Direct carbonylation of glycerol with CO2 to glycerol carbonate over Zn/Al/La/X (X = F, Cl, Br) catalysts: The influence of the interlayer anion

Article abstract of DOI:10.1016/j.molcata.2015.03.012

Abstract Halogen anions (F^-, Cl^- and Br^-) modified Zn-Al-La (Zn/Al/La = 4:1:1) mixed oxides derived from calcination of re-constructed hydrocalcites were prepared and their catalytic activities for the synthesis of glycerol carbonate (GC) form glycerol and CO₂ were investigated. The re-formed hydrotalcites with the introduction of halogen anions formed more thin plates which improved the specific surface area that lead to more exposured active sites. The binding energies of Zn 2p_3/2 increased with the introduction of halogen anions while the basicity decreased with the change of the catalysts' composition. The halogen anions entered hydrotalcites followed the sequence of: F^- > Cl^- > Br^-. The halogen anions modified catalysts showed high catalytic activities toward the reaction. The improved catalytic activities were suggested to be closely associated with the structure properties, such as large specific surface area, high surface content of Zn atoms, higher binding energy of Zn 2p_3/2 and high dispersion of metal oxides, which could provide more "accessible" active sites for the reaction. In addition, the changing of halogen anions may also had some influence on the catalytic activity; the introduction of Cl^-had more advantageous for the reaction.

Full text of DOI:10.1016/j.molcata.2015.03.012

Accepted Manuscript
Title: Direct carbonylation of Glycerol with CO2 to glycerol
carbonate over Zn/Al/La/X (X= F, Cl, Br) Catalysts: the
influence of the interlayer anion
Author: Hongguang Li Chunling Xin Xi Jiao Ning Zhao
Fukui Xiao Lei Li Wei Wei Yuhan Sun
PII:
S1381-1169(15)00116-8
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2015.03.012
MOLCAA 9451
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
8-10-2014
11-3-2015
19-3-2015
Please cite this article as: Hongguang Li, Chunling Xin, Xi Jiao, Ning Zhao,
Fukui Xiao, Lei Li, Wei Wei, Yuhan Sun, Direct carbonylation of Glycerol
with CO2 to glycerol carbonate over Zn/Al/La/X (X= F, Cl, Br) Catalysts: the
influence of the interlayer anion, Journal of Molecular Catalysis A: Chemical
http://dx.doi.org/10.1016/j.molcata.2015.03.012
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Direct carbonylation of Glycerol with CO2 to glycerol carbonate over
Zn/Al/La/X (X= F, Cl, Br) Catalysts: the influence of the interlayer
anion
Hongguang Lia,b, Chunling Xina,b, Xi Jiaoa,b, Ning Zhaoa*, Fukui Xiaoa, Lei Lia, Wei
Weic, Yuhan Sund
a
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese
Academy of Sciences, Taiyuan 030001, China.
b University of Chinese Academy of Sciences, Beijing 100049, China.
c Center for Greenhouse Gas and Environmental Engineering, Shanghai Advanced
Research Institute, Chinese Academy of Sciences, Shanghai, 201210, China.
d
Low Carbon Conversion Center, Shanghai Advanced Research Institute, Chinese
Academy of Sciences, Shanghai 201210, China.
* Corresponding author. Fax: +86-0351-4041153.
E-mail: zhaoning@sxicc.ac.cn (N. Zhao).
1

Graphic Abstract
2

Highlights
ꢀ The halogen anions enter hydrotalcites follow the sequence of F- > Cl- > Br-.
ꢀ The basicity decreases with the change of the catalysts’ composition.
ꢀ The binding energies of Zn 2p3/2 increase with the introduction of halogen anions.
ꢀ The catalytic activity improves by the introduction of halogen anions.
3

