Synthesis of glycerol carbonate by transesterification of glycerol with dimethyl carbonate over MgAl mixed oxide catalysts

Article abstract of DOI:10.1016/j.apcata.2013.07.020

A series of hydrotalcite-like layered double hydroxides (LDHx) with different Mg/Al atomic ratios (x = 2-6) were prepared by using the co-precipitation method. Further calcination yields mixed oxides with tunable basicity. The basicity of the calcined LDHx (LDOx) strongly depends on the Mg/Al ratio and the calcination temperature. The resulting LDOx materials were used as solid base catalysts and evaluated in the transesterification between glycerol and dimethyl carbonate without use of organic solvent. The correlation between the basic properties of the solid catalysts and the catalytic performance was investigated. The activity of the LDOx catalysts was demonstrated to be proportional to the surface density of basic sites. LDO2 calcined at 600 C exhibited maximum activity for the transesterification reaction. The beneficial effect of the optimum ratio of Mg/Al = 2 is related to its high total basicity. The LDO2 catalyst can be readily recycled while maintaining high catalytic activity and selectivity of glycerol carbonate.

Full text of DOI:10.1016/j.apcata.2013.07.020

Applied Catalysis A: General 467 (2013) 124–131
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Synthesis of glycerol carbonate by transesterification of glycerol with
dimethyl carbonate over MgAl mixed oxide catalysts
Peng Liua,∗, Margherita Derchib, Emiel J.M. Hensena,∗
a Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, P.O. Box 513, Eindhoven, The Netherlands
b Dipartimento di Ingegneria Chimica, Civile e Ambientale, Università degli Studi di Genova, I-16145 Genova, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of hydrotalcite-like layered double hydroxides (LDHx) with different Mg/Al atomic ratios (x = 2–6)
were prepared by using the co-precipitation method. Further calcination yields mixed oxides with tun-
able basicity. The basicity of the calcined LDHx (LDOx) strongly depends on the Mg/Al ratio and the
calcination temperature. The resulting LDOx materials were used as solid base catalysts and evaluated
in the transesterification between glycerol and dimethyl carbonate without use of organic solvent. The
correlation between the basic properties of the solid catalysts and the catalytic performance was inves-
tigated. The activity of the LDOx catalysts was demonstrated to be proportional to the surface density of
basic sites. LDO2 calcined at 600 ◦C exhibited maximum activity for the transesterification reaction. The
beneficial effect of the optimum ratio of Mg/Al = 2 is related to its high total basicity. The LDO2 catalyst
can be readily recycled while maintaining high catalytic activity and selectivity of glycerol carbonate.
© 2013 Elsevier B.V. All rights reserved.
Received 20 April 2013
Received in revised form 23 June 2013
Accepted 3 July 2013
Available online 19 July 2013
Keywords:
Heterogeneous catalysis
Transesterification
Hydrotalcite
Layered double hydroxides
Glycerol carbonate
1. Introduction
of glycerol with alkyl carbonates. Although carbonylation with
urea has been described by using homogeneous inorganic salts,
Since the demand for biomass-derived biofuels is increasing
globally, the utilization of surplus byproduct glycerol as a platform
chemical represents a promising opportunity to obtain value-
added products from renewable raw materials [1–3]. Glycerol
carbonate (GLC) is one of the glycerol derivatives that attracts sig-
nificant scientific and industrial interest due to its low toxicity, low
flammability, biodegradability and high boiling point [4,5]. GLC has
found applications as a high boiling polar solvent, an intermedi-
ate in organic synthesis and in the synthesis of polycarbonates,
polyurethanes, glycidol-based polymers and surfactants [5]. The
direct carbonylation of glycerol with CO2 to GLC, involving two
abundant waste streams, is the most desirable route to replace the
traditional reaction of glycerol with harmful phosgene. Attempts
have been made to use homogeneous tin complexes, however, the
unfavorable thermodynamic equilibrium and low yields limit the
applicability of such processes [6,7]. The direct reaction of glyc-
erol with supercritical CO2 was also attempted using zeolites as
catalysts [8], but there is no evidence that direct CO2 insertion
occurred as GLC was only produced when another organic car-
bonate was added as a reactant. The alternative routes include
the carbonylation of glycerol with urea and the transesterification
solid oxides and supported nanoparticles as catalysts, this reaction
must be performed at high temperature (>120 ◦C) and low pressure
(30–50 mbar) or under dry inert gas flow to shift the equilibrium
toward GLC formation by isolating produced ammonia continu-
ously from the gaseous phase [9–12]. A more attractive route for
industrial application would be transesterification between glyc-
erol and alkyl carbonates, such as dimethyl carbonate (DMC) and
diethyl carbonate (DEC), because of the milder and greener pro-
cess conditions [4]. Base catalysis seems more promising than acid
catalysis for this transesterification reaction, because higher con-
version can be achieved by using basic catalysts [13]. Regarding
base catalysis solid bases are in particular interesting due to the
fact that corrosion problem and waste formation can be avoided
and, furthermore, solid catalysts can be easily recovered and reused
[14,15].
