Selective Cleavage of C?O Bonds in Lignin Catalyzed by Rhenium(VII) Oxide (Re2O7)

Article abstract of DOI:10.1002/cplu.201700547

The selective cleavage of C?O bonds in typical model lignin β-O-4 compounds and deconstruction of a realistic lignin feedstock catalyzed by Re₂O₇ is described. High yields of C?O cleavage products (up to 97.8 %) from model compounds and oils (76.3 %) from organosolv pinewood lignin were obtained under mild conditions. Evidence for the pathway of this catalytic process is also provided.

Full text of DOI:10.1002/cplu.201700547

Accepted Article
Title: Selective cleavage of C-O bond in lignin catalyzed by a simple
rhenium oxide catalyst
Authors: Changzhi Li, Zaojuan Qi, Bo Zhang, jianwei Ji, Tao Dai,
Haiwei Guo, Aiqin Wang, and Lican Lu
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10.1002/cplu.201700547
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Selective cleavage of C-O bond in lignin catalyzed by a simple
rhenium oxide catalyst
Zaojuan Qi,†a,b Bo Zhang,†b Jianwei Ji,b,c Xinxin Li,b Tao Dai,b,d Haiwei Guo,b,d Aiqin Wang,b Lican Lu,*a
Changzhi Li*b
Dedication ((optional))
Abstract: The selective cleavage of C-O bond in typical β-O-4 lignin
model compounds and deconstruction of realistic lignin feedstock
catalyzed by simple Re2O7 has been described. High yields of C-O
cleavage products (up to 97.8%) from model compounds and liquid
oils (76.3%) from organosolv pinewood lignin were obtained
respectively under mild condition. The pathway proof for this
catalytic process has also been provided.
promising metal for both lignin depolymerization[9] and
deoxydehydration of alcohols and polyols[10]
presumably
,
because of its unique Lewis acidity, large number of accessible
oxidation states and its specific ability to transfer oxygen atom
between rhenium-oxo complexes and substrates. The
representative example is that MeReO3, a Re (VII) complex,
exhibited the ability to cleave of C-O bond of lignin β-hydroxy
ethers without involving any external redox species through the
reduction of ReVII to ReV by the substrate itself to generate the
catalytically active species MeReO2.[9] However, it is mainly
devoted to model compounds study and meets huge challenge
in depolymerization of the realistic lignin feedstocks.[11] In that
regard, the realization of the biorefinery concept hinges on the
development of highly active, and easily available catalysts,
which could provide promising solutions for the large-scale
application.
In recent years, rhenium oxide Re2O7 has been increasingly
used as a sustainable, easy-to-handle, and low-cost catalyst in
organic synthesis.[12] Although many excellent synthetic
methodologies have been developed by using Re(VII)-oxo
complexes as the catalysts, the reaction scopes are typically
limited to direct C-O, C-N formation, and deoxydehydration.[13]
As part of our ongoing interest in exploring rhenium oxides
utilization in lignin depolymerisation, we report in this paper the
easily available Re(VII) oxide (Re2O7) catalyst has the
outstanding performance for the cleavage of C-O bond in both
lignin model compounds (Scheme 1) and realistic lignin
feedstocks. It should be noted that the application of Re2O7 in
lignin remains unexplored.
With the gradual diminishing of fossil fuel reserves and
decreasing availability of energy resources, much attention is
focused on utilizing lignocellulosic biomass,
a renewable
resource that has the potential to serve as a feedstock for the
production of commodity chemicals and fuels.[1] As one of the
major components in lignocellulosic biomass, lignin is
a
complicated three-dimensional amorphous polymer consisting of
methoxylated phenylpropanoid units of various types, and is the
most abundant renewable aromatic source in nature.[2] In recent
years, the big potential for the conversion of lignin to a spectrum
of aromatic compounds via catalytic approach is increasingly
recognized as a promising strategy, which maybe an alternative
to aromatic compounds that are traditionally obtained from fossil
resources.[3] An important challenge in catalytic valorization
concepts, however, is predominantly related to the recalcitrant
polymer structure of lignin.[4] Thus much efforts has been made
to develop homogeneous catalysts such as organometallic
complexes[5] for the selective cleavage of the typical lignin ether
linkages in model compounds. The research efforts in
heterogeneous catalysis using supported noble metals,[6]
carbides[7] and bimetallic catalysts[8] have also been developed
for the depolymerization of lignin into small aromatic chemicals.
