Structure of Al-Lewis site in beta zeolite active in the Meerwein-Ponndorf-Verley reduction of ketone to alcohol

H-beta zeolites differing in removal of the template by calcination in an oxygen or in a stream of ammonia, and in their subsequent steam treatment at various temperatures, as well as beta zeolites exchanged with alkaline metal ions, were investigated in the MPV reduction of 4-tert-butyl cyclohexanone to 4-tert-butyl hexanol using isopropanol. FTIR of the OH groups, of the C=N groups of adsorbed d(3)-acetonitrile, and of the framework T-O-T antisymmetric mode was used to estimate the concentration of the individual Al-Lewis sites. XPS was employed to monitor the migration of the Al species into the surface layers of the zeolite crystals. The VHF band at 3780 cm(-1) and skeletal T-O-T bands at 901, 890, and 882 cm-1 were found for steam-treated (ST) H-beta. Both alkali metal ion exchange and treatment of zeolite by gaseous ammonia annihilated the bands at 3780 and 882 cm-1, but the bands at 901 and 890 cm-1 (siloxane bridges) were retained. While the bands at 3780 and 882 cm-1 correspond to one "partly" framework Al species bearing an OH group, we suggest that the band at 901 cm(-1) reflects perturbation of the framework due to the presence of highly acidic extraframework AIOx counter ions exhibiting Lewis properties inside the zeolite channels. The inability to replace these AIOx complex ions by alkali metal ion exchange indicates their complex structure. These extraframework AlOx species localized inside the zeolite void volume of beta zeolites represent highly active sites in the stereoselective MPV reduction of ketone to cis-alcohol, but their migration to the outer surface of zeolite crystals provides higher yields of trans-alcohol. (C) 2002 Elsevier Science (USA).