Process of lignin oxidation in an ionic liquid coupled with separation

Article abstract of DOI:10.1039/c3ra40391b

A novel approach has been developed in order to use lignin as a renewable resource for the production of a high added-value aromatic aldehyde. The concept is based on the use of an ionic liquid as a reversible medium coupled with the separation process, which prevents the aromatic aldehyde products from oxidizing and increases their yields. The conversion of lignin reached 100%, and the total yield of the aromatic aldehydes (vanillin, syringaldehyde and p-hydroxybenzaldehyde) was 29.7% in the coupled process. In addition, the mixture of product and IL phase was easily separated, and the IL phase demonstrated good reusability. Hence, a clean and environmentally friendly strategy for overall utilization of lignin and preparation of an aromatic aldehyde is developed.

Full text of DOI:10.1039/c3ra40391b

RSC Advances
View Article Online
View Journal | View Issue
COMMUNICATION
Process of lignin oxidation in an ionic liquid coupled
with separation3
Cite this: RSC Advances, 2013, 3, 5789
Shiwei Liu,*ab Zhongliang Shi,a Lu Li,a Shitao Yu,a Congxia Xiec and Zhanqian Songa
Received 11th November 2012,
Accepted 19th February 2013
DOI: 10.1039/c3ra40391b
www.rsc.org/advances
A novel approach has been developed in order to use lignin as a
renewable resource for the production of a high added-value
aromatic aldehyde. The concept is based on the use of an ionic
pharmaceutical drugs and agricultural pesticides.3,4 The catalytic
wet air oxidation process using air (or oxygen) and supported
catalysts has been indicated as one of the most promising
processes to convert lignin into aromatic aldehydes.5–7 However,
this process still has some shortcomings including a complicated
technique, the non-recyclability of the catalyst and serious
environmental pollution. Especially, the selectivity and final yield
of the products are poor because aromatic aldehydes cannot be
separated from the reaction mixture during the process of the
reaction, which leads them to over-oxidize to form carboxylic
acidic derivatives, such as vanillic acid, syringic acid and minor
quantities of carboxylic acidic derivatives etc.8 Otherwise, the
dissolution of lignin is critically important for its efficient
valorization, but remains a challenge because of the particular
property of the structure of lignin which resists chemical or
enzymatic degradation.9,10 When lignin is dissolved or swollen in
the solvent, the connecting points of the phenylpropane building
units are exposed and the reagents are easily in contact with them;
as a result the degradation continues smoothly.9 Therefore,
finding a solvent which can efficiently dissolve lignin and has
good reusability is also a prerequisite for its oxidative degradation.
Recently, the effectiveness of ionic liquids (ILs) as solvents for
the dissolution of wood and lignocellulosic biomass has been
demonstrated.10–15 Because of their good thermal stability and
negligible vapor pressure, they have been regarded as ‘‘green
solvents’’ that could potentially replace volatile organic sol-
vents.16,17 In a pioneering study by Rogers et al., it was reported
that 1-butyl-3-methylimidazolium chloride [C4mim]Cl was capable
of dissolving up to 10 wt% cellulose,18 due to the disruption of
intramolecular H-bonding by nonhydrated Cl-ions.19,20 Since this
study, more researchers have used ILs to investigate the
dissolution of biomass including lignin,21,22 and based on these
initial investigations, the use of ILs as solvents for the oxidative
degradation of lignin or lignin-related model compounds with
molecular oxygen has also been achieved.12,23–26 However, the
yields of the desired aromatic aldehydes are poor. One main
reason is that they are prone to over oxidation, as they are not
separated from the reaction mixture. Herein, several ILs were
synthesized, characterized and used to constitute a coupled
liquid as
a reversible medium coupled with the separation
process, which prevents the aromatic aldehyde products from
oxidizing and increases their yields. The conversion of lignin
reached 100%, and the total yield of the aromatic aldehydes
(vanillin, syringaldehyde and p-hydroxybenzaldehyde) was 29.7%
in the coupled process. In addition, the mixture of product and IL
phase was easily separated, and the IL phase demonstrated good
reusability. Hence, a clean and environmentally friendly strategy
for overall utilization of lignin and preparation of an aromatic
aldehyde is developed.