Among the wide range of solid bases, hydrotalcite
(Mg6Al2(OH)16CO3·nH2O) type layered double hydroxides (LDH)
attract significant industrial interest [16,17] and are the subject
of many academic investigation [18]. Being basic in nature, LDHs
have been widely studied for various base-catalyzed or -assisted
reactions such as alkylation, Michael addition, Claisen-Schmidt
condensation, Knoevenagel condensation, aldol condensation,
transesterification, hydrogenation, olefin epoxidation and alcohol
oxidation [19–26]. Through controlled thermal decomposition,
LDHs are converted to mixed oxides (LDO) with high specific
surface areas, homogeneous dispersion of metal cations and
∗
Corresponding authors. Tel.: +31 40 247 2833; fax: +31 40 245 5054.
E-mail addresses: pliu503@hotmail.com, p.liu@tue.nl (P. Liu),
e.j.m.hensen@tue.nl (E.J.M. Hensen).
0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2013.07.020

P. Liu et al. / Applied Catalysis A: General 467 (2013) 124–131
125
strong Lewis basic sites [27,28]. The basic properties of the LDO
2.2. Catalyst characterization
depend on the composition of metal cations in the precursor LDH.
Interestingly, the catalytic activity of LDO can also be enhanced by
rehydration in an inert atmosphere, which results in reconstruc-
tion of the original layered structure, with Brönsted basic sites
OH− as the charge compensating anions in the interlayer [29,30].
Consequently, the facile adjustment of the type of basic sites and
surface base strength makes LDH versatile precursors for optimal
solid base catalysts for a wide range of base-catalyzed reactions.
Recently, LDH-based solid base catalysts have shown promise
for transesterification of glycerol to value-added GLC [31–38].
Uncalcined MgAl LDHs with co-existent hydromagnesite phase
were found to be active catalysts for GLC synthesis from glyc-
erol and DMC [31,32], but a larger amount of catalyst (∼50 wt%)
and a harmful solvent (N,N-dimethyl formamide) are needed for
this reaction. Medina and co-workers reported that rehydrated
MgAl LDH showed higher activity than the calcined MgAl LDO
for transesterification of glycerol with DEC [33–35], but the active
reconstructed LDH catalysts often deactivate rapidly due to the
instability of surface Brönsted basic sites. Carbon nanofiber sup-
ported MgAl LDH, however, exhibited a different activity trend with
calcined LDO showing higher activity than the rehydrated sam-
ples, although the calcined catalyst also deactivated upon reuse
and showed lower GLC selectivity (<90%) [36]. Zr-doping MgAl
LDH (Mg/Al/Zr = 3/1/1) showed optimal GLC yield when calcined
at 650 ◦C, but without evidence on catalyst stability [37]. Although
useful MgAl LDH-based catalysts were reported, to the best of our
knowledge the effects of Mg/Al atomic ratio on the basicity of cal-
cined MgxAl LDH (LDOx) and their catalytic performance in the
transesterification of glycerol with DMC to GLC remain unknown
[34]. It remains a challenging task to develop more efficient and
reusable catalysts for synthesis of GLC under mild and solvent-free
conditions.
We have recently reported on the facile adjustment of the basic-
ity of MgAl LDH by variation of the Mg/Al atomic ratio to render
optimum MgxAl LDH supported gold nanoparticle catalysts for one-
pot tandem reactions [26]. The basicity of the MgxAl LDH supports
varied with the Mg/Al ratio (x = 2, 3, 4, 5, 6), which tunes the alco-
hol dehydrogenation activity. Herein, we will show that the LDOx
samples obtained by calcination of the MgxAl LDH precursors are
efficient solid base catalysts for GLC synthesis from glycerol and
DMC with GLC selectivity up to 100% under moderate (100 ◦C)
and solvent-free conditions. Their activity can be tailored by sim-
ple adjustment of the calcination temperature. The correlations
between the textural, chemical structures and the basic properties
of the LDOx catalysts and the catalytic performance in the glyc-
erol transesterification were investigated. Moreover, the optimal
catalyst and reaction conditions were also determined.
XRD was performed on a Bruker Endeavour D4 with Cu K␣ radi-
ation (40 kV and 30 mA). The BET surface areas were recorded on a
Tristar 3000 automated gas adsorption system. The samples were
degassed at 180 ◦C for 3 h prior to analysis. The metal composi-
tion of the LDHx was determined by ICP-OES after an aliquot of
the sample was dissolved in HNO3. Magic-angle spinning (MAS)
27Al NMR spectra were recorded on a Bruker DMX500 spectrome-
ter equipped with a 4-mm MAS probe head operating at an Al NMR
frequency of 130 MHz. The 27Al chemical shift is referenced to a sat-
urated Al(NO3)3 solution. In a typical experiment sample (10 mg)
was packed in a 2.5 mm zirconia rotor. The MAS sample rotation
speed was 25 kHz. The relaxation time was 1 s and the pulse length
was 1 ␮s.
X-ray photoelectron spectroscopy (XPS) measurements were
performed using
a Thermo Scientific K-Alpha spectrometer,
equipped with a monochromatic small-spot X-ray source and a
180◦ double focusing hemispherical analyzer with a 128-channel
detector. Spectra were obtained using an aluminum anode (Al
K␣ = 1486.6 eV) operating at 72 W and a spot size of 400 ␮m. Survey
scans were measured at a constant pass energy of 200 eV and region
scans at 50 eV. The background pressure was 2 × 10−9 mbar and
during measurement 3 × 10−7 mbar Argon because of the charge
compensation dual beam source. Data analysis was performed
using CasaXPS software. The binding energy was corrected for sur-
face charging by taking the C 1s peak of contaminant carbon as a
reference at 284.5 eV.
