Preparation and thermal expansion behavior of an EDI-type SAPO with dehydration-induced negative thermal expansion

This study investigates the thermal stability and thermal expansion behavior of K-EDI-type SAPO and ion-exchanged EDI-type SAPO. K-EDI-SAPO was synthesized and subsequently subjected to ion exchange using various chloride solutions. ICP-OES analysis confirmed that the framework composition (Si, Al, P) remained unchanged, indicating that only pore cations were replaced. Monovalent ions were almost completely exchanged, except Li+ (∼76 %), while divalent Ca2+ and Mg2+ showed lower rates (∼80 and 45 %, respectively). XRD patterns revealed successful synthesis of single-phase K-EDI-SAPO, with peak shifts after ion exchange reflecting changes in the lattice parameters. Rb-exchange slightly increased lattice volume, whereas Ca2+ exchange reduced crystallinity. Thermal expansion studies showed giant negative thermal expansion (NTE) for K-EDI-SAPO (−342 ppm/K) and significant NTE for Rb-EDI-SAPO (−99 ppm/K). Other ion-exchanged samples collapsed upon heating, attributed to stronger Coulombic interactions between small or divalent cations and framework oxygen. TG analysis linked dehydration behavior to lattice contraction, demonstrating that Rb substitution broadens the NTE temperature window.