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J.-H. Park et al. / Applied Catalysis A: General 433–434 (2012) 35–40
Scheme 1. Synthesis of glycerol carbonate (GLC) from glycerol and urea.
2.1. Glycerol carbonate synthesis
spectra of catalyst samples were recorded on a Nicolet FT-IR spec-
trometer (iS10, USA) equipped with a SMART MIRACLE accessory
over a range of 400–4000 cm−1 at a resolution of 2 cm−1. X-ray
diffraction (XRD) was measured using a Shimadzu XRD-6000 with
a Cu K␣ radiation source (40 kV and 30 mA).
Glycerol (23.0 g, 250 mmol), urea (15.0 g, 250 mmol), and ZnCl2
(0.68 g, 5 mmol) were loaded into a 100 mL round-bottomed flask
equipped with a condenser and an electric heater. The top of the
condenser was connected to an aspirator, and the pressure inside
the flask was controlled using a vacuum release valve and a vacuum
gauge. The flask was then heated to a specified reaction tempera-
ture, and the vacuum inside was maintained at 2.67 kPa. After the
reaction was completed, the flask was cooled to room tempera-
ture and the solid materials were isolated through filtration. The
remaining solution was analyzed using a HPLC after the addition of
water and t-butanol as an external reference.
3. Results and discussion
3.1. Catalyst screening
The catalytic activities of various zinc compounds were evalu-
ated for the reaction of glycerol with urea to produce GLC at 150 ◦C
for 2 h with a urea/glycerol molar ratio of 1. To shift the equilib-
rium toward the formation of GLC, NH3 was removed as soon as it
formed using an aspirator (2.67 kPa). As listed in Table 1, the reac-
tion proceeded even in the absence of a catalyst, but the yield and
selectivity of GLC were only 29.5 and 56.2%, respectively (entry 1).
13C NMR and liquid chromatographic analyses of the product mix-
ture showed that glycerol carbamate, an intermediate of GLC, was
produced in large quantities along with trace amounts of diglycerol.
The GLC yield and selectivity, however, were greatly enhanced by
the use of a zinc compound as the catalyst. For instance, the use of
2 mol% ZnCl2 with respect to glycerol produced GLC in yield of 80.2%
ZnBr2 and ZnI2, whereas ZnF2 resulted in a much lower GLC yield
and selectivity, possibly due to the low solubility of ZnF2 in glycerol.
For comparison, the activities of other zinc compounds including
Zn(NO3)2·6H2O, Zn(OAc)·2H2O, and ZnO were also evaluated. As
shown in Table 1, Zn(NO3)2·6H2O exhibited the highest conversion
of glycerol (85.3%) although the GLC selectivity (93.2%) was slightly
lower than that of ZnCl2 (99.7%). By contrast, Zn(OAc)2·2H2O and
ZnO exhibited much reduced activities compared with other zinc
compounds tested, producing GLC in yields of 44.3 and 47.9%,
respectively. The origin of the different catalytic activity of ZnX2
(X = halide, NO3, OAc) with the variation of X is not clear at the
moment, but the Lewis acidity of ZnX2 and ease of Zn X dissocia-
tion are likely to be responsible for the activity difference.
2.2. Isolation and characterization of intermediate catalytic
species
2.2.1. Zn(NH3)2Cl2
Glycerol (23.0 g, 250 mmol), urea (15.0 g, 250 mmol), and ZnCl2
(7.1 g, 52.0 mmol) were loaded into a 100 mL round-bottomed
flask and heated to 130 ◦C at 2.67 kPa. When the reaction mixture
became transparent after approximately 30 min at 130 ◦C, the flask
was cooled to room temperature and 100 mL of MeOH was added to
precipitate the solid material, which was identified as ZnCl2(NH3)2
by XRD, FT-IR, and elemental analyses. Isolated yield: 77.5%. Anal.
Calcd for ZnCl2(NH3)2: Zn, 39.3%; N, 16.8%; H, 1.2%. Found: Zn,
38.0%; N, 16.2%; H, 1.2%.
2.2.2. Zn(C3H6O3)
Glycerol (23.0 g, 250 mmol), urea (15.0 g, 250 mmol), and ZnCl2
(0.71 g, 5.2 mmol) were added to a 100 mL round-bottomed flask
and heated to 150 ◦C at 2.67 kPa. After 2 h of the reaction, the solu-
tion was cooled to room temperature and 100 mL of MeOH was
added to precipitate the dissolved zinc complex, Zn(C3H6O3). Iso-
lated yield: 38.2%. Anal Calcd for Zn(C3H6O3): Zn, 42.1%; C, 23.2%;
H, 3.9%. Found: Zn, 42.1%; C, 24.5%; H, 3.9%.
2.3. Instrumentation
3.2. Effect of catalyst loading
Quantification of the reaction products was made on a Waters
HPLC equipped with an Aminex HPX-87H column (Biorad) and a RI
detector (Waters 410). The mobile phase used was a 5 mM H2SO4
aqueous solution and the flow rate was set at 0.6 mL/min. For the
quantitative analysis, an external standard method was used. FT-IR
The effect of catalyst loading on the yield of GLC was investigated
using ZnCl2 as the catalyst at 150 ◦C for 2 h with a reduced pressure
of 2.67 kPa. As shown in Fig. 1, the GLC yield and glycerol conversion
increased continuously with an increasing ZnCl2/glycerol ratio up
Table 1
Activities of various zinc catalysts for the glycerolysis of urea.a
Entry
Catalyst
Conversion (%)
GLC yield (%)
GLC selectivity (%)
1
2
3
4
5
6
7
8
–
ZnF2
ZnCl2
ZnBr2
52.5
76.4
80.4
81.4
80.9
85.3
67.2
69.9
29.5
64.3
80.2
79.2
76.9
79.6
44.3
47.9
56.2
84.1
99.7
97.2
94.9
93.2
66.0
68.5
ZnI2
Zn(NO3)2·6H2O
Zn(OAc)2·2H2O
ZnO
a
Reaction condition: glycerol = 250 mmol, urea = 250 mmol, catalyst/glycerol = 2 mol%, P = 2.67 kPa, T = 150 ◦C, t = 2 h.