ISOMORPHOUS SUBSTITUTION IN ZEOLITES - ACIDIC AND CATALYTIC PROPERTIES OF [B]-, [B,AL]- AND [AL]-ZSM-5

Molecular sieves with ZSM-5-structure containing boron and/or aluminium in the framework have been synthesized using two different sources of silicon. The amount of aluminium in the [B, Al]-ZSM-5-sample was comparable with that of boron. Using B-11 MAS NMR in tandem with IR studies led to the conclusion that boron was present in the tetrahedrally coordinated framework positions. As shown by TPD and IR studies, boron incorporated into the framework positions is responsible for the generation of weak acid centres. A comparative study of the catalytic properties of the [B,Al]-ZSM-5-samples, using three different reactions, revealed that Al alone is responsible for the catalytic activity. The shape selectivity is a function of the boron content and not of the crystallite size.