H. Liu et al. / Inorganica Chimica Acta 428 (2015) 32–36
33
where nHMF was the moles of HMF; nfructose was the initial moles of
fructose; MHMF and Mfructose were the mole mass of HMF and fruc-
tose, respectively. mHMF was the mass of HMF calculated by HPLC;
mfructose was the initial mass of fructose.
because of not fulfilling the principle of green chemistry. Similarly,
satisfactory yield of HMF has been achieved by using liquid acids as
the catalyst (e.g., H2SO4 and HCl). However, their strong acidity and
high corrosiveness result in the pollution of the environment and
the corrosion of the equipment. Therefore, it is essential to find a
homogeneous catalyst with high catalytic performance and
friendly environment.
3. Results and discussion
Thiosulfate anion (S2O23ꢀ) has a similar structure as sulfate
anion (SO24ꢀ), which can be seen that an oxygen atom in SO24ꢀ is
substituted by a sulfur atom. Inspired by this, homogeneous
catalysis of fructose to HMF in ionic liquids (ILs) associated with
thiosulfate has been explored, aiming to attain a green homoge-
neous catalyst with high activity and commercial availability.
Herein, a series of sulfur oxyacid salts with weak acidity or weak
alkalinity, such as Na2SO4, (NH4)2SO4, (NH4)2S2O3 and Na2S2O3,
were applied to the dehydration of fructose to HMF in this paper.
Moreover, the influences of various process parameters on HMF
yields as well as the recycle of the catalytic system were
investigated.
3.1. Effect of catalyst
First, two sulfate salts (Na2SO4 and (NH4)2SO4) and two thiosul-
fate salts (Na2S2O3ꢁ5H2O and (NH4)2S2O3) were applied to the pro-
duction of HMF from fructose in [Bmim]Cl. The results are listed in
Table 1. As seen from Table 1, the HMF yields were depended on
the catalyst largely. In the absence of catalyst, only 17.2% yield of
HMF was obtained, indicating that the auto-catalysis of [Bmim]Cl
for the dehydration of fructose was quite poor. An desired yield
of 81.6% was achieved when (NH4)2S2O3 was employed whereas
only moderate yield was obtained with (NH4)2SO4. Notably, when
Na2S2O3ꢁ5H2O were used as the catalyst, the HMF yield was even
lower than that without catalyst.
A clear relationship between the nature of the catalysts and the
HMF yields is observed. Compared with the sodium salts (entries 2
and 3), the ammonium salts (entries 4 and 5) with stronger acidity
favored the conversion of fructose to HMF because that the dehy-
dration of fructose to HMF was an acid-catalyzed process. Interest-
ingly, though the acidity of (NH4)2SO4 was stronger than
(NH4)2S2O3, the catalytic performance of (NH4)2S2O3 was even
higher. As observed in Fig. 1, S2O23ꢀ has the similar structure as
SO24ꢀ, but it was reported that the sulfate anion could shift the
pyranose-furanose equilibrium, thus hindering the dehydration
reaction [19]. Therefore, (NH4)2S2O3 was chosen as the catalyst
for the conversion of fructose to HMF and the reaction conditions
were further optimized.
2. Material and methods
2.1. Materials
HMF (99% purity) was purchased from Wutong Spice Co., Ltd.
