309
bisphenol having diaromatic-substituted moiety as 4,4-bis(4-hydroxyphenyl)valeric
acid, prepared on condensation of LA with phenol under acid-catalysed reaction
(Scheme 7) [82]. Many catalysts are reported for this conversion. Initially, the reaction was reported with Bronsted mineral acids (H 2 SO 4 and HCl) [83]. However,
these are toxic and corrosive, and hence homogeneous mineral acids were replaced
by insoluble heterogeneous catalysts.
The insoluble cesium-substituted Keggin-type heteropolyacid such as
Cs 1.5 H 4.5 P 2 W 18 O 62 showed 70.5% yield of DPA with 88% selectivity of p,p′-DPA
isomer at 150 °C, molar ratio of phenol to LA 4:1 for 10 h [84]. The heteropolyacid
catalyst revealed good reusability for three repetitive runs.
A water-tolerant solid acid catalyst, i.e. mesoporous H 3 PW 12 O 40 /SBA-15
(obtained from sol-gel co-condensation), has been reported for DPA synthesis [85–
87]. The H 3 PW 12 O 40 /SBA-15 has high surface area, well-defined pore size and uniform distributed catalytic sites. The SBA with 15.7% loading of H 3 PW 12 O 40 showed
better activity towards the reaction, 80% conversion of levulinic acid with 2.9:1
ratio of DPA (p,p′) isomer to DPA (o,p′) isomer. The high pore size of catalytic
material facilitates the accessibility of the reactive sites in the framework for the
reactants. The H 3 PW 12 O 40 /SBA-15 catalyst is regenerated by calcination (420 °C)
and was recycled for five reaction cycles with almost similar catalytic activity.
Sels research group reported a new class of acid catalyst, i.e. sulfonated hyperbranched poly(arylene oxindoles) (SHPAOs) in combination with thiol co-catalyst.
Using SHPAO catalyst, various thiol compounds, e.g. benzylthiol, ethanethiol,
1-propanethiol, 1-butanethiol, 2-propanethiol and 2-methyl-2-propanethiol, were
explored at 200 °C for 16 h under inert atmosphere and absence of solvent [88, 89].
The less sterically hindered co-catalyst/additive ethanethiol showed high yield of
DPA (52.9%) with high selectivity of p,p′ isomer (19.5 ratio of the p,p′-DPA to the
o,p′-DPA isomer). The role of thiol promoter in catalytic mechanism cycle is mentioned in Scheme 8.
Scheme 6 Mechanistic pathway for Agvls formation from LA with anisole using β-zeolite
Levulinic Acid- and Furan-Based Multifunctional Materials: Opportunities…
bisphenol having diaromatic-substituted moiety as 4,4-bis(4-hydroxyphenyl)valeric
acid, prepared on condensation of LA with phenol under acid-catalysed reaction
(Scheme 7) [82]. Many catalysts are reported for this conversion. Initially, the reaction was reported with Bronsted mineral acids (H 2 SO 4 and HCl) [83]. However,
these are toxic and corrosive, and hence homogeneous mineral acids were replaced
by insoluble heterogeneous catalysts.
The insoluble cesium-substituted Keggin-type heteropolyacid such as
Cs 1.5 H 4.5 P 2 W 18 O 62 showed 70.5% yield of DPA with 88% selectivity of p,p′-DPA
isomer at 150 °C, molar ratio of phenol to LA 4:1 for 10 h [84]. The heteropolyacid
catalyst revealed good reusability for three repetitive runs.
A water-tolerant solid acid catalyst, i.e. mesoporous H 3 PW 12 O 40 /SBA-15
(obtained from sol-gel co-condensation), has been reported for DPA synthesis [85–
87]. The H 3 PW 12 O 40 /SBA-15 has high surface area, well-defined pore size and uniform distributed catalytic sites. The SBA with 15.7% loading of H 3 PW 12 O 40 showed
better activity towards the reaction, 80% conversion of levulinic acid with 2.9:1
ratio of DPA (p,p′) isomer to DPA (o,p′) isomer. The high pore size of catalytic
material facilitates the accessibility of the reactive sites in the framework for the
reactants. The H 3 PW 12 O 40 /SBA-15 catalyst is regenerated by calcination (420 °C)
and was recycled for five reaction cycles with almost similar catalytic activity.
Sels research group reported a new class of acid catalyst, i.e. sulfonated hyperbranched poly(arylene oxindoles) (SHPAOs) in combination with thiol co-catalyst.
Using SHPAO catalyst, various thiol compounds, e.g. benzylthiol, ethanethiol,
1-propanethiol, 1-butanethiol, 2-propanethiol and 2-methyl-2-propanethiol, were
explored at 200 °C for 16 h under inert atmosphere and absence of solvent [88, 89].
The less sterically hindered co-catalyst/additive ethanethiol showed high yield of
DPA (52.9%) with high selectivity of p,p′ isomer (19.5 ratio of the p,p′-DPA to the
o,p′-DPA isomer). The role of thiol promoter in catalytic mechanism cycle is mentioned in Scheme 8.
Scheme 6 Mechanistic pathway for Agvls formation from LA with anisole using β-zeolite
Levulinic Acid- and Furan-Based Multifunctional Materials: Opportunities…