G. Mitran et al. / Applied Catalysis A: General 514 (2016) 71–82
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that, under the same flow of He, the temperature was raised from
40 ◦C to 700 ◦C at 10 ◦C/min, analyzing the desorbed ammonia. The
weak, medium, and strong acidities were assigned to the peaks of
NH3-TPD profiles at temperatures lower than 350 ◦C, between 350
and 500 ◦C, and above 500 ◦C, respectively [23].
2.3. Catalytic experiments
The catalytic reaction was checked in the p-xylene (>99% purity,
purchased from Merck) oxidative cross-coupling over supported
vanadia catalysts. The tests were performed in a fixed-bed glass
reactor (15 mm i.d.) in the temperature range from 200 to 400 ◦C.
To each reaction temperature the system was kept half an hour
until reaching the system steady state. The reactions were con-
ducted under a PX hourly space velocity (HSV) of 5.2 ml/gcat h and
for a molar ratio PX to O2 of 1.5. The reactant and product mixtures
were analyzed by a gas chromatograph equipped with a hydrogen
flame ionization detector (FID). The products were identified by
mass spectrometry with a GC/MS using a Trace GC 2000 coupled
with DSQ MS from Thermo Electron Corporation equipped with FID
detector, a capillary column with DB5 stationary phase (30 m length
and 0.324 mm diameter), and highly pure N2 (99.999%) as carrier
gas. CO2 was analyzed with a Thermo Finnigan Gas-Chromatograph
equipped with a thermal conductivity detector (TCD). The products
identification was also carried out by 1H NMR spectroscopy. The
spectra were collected with a Bruker FOURIER 300 spectrometer in
CDCl3 solvent. The conversion was calculated based on the p-xylene
consumption.
3. Results and discussion
3.1. Catalysts characterization
The XRD patterns of VOx/Al2O3 catalysts are shown in Fig. 1.
No diffraction lines corresponding to vanadium compounds were
detected in the patterns of the samples prepared by carbonate
method, suggesting that the VOx species are highly dispersed on
the alumina surface. Within the detection limits of XRD, small
crystallites (<5 nm) cannot generate X-ray diffraction patterns. The
alumina prepared by citrate and urea methods is amorphous. How-
ever, diffraction lines due to the presence of V2O5 were evidenced
in the samples prepared by citrate and urea methods. The charac-
teristic peaks of ␥-alumina phase (2ꢁ = 37.4◦, 39.4◦, 45.9◦ and 66.3◦
for (3 1 1), (2 2 2), (4 0 0) and (4 4 0)) and of crystalline V2O5 (2ꢁ = 15,
20, 22, 26, 31 and 33◦) were detected [24]. The characteristic lines
of the AlVO4 phase at 21, 26, 28, 30◦ [25] interfered with those of
V2O5, making the identification of the vanadate phase more diffi-
cult. The lines corresponding to vanadia at 2ꢁ = 15, 20, 22, 26, 31◦,
were more intense for sample prepared by citrate method com-
pared with urea method. No change in the XRD patterns of samples
prepared by carbonate method, were observed after calcination to
600 ◦C. XRD analysis of the spent catalysts indicated a good struc-
tural stability, and no change in the diffraction patterns or relative
intensities of the identified lines has been detected for the spent
catalysts.
Fig. 6. Effect of reaction temperature and catalyst acidity on p-xylene conversion
(a) and biaryl selectivity (b) over alumina supported vanadia catalysts prepared by
carbonate method (5.2 ml PX/gcat h).
Raman spectra were collected with a LabRAM HR UV–vis–NIR
(200–1500 nm) Raman Microscope Spectrometer from Horiba
Jobin Yvon equipped with a DL785-100 laser emitting at 540 nm.
The photons scattered by the sample were dispersed by a 1200
lines/mm grating monochromator and simultaneously collected on
a CCD camera. The collection optic was set at 15× objective. Each
spectrum corresponds to the sum of 30 acquisitions each of them
during 10 s.
respond to 4–5 nm for alumina prepared by carbonate method and
did not vary significantly when vanadia is added.
The NH3-TPD profiles were obtained using a Micromeritics
Auto-ChemII apparatus equipped with a programmable temper-
ature furnace and TCD detector. 0.05 g of sample was purged for
2 h with 40 mL/min He at 500 ◦C, after the increase of temperature
from ambient to 500 ◦C using a heating rate of 5 ◦C/min, followed
by lowering temperature to 40 ◦C. Then, 3% NH3/He was flushed for
30 min at 40 ◦C, and purged with 40 mL/min He at 40 ◦C for 1 h. After
N2 adsorption–desorption isotherms for V-Al catalysts are
shown in Fig. 2. All isotherms for catalysts prepared by carbonate
method exhibited the typical type IV shape, which is characteris-
tic of a mesoporous texture. H3-type hysteretic loops, according
to IUPAC classification, indicate the presence of slit-like pores. The
capillary condensation step was gradually shifted to higher relative
pressures when the calcination temperature increased from 400 to