Synthesis of Hydrotalcite-Like Compounds and their Physico-Chemical Properties—The Systems Mg²⁺-Al³⁺-SO ²⁻₄ and Mg²⁺-Al³⁺-CrO ²⁻₄

Abstract

Hydrotalcite-like compounds [Mg_{I − x}Al_x(OH)₂]^x+ [A ²⁻ _x/2 , mH₂O]^x−, where A²⁻ is SO ²⁻₄ or CrO ²⁻₄ and x ≇ 0.25, were prepared by a coprecipitation method and their physico-chemical properties were studied by X-ray diffraction, thermal analysis, i.r. absorption spectra and acidity-basicity measurement.

The compounds including SO ²⁻₄ and CrO ²⁻₄ were analogous to an orthorhombic hydrotalcite and the lattice constants, a₀ and c₀ were 3.05 and 25.97 Å, and 3.05 and 26.48 Å, respectively. The crystallite size and strain in the 003 direction were 127 Å and 4.77 × 10⁻² for the SO ²⁻₄ system or 83 Å and 6.60 × 10⁻² for the CrO ²⁻₄ system, indicating a largely distorted microcrystallite. Two endothermic peaks observed at 240 and 455°C for the SO₄²⁻ system and at 230 and 460°C for the CrO ²⁻₄ system are ascribed to the eliminations of interlayer water (mH₂O) and structural water, respectively. The compound including SO ²⁻₄ formed MgO by calcination at 500°C and MgSO₄ and MgAl₂O₄ by calcination at 800°C while the compound including CrO₄²⁻ formed MgO at 400°C and MgAl_{2 − x}Cr_xO₄ at 800° I.r. study of the SO ²⁻₄ compound indicated that the bonding of SO ²⁻₄ was a bridge type. The highest acid and base strengths of the SO₄²⁻ compound were H₀ ≤ 1.5 and H₋ ≥ 12.2, the acidity and basicity being 0.3 and 0.1 mmol/g, respectively.