Selective catalytic conversion of guaiacol to phenols over a molybdenum carbide catalyst

Article abstract of DOI:10.1039/c5cc01900a

An activated carbon supported α-molybdenum carbide catalyst (α-MoC_1-x/AC) showed remarkable activity in the selective deoxygenation of guaiacol to substituted mono-phenols in low carbon number alcohol solvents. Combined selectivities of up to 85% for phenol and alkylphenols were obtained at 340°C for α-MoC_1-x/AC at 87% conversion in supercritical ethanol. The reaction occurs via consecutive demethylation followed by a dehydroxylation route instead of a direct demethoxygenation pathway.

Full text of DOI:10.1039/c5cc01900a

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Selective Catalytic Conversion of Guaiacol to Phenols over Molybdenum
Carbide Catalyst
Cite this: DOI: 10.1039/x0xx00000x
↓
↓
Rui Ma , Kai Cui , Le Yang, Xiaolei Ma, Yongdan Li*
Received 00th January 2012,
Accepted 00th January 2012
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin),
Tianjin Key Laboratory of Applied Catalysis Science and Technology, State Key
Laboratory of Chemical Engineering (Tianjin University), School of Chemical
Engineering, Tianjin University, Tianjin 300072, (China)
DOI: 10.1039/x0xx00000x
www.rsc.org/
↓
These authors contribute equally to this work.
*
ydli@tju.edu.cn
An activated carbon supported αꢀmolybdenum carbide catalyst
αꢀMo1ꢀxC/AC) showed remarkable activity on the selective Mo
deoxygenation of guaiacol to substituted monoꢀphenols in low selectivity in the HDO of guaiacol.
carbon number alcohol solvents. Combined selectivities of up to obtained phenol and methylated phenols from guaiacol over a
Recently, molybdenumꢀbased hydrotreating catalysts e. g., Mo
2
C,
(
2
N, MoS and MoO etc. have shown good activity and
2
3
7ꢀ10
Jongerius and coworkers
o
8
8
5% to phenol and alkylphenols were obtained at 340 C for αꢀ supported Mo
Mo1ꢀxC/AC at 87% conversion in supercritical ethanol. The However, the hydrogenation of the aromatic ring also happened.
reaction happens via a consecutive demethylation followed by Similarly, Prasomsri et al. showed that MoO is active and selective
dehydroxylation route instead of a direct demethoxygenation for a direct C–O bond cleavage of guaiacol in a vapourꢀphase over a
2
C catalyst, with a total selectivity of 69% phenolics.
3
pathway.
packedꢀbed flow reactor, producing phenol and hydrocarbons with
o
selectivities of 29.3% and 53.5%, respectively, at 320 C. The
Lignin is becoming an important renewable feedstock for the stability of their catalyst was poor at further higher temperatures i.e.
o
9a
sustainable production of fuels and valueꢀadded chemicals and has > 350 C. Veryasov et al. reported that a novel urchinꢀlike
1
received considerable attention in the last decade. We recently crystalline MoS showed a high activity on the HDO of liquefied
2
reported Kraft lignin can be completely ethanolysed to small wood sample, where the oxygen content decreased from 43.3% to
o
10
molecules in supercritical ethanol over an αꢀMo1ꢀxC/AC catalyst in 8.2% at 300 C under 8.0 MPa H pressure.
2
[
2]
an inert atmosphere. However, the reaction pathways in the
Here, we report the catalytic deoxygenation of guaiacol to monoꢀ
ethanolysis is complex and difficult to be followed. Furthermore, in oxygenated phenols over αꢀMo1ꢀxC/AC catalyst with high selectivity
the utilization of lignin derived compounds, the high oxygen content in popular solvents without the hydrogenation of benzene rings
2, 3
and poor chemical stability are often the limiting factors.
(Scheme 1).
Therefore, obtaining information from the reactions of model
compounds is often meaningful to the utilization of the lignin
4
derived compounds.
Guaiacol, which contains simultaneously hydroxyl and methoxyl
functional groups, has been typically used as a model compound in
5ꢀ8
the study of lignin valorization. Early works indicated that sulfided
CoMo and NiMo catalysts and supported metal hydrogenation
catalysts (Ni, Ru, Pt and Pd) have activity in the
hydrodeoxygenation (HDO) of guaiacol under high H pressure and Scheme 1: Reaction pathway for the deoxygenation of guaiacol.
2
5ꢀ7
temperature.
However, aromatic ring hydrogenation happened
simultaneously and the employment of the noble metal–based
catalysts could significantly raise cost. For the sulfided catalysts,
The catalyst used in this work has a Mo content of 30 wt %, a
2
ꢀ1
5
BETꢀspecific surface area of 749 m g , micropore and mesopore
continuous addition of sulfur is required in the reactant stream.
