Detection of Lewis acid sites using NO as probe molecule

EPR spectra of H-ZSM-5 activated at 773 and 1073 K for non-treated and hydr othermally treated samples were measured without loading and loaded with tw o molecules of NO per three aluminum atoms of the zeolite framework. In acc ordance 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 fo und that, inspite of the loading with NO, this signal is absent for the hyd rothermally treated sample activated at 773 K, while it appears for the cor responding untreated sample and the treated and non-treated samples activat ed at 1073 K. Taking into account the time dependence of the signal intensi ty, we must conclude that only in the latter two cases, stable Lewis acid s ites (SLAS) are formed. This finding should provide an additional aspect fo r the explanation of the enhancement of the catalytic activity of H-ZSM-5 t hrough a mild hydrothermal treatment. (C) 2000 Elsevier Science B.V. All ri ghts reserved.