Dissociation of Acetaldehyde on β-Mo_2C To Yield Ethylidene and Oxo Surface Groups: A Possible Pathway for Active Site Formation in Heterogeneous Olefin Metathesis

摘要

The initiating and propagating intermediates in homogeneous olefin metathesis are metal alkylidenes, as described by the Herisson—Chauvin mechanism. It is generally assumed that the same mechanism holds for heterogeneously catalyzed olefin metathesis. However, it is not clearly known how the initiating site forms at the surface, although several mechanisms have been proposed over the past three decades. Vibrational spectroscopy investigations related to this question are extremely challenging due to the small number of active sites. For example, dynamic counting experiments revealed that < 1 % of Mo atoms in a highly dispersed Mo/Al_2O_3 catalyst are active for propene metathesis. We show that acetaldehyde undergoes carbonyl bond-specific dissociation on β-Mo_2C to yield surface ethylidene—oxo species. This novel surface reaction permits the first measurement of the complete infrared spectrum of one of the Herisson—Chauvin intermediates in propene metathesis. Since alkylidene-functionalized molybdenum carbide is active for olefin metathesis, our results also suggest a possible route for initiator formation in heterogeneous olefin metathesis.