Guaiacol demethoxylation catalyzed by Re2O7 in ethanol

Article abstract of DOI:10.1016/j.cattod.2019.07.018

Re₂O₇ is used to convert guaiacol in alcohols at 280–320 °C. In ethanol, guaiacol is deoxygenated and alkylated, and the major products are phenol and alkylphenols (including ethylphenol, diethylphenol, diisopropylphenol, di-tert-butylphenol and 2,6-di-tert-butyl-4-ethylphenol), accounting for 97 mol% of all products after 6 hour reaction at 320 °C. Both catechol and phenol are the intermediates of guaiacol demethoxylation. Among the substituents, ethyl is directly provided by ethanol while isopropyl and tert-butyl are formed by the addition of methyl to ethyl step by step. In addition, Re₂O₇ has negligible activity for the saturation of benzene ring so it does not cause considerable over-consumption of reductant. The actual catalyst for guaiacol demethoxylation is likely a Re^IV?VI species.

Full text of DOI:10.1016/j.cattod.2019.07.018

Accepted Manuscript
Title: Guaiacol demethoxylation catalyzed by Re2O7 in
ethanol
Authors: Fei Yan, Yushuai Sang, Yunfei Bai, Kai Wu, Kai Cui,
Zhe Wen, Fuhang Mai, Zewei Ma, Linhao Yu, Hong Chen,
Yongdan Li
PII:
S0920-5861(19)30378-5
DOI:
Reference:
https://doi.org/10.1016/j.cattod.2019.07.018
CATTOD 12347
To appear in:
Catalysis Today
Received date:
Revised date:
Accepted date:
14 March 2019
2 July 2019
16 July 2019
Please cite this article as: Yan F, Sang Y, Bai Y, Wu K, Cui K, Wen Z, Mai F, Ma Z, Yu
L, Chen H, Li Y, Guaiacol demethoxylation catalyzed by Re2O7 in ethanol, Catalysis
Today (2019), https://doi.org/10.1016/j.cattod.2019.07.018
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Guaiacol demethoxylation catalyzed by Re2O7 in ethanol
Fei Yan,[b] Yushuai Sang,[b] Yunfei Bai,[b] Kai Wu,[b] Kai Cui,[b] Zhe Wen,[b] Fuhang
Mai,[b] Zewei Ma,[b] Linhao Yu,[b] Hong Chen* [a] and Yongdan Li* [b,c]
[a] School of Environmental Science and Engineering, Tianjin University, Tianjin
300072, China
[b] State Key Laboratory of Chemical Engineering (Tianjin University), Tianjin Key
Laboratory of Applied Catalysis Science and Technology, School of Chemical
Engineering, Tianjin University, Collaborative Innovation Center of Chemical Science
and Engineering (Tianjin), Tianjin, 300072, China
[c] Department of Chemical and Metallurgical Engineering, School of Chemical
Engineering, Aalto University, Kemistintie 1, Espoo, P.O. Box 16100, FI-00076 Aalto,
Finland
* All correspondence should be addressed to Hong Chen. Email: chenhong_0405@tju.edu.cn; Tel: +86-22-27409405

Graphical abstract
Highlights
 Re2O7 catalyzes the deoxygenation of guaiacol in ethanol without H2
input.
 Guaiacol is mainly converted to phenol and alkylphenols via
deoxygenation and alkylation.
 The saturation of benzene ring occurs but is negligible.
o
ABSTRACT: Re2O7 is used to convert guaiacol in ethanol at 320 C. Guaiacol is
deoxygenated and alkylated, and the major products are phenol and alkylphenols
(including ethylphenol, diethylphenol, diisopropylphenol, di-tert-butylphenol and 2,6-di-
tert-butyl-4-ethylphenol), accounting for 97 mol% of all products. Both catechol and
phenol are the intermediates of guaiacol demethoxylation. Among the substituents, ethyl is
directly provided by ethanol while isopropyl and tert-butyl are formed by the addition of
methyl to ethyl step by step. In addition, Re2O7 has negligible activity for the saturation of

benzene ring so it does not cause considerable over-consumption of reductant. The actual
catalyst for guaiacol demethoxylation is likely a ReIV-VI species.
Keywords: guaiacol, demethoxylation, alkylation, ethanol, Re2O7
1. Introduction
Due to the depletion of fossil fuel reserves, converting renewable and abundant biomass
into fuels and chemicals becomes important. Lignin, cellulose and hemicellulose are the
three major components of lignocellulosic biomass and are produced with a large amount
annually by the biosphere [1, 2]. Lignin is a polymer composed of substituted phenol
monomers thus is a promising resource for aromatic chemicals [3-5]. In lignin
depolymerization reaction, the condensation of the produced lignin fragments and
monomers has to be suppressed [6-8], so it is usually carried out under reductive conditions,
either with hydrogen or a hydrogen donor [9-13]. For example, Barta et al. reported that a
mixed metal oxide catalyst converted organosolv lignin into a number of substituted
o
cyclohexanes with methanol as solvent at 300 C [9]. However, the hydrogenation of
benzene ring in this reaction caused the undesirable over-consumption of hydrogen. Xu
and his coworkers reported that Ni/C decomposed birch wood lignin in alcohols at 200 oC,
obtaining a 50% conversion after 6 h [10]. We reported that WP/AC catalyzed the
depolymerization of Kraft lignin to mono-phenols in ethanol/water mixture [14]. In
addition, we found that several Mo-based catalysts, including MoC1-x/AC, Mo/Al2O3,
MoC1-x/Cu-MgAlOz, Mo2N/Al2O3, and modified MoO3, converted Kraft lignin into value-