1. Introduction
With the gradual diminishing of fossil fuel reserves and growing
concerns about global warming, finding ways of exploiting feasible
pathways for the replacement of the petroleum-based chemicals
are highly desirable.1 To meet the growing demand for this,
biomass can serve as a sustainable source of renewable fuels and
chemicals.2 Lignin, one of the main components of biomass in
the biorefining process, is usually discarded as non-cellulosic
waste in industry. It is an extremely complex three-dimensional
macromolecule with an irregular structure, which results from
random dehydrogenative polymerization of phenylpropane build-
ing units, including coniferyl, sinapyl and p-coumaryl alcohols.
Therefore, lignin can be converted into aromatic aldehydes such
as p-hydroxybenzaldehyde, vanillin and syringaldehyde by the
catalytic oxidation degradation process. These aromatic aldehydes
are widely used in flavoring as chemical intermediates for
aCollege of Chemical Engineering, Qingdao University of Science and Technology,
Qingdao 266042, China
bState Key Laboratory of Pulp and Paper Engineering, South China University of
Technology, Guangzhou 510640, China
cCollege of Chemistry and Molecular Engineering, Qingdao University of Science and
Technology, Qingdao 266042, China
Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra40391b
3
This journal is ß The Royal Society of Chemistry 2013
RSC Adv., 2013, 3, 5789–5793 | 5789

View Article Online
Communication
RSC Advances
disc): n 2953.55, 2849.40, 1639.82, 1495.84, 1227.21, 1047.90,
808.09, 676.39; 1H NMR (500 MHz, D2O): d 8.61–8.60(d, 2H, J = 6.5
Hz), 8.36–8.33(t, 2H, J = 7.5 Hz), 7.88–7.85(t, 2H, J = 7 Hz), 4.67(s,
3H), 3.39–3.34(m, 6H, J = 5.5 Hz); 13C NMR (125 MHz, D2O): d
145.14, 144.94, 127.82, 52.60, 47.98; 31P NMR (202 MHz, D2O): d
3.10. [mmo][Me2PO4]: IR (KBr disc): n 3008.35, 2954.97, 1653.34,
process of reaction–separation for the oxidative degradation of
lignin to produce aromatic aldehydes.
2. Experimental
2.1 Chemicals and instruments
1
1472.52, 1229.10, 1048.76, 925.93, 809.76, 538.97; H NMR (500
Lignin (organosolv lignin from Sigma Aldrich, product No. 371017.
It is extracted from mixed hardwoods (maple, birch, and poplar)
by an organosolv process using aqueous ethanol), trimethyl
phosphite (Albright & Wilson), 1-methylimidazole (99.8 wt%,
Sigma Aldrich), 1,3-propane sultone (1,2-oxathiolane-2,2-dioxide,
99.7 wt%), and other chemicals (analytic purity) were commer-
cially available and used without further purification.
IR spectra were recorded by a Nicolet 510P FT-IR spectrometer
in the range 4500–400 cm21. NMR spectra were taken by a Bruker
AV500 Fourier Transform spectrometer with reference to SiMe4,
using solvent D2O containing 5 wt% of the sample. Qualitative
analysis of the product sample was recorded by a Shimadzu LC-
MS 8040 equipped with a column of an Agilent C18 (1.8 mm, 2.1
mm 6 100 mm) and a UV detector set at 280 nm. The mobile
phase was a mixture of ammonium acetate (5 mmol l21) and
acetonitrile (100%). Quantitative analysis of the product sample
was recorded by an Agilent HP-LC1100 equipped with a column of
Dikma ODSC18 (5.0 mm, 4.6 mm 6 250 mm), and an Agilent1200
detector set at 280 nm. The mobile phase was a mixture of
acetonitrile (8.5%), deionized water (90%) and acetic acid (1.5%).27
MHz, D2O): d 3.83(t, 4H, J = 2.5 Hz), 3.37–3.35(s, 6H, J = 11 Hz),
3.30–3.28(t, 4H, J = 10 Hz), 3.04(s, 6H); 13C NMR (125 MHz, D2O): d
60.85, 60.46, 52.58, 51.53; 31P NMR (202 MHz, D2O): d 3.12.
