DISTRIBUTION OF COBALT IN COAPO-5 AND COAPO-11 STUDIED BY P-31 NMR

AlPO-5 and AlPO-11 containing cobalt were studied by diffuse reflectan ce electronic absorption spectroscopy in combination with P-31 NMR. Qu antitative P-31 NMR and relaxation measurements (Carr-Purcell-Gill-Mei boom procedure) were performed on calcined samples with increasing amo unts of paramagnetic cobalt on lattice positions. The isomorphous subs titution of aluminium by cobalt leads to NMR-invisible phosphorus (up to 30%); at least the first and third sphere (P) have to become NMR in visible to explain the observed losses for the lowest cobalt loadings. The amount of undetectable phosphorus, however, does not increase pro portionally to the cobalt content, suggesting clustering of cobalt on lattice positions, as previously reported for CoAPO-50. The P-31 T2 re laxation behaviour of NMR-detectable phosphorus is also influenced by framework cobalt. The measured T2 decays were fitted with a biexponent ial Lorentzian function. Both the T2 values, as well as the relative c ontributions, change with increasing amounts of lattice cobalt. Impreg nated 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.