Epoxidation of α,β-unsaturated carbonyl compounds over various titanosilicates

The epoxidation of a variety of electron-deficient α,β-unsaturated carbonyl compounds has been studied using dilute H₂O₂ and titanium-containing zeolites under liquid-phase conditions. The influence of the reaction medium and the structure of titanium-containing silicates (Ti-β(OH), Ti-Al-β(OH), Ti-β(F), TS-1, TS-2, Ti-MCM-22, and Ti-MCM-41) have been investigated. The weak basic acetonitrile solvent shows better activity and selectivity for epoxide than for other solvents used in this reaction over large-pore zeolite Ti-β. Among the various titanosilicates studied, aluminum-free Ti-β exhibits the best activity and H₂O₂ selectivity for cyclic α,β-unsaturated ketone, whereas Ti-β and TS-1 exhibit similar activities for open-chain α,β-unsaturated carbonyl compounds. The medium-pore TS-1, TS-2, and Ti-MCM-22 exhibit lower activities for the oxidation of cyclic ketones due to their diffusion limitation. For unsaturated ketones, epoxides are selectively formed, whereas unsaturated aldehydes mainly produce carboxylic acid. Branching at α- and/or β-carbon influences the reactivity of the carbonyl compounds considerably. However, the Ti-β/H₂O₂ catalytic system fails to oxidize substrates like α,β-unsaturated acids, α,β-unsaturated esters, and isophorone.