[mtetn][Me2PO4]: IR (KBr disc): n 2991.71, 2953.56, 1649.85,
1460.69, 1228.89, 1047.47, 807.75, 538.89; 1H NMR (500 MHz,
D2O): d 3.30–3.29(d, 6H, J = 10.5 Hz), 3.08–3.06(q, 6H, J = 7.5 Hz),
2.67(s, 9H), 1.06–1.03(q, 6H, J = 7 Hz); 13C NMR (125 MHz, D2O): d
55.68, 52.64, 46.31, 7.10, 7.05; 31P NMR (202 MHz, D2O): d 23.29.
2.3 Catalytic experiments
The reaction phase (white) contains IL, liginin, and the catalyst
CuSO4 and is represented in the bottom half of the batch reactor
in . The extraction phase (gray) contains methylisobutylketone and
is represented in the top half of the batch reactor. Oxidant O2,
through its distributor (total volume 6 ml, including pipeline),
passes directly into the reaction layer (Fig. 2).
The heterogeneous catalytic oxidation degradation process of
lignin was carried out in a 100 ml high pressure reactor equipped
with an oxygen distributor, a stirrer and a thermometer. 50 g of an
IL solution of lignin dissolved at a concentration of 5 wt% and a
certain quantity of catalyst CuSO4 were introduced into the
reactor. The heating program was started under a slight N2
pressure. When the solution in the reactor reached the desired
temperature, N2 was added through its catheter until a total
pressure of 2.36 MPa was attained (in order to prevent O2 touching
with the extraction phase, N2 pressure was determined according
to the Clapeyron–Clausius equation), and then O2 was added
through its catheter linking the distributor until its pressure
balanced with the N2 pressure and reached 2.5 MPa. O2 pressure
in the reactor was kept at 2.5 MPa by continuous flushing of O2 as
2.2 IL preparation
Trimethyl phosphite (84.5 g, 0.6 mol) was added dropwise in
1-methylimidazole (41.1 g, 0.5 mol) at 150 uC within 2 h and then
reacted for 10 h at 115 uC. The resultant mixture was washed with
ether three times, and the residual ether was removed under
vacuum, then the IL, 1,3-dimethylimidazolium dimethyl phos-
phate [mmim][Me2PO4], was dried under high vacuum at 70 uC for
4
h (106.8 g, yield 95.8%). Other ILs, [mPy][Me2PO4],
[mtetn][Me2PO4] and [mmo][Me2PO4], were synthesized according
to the same process (Fig. 1). [mmim][Me2PO4]: IR (KBr disc): n
2954.05, 2849.13, 1650.90, 1578.01, 1462.75, 1229.58, 1181.03,
1048.28, 805.90, 751.42, 621.82; 1H NMR (500 MHz, D2O): d 8.47(s,
1H), 7.23(d, 2H, J = 1 Hz), 3.70–3.68(d, 6H, J = 12.5 Hz), 3.31–3.29(t,
6H, J = 11 Hz); 13C NMR(125 MHz, D2O): d 136.43, 123.28, 52.45,
35.50; 31P NMR (202 MHz, D2O): d 29.85. [mPy][Me2PO4]: IR(KBr
Fig. 2 Batch process for the production of aromatic aldehydes from lignin with
Fig. 1 The structure of the synthesized ILs.
5790 | RSC Adv., 2013, 3, 5789–5793
simulated countercurrent extraction.
This journal is ß The Royal Society of Chemistry 2013

View Article Online
RSC Advances
Communication
Table 1 Effects of different ILs on the reaction resultsa
a supplement because of its consumption during the reaction.
When the reaction was finished, the mixture was separated and
the IL layer was extracted with methylisobutylketone three times.
