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

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. It seems that during the dealumination of ZSM-12 the acid “washes” all the extraframework Al atoms. This was not the case for β zeolite since the as-synthesized and dealuminated β zeolites with Si/Al ≤ 80 possess Lewis sites. By coupling results from the NH3-STPD and FT-IR studies, we found that the Lewis sites of β zeolites are of weak strength since chemisorbed ammonia does not remain on those sites at temperatures higher than about 300 °C. These results are in very good agreement with the Al-27 NMR data which indicate mostly the presence of tetrahedral sites for ZSM-12; an extremely small abundance of octahedral sites were detected only with Si/Al ratios < 40. Si-29 NMR data can be described by at least three distinctly different Si sites. On the basis of the chemical shift and intensity distribution data, two of these sites can be assigned to Q4 and the remaining Si site is consistent with being a Q3 site. For β zeolite Al-27 NMR spectra show a relatively high concentration of octahedral sites, thus supporting the presence of Lewis sites. Furthermore, the NMR peaks for the tetrahedral sites in β zeolite are asymmetric and broad compared to ZSM-12 samples, indicating the presence of two or more different tetrahedral or distorted tetrahedral sites. The Si-29 NMR spectra of β zeolite are qualitatively similar to those observed for the ZSM-12 samples.