5, 1, and 1.5. The
x
Zn
1
TPA catalysts were calcined at different temperatures
ranging from 300 to 600 °C. These catalysts are denoted as
Zn TPA-300 to 600. The number indicates the calcination
1
temperature.
Characterization of catalysts
Results and discussion
Fourier transform infrared (FT-IR) spectra were recorded on
a Bio-rad Excalibur series spectrometer using a KBr disc
method.
Catalyst characterization
The structural and surface characteristics of the Zn TPA
x
catalysts were evaluated by different spectroscopic and non-
spectroscopic techniques such as X-ray diffraction, FT-IR
spectroscopy, Laser Raman spectroscopy, TPD of ammonia
and pyridine adsorbed FT-IR spectroscopy.
The nature of the acid sites (Bronsted and Lewis) of
the catalysts was determined by FT-IR spectroscopy with
chemisorbed pyridine. The pyridine adsorption studies were
carried out in the diffuse reflectance infrared Fourier trans-
form (DRIFT) mode. Prior to the pyridine adsorption the cata-
lysts were degassed under vacuum at 200 °C for 3 h followed
by suspending dry pyridine. Then, the excess pyridine was
removed by heating the sample at 120 °C for 1 h. After
XRD patterns
The XRD patterns of the Zn TPA catalysts along with pure
TPA are shown in Fig. 1. Pure TPA showed characteristic XRD
x
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Fig. 1 XRD patterns of Zn
x
TPA catalysts (a) TPA, (b) Zn0.5TPA,
Fig. 2 FT-IR patterns of Zn
c) Zn TPA, and (d) Zn1.5TPA.
x
TPA catalysts (a) TPA, (b) Zn0.5TPA,
(
c) Zn TPA, and (d) Zn1.5TPA. (#) TPA, (*) ZnTPA.
1
(
1
patterns of Keggin ion at 2θ values of 10.28°, 17.93°, 23.13°,
3
4
2
5.42°, 29.49°, and 37.74°. All the catalysts showed the
characteristic XRD patterns related to the Keggin ion of
heteropoly tungstate. However, in the case of Zn TPA cata-
x
lysts with increase in Zn content from 0.5 to 1.5, the intensi-
ties of the characteristic diffraction peaks related to Keggin
ion decreased and additional peaks were observed. This
x
result suggests that the Zn TPA catalysts consist of the parent
TPA and crystalline salt of Zn TPA. From the literature a shift
x
in XRD patterns was observed when the protons of TPA
3
+
+ 24,36
exchanged with metal ions such as Sm and Cs .
Simi-
larly, in the case of Zn TPA catalysts with increase in Zn con-
x
tent from 0.5 to 1, the intensity of the characteristic peak at
1
0.28° decreased and shifted to lower angles at 8.86°. With
further increase in Zn content, the characteristic peak at
Fig. 3 Laser Raman spectra of spectra of Zn
(b) Zn0.5TPA, (c) Zn TPA, and (d) Zn1.5TPA.
x
TPA catalysts (a) TPA,
1
0.28° completely disappeared, which indicates an expansion
2
4
1
of unit cell volume.
Fourier transform infrared spectroscopy
−
1
at 1015 and 990 cm are asymmetric and symmetric vibra-
tions of WO , respectively. The Raman bands at 933 and
The FT-IR spectra of bulk TPA and Zn exchanged tung-
stophosphoric acid catalysts are shown in Fig. 2. TPA
exhibited four bands in the finger print region 1100–500 cm .
t
−
1
530 are ascribed to asymmetric and symmetric stretching
vibrations of W–O –W and W–O –W, respectively. The band
c
e
The characteristic Keggin ion peaks of TPA observed at 1082,
−
1
−
1
observed at 231 cm is related to W–O–W bending mode
9
88, 890, and 792 cm were attributed to P–Oa (Oa – an
oxygen atom bound to three W atoms and to P), WO (t –
terminal oxygen), W–O –W (O – a corner sharing bridging
oxygen atom), and W–O –W (O – an edge-sharing bridging
−
1
and the band at 215 cm might be due to the coupling
t
3
4
between two bending mode vibrations. Zn exchanged hete-
ropoly tungstate catalysts also showed all the characteristic
bands related to the Keggin ion, confirming the existence of
the primary Keggin structure of heteropoly tungstate.
c
c
e
e
3
7
x
oxygen atom), respectively. All the Zn TPA catalysts showing
the characteristic bands of Keggin heteropoly tungstate indi-
cate that the primary Keggin structure remained unchanged
even after exchanging the protons of TPA with Zn ions.
