Ionic liquids catalyzed lignin liquefaction: Mechanistic studies using TPO-MS, FT-IR, RAMAN and 1D, 2D-HSQC/NOSEY NMR

Article abstract of DOI:10.1039/c6gc00771f

Valorization of a profusely available alternate resource, biomass and in particular its 3-D intricate component lignin into low molecular weight aromatic products are used as platform chemicals and fuel additives, and developing a low temperature catalytic process is imperative in preserving atom efficiency. Ionic liquids, due to their unique properties, offer an advantage to develop such methods under milder conditions. Herein, we show use of -SO₃H functionalized imidazolium based various recyclable Br?nsted acidic ionic liquids (BAILs) in catalytic quantity under ambient pressure at 120 °C for depolymerization of lignin (60 000 g mol^-1) into THF soluble products with high efficiency (78% yield, 95% ± 5% mass balance). The decoding of this efficiency by 1D and 2D (HSQC/NOSEY) NMR, FT-IR and RAMAN studies exemplify that the -OH group(s) interact with the electron deficient BAIL cation. The mechanistic insights unraveled in this study open a plethora of opportunities to design catalysts for developing efficient processes.

Full text of DOI:10.1039/c6gc00771f

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DOI: 10.1039/C6GC00771F
Ionic liquids catalyzed lignin liquefaction: Mechanistic
studies using TPO-MS, FT-IR, RAMAN and 1D, 2D-
HSQC/NOSEY NMR
Sandip K. Singha,b and Paresh L. Dhepea,b*
aCatalysis & Inorganic Chemistry Division, CSIR- National Chemical Laboratory, Dr. Homi
Bhabha Road, Pune-411 008, India. E-mail, pl.dhepe@ncl.res.in; Tel, +91-20 2590 2024; Fax,
+91-20 2590 2633.
bAcademy of Scientific and Innovative Research (AcSIR), New Delhi 110025, India.
ABSTRACT. Valorization of profusely available alternate resource, biomass and in particular
its 3-D intricate component lignin into low molecular weight aromatic products those are used as
platform chemicals and fuel additives, by developing low temperature catalytic process is
imperative to preserve atom efficiency. Ionic liquids due to their unique properties offer an
advantage to develop such methods under milder conditions. Herein we show, use of –SO3H
functionalized imidazolium based various recyclable Brønsted acidic ionic liquids (BAILs) in
catalytic quantity under ambient pressure at 120 oC for depolymerization of lignin (60000 g/mol)
into THF soluble products with high efficiency (78% yield, 95±5% mass balance). The decoding
of this efficiency by 1D and 2D (HSQC/NOSEY) NMR, FT-IR and RAMAN studies exemplify
that the –OH group(s) interact with electron deficient cation of BAIL. The mechanistic insights

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unraveled in this work open plethora of opportunities to design catalysts for developing efficient
processes.
Introduction.
The non-edible lignocellulosic biomass (agriculture waste) made up of cellulose, hemicelluloses
and lignin is the only alternate renewable resource of C, H and O elements available to us.
Lignin (15-25 dry wt%), a cross-linked 3-D amorphous phenolic polymer accessible in plenty as
a by-product from paper & pulp industry, is drawing lot of interest as an alternate resource for
the synthesis of chemicals and fuels.1-4 To attain the ambitious targets set for the synthesis of at
least fraction of chemicals from biomass in near future and to augment the prospects of cellulose
to ethanol process becoming lucrative, it becomes imperative to convert lignin into chemicals by
developing competent methods.3, 5, 6
To invigorate the efforts in valorizing lignin, series of catalytic processes based on homogeneous
o
o
base (>260 C)7, 8; supported metal catalysts (200-450 C; H2, 40-180 atm)2, 9, 10; stand-alone
homogeneous acids (H3PO4) or along with metal catalyst11 etc. are developed to yield organic
solvent soluble low molecular weight compounds. Moreover, exploitation of supercritical water
(Tc=373.6 oC; Pc=246.7 atm)12-14 as well is known to yield aromatic compounds along with char,
tar and gas (CO/CO2/CH4/alkanes) formation. Solid acid based high temperature cracking (>300
oC)15 and hydrolysis (<250 oC)16, 17 is too known to yield aromatic monomers and gases. Of late,
employment of ionic liquids (ILs) in the biomass conversions is intensifying since ILs are
regarded as green solvents and have exploitable tunable properties.18, 19 Use of ILs chiefly as a
solvent is also identified in the depolymerization of lignin to facilitate the action of catalyst.20-22
A recent study suggests that ionic liquid ([EMIM][CF3SO3]) and metal nitrate (Mn(NO3)2) can

