C. Wen et al. / Applied Catalysis A: General 458 (2013) 82–89
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such as mechanical strength, thermal stability and larger surface
area. Binary SiO2–TiO2 used as catalyst in the selective oxidation
eration of new catalytic sites [22,23]. However, the preparation
of the catalyst is complicated, and usually requires organic sol-
vents and surfactant which are always harmful to the environment.
Boccuzzi et al. [24,25] studied the Cu/SiO2 and Cu/SiO2–TiO2 cat-
alysts in a polymerization reaction and found that morphological
and surface properties of the copper phase were essentially the
same for all the catalysts investigated. An intriguing observation
was that the TiO2 phase presented in Cu/SiO2–TiO2 formed TiOx
suboxides and covered a fraction of the exposed Cu metal sites by
reduction at higher temperature. They also observed the electron
transfer from TiO2 to Cu directly, which can be interpreted as the
strong interactions between copper and titanium. Although using
binary oxide as support can improve the catalytic property in many
different aspects, few attempts have been done to investigate the
interaction between the active sites and the binary oxide supports.
Furthermore, the application scope for catalysts with binary oxide
support is still scarce; there are few reports of the applications in
the hydrogenation reactions especially for the ester hydrogenation.
Ground on the above discussions, it is expected that the copper
based catalyst with Si–Ti binary-oxide support may exhibit inti-
mate interaction between copper species and the supports, and the
electronic interaction between copper and titanium will accommo-
date the texture of the catalyst and further enhance the catalytic
performance. In our previous study, hydrogenation of DMO is sys-
tematically investigated based on the Cu/SiO2 catalysts and a high
yield of EG can be obtained under optimized conditions [11–15].
However, it is easy to deactivate especially for the long-term run-
ning due to the poor interaction between copper and silica [26]. In
addition, to the best of our knowledge, most of the catalysts used in
this reaction are based on unitary SiO2 as support which shows low
mechanical strength and poor stability in alkaline environment.
These drawbacks greatly restrict its industrial application. Herein,
we fabricate a high-efficient and stable catalyst with a novel Si–Ti
binary oxide as support in selective hydrogenation of DMO to EG
in an attempt to investigate the enhancement mechanism in the
catalytic performance caused by the binary oxide support.
cuprammonia until the pH value reached 6–7. After then, the filtrate
is washed with deionized water for five times and dried at 393 K
overnight. Finally, the catalyst precursors are calcined in static air
at 723 K for 4 h, then pelletized, and grounded to 40–60 meshes.
For comparison, Cu/SiO2 and Cu/TiO2 both containing 10 wt.% Cu
are synthesized using the similar method as Cu/SiO2–TiO2 without
the addition of titania-sol or silica-sol, respectively. All the catalyst
precursors are then reduced at 573 K for 4 h under the 5% H2/Ar
(v/v) atmosphere.
2.2. Catalyst characterization
Specific surface areas of the catalysts are measured by nitrogen
adsorption–desorption method at 77 K (Micromeritics Tristar ASAP
3000) using Brunauer–Emmett–Teller (BET) method. The copper
loadings are determined by the inductively coupled plasma method
(ICP, thermo E.IRIS). The wide-angle XRD patterns were collected
on a Bruker D8 Advance X-ray diffractometer using nickel-filtered
Cu K˛ radiation (ꢀ = 0.15406 nm) with a scanning angle (2ꢁ) range
of 20–90◦, a scanning speed of 2◦ min−1, and a voltage and current
of 40 kV and 40 mA, respectively. TEM micrographs are obtained
on a JOEL JEM 2010 transmission electron microscope. Temperate
programmable reduction (TPR) profiles were obtained on a Tianjin
XQ TP5080 auto-adsorption apparatus. 30 mg of the calcinated cat-
alyst was outgassed at 473 K under Ar flow for 2 h. After cooling to
H2/Ar (v/v), and the sample was heated to 773 K at a ramping rate of
10 K min−1. The H2 consumption was monitored by a TCD detector.
The metallic Cu surface area was measured by decomposition of
N2O at 323 K using a pulsed method with N2 as the carrier gas. The
consumption of N2O was detected also by a TCD detector. The spe-
cific area of metallic copper was calculated from the total amount
of N2O consumption with 1.46 × 1019 copper atoms per m2 [28]. X-
ray photoelectron spectroscopy (XPS) was recorded with a Perkin
Elmer PHI 5000 C ESCA system equipped with a hemispherical elec-
tron energy analyzer. The Mg-K␣ (1253.6 eV) anode was operated
at 14 kV and 20 mA. The carbonaceous C 1s line (284.6 eV) was used
as the reference to calibrate the binding energies (BEs).
2.3. Activity measurements.
The catalytic activity test was conducted using a fixed-bed reac-
tor. Typically, 2.0 g of catalyst (40–60 meshes) sample was packed
into a stainless steel tubular reactor (i.d., 10 mm) with the ther-
mocouple inserted into the catalyst bed for better control of the
actual pretreatment and reaction temperature. Catalyst activation
was performed at 573 K for 4 h with a ramping rate of 2 K min−1
from the room temperature. After cooling to the reaction temper-
ature, 15 wt.% of DMO (purity > 99%) in methanol and H2 were fed
into the reactor at a H2/DMO molar ratio of 100 and a system pres-
sure of 3.0 MPa. The reaction temperature was first set at 473 K and
the LHSV of DMO was set in the range from 0.15 to 0.8 h−1. The
products were condensed, and analyzed on a gas chromatograph
(Finnigan Trace GC ultra) fitted with an AT.FFAP capillary column
and a flame ionization detector (FID).
2. Experimental
2.1. Catalyst preparation
All the reagents are purchased from Sinopharm Chem. Reagent
Co., Ltd. without further purification, unless otherwise specified.
To obtain the Si–Ti binary oxide support, a titania sol is synthe-
sized via a facile method. Firstly, 10 ml of Ti(OBu)4 is added into
30 ml of ethanol solution which contains 2 ml of acetic acid and the
solution is kept magnetic stirring for 0.5 h. Then, small amount of
nitric acid is added to control the pH value below 3.0. Finally, 10 ml
of ethanol and 1 ml of deionized water are added to the solution
and the TiO2 sol are obtained after the mixture was kept stirring
for 1 h.
The Cu/SiO2–TiO2 catalyst precursor with 10 wt.% copper load-
ings and Si/Ti molar ratio of 3:1 is prepared by ammonia
evaporation method (AE) [27]. Firstly, 1.9 g of Cu (NO3)2·3H2O are
dissolved into 100 ml of deionized water, then, about 11 ml of aque-
ous ammonia (25 wt.%) are added to the above solution and the pH
is adjusted to 11.0. After that, mixed sols containing 12 ml of silica
sol (JN30, Qingdao Haiyang Chem. Co., Ltd.) and 15 ml of titania sol
containing 1.25 g of TiO2 are added dropwise into the solution. The
as-obtained suspensions are kept stirring for 2 h at room tempera-
ture, and then the temperature is risen to 363 K to decompose the
3. Results
3.1. Structural and textural properties
The textural properties and the chemical compositions of the
three catalysts are listed in Table 1. The BET surface area of Cu/TiO2
is only 48 m2 g−1, much smaller than that of Cu/SiO2. When the
Si–Ti binary oxide is used as support, the catalyst shows a large
specific surface area, 238 m2 g−1, and an average pore volume of
0.4 cm3 g−1. It is interesting to find that the specific surface area