Highly selective catalytic hydrodeoxygenation of Caromatic-OH in bio-oil to cycloalkanes on a Ce-Ni-W-B amorphous catalyst

Article abstract of DOI:10.1039/c4ra04364b

This study focused on the preparation of Ce-Ni-W-B amorphous catalysts and the effect of Ce content on their catalytic activities in the hydrodeoxygenation (HDO) of phenols in bio-oil. Adding the promoter Ce could increase the content of Ni⁰ and WO₃ on the Ce-Ni-W-B catalyst surface, leading to the improvement of the deoxygenation activity, but excess Ce would cover some active sites, resulting in a reduction of the catalytic activity. Because of the amorphous structure and the electron transfer between Ni⁰ and B⁰, these catalysts possess very high hydrogenation activity, making the HDO of phenols on these amorphous catalysts proceed with a hydrogenation-dehydration route, which not only decreases the aromatic content in the product but also the reaction temperature. With an optimal Ce content (2.5 mol%), the total aromatic selectivity reduced to 1.0% and the HDO reaction temperature decreased to 498 K. This research provides a high activity catalyst for transforming phenols into cycloalkanes. the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra04364b

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Highly selective catalytic hydrodeoxygenation of
Caromatic–OH in bio-oil to cycloalkanes on a
Cite this: RSC Adv., 2014, 4, 37288
Ce–Ni–W–B amorphous catalyst
ab
a
a
a
ab
*
*
Zhiqiang Qiao, Kun Zhang, Pengli Liu, Yunquan Yang
Weiyan Wang,
and Kui Wua
This study focused on the preparation of Ce–Ni–W–B amorphous catalysts and the effect of Ce content on
their catalytic activities in the hydrodeoxygenation (HDO) of phenols in bio-oil. Adding the promoter Ce
could increase the content of Ni0 and WO3 on the Ce–Ni–W–B catalyst surface, leading to the
improvement of the deoxygenation activity, but excess Ce would cover some active sites, resulting in a
reduction of the catalytic activity. Because of the amorphous structure and the electron transfer
between Ni0 and B0, these catalysts possess very high hydrogenation activity, making the HDO of
phenols on these amorphous catalysts proceed with a hydrogenation-dehydration route, which not only
decreases the aromatic content in the product but also the reaction temperature. With an optimal Ce
content (2.5 mol%), the total aromatic selectivity reduced to 1.0% and the HDO reaction temperature
decreased to 498 K. This research provides a high activity catalyst for transforming phenols into
cycloalkanes.
Received 10th May 2014
Accepted 11th August 2014
DOI: 10.1039/c4ra04364b
www.rsc.org/advances
sulde catalysts promoted by cobalt (Co) or nickel (Ni) were
investigated extensively.9–14 One of the worst inherent disad-
1. Introduction
vantages of these suldes is the negative effect of oxygen for its
suldation level.15,16 Noble metal catalysts possess high hydro-
genation activity,17–20 favoring mild required reaction condi-
tions and a low aromatics content in the nal product,8,21 but
the high cost prevents their wide application. Recently, some
other catalysts such as transition metal phosphides,22–25
nitrides26,27 and metal catalyst28,29 also show high HDO activity.
For these above catalysts, except for some noble metal catalysts,
a serious problem worthy to be pointed out is that the HDO
reaction temperature is always higher than 573 K,12,23,28–30
resulting in a high energy consumption and high content of
benzene/aromatics in the products. However, aromatics were
conrmed to be the environmental carcinogens. If the fuel
contained much aromatics such as benzene and its derivatives,
there would exhaust some aromatics to the air when this fuel
Bio-oil, derived from biomass by liquefaction or pyrolysis, has
been considered as the most potential replacement or supple-
ment for conventional transportation fuels due to growing
energy consumption, the depletion of non-renewable fossil fuel
resources and the global greenhouse effect.1–3 However, this bio-
oil contains numerous kinds of oxygenic compounds including
phenols, aldehydes and esters, contributing to its high oxygen
content, which leads to a poor heating value, thermal instability
and tendency of polymerization during storage and trans-
portation.4,5 Therefore, for a satisfactory result, the oxygen
content in bio-oil has to be reduced. Catalytic hydro-
deoxygenation (HDO) is deemed to be a particularly effective
method to remove the oxygen and increase the H/C atomic ratio
of the bio-oil.
