OXYDEHYDROGENATION OF PROPANE OVER MG-V-SB-OXIDE CATALYSTS .2. REACTION-KINETICS AND MECHANISM

Recently we reported that Mg4V2Sb2Ox is selective for propane and n-bu tane oxydehydrogenation at low hydrocarbon conversion, and that propan e is oxidized in parallel reactions to propylene and COx. We report no w on the kinetics of propane and propylene oxidations over this cataly st. The partial oxidations of propane and propylene and zero-order in oxygen, whereas deep oxidations of both hydrocarbons are half-order. T his difference in reaction order indicates that different forms of rea ctive oxygen are involved in the partial and deep oxidation reactions. Presumably, nucleophilic lattice oxygen partakes in the partial oxida tion, while electrophilic dissociatively adsorbed oxygen is involved i n deep oxidation. A single activated surface adsorbed state of the hyd rocarbons is thought to be involved in both the partial and deep oxida tion reactions. An interpretation of the observed reaction kinetics in context of the Mg4V2Sb2Ox, solid state chemistry, and the partial oxi dation literature in general, suggests that selective oxydehydrogenati on of propane occurs on isolated (Sb-O-V-O-Sb) sites, deep oxidation o n multiple vicinal vanadium sites (Sb-O-V-O-V-O-Sb), and partial oxida tion of propylene to acrolein on subsurface V-promoted antimony sites (Sb-O-Sb). Therefore, improved selectivity of desired intermediates (p ropylene/acrolein) should be achieved by further lowering the vanadium concentration and/or through key solid state positioning of the vanad ium in the catalyst lattice. Alternatively, selective doping to electr onically decrease the electrophilicity of the waste forming sites and its appended oxygen should also help depress the waste forming reactio n channels in favor of the desired partial oxidation channels. Finally it is anticipated that higher useful product yields would be attained with a compositionally optimized Mg-V-Sb-oxide catalyst by opting for a more stable, isolatable intermediate, e.g., acrylonitrile, by react ing propane in the presence of ammonia and oxygen(air) over this catal yst.