258
types of ILs, but only about 1000 types of ionic liquids have been synthesized to
date. The drawbacks of ionic liquids are high preparation cost, active phase leaching,
and poor recyclability that can be overcome by solid-phase ILs [5].
Depolymerization of cellulose is a critical step to obtain platform chemicals such
as fructose, glucose, xylose, etc. Cellulose is insoluble in most of the common
industrial solvents due to its high degree of crystallinity, thus hampering the catalytic
activity in the depolymerization step and demanding for harsh reaction conditions.
Due to this reason, depolymerization of cellulose to glucose under mild reaction
conditions still remains a challenge [70]. ILs are excellent solvents for cellulose
dissolution that can be helpful in the depolymerization of cellulose through
hydrolysis, alcoholysis, etc. The key advantages of ILs are efficient functionalization,
promising reusability, and lower amount requirement for the reaction. The cost of
functionalized ILs is a crucial factor that needs due consideration. Depending on the
solubility of biomass molecule in ILs, a careful selection of coupled catalyst-solvent
system is necessary. An understanding of molecular interaction between the
feedstock molecule, catalyst, and solvent is necessary for the development of new
catalytic systems. The post-reaction separation technologies for catalyst, solvent,
and biomass products are necessary. There is a limited success for the
depolymerization of lignin using ILs. Therefore, more opportunities are available
for research in this domain [71]. Even though the ILs have several advantages over
conventional catalysts, handling of ILs is difficult owing to its viscous nature. Also,
the leaching of ILs in reaction mixture limits their practical applications in biomass
conversion. To address these issues, researchers have combined ILs with solid
catalyst and developed a new class of functionalized catalyst, namely, “solid-state
ILs.” The ILs are deposited in the form of a film on the support material with a high
specific surface area, thus creating a homogeneous environment for reactions [5].
This way, the use of solid-state ILs is more practical in biomass conversion.
Supported IL-Based Catalysts
The ionic liquid immobilized on solid support offers several advantages than using
the ILs themselves. Both inorganic and organic materials can be used as supports.
Inorganic supports offer better thermal stability and low on cost and easy to prepare,
i.e., alumina and silica [72]. Various carbohydrates (cellulose, sucrose, glucose)
were converted to furans with a functional ILs supported on silica nanoparticles
with different acidity. The dual acid-functionalized supported ionic liquid (SIL)
(IL-SO 3 H-HSO 4 /SiO 2 ) was effective for high conversion (99%) of sugars. The
reusability of the ILs/SiO 2 was also investigated for dehydration of fructose with
IL-SO 3 H-HSO 4 /SiO 2 at the optimized conditions. Consistent conversion (99%) of
fructose was obtained for five reaction cycles that suggest no leaching of immobilized
functional groups (SO 3 H and –HSO 4 ) during the repeated process [73]. ILs
immobilized on mixed metal oxides, and silica gels are investigated for biodiesel
synthesis from waste cooking oil. The mixed oxide catalysts (Mg + Al), in its
pristine form, exhibited low activity for the conversion of waste cooking oil by
R. Bhoi et al.
types of ILs, but only about 1000 types of ionic liquids have been synthesized to
date. The drawbacks of ionic liquids are high preparation cost, active phase leaching,
and poor recyclability that can be overcome by solid-phase ILs [5].
Depolymerization of cellulose is a critical step to obtain platform chemicals such
as fructose, glucose, xylose, etc. Cellulose is insoluble in most of the common
industrial solvents due to its high degree of crystallinity, thus hampering the catalytic
activity in the depolymerization step and demanding for harsh reaction conditions.
Due to this reason, depolymerization of cellulose to glucose under mild reaction
conditions still remains a challenge [70]. ILs are excellent solvents for cellulose
dissolution that can be helpful in the depolymerization of cellulose through
hydrolysis, alcoholysis, etc. The key advantages of ILs are efficient functionalization,
promising reusability, and lower amount requirement for the reaction. The cost of
functionalized ILs is a crucial factor that needs due consideration. Depending on the
solubility of biomass molecule in ILs, a careful selection of coupled catalyst-solvent
system is necessary. An understanding of molecular interaction between the
feedstock molecule, catalyst, and solvent is necessary for the development of new
catalytic systems. The post-reaction separation technologies for catalyst, solvent,
and biomass products are necessary. There is a limited success for the
depolymerization of lignin using ILs. Therefore, more opportunities are available
for research in this domain [71]. Even though the ILs have several advantages over
conventional catalysts, handling of ILs is difficult owing to its viscous nature. Also,
the leaching of ILs in reaction mixture limits their practical applications in biomass
conversion. To address these issues, researchers have combined ILs with solid
catalyst and developed a new class of functionalized catalyst, namely, “solid-state
ILs.” The ILs are deposited in the form of a film on the support material with a high
specific surface area, thus creating a homogeneous environment for reactions [5].
This way, the use of solid-state ILs is more practical in biomass conversion.
Supported IL-Based Catalysts
The ionic liquid immobilized on solid support offers several advantages than using
the ILs themselves. Both inorganic and organic materials can be used as supports.
Inorganic supports offer better thermal stability and low on cost and easy to prepare,
i.e., alumina and silica [72]. Various carbohydrates (cellulose, sucrose, glucose)
were converted to furans with a functional ILs supported on silica nanoparticles
with different acidity. The dual acid-functionalized supported ionic liquid (SIL)
(IL-SO 3 H-HSO 4 /SiO 2 ) was effective for high conversion (99%) of sugars. The
reusability of the ILs/SiO 2 was also investigated for dehydration of fructose with
IL-SO 3 H-HSO 4 /SiO 2 at the optimized conditions. Consistent conversion (99%) of
fructose was obtained for five reaction cycles that suggest no leaching of immobilized
functional groups (SO 3 H and –HSO 4 ) during the repeated process [73]. ILs
immobilized on mixed metal oxides, and silica gels are investigated for biodiesel
synthesis from waste cooking oil. The mixed oxide catalysts (Mg + Al), in its
pristine form, exhibited low activity for the conversion of waste cooking oil by
R. Bhoi et al.