Collect. Czech. Chem. Commun. 1993, 58, 2474-2488

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

Rudolf Zahradník, Pavel Hobza, Blanka Wichterlová and Jiří Čejka

J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, 182 23 Prague 8, Czech Republic


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.