Solid base catalysts derived from Ca-Al-X (X = F-, Cl- and Br-) layered double hydroxides for methanolysis of propylene carbonate

Article abstract of DOI:10.1039/c7ra10832j

The Ca-Al and Ca-Al-X (X = F^-, Cl^- and Br^-) catalysts were prepared via thermal decomposition of Ca-Al layered double hydroxides (LDHs), and tested for methanolysis of propylene carbonate (PC) to produce dimethyl carbonate (DMC). The catalytic performance of these catalysts increased in the order of Ca-Al-Br^- < Ca-Al < Ca-Al-Cl^- < Ca-Al-F^-, which was consistent with the strong basicity of these materials. The recyclability test results showed that the addition of Al and halogens (F^-, Cl^- and Br^-) not only stabilized the CaO but also improved the recyclability of the catalysts. Particularly, the Ca-Al-F^- catalyst exerted the highest stability after 10 recycles. These catalysts have an important value for the exploitation of DMC synthesis by transesterification of PC with methanol.

Full text of DOI:10.1039/c7ra10832j

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PAPER
ꢀ
Solid base catalysts derived from Ca–Al–X (X ¼ F ,
ꢀ
ꢀ
Cl and Br ) layered double hydroxides for
Cite this: RSC Adv., 2018, 8, 785
methanolysis of propylene carbonate†
ab
a
a
a
ab
a
Yunhui Liao,
Feng Li,* Yanfeng Pu, Feng Wang, Xin Dai, Ning Zhao
a
and Fukui Xiao*
ꢀ
ꢀ
ꢀ
The Ca–Al and Ca–Al–X (X ¼ F , Cl and Br ) catalysts were prepared via thermal decomposition of Ca–Al
layered double hydroxides (LDHs), and tested for methanolysis of propylene carbonate (PC) to produce
dimethyl carbonate (DMC). The catalytic performance of these catalysts increased in the order of Ca–
ꢀ
ꢀ
ꢀ
Al–Br < Ca–Al < Ca–Al–Cl < Ca–Al–F , which was consistent with the strong basicity of these
Received 30th September 2017
Accepted 18th December 2017
ꢀ
ꢀ
ꢀ
materials. The recyclability test results showed that the addition of Al and halogens (F , Cl and Br ) not
ꢀ
only stabilized the CaO but also improved the recyclability of the catalysts. Particularly, the Ca–Al–F
DOI: 10.1039/c7ra10832j
catalyst exerted the highest stability after 10 recycles. These catalysts have an important value for the
rsc.li/rsc-advances
exploitation of DMC synthesis by transesterification of PC with methanol.
34
carbonate, Lu et al. added the Al into the CaO by an extrusion
method, which effectively improved the stability of CaO. Koc ´ı k
Dimethyl carbonate (DMC), as a green organic ne chemical et al. prepared the Ca–Al oxide catalysts by coprecipitation
1
Introduction
35
intermediate, is widely used in carbonylation, methylation and method for the biodiesel production, and found that the ob-
1,2
methoxylation reactions. Several ways are developed for DMC tained catalysts was stable with nearly no calcium leaching.
3
–5
synthesis, such as oxidative carbonylation of methanol, trans-
In addition, it is well known that the basicity of the catalysts
6–9
esterication of methanol with cyclic carbonate, alcoholysis of plays an important role in the transesterication of PC with
1
0–12
20
urea
and direct synthesis from carbon dioxide and meth- methanol. It is possible to modulate the basicity of the cata-
As compared to other routes, DMC synthesis by trans- lysts by modication of halogen anions. Dai et al. employed
1
3–16
36
anol.
ꢀ
ꢀ
ꢀ
esterication of propylene carbonate (PC) with methanol under the BaX
2
(X ¼ F , Cl and Br )-promoted Y
2
O
3
catalysts for the
-TPD results
indicated that the halogen anions effectively improved the
Several catalysts, such as alkali metals, KF supported basicity of the catalysts, thereby improved the conversion rate of
mild conditions is a promising and environmentally benign oxidative dehydrogenation of ethane, and the CO
2
1
7,18
route, which has earned much attention of researchers.
