The Impact of Missing Linker Defects in UiO-66 on Adsorption and Diffusion of Isopropyl Alcohol

Metal–organic framework (MOF) UiO-66 and its variants have been used for applications ranging from gas adsorption to catalyzing chemical warfare agent degradation. Intrinsic point defects, most commonly missing linkers, are usually present in UiO-66. At high concentrations, defects may significantly impact the material’s properties, such as adsorption, transport, and catalytic activity. A quantitative description of how intrinsic defects affect adsorption and transport of guest molecules is required to design tailored materials. In this work, we identify how different arrangements of missing linker defects impact adsorption and diffusion of isopropyl alcohol (IPA). IPA is an ideal test molecule to quantify the impact of missing linker defects on adsorption and diffusion of polar molecules because it forms hydrogen bonds with other IPA molecules and the framework atoms, similar to some classes of chemical warfare agents and their simulants. We have generated 25% missing linker defect structures having ordered and nonordered arrangements of defects. We found that adsorption isotherms are fairly insensitive to the specific arrangement of defects. In contrast, diffusivities can depend strongly on ordering of the defects. Specifically, we found that structures that contain percolating defects, which allow diffusion through the material traversing only defective windows, exhibit faster diffusivities and lower diffusion barriers compared with structures having nonpercolating defects. All defective structures exhibited faster diffusivities at low to moderate IPA loadings than pristine UiO-66. At high IPA loading, diffusivity values in nonpercolating defective structures are less than in pristine UiO-66. This decrease at high loadings is due to IPA forming hydrogen-bonding ring-like structures facilitated by the larger defective pores. We show that an experimentally synthesized defective UiO-66 having a bcu net topology has percolating diffusion pathways, which serves as a proof-of-concept that it is possible to synthesize of MOFs having percolating diffusion pathways.