Tungsten Carbide: A Remarkably Efficient Catalyst for the Selective Cleavage of Lignin C?O Bonds

Article abstract of DOI:10.1002/cssc.201600901

A remarkably effective method for the chemoselective cleavage of the C?O bonds of typical β-O-4 model compounds and the deconstruction of lignin feedstock was developed by using tungsten carbide as the catalyst. High yields of C?O cleavage products (up to 96.8 %) from model compounds and liquid oils (up to 70.7 %) from lignin feedstock were obtained under low hydrogen pressure (0.69 MPa) in methanol. The conversion efficiency was determined to a large extent by solvent effects and was also affected by both the electronic and steric effects of the lignin model compounds. In situ W₂C/activated carbon (AC)-catalyzed hydrogen transfer from methanol to the substrate was proposed to be responsible for the high performance in methanol solvent. The conversion of 2-(2-methoxyphenoxy)-1-phenylethanol showed that the catalyst could be reused five times without a significant loss in activity for C?O bond cleavage, whereas the selectivity to value-added styrene increased markedly owing to partial oxidation of the W₂C phase according to X-ray diffraction, Raman spectroscopy, and transmission electron microscopy characterization. 2 D-HSQC-NMR spectroscopy analysis showed that W₂C/AC exhibited high activity not only for β-O-4 cleavage but also for the deconstruction of more resistant α-O-4 and β-β linkages, so that a high yield of liquid oil was obtained from lignin. Corn stalk lignin was more liable to be depolymerized than birch lignin owing to its loosened structure (scanning electron microscopy results), larger surface area (BET results), and lower molecular weight (gel-permeation chromatography results), whereas its liquid oil composition was more complicated than that of birch wood lignin in that the former lignin contained more p-hydroxyphenyl units and the former contained noncanonical units.

Full text of DOI:10.1002/cssc.201600901

DOI: 10.1002/cssc.201600901
Full Papers
Tungsten Carbide: A Remarkably Efficient Catalyst for the
Selective Cleavage of Lignin CÀO Bonds
Haiwei Guo+,[a, b] Bo Zhang+,[a] Changzhi Li,*[a] Chang Peng,[a, b] Tao Dai,[a, b] Haibo Xie,[c]
Aiqin Wang,[a] and Tao Zhang*[a]
A remarkably effective method for the chemoselective cleav-
age of the CÀO bonds of typical b-O-4 model compounds and
the deconstruction of lignin feedstock was developed by using
tungsten carbide as the catalyst. High yields of CÀO cleavage
products (up to 96.8%) from model compounds and liquid oils
(up to 70.7%) from lignin feedstock were obtained under low
hydrogen pressure (0.69 MPa) in methanol. The conversion effi-
ciency was determined to a large extent by solvent effects and
was also affected by both the electronic and steric effects of
the lignin model compounds. In situ W2C/activated carbon
(AC)-catalyzed hydrogen transfer from methanol to the sub-
strate was proposed to be responsible for the high per-
formance in methanol solvent. The conversion of 2-(2-meth-
oxyphenoxy)-1-phenylethanol showed that the catalyst could
rene increased markedly owing to partial oxidation of the W2C
phase according to X-ray diffraction, Raman spectroscopy, and
transmission electron microscopy characterization. 2D-HSQC-
NMR spectroscopy analysis showed that W2C/AC exhibited
high activity not only for b-O-4 cleavage but also for the de-
construction of more resistant a-O-4 and b-b linkages, so that
a high yield of liquid oil was obtained from lignin. Corn stalk
lignin was more liable to be depolymerized than birch lignin
owing to its loosened structure (scanning electron microscopy
results), larger surface area (BET results), and lower molecular
weight (gel-permeation chromatography results), whereas its
liquid oil composition was more complicated than that of
birch wood lignin in that the former lignin contained more
p-hydroxyphenyl units and the former contained noncanonical
be reused five times without a significant loss in activity for CÀ units.
O bond cleavage, whereas the selectivity to value-added sty-
Introduction
Lignocellulose is the most abundant form of biomass that has
the potential to serve as a sustainable feedstock to replace
fossil resources for the production of future chemicals and bio-
fuels.[1] As one of the three major components in lignocellu-
lose, lignin accounts for 10–35% by weight and up to 40% by
energy of lignocellulosic biomass, and the conversion of lignin
into value-added products will greatly improve the economics
of the overall biorefinery process.[2] However, in comparison
with the other two components (i.e., cellulose and hemicellu-
lose) of lignocellulosic materials, lignin is a more complicated
three-dimensional amorphous polymer consisting of methoxy-
lated phenylpropanoid units of various types and is relatively
intractable.[3] It is typically regarded as a low-value byproduct
in most current biorefinery processes. Considering the aromat-
ic features of lignin, recent advances have demonstrated the
potential for the conversion of lignin into a spectrum of aro-
matic compounds by catalysis, and these aromatics may serve
as alternatives to traditional aromatic compounds obtained
from fossil resources.[2,4]
Among the various reported strategies for the chemical dis-
assembly of lignin,[5] one of the most popular and efficient ap-
proaches is hydrocracking by using supported noble metals
such as Ru,[6] Pd,[7] Pt,[8] Rh,[9] Au,[10] and Re[11] as catalysts. In
lignin systems, the aryl CÀO bonds are not very active, and
high temperatures and high hydrogen pressures are usually re-
quired for their cleavage. The harsh reaction conditions, in
turn, lead to undesired concurrent hydrogenation of the aro-
matic rings and cleavage of aliphatic CÀO bonds, which afford
a mixture of low depolymerized lignin products, phenols, cy-
clanes, and some other repolymerization products. A major
challenge is the selective cleavage of the aryl CÀO bond rather
than overhydrogenation of the aromatic ring.[12] Moreover,
owing to the scarcity and high costs of noble metals, it is
highly desirable to develop cheap and efficient catalysts for
the production of aromatics from lignin.