19
37
ꢀ
ꢀ
2
0
21
22,23
catalysts, zeolites, ion exchange resins,
double metal the catalytic reaction. Au et al. reported the F and Br
metal oxides and mixed metal modied La catalyst for the oxidative coupling of methane,
have been investigated for the transesterication of and the CO adsorption characterization also demonstrated
PC with methanol. However, the high reaction temperature that introduction of F and Br increased the basicity of the
403–443 K) restricts the DMC yield due to the equilibrium catalysts. Wu et al. discovered that the uorine-modied Mg–
limitation. Therefore, efficient catalysts with high activity at Al mixed oxides as a solid base with variable basic sites and
low temperature are highly demanded. It was reported that CaO tunable basicity were effective for the synthesis of propylene
1
7
24,25
cyanides, hydrotalcites,
O
2 3
2
6–29
oxides
2
ꢀ
ꢀ
38
(
30
31
was effective for the reaction at room temperature, but the glycol methyl ether (PM) from methanol and propylene oxide
recyclability was poor due to the leaching of the active phase in (PO). Gao et al. synthesized a series of uorine-containing Cu/
39
the reaction medium which limited its industrial applica- Zn/Al/Zr hydrotalcite-like compound catalysts which were tested
3
2,33
tion.
In order to improve the recyclability of the CaO for the for CO
2
hydrogenation to methanol. It was found that intro-
ꢀ
synthesis of glycerol carbonate from glycerol and dimethyl duction of F enhanced the surface basicity, and signicantly
increased the selectivity of methanol.
Layered double hydroxides (LDHs) can be represented by the
a
2+
3+
x+
nꢀ xꢀ
2+
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese
formula of [M1ꢀx
and M are divalent and trivalent metal, respectively; the value
of x is equal to the molar ratio of M /(M + M ) and A is the
M
x
(OH)
2 x/n 2
] [(A ) ] $mH O, where M
Academy of Sciences, 27# South Taoyuan Road, Taiyuan 030001, P. R. China.
3+
E-mail: lifeng2729@sxicc.ac.cn; xiao@sxicc.ac.cn
b
3
+
2+
3+
nꢀ
University of Chinese Academy of Sciences, Beijing 100049, P. R. China
anion which has the exchangeable capacity in compensating
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c7ra10832j
40,41
charge position.
Aer calcination, the obtained mixed
RSC Adv., 2018, 8, 785–791 | 785
1
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oxides possess enough Lewis base sites and uniform distribu-
tion. With the intercalation of different cations and anions, a JSM-7001F scanning electron microscope.
The surface morphologies of the catalysts were collected with
42
the basicity of the catalysts can be modulated.
The Fourier transform infrared (FT-IR) spectra were acquired
In this work, a series of Ca–Al mixed oxide catalysts were using a Nicolet Nexus 470 FT-IR spectrometer range from 400 to
ꢀ
ꢀ
ꢀ1
ꢀ1
synthetized by thermal decomposition of Ca–Al–X (X ¼ F , Cl
4000 cm with a 4 cm resolution.
ꢀ
and Br ) layered double hydroxides (LDHs), and were tested for
Thermogravimetric (TG) analyses were measured with
the transesterication of PC with methanol. The effect of Al on Rigaku TG 8120 equipment. The samples were heated at a rate
ꢀ
1
the stabilization of the active CaO component and the inuence of 10 K min
from room temperature to 1173 K in N
2
ꢀ
ꢀ
ꢀ
of the halogen anions (F , Cl and Br ) on the basicity of the Ca– atmosphere.
Al mixed oxide catalysts were investigated. This work has mean- The N2 adsorption–desorption isotherms measurement at
ingful and practical applicative values for the stabilization and 77 K employed the Brunauer–Emmett–Teller method. The
basicity regulation of CaO catalyst, and provides a new idea for specic surface areas of catalysts were determined from the
the stabilization of the catalyst applied to the transesterication. nitrogen adsorption isotherms.
