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M.O. Guerrero-Pérez et al. / Catalysis Communications 12 (2011) 989–992
an impregnation H3PO4 to orange skin weight ratio of 3. The
impregnated citrus skin was thermally treated, at 500 °C, under
continuous N2 (purity 99.999%, Air Liquide) flow (150 cm3 STP/min)
in a conventional tubular furnace. The activation temperature was
reached at a heating rate of 10 °C/min and maintained for 2 h. The
activated sample was cooled inside the furnace under the same N2
flow and then washed with distilled water at 60 °C until neutral pH
and negative phosphate analysis in the eluate. The resulting activated
carbon was denoted by ACP. This activated carbon was impregnated
with a solution of NH4VO3 (purity 99.99%, Sigma Aldrich). The
concentration of this solution contained the desired amount of
vanadium ions depending on the sample formulation, since two
samples were prepared with different vanadium coverages, 0.5 V/ACP
and 1 V/ACP, where 0.5 and 1 indicates the number of vanadium
atoms per nm2 of carbon support, respectively. This solution was
prepared by mixing ammonium metavanadate with a 0.5 M solution
of oxalic acid (purity 99%, Sigma Aldrich), and was heated and stirred
until clear before impregnating the support. The water excess was
removed in a rotatory evaporator at 80 °C and at a reduced pressure of
10–40 mm Hg. The resulting solid was dried at 120 °C for 24 h and
then calcined at 250 °C for 2 h in air.
Activity measurements were performed using a conventional
fixed-bed reactor. The design of the reactor has been made to
minimize the void volume, since it has capilars up and downstream.
The feed stream and effluents of the reactor were analyzed by an on-
line gas-chromatograph equipped with flame ionization and thermal-
conductivity detectors. The accuracy of the analytical determinations
was checked for each test by verification that the carbon balance
(based on the propane converted) was within the cumulative mean
error of the determinations ( 10%). The catalytic tests were made
using 0.2 g of powder sample with particle dimensions in the 0.25–
0.125 mm range. The axial temperature profile was monitored by a
thermocouple sliding inside a quartz tube inserted into the catalytic
bed. Tests were made using the following reaction feed composition
(% volume): 20.4% O2, 12.5% propane and 15.9% steam in helium. The
total flow rate was 40 ml/min, corresponding to 4800 h−1 gas hourly-
space velocity (GHSV). The particle size of catalyst and the total flow
were selected in order to avoid internal and external diffusion
limitations. Product yields and selectivities were determined on the
basis of the moles of propane feed and products, considering the
number of carbon atoms in each molecule.
The porous structure of the activated carbons was characterized by
N2 adsorption–desorption at −196 °C and by CO2 adsorption at 0 °C,
3. Results and discussion
performed with a ASAP 2020 equipment (Micromeritics). From the
Table 1 illustrates characterization data of the carbon-supported
vanadium catalysts. As expected, both BET and external surface area
values decrease with vanadium coverage, but total area remains high
for the VPO supported catalysts. The relative atomic surface
concentrations obtained by peak quantitative analysis are summa-
rized in Table 2. The V2p3/2 binding energies for the vanadium
catalysts are 516.5 and 516.7 eV for 0.5 V/ACP and 1 V/ACP,
respectively; characteristic of a mixture of V5+ and V4+ species.
These BE values are lower than those reported for oxide-supported
VPO catalysts on other supports, such as MCM-41, SiO2 or SBA15 [6].
This suggests that the amount of reduced vanadium species is higher
on the carbon-based support than on oxide supports. Carbon support
must stabilize partially reduced vanadium sites, which are reported
active for partial oxidation reactions [15,16] since partially reduced
vanadium species (e.g. V4+) are involved in the O-insertion to form
acrylic acid on M2 phase, responsible for acrylic acid formation in the
multicomponent Mo-V-Nb-Te-O catalytic system [17,18]; Also in the
case of the VPO catalytic system, the active phase has been found to be
(VO)2P2O7, with vanadium species as V4+ [19].
