Mordenite Acidity: Dependence on the Si/Al Ratio and the Framework Aluminum Topology. Part 2. Acidity Investigations

University of Leipzig, Leipzig, Saxony, Germany
The Journal of Physical Chemistry (Impact Factor: 2.78). 10/1992; 96(21). DOI: 10.1021/j100200a051


The acidity of a series of mordenites dealuminated by acid refluxing was studied by calorimetric measurements of the NH3 chemisorption, protein magic angle spinning nuclear magnetic resonance spectroscopy, and temperature-programmed desorption of NH3 and NH4+ ion exchange. The number of bridged OH groups determined by H-1 MAS NMR corresponds to the number of tetrahedrally coordinated Al atoms. Starting with a low Al content (a high Si/Al ratio), the number of the strong acidic sites increases with increasing Al number of the framework up to 4.6 Al atoms/u.c. (Si/Al ratio of 9.5) and then decreases. The decrease of the strong acidity above 4.6 Al atoms/u.c. may be explained by the appearance of an aluminum atom in the second coordination sphere of the Si-OH-Al group. The experimentally found value of the maximum (at m = 0.096) in the curve of the strong acidity versus the aluminum molar fraction coincides with a value calculated by Barthomeuf from the topological Al density of mordenite, thus confirming her theoretical concept of zeolite acidity.

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    • "Furthermore, using UV–Vis diffuse reflectance spectroscopy (UV–VisDR) with iodine as a probe, Choi et al. (Choi et al., 1996) reported similar tendencies, where large red-shifts in aluminum rich zeolites with high Si/Al ratio were related to decreasing values of δ O . On the other hand, the strength of acid sites was recognized by several authors (Lohse et al., 1989; Martra et al., 2002; Mirodatos et al., 1976; Parrillo and Gorte, 1993; Stach et al., 1992; Teraishi and Akanuma, 1997; Tsutsumi et al., 1975) to be influenced by Si/Al ratio. In Si–OH–Al, Stach et al. (Stach et al., 1992) explained this phenomenon to be related to the higher electronegativity of Si when compared to Al. "
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