Gas phase hydrodeoxygenation of anisole and guaiacol to aromatics with a high selectivity over Ni-Mo/SiO2

Article abstract of DOI:10.1016/j.catcom.2017.09.011

A very active Ni-Mo bimetallic catalyst supported on SiO₂ was prepared for the hydrodeoxygenation (HDO) reaction of anisole and guaiacol under a H₂ partial pressure of 83 kPa. Generally, transition metal and their oxides were thought to have less HDO activity than others, such as sulfides, carbides, phosphides. However, this work achieved a novel deoxygenation result, where a high conversion (98.7%) at 673–693 °C and atmospheric pressure for the model compounds and high selectivity (> 96%) to aromatic hydrocarbons (BTX) were obtained. Furthermore, the methyl transfer reaction is greatly promoted by abundant acid sites of the catalyst, leading to a low carbon-loss. The present work provides a promising way and new insight for an economic and efficient HDO process for the lignin-derived thermal degradation products.

Full text of DOI:10.1016/j.catcom.2017.09.011

Accepted Manuscript
Gas phase hydrodeoxygenation of anisole and guaiacol to
aromatics with a high selectivity over Ni-Mo/SiO2
Tao He, Xinxin Liu, Yuanzheng Ge, Dezhi Han, Jianqing Li, Zhiqi
Wang, Jinhu Wu
PII:
S1566-7367(17)30384-9
doi: 10.1016/j.catcom.2017.09.011
CATCOM 5193
DOI:
Reference:
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
9 May 2017
11 September 2017
11 September 2017
Please cite this article as: Tao He, Xinxin Liu, Yuanzheng Ge, Dezhi Han, Jianqing
Li, Zhiqi Wang, Jinhu Wu , Gas phase hydrodeoxygenation of anisole and guaiacol to
aromatics with a high selectivity over Ni-Mo/SiO2. The address for the corresponding
author was captured as affiliation for all authors. Please check if appropriate.
Catcom(2017), doi: 10.1016/j.catcom.2017.09.011
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ACCEPTED MANUSCRIPT
Gas phase hydrodeoxygenation of anisole and guaiacol to aromatics with a
high selectivity over Ni-Mo/SiO2
a,†,*
a,b,†
a,b
a
a,b
a
Tao He
, Xinxin Liu
, Yuanzheng Ge , Dezhi Han , Jianqing Li , Zhiqi Wang ,
a
Jinhu Wu
a
Key Laboratory of Biofuels, Qingdao Institute of Bioenergy and Bioprocess Technology,
Chinese Academy of Sciences, Qingdao 266101, P R China
b
University of Chinese Academy of Sciences, Beijing 100049, P R China
*
Corresponding author: Email: hetao@qibebt.ac.cn, Tel: +86-532-80662763
†
The authors contributed equally to the work.

ACCEPTED MANUSCRIPT
Abstract
A very active Ni-Mo bimetallic catalyst supported on SiO2 was prepared for the
hydrodeoxygenation (HDO) reaction of anisole and guaiacol under a H2 partial pressure of 83
kPa. Generally, transition metal and their oxides were thought to have less HDO activity than
others, such as sulfides, carbides, phosphides. However, this work achieved a novel
deoxygenation result, where a high conversion (98.7%) at 673-693 ℃ and atmospheric
pressure for the model compounds and high selectivity (more than 96%) to aromatic
hydrocarbons (BTX) were obtained. Furthermore, the methyl transfer reaction is greatly
promoted by abundant acid sites of the catalyst, leading to a low carbon-loss. The present
work provides a promising way and new insight for an economic and efficient HDO process
for the lignin-derived thermal degradation products.