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mediate depolymerization of lignin to yield aromatic products.21 Few other reports as well
scrutinize the role of different ionic liquids on the structures of pre-treated lignins.23-30
While in most of the works, ILs are used as solvents along with other catalysts20, 21, 31-33, the
methodical investigations on the correlation of lignin properties with products and mechanistic
studies on the interaction between lignin and catalyst is poorly done. Moreover, use of ILs on
larger scale as solvent would be costly and due to their viscous nature can become a hurdle in
achieving higher yields. One acknowledged limitation in majority of the above studies is
employment of model compounds (dimer/trimer) to demonstrate the competency of method.34, 35
This is because lignins have very high molecular weights (>3000 Da) and those consists of
difficult to cleave diversified linkages (β-O-4, α-O-4, β-5, β-1, β-β, 4-O-5 etc.; ESI, Section S1,
Table S1, Fig. S1†), which all are not present in most of the model compounds.36, 37 Additionally,
in reality, impurities (Na, S, ignition residue) associated with lignin can pose a challenge in reuse
of catalysts with the matching efficiency. Furthermore, use of high temperatures leads to
formation of degradation products (char, tar) and thus it is necessary to achieve conversions at
lower temperatures to accomplish higher atom efficiency by avoiding formation of char, tar and
gases.
Considering these shortcomings; it was contemplated to make use of ILs in catalytic amount
for the depolymerization of (real) lignin without addition of supplementary catalyst in presence
of water and alcohols. To accomplish this, exploitation of Brønsted acidic ionic liquids (BAILs)
in depolymerization of lignin by virtue of cleavage of ether (β-O-4 etc.) bonds due to presence of
–SO3H groups was contemplated. Given that both, BAIL and substrate are polar, those can form
homogeneous phase in polar solvent (water/alcohols) and thus, diffusion limitations can be
conquered to enhance the reaction rate. Besides, after reaction it was hypothesized that easy

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separation of BAILs from products (organic solvent extraction) will facilitate easy recycling of
catalyst. Furthermore, despite its significance, current knowledge on the fundamental
understanding on the depolymerization of lignin is limited38, and hence robust evaluation of
results to ascertain the reaction pathway is crucial. Subsequently in this work, reasonable
deliberations on the mechanism through studies of ILs for their interaction with (real) lignin,
structures & functional groups etc. were made by meticulous characterization of lignin and
products.
In view of this, diverse imidazolium based BAILs and IL such as, 1-methyl-3-(3-sulfopropyl)-
imidazolium hydrogensulfate, [C3SO3HMIM][HSO4]; 1-methyl-3-(3-sulfopropyl)-imidazolium
p-toluenesulfonate, [C3SO3HMIM][PTS]; 1-methyl-3-(3-sulfopropyl)-imidazolium chloride,
[C3SO3HMIM][Cl]; and 1-butyl-3-methylimidazolium chloride, [BMIM][Cl] were synthesized
(ESI, Section S2, Scheme S1, Fig. S2†) and used for liquefaction of lignin. The analytical
techniques (CHNS, NMR (1H, 13C), FT-IR, TGA and UV-Vis) (ESI, Section S2, Table S2, Figs.
o
S3-S5†) disclosed that ILs are obtained in pure form, are stable until 250 C and have different
acid strength.
Experimental Section.
Synthesis procedure of BAILs and IL. Various imidazolium based BAILs such as, 1-methyl-3-
(3-sulfopropyl)-imidazolium
hydrogensulfate,
[C3SO3HMIM][HSO4];
1-methyl-3-(3-
sulfopropyl)-imidazolium p-toluenesulfonate, [C3SO3HMIM][PTS]; and 1-methyl-3-(3-
sulfopropyl)-imidazolium chloride, [C3SO3HMIM][Cl] were synthesized in a two-step method.39-
42
Along with BAILs, neutral IL, 1-butyl-3-methylimidazolium chloride [BMIM][Cl] was also
synthesized (ESI, Section S2, Scheme S1, Fig. S2†).

