One step catalytic conversion of cellulose to sustainable chemicals utilizing cooperative ionic liquid pairs

Article abstract of DOI:10.1039/c1gc15597k

A 100% conversion of cellulose to industrially useful chemicals is, for the first time, achieved in a single pot reaction by the use of cooperative ionic liquid pairs for combined dissolution and catalytic degradation of cellulose, which overcomes the long intrinsic phase problem in the conversion of biomass to chemicals.

Full text of DOI:10.1039/c1gc15597k

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Green Chemistry
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COMMUNICATION
One step catalytic conversion of cellulose to sustainable chemicals utilizing
cooperative ionic liquid pairs†
Jinxing Long,a Bin Guo,a Xuehui Li,*a Yanbin Jiang,a Furong Wang,a Shik Chi Tsang,*b Lefu Wanga and
Kai Man K. Yub
Received 24th May 2011, Accepted 30th June 2011
DOI: 10.1039/c1gc15597k
A 100% conversion of cellulose to industrially useful chem-
icals is, for the first time, achieved in a single pot reaction
by the use of cooperative ionic liquid pairs for combined
dissolution and catalytic degradation of cellulose, which
overcomes the long intrinsic phase problem in the conver-
sion of biomass to chemicals.
Thus, there is an urgent need to develop novel processes that can
carry out the dissolution and catalysis of cellulose degradation
under mild conditions for bio-refinery.
The dramatic dissolution of cellulose in alkyl-imidazolium
ionic liquids (ILs) has been previously reported by Rogers
et al..7 The solubility of cellulose in bmimCl was shown to reach
10 wt%, which opens up a new method for efficient cellulose
utilization. Zhao explored an efficient IL system which used
mineral acids as catalysts for the hydrolysis of cellulose and
lignocelluloses to sugar.8 The catalytic hydrolysis of cellulose dis-
solved in bmimCl can also be realized using solid acid catalysts.9
Some functionalized ILs have been previously examined for
the dissolution, hydrolysis and pretreatment of cellulose.10 ILs
have also been extensively investigated as solvents for the
conversion of cellulose to biochemicals such as 5-hydromethyl
furfural (HMF), a versatile intermediate for chemical industry.11
For example, the combination of emimCl and some metal
chlorides has been shown to efficiently and selectively catalyze
the conversion of glucose to HMF.12 Cellulose and corn stover
could be selectively converted to HMF in DMA-LiCl/emimCl
as solvent, using CrClx-HCl as catalyst (yields of 54% and
48% achieved, respectively).13 Other metal salts, such as FeCl2
and CoSO4, in acidic ILs have also proven useful in this
process.14 ILs therefore clearly have a key role to play in the
production of biofuels and biochemicals. However, most of
the processes described above require adding water, which has
already been affirmed as the chief obstacle for the dissolution
of cellulose in ILs.7 The reliance on large quantities of metal
salts may also limit these processes due to separation problems
and environmental concerns. Furthermore, the dissolution of
cellulose in ILs is often a tedious and difficult process. The ratios
of cellulose to IL solvent used in many reports are usually far
lower than its solubility in the interest of getting a completely
homogeneous solution for full interfacial contacting between
cellulose molecules and the heterogeneous or homogeneous
catalysts, which undoubtedly lowers the efficiency of reaction.
In this paper, we demonstrate the use of novel IL pairs with
the combined feature of dissolution and controlled catalytic
degradation of cellulose in a recyclable batch reactor (Fig. 1).
One IL is selected to achieve cellulose dissolution, while the other
is used for the catalytic conversion of the dissolved cellulose
Fossil fuels have been our primary source of chemicals and
energy over the past two centuries. However, fossil resources
are becoming increasingly unreliable and their use has gener-
ated enormous problems for our biosphere.1 It is increasingly
accepted that the utilization of organic carbohydrate biomass
derived from solar power will be a more sustainable source of
chemicals and energy in the future.2 Cellulose is the cheapest and
most abundant biomass, with an estimated annual biosphere
production of 90 ¥ 109 metric tons.3 Currently, the key challenge
facing cellulose utilization lies in the lack of solubility of the
macromolecule in common solvents, due to its strong intra- and
inter-molecular hydrogen bonds.4 This insolubility precludes
its efficient conversion to small molecules of wider interest.
The use of some promising solvents may be precluded by
cost, toxicity, instability and recyclability issues.5 Thus, current
methods of cellulose processing are generally very aggressive
and have poor efficiency. For instance, gasification requires hard
curing processes such as the Fischer–Tropsch synthesis process.
