Catalytic studies with dealuminated Y zeolite. Part 3.—n-Hexane cracking

Abstract

The activity of dealuminated HY zeolite catalysts for n-hexane cracking has been studied at 673 K under continuous-flow conditions and in the range 536–673 K under pulsed-flow conditions. Catalysts were dealuminated hydrothermally at different temperatures and partial pressures of steam, or chemically by SiCl4 treatment at different temperatures; further catalysts were prepared by extracting non-framework aluminium from those prepared by hydrothermal treatment using aqueous solutions of EDTA salts. In all cases, with the exception of one catalyst under pulse-flow conditions, extraction of non-framework aluminium resulted in increased catalytic activity, showing that detrimental effects of such aluminium on catalytic activity outweighed any beneficial effects. Plots of activity vs. structural aluminium content, AlF, after different reaction times were bell-shaped, possessing activity maxima at ca. 29 AlF per unit cell for hydrothermally dealuminated catalysts and at ca. 16 AlF per unit cell for those dealuminated with SiCl4. Furthermore, the AlF content for maximum activity with hydrothermally dealuminated catalysts appeared to be independent of the reaction studied, in that published data for the disproportionation of ethylbenzene and toluene also gave maxima at ca. 29 AlF per unit cell. The mechanism of n-hexane cracking is discussed in terms of protolytic fission via non-classical carbonium ion formation on single, strong Bronsted acid sites, with subsequent β-scission of carbenium ions on two sites. Cracking mechanism ratios show that products from protolytic fission predominate with low-activity catalysts, whereas those from β-scission predominate with high-activity catalysts.