Selective Utilization of the Methoxy Group in Lignin to Produce Acetic Acid

Article abstract of DOI:10.1002/anie.201706846

Selective transformation of lignin into a valuable chemical is of great importance and challenge owing to its complex structure. Herein, we propose a strategy for the transformation of methoxy group (-OCH₃) which is abundant in lignin into pure highly valuable chemicals. As an example to apply this strategy, a route to produce acetic acid with high selectivity by conversion of methoxy group of lignin was developed. It was demonstrated that the methoxy group in lignin could react with CO and water to generate acetic acid over RhCl₃ in the presence of a promoter. The conversions of methoxy group in the kraft lignin and organosolv lignin reached 87.5 % and 80.4 %, respectively, and no by-product was generated. This work opens the way to produce pure chemicals using lignin as the feedstock.

Full text of DOI:10.1002/anie.201706846

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Accepted Article
Title: Selective Utilization of Methoxyl Group in Lignin to Produce
Acetic Acid
Authors: Qingqing Mei, Huizhen Liu, Xiaojun Shen, Qinglei Meng,
Hangyu Liu, Junfeng Xiang, and Buxing Han
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Angewandte Chemie International Edition
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COMMUNICATION
Selective Utilization of Methoxyl Group in Lignin to Produce
Acetic Acid
Qingqing Mei[a,b], Huizhen Liu*[a,b], Xiaojun Shen[a,b], Qinglei Meng , Hangyu Liu[a,b], Junfeng Xiang[a],
[a]
Buxing Han*[a,b]
Abstract: Selective transformation of lignin into a valuable chemical
is of great importance and challenge due to its complex structure.
Herein, we propose the strategy for the transformation of methoxyl
complex structure of lignin. The depolymerized product usually
contains numerous compounds.
Lignin is abundant of methoxyl group[1a], as shown in Figure
S1. However, selective utilization of methoxyl group in lignin to
produce valuable compounds has not received attention. Acetic
acid is an important commodity chemical with a broad range of
applications, such as producing vinyl acetate monomer (VAM)
and aceticanhydride, and as a solvent for purified terephthalic
3
group (-OCH ) which is abundant in lignin into pure highly valuable
chemicals. As an example to apply this strategy, the route to
produce acetic acid with high selectivity by selective conversion of
methoxyl group of lignin was developed. It was demonstrated that
the methoxyl group in lignin could react with CO and water to
[
9]
generate acetic acid over RhCl
3
in the presence of promoter. The
acid (PTA) production . Currently, acetic acid is produced in
industry mainly by carbonylation of methanol[10]
conversions of methoxyl group in the kraft lignin and organosolv
lignin could reach 87.3% and 80.4%, respectively, and no by-product
was generated. This work opens the way to produce pure chemicals
using lignin as the feedstock.
.
Herein, we propose the strategy that a specified class of
group (methoxyl group) in lignin is transformed into highly
valuable pure chemical. As an example, the route to produce
acetic acid by selective conversion of methoxyl group of lignin
was developed. It was demonstrated that 87.3% and 80.4% of
methoxyl group in the kraft lignin and organosolv lignin could be
transformed into acetic acid, respectively. The reaction could be
[
1]
Biomass resources are abundant and renewable . Utilization of
biomass as raw materials to produce useful chemical
compounds and fuels can liberate us from the reliance on fossil
[
2]
o
resource, and many researches have been conducted .
Lignocellulose is the most abundant form of biomass, with an
carried out at 120 C, which was lower than that for the
carbonylation of methanol to acetic acid (about 150–200 C)[11].
o
[
3]
annual production of around 170 billion metric tons , about 20%
of which is lignin. Large amount of lignin is produced in the pulp
and paper industry as by-product. Simply burning lignin is
Kraft lignin, which is a by-product of paper industry, was first
studied. The lignin contained 11.0 wt% of methoxyl group
determined by the iodine stoichiometry titration method[ , and
the apparatus and procedures are discussed in Supplementary
Information (Figure S2 and Figure S3). The catalyst system for
the production of acetic acid from the lignin was explored. It is
known that Rh-based catalyst was efficient for carbonylation
12]
[
4]
increasingly recognized as a waste of its potential , and efficient
utilization of lignin is of great importance for sustainable
development of our society. Over the past few decades,
researches on the production of alternative fuels, platform
compounds,value-added chemicals, and materials from lignin
reaction, and metal halides can cleave ether bond at appropriate
[
5]
have grown rapidly . Overall, utilization of lignin is mainly at the
research stage, and there have been few industrial
applications[ . So far, general routes to convert lignin into
valuable chemicals and fuels focus on breaking it into low-
condition[ . Using RhCl
13]
as the catalyst, the performances of
3
different metal halide promoters and solvents were studied, and
the results obtained at different conditions are presented in
Table 1. In this work, the yield of acetic acid is defined as the
percentage of the methoxyl group in the lignin that converted
into acetic acid. The output is defined as the grams of acetic
acid produced from per gram of lignin. No gaseous product was
generated in the reaction.
