Brønsted and Lewis Acid Sites in Dealuminated ZSM-12 and β Zeolites Characterized by NH3-STPD, FT-IR, and MAS NMR Spectroscopy

Abstract

Infrared spectroscopy, nuclear magnetic resonance, and NH3-stepwise temperature-programmed desorption (STPD) were used to study the acidity characteristics of dealuminated ZSM-12 and β zeolites in a wide range of Si/Al ratios. For all samples, the STPD experiments revealed the presence of five distinct peaks. A 1:1 relation between the Al atoms of each sample and ammonia determined from the STPD experiments was derived, thus indicating that this technique can probe accurately all acid sites for zeolites with Si/Al ≥ about 20. A careful deconvolution of the FT-IR peaks indicated that only the as-synthesized ZSM-12 sample possessed a small number of Lewis acid sites. We found that ammonia desorbs completely from those Lewis sites upon an increase of temperature above 150 °C. It is remarkable to note that all the dealuminated ZSM-12 samples do not possess any Lewis acid sites. This is in contrast to what is commonly known since during dealumination framework aluminun atoms move to extraframework locations. I...