Metal-organic frameworks as adsorbents for hydrogen purification and pre-combustion carbon dioxide capture

Abstract

Selected metal-organic frameworks exhibiting representative properties-high surface area, structural flexibility, or the presence of open metal cation sites-were tested for utility in the separation of CO₂ from H₂ via pressure swing adsorption. Single-component CO₂ and H₂ adsorption isotherms were measured at 313 K and pressures up to 40 bar for Zn₄O(BTB)₂ (MOF-177, BTB^3- = 1,3,5-benzenetribenzoate), Be₁₂(OH)₁₂(BTB)₄ (Be-BTB), Co(BDP) (BDP^2-= 1,4-benzenedipyrazolate), H₃[(Cu₄Cl)₃(BTTri)₈] (Cu-BTTri, BTTri³⁻ = 1,3,5-benzenetristriazolate), and Mg₂(dobdc) (dobdc⁴⁻ = 1,4-dioxido-2,5-benzenedicarboxylate). Ideal adsorbed solution theory was used to estimate realistic isotherms for the 80:20 and 60:40 H₂/CO₂ gas mixtures relevant to H₂ purification and precombustion CO₂ capture, respectively. In the former case, the results afford CO₂/H₂ selectivities between 2 and 860 and mixed-gas working capacities, assuming a 1 bar purge pressure, as high as 8.6 mol/kg and 7.4 mol/L. In particular, metal-organic frameworks with a high concentration of exposed metal cation sites, Mg₂(dobdc) and Cu-BTTri, offer significant improvements over commonly used adsorbents, indicating the promise of such materials for applications in CO₂/H₂ separations.