248
wood is hydrolyzed using H 2 SO 4 and then hydrogenated over metal Ru and Pt catalyst in an integrated process. The process was carried out at 433 K [18].
2.2 Advanced Heterogeneous Catalysts
for Biomass Conversion
Heterogeneous catalysts are characterized by size, shape, and porosity. The nanosize metal catalyst shows improved properties such as selectivity, durability, reusability, and other such properties compared to their bulk counterparts [19]. This is
beneficial for biomass conversion as it results in improved interaction between
active sites and reacting molecules. The porosity of the solid catalyst is an important
tunable property that can influence the product selectivity. The widely used porous
catalysts for biomass processing are zeolites and silica. Multipore catalysts (micro,
meso, and macro) are preferred over monopore catalyst to overcome certain drawbacks of monopore catalyst such as mass transfer limitation resulting in poor selectivity, catalyst deactivation, and low conversion.
2.2.1 Zeolites, Silica, and Metal Oxides
Porous materials are classified as micropore (diameter up to 2 nm), mesopore
(diameter between 2 and 50 nm), and macropore (diameter above 50 nm). Zeolites
are microporous materials with tunable Brønsted and Lewis acidity that makes them
suitable candidates for biomass conversion. For example, sugarcane bagasse has
been fast pyrolyzed to aromatics (yield 12.4%) and olefins (yield 10.9%) using
zeolite catalysts. Also, HZSM zeolites are used for catalytic hydrolysis of furfuryl
alcohol to levulinic acid with a 70% yield. Zeolites are also used for various
conversion processes such as upgrading glycerol to valuable chemicals, aromatic
hydrocarbons from furans, phenolic components from lignin, etc. [2]. Zeolites as
support can accommodate various metals, and their oxides and can be used in
application such as biomass pyrolysis (Fe/HZSM-5), glycerol upgradation (Sn/
HZSM-5), and production of other biomass-derived components (Pt/(NH 4 -USY).
Zeolite suffers from mass transfer limitations due to its microporous nature. This
can be overcome by the use of silica that has a mesoporous structure. Silica can
accommodate different functional groups and has a high BET surface area
(400–1400 m
2
/g), tunable pore diameters, and optimized acid sites [20]. Therefore,
the mesoporous silica catalysts are ideal candidates for the conversion of bulky
biomass molecules. For example, glucose is converted to 5-hydroxymethylfurfural
(5-HMF) over SiO 2 -Al 2 O 3 catalyst. Similarly, lignin can be converted to aromatic
hydrocarbon using silica-based catalysts [21]. Aluminum-doped silica catalyst with
varying silica-alumina ratio was used for the conversion of lignin to aromatics. For
example, Al-SBA-15 (Si/Al = 50) was used for pyrolysis of corncob to selectively
R. Bhoi et al.
wood is hydrolyzed using H 2 SO 4 and then hydrogenated over metal Ru and Pt catalyst in an integrated process. The process was carried out at 433 K [18].
2.2 Advanced Heterogeneous Catalysts
for Biomass Conversion
Heterogeneous catalysts are characterized by size, shape, and porosity. The nanosize metal catalyst shows improved properties such as selectivity, durability, reusability, and other such properties compared to their bulk counterparts [19]. This is
beneficial for biomass conversion as it results in improved interaction between
active sites and reacting molecules. The porosity of the solid catalyst is an important
tunable property that can influence the product selectivity. The widely used porous
catalysts for biomass processing are zeolites and silica. Multipore catalysts (micro,
meso, and macro) are preferred over monopore catalyst to overcome certain drawbacks of monopore catalyst such as mass transfer limitation resulting in poor selectivity, catalyst deactivation, and low conversion.
2.2.1 Zeolites, Silica, and Metal Oxides
Porous materials are classified as micropore (diameter up to 2 nm), mesopore
(diameter between 2 and 50 nm), and macropore (diameter above 50 nm). Zeolites
are microporous materials with tunable Brønsted and Lewis acidity that makes them
suitable candidates for biomass conversion. For example, sugarcane bagasse has
been fast pyrolyzed to aromatics (yield 12.4%) and olefins (yield 10.9%) using
zeolite catalysts. Also, HZSM zeolites are used for catalytic hydrolysis of furfuryl
alcohol to levulinic acid with a 70% yield. Zeolites are also used for various
conversion processes such as upgrading glycerol to valuable chemicals, aromatic
hydrocarbons from furans, phenolic components from lignin, etc. [2]. Zeolites as
support can accommodate various metals, and their oxides and can be used in
application such as biomass pyrolysis (Fe/HZSM-5), glycerol upgradation (Sn/
HZSM-5), and production of other biomass-derived components (Pt/(NH 4 -USY).
Zeolite suffers from mass transfer limitations due to its microporous nature. This
can be overcome by the use of silica that has a mesoporous structure. Silica can
accommodate different functional groups and has a high BET surface area
(400–1400 m
2
/g), tunable pore diameters, and optimized acid sites [20]. Therefore,
the mesoporous silica catalysts are ideal candidates for the conversion of bulky
biomass molecules. For example, glucose is converted to 5-hydroxymethylfurfural
(5-HMF) over SiO 2 -Al 2 O 3 catalyst. Similarly, lignin can be converted to aromatic
hydrocarbon using silica-based catalysts [21]. Aluminum-doped silica catalyst with
varying silica-alumina ratio was used for the conversion of lignin to aromatics. For
example, Al-SBA-15 (Si/Al = 50) was used for pyrolysis of corncob to selectively
R. Bhoi et al.