Metal Catalyst-Free Oxidative C?C Bond Cleavage of a Lignin Model Compound by H2O2 in Formic acid

Article abstract of DOI:10.1002/cssc.201903180

Selective cleavage of the β-O-4 ether bond of lignin to produce aromatics is one of the most important topics for the sustainable production of chemicals from biomass. A simple system has been developed for C_α?C_β bond cleavage of a β-O-4 ketone-structured lignin model compound (LMC) by H₂O₂ in formic acid under metal catalyst-free conditions. By using this simple system, with H₂O₂, formic acid, and mineral acid catalyst, over 90 % product yield is achieved in 6 h at room temperature. The reaction proceeds through the classic Baeyer–Villiger oxidation and in situ-generated performic acid serves as the key oxidant. The cleavage of alcoholic LMCs by using the presented method in a two-step process is also demonstrated.

Full text of DOI:10.1002/cssc.201903180

Accepted Article
Title: Metal Catalyst-Free Oxidative C−C Bond Cleavage of a Lignin
Model Compound by H2O2 in Formic acid
Authors: Yugen Zhang and Xiukai Li
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10.1002/cssc.201903180
ChemSusChem
COMMUNICATION
Metal Catalyst-Free Oxidative C−C Bond Cleavage of a Lignin Model
Compound by H2O2 in Formic acid
Xiukai Li*[a] and Yugen Zhang*[a]
Abstract: Selective cleavage of the β-O-4 ether bond of lignin to
produce aromatics is one of the most important topics of the
sustainable production of chemicals from biomass. We demonstrate
a simple system for Cα-Cβ bond cleavage of a β-O-4 ketone structured
lignin model compound (LMC) by H2O2 in formic acid under metal
catalyst-free conditions. In the system simply with H2O2, formic acid,
and mineral acid catalyst, over 90% of product yield could be achieved
in 6 h at room temperature. The reaction proceeds through the classic
Baeyer-Villiger oxidation (BVO) and the in situ generated performic
acid is the key oxidant. The cleavage of alcohol LMC by the present
method in a two-step process is also successfully demonstrated.
tandem reactions for more difficult Cα-Cβ bond cleavage via the
ketone-structured LMC (Scheme 1). As a good example, Wang
et al.[11a] developed a two-step process for Cα-Cβ cleavage of LMC.
The β-O-4 alcohol LMC was oxidized to β-O-4 ketone LMC by
pressurized oxygen with the assist of VOSO4/TEMPO catalysts,
and then cleaved by pressurized oxygen in presence of
organometallic Cu complex.
Scheme 1. Two-step strategy for Cα-Cβ cleavage of β-O-4 alcohol LMC via the
β-O-4 ketone LMC.
Due to the increasing concerns on energy security and
environment pollution in the recent years, there have been
particular interests for the sustainable production of chemicals
and fuels from renewable biomass.[1] Non-edible lignin, the
second most abundant biomass in nature, is the largest source of
aromatics in world.[2] For lignin, the β-O-4 ether bond is the most
abundant linkage (> 50% in total) in its polymeric structure
(Scheme 1).[3] It would be significant to depolymerise these
sustainable lignin materials into aromatics or other small molecule
chemicals.[4] However, because of the complex structure of lignin
and the rather stable chemical bonds, only limited selectivity and
yield were achieved from raw lignin. Efficient methods for β-O-4
ether bond breaking and C-C bond breaking are nessisary for
lignin utilization. In this context, β-O-4 lignin model compounds
(LMCs) have been widely studied for better understanding of the
chemistry and challenges of real lignin.