Temperature programmed desorption of CO2 (CO2-TPD) exper-
iments were carried out for the measurement of basicity of the
catalysts. After the catalyst (50 mg) was pre-treated at 500 ◦C for
1 h under He stream (50 mL/min), it was cooled down to 90 ◦C
and CO2 (10 vol%) was introduced for adsorption at this temper-
ature for 0.5 h. After the catalyst was swept with He for 60 min to
remove the physisorbed CO2 from catalyst surface, the tempera-
ture was increased linearly with rate of 10 ◦C/min in He and the
signal of CO2 (M/e = 44) was recorded by online mass spectrometry
(quadrupole mass spectrometer, Balzers TPG-300). The amount of
CO2 was quantified by a calibration curve, which was established
by thermal decomposition of known amounts of NaHCO3.
Temperature programmed surface reactions (TPSR) of pre-
adsorbed isopropanol were carried out as follows: after the LDOx
catalyst (50 mg) was pre-treated at 500 ◦C for 1 h under He stream
(50 mL/min), it was cooled to room temperature and isopropanol
vapor was introduced for adsorption at room temperature for 0.5 h.
After the catalyst was swept with He for 60 min, the temperature
was increased linearly with rate of 10 ◦C/min in He and the sig-
nals of H2 (M/e = 2), acetone (M/e = 43 and M/e = 58), and propene
(M/e = 41) were recorded by online mass spectrometry (quadrupole
mass spectrometer, Balzers TPG-300) simultaneously.
2. Experimental
2.3. Catalytic activity measurements
2.1. Catalyst preparation
The transesterification of dimethyl carbonate and glycerol to
glycerol carbonate was carried out using a 10 mL glass tube reactor
with a reflux condenser. Typically, the reactor was charged with
5 mmol of glycerol, 15 mmol of dimethyl carbonate, 2 mmol of 1,4-
butanediol (internal standard), and 10 wt% of catalyst (46 mg). The
resulting mixture was stirred at 100 ◦C for 2 h then 5 mL of ethanol
was added to quench the reaction. The used catalyst was sepa-
rated by centrifugation, washed with ethanol and dried at 200 ◦C.
Recovered catalyst can be reused in the next run under the same
conditions. The reaction products were quantitatively analyzed by
a Shimadzu QP5050 GC-MS (Stabilwax column, 30 m × 0.32 mm,
df = 0.5 ␮m) using an internal standard technique.
A series of MgxAl LDH (x = 2, 3, 4, 5 and 6) precursors were pre-
pared by co-precipitation method as follows. Appropriate amounts
of Mg(NO3)2·6H2O (20–60 mmol) and Al(NO3)3·9H2O (10 mmol)
were dissolved in 100 mL of deionized water and added into a
three-necked glass flask (500 mL) which initially contained Na2CO3
(20 mmol) in 200 mL of deionized water. The pH was controlled
by adding a 2 M NaOH solution and kept at 10.0 0.5. Both solu-
tions were added dropwise under vigorous stirring. The slurry was
stirred overnight at room temperature, then the precipitate was
filtered and washed intensively until neutral filtrate. The solid was
dried at 110 ◦C overnight to yield the LDHx precursors, which were
further calcined at 500 ◦C in air for 6 h to yield the LDOx catalysts.

126
P. Liu et al. / Applied Catalysis A: General 467 (2013) 124–131
Fig. 2. 27Al MAS NMR spectra of the LDHx precursors.
Fig. 1. XRD patterns of the LDHx precursors.
3. Results and discussion
3.1. Catalyst preparation and characterization
To study the influence of the “coprecipitation–calcination”
process on the structural and chemical properties of the LDHx
precursors and LDOx catalysts, various physico-chemical char-
acterization techniques such as XRD, N2 physisorption, ICP-OES,
27Al MAS NMR and XPS were employed. Fig. 1 shows the XRD
patterns of the LDHx precursors. All patterns contain the character-
istic peaks corresponding to a well-defined hydrotalcite structure
[16], although the peak intensities were somewhat decreased with
the Mg/Al ratio increasing. This result indicates that by the co-
precipitation method the Mg and Al cations can be successfully
incorporated into the LDH layers in a relative wide range of atomic
ratio (Mg/Al = 2–6). The surface area, pore volume and metal com-
position of the LDHx precursors are collected in Table 1. Evidently,
an increase in the Mg/Al atomic ratio of the LDH from 2 to 6 led to a
decrease of the LDH surface area and pore volume, indicating that
lower Mg/Al ratios can result in a more open LDH structure.
Fig. 2 shows the 27Al MAS NMR spectra of the LDHx precursors.
The signals centered at ca. 9.6 ppm are attributed to six-coordinated
(octahedral or AlO6) aluminum [38]. All spectra of the LDHx precur-
sors are characterized by a single group of AlO6 signals, indicating
that Al predominantly occupies octahedral sites. This result is in
good agreement with the XRD results. Table 1 also shows the sur-
face Mg/Al atomic ratios of the LDHx precursors. Interestingly, there
are differences between the Mg/Al ratio in the bulk and at the sur-
face. XPS data suggests a surface enrichment of Al for all LDHx
samples.
