Directly Microwave-Accelerated Cleavage of C?C and C?O Bonds of Lignin by Copper Oxide and H2O2

Article abstract of DOI:10.1002/cssc.202000502

Model erythro, phenolic, and nonphenolic lignin β-O-4 dimer compounds are treated with copper oxide and H₂O₂ at the electronic field maximum position of a single-mode 2.45 GHz microwave system equipped with a cavity resonator. The products obtained through microwave heating and oil-bath heating with the same reaction vessel and temperature profile are quantitatively compared. Dimer degradation is found to proceed through consecutive elementary reactions. The phenolic dimer is dehydroxylated and this is followed by the spontaneous cleavage of C_α?C_β and C?O?C bonds to produce guaiacol, vanillin, and vanillic acid. The reaction of the nonphenolic dimer produces veratric acid, veratraldehyde, and guaiacol. Microwave irradiation accelerates cleavage of the side chain and the oxidation of vanillin to vanillic acid. However, no acceleration of veratraldehyde oxidation to veratric acid or aromatic ring cleavage to produce dicarboxylic acids is observed. The selective acceleration of elementary reactions during the degradation of model lignin compounds indicates that microwaves interact with reaction intermediates that are sensitive to electromagnetic waves.

Full text of DOI:10.1002/cssc.202000502

Chemistry–Sustainability–Energy–Materials
Accepted Article
Title: Microwave directly accelerated cleavage of C–C and C–O bonds
of lignin by copper oxide–peroxide reaction
Authors: Chen Qu, Keigo Ito, Isamu Katsuyama, Tomohiko Mitani,
Keiichiro Kashimura, and Takashi Watanabe
This manuscript has been accepted after peer review and appears as an
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To be cited as: ChemSusChem 10.1002/cssc.202000502
Link to VoR: https://doi.org/10.1002/cssc.202000502
01/2020

10.1002/cssc.202000502
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Microwave directly accelerated cleavage of C–C and C–O bonds
of lignin by copper oxide–peroxide reaction
Chen Qu,[a] Keigo Ito,[a] Isamu Katsuyama,[a] Tomohiko Mitani,[b] Keiichiro Kashimura,[c] and Takashi
Watanabe*[a]
Abstract: Model erythro, phenolic and non-phenolic lignin β-O-4
dimer compounds were reacted with copper oxide and H2O2 at the
electronic field maximum position of a single-mode 2.45-GHz
microwave system equipped with a cavity resonator. The reaction
products obtained via microwave heating and an oil bath heating in
the same reaction vessel and same temperature profile were
quantitatively compared. Dimer degradation proceeded through
consecutive elementary reactions. The phenolic dimer was
dehydroxylated and followed by the spontaneous cleavage of C–C
and C–O–C bonds to produce guaiacol, vanillin, and vanillic acid.
The reaction of the non-phenolic dimer produced veratric acid,
veratraldehyde, and guaiacol. Microwave irradiation accelerated
been reported that the microwave-assisted depolymerization of
lignin requires significantly less reaction time and/or
increases the depolymerization yield over that of traditional
–
7]
methods.[5
Zhu et al. reported that MW heating
contributed to the selective cleavage of C–C bonds for the
degradation of several β-O-4 model compounds, such as 2-
(2’-methoxyphenoxy)-4’-hydroxyl-1-phenylethanol, in the
presence of Pd/C or ferric sulfate as a catalyst.[8,9] The
degradation products obtained with microwave heating
(MWH) and conventional heating (CH) were then compared
.
The authors reported that the heating method affected the
reaction products. Pan et al. reported that with the
cleavage of the side chain and the oxidation of vanillin to vanillic acid. assistance of MWH, degradation yields of the α-O-4 and β-
However, no acceleration of veratraldehyde oxidation to veratric acid
or aromatic ring cleavage to produce dicarboxylic acids was
observed. The selective acceleration of elementary reactions during
the degradation of model lignin compounds indicated that
microwaves interacted with reaction intermediates that were
sensitive to electromagnetic wave.