Baeyer-Villiger oxidation (BVO) by peroxides is a classic
method to form an ester from a ketone. Inserting an oxygen atom
to LMC substrate by BVO would enable easier dissociation of the
inert Cα-Cβ bond of β-O-4 LMC. In 2013, Stahl et al.[11b]
demonstrated the cleavage of Cα-Cβ bond of ketone LMC by H2O2
under basic condition (2 M NaOH) in THF/methanol (1/1) mixed
solution. Veratric acid in 88% yield and guaiacol in 42% yield was
achieved from this system. In 2017, Zhang et al.[8a] used sole 3-
chloroperbenzoic acid or H2O2 in combination with (PhCH2Se)2
catalyst for Baeyer-Villiger oxidation of ketone LMC. The acetal
ester and aryl ester products from BVO were further dissociated
by alcoholysis in K2CO3/alcohol mixture. The breakthrough of
previous work inspired us to look for a simpler system for Cα-Cβ
bond cleavage of β-O-4 LMC. In this work, ketone LMC was
cleaved directly by H2O2, a green and liquid oxidant,[12] in formic
acid under metal catalyst-free conditions, and over 85% product
yield was achieved in 4 h at 100 °C. With mimeral acid catalyst,
the reaction was efficient at room temperature (25 °C) and over
90% of product yield could be achieved in 6 h.
Oxidation is one of the most important chemistry for lignin
depolymerisation. Systems with catalysts like organometallics,[5]
simple metal irons,[6] and metal oxides[7] have been developed for
oxidative lignin depolymerisation or LMCs cleavage, resulting in
poor to good product yields. The breaking of the β-O-4 ether bond
could happen at Cα-Cβ or Cβ-O bonds. Generally, the Cα-Cβ bond
is more difficult to be cleaved than the Cβ-O bond due to the higher
energy barrier.[8] Oxidation of Cα-OH to ketone is of particular
significance for LMCs cleavage.[9] Computational and
experimental studies indicate that the β-O-4 ether bond shows
In view that the selective oxidations of β-O-4 alcohol LMC to
ketone LMC have been well documented and high product yields
could be achieved,[8b, 9, 11a] a ketone-structured LMC, 2-phenoxy-
1-phenylethanone (1), was employed as substrate directly in our
work. In our initial study, we tried to cleave (1) directly by H2O2 in
various solvents without addition of any acid, base, or metal
catalyst (Table 1). Over 90% conversions were achieved in C1 ~
C4 organic acids (Entries 1 ~ 4, Table 1) in 4 h at 100 °C. The
conversions were dropped to less than 10% when other solvents
were used, (Entries 5 ~ 8, Table 1). Phenoxymethyl benzoate (2),
benzoic acid (3), and phenol (4) were the major products for all
solvents. Phenyl formate and benzaldehyde were produced in <
1% yield, thus, they were not included in Table 1. Some
unidentified products were also detected in our GC analysis. With
increasing length of the carbon chain of acid, the yield for (2)
increased while the yields for (3) and (4) decreased. In formic acid
solvent, full conversion was achieved with 85.9% yield for (3) and
18.4% yield for (4). In butyric acid solvent, (2) was produced at
higher tendency to be cleaved when the Cα-OH was oxidized to
10]
ketone.[9a,
When the Cα-OH was oxidized to ketone, the
dissociation energy for the Cβ-O bond reduced from 69.2 kcal mol-
1 to 55.9 kcal mol-1.[10] Various oxidation methods have therefore
been developed for the transformation of the β-O-4 alcohol LMC
to the β-O-4 ketone LMC,[8b, 9, 11] and over 90% product yields
could be achieved.[9c, e, 11] There have also been fewer reports on
[a] Dr. X. Li, Dr. Y.G. Zhang.
Institute of Bioengineering and Nanotechnology, 31 Biopolis Way, The
Nanos, Singapore 138669, Singapore. E-mail: xkli@ibn.a-star.edu.sg,
ygzhang@ibn.a-star.edu.sg
Supporting information for this article is given via a link at the end of the
document.
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10.1002/cssc.201903180
ChemSusChem
COMMUNICATION
A
61.8% yield while (3) and (4) were produced at the yields of 17.8%
and 1.9%, respectively. The changes in conversions and product
yields with respect to different solvents signify that organic acids
play profound roles in ketone LMC cleavage by H2O2.
