318
Heteropolyacid (HPA)-Based Catalysts
Heteropolyacids are a class of strong acids having certain metal or non-metal-based
oxo-anionic clusters in the system. Due to their excellent physicochemical properties, Keggin structure, thermal stability and high acidity, they have been extensively
reported as catalysts for acid-catalysed organic transformations including dehydration reactions [118–120]. Xiao et al. demonstrated that H 3 PW 12 O 40 and H 4 SiW 12 O 40
are highly efficient catalysts for this dehydration reaction using 1-butyl-3methylimidazolium chloride [BMIM]Cl as a solvent. HMF was obtained with 99%
yield in only 5 min at 80 °C under microwave irradiation condition (Table 6, entry
1) [121].
Jiang group reported a novel water-tolerant C 16 H 3 PW 11 CrO 39 catalyst, prepared
from cetyltrimethyl ammonium salt of transition metal (Cr
(III)
)-substituted polyoxometalate (C16-cetyltrimethyl ammonium). The novel catalyst showed moderate
performance even at high concentration of fructose (30 wt%) (Table 6, entry 2)
[122]. The high catalytic activity at concentrated fructose solution is due to the
coexistence of Lewis and Bronsted acidic sites and also the hydrophobic groups
present in the catalyst.
Wang research group found an economic and environmental friendly method for
obtaining HMF from fructose using Cs 2.5 H 0.5 PW 12 O 40 catalyst in a biphasic system
(water + MIBK). Seventy-four per cent yield of HMF with 94.75% conversion of
fructose at 115 °C for 1 h was reported, and the catalyst was reported to be effective
up to high concentrations of fructose (50 wt%) as well (Table 6, entry 3) [123].
Silver ion exchanged phosphotungstic acid (Ag 3 PW 12 O 40 ) also proved to be an
excellent catalytic system for fructose dehydration to HMF (Table 6, entry 4) [124].
Scheme 15 Conversion of glucose to HMF using combination of Al 2 O 3 and CuCl 2 in
DMSO medium
S. Gundekari et al.
Heteropolyacid (HPA)-Based Catalysts
Heteropolyacids are a class of strong acids having certain metal or non-metal-based
oxo-anionic clusters in the system. Due to their excellent physicochemical properties, Keggin structure, thermal stability and high acidity, they have been extensively
reported as catalysts for acid-catalysed organic transformations including dehydration reactions [118–120]. Xiao et al. demonstrated that H 3 PW 12 O 40 and H 4 SiW 12 O 40
are highly efficient catalysts for this dehydration reaction using 1-butyl-3methylimidazolium chloride [BMIM]Cl as a solvent. HMF was obtained with 99%
yield in only 5 min at 80 °C under microwave irradiation condition (Table 6, entry
1) [121].
Jiang group reported a novel water-tolerant C 16 H 3 PW 11 CrO 39 catalyst, prepared
from cetyltrimethyl ammonium salt of transition metal (Cr
(III)
)-substituted polyoxometalate (C16-cetyltrimethyl ammonium). The novel catalyst showed moderate
performance even at high concentration of fructose (30 wt%) (Table 6, entry 2)
[122]. The high catalytic activity at concentrated fructose solution is due to the
coexistence of Lewis and Bronsted acidic sites and also the hydrophobic groups
present in the catalyst.
Wang research group found an economic and environmental friendly method for
obtaining HMF from fructose using Cs 2.5 H 0.5 PW 12 O 40 catalyst in a biphasic system
(water + MIBK). Seventy-four per cent yield of HMF with 94.75% conversion of
fructose at 115 °C for 1 h was reported, and the catalyst was reported to be effective
up to high concentrations of fructose (50 wt%) as well (Table 6, entry 3) [123].
Silver ion exchanged phosphotungstic acid (Ag 3 PW 12 O 40 ) also proved to be an
excellent catalytic system for fructose dehydration to HMF (Table 6, entry 4) [124].
Scheme 15 Conversion of glucose to HMF using combination of Al 2 O 3 and CuCl 2 in
DMSO medium
S. Gundekari et al.