Imidazolium-Based Ionic Liquids as Efficient Reagents for the C?O Bond Cleavage of Lignin

Article abstract of DOI:10.1002/cssc.201701668

The demethylation of lignin in ionic liquids (ILs) was investigated by using pure lignin model monomers and dimers together with dioxane-isolated lignins from poplar, miscanthus, and maize. Different methylimidazolium ILs were compared and the samples were treated with two different heating processes: microwave irradiation and conventional heating in a sealed tube. The conversion yield and influence of the treatment on the lignin structure were assessed by ³¹P NMR spectroscopy, size-exclusion chromatography, and thioacidolysis. The acidic methylimidazolium IL [HMIM]Br was shown to be an effective combination of solvent and reagent for the demethylation and depolymerization of lignin. The relatively mild reaction conditions, the clean work-up, and the ability to reuse the IL makes the described procedure an attractive and new green method for the conversion of lignin to produce phenol-rich lignin oligomers.

Full text of DOI:10.1002/cssc.201701668

Accepted Article
Title: Imidazolium based Ionic liquids as efficient reagents for lignin C-
O bond cleavage
Authors: Marina Thierry, Amel Majira, Bruce Pégot, Laurent Cézard,
Flavien Bourdreux, Gilles Clément, François Perreau,
Stéphanie Boutet-Mercey, Patrick Diter, Giang Vo-Thanh,
Catherine Lapierre, Paul-Henri Ducrot, Emmanuel Magnier,
Stephanie Baumberger, and betty cottyn
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the VoR from the journal website shown below when it is published
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To be cited as: ChemSusChem 10.1002/cssc.201701668
Link to VoR: http://dx.doi.org/10.1002/cssc.201701668
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Imidazolium based Ionic liquids as efficient reagents for lignin C-
O bond cleavage
M. Thierry,[a] A. Majira,[a] B. Pégot,[b] L. Cezard,[a] F. Bourdreux,[b] G. Clément,[a] F. Perreau,[a] S. Boutet-
Mercey,[a] P. Diter,[b] G. Vo-Thanh,[c] C. Lapierre,[a] P.-H. Ducrot,[a] E. Magnier,[b] S. Baumberger,[a] B.
Cottyn*[a]
1).[2] Many efforts have been dedicated to improve the usability of
lignins for the production of value-added aromatic products by
depolymerisation or structure modifications.[ 3 ] An important
feature is also the relatively low amount of free phenol groups in
Abstract: Lignin chemical demethylation in ionic liquids was
lignins, as most of the phenol oxygen atoms are involved in
investigated using pure lignin-model monomers and dimer together
interunit linkages.[4]
with dioxan isolated lignins from poplar, miscanthus and maize.
Different methylimidazolium ionic liquids (ILs) were compared,
according to two different heating processes, microwave irradiation or
conventional heating in sealed tube. The conversion yield and
influence of the treatments on lignin structure were assessed by 31
P
NMR, size exclusion chromatography and thioacidolysis. The acidic
IL [HMIM][Br] was shown to be an effective combination
solvent/reagent for the demethylation and even depolymerisation of
lignin. The relatively mild reaction conditions, the clean work-up, and
the ability to reuse the ionic liquid made the described procedure an
attractive and new green method for the lignin conversion to produce
phenol-rich lignin oligomers.
Introduction
Lignins are abundant cell wall biopolymers essentially made of
monomethoxylated guaiacyl (G) and dimethoxylated syringyl (S)
p-hydroxyphenylpropane (H) units (Fig. 1) in ratios depending on
their botanical origin. Due to their aromatic structure, lignins are
considered as the main natural source of phenolic molecules and
could thereby be an alternative to fossil-oil based aromatics. G
and S units are either linked by labile β-O-4 aryl-alkyl ether bonds,
that are the most frequent interunit bonds in native lignins and the
main targets of lignin chemical depolymerisation processes,[1] or
by resistant interunit bonds, such as 5-5, β-5 and β-β linkages (Fig.
Figure 1. Schematic view of G and S phenylpropane units linked together with
labile β-O-4 and resistant bonds (5-5, β-5 or β-β)
Therefore, a promising solution to enhance the chemical reactivity
of lignins is based on aryl-alkyl ether cleavage in order to generate
new
reactive
free
phenol
groups
and/or
induce
depolymerisation.[ 5 ] The so formed oligomers, richer in free
phenolic groups and also potentially exhibiting catechol moieties
could thereby be used as biobased antioxidants for cosmetic and
packaging applications or to substitute phenol in polymer
chemistry.[6] A major drawback of usual dealkylation procedures
however lies in the need of excess of reagents and/or high
temperatures, favouring side reactions such as recondensation
through intra- and intermolecular coupling reactions. These
reactions are due to the reactivity of the substituted aromatic rings
and induce reticulation of the polymer, and have been described
in almost all processes dealing with lignin modification.[7] The poor
solubility of most of the lignins in conventional organic solvents is
also a feature to be taken in account for the design of new lignin
modification processes.
[a]
M. Thierry, A. Majira, L. Cézard, Dr. G. Clément, Dr. F. Perreau, S.
Boutet-Mercey, Prof. C. Lapierre, Dr. P.H. Ducrot, Prof. S.
Baumberger, Dr. B. Cottyn
Institut Jean-Pierre Bourgin, INRA, Agro Paris Tech, CNRS,
Université ParisSaclay, 78000 Versailles, France.
E-mail: betty.cottyn@inra.fr
[b]
[c]
Dr. B. Pégot, F. Bourdreux, Dr. P. Diter, Dr. E. Magnier
Université de Versailles St-Quentin, ILV, UMR CNRS 8180
45 avenue des Etats-Unis, 78035 Versailles, France.
