NH3 as a probe molecule for NMR and IR study of zeolite catalyst acidity

The measurement of the numbers of Bronsted and Lewis acid sites has been performed quantitatively using NH3 as an infrared molecular probe. The assignment of the deformation NH4+ and NH3 bands has been carried out by comparing the proton MAS NMR spectra with the corresponding infrared spectra in dealuminated acid ZSM-5 (DHZ). It has been shown that the total number of Bronsted sites is equal to the number of OH bridging a silicon to an aluminum in a (4)Q(1A1) cluster. This represents a variable fraction of the content in framework aluminum (FA1), depending on the zeolite composition. The bridging OH in (4)Q(nAI), n > 1, clusters are not Bronsted sites, in agreement with the results of a recent REDOR study. The number of Lewis sites (L:NH3) is a fraction of the nonframework aluminum content (NFAl). The Lewis sites dispersion ratio L:NH3/NFAl is between 75 and 40%, depending upon the temperature (115 or 175 degrees C) used for outgassing the sample after NH3 adsorption. The quantitative results obtained with NH3 IR are compared to the qualitative results obtained earlier by low-temperature CO IR. This comparison shows that, in DHZ as well as in USY, two types of Bronsted sites exist. The strongest Bronsted sites are related to FAl1, a computable number representing a bridging OH in a (4)Q(1Al) environment with no next-nearest-neighbor aluminum.