O-4 lignin model dimer
were higher than those obtained from CH-assisted reactions
They screened various ionic liquid and alkali metal salts in
s and some monomeric compounds
.
s
their study of lignin model compound degradation and
proposed degradation mechanisms. [10,11] Liu et al. reported
efficient cleavage of lignin–carbohydrate complexes and
ultrafast extraction of lignin oligomers from wood biomass by
microwave-assisted treatment with deep eutectic solvent. [12]
Our group developed a method for the direct production of
vanillin and vanillic acid from wood particles using copper oxide
Introduction
[13]
(CuO) and peroxide (H2O2) under alkaline conditions.
Concerns about the environmental impact of fossil fuels and the
limited long-term supply of petroleum have stimulated the search
for new energy resources. Woody biomass is a natural resource
with the potential to meet this challenge. [1] Lignin is one of the
main components of woody biomass. It is considered as a
source energy and fine chemicals through various processes. [2]
It is important to develop a new, efficient, and cost-effective
depolymerization method for the utilization of woody biomass.
Woody biomass degradation usually requires elevated
temperatures. Microwave (MW) energy generates heat by
inducing the rotation of molecules in an electromagnetic field. It
is known that MW irradiation also expedites some organic
reactions compared to conventional conductive heating.[3,4] It has
Obvious microwave acceleration effects were observed in a
precise comparison of the woody biomass depolymerization
reactions performed with MWH and CH using the same reaction
conditions and reaction vessel.
The effects of electromagnetic waves on lignin
degradation have not been well studied closely at the
molecular level. MW photons correspond to a frequency of
2.45 GHz is 0.0016-eV. This is not a sufficient amount of
[14,15]
energy to break any chemical bonds.
However, we
found that MW irradiation accelerated the reactions that
produced vanillin and vanillic acid from wood particles when
[13]
CuO and H2O2 were used under alkaline conditions.
There were two plausible explanations for the observed
yield enhancement. (1) MW irradiation heated water molecules
in the wood cell walls and promoted wood swelling. This, in turn,
assisted the impregnation of the oxidative catalysts and solvents
into the cell walls, leading to the enhanced cleavage of ether
and C–C bonds in lignin. (2) MW irradiation directly
accelerated the scission of covalent bonds in lignin. In this
study, we focused on the second possibility and
investigated whether MW irradiation directly accelerated the
cleavage of the covalent C–C and C–O bonds in lignin
using β-O-4 lignin dimer model compounds (Figure 1). To
date, this is the first study to provide direct evidence of the
acceleration of bond cleavage in lignin by MW irradiation
using precisely controlled MW equipment. The findings of
this investigation will facilitate the optimization of MW-
[a] Dr. C. Qu, K. Ito, Dr. I. Katusyama, Prof. T. Watanabe,
Laboratory of Biomass Conversion, Research Institute for Sustainable
Humanosphere, Kyoto University
Gokasho, Kyoto 611-0011, Japan
E-mail: twatanab@rish.kyoto-u.ac.jp
[b] Dr. T. Mitani,
Laboratory of Applied Radio Engineering for Humanosphere, Research
Institute for Sustainable Humanosphere, Kyoto University,
Gokasho, Uji, Kyoto 611-0011, Japan
[c] Dr. K. Kashimura
Information Technology Section, General Education Division, College of
Engineering, Chubu University
1200 Matsumoto-cho, Kasugai, Aichi 487-8501, Japan
Supporting information for this article is given via a link at the end of the
document.
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assisted lignin volarization strategies for future industrial
applications.
2. Comparison of elementary reactions during the
degradation of lignin dimer model compounds with MWH
and CH assistance
The gas chromatography–mass spectrometry (GC–MS) spectra
of the phenolic lignin dimer (1a) degraded at 150 °C for 20 min
with MWH and CH are shown in Figure 3. The GC–MS spectra
of the non-phenolic lignin dimer (1b) degraded at 170 °C for 20
min with MWH and CH are shown in Figure 4. Distinct
differences were observed in the reactivities of the phenolic and
non-phenolic lignin β-O-4 model compounds in the presence of
CuO and H2O2 under alkaline conditions. The main degradation
products of the phenolic lignin dimers were vanillin, vanillic acid,
and guaiacol. The main degradation products of the non-
phenolic lignin dimer were veratraldehyde, veratric acid, vanillin,
vanillic acid, and guaiacol. Acetoveratrone and several
dicarboxylic acids, including malonic acid, succinic acid, maleic
acid, malic acid, and fumaric acid, were also detected. The
identities of the products were confirmed by comparing their
spectra to those of authentic samples. High-molecular-weight
products were also detected by GC–MS.
