Chromium Oxide-Catalyzed Disproportionation of Chlorodifluoromethane: A Mechanism Study

The facile dispropartionation of CHF 2Cl over chrome(III) oxide-based catalysts has allowed the study of catalyst activation and disproportionation as separate processes. Using both a conventional plug-flow microreactor and a TAP reactor, we have found evidence that chrome oxides, in the presence ofCHF 2Cl, undergo at least two separate surface transformations before becoming catalytically active toward disproportionation. The first transformation involves a reductive deoxygenation of high-valent surface-Cr species resulting in the formation of CO 2 and other oxidized products. This reduced surface then reacts with CHF 2Cl to form CO and a halogenated, catalytically active surface. It is proposed that catalysis occurs on coordinatively unsaturated halogenated Cr 3+ active sites. An increase in Lewis acidity of the catalyst surface was shown to accompany each transformation step. A brief kinetic study of the disproportionation gave evidence that the reaction does not proceed by a Rideal-Eley mechanism. Examination of initial product distributions, arising from a CHF 3-activated catalyst, gave evidence that is consistent with a monomolecular halogen-exchange mechanism.