ACCEPTED MANUSCRIPT
1
. Introduction
Lignin is an aromatic bio-polymer mainly connected by aryl ethers, it can be a renewable
source for aromatic compounds production. However, during the lignin thermal degradation,
most of the O atoms retain in the product molecules, leading to high oxygen content and poor
chemical properties[1-2]. The removal of O via catalytic HDO reaction attracted much
attention in the past decades[2-6].
Noble metals Pt, Pd, Ru show high activity and stability under 0.5-4 Mpa H2 pressure，but
undesired ring saturation to cyclanes usually coexists, leading to high H2 consumption[7,8].
Traditional Ni, Mo, Co sulfides are extensively studied[9-11] under high H2 pressure
(2-8MPa). Transition metal phosphides Ni2P, Co2P and carbides W2C, Mo2C have been found
to have better HDO activity and good selectivity to benzene in gas phase HDO reaction. Zhao
et al.[12] found that the major products from HDO of guaiacol using N2P were benzene and
phenol, with a conversion of 80%, benzene selectivity of 60%. Lee et al.[13] firstly reported
that Mo2C could effectively catalyze vapor phase anisole HDO reaction, with benzene
selectivity of 92∼96% under H2/anisole molar ratio of 110. Lu et al.[14] reported that ordered
mesoporous W2C catalyst had a higher than 96% benzene selectivity in vapor phase anisole
HDO. Recently, Román-Leshkov et al.[15,16] found that MoO3 can selectively cleave the C–
O bond, MoO3/ ZrO2 was the optimal catalyst giving the cresol conversion of 78% and BTX
(benzene, toluene, xylene) selectivity of 99%, but MoO3/SiO2 only gave a conversion of 13%.
Although, the transition metal oxides deactivated in 5-7 h, the mild condition provides a
promising way for an economic and efficient HDO process.
In this contribution, a bimetallic catalyst Ni/Mo oxides was prepared, SiO2 with high
surface area was used as the support to disperse the active metal and enhance the
metal-support interaction. Anisole, phenol and guaiacol were used as the lignin-derived HDO
model compounds. The fabricated Ni-Mo/SiO2 catalyst exhibited high selectivity (>96%) to
BTX and high conversion (99%) of model compounds under low H2 partial pressure of 83
kPa. Commonly, transition metal and their oxides were thought to have less HDO activity
than others, however, the bimetallic catalyst exhibited the highest BTX yield reported to date.

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2
. Experimental
2
.1.Catalyst preparation
The catalyst with 15wt% NiO and 15wt% MoO3 was prepared by a two-step
incipient wetness impregnation method, as detailed in supplementary material.
2
.2.Catalytic HDO tests
The catalytic HDO experiments were conducted in a fixed bed stainless steel reactor
(
internal diameter 8 mm, length 300 mm) surrounded by a temperature-controlled electrical
oven. Anisole and guaiacol were fed with a syringe pump, while phenol was dissolved in
benzene. The molar ratio of H2 to liquid feed (mol/mol) was kept constant to the value of 5.
The reactor outlet was connected to a condenser and two receiving bottles. Then the line was
kept at 100 ℃ and connected to a gas chromatograph (GC) online.
Prior to HDO reaction, the catalyst was reduced with H2 flow rate of 100 ml/min at 450 ℃
for 30 min. The liquid products were qualitatively analyzed by gas chromatography−mass
spectrometry (GC-MS, Agilent 7890A-5975C), and then quantitatively analyzed using an
offline GC equipped with the same type capillary column and FID detector. The
non-condensable gas was quantified by online GC.
The following relationships for the conversion, selectivity to aromatics and product yield
were used:
moles of reactant consumed
Conversion (%) 
100
(1)
moles of reactant fed
Selectivity toaromatic hydrocarbons (%)
moles of aromatic hydrocarbons produced
(2)

100
moles of totalcondensable organic product
The weight of certain product
Yield (%) 
100
(3)
The weight of liquid reactant fed
3
. Results and discussion
3
.1.Activity test and product distribution

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Fig. 1 shows the anisole HDO catalytic results obtained over the investigated Ni-Mo/SiO2
catalyst in the temperature range of 380-420 ℃ and after 60 min on time on stream (TOS).
The anisole conversion and selectivity to aromatic hydrocarbons increase with reaction
temperature. Above 390 ℃, the anisole conversion is more than 98.3%, and the hydrocarbons
selectivity is higher than 97.5%. It can also be seen in Fig. 2 that the performance of the
-
1
catalyst is stable with weight hourly space velocity (WHSV) in the range of 0.75 to 3 h . In
the subsequent HDO reaction, the experimental conditions were fixed at T=410℃, WHSV
-
1
=
1.5 h , atmospheric pressure and H2/liquid-feed molar ratio of 5.
Table 1: Conversion (%), product yield (%), selectivity to aromatic hydrocarbons (%) and
condensable organics composition (mol%) for the anisol, guaiacol and phenol HDO reactions.
Feed
Anisole
99.35
Guaiacol
99.79
Phenol
99.30
Conversion (%)
Product yield (%)
Gas(CH4)
Condensable organics
water
9.23
72.91
14.7
6.28
62.96
26.34
negligible
79.23
16.35
Selectivity to aromatic
hydrocarbons (%)
9
8.55
97.53
99.27
99.27
Condensable organics composition (mol%)
Benzene
Toluene
m-Xylene
p-Xylene
o-Xylene
Anisole
Phenol
Trimethylbenzene
66.23
24.46
0.09
4.83
1.43
0.42
0.40
0.94
0.10
0.33
0.21
0.08
0.00
0.07
0.24
0.18
64.33
27.43
0.36
2.74
1.77
0.83
0.86
0.39
0.05
0.40
0.18
0.13
0.3
0.73
2-Methylanisole
Ethyltoluene
o-Cresol
p-Cresol
Guaiacol
2
,6-Dimethylphenol
Tetramethylbenzene
,4-Dimethylphenol
0.00
0.11
0.11
2

