Ab Initio Simulation of Lewis Sites in Mordenite and Comparative Study of the Strength of Active Sites via CO Adsorption

Ab initio density-functional investigations of intrazeolite active centers have been performed for mordenite. We investigate the structure and properties of bare Al 3+ cations bound to the framework and originating from extra-framework Al(H 2 O) 6 3+ cations dehydrated at severe thermal treatment of the zeolite. A bare Al 3+ cation in the zeolite causes either a limited local relaxation or is trapped in a small ring of the zeolite structure without substantial relaxation of the framework. Such a cation behaves as a strong Lewis site. The strength of trapped Al 3+ is evaluated via the adsorption of CO and compared to a variety of zeolite active centers such as Brønsted sites, Na-counterions, and surface silanols. An overall agreement between experimental and calculated shifts of the CO stretching frequency is observed. The comparison of bare and hydrated Al 3+ and Ga 3+ cations shows that stretching frequencies blue-shifted to ∼2230 cm -1 can originate only from adsorption on bare cations. In zeolites not containing any exchanged extra-framework cations the presence of stretching bands at ∼2230 thus cm -1 evidences the presence of highly active bare Al 3+ trapped in five-membered rings of the zeolite framework.