Fluorinated Mg–Al Hydrotalcites Derived Basic Catalysts for Transesterification of Glycerol with Dimethyl Carbonate

Article abstract of DOI:10.1007/s10562-017-2013-9

Abstract: A series of fluorinated Mg–Al hydrotalcite-like (HTl) compounds were prepared by introduction of different amount of (AlF₆)^3? into HTl sheets via co-precipitation method. The fluorine-modified Mg–Al mixed oxides (CHT-F) with mesoporous structure were then acquired by thermal decomposition of as-prepared HTl precursors. The influence of fluorine modifier on the physicochemical properties of CHT-F samples was studied in detail and the results demonstrated that their structure and basicity were strongly related to the fluorine content. Simultaneously, the CHT-F samples were test for the glycerol carbonate (GC) production via transesterification between glycerol and dimethyl carbonate without organic solvent. For these catalysts, the role of basic sites was examined based on reactant conversion and product selectivities in overall reaction as well as in side reaction identified in this work. The catalytic results showed that the activity of CHT-F catalysts depended on the total number of surface basic sites. However, further study demonstrated that the undesired product was catalyzed by strong basic sites, resulting in decrease of GC selectivity. Thus, the introduction of appropriate amount of (AlF₆)^3? into the HTl structure can promote the production of GC, and the best catalytic performance was obtained over the catalyst with F:Al = 1.0. Besides, various parameters including reaction time, reaction temperature and catalyst amount were investigated to optimize the reaction conditions. Furthermore, these CHT-F catalysts possessed high stability on the basis of reusability test and catalyst characterization. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-017-2013-9

Catal Lett (2017) 147:1181–1196
DOI 10.1007/s10562-017-2013-9
Fluorinated Mg–Al Hydrotalcites Derived Basic Catalysts
for Transesterification of Glycerol with Dimethyl Carbonate
Xuelan Zhang1,2 · Dengfeng Wang1 · Jie Ma1 · Wei Wei2
Received: 9 January 2017 / Accepted: 21 February 2017 / Published online: 9 March 2017
© Springer Science+Business Media New York 2017
Abstract A series of fluorinated Mg–Al hydrotalcite-like
(HTl) compounds were prepared by introduction of differ-
ent amount of (AlF6)3− into HTl sheets via co-precipita-
tion method. The fluorine-modified Mg–Al mixed oxides
(CHT-F) with mesoporous structure were then acquired
by thermal decomposition of as-prepared HTl precursors.
The influence of fluorine modifier on the physicochemical
properties of CHT-F samples was studied in detail and the
results demonstrated that their structure and basicity were
strongly related to the fluorine content. Simultaneously, the
CHT-F samples were test for the glycerol carbonate (GC)
production via transesterification between glycerol and
dimethyl carbonate without organic solvent. For these cata-
lysts, the role of basic sites was examined based on reactant
conversion and product selectivities in overall reaction as
well as in side reaction identified in this work. The cata-
lytic results showed that the activity of CHT-F catalysts
depended on the total number of surface basic sites. How-
ever, further study demonstrated that the undesired product
was catalyzed by strong basic sites, resulting in decrease
of GC selectivity. Thus, the introduction of appropriate
amount of (AlF6)3− into the HTl structure can promote the
production of GC, and the best catalytic performance was
obtained over the catalyst with F:Al=1.0. Besides, vari-
ous parameters including reaction time, reaction tempera-
ture and catalyst amount were investigated to optimize the
reaction conditions. Furthermore, these CHT-F catalysts
possessed high stability on the basis of reusability test and
catalyst characterization.
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-017-2013-9) contains supplementary
material, which is available to authorized users.
* Xuelan Zhang
mhszxl@sxicc.ac.cn
* Dengfeng Wang
bitwdf@163.com
1
College of Chemistry, Chemical Engineering and Materials
Science, Zaozhuang University, Zaozhuang 277160, China
2
CAS Key Lab of Low-Carbon Conversion Science
and Engineering, Shanghai Advanced Research Institute,
Chinese Academy of Sciences, Shanghai 201210, China
Vol.:(0123456789)
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X. Zhang et al.
Graphical Abstract
Among them, transesterification of glycerol with dimethyl
carbonate (DMC) is considered as one of the most prospec-
tive and simplest process for the GC synthesis in compari-
son with the reaction of glycerol with carbon dioxide, car-
bon monoxide, phosgene, urea, diethyl carbonate, ethylene
carbonate [2, 6]. Although homogeneous basic catalysts
including KOH, NaOH and K2CO3 are valuable catalysts
for the GC production, the separation of them from the
products is a great inconvenience for industrial production
[7]. So far, various heterogeneous catalysts such as immobi-
lized lipase Novozym 435, Merrifield resin-supported pol-
yamines and KNO3/CaO have been used for this reaction.
Unfortunately, they all have some corresponding drawbacks
such as long reaction time, inconvenient removal of THF
solvent, high cost, a complex catalyst preparation process,
and easy leaching [8–10].
Keywords Fluorinated hydrotalcite · Glycerol carbonate ·
Transesterification · Dimethyl carbonate · Basic property
1 Introduction
Recently, biodiesel has been drawing extensive attention
as a renewable clean energy due to its great potential as
an alternative to fossil fuels [1]. So, its production capac-
ity increased remarkably. However, a large amount of
glycerol is generated as an undesired product during the
transesterification process, which makes for a large surplus
in current glycerol market [1, 2]. Since the production of
biofuels from biomass is increasing, the surplus production
of glycerol is expected to continue for years to come [3].
Therefore, the application for the low-cost glycerol need be
explored by converting it into useful chemicals. One impor-
tant glycerol derivative is glycerol carbonate (GC), which
is widely used in fields of organic synthesis, gas separation,
detergents. More important, due to its low toxicity, low
evaporation rate, low flammability, and moisturizing abil-
ity, GC is used as wetting agent for cosmetics and carrier
solvent for medical preparations [4, 5]. Up to now, several
synthetic routes for synthesis of GC have been reported.
Hydrotalcite-like compounds (HTlcs) are the most stud-
ied catalyst precursors in many catalytic processes. These
compounds could be described by the general formula
[M2+ M3+(OH)2]x+(An−)x/n·mH2O, where M2+ and M3+
1−x
represent divalent and trivalent cations, respectively, and
An− stands for a charge-balancing anion [11]. The ther-
mal decomposition of HTlcs can give the mixed oxides,
which have many advantages including high surface area,
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Fluorinated Mg–Al Hydrotalcites Derived Basic Catalysts for Transesterification of Glycerol…
1183
high stability against sintering and strong Lewis basic sites.
More important, the basic properties of these mixed oxides
could be tunable by the composition of metal cations in
the HTlcs [12–15]. As a result, the facile adjustment of the
type of basic sites and basic strength makes HTlcs precur-
sors as valuable solid base catalysts for the GC synthesis.
