The Kinetics of Propylene Metathesis Catalyzed by a Molybdenum (100) Single Crystal

Abstract

It is shown, using a high-pressure reactor incorporated into an ultrahigh-vacuum chamber, that an initially clean molybdenum (100) single crystal can catalyze propylene metathesis. The reaction is found to be first-order in propylene pressure which proceeds with a high activation energy (∼65 kcal/mol). Auger spectroscopy reveals the presence of a thick (∼10 monolayers) carbonaceous layer on the surface of the catalyst following reaction. It is further demonstrated, using restart reactions, that metathesis proceeds in presence of this layer. However, the reaction prefactor is ∼ 8 × 1012 site−1 s−1 so that the reaction appears not to be drastically inhibited by the presence of the carbonaceous layer, rather, the high reaction activation energy is primarily responsible for the low rate. Methylcyclopropane and hydrogenolysis products are also formed in the reaction, an observation that is consistent with the reaction proceeding at the surface via the "carbene" mechanism.