Phenols are oen selected as the model compounds for the
HDO of bio-oil because they account for a large proportion in
the bio-oil and the Caromatic–OH bond is stronger than the other
C–O bonds.6,7 Generally, phenols HDO mechanism includes
direct hydrogenolysis (DDO) route yielding aromatic products
and hydrogenation-dehydration (HYD) route producing cyclo-
alkanes,8,9 as shown in Fig. 1. Conventional molybdenum
aSchool of Chemical Engineering, Xiangtan University, Xiangtan, Hunan, 411105, P. R.
China. E-mail: wangweiyan@xtu.edu.cn
bNational
& Local United Engineering Research Center for Chemical Process
Simulation and Intensication, Xiangtan University, Xiangtan 411105, P. R. China.
Fig. 1 Reaction network for the HDO of phenols.
E-mail: yangyunquan@xtu.edu.cn
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combusted not completely, which would threaten the human VISTAMPX. The surface composition and surface electronic
health terribly. Hence, the quality standards for clean fuel were state were analyzed by X-ray Photoelectron Spectroscopy (XPS)
become stricter with the increasing demand of environmental using Kratos Axis Ultra DLD instrument at 160 eV pass energy.
protection and World Fuel Oil Regulation IV limited the Al Ka radiation was used to excited photoelectrons. The binding
benzene and aromatics content in gasoline within 1% and 35%, energy value of each element was corrected using C1s ¼ 284.6 eV
respectively. Therefore, how to develop an innovative HDO as a reference. The XP spectra of each element was deconvo-
catalyst with high activity, non-sulfurization, non-noble metal is luted using a Gaussian–Lorentz curve-tting program. The
still a signicant challenge.
surface composition of each sample was calculated using the
Previous studies8,31–33 had demonstrated that if the catalyst corresponding peak areas of Ni, W, B, Ce and the sensitivity
has a high hydrogenation activity to facilitate the HDO of factors for these elements.
phenols with the route of hydrogenation of the aromatic ring
followed by dehydration of the formed alcohols, it would
decrease the required HDO temperature, which can realize the
energy saving and consumption reduction. Li et al. had reported
that adding Ce into Ni–B amorphous catalyst could increase its
and dodecane were placed into the autoclave. The total weight
thermal stability, surface area and catalytic activity.34 We had
veried that La-promoted Ni–W–B amorphous catalyst has a
2.3 Catalyst activity measurement
The catalyst activity tests were carried out in a 300 mL sealed
autoclave. The fresh catalyst (0.1 g or 0.3 g), phenols (0.12 mol)
of substrate and dodecane was 100 g. Air in the autoclave was
evacuated by pressurization-depressurization cycles with
¨
high HDO activity, but its thermal stability and Bronsted acid
nitrogen and subsequently with hydrogen. The mixture was
heated at 10 K minꢀ1 to the desired temperature, then pres-
surized with hydrogen to 4.0 MPa and stabilized the stirring
speed at 700 rpm. During the reaction, liquid samples were
withdrawn from the reactor and identied by Agilent 6890/
5973N GC-MS. The amounts of substrate and products were
analyzed by Agilent 7890 gas chromatography using a ame
ionization detector (FID) with a 30 m AT-5 capillary column.
