Molecular Understanding for the Adsorption of Water and Alcohols in Hydrophilic and Hydrophobic Zeolitic Metal-Organic Frameworks

A molecular simulation study is reported for the adsorption of water and alcohols (methanol and ethanol) in two metal-organic frameworks (MOFs) topologically similar to rho-zeolite; one is a hydrophilic Na+-exchanged rho-zeolite-like MOP (Na-rho-ZMOF), and the other is a hydrophobic zeolitic-imidazolate framework-71 (ZIF-71). The adsorption isotherms in Na-rho-ZMOF are type I as a consequence of the high affinity of the nonframework Na+ ions and ionic framework. The adsorption capacity decreases in the order of water > methanol > ethanol. Water is adsorbed more closely in the window region, whereas methanol and ethanol are populated in the a cage due to steric effect. In water/methanol and water/ethanol mixtures, water adsorption increases continuously with increasing pressure and replaces alcohols competitively at high pressures. In ZIF-71, the framework-adsorbate affinity is relatively weaker and type V adsorption is observed. Water has vanishingly small adsorption at low pressures and a sharp increase in adsorption at 22 kPa due to capillary condensation. Methanol and ethanol exhibit cluster-growth adsorption, followed by continuous pore filling. The adsorption in ZIF-71 increases in the order of water < methanol < ethanol at low pressures; however, the opposite order is observed at high pressures because of entropy effect. In water/alcohol mixtures, alcohols are selectively more adsorbed at low pressures but surpassed by water with increasing pressure. The framework charges have a substantial effect on adsorption in Na-rho-ZMOF, but not in ZIF-71. This study provides a molecular understanding for the adsorption of water and alcohols in two zeolitic MOFs with the identical topology and reveals the significantly different adsorption mechanisms.