Abstract :
Halogen anions (F-, Cl- and Br-) modified Zn-Al-La (Zn/Al/La=4:1:1) mixed oxides
derived from calcination of re-constructed hydrocalcites were prepared and their
catalytic activities for the synthesis of glycerol carbonate (GC) form glycerol and CO2
were investigated. The re-formed hydrotalcites with the introduction of halogen
anions formed more thin plates which improved the specific surface area that lead to
more exposured active sites. The binding energies of Zn 2p3/2 increased with the
introduction of halogen anions while the basicity decreased with the change of the
catalysts’ composition. The halogen anions entered hydrotalcites followed the
sequence of: F- > Cl- > Br-. The halogen anions modified catalysts showed high
catalytic activities toward the reaction. The improved catalytic activities were
suggested to be closely associated with the structure properties, such as large specific
surface area, high surface content of Zn atoms, higher binding energy of Zn 2p3/2 and
high dispersion of metal oxides, which could provide more “accessible” active sites
for the reaction. In addition, the changing of halogen anions may also had some
influence on the catalytic activity; the introduction of Cl– had more advantageous for
the reaction.
Key words: Glycerol carbonylation; CO2; ZnAlLa; Halogen anions modified
hydrotalcites
4

1. Introduction
Biodiesel, a renewable clean fuel, has received much attention and as a result, its
production increases greatly in recent years[1]. However, a large amount of glycerol,
byproduct of biodiesel production, has been produced during the last decade. Excess
market-supply of glycerol is not favorable for the development of biodiesel industry[1,
2]. Consequently, new applications for glycerol need to be developed and/or the
existing routes need to be expanded. Glycerol carbonate (GC) can be used as a novel
component for gas separation membranes, polyurethane foams and surfactant as well
as the component in coatings, paints and detergents, etc.[3, 4]. Therefore, the
synthesis of glycerol carbonate from glycerol becomes very meaningful.
At present, the main methods for the synthesis of GC form glycerol include the
transesterification of glycerol with carbonates, carbonylation of glycerol with urea,
direct carbonation of glycerol with CO2, as well as the reaction of glycerol
halo-derivatives with alkaline (hydrogen) carbonates[2, 5]. Transesterification process
involves the feed of dimethyl, diethyl or propylene carbonate[6-9], which suffered
from the economic disadvantage when compared with urea as feed. Although many
heterogeneous catalysts showed good catalytic activity for GC synthesis from glycerol
and urea[9-12], the reaction of glycerol with urea must be conducted at the pressure
below 20-30 mbar in order to remove the formed ammonia. Moreover, the
purification process is also complicated[13].
Compared with the above-mentioned synthesis routes, the direct carbonation of
glycerol with CO2 is a green process that would convert two cheap materials into a
valuable product with water as by-product. However, most of catalysts such as
zeolites, basic ion-exchange resins, tin complexes, CeO2-Al2O3 and CeO2-Nb2O5
showed low catalytic activities even under high pressure because of the
thermodynamic limitation[14-18]. Therefore, acetonitrile was introduced as coupling
agent in the process to obtain enhanced results[19], and the catalytic systems of
ZnO-La2O2CO3 and Cu/La2O3 oxides showed enhanced catalytic activities[20, 21].
Although many research works were reported, the development of more efficient solid
5

catalysts is still a challenge.
It is reported that the mixed metal oxides, derived from calcinations of
x+
hydrotalcite-like compounds (HTlcs) (M2+ M3+ (OH)2 (An−)x/n·yH2O), possess
1−x
x
homogeneous dispersion of M2+ and M3+ with an atomic level, high specific surface
area and strongly basic properties. In addition, acid-base properties of the catalysts
can be modified by changing cations in the layers, the compensating anions or
activation method[22]. As a result, the mixed metal oxides obtained from calcinations
of hydrotalcites have been studied for various catalytic reactions including the
synthesis of GC by transesterification or carbonylation with urea[6, 9, 23-25].
In our previous research, the effect of Zn:La atomic ratios and calcination
temperatures on the activity of glycerol carbonate synthesis were investigated [20].
Therefore, in order to further improve the catalytic performance, the Zn-La containing
hydrotalcites (Zn/Al/La=4:1:1) were prepared and tested for the reaction. In addition,
for exploration the influence of introduction of different compensating anions on the
physico-chemical and catalytic properties, a series of halogen (F, Cl, Br) modified
Zn-Al-La (Zn/Al/La=4:1:1) mixed oxides derived from calcinations of re-constructed
hydrocalcite compounds were prepared and characterized by XRD, FTIR, TG, BET,
XPS, CO2-TPD. The catalytic activities for the synthesis of GC form glycerol and
CO2 were investigated for the first time. The influence of the halogen anions on
physicochemical properties, and the relationship between the catalytic activities and
physicochemical properties were discussed.
2. Experimental
2.1. Catalyst preparation
La modified Zn-Al hydrotalcite with the atomic ratio of Zn:Al:La = 4:1:1 were
synthesized by co-precipitation method. Typically, two aqueous solutions, a solution
of Zn (II), Al (III), La (III) nitrates and a mixed solution of NaOH and Na2CO3
precipitant, were added dropwise to 100 mL of deionized water under vigorous
o
stirring with a constant pH of 10. The resulting slurry was aged at 60 C for 15 h
under stirring and then filtered and washed with deionized water. The obtained solid
6

