Using reduced catalysts for oxidation reactions: Mechanistic studies of the "Periana-Catalytica" system for CH4 oxidation

Abstract

Designing oxidn. catalysts based upon CH activation with reduced, low oxidn. state species is challenging given the proclivity for catalyst deactivation by over-oxidn. This dilemma has been recognized in the Shilov system where reduced Pt is used to catalyze methane functionalization. Thus, it is generally accepted that the key to replacing Pt in that system with more practical oxidants is ensuring that the oxidant does not over-oxidize the reduced Pt^(II) catalyst. The "Catalytica" system, which utilizes (bpym)Pt^(II)Cl_2 in concd. H_2SO_4 solvent at 200 °C, is a highly stable catalyst for the selective, high yield oxyfunctionalization of methane. In lieu of the over-oxidn. dilemma, the high stability and obsd. rapid oxidn. of (bpym)Pt^(II)Cl_2 to Pt^(IV) in the absence of methane would seem to contradict the originally proposed mechanism involving CH activation by a reduced Pt^(II) species. Mechanistic studies show that the originally proposed mechanism is incomplete and that, while CH activation proceeds with Pt^(II), the system provides a soln. to the over-oxidn. dilemma. Importantly, contrary to the accepted view (to minimize Pt^(II) over-oxidn.), these studies indicate that increasing the rate of Pt^(II) oxidn. could actually increase the rate of catalysis and enhance system stability if catalysts are designed to facilitate a rapid Pt^(IV) + Pt^(II)-R reaction. The mechanistic basis for this counterintuitive prediction could help to guide the design of new catalysts for alkane oxidn. that operate by CH activation.

Additional details