4b]
[
6]
molecular-weight compounds , which are further converted into
[
7]
valuable products . In addition, many researches have been
carried out to transform lignin model compounds into chemicals
[
8]
or fuels . Although satisfactory results have been achieved with
different model compounds, the catalytic systems are usually not
efficient for conversion of real lignin. Moreover, it is a great
challenge to get a chemical with high selectivity because of the
4
LiI was efficient promoter in the presence of LiBF , and the
activity of LiBr was much lower, while LiCl, NaI, KI, CsI were not
effective for the reaction (Table 1, entries 1-6). Solvents often
affect a reaction significantly[14]. The reaction did not occur in
water or DMSO (Table 1, entries 7 and 8). Acetic acid was
produced in toluene, cyclohexane, benzene and chlorobenzene
[
a]
Dr. Q. Mei, Prof. Dr. H. Liu, Dr. X. Shen, Dr. Q. Meng, Dr. H. Liu,
Dr.Prof. J. Xiang, Prof. Dr. B. Han.
Beijing National Laboratory for Molecular Sciences, CAS Key
Laboratory of Colloid and Interface and Thermodynamics, CAS
Research/Education Center for Excellence in Molecular Sciences,
Institute of Chemistry,Chinese Academy of Sciences, Beijing
(Table 1, entries 1 and 9-11), and toluene was the optimum
organic solvent with a yield of 78.8%, i.e., 0.168 g of acetic acid
could be produced from 1 g of lignin considering that lignin
contained 11.0 wt% of methoxyl group. Acetic acid is the only
product (the NMR spectra are given in Figure S4), and the mass
balance was 99.7% at this condition (See supporting information
1
00190 (P. R. China).
E-mail: liuhz@iccas.ac.cn (Huizhen Liu); hanbx@iccas.ac.cn
Buxing Han).
(
[
b]
Dr. Q. Mei, Prof. Dr. H. Liu, Dr. X. Shen, Dr. H. Liu, Prof. Dr. B. Han.
School of Chemistry and Chemical Engineering, University of
Chinese Academy of Sciences, Beijing 100049 (P. R. China).
for details). LiBF
4
was necessary for the reaction and no acetic
acid was produced in the absence of LiBF
4
(Table 1, entry 12).
Ionic liquids (ILs) have been widely used as reaction solvents[
.
15]
Supporting information for this article is given via a link at the end of
the document.
The reaction was also conducted in different ILs (Table 1,
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COMMUNICATION
Table 1. Results for the reaction of lignin, CO and H
different catalyst systems.
2
O to acetic acid in
[
a]
Output of acetic
acid (g/g)
Yield of acetic
acid (%)
Entry
Solvent
Additive
[
b]
[c]
Figure 1. Effects of amount of lignin (a) and reaction time using 0.2 g kraft
1
2
toluene
toluene
toluene
toluene
toluene
toluene
LiBr+LiBF
4
0.045
21.1
0
lignin (b) on the reaction. Reaction conditions: 0.01 g RhCl
4 2
mmol LiBF , 5 MPa CO, 1.5 mL toluene, 15 mmol H O, 140 C, 12 h.
3
, 2 mmol LiI, 1
o
LiCl+LiBF
4
0
3
LiI+LiBF
4
0.168
78.8
0
entries 13-17). The reaction could proceed smoothly without
4
NaI+LiBF
4
0
adding LiBF
tetrafluoroborate) and BMimBF
acetate), which contains BF -
3.1% in HMimBF
produced from 1 g of lignin (Table 1, entry 13). The yield of
acetic acid was 83.7% in BMimBF (Table 1, entry 14). However,
4
in HMimBF
4
(1-hexyl-3-methylimidazolium
(1-butyl-3-methylimidazolium
5
KI+LiBF
4
0
0
4
4
. The yield of acetic acid was
4
, i.e., 0.177 g of acetic acid could be
6
CsI+LiBF
4
0
0
0
8
7
H
2
O
LiI+LiBF
LiI+LiBF
LiI+LiBF
LiI+LiBF
4
0
4
8
DMSO
cyclohexane
benzene
4
0
0
no acetic acid was generated in BMimCl (1-butyl-3-
methylimidazolium chloride) and BMimOAc (1-butyl-3-
9
4
4
4
0.165
0.114
0.108
0
77.5
53.5
50.7
0
methylimidazolium acetate) without LiBF
and 16). The reaction also occurred in BMimCl (1-butyl-3-
methylimidazolium chloride) with LiBF , and 74.7% yield of
4
(Table 1, entries 15
10
4
11
chlorobenzene LiI+LiBF
acetic acid was obtained, i.e., 0.159 g of acetic acid could be
produced from 1 g of lignin (Table 1, entry 17). The results
12
13
14
toluene
LiI
LiI
-
further indicated that BF
4
was necessary for the reaction. The
HMimBF
4
0.177
0.174
0
83.1
81.7
0
blank test demonstrated that acetic acid could not form without
lignin (Table 1, entry 18). We also carried out the reaction
without RhCl , and acetic acid was not produced, but CH I was
3 3
BMimBF
4
LiI
yielded (Table 1, entry 19). It can be known from Entries 12 and
15
BMimCl
LiI
-
19 that BF
4
could catalyze the reaction of lignin and LiI to form
1
6
7
BMimOAc
BMimCl
toluene
LiI
0
0
CH
3
I, which is an intermediate for the generation of acetic acid.
