Incorporation of transition metal ions in aluminophosphate molecular sieves with AST structure

Transition metal ion containing aluminophosphate molecular sieves with AlPO 4-16 (AST) structure have been synthesized using the classic template, quin uclidine, and the corresponding salts of V4+, Co2+, Cr3+, and Mn2+. The cub ic AST form was the main product. but in the presence of fluoride ions, the tetragonal AST form could also be obtained. The AST materials were charact erized with X-ray diffraction, thermal analysis, scanning electron microsco py, electron spin resonance, infrared spectroscopy, and diffuse reflectance spectroscopy in the UV-vis-NIR region. Spectroscopic measurements showed t hat small amounts of Co2+ and Mn2+ can be incorporated in the framework. Th e diffuse reflectance spectra are indicative for the presence of two types of Co2+, most probably taking the two framework positions T-1 and T-2 with a T-1:T-2 occupation ratio of 3-8. In contrast, V4+ is present in defect si tes by coordination to two (or three) framework oxygen atoms, The incorpora tion of Cr3+ in the AST framework is also uncertain and two surface Cr3+ sp ecies were observed: clusters similar to the bulk Cr2O3 oxide and isolated Cr3+ ions in strongly distorted octahedral coordination. These spectroscopi c observations were confirmed by thermal analysis results, which showed tha t the degree of metal ion incorporation is inversely proportional with the phase transition temperature of AST to berlinite. Furthermore, Co2+ and V4 are very stable in the AST structure and difficult to oxidize upon heating .