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Liquefaction of lignin. Depolymerization reactions of lignin were conducted at desired
temperature for required time in a batch mode reactor of 100 mL capacity (Parr Autoclave,
USA). Unless specified, in all the reactions, lignin (2 wt.% solution H2O:CH3OH, 1:5 v/v, 30
mL) & catalyst (0.5g) were charged and the reactor was heated to desired temperature under
slow stirring (100 rpm). After attaining the desired temperature, stirring was increased to 1000
rpm and this time was considered as starting time of reaction. At this temperature the pressure
generated was 0.35 MPa due to presence of water and methanol. After completion of reaction,
reactor was allowed to cool to room temperature under the flow of air and slow stirring (100
rpm) (ESI, Section S3†).
Analytical methods. The organic solvent (THF, EtOAc, DEE) soluble products were analysed
using gas chromatography (GC), gas chromatography-mass spectroscopy (GC-MS), liquid
chromatography-mass spectroscopy (LC-MS) and gel permeation chromatography (GPC)
techniques (ESI, Section S3†).
Result and Discussion.
Depolymerization of lignin. When lignin depolymerization experiment was conducted in
water:methanol (1:5 v/v) solvent system at lower temperature of 120 oC (than reported earlier)2, 7-
16
for 1 h, among all the BAILs evaluated, [C3SO3HMIM][HSO4] achieved the best yield (78%,
Fig. 1) for THF soluble products (ESI, Section S3, Fig. S6†) while complete conversion of lignin
was seen. The insoluble fraction was composed of mainly larger molecules but undoubtedly with
lower molecular weight than lignin. Although, other BAILs were too active, those presented
lower yields (24-42%) compared to [C3SO3HMIM][HSO4]. Interestingly, neutral IL,
[BMIM][Cl] as expected showed inferior yield (18%) compared to BAILs because of absence of

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–SO3H group, yet it offered improved yield than non-catalytic reaction (4%). This is owing to
the fact that the hydrogen attached to the carbon linking two nitrogens (NCHN) is acidic. Since,
in all the experiments same quantity of catalyst (0.5 g) was used, it was expected that due to
disparity in H+ concentration at 0.5 g loading of catalyst (H2SO4, 10.2 mmol;
[C3SO3HMIM][HSO4], 3.32 mmol; [C3SO3HMIM][PTS], 1.33 mmol) variation in activity was
observed (98, 78 and 42% yield). To nullify this effect, reactions were performed using
comparable H+ concentration (2.78 mmol) in the reaction mixture by altering the catalyst
quantities and the following order of activity was seen,
[H2SO4] (96%) > [C3SO3HMIM][HSO4] (52%) > [C3SO3HMIM][PTS] (49%).
This shows that if H+ concentration is kept constant, analogous activity (52, 49%) can be
observed with all the ILs with varying anions. This implies that the anion does not provide any
contribution to the activity (ESI, Section S3†).
100
90
80
70
60
50
40
30
20
10
0
Catalyst
Fig. 1 Depolymerisation of lignin. Reaction condition: lignin (2 wt.% solution H2O:CH3OH, 1:5
v/v), 120 ˚C, 1 h, 1000 rpm. Values are average of three reactions with ±3% error observed.

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Effects of reaction temperature and time on activity are discussed in ESI, along with
achievement of high mass balance of 95±5% in all the reactions (ESI, Section S3, Fig. S7†). The
chromatographic technique coupled with mass spectroscopy (GC, GC-MS, LC-MS; ESI, Section
S3, Figs. S8-S10†) confirmed the depolymerization of lignin and identified the low molecular
weight aromatic products extracted in THF (102-220 gmol-1, ESI, Fig. S11†). The finding was
corroborated by injecting commercially procured chemicals (identified by above techniques).
Besides formation of expected products, the appearance unexpected products such as p-cymene
and butylated hydroxytoluene is possible because of acid catalyzed re-arrangement reactions of
lignin fractions.16, 43 The product distribution of the GC-FID identified low molecular weight
products (m. wt. 124-196 g mol-1) is given in Table 1. However, few of the GC-MS identified
products are still not quantified since those are commercially not available and hence their
calibration could not be carried out. Based on the results from GC analysis it is estimated that ca.
45% yield of the products extracted in THF is having molecular weight in the range of 124-196 g
mol-1. This suggests that out of 78% THF soluble products formed, 45% products are low
molecular weight GC detected and quantified products.
Table 1. Summary on the quantification of GC and GC-MS identified low molecular weight
products in the THF soluble product.
Low molecular
weight products
distribution (wt %)
Molecular weight
(g mol-1)
Sr.
No.
Aromatic
monomers
Structure
1
p-cymene
134
124
4.8
2
2-methoxyphenol
22.3

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1,4-dimethoxy
benzene
3
138
24.8
2-methoxy-4-
(propenyl)
phenol
4
5
164
152
23.0
8.1
4-hydroxy-3-
methoxy
benzaldehyde
1-(-hydroxy-3-
methoxy phenyl)
ethanone
6
7
166
182
8.0
5.7
methyl
4-
hydroxy-3-
methoxybenzoate
4-hydroxy-3,5-
dimethoxy
benzaldehyde
8
9
182
196
2.2
1.2
1-(4-hydroxy-
3,5-dimethoxy
phenyl) ethanone