[a] H. Guo,+ Dr. B. Zhang,+ Dr. C. Li, C. Peng, T. Dai, Prof. A. Wang,
Prof. T. Zhang
State Key Laboratory of Catalysis
Dalian Institute of Chemical Physics, Chinese Academy of Sciences
Dalian 116023 (China)
E-mail: licz@dicp.ac.cn
taozhang@dicp.ac.cn
[b] H. Guo,+ C. Peng, T. Dai
University of Chinese Academy of Sciences
Beijing, 100049 (China)
[c] Prof. H. Xie
Department of Polymeric Materials & Engineering,
College of Materials & Metallurgy
Guizhou University
Guiyang, 550025 (China)
[+] These authors contributed equally to this work.
Supporting Information for this article can be found under:
http://dx.doi.org/10.1002/cssc.201600901.
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Carbides can be formed by carburizing metals from
groups 4–6, which are much cheaper than noble metals, but
they show catalytic performances similar to those of platinum-
group metals in a variety of reactions involving hydrogena-
tion.[13] Inspired by this property, we first reported that nickel
promoted tungsten carbide supported on active carbon
(Ni-W2C/AC) could replace noble metals to catalyze selective
CÀC cleavage of cellulose to ethylene glycol through consecu-
tive retro-aldol and hydrogenation reactions.[14] Building on the
above results, we further found that Ni-W2C/AC could catalyze
the direct hydrocracking of raw woody biomass into two
groups of chemicals, namely, diols from the carbohydrate frac-
tion and monophenols from the lignin fraction.[15] Though this
bimetallic catalyst exhibited competitive activity relative to
that of noble metal catalysts, the precise role of tungsten car-
bide in the hydrocracking of lignin is still unclear, particularly
the pathway for cleavage of the CÀO bond in lignin. In this
paper, starting from 2-(2-methoxyphenoxy)-1-phenylethanol,
a typical lignin model compound, we report the selective
cleavage of the CÀO bonds in b-O-4 lignin model compounds
by using tungsten carbide as the catalyst without the addition
of a second metal and extension to the catalytic conversion of
realistic lignin. This catalytic process features low hydrogen
pressure and high aromatic selectivity without destroying the
aromatic rings in the cleavage of the CÀO bonds in both lignin
feedstocks and model compounds.
Table 1. Hydrogenolysis of 2-(2-methoxyphenoxy)-1-phenylethanol cata-
lyzed by W2C/AC under different reaction conditions.[a]
Entry
Temp. Conv.[b] Yield [%][b]
[8C]
[%]
ethylbenzene styrene E+S[c] guaiacol
1
2
3
4
200
220
240
260
280
260
260
240
23.3
2.3
3.1
16.0
38.5
59.8
8.5
18.3
41.6
83.4
96.8
93.5
92.7
18.5
77.8
17.4
39.8
83.7
93.0
89.5
89.2
6.9
54.2
97.3
99.8
100
100
43.9
100
23.6
88.4
91.9
46.5
7.1
5
1.6
6[d]
7[e]
8[f]
46.2
11.4
24.3
53.5
75.1
[a] Reactions conditions: unless otherwise specified, a stainless-steel auto-
clave was charged with the substrate (100 mg), 30 wt% W2C/AC
(100 mg), and methanol (30 mL), and the vessel was pressurized with H2
(0.69 MPa); the reactions were performed at different temperatures for
2 h. [b] The conversion of 2-(2-methoxyphenoxy)-1-phenylethanol and
yields of the products were determined by GC-FID with mesitylene as an
internal standard. [c] E+S is the abbreviation of ethylbenzene plus sty-
rene. [d] 30 wt% W2C/AC (50 mg) was used. [e] No catalyst was em-
ployed; considerable amounts of etherification products, dipolymers, and
other byproducts were detected. [f] H2 (4 MPa) was the initial pressure.
the lowest yield of styrene (1.6%) were obtained (Table 1,
entry 5). Apparently, the selectivity to ethylbenzene/styrene
was thoroughly reversed relative to that observed at low tem-
peratures. The shift in selectivity from styrene to ethylbenzene
indicates that higher temperatures favor hydrogenation of the
alkene group in styrene. Increasing the catalyst amount
(Table 1, entry 4 vs. 6) and the hydrogen pressure (Table 1,
entry 3 vs. 8) also benefitted the hydrogenation reaction. On
the whole, the yields of the CÀO bond-cleavage products (E+
S and guaiacol) were higher than 83% with almost complete
conversion (>97%) if the temperature was over 2408C. In
sharp comparison, thermal reaction without a catalyst at
2608C only afforded ethylbenzene, styrene, and guaiacol in
yields of 7.1, 11.4, and 6.9%, respectively (Table 1, entry 7). In
the reactions performed at low temperatures (Table 1, en-
tries 2, 3, and 8) or without a catalyst (Table 1, entry 7), com-
pound 1 underwent dihydroxylation, etherification, or deme-
thoxylation to produce 1-methoxy-2-phenethoxybenzene,
1-methoxy-2-(2-methoxy-2-phenylethoxy)benzene, or 2-phen-
oxy-1-phenylethanol as the major byproduct. Notably, in all re-
actions, no cycloalkane product was detected, which suggests
that W2C holds promise as a substitute for noble metals in the
hydrocracking of lignin b-O-4 bonds without destroying the ar-
omatic rings. In contrast, hydrogenolysis of aryl ethers with
noble metal catalysts typically produces a mixture of arenes,
cycloalkanes, and phenols.[16]
Results and Discussion
Preliminary study with b-O-4 model compound
2-(2-Methoxyphenoxy)-1-phenylethanol (1), a common b-O-4
dimeric model compound of lignin, was first investigated to
explore the activity of W2C/AC for the cleavage of CÀO bonds.
