Synthesis of glycerol carbonate from glycerol and urea using zinc-containing solid catalysts: A homogeneous reaction

Article abstract of DOI:10.1016/j.jcat.2012.10.001

Zinc-containing solid catalysts (zinc oxide, smectite, hydrotalcite) and several inorganic zinc salts were used to produce glycerol carbonate from glycerol and urea under solvent-free conditions at 130 °C and at a reduced pressure of 3 kPa. The leaching of Zn species was observed to occur for the solid catalysts, and the carbonate yield was shown to be correlated with the amount of zinc species dissolved into the liquid phase with a single relationship in common for all the catalysts employed. The reaction was also indicated to continue in the liquid phase alone, after the solid catalysts were removed from the reaction mixtures by filtration. The results obtained reveal that the reaction takes place homogeneously in the liquid phase irrespective of the parent solid catalysts used. Possible structure of active Zn species was discussed from the results of reaction runs under different conditions and Fourier transform infrared spectroscopy measurements of the liquid phase after the reaction.

Full text of DOI:10.1016/j.jcat.2012.10.001

Journal of Catalysis 297 (2013) 137–141
Contents lists available at SciVerse ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Synthesis of glycerol carbonate from glycerol and urea using zinc-containing
solid catalysts: A homogeneous reaction
⇑
Shin-ichiro Fujita, Yuki Yamanishi, Masahiko Arai
Division of Chemical Process Engineering, Faculty of Engineering, Hokkaido University, Sapporo 060-8628, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Zinc-containing solid catalysts (zinc oxide, smectite, hydrotalcite) and several inorganic zinc salts were
used to produce glycerol carbonate from glycerol and urea under solvent-free conditions at 130 °C and
at a reduced pressure of 3 kPa. The leaching of Zn species was observed to occur for the solid catalysts,
and the carbonate yield was shown to be correlated with the amount of zinc species dissolved into the
liquid phase with a single relationship in common for all the catalysts employed. The reaction was also
indicated to continue in the liquid phase alone, after the solid catalysts were removed from the reaction
mixtures by filtration. The results obtained reveal that the reaction takes place homogeneously in the
liquid phase irrespective of the parent solid catalysts used. Possible structure of active Zn species was dis-
cussed from the results of reaction runs under different conditions and Fourier transform infrared spec-
troscopy measurements of the liquid phase after the reaction.
Received 12 March 2012
Revised 19 September 2012
Accepted 1 October 2012
Available online 5 November 2012
Keywords:
Glycerol carbonate
Glycerol
Urea
Solid zinc catalyst
Homogeneous reaction
Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction
example, Rubio-Marcos et al. reported a conversion of about 70%
with close 100% selectivity with a Co3O4/ZnO catalyst at 145 °C
The synthesis of glycerol carbonate from glycerol and urea
(Scheme 1) is currently attracting much attention from viewpoint
of sustainable chemistry. This synthetic reaction has several mer-
its: one can (a) utilize and reduce undesired CO2 emitted from var-
ious sources, (b) utilize glycerol formed as a by-product in the
manufacture of biodiesels [1], (c) produce a value added organic
product of glycerol carbonate that can find practical applications
[2,3], and (d) replace a current route for the production of glycerol
carbonate using such a toxic compound as phosgene [4]. Urea can
be produced from CO2 and ammonia [5], and so the synthesis of
glycerol carbonate from glycerol and urea is one of indirect chem-
ical transformations of CO2 into organic compounds. The authors
have investigated direct [6–10] and indirect utilization of CO2 as
a feedstock, including chemical reactions with urea [11,12].
Various base catalysts have been tested for the synthesis of
glycerol carbonate from glycerol and urea in the literature. Such
inorganic salts as ZnSO4 [13,14] and MgSO4 [15,16] are effective
homogeneous catalysts. Compared to these homogeneous ones,
heterogeneous catalysts may be more useful for post-reaction pro-
cedures like easy recovery and reuse of catalysts. Several previous
authors studied the performance of solid catalysts including La2O3
[17], Au nanoparticles [18], Co3O4/ZnO [19], metal oxides (MgO,
CaO) [20] and mixed metal oxides (Al/MgOx, Al/LiOx) prepared
for 4 h [19]. Although those useful catalysts are reported, it is still
a challenging task to develop new heterogeneous catalysts having
a high performance under milder conditions.
