Kinetics and mechanism of cyclohexane oxidation on MnAPO-5 catalysts

Kinetic and isotopic measurements have led to a detailed description of the elementary steps required for cyclohexane (RH) reactions with O2 on MnAPO-5 catalysts. Cyclohexyl hydroperoxide (ROOH) is an intermediate in cyclohexanol (ROH) and cyclohexanone [R(H)O] formation. Combined rates of ROH + R(H)O synthesis are first order in ROOH concentration and proportional to the number of redox-active framework Mn sites. Taken together with UV–visible evidence for Mn2+ as the most abundant active structure during steady-state catalysis, these data indicate that ROOH decomposition on Mn2+ is a kinetically relevant step. C6H12/C6D12 kinetic isotope effects (KIE) for ROOH decomposition as a function of ROOH concentration are 2.5 at 403 K, consistent with OH bond cleavage at Mn2+OH in this elementary step. A catalytic ROOH decomposition cycle proceeding via adsorbed intermediates without the involvement of free radicals or their bimolecular termination accounts for measured alcohol/ketone product ratios. Mn3+ species, initially present in air-treated MnAPO-5, activate RH bonds and lead to shorter initial induction periods for ROOH-mediated pathways. tert-Butyl hydroperoxide (TBHP) led to higher ROOH synthesis rates via H abstraction from RH to form ROOH, without influencing ROOH decomposition rate constants. In the absence of TBHP, ROOH formation occurs predominantly through activation of CH bonds in RH by ROO∗ species in a step that gives a KIE value of 6.8, consistent with such activation steps. These findings are expected to be also relevant for related RH oxidation reactions on materials containing redox-active sites such as inorganic solids or solvated cations. The proposed sequence of elementary steps illustrates the difficulties in interpreting effects of isotopic identity on rates and of spatial constraints on regioselectivity without rigorous assessment of the identity and kinetic relevance of elementary steps, and also the risk of using overall KIE values as phenomenological inference for a certain mechanism, particularly for sequential pathways, such as the ROOH formation and ROOH decomposition steps discussed here.