Acid sites in Al-ZSM-22 and Fe-ZSM-22

Abstract

A crystalline ferrisilicate zeolite having the framework structure of ZSM-22 is reported. The ferrisilicate was characterized by XRD, SEM t.g.a. t.p.d. of NH 3 , Ft i.r., XPS, and hydrocarbon sorption measurements. XRD patterns of as-synthesized as well as calcined ferrisilicate zeolite matched well with that of ZSM-22 zeolite except for some minor changes in the peak intensities. The Fe 3+ ions in the ferrisilicate zeolite were in the trivalent oxidation state as confirmed by XPS study. The white color of the as-synthesized as well as that of calcined ferrisilicate sample, ion-exchange capacity, and hydrocarbon sorption data have indicated that extraframework material was not present in major amount. The O 1s XPS spectra, however, showed the presence of small amounts of extraframework iron. The acidic properties of the ferrisilicate zeolite were studied by t.p.d. of NH 3 , Ft i.r. spectra in the OH as well in the pyridine regions, and the N 1s XPS spectra of chemisorbed pyridine. I.r. spectroscopy revealed a distinct absorption band at 3628 cm −1 assigned to structural OH groups of Fe-ZSM-22 associated with lattice iron. T.p.d of NH 3 and N 1s XPS spectra of chemisorbed pyridine showed that Bronsted acid sites present in ferrisilicate zeolite are weaker than those present in the Al-ZSM-22 zeolite. From the comparison of these results with those obtained with Al-ZSM-5 and Al-ZSM-22 zeolites, the following order for the acidic strength of the framework hydroxyls is derived: Al-ZSM-22 > Fe-ZSM-22 ≥ Al-ZSM-5