o
were dried at 80 C overnight to yield the synthesized hydrotalcite (denoted as HT)
and then calcined in air by heating at 5 oC min-1 up to 450 oC over 5 h under a flowing
stream of pure nitrogen to obtain the corresponding mixed oxides (HT-450).
The halogen-containing Zn-Al-La hydrotalcites (HT-X, X= F, Cl, Br) were
prepared using the ‘‘memory effect’’ according to the literature[26]. Typically, the
as-prepared HT-450 (1.0 g) were treated separately with an aqueous solution of NH4F
(0.1 mol/L, 150 mL) under N2 atmosphere with stirring at room temperature for 48 h,
and the pH values of all solutions were adjusted to 8.0 by addition of appropriate
amounts of NH4OH. Then the resulting products were filtered, washed extensively
with deionized water and dried at 80 oC. The obtained HT-Xs were further calcined at
o
500 C for 5 h under a flowing stream of pure nitrogen and they were denoted as
o
CHT-X. For comparison, Zn-Al-La hydrotalcite was also calcined at 500 C and the
obtained mixed oxide was denoted as CHT.
2.2. Characterization of Catalysts
X-ray diffraction (XRD) patterns were recorded on a Panalytical X’Pert Pro
X-ray diffractometer with Cu Kα radiation over the range of 10–80°. The Fourier
Transform Infrared spectra (FT-IR) of the prepared catalysts were obtained using a
Nicolet NEXUS 470 FT-IR spectrometer by the KBr disc method over a range of
400–4000 cm-1 with a 4 cm-1 resolution. The surface area of samples was determined
o
by N2 adsorption-desorption at liquid nitrogen temperature -196 C using a
Micromeritics Tristar 3000 instrument. Thermogravimetric analysis (TGA) was
performed on Rigaku thermogravimetric analyzer with 60 mL min-1 of air flow from
o
o
30 to 800 C at a heating rate of 10 C min-1. The morphology of the samples was
performed on a JEOL JSM-7001F scanning electron microscope (SEM) with an
accelerating voltage of 10.0 kV.
X-ray photoelectron spectroscopy (XPS) measurements were performed over a
Kratos AXIS ULTRA DLD spectrometer equipped with Al Kα radiation (hν = 1486.6
eV) under ultrahigh vacuum. The binding energies were calibrated internally by
adventitious carbon deposit C (1s) with Eb = 284.8 eV.
7

The surface basicity of the catalysts were measured by temperature programmed
desorption of CO2 (CO2-TPD) and carried out on TPD flow system equipped with an
MS detector (Omnistar, PFEIFFER VACUUM). Each sample (0.1 g) was pre-treated
o
o
in Ar flow at 500 C. The adsorption of CO2 was performed at 40 C and then
followed by Ar purge to remove the physisorbed CO2. Desorption process was
performed at a heating rate of 10 oC min-1from 40 oC to 500 oC.
2.3 Evaluation of Catalysts
The GC synthesis was carried out in a 50 mL stainless-steel autoclave equipped
with a magnetic stirrer. In a typical experiment, 4.60 g glycerol, 0.14 g catalyst and
5.0 mL CH3CN were added into the reactor. Then CO2 was introduced into the
o
autoclave with an initial pressure of 4.0 MPa. The reaction was conducted at 170 C
for 12 h. For quantitative analysis, an external standard-calibration factor method was
used and acetonitrile was used as external standard[20]. Considering the low
solubility of glycerol and the products in acetonitrile and the high solubility in ethanol,
ethanol was also added for the dilution of the reaction mixtures. Qualitative analysis
was conducted on a gas chromatograph (FID detector) with a capillary column
SHIMADZU CBP/20.
3. Results and discussion
3.1 Textural and structural properties of the prepared materials
The XRD patterns of the prepared precursors are shown in Fig. 1. Besides the
typical peaks of HTlcs in HT, the appearance of ZnO and La2(CO3)2(OH)2 H2O can be
ascribed to the high atomic ratio of M2+/M3+ (Zn/Al=4) and the large ionic radius of
La3+, respectively. Previous reports suggested that the atomic ratio of M2+/M3+
between 2 to 4 was favorable for the synthesis of hydrotalcite-like structure[25, 27]
and Benito et al. [28] have reported that it was impossible to hinder the ZnO
segregation for Zn/Al HTlcs when the Zn2+/Al3+ atomic ratio was higher than 2. For
HT-450, the hydrotalcite structure collapses and forms the corresponding metal
oxide/carbonate. In addition, the original layered structure re-appeares from the
8

