Synthesis of glycerol carbonate from glycerol and dimethyl carbonate catalyzed by K2CO3/MgO

Article abstract of DOI:10.1007/s11164-011-0443-3

A series of M/MgO (M = CaO, KNO₃, KOH, K₂CO ₃) catalysts were prepared by a dry impregnation method and used for synthesis of glycerol carbonate from glycerol and dimethyl carbonate. It was found that K₂CO3/MgO was the most efficient catalyst, with a glycerol carbonate yield of approximately 99% under the conditions: DMC/glycerol molar ratio 2.5:1, catalyst/raw material weight ratio 1%, reaction time 2 h, and reaction temperature 80 °C. FTIR, BET, TEM, and XRD were used for characterization of the catalyst and showed that the active sites seemed to be K₂O formed on the K₂CO3/MgO catalyst. Finally, a recycling experiment showed that the catalyst was relatively stable and could be reused up to four times, at least, by regeneration. Springer Science+Business Media B.V. 2011.

Full text of DOI:10.1007/s11164-011-0443-3

Res Chem Intermed (2012) 38:1069–1077
DOI 10.1007/s11164-011-0443-3
Synthesis of glycerol carbonate from glycerol
and dimethyl carbonate catalyzed by K CO /MgO
2
3
Meimei Du
Tao Geng
•
Qiuxiao Li
Yajie Jiang
•
Wantian Dong
•
•
Received: 16 October 2011 / Accepted: 12 November 2011 / Published online: 26 November 2011
Ó Springer Science+Business Media B.V. 2011
Abstract A series of M/MgO (M = CaO, KNO , KOH, K CO ) catalysts were
2
3
3
prepared by a dry impregnation method and used for synthesis of glycerol carbonate
from glycerol and dimethyl carbonate. It was found that K CO /MgO was the most
2
3
efficient catalyst, with a glycerol carbonate yield of approximately 99% under the
conditions: DMC/glycerol molar ratio 2.5:1, catalyst/raw material weight ratio 1%,
reaction time 2 h, and reaction temperature 80 °C. FTIR, BET, TEM, and XRD
were used for characterization of the catalyst and showed that the active sites
seemed to be K O formed on the K CO /MgO catalyst. Finally, a recycling
2
2
3
experiment showed that the catalyst was relatively stable and could be reused up to
four times, at least, by regeneration.
Keywords Glycerol carbonate ꢀ Transesterification ꢀ Supported catalyst ꢀ
Dimethyl carbonate
Introduction
Glycerol is becoming an abundant and inexpensive natural resource since the rapid
growth of global biodiesel industry, so exploration of its further processing has
attracted much attention [1–4]. Among all the derivatives of glycerol, glycerol
carbonate (GC), with many potential applications, is the most attractive. It can be
widely used as an adhesive, surfactant, and elastomer, and is also an excellent
solvent in the paint industry. In addition, its high reactivity enables its use as an
intermediate in the synthesis of many valuable chemicals [5–8].
M. Du ꢀ Q. Li (&) ꢀ T. Geng ꢀ Y. Jiang
China Research Institute of Daily Chemical Industry, Taiyuan 030001, Shanxi, China
e-mail: liqiuxiao@sina.com
W. Dong
Shanghai Fine Chemical Co. Ltd, Shanghai 201505, China
123

