In acid catalysis, both the homogeneous and heterogeneous
methods proceed via basically a similar reaction mechanism, with
the Brønsted option being preferential. The acidity should be of the
Brønsted type, production of water and glycerol during the transesterification
process have previously been reported to significantly
deactivate the Lewis acid sites. Initially, a carbonyl group is protonated
in order to enhance its electrophilicity thereby making it
more ready for nucleophilic attack (Scheme 3). The rate of attack
between free alcohol and the Brønsted sites adsorbed carboxylic
acid is usually the rate determining step with most solid acids.
Thus, the mechanism is of the EleyeRideal type. However, where
Lewis sites participate, their strength determines the slowest step.