A continuous-flow apprach to alkene epoxidation catalysed by Polystyrene 2-(Aminomethyl)Pyridine supported Mo(VI) complex

Epoxides are raw materials for a broad range of products, from pharmaceuticals to plastics and paints to adhesives. The production of epoxides often uses peracids including peracetic acid and m-chloroperbenzoic acid in batch reactions. The employment of peracids is not an environmentally friendly synthetic procedure since equivalent amounts of acid wastes are produced. Hence, there is a strong need for cleaner catalytic epoxidation methods that employ safer oxidants and produces little waste. There have been considerable amount of research efforts on developing stable heterogeneous catalysts for epoxidation by immobilisation of catalytically active metal species on organic or inorganic materials such as polymers, ion-exchange resins, alumina; zeolite and silica. Polymers have gained attention as suitable supports for transition metal catalysts as they are inert, nontoxic, insoluble and often recyclable. In this work, we report a new process which is considered to be clean as it employs an efficient and selective Polystyrene 2-(aminomethyl) pyridine supported molybdenum complex, i.e. Ps.AMP.Mo as a catalyst for epoxidation of 4-vinyl-1-cyclohexene. The process uses environmentally benign tert-butyl hydroperoxide (TBHP) as a terminal oxidant. Experiments have been carried out to study the effect of reaction temperature, catalyst loading and feed molar ratio of alkene to TBHP on the conversion of TBHP to 4-vinyl-1-cyclohexene 1,2-epoxide for optimisation of reaction conditions in a batch reactor. The long term stability of the heterogeneous catalyst has been evaluated by recycling a sample of the catalyst several times in batch experiments. The extent of Mo leaching from the polymer supported catalyst has been investigated by isolating any residue from reaction supernatant studies after removal of the heterogeneous catalyst and using the residue as potential catalyst for epoxidation reaction. Furthermore, the efficiency of the heterogeneous catalyst for continuous epoxidation studies have been assessed using a FlowSyn continuous flow reactor by studying the effect of reaction temperature, feed molar ratio of alkene to TBHP and feed flow rate on the conversion of the oxidant and the yield of epoxide. The continuous flow epoxidation using FlowSyn reactor has shown considerable time savings, high reproducibility and selectivity along with remarkable improvements in catalyst stability compared to reactions carried out in a batch reactor.