Propylene oxide hydrogen-oxygen route

The direct oxidation of propylene on silver catalysts has been intensively investigated, but has failed to provide results with commercial potential. Selectivities are generally too low and the isolation of propylene oxide is complicated by the presence of many by-products. The best reported selectivities are in the range 50-60% for less than 9% propylene conversion. The relatively low selectivity arises from the high temperature necessary for the silver catalysts, the radical nature of molecular oxygen, as well as the allylic hydrogens in propylene. Thus alternative routes have been studied based on the use of oxidants able to act heterolytically under mild conditions. Hypochlorous acid (chlorine+water) and organic hydroperoxides fulfill these requirements and their use has led to the introduction of the chlorohydrin (Box 2) and the hydroperoxide processes, both currently employed commercially. 

Thus, a preliminary analysis of olefin production pathways can be performed based on the methane-to-ethylene ratio and on temperature dependence of the (C3 = )-to-(C2 =) ratio. A more detailed elaboration can be reached from experiments with varied oxygen concentration and from the detailed analysis of the product distribution (including hydrogen formation). However, ethylene formation itself is strong evidence for the contribution of the radical route in product formation. The analysis of experimental data about product distribution during propane oxidation (Kondratenko et al, 2005) demonstrates that over rare-earth oxide catalysts radical route is prevailing in olefin formation. On the other hand, over supported Y-containing catalysts, propylene... 

New routes to hydrogen peroxide new methods for direct synthesis of hydrogen peroxide (from hydrogen and oxygen) in a controlled, safe manner could provide a lower cost oxidant that reduces the use of chlorine. For example, in situ generation of hydrogen peroxide can be used to produce propylene oxide in place of the chlorohydrin route and... 

Finally, titanium silicates have also been extensively investigated for the epoxida-tion of olefins. The reaction of ethylene over a silver-supported catalyst to ethylene oxide is one of the few large-scale industrial oxidation reactions with molecular oxygen as the oxidant. Numerous studies have shown TS-1 to be effective at selectively forming propylene oxide (PO) from propylene using hydrogen peroxide as the oxidant. This is a more environmentally friendly route to PO than the currently used chlorhydrin route, and it is likely that this process will see commercialization in the near future. 

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