The reaction mixture was stirred and heated at reflux for 24 h.
After the reaction was complete, the CH2Cl2 was decanted and the
solid sample was washed with CH2Cl2 (330 mL) and dried to give
[PzS]Cl as a white powder. Analogous IL [PzS-H]Cl2 was prepared in
a similar procedure by using a 2:1 molar ratio of chlorosulfonic
acid and pyrazine. Analogous IL [MimS]Cl was prepared in a similar
procedure by using methylimidazole instead of pyrazine. The char-
acterization data of the synthesized ILs are presented in the Sup-
porting Information.
3. Conclusions
This study focused on the effects of the solvent and the
Brønsted/Lewis acid sites of ionic hybrid catalysts on the dehy-
dration of glucose to HMF. A series of sulfonic-acid-functional-
ized pyrazine HPA-based ionic hybrid catalysts was prepared
and examined for the conversion of glucose to HMF. The maxi-
mum total yield of HMF and LGA (71%) was achieved by using
[PzS]H2PW (B/L acid sites 3:2) in THF/H2O–NaCl (v/v 5:1). The
introduction of heteropolyanions in the catalyst facilitated the
direct conversion of glucose and intermediate LGA to HMF.
The synergy of the Brønsted/Lewis acid sites was more favora-
ble for the formation of HMF. Moreover, the effect of the water
content in the solvent was systematically surveyed. It was
found that a small amount of water could inhibit the dehydra-
tion of LGA to LGO and enhance the direct conversion of LGA
to HMF. Additionally, the direct dehydration of glucose to HMF
was not negligible. The encounter complexes between small
amounts of water molecules and the sulfonic acid catalyst
were likely to promote proton transfer through effective hy-
drogen bonding and ionic interactions, and thus improved the
selectivity for HMF. This work may be helpful to understand
the dehydration of glucose to HMF catalyzed by HPA ionic cat-
alysts and to develop effective HPA catalysts to produce valua-
ble chemicals from biomass.
The sulfonic-acid-functionalized imidazole IL MimPS was synthe-
sized according to a literature procedure.[20b] Methylimidazole
(0.1 mol) and 1,3-propane sulfone (0.1 mol) were added to toluene
(20 mL) in a 100 mL round-bottomed flask. The mixture was stirred
for 24 h at 508C under a nitrogen atmosphere, then the formed
precipitate was filtered and washed with CH2Cl2 (330 mL) and
dried to give MimPS as a white solid (yield: 96%).
HPA-based ionic hybrids were prepared by a reaction between an
IL and HPA. Typically, [PzS]Cl (5.0 mmol) and H3PW12O40 (5.0 mmol)
were dissolved in deionized water (20 mL). Aqueous H3PW12O40
was added dropwise to the [PzS]Cl solution, and then stirred at RT
for 8 h. The formed precipitate was filtered and washed with de-
ionized water, then dried to give [PzS]H2PW. In accordance with
the above procedure, [PzS]2HPW and [PzS]3PW were prepared by
using the corresponding molar ratio of reactants. Analogous hy-
brids [PzS-H]HPW, [MimPS]H2PW, and [MimS]H2PW were prepared
by using the corresponding IL precursors. The characterization
data for the HPA-based ionic hybrids are presented in the Support-
ing Information.
Experimental Section
Glucose Dehydration Reaction
Materials and Methods
The catalytic reaction proceeded in a 100 mL steel autoclave lined
with Teflon. In a typical experiment, a mixture of glucose (0.1 g),
catalyst (60 mmol), solvent (12 mL), and NaCl (0.37 g) were added
to the autoclave, which was then purged three times with nitro-
gen, heated, and stirred in an oil bath for specified time, then the
reaction mixture was cooled to RT immediately. The mixture pre-
sented two layers: the upper layer was the organic phase that con-
tained desired products HMF and LGO; the bottom layer was an
aqueous phase in which NaCl, traces of HMF, unreacted glucose,
the intermediate LGA, and the byproduct humins were dissolved.
The qualitative analysis of LGA was carried out by using a gas chro-
matography-mass spectrometry instrument (GC-MS; Perkin–Elmer,
Clarus SQ 8) equipped with a capillary column, TC-1701 (GL Scien-
ces, length 60 m, i.d. 0.25 mm and film thickness 0.25 mm). LGA
and glucose were analyzed by using a HPLC instrument (Shimadzu
LC-10A) equipped with a HPX-87H column and a refractive index
detector. Separation was achieved at 308C with 5 mm H2SO4 as the
mobile phase at a flow rate of 0.6 mLminÀ1. LGO and HMF were
All chemicals, including glucose, THF, pyrazine, phosphotungstic
acid, 1,3- propanesultone, and LGA, were purchased from Sino-
pharm Chemical Reagent Co. and used without further purification.
Elemental analyses were performed by using a CHN elemental ana-
lyzer (Vario EL cube) and an inductively coupled plasma spectrom-
1
eter (TCP, Jarrell-Ash 1100). The H NMR spectra were obtained at
400 MHz by using the Hahn echo at RT. The spin rate was 10 kHz.
Scanning electron microscopy (SEM; Hitachi S-4800, accelerating
voltage: 5 kV) accompanied by energy-dispersive X-ray spectrome-
try (EDS; accelerating voltage: 20 kV) was used to study the mor-
phology and the elemental distribution. FTIR spectra (KBr discs)
were recorded by using a Nicolet 360 FT-IR instrument in the
4000–400 cmÀ1 region.
The acidity of the solid samples was measured by using a Ham-
mett acidity analysis according to a literature procedure.[22] The
Hammett indicator (p-nitroaniline, 25 mg) and the catalyst (50 mg)
were added to deionized water (10 mL and the mixture was stirred
at RT for 12 h, then the solid was separated by centrifugal filtration.
The solution was measured by using a UV spectrophotometer at
l=380 nm. Each test was done in triplicate and the reported data
is the average value.
analyzed by using
a gas chromatograph (GC6890, Agilent)
equipped with a flame ionization detector. The temperature of the
oven was kept at 408C for 5 min, then heated to 2408C at a rate
of 7.58CminÀ1 and held this temperature for further 15 min. Under
optimized conditions, the recycling tests of the water phase in the re-
action mixture are described in the Supporting Information. Each test
was done in triplicate and the reported data is the average value.
Catalyst Preparation
The synthesis strategy of sulfonic-acid-functionalized HPA-based
ionic hybrid catalysts is illustrated in Scheme 2. The sulfonic-acid-
functionalized pyrazine IL monomer [PzS]Cl was prepared accord-
ing to previous reports.[21] The main experimentation process is as
follows. Chlorosulfonic acid (0.01 mol) and pyrazine (0.01 mol)
were added to CH2Cl2 (20 mL) in a 50 mL round-bottomed flask.
Acknowledgements
The authors thank the National Natural Science Foundation of
China (nos. 21506118, 21476132, and 51536009), the Provincial
ChemistryOpen 2018, 7, 824 –832
831
ꢀ 2018 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim