Neutron Powder Diffraction Investigation of Zeolite heulandite in its acid D-form and in its rehydrated D2O-form.

Acidic zeolites are widely used in the chemical and petroleum industry because of their catalytic activity, remarkable reaction selectivity, and excellent chemical and thermal stability. The principal acidity mechanism in these materials is the donation of Brønsted acid protons from bridging framework hydroxyls. Therefore, the location of these hydroxyl groups in acidic zeolites provides a basis for the interpretation of these properties. The aim of this work is to determine position and concentration of hydroxyl groups in calcined D-heulandite via neutron powder diffraction. Neutron powder patterns of a natural heulandite (Si/Al = 3.1) were collected at 2.25 K at the D2B line (ILL, Grenoble) in collaboration with prof. Thomas Armbruster, Laboratorium für Chemische und Mineralogische Kristallographie, University of Bern, Switzerland. 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 [102]; the other was on O6, occupied to 30% and headed toward the 10-membered ring channel running parallel to the c-axis.