added chemicals with high yields in ethanol [15-20]. Song et al. reported that a four-step
strategy produced terephthalic acid form corn stover lignin with a 15.5 wt% yield, in which
Ru/C, MoOx/AC, Pd(OAc)2 and Co-Mn-Br system are used consecutively to achieve lignin
depolymerization, product demethoxylation, carbonylation and oxidation [21].
The products of lignin depolymerization are always highly substituted, especially for the
phenols. For example, both pyrolysis and catalytic depolymerization of lignin produce
substituted phenols with functional groups such as alkyl, methoxyl and aldehyde [13, 14,
22]. Guaiacol is a common product in lignin depolymerization. Conventional hydrotreating
catalysts catalyze the hydrodeoxygenation (HDO) of guaiacol but always cause undesirable
saturation of benzene ring [23-26]. In comparison, we reported that MoC1-x/AC and MoO3
catalyzed the selective deoxygenation and alkylation of guaiacol to alkylphenols in ethanol
without hydrogen input or benzene ring saturation [27, 28]. Zhang et al. reported that a
single step strategy converted 4-propylguaiacol to phenol with a 65% yield, in which Pt/C
and H-ZSM-5 were used together to achieve the hydro-demethoxylation and the de-
alkylation of 4-propylguaiacol, respectively [29].
In addition to MoO3, V2O5 is also an effective oxide catalyst for the deoxygenation of
guaiacol [30, 31]. Hence, it is reasonable to consider other oxide as the catalyst for the
deoxygenation of guaiacol. Leiva et al. used ReOx/SiO2, ReS2/SiO2 and Re/SiO2 for the
HDO of guaiacol at 300 oC under 5 MPa H2, and found that ReOx/SiO2 was the most active
one with benzene, toluene, xylene and cyclohexane as the major products of guaiacol at
100% conversion [32]. Jung et al. used bimetallic RuRe particles supported on activated
carbon, carbon black, or multiwalled carbon nanotube for the HDO of guaiacol at 200 oC
under 2 MPa H2 [33]. The major products are cyclohexane, cyclohexanol and 2-

methoxycyclohexanol. Therefore, we chose Re2O7 as catalyst and found that Re2O7 is
active for the demethoxylation of guaiacol. In contrast to the work of Leiva et al. and Jung
et al., benzene ring saturation is also observed in this work but negligible. The main
products of guaiacol demethoxylation are phenol and alkylphenols. These alkylphenols can
be converted to value-added products. For example, alkylphenols are used intensively in
the polymer industry as antioxidants, UV absorbers and thermal stabilizers [34]. 2-
Ethylphenol and 3-ethylphenol are the starting materials for photochemicals while 4-
ethylphenol is employed to obtain 4-vinylphenol, an intermediate in the synthesis of
pharmaceuticals, dyes, and various antioxidants used in the manufacture of rubbers and
polymers [35]. 2-tert-Butylphenol can be used to produce cis-2-tert-butylcyclohexanol
which is utilized in perfume and fragrance industry [36-38].
2. Experimental
2.1. Materials
The chemicals were purchased from commercial suppliers and used as received. AR
reagent grade chemicals and solvents including guaiacol, methanol, ethanol, n-propanol,
n-butanol, catechol, anisole, and phenol were purchased from Guangfu Inc. Rhenium oxide
(Re2O7) was purchased from Zhuzhou Kete Industries CO., LTD.
2.2. Catalyst activity measurements
The catalytic reactions were carried out in a batch reactor (Parr Instruments, 300 mL). In a
typical model compound conversion experiment, the reactor was loaded with 0.1 g catalyst,
1 g substrate, 60 mL solvent, sealed and purged with N2 five times. The reactor was then
heated to the desired reaction temperature within 1.5 h and kept at this temperature for the

