UV-Raman Fingerprint of Brønsted Sites in MFI Zeolites: A Useful Marker in Dealumination Detection

In the present work, a detailed UV-Raman characterization of a set of protonic MFI zeolites with variable Si/Al ratio is presented. A new vibrational feature was detected: interestingly, such peak is present only when samples are carefully activated. Moreover, its intensity is quantitatively related to the Al content of the framework through a direct law. On these bases, the origin of the vibrational mode can be attributed to the Brønsted acid site. The assignment was further supported by means of exchange experiments (with deuterium, sodium and ammonium), demonstrating the relevant contribution of the Brønsted proton in this peculiar vibration. Finally, by applying a steaming-like treatment (known to be able to induce dealumination), the observed decrease in the mode intensity was related to the leaching of Al atoms out of the framework, i.e., to the destruction of a fraction of the former Brønsted sites population. In this regard, the monitoring by UV-Raman of such signal could represent an useful marker in detecting steaming and/or reaction induced dealumination in acidic zeolites.