140
R. Mi et al. / Journal of Catalysis 370 (2019) 138–151
oxynitrate and urea solution was transferred into a 100 mL Teflon-
lined autoclave. Hydrothermal treatment was carried out at 473 K
under autogenous pressure for 6 h. The resulting precipitate was
recovered by centrifugation, fully washed with deionized water
until a neutral pH and finally dried at 383 K overnight. The white
powder was then calcined in air at 773 K for 4 h with a heating rate
of 1 K/min.
(spectra will reach steady in about 40 min), the sample was flushed
with Ar (30 mL/min) for another 40 min until the spectra reaching
steady again. All spectra shown in figure were obtained using dif-
ference spectra method.
2.3. Catalytic tests
ZrO2-supported Yb2O3 catalysts were prepared by wet impreg-
nation method. 3 g ZrO2 was impregnation for over 12 h with mag-
netic stirring using 30 mL aqueous solution of Yb(NO3)3ꢀ5H2O
(99.99%, Energy Chemical). The concentration of Yb3+ was varied
from molar ratio of Yb/(Yb + Zr) various from 2.5 to 20%. The sus-
pension was dried at 333 K, 10 kPa in a rotary evaporator with
60 r/min and after that in an oven at 383 K overnight. The powder
was calcined in air at 773 K for 4 h with a heating rate of 1 K/min.
The powder was denoted as n Yb/Zr, where ‘‘n” represented the
molar ratio of Yb/(Yb + Zr). The actual loading of Yb was analyzed
by X-ray fluorescence (XRF) using a Bruker S8 tiger WDXRF
spectrometer.
Reactions were carried out at atmospheric pressure in a tubular
stainless fixed-bed reactor of 7 mm inner diameter and 700 mm
height. The catalyst bed consisted of a mixture of 1 g of catalyst
(40 mesh) and 1 g of inert quartz sand (40 mesh), which was fixed
at the center of the reactor with quartz wool. In a typical process,
the catalyst was treated for 2 h in a nitrogen flow at 723 K first. The
reactant, BDO (>99.0%, Sinopharm Chemical Reagent) was continu-
ously pumped from vessel by a calibrated digital pump (Series II
Digital Pump, SSI) to a gasifier (553 K), and then the BDO gas was
mixed with 30 mL/min nitrogen as carrier gas in a mixer (573 K)
into the fixed bed reactor. The reactor was placed in
a
temperature-controlled electrically heated furnace and heated to
the reaction temperature, which was monitored with thermocou-
ples placed inside the reactor touching the inlet of the catalyst
bed. The temperature of the catalyst bed can be controlled
to 1 K. Reactor effluent gas lines were heated to 573 K with heat-
ing tapes to prevent possible condensation. The reactor outlet
products were passed through a condenser to the gas-liquid sepa-
rator and weighed at regular intervals. The condensate was quali-
fied and quantified by off-line gas chromatography (Fuli 9790II)
using retention time and internal standard method, respectively.
Outlet gas from gas-liquid separator was measured by a gas flow
meter. A small portion of gas was subjected to GC–MS (Shimadzu
GCMS-QP2010 Plus) to detect the possible gaseous by-products.
Essentially, only BDO, BTO, THF, 1-butanol, 2BT1O and GBL were
detected in the reactor effluent mixture, and no gaseous by-
product was detected. The results including BDO conversion, BTO
selectivity, THF selectivity and BTO yield were calculated using
the following equation:
2.2. Catalysts characterization
X-ray diffraction (XRD) patterns were collected on a Shimadzu
XRD-6100 X-ray diffractometer with a Cu K
a (k = 0.15406 nm)
radiation source at 40 kV and 30 mA to identify the crystal struc-
ture of each catalyst. The 2h angles were scanned from 10 to 80°
with a step size of 0.02° and scan speed of 7°/min. The average
crystallite sizes (D) were estimated by the Scherrer equation,
D = 0.90 k/b cos h, where h was the diffraction angle and b was
the full width at half maxima (FWHM).
