Influence of the framework composition of commerical HFAU zeolites on their activity and selectivity in m-xylene transformation

Abstract

m-Xylene transformation was carried out at 623 K over a series of commercial HFAU zeolites with framework Si/Al ratios in the 4–100 range. Nitrogen adsorption at 77 K shows that all the samples present, besides the micro-, ultramicro- and mesopores result from the collapse of part of the micropore walls during dearamination. The acidity of the samples was characterized by pyridine adsorption followed by IR spectroscopy. Contrary to what is generally found, the acid strength increases with 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 extraframework aluminium species. The very weak acidity of the more dealuminated samples seems to result from the presence of adjacent OH groups. This large difference in acid strength explains the very large difference in activity of the acid sites of the slightly and strongly dealuminated samples. With all the samples, the classical monomolecular isomerization pathway is accompanied with a bimolecular pathway involving, successively, xylene disproportionation and transalkylation of trimethylbenzenes with m-xylene. The greater the acid site density, the less significant is the proportion of bimolecular isomerization. However, this selectivity change is due to changes in acid strength rather than in site density. On the very strong acid sites of the slightly dealuminated samples the diphenylmethane intermediates of disproportionation would be rapidly transformed into coke precursors and not into the expected toluene and trimethylbenzene products, which would not be the case on the weak acid sites of the strongly dealuminated samples.