Highly selective conversion of guaiacol to: Tert -butylphenols in supercritical ethanol over a H2WO4 catalyst

Article abstract of DOI:10.1039/c8ra07962e

The conversion of guaiacol is examined at 300 °C in supercritical ethanol over a H₂WO₄ catalyst. Guaiacol is consumed completely, meanwhile, 16.7% aromatic ethers and 80.0% alkylphenols are obtained. Interestingly, tert-butylphenols are produced mainly with a high selectivity of 71.8%, and the overall selectivity of 2,6-di-tert-butylphenol and 2,6-di-tert-butyl-4-ethylphenol is as high as 63.7%. The experimental results indicate that catechol and 2-ethoxyphenol are the intermediates. Meanwhile, the WO₃ sites play an important role in the conversion of guaiacol and the Br?nsted acid sites on H₂WO₄ enhance the conversion and favour a high selectivity of the tert-butylphenols. The recycling tests show that the carbon deposition on the catalyst surface, the dehydration and partial reduction of the catalyst itself are responsible for the decay of the H₂WO₄ catalyst. Finally, the possible reaction pathways proposed involve the transetherification process and the alkylation process during guaiacol conversion.

Full text of DOI:10.1039/c8ra07962e

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Highly selective conversion of guaiacol to tert-
butylphenols in supercritical ethanol over a H2WO4
catalyst†
Cite this: RSC Adv., 2019, 9, 2764
Fuhang Mai,ab Kai Cui,ab Zhe Wen,ab Kai Wu,ab Fei Yan,ab Mengmeng Chen,ab
d
and Yongdan Liabc
*
Hong Chen
The conversion of guaiacol is examined at 300 ꢀC in supercritical ethanol over a H2WO4 catalyst. Guaiacol is
consumed completely, meanwhile, 16.7% aromatic ethers and 80.0% alkylphenols are obtained.
Interestingly, tert-butylphenols are produced mainly with a high selectivity of 71.8%, and the overall
selectivity of 2,6-di-tert-butylphenol and 2,6-di-tert-butyl-4-ethylphenol is as high as 63.7%. The
experimental results indicate that catechol and 2-ethoxyphenol are the intermediates. Meanwhile, the
WO3 sites play an important role in the conversion of guaiacol and the Brønsted acid sites on H2WO4
enhance the conversion and favour a high selectivity of the tert-butylphenols. The recycling tests show
that the carbon deposition on the catalyst surface, the dehydration and partial reduction of the catalyst
itself are responsible for the decay of the H2WO4 catalyst. Finally, the possible reaction pathways
proposed involve the transetherification process and the alkylation process during guaiacol conversion.
Received 25th September 2018
Accepted 20th December 2018
DOI: 10.1039/c8ra07962e
rsc.li/rsc-advances
guaiacol conversion focused mainly on sulded CoMo and
NiMo catalysts,13,14 and supported noble metal catalysts.15,16
1 Introduction
However, the high H2 consumption, high temperature and
aromatic ring saturation are not favoured parameters.17 There-
fore an alternative catalyst with a relatively strong deoxygen-
ation activity and a weak aromatic ring saturation capacity is
called for.5 Transition metals18 and their oxides,19,20 phos-
phides,21,22 nitrides,23,24 and carbides9,11,25–27 therefore have been
examined. Metal phosphide catalysts were found to show
excellent activity but tungsten phosphide (WP) showed a rela-
tively disappointing result for gaseous phase hydro-
deoxygenation (HDO) of guaiacol.21 Compared with the
conventional CoMo/Al2O3 catalyst, a higher conversion and
a higher selectivity towards phenolics were obtained in the HDO
of guaiacol over W2C/CNF at 55 bar H2 pressure.11 However,
metal carbide catalysts are easily oxidized, resulting in
deactivation.28
Recently, the conversion of model compounds into value-
added chemicals in supercritical alcohol was reported.26,29
Yang et al. reported the transetherication of guaiacol to o-
ethoxyphenol over a g-Al2O3 catalyst in supercritical ethanol.30
Ma et al.9 reported the catalytic conversion of guaiacol in
supercritical ethanol over a-MoC1ꢁx/AC without the addition of
gaseous hydrogen. Phenols and alkylphenols were obtained as
the main products with an overall selectivity of 85%. More
recently, Cui et al.28 reported the selective conversion of guaiacol
over a MoO3 catalyst to produce various alkylphenols in ethanol.
