THE EFFECT OF AL, FE, AND IN SUBSTITUTION IN THE MFI SILICATE STRUCTURE ON THE AROMATIC HYDROCARBON TRANSFORMATION - SI-OH-M SITE STRENGTH

The conversion of toluene in toluene disproportionation and its alkyla tion with ethylene over metallosilicates (Al, Fe, In) of the MFI struc ture has been found to be proportional to the concentration of the ind ividual framework bridging Si-OH-M groups, and when related to one Si- OH-M site, it increases in the sequence of the isomorphous substitutio n of Si for In < Fe < Al. This clearly reflects the increasing acid st rength of the Si-OH-M group from In, Fe, to Al. As the alkylation of t oluene with isopropanol is controlled by the desorption/transport rate of the bulky propyltoluenes from the molecular sieve channels, no cor relation between the molecular sieve acidity and conversion-selectivit y for this reaction has been found. All metallosilicates (Al, Fe, In) exhibit para-shape selectivity; however, no definite conclusion on the effect of the acidity on the para-shape selectivity can be drawn. Alt hough the Al-silicates contain Si-OH-M sites with the highest acid str ength, the deactivation of Fe and In-silicates by coking is greater co mpared to Al analogs, likely due to the higher relative amount of extr aframework metal species.