W. Liu et al.
renewable clean energy have been extensively concerned.
In this respect, biomass, which is affluent, pollution-free
and widespread, provides an ideal substitute to fossil
resources for the production of fine chemicals and fuels [1–
4]. Recently, an increasing efforts have been devoted
towards transforming biomass into 5-hydroxymethylfurf-
ural (HMF), which is one of vitally important versatile
intermediates for preparing a broad of valuable chemicals
[5–11], polymers [12], pharmaceutical [13].
The abundance of differently functionalized ionic liq-
uids are incorporated PEG chain into cationic units, gen-
erating an attractive group of solvents or catalysts that find
applications across a range of disciplines, including
extractions, gas separations, carbohydrate dissolution,
organic synthesis and catalysis [39]. Recent studies have
demonstrated that ether-functionalized ILs tend to present
lower viscosity and reduced toxicity than their aliphatic-
substituted counterparts [40, 41]. Considering the above
factors, we attempted to prepare polyethylene glycol-400-
As a sustainable precursor for monomers and fuel
components, HMF can be synthesized from all types of C6
carbohydrates, including monomeric and polymeric sac-
charides, among which conversion of fructose to HMF is of
significant interest with acidic catalysts. Over the past few
years, all sorts of mineral or organic acids catalysts for
direct production of HMF from fructose have been exten-
sively studied in water [14] and various aprotic organic
solvents [15, 16]. However, the homogenous acid catalysts
show innate drawbacks in corrosion and non-recyclability.
Recently, there were several novel heterogeneous solid
acid catalysts which were employed for the conversion of
fructose to HMF under different conditions, such as silica
materials [17, 18], acid-functionalized carbons [19, 20],
acidic resins [21], zeolites [22, 23], functionalized metal–
organic frameworks (MOFs) [24], graphene oxide (GO)
[25], polyoxometalates (POMs) [26]. Generally, heteroge-
neous catalysis is preferred over homogeneous catalysis
because of the ease of catalyst separation and its reus-
ability. However, soluble polymers and insoluble humins
formed can deposit in the catalyst pores, leading to the
partial deactivation of the catalysts. This suggests a need to
explore efficient homogeneous catalytic processes for HMF
production from fructose.
functionalized dicationic acidic ionic liquids (PEG400
-
DAILs) to broaden homogeneous supported ionic liquids
catalysts applications in the dehydration of fructose into
HMF. In the present study, a variety of PEG400-DAILs
were synthesized and used as catalysts for dehydration of
fructose into HMF. Moreover, the influence of different
anion moieties on the catalytic activity of ionic liquids
were discussed and the effects of various process param-
eters were investigated in detail.
2 Experimental
2.1 Materials and Equipment (General Remark)
1,3-Propanesultone, phosphotungstic acid (H3PW12O40)
and phosphomolybdic acid (H3PMo12O40) were purchased
from shanghai Aladdin Industrial Inc.; HMF used in the
study was obtained from Sigma-Aldrich Co. LLC.; inulin
was obtained from Alfa Aesar; fructose, sucrose, glucose,
cellobiose, polyethylene glycol-400, imidazole, sodium
ethoxide, potassium carbonate, sodium hydroxide, were
purchased from Sinopharm Chemical Reagent Co. Ltd.; All
other reagents and solvents were of analytical grade and
used without further purification unless otherwise stated.
Deionized water was produced by a laboratory water-
purification system (RO DI Digital plus). FT-IR spectra
were recorded on a Nicolet 360 FT-IR instrument (KBr
discs) in the 4,000–500 cm-1 region. 1H NMR spectra
were measured with a Bruker DPX 300 (400 MHz) spec-
trometer and tetramethylsilane (TMS) was used as internal
standard. UV–Vis spectra were performed on TU-1901
spectrophotometer in water.
Interestingly, ionic liquids (ILs) have been defined as a
sort of powerful reaction medium as well as catalysts for
the dehydration of hexose to HMF [27–34], while the use
of innovative catalysts containing ionic liquids have been
scarce and subject to intense research. Lee et al. [35]
reported that functionalized mesoporous silica nanoparti-
cles with both sulfonic acid and ionic liquid were applied
as effective and recyclable catalysts for generating HMF
from fructose. Shi et al. [36] first developed a novel class of
fiber supported ionic liquid to catalyze fructose dehydra-
tion into HMF and showed excellent catalytic activity.
Recently, Jadhav et al. [37, 38] prepared a series of
homogeneous supported dicationic ionic liquids, in which
each IL contains short oligo (ethylene glycol) linkers.
These ionic liquids showed high catalytic activity for
selective dehydration of fructose into HMF. At present, the
functionalized ionic liquid catalysts still exist some defi-
ciencies, thus we need to make further constant efforts to
modify the kind of catalysts to improve catalyst activity for
fructose conversion into HMF.
2.2 Catalysts Preparation
The IL 4 was prepared according to literature procedure
with slight modifications (Fig. 1) (see the Supporting
Information for details).
The PEG400-DAILs used in the study were synthesized
by the treatment of IL 4 or 5 with acid in water. Typically,
the ionic liquid of [4ꢀ2H][HSO4]2 was prepared as follows:
the H2SO4 (0.7 mL, 12 mmol) was added to the IL 4
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