Incorporation of transition metal ions in aluminophosphate molecular sieves with AST structure
Faculty of Sciences. Chemistry
The journal of physical chemistry : B : condensed matter, materials, surfaces, interfaces and biophysical. - Washington, D.C.
, p. 2677-2686
University of Antwerp
Transition metal ion containing aluminophosphate molecular sieves with AlPO(4)-16 (AST) structure have been synthesized using the classic template, quinuclidine, and the corresponding salts of V(4+), Co(2+), Cr(3+), and Mn(2+). The cubic 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 characterized with X-ray diffraction, thermal analysis, scanning electron microscopy, electron spin resonance, infrared spectroscopy, and diffuse reflectance spectroscopy in the UV-vis-NIR region. Spectroscopic measurements showed that small amounts of Co(2+) and Mn(2+) can be incorporated in the framework. The diffuse reflectance spectra are indicative for the presence of two types of Co(2+), 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, V(4+) is present in defect sites by coordination to two (or three) framework oxygen atoms, The incorporation of Cr(3+) in the AST framework is also uncertain and two surface Cr(3+) species were observed: clusters similar to the bulk Cr(2)O(3) oxide and isolated Cr(3+) ions in strongly distorted octahedral coordination. These spectroscopic observations were confirmed by thermal analysis results, which showed that the degree of metal ion incorporation is inversely proportional with the phase transition temperature of AST to berlinite. Furthermore, Co(2+) and V(4+) are very stable in the AST structure and difficult to oxidize upon heating.