Degradation of lignin with aqueous ammonium-based ionic liquid solutions under milder conditions

Article abstract of DOI:10.1039/C8NJ05185B

This study investigates the performance of two aqueous ionic liquids (ILs), dimethylbutylammonium acetate ([DMBA][Ac]) and dimethylbutylammonium butanoate ([DMBA][B]), solutions for depolymerizing alkali lignin into valuable phenolic compounds. The favorable operation conditions, including reaction temperature and reaction time, are explored. The extent of depolymerization of the lignin is evaluated by analysis with gel permeation chromatography (GPC). The results show that the average molecular weights of the depolymerized lignin samples can be reduced by as high as 93.8% and 86.8% after treating with the aqueous [DMBA][Ac] and [DMBA][B], respectively. Moreover, the aromatic chemical species in the depolymerized solutions are identified by using gas chromatography?mass spectrophotometry (GC-MS). The confirmation of the chemical species is further made by using a series of spectroscopic techniques, such as FT-IR, and ¹H NMR and ¹³C NMR spectroscopy. Promising results have been achieved for the depolymerization of the lignin into valuable chemicals by using the proposed green media, aqueous solutions of ionic liquids [DMBA][Ac] and [DMBA][B], under milder conditions.

Full text of DOI:10.1039/C8NJ05185B

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Degradation of Lignin with Aqueous Ammonium-Based Ionic Liquid SolutionVsiew Article Online
DOI: 10.1039/C8NJ05185B
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under Milder Condition
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Leta Deressa Tolesa, Bhupender S. Gupta, and Ming-Jer Lee*
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Department of Chemical Engineering, National Taiwan University of Science and
Technology, 43 Keelung Road, Section 4, Taipei 106-07, Taiwan.
Abstract
This study investigates the performance of two aqueous ionic liquids (ILs),
dimethylbutylammonium
acetate
([DMBA][Ac])
and
dimethylbutylammoniumbutanoate ([DMBA][B]), solutions for depolymerizing alkali
lignin into valuable phenolic compounds. The favorable operation conditions,
including reaction temperature and reaction time, are explored. The extent of
depolymerization of the lignin is evaluated by analysis with gel permeation
chromatography (GPC). The results show that the average molecular weights of the
depolymerized lignin samples can be reduced by as high as 93.8 % and 86.8 % after
treating with the aqueous [DMBA][Ac] and [DMBA][B], respectively. Moreover, the
aromatic chemical species in the depolymerized solutions are identified by using Gas
Chromatography-Mass Spectrophotometer (GC-MS). The confirmation of the
chemical species is further made by using series of spectroscopic techniques, such as
1
FT-IR, H NMR, and 13C NMR. Promising results have been achieved for the
depolymerization of the lignin into valuable chemicals by using the proposed green
media, aqueous solutions of ionic liquids [DMBA][Ac] and [DMBA][B], under
milder condition.
Keywords: lignin; aqueous ionic liquid; depolymerization; aromatic chemicals
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Introduction
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Lignin has been recognized as a potential resource for producing valuable
aromatic chemicals in the recent years. Additionally, various applications of the lignin
such as an additive in plastic, biofuel, and bioactive compounds have also been
reported in literature.1-7 It is one of the major components in the biomass, found from
woody materials, agricultural waste, and also produced as a byproduct in paper or
pulp industries.8,9 Its readily availability in enormous amount makes it as a potential
candidate to fulfill our future demand of valuable chemicals as well as energy.
Although the importance of the lignin material is well noticed worldwide in the
general practice, it is mainly burned in the atmosphere to get energy and only a few
amount of it is used for the other purposes.2 Therefore, developing an efficient method
to utilize those lignin raw-materials as a feedstock for producing the specific
chemicals could be highly advantageous. However, the inherent complexity and the
heterogeneity in the structure make its solvation and controlled depolymerization to
be a challenging task and also prevent us to have a deep understanding on the
mechanism of lignin depolymerization.
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The main challenge in selective depolymerization of lignin arises from its
polymeric network, which is formed by the strong C-C and C-O-C linkages. The
presence of those strong linkages limits the depolymerization process. However, apart
from the challenges associated with the depolymerization of the lignin, considerable
amount of researches have been conducted to convert lignin into low molecular
weight chemicals.10-15 Among several others, pyrolysis,16,17, hydrolysis,18,19 treatments
with super- or sub-critical solvents,20-24 hydrogenolysis, and catalysis
have been
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proposed to solvate and depolymerize the lignin. However, most of the reported
methods used conventional volatile organic solvents with homogeneous or
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heterogeneous catalyst. In addition, the proposed methods, such as pyrolysis, require
high energy input and lead to the non-selective depolymerization of the lignin and
produce high amount of char in the final product. The use of subcritical water shows
promising results in dissolution of lignin, but overall process is very complex and also
energy consumption is still high.
