483
and found out conversion of glycerol increased from 65% to 100% at 110 °C where
no further increment in conversion could be observed above 110 °C. However, temperature upsurge had no effect on the yield of TAG. This finding is a clear indication
that equilibrium is attained and optimum temperature is reached. In another study
over polymer-supported WO 3 catalyst, it showed increasing temperature (50–110 °C)
increased conversion of glycerol (58.8–98%) along with TAG selectivity (5–70%).
Based on the catalyst system, the optimum temperature should be taken, for example, ion-exchange resins are not much thermally stable at temperatures above 120 °C
and are prone to be deactivated which results in low catalytic activity [142]. Based
on the above investigations, we can summarize that with increasing temperature,
rate also increases, that is, the glycerol conversion increases with gradual upsurge in
DAG and TAG selectivity at the cost of MAG.
Different investigations showed increment in catalyst loading facilitates the acetylation of MAG and DAG to TAG. This probably may have emerged from the increment in number of accessible active acid sites in reaction mixture. In the glycerol
esterification with acetic acid over sulphated activated carbon, it was observed that
an increase in catalyst loading (0.2–0.8 g) causes linear increment in conversion of
glycerol and selectivity of TAG. Further increment in catalyst amount resulted in no
significant surge in conversion or selectivity which indicates the saturation. In a
similar way, in another investigation involving zirconia-supported silicotungstic
acid, it showed that on increasing the amount of catalyst from 0.1 to 0.2 g, conversion of glycerol escalated from 97.7% to 100% and on further increasing the amount
of catalyst (0.3 g), it resulted in higher selectivity for DAG and TAG. However, catalyst loading over 0.3 g exhibited no significant effect on either conversion or selectivity. According to Zhu et al. [149] and Ghoreishi et al. [165], acetylation does not
mainly depend on catalyst as the production of MAG can equally proceed without
the presence of catalyst. Therefore, an increased catalyst amount in general promotes higher selectivity towards DAG and TAG. This also confirms the fact that a
three-step glycerol esterification actually takes place.
The molar ratio of the reactants is an influencing parameter for this reaction.
From the equation stoichiometry, it is evident that 3 mol of acetic acid is needed to
react completely with 1 mol glycerol for the production of TAG. So the excess
amount of acetic acid facilitates DAG and TAG formation. Acetic acid in excessive
amount moves the equilibrium towards higher selectivity of DAG and TAG. In an
observation by Popova et al. [166], excess glycerol to acetic acid molar ratio (1:10)
increased the DAG and TAG selectivity and lowered the reaction time for reaching
equilibrium. While in other reports involving Amberlyst resin, it was showed that
high amount of acetic acid lags the formation of product and reaction equilibrium
[166]. Mufrodi et al. [167] showed that the conversion of glycerol increased from
0.28% to 98.5% upon increasing the molar ratio of glycerol to acetic from 1:1 to 1:6
accompanied by increase in selectivity for DAG and TAG at the price of MAG. All
these studies suggest that the increase in amount of acetic acid increases conversion
of glycerol as well as formation of acetyl ester.
The influence on reaction time was reviewed to investigate the time for reaching
equilibrium. According to Ghoreishi et al. [165], there is a linear relationship
Catalytic Conversion of Biomass-Derived Glycerol to Value-Added Chemicals
and found out conversion of glycerol increased from 65% to 100% at 110 °C where
no further increment in conversion could be observed above 110 °C. However, temperature upsurge had no effect on the yield of TAG. This finding is a clear indication
that equilibrium is attained and optimum temperature is reached. In another study
over polymer-supported WO 3 catalyst, it showed increasing temperature (50–110 °C)
increased conversion of glycerol (58.8–98%) along with TAG selectivity (5–70%).
Based on the catalyst system, the optimum temperature should be taken, for example, ion-exchange resins are not much thermally stable at temperatures above 120 °C
and are prone to be deactivated which results in low catalytic activity [142]. Based
on the above investigations, we can summarize that with increasing temperature,
rate also increases, that is, the glycerol conversion increases with gradual upsurge in
DAG and TAG selectivity at the cost of MAG.
Different investigations showed increment in catalyst loading facilitates the acetylation of MAG and DAG to TAG. This probably may have emerged from the increment in number of accessible active acid sites in reaction mixture. In the glycerol
esterification with acetic acid over sulphated activated carbon, it was observed that
an increase in catalyst loading (0.2–0.8 g) causes linear increment in conversion of
glycerol and selectivity of TAG. Further increment in catalyst amount resulted in no
significant surge in conversion or selectivity which indicates the saturation. In a
similar way, in another investigation involving zirconia-supported silicotungstic
acid, it showed that on increasing the amount of catalyst from 0.1 to 0.2 g, conversion of glycerol escalated from 97.7% to 100% and on further increasing the amount
of catalyst (0.3 g), it resulted in higher selectivity for DAG and TAG. However, catalyst loading over 0.3 g exhibited no significant effect on either conversion or selectivity. According to Zhu et al. [149] and Ghoreishi et al. [165], acetylation does not
mainly depend on catalyst as the production of MAG can equally proceed without
the presence of catalyst. Therefore, an increased catalyst amount in general promotes higher selectivity towards DAG and TAG. This also confirms the fact that a
three-step glycerol esterification actually takes place.
The molar ratio of the reactants is an influencing parameter for this reaction.
From the equation stoichiometry, it is evident that 3 mol of acetic acid is needed to
react completely with 1 mol glycerol for the production of TAG. So the excess
amount of acetic acid facilitates DAG and TAG formation. Acetic acid in excessive
amount moves the equilibrium towards higher selectivity of DAG and TAG. In an
observation by Popova et al. [166], excess glycerol to acetic acid molar ratio (1:10)
increased the DAG and TAG selectivity and lowered the reaction time for reaching
equilibrium. While in other reports involving Amberlyst resin, it was showed that
high amount of acetic acid lags the formation of product and reaction equilibrium
[166]. Mufrodi et al. [167] showed that the conversion of glycerol increased from
0.28% to 98.5% upon increasing the molar ratio of glycerol to acetic from 1:1 to 1:6
accompanied by increase in selectivity for DAG and TAG at the price of MAG. All
these studies suggest that the increase in amount of acetic acid increases conversion
of glycerol as well as formation of acetyl ester.
The influence on reaction time was reviewed to investigate the time for reaching
equilibrium. According to Ghoreishi et al. [165], there is a linear relationship
Catalytic Conversion of Biomass-Derived Glycerol to Value-Added Chemicals