Greener and sustainable approach for the synthesis of commercially important epoxide building blocks using polymer-supported Mo(VI) complexes as catalysts

摘要

The growing concern for the environment, increasingly stringent standards for the release of chemicals into the environment and economic competitiveness have prompted extensive efforts to improve chemical synthesis and manufacturing methods as well as development of new synthetic methodologies that minimise or completely eliminate pollutants. As a consequence, more and more attention has been focused on the use of safer chemicals through proper design of clean processes and products. Epoxides are key raw materials or intermediates in organic synthesis, particularly for the functionalisation of substrates and production of a wide variety of chemicals such as pharmaceuticals, plastics, paints, perfumes, food additives and adhesives. The conventional methods for the industrial production of epoxides employ either stoichiometric peracids or chlorohydrin as an oxygen source. However, both methods have serious environmental impact as the former produces an equivalent amount of acid waste, whilst the later yields chlorinated by-products and calcium chloride waste. Hence, a greener and efficient route for catalytic epoxidation that could improve manufacturing efficiency by reducing operational cost and minimising waste products is highly desired. In this chapter, a greener alkene epoxidation process using molybdenum (Mo) based heterogeneous catalyst and tert-butyl hydroperoxide (TBHP) as an oxidant has been presented. A polystyrene 2-(aminomethyl)pyridine supported molybdenum(VI) complex, i.e. Ps.AMP.Mo and a polybenzimidazole supported Mo(VI) complex, i.e. PBI.Mo have been successfully prepared and characterised. The catalytic activities of the polymer supported Mo(VI) complexes have been evaluated for epoxidation of 1-hexene and 4-vinyl-1-cyclohexene (4-VCH) in a batch reactor. Experiments have been carried out to study the effect of reaction temperature, feed molar ratio of alkene to TBHP and catalyst loading on the yield of epoxide for optimisation of reaction conditions in a batch reactor. The long term stability of the polymer supported Mo(VI) catalysts have been evaluated by recycling the catalysts several times in batch experiments using conditions that form the basis for continuous epoxidation studies. The extent of Mo leaching from each polymer supported catalyst has been investigated by isolating any residue from reaction supernatant studies after removal of heterogeneous catalyst and using the residue as potential catalyst for epoxidation. The efficiency of Ps.AMP.Mo catalyst has been assessed for continuous epoxidation of 1-hexene and 4-vinyl-1-cyclohexne with TBHP as an oxidant using a FlowSyn reactor by studying the effect of reaction temperature, feed molar ratio of alkene to TBHP and feed flow rate on the conversion of TBHP and the yield of epoxide. The catalysts were found to be active and selective for batch and continuous epoxidation of alkenes using TBHP as an oxidant. The continuous epoxidation in a FlowSyn reactor has shown considerable time savings, high reproducibility and selectivity along with remarkable improvement in catalyst stability compared to the reactions carried out in a batch reactor.