Lignin oxidation and depolymerisation in ionic liquids

Article abstract of DOI:10.1039/c5gc01950h

The depolymerisation of lignin directly in the black liquor was studied, comparing two ionic liquids as extracting solvents (butylimidazolium hydrogen sulphate and triethylammonium hydrogen sulphate), under oxidising conditions. H₂O₂

Full text of DOI:10.1039/c5gc01950h

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Lignin oxidation and depolymerisation in
ionic liquids†
Cite this: DOI: 10.1039/c5gc01950h
R. Prado,*a A. Brandt,b X. Erdocia,c J. Hallet,b T. Weltona and J. Labidic
The depolymerisation of lignin directly in the black liquor was studied, comparing two ionic liquids as
extracting solvents (butylimidazolium hydrogen sulphate and triethylammonium hydrogen sulphate),
under oxidising conditions. H2O2 was chosen as the oxidant agent. It was observed that lignins derived
from butylimidazolium hydrogen sulphate were more susceptible to degradation. The main degradation
products found in the extracted oils were aromatic acids, such as vanillic acid, benzoic acid and 1,2-
benzenedicarboxylic acid.
Received 19th August 2015,
Accepted 21st September 2015
DOI: 10.1039/c5gc01950h
www.rsc.org/greenchem
ration through its combustion. However, lignin’s structure is
unique in nature and it has the potential to be an exceptional
Introduction
According to the World Energy Council, about 82% of the renewable source of aromatic compounds, if an economic
world’s energy needs are currently supplied from fossil means of extraction and depolymerisation can be developed.
resources such as petroleum, natural gas and coal.1 The eco- Lignin valorisation to high added value platform chemicals
logical disadvantages of this have come into prominence as could provide a qualitative leap to the exploitation of this side
the use of fossil energy sources suffers a number of ill conse- stream of the pulp and paper industries and in future second
quences for the environment, including greenhouse gas emis- generation bioethanol plants;6 biorefinery plant efficiency and
sions that are causing the climate change, air pollution, acid profitability strongly rely on the maximum valorisation of the
rain, etc.2 These resources are not only used as energy sources, lignin.7 In addition, selective and controlled oxidation of
but also for the production more than 2500 different oil-based lignin, would be extremely valuable as it is believed to be the
products. Crude oil is the major source for plastics, fibres and only economically viable stream of renewable aromatic com-
dyes. Approximately 10% of world-wide natural gas consump- pounds for large scale production.8
tion, 21% of the liquefied natural gas, and 4% of crude oil are
Lignin conversion into simple aromatic products is a
used for chemicals production.3 In order to reduce depen- difficult task due to lignin’s recalcitrant nature, its complex
dence on fossil fuels renewable sources and technologies have structure, variability of sources and extraction processes. There
been widely investigated. One of these technologies is the con- are some depolymerisation processes that are already under
version of biomass to fuels and chemicals in the so-called research namely pyrolysis of the isolated lignins, catalytic
“Integrated Biorefinery”.4
hydrogenolysis, alkaline hydrolysis, supercritical water and
Lignocellulosic biomass is constituted principally of cellulose, solvent depolymerisation.9–11 The C–C linkages of lignin are
lignin and hemicelluloses, in varying proportions depending on resistant towards cleavage and majority of these survive regard-
the individual biomass source, e.g., species, growing season, and less of the depolymerisation process. Thus, depolymerisation
geographic location. Lignocelluloses are a renewable source of of the isolated lignins results in a complex mixture of aromatic
materials and chemicals. Lignin due to its structure is the most compounds in which the individual mass fraction of each
abundant renewable source of aromatic compounds on earth.5
compound barely exceeds few percent. In addition, the depoly-
In the past and until now, the most common lignin appli- merisation of lignin depends on the source of lignin, during
cation in the pulp and paper industry has been energy gene- the pulping processes a chemical modification of lignin
occurs, with the formation of different structures depending
on the pulping conditions, for example it is very hard to
aDepartment of Chemistry, Imperial College London, UK.
