PHYSICOCHEMICAL AND CATALYTIC PROPERTIES OF AL-SUBSTITUTED, GA-SUBSTITUTED AND FE-SUBSTITUTED MESOPOROUS MATERIALS RELATED TO MCM-41

Al-, Ga-, and Fe-substituted mesoporous materials related to MCM-41 we re hydrothermally synthesized. The samples were characterized by XRD, SEM, MIR, TA, Si-29 and Al-Z7 MAS NMR, ESR, and IR spectroscopy of ads orbed pyridine. The results reveal the substitution of Si in tetrahedr al positions in the pore walls by Al, Ga, and Fe. In as-synthesized Al -MCM-41, aluminum is tetrahedrally coordinated only. Calcination cause s the formation of octahedrally coordinated aluminum at the expense of framework aluminum. Al-27 satellite transition spectra could be recor ded from which the isotropic chemical shift delta(iso) and the structu re-sensitive mean quadrupole coupling constant C-Q were determined. Th ese results show that the tetrahedrally coordinated aluminum atoms in the pore walls of mesoporous molecular sieves and in the framework pos itions of zeolites are closely related. From the delta(iso) values, al so the mean intertetrahedral T-O-T angles theta at the framework Al we re estimated. For the as-synthesized sample, theta amounts to 159 degr ees and for the H-form to 142 degrees, i.e. Al is highly 'stressed' in the as-synthesized MCM-41. This stress is considerably reduced after calcination which is accompanied by strong dealumination. The acidity was studied by temperature-programmed FTIR spectroscopy of chemisorbed pyridine. Both Bronsted and Lewis sites of different strength are fou nd in all substituted samples. Bronsted acidity conclusively confirms the isomorphous substitution of Si by Al, Ga, and Fe in the MCM-41 fra mework. The samples show a remarkable catalytic activity in the aceton e conversion which decreases in the order Al>Ga>Fe. This is in line wi th the decrease of the strength and of the number of acid sites in the above order. Changes of the acidity also influence the selectivity be haviour. Thus, formation of isobutene is favoured by increased acidity .