Mild selective oxidative cleavage of lignin C-C bonds over a copper catalyst in water

Article abstract of DOI:10.1039/d1gc02102h

The conversion of lignin into aromatics as commodity chemicals and high-quality fuels is a highly desirable goal for biorefineries. However, the presence of robust inter-unit carbon-carbon (C-C) bonds in natural lignin seriously impedes this process. Herein, for the first time, we report the selective cleavage of C-C bonds in β-O-4 and β-1 linkages catalyzed by cheap copper and a base to yield aromatic acids and phenols in excellent yields in water at 30 °C under air without the need for additional complex ligands. Isotope-labeling experiments show that a base-mediated Cβ-H bond cleavage is the rate-determining step for Cα-Cβ bond cleavage. Density functional theory (DFT) calculations suggest that the oxidation of β-O-4 ketone to a key intermediate, i.e., a peroxide, by copper and O2 lowers the Cα-Cβ bond dissociation energy and facilitates its subsequent cleavage. In addition, the catalytic system could be successfully applied to the depolymerization of various authentic lignin feedstocks, affording excellent yields of aromatic compounds and high selectivity of a single monomer. This study offers the potential to economically produce aromatic chemicals from biomass.

Full text of DOI:10.1039/d1gc02102h

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Mild selective oxidative cleavage of lignin C–C
bonds over a copper catalyst in water†
Cite this: DOI: 10.1039/d1gc02102h
Yuzhen Hu,a,c Long Yan,a Xuelai Zhao,d Chenguang Wang,
*
a,d Song Li,a,c
a,b
Xinghua Zhang, a,b Longlong Maa,b and Qi Zhang
*
The conversion of lignin into aromatics as commodity chemicals and high-quality fuels is a highly desir-
able goal for biorefineries. However, the presence of robust inter-unit carbon–carbon (C–C) bonds in
natural lignin seriously impedes this process. Herein, for the first time, we report the selective cleavage of
C–C bonds in β-O-4 and β-1 linkages catalyzed by cheap copper and a base to yield aromatic acids and
phenols in excellent yields in water at 30 °C under air without the need for additional complex ligands.
Isotope-labeling experiments show that a base-mediated Cβ–H bond cleavage is the rate-determining
step for Cα–Cβ bond cleavage. Density functional theory (DFT) calculations suggest that the oxidation of
β-O-4 ketone to a key intermediate, i.e., a peroxide, by copper and O2 lowers the Cα–Cβ bond dis-
sociation energy and facilitates its subsequent cleavage. In addition, the catalytic system could be suc-
cessfully applied to the depolymerization of various authentic lignin feedstocks, affording excellent yields
of aromatic compounds and high selectivity of a single monomer. This study offers the potential to econ-
omically produce aromatic chemicals from biomass.
Received 13th June 2021,
Accepted 3rd August 2021
DOI: 10.1039/d1gc02102h
rsc.li/greenchem
β-O-4 linkage is the major challenge for producing aromatic
1. Introduction
chemicals from lignin.7 In targeting its cleavage, researchers have
Lignin, a heterogeneous aromatic biopolymer, is one of the developed pyrolysis,9–12 acid or alkali hydrolysis,13–16 reduction17–20
main constituents of lignocellulosic biomass, comprising and oxidation21–27 methods, each with its own advantages and
roughly 30% on a weight basis and 40% on an energy basis.1,2 limitations. From a chemical point of view, oxidative conversion of
It is constructed by three main phenyl propane units, namely lignin has several attractive advantages, including but not limited
p-coumaryl (H), coniferyl (G), and sinapyl (S).3–5 The aromatic to, the relatively low conversion temperatures and pressures, and
structure offers the potential to convert it into commodity the production of highly functional aromatic compounds (such as
chemicals and high-quality fuels, which exhibits bright pro- aldehydes, esters and organic acids), which can be directly used as
spects for sustainable development given the concerns at fine and platform chemicals.1,2,7
present over limited fossil resource reserves.1,6
The oxidative depolymerization of β-O-4 linkages can be
However, these phenyl propane units are linked via various C– achieved by breaking the C–O or C–C bonds. Much current
O and C–C bonds, including β-O-4, β-1, β–β, 4-O-5, α-1, 5–5′ and so research has focused on their depolymerization through the clea-
on.3–5,7 Among these linkages, the β-O-4 structure accounts for vage of C–O bonds.22,23,28–32 Inter-unit C–C bonds have higher
approximately 43% to 65% of all the bonds in softwood and hard- dissociation energies and the selective cleavage of these bonds
wood native lignin.1,2,8 Therefore, the efficient cleavage of the remains a significant challenge.3,33 Therefore, an efficient cata-
lytic system that can selectively break C–C linkages in lignin
should be developed. Rossi and colleagues had previously shown
that a copper system (composed of CuCl and pyridine) catalyzed
aGuangdong Provincial Key Laboratory of New and Renewable Energy Research and
Development, CAS Key Laboratory of Renewable Energy, Guangzhou Institute of
the aerobic oxidation of diols, highlighting the potential of
Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, PR China.
