Polymer-supported Mo and V cyclohexene epoxidation catalysts: Activation, activity, and stability

Abstract

The immobilisation of MoO 2acac 2 and VOacac 2 on a polystyrene support carrying a hydroxypropylated aminomethylpyridine ligand occurs via a ligand exchange reaction involving displacement of the acetylacetonate (acac) groups. In the case of the molybdenum species, the resulting blue polymer complex contains both MoO and MoOMo groups and immobilised ligand-metal bonds. Activation of the resulting complexes to form bright yellow species by pretreatment with excess t-butyl hydroperoxide is required before the supported complexes can become active catalysts for the epoxidation of cyclohexene by t-butyl hydroperoxide. In situ activation does not occur. It is believed that in the case of molybdenum activation involves oxidation of Mo(V) to new supported Mo(VI) species and that this is inhibited in the presence of alkene. Both polymer-immobilised Mo and V systems are catalysts of the epoxidation reaction, but the Mo-based system is considerably more active. Epoxidation reactions are characterised by an induction period followed by a relatively long linear conversion/time relationship. The induction period is probably associated with formation of the reactive intermediate which constitutes the true catalytic species, and this probably involves the reaction of the immobilised Mo(VI) complex with cyclohexene or traces of its epoxide. Polymer complexes subjected to more severe activation conditions display shorter induction periods and faster rates of epoxidation as a result of formation of more Mo(VI) centres. Additional Mo(VI) centres do not appear to be generated during epoxidation reactions. Mo and V are leached from their supports during activation. More severe activation conditions cause more metal leaching but the amounts lost are small (always < 1.6%). The supported Mo complex remains an active catalyst when recycled up to nine times. Apart from an initial small drop, the activity remains essentially constant on reuse. Mo is also leached during epoxidations but the leached species make no significant contribution as catalyst-immobilised complexes are the active catalysts. The amounts of Mo leached are small (<0.4% per cycle) and fall progressively to essentially zero after nine cycles. The immobilised Mo complex offers the prospect of a technologically useful alkene epoxidation catalyst for continuous liquid phase oxidations using t-butyl hydroperoxide.