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As an ideal solvation and depolymerization medium for the lignin, it should be
non-volatile, biodegradable, easy to recycle and thermally stable. Ionic liquid, a new
user tailor medium, emanates as an alternative solvent and appears to fulfill all those
expectations of an ideal medium for lignin research. In the last decade, using ionic
liquids in the lignin researches is gaining great attention.30,31 The main advantage
associated with the use of ionic liquid is that its structure and thus properties can be
tuned to increase the interactions and the reaction site with lignin.
The suitability of the ILs to solvate the range of lignocellulosic material at
comparatively milder condition is well documented in the literature.32 Recently, Singh
et al.33 reported that the use of ionic liquid ([C3SO3HMIM][HSO4]), formed by
combination of the imidazolium as a cation and sulfate as an anion, for the
depolymerization of lignin yielded high concentration of low molecular weight
compounds at 120 °C. Xue et al.3 have demonstrated that by using
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imidazolium-based ILs, such as 1-methyl-3-benzylimidazolium chloride
([Bnmim][Cl]) and 1-methyl-3 benzylimidazoliumtrichloroacetate ([Bnmim][TFA])
with HCl as a catalyst, about 75.5 % (mass basis) of the lignin can be depolymerized
at the optimized reaction condition. Moreover, the fractionate solution is found to
contain about 60 wt% of low molecular weight phenolic compounds. The
fractionation of about 40 % lignin in the aqueous solutions of IL, such as
1-ethyl-3-methylimidazolium acetate [C2mim][OAc], was also observed from the
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reaction taking place at 150 ºC for 3 hours. Some other studies also highlighted the
potential of the imidazoilum-based ionic liquids for depolymerization of lignin in
cooperating with an external catalyst or promoter. Moreover, to avoid using an
external catalyst or promoter, the IL such as 1-H-3-methylimidazolium chloride with
bi-functional activity (solvent + degradation agent) for depolymerization of lignin was
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also reported in literature.3
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Since imidazolium-based ionic liquids are categorized as non-biodegradable in
nature3 ,38, the use of the non-immidazolium based ionic liquids for improving the
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utilization of the lignin is suggested. For example, Prado et al. utilized ionic liquid
([ET3NH][HSO4]), derived from ammonium as a cation and sulfate as a anion, as a
solvent for the lignin from willow biomass. The lignin in the ionic liquid solutions
was depolymerized by using homogeneous H2O2 as well as heterogeneous TiO2
catalysts. The depolymerized solution catalyzed by TiO2was found rich in phenolic
derived compounds. Kim et al.40 have shown that the new class of non-imidazolium
IL, choliniumlysinate ([Ch][Lys]), was applicable to extract lignin from the switch
grass. In order to fractionate the lignin, they have used isopropyl alcohol (IPA) as a
hydrogen transfer catalyst. As the reaction was operated at 300 °C, about 27 wt% of
monomers was produced with low char formation in the depolymerized solution.
Moreover, they have also found that on prolong heating the solution in the presence of
Ru/C catalyst, the depolymerization of lignin was significantly enhanced and yielded
low molecular weight value-added chemicals with greater extent of alkylation on the
aromatic ring. The reaction is initiated by dehydrogenation of the ether bond via
formation of Ru(0) complex which leads to Ru-enolateformation which yields
phenolic monomer through reductive elimination. Our literature survey reveals that
the ILs may serve as an important role in the utilization of the enormously available
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lignin materials. It is reported that the anion of the ionic liquids stabilDizOeI:s10.t1h03e9/C8NJ05185B
hydroxyl group of lignin which speed up the cleavage of C-O bond. In addition to the
coordination ability of an IL anion for lignin depolymerization, they act as
nucleophiles by attacking the highly electron deficient protonated C–O bonds.41 Some
studies showed the suitability of imidazolium-based ionic liquids in combination with
heterogeneous catalyst for the depolymerization of the lignin, the reports on the
non-imidazolium-ammonium-based ionic liquids, which could serve as both a solvent
and a degradation agent for the lignin depolymerization process, are rather scarce in
the literature. Most of the references related to the use of ionic liquids for
depolymerization of lignin are summarized in Table 1.
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Table 1
Ionic liquid assisted depolymerization of lignin at various reaction conditions.
Types of lignin
Oak wood lignin
Organosolv lignin
Klason lignin
Ionic liquid
Reaction condition
110–150 ºC,
Reference
[36]
[8]
[HMIM][Cl]
[Emim][ABS]
[Bmim][MeSO4]
[Bmim][MeSO4]
[Emim][ABS]
[C2mim][OAc]
[DIPEA][Ac]
[DIPEA][B]
180 °C, 6 h
180 °C, 6 h
[8]
Organosolv lignin
Klason lignin
180 °C, 6 h
[8]
180 °C, 6 h
[8]
Alkali lignin
110–170 °C, 1–16 h
100-170 ºC, 0.5-1.5 h
100-170 ºC, 0.5-1.5 h
150 °C, 3 h
[42]
[43]
[43]
[44]
Alkali lignin
Alkali lignin
Enzymatic lignin
[C4mim][Cl]
In the present study, we investigate the performance of two ammonium-based ILs,
dimethylbutylammonium acetate ([DMBA][Ac]) and
dimethylbutylammoniumbutanoate ([DMBA][B]), for the degradation of the alkali
lignin. The favorable operating conditions, including reaction temperature and time,
are explored for the lignin depolymerization in several specific concentrations of the
aqueous ILs solutions. The depolymerized solutions are characterized by using series
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of techniques, including FTIR, GPC, C and H NMR, and GC-MS. We select these
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two ionic liquids ([DMBA][Ac] and [DMBA][B]) to get more insight into the
behavior of non-symmetrical ammonium ion on the depolymerization of the lignin.
Also, these two ionic liquids having common ammonium cation and different anions
of varying alkyl chain length can enhance the knowledge on the effect of increasing
hydrophobicity on the results of lignin depolymerization.
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Materials and methods
Materials
Alkali lignin with low sulfonate content (Sigma-Aldrich, USA) was used for
depolymerization process. Acetic acid (mass fraction purity > 0.998, Sigma-Aldrich,
USA), butyric acid (mass fraction purity > 0.99, Sigma-Aldrich, USA), and
dimethylbutylamine (mass fraction purity > 0.99, Sigma-Aldrich, USA) are the raw
materials for the synthesis of the ionic liquids. The ionic liquids ([DMBA][Ac] and
[DMBA][B]) were synthesized in our laboratory according to the method reported in
literature.45,46,47 While sodium nitrate (mass fraction purity > 0.99, Sigma-Aldrich,
USA) was used as a mobile phase for GPC analysis, pullulan (Mw ∼10000,
Sigma-Aldrich, USA) was served as a standard for calibrating GPC. Methanol (mass
fraction purity > 0.999, Sigma-Aldrich, USA) was employed to sample preparation
for GC-MS analysis. Dimethyl sulfoxide-d6 (mass fraction purity > 0.999,
Sigma-Aldrich, USA) was taken as a solvent for the preparation of samples for NMR
analysis. All the chemicals were used as obtained from the supplier without further
purification. Double-distilled deionized water was prepared by using NANO
Pure-Ultra pure water purifying system at the resistivity of 18.3 M Ω-cm.
Depolymerization of alkali lignin
A modified high pressure and temperature autoclave reactor (Model: series 4576,
T316 stainless steel, Parr Instrument Co., USA) was used for the depolymerization of
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the alkali-lignin. This reactor is commonly used for high-pressure and
high-temperature studies. The apparatus consists of an equilibrium cell with a 187
cm3 maximum working capacity. Automatic temperature control was provided by a
heater surrounded the cell together with a cooling water system. The cell is connected
to a controller series type 4843 (Parr Instrument Co., USA), which consists of a
digital pressure display and a high-temperature cutoff (J-type thermocouple) for safety
purposes. Stirring was accomplished by a variable speed stirrer with a magnetic drive.
Two valves are attached to the cell head: (a) a sample inlet valve which is connected
to a burette (made up of IWAKI code 7740 glass, USA) and (b) the gas release valve
which is connected to a vacuum pump. Before loading liquid mixtures into the cell, it
is necessary to vacuum the system thoroughly to evacuate the air and traces impurities
inside the cell.43 The freshly prepared solution of the alkali lignin in 30, 50 or 80 wt%
of the aqueous ILs solution was injected into the autoclave reactor and heated at a
given temperature over a preset reaction time. After completion of the reaction, the
autoclave reactor was cooled down to the room temperature and then the sample was
withdrawn from the reactor for analysis. Before conducting the next experimental run,
the reactor was washed many times, dried overnight, and then vacuumed.
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Characterization of depolymerized samples
The FT-IR spectrum of each sample was measured by using Nicolet FTIR-iS10
instrument with the attenuated total reflectance (ATR) method in the range of 4000 -
650 cm-1 and collected at a resolution of 4 cm-1 with 200 scans. At least 5 repeated
measurements are made for each sample and the average of those repeated
measurements is considered as a final result.
The molecular weight distributions of lignin before and after depolymerization
were analyzed by gel permeation chromatography (GPC, Aqueous phase, Waters 1525
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HPLC Pump). An aqueous solution of NaNO3 (0.05 M) was used as a mobile phase
with a flow rate of 0.5 mL.min-1. Prior to the analysis of the samples, the GPC was
calibrated by using pullulan as a standard. Each sample was prepared by dissolving 2
mg of pre-treated and post-treated samples of lignin in per milliliter of degassed
distilled water. The mobile phase and samples were filtered by using a 0.2 μm
membrane filter. The filtered solution is injected into the GPC with an air tight
glass-made micro syringe.
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To identify the species obtained after depolymerization of the lignin, the GC-MS
spectra of the post-treated samples of lignin were measured by using a chromatograph
(HP model 6890 series GC system). This GC-MS was equipped with a 5973 mass
selective detector and Agilent, DB-5MS capillary column (size 30 m x 0.25 mm,
J&W Scientific). The temperature program was set as 523 K to 573 K with heating
rate of 10 ºC.min-1. Helium is used as a carrier gas with flow rate of 1.0 mL.min-1. The
National Institute of Standards and Technology (NIST) library of mass spectroscopy
was used for identification of the compounds.
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The H NMR and CNMR spectra for the depolymerized lignin in 50 wt% and
80 wt% of [DMBA][Ac] aqueous solutions or 30 wt%, 50 wt% and 80 wt% of
[DMBA][B] aqueous solutions, after the treatment at 180 ºC for 60 min, were
measured by using a Bruker-Biospin 500 MHz instrument. The samples were
prepared in deuterated dimethyl sulfoxide (D6-DMSO). Each NMR spectrum was
recorded by using tetramethylsilane (TMS) as an internal standard and with an
average of 120 scans. The operation parameters of the NMR analyses are pulse
sequence: zg30; relaxation delay: 2.00000000 sec; acquisition time: 3.0277631 sec.
Results and discussion
The main objective of this study is to investigate the effect of operating
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parameters on the solvation and depolymerization of the alkali lignin by uDsiOnI:g10.t1h03e9/C8NJ05185B
aqueous solutions of ammonium-based ILs ([DMBA][Ac] and [DMBA][B]). For this
purpose, the aqueous solutions of lignin (0.1g.mL-1) containing 50 and 80 wt% of
[DMBA][Ac] and 30, 50, 80 wt% of [DMBA][B] are prepared. Each prepared
solution is heated under different temperatures (90, 120, 150 and 180 °C) and periods
of reaction time (30, 60 and 120 min). After the treatment at pre-specified reaction
condition (temperature and time interval), the product sample is withdrawn from the
autoclave reactor for characterization. On the basis of the results obtained from a
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series of analyses, including GPC, FT-IR, GC-MS, H NMR and 13C NMR, the
performances of the selected ionic liquids for depolymerizing lignin into low
molecular weight compounds are studied and the molecular structures of the major
species obtained from the depolymerization of lignin are identified.
Molecular weight distribution
To evaluate the performance of the aqueous ionic liquid solutions for the
depolymerization of the lignin, we monitor the change in average molecular weight
(Mw) of the samples of lignin after the treatment under different reaction temperatures
and time intervals. The results are compared with the Mw of the lignin sample in
pristine solution. GPC was used to determine the molecular weight distribution of the
samples before and after treatment.48 The results of GPC analysis for the samples of
depolymerized lignin in the aqueous solutions of [DMBA][Ac] and [DMBA][B] are
graphically presented in electronic supplementary information (ESI), Fig. S1 and Fig.
S2, respectively. In some cases, more than one peak are shown on their GPC graphs.
The corresponding molecular weight (Mw), poly-dispersity index (PDI), and the
retention time (RT) of each peak obtained from each experimental run are reported in
Table 2. Prior to any treatment, the Mw of the lignin in aqueous solution is found to be
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6436 g.mol-1.
Effect of the temperature on lignin depolymerization
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According to the tabulated values in Table 2, the average Mw of the major species in
the products are about 1369, 1337, 1279, 1130 g.mol-1 after the treatment by 50 wt%
aqueous solution of [DMBA][Ac] at 90, 120, 150, and 180 ˚C for 60 min, respectively.
As also illustrated in Fig. 1, increasing reaction temperature obviously enhances the
depolymerization of the lignin in the aqueous solutions of [DMBA][Ac]. Similar trend
of temperature-effect is also revealed in Fig. 1, Table 2, and Fig. S2 by using the
aqueous [DMBA][B] solution. However, the average Mw of [DMBA][B] system is
generally greater than those of [DMBA][Ac] system under the same reaction
condition. For example, after the treatment at 180 ˚C for 60 min, the average Mw of
depolymerized lignin in 80 wt% [DMBA][B] aqueous solution is 912 g.mol-1 (reduced
by about 86% after treatment), while that in 80 wt% [DMBA][Ac] aqueous solution
decreases down to 437 g.mol-1 (reduced by about 93%). It appears that the ionic
liquids with less hydrophobic ion may provide better performance for the lignin
depolymerization.
