264
3 Current Challenges and Future Opportunities
In order to reduce the dependence on fossil fuels, large-scale production of fuels and
chemicals from lignocellulosic biomass is necessary. But this is very challenging
due to variations in supply, composition, and properties. Many technologies devised
at laboratory/pilot plant scale are technically viable but economically not feasible.
To make the process economically feasible, a multidisciplinary approach, including
chemistry, material science, and process engineering, is required. The efficiency of
various technologies and yield of the products depends on many factors such as type
of biomass, pretreatment method, biomass composition, catalyst activity, etc. In the
following section, the major concerns regarding technologies and the use of catalysts
for biomass conversion are discussed.
3.1 The Selectivity of Desirable Products
It is essential to tune the porosity and structure of the catalyst to selectively produce
the desired products. The pore dimension of the solid catalyst influences the
diffusion properties of the reacting species. Zeolites ordered mesoporous silica and
metal-organic frameworks (MOFs) are the materials that can be used as a catalyst
for biomass conversion. These materials also act as support for metals and metal
oxides with excellent stabilizing effects. One of the reasons for low conversion and
selectivity of complex and viscous biomass molecules is the inaccessibility of active
catalyst sites. The active sites are located inside the porous catalyst materials and
become difficult to access due to diffusional limitations. Due to this reason,
multimode porous materials are preferred over single-mode porous materials for
enhanced selectivity. For example, large biomass molecules (lignin, triglycerides,
polysaccharides) cannot access the catalytically active sites in zeolites due to small
pore diameters. In that case, catalyst materials with mixed pores (micro, meso, and
macro) such as hierarchical zeolites are useful. Zeolites offer numerous advantages,
such as acidity, porosity, and shape selectivity. Of these, tunable Brønsted/Lewis
acidity is a vital feature for biomass conversion and can influence conversion and
selectivity. Both Brønsted and Lewis acidities are required for effective biomass
conversion. For example, during the conversion of 2,5-dimethylfuran, zeolites with
strong Lewis acid sites (Na-Y) favored the formation of toluene, whereas Brønsted
acid zeolites (H-β and H-Y) favored p-xylene formation.
Another parameter that can affect the selectivity of the desired product is the
biomass to catalyst ratio. In fast catalytic pyrolysis of Pongamia pinnata seeds using
USY zeolites, more aromatic hydrocarbons were produced (4.7% to 52.5%) when
the ratio of biomass to catalyst increased for five times [2]. Mesoporous SiO 2 -based
materials (SBA, KIT, MSU, COK) offer similar properties to zeolites but with larger
pores. Larger pores can offer access to active catalyst sites, thereby enhancing the
biomass conversion. During the condensation of furfuryl alcohol to butyl levulinate,
R. Bhoi et al.
3 Current Challenges and Future Opportunities
In order to reduce the dependence on fossil fuels, large-scale production of fuels and
chemicals from lignocellulosic biomass is necessary. But this is very challenging
due to variations in supply, composition, and properties. Many technologies devised
at laboratory/pilot plant scale are technically viable but economically not feasible.
To make the process economically feasible, a multidisciplinary approach, including
chemistry, material science, and process engineering, is required. The efficiency of
various technologies and yield of the products depends on many factors such as type
of biomass, pretreatment method, biomass composition, catalyst activity, etc. In the
following section, the major concerns regarding technologies and the use of catalysts
for biomass conversion are discussed.
3.1 The Selectivity of Desirable Products
It is essential to tune the porosity and structure of the catalyst to selectively produce
the desired products. The pore dimension of the solid catalyst influences the
diffusion properties of the reacting species. Zeolites ordered mesoporous silica and
metal-organic frameworks (MOFs) are the materials that can be used as a catalyst
for biomass conversion. These materials also act as support for metals and metal
oxides with excellent stabilizing effects. One of the reasons for low conversion and
selectivity of complex and viscous biomass molecules is the inaccessibility of active
catalyst sites. The active sites are located inside the porous catalyst materials and
become difficult to access due to diffusional limitations. Due to this reason,
multimode porous materials are preferred over single-mode porous materials for
enhanced selectivity. For example, large biomass molecules (lignin, triglycerides,
polysaccharides) cannot access the catalytically active sites in zeolites due to small
pore diameters. In that case, catalyst materials with mixed pores (micro, meso, and
macro) such as hierarchical zeolites are useful. Zeolites offer numerous advantages,
such as acidity, porosity, and shape selectivity. Of these, tunable Brønsted/Lewis
acidity is a vital feature for biomass conversion and can influence conversion and
selectivity. Both Brønsted and Lewis acidities are required for effective biomass
conversion. For example, during the conversion of 2,5-dimethylfuran, zeolites with
strong Lewis acid sites (Na-Y) favored the formation of toluene, whereas Brønsted
acid zeolites (H-β and H-Y) favored p-xylene formation.
Another parameter that can affect the selectivity of the desired product is the
biomass to catalyst ratio. In fast catalytic pyrolysis of Pongamia pinnata seeds using
USY zeolites, more aromatic hydrocarbons were produced (4.7% to 52.5%) when
the ratio of biomass to catalyst increased for five times [2]. Mesoporous SiO 2 -based
materials (SBA, KIT, MSU, COK) offer similar properties to zeolites but with larger
pores. Larger pores can offer access to active catalyst sites, thereby enhancing the
biomass conversion. During the condensation of furfuryl alcohol to butyl levulinate,
R. Bhoi et al.