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 be en 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 inf rared 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 bridg ing a silicon to an aluminum in a (4)Q(1A1) cluster. This represents a variable fraction of the content in framework aluminum (FA1), dependi ng on the zeolite composition. The bridging OH in (4)Q(nAI), n > 1, cl usters are not Bronsted sites, in agreement with the results of a rece nt REDOR study. The number of Lewis sites (L:NH3) is a fraction of the nonframework aluminum content (NFAl). The Lewis sites dispersion rati o L:NH3/NFAl is between 75 and 40%, depending upon the temperature (11 5 or 175 degrees C) used for outgassing the sample after NH3 adsorptio n. The quantitative results obtained with NH3 IR are compared to the q ualitative results obtained earlier by low-temperature CO IR. This com parison shows that, in DHZ as well as in USY, two types of Bronsted si tes exist. The strongest Bronsted sites are related to FAl1, a computa ble number representing a bridging OH in a (4)Q(1Al) environment with no next-nearest-neighbor aluminum.