E-mail: r.prado-garcia13@imperial.ac.uk
depolymerise kraft lignin due to the percentage of refractory
C–C linkages between the propylphenol monomers being sig-
nificantly higher than in the proto lignin initially present in
the wood.12,13
Recently, ionic liquids have been studied for biomass
delignification and further treatments. In addition, solubilisation
bDepartment of Chemical engineering, Imperial College London, UK
cChemical and Environmental Engineering Department, University of the Basque
Country, Donostia-San Sebastián, Spain
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c5gc01950h
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of lignin is under study; several ionic liquids have been methanol, (2-methoxyphenyl)methanol, (3,4-dimethoxyphenyl)
used with this aim with good results compared to conventional methanol and (3-phenoxyphenyl)methanol) and lignin.
solvents,7 such as 1-butyl-3-methylimidazolium chloride Substituted benzyl alcohols were selectively oxidized to their
[C4C1im][Cl], 1-allyl-3-methylimidazolium chloride [C2C1im][Cl],14 corresponding aldehydes; benzaldehyde, 4-methoxybenzalde-
1,3-dimethylimidazolium methylsulfate [C1C1im][MeSO4],6 and hyde, 2-methoxybenzaldehyde, 3,4-dimethoxybenzaldehyde
1-butyl-3-methylimidazolium methylsulfate [C4C1im][MeSO4].5 and 3-phenoxybenzaldehyde were obtained in 93%, 86%, 77%,
Recently, it has been demonstrated that anions mostly affect 84% and 80% isolated yields respectively, under the optimum
the lignin reactivity in the order alkylsulphonates > lactates > conditions. Lignin was treated under the same conditions
acetates > chlorides > phosphates. This is explained by the (time and temperature), achieving 72% lignin conversion; the
nucleophilicity of the electronegative part of the anion.15 The main products were identified as syringaldehyde, vanillin,
cation has not yet been shown to have any great effect on the p-hydroxybenzaldehyde mixed with a small amount of 2,6-
reactivity of the ionic liquid.
dimethoxy-1,4-benzoquinone.19 The toxicity of the imidazo-
Due to its complex structure, there are few works that use lium derived ILs has been recently investigated.21 It was
lignin as a starting material to study its depolymerisation. demonstrated that the toxicity varies more with the cation
However, there are many authors that have studied lignin than with the anion, allowing for the potential to optimise
model compounds in order to elucidate the reaction mecha- both the biomass processing performance while minimising
nism and the proper conditions to extend to the reaction to the toxicity of the ionic liquid.
the whole lignin. The depolymerisation of lignin model com-
In the work reported here, two ILs, butylimidazolium hydrogen-
pounds and lignin in ILs has been studied under reductive sulphate [HC4im][HSO4] and triethylammonium hydrogen-
and oxidative conditions. The reductive conditions were sulphate [Et3NH][HSO4] were used for the delignification of
achieved by adding Lewis and Brönsted acids as catalysts; Miscanthus Giganteus and subsequent lignin depolymerization
however, these reactions were only briefly studied because of by oxidation with H2O2 directly applied to the black liquors.
the low reactivity showed by the lignin despite the good yields Due to imidazolium derived ILs toxicity we compare the
obtained on lignin model compounds.16 Oxidation has been obtained results with the ammonium derived IL in order to
more extensively studied, in most cases coupling a metal cata- get a greener processes. H2O2 efficiency for lignin oxidation
lyst such as Fe, Mn, Co, V and on oxidant such as O2 or H2O2, was previously demonstrated by Wiermans et al.22
in many different ILs under different conditions. The role of
the ILs was as both reaction solvent and/or catalyst.8,15,17
Jiang et al.,18 and Zhu et al.19 worked on the reactivity of
lignin model compounds under different conditions and then
Results and discussion
Characterization of Miscanthus Giganteus fibres was done
according to TAPPI standard methods.23 Moisture content
(8.80 0.03 wt%) was determined after drying the samples at
105 °C for 24 h (TAPPI T264 cm-97). Chemical composition,
given on an oven dry weight basis, was the following: 4.2
0.5% ash (TAPPI T211 om-93), 12.4 0.2% hot water soluble
they applied the optimum conditions to lignin itself.