desired reaction time with stirring at 500 rpm. The reaction time was denoted as 0 h when
the prescribed reaction temperature was reached. After the reaction, the reactor was firstly
o
cooled in air by removing the heating jacket. On reaching below 150 C, the reactor was
further cooled by immersing it into cold-water. The post-reaction solution and the spent
catalyst were separated by filtration. The solution mixture was analyzed with an Agilent
Technologies 6890N gas chromatography (GC) equipped with a HP-5 MS capillary
column (Agilent, 30 m × 0.25 mm × 0.25 μm) and a FID detector with anisole as the
internal standard. However, when the substrate was anisole, tridecane was chosen as the
internal standard. The GC parameters were: inlet temperature 280 oC, detector temperature
300 oC, split ratio 1:50. The oven temperature ramped from 45 oC to 105 oC at 15 oC min−1,
o
o
and then ramped to 280 C at 6 C min−1. Products were identified with a gas
chromatography–mass spectroscopy (GC-MS, Agilent Technologies, model 5973). The
mass spectrum acquired with the GC-MS was retrieved in the NIST Mass Spectral Library
to identify the structure of each product.
Substrate conversion, product yield and mass balance were calculated according to Eqs (1),
(2) and (3). In these equations, n represents the amount of the species in moles and substrate
represents lignin model compounds including guaiacol, catechol, phenol, and anisole.
These values were determined by averaging the data measured in three times.
푛
−푛
푓푖푛푎푙 푠푢푏푠푡푟푎푡푒
푖푛푖푡푖푎푙 푠푢푏푠푡푟푎푡푒
퐶표푛푣푒푟푠푖표푛 =
× 100%
(1)
푛
푖푛푖푡푖푎푙 푠푢푏푠푡푟푎푡푒
푛
푝푟표푑푢푐푡 푖
푌푖푒푙푑푖 =
× 100%
(2)
푛
푖푛푖푡푖푎푙 푠푢푏푠푡푟푎푡푒
∑
+ 푛
푛
푓푖푛푎푙 푠푢푏푠푡푟푎푡푒
푀푎푠푠 푏푎푙푎푛푐푒 =
푝푟표푑푢푐푡 푖 × 100%
(3)
푛
푖푛푖푡푖푎푙 푠푢푏푠푡푟푎푡푒

2.3. Characterizations
To analyze the Re-4f state of the spent catalyst, X-ray photoelectron spectroscopy (XPS)
was recorded using a PHI1600 XPS system with Al-Kα radiation.
2.4. Leaching test
A vial was loaded with 4 mL post-reaction solution of guaiacol demethoxylation reaction.
o
Then it was heated to and kept at 100 C overnight to get rid of volatile organics. 4 mL
CH2Cl2 was added into the vial to dissolve the residue. Then 15 mL H2O was added into
the vial to extract the leaching rhenium from the CH2Cl2 phase with the assistance of
ultrasonic for 1 h. The vial was then left still for 2 h. Finally, the water phase was analyzed
with ICP-OES to determine the amount of leaching rhenium. Note: we assume that water
can extract most rhenium from the CH2Cl2 phase because rhenium species are quite soluble
in water.
3. Results and Discussion
3.1. Guaiacol demethoxylation
The GC-MS chromatogram of the post-reaction solution obtained in guaiacol
demethoxylation reaction is shown in Figure 1. 41 peaks (P1-P41) were identified. P1-P4,
P6 and P7, including alcohol, acid, acetal, and esters, were confirmed to be the products of
ethanol by heating ethanol and Re2O7 under the same conditions. Except the internal
standard (P11) and guaiacol (P15), the other 33 compounds (P5, P8-P10, P12-P14, P16-
P41) were only detected in guaiacol demethoxylation reaction, so they are the products of
guaiacol. Phenols, including phenol, ethylphenol, isopropylphenol and tert-butylphenol,

formed via the demethoxylation and alkylation of guaiacol, are the major products. These
alkylphenols were also observed in our previous work with MoO3 as catalyst [28]. Among
the 33 products, 11 products (P5, P8-P10, P17-P19, P21, P23, P38-P39) were only detected
with GC-MS but not quantified because of their weak GC signal. The presence of trace
amount of octane, ethylcyclohexane and 1-ethyl-1-cyclohexene demonstrates that the
catalyst has negligible activity for the saturation of benzene ring and the cleavage of
cyclohexane ring. Hence, this catalyst does not cause over-consumption of the reductant,
i.e. ethanol in this reaction.
Figure 2 presents the conversion, mass balance and yields of 10 kinds of products from
guaiacol demethoxylation and defines the classification of products. Overall, 18 products
are quantified with GC. Among the 18 products, phenol and alkylphenols (including
ethylphenol, diethylphenol, diisopropylphenol, di-tert-butylphenol and 2,6-di-tert-butyl-4-
ethylphenol) are the major products. Demethoxylation and alkylation are involved in the
formation of phenol and alkylphenols. 93.1% guaiacol is converted before 1.5 h. At 6 h, a
98.0% conversion is achieved. The mass balance increases in the first 2 h from 72.7% to
85.3% and then decreases to 73.8% at 6 h. As mentioned above, 11 products were only
detected with GC-MS but not enough to quantify. In addition, a number of small GC-MS
peaks cannot be identified still (Figure 1). The two reasons contributes to the low mass
balance. The yields of phenol and ethylphenol show increasing trends with the maxima
being 13.8% and 21.0% at 6 h, respectively. The yields of diethylphenol, diisopropylphenol,
di-tert-butylphenol and 2,6-di-tert-butyl-4-ethylphenol increase in the first 2 h, reaching
20.4%, 17.1%, 6.8% and 3.0% respectively, and then decrease because of further alkylation
and repolymerization. The yield of 4-ethyl-2-methoxyphenol drops continuously and

