Two-Step, Catalytic C-C Bond Oxidative Cleavage Process Converts Lignin Models and Extracts to Aromatic Acids

Article abstract of DOI:10.1021/acscatal.6b02049

We herein report a two-step strategy for oxidative cleavage of lignin C-C bond to aromatic acids and phenols with molecular oxygen as oxidant. In the first step, lignin β-O-4 alcohol was oxidized to β-O-4 ketone over a VOSO₄/TEMPO [(2,2,6,6-tetramethylpiperidin-1-yl)oxyl)] catalyst. In the second step, the C-C bond of β-O-4 linkages was selectively cleaved to acids and phenols by oxidation over a Cu/1,10-phenanthroline catalyst. Computational investigations suggested a copper-oxo-bridged dimer was the catalytically active site for hydrogen-abstraction from C_β-H bond, which was the rate-determining step for the C-C bond cleavage.

Full text of DOI:10.1021/acscatal.6b02049

Letter
pubs.acs.org/acscatalysis
Two-Step, Catalytic C−C Bond Oxidative Cleavage Process Converts
Lignin Models and Extracts to Aromatic Acids
Min Wang, Jianmin Lu, Xiaochen Zhang, Lihua Li, Hongji Li, Nengchao Luo, and Feng Wang*
†
†
†
†
†,‡
†,‡
,†
†State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese
Academy of Sciences, Dalian 116023, P. R. China
‡
University of Chinese Academy of Sciences, Beijing 100049, P. R. China
*
S Supporting Information
ABSTRACT: We herein report a two-step strategy for
oxidative cleavage of lignin C−C bond to aromatic acids and
phenols with molecular oxygen as oxidant. In the first step,
lignin β-O-4 alcohol was oxidized to β-O-4 ketone over a
VOSO /TEMPO [(2,2,6,6-tetramethylpiperidin-1-yl)oxyl)]
4
catalyst. In the second step, the C−C bond of β-O-4 linkages
was selectively cleaved to acids and phenols by oxidation over
a Cu/1,10-phenanthroline catalyst. Computational investiga-
tions suggested a copper-oxo-bridged dimer was the catalyti-
cally active site for hydrogen-abstraction from C −H bond,
β
which was the rate-determining step for the C−C bond
cleavage.
KEYWORDS: lignin, biomass, oxidation, copper, C−C cleavage
a
he depolymerization of lignin will provide a renewable1
Scheme 1. Two-Step Strategy for β-O-4 Cleavage
T
way of producing substituted aromatics for our society.
The valorization of this complex natural polymer must
selectively break its major linkages down. The β-O-4 bond
1c
accounts for ca. 50% of all the lignin linkages. Targeting its
2
3
cleavage, researchers have developed solvolysis, pyrolysis,
4
5
reduction, and oxidation methods, as well as their
combinations. The most-studied approach is the catalytic
reduction of the C −O bond. We recently reported that birch
wood could be converted to phenols via C−O cleavage by a
6
7
β
8
tandem alcoholysis-hydrogenolysis strategy.
While most efforts have involved cleavage of the C −O
β
a
bond, we focused on the C −C bond in this study. The
The bond energy was obtained from density functional theory (DFT)
α
β
calculation using 2l (eq 2) as model.
selective cleavage of C−C bond is a challenge because of its
nonpolarity and robustness. Although some works have
reported the C −C cleavage of β-O-4 models without C −
was cleaved using stoichiometric CuCl/TEMPO (43% yield,
1
copper(I) trifluoromethanesulfonate with 1 equiv of TEMPO
and 10 equiv of 2,6-lutidine (52% yield, 100 °C, 40 h), or
under photoirradiation (70% yield). However, the major
products of these studies were aldehydes.
Substituted aromatic acids are very useful chemical
intermediates, and currently, they are produced by hydrocarbon
oxidation. Thus, developing a sustainable production route
for acids from biomass is particularly interesting. Recently our
group and others found that the copper catalysts could oxidize
α
β
γ
13
9
00 °C, 40 h, in pyridine), or using catalytic amount of
OH, the oxidative cleavage of the lignin C −C bond,
α
β
especially in the presence of C −OH, to afford acid is still
γ
14
1
0
11
unsuccessful. Toste’s group observed an unusual C−O
bond cleavage to afford an enone product using vanadium oxo-
complex catalysts under aerobic conditions. Hanson and
15
1
2
Baker studied the oxidative cleavage of C−O and C−C
bond in methoxyl-substituted arylglycerol β-aryl ether (β-O-4
16
linkage model with C −OH) with vanadium catalysts, and β-O-
γ
4
ketone was the dominant product or reaction intermediate.