Jiang et al.20 studied the selective oxidation of primary alco-
hols into aldehydes or acids. First, methoxybenzylalcohol was
used to check the optimum conditions.18 The influence of
cocatalyst and IL structure was studied. It was observed that
the aldehyde was obtained in 90% yield with 99% selectivity
without adding the cocatalyst and using [C4C1im][PF6] as the
IL. To obtain the acid selectively Cu(II) 2-ethylhexanoate was
added as cocatalyst and 1-hexyl-3-methylimidazolium triflate
[C6C1im][OTf] was used as the solvent, but only 77% yield was
obtained with 87% selectivity. With pyridinium ILs a complex
mixture of products was obtained.18 Then lignin model com-
pounds such as veratryl alcohol and vanillyl alcohol were oxi-
matter (TAPPI T207 om-93), 36.8
0.7% aqueous NaOH
soluble matter (TAPPI T212 om-98), 7.9
benzene extractives (TAPPI T204 cm-97), 18.4
0.4% ethanol–
0.4% lignin
(TAPPI T222 om-98), 30.8 2% hemicelluloses24 and 36.2
0.2% α-cellulose.25
Reaction yields
dized under the same conditions to establish if the behavior of The reaction yields and S/G ratios (S = syringyl-like lignin struc-
the catalyst could be extended to other phenolic structures. For tures; G = guaiacyl-like lignin structures) of the obtained
vanillyl the primary alcohol was selectively oxidized and not lignins are detailed in Table 1. [HC4im][HSO4] is more reactive
the phenolic alcohol. For both veratryl and vanillyl alcohols, than [Et3NH][HSO4], as the lignin yield after pretreatment was
the oxidations stop at the aldehyde with 96% and 94% selecti- higher. In addition, BSL (lignin obtained by [HC4im][HSO4]
vity and 94% and 82% yields, respectively.20
pretreatment) was more reactive under H2O2 conditions than
The use of nanoparticles as the catalyst was studied on ESL (lignin obtained by [Et3NH][HSO4] pretreatment), as the
lignin model compounds and lignin oxidation by Zhu et al. by lignin yield showed that degradation was higher, though H2O2
dispersing metal nanoparticles in the IL.19 Palladium nano- treatment degraded both lignins, and the degradation
particles as catalyst, pyridinium salt of iron bis(dicarbollide) depended on H2O2 concentration. In general, it was observed
as cocatalyst, O2 as oxidant and [C4C1im][MeSO4] or [C4C1im]- that the lignin yield decreased with H2O2 concentration, oil
[PF6] as ILs were used for the oxidation of benzyl alcohol yield was higher for 10% H2O2 concentration and IL recovery
with different substituents (benzylalcohol, (4-methoxyphenyl) was not affected by the treatment, being around 85%.