Duplicate or triplicate experiments were performed and the
average of these tests is reported here. The errors for conversion
values were typically within plus/minus 5.0 mol%. Conversion
¼ (the amount of aromatic-ring change during reaction/total
amount of aromatic-ring) ꢂ 100%; selectivity ¼ (C atom in each
product/total C atom in the products) ꢂ 100%; deoxygenation
degree (DD, wt%) is dened as [1 ꢀ oxygen content in the nal
organic compounds/total oxygen content in the initial material]
ꢂ 100%. Carbon balance is better than 96 ꢃ 3% in this work.
sites needed to be further improved.31 Therefore, to overcome
these shortcomings, in this study, we synthesized a series of Ce
promoted Ni–W–B amorphous catalysts and focused on the
effect of Ce on their HDO performances using phenol deriva-
tives as the model substrate.
2. Experimental section
2.1 Catalyst preparation
Ce-promoted Ni–W–B amorphous catalysts were prepared by
the following steps: Sodium tungstate (2.94 g), nickel nitrate
and cerium nitrate were dissolved in 100 mL aqueous solution,
where the Ni/W molar ratio in each case was 1.1 : 1. This
mixture solution was placed in a 250 mL three-necked ask. An
80 mL sodium borohydride aqueous solution (1 mol Lꢀ1) was
added dropwise to the three-necked ask with vigorous agita-
tion at 273 K. Then, the black precipitate was produced, which
was washed with ultra-pure water several times until the pH ¼ 7
to remove the soluble boron species and Na+ ions. This was
followed by washing with absolute ethanol several times to
remove the residual water and water-soluble impurities. Finally,
the resulting product was dried under vacuum at 323 K for 8
hours. The resulting material was donated as X Ce where X
represented the molar ratio of Ce3+/W6+ in the form of X:1000 in
3. Results and discussion
3.1 Characterization of the amorphous catalysts
Fig. 2 shows the XP spectra of the Ni–W–B and Ce–Ni–W–B
catalysts in Ni 2p, W 4f, B 1s and Ce 3d levels, respectively. Each
of the spectra was deconvoluted, and the relative content of
each state was calculated according to the corresponding peak
area. Because the elements on catalyst surface were inevitable to
be oxidized during synthesis and characterization, peaks for
oxidized metal and B species dominated the XPS spectra. The Ni
2p spectra of the Ni–W–B or Ce–Ni–W–B consists of a small peak
at 852.5 eV, assigning to metallic nickel (Ni0),35–39 and of three
major peaks at higher binding energies (>854.0 eV) to Ni2+
the initial solution. Ni–Mo–S/g-Al2O3 (g-Al2O3, 156.2 m2 gꢀ1
)
was also prepared for comparison using a mixture of 3.0 wt%
carbon disulde in cyclohexane, as described in the previous
literature.30
2.2 Catalyst characterization
Specic surface area was measured by a Quantachrome's NOVA- species.35–39 Two peaks around 188.0 eV and 192.0 eV are
2100e Surface Area instrument by physisorption of nitrogen at observed in the B 1s spectra, corresponding to B0 and B3+,39–41
77 K. X-ray diffraction (XRD) test was carried out on a D/max respectively. In comparison with the standard binding energies
2550 18 KW Rotating anode X-ray Diffractometer with Cu Ka (l of pure elemental Ni (853.0 eV) and B (187.1 eV),39 the binding
¼ 1.5418 A) radiation (40 kV, 300 mA). The 2q was scanned over energy of Ni0 in the obtained samples shis negatively by
˚
the range of 10–80ꢁ at a rate of 10ꢁ per min to identify the approximately 0.5 eV, whereas the binding energy of B0 shis
amorphous structure. Bulk compositions were identied by positively by 0.9 eV, suggesting that a partial electron transfers
Inductively Coupled Plasma analysts (ICP) on
a
Varian from elemental boron to metallic nickel, which is in agreement
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NaBH4 than that of Ni2+ with NaBH4. The Ni0/Ni2+ molar ratio
for Ni–W–B is 2.9 : 97.1, suggesting that only a minority of
nickel presents in a metallic form. The addition of Ce promoter
has a great effect for the surface composition. Aer adding Ce
into Ni–W–B, both the W content and B0 content of the catalyst
surface are increased. The Ni0 content increases from 2.9% for
Ni–W–B to 16.9% for 50-Ce, and then decreases to 10.7% for 70-
Ce. Although the Ni/W atomic ratio on the catalyst surface
decreases with Ce content, it is still much higher than that in
bulk composition. The XPS characterization demonstrated that
all of W in the prepared catalyst was only existed in the form of
WO3. According to the reports by X. Li et al.45 that WO3 in WO3/
¨
¨
TiO2 supported catalyst acted as Bronsted acid and the Bronsted
acid was inchmeal enhanced with the increase of WO3 loading.