copper to catalyze C–C bond cleavage.34 Recent work showed that
E-mail: zhangqi@ms.giec.ac.cn, wangcg@ms.giec.ac.cn; Fax: +86 020 87057789;
copper-based catalytic systems, such as CuCl/TEMPO/2,6-luti-
Tel: +86 020 87057789
dine/toluene,35 Cu(OAc)2/1,10-phenanthroline/methanol,36 Cu
bKey Laboratory of Energy Thermal Conversion and Control of Ministry of Education,
School of Energy and Environment, Southeast University, Nanjing 210096, PR China
cUniversity of Chinese Academy of Sciences, Beijing 100049, PR China
dDepartment of Thermal Science and Energy Engineering, University of Science and
Technology of China, Hefei, 233022, PR China
(OAc)2/BF3·OEt2/methanol,37
CuBr2/N-methyl
TBD/DMSO,38
CuCl2/N-iodosuccinimide/DMSO,39 etc., were efficient in cleaving
C–C bonds in β-O-4 ketones, affording acids, esters and phenols.
However, organic solvents (such as methanol, pyridine,
toluene, acetonitrile, DMSO, etc.) and complex ligands (such as
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1gc02102h
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the
abovementioned
2,6-lutidine,
1,10-phenanthroline, these compounds were confirmed by 1H-NMR and 13C-NMR
N-methyl TBD, etc.) are usually indispensable for increasing the (Fig. S1–S6†).
solubility and generating active copper complex cores.33,35–43
From the atom-economical and green chemistry viewpoint, the
2.2 General procedure for the cleavage reaction
copper-catalyzed aerobic oxidation of lignin linkages in water
and without any additional complex ligands would be more
ideal, as it avoids the use of toxic organic solvents, which are
often hazardous and generate waste.1,44–48 Besides, it is known
that phenols are easily oxidized or undergo repolymerization
reactions under oxidative conditions to create products that are
not desirable.49 Previous work on the oxidative cleavage of
lignin models usually showed low yields for phenols.36,50–52
Thus, if the oxidative cracking of lignin linkages can be carried
out at mild ambient temperature and pressure, the yield of the
desired monomers could be fully maximized.
Herein we report a mild aerobic lignin depolymerization
system based on an inexpensive commercially available copper
salt and a base. The catalytic system can selectively cleave the
C–C bonds of a series of β-O-4 and β-1 lignin model com-
pounds into acids, phenols and aldehydes under ambient con-
ditions in water (Scheme 1). The mechanism for the cleavage
of C–C bonds was studied via isotope-labeling experiments
and DFT calculations. Encouraged by the success of the reac-
tion system in the cleavage of the model compounds, the
applicability of both CuCl and NaOH in the degradation of
real lignin raw materials was studied, affording excellent yields
of aromatic compounds and high selectivity of a single
monomer. This study provides an attractive option for produ-
cing value-added chemicals from renewable lignin with a
simple and economical catalyst under mild conditions.
