1,1,3,3-Tetramethylguanidinium hydrogen sulphate (TMG·HSO4) ionic liquid in carbon dioxide enriched water: A highly efficient acidic catalytic system for the hydrolysis of cellulose

Article abstract of DOI:10.1039/c4ra11906a

1,1,3,3-tetramethylguanidinium hydrogen sulphate (TMG·HSO₄) ionic liquid was used for the hydrolysis of cellulose using water enriched with carbon dioxide as the reaction media. The addition of carbon dioxide enhanced the reaction rates significantly and afforded glucose and total reducing sugars in 26 and 72% yields, respectively. The addition of carbon dioxide might play a dual role, it reduces the viscosity of the medium for better mass transfer and also reacts with water to give carbonic acid which subsequently dissociates to the hydronium ion and enhances the acidity of the medium. The developed ionic liquid was easily recovered and reused for a subsequent four runs without any significant change in the catalytic efficiency. This journal is

Full text of DOI:10.1039/c4ra11906a

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1,1,3,3-Tetramethylguanidinium hydrogen sulphate
(
TMG$HSO ) ionic liquid in carbon dioxide enriched
4
Cite this: RSC Adv., 2014, 4, 58238
water: a highly efficient acidic catalytic system for
the hydrolysis of cellulose†
Received 7th October 2014
Accepted 29th October 2014
Subodh Kumar, Raj Kumar Singh and Suman L. Jain*
DOI: 10.1039/c4ra11906a
www.rsc.org/advances
1
4
,1,3,3-tetramethylguanidinium hydrogen sulphate (TMG$HSO ) ionic economical due to their reduced or even no corrosion, milder
liquid was used for the hydrolysis of cellulose using water enriched reaction conditions and ease of catalyst recovery. Conventional
with carbon dioxide as the reaction media. The addition of carbon strong solid acid catalysts such as H-form zeolites, transition-
dioxide enhanced the reaction rates significantly and afforded glucose metal oxides, cation-exchange resins, supported solid acids
and total reducing sugars in 26 and 72% yields, respectively. The and heteropoly compounds, niobic acid, H-mordenite, Naon
1,2
addition of carbon dioxide might play a dual role, it reduces the and amberlist have been tried but showed poor efficiency then
viscosity of the medium for better mass transfer and also reacts with amorphous carbon materials bearing SO H, COOH and OH
water to give carbonic acid which subsequently dissociates to the functions were used but the glucose yield only reached up to
3
3
4
hydronium ion and enhances the acidity of the medium. The devel- 4%. Nanostructured solid acid catalysts were also developed.
oped ionic liquid was easily recovered and reused for a subsequent The main drawback of these methods is the generation of lots of
four runs without any significant change in the catalytic efficiency.
waste during the synthesis of these catalysts. Recently graphene
oxide, a two-dimensional carbon lattice decorated by abundant
oxygen functionalities, is demonstrated to be an efficient green
Depleting petroleum resources and the increasing demand and
price of the petroleum products, has forced researchers to
utilize sustainable natural resources for the production of
alternative fuels (bio-ethanol, biofuels). One such important
renewable resource is cellulose which is an abundantly
underutilized biomass polysaccharide component consisting of
a chain of b-(1,4)-linked glucose residues. For the commercial
production of bio-ethanol, efficient and economical cellulose
depolymerisation to produce glucose is an essential step to
achieve the high conversion of cellulose to bio-ethanol through
fermentation. The rst important obstacle encountered in
hydrolysis, processing, fusibility, and functionalization of
cellulose is the insolubility of the cellulose in water and in
common traditional organic solvents except under extreme
conditions due to its molecular and supra molecular rigid
structure as a consequence of hydrogen bonding. A second
important drawback is the need for severe conditions such as
the use of dilute sulphuric acid at high temperatures. To
combat these issues, many solvent systems and catalytic strat-
egies have been reported for the hydrolysis of cellulose. Solid
acid catalysts have been found to be advantageous and
5
catalyst towards selective hydrolysis of cellulose to glucose.
Hydrolysis of cellulose into saccharides was also done utilizing
magnetically separable functionalized graphene. It was
observed that complete hydrolysis of cellulose takes place into
glucose and small (4–5 unit size) oligomers using low (1 : 1)
6
catalyst to cellulose ratio. As a consequence of the excellent
7,8
work done by Rogers et al. who showed that the ionic liquid
+
ꢀ
(
4
IL) 1-butyl-3-methylimidazolium chloride ([C mim] Cl ) is a
powerful solvent for cellulose to make homogeneous solution,
many efforts have been done to use the ionic liquids for the
cellulose hydrolysis. Acidic catalyst in ionic liquid was demon-