The reaction fragments produced from the phenolic and non-
phenolic dimer model compounds were the same between MWH
and CH, indicating that the mechanisms of MWH and CH were
essentially the same. Kim et al. and Zhu et al. reported that the
reaction products obtained from the degradation of sulfonate
lignin in the presence of potassium hydroxide or methanol using
MWH were entirely different from those obtained when CH was
used. They speculated that the differences between the reaction
products arose from different reaction mechanisms. However,
the CH and MWH reaction conditions were not identical in their
experiments, because the MW reactor stopped the reaction at
the maximum holding pressure. [8,16]
Results and Discussion
1. MWH- and CH-assisted degradation of lignin dimer model
compounds
Lignin is a heterogeneous aromatic polymer composed of
phenylpropane units connected by C–O–C and C–C bonds.
Lignin dimer model compounds with β-O-4 structures with the
most abundant linkage type in lignin were selected for the study.
A phenolic β-O-4 dimer model (erythro form) compound (1a)
was selected to represent lignin substructures with free phenolic
hydroxyl groups at their terminal ends. A non-phenolic β-O-4
dimer compound (erythro form) (1b) was used to model lignin
substructures with guaiacyl nuclei joined to adjacent phenyl
propane units via ether linkages (Figure 1).
The lignin model compounds were reacted with CuO and
H2O2 and heated by either MWH or CH in an alkaline medium.
The same reaction time, temperature profile, stirring speed, and
reaction vessel were used for both heating types (Figure S1).
The reactions allowed us to precisely evaluate the effects of
each heat source.
A MW cavity heating system was used for the MW heating
study. Microwaves are electromagnetic waves. With this reactor,
the maximum electronic field (Emax) and the maximum magnetic
field (Hmax) positions could be separated. This allowed us to
compare the lignin depolymerization reactions in the maximum
MW electronic field (Emax) to those of a conventionally heated
process using an oil bath. In this study, the MW reactions were
located in the Emax position (Figure 2). The input MW power and
reflected MW power were recorded, and the reverse power was
not allowed to exceed ~10% of the forward power (Figure S1).
Figure 1. Chemical structures of lignin β-O-4 model compounds used in this
study.
Figure 2. (a) Cavity resonator. The sample was placed at the Emax position (H
= 0) in the single-mode MW heating apparatus. (b) Schematic illustration of
the 2.45-GHz MW heating system.
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Figure 4. TIC of non-phenolic dimer lignin model compound degradation
products obtained with MWH (blue) and CH (red) at 170 °C for 20 min.
of a phenoxy radical and a cinnamaldehyde-like intermediate,
followed by C–C bond cleavage via a retro-aldol reaction that
does not involve oxygen (Figure 5A). [22,23] Degradation of the
phenolic β-O-4 lignin dimer in the presence of CuO and H2O2
could proceed in both pathways after formation of the quinone
methide intermediate to produce vanillin (2a), vanillic acid (3a),
and guaiacol (4). We found that MWH provided higher yields of
these monomers than CH (Figure 6A–C).
Under aerobic alkaline conditions, o- and p-quinone
intermediates were generated from the phenolic β-O-4 lignin
dimer and decomposed into dicarboxylic acids via aromatic ring
opening and the formation of muconic acid derivatives (Figure
5B). Oxidation by hydrogen peroxide and aromatic ring opening
Figure 3. (a) Total ion chromatogram (TIC) of phenolic dimer lignin model
compound degradation products obtained with MWH (blue) and CH (red) at
150 °C for 20 min. (b) TIC of the dicarboxylic acid fraction of the phenolic
dimer degradation products obtained with MWH at 150 °C for 20 min.
to produce dicarboxylic acids have been observed under mildly
[24,25]
acidic conditions.
