Structural Characterization and Hydrochlorofluorocarbon Reactivity on Zinc-Exchanged NaX

Abstract

Solid-state MAS NMR and synchrotron X-ray powder diffraction have been used to study the reactivity and structure of Zn 2+ -exchanged NaX. Dehydrated ZnX with a Zn:Na ratio of 3.6:1 was refined in the space group Fd 3 m . The zinc cations were located in four distinct positions all along the [111] direction. Residual sodium cations were found in the supercage at the site II position. A tetrahedral extraframework aluminum species, AlO 4 5-, was refined in the center of the sodalite cage; this anion was also observed by 27 Al MAS NMR. The reactions of hydrochlorofluorocarbon (HCFC)-124a (CF 2 HCF 2 Cl) over NaX, and two Zn 2+ -exchanged NaX samples, were studied with NMR and a mass spectrometry (MS)-equipped catalysis unit. The products of both the dehydrofluorination and dehydrochlorination reactions are observed over all zeolites: the unsaturated products (CF 2 CF 2 and CF 2 CFCl) desorb from the zeolites and are detected by MS. In contrast, saturated products are primarily observed when the reaction is followed with 19 F NMR in sealed ampules. Both CF 2 CF 2 and HFC-125 (CF 3 CF 2 H) are observed for reactions over NaX, while on Zn 2+ exchange, CF 2 CF 2 is no longer observed. Instead, a resonance is observed at −123 ppm, which is tentatively assigned to (CF 2 ) n groups. No evidence is seen by 27 Al and 19 F NMR for attack of the zeolite framework when the reactions are performed at low temperatures. An in situ X-ray diffraction study of HCFC-124a loaded ZnX shows significant structural changes at the same temperature as the onset of HCFC reactivity.