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Reaction condition, lignin (2 wt.% solution H2O:CH3OH, 1:5 v/v), [C3SO3HMIM][HSO4] 0.5g
catalysts, 120 ˚C, 1 h, 1000 rpm.
While with all BAILs similar peak patterns (GC-MS, ESI, Fig. S8†) were observed yet,
differences in peak intensities due to disparity in yields (Fig. 1) are apparent. Contrary to this,
GC-MS profile (ESI, Fig. S8f†) of H2SO4 catalyzed reaction showed only few peaks compared
with [C3SO3HMIM][HSO4] signifying that BAIL has superior capability to yield low molecular
weight aromatic products even if H2SO4 exhibits higher yield (98%) than BAILs (Fig. 1). Owing
to insolubility of few THF soluble products in other extracting solvents such as EtOAc and DEE
because of dissimilarity in Hansen solubility parameter (HSP, ESI, Section S3, Table S3†) lower
yields of 36% and 14%, respectively were observed (ESI, Fig. S6†). This finding can be
substantiated by evaluating the GC-MS profiles of the products extracted in these solvents with
THF (ESI, Figs. S8d-e†). Incidentally, this divergence in solubility facilitates the separation of
products from each other.
The comparable performance in minimum five recycles runs Fig. 2 (ESI, Section S3†) of
[C3SO3HMIM][HSO4] and detailed characterization of recovered BAILs by CHNS, NMR (1H,
13C), FT-IR and TGA studies (ESI, Table S4, Figs. S12-S13†) divulge that the catalysts were
stable under reaction conditions.

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Fig. 2 Results for the recycle study using [C3SO3HMIM][HSO4] catalyst. Reaction condition:
lignin (2 wt.% solution H2O:CH3OH, 1:5 v/v) and catalyst [C3SO3HMIM][HSO4] (0.5 g), 120
˚C, 1 h, 1000 rpm.
Mechanistic studies.
Albeit the importance of depolymerization, investigations on the mechanistic insights on the
roles of interaction between lignin and catalyst, linkages in lignin, cation/anion, concentration of
H+, etc. are largely not performed and hence deserves attention.
Lignin characterization. In this work depolymerization studies were performed with real lignin
and since it is recognized that the properties of lignins are dependent on the type/age of plant and
isolations procedures, it was obligatory to investigate those and therefore, lignin44 was
systematically characterized by assorted thermal/spectroscopic techniques (ESI, Sections S3-S4,
Tables S3, S5, Figs. S14-S16†). To understand the TGA results, TPO-MS studies were
undertaken and it was observed (Fig. 3) that lignin decomposes in a wide range of temperature,
probably because it contains various linkages, functional groups and bond dissociation enthalpies
etc. (ESI, Section S1, Table S1, Fig. S1†).45, 46 It is worth noting that when experiment was
performed with 5% oxygen (v/v), intense peaks for gaseous products (CO/CO2/CH4/H2 etc. ESI,
o
Section S4, Table S6, Equations S1-S16†) were observed at lower temperatures (<550 C), but
o
formation of these products was less under helium atmosphere even until 800 C. This is usual

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since oxygen helps degradation. Importantly, observance of ozone under both the conditions is
possibly due to radical mechanism under thermal conditions. However, for the explanations on
the formation of ozone,47 typically radical mechanism is discussed.
Product(s) stability. The foremost cause to obtain poor yields of low molecular weight products
in lignin depolymerization reactions is the likelihood of repolymerization of products at high
temperatures.2, 3, 32, 48, 49 To comprehend this, commercially available chemicals (identified by
GC/GC-MS/LC-MS) were acquired, mixed along with [C3SO3HMIM][HSO4] in water:methanol
(1:5 v/v) and scrutinized for their stability (ESI, Section S4, Fig. S17†). The change in the
concentrations of these chemicals was monitored by GC and LC analysis. Whilst the study was
o
performed at 120 C for 1 h, slight change (±3%) in the peak areas was seen however, large
decline (10-30%) in the peak areas for few chemicals due to char formation was noticed when
the study was performed at 170 oC. These results explicate the decline in yields for THF soluble
products when reactions were done at higher temperatures.
Correlation between lignin structure and products.
As disclosed earlier, substantiation for formation of depolymerized products was done by
spectroscopic techniques (ESI, Figs. S8-S10†). Nonetheless, to perceive the changes transpiring
in lignin structure and existence of several functional groups in both, lignin and product on bulk
and molecular level was done using several techniques (GPC, DSC, UV-Vis, FT-IR, 1D and 2D
(HSQC) NMR).
Though, irrefutable estimation of molecular weight of lignin and products from GPC analysis
is not feasible owing to multifarious structure of lignin and non-availability of standards for
calibration, it is noticeable from the GPC profiles (ESI, Fig. S18†) that lignin undergoes
depolymerisation as emergence of new major peak with solvent is detected in products.