The metal (Ca and Al) elemental chemical analysis was
measured with the inductively coupled plasma-optical (ICP)
2
Experimental
emission spectroscopy (Thermo iCAP 6300). Halogen anions
were measured with an ionic chromatography (881 Compact IC
Pro, Metrohm) with conductivity detection.
X-ray photoelectron spectroscopy (XPS) was analyzed by Axis
Ultra DLD spectrometer with a monochromatic Al Ka (1486.8
eV) source under ultrahigh vacuum. The binding energies were
2.1 Catalysts preparation
The Ca–Al LDH with Ca/Al molar ratio of 2 was prepared by
coprecipitation method. Typically, appropriate amounts of
CaCl and AlCl $6H O (the molar ratio of Ca/Al was 2) were
2 3 2
dissolved in 100 mL deionized decarbonated water to prepare
solution A, while appropriate amounts of NaOH were dissolved
in 100 mL deionized decarbonated water to prepare solution B.
Subsequently, both solutions were added dropwise by peri-
staltic pump to 250 mL water–ethanol (2 : 3, v/v) solution at
calibrated internally using the C 1s peak with E
b
¼ 284.8 eV. The
experimental error was within ꢂ0.1 eV.
CO temperature programmed desorption (CO -TPD) was
2
2
performed on Builder PCA-1200 chemical adsorption instru-
ment with a thermal conductivity detector using He as carrier
gas. The catalyst was placed in a quartz U-tube reactor and pre-
treated under helium ow at 1073 K for 1 h, and then cooled to
room temperature under helium ow. The adsorption process
333 K under vigorous stirring in inert atmosphere (N ), and the
2
pH was kept at 10. The suspended liquid was aged at 333 K for
4 h. Aer ltration, the precipitate was washed with deionized
2
decarbonated water until the pH of the ltrate was near 7. Then,
the ltration cake was dried at 353 K in vacuum for 24 h. Finally,
ꢀ
1
was performed with CO
2
(30 mL min ) at room temperature for
3
0 minutes, and desorption process was measured from room
the catalyst was obtained by calcination in N
073 K for 6 h. The precursor and the calcined catalyst were
labelled as CAP-2 and CA-2, respectively.
2
atmosphere at
ꢀ
1
temperature to 1173 K at a heating rate of 10 K min under
helium ow. The thermal conductivity detector (TCD) was
operated in differential mode and the signal transferred to
a data acquisition computer.
1
ꢀ
ꢀ
ꢀ
The Ca–Al–X (X ¼ F , Cl and Br ) LDHs were prepared by the
coprecipitation method. Typically, appropriate amounts of
calcium and aluminum chloride salts (the molar ratio of Ca/Al
was 2) were dissolved in a 100 mL deionized decarbonated
water (solution C). Then, appropriate amounts of NaOH and Na– 2.3 Catalytic reaction
ꢀ
ꢀ
ꢀ
X (X ¼ F , Cl and Br ) were dissolved in a 100 mL deionized
decarbonated water (solution D). The molar ratio of Ca/X was 2.5.
The following steps were the same as the preparation methods of
CAP-2 and CA-2. The precursors and calcined catalysts were
The DMC synthesis from transesterication of methanol with
PC can be depicted in Scheme 1. All catalysts were pre-treated at
2
1073 K in N atmosphere for 1 h before reaction. The trans-
esterication was carried out in a three neck ask with Dimroth
condenser containing designed amount (mmol) of reactants
and catalysts. The products were analyzed by gas chromatog-
raphy (GC-950) equipped with TCD detector. The separation
column was made of F 3 mm ꢃ 3 m stainless steel column
ꢀ
ꢀ
ꢀ
labelled as CAP-X and CA-X (X ¼ F , Cl and Br ), respectively.
For comparison, the CaO catalyst was prepared by the
coprecipitation method. Typically, appropriate amounts of
calcium chloride salts were dissolved in a 100 mL deionized
decarbonated water (solution E). Then, appropriate amounts of
NaOH were dissolved in a 100 mL deionized decarbonated
water (solution F). The following steps were the same as the
preparation methods of CA-2.

