Synthesis of glycerol carbonate over porous La-Zr based catalysts: The role of strong and super basic sites

Article abstract of DOI:10.1016/j.jallcom.2018.03.392

As important glycerol derivatives, glycerol carbonate (GC) and glycidol (GD) have attracted increasing attention in recent years. In the present work, a series of new solid base catalysts were developed to catalyze the conversion of glycerol to afford GC and GD. These catalysts were prepared by loading KF on the porous La-Zr solid base catalyst and characterized using a series of methods. A large number of various basic sites were generated upon loading KF onto the La-Zr-600 support. The weak basic sites were assigned to the surface hydroxyl groups produced during the formation of LaOF, while the strong and super basic sites were related to the Lewis base produced due to the interaction between KF and the La-Zr-600 support. A glycerol conversion of 91.77% and a GC selectivity of 99% were obtained over 0.3KF/La-Zr, which displayed the best catalytic performance under the optimal reaction conditions. The excellent activity of these catalysts was attributed to the presence of the strong and super basic sites, which favor the transesterification of glycerol with dimethyl carbonate. The production of GD from GC decarbonylation was unfeasible at low temperature using the newly developed catalysts. A plausible reaction mechanism has been proposed based on the experimental results and characterization.

Full text of DOI:10.1016/j.jallcom.2018.03.392

Accepted Manuscript
Synthesis of glycerol carbonate over porous La-Zr based catalysts: The role of strong
and super basic sites
Xianghai Song, Donghui Pan, Yuanfeng Wu, Pin Cheng, Ruiping Wei, Lijing Gao, Jin
Zhang, Guomin Xiao
PII:
S0925-8388(18)31269-6
10.1016/j.jallcom.2018.03.392
JALCOM 45622
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 1 September 2017
Revised Date: 28 March 2018
Accepted Date: 29 March 2018
Please cite this article as: X. Song, D. Pan, Y. Wu, P. Cheng, R. Wei, L. Gao, J. Zhang, G. Xiao,
Synthesis of glycerol carbonate over porous La-Zr based catalysts: The role of strong and super basic
sites, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.03.392.
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ACCEPTED MANUSCRIPT
Graphical Abstract
Preparation of glycerol carbonate from glycerol over KF/La-Zr solid base

ACCEPTED MANUSCRIPT
Synthesis of glycerol carbonate over porous La-Zr based catalysts:
The role of strong and super basic sites
Xianghai Song, Donghui Pan, Yuanfeng Wu, Pin Cheng, Ruiping Wei, Lijing Gao, Jin Zhang,
Guomin Xiao*
School of Chemistry and Chemical Engineering, Southeast University, Nanjing, China
Abstract
As important glycerol derivatives, glycerol carbonate (GC) and glycidol (GD) have attracted
increasing attention in recent years. In the present work, a series of new solid base catalysts were
developed to catalyze the conversion of glycerol to afford GC and GD. These catalysts were
prepared by loading KF on the porous La-Zr solid base catalyst and characterized using a series of
methods. A large number of various basic sites were generated upon loading KF onto the
La-Zr-600 support. The weak basic sites were assigned to the surface hydroxyl groups produced
during the formation of LaOF, while the strong and super basic sites were related to the Lewis
base produced due to the interaction between KF and the La-Zr-600 support. A glycerol
conversion of 91.77% and a GC selectivity of 99% were obtained over 0.3KF/La-Zr, which
displayed the best catalytic performance under the optimal reaction conditions. The excellent
activity of these catalysts was attributed to the presence of the strong and super basic sites, which
favor the transesterification of glycerol with dimethyl carbonate. The production of GD from GC
decarbonylation was unfeasible at low temperature using the newly developed catalysts. A
plausible reaction mechanism has been proposed based on the experimental results and
characterization.
Keywords: Glycerol, Glycidol, Glycerol carbonate, KF/La-Zr, Basic sites
1
. Introduction
The biodiesel industry has rapidly increased worldwide over the last decades in an effort to reduce
our dependence on fossil fuels. However, the expansion in biodiesel production has generated
huge amounts of glycerol as a by-product, which has seriously hampered the development of the
biodiesel industry. The conversion of glycerol to high value-added products is a perfect way to
utilize glycerol by-products formed during production of biodiesels and has attracted a
considerable amount of attention [1-3]. Several conversion processes have been reported for the
synthesis of valuable glycerol derivatives [4, 5]. Among the various derivatives of glycerol,
glycerol carbonate (GC) and glycidol (GD) are of great importance due to their wide ranging
applications. GC can be used for the manufacturing of coatings, biolubricants, polycarbonates,
polyesters, and polyurethanes due to its low flammability, low toxicity, and biodegradability [6-8].
1