calcined material after rehydration for HT-X while the ZnO peak intensity decreases.
The re-appearance of the diffraction peaks of HTlcs in HT-X can be attributed to the
“memory effect” of hydrotalcite structure[26]. The weak diffraction peak intensity
and the wide peak shape of hydrotalcite in HT-F suggest the re-formed HTlcs with
small grain size. Compared with HT, the peak intensity of HTlcs increases for HT-Cl
and HT-Br, while decreases for HT-F. HT-Cl has the strongest peak intensity of HTlcs.
The results indicate an increasing in crystallinity for HT-Cl and HT-Br which can be
ascribed to the Ostwald ripening effect, as described in previous report[25].
It can be seen from Fig. 2 that all the FT-IR spectra are rather similar. The bands
at 3435 and 1637 cm-1 can be ascribed to the stretching of OH groups attached to
metal ions as well as the water molecules in the interlays space[29]. The band at
around 1362 cm-1 can be attributed to the v3 mode of interlayer carbonate species,
while 1421 cm-1 can be assigned to the anti-symmetric stretching mode of the outer
2-
layer CO3 [27]. It can be found that HT-F shows the lowest peak intensity for
interlayer carbonate species (at 1362 cm-1) which can be ascribed to the fact that some
2-
parts of CO3 was substituted by F- or OH-. Previous report described that the
anion-exchange selectivity in the HTlcs follows the sequence as: Br- < Cl- < F- < OH-
2-
< CO3 [30]. Therefore, compared with HT-F, the increasing of peak intensity for
interlayer carbonates species in HT-Cl and HT-Br can be ascribed to the lower
anion-exchange ability of Cl- and Br- than F- which lead to a smaller amount of
halogen anions (Cl- and Br-) entered the interlayer of hydrotalcite. Moreover, the
bands at 400-900 cm-1 clearly show a typical pattern of hydrotalcite, where strong
bands at 615, 552 and 429 cm-1 are assigned to the stretching mode of Zn−OH and
Al−OH [27, 31], respectively. Compared with HT, the strong peak intensities of
Zn−OH and Al−OH in HT-X indicate that more OH- groups entered the interlayer of
hydrotalcite because of the stronger ability of anion-exchange for OH- than halogen
anions.
The scanning electron microscopy images (Fig. 3) shows typical plate-shaped
crystals which suggests the formation of layered structure[32]. Compared with HT,
the thicknesses of platelets for HT-F are much smaller, indicating that the rebuilding
9