The selective hydrolysis of biomass polysaccharides produces
monosaccharides, which are platform chemicals that need to be
processed intensively for further utilization; pyrolysis requires
extremely high processing temperatures and pressures, and still
yields a poor interfacial contact of cellulose with the catalyst.6
aSchool of Chemistry and Chemical Engineering, Pulp & Paper
Engineering State Key Laboratory of China, South China University of
Technology, Guangzhou, 510640, P. R. China. E-mail: cexhli@
scut.edu.cn; Fax: +86 20 8711 4707; Tel: +86 20 8711 4707
bWolfson Catalysis Centre, Inorganic Chemistry Laboratory, University
of Oxford, Oxford, UK, OX1 3QR. E-mail: edman.tsang@
chem.ox.ac.uk; Fax: +44 186 528 2610; Tel: +44 186 528 2610
† Electronic supplementary information (ESI) available: raw materials,
experimental and separation processes, molecular weight distribution of
hexane, ethyl acetate and methanol soluble products and residues, SEM
of raw cellulose and residues, TG-DSC of ILs pairs and GC-MS analysis
of products. See DOI: 10.1039/c1gc15597k
2334 | Green Chem., 2011, 13, 2334–2338
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The SEM image shows that the surface of the raw cellu-
lose is regular, smooth and plump (Fig. 2a and Fig. S2a†).
After regeneration, it displays a rough, but conglomerated
texture with fused fibers and a homogeneous macrostructure
(Fig. 2b), without obvious loss in mass. This indicates that
the dissolved cellulose is capable of regenerating from CH3OH
without significant changes in the degree of polymerization
or the polydispersity. It is very consistent with previously
reported results.7 When the temperature was elevated to 473 K
(Table 1, entry 3), the interlaminar hydrogen bonds of cellulose
were ruptured by thermal effects, as indicated by the broken
thin layers of cellulose residue observed (Fig. 2c and Fig. S2b†).
The surface of the residue obtained from the acidic mimHSO4
IL catalyst system at 473 K (Table 2, entry 2, Fig. 2d) was
found to be quite different from that described above (Fig.
2a–c). It was loose, flocculent, and featured many uniform
gibbosities, generated by the slow dissolution and rapid catalytic
degradation of bulk insoluble cellulose. The SEM images
therefore indicate significantly that the liquefaction process of
cellulose in the presence of cooperative ILs is a dissolution-in
situ catalysis process. This allows more cellulose to be converted
than would be anticipated on the basis of its solubility.
Fig. 1 Catalytic degradation of cellulose in cooperative IL pairs.
to low-molecular weight products. During the dissolution and
catalytic reaction, an immiscible organic solvent is placed on top
of the IL pair solvent system to extract the small soluble organic
products (MW < 300 g mol-1). This single-pot, combined
solvent-catalyst system can drive the dissolution and catalytic
decomposition processes in situ, allowing complete conversion
of more cellulose than its solubility normally permits.
The synthesis of the ILs used (Scheme S1†) and the separation
process used to isolate the products (Fig. S1†) is described in
the Electronic Supplementary Information (ESI†) of this paper.
Table 1 summarizes the degradation effects of blended ILs at
various temperatures of commercially available cellulose. We
show that the degree of degradation of the microcrystalline
cellulose can reach up to 59.2 wt% in bmimCl at 473 K
(Table 1 entry 3). This is attributed not only to the partial
destruction of hydrogen bonds in cellulose by this IL but also
some thermal degradation. The complete conversion of cellulose
was achieved when the acidic IL (C4H8SO3HmimHSO4) was
used with bmimCl at 473 K as shown in Table 1, entry 4. The
highly beneficial effect of the acidic IL resulted in a very effective
process, however, this effect was temperature dependent (Table
1, entries 4, 6, 7, 8). Significant variations in conversion where
observed for different ILs solvent mixtures at 433 K (Table
1, entries 8–10), with emimAc achieving a greater cellulose
solubility than bmimCl and bmimAc,11d which is likely to
account for its higher efficiency.
Fig. 2 The SEM images of cellulose and residue (a) raw cellulose; (b)
regeneration cellulose (see ESI†); (c) residue from non-catalytic system
at 473 K (Table 1, entry 3); (d) residue from mimHSO4 catalytic system
at 473 K (Table 2, entry 2).
The relationship between the acidity of the 2nd IL and
its catalytic activity is clearly shown in Table 2. The acidity
was directly related to the catalytic degradation of cellulose.
C4H8SO3HmimHSO4, the strongest acid of the IL tested, showed
the highest catalytic performance and yielded the lowest average
molecular weight products (Table 2).
