Carbon monoxide contamination in H2 streams is a significant problem that is currently addressed largely by energetically costly cryogenic distillation. Metal–organic frameworks are a class of porous materials that have been studied extensively as adsorbants for a variety of gas separations, but to date cannot adequately adsorb low concentrations of CO in CO/H2 mixtures. A new metal–organic framework, Fe-BTTri ([Fe(MeOH)6]3[(Fe4Cl)3(BTTri)8]2, H3BTTri = 1,3,5-tris(1H-1,2,3-triazol-5-yl)benzene)), is reported as extremely effective for trace CO removal from CO/H2 mixtures, displaying unprecendentedly high selectivity for CO. This framework displays significant CO adsorption capacity at very low pressures, and also exhibits readily reversible CO binding. As characterized by a variety of spectroscopic methods, Fe-BTTri utilizes a unique spin state change mechanism to bind CO, in which the coordinatively unsaturated, high-spin Fe2+ sites of the framework convert to low-spin Fe2+ centers upon CO binding. CO desorption converts the Fe2+ sites back to high-spin centers, which likely contributes to the easy regeneration and recyclability of the material. Importantly, this spin change is selective for CO and not observed in the presence of H2, resulting in unprecedentedly high selectivity for CO adsorption. To the best of our knowledge, this is the first time an adsorbate-induced spin transition has been observed in a metal–organic framework.

Reversible, low-concentration carbon monoxide binding in a metal–organic framework utilizing a unique spin state change mechanism

251st National Meeting (2016)

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