J . Phys. Chem. 1994, 98, 7848-7853
7848
Reversible Migration of Lithium in Montmorillonites Rafael Alvero,' Maria D. Alba, Miguel A. Castro, and Jose M. Trill0 Departamento de Quimica Inorg@ica, Instituto de Ciencia de Materiales, Universidad de Sevilla, Consejo Superior de Investigaciones Cientificas, P.O. Box 874, 41 080 Sevilla, Spain Received: November 15, 1993; In Final Form: April 12, 1994'
The migration of lithium cations into the lattice of dioctahedral clays upon heating, considered up to now as irreversible, has been investigated. The samples under examination are two lithium-saturated montmorillonites, selected with different charge-deficit distributions between the tetrahedral and octahedral sheets, to obtain direct information on the structural changes occurring in both layers. Reexpansion of these samples previously collapsed at 300 O C has been attained, for the first time, under high vapor water pressures. A structural characterization of the smectites under the different treatments has been monitored by means of X-ray diffraction, XRD, Fourier transform infrared spectroscopy, FTIR, and 29Si, 27Al, and 7Li magic angle spinning nuclear magnetic resonance, MAS-NMR. The effect that Li+ ions exert on the quadrupolar coupling constant of tetrahedral A1 ions has been shown. Direct experimental evidence on the location of lithium ions in the hexagonal holes of the collapsed structure is provided. Additionally, generation of protons during the rehydration process is concluded.
Introduction Montmorilloniteis a dioctahedral member of the smectite group of clay minerals. Its framework consists of a layered structure with individual negative-charged plates separated by an interlayer space within which reside charge-balancing, exchangeable hydrated cations. Each layer comprises two sheets of inward pointing Si04 tetrahedra which sandwich a third sheet of octahedrally coordinated cations. In an ideal uncharged structure, containing only trivalent octahedral cations, two of the three possible octahedral sites in each unit cell are occupied, the third position being vacant. The presence of isomorphous-substituted cations of lower charge in the tetrahedral and octahedral sheets gives the layer the above-mentioned negative charge.' Precise knowledge of the interaction mechanisms between the smectite lattice and the cations present in its interlayer space after thermal and hydrothermal treatments is greatly desirable for the design of new materials, including radioactive waste repository components and solid acid catalysts.2 Within this general approach, a classical subject not yet resolved is that of the interaction between interlayer cations of small radius (
