The structure of a dehydrated partially magnesium exchanged zeolite a (∼Mg 2Na 8A) by neutron profile refinement

Abstract

The structure of a dehydrated sample of partially magnesium-exchanged zeolite A [∼Mg 2Na 8A] has been determined by Rietveld profile refinement methods. The space group employed was Fm 3c and a refined to 24.4244(14) Å. The final R pw was 10.9%. Exchangeable cations were located in two sites, the first being on the threefold axis on the α-cage side of the 6-ring, while the second lies in the plane of the 8-ring. The magnesium ions and most of the sodium ions lie on the first site while the remaining sodium ions lie on the second. A small amount of water ( ∼ 3 4 H 2O per α-cage) remained in the sample despite the vigorous dehydration conditions employed; this water lies on the threefold axis, further into the α-cage than the exchangeable cations in 6-ring sites, and is coordinated to some of the magnesium cations. Probably as a result of damage to the sample during the Mg 2+ ion exchange, the peaks were broader than those found in some of the previous studies of ion-exchanged zeolite A samples. Attempts at refinement using Lorentzian, rather than Gaussian, peak-shape functions produced better fits to the experimental diffraction profile ( R pw = 7.9%) but did not improve the esd's in the atomic parameters.