It was found that in all trials (Table 1), compound 1 was con-
verted into styrene, ethylbenzene, and guaiacol as the domi-
nant products in methanol solvent under a low hydrogen pres-
sure (Scheme 1). The catalytic activity and the selectivity to the
products highly depended on the reaction temperature. A low
temperature (2008C) led to low conversion (23.3%) with sty-
rene and guaiacol as the primary products (Table 1, entry 1),
and the yield of ethylbenzene was only 2.3%. Upon increasing
the reaction temperature (Table 1, entries 1–5), not only the
conversion of the substrate but also the yield of ethylbenzene
remarkably increased. For instance, compound 1 was nearly
completely consumed at 2608C in 2 h and afforded ethylben-
zene and guaiacol in yields up to 88.4 and 93.0%, respectively
(Table 1, entry 4). The total yield of ethylbenzene plus styrene
(E+S) was 96.8% at this temperature. Upon performing the re-
action at 2808C, the highest yield of ethylbenzene (91.9%) and
Scheme 1. CÀO bonds cleavage of 2-(2-methoxyphenoxy)-1-phenylethanol catalyzed by W2C/AC.
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Effects of solvent
As determined by GC with a flame-ionization detector (FID)
and LC–UV results, no byproducts in the water reaction system
were detected other than butane-1,3-diyldibenzene (2.7%, sty-
rene dipolymerization product), 1-methoxy-2-phenethoxyben-
zene (8.7%, dehydroxylation product of compound 1), and 2-
phenoxy-1-phenylethanol (trace, methoxy group release prod-
uct of compound 1). The poorest result can probably be attrib-
uted to the conversion of the initially formed intermediates,
such as styrene, into insoluble products by polymerization.
It was interesting to find that small-molecule alcohols could
significantly promote the conversion efficiency, which was
evinced by a 75.5% yield of guaiacol, associated with a 61.2%
total yield of ethylbenzene and styrene if ethanol was used as
the solvent (Table 2, entry 5). Furthermore, the yield of guaiacol
was increased to 93.0% and the total yield of ethylbenzene
and styrene was up to 96.8% if methanol was employed
(Table 2, entry 6). It was previously reported that Cu-doped
porous metal oxides catalyzed the reforming of methanol
above the supercritical temperature (239.68C) to produce hy-
drogen.[18] Wang and Rinaldi also reported the hydrogen trans-
fer of 2-propanol with Raney Ni for the upgrade of bio-oils.[5d]
We therefore envisaged the same scenario in our experiments.
This speculation gained support from the following experi-
ment. A control reaction with W2C as the catalyst in methanol
in the absence of the substrate was conducted at 2608C for
2 h, and 0.69 MPa argon was used instead of H2 as the initial
atmosphere. After the reaction, the gases were captured and
analyzed by GC with a thermal conductivity detector (TCD). It
was interesting to find that besides the dominant argon com-
ponent, minor components of H2 and CO were detected (Fig-
ure S1), whereas these two gases were not formed in the ab-
sence of the catalyst. Thus, hydrogen must have originated
from methanol by the reforming reaction as proposed in
Ref. [18b]. The resulting hydrogen source provides the reduc-
ing equivalents necessary both to cleave the aromatic ethers
and to reduce styrene further. This catalytic disassembly of
compound 1 through hydrogen transfer from methanol was
also confirmed by a reaction performed without any hydrogen
gas (Table 2, entry 7). In this trial, 71.1% styrene was accompa-
nied by 13.5% ethylbenzene (styrene hydrogenation product)
and 98.7% guaiacol. Notably, styrene is much more valuable
than ethylbenzene, as it is a precursor to many commercially
significant products, including polystyrene, acrylonitrile buta-
diene styrene (ABS), styrene–butadiene rubber, styrene–
butadiene latex, styrene–acrylonitrile resin, and unsaturated
polyesters used in rubbers, plastics, insulation, and fiber-
glass.[19] Styrene is mainly produced by the dehydrogenation of
ethylbenzene in the presence of an iron oxide catalyst at high
temperatures (600–6508C), but this process suffers from high
energy consumption and a low yield of styrene owing to ther-
modynamic limitations of the endothermic reaction and depo-
sition of dehydrogenated carbonaceous species.[20] The above
result provides a potential route to value-added styrene from
lignin.
The influence of solvent on the performance of chemical reac-
tions is basically determined by the nature of the solvent and
its dissolution ability. To realize a higher conversion for the re-
action, several solvents such as THF, toluene, dioxane, and
small-molecule alcohols, which were reported to solubilize lig-
nin,[16c,17] were screened for W2C/AC-catalyzed b-O-4 cleavage.
Importantly, THF, toluene, and dioxane are usually not pre-
ferred solvents for hydrogenation reactions, as they are liable
to be destroyed. One of the advantages in this catalytic system
is that the moderate catalytic activity of W2C/AC and the low
hydrogen pressure could leave these solvents untouched.
