Author's personal copy Mobility of acidic protons in zeolites: A neutron diffraction study of D-heulandite

Abstract

a b s t r a c t Neutron Rietveld refinement of a natural heulandite (Si/Al = 3.1) in its deuterium form was performed in space group C2/m. Two Brønsted acid sites were identified. One was on framework oxygen O1, occupied to 20% and headed toward the center of the 8-membered ring channel running parallel to [1 0 2]; the other was on O6, occupied to 30% and headed toward the 10-membered ring channel running parallel to the c-axis. Three other extraframework sites, located around a distance of 3 Å from the framework oxygens, were attributed to reabsorbed H 2 O molecules. On the whole, about 3.2 Brønsted acid sites were located representing about 37% of the value expected on the basis of the aluminium content. This discrepancy is attributed to proton transfer from th