Prof. G. Vo-Thanh
ICMMO, UMR CNRS 8182
Université Paris-Sud 11, 91405 Orsay, France.
A way for bypassing the solubility problem may lie in the use of
ionic liquids (ILs).[8] Indeed, the potential of ILs in green chemistry
processes has been proven and their good solvating properties,
low vapor pressures, and high thermal stabilities make them good
Supporting information for this article is given via a link at the end of
the document.
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candidates to substitute a wide range of organic solvents.
Moreover, they have been described as promising solvents to
assist lignocellulosic biomass fractionation and/or lignins
extraction.[9] On the other hand, the promising cleavage of methyl
aryl ethers using ILs in the presence or not of acidic catalysts has
already been described in the literature.[10] In addition, according
to their structure, ILs may exhibit an acidic behaviour[11] that may
enhance their efficiency for methyl-aryl ether cleavage; the mostly
advocated chemical mechanism for this transformation being
indeed a SN2-like process of the phenol moiety by the IL’s anion
through electrophilic activation. However, to our knowledge, only
one reported procedure has been conducted on ortho-substituted
methyl ethers, a common structure found in lignins that may
influence the course of the reaction.[12]
In this paper, we describe an efficient cleavage of alkyl-aryl ether
bonds in lignins using ILs under microwave irradiation or with
conventional heating in sealed tube. After optimization of the
process on model compounds related to lignins, its
implementation to isolated lignins will prove its ability to partially
depolymerise lignins and to increase their free phenol content. As
compared to published data about IL-induced demethylation of
alkyl aryl ethers (Table 1), this is the first time to our knowledge
that processes combining IL with MW irradiation or sealed tube
heating (pressure tubes, PT) have been applied to isolated lignin.
Results and Discussion
1. Alkyl-aryl ether bond cleavage on model compounds
Our first goal was the cleavage of methyl-aryl ether bonds, and
vanillin 1 was chosen as model compound exhibiting the same
substitution pattern of the aromatic ring as the G units
encountered in lignins. Methylimidazolium-based ionic liquids
(MIM-ILs) were selected since they have been shown to be good
solvents for lignocellulose dissolution,[13] used in a wide range of
chemical processes. Moreover, the hydrogen in position 2 of the
imidazolium ring was expected to play a role during the reaction
as an acidic catalyst. From a practical point of view, they are easy
to prepare and, for some of them, commercially available.
Microwave irradiation (MW) was first tested as heating process,
since ILs are suitable for the microwave assisted organic
synthesis.[14] The reaction was also conducted in a sealed tubes
under conventional heating in order to compare with microwave
irradiation. The results obtained were presented in Table 2.
Table 1. Previous studies relative to demethylation of alkyl aryl ethers in ionic liquids
Entry
Substrate
IL
Temp (°C)
Time
yield (%)
Ref N°
1
2
3
4
5
6
anisole
[HMIM][Br]-HBr
[BMIM][BF4]
[BMIM][BF4]/MgI2
[BMIM][Br]
RT
115
50
9h
9h
62
97
15c
15a
15d
16
Conventional
heating
2-methoxynaphtalene
Alkyl aryl ethers
4h
85 to 92
92
1-methoxynaphtalene
4-methoxyphenol
Alkyl aryl ethers
210
100
110
40min
4*30s
30s, cycles
MW
[BPy][Br]
77
15b
12
[BPy][Br]
71 to 95
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Table 2. Demethylation of vanillin in various MIM-Ils
IL
(equiv)
Heating
process
Temp
(°C)
Time
Product 2
yield[a] (%)
Residual
vanillin[a] (%)
Entry
R
R’
X
1
2
C4H9
C4H9
C4H9
C4H9
C4H9
C4H9
C4H9
C4H9
C4H9
C4H9
C8H17
H
H
H
Br
Br
6
6
MW
MW
MW
MW
MW
MW
MW
MW
MW
MW
MW
MW
MW
PT
110
110
150
110
110
110
110
110
110
110
110
110
110
110
1min
10s
10s
10s
10s
10s
10s
10s
10s
10s
10s
10s
10s
5h
24
47
12
24
29
3
3
14
8
3
H
Br
6
4
H
I
6
37
37
17
100
100
100
27
65
11
0
5
H
Cl
6
6
H
F
6
7
H
OTf
HSO4
NTf2
Br
6
0
8
H
6
0
9
H
6
0
10
11
12
13
14c
CH3
H
6
22
7
Br
6
H
Br
6
56
61
83
H
H
Br
12
12
H
H
Br
0
[a] Calculated percentages after compounds isolation by flash chromatography.
Firstly, the reactions were conducted under microwave irradiation
using 6 equivalents of [BMIM][Br] for one minute at 110°C.[15]
Despite an almost quantitative conversion, the desired catechol 2
was isolated in only 24% yield (entry 1). Decreasing reaction time
to only 10s allowed a better yield (47%, entry 2). When using
[BMIM][Br], 4-hydroxy-3-butoxybenzaldehyde was also identified
as an abundant reaction product. Migration of the butyl group from
the IL to the demethylated intermediates has already been
described in the literature in closely related cases.[ 16 ] Higher
temperature led to undesired reactions with a more important
insoluble fraction (entry 3). Counter anion effect was then
evaluated, and the best result was obtained with the bromide
anion confirming the necessity of using a good nucleophile as
imidazolium counter anion (entries 2, 4-9). The substitution at
position 2 (entry 10) as well as the increase of the alkyl chain
length at position 1 (entry 11) was deleterious to the yield. On the
other hand, when acidic [HMIM][Br] was used, the yield raised up
to 56% (entry 12). Using 12 equiv. of IL, demethylation took place
in 61% yield with total conversion of the starting vanillin (entry 13).
