Bronsted and Lewis acid sites in dealuminated ZSM-12 and beta zeolites characterized by NH3-STPD, FT-IR, and MAS NMR spectroscopy

Infrared spectroscopy, nuclear magnetic resonance, and NH3-stepwise tempera ture-programmed desorption (STPD) were used to study the acidity characteri stics of dealuminated ZSM-12 and beta zeolites in a wide range of Si/Al rat ios. For all samples, the STPD experiments revealed the presence of five di stinct peaks. A 1:1 relation between the Al atoms of each sample and ammoni a determined from the STPD experiments was derived, thus indicating that th is technique can probe accurately all acid sites for zeolites with Si/Al gr eater than or equal to about 20. A careful deconvolution of the FT-IR peaks indicated that only the as-synthesized ZSM-12 sample possessed a small num ber of Lewis acid sites. We found that ammonia desorbs completely from thos e Lewis sites upon an increase of temperature above 150 degrees C. It is re markable to note that all the dealuminated ZSM-12 samples do not possess an y Lewis acid sites. This is in contrast to what is commonly known since dur ing dealumination framework aluminun atoms move to extraframework locations . It seems that during the dealumination of ZSM-12 the acid "washes" all th e extraframework Al atoms. This was not the case for beta zeolite since the as-synthesized and dealuminated beta zeolites with Si/Al less than or equa l to 80 possess Lewis sites. By coupling results from the NH3-STPD and FT-I R studies, we found that the Lewis sites of beta zeolites are of weak stren gth since chemisorbed ammonia does not remain on those sites at temperature s higher than about 300 degrees 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 det ected only with Si/Al ratios < 40. Si-29 NMR data can be described by at le ast three distinctly different Si sites. On the basis of the chemical shift and intensity distribution data, two of these sites can be assigned to Q(4 ) and the remaining Si site is consistent with being a Q(3) site. For beta zeolite Al-27 NMR spectra show a relatively high concentration of octahedra l sites, thus supporting the presence of Lewis sites. Furthermore, the NMR peaks for the tetrahedral sites in beta zeolite are asymmetric and broad co mpared to ZSM-12 samples, indicating the presence of two or more different tetrahedral or distorted tetrahedral sites. The Si-29 NMR spectra of beta z eolite are qualitatively similar to those observed for the ZSM-12 samples.