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M. Du et al.
Several methods have been described for synthesis of GC, including reaction of
glycerol with carbon dioxide using an Sn catalyst, which was found to be both
directly and environmentally friendly but of high cost, and the catalyst is of low
activity [9, 10]. Carbonylation of glycerol with carbon monoxide and oxygen or
hazardous phosgene has also been reported, but these methods have the drawbacks
of rigorous reaction conditions and the toxicity of the starting materials [11, 12].
Synthesis of glycerol carbonate from glycerol and dimethyl carbonate (DMC), an
environmentally benign material, is an alternative means of producing glycerol
carbonate under much milder conditions without the production of problematic side
products, so it has aroused much interest [13]. Seeking highly efficient catalysts is
the keystone of this base-catalyzed reaction. Compared with homogeneous alkaline
catalysts, for example NaOH or KOH, heterogeneous catalysts are preferred because
they are less corrosive, safer, less expensive, and more environmentally friendly
[
14–19]. Several heterogeneous alkaline catalysts have been investigated for
synthesis of glycerol carbonate by transesterification of glycerol with DMC. CaO,
the traditional solid base catalyst was found to be a good catalyst for production of
glycerol carbonate, because its basicity is as high as that of traditional homogeneous
catalysts and its sources such as calcium carbonate and calcium acetate are less
expensive and less toxic; CaO is, however, very prone to deactivation in air and
readily forms suspensions in the reaction mixture, resulting in poor reusability [14].
Other catalysts, for example lipase, hydrotalcite, and ionic liquids have also been
investigated [15–17], but they either required long reaction times or needed an
additional solvent environment. Recently, Bai et al. [18] reported KF-modified
hydroxyapatite as a highly active catalyst for the synthesis of GC from glycerol and
DMC. The yield of GC reached nearly 99%, but KF is a very toxic substance and its
safety should be carefully considered in industrial applications. Mg/Al/Zr mixed
oxide catalysts have also been investigated recently [19], but reactions catalyzed by
these catalysts must be conducted in a large excess of DMC. Development of
suitable non-toxic solid alkaline catalytic systems with high activity which are
easily recoverable and recyclable is therefore of great importance.
To the best of our knowledge, there is no report of the application of MgO as a
catalyst support for glycerol carbonate production. In work discussed in this paper, a
series of M/MgO (M = CaO, KNO , KOH, K CO ) catalysts were prepared and
3
2
3
used for synthesis of glycerol carbonate by transesterification of glycerol with
dimethyl carbonate. IR, BET, TEM, and XRD were used for characterization of the
catalyst. The activity of the catalysts was investigated and compared with that of the
conventional catalysts CaO, K CO , and KOH. The stability of the synthesized
2
3
K CO /MgO catalyst was also evaluated.
2
3
Experimental
Catalyst preparation
All catalysts were prepared by a dry impregnation method. In all cases, a specific
amount of MgO carrier was first preheated at 500 °C for 5 h to remove physisorbed
1
23

Synthesis of glycerol carbonate
1071
water and then impregnated with 20 wt% Ca(CH COOH) , KNO , KOH, and
3
2
3
K CO solutions, dried at 110 °C for 10 h, and then calcined at 600 °C in air for
2
3
3
h.
Catalyst characterization
FTIR spectra were recorded with a Hitachi (Japan) model 273-30 spectrometer over
-
1
the wavenumber range 4,000–400 cm . The KBr pellet technique was used to
prepare samples. All measurements were conducted at room temperature.
Nitrogen adsorption–desorption isotherms were determined by use of a
Micromeritics ASAP 2010 at 77 K. Before the measurements, all samples were
-
4
degassed at 473 K under high vacuum (10 ) for 2 h. The specific surface area of
samples was calculated by use of the Brunauer–Emmett–Teller (BET) method. The
pore volume was given at p/p = 0.990. The pore size distribution was calculated
0
by use of the Barrett–Joyner–Halenda (BJH) method.
Microstructure characterization was carried out by use of a Jeol JEM-1011
transmission electron microscope (TEM) with an accelerating potential of 100 kV.
X-ray diffraction (XRD) patterns of the catalysts were recorded with a Rigaku
˚
MiniFlex II using Cu Ka radiation (1.5418 A) at 30 kV and 15 mA. The
measurements were obtained over a 2h range of 20–70° with a step size of 0.01°
at a scanning speed of 8°/min.
Synthesis of glycerol carbonate
Glycerol carbonate was synthesized in a 100-mL round-bottomed four-neck
jacketed glass reactor fitted with a mechanical stirrer, a reflux condenser, and a
thermometer. In a typical experiment, 0.1 mol glycerol (99%, AR grade), 0.25 mol
DMC (99.5%, AR grade), and the catalyst (1 wt%, based on the mass of raw
material) were successively added into the reactor. Transesterification was
performed at a stirring speed of 250 rpm at 80 °C for 2 h. After the reaction, the
catalyst was isolated by filtration and collected for reuse. To test the leakage of K
from the catalyst in the reaction, the K content in the reaction mixture was analyzed
by ICP using an iCAP6300 (Thermo Scientific, USA).
Product analysis
After separating the catalyst, the products were analyzed by gas chromatography
(
(
Agilent GC-6890) with flame ionization detection (FID) using an HP1701
30 m 9 0.32 mm 9 0.25 lm) capillary column. Injector and detector tempera-
tures were 280 and 300 °C, respectively. Oven temperature started at 80 °C and was
-
1
increased at 12 ° min to 250 °C which was held 2 min. Tetraethylene glycol was
used as the internal standard. Conversion of glycerol (X ), and yield of glycerol
carbonate (YGC) were expressed as:
G
123