Fig. 3. XRD patterns of the LDOx catalysts.
where Al3+ cation is dissolved in the lattice to form a solid solu-
tion [39]. The peak intensities increase with the Mg/Al ratio. These
mixed oxides possess higher surface area and pore volume than the
LDHx precursors (Table 2). Fig. 4 shows that the pore size distribu-
tions varied considerably among the calcined LDOx catalysts, with
average pore diameters between 9 and 17 nm. XPS analysis reveals
that LDOx catalysts have much lower surface Mg2+ concentration
than the LDHx precursors (Table 2), suggesting that calcination can
further promote the migration of Al3+ from the LDH bulk to the
surface of the mixed oxides. Fig. 5 presents the O 1s XP spectra for
these LDOx catalysts. Three contributions are distinguished after
deconvolution, i.e. at 532.9, 531.5 and 529.8 eV. The higher binding
energy (BE) peak at ∼533 eV is assigned to surface adsorbed H2O.
The BE peak at 531.5 eV is attributed to the surface metal-hydroxide
After calcination at 500 ◦C, LDOx catalysts were obtained. XRD
patterns (Fig. 3) indicate that all catalysts have a MgO-like structure
Table 1
Textural and chemical properties of LDHx precursor.
Table 2
SBET (m2/g)
Pore volume
(cm3/g)
Mg/Al atomic ratio
Textural and chemical properties of the LDOx catalysts.
Sample
Sample
SBET (m2/g)
Pore volume
(cm3/g)
Pore diameter
(nm)a
Mg/Al molar
ratiob
in bulka
in surfaceb
LDH2
LDH3
LDH4
LDH5
LDH6
103
0.41
0.52
0.22
0.20
0.02
1.99:1.00
2.95:1.00
3.69:1.00
4.60:1.00
5.66:1.00
1.89:1.00
2.72:1.00
3.52:1.00
4.31:1.00
5.03:1.00
LDO2
LDO3
LDO4
LDO5
LDO6
219
197
178
158
155
0.52
0.53
0.64
0.65
0.50
9.5
10.8
14.4
16.5
12.9
1.66:1.00
2.46:1.00
2.89:1.00
3.63:1.00
3.46:1.00
97
62
53
16
a
a
Determined by ICP-OES.
Evaluated by XPS.
Cylindrical pore diameter (PD); PD = 4000 × pore volume/SBET
.
b
b
Evaluated by XPS.

P. Liu et al. / Applied Catalysis A: General 467 (2013) 124–131
127
Fig. 4. Pore size distribution of the LDOx catalysts from N2 desorption branch using
BJH method.
species. The lower BE peak at 529.8 eV is characteristic for surface
O2− species [40]. The significant increase in O2− character is con-
sistent with the presence of strong Lewis basic sites in the LDOx
catalysts.
Fig. 6A shows a representative profile of CO2 release from LDO2
during thermal treatment. Since the LDO2 catalyst was prepared
by calcination of LDH2 at 500 ◦C for 6 h, decomposition of the LDH
should be essentially complete [39,41]. However, calcined LDO can
absorb water and CO2 during sample exposure in air, as also evident
from the XPSresults. Therefore, theLDOx catalysts werepre-treated
under helium at 500 ◦C for 1 h before CO2 adsorption. By CO2-TPD
measurements (Fig. 6B), CO2 adsorption volumes and surface basic
site densities can be evaluated (Table 3). The range of CO2 adsorp-
tion volumes is between 0.27 and 0.51 mmol/g, with LDO5 and
LDO2 showing the lowest and the highest value, respectively. These
CO2 adsorption volumes are similar to that of previously reported
MgAl mixed oxides [27,42]. The surface basic site densities increase
in the order LDO5 < LDO4 < LDO3 < LDO6 < LDO2. The range of the
density of surface basic site is between 1.7 and 2.3 ␮mol/m2, in good
agreement with previously reported values for calcined hydrotal-
cites [27].
Fig. 6. (A) Release of CO2 from the LDO2 catalyst during calcination in flowing He
with a heating rate 5 ◦C/min. (B) CO2-TPD profiles of the LDOx catalysts.
The observed CO2 desorption peaks can be used to interpret
the relationship between the base strength and the calcination
temperature. Higher desorption temperature and higher adsorp-
tion volume point to higher base strength. In all cases, a broad
desorption band is observed between 100 and 500 ◦C (Fig. 6B),
which can be deconvoluted into three main peaks at about 170 ◦C
(weak strength), 230 ◦C (medium strength) and 300–350 ◦C (high
strength). The medium temperature peak is more intense (38–48%
of total area, Table 3), except for LDO4. These medium-strength
sites are mainly associated with Mg–O pairs [42]. The low temper-
ature peak can be ascribed to adsorption at surface Brönsted basic
site (OH− groups). The high temperature peak is attributed to the
strong Lewis basic sites of O2− anions. Apparently, the base strength
distribution of the LDOx catalysts varied as a function of the Mg/Al
ratio, with LDO2 showing more and stronger high-strength basic
sites (27%, 350 ◦C) than the other LDOx containing higher Mg con-
tent.