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In the thermogram obtained from DSC analysis of lignin (Fig. 4a), peaks were recorded at 230
o
and 245 C, due to melting (endothermic) and crystallization/char (exothermic), respectively.
Nonetheless, nonappearance of these peaks in the products and observance of multiple new
peaks (Fig. 4b) denote that lignin was depolymerised into products having diverse transition
temperatures. Additionally, it is advocated that the products are low boilers (<300 oC) as no peak
for crystallization was spotted and subsequent to analysis, sample pan was empty. This finding is
in line with the products ascertained from spectroscopic analysis.
Efforts were undertaken to identify functionalities (conjugation, function groups) by UV-Vis
(ESI, Fig. S19†) absorption measurement study and as seen, in both the samples, existence of
three peaks (280, 230, 206 nm) due to occurrence of various types of chromophores/substituent
in aromatic compounds proved that almost all the functionalities are preserved in products, as
such present in lignin. Nevertheless, to delve more into this, FT-IR study for lignin and products
was carried out (ESI, Section S4, Fig. S20†). The emergence of two new peaks for phenolic C-O
stretch in phenol (1180 cm-1) and enhancement in intensity for few peaks along with decrease in
peak intensity for ether linkages (1040 and 1125 cm-1) in the FT-IR spectrum of products
exhibits depolymerization of lignin.

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(b)
DOI: 10.1039/C6GC00771F
(a)
m/z=32
m/z=48
m/z=18
m/z=2
m/z=16
m/z=64
m/z=78
m/z=44
m/z=28
50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800
Temperature (oC)
50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800
Temperature (oC)
m/z=92
m/z=32
(d)
(c)
m/z=48
m/z=2
m/z=16
m/z=64
m/z=78
m/z=44
m/z=28
50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800
Temperature (oC)
50 100150200250300350400450500550600650700750800
Temperature (oC)
Fig. 3 Temperature programme oxidation-mass spectroscopy (TPO-MS) of lignin. (a) TCD-
(95%He+5%O2 v/v), (b) MS profile-(95%He+5%O2 v/v) (c) TCD-100%He (d) MS profile-100% He.
The different m/z denotes following: 2:H2, 16:CH4, 18:H2O, 28:CO, 32:O2, 44:CO2 etc.
2
0
(a)
-2
-4
-6
50 75 100 125 150 175 200 225 250 275 300 325 350
Temperature (OC)
(b)
-2
-4
-6
-8
-10
-12
50
75
100 125 150 175 200 225 250 275 300
Temperature (OC)
Fig. 4 DSC analysis. (a) lignin (b) THF soluble products obtained in [C3SO3HMIM][HSO4] catalyzed
reaction.

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To ascertain the linkages and functional groups present in lignin and products, advance analytical
DOI: 10.1039/C6GC00771F
tool, 2D (HSQC) NMR was employed (Fig. 5, ESI, Section S4, Tables S7a-b, Fig. S11†). The HSQC
spectrum shows signals for oxygenated aliphatic and aromatic lignin structures in the characteristic
regions of δC/δH 50-90/2-6 and 100-140/6-8 ppm, respectively.50-55 In lignin sample, strong signal for –
OR groups (etherified) was observed at δC/δH 56/3.2-4.0 ppm (Fig. 5a). Nonetheless, in products, the
intensity of this signal was decreased (Fig. 5d) manifesting that during depolymerisation, most of the
ether linkages were cleaved. However, presence of few –OR groups in products is in line with the
products identified (ESI, Figs. S8 and S11†) with –OCH3 groups. Likewise, in products (Fig. 5d), low
intensity signals (δC/δH 62-64/3.2-3.65 ppm) for Cγ-Hγ was observed compared with lignin sample (Fig.
5a) hinting decrease in concentration of these species. On the contrary increase in intensity for signal
(δC/δH 60-62/3.2-3.5 ppm) assigned to Cγ-Hγ specifies emergence of new moieties in products. The
nonappearance of signals for Aβ (δC/δH 84.2/4.3 ppm) for Guaiacol moiety (G) and Bα (δC/δH 85.5/4.6
ppm) in products (Fig. 5e) again accentuate that lignin (Fig. 5b) undergoes depolymerisation. Generally,
the cross-signals observed at δC/δH 100-140/6-8 ppm corresponds to aromatic moieties such as, p-
hydroxyphenol (H), guaiacyl (G) and syringyl alcohol (S) (Fig. 5). While in lignin sample (Fig. 5c),
prominent signals for ‘G’ units are observed (δC/δH 111/7.4, 115.4-115.7/6.7-7.0 ppm for C2-H2, C5-H5
and C6-H6 in ‘G’ units), those were decreased in products (Fig. 5f) due to cleavage of –OR bonds. The
observance of new signals for ‘H’ units (δC/δH 125.4-127.1/6.0-7.1 ppm) in products (Fig. 5f) again
emphasize that lignin is depolymerised. The same observation is made for Aβ (δC/δH 84.2/4.3 ppm) as
reviewed above. Since similar 2D NMR results were also observed when other BAILs were used in
these reactions, as is also found from GC-MS analysis (ESI, Fig. S8†) it is advocated that anions do not
influence the course of reaction. Moreover, H+ concentration study (ESI, Section S3†) also exhibit that
anions do not impact activity. In view of this, it is compelling to study the interaction of cation,
imidazolium with lignin.