2
+
Temperature programmed desorption of ammonia
Acidity of the Zn TPA catalysts were measured by TPD of NH
x
3
Laser Raman spectroscopic studies
and the patterns are shown in Fig. 4. The acidity values calcu-
lated based on desorbed ammonia are presented in Table 1.
The patterns showed three desorption peaks in the range of
120–700 °C. The main desorption peaks were observed in the
Fig. 3 shows the Raman spectra of Zn
with bulk TPA. Pure crystalline TPA showed characteristic
bands at 1015, 990, 933, 530, 231, and 215 cm . The bands
x
TPA catalysts along
−
1 38
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Fig. 4 Temperature programmed desorption of ammonia profiles of
x 1
Zn TPA catalysts (a) Zn0.5TPA, (b) Zn TPA, and (c) Zn1.5TPA.
Fig. 5 Pyridine adsorbed FT-IR spectra of Zn
x
TPA catalysts (a) TPA, (b)
Zn0.5TPA, (c) Zn
1
TPA, and (d) Zn1.5TPA.
Table 1 Acid strength distribution of Zn
x
TPA catalysts
−
1
Acidity (mmol g
)
−1
39
1
530 and 1542 cm , characteristic of Bronsted acidic sites.
Catalyst
Weak
Moderate
Strong
Total acidity
−1
Another intense band is located at 1485 cm , which is a
combined band originating from pyridine bonded to both
the Bronsted and Lewis acid sites. In the case of Zn
Zn0.5TPA
0.513
1.111
0.071
—
—
—
0.032
0.155
0.927
0.782
0.184
0.032
1.866
1.411
0.563
1.305
0.005
0.083
2.411
2.677
1.561
2.087
0.189
0.115
1
Zn TPA
33
Zn1.5TPA
exchanged TPA catalysts, the band related to the Lewis acidic
Zn
1
Zn
1
Zn
1
TPA-400
TPA-500
TPA-600
−
1
site is clearly seen at 1448 cm and its intensity increased
with increase in Zn content in the sample. The intensity of
the band related to Bronsted acidic sites decreased with
increase in Zn content indicating the generation of Lewis
acidic sites in Zn TPA catalysts. The results of TPD of NH
x
3
range of 120–300 °C and 300–450 °C corresponding to weak
and moderate acidic sites, respectively. Another strong
desorption peak observed in the range of 450–700 °C was
related to strong acidic sites. With increase in exchangeable
Zn ions from 0.5 to 1, the desorption peak related to strong
acidic sites shifted to lower temperatures and the intensity of
the desorption peak corresponding to weak acidic sites
increased. Further increase in Zn content to 1.5 shows that
the intensity of the desorption peak related to strong acidic
sites decreased and at the same time the intensity of the
desorption peak related to moderate acidic sites also
increased. This suggested that the number of moderate
acidic sites, which are probably Lewis acidic, increased with
Zn content (Table 1). Compared to other catalysts partially
also support the presence of Lewis acidic sites as Zn
x
TPA
(
x = 0.5–1.5) catalysts exhibited a strong desorption peak
related to moderate acidic sites.
Activity measurements
Preparation of glycerol carbonate from urea and glycerol.
The activity of zinc exchanged tungstophosphoric acid
catalysts was evaluated for the synthesis of glycerol carbonate
and the results are presented in Table 2. The catalytic activity
of ZnO was also studied for comparison. When reaction was
1
exchanged Zn TPA catalysts exhibited high acidity.