of the layered structure which is consistent with the result of XRD measurements. In
addition, some bulks formed by the growth and/or accumulation of plate-shaped
crystals during the process of Ostwald ripening are found in HT-Cl and HT-Br which
are accordance with the XRD results (Fig. 1).
Fig. 4 presents the TG-DTG profiles of the precursors. Initially, all the samples
o
behave similarly with respect to the weight loss between 70 to 200 C, which
corresponds to the removal of physically adsorbed and interlayer water molecules.
The removal of both the hydroxyl groups from the HTl network and the carbonate
o
anions from the interlayer are recorded at around 200-300 C which results in the
formation of corresponding metal carbonate. Finally, the decomposition of metal
carbonates formed during the former steps is observed between 300 to 500 oC, leading
o
to the formation of metal oxide or halide. For HT, the weight loss at around 690 C
can be ascribed to the decomposition of La2O2CO3 [20].
The weight losses of each stage obtained from the TG curves are listed in Table 1.
Compared with HT, the weight losses of physically adsorbed and interlayer water
increase for HT-X samples, especially for HT-F, which can be ascribed to the
formation of more amounts of hydrotalcites (as shown in XRD results, Fig. 1). In
addition, the formation of hydrogen bonds between water molecules and interlayer
halogen anions also leads to the increasing of the amount of interlayer water. The
weight loss of second stage ascribed to the decomposition of hydrotalcites also
increases for HT-X samples, especially for HT-Cl, which can be attributed to the
re-formation of more hydrotalcites according to the results of XRD and FT-IR.
Furthermore, for the decomposition of the formed metal carbonate (third stage), the
weight loss decreases for HT-X, especially for HT-Cl, which can be ascribed to the
following reasons: firstly, the amount of carbonate species in the interlayer decreases
for the re-formed hydrotalcite of HT-X. Moreover, as described in previous report[33],
the dehalogenation process seems more difficult than dehydroxylation and
decarbonation process which then leads to the decrease of weight loss.
Fig. 5 shows the XRD patterns of the samples after calcination. It is observed that
o
upon thermal decomposition at 500 C, the HTl structure collapses and forms the
10

corresponding metal oxide/carbonate/halide. Some strong diffraction peaks due to
ZnO phase appear in all samples. The most obvious difference is the existence forms
of La. For CHT and CHT-Br, La exists in the form of La2O2CO3 while LaOF and
LaOCl become the main phases in CHT-F and CHT-Cl, respectively. The different
existence forms of La may have some influence on the surface basicity[34]. In
addition, diffraction peaks related to bromide are not found in CHT-Br, indicating that
the formed bromide may exist in an amorphous state. Furthermore, compared with
CHT, the stronger peak intensity of La2O2CO3 indicates La2O2CO3 may exist in the
form of aggregation in CHT-Br.
The textures of the calcined samples are listed in Table 2. Compared with CHT,
the specific surface areas increase significantly from 36.4 to 61.4 m2 g-1 with the
introduction of halogen anions. The pore volume and pore radius also increase with
the introduction of halogen anions, especially for CHT-Cl. The increasing of specific
surface area, pore volume and pore radius for CHT-X can be ascribed to the followed
reasons: firstly, the re-formed hydrotalcites with the introduction of halogen anions
have more amounts of hydrotalcite structures which results in more amounts of open
porous networks after thermal treatment. In addition, compared with HT, the
decreasing of the amounts of CO3 (2-) leads to a two times of OH- groups (-) enter
2-
the interlayer of halogen anions modified hydrotalcites. Then the dehydroxylation
process may result in the formation of more amounts of open-framework structure
which is also contributed to the increasing of the surface area, pore volume and pore
radius.
o
The particles of the samples calcined above 500 C maintained morphology
similar to that of the precursors, though some crystals were destroyed (Fig. 6).
Compared with uncalcined samples (Fig. 3), more homogeneous and thin plates are
observed in CHT-X catalysts which can be ascribed to the dehydroxylation and
decarbonation during the decomposition process that results in the disappearing of the
superimposed bulks, formed during the process of Ostwald ripening. In addition, it
can be seen that the thicknesses of plates in CHT-X are much smaller than CHT which
is responsible for the increasing of specific surface area for CHT-X (Table 2).
11

