Acid-Catalyzed Hydrolysis of Cellulose
(2.1 mL) at room temperature. An aqueous solution of arabinose
(700 mL of 10 mgmLÀ1 arabinose in water) serving as an internal
standard was added to this mixture. The sample was then centri-
fuged, and the supernatant fluid (500 mL) was treated with a
mixed-bed resin (hydrogen and hydroxide form) to remove the IL.
The sample with the solid ion-exchange resin was then centrifuged
again, and the supernatant was analyzed by applying either high-
performance liquid chromatography using a refractive index detec-
tor (HPLC-RID) or high-performance anion-exchange liquid chroma-
tography using a pulsed amperometric detector (HPAEC-PAD). The
remaining sample not treated with ion-exchange resin was extract-
ed with ethyl acetate (5 extractions using 5 mL ethyl acetate) in a
pear-shaped flask, treated with excess sodium carbonate (NaCO3)
to remove water, centrifuged, and analyzed by using gas chroma-
tography (GC) equipped with a mass spectrometer (MS) and flame
ionization detector (FID).
Reactions in which water was added periodically to the reaction
mixture were carried out on the deck of the Symyx CMR. In a typi-
cal reaction, [Emim][Cl] [353 mL, (400Æ20) mg] at 383 K was dis-
pensed manually with a micropipette into a 4 mL screw-top glass
vial containing a magnetic stir bar rotating at 250 rpm. Avicel
[(20Æ0.5) mg] was added to the reactor and allowed to dissolve at
383 K for 3 h, resulting in a clear solution. The reactor temperature
was adjusted to the reaction temperature, and the contents of the
reactor was equilibrated for 1 h. A premade solution of 1.66m HCl
(23.2 mL) was then added to the reactor, and the reaction was initi-
ated. After 10, 20, 30, and 60 min, 80, 40, 60, and 100 mL of water,
respectively, was added to the reactor which was then resealed.
Reactors were removed from the heated CMR deck at specified
time intervals and quenched with running room-temperature
water for 1 min. The reactor contents were then diluted with ultra-
pure water (300 mL), and an aqueous solution of arabinose was
added (400 mL of 10 mgmLÀ1 arabinose in water), which served as
an internal standard. The sample was then centrifuged and the su-
pernatant fluid (500 mL) was treated with the mixed-bed resin and
centrifuged. The supernatant of the sample treated with the solid
ion-exchange resin was analyzed by using HPLC-RID or HPAEC-
PAD.
of cellobiose hydrolysis is a strong function of water concen-
tration, acid strength and concentration, and temperature. The
applicability of these results to cellulose is, however, not fully
apparent because there are notable differences between cellu-
lose and cellobiose. For example, it has been reported that the
hydrolysis of dissolved cellulose (glucan) could be limited by
the accessibility of glycosidic linkages because the glucan
strands take on a random coil structure.[10] Likewise, evidence
has been presented that suggests that the rate of glucan hy-
drolysis is dependent on its degree of polymerization because
the glycosidic bonds near the ends of a chain of cellulose react
twice as fast as those in the interior of the chain.[11]
The present study was undertaken with the aim of develop-
ing a deeper understanding of the factors controlling the acid-
catalyzed hydrolysis of cellulose dissolved in an IL. Among the
questions addressed were the kinetics of hydrolysis, including
the temporal evolution of all products, the effects of acid
strength and concentration, and the effects of water addition
on the kinetics of cellulose hydrolysis, glucose dehydration,
and formation of humins. Microcrystalline cellulose (Avicel) was
used as the substrate and reactions were carried out in either
[Bmim][Cl] or [Emim][Cl].
Experimental Section
Materials
Unless stated otherwise, materials were used as received. The ILs
1-ethyl-3-methylimidazolium chloride ([Emim][Cl], 98% purity) and
1-butyl-3-methylimidazolium chloride ([Bmim][Cl], 98% purity)
were purchased from Iolitec, Germany. Avicel (microcrystalline cel-
lulose, PH-101, DP<350), cellobiose (CB), arabinose (Ar), methane
sulfonic acid (100% purity), 5-hydroxymethylfurfural (5-HMF, 99%
purity), trifluoroacetic acid (100% purity), acetic acid (100% purity),
phosphoric acid (100% purity), mixed-bed resin TMD-8 (hydrogen
and hydroxide form), and 1-butyl-3-methylimidazolium acetate
([Bmim][CH3COO], 95% purity) were purchased from Sigma Aldrich,
USA. Glucose (G, USP Grade) was purchased from Hyclone, USA.
Sulfuric acid (98% purity) was purchased from Acros. Hydrochloric
acid (37% purity), HPLC grade acetonitrile, and ethyl acetate were
purchased from Fisher Scientific, USA.
Analytical techniques
HPLC-RID analyses were performed on a Shimadzu instrument
equipped with a Biorad Aminex HPX-87H column maintained at
333 K and eluted using a 0.01n H2SO4 mobile phase flowing at
0.6 mLminÀ1. Products were identified by comparing retention
times with those of pure substances. Quantification was deter-
mined by dividing the integrated peak areas of hydrolysis products
(cellobiose, glucose, or 5-HMF) by the integrated peak area of the
internal standard (arabinose) and converting the area ratio to a
molar concentration using a seven-point calibration curve.
HPAEC-PAD analyses of oligosaccharides were performed on a
Dionex ICS-3000 system equipped with a CarboPac PA200 column
(3ꢁ150 mm) maintained at 303 K. Analyses were conducted utiliz-
ing a gradient mobile phase flowing at 0.6 mLminÀ1, in which the
concentration of NaOH changed from 30 to 100mm over a 20 min
period.
Matrix-assisted laser-desorption ionization time-of-flight mass spec-
troscopy (MALDI-TOFMS) analyses were conducted on a Shimadzu
Biotech AXIMA system operating in reflection mode using 2,5-dihy-
droxylbenzoic acid in 70% acetonitrile as the matrix.
Samples extracted into ethyl acetate were injected into a Varian
CP-3800 gas chromatograph equipped with a FactorFour Capillary
Column (UF-5ms, 30 mꢁ0.25 mm, 0.25 mm, P/N CP8944) and ana-
Cellulose hydrolysis
The hydrolysis of cellulose was performed by using a Symyx core
module robot (CMR) equipped with a positive displacement tip. In
a typical experiment, [Bmim][Cl] (15 mL) was dispensed into a
20 mL vial at 373 K using magnetic tumble stirring at 400 rpm. As
the IL was dispensed, microcrystalline cellulose (750 mg, Avicel)
was added to ensure the substrate had good contact with the stir
bar. Once the cellulose and IL had been added, the solution was
stirred for 3 h at 373 K to assure complete dissolution of the cellu-
lose, as determined by the formation of a clear solution. After dis-
solution, the temperature was adjusted to the reaction tempera-
ture (usually 363 K) and allowed to equilibrate for 1 h. The mineral
acid was diluted with water obtained from a Milli-Q ultrapure
water purification system. The diluted acid solution was then
added to the IL solution to initiate the reaction. Samples (700 mL)
were taken at specified intervals using the Symyx CMR positive dis-
placement tip and added to vials containing ultrapure water
ChemSusChem 2011, 4, 1166 – 1173
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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