In the present work, the authors intended to apply zinc-
containing mesoporous smectite-like materials as heterogeneous
catalysts for the production of glycerol carbonate from glycerol
and urea at a temperature of 130 °C under solvent-less conditions.
Those porous catalytic materials can be prepared by a simple
hydrothermal method [22,23] and show interesting catalytic activ-
ities in direct/indirect CO2 fixation reactions [10,24,25] and others
[26–30]. In addition to those smectite-like catalysts, zinc oxide and
a zinc-containing hydrotalcite have also been tested for compari-
son. It has been observed for these three catalysts that a zinc-
containing species is dissolved into in the liquid phase, and the
product yield is well correlated with the amount of the zinc species
dissolved. Therefore, the reaction should proceed homogeneously
with the same active zinc species present in the liquid phase, con-
trary to our beginning expectation. The formation and structure of
these active zinc species have been studied by making reaction
runs under different conditions and Fourier transform infrared
spectroscopy (FTIR) measurements of the organic liquids after
the reaction runs. It has been observed that the coexistence of
glycerol and urea is required for the leaching of zinc species from
the parent solid catalysts into the liquid phase. The same zinc
complex coordinated with N@C@O is likely to form irrespective
of the parent solid catalysts used and active for the homogeneous
transformation of glycerol and urea to glycerol carbonate.
from hydrotalcites [20], and
c-zirconium phosphate [21]. For
⇑
Corresponding author. Fax: +81 11 706 6594.
E-mail address: marai@eng.hokudai.ac.jp (M. Arai).
0021-9517/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jcat.2012.10.001

138
S.-i. Fujita et al. / Journal of Catalysis 297 (2013) 137–141
the reaction was also examined by FTIR (JASCO FTIR-620). It was
separated from the catalysts by filtration and then subjected to
FTIR measurement under ambient conditions.
O
OH
OH
OH
O
O
+
O
+
2NH3
H2N NH2
HOH2C
3. Results and discussion
Scheme 1. Synthesis of glycerol carbonate from glycerol and urea.
The three different solid catalysts of ZnO, SM(Zn), and HT(Zn)
used in the present work indicated Brunauer–Emmett–Teller
(BET) surface areas of 3.5, 248, and 45.7 m2 gÀ1. Before the detailed
results with the three solid catalysts are presented, the activities of
several heterogeneous and homogeneous catalysts are compared
(Table 1). The selectivity to glycerol carbonate was <100%, mainly
due to the oligomerization of the product. It was found that ZnCl2,
ZnBr2, and ZnSO4 gave conversion values >80% and selectivity lev-
els >90% (entries 4–6), but (CH3COO)2ZnÁ2H2O was less active and
selective (entry 7). After the reaction with ZnBr2, the reaction solu-
tion was colored, indicating the decomposition of the catalyst and
the formation of Br2. Compared with zinc halides and sulfate, ZnO
and SM(Zn) showed smaller activity and selectivity values (entries
1, 2); the activity of HT(Zn) was good, but the selectivity was not so
high (entry 3). However, Table 1 indicates that the differences in
the performance among those heterogeneous and homogeneous
2. Experimental
2.1. Catalyst samples and characterizations
Several catalysts were used in the present work. ZnCl2, ZnBr2,
ZnSO4, and (CH3COO)2ZnÁ2H2O were purchased from Wako Pure
Chemical Industries. ZnO purchased from Kanto Chemical Com-
pany was also used as a catalyst.
A Zn-containing mesoporous smectite-like material was pre-
pared by a hydrothermal method. The detailed procedures were
described elsewhere [22,23]. In brief, an aqueous solution of ZnCl2
was dropped to an aqueous solution of water glass (Nippon Chem-
ical Industrial) and NaOH (Wako) while stirring. The Si:Zn molar
ratio in the mixture was adjusted to 8:6. After further stirring over-
night, the solid material was separated by centrifuge. The resultant
solid material was mixed with distilled water, and the mixture was
treated in an autoclave at 150 °C under autogenous pressure for
5 h. This catalyst will be abbreviated as SM(Zn) in the following.