We found that dicarboxylic acids,
3. Lignin model compound reaction mechanisms
including malonic acid, maleic acid, succinic acid, fumaric acid,
and malic acid, were produced in the CuO/H2O2 system under
alkaline conditions with either MWH or CH. Unlike the production
of guaiacol, vanillin, and vanillic acid, yields of the dicarboxylic
acids were not higher with MWH. Repolymerization was
observed over the course of the reactions.
Degradation of the lignin model compounds involved several
consecutive reactions: (1) the cleavage of Cα and Cβ aryl ether
linkages; (2) the cleavage of the C–O bond in the side chain; (3)
the cleavage of the C–O bond in the methoxy group at the C4
position; (4) oxidation reaction of the aldehydes, (5) aromatic
ring opening, and (6) repolymerization reactions (Table S1).
In the alkaline medium, the phenolic β-O-4 lignin model
compound was oxidized to a quinone methide intermediate after
deprotonation of the phenolic hydroxyl group. According to the
mechanism proposed by Gierer, the quinone methide is
transformed into a peroxy anion, which is then converted into a
dioxetane in the presence of an oxidant. This leads to the
cleavage of the C–C bond to yield a phenolic aldehyde (Figure
5A).[17–21] On the other hand, the mechanism proposed by
Tarabenko for the anaerobic alkaline oxidation of the phenolic β-
O-4 lignin model compound involves the consecutive formation
It has been proposed that degradation of the non-phenolic β-O-4
dimer under alkaline conditions proceeds via the formation of an
epoxide intermediate through the cleavage of the C–C bond
[26–30]
and the generation of phenolic units.
The reaction with
CuO and H2O2 produced veratraldehyde (2b) and veratric acid
(3b) as major degradation products. This suggested that this
reaction proceeded via epoxide formation, as shown in Figure 5
(Route I Major). Remarkable accelerating effects of MWH were
observed in these reactions (Figure 7C and D). Veratric acid
formed through the oxidation of veratraldehyde, but MWH did
not accelerate this reaction. The methoxy group at the C4
position was removed during the CuO/H2O2 reactions. This was
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verified experimentally using veratraldehyde as the starting
material. Vanillin was produced from veratraldehyde, but no
MW-accelerated effects were observed in the demethylation
step. MWH had a slight accelerating effect on the oxidation of
vanillin to vanillic acid. In addition to the major route through the
epoxide, a separate reaction pathway following the removal of
the methoxy group was possible (Route II Minor). This was a
minor pathway, because only a small amount of vanillin, vanillic
acid, and trace amounts of the dicarboxylic acids were produced
(Figure 7A, B, and F). Because no -ketone derivatives were
detected throughout the reactions, degradation pathway starting
from formation of -ketone is not included in Figure 5.
Unassigned peaks at retention time 19.4, 20.6 and 20.7 min in
Figure 4 are estimated as -carboxylic compounds by mass
fragment patterns. One possible route for this reaction is
oxidation of -position to aldehyde, subsequent -ether cleavage
and further oxidation of the aldehyde to carboxylic acid. [31]
Figure 5. Proposed reaction mechanisms in the MW system. (a) Cleavage of the phenolic β-O-4 model compound; (b) formation of various dicarboxylic acids;
and (c) cleavage of the non-phenolic dimer compound under alkaline conditions in the presence of CuO and H2O2.[22–31]
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4. Time course of lignin model compound degradation and
temperature-related effects of MWH and CH
reacted in the absence of CuO and H2O2, with the CuO catalyst
alone, or with H2O2 in the absence of CuO (Figure S5 and Table
S2). The results demonstrated that the combination of the
catalyst and H2O2 was necessary to promote degradation.