vanishes at 3 h, indicating the possible demethoxylation of it to alkylphenols. In addition,
7 other products as shown in Figure 2b were also quantified, with the maximum yield of
each less than 2.5% throughout the 6 h and the maximum sum of their yields being 5.58%
at 2 h. Among ethylphenols, the yield of 2-ethylphenol is always higher than that of 3-
ethylphenol because ortho-position is preferential to meta-position in the alkylation of
phenol (Figure 3). The yield of 2,5-diethylphenol is always more than five times of the
yield of 3,5-diethylphenol. The yield of 2,4-diisopropylphenol is always more than five
times of the yield of 2,6-diisopropylphenol. The yield of 2,6-di-tert-butylphenol is always
more than twice of 2,5-di-tert-butylphenol. Phenol and alkylphenols (including
ethylphenol, diethylphenol, diisopropylphenol, di-tert-butylphenol and 2,6-di-tert-butyl-4-
ethylphenol) are the major products of guaiacol demethoxylation and accounts for 97 mol%
of all products at 6 h.
3.2. The influence of reaction temperature and solvent
Figure 4 presents the results of guaiacol demethoxylation at different temperatures. With
the temperature increasing, the conversion of guaiacol increases rapidly from 53.3% to
o
96.5%. At 320 C, the mass balance reaches the maximum, 77.8%. The yield of phenol,
ethylphenol, diethylphenol, diisopropylphenol, di-tert-butylphenol, 2,6-di-tert-butyl-4-
ethylphenol, and other products increase continuously with the increasing of temperature
and the maxima are 12.5%, 19.9%, 19.4%, 13.1%, 4.5%, 1.4% and 3.5%, respectively. The
yield of 4-ethyl-2-methoxyphenol decreases continuously with the increasing of
temperature. The yield of 2-ethoxyphenol & 1-ethoxy-4-methoxybenzene shows a volcano
curve with the maximum being 1.9% at 300 oC.

Figure 5 presents the results of guaiacol demethoxylation in different alcohols. When
o
guaiacol is converted in methanol at 280 C for 4 h, the conversion of guaiacol is 24.5%
and the mass balance is 81.3%. The major products are phenol and alkylphenol and the
yield of them are 4.5%. In addition, the yield of 4-methylguaiacol is 1.3%. When guaiacol
o
is converted in ethanol, n-propanol and n-butanol at 320 C for 3 h, the conversion of
guaiacol decreases with the increasing of carbon chain length of the solvent. The highest
conversion, 96.5%, is achieved in ethanol. In the three solvents, the major products are
phenol and alkylphenols of which the yield are 70.8%, 8.3% and 10.5% for ethanol, n-
propanol and n-butanol, respectively. Therefore, the main reactions of guaiacol in the four
alcohol solvents are demethoxylation and alkylation. However, we must clarify that many
peaks in the gas chromatography of the post-reaction solution of guaiacol demethoxylation
reaction in n-propanol or n-butanol can not be identified and quantified (Figure S5 &S6).
As a result, the mass balances of the two reactions are not calculated.
3.3. The conversion of other intermediates
Catechol, phenol and anisole are three possible intermediates in guaiacol deoxygenation,
so they are tested with Re2O7 in ethanol. The results are shown in Table 1. Catechol is
converted to the same products to those of guaiacol but the main products are 2,5-
diethylphenol, 2,4-diisopropylphenol, 2,6-di-tert-butylphenol and 2,6-di-tert-butyl-4-
ethylphenol with individual yields being 5.2%, 10.1%, 5.8% and 4.1%, respectively. Hence,
catechol is likely the intermediate of guaiacol demethoxylation. The post-reaction solution
of this reaction is brown and turbid, which is the sign of condensation. Liquid
chromatography (LC)-MS test shows that the molecular weight of an oligomer is as high
as 447.3 (Figure 6). Consequently, the mass balance of this reaction is only 36.4%. With

phenol as the substrate, the conversion is only 39.5%, which indicates the low reaction rate
of phenol, but the mass balance is very good, being 92.9%. Only four products were
detected by GC and they are 2-ethylphenol, 4-ethylphenol, 2,5-diethylphenol and 2,6-
diisopropylphenol with yields being 21.4%, 5.1%, 5.5% and 0.5%, respectively. 21.8%
anisole is converted to alkylbenzene, monophenols and alkylated anisoles, indicating that
both the CH3-O and the aryl-O bonds in anisole can be cleaved. The main product is phenol
with a 13.6% yield.
3.4. Catalyst leaching and recyclability and the acidity of Re2O7
The post-reaction solution of guaiacol demethoxylation in ethanol at 320 oC for 1.5 h was
analyzed with ICP-OES. The rhenium concentration of this solution is 5.2 mg/L which is
equivalent to 0.37% loss of initial Re2O7.
For the recycling experiment, two reactions at the same conditions (0.1 g Re2O7, 1 g
guaiacol, 60 mL ethanol, 320 oC, 1.5 h) were carried out and the used catalysts were washed
with ethanol, dried at 100 oC overnight and then mixed. Then 0.1 g used catalyst was reused
in guaiacol demethoxylation reaction at the same conditions. The conversion and mass
balance of this reaction of this reaction are 24.5% and 79.0%. Only phenol, 2-ethylphenol,
and 2,5-dimethylphenol are detected as the products of guaiacol by GC and their yields are
1.5%, 1.4% and 0.6%, respectively. Therefore, the catalyst is not directly recyclable.
Usually, the activity of metal oxide catalysts can be recovered by simple calcination [28,
39], so the activity of used Re2O7 is likely to be recovered by calcination. However, the
o
boiling point of Re2O7 is 360 C, which means its calcination must be carried out under
special conditions. Currently, we are unable to calcine it.

