INFLUENCE OF THE FRAMEWORK COMPOSITION OF COMMERCIAL HFAU ZEOLITES ONTHEIR ACTIVITY AND SELECTIVITY IN M-XYLENE TRANSFORMATION

m-Xylene transformation was carried out at 623 K over a series of comm ercial HFAU zeolites with framework Si/Al ratios in the 4-100 range. N itrogen adsorption at 77 K shows that all the samples present, besides the micro-, ultramicro- and mesopores result from the collapse of par t of the micropore walls during dealumination. The acidity of the samp les was characterized by pyridine adsorption followed by IR spectrosco py. Contrary to what is generally found, the acid strength increases w ith the density of framework aluminium atoms, hence of protonic sites. The strong acidity of the less dealuminated samples (Si/Al between 4- 16) is due to an interaction of framework protonic sites with extrafra mework aluminium species. The very weak acidity of the more dealuminat ed samples seems to result from the presence of adjacent OH groups. Th is large difference in acid strength explains the very large differenc e in activity of the acid sites of the slightly and strongly dealumina ted samples. With all the samples, the classical monomolecular isomeri zation pathway is accompanied with a bimolecular pathway involving, su ccessively, xylene disproportionation and transalkylation of trimethyl benzenes with m-xylene. The greater the acid site density, the less si gnificant is the proportion of bimolecular isomerization. However, thi s selectivity change is due to changes in acid strength rather than in site density. On the very strong acid sites of the slightly dealumina ted samples the diphenylmethane intermediates of disproportionation wo uld be rapidly transformed into coke precursors and not into the expec ted toluene and trimethylbenzene products, which would not be the case on the weak acid sites of the strongly dealuminated samples. (C) 1998 Elsevier Science B.V.