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Interaction studies.
Since, BAILs displayed better performance than other catalysts (Fig. 1, ESI, Section S3†), it is
anticipated that because of their ionic nature BAILs can interact with lignin moieties which would
facilitate cleavage of C-O-C (ether/ester) bonds. Moreover, catalytic results point out that lignin may
1
interact more with cation than anion. To unravel this phenomenon, H NMR studies were performed
with monomers (cumene, vanillin), dimer (guaiacol glyceryl ether) and polymer (lignin) (Fig. 6, ESI,
Section S4, Tables S7a-b†). In the spectra recorded for mixture of IL and vanillin (Fig. 6b), chemical
shifts of proton in IL (ppm:9.439.36 (NCHN, ‘h’), shielding) and vanillin (ppm:10.2710.60 (-
OH, ‘o’), 6.987.10 and 6.947.06 (doublet, Ar-H, ‘k’), deshielding] compared to neat spectra (of IL
and vanillin) were observed. Additionally, broadening of –OH peak is indisputably visible in the
mixture compared with neat sample. This effect is best explained on the basis of transfer of electron
density (through ‘O’ of –OH) of vanillin to electron deficient imidazolium ring (cation of IL). To
eliminate the possibility of involvement of pi electrons, NMR spectra were recorded with cumene
(without –OH group) and as anticipated no shift in proton peaks was observed (Fig. 6a) which
reaffirmed that IL interacts with substrates having –OH groups via ‘O’.
In a vanillin concentration study (ESI, Section S4.4.b, Figs. S21a, b), maximum (1H) chemical shift
was observed at 0.5 and 1 molar ratio (vanillin/IL) for vanillin and later at higher molar ratios (1.5, 3
and 5) it declined. Nevertheless, at the same time continuous increase in chemical shift in IL was seen
(Fig. 6f). These contrasting results were exhibited due to change of concentration of both vanillin and IL
in DMSO-d6. Until reaching stoichiometric amount of vanillin to IL, increase in chemical shift was
observed but when surplus (above stoichiometric >1 molar ratio) vanillin 1H 1D NMR studies (Fig. 6b).
The 2D (HSQC) NMR study performed for neat lignin and mixture of lignin with IL also demonstrate
similar observations. For e.g. signal for NCHN in IL was shifted to upfield (δC/δH
136.55/9.72136.57/9.47 ppm for ‘h’) after addition of lignin (Fig. 7b). Thus, it can be affirmatively
inferred from NMR studies that lignin interacts with cation of IL via ‘O’ of –OH groups was present it
had limitations to interact with IL and thus chemical shifts were averaged out towards neat. But as these

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interactions are rapid, IL was competent of interacting with numerous vanillin molDeOcIu: 1l0e.s103a9n/Cd6GtCh0u0s771F
displayed uninterrupted increase in chemical shift. Next, interaction study was performed with dimer,
guaiacol glyceryl ether (Fig. 6c) and analogous trend in chemical shift was observed for IL
(ppm:9.429.35 (NCHN, ‘h’), shielding) and ether (, ppm:4.945.06 (-OH, ‘o’), 4.654.80 (-OH,
‘n’), deshielding) as found with vanillin. In case of lignin, low intensity peaks for –OH and Ar-H were
visible (Fig. 6d) owing to its polymeric nature. Yet, an apparent shift in NCHN peak was visible. This is
because lignin is comprised of higher concentration of –OH and other functional groups (than other
studied molecules here) which can interact with IL. The similar observation was made during vanillin
study.
To underpin the explanations derived from 1D (1H) NMR, interactions were studied with the help of
2D (HSQC) NMR. As seen from Fig. 7, the signal for NCHN in IL was shifted to upfield (δC/δH
136.55/9.75136.74/9.37 ppm for ‘h’) after addition of vanillin. Similarly, signal for Ar-H in vanillin
(Fig. 7a) was shifted to downfield (δC/δH 115.4/6.97115.63/7.11 ppm for ‘k’). These results reinforce
the observations made from in the RAMAN studies (ESI, Fig. S22†), characteristic peaks for stretching
mode of C=O were visible at 1661, 1667 and 1692 cm-1 for neat vanillin, but those were shifted to
positive wavenumber (1667, 1699, 1758, 1770 cm-1) after addition of IL.

