Morphology, Activation, and Metal Substitution Effects of AlPO₄-5 for CO₂ Pressure Swing Adsorption

Aluminophosphate, AlPO₄-5, an AFI zeotype framework consisting of one-dimensional parallel micropores, and metal-substituted AlPO₄-5 were prepared and studied for CO₂ adsorption. Preparation of AlPO₄-5 by using different activation methods (calcination and pyrolysis), incorporation of different metals/ions (Fe, Mg, Co, and Si) into the framework using various concentrations, and manipulation of the reaction mixture dilution rate and resulting crystal morphology were examined in relation to the CO₂ adsorption performance. Among the various metal-substituted analogs, FeAPO-5 was found to exhibit the highest CO₂ capacity at all pressures tested (up to 4 bar). Among the Fe-substituted samples, xFeAPO-5, with x being the Fe/Al₂O₃ molar ratio in the synthesis mixture (range of 2.5:100-10:100), 5FeAPO-5 exhibited the highest capacity (1.8 mmol/g at 4 bar, 25°C) with an isosteric heat of adsorption of 23 kJ/mol for 0.08-0.36 mmol/g of CO₂ loading. This sample also contained the minimum portion of extra-framework or clustered iron and the highest mesoporosity. Low water content in the synthesis gel led to the formation of spherical agglomerates of small 2D-like crystallites that exhibited higher adsorption capacity compared to columnar-like crystals produced by employing more dilute mixtures. CO₂ adsorption kinetics was found to follow a pseudo-first-order model. The robust nature of AlPO₄-5-based adsorbents, their unique one-dimensional pore configuration, fast kinetics, and low heat of adsorption make them promising for pressure swing adsorption of CO₂ at industrial scale.