Hydrothermal crystallization mechanism of sodium beidellite from amorphous gel

Abstract

Beidellite is a dioctahedral member of the smectite group and sodium beidellite can be described by the general formula, NaxA12(Si4_xAlx)O10(OH)2.nH20. Natural beidellite has an average layer charge of 0.33, although theoretically values range from 0.2 to 0.6 [1]. Beidellite has been hydrothermally synthesized from a gel by several researchers [1-5]. Their methods can be classified into two groups depending on the silica source for the gel preparation: the procedure of Luth and Ingamells [6] used colloidal silica, and that of Hamilton and Henderson [7] used tetraethyl orthosilicate (TEOS). Kloprogge et al. [1, 5] used gels prepared from TEOS to prepare sodium beidellite and presented the properties of synthesized sodium beidellite and its stability in the Na20-A1203-SiO2-H20 system. They reported that pure phase of beidellite was obtained by hydrothermal treatment of the gel only in water, but low-quartz co-existed with beidellite under basic conditions. Furthermore, kaolinite and low-quartz were crystallized at low temperatures below 250 °C but not sodium beidellite. Koizumi and Roy [2] used colloidal silica to prepare gels and showed the formation of wellcrystallized beidellite at 300-350 °C by reaction of a small amount of water with the gel with ideal beidellite composition. Plee et al. [3] synthesized sodium beidellite under basic conditions from a gel prepared by using colloidal silica. This report disagreed with the results obtained by Kloprogge et al. [1, 5]. The aim of the work reported here was to clarify the crystallization mechanism of sodium beidellite from a gel prepared from colloidal silica. The starting materials were Al(NO3)3.9H20, NaNO3 (reagent grade, Wako Pure Chemical Industries Ltd, Japan) and Ludox colloidal silica (HS-40; E.I. du Pont Nemours & Co., USA), which contains 40% SiO2 and a small amount of Na with SiOJNa20 weight ratio of 95. Starting gel (gel 1) was prepared to obtain the composition of Na0.4A12 (Si3.6A10.4) O10(OH)2'nH20. Firstly, a solution was prepared by dissolving 150.052 g of Al(NO3)3.9H20