363
reagent is covalently linked to solid support [62], the product can be easily separated from the reaction mixture. Usually, porous support is preferable as it provides
higher surface area. Thus, insoluble support provides powerful means to facilitate
synthesis. There are several examples of some reactions which are difficult to carry
out in homogeneous medium or offer very poor yields which can be effectively carried out with solid-supported reagents under mild reaction conditions with high
selectivity.
Silica gel-supported reagents are reported to recover valuable scaffolds [62].
Silica is a low-cost significant inorganic porous solid which is the widely accepted
support due to its high surface area and chemical and mechanical stability and the
feasibility of surface functionalization [29, 100, 125–129]. Moreover, they possess
excellent mechanical stability. Pasquale et al. [29] performed a study of the catalytic
activity toward synthesis of ethyl levulinate via esterification of levulinic acid with
ethanol over silica-based catalyst. Yang and coworkers [129] studied the synthesis
of alkyl levulinates over perchloric acid-decorated nano-porous silica. Enumula
et al. demonstrated [125] the use of tungsten oxide-based silica catalyst in alkyl
levulinate under continuous mode vapor-phase conditions. Malero et al. [127] performed the synthesis of ethyl levulinates over sulfonic mesoporous silica catalyst.
Furthermore, they could reuse the catalyst up to three runs without its regeneration.
Ramli et al. [100] carried out the synthesis of methyl levulinate and obtained 69%
yield over sulfated silica under mild conditions. Chermahini and Nazeri [128] used
Al-MCM-41 as the solid acid catalyst for the synthesis of isobutyl and butyl levulinates. Maggi et al. [126] have successfully utilized silica-supported sulfonic acid
catalysts for ethyl levulinate and achieved excellent yield and selectivity for ethyl
levulinate. Tungsten oxide-based SBA-16 silica catalyst was found to exhibit better
acidity and afforded subsequent yield of levulinate ester, owing to dispersion of
dynamic acidic sites of tungsten oxide over larger surface area of SBA-16 [125].
The authors concluded that sulfonic acid mesostructured silica exhibits promising
catalytic features such as higher surface area and higher hydrophobicity which lead
to higher activity and better recyclability than various homogeneous sulfonic acid
catalysts under study (H 2 SO 4 , PTSA, propylsulfonic acid). It is noteworthy to mention that the silica-based catalyst leads to form the H– bonding in polar solvent (like
alcohol in esterification of levulinic acid with alcohol), leading to dissolution of the
active functionality of silica-supported catalyst, consequently causing decrease in
activity and reusability of the catalyst [29].
Heteropolyacids (HPAs) are well-known as acid catalysts in view of their significant high acidity, redox properties, catalytic activity, pseudo-liquid behavior, and
physicochemical properties [130]. HPAs are well-known for its Brønsted acidity.
Especially, in organic media, the molar catalytic activity of HPAs is extremely
higher as compared to mineral acid like sulfuric acid [131–133]. This feature makes
possible to carry out the catalytic reaction through “chemie douce” approach, i.e., a
soft chemical route involving lower catalyst concentration and/or at a lower temperature. Moreover, no side reactions occur during HPAs-assisted sulfonation, chlorination, nitration, etc. which are obvious with mineral acids [131, 132]. However,
these solid acids are lacking in terms of surface area, thermal stability, uneven
Solid Acid-Catalyzed Esterification of Levulinic Acid for Production of Value-Added…
reagent is covalently linked to solid support [62], the product can be easily separated from the reaction mixture. Usually, porous support is preferable as it provides
higher surface area. Thus, insoluble support provides powerful means to facilitate
synthesis. There are several examples of some reactions which are difficult to carry
out in homogeneous medium or offer very poor yields which can be effectively carried out with solid-supported reagents under mild reaction conditions with high
selectivity.
Silica gel-supported reagents are reported to recover valuable scaffolds [62].
Silica is a low-cost significant inorganic porous solid which is the widely accepted
support due to its high surface area and chemical and mechanical stability and the
feasibility of surface functionalization [29, 100, 125–129]. Moreover, they possess
excellent mechanical stability. Pasquale et al. [29] performed a study of the catalytic
activity toward synthesis of ethyl levulinate via esterification of levulinic acid with
ethanol over silica-based catalyst. Yang and coworkers [129] studied the synthesis
of alkyl levulinates over perchloric acid-decorated nano-porous silica. Enumula
et al. demonstrated [125] the use of tungsten oxide-based silica catalyst in alkyl
levulinate under continuous mode vapor-phase conditions. Malero et al. [127] performed the synthesis of ethyl levulinates over sulfonic mesoporous silica catalyst.
Furthermore, they could reuse the catalyst up to three runs without its regeneration.
Ramli et al. [100] carried out the synthesis of methyl levulinate and obtained 69%
yield over sulfated silica under mild conditions. Chermahini and Nazeri [128] used
Al-MCM-41 as the solid acid catalyst for the synthesis of isobutyl and butyl levulinates. Maggi et al. [126] have successfully utilized silica-supported sulfonic acid
catalysts for ethyl levulinate and achieved excellent yield and selectivity for ethyl
levulinate. Tungsten oxide-based SBA-16 silica catalyst was found to exhibit better
acidity and afforded subsequent yield of levulinate ester, owing to dispersion of
dynamic acidic sites of tungsten oxide over larger surface area of SBA-16 [125].
The authors concluded that sulfonic acid mesostructured silica exhibits promising
catalytic features such as higher surface area and higher hydrophobicity which lead
to higher activity and better recyclability than various homogeneous sulfonic acid
catalysts under study (H 2 SO 4 , PTSA, propylsulfonic acid). It is noteworthy to mention that the silica-based catalyst leads to form the H– bonding in polar solvent (like
alcohol in esterification of levulinic acid with alcohol), leading to dissolution of the
active functionality of silica-supported catalyst, consequently causing decrease in
activity and reusability of the catalyst [29].
Heteropolyacids (HPAs) are well-known as acid catalysts in view of their significant high acidity, redox properties, catalytic activity, pseudo-liquid behavior, and
physicochemical properties [130]. HPAs are well-known for its Brønsted acidity.
Especially, in organic media, the molar catalytic activity of HPAs is extremely
higher as compared to mineral acid like sulfuric acid [131–133]. This feature makes
possible to carry out the catalytic reaction through “chemie douce” approach, i.e., a
soft chemical route involving lower catalyst concentration and/or at a lower temperature. Moreover, no side reactions occur during HPAs-assisted sulfonation, chlorination, nitration, etc. which are obvious with mineral acids [131, 132]. However,
these solid acids are lacking in terms of surface area, thermal stability, uneven
Solid Acid-Catalyzed Esterification of Levulinic Acid for Production of Value-Added…