NH3 as a Probe Molecule for NMR and IR Study of Zeolite Catalyst Acidity
ABSTRACT The measurement of the numbers of Brönsted 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 Brönsted sites is equal to the number of OH bridging a silicon to an aluminum in a 4Q(1Al) cluster. This represents a variable fraction of the content in framework aluminum (FAl), depending on the zeolite composition. The bridging OH in 4Q(nAl), n > 1, clusters are not Brönsted 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 °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 Brönsted sites exist. The strongest Brönsted sites are related to FAl1, a computable number representing a bridging OH in a 4Q(1Al) environment with no next-nearest-neighbor aluminum.
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ABSTRACT: The detection and rationalization of the coordination of low concentrations of ammonia within Na-zeolite A is carried out by the application of inelastic neutron scattering (INS), using inverse geometry time of flight spectrometry to study the partial phonon density of states of in situ ammoniated Na-zeolite A. The experimental spectra are subsequently characterised by density functional calculations, incorporating pre-optimisation by geometric simulation. We find that at a concentration of four ammonia molecules per alpha-cage, the ammonia molecule coordinates with extra-framework Na(+) cations and gives rise to three structured regions in the INS spectrum. We show that these regions correspond to translational, librational and tilting motions of the ammonia molecule. These results are in agreement structurally with previous studies of ammonia within a zeolite, and thus show that INS is a valid technique for such investigations.Physical Chemistry Chemical Physics 09/2010; 12(33):9661-6. · 3.83 Impact Factor
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ABSTRACT: Because of their use as catalysts, the volume of papers published on the nature of the acid sites in zeolites is impressive. The number of acid sites is correctly estimated and their chemical nature is known. There are however numerous domains still largely unexplored, such as the structural description (topology) of the disordered surface of a real catalyst. 29Si and 27Al REDOR NMR from chemisorbed ammonia provide the spatial arrangement of the base with respect to either the Si/Al distribution in the lattice or the coordination in the nonframework Al nano-particles, shedding light on the nature of the Brønsted and Lewis sites. In particular they explain the main features ruling the strength of the Brønsted sites and the ‘dispersion’ of Lewis sites, that is the ratio of the number of sites to the number of nonframework aluminum. Copyright © 1999 John Wiley & Sons, Ltd.Magnetic Resonance in Chemistry 11/1999; 37(13):S118 - S125. · 1.53 Impact Factor
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ABSTRACT: Ammonia adsorption studies reveal that the observed Lewis acidity in the zeolite MCM-22 is derived from at least two types of framework aluminum sites (AlF), that is, octahedral AlF and three-coordinate AlF. Comparative ammonia or trimethylphosphine (TMP) adsorption experiments with MCM-22 confirm that octahedral Al species gives rise to the signal at delta(iso) approximately 0 in the 27Al NMR spectrum; this is a superposition of two NMR signals from the different Al species on the water-reconstructed zeolite surface. A sharp resonance assigned to framework Al reversibly transforms on ammonia adsorption to delta(iso)27Al approximately 55 from tetrahedral AlF, while the broad peak is assigned to nonframework aluminum which results from hydrothermal treatment. This study also demonstrates the effectiveness of 27Al magic angle spinning (MAS) and multiple quantum (MQ) MAS NMR spectroscopy as a technique for the study of zeolite reactions.Chemistry 02/2002; 8(1):162-70. · 5.83 Impact Factor