361
amount of cross-linker. Higher cross-linking causes more stiffness and lesser swelling. The gel type Dowex 50Xx2 affords higher yield than macroporous resins in
polar media (e.g., esterification of levulinic acid with alcohol) due to swelling which
causes enlargement of the pore size, recurrence of new pores, and subsequent
improvement in mass transfer of reactants and surface area. Trombettoni et al. [77]
used cationic sulfonated resins (Aquivion mP90, micronized pellets of perfluorosulfonic polymer) for esterification of levulinic acid with long-chain alcohols. Ramli
et al. [98] have made use of Amberlyst-15 resin as a solid acid catalyst for alkyl
levulinate compounds. SO 3 H and SO 4 functional groups present in Amberlyst-15
were found to play a crucial role in esterification of levulinic acid. Tejero et al. [76]
have carried out an esterification of levulinic acid with butanol by employing Dowex
50Wx2, a gel-type resin. Several other resin-based catalysts such as Amberlystbased resins, Purolite, Dowex, and polystyrene-supported PTSA are reported for
levulinate synthesis. In esterification reaction, the resin which prevents adsorption
of byproduct (water molecules) from its own surface performs better and offers
higher yield. Adsorption of water declines the catalytic activity and slows down the
reaction in the case of resins like Amberlyst and polystyrene-supported PTSA [77,
100]. Amberlyst-70 is found efficient among all macroporous Amberlyst catalysts
for levulinate synthesis and found to exhibit exceptional thermal stability and flexibility among all Amberlyst resins [76]. Use of Amberlyst-15 and Amberlyst-70
type of resins, in the reaction medium, is very simple and is less expensive than
other resin-based catalysts; hence, they are widely used. So far, inorganic ionexchange resins like tetravalent metal acid salts are not much explored for levulinic
acid esterification reaction.
Resins play a vital role in maintaining appropriate flexibility, hydrophobicity,
and porosity in reaction media. However, they could not withstand high boiling
stiffness and disintegrate at high temperatures. Moreover, high cost, sluggish thermostability, tendency to form H-bonds, and destabilization of active groups are
some of the major challenges associated with such materials [71, 101–103].
6.3 Zirconia
Zirconia (ZrO 2 ) is the metal oxide-based solid acid catalyst, specifically zirconium
metal, which of great interest as it possesses higher catalytic activity with typical
characteristic porosity. Such porosity provides easy access to the long-chain and
bulky molecules and facilitates smooth mass transport on the pore surfaces [104–
108]. It also possesses semiconducting nature, anticorrosion property, and reducing
nature [109]. Both Brønsted and Lewis acidities are available in zirconia [109, 110].
Various zirconia-based catalysts have been explored for the synthesis of levulinate
esters. Su et al. have synthesized zirconia-supported phosphotungstic heterogeneous solid acid catalyst which is utilized for the production of methyl levulinate
from LA. The yield of 99 mol% methyl levulinate was observed at 65 °C, 1:7 LA to
methanol ratio in 3-h reaction time [107].
Solid Acid-Catalyzed Esterification of Levulinic Acid for Production of Value-Added…
amount of cross-linker. Higher cross-linking causes more stiffness and lesser swelling. The gel type Dowex 50Xx2 affords higher yield than macroporous resins in
polar media (e.g., esterification of levulinic acid with alcohol) due to swelling which
causes enlargement of the pore size, recurrence of new pores, and subsequent
improvement in mass transfer of reactants and surface area. Trombettoni et al. [77]
used cationic sulfonated resins (Aquivion mP90, micronized pellets of perfluorosulfonic polymer) for esterification of levulinic acid with long-chain alcohols. Ramli
et al. [98] have made use of Amberlyst-15 resin as a solid acid catalyst for alkyl
levulinate compounds. SO 3 H and SO 4 functional groups present in Amberlyst-15
were found to play a crucial role in esterification of levulinic acid. Tejero et al. [76]
have carried out an esterification of levulinic acid with butanol by employing Dowex
50Wx2, a gel-type resin. Several other resin-based catalysts such as Amberlystbased resins, Purolite, Dowex, and polystyrene-supported PTSA are reported for
levulinate synthesis. In esterification reaction, the resin which prevents adsorption
of byproduct (water molecules) from its own surface performs better and offers
higher yield. Adsorption of water declines the catalytic activity and slows down the
reaction in the case of resins like Amberlyst and polystyrene-supported PTSA [77,
100]. Amberlyst-70 is found efficient among all macroporous Amberlyst catalysts
for levulinate synthesis and found to exhibit exceptional thermal stability and flexibility among all Amberlyst resins [76]. Use of Amberlyst-15 and Amberlyst-70
type of resins, in the reaction medium, is very simple and is less expensive than
other resin-based catalysts; hence, they are widely used. So far, inorganic ionexchange resins like tetravalent metal acid salts are not much explored for levulinic
acid esterification reaction.
Resins play a vital role in maintaining appropriate flexibility, hydrophobicity,
and porosity in reaction media. However, they could not withstand high boiling
stiffness and disintegrate at high temperatures. Moreover, high cost, sluggish thermostability, tendency to form H-bonds, and destabilization of active groups are
some of the major challenges associated with such materials [71, 101–103].
6.3 Zirconia
Zirconia (ZrO 2 ) is the metal oxide-based solid acid catalyst, specifically zirconium
metal, which of great interest as it possesses higher catalytic activity with typical
characteristic porosity. Such porosity provides easy access to the long-chain and
bulky molecules and facilitates smooth mass transport on the pore surfaces [104–
108]. It also possesses semiconducting nature, anticorrosion property, and reducing
nature [109]. Both Brønsted and Lewis acidities are available in zirconia [109, 110].
Various zirconia-based catalysts have been explored for the synthesis of levulinate
esters. Su et al. have synthesized zirconia-supported phosphotungstic heterogeneous solid acid catalyst which is utilized for the production of methyl levulinate
from LA. The yield of 99 mol% methyl levulinate was observed at 65 °C, 1:7 LA to
methanol ratio in 3-h reaction time [107].
Solid Acid-Catalyzed Esterification of Levulinic Acid for Production of Value-Added…