Finally the reaction was conducted in a sealed tube under
conventional heating leading to the formation of the desired
product in 83% yield (entry 14). However, a longer reaction time
was needed (5 h) to reach a complete conversion, but the reaction
was cleaner with no formation of insoluble fraction. The use of
hydrobromic acid for the demethylation process was already
described but it gave only poor yield. [17] This result points out the
crucial role of imidazolium in the mechanism of the reaction.[18]
According to these preliminary results, the following studies were
carried out in [HMIM][Br] under MW irradiation for 10s at 110°C or
in PT at 110°C.[19]
The same reaction was thereafter performed on commercially
available guaiacylglycerol-β-guaiacyl ether 3 (Figure 2) as a
model of β-O-4 bonds, that are the most encountered alkyl-aryl
ether groups found in lignins after methyl ethers. This compound
has indeed already been used in previous works to study the
acidolysis of β-O-4 bonds.[20] The reaction in sealed tube was first
tested with a short reaction time to avoid as far as possible
recondensation reactions. The substrate 3 totally disappeared in
only 5 minutes. The PT treatment with this acidic IL induced the
cleavage of the β-O-4 bond, yielding to 84% of the acidolysis
ketone 4 and guaiacol 5. With a longer PT treatment (20 minutes)
the demethylation reaction occurred in addition to β-O-4 cleavage
yielding in 78% yield the reduced and demethylated compound 6
and catechol. The low yields obtained for molecules 5 and 8 can
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be explained by the reaction work-up, the ionic liquid taking them
away in the aqueous phase. On the other hand, using microwave
irradiation, the reaction afforded directly ketone 6 in 95% yield.
GC-MS analyses were performed to confirm the formation of
ketone 6 in both cases and spectroscopic data were similar to
those previously described in the literature.[21] Presence of over
oxidized compounds such as α,β-diketone 7 (Figure 3) was also
detected through GC-MS experiments and may explain the
formation of the reduced compound 6. The presence of such
compounds was already reported in Lundquist’s work on lignin
acidolysis.[22]
to the same catechol derivative 6 and catechol 8 with even higher
reaction yield. The differences observed between the two
processes can be explained by the specificities of the heating
modes and devices. With MW, the absorption of the
electromagnetic wave energy by IL allows a rapid rise of
temperature in the medium. Stabilization of polar molecules or the
transition state by magnetic field also has a direct effect on the
reaction kinetics. Moreover, the employed IL may be present in
two forms, ionic and methylimidazole/HBr. In the PT, the
equilibrium is displaced, more HBr is released leading to a
lowering of the bromide anion’s reactivity, thus increasing the
reaction time in PT.
2.Application to dioxan-isolated lignin samples
2.1. Experimental strategy and lignin characteristics
The preliminary studies carried out with vanillin and with the β-O-
4 dimer allowed us to establish that the treatment with [HMIM][Br]
IL efficiently induced the demethylation of aryl methyl ether
groups and the cleavage of ether bonds representative of the
lignin β-O-4 interunit linkage. The next and most important step
regarding the targeted application was to evaluate its impact on
lignins. Upon lignin treatment, both bond cleavage and
condensation reactions were likely to occur, leading to the
formation of low-molar-mass compounds (monomers and
oligomers) and polymers, respectively. In order to assess the
efficiency of the treatment to depolymerize lignin and to identify
markers of the depolymerisation mechanisms, ethyl acetate
extractable compounds (“soluble fraction”) were analyzed by GC-
MS, SEC and 31P NMR. Ethyl acetate was selected as extraction
solvent due to its non-miscibility with aqueous IL and to the good
solubility of lignin low-molar-mass oligomers expected to be
released by the treatment. This analysis was completed by the
analysis of the residual lignin fraction (“insoluble fraction”) by
thioacidolysis, in order to assess the proportion of residual β-O-4
ether bonds with respect to carbon-carbon bonds, and to bring
evidences for demethylation in the polymer. The mass balance
between the soluble and insoluble fraction was used as marker of
the ratio between depolymerisation and condensation reactions.
In view of this analytical strategy, dioxan-isolation was selected to
obtain the starting lignin material from plants. Indeed, dioxan-
isolated lignins combined several advantages compared to other
types of isolated lignins: they keep most structural features of the
native lignins, in particular a high proportion of β-O-4 bonds, they
show low contamination degree by carbohydrates likely to
interfere with lignin analysis and they contain an abundant ethyl
acetate extractable fraction (about half of the lignin mass, Table
3) useful for the investigation of depolymerisation mechanisms.
Three different plants representative of lignocellulosic biomass of
agronomical and industrial interest for biorefinery (poplar wood,
maize and miscanthus stems) were selected as starting materials
for lignin isolation (Populus trichocarpa, cv. Fritzi Pauley; Zea
mays, F2 line; Miscanthus giganteus). Though obtained in the
Figure 2. Reaction of the β-O-4 dimer 3 in [HMIM][Br] IL (12 eq) in sealed tube
(PT, 5 or 20 minutes) or in microwave (MW).
Figure 3. Schematic view of α,β-diketone 7.