Moreover, compared with CHT, the formed more thin plates in CHT-X improve the
metal dispersion which then favors the exposure of more active sites.
3.2 XPS investigations
The calcined samples were analyzed by XPS, and the binding energies (BE) of
Zn2p3/2, Al2p and La3d are presented in Table 3. For all samples, peaks are found at
around 834.4–835.3 eV (BE) for La3d. The Zn2p3/2 peak at 1020.8–1021.3 eV can be
assigned to the Zn2+ species on the catalyst surface. It can be seen that the BE of
Zn2p3/2 and La3d increase with the introduction of halogen anions, especially for
CHT-F. The results can be ascribed to the strong electronegativity of halogen anions
which results in more electrons transfer form zinc and lanthanum atoms to halogen
atoms. In addition, the formation of LaOF and LaOCl is also favorable for the
increase of BE of La atoms. Moreover, the BE of Zn2p3/2 and La3d decrease with the
change of anions form F- to Br- which can be ascribed to the decrease of the
electronegativity of Br-. The high BE of Zn2p3/2 in CHT-X can promote the activation
of glycerol[20].
Surface compositions of the catalysts, as determined by XPS, are also shown in
Table 3. It can be seen that the surface content of Zn2+ in CHT-X is higher than that in
CHT, which can be related to more amounts of re-formed hydrotalcites in HT-X
samples that agree with the results of XRD (Fig. 1) and SEM (Fig. 6). More
hydrotalcites improve the surface area (Table 2) and metal dispersion which leads to
more amounts of metal active sites exposure on the surface. In addition, the
decreasing of surface Zn2+ content with the change of halogen anions form F- to Br-
can be ascribed to the aggregation of ZnO particles which is confirmed by the strong
peak intensity of ZnO in CHT-Cl and CHT-Br (Fig. 4). Moreover, the surface content
of halogen anions decreases form 4.8 to 0.9% with the anions change from F- to Br-.
Previous reports described that the anion-exchange selectivity in the HTlcs follows
the sequence of: Br- < Cl- < F- [30] and the stability of formed halogen-containing
HTlcs also follows the above sequence[35]. Therefore, the decreasing of surface
content of halogen anions can be related to the selectivity of anion-exchange and the
stability of the re-formed hydrotalcites.
12

3.3 The basicity of prepared catalysts
The basicity of the catalysts was measured by CO2-TPD (Fig. 7). According to
the previous reports[8, 34], CO2 desorption peaks can be roughly divided into three
regions, e.g. the weakly (50-150 oC), moderately (150-400 oC) and strongly (> 400 oC)
basic sites. The weakly basic sites were related to OH- groups, moderately basic sites
were ascribed to metal–oxygen pairs (such as Zn–O, Al–O, and La–O) and strongly
basic sites were associated with low-coordination oxygen atoms[34, 36]. All profiles
in Fig. 6 can be deconvoluted into four Gaussian peaks assigned to weakly (α peak)
and moderately basic sites (β, γ and δ peak), and the peak area can be considered to be
the amount of the basic sites. Compared with CHT, the decreasing of α peak area in
CHT-X suggests the decrease of the amount of weakly basic sites with the
introduction of halogen anions which is in accordance with the result of previous
reports[25, 37]. In addition, the total peak area of the sum of β, γ and δ decrease for
CHT-X catalysts, especially for CHT-Cl, which indicates the decrease of the amount
of moderately basic sites. The phenomenon can be explained as follows:
The more amounts of moderately basic sites in CHT can be related to the
formation of La2O2CO3. Previous reports described that the introduction of La may
enhance the basicity of catalysts[38] via the formation of La2O2CO3 [39]. However,
although CHT-X shows large surface area, the amount of moderately basic sites in
CHT-X decreases which suggests that the amount of basic sites is not only correlated
to the surface area of the catalysts [20, 36]. In addition, as shown in Fig. 4, the
existence forms of La changes with the introduction of halogen anions which may
have some influence on the surface basicity[34]. Therefore, the formation of LaOF
and LaOCl may be responsible for the reduction of the amount of moderately basic
sites. For CHT-Br, the decrease of the amount of moderately basic site may be related
to the aggregation of La2O2CO3 (Fig. 5). Moreover, the formed amorphous bromide
may also account for the decreasing of the amount of moderately basic site for
CHT-Br.
3.4 Catalytic activity
The catalytic performances for GC synthesis of the prepared catalysts are
13

