Mixed-linker UiO-66: structure–property relationships revealed by a combination of high-resolution powder X-ray diffraction and density functional theory calculations

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.

Band gap modulation in zirconium-based metal–organic frameworks by defect engineering

“…In the present Please do not adjust margins work, we focus instead on providing proof of concept that the band gap of UiO-66 can be systematically modulated through defect engineering. Several reports in the literature [16][17][18][19][20] have shown that replacement of BDC linker with aminoterephthalate (ABDC) in UiO-66 reduces the band gap from about 4.0 eV to about 2.8-2.9 eV, mainly thanks to the generation of a new highest occupied crystalline orbital (HOCO), predominantly localised on the organic linker. Inspired by these reports, we set out to investigate whether a similar result could be achieved by grafting of a range of commercially available and inexpensive amino-functionalised benzoic acids (ABAs, Figure 1) at defective sites, and how the relative arrangement of the carboxylic and amine groups within the phenyl ring would affect the electronic structure of the framework.…”

“…A trend depending on the position and on the number of NH 2 can be noted, which is consistent with that experimentally observed by UV-Vis spectroscopy. For the pristine, defect-free, UiO-66 crystal the Highest Occupied Orbital (corresponding to the valence band) is a localised orbital composed of the oxygen p orbitals, 19 while the Lowest Unoccupied Orbital (corresponding to the conduction band) is a fully delocalised orbital between the inorganic node and the organic ligand. 18 A similar situation can be found in the HOMO and LUMO of the single-cluster model for defect free UiO-66 ( Figure S53), which validates our computational model.…”

Synthetic Strategies and Structural Arrangements of Isoreticular Mixed‐Component Metal–Organic Frameworks

“…Post‐synthetic linker exchange (PSE, c ) is an alternative PSM strategy to produce A ‐ and C ‐type MOFs. If the pre‐synthesized material is suspended in a solution containing only a second linker and no additional metal precursor, linker molecules in the existing framework can be exchanged .…”

“…In addition, the incorporation of the second linker might not be achieved even though it is present in the reaction solution. In the majority of reported literature, A is the targeted arrangement when using route a , but only few reports presented a clear evidence for the random incorporation of both linker molecules …”

“…The similarity of different linkers leads in most cases to only very small differences in the resulting unit cell. However, if the difference of the unit cell volume of the single‐linker frameworks is sufficiently large, high‐resolution PXRD measurements allow for the identification of an equal linker distribution within the framework according to Vegard's law . In an ideal solid solution, the positions of the reflections shift gradually with varying linker ratios (see Figure ).…”