492
[189]. Similarly, pure SnO 2 showed lower glycerol conversion than the promoted
(molybdenum and tungsten) SnO 2 prepared by wet impregnation method [168].
Promotion of Mo and W induces surface area, acid sites, redox property as well as
lattice defect. Metal oxide can be prepared by thermal treatment to obtain better
physicochemical properties, but pure metal oxide suffers due to the agglomeration
or sintering of particles at higher temperature which leads to the formation of larger
crystallites. In the case of mixed oxide, the cooperative nature of the cations resists
the agglomeration and sintering process, and their mutual interactions can stop the
individual crystallization and particle growth which is responsible for obtaining
higher surface of mixed oxide than pure oxide. The incorporation of Mo and W
leads to generation of excess positive or negative on Sn-O-M (M = Mo, W) bond
which causes an improvement of acid amount into the SnO 2. Additionally, promoted
sample is mainly enriched with Brønsted acidic site rather than Lewis acidic sites;
that’s why it follows ketal mechanism and resulting formation of solketal as a selective product. A further improvement of glycerol conversion is observed when SO 4
2−
is added into the SnO 2 [170]. This observation could be due to the existence of
surface superacid sites along with improvement of surface area. However, the catalyst suffers during regeneration because of the loss of sulphur content which actually provides Brønsted acid sites and helps to improve surface area by decreasing
crystallite size.
Niobium-based catalyst is also active towards glycerol acetalization [190–192].
Recently, it has been found that the glycerol acetalization performance changes with
changing the niobium precursor for preparing same niobium oxyhydroxide catalyst
[191]. For the glycerol acetalization, three niobium oxyhydroxide catalysts can be
prepared with two different niobium precursor (ammonium niobium oxalate (NH 4 -
[NbO(C 2 O 4 ) 2 (H 2 O)](H 2 O)n) or niobium chloride (NbCl 5 )) and CTAB. It was
obtained that the ammonium niobium oxalate precursor has the larger CTAB incorporation capacity into the niobium oxyhydroxide than the niobium chloride precursor which causes reduction of surface area but improvement of the surface
hydrophobicity of the former precursor. The CTAB is likely to bind with surface
hydroxyl group resulting in decreasing of surface acidity of niobium oxyhydroxide
catalyst prepared from ammonium niobium oxalate or niobium chloride precursor
with CTAB. The niobium oxyhydroxide catalyst prepared without surfactant exhibits higher acidity than the CTAB-containing catalyst but suffers due to the lack of
hydrophobicity. So, acidity with hydrophobicity is essential for obtaining better
glycerol conversion. However, niobium oxyhydroxide catalyst prepared from niobium chloride precursor could not perform well despite having high surface area as
its hydrophobicity and acidity are lower than the catalyst prepared from ammonium
niobium oxalate precursor (Table 6).
Effect of various reaction parameters: Glycerol acetalization is an exothermic
reaction. A high temperature can shift the reaction equilibrium towards the reactant
side resulting in a decrease in glycerol conversion. So, moderate temperature (room
temperature to 353 K) is highly desirable for getting maximum conversion. On the
other hand, the yield of a product is highly dependent on the molar ratio of the reactants. As we know, the lower acetone concentration in acetalization favours the
K. Bhaduri et al.
[189]. Similarly, pure SnO 2 showed lower glycerol conversion than the promoted
(molybdenum and tungsten) SnO 2 prepared by wet impregnation method [168].
Promotion of Mo and W induces surface area, acid sites, redox property as well as
lattice defect. Metal oxide can be prepared by thermal treatment to obtain better
physicochemical properties, but pure metal oxide suffers due to the agglomeration
or sintering of particles at higher temperature which leads to the formation of larger
crystallites. In the case of mixed oxide, the cooperative nature of the cations resists
the agglomeration and sintering process, and their mutual interactions can stop the
individual crystallization and particle growth which is responsible for obtaining
higher surface of mixed oxide than pure oxide. The incorporation of Mo and W
leads to generation of excess positive or negative on Sn-O-M (M = Mo, W) bond
which causes an improvement of acid amount into the SnO 2. Additionally, promoted
sample is mainly enriched with Brønsted acidic site rather than Lewis acidic sites;
that’s why it follows ketal mechanism and resulting formation of solketal as a selective product. A further improvement of glycerol conversion is observed when SO 4
2−
is added into the SnO 2 [170]. This observation could be due to the existence of
surface superacid sites along with improvement of surface area. However, the catalyst suffers during regeneration because of the loss of sulphur content which actually provides Brønsted acid sites and helps to improve surface area by decreasing
crystallite size.
Niobium-based catalyst is also active towards glycerol acetalization [190–192].
Recently, it has been found that the glycerol acetalization performance changes with
changing the niobium precursor for preparing same niobium oxyhydroxide catalyst
[191]. For the glycerol acetalization, three niobium oxyhydroxide catalysts can be
prepared with two different niobium precursor (ammonium niobium oxalate (NH 4 -
[NbO(C 2 O 4 ) 2 (H 2 O)](H 2 O)n) or niobium chloride (NbCl 5 )) and CTAB. It was
obtained that the ammonium niobium oxalate precursor has the larger CTAB incorporation capacity into the niobium oxyhydroxide than the niobium chloride precursor which causes reduction of surface area but improvement of the surface
hydrophobicity of the former precursor. The CTAB is likely to bind with surface
hydroxyl group resulting in decreasing of surface acidity of niobium oxyhydroxide
catalyst prepared from ammonium niobium oxalate or niobium chloride precursor
with CTAB. The niobium oxyhydroxide catalyst prepared without surfactant exhibits higher acidity than the CTAB-containing catalyst but suffers due to the lack of
hydrophobicity. So, acidity with hydrophobicity is essential for obtaining better
glycerol conversion. However, niobium oxyhydroxide catalyst prepared from niobium chloride precursor could not perform well despite having high surface area as
its hydrophobicity and acidity are lower than the catalyst prepared from ammonium
niobium oxalate precursor (Table 6).
Effect of various reaction parameters: Glycerol acetalization is an exothermic
reaction. A high temperature can shift the reaction equilibrium towards the reactant
side resulting in a decrease in glycerol conversion. So, moderate temperature (room
temperature to 353 K) is highly desirable for getting maximum conversion. On the
other hand, the yield of a product is highly dependent on the molar ratio of the reactants. As we know, the lower acetone concentration in acetalization favours the
K. Bhaduri et al.