Systematic Ligand Modulation Enhances the Moisture Stability and Gas Sorption Characteristics of Quaternary Metal-Organic Frameworks

摘要

Complex metal-organic frameworks (MOFs) that maintain high structural order promise sophisticated and tunable properties. Here, we build on our strategy of using combinations of structurally distinct ligands to generate a new isoreticular series of ordered quaternary Zn_4O-carboxylate MOFs. Rational design of the framework components steers the system toward multicomponent MOFs and away from competing phases during synthesis. Systematic ligand modulation led to the identification of a set of frameworks with unusually high stability toward water vapor. These frameworks lose no porosity after 100 days' exposure to ambient air or 20 adsorption-desorption cycles up to 70% relative humidity. Across this series of frameworks, a counterintuitive relationship between the length of pendant alkyl groups and framework stability toward water vapor emerges. This phenomenon was probed via a series of gas and vapor adsorption experiments together with Grand Canonical Monte Carlo (GCMC) simulations, and could be rationalized on the basis of the propensity of the frameworks to adsorb water vapor and the proximity of the adsorbed water molecules to the water-sensitive metal clusters. Systematic variation of the pore volume and topography also tunes the CO_2 and CH_4 gas adsorption behavior. Certain of these materials display increases in their adsorption capacities of 237% (CO_2) and 172% (CH_4) compared to the parent framework.