Hydrodeoxygenation of p-cresol on unsupported Ni-P catalysts prepared by thermal decomposition method

Article abstract of DOI:10.1016/j.catcom.2013.07.003

Unsupported Ni-P catalysts were prepared from the mixed precursor of NiCl₂ and NaH₂PO₂ by thermal decomposition method, and their catalytic activities were measured using the hydrodeoxygenation (HDO) of p-cresol as probe.

Full text of DOI:10.1016/j.catcom.2013.07.003

Catalysis Communications 41 (2013) 41–46
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Hydrodeoxygenation of p-cresol on unsupported Ni–P catalysts
☆
prepared by thermal decomposition method
Weiyan Wang, Kun Zhang, Huan Liu, Zhiqiang Qiao, Yunquan Yang ⁎, Kai Ren
School of Chemical Engineering, Xiangtan University, Xiangtan City, Hunan 411105, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Unsupported Ni–P catalysts were prepared from the mixed precursor of NiCl2 and NaH PO2 by thermal de-
2
Received 20 April 2013
Received in revised form 6 June 2013
Accepted 2 July 2013
composition method, and their catalytic activities were measured using the hydrodeoxygenation (HDO) of
p-cresol as probe. The effects of the H PO /Ni molar ratio in the precursor and the thermal decomposition
2 2
temperature on the catalyst purity, crystallite size and HDO activity were studied. The HDO of p-cresol on
these Ni–P catalysts proceeded with two parallel pathways yielding methylbenzene and methylcyclohexane
as final products. The higher HDO catalytic activity of the catalyst was attributed to its bigger crystallite size
−
2+
Available online 9 July 2013
Keywords:
Hydrodeoxygenation
2
and purer phase of Ni P.
Ni
2
P catalyst
Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.
Thermal decomposition
p-cresol
Bio-oil
1
. Introduction
Owing to the decline in the availability of crude oil resources,
into the feed to maintain the HDO activity of sulfide catalysts
[
19]. Noble metal catalysts exhibited higher HDO activity than
sulfide catalysts, but the high cost prevents their wide practical ap-
plication. However, metal phosphide catalysts had shown high ac-
tivity for the HDS and HDN reactions as compared to the traditional
Co(Ni)–Mo–S catalysts and possessed higher thermal stabilities
as compared to nitride catalysts [20]. Yang et al. [21] had reported
that CoMoP/MgO catalyst was a good candidate for the HDO of
phenol, and the deoxygenation rate was increased drastically
with the reaction temperature. Recently, Zhao et al. [11] had
found that metal phosphide catalyst was superior active for the
HDO reactions as compared to the traditional molybdenum sulfide
bio-oil from the fast pyrolysis of lignocellulosic biomass was consid-
ered to be an alternative source for green and renewable fuel because
it could reduce the world's dependency on fossil fuel and contribute
no new carbon dioxide to the atmosphere [1,2]. However, this bio-oil
contains very important amount of oxygenated compounds, which
results in its immiscibility with crude oil and low heating value [3].
To expand the utilization of bio-oil as supplements or replacements
for gasoline or fossil diesel, it is necessary to selectively remove oxy-
gen [4]. Hydrodeoxygenation (HDO), a promising technology, has
been intensively investigated for the upgrading bio-oil in recent
years [5].
There have appeared many HDO catalysts in the literatures, in-
cluding noble catalysts [6,7], sulfide catalysts [8,9], phosphide cata-
lysts [10,11] and others [12,13]. Of these catalysts, sulfide catalysts
are the traditional hydrotreating catalysts, which were widely applied
in hydrodesulfurization (HDS) [14,15], hydrodenitrogenation (HDN)
catalysts and the commercial 5% Pd/Al
had also confirmed that the activity of metal phosphides followed
the order Ni P N WP N MoP N CoP N FeP for the HDO of a biofuel
2 3
O catalyst. Moreover, they
2
model compound [22].
2
At present, the prepared method of Ni P catalyst for the HDO
reaction was mainly concentrated on the TPR method [10,11].
This method requires a high temperature (600–650 °C) and a
long treatment time due to the strong P–O bond in the phosphate,
which leads to a serious agglomeration and relatively low activity.
[16,17] and hydrodeoxygenation (HDO) [8,9,18]. But for HDO, be-
cause of a very low of sulfur in the bio-oil and a negative effect of ox-
ygen for the sulfide structure, it needs to add some sulfiding agent
2
Ni P catalysts prepared by thermal decomposition had exhibited a
high activity in the hydrodesulfurization (HDS) [23,24]. It had
been also reported that there was a strong interaction with the
Ni
great influence for the catalytic activity of the supported catalysts
25,26]. Therefore, to eliminate the effects of supports on the cata-
lytic properties, we synthesized unsupported Ni P catalyst from
the mixed precursors of nickel chloride and sodium hypophosphite by
thermal decomposition and focused on the effects of the preparation
2
P particles and supports, and the supports' performance had
☆
This is an open-access article distributed under the terms of the Creative Commons
Attribution-NonCommercial-No Derivative Works License, which permits non-commercial
use, distribution, and reproduction in any medium, provided the original author and source
are credited.
[
2
⁎
Corresponding author. Tel.: +86 731 58298581; fax: +86 731 58293284.
E-mail address: yangyunquan@xtu.edu.cn (Y. Yang).
1566-7367/$ – see front matter. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2013.07.003