Because of the low boiling point of Re2O7, it is impossible to characterize its acidity with
NH3-TPD or pyriding-IR. When Re2O7 (0.05 g) was put into water (10 mL), it dissolved
completely. The pH of this solution was measured with pH meter and the value was 3.21,
which implies that Re2O7 is acidic. The acidity of Re2O7 is possibly the reason why
alkylation accompanies the demethoxylation of guaiacol.
3.5. Possible reaction mechanism
Based on the results of different substrates and the structures of products, we propose the
routes for the demethoxylation and alkylation of guaiacol as shown in Scheme 1. Guaiacol
is converted into phenol or catechol by the cleavage of the aryl-OCH3 or the CH3-O bond
(Scheme 1a). As shown in Table 1, the reaction rate of catechol is higher than that of phenol.
Therefore, phenol is detected in the post-reaction solution of guaiacol demethoxylation
because of its low reaction rate while catechol is not detected because of its high reaction
rate. Since phenol is a product of catechol deoxygenation and catechol are converted to
more products (including Group 1 & 2 in Scheme 1a) than phenol (Group 1 in Scheme
1a), there must be two routes for catechol conversion: 1. Catechol is converted to phenol
by removing –OH; 2. Catechol is converted to the products in Group 2 by deoxygenation
and alkylation, consecutively. The produced phenol is then alkylated into the products in
Group 1. It is obvious that ethyl in the products are provided by ethanol. However, no
isopropanol and tert-butylphenol are observed in the post-reaction solution (Figure 1), so
isopropyl and tert-butyl are not provided by the two alcohols. In the MoO3 catalyzed
deoxygenation of guaiacol in ethanol, we observed that ethylphenol was further alkylated
to isopropylphenol and tert-butylphenol by the methyl supplied by supercritical ethanol
[28]. Hence, isopropylphenol and tert-butylphenol in this work are formed via the

alkylation of ethylphenol with methyl (Scheme 1b) generated by hot ethanol [40]. The
hydrogen atom on the α-carbon of ethylphenol is removed and then methyl is attached to
this carbon, so ethylphenol is converted to isopropylphenol. Similarly, isopropylphenol is
converted to tert-butylphenol. We also propose the routes for the formation of some minor
products (Scheme 2). 2-Ethylphenol is deoxygenated into ethylbenzene, which is further
alkylated into di- and tri-ethylbenzene by ethanol or transfer-hydrogenated into 1-ethyl-1-
cyclohexene and ethylcyclohexane (Scheme 2a). Ethylcyclohexane is further cleaved to
octane. The methoxyl of guaiacol is removed so guaicol is converted to phenol and
methanol (Scheme 2b). Phenol is further etherificated into ethoxybenzene by ethanol.
Ethanol transetherificates guaiacol into 2-ethoxyphenol. 1,2-Dimethoxybenzene is formed
by the intermolecular transalkylation between guaiacol molecules [41] or by the direct
etherification of guaiacol with in-situ produced methanol.
3.4. Characterization of used catalysts
The used catalysts were characterized with XPS (Figure 7). The Re 4f XPS signal contains
contribution from Re7+, Re6+ and Re4+ with the 4f7/2 bands located at 48.18, 47.20 and 44.94
eV, respectively. Since the Re7+ in Re2O7 can be reduced to Re6+ and even Re4+ in the
mixture of guaiacol and ethanol, the percent of superficial Re7+ should have been 0 or at
least decreased with time increasing. However, the percent of superficial Re7+ increases
with time increasing. That is because the used Re2O7 was handled in air and was oxidized
by O2. This explains why Re7+ is detected in the used catalysts. Hence, ReIV-VI is possibly
the active species for guaiacol demethoxylation. In addition, ReIV−VI is also observed in the
C−O bond cleavage of benzyl phenyl ether catalyzed by ReOx/AC [42].

