The effect of the Si/Al ratio of CHA-zeolites on thermal expansion behaviors

Chabazite-type (CHA) zeolites are attractive catalytic materials, but their potential as framework-type negative thermal expansion (NTE) materials has been little explored. This study systematically investigates the effect of Si/Al ratio on the thermal expansion behavior of H-form CHA zeolites. H-CHA samples with Si/Al ratios from 2.8 to 9.2 were synthesized by controlled hydrothermal routes and converted to the H form via NH 4 Cl ion exchange without changing framework composition. X-ray diffraction confirmed preservation of the CHA structure before and after ion exchange. Thermogravimetric analysis showed that low Si/Al samples were highly hydrophilic and retained large amounts of water, while higher Si/Al samples absorbed significantly less water. High-temperature XRD revealed strong hysteresis in lattice parameters for low Si/Al samples due to water adsorption–desorption, whereas high Si/Al samples exhibited negligible hysteresis and clear framework-derived NTE. The volumetric thermal expansion coefficient became increasingly negative with increasing Si/Al ratio, reaching − 14.4 ppm/K at Si/Al = 9.2. This behavior is attributed to reduced proton concentration at high Si/Al ratios, which enhances rigid unit mode flexibility. These results identify Si/Al ratio as a key parameter for designing robust, low-moisture-sensitive NTE CHA zeolites.