The extraction solution and the extraction phase were mixed, and
the contents of p-hydroxybenzaldehyde (PHB), vanillin (VAN) and
syringaldehyde (SYR) were analyzed by high performance liquid
chromatography. The residual lignin was obtained from the
extracted phase by centrifugation. 30 ml water was added to the
extracted phase, and then the precipitated lignin was centrifuged,
washed and dried to determine its conversion. The residual lignin
was dissolved in the sodium hydroxide solution and a UV
spectrophotometer was used to determine lignin content at a
wavelength of 280 nm.28 The filtrate was distilled under vacuum
to remove the water and to obtain the IL phase which
contained the catalyst and was reused in the recycle experi-
mental. Reactant conversion and product yield were calculated
Yproduct (%)
Entry
ILs
Xreactant (%)
VAN
SYR
PHB
1b
2
3
—
100
100
100
92.6
86.0
100
10.2
14.7
14.5
12.3
11.2
10.5
5.6
8.8
8.7
7.1
6.5
5.4
3.5
6.2
5.9
5.4
4.4
3.3
[mmim][Me2PO4]
[mPy][Me2PO4]
[mtetn][Me2PO4]
[mmo][Me2PO4]
[mmim][Me2PO4]
4
5
6c
a
Lignin 2.5 g, solvent IL 47.5 g, catalyst CuSO4 0.025 g, O2 pressure
2.5 MPa, T = 175 uC, t = 1.5 h. The solvent was 47.5 g, 2.0 mol l21
NaOH aqueous solution, methylisobutylketone was unused, and the
other conditions were the same as a. Methylisobutylketone was not
b
c
used, and the other conditions were the same as a.
Freactant, in{Freactant, out
metal without a substrate.23 An O2–[mmin] intermediate (Fig. 3,
(4)) may be generated from the IL, which enhances the oxygen
contact of lignin and accelerates the generation of the inter-
mediate of the quinone methide radical (Fig. 3, (3)) and quinone
methide hydroperoxide (Fig. 3, (5)).29 With the combined effects of
the above mentioned factors, the proposed mechanism of
oxidation of lignin catalyzed by CuSO4 in IL is depicted in
Fig. 3. Secondly, the used IL is a weak alkali and its basicity is
lower than that of NaOH. The weaker alkali environment usually
increases the oxidative ability of the active oxygen,30 as a result the
degradation in IL is easier than that in NaOH.31 Otherwise, the
degradation of lignin and the extraction of aromatic aldehydes
simultaneously happened in the coupled process. Once lignin is
oxidized to form aromatic aldehydes, they are extracted by
methylisobutylketone and separated from the IL layer, which
avoids their deep oxidation reaction and increases their yields. The
result from the experimental without using methylisobutylketone
as an extractant was also determined to show the effect of the
coupled system (Entry 6). Among all the ILs which were
investigated, ILs with an aromatic ring in the cation showed
better properties for the oxidative degradation of lignin. This is
due to the fact that these ILs have a better solubility to lignin,
which makes the connecting points of the phenylpropane building
units exposed, and then accelerates the process of degradation.
as
follows:
Xreactant(%)~
|100%,
Freactant, in
|100%, Fi,in and Fi,out are the weight of
Fproduct, out
Yproduct(%)~
Freactant, in
the i species for reactant and product at the inlet and outlet of the
reactor, respectively.
3. Results and discussion
3.1 Effects of different ILs on the reaction results
For the reaction–separation coupled process for the oxidative
degradation of lignin, solvents with good solubility to aromatic
aldehydes, such as acetone, methylisobutylketone, toluene, benzyl
chloride, trichloroethane and n-butanol, were investigated. Their
solubility in IL [mmim][Me2PO4] and their effect on IL solubility to
lignin at room temperature was monitored. The results showed
that n-butanol and trichloroethane had good solubilities in ILs,
which made the phase separation become infeasible. Toluene and
benzyl chloride decreased the lignin solubility in ILs and made the
lignin separate out from the IL phase. Acetone and methylisobu-
tylketone did not have the above phenomena and had good
performances to build the coupled process of reaction–separation
with IL. However, acetone is easy to lose in the processes of
separation and reuse, because its saturated vapor pressure is
higher than that of methylisobutylketone under atmospheric
pressure. Therefore, methylisobutylketone was selected and used
for the coupled process of reaction–separation.
