S. Wang et al. / Journal of Molecular Catalysis A: Chemical 398 (2015) 248–254
249
Scheme 1. Transesterification of phenol with DMC to DPC.
2
. Experimental
the total specific surface area was calculated by the multi-point
Brunauer–Emmett–Teller (BET) method. The total pore volume was
2.1. Catalyst preparation
determined from the amount adsorbed at the relative pressure of
◦
0
.99. Prior to N2 adsorption, the samples were evacuated at 300 C
All the reagents (high purity) were purchased from the
for 3 h.
The Mo element contents were determined by inductively cou-
pled plasma-atomic emission spectrometry (ICP-AES) using argon
as the support gas.
chemical vendors without further purification. Tetraethyl ortho-
silicate (28%), ethanol (99.7%), 1,2-propanediol (99%), ammonium
heptamolybdate (99%) and ammonia hydroxide (28–30%) were
purchased from Sigma-Aldrich. Deionized water was used in the
synthesis.
2.3. Activity tests
1
5 wt% MoO /SiO catalysts were prepared with sol–gel method
3 2
[
21], sol–gel and hydrothermal treatment method, and incipient
MPC was synthesized and purified in the laboratory. The 99.8%
purity was determined by high performance liquid chromatogra-
phy.
wetness impregnation method, respectively.
2
.1.1. Sol–gel method
Tetraethyl orthosilicate (20 mL), ethanol (20 mL) and 1,2-
The catalytic activity for the liquid-phase disproportionation of
MPC to DPC was tested using 100 mL three-neck round-bottomed
flask, equipped with a magnetic stirring, a nitrogen inlet and a frac-
tionating column connected to a liquid dividing head. Typically,
under nitrogen atmosphere, 150 mmol MPC and desired catalyst
propanediol (5.0 g) of (NH ) Mo7·4H O solution (1.2 g) were mixed
4
6
2
together, and then added drop-wise into ammonium hydroxide
◦
aqueous solution (50 mL, pH 8.5) under vigorous stirring at 45 C.
After stirring for 1 h, the mixture was divided into two parts of A and
B. Part A was aged at room temperature (25 C) for 24 h. Afterwards,
the resulting solid of part A was dried at 110 C overnight, followed
◦
were introduced into the flask. The mixture was heated to 200 C
◦
slowly and then the temperature was kept for the reaction under
atmospheric pressure. During the reaction, DMC was distilled off
by the liquid dividing head attached to a receiver flask.
◦
◦
by calcination in air at 500 C for 5 h. The sample was named as
M-SG.
The product distribution was defined by GC–MS on a HP-
6
890/5973 system. The reaction mixture was quantitatively
2
.1.2. Sol–gel and hydrothermal treatment method
The obtained part B by sol–gel method was transferred into
analyzed by GC system (Agilent Technologies 7820A) with an FID
detector and a DB-35 capillary column (30 m × 320 m × 0.25 m)
to determine the conversion and selectivity. The GC results were
calculated using a correction factor normalization method.
◦
Teflon-lined autoclave (100 mL) and aged at 150 C for 24 h. The
resulting solid of part B was dried at 110 C overnight, and calcined
at 500 C for 5 h. The sample was named as M-SGH.
◦
◦
3. Results and discussions
2.1.3. Incipient wetness impregnation method
SiO2 support was synthesized by the abovementioned sol–gel
3.1. Catalyst characterization
method without addition of (NH ) Mo7·4H O and calcined at
4
6
2
◦
5
00 C for 5 h. Thereafter, impregnated sample was obtained by
3.1.1. XRD analysis
adding stoichiometric amounts of (NH ) Mo7·4H O solution to the
The XRD patterns of all samples are shown in Fig. 1. For M-I, only
small amounts of broad characteristic peaks of crystalline ␣-MoO3
phases could be observed from XRD (JCPDS No. 05-0508), indicating
4
6
2
SiO support. The sample was placed in a rotary evaporator to evac-
2
◦
uate excess water and then calcined at 500 C for 5 h, which was
referred to M-I.
that MoO is few aggregations and relatively highly dispersed in M-
3
For comparison, pure MoO3 was also prepared by calcination of
I. However, for M-SG and M-SGH, no crystalline MoO3 diffraction
peaks are observed, showing that the dispersions of MoO3 in M-SG
and M-SGH are even higher than that of M-I.
◦
(
NH ) Mo7·4H O at 500 C for 5 h.
4
6
2
2.2. Characterization
3.1.2. FT-IR analysis
Powder X-ray diffraction (XRD) patterns were collected
The FT-IR spectra of all samples are showed in Fig. 2. For M-
on X’pert PRO diffractiometer (PANalytical) using Cu K␣
I, M-SG and M-SGH, the intensive absorption vibration peaks at
1094, 802 and 467 cm are observed, which are ascribed to the
asymmetric stretching, symmetric stretching and bending modes
◦
−1
(
ꢀ = 1.54056 A˚ ) radiation (40 kV, 45 mA), a scan range of 5–90 and
◦
a scanning velocity of 1.2 /min.
Fourier transfer infrared spectra (FT-IR) were recorded on a
Nicolet MX-1E spectrometer (USA) using KBr pellets.
The specific surface area and pore volume were obtained by
of Si O Si, respectively [27]. The typical bands of crystalline
␣-MoO3 at 992, 875 and 611 cm , corresponding to the vibra-
tions of Mo O terminal stretching, the asymmetric vibrations of
Mo O Mo bridging bonds and the symmetric Mo O Mo stretch-
ing, respectively [28,29], are not observed in the three samples,
indicating that MoO3 is highly dispersed, which is agreement with
−
1
◦
N2 adsorption isotherms at −196 C using NOVA1000e adsorp-
tion analyzer (Quantanchrome, USA). From the adsorption branch
of isotherm curves at the P/P0 range between 0.05 and 0.35,