Most of the propylene oxide manufactured for production of propylene glycol- derived polymers and other applications, is the product of either the chlorohydrin process which produces huge amounts of chloride waste or the hydroperoxide process that produces much more of the by-product, t-BuOH, than the main product, propylene oxide. The market for ethylene oxide is even larger, and the dominant process for its manufacture is a high temperature and pressure, gas phase O2 oxidation reaction that produces more CO2 as a waste product than any other chemical manufacturing processes. CEBC researchers have been developing an alternative to these commercial processes which will have a reduced environmental impact.
The new CEBC light olefin process has excellent phase equilibrium and mechanism-based design features that suggest it can be competitive in the market place. The system was first developed at CEBC for the conversion of propylene to propylene oxide. It uses a long-known, outstanding Lewis acid oxidation catalyst, methyl trioxorhenium, CH3ReO3, in methanol solution with safe 50% aqueous hydrogen peroxide as the oxidant, operates in the mild temperature range from 20 to 40 °C, under ~15 atmospheres of nitrogen gas pressure, and proceeds to completion in <2 hours, with >95% selectivity in favor of propylene oxide over solvolysis products. Further, no substrate or solvent burning is observed. The main by-product is water, accompanied by 0.5-2% 1-methyoxy-2-propanol. The exceptional conversion and yield result from reaction conditions designed to condense the gaseous propylene substrate into the liquid reaction medium, using the same principle that creates CXLs.
Within the range of these studies, methyl trioxorhenium, MTO, catalyzes epoxidation reactions more rapidly than other tested catalysts, and to the extent that the epoxidation occurs via the Lewis acid mechanism, there is no accompanying 1-electron radical chemistry to generate other by-products. Ongoing studies on the durability of the MTO catalyst have used UV/Vis spectroscopy to demonstrate the catalyst stability in the reaction solution. In the absence of substrate and with excess H2O2, changes in absorbance of less than 2% were observed over a period of a week. Short term stability has also been confirmed by ReactIR spectroscopy.
More details may be found in Lee, H-J.; Shi, T-P.; Busch, D.H.; Subramaniam, B. "A Greener, Pressure Intensified Propylene Epoxidation Process with Facile Product Separation," Chemical Engineering Science 2007 62 7282-7289, DOI: 10.1016/j.ces.2007.08.018.