According to previous work, uncalcined Mg–Al hydrotal-
cite samples could be used as base catalysts for this reac-
tion with THF as solvent [16]. Alvarez and his co-workers
found that calcined Mg–Al mixed oxides from hydrotalcite
(HT) could improve this reaction and 76% of GC yield was
achieved [17]. Calcined Mg–Al–Zr, calcined Mg–Zr–Sr
and doping transition metals cations into Mg–Al hydrotal-
cites have been also tested as catalysts for this transesteri-
fication reaction at reaction temperatures in the range of
75–100°C [12, 18, 19]. Besides, Li-doped Mg–Al mixed
oxide showed high catalytic ability, giving 100% glycerol
conversion. However, the catalyst stability was not investi-
gated [20]. More recently, calcined Ca–Al double hydrox-
ides were used to promote this reaction [21], but Granados-
Reyes et al. found that the GC selectivity was only 65–75%
[22]. Thus, it is still a challenging task to develop more
efficient and reusable catalysts for synthesis of GC avoid-
ing side reactions such as the decarboxylation of GC, form-
ing the dominant side product, glycidol (GD) [19]. At the
same time, the previous works suggested that the amount of
basic sites were important for the GC synthesis. Neverthe-
less, the role of basic sites in decarbonylation of GC and in
the overall reaction performance, i.e. selectivity of GC have
been unclarified.
fluorine ions could be directly incorporated into HTl sheets
via a novel synthesis method with partial replacement of
the (Al(OH)6)3− octahedra by (AlF6)3− [26, 27]. Thereby,
fluorinated HTlcs can be easily acquired with a wide range
in fluorine additive content.
In the present research, the fluorine ions were incor-
porated as a part of the Mg–Al hydrotalcite layers with a
large rang of fluorine amount via co-precipitation pro-
cess using a constant-pH method. The fluorine-modified
Mg–Al mixed oxide catalytic materials with different fluo-
rine amount were then obtained by thermal decomposition
of fluorinated Mg–Al HTlcs. To achieve accurate insights
into the role of basic sites in the reaction, the side reaction
influencing the yield of GC was identified, and every reac-
tion path in the network was separately studied. The rela-
tionship between conversion of glycerol, selectivity of GC
and the basic property of catalytic materials, measured by
CO2-TPD, were estimated to clarify the function of the base
sites during the reaction, providing a strategy to rationally
maximize GC yield. Moreover, the optimal catalyst and
reaction conditions were also attempted. Finally, on the
basis of results of reusability test and catalyst characteri-
zation, a stable heterogeneous catalyst with high reactivity
was successfully obtained.
2 Experimental
2.1 Preparation of Catalysts
On the other hand, for the HTlcs, their base properties
can also be tailored by the variation of interlayer anions
[23]. As a basic anion, fluorine ion can be used to promote
many base-catalytic reactions. It has been reported that
fluorine-containing hydrotalcite-derived mixed oxides are
effective solid base catalysts for O-methylation of phenol
with DMC [24]. Gao et al. have disclosed that the incor-
poration of fluorine into Cu-based catalysts could enhance
their surface basicity [25]. However, to our best knowledge,
the influence of fluorine doping on the basicity of calcined
hydrotalcites and their catalytic performance towards GC
synthesis via transesterification have not been reported.
Meanwhile, in previous work, the fluorine ions were intro-
duction into the hydrotalcite-like (HTl) layers using the
so called “memory effect” (the calcination-reconstruction
process), and the calcined sample can be recovered an
HTl structure after treatment in aqueous solutions con-
taining the fluorine anions under N2 atmosphere with stir-
ring. Obviously, this preparation process is complex and
time consuming, as well as the specific surface reduced
greatly after reconstruction of the material [23–25]. In the
HTl structure, the M3+ is encircled by six oxygen atoms of
the OH− anions as first neighbors and it is likely that the
Fluorinated Mg–Al hydrotalcite-like compounds with
atomic ratio Mg:Al=3:1 were synthesized by a co-precip-
itation procedure at room temperature using a constant-pH
method. Typically, a 100 mL aqueous solution containing
appropriate amounts of Mg(NO3)2·6H2O, Al(NO3)3·9H2O
and Na3AlF6, and a second 100 mL aqueous solution con-
taining NaOH and Na2CO3 were simultaneously added
dropwise to 100 mL of deionized water under vigorous
stirring at room temperature. The pH during precipitation
was at a constant value of 10.0 0.1 by addition suitable
amount of 2 M NaOH aqueous solution. The resulting gel-
like slurry was aged at 60°C for 6 h and then filtered and
washed with deionized water until pH 7. The filter cakes
were dried at 100°C in air for 12 h to obtain the HTlcs.
Thus, the source of aluminum was either aluminum nitrate
or a mixture of aluminum nitrate and sodium hexafluoro-
aluminate. Finally, the HTl precursors were calcined in
oven at 500°C for 7 h under a flowing stream of pure N2
to obtain the fluorine-modified Mg–Al mixed oxides. The
as-prepared HTlcs were referred to as HT-nF, where n was
the nominal F/Al atomic ratio in the starting solution (see
Table 1), and corresponding mixed oxides were denoted as
CHT-nF.
1 3

1184
X. Zhang et al.
Table 1 Structure, composition
as well as weigh loss of the
fluorinated HTl precursors
Sample
F/Al atomic Lattice parametersb
Weight lossc (%)
SBET (m2/g)
Pore
ratioa
(Å)
volume
(cm3/g)
a
c
Total
Second
HT-0F
0
3.048
3.048
3.048
3.048
3.030
23.307
23.066
23.054
23.026
23.002
31.6
31.2
30.8
29.9
27.5
21.4
17.8
14.7
13.2
12.6
21
37
42
26
33
0.12
0.29
0.34
0.17
0.21
HT-0.5F
HT-1.0F
HT-1.5F
HT-2.0F
0.5
1.0
1.5
2.0
aThe nominal atomic ratio in the synthesis mixture
bDetermined by XRD, a=2d (110), c=3d (003)
cThe weight losses measured in TG experiments for total and second steps
2.2 Characterization
The basic properties of the catalysts were studied by
CO2 temperature-programmed desorption (CO2-TPD).
Before the experiments, catalysts (0.1 g, 40–60 mesh) were
pretreated in Ar flow at 500°C for 2 h, then, cooled to room
temperature. Subsequently, they were saturated with pure
CO2 (30 L/min) using a six-way valve for 3 h and then
flushed with Ar (40 L/min) flow to eliminate all physical
adsorbed molecules. Afterward, the CO2-TPD tests were
started from 30 to 700°C with a heating rate of 10°C/
min under Ar flow (40 L/min), and the desorbed CO2 was
detected using an AMETEK mass spectrometer. CO2 peak
area was quantitatively calibrated by injecting CO2 pulses.
Thermogravimetric analysis (TGA) was conducted on a
NETZSCH STA-409 thermal analyzer. The samples were
heated from 30 to 700°C at 10°C/min under nitrogen
(30 mL/min).
