Local Environment of Phosphorus Atoms in CoAPO4-n Molecular Sieves: A 31P NMR Study

Abstract

A spin-echo mapping technique has been used to obtain the 31P NMR spectra of various cobalt-substituted aluminophosphate molecular sieves. The technique involves recording Hahn-echo spectra at different carrier frequencies in order to observe the signal within a broad spectral region. In contrast to the more commonly used magic angle spinning (MAS) or simple Hahn-echo methods, all phosphorus nuclei could be detected, even in the case of samples with high Co contents. Resonances are usually broad and range between about −500 and 10 000 ppm/H3PO4, but the resolution is sufficient to access the local structure around the 31P nuclei. The technique is particularly interesting since the presence of signals above 500 ppm is direct proof for the framework cobalt siting. Indeed, such signals were not observed for Co-impregnated samples and are thus characteristic of framework Co species. The different NMR lines of the spectra have been assigned to various P(nCo) environments in the structure, and their shift was f...