Oxidative depolymerization of lignin in ionic liquids

Article abstract of DOI:10.1002/cssc.200900242

Beech lignin was oxidatively cleaved in ionic liquids to give phenols, unsaturated propylaromatics, and aromatic aldehydes. A multiparallel batch reactor system was used to screen different ionic liquids and metal catalysts. Mn(NO₃)₂ in 1-ethyl-3-methylimidazolium trifluoromethanesulfonate [EMIM][CF₃SO₃] proved to be the most effective reaction system. A larger scale batch reaction with this system in a 300mL autoclave (11g lignin starting material) resulted in a maximum conversion of 66.3% (24h at 100 °C, 84 × 10⁵ Pa air). By adjusting the reaction conditions and catalyst loading, the selectivity of the process could be shifted from syringaldehyde as the predominant product to 2,6-dimethoxy-1,4-benzoquinone (DMBQ). Surprisingly, the latter could be isolated as a pure substance in 11.5 wt% overall yield by a simple extraction/crystallization process.

Full text of DOI:10.1002/cssc.200900242

DOI: 10.1002/cssc.200900242
Oxidative Depolymerization of Lignin in Ionic Liquids
Kerstin Stꢀrk, Nicola Taccardi, Andreas Bçsmann,* and Peter Wasserscheid*[a]
Beech lignin was oxidatively cleaved in ionic liquids to give
phenols, unsaturated propylaromatics, and aromatic aldehydes.
A multiparallel batch reactor system was used to screen differ-
ent ionic liquids and metal catalysts. Mn(NO3)2 in 1-ethyl-3-
methylimidazolium trifluoromethanesulfonate [EMIM][CF3SO3]
proved to be the most effective reaction system. A larger scale
batch reaction with this system in a 300 mL autoclave (11 g
lignin starting material) resulted in a maximum conversion of
66.3% (24 h at 1008C, 84ꢁ105 Pa air). By adjusting the reaction
conditions and catalyst loading, the selectivity of the process
could be shifted from syringaldehyde as the predominant
product to 2,6-dimethoxy-1,4-benzoquinone (DMBQ). Surpris-
ingly, the latter could be isolated as a pure substance in
11.5 wt% overall yield by a simple extraction/crystallization
process.
Introduction
Today, the crude oil-based production of fuels and chemicals is
considered to be unsustainable due to the foreseeable deple-
tion of its raw material basis. Consequently, the efficient use of
renewable resources has become a global scientific, economi-
cal, and political issue. Moreover, a great amount of CO2 is re-
leased into the atmosphere by burning crude oil-based fuels,
which is promoting global warming. Therefore, the develop-
ment of new sustainable chemical processes and technologies
based on biomass is a fundamental task for chemists and
chemical engineers for the forthcoming years. Biomass can be
considered as a non-fossil organic material made from CO2 and
water through photosynthesis using sunlight.
the plant species it is obtained from. Over the years, different
methods have been explored and reported for the depolymeri-
zation of lignin. Its structure has been cleaved enzymatically,[7]
hydrogenolytically,[8] and oxidatively. Oxidative approaches in-
cluded the use of oxidizing agents like potassium permanga-
nate,[9] nitrobenzene,[10] hydrogen peroxide,[11] or pure
oxygen.[12] Oxidations using molecular oxygen are commonly
carried out in alkaline solutions using copper or iron salts to
activate the pure oxygen. In recent studies, perovskite-type
oxides were also used as catalysts for the wet aerobic oxida-
tion of lignin.[13] The addition of a catalyst was necessary in
order to increase the selectivity for monomeric aromatics. The
products were aromatic aldehydes, for example, vanillin, syrin-
galdehyde, and p-hydroxybenzaldehyde (Figure 1). These sub-
stances are all valuable intermediates for flavors, agrochemi-
cals, performance chemicals, and other fine chemicals.
In principle, a number of well-known refinery processes can
be adapted to a biogenic feedstock basis leading to biorefinery
concepts.[1] However, choosing the right source of biogenic
feedstock is crucial to avoid socioeconomic problems. For ex-
ample, the use of edible biomass to produce biofuels is ques-
tioned today[2] and research efforts into second generation bio-
fuels clearly aim to avoid this problematic “eat or drive” con-
flict.
