Acidity and active sites of Al-MCM-41

The influence of the aluminum contents of MCM-41 (Si/Al ratio varied betwee n 2.7 and 69) on the coordination of Al, on the acidity, and on the catalyt ic properties is studied by Al-27 MAS NMR, temperature programmed desorptio n of ammonia, and the conversion of acetone. Based on TPDA results, an assi gnment of the peaks of desorption of ammonia is proposed. With low Al conte nts, the concentration of strong Bronsted acid sites, which are attributed to tetrahedral aluminum, increases with growing Al amounts. At higher Al co ntents, however, the number of strong acid sires decreases again. Besides u sual strong Bronsted sites, TPDA reveals the existence of weak Bronsted and Lewis sites and of Lewis sites of a high acidic strength. Separation betwe en the two types of sites of weak acidity is incomplete. In the temperature programmed decomposition of NH4-exchanged MCM-41 samples , strong Lewis acid sites temporarily re-adsorb ammonia generated by the th ermal decomposition of the NH4+ ions located at the Bronsted sites. Ammonia readsorption 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 Bronsted sites. To evaluate the ac idity of Al-MCM-41, recording of decomposition profiles has been extended t o Ga- and Fe-MCM-41. Catalytic activity in the conversion of acetone reaches its maximum with th e sample based on Al-MCM-41 with the molar Si/Al ratio of 6.85. Highly favo red formation of isobutene points to a comparably high acidic strength of t he active sites. (C) 1999 Elsevier Science B.V. All rights reserved.