Acid promoted C-C bond oxidative cleavage of β-O-4 and β-1 lignin models to esters over a copper catalyst

Article abstract of DOI:10.1039/c6gc02970a

Depolymerisation of lignin to aromatics is a challenging task. We herein report that a Cu(OAc)₂/BF₃·OEt₂ catalyst is effective in simultaneously cleaving C-C bonds in β-1 and β-O-4 ketones, yielding esters and phenols. In-depth studies show that C-H bond activation is the rate determining step for C-C bond cleavage. BF₃·OEt₂ promotes the reaction via activating the β-C-H bond. This study offers the potential to obtain aromatic esters from lignin.

Full text of DOI:10.1039/c6gc02970a

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Heterogeneous catalytic oxidation for lignin valorization into valuable
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Acid promoted C–C bond oxidative cleavage of β-O-4 and β-1
lignin models to esters over copper catalyst
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
a, b
a
a, b
a, b
a
M. Wang, L. H. Li, J. M. Lu, H. J. Li, X. C. Zhang, H. F. Liu, N. C. Luo and F. Wang *
DOI: 10.1039/x0xx00000x
2 3 2
Depolymerization of lignin into aromatics is a challenge task. We herein report a Cu(OAc) /BF ·OEt catalyst is effective to
simultaneously cleave C-C bond in β-1 and β-O-4 ketone, yielding esters and phenols. In-depth studies show C−H bond
www.rsc.org/
activation is the rate-determining step for the C–C bond cleavage. BF
bond. This study shows potentials to get aromatic esters from lignin.
3 2
·OEt promotes the reaction via activating the β-C−H
report the oxidative cleavage of C–C bond in β-O-4 ketone to
6
1
acid over Cu(OAc) /1,10-phenanthroline catalyst, but it
Introduction
2
shows poor efficiency for β-1 scission although they are similar
Lignin is a renewable aromatic biopolymer abundant in plants.
The depolymerization of lignin is a promising way to provide
aromatics for our society in the future, and thus has aroused
in structure.
1
-5
an intense focus for both industrial and academic research.
The valorization of this complex natural polymer must
selectively break its major linkages down. The β-O-4 structure
accounts for ca. 50% of all the lignin linkages. Targeting its
6
, 7
8-
cleavage, researchers have developed solvolysis, pyrolysis,
1
1
12-28
29-47
reduction
and oxidation methods,
and their
4
8-52
combinations.
Besides of β-O-4, other linkages, for example β-1 which is
Scheme 1. Simultaneous cleavage of β-O-4 and β-1 ketone linkage to esters and
5
3
phenols.
prevalent in hardwood, are rarely explored. Vanadium-based
catalyst is widely used for the oxidative cleavage of the β-O-4
3
8-45
Herein, on continuance of our work on C-C bond
6
linkages, but it is ineffective for the β-1.
reported the cleavage of β-1
Baker and Hanson
over copper(I)
1-63
cleavage,
2 3 2
we found a Cu(OAc) /BF ·OEt catalytic system
was active for the C
α
–C bond cleavage of β-1 and β-O-4
β
trifluoromethanesulfonate catalyst with 1 equiv. of TEMPO
and 10 equiv. of 2,6-lutidine, and aldehyde and quinone were
ketone. Detailed characterization and DFT calculations
revealed the acid activates the C–H bond via keto-enol
tautomerism and thus promotes the C–C bond cleavage. More
importantly, esters formed with alcohol solvent. A wide range
of methoxyl-substituted lignin models and C1-C8 aliphatic
alcohols were converted to aromatic esters with 43-98%
yields.
5
4
the major products.
For the complete lignin
depolymerization, it is necessary to simultaneously cleave
different kinds of the linkages.
Substituted aromatic esters are very useful chemical inter-
mediates, and nowadays are produced based on petroleum-
based feedstocks. Thus developing a sustainable production
route for esters from biomass is particularly interesting. The
recent development of a method for the chemoselective
aerobic oxidation of Cα alcohols to ketones in native lignin,
enables direct investigation of the reactivity of oxidized
Experimental section
Reaction procedures
4
8, 49, 55-58
The catalytic reactions were performed in a 10-mL autoclave
reactor with an internal Teflon insert. Typically, 0.2 mmol of
lignin.
Several groups have studied the oxidative
cleavage of the β-O-4 ketone models to aldehyde, acids,
5
9, 60
substrate, 0.04 mmol of Cu(OAc)
2 3 2
, 0.04 mmol of BF ·OEt and 2
alkyloxy acetophenones and amides.
Very recently, we
mL of alcohol were added to the reactor. Then, the reactor
o
was charged with 0.4 MPa O and heated to 100 C under
2
a.
magnetic stirring. When the reaction completed, the reaction
mixture was diluted with 4 mL of methanol for GC-MS analysis.
