SOLID-STATE NA-23 AND AL-27 MAS NMR-STUDY OF THE DEHYDRATION OF NA2O.AL2O3.3H2O

The dehydration of Na2O·Al2O3·3H2O was investigated as a function of dehydration temperature (100-600 °C) and dehydration time, using solid-state 23Na and 27Al nuclear magnetic resonance with high-speed magic-angle spinning (MAS). 23Na and 27Al MAS NMR spectra were recorded at field strengths of 8.4, 11.7, and 14.0 T. The field dependence of the NMR measurements aid in determining the presence of a number of chemically distinct sodium and aluminum sites. The MAS NMR results indicate that both untreated and dehydrated Na2O·Al2O3·3H2O contain three chemically distinct sodium sites and one aluminum site. Simulation of the observed 23Na and 27A1 spectra yields quadrupole coupling constants and asymmetry parameters for each sodium and aluminum site which, when used in combination with isotropic chemical shift measurements, permit various structures to be assigned to the sodium and aluminum sites. Water weight-loss measurements indicate that all structural changes observed in both the 23Na and 27Al NMR spectra are related to the progress of dehydration, although not all the water is removed, even after heating at 598 °C for extended times. For untreated Na2O·Al2O3·3H2O, the two major types of sodium sites are assigned to pseudooctahedral structures, while the aluminum site is assigned to a four-coordinate aluminum and the local chemical environment, A1O2(OAl)2, i.e., with two coordinating oxygen atoms that are bonded to other aluminum atoms and two that are not. For dehydrated Na2O·Al2O3·3H2O, the major sodium site is assigned to a pseudotrigonal-bipyramidal structure, while the aluminum site is assigned to the tetrahedral AlO4 environment. Minor sodium ion components in both hydrated and dehydrated Na2O·Al2O3·3H2O may exist in zones of irregular, partially dehydrated sites of the sodium oxide type. A bulk chemical structural model of untreated Na2O·Al2O3·3H2O consistent with these structural assignments is one in which aluminum atoms are covalently linked together via oxygen bridges to form rings or polymer chains. A bulk structural model of dehydrated Na2O·Al2O3·3H2O that is proposed is based on monomeric AlO4 tetrahedra linked by uniformly distributed five-coordinate sodium ions. This bulk structure differs from that of the stable forms of dehydrated sodium alumínate, established by X-ray diffraction, in which both sodium and aluminum ions are tetrahedrally coordinated, indicating that NaAIO2 cannot be formed directly by dehydration of Na2O·Al2O3·3H2O with use of the dehydration temperatures of this study. © 1990, American Chemical Society. All rights reserved.