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

Abstract

Hydrotalcite-like compounds [Mg I − xAlx(OH)2]x+ [A 2−x/2, m H 2 O]x−, where A2− is SO 42− or CrO 42− 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 42− and CrO 42− 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 42− system or 83 Å and 6.60 × 10−2 for the CrO 42− system, indicating a largely distorted microcrystallite. Two endothermic peaks observed at 240 and 455°C for the SO 42− system and at 230 and 460°C for the CrO 42− system are ascribed to the eliminations of interlayer water (m H 2 O) and structural water, respectively. The compound including SO 42− 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 42− formed MgO at 400°C and MgAl 2 − xCrxO 4 at 800° I.r. study of the SO 42− compound indicated that the bonding of SO 42− was a bridge type. The highest acid and base strengths of the SO 42− compound were H 0 ≤ 1.5 and H− ≥ 12.2, the acidity and basicity being 0.3 and 0.1 mmol/g, respectively.