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

Abstract

A theoretical and experimental study of metallosilicates with MFI structure revealed the order Al > Ga > Fe > In > B in the acid Si-OH-M framework site strength. Ab initio calculations of the energy and geometry characteristics in a simple structural unit modelling the isomorphous 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 the acid site strength of Al-, Fe- and In-silicates, thus, completing the acidity sequence of metallosilicates synthesized so far. Moreover, the study confirmed rather short-range effects resulting from isomorphous substitution of a heteroatom into the molecular sieve framework.