42
W. Wang et al. / Catalysis Communications 41 (2013) 41–46
condition of the catalysts on their catalytic activities in the HDO of
p-cresol.
3. Results and discussion
3.1. Characterization of the Ni–P catalysts
2
. Experimental
Fig. 1(a) shows the powder XRD patterns of Ni–P catalysts pre-
−
2+
2
.1. Catalyst preparation
pared with different ratio of H
2
PO
2
/Ni . The thermal decomposition
−
of H
PH
and P can reduce Ni to form Ni
2
PO
, P, PO
2
usually undergoes disproportionation reactions to produce
2
−
3−
All solvents and reagents were obtained from Sinopharm
3
3
4 3
and PO , in which the low valent products such as PH
2
+
Chemical Reagent Co., Ltd. in high purity (≥99%) and used without
further purification. Unsupported nickel phosphide catalysts were
2
P [23]. Ni-P-1-300 presented sever-
al sharp diffraction peaks at 2θ = 40.6°, 44.5°, 47.3°, 54.0°, 54.9° and
74.6°, attributing to the 111, 201, 210, 002, 211 and 400 reflections
synthesized by directly thermal decomposition method. NiCl
4 mmol) and a certain amount of NaH PO were dissolved in
2
(
2
2
of the crystalline Ni
Ni-P-1-300 had a good crystallinity. When the ratio of H
was increased to 3 or 5, some slight peaks corresponding to Ni
was observed, besides the main phase of Ni P, but the peak intensity
of Ni P was increased, which indicated that the crystallite size of Ni–P
catalysts calculated from the XRD pattern was increased by increasing
2
P (JCPDS No. 74-1385) [27]. It indicated that
−
2+
water under ultrasonic environment. Subsequently, a solid precur-
sor was obtained after the solution was evaporated slowly. Then,
the precursor was directly heated to the required temperature
2 3
PO /Ni
5 4
P
2
for 1.0 h in a flowing 30 mL/min N
2
. Finally, the product was
2
cooled to ambient temperature in nitrogen atmosphere, washed
with deionized water followed by ethanol and dried at 60 °C
for 3 h in a vacuum oven. The prepared catalysts were denoted
as Ni–P–X–T, where X and T represented the molar ratio of
−
2+
−
2+
the ratio of H
was increased to 10, the peak intensity of Ni
but the intensities of peaks of Ni was increased. These suggested
2
PO
2
/Ni . However, when the ratio of H
2 2
PO /Ni
2
P was decreased greatly,
5 4
P
−
2+
H
2
PO
2
/Ni
in the precursor and the thermal treatment tempera-
that the main phase was transferred from Ni
resulted from that the high ratio of H
PH for other reactions [25].
2
P to Ni
5
P
4
, which was
−
2+
ture, respectively.
2
PO
2
/Ni
provided excessive
3
2
.2. Catalyst characterization
X-ray diffraction (XRD) measurements were carried on a D/max2550
1
8KW rotating anode X-ray diffractometer with monochromatic Cu Kα
(
a)
radiation (λ = 1.5418 Å) at a voltage and a current of 40 kV and
00 mA, respectively. The crystallite size of the sample was quantified
Ni P (JPCDS No. 18-0883)
5
4
3
Ni P (JPCDS No. 74-1385)
2
from the XRD pattern using the whole pattern fitting method.
The specific surface area was measured by a Quantachrome's
NOVA-2100e Surface Area instrument by physisorption of nitrogen
at −196 °C. The elemental content of the sample was identified by
inductively coupled plasma analysts (ICP) on a Varian VISTAMPX.
The morphologies of catalysts were determined by transmission
electron microscopy (TEM) on a JEOL JEM-2100 transmission elec-
tron microscope with a lattice resolution of 0.19 nm and an acceler-
ating voltage of 200 kV.
Ni-P-10-300
Ni-P-5-300
Ni-P-3-300
Ni-P-1-300
2
.3. Catalyst activity measurement
The HDO activity tests were carried out in a 300-mL sealed au-
toclave. The prepared catalyst (0.15 g) without any further treat-
ment, p-cresol (13.5 g) and dodecane (86.5 g) were placed into
the autoclave. Air in the autoclave was evacuated by pressuriza-
tion–depressurization cycles with nitrogen and subsequently with
hydrogen. The system was heated to desired temperature then
pressurized with hydrogen to 4.0 MPa and adjusted the stirring
speed to 900 rpm. During the reaction, liquid samples were with-
drawn from the reactor and analyzed by Agilent 6890/5973 N
GC-MS and 7890 gas chromatography using a flame ionization de-
tector (FID) with a 30-m AT-5 capillary column. For the HDO of
phenols, there exist two main pathways involving direct C–O
bond rupture (DDO route), yielding aromatic products, and the
other via the pre-hydrogenation of the aromatic ring producing
cycloalkenes and cycloalkanes (HYD route). The DDO/HYD ratio
and deoxygenation rate for each experiment were calculated as
follows:
0
10
20
30
40
50
60
70
80
90
2
Theta (degree)
(
b)
Ni P (JPCDS No. 18-0883)
5
4
Ni P (JPCDS No. 74-1385)
2
o
3
50 C
o
300 C
moles of methylbenzene
DDO=HYD ¼
o
2
50 C
moles of methylcyclohexane and 3‐methylcyclohexene
0
10
20
30
40
50
60
70
80
90
Deoxygenation rate ðwt%Þ
2 Theta (degree)
ꢀ
ꢁ
oxygen content in the final organic compounds
total oxygen content in the initial material
2
Fig. 1. XRD patterns of Ni–P catalysts prepared (a) with different ratio of H PO
/Ni2+
and (b) under different temperature.
−
¼
1−
ꢀ 100 %
2