Noteworthy, no brominated phenylpropane molecules were
detected in these reactions, demonstrating that the chemical
mechanism leading to the cleavage of the β-O-4 bond is different
to that of the methyl-aryl ether cleavage. β-O-4 bond acidolysis
may result from a dehydration process, due to the presence of the
hydroxyl group in benzylic position and the acidity of the reaction
medium, affording the corresponding enol ether which is
thereafter hydrolysed to the final ketone, as postulated in previous
works.[22] Similarly, MW-assisted IL treatment gave rise in only 10s
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same isolation conditions, the corresponding dioxan-isolated
lignins (initial samples) exhibited differences in thioacidolysis
yields (2 and 3 times lower proportion of β-O-4 linked units in
maize than in miscanthus and poplar respectively; Table 4),
reflecting lignin structural variations according to botanical
origin.[23] Their respective ethyl acetate-soluble fractions showed
similar molar mass distributions (Figure 5) but a lower content in
phenolic OH for the poplar lignin (Table 5). In order to assess the
influence of botanical origin on lignin reactivity towards the
treatment, the three lignins were subjected to the [HMIM][Br]
treatment, either in microwave-assisted conditions or in sealed
tube under inert atmosphere with a short reaction time (20 or 40
min) to limit recondensation reactions.
fractions remaining close to 50/50 after treatment. However, this
treatment led to a global material loss of 26-35%, which could be
explained by the generation of volatile substances and their loss
in non-sealed tubes. Beside the formation of insoluble
compounds with PT and material loss with MW, interesting
qualitative changes in the composition of the soluble fraction were
likely to occur. Therefore the ethyl acetate-extracts were analyzed
by size-exclusion and gas chromatography.
2.2. Mass balance of the soluble and insoluble fractions
Comparison of the amounts of soluble and insoluble fractions
recovered from the initial and treated samples (Table 3) showed
that PT treatment lead to an increase of the insoluble amount,
whatever the lignin and the reaction time.
Table 3. Mass balance (mg) of soluble and insoluble fractions in ethyl
Figure 4. HPSEC-PDA profiles of the demethylation reactions using (A)
microwave irradiation or (B) pressure tube on (a) maize (b) poplar or (c)
miscanthus. The axes represent the elution time (min) horizontally and the
absorbance at 280 nm (a.u.) vertically.
acetate before and after treatment of DL lignins in [HMIM][Br]
Initial
mass
soluble EtOAc insoluble EtOAc
DL sample
fraction
fraction
Poplar DL samples
Initial
200
200
200
200
98
60
61
65
102
70
MW
PT (20 min)
PT (40 min)
132
125
Miscanthus DL
samples
Initial
MW
200
200
200
200
120
71
80
70
PT (20 min)
PT (40 min)
Maize DL samples
Initial
55
135
125
57
200
200
200
200
106
75
94
73
MW
Figure 5. Estimated Mass- and Number-Average Molar Mass (Mw and Mn,
g.mol-1) by size-exclusion chromatography
PT (20 min)
PT (40 min)
56
142
140
57
2.4. Evidence of demethylation brought by thioacidolysis of the
soluble and insoluble fractions
Concomitant to a decrease of the soluble fraction amount, this
evolution was diagnostic of condensation reactions. It was found
more pronounced for the miscanthus and maize samples than for
the poplar one (69% and 51% insoluble fraction increase for
miscanthus and maize, respectively, against 22% for poplar).
Interestingly, the MW treatment did not lead to apparent
recondensation, the proportion between soluble and insoluble
Thioacidolysis specifically cleaves lignin β-O-4 interunit bonds,
releasing H, G and S thioethylated species from H, G and S lignin
units only involved in labile β-O-4 interunit bonds.[24] Thus, the
thioacidolysis yield (µmol of thioethylated monomers recovered
from 1 g lignin) is indicative of the proportion of β-O-4 in the
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Table 4. Amount of guaiacyl G and syringyl S monomers released by
thioacidolysis of DL fractions before (initial DL) and after microwave (MW)- or
pressure tube (PT)-assisted treatments in [HMIM][Br] IL.
sample, relative to the other types of bonds, resistant to
thioacidolysis. Whatever the lignin soluble or insoluble fractions,
this yield was found dramatically reduced by all the treatments
(Table 4), compared to the initial lignins. The lowest final yields
were obtained for the soluble fractions and with the MW treatment
(4-5 µmol.g-1 against 7-19 µmol.g-1 for the PT treatments). This
result was found consistent with the cleavage of β-O-4 bonds but
could also be explained by the formation of new bonds resistant
to thioacidolysis. One main advantage of thioacidolysis is to avoid
side reactions and to preserve the structure of the aromatic phenyl
rings of lignins. Thus, the structure of the thioethylated released
products can be used to assess the structure of the initial lignin
unit, in particular their methoxylation degree. Since H species
(released from non methoxylated units) were recovered in very
low amounts or even as trace components (<1%), only G and S
species (carrying one and two methoxy groups, respectively)
were taken into account for lignin structural investigation. An
increase of the S/G unit ratio in all soluble and insoluble lignin
fractions upon treatment was observed. This indicated that the
lignin structural alterations induced by the treatments more
specifically involved G units. In addition to the conventional G and
S thioacidolysis monomers, the GC-MS analyses of thioacidolysis
mixtures revealed the occurrence of catechol (C), of 5-
hydroxyguaiacyl (5-OH G) and of 3,5-dihydroxyphenyl (3,5-OH-
P) monomers released from the [HMIM][Br] IL treated samples.
The recovery yields of these catechol derivatives often exceed
those of the conventional monomers, as revealed by the relative
importance of the corresponding peaks on the GC-MS traces (Fig.
