Mechanism of alcohol-water separation in metal-organic frameworks

The metal-organic framework Zn₂(BDC)₂(TED) (1) has been reported to be water-stable and highly selective toward the adsorption of water and alcohols, suggesting the application of this material as a separation membrane for the production of bioethanol. We have studied the adsorption mechanism of water, methanol, ethanol, and dimethylether in this framework by means of density-functional theory with corrections for London dispersion. We show that the combination of the hydrogen bond between the hydroxyl group in ethanol with the oxy group in 1 and the van der Waals interaction between the ethanol alkyl chain with the phenyl ring in 1 is responsible for the preferential adsorption of ethanol over water in the framework. The calculated enthalpy of adsorption for the four compounds in 1 is in excellent agreement with experimental results. We further note that the computational approach has to be chosen with care: It is essential to account for London dispersion interactions, as well as the use of large models, preferably the full periodic structure, to obtain correct adsorption geometries and energies.