The oxidation of β-O-4 alcohol to β-O-4 ketone lowers the
−
1
C −O bond energy from 247.9 to 161.1 kJ mol but increases
β
−1
the C −C bond energy from 264.3 to 294.2 kJ mol , making
Received: July 21, 2016
Revised: August 8, 2016
α
β
11,12
the C −C bond harder to be converted (Scheme 1).
α
β
Bypassing the β-O-4 ketone intermediates, the C −C bond
α
β
©
XXXX American Chemical Society
6086
DOI: 10.1021/acscatal.6b02049
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ACS Catalysis
Letter
17
C−C bond of ketones to acids and its derivatives. We thus
investigated the possibility of oxidizing β-O-4 ketone to acids.
Herein we report a two-step strategy for converting lignin
model compounds to aromatic acids and phenols (Scheme 1).
In the first step, the oxidation of lignin β-O-4 alcohol to β-O-4
ketone was realized over a VOSO /TEMPO catalyst. In the
4
second step, a copper/1,10-phenanthroline complex was highly
active and selective for oxygenation of C −H bond to a
β
hydroxyl ketone-like intermediate. This oxidation lowers the
−
1
C −C bond energy by ca. 100 kJ mol and facilitates its
α
β
cleavage. A wide range of methoxyl-substituted β-O-4 ketones
were converted to aromatic acids with 80−95% yields. The β-
O-4 ketone with C −OH was also successfully oxidized to acid.
γ
We commenced our study with catalytic oxidation of lignin
β-O-4 alcohol to β-O-4 ketones (Table 1). Previous studies on
a
Table 1. Oxidation of Lignin β-O-4 Alcohol Models
Figure 1. Partial 2D HSQC NMR spectra of organosolv Alcell lignin
(a) before and (b) after oxidation.
a
0.5 mmol β-O-4 alcohol, 0.1 mmol VOSO4 (20%), 0.1 mmol
TEMPO (20%), 0.4 MPa O , 2 mL MeCN, 100 °C, 12 h.
2
benzyl hydroxyl groups to ketones have been widely
6
b,c,18
19
documented.
Based on our work on alcohol oxidation,
we found VOSO /TEMPO to be a selective catalyst for
4
converting β-O-4 alcohols to ketones. Model compounds with
different methoxyl substituents were oxidized to the ketones
(
2a−2f) with 80−98% yields. Only a minor amount of C −C /
β
α
β
C -O cleavage occurred, and the selectivity is different
compared to the vanadium oxo-complex catalysts of Hanson
9
c,11a,12a,13
and Toste.
Phenolic lignin model was also success-
fully oxidized to the corresponding ketone (2g) with 82% yield.
The oxidation of lignin models with C −OH (1k) gave a
γ
Figure 2. Catalyst screen. 0.2 mmol 2a, 0.04 mmol Cu, 0.04 mmol
mixture of 2k and 2h. 2h may be formed via a retro-aldol
reaction of 2k (eq 1). In addition, when decreasing the catalyst
ligand, 2 mL MeOH, 0.4 MPa O , 80 °C, 2 h. The major product for
2
CuO, CuCl , Cu(NO ) , and CuSO was the α-keto ester (phenyl 2-
2
3
2
4
oxo-2-phenylacetate).
Cu(OAc) showed the highest activity with 40% conversion.
2
The conversion was further increased when nitrogen-containing
ligands were added with Cu(OAc) . 1,10-Phenanthroline (L4)
2
amount to 5 mol %, nearly the same yield of ketone (2a) can
also be obtained with longer reaction time (24 h). The catalyst
was also tested on an organosolv Alcell lignin, which was rich in
syringl units and consisted of a high proportion of β-O-4
and 2,2′-bipyridine (L5) were among the best two ligands for
the oxidation, with 93% and 92% conversion, respectively.
However, Cu(OAc) /L4 catalyst was inactive toward direct
2
oxidation of β-O-4 alcohols to acids.
6
b,18c
linkages.
Based on the Aα integrals of the 2D HSQC
We next investigated the Cu(OAc) /L4 catalyst in the
2
NMR spectra (Figure 1), approximately 36% of β-O-4 was
oxidized.
