Tuning CO2 Binding in Metal–Organic Framework Materials by Varying Metal Identity and Oxidation State

Abstract

CO2 adsorption from distributed and point sources has attracted significant interest in the recent past, and advanced sorbent design remains a fruitful avenue of research, yet structure–property relationships that capture the effect of an adsorption site coordination environment on binding characteristics remain (for the most part) elusive. Metal–organic framework (MOF) materials carry adsorption sites in highly well-defined coordination environments and present an opportunity for relating specific features of an open-metal coordination environment (such as oxidation state) with sorbate binding characteristics. In this work, we investigate CO2 adsorption onto isostructural Al-, Fe-, and Cr-containing MIL-100(M) materials to clearly demonstrate the oxidation state-dependent binding of CO2 to some metals but not others. Specifically, Cr2+ sites on MIL-100 nodes─found challenging to access in the prior literature─are demonstrated to exhibit remarkably stronger binding to CO2 compared to their Cr3+ counterparts. Results presented provide a template for tuning CO2 adsorption properties by systematically varying metal identity and oxidation state in MOFs specifically and CO2 capture sorbents more generally.