6). This result definitely established that demethylation reactions
occurred in lignins during the IL treatment, whether
depolymerisation took place or not. Indeed, the demethylated
units were detected after all treatments, even when no significant
change in Mn and Mw were observed. Thus, whatever the
treatment, ether bond cleavages were shown to take place, within
methyl-aryl-ether and β-O-4 structures. An expected
consequence of these cleavages was the release of phenolic
hydroxyls, which was further investigated by 31P NMR.
Soluble fraction
Insoluble fraction
DL sample
yield
(G+S)
µmol/g
S/G
molar
ratio
yield
(G+S)
µmol/g
S/G
molar
ratio
Poplar DL samples
Initial
1171
4
2.33
3.10
3.03
2.92
-
-
MW
-
-
PT (20 min)
PT (40 min)
Miscanthus DL samples
Initial
19
37
21
3.82
3.59
16
807
4
0.91
1.54
2.79
1.49
-
-
MW
-
-
PT (20 min)
PT (40 min)
Maize DL samples
Initial
9
13
18
3.64
2.04
10
411
5
1.28
1.72
1.52
1.83
-
-
-
MW
-
PT (20 min)
PT (40 min)
10
7
18
9
2.40
2.86
-
Not measured
2.5. Increase in the phenolic/aliphatic OH ratio in the soluble
fraction shown by 31P NMR
31P NMR after phosphitylation provides a way to determine the
amount of functional groups in lignin, in particular hydroxyl groups
accessible to the derivatisation reagent. Comparison of the
functional OH group distributions of the soluble fractions before
and after IL-treatments (Table 5) showed a systematic increase
in the phenolic / aliphatic OH proportion, which reached for all
lignin a maximum with the MW treatment. Though an increase in
phenolic OH amount was expected from ether bond cleavages,
the increase in the phenolic/aliphatic OH ratio mainly resulted
from the severe decrease in the aliphatic OH amount. This
decrease could be due to dehydration phenomenon but was also
fully consistent with the depolymerisation mechanism observed
for β-O-4 structures with formation of the acidolysis ketones and
loss of one or two OH groups adjacent to the β-O-4 linkage (Figure
2). Accordingly, the lowest aliphatic OH amounts were obtained
for the MW treatment, which was shown to induce more extensive
depolymerisation. Thus, 31P NMR analysis confirmed that
structural changes took place within all the lignins upon
treatments, whatever their botanical origin, and that MW
treatment induced lignin depolymerisation through β-O-4
cleavage.
Figure 6. Lignin demethylation proofs on GC-chromatograms after
thioacidolysis, example of miscanthus samples (red) before and (green) after
microwave treatment.
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Experimental Section
Table 5. 31P NMR analysis[a] of the lignin samples before and after
demethylation
General Materials and methods
Phenolic/
Aliphatic
ratio
Aliphatic
OH
Phenolic
OH
DL sample
Guaiacylglycerol-β-guaiacyl ether (GG 99 %) was purchased from Tokyo
Chemical Industry (Belgium). Ionic liquids [BMIM][OTf], [BMIM][HSO4],
[BMIM][NTf2], [BDMIM][Br], [OMIM][Br] were purchased from Solvionic
(France). [BMIM][I], [BMIM][Br], [BMIM][Cl], [BMIM][F], [HMIM][Br] were
synthesized in the lab following previously reported procedures
(supporting information S1 and S2).[25] Ethyl acetate, cyclohexane, diethyl
ether were purchased from Carlo Erba Reagents (France) and were used
as received. All other reagents were purchased from Sigma-Aldrich
Chemical Co. (USA) and were used as received.
Poplar DL samples
Initial
6.22
0.01
0.43
0.24
1.75
2.52
3.52
2.61
0.3
252
8
MW
PT (20 min)
PT (40 min)
Miscanthus DL samples
Initial
11
5.21
0.01
0.24
0.48
2.53
2.63
2.34
3.29
0.5
263
9
Microwave experiments were conducted in an Anton Paar Monowave 300
instrument using the following method: P=300W, Tmax= 110°C, Ramp 30s,
Hold 10s, Full air cooling ON, Stirring ON. TLC experiments were
performed on an aluminium strip coated with Silica Gel 60 F254 from
Macherey-Nagel, revealed under UV-light (254 nm), then in the presence
of a 5% w/w ethanolic solution of phosphomolybdic acid. Evaporations
were conducted under reduced pressure at temperatures below 35°C
unless otherwise stated. Column chromatography (CC) was carried out
with an automated flash chromatography PuriFlash system and pre-
packed INTERCHIM PF-30SI-HP (30 μm silica gel) columns. 1H, 13C and
31P NMR spectra were recorded in CDCl3 or CD3OD at 400, 100 and 162
MHz respectively on a Bruker Ascend instrument. Chemical shifts are
reported in parts per million relative to internal references (solvent signal
or phosphorylated cyclohexanol). High-resolution mass spectra (HRMS)
were recorded on a Bruker Impact II instrument, the accurate masses are
reported for the molecular ion [M-H]- and are reported with an error <2 ppm.
MW
PT (20 min)
PT (40 min)
Maize DL samples
Initial
7
4.23
0.01
0.31
0.10
2.55
4.22
2.57
1.81
0.6
422
9
MW
PT (20 min)
PT (40 min)
18
[a]Amount (mmol.g-1) of aliphatic and phenolic (syringyl, guaiacyl and
phenol) OH groups in lignin samples by quantitative 31P NMR analysis.
