Synthesis of dipropyl carbonate over calcined hydrotalcite-like compounds containing la

Article abstract of DOI:10.1016/j.apcata.2013.05.026

Dipropyl carbonate (DPC) was selectively synthesized via transesterification of dimethyl carbonate (DMC) and n-propanol over Mg-Al composite oxide containing La catalysts. The catalysts were prepared by calcining the precursors of hydrotalcite-like compounds (HTLcs) from co-precipitation method. The effect of La content (n_Mg:n _Al:n_La = 3:1:x) in the catalyst on the synthesis of DPC was investigated. And the catalyst exhibited the highest catalytic activity when x was tuned to 0.7. Under the optimized reaction conditions, the DMC conversion and DPC selectivity were 98.4% and 95.4%, respectively. These catalysts were characterized by thermogravimetry differential thermal analysis (TG-DTA), X-ray diffraction (XRD), nitrogen adsorption-desorption, Fourier transform infrared (FT-IR), temperature programmed desorption with CO₂ (CO ₂-TPD) and scanning electron micrograph (SEM). It was clarified that the amount of basic sites of moderate strength, formed by chelating bidentate carbonate, was enhanced with the La content increasing until x = 0.7, which were the catalytically-active sites for this reaction. But when La content x reached 1.0, these sites were transformed to bridging bidentate carbonate, leading to a loss of the catalytic activity to some extent. These findings indicated that La content controlled the catalytic performance of the composite oxides catalysts via structure change of the basic sites. The catalyst was easily prepared and handled, easily separated from the reaction medium and could be reused many cycles.

Full text of DOI:10.1016/j.apcata.2013.05.026

Applied Catalysis A: General 464–465 (2013) 142–148
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Synthesis of dipropyl carbonate over calcined hydrotalcite-like
compounds containing La
Qingxiang Maa,b, Tiansheng Zhaob,∗, Ding Wanga, Wenqi Niua,
Peng Lva, Noritatsu Tsubakia,∗∗
a Department of Applied Chemistry, School of Engineering, University of Toyama, Gofuku 3190, Toyama 930-8555, Japan
b State Key Laboratory Cultivation Base of Natural Gas Conversion, Ningxia University, Yinchuan 750021, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Dipropyl carbonate (DPC) was selectively synthesized via transesterification of dimethyl carbonate (DMC)
and n-propanol over Mg–Al composite oxide containing La catalysts. The catalysts were prepared by cal-
cining the precursors of hydrotalcite-like compounds (HTLcs) from co-precipitation method. The effect of
La content (nMg:nAl:nLa = 3:1:x) in the catalyst on the synthesis of DPC was investigated. And the catalyst
exhibited the highest catalytic activity when x was tuned to 0.7. Under the optimized reaction conditions,
the DMC conversion and DPC selectivity were 98.4% and 95.4%, respectively. These catalysts were char-
acterized by thermogravimetry differential thermal analysis (TG-DTA), X-ray diffraction (XRD), nitrogen
adsorption–desorption, Fourier transform infrared (FT-IR), temperature programmed desorption with
CO2 (CO2-TPD) and scanning electron micrograph (SEM). It was clarified that the amount of basic sites of
moderate strength, formed by chelating bidentate carbonate, was enhanced with the La content increas-
ing until x = 0.7, which were the catalytically-active sites for this reaction. But when La content x reached
1.0, these sites were transformed to bridging bidentate carbonate, leading to a loss of the catalytic activ-
ity to some extent. These findings indicated that La content controlled the catalytic performance of the
composite oxides catalysts via structure change of the basic sites. The catalyst was easily prepared and
handled, easily separated from the reaction medium and could be reused many cycles.
Received 22 March 2013
Received in revised form 13 May 2013
Accepted 19 May 2013
Available online 6 June 2013
Keywords:
Dipropyl carbonate
Transesterification
Hydrotalcite-like compounds
Composite oxide
DMC
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
dimethylsulfate and generating the by-products hydrochloric acid.
