Using Reduced Catalysts for Oxidation Reactions: Mechanistic Studies of the “Periana-Catalytica” System for CH₄ Oxidation

Designing oxidation catalysts based
on CH activation with reduced,
low oxidation state species is a seeming dilemma given the proclivity
for catalyst deactivation by overoxidation. This dilemma has been
recognized in the Shilov system where reduced PtII is used
to catalyze methane functionalization. Thus, it is generally accepted
that key to replacing PtIV in that system with more practical
oxidants is ensuring that the oxidant does not over-oxidize the reduced
PtII species. The “Periana-Catalytica” system,
which utilizes (bpym)­PtIICl2 in concentrated
sulfuric acid solvent at 200 °C, is a highly stable catalyst
for the selective, high yield oxy-functionalization of methane. In
lieu of the over-oxidation dilemma, the high stability and observed
rapid oxidation of (bpym)­PtIICl2 to PtIV in the absence of methane would seem to contradict the originally
proposed mechanism involving CH activation by a reduced PtII species. Mechanistic studies show that the originally proposed mechanism
is incomplete and that while CH activation does proceed with PtII there is a solution to the over-oxidation dilemma. Importantly,
contrary to the accepted view to minimize PtII overoxidation,
these studies also show that increasing that rate could increase the
rate of catalysis and catalyst stability. The mechanistic basis for
this counterintuitive prediction could help to guide the design of
new catalysts for alkane oxidation that operate by CH activation.