2.2.1 Procedure for the cleavage reaction of the β-O-4 and
β-1 model compounds. A pressure tube was equipped with a
magnetic stir bar and was charged with the model compound
(0.1 mmol), catalyst (0.05 mmol), NaOH (0.4 mmol) and H2O
(2.5 mL). The tube was sealed and then heated at 30 °C for
5.5 h. After the reaction, the mixture was acidified with hydro-
chloric acid solution to pH = 2 and extracted with dichloro-
methane (3 × 5 mL). The organic layers were combined and
dried over anhydrous Na2SO4. The products were subsequently
characterized and quantified using gas chromatography/mass
spectrometry (GC-MS) and gas chromatography (GC).
2.2.2 Procedure for the cleavage reaction of the authentic
lignin material. Oxidation experiments were performed in a
50 mL autoclave (type 316 stainless steel, Anhui Kemi
Machinery Technology Co., Ltd) charged with 0.5 g of auth-
entic lignin sample, catalyst (0.5 mmol), and NaOH (15 mmol)
and H2O (25 mL) as the solvent. The reactor was pressurized
with 5 bar of air, and heated to the desired reaction tempera-
ture. After the reaction, 5 mL of the reaction solution was
added to a 15 mL glass vial, acidified with hydrochloric acid
solution to pH = 2 and extracted with dichloromethane (3 ×
10 mL). The organic layers were combined and dried over
anhydrous Na2SO4. The products were subsequently character-
ized and quantified using GC-MS and GC.
2.3 Density functional theory (DFT) calculations
The Cα–Cβ, Cβ–O and Cβ–H bond dissociation energies (BDE)
were calculated using the DFT method.54,55 The geometry
optimizations of the lignin model compounds used in this
article were performed using DFT with the M06-2X hybrid
2. Experimental
2.1 Procedure for the synthesis of β-O-4 model compounds
All β-O-4 dimeric lignin model compounds were prepared exchange–correlation functional and the 6-311 basis set using
based on previously reported procedures.53 The structures of the Gaussian 09 suite of programs.56
Scheme 1 The C–C bond cleavage of β-O-4 and β-1 ketone linkages to benzoic acid, phenol, and benzaldehyde.
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Optimization of the amount of NaOH for the cleavage of 1a is
given in the ESI (see Table S1†).
3. Results and discussion
3.1 Model compound studies
In addition to the alkali, the copper catalyst also plays an
The initial reactions were performed using a typical and widely important role in the cleavage of 1a. Only a small amount of
used lignin β-O-4 model compound, 2-phenoxy-1-phenyl- the desired products c and d (<2%) were obtained without
ethanone (1a), as the starting material under air conditions at adding copper (Table 1, entry 1). Then, numerous copper salts
room temperature (30 °C) for 5.5 h in the presence of CuCl. or oxides were screened to potentially enhance the conversion
Reports46,57,58 in the literature have indicated that lignin tends of 1a. Among the Cu(II) catalysts (Table 1, entries 9–13), CuO
to be partly soluble in alkaline solution. Therefore, a range of and Cu(OAc)2 showed moderate activity with 49% conversion
bases was screened to potentially enhance the conversion of of 1a, while Cu(OH)2 was ineffective for the 1a oxidation.
the lignin model compound 1a in water over a CuCl catalyst Regarding CuCl2 and CuSO4, about 80% and 71% of 1a were
(Table 1, entries 3–8). The product distribution analysis shows converted, respectively. Unexpectedly, Cu(I) performed much
that two kinds of typical products can be obtained in parallel better than Cu(II). Over CuCl, almost complete conversion of
from the oxidation of 1a: benzoic acid (c) and phenol (d). The model 1a could be achieved with 85.12% yield of benzoic acid
bases showed an obvious effect on the reaction. The reaction and 89.03% yield of phenol even under 1 atm aerobic environ-
could not proceed without adding a base (Table 1, entry 2), ment at 30 °C for 5.5 h (Table 1, entry 3). However, Cu2O and
indicating the crucial role of the alkaline conditions. The CuBr showed lower activities (Table 1, entries 14 and 15), poss-
activity increased with the base strength. The weak bases, such ibly due to poor solubility in water. In addition to copper cata-
as NH3·H2O and NaOAc, showed little effect on the reaction lysts, various other commonly used metal salt catalysts in the
(Table 1, entries 7 and 8). NaOH and KOH were favored for oxi- literature26,28,43,44,46 were tested for the conversion of 1a
dative cleavage reaction (Table 1, entries 3 and 4), and NaOH (Table 1, entries 16–19). ZnCl2 had no activity for β-O-4 linkage
showed the best performance with 96.96% conversion of 1a cleavage. Regarding MnCl2, AlCl3 and FeCl3, about 40% of 1a
(Table 1, entry 3). Therefore, NaOH was chosen as the base was converted, however, and only low yields of the desired
due to its excellent performance and cheaper price than KOH. product benzoic acid and phenol (<10%) were obtained,
suggesting that 1a was possibly transformed into (un-
identified) byproducts, e.g., polymers according to previous
reports.7,37,50 Moreover, poor product yields were detected
under an Ar atmosphere with a pronouncedly decreased con-
version (Table 1, entry 20), confirming the necessity of atmos-
pheric oxygen in the cleavage of the β-O-4 linkage.