100
80
60
40
20
0
Conv.
Yield of (2)
Yield of (3)
Yield of (4)
Table 1. Oxidative cleavage of lignin model compound (1) in different solvents.
Yield
(2)
0.0
48.0
54.3
61.8
0.0
1.8
0.4
0.0
/ %
(3)
Conv.
/ %
Entry
Solvent
0.0
0.5
1.0
1.5
2.0
(4)
Amount of formic acid / mL
1
2
3
4
5
6
7
8
formic acid
acetic acid
propionic acid
butyric acid
DMSO
t-butanol
IBMK
H2O
100.0
93.2
99.0
97.8
9.9
6.4
3.8
< 1.0
85.9
30.8
31.0
17.8
4.8
6.1
0.0
0.0
18.4
8.7
1.4
1.9
8.1
8.1
7.4
0.0
B
100
80
60
40
20
0
Conv.
Yield of (2)
Yield of (3)
Yield of (4)
Reaction conditions: 2-phenoxy-1-phenylethanone (1): 107 mg (0.5 mmol),
H2O2 (31%): 0.5 mL, solvent 2 mL, 100 °C, 4 h.
As high conversion and good product yields were achieved for
(1) cleavage in formic acid, the amount of formic acid and other
variables such as reaction temperature, reaction time, and
amount of H2O2 were further studied. Figure 1A shows that the
amount of formic acid had a significant influence on the reaction.
The conversion of (1) and the yield for (3) increased with
increasing amount of formic acid. Over 90% conversion for (1)
and over 80% yield for (3) were achieved when the amount of
formic acid exceeded 1 mL. However, in a comparison reaction
with 1 mL of formic acid plus 1 mL of DMSO as solvent, the
conversion of (1) was less than 5%. Thus, it is concluded that high
concentration of formic acid is essential for this reaction. Figure
1B shows the effect of reaction temperature on the reaction.
Though over 90% conversion was achieved in 4 h at room
temperature (25 °C), the dominant product was acetal ester (2) at
69.1% yield and the cleavage products were at respectively
18.2% yield for (3) and 7.3% yield for (4). With temperature rise,
the yield for (2) decreased drastically, and the yield for (3)
increased accordingly and reached 85.9% at 100 °C. Further
increase in temperature did not affect the conversion and yields
for (3) and (4) significantly. The optimal reaction time at 100 °C
was 4 h and the optimal loading amount of H2O2 was 0.5 mL
(Figures S1). The reaction performed at room temperature was
slower but would be helpful to understand the reaction mechanim
and the evolution of products. Figure 1C demonstrates that the
conversion increased quickly to 92.2% in the first 4 h at room
temperature (25 °C). The yield for (2) reached 69.1% in 4 h and
then decreased gradually at prolonged reaction time. The yield for
(3) increased continuously and reached 53.9% in 65 h. The yield
for (4) was always less than 10% due to the formation of side
products. The reverse trends in yields for (2) and (3) signify that
(2) is an intermediate for the reaction and that the transition of (2)
to (3) is kinetic controlled.
20
40
60
80
100
120
140
Temperature / C
C
100
80
60
40
20
0
Conv.
Yield of (2)
Yield of (3)
Yield of (4)
0
20
40
60
Time / h
Figure 1. (A) Lignin model compound (1) cleavage by H2O2 in different amount
of formic acid at 100 °C for 4 h, (B) Lignin model compound (1) cleavage by
H2O2 in 2 mL formic acid for 4 h at different reaction temperatures, and (C) lignin
model compound (1) cleavage by H2O2 in 2 mL formic acid for different reaction
time at room temperature (25 °C). Other reaction conditions: substrate (1) 107
mg (0.5 mmol), H2O2 (31%) 0.5 mL.