The rate constant of non-phenolic dimer degradation reaction
by MWH and CH at 170 °C were 4.1 ·10-3·s-1 and 3.3 ·10-3·s-1
respectively. The rate constant for labile phenolic dimer by MWH
and CH was determined at 150 °C to give 3.0 ·10-3·s-1 and 2.6 ·
10-3·s-1 respectively. Furthermore, the dielectric loss factors of
substances in the reaction system affect the energy
consumption efficiency. Dielectric loss factor of the alkaline
solution used in this reaction system is higher than water,
indicating that the MWH system is advantageous than CH in
terms of energy efficiency in addition to the acceleration of bond
cleavage in lignin.
The phenolic lignin dimer model was degraded at 130, 140, and
150 °C for 20 min with MWH and CH. The yields of vanillin and
vanillic acid with MWH were 1.6- and 1.4-fold higher,
respectively, than they were with CH at 150 °C (Figure 6). Yields
of the major reaction products vanillin, vanillic acid, and guaiacol
increased with increasing reaction temperature when MWH was
used. However, the yields of these products did not increase
noticeably with increasing temperature when CH was used. The
time course of degradation differed in the two reaction systems.
The MWH-assisted reactions generated higher yields during the
first 10 minutes, but the yields of the MWH- and CH-assisted
reactions were nearly equal at 15 minutes. The yields of the
MWH-assisted reactions then increased until completion of the
experiment, while those of the CH-assisted reactions showed
slightly dropping. (Figure 8).
The non-phenolic β-O-4 lignin dimer compound was reacted
at 130, 150, and 170 °C for 20 min with MWH and CH (Figure 7
and Figure S6). The non-phenolic dimer completely
decomposed in the MWH-assisted reaction at 170 °C and
generated decomposition products in higher yields than the CH-
assisted reaction. With CH at 170 °C, approximately 3.4% of the
non-phenolic dimer remained at the end of the experiment. The
yield of veratric acid obtained with MWH at 170 °C was 2.28
times higher than that obtained with CH. The yields of vanillin
and vanillic acid with MWH at 170 °C were 25% and 9.6% higher,
respectively, than those obtained with CH (Figure 7). The
dependence of the veratraldehyde and guaiacol yields on
temperature with MWH differed from that observed with CH. The
time course of non-phenolic lignin dimer degradation is shown in
Figure 9. The yields of guaiacol and veratric acid obtained with
MWH and CH differed markedly, particularly after 10 min.
Figure 6. Yields of phenolic lignin dimer degradation products obtained with
MWH (blue) and CH (red) at 130 °C, 140 °C, and 150 °C for 20 min. (a)
Vanillin; (b) vanillic acid; (c) guaiacol; (d) dicarboxylic acids: malonic acid,
maleic acid, succinic acid, fumaric acid, and malic acid
To further clarify the degradation process, veratraldehyde was
used as a starting compound. During the 10-min reaction with
CuO and H2O2 at 150 °C, the decomposition of veratraldehyde
yielded veratric acid, vanillin, and vanillic acid with both MWH
and CH (Figure 10). This indicated that veratraldehyde served
as a reaction intermediate.
The production of vanillin and vanillic acid from
veratraldehyde indicated the removal of the methoxy group at
the C4 position. The cleavage of the methoxy C–O bond
produced a phenolic nucleus, and the subsequent reaction
pathway proceeded from the phenolic lignin units. There was a
significant difference between the yields of veratraldehyde
conversion to vanillin obtained with MWH (5.5%) and CH (6.2%).
Vanillin and guaiacol were also subjected to degradation with
MHW and CH at 150 °C for 10 min. Vanillic acid was generated
via the oxidation of vanillin, with slight acceleration by MWH.
Dicarboxylic acids were produced from the vanillin and guaiacol
degradation. Veratric acid was stable at 150 °C for 10 min with
either MWH or CH. No dicarboxylic acids or other degradation
products were detected in the reaction products of veratric acid.
In a control experiment, the lignin dimer model compounds were
Figure 7. Yields of non-phenolic lignin dimer degradation products obtained
with MWH (blue) and CH (red) at 130 °C, 150 °C, and 170 °C for 20 min. (a)
vanillin, (b) vanillic acid, (c) veratric acid, (d) veratraldehyde, (e) guaiacol, and
(f) succinic acid.