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-
1
2
Reaction conditions: T=410 ℃, P=1 atm, WHSV=1.5 h , H /anisole (mol/mol)=5, TOS=60 min.
Detailed results of the anisole and guaiacol catalytic HDO reactions are listed in Table 1.
As shown, the conversion is as high as 99.35% and 99.72% for anisole and guaiacol
respectively. Seventeen types of organic condensable compounds were detected and
quantified, including benzene, toluene, xylene, trimethylbenzene, ethyltoluene,
2
2
-methylanisole, anisole, phenol, 2-methylanisole, o-cresol, p-cresol, guaiacol,
,6-dimethylphenol, tetramethylbenzene, 2,4-dimethylphenol. For anisole HDO reaction, the
selectivity to aromatic hydrocarbons reaches 98.55%, and it achieves 97.53% for guaiacol.
The comparison of the present work with literature is also shown in Table 2. It is noted that
the BTX selectivity is relatively high. This is an encouraging result considering that BTX are
high atom-economy and high value-added products. These results demonstrate that the
bimetallic Ni-Mo/SiO2 catalyst has very good HDO activity and BTX selectivity, and it
further proves Román-Leshkov’s opinion on MoOx HDO activity[16].
In the present work, Ni loading and reaction temperature dramatically enhance both the
HDO conversion and aromatic hydrocarbons selectivity. It is believed that nickel promotes
the splitting of molecular hydrogen to active atomic hydrogen species, where the split-over
hydrogen can then attack and cleave the Caro-O bond[19]. It is also noteworthy that the
over-hydrogenation products of cyclohexane and cyclohexene were not observed in all tests
due to the low H2 pressure of 83 kPa and selective adsorption on the catalyst surface. In the
non-condensable gas, methane is the main product with a yield of 9.23% for anisole and 6.28%
for guaiacol, where trace amounts of C -C alkanes are negligible. Thus, the decomposition of
2
5
aromatic ring to C2-C3 alkanes hardly occurred.
Table 2: Comparison of HDO results over several catalysts reported in the literature.
BTX
T/P
Conversion
(%)
Catalyst
Molecule
Selectivity
%)
Stability
(
℃/atm)
400/1
320/1
(
Activity lose ~30%,
within 150 min
Activity lose ~30%,
within 60 min, at 400 ℃
Fe/SiO2 [5]
MoO3 [16]
Guaiacol
Anisole
~100
78.7
38.0
81.7

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Activity lose <10%,
within 120 min
Activity lose ~10%,
within 120 min
Activity lose ~10%,
within 120 min
Pd-Fe/C [17]
MoO3/ZrO2 [18]
This work
Guaiacol
Anisole
Anisole
450/1
320/1
410/1
~100
~63.5
99.3
83.2
~49.5
97.1
3
.2.Proposed reaction mechanism
Water was detected as the dominant oxygen-containing compound in the condensable
products with a yield of 14.7% for anisole and 26.34% for guaiacol, whereas carbon dioxide
and methanol were not found. This indicates that the initial step is neither the phenolic Caro-O
bond scission nor the decarbonation reaction. In addition, apart from benzene, the total
concentration of toluene and xylene is found to be higher than 30% for both anisole and
guaiacol conversion. The latter suggests for a methyl transfer reaction. Phenol may be an
intermediate product of anisole HDO conversion, because the etheric bond CH3-OPh energy
(268±7 kJ/mol) is much lower than that of aryl ether bond Caro-OCH3 (417±5 kJ/mol).
Anisole HDO mechanism is depicted in Fig. 3. The homolytic cleavage of weakest etheric
bond CH3-OPh occurs firstly, forming methyl radical CH3• and phenoxy radical. The phenoxy
radical is more active and has very short lifetime[2], and is further transformed into benzene
and water via hydrogenolysis reaction at elevated temperatures. Simultaneously,
trans-alkylation reaction occurs, partial methyl radicals adhere to the aromatic ring forming
toluene, xylene and other alkylbenzenes. In order to further prove the anisole HDO
mechanism, phenol HDO experiments were conducted at the same experimental conditions,
and the obtained results are also presented in Table 1. As expected, there was only negligible
amounts of methane detected in the gas products, mainly from impurities. The condensable
organic products were mainly composed of benzene (99.27%) and no other trans-alkylation
product was detected. The phenol HDO results provide an important counterevidence for the
ether bond homolytic cleavage and methyl transfer mechanism.
It is estimated that about 36-40% of the methyl groups were transferred to the benzene ring
during anisole and guaiacol HDO process, and this ratio is relatively high compared with
previous works[16,18]. Alkylated aromatics are more valuable and imply a lower carbon loss.
NH3-TPD spectras are shown in Fig. S1, where abundant acid sites in the present catalyst