The increase of W content on the catalyst surface means the
¨
more Bronsted acid sites.
The bulk composition, surface composition and the content
of d As shown in Fig. 3a, fresh Ni–W–B catalyst shows a broad
diffraction peak around 2q ¼ 45ꢁ, characterizing a typical
amorphous structure.35,40,41,44 Other catalysts also present the
similar XRD pattern as Ni–W–B, indicating that the addition of
Ce promoter does not destroy the amorphous structure.
However, the intensity of this peak (2q ¼ 45ꢁ) increases rstly
and then decreases with the increase of Ce, suggesting that the
amorphous degree of these catalysts are changed, which is
mainly caused by the following two reasons. As shown in Table
1, the addition of Ce increases the content of Ni0 and B0 on the
catalyst surface, enhancing the particle agglomeration because
of the strong interaction between Ni0 and B0,42 which leads to a
low amorphous degree. On the other side, Ce oxides in the
catalyst, served as a support for the element homogeneous
dispersion, hinders the particle agglomeration, which results in
a high amorphous degree.31,34,35 The particle agglomeration
leads to produce large particles and then decrease the surface
area of sample whereas Ce oxides promote the particle disper-
sion and then increase its surface area. With these two opposite
effects, only a negligible change in the surface area of Ce–Ni–W–B
is registered. It is in the order of 50-Ce (47.1 m2 gꢀ1) z 25-Ce
(47.6 m2 gꢀ1) > Ni–W–B (45.4 m2 gꢀ1) > 70-Ce (35.3 m2 gꢀ1). The
low surface area of 70-Ce might be attributed to the insertion of
excess Ce oxides in the pore channels of Ni–W–B catalyst.
To further identify the amorphous structure of the prepared
catalysts, the fresh catalysts and dodecane were placed into the
autoclave and then heat-treated at 523 K or 573 K for 10 h under
the condition of 4.0 MPa hydrogen pressure. The corresponding
XRD patterns are showed in Fig. 3b and c. The intensity of the
Fig. 2 XP spectra of XP spectra of Ni 2p, B 1s, W 4f and Ce 3d levels of
Ni–W–B and Ce–Ni–W–B samples.
with the previous studies.35,39–43 This electron transfer, making
the Ni0 electron-enriched and the B0 electron-decient, will
enhance the hydrogenation activity of the catalyst.44 Each of the
W 4f XP spectra of these samples is deconvoluted into two peaks
at around 35.2 eV and 37.3 eV, corresponding to W 4 f7/2 and W
4 f5/2 doublets of WO3,35,40 respectively, which implies that
WO4 can not be reduced to W0. Two peaks at 881.7 eV and
2ꢀ
886.1 eV appeared in the Ce 2p XP spectra are assigned to CeO2
and Ce2O3,34 respectively, indicating that the promoter Ce exists
in the form of oxide species in the catalyst. It had reported
previously by H. Li et al.34 that Ce oxide species in Ce–Ni–B
amorphous alloy could act as support. Hence, the presence of
Ce oxides in the Ce–Ni–W–B catalysts can prevent the particles
from aggregating, which would be signicantly improved the
surface area and thermal stability of the catalyst.