4. Conclusions
Re2O7 catalyzes the demethoxylation and alkylation of guaiacol in ethanol. Guaiacol is
mainly converted to phenol and alkylphenols (making up 97 mol% of all products). The
produced alkylphenols include ethylphenol, diethylphenol, diisopropylphenol, di-tert-
butylphenol and 2,6-di-tert-butyl-4-ethylphenol. Both catechol and phenol are the
intermediates of guaiacol demethoxylation. Among the substituents, ethyl is directly
provided by ethanol while isopropyl and tert-butyl are formed by the addition of methyl to
ethyl step by step. Although Re2O7 catalyzes the saturation of benzene ring but it does not
cause considerable over-consumption of ethanol. The actual catalyst for guaiacol
demethoxylation is possibly a ReIV-VI species.
Acknowledgements
The financial support from the Natural Science Foundation of China under Contract No.
21336008 and 21690083, the Ministry of Science and Technology of China under Contract
No. 2011DFA41000 are gratefully acknowledged. This research was also supported in part
by the Program of Introducing Talents to the University Disciplines under File No. B06006
and the Program for Changjiang Scholars and Innovative Research Teams in Universities
under File No. IRT 0641.

Reference
[1] M. Stocker, Biofuels and Biomass-To-Liquid Fuels in the Biorefinery: Catalytic Conversion of
Lignocellulosic Biomass using Porous Materials, Angew. Chem. Int. Ed. 47 (2008) 9200-9211.
[2] K.V. Sarkanen, C.H. Ludwig, Lignins : Occurrence, Formation, Structure and Reactions, Wiley,
New York, 1971.
[3] S.K. Hanson, R.T. Baker, Knocking on Wood: Base Metal Complexes as Catalysts for Selective
Oxidation of Lignin Models and Extracts, Acc. Chem. Res. 48 (2015) 2037-2048.
[4] A.L. Marshall, P.J. Alaimo, Useful Products from Complex Starting Materials: Common
Chemicals from Biomass Feedstocks, Chem. Eur. J. 16 (2010) 4970-4980.
[5] R. Singh, A. Prakash, S.K. Dhiman, B. Balagurumurthy, A.K. Arora, S.K. Puri, T. Bhaskar,
Hydrothermal conversion of lignin to substituted phenols and aromatic ethers, Bioresour. Technol.
165 (2014) 319-322.
[6] T. Voitl, P.R. von Rohr, Oxidation of Lignin Using Aqueous Polyoxometalates in the Presence of
Alcohols, ChemSusChem 1 (2008) 763-769.
[7] P.J. Deuss, M. Scott, F. Tran, N.J. Westwood, J.G. de Vries, K. Barta, Aromatic Monomers by in
Situ Conversion of Reactive Intermediates in the Acid-Catalyzed Depolymerization of Lignin, J. Am.
Chem. Soc. 137 (2015) 7456-7467.
[8] J.A. Onwudili, Influence of reaction conditions on the composition of liquid products from two-
stage catalytic hydrothermal processing of lignin, Bioresour. Technol. 187 (2015) 60-69.
[9] K. Barta, T.D. Matson, M.L. Fettig, S.L. Scott, A.V. Iretskii, P.C. Ford, Catalytic disassembly of an
organosolv lignin via hydrogen transfer from supercritical methanol, Green Chem. 12 (2010) 1640-
1647.
[10] Q. Song, F. Wang, J. Cai, Y. Wang, J. Zhang, W. Yu, J. Xu, Lignin depolymerization (LDP) in
alcohol over nickel-based catalysts via a fragmentation-hydrogenolysis process, Energ. Environ.
Sci. 6 (2013) 994-1007.
[11] T.D. Matson, K. Barta, A.V. Iretskii, P.C. Ford, One-Pot Catalytic Conversion of Cellulose and
of Woody Biomass Solids to Liquid Fuels, J. Am. Chem. Soc. 133 (2011) 14090-14097.
[12] G. Warner, T.S. Hansen, A. Riisager, E.S. Beach, K. Barta, P.T. Anastas, Depolymerization of
organosolv lignin using doped porous metal oxides in supercritical methanol, Bioresour. Technol.
161 (2014) 78-83.
[13] P. Ferrini, R. Rinaldi, Catalytic Biorefining of Plant Biomass to Non-Pyrolytic Lignin Bio-Oil and
Carbohydrates through Hydrogen Transfer Reactions, Angew. Chem. Int. Ed. 53 (2014) 8634-8639.
[14] X. Ma, Y. Tian, W. Hao, R. Ma, Y. Li, Production of phenols from catalytic conversion of lignin
over a tungsten phosphide catalyst, Appl. Catal. A-Gen. 481 (2014) 64-70.
[15] R. Ma, W. Hao, X. Ma, Y. Tian, Y. Li, Catalytic Ethanolysis of Kraft Lignin into High-Value Small-
Molecular Chemicals over a Nanostructured alpha-Molybdenum Carbide Catalyst, Angew. Chem.
Int. Ed. 53 (2014) 7310-7315.
[16] X. Ma, K. Cui, W. Hao, R. Ma, Y. Tian, Y. Li, Alumina supported molybdenum catalyst for lignin
valorization: Effect of reduction temperature, Bioresour. Technol. 192 (2015) 17-22.
[17] X. Ma, R. Ma, W. Hao, M. Chen, F. Yan, K. Cui, Y. Tian, Y. Li, Common pathways in ethanolysis
of Kraft lignin to platform chemicals over molybdenum-based catalysts, Acs. Catal. 5 (2015) 4803-
4813.

