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

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 in n-hexane was identical on both catalysts (kprim/ksec = 0.10−0.11) and similar to that predicted from relative C−H bond energies in n-hexane using Evans−Polanyi relations. Spatial constraints within MnAPO-18 did not lead to any preference for terminal oxidation or to hexanoic acid as the main product, in contradiction with previous reports on materials with identical structure. The lack of specific regioselectivity on MnAPO-18 is not unexpected, in view of its large intracrystalline cages, of the accepted involvement of ROOH intermediates, and of the lack of diffusional constraints on the rates of n-hexane oxidation on MnAPO-18 catalysts.