Among all the transition elements, in the field of biomass
conversion, rhenium has rapidly emerged as one of the most
Scheme 1. Cleavage of β-O-4 model comopounds catalyzed by Re2O7
catalyst.
[a]
[b]
[c]
[d]
Z. Qi, Prof. L. Lu
College of Chemistry,
Xiangtan University,
Xiangtan, Hunan, 411105, China,
E-mail: lulican2008@126.com;
Z. Qi, Dr. B. Zhang, Dr. J. Ji, X. Li, T. Dai, H. Guo, Prof. A. Wang, Dr.
C. Li,
State Key Laboratory of Catalysis
Dalian Institute of Chemical Physics, Chinese Academy of Sciences,
Dalian 116023, China; E-mail: licz@dicp.ac.cn;
Dr. J.Ji,
Shaanxi Key Laboratory of Catalysis, School of Chemistry and
Environment Science,
Shaanxi University of Technology,
Hanzhong, 723001, China;
Our initial investigation began with lignin model compounds
due to the recalcitrant nature of lignin. Noting that β-O-4
linkage represents the predominant interconnecting bond
(45.0% to 60.0%, depending on the wood type),[3c] 2-(2-
methoxyphenoxy)-1-phenyl ethanol (compound 1), a common
β-O-4 dimeric model compound of lignin,[12] was initially
studied to explore the activity of various catalysts for the
cleavage of C-O bonds. As our interest is in the application of
rhenium oxide towards cleavage of lignin ether linkages, we
started our experiment using Re2O7 as the catalyst. As shown
H. Guo, T. Dai,
University of Chinese Academy of Sciences,
Beijing, 100049, China.
o
in Table 1, no reaction occurred at 200 C in the absence of
the catalyst (entry 1). In sharp comparison, 5 mol% Re2O7
afforded complete conversion with 2-phenylethanol (71.2%)
†These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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Table 1. Hydrogenolysis of 2-(2-Methoxyphenoxy)-1-phenyl
Lewis acidity favors the reaction and high yields of the target
products can be obtained only if using Re2O7 as the catalyst.
Encouraged by the good performance of Re2O7 in selectively
cleavage of the aryl-ether linkages and the simple process, the
substrate scope in the reaction was subsequently examined. A
series of 2-aryloxy-1-arylethanols with different substituent
groups in R1-R3 positions were employed as the substrates for
the cleavage of β-O-4 bonds catalyzed by Re2O7 in THF solvent,
and the results are summarized in Table 2. These substrates
have similar chemical groups and are detected in different
natural lignins.[16] For example, 2-(2,6-dimethoxyphenoxy)-1-
phenylethanol with two methoxyl groups (substrate 3 in Table 2),
typical syringyl unit, is a major moiety in birch lignin.[14b] 2-
Phenoxy-1-phenylethanol (Entry 2) without methoxyl substitution
on phenyl ring is one component in grass lignin.[17] All the
investigated substrates provided 2-phenylethanol and phenol
derivatives (compounds a and b in Table 3) as the major
products, suggesting that the aryl C-O bonds of the substrates
were selectively cleaved with the aryl rings intact. The
conversions of all the substrates are higher than 94.0% whilst
the yields of the aromatics varied. Lignin model compounds with
the methoxyl substitution in R1 position (substrates 4-6) did not
produce ethylbenzene (compounds c), while the compounds
without methoxyl group in R1 position (substrates 1-3) afforded
ethylbenzene with the yields of 1.3%-9.5%, suggesting that
methoxyl group in R1 position favoured the target products a and
b. Electronic effect may play important role for the phenomenon
because products a with additional electron-donating group (-
OCH3) is not easy to be further reduced. However, methoxyl
ethanol catalyzed by different catalysts in THF.[a]
Yield [%][b]
1a 1b
Temp. Time Conv.
Entry Cat.
