Synthesis of ionones on solid Brønsted acid catalysts: Effect of acid site strength on ionone isomer selectivity

The effect of Brønsted acid site strength on the liquid-phase conversion of pseudoionone to ionone isomers (α-, β- and γ-ionone) was studied on resin Amberlyst 35W, silica-supported heteropolyacid (HPAS) and silica-supported triflic acid (TFAS). Catalyst acidity was probed by temperature-programmed desorption of NH3 coupled with infrared spectra of adsorbed pyridine. The initial pseudoionone conversion rate followed the order: TFAS > Amberlyst 35W ≈ HPAS. Synthesis of the three ionone isomers occurred via a common cyclic carbocation intermediate formed from the activation of the pseudoionone molecule on Brønsted acid sites. Initial ionone mixtures containing a α:β:γ isomer distribution of about 40:20:40 were formed, irrespective of the acid site strength. But the ionone mixture composition changed with the progress of the reaction because γ-ionone was consecutively converted to α-ionone on HPAS and Amberlyst 35W, whereas the stronger acid sites of TFAS converted γ-ionone to β-ionone.