High adsorption capacities and two-step adsorption of polar adsorbates on copper-benzene-1,3,5-tricarboxylate metal-organic framework

This work discusses the adsorption of polar and apolar molecules on the copper-benzene-1,3,5-tricarboxylate (Cu-BTC) metal-organic framework. Vapor phase adsorption isotherms of various polar adsorbates such as methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 1-hexanol, water, acetone, acetonitrile, tetrahydrofuran, and N,N-dimethylformamide, as well as some apolar adsorbates such as n-hexane, n-heptane, m-xylene, and cyclohexane, on the Cu-BTC framework are presented. We show that water exposure of the Cu-BTC framework has an adverse effect on the uptake capacity. However, with minimized water exposure, we find high adsorption capacities, exceeding 0.65 cm ³/g for all adsorbates with the exception of water, and we show that small polar adsorbates exhibit a two-step adsorption behavior. This behavior is further studied using molecular simulation and proposed to occur due to the presence of the various Cu-BTC cages. The cages containing the exposed coordinatively unsaturated copper sites have a more polar character, while the other cages behave in a more apolar way, causing a two-step adsorption behavior depending on the character of the adsorbate.