Investigating Competing Effects of Pore Size and Hydrophobicity on Water Adsorption in Nanoporous Metal−Organic Frameworks

Abstract Metal−organic frameworks (MOFs) are a highly tunable class of porous materials with potential in various applications. One promising area of research is water adsorption, but challenges remain related to stability and control over water uptake properties. In this study, we report the synthesis of NU-2020, which is structurally analogous to MIL-101 but incorporates a ditopic three-dimensional linker (3DL), bicyclo[2.2.2]octane-1,4-dicarboxylic acid (BODCA), with a sterically bulky and higher symmetry core. We investigated how incorporating a 3DL affects both the stability and water adsorption capacity of MIL-101. The incorporation of BODCA is expected to uniformly decrease pore sizes and increase pore hydrophobicity due to increased linker protrusion into the pore space and the presence of additional hydrophobic carbon and hydrogen atoms. These two modifications to the pores are competing effects that drive the relative humidity at which water uptake occurs in different directions. Decreasing the pore sizes is expected to shift the adsorption step to lower relative humidity, and increasing hydrophobicity is expected to shift the step to higher relative humidity. We employed a combination of thermogravimetric analysis, variable-temperature X-ray diffraction, and porosity measurements to characterize the thermal stability of MIL-101 and NU-2020 and evaluated the water uptake properties through a combination of standardized accelerated aging, contact angle measurements, and water sorption isotherms. Our findings indicate that incorporating a 3DL increases the framework’s thermal stability and increases the water adsorption capacity in NU-2020 relative to MIL-101. Moreover, in this system, the two competing effects appear to be negating one another, resulting in minimal shifts in the water isotherm steps.