Specific surface area was measured at 77 K using a BELSORP-
Max instrument. Prior to the measurement, the sample was treated
using a BELPREP-vac Ⅱ at 573 K for 3 h. The specific surface area
was determined by the Brunauer–Emmett–Teller (BET) method
and the pore size distribution was calculated by Barrett–Joyner–
Halenda (BJH) method.
Acid sites on the catalyst surfaces were probed by NH3 temper-
ature programmed desorption using a Quantachrome ChemBET
Pulsar chemisorption analyzer with a thermal conductivity detec-
tor (TCD). NH3-TPD was performed by heating the sample to
823 K with a heating rate of 10 K/min. The desorbed NH3 was mon-
itored by TCD. Prior to the test, samples (ca. 100 mg) were placed
in a U-shaped glass tube, purged with He flow (30 mL/min) for
10 min and then heated to 823 K (10 K/min) in He flow (30 mL/
min) and held at this temperature for 60 min before the tempera-
ture was lowered to 323 K. After this pretreatment, 5 mol % NH3/Ar
was passed over samples with a flow rate of 30 mL/min for 60 min
and then switched back to He flow for 60 min. CO2-TPD measure-
ments were performed following the aforementioned process, but
using CO2 instead of NH3 to probe the basic sites of samples.
In situ Diffuse Reflectance Infrared Fourier Transform Spec-
troscopy (DRIFTS) of m-ZrO2 and Yb2O3 were performed on a Nico-
let iS50 spectrometer (Thermo Scientific) equipped with a DiffusIR
Reflectance Accessory (Pike Technology), which consists of a heat
chamber, DRIFT cell with ZnSe window and an MCT-A detector.
All spectra were collected in Kubelka-Munk units and were aver-
aged over 32 scans at 4 cmꢁ1 resolution at desired temperature.
In a typical process, the powder sample was placed in a DRIFTS cell
and set at 1 mm lower than cell surface by using sample press
stick. The cell was heated to 723 K in Ar flow (30 mL/min) and held
at this temperature for 60 min before the temperature was con-
trolled to desired temperature (373, 473, 573 673 K) under Ar flow.
After stabilization of the spectrum, the background spectrum was
collected. The BDO molecule was introduced by flowing 30 mL/
min Ar through a room-temperature saturator, and the spectra
were began to be recorded every 30 s, simultaneously. Afterward
moles of BDO consumed
moles of BDO in the feed
BDO Conversion ð%Þ ¼
BTO Selectivity ð%Þ ¼
THF Selectivity ð%Þ ¼
ꢂ 100%
moles of BTO produced
moles of BDO consumed
ꢂ 100%
ꢂ 100%
moles of THF produced
moles of BDO consumed
BTO Yield ð%Þ ¼ BDO Conversion ꢂ BTO Selectivity
All of the catalytic data were reported after 2 h reaction. The
catalytic performance of BDO dehydration were investigated at
temperature range of 573–723 K and W/F of 0.019 gꢀhꢀmlꢁ1 over
all the catalysts.
3. Results and discussion
3.1. Structural and textural characteristics
Fig. 1 shows the nitrogen adsorption–desorption isotherm and
pore size distributions of n Yb/Zr catalysts with different molar
ratio of Yb / (Yb + Zr). It can be seen from Fig. 1(a) that all isotherms
belong to type IV with H1 hysteresis loops, which is typical for
mesopores materials. The gentle slope of the capillary condensa-
tion step indicates non-uniformity and disorder of mesopores.
The impregnation of Yb on the mesopores m-ZrO2 resulted in a nar-
rowing of the hysteresis loops and a slight shift of the condensation
step toward lower relative pressure, indicating lower pore volume
and pore diameter. Indeed, Fig. 1(b) exhibits narrow pore size