484
between the glycerol conversion rate and increase in time in a 10 h reaction.
Selectivity for TAG reached 70% at 10 h time and selectivity for DAG reached 50%
at 6 h, while the MAG selectivity gradually decreased. Zhu et al. [149] showed that
conversion of glycerol reached 93% within 30 min and 100% within 2 h over silicotungstic catalyst supported on zirconia, while selectivity of MAG was higher than
TAG at 30 min. As the time progressed, yield for DAG and TAG increases. So it is
quite clear that increment in reaction time increases glycerol conversion as well as
selectivity towards DAG and TAG, but this also depends on the catalyst system and
reaction conditions chosen during the esterification reaction.
6 Catalytic Acetalization of Glycerol
Production of cyclic acetals and ketals from glycerol with aldehydes and ketones is
another strategy to promote the concept of circular economy in between biodiesel
production and glycerol upgradation. The glycerol acetalization with aldehydes or
ketones yields five- and six-membered cyclic acetals or ketals. These cyclic oxygenated compounds are one of the major potential fuel additive candidates. Such additives are also known as anti-knocking agent, used in the formulation of diesel,
gasoline and biodiesel as they can significantly enhance fuel rating and cold flow
properties besides achieving the required flash point and oxidation stability for
long-term biodiesel storage [14, 168–170]. In addition, theses additives can effectively cut down the emission of hydrocarbons, carbon monoxide, unregulated aldehydes, etc. released by the uncontrolled combustion of diesel fuel. Among various
fuel additives, solketal is the most effective, synthesized by the acetalization of
glycerol with acetone. Melero et al. [171] performed an experiment with different
oxygenated compounds obtained from glycerol, including triacetin, solketal, a mixture of glycerol esters and mixture of tert-butylated glycerol. There they showed
that the solketal and tert-butylated glycerol are able to reduce viscosity and density
of the fuel effectively. Apart from that, blending of solketal with biodiesel meets its
established requirements for maximum pour point at a temperature −4 to −7 °C,
higher than the other additives. Based on several literature reports, glycerol acetalization process can be classified into three generations (Scheme 13): (a) firstgeneration glycerol acetalization, (b) second-generation glycerol acetalization and
(c) third-generation glycerol acetalization.
Amin et al. [172] recently showed the annual operation cost of a plant through a
pie chart where 432 tons of glycerol is consumed with the production of 620.9 tons
of solketal per year and solketal production cost was 12.29 US$/kg (Fig. 2).
Therefore, more attention and investigation should be needed to minimize its cost of
production.
Glycerol acetalization reaction with acetone is acid-catalysed reversible reaction
where water is formed as co-product along with formation of five- and six- membered
cyclic ketals [14]. Therefore, hydrophobic acid catalyst is highly desirable for getting maximum glycerol conversion and five-membered selectivity. In this reaction,
K. Bhaduri et al.
between the glycerol conversion rate and increase in time in a 10 h reaction.
Selectivity for TAG reached 70% at 10 h time and selectivity for DAG reached 50%
at 6 h, while the MAG selectivity gradually decreased. Zhu et al. [149] showed that
conversion of glycerol reached 93% within 30 min and 100% within 2 h over silicotungstic catalyst supported on zirconia, while selectivity of MAG was higher than
TAG at 30 min. As the time progressed, yield for DAG and TAG increases. So it is
quite clear that increment in reaction time increases glycerol conversion as well as
selectivity towards DAG and TAG, but this also depends on the catalyst system and
reaction conditions chosen during the esterification reaction.
6 Catalytic Acetalization of Glycerol
Production of cyclic acetals and ketals from glycerol with aldehydes and ketones is
another strategy to promote the concept of circular economy in between biodiesel
production and glycerol upgradation. The glycerol acetalization with aldehydes or
ketones yields five- and six-membered cyclic acetals or ketals. These cyclic oxygenated compounds are one of the major potential fuel additive candidates. Such additives are also known as anti-knocking agent, used in the formulation of diesel,
gasoline and biodiesel as they can significantly enhance fuel rating and cold flow
properties besides achieving the required flash point and oxidation stability for
long-term biodiesel storage [14, 168–170]. In addition, theses additives can effectively cut down the emission of hydrocarbons, carbon monoxide, unregulated aldehydes, etc. released by the uncontrolled combustion of diesel fuel. Among various
fuel additives, solketal is the most effective, synthesized by the acetalization of
glycerol with acetone. Melero et al. [171] performed an experiment with different
oxygenated compounds obtained from glycerol, including triacetin, solketal, a mixture of glycerol esters and mixture of tert-butylated glycerol. There they showed
that the solketal and tert-butylated glycerol are able to reduce viscosity and density
of the fuel effectively. Apart from that, blending of solketal with biodiesel meets its
established requirements for maximum pour point at a temperature −4 to −7 °C,
higher than the other additives. Based on several literature reports, glycerol acetalization process can be classified into three generations (Scheme 13): (a) firstgeneration glycerol acetalization, (b) second-generation glycerol acetalization and
(c) third-generation glycerol acetalization.
Amin et al. [172] recently showed the annual operation cost of a plant through a
pie chart where 432 tons of glycerol is consumed with the production of 620.9 tons
of solketal per year and solketal production cost was 12.29 US$/kg (Fig. 2).
Therefore, more attention and investigation should be needed to minimize its cost of
production.
Glycerol acetalization reaction with acetone is acid-catalysed reversible reaction
where water is formed as co-product along with formation of five- and six- membered
cyclic ketals [14]. Therefore, hydrophobic acid catalyst is highly desirable for getting maximum glycerol conversion and five-membered selectivity. In this reaction,
K. Bhaduri et al.