Detection of Lewis acid sites using NO as probe molecule

Abstract EPR spectra of H-ZSM-5 activated at 773 and 1073 K for non-treated and hydrothermally treated samples were measured without loading and loaded with two molecules of NO per three aluminum atoms of the zeolite framework. In accordance with the literature, the strong EPR signal with g =1.98 was ascribed to NO adsorbed on “true” Lewis acid sites (LAS). Interestingly, we have found that, inspite of the loading with NO, this signal is absent for the hydrothermally treated sample activated at 773 K, while it appears for the corresponding untreated sample and the treated and non-treated samples activated at 1073 K. Taking into account the time dependence of the signal intensity, we must conclude that only in the latter two cases, stable Lewis acid sites (SLAS) are formed. This finding should provide an additional aspect for the explanation of the enhancement of the catalytic activity of H-ZSM-5 through a mild hydrothermal treatment.