[18] M. Chen, X. Ma, R. Ma, Z. Wen, F. Yan, K. Cui, H. Chen, Y. Li, Ethanolysis of Kraft Lignin over a
Reduction-Modified MoO3 Catalyst, Ind. Eng. Chem. Res. 56 (2017) 14025-14033.
[19] M. Chen, W. Hao, R. Ma, X. Ma, L. Yang, F. Yan, K. Cui, H. Chen, Y. Li, Catalytic ethanolysis of
Kraft lignin to small-molecular liquid products over an alumina supported molybdenum nitride
catalyst, Catal. Today 298 (2017) 9-15.
[20] F. Yan, R. Ma, X. Ma, K. Cui, K. Wu, M. Chen, Y. Li, Ethanolysis of Kraft lignin to platform
chemicals on a MoC1-x/Cu-MgAlOz catalyst, Appl. Catal. B-Environ. 202 (2017) 305-313.
[21] S. Song, J. Zhang, G. Gözaydın, N. Yan, Production of Terephthalic Acid from Corn Stover Lignin,
Angew. Chem. 131 (2019) 4988-4991.
[22] M. Zabeti, J. Baltrusaitis, K. Seshan, Chemical routes to hydrocarbons from pyrolysis of
lignocellulose using Cs promoted amorphous silica alumina catalyst, Catal. Today 269 (2016) 156-
165.
[23] M. Ishikawa, M. Tamura, Y. Nakagawa, K. Tomishige, Demethoxylation of guaiacol and
methoxybenzenes over carbon-supported Ru-Mn catalyst, Appl. Catal. B-Environ. 182 (2016) 193-
203.
[24] Q. Bu, H.W. Lei, A.H. Zacher, L. Wang, S.J. Ren, J. Liang, Y. Wei, Y.P. Liu, J. Tang, Q. Zhang, R.
Ruan, A review of catalytic hydrodeoxygenation of lignin-derived phenols from biomass pyrolysis,
Bioresour. Technol. 124 (2012) 470-477.
[25] V.N. Bui, D. Laurenti, P. Afanasiev, C. Geantet, Hydrodeoxygenation of guaiacol with CoMo
catalysts. Part I: Promoting effect of cobalt on HDO selectivity and activity, Appl. Catal. B-Environ.
101 (2011) 239-245.
[26] V.N. Bui, D. Laurenti, P. Delichère, C. Geantet, Hydrodeoxygenation of guaiacol: Part II:
Support effect for CoMoS catalysts on HDO activity and selectivity, Appl. Catal. B-Environ. 101
(2011) 246-255.
[27] R. Ma, K. Cui, L. Yang, X. Ma, Y. Li, Selective catalytic conversion of guaiacol to phenols over
a molybdenum carbide catalyst, Chem. Commun. 51 (2015) 10299-10301.
[28] K. Cui, L. Yang, Z. Ma, F. Yan, K. Wu, Y. Sang, H. Chen, Y. Li, Selective conversion of guaiacol
to substituted alkylphenols in supercritical ethanol over MoO3, Appl. Catal. B-Environ. 219 (2017)
592-602.
[29] J. Zhang, L. Lombardo, G. Gözaydın, P.J. Dyson, N. Yan, Single-step conversion of lignin
monomers to phenol: Bridging the gap between lignin and high-value chemicals, Chinese J. Catal.
39 (2018) 1445-1452.
[30] J. Filley, C. Roth, Vanadium catalyzed guaiacol deoxygenation, J. Mol. Catal. A: Chem. 139
(1999) 245-252.
[31] F. Yan, Y. Bai, Y. Sang, L. Yu, K. Wu, K. Cui, Z. Wen, F. Mai, Z. Ma, H. Chen, Y. Li, Facile
Preparation of Haggite by Reducing V2O5 in Guaiacol/Methanol Solution, Inorg. Chem. 57 (2018)
8705-8708.
[32] K. Leiva, N. Martinez, C. Sepulveda, R. Garcia, C.A. Jimenez, D. Laurenti, M. Vrinat, C. Geantet,
J.L.G. Fierro, I.T. Ghampson, N. Escalona, Hydrodeoxygenation of 2-methoxyphenol over different
Re active phases supported on SiO2 catalysts, Appl. Catal. A-Gen. 490 (2015) 71-79.
[33] K.B. Jung, J. Lee, J.-M. Ha, H. Lee, D.J. Suh, C.-H. Jun, J. Jae, Effective hydrodeoxygenation of
lignin-derived phenols using bimetallic RuRe catalysts: Effect of carbon supports, Catal. Today 303
(2018) 191-199.
[34] A. Wali, J. Das, S.M. Pillai, M. Ravindranathan, Disproportionation, isomerization and de-tert-
butylation of 2,6-di-tert-butylphenol catalyzed by H-MCM-41, Green Chem. 4 (2002) 587-591.
[35] M.E. Sad, H.A. Duarte, C.L. Padró, C.R. Apesteguía, Selective synthesis of p-ethylphenol by
gas-phase alkylation of phenol with ethanol, Appl. Catal. A-Gen. 486 (2014) 77-84.