Page 17 of 29
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DOI: 10.1039/C6GC00771F
Lignin
THF soluble products
80
82
(e)
84
86
88
90
2.5 2.0
6.0
5.5
5.0
4.5
4.0
3.5
3.0
1H (ppm)
Fig. 5 2D (HSQC) NMR spectra of lignin (a, b, c) and of THF soluble products (d, e, f) obtained in
[C3SO3HMIM][HSO4] catalyzed reaction. All the spectra were measured after dissolving samples in
DMSO-d6.
Simultaneously, in the mixture, negative shift in wavenumber (3090 to 3068 cm-1) for C-H (NCHN)
of imidazolium ring (cation of IL) was noticed. The observed positive shift was because of transfer of
electron density from vanillin to imidazolium via aromatic ring and ‘O’ of -OH, which enhances the
double bond character of C=O as it is located to para position to –OH. Similarly, with increase in

Green Chemistry
Page 18 of 29
+
electron density on imidazolium,  character is weakened and accordingly negative DsOhI:if1t0.1i0s3V9ies/wCp6AorGttiCctlee0d0O7n.7lin1Fe
These results yet again endorse the fact that –OH groups interact with imidazolium. Albeit from 1D
NMR and RAMAN studies it was apprehended that –OH groups interact with immidazolium, however
to find out the mode of interaction; either parallel or perpendicular (ESI, Fig. 23S†), studies on 2D
(NOSEY) NMR were undertaken for a mixture of vanillin and IL as this technique allows to gather
information for an interaction between H-H (inter or intra molecular) if they are in the close vicinity
(<5Å) with each other. As observed (Fig. 7c), we could only detect intra-molecular H-H interaction and
consequently it is believed that predominantly –OH and immidazolium interact with each other in a
perpendicular mode.
(a)
(b)
b
d
i
{[BMIM][Cl]+cumene}
l
{[BMIM][Cl]+vanil lin}
d
n
a
a
k
l
e
g
m
f
e
h
g
f
c
b
h
k
c
i
k
o
H
O
i
i
n
i
m
l
l
k
O
n
l
O
[cumene]
o
i
H
l
o
[vanillin]
k
m
k
d
d
g
f
g
Cl
f
Cl
e
c
e
N
N
N
N
c
d
d
h
h
h
[BMIM][Cl]
[BMIM][Cl]
e
e
g
f
g
f
h
c
c
11 10
9
8
7
6
5
4
3
2
1
11 10
9
8
7
6
5
4
3
2
1
Chemical Shift (ppm)
Chemical Shift (ppm)

Page 19 of 29
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DOI: 10.1039/C6GC00771F
Fig. 6 1D 1H NMR spectra. a) neat [BMIM][Cl], neat cumene and mixture of [BMIM][Cl] & cumene;
b) neat [BMIM][Cl], neat vanillin and mixture of [BMIM][Cl] & vanillin; c) neat [BMIM][Cl], neat
guaiacol glyceryl ether and mixture of [BMIM][Cl] & guaiacol glyceryl ether; d) neat [BMIM][Cl], neat
lignin and mixture of [BMIM][Cl] & lignin. All the spectra were recorded in DMSO-d6.

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DOI: 10.1039/C6GC00771F
[BMIM][Cl]
[vanillin]
[vanillin]
[BMIM][Cl]
(a)
H
H
O
O
Cl
Cl
g
N
g
N
f
f
n
n
b
b
e
e
N
N
m
m
c
l
c
l
d
d
k
k
h
h
O
O
O
O
i
i
H
H
o
o
10
15
20
25
30
35
40
45
50
10
15
(a)
(b)
(c)
20
25
30
35
40
45
50
20
25
30
35
40
45
50
0.0
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0.0
3.0
3.0
2.5
2.0
1.5
1.0
0.5
2.5
2.0
1.5
1.0
0.5
1H (ppm)
1H (ppm)
1H (ppm)
50
50
48
56
50
48
56
48
56
64
64
64
72
80
72
80
72
80
90
3.0
90
3.0
90
3.0
6.0
5.5
5.0
4.5
4.0
3.5
6.0
5.5
5.0
4.5
1H (ppm)
4.0
3.5
5.5
5.0
4.5
1H (ppm)
4.0
3.5
6.0
1H (ppm)
100
110
100
110
100
110
120
130
140
120
130
140
120
130
140
9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0
10.0
6.5
9.5 9.0 8.5 8.0 7.5 7.0
1H (ppm)
6.0
10.0
10.0
10.0
9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0
1H (ppm)
1H (ppm)
180
180
190
200
180
190
190
200
6.0 10.0
200
6.0
6.5
6.5
9.5 9.0 8.5 8.0 7.5 7.0
1H (ppm)
9.5 9.0 8.5 8.0 7.5 7.0
1H (ppm)
6.0
10.0
6.5
9.5 9.0 8.5 8.0 7.5 7.0
1H (ppm)