Table 1 Catalyst and base screening for the cleavage of β-O-4 lignin
model compound 1a
Further insight into the reaction conditions was obtained
using 1a. Fig. 1(I) shows that with the reaction time increased
from 0.5 to 5.5 h, the yields of c and d at 30 °C increased from
1.99% and 4.54% to the maximum yields of 85.12% and
89.03%, respectively. A further increase in the reaction time to
10 h led to a significant decrease in yields, although 100% con-
version of 1a could be achieved. Fig. 1(I) also shows that reac-
tant 1a was rapidly transformed within 10 min at higher temp-
eratures. A conversion of 62.36% was obtained after 10 min of
reaction at 60 °C. Further increasing the temperature
enhanced the conversion of 1a but also led to a notable
decline in yields and selectivity of c and d (see Table S2†),
probably due to degradation or repolymerization at high temp-
eratures according to previous reports.42,50 These results
demonstrate the importance of mild conditions in avoiding
unnecessary condensation and decomposition and provide
strong motivation to develop milder lignin oxidation methods.
Natural lignin usually contains various substituted func-
tional groups (especially OMe, phenolic OH, and γ-CH2OH
groups), which may affect the activation of linkages.1,7 A series
of other lignin β-O-4 model substrates 2–5a was then tested in
this catalytic system, and all of them could be almost comple-
tely converted within 4 to 18 h with high yields of aromatic
acids, phenols or aromatic aldehydes (see Fig. 1(II)).
Conversion of 97.42% of 2a was obtained at 30 °C after 4 h,
and generated corresponding acid and phenol in high yields
Yields (mol%)
Conversion
(%)
c (benzoic
acid)
d
Entry
Catalyst
Base
(phenol)
1
2
3
4
5
6
7
8
None
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuO
Cu(OAc)2
Cu(OH)2
CuCl2
CuSO4
Cu2O
CuBr
MnCl2
AlCl3
FeCl3
ZnCl2
CuCl
NaOH
None
NaOH
KOH
8.07
0.75
0.16
0
1.73
0
96.96
83.55
49.18
18.52
23.32
27.86
49.13
49.83
17.99
80.23
71.15
49.81
71.03
35.26
45.78
38.23
12.37
7.64
85.12
67.33
2.62
1.82
0.89
1.11
19.94
8.53
10.78
51.04
60.01
22.48
48.55
2.39
9.54
8.56
0
89.03
70.02
5.51
2.33
2.98
2.56
16.41
6.12
7.45
45.22
55.14
19.53
42.9
4.56
7.20
6.39
0
Na2CO3
CsCO3
NH3·H2O
NaOAc
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
9
10
11
12
13
14
15
16
17
18
19
20a
0.54
2.73
General conditions: 1a (0.1 mmol), catalyst (0.05 mmol), base
(0.4 mmol), water solvent (2.5 mL), air (1 atm), 30 °C for 5.5 h. a Ar
atmosphere (1 atm) instead of air.
