Reversible CO Scavenging via Adsorbate-Dependent Spin State Transitions in an Iron(ii)-Triazolate Metal-Organic Framework


A new metal-organic framework, Fe-BTTri (Fe3[(Fe4Cl)3(BTTri)8]2·18CH3OH, H3BTTri =1,3,5-tris(1H- 1,2,3-triazol-5-yl)benzene)), is found to be highly selective in the adsorption of CO over a variety of other gas molecules, making it extremely effective, for example, in the removal of trace CO from mixtures with H2, N2, and CH4. This framework not only displays significant CO adsorption capacity at very low pressures (1.45 mmol/g at just 100 bar), but, importantly, also exhibits readily reversible CO binding. Fe-BTTri utilizes a unique spin state change mechanism to bind CO in which the coordinatively unsaturated, high-spin FeII centers of the framework convert to octahedral, low-spin FeII centers upon CO coordination. Desorption of CO converts the FeII sites back to a high-spin ground state, enabling the facile regeneration and recyclability of the material. This spin state change is supported by characterization via infrared spectroscopy, single crystal X-ray analysis, Mössbauer spectroscopy, and magnetic susceptibility measurements. Importantly, the spin state change is selective for CO and is not observed in the presence of other gases, such as H2, N2, CO2, CH4, or other hydrocarbons, resulting in unprecedentedly high selectivities for CO adsorption for use in CO/H2, CO/N2, and CO/CH4 separations and in preferential CO adsorption over typical strongly adsorbing gases like CO2 and ethylene. While adsorbate-induced spin state transitions are well-known in molecular chemistry, particularly for CO, to our knowledge this is the first time such behavior has been observed in a porous material suitable for use in a gas separation process. Potentially, this effect can be extended to selective separations involving other π-acids.



Keywords and Phrases

Adsorbates; Adsorption; Bins; Carbon dioxide; Characterization; Crystalline materials; Ethylene; Ground state; Infrared spectroscopy; Magnetic susceptibility; Organometallics; Porous materials; Separation; Single crystals; Trace elements; X ray analysis; Co-ordinatively unsaturated; Gas separation process; High-spin ground state; Magnetic susceptibility measurements; Metal organic framework; Single crystal X-ray analysis; Spin state transition; Ssbauer spectroscopies; Spin dynamics

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Article - Journal

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© 2016 American Chemical Society (ACS), All rights reserved.

Publication Date

01 May 2016