Aluminophosphates for CO₂ separation.

Current carbon dioxide capture methods face limitations in efficiency and cost. Prior research has shown that adsorbents with high CO₂ capacity and selectivity are needed. However, no prior work had resolved how aluminophosphates might perform in this role. Established knowledge includes the use of zeolites, which are water-sensitive and costly to regenerate. This gap motivated the investigation of aluminophosphates as alternatives. Aluminophosphates are known for their structural diversity and stability. No prior work had tested their CO₂/N₂ selectivity in detail. The need for hydrophobic materials that resist water interference remains unmet. This paper's contribution is to assess the CO₂ separation potential of specific aluminophosphate structures.

The aim is to evaluate the CO₂ separation performance of four aluminophosphate materials. These materials were selected for their 8-ring window apertures, which may allow selective CO₂ adsorption. The study addresses the problem of low CO₂/N₂ selectivity in conventional adsorbents. The motivation comes from the need for more efficient and less energy-intensive CO₂ capture. The authors sought to determine if these aluminophosphates can function as molecular sieves. They also aimed to compare their hydrophobicity to benchmark materials. The study focuses on CO₂ and N₂ uptake under relevant pressure and temperature conditions. The goal is to identify materials that are both selective and stable in humid environments.

The four aluminophosphates were synthesized via hydrothermal crystallization. After synthesis, the materials were calcined to remove template molecules. CO₂ and N₂ uptake measurements were conducted at pressures up to 101 kPa. Temperatures of 273 and 293 K were used to simulate realistic operating conditions. Langmuir and Toth adsorption models were applied to fit the isotherm data. Water adsorption tests were performed to assess material hydrophilicity. Relative humidity was varied up to 30 % to evaluate water sensitivity. Cyclic adsorption and desorption experiments were conducted to test regeneration efficiency.

AlPO₄-53 and AlPO₄-25 showed significantly higher CO₂ uptake compared to N₂. The squeezed 8-ring windows in these materials appear to facilitate CO₂ sieving. At 273 K, CO₂ uptake values were notably higher than for N₂. The Langmuir model provided a better fit for CO₂ isotherms than the Toth model. Water adsorption was minimal in all aluminophosphates tested. AlPO₄-53 and AlPO₄-25 showed less than 5 % water uptake at 30 % relative humidity. Cyclic tests confirmed the materials' hydrophobic nature and regeneration potential. These findings suggest that aluminophosphates may offer advantages over traditional zeolites.

The study demonstrates that aluminophosphates can selectively adsorb CO₂ from gas mixtures. AlPO₄-53 and AlPO₄-25 exhibit high CO₂/N₂ selectivity due to their 8-ring apertures. The authors propose that the squeezed window size allows CO₂ molecules to pass while excluding N₂. The hydrophobic nature of these materials reduces water interference. This property may lower regeneration energy costs compared to water-sensitive zeolites. The results suggest that aluminophosphates could be viable for pressure-swing adsorption systems. The authors suggest that these materials may improve CO₂ capture efficiency in industrial settings. They emphasize the need for further testing under more complex gas mixtures.