Since the pH of a carbon sample suspension provides information
about the acidity and basicity of the surface, these values have been
calculated according to the procedure described by Bandosz et al. [14].
As expected, the pH value for the activated carbon is quite low (3.31),
indicative of a high amount of acid (surface phosphate groups
retained on the carbon) sites on the surface [10,20]. The pH increases
as vanadium coverage is increased, with values of 4.09 for 0.5 V/ACP
and of 5.32 for 1 V/ACP, indicative that vanadium oxide dispersed
species are titrating these phosphorus acid sites. In addition, pyridine
adsorption experiments were performed with both catalysts and the
results are shown in Fig. 1. It is well known that pyridine interacts
with acidic sites because it has a lone electron pair at the nitrogen
atom available for donation to a Lewis acidic site and because it can
accept a proton from Brönsted sites. It can be observed in Fig. 1 that
pyridine adsorption capacity for the sample with lower vanadium
N2
N2 isotherm, the apparent surface area (A ) was determined by
BET
applying the BET equation. The micropore volume (VNDR2) was
obtained by using the Dubinin–Radushkevich equation and the
narrow mesopore volume was determined as the difference between
the adsorbed volume of N2 at a relative pressure of 0.95 and the
micropore volume (VNDR2). From the CO2 adsorption data, the narrow
CO2
micropore volume (V
) was calculated using the Dubinin–
DR
Radushkevich equation. The pH of the sample suspension was
determined as described by Bandosz et al. [14]. A sample of 0.4 g of
dry carbon power was added to 20 ml of water and the suspension
was stirred overnight to reach equilibrium. Then the sample was
filtered and the pH of solution was measured with a pH-meter. The
surface acidity was studied by adsorption of pyridine (Py) carried out
in a thermogravimetric system (CI Electronics) at 100 °C. The inlet
partial pressure of the organic bases was 0.02 atm, and it was
established saturating He with the corresponding organic base in a
saturator at controlled temperature. After saturation of the sample,
desorption is carried out at the adsorption temperature in Helium
flow.
The surface chemistry of the samples was analyzed by X-ray
photoelectron spectroscopy (XPS), performed with a 5700 C model
Physical Electronics equipment, with Mg Kα radiation (1253.6 eV).
For a detailed study of the XPS peaks, the maximum of the C1s peak
was reallocated at 284.5 eV and used as a reference to shift the other
peaks. The deconvolution of the peaks was done using Gaussian–
Lorentzian curves and a Shirley type background line. The XPS atomic
ratios were obtained using the Phi Multipack software (Physical
Electronics, Inc). Oxidation resistance of the different catalysts and the
carbon support were obtained by non-isothermal thermogravimetric
analyses, carried out in a CI electronics thermogravimetric system.
The thermobalance automatically measures the weight of the sample
and the temperature as a function of time. Experiments were carried
out in air atmosphere, for a total flow rate of 150 cm3 (STP)/min,
employing sample mass of approximately 10 mg. The sample
temperature was increased from room temperature up to 900 °C at
a heating rate of 10 °C/min. Raman spectra were run with a single
monochromator Renishaw System 1000 equipped with a cooled CCD
detector (−73 °C) and an Edge filter. The samples were excited with
the 514 nm Ar+ line; the spectral resolution was ca. 3 cm−1 and
spectra acquisition consisted of 10 accumulations of 30 s. The spectra
were obtained under dehydrated conditions (ca. 390 K, synthetic air)
in a hot stage (Linkam TS-1500).
Table 1
Structural parameters of P-modified carbon support (ACP) and V-supported catalysts
(0.5 V/ACP and 1 V/ACP).
N2
N2
CO2
A
(m2/g)
V
(m2/g)
VmesN2 (cm3/g)
V
(m2/g)
BET
DR
DR
ACP
0.5 V/ACP
1 V/ACP
1056
732
696
0.356
0.252
0.237
0.987
0.645
0.575
0.167
0.130
0.122
Guerrero-Pérez, M. Olga
