Lewis Acidity Inherent to the Framework of Zeolite Mordenite

Despite being used extensively as heterogeneous catalysts and supports at the academic and industrial levels alike, the nature of aluminum-based Lewis acidity in zeolites is not completely understood. In this study, we discovered a correlation between Lewis acidity and aluminum structure in the framework of zeolite mordenite. The amount of octahedrally coordinated aluminum in these samples present under wet conditions correlates to the number of Lewis acid sites as detected by Fourier-transform infrared spectroscopy of adsorbed probe molecules in the fully dehydrated state. We illustrate that these aluminum species, which are better considered as framework-associated and not extra-framework aluminum, have Lewis acidic properties when treated under vacuum at high temperature. This observation constitutes important progress in understanding the structure of Lewis acid sites. The Lewis acid sites are framework aluminum species which undergo a change in coordination, from tetrahedral to octahedral, depending on the charge-balancing cation and degree of hydration. These Lewis acid sites exist in the case of charge balancing by protons.