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 cm3/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.