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Fig. 1 (I) Optimization of reaction conditions for 1a conversion. (II) Exploration of substrate scope. General conditions: Substrate (0.1 mmol), CuCl
catalyst (0.05 mmol), NaOH (0.4 mmol), solvent (2.5 mL), air (1 atm).
of 85.56% and 91.28%, respectively, while 3a and 4a posses- pounds are listed in Table 2. Optimized structures for the
sing two or three methoxy groups in the aromatic units were lignin model compounds are provided in the ESI Fig. S8–S13.†
oxidized more slowly, probably owing to the steric hindrance
The BDEs of the C–O and C–C bonds in models 1–4a are
effect.7,50 The β-O-4 lignin model with phenolic OH and approximately 60–65 and 81–86 kcal mol−1 (Table 2, entries
γ-CH2OH (1-(4-hydroxy-3,5-dimethoxyphenyl)-2-(2-methoxyphe- 1–4), respectively while the β-1 linkage has a moderate C–C
noxy)propane-1,3-diol, 5a) was also successfully broken down BDE of 76 kcal mol−1 (Table 2, entry 5). Breaking the Cβ–H
to guaiacol and syringaldehyde in 83.57% and 76.06% yields, bonds in the β-O-4 ketone and β-1 ketone require approxi-
respectively. But a higher reaction temperature (50 °C) was mately 90 kcal mol−1 and 106 kcal mol−1, respectively.
required. Except for β-O-4 compounds, β-1 ketone lignin Combining the above experimental results that the complete
model (1,2-diphenylethanone, 1b) was also successfully broken conversion of 1b requires a higher temperature (50 °C) than
down into benzoic acid (c) and benzaldehyde (e) with 94.38% the conversion of 1–4a, it is speculated that Cβ–H may have
conversion under air conditions at 50 °C for 10 h in the pres- played an important role in the first reaction step (see also dis-
ence of Cu(I), and the third product was a trace amount cussion below).
1
(3.02%) of the C–H bond oxidation product diphenylethane-
dione (f).
Then, the H-NMR analysis was performed to elucidate the
structural transformation of model compound 1a before and
after oxidative reaction (Fig. S15†). The strong signal corre-
sponded to Cβ–H (δ = 5.58) of substrate 1a disappeared com-
pletely after 5.5 h of reaction, demonstrating the effective
breakage of the Cβ–H bond. Together with other significant
3.2 Mechanism study
Mechanistic insights into the depolymerization process are
discussed below. It is apparent that benzoic acid c and phenol
d were generated from the breakage of the Cα–Cβ and/or Cβ–O
bonds in models 1–3a. To investigate the cleavage mechanism
of the chemical bonds in lignin β-O-4 and β-1 ketone, DFT cal-
culations were initially performed with Gaussian 09. The calcu-
lated bond dissociation energies of the Cα–Cβ, Cβ–O and Cβ–H
bonds from the optimized geometries of lignin model com-
1
changes of the H-NMR signals, it is considered that the main
structure of 1a was significantly changed due to the breaking
of the specific bonds over CuCl and NaOH. To verify this con-
clusion, the 1H-NMR analysis was conducted using com-
pounds 1a and 1a-d2 (hydrogen atoms in Cβ–H substituted by
deuterium) as the substrates. The signals before and 4 h after
1
the reaction are shown in Fig. 2. The H spectra of the main
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Table 2 Bond dissociation energies of Cα–Cβ, Cβ–O and Cβ–H in lignin
compounds
detected, indicating that no stable intermediates were formed
during the β-O-4 ketone oxidative cleavage reaction.
Furthermore, the product signals of 1a-d2 were obviously
weaker than that of 1a.
Dissociation energy (kcal
mol−1
)
Then the isotope effect was investigated. The time-on-
course process for 1a and 1a-d2 fragmentation at the above-
mentioned conditions were recorded in Fig. 3. Different from
1a fragmentation, products c and d were formed slower when
1a-d2 was the substrate. This phenomenon is caused by the
difference of H and D species.61 When the independent reac-
tions of substrates 1a and 1a-d2 were compared, a primary
kinetic isotope effect (KIE) was obtained with a kH/kD = 2.2,
indicating that the cleavage of Cβ–H is the rate-limiting step
during the cleavage of the β-O-4 linkages. As mentioned above,
the reaction could not proceed without adding a base. Then
we infer that NaOH played a significant role in the first reac-
tion step, i.e., Cβ–H cleavage. Thereafter, CuCl and O2 were
indispensable for the next steps.
