ACID SITES IN AL-ZSM-22 AND FE-ZSM-22

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 NH3, Fti.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 Fe3+ 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 O1s 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 NH3, Fti.r. spectra in the OH as well in the pyridine regions, and the N1s 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 NH3 and N1s 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 greater-than-or-equal-to Al-ZSM-5.