Structural and functional characterization of redox Mn and Co sites in AlPO materials and their role in alkane oxidation catalysis

The number of redox-active and inactive Mn and Co species in MeAPO-5 and MeAPO-18 (Me = Mn, Co) was measured from H-2 consumption rates during H2 temperature-programmed reduction (H-2-TPR), and their structure and oxidation state were probed at identical conditions by UV-visible and X-ray absorption spectroscopies. H-2 consumption, loss of Me3+ UV-visible features, and a decrease in X-ray absorption edge energy occurred concurrently and at similar rates, indicating excellent agreement between these techniques. No H2O or CO2 were detected during treatment in H-2 or CO, respectively, indicating that reduction from Me3+ to Me2+ occurred by introduction of protons. These protons are fully removed by treatment in 02 to 773 K, and O-2-H-2 redox cycles involving reversible proton formation suggest that cations reside within AIPO framework positions, in which protons reside as charge balancing cations at Me2+-O-P bridges. H-2 consumption rates measured during H-2-TPR could be accurately described by Arrhenius-type behavior, and H-2/Me ratios showed that only a fraction of all Me cations undergo reversible redox cycles. This fraction was 0.86 for MnAPO-18 (atomic Mn/P = 0.05); it decreased from 0.68 to 0.40 for MnAPO-5 as Mn/P ratios increased from 0.028 to 0.10. For CoAPO-18 with 0.028 Co/P and CoAPO-5 with 0.40 Co[P, the fractions of redox sites were 0.64 and 0.40, respectively. UV-visible spectra showed no detectable Me3+ features after thermal treatment in H-2. Thus, cations that do not undergo redox cycles remain as permanently divalent cations throughout O-2-H-2 cycles. The redox-active fraction also decreased during repeated H-2-O-2 redox cycles above 773 K, indicating that redox cations convert into permanently divalent sites. This conversion coincides with the evolution of H2O during H-2 treatments above 773 K. These findings together with the effects of Me/P ratio on the redox fraction are consistent with a mechanism in which OH groups at divalent framework cation sites recombine to form H2O and an oxygen vacancy. Cyclohexanol and cyclohexanone formation rates during liquid-phase cyclohexane oxidation with O-2 on MnAPO-5 samples increased linearly with the number of redox-active sites, suggesting that elementary steps of cyclohexane oxidation involve cycling between Me2+ and Me3+, and require cation sites able to reversibly form charge balancing cationic species.