Entry
1
Compound
Cα–Cβ
Cβ–O
Cβ–H
81.09
64.51
89.57
2
3
84.49
83.54
61.12
64.34
90.09
90.38
4
5
86.63
76.35
59.77
91.90
Due to the huge difference (approximately 20 kcal mol−1) in
dissociation energy between the Cα–Cβ and Cβ–O bonds, it was
speculated that the Cβ–O bond in β-O-4 ketone linkages is
broken first to form 2-hydroxy-1-phenylethanone and sodium
phenolate as reported.40 However, 2-hydroxy-1-phenylethanone
was not detected as a product in the present catalytic system.
When it was used as a substrate, only 9.75% conversion was
obtained, affording a 2.02% yield of benzoic acid over the
CuCl catalyst after 5.5 h at the identical conditions as the
abovementioned conditions for compounds 1–4a conversion
(Scheme 2, eqn (1)), suggesting that phenols and benzoic acids
were not formed via the Cβ–O ether bond direct cleavage.
Instead, it appears more likely that the two phenyl units were
generated from the breakage of Cα–Cβ bonds. Thus, phenyl
formate was speculated as a transient intermediate result from
the breakage of the Cα–Cβ bond and was facilely converted to
phenol and formic acid in the presence of NaOH and CuCl
(Scheme 2, eqn (2)). Regarding the β-5 model compound con-
version, diphenylethanedione (f) was observed as a minor
product, but cannot be converted to benzoic acid and benz-
aldehyde (Scheme 2, eqn (3)), indicating that it was not the
reaction intermediate, but a by-product.
—
106.38
Bond dissociation energies (BDEs) of lignin model compounds
determined by density functional theory (DFT).
In order to determine whether the aerobic breakage of 1a
undergoes a free radical process or not, 0.1 mmol (1.0 equiv.)
TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) was added as a
radical scavenger before the oxidation reaction. It was found
that the yields of c and d significantly reduced from 85.12 and
89.03% to 21.63 and 15.32%, respectively (Table 3). If the
amount of TEMPO was reduced to 0.05 mmol (0.5 equiv.), the
yields of c and d can be restored to a certain extent, reaching
43.95 and 36.45%, respectively. Similar results were obtained
when using 1b as the substrate (Table 3). Therefore, it was con-
sidered that the reaction was significantly inhibited but not
completely hindered, suggesting that the Cα–Cβ cleavage reac-
Fig. 2 1H NMR spectra of 1a and 1a-d2 before and after 4 h reaction in
0.6 mL of DMSO as the solvent. Conditions: Substrate (0.1 mmol), CuCl
catalyst (0.05 mmol), NaOH (0.4 mmol), water solvent (2.5 mL), air (1
atm), 30 °C, 4 h.
cleavage products benzoic acid c and phenol d standards are tion involves a free radical mechanism, which may be the
also provided for reference. As shown in Fig. 2, the character- dominant path.
istic peaks of c and d signal appeared in the 4 h reaction
On the basis of the above-described explorations combined
mixture from both 1a and 1a-d2. No other products were with the knowledge in the literature, two plausible reaction
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Fig. 3 Kinetic Isotope Effects (KIEs) with deuterium labelled compound 1a-d2 (I), the linear fit of concentration against the reaction time of models
1a (II) and 1a-d2 (III). Conditions: Substrate (0.1 mmol), CuCl catalyst (0.05 mmol), NaOH (0.4 mmol), water solvent (2.5 mL), air (1 atm), 30 °C.