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It can also be used as a solvent due to its high boiling point [9]. On the other hand, GD is utilized
as a stabilizer, plastic modifier, fire retardant, and surfactant [10]. In addition, GD can also be used
as a raw material for the preparation of polyglycerol, glycidyl ethers, and propanediols [11-13].
Over the past few years, extensive research has been carried out on the catalytic conversion of
glycerol to GC. For example, the transesterification of glycerol with dimethyl carbonate (DMC)
has been widely reported in the production of GC [14-16]. However, only a few studies on the
synthesis of GD from glycerol have been reported to date. Malkemus et al. first patented the
production of GD from GC using a metal salt as a homogeneous catalyst, achieving GD yields of
o
8
0%–90% at high temperature (175–225 C) and reduced pressure [17]. Later, zeolite-A was used
o
as a heterogeneous catalyst for the patented synthesis of GD from GC at 3.5 kPa and 183 C [18].
In the presence of a solvent and the absence of active hydrogen, anhydrous Na SO has been
2
4
o
shown to be effective for the preparation of GD at 200 C and 2.7 kPa [19]. Moreover, an ionic
liquid has also been found to be useful in the production of GD from GC [20]. Bolívar-Diaz et al.
selectively transformed GC into GD under mild conditions using a ZSM-5 zeolite catalyst and a
zinc oxide-supported nanoscale cobalt oxide catalyst [21].
Accordingly, to prepare GD directly from glycerol via the decarbonylation of GC is of great
interest and thus has recently attracted a great deal of research attention. Kelkar et al. reported the
first preparation of GD from glycerol with high GD selectivity (78%) using tetramethylammonium
o
hydroxide as a catalyst at 80 C [22]. Later, they found that a 1,4-diazabicyclo[2.2.2]octane based
ionic liquid was also highly active in the decarbonylation of GC obtained from the
transesterification of glycerol to give GD [23]. A Mg/Zr/Sr mixed oxide was proven to be active
for the production of GD and achieved a GD yield of 40% [24]. In addition, a KF/sepiolite catalyst
was also developed by Algoufi et al., which was used for the production of GD from glycerol [25].
To date, the research in this area is still limited and thus, further study is needed.
The preparation of GD directly from glycerol via the decarbonylation of GC involves two steps as
shown in Scheme 1: (1) The transesterification of glycerol with DMC to produce GC and (2) the
decarbonylation of GC to yield GD. The transesterification of glycerol can be easily achieved in
the presence of a basic catalyst [26, 27]. However, the decarbonylation of GC requires special
conditions, such as high temperature [17, 20], reduced pressure [18] or the requirement a use of
solvent [28].
Some studies on the preparation of GD at low temperature have been reported [22-25]. In this
work, we aimed to develop a new type of catalyst comprised of various basic sites, which
exhibited both excellent activity for glycerol transesterification and good selectivity towards GD.
Various catalysts, including metal oxides and alkaline metal salts were investigated. The results
2

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indicated that loading La-Zr with KF resulted in the best catalytic performance and the highest GD
selectivity. However, analysis of the product showed that GC was the only product of the reaction.
GD that was formed was derived from the decomposition of GC in the injection port of the gas
chromatograph due to its high temperature. It seems that the preparation of GD at low temperature
is not possible.
Scheme 1 Preparation of GD from glycerol and dimethyl carbonate.
2
2
. Experimental section
.1 Materials
La(NO ) ·6H O (AR), Zr(NO ) ·5H O (AR), CaO (AR) were purchased from Sinopharm
3
3
2
4 4
2
®
Chemical Reagent Co., Ltd. PEG-PPG-PEG Pluronic P-123 (Mn~5800) was purchased from
Sigma-Aldrich. SrO (AR), K CO (AR) and KF anhydrous (AR) were obtained from Aladdin
2
3
Industrial Corporation, Shanghai, China. Glycerol (AR), dimethyl carbonate (DMC) (AR) were
obtained from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. Glycidol (96%) was
purchased from Heowns Biochem Technologies Co., Ltd. Glycerol carbonate (96.6%) was
supplied by Accela ChemBio Co. Ltd.
2
.2 Catalyst preparation
2
.2.1 Preparation of porous La-Zr-600
The porous La-Zr mixed oxides were prepared by a co-precipitation method with P123 as model
template. In the experiment, P123 (2.41 g) was dissolved in 160 mL of deionized water and stirred
for another 30 minutes after dissolution. Then the system was acidified with 1 M HNO to PH=1-2.
3
La(NO ) ·6H O (6.93 g), Zr(NO ) ·5H O (6.87 g) were added to the solution mentioned above
3
3
2
4 4
2
under vigorous stirring for 3 h. Subsequently, the solution was basified with K CO (40 wt.%) to
2
3
o
PH=9-10. The obtained mixture was kept at 40 C for 12 h, and then hydrothermal treated at 180
o
C for 24 h. The resulted solid was separated by centrifugation and washed with a large amount of
o
deionized water to PH=7. Finally the obtained solid was dried at 120 C for 12 h and then
o
annealed at 600 C for 5 h. The resulted catalyst was marked as La-Zr-600.
2
.2.2 Preparation of La-Zr-600 supported KF
La-Zr-600 supported KF was prepared by wet impregnation. Typically, calculated amount of
anhydrous KF (0.30 g), La-Zr-600 (1.02 g) and 20 mL deionized water were placed into a 100 mL
flask. The resulted mixture was stirred at room temperature for 12 h, after which the water was
o
removed by evaporation at reduced pressure. After drying at 100 C for 12 h, the obtained sample
o
was annealed at 500 C for 5 h. The resulted catalyst was denoted as xKF/La-Zr, where x
3

