491
groups [14]. An oxidizing treatment with HNO 3 leads to increase in the population
of acid sites, which is much higher than the H 2 SO 4 -treated catalyst as HNO 3 is the
strong oxidizing agent. Notably, after this treatment, HNO 3 -treated catalyst is not
able to reach the acetalization performances shown by H 2 SO 4 -treated catalyst. The
better performances of H 2 SO 4 -treated catalyst are related to the generation of
strongly acidic −SO 3 H groups on the surface. The effect of acid treatment is more
pronounced when we move forwards to the higher concentration [14, 187]. The
HNO 3 treatment with high concentration is accompanied by the reduction of pore
volume and surface area along with formation of a large number of acid sites [14].
However, surface area and pore volume are preserved even after treatment with high
concentration of H 2 SO 4 with the generation of a greater number of −SO 3 H. Therefore,
H 2 SO 4 -treated carbon catalyst appeared as a better candidate for glycerol acetalization reaction.
The second factor which influences the glycerol acetalization is the hydrophobicity of the surface. Carbon surface is generally hydrophobic in nature, but incorporation of heteroatoms alters their hydrophobicity. As we know, the HNO 3
treatment forms a large number of oxygen functionalities whose concentration is
significantly higher than the H 2 SO 4 -treated sample; that’s why H 2 SO 4 -treated sample is more hydrophobic and responsible for the highest activity towards glycerol
acetalization through protecting acid sites from water. Despite having high amount
of strong acid (-SO 3 H) sites on the carbon surface, it won’t be able to perform well
in glycerol acetalization reaction as higher concentration of −SO 3 H group makes
the surface hydrophilic which favours the reverse reaction as well as diffusion of
water to the pores [14, 187]. Taking this into account, a balance is necessary in
between hydrophobicity and acidity for obtaining maximum glycerol conversion.
Metal oxide catalyst: The metal oxides are the most vibrant catalyst in the field
of heterogeneous catalysis as they cover majority of the reactions and serve as
prominent candidate since 1950. The oxide surface consists of defects and environments (kinks, steps, terraces), which control the catalytic performances. The salient
features for controlling catalytic activity are the point and extended defect structure,
crystalline phase, atomic compositions and electronic defects. Apart from this,
porous structure, electrical conductivity, thermal stability, diffusion acting on sintering and lattice oxygen anion mobility are also taken into consideration [188]. Solid
acid oxide catalyst is being used in petroleum industry when the green chemistry
concept is made to stop harmful effect of a chemical process on the environment. A
large number of solid acid oxide catalysts have been used for acetalization reaction
such as ZrO 2 , promoted ZrO 2 [189], promoted SnO 2 [168, 170], Nb 2 O 5 [190], modified niobium oxyhydroxide [191], Nb 2 O 5 -Al 2 O 3 [192] and phosphate [193] catalyst
as they are acidic as well as have excellent water tolerance power. When glycerol
acetalization is done over ZrO 2 and promoted ZrO 2 (WOx/ZrO 2 , MoOx/ZrO 2 ,
SO4
2−
/ZrO 2 ), glycerol conversion follows the order of SO4
2−
/ZrO 2 → MoOx/
ZrO 2 → WOx/ZrO 2 → ZrO 2 [189]. The highest activity may be due to the highest
surface area and large number of available acid sites. It has also been noticed that all
promoted ZrO 2 displayed >80% glycerol conversion, unlike the case of ZrO 2 (10%).
This may be due to the greater number of acid sites on the promoted ZrO 2 catalyst
Catalytic Conversion of Biomass-Derived Glycerol to Value-Added Chemicals
groups [14]. An oxidizing treatment with HNO 3 leads to increase in the population
of acid sites, which is much higher than the H 2 SO 4 -treated catalyst as HNO 3 is the
strong oxidizing agent. Notably, after this treatment, HNO 3 -treated catalyst is not
able to reach the acetalization performances shown by H 2 SO 4 -treated catalyst. The
better performances of H 2 SO 4 -treated catalyst are related to the generation of
strongly acidic −SO 3 H groups on the surface. The effect of acid treatment is more
pronounced when we move forwards to the higher concentration [14, 187]. The
HNO 3 treatment with high concentration is accompanied by the reduction of pore
volume and surface area along with formation of a large number of acid sites [14].
However, surface area and pore volume are preserved even after treatment with high
concentration of H 2 SO 4 with the generation of a greater number of −SO 3 H. Therefore,
H 2 SO 4 -treated carbon catalyst appeared as a better candidate for glycerol acetalization reaction.
The second factor which influences the glycerol acetalization is the hydrophobicity of the surface. Carbon surface is generally hydrophobic in nature, but incorporation of heteroatoms alters their hydrophobicity. As we know, the HNO 3
treatment forms a large number of oxygen functionalities whose concentration is
significantly higher than the H 2 SO 4 -treated sample; that’s why H 2 SO 4 -treated sample is more hydrophobic and responsible for the highest activity towards glycerol
acetalization through protecting acid sites from water. Despite having high amount
of strong acid (-SO 3 H) sites on the carbon surface, it won’t be able to perform well
in glycerol acetalization reaction as higher concentration of −SO 3 H group makes
the surface hydrophilic which favours the reverse reaction as well as diffusion of
water to the pores [14, 187]. Taking this into account, a balance is necessary in
between hydrophobicity and acidity for obtaining maximum glycerol conversion.
Metal oxide catalyst: The metal oxides are the most vibrant catalyst in the field
of heterogeneous catalysis as they cover majority of the reactions and serve as
prominent candidate since 1950. The oxide surface consists of defects and environments (kinks, steps, terraces), which control the catalytic performances. The salient
features for controlling catalytic activity are the point and extended defect structure,
crystalline phase, atomic compositions and electronic defects. Apart from this,
porous structure, electrical conductivity, thermal stability, diffusion acting on sintering and lattice oxygen anion mobility are also taken into consideration [188]. Solid
acid oxide catalyst is being used in petroleum industry when the green chemistry
concept is made to stop harmful effect of a chemical process on the environment. A
large number of solid acid oxide catalysts have been used for acetalization reaction
such as ZrO 2 , promoted ZrO 2 [189], promoted SnO 2 [168, 170], Nb 2 O 5 [190], modified niobium oxyhydroxide [191], Nb 2 O 5 -Al 2 O 3 [192] and phosphate [193] catalyst
as they are acidic as well as have excellent water tolerance power. When glycerol
acetalization is done over ZrO 2 and promoted ZrO 2 (WOx/ZrO 2 , MoOx/ZrO 2 ,
SO4
2−
/ZrO 2 ), glycerol conversion follows the order of SO4
2−
/ZrO 2 → MoOx/
ZrO 2 → WOx/ZrO 2 → ZrO 2 [189]. The highest activity may be due to the highest
surface area and large number of available acid sites. It has also been noticed that all
promoted ZrO 2 displayed >80% glycerol conversion, unlike the case of ZrO 2 (10%).
This may be due to the greater number of acid sites on the promoted ZrO 2 catalyst
Catalytic Conversion of Biomass-Derived Glycerol to Value-Added Chemicals