The products were identified and quantified using gas
chromatography-mass spectrometry (GC-MS) and an Agilent
State Key Laboratory of Catalysis, Dalian National Laboratory for Clean
Energy,Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian
116023 (China). E-mail: wangfeng@dicp.ac.cn.
Electronic Supplementary Information (ESI) available: reaction date, GC, MS and
†
NMR spectra, XRD characterization. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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7890A/5975C instrument equipped with an HP-5 MS column
(30 m in length, 0.25 mm in diameter). Conversion and yield We first study the oxidation cleavage of the oxidized linkage β-
were determined using p-xylene as the internal standard.
O-4 and β-1 ketone, separately (Table 1). Lignin is tended to
partly soluble in polar solvent, especially in alcohols.
Nuclear magnetic resonance (NMR) experiment
Therefore, methanol is used as solvent. Cu(OAc)
2
showed
0
.05 mmol of β-O-4 ketone model
were dissolved in 0.6 mL of CDCl
were obtained with Bruker AVANCE III 400 MHz the Cu(OAc)
spectrometer at room temperature with tetramethylsilane the cleavage of β-O-4 ketone. However, it showed low activity
2
and 0.05 mmol of BF
3
·OEt
2
moderate conversion of
1
and 2. 63% percentage of e product
1
13
3
. H and C NMR spectra formed via the C
β
–H bond oxidation of 1. Previously, we found
a
2
/1,10-phenanthroline complex was effective for
used as the internal reference.
in the cleavage of β-1 ketone, and the major product was the
C-H bond oxidation product ( , 40% percentage). The
combination of acid with Cu(OAc) apparently increased the
and
method. The DFT calculations were performed using the were converted with 98% and 86% conversion, respectively, at
e
Density functional theory (DFT) calculation
2
The C–H bond dissociative energy was calculated by DFT activity. BF ·OEt showed the best performance. Both
1
2
3
2
6
4
o
Vienna ab initio simulation package (VASP) with project 90 C for 2 h. Notably, ester formed as the major product.
augmented wave (PAW) potential and the Perdew-Burke- Further optimization of the reaction conditions lead to
6
5
Ernzerhof (PBE) functional. A plane wave cutoff of 400 eV completely conversion of
was used. The β-O-4 and β-1 ketone models and their phenol (Table 2).
interaction with BF were fully relaxed in a cubic box with a
1
and
2
to methyl benzoate and
3
a
Table 2 Substrate scope of various oxidized lignin models.
side length of 20 Angstroms until the residual forces were less
than 0.02 eV/Å.
Results and discussion
a
Table 1. Catalyst screen for the oxidation of oxidized lignin models.
Lignin model 2 as substrate
Product distribution / %
Conversion
Entry
additive
/
%
a
b
c
d
f
a
1
2
3
4
5
6
-
48
97
36
48
83
86
0
0
0
0
0
0
0
0
0
0
0
0
44
48
45
11
5
3
3
40
35
42
35
38
39
0.2 mmol substrate, 0.04 mmol Cu(OAc)
2
, 0.4 mmol BF
3 2
·OEt , 2 mL MeOH,
o
.4 MPa O , 100 C, 3 h. The results were determined by GC-MS.
0
2
Phen
HAc
2
Native lignin contains various functional groups (in
particular CH O–) in the aromatic units, which may affect the
activation of linkages. We next investigated the
Cu(OAc) /BF ·OEt catalytic system in the oxidation of CH O–
substituted lignin models. A series of CH O–substituted β-O-4
HNO
3
32
49
51
3
H
2
SO
4
BF
3
·
OEt
2
6
2
3
2
3
Lignin model 1 as substrate
Product distribution / %
3
Conversion
linkage models were all completely converted to esters with
Entry
additive
/
%
a
b
5
c
d
1
e
77-96% yields (Table 2). β-O-4 lignin models with C -OH were
γ
7
8
9
-
6
20
23
21
30
27
22
12
28
22
11
4
63
40
61
15
16
28
also successfully broken down to esters with 43% yields (76%
yields in ethanol solvent). It is known that phenol is easily
oxidized and previous work on the oxidative cleavage of lignin
models usually showed low yields for phenols. In this study, a
moderate to high yields of the phenols were obtained (46-95%
yields), except for the highly active 2, 6-dimethoxyl phenol.
Phen
HAc
99
45
52
62
98
8
1
5
1
1
1
1
0
1
2
HNO
3
10
5
32
49
45
H
2
SO
4
BF
3
·
OEt
2
<1
5
3
CH O–substituted β-1 lignin models were completely
Reaction conditions: 0.2 mmol 1, 0.2 mmol 2, 0.04 mmol Cu, 0.04 mmol ligand, converted into esters and aldehydes (Table 3). β-1 lignin
a
o
0
2
.4 mmol acid, 2 mL MeOH, 0.4 MPa O , 90 C, 2 h. Phen : 1,10-phenanthroline.
