Unraveling the molecular mechanism for enhanced gas adsorption in mixed-metal MOFs via solid-state NMR spectroscopy

Abstract

The incorporation of multiple metal ions in metal–organic frameworks (MOFs) through one-pot synthesis can induce unique properties originating from specific atomic-scale spatial apportionment, but the extraction of this crucial information poses challenges. Herein, nondestructive solid-state NMR spectroscopy was used to discern the atomic-scale metal apportionment in a series of bulk Mg 1− x Co x -MOF-74 samples via identification and quantification of eight distinct arrangements of Mg/Co ions labeled with a 13 C-carboxylate, relative to Co content. Due to the structural characteristics of metal-oxygen chains, the number of metal permutations is infinite for Mg 1− x Co x -MOF-74, making the resolution of atomic-scale metal apportionment particularly challenging. The results were then employed in density functional theory calculations to unravel the molecular mechanism underlying the macroscopic adsorption properties of several industrially significant gases. It is found that the incorporation of weak adsorption sites (Mg 2+ for CO and Co 2+ for CO 2 adsorption) into the MOF structure counterintuitively boosts the gas adsorption energy on strong sites (Co 2+ for CO and Mg 2+ for CO 2 adsorption). Such effect is significant even for Co 2+ remote from Mg 2+ in the metal-oxygen chain, resulting in a greater enhancement of CO adsorption across a broad composition range, while the enhancement of CO 2 adsorption is restricted to Mg 2+ with adjacent Co 2+ . Dynamic breakthrough measurements unambiguously verified the trend in gas adsorption as a function of metal composition. This research thus illuminates the interplay between atomic-scale structures and macroscopic gas adsorption properties in mixed-metal MOFs and derived materials, paving the way for developing superior functional materials.