The complete conversion of cellulose in the IL pairs of
C4H8SO3HmimHSO4 and bmimCl at 473 K described above
Table 1 Catalytic conversion of microcrystalline cellulose in cooperative IL pairsa
Entry
1st IL (g)
2nd IL (mmol)
Temp. (K)
Time (min)
Degradation (%)
1
2
3
—
—
—
3.5
—
3.5
3.5
3.5
3.5
3.5
3.5
3.5
473
473
473
473
473
493
453
433
433
433
15
15
15
15
15
0
15
15
15
15
£5
£5
bmimCl
bmimCl
bmimCl
bmimCl
bmimCl
bmimCl
bmimAc
emimAc
59.2
100
72.5
100
83.4
53.6
54.4
62.6
4
5b
6
7
8
9
10
a 5.0 g cellulose, 20 g 1st IL, 3.5 mmol C4H8SO3HmimHSO4 as 2nd IL, 10 mL CH3OH, 80 mL hexane; heated at 5 K min-1 from room temperature
to the reaction temperature. b Recycled 10 times.
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Table 2 Effect of acidic ILs on the degradation of cellulose and the average molecular weight of the products in methanol phase
Peak I
Mn
Peak II
Mn
Entry
Catalyst
pH
Degradation (%)
Mw
Mz
Mw
Mz
1
2
3
4
C4H8SO3HmimHSO4
mimHSO4
bmimHSO4
0.09
0.43
0.73
1.79
100
788
861
950
954
1025
1115
1196
1217
1500
1594
1619
1692
97
92
107
111
99
103
106
113
117
96.4
95.2
73.8
104
109
114
C2H4COOHmimCl
Reaction conditions: 20 g bmimCl, 3.5 mmol acidic IL, 10 mL CH3OH, 5.0 g cellulose, 80 mL hexane, 473 K, 15 min.
was further studied. Detailed product analysis was carried out
using methanol, hexane and ethyl acetate solvent extractions.
Interestingly, the products derived from our IL system indicate
that the process is significantly more selective than many
traditional high temperature degradation processes.15 A key
observation was the large quantity of low molecular weight,
volatile products (equivalent to 81.5% cellulose conversion) that
could be extracted using the above solvents. These degradation
products had an average Mn value of 97 in the methanol extract
(Fig. 3a at 473 K and Table 2, entry 1), 229 in the hexane
extract and 256 in the ethyl acetate extract (Table S1†). A
number of lower boiling polyols, aldehydes, ketones, ethers,
furans and their derivatives, olefins, methyl esters, and other
oxygenates were identified by GC-MS in these extracts (see
Fig. S3† and Table S2†), however, 2-(diethoxymethyl)furan was
found to be the main component, with 45.8 wt% of the cellulose
selectively converted to it. This compound is therefore presumed
to be more kinetically stable than the other identified products.
We therefore show for the first time that the cooperative ILs
pair can degrade cellulose into just one principal product, 2-
(diethoxymethyl)furan. This compound is a very useful platform
chemical and can be readily converted to other chemicals of
interest both as fuels and in the pharmaceutical industry.16
The molecular weight distributions of the non-volatilizable
phase generated at various reaction temperatures after extrac-
tion were analyzed and are presented in Fig. 3b and Table S3†.
These products and residues were characterized by a much
smaller average molecular weight than feedstock suggesting
that the IL pairs indeed induce the intensive degradation of
microcrystalline cellulose (Mw of 2.94 ¥ 104). When a higher
temperature was used, smaller average molecular weights and
narrower molecular weight distributions were obtained. At the
same time, it can be seen that there was a slight decrease on
the average molecular weight and a little narrowness of the
molecular weight distributions for both CH3OH soluble fraction
and residues with prolonged reaction time at 433 K (Fig. S4 and
S5†).
One important concern in the development of a new coop-
erative ILs method for the industrial processing of cellulose
is the long term stability of the IL pairs under the reaction
conditions.17 Two distinct phases were formed when the IL pairs
were mixed with methanol and hexane, with hexane forming
a colorless and discrete top phase (Fig. 4A). The lower phase
(ILs and methanol) became puce in color while the mixture
was heated to 473 K, but, the top phase was still colorless
(Fig. 4B). The imidazolium cooperative IL pairs were found
to be stable, with no decomposition products detected by GC-
MS analysis. This is consistent with our previous study.18
A
change in color of IL pairs upon heating has been observed
previously, and is attributed to the presence of trace quantities
of heat sensitive impurities in the ILs.19 Before the conversion
of cellulose, the lowest (solid) phase represents un-dissolved
cellulose, the middle phase is a mixture of methanol, ILs and
the dynamic, dissolved cellulose resulting from the effect of the
1st IL, and no cellulose is present in the upper hexane phase, as
confirmed by simple evaporation of this phase (Fig. 4C). After
dissolution (1st IL) combined with catalytic degradation (2nd
IL), the top hexane phase becomes brown in color, as it extracts
compounds produced by the degradation of cellulose (Fig. 4D).
