M.-L. Frauwallner et al. / Applied Catalysis A: General 394 (2011) 62–70
63
denum nitride (Mo2N), and a sulfide Ni–Mo/Al2O3 catalyst. A
kinetic evaluation for toluene hydrogenation using this catalyst is
included.
matograph coupled with mass spectrometer (GC–MS) QP-5000
attached to a GC-17A (Shimadzu) equipped with a Petrocol column
with the following characteristics: length: 100 m, inner diameter:
0.25 mm, film thickness: 0.5 m. Allowed identifying and quan-
tifying the liquid products. The separation of these products was
achieved by the following temperature program: 38 ◦C as initial
temperature, ramp 1: 3.1 ◦C/min up to 100 ◦C, ramp 2: 10 ◦C/min up
to 200 ◦C (hold for 20 min). The MS worked with the following set-
tings: solvent cut time: 4.5 min, interval: 0.21 s, start m/z: 29–250,
and scan speed: 1000. A HP 6890 Series GC System (Hewlett
Packard) with an incorporated PONA column with the following
characteristics: length: 50 m, inner diameter: 200 m, film thick-
ness: 0.5 m, and a flame ionization detector (FID) analyzed the
gas stream. The separation of these products was achieved by the
following temperature program: 38 ◦C as initial temperature (hold
for 16.5 min), ramp 1: 3.1 ◦C/min up to 100 ◦C, ramp 2: 50 ◦C/min
up to 200 ◦C, and post-run at 200 ◦C (hold for 5 min).
2. Experimental
2.1. Catalysts and materials
The carbidic catalyst was synthesized in
a continuous
laboratory-scale unit using MoO3 (99%, Fluka), toluene CHROMA-
SOLVfor HPLC (99.9%, Sigma Aldrich), and industrial hydrogen (99%,
Praxair). For the preparation of the catalyst, toluene was pumped
to the unit and mixed with hydrogen before entering to the reactor
where MoO3 was previously loaded. Operating pressure was kept at
6.0 MPa during the preparation, toluene weight hourly space veloc-
ity (WHSV) was 0.35 h−1, and a particular temperature ramp-up
procedure was employed. During the first 24 h temperature was
set at 523 K; the following 24 h temperature was kept at 598 K;
and finally, temperature was set at 673 K and maintained between
24 and 72 h. The commercial molybdenum carbide (Mo2C) cata-
lyst was purchased from Sigma Aldrich and employed as received;
the molybdenum nitride (Mo2N) catalyst was prepared according
to the methodology reported by Afanasiev [24], using ammo-
nium molybdate tetrahydrate (AHM) (NH4)6Mo7O24·4H2O and
hexamethylentetramine (HMT) (both reagents high purity grade
from Sigma Aldrich); and the supported Ni–Mo/Al2O3 catalyst
(8 wt% Mo and 2.3 wt% Ni) was prepared by an incipient wetness
impregnation methodology [25], using nickel acetate Ni(OCOCH3)2
from Sigma Aldrich (98%) and AHM. Toluene was the model
molecule selected for hydrogenation activity tests. Industrial grade
hydrogen (99%), oxygen, and nitrogen were supplied by Praxair.
Supported Ni–Mo/Al2O3 pre-sulfiding agents were carbon disulfide
CS2 (99.9%) and thiophene (99%) both from Sigma–Aldrich.
2.4. Catalytic activity experiments
2.4.1. Activity tests of the in situ prepared molybdenum carbide
catalyst
The molybdenum carbide was prepared and its activity evalu-
ated in the same reactivity unit where it was prepared, without
any contact with air. Toluene hydrogenation was used to estimate
the hydrogenation capability of the catalyst. After preparation,
conditions for hydrogenation tests were set. For every run, total
pressure in the unit was 2.76 MPa and H2 flow was 80 sccm to
ensure an excess of partial pressure of H2 in the system. The
H2/toluene ratio (at pump conditions) was 917 cc/cc for every
run. Evaluated reaction temperatures were: 423, 448, 473, 498,
523, 573, and 598 K at two different weight hourly space veloc-
ities (WHSV): 1.6 and 0.35 h−1. For each condition at least two
(2) mass balances were completed at steady state conditions with
their corresponding conversion and selectivity calculations. Time
of reaction with the same catalyst, after reaching steady state con-
ditions, was 1005 h.
2.2. Characterization of the solids
All catalysts except the supported Ni–Mo/Al2O3 were char-
acterized using the X-Ray diffraction technique and Raman
Due to the pyrophoric nature of these materials, a passivation
procedure was required after reaction. This process consisted of
flowing a mixture of 200 sccm of N2 and 10 sccm of O2 through the
unit at atmospheric pressure and room temperature for 2 h.
spectroscopy.
A Rigaku Multiflex X-ray diffractometer was
employed in the scanning angle 2-theta (2Â) from 0◦ to 90◦
range. JADE 6.5 Materials Data Inc software was employed for
the identification of the species present in the solid. Raman spec-
troscopy was performed using a Labram system model Dilor
micro-Raman equipped with a 20 mW Ar laser emitting at 488 nm
and a holographic notch filter made by Kaiser Optical Systems,
Inc. (model supertNotch-Plus) with a 256 × 1024-pixel charge-
coupled device (CCD) used as the detector. All measurements were
carried out at room temperature with no special sample prepara-
tion. Textural characterization of the catalysts in terms of surface
area was performed using a Tristar II 3020, from Micromerit-
ics Instruments. These values were determined from nitrogen
adsorption–desorption isotherms measured at 77 K. All samples
were pre-treated at 150 ◦C overnight under N2 flow prior to N2
adsorption. SEM-EDAX was performed to the synthesized molyb-
denum carbide catalyst after reaction on a Philips FEI XL-30 ESEM
from Philips Electron Optics with EDS acquisition and analysis soft-
ware EDAX Genesis Spectrum V5.2. Micrographs were taken with
an XR beam of 20 kV and a spot size of 3.0 nm. The sample prepa-
ration consisted of placing a small amount of the solid sample into
a holder with a conductive carbon adhesive to keep the sample in
place.
2.4.2. Kinetic evaluation
A kinetic evaluation of the toluene hydrogenation reaction was
carried out to determine the reaction order and activation energy of
the newly synthesized molybdenum carbide catalyst. The reaction
order with respect to toluene concentration was estimated at 523 K
with 1.6, 2 and 3 h−1 as space velocities.
2.4.3. Additional catalytic formulations
Commercial molybdenum carbide (Mo2C), molybdenum nitride
(Mo2N), and supported Ni–Mo/Al2O3 were tested for toluene
hydrogenation in order to compare their activity with the novel
molybdenum carbide catalyst presented in this work. For all experi-
ments the total pressure was 2.76 MPa and the H2 flow was 80 sccm
with a H2/toluene ratio (at pump conditions) of 917 cc/cc. Table 1
summarizes the evaluated conditions for each catalyst.
The commercial molybdenum carbide was activated before per-
forming the activity test to remove the oxide layer formed on the
surface from the passivation process performed by the manufac-
turer. The activation consisted of flowing hydrogen (120 sccm) at
atmospheric pressure and 673 K for three (3) hours.
2.3. Characterization of the products
To achieve the most active phase, the homemade supported
Ni–Mo/Al2O3 was pre-sulfided in two steps:
In order to calculate conversion and selectivity values,
characterization of liquid and gas products was done. A gas chro-