Table 1
Comparison of the performance among several heterogeneous and homogeneous
catalysts for the synthesis of glycerol carbonate from glycerol and urea.a
A
Zn-containing hydrotalcite material was also prepared
Entry
Catalyst
Conversion (%)
Selectivity (%)
TOFb (hÀ1
)
according to the literature [20]. Zn(NO3)2Á6H2O (Wako) and
Al(NO3)2Á9H2O (Wako) were mixed in distilled water with a Zn:Al
molar ratio of 2:1. This aqueous solution was added to another
solution of Na2CO3 (Wako) and NaOH (Wako) with a molar ratio
of 3:7 while stirring, and the resultant mixed solution was allowed
to stand at 65 °C for 18 h. Then, the precipitate obtained was sep-
arated by centrifuge and washed with distilled water; this was re-
peated four times, and the solid material obtained was dried at
110 °C for 12 h. It was then used as a catalyst for the title reaction.
This sample will be referred to as HT(Zn) hereinafter.
1
2
3
4
5
6
7
ZnO
SM(Zn)
HT(Zn/Al)
ZnCl2
ZnBr2
ZnSO4
(CH3COO)2ZnÁ2H2O
61
65
82
84
82
81
65
69
75
80
97
96
92
80
2.3
5.3
6.2
7.5
12
8.0
7.7
a
Reaction conditions: Catalyst 0.25 g, glycerol 50 mmol, urea 50 mmol, tem-
perature 130 °C, pressure 3 kPa, time 3 h.
b
Apparent turnover frequency = (moles of glycerol carbonate formed)/((moles of
Zn used)Á(reaction time)).
The structural features of solid catalysts of ZnO, SM(Zn), and
HT(Zn) used were examined by X-ray diffraction (XRD) and nitro-
gen adsorption using JOEL JDX-8020 and Quantachrome NOVA
1000, respectively.
2.2. Reaction runs and leaching tests
A 100 cm3 pear shape flask with a condenser was loaded with
glycerol (Wako) in 50 mmol (4.6 g), urea (Wako) in 50 mmol
(3.0 g), and solid catalyst in 0.25 g. After the pressure was reduced
to 3 kPa by an aspirator, the reaction mixture was heated in an oil
bath to 80 °C and kept at this temperature for 30 min while stirring
by a magnetic stirrer. Then, the reaction mixture was further
heated and kept at 130 °C for 180 min. After the reaction, the solid
catalyst was separated by filtration, and the liquid phase was
diluted by ethanol (Wako) and analyzed by a gas chromatography
with a flame ionization detector (Shimadzu GC-14B with ZB-50
column). Anisole (Wako) was used as an internal standard.
As described in the following, the leaching of Zn from the Zn-
containing solid catalysts was observed to occur. The amount of
Zn species dissolved into the liquid phase after the reaction was
measured by chelatometry. The liquid phase was separated from
the solid materials by filtration. A small quantity of an indicator
of blue tetrazolium (Aldrich) was added to this liquid sample. A
0.01 M ethylenediaminetetraacetic acid (EDTA, Aldrich) solution
was dropped to the liquid to find an end point when the liquid
changed in color from red to blue. The leaching of Zn was then
determined from the amount of EDTA used. The liquid phase after
2θ (degree)
Fig. 1. XRD patterns of Zn-containing solid catalyst samples used: (a) ZnO, (b)
smectite, and (c) hydrotalcite before and after activity measurements in glycerol
carbonate synthesis.

S.-i. Fujita et al. / Journal of Catalysis 297 (2013) 137–141
139
Table 2
100
80
Results in the presence and absence (filtrate) of solid catalysts in synthesis of glycerol
carbonate from glycerol and urea.a
Catalyst
(a) ZnO
Time
(h)
Conversion
(%)
Selectivity
(%)
Dissolved Zn
(mmol)
60
1
52
78
81
58
74
74
0.39
7b
7c
0.18
0.36
40
20
0
(b) SM(Zn)
(c) HT(Zn)
1
33
87
87
61
86
86
7b
7c
0.23
0.41
1
43
92
88
60
80
82
7b
7c
0.24
0
1
2
3
4
a
Amount of Zn used (mmol)
Reaction conditions: Catalyst 0.25 g, glycerol 50 mmol, urea 50 mmol, tem-
perature 130 °C, pressure 3 kPa, time 3 h.
b
Fig. 2. The yield of glycerol carbonate as a function of the amount of Zn included in
the catalyst used. (s) ZnO, (h) SM(Zn), and (d) HT(Zn).