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represented the mass ratio of KF to La-Zr-600.
.3 Catalyst characterization
X-ray diffraction (XRD) patterns of the catalysts were obtained using a Rigaku D/max-A
2
−
1
diffractometer. The scanning range (2θ) was within 5-80°, at a scanning rate of 20° min and a
step function of 0.02.
N adsorption–desorption isotherms were performed on a Beishide 3H-2000 analyser by static N2
2
o
physisorption at -196 C. The surface area of the prepared catalysts was calculated based on
multipoint Brunauer–Emmett–Teller (BET) method. Pore size and pore volume was calculated
from desorption branches of the isotherms through Barrett–Joyner–Halenda (BJH) method.
The surface functional groups were determined by fourier transform infrared spectrometer (FT-IR)
-1
on a Nicolet 5700 spectrometer, in the wavenumber range of 400-4000 cm with KBr as a
reference for the measurements.
Thermogravimetric (TG) analysis was conducted using a TG 209 F3 Tarsus instrument. In a
typical procedure, about 5 mg catalyst was placed into an aluminum pan which was then treated
on the instrument under air atmosphere from 50 to 800 ◦C at a heating rate of 10 ◦C per minute.
Temperature programmed desorption (CO -TPD) was performed to determine the basicity of the
2
o
catalyst. The moisture and other adsorbed gases were removed by pretreating the sample at 300 C
−
1
for 1 h in a flow of He (30 mL min ). Subsequently, the sample was cooled down to 50 °C, and
exposed to pure CO for 30 minutes. The physically adsorbed CO was excluded by purging the
2
2
−
1
o
sample with He flow (30 mL min ) for 1 h. Thereafter, the sample was heated to 700 C at a rate
o
−1
of 10 C min and the desorbed CO was detected by a thermal conductivity detector.
2
The TG-MS was carried out on a 409PC thermal analyzer (Netzsch, Germany) coupled with a
QMS403C instrument (Netzsch, Germany). In the experiment, 10 mg sample was heated at 10 °C
−
1
min from 40 to 600 °C in argon. Mass scanning was performed in the range m/z 2-200.
X-ray photoelectron spectroscopy (XPS) was measured on a Thermo Fisher Scientific ESCALAB
2
50Xi X-ray photoelectron spectrometer, using nonmonochromatized Mg Kα radiation (1253.6 eV)
as the X-ray source. The precision of the binding energy values were within 0.1 eV. The binding
energy values were calibrated by referencing the C 1s signal (285.0 eV).
2
.4. Catalytic activity test
The preparation of GD from glycerol and DMC was performed in a 50 mL round bottomed flask
equipped with a thermometer, a magnetic stirring bar and a rectifying column connected to a
liquid dividing head. In a typical experiment, 50 mmol (4.62 g) of glycerol and 100 mmol (9.05 g)
of DMC were placed into the flask, followed by 0.14 g (3 wt.%) of catalyst. Subsequently, the
reaction system was heated to the desired temperature for a designed time. During the reaction, the
4

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byproduct methanol was separated in suit to facilitate the reaction. After the reaction was
completed, the catalyst was separated from the reaction system by centrifugation. The product was
analyzed using a gas chromatograph (GC-6890, China), equipped with a flame ionization detector
and a capillary column (SE-30, 30 m × 0.25 mm). 2-Butoxy ethanol was added to the product as
an internal standard for the quantification analysis.
3
. Results and discussion
Fig. 1 XRD patterns of various catalysts. ꢀ La O CO , ꢁ ZrO , ꢂ LaOF, ꢃ KF, ꢄ▽
2
2
3
2
KLaF , × K CO (H O) .
4
2
3
2
1.5
3
.1 Characterization of catalysts
3
.1.1 XRD characterization
The XRD patterns of La-Zr-600 and La-Zr-600 loaded with different amounts of KF are presented
in Fig. 1. La-Zr-600 exhibits the typical characteristic peaks of La O CO (JCPDS Card No.
2
2
3
8
4-1963) and the small characteristic peaks of ZrO (JCPDS Card No. 49-1642). The good
2
dispersibility of ZrO in the catalyst may account for its small peaks when compared with
2
La O CO . After loading with 30 wt.% KF, the peaks for La O CO are completely replaced by the
2
2
3
2
2
3
peaks of LaOF (JCPDS Card No. 44-0121) in 0.3KF/La-Zr, indicating the product of the reaction
between La O CO and KF was LaOF (Scheme 2) [29]. The new LaOF species may be
2
2
3
responsible for the high activity and GD selectivity. However, the peaks for ZrO were almost
2
unchanged, which means ZrO may not react with KF. New characteristic diffraction peaks for
2
KLaF (JCPDS Card No. 75-1927) are observed in the 0.7KF/La-Zr catalyst. Besides, the intensity
4
of the diffraction peaks for KLaF increases upon increasing the loading amount of KF and the
4
5

