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

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 per formed in the Cmcm space group. Four Br phi nsted acid sites were recognize d. 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, abo ut half of the acid sites vibrate in the 12-ring channel, and half in the s maller channels. The occupancy of these sites varies in the range 7-23%, an d all the OD groups show regular site geometry. The concentration of Br phi nsted 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 d ehydroxylation process occurring during calcination. Few other extraframewo rk sites, with low occupancy and at large distances from the framework oxyg en atoms, were found, and are attributed to reabsorbed H2O molecules. (C) 2 000 Elsevier Science B.V. All rights reserved.