The bulk composition, surface composition and the content
of different valence element of Ce–Ni–W–B are showed in Table
1. Compared with the bulk composition, the Ni/W molar ratio
on the surface of Ni–W–B or Ce–Ni–W–B is much higher, indi-
cating that Ni enriches on the catalyst surface. This Ni enrich-
ment might originate from the faster reaction rate of W6+ with
Table 1 Bulk composition and surface composition of Ce–Ni–W–B samples
Ni (%)
B (%)
B0
W (%)
W0
Ce (%)
Ce3+
Catalyst
Bulk composition
Surface composition
Ni0
Ni2+
B3+
W6+
Ce4+
0-Ce
Ni1.03W1.00B4.82
Ni10.44W1.00B5.53
2.9
97.1
94.4
83.1
89.3
11.4
15.6
20.1
21.6
88.6
85.4
79.9
78.4
0
0
0
0
100
100
100
100
—
—
25-Ce
50-Ce
70-Ce
Ce0.02Ni1.01W1.00B2.34
Ce0.05Ni1.05W1.00B1.53
Ce0.07Ni0.97W1.00B1.02
Ce 0.02Ni4.24W1.00B2.48
Ce 0.05Ni2.06W1.00B1.83
Ce 0.07Ni1.95W1.00B1.21
6.6
16.9
10.7
26.8
43.6
39.0
73.2
56.4
61.0
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still amorphous in the range of measurement temperature. 25-
Ce shows none diffraction peak expect for a very weak and broad
one. Fig. 3c also shows that the intensity of the peak around 2q
¼ 45ꢁ is decreased with the Ce content, suggesting that the
thermal stability of Ce–Ni–W–B amorphous catalysts is
increased with Ce content, which is consistent with the previous
investigation.34 This positive effect of Ce on the thermal stability
is mainly attributed to the support effect of Ce oxides. Ce oxides,
acted as support, inhibit the migration of the particles effec-
tively, which is essential for the crystallization. Compared with
La–Ni–W–B amorphous catalyst in our previous study,31 it can
conclude that Ce has a better effect for improving the thermal
stability of Ni–W–B amorphous catalyst.
3.2 Catalyst activity
Generally, the HDO of phenols compounds reacts through two
routes: (i) direct C–O bonds scission (DDO) yielding aromatic
products and (ii) pre-hydrogenation of the aromatic ring and
then dehydration producing cycloalkenes and cycloalkanes
(HYD). The p-cresol conversion and product selectivity as a
function of time in the HDO of p-cresol on 25-Ce amorphous
catalyst at 498 K are showed in Fig. 4. The p-cresol conversion
reaches to 99.8% with a selectivity of 82.8% 4-methylcyclohex-
anol within 1 hour. Then, the HDO of p-cresol converts to the
HDO of 4-methylcyclohexanol. Aer 10 hours, the deoxygen-
ation degree reached to 100% with a very little of toluene in the
products. This indicates that 25-Ce has a high hydrogenation
activity and the deoxygenation reaction is the limited step. For
comparison, Ni–Mo–S/g-Al2O3, as traditional and industrial
catalyst, was also selected as catalyst for the HDO of p-cresol. As
shown in Fig. 5, no any oxygen-containing compound is found
in the products and the 4-methylcyclohexene selectivity is very
low during the whole process. Aer reacting at 573 K for 10
hours, p-cresol conversion is only 47.1% with a selectivity of
58.3% methylcyclohexane, whereas the selectivity of toluene
obtained by the direct deoxygenation route reaches to 39.4%,
indicating that this sulde catalyst has low hydrogenation but
high dehydration at 573 K. According to the above results, it can
Fig. 3 XRD patterns of Ni–W–B and Ce–Ni–W–B catalysts without
heat-treatment (a), heat-treatment at 523 K (b) and 573 K (c).
peak around 2q ¼ 45ꢁ is gradually strengthened due to a certain
level of agglomeration with heat-treated temperature. When
heat-treated at 523 K, no obvious diffraction peak can be
observed, indicating that the prepared Ce–Ni–W–B catalysts
remain the amorphous structure. When the heat-treated
temperature is increased to 573 K, Ni–W–B and 12-Ce shows two
diffraction peak at 2q ¼ 45ꢁ and 52ꢁ, which corresponds to
Ni(111) and Ni(200),39,46 respectively. This serves as evidence
that some metallic Ni had begun to transform from amorphous
Fig. 4 p-Cresol conversion and the product selectivity versus time on
into crystalline, while the other compounds in Ce–Ni–W–B were 25-Ce. Reaction conditions: 0.10 g catalyst, temperature 498 K.