ACCEPTED MANUSCRIPT
peaks for LaOF almost disappear in 1.2KF/La-Zr, revealing LaOF can further react with KF to
form KLaF (Scheme 2). Moreover, the diffraction peaks for KF appear in 1.0KF/La-Zr and
4
increase upon increasing the loading amount of KF. This means that to some extent, KF was
dispersed in the catalyst and interacts with the support, and beyond this extent the excess KF will
agglomerate on the surface of the support. The possible reaction between the La-Zr-600 support
and KF may occur as follows:
Scheme 2 Possible reaction formula between KF and La-Zr-600.
The existence of K CO was confirmed by the diffraction peaks observed in the XRD patterns
2
3
(
Scheme 2). The formation of LaOF from the reaction between La O (or La O CO ) and KF has
2 3 2 2 3
been reported previously [30, 31]. However, K O that was found in the second step was not
2
detected and this may be due to the high dispersion degree of K O in the catalyst. The presence of
2
KCaF and K O on CaO loaded with KF may serve as a reference for the second reaction [32].
3
2
The strong base K O may also account for the high activity of the La-Zr-600 supported KF.
2
Fig. 2 N adsorption-desorption isotherms of various La-Zr catalysts.
2
6

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3
.1.2 Morphologies characterization
Table 1 Textural parameters of the prepared catalysts
2
3
Catalyst
BET surface area (m /g)
Pore size (nm)
12.97
Pore volume (cm /g)
La-Zr-600
52.21
14.49
10.41
6.30
0.17
0.16
0.12
0.04
0.05
0
0
1
1
.3KF/La-Zr
.5KF/La-Zr
.0KF/La-Zr
.2KF/La-Zr
43.54
44.22
26.34
7.31
26.82
The morphologies of the as-prepared catalysts were characterized using N adsorption-desorption
2
isotherms and the results displayed in Fig. 2. Clearly, La-Zr-600 exhibits a type IV isotherm with a
hysteresis loop at P/P = 0.59–0.99, indicating the presence of mesopores [33]. In addition, a sharp
0
increase in the N adsorption amount was observed at low pressure, implying the existence of
2
micropores [34]. The formation of mesopores and micropores was attributed to the removal of the
soft template (P123). The surface area of the catalysts decreases sharply upon loading with KF from
2
2
5
2.21 m /g for La-Zr-600 to 6.30 m /g for 1.0KF/La-Zr, as shown in Table 1. The decreased
surface area may be related to the blocking of the pores in the catalysts by the loaded KF. In
addition, the pore volume decreases upon increasing the loading amount of KF, which was in
agreement with the change in the surface area. However, the pore size of the catalysts first
increased and then decreased upon increasing the loading amount of KF. Considering the pores are
blocked by KF, the larger pore size can be attributed to the void space between the catalyst
particles [35, 36]. Moreover, KF on the La-Zr-600 surface began to agglomerate when the content
of KF reached a particular amount, as described in the XRD analysis. This agglomeration may
account for the reduced pore size of the catalysts upon increasing the amount of KF.
3
.1.3 FT-IR characterization
The surface species of the as-prepared catalysts were also investigated using FT-IR spectroscopy
-1
and the results are shown in Fig. 3. The absorption bands within 700–1500 cm are attributed to
2
-
-1
various vibration modes of CO3 [32]. The bands around 873, 1089 and 1398 cm are assigned to
the characteristic bands of La O CO [37-39]. All the La-Zr-600 catalysts display an absorption
2
2
3
-1
band at around 1464 cm upon the addition of KF. The new band may be related to the C-O
stretching mode of K CO , which was produced due to the reaction of KF and La O CO , as
2
3
2
2
3
-1
shown in Scheme 2 [30]. The bands at 3000–3500 and 1663 cm are associated with the O-H
stretching mode and O-H bending vibration of the hydroxyl groups, respectively [26]. Moreover,
7

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the peak area of these bands increases upon increasing the loading amount of KF, which was in
-
1
accordance with the deliquescent properties of KF. Besides, a small absorption band at 2349 cm ,
which was assigned to the adsorbed CO , was also observed and indicated the existence of strong
2
basic sites. The FT-IR results match well with the XRD analysis.
Fig. 3 FT-IR spectra of (a) La-Zr-600, (b) 0.3KF/La-Zr, (c) 0.5KF/La-Zr, (d) 0.7KF/La-Zr, (e)
1.0KF/La-Zr, (f) 1.2KF/La-Zr.
3
.1.4 TG characterization
Fig. 4 TG curves of La-Zr-600, 1.0KF/La-Zr-rt (1.0KF/La-Zr without calcination), 1.0KF/La-Zr.
8