To obtain more insights into the catalyst structure-basicity cor-
relations, we compared the TPSR of pre-adsorbed isopropanol on
LDOx samples (Fig. 7). It can be clearly seen that the dehydro-
genation activity, being related to the catalyst basicity, strongly
depends on the Mg/Al ratio (Fig. 7A). The LDO2 catalyst showed sig-
nificantly higher activity of dehydrogenation at lower temperature
(100–200 ◦C), consisting with the presence of stronger basic sites in
the LDO2 sample as indicated by CO2-TPD at higher temperature. It
is worthwhile noting that propene evolution (Fig. 7B) was similar to
acetone evolution (Fig. 7C), although the former showed relatively
weaker intensity. This indicates that besides the dehydrogenation
occurring over various basic sites, the dehydration can also occur
on the acidic Al3+ sites, consistent with previous results [27,43].
Fig. 5. O 1s XP spectra for the LDH2 precursor and the LDOx catalysts.

128
P. Liu et al. / Applied Catalysis A: General 467 (2013) 124–131
Table 3
Basic properties of the LDOx catalysts.a
Sample
CO2 desorption peaks (area%)
Total CO2 adsorption
(mmol/g)
Basic site density
(␮mol/m2)
∼170 ◦
C
∼230 ◦
C
>300 ◦
C
LDO2
LDO3
LDO4
LDO5
LDO6
28
36
45
33
40
45
48
38
45
41
27
16
17
22
19
0.51
0.38
0.32
0.27
0.33
2.3
1.9
1.8
1.7
2.1
a
Determined by adsorption and TPD of CO2.
The foregoing characterization results demonstrate that the
tunability of Mg/Al atomic ratio can be used to fine-tune the struc-
tural, textural and chemical properties of the calcined LDHs. The
“coprecipitation–calcination” process yields LDOx catalysts with
high surface area and pore volume. The Mg/Al ratio also has an influ-
ence on the surface basic site density and alcohol dehydrogenation
activity, thus potentially affecting the catalytic performance in glyc-
erol transesterification.
3.2. Catalytic performance
3.2.1. Effect of Mg/Al ratio
The transesterification of glycerol with dimethyl carbonate was
chosen as the model reaction to evaluate the catalytic performance
of the various LDOx catalysts (Scheme 1). Diethyl carbonate was
not chosen as the substrate mainly due to higher reaction temper-
atures required for its conversion, thus resulting in lower glycerol
carbonate selectivity [33–36]. The reaction was initially performed
with 10 wt% of LDOx and 3 equiv. of DMC at 100 ◦C under organic
solvent-free conditions.
As shown in Fig. 8, the GLC yield after 2 h reaction differed signif-
icantly among the LDOx catalysts. LDO5 gave the lowest yield of GLC
(42%), whereas LDO2 achieved the highest yield (56%). The catalytic
activity increased in the order LDO5 < LDO4 < LDO3 < LDO6 < LDO2,
which is in good agreement with the trend of surface basic site
density. The higher activity of LDO2 is also consistent with pre-
viously reported results, which showed similar activity trend, i.e.
LDO2 > LDO3 > LDO4 [34]. It is noteworthy that all LDOx catalysts
gave 100% GLC selectivity. The GLC yield was negligible in the
absence of catalyst.
OH
OH
O
O
10 wt% LDOx
100 oC
+
OH
O
O
+ 2CH3OH
O
O
OH
Scheme 1. Synthesis of GLC by transesterification of glycerol with DMC.
Fig. 8. Catalytic performance of the LDOx catalysts for the synthesis of glycerol
carbonate. Reaction conditions: 5 mmol glycerol, 10 wt% of LDOx catalyst, 15 mmol
DMC, 2 mmol 1,4-butanediol, 100 ◦C, 2 h.
Fig. 7. Evolution of H2 (A), propene (B) and acetone (C) during TPSR of pre-adsorbed
isopropanol for the LDOx catalysts.

P. Liu et al. / Applied Catalysis A: General 467 (2013) 124–131
129
Fig. 9. Effect of calcination temperature of the LDOx catalysts on the catalytic activ-
ity of glycerol transesterification with DMC. Reaction conditions: 5 mmol glycerol,
10 wt% of LDOx catalyst, 15 mmol DMC, 2 mmol 1,4-butanediol, 100 ◦C, 2 h.
Fig. 10. CO2-TPD profiles of the LDO2 samples calcined at different temperatures.
Clearly, the good correlation between the catalytic activity and
the surface basic site density of LDOx catalysts points to the impor-
tance of catalyst basicity in the transesterification reaction. It is
well known that the basicity of calcined LDH depends not only
on the metal composition but also on the calcination temperature
[37,40,42]. Therefore, it is possible to further improve the basicity
and catalytic activity of LDOx catalysts by varying the calcination
temperature.
3.2.2. Effect of catalyst calcination temperature
To obtain optimum transesterification activity, various LDOx
catalysts were calcined at different temperatures (300–700 ◦C)
before their use in the reaction. The results are presented in Fig. 9.
Evidently, each LDOx catalyst has its own optimum calcination tem-
perature. LDO3 showed optimal activity when calcined at 400 ◦C.