Table 2 Glycerol conversion and glycerol carbonate selectivity over
x
Zn TPA catalysts
FT-IR pyridine adsorption studies
Selectivity (%)
Conversion of
glycerol (%)
a
Pyridine adsorbed FT-IR spectroscopy can be used to investi-
gate the presence of Bronsted and Lewis acidic sites in a cata-
lyst. Pyridine adsorbed FT-IR spectra of Zn TPA catalysts are
shown in Fig. 5. The pyridine-adsorbed FT-IR spectra showed
various features in the region of 1400–1600 cm due to the
stretching vibrations of M–N (metal–nitrogen) and N–H
Catalyst
Glycerol carbonate Others
TON
TPA
Zn0.5TPA
40.4
60.4
69.2
58.1
34.0
75.7
100
99.4
98.6
91.2
19.3
24.3
—
0.6
1.4
8.8
80.7
126
191
221
188
31
x
1
Zn TPA
−
1
Zn1.5TPA
ZnO
Without catalyst 10.5
—
(
pyridinium ion). The band assigned to the pyridinium ion is
a
−
1
−1
5-(Hydroxymethyl)oxazolidin-2one, (2-oxo-1,3-dioxolan-4-yl)methylcarbamate,
and glycedol. Reaction conditions: glycerol (2 g), urea (1.306 g),
catalyst weight (0.2 g), reaction temperature (140 °C), and reaction
time (4 h).
recorded at 1536 cm and the other one at 1449 cm associ-
ated with the pyridine adsorbed onto Lewis-type acid sites.
The FT-IR spectra of TPA shows typically intense bands at
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carried out in the absence of a catalyst, conversion of glycerol
was only 10.5% and selectivity towards glycerol carbonate
was 19.3%. However, in the presence of ZnO conversion of
glycerol is improved to 34% with 91.2% selectivity towards
glycerol carbonate. However, the TPA showed about 40%
glycerol conversion with 75.7% selectivity towards glycerol
carbonate. The exchange of Zn with the protons of TPA
resulted in a substantial enhancement in activity. The
presence of Zn led to an increase in glycerol conversion from
4
0.4% to 60.4%. Further increase in Zn content from 0.5 to 1
shows that the conversion of glycerol increased from 60.4%
to 69.2% and there is no appreciable change in selectivity
towards glycerol carbonate. Further increase in Zn content to
1
.5 shows that the conversion of glycerol decreased to 58.1%.
Among all catalysts, Zn TPA showed high activity compared
to its counter parts. In order to know the efficiency of
1
Zn TPA catalysts, the turnover number (TON) was calculated
x
Fig. 6 Effect of Zn TPA catalyst calcination temperature on glycerol
1
and the corresponding values are presented in Table 2. The
TON and selectivity towards glycerol carbonate depend upon
conversion.
2
+
the presence of Zn ions in the secondary structure. The
TON of pure TPA was low compared to Zn TPA catalysts.
With the increase in Zn content in TPA, the TON values
x
2
+
increased and maximum value was obtained for Zn
catalysts.
1
TPA
The variation in the catalytic activity of Zn TPA catalysts
x
can be explained based on the acidic properties of the cata-
2
+
lysts. Activity of Zn
x
TPA catalysts depended on the Zn
exchanged with the protons of TPA. Partial exchange of TPA
2
+
protons with Zn results in generation of Lewis acidic sites
along with Bronsted acidic sites. The high activity is mainly
related to the presence of Lewis acidic sites originated from
the presence of Zn in the secondary structure of TPA. The
1
partial exchanged catalyst Zn TPA exhibited high activity as it
contains more number of acidic sites.
Effect of catalyst calcination temperature on glycerol
1
conversion. Zn TPA catalysts were calcined at different
1
Fig. 7 XRD patterns of Zn TPA catalysts calcined at (a) 300 °C, (b) 400 °C,
temperatures in the range of 300–600 °C and studied for
their activity for glycerol carbonate synthesis. The activity of
the catalysts are presented in Fig. 6. From the figure it is
noticed that with the increase in calcination temperature
from 300 to 600 °C, conversion of the glycerol decreased
from 69.2 to 33.9%. Selectivity towards glycerol carbonate
also followed the same trend. In order to know the variation
in the catalytic activity and selectivity, these catalysts were
further characterized by different spectroscopic techniques.
(c) 500 °C, and (d) 600 °C.
in calcination temperature to 600 °C shows that intense
peaks related to crystalline WO
indicates complete decomposition of Keggin ion at high
calcination temperature. From the XRD results it can be
x
concluded that the Keggin structure of Zn TPA is thermally
4
0
3
were noticed.
This
Characterization of Zn TPA catalysts calcined at different
stable below 400 °C.