The maximum yield of aldehydes was about 15 wt% based
on the initial lignin, at a conversion of 73%.[12] However, all
these methods suffer from the fact that only complex mixtures
of the valuable products can be obtained.
Wood is a very interesting, non-edible source of biomass. It
consists mainly of cellulose, hemicelluloses, and lignin, the
quantities of which vary depending on the plants of origin.[3]
Different approaches have already been reported for the indus-
trial use of wood in the context of biorefinery concepts.[4] All
of them have in common that their efficiency and profitability
strongly rely on the maximum valorization of the lignin. This
emphasizes the importance of convincing routes to transform
lignin into valuable products. Furthermore, lignin is the main
byproduct of the pulp and paper industry, in which about
50 Million tonnes of lignin are produced per year.[5] The largest
part of this lignin is burned today to cover the energy needs
of the paper mills. In the future, one can expect that this ener-
getic use might be less desirable given the fact that lignin is
the only major renewable source of the aromatic substances.
Lignin is an irregular macromolecular network of phenylpro-
pane units.[6] Its monomeric composition depends strongly on
Herein, we report our results on oxidative lignin depolymeri-
zation in ionic liquids (ILs). Ionic liquids have been defined as
salts with melting point below 1008C and have attracted
steadily growing interest in recent years due to their often
unique property profiles.[14]
In particular, it has been observed that ILs can dissolve
larger amounts of lignin compared to other solvents common-
ly used for lignin processing.[15] In fact, up to 500 g lignin per
[a] K. Stꢀrk, Dr. N. Taccardi, A. Bçsmann, Prof. P. Wasserscheid
Lehrstuhl fꢁr Chemische Reaktionstechnik,
Friedrich-Alexander Universitꢀt Erlangen-Nꢁrnberg
Egerlandstr. 3, 91058 Erlangen (Germany)
Fax: (+49)9131 85-27421
E-mail: Peter.Wasserscheid@crt.cbi.uni-erlangen.de
Andreas.Boesmann@crt.cbi.uni-erlangen.de
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.200900242.
ChemSusChem 2010, 3, 719 – 723
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719

A. Bçsmann, P. Wasserscheid et al.
Figure 1. Aromatic aldehydes as products of lignin oxidation.
Figure 2. 10-fold batch screening reactor system as used in this work to
screen for suitable ionic liquid/metal salt combinations to oxidatively depo-
lymerize lignin.
kg of ionic liquid have been demonstrated to dissolve in the
ionic liquids 1,3-dimethylimidazolium methylsulfate [MMIM]-
[MeSO4] and 1-butyl-3-methylimidazolium trifluormethanesul-
fonate [BMIM][CF3SO3] after 24 h incubation at 908C.[15c] In ad-
dition, for our specific project, the solubility of oxygen in the
ionic liquid is relevant. The oxygen solubility has been report-
ed to be low.[16] This makes the reaction under elevated pres-
sure necessary but reduces the risk of non-selective over-oxida-
tion. The possibility to direct the selectivity of the oxidative de-
polymerization process towards the most interesting products
by selecting the most suitable ionic liquid structure is particu-
larily attractive. Moreover, adjusting the right oxygen partial
pressure and the right conditions effecting oxygen transfer
into the ionic liquids are further important parameters to opti-
mize the product spectrum.
higher lignin conversion compared to the respective solutions
with iron and copper salts. The most promising system identi-
fied from a screening of 40 systems was a combination of
[EMIM][CF3SO3] with dissolved Mn(NO3)2 as the catalyst.
The following set of experiments were aimed to confirm the
screening results by an ionic liquid optimization study in a
300 mL batch autoclave (Parr Instruments). Four different ionic
liquids were tested in combination with Mn(NO3)2. The results
of these catalytic runs are presented in Table 1. [EMIM][EtSO4]
was selected as the second best candidate in the screening ex-
periments. [MMIM][MeSO4] was chosen for the larger scale test
due to its ability to dissolve large amounts of lignin. [EMIM]-
[MeSO3] was tested due to its structural similarity with [EMIM]-
[CF3SO3].
