Acid-Catalyzed Dehydration of Fructose
83, 70, 63, and 43% after 10 min, respectively; this can be at-
tributed to the decrease in the ratio of catalytic active sites
(SO3H groups) in CSS to fructose concentration. However, with
prolonged reaction times, high 5-HMF yields were obtained in
all cases. For example, for the case of 0.2 g fructose being
loaded, 71% of 5-HMF yield was obtained in 90 min reaction
time. Therefore, the reaction with prepared CSS catalyst in an
ionic liquid is applicable to high feedstock concentrations.
Conclusions
Carbon materials prepared by incomplete hydrothermal car-
bonization of cellulose and post-treatment sulfonation or KOH
activation resulted in highly active solid acid catalysts with ꢀ
SO3H, ꢀCOOH, and phenolic ꢀOH groups. The catalysts were
effective for the catalytic transformation of fructose into 5-HMF
(5-hydroxymethylfurfural). With CSS (carbonaceous sulfonated
solid), a 5-HMF yield of 83% could be obtained in [BMIM][Cl]
(1-butyl-3-methylimidazolium chloride) at 808C for 10 min. Cat-
alyst a-CSS (activated CSS) exhibited a somewhat lower activity
than that of CSS, even though a-CSS had a much larger surface
area than that of CSS. The lower activity of the a-CSS catalyst
compared with that of the CSS catalyst can be attributed to
the lower concentration of ꢀSO3H groups. The use of an ionic
liquid with CSS resulted in the reaction being stable for five
cycles and this combination could be applied to the conver-
sion of high-concentration fructose solutions (ca. 20 wt%).
Recycling of CSS
To investigate the activity and stability of the CSS catalyst in
the fructose–ionic liquid reaction system, recycling of the CSS
catalyst was studied for five cycles (Figure 7). Experiments
Experimental Section
Materials: Fructose (99%), sulfuric acid (99%), ethyl acetate, etha-
nol, and potassium hydroxide were used as received from Guangfu
fine chemicals research institute (Tianjin). [BMIM][Cl] (99%) was ob-
tained from Henan Lihua Pharmaceutical (Xinxiang). Microcrystal-
line cellulose (pharmaceutical grade, ca. 50 mm) was obtained from
the Boya company (Tianjin), and 5-HMF (98%) was purchased from
the Acros Organics company (Geel).
Preparation of carbonaceous solid acid catalysts: The cellulose-de-
rived solid acid catalysts were prepared by hydrothermal treatment
of cellulose and sulfonation according to the following two-step
procedure. Typically, cellulose (6 g) was dispersed in water (50 mL)
and the mixture was then transferred to a 100 mL stainless-steel
autoclave and heated to 2508C; conditions were maintained for
4 h at the autogenous pressure. The resulting carbonaceous solid
materials were denoted CS.
The CS samples were recovered by centrifugation and washed
with water and ethanol several times and dried at 808C for 12 h in
a vacuum oven. The CS samples were then heated in concentrated
sulfuric acid at 2008C under a nitrogen atmosphere. After heating
for 12 h and cooling to room temperature, the black precipitate
was washed with distilled water until sulfate ion impurities could
no longer be detected in the washings. The obtained sulfonated
black materials were dried for 12 h at 808C and ground into
powder. The resulting carbonaceous solid materials were denoted
CSS.
To increase the specific surface area of the prepared carbonaceous
solid acid catalyst, CS was chemically activated by using KOH
before sulfonation. Briefly, the CS sample and KOH were mixed in
a weight ratio of KOH/CS of 4 and then the mixture was heated to
6008C under a flow of nitrogen and maintained under these condi-
tions for 1 h. The samples were then thoroughly washed repeated-
ly with a 1 molLꢀ1 aqueous solution of HCl to remove inorganic
salts and washed with distilled water until a neutral pH of the
washings was obtained. The wet solids were filtered to remove
excess water and dried at 808C for 12 h. The dried solids were sub-
jected to the sulfonation procedure described above. The sulfonat-
ed catalyst thus synthesized was denoted a-CSS.
Figure 7. Recycling of the catalytic system. Reaction conditions: fructose
(0.10 g), [BMIM][Cl] (1 g), CSS (0.05 g), 808C, 20 min.
were carried out at 808C for 20 min. The product 5-HMF was
effectively separated from the solvent mixture after the reac-
tion by extraction with ethyl acetate (8ꢁ6 mL), since [BMIM]
[Cl] and fructose were insoluble in ethyl acetate and 5-HMF
was the sole product in the ethyl acetate phase, as noted by
Hu et al.[24] The number of mols of 5-HMF extracted with ethyl
acetate was taken to represent the amount of 5-HMF formed
during the reaction. After the extraction, the reaction mixture
was heated at 608C for 24 h in a vacuum oven to remove
water and residual ethyl acetate and then it was used directly
in the next run by adding an equal amount of fresh fructose. It
can be seen from Figure 7 that a 5-HMF yield of 77% was ob-
tained for the first use of CSS catalyst and this only decreased
slightly to 72% 5-HMF after five successive recycles. Residual
humins or water-soluble products, if present in the reaction
solvent, did not seem to have any significant effect on the
yields obtained with the reaction solvents and recycled cata-
lysts. Thus, the recycled catalytic system gave comparable 5-
HMF yields; this indicated that the CSS catalyst retained good
activity for the conversion of fructose into 5-HMF after repeat-
ed use.
Catalyst characterization: The catalysts were characterized by XRD
(D/max-2500, RIGAKG), SEM (S4800, HITACHI), FTIR spectroscopy
ChemSusChem 0000, 00, 1 – 7
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