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Table 1 H2O2 depolymerisation treatment yields
Sample
Treatment
Lignin %
Oil %
S/G
BSL
Not treated
14.64 0.03
1.48
3.16
11.29
17.39
11.7 0.4
4.39
7.56
7.52
10.46
0.94
0.69
0.58
0.51
0.90
1.11
0.78
0.88
0.82
0.85
BSL_10
BSL_5
BSL_2
BSL_1
ESL
ESL_10
ESL_5
ESL_2
ESL_1
[HC4im][HSO4] H2O2 10% 1 h 120 °C
[HC4im][HSO4] H2O2 5% 1 h 120 °C
[HC4im][HSO4] H2O2 2% 1 h 120 °C
[HC4im][HSO4] H2O2 1% 1 h 120 °C
Not treated
[Et3NH][HSO4] H2O2 10% 1 h 120 °C
[Et3NH][HSO4] H2O2 5% 1 h 120 °C
[Et3NH][HSO4] H2O2 2% 1 h 120 °C
[Et3NH][HSO4] H2O2 1% 1 h 120 °C
16.05
10.08
11.44
12.15
18.50
9.40
9.19
4.24
Table 2 ATR-IR spectra band assignment for BSL
Although, lignin degradation was higher for [HC4im][HSO4]
than for [Et3NH][HSO4], the oil yields were similar for both
ILs. The S/G ratio was affected by H2O2 treatment, according to
the results syringol is more easily oxidised than guaicyl, as the
S/G ratio decreased after oxidation.
Band
(cm−1
)
Functional group assignment
CH stretching on CH3 and CH2
3143
2936
1704
Carbonyl stretching in un-conjugated ketones and
conjugated carboxyl
Lignin characterization
1596
1512
1455
1425
1209
1115
1029
Aromatic skeletal vibrations
Spectroscopic characterization
Treatment with [HC4im][HSO4]. Residual lignins were charac-
terized by ATR-IR (Fig. 1). All the spectra up to 5% H2O2 con-
centration showed the same bands assigned as is shown in
Table 2. However, the relative intensity of the bands changed
in the different samples. It was observed that with increasing
H2O2 concentration the intensity of the band at 1704 cm−1
related to CO group increased, which means that lignin could
be oxidising, and the intensity of the band at 1512 cm−1
CH asymmetric deformation
Aromatic skeletal vibrations
Guaicyl ring breathing with CO stretching
–CH in plane deformation in syringyl ring
–CH in plane deformation in guaiacyl and CO
deformation in primary alcohols
834
Aromatic out of plane deformation
decreased. This behaviour is relatively proportional up to 5% ing on CH2 and CH3 groups was also observed; the formation
H2O2, while at 10% H2O2 lignin lost its structure and its spec- of these bands can be influenced by IL contamination.26,27
trum showed the formation of 3 new bands; the band at the
Treatment with [Et3NH][HSO4]. After H2O2 treatment the
band at 1266 cm−1 was assigned to the guaiacyl ring and CO same bands as for the initial lignin were observed (Fig. 2), they
stretching, the band at 2854 cm−1 was assigned to CH stretch- were assigned as shown in Table 3.28,29 Apart from the
ing on CH2 and CH3 groups and the band at 804 cm−1 was common bands, slight changes were observed for residual
assigned to CH harmonics on CH2 and CH3 groups, an inten- lignin after 10% H2O2 treatment;
a band appeared at
sity increase of the band at 2936 cm−1 assigned to CH stretch- 1152 cm−1 assigned to –OSO3 due to IL contamination and the
Fig. 1 ATR-IR spectra of residual lignins after H2O2 treatments for
Fig. 2 ATR-IR spectra of residual lignins after H2O2 treatment for
[HC4im][HSO4].
[Et3NH][HSO4].
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Table 3 ATR-IR spectra band assignment for ESL
Band
(cm−1
)
Functional group assignment
CH stretching on CH3 and CH2
2936
2840
1696
Carbonyl stretching in un-conjugated ketones and
conjugated carboxyl
1596
1512
1455
1424
1325
1209
1114
1027
Aromatic skeletal vibrations
CH asymmetric deformation
Aromatic skeletal vibrations
Syringyl ring breathing with CO stretching
Guaicyl ring breathing with CO stretching
–CH in plane deformation in syringyl ring
–CH in plane deformation in guaiacyl and CO
deformation in primary alcohols
Fig. 3 MWD profiles for H2O2 treatment lignins.