The applied 300 mL stainless steel autoclave was equipped
with a gas entrainment stirrer. For the experiments, a 10%
lignin solution in ionic liquid was pressurized with compressed
air to 80ꢁ105 Pa at 1008C. After 24 h reaction time, the reactor
was cooled down to room temperature and the pressure was
released. The unreacted lignin was precipitated from the reac-
tion mixture by addition of water. The lignin was filtered off
and oven-dried. Lignin conversion was calculated using Equa-
tion (1):
Results and Discussion
In an initial series of screening experiments, various ionic liq-
uids (1-ethyl-3-methylimidazolium-methansulfonate, 1-ethyl-3-
methylimidazolium-ethansulfonate, 1-ethyl-3-methylimidazoli-
um-bis(trifluoromethylsulfonyl)imide, 1-ethyl-3-methylimidazoli-
um-octylsulfate, 1-ethyl-3-methylimidazolium-trifluoromethan-
sulfonate, and 1-ethyl-3-methylimidazolium-tosylate) together
with different Fe (Fe2(SO4)3, FeCl3), Cu (CuSO4, CuCl2), and Mn
(MnSO4, MnCl2, Mn(NO3)2) salts
as catalysts, were tested in the
multiparallel
batch
reactor
Table 1. Oxidation of lignin using Mn(NO3)2 as catalyst in different ionic liquids.
system (Figure 2). Reaction con-
ditions in these screening experi-
ments were 1008C, 80ꢁ105 Pa
compressed air, and a reaction
time of 17 h.
IL
Conversion Product extracted
Product extracted Product extracted
with DCM [wt%][a] with ethylacetate
[wt%][a]
Total extracted
product
[%]
with toluene
[wt%][a]
[wt%][a]
[EMIM][CF3SO3] 54.6
[MMIM][MeSO4] 32.3
[EMIM][EtSO4] 30.0
[EMIM][MeSO3] 27.1
5.9
3.5
7.7
3.1
26.9
5.3
14.3
6.9
19.8
5.7
6.8
52.6
14.5
28.8
17.1
As the key result of these
screening experiments, it was
found that the solution of man-
ganese salts in ionic liquids, es-
pecially Mn(NO3)2, resulted in a
7.1
[a] Refers to weight of initial lignin. Conditions: lignin (4 g), IL (40 g), 1008C, oxidant: 80ꢁ105 Pa synthetic air
(oxygen partial pressure=16.7ꢁ105 Pa), 24 h reaction time, 1000 rpm.
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Oxidative Depolymerization of Lignin
has an influence on the course of the oxidative depolymeriza-
tion reaction and the resulting product distribution. The prod-
uct mixtures obtained in the four different ionic liquids are
listed in Table 2. 2,6-dimethoxy-1,4-benzoquinone (DMBQ) was
not present in the product mixture of [MMIM][MeSO4] but in
all other experiments. Furthermore, vanillic acid was only de-
tected in the product mixture obtained in [EMIM][CF3SO3].
An obvious question that arises from the experiments
shown in Table 1 is whether the nitrate salt itself plays an
active role in the oxidation chemistry, that is, whether the
lignin oxidation occurs mainly from the gaseous oxygen or
only from the oxygen of the nitrate anion. To obtain a first hint
to answer this important question, we reduced the metal salt
loading by a factor of 10 (from 20 wt% to 2 wt%). One would
expect a greatly reduced conversion if the nitrate plays the
role of a stoichiometric oxidant. Table 3 shows the comparison
of two experiments that are identical except for the catalyst
loading. It can be stated that catalyst loading does not signifi-
cantly influence lignin conversion. However, the selectivity was
found to be greatly affected by the amount of catalyst. Table 4
lists the GC–MS identified products from the DCM extraction
after the reaction at 1008C and 84ꢁ105 Pa pressure of synthet-
ic air. Vanillin, syringaldehyde, DMBQ, and only small quantities
of syringol could be identified in the case of a low catalyst
loading.
weight of precipitated lignin
ð1Þ
Conversion ¼
ꢀ 100
weight of initial lignin
As expected from the screening experiments, the highest
conversion of 54.6% was achieved in [EMIM][CF3SO3], followed
by 32.3% in [MMIM][MeSO4]. Note that the solubility of lignin
in [EMIM][CF3SO3] and [MMIM][MeSO4] is similar and higher
than in the two other ionic liquids.[14c] This shows that lignin
solubility is not the only influencing factor for the efficiency of
oxidative depolymerization in these systems. Interestingly,
using the ionic liquids [EMIM][CF3SO3] and [EMIM][EtSO4], the
mass balance could be closed by a sequence of extractions.