It would make serious corrosion of reaction equipment and envi-
Dipropyl carbonate (DPC), containing both propoxy and car-
bonyl groups, is an important synthesis intermediate as a useful
propylating and propoxycarbonylating agent for synthesis of many
organic compounds. It is also widely used as electrolyte in lithium
ion batteries for its high dielectric constants [1]. Compared to
dimethyl carbonate (DMC) and diethyl carbonate (DEC), DPC
has larger molecular weight, higher flash point and stronger
antioxidation than that of DMC and DEC, it can be used as
a more stable and safer electrolyte for lithium ion batteries
in extreme environment such as high temperature and oxygen
environment [2].
ronmental pollution. Meanwhile, the conversion of reactants is very
low [3,4]. Therefore, a new approach of synthesis of DPC becomes
more attractive to researchers. Recent years, DMC has been paid
extensive attention as an important green chemical material which
is nontoxic and easily biodegradable; it can be used as esterifica-
tion and transesterification reagent instead of dimethylsulfate and
phosgene in carbonylation process. Both symmetric and asymmet-
ric organic carbonates can be synthesized via transesterification
reactions using DMC as a raw material [5–12]. In general, trans-
esterification reactions are carried out in the liquid phase using
homogeneous catalysts such as bases, organic Mo, Sn or Ti com-
pounds, and so on [13–16]. However, this process has an obvious
disadvantage due to the difficulty of separation. Therefore, develop-
ment of active solid catalysts is desirable. As far as we know, there
are only few researches reported on the DPC synthesis by transes-
terification with DMC and n-propanol. Li et al. investigated the DPC
synthesis from DMC and propanol catalyzed over KNO3/MCM-48
by transesterification. Although the activity was high, the prepa-
ration of the catalysts was very complicated while mesoporous
MCM-48 was not stable [17]. Chen et al. studied the synthesis of
Traditionally, DPC was synthesized by the reaction of phosgene
or dimethylsulfate with propanol. But this synthesis method has
many disadvantages such as reactant containing toxic phosgene,
∗
Corresponding author. Tel.: +86 951 206 2237; fax: +86 951 206 2237.
Corresponding author. Tel.: +81 76 445 6846; fax: +81 76 445 6846.
E-mail addresses: zhaots@nxu.edu.cn (T. Zhao), tsubaki@eng.u-toyama.ac.jp
∗∗
(N. Tsubaki).
0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2013.05.026

Q. Ma et al. / Applied Catalysis A: General 464–465 (2013) 142–148
143
methyl propyl carbonate (MPC) over supported TiO2/Al2O3 via gas-
phase transesterification [18], but DPC was the co-product with
low selectivity in the process. In their reports, base sites were
responsible for the catalytic activity during the transesterificati
on reaction.
Hydrotalcite (HT) or hydrotalcite-like compounds (HTLcs) are
layered double hydroxides belonging to anionic clays. The general
formula of these compounds can be represented as:
were measured at a Rigaku D/MAX2200PC X-ray differactometer
using Cu K␣ radiation. Thermogravimetric analysis (TGA) system
was performed with DTG-60 instrument under air atmosphere
(50 ml/min) with a heating rate of 10 ◦C/min from room temper-
ature to 800 ◦C to estimate thermal decomposition behavior of
catalyst. The basicity of catalysts was studied by the temperature
programmed desorption (TPD) with CO2. The TPD of CO2 was car-
ried out between 50 and 700 ◦C under a helium flow (30 ml/min)
with a heating rate of 10 ◦C/min. Before the test, the samples
were pretreated under helium atmosphere at 500 ◦C for 1 h, then,
cooled to 50 ◦C, and exposed to pure CO2 (30 ml/min) for 0.5 h.
The Fourier transform infrared spectroscopy (FT-IR) spectra were
recorded using the KBr pellet technique on a FT-IR spectrometer in
the 4000–400 cm−1 range. SEM images were obtained at an ampli-
fying time of 5000 on a JEOL JSM-6360LV instrument.