As shown in Table 2, although over 99% conversion was
achieved in all cases by using the various solvents, under the
identified conditions the product distribution was sensitive to
the solvent selected. The results show that the yields of guaia-
col (62.8–65.1%; Table 2, entries 1–3) were similar in the cases
Table 2. W2C/AC-catalyzed hydrogenolysis of 2-(2-methoxyphenoxy)-1-
phenylethanol in different solvents.[a]
Entry
Solvent
Conv. Yield [%]
[%]
ethylbenzene styrene E+S guaiacol
1
THF
100
99.5
15.6
4.1
35.0
31.2
21.7
1.0
22.0
8.5
50.6
35.3
58.9
1.8
61.2
96.8
84.6
28.1
62.8
65.1
64.7
51.8
75.5
93.0
98.7
29.7
2
3
4
dioxane
toluene
H2O
100
100
100
37.2
0.8
39.2
5
6
7[b]
8[c]
ethanol
methanol
methanol 100
methanol 34.8
99.8 88.4
13.5
3.1
71.1
25.0
[a] Reaction conditions: unless otherwise specified, a stainless-steel auto-
clave was charged with the substrate (100 mg), 30 wt% W2C/AC
(100 mg), and solvent (30 mL), and the reactions were conducted under
an atmosphere of H2 (0.69 MPa) at 2608C for 2 h. [b] The initial atmos-
phere was argon (0.69 MPa) instead of H2. [c] WO3 with an equal amount
of tungsten was used as the catalyst; the reaction time was 2 h, and
other conditions were the same as those outlined in entry 6.
of THF, toluene, and dioxane; however, the yields of ethylben-
zene and styrene were variously dependent on the nature of
the solvent. Ethylbenzene was provided in lower yields in THF
and dioxane (15.6 and 4.1%, respectively) than in toluene
(37.2%), which implied that toluene was the preferred solvent
for the styrene hydrogenation reaction. Nevertheless, severe
side reactions occurred in the above three organic solvents,
which resulted in the production of large amount of byprod-
ucts, including 2-phenylacetaldehyde, (E,Z)-(2-methoxyvinyl)-
benzene, 2-benzyl-1,3-dioxolane, 1-methoxy-2-phenethoxy-
benzene, and (E,Z)-1-methoxy-2-(styryloxy)benzene, as well as
some unknown compounds (Figures S2–S4 in the Supporting
Information), besides the target products. In chemical reac-
tions, water is often chosen as a solvent because of its low
cost, availability, and environmentally benignity, and we thus
tested the reaction in water as the solvent. Unfortunately, this
reaction only afforded trace amounts of ethylbenzene and sty-
rene, as well as the lowest yield of guaiacol (Table 2, entry 4).
One of the advantages of using methanol as the solvent
rather than ethanol is that cheaper methanol gives higher
target product selectivity. Upon using ethanol as the solvent,
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a considerable portion of byproducts, such as n-butylbenzene,
1-ethoxyethylbenzene, 2-ethoxyethylbenzene, 2-ethoxyphenol,
1-ethoxy-3-butenylbenzene, 2-phenylethanol, as well as
4-phenyl-2-butanol, were formed on the basis of GC–MS meas-
urements (Figure S5). These compounds might be generated
from the reactions of ethanol with intermediates/products by
esterification and/or alkylation.
not have a straightforward orderliness. Although we have no
clear explanation for the results, they might arise from a combi-
nation of stereochemical and electronic effects. The methoxy
groups in the R2 and R3 positions (i.e., substrates 3 and 6) pro-
vided the lowest yields of product C (2,6-dimethoxylphenol),
whereas 3-methoxybenzene-1,2-diol and other byproducts de-
rived from 2,6-dimethoxylphenol were detected, which implied
that this compound was unstable in our catalytic system.
Taking the results all together, W2C/AC in methanol showed ex-
cellent activity in the cleavage of various b-O-4 linkages.
Conversion of various lignin b-O-4 model compounds
A series of 2-aryloxy-1-arylethanols with different substituents
in the R1, R2, and R3 positions were employed as substrates
(Table 3) for cleavage of b-O-4 bonds catalyzed by W2C/AC
under the same conditions as those outlined in entry 4 of
Table 1. All these substrates (Table 3) have similar chemical
groups and are found in different natural lignins.[3] For exam-
ple, 2-(2-methoxyphenoxy)-1-phenylethanol (substrate 1 in
Catalyst stability and b-O-4 cleavage pathway
Catalyst reusability is one of the key factors affecting the po-
tential use of a catalytic process. Therefore, W2C/AC was recy-
cled over five runs in the conversion of model compound 1 to
test its stability. It can be seen in Figure 1 that the conversion
over the catalyst was maintained at 100% even in
the fifth run, and in parallel, the total yields of ethyl-
Table 3. W2C/AC-catalyzed CÀO bond cleavage of various 2-aryloxy-1-aryethanols.[a]
benzene plus styrene remained nearly constant over
the five runs (from 89.9% to 86.5%), which suggest-
ed that the activity for cleavage of b-O-4 linkages
was basically maintained, although the yield of
guaiacol exhibited a slight decrease owing to deme-
thoxylation and etherification side reactions, as trace
amounts of phenol and 1,2-dimethoxybenzene ap-
peared in the recycling runs. This result differs from
that outlined in our previous report for the Ni-W2C/
AC catalyzed processing of cellulose in water solven-
t.[14a,23] In that study, the catalyst was quickly deacti-
vated in repeated runs owing to leaching of active
species and oxidation of the W2C phase. It is interest-
ing to point out that the W loadings of the fresh cat-
alyst and the five-run catalyst were 30.2 and
27.0 wt%, respectively, according to inductively cou-
pled plasma (ICP) analysis. This observation indicated
Substrate
R1
R2
R3
Conv.
[%]
Yield [%]
A
B
C
1
2
3
4
5
6
H
H
H
OCH3
OCH3
OCH3
OCH3
H
OCH3
H
OCH3
OCH3
H
H
OCH3
H
H
99.8
100
100
100
100
100
88.4
44.1
65.2
56.5
71.5
82.8
8.5
93.0
70.7
54.1
86.2
78.0
48.4
15.3
17.8
0
0
0
OCH3
[a] Reaction conditions: a stainless-steel autoclave was charged with the substrate
(100 mg), 30 wt% W2C/AC (100 mg), and methanol (30 mL); the reactions were con-
ducted under an atmosphere of H2 (0.69 MPa) at 2608C for 2 h.