They proposed that the higher alkylphenols formed via
a consecutive substitution route. Hansen et al.29 performed the
catalytic conversion of 5-hydroxymethylfurfural to 2,5-
The depletion of fossil fuels and the growing demand of energy
and chemicals call for practical technologies for biomass,
especially lignin, utilization. Lignin is an important component
and accounts for 10–35% by weight, and up to 40% by energy in
lignocellulosic biomass.1 The available aromatic structure of
lignin renders it an attractive material for production of
aromatic chemicals. However, due to the strong linkages in its
complex structure, only a small amount of lignin is utilized as
a low-grade fuel or as a low-value building material additive
(lignosulfonate).2 Up to now, a number of techniques have been
explored to depolymerize lignin, including pyrolysis,3 hydro-
genolysis/hydrogenation,4,5 oxidation,6 hydrolysis,7 reforming8
and ethanolysis.9,10
Guaiacol, which contains both hydroxyl and methoxyl
groups, is oen chosen as a model compound to verify reaction
mechanism during lignin depolymerization.11,12 Early works on
aState Key Laboratory of Chemical Engineering (Tianjin University), Tianjin Key
Laboratory of Applied Catalysis Science and Technology, School of Chemical
Engineering, Tianjin University, Tianjin 300072, China
bCollaborative Innovation Center of Chemical Science and Engineering (Tianjin),
Tianjin 300072, China
cDepartment of Chemical and Metallurgical Engineering, School of Chemical
Engineering, Aalto University, Kemistintie 1, Espoo, P. O. Box 16100, FI-00076
Aalto, Finland
dSchool of Environmental Science and Engineering, Tianjin University, Tianjin 300072,
China. E-mail: chenhong_0405@tju.edu.cn; Tel: +8613920176659
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ra07962e
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dimethylfuran and 2,5-dimethyltetrahydrofuran over a Cu- were pressed into thin wafers and treated under vacuum (10ꢁ3
doped porous metal oxide (Cu-PMO) in supercritical meth- torr) at 423 K for 2 h. Aer cooling to room temperature, the
anol. More importantly, supercritical alcohols such as meth- samples were exposed to pyridine steam for 30 min. Then, the
anol, ethanol and isopropanol were found to be also efficient for spectra (32 scans, 8 cmꢁ1 resolution) were recorded aer evac-
lignin depolymerization.30–33
uation for 30 min at room temperature.
The binary catalyst composed of H2WO4 and Ru/C showed
a high activity for the one-pot conversion of cellulose to ethylene
glycol.34 Tai et al.35 proposed that H2WO4 was dissolved partially
in water under the reaction conditions and transformed into
HxWO3, which served as the active site. Jongerius et al.11 found
that WOx phase over the W2C/CNF catalyst formed in the reac-
tion process had a benecial effect on the catalytic activity for
gaseous phase HDO (55 bar H2) of guaiacol. In this work, we
report the utilization of H2WO4 as a catalyst in the selective
conversion of guaiacol in supercritical ethanol. A number of
aromatic ethers and alkylphenols are formed with high selec-
tivity, especially for 2,6-di-tert-butylphenol and 2,6-di-tert-butyl-
4-ethylphenol.
2.3 Catalytic activity measurements
H2WO4 was directly applied in guaiacol conversion reactions
without any treatment. The WO3 catalyst sample for compar-
ison was prepared with calcining AMT at 500 ꢀC for 4 h in static
air.36 All the reactions were carried out in a 250 mL batch reactor
(MS 250, Anhui Kemi Machinery Technology Co., LTD.). In
a typical test, 1.0 g guaiacol was dissolved in 80 mL ethanol as
the reactant. The reactant and 0.5 g catalyst were added into the
autoclave. Aer leakage test, the reactor was purged 5 times
with ultrapure N2 to remove the remaining air. The reactor was
heated to the reaction temperature (260–310 ꢀC) with a heating
rate of 2.8 ꢀC minꢁ1, and kept for the desired time with stirring
at 600 rpm. The liquid products and the spent catalyst were
separated and recovered aer the reaction.