pathways are proposed for aerobic oxidative cleavage of the
lignin β-O-4 and β-1 linkages (Scheme 3). The initial step
involves a base-catalyzed enolization reaction of ketone 1a (or
1b) to form enolate g and hydrogen oxide, the rate-limiting
step in the reaction pathway. In the presence of a base, the
enolization reaction can quickly reach equilibrium, which is
why the cleavage of β-O-4 and β-1 ketone can occur even under
mild room temperature conditions. The enolate g may sub-
sequently undergo aerobic oxidation via a copper-catalyzed
single electron transfer (SET) route.62 Meanwhile, Cu(I) ion is
also easily oxidized to Cu(II) ion by atmospheric oxygen, and
Cu(II) in turn is an effective single-electron oxidant.39,62
Scheme 2 Control experiments of the possible intermediates or side
Considering that both Cu(I) and Cu(II) salts had induced the
cleavage of 1a to c and d (see Table 1, e.g., entries 3 and 12),
the main role of the copper in the early reaction stages might
be to oxidize the enolate g to the peroxide intermediate h.
Such peroxide intermediates are often related to aerobic oxi-
dation reactions according to previous studies.59,60 In pathway
A (the radical pathway), the peroxide intermediate h might be
homolytically cleaved into the oxygen-centered radical i cata-
lyzed by copper, which could undergo rapid C–C fragmenta-
tion according to previous reports by Wang et al.63 and
Schoenebeck et al.62 The benzaldehyde radical j is then oxi-
dized to final benzoic acid product c in the presence of Cu(I)
and O2. While another C–C fragmentation product is ester k or
benzaldehyde e (for β-O-4 and β-1, respectively). In the pres-
ence of a base, ester k is facilely converted to final product
phenol d and small molecule carboxylic acid via hydrolysis
(Scheme 2, eqn (2)). In pathway B (the anionic path), h could
generate dioxetane l, which could further proceed ring-
products.
Table 3 Oxidation of lignin model compounds with TEMPO
Yield (%)
Substrate
Quantity (equiv.)
Conversion (%)
c
d(e)
1a
—
96.96
40.02
69.30
94.38
36.02
55.65
85.12
21.63
43.95
82.53
14.26
32.38
89.03
15.32
36.45
76.65
15.35
24.11
1.0
0.5
—
1.0
0.5
1b
Conditions: Lignin model (0.1 mmol), CuCl (0.1 mmol), NaOH
(0.4 mmol), 2.5 ml H2O, 30 °C and 5.5 h for 1a, 50 °C and 10 h for 1b.
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Scheme 3 Possible mechanism of the copper(I) and base catalyzed oxidative cleavage of β-O-4 and β-1 lignin linkages. The bond dissociation
energy (BDE) values of Cα–Cβ refer to the compounds with R = phenoxy, X = H, obtained with the M06-2X.
opening to generate benzoic acid c and ester k or benz- the desired monomers.64 Therefore, compared with the model
aldehyde e (for β-O-4 and β-1, respectively).39 The ester k is compounds, higher temperature and pressure are required in
readily converted to the final product d as mentioned above. the cleavage reaction of the authentic lignin. Thus, the oxi-
The whole mechanism shows that the role of NaOH in activat- dative depolymerization of the authentic lignin was performed
ing the substrates and the role of copper in electron transfer at optimized conditions at 160 °C under 5 bar air pressure for
and oxygen transfer are the key points in this mild aerobic C–C 60 min according to Fig. S16–18.† The oxidation of corn stover
cleavage.
lignin mainly yielded the aromatic aldehydes vanillin and syr-
DFT calculations (see Scheme 3) show that the BDEs of the ingaldehyde, along with other compounds such as vanillic
Cα–Cβ bond from substrate 1a to intermediate h (X = H) signifi- acid and syringic acid. According to the typical functional
cantly decrease from 81.09 kcal mol−1 to 31.38 kcal mol−1. The groups, products can be divided into three categories: aromatic
subsequent C–C fragmentation from the corresponding aldehydes, acids and ketone compounds. The monomer yields
oxygen-centered radical i in pathway A can be very easy, requir- and selectivity of the main product vanillin and syringaldehyde
ing less than 10 kcal mol−1 dissociation energy. The dis- from different substrates are illustrated in Fig. 4. The oxidation
sociation energy of C–C cracking from the dioxetane l in of pine yielded only G-type aromatics (vanillin, vanillic acid
pathway B was calculated to be much higher (22.05 kcal and acetovanillone), which is consistent with the intrinsic
mol−1). Thus the above calculations suggest that the radical nature of softwoods, with high selectivity towards vanillin
route is more favorable than the anionic route.
