Abstract
The use of mixed-linker metal-organic frameworks (MIXMOFs) is one of the most effective strategies to modulate the physical-chemical properties of MOFs without affecting the overall crystal structure. In many instances, MIXMOFs have been recognized as solid solutions, with random distribution of ligands, in agreement with the empirical rule known as Vegard's law. In this work, we have undertaken a study combining high-resolution powder X-ray diffraction (HR-PXRD) and density functional theory (DFT) calculations with the aim of understanding the reasons why UiO-66-based amino- and bromo-functionalized MIXMOFs (MIXUiO-66) undergo cell expansion obeying Vegard's law and how this behaviour is related to their physical-chemical properties. DFT calculations predict that the unit cell in amino-functionalized UiO-66 experiences only minor expansion as a result of steric effects, whereas major modification to the electronic features of the framework leads to weaker metal-linker interaction and consequently to the loss of stability at higher degrees of functionalization. For bromo-functionalized UiO-66, steric repulsion due to the size of bromine yields a large cell expansion, but the electronic features remain very similar to pristine UiO-66, preserving the stability of the framework upon functionalization. MIXUiO-66 obtained by either direct synthesis or by post-synthetic exchange shows Vegard-like behaviour, suggesting that both preparation methods yield solid solutions, but the thermal stability and the textural properties of the post-synthetic exchanged materials do not display a clear dependence on the chemical composition, as observed for the MOFs obtained by direct synthesis.
Highlights
We have undertaken a study combining high-resolution powder X-ray diffraction (HR-PXRD) and density functional theory (DFT) calculations with the aim of understanding the reasons why UiO-66-based amino- and bromofunctionalized mixed-linker metal–organic frameworks (MIXMOFs) (MIXUiO-66) undergo cell expansion obeying Vegard’s law and how this behaviour is related to their physical–chemical properties
8093 Zurich, Switzerland e Department of Chemical and Biomolecular Engineering, University of California, Berkeley, CA 94720, USA † Electronic supplementary information (ESI) available: Tables with amounts of reagents used in the syntheses; results of quantitative NMR analysis; high-resolution powder X-ray diffraction (HR-PXRD) patterns; TG curves; nitrogen sorption isotherms; details of density functional theory (DFT) calculations
Quantitative 1H-NMR analysis was performed to determine the relative amounts of ligands in the MIXUiO-66, showing that the incorporation of both ABDC and BBDC into the solid was slightly preferential with respect to based on either terephthalic acid (BDC) (Tables S3 and S4, ESI†)
Summary
We have undertaken a study combining high-resolution powder X-ray diffraction (HR-PXRD) and density functional theory (DFT) calculations with the aim of understanding the reasons why UiO-66-based amino- and bromofunctionalized MIXMOFs (MIXUiO-66) undergo cell expansion obeying Vegard’s law and how this behaviour is related to their physical–chemical properties. One of the pieces of evidence normally used to support the solid solution hypothesis in MIXMOFs is the observation of a gradual shift of reflections in the powder X-ray diffraction (PXRD) patterns associated with varying ligand ratios.[16,20] The shift is due to the adaptation of the structure to the presence of linkers with different properties, leading to either shrinkage or expansion of the unit cell.
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