2 Materials and methods
2.1 Materials
2.4 Analysis of products
Analytical grade (AR) chemicals and solvents, including guaia-
col, phenol, catechol, anisole, ethanol, were purchased from
Tianjin Guangfu Technology Development Co., Ltd. and were
used without further purication. 2-Ethoxyphenol (AR) was
obtained from Heowns (Tianjin) and used as received.
Commercial ammonium metatungstate (AMT, (NH4)6-
H2W12O40$xH2O) and H2WO4 were purchased from Aladdin
Industrial Corporation. The water used was provided by an
ultrapure water purication system (UPH-1-10).
nðguaiacolÞ
ꢁ nðguaiacolÞ
initial
initial
residual
Cð%Þ ¼
ꢂ 100%
(1)
(2)
(3)
nðguaiacolÞ
nðaromatic productÞ
consumed
Sð%Þ ¼
ꢂ 100%
nðguaiacolÞ
M (%) ¼ SSi
quantified aromatics
2.2 Catalyst characterization
The organic phase containing liquid products was injected
directly with a syringe into a gas chromatograph-mass spec-
trometer (GC-MS) system for product qualitative analysis. The
product was further analysed quantitatively with a GC equipped
with a ame ionization detector (FID). The detailed parameters
used for the analysis can be found in the previous work.27,28 The
identication of the liquid products was achieved through the
standard spectrogram database (NIST02). An internal standard
method was performed to analyse and quantify the product
yield. The subsequent calculation within this context are mainly
focused on the aromatic compounds. Hence, the calculation,
including guaiacol conversion (C (%)), a specic aromatic
product selectivity (S (%)) and mass balance (M (%)), is based on
the variations of guaiacol and aromatic compounds before and
aer the reaction (equation (1)–(3)). In these equations, n
presents the amount of the aromatic species in moles. The
conversion and yield are determined with averaging the data
measured in three times and the error is less than 5%.
The structure of the fresh and spent catalyst samples were
identied with an X-ray diffraction (XRD) technique (Bruker
AXS, D8-S4), operated at 40 kV and 40 mA, using a Cu-Ka
monochromatized radiation source. Diffraction data was
recorded between 2q of 10ꢀ and 80ꢀ at a rate of 8ꢀ minꢁ1. X-ray
photoelectron spectroscopy (XPS) was performed with a PHI-
1600 ESCA system spectrometer using Mg Ka as the X-ray
source under a residual pressure of 5 ꢂ 10ꢁ6 Pa, with the
binding energy calibrated using C1s at 284.6 eV as the standard.
Thermogravimetric (TG) curves were measured with a Perki-
nElmer Diamond analyzer in air from 100 ꢀC to 900 ꢀC with
a heating rate of 10 ꢀC minꢁ1. Raman spectra were obtained
using a Renishaw inVia reex spectrometer with a 523 nm argon
laser excitation source. The resistivity of the H2WO4 and WO3
catalysts were tested. The catalyst powder was pressed into
a thin wafer on a standard abrasive tool and then was tested in
a tube furnace at 300 ꢀC. NH3 temperature programmed
desorption (NH3-TPD) was performed in a xed bed reactor
using a Thermo Nicolet IS10 FT-IR as the NH3 detector.
Nitrogen adsorption measurements at 77 K were performed on
an automatic surface analyzer (QuadraSorb Station 3). Pyridine
3 Results and discussion
3.1 Catalysts characterization
Fourier transform infrared spectroscopy (FT-IR) spectra were The XRD patterns of the WO3 catalyst, fresh H2WO4 catalyst and
recorded in a Nicolet iS50 FT-IR spectrometer. The samples spent catalysts are illustrated in Fig. 1. For the mesoporous
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Fig. 3 The pyridine adsorption infrared spectra of the WO3 catalyst,
fresh H2WO4 catalyst and spent catalysts.