[oC]
200
200
200
200
120
150
180
200
200
[h]
3
3
1
8
3
3
3
3
3
3
3
3
3
3
3
3
3
[%][b]
0
1
--
0
0
2
3
4
5
6
7
8
9
Re2O7
Re2O7
Re2O7
Re2O7
Re2O7
Re2O7
ReO2
ReO3
99.9
10.6
99.9
8.4
42.1
51.3
0
71.2
4.8
69.9
2.4
16.7
27.8
0
97.8
8.0
99.3
5.4
25.8
39.1
0
0
0
0
10
11
12
13
14
15
16
17
CH3ReO3 200
19.2
15.8
27.0
9.1
0
0
0
0
0
11.5
3.0
11.2
2.9
0
WO3
MoO3
Fe2O3
CuO
ZnO
MgO
CeO2
200
200
200
200
200
200
200
0
0
0
0
0
0
0
0
0
0
[a]The reaction condition is: 100 mg substrate, 5 mol% catalyst and 15 mL
THF were added in a stainless-steel autoclave, this mixture was charged with
10 bar H2.[b]The conversion of substrate and yields of the products were
determined by GC-FID with mesitylene as an internal standard.
Table 2. Re2O7 catalyzed C-O bond cleavage of various 2-
Aryloxy-1-aryethanols in THF. [a]
and guaiacol (97.8%) as the dominant products under 10 bar
hydrogen for 3 h (entry 2). Shorter reaction time could not
guarantee the completion of the reaction (entry 3), while
prolonging the reaction time to 8 h provided similar results and
on the other hand may induce side reactions (entry 2 vs 4). The
catalytic activity highly depended on the reaction temperature.
Low temperature (120 oC) led to very poor conversion (8.4%)
with 2.4 % yield of 2-phenylethanol and 5.4 % yield of guaiacol
(entry 5). With the increasing of the reaction temperature, not
only the conversion of the substrate, but also the yields of the
target products 2-phenylethanol and guaiacol were remarkably
enhanced (entries 5-7), suggesting that Re2O7 catalyzed C-O
bond cleavage is a thermodynamic controlling reaction,[14] and
the reaction temperature is one of the key factors that controlling
the overall rate. Taking economy and energy consumption into
consideration, 200 °C was selected as the suitable reaction
temperature. Interestingly, rhenium oxides with different rhenium
valences such as ReO2 and ReO3 were inactive for the cleavage
of β-O-4 bond (entries 8-9), implying that Re2O7 with the highest
rhenium valence Re(VII) was essential for this reaction. This
probably because that the chemical property of high valence of
Re2O7 is more reactive than other Re species particularly
rhenium oxide with low Re valence (ReO2 and ReO3).[15] Under
the same condition rhenium complexes CH3ReO3 exhibited very
low conversion (entry 10). For the sake of comparison, other
acidic metal oxides, such as WO3, MoO3 and Fe2O3 showed
much poorer conversion with less than 12.0 % yield of the target
products (entries 11-13). We also tried alkaline oxides such as
CuO, ZnO, MgO and CeO2, even worse, no conversion was
observed (entries 14-17). The above results suggested that
Yield [%][b]
b
Conv.
[%][b]
99.9
Sub. R1
R2
R3
a
c
1
2
3
4
5
6
H
H
H
OCH3
OCH3
H
H
H
71.2
80.9
73.0
71.1
74.4
63.2
97.8
95.9
87.4
87.6
97.3
78.7
1.3
9.5
6.7
0
0
0
98.2
OCH3 OCH3 94.2
H
H
H
99.9
99.9
OCH3 OCH3
OCH3 OCH3 OCH3 99.9
[a] The reaction condition is: 100 mg substrate, 5 mol% Re2O7 and 15 mL THF
were added in a stainless-steel autoclave, this mixture was charged for 3 h
with 10 bar H2 at 200 oC. [b] The conversion of substrates and yields of the
products were determined by GC-FID with mesitylene as an internal standard.
groups in R2 and R3 positions showed some diversity in
hydrogenolysis behaviour which did not have a straightforward
orderliness. The methoxyl groups in R2 and R3 positions (entry 6)
provided the relative lower yields of products a and b (2,6-
dimethoxylphenol), probably owing to the effect of steric
hindrance. It was found that the yield of b is slightly higher than
the total yield of
a and c, which might due to further
transformation of a and other side reactions. To confirm this
speculation, the stability of the products 2-phenylethanol (1a)
and guaiacol (1b) was tested under the reaction condition in the
presence of Re2O7. The results showed that the no reaction took
place on guaiacol; in contrast, 21.6 % conversion of 2-
phenylethanol and 5.8 % yield of ethylbenzene were obtained.