[M(II)1−xM(III)x(OH)2] · An− · mH2O
x/n
Their layer structure is very similar to that of brucite, where part
of Mg2+ cations or divalent cations represented as [M(II)] could
be isomorphously substituted replaced by Al3+ or other trivalent
metal cations represented as [M(III)], forming positively charged
layers. The excess positive charge of brucite-like sheets is compen-
sated by the inter-layer anions (An−) such as CO3
or OH−. The
2−
2.3. Catalytic activity tests
advantage of this kind of HTLcs is that the basic properties can be
easily controlled by the M3+/(M2+ + M3+) ratio, and the stoichiomet-
ric coefficient (x) may be assorted over a wide range (0.25 < x < 0.44)
[19,20]. In addition, the thermal decomposition of these com-
pounds at above 450 ◦C results in the formation of basic compound
oxides with high surface areas, which show high catalytic activ-
ity in many reactions such as aldol condensation, Michael addition,
transesterification and so on [21–26].
In this study, we prepared Mg–Al composite oxides containing
La from hydrotalcite-like precursor via co-precipitation method.
The physicochemical properties of the catalysts were studied by
XRD, BET, TG-DTA, FT-IR, CO2-TPD and SEM. The catalytic perfor-
mance of catalysts for the DPC synthesis via the transesterification
of DMC with propanol was investigated. The reaction can usually
be completed via the following two steps:
The catalytic activities of the composite oxides xLHTs for the
DPC synthesis via transesterification were evaluated in a one-neck
round bottom flask with a rotary evaporator. The freshly prepared
powder catalysts were placed in the flask along with liquid reactant
of DMC/n-propanol with an initials molar ratio of 1:3. The reac-
tion was conducted at 90 2 ◦C at an atmospheric pressure. The
reaction time was 6 h. The dosage of the catalyst was defined as
weight percentage of the reactants. The co-product of methanol
was evaporated and separated from reaction system during the
reaction.
The products were analyzed on a SP-2100 gas chromatography
with a flame ionization detector (FID) and a separation column
packed with polyglycol 20 M. The column was temperature-
programmed from 100 ◦C to 150 ◦C.
CH3OCOOCH3 + C3H7OH → C3H7OCOOCH3 + CH3OH
C3H7OCOOCH3 + C3H7OH → C3H7OCOOC3H7 + CH3OH
(1)
(2)
3. Results and discussion
3.1. Catalyst characterization
2. Experimental
3.1.1. TG-DTA analysis
The thermal decomposition behavior of La-HTLcs is studied. The
TG-DTA curves of samples are shown in Fig. 1. The TG profile indi-
cates two major weight losses at around 150 ◦C and 400 ◦C. The first
weight loss is due to the elimination of physical adsorbed water
and CO2, interlayer water molecules, along with small amounts
of weakly bound OH groups, and the weight loss is about 10%.
This weight loss is associated with an endothermic peak around
200 ◦C with an endothermic shoulder peak around 85 ◦C, cor-
responding to the dehydroxylation of Al OH and the physical
adsorbed molecules, respectively [27]. The second weight loss is
corresponded to dehydroxylation of samples. This weight loss is
accompanied by an endothermic band for 0La-HTLcs and 0.1 La-
HTLcs. This endothermic band is due to the dehydroxylation of
Mg OH. However, there are two endothermic bands for other sam-
ples. The first endothermic band belongs to dehydroxylation of
Mg OH and the second endothermic band is due to dehydrox-
ylation of La OH [28]. Above 700 ◦C, no obvious weight losses
were observed, but very small exothermic peaks could be observed
because the crystallization of new phase took place [29].
