Chemical Vapor Deposition of Trimethylaluminum on Dealuminated Faujasite Zeolite

摘要

Chemical vapor deposition of trimethylalumi- num (TMA) was explored as an approach for the preparation of model faujasite-type catalysts containing extraframework aluminum. The decomposition of the grafted organoaluminum species was investigated in hydrogen and oxygen atmosphere. The process of grafting Al-containing species and the associated changes of the zeolite hydroxyl groups were followed by in situ FTIR spectroscopy. The state of intrazeolite Al atoms, the changes in zeolite structure and acidity caused by the CVD procedure as well as by subsequent treatment were analyzed in detail by ~1H, ~(29)Si, and ~(27)Al MAS NMR, COads IR, H/D exchange of acidic hydroxyl groups with perdeuterobenzene, and propane cracking. Reaction of an extraframework aluminum-free high-silica faujasite zeolite with TMA leads to nearly complete substitution of the bridging hydroxyl groups with Al species. The reaction, however, does not produce uniform homogeneously distributed species. Because of the high reactivity of TMA, the zeolite lattice is partially decomposed resulting in its partial dealumination and formation of stable Si-CH3 moieties. The exact conditions of post-CVD treatment influence strongly the chemical and catalytic properties of the zeolites. The strongest increase of the propane conversion rate was observed when grafted TMA species were decomposed in H2 at high temperature. Such zeolite displays much higher activity per Bronsted acid site in propane cracking than a commercial ultrastabilized Y zeolite. It is proposed that the activity enhancement is related to strong polarization of a fraction of the zeolite Bronsted acid sites by Lewis acid sites formed by the hydrogenolysis of grafted TMA complexes.