A new type of nonsulfide hydrotreating catalyst: Nickel phosphide on carbon

Article abstract of DOI:10.1039/b415838e

Nickel phosphide on carbon is successfully synthesized by temperature-programmed reduction as verified with X-ray diffraction and extended X-ray absorption fine structure measurements; it shows superior activity, selectivity, and stability for sulfur removal from the refractory compound 4,6-dimethyldibenzothiophene with a steady-state conversion of 99%, which is much higher than that of a commercial NiMoS/γ-Al₂O₃ catalyst of 68%. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b415838e

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A new type of nonsulfide hydrotreating catalyst: nickel phosphide on
carbon
Yuying Shu and S. Ted Oyama*
Received (in Cambridge, UK) 14th October 2004, Accepted 22nd November 2004
First published as an Advance Article on the web 12th January 2005
DOI: 10.1039/b415838e
for the phosphide and low-temperature O2 chemisorption for the
sulfide. The molar space velocity (moles of 4,6-DMDBT/moles of
sites h) was 0.88 h21 for all the catalysts. Hydrotreating samples
were collected in sealed septum vials and were analyzed offline
with a gas chromatograph (Hewlett-Packard, 5890A) equipped
with a 0.32 mm i.d. 6 50 m fused silica capillary column (CPSIL-
5CB, Chrompack, Inc.) and a flame ionization detector.
Nickel phosphide on carbon is successfully synthesized by
temperature-programmed reduction as verified with X-ray
diffraction and extended X-ray absorption fine structure
measurements; it shows superior activity, selectivity, and
stability for sulfur removal from the refractory compound
4,6-dimethyldibenzothiophene with a steady-state conversion of
99%, which is much higher than that of a commercial NiMoS/
c-Al2O3 catalyst of 68%.
X-ray diffraction patterns of the fresh Ni2P/C sample and Ni2P
PDF reference are presented in Fig. 1. The pattern of the Ni2P/C
sample shows a broad feature at 2h y 25u due to the carbon
support. At higher angles, peaks due to Ni2P are visible, indicating
the formation of nickel phosphide on carbon.
Hydrotreating research has become an important subject of
environmental catalysis studies worldwide because of more
stringent environmental regulations and the decreasing quality of
available petroleum feedstocks. Recently a new class of high-
activity hydrotreating catalysts, the transition metal phosphides,
has been reported.1 The preparation of highly dispersed phosphide
phases on carriers such as SiO2 and c-Al2O3 has been described.2–7
These catalysts have been shown to be effective for sulfur removal
from thiophene and dibenzothiophene (DBT). Here we present a
new type of supported phosphide catalyst, nickel phosphide on
carbon, which is very active for treating 4,6-dimethyldibenzothio-
phene (4,6-DMDBT),which is representative of the least reactive
sulfur fraction in petroleum.8–10
The hydrotreating reaction results for the Ni2P/C, Ni2P/SiO2
and a NiMoS/c-Al2O3 catalyst in the hydrodesulfurization (HDS)
of 4,6-DMDBT, hydrodenitrogenation (HDN) of quinoline,
dehydrogenation (DeHYD) and hydrogenation (HYD) of tetralin
are summarized in Table 1. The data were taken after 100 h of
time on stream. Mass balances were 100 ¡ 5% for each reaction
type. The reaction of 4,6-DMDBT on the Ni2P/C occurs with a
very high and stable conversion of 99%, which is much higher than
that of the Ni2P/SiO2 catalyst with 76% or the NiMoS/c-Al2O3