(73.66%). From eucalyptus, an excellent (38.61 wt%) monomer
yield was obtained, comprising 23.96 wt% single product syr-
ingaldehyde. Fig. S19† shows a gas chromatogram of the
3.3 The conversion of authentic lignin feedstocks
The results obtained with the lignin model compounds monomeric products after eucalyptus conversion.
encouraged us to apply the methodology to the conversion of
To explain the structural changes occurring, eucalyptus and
real lignin. Lignocellulosic feedstocks, viz. hardwood eucalyp- the oxidative depolymerization products (the organic oil) were
tus, softwood pine, herb corn stover, bamboo, pennisetum analysed through two-dimensional heteronuclear single
and bagasse, were subjected to alkaline aerobic oxidation with quantum coherence nuclear magnetic resonance (2D
CuCl. Due to the inherent complexity of the real lignin struc- HSQC-NMR, Fig. 5). In the aliphatic regions (Fig. 5(I) and (II)),
ture and the variety of different linkages and subunits depend- the native lignin consists of abundant β-O-4 linkages (A), calcu-
ing on plant type and high temperature (typically ≥160 °C) lated as 51.2/100 C9,66 along with relatively small amounts of
together with proper O2 or air pressure must be taken to par- β–β (B) and β-5 (C). After oxidative depolymerization, the
tially dissolve lignin into small pieces to maximize the yield of signals of these linkages were extremely small, demonstrating
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Fig. 6 The FTIR spectrum of the residue after mild alkaline aerobic oxi-
dation. Reaction conditions: 0.5
(15 mmol) and H2O (25 mL) as the solvent, 160 °C, 5 bar air (at RT),
60 min.
g wood, 0.5 mmol CuCl, NaOH
Fig. 4 Monomer yields (on lignin basis) from CuCl catalyzed oxidation
of various lignocellulosic feedstocks. Reaction conditions: 0.5 g wood,
0.5 mmol CuCl, NaOH (15 mmol) and H2O (25 mL) as the solvent,
160 °C, 5 bar air (at RT), 60 min.
The Fourier transformation infrared spectrum (FTIR, Fig. 6)
of the residue after the reaction exhibited several functional
groups commonly found in cellulose and hemicellulose67,68
(Table S4†), indicate that most of the cellulose and part of the
hemicellulose may be still in the residue. Then the compo-
sitional analysis of the residue was conducted based on the
Laboratory Analytical Procedure (LAP)69 of the National
Renewable Energy Laboratory (NREL). Delignification (%) and
that these bonds have been successfully disrupted, especially
for β-O-4 linkages. From the aromatic region (Fig. 5(III) and
(IV)), the S and G peaks reduced and the S′ became the most
prominent signals in the organic oil, supporting that the euca-
lyptus lignin was mainly degraded into large amounts of
benzylic ketone analogs by this catalytic system.
Fig. 5 2D HSQC-NMR spectra of eucalyptus wood lignin sample before (I and III) and after (II and IV) aerobic oxidation. Cross peaks are assigned
according to the previous literature.65,66 Reaction conditions: 0.5 g eucalyptus wood, 0.5 mmol CuCl, NaOH (15 mmol) and H2O (25 mL) as the
solvent, 160 °C, 5 bar air (at RT), 60 min.
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hemicellulose dissolution (%) of different biomass sources in
mild alkaline oxidation reaction are illustrated in Table S5.†
More than 80% of lignin and 60% of hemicelluloses can be
extracted at quite a short time of 1 h and a very low degree of
cellulose solubilisation. The mild alkaline aerobic oxidation
process simultaneously realizes the valorisation of lignin and
biological pretreatment, which reduced the recalcitrance of
lignocellulosic biomass. After the reaction, cellulose and hemi-
cellulose sediments can be separated by acidification and sub-
sequent filtration. The lignin-free cellulose is a good starting
material for biochemical processes.17 These promising results
provide an attractive option for a truly integrated and economi-
cally viable lignocellulosic biorefinery in the years to come.70
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