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Figure 8. Time course for the production of (a) vanillin, (b) vanillic acid, and (c)
guaiacol from the phenolic dimer compound with MWH (blue) and CH (red) at
150 °C for 10-20 min.
Figure 9. Time course for the production of (a) vanillin, (b) vanillic acid, (c)
guaiacol, (d) veratric acid, and (e) veratraldehyde from the non-phenolic dimer
model compound with MWH (blue) and CH (red) at 170 °C for 10-20 min.
Figure 10. TIC of degradation products. (a) veratraldehyde, (b) vanillin, (c) guaiacol, and (d) veratric acid obtained at 150 °C after 10 min with MWH.
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5. Mechanistic insight for the selective acceleration of
elementary reactions by MW irradiation
Experimental Section
Chemicals
In this study, we found that the cleavage of C–C and C–O–
C bonds in lignin side chains and the oxidation of vanillin
were selectively accelerated. However, no acceleration of
aromatic ring C–C cleavage was observed. The selective
acceleration of elementary reactions indicated that
microwaves interacted with reaction intermediates that were
sensitive to electromagnetic wave. The mechanism of
promotion could be explained by selective heating, a
thermal effect, and/or MW specific effect (non-thermal
effect). In both cases, acceleration could be attributed to
either an increased dielectric loss factor in the transition
state that generated thermal effects or to non-thermal
effects, such as perturbation of the dipole moments of the
reaction intermediates by microwaves. If electromagnetic
waves affected their dipole moments, it would induce
rotation of the molecules. It has been suggested that MW
specific effects arise from the perturbation of the bonding
angles in reactive species, although such effects await
confirmation in future studies. [15] A number of MW-specific
effects have been reported in organic syntheses, including
enantioselectivity.[32] This phenomenon has been ascribed
to an increase in frequency factor (A) by MW irradiation. In
our reaction system, MWH generated larger A values than
CH during the degradation of both the phenolic and non-
phenolic dimers (Table S3). This result was consistent with
non-thermal effects on elementary reactions. However, this
conclusion awaits confirmation, because microscale
increases in temperature would affect the Arrhenius factors.
1-(4-Hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)-1,3-propanediol
(1a, phenolic dimer), 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)-
1,3-propanediol (1b, non-phenolic dimer), hydrogen peroxide (30 wt.%),
copper (II) oxide (99.9%), 4-hydroxy-3-methoxybenzaldehyde (2a,
vanillin), 4-hydroxy-3-methoxybenzoic acid (3a, vanillic acid), ferulic acid,
malonic acid, succinic acid, maleic acid, malic acid, and fumaric acid
were obtained from Wako Pure Chemical Co. (Osaka, Japan) and used
as received. 1,3,5-Triisopropyl benzene (TIB), N,O-bis(trimethylsilyl)
trifluoroacetamide with 1% trimethylchlorosilane (BSTFA-TMCS), 2-
methoxyphenol (4, guaiacol), 3,4-dimethoxybenzaldehyde (2b,
veratraldehyde), and 3,4-dimethoxybenzoic acid (3b, veratric acid) were
purchased from TCI (Tokyo, Japan) and used as received.
MWH and CH experimental conditions
The cavity resonator (single-mode MW) heating system (Figure 2)
consisted a waveguide, a semiconductor amplifier, a three-stub tuner, a
plunger, and an isolator. The iris, which had a 50-mm slit parallel to the
electric field, was used for concentrating the microwaves in the TE103
-
mode cavity. The system enabled spatial separation of the electronic and
magnetic fields that comprised the microwaves. The sample was placed
in the maximum electric field position, and heated using 2.45-GHz
microwave radiation. The internal temperature of the reaction vessel was
monitored with a fiber optic thermometer (Neoptix, Québec City, Canada).
The internal temperature and MW power data were recorded on a data
logger. The structures of the lignin dimer model compounds (1a and 1b)
were confirmed by nuclear magnetic resonance (NMR) spectroscopy
prior to use in the depolymerization experiments (Supporting information).
