E.J. Lee, et al.
Molecular Catalysis 467 (2019) 111–119
WO
WO
x
x
/C catalysts were prepared by a wetness impregnation method.
/Z, WO /CZ, and WO /C catalysts were characterized by XRF, ni-
helium (30 ml/min) to saturate acid sites of the catalyst. Physisorbed
x
x
ammonia was removed at 100 °C for 1 h under a flow of helium (50 ml/
trogen adsorption–desorption, FE-SEM, SEM-EDX, XRD, Raman, H
2
-
min). After cooling the catalyst, NH -TPD measurement was conducted
3
TPR, NH
oxygen was carried out over WO
continuous flow reactor, and their catalytic activities were compared.
3
-TPD, and CO
2
-TPD analyses. Propylene epoxidation by
within the temperature range of 50–600 °C under helium flow (50 ml/
x
/Z, WO /CZ, and WO /C catalysts in a
x
x
min). Thermal conductivity detector (TCD) was used to detect desorbed
NH
3
. Basicity of the catalysts was measured by CO -TPD experiments.
2
Experimental procedures for CO
2
-TPD were identical to those for NH
3
-
2
. Experimental
.1. Catalysts preparation
The CeZrO support was prepared by using a co-precipitation
TPD, except that CO
cule.
2
instead of NH was employed as a probe mole-
3
2
2.3. Propylene epoxidation by oxygen
2
method. Known amounts of cerium precursor (Ce(NO ·6H O, 5 mol%)
and zirconium precursor (ZrO(NO ·xH O, 95 mol%) were dissolved in
distilled water. After dissolving all the precursors, ammonium hydro-
3
)
3
2
The catalytic activity of WO
x
/Z, WO
x
/CZ, and WO /C catalysts in
x
3
)
2
2
the propylene epoxidation by oxygen was evaluated in a continuous
flow fixed-bed reactor at 400 °C for 6 h under atmospheric pressure.
Prior to the catalytic reaction, each catalyst (0.3 g) was pretreated at
xide solution (NH OH, Sigma–Aldrich) was added dropwise into the
4
cerium-zirconium precursor solution till the pH value of solution
reached ca. 8. The solution was vigorously stirred at 25 °C for 6 h. This
precipitate was washed with ethanol and distilled water to remove the
residual organic materials. Then support powder was dried at 110 °C for
4
00 °C for 1 h with a nitrogen stream (45 ml/min). Feed stream of
propylene (10 ml/min) and oxygen (5 ml/min) was continuously fed
into the reactor together with nitrogen carrier (45 ml/min). Total feed
rate with respect to catalyst weight was maintained at 12,000 ml/h · g.
Reaction products were periodically sampled and analyzed with gas
chromatographs (Younglin, ACME 6100) equipped with a thermal
conductivity detector (Molsieve 5 A and Porapak N columns) and a
flame ionization detector (DB-1 column). Conversion of propylene and
selectivity for propylene oxide were calculated on the basis of carbon
balance as follows. Yield for propylene oxide was calculated by multi-
plying conversion of propylene and selectivity for propylene oxide.
2
4 h and calcined at 500 °C for 3 h in an air stream to yield CeZrO
2
support. The prepared CeZrO
parison, ZrO and CeO supports were also prepared by using the same
method described above and denoted as Z and C, respectively.
The WO /CeZrO catalysts were prepared by using an impregnation
2
support was denoted as CZ. For com-
2
2
x
2
method. An appropriate amount of ammonium (para) tungstate hydrate
(
(
(NH
4
)
10
H
2
(W
2
O
7
)
6
·xH
2
O, Sigma-Aldrich) was solved in distilled water
support was then introduced into the
20 ml). The prepared CeZrO
2
mole of propylene reacted
mole of propylene supplied
solution under vigorous stirring, and the mixture was stirred at 60 °C for
Conversion of propylene(%) =
(1)
6
h. The resulting solid was dried at 110 °C for 24 h. The dried product
was calcined at 800 °C for 3 h to yield WO /CeZrO catalyst. The pre-
pared WO /CeZrO catalyst was denoted as WO /CZ. For comparison,
WO /ZrO and WO
x
2
mole of propylene oxide formed
mole of propylene reacted
x
2
2
x
Selectivity for propylene oxide(%) =
x
x
/CeO catalysts were also prepared by using the
2
(
2)
same method described above and denoted as WO
x
/Z and WO /C, re-
x
spectively.
3
. Results
2.2. Characterization of catalysts
3
.1. Catalytic performance of ceria & zirconia-supported tungsten oxides
WO contents and Zr/Ce ratios of each catalyst were determined by
x
X-ray fluorescence (XRF) analysis. Surface areas and average pore
3.1.1. Effect of reaction temperature on the propylene epoxidation
diameter of WO
x
/Z, WO
x
/CZ, and WO /C catalysts were obtained by
x
Fig. 1 shows the effect of the reaction temperature on the catalytic
nitrogen adsorption-desorption measurements (BELSORP mini II).
Before the analyses, all the catalysts were degassed using a rotary va-
cuum pump at 150 °C for 3 h to remove impurities and moisture.
Surface morphologies of the catalyst were examined by FE-SEM
activity of the WO
x
/Ce0.3Zr0.7
O catalysts. The tungsten loading was
2
fixed at 7 wt% and the calcination temperature of the catalyst was
7
00 °C. It can be seen that the catalytic acrivity was sensitive to the
reaction temperature. The conversion of propylene increased from 2.3%
(
Scanning Electron Microscope) analyses (JSM-6700 F, Jeol). To con-
firm the detailed distribution of metal species of the catalysts, energy
dispersive X-ray spectroscopy (EDX) mapping analyses were conducted
(
JSM-6700 F, Jeol). X-ray diffraction patterns of supports (Z, CZ, and C)
and catalysts (WO
x
/Z, WO
x
/CZ, and WO /C) were taken with an X-ray
x
diffractometer (Rigaku, D-MAX2500-PC) operated at 50 kV and 100 mA
using Cu-Ka radiation (λ = 0.154056 nm). Raman spectra were ob-
tained with a Raman spectrometer (DXR2xi, Thermo) using an argon
ion (λ = 532 nm). Reduction behavior of supports (Z, CZ, and C) and
catalysts (WO
x
/Z, WO
x
/CZ, and WO
x
/C) were investigated by H -TPR
2
(
temperature programmed reduction) experiments using a conventional
flow system equipped with a TCD (thermal conductivity detector) at
temperatures ranging from 50 to 900 °C with a ramping rate of 10 °C/
min under a mixed stream of H
2
(2 ml/min) and N (20 ml/min). 50 mg
2
of each catalyst was loaded into the U-shaped quartz reactor. The
acidity of the catalysts was measured by NH -TPD (Temperature
3
Programmed Desorption) experiments (BEL Japan, BELCAT-B). In order
to remove any physisorbed organic molecules, 0.05 g of each catalyst
was treated in TPD apparatus at 200 °C for 2 h with a stream of helium
Fig. 1. Effect of the reaction temperature on the catalytic performance of the
(
50 ml/min). After cooling the catalyst to 25 °C, NH (50 ml) gas was
3
WO
x
/Ce0.3Zr0.7
O catalysts.
2
then introduced into the reactor at room temperature in a stream of
112