Fig. 1 XRD patterns of the WO3 catalyst, fresh H2WO4 catalyst and
spent catalysts.
about 7.5%, which was almost identical to the theoretical value
(7.2%).39,40 For the spent samples, there is one main peak
H2WO4 and WO3 samples (Table S1†), their diffraction peaks
corresponded to the standard H2WO4 (WO3$H2O) phase (JCPDS
no. 84-0886) and the standard WO3 (JCPDS no. 71-2141) phase,
respectively. However, the diffraction peaks of the spent H2WO4
samples (rst run-15 min (the H2WO4 catalyst was recovered
from the rst run aer 15 minutes without any treatment) and
rst run) were basically in accord with those of WO3 catalyst,
indicating that H2WO4 was transformed into WO3 under the
reaction condition in the rst run. The change was again
conrmed through Raman spectra (Fig. S1†). For the spent
samples, the strength of the X-ray diffraction peaks at 23.6ꢀ and
24.4ꢀ declined sharply as the cycle time increased, indicating
that the spent catalyst may be partly reduced into W18O49 or
WO2.72 phase,37,38 and the reduction process was further proved
through the XPS proles (Fig. S2†).
ꢀ
ꢀ
ꢀ
respectively located at 491 C, 511 C and 550 C for different
runs, corresponding to the accumulation of carbon.20 With the
increase of number of cycle time, the mass loss of carbon on the
spent catalysts reached 5.9%, 6.5% and 9.2%, respectively. The
phenomenon was consistent with the results from Raman
spectra. From the Fig. S1,† we can see that as the cycle time
increased, the two small bands centered at 1586 cmꢁ1 and
1318 cmꢁ1 in Raman spectra became stronger, showing the
accumulation of carbon on the spent catalyst.41
The pyridine FT-IR spectra of the H2WO4 and WO3 samples
are shown on Fig. 3. Brønsted acid sites (at 1546.7 cmꢁ1) and
Lewis acid sites (between 1442 cmꢁ1 and 1448 cmꢁ1) simulta-
neously exist on the H2WO4 catalyst, while there are only Lewis
acid sites on the WO3 sample. It is well known that WO3 may be
a component in the H2WO4 sample. Further, the Brønsted acid
sites over the spent H2WO4 catalyst which was recovered from
the third run were completely disappeared, while the Lewis acid
sites remained. The surface acidity of the fresh and spent
catalysts were further characterized with an NH3-TPD method
in a temperature rage 100–550 ꢀC, and the results are depicted
The TG curves of the fresh and spent catalysts in air atmo-
sphere are depicted in Fig. 2. For the fresh H2WO4 catalyst, it
can be seen that there is one peak located at below 300 ꢀC due to
the lattice water that was the fresh H2WO4 catalyst dehydrated
between 170 ꢀC and 260 ꢀC, and the nal mass loss reached
Fig. 2 (a) TG and (b) DTG profiles of the fresh and spent catalysts.
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Fig. 5 TIC of the liquid products obtained from guaiacol conversion
over H2WO4 in ethanol at 300 ꢀC for 6 h. The peaks marked with
numbers correspond to the products in Tables 1 and S2.† Reaction
conditions: guaiacol (1.0 g), catalyst (0.5 g H2WO4), 600 rpm, 0 MPa
(gauge) initial N2 pressure at room temperature, 80 ml ethanol. The
compound marked with 13 is phenol, which is used as an internal
standard to calculate the yield of products.
Fig. 4 NH3-TPD profiles of the WO3 catalyst, fresh H2WO4 catalyst
and spent catalysts.
in Fig. 4. The area and position of the peaks in the TPD curves
are correlated to the acid amount and the acid strength,
respectively.42,43 For the fresh the H2WO4 sample, two distinct
desorption peaks were observed at 110 ꢀC and 380 ꢀC, ascribing
to the NH3 desorption from the weak and strong acid sites,
respectively.44 For the spent catalysts, the peak ascribed to the
strong acid sites disappeared and a peak corresponding to
medium strength acid sites between 200 ꢀC and 300 ꢀC formed.
Nevertheless, as the increase of the cycle time, the amount of
medium acid sites underwent an obvious decrease. Overall,
with the increase of number of cycle time, the acid sites over the
H2WO4 catalyst gradually decrease, especially for the Brønsted
acid sites. Noticeably, the resistivity of the H2WO4 and WO3
catalysts uctuated between 106 and 107, which belonged to the
semiconductor materials. It proved that the results from the
pyridine FT-IR spectra and NH3-TPD spectra were reliable to
some extent.
