482
nucleophilic attack on the hydroxyl group of the MAG that is formed produces
DAG which again releases water as a by-product. Finally, the remaining oxygen
lone pair electrons of the last hydroxyl group in DAG experience a third nucleophilic attack to produce TAG followed by more water. Normally three moles of
water molecule are formed as a result of the consecutive esterification steps in the
formation of TAG.
5.2 Effects of Different Reaction Parameters: Reaction
Temperature, Catalyst Loading, Molar Ratio
and Reaction Time
In the presence of an acid catalyst, glycerol acetylation with acetic acid comprises a
three-step reversible equilibrium reaction from monoacetin to triacetin. The reaction insights and behaviour under optimum conditions have been thoroughly discussed by many researchers in order to develop thermodynamic and kinetic models.
By developing these models with the help of experimental and mechanistic data, the
consequences of various reaction parameters over reaction rate were demonstrated
[163]. Thermodynamic parameters like activation energy and Gibbs free energy
were estimated under different reaction conditions. Esterification of glycerol with
acetic acid pursues first-order rate law kinetics, and the mathematical expression
can be revealed by applying the Langmuir-Hinshelwood-Hougen-Watson model,
where the rate-determining step is the surface reaction. Further thermodynamic
investigation demonstrated that Gibbs free energies corresponding to the formation
of MAG and DAG were rather low than TAG which justifies the reduced selectivity
of TAG. However, if high acetic acid amount is used compared to glycerol, the reaction equilibrium can be shifted to obtain satisfactory value of TAG. The quantitative
relation between the reactant amount and the formed product amount can be easily
found out from the balanced equation of the three successive steps for esterification
of glycerol with acetic acid. However, this provides no information regarding the
reaction rate. Study on chemical kinetics reveals the rate of the reaction is dependent on various parameters which include reaction temperature, reactant concentrations and catalyst loading. This reveals the insights about the correct parameter that
should be subjected to modification in order to achieve high yield. The study conducted by Ghoreish et al. [164] revealed the increase in temperature from 50 to
110 °C increased the conversion of glycerol with high selectivity towards DAG and
TAG. According to their study, high temperature enhances the conversion of glycerol as well as DAG and TAG formation, whereas the MAG formation is enhanced
by low temperatures. Another study involving silicotungstic acids supported on zirconia revealed rising temperature (60–120 °C) increased conversion of glycerol
(54.4–100%) linearly with a gradual increment in DAG and TAG at the consumption of MAG under endothermic condition [149]. Khayoon et al. [153] investigated
yttrium-functionalized SBA-3 catalyst in the temperature range of 90 °C to 110 °C
K. Bhaduri et al.
nucleophilic attack on the hydroxyl group of the MAG that is formed produces
DAG which again releases water as a by-product. Finally, the remaining oxygen
lone pair electrons of the last hydroxyl group in DAG experience a third nucleophilic attack to produce TAG followed by more water. Normally three moles of
water molecule are formed as a result of the consecutive esterification steps in the
formation of TAG.
5.2 Effects of Different Reaction Parameters: Reaction
Temperature, Catalyst Loading, Molar Ratio
and Reaction Time
In the presence of an acid catalyst, glycerol acetylation with acetic acid comprises a
three-step reversible equilibrium reaction from monoacetin to triacetin. The reaction insights and behaviour under optimum conditions have been thoroughly discussed by many researchers in order to develop thermodynamic and kinetic models.
By developing these models with the help of experimental and mechanistic data, the
consequences of various reaction parameters over reaction rate were demonstrated
[163]. Thermodynamic parameters like activation energy and Gibbs free energy
were estimated under different reaction conditions. Esterification of glycerol with
acetic acid pursues first-order rate law kinetics, and the mathematical expression
can be revealed by applying the Langmuir-Hinshelwood-Hougen-Watson model,
where the rate-determining step is the surface reaction. Further thermodynamic
investigation demonstrated that Gibbs free energies corresponding to the formation
of MAG and DAG were rather low than TAG which justifies the reduced selectivity
of TAG. However, if high acetic acid amount is used compared to glycerol, the reaction equilibrium can be shifted to obtain satisfactory value of TAG. The quantitative
relation between the reactant amount and the formed product amount can be easily
found out from the balanced equation of the three successive steps for esterification
of glycerol with acetic acid. However, this provides no information regarding the
reaction rate. Study on chemical kinetics reveals the rate of the reaction is dependent on various parameters which include reaction temperature, reactant concentrations and catalyst loading. This reveals the insights about the correct parameter that
should be subjected to modification in order to achieve high yield. The study conducted by Ghoreish et al. [164] revealed the increase in temperature from 50 to
110 °C increased the conversion of glycerol with high selectivity towards DAG and
TAG. According to their study, high temperature enhances the conversion of glycerol as well as DAG and TAG formation, whereas the MAG formation is enhanced
by low temperatures. Another study involving silicotungstic acids supported on zirconia revealed rising temperature (60–120 °C) increased conversion of glycerol
(54.4–100%) linearly with a gradual increment in DAG and TAG at the consumption of MAG under endothermic condition [149]. Khayoon et al. [153] investigated
yttrium-functionalized SBA-3 catalyst in the temperature range of 90 °C to 110 °C
K. Bhaduri et al.