3.2 Effects of reaction conditions on the reaction results
As shown in Table 1, compared with the traditional solvent of
NaOH aqueous solution (Entry 1), the investigated ILs were better
solvents for the oxidative degradation reaction. Especially, ILs
[mmim][Me2PO4] and [mPy][Me2PO4] exhibited outstanding prop-
erties with 100% conversion and nearly 30% total yield of aromatic
aldehydes (Entries 2 and 3). Furthermore, the amount of
byproducts produced was obviously smaller in the coupled process
than that of NaOH (Entry 1). For example, among the determined
compounds, the yield of syringate decreased from 8.3% to 1.1%,
3,5-dimethoxybenzoic acid from 4.9% to 0.4%, and p-hydroxyben-
Table 2 shows the effects of reaction conditions on the oxidative
degradation reaction. It was obviously seen that the yields of
products became higher with increasing the IL dosage from 2.0 g
to 3.0 g. However, when lignin dosage was more than 2.5 g (Entry
3), the yields of products did not obviously increase, but the
conversion of lignin decreased to 94.2%, which meant that the
degradation reaction did not complete at the given time. O2
pressure was very important for the oxidative degradation reaction.
When O2 pressure was 2.0 MPa, the conversion of lignin was less
than 85% under the given conditions (Entry 4). When the O2
pressure was more than 2.5 MPa, the conversion of lignin reached
100% (Entry 5), but the yields of products obviously decreased.
The above results may be due to the fact that, the higher the O2
pressure, the stronger the oxidative ability of the active oxygen,
which leads to aromatic aldehydes oxidizing thus decreasing their
zoic acid was not detected in the coupled process (see ESI
3
S-Fig. 12 and 13). This good reaction result may be explained by
the following three points. Firstly, the properties of the IL greatly
affect the catalytic activity of the transition metal salt both by
stabilizing the reactive intermediate and by favoring the coordina-
tion of the substrate to the metal over the direct oxidation of the
This journal is ß The Royal Society of Chemistry 2013
RSC Adv., 2013, 3, 5789–5793 | 5791

View Article Online
Communication
RSC Advances
Fig. 3 Proposed mechanism of lignin oxidation in IL [mmim][Me2PO4].
yields. The effects of reaction temperature and time on the
oxidative degradation reaction are also shown in Table 2 (Entries 2
and 6–9). It was found that both were very important for the
degradation reaction. With an increase of reaction temperature
from 155 uC to 195 uC, the conversion of lignin significantly
increased. However, when the reaction temperature reached 195
uC (Entry 7), the yields of products decreased. With increasing
reaction time (Entries 2, 8 and 9), similar reaction results were
obtained. Therefore, it was indicated that a higher reaction
temperature and longer reaction time were favorable to the deep
oxidation of aromatic aldehydes, which made the yields of
products decrease obviously. Otherwise, under harsh reaction
conditions, such as increasing reaction temperature and O2
pressure, or prolonging the reaction time, the results showed that
the yields of vanillin and syringaldehyde decreased even more
sharply than that of p-hydroxybenzaldehyde, which indicated that
vanillin and syringaldehyde were more easily oxidized than
p-hydroxybenzaldehyde. This is because of the electronic effect.
Compared with p-hydroxybenzaldehyde, the methoxy group of
vanillin or syringaldehyde, as an electron donor, increases the
electron cloud density of the benzene ring, which makes its
oxidation easier. Based on the above results, the optimum
conditions were obtained: IL [mmim][Me2PO4] 47.5 g, lignin 2.5
g, O2 pressure 2.5 MPa, reaction temperature 175 uC, and reaction
time 1.5 h. Under these conditions, the conversion of lignin
reached 100%, and the total yield of products was nearly 30%.
