Location of Brønsted sites in D-mordenites by neutron powder diffraction

Abstract

Neutron Rietveld refinements of one natural mordenite (Si/Al=5.5) and two synthetic mordenites (Si/Al=5.6 and 10.0) in their deuterium form were performed in the Cmcm space group. Four Brønsted acid sites were recognized. One was on framework oxygen O6, heading toward the side pocket; another was on O9, heading toward the center of the eight-ring; the other two were on O5 and O10, pointing toward the center of the 12-ring. On the whole, about half of the acid sites vibrate in the 12-ring channel, and half in the smaller channels. The occupancy of these sites varies in the range 7–23%, and all the OD groups show regular site geometry. The concentration of Brønsted acid sites found is around 50–60% of that which would be expected on the basis of the aluminum content. This discrepancy is attributed to the dehydroxylation process occurring during calcination. Few other extraframework sites, with low occupancy and at large distances from the framework oxygen atoms, were found, and are attributed to reabsorbed H 2O molecules.