Table 3) is a major b-O-4 moiety in hardwoods such as birch.[21]
2-Phenoxy-1-phenylethanol (substrate 2) without methoxy
substitution on the phenyl ring is one component of grass
lignin.[22] After reaction, it was found that ethylbenzene and
phenol derivatives (compounds A and C in Table 3) were the
dominant products in all cases, which suggested that, in the
W2C/methanol system, the aryl CÀO bonds of the substrates
were selectively cleaved and the aryl rings were left intact in
the products. The conversions of all substrates were higher
than 99% in our reaction system; however, the yields of the ar-
omatics varied (Table 2). Product B was not detected for the
lignin model compounds with a methoxy group in the R1 posi-
tion (i.e., substrates 4–6), whereas those without a methoxy
group in the R1 position (i.e., substrates 1–3) afforded B in
yields of 8.5 to 17.8%, which suggested that a methoxy group
in the R1 position accelerated the hydrogenation of the alkene
in the para position. Electronic effects may have played an im-
portant role in this phenomenon, because product B with an
electron-donating methoxy group favored adsorption of the
catalyst. However, methoxy groups in the R2 and R3 positions
showed some diversity in hydrogenolysis behavior, which did
that leaching of the tungsten species was negligible in the
methanol system, which might be an important reason for the
good reusability of the catalyst for cleavage of b-O-4 linkages.
Figure 1. Recycling results for the W2C/AC-catalyzed CÀO bond cleavage of
2-(2-methoxyphenoxy)-1-phenylethanol. E+S is the abbreviation of the total
yield of ethylbenzene plus styrene.
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Scheme 2. Proposed reaction pathway for the W2C/AC-catalyzed deconstruction of 2-(2-methoxyphenoxy)-1-phenylethanol in alcohol solvent.
It was also found that the hydrogenation activity of the cata-
lyst gradually decreased in the recycling runs according to the
change in the yields of ethylbenzene (from 81.3% in the first
run to 28.3% in the fifth run) and styrene (from 8.5% in the
first run to 58.2% in the fifth run). Considering that styrene has
higher value than ethylbenzene, a greater amount of styrene
obtained in the recycling runs is beneficial for the production
of value-added chemicals from lignin. X-ray diffraction (XRD)
examinations demonstrated that the active W2C phase on the
after-run catalyst was slightly oxidized (shown in Figure S6).
The Raman spectra of the catalysts also confirmed this obser-
vation (see Figure S7 for the Raman spectra and discussion).
Transmission electron microscopy (TEM) images shown in Fig-
ure S8 indicate that large W2C particles (10–20 nm) were
formed on the AC supports for the fresh catalyst, whereas after
it was used over five runs, W2C nanoparticles were gradually
transformed into WOx nanofibers (10 nm width and 100 nm
long), which is consistent with the XRD and Raman spectrosco-
py results. We therefore envisage that the increased yield of
styrene can be attributed to partial oxidation of W2C, which re-
strains the hydrogenation ability of the catalyst. Comparison
experiments performed with the use of WO3 and W2C as the
catalysts for the hydrogenation of styrene confirmed this spec-
ulation. Under the reaction conditions of Table 3, W2C afforded
a much higher yield (72.4%) of ethylbenzene than WO3
(18.0%). Furthermore, the reaction of model compound 1 by
using WO3 (tungsten amount equal to that in W2C/AC) in place
of W2C/AC provided a much lower conversion of 34.8%, and
the yields of ethylbenzene, styrene, and guaiacol were 3.1,
25.0, and 29.7%, respectively (Table 2, entry 8 vs. 6). These re-
sults indicate that W2C plays a key role in cleavage of the b-O-
4 linkage in model compound 1.
thoxy-2-phenethoxybenzene, it was found that (E)- and (Z)-1-
methoxy-2-(styryloxy)benzene appeared in the GC–MS spec-
trum (Figure S9), whereas these two compounds could not be
detected at higher reaction temperatures; this implies that
they are active intermediates that readily undergo subsequent
hydrogenolysis to styrene and guaiacol. Taking into account all
intermediates and products formed in the above reactions
(Figures S5 and S9), the overall reaction pathway for the W2C-
catalyzed conversion of b-O-4 model compound 1 is proposed
in Scheme 2. It proceeds by initial dehydration of compound
1 to form intermediate 7 (including the E and Z isomers),
which is followed by hydrogenolysis of 7 to produce styrene
and guaiacol and the hydrogenation of styrene to obtain ethyl-
benzene. Hydrogenolysis of 7 is proposed as the major route
to the target products, because styrene and guaiacol are the
dominant products at 2008C (Table 1, entry 1). Hydrogenation
of 7 to 8 followed by hydrogenolysis of 8 to ethylbenzene and
guaiacol is a possible minor route, as compound 8 was detect-
ed. Moreover, compound 8 can produce 2-phenylethanol by
hydrolysis according to Figure S5. Reaction of compound 8
provides evidence for the proposed reaction pathway. Under
the conditions of Table 1, entry 4, compound 8 gives ethylben-
zene (3.0%), guaiacol (4.9%), and 2-phenylethanol (trace) as
the products with a conversion of 7.1%. Hydrolysis of 7 can
generate guaiacol and 9, which tends to isomerize into 19.
Further, direct hydrogenolysis of compound 1 occurs to pro-
duce guaiacol and 1-phenylethanol, and the latter is then
etherified to 17 (Table S1). Other side reactions for the mecha-
nism include polymerization of styrene to 11 and the deme-
thoxylation, hydrogenolysis, and etherification of compound
1 to give 12, 13, and 14, respectively.