2.1. Catalyst preparation
Hydrotalcite-like samples of the catalyst were synthesized by
the co-precipitation method. 0.03 mol (7.69 g) Mg(NO3)2·6H2O,
0.01 mol (3.75 g) Al(NO3)3·9H2O and the fixed amount of
La(NO3)3·H2O (AR) were dissolved in deionized water of 400 mL at
the molar ratios of Mg:Al:La = 3:1:0, 3:1:0.1, 3:1:0.4, 3:1:0.7, 3:1:1,
respectively. Then an aqueous solution of Na2CO3 and NaOH (2:1,
Na+: 1.5 mol/L) was slowly dropped at a constant rate of 3 mL/min
into the solution of nitrates under vigorous stirring until the pH
value reached 10. The slurry was continuously stirred for 0.5 h and
then kept at room temperature for 20 h. The slurry was then fil-
trated and washed with deionized water until the pH value reached
7.0. The resulting cake was dried at 80 ◦C for 10 h to obtain the
HTLcs samples. Finally, the catalysts were calcined in air at 650 ◦C
for 5 h with a heating rate of 10 ◦C/min. The obtained HTLcs samples
and composite oxide catalysts were denoted as xLa-HTLcs before
calcination and xLHTs after calcination, respectively, where x rep-
resented the added molar amount of La.
3.1.2. Nitrogen adsorption–desorption analysis
2.2. Catalyst characterization
The N2 adsorption–desorption isotherms were used
to determine the pore structure of the catalysts. The N2
adsorption–desorption isotherms and the corresponding pore
size distribution of the xLHTs catalysts are shown in Fig. 2, respec-
tively. All the samples exhibit typical type-IV adsorption isotherms
with clear hysteresis loops at higher relative pressure, suggesting
that mesopores appear in the samples. The hysteresis loops have
Nitrogen adsorption measurements were carried out at −196 ◦C
using a Quantachrome Instruments NOVA 2200e, while BET sur-
face area and the pore size distribution were determined from
the isotherms. The sample was out-gassed at 200 ◦C for 2 h before
each test. Powder X-ray diffraction (XRD) patterns of the samples

144
Q. Ma et al. / Applied Catalysis A: General 464–465 (2013) 142–148
100
80
100
a
b
0
0
TG
TG
DTA
DTA
80
60
40
20
-20
-40
-60
-80
-20
-40
-60
-80
60
40
20
0
0
0
100 200 300 400 500 600 700 800
0
100 200 300 400 500 600 700 800
Temperature(oC)
Temperature(oC)
100
80
100
80
c
d
0
0
TG
TG
DTA
-20
-40
-60
-80
-20
-40
-60
-80
DTA
60
60
40
40
20
20
100 200 300 400 500 600 700 800
0
100 200 300 400 500 600 700 800
Temperature( oC )
Temperature(oC)
100
80
e
0
TG
DTA
-20
-40
-60
-80
60
40
20
100 200 300 400 500 600 700 800
Temperature(oC)
Fig. 1. TG-DTA curves of xLa-HTLcs samples (a) x = 0, (b) x = 0.1, (c) x = 0.4, (d) x = 0.7, (e) x = 1.0.
features of H3 type (IUPAC classification). Type H3 hysteresis is
usually associated with solids consisting of platy shaped particles.
This is in accordance with the traditional view of these types of
mixed oxides. Meanwhile, the hysteresis loops become narrow
with the La content increase. This indicates that the pore becomes
regular. The pore size distribution curve of the catalysts is gradually
broadened as the La additive amount is increased.
formation of new La2O3 structure. Along with the increased La
additive amount, the BET surface area and pore volume reach
115.1 m2 g−1 and 0.734 cm3 g−1,respectively, when x = 0.7. How-
ever, when x reached 1.0, the surface area of xLHTs drops to
31.4 m2 g−1
.
3.1.3. XRD analysis
Table 1 shows the nitrogen adsorption measurement data of the
composite oxide catalysts xLHTs. It can be seen that the BET sur-
face area increases from 77.4 to 115.1 m2 g−1 and the pore volume
increases from 0.29 to 0.73 cm3 g−1 with the La content increas-
ing from x = 0 to x = 0.7. The BET surface area and the pore volume
are dramatically decreased when the x reaches 1.0, because of
The XRD patterns of all La-HTLcs samples are showed in Fig. 3.
