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A maximum Al 3+ -substitution has been demonstrated to be 45 mole% of (M + Al) in the brucite layer of hydrotalcites. The chemical composition of the highly substituted hydrotalcites can be typically represented by [M 0 .55Alo.45(OH) where M = Mg, Ni, Zn, and Co. It showed the small lattice parameters of ao 3.05-2.98 A in the hexagonal lattice, which corroborates Al 3+ -substitution in the brucite layer. The simultaneous thermal analyses (TG and DTA) and mass spectrometry (MS) study have been performed. The highly Al 3+ -substituted hydrotalcites also showed quite different isotherms for the CO 2 adsorption. These materials adsorbed CO 2 gas by removing water within the interlayer and showed the selectivity for CO 2 adsorption: Cu-Al ~Z n -A l < Co-Al < Mg-Al < Ni-Al. The Mg-Al and Co-Al hydrotalcite-like compounds showed a doubled amount of CO 2 by removing carbonate ions within the interlayer.

Selective cation exchange in tobermorites with three levels of A13+ and Na+ substitution for Si4+ has been investigated. The cation exchange selectivity for a tobermorite with 2 mol% A13+ and 0 mol% Na+ substitution increased as follows:MgZf > Ba2+ > SrZ+. Cation exchange in the above tobermorite is postulated to take place mainly from edge and planar surface sites and apparently from interlayer Ca2+ sites. Tobermorites with 10 and 15 mol% A13+ (and Na+) substitution have additional Na' exchange sites in the interlayers and show the following selectivity: Ba2+ > SrZ+ > Mgz+. The cation exchange selectivity in the substituted tobermorites can be explained by the hydrated radii of cations and the steric limitations of the ion exchange cavity in tobermorite; the latter was determined by 27AI and 29Si magic angle spinning nuclear magnetic resonance spectroscopy. These basic selective cation exchange studies of substituted tobermorites are of relevance in nuclear waste treatment and disposal. [

A1 and 29Si magic angle spinning nuclear magnetic resonance spectroscopy revealed that the maximum amount of Al that can be substituted for Si in the tobermorite structure is 15 to 20 mol%. Powder X-ray diffraction and cation exchange studies corroborate the above finding. Anomalous tobermorite structures resulted in all cases, and hydrogarnet appeared beyond the 15 to 20 mol% A1 substitution limit for Si. The sorption of water molecules by synthetic [A13+ + Na+]-substituted tobermorites and their cationexchanged forms heated at 200°C under vacuum (>lo-' torr) was measured at 25°C in order to probe the nature of the ion-exchange cavity. Samples with 530 mol% of Al substitution for Si showed isotherms of Brunauer, Demming, Demming, Teller (BDDT) type 1 and gave better linearity with the Langmuir plot than with the BET plot. Samples with 2 3 5 mol% of A1 substitution for Si showed BDDT type I1 isotherms and gave better linearity with the BET than with the Langmuir equation. The Langmuir monolayer capacity was found to depend on the Al content and on the cationic form. The Li+-and Na+-exchanged tobermorites with about 20 mol% of Al substitution for Si showed the highest Langmuir monolayer water sorption capacity. The Cs+-exchanged tobermorites showed a smaller capacity than the Li+-or Na+-exchanged samples, which can be ascribed to clogging of the ion-exchange cavity by the large Cs+ ions.[

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