This is possibly the major reason that the yield of 2-
phenylethanol was lower than guaiacol. To further diversify the
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scope of β-O-4 -derived chemicals accessible, 1-(4-hydroxy-3-
experiments under lower hydrogen pressure of 1 bar and 5 bar
were conducted (Figure S5). These two reactions also provided
very high conversion, whilst the products distributions were
much different from that obtained under 10 bar hydrogen
pressure (Figure S2). The reaction under 1 bar hydrogen
pressure afforded new product phenylacetaldehyde with a high
yield of 61.5%, whereas the yield of 2-phenylethanol was only
3.8%. Increasing hydrogen pressure to 5 bar also detected high
yield of phenylacetaldehyde and 14.2% yield of 2-phenylethanol.
The intermediate styrene were also examined in both systems.
In comparison, 10 bar hydrogen pressure provided much higher
methoxyphenyl)-2-(2-methoxyphenoxy)propane-1,3-diol,
a
typical β-O-4 model compounds with γ-OH group which reflect
the natural lignin structure was tested, guaiacol with the yield of
38 % and some aromatic compounds (yield: <1%) were obtained
with 100% conversion (Figure S1). It’s deserve to point out that
in all above reactions, no cycloalkane product was detected,
indicating that Re2O7 showed a good property in hydrocracking
of lignin β-O-4 bonds without destroying the aromatic rings.
To explore the principal conversion route of β-O-4 cleavage,
several mechanism verification experiments were conducted
(equations 1-5). First, analysis of the liquid products suggested
that besides the listed target products, (E)- and (Z)- 1-methoxy-
2-(styryloxy)benzene (compound 7) were observed in the
conversion of compound 1 (Figures S3 and S4). Whereas these
two isomers could not be detected at higher reaction
temperatures, implying that they are active intermediates which
readily undergo subsequent hydrolysis to 2-phenylethenol
(compound 15) and guaiacol. The former one with enol structure
was easily liable to convert to phenylacetaldehyde via
isomerization reaction. To confirm this point, we have
synthesized compound (E)- (2-phenoxyvinyl) benzene
(compound 8), which was then catalyzed by Re2O7 under the
same conditions in Table 2. Ethylbenzene and 2-phenylethanol
with the yields of 14.8 % and 8.1 % were observed (equation 1),
indicating that both hydrogenolysis and hydrolysis of compound
7 may occur. Furthermore, under this system 2-phenylethanol
could convert to ethylbenzene (equation 2). We therefore
conclude that 2-phenylethanol comes from hydrolysis reaction,
whilst ethylbenzene may be from both hydrolysis and
hydrogenolysis reactions. With the aim to confirm the
hydrogenolysis reaction, substrate 9 with blocked Cβ position (by
two methyl groups) has been synthesized and was used in the
reaction under the same condition (equation 3). Hydrogenolysis
products such as 10.3% yield of 10, 12.9% yield of 11, 8.2%
yield of 12 and 15.7% yield of phenol were obtained, suggesting
that direct hydrogenolysis of compound 7 occurred. The above
results confirmed that the reaction proceeded simultaneously
through both hydrolysis and hydrogenolysis reaction. Since
Table 2 showed that compounds a were the dominant products
with trance ethylbenzene, it is therefore logical that the
hydrolysis reaction is the major pathway. It is deserve to point
out that aryl ethers 13 and 14 without Cα-OH groups failed to be
cleaved under optimized conditions (equations 4-5), supporting
that Cα-OH groups plays an important role in overall
transformation, which probably reduced bond energy of β-O-4
linkage,[18] meanwhile, the presence of Cα-OH favours
dehydration of compound 1 to generate intermediates 7. To gain
more insight into the apparent reaction pathway, two
yield
of
2-phenylethanol
(71.2%)
without
any
phenylacetaldehyde. In all above three reactions, the yields of
guaiacol were higher than 90%. We therefore concluded that the
cleavage
of
β-O-4
first
generate
guaiacol
and
phenylacetaldehyde, the latter one was further hydrogenated to
2-phenylethanol. Based on the above discussion, the overall
reaction pathway for the Re2O7-catalyzed conversion of β-O-4
model compound 1 is proposed in Scheme 2. The major
pathway started by an initial dehydration of compound 1 to form
intermediate 7 (including (E) and (Z) isomers) followed by
hydrolysis of
7 to produce guaiacol and 15, the latter
intermediate with enol structure then fast isomerized to
phenylacetaldehyde, which was further hydrogenated to 2-
phenylethanol.
