Ti(III)APO-5 materials as selective catalysts for the allylic oxidation of cyclohexene: Effect of Ti source and Ti content

Abstract

Different TAPO-5 materials, prepared from Ti(III) chloride, have been tested as catalysts in the oxidation of cyclohexene with hydrogen peroxide under anhydrous conditions. Solid TiCl3 was shown to render better synthesis reproducibility and higher catalytic activity of the resultant materials compared to TiCl3 aqueous solution. The synthesis of these materials was carried out under N2 atmosphere to preserve the initial oxidation state of Ti during the crystallization process. As a consequence, the so-called Ti(III)APO-5 materials have Ti environments different to those found in conventional TAPO-5 (Ti(IV)APO-5). Indeed, their catalytic activity in the oxidation of cyclohexene markedly overcomes that of the Ti(IV)APO-5 at any Ti content and after any reaction time. Turnover number (TON) of Ti(III)APO-5 samples exponentially increases as Ti content decreases, the Ti-poorer sample (Ti/(Ti+Al+P) molar ratio of 0.003) reaching TON values higher than 200 after 6h of reaction at 343K and higher than 550 after 24h. The interest of Ti(III)APO-5 catalysts lies on their high selectivity to products formed through allylic oxidation of cyclohexene. The main product of this reaction, 2-cyclohexenyl hydroperoxide, was obtained with more than 80% selectivity over Ti(III)APO-5 catalysts. This behavior is in contrast with the well-known strong tendency of the more active Ti-zeolites to epoxidize the double bond.