Efficient and selective molybdenum based heterogeneous catalyst for alkene epoxidation using batch and continuous reactors
- Publisher: Royal Society of Chemistry
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. There has been considerable effort to develop alternative alkene epoxidation methods by employing an oxidant such as tert-butyl hydroperoxide (TBHP) as it is environmentally benign, safer to handle and possesses good solubility in polar solvents. A notable industrial implementation of alkene epoxidation with TBHP was the Halcon process that employed soluble molybdenum(VI) as a catalyst for liquid phase epoxidation of propylene to propylene oxide. However, homogenous catalysed alkene epoxidation has several drawbacks including deposition of catalyst on the reactor walls and increased difficulties in separation of catalyst from the reaction mixture. In this work, an efficient and selective polystyrene 2-(aminomethyl)pyridine supported molybdenum complex (Ps.AMP.Mo) and a polybenzimidazole supported molybdenum complex (PBI.Mo) have been used as catalysts for epoxidation of 4-vinyl-1-cyclohexene (i.e. 4-VCH) using TBHP as an oxidant in batch and continuous reactors. An extensive assessment of the catalytic activity, stability and reusability of the catalysts has been conducted in a classical 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 1,2-epoxyhexane and 4-vinyl-1-cyclohexane 1,2-epoxide (4-VCH 1,2-epoxide) to optimise the reaction conditions in a batch reactor. A detailed evaluation of molybdenum (Mo) leaching from the polymer supported catalyst has been investigated by isolating any residue from reaction supernatant solutions and then using these residues as potential catalyst in epoxidation reactions. 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 corresponding 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.