To explore the principal conversion route of b-O-4 cleavage,
we reanalyzed the liquid products of the low-temperature ex-
periment in Table 1, entry 1. Besides the listed target products
and two byproducts, 2-phenoxy-1-phenylethanol and 1-me-
Conversion of realistic lignin feedstocks
The excellent results obtained with the model compounds mo-
tivated us to investigate the conversion of realistic lignin feed-
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Table 4. Quantified liquid components obtained from the W2C/AC-catalyzed valorization of BWL and CSL.[a]
Monomer produced
Unit type
Yield [wt%]
BWL
Select.[b] [wt%]
BWL
CSL
CSL
guaiacylpropanol
guaiacylethanol
syringylpropane
syringylpropene
guaiacylethane
3,5-dimethoxy-4-hydroxyphenyl ethanone
guaiacylpropane
2,6-dimethoxyphenol
2,6-dimethoxy-4-methylphenol
1,2,3-trimethoxy-5-methylbenzene
4-ethylphenol
G
G
Sy
Sy
G
Sy
G
Sy
Sy
Sy
H
H
0.46
0.68
0.92
0.77
0.11
0.22
trace
trace
trace
trace
0.06
0
0
11.5
17.1
23.1
19.3
5.3
0
0.24
1.08
0.45
0.39
0
0.68
0.71
0
2.6
12.1
5.0
4.3
0
7.6
7.9
0
2.7
trace
trace
trace
trace
1.4
0
0
0.41
0.17
0
0.05
1.57
0.87
2.40
8.96
70.7
12.7
4.5
1.8
0
0.6
17.5
9.6
26.8
4-methylphenol
benzene analogues
0
–
wax
p-coumarate
ferulate
ferulate
trace
0.41
0.37
0
trace
10.30
9.2
0
0
fatty acid methyl esters
methyl hydrocinnamate
methyl 3-(4-hydroxy-3-methoxyphenyl)propanoate
methyl 3-(4-hydroxy-3-methoxyphenyl)propanoic acid
total monomers
0
3.98
53.0
7.5
oil yield
monomers in oil
[a] Reaction condition: a stainless-steel autoclave (Parr, 100 mL) was charged with lignin (200 mg), catalyst (200 mg), and methanol (60 mL) with an initial
H2 pressure of 0.69 MPa and reacting at 2608C for 2 h. [b] Selectivity in total monomers.
stocks. One typical grass lignin extracted from corn stalk by
the alkaline twin-screw extrusion process (denoted CSL in
Table 4) and one typical wood lignin extracted from birch
wood by the organosolv process (denoted BWL in Table 4)
were selected as substrates to be depolymerized by the W2C/
AC catalyst. Table 4 illustrates the yields of the phenolic oils
and the monomer components of the oils. Interestingly, these
two lignins were effectively converted into liquid oil in high
yields of 70.7 and 53.0 wt%, respectively. The short growth
period of corn stalk produces lignin with a looser structure
[Figure S10, scanning electron microscopy (SEM) images] and
a lower molecular weight [Table 5, gel-permeation chromatog-
raphy (GPC) results] than birch wood lignin, and these are im-
Regarding the monomer composition, BWL yielded a broad
variety of phenolic compounds, including guaiacylpropanol
(11.5 wt% of the monomers), guaiacylethanol (17.1 wt%), syrin-
gylpropane (23.1 wt%), and syringylpropene (19.3 wt%) as the
main products, which are derived from lignin guaiacyl (G) and
syringyl (Sy) units. Other G and Sy monomers include guaiacyl-
ethane (5.3 wt%), 3,5-dimethoxy-4-hydroxyphenyl ethanone
(2.7 wt%), and guaiacylpropane (trace). 2,6-Dimethoxyphenol,
2,6-dimethoxy-4-methylphenol, and 1,2,3-trimethoxy-5-methyl-
benzene are also derived from Sy-type precursors. In addition,
a small amount of the p-hydroxyphenyl (H) unit 4-ethylphenol
(1.4 wt% of the monomers) was detected in the liquid oil. Typi-
cally, hardwood lignin belongs to type G–Sy lignin, which com-
prises a mixture of Sy and G units along with a trace amount
of the H unit. The BWL conversion results are in good agree-
ment with the composition of hardwood lignin. For the liquid
oil of CSL, the monomer composition is much different and
more complicated than that of BWL. First, more H units
(4.5 wt% 4-ethylphenol and 1.8 wt% 4-methylphenol) are com-
prised in the monomers, which leads to a Sy/G/H molar ratio
of 42:36:22. It is known that corn stalk lignin is type H–G–Sy
lignin with Sy, G, and H contents of 25–50, 25–50, and 10–
25%, respectively.[2a] Our result for the Sy/G/H molar ratio fits
well with the literature. Second, in comparison with less nonca-
nonical subunits contained in birch wood lignin, a much
higher content of the p-coumarate unit methyl hydrocinna-
mate (17.5 wt% of monomers) along with trace amounts of
fatty acid methyl esters were found in corn stalk lignin prod-
ucts. In addition, a dramatically higher amount of two ferulate-
derived units, methyl 3-(4-hydroxy-3-methoxyphenyl)propa-
noate (9.6 wt%) and methyl 3-(4-hydroxy-3-methoxyphenyl)-
propanoic acid (26.8 wt%), were detected, which are in agree-
Table 5. GPC results of BWL and CSL before and after reaction.
Lignin sample
Mw
Mn
Mw/Mn
fresh BWL
BWL residue
fresh CSL
2757
937
1680
715
920
740
740
191
3.00
1.27
2.27
3.70
CSL residue
portant reasons why the oil yield from CSL is higher than that
from BWL. Furthermore, the difference in the oil yield is proba-
bly due to the fact that each lignin resource has inherent varia-
bility in the chemical composition and pretreating may often
make it even more complex. BET analysis of these two lignins
showed that CSL had a larger surface area (20.3 m2 gÀ1) than
BWL (11.1 m2 gÀ1), which is also in agreement with the conver-
sion results.
