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

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
Zn4O–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 CO2 and CH4 gas adsorption behavior.
Certain of these materials display increases in their adsorption capacities
of 237% (CO2) and 172% (CH4) compared to the
parent framework.