Organic-free synthesis of silicoaluminophosphate zeotype membranes with tunable framework charge density†

Microporous crystalline membranes utilizing strong adsorptive affinity for separation, especially at elevated temperatures, are needed for separating mixtures containing polar and nonpolar molecules with similar sizes. Zeotype membranes with tunable framework charge densities (FCDs) may serve as a promising candidate for separating such mixtures under industrially relevant conditions. Typically, synthesis of zeotype membranes requires organic compounds as structure-directing agents (SDAs), increasing synthesis cost and complexity and thus potentially restricting their large-scale use. This work reports, for the first time, organic-free synthesis of silicoaluminophosphate (SAPO) zeotype membranes with LTA topology by secondary growth. By varying framework heteroatom (Si) concentrations, the FCD of LTA-type SAPO membranes was tuned between 0.30 and 0.37 with respect to charge-density matching with the inorganic SDA, Na⁺ cation. Highly charged SAPO membranes showed good separation performance of ammonia (NH₃) over non-polar N₂ and H₂ gases, with selectivities of 32.5 and 9.9, respectively, and excellent thermal stability over 100 h at 150 °C. This inorganic synthesis strategy might be applicable to the fabrication of other types of zeotype membranes with adjustable compositions for affinity-based separations.