The Sites of Molecular and Dissociative Hydrogen Adsorption in High-Silica Zeolites Modified with Zinc Ions. III DRIFT Study of H<Subscript>2</Subscript> Adsorption by the Zeolites with Different Zinc Content and Si/Al Ratios in the Framework The sites of molecular and dissociative hydrogen adsorption in high- silica zeolites modified with zinc ions. III DRIFT study of H2 adsorption by the zeolites with different zinc content and Si/Al ratios in the framework V.B. Kazanskya,�, A.I. Serykha, B.G. Andersonb and R.A. van Santenb aZelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky prospect 47, Moscow 119991, Russia bEindhoven University of Technology, P.O. Box 513 5600 MB Eindhoven, The Netherlands Received 24 September 2002; accepted 3 April 2003 DRIFT study of dihydrogen adsorbed at 77K by the zinc-modified hydrogen forms of mordenite and ZSM-5 zeolites with the different Si/Al ratios in the framework indicated the existence of several adsorption sites connected with the modifying zinc ions. The fraction of the sites of the strongest perturbation of adsorbed molecular hydrogen with the H–H stretching frequency of ca. 3930–3950 cm�1 is the highest at the highest Si/Al ratios of 41 or 80. These sites dissociatively adsorb hydrogen at moderately elevated temperatures. Adsorption of hydrogen with the higher H–H stretching frequency of about 3970–4000 cm�1 predominates at the lower Si/Al ratios. It does not result in the dissociation of adsorbed molecules. It was concluded that the most active sites are most probably connected with the zinc ions, which compensate two distantly separated negatively charged ½AlO4�� tetrahedra localized in the adjacent five- or six-membered rings on the walls of the large channels of the pentasil’s framework. The sites of the weaker perturbation of adsorbed hydrogen are most probably connected with Znþ2 ions at the conventional exchangeable cationic positions with two aluminum atoms in the same five- or six-membered ring.