Scheme 2
.
Proposed reaction pathway for Re2O7 catalyzed 2-(2-
methoxyphenoxy)-1-phenyl ethanol.
The excellent results obtained from model compounds
motivated us to investigate the conversion of realistic lignin
feedstock. Pinewood lignin, one typical softwood lignin, was
isolated via the organosolv process because this isolation
process provided lignin with structure more similar to the native
lignin.[2] Fourier transform infrared spectroscopy (FTIR)
characterization suggested that the isolated organosolv
pinewood lignin (OPL) shared the basic skeleton of lignin and
had a mixture of dominant guaiacol (G) and p-hydroxyphenyl (H)
units (Figure S6 and Table S1), which is also strongly supported
by 2D-Heteronuclear Single QuantumCoherence (HSQC) NMR
analysis (Figure 1b). For the conversion of realistic lignin, small
alcohol favors the dissolution of the feedstock and therefore is
benefit for lignin depolymerization.[19] Methanol is thus chosen as
a solvent to investigate the conversion of realistic lignin. Table
S2 illustrates the phenolic oil yields and the monomer
components of the oils. The results showed that pinewood lignin
could be effectively converted to liquid oil with high yield up to
76.3 wt%. Regarding the monomer composition, pinewood lignin
yielded
a broad variety of phenolic compounds, being
guaiacylpropene, guaiacylethanol, 4-oxhydryl-phenoland phenol
as the main products (Table S2), which are derived from lignin
guaiacyl (G) and p-hydroxyphenyl (H) units. Other G monomers
include guaiacylethane, guaiacol, guaiacylpropane, methyl-4-
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hydroxy-3-methoxybenzoate and trace 4-methoxyphenol and 4-
ethylphenol. Noting that depolymerization occurred during lignin
isolation process, as a comparison, raw pinewood powder (40
mesh) was also submitted to deconstruction catalyzed by Re2O7
at 200 °C under 3 MPa H2 for 8 h. 18.8 wt% lignin oil was
obtained, the distribution of monomer products being similar to
that obtained from isolated lignin (Table S2). It should be noted
that no cycloalkane was detected in the liquid oil, which further
identified that Re2O7 is highly selective in activation of the
linkages between the primary units in lignin without destroying
aromatic ring.
In order to better understand the change of the lignin structure
before and after reaction, 2D-HSQC NMR spectroscopy which
has been regarded as an extremely powerful tool to provide
detailed insight into the different ether linkages and aromatic
units in the lignin structure was performed. In the side-chain
region of 2D-HSQC NMR spectra (Figure 1a), OPL had a very
high content of β-O-4 aryl ether linkages (up to 43.9 % for A: β-
O-4 and A’: β-O-4’) with phenylcoumaran (B: β-5, 42.1 %) and
resinol (C: β-β, 14.0 %). After reaction, gratifyingly, most A and
C as well as certain amount of B linkages disappeared in the
side-chain region of the oil (Figure 1c), indicating that the three
major linkages of A, B, and C were attacked by Re2O7. The
molar ratios of A, B and C in liquid oil (39.4% : 50.5% : 10.1%)
suggested that the lowest amount of B (β-5) was cleaved
possibly because it possessed the highest bond dissociation
enthalpy.[20] According to aromatic region of 2D-HSQC NMR
characterization, bio-oil provided G:H molar ratio of 98.2 : 1.8
(Figure 1d), which is in good agreement with the composition of
OPL (G:H molar ratio of 97.9 : 2.1 in Figure 1b). Furthermore, no
S unit was detected in both samples (Figures 1b and 1d),
suggesting that pinewood lignin belongs to G-type lignin which
comprises a dominant amount of G units along with trace
amount of H units.
Based on the conversion results and HSQC analysis, Re2O7-
catalyst shows high activity not only in β-O-4 cleavage but also
in the cleavage of more resistant β-5 and β-β linkages of
pinewood
lignin.