Fig. 3 Molecular weight distributions (a) products extracted by
methanol and (b) residues.
The products of about 18.5 wt% of the cellulose was found
to be mainly composed of polar but non-volatile, saccharide-
degraded products, and these were retained in the methanol
phase (Fig. 3a) when the IL pair of C4H8SO3HmimHSO4 and
bmimCl at 473 K was used. These products were immiscible
with hexane and ethyl acetate but were miscible and extractable
with methanol. The Mn value of these polar, methanol soluble
compounds was typically 788 (Table S3†). These products were
the acid-cleaved fragments of cellulose, possibly representing
intermediate fragments before further conversion to the final
kinetically stable product, 2-(diethoxymethyl)furan.
Fig. 4 Mixture of cooperative IL pairs with cellulose. A: Before heating,
3.5 mmol C4H8SO3HmimHSO4, 20 g bmimCl, 10 mL CH3OH and
80 mL hexane; B: After heating of A; C: Before reaction, 3.5 mmol
C4H8SO3HmimHSO4; 20 g bmimCl, 5.0 g cellulose; 10 mL CH3OH and
80 mL hexane; D: After reaction of C at 473 K.
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TG analyses of the ILs after 1, 5 and 10 repeated runs indicated
that negligible changes in the thermal properties of the IL pairs
had occurred (Fig. S6†). It is noted that about 10% loss in the
mass of the recycled IL pairs before 430 K was attributed to the
loss or vaporization of impurities from the cellulose degradation
process. The presence of these impurities did not appear to affect
the thermal decomposition of the IL pairs. There was a small
decrease in the activity of the ILs when they were recycled for
the degradation of 10 fresh batches of cellulose (see Table 1,
entry 5). The reason for deactivation could not be assigned to
the instability of ILs. Instead, the physical loss of the IL pairs
due to solvent extractions was one likely possibility.
tive IL pairs is reported here for the first time. An excep-
tional 100% conversion of cellulose was achieved, using the
C4H8SO3HmimHSO4/bmimCl pair at 473 K. Under these
conditions, 45.8 wt% of the cellulose was selectively converted to
2-(diethoxymethyl)furan, an important and useful intermediate.
The dragging of the dissolution equilibrium, combined with
the rapid, in situ acid-catalyzed degradation of bulk insoluble
cellulose, overcame the long intrinsic problem of cellulose insol-
ubility encountered in the conversion of biomass to biochemical.
Thus, this highly efficient and selective chemical process, which
operates under mild operational conditions, has great potential
to facilitate the future utilization of renewable carbon sources.
A detailed analysis of the products formed during the course
of decomposition gave valuable insight into the degradation
mechanism. As shown in Scheme 1, the degradation of cellulose
by cooperative IL pairs is the result of dissolution followed by
a rapid, acid-catalyzed liquefaction and hydrolysis. According
to reported hydrolysis processes,6a,12,20 cellulose is thought to
be first hydrolyzed to glucose (a) and then isomerized to
fructose (b). This is then thought to be the key intermediate in
the production of 5-hydromethyl-furan (HMF) (c) and related
compounds. Further catalytic conversions of HMF result in
the generation of levulinic acid (d) and furfural (e), both of
which are detected in the system described here (Fig. S3 and
Table S2†). The liquefaction process is thought to occur via the
catalytic degradation of multiple structural units,20b,c,21 as shown
in Scheme 1. Dehydration of cellulose is catalyzed by acidic
ILs (2nd IL) generating methanol-soluble fractions with average
molecular weights of about 1025. This intermediate fraction is
then catalytically degraded to polyols (f ), which readily undergo
esterification reactions with organic acids such as levulinic acid
(d), generated during the above hydrolysis process, to give
chemical species such as (g). Dehydration of (g) can be catalyzed
bythe acidicIL andbondcleavage then leads to methyllevulinate
(i) and the intermediate (j). The product (i) was identified in our
cooperative IL system. The dehydration of (j) may generate (k),
which we speculate is the precursor of the predominant product
of the described system, 2-(diethoxymethyl)furan (l), as well as
the cellulose fragment (m). Further bond cleavage of (m) may
produce butyl acetate (n).
Acknowledgements
The authors gratefully acknowledge the financial support of the
Natural Science Foundation of China (Grant No. 20876055 and
21076085) and the Fundamental Research Funds for the Central
Universities of China, SCUT.
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Scheme 1 Proposed mechanism for the degradation of cellulose by
cooperative ILs.
In conclusion, a novel method for the one-pot, low-
temperature, catalytic conversion of sustainably sourced cel-
lulose to useful biochemical in the presence of coopera-
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