The β-O-4 ketone (2a) oxidation was then conducted over
several Cu(I), Cu(II) salts, and Cu-oxide catalysts (Figure 2).
oxidation of β-O-4 ketones with CH O groups. With CH O
3
3
2
0
groups either on the acetophenone part or on the phenoxyl
part, β-O-4 ketones were converted to acids with 85−99%
yields (Table 2). In some cases, higher reaction temperatures
6
087
DOI: 10.1021/acscatal.6b02049
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ACS Catalysis
Letter
a
Table 2. Oxidation of Lignin β-O-4 Ketone Models
Figure 3. (a) X-ray structure of Cu(OAc) /L4 complex and (b) its
2
differential charge density (DCD), which was defined as the difference
in the electron density with and without L4 ligand. The blue and
yellow regions in (b) indicate a charge increase and decrease,
respectively. (c) Electron paramagnetic resonance (EPR) spectra (at
7
7 K) and (d) cyclic voltammetry (CV) of Cu(OAc) and Cu(OAc) /
2 2
L4.
a
0
.2 mmol β-O-4 ketone models, 0.04 mmol Cu(OAc) , 0.04 mmol
2
L4, 2 mL MeOH, 0.4 MPa O , 80 °C, 3 h.
2
g⊥ signals of Cu(II) appeared. After coordination with L4, the
g peak apparently shifted to high magnetic field along with
⊥
(
2i and 2j, 100 °C) were needed to achieve 100% cleavage of
the β-O-4 ketone. Lignin model with C −OH (2l) was also
five-line hyperfine splittings (Figure 3c), which was caused by
the two coordinated nitrogen atoms. CV characterization
showed the oxidation potential of copper greatly decreased
from 0.5 V vs SCE to −0.04 V vs SCE after coordinating with
L4 (Figure 3d). Bader charge analysis from DFT calculation
indicated 0.31 electrons were transferred from L4 ligand to
copper acetate. The DCD result revealed an anisotropic
distribution of copper 3d electrons, with a decrease in the
four-coordinated direction and increase in the unoccupied
perpendicular direction (Figure 3b). The increase of the
electron density in the unoccupied site was beneficial for the
adsorption and activation of oxygen. This may account for the
γ
converted with 100% conversion and 92% yield for acid (eq 2).
enhanced catalytic performance of Cu(OAc) /L4 complex.
2
We then explored the oxygen activation by Cu(OAc) /L4. In
2
The C −OH made the C −C bond more resistant to
γ
α
β
the enzyme, oxygen is activated to copper-oxo-bridged oxygen
22
oxidation, and higher reaction temperature was thus required
dimers, which is active in H-abstraction. We calculated the
(
100 °C for 2i vs 150 °C for 2l). Phenolic β-O-4 lignin model
reaction energy for the oxygen activation by Cu(OAc) /L4
2
(2g) majorly sustained C −C bond cleavage. In addition,
α
β
(Figure 4), and we found the copper-oxo-bridged dimer was
when decreasing the catalyst amount to 5 mol %, β-O-4 ketone
2a) was also effectively cleaved with longer reaction time (6
(
h). Note that the phenols with methoxyl groups were obtained
in low yields. Attempts to perform the β-O-4 alcohol cleavage
in a one-pot, two-step procedure without switching solvents
were unsuccessful.
The detection of gas products showed CO was generated
Figure S1). A minor amount of benzoylformic acid (3a) was
2
(
detected (Figure 2), but using it as the reactant gave no benzoic
acid (eq 3), indicating that it was not the reaction intermediate.
Phenol formate was detected in the initial stage but disappeared
after reaction, suggesting that the C −C in 2a was first cleaved
α
β
to phenol formate, followed by C −O cleavage to phenol. A
β
kinetic isotopic experiment (eq 4) showed a large value of k /
H
kD (3.1), indicating the C −H oxidation could be the rate-
β
determining step for the C −C cleavage.
α
β
We obtained the single crystal of the Cu(OAc) /L4 complex.
2
Figure 4. DFT calculation of the reaction energy for the molecular
The X-ray diffraction showed copper and L4 formed a chelating
oxygen activation by Cu(OAc) /L4 and H abstraction by copper−
2
21
structure (Figure 3a). In EPR spectra, well-resolved g// and
oxygen center.
6
088
DOI: 10.1021/acscatal.6b02049
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ACS Catalysis
Letter
more favorable than the copper-superoxide monomer (−3.78
eV vs −1.16 eV). The ligands, such as L3, L7, and L8, showed
poor catalytic results, which were due to the steric hindrance
Notes
The authors declare no competing financial interest.
23
for forming the copper-oxo-bridged dimer. The H-abstraction
by the copper−oxygen monomer and dimer was further
calculated by DFT (Figure 4). The results showed that
copper-oxo-bridged dimer was more active than the monomer
with a larger reaction energy (−2.18 eV for dimer vs −0.42 eV
for monomer).
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (projects 21422308, 21273231,
1403216) and the Dalian Excellent Youth Foundation
2
(
2014J11JH126).
We come to a tentative reaction mechanism (Scheme 2).
First, the oxidation of C −OH alcohol to a ketone activates the
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DOI: 10.1021/acscatal.6b02049
ACS Catal. 2016, 6, 6086−6090