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DOI: 10.1039/C C0190
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3
ꢀ1
volumes 0.12 and 0.59 cm g , respectively, and a mean pore
diameter of 7.9 nm. The XRD and TEM analysis data were
14
Ethanol
Ethanol
Ethanol
Ethanol
340
340
340
340
1
4
4
4
97
87
90
91
36
86
84
85
12 28 40
15 31 35
17 30 37
14 32 39
7
3
2
3
10
3
4
h
1
5
i
j
1
6
2
presented in our previous work. The catalytic performance of the αꢀ
17
3
Mo1ꢀxC/AC catalyst was examined in a number of solvents, i.e.
a
Reaction conditions: guaiacol (2.0 g), catalyst (0.5 g), solvent (60
methanol, ethanol, isopropanol, tetralin, nꢀhexane and water at 340
o
ml), initial N
4 h, stirred at 400 rpm, the yields were calculated by mole. for
methanol, tetralin, nꢀhexane solvent, R = CH ; for ethanol, R =
2
pressure at room temperature 0 MPa (gauge), 340 C,
o
b
C under 0 MPa N (gauge and being the initial pressure at room
2
3
temperature) (Table 1). In the organic solvents, i.e. methanol,
ethanol, isopropanol, tetralin and nꢀhexane, monoꢀoxygenated
products such as phenol (2), alkyl phenol (3) and dialkyl phenol (4)
were obtained as the major products with a high selectivity, while
small amounts of transetherification products (5) and alkyl guaiacols
c
d
CH
3 2 3 2
CH , for isopropanol, R = (CH ) CH. M: Mass balance. C:
e f g
Conversion of 1. S: Selectivity. R:H. the catalyst used in entry 6
was MoO /AC. First reuse, Second reuse, Third reuse.
2
h
i
j
Interestingly, αꢀMo1ꢀxC/AC performs high conversions of
molecule 1 in alcohol solvents (methanol, ethanol, and isopropanol)
due to the similar nature of the hydrogen donor and the high
(
6) were also measured in the product. The substituent group in the
oꢀposition of the phenolic hydroxyl group depends on the solvent,
1
1
diffusivity of the molecules in the supercritical conditions. For
example, guaiacol was converted to phenolic compounds with a
viz. CH for methanol, tetralin and nꢀhexane, CH CH for ethanol
3
2
3
and (CH ) CH for isopropanol. Neither full deoxygenation products
3
2
2
o
conversion of 87% at 340 C for 4 h in ethanol, while it afforded a
(benzene, toluene) nor ringꢀhydrogenation products (cyclohexane,
lower conversion of 53% in tetralin (entry 3 Table 1), which was
also widely used as a hydrogenꢀdonor solvent. This may be due to
the weak polarity of tetralin compared to the ethanol solvent. For the
alkane solvent, i.e. nꢀhexane (entry 4), a conversion of only 43% was
achieved. Meanwhile, the selectivity of alkyl phenols obtained in the
organic solvents shows a trend of in alcolhols (methanol, ethanol,
isopropanol) > in nꢀhexane > in tetralin. In a sharp contrast, catechol,
which was not detected in the products with organic solvents, was
formed with a selectivity of 94% in water (entry 5). The structural
cyclohexene, cyclohexanone, etc.) were observed in this work,
indicating that the αꢀMo1ꢀxC/AC catalyst has excellent selectivity to
partially deoxygenated compounds. Besides, ESIꢀMS analysis of the
reaction mixture obtained from the Entry 7 revealed some peaks of
higher molecular weight in the product mixtures (Figure S2), but
these products could not be identified. These results showed great
difference from those reported in literature, where fully
deoxygenated products and hydrogenation products were obtained
over conventional HDO catalysts (CoMo and NiMo catalysts etc.)
and the transetherification products were produced as the main
transformation of Mo1ꢀxC into MoO in the high temperature water
2
may contribute to this result (Fig. S1). To verify this assumption, a
5
ꢀ10
products with γꢀAl O as the catalyst.
2
3
control experiment was carried out using MoO /AC as the catalyst
2
(
entry 6). It turned out that the product distribution is similar with
a
Table 1：Deoxygenation of guaiacol (1) in different solvent .
that achieved with Mo1ꢀxC/AC (entry 5).
Reaction temperature has a profound effect on the conversion and
product distribution. In the investigated temperature range (280ꢀ340
o
C), an increase in the reaction temperature results in an
improvement of the conversion of 1 from 22% to 87% and a higher
selectivity towards total phenolic compounds (2, 3, 4) from 63% to
8
5% in ethanol solvent, which was much higher than that typically
obtained with CoMo/Al O , Mo C/CNF and Mo N/AC as catalysts
2
3
2
2
in similar solvent and temperature but with hydrogen in the
[
7, 8]
atmosphere.