Power X-ray diffraction (XRD) experiments were car-
ried out on a Rigaku Miniflex diffractometer using a Cu tar-
get with a Ni filter in a 2θ range of 5–80°.
Nitrogen adsorption–desorption measurements were
performed at −196°C on a Micromeritics ASAP-2020
instrument (Norcross, GA), using static adsorption proce-
dures. The samples were degassed at 100 and 200°C prior
to the measurement for the HTlcs and mixed oxide sam-
ples, respectively. The specific surface area and pore vol-
ume were calculated by the BET isotherm and BJH model,
respectively.
2.3 Catalytic Test
Transesterification of glycerol and DMC was carried out in
a 50 mL round-bottom flask with a reflux condenser and a
magnetic stirrer. Typically, 20 mmol of glycerol, 60 mmol
of DMC and 0.3 g catalysts were put into the flask. Then,
the flask was heated to 110°C in a short time and kept for
3 h with magnetic stirring. During the reaction, the tem-
perature error was less than 2°C. After reaction, the reac-
tion mixture was centrifuged to separate the solid catalyst.
Finally, after 4 mmol of 1,4-butanediol (internal stand-
ard) was added, the reaction products were quantitatively
analyzed using a gas chromatograph (GC-2014ATFSPL,
Shimadzu) with a CBP-20 capillary column and a flame
ionization detector (FID). The column was temperature-
programmed from 80 to 240°C. Meanwhile, all products
detected in the liquid were identified by a GC-MS (Agilent
7890-7000B) equipped with an HP-5MS capillary column
(30 m).
Field-emission scanning electron microscopy (SEM)
was performed on a Hitachi S4800 scanning electron
micrograph operated at 20.00 kV.
The chemical compositions of catalytic materials were
measured by an inductively coupled plasma-optical (ICP)
emission spectroscopy using Thermo iCAP 6300. Besides,
ion-selective electrode analysis was used to determine the
content of fluorine in the samples.
X-ray photoelectron spectroscopy (XPS) measure-
ments were conducted using a Thermo Scientific k-Alpha
spectrometer equipped with Al anode (Al kα=1486.6 eV)
operating at 72 W and spot size of 400 μm under ultrahigh
vacuum (10−7 Pa). The binding energies were calibrated
internally for surface charging by adventitious carbon
deposit C (1 s) with Eb =284.5 eV.
The single pulse 19F MAS NMR single excitation spec-
tra were recorded on a Brucker Advance III spectrometer.
The 19F MAS NMR spectra were acquired by operating the
spectrometer at 375.5 MHz, using π/2 pulses of 6 ms with
a recycle delay of 1 s, and 19F chemical shifts were refer-
enced to those of CFCl3 at 0 ppm.
In case of GC decomposition reaction, a mixture of the
GC (20 mmol) and 0.3 g of catalyst was set to react for 3 h
at 110°C in a stainless steel batch reactor. Once cooled
to room temperature, the reactor was dipped in a cooling
bath at −50°C, and vented. The released gas was ana-
lyzed by GC-MS. Then the autoclave was allowed to return
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Fluorinated Mg–Al Hydrotalcites Derived Basic Catalysts for Transesterification of Glycerol…
1185
gradually to room temperature and the liquid reaction mix-
ture was analyzed by GC and GC-MS.
were related to (003), (006), (009), (015), (018), (110), and
(113) crystal planes. At the same time, no other diffraction
peaks were found for fluorine-containing precursors [11].
So, this phenomenon revealed that no crystalline fluorine
compounds existed on the surface of HTlcs. However,
it could be observed that the peaks intensities and sharp-
ness of (003) and (006) planes decreased gradually, indi-
cating the decrease of the crystallinity of material as the
increase of the fluorine content in HTl precursors. Clearly,
when n increased to 2.0, the diffraction peaks of samples
were much weaker, implying the deterioration of HTl phase
due to the insert of excess amount of (AlF6)3−. Besides, the
position of the XRD peak assigned to (003) plane shifted
slightly from lower to higher 2θ values with the increase
of fluorine content. This suggested that the attraction
between the anions and the brucite-like layers was stronger
with the increasing fluorine content. Consequently, the dis-
tance between the layers decreased [26]. In addition, the
HTl precursors could be described by two crystallographic
parameters a and c, and the results are listed in Table 1. It
could be found that except for HT-2.0F, the cell parameters
a (=2d110) of all the fluorine-containing precursors were
identical to that of fluorine-free precursor (HT-0F). Gener-
ally speaking, the lattice parameter a reflected the atomic
distribution in the (110) crystal plane and closely depended
on the Mg/Al molar ratio in samples [11, 24]. Thus, com-
bined the above results based on chemical analysis, the
lower content of Al3+ in HT-2.0F compared to other sam-
ples gave rise to the decrease of the positive charge den-
sity in the layers. As a result, HT-2.0F precursor presented
lower value of parameter a than that of others [26]. The
parameter c (=3d003) was a reflection of the thickness of
the HTl layer and the interlayer distance, which was related
to the hydrated anion size, the thickness of the HTl layer,
and electrostatic force in the HTl interlayer [11, 24]. Evi-
dently, it could be seen that the parameter c decreased on
increasing of fluorine content in the present case. For the
3 Results and Discussion
3.1 Textural and Structural Properties of the Prepared
Materials
The results of chemical analysis are shown in Table 2.
Except for CHT-2.0F, the Mg/Al atomic ratioes in all of
the samples were identical. As to CHT-2.0F, its Al con-
tent was lower compared to that in other catalytic materi-
als. Meanwhile, for all the catalysts, the F/Al atomic ratio
was deviated from the nominal values in starting solutions
(see Table 1) and the deviation gradually increased with the
increase of n from 0.5 to 2.0. Especially, the F/Al atomic
ratio in CHT-2.0F sample was much lower than that in
other fluorine-modified catalysts. These results were asso-
ciated with the stability of (AlF6)3− in aqueous solution.
According to previous report, there was an ionic equilib-
rium between (AlF6)3− and (AlF4)− in aqueous aluminum-
sodium fluoride mixtures. Under alkaline conditions for
synthesis of HTlcs using co-precipitation method, only
(AlF6)3− could be incorporated into HTl structure [26]. In
order to follow the Le-Chatelier’s theory, (AlF6)3− species
were generated incessantly during the co-precipitation pro-
cess. When HT-2.0F sample was prepared, excess amount
of (AlF6)3− was added in the solution. Unavoidably, some
(AlF4)− species, which could not incorporate into the HTl
sheet, might still exist in mother solution, leading to the
lower aluminum and fluorine content in CHT-2.0F sample
compared with the nominal values [28].
