Infrared study of CO chemisorbed on Bronsted and Lewis sites in dealuminated acid Y and ZSM-5 zeolites

The potentiality of the infrared study of the CO stretching mode with respect to Bronsted acidity has been applied to dealuminated acid ZSM-5 (DHZ), ultrastable Y (USY), fluorinated USY (USYnF), and Y zeolites (DKY). The interaction of CO with OH, bridging framework silicon to a framework aluminum with zero next nearest neighbor aluminums (FAl1), yields a band at 2173 cm(-1) in DHZ and at 2177 cm(-1) in USY and USYnF with different extents of dealumination. These OH's are the most acidic. The 2173 or 2177 cm(-1) band can be separated from a band at 2163 cm(-1) which arises from CO interacting within other framework bridging OH in Si-nAl (n > 1) clusters or with nonframework OH. In contrast, CO chemisorbed on DHY yields a broad band revealing a broad distribution of OH from which it is not possible to extract any quantitative information on the strong Bronsted acid sites. Lewis sites interacting with CO are observed on DHY but they have not been observed on DHZ or USYnF, maybe because the preliminary outgassing temperature in the IR cell was not high enough (similar to 500 degrees C). Again,this study outlines the fact that CO allows distinctions which are not revealed by the OH stretching vibrations in dealuminated zeolites.