In those cases, ringꢀhydrogenation happened as an
important side reaction. Furthermore, with the increase of
o
temperature from 280 to 320 C, the 4’s selectivity slightly rises
c
d
e
Entry
Solvent
T/
C
340
t/
h
4
4
4
4
4
4
4
4
4
4
5
3
2
M
C
S
/ %
from 34% to 41%. However, the further increase of reaction
temperature leads to a slightly decrease of 4’s selectivity.
o
/ % / %
2
3
4
5
0
2
0
0
94f
93
3
3
5
7
3
6
7
7
6
4
0
0
4
6
9
1
2
3
4
5
Methanol
Isopropanol 340
Tetralin
nꢀhexane
Water
91
96
95
90
98
97
92
91
88
82
93
93
95
85
84
53
43
36
33
87
56
35
22
89
71
44
16 32 36
15 34 38
Data in entry 11 to entry 14 showed the effect of reaction time on
the reaction. The conversion increases with the increase of the
reaction time and the highest conversion reached 89% after 5 h,
while the selectivity towards total phenolic compounds (2, 3, 4)
showed a slightly increase from 80% to 86%. Compared to the slight
change of the selectivity of phenolic compounds, the selectivities of
340
340
340
340
340
320
300
280
340
340
340
84
31 55
5
5
5
0
0
0
0
f
0
0
g
6
Water
7
8
9
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
15 32 38
12 29 41
10 26 38
1
1
1
1
0
1
2
3
7
22 34
12 5 and 6 decreased obviously with the increase of the reaction time.
14 33 39
14 32 38
14 30 37
4
5
8
4
6
2
| J. Name., 2012, 00, 1-3
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DOI: 10.1039/C5CC01900A
reaction pathway of guaiacol is that catechol is a primary product
followed by the hydroxyl group removal to form phenol (Scheme 1).
However, it was reported that direct demethoxygenation to phenol
was the reaction pathway of the guaiacol conversion over the Mo C
2
8
catalyst because no catechol was detected. A reasonable explanation
is that the absence of catechol may be attributed to a full conversion
of consecutive reactions (entry 18).
In summary, the αꢀMo1ꢀxC/AC catalyst is an effective and durable
catalyst for the deoxygenation of guaiacol to phenols in alcohol
solvents under an inert atmosphere. Higher temperature favours the
conversion and the selectivity to phenols. The reactions take place
via demethylation at the methoxy group followed by deoxygenation
and transalkylation. Compared to the hydrogenation catalysts, no
Scheme 2. Deoxygenation of ligninꢀderived model compounds on αꢀ complete deoxygenation and benzene ringꢀhydrogenation products
Mo1ꢀxC/AC catalyst in ethanol. (Reaction conditions: see entry 7 in
Table 1)
are produced, which make it an excellent catalyst for the production
of phenolic compounds from lignin depolymerization stream.
This work was financially supported by the National Science
Foundation of China (21336008) and the Ministry of Science and
Technology of China (2011DFA41000).
o
The reusability of the catalyst was tested for the reaction at 340 C
for 4 h in the ethanol solvent (Table 1, entry 15ꢀ17). After each run,
the catalyst was recovered with a centrifugation technique and used
directly in the next run without any treatment. The recovery of each
catalyst after every run was close to 100% while the particle size of
recovered catalyst decreased slightly (Table S1). It is obvious that
similar conversion and selectivity towards the products were
achieved in the three cycles. The XRD patterns of the recovered
Mo1ꢀxC catalysts showed that the Mo1ꢀxC nanoparticles still retained
their small sizes (< 5 nm, calculated by Scherrer Equation) (Fig. S1),
thus indicating that the catalysts can be reused at least for 3 times
without noticeable loss of activity.
Notes and references
1
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In addition to guaiacol, the reactions with catechol, phenol and
anisole as the reactant were examined with ethanol as the solvent. As
shown in Scheme 2, full conversion of catechol was achieved after 4
o
h at 340 C over the catalyst (Entry 18). Phenol, ethyl phenol, and
2
012, 5, 6383; (e) M. R. Sturgeon, et al., Green Chem. 2014,
6, 824.
diethyl phenol were formed as the major product with selectivities of
1
1
4%, 31% and 39%, respectively. This result is in accordance with
that obtained from guaiacol as the reactant under the same condition
Table 1, entry 7). This indicates that catechol was formed as an
4
(a) C. Zhao, J. A. Lercher, Angew. Chem. 2011, 124, 6037; (b)
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5
15, 249; (c) M. V. Galkin, S. Sawadjoon, V. Rohde, M.
intermediate in guaiacol deoxygenation (Scheme 1). In contrast, the
reaction with anisole as the reactant (Scheme 2, entry 19) under the
same conditions only showed a conversion of 26% with phenol as
the major product with 65% selectivity. In the reaction with phenol
as the reactant (Figure 1, entry 20), ethyl phenol and diethyl phenol
were found as the major products with selectivities of 58% and 40 %,
respectively, but the conversion was only 46%. The product
distribution from the phenol conversion was very different from that
with guaiacol as the feedstock, indicating that phenol should not be
the intermediate in the deoxygenation of guaiacol. Furthermore,
methanol, which was considered as one of the product of the
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deoxygenation of guaiacol. Moreover, gas products of the guaiacol
conversion were identified with a gas mass spectrometer and
methane was indeed produced. Therefore, it can be deduced that the
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The maximum reaction pressures are 16.8 MPa, 13.9 MPa,
o
1
4.7 MPa for methanol, ethanol, and isopropanol at 340 C,
respectively, which are beyond their critical point.
o
1
3
The reaction pressure is 0.8 MPa for tetralin at 340 C.
4
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