ISOMORPHOUS SUBSTITUTION OF SI FOR AL, GA, FE, IN AND B IN MOLECULAR-SIEVES OF MFI STRUCTURE - A QUANTUM-CHEMICAL, AMMONIA DESORPTION AND CATALYTIC ACTIVITY STUDY OF FRAMEWORK SI-OH-M ACID SITE STRENGTH

A theoretical and experimental study of metallosilicates with MFI stru cture revealed the order Al > Ga > Fe > In > B in the acid Si-OH-M fra mework site strength. Ab initio calculations of the energy and geometr y characteristics in a simple structural unit modelling the isomorphou s substitution were carried out for Al, Ga, In and B as heteroatoms in silicates. Experimental data on toluene alkylation with ethylene and temperature-programmed desorption of ammonia allowed to distinguish th e acid site strength of Al-, Fe- and In-silicates, thus, completing th e acidity sequence of metallosilicates synthesized so far. Moreover, t he study confirmed rather short-range effects resulting from isomorpho us substitution of a heteroatom into the molecular sieve framework.