The reaction was conducted in the presence of catalyst for 1 h, it was separated
by filtration, and then the filtrate was kept to stand under the reaction conditions
for 6 h, total 7 h.
c
The reaction was conducted in the presence of catalyst for 7 h.
0.3
(a)
0.2
0.1
0
0
1
2
3
4
Amount of Zn used (mmol)
100
(b)
Wave number (cm-1)
ZnSO4
80
60
40
ZnCl2
ZnBr2
Fig. 4. FTIR spectra collected in the presence and absence of Zn-containing solid
catalysts. (a) glycerol + urea, (b) glycerol + urea + ZnO, (c) glycerol + urea + SM(Zn),
(d) glycerol + urea + HT(Zn).
(AcO)2Zn
20
0
these two mesoporous catalyst samples, but some structural
changes occurred in the course of reaction. The absence of the for-
mation of zinc glycerolate might result from the presence of Si and
Al atoms in their layer structures.
0
0.1
0.2
0.3
0.5
0.6
0.4
Amount of Zn dissolved (mmol)
Fig. 2 shows the changes in the yield of glycerol carbonate using
the three solid catalysts by varying the catalyst amount. The in-
creases in the yield were marginal above certain amounts of the
catalysts used. Thus, the yield of glycerol carbonate was not simply
determined by the catalyst amount. Zhao et al. used ZnO for the
reaction of urea and methanol to produce dimethyl carbonate
and showed that ZnO was converted to a zinc complex of (NH3)2-
Zn(NCO)2, in which two molecules of ammonia were weakly coor-
dinated to Zn atom [32]. This Zn complex was proposed to be the
active species for the dimethyl carbonate synthesis from methanol
and urea. These results strongly suggest that similar Zn complexes
were also produced in the present reaction system and partici-
pated in the reaction. So, the possibility of leaching of Zn species
from these solid catalysts was studied. The reaction runs were con-
ducted with different amounts of catalysts while keeping the other
reaction conditions unchanged. The amounts of Zn species dis-
solved into the organic liquid phase were also measured. The
amount of dissolved Zn species is plotted against the initial
amount of Zn included in the catalysts used in Fig. 3a. The amount
of dissolved Zn species increased significantly with the amount of
catalyst used when it was small (<1 mmol). But, when the catalyst
Fig. 3. (a) Relationship between the amount of Zn species dissolved and the initial
amount of Zn included in the parent catalysts. (b) Relationship between the yield of
glycerol carbonate (GC) and the amount of Zn species dissolved in the liquid phase.
(s) ZnO, (h) SM(Zn), (d) HT(Zn), and (j) zinc salts.
catalysts examined are not large, and so there is still room to fur-
ther examine the performance of those solid catalysts.
Fig. 1 gives XRD patterns of the three solid catalysts before and
after a reaction run. Surprisingly, the XRD measurements of ZnO
showed that, on the reaction run, ZnO changed almost completely
to a certain species, which is zinc glycerolate, C3H6O3Zn, according
to the literature XRD data [31]. It was further observed that the
catalyst weight increased after the reaction. The XRD patterns of
SM(Zn) were almost the same before and after one reaction run,
but the XRD peaks of SM(Zn) gradually became broader and weak-
er after repeating several reaction runs (not shown), strongly sug-
gesting that its crystallinity became lowered. No peak was
observed in the XRD pattern of HT(Zn) after the reaction. Thus,
XRD results did not indicate the formation of zinc glycerolate for

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S.-i. Fujita et al. / Journal of Catalysis 297 (2013) 137–141
Zn species have the same structure (thus the same specific
performance) irrespective of the parent Zn-containing solid and
homogeneous catalysts used.
The homogeneous nature of the present glycerol carbonate
synthesis was further studied with the liquid phase alone after
the solid–liquid reaction. Table 2 gives the results with the three
solid catalysts, with which a reaction run was conducted for 1 h,
the catalyst was separated from the reaction mixture by filtration,
and the liquid phase was further kept to stand for 6 h under the
same reaction conditions. The conversion and selectivity obtained
with the filtrates were the same as in the reactions carried out for
7 h in the presence of catalysts. These results also indicate that the
reaction proceeds homogeneously with the Zn species dissolved in
the liquid phase. Table 2 gives another interesting fact that the
amount of Zn dissolved in 7 h was smaller than that in 1 h for
the three solid catalysts. This indicates a possibility that the
dissolved Zn species may redeposit on the parent catalysts and/
or precipitate separately in a long reaction time (at a higher
conversion). The Zn leaching occurs for the three Zn-containing
solid catalysts, but the extent of leaching and the extent of redepo-
sition/precipitation depend on the catalysts used.