lled with GDX-203 stationary phase. The conversion of PC and
selectivity of DMC were calculated by the following equations:
m
PC1=MPC ꢀ mPC2=MPC
The conversion of PC ¼
ꢃ 100%
m
PC1=MPC
(1)
2.2 Catalysts characterization
The X-ray powder diffraction (XRD) was performed on a Bruker
D8 Advance (Germany) diffractometer, using Cu Ka radiation at
mDMC=MDMC
The selectivity of DMC ¼
ꢃ 100%
m
PC1=MPC ꢀ mPC2=MPC
ꢁ
ꢀ1
4
0 kV and 50 mA. The scan rate was 3 min in the 2q range
(2)
ꢁ
ꢁ
from 5 to 80 .
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Scheme 1 Reaction scheme for the synthesis of DMC.
mDMC is the mass of formed DMC, MDMC is the molecular weight
of DMC, mPC1 is the mass of PC in the feeds, mPC2 is the mass of
PC in the products, and MPC is the molecular weight of PC.
Fig. 2 Infrared spectra of (a) CAP-2, CAP-X and (b) CA-2, CA-X
samples.
3
Results and discussion
3.1 Characterization of the catalysts
The XRD technique is employed to characterize the structure of
LDH precursors, and the patterns of the CAP-2 and CAP-X
samples are shown in Fig. 1a, in which diffraction peaks
correspond to (002), (004), and (020) planes of the Ca–Al LDHs
symmetric, which suggests that the vibration of M–O and M–OH
is modied owing to the anionic group in the layered structure
of LDH.
Aer calcination, great changes are found for the CA-X
43
are observed, suggesting the well-developed layered structure.
samples (Fig. 2b) e.g. the intensity of the band at 3460 and
The TG-DTG curves of the samples are shown in Fig. S1.† The
ꢀ1
1
621 cm assigned to the vibration band of hydroxyl groups
ꢀ
thermal stability of LDH precursors are in the order of CAP-F >
decrease, suggesting that the water in the interlayer is
ꢀ
ꢀ
CAP-Cl > CAP-2 > CAP-Br .
10
removed. The bands at around 447, 571, 673, 792 and
Aer calcination at 1073 K, the phases of the LDH disappear,
ꢀ1
8
52 cm ascribed to the characteristic vibration of Ca–O and
while cubic phase of CaO and mayenite are observed as shown
Al–O shi to the higher wave number due to the phase trans-
ꢀ
in Fig. 1b. Specially, the CaF
2
is detected for the CA-F sample.
48
formations. The spectrum is dominated by a strong band at
The infrared spectra are employed to study the structural
features of the samples. For all CAP-X samples (Fig. 2a), the
ꢀ1
8
52 cm and the intensity of this band increases in the order of
ꢀ
ꢀ
ꢀ
ꢀ
CA-F > CA-Cl > CA-2 > CA-Br . For the CA-F sample, the
highest intensity of this absorption band indicate the change in
the environment of M–O (M ¼ Ca and Al) systems due to the
ꢀ
1
absorption band at 3644 cm
due to the O–H stretching
10
vibration of free water is observed. The broad band at
455 cm can be ascribed to the stretching mode of hydroxyl
groups, both from the layers and interlayer water molecules.
Moreover, the band at 1621 cm attributed to the bending
ꢀ1
3
ꢀ
introduction of F which may increase the stability of the M–O
44
group.
ꢀ
1
The textural parameters and compositions of the catalysts
are summarized in Table 1. Compared to the CA-2 catalyst, the
specic surface areas and pore volume of the catalysts increase
45
deformation of molecular water is presented. The band
attributed to the asymmetric stretching of C–O bonds in
ꢀ
1
46,47
carbonate ions is also observed at around 1400 cm
.
In fact,
ꢀ
ꢀ
ꢀ
ꢀ
with introduction of F and Cl . The CAP-F and CAP-Cl LDH
precursors have more united structures, which result in more
amounts of small and uniform particle size aer thermal
treatment as supported by the SEM analysis (Fig. S2†), and thus
lead to the increased specic surface areas and pore volume.