1
LiI+LiBF
LiI+LiBF
LiI+LiBF
4
4
4
0.159
0
74.7
0
The effects of different factors on the reaction were studied
(Figure 1 and Figure S5). The effect of amount of lignin on the
8[d]
1
yield and output is demonstrated in Figure 1a. The yield and
output of acetic acid decreased continuously with increasing
amount of lignin. This is understandable because the yield and
output was on the lignin basis (Figure 1a). The yield and output
of acetic acid increased gradually with reaction time prolonging
at beginning, and became almost unchanged after 12 h (Figure
1
9[e]
toluene
0
0
[
0
a] Reaction conditions: 0.01 g RhCl
.2 g kraft lignin containing 11.0 wt% of methoxyl group determined by the
3
, 2 mmol metal halide, 1mmol LiBF
4
,
[
12]
o
o
reported method , 120 C in ionic liquids and 140 C in organic solvents,
.5 mL solvent, 12 h. [b] Output is the grams of acetic acid produced per
gram of lignin. [c] Yield is the percentage of the methoxyl group in the lignin
that converted into acetic acid. The amount of acetic acid was obtained by
NMR method using 1,3,5-trioxane as an internal standard. [d] Without lignin.
1
1b). The effects of reaction temperature, CO pressure and the
amount of water on the yield and output of acetic acid were also
studied, and results are given in Figure S5. No acetic acid was
3
No acetic acid was produced. [e] Without RhCl . No acetic acid was
produced, but CH I was detected.
o
produced at 100 C and the yield of acetic acid was only 8.4% at
3
o
1
10 C, i.e., only 0.018 g of acetic acid could be produced from 1
g of lignin. The yield of acetic acid increased sharply to 55% at
o
1
20 C, while it increased slowly as the temperature increased
o
o
o
from 140 C to 160 C. At 160 C, the yield reached 87.5%, i.e.,
.186 g of acetic acid could be produced from 1 g of lignin
Figure S5a). The yield and output of acetic acid increased
We characterized the lignin before and after reaction by two-
dimensional heteronuclear single quantum coherence nuclear
magnetic resonance (2D-HSQC NMR)[ , and the spectra are
shown in Figure 2. It is known that the signal of methoxyl group
in lignin is at 56.4/3.70 ppm[ , and the corresponding region
was colored with yellow in Figure 2. It can be known that the
signal of methoxyl group dramatically decreased after reaction.
Meanwhile, the signal of the aromatic region also indicates that
0
(
16]
continuously with increasing CO pressure at beginning, and was
independent of the pressure after 5 MPa (Figure S5b). Water
was necessary for the reaction, but too much water in the
reaction system was not favorable to the reaction (Figure S5c).
16b]
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aryl methyl ether structures (guaiacyl units, colored with green)
transformed to phenol structures (catechyl units, colored with
brown). The lignin was modified to be rich of hydroxyl, which
could be advantageous and feasible for further usage and
modification.
We performed the experiments using deuterated toluene
(toluene-d ) instead of toluene in the reaction of the two lignins.
8
NMR analysis was carried out before and after reaction, and the
results are provided in the Figure S7 and Figure S8, respectively.
It can be concluded from the results that the solubility of the
lignins in toluene before and after reaction was
negligible. The results also showed that CH
was produced even deuterated toluene was used as
the solvent, confirming that the methyl in CH COOH is
3
COOH
3
from lignin. This is consistent with the conclusion
obtained from the blank experiment that no acetic acid
was produced without lignin (entry 18 of table 1).
The GPC traces of the two lignins before and after
the reaction were given in Figure S9 and Figure S10,
respectively. The results showed the GPC traces
before and after the reaction were similar, indicating
there were no detectable depolymerisation products.
We also analyzed the liquid phase (toluene phase) of
the reaction mixture by HPLC (Figures S11 and S12).