strated as an efficient system for hydrolysis of cellulosic with
9
improved total reducing sugars yield under mild conditions. Li
and Zhao summarizes the hydrolysis conditions and yields of
+
ꢀ
model substrate Sigma cell cellulose in solvent [C
4
mim] Cl
10
with H SO as the catalyst. To increase the efficiency of solid
2 4
acid catalysts, the hydrolysis reaction was also performed in
ionic liquids. In this regard, the macroporous styrene divinyl-
benzene resins functionalized with sulfonic groups (–SO
3
H)
proves to be powerful catalyst for the selective depolymerisation
11–13
of cellulose dissolved in ionic liquids.
However, due to
higher cost and higher consumption of solid acid catalyst with
Chemical Sciences Division, CSIR-Indian Institute of Petroleum, Dehradun-248005, ionic liquid makes this process unfeasible for the commercial
India. E-mail: suman@iip.res.in; Fax: +91-135-2660202; Tel: +91-135-2525788
production. Also the high activity of IL leads to further degra-
dation of the reducing sugars and thus making their isolation
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c4ra11906a
1
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Scheme 1 Acidic hydrolysis of cellulose.
4
Scheme 2 Synthesis of TMG$HSO ionic liquid.
tough. In addition to that some work on the acidic ionic liquid
graed over the nanoporous polymeric material was also
14
15
reported.
Mellvaine et al.
reported the use of 1-
methylimidazolium-3-ethylsulfonic acid triate as catalyst for
the depolymerisation of cellulose in an ionic liquid. Combina-
tion of solid acid catalyst, Amberlyst 15 DRY in ionic liquid (1-
butyl-3-methylimidazolium chloride) and microwave irradia-
tion provides a quite effective method which is useful for
depolymerisation of natural polymers such as cellulose.
However this solid catalyst is not really heterogeneous as acidity
16
is being leached from it into IL. Nowadays the efforts are going
to tune the activity of the ionic liquid which can work as effi-
cient catalyst for the cellulose hydrolysis. Therefore, the devel-
opment of highly active, selective and low cost new ionic liquids
would be an emerging eld for the conversion of cellulose into
alternative fuels.
Protic ionic liquids, especially those based on the 1,1,3,3-
tetramethylguanidine (TMG) are synthesized through simple
neutralizing of equimolar TMG with acids have been found to
be useful as catalysts for various organic transformations
4
Fig. 1 TGA of TMG$HSO ionic liquid.
sulphate ionic liquid (TMG$HSO ) was found to be stable up to
4
ꢁ
ꢁ
2
50 C and then degraded between 250–300 C temperature
range.
Furthermore, successful synthesis and identity of the
1
13
synthesized ionic liquid were conrmed by H and C NMR
spectroscopy (Fig. S1†).
1
7
18
including Henry reaction, one-pot synthesis of pyran,
19
synthesis of 3,4-dihydropyridin-2-(1H)-ones and direct Aldol
reaction.
Inspired by literature reports on acid catalyzed reactions in
20
TMG$HSO ionic liquid catalyzed
21
4
CO
viscosity of CO
cient acidic catalytic system including 1,1,3,3-tetramethylgua-
nidinium hydrogen sulphate ionic liquid (TMG$HSO ) prepared
2
enriched high temperature liquid water and study on the
22
hydrolysis of cellulose
Effect of CO on cellulose hydrolysis
2
2
–ionic liquid system herein we report an effi-
4
At rst, the reaction of cellulose was carried out in water using
from the reaction of 1,1,3,3,-tetramethylguanidine and sul-
phuric acid in 1 : 1 molar ratio for hydrolysis of cellulose in
ꢁ
TMG$HSO
4
as catalyst without adding CO
2
. At 160 C for 3 h the
yield of total reducing sugar and glucose was found to be 48%
and 14% respectively. However using CO under 25 bar pressure
water using CO as additive at higher temperature (Scheme 1).
2
2
to the reaction mixture, the yield of TRS and glucose was
Synthesis and characterization of
signicantly increased and reached up to 72% and 26% respec-
21,22
tively. As suggested in the literature,
2
addition of CO might be
TMG$HSO ionic liquid
4
reducing the viscosity of reaction medium for better mass
In a typical experiment, TMG (20 mmol) was added in 100 mL of transfer and also it reacts with water at high temperature to yield
ethanol followed by the drop wise addition of H
in 30 mL of ethanol) at 0 C under stirring. The stirring was hydronium ion concentration and in turn acidity of the medium.
2
SO
4
(20 mmol carbonic acid, which subsequently dissociates to increase the
ꢁ
continued for another 2 h. A white semi-solid material was
isolated by decantation and washed thoroughly with ethanol, under different CO2 pressure conditions. It is evident from
dried under vacuum for 6 h and then characterized by elemental Fig. 2 that beyond 25 bar pressure of CO the yield of glucose
unchanged. This is most likely due to the saturation of the
Elemental analysis of the 1,1,3,3-tetramethylguanidinium reaction medium with carbon dioxide.
hydrogen sulphate ionic liquid (TMG$HSO To ascertain the effect of CO on the acidity of reaction