The XRD patterns of HTl precursors are shown in
Fig. 1a. Evidently, all the samples exhibited similar profiles
to the pure HT, exhibiting seven feature diffraction peaks
at about 2θ of 11°, 23°, 34°, 38°, 46°, 60° and 61°, which
Table 2 The chemical composition and physicochemical properties of the CHT-nF catalysts
Sample
Atomic ratio in
solidsa
Lattice param- SBET (m2/g)
Pore volume Amounts of basic sites (μmol/g) and contributionc
eters (Å)b
(cm3/g)
Mg:Al
F:Al
Total
W.
M.
S.
11.2 (5.8)
71.0 (29.8)
100.4 (37.3)
113.1 (45.7)
120.2 (57.0)
CHT-0F
2.69:1
2.69:1
2.69:1
2.69:1
2.77:1
0
4.162
4.164
4.168
4.173
4.186
207
192
177
151
126
0.89
0.81
0.73
0.63
0.57
193.5
238.3
269.2
247.5
210.8
68.1 (35.2)
47.4 (19.9)
44.7 (16.6)
26.0 (10.5)
17.7 (8.4)
114.2 (59.0)
119.9 (50.3)
124.1 (46.1)
108.4 (43.8)
72.9 (34.6)
CHT-0.5F
CHT-1.0F
CHT-1.5F
CHT-2.0F
0.45:1
0.94:1
1.41:1
1.59:1
W., weak basic sites; M., moderate basic sites; S., strong basic sites
aValues measured by ICP
bDetermined by XRD, and calculated from the position of the 2θ=43° peak in Fig. 2b
cThe value in parentheses was the contribution of single basic sites to the number of total basic site
1 3

1186
X. Zhang et al.
(a)
(b)
o
43
o
62
o
35
CHT-2.0F
CHT-1.5F
HT-2.0F
HT-1.5F
HT-1.0F
CHT-1.0F
CHT-1.0F-fourth
HT-0.5F
HT-0F
CHT-0.5F
CHT-0F
20
30
50
60
70
420
θ
(o)
10
20
30
40
50
60
70
2θ
(o)
Fig. 1 XRD patterns of the HT-nF precursors (a); CHT-nF mixed oxide catalysts (b)
fluorine-containing materials, fluorine could form strong
hydrogen bonds with hydrogen atoms of water [26]. Thus,
it seemed that the decrease in c value could be attributed to
the stronger attraction between the interlayer anions and the
HTl layer.
the XRD characterization. As expected, the 19F NMR spec-
tra of HT-1.0F and HT-2.0F in Fig. 2 are more illustrative.
No matter how much the amount of fluorine that was intro-
duced into HTl structure, two peaks for fluorine-containing
samples could be observed. The first peak at −174.8 ppm
corresponds to Na3AlF6 [25], whose spectrum was also
provided in Fig. 2. for reference. Therefore, the main peak
at −174.8 ppm was attributed to the (AlF6)3− octahedra,
Figure 1b gives the XRD patterns of the catalytic mate-
rials after calcination. Obviously, calcination of precursors
at 500°C resulted in the collapse of the HTl structure, for
no characteristic diffraction peaks of HTls appeared in the
XRD patterns. Thus, fluorine-modified Mg–Al mixed oxide
catalysts were successfully prepared. Besides, it was inter-
esting to note that new diffraction peaks due to the peri-
clase MgO-like structure (PDF No. 45-946) was detectable
at 2θ=35°, 43°, and 62° for all the mixed oxide catalysts,
regardless of presence or absence of fluorine. No reflections
owing to other metal oxides or fluorides appeared, display-
ing that they might be homogeneously dispersed in an
amorphous state. In addition, the crystallographic param-
eters corresponding to the position of the 2θ=43° of this-
like MgO structure were calculated in this research (see
Table 2). Apparently, it was seen that all these values were
less than that of pure MgO (4.21 Å) [29]. This was due to
that other metal oxides and fluorides had incorporated into
the MgO lattice to generate a solid solution. Therefore, it
might be beneficial for improving their catalytic stability
during the reaction.
-174.8
-190.5
HT-2.0F
HT-1.0F
Na AlF
3
6
70
0
-70
-140
-210
-280
Chemical Shift (ppm)
According to the analysis above-mentioned, it was not
evident for the presence of fluorine in the samples based on
Fig. 2 19F MAS NMR spectrum of precursor materials
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Fluorinated Mg–Al Hydrotalcites Derived Basic Catalysts for Transesterification of Glycerol…
1187
although this peak was much broader than that of Na3AlF6.
Besides, in the spectra of fluorinated HTlcs, another NMR
peak at −190.5 ppm indicated a second type of fluorine
coordination. This was not surprising because that not only
of F− but also of OH− existed in network of HTl precursors
(see Fig. S1). Thus, OH− could influence the chemical shift
of fluorine. In addition, the inclusion of (AlF6)3− into lay-
ered structure took place most probably randomly. So, the
first coordination sphere of fluorine included both Al3+ and
Mg2+ cations, which was also proved by other researchers
[26, 28].
weight loss at 389–425°C might be attributed to the dehy-
droxylation of layers and the loss of anions, leading to the
collapse of the layered structure [11, 24]. Consequently,
fluorinated Mg–Al mixed oxides with the periclase struc-
ture were successfully prepared by the complete decompo-
sition of HT-nF at 500°C. More important, the introduction
of fluorine into HTlcs clearly gave rise an evident difference
for the second step: compared to pure Mg–Al hydrotalcite
(HT-0F), this step took place at higher temperature for the
fluorine-containing samples because of the stronger inter-
action between layers and anions as the result of the pres-
ence of fluorine, which was in accordance with the XRD
results [28]. Furthermore, the weight losses determined by
TG characterization for the total and second steps are com-
pared in Table 1. It could be seen that the HT-0F sample
lost more weight than that of fluorine-containing samples.
Meanwhile, the weight loss for the second step decreased
as increase of n from 0 to 2.0; especially, HT-0F sample
lost about 10% more weight than that of HT-2.0F sample,
which indicated that dehydroxylation or decarbonylation
occurred instead of defluorination for these fluorine-con-
taining materials. All these phenomena were in line with
the results reported in previous work, and the changes con-
cerning the thermal decomposition process might modify
the textural properties of the final fluorinated Mg–Al mixed
oxide catalysts [7, 26].
In order to clarify the decomposition process during cal-
cination, all the HTl precursors were analyzed by the TG
technique and their weight loss and weight loss rate curves
are shown in Fig. 3. In general, two major steps were
observed for all the samples. The first step at 80–225°C
included two overlapped peaks, which could be associated
with the elimination of physically absorbed and interlayer
water [11]. Obviously, it could be found that the addition
of (AlF6)3− into the HTl structures increased the tempera-
ture of removal of interlayer water, which was due to that
fluorine could enhance the strength of hydrogen bonds with
hydrogen atoms of water [26]. Subsequently, the second
The morphologies of the HTl precursors are displayed in
Fig. 4. It could be found that all the materials exhibited typ-
ical well-developed and plate-shaped crystals with a size of
50–250 nm, which indicated the formation of layered struc-
ture [28]. Meanwhile, the platelet-like crystals were homo-
geneous and dense stacking. As to the pure Mg–Al HT
(HT-0F) precursor, it possessed mainly hexagonal plate-
shaped crystals, which was in line with the result in the pre-
vious report [12]. With respect to fluorine-containing pre-
cursor, some fragments appeared when more (AlF6)3− was
incorporated into the HTl layers, and the hexagonal plate-
let-like structure seemed to be vague for HT-2.0F sample.
