Identity of two types of strong Bronsted acid sites in mazzite revealed by CO probe: IR study and periodic DFT modeling

The nature of catalytic Bronsted sites in mazzite is clarified by molecular modeling combined with spectroscopy. Density Functional Theory study for periodic models of high-silica mazzite evidence that most stable bridging hydroxyls, noticeably binding CO probe, fall into two categories: Bronsted sites located in larger channels, characterized by higher OH frequency of bare hydroxyl with very large redshift upon CO interaction, and lower-frequency sites located in smaller channels, showing lower redshift. This fully corresponds to two bands obtained for OH stretch in IR spectra. Very good agreement between theory and experiment found in this work not only confirms that Bronsted sites studied here belong to the strongest acid sites among known zeolites but also clarifies their identity in mazzite. Location of sites with exceptionally large red shift upon CO adsorption at 12-T wide channel very well conforms to both intuitive expectations and predictions for other zeolites from former studies.