Page 66 - Refining Biomass Residues for Sustainable Energy and Bioproducts

40 Refining Biomass Residues for Sustainable Energy and Bioproducts

mixture is settled for phase separation, and two separate liquid phases are obtained:
one is ester and the other is crude glycerol. The base-catalyzed transesterification is
faster than acid-catalyzed reaction, which is one of the advantages with alkali-
catalyzed reaction (Bharti et al., 2014a; Kumar et al., 2017b). Generation of an alk-
oxide and a protonated catalyst, after the reaction of base with the alcohol, is the
initial step of this reaction. The formation of alkyl ester and its corresponding anion
of the diglyceride occurs after the attack of alkoxide (nucleophile) at the carbonyl
group of triglyceride, which generates a tetrahedral intermediate. Finally, a mixture
of alkyl esters and glycerol is obtained after the conversion of generated diglycerides
and monoglycerides by the same mechanism. The alkali catalyst such as alkoxides of
alkaline metal (CH 3 ONa) are very active and well-known catalyst as it gives a higher
yield ( . 98%) in quick reaction time at very low concentration (0.5 mol%)
(Demirbas, 2009). While the catalytic efficiency is reduced in the presence of water,
blocking its application in industrial processes (Schuchardt et al., 1998). Ramadhas
et al. (2004) have reported several sodium methoxide catalysts, which were used in
the process of transesterification at a large scale. The catalytic rate and efficiency of
sodium methoxide in transesterification reaction of methanol oil is very high. The
preparation of methoxide anion is done by dissolving the clean metals in anhydrous
methanol. A 0.5 2 M concentration of sodium methoxide in methanol rapidly com-
pletes the transesterification as compared to other transesterification agents. A similar
concentration of potassium methoxide makes the transesterification of triglyceride
quicker than the sodium methoxide (Ramadhas et al., 2004). But due to an inherent
high heat of reaction with methanol, there is a safety issue, so sodium methoxide is a
preferable catalyst in methanol as compared to potassium methoxide.

2.4.1.3 Enzyme-catalyzed transesterification

Application of enzymes or biological catalyst is a recent technique for the production of
biodiesel from microbial oils or lipids by transesterification (Shieh et al., 2003; Khosla
et al., 2017). Recently, three diverse lipases from Chromobacterium viscosum, Candida
rugosa,and Porcine pancreas were selected for transesterification of oil in a solvent-
free environment for the production of biodiesel; out of which only lipase from C. visco-
sum was reported to produce a substantial yield (Shah et al., 2004). For enhancing the
yield of biodiesel from 62% to 71%, the lipase from C. viscosum was immobilized on
Celite-545. This immobilized lipase can also be applied for the ethanolysis of microbial
lipids. It was observed that the combined optimization of process parameters of transes-
terification and immobilization of lipases increased the yield of biodiesel at a certain
instant (Shah et al., 2004). Even though the enzyme-based transesterification methods
have been reported in several new publications and patents, still its commercial level
application is not fully developed. In order to apply enzymatic transesterification at an
industrial level, the basic characteristic of enzymes such as solvent tolerance, working
temperature, pH, and source of enzyme should be optimized. The yield as well as effi-
ciency of enzymatic transesterification is still lagging as compared to the alkali-
catalyzed transesterification (Schuchardt et al., 1998). Optimistically, it can be expected
that in future it will present itself as a better technique for the production of biodiesel,
due to its readily availability and ease to handle.