Synthesis, characterization and catalysis of (Co, V)-, (Co, Cr)- and (Cr, V)APO-5 molecular sieves

A series of MAPO-5 and (M, N)APO-5 (with M and N = Co2+, Cr3+ and V4+) molecular sieves have been hydrothermally synthesized and characterized. The as-synthesized and calcined samples were investigated with XRD, ICP, DRS UV–vis spectroscopy, ESR, 27Al MAS NMR techniques to explore the valence, coordination and location of the incorporated transition metal ions. It was shown that isomorphous substitution of Co2+ for Al3+ convincingly occurred, and V4+ also anchored to the sample by coordinating to two framework oxygen atoms and most probably occupying P5+ sites, while no unambiguous spectroscopic evidence was provided for the incorporation of Cr3+ in the lattice, although chemical analysis suggests this possibility in the case that Co2+ or V4+ are added to the synthesis gel. The preferential sequence for the incorporation of these transition metal ions in the AFI framework is: Co2+ > V4+ > Cr3+. After calcination, almost all V4+ ions were oxidized to V5+, whereas Cr3+ ions were oxidized to Cr5+ and Cr6+. As catalysts, the (M, N)APO-5 molecular sieves are highly active for the selective oxidation of cyclohexane with a high cyclohexanone/cyclohexanol ratio in the presence of H2O2. The reaction temperature and solvent significantly influenced the catalytic behavior. Although some leaching of the transition metal ions was observed, the catalytic performance remained stable at least within five repeated runs.