Apart from 1.0 La-HTLcs, all the samples exhibited similar profiles
to the pure hydrotalcite, performing six feature diffraction peaks of
(0 0 3), (0 0 6), (0 1 2) (0 1 5), (0 1 8), (1 1 0) and (1 1 3), which indicate
the crystalline formation of layered double hydroxide structure of
HT [30,31]. Peaks intensities and sharpness of (0 0 3) and (0 0 6)
planes, which are directly proportional to the crystallinity of mate-
rial, were observed to decrease on increasing the La content of
hydrotalcite. And its crystallinity decreases with the La content
addition. Finally, such HTLcs structure disappeared when the La
additive amount reached x = 1.0.
Table 1
Specific surface area and pore parameter of xLHTs catalysts.
Catalysts
Surface area
Pore volumea
Average pore sizea
(nm)
(m2 g−1
)
The XRD patterns of the xLHTs are presented in Fig. 4. It can be
observed that the layered crystalline structure of HTLcs is destroyed
during calcination. For xLHTs of x = 0, 0.1, 0.4 and 0.7, the struc-
ture of samples changes from orthorhombic to cubic, corresponding
to the transformation of a two-dimensional layered structure to a
three-dimensional structure and it is converted into a type of Mg–Al
0LHTs
77.4
101.5
105.5
115.1
31.4
0.29
0.46
0.61
0.73
0.20
14.3
16.0
20.5
24.1
25.0
0.1LHTs
0.4LHTs
0.7LHTs
1.0LHTs
a
Calculated based on adsorption branch of isotherm.

Q. Ma et al. / Applied Catalysis A: General 464–465 (2013) 142–148
145
(a)500
0LHTs
Mg(Al)O
La2O3
0.1LHTs
0.4LHTs
0.7LHTs
1.0LHTs
400
300
200
100
0
0LHTs
0.1LHTs
0.4LHTs
0.7LHTs
1.0LHTs
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/P0)
20
40
60
80
2
1.2
(b)
0LHTs
Fig. 4. XRD patterns of xLHTs calcined at 650 ◦C.
0.1LHTs
0.4LHTs
0.7LHTs
1.0LHTs
1.0
0.8
0.6
0.4
0.2
0.0
two bands because of lowering in the local symmetry of the car-
bonate anion in the interlayer space [34]; the extremely weak
band at 1060 cm−1 is attributed to the v1 mode of carbonate
species. The bands observed at 856 cm−1, 680 cm−1 and 443 cm−1
are due to the Mg OH stretching and Mg OH M (Al, La) bending
vibration [27].
3.1.5. CO2-TPD analysis
The surface basicity of composite oxides catalysts was measured
by CO2-TPD and the basic species could be assigned according to
the temperature at which peaks appeared. The CO2-TPD curves are
shown in Fig. 6. The basic species was studied by means of CO2-TPD
peak-differentiation-imitating analysis. All the profiles can be split
into three Gaussian peaks. As reported by León, the formation of
different species stems from the presence of sites with difference
basic strength in the CO2 adsorption and desorption process: bicar-
bonates (OH, weak base sites), bidentate carbonates (bridging and
chelating, Mm+–O2− pairs, medium-strength base sites) or uniden-
tates (O2−, strong base sites). Associating these types of species with
the CO2 desorption temperature, it is clear that weak base sites cor-
respond to the range 20–100 ◦C, bridging bidentate carbonate with
100–200 ◦C, chelating bidentate carbonate with 200–300 ◦C and
unidentate carbonate with temperatures over 300 ◦C [35]. Accord-
ing to information above, it is clarified that all the three peaks be
assigned to weak basic sites, moderate basic sites and strong basic
sites, respectively. However, the moderate basic sites of 1.0LHTs
belong to bridging bidentate carbonate, while the moderate basic
sites of other catalysts belongs to chelating bidentate carbonate.