16.7%. Alkylphenols were obtained as the main products with
an overall selectivity of 80.0%. The alkylphenols, including
ethylphenol, iso-propylphenols, tert-butylphenols and neo-pen-
tylphenols, exhibited selectivities of 0.4%, 6.3%, 71.8%, 1.5%,
respectively.
The alkylphenols formed through selective deoxygenation
and alkylation have been conrmed in supercritical ethanol
system.28 Three intensive peaks exist in the TIC of liquid prod-
ucts, which correspond to 2,6-di-tert-butylphenol (P36, P37) and
2,6-di-tert-butyl-4-ethylphenol (P45). The sum selectivity of 2,6-
Table 1 Product distribution of the liquid products obtained from
guaiacol conversion. a
Type
Molecule
Selectivity
3.2 Product distribution
The effects of the reaction parameters such as reaction time,
temperature and solvent amount were explored and nally the
typical conversion reaction of guaiacol over the H2WO4 catalyst
was performed in 80 mL ethanol at 300 ꢀC for 6 h (Fig. S3–S5†).
The total-ion chromatogram (TIC) of the products derived from
the GC-MS is shown in Fig. 5. 45 compounds (P1–P12, P14–46)
were identied aer the reaction, and they can be classied into
two categories: (1) aliphatic compounds; (2) aromatic
compounds. Generally, P1–12, P14–P16 and P18, including
aliphatic hydrocarbons and acetal, yielded up to 1535.2 mg and
were conrmed to be formed from ethanol with a blank reac-
tion. The detailed molecular structures are listed in Table S2.†
In addition to the internal standard (phenol, P13) and the
reactant (guaiacol, P17), the other 28 aromatic products (P19–
46) were only detected in the guaiacol conversion. The results
are listed in Table 1. In terms of aromatic products, there are
two sorts: (1) aromatic ethers and (2) alkylphenols. The
aromatic ethers were composed of 8 molecules, all containing
either ethoxy group or methoxy group, with a total selectivity of
Aromatic ethers
16.7%
Alkylphenols
80.0%
a
The numbers in the table correspond to those product peaks marked
in the TIC (Fig. 5).
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Table 2 Conversion of model compounds over H2WO4 in supercrit-
when anisole was used as the reactant. Hence, catechol and 2-
ethoxyphenol are likely the intermediates of the guaiacol
conversion over the H2WO4 catalyst.
ical ethanol. a
Feedstock
Cb/% Mc/% S1
S2
S3
S4
S5
S6
d
Guaiacol
Catechol
2-Ethoxyphenol 88.1 92.1
Phenol
Anisole
>99
>99
96.7
88.4
16.7 80.0 71.6 2.10 28.5 35.2
19.8 68.5 63.7 3.20 26.7 28.0
3.4 Catalytic activity for H2WO4 or WO3
The comparison results for conversion of guaiacol over different
catalysts are given in Table 3. When WO3 was utilized as the
catalyst, 91.8% guaiacol was consumed and the products were
still aromatic ethers and alkylphenols. However, the overall
selectivity of aromatic ethers was as high as 67.0%, in which 2-
ethoxyphenol was the main product with the selectivity of
46.0%. The overall selectivity of alkylphenols was less than half
of that obtained over the H2WO4 catalyst. Obviously, WO3
exhibited a lower yield of alkylphenols than that obtained with
H2WO4 as catalyst. No aromatic product was detected when
H3PO4 was separately used as the catalyst in the conversion of
guaiacol. However, when the WO3 combined with H3PO4 was
utilized as the catalyst, guaiacol was completely converted, but
the mass balance was 85.3%, which was because Brønsted acid
sites facilitated the repolymerization of aromatics.5,45 The
combined selectivity of aromatic ethers markedly decreased to
45.8%, in which 2-ethoxyphenol was still a main product but its
selectivity decreased to 17.9%. On the contrary, the total selec-
tivity of alkylphenols underwent an increase to 39.5%, and the
tert-alkylphenols were still obtained as the dominating alkyl-
phenols with a combined selectivity of 31.8%.