834
Aromatic out of plane deformation
10% H2O2, where the peaks were wider, which agrees with pre-
vious polydispersity results.31,32
band at 1696 cm−1 was slightly displaced due to oxidation of
the lignin.30
In general it was observed that IL contamination was more
evident in the samples with [HC4im][HSO4] as solvent, and
only above 10% H2O2 concentration.
Fig. 4 shows the MWD of residual lignin samples for
[Et3NH][HSO4], in this case the MWD profile was the same for
residual lignins and initial lignin, the chromatograms were
displaced to longer retention time, for 5% H2O2 the highest
displacement was achieved, this agreed with the fact that 5%
H2O2 lignin had the lowest Mw.
Chromatographic characterization
Residual lignins were characterized by pyrolysis GC-MS to
analyze changes in the structure caused by H2O2 treatment.
The peaks from 7 min to 23 min were assigned to different
phenolic monomers which are part of the lignin, from 23 min
the peaks were assigned to different acids and long chain
molecules. Analyzing the chromatograms (Fig. 5) of the
[HC4im][HSO4] treated samples shows that the residual lignin
structure after 1% and 2% H2O2 treatments was not highly
affected, but with increasing H2O2 concentration a decrease of
the peak at 11.224 min assigned to butylimidazole was
observed. The proportion of phenolic monomers decreased
and the relative intensity of the peaks assigned to acid com-
pounds that are part of residual lignin structure after H2O2
treatment increased.
Residual lignins obtained after H2O2 treatment were character-
ized by HPSEC, the obtained results of Mw (average molecular
weight), Mn (average molecular number) and polydispersity are
shown in the Table 4. For both ILs it was observed that lignin
was degraded by the action of H2O2 because its Mw decreased
with increasing concentration of H2O2 up to 5%, above which
it increased; this could be due to re-polymerisation reactions
which are common for lignin. The decrease of Mw was higher
for [HC4im][HSO4] derived lignins, which suggests that H2O2
effectiveness for lignin degradation is higher for [HC4im]
[HSO4] than for [Et3NH][HSO4]. In Fig. 3 the molecular weight
distribution (MWD) of residual lignins obtained from [HC4im]
[HSO4] is shown. The chromatograms were displaced to longer
retention time which means lower Mw. It was observed that the
MWD profile of residual lignins was similar to the initial
lignin for low concentrations of H2O2, but it changed for 5 and
Table 4 HPSEC results for lignins and residual lignins after H2O2
treatment
Samples
Mw
6859
Mn
Polydispersity
BSL
1273
882
738
507
667
1643
1303
1325
1151
1242
5.39
5.67
6.29
3.10
5.19
6.17
7.37
6.62
6.55
6.56
BSL_1
BSL_2
BSL_5
BSL_10
ESL
ESL_1
ESL_2
ESL_5
ESL_10
5005
4642
1573
3464
10 142
9613
8771
7546
8156
Fig. 4 MWD of 22 h lignin treated with H2O2.
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Oil characterization
The obtained oils were characterized by GC-MS (Fig. 7 and
Fig. 8). For [HC4im][HSO4] pretreatment derived oils; it was
observed that H2O2 concentration affects the compounds com-
posing its extracted oils, at low H2O2 concentration the oil is
mainly composed of typical lignin phenolic compounds, such
as guaiacol and syringol, but when the H2O2 concentration is
increased, oxidised compounds are more abundant, such as
vanillic acid (Table 5). At low H2O2 concentration, the main
peak at 17.350 min of the chromatograms was assigned to
butylated hydroxytoluene that is a stabiliser for MeTHF, but it
was negligible for the 5 and 10% treatments with H2O2. Since
the same quantity of solvent was used for all the extractions,
this suggests that the proportion of lignin derived compounds
in the oil increased considerably with the increase of H2O2
concentration. Apart from calibrated compounds, the oil was
composed of other lignin derived products, such as benzoic
acid and 3-phenyl-2-propenoic acid, hemicellulose derived
Fig. 5 Pyrolysis GC-MS chromatograms of lignins after H2O2 treatment. compounds, such as 2(5H)-furanone and IL derived com-
pounds such as 1-butyl-2,4,5-trioxoimidazolidine. In general
oils after 5 and 10% H2O2 treatments were composed of a
For the residual lignins obtained from [Et3NH][HSO4], there
are several changes observed in the proportion of groups that
form the lignin. It was observed that lignin after 5% H2O2
treatment showed a higher proportion of acid derived com-
pounds, in addition, the proportions of the aromatic and
sugar derived compounds showed lower values. The observed
changes indicated that the residual lignin was oxidising by the
action of H2O2 and losing phenol derived monomers from its
structure (Fig. 6).