ICP results show that the bulk metal compositions of the cata-
lysts are similar to that in the mother liquor, indicating that the
metal ions precipitated completely in the preparation process.
While XPS results indicate that the Ca is enriched on the surface
which is more remarkable for the halogen anions modied
catalysts. Thus, it can be concluded that the surface composi-
tions of the catalysts can be modulated by modication of the
it is inevitable to suffer some carbonates due to high alkaline
48,49
surface of the samples.
In addition, the vibrations at around
ꢀ
1
782, 578, 526 and 427 cm are assigned to the characteristic
48
vibration of M–O and M–OH (M ¼ Ca and Al). It is noteworthy
that the shape of the absorption band centered at around 578
ꢀ
1
and 526 cm
for CAP-2 sample is asymmetric, while the
halogen anions modied CAP-X samples are relatively
ꢀ
halogen anions. Specially, the F shows the most signicant
impact on the surface Ca : Al atomic ratio.
The CO -TPD proles are shown in Fig. 3. For the pure CaO
2
catalysts, only one desorption peak ascribed to the strong basic
sites is observed. Obviously, with the addition of Al and halogen
anions, three broad desorption peaks for each catalyst are
2
observed, which indicate the different adsorption sites for CO .
For the CA-2 catalyst, the weak (a peak), moderate (b peak) and
ꢀ
strong (g peak) basic sites can be ascribed to basic OH group,
3
+
2ꢀ
2+
2ꢀ
2ꢀ
Fig. 1 XRD patterns of (a) CAP-2, CAP-X and (b) CA-2, CA-X samples. Al –O
and Ca –O
pairs and unsaturated O
anions,
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a
Table 1 Textural parameters and compositions of the catalysts
b
Compositions (mol%)
2
ꢀ1
3
ꢀ1
Catalysts
S
BET (m g
)
V
pore (cm g
)
Ca
Al
X
Ca : Al
ꢀ
CA-F
45.0
40.3
32.7
37.6
0.38
0.33
0.22
0.26
52.9 (54.8)
53.0 (54.1)
52.8 (53.7)
66.6 (66.9)
26.6 (25.7)
26.4 (26.1)
26.6 (26.4)
33.4 (33.1)
20.5 (19.5)
20.6 (19.8)
20.6 (19.9)
0
1.99 (2.13)
2.01 (2.07)
1.98 (2.03)
1.99 (2.02)
ꢀ
ꢀ
CA-Cl
CA-Br
CA-2
a
ꢀ
ꢀ
ꢀ
b
X ¼ F , Cl and Br . The values outside and inside the parentheses were obtained by ICP or ionic chromatography and XPS measurements,
respectively.
4
9,50
32,33
respectively.
While the CA-X catalysts are consisted of more consistent with the previously reported literature.
Upon
ꢀ
complex basic groups e.g. the basic OH group can result in the addition of Al into the CaO, although the PC conversion of the
weak basic site, the Al –X , Ca –X , Al –O and Ca –O
3
+
ꢀ
2+
ꢀ
3+
2ꢀ
2+
2ꢀ
CA-2 catalyst was reduced to 53.7%, the decrement of PC
pairs can lead to the moderate basic site, the coordinatively conversion aer 10 recycles was only 11.8%, which suggest that
ꢀ
2ꢀ
unsaturated X and O anions can result in the strong basic the Al effectively improved the stability of CaO. Furthermore,
3
8,51,52
site.
