Structural and Functional Characterization of Redox Mn and Co Sites in AlPO Materials and Their Role in Alkane Oxidation Catalysis

Abstract

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 H 2 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 Me 3+ 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 H 2 O or CO 2 were detected during treatment in H 2 or CO, respectively, indicating that reduction from Me 3+ to Me 2+ occurred by introduction of protons. These protons are fully removed by treatment in O 2 to 773 K, and O 2 -H 2 redox cycles involving reversible proton formation suggest that cations reside within AlPO framework positions, in which protons reside as charge balancing cations at Me 2+ -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 Me 3+ 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 H 2 O 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 H 2 O 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 Me 2+ and Me 3+ , and require cation sites able to reversibly form charge balancing cationic species.

Selective Catalytic Oxidation of Organosulfur Compounds with tert‐Butyl Hydroperoxide

“…The amount of H 2 consumed corresponds to one-electron reduction of Co 3 + of a fraction of all Co atoms present in the sample, [41] consistent with X-ray absorption, [40] IR, and UV-visible [42][43][44] spectral evidence for some permanently divalent cations. Measured H 2 /Co consumption ratios (0.18) were much smaller than expected if all Co atoms in the sample underwent a one-electron reduction (0.5).…”

“…Previous studies have reported similar fractions of redox-active Co cations (40 %) in CoAPO-5, similarly prepared. [41] Diffuse reflectance UV-visible spectroscopy of CoAPO-5 and Co/H-Y samples: UV-visible spectroscopy can be used to determine the valency and coordination of Co cations in AlPO structures. CoAPO-5 is bright blue after synthesis, indicating the presence of tetrahedral Co 2 + cations.…”

Structure of the Catalytic Active Sites in Vanadium-Doped Aluminophosphate Microporous Materials. New Evidence from Spin Density Studies

“…3 It is well known that these divalent ions usually substitute for trivalent Al framework atoms as tetrahedral sites, resulting in a Br nsted functionality (necessary to compensate the negative charge delocalized on the framework) which disappears after a reversible oxidation to the trivalent state. 10,[14][15][16][17][18] More complex is the situation when tetravalent ions are inserted in the AlPOs framework, as in the case of Ti 4+ or V 4+ ions.…”

Catalytic Performance of MgO-Supported Co Catalyst for the Liquid Phase Oxidation of Cyclohexane with Molecular Oxygen

“…Compared with the 3.0%Co/MgO catalyst, the reduction peak of the 5.0%Co/MgO catalyst moves to a lower temperature. This result indicates the better reduction property of 5.0%Co/MgO, which is favorable to improve its catalytic activity, since the decomposition of reaction intermediate cyclohexyl hydroperoxide (CHHP) is proposed to involve one-electron redox cycles [26,30]. In the O2-TPD profiles of 0.2%Co/MgO, 0.5%Co/MgO, 1.0%Co/MgO and 3.0%Co/MgO ( Figure 6), there are two broad peaks at 95~500 °C, and only one peak is observed at < 290 °C for the 5.0%Co/MgO sample.…”