28.5 62.5 55.8
—
—
—
22.4 24.8
84.8 73.5
43.7 69.2
56.1e
2.3f
—
—
—
—
—
12.9g
—
—
a
Reaction conditions: feedstock (1.0 g), catalyst (0.5 g), 300 ꢀC, 6 h,
600 rpm, 80 ml ethanol, 0 MPa (gauge) initial N2 pressure at room
b
c
d
temperature. C: conversion. M: carbon balance. S1: aromatic
ethers, S2: alkylphenols, S3: tert-butylphenols, S4: 2-ethoxyphenol, S5:
2,6-di-tert-butylphenol, S6: 2,6-di-tert-butyl-4-ethylphenol. Selectivity:
1-ethoxybenzene (53.5%), 4-ethyl-1-ethoxybenzene (2.6%). Selectivity:
2-ethylphenol (2.3%).
e
f
g
Selectivity: 1-ethoxybenzene (1.0%), di-
methylanisole (7.6%), 4-ethylanisole (4.3%).
di-tert-butylphenol (28.5%) and 2,6-di-tert-butyl-4-ethylphenol
(35.2%) was as high as 63.7%, which accounts almost 79.6%
of the yield of the alkylphenols. Notably, no completely deoxy-
genated product or ring hydrogenation product was detected.
Besides, neither phenol nor catechol was found in the liquid
products. Compared with the a-MoC1ꢁx/AC and MoO3 catalysts
reported in our previous work,9,28 the advantage of high selec-
tivity to tert-butylphenols, especially 2,6-di-tert-butylphenol and
2,6-di-tert-butyl-4-ethylphenol, provides the prospect of lignin
depolymerization into high-valued chemicals.
3.5 Recycle tests
3.3 Reactions with possible intermediates as reactants
The reusability of the H2WO4 catalyst was tested and the results
Tests with potential intermediates of guaiacol conversion are listed in Table S3.† The spent H2WO4 catalyst was separated
reaction, i.e. catechol, 2-ethoxyphenol, phenol and anisole, as from the liquid products and was directly reused in the next run
reactants were carried out under the same condition. The without treatment. The conversion was still as high as 86.0% in
results are presented in Table 2. Catechol was completely the third run despite there was a slight decline. In addition, the
consumed, and the product distribution was almost identical product was still aromatic ethers and alkylphenols but the
with that from the reaction with guaiacol, indicating catechol distribution experienced a signicant change. As the cycle time
was likely an intermediate. In Fig. S3b,† the selectivity of 2- increased, the overall selectivity of aromatic ethers increased
ethoxyphenol underwent a gradual decrease with time increase, markedly while that of alkylphenols underwent an obvious
indicating 2-ethoxyphenol may be also an intermediate. The decline. In the second run the yield of aromatic ethers was close
product catagories of using 2-ethoxyphenol as a reactant were to that of alkylphenols while the overall selectivity of aromatic
indeed identical to those from guaiacol conversion although ethers increased markedly to 82.1% but that of alkylphenols
12% of 2-ethoxyphenol was not converted and a drop in overall dropped to 11.8% in the third run. Compared with the rst run,
selectivity of alkylphenols were observed. However, the product the selectivities of 2,6-di-tert-butylphenol and 2,6-di-tert-butyl-4-
distribution from phenol conversion was very different from ethylphenol reduced by 90% and 82% while that of 2-ethox-
that with guaiacol as the reactant, indicating that phenol is not yphenol increased by more than 26 times in the third run.
an intermediate. Similarly, only a small amount of 1-ethox- Further, the decay in activity of the third run decreased more
ybenzene, 4-ethylanisole and di-methylanisole were detected obviously than that of the second run.
Table 3 Conversion of guaiacol over different catalysts. a
d
Catalyst
Cb/%
Mc/%
S1
S2
S3
S4
2.10
46.0
17.9
S5
S6
H2WO4
WO3
0.5 g WO3 + 0.1 g H3PO4
>99
91.8
>99
96.7
98.9
85.3
16.7
67.0
45.8
80.0
31.8
39.5
71.6
28.4
31.8
28.5
4.7
2.3
35.2
21.1
25.2
a
Reaction conditions: guaiacol (1.0 g), catalyst (0.5 g), 300 ꢀC, 6 h, 600 rpm, 80 ml ethanol, 0 MPa (gauge) initial N2 pressure at room temperature.
b
C: conversion. c M: mass balance. d S1: aromatic ethers, S2: alkylphenols, S3: tert-butylphenols, S4: 2-ethoxyphenol, S5: 2,6-di-tert-butylphenol, S6:
2,6-di-tert-butyl-4-ethylphenol.