Besides, the dispersion of platelets for HT-0F sample was
better compared to that of fluorine-modified samples and
it became worse and worse as n increased from 0 to 2.0.
Furthermore, energy-dispersive X-ray spectroscopy (EDX)
analysis of the particles of the HT-1.0F sample (Fig. 4f)
displayed the presence of Mg, Al, O, F and C, which also
indicated that fluorine had been successfully introduced
into HTl layers. The crystals of the mixed oxides obtained
at 500°C showed the similar morphology to that of the pre-
cursors, but some crystals were destroyed during calcina-
tion (see Fig. 5). Moreover, it was not easily to distinguish
the individual platelets and neighboring particles seemed to
be sintered.
HT-2.0F
105
425
225
HT-1.5F
HT-1.0F
94
220
216
415
101
407
HT-0.5F
80
199
396
389
HT-0F
86
180
120 240 360 480 600
Temperature (oC)
The N2 adsorption–desorption isotherms were recorded
to measure the pore structure of the catalytic materials. The
Fig. 3 Thermogravimetry and differential thermogravimetry (TG-
DTG) profiles of HT-nF precursors
1 3

1188
X. Zhang et al.
Fig. 4 SEM images of uncal-
cined samples: HT-0F (a); HT-
0.5F (b); HT-1.0F (c); HT-1.5F
(d); HT-2.0F (e); corresponding
EDX spectrum of the HT-1.0F
revealing the presence of Mg,
Al, F, O and C (f)
N2 adsorption–desorption isotherms and the correspond-
ing pore size distribution of the CHT-nF catalysts are illus-
trated in Fig. 6a, b, respectively. Clearly, all the catalysts
presented typical type IV adsorption isotherms with clear
hysteresis loops at higher relative pressure, and their pore
sizes were widely distributed in the range of 5–100 nm.
These results suggested that the mesoporous structures
were formed in all the samples during thermal treatment.
More important, it could be seen that these hysteresis
loops were narrow. So, their mesoporous pores were regu-
lar. Moreover, all the CHT-nF samples possessed H3 hys-
teresis loops (IUPAC classification), which was related to
materials consisting of platy-shaped particles. This was in
line with the view of mixed oxides derived from HTl com-
pounds [30]. However, their pore size distribution profiles
were gradually broadened on increasing of fluorine con-
tent in the catalysts. Table 2 gives the nitrogen adsorption
analysis data of the CHT-nF catalysts. It could been found
that the BET specific surface areas (SBET) and pore volume
were much higher than that of corresponding HTl precur-
sors (see Table 1). These results were mainly ascribed to a
large amount of H2O and CO2 evolved from the precursors
during the second weight loss, according to the TG analy-
sis [24]. Thus, this might improve their catalytic ability for
the GC synthesis. In addition, it should be mentioned that
both the BET surface areas and pore volume were gradu-
ally decreased as the increase of fluorine additive amount.
According to the previous report, this might be because of
the decrease of the dispersion of platelets for precursors
as shown in Fig. 4 [28]. At the same time, the decrease of
weight loss shown in TG analysis indicated that the amount
of H2O and CO2 evolved from the precursors during calci-
nation decreased as the n increased from 0 to 2.0. Conse-
quently, less open porous networks were formed after heat
treatment, which undoubtedly lower their surface area and
pore volume.
3.2 XPS Investigations
The mixed oxides of CHT-nF catalysts were characterized
by XPS, and the binding energies (BE) of Mg 1s, Al 2p
and F 1s bands are presented in Table 3. It was well known
1 3

Fluorinated Mg–Al Hydrotalcites Derived Basic Catalysts for Transesterification of Glycerol…
1189
Fig. 5 SEM images of CHT-nF
mixed oxides
that shifts in the photoelectron (binding) energy of the peak
maximum value implied the changes in the chemical envi-
ronment (valence) of an atom [28]. When nominal F/Al
atomic ratio <1.0, the variation of the BE of Mg 1s and Al
2p bands was small as the increase of fluorine content. In
contrast, with further increase of amount of fluorine addi-
tive, these two bands moved gradually to higher binding
energies. These results demonstrated the decrease of the
electron density in the Mg2+ and Al3+ nucleus. Besides,
the BE of F 1s changed from 684.5 to 685.3 eV, but there
was no direct relationship between the BE and the amount
of fluorine in catalysts. As the mentioned above, the first
coordination sphere of fluorine contained Mg2+ and Al3+
cations. Thus, the electronegativity differences between
oxygen and fluorine led to the gradual shift in the BE of
Mg 1s and Al 2p bands [31].
chemical compositions presented in Table 2, it could be
found that the surface was significantly depleted of Mg,
but enriched with Al for these catalytic materials. Similar
phenomenon had been reported in a previous literature, in
which F:Al atomic ratios in the surface decreased consid-
erably when fluorinated Cu–Zn–Al–Zr mixed oxides were
prepared [28].
3.3 The Surface Basicity of Catalysts
Generally speaking, the surface basic property of solid
base catalysts could be evaluated by two important param-
eters: basic strength and total basicity (the amount of basic
sites per unit weight of catalyst) [32]. Here, the informa-
tion about the surface basicity of calcined catalysts was
obtained by using the CO2-TPD characterization. Higher
desorption temperature was corresponding to stronger
basic strength and higher amount of desorped CO2 stood
for more total basicity. The CO2-TPD curves obtained
At the same time, XPS technique was also used to deter-
mine the surface compositions of the CHT-nF samples
and the results are listed in Table 3. Compared with their
1 3

1190
X. Zhang et al.
700
600
500
400
300
200
100
0
CHT-2.0F
466
450
400
(a)
CHT-0F
200
302
CHT-0.5F
CHT-1.0F
CHT-1.5F
CHT-2.0F
CHT-1.5F
296
285
436
CHT-1.0F
416
CHT-0.5F
CHT-0F
0.0
0.2
0.4
0.6
0.8
1.0
271
265
Relative pressure (P/P)
0
(b)
CHT-0F
1.8
1.5
1.2
0.9
0.6
0.3
0.0
CHT-0.5F
CHT-1.0F
CHT-1.5F
CHT-2.0F
405
100 200 300 400 500 600
Temperature (oC)
Fig. 7 CO2-TPD of CHT-nF mixed oxide catalysts
to the coordinatively unsaturated O2− ions [12, 24, 33].
