Thermally induced conversion of Mg2+ cation exchanged LTA, FAU, GIS and SOD zeolites: Syntheses and characterization of γ-cordierite, a new Mg2Al4Si5O18 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 ?-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 ?-cordierite, a new Mg2Al4Si5O18 polymorph. ?-Cordierite was formed as a transformation product from the investigated zeolite precursors, except for the Mg-SOD zeolite. Structurally the ?-cordierite phase is a modulated, Mg-stuffed derivative of cristobalite with the approximate composition Mg2Al4Si5O18, which precedes the appearance of ?-cordierite. The orthorhombic unit cell dimensions calculated from X-ray powder diffraction pattern are: a = 16.387(7) ?, b = 8.835(4) ?, c = 7.853(3) ?. ?-Cordierite has a narrow range of temperature stability (1100-1200 ?C) and during prolonged temperature/time treatment, it is polymorphously transformed to the ?-cordierite phase.