Preferential Siting of Aluminum Heteroatoms in the Zeolite Catalyst Al-SSZ-70.

Abstract

The adsorption and reaction properties of heterogeneous zeolite catalysts (e.g. for catalytic cracking of petroleum, partial oxidation of natural gas) depend strongly on the types and distributions of Al heteroatoms in the aluminosilicate frameworks. The origins of these properties have been challenging to discern, owing in part to the structural complexity of aluminosilicate zeolites. Herein, combined solid-state NMR and synchrotron X-ray powder diffraction analyses show the Al atoms locate preferentially in certain framework sites in the zeolite catalyst Al-SSZ-70. Through-covalent-bond 2D 27 Al{29 Si} J-correlation NMR spectra allow distinct framework Al sites to be identified and their relative occupancies quantified. The analyses show that 94 % of the Al atoms are located at the surfaces of the large-pore interlayer channels of Al-SSZ-70, while only 6 % are in the sub-nm intralayer channels. The selective siting of Al atoms accounts for the reaction properties of catalysts derived from SSZ-70.