Upon introduction of fluorine into Mg–Al mixed oxides,
the metal-fluorine pairs and the coordinatively unsaturated
F− ions were also presence on the surface of the fluorine-
containing samples. Alike to mechanisms of basic sites of
CHT-0F sample, the weak, moderate and strong basic sites
of fluorine-containing catalytic materials might be assigned
to the OH− group, metal-oxygen and metal-fluorine pairs,
and unsaturated O2− and F− ions, respectively [24, 25].
Although no direct correlation between the desorption tem-
perature of weak basic sites and the fluorine content could
be found for fluorine-modified catalysts, their moderate and
strong basic sites shifted gradually to higher temperature
from CHT-0F to CHT-2.0F samples. As stated above, these
results might be because of the new basic sites of metal-
fluorine pairs and F− ions formed in fluorine-modified sam-
ples, which enhanced their moderate and strong basic sites,
respectively.
0
20
40
60
80
100
Pore diameter (nm)
Fig. 6 N2 adsorption–desorption isotherms (a); and pore-size distri-
butions (b) of CHT-nF catalysts
Table 3 XPS results of CHT-nF catalysts
Sample
Binding energy of peak maxi-
mum (eV)
Atomic ratioa
Mg 1s
Al 2p
F 1s
Mg:Al
F:Al
CHT-0F
1302.6
1302.6
1302.7
1303.1
1303.5
73.9
73.9
74.0
74.3
74.7
–
2.47:1
2.02:1
2.03:1
1.96:1
2.01:1
0:1
CHT-0.5F
CHT-1.0F
CHT-1.5F
CHT-2.0F
684.5
685.3
685.0
684.8
0.43:1
0.84:1
1.03:1
1.12:1
The basicity and the distribution of different basic sites
on various fluorinated Mg–Al mixed oxide catalysts are
summarized in Table 2. It was found that the contribution
of the weak basic sites to the total basic sites decreased
gradually with rise of fluorine content. This might be
mainly due to the substitution of hydroxyl anions by fluo-
rine anions. In contrast, their proportions of both moder-
ate and strong basic sites were more than that of fluorine-
free sample. Although the amount of strong basic sites of
CHT-F monotonically increased on increasing of fluorine
content, the number of moderate basic sites increased first
aOn the surface of catalyst
for the fluorine-free and fluorine-modified catalysts are
shown in Fig. 7. Clearly, all the profiles can be divided into
three Gaussian peaks, which could be ascribed to weak
(<200°C), moderate (200–400°C) and strong basic sites
(>400°C), respectively. For fluorine-free sample (CHT-
0F), the weak and moderate basic sites were associated with
the surface hydroxyl group and the metal-oxygen pairs,
respectively. As to the strong basic sites, they were related
1 3

Fluorinated Mg–Al Hydrotalcites Derived Basic Catalysts for Transesterification of Glycerol…
1191
and reached the maximum until n=1.0, and then decreased.
Theoretically, addition of fluorine could promote the num-
ber of metal-fluorine pairs on the catalyst surface, which
could result in enhancing the density of moderate basic
sites in fluorine-modified samples. Nevertheless, the strong
basic sites (O2− and F−) in fluorine-modified samples were
mainly associated with the break of metal-oxygen and
metal-fluorine pairs. With the increase of fluorine additive,
unsaturated O2− and F− were inclined to aggregate on the
catalyst surface at the cost of metal-oxygen and metal-fluo-
rine pairs, leading to increase of electronic density of strong
basic site [28]. So, the metal-oxygen and metal-fluorine
pairs in the mixed oxides would gradually convert to unsat-
urated O2− and F− ions as the fluorine content increased.
Thus, the presence of fluorine not only improved the basic
strength, but also enhanced the amounts of strong basic
sites of CHT-F catalytic materials. It was worth mentioning
that the number of strong basic sites of samples followed
the order: CHT-2.0F>CHT-1.5F>CHT-1.0F>CHT-
0.5F>CHT-0F. Furthermore, one could see that the rela-
tion of total basicity of samples with the amount of fluo-
rine additive was similar to that of number of moderate
basic sites. This was due to that the influence of the insert
of (AlF6)3− on the total basic density were double-faced.
On one hand, the generation of metal-fluorine pairs and
the coordinatively unsaturated F− ions could increase the
amounts of moderate and strong basic sites, respectively.
On the other hand, the specific surface area decreased on
increasing of fluorine content, resulting in decrease of the
exposed basic sites. The former might be a dominant factor
when the atomic ratio of F/Al was less 1.0, while the lat-
ter would surpass the former if n further increased. There-
fore, the variation exhibited a volcanic shape and their sur-
face basic densities decreased in the following sequence:
CHT-1.0F>CHT-1.5F>CHT-0.5F>CHT-2.0F>CHT-0F.
3.4 Catalytic Performance
3.4.1 The Reaction System of Glycerol and DMC
to Synthesis GC Identified in this Work
The chemical components of the reaction mixture were
identified by means of GC-MS and quantified by GC analy-
sis. In addition to the major reaction path and product gen-
erally reported (Scheme 1), GD was detected by GC-MS
as another product in this work (see Fig. S2(a)). It was pro-
duced by decarbonylation of GC, accompanying a release
of CO2. As a result, the GC selectivity would decrease
correspondingly with the formation of GD. Rokichi et al.
reported that diglycerol tricarbonate could be produced
from the reaction of GC with DMC when using DMC in
a much (tenfold) excess and for a long reaction time (48 h)
[34]. However, this product was not detected in the present
case. This might be because that the DMC/glycerol molar
ratio was only three, much lower than ten. Furthermore, the
reaction time was also much lower than 48 h in the present
research.
3.4.2 The Catalytic Perfzormance for the Synthesis of GC
from Glycerol and DMC
Table 4 gives the glycerol conversion and product yield in
the absence of a catalyst (blank) and in the presence of the
fluorine-modified mixed oxides. Obviously, the reaction of
glycerol and DMC hardly took place without a catalyst. For
the mixed oxide catalysts, it was important to note that the
amount of fluorine presented a significant impact on cata-
lytic results. When CHT-0F sample was used as the cata-
lyst, only trace of GD was found under the reaction condi-
tions of this work. However, the glycerol conversion over
it was only 57.8%, relatively low in comparison with that
of fluorine-containing mixed oxide samples. As a result,
GC yield was far from satisfactory. CHT-0.5F could give
83.7% yield of GC, but the GC selectivity was lower than
that achieved over pure Mg–Al mixed oxide. The best
catalytic performance in terms of glycerol conversion was
Scheme 1 The reaction system
O
of glycerol and DMC: identified
O
OH
reaction pathways in this work
O
O
Catalyst
+
HO
OH
+
2CH3OH
O
O
OH
GC
DMC
Glycerol
Catalyst
O
-
CO2
OH
GD
1 3

