Investigation of paramagnetic catalysts by solid state NMR spectroscopy

Abstract

A spin echo mapping (SEM) technique has been used to record the total 31P-NMR information in paramagnetic phosphate-based catalysts. For these materials, conventional NMR techniques give poor information and a great proportion of 31P nuclei cannot be detected due to broad and shifted signals. By contrast to conventional sequences, all 31P nuclei are detected using a mapping technique. Under specific conditions, the method is quantitative and gives direct information on the nature, the amount and the oxidation state of neighboring paramagnetic species. For vanadium phosphorus oxide catalysts (VPO), the technique is particularly interesting for characterizing intermediates formed during catalytic reactions, especially when the latter are amorphous and cannot be detected by XRD. Moreover, 31P-NMR spectra recorded at various temperatures give precious information on the magnetic properties of these materials. In the case of cobalt-substituted aluminophosphate molecular sieves, P/Co ratios as well as the various populations P( nCo, 4- nAl) can be derived from 31P-NMR spectra. Such parameters, which cannot be obtained by other spectroscopic techniques, are of particular importance for the understanding of the acidic and redox properties of these catalysts.