CATALYTIC OXIDATION OF PROPYLENE. 11. AN INVESTIGATION OF THE KINETICS AND MECHANISM OVER IRON-ANTIMONY OXIDE

Article abstract of DOI:10.1021/j100411a012

The kinetics of propylene oxidation over iron-antimony oxide (atomic ratio Sb/Fe = 4) were determined over the temperature range 325-475 deg C, at 25 deg C intervals by using a single-pass flow reactor.Propylene labeled selectively with D or ¹³C in a variety of positions and oxygen-18 were used as tracers to probe the mechanistic details of the reaction.Mass spectrometry and NMR were used to analyze the isotopic content of the acrolein produced.The rate of the reactions was dependent upon the partial pressures of both propylene and oxygen over the entire temperature range.The kinetics could be explained in terms of the coupled reactionof catalyst reduction and reoxidation.The kinetics suggested that the rate of catalyst reoxidation is slower than the rate of reduction and the oxidation of propylene takes place on a partially reduced catalyst surface.Deuterated propylene studies showed that the rate-determining step is the abstraction of an allyl hydrogen from propylene to form a ?-allyl intermediate at all temperatures.Subsequent addition of oxygen to the ?-allyl intermediate before the abstraction of a second hydrogen yields a ?-allyl species.This ?-allyl species is subsequently converted to the product acrolein.Oxygen-18 data showed that 1.6 and 2.6 layers of oxygen participate in the formation of both acrolein and carbon dioxide at 400 and 450 deg C, respectively.This suggested that lattice oxygen is the selective oxygen for product formation.At lower temperatures (<350 deg C), only 0.8 layer of oxygen participates in the formation of products.This suggested that adsorbed oxygen may become the possible source of selective oxygen for product formation at low temperatures.Both kinetics and tracer studies showed that carbon dioxide is formed exclusively trought the further oxidation of acrolein.