The Ultimate Challenge Direct Oxidation of Propane to PO

A one-stage process for the manufacture of PO from propane instead of propene would have substantial economical advantages. In one patent, a catalyst composed of Ag/Cl/NaN03/La/Cr/BaC03 is claimed that gives 10% propane conversion with a PO selectivity of 8% at 480 °C, resulting in a PO space-time-yield of0.002 gpo gcat h however, this catalyst deactivates very rapidly [45a]. 

An alternative approach is to combine the two steps, that is, the oxidative dehydrogenation of propane to propene and the epoxidation of propene to PO, in a single reactor, with two sequential catalytic beds, but with similar reaction conditions for the two steps [45b]. The problem is that most of the active and selective catalysts in propane ODH operate at temperatures that are too high for propene epoxidation. 

Many problems remain to be solved for these fascinating processes that directly oxidize propene with green oxidants in the gas phase (i) the rapid deactivation of the catalyst, due to the accumulation of heavy compounds, precursors for coke formation (ii) the need for gas-phase promoters, for example, NO, or chlorocarbons, which act to moderate activity and enhance PO selectivity even in the very first patents issued in this field, this was claimed to be a key feature for optimal performance [46] and (iii) the low space-time-yield achieved, due to the low conversion of propene and/ or the low residence time. 

For conversions lower than 5%, very high selectivity for PO based on propene can be obtained (e.g., higher than 90%) with the O2/H2 mixture (HOPO), whereas in the presence of O2 alone the selectivity is not higher than 50-60% even at very low propene conversion. In general, yields for the direct oxidation of propene are lower than 5%. As shown clearly in [43a], if all the results achieved in the gas-phase epoxidation of propene with various oxidants, that is, O2, O2 + H2, HP vapors or N2O, are compiled in a cumulative plot of PO selectivity versus propene conversion, a limit curve can easily be drawn up, which seems to indicate that the conditions needed to increase propene conversion are not compatible with good PO selectivity. Moreover, selectivity to PO with respect to hydrogen is still too low. 

Presently, the scientific community is making every effort to study irmovative catalytic materials that combine morphological features suited to a fast counterdiffusion of PO, surface properties designed to favor the desorption of PO and limited side-reactions deriving from acid or base-catalyzed reactions - especially those leading to heavy product accumulation - as well as to stabilizing metal nanopartides. 

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