Fig. 7 GC-MS Chromatograms of H2O2 treatment derived oils for
[HC4im][HSO4].
Fig. 6 Pyrolysis GC-MS chromatogram of 22 h pretreatment lignins
Fig. 8 GC-MS chromatogram of 22 h pretreatment and H2O2 treatment
after H2O2 treatment.
oils for [Et3NH][HSO4].
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Table 5 H2O2 treatment oils calibrated compounds
Experimental
R.T.
Compounds
BSO_10 BSO_5 BSO_2 BSO_1
Materials and methods
All lignin samples were characterized by attenuated-total
reflectance infrared spectroscopy (ATR-IR) by direct transmit-
tance in a single-reflection ATR System (ATR top plate fixed to
an optical beam condensing unit with ZnSe lens) with an MKII
Golden Gate SPECAC instrument. Spectral data were recorded
as 30 scans in a range of 4000–700 cm−1 and resolution of
5.709
6.587
7.430
9.412
Phenol
0.260
0.012
0.335
0.184
0.026
0.011
0.019
—
0.080
—
0.105
—
1.294
—
0.161
—
0.962
—
0.023
0.222
—
0.007
0.003
o-Cresol
Guaiacol
Catechol
0.363
0.028
0.026
0.037
—
0.006
0.003
11.338 3-Methoxycatechol
13.995 Syringol
15.170 Vanillin
17.025 Acetovanillone
17.677 4-Hydroxy-3-
methoxyphenyl acetone
18.452 Vanillic acid
19.410 Syringaldehyde
20.523 3,5-Dimethoxy-4-
hydroxy acetophenone
—
0.709
0.018
0.049
0.015
4 cm−1
.
—
Lignins were subjected to High Performance Size Exclusion
Chromatography (HPSEC) to evaluate lignin molecular weight
(MW) and MW distribution (MWD) using a JASCO instrument
equipped with an interface (LC-NetII/ADC) and a reflex index
detector (RI-2031Plus). PolarGel-M column (300 × 7.5 mm) and
PolarGel-M guard (50 × 7.5 mm) were employed. Dimethyl-
formamide + 0.1% lithium bromide was the eluent. The flow
8.341
0.007
—
3.514
—
—
—
0.011
0.022
—
—
0.004
complex mixture of different compounds, which came from rate was 0.7 mL min−1 and the analyses were carried out at
lignin, sugars and IL.33–35
40 °C. Calibration was made using polystyrene standards
For the oils derived from [Et3NH][HSO4] pretreatment, it (Sigma-Aldrich) ranging from to 266 to 70 000 g mol−1
was observed that a peak which corresponded to butylated GC-MS analysis was performed to identify and to quantify
.