Table 2 Effects of reaction conditions on the reaction resultsa
Yproduct (%)
Entry Lignin (g) P (MPa) T (uC) t (h) X
(%) VAN SYR PHB
reactant
1
2
3
4
5
6
7
8
9
2.0
2.5
3.0
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.0
3.0
2.5
2.5
2.5
2.5
175
175
175
175
175
155
195
175
175
1.5 100
1.5 100
14.3 8.5 6.9
14.7 8.8 6.2
14.8 8.6 6.5
9.9 3.7 2.0
10.2 4.3 5.4
10.5 4.9 2.5
11.7 5.0 5.4
11.4 4.8 2.1
12.5 5.3 5.2
1.5
1.5
94.2
82.3
1.5 100
1.5
86.0
1.5 100
1.0
88.3
2.0 100
a
Solvent: IL [mmim][Me2PO4] 47.5 g. Catalyst: CuSO4. Extractant:
Methylisobutylketone.
5792 | RSC Adv., 2013, 3, 5789–5793
This journal is ß The Royal Society of Chemistry 2013

View Article Online
RSC Advances
Communication
Table 3 Reusability of the IL phase
References
1 J. R. Rostrup-Nielsen, Science, 2005, 308, 1421–1422.
2 J. N. Chheda and J. A. Dumesic, Catal. Today, 2007, 123, 59–70.
3 P. Sridhar, J. D. Araujo and A. E. Rodrigues, Catal. Today, 2005,
105, 574–581.
4 F. G. Sales, L. C. A. Maranhao, N. M. L. Filho and C. A.
M. Abreu, Ind. Eng. Chem. Res., 2006, 45, 6627–6631.
5 A. L. Mathias and A. E. Rodrigues, Holzforschung, 1995, 49,
273–278.
Yproduct (%)
t (h)
Xreactant (%)
VAN
SYR
PHB
1
2
3
4
5
6
100
100
100
100
100
100
14.7
14.8
14.5
14.7
14.6
14.6
8.8
8.7
8.5
8.9
8.6
8.5
7.2
7.3
7.0
7.4
7.3
7.2
6 J. D. P. Araujo, C. A. Grande and A. E. Rodrigues, Chem. Eng.
Res. Des., 2010, 88, 1024–1032.
7 P. C. R. Pinto, E. A. Borges da Silva and A. E. Rodrigues, Ind.
Eng. Chem. Res., 2011, 50, 741–748.
3.3 Reusability of the IL phase
8 M. A. Longo and M. A. Sanroman, Food Technol. Biotechnol.,
2006, 44, 335–353.
9 J. Zakzeski, P. C. A. Bruijnincx, A. L. Jongerius and B.
M. Weckhuysen, Chem. Rev., 2010, 110, 3552–3599.
10 M. Ohkuma, Y. Maeda, T. Johjima and T. Kudo, RIKEN Rev.,
2001, 42, 39–42.
11 J. Zakzeski, A. L. Jongerius and B. M. Weckhuysen, Green
Chem., 2010, 12, 1225–1236.
12 M. Zavrel, D. Bross, M. Funke, J. Buchs and A. C. Spiess,
Bioresour. Technol., 2009, 100, 2580–2587.
13 I. Kilpelainen, H. B. Xie, A. King, M. Granstrom, S. Heikkinen
and D. S. Argyropoulos, J. Agric. Food Chem., 2007, 55,
9142–9148.
14 Y. Pu, N. Jiang and A. J. Ragauskas, J. Wood Chem. Technol.,
2007, 27, 23–33.
15 A. Brandt, J. P. Hallett, D. J. Leak, R. J. Murphy and T. Welton,
Green Chem., 2010, 12, 672–679.
16 R. A. Sheldon, R. M. Lau, M. J. Sorgeddrager and F. van
Rantwijk, Green Chem., 2002, 4, 147–151.