summarized in Table 4. The reaction conditions are similar with the previous
report[20]. Acetonitrile was employed since previous reports suggested that
acetonitrile is an effective coupling agent for the process[19]. However, monoacetin
and diacetin are also detected as by-products which result from the reaction of
glycerol and acetic acid derived from hydrolysis of acetonitrile[20]. Although the
formation of acetins as byproducts cannot be avoided, the wide applications of the
byproducts may reduce the cost of the synthesis route, e.g. monoacetin can be used in
the manufacture of dynamite and tanning leather, as a solvent for various dyes, as
food additive, plasticisers and softening agents. While diacetin can be used as solvent,
softening agent and fuel additives for viscosity reduction[40, 41].
Compared with CHT, the catalytic activities are improved with the introduction
of halogen anions. A maximum glycerol conversion of 35.5% with GC yield of 16.0%
is obtained over CHT-Cl. In addition, the higher selectivity towards acetins than GC
can be ascribed to the impurity of glycerol and acetonitrile which were analytical
agents and the fact that there was no dehydration processing. Similar results were
obtained in previous report[42]. Furthermore, the selectivity toward GC increases
with the introduction of halogen anions. The improvement of the catalytic activities
with the introduction of halogen anions can be speculated from the catalyst
characteristics.
The increasing of diffraction peak intensity of hydrotalcite in the halogen anions
modified samples (Fig. 1) indicates that more amounts of hydrotalcites were formed
which enhances the dispersion of active components and then improves the catalytic
activities. In association with the result of the N2 adsorption, the introduction of
halogen anions improves the specific surface area (Table 2) which favors the exposure
of more active sites. In addition, the SEM images shown in Fig. 5 also suggest that the
formed thin plates in CHT-X favors the enhancement of the dispersion and exposure
of active sites. Furthermore, considering the results of XPS, the increasing of the BE
of Zn2p3/2 in CHT-X favors the activation of glycerol[20], and the increasing of the
surface Zn content in CHT-X is also helpful to improve the glycerol activation.
Previous reports described that moderately basic sites is favorable for the
14

synthesis of GC from glycerol and CO2, due to the strong ability of moderately basic
sites for the activation of CO2 and hydrolysis of acetonitrile[20, 43]. In our
experiments, although the amounts of moderately basic sites decrease for CHT-X (Fig.
6), the increasing of the catalytic activities may indicate that the amount of
moderately basic sites is not the key factor for catalytic activity. The decreasing of the
amount of moderately basic sites may not favorable for acetonitrile hydrolysis which
may results in the decreasing of the formation of acetins and the increasing of the
selectivity of GC.
The improvement of the catalytic activities for CHT-X may also be associated
with the halogen anions. Previous research results suggested that different anions had
great influence on the catalytic activities and the introduction of Cl- and Br- obviously
improved the catalytic activities[44]. In addition, Park et al. have reported that the Cl-
could be considered as Lewis base to activate glycerol[45]. Therefore, the
improvement of the catalytic activities may be related to the halogen anions which
may activate glycerol.
The stability of CHT-Cl was investigated and the result is shown in Table 4. It can
be found that the glycerol conversion and GC yield decrease which can be ascribed to
the loss of the active component (such as Zn2+ and Cl-) by leaching[12, 46]. In
addition, the selectivity of GC increases from 45.2% to 52.3%, which can be
attributed to the existence of zinc glycerolate on the surface of the used catalyst as
described in our previous report[46]. The result indicates that the stability of the
halogen anions modified catalysts is not good, it is necessary to develop highly
efficient and stable catalyst.
4. Conclusion
Halogen anions (F-, Cl-, Br-) modified Zn/Al/La catalysts derived from
hydrotalcite-like compounds were prepared and tested for glycerol carbonate
synthesis from glycerol carbonylation with CO2. It is found that the introduction of
halogen anions has significant influence on the physicochemical and catalytic
properties of the Zn/Al/La catalysts. The following conclusions may be drawn:
15

(1) The specific surface area increase with the introduction F-, Cl- and Br- anions. The
binding energy of Zn2p3/2 increase for the halogen anions modified samples, while
the catalyst basicity decreases with the formation of LaOF and LaOCl. In addition,
the ability of halogen anions enter the hydrotalcite structure follows the sequence
of: Br- < Cl- < F-.
(2) The catalytic activity is improved with the introduction of halogen anions, and the
best catalytic performance is obtained on CHT-Cl.
(3) The catalytic activity may associate with the specific surface area, surface content
of Zn as well as binding energy of Zn atoms. The surface basicity has some
influence on the selectivity of glycerol carbonate. In addition, the halogen anions
also have great influence on the catalytic activities.
Acknowledgements
This work was financially supported by National Natural Science Foundation of
China (21306217).
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Captions of the illustration
Fig. 1. XRD patterns of prepared HT and HT-X hydrotalcites. *: ZnO, ◊: La2(CO3)2(OH)2 H2O
(46-0368), : La2CO5 (23-0320), ο: La2O2CO3 (37-0804).
Fig. 2. FT-IR spectra of prepared HT and HT-X hydrotalcites.
Fig. 3. SEM images of HT, HT-F, HT-Cl and HT-Br samples.
Fig. 4. TG-DTG curves of prepared HT and HT-X hydrotalcites.
Fig. 5. XRD patterns of calcined catalysts. *: ZnO, ο: La2O2CO3 (37-0804), ♦ : La2O2CO3
(48-1113), : LaOF (05-0470), ∆: LaOCl (34-1494).
Fig. 6. SEM images of calcined samples.
Fig. 7. The CO2-TPD profiles of CHT and CHT-X catalysts. The values of the peak (β, γ and δ)
area of CHT, CHT-F, CHT-Cl and CHT-Br were multiplied by 1010 and the results are 3.019, 1.311,
1.100 and 1.469 (a.u.)/g, respectively.
19