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DOI: 10.1039/C6GC00771F
0
2
(e&a)
(h&a)
(h&c)
(g&b)
(g&c)
(e&b)
(e&c)
(h&b)
(d)
(l&i/d)
(b)
(h&d)
(h&e)
(a)
(c)
(f&d)
4
6
(h & )
(o & k)
(k& )
(g&e)
(k)
(e)
(i)
(i)
(l&n)
(n)
8
(l, m)
(o)
(f, g)
8
10
(h)
11
0
11
10
9
7
6
5
4
3
2
1
1H (ppm)
[vanillin]
(c)
[BMIM][Cl]
H
n
O
g
f
H
a
c
H
H
l
H
m
H
H
b
H
e
H
H
H
H
H
N
N
H
Cl
O
i
H
H
k
H
H
H
H
h
O
o
d
H
H
H
Fig. 7 2D (HSQC) NMR and 2D (NOSEY) NMR. (a) 2D (HSQC) NMR of neat [BMIM][Cl],
neat vanillin and mixture of [BMIM][Cl] & vanillin; (b) 2D (HSQC) NMR of neat [BMIM][Cl],
neat lignin and mixture of [BMIM][Cl] & lignin. (c) 2D (NOSEY) NMR of mixture of
[BMIM][Cl] & vanillin. All the spectra were recorded in DMSO-d6.
This is evident since steric hindrance between alkyl chains, methoxy group etc. would
destabilize parallel arrangement. Moreover, aromatic rings would repel each other and this will
lead to almost perpendicular interaction. These results also match well with the results obtained
with 1D 1H NMR of cumene analysis where no shift in  values was noticed because of absence
of –OH groups and that there is a least prospects of pi-pi interaction between aromatic rings in
cumene and immidazolium owing to perpendicular interaction.

Page 23 of 29
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DOI: 10.1039/C6GC00771F
Based on the results a mechanistic pathway (Fig. 8) is advocated wherein it is proposed that
BAILs interact with lignin molecules via ‘O’ of –OH groups and make a virtual binding with the
substrate. This helps in easy supply of H+ available with BAIL for protonation of ‘O’ of ether
linkages which exist in lignin with almost 70% of all linkages. Once this is achieved, cleavage of
C-O-C bond and attack of water on the cation will take place to yield hydrolysis products with
higher concentration of –OH groups which is again discussed through FT-IR study. For more
details on the mechanism please refer ESI (Section S4.4.c).

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Page 25 of 29
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Fig. 8 The proposed mechanism. (A) β-O-4, (B) α-O-4, (C) 4-O-5, (D) α-O-γ linkage present in
the lignin molecules to form monomers and dimer units and (E) interaction of lignin with
[C3SO3HMIM][HSO4] BAIL.
Conclusions.
The superior activity with high atom efficiency accomplished with recyclable BAILs under
milder conditions for yielding 78% THF soluble products from (real) lignin is correlated to their
interaction with each other via transfer of electron density from ‘O’ of –OH groups present on
lignin to imidazolium ring. This conclusion is substantiated by meticulous studies performed
with the help of RAMAN and 1D & 2D (HSQC/NOSEY) NMR. These characterizations also
helped comprehend the mode of interaction as (preferably) perpendicular. The FT-IR, NMR and
GC-MS techniques could assertively identify the formation of different products with diversity in
functional groups and a unique temperature dependent product and lignin stability study was
monitored using thermal and chromatographic techniques. After compilation of all the data,
hydrolysis mechanism is proposed for depolymerisation of lignin through protonation of C-O-C
linkages. The work described here should assist tuning reaction conditions for proficient
conversions.
Acknowledgment.
We would like to acknowledge Mr. Babasaheb M. Matsagar for help in synthesis of ILs; Ms. A.
K. Deepa for helpful discussions on lignin work; Dr. C. V. V. Satyanarayana for help in TGA
and TPO-MS analysis; Dr. P. R. Rajmohanan for help in NMR analysis; Dr. Nandini Devi for

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helpful discussions and suggesting corrections in the manuscript. SKS thanks Council of
Scientific and Industrial Research (CSIR), India for Research Fellowship.
Footnote.
†Electronic supplementary information (ESI) available. Methods, synthesis of ILs,
Characterizations of IL, lignin, reaction mixture, organic solvent soluble products, identification
of products.
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Table of Contents:
DOI: 10.1039/C6GC00771F
Water + methanol,
120˚C, 1 h
Interaction
(a)
(b)
lignin
Depolymerised
products
Reusable Brønsted acidic ionic liquids are shown to depolymerize lignin into value-added
depolymerized products with 78% yield.