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time, additional solvent and a high reactant mole ratio. Therefore, it
was urgent to develop a highly efficient catalyst for this reaction.
Ceria is a basic catalyst. Although it is not very reactive in
transesterification reactions, it is a good candidate to improve the
basicity of solid basic catalyst, therefore enchaining the catalytic
activity in base-catalyzed reaction [20,21]. The utilization of Ce-
MgO in the synthesis of dimethyl carbonate (DMC) from ethylene
carbonate and methanol resulted in 95% DMC selectivity and 64%
DMC yield, which were far superior to the MgO (DMC yield was
only 49%) [22], Magnesium incorporation into cerium generated
the strongest basic sites, compared with other alkali or alkaline
earth metal ion, because of the higher oxygen vacancy concentra-
tion created simultaneously [23,24].
Ce-NiO catalysts have been applied in many fields such as CH4
oxidation [25], dry reforming of methane [26], H2 production [27]
and CO oxidation [28]. But it has not been reported as a catalyst
in many reactions. Besides, to the best of our knowledge, the per-
formance of Ce-NiO catalysts was not investigated in the synthesis
of GC from GL and DEC. Thus, in this paper, Ce-promoted NiO as
solid base catalyst was simply prepared by a co-precipitation
method and characterized by various techniques. The effects of
reaction parameter on the catalytic performance including calci-
nation temperature, Ce/Ni atomic ratio, reaction temperature,
catalyst amount, GL/DEC mole ratio, reaction time as well as the
catalyst recyclability was studied in details.
min. The mean crystal size of NiO and CeO2 was estimated ac-
cording to main plans (0 1 2) of NiO and (111) of CeO2, by Scherrer
equation [29].
Microscopic morphology of the catalysts was characterized by
field emission scanning election microscopy (FESEM). The metal
elements distribution was recorded by energy dispersive X-ray
spectrum (EDX).
X-ray photoelectron spectroscopy (XPS) measurements were
conducted on an ESCALAB-250 (Thermo-VG Scientific, USA) spec-
trometer equipped with Al Ka (1486.6 eV) irradiation. The binding
energies value were calibrated internally based on the adventitious
carbon deposit C (1s) peak at 284.6 eV.
Specific surface area (BET) and pore characteristics of the cata-
lyst were detected by N2 physisorption at liquid nitrogen temper-
ature (-196 ꢀC) using a Beishide 3H-2000 analyzer after the catalyst
was degassed at 200 ꢀC for 12 h.
Basic sites of the catalyst were measured by temperature-
programmed desorption of CO2 (CO2-TPD) performed using the
TP-5056 equipment connected to a TCD detector. The catalysts
(0.10 g) were first pretreated in Helium atmosphere at 200 ꢀC for
1 h, then cooled down to 30 ꢀC maintained in the CO2 flowing gas
for 30 min and purged with He for 50 min in order eliminate the
physically adsorbed CO2. Subsequently, the sample was heated at a
linear rater of 10 ꢀC/min to 800 ꢀC in order to acquire the CO2
desorption curve.
Fourier transform infrared (FT-IR) spectra of the catalysts were
detected on a Nicolet 5700 spectrometer using the KBr disc method
and the wavenumber range of the spectra was obtained from 400 to
4000 cmꢁ1 with a 2 cmꢁ1 resolution.
2. Experimental
2.1. Chemicals
Thermogravimetric (TG) analysis of the catalyst was conducted
in the range of 30e800 ꢀC under dry air atmosphere on a thermal
analyzer SDT Q600 by using 10 mg catalysts with a heating rate of
20ꢀ/min.
Ce(NO3)3$6H2O, Ni(NO3)2$6H2O, Na2CO3 and ethylene glycol
monobutyl ether were purchased from Aladdin Industrial Corpo-
ration, shanghai, China, GL, DEC and ethanol were purchased from
Sinopharm Chemical Reagent Co. Shanghai, China. All the reagents
were of analytical grade without further purification.
2.4. Catalytic reaction
2.2. Catalyst preparation
Transesterification of DEC with GL to produce GC, depicted in
Scheme 1a, was performed in a round bottom flask equipped with a
magnetic stirrer and a thermocouple.
The Ce-NiO with different Ce/Ni ratio was prepared by co-
precipitation method. In a typical process, a mixed aqueous so-
lution (Ce(NO3)3$6H2O, Ni(NO3)2$6H2O) and 3 mol/L Na2CO3 were
simultaneously added dropwise to 10 mL deionized water under
vigorous stirring to achieve pH value of 9.0. After 12 h reaction, the
precipitate was separated via filtration, washed with deionized
water to remove sodium ion until the precipitate was nearly
neutral. Afterwards, the precipitate was dried at 80 ꢀC for 10 h, and
then treated in static ambient air at 400 ꢀC for 5 h. Finally, the as-
prepared Ce-NiO catalyst was grind into the designated size and
used for GL transesterification. The Ce-NiO catalysts with different
composition were denoted as xCeNiO-400, where, x represents the
Ce/Ni atomic ratio (x ¼ 0.2, 0.4, 0.6, 0.8, and 1.0, respectively). To
study the influence of the calcination temperature, the 0.2CeNiO
catalysts were calcined at different temperature (from 300 to
600 ꢀC) and the catalysts were denoted as xCeNiO-T (x ¼ 0.2,
T ¼ 300, 400, 500, 600, respectively), where T represents the
pretreatment temperature. For comparison, the pure NiO and
CeO2 were synthesized under the same conditions without addi-
tion of Ce(NO3)3$6H2O or Ni(NO3)2$6H2O, and calcined at 400 ꢀC
for 5 h.
Typically, GL, DEC and catalyst were charged into the glass
reactor heated by oil bath with a set point. During the reaction, the
system was stirred variously. After 6 h reaction, the system was
cooled down to room temperature. The recovered catalyst from
mixture was separated via centrifugation, washed with ethanol to
remove organic components, and dried at 40 ꢀC in an oven over-
night for further research. The catalyst-free liquid (ethylene glycol
monobutyl ether as the internal standard) was collected and
detected by flame ionization detector Gas chromatograph (Teng
Hai, Shandong, China, GC-6890) equipped with a SE-54 capillary
column (30 m ꢂ 0.32 mm ꢂ 1
mm).
GL conversion was defined as the ratio of C-based mole of all the
products to C-based mole of initially added GL. GC selectivity can be
2.3. Catalyst characterization
X-ray diffraction (XRD) patterns of the Ce-NiO were recorded in
2q a
range of 5-85ꢀ on the Rigaku D/max-A instrument using Cu k
radiation performed at 40 kV and 20 mA with a scan step of 20ꢀ/
Scheme 1. (a) one-pot synthesis of GC from GL, (b) GC decarbonylation to glycidol.