W. Wang et al. / Catalysis Communications 41 (2013) 41–46
43
The effects of thermal temperature on the sample phases are
shown in Fig. 1(b). The peak intensity of Ni P was increased firstly
and then decreased when the temperature rose from 250 °C to
50 °C. Heat treatments at high temperatures (300 °C and 350 °C)
led to the formation of Ni , besides the main phase of Ni P. This
Table 1
Surface area and composition of Ni–P catalysts.
2
2
Catalyst
Surface area (m
/g)
Ni/P atomic ratio
Crystallite size (nm)
3
Ni-P-1-300
Ni-P-3-300
Ni-P-5-250
Ni-P-5-300
Ni-P-5-350
Ni-P-10-300
6.2
6.5
6.8
5.2
5.3
6.7
1.94
1.90
1.96
1.84
1.75
1.52
22.1
29.8
35.1
36.8
37.2
33.2
P
5 4
2
was mainly caused by the following reason. At the higher reaction
−
temperature, the disproportionation of H
ing to generate adequate PH
at 350 °C [23]. Therefore, there had optimum reaction temperature
2
PO
2
was very faster, lead-
3
for the formation of Ni
5
4
P , especially
−
2+
and H
2 2 2
PO /Ni ratio for the maximum pure Ni P with big crystallite
size.
The morphologies of Ni–P catalysts were observed by TEM and
showed in Fig. 2. These TEM images showed that some particles
aggregated each other in a certain extent, which might cause by a
big surface energy of nano-particles. Many particles with the size
of about 30–40 nm could be clearly seen in the TEM images of
Ni-P-3-300 and Ni-P-5-300, but Ni-P-10-300 image displayed some
blurry and irregular particles. The HRTEM image of Ni-P-5-300
showed many clear stripes. The distance of neighboring stripes was
measured to be 0.51 nm. This was very close to 0.507 nm of (111)
with lower Ni/P atomic ratio were generated. Hence, heating the pre-
−
2+
cursor with low H
250 °C) could obtain purer phase of Ni
To investigate the valence state of Ni and P, Ni-P-5-300 catalyst
2
PO
3
/Ni
molar ratio (1–3) at low temperature
(
2
P.
was characterized the by XPS, as show in Fig. 3. Some oxides were un-
avoidably produced on the catalyst surface during the preparation
and characterization. In the level of Ni 2p, the peak centered at
δ+
8
8
53.0 eV was attributed to Ni
56.4 eV corresponded to Ni
2
in the Ni P phase, and the peak at
2+
in surface NiO [28,29]. In the level of
2
plane of Ni P form [25].
P 2p, two peaks around 129.6 and 133.7 eV were exhibited, corre-
Specific surface areas and Ni/P atomic ratios of Ni–P catalysts pre-
δ−
5+
sponding to P in Ni
28,29]. These XPS results were well consistent with previous investi-
gations [25,28,29].
2
P and P
in some surface phosphorus oxides
pared under different conditions are listed in Table 1. The tempera-
[
−
2+
ture and the molar ratio of H
2
PO
3
/Ni
did not affect the specific
surface area significantly but obviously affect the Ni/P atomic ratio
of Ni–P catalysts. The surface area of each as-prepared Ni–P catalyst
2
was close to 6.0 m /g. All of Ni/P atomic ratios were lower than 2. It
3.2. Hydrodeoxygenation of p-cresol on Ni–P catalysts
was also found that the Ni/P atomic ratio of Ni–P catalyst was de-
−
2+
creased with the increases of the temperature and the H
PO
2 3
/Ni
The changes of p-cresol and product concentrations in the HDO of
p-cresol on Ni-P-1-300, Ni-P-3-300, Ni-P-5-300 and Ni-P-10-300
molar ratio. These suggested that other Ni–P species (e.g. Ni
5 4
P )
Fig. 2. TEM images of Ni–P catalysts.