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Singh et al.49 indicated that processing the lignin at higher reaction temperatures
may not only increase the production cost but also lead to formation of unnecessary
by products, which may become more difficult in the further separation. Additionally,
Opriset al.50 conducted research on the fragmentation of lignin over magnetically
recyclable composite in a temperature range of 100-230 °C. Analyzing the fragmented
intermediates by GC-MS, they found the formation of dimers and trimers at lower
reaction temperatures. At 180 °C good performance was achieved, but rising the
temperature from 180 to 230 °C could decrease the yield of the product. In this study,
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Table 2
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GPC results of depolymerized lignin in aqueous solutions of [DMBA][Ac] or
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[DMBA][B] at various reaction temperatures (T) and time intervals (t).
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t
Lignin in 50 wt% [DMBA][Ac]
Lignin in 50 wt% [DMBA][B]
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51
52
53
54
55
56
57
58
59
60
(oC) (min)
Mw
PDI
RT
remark
Mw
PDI
RT
average
Mw
average
Mw
(min)
(min)
25.07
26.73
25.13
90
60
60
1937
801
1.10 24.90
1.02 26.05
1.07 24.84
1.02 26.55
1.12 25.02
1.01 26.28
1.05 25.00
1.01 26.50
1.05 25.13
Peak 1
Peak 2
Peak 1
Peak 2
Peak 1
Peak 2
Peak 1
Peak 2
Peak 1
Peak 2
Peak 1
Peak 2
3632
934
1.20
1.07
1.11
1369
137.5
1279
1208
2283
120
150
180
180
180
1909
766
1880
60
1820
738
1627
1.19
25.13
30
1671
745
1826
758
1.13
1.01
1.20
25.02
26.05
26.88
1292
60
1527
734
1202
1130.5 1.01 26.65
1.13 25.02
120
1787
734
851
1.05
26.99
1260.5 1.01 26.30
Lignin in 80 wt% [DMBA][Ac] Lignin in 80 wt% [DMBA][B]
Mw
PDI
RT
remark
Mw
PDI
RT
average
Mw
average
Mw
(min)
26.50
26.53
26.57
(min)
26.07
26.08
26.31
180
30
60
476
437
399
1.09
1.06
1.04
Peak 1
Peak 1
Peak 1
1055
912
1.16
1.05
1.11
120
983
Lignin in 30 wt% [DMBA][B]
Mw
PDI
RT
remark
average
Mw
(min)
27.40
27.42
24.38
27.35
180
30
60
1527
1475
4283
1245
1.26
1.23
1.07
1.21
Peak 1
Peak 1
Peak 1
Peak 2
120
2764
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1400
1200
50 wt % [DMBA][Ac]
50 wt % [DMBA][B]
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60
80
100
120
140
T (oC)
160
180
Fig. 1 The effect of reaction temperature on the averaged molecular weight of the
products of lignin depolymerization in the aqueous solutions of 50 wt% ILs at t = 60
min.
lower molecular weight compounds were also produced at 180oC. The analysis with
GC-MS later confirms that aromatic valuable chemicals are produced from the lignin
depolymerization at 180 °C.
Effect of the reaction time on lignin depolymerization
As shown in Table 2 and Fig. 2, the average Mw of the product can be
significantly reduced to 476 g.mol-1 by heating the lignin in higher concentration (80
wt%) of aqueous [DMBA][Ac] solution at 180 ˚C for 30 min. Increasing the reaction
time to 60 min and120 min, the values of average Mw further decrease down to 437
and 399 g.mol-1, respectively. The reduction in the average Mw is as high as by about
94% after the treatment of lignin in 80 wt% [DMBA][Ac] aqueous solution at 180 ˚C
for 120 min as comparing with the pristine lignin (6436 gmol-1). This indicates that
[DMBA][Ac] efficiently enhances the depolymerization of the lignin.
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However, increasing reaction time appears not to always decrease the DaOvI:e1r0a.1g03e9/C8NJ05185B
Mw of the products as presented in Table 2 and Fig. 2. For example, the average Mw
of the products are about 1208, 1130, and 1260 g.mol-1 for the lignin in 50 wt%
[DMBA][Ac] aqueous solution at 180 ˚C after processing 30, 60, and 120 min,
respectively. It reveals that the extent of depolymerization increases as the increase of
reaction time from 30 to 60 min, but further increasing reaction time to 120 min,
instead of enhancing the depolymerization, the repolymerization of the fractionate
species takes place.51 Similar behavior is also clearly found from the case of using 30
wt% of [DMBA][B] aqueous solution as shown in Fig. 3.