Plant material. Isolation of DL fractions. Dioxan lignin fractions were
isolated from three different plants representative of lignocellulosic
biomass, prepared from poplar wood (Populus trichocarpa, cv. Fritzi
Pauley), mature maize (Zea mays, F2 line) or miscanthus (Miscanthus
giganteus) stems according to a published procedure.[26] About 30g of
sample was suspended in 50mL of a dioxan: 0.2 M HCl aqueous solution
(9:1, v/v) mixture. The suspension was heated at 100°C. After 30 minutes,
the cooled reaction mixture was filtered over a Büchner funnel, and the
residue was washed three times with 10 mL of a dioxan:HCl mixture (9:1,
v/v). All the filtrates containing the dissolved lignins were pooled, and the
pH of the resulting solution was adjusted to pH 3-4 (saturated NaHCO3
aqueous solution). The solution was concentrated under reduced pressure
at 45°C to about 10 mL. The concentrated DL solution was then injected
into about 100 mL of cold water under magnetic stirring. The lignin
precipitate was recovered by centrifugation (30 min at 2,000g and 10°C),
washed with pure water, centrifuged again, and freeze-dried to recover a
purified DL fraction (supporting information S5).
Conclusions
In this paper, we addressed the possibility to use new processes
based on ionic liquids to obtain phenols enriched oligomers from
lignin in an environmentally friendly manner. We have shown that
acidic IL [HMIM][Br] is an effective combination solvent/reagent
for the demethylation and even depolymerisation of lignin. Among
the different conditions tested, the treatment under microwave
showed several advantages: the short reaction time, the limited
formation of insoluble material and the final highest
phenolic/aliphatic OH ratio. The relatively mild reaction conditions
(low temperature compared to other lignin catalytic conversion
processes3c), the clean work-up, the stability of the products and
the ability to reuse the ionic liquid made the procedure described
an attractive method for the lignin conversion. Applying different
conditions in terms of time and processes allowed recovering
phenolic oligomers with higher proportion of free phenolic groups.
Such bio-based extracts could be advantageously used for their
tunable functional properties, in particular as antioxidants in
cosmetics and packaging applications.
GC-MS analysis. In a 200 µl GC-MS vial fitted with a Teflon-lined screwcap,
20 µl of ethyl acetate solutions (1mg/ml previously dried on sodium sulfate)
were silylated with 100 µl of bistrimethylsilyl-trifluoroacetamide (BSTFA)
and 10 µl of GC-grade pyridine. The silylation was completed within few
minutes at room temperature. The GC-MS analyses were performed on
an Agilent Instrument, with a poly(dimethylsiloxane) column (30 m × 0.25
mm; Rxi-5Sil, RESTEK), working in the temperature program mode from
70 to 330 °C at +30°C min-1, during 20 minutes, with helium as carrier gas,
a moving needle type injector and a flame ionization detector. The
chromatographic system was combined to a quadrupole spectrometer
operating in the electron impact mode (70 eV), with a source at 230°C and
an interface at 300°C, and with a 50 to 800 m/z scanning range (supporting
information S6).
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Quantitative 31P NMR and samples preparation. Derivatization of the lignin
acetate (2* 10 mL). The combined ethyl acetate extracts were dried on
MgSO4 and concentrated under reduced pressure. The resulting 3,4-
dihydroxybenzaldehyde 2 and the residual vanillin were purified by flash
column chromatography (elution with 50 to 100% ethyl acetate in
cyclohexane).
samples
with
2-chloro-4,4’,5,5’-tetramethyl-1,3,2-dioxaphospholane
(TMDP, Sigma-Aldrich, France) was performed according to Granata et al.
(1995). Lignin samples (20 mg) were dissolved in 400 µL of a mixture of
anhydrous pyridine and deuterated chloroform (1.6:1 v/v). Then was
added 150 µl of a solution containing cyclohexanol (6 mg/mL) and
chromium(III)acetylacetonate (3.6 mg/mL), which served as internal
standard and relaxation reagent respectively. NMR spectra were acquired
on a Bruker Biospin Avance III 400 MHz spectrometer. A total of 128 scans
were acquired with a delay time of 6 s between successive pulses. The
spectra were processed using Topspin 3.1. All chemical shifts are reported
relative to the product of TMDP with cyclohexanol, which has been
observed to give a doublet at 145.1 ppm referenced from the water-TMDP
signal (132.2 ppm). The content of hydroxyl groups (in mmol.g-1) was
calculated on the basis of hydroxyl groups contained in the internal
reference cyclohexanol and by integration of the following spectral regions:
aliphatic hydroxyls (150.8–146.4 ppm), condensed phenolic units (145.8-
143.8 ppm; 142.2-140.2 ppm), syringyl phenolic hydroxyls (143.8-142.2
ppm), guaiacyl phenolic hydroxyls (140.2–138.2), p-hydroxyphenyl
phenolic hydroxyls (138.2–137.0 ppm), and carboxylic acids (136.6-133.6
ppm) (supporting information S7).
Rf =0.32 (cyclohexane/ethyl acetate 1:1); brown spot; 1H NMR (400 MHz,
CD3OD, 25°C): δH 6.91 (d, 3J(H/H)=8.0 Hz, 1H, H5), 7.31 (s+d, 3J(H/H)=8.0
Hz, 2H, H2 and H6), 9.69 (s, 1H, CHO); 13C NMR (100 MHz, CD3OD, 25°C):
δC 117.21, 125.03, 117.43 (C2, C5 and C6), 130.45, 146.62, 153.14 (C1, C3
C4), 191.85 (CO); HRMS: m/z [M-H]- calcd for C7H5O3: 137.0239, found:
137.0243.
Thioacidolysis. Thioacidolysis of lignins (5 mg) was carried out according
to the literature,[27] using heneicosane (C21H44, Fluka) as internal standard
(IS). Lignin-derived p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S)
thioacidolysis monomers were analyzed as their trimethylsilyl derivatives
Procedure for vanillin in sealed tube, data reported in Table 1.