44
W. Wang et al. / Catalysis Communications 41 (2013) 41–46
the end of a 10-h run was only 5.0%. In addition, the previous
Ni 2p
study has found that the hydrogenation of p-cresol was easier
than the hydrogenation of methylbenzene [30]. Under the presence
of p-cresol, the hydrogenation of methylbenzene would be much
slower due to the competitive adsorption of p-cresol on the cata-
lyst surface. Therefore, the hydrogenation of methylbenzene was
concluded to be negligible under the studied conditions. Şenol et
al. [19] had also reported this similar result. Compared with the
DDO/HYD ratio, it showed that the p-cresol HDO proceeded with
the DDO pathway being the primary route for Ni-P-10-300 while
Ni-P-5-300 favored the HYD pathway.
The effects of reaction temperature on the HDO of p-cresol over
Ni-P-5-300 were summarized in Table 2. Both the conversion and
the deoxygenation rate were increased with temperature, indicating
that high temperature was beneficial to deoxygenation in the mea-
sured range. However, it does not completely true that the higher
temperature corresponded to the higher conversion. Our previous
study had revealed that there exists an exothermic reversible reaction
equilibrium effect in the reaction networks for the HDO of p-cresol
based on a thermodynamic calculation [31]. Compared with Ni-P-
8
68
864
860
856
852
848
844
Electron Binding Energy (eV)
P 2p
5
-300 catalyst, when the thermal decomposition temperature was
decreased to 250 °C or increased to 350 °C, the prepared catalyst
showed lower conversion but higher HYD selectivity. The deoxy-
genation rate on Ni-P-5-250 and Ni-P-5-350 was 66.1% and 72.1%,
respectively, being lower than that on Ni-P-5-300. Therefore, it
could conclude that there existed an optimal thermal decomposi-
tion temperature to make the Ni–P catalyst to be the maximum
activity.
As shown in Table 1, for the different thermal decomposition tem-
perature, the crystallite size of these catalyst decreases with the order
of Ni-P-5-350 (37.2 nm) N Ni-P-5-300 (36.8 nm) N Ni-P-5-250
(
35.1 nm) while the Ni/P atomic ratio decreases in the order of
Ni-P-5-250 (1.96) N Ni-P-5-300 (1.84) N Ni-P-5-350 (1.75). From
Table 2, Ni-P-5-300 exhibited the highest activity and Ni-P-5-250
showed the lowest activity in the HDO of p-cresol. It seems that
the big crystallite size of the Ni–P catalyst corresponds to the
high HDO activity, which might be explained by the following rea-
son. The relatively larger particle size facilitates the adsorption of
p-cresol and thus was favorable for the conversion of p-cresol
1
44 142 140 138 136 134 132 130 128 126 124
Electron Binding Energy (eV)
Fig. 3. XP spectra of Ni 2p and P 2p levels of Ni-P-5-300.
[
32]. However, the Ni/P atomic ratio of the catalyst also had a
great effect on its HDO activity. Compared with Ni-P-5-300,
Ni-P-5-350 exhibited lower HDO activity, which was resulted
catalysts at 325 °C versus reaction time are displayed in Fig. 4.
The HDO products were methylbenzene, methylcyclohexane and
from the lower Ni/P atomic ratio causing by the more Ni
5 4
P . Form
the crystallite size, Ni/P atomic ratio and HDO activity of Ni–P catalysts
−
2+
3
-methylcyclohexene without any oxygen-containing compounds.
2 2
prepared with different of H PO /Ni molar ratio, it could also be con-
This indicated that Ni–P catalysts prepared by thermal decomposi-
tion method possessed high deoxygenation activity. According
to the concentration of p-cresol, it was obvious to see that the ac-
tivity of Ni–P catalyst was increased in the order of Ni-P-10-
cluded that the HDO activity depended on the crystallite size and Ni/P
atomic ratio. For example, Ni-P-1-300 had smaller crystallite size but
displayed higher HDO activity than Ni-P-10-300. Therefore, the bigger
2
crystallite size and the purer phase of Ni P contributed to its higher
3
00 b Ni-P-1-300 b Ni-P-3-300 b Ni-P-5-300, indicating that the
HDO activity.
−
2+
catalyst prepared with a suitable H
2
PO
3
/Ni
molar ratio had the
maximal HDO activity. Both the concentrations of methylbenzene
and methylcyclohexane were increased with p-cresol conversion,
and 3-methylcyclohexene in trace amounts was detected during
reaction, which suggested that the reaction network for p-cresol
HDO included two parallel pathways. One was proceeded by the
pre-hydrogenation of the aromatic ring, the rapid elimination of
water and the subsequent hydrogenation to aliphatic hydrocarbons
4. Conclusion
Unsupported Ni–P catalysts were directly synthesized by thermal
decomposition method. The prepared conditions had little effect on
the surface area of the catalyst but great effect on its purity and crys-
−
2+
tallite size. Heating the precursor with a suitable H
2
PO
3
/Ni
molar
(
HYD route), and the other was the direct C–O bond scission
ratio at low temperature (250 °C) could obtain pure phase of Ni P
2
yielding aromatic products (DDO route). To determinate whether
methylcyclohexane and 3-methylcyclohexene were (all or partly)
with a small crystallite size. The HDO of p-cresol on these Ni–P cata-
lysts reacted though two parallel pathways: hydrogenation and dehy-
dration route (HYD) and direct deoxygenation route (DDO), and the
produced from the further hydrogenation of methylbenzene in H
2
−
2+
atmosphere, the activity of Ni-P-5-300 in the hydrogenation
of methylbenzene was tested under the same conditions as
the HDO of p-cresol. The results showed that the conversion of
methylbenzene to methylcyclohexane or 3-methylcyclohexene at
ratio of DDO/HYD was changed with the H
PO
2 3
/Ni
molar ratio in
the precursor and the thermal decomposition temperature. The
conversion and the deoxygenation rates were reached to 85.0%
and 83.4%, respectively. Ni-P-5-300 exhibited higher HDO catalytic