MALDI-TOF
mass
spectrometry
characterization offered another support on this point. It was
found in Figure S7 that the obtained oil liquid from OPL is
exclusively in the range of m/z 0-600 with most of the intensive
peaks in the range of 100-400 m/z. Considering that typical G
and H units for instance guaiacylethanol and 4-propylphenol
have molecular weights of 210 and 136 g/mol, it could therefore
be concluded that the obtained products from OPL are including
monomers, dimers, trimers and tetramers with the former two as
the main products. In contrast, the original OPL showed a Mw of
3294 g/mol, which was much higher than Mw of the oil fraction
(452g/mol) according to GPC analysis (Table S3), indicating that
Re2O7 has outstanding ability to catalyze the cleavage of the C-
O linkages between the primary units in lignin.
Experimental Section
Isolation of pine wood lignin via organosolv process: The
organosolv pine lignin (OPL) was prepared as described in our
previous paper. [7] The treatment for lignin extraction consisted of
the digestion of pine wood powder in a mixture of ethanol–water
(70 wt %) at 200 oC for 90 min in a stainless-steel autoclave
(Parr, 100 mL) with constant stirring at 700 rpm. The solid (10
g)-to-liquid (60 g) ratio was 1 : 6 (w/w). After the high-
Figure 1. 2D-HSQC-NMR spectra of organosolv pinewood lignin
(a, b) and pinewood oil (c, d) in DMSO-d6/pyridine-d5 (4:1 v/v).
temperature extraction, the mixture was cooled to room
temperature and was filtered; and two volumes of acidified water
(pH = 2) were added to the liquid fraction in order to precipitate
the lignin. The precipitated lignin was then separated by
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o
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centrifugation (4000 rpm, 20 min) and dried at 80 C for 24 h to
obtain 0.4 g organosolv pine lignin.
Typical procedure for C-O cleavage of lignin model
compounds: The catalytic conversion of lignin model
compounds was carried out in a stainless-steel autoclave (Parr,
o
75 mL) with an initial H2 pressure of 10 bar and 200 C for 3 h.
[4]
[5]
Typically, lignin model compound (100 mg), Re2O7 (10 mg), and
THF (15 mL) were added in a stainless-steel autoclave (Parr, 75
mL), the autoclave was then charged with an initial H2 pressure
of 10 bar and stirred (800 rpm) at 200 oC for 3 h. After reaction,
the reaction mixture was cooled to room temperature and was
filtered. The liquid phase was analyzed by GC-FID and was
quantified by internal standard method (standard: mesitylene,
HP-5 column, 30 m × 0.32 mm × 0.25 µm).
a) M. C. Haibach, N. Lease, A. S. Goldman, Angew. Chem. Int. Ed.
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Typical procedure for depolymerization of lignin feedstock：
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Han, S. Yang, Y. Wang, Nat. Commun. 2016, 7, 11162-11170; b) Y.
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The catalytic conversion of lignin material was carried out in a
stainless-steel autoclave (Parr, 100 mL) with an initial H2
pressure of 10 bar and 200 oC for 6 h. Typically, lignin (100 mg),
Re2O7 (10 mg), and methanol (30 mL) were charged in the
autoclave and stirred at rate of 800 rpm. After reaction, the
reaction mixture was cooled to room temperature and was
filtered. The filtrate was concentrated under reduced pressure at
35 oC to obtain liquid oil, weighed. The liquid oil was then diluted
to 2 mL with methanol, the monomer products in liquid oil were
analyzed and quantified by internal standard method (standard:
mesitylene) with HP 5973 GC–MS (HP-5 column, 30 m × 0.32
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Support from the National Natural Science Foundation of China
(21506214, 21690083, 21473187, 21690080), the Strategic Priority
Research Program of the Chinese Academy of Sciences (XDB17020100),
China Postdoctoral Science Foundation (2017M611279), the Natural
Science Foundation of Shaanxi Province (2017JQ2029) and Shaanxi
University of Technology doctoral Foundation (SLGKYQD-2) is gratefully
acknowledged. The Project-sponsored by SRF for ROCS, SEM.
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Keywords: lignin depolymerization • rhenium oxide • C-O bond
cleavage
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Entry for the Table of Contents (Please choose one layout)
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Re2O7-catalyzed selectively
cleavage of lignin C-O bonds
to produce aromatic
chemicals is reported for the
first time.
Zaojuan Qi,† Bo Zhang,† Jianwei Ji,
XinXin Li, Tao Dai, Haiwei Guo, Aiqin
Wang, Lican Lu,* and Changzhi Li*
Page No. – Page No.
Selective cleavage of C-O bond in
lignin catalyzed by a simple rhenium
oxide catalyst
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