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The thermal behavior of La-Zr-600 and La-Zr-600 loaded with KF was investigated by
thermogravimetric analysis and the results are displayed in Fig. 4. Obviously, La-Zr-600 was quite
o
o
stable before 480 C and there was a slight weight loss of 0.91% between 480 and 600 C.
o
o
Considering the sample was treated at 600 C, the weight loss within 480–600 C may be
attributed to the decomposition of La (CO ) formed in the cooling process of the sample due to
2
3 3
o
the absorption of CO in the air [39]. The marked decrease in weight above 700 C should be
2
ascribed to the decomposition of La O CO and the structural carbonate [30, 40].
2
2
3
thermogravimetric analysis–mass spectroscopy was performed to verify the products of the
o
decomposition process, as shown in Fig S1. Clearly, H O and CO were detected at around 480 C
2
2
o
and a large amount of CO is detected above 700 C. Thus, the products of the catalyst
2
decomposition were CO , La O , and a small amount of water. Both 1.0KF/La-Zr and
2
2
3
o
1
.0KF/La-Zr-rt present a weight loss of about 3.8% below 100 C, which was attributed to the
adsorbed water due to the facile deliquescence of KF. When compared with 1.0KF/La-Zr,
o
1
.0KF/La-Zr-rt exhibits a weight loss of 1.12% between 200–300 C, indicating the interaction of
KF and La-Zr-600. Similarly, a decrease in weight at high temperature was also observed in these
o
o
two samples, while the beginning temperature shifted to 480 C, which was about 220 C lower
than that of La-Zr-600. The lower decomposition temperature may be related to the formation of
easily decomposed species, as indicated by XRD analysis. Besides, the interaction between KF
and the support may also lead to the decreased decomposition temperature.
9

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Fig. 5 CO -TPD profiles of the prepared KF/La-Zr catalysts.
2
Table 2 CO -TPD data for various KF/La-Zr catalysts
2
Weak base
strong base
Super base
c
Catalyst
TBA
a o
b
a
o
b
a
o
b
T ( C)
146.3
139.9
162.8
141.1
Area
T ( C)
562.3
575.9
590.0
575.3
Area
T ( C)
639.8
644.5
627.3
626.8
Area
0
0
1
.3KF/La-Zr
.5KF/La-Zr
.0KF/La-Zr
.2KF/La-Zr
605.36
450.27
394.87
506.03
626.63
965.76
2834.82
2048.26
2537.58
2019.84
4066.81
3464.29
5445.63
5105.28
2513.18
2579.41
1
a
b
c
Temperature of desorption peak. CO desorption peak area of various basic sites. Total CO
2
2
desorption peak area of various basic sites.
3
.1.5 CO -TPD characterization
2
The basicity of the catalyst is responsible for the transesterification of glycerol and the subsequent
decarboxylation of GC to give GD. Hence, CO -TPD was performed to investigate the basicity of
2
the as-prepared KF/La-Zr catalysts (Fig. 5)., The magnified profiles of KF and La-Zr-600 are
displayed in Fig. 5a to fully understand the basic properties of the two samples due to their poor
signals when compared with the KF/La-Zr catalysts. Clearly, almost no CO desorption peak was
2
observed for pure KF, indicating the absence of basicity in the pure KF sample. The La-Zr-600
o
sample exhibits evident multi-desorption peaks of CO at around 158, 231, 405, and 528 C,
2
implying the presence of weak, moderate and strong basic sites in the sample. There are few basic
sites on the surface of La O or La O CO , thus the basicity of La-Zr-600 may originate from the
2
3
2
2
3
La-Zr solid solution [40]. According to the TGA, after loading with KF the catalyst decomposes
1
0

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o
obviously within 480–700 C. Thus, a blank experiment (1.0KF/La-Zr-b: 1.0KF/La-Zr without
CO adsorption) was performed to investigate the influence of the catalyst decomposition process
2
on the CO -TPD profiles. Though 1.0KF/La-Zr-b (blank experiment) presents desorption peaks in
2
the zoomed in profile, these peaks can be ignored when compared with that of 1.0KF/La-Zr after
CO adsorption. Hence the effect of the catalyst decomposition process was almost negligible.
2
After loading with KF, the KF/La-Zr catalysts present intense CO desorption peaks (Fig. 5a),
2
suggesting the strong interaction between KF and the La-Zr-600 support. These basic sites may
originate from the new species formed due to the reaction between KF and La-Zr-600. Besides,
the interaction between KF and the La-Zr-600 support may also be responsible for these basic
sites.
Deconvolution of the CO -TPD profiles was performed to further understand the basic properties
2
of the catalysts (Fig. 5b). Clearly, all the samples present three peaks at around 150, 560 and 630
o
C, which are attributed to desorption of CO from the weak, strong and super basic sites,
2
respectively [30]. The weak basic sites are associated with the Brønsted base sites, while the
strong and super basic sites should be related to the Lewis base sites. According to the literature,
the weak basic sites are attributed to the surface hydroxyl groups, which are produced during the
formation of LaOF [29], as demonstrated by XRD and IR analyses. In addition, LaOF may also be
-
responsible for the basic sites in the catalyst [31]. The electronegativity of F is higher than lattice
2
-
-
oxygen (O ), which results in the pull of the negative charge of lattice oxygen towards the F ion
and the formation of strong basic sites [29, 33]. Thus, the strong basic sites should be derived from
the interaction between the highly dispersed KF and the La-Zr-600 support. In addition, the
produced K O may also account for the strong basic sites, as depicted in Scheme 2. The amount of
2
alkaline and temperature of the desorption peaks observed for the various catalysts are
summarized in Table 2. The amount of strong basic sites increases upon increasing the KF loading
to 50% (1.0KF/La-Zr) with a shift in the desorption peak to higher temperature. A further increase
in the KF loading leads to a decrease in the desorption temperature. This phenomenon may be due
to the complicated interaction between the KF and the support. Considering all the catalysts
loaded with KF exhibit a high glycerol conversion when compared with La-Zr-600, the strong and
super basic sites should play key roles in the transesterification of glycerol to afford GC and GD.
1
1

