Reactant Selectivity and Regiospecificity in the Catalytic Oxidation of Alkanes on Metal-Substituted Aluminophosphates

Abstract

The rate of n-hexane reactions with O2 increased in parallel with the concentration of hexyl hydroperoxide (ROOH) intermediates and with the number of Mnredox sites in microporous MnAPO-5 and MnAPO-18 catalysts. These data confirmed the catalytic nature of oxidation pathways and the mechanistic resemblance between n-alkane and cycloalkane oxidation pathways. Cyclohexane oxidation turnover rates were higher on MnAPO-5 than on MnAPO-18, because small channels in the latter inhibit contact between reactants and Mn active centers. In contrast, n-hexane oxidation turnover rates (per redox-active Mn center) were similar on MnAPO-5 and MnAPO-18, because smaller n-hexane reactants diffuse rapidly and contact active sites in both microporous structures. MnAPO-18 is able to select reactants based on their size, but no regiospecificity was detected on MnAPO-18 or MnAPO-5 for n-hexane oxidation to alkanols, aldehydes, and ketones (7−8% terminal selectivity). The relative reactivity of primary and secondary C−H bonds ...