Abstract
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.
Highlights
The severe environmental concern related to the greenhouse effects is mainly attributed to gases that absorb and emit radiation within the thermal infrared range
The objective was to examine whether remaining carbon species would act favorably toward CO2 adsorption via interaction of CO2 with both AlPO4-5 and carbon surfaces, or complete removal of structure-directing agent (SDA) would be preferred due to maximizing the pore volume and carbon-free AlPO45 surface
In our previous work we have shown that a critical parameter that strongly affects crystal morphology upon AlPO4-5 growth is the water content in the synthesis mixture (Karanikolos et al, 2008)
Summary
The severe environmental concern related to the greenhouse effects is mainly attributed to gases that absorb and emit radiation within the thermal infrared range. The reason is that multiple factors need to be met at the same time, i.e., adsorbents need to exhibit (i) high capacity; selectivity compared to other components such as NOx, SO2, and H2O vapor; fast adsorption/desorption kinetics; and low energy consumption; and (ii) chemical and thermal stability, sustainable performance for many cycles, low manufacturing cost, and mechanical robustness at large scale. We assess the impact of various factors on CO2 sorption, namely, (a) the effect of various heteroatoms used in isomorphic substitution in the AlPO4-5 framework at varying concentrations, (b) the pore activation method and in particular the affinity of remnant carbon species present into the inner pore surface after structure-directing agent (SDA) removal by two different thermal treatment methods, i.e., partial oxidation and pyrolysis, and (c) the impact of AlPO4-5 crystal morphology and hydrothermal synthesis mixture composition and in particular the water content in the mixture. The procedure was repeated for three different temperatures, namely, 25, 45, and 60◦C
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