The results in Figure 1 suggest that the cleavage of (1) by
H2O2 is efficient in organic acids such as formic acid; however,
the reaction was sluggish in other types of solvents as have been
reported in Table 1. Thus, the reaction in formic acid was
monitored by NMR (Figure 2) to better understand the roles of this
particular solvent. Blank experiment show that there was no
reaction happened between LMC (1) and formic acid in the
absence of H2O2 (Figure 2 a). In the LMC (1)/H2O2/formic acid
system (Figure 2, b ~ f), new signal ( 8.21) attributable to
performic acid in addition to that of formic acid ( 7.99) was
observed. The -CH2- signal ( 5.30) for substrate (1) diminished
gradually with time while the -CH2- signal ( 5.83) for (2) increased
accordingly. In Table 1, it has been shown that the reaction cannot
proceed in aquous H2O2 probably because of the poor solubility
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10.1002/cssc.201903180
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of the substrate. However, in homogenoeus DMSO and t-butanol
systems (Entries 5 & 6, Table 1), the conversions of (1) by H2O2
were still much lower (< 10%). Thus, it is concluded that the in situ
generated performic acid in stead of H2O2 should be the oxidation
species for this reaction. Based on the results of kinetic study and
NMR analysis, we propose the reaction should follow the classic
Baeyer-Villiger oxidation mechanism (Scheme 2): performic acid
attacks the carbonyl group of (1) to form the Criegee intermediate,
then to acetal ester (2); the dissociation of (2) leads to benzoic
acid (3) and phenol (4). Organic peracids have stronger -O-O-
bonds than H2O2 because of the electron-withdrawing effect of the
carbonyl group.[13] Moreover, the organic acids and peracids
provide strong acidic environments that are favorable for the BVO
and the dissocaition of the intermediate product (2). With
increasing length of the carbon chain, the acidity of organic acid
decreases and the dissociation of the intermediate becomes
slower as have been shown in Table 1. Similar effects of organic
acid on BVO have also been observed in our previous study on
furfural oxidation to maleic acid by H2O2.[13] Using in situ
generated perorganic acids as oxidants is a method for biomass
varitization, however, the extents of oxidations and the
selectivities for products depend significantly on the type and
concentration of the organic acid employed in the reaction.[13-14]
The reaction conducted at room temperature would be energy
efficient. The results in Figure 1 show that the BVO of ketone LMC
(1) to (2) by performic acid is efficient in a broad range of
temperature, however the dissociation of the intermediate (2) to
(3) and (4) was kinetic controlled and slow at room temperature.
Inspired by the result that the reaction was faster in the stronger
formic acid solvent than in other weaker organic acids (Table 1),
various mineral acids with different strength was added into the
system to accelerate the reaction at room temperature. The
amounts of H+ were 0.1 mmol for all added acids. From Table 2,
it can be seen that the dissociation of (2) is correlated with the
strength of the acid additive. With the strongest F3CSO3H and
HOCl4 acids, respectively 96.3% and 86.2% yields for (3) were
achieved and less than 2% for (2) was remained. With medium
strength H2SO4 added, the yield for (3) increased to 62.6% and
the yield for (2) dropped to 21.4%. When acids with lower strength
such as HNO3, H3PO4, and solid Amberlyst-15 were used, the
yield for (2) decreased by less than 10%. When hydrochloric acid
was used as an additive, the conversion was still high but the
selectivities for products were low and a lot of side products were
observed. This is probably because hydrochloric acid was
destroyed in the strong oxidizing environment. Thus, a strong
protonic acid is favorable to catalyze the conversion of (1) to (3)
at room temperature.
Table 2. Oxidative cleavage of lignin model compound (1) by H2O2 in
formic acid with mineral acid additives at room temperature.