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seem to play an important role in methyl transfer reaction[15,19,20].
3
.3.Catalyst deactivation analysis and regeneration stability
It is observed in Fig. S2 that the HDO conversion and BTX selectivity maintained at a
steady level at the first 70-90 min of reaction and then decreased, meanwhile the phenol
selectivity increased, and some heavy products (yellow color) appeared. These observations
present evidence for catalyst deactivation. The heavy products of anisole HDO in the
deactivation stage (120 min on TOS) were analysed by GC-MS as shown in Fig. S3. The
increase in the aromatic ring number can be easily deduced from Fig. S3, thus, it is rather
predictable that products with even higher ring numbers would condense on the catalyst
surface. As revealed by the results shown in Fig. S1, carbon deposition would likely cover
part of the active acid sites, thus potentially leading to the deactivation of catalyst.
X-ray diffraction (XRD) analysis of the fresh and spent catalysts was performed in order to
study the Ni and Mo chemical states (crystal phases) during the reaction. As shown in Fig. S4,
for the fresh catalyst, Ni and MoO2 phases are present, whereas a NiO phase emerges for the
spent catalyst. Also, X-ray photoelectron spectroscopy (XPS) studies were conducted in order
to probe the evolution of the oxidation states of surface Mo species with reaction (Fig. S5).
4
+
5+
4+
Mo and Mo species were found to co-exist on the catalyst surface, the proportion of Mo
is about 23~30%, and the reaction obviously did not change the nature of Mo surface species.
5
+
3+
It was reported [15] that Mo and Mo play an important role in the HDO reaction due to the
presence of oxygen vacancies in the MoOx crystal phase.
The spent catalyst was regenerated by combustion in air flow (80 ml/min) at T=550 ℃.
Fourteen regeneration cycles were performed using anisole as the feedstock material, and the
obtained results are shown in Fig. 4. It is observed that the anisole conversion and
hydrocarbons selectivity are increased slightly after the first regeneration cycle, and then
reach a plateau. The regeneration catalyst exhibits practically stable performance for short
TOS.
4
. Conclusions
In summary, the present SiO2-supported Ni-Mo bimetallic catalyst shows high conversion

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and selectivity to Caro-O bond cleavage in the gas phase HDO reactions of anisole, phenol and
guaiacol, giving BTX selectivity more than 96%. The low H2 consumption and carbon loss
results are encouraging and novel. It is estimated that about 36-40% of the methyl radicals are
transferred to the benzene ring during anisole and guaiacol HDO processes due to abundant
suitable surface acid sites. Major precursors of coke such as biphenyl, anthracene and
phenanthrene were detected after catalyst deactivation. The spent catalyst can be regenerated
o
by combustion in air at 550 C, and it kept stable performance after fourteen regeneration
cycles with short TOS. We have further proved that the use of higher H2 pressure would
prolong catalyst’s lifetime.
Acknowledgements
Financial support from the National Natural Science Foundation of China through contract
(Grant no. 11375249) is greatly acknowledged.
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Figure captions:
Fig. 1. Effect of reaction temperature on anisole conversion. Reaction conditions: P=1 atm,
-
1
WHSV=1.5 h , H2/anisole(mol/mol)=5, TOS=60 min.
-
1
Fig. 2. Effect of WHSV(h ) on anisole conversion. Reaction conditions: P=1 atm, T=410 ℃,
H2/anisole(mol/mol)=5, TOS=60 min.
Fig. 3. Proposed reaction pathway for anisole HDO.
Fig.4. Catalyst regeneration performance for anisole HDO conversion. Reaction conditions:
-
1
T=410 ℃, P=1 atm, WHSV=1.5 h , H2/anisole (mol/mol)=5, TOS=60 min.

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Fig. 1.

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Fig. 2.

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Fig. 3.

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Fig. 4.

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Highlight




Ni-Mo/SiO2 shows >96% selectivity to BTX in anisole gas phase HDO reaction.
Methyl transfer reaction is promoted by abundant acid sites of the catalyst.
No obvious active phase change in the catalyst after the reaction.
Low H2 consumption and carbon loss, easy regeneration are the traits of the process.