364
dispersion of the positive charges, solubility in the polar solvent, and porosity. They
exhibit very low surface area (typically 1–10 m
2
g
−1
) and low porosity (<0.1cm
3
g
−1
).
To overcome these difficulties, these salts are supported on various supports such as
titania, zirconia, mesoporous silica, etc. and exchanging the HPAs protons with suitable metal cations [29, 134, 135]. To avoid the leaching issue, hydrophobic
graphene- anchored HPAs compounds are reported, which furnish well acidity and
are considered as good graphene-based compounds anchored onto HPAs. The
HPAs-supported catalysts furnish well acidity and are regarded as good candidates
for better conversion of the levulinate compounds, due to their improved surface
area and free dynamic active sites required to facilitate the reaction.
Quereshi et al. have prepared zirconia-supported Keggin silicotungstic acid to
produce ethyl levulinate from LA and get more than 90 mol% ethyl levulinate at
110 °C in 30 min using a microwave reactor [136]. Pasha and Raj [137] synthesized
Zr exchanged phosphotungstic acid with retention of Keggin’s structure of TPA and
applied to the esterification reaction of LA and ethanol. The authors have successfully demonstrated the use of zirconia to enhance the catalytic activity of the HPAs.
Wu et al. [138] have studied extensively porous 3D graphene aerogel-anchored
tungstophosphoric acid catalyst for the synthesis of ethyl levulinate. Shimizu et al.
[139] prepared Cs
+
, Ag
+
Al
3+
,Ti
4+
, Y
3+
, Zr
4+
, Sn
4+
, and Hf
4+
salts of tungstophosphoric acid. Among the studied catalysts, Zr0.75TPA has been found to be the best
catalyst for the conversion of LA to ethyl levulinate with a yield of 91% in 2 h and
at 120 °C. The catalyst also showed five-time reusability. Zhou et al. [140] have
assessed the catalytic activity of silver or ammonium co-doped phosphotungstic
acid for the synthesis of ethyl levulinate. The catalytic activity of Al-MCM-41 and
zirconia-supported dodeca-tungstophosphoric acid for the synthesis of hexyl levulinate and ethyl levulinate, respectively, is also reported [98, 141, 142]. These catalysts displayed excellent recyclability.
Zheng et al. [143] and Wu et al. [144] have undertaken catalytic studies of tungstophosphoric acid supported on graphene oxide and MCM-41 catalysts, respectively, in esterification of LA to synthesize ethyl levulinate. Luan et al. [145]
demonstrated the synthesis of alkyl levulinates (ethyl, methyl, and isobutyl levulinates) using organic salt of H 4 SiW 12 O 40 . Dharne and Bokade [18] performed the
synthesis of butyl ester from levulinic acid over dodeca-tungstophosphoric acidtreated clay (K − 10). H 4 SiW 12 O 40 catalyst is reported for the synthesis of several
levulinates by Vilanculo et al. [146]. Numerous catalysts are reported for such as
heteropolyacid-supported silicalites, heteropolyacid-supported STA-AlSBA-15,
and tungstosilicic acid H 4 SiW 12 O 40 -SiO 2 for an efficient synthesis of alkyl levulinates [147–149]. In all cases, the heteropolyacid-supported catalysts performed
better than the sole support (Al-SBA15/silica/K10/MCM-41/Al-MCM-41/desilicated H-ZSM-5, etc.).
Several sulfated metal oxides (SnO 2 , ZrO 2 , Nb 2 O 5 , TiO 2 ) have also been used as
solid acid catalysts for esterification reactions including zeolites. Since the last
decade, we have been working in the area of zeolite-/mesozeolite-catalyzed organic
transformations including an esterification reaction of levulinic acid for the
K. C. Maheria et al.
dispersion of the positive charges, solubility in the polar solvent, and porosity. They
exhibit very low surface area (typically 1–10 m
2
g
−1
) and low porosity (<0.1cm
3
g
−1
).
To overcome these difficulties, these salts are supported on various supports such as
titania, zirconia, mesoporous silica, etc. and exchanging the HPAs protons with suitable metal cations [29, 134, 135]. To avoid the leaching issue, hydrophobic
graphene- anchored HPAs compounds are reported, which furnish well acidity and
are considered as good graphene-based compounds anchored onto HPAs. The
HPAs-supported catalysts furnish well acidity and are regarded as good candidates
for better conversion of the levulinate compounds, due to their improved surface
area and free dynamic active sites required to facilitate the reaction.
Quereshi et al. have prepared zirconia-supported Keggin silicotungstic acid to
produce ethyl levulinate from LA and get more than 90 mol% ethyl levulinate at
110 °C in 30 min using a microwave reactor [136]. Pasha and Raj [137] synthesized
Zr exchanged phosphotungstic acid with retention of Keggin’s structure of TPA and
applied to the esterification reaction of LA and ethanol. The authors have successfully demonstrated the use of zirconia to enhance the catalytic activity of the HPAs.
Wu et al. [138] have studied extensively porous 3D graphene aerogel-anchored
tungstophosphoric acid catalyst for the synthesis of ethyl levulinate. Shimizu et al.
[139] prepared Cs
+
, Ag
+
Al
3+
,Ti
4+
, Y
3+
, Zr
4+
, Sn
4+
, and Hf
4+
salts of tungstophosphoric acid. Among the studied catalysts, Zr0.75TPA has been found to be the best
catalyst for the conversion of LA to ethyl levulinate with a yield of 91% in 2 h and
at 120 °C. The catalyst also showed five-time reusability. Zhou et al. [140] have
assessed the catalytic activity of silver or ammonium co-doped phosphotungstic
acid for the synthesis of ethyl levulinate. The catalytic activity of Al-MCM-41 and
zirconia-supported dodeca-tungstophosphoric acid for the synthesis of hexyl levulinate and ethyl levulinate, respectively, is also reported [98, 141, 142]. These catalysts displayed excellent recyclability.
Zheng et al. [143] and Wu et al. [144] have undertaken catalytic studies of tungstophosphoric acid supported on graphene oxide and MCM-41 catalysts, respectively, in esterification of LA to synthesize ethyl levulinate. Luan et al. [145]
demonstrated the synthesis of alkyl levulinates (ethyl, methyl, and isobutyl levulinates) using organic salt of H 4 SiW 12 O 40 . Dharne and Bokade [18] performed the
synthesis of butyl ester from levulinic acid over dodeca-tungstophosphoric acidtreated clay (K − 10). H 4 SiW 12 O 40 catalyst is reported for the synthesis of several
levulinates by Vilanculo et al. [146]. Numerous catalysts are reported for such as
heteropolyacid-supported silicalites, heteropolyacid-supported STA-AlSBA-15,
and tungstosilicic acid H 4 SiW 12 O 40 -SiO 2 for an efficient synthesis of alkyl levulinates [147–149]. In all cases, the heteropolyacid-supported catalysts performed
better than the sole support (Al-SBA15/silica/K10/MCM-41/Al-MCM-41/desilicated H-ZSM-5, etc.).
Several sulfated metal oxides (SnO 2 , ZrO 2 , Nb 2 O 5 , TiO 2 ) have also been used as
solid acid catalysts for esterification reactions including zeolites. Since the last
decade, we have been working in the area of zeolite-/mesozeolite-catalyzed organic
transformations including an esterification reaction of levulinic acid for the
K. C. Maheria et al.