LDO4, LDO5 and LDO6 achieved maximum GLC yield after calcined
at 500 ◦C. While 600 ◦C is the optimum calcination temperature for
LDO2 catalyst, the other LDOx catalysts show decreased activity
after being calcined at 600 ◦C. Clearly, calcination at 700 ◦C led to
substantial decrease of the activity for all catalysts. In all cases, GLC
selectivity is 100% without any byproducts. Among various LDOx
catalysts and various calcination temperatures, LDO2 calcined at
600 ◦C exhibited the highest catalytic activity.
It can be speculated that the activity for catalysts calcined at
relatively low temperature (300 ◦C) is mainly related to weak OH−
sites formed by the decomposition of surface bicarbonate species.
Since calcination at 400 ◦C will only lead to partial decomposition,
the activity is attributed to both weakly basic OH− sites and the
medium-strength Mg–O pair sites. After complete decomposition
at higher temperature (500–700 ◦C), the activity can be ascribed to
three kinds of basic sites, i.e. weak OH− sites, medium Mg–O pair
sites and strong O2− sites. For the activity decrease of LDOx catalysts
at high calcination temperature, it is reasonable to state that the
Lewis acidic nature of Al3+ cations may suppress basicity. This spec-
ulation is supported by the fact that the surface Al3+ concentration
increased with calcination temperature, and the CO2 adsorption
volume was lowered for LDO2 upon calcination at 700 ◦C (Fig. 10).
In the case of the optimum LDO2 catalyst, the basicity is signifi-
cantly improved, probably due to the presence of a large amount of
weak-strength OH− sites and medium-strength Mg–O basic sites
and lack of Lewis acidic sites of Al3+ when calcined at 600 ◦C.
3.2.3. Influence of reaction conditions
The reaction conditions were further optimized for the LDO2
(@600 ◦C) catalyst. Fig. 11A shows the effect of reaction temper-
ature. The GLC yield was increased with reaction temperature up
to 100 ◦C and, thereafter, there was a significant decrease in yield.
Fig. 11. Effect of reaction temperature (A), catalyst amount (B) and reaction time
(C) on catalytic performance of LDO2 (@600 ◦C) catalyst. Typical reaction conditions:
5 mmol glycerol, 10 wt% of LDO2, 15 mmol DMC, 2 mmol 1,4-butanediol, 100 ◦C, 2 h.

130
P. Liu et al. / Applied Catalysis A: General 467 (2013) 124–131
Fig. 12. Reusability of LDO2 (@600 ◦C) catalyst. Reaction conditions: 5 mmol glyc-
erol, 15 mmol DMC, 2 mmol 1,4-butanediol, 100 ◦C, 2 h; for the first cycle, 10 wt% of
LDO2 catalyst was added.
The LDO2 catalyst showed maximum activity at a reaction temper-
ature of 100 ◦C. The decreased GLC yield (with 100% selectivity) at
high temperature suggests that side-reactions (dehydrogenation
and condensation) by the byproduct methanol may occur on the
basic sites. Fig. 11B presents the influence of the catalyst amount
on transesterification reaction. Surprisingly, the GLC yield was not
increased when the amount of catalyst was increased. Compared
to 10 wt% of catalyst, the activity was even lower when 20 wt%
of catalyst was used. When 15 wt% of catalyst was used the GLC
yield was similar. This is mainly due to the increased resistance of
mass transfer in the solid catalyst-glycerol-DMC triphase system,
because catalyst aggregation was clearly observed when catalyst
amount increased to more than 10 wt%.
The optimum reaction temperature and catalyst amount were
determined to be 100 ◦C and 10 wt%, respectively. Under these con-
ditions, the effect of reaction time was also investigated and the
results are shown in Fig. 11C. Clearly, the transesterification reac-
tion has no apparent induction period. The reaction achieved 96%
conversion of glycerol after 5 h but with a slight decrease in GLC
selectivity (97%). Glycerol dicarbonate was identified to be the only
by-product, which is generally reported as the main by-product for
GLC synthesis by transesterification of glycerol with alkyl carbo-
nates and the selectivity can reach up to 30–80% by other calcined
or rehydrated LDH catalysts [33–36]. The decrease in GLC selectiv-
ity is indicative of some deactivation of the LDO2 catalyst and could
be attributed to a strong adsorption of side products on the basic
sites during the reaction.
Fig. 13. XRD patterns (A) and XP spectra (B) of the fresh (a) and fifth recycled (b)
LDO2 (@600 ◦C) catalyst.
the reaction. From the structural information of the recycled LDO2
catalyst, the reconstruction of the layered LDH2 structure by the
“memory effect” can be excluded. This is mainly due to the anhy-
drous conditions we applied during reactions and catalyst recovery
processes. Significantly, the optimum LDO2 catalyst showed higher
stability than previously reported rehydrated LDH catalysts in the
transesterification of glycerol to GLC [33,34].
3.2.5. Activity comparison of the LDO2 catalyst with other
reference catalysts
Since most previous studies use quite different reaction condi-
tions, it is difficult to directly compare the activity of our LDO2
catalyst with previously reported base catalysts. Therefore, we
prepared and compared several representative reference catalysts
under our reaction conditions and the results are shown in Table 4.