1
10
100
Pore diameter (nm)
Fig. 2. N2 adsorption–desorption isotherms (a) and pore distribution (b) of xLHTs
catalysts.
composite oxide. As the La additive amount increased to x = 1.0, the
XRD spectrum displayed the phase of La oxide only.
3.1.4. FT-IR analysis
FT-IR spectra for the samples of xLa-HTLcs are shown in the
Fig. 5. All the samples show rather similar spectra. The broad
band is recorded around 3450–3500 cm−1 due to the stretching
vibrations of structural hydroxyl groups, which exist in the HTLcs
layers and the water molecules in the interlays space [32,33]. A
rather weak band at 1640 cm−1 is attributed to the deformation
vibrations of interlayer water molecules [31]. The bands around
1350–1390 cm−1 is ascribed to the v3 mode of interlayer carbon-
ate species. For the samples 0La-HTLcs, 0.1La-HTLcs, 0.4La-HTLcs
and 0.7La-HTLcs, it is found that this single vibration splits into
a
b
c
d
e
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber(cm-1)
Fig. 5. FT-IR spectra of xLa-HTLcs samples (a) x = 0, (b) x = 0.1, (c) x = 0.4, (d) x = 0.7,
(e) x = 1.0.
Fig. 3. XRD patterns of xLa-HTLcs.

146
Q. Ma et al. / Applied Catalysis A: General 464–465 (2013) 142–148
and the moderate basic sites are replaced by bridging bidentate
carbonate sites.
1.0LHTs
0.7LHTs
3.1.6. SEM analysis
SEM was applied to study the morphological properties of the
as-synthesized catalysts. SEM images of catalysts calcined at 650 ◦C
are compared in Fig. 7. It can be observed that the catalyst of (a)
0LHTs, (b) 0.1LHTs, (c) 0.4LHTs and (d) 0.7LHTs exhibits a layered
and platelet-like structure, while the structure of (e) 1.0LHTs is dif-
ferent. For the 0LHTs catalyst, it is clear that its surface of platelet
is smooth, which is the same as reported in Refs. [36,37]. However,
the platelet surface becomes rough and fluffy along with the La con-
tent increasing and it no longer has the platelet structure when La
content is as high as 1.0. This result is well in accordance with the
result of XRD analysis.
0.4LHTs
0.1LHTs
0LHTs
0
100
200
300
400
500
600
Temperature(oC)
Fig. 6. Temperature-programmed desorption using CO2.
3.2. Catalytic performance
3.2.1. The effects of La content
It can be clearly observed that the moderate basic strength is
enhanced with the La content increasing and it reaches the highest
desorption temperature when the x is 0.7. But when the La content
x is as high as 1.0, the chelating bidentate carbonate sites disappear
The catalytic performance of xLHTs for the DPC synthesis is
compared in Table 2. It is apparent that the as-prepared compos-
ite oxide catalysts derived from different HTLcs precursors exhibit
different catalytic performance in the transesterification reaction.
Fig. 7. SEM images of the catalysts: (a) 0LHTs, (b) 0.1LHTs, (c) 0.4LHTs (d) 0.7LHTs (e) 1.0LHTs.

Q. Ma et al. / Applied Catalysis A: General 464–465 (2013) 142–148
147
Table 2
100
Activities of xLHTs for the DPC synthesis.
Catalysts
DMC conv. (%)
DPC sel. (%)
DPC yield (%)
90
80
70
60
50
0LHTs
98.2
98.1
99.2
98.4
98.0
71.3
72.7
93.6
95.4
85.1
70.0
71.3
92.9
93.9
83.4
0.1LHTs
0.4LHTs
0.7LHTs
1.0LHTs
DMC conv.
DPC sel.