The present results demonstrate that the production of glycerol
carbonate from glycerol and urea is catalyzed by a certain Zn spe-
cies dissolved from the solid catalysts into the organic liquid phase.
FTIR measurements were made for the liquid phase after the reac-
tion runs to examine structural features of the dissolved Zn species
present therein. Fig. 4a and b shows FTIR spectra for the liquid
phases of glycerol and urea in the absence and presence of ZnO,
respectively, after the same thermal history as in the reaction runs.
Additional absorption bands were observed to exist at 2216, 1791,
1713, 1186, and 778 cmÀ1 in the presence of ZnO. These absorption
Wave number (cm-1)
Fig. 5. FTIR spectra collected in the presence and absence of Zn-containing
homogeneous catalysts. (a) glycerol + urea, (b) glycerol + urea + ZnCl2, (c) glyc-
erol + urea + (CH3COO)2Zn, and (d) glycerol + urea + ZnSO4.
amount was further increased (>1 mmol), the amount of dissolved
Zn species did not change so much. Fig. 3b shows the yield of glyc-
erol carbonate formed against the amount of dissolved Zn species.
A single relationship can be seen in common for ZnO, SM(Zn) and
HT(Zn) catalysts. Fig. 3b also gives the glycerol carbonate yield ob-
tained with the homogeneous catalysts against the amount of Zn in
the reaction mixture. It should be noted that the relationship for
the solid catalysts can also well correlate the glycerol carbonate
yields obtained with the homogeneous catalysts. Those results
strongly suggest that the synthesis of glycerol carbonate is cata-
lyzed by Zn species dissolved in the liquid phase, and such active
ZnO + 2(NH2)2CO
-H2O
O
H3N
H3N
O
O
NCO
NCO
Zn
1
HO
-NH3
-NH3
HO
OH
+(NH2)2CO
OH
O
H
H
NH
Zn
O
O
O
NCO
NCO
O
NCO
NH3
-NH3
NCO
NCO
Zn
H
Zn
OH
O
HO
NH3
H
OH
4
2
OH
C3H6O3Zn
O
O
NH
Zn
C
O
NCO
NH3
H
O
N
H
Zn
O
NCO
OH
NH3
3
OH
OH
Scheme 2. Reaction mechanisms proposed for the synthesis of glycerol carbonate from glycerol and urea with Zn-containing catalyst.

S.-i. Fujita et al. / Journal of Catalysis 297 (2013) 137–141
141
bands can be assigned to such functional groups according to the
literature: 2216 cmÀ1 to NCO [32], 1791, 1186, and 778 cmÀ1 to
glycerol carbonate [33], 1713 cmÀ1 to intermediate carbamate spe-
cies [33]. The same FTIR measurements were also made for the li-
quid phases containing homogeneous catalysts of ZnCl2,
(CH3COO)2Zn, and ZnSO4. Fig. 5 shows FTIR spectra obtained, indi-
cating that new absorption bands appeared at very similar fre-
quencies in the presence of the homogeneous catalysts as
observed with the Zn-containing solid ones (Fig. 4). Therefore,
the same Zn-containing complex should also exist in the liquid
phases with the homogeneous catalysts, and it should be an active
species irrespective of the parent heterogeneous and homogeneous
catalysts used.
hydrotalcite) under solvent-free conditions at 130 °C and at 3 kPa
proceeds homogeneously but not heterogeneously. The constituent
Zn species may dissolve into the liquid phase by an action of both
glycerol and urea. The dissolved Zn species are active ones for the
reaction, which has the same structure irrespective of the parent
solid catalysts used. The active species could be a complex of a
Zn atom coordinated with N@C@O. For homogeneous catalysts
such as ZnCl2, (CH3COO)2Zn, and ZnSO4, the same Zn complex is
likely to form and be an active species for the transformation of
glycerol and urea to glycerol carbonate.
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The synthesis of glycerol carbonate from glycerol and
urea using Zn-containing solid catalysts (zinc oxide, smectite,