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ment with the characterization results of Ralph and
co-workers.[24] These results suggest that corn stalk lignin con-
tains a higher amount of noncanonical subunits than birch
wood lignin.
lignin, Figure 2 (botom) shows that the molecular weight distri-
bution of the oil is much different from that of birch wood
lignin. Not only were plenty of signals below m/z=600 detect-
ed, but more interestingly, an abundance of higher molecular
weight compounds appeared in the m/z=600–1100 range,
which implies that the CSL product distribution is much more
complicated. This result is reasonable taking into account that
corn stalk lignin contains more H units and noncanonical subu-
nits than birch wood lignin.
The total percentage of monomers in the oil (7.5 wt% for
BWL, 12.7 wt% for CSL; see Table 4) suggests that there are
more products in the oil that could not be detected by GC–
MS, a technique that can only detect compounds with low
boiling points. To analyze the oil composition in more detail,
orbitrap mass spectrometry was performed. Figure 2 shows
the total ion current (TIC) traces of the BWL and CSL oil prod-
ucts. The obtained products from BWL are exclusively in the
m/z=0–600 range, and no signal is detected after m/z=600
(Figure 2, top). Considering that typical Sy, G, and H units, for
instance, syringylpropane, guaiacylethanol, and 4-propylphe-
nol, have molecular weights of 168, 210, and 136 gmolÀ1, re-
spectively, it can be concluded that the obtained products
from BWL include monomers, dimers, trimers, and tetramers
with the first three as the main products. Regarding corn stalk
Table 5 shows the GPC number average molecular weight
(Mn), weight average molecular weight (Mw), and polydispersity
(Mn/Mw) results of the fresh and residue lignins (relative to
polystyrene standards). The residual lignin molecular weight
decreased in both cases relative to that of the corresponding
untreated lignins, which proved that depolymerization had oc-
curred. Riedl et al. and Chakar and Ragauskas reported that
during the degradation process, repolymerization (condensa-
tion) of lignin with active intermediates such as quinone me-
thide and other radical phenolic species generated modified
Figure 2. MS (ESI+) spectra of BWL liquid oil (top) and CSL liquid oil (bottom) (LTQ Orbitrap Elite, m/Dm50 =60000 at m/z=400).
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structures with higher molecular weights than the initial com-
pounds.[22,25] The decreased molecular weight of residue lignin
in our case suggests that W2C would not induce a severe repo-
lymerization reaction, which is one of the advantages of this
catalytic system.
As shown in Figure 3 (top, left), cross-signals from the sy-
ringyl units are observed as prominent signals for the C2,6–H2,6
correlation at approximately dC/dH =103.3/6.6 and 105.3/
7.3 ppm, whereas the guaiacyl unit correlations for C2–H2 are
detected at dC/dH =110.7/7.3 ppm, for C5–H5 at dC/dH =114.9/
6.8 ppm, and for C6–H6 at dC/dH =118.6/6.8 ppm. Interestingly,
signals related to the Sy units are more intense than those as-
sociated with the guaiacyl units, and the ratio of Sy/G is ap-
proximately 4.9. In the side-chain region, different signals can
also be observed in the birch lignin. For example, in Figure 3
To estimate the structural change of lignin before and after
reaction, and to better understand the mechanism of CÀO
bond cleavage, fresh organsolv birch lignin (BWL) and the resi-
due after the reaction were characterized by 2D-HSQC-NMR
spectroscopy.
Figure 3. 2D-HSQC-NMR spectra of organosolv birch lignin in [D6]DMSO/[D5]pyridine (4:1 v/v) before the reaction (top, left and right) and the resulting residue
mixture after treatment with our catalytic system (bottom, left and right).
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(top, right), BWL has a very high content of b-O-4 aryl ether
linkages [up to 64.6% for A (b-O-4) and 16.5% for A’ (b-O-4’),
which is produced from pretreatment under acidic conditions
with ethanol][26] with 0.7% resinol (B: b-5) and 18.2% phenyl-
coumaran (C: b-b). We also find from Figure 3 (top, right) that
A is the dominant linkage in birch wood lignin, and the Ca–Ha,
Cb–Hb, and Cg–Hg correlations are observed at dC/dH =72.0/5.0,
86/4.2, and 60/3.5 ppm, respectively. In addition, a small
amount of B is observed in Figure 3 (top, right) with Ca–Ha,
Cb–Hb, and Cg–Hg correlations at dC/dH =87.1/5.5, 52.8/3.5, and
62.6/3.8 ppm, respectively, whereas signals from C are ob-
served with Ca–Ha Cb–Hb, and double Cg–Hg correlations at
dC/dH =85.0/4.7, 53.6/3.1, 71.0/3.8, and 71.0/4.2 ppm, respec-
tively. After the reaction, the disappearance of the A, B, and C
linkages of the residue is quite noticeable (Figure 3; bottom,
left), which indicates these three linkages of A (b-O-4), B (a-O-
4), and C (b-b) are attacked by the W2C/AC catalyst, and cleav-
age of the CÀO bond of b-O-4 is the major contribution. Inter-
estingly, several cross-peaks at dC/dH =124–131/6.9–7.8 ppm
arise in the residue (Figure 3; bottom, right), probably owing
to side reactions and the recombination of highly reactive radi-
cal intermediates. Additionally, after the reaction, the area of
Sy and G in the lignin residue spectrum is significantly de-
creased (Figure 3 bottom, right vs. Figure 3 top, left), which
suggests that most of the Sy and G aromatic structures are
converted or released in the oil products. This above result is
in good agreement with the orbitrap mass spectrometry and
GPC results.