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Fig. 6 XPS spectra of La-Zr-600 and 1.0KF/ La-Zr, (a) full spectra, (b) La3d, (c) Zr3d, (d) O1s.
3
.1.6 XPS characterization
XPS characterization was employed to gain further insight into the chemical state of the species
present on the surface of the catalysts (Fig. 6). The characteristic peaks for the elements La, Zr, O
and C were observed in La-Zr-600, which was in good agreement with the XRD results. After
loading with KF, the signals for K and F were detected in 1.0KF/La-Zr, implying KF was
successfully loaded onto the support (Fig. 6a). The La 3d spectra of La-Zr-600 presents two peaks
3
+
at 855.0 and 838.1 eV, which were assigned to the characteristic 3d3/2 and 3d5/2 levels of La ,
respectively (Fig. 6b) [41, 42]. The doublet structures of these two peaks are attributed to multiplet
splitting [43]. However, the La 3d spectrum observed for 1.0KF/La-Zr was completely different
from that observed for La-Zr-600. Two main peaks are observed at 852.4 and 835.6 eV, which
were accompanied by some small peaks at 856.3, 839.5 and 828.7 eV, in the La 3d spectrum of
1
.0KF/La-Zr. The complicated peak shape of La 3d may be related to the co-existence of various
La species, such as La O , La O CO , LaOF, and KLaF, which were in accordance with the XRD
2
3
2
2
3
results. The Zr 3d peaks observed at 183.7 and 181.3 eV in La-Zr-600 (Fig. 6c) are assigned to the
1
2

ACCEPTED MANUSCRIPT
4
+
3
d3/2 and 3d5/2 levels of Zr , and are derived from the spin–orbit splitting [44]. In comparison to
La-Zr-600, the Zr 3d spectrum of 1.0KF/La-Zr was almost unchanged with only a little shift to the
lower binding energy by 0.5 eV. The shift in the binding energy may be associated with the
interaction between ZrO (or La-Zr solid solution) and KF. The O1s spectrum for La-Zr-600 can
2
be roughly divided into two peaks (Fig. 6d). The peak at around 529.1 eV was attributed to the
2
-
lattice oxygen (O ) of La O and ZrO [39, 44, 45], while the peak centered at 531.1 eV was
2
3
2
2
-
assigned to the CO3 in La O CO [46, 47]. After being treated with KF, 1.0KF/ La-Zr presents an
2
2
3
intense peak at 530.3 eV, which was due to the generation of a new environment around the
surface oxygen atoms, which may be related to the new LaOF species, as depicted in the XRD
analysis [29]. According to the characterization mentioned above, it seems that the interaction
between KF and La-Zr-600 mainly occurs on the La species.
3
.2 Catalytic activities
3
.2.1 Catalyst screening
a
Table 3 Catalytic activity of various catalysts for the production of GD and GC
d
d
Catalyst
KF
Gly. conv. (%)
19.93
GD Sel. (%)
26.70
7.09
GC Sel. (%)
73.30
GC yield (%)
14.61
CaO
SrO
81.85
92.91
76.05
50.64
4.36
95.64
48.44
b
La O
70.00
22.00
0
77.00
53.90
2
3
La-Zr-600
15.97
100.00
68.91
15.97
c
KF+La-Zr-600
38.43
31.09
46.89
26.48
0
.3KF/La-Zr
91.77
53.11
48.74
a
o
b
Reaction conditions: Glycerol 50 mmol, DMC 100 mmol, catalyst 3 wt.%, 80 C, 60 min. 120
o
c
d
C, DMF as solvent, 90 min, Reference [28]. A mixture of 0.03 g KF and 0.10 g La-Zr-600.
The yield and selectivity are calculated based on gas chromatography.
According to the literature, basic catalysts are active for the transesterification of glycerol to afford
GC, and strong bases and alkaline metal salts are beneficial for the decarbonylation of GC to give
GD. Thus, various catalysts have been utilized to catalyze the production of GD from glycerol,
which are presented in Table 3. Clearly, KF itself displays poor glycerol conversion and GD
selectivity. The glycerol conversion of CaO and SrO is moderate, but the selectivity towards GD is
low. According to the literature, La O presents a glycerol conversion of 70% and GD selectivity
2
3
1
3

