The dependence on time-onstream of the catalytic behavior of (VO)2P2O7 and α1-VPO5 with preadsorbed NH3 in aerobic interaction with propane (I) is reported. The rate of I conversion and the selectivity to acrylonitrile (II) depends on the surface coverage with NH3. Three different limiting surface situations for vanadyl pyrophosphate as a function of the amt. of adsorbed NH3 were found: (a) for high surface coverage with adsorbed NH3, I is oxidatively dehydrogenated to propylene, but is not further oxidized to II or to carbon oxides; (b) for an intermediate surface coverage with adsorbed NH3, the selectivity to II is max.; and (c) for a very low surface coverage with adsorbed NH3, the rate of I depletion is higher, but mainly carbon oxides are formed. The dependence of the activity on the surface coverage with NH3 was attributed to a competitive adsorption between NH3 and O on coordinatively unsatd. vanadyl sites. In addn., the comparison of the evolution of the catalytic behavior of both (VO)2P2O7 and α1-VPO5 in I conversion in the presence and absence of gaseous O suggests a possible role of adsorbed O species in the mechanism of I activation and selective transformation to II. [on SciFinder(R)]
Acrylonitrile from propane on vanadyl pyrophosphate with preadsorbed ammonia. 1. Role of competitive adsorption phenomena in determining selectivity
CENTI, Gabriele;
1993-01-01
Abstract
The dependence on time-onstream of the catalytic behavior of (VO)2P2O7 and α1-VPO5 with preadsorbed NH3 in aerobic interaction with propane (I) is reported. The rate of I conversion and the selectivity to acrylonitrile (II) depends on the surface coverage with NH3. Three different limiting surface situations for vanadyl pyrophosphate as a function of the amt. of adsorbed NH3 were found: (a) for high surface coverage with adsorbed NH3, I is oxidatively dehydrogenated to propylene, but is not further oxidized to II or to carbon oxides; (b) for an intermediate surface coverage with adsorbed NH3, the selectivity to II is max.; and (c) for a very low surface coverage with adsorbed NH3, the rate of I depletion is higher, but mainly carbon oxides are formed. The dependence of the activity on the surface coverage with NH3 was attributed to a competitive adsorption between NH3 and O on coordinatively unsatd. vanadyl sites. In addn., the comparison of the evolution of the catalytic behavior of both (VO)2P2O7 and α1-VPO5 in I conversion in the presence and absence of gaseous O suggests a possible role of adsorbed O species in the mechanism of I activation and selective transformation to II. [on SciFinder(R)]Pubblicazioni consigliate
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