Transesterification reactions as a means to produce esters: A critical review

Esters are important chemicals used in fine and bulk chemical industry with numerous applications: solvents, paints, varnishes, dyes, and cosmetics. Ester formation is dominated mainly by Fischer esterification and transesterification. Fischer reaction is generally used for ester production, but in certain cases, transesterification can be used with advantage. It is useful when the acid is less soluble in the alcohol or in the solvent used, thus forming two layers. Water formation creates purification problems during esterification reactions due to azeotrope formation and transesterification can be useful in such cases. Commercially, cheaply available methyl and ethyl esters can be conveniently used as raw materials for value added ester production by transesterification. Transesterification is also useful when the parent acids are highly reactive and pose difficulty in separation. Transesterifications are slow reactions and a catalyst is used: acids and bases, ion exchange resins, zeolites, and clays. Homogeneous catalysts were used in the past which are now replaced by their heterogeneous counterparts. Heterogeneous catalysts offer added advantages like reusability, lesser corrosion, and ease of separation. Transesterification can be commercially used to produce a number of esters of industrial importance like acrylics and biodiesel. This review considers all these aspects in considerable detail. A large literature set was scanned and its judicious extract is presented.