Dynamic Acidity in Defective UiO-66

The metal organic framework (MOF) material UiO-66 has emerged as one of the most promising MOF materials due to its thermal and chemical stability and its potential for catalytic applications. Typically, as-synthesised UiO-66 has a relatively high concentration of missing linker defects. The presence of these defects has been correlated with catalytic activity but characterisation of defect structure has proved elusive. We refine a recent experimental determination of defect structure using static and dynamic first principles approaches, which reveals a dynamic and labile acid centre that could be tailored for functional applications in catalysis.Comment: 5 figure

A new approach to enhancing the CO2 capture performance of defective UiO-66 via post-synthetic defect exchange

Zirconium-based metal-organic frameworks (Zr-MOFs) are a subclass of MOFs known for their remarkable stability, especially in the presence of water. This makes them extremely attractive for practical applications, including CO2 capture from industrial emission sources; however, the CO2 adsorption capacity of Zr-MOFs is moderate compared to that of the best performing MOFs reported to date. Functionalization of Zr-MOFs with amino groups has been demonstrated to increase their affinity for CO2. In this work, we assessed the potential of post-synthetic defect exchange (PSDE) as an alternative approach to introduce amino functionalities at missing-cluster defective sites in formic acid modulated UiO-66. Both pyridine-containing (picolinic acid and nicotinic acid) and aniline-containing (3-aminobenzoic acid and anthranilic acid) monocarboxylates were integrated within defective UiO-66 with this method. Non-defective UiO-66 modified with linkers bearing the same amino groups (2,5-pyridinedicarboxylic acid and 2-aminoterephthalic acid) were prepared by classical post-synthetic ligand exchange (PSE), in order to compare the effect of introducing functionalities at defective sites versus installing them on the backbone. PSDE reduces the porosity of defective UiO-66, but improves both the CO2 uptake and the CO2/N2 selectivity, whereas PSE has no effect on the porosity of non-defective UiO-66, improving the CO2 uptake but leaving selectivity unchanged. Modification of defective UiO-66 with benzoic acid reveals that pore size reduction is the main factor responsible for the observed uptake improvement, whereas the presence of nitrogen atoms in the pores seems to be beneficial for increasing selectivity

Dynamic interplay between defective UiO‐66 and protic solvents in activated processes

UiO-66, composed by Zr-oxide inorganic bricks [Zr-6(mu(3)-O)(4)(mu(3)-OH)(4)] and organic terephthalate linkers, is one of the most studied metal-organic frameworks (MOFs) due to its exceptional thermal, chemical, and mechanical stability. Thanks to its high connectivity, the material can withstand structural deformations during activation processes such as linker exchange, dehydration, and defect formation. These processes do alter the zirconium coordination number in a dynamic way, creating open metal sites for catalysis and thus are able to tune the catalytic properties. In this work, it is shown, by means of first-principle molecular-dynamics simulations at operating conditions, how protic solvents may facilitate such changes in the metal coordination. Solvent can induce structural rearrangements in the material that can lead to undercoordinated but also overcoordinated metal sites. This is demonstrated by simulating activation processes along well-chosen collective variables. Such enhanced MD simulations are able to track the intrinsic dynamics of the framework at realistic conditions

The remarkable amphoteric nature of defective UiO-66 in catalytic reactions

One of the major requirements in solid acid and base catalyzed reactions is that the reactants, intermediates or activated complexes cooperate with several functions of catalyst support. In this work the remarkable bifunctional behavior of the defective UiO-66(Zr) metal organic framework is shown for acid-base pair catalysis. The active site relies on the presence of coordinatively unsaturated zirconium sites, which may be tuned by removing framework linkers and by removal of water from the inorganic bricks using a dehydration treatment. To elucidate the amphoteric nature of defective UiO-66, the Oppenauer oxidation of primary alcohols has been theoretically investigated using density functional theory (DFT) and the periodic approach. The presence of acid and basic centers within molecular distances is shown to be crucial for determining the catalytic activity of the material. Hydrated and dehydrated bricks have a distinct influence on the acidity and basicity of the active sites. In any case both functions need to cooperate in a concerted way to enable the chemical transformation. Experimental results on UiO-66 materials of different defectivity support the theoretical observations made in this work