ACCEPTED MANUSCRIPT
o
of 22%, while the reaction needed to be performed at 120 C and in the presence of DMF. It has
been reported that the presence of ZrO could improve the basicity of La O [38]. However, when
2
2
3
La-Zr-600 was used as the catalyst, both the glycerol conversion and GD selectivity were poor,
o
which may be attributed to the low reaction temperature (80 C). The combination of KF and
La-Zr-600 exhibits a glycerol conversion of 38.43% and GD selectivity of 31.09%, indicating the
synergistic effect between KF and La-Zr-600. Moreover, when KF was loaded onto La-Zr-600, the
conversion of glycerol increased markedly to 91.77% and the selectivity towards GD also
increased to 46.89%. According to the CO -TPD results, extensive strong and super basic sites are
2
generated on La-Zr-600 after being loaded with KF. Thus, the better glycerol conversion and high
selectivity of GD should be related to the strong and super basic sites on the surface of
0
.3KF/La-Zr as a result of the interaction of KF and the La-Zr-600 support.
3
.2.2 Identification of product
Considering many of the literature methods used to produce GD require special conditions (high
o
temperature and reduced pressure), the appearance of GD at 80 C under atmosphere pressure
1
13
seems to be unbelievable. Thus, H- and C-NMR spectroscopy was performed to confirm that
GD was indeed produced during the reaction, After removing DMC and methanol by evaporation,
1
13
the product of 0.3KF/La-Zr was isolated and subjected to H-NMR and C-NMR analysis (Fig. 7a
1
and b). The H-NMR spectrum shows that the product was almost pure GC; the peaks
1
3
corresponding to GD at δ = 2-3 were absent (Fig. S2). Furthermore, the C-NMR spectrum of the
product presented an intense peak at δ =155.64, which was the characteristic peak of the carbonyl;
again the peaks corresponding to GD were not observed (Fig. S3). These results show that no GD
is produced in the reaction. To further confirm the result, the reaction process was monitored by
1
H-NMR spectroscopy to determine the reaction pathway (Fig. 7c). The peak intensity of glycerol
(
Fig. S4) decreased as the reaction proceeded and the peak intensity of GC increased as the
reaction time progressed, while no peaks for GD were observed during this process, suggesting no
GD was produced during the reaction. In addition, FT-IR spectroscopy also shows the product
produced was GC and not GD (Fig. 7d), as indicated by the sharp peak corresponding to the
-1
1
carbonyl group at around 1750 cm . Moreover, no GD was detected by H-NMR spectroscopy
o
even when the temperature was increased to 100 C. According to the literature, the preparation of
1
4

ACCEPTED MANUSCRIPT
o
GD was conventionally carried out at high temperature (175–200 C) [17-19]. The GD detected on
the gas chromatograph was a result of the decomposition of GC due to the high temperature of the
o
injection port (270 C). Thus, the product of the reaction over 0.3kF/La-Zr was GC and the
synthesis of GD at low temperature seems to be impossible over this catalyst.
1
Fig. 7 (a) H-NMR of the product over 0.3kF/La-Zr, (600 MHz, DMSO-D6) δ = 5.33 (s, 1H), δ =
4
.78-4.82 (m, 1H), δ = 4.50 (t, 1H), δ = 4.28-4.30 (m, 1H), δ = 3.65-3.68 (dd, 1H), δ = 3.50-3.53
1
3
1
(
dd, 1H). (b) C-NMR δ = 155.64, 77.49, 66.33, 61.05. (c) H-NMR of the reaction at different
time, the number 1, 2, 3 and 4 represents methanol, DMC, glycerol and GC, respectively. (d)
FT-IR of the product.
1
5

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3
.2.3 Effects of reaction parameters
Table 4 Effects of reaction parameters
Catalyst
Catalyst
Time (min)
Temperature
Glycerol Conv. GC Sel. (%)
(%)
o
loading (%)
( C)
0
0
0
0
0
0
0
0
0
0
0
0
.1KF/La-Zr
.3KF/La-Zr
.5KF/La-Zr
.7KF/La-Zr
.3KF/La-Zr
.3KF/La-Zr
.3KF/La-Zr
.3KF/La-Zr
.3KF/La-Zr
.3KF/La-Zr
.3KF/La-Zr
.3KF/La-Zr
3
90
90
90
90
90
90
90
90
90
20
40
60
80
80
80
80
80
80
80
70
90
80
80
80
85.22
90.70
91.21▽
91.44▽
74.43
91.51
92.61
36.30
91.42
47.55
73.52
91.77
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
3
3
3
0.5
1
5
1
1
1
1
1
The La-Zr-600 supported KF catalysts were further studied to obtain the optimum reaction
parameters and the results presented in Table 4. Clearly, the KF loading amount has a significant
effect on the glycerol conversion, which increases upon increasing the loading amount of KF from
8
5.22% (0.1KF/La-Zr) to 90.70% (0.3KF/La-Zr). Further increasing the KF loading amount does
not enhance the conversion of glycerol as demonstrated by a glycerol conversion of 91.44% over
.7KF/La-Zr. The results suggest the amount of KF is crucial for glycerol conversion to afford GC.
0
The effects of the catalyst amount were also performed to obtain the optimal catalyst loading.
Similarly, the glycerol conversion increases upon increasing the catalyst loading and reaches 91.51%
at a catalyst loading of 1 wt.% of. More catalyst does not increase the glycerol conversion
considerably as indicated by the glycerol conversion of 92.61% at a catalyst loading of 5 wt.%.
Considering the significant dependence of the reaction on temperature, the effects of the reaction
temperature were also investigated. The glycerol conversion increases from 36.30 to 91.51% upon
o
increasing the reaction temperature from 70 to 80 C. A further increase in temperature did not
o
significantly enhance the conversion of glycerol. Thus, 80 C was determined to be suitable for the
reaction. According to the literature, a longer reaction time is beneficial for the transesterification
of glycerol to give GC. The conversion of glycerol enhanced from 47.55 to 91.77% when the
1
6

