ACRYLONITRILE FROM PROPANE ON (VO)2P2O7 WITH PREADSORBED AMMONIA .1. ROLE OF COMPETITIVE ADSORPTION PHENOMENA IN DETERMINING SELECTIVITY

The dependence on time-on-stream of the catalytic behavior of (VO)2P2O7 and α1-VPO5 with preadsorbed ammonia in the aerobic and anaerobic interaction with propane is reported. The rate of propane conversion and the selectivity to acrylonitrile depends on the surface coverage with ammonia. Three different limiting surface situations for vanadyl pyrophosphate as a function of the amount of adsorbed ammonia were found: (a) for high surface coverage with adsorbed ammonia, propane is oxidatively dehydrogenated to propylene, but is not further oxidized to acrylonitrile or to carbon oxides; (b) for an intermediate surface coverage with adsorbed ammonia, the selectivity to acrylonitrile is maximum; and (c) for a very low surface coverage with adsorbed ammonia, the rate of propane depletion is higher, but mainly carbon oxides are formed. The dependence of the activity on the surface coverage with ammonia was attributed to a competitive adsorption between ammonia and oxygen on coordinatively unsaturated vanadyl sites. In addition, the comparison of the evolution of the catalytic behavior of both (VO)2P2O7 and αI-VPO5 in propane conversion in the presence and absence of gaseous oxygen suggests a possible role of adsorbed oxygen species in the mechanism of propane activation and selective transformation to acrylonitrile. © 1993 Academic Press, Inc.