Synthesis of Hydrotalcite-Like Compounds and their Physico-Chemical Properties—The Systems Mg²⁺-Al³⁺-SO₄²⁻ and Mg²⁺-Al³⁺-CrO₄²⁻

Abstract

Abstract Hydrotalcite-like compounds [Mg I − x Al x (OH) 2 ] x + [A 2− x /2 , m H 2 O] x − , where A 2− is SO 4 2− or CrO 4 2− 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 4 2− and CrO 4 2− were analogous to an orthorhombic hydrotalcite and the lattice constants, a 0 and c 0 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 4 2− system or 83 Å and 6.60 × 10 −2 for the CrO 4 2− system, indicating a largely distorted microcrystallite. Two endothermic peaks observed at 240 and 455°C for the SO 4 2− system and at 230 and 460°C for the CrO 4 2− system are ascribed to the eliminations of interlayer water ( m H 2 O) and structural water, respectively. The compound including SO 4 2− formed MgO by calcination at 500°C and MgSO 4 and MgAl 2 O 4 by calcination at 800°C while the compound including CrO 4 2− formed MgO at 400°C and MgAl 2 − x Cr x O 4 at 800° I.r. study of the SO 4 2− compound indicated that the bonding of SO 4 2− was a bridge type. The highest acid and base strengths of the SO 4 2− compound were H 0 ≤ 1.5 and H − ≥ 12.2, the acidity and basicity being 0.3 and 0.1 mmol/g, respectively.