Evidently, the LDH2 precursor showed much lower activity than
LDO2 catalyst, mainly due to the presence of only weak-strength
basic sites in the uncalcined sample. LDO2 outperforms MgO under
identical conditions, further confirming the beneficial effect of the
optimum ratio of Mg/Al = 2 on catalyst basicity and activity. CaO and
3.2.4. Stability of LDO2 catalyst
In order to determine stability of the LDO2 catalyst, the spent
catalyst was recycled in the glycerol transesterification reaction.
The results are presented in Fig. 12. The solid catalyst could be
readily separated from the reaction mixture by centrifugation
and used in next cycle after washing and drying. To our delight,
LDO2 could be recycled and reused at least five times without
significant loss of activity and selectivity, and the GLC selectivity
retained 100%, even after the repetitive use. Subsequently, the fifth
recycled catalyst was tested by XRD and XPS. XRD results con-
firm retention of the original MgO-like structure (Fig. 13A). XPS
evidences a different surface metal composition for the recycled
catalyst with Mg/Al = 1.58:1.00. Compared to the fresh catalyst
(Mg/Al = 1.63:1.00), an increase in surface Al concentration was
observed, indicating that more Al3+ in the bulk phase gradually
transferred to the surface. Fig. 13B shows the O 1s XP spectra for
the fresh and fifth recycled LDO2 catalyst. The decrease in O2−
character (529.8 eV) for the reused catalyst supports the specula-
tion that side products may adsorb on the strong basic sites during
Table 4
Comparison of the catalytic activity of LDO2 with other reference catalysts for the
synthesis of glycerol carbonate.a
Catalyst
Time (h)
GLC yield (%)
GLC selectivity (%)
LDO2b
LDH2
MgOb
CaOc
2
2
2
1
1
66
24
50
79
70
100
100
100
81
NaOHd
70
a
Reaction conditions: 5 mmol glycerol, 15 mmol DMC, 2 mmol 1,4-butanediol,
and 10 wt% of catalyst, 100 ◦C.
b
Calcined at 600 ◦C.
c
Calcined at 900 ◦C.
d
2 wt% of catalyst was used.

P. Liu et al. / Applied Catalysis A: General 467 (2013) 124–131
131
NaOH exhibited much higher activity but with significantly lower
GLC selectivity, in agreement with previously reported results
[13,44]. Moreover, CaO cannot be reused unless regenerated at
high temperature (900 ◦C) [13]. Based on the above results, we can
conclude that the LDO2 calcined at 600 ◦C is a stable and practical
catalyst for glycerol carbonate synthesis by glycerol transesterifi-
cation with dimethyl carbonate under solvent-free conditions.
[7] J. George, Y. Patel, S.M. Pillai, P. Munshi, J. Mol. Catal. A: Chem. 304 (2009) 1–7.
[8] C. Vieville, J.W. Yoo, S. Pelet, Z. Mouloungui, Catal. Lett. 56 (1998) 245–247.
[9] M. Aresta, A. Dibenedetto, F. Nocito, C. Ferragina, J. Catal. 268 (2009) 106–114.
[10] M.J. Climent, A. Corma, P.D. Frutos, S. Iborra, M. Noy, A. Velty, P. Concepción, J.
Catal. 269 (2010) 140–149.
[11] C. Hammond, J.A. Lopez-Sanchez, M.H.A. Rahim, N. Dimitratos, R.L. Jenkins, A.F.
Carley, Q. He, C.J. Kiely, D.W. Knight, G.J. Hutchings, Dalton Trans. 40 (2011)
3927–3937.
[12] S.-i. Fujita, Y. Yamanishi, M. Arai, J. Catal. 297 (2013) 137–141.
[13] J.R. Ochoa-Gómez, O. Gómez-Jiménez-Aberasturi, B. Maestro-Madurga, A.
Pesquera-Rodríguez, C. Ramírez-López, L. Lorenzo-Ibarreta, J. Torrecilla-Soria,
M.C. Villarán-Velasco, Appl. Catal. A: Gen. 366 (2009) 315–324.
[14] H. Hattori, Chem. Rev. 95 (1995) 537–558.
4. Conclusions
[15] G. Busca, Chem. Rev. 110 (2010) 2217–2249.
[16] F. Cavani, F. Trifiró, A. Vaccari, Catal. Today 11 (1991) 173–301.
[17] D. Tichit, B. Coq, Cattech 7 (2003) 206–217.
[18] D.P. Debecker, E.M. Gaigneaux, G. Busca, Chem. Eur. J. 15 (2009) 3920–3935.
[19] B.F. Sels, D.E. De Vos, P.A. Jacobs, Catal. Rev. Sci. Eng. 43 (2001) 443–488.
[20] F. Figueras, Top. Catal. 29 (2004) 189–196.
[21] D. Tichit, C. Gérardin, R. Durand, B. Coq, Top. Catal. 39 (2006) 89–96.
[22] Z.P. Xu, J. Zhang, M.O. Adebajo, H. Zhang, C. Zhou, Appl. Clay Sci. 53 (2011)
139–150.
[23] P. Liu, H. Wang, Z. Feng, P. Ying, C. Li, J. Catal. 256 (2008) 345–348.
[24] P. Liu, C. Wang, C. Li, J. Catal. 262 (2009) 159–168.
[25] P. Liu, Y. Guan, R.A. van Santen, C. Li, E.J.M. Hensen, Chem. Commun. 47 (2011)
11540–11542.
