Page 115 - Refining Biomass Residues for Sustainable Energy and Bioproducts

Application of heterogeneous acid catalyst derived from biomass for biodiesel process 89

monoglyceride, and glycerol in which process an alkyl ester is liberated at every
stage. Acid catalyst and base-catalyst transesterification follow different mechan-
isms. Fig. 4.1 shows the chemical reaction of an acid-catalyzed transesterification
0
0
process in which R ,Rv, and R v are the long chains of fatty acids present in trigly-
cerides, and R is the alkyl group of an alcohol. Amid the reaction, the association
of catalyst substrate is the most imperative step for the protonation of carbonyl oxy-
gen. This consecutively builds the electrophilicity of the adjacent carbon atom,
making it more vulnerable to nucleophilic attack.
On the contrary, during esterification reaction, the catalyst interacts with FFA
for the protonation of carboxylic moiety, as shown in Fig. 4.2. The steric deterrent
characteristic of both the fatty acid and alcoholic species is the essentially affecting
component at the subatomic level (Hoydonckx et al., 2004). This liberates a proton
and protonated acid molecule leading to the removal of one molecule of water.
Further, the hydroxyl group of alcohol is affected by the nucleophilic attack of oxy-
gen. This results in the formation of a tetrahedral intermediate that finally deproto-
nates and generates an alkyl ester.

4.3 Preparation of heterogeneous acid catalyst from
biomass

Biomass is an excellent option to prepare low-cost heterogeneous acid catalysts.
These catalysts have polycyclic aromatic carbon sheets with acidic groups of higher

Figure 4.1 Chemical reaction pathway of an acid catalyzed transesterification.