Distribution of cobalt in CoAPO-5 and CoAPO-11 studied by 31P NMR

AlPO-5 and AlPO-11 contg. Co were studied by diffuse reflectance electronic absorption spectroscopy in combination with 31P NMR. Quant. 31P NMR and relaxation measurements (Carr-Purcell-Gill-Meiboom procedure) were performed on calcined samples with increasing amts. of paramagnetic Co on lattice positions. The isomorphous substitution of Al by Co leads to NMR-invisible P (up to 30%); at least the 1st and 3rd sphere (P) have to become NMR invisible to explain the obsd. losses for the lowest Co loadings. The amt. of undetectable P, however, does not increase proportionally to the Co content, suggesting clustering of Co on lattice positions, as previously reported for CoAPO-50. The 31P T2 relaxation behavior of NMR-detectable P is also influenced by framework Co. The measured T2 decays were fitted with a biexponential Lorentzian function. Both the T2 values, as well as the relative contributions, change with increasing amts. of lattice Co. Impregnated samples (contg. extralattice Co) behave essentially like normal aluminophosphates. This is probably due to the poor dispersion of Co throughout the crystal in the impregnated samples. CrAPO-11 and VAPO-11 aluminophosphates were similarly studied. [on SciFinder (R)]