Leveraging Ligand Steric Demand to Control Ligand Exchange and Domain Composition in Stratified Metal-Organic Frameworks.

Incorporating diverse components into metal-organic frameworks (MOFs) can expand their scope of properties and applications. Stratified MOFs (sMOFs) consist of compositionally unique concentric domains (strata), offering unprecedented complexity through partitioning of structural and functional components. However, the labile nature of metal-ligand coordination handicaps achieving compositionally distinct domains due to ligand exchange reactions occurring concurrently with secondary strata growth. To achieve complex sMOF compositions, characterizing and controlling the competing processes of new strata growth and ligand exchange are vital. This work systematically examines controlling ligand exchange in UiO-67 sMOFs by tuning ligand sterics. We present quantitative methods for assessing and visualizing the outcomes of strata growth and ligand exchange that rely on high-angle annular dark-field images and elemental mapping via scanning transmission electron microscopy-energy dispersive X-ray spectroscopy. In addition, we leverage ligand sterics to create 'blocking layers' that minimize ligand exchange between strata which are particularly susceptible to ligand exchange and inter-strata ligand mixing. Finally, we evaluate strata compositional integrity in various solvents and find that sMOFs can maintain their compositions for >12 months in some cases. Collectively, these studies and methods enhance understanding and control over ligand placement in multi-domain MOFs, factors that underscore careful tunning of properties and function.

The ionic liquid [C4mpy][Tf2N] induces bound-like structure in the intrinsically disordered protein FlgM.

The A. aeolicus intrinsically disordered protein FlgM has four well-defined α-helices when bound to σ28, but in water FlgM undergoes a change in tertiary structure. In this work, we investigate the structure of FlgM in aqueous solutions of the ionic liquid [C4mpy][Tf2N]. We find that FlgM is induced to fold by the addition of the ionic liquid, achieving average α-helicity values similar to the bound state. Analysis of secondary structure reveals significant similarity with the bound state, but the tertiary structure is found to be more compact. Interestingly, the ionic liquid is not homogeneously dispersed in the water, but instead aggregates near the protein. Separate simulations of aqueous ionic liquid do not show ion clustering, which suggests that FlgM stabilizes ionic liquid aggregation.

Identifying UiO-67 Metal-Organic Framework Defects and Binding Sites through Ammonia Adsorption.

Ammonia is a widely used toxic industrial chemical that can cause severe respiratory ailments. Therefore, understanding and developing materials for its efficient capture and controlled release is necessary. One such class of materials is 3D porous metal-organic frameworks (MOFs) with exceptional surface areas and robust structures, ideal for gas storage/transport applications. Herein, interactions between ammonia and UiO-67-X (X: H, NH2 , CH3 ) zirconium MOFs were studied under cryogenic, ultrahigh vacuum (UHV) conditions using temperature-programmed desorption mass spectrometry (TPD-MS) and in-situ temperature-programmed infrared (TP-IR) spectroscopy. Ammonia was observed to interact with µ3 -OH groups present on the secondary building unit of UiO-67-X MOFs via hydrogen bonding. TP-IR studies revealed that under cryogenic UHV conditions, UiO-67-X MOFs are stable towards ammonia sorption. Interestingly, an increase in the intensity of the C-H stretching mode of the MOF linkers was detected upon ammonia exposure, attributed to NH-π interactions with linkers. These same binding interactions were observed in grand canonical Monte Carlo simulations. Based on TPD-MS, binding strength of ammonia to three MOFs was determined to be approximately 60 kJ mol-1 , suggesting physisorption of ammonia to UiO-67-X. In addition, missing linker defect sites, consisting of H2 O coordinated to Zr4+ sites, were detected through the formation of nNH3 ⋅H2 O clusters, characterized through in-situ IR spectroscopy. Structures consistent with these assignments were identified through density functional theory calculations. Tracking these bands through adsorption on thermally activated MOFs gave insight into the dehydroxylation process of UiO-67 MOFs. This highlights an advantage of using NH3 for the structural analysis of MOFs and developing an understanding of interactions between ammonia and UiO-67-X zirconium MOFs, while also providing directions for the development of stable materials for efficient toxic gas sorption.

Tuning the Lewis acidity of metal-organic frameworks for enhanced catalysis.

The kinetics of hydrolysis of dimethyl nitrophenyl phosphate (DMNP), a simulant of the nerve agent Soman, was studied and revealed transition metal salts as catalysts. The relative rates of DMNP hydrolysis by zirconium and hafnium chlorides are in accordance with their Lewis acidity. In situ conversion of zirconium chloride to zirconium oxy-hydroxide was identified as the key step. We propose a precursor-MOF activity relationship.