1
temperatures. The XRD patterns of Zn
1
TPA calcined at
Stability of the Keggin ion of Zn TPA with increase in calci-
1
various temperatures are shown in Fig. 7. The catalyst
calcined at 300 °C showed the characteristic peaks of Keggin
ion as described earlier. In the case of an increase in calcination
temperature to 400 °C, apart from the characteristic Keggin
patterns, additional peaks related to WO3 were also
nation temperature was further confirmed by Laser Raman
spectroscopy. Fig. 8 shows the Laser Raman spectra of Zn TPA
1
catalysts calcined at various temperatures. Catalysts calcined
at lower temperatures (300 and 400 °C) showed characteristic
−
1
bands related to Keggin ion at 990 and 1011 cm correspond-
4
0
41
observed. This is due to partial decomposition of the
Zn TPA catalysts. In the case of the catalyst calcined at
00 °C, no patterns related to Keggin ion was observed and
patterns related to only WO were observed. Further increase
ing to asymmetric and symmetric vibrations of WOt.
1
However, in the catalysts calcined at higher temperatures
5
(500 and 600 °C), bands related to Keggin ion were absent.
−
1
3
New bands were observed at 824, 720, and 265 cm which
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WO . From these studies it can be noted that catalysts with a
3
Keggin structure possess more number of acidic sites.
The high activity of the catalyst calcined at 300 °C is
mainly due to the presence of high acidity associated with
the intact Keggin ion of Zn TPA. The catalyst calcined at high
1
temperature showed low activity as their acidity decreased
due to the decomposition of the Keggin structure with
2
+
temperature. The exchange of Zn with the protons of TPA
led to increased acidity with the generation of Lewis acidic
sites. Calcination of the catalysts at different temperatures
results in variation of acidity with change in the structural
characteristics.
Optimization of reaction parameters
1
Fig. 8 Laser Raman spectra of Zn TPA catalysts calcined at (a) 300 °C,
Influence of the reaction time. Fig. 10 shows the time on
stream analysis of glycerol carbonate synthesis over Zn TPA
1
(
b) 400 °C, (c) 500 °C, and (d) 600 °C.
catalysts at a reaction temperature of 140 °C. Conversion of
4
0
glycerol carbonate increased continuously with time up to
are characteristic bands of tungsten oxide. These results
suggest that the Keggin ion of Zn TPA is decomposed to its
6
h. Initially conversion of glycerol was only 38.7% at 1 h of
1
reaction time and increased to 73.2% after 6 h of reaction
time. However, glycerol carbonate selectivity was achieved at
a maximum reaction time of 4 h and decreased slightly at a
longer reaction time.
constituent metal oxide at higher temperatures. These results
are in agreement with the results obtained from XRD
analysis.
1
The patterns of TPD of ammonia of the Zn TPA catalysts
Effect of reaction temperature. The effect of reaction
temperature on conversion of glycerol and the selectivity to
glycerol carbonate are shown in Fig. 11. Conversion of
glycerol was very low (10%) when reaction was carried out
below 120 °C. However, a drastic increase in the conversion
of glycerol up to 69.2% was observed when reaction
temperature increased to 140 °C. Selectivity towards glycerol
carbonate also increased from 93.3 to 99.4% with increase in
reaction temperature from 100 to 140 °C. Further increase in
reaction temperature to 160 °C shows that there is no
appreciable increase in glycerol conversion; conversely the
selectivity towards glycerol carbonate was decreased.
calcined at various temperatures are shown in Fig. 9. The
catalyst calcined at 300 °C showed three unresolved peaks
with high intensity. This indicates the presence of maximum
acidity. However, the catalyst calcined at 400 °C showed two
desorption peaks related to weak and strong acidic sites. The
desorption peak related to moderate acidic sites was not
observed for the catalyst calcined at 400 °C and exhibited low
acidity compared to the catalyst calcined at 300 °C. This is
due to partial degradation of Keggin units of Zn
1
TPA at
4
00 °C. Further increase in catalyst calcination temperature
up to 600 °C shows that there was no distinct ammonia
desorption peak. This might be due to the complete decom-
position of Zn TPA catalysts into its constituent metal oxide
1
Fig. 9 Temperature programmed desorption of ammonia patterns of
Fig. 10 Effect of reaction time on glycerol conversion and glycerol
carbonate selectivity. Reaction conditions: glycerol (2 g), urea (1.306 g),
catalyst weight (0.2 g), and reaction temperature (140 °C).
Zn
1
TPA catalysts calcined at (a) 300 °C, (b) 400 °C, (c) 500 °C, and
(
d) 600 °C.
