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

Abstract

A1PO-5 and A1PO-11 containing cobalt were studied by diffuse reflectance electronic absorption spectroscopy in combination with 31P NMR. Quantitative 31P NMR and relaxation measurements (Carr—Purcell—Gill—Meiboom procedure) were performed on calcined samples with increasing amounts of paramagnetic cobalt on lattice positions. The isomorphous substitution of aluminium by cobalt leads to NMR-invisible phosphorus (up to 30%); at least the first and third sphere (P) have to become NMR invisible to explain the observed losses for the lowest cobalt loadings. The amount of undetectable phosphorus, however, does not increase proportionally to the cobalt content, suggesting clustering of cobalt on lattice positions, as previously reported for CoAPO-50. The 31P T 2 relaxation behaviour of NMR-detectable phosphorus is also influenced by framework cobalt. The measured T 2 decays were fitted with a biexponential Lorentzian function. Both the T 2 values, as well as the relative contributions, change with increasing amounts of lattice cobalt. Impregnated samples (containing extralattice cobalt) behave essentially like normal aluminophosphates. This is probably due to the poor dispersion of cobalt throughout the crystal in the impregnated samples.