Yield %
Acid additive
(concentration)
Entry
Conv. %
(2)
(3)
(4)
1
2
3
4
5
6
7
8
-
92.2
99.5
98.6
94.8
96.1
96.1
98.1
94.6
69.1
0.5
18.2
96.3
86.2
62.6
27.3
28.8
20.1
20.6
7.3
F3CSO3H (98%)
HOCl4 (70%)
H2SO4 (98%)
HNO3 (65%)
H3PO4 (85%)
HCl (37%)
28.9
35.4
35.2
11.0
12.0
2.2
1.7
21.4
65.2
61.6
28.9
66.0
Amberlyst-15
8.0
Reaction conditions: substrate (1) 107 mg (0.5 mmol), H2O2 (31%) 0.5 ml, acid
Figure 2. Lignin model compound LMC (1) cleavage by H2O2 in formic acid at
room temperature (25 °C) in the absence of H2O2 (a) and in the presence of
H2O2 (b ~ f). Substrate (1) 107 mg (0.5 mmol), H2O2 0.5 mL, formic acid 2 mL.
additive 0.1 mmol of H+, formic acid 2 mL, 25 °C, 4 h.
H2SO4 is safer and cheaper than F3CSO3H and HOCl4,
therefore, more applicable in large scale synthesis. The loading
amount and the reaction time for H2SO4 as a catalyst were further
optimized. As depicted in Figure 3A, the yield for (3) increased
with increasing loading amount of H2SO4. Over 80% of (3) was
produced when the amount of loaded H2SO4 exceeded 30 µL
(0.15 mmol). With 30 µL of H2SO4 added (Figure 3B), the yield for
(3) increased continuously with time and reached 94.4% in 8 h.
There was an optimal reaction time for phenol (4). The 43.5%
yield of (4) maximized at 2 h, then started to decrease at longer
reaction time. Thus, under optimized conditions with H2SO4 as a
catalyst, product yields could be achieved comparable to that from
the cases with strong F3CSO3H and HOCl4 as catalysts.
Scheme 2. The proposed reaction mechanism for lignin model compound (1)
cleavage by in situ generated performic acid.
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10.1002/cssc.201903180
ChemSusChem
COMMUNICATION
catalyze the conversion of (1) to (2), but a strong acid is required
for the dissociation of (2) to (3) and (4). The strong acid additive
could be added at the initial stage or several hours after starting
the reaction. Raising the reaction temperature is another method
to promote this rate-limiting step as that has been shown in Figure
1.
A
100
80
60
40
20
0
Conv.
Yield of (2)
Yield of (3)
Yield of (4)
Methoxy group is the most common substituent at the
aromatic rings in lignin. For a β-O-4 model compound with the
methoxy group at the ortho- position of phenoxy part (LMC3), up
to 93.8% yield for benzoic acid and 20.2% yield for 2-methoxy
phenol were achieved with full conversion at room temperature
for 16 h (Table S1). However, for a model compound with
methoxy groups at the meta- and oposite- positions of the
phenylethanone part (LMC4), the reaction was sluggish which is
due to the electron-donating nature of the CH3O- group and
affects the nucleophile attack of performic acid to ketone.
The results for ketone LMC cleavage by BVO in simple
H2O2/formic acid/H2SO4 system is encouraging. Since alcohol
lignin is prevalent in natural than the ketone lignin, we further
demonstrated the conversion of alcohol LMC by our method. The
direct cleavage of the alcohol LMC in the typical H2O2/formic
acid/H2SO4 system resulted in high conversion but broad product
distribution from (1), (3), and (4) to other side products. Thus, we
developed the two-step reaction route as illustrated in Scheme 3.
In the first step, alcohol LMC was oxidized by TMPO/O2 in
acetonitrile/H2O solvent[11b] and 91% yield for ketone LMC was
achieved in 24 h at 45 °C. With crude ketone LMC for the second
step reaction, 70.8% yield for (3) and 33.3% yield for (4) were
obtained in 4 h at room temperature from the H2O2/formic
acid/H2SO4 system in this single step. These results provide
additional support for the two-step lignin conversion strategy
proposed in Scheme 1.
