Diamine Functionalization of a Metal-Organic Framework by Exploiting Solvent Polarity for Enhanced CO2 Adsorption

Diamine-appended metal-organic frameworks (MOFs) exhibit exceptional CO2 adsorption capacities over a wide pressure range because of the strong interaction between basic amine groups and acidic CO2. Given that their high CO2 working capacity is governed by solvent used during amine functionalization, a systematic investigation on solvent effect is essential but not yet demonstrated. Herein, we report a facile one-step solvent exchange route for the diamine functionalization of MOFs with open metal sites, using an efficient method to maximize diamine loading. We employed an MOF, Mg-2(dobpdc) (dobpdc(4-) = 4,4'-dioxido-3,3'-biphenyldicarboxylate), which contains high-density open metal sites. Indirect grafting with N-ethylethylenediamine (een) was performed with a minimal amount of methanol (MeOH) via multiple MeOH exchanges and diamine functionalization, resulting in a top-tier CO2 adsorption capacity of 16.5 wt %. We established the correlation between N,N- dimethylformamide (DMF) loading and infrared peaks, which provides a simple method for determining the amount of the remaining DMF in Mg-2 (dobpdc). All interactions among Mg, DMF, diamine, and solvent were analyzed by van der Waals (vdw)- corrected density functional theory (DFT) calculations to elucidate the effect of chemical potential on diamine grafting.