A facile recrystallization strategy for fabrication of nanocrystalline microspheres of sulfatic sodalite with high thermal stability

Abstract

Sodalite as one prototype of zeolite-type crystal structures contains a β-cage with a pore size of ∼2.7 Å and can capture small-sized molecules such as ammonia and water. However, the low thermal stability of sodalite stemming from the presence of hydroxyl groups in the aluminosilicate framework adversely affects its high-temperature applications. Herein, a novel strategy for synthesizing a sulfatic modification of sodalite (S-sodalite) without any hydroxyl group in the framework, except water molecules in the cage, has been successfully developed. S-Sodalite was hydrothermally recrystallized from a gel, which was first digested from basic sodalite (B-sodalite) by sulfuric acid (H₂SO₄) and neutralized by alkali. Thermogravimetric analysis in combination with powder X-ray diffraction shows that the framework of S-sodalite has exceptionally high thermal stability up to at least 900 °C, which is attributed to the incorporation of the SO₄²⁻ group rather than OH⁻ into the structure. The robust framework of S-sodalite avoids the 14.5% volume expansion accompanying the conversion of hydrosodalite to anhydrous sodalite, which can cause cracking and affect applications. In addition, nitrogen (N₂) adsorption–desorption measurements show that the specific surface areas of the nanocrystalline microspheres of S-sodalite are greatly enhanced to 48.8 m² g⁻¹ from 0.8 m² g⁻¹ of micro-sized B-sodalite. The adsorbed amount of ammonia (NH₃) reaches 1.78 mmol g⁻¹ at 25 °C under a pressure of 1 bar. Therefore, the new strategy for the synthesis of hydrophilic S-sodalite with exceptionally high thermal stability paves the way for its potential applications of water removal from mixed gases at elevated temperatures.