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Effect of ionic liquid concentration on lignin depolymerization
As seen from Figs. 2 and 4, the aqueous ionic liquids are able to degrade the
lignin to smaller compounds and the average Mw’s of the depolymerized lignin
decrease with increasing the concentration of ionic liquids [DMBA][Ac] and
[DMBA][B], respectively, as the reaction time intervals are no greater than 60 min.
The average Mw of the lignin decreases by 76% when the lignin is heated at 180˚C for
30 min in 30 wt % of [DMBA][B] aqueous solution. Increasing the concentration of
[DMBA][B] to 50 wt% under the same reaction time the average Mw decreases by
80%. Further increasing the concentration of the ionic liquid to 80 wt% the average
Mw of lignin reduces by 84%. However, repolymerization is shown in Fig. 4 in case
of using 80 wt% of [DMBA][B] aqueous solution for 120 min at 180 ˚C. This
phenomenon is not found as the concentrations of the IL are 30 wt% and 50 wt%.
Repolymerization was also shown in Fig. 2 for the case of using 50 wt% [DMBA][Ac]
aqueous solution at 180oC for 120 min.
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1200
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60
50 wt % [DMBA][Ac]
80 wt % [DMBA][Ac]
600
400
20
40
60
80
100
120
t (min)
Fig. 2 The effect of reaction time on the averaged molecular weight of the products of
depolymerized lignin in the aqueous solutions of [DMBA][Ac] at T = 180 °C.
2700
30 wt % [DMBA][B]
50 wt % [DMBA][B]
80 wt % [DMBA][B]
2400
2100
1800
1500
1200
900
20
40
60
80
100
120
t (min)
Fig. 3 The effect of reaction time on the averaged molecular weight of the products of
depolymerized lignin in the aqueous solutions of [DMBA][B] at T = 180 °C.
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1800
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1400
1200
1000
800
t = 30 min
t = 60 min
t = 120 min
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60
30
40
50
60
70
80
CIL (wt %)
Fig. 4 The effect of concentration of [DMBA][B] on the averaged molecular weight
of the products of depolymerized lignin in the aqueous solutions of [DMBA][B] at
various reaction time intervals and T = 180 ˚C.
Structure identification for the depolymerized lignin
FT-IR
After confirming that these selected ammonium-based ionic liquids ([DMBA][Ac]
and [DMBA][B]) are able to assist the depolymerization of lignin, we attempt to
identify the major functional groups in the depolymerized species by using FT-IR.
The FT-IR spectra for the samples of lignin treated by [DMBA][Ac] and [DMBA][B]
are given in Fig. 5(a-c) and Fig. 6(a-d), respectively. Based on those FT-IR spectra,
various possible functional groups are identified and compiled in Table S1. It can be
seen from Fig. 5 (a-c) that the FT-IR spectra of all the samples of depolymerized
lignin treated by [DMBA][Ac] possess the characteristic peak in the region of
3382-3358 cm-1, which indicates the presence of hydroxyl group in the depolymerized
species.8,17 The vibrational spectrum for C-H group is also found from the peak in the
region 2909-2908 cm-1.49 Additionally, the presence of aromatic compounds is noted
by their characteristic peaks around 1648-1631 cm-1. A small spectral peak is also
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observed in the lower wavenumber region (1170-1028 cm-1), which indicDaOteI:s10.t1h03e9/C8NJ05185B
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existence of species with Ar-O linkages of ether or alcohol.26
8
9
Similar spectral peaks are noted in the spectra of lignin samples treated by the
aqueous [DMBA][B] solutions over the entire experimental conditions as shown in
Fig. 6 (a-d). The presence of hydroxyl functional group is noted which corresponds to
the peak in the wavenumber region of 3408-3358 cm-1.52 The intense spectral peak
appeared around 1648 – 1631 cm-1 is the characteristic peak for benzene ring. The
stretch of C-H bond is observed at 2921 cm-1 and C-O stretch is also found in the
wavenumber region of 1170-1068 cm-1. Comparing the untreated alkali lignin and
aqueous ionic liquid treated lignin, the later somewhat shift towards higher
wavenumber. Overall, our FT-IR analyses indicate that the lignin treated by the
aqueous [DMBA][Ac] and [DMBA][B] solutions produces a wide variety of
polyhydroxy and alkoxy aromatic compounds.
