Structural and textural characteristics of systems based on layered Mg-Al hydroxides

Abstract

Hydrotalcite-type layered Mg-Al hydroxides with Mg/Al ≥ 2 and related oxide systems have been synthesized and characterized. During the synthesis of the Mg-Al hydroxides with Mg/Al = 2 and 3, the interplanar spacings decrease because of the replacement of the interlayer NO 3 − anion by the CO 3 2− anion. Morphologically, the initial materials are aggregates of ∼1-μm oblong planar particles with a layered structure. The transverse dimension of the plates is 50–100 nm, and their thickness is 10–30 nm. As a rule, these planar particles are heavily distorted. Heat treatment of the Mg-Al hydroxides in the temperature range from 450 to 600°C causes the separation of the layered structures into planar domains of epitaxial MgAl2O4/MgO structures, which survive heat treatment at 900°C. During this heat treatment at 900°C, the greater part of the sample breaks down into two-dimensional particles of defective MgO and MgAl2O4 phases whose size is up to 100 nm. The oxide compositions synthesized from the layered Mg-Al hydroxides have a large pore volume and a fairly high specific surface area, which are retained upon heat treatment at 1000–1100°C.