Sodium sulfate anhydrous (Na2SO4, A.R. grade), sodium thiosulfate
pentahydrate (Na2S2O3ꢁ5H2O, A.R. grade), ammonium sulfate
((NH4)2SO4, A.R. grade), fructose (B.R. grade), methanol (CH3OH,
L.C. grade), ethyl acetate (CH3COOC2H5, A.R. grade), dimethyl sulf-
oxid ((CH3)2SO, A.R. grade) and N,N-dimethylformamide
(HCON(CH3)2, A.R. grade) were purchased from Sinopharm Chem-
ical Reagent Co., Ltd. Ammonium thiosulfate ((NH4)2S2O3, 99% pur-
ity) was purchased from Aladdin. Potassium hydrogen phthalate
(C8H5KO4) and sodium tetraborate (Na2B4O7) were purchased from
Shanghai Rex Co-Perfect Instrument Co., Ltd. 1-Butyl-3-methylimi-
dazolium chloride ([Bmim]Cl), 1-octyl-3-methylimidazolium chlo-
ride ([Omim]Cl), 1-butyl-3-methylimidazolium tetrafluoroborate
([Bmim]BF4), 1-butyl-3-methylimidazolium hexafluorophosphate
([Bmim]PF6) were purchased from Shanghai Cheng Jie Chemical
Co., Ltd. [(C8H17)2N(CH3)2]Cl (D8Cl) and [(C12H25)2N(CH3)2]Cl
(D12Cl) were purchased from Xiamen Pioneer Technology Inc. All
reagents were used as received without further purification.
3.2. Effect of catalyst dosage
Table 2 shows the effect of catalyst dosage on the HMF yield.
Only 17.2% yield of HMF was obtained in the absence of (NH4)2S2-
O3. Nevertheless the yield of HMF increased to 40.0% when 1 mg
(NH4)2S2O3 was added. The result confirms the addition of (NH4)2
S2O3 can promote the conversion of fructose to HMF. When the
amount of (NH4)2S2O3 increased to 5 mg, the HMF yield boosted
to 81.6%. The high catalyst dosage facilitated the production of
HMF from fructose, possibly because the kinetic of fructose dehy-
dration was faster in the case of higher catalyst dosage [20]. How-
ever, the yields of HMF dropped off when the amount of catalyst
was further increased. The decrease might be ascribed to the fact
that the over-using catalyst accelerated the side reactions such as
the rehydration and condensation of HMF [21]. Thus, (NH4)2S2O3 of
5 mg was chosen for subsequent experiments.
2.2. Experimental methods
2.2.1. Typical procedure
Fructose (0.1 g) and ionic liquid (0.5 g) were put into the reac-
tion bulb and heated in a phase at reaction temperature. Then, add-
ing a given amount of catalysts into the mixture, the reaction
started with a magnetic stirrer. After the desired time, the mixture
was cooled quickly and diluted using pure water. The solution was
filtered through a syringe filter before HPLC analysis.
Fig. 2 depicts the effect of the dosages of (NH4)2S2O3 on the HMF
yields with reaction running. At low dosage (1 mg) of catalyst, the
2.2.2. Analysis
The sample was analyzed by means of high performance liquid
chromatography (HPLC). The HPLC setup was equipped with two
Varian ProStar210 pumps, an Agilent TC-C (18) column, and a Var-
ian ProStar325 UV–Vis detector. The measured absorbance wave-
length was 283 nm. A solution of methanol and H2O in the ratio
50:50 (v/v) was used as the mobile phase at 0.8 mL/min flow rate.
The column oven temperature was 35 °C. The amount of HMF was
calculated by using an external standard method.
Table 1
Effect of catalyst on the HMF yield.
Entry
Catalyst
pHa (T = 293.15 K)
HMF yield (%)
1
2
3
4
5
none
Na2SO4
Na2S2O3ꢁ5H2O
(NH4)2SO4
(NH4)2S2O3
–
17.2
34.3
2.3
59.0
81.6
7.31
7.80
4.97
5.54
For fructose, HMF yield was defined as follows:
Conditions: fructose 0.1 g, Catalyst 0.034 mmol, [Bmim]Cl 0.5 g, 120 °C, 50 min.
a
pH value was measured by PHS-3C precision pH/mV meter. The pH meter
standardized with C8H5KO4 (pH 4.0, T = 293.15 K) and Na2B4O7 (pH 9.0,
T = 293.15 K) before using.
nHMF
mHMF ꢂ Mfructose
MHMF ꢂ mfructose
Yeild ¼
ꢂ 100% ¼
ꢂ 100%
nfructose