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Table 2 Comparison of Ni–Mo–S/g-Al2O3 and Ce–Ni–W–B amor-
phous catalysts with different Ce content in the HDO of p-cresola
Catalysts
Sb
Ni–W–B 12-Ce 25-Ce 50-Ce 70-Ce
Conversion, mol%
47.1 100
100
99.7
99.1
97.7
Products distribution, mol%
4-Methylcyclohexanol
3-Methylcyclohexene
Methylcyclohexane
Toluene
0
2.3
58.3
39.4
44.1
64.6
0.9
33.9
0.6
57.8
0.2
41.3 97.4
0.7 1.0
41.8 98.1
1.4
0.2
3.4
0.2
94.9
1.5
5.5
5.6
78.9
10.0
91.6
DD, wt%
35.6
95.3
a
Reaction conditions: 0.30 g catalyst, 12.74 g p-cresol, 87.26 g dodecane,
b
temperature 498 K, time 1 h. Reaction conditions: 0.60 g Ni–Mo–S/g-
Al2O3 catalyst, 12.74 g p-cresol, 87.26 g dodecane, temperature 573 K,
time 10 h.
Fig. 5 p-Cresol conversion and the product selectivity versus time on
Ni–Mo–S/g-Al2O3. Reaction conditions: 0.60 g catalyst, temperature
573 K.
end products are 4-methylcyclohexanol, 4-methylcyclohexene,
methylcyclohexane and toluene. Ni–W–B exhibits a high
conversion (100%) but a low deoxygenation degree (35.6%) at
498 K. Aer adding promoter Ce to Ni–W–B, the deoxygenation
be concluded that the dominant route for p-cresol HDO on activity of the catalyst and the product distribution are signi-
Ce–Ni–W–B is HYD. That was, p-cresol is hydrogenated to cantly changed. The deoxygenation degree of Ce–Ni–W–B is
4-methylcyclohexanol rstly and then dehydrated. The energy decreased with the order of 25-Ce (98.1%) > 50-Ce (95.3%) >
barrier for phenolic hydroxyl is much higher than that for 70-Ce (91.6%) > 12-Ce (41.8%) > Ni–W–B (35.6%) and the
aliphatic hydroxyl. Thus, for the HDO of p-cresol, HYD route can selectivity of 4-methylcyclohexanol presents an opposite trend.
not only decrease the reaction temperature but aromatic The deoxygenation degree reaches to 98.1% and the selectivity
content in the product.
of 4-methylcyclohexanol decreases to 1.4% on 25-Ce at 498 K for
As shown in Fig. 4, the high conversion and high 4-methyl- 1 hour, showing that the Ce–Ni–W–B amorphous catalyst has
cyclohexanol selectivity within 1 hour indicates a high hydro- much higher HDO activity than the previous catalysts.22,30,33
genation activity but a low deoxygenation activity for 25-Ce
For the HDO on Ce–Ni–W–B amorphous catalysts, the main
amorphous catalyst. The high hydrogenation activity is attrib- reaction route is HYD (p-cresol / 4-methylcyclohexanol / 3-
uted to the amorphous structure and electron transfer between methylcyclohexene / methylcyclohexane). Under the studied
Ni0 and B0 that facilitated the adsorption of hydrogen on the Ni reaction conditions, the high conversion of p-cresol on Ni–W–B
metal sites with higher electron density to form Hꢀ species and and Ce–Ni–W–B catalysts is mainly attributed to the unique
thus enhanced its hydrogenation activity.35,42 For Ce–Ni–W–B amorphous structure and the electronic interaction between
¨
catalysts, WO3 on the surface acts as Bronsted acid site. The metallic Ni and the elemental B. The addition of Ce promoter
¨
dehydration of 4-methylcyclohexanol occurs on these Bronsted leads to produce more metallic Ni than that without any
acid sites, and the deoxygenation rate depends on the number promoter. This metallic Ni is electron-enriched, which could
¨
of Bronsted acid site. Compared the hydrogenation activity with increase the back donation of electrons to the p* (anti-bonding)
the deoxygenation activity, it can conclude that the available of aromatic ring in chemisorbed phenol, making the aromatic
Bronsted acid sites for deoxygenation are not enough while the ring activated towards hydrogenation.47 However, in this study,
¨
hydrogenation active sites are saturated. This indicates that the the end aim is a high deoxygenation degree but not a high
¨
matching of Ni metal sites and WO3 Bronsted acid sites might conversion, and the deoxygenation is usually occurred on
¨
be unsuitable and the presence of more WO3 active sites would Bronsted acid sites. When adding Ce promoter, WO3, acted as
¨
favor the high deoxygenation degree. To verify this deduction, Bronsted acid site, was increased, leading to the high deoxy-
the effect of catalyst amount for the HDO of p-cresol was studied genation degree. But, high Ce amount is harmful for the
by adding 3 fold amounts of 25-Ce catalyst. As a result, the activity. As shown in Table 2, the p-cresol conversion is gradu-
deoxygenation degree increases from 17.8% (calculated from ally decreases from 99.7% for 25-Ce to 97.7% for 70-Ce and the
¨
Fig. 5) to 98.1% (in Table 2), which demonstrates that Bronsted deoxygenation degree is decreased from 98.1% for 25-Ce to
acid sites on Ce–Ni–W–B amorphous catalyst surface needs to 91.6% for 70-Ce. The XPS characterization results showed that
be improved. In addition, the catalyst surface enriches with Ni, Ce was existed in the form of oxides, which is inactive for the
but only metal Ni possesses the high hydrogenation activity. hydrogenation. On the other hand, excess Ce oxides blocked the
Therefore, the HDO activity of Ce–Ni–W–B amorphous catalyst pore channels of Ni–W–B, leading to a low surface area and low
can be further enhanced by increasing the metal Ni0 and WO3 amount of available active sites. Hence, the reduction in
sites or regulating the matching between these two site types.
conversion and deoxygenation degree on 70-Ce is resulted from
The effect of the Ce content for the catalytic activity of the coverage of surface active sites by Ce oxides and the decrease
Ce–Ni–W–B in the HDO of p-cresol is presented in Table 2. The of surface area.
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According to World Fuel Oil Regulation, the less the
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aromatic content is the better. For the HDO of p-cresol, the
products (methylcyclohexane and toluene) distribution
depends on the molecular adsorption modes on the catalyst
surface. Vertical orientation that p-cresol is adsorbed on the
catalyst via the hydroxyl group facilitates the removal of oxygen
to form toluene while coplanar adsorption that aromatic ring
adsorbed on the catalyst followed saturating to yield 4-methyl-
cyclohexanol as an intermediate.48 Table 2 shows the toluene
selectivity is increased from 0.6% on Ni–W–B to 10.0% on 70-Ce,
indicating that the vertical orientation adsorption is enhanced.
This change accounting for effect of Ce content can be attrib-
uted to Ce(III) species. Ce(III) species in Ce–Ni–W–B amorphous
catalysts enhances the hydrophilicity of the catalysts, which is
benecial to accept the lone electron pair on the oxygen atom
Fig. 7 XRD patterns of 25-Ce catalyst after reaction (a) and before
from phenolic hydroxyl.49 That is, Ce(III) species adsorb p-cresol reaction (b).
via vertical orientation mode. Therefore, the higher the content
of Ce was in the catalyst surface, the more the Ce(III) species was,
and then the higher is toluene selectivity.
corresponding cycloalkanes in selectivity no less than 98.0%.