hydroxytoluene (Fig. 8) appeared again in all the chromato- the monomers present in the oil. Thus, the oil was dissolved
grams; this compound was used as a qualitative indicator of in ethyl acetate (HPLC grade). The solution was injected in an
the composition of the oil. It was observed that for 1% H2O2 Agilent GC (7890A)-MS (5975C inert MSD with Triple-Axis
and 2% H2O2 treatment, the main peak corresponded to Detector) equipped with a capillary column HP-5MS (5%-
butylated hydroxytoluene, but with increasing H2O2 concen- phenyl)-methylpolysiloxane, 60 m × 0.32 mm). The tempera-
tration the relative abundance of this peak decreased, so the ture program started at 50 °C then, the temperature was raised
oil was enhanced with lignin derived compounds (Fig. 8). to 120 °C at 10 °C min−1, held for 5 min, raised to 280 °C at
Table 6 shows the concentration of different phenolic com- 10 °C min−1, held for 8 min, raised to 300 °C at 10 °C min−1
pounds that were previously calibrated. The highest phenolics and held 2 min. Helium was used as carrier gas. Calibration
concentration was obtained for 5% of H2O2. In all cases, curves were done using pure compounds (Sigma-Aldrich) –
phenolic acids, such as vanillic acid, 4-hydroxy-3-methoxyethyl phenol, o-cresol, m-cresol, p-cresol, guaiacol, catechol,
ester, benzoic acid and 1,2-benzenedicarboxylic acid, mono- 4-methylcatechol, syringol, acetovanillone, syringaldehyde,
(2-ethylhexyl) ester were the main products. It was observed acetosyringone and 4-hydroxy-3-methoxyphenylacetone.
that not only phenolic products derived from lignin but
In order to determine the influence of the process on the
also hemicellulose derived compounds, such as maleic an- nature of the ionic liquid and elucidate how the chemical
hydride and furanone and other degraded structures of lignin structure of the [HC4im][MeSO4] is affected ATR-IR espectra
as hexadecanoic acid or octadecanoic acid were present. and NMR spectra were recorded at 30 °C on a Bruker Avance
In general, the obtained compounds were highly oxidised 600 MHz equipped with a z-gradient BBI probe. Typically,
(Table 6).33–35
40 mg of sample were dissolved in DMSO-d6. The spectral
Table 6 GC-MS calibrated characterization of oils
R.T.
Compounds
ESO_1
ESO_2
ESO_5
ESO_10
5.709
7.430
9.412
Phenol
Guaiacol
Catechol
3-Methoxycatechol
4-Hydroxybenzaldehyde
Syringol
Vanillin
Acetovanillone
Veratrol
4-Hydroxy-3-methoxyphenyl acetone
Vanillic acid
Syringaldehyde
3,5-Dimethoxy-4-hydroxy acetophenone
0.0932
0.2961
0.0412
—
—
0.0049
—
0.0108
0.0321
—
0.0637
—
0.1271
0.8387
0.0285
0.0368
—
0.0211
—
0.0187
—
0.1492
1.2367
0.0171
0.0683
0.0677
0.4434
0.0193
0.0071
—
0.1141
1.0246
—
0.0320
—
0.3752
0.0323
0.0290
—
0.0816
0.1209
0.0203
0.0168
11.338
14.225
3.995
15.170
17.025
17.275
17.677
18.452
19.410
20.523
0.0071
0.3847
—
0.0537
0.4151
—
—
—
0.0074
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widths were 5000 and 25 000 Hz for the 1H and 13C dimen- possibility of its reutilization. The best phenolic yield results
sions, respectively.
were obtained for 5% H2O2 and guaiacol was the main com-
ponent of the oil.
Synthesis of ionic liquid
[HC4im][HSO4] was synthesized by drop-wise addition of
H2SO4 to butylimidazole using a ratio of 1 : 1, while the flask
was cooled in an ice bath. The mixture was magnetically
stirred for 24 h and dried under vacuum.
[Et3NH][HSO4] was synthesized by drop-wise addition of
5 M H2SO4 on triethylamine using a ratio of 1 : 1, while the
flask was cooled in an ice bath. The mixture was magnetically
stirred for 24 h and dried under vacuum.
Acknowledgements
Authors would like to thank the Department of Education,
Universities and Investigation of the Basque Government for
financially supporting this work (project IT672-13).
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