358
intermediates [67, 68]. Khiratkar et al. used ionic liquid-based phase-transfer catalyst (sulfonic acid-functionalized benzimidazolium-based poly-IL) for the synthesis
of ethyl levulinate, while Bhanage and coworkers have used imidazole-based IL
(1-methyl-imidazolium hydrogen sulfate) as both a catalyst and a solvent for the
synthesis of levulinates [69].
It is worthy to mention that homogeneous catalysis shows excellent catalytic
activity in esterification reaction. However, major drawbacks associated with such
homogeneous system are difficulty in isolation, corrosiveness of reactor, high reaction temperature, waste disposal, toxicity, non-reusability, non-regeneration, etc.
Such practical difficulties necessitate the need for replacement of such conventional
homogeneous toxic and corrosive acid catalysts with recyclable strong solid acid
(heterogeneous) catalysts in synthetic organic reactions. Thus, the development of
cost-effective and robust solid acid catalyst via energy-efficient synthetic protocols
to improve the yield of levulinic esters is currently of great research interest.
6 Solid Acids as Catalysts for Esterification of Levulinic Acid
According to the name, the solid acid catalysts are the solid materials which work
similar to the liquid catalyst. They provide active surface having active hydrogen
ions and act as a hydrogen donor to produce reaction intermediates, which is further
converted to the desired products by giving back the hydrogen ions [70]. The important properties of solid acid catalysts consist of their acidity in terms of Brønsted
and Lewis, the strength of these acid sites in terms of NH 3 -TPD, and the surface
properties, i.e., morphology, surface area, and pore size [71].
Reactions of LA with various alcohols using several solid acid catalysts have
been reported, such as various metal oxide (SnO 2 , ZrO 2 , Nb 2 O 5 , TiO 2 , etc.)-supported catalysts, various zeolites (HBEA, HZSM-5, H-Y, HMOR, MCM-22, etc.)
(Scheme 8), metal organic frameworks, ion exchange resins, graphene oxides, etc.
[72–77]. The subsequent text covers discussion on major class of heterogeneous
catalysts that are currently in practice for esterification of levulinic acid.
6.1 Metal Organic Frameworks (MOFs)
Porous materials have grabbed significant levels in modern science and technology.
They are strategic materials and have contributed significantly in the area of petroleum, catalysis, gas separation, or nuclear storage. Currently, intense research has
been generated for the discovery of new porous materials with larger pores, in order
to cope up with the demand of industry for obtaining special dimensions and shapes.
Metal organic frameworks, abbreviated as MOFs, are such collection of compounds
which contain metal ions linked through molecular species, which first appeared in
the literature in 1997. However, co-ordinatively bridged metal centers with organic
K. C. Maheria et al.
intermediates [67, 68]. Khiratkar et al. used ionic liquid-based phase-transfer catalyst (sulfonic acid-functionalized benzimidazolium-based poly-IL) for the synthesis
of ethyl levulinate, while Bhanage and coworkers have used imidazole-based IL
(1-methyl-imidazolium hydrogen sulfate) as both a catalyst and a solvent for the
synthesis of levulinates [69].
It is worthy to mention that homogeneous catalysis shows excellent catalytic
activity in esterification reaction. However, major drawbacks associated with such
homogeneous system are difficulty in isolation, corrosiveness of reactor, high reaction temperature, waste disposal, toxicity, non-reusability, non-regeneration, etc.
Such practical difficulties necessitate the need for replacement of such conventional
homogeneous toxic and corrosive acid catalysts with recyclable strong solid acid
(heterogeneous) catalysts in synthetic organic reactions. Thus, the development of
cost-effective and robust solid acid catalyst via energy-efficient synthetic protocols
to improve the yield of levulinic esters is currently of great research interest.
6 Solid Acids as Catalysts for Esterification of Levulinic Acid
According to the name, the solid acid catalysts are the solid materials which work
similar to the liquid catalyst. They provide active surface having active hydrogen
ions and act as a hydrogen donor to produce reaction intermediates, which is further
converted to the desired products by giving back the hydrogen ions [70]. The important properties of solid acid catalysts consist of their acidity in terms of Brønsted
and Lewis, the strength of these acid sites in terms of NH 3 -TPD, and the surface
properties, i.e., morphology, surface area, and pore size [71].
Reactions of LA with various alcohols using several solid acid catalysts have
been reported, such as various metal oxide (SnO 2 , ZrO 2 , Nb 2 O 5 , TiO 2 , etc.)-supported catalysts, various zeolites (HBEA, HZSM-5, H-Y, HMOR, MCM-22, etc.)
(Scheme 8), metal organic frameworks, ion exchange resins, graphene oxides, etc.
[72–77]. The subsequent text covers discussion on major class of heterogeneous
catalysts that are currently in practice for esterification of levulinic acid.
6.1 Metal Organic Frameworks (MOFs)
Porous materials have grabbed significant levels in modern science and technology.
They are strategic materials and have contributed significantly in the area of petroleum, catalysis, gas separation, or nuclear storage. Currently, intense research has
been generated for the discovery of new porous materials with larger pores, in order
to cope up with the demand of industry for obtaining special dimensions and shapes.
Metal organic frameworks, abbreviated as MOFs, are such collection of compounds
which contain metal ions linked through molecular species, which first appeared in
the literature in 1997. However, co-ordinatively bridged metal centers with organic
K. C. Maheria et al.