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M. Du et al.
Consumed amount of glycerol (mol)
Charged amount of glycerol (mol)
Amount of glycerol carbonate (mol)
Charged amount of glycerol (mol)
XG ¼
YGC
¼
Results and discussion
Catalyst characterization
FTIR spectroscopy, an effective tool for obtaining semi-quantitative structural
information about complex solids [20], was used to investigate the functional groups
of the K CO /MgO catalyst, and of K CO and the carrier MgO. All three samples
2
3
2
3
were calcined at 600 °C for 3 h. The FTIR spectra are shown in Fig. 1. Two strong
-
bands at 1,457 and 1,385 cm were obtained from K CO ; these bands, which
1
2
3
were attributed to carbonate species [21–23], were also observed in the spectrum of
the synthesized K CO /MgO catalyst, indicating that the K CO was successfully
loaded on to the surface of the MgO.
2
3
2
3
The measured BET surface area, pore volume, and pore diameter are shown in
Table 1. The BET surface areas and the pore volumes and pore diameters decreased
with loading of CaO, KNO and KOH, indicating these active sites were well
3
dispersed on MgO. In contrast, BET surface areas increased on loading with K CO .
2
3
This might be because of chemical reaction between absorbed K CO and MgO, it
2
3
?
seems that the K ions of the K CO could become inserted into the vacant sites of
2
3
2 3 2 3
Fig. 1 FTIR spectra of K CO , MgO, and K CO /MgO
1
23

Synthesis of glycerol carbonate
1073
Table 1 BET surface areas,
pore volumes, and pore
diameters of the prepared
catalysts
Catalyst
BET surface area
(
Pore volume
3
(cm /g)
Pore diameter
(nm)
2
m /g)
MgO
19.16
16.85
8.74
0.0690
0.0391
0.0175
0.0058
0.1073
14.7
9.4
CaO/MgO
KNO
KOH/MgO
CO /MgO
3
/MgO
7.1
3.21
7.6
K
2
3
25.74
24.3
2 3
Fig. 2 TEM images of a MgO and b K CO /MgO
MgO, accelerating dispersion and decomposition of K CO to form strong basic
3
2
sites of K O either during the impregnation process or later [24]. These processes
2
induced recrystallization and the formation of more new small crystals in the large
pores or on the MgO surface, which might mainly contribute to the higher BET
surface area, pore volume, and pore size.
TEM of MgO and K CO /MgO is shown in Fig. 2a, b. It can be seen that after
2
3
impregnation with K CO followed by calcination at 600 °C, agglomeration of
2
3
MgO particles is obvious, and formation of new small crystals on the surface of the
MgO can also be observed. This result agrees well with that of BET.
XRD analysis (Fig. 3) of the synthesized K CO /MgO catalyst was also carried
2
3
out. The results showed that the difference between the catalyst and the carrier MgO
was obvious, as shown by the appearance of new peaks. In addition to the typical
diffraction peaks of K CO at 2h = 25, 32, 38, and 41° [23], the new phase of K O
2
3
2
could also be observed clearly at 2h = 31° [25, 26]. Accordingly, it is reasonable to
suppose that part of the loaded K CO should be decomposed under the activation
conditions, depending on whether or not the K CO interacts with the MgO support,
2
3
2
3
which further confirms the results of BET and TEM.
Screening of highly active supported catalysts for synthesis of glycerol
carbonate
Magnesium oxide has been widely used as a basic carrier in industrial processes,
with excellent catalytic activity [24, 27–29]. In this work, MgO supported by CaO
123

