Highly Selective Epoxidation Of Alkenes With Air Catalyzed By Simple Co²⁺-exchanged Molecular Sieves

The catalytic epoxidation of alkenes has been a subject of growing interests in the fine chemistry and modern fundamental industry, since epoxides are key building blocks in organic synthesis. Traditionally, epoxides are produced by the epoxidation of alkenes with stoichiometric amount of peroxy-acids as oxidants. However, peroxy-acids are very expensive and hazardous, and the acidic waste is environmentally undesirable. Molecular oxygen is widely accepted as a green oxidant due to the advantages of atom economy and low cost. Due to triplet ground state of O2disfavoring reactions with singlet organic compounds, great efforts have focused on the discovery of efficient catalysts.In the present work, some transition metal exchanged zeolites, such as M-ZSM-5, M-Beta, M-MOR, M-A, M-X, and M-Y, have been employed to catalyze the epoxidation of alkenes with air. The structure of catalysts are characterized by powder X-ray diffraction (XRD), IR, UV-vis, SEM, BET, and X-ray fluorescence (XRF) techniques. The results show that the selective epoxidation of styrene and a-pinene with dry air can be efficiently conducted over simple Co2+-exchanged zeolites in the presence of N,N-dimethylformamide (DMF) as the solvent and tert-butyl hydroperoxide (TBHP) or cumene hydroperoxide (CHP) as the initiator, in which Co-ZSM-5and Co-Beta without any organic Schiff-base ligand or additive behave as highly efficient catalysts. Co-ZSM-5and Co-Beta achieve88.4-92.4mol%conversion with the overall selectivity of94.6-96.6%for the epoxidation of styrene, and92.8-90.6mol%conversion with the selectivity of86.1-88.3%for the epoxidation of a-pinene, respectively. Meanwhile, some factors inclusive of the source of cobalt, cobalt content, reaction time, reaction temperature, oxidants, solvents, and initiators are investigated in detail. Recycling studies and control experiments show the stability and recyclability of Co2+-zeolites in the epoxidation reaction.Based on the above work, our new progress firstly reports the catalytic epoxidation of alkenes with air over M-SAPO-5and M-SAPO-34molecular sieve catalysts, prepared simply by ion-exchange method. Both Co-SAPO-5and Co-SAPO-34exhibit superior activities for the epoxidation of styrene to achieve ca.92.0-91.9mol%conversion with89.5-90.5%selectivity of epoxide, compared to the other transition metal exchanged catalysts. However, for the catalytic epoxidation of some carbocyclic alkenes like a-pinene and cyclooctene both display less efficiency.