The aqueous solution was first contacted with toluene, then
with dichloromethane (DCM) and finally with ethylacetate (see
Experimental Section for details). Thus, full access to the whole
product spectrum can be realized in these systems.
The reason for an open mass balance in the other systems
could be the formation of gaseous products like CO2 or the
formation of products which are highly water soluble (e.g., un-
substituted phenols). The gas phase was not analyzed for CO2,
but in independent experiments, we could indeed show that
small amounts of phenols were formed in the methylsulfonate
and methylsulfate systems. These phenols could be extracted
from the aqueous phase using trioctylamine as reactive ex-
tracting agent. Here, extraction occurred in form of the respec-
tive trioctylammonium phenolate salts. However, the phenols
found for these two ionic liquids were not enough to close the
mass balance, indicating that other products were formed in
these systems that could not be extracted from the aqueous
phase. The identification and quantification of these products
is currently underway in our laboratories.
With a high catalyst amount, the main product was DMBQ.
Most interestingly, DMBQ (Figure 3) was isolated from the ex-
periment with 20% catalyst loading as a pure substance (>
95%) by precipitation in ethanol, which was identified by NMR
spectrscopy and GC–MS (comparison with pure reference
sample; see the Supporting Information for the corresponding
spectra). In contrast, with the low catalyst loading of 2 wt% of
Mn(NO3)2, DMBQ could be detected as a very small peak in the
GC; however, it was not enough to isolate DMBQ using the de-
In all experiments, no degradation of the ionic liquids was
observed by NMR spectroscopy and no degradation products
could be detected by GC–MS. An attempt to recycle the ionic
liquid after the oxidation experi-
ment has been performed and
showed the principle feasibility
Table 2. Products obtained by extraction with different solvents analyzed by GC–MS.
of ionic liquid reuse. For our re-
cycling experiment, the acidic
aqueous solution obtained after
product extraction was neutral-
ized with CaCO3 and the formed
CaSO4 was filtered off. From the
remaining solution, water was
evaporated, which led to more
CaSO4 precipitation. Therefore,
our so far applied IL recovery
procedure required multiple fil-
tration steps with the corre-
sponding ionic liquid losses to
the filter materials leading to in-
complete IL recovery. Attempts
to improve the IL recovery pro-
cedure are ongoing in our labo-
ratory.
IL
Extracting
agent
Retention time
[min]
m/z
Substances
[EMIM][CF3SO3] toluene
DCM
4.8, 8.4
157, 169
n.i.,[a] DMBQ
syringol, vanillin, DMBQ, n.i., syringalde-
hyde
n.i., coniferyl-fragment, sinapinic acid
n.i., vanillin, syringaldehyde
syringol, vanillin, syringaldehyde
vanillic acid-COOH, syringol, vanillin, n.i.
5.8 ,6.5, 8.4, 9.3, 154, 152,169,
9.5
ethylacetate 3.9, 8.6, 11.3
170, 183
129, 151, 226
143, 153, 183
154, 153, 183
[MMIM][MeSO4] toluene
DCM
4.3, 6.5, 9.5
5.8, 6.5, 9.5
ethylacetate 3.2, 5.8, 9.5, 10.9 124, 154, 183,
199
[EMIM][EtSO4] toluene
5.8, 6.5, 8.4, 8.6, 154, 153, 169,
9.5, 11.3
5.8, 6.5, 8.4, 9.5
syringol, vanillin, DMBQ, syringic acid, sy-
ringaldehyde, sinapinic acid
syringol, vanillin, DMBQ, syringaldehyde
197, 183, 226
154, 153, 169,
183
DCM
ethylacetate 3.2, 5.9, 9,5
[EMIM][MeSO3] toluene 4.8, 6.5, 8.4, 9.5
DCM
124, 154, 183
vanillic acid-COOH, syringol, syringalde-
hyde
n.i., vanillin, DMBQ, syringaldehyde
157, 153, 169,
183
3.2, 4.8, 5.8, 6.5, 124,157, 153,
8.4, 9.3, 9.5
vanillic acid-COOH, n.i., vanillin, DMBQ, n.i.,
syringaldehyde
vanillic acid-COOH, syringol
169, 170, 183
124, 154
ethylacetate 3.2, 5.8
Our catalytic results indicate
that the anion of the ionic liquid
[a] n.i.=non-identified lignin fragment.
