Thermally induced conversion of Mg²⁺ cation exchanged LTA, FAU, GIS and SOD zeolites: Syntheses and characterization of γ-cordierite, a new Mg<inf>2</inf>Al<inf>4</inf>Si<inf>5</inf>O<inf>18</inf> polymorph

Abstract

The thermal behaviours of fully Mg2+ cation exchanged LTA, FAU (X and Y species), GIS and SOD zeolites were investigated in the temperature range from ambient to 1400°C. The initial zeolite frameworks collapsed into amorphous intermediate substances after air heating between 800-850°C. Prolonged annealing of these products above 850°C induced recrystallization into sapphirine-like phases (Mg-LTA and Mg-FAUx zeolites) and μ-cordierite (Mg-FAUy and Mg-GIS zeolites) phases. These phases were unstable during further temperature/time treatment and between 1000-1100°C they polymorphously transformed into γ-cordierite, a new Mg2Al4Si5O18 polymorph. γ-Cordierite was formed as a transformation product from the investigated zeolite precursors, except for the Mg-SOD zeolite. Structurally the γ-cordierite phase is a modulated, Mg-stuffed derivative of cristobalite with the approximate composition Mg2Al4Si 5O18, which precedes the appearance of α-cordierite. The orthorhombic unit cell dimensions calculated from X-ray powder diffraction pattern are: a = 16.387(7) Å, b = 8.835(4) Å, c = 7.853(3) Å. γ-Cordierite has a narrow range of temperature stability (1100-1200°C) and during prolonged temperature/time treatment, it is polymorphously transformed to the α-cordierite phase.