1192
X. Zhang et al.
Table 4 The catalytic
performance for GC synthesis
by transesterification of glycerol
with DMC
Entry
Catalyst
Glycerol conversion Selectivity (%)
(%)
Yield (%)
GC
3.2
57.7
82.4
95.3
80.5
59.4
GC
GD
GD
1
2
3
4
5
6
–
3.2
100
0
0
CHT-0F
CHT-0.5F
CHT-1.0F
CHT-1.5F
CHT-2.0F
57.8
83.7
100
91.3
71.2
99.8
98.4
95.3
88.2
83.4
0.2
1.6
4.7
11.8
16.6
Trace
1.3
4.7
10.8
11.8
Reaction conditions: glycerol: 20 mmol (1.84 g), DMC: 60 mmol (5.4 g), catalyst: 0.3 g, temperature:
110°C, time: 3 h
obtained for CHT-1.0F sample, showing 100% glycerol
conversion. More important, this catalyst could effectively
suppressed the production of GD and a good GC selectiv-
ity of 95.3% could be achieved. Consequently, it exhibited
the highest GC yield among all the catalytic materials. As
the fluorine content further increased, both the glycerol
conversion and GC selectivity reduced sharply. Specially,
the EC conversion was only 71.2% when CHT-2.0F cata-
lyst was used. In contrast, its selectivity towards undesired
side product (GD) were very high (16.6%). So, the yield of
GC was only 59.4%, revealing that too high content of fluo-
rine was not beneficial for achieving high GC yield. Impor-
tantly, it was worth to found that for these catalysts, their
catalytic activity followed this order: CHT-1.0F>CHT-
1.5F>CHT-0.5F>CHT-2.0F>CHT-0F. Nevertheless, the
GC selectivity obtained over them decreased in sequence of
CHT-0F>CHT-0.5F>CHT-1.0F>CHT-1.5F>CHT-2.0F.
Thus, the results of catalytic test revealed that the incorpo-
ration of fluorine into Mg–Al mixed oxides could drasti-
cally enhance glycerol conversion, but the selectivity of GC
gradually decreased as the increase of fluorine content.
the GC conversion was closely related to the fluorine con-
tent. The conversion of GC was very modest over CHT-
0.5F catalyst, whereas CHT-2.0F gave the highest conver-
sion. Considering the amount of strong basic sites increased
with the fluorine content increasing from n=0 to n=2.0,
the catalytic ability of catalysts might be originated from
their strong basic sites. Indeed, a good correlation between
the catalytic activity and the number of strong basic sites
could be observed in Fig. 8a. Therefore, the undesired side
reaction of GC decomposition was possibly prevented by
weakening the density of strong basic sites.
3.4.4 The Role of Basic Sites in the Transesterification
of Glycerol and DMC
For catalysts, it was like that high BET surface areas could
help them exhibit higher catalytic activity. As listed in
Table 1, it could be seen that for the CHT-F catalytic sam-
ples, their BET surface area decreased as the increase of
fluorine content. However, their catalytic activity followed
this order: CHT-1.0F>CHT-1.5F>CHT-2.0F>CHT-
0.5F>CHT-0F. Thus, surface area of catalysts was not
the sole factor that influenced their catalytic ability in the
present work. In the course of careful investigation on fac-
tors influencing GC conversion, there was a general trend
observed between the reactivity and the total amount of
basic sites (Fig. 8b). This indicated that the total basic-
ity of all three types of basic sites played a crucial role in
glycerol conversion and the catalytic activity of these cat-
alysts depended on their total surface basic density. The
importance of basic property of catalytic materials in this
or similar reaction has been concluded by other studies. P.
Unnikrishnan and D. Srinivas reported that the transesteri-
fication of propylene carbonate with methanol to produce
DMC could be improved over the catalyst with high surface
basicity [35]. Recently, Liu et al. stated that the high activ-
ity of Ni-modified Mg–Al catalyst could be attributed to its
high surface basic density [12].
3.4.3 The Catalytic Performance for the Synthesis of GD
from Decarbonylation of GC
As described earlier, the desired target product GC could
decompose into GD, accompanying a release of CO2. In
order to understand the nature of this reaction in light of the
basic property of the catalysts, all the materials were used
to catalyze the decarbonylation of GC. The GC-MS anal-
ysis of the released gas showed the sole presence of CO2
(see Fig. S2(b)). Meanwhile, the analysis of residual liquid
mixture in the autoclave revealed that GD was by-product,
confirming that the decarbonylation of GC occurred as pre-
sented in Scheme 1. Importantly, the analysis of gas and
liquid phases also offered the possibility to determine the
conversion of GC. As shown in Fig. 7a, the GC conversion
after 3 h reaction differed significantly among the CHT-nF
catalysts. When CHT-0F was used as catalyst, GC hardly
decomposed into GD. For the fluorine-containing samples,
However, it was important to remark that regarding
GC selectivity, fluorine-modified Mg–Al mixed oxide
1 3

Fluorinated Mg–Al Hydrotalcites Derived Basic Catalysts for Transesterification of Glycerol…
1193
product GC would be consumed drastically in presence of
catalysts with high density of strong basic sites, undoubt-
edly resulting in the decrease of GC selectivity in the
overall reaction. To further verify whether the GC selec-
tivity in reaction of glycerol and DMC could be influ-
enced by the strong basic sites of the catalytic materials,
we illustrate the plot of GC selectivity and the amount
of strong basic sites in the Fig. 8c. As expected, the
trend for GC selectivity obtained over the catalysts was
observed to be in contrast with the order of their basic
density of strong basic sites. This was consistent with the
results in previous work, in which the GC selectivity also
decreased with rise of basic strength of strong basic sites
when several NaOH/γ-Al2O3 was used as basic catalyst
for this reaction [36]. Therefore, it could be conclude
that the improvement of the catalytic performance of
fluorine-modified Mg–Al mixed oxide catalysts towards
GC synthesis by transesterification of glycerol and DMC
was owing to the modification of the basic properties by
introduction of a appropriate amount of fluorine additive.
120
90
(a)
24
16
8
60
30
0
F
CHT-0F
CHT-1.0F
CHT-0.5F
CHT-1.5 CHT-2.0F
270
240
210
180
150
100
75
50
25
0
(b)
3.4.5 Effect of Reaction Conditions
None
CHT-0F
CHT-0.5F
CHT-1.0F
CHT-1.5F
The reaction conditions were further optimized for the
CHT-1.0F catalyst. As shown in Fig. 9a, the transestifica-
tion of reaction achieved 100% conversion of glycerol after
3 h. The GC yield increased rapidly with the reaction time
up to 3 h, and then a marginal decrease was observed on
further extension of the reaction time to 4 h. This might be
because that small amount of GD was formed by decompo-
sition of GC.18 So, the suitable reaction time was chosen as
3 h.