DPC yield
(Reaction conditions: n(n-propanol):n(DMC) = 3:1; m(catalyst):m(reactant) = 0.03;
90 ◦C;6 h)
Under identical reaction conditions, the Al–Mg composite oxide
catalyst from the HTLcs precursor without La addition indicates that
the DMC conversion and the DPC selectivity are 98.2% and 71.3%,
respectively. The by-product is methyl propyl carbonate (MPC).
With the La content increasing, the selectivity of DPC increases.
And when x is 0.7, the DPC selectivity reaches a maximum of 95.4%.
However, when the La content further reaches 1.0, the selectivity
of DPC turns to decrease. Generally, the catalytic performance of
the catalysts is not only related to their catalytic base sites, but also
correlated with their structure. It was reported that the calcined
Al–Mg hydrotalcite showed the catalytic activity of alkylation via
the base active sites [24]. The current activity tests suggest that La
additive in the Al–Mg composite oxides from HTLcs precursor can
remarkably improve the product selectivity toward DPC synthesis
via the transesterification of DMC and propanol. And this enhance-
ment is owing to the adjustment of the base sites properties by La
additive.
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
Catalyst dosage (wt%)
Fig. 9. Effect of the catalyst dosage on the activity.
100
90
80
70
60
50
DMC conv.
DPC sel.
DPC yield
3.2.2. The effect of the reaction time
Fig. 8 shows the effect of reaction time on DPC production. It is
indicated that the conversion of DMC and the selectivity of DPC
increase gradually with the reaction time increasing within the
first 6 h. Meanwhile, the selectivity of MPC reaches 96.4% after 1 h,
but decreases rapidly with the reaction proceeding, suggesting that
MPC is the intermediate product in this reaction.
1
2
3
4
5
6
Times
Fig. 10. Reuse tests of 0.7LHTs.
conversion and the DPC selectivity are almost unchanged. These
findings indicate that the catalysts have good reusability toward
the DPC synthesis via the transesterification between DMC and
propanol.
3.2.3. The effect of the catalyst dosage
The effect of 0.7LHTs dosage on the catalytic performance is
presented in Fig. 9. It is indicated that the DMC conversion does
not obviously change with the increased catalyst dosage. When
the dosage is kept above 2.0%, the catalysts sustain high and stable
performance.
4. Conclusions
3.2.4. The reusability of the catalyst
The Mg–Al and La–Mg–Al composite oxide catalysts were pre-
pared via calcining the precursors of hydrotalcite-like compounds
(HTLcs) from co-precipitation method and they exhibited high cat-
alytic activities for the DPC synthesis via transesterification of DMC
and propanol. When the HTLcs precursor with a La:Mg:Al molar
ratio of 0.7:3:1 was calcined at 650 ◦C, the obtained catalyst dis-
played the best catalytic activity. Under the stipulated reaction
conditions, the maximum conversion of DMC and selectivity of DPC
reached above 98% and 95.4%, respectively.
The reusability of the catalyst was also tested over 0.7LHTs.
The catalyst was continuously tested for six times and the results
are displayed in Fig. 10. After each reaction the catalyst was
filtered, dried and then reused. It is clear that both the DMC
100
80
The catalyst characterization results suggested that the La
content played an important role in tuning the structural and mor-
phological properties of the catalysts. The platelet surface became
rough and fluffy along with the La content increasing and it no
longer had the platelet structure when La content reached 1.0. It
was also found that the moderate strength basic sites enhanced
with the La content increasing and it reached to the highest desorp-
tion temperature when the x rose to 0.7. As the La content x
increased to 1.0, the chelating bidentate carbonate sites disap-
peared and the moderate basic sites were replaced by bridging
bidentate carbonate sites. The moderate strength basic sites were
the catalytic centers in this reaction.
DMC conv.
60
DPC sel.
DPC yield
MPC sel.
40
20
MPC yield
0
0
2
4
6
8
10
Reaction time (h)
Fig. 8. Effect of reaction time.

148
Q. Ma et al. / Applied Catalysis A: General 464–465 (2013) 142–148
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