The ionic liquid was vacuum dried at room temperature before use. Vanillin
(100 mg, 0.66 mmol) and [HMIM][Br] (1.27 g, 12 equivalents relatively to
vanillin) were added into an Ace pressure tube (Ace glass Inc., Sigma
Aldrich) with a magnetic stirrer under inert atmosphere. The tube was
sealed and the mixture was stirred in an oil bath at 110 °C. After 5 hours,
water (10 mL) and ethyl acetate (10 mL) were added. The layers were
separated, the aqueous layer was extracted with ethyl acetate (2* 10 mL).
The combined ethyl acetate extracts were dried on MgSO4 and
concentrated under reduced pressure. The crude mixture was purified by
flash column chromatography (elution with 50 to 100% ethyl acetate in
cyclohexane) to yield 83% of 3,4-dihydroxybenzaldehyde 2.
by
a gas chromatography−mass spectrometry (GC-MS) instrument
(Saturn 2100, Varian) equipped with a poly(dimethylsiloxane) column (30
m × 0.25 mm; SPB-1, Supelco) and using the following heating program:
40 to 180 °C at 30 °C min−1, then 180 to 260 °C at 2 °C min−1. The mass
spectrometer was an ion trap with a ionization energy of 70 eV and positive
mode detection. Quantitative determination of H, G, and S monomers was
performed from ion chromatograms reconstructed at m/z 239, 269, and
299, respectively, as compared to the IS signal measured from the ion
chromatogram reconstructed at m/z (57 + 71 + 85). The molar yield of the
detected thioethylated monomers was calculated on the basis of the
Klason lignin content of the sample, determined according to a published
procedure.[28]
Procedure for Guaiacylglycerol-β-guaiacyl ether 3 under microwave
irradiation, data reported in Figure 2.
Molar Mass (MM) Distribution. The mass-average MM of each lignin
The ionic liquid was vacuum dried at room temperature before use.
Guaiacylglycerol-β-guaiacyl ether 3 (100 mg, 0.31 mmol) and [HMIM][Br]
(610 mg, 12 equivalents relatively to 3) were placed in an Anton Paar 30
mL reaction tube with a magnetic stirrer. The mixture was irradiated with
P=300 W, Tmax=110°C, Ramp 30 sec, Hold 10 sec, with full air cooling
and stirring. At the end of the reaction, water (10 mL) and ethyl acetate (10
mL) were added. The layers were separated, the aqueous layer was
extracted with ethyl acetate (2* 10 mL). The combined ethyl acetate
extracts were dried on MgSO4 and concentrated under reduced pressure.
The resulting mixture was purified by flash column chromatography
(elution with 50 to 100% ethyl acetate in cyclohexane) to yield 95% of
product 6. The product was characterized by NMR and mass spectrometry.
sample
was
estimated
via
high-performance
size-exclusion
chromatography (HPSEC) using a styrene - divinylbenzene PL-gel column
(Polymer Laboratories, 5 µm, 100 Å, 600 mm x 7.5 mm I.D.) with
photodiode array detector (Dionex Ultimate 3000 UV/vis detector) set at
280 nm UV and THF (1 mL.min−1) as eluent. Samples were solubilized in
THF, filtered on PTFE membrane 0.45 μm before analysis by HPSEC.
Polymerisation degrees were assessed from the apparent molar masses
determined by a calibration curve based on polyethylene oxide standards
(Igepal, Aldrich) and injection of pure coniferyl alcohol monomers and
dimers.
Synthetic procedures
9
Procedure for vanillin under microwave irradiation, data reported in
Table 1.
7
O
1
6
5
2
3
The ionic liquid was vacuum dried at room temperature before use. Vanillin
(100 mg, 0.66 mmol) and the appropriate quantity of ionic liquid (6 or 12
equivalents, see Table 1) were placed in an Anton Paar 30 mL reaction
tube with a magnetic stirrer. The mixture was irradiated with P=300 W,
Tmax=110°C, Ramp 30 sec, Hold 10 sec, with full air cooling and stirring.
At the end of the reaction, water (10 mL) and ethyl acetate (10 mL) were
added. The biphasic solution was filtered to remove solid residues, if any.
The layers were separated, the aqueous layer was extracted with ethyl
OH
4
OH
6
Rf =0.33 (cyclohexane/ethyl acetate 1:1); 1H NMR (400 MHz, CD3OD,
25°C): δH 2.10 (s, 3H, CH3-9), 3.55 (s, 2H, CH2-7), 6.52 (dd, 3J(H/H)=8.0 Hz
and 4J(H/H)=2.0 Hz, 1H, H6), 6.63 (dd, 4J(H/H)= 2.0 Hz, 1H, H2), 6.71 (d,
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10.1002/cssc.201701668
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3J(H/H)=8.0 Hz, 1H, H5); 13C NMR (100 MHz, CD3OD, 25°C):
δC 27.44
Procedure for Dioxan extracted Lignin samples under microwave
irradiation, data reported in Tables 2, 3, 4.
(C9), 49.59 (C7), 115.12, 116.06, 120.43 (C2, C5 and C6), 125.87, 145.14,
145.65 (C1, C3 C4), 208.85 (CO); HRMS: m/z [M-H]- calcd for C9H9O3:
165.0552, found: 165.0560.