In order to investigate the possibility of recycling the IL
[mmim][Me2PO4], a recycling experiment was conducted under
optimum conditions. The reusability of the IL phase including
catalyst CuSO4 is shown in Table 3. The IL phase could be reused
six times without an obvious decrease in its performance. This
indicated that the IL phase had good reusable performance in the
reaction. The good reusability of the IL is attributed to its good
thermal stability and good solubility for the catalyst, which can
help to avoid the loss of the IL and catalyst during the reaction
and separation processes.
In order to investigate the integrity of the repeatedly utilized IL,
the obtained low phase during oxidation degradation of lignin was
extracted three times with methylisobutylketone and then distilled
to remove the residual methylisobutylketone. The residue which
was mainly composed of IL [mmim][Me2PO4] was obtained and
characterized by IR and NMR. The spectra are given in the ESI (S-
3
Fig. 1–5). The results showed that IR and NMR spectra of the six
time-reused IL [mmim][Me2PO4] were identical to that of the
unused IL. It was demonstrated that the structure of the reused IL
was unspoiled.
17 S. H. Lee, T. V. Doherty, R. J. Linhardt and J. S. Dordick,
Biotechnol. Bioeng., 2009, 102, 1368–1376.
18 R. P. Swatloski, S. K. Spear, J. D. Holbrey and R. D. Rogers, J.
Am. Chem. Soc., 2002, 124, 4974–4975.
19 R. D. Rogers and K. R. Seddon, Science, 2003, 302, 792–793.
20 N. Sun, H. Rodriguez, M. Rahman and R. D. Rogers, Chem.
Commun., 2011, 47, 1405–1421.
21 N. Muhammad, Z. Man and M. A. B. Khalil, Holz Roh- Werkst.,
2012, 70, 125–133.
22 W. Y. Li, N. Sun, B. Stoner, X. Y. Jiang, X. M. Lu and R.
D. Rogers, Green Chem., 2011, 13, 2038–2047.
23 J. Zakzeski, P. C. A. Bruijnincx and B. M. Weckhuysen, Green
Chem., 2011, 13, 671–680.
24 S. Y. Jia, B. J. Cox, X. W. Guo, Z. C. Zhang and J. G. Ekerdt, Ind.
Eng. Chem. Res., 2011, 50, 849–855.
25 K. Stark, N. Taccardi, A. Bosmann and P. Wasserscheid,
ChemSusChem, 2010, 21, 719–723.
26 J. B. Binder, M. J. Gray, J. F. White, Z. C. Zhang and J.
E. Holladay, Biomass Bioenergy, 2009, 33, 1122–1130.
27 J. M. Lobbes, H. P. Fitznar and G. Kattner, Anal. Chem., 1999,
71, 3008–3012.
4. Conclusions
A coupled process of reaction–separation composed IL and
methylisobutylketone exhibited good performance in the oxidative
degradation reaction of lignin. Compared with the traditional
solvent of NaOH aqueous solution, the coupled process avoided
the deep oxidation of the products and significantly improved the
lignin conversion and the yield of aromatic aldehydes. Otherwise,
the mixture of product and IL phase was easily separated, and the
IL phase containing the catalyst CuSO4 had a good reusability.
Hence, a clean and environmentally friendly strategy for overall
utilization of lignin and preparation of aromatic aldehyde is
developed.
Acknowledgements
28 M. Zabkova, E. A. Borges da Silva and A. E. Rodrigues, J. Membr.
Sci., 2007, 301, 221–237.
29 M. Yang, A. H. Xu and H. Z. Du, J. Hazard. Mater., 2007, 139,
86–92.
30 T. Y. Liu, C. L. Chen and C. W. Chi, Mutat. Res., Genet. Toxicol.,
1996, 367, 25–31.
The authors are grateful for the financial support from the
Natural Science Foundation of China (31100430), and the
Open Foundation from State Key Laboratory of Pulp and Paper
Engineering of South China University of Technology (201120).
31 F. Cui and D. Dolphin, Bioorg. Med. Chem., 1995, 3, 471–477.
This journal is ß The Royal Society of Chemistry 2013
RSC Adv., 2013, 3, 5789–5793 | 5793