[36] B. Silberova, L. Cerveny, Hydrogenation of 2-tert-butylphenol over Ni catalyst, React. Kinet.
Catal. Lett. 67 (1999) 29-33.
[37] O.M. Kut, U.R. Daetwyler, G. Gut, Stereoselective hydrogenation of 2-tert-butylphenol to cis-
2-tert-butylcyclohexanol. 1. Equilibrium composition of the system 2-tert-butylphenol/2-tert-
butylcyclohexanone/cis- or trans-2-tert-butylcyclohexanol and hydrogen, Ind. Eng. Chem. Res. 27
(1988) 215-218.
[38] O.M. Kut, U.R. Daetwyler, G. Gut, Stereoselective hydrogenation of 2-tert-butylphenol to cis-
2-tert-butylcyclohexanol. 2. Kinetics of the liquid-phase hydrogenation of 2-tert-butylphenol over
nickel, cobalt, and noble metal catalysts, Ind. Eng. Chem. Res. 27 (1988) 219-225.
[39] F. Yan, Z. Wen, K. Wu, K. Cui, F. Mai, Z. Ma, Y. Sang, Y. Bai, H. Chen, Y. Li, Deoxyalkylation of
guaiacol using haggite structured V4O6(OH)4, Catal. Sci. Technol. 9 (2019) 1922-1932.
[40] Z.F. Xu, K. Xu, M.C. Lin, Thermal Decomposition of Ethanol. 4. Ab Initio Chemical Kinetics for
Reactions of H Atoms with CH3CH2O and CH3CHOH Radicals, J. Phys. Chem. A 115 (2011) 3509-
3522.
[41] L. Nie, B. Peng, X. Zhu, Vapor-Phase Hydrodeoxygenation of Guaiacol to Aromatics over
Pt/HBeta: Identification of the Role of Acid Sites and Metal Sites on the Reaction Pathway,
ChemCatChem 10 (2018) 1064-1074.
[42] B. Zhang, Z. Qi, X. Li, J. Ji, W. Luo, C. Li, A. Wang, T. Zhang, ReOx/AC-Catalyzed Cleavage of C–
O Bonds in Lignin Model Compounds and Alkaline Lignins, ACS Sustainable Chem. Eng. 7 (2019)
208-215.

Figure 1. The GC-MS chromatogram of the post-reaction solution of guaiacol
deoxygenation in ethanol and the structures of identified compounds.

Figure 2. (a) The results of guaiacol deoxygenation in ethanol at 320 oC for different
time. (b) The classification of products. Note: Four, three and two peaks are assigned to
[a] 2,5-diethylphenol, [b] 2,4-diisopropylphenol and [c] 2,6-di-tert-butylphenol by the
NIST02 library, respectively.

Figure 3. The yields of alkylphenols obtained in guaiacol deoxygenation.

Figure 4. The results of guaiacol demethoxylation in ethanol at different temperatures for
3 h.

Figure 5. The results of guaiacol demethoxylation in different alcohols. (a) The
quantified products obtained in methanol, n-propanol and n-butanol, (b) the yield of
products. Note: in image b, “Other“ includes all the quantified products except phenol
and alkylphenols. Reaction conditons for methanol: 280 oC, 4h. Reaction conditions for
ethanol, n-propanol and n-butanol: 320 oC, 3 h.

Table 1. The results of the reactions with cathechol, phenol and anisole as substrates in
ethanol at 320 oC for 6 h.
Substrate
Yield/%
1.4%
1.7%
1.3%
0.2%
5.2%[a]
1.6%
4.1%
10.1%[b]
X 100.0%
M.B. 36.4%
0.8%
1.6%
5.1%
5.8%[c]
1.6%
0.5%
1.0%
X 39.5%
M.B. 92.9%
21.4%
5.5%[d]
X 21.8%
M.B. 95.0%
0.1%
13.6%
0.4%
0.5%
0.3%
0.1%
1.7%

Note: In the post-reaction solution of catechol, four, three and two peaks are assigned to [a] 2,5-
diethylphenol, [b] 2,4-diisopropylphenol and [c] 2,6-di-tert-butylphenol by the NIST02 library,
respectively. [d] In the post-reaction solution of phenol, two peaks are assigned to 2,5-
diethylphenol. X: conversion; M.B.: mass balance.
Figure 6. ESI-MS analysis of the post-reaction solution acquired in the deoxygenation of
catechol at 320 oC for 6 h in ethanol.

Scheme 1. (a) Possible reaction pathways for guaiacol in ethanol; (b) Reaction pathways
of lower alkylphenols to higher alkylphenols . *: Activated atom.

Scheme 2. Possible pathways for the formation of (a) hydrocarbons (b) ethers.

Figure 7. XPS of the Re (4f) energy region of the spent Re2O7 samples used in guaiacol
deoxygenation at 320 oC for different time.