Kinetic and mechanistic study of the synthesis of ionone isomers from pseudoionone on Brønsted acid solids

Abstract

The kinetics of the liquid-phase synthesis of α-, β- and γ-ionones from pseudoionone was studied on Brønsted acid solids. Four silica-supported tungstophosphoric acid catalysts containing different heteropolyacid loadings, as well as a silica-supported triflic acid sample and a commercial resin (Amberlyst 35W) were tested in a batch reactor at 343–383 K under autogenous pressure. The final composition of the ionone isomer mixture depended on the catalyst acidic properties and operational conditions. The reaction pathways leading to the three ionone isomers were elucidated by postulating a heterogeneous Langmuir-Hinshelwood-Hougen-Watson (LHHW) kinetic model. First order rate expressions, participation of a single Brønsted acid site in each reaction step and a cationic cyclic intermediate shared by the three ionone isomers were the main model assumptions. It was found that α-, β- and γ-ionones form directly from pseudoionone by cyclization. However, the final concentration of α- and β-ionones is enhanced in consecutive pathways involving the isomerization of γ-ionone. The relative importance of the isomerization steps and the selective formation of α- or β-ionone depend on the Brønsted acid site strength and reaction temperature.