W. Wang et al. / Catalysis Communications 41 (2013) 41–46
45
(a) 0.5
(b) 0.5
p-Cresol
p-Cresol
Methylcyclohexane
Methylcyclohexane
3-Methylcyclohexene
Toluene
0
0
0
0
0
.4
.3
.2
.1
.0
0.4
0.3
0.2
0.1
0.0
3-Methylcyclohexene
Toluene
0
1
2
3
4
5
6
7
8
9
10
0
1
2
3
4
5
6
7
8
9
10
Time (h)
Time (h)
(c) 0.5
(d) 0.5
p-Cresol
p-Cresol
Methylcyclohexane
-Methylcyclohexene
Toluene
Methylcyclohexane
3-Methylcyclohexene
Toluene
0
0
0
0
0
.4
.3
.2
.1
.0
0.4
0.3
0.2
0.1
0.0
3
0
1
2
3
4
5
6
7
8
9
10
0
1
2
3
4
5
6
7
8
9
10
Time (h)
Time (h)
Fig. 4. The HDO of p-cresol on (a) Ni-P-1-300, (b) Ni-P-3-300, (c) Ni-P-5-300 and (d) Ni-P-10-300 catalysts. Reaction conditions: initial concentration of p-cresol 0.46 mol/L, 0.20 g
catalyst, temperature 325 °C.
Table 2
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Acknowledgements
[
This research was supported by a specialized research fund for
the Doctoral Program of Higher Education (20124301120009), the
Natural Science Foundation of Hunan Province (13JJ4048), the Na-
tional Students' Innovation and Entrepreneurship Training Program
[
[
(
201210530011) and the Scientific Research Fund of Hunan Provin-
[
cial Education Department (12C0392).

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