1074
M. Du et al.
×
K CO /MgO
*
××
2
3
×
×
*
K CO3
2
K O
2
MgO
2
0
30
40
50
60
70
2
θ (°)
2 3
Fig. 3 X-ray diffraction patterns of MgO and K CO /MgO
Table 2 Screening of highly active supported catalysts for synthesis of glycerol carbonate
Entry
Catalyst
Conversion of
glycerol (%)
Yield of glycerol
carbonate (%)
1
2
3
4
5
6
7
8
9
CaO
97.4
20.3
67.5
68.0
98.5
97.8
99.0
98.7
96.8
96.5
15.2
64.4
64.7
94.7
95.2
98.6
98.3
96.3
MgO
CaO/MgO
CaO/MgO
a
b
KNO
3
/MgO
KOH/MgO
K
2
CO
c
3
/MgO
KOH
c
3
K
2
CO
Reaction conditions: temperature, 80 °C; molar ratio of DMC to glycerol, 2.5; glycerol, 0.1 mol; reaction
time, 2 h; amount of catalyst, 1 wt% (on the basis of the mass of raw material)
a
The amount of catalyst: 2.0 wt%
b
Reaction time 3 h
c
Reaction time 40 min
and potassium compounds (KNO , KOH, and K CO ) was prepared and used for
3
2
3
the synthesis of glycerol carbonate from glycerol and DMC.
As shown in Table 2, the catalytic performance of MgO was affected by the
nature of the active component. Although it has been reported that CaO/MgO can be
used as an efficient catalyst for production of biodiesel by transesterification [27],
this supported catalyst had poor activity in the synthesis of glycerol carbonate.
Considering that the low catalytic activity may be because of the low CaO loading,
we increased the amount of the catalyst to 2 wt% (on the basis of the mass of raw
1
23

Synthesis of glycerol carbonate
1075
material), however, both the conversion of glycerol and the yield of glycerol
carbonate remained unchanged.
We then used MgO loaded with a series of potassium salts (KNO , KOH, and
3
K CO ) to catalyze the reaction; surprisingly, high conversion and yield were
2
3
obtained. From Table 2, one can observe that, in the presence of supported catalysts
KNO /MgO, KOH/MgO and K CO /MgO, the glycol conversions are all higher
3
2
3
than 97%, comparable with that of the traditional catalyst CaO. Under the same
reaction conditions, using MgO, we obtained conversions of only 20.3%, indicating
that these basic active components KNO , KOH, and K CO had a positive effect on
3
2
3
the MgO after impregnation or calcination, resulting in excellent catalytic activity.
Because KNO /MgO needed more time to reach the equilibrium state and KOH/
3
MgO was almost as active as CaO, K CO /MgO was the best and most active
3
2
catalyst for high glycerol conversion (99.0%) and good selectivity for glycerol
carbonate (almost 100%). For comparison, the catalytic activity of the homogenous
catalysts, KOH and K CO , were also examined in the transesterification of glycerol
2
3
and DMC (Table 2, entries 8 and 9), and conversion of glycerol reached 98.7 and
9
was almost the same as the homogenous catalyst of KOH and K CO .
6.8%, respectively. This showed that the catalytic performance of K CO /MgO
2 3
2
3
The effect of the amount of K CO /MgO catalyst on the reaction was also
2
3
investigated; the results are shown in Fig. 4. It is apparent that the yield of glycerol
carbonate increased gently with increasing amount of catalyst; this may be
attributed to the availability of more active sites for the reaction when the amount of
catalyst is increased. However, even when the amount of catalyst was as low as 0.2
wt%, the yield of glycerol carbonate reached approximately 95%. This result
9
9
9
9
9
9
8
7
6
5
0.2
0.4
0.6
0.8
1.0
Catalyst amount (based on mass of raw material) (%)
Fig. 4 Effect of the amount of K
conditions: temperature, 80 °C; molar ratio of DMC to glycerol, 2.5; glycerol, 0.1 mol; reaction time,
h)
2 3
CO /MgO catalyst on activity in glycerol transesterification. (Reaction
2
123

