Enhancing Van der Waals Interactions of Functionalized UiO‐66 with Non‐polar Adsorbates: The Unique Effect of para Hydroxyl Groups

Abstract

UiO-66 is a highly stable metal-organic framework (MOF) that has garnered interest for many adsorption applications. For small, nonpolar adsorbates, physisorption is dominated by weak Van der Waals interactions limiting the adsorption capacity. A common strategy to enhance the adsorption properties of isoreticular MOFs, such as UiO-66, is to add functional groups to the organic linker. Low and high pressure O isotherms were measured on UiO-66 MOFs functionalized with electron donating and withdrawing groups. It was found that the electron donating effects of -NH , -OH, and -OCF groups enhance the uptake of O . Interestingly, a significant enhancement in both the binding energy and adsorption capacity of O was observed for UiO-66-(OH) -p, which has two -OH groups para from one another. Density functional theory (DFT) simulations were used to calculate the binding energy of oxygen to each MOF, which trended with the adsorption capacity and agreed well with the heats of adsorption calculated from the Toth model fit to multi-temperature isotherms. DFT simulations also determined the highest energy binding site to be on top of the electron π-cloud of the aromatic ring of the ligand, with a direct trend of the binding energy with low pressure adsorption capacity. Uniquely, DFT found that oxygen molecules adsorbed to UiO-66-(OH) -p prefer to align parallel to the -OH groups on the aromatic ring. Similar effects for the electron donation of the functional groups were observed for the low pressure adsorption of N , CH , and CO .

Electronic Structure Modeling of Metal–Organic Frameworks

“…Linker substitutions and metal substitutions also offer a route to tuning the local interactions between MOFs and small molecular guests. − The conformation and selectivity of binding, approximated in DFT methodologies by the relative exothermicity, influence the performance of MOFs for functions like catalysis or separations. ,,, Linker substitutions may be introduced postsynthetically by introducing new molecules of the appropriate size and symmetry into a defective MOF system. UiO-66, for example, features high quantities of linker vacancies, , which can be saturated by the introduction of functional ligands; monodentate 3-mercaptoisobutyric acid was introduced into defective UiO-66 to incorporate a thiol functionality for cation trapping .…”

Performance enhancement strategies for surface plasmon resonance sensors in direct glucose detection using pristine and modified UiO-66: effects of morphology, immobilization technique, and signal amplification

“…The pristine UiO-66 has the highest SSA among all the samples (1784.0 m 2 g À1 ) and this value is higher than the SSAs of other reported UiO-66, which varied in the range of 826.05-1710 m 2 g À1 . [62][63][64][65][66][67][68] For UiO-66/Au samples, the hybridization with AuNps changes the SSA of UiO-66. The larger the AuNps colloid added, the smaller the SSA value.…”