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

V.K. DÍEZ; C.R. APESTEGUÍA; J.I. DI COSIMO

CATALYSIS TODAY

Elsevier

Lugar: Amsterdam ; Año: 2010 vol. 149 p. 267 - 274

0920-5861

The effect of Brønsted acid site strength on the liquid-phase conversion of pseudoionone to ionone isomers (a-, b- and g-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 a:b:g 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 g-ionone was consecutively converted to a-ionone on HPAS and Amberlyst 35W, whereas the stronger acid sites of TFAS converted g-ionone to b-ionone.