The product mixtures include very low concentrations of
benzene/aromatics. For the methoxyl substituted phenols, the
methoxyl group was hydrolyzed to give small molecules such as
methanol and methane, leading to a selectivity of 85% cyclo-
alkanes. Notably, when the conversion reaches to nearly 100%,
the reaction time for m-substituted and p-substituted phenols
was shorter than that for o-substituted phenols. In fact, phenols
are adsorbed preferentially via the C–O bond, adjacent groups
will interfere this process, causing the difference in the
conversion. The above results therefore indicate that Ce–Ni–W–B
amorphous catalyst system is applicable to the HDO of various
phenol derivatives into cycloalkanes.
Fig. 6 shows the recycling tests using 25-Ce catalyst at 498 K.
the conversion was decreased while both 3-methylcyclohexene
selectivity and toluene selectivity were increased with the
increase of recycling numbers. This suggested that the hydro-
genation activity of 25-Ce was declining. Aer ve recycle reac-
tions, the structure of 25-Ce catalyst was characterized by XRD,
as shown in Fig. 7. Compared with fresh catalyst, three peak
were observed at 2q ¼ 45ꢁ, 52ꢁ and 76ꢁ in the XRD pattern of 25-
Ce catalyst aer reaction, corresponding to Ni(111), Ni(200) and
Ni(220),46 but these peaks were not very sharp, which indicated
that only partial amorphous Ni was transformed to crystalline
Ni. This transformation made the hydrogenation activity of the
catalyst reduced,44,50 which can be prevent efficiently by
preparing supported catalyst.
Table 3 HDO of phenol derivatives on 25-Cea
Based on the excellent performance of 25-Ce in the HDO of p-
cresol, the generality of this catalyst was investigated through a
series of phenol derivatives. The results are summarized in
Table 3. All the phenols evaluated in this study, except for
methoxyl substituted phenols, are efficiently converted to the
Run
Substrate
Time (h)
Conversion (%)
Selectivity (%)
1
2
1.0
1.5
100
100
3
1.0
100
4
5
1.5
2.0
98.2
100
6
7
8
4.0
2.0
6.0
94.5
100
100
9
2.0
99.5
a
Fig. 6 Recycling tests of 25-Ce for the HDO of p-cresol. Reaction
conditions: 0.3 g catalyst, 12.74 g p-cresol, 87.26 g dodecane,
temperature, 1 h reaction time.
Reaction conditions: 0.30 g catalyst, 0.12 mol substrate, the weight of
substrate and dodecane was 100 g, temperature 498 K.
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Compared with the sulphide catalysts and metal phosphides research Fund for the Doctoral Program of Higher Education
in other studies,12,22,51 the HDO temperature of p-cresol on these (20124301120009) and Natural Science Foundation of Hunan
Ce–Ni–W–B amorphous catalyst is much lower. Since sulphide Province (13JJ4048).
catalysts and metal phosphides have a low hydrogenation
activity, the hydrogenation of p-cresol on these catalysts into 4-
methylcyclohexanol is hard, not to mention this hydrogenation
at a low temperature. On the other hand, the oxygen removed
Notes and references
from the phenolic hydroxyl group is much harder than that
from the alcoholic hydroxyl group because of its higher energy
barrier. If it wants to reach the goal of high deoxygenation
degree on sulphides and metal phosphides catalysts, the reac-
tion temperature must be increased. But the direct C–O bonds
scission (DDO route) yielding aromatic is easy to occur at high
temperature, leading to high aromatic selectivity. In contrast,
noble metal Pt possesses a high hydrogenation activity, adopt-
ing it as a catalyst for the HDO of phenols can make the HDO
reaction proceeded with the hydrogenation-dehydration route,
leading to a low aromatics selectivity and reaction tempera-
ture.8,33 Ni based amorphous catalyst, possessing the same or
ever higher hydrogenation activity as noble metal Pt catalyst, is
widely applied into the hydrogenation.35–39 In this case, we select
amorphous metal Ni and WO3 as active sites to prepare Ni–W–B
catalyst and add promoter Ce to further increase its HDO
activity. The metal Ni content and WO3 are increased by adding
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