Tuning the Mechanical Response of Metal−Organic Frameworks by Defect Engineering

The incorporation of defects into crystalline materials provides an important tool to fine-tune properties throughout various fields of materials science. We performed high-pressure powder X-ray diffraction experiments, varying pressures from ambient to 0.4 GPa in 0.025 GPa increments to probe the response of defective UiO-66 to hydrostatic pressure for the first time. We observe an onset of amorphization in defective UiO-66 samples around 0.2 GPa and decreasing bulk modulus as a function of defects. Intriguingly, the observed bulk moduli of defective UiO-66­(Zr) samples do not correlate with defect concentration, highlighting the complexity of how defects are spatially incorporated into the framework. Our results demonstrate the large impact of point defects on the structural stability of metal–organic frameworks (MOFs) and pave the way for experiment-guided computational studies on defect engineered MOFs

Defect-dependent colossal negative thermal expansion in UiO-66(Hf) metal-organic framework

Thermally-densified hafnium terephthalate UiO-66(Hf) is shown to exhibit the strongest isotropic negative thermal expansion (NTE) effect yet reported for a metal-organic framework (MOF). Incorporation of correlated vacancy defects within the framework affects both the extent of thermal densification and the magnitude of NTE observed in the densified product. We thus demonstrate that defect inclusion can be used to tune systematically the physical behaviour of a MOF.Comment: 8 pages, 4 figures, revise

Influence of a confined methanol solvent on the reactivity of active sites in UiO-66

UiO-66, composed of Zr-oxide bricks and terephthalate linkers, is currently one of the most studied metal-organic frameworks due to its exceptional stability. Defects can be introduced in the structure, creating undercoordinated Zr atoms which are Lewis acid sites. Here, additional BrOnsted sites can be generated by coordinated protic species from the solvent. In this Article, a multilevel modeling approach was applied to unravel the effect of a confined methanol solvent on the active sites in UiO-66. First, active sites were explored with static periodic density functional theory calculations to investigate adsorption of water and methanol. Solvent was then introduced in the pores with grand canonical Monte Carlo simulations, followed by a series of molecular dynamics simulations at operating conditions. A hydrogen-bonded network of methanol molecules is formed, allowing the protons to shuttle between solvent methanol, adsorbed water, and the inorganic brick. Upon deprotonation of an active site, the methanol solvent aids the transfer of protons and stabilizes charged configurations via hydrogen bonding, which could be crucial in stabilizing reactive intermediates. The multilevel modeling approach adopted here sheds light on the important role of a confined solvent on the active sites in the UiO-66 material, introducing dynamic acidity in the system at finite temperatures by which protons may be easily shuttled from various positions at the active sites

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

We report a defect-engineering approach to modulate the band gap of zirconium-based metal-organic framework UiO-66, enabled by grafting of a range of amino-functionalised benzoic acids at defective sites. Defect engineered MOFs were obtained by both post-synthetic exchange and modulated synthesis, featuring band gap in the 4.1-3.3 eV range. First principle calculations suggest that shrinking of the band gap is likely due to an upward shift of the valence band energy, as a result of the presence of light-absorbing monocarboxylates. The photocatalytic properties of defect-engineered MOFs towards CO2 reduction to CO in the gas phase and degradation of Rhodamine B in water were tested, observing improved activity in both cases, in comparison to a defective UiO-66 bearing formic acid as the defect-compensating species

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

We report a defect-engineering approach to modulate the band gap of zirconium-based metal-organic framework UiO-66, enabled by grafting of a range of amino-functionalised benzoic acids at defective sites. Defect engineered MOFs were obtained by both post-synthetic exchange and modulated synthesis, featuring band gap in the 4.1-3.3 eV range. First principle calculations suggest that shrinking of the band gap is likely due to an upward shift of the valence band energy, as a result of the presence of light-absorbing monocarboxylates. The photocatalytic properties of defect-engineered MOFs towards CO2 reduction to CO in the gas phase and degradation of Rhodamine B in water were tested, observing improved activity in both cases, in comparison to a defective UiO-66 bearing formic acid as the defect-compensating species