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3.6 Active sites
Tai et al.34 reported the selective conversion of cellulose to
ethylene glycol over H2WO4 and Ru/C catalyst. They found that
H2WO4 transformed into HxWO3, which was the active species
for the conversion of cellulose. However, H2WO4 was not active
for guaiacol conversion when we attempted to utilize H2O as
solvent. In addition, the diffraction peaks of the spent H2WO4
sample which was recovered aer reaction at once were basi-
cally in accord with those of WO3 catalyst. The phenomenon
was obviously different from the results reported in the previous
work.35 Notably, a higher guaiacol conversion and a higher
selectivity of the alkylphenols were obtained over the H2WO4
catalyst than the WO3 catalyst. Besides, Brønsted acid (H3PO4)
showed no catalytic activity for guaiacol conversion but the WO3
in combination with the H3PO4 showed a better performance
than the WO3 catalyst. Hence, the difference in catalytic activity
between H2WO4 catalyst and WO3 catalyst may be related to the
presence of Brønsted acid sites. We speculated that WO3 sites
played an important role in the conversion of guaiacol, and the
Brønsted acid sites on H2WO4 rather than the HxWO3 sites
enhanced the conversion and favoured a high selectivity of the
alkylphenols, especially for tert-butylphenols. The interaction
effect between metal sites and acid sites have been
reported.16,46–48
Scheme 1 The deoxygenation and alkylation pathways of guaiacol
conversion.
happens subsequently through the catalytic cleavage of the
oxygen–carbon bond (C(sp3)–OAr) on the H2WO4 surface.5 The
obtained phenoxide ion then reacts with the ethoxy or proton to
produce the 2-ethoxyphenol and catechol intermediates. For the
2-ethoxyphenol intermediate, it is transformed into aromatic
ethers and catechol soon. For the catechol intermediate, it also
reacts with ethanol and produced the 2-ethoxyphenol interme-
diate. Meanwhile, the deoxygenation and alkylation steps are
carried out. The catechol adsorbs on the H2WO4 surface, and
one of the hydroxyl groups undergoes deprotonation, producing
the electronegative phenolate ion (Scheme 1(2)). Concurrently,
the electropositive hydrogen ion is adsorbed on the H2WO4
surface. The unstable phenolate ion (Scheme 1(2)) forms an
equivalently charged compound (Scheme 1(3)) containing
a ketone group through a resonance hybridization step.50 The
formed unstable intermediate (Scheme 1(3)) is adsorbed on the
H2WO4 surface by hydrogen bond (Scheme 1(4)). Dai et al.51
investigated direct conversion of cyclohexane to adipic acid over
a bifunctional H2WO4/TS-1 catalyst, and also proposed the
partial deoxygenation of 1,2-cyclohexanediol involved the acti-
vation of cyclohexanone via protonation and tautomerisation.
The detail is shown in Scheme S1.† Subsequently, the inter-
mediate (Scheme 1(4)) undergoes a dehydroxylation process.
The hydrogen ions on H2WO4 surface, including the adsorbed
hydrogen ions and the inherent hydrogen ions over H2WO4
catalyst, can react with the electronegative hydroxyl groups to
remove the hydroxyl group in the form of H2O. The interme-
diate (Scheme 1(4)) can be regarded as the product of partial
hydrogenation of the benzene ring, which is benecial to the
subsequent dehydroxylation due to the lower bond dissociation
energy.16,49 Therefore, the dehydroxylation reaction becomes
relatively easy to proceed. The obtained product (Scheme 1(5))
reacts with the adsorbed methyl or ethyl groups, leading to the
formation of primitive alkylphenol. The methyl group can be
formed form the demethylation step during guaiacol conver-
sion and the evolved from the ethanol medium.28 Subsequently,
the higher alkylphenols, including iso-propylphenols, tert-
butylphenols and neo-pentylphenols, are formed through
a series of consecutive substitution steps with methyl or ethyl
3.7 The mechanism of deoxygenation and alkylation
Two alternative mechanisms have been proposed to describe
deoxygenation of phenolics: (1)
a direct HDO produces
aromatics with water formation.11,13 (2) an indirect hydrogena-
tion of the benzene ring happens, followed by hydroxyl dehy-
dration to form
a
double bond and hydrogenated
hydrocarbons.15,20 No ring saturation product was detected in
this work, indicating that the direct HDO reaction occurred.
