Acidity and catalytic behavior of substituted MCM-48

摘要

Al-, Ga-, and Fe-MCM-48 molecular sieves were synthesized and characterized by XRD, TGA, Si-29-, Al-27-, Ga-71-MAS NMR, EPR, TPDA, and FTIR spectroscopy as well as by microcalorimetry of ammonia. Catalytic activity and selectivity were studied by the conversion of acetone. The substituting elements are, in part, incorporated into framework positions giving rise to substantial Bronsted acidity. Lewis acidity is probably generated by extra-framework species. Temperature maxima of TPDA decrease in the order: 460 degrees C (Al) > 430 degrees C (Ga) > 410 degrees C (Fe). TPDA and FTIR results prove the high thermal stability of a part of acid sites. The average acidic strength of sites is lower than that observed for acidic zeolites. Nevertheless, small portions of very strong acid sites may occur. The acidic strength of Bronsted sites differs according to on the nature of the substituting element. The majority of sites has a medium-to-weak strength, but very weak and strong sites are also present. As thermal desorption shows, a broad range of Lewis acid strength is present. Samples are catalytically active and fairly stable after an initial activity loss. Fe-MCM-48 shows stable catalytic activity. (C) 1998 Academic Press. [References: 49]