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Fig. 11 Effect of reaction temperature on glycerol conversion and
glycerol carbonate selectivity. Reaction conditions: glycerol (2 g), urea
Fig. 13 Effect of catalyst weight on glycerol conversion and glycerol
carbonate selectivity. Reaction conditions: glycerol (2 g), urea (1.306 g),
reaction temperature (140 °C), and reaction time (4 h).
(
1.306 g), catalyst weight (0.2 g), and reaction time (4 h).
Effect of glycerol to urea molar ratio. Glycerol to urea
give other by-products like (2-oxo-1,3-dioxolan-4-yl)methyl-
molar ratio is an important parameter to obtain high glycerol
conversion and selectivity towards glycerol carbonate. Fig. 12
shows the influence of glycerol to urea molar ratio. As shown
in the figure, the conversion of glycerol increased from 48.6
to 69.2% with decrease in glycerol to urea molar ratio from
2
2
carbamate. These results suggest that at moderate concen-
tration of glycerol, the major product is glycerol carbonate.
Effect of catalyst weight. Fig. 13 shows the effect of
catalyst weight on the glycerol conversion and glycerol
carbonate selectivity. Increase in catalyst loading from 3 to
2
: 1 to 1 : 1. The selectivity towards glycerol carbonate also
5
% conversion of glycerol increased from 56.6 to 69.2% and
increased up to 99.4%. Further decrease in glycerol to urea
molar ratio shows that there was no marginal increase in the
glycerol conversion and the selectivity towards glycerol
carbonate was decreased. The increase in selectivity towards
the by-product can be explained as follows: i) formation of
glycedol due to decarbonylation of glycerol, ii) at higher
concentration of urea, glycerol carbonate reacts with urea to
selectivity increased up to near 100%. Further increase in
catalyst loading from 5 to 10% results in only a slight increase
in glycerol conversion, and selectivity towards glycerol
carbonate was decreased to 91.4% and formation of glycedol
was observed. At high concentration of catalysts decarbonylation
of glycerol carbonate favors forming glycedol. These results
suggest that the optimum loading of the catalyst is 5% to
Fig. 12 Influences of glycerol and urea molar ratio on glycerol
conversion and glycerol carbonate selectivity. Reaction conditions:
catalyst weight (0.2 g), reaction temperature (140 °C), and reaction
time (4 h).
1
Fig. 14 Reusability of Zn TPA catalysts for carbonylation of glycerol.
Reaction conditions: glycerol (2 g), urea (1.306 g), catalyst weight
(0.2 g), reaction temperature (140 °C), and reaction time (4 h).
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Table 3 Comparison of catalytic activity of the Zn TPA catalysts with other reported solid acid catalysts
Catalyst
Zn TPA
Sm0.66TPA
SW21
Sn–β zeolite
Zr–P
Reaction temperature (°C)
Reaction time (h)
Glycerol conversion (%)
Glycerol carbonate yield (%)
Ref.
1
140
140
140
145
140
150
150
4
4
4
5
3
4
4
69.2
49.5
52.1
70
80
80
68.8
42.3
49.7
26
76
39
Present work
24
14
10
25
26
26
2
Au/Fe O
3
2.5% Au/Nb
O
2 5
66
21
obtain high conversion of glycerol with high selectivity towards
glycerol carbonate.
Reusability of catalysts. Recycling experiments were
carried out to investigate the stability and reusability of the
resulted in generation of Lewis acidic sites along with
Bronsted acidic sites. The acidity of the catalysts was related
not only to the amount of Zn in TPA but also to its stability.
High temperature calcination destabilized the Keggin ion of
Zn
1
TPA catalysts. In each cycle, the used catalyst was
x 1
Zn TPA. Catalytic activity of Zn TPA also depends on reaction
separated by filtration, washed with methanol to remove the
products adhering to the surface of the catalyst, dried in an
oven at 100 °C and reused for the next run. Fig. 14 shows the
temperature, catalyst loading and glycerol to urea molar
ratio. The catalyst is reusable without any pre-treatment.
reusability results of the Zn TPA catalysts for the synthesis of
1
Acknowledgements
glycerol carbonate. Fresh catalysts exhibited 69.2% of glycerol
conversion and 99.4% selectivity for glycerol carbonate. The
selectivity towards glycerol carbonate was almost constant in
each recycle. However, a marginal decrease in glycerol
conversion was observed after the third recycle of the
The authors thank the Council of Scientific Industrial
Research (CSIR), India, for the financial support in the form
of the Indus magic (CSC-0123) project under the 12th Five
Year Program.
catalyst. These results indicate the efficiency of the Zn TPA
1
catalysts. Further studies are required to know the reasons
for marginal decrease in activity.
