Acidity and active sites of Al-MCM-41

The influence of the aluminum contents of MCM-41 (Si/Al ratio varied between 2.7 and 69) on the coordination of Al, on the acidity, and on the catalytic properties is studied by 27Al MAS NMR, temperature programmed desorption of ammonia, and the conversion of acetone. Based on TPDA results, an assig...

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Bibliographic Details
Published in:	Applied catalysis. A, General General, 1999-08, Vol.184 (1), p.49-60
Main Authors:	Kosslick, H, Lischke, G, Parlitz, B, Storek, W, Fricke, R
Format:	Article
Language:	eng
Subjects:	Catalysis Catalysts: preparations and properties Chemistry Exact sciences and technology General and physical chemistry Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry
Online Access:	Get full text
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Summary:	The influence of the aluminum contents of MCM-41 (Si/Al ratio varied between 2.7 and 69) on the coordination of Al, on the acidity, and on the catalytic properties is studied by 27Al MAS NMR, temperature programmed desorption of ammonia, and the conversion of acetone. Based on TPDA results, an assignment of the peaks of desorption of ammonia is proposed. With low Al contents, the concentration of strong Brønsted acid sites, which are attributed to tetrahedral aluminum, increases with growing Al amounts. At higher Al contents, however, the number of strong acid sites decreases again. Besides usual strong Brønsted sites, TPDA reveals the existence of weak Brønsted and Lewis sites and of Lewis sites of a high acidic strength. Separation between the two types of sites of weak acidity is incomplete. In the temperature programmed decomposition of NH 4-exchanged MCM-41 samples, strong Lewis acid sites temporarily re-adsorb ammonia generated by the thermal decomposition of the NH 4 + ions located at the Brønsted sites. Ammonia re-adsorption at Lewis sites results in a shift of the maximum of the TPDA peak to a higher temperature. Calcination of samples leads to the formation of strong Lewis sites at the expense of Brønsted sites. To evaluate the acidity of Al-MCM-41, recording of decomposition profiles has been extended to Ga- and Fe-MCM-41. Catalytic activity in the conversion of acetone reaches its maximum with the sample based on Al-MCM-41 with the molar Si/Al ratio of 6.85. Highly favored formation of isobutene points to a comparably high acidic strength of the active sites.
ISSN:	0926-860X 1873-3875