Investigation of paramagnetic catalysts by solid state NMR spectroscopy

A spin echo mapping (SEM) technique has been used to record the total P-31- NMR information in paramagnetic phosphate-based catalysts. For these materi als, conventional NMR techniques give poor information and a great proporti on of P-31 nuclei cannot be detected due to broad and shifted signals. By c ontrast to conventional sequences, all P-31 nuclei are detected using a map ping technique. Under specific conditions, the method is quantitative and g ives direct information on the nature, the amount and the oxidation state o f neighboring paramagnetic species. For vanadium phosphorus oxide catalysts (VPO), the technique is particularly interesting for characterizing interm ediates formed during catalytic reactions, especially when the latter are a morphous and cannot be detected by XRD. Moreover, P-31-NMR spectra recorded at various temperatures give precious information on the magnetic properti es of these materials. In the case of cobalt-substituted aluminophosphate m olecular sieves, P/Co ratios as well as the various populations P(nCo. 4-nA l) can be derived from P-31-NMR spectra. Such parameters, which cannot be o btained by other spectroscopic techniques, are of particular importance for the understanding of the acidic and redox properties of these catalysts. ( C) 1999 Elsevier Science B.V. All rights reserved.