Location of Brønsted sites in mordenite

Abstract

Crystal structure refinements of mordenites indicate that partial (Si,Al) order in the tetrahedral sites regularly recurs within the observed range (6–9) Al/unit cell. Consequently, for a given Si/Al ratio, it is possible to determine the maximum number of OH groups that can be allocated on each framework oxygen. Assuming that only one proton can be bonded at the same time to the oxygens of a tetrahedron, structural data indicate that it is highly probable that Brønsted sites are on framework oxygens 02 and 07, heading toward the center of the 12-ring, and on 09, heading toward the center of the 8-ring, each with about the same population. The intensities of the i.r. stretching bands (around 3,600 cm⁻¹ wavenumbers) of the hydroxyls Si(OH)Al agree well with this result.