References
Comparison of Zn TPA catalysts with reported solid acid
catalysts. Table 3 shows the comparison of catalytic activity
1 M. Pagliaro, R. Ciriminna, H. Kimura, M. Rossi and
C. D. Pina, Angew. Chem., Int. Ed., 2007, 46, 4434–4440.
2 C. H. Zhou, J. N. Beltramini, Y. X. Fan and G. Q. Lu, Chem.
Soc. Rev., 2008, 37, 527–549.
1
of the Zn
1
TPA catalysts with other reported solid acid
catalysts. Gold supported on acidic supports such as Fe O
2
3
and Nb
temperature (150 °C).
2
O
5
exhibited high conversion at a high reaction
3 A. Behr, J. Eilting, K. Irawadi, J. Leschinski and F. Lindner,
Green Chem., 2008, 10, 13–30.
2
6
However, the desired glycerol
carbonate yield was very low. Sn-β zeolite exhibited 70% of
glycerol conversion. The major drawback of this catalyst took
a long reaction time (5 h) for reasonable glycerol conversion.
Among all the different solid acid catalysts zirconium phos-
phate exhibited 80% of glycerol conversion with 100% selec-
4 D. T. Johnson and K. A. Taconi, Environ. Prog., 2007, 26,
338–348.
5 A. E. D. Alvarez, J. Francos, B. L. Barreira, P. Crochet and
V. Cadierno, Chem. Commun., 2011, 47, 6208–6227.
6 Y. Gu and F. Jerome, Green Chem., 2010, 12, 1127–1138.
7 A. Tsuji, K. T. V. Rao, S. Nishimura, A. Takagaki and
K. Ebitani, ChemSusChem, 2011, 4, 542–548.
2
5
tivity towards glycerol carbonate. The high activity of this
catalyst is due to the presence of both acidic and basic sites.
The present catalyst Zn
sion than the previously reported catalysts such as tin–tungsten
1
TPA exhibited higher glycerol conver-
8 M. Balaraju, V. Rekha, B. L. A. Prabhavathi Devi, R. B. N. Prasad,
P. S. S. Prasad and N. Lingaiah, Appl. Catal., A, 2010, 384,
107–114.
9 B. Katryniok, S. Paul, V. B. Baca, P. Reye and F. Dumeignil,
Green Chem., 2010, 12, 2079–2098.
1
4,24
mixed oxide and Sm exchanged TPA catalysts.
lysts exhibited about 69% glycerol conversion with near
00% selectivity towards glycerol carbonate. The present cata-
Zn TPA cata-
1
1
lyst is reusable and exhibited high activity compared to other
solid acid catalysts.
10 M. J. Climent, A. Corma, P. D. Frutos, S. Iborra, M. Noy,
A. Velty and P. Concepcion, J. Catal., 2010, 269, 140–149.
1
1 J. R. O. Gomez, O. G. J. Aberasturi, B. M. Madurga,
A. P. Rodriguez, C. R. Lopez, L. L. Ibarreta, J. T. Soria and
M. C. V. Velasco, Appl. Catal., A, 2009, 366, 315–324.
Conclusions
Zinc exchanged heteropoly tungstate catalysts were prepared
12 J. Hu, J. Li, Y. Gu, Z. Guan, W. Mo, Y. Ni, T. Li and G. Li,
Appl. Catal., A, 2010, 386, 188–193.
with retention of the Keggin ion structure. Zn TPA catalysts
x
are efficient catalysts for the synthesis of glycerol carbonate
13 T. Mizuno, T. Nakai and M. Mihara, Heteroat. Chem.,
2010, 21, 99–102.
from glycerol and urea. The Zn
x
TPA catalyst activity depends
upon the exchangeable Zn content which directs the overall
acidity of the catalysts. Exchange of protons of TPA with Zn
14 K. Jagadeeswaraiah, C. R. Kumar, P. S. S. Prasad, S. Loridant
and N. Lingaiah, Appl. Catal., A, 2014, 469, 165–172.