CO -TPD peaks area are listed in Table 2. The amounts of the increased with modication of F and Cl . The CA-F catalyst
strong and total basic site of the catalysts increase in the same possessed the high catalytic performance with the PC conver-
The CO uptakes on each catalyst calculated from the the catalytic performance and recyclability of the catalysts both
2
ꢀ
ꢀ
ꢀ
2
ꢀ
ꢀ
ꢀ
order of CA-F > CA-Cl > CA-2 > CA-Br . Since the ionic radii of sion of 65.9% and DMC selectivity of 95.3%, which was similar
halogen anions increase in the order of F < Cl < Br , the to the CaO catalyst. Moreover, the catalyst still possessed
supercial F is more effective to substitute O than Cl and a higher PC conversion (60.1%) aer 10 recycles, and the
Br , which may accounts for the highest basicity of the CA-F
catalyst. Besides, the CO desorption peak ascribed to the Br catalyst, the PC conversion was lower than the CA-2 catalyst
strong basic site of CA-F catalyst shis to higher temperature, in spite of the increased stability.
suggesting that the strong basicity of the catalyst increase with
modication of F . It can be interpreted as the generation of the high catalytic performance. Previous reports described that
ꢀ
ꢀ
ꢀ
ꢀ
2ꢀ
ꢀ
ꢀ
ꢀ
decrement of PC conversion was only 5.8%. While for the CA-
53
ꢀ
2
ꢀ
ꢀ
During the reaction, the catalyst modied by the F exerted
ꢀ
ꢀ
2ꢀ
38,51,52
+
ꢀ
coordinatively F and O unsaturated anions.
the active K and F presented on the surface of KF/Al
2 3
O
catalyst were responsible for the transesterication of PC with
3.2 Catalytic performance and recyclability of the catalysts
The effect of reaction parameters on catalytic performance of
the catalysts was investigated (Fig. S3†) and the catalytic results
under the optimized condition were shown in Fig. 4. The pure
CaO catalyst possessed good catalytic performance with the PC
conversion of 68.8% and the DMC selectivity of 95.6% (Fig. 4a).
However, the PC conversion decreased to 35.5% aer 10 recy-
cles (Fig. 4b), suggesting the poor recyclability which is
2
Table 2 The CO -TPD results of the catalysts
ꢀ
1
CO
2
uptakes (mmol g )
Total basicity
(mmol g )
ꢀ1
Catalysts
a
b
g
ꢀ
CA-F
0.11
0.05
0.06
0.10
0
0.34
0.33
0.23
0.26
0
0.56
0.49
0.36
0.44
0.60
1.01
0.87
0.65
0.80
0.60
ꢀ
ꢀ
CA-Cl
CA-Br
CA-2
CaO
Fig. 4 The catalytic performance and recyclability of the CA-X, CA-2
and CaO catalysts. Reaction conditions: n(methanol)/n(PC) ¼ 12,
catalyst weight ¼ 2 wt% of total reactants, T ¼ 333 K, t ¼ 2 h.
2
Fig. 3 CO -TPD profiles of the catalysts.
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2
0,54,55
56
methanol.
[
Zhu et al. demonstrated that the basic Al– g as shown in Fig. 5. It indicates that although the trans-
ꢀ
OH/F] species was effective for the activation of methanol. esterication of PC with methanol occurred over various basic
5
7
58
Ando et al. and Chen et al. also emphasized the signicant sites, the strong basic site g plays main role on the reaction.
ꢀ
effect of coordinately unsaturated F as the basic sites, which
effectively activated the methanol. According to the results of
ꢀ
CO
2
-TPD, the introduction of F greatly increased the amounts
3
.3 A discussion of the reaction mechanism
of the basic sites, and thus signicantly increased the catalytic
performance of the catalysts. The catalysts with high basicity Based on previous researches
18,26,60
and the results in this work,
lowered the free energy of the transesterication, which accel- a possible reaction mechanism of DMC synthesis from PC with
erated the generation and desorption of the products from the methanol is proposed and depicted in Scheme 2. The surface
active basic sites of the catalysts, and thus increased the PC basic sites of catalysts activated the methanol, which cleaved PC
59
conversion.
to form 2-methyl-hydroxyethyl methyl carbonate as an inter-
Associated with the CO
2
-TPD characterization results, the mediate. The intermediate further reacted with the activated
catalysts are consisted of different basic sites. Therefore, the methanol to produce DMC and PG as product. For this reaction,
relationship between PC conversion and different basic sites of the main role of solid base catalyst is to activate CH OH through
3
d+
the catalyst is investigated. It is noticeable that the PC conver- the abstraction of H by basic site. It is possible that the higher
sion is linearly related with the amounts of the strong basic sites the basicity of the catalyst, the more negative is the charge of
dꢀ
CH
3
O
, and thus easily promoted the reaction activity of DMC
synthesis.
