Structural studies of Na-montmorillonite exchanged with Fe²⁺, Cr³⁺, and Ti⁴⁺ by N₂ adsorption and EXAFS

The structures of Fe²⁺-, Cr³⁺-, and Ti ⁴⁺-modified montmorillonite prepared from ion exchange of the Na-clay with Fe²⁺, Cr³⁺, and Ti⁴⁺ were investigated. Conventional BET surface area and spectroscopic analysis by extended adsorption fine structure (EXAFS) were applied. It was shown that the BET surface area of Na-clay was similar to that of Fe-clay, but somewhat different from those of Cr- and Ti-clay; it decreased in the order Na- > Fe- > Ti- > Cr-montmorillonite. This sequence appeared to be consistent with the ion size Na⁺ (0.95 nm)>Fe²⁺ (0.65 nm)>Cr³⁺ (0.62 nm), except for Ti⁴⁺ (0.69 nm). EXAFS data showed that some Si atoms within montmorillonite were replaced by Ti atoms and that a neostructure of titanium oxide was formed.