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 Al3+ cations bound to the framework and originating from extra-framework Al(H2O)(6)(3+) cations dehydrated at severe thermal treatment of the zeolite. A bare Al3+ 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 Al3+ is evaluated via the adsorption of CO and compared to a variety of zeolite active centers such as Bronsted 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 Al3+ and Gall cations shows that stretching frequencies blue-shifted to similar to2230 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 similar to2230 thus cm(-1) evidences the presence of highly active bare Al3+ trapped in five-membered rings of the zeolite framework.