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721

A. Bçsmann, P. Wasserscheid et al.
Conclusions
Table 3. Catalyzed synthetic air-oxidation of Organosolv-lignin in [EMIM]-
[CF3SO3] with different catalyst loadings of Mn(NO3)2 at a temperature of
1008C and an oxygen partial pressure of 14ꢁ105 Pa.
In summary, we have developed a method for the oxidative
depolymerization of lignin in ionic liquid using Mn(NO3)2 as the
catalyst. The highest lignin conversion could be reached in the
system [EMIM][CF3SO3]/Mn(NO3)2. The reaction proceeded to
more than 63% lignin conversion for both the 2 wt% and
20 wt% catalysts. At high catalyst loading, DMBQ was formed
as the main product. This compound could be isolated from
the reaction mixture as a pure substance in 21.0% selectivity
and 11.5 wt% overall yield with respect to the amount of
lignin added to the reactor. DMBQ is reported to be of rele-
vance in the context of anti-tumor drugs.
Catalyst
[g]
Catalyst
[wt%][a]
Conversion Product extracted with DCM
[%]
[wt%][a]
2.22
0.22
20.0
2.0
66.3
63.0
32.5
27.2
[a] Refers to the weight of initial lignin. Conditions: lignin (11.2 g), IL
(111.1 g), 1008C, oxidant: 84ꢁ105 Pa synthetic air (oxygen partial pres-
sure=17.5ꢁ105 Pa), 24 h reaction time, 1000 rpm.
Table 4. Products obtained by extraction with DCM analyzed by GC–MS.
IL/catalyst
Extracting
agent
Retention
time [min]
m/z
Products
Experimental Section
Materials
[EMIM][CF3SO3]/
20 wt%[a] Mn(NO3)2
DCM
6.7
8.8
9.7
152
169
183
vanillin
DMBQ
syringaldehyde
The Organosolv-beech-lignin was donated by the Fraunhofer ICT
Pfinztal. The ionic liquids were purchased from Merck.
[EMIM][CF3SO3]/
2 wt%[a] Mn(NO3)2
DCM
5.9
6.7
8.6
9.7
154
153
169
183
syringol
vanillin
DMBQ
syringaldehyde
Analysis
GC–MS analyses were carried out using a Varian GC 2900 coupled
to a Varian MS 2100T. The column was a Varian FactorFour VF-5ms,
30 mꢁ0.25 mm, DF=0.25 mm. The temperature program was a
ramp from 1008C to 3208C at 10 Kminꢁ1 with a gas flow of
1 mLminꢁ1 He.
[a] Refers to the weight of initial lignin.
Oxidation of lignin
Screening experiments
The screening experiments were conducted in a 10-fold autoclave
system. For the experiments, each reactor of a nominal volume of
21 mL was filled with the reaction mixture consisting of lignin
(250 mg), ionic liquid (2.5 g), and catalyst (50 mg). The reactors
were heated to 1008C and pressurized with air to 80ꢁ105 Pa. After
a reaction time of 17 h, the system was allowed to cool to room
temperature and the pressure was released.
Figure 3. Structure of 2,6-dimethoxybenzoquinone.
Variation of ionic liquid
scribed procedure. Therefore, we estimated that the amount of
DMBQ formed with low catalyst loading was lower by at least
a factor 20.
The oxidation of lignin with different ionic liquids was carried out
in a 250 mL batch-autoclave (Hastelloy-C) with a gas entrainment
stirrer. For the experiments, lignin (4.0 g) was dissolved in ionic
liquid (40 g). 20 wt% (with respect to lignin) of the catalyst
Mn(NO3)2 was added to this solution. The resulting solution was
transferred into the reactor. The autoclave was pressurized with
synthetic air at room temperature to a pressure of 65ꢁ105 Pa, the
stirrer was set at 1000 rpm, and the reactor was heated up to
1008C. The total pressure after heating up the reactor was set to
80ꢁ105 Pa. After 24 h reaction time, the reactor was cooled down
to room temperature and the pressure was released.