0
10
20
30
40
50
60
H2SO4 / L
B
100
80
60
40
20
0
Conv.
Yield of (2)
Yield of (3)
Yield of (4)
0
2
4
6
8
10
12
14
16
Time / h
Figure 3. (A) Lignin model compound (1) cleavage by H2O2 in formic acid with
different amount of H2SO4 for 4 h at room temperature (25 °C), and (B) Lignin
model compound (1) cleavage by H2O2 in formic acid with 30 µL of H2SO4 at
room temperature (25 °C). Other reaction conditions: substrate (1) 107 mg (0.5
mmol), H2O2 (31%) 0.5 mL, formic acid 2 mL.
Scheme 3. The two-step strategy for alcohol LMC cleavage via the ketone LMC
intermediate. Step 1: alcohol LMC 107 mg (0.5 mmol), TEMPO 10 mg, O2 50
psi, acetonitrile 2.5 mL, H2O 130 µL, HNO3 (65%) 5 µL, HCl (37%) 5 µL, 45 °C,
24 h. Step 2: crude LMC (1) from previous step, H2O2 (31%) 0.5 mL, formic acid
2 mL, H2SO4 (30 µL), room temperature (25 °C), 4 h.
H2SO4
Conv.
Yield for (2)
Yield for (3)
Yield for (4)
100
80
In summary, we have demonstrated a very simple system for
C-C cleavage of ketone LMC by Baeyer-Villiger oxidation (BVO)
under metal catalyst-free conditions. In the system with ketone
LMC, H2O2 and formic acid reagents, over 85% product yield was
achieved in 4 h at 100 °C. Catalytic ammount of mineral acids
could accelerate the reaction and over 90% of product yield could
be achieved in 6 h at room temperature (25 °C). Mechanic study
indicated that the in situ generated performic acid is responsible
for the BVO of ketone LMC. Strong mineral acids accelerate the
dissociation of the acetal ester intermediate to form small
molecule aromatics. The cleavage of alcohol LMC by the present
method was also demonstrated in a two-step reaction process.
60
40
20
0
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Time / h
Figure 4. Lignin model compound (1) cleavage by H2O2 in formic acid. H2SO4
(30 µL) was added 4 h after starting the reaction. Reaction conditions: substrate
(1) 107 mg (0.5 mmol), H2O2 (31%) 0.5 mL, formic acid 2 mL, room temperature
(25 °C).
To further clarify the role of proton in each step of (1) oxidative
cleavage to (3) and (4) by H2O2, we added H2SO4 at the reaction
time 4 hours after starting the reaction (Figure 4). About 92.2%
conversion of (1) was attained in 4 h at room temperature without
any mineral acid added, and the yields for (2), (3), and (4) were
69.1%, 18.2% and 7.3%, respectively. In 10 min after 30 µL of
H2SO4 was added, the yield for (2) dropped from 69.1% to 20.8%
while the yield for (3) and (4) increased rapidly from 18.2% to
77.1% and 7.3% to 40.0%. It is clear that (2) dissociation to (3) is
the rate-limiting step in the whole process. A weak acid is able to
Experimental Section
Lignin model compounds synthesis.2-phenoxy-1-phenylethanone
(LMC (1)) and 2-phenoxy-1-phenylethanol (LMC (2)) were synthesized
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Keywords: biomass • oxidation • lignin • formic acid • hydrogen
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10.1002/cssc.201903180
ChemSusChem
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
In the system simply with H2O2,
formic acid, and mineral acid
catalyst, over 90% of product yield
could be achieved from Cα-Cβ bond
Xiukai Li* and Yugen Zhang*
Page No. – Page No.
cleavage of
structured lignin model compound
in 6 h at room temperature.
a
β-O-4 ketone
Metal Catalyst-Free Oxidative C−C
Bond Cleavage of a Lignin Model
Compound by H2O2 in Formic acid
This article is protected by copyright. All rights reserved.