Calculated magnetic properties for the characterization of zeolite active sites.

Abstract

Detailed structures of zeolite catalysts, including Al and cation distribution, framework and catalytic site geometries, are not fully accessible from experiment. Since the magnetic properties of framework elements and extra-framework cations are strongly dependent on their environment, the combined use of magic angle spinning NMR or ESR techniques with quantum chemical calculations is very useful to establish the local structure around specific sites. General effects of Al and B substitution in the zeolite framework, coupled with H(+) or Na(+) counter-ions, on the (29)Si, (27)Al and (11)B NMR spectra were studied, using a density functional theory (DFT)-based methodology, for a model of the zeolite mazzite. In agreement with experiment, the exchange of Na(+) by H(+) in the boron compound is accompanied by a change of B coordination from tetrahedral to trigonal, with a characteristic downfield shift of around 10 ppm, whereas the presence of water restores the tetrahedral boron and its NMR chemical shift. Further, ESR spectra of zeolites exchanged with open-shell cations provide useful data on the metal coordination and its reactivity compared with calculated model ESR properties. The ESR hyperfine coupling constants, calculated using DFT, for models of different Cu sites of a Cu(II)-Y zeolite, with and without H(2)O or NH(3), show a clear correlation between the Cu spin population and its coordination, involving the participation of the zeolite framework in the reactivity.