P-31 SPIN-LATTICE RELAXATION IN COBALT-CONTAINING ALUMINOPHOSPHATE MOLECULAR-SIEVES

Phosphorus spin-lattice relaxation was studied in aluminophosphate mol ecular sieves containing various concentrations of either framework or non-framework cobalt. The behaviour of nuclear magnetisation in the p resence of these paramagnetic centres was described successfully in th e limit of no spin-diffusion. The diffusionless regime was strongly in dicated with non-exponential magnetisation recovery and was therefore easy to recognise. According to the model, spin-lattice relaxation rat es depend on the square of cobalt concentration. Measured relaxation r ates agreed well with calculations if effective cobalt concentration w as considered rather than the average one. The latter was obtained by bulk elemental analysis, while the former was extracted from cobalt co ncentration depth-profiles measured with Auger electron spectroscopy. These measurements indicated that in impregnated samples containing no n-framework cobalt there could be much more cobalt near the crystal su rface than within the crystal. Because high cobalt concentration can l ead to an invisible phosphorus, only nuclei deep within the crystal co ntribute to the NMR signal. In such a case, the effective concentratio n is simply the concentration of cobalt far from the crystal surface. In our case, two impregnated samples with different bulk cobalt concen trations exhibited equal relaxation rates. Previously, such a case was misinterpreted as a case, in which nuclear spin-lattice relaxation wa s independent of cobalt concentration. AES measurements, however, reve aled, that although average concentrations of the two samples were dif ferent by a factor of two, their effective concentrations were equal a nd thus in complete agreement with observed relaxation rates. (C) 1998 Elsevier Science B.V. All rights reserved.