NH{sub 3} as a probe molecule for NMR and IR study of zeolite catalyst acidity
- Univ. of Wisconsin, Milwaukee, WI (United States)
The measurement of the numbers of Broensted and Lewis acid sites has been performed quantitatively using NH{sub 3} as an infrared molecular probe. The assignment of the deformation NH{sub 4}/{sup +} and NH{sub 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 Broensted sites is equal to the number of OH bridging a silicon to an aluminum in a {sup 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 {sup 4}Q(nAl), n > 1, clusters are not Broensted sites, in agreement with the results of a recent REDOR study. The number of Lewis sites (L:NH{sub 3}) is a fraction of the nonframework aluminum content (NFAl). The Lewis sites dispersion ratio L:NH{sub 3}/NFAl is between 75 and 40%, depending upon the temperature (115 or 175{degree}C) used for outgassing the sample after NH{sub 3} adsorption. The quantitative results obtained with NH{sub 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 Broensted sites exist. The strongest Broensted sites are related to FAl{sub 1}, a computable number representing a bridging OH in a {sup 4}Q(1Al) environment with no next-nearest-neighbor aluminum. 34 refs., 7 figs., 3 tabs.
- OSTI ID:
- 501899
- Journal Information:
- Journal of Physical Chemistry B: Materials, Surfaces, Interfaces, amp Biophysical, Journal Name: Journal of Physical Chemistry B: Materials, Surfaces, Interfaces, amp Biophysical Journal Issue: 10 Vol. 101; ISSN 1089-5647; ISSN JPCBFK
- Country of Publication:
- United States
- Language:
- English
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