2
+
2976 | Catal. Sci. Technol., 2014, 4, 2969–2977
This journal is © The Royal Society of Chemistry 2014

View Article Online
Catalysis Science & Technology
Paper
1
1
1
5 C. G. Sancho, R. M. Tost, J. M. M. Robles, J. S. Gonzalez,
27 C. Vieville, J. W. Yoo, S. Pelet and Z. Mouloungui, Catal.
Lett., 1998, 56, 245–247.
28 G. Rokicki, P. Rakoczy, P. Parzuchowski and M. Sobiecki,
Green Chem., 2005, 7, 529–539.
29 J. W. Yoo and Z. Mouloungui, Stud. Surf. Sci. Catal.,
2003, 146, 757–760.
30 T. Okutsu, JP Pat. 039347, 2007.
A. J. Lopez and P. M. Torres, Catal. Today, 2011, 167,
8
4–90.
6 M. O. Sonnati, S. Amigoni, E. P. T. de Givenchy,
T. Darmanin, O. Choulet and F. Guittard, Green Chem.,
2
013, 15, 283–306.
7 J. Rousseau, C. Rousseau, B. Lynikaite, A. Sakcus,
C. de Leon, P. Rollin and A. Tatibouet, Tetrahedron,
31 J. H. Park, J. S. Choi, S. K. Woo, S. D. Lee, M. Cheong,
H. S. Kim and H. Lee, Appl. Catal., A, 2012, 433–434, 35–40.
32 F. R. Marcos, V. C. Casilda, M. A. Banares and
J. F. Fernandez, J. Catal., 2010, 275, 288–293.
33 J. Li, X. Wang, W. Zhu and F. Cao, ChemSusChem, 2009, 2,
177–183.
2
009, 65, 8571–8581.
1
1
2
2
8 M. Aresta, A. Dibenedetto, F. Nocito and C. Pastore, J. Mol.
Catal. A: Chem., 2006, 257, 149–153.
9 T. Mizuno, T. Nakai and M. Mihara, Heteroat. Chem.,
2
010, 21, 541–545.
0 F. S. H. Simanjuntak, T. K. Kim, S. D. Lee, B. S. Ahn,
H. S. Kim and H. Lee, Appl. Catal., A, 2011, 401, 220–225.
1 J. B. Li and T. Wang, J. Chem. Thermodyn., 2011, 43,
34 C. R. Kumar, N. Rambabu, N. Lingaiah, P. S. S. Prasad and
A. K. Dalai, Appl. Catal., A, 2014, 471, 1–11.
35 H. Yamamoto, Proc. Jpn. Acad., Ser. B, 2008, 84, 134–146.
36 A. Corma, A. Martinez and C. Martinez, J. Catal., 1996, 164,
422–432.
37 D. Wang, S. Lu, P. J. Kulesza, C. M. Li, R. D. Marco and
S. P. Jiang, Phys. Chem. Chem. Phys., 2011, 13, 4400–4410.
38 G. Busca and J. Raman, Spectroscopy, 2002, 33, 348–358.
39 C. R. Kumar, P. S. S. Prasad and N. Lingaiah, J. Mol. Catal.
A: Chem., 2011, 350, 83–90.
7
31–736.
2
2
2
2 M. Okutsu and T. Kitsuki, WO Pat. 50415, 2000.
3 A. G. Shaikh and S. Sivaram, Chem. Rev., 1996, 96, 951–976.
4 C. R. Kumar, K. Jagadeeswaraiah, P. S. S. Prasad and
N. Lingaiah, ChemCatChem, 2012, 4, 1360–1367.
2
2
5 M. Aresta, A. Dibenedetto, F. Nocito and C. Ferragina,
J. Catal., 2009, 268, 106–114.
6 C. Hammond, J. A. L. Sanchez, M. H. A. Rahim,
N. Dimitratos, R. L. Jenkins, A. F. Carley, Q. He, C. J. Kiely,
D. W. Knight and G. J. Hutchings, Dalton Trans., 2011, 40,
40 G. Liu, X. Wang, X. Wang, H. Han and C. Li, J. Catal.,
2012, 293, 61–66.
41 T. Takashima, R. Nakamura and K. Hshimoto, J. Phys. Chem.
C, 2009, 113, 17247–17253.
3
927–3937.
This journal is © The Royal Society of Chemistry 2014
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