CHT-2.0F
102
96
90
84
78
120
90
(c)
60
30
The reaction of glycerol and DMC was conducted in
the temperature range of 40–130°C, and the effect of reac-
tion time was shown in Fig. 9b. Clearly, the yield of GC
increased slowly with the temperature up to 80°C, but
increased sharply thereafter. The rapid increase of GC yield
above 80°C might attributed in part to vaporization loss of
co-produced methanol from the reaction, which would be
favorable for shifting the reaction towards the synthesis of
GC [9]. Specially, the GC yield reached its maximum of
95.3% at 110°C, and then the consumption of GC in the
further reaction surpassed gradually the formation of GC
from glycerol and DMC. Thus, these results revealed that
the appropriate reaction temperature was 110°C.
F
None
CHT-0F
CHT-1.5
CHT-1.0F
CHT-0.5F
CHT-2.0F
Fig. 8 The relationship between GC conversion and density of strong
basic sites for the decarbonylation of GC (a). The correlation between
glycerol conversion and the total basic sites density for the overall
reaction of glycerol and DMC (b). The correlation between GC selec-
tivity and density of strong basic sites for the overall reaction of glyc-
erol and DMC (c)
showed inferior performance in comparison with fluo-
rine-free sample (CHT-0F). As the discussion above, the
addition of fluorine in mixed oxide catalysts was found
to drastically promote their strong basic sites and conse-
quently influence reaction paths, GC yield and selectivity.
Indeed, the catalytic tests above had been confirmed that
the side reaction (decarbonylation of GC) was catalyzed
greatly by the strong basic site. So, the desired target
The effect of catalyst amount was illustrated in Fig. 9c.
As the catalyst amount increased, the glycerol conversion
increased and it reached maximum value when the cata-
lyst amount was 0.3 g. However, further increase of cata-
lyst amount led to a slight decrease of GC yield due to the
side reaction. Therefore, the preferable catalyst amount was
0.3 g.
1 3

1194
X. Zhang et al.
100
80
60
40
20
100
90
80
70
60
50
100
80
(a)
Glycerol conversion
GC yield
60
1
2
3
4
5
Reaction time (h)
40
100
80
60
40
20
0
(b)
20
0
1
2
3
4
Cycle number
Glycerol conversion
GC yield
Fig. 10 Reusability of the CHT-1.0F catalyst for the GC synthesis
from glycerol and DMC. Reaction conditions: 110°C, 3 h, 0.3 g cata-
lyst, 20 mmol glycerol, 60 mmol DMC
40
60
80
100 120
Reaction temperature (oC)
100
90
80
70
60
50
(c)
also confirmed that the resused catalyst exhibited original
MgO-like structure without appearance of magnesium car-
bonate (Fig. 1b). Furthermore, for CHT-1.0F-fourth cata-
lyst, its BET surface area (173 m2/g) was only change little
compared with that of the fresh catalyst (177 m2/g). There-
fore, the CHT-1.0F was stable for glycerol transesterifica-
tion with DMC, to yield GC.
Glycerol conversion
GC yield
0.1
0.2
0.3
0.4
0.5
Catalyst amount (g)
3.4.7 Activity Comparison of the CHT‑1.0F Catalyst
with Other Reference Catalysts
Fig. 9 Effect of reaction conditions on the GC synthesis over CHT-
1.0F catalyst. Effect of reaction time (a): reaction temperature,
110°C; catalyst amount, 0.3 g; 20 mmol glycerol; 60 mmol DMC.
Effect of reaction temperature (b): reaction time, 3 h; catalyst amount,
0.3 g; 20 mmol glycerol; 60 mmol DMC. Effect of catalyst amount
(c): reaction temperature, 110°C; reaction time, 3 h; 20 mmol glyc-
erol; 60 mmol DMC
It was difficult to directly compare the activity of this CHT-
1.0F catalyst with other basic catalysts that reported in pre-
vious literature, for their catalytic performances were esti-
mated under quite different reaction conditions. So, several
representative references catalysts were prepared and tested
under the reaction conditions of this work and the results
are shown in Table 5. Apparently, the HT-1.0F precursor
exhibited inferior activity compared with that of CHT-1.0F
catalyst. The reason might be as follows: on the one hand,
the former possessed very lower surface area (42 m2/g) than
that of CHT-1.0F (177 m2/g). On the other hand, accord-
ing to the traditional view of this material, the basic den-
sity available on the surface of HT-1.0F precursor should
be much weaker compared to the latter. Besides, under the
same reaction conditions, CHT-1.0F was more active com-
pared with MgO, implying the improvement effect of fluo-
rine on catalyst basicity and activity. More important, CHT-
1.0F sample gave higher glycerol conversion and GC yield
compared to CHT-Ni and CHT-Zr catalysts, which revealed
that addition of fluorine was more favorable for improving
the activity of Mg–Al mixed oxide towards GC synthe-
sis. As to CaO and NaOH, they also presented high activ-
ity for GC synthesis, but the GC selectivity was very low,
3.4.6 Stability of CHT‑1.0F Catalyst
As the main aspect of the heterogeneous catalyst, reusabil-
ity of catalyst plays an important role in practical industrial
production for the GC. So, the reusability of the catalyst
concerning CHT-1.0F was studied. After reaction, the used
catalyst was collected by filtration. Then, it was washed
with methanol several times to eliminate the absorbed
organic product, dried in oven at 80°C and directly reused
for the next run. The results shown in Fig. 10 indicated that
CHT-1.0F sample could be recycled and reused at least four
times without significant loss of activity and selectivity. In
order to investigate the structure change of catalyst, several
characterization techniques were used to analyze the recov-
ered catalyst after reused four times (CHT-1.0F-fourth).
SEM disclosed that the platelet -shaped morphology was
unchanged for it, but many smaller sized particles were
detected on its surface (see Fig. 5). At the same time, XRD
1 3

Fluorinated Mg–Al Hydrotalcites Derived Basic Catalysts for Transesterification of Glycerol…
1195
Table 5 Comparison of catalytic activity of CHT-1.0F with other
In addition, CHT-F catalyst presented high structure stabil-
ity and its catalytic activity remained almost constant after
used four times. Thereby, it has a promising prospect in
production of GC on the industrial scale.
reference catalysts for the synthesis of GC
Catalyst
Time (h)
Tempera-
ture (°C)
GC yield (%)
GC
selectiv-
ity (%)
Acknowledgements Authors acknowledge the financial support
from “Strategic Priority Research Program-Climate Change: Car-
bon Budget and Related Issues” of the Chinese Academy of Sci-
ences Grants (XDA05010108), National Natural Science Foundation
of China (21401164), the Natural Science Foundation of Shandong
Province, China (ZR2016BL21, ZR2013BL019), and National Train-
ing Programs of Innovation and Entrepreneurship for Undergraduates
(311190805, 311190806, 201510904007).
CHT-1.0F
HT-1.0F
MgO
3
3
2
1
1
2
2
110
110
100
100
100
100
100
95.3
25.3
57.2
81.4
76.9
82.6
80.1
95.3
100
100
78.2
68.5
90.4
86.9
CaO
NaOH
CHT-Nia
CHT-Zrb
Reaction conditions: 20 mmol glycerol, 60 mmol DMC, and 0.3 g
catalyst
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