Tuning CO2 Binding in Metal-Organic Framework Materials through Control over Metal Identity and Oxidation State

Despite the fact that CO₂ adsorption from point sources and ambient air are critical separations applications that have attracted the discovery and development of a range of proficient adsorbent materials, a clear understanding of atomic-level features that determine the strength of CO₂ binding still remains unclear. Metal–organic framework materials (MOFs) are endowed with multinuclear nodes that carry metals in highly well-defined coordination environments, and provide an opportunity to clearly elucidate the relationship between specific aspects of metal coordination environment and sorbate binding characteristics. In this work, we study CO₂ adsorption onto oxo-bridged trimers on MIL-100 to show that metal oxidation identity and oxidation state can be used to systematically tune CO₂ binding strength.

Five isostructural MIL-100(M) materials were synthesized (M = Cr, Fe, Al, Sc, Mn) and procedures for accessing M²⁺ and M³⁺ sites were developed. All MIL-100(M) variants exhibited selective binding onto M²⁺ sites compared to M³⁺ sites, and the relative binding strengths onto reduced metal sites were found to be dependent on metal identity. The higher relative CO₂ binding strength of specific metals such as chromium were rationalized based on nuclear screening effects and distortion of the octahedral coordination environment through the Jahn-Teller effect, and further supported using Density Functional Theory calculations. Our study uses a prototypical metal-organic framework material to clearly and rigorously elucidate relationships between atomic level structure and CO₂ binding characteristics that can be used more broadly in CO₂ sorbent design.