catalyst with 68%. The Ni2P/SiO2 catalyst was previously reported
to be the most active among phosphide catalysts for the
desulfurization of DBT.1 There are three major products formed
from 4,6-DMDBT on these catalysts: (1) 3,39-dimethylbiphenyl
(DMBP) from direct desulfurization (DDS), (2) 3-(39-methylcy-
clohexyl)toluene (MCHT), and (3) 3,39-dimethylbicyclohexyl
(DMBCH) from hydrogenation (HYD) followed by desulfuriza-
tion. Compared with the NiMoS/c-Al2O3 and Ni2P/SiO2 catalysts,
the Ni2P/C catalyst gave a lower DMBP selectivity of 10% and a
The carbon-supported nickel phosphide (Ni2P/C) was prepared
by the temperature-programmed reduction (TPR) in H2 of a nickel
phosphate precursor.3,5,7 The carbon used in this study had a
surface area of 250 m2/g (Vulcan 4pc, XC72R). X-ray diffraction
(XRD) patterns of the synthesized samples were obtained with a
Scintag XDS-2000 powder diffractometer operated at 45 kV and
40 mA, using Cu Ka monochromatized radiation. The active
phase in the catalyst was further examined using extended X-ray
absorption fine structure (EXAFS) spectra, measured at the X-18B
beamline in Brookhaven National Laboratory with a 2.5-GeV ring
energy and a 400-mA ring current. The fresh sample was
pretreated in H2 flow at 723 K and transferred to a glass cell
with Kapton windows. The spent sample was taken from the
hydroprocessing reactor, washed with hexane, and then pretreated
in He flow at 613 K without exposure to air and similarly
transferred to a glass cell.5,7 The activity of the catalysts was
studied in a three-phase packed-bed reactor operated at 3.1 MPa
and 613 K with a model liquid containing 500 ppm sulfur as 4,6-
DMDBT, 3000 ppm sulfur as dimethyl disulfide (DMDS),
200 ppm nitrogen as quinoline, 1 wt% tetralin, 0.5 wt% n-octane
(internal standard), and balance n-tridecane. Quantities of catalysts
loaded in the reactor corresponded to the same amount of ex situ
chemisorption sites (70 mmol), as measured by CO chemisorption
Fig. 1 X-ray diffraction patterns of the synthesized Ni2P/C sample and a
*oyama@vt.edu
reference Ni2P (PDF 74-1385).
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Chem. Commun., 2005, 1143–1145 | 1143

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a
Table 1 Hydrotreating performance of Ni2P/C catalyst and its comparison with commercial NiMoS/c-Al2O3 and Ni2P/SiO2
Conversion (%)
Selectivity (%)
Ni2P/C Ni2P/SiO2 NiMoS/c-Al2O3
Reactants
Type
Ni2P/C Ni2P/SiO2 NiMoS/c-Al2O3 Product
4,6-DMDBT HDS
99
76
68
3,39-Dimethylbiphenyl 10
3-(39-Methylcyclohexyl)toluene 51
44
42
14
52
41
4
2
1
69
21
10
24
58
18
77
23
0
0
0
26
56
18
3,39-Dimethylbicyclohexyl
Propylcyclohexane
Propylbenzene
ortho-Propylaniline
5,6,7,8-Tetrahydroquinoline
1,2,3,4-Tetrahydroquinoline
Naphthalene
39
61
39
0
0
0
21
51
28
Quinoline
HDN
HYD
100
0
92
6
94
0
Tetralin
DeHYD
HYD
3
10
23
10
7
19
trans-Decalin
cis-Decalin
a
Reaction conditions: T 5 613 K (340 uC), and P 5 3.1 MPa (450 psig); liquid feed 5 5 cm3 h21; gas flow 5 150 cm3 (NTP) min21
.
higher DMBCH selectivity of 39%, indicating that it favors the
hydrogenation pathway. The Ni2P/C catalyst also showed a
superior ability for nitrogen removal with an HDN conversion of
100%, which is higher than that of the Ni2P/SiO2 of 92% or
NiMoS/c-Al2O3 of 94%. The HDN products are propylcyclohex-
ane and propylbenzene. But for the DeHYD and HYD of tetralin,
the Ni2P/C catalyst exhibits less activity with a conversion of 13%.