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M. Du et al.
reiterates the high activity of the K CO /MgO catalyst for transesterification of
2
3
glycerol with dimethyl carbonate to yield glycerol carbonate.
Recycling of the catalyst
Recycling experiments were carried out to investigate the stability of the prepared
K CO /MgO catalyst. At first, used catalyst was directly reused after washing with
2
3
DMC and drying at 110 °C for 5 h. However, a remarkable reduction in activity
occurred and the yield of glycerol carbonate achieved was only 45.7% in the secondrun.
Because deactivation of the catalyst may have been caused by products adhering
during the reaction, the K CO /MgO was calcined at 600 °C for 3 h after each run
2
3
and then reused. After this treatment, the yield of glycerol carbonate slowly
decreased with increasing number of times reused (Fig. 5).
To discover the reason for the decrease in catalytic activity despite the
regeneration procedure, the potassium content of the product mixture after the 1st
run was investigated by ICP, and the result was 0.048% (mass concentration), which
indicated leakage of K from the catalyst during the reaction. The very small amount
leaching accorded with the slow decrease of glycerol carbonate yield (one can
observe from Fig. 5 that the yield of glycerol carbonate was still [83% in the 4th
run). However, because yield decrease continuously from run to run, it is reasonable
that potassium leaching is another important reason for the catalyst’s deactivation.
The results discussed above suggest the K CO /MgO catalyst was probably
2
3
acting as a homogeneous–heterogeneous catalytic system [23]. Although not as
good as a purely heterogeneous catalyst, it was of relatively good stability, because
it retained its high activity when reused at least four times when calcined again after
reaction.
100
8
6
4
2
0
0
0
0
0
cycle 1
cycle 2
cycle 3
cycle 4
cycle 5
Fig. 5 Recycle test of catalysts (reaction conditions: temperature, 80 °C; molar ratio of DMC to
glycerol, 2.5; glycerol, 0.1 mol; reaction time, 2 h; amount of catalyst, 1 wt%)
1
23

Synthesis of glycerol carbonate
1077
Conclusion
MgO supported by CaO and a series of potassium compounds (KNO , KOH, and
3
K CO ) was adopted as alkaline catalyst for synthesis of glycerol carbonate from
2
3
glycerol and DMC. It was found that MgO loaded with potassium salts, especially
K CO /MgO, had high catalytic activity, with a glycerol carbonate yield of
2
3
approximately 99% under mild reaction conditions. The catalyst was reasonably
stable and could be reused at least four times after re-activation. One can expect its
use as an alternative to conventional alkaline catalysts for synthesis of glycerol
carbonate by transesterification of glycerol with DMC.
Acknowledgments The authors are grateful for financial support from the Natural Science Foundation
of Shanxi (2010021013-1) and the Patent Transformation Project of Taiyuan (1008).
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