The results indicated that acetic acid was the only
detectable product. The result was consistent with the
conclusion obtained from NMR analysis that acetic
acid was the only product (Figure S4).
Anisole is the simplest model compound of lignin
that contains a methoxyl group. We also studied the
reaction using anisole (Scheme 1). Acetic acid and
phenol were produced, and the conversion of anisole
could reach 91.2% without any other product. This is
consistent with the results that methoxyl group in lignin
can be transformed into acetic acid selectively.
Scheme 1. The reaction of anisole as methyl source to produce
Figure 2. The 2D-HSQC NMR spectra of the lignin before (left) and after
acetic acid. Reaction conditions: 0.01 g RhCl
MPa CO, 15 mmol H O.
3 4
, 2 mmol LiI, 1.5 mL HMinBF , 5
(right) reaction. Reaction conditions: 0.01 g RhCl
3
, 2 mmol LiI, 1 mmol LiBF
4
,
2
o
0
.2 g kraft lignin, 5 MPa CO, 1.5 mL toluene, 15 mmol H O, 140 C, 12 h.
2
On the basis of the experimental results and the related
knowledge in the literature[18], we propose the possible reaction
mechanism, which is shown schematically in Scheme 2. The
active component of the catalyst is rhodium carbonyl generated
The reaction was also conducted with organosolv lignin. The
organosolv lignin was obtained by treating milled bamboo with
1
,4-dioxane/water mixed solvent using the method developed by
Björkman[ . The methoxyl content in the organosolv lignin was
17]
in situ[18]. LiBF
4
catalyzed the reaction of methoxyl group and LiI
I and lithium phenolate, as can be known from
Entries 12 and 19 of Table 1. In addition, trace amount of CH
1
4.1wt% determined by iodine stoichiometry titration method[12]
.
to form CH
3
o
At 140 C, the yield of acetic acid was 80.4%, i.e., 0.219 g of
acetic acid could be produced from 1 g of organosolv lignin, and
the mass balance was 99.1% at this condition. The organosolv
lignin before and after reaction were also characterized by 2D-
HSQC NMR (Figure S6). Similarly, the signal of methoxyl group
dramatically decreased after reaction and the signal of the
aromatic region also indicates that aryl methyl ether structures
3
I
was detected in the reaction mixture (Figure S4), which supports
this argument. The sequence involves the oxidative addition of
CH I to Rh species, CO-insertion, then reductive elimination of
3
acetyl iodide, followed by its hydrolysis to acetic acid and HI.
Finally, HI reacts with lithium phenolate and regenerates LiI. It is
known that iodide is
a
necessary component for the
(guaiacyl units, syringyl units and ferulates, colored with green,
carbonylation of methanol to acetic acid, and iodide serves as a
promoter[9-10]. However, other iodide sources in our work such as
NaI, KI and CsI were not effective for the reaction of lignin, CO
purple and light yellow, respectively) transformed to phenol
structures (catechyl units and 5-hydroxyguaiacyl units, colored
with brown and black, respectively). This indicates that the
method is applicable to different types of lignin.
and H
2
O to form acetic acid (Table 1, entries 2-4). The main
reason may be that NaI, KI and CsI could promote the
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COMMUNICATION
carbonylation reaction but could not break the ether bond. So LiI
acts as two roles. One role is breaking the ether bond and the
other is promoting the carbonylation of methyl iodide to acetic
acid.
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methyl source to produce acetic acid
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In summary, we have proposed the strategy to produce pure
valuable chemical using lignin as the feedstock, in which a
specified class of group is utilized in the reaction instead of
depolymerization of lignin. To demonstrate the feasibility of this
method, the route to produce acetic acid using the methoxyl
group in lignin as the feedstock have been developed. For
producing acetic acid from lignin, CO and water, the yields can
reach 87.3% and 80.4%, respectively, when kraft lignin and
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[
[
9]
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from lignin. We believe that the strategy that transforms a class
of group in lignin selectively into product with high purity can be
used to produce other chemicals, which needs many researches
in the future.
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Keywords: Transformation • lignin• carbon monoxide • water •
acetic acid
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Entry for the Table of Contents
COMMUNICATION
Qingqing Mei, Huizhen Liu*, Xiaojun
Shen, Qinglei Meng, Hangyu Liu,
Junfeng Xiang, Buxing Han*
Page No. – Page No.
Selective Utilization of Methoxyl
Group in Lignin to Produce Acetic
Acid
A strategy for the transformation of methoxyl group in lignin is proposed. The route
to produce acetic acid by selective reaction of methoxyl group in lignin with CO and
water over RhCl
3
is developed. The conversion of methoxyl group in the lignin can
reach 87.3% with 100% selectivity to acetic acid.
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