Table 1
Weight loss of the prepared HT and HT-X hydrotalcites.
Table 2
Physical properties of CHT and CHT-X catalysts.
Table 3
The binding energies and surface composition of the calcined samples.
Table 4
The catalytic activities of various catalysts for the synthesis of GC.
20

Fig. 1. XRD patterns of prepared HT and HT-X hydrotalcites. *: ZnO, ◊: La2(CO3)2(OH)2 H2O
(46-0368), : La2CO5 (23-0320), ο: La2O2CO3 (37-0804).
Fig. 2. FT-IR spectra of prepared HT and HT-X hydrotalcites.
21

Fig. 3. SEM images of HT, HT-F, HT-Cl and HT-Br samples.
Fig. 4. TG-DTG curves of prepared HT and HT-X hydrotalcites.
22

Fig. 5. XRD patterns of calcined prepared catalysts. *: ZnO, ο: La2O2CO3 (37-0804), ♦ :
La2O2CO3 (48-1113), : LaOF (05-0470), ∆: LaOCl (34-1494).
Fig. 6. SEM images of calcined samples.
23

Fig. 7. The CO2-TPD profiles of CHT and CHT-X catalysts. The values of the peak (β, γ and δ)
area of CHT, CHT-F, CHT-Cl and CHT-Br were multiplied by 1010 and the results are 3.019, 1.311,
1.100 and 1.469 (a.u.)/g, respectively.
24

Table 1. Weight loss of the prepared HT and HT-X hydrotalcites.
Weight Loss (%)*
Samples
First
(< 200 oC)
Second
(200-300 oC)
Third
(300-500 oC)
HT
7.5
11.1
8.6
5.0
5.8
7.6
6.5
8.7
5.6
3.8
7.2
HT-F
HT-Cl
HT-Br
8.3
*Obtained from the TG-DTG curves of Fig. 4.
25

Table 2. Physical properties of CHT and CHT-X catalysts.
Surface area
(m2/g)
Pore volume
(cm3/g)
Sample
Pore radius (nm)
CHT
36.4
46.2
53.1
61.4
0.069
0.13
0.20
0.16
5.6
6.6
8.1
5.3
CHT-F
CHT-Cl
CHT-Br
26

Table 3. The binding energies and surface composition of calcined samples.
Binding Energy (eV)
Surface percentage (at.%)
Samples
Zn
La
Al
X
-
Zn
La
Al
X
-
CHT
1020.8
834.4
74.6
17.9
4.4
3.8
CHT-F
CHT-Cl
CHT-Br
1021.3
1021.1
1021.0
835.3
835.0
834.8
74.3
74.5
74.1
684.9
199.2
69.2
26.1
23.6
22.7
2.5
2.7
4.1
6.2
6.8
6.2
4.8
2.8
0.9
X represents F, Cl and Br in CHT-F, CHT-Cl and CHT-Br, respectively.
Table 4. The catalytic activities of various catalysts for the synthesis of GC.
Glycerol
Selectivity (%)
GC Yield
(%)
Sample
Conversion (%)
Monoacetin
50.9
Diacetin
5.2
GC
43.8
46.1
CHT
30.4
30.6
13.3
14.1
CHT-F
46.9
7.0
CHT-Cl
CHT-Br
35.5
33.6
26.9
47.0
48.9
44.8
7.7
6.8
2.8
45.2
44.2
52.3
16.0
14.8
14.0
Used CHT-Cl
Reaction condition: Glycerol 4.60 g, CH3CN 5.0 mL, catalyst 0.14 g, Initial pressure PCO2=4.0
MPa, 170 oC, 12 h.
27