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 Zn2+-exchanged NaX. Dehydrated ZnX with a Zn:Na ratio of 3.6:1 was refined in the space group Fd3m. 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, AlO4 5-, was refined in the center of the sodalite cage; this anion was also observed by 27Al MAS NMR. The reactions of hydrochlorofluorocarbon (HCFC)-124a (CF2HCF2Cl) over NaX, and two Zn2+-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 (CF2CF2 and CF2CFCl) desorb from the zeolites and are detected by MS. In contrast, saturated products are primarily observed when the reaction is followed with 19F NMR in sealed ampules. Both CF2CF2 and HFC-125 (CF3CF2H) are observed for reactions over NaX, while on Zn2+ exchange, CF2CF2 is no longer observed. Instead, a resonance is observed at −123 ppm, which is tentatively assigned to (CF2) n groups. No evidence is seen by 27Al and 19F 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.

New Force Field for Na Cations in Faujasite-Type Zeolites

“…The agreement is excellent with the exception of distances involving Na(I), which is often reported by experiments to reside in the center of the hexagonal prism. ,,, Our force field results suggest that Na(I) is not located at the center of the hexagonal prism, but rather is in one of two symmetric S I sites displaced by about 0.6 Å along the [111] direction. This displacement has been observed recently by Engelhardt 20 using DOR 23 Na NMR and confirmed by synchrotron X-ray powder diffraction for Zn cations in zeolite X . This explains why our Na(I)−O(3) and Na(I)−Na(I‘) distances are generally shorter than most experimental findings but are in agreement with Engelhardt's results.…”

“…This displacement has been observed recently by Engelhardt 20 using DOR 23 Na NMR and confirmed by synchrotron X-ray powder diffraction for Zn cations in zeolite X. 21 This explains why our Na(I)-O(3) and Na(I)-Na(I′) distances are generally shorter than most experimental findings but are in agreement with Engelhardt's results.…”

“…However, we believe that the average T-site method is unrealistic for modeling cations in zeolites, because of the close proximity between cations and frame atoms. In particular, an average T-site model fails to account for the different site III‘ positions and occupancies, ,, and assigns an erroneous symmetric position to cations in site I. − In addition, vibrational spectra of CO in Na−Y detect three different infrared (IR) frequencies for CO adsorbed near Na(II), corresponding to six-rings containing one, two, or three aluminum atoms . In an effort to model the framework charge distribution more faithfully, we develop a force field that contains different partial charges on silicon and aluminum.…”

Crystallographic Study of the Reaction of Zinc Vapor with Fully Cd2+-Exchanged Zeolite X. Complete Reduction of Cd2+ by Zn, Extraction of SiO44- and AlO45- from the Zeolite Framework, and Reduction of Si4+ to Si

“…The Al-O distances of the tetrahedral orthoaluminate ions in the structure of dehydrated Zn 2+ -exchanged zeolite X 22 are 1.79-(6) Å; that these are orthoaluminate and not orthosilicate ions has been verified by solid-state NMR. 25 In Ni-X, also prepared by aqueous ion-exchange, the T-O distance, 1.75(3) Å, also suggest that T is Al. 23 The high esds of all of the above bond lengths decrease the certainty of these assignments for the current structure.…”

Crystal Structure of a Zinc Sorption Complex of Cd2+-Exchanged Zeolite X Containing Tetrahedral Cd2+4(μ3-Zn0Cd2+Zn0)4 Clusters

“…The Na + ions in zeolite X can be completely and stoichiometrically exchanged by group IIA metal cations. − Among the dipositive transition metal cations, however, only (Mn 2+ ) and (Cd 2+ ) have been so simply exchanged into zeolite X. More complex outcomes are seen when aqueous ion exchange is attempted with (Co 2+ ), (Pb 2+ ), and (Zn 2+ ), − to cite three examples. Protons from the hydrolysis of these cations can attack zeolite frameworks, and hydroxide ions from this same hydrolysis may accompany cations into the zeolitic structure.…”