ACCEPTED MANUSCRIPT
reaction time was increased from 20 min to 60 min, and then remained almost unchanged upon
further increasing the reaction time. Therefore, 60 min was determined as the optimal reaction
o
time. In summary, the optimal reaction conditions are 0.3KF/La-Zr, 1 wt.% catalyst l, 80 C and
6
0 min.
Fig. 8 Reusability of 0.3KF/La-Zr in the synthesis of GC. Reaction conditions: Glycerol 50 mmol,
o
DMC 100 mmol, catalyst 1 wt.%, 80 C, 60 min.
3
. 3 Reusability of the catalyst
Reusability is the main criterion used to assess heterogeneous catalysts. The reuseability of
0
.3KF/La-Zr is displayed in Fig. 8. The catalyst was recovered using simple filtration and
o
calcination at 500 C for 5 h to remove the adsorbed organic compounds, and then used for the
next run. Clearly, the activity of 0.3KF/La-Zr for glycerol transesterification decreases upon
increasing the cycle number. In the second cycle, the conversion of glycerol drops from 91.77 to
7
3.83%, and in the third run to 54.14%, suggesting the gradually deactivation of the catalyst.
However, the selectivity of GC was almost unchanged during this process. The observed
deactivation for glycerol conversion was mainly attributed to the loss of the strong and super basic
sites, which results from the leaching of KF. KF can serve as a catalyst for GC decarbonylation to
1
7

ACCEPTED MANUSCRIPT
afford GD at high temperature, as proved by the high selectivity of GD on gas chromatography.
This means the KF leaching into the reaction solution during the reaction process was occurring.
Considering the stability of the support, the catalyst was regenerated by reloading KF on the
o
recovered support followed by calcination at 500 C for 5 h. After this regeneration step, the
catalyst exhibited a glycerol conversion of 90.18% and GC selectivity of 99% in the fourth cycle,
suggesting the catalyst can be easily regenerated by reloading KF onto the recovered support.
Fig. 9 Plausible reaction mechanism for synthesis of GC and GD from glycerol.
3
.4 Reaction mechanism
The reaction of glycerol with dimethyl carbonate to afford GD involves two steps, as shown in
Scheme 1. The first step can be easily achieved in the presence of a basic catalyst. However, the
decarbonylation of GC is rather difficult. Based on the experimental and characterization results,
the strong and super basic sites play a critical role in the first step, however, temperature seems to
be the decisive factor in the second step. By combining the literature precedent [24, 28, 48] and
our experimental and characterization results, a plausible reaction mechanism was proposed and
displayed in Fig. 9. Firstly, the hydroxyl groups of glycerol are adsorbed and activated by the
basic sites of the catalyst through hydrogen bonding. Meanwhile, dimethyl carbonate is activated
4
+
by the Lewis acid sites (Zr ) in the catalyst. Thereafter, the activated hydroxyl groups undergo
1
8

ACCEPTED MANUSCRIPT
nucleophilic attack on the carbonyl group of dimethyl carbonate, accompanied by the elimination
of one molecule of methanol to give intermediate (I). Then, the activated hydroxyl groups and
carbonyl groups of intermediate (I) undergo an intramolecular nucleophilic substitution along with
the elimination of another molecule of methanol to give the product, GC. However, GC cannot be
activated by the strong and super basic sites at low temperature to conduct a decarbonylation
reaction to afford GD. However, at high temperature and in the presence of a metal salt, such as
KF, GC can lose one molecule of CO to give GD.
2
Conclusions
A series of novel catalysts were developed by loading KF onto a porous La-Zr-600 solid support
and their catalytic performance investigated for glycerol conversion to prepare GC and GD. The
characterization results showed that large number of weak, strong and super basic sites were
generated upon loading KF on the La-Zr support. The weak basic sites were proposed to be
associated with the Brønsted base generated during the reaction between KF and the La-Zr-600
support, while the strong and super basic sites were assigned to the Lewis base originating from
the interaction between KF and the La-Zr-600 support. The high activity of the catalyst was
related to the large number of strong and super basic sites generated on the catalyst’s surface. The
large amount of GD detected by gas chromatograph was attributed to the decomposition of GC
due to the high temperature of the injection port. It was found that the strong and super basic sites
play a key role in the transesterification of glycerol, while high temperature seems to be essential
for the decarbonylation of GC to give GD. Finally, the catalyst can be easily regenerated by
simply reloading KF onto the recovered catalyst.
Acknowledgments
This work was financially supported by National Natural Science Foundation of China (No.
2
1276050, 21676054 and 21406034), Natural Science foundation of Jiangsu (No. BK20161415),
Open Found of Jiangsu key laboratory (No. JSBEM21409). We thank International Science
Editing (http://www.internationalscienceediting.com ) for editing this manuscript.
1
9

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1
2
. Porous La-Zr solid base was prepared with P123 as soft template.
. Abundant various basic sites were generated upon loading with KF on La-Zr
support.
. KF/La-Zr catalysts displayed good catalytic performance in the preparation of
glycerol carbonate.
. The high catalytic performance was related to the strong and super basic sites.
3
4