NH3as a Probe Molecule for NMR and IR Study of Zeolite Catalyst Acidity

The measurement of the numbers of Brönsted and Lewis acid sites has been performed quantitatively using NH 3 as an infrared molecular probe. The assignment of the deformation NH 4 + and NH 3 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 Brönsted sites is equal to the number of OH bridging a silicon to an aluminum in a 4 Q(1Al) cluster. This represents a variable fraction of the content in framework aluminum (FAl), depending on the zeolite composition. The bridging OH in 4 Q( n Al), n > 1, clusters are not Brönsted sites, in agreement with the results of a recent REDOR study. The number of Lewis sites (L:NH 3 ) is a fraction of the nonframework aluminum content (NFAl). The Lewis sites dispersion ratio L:NH 3 /NFAl is between 75 and 40%, depending upon the temperature (115 or 175 °C) used for outgassing the sample after NH 3 adsorption. The quantitative results obtained with NH 3 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 Brönsted sites exist. The strongest Brönsted sites are related to FAl 1, a computable number representing a bridging OH in a 4 Q(1Al) environment with no next-nearest-neighbor aluminum.