Besides, no phenol or catechol was detected in the product,
showing that the deoxygenation and the alkylation reactions
performed continuously.28 However, theoretical calculation
shows that the bond dissociation energy of Caromatic–OH bond is
about 84 kJ molꢁ1 higher than that of the Caliphatic–OH.16 Thus,
the direct deoxygenation path may involve a partial hydroge-
nation of the benzene ring. A similar reaction pathway was
proposed for gaseous phase HDO of phenol on a CoMo catalyst
and gaseous phase HDO of anisole over a Pt/H-beta catalyst.16,49
The conversion and product distribution from the different
reactant (catechol, 2-ethoxyphenol, phenol and anisole) indi-
cated that catechol and 2-ethoxyphenol were likely the inter-
mediates of the guaiacol conversion over the H2WO4 catalyst. To
sum up, the possible deoxygenation and alkylation pathways of
guaiacol conversion are proposed as Scheme 1. With the catal-
ysis of H2WO4, the demethylation of guaiacol takes place
primarily. The methoxy group (–OMe) in guaiacol forms
a hydrogen bond with the acid site over the H2WO4 catalyst, and
adsorbs on the H2WO4 catalyst surface.13,14 The guaiacol mole-
cule possesses three types of oxygen–carbon bonds: C(sp3)–OAr,
C(sp2)–OMe and C(sp2)–OH with the bond energies about 247,
356 and 414 kJ molꢁ1 respectively.13 The catalytic cleavage of the
C(sp3)–OAr is much easier. Hence, the demethylation step
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groups supplied by the ethanol medium on the primitive deposition on the catalyst surface, the dehydration and partial
alkylphenol. Finally, the obtained alkylphenol desorbs from the reduction of the catalyst itself.
catalyst surface. Notably, no completely deoxygenated
compound was detected, because the alkylphenols are more
difficult to achieve complete deoxygenation than phenol.13,52
Conflicts of interest
Throughout the reaction, the supercritical ethanol not only There is no conicts to declare.
severs as a hydrogen-donor for HDO of guaiacol, but plays a role
of capping agent to stabilize the highly active phenolic inter-
mediates by the consecutive alkylation steps.32,33
Acknowledgements
The nancial support from the Ministry of Science and Tech-
nology of China (2011DFA41000) and the National Natural
Science Foundation of China (21336008, 21808163 and
21690083) is gratefully acknowledged.
3.8 The reason of deactivation
The XRD proles (Fig. 1) proved that the H2WO4 catalyst
dehydrated into WO3, and further reduced into W18O49 or
WO2.72 phase. The TG proles (Fig. 2) also conrmed the
H2WO4 dehydration process. The transfoꢀrmation of H2WO4 to
WO3 in a temperature range of 200–310 C have also been re-
ported.39,40,53 The Raman spectra (Fig. S1†) and TG proles
(Fig. 2) veried that the accumulation of carbon on the spent
H2WO4 catalyst was gradually increased with the increase of
number of cycle time. Further, we concluded from pyridine FT-
IR spectra (Fig. 3) and NH3-TPD proles (Fig. 4) that with the
increase of number of cycle time, the acid sites over the
H2WO4 catalyst gradually decrease and the Brønsted acid sites
were completely disappeared aer third run. Notably, the
decrease in the activity of the third run was more obvious than
that of the second run. This may be due to the partial reduc-
tion of the spent catalyst during the recycle tests and the
gradual accumulation of carbon deposited on the spent cata-
lyst. The carbon covered the active sites on the catalyst surface,
causing a further decrease in catalytic activity. Overall, the
carbon deposition on the catalyst surface, the dehydration and
partial reduction of the catalyst itself were responsible for the
decay of the catalyst. However, the technique that synthesis
H2WO4 through WO3 has already been realized in the recovery
of waste tungsten materials.54,55
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