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a)
T = 150 oC, t = 60 min
T = 120 oC, t = 60 min
T = 90 oC, t = 60 min
3500 3000 2500 2000 1500 1000
Wavenumber (cm-1)
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b)
T = 180 oC, t =120 min
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T = 180 oC, t = 60 min
T = 180 oC, t = 30 min
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Wavenumber (cm-1)
c)
T = 180 oC, t = 120 min
T = 180 oC, t = 60 min
T = 180 oC, t = 30 min
3500 3000 2500 2000 1500 1000
Wavenumber (cm-1)
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Fig. 5 FT-IR spectra of commercial alkali lignin and the depolymerized lignin treated
by the 50 wt% (a,b) and 80 wt% (c) of [DMBA][Ac] aqueous solutions at different
experimental conditions.
a)
T = 180 oC, t = 30 min
T = 180 oC, t = 60 min
T = 180 oC, t = 30 min
3500 3000 2500 2000 1500 1000
Wavenumber (cm-1)
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T = 150 oC, t = 60 min
b)
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T = 120 oC, t = 60 min
T = 90 oC, t = 60 min
3500 3000 2500 2000 1500 1000
Wavenumber (cm-1)
c)
T = 180 oC, t = 30 min
T = 180 oC, t = 60 min
T = 180 oC, t = 30 min
3500 3000 2500 2000 1500 1000
Wavenumber (cm-1)
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d)
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60
T = 180 oC, t = 120 min
T = 180 oC, t = 60 min
T = 180 oC, t = 30 min
3500 3000 2500 2000 1500 1000
Wavenumber (cm-1)
Fig. 6 FT-IR spectra of the depolymerized lignin treated by the 30 wt% (a), 50 wt% (b,
c) and 80 wt% (d) of [DMBA][B] aqueous solutions at different experimental
conditions.
GC-MS
As mentioned previous, this study used GC-MS to identify the chemical species
after the depolymerization of lignin in aqueous [DMBA][Ac] and [DMBA][B]
solutions. The samples to be analyzed are the depolymerized solutions of lignin in 50
wt% and 80 wt% [DMBA][Ac] and in 30 wt%, 50 wt% and 80 wt% [DMBA][B] at
180 °C for 60 min. The results of measurements, including all the identified
compounds with corresponding molecular weight as well as retention time, are
compiled in Table 3. The mass spectra with the structures of the identified molecules
are shown in Fig. S3. From the provided values of the identified compounds in Table
3, we can see that a variety of valuable aromatic compounds are identified from the
samples treated by both two ionic liquids and those compounds are almost identical
over the entire investigated concentrations of the ionic liquids. The results also
confirm that these two selected ionic liquids not only promote the depolymerization
process of the lignin but also produce a variety of valuable chemical species.
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Table 3
Lignin-derived monomeric compounds in the depolymerized lignin treated by the
aqueous ionic liquid solutions at 180 °C for 60 min
8
9
IL Conc.
RT
Identified compound
Relative Mw
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41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(wt%)
(min)
area (%)
Lignin in aqueous [DMBA][Ac]solution
o-Guaiacol
16.4
14.3
12.8
8.6
50
80
4.5
7.4
6.4
3.5
6.4
124
164
166
208
122
Isoeugenol
p-Propylguaiacol
1,2,3-Trimethoxy-5-(1-propenyl)benzene
1,2-(Methylendioxy)benzene
3.2
17.6
2.1
6.2
6.4
2-Allylphenol
134
164
166
152
13.2
13.0
2.8
3-Allyl-6-methoxyphenol
p-Propylguaiacol
6.42 3-Hydroxy-4-methoxy- benzaldehyde
Lignin in aqueous [DMBA][B]solution
8.5
10.8
7.4
1.8
9.4
12
30
50
4.5
8.1
6.4
8.6
4.5
8.1
6.4
8.0
5.3
4.5
3.3
3.6
8.1
6.4
2.6
o-Guaiacol
124
164
166
252
124
164
166
240
252
124
134
136
164
166
208
2-methoxy-4-(1-propenyl)- Phenol
p-Propylguaiacol
(E)- 4,4'-(3-Ethenyl-1-propene-1,3-diyl)bis-phenol
o-Guaiacol
2-Methoxy-4-(1-propenyl)- phenol
p-Propylguaiacol
9.6
2.4
2.8
11.2
14.3
8.9
12.5
9.8
2.6
1,4-Diol-1,4-diphenyl-2-butene
(E)-4,4'-(3-Ethenyl-1-propene-1,3-diyl)bis-phenol
o-Guaiacol
80
2-Allylphenol
p-Propylphenol
2-Methoxy-4-(1-propenyl)- phenol
p-Propylguaiacol
1,2-Dimethoxy-4-(1-methoxy-1-propenyl)benzene
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NMR analysis
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In the present study we used nuclear magnetic resonance spectroscopy (1H NMR
8
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13
and C NMR) to further reconfirm the identified species, which are produced from
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the lignin treated by 50 wt% of [DMBA][Ac] or [DMBA][B] aqueous solution at 180
°C for 60 min. The 1H NMR spectra of the depolymerized lignin by using the aqueous
[DMBA][Ac] and [DMBA][B] solutions are depicted in Fig. S4 and Fig. S5,
13