The ionic liquid was vacuum dried at room temperature before use. DL
samples (200 mg) and [HMIM][Br] (1g, excess to dissolve DL) were placed
in an Anton Paar 30 mL reaction tube with a magnetic stirrer. The mixture
was irradiated with P=300 W, Tmax=110°C, Ramp 30 sec, Hold 10 sec,
with full air cooling and stirring. At the end of the reaction, water (10 mL)
and ethyl acetate (10 mL) were added. The layers were separated, the
aqueous layer was extracted with ethyl acetate (2* 10 mL). The combined
ethyl acetate extracts were dried on MgSO4 and concentrated under
reduced pressure. The crude mixture was analysed by 31P NMR, HPSEC
and thioacidolysis.
Procedure for Guaiacylglycerol-β-guaiacyl ether 3 in sealed tube 5
minutes, data reported in Figure 2.
The ionic liquid was vacuum dried at room temperature before use.
Guaiacylglycerol-β-guaiacyl ether 3 (100 mg, 0.31 mmol) and [HMIM][Br]
(610 mg, 12 equivalents) were placed into the Ace pressure tube with a
magnetic stirrer under inert atmosphere. The tube was sealed and the
mixture was stirred in an oil bath at 110 °C. After 5 minutes, water (10 mL)
and ethyl acetate (10 mL) were added. The layers were separated, the
aqueous layer was extracted with ethyl acetate (2* 10 mL). The combined
ethyl acetate extracts were dried on MgSO4 and concentrated under
reduced pressure. The crude mixture was purified by flash column
chromatography (elution with 50 to 100% ethyl acetate in cyclohexane) to
yield 84% of product 4. The product was characterized by NMR and mass
spectrometry.
Procedure for Dioxan extracted Lignin samples in sealed tube, data
reported in Tables 2, 3, 4.
The ionic liquid was vacuum dried at room temperature before use. DL
samples (200 mg) and [HMIM][Br] (1g, excess to dissolve DL) were placed
into the Ace presssure tube with a magnetic stirrer under inert atmosphere.
The tube was sealed and the mixture was stirred in an oil bath at 110 °C.
After 20 or 40 minutes, water (10 mL) and ethyl acetate (10 mL) were
added. The layers were separated, the aqueous layer was extracted with
ethyl acetate (2* 10 mL). The combined ethyl acetate extracts were dried
on MgSO4 and concentrated under reduced pressure. The crude mixture
was analysed by 31P NMR, HPSEC and thioacidolysis.
Recycling of the ionic liquid.
The aqueous layer, containing the unreacted ionic liquid [HMIM][Br], was
kept and concentrated under vacuum. The resulting ionic liquid was
washed with ether. An NMR spectrum was realized showing the recovery
of a very clean ionic liquid. 75% of the starting [HMIM][Br] was recovered
and usable for further similar reactions.
Rf =0.43 (cyclohexane/ethyl acetate 1:1); 1H NMR (400 MHz, CD3OD,
25°C): δH 3.82 (s, 3H, OCH3), 3.62 (s, 2H, CH2-7), 4.26 (s, 2H, CH2-9), 6.66
(d, 3J(H/H)=8.2 Hz, 1H, H6), 6.73 (d, 3J(H/H)=8.2 Hz, 1H, H5), 6.80 (s, 1H,
H2); 13C NMR (100 MHz, CD3OD, 25°C): δC 45.64 (CH2-7), 56.46 (CH3),
67.59 (CH2-9), 114.23, 115.06, 122.83 (C2, C5 and C6), 125.97, 147.18,
147.65 (C1, C3 C4), 208.95 (CO); HRMS: m/z [M-H]- calcd for C10H11O4:
195.0657, found: 195.0656.
Acknowledgements
Procedure for Guaiacylglycerol-β-guaiacyl ether 3 in sealed tube 20
minutes, data reported in Figure 2.
The IJPB benefits from the support of the LabEx Saclay Plant
Sciences-SPS (ANR-10-LABX-552 0040-SPS). This project has
received funding from the Bio Based Industries Joint Undertaking
under the European Union’s Horizon 2020 research and
innovation programme under grant agreement No 720303.
The ionic liquid was vacuum dried at room temperature before use.
Guaiacylglycerol-β-guaiacyl ether 3 (100 mg, 0.31 mmol) and [HMIM][Br]
(610 mg, 12 equivalents) were placed into the Ace presssure tube with a
magnetic stirrer under inert atmosphere. The tube was sealed and the
mixture was stirred in an oil bath at 110 °C. After 20 minutes, water (10
mL) and ethyl acetate (10 mL) were added. The layers were separated,
the aqueous layer was extracted with ethyl acetate (2* 10 mL). The
combined ethyl acetate extracts were dried on MgSO4 and concentrated
under reduced pressure. The crude mixture was purified by flash column
chromatography (elution with 50 to 100% ethyl acetate in cyclohexane) to
yield 78% of product 6.
Keywords: biomass • dealkylation • green chemistry • ionic
liquids • lignin
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Entry for the Table of Contents
FULL PAPER
M. Thierry, A. Majira, B. Pégot, L. Cezard,
F. Bourdreux, G. Clément, F. Perreau, S.
Boutet-Mercey, P. Diter, G. Vo-Thanh, C.
Lapierre, P.-H. Ducrot, E. Magnier, S.
Baumberger, B. Cottyn*
UNCONVENTIONNAL
HEATING METHODS
Value-added
polyols
Isolated lignin
IONIC LIQUID
Raw material
Page No. – Page No.
Lignin chemical demethylation in ionic liquids was investigated using pure lignin-
model monomers and dimer together with dioxan isolated lignins from poplar,
miscanthus and maize. Different methylimidazolium ionic liquids were compared,
according to two different heating processes, microwave irradiation and conventional
heating in sealed tube.
Ionic liquids as efficient reagents for
lignin C-O bond cleavage
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