Mind the Gap: The Role\nof Mass Transfer in Shaped\nNanoporous Adsorbents for Carbon Dioxide Capture

Adsorptive\nseparations by nanoporous materials are major\nindustrial\nprocesses. The industrial importance of solid adsorbents is only expected\nto grow due to the increased focus on carbon dioxide capture technology\nand energy-efficient separations. To evaluate the performance of an\nadsorbent and design a separation process, the adsorption thermodynamics\nand kinetics must be known. However, although diffusion kinetics determine\nthe maximum production rate in any adsorption-based separation, this\naspect has received less attention due to the challenges associated\nwith conducting diffusion measurements. These challenges are exacerbated\nin the study of shaped adsorbents due to the presence of porosity\nat different length scales. As a result, adsorbent selection typically\nrelies mainly on adsorption properties at equilibrium, i.e., uptake\ncapacity, selectivity and adsorption enthalpy. In this Perspective,\nbased on an extensive literature review on mass transfer of CO<sub>2</sub> in nanoporous adsorbents, we discuss the importance and limitations\nof measuring diffusion in nanoporous materials, from the powder form\nto the adsorption bed, considering the nature of the process, i.e.,\nequilibrium-based or kinetic-based separations. By highlighting the\nlack of and discrepancies between published diffusivity data in the\ncontext of CO<sub>2</sub> capture, we discuss future challenges and\nopportunities in studying mass transfer across scales in adsorption-based\nseparations.