respectively, and the C NMR spectra of the depolymerized lignin in the aqueous
[DMBA][Ac] and [DMBA][B] solutions are shown in Fig. S6 and Fig. S7,
respectively. From the spectra of NMR of both aqueous ionic liquids solution assisted
lignin depolymerization, peaks are observed at δH (0.8-1.8 ppm, 2.1 – 2.5 ppm and
6.9-7.4 ppm) and δC (13-19 ppm, 39-40 ppm, 48-50 ppm, 128-134 ppm and 167-168
1
13
13
ppm). From peak assignment on H, and C-NMR spectra, The results of C NMR
of depolymerized lignin in aqueous [DMBA][Ac] solution presents a chemical shift at
167.99 ppm, which is an indication of presence of carbonyl compound. A
characteristic peak of aromatic compounds is also observed at chemical shifts of
133.6-128 ppm and 6.9 ppm on the spectra of 13C NMR and 1H NMR, respectively. In
addition to the aromatic compounds, C-C and protons on methyl group are observed
13
1
in the range of aliphatic regions, 40-13 ppm on C NMR and 0.8-2.5 ppm on H
13
NMR, respectively. A spectral peak of C NMR is noted at a chemical shift of 50
ppm, which shows the presence of electronegative element adjacent to carbon,
especially alkoxy group.
Similar functional groups are also found from the samples of depolymerized
lignin by using the aqueous [DMBA][B] solutions. For instance, the presence of
carbonyl and alkoxy functional groups is confirmed by the chemical shifts of 168 ppm
and 48 to 50 ppm on the spectra of 13C NMR, respectively. The aromatic group is also
identified by the chemical shifts of 133-128 ppm on 13C NMR spectrum and 7.4 ppm
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on H NMR spectrum. Generally, from the spectral analyses of C NMRDaOnI: d10.10H39/C8NJ05185B
NMR the presence of hydroxyl, alkoxy, aromatic and aliphatic groups are confirmed.
The results obtained from the analyses of NMR spectra are consistent with those from
FT-IR.
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Conclusions
The possibility of using aqueous ionic liquid ([DMBA][Ac] or [DMBA][B])
solutions for the production of valuable chemicals from lignin is investigated. The
performance of depolymerization of the lignin is found to be substantially influenced
by reaction temperature, reaction time, and the concentration of the ionic liquid. The
samples of depolymerized lignin by using the aqueous ionic liquid solution are
characterized by GPC, GC-MS, FT-IR, and NMR. Promising results have been
achieved when the depolymerization of the lignin was conducted at 180 °C for 60 min.
The depolymerizing ability of both two aqueous ionic liquid solutions to disconnect
C–O–C linkages are found even at low concentration of ionic liquid. According to the
GPC analysis, the average Mw’s of the depolymerized lignin are reduced by about
82.4% and 93% in 50 wt% and 80 wt% aqueous [DMBA][Ac] ionic liquid solutions,
respectively, and reduced by 77%, 81.3% and 86% in 30 wt%, 50 wt%, and 80 wt%
aqueous [DMBA][B] ionic liquid, respectively, as the lignin was treated at 180 °C for
60 min. The analyses results from GC-MS, FT-IR, and NMR confirm the presence of
aromatic polyhydroxyl compounds in the depolymerized lignin. It is concluded that
these two investigated aqueous ionic liquid solutions have great potential to degrade
the lignin into phenolic compounds under milder reaction condition.
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Acknowledgments
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The authors are grateful for financing provided by the Ministry of Science and
Technology (MOST), Taiwan, through grant MOST 105-2221-E011-144-MY3,
MOST 105-2811-E-011-012, Mettu University and the international student
scholarship from NTUST.
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Corresponding Author
M. J. Lee
Department of Chemical Engineering, National Taiwan University of Science and
Technology, 43 Keelung Road, Section 4, Taipei 106-07, Taiwan.
E-mail: mjlee@mail.ntust.edu.tw
Conflicts of interest
There are no conflicts to declare.
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References
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DOI: 10.1039/C8NJ05185B
Degradation of Lignin with Aqueous Ammonium-Based Ionic Liquid
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Solutions under Milder Condition
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Leta Deressa Tolesa, Bhupender S., Gupta, and Ming-Jer Lee*
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Department of Chemical Engineering, National Taiwan University of Science and
Technology, 43 Keelung Road, Section 4, Taipei 106-07, Taiwan.
Graphical Abstract
Ammonium-based ionic liquids can be served as solvent and promoter for
lignin depolymeriazation.