Water Content and Particle Texture of Synthetic Hydrotalcite-like Layered Double Hydroxides

Abstract

Hydrotalcite-like layered double hydroxides (LDHs) with compositions corresponding to [Mg1-xAlx(OH)2](CO3)x/2.nH2O, where (1 - x)/x ≈ 2, 3, and 4, were prepared by complementary variable- and constant-pH coprecipitation methods. In the commonly used variable pH process, LDH precipitation was initiated at the elevated pH value of the starting carbonate solution and terminated at pH 10. In contrast, the constant pH method allowed the entire precipitation process to be carried out at pH 10. Both synthesis methods yielded air-dried LDH carbonates containing two types of water, namely, interparticle pore water, wherein water is condensed between aggregated crystal platelets, and surface water, which is bound to intragallery and external basal planes. Interparticle pore water was readily removed by heating to 60 °C, but the temperature for complete removal of surfaces water increased from 240 to 280 °C (5 °C/min) with increasing Al3+ content. A bimodal loss of surface water was consistent with the presence of “intrinsic” surface water bound to intracrystal gallery surfaces and “extrinsic” surface water adsorbed at the external surfaces of the crystallites. The particle textures of the LDH reaction products, as reflected in crystal morphologies, surface areas, and interparticle pore size distributions, were highly dependent on the preparation method and the layer charge density. Fine grained crystals with rough surfaces and relatively high surface areas were obtained by the variable pH method, whereas the constant pH method afforded larger, well-formed hexagonal crystals. All of the products prepared by the variable pH method exhibited mesopores with radii in the range 50-300 A. In contrast, the constant pH method gave Mg3Al- and Mg4Al-LDH carbonate crystals with narrow mesopore distributions near 20 Å radius. TEM images provided evidence for the accommodation of interparticle pore water in voids formed by edge-face crystal aggregation. Cofacial layer stacking disorders also were observed that contribute both to the binding of extrinsic surface water and to the formation of mesopores. © 1995, American Chemical Society. All rights reserved.