2
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γ
models with C -OH were also successfully broken down to Keto-Enol tautomerism of ketone and make the β-H active.
esters with 87% yields.
Isotopic experiment showed a large k
−H oxidation is the rate-determining step for C−C bond
cleavage. The BF ·OEt may activate the C −H bond via the
coordination with oxygen atom in carbonyl group. DFT
calculation demonstrated the C −H bond energy decreased by
, respectively, after interaction
(Fig. 1). NMR results further confirmed this point. The
coordination of carbonyl group with BF
induced the β−H and
α−C shiꢀ to lower magneꢁc field due to the deshielding effect
Fig. 2).
H D
/k (6.7, eq.1), indicating
C
β
a
Table 3. Substrate scope of various oxidized lignin models.
3
2
β
β
-1
1
24 and 29 kJ mol for
1 and 2
with BF
3
3
(
a
0
.2 mmol substrate, 0.04 mmol Cu(OAc)
2
, 0.4 mmol BF
3 2
·OEt , 2 mL MeOH,
o
, 100 C, 3 h. The results were determined by GC-MS.
0
.4 MPa O
2
The solvent shows significant effect on the solubility of the
native lignin and thus its depolymerization. Various aliphatic
alcohols were then used as solvent. The linkage cleavage
proceeded successfully in the aliphatic alcohols including
ethanol, 1-propanol, 1-butanol, iso-butanol, 1-pentanol, and 1-
hexanol and 1-octanol, forming the corresponding esters with
Then, we investigated the reaction mechanism. Methyl
benzoylformate was detected as minor product, but cannot be
converted to methyl benzoate (eq. 2), indicating that it was
not the reaction intermediate. Nearly no CO
2
was detected
82-94% yields together with 40-87% yields of phenol (Table 4).
(
Fig.S1), and HCOOMe was detected by GC-MS (Fig.S2),
The result expands the possible use of this catalytic system to
produce various esters from lignin.
β
indicating the the C was dominantly converted into formic
acid instead of CO
2
. The benzoic acid was facilely converted to
·OEt (eq. 3). 2-
a
Table 4. Oxidative cleavage of lignin ketone models in various alcohols.
methyl benzoate in the presence of BF
3
2
Hydroxy-2-phenylacetophenone (eq. 4) and 1, 2-
diphenylethanedione (eq. 5) can’t be cleaved by the
2 3 2
Cu(OAc) /BF ·OEt
, indicating the C−C bond cleavage is not via
the hydroxyl ketone structure.
3
Fig. 1. The C−H bond dissociaꢁve energy before and aꢀer interacꢁon with BF .
a
0
.2 mmol substrate, 0.04 mmol Cu(OAc) , 0.4 mmol BF
2
3 2
·OEt , 2 mL
o
, 100 C, 6 h. The results were determined by GC-MS.
alcohol, 0.4 MPa O
2
Based on the above-mentioned results, we proposed a
tentative mechanism for C−C bond cleavage (Scheme 2).
Considering that, the C–C cleavage reaction did not occur
3 2
The promoting effect of BF ·OEt on the C−C bond cleavage
is further studied. It is well know that the acid can promote the
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under Ar atmosphere (not shown), it may involve the
formation and decomposition of hydroperoxide intermediate
Acknowledgements
6
6
This work was supported by the National Natural Science
Foundation of China (21603219, 21422308, 21273231), the
Doctor Startup Foundation of Liaoning Province (20141148)
and by the Strategic Priority Research Program of Chinese
Academy of Sciences (XDB17020300). Computing resources
from the National Supercomputing Center in Shenzhen,
People’s Republic of China, and the National Supercomputing
Center Tianjin, People’s Republic of China, are gratefully
acknowledged.
as reported. In the initial step, the BF
−H bond via the coordination with oxygen atom in carbonyl
group groups. C −H bond is oxidized by the activated oxygen to
give hydroperoxide intermediate
). Then, O−O bond
cleavage in generates oxygen-centered radicals ( ) which
facilitates the C –C bond cleavage via single electron transfer,
generating benzoic acid. The BF ·OEt in situ catalyzes the
3 2
·OEt activates the
C
β
β
(
A
A
B
α
β
3
2
esterification of acid with alcohol solvent to ester and the
acetalation of aldehyde.
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Green Chemistry
Page 6 of 6
View Article Online
DOI: 10.1039/C6GC02970A
The combination of Cu(OAc)
2
and BF
3
·OEt is effective for the simultaneous C-
2
C bond cleavage of β-1 and β-O-4 ketone, yielding esters and phenols. In-
depth studies show C−H bond activation is the rate-determining step for the C–
C bond cleavage and BF
3
·OEt promotes the reaction via activating the β-C−H
2
bond.