nols and liquid oils in high yields. W2C/AC exhibited promising
potential for the substitution of noble metals in the cleavage
of the major aryl ether bonds of lignins without destroying the
aromatic rings. Methanol was the preferred solvent because
W2C/AC catalyzed the in situ hydrogen-transfer reaction from
methanol to the substrate. W2C/AC exhibited high activity not
only in b-O-4 cleavage but also in the deconstruction of more
resistant a-O-4 and b-b linkages in lignin. More importantly,
this catalyst could be reused five times without a significant
loss in CÀO bond-cleavage activity. These features make this
simple catalytic process highly effective and environmentally
green. Furthermore, the high yield of the aromatic bio-oil ob-
tained from lignin without the formation of cycloalkanes is of
great significance for the cost-efficient production of aromatic
chemicals from biomass.
Experimental Section
Materials and analysis methods
WO3 was purchased from Aladdin Chemistry Co., Ltd., Shanghai,
China. All lignin model compounds, b-hydroxy ether substrates,[28]
and the W2C/AC catalyst[14a] were prepared as described in our pre-
vious paper. Elemental analysis of the leached (or dissolved) cata-
lyst was accomplished by using a Thermo iCAP Model 6300 induc-
tively coupled plasma atomic emission spectrometer (ICP-AES). X-
ray diffraction (XRD) patterns of the catalysts were recorded with
a PW3040/60 X’Pert PRO (PANalytical) diffractometer equipped
with a CuKa radiation source (l=0.15432 nm). The Raman spectra
were recorded with a Horiba Jobin Yvon T64000 system (Horiba,
Japan) equipped with a confocal microscope. The molecular
weights of the lignin and residues were analyzed with a gel-perme-
ation chromatograph (Viscotek TDAmax). NMR spectra were re-
corded with a Bruker Avance DPX-400 spectrometer at 298 K. De-
tailed synthesis procedures and characterization methods as well
as supporting figures are provided in the Supporting Information.
On the basis of the conversion results and HSQC analysis,
one can conclude that W2C/AC shows high activity not only in
b-O-4 cleavage but also in the cleavage of the more resistant
a-O-4 and b-b linkages for lignin conversion, so as to obtain
a high yield of liquid oil products. Notably, no cycloalkane was
detected in the two liquid oils, which further confirms that
W2C/AC is highly selective in the cleavage of aryl CÀO bonds
in lignin.
Typical procedure for CÀO cleavage of lignin model com-
For a catalytic process, a high substrate concentration is ex-
pected to be valuable in facilitating the cost-efficient conver-
sion of biomass into biobased products. We tested the conver-
sion of CSL with a higher concentration (3 g lignin in 60 mL
methanol), and an oil yield of 42.0 wt% was obtained (for de-
tails, see Table S3). Despite the fact that this result is lower
than that obtained with a lower concentration of realistic
lignin, it is fairly good taking into account the much higher
substrate concentration. It should be noted that although sev-
eral methods[27] have been developed for the fractionation of
phenolic compounds from a lignin depolymerization mixture,
because of the complicated composition of the oil and the
similar properties of the phenolic products, it is still a big chal-
lenge to isolate target chemicals selectively from the reaction
mixture.
pounds
The catalytic conversion of lignin model compound was performed
in a stainless-steel autoclave (Parr, 100 mL) with an initial H2 pres-
sure of 0.69 MPa. Typically, the stainless-steel autoclave was
charged with the lignin model compound (100 mg), catalyst
(100 mg), and methanol (30 mL), and the autoclave was then
charged with an initial H2 pressure of 0.69 MPa; the mixture was
stirred (800 rpm) at 2608C for 2 h. After the reaction, the mixture
was cooled to room temperature and filtered. The liquid phase
was analyzed by GC-FID and was quantified by the internal stan-
dard method (standard: mesitylene, HP-5 column, 30 mꢁ
0.32 mmꢁ0.25 mm).
Typical procedure for the W2C/AC-catalyzed depolymeriza-
tion of lignin feedstocks
The catalytic conversion of lignin material was performed in a stain-
less-steel autoclave (Parr, 100 mL) with an initial H2 pressure of
0.69 MPa and 2608C for 2 h. Typically, the autoclave was charged
with lignin (200 mg), catalyst (200 mg), and methanol (60 mL), and
the mixture was stirred at a rate of 800 rpm. After the reaction, the
mixture was cooled to room temperature and filtered. The solid
Conclusions
In summary, by using the cheap W2C/activated carbon (AC)
catalyst, it was possible to convert various lignin model com-
pounds and realistic lignin materials selectively into monophe-
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Acknowledgements
Support from the National Natural Science Foundation of China
(21473187, 21506214) and the China Postdoctoral Science Foun-
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Received: July 7, 2016
Published online on && &&, 0000
&
ChemSusChem 2016, 9, 1 – 11
www.chemsuschem.org
10
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

FULL PAPERS
Breaking up: Tungsten carbide shows
remarkable activity in selective CÀO
bond cleavage of typical b-O-4 model
compounds and deconstruction of
lignin feedstocks. High yields of CÀO
cleavage products from model com-
pounds (up to 96.8%) and from lignin
feedstock (up to 70.7%) are obtained
without destroying the aromatic rings.
H. Guo, B. Zhang, C. Li,* C. Peng, T. Dai,
H. Xie, A. Wang, T. Zhang*
&& – &&
Tungsten Carbide: A Remarkably
Efficient Catalyst for the Selective
Cleavage of Lignin CÀO Bonds
ChemSusChem 2016, 9, 1 – 11
www.chemsuschem.org
11
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