F. Jiang et al. / Journal of Molecular Catalysis A: Chemical 334 (2011) 8–12
9
Fig. 1. Ionic liquids used in the experiments. (A) [Bmim]Cl, (B) [C1COOHmim]Cl, (C)
[Bmim]HSO4, (D) [C4SO3Hmim]Cl, and (E) [C4SO3Hmim]HSO4.
stand the underlying reaction mechanism. Based on the knowledge
of the characterization results, we propose a simplified reaction
model, and perform kinetic studies to obtain important reaction
parameters. To our knowledge, it is the first time to report in situ
study of 13C NMR to monitor the hydrolysis process of cellulose
with the acidic ionic liquids as catalysts.
Fig. 2. Yield of TRS/glucose/HMF at 100 ◦C (1 h) with different acid ionic liquids as
catalysts in the solvent of [Bmim]Cl. (a) Blank, (b) [C1COOHmim]Cl, (c) [Bmim]HSO4,
(d) [C4SO3Hmim]Cl, and (e) [C4SO3Hmim]HSO4.
2.3. Characterization
2.3.1. IR
2. Experimental
The interaction of pyridine molecule with the acidic sites of
studied ionic liquids was studied by infrared spectroscopy at room
temperature. The probe molecule of pyridine was mixed with the
ionic liquids with the ratio of 1:5, and the mixture was spreaded
into the liquid film between KBr windows. All the IR spectra were
recorded on a Bruker Vector 22 spectrometer with an MCT detector
at a resolution of 1 cm−1 and 32 scans.
2.1. Materials
The structures of the ionic liquids used for this study
are displayed in Fig. 1. 1-Butyl-3-methylimidazolium chlo-
ride ([Bmim]Cl), 1-carboxyethyl-3-methylimidazolium chloride
([C1COOHmim]Cl), 1-butyl-3-methylimidazolium hydrogensulfate
([Bmim]HSO4), 1-butyl sulfonic acid-3-methylimidazolium chlo-
ride ([C4SO3Hmim]Cl), 1-butyl sulfonic acid-3-methylimidazolium
hydrogensulfate ([C4SO3Hmim] HSO4) were provided from Meis-
ibei company, China, and purities of all ILs were ca. 98%. We
speculate that [C1COOHmim]Cl and [Bmim]HSO4 belong to the
weakly acidic ILs since it− is well known that the function
group of –COOH and HSO4 are relatively weak acids, whereas
[C4SO3Hmim]Cl and [C4SO3Hmim]HSO4 are suggested to be rel-
atively strong ILs because –SO3H group are widely used to modify
the acidity of the polymers, leading to the strong acidic catalysts.
To remove the water content in the ILs, the ILs have been kept in
the vacuum drying oven at 50 ◦C, and reactant of microcrystalline
cellulose (AR, ACROS) is dried at 100 ◦C oven overnight before use.
2.3.2. NMR
Liquid-state 13C nuclear magnetic resonance (NMR) spectra
were obtained on a Varian DRX-400 spectrometer. The resonance
frequency employed in 13C NMR was 100.6 MHz. The sample was
added with 10% DMSO-d6 in the sample tube, and the chemical
shifts were referenced to tetramethylsilane (TMS). In the standard
sample measurements, 4% of glucose or cellulose was dissolved in
the [Bmim]Cl. The samples were heated at 90 ◦C to reduce the vis-
cosities of the mixture. In contrast, the in situ NMR experiment was
carried out in a home-made glass NMR sample tube, with the tem-
perature accurately controlled. The sample tube was loaded into
NMR machine and heated quickly to 80 ◦C and reacted at that tem-
perature for 24 h. In situ 13C NMR spectra were recorded during the
reaction with the accumulation of 800 scans.
2.2. Hydrolysis reaction
The cellulose hydrolysis was carried out in a round bottom
flask that was heated in the oil-bath in the range from 80 ◦C to
120 ◦C. Typically, 0.4 g microcrystalline cellulose was added into
8.0 g [Bmim]Cl solvent that was preheated under vigorous stirring,
followed by the addition of desired 1 ml acidic ionic liquid cata-
lyst, 0.08 g H2O and 2 ml co-solvent of dimethylformamide (DMF)
at the reaction temperature. Blank experiment was also performed
without the addition of the acidic IL catalyst for comparison. The
reaction was then vigorously stirred until a transparent solution
was obtained. At different time intervals, 0.11 g samples (0.1 ml)
were extracted, and quenched immediately with 0.9 ml NaOH solu-
tion (0.03 mol/l). The solution was centrifuged at 15,000 rpm for
10 min and further employed for the product analysis. The anal-
ysis of total reducing sugar (TRS) was based on the adsorption
spectroscopy, and the details are elaborated in the literature [12].
In a typical measurement, 0.5 ml resultant solution was mixed
with 0.5 ml predetermined 3,5-dinitrosalicylic acid (DNS) reagent,
heated for 10 min at 100 ◦C and cooled to room temperature. 4 ml
deionized water was then added to the sample, and the mixture
was subjected for the quantitative analysis on a JASCO V-550 spec-
trophotometer at 520 nm with a slit width of 0.1 mm.
3. Results and discussion
The cellulose hydrolysis reaction results (100 ◦C, 1 h) over the
various acidic IL catalysts are exhibited in Fig. 2. For the blank
experiment without the presence of acidic IL, i.e., the reaction in
the solvent of [Bmim]Cl, the yield of TRS is negligible. Indeed, lit-
tle activity is observed even when the reaction is prolonged to
100 h, confirming that [Bmim]Cl is not active for the production
of reducing sugars. In contrast, the presence of weakly acidic ILs of
[C1COOHmim]Cl and [Bmim]HSO4 catalyze the hydrolysis of cel-
lulose, and TRS of 3% and 5% are achieved after 1 h reaction. We
note further reaction time at 24 h enhance the release of reduc-
ing sugars to 17% and 28% over [C1COOHmim]Cl and [Bmim]HSO4,
respectively. It is remarkable that the presence of the sulfonic
acid group in the ILs, i.e., [C4SO3Hmim]Cl and [C4SO3Hmim]HSO4,
greatly increase the reaction rates of the cellulose hydrolysis, and
the hydrolysis are nearly complete in 1 h. TRS of 95% and 85% are
observed on [C4SO3Hmim]Cl and [C4SO3Hmim]HSO4, respectively.
The major products of the cellulose transformation, as revealed by
chromato analysis, are glucose and HMF, further careful inspec-
tion discloses a variety of organic substances (containing carbon,