Organic template-free synthesis of an open framework silicoaluminophosphate (SAPO) with high thermal stability and high ionic conductivity†
An open-framework silicoaluminophosphate (SAPO) material with a composition of Cs2(Al0.875Si0.125)4(P0.875Si0.125O4)4(HPO4) was synthesized in an organic template-free cesium-containing system under hydrothermal conditions. The material was resolved to be isostructural with an open-framework aluminophosphate (AlPO) material AlPO-CJ19, which was previously synthesized using pyridine as the solvent and 2-aminopyridine as the organic template via a solvothermal route. The substitution of organic templates by alkaline cations not only makes the current synthesis protocol more favorable from an environmental point of view but also leads to a significantly improved thermal stability of this structure. The framework remained completely stable at a temperature of 510 °C. A high-temperature polymorph was discerned at a temperature higher than 540 °C, the structure of which is revealed to crystallize in the P1c1 space group. UV-vis results reveal that cobalt and copper cations could be readily incorporated through simple ion-exchange treatment. The conductivity of the material is 1.70 × 10−4 S cm−1 (293 K, 30% relative humidity) as revealed by the alternating current (A.C.) impedance spectroscopy analysis. This is the first report of the synthesis of a silicon-containing phase with an AlPO-CJ19 structure.