[26] P. Liu, C. Li, E.J.M. Hensen, Chem. Eur. J. 18 (2012) 12122–12129.
[27] A.L. Mckenzie, C.T. Fishel, R.J. Davis, J. Catal. 138 (1992) 547–561.
[28] A. Corma, V. Fornés, F. Rey, J. Catal. 148 (1994) 205–212.
[29] J.C.A.A. Roelofs, D.J. Lensveld, A.J. van Dillen, K.P. de Jong, J. Catal. 203 (2001)
184–191.
[30] F. Winter, X. Xia, B.P.C. Hereijgers, J.H. Bitter, A.J. van Dillen, M. Muhler, K.P. de
Jong, J. Phys. Chem. B 110 (2006) 9211–9218.
[31] A. Takagaki, K. Iwatani, S. Nishimura, K. Ebitani, Green Chem. 12 (2010)
578–581.
In summary, we have shown that the Mg/Al atomic ratio of
hydrotalcite-like LDHs strongly affects the basicity and catalytic
activity of calcined LDOx mixed oxides for the transesterification of
glycerol with dimethyl carbonate to glycerol carbonate. The basic-
ity of LDOx can be fine-tuned by simple adjustment of the Mg/Al
ratio and the calcination temperature. The catalytic performance of
the LDOx catalysts depends on the Mg/Al ratio, the surface basicity
and the reaction conditions. A good correlation between catalytic
activity and surface basic site density of the LDOx catalysts has
been established. LDO2 catalyst calcined at 600 ◦C shows maximum
activity, which has been attributed to the beneficial effect of the
optimum ratio of Mg/Al = 2 on enhancement of catalyst basicity.
The optimum LDO2 catalyst can be readily recovered and reused
with no significant loss of performance. These findings provide
insights into the structure-basicity-activity relationship of the cal-
cined hydrotalcite catalysts, and make it possible to design more
efficient solid base catalysts for various base-catalyzed reactions.
[32] A. Kumar, K. Iwatani, S. Nishimura, A. Takagaki, K. Ebitani, Catal. Today 185
(2012) 241–246.
[33] M.G. Álvarez, A.M. Segarra, S. Contreras, J.E. Sueiras, F. Medina, F. Figueras,
Chem. Eng. J. 161 (2010) 340–345.
Acknowledgments
[34] M.G. Álvarez, R.J. Chimentao, F. Figueras, F. Medina, Appl. Clay Sci. 58 (2012)
16–24.
[35] M.G. Álvarez, M. Pliskova, A.M. Segarra, F. Medina, F. Figueras, Appl. Catal. B:
Environ. 113–114 (2012) 212–220.
[36] M.G. Álvarez, A.M. Frey, J.H. Bitter, A.M. Segarra, K.P. de Jong, F. Medina, Appl.
Catal. B: Environ. 134–135 (2013) 231–237.
[37] M. Malyaadri, K. Jagadeeswaraiah, P.S.S. Prasad, N. Lingaiah, Appl. Catal. A: Gen.
401 (2011) 153–157.
This research was financially supported by Programme Strategic
scientific Alliances between the Netherlands and China (Grant No.
2008DFB5-130).
References
[38] A. Vyalikh, F.R. Costa, U. Wangenknecht, G. Heinrich, D. Massiot, U. Scheler, J.
Phys. Chem. C 113 (2009) 21308–21313.
[1] M. Pagliaro, R. Ciriminna, H. Kimura, M. Rossi, C.D. Pina, Angew. Chem. Int. Ed.
46 (2007) 4434–4440.
[39] F. Millange, R.I. Walton, D. O’Hare, J. Mater. Chem. 10 (2000) 1713–1720.
[40] D.G. Cantrell, L.J. Gillie, A.F. Lee, K. Wilson, Appl. Catal. A: Gen. 287 (2005)
183–190.
[41] P. Liu, M. Derchi, E.J.M. Hensen, Appl. Catal. B: Environ. (2013), http://dx.doi.
org/10.1016/j.apcatb.2013.07.010.
[42] J.I.D. Cosimo, V.K. Díez, M. Xu, E. Iglesia, C.R. Apesteguía, J. Catal. 178 (1998)
499–510.
[43] C.T. Fishel, R.J. Davis, Langmuir 10 (1994) 159–165.
[44] R. Bai, Y. Wang, S. Wang, F. Mei, T. Li, G. Li, Fuel Process. Technol. 106 (2013)
209–214.
[2] C.-H. Zhou, J.N. Beltramini, Y.-X. Fan, G.Q. Lu, Chem. Soc. Rev. 37 (2008)
527–549.
[3] A. Behr, J. Eilting, K. Irawadi, J. Leschinski, F. Lindner, Green Chem. 10 (2008)
13–30.
[4] J.R. Ochoa-Gómez, O. Gómez-Jiménez-Aberasturi, C. Ramírez-López, M. Belsué,
Org. Process Res. Dev. 16 (2012) 389–399.
[5] M.O. Sonnati, S. Amigoni, E.P.T. de Givenchy, T. Darmanin, O. Choulet, F. Guit-
tard, Green Chem. 15 (2013) 283–306.
[6] M. Aresta, A. Dibenedetto, F. Nocito, C. Pastore, J. Mol. Catal. A: Chem. 257 (2006)
149–153.