) reveals C ¼
To see the effect of pressure, the reaction was performed
2
1
13
analysis and H and C NMR spectroscopy (Scheme 2).
4
2
3
5.71%, H ¼ 6.59%, N ¼ 14.12%, S ¼ 9.53%, which was found medium, we also determined the acidity of the ionic liquid in
to be in good agreement with the calculated values: C ¼ 32.79%, water at higher temperature before and aer the addition of
H ¼ 7.57%, N ¼ 14.46%, S ¼ 10.51%. Thermal stability of the CO taking 4-nitroaniline as an indicator by using UV-visible
2
catalyst was determined by thermo gravimetric analysis (TGA) spectroscopy (Fig. 3). The absorption spectra of IL having 4-
ꢁ
ꢀ1
under nitrogen atmosphere at 10 C min of heating rate. As nitroaniline in water with or without adding CO
2
is shown in
shown in Fig. 1, the 1,1,3,3-tetramethylguanidinium hydrogen Fig. 3. It is evident that absorbance of 4-nitroaniline is
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Fig. 4 Effect of temperature on TRS and glucose yield.
2
Fig. 2 Effect of CO pressure on TRS and glucose yield.
Fig. 3 UV-vis spectra of ionic liquid in water.
4
Fig. 5 Effect of TMG$HSO concentration on TRS and glucose yield.
decreased because of protonation of 4-nitroaniline, indicating Fig. 5, when the dosage of IL is increased, TRS yield also
the higher acidity of the reaction mixture in presence of CO2.
increases at a certain point. Beyond this point, the yield of TRS
becomes independent on the ionic liquid. Based on these
results, the optimum conditions for hydrolysis of cellulose to
produce maximum yield of TRS with effective yield of glucose
Effect of temperature
The effect of reaction temperature was studied to obtain the
optimum temperature condition for the hydrolysis of cellulose
in synthesized acidic ionic liquid as catalyst. To conrm the
effect of hydrolysis temperature on the yield of TRS, we studied
was as follows: amount of ionic liquid (2.0 g), temperature
ꢁ
1
60 C, CO
2
pressure 25 bar and reaction time 3 h.
the hydrolysis of cellulose at different temperature i.e. 80, 100, Effect of reaction time
ꢁ
ꢁ
1
20, 140, 160 and 180 C (Fig. 4). The hydrolysis rate at 160 C
The TRS yields kept nearly constant and had a slowly decrease
from the 4th hour due to the dehydration of monosaccharide
such as glucose. It also observed that longer heating times
produced excessive charring of the sample, giving black resi-
dues, and thus lowering the TRS and glucose yields (Fig. 6).
was highest than the rate at other temperatures. The yield of
ꢁ
TRS was found to be 72% in 3 h at 160 C. While 58% of TRS
ꢁ
ꢁ
yield was produced at 140 C. At 80 C the yield of TRS was
found to be very low i.e. 36.2%. There is no doubt that reaction
temperature has a great effect on the hydrolysis of cellulose. A
higher hydrolysis temperature can shorten the reaction time
and produced higher TRS yield. Meanwhile, the degradation of
glucose begins earlier as shown in Fig. 4.
Reuse of the catalyst
The reusability of the catalyst was tested by using the same
catalyst sample in four catalytic cycles using 2.0 g of catalyst at
Effect of the TMG$HSO
4
ionic liquid concentration
ꢁ
1
60 C applying 25 bar pressure of CO for 3 h, and the results of
2
The effect of acidic ionic liquid concentration on the hydrolysis these experiments are shown in Fig. 7. The yield of glucose and
of cellulose was studied. The amount of IL used was 0.1 g, 0.5 g, TRS in these recycling experiments are obtained as 26% and
1.0 g, 1.5 g, 2.0 g and 2.5 g respectively. As it was depicted in 72% (rst run), 26% and 71.5% (second run), 25% and 71%
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of Parr reactor. The reactor is then pressurized by carbon
ꢁ
dioxide up to 25 bar and heated at 160 C for 3 h. Aer being
cooled the reaction mixture at room temperature, the CO is
2
vented out and the mixture was concentrated under reduced
pressure. The resulting residue was diluted with methanol and
the solid material which was settled down was separated by

ltration. The methanol layer was concentrated under reduced
pressure to recover the ionic liquid. The recovered ionic liquid
was reused for subsequent runs. The solid thus obtained was
mixed with water to dissolve the reducing sugars. The yield of
total reducing sugars (TRS) was found to be 72% and the
selectivity towards glucose was found to be 26%. Yield and
selectivity is calculated with the help of calibration curves
generated with commercially available standards using HPLC.
Detailed information on the analytical method used to quantify
glucose and TRS has been given in the ESI† le.
Fig. 6 Effect of time on TRS and glucose yield.
Acknowledgements
We kindly acknowledge Director IIP for his kind permission to
publish these results. Analytical Science Division of the Institute
is acknowledged for providing NMR and TGA analyses. SK is
thankful to CSIR, New Delhi for his research fellowship.
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