Alkaline degradation of each lignin model compound (20 mg) was
performed in a 2 N NaOH solution (5 mL) with CuO (20 mg) and 30%
H2O2 (250 μL) in the cavity system. The reagents were placed in a 12-mL
glass container with a silicon cover (Milestone General, Kawasaki,
Japan). The MWH reaction was performed in the E maximum (Emax) field
(N = 3) with constant magnetic stirring at 600 rpm. The reaction
temperature was increased from room temperature to 130–170 °C within
5 min and maintained at that temperature for a total of 20 min. The
reaction mixtures were then cooled to 50 °C (Figure S1).
Conclusions
Phenolic and non-phenolic β-O-4 lignin dimer model
compounds were reacted with copper oxide and H2O2 using
CH and microwave (MW) heating in the maximum
electronic field position. CH and MWH generated the same
reaction products, but the reaction rates in the two reaction
systems differed. MW irradiation accelerated side chain
cleavage and the oxidation of vanillin to vanillic acid.
However, no acceleration was observed in the oxidation of
veratraldehyde to veratric acid or aromatic ring cleavage for
the production of dicarboxylic acids. The selective
acceleration with MWH could not be explained by the
activation of the CuO catalyst. The selective acceleration of
elementary reactions during the degradation of lignin model
compounds indicated that microwaves interacted with
elementary reaction intermediates that were sensitive to
electromagnetic waves. These results provide new insights
into the MW effect on the intermediates involved in
consecutive reactions pathways. This study is also
important for lignin volarization due to the high reactivity of
interunit linkages and aromatic rings in lignin.
Figure S2 contains a flow chart of the lignin compound degradation
process and subsequent workup. A saturated Na2SO3 solution was
added to the intermediate and final reaction mixtures to quench
unreacted H2O2. The mixtures were filtered through cotton to remove
CuO and acidified to pH ~2.0 using concentrated hydroxide chloride
(35%). The acidified products were then extracted into ethyl acetate. The
extracts were dried over MgSO4 and filtered, and the solvent was
evaporated under reduced pressure. CH experiments were conducted
under conditions that were otherwise identical to those used for MWH
experiments. The reactions were performed at the same agitation rate
and internal temperatures in the same reaction vessel using an oil bath
(TBX 203HA, Advantec Toyo Kaisha Ltd., Tokyo, Japan) (Figure S3).
Gas chromatography–mass spectrometry (GC–MS) analysis
The products were derivatized using BSTFA-TMCS and quantified from
the total ion chromatograms (TIC) of GC–MS using 1,3,5-triisopropyl
benzene as an internal standard. GC–MS analysis was performed on a
QP2010SE GC–MS system (Shimadzu, Kyoto, Japan) using a 30 m ×
0.25 mm (i.d.) DB-5 MS ultra-inert column with a 0.25-μm film thickness
(Agilent Technologies). Helium was used as the carrier gas at a flow rate
of 1.6 mL/min. Injection was performed in split mode using a 1:10 split
ratio and an injection volume of 1.0 μL. The mass spectrometer was
operated in electron impact ionization (EI) mode with an ionizing energy
of 70 eV. The mass scan range was m/z 30–700. The column
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temperature program began at an initial temperature of 50 °C for 3 min.
The temperature was increased to 300 °C at a rate of 3 °C/min, then held
at 300 °C for 7.5 min. Calibration curves were constructed using
authentic standards. The identities of the reaction products were
confirmed by comparing the mass peaks with those of the authentic
standards and quantified using the calibration curves.
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Acknowledgments
This work was supported by Analysis and Development System
for Advanced Materials (ADAM), Mission 2 and Mission 5-2
Research Programs of RISH, Kyoto University.
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Keywords: microwave effect • lignin model compounds •
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10.1002/cssc.202000502
ChemSusChem
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Text for Table of Contents
Dr. Chen Qu,[a] Keigo Ito,[a] Dr. Isamu
Katsuyama, [a] Dr. Tomohiko Mitani,[b]
Keiichiro Kashimura,[c] and Prof. Takashi
Watanabe*[a]
Page 1–Page 8
Microwave directly accelerated
cleavage of C–C and C–O bonds of
lignin by copper oxide–peroxide
reaction
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