Fig. 5 The relationship between PC conversion and amounts of the
strong basic site g. Reaction conditions: catalyst weight ¼ 2 wt% of Fig. 6 (a) XRD patterns and (b) CO
2
-TPD profiles of the fresh and used
ꢀ
total reactants, n(methanol)/n(PC) ¼ 12, T ¼ 333 K, t ¼ 2 h.
CaO and CA-F catalysts.
Scheme 2 Possible reaction mechanism of DMC synthesis.
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Table 3 The bulk compositions of the fresh and used catalysts
Acknowledgements
Bulk compositions (mol%)
This work was supported by the Science Foundation for Young
Scientists of Shanxi Province, China (No. 201701D221052),
Natural Science Foundation of Shanxi Province, China (No.
a
a
b
Catalysts
Ca
Al
F
ꢀ
Fresh CA-F catalyst
52.9
51.5
26.6
28.3
20.5
20.2
201601D102006) and the Key Science and Technology Program
ꢀ
Used CA-F catalyst
of Shanxi Province, China (MD2014-09, MD2014-10).
a
b
Determined by the ICP. Determined by the ionic chromatography.
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Fig. 6 shows the XRD patterns and CO
2
-TPD proles of the fresh
ꢀ
and used CaO and CA-F catalysts. For the pure CaO catalyst,
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
the diffraction peaks at 2q of 32.2 , 37.4 , 53.8 , 64.2 and 67.4
corresponded to the phase of CaO were weakened aer 10
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2
3
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adsorption capacity of the CO on the surface of the CaO catalyst
2
59,61
signicantly decreased aer 10 recycles (Fig. 6a).
Therefore,
1
the dramatically decreased catalytic activity of the CaO catalyst
could be ascribed to the decreased amounts of the surface basic
P. Kumar, V. C. Srivastava and I. M. Mishra, Renewable
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sites. For the CA-F catalyst, the intensity of CaO phase and the
2
CO adsorption capacity did not decrease signicantly. More-
over, the elemental analysis (Table 3) shows that the content of
600–605.
F and Ca remain almost unchanged aer 10 recycles, suggesting
ꢀ
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Conclusions
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1
1
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In this work, a series of Ca–Al solid base catalysts were prepared
via thermal decomposition of Ca–Al LDHs, and were effectively
for the transesterication of PC with methanol. The character-
ization results demonstrated that Al and different halogen
anions (F , Cl and Br ) controlled the structural and chemical
properties of the catalysts and affected the catalytic perfor-
mance for the transesterication reaction. The PC conversion
ꢀ
ꢀ
ꢀ
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1
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was linear to the strong basic amounts of the catalysts. The CA-
ꢀ
F
catalyst possessed the highest surface basicity and exerted
1
1
1
1
1
2
2
2
2
2
5 L. Chen, S. P. Wang, J. J. Zhou, Y. L. Shen, Y. J. Zhao and
the highest catalytic performance. The highest PC conversion of
5.9% and DMC selectivity of 95.3% had been reached with
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6
6 P. Kumar, P. With, V. C. Srivastava, K. Shukla, R. Glaser and
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halogen anions (F , Cl and Br ) effectively improved the
recyclability of catalysts. The CA-F catalyst maintains the
highest activity aer 10 recycles and the decrement of PC
ꢀ
ꢀ
ꢀ
241, 34–44.
ꢀ
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chemical compositions of the use CA-F catalyst aer 10 recy-
ꢀ
2
cles showed the high recyclability and stability. The work re-
ported herein is expected to advance the preparation method of
high-efficiency solid base catalysts used in the trans-
esterication of propylene carbonate with methanol, which has
good prospects for industrial application with many advantages
such as easy catalyst separation and better recyclability.
1
1
Conflicts of interest
There are no conicts to declare.
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