Comparative Studies on the Synthesis of Na-magadiite, Na-kenyaite and RUB-18 Phases

Abstract

Ludox-AS40, Colloidal silica, Ludox HS-40 and Fumed silica were used as silica sources, in order to prepare well crystalline Na-magadiite and Na-kenyaite layered silicates. The influence of reaction times, NaOH concentration and water contents in the mixture gel were investigated. Pure Na-magadiite phase was obtained after a hydrothermal treatment of Na₂O/5SiO₂/122H₂O gel, at temperatures below 150°C for a period of 2 days reaction, independently of the silica source. However, better crystallinity was achieved using Ludox AS-40. Pure Na-kenyaite was formed only from Fumed silica at 170°C and after 2 days of reaction. The water content affected the type of layered silicates when fumed silica was used. Indeed, pure Na-kenyaite was obtained at 150°C and after 2 days of reaction using a gel of Na₂O/5SiO₂/24H₂O. Meanwhile, Na-magadiite was formed for a content of water above 70. A minimum content of NaOH (0.25Na₂O) was required to obtain Na-magadiite from Fumed silica and colloidal silica. The control of the transformation rate of Na-magadiite to Na-kenyaite or vice versa can be achieved by increasing the synthesis temperatures, or the gel compositions. We were able to block the transformation of Na-magadiite to Na-kenyaite by adding an organic solvent dioxane, and pure RUB-18 phase was obtained at temperatures above 130°C after 1 day of reaction.