DMBQ is mentioned in the literature as a potential anti-
tumor agent and as a synthon in organic chemistry.[17] In an
optimized protocol, DMBQ (0.46 g ) was isolated from lignin
(4.0 g), which corresponds to a yield of 11.5 wt%. The selectivi-
ty to isolated DMBQ was calculated according to Equation (2):
S ¼ mDMQ, isolated=ðm0,ligninꢁmligninÞ ¼ 0:46=ð4:0ꢁ1:84Þ ¼ 0:21
ð2Þ
Variation of catalyst loading
and was determined to be 21.0%.
It was assumed that DMBQ is formed by the overoxidation
of syringaldehyde. This was shown in a separate experiment in
which syringaldehyde was oxidized under standard reaction
conditions to give DMBQ as the only product.
Experiments to determine the influence of the catalyst loading
were carried out in a 300 mL stainless steel batch-autoclave
equipped with a gas entrainment stirrer. For these experiments,
lignin (11.12 g) was dissolved in ionic liquid (111.12 g). 20 wt% or
722
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ChemSusChem 2010, 3, 719 – 723

Oxidative Depolymerization of Lignin
2 wt% (with respect to lignin) of the catalyst Mn(NO3)2 were added
to this mixture and the resulting solution was transferred to the
autoclave, which was pressurized at room temperature to 70ꢁ
105 Pa. The stirrer was set at 1000 rpm and the reactor was heated
up to 1008C. The total pressure after heating up the reactor was
set to 84ꢁ105 Pa. After 24 h reaction time, the reactor was cooled
down to room temperature and the pressure was released.
Keywords: aldehydes · ionic liquids · lignin · manganese ·
oxidation
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Oxidation of syringaldehyde
The oxidation of syringaldehyde was carried out in a 250 mL
batch-autoclave (Hastelloy-C) with a gas entrainment stirrer. For
this experiment, syringaldehyde (0.5 g) was dissolved in ionic liquid
(40 g). 20 wt% (referred to syringaldehyde) of the catalyst
Mn(NO3)2 was added to this solution and then put into the reactor.
The synthetic air was introduced to a cold pressure of 65 ꢁ105 Pa,
the stirrer was set at 1000 rpm, and then the reactor was heated
up to 1008C. The total pressure after heating up the reactor was
80ꢁ105 Pa. After 24 h reaction time, the reactor was cooled down
to room temperature and the pressure was released.
Work-up procedure and analytics
The work-up procedure is exemplified for a reaction of lignin (ap-
proximately 11 g) in ionic liquid (111 g).
After the reaction, the lignin was precipitated from the reaction
mixture by adding distilled water (500 mL). The precipitated lignin
was then filtered, dried in an oven, and weighed. The given con-
version was calculated using Equation (1).
The filtrate was first extracted with toluene, then it was acidified
with sulfuric acid (pH 1) and extracted with dichloromethane
(DCM) and ethylacetate. The different extracted product solutions
were dried by evaporation of the solvent and the amount of prod-
uct was weighed. The products were analyzed by GC–MS.
Acknowledgements
[17] a) S. Hayashi, H. Ueki, H. Aoki, K. Tanaka, F. Kanji, K. Junjiro, M. Kozo,
Chem. Pharm. Bull. 1963, 11, 948–951; b) E. Jones, Ekundayo, Olusegun,
Kingston, G. I David, J. Nat. Prod. 1981, 44, 493–4944; c) The Chemistry
of the Quinonoid Compounds (Eds.: S. Patai, Z. Rappoport), Vol. 2, Parts
1 and 2, Wiley, NY, 1988.
The authors wish to acknowledge support by the FNR-project
“NaWaRo” and all project partners in particular, Dr. Jochen Mi-
chels from DECHEMA and Dr. Gerd Unkelbach from Frauenhofer
ICT. Additional support by the German Science Foundation
through its “Leibniz-Program” and through its Excellence Cluster
“Engineering of Advanced Materials” is additionally acknowl-
edged.
Received: October 14, 2009
Revised: December 26, 2009
Published online on May 17, 2010
ChemSusChem 2010, 3, 719 – 723
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org
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