Fig. 2 presents the time course of HDS activity on the Ni2P/C,
Ni2P/SiO2 and NiMoS/c-Al2O3 catalysts. As can be seen, the Ni2P/
C gives 4,6-DMDBT conversions of close to 100% and no
deactivation in the prolonged 110 h of time on-stream. The
conversions on both the Ni2P/SiO2 and NiMoS/c-Al2O3 catalysts
are lower. The Ni2P/SiO2 catalyst shows deactivation, and the 4,6-
DMDBT conversion decreases from an initial 96% to 76%.
The reason for deactivation is probably the loss of active sites as
revealed from chemisorption data discussed in our earlier study.5
Fig. 3 Nickel K-edge EXAFS of the fresh and spent Ni2P/C samples.
Overall, in terms of activity, selectivity, and stability, the Ni2P/C
was superior to the other catalysts.
fresh Ni2P/C sample displays two distinct peaks, whose positions
correspond to the Ni–P and Ni–Ni distances in bulk Ni2P (Ni–P:
R 5 0.18 nm; Ni–Ni: R 5 0.23 nm). This again demonstrates the
formation of the Ni2P on the carbon support, and is consistent
with the XRD results. The spectrum of the spent Ni2P/C sample is
different in comparison with that of the fresh sample, showing a
diminution in the intensity of the Ni–Ni peak and a shift to a lower
distance. It is likely that another nickel compound is formed as a
consequence of the reaction. Earlier studies5,7 indicate that the
change could be due to development of intensity in the Ni–S
distance region in between the Ni–P and Ni–Ni distances. Even
though no distinct Ni–S peak is seen, a feature in that region
would give rise to the broad signals actually observed.5,7 Thus, the
active catalyst is probably a Ni–P–S surface phase on the outer
region of a Ni2P crystallite core.
To better characterize the nickel phosphide on the carbon
support and elucidate the possible active phase involved in the
hydrotreating reaction, both the fresh and spent Ni2P/C samples
were analyzed by EXAFS spectroscopy, and the resulting Fourier-
transformed Ni K-edge EXAFS spectra are shown in Fig. 3. The
We have thus demonstrated that the nickel phosphide on
carbon (Ni2P/C) is a promising hydrotreating catalyst and is able
to effectively remove the sulfur from the tenacious 4,6-DMDBT
compound.
The authors acknowledge support from the US Department of
Energy through Grant DE-FG02-963414669. They are also
grateful for the use of beamline X18B at the National
Synchrotron Light Source at Brookhaven National Laboratory
under Grant 4513, and to Yong-Kul Lee for his assistance with the
reaction experiments.
Fig. 2 Hydrodesulfurization performance of (a) Ni2P/C, (b) Ni2P/SiO2,
and (c) NiMoS/c-Al2O3.
1144 | Chem. Commun., 2005, 1143–1145
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Yuying Shu and S. Ted Oyama*
3 X. Wang, P. Clark and S. T. Oyama, J. Catal., 2002, 208, 321.
4 C. Stinner, Z. Tang, M. Haouas, Th. Weber and R. Prins, J. Catal.,
2002, 208, 456.
5 S. T. Oyama, X. Wang, Y. K. Lee, K. Bando and F. G. Requejo,
J. Catal., 2002, 210, 207.
Environmental Catalysis and Nanomaterials Laboratory, Department of
Chemical Engineering (0211), Virginia Polytechnic Institute and State
University, Blacksburg, VA 24061, USA. E-mail: oyama@vt.edu;
Fax: 1 540 231 5022; Tel: 1 540 231 5309
6 V. Zuzzaniuk and R. Prins, J. Catal., 2003, 219, 85.
7 S. T. Oyama, X. Wang, Y. K. Lee and W. J. Chun, J. Catal., 2004, 221,
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Notes and references
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10 M. Egorova and R. Prins, J. Catal., 2004, 225, 417.
1 S. T. Oyama, J. Catal., 2003, 216, 343.
2 D. C. Philips, S. J. Sawhill, R. Self and M. E. Bussell, J. Catal., 2002,
207, 266.
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Chem. Commun., 2005, 1143–1145 | 1145