Evaluating metal-organic frameworks for post-combustion carbon dioxide capture via temperature swing adsorption

Abstract

Two representative metal-organic frameworks, Zn₄O(BTB)₂ (BTB³⁻ = 1,3,5-benzenetribenzoate; MOF-177) and Mg₂(dobdc) (dobdc⁴⁻ = 1,4-dioxido-2,5-benzenedicarboxylate; Mg-MOF-74, CPO-27-Mg), are evaluated in detail for their potential use in post-combustion CO₂ capture via temperature swing adsorption (TSA). Low-pressure single-component CO₂ and N₂ adsorption isotherms were measured every 10 °C from 20 to 200 °C, allowing the performance of each material to be analyzed precisely. In order to gain a more complete understanding of the separation phenomena and the thermodynamics of CO₂ adsorption, the isotherms were analyzed using a variety of methods. With regard to the isosteric heat of CO₂ adsorption, Mg₂(dobdc) exhibits an abrupt drop at loadings approaching the saturation of the Mg²⁺ sites, which has significant implications for regeneration in different industrial applications. The CO₂/N₂ selectivities were calculated using ideal adsorbed solution theory (IAST) for MOF-177, Mg₂(dobdc), and zeolite NaX, and working capacities were estimated using a simplified TSA model. Significantly, MOF-177 fails to exhibit a positive working capacity even at regeneration temperatures as high as 200 °C, while Mg₂(dobdc) reaches a working capacity of 17.6 wt % at this temperature. Breakthrough simulations were also performed for the three materials, demonstrating the superior performance of Mg₂(dobdc) over MOF-177 and zeolite NaX. These results show that the presence of strong CO₂ adsorption sites is essential for a metal-organic framework to be of utility in post-combustion CO₂ capture via a TSA process, and present a methodology for the evaluation of new metal-organic frameworks via analysis of single-component gas adsorption isotherms.