Recyclable solid catalysts for epoxidation of alkenes: Amino- and oniumsilica-immobilized [HPO4{W2O2(μ-O2)2(O2)2}]2− anion

We designed solid catalysts for liquid-phase epoxidation based on functionalized silica Si(CH2)3Q+ [Q: NH3, NEt3, NC5H5, PPh3] and [HPO4{W2O2(μ-O2)2(O2)2}]2−. The approach that we adopted allowed us to avoid the use of chlorocarbon solvent and enabled catalyst recycling. By using supports with 4 different linking chains between the anion and silica and different surface lipophilicities, we followed their influence on catalyst activity in the epoxidation of cyclooctene and (R)-limonene by H2O2 in t-BuOH. All solids were active in cyclooctene epoxidation (conversion up to 100%; epoxide selectivity 100%; TOF 2-4 h−1 anion−1). The degree of surface coverage by organic functions was crucial for recycling performance. Catalysts with low densities of organic functions and hydrophilic surfaces were easily deactivated. End-capping improved their stability but decreased their activity. Catalysts with dense coverage of onium groups and the active site in a hydrophobic chloropropyl environment demonstrated high activity and excellent recycling stability. Less promising results were obtained in the epoxidation of (R)-limonene.