480
(MAG), diacetin (DAG) and triacetin (TAG). Among the various uses of these products, MAG is normally used in cryogenics (resists freezing), solvents in dye industry, raw material for biodegradable polyester production, explosive production,
humectant to retain moisture in cosmetics, etc. DAG is used as solvent and plasticizers, softening agents, raw material for biodegradable polyester production, cryogenics processes, etc. TAG is primely used as additive for fuel blend which improves
the efficiency for burning. Blending of TAG with biodiesel improves the performance of direct-injection diesel engine. Apart from the above important use, it is
also used as antimicrobial agent and emulsifying agent in pharmaceuticals [138,
141]. In terms of acylation agent, acetic acid (AcOH) and acetic anhydride (Ac 2 O)
have been widely used. For the production of TAG, Ac 2 O is a more thermodynamically preferable alternative than AcOH as the reaction between glycerol and Ac 2 O
goes ahead exothermally and results in negative Gibbs free energy, while the reverse
is observed for acetylation of glycerol with AcOH. The acetylation of glycerol with
Ac 2 O over zeolite beta and K-montmorillonite can yield 100% TAG under mild
reaction conditions [142]. However, the use of Ac 2 O generates elevated heat of reaction, sometimes violent reactions, and apart from this, no effective changes can be
seen for thermodynamic parameters even at different ambient temperature conditions. Furthermore, Ac 2 O is used as a raw material to produce narcotics which
restricts its use in many countries. So the use of Ac 2 O as an acylation agent has
become costlier and tough in comparison to AcOH which is cheap and readily available. Along with this, exclusively synthesizing TAG cuts down the production of
other two analogues (MAG and DAG) that have important application to industry.
Esterification is generally carried out involving mineral acids (homogeneous
catalyst) to structure glycerol esters (MAG, DAG and TAG). These mineral acids
like hydrofluoric acid, hydrochloric acid and sulphuric acid are highly toxic, corrosive and hazardous, and their usage brings in recovery difficulties and high-energy
multistep processes [143]. The disposal of these acids is another challenging task
which might lead to environmental problems. So the use of friendly heterogeneous
catalysts is more justified as they can be designed accordingly to attain high yield
and effortless regeneration. The catalyst systems used for glycerol esterification
include heteropolyacids, mixed oxides, ion-exchange resins, ionic liquids, acidfunctionalized zeolites, carbon-based catalysts, etc. The catalytic activity of various
catalysts employed for glycerol esterification with acetic acid is summarized in
Table 4.
5.1 Route of the Esterification of Glycerol with Acetic Acid
The esterification of glycerol with acetic acid is a reversible equilibrium reaction for
synthesizing acetylated esters which proceeds in three successive steps. Firstly, the
protons present within the catalyst get attached to the acetic acid oxygen lone pair
electron, and then nucleophilic attack of the hydroxyl group takes place by electrophiles which releases water to form MAG. Secondly more acetylation by a second
K. Bhaduri et al.
(MAG), diacetin (DAG) and triacetin (TAG). Among the various uses of these products, MAG is normally used in cryogenics (resists freezing), solvents in dye industry, raw material for biodegradable polyester production, explosive production,
humectant to retain moisture in cosmetics, etc. DAG is used as solvent and plasticizers, softening agents, raw material for biodegradable polyester production, cryogenics processes, etc. TAG is primely used as additive for fuel blend which improves
the efficiency for burning. Blending of TAG with biodiesel improves the performance of direct-injection diesel engine. Apart from the above important use, it is
also used as antimicrobial agent and emulsifying agent in pharmaceuticals [138,
141]. In terms of acylation agent, acetic acid (AcOH) and acetic anhydride (Ac 2 O)
have been widely used. For the production of TAG, Ac 2 O is a more thermodynamically preferable alternative than AcOH as the reaction between glycerol and Ac 2 O
goes ahead exothermally and results in negative Gibbs free energy, while the reverse
is observed for acetylation of glycerol with AcOH. The acetylation of glycerol with
Ac 2 O over zeolite beta and K-montmorillonite can yield 100% TAG under mild
reaction conditions [142]. However, the use of Ac 2 O generates elevated heat of reaction, sometimes violent reactions, and apart from this, no effective changes can be
seen for thermodynamic parameters even at different ambient temperature conditions. Furthermore, Ac 2 O is used as a raw material to produce narcotics which
restricts its use in many countries. So the use of Ac 2 O as an acylation agent has
become costlier and tough in comparison to AcOH which is cheap and readily available. Along with this, exclusively synthesizing TAG cuts down the production of
other two analogues (MAG and DAG) that have important application to industry.
Esterification is generally carried out involving mineral acids (homogeneous
catalyst) to structure glycerol esters (MAG, DAG and TAG). These mineral acids
like hydrofluoric acid, hydrochloric acid and sulphuric acid are highly toxic, corrosive and hazardous, and their usage brings in recovery difficulties and high-energy
multistep processes [143]. The disposal of these acids is another challenging task
which might lead to environmental problems. So the use of friendly heterogeneous
catalysts is more justified as they can be designed accordingly to attain high yield
and effortless regeneration. The catalyst systems used for glycerol esterification
include heteropolyacids, mixed oxides, ion-exchange resins, ionic liquids, acidfunctionalized zeolites, carbon-based catalysts, etc. The catalytic activity of various
catalysts employed for glycerol esterification with acetic acid is summarized in
Table 4.
5.1 Route of the Esterification of Glycerol with Acetic Acid
The esterification of glycerol with acetic acid is a reversible equilibrium reaction for
synthesizing acetylated esters which proceeds in three successive steps. Firstly, the
protons present within the catalyst get attached to the acetic acid oxygen lone pair
electron, and then nucleophilic attack of the hydroxyl group takes place by electrophiles which releases water to form MAG. Secondly more acetylation by a second
K. Bhaduri et al.