Effect of fluoride ions on the acidic and catalytic properties of beta zeolite

Abstract

Fluorided beta Zeolite samples containing 0–1.6 wt.% F have been obtained by treating the original NH₄-beta zeolite with different quantities of NH₄F solution followed by drying and calcination. The samples were characterised by X-ray diffraction (XRD), FIR, NMR, X-ray photoelectron spectroscopy (XPS), chemical analysis and catalytic studies. XRD indicated that severe treatment with NH₄F results in the formation of (NH₄)₂SiF₆ and (NH₄)₃AIF₆ species that decompose on heating at 723 K to fluorocomplexes of Si and Al and occluded fluoride ions. IR spectroscopy showed that flouridation results not only in an increase and then decrease in the relative intensity of the band at 3610 cm^–1, associated with structural Brønsted acid sites (with respect to the 3740 cm^–1 band), but also an initial increase and then decrease in the Brønsted-to-Lewis (B/L) ratio of the zeolite. An Al 2p XPS study indicated the presence of two component peaks in fluorided zeolites. The peak at lower binding energy, E_b, corresponds to the zeolitic Al framework atoms (not interacting with fluoride ions) and that at higher E_b to the Al atoms interacting with fluoride ions. However, the F 1s XPS study showed the presence of three component peaks, at low, medium and high E_b, corresponding to fluoride ions interacting with Al atoms, occluded/trapped fluoride ions and fluoride ions interacting with Si atoms. The N 1s XP spectra also exhibited three component peaks in both fluorided and unfluorided beta zeolites that were previously assigned to pyridine chemisorbed on Lewis acid sites and relatively weak and strong Brønsted acid sites. A simultaneous decrease in the relative intensity of the strong Brønsted component and an increase in the relative intensity of the Lewis acid component as a function of fluorine content was observed in the N 1s XP spectra. The ²⁷ Al NMR study showed that octahedral Al species present in the starting material were transformed into tetrahedral Al species by fluoridation suggesting that some octaheral Al atoms belong to the zeolite framework. Catalytic activity data suggested that the beta zeolite with 0.3% F had a higher B/L ratio and higher catalytic activity for n-hexane and cyclohexene conversion reactions. In the light of IR, XPS and NMR data and the catalytic activity study, it is concluded that an optimum fluoride ion concentration is needed to improve the catalytic activity of beta zeolite and the inrease in catalytic activity is mainly due to the increase in the acidic strength of the hydroxy groups remaining after fluoridation.