Missing Linker Defects Research Articles

Vibrational spectroscopy investigation of defects in Zr- and Hf-UiO-66.

Defect engineering in metal–organic framework compounds has allowed for improvements in catalysis-based functionalities, gas sensing, and gas storage. Metal–organic framework UiO-66 compounds with Zr- and Hf-based metal secondary building units were studied with Raman and infrared vibrational spectroscopy. Missing linker and missing cluster defects were engineered into the crystal structure via a modulated synthesis technique. Missing cluster defects in Hf-UiO-66 are first characterized by powder X-ray diffraction (PXRD) whereby two low-angle peaks were fit to extract the relative quantity of reo topology in four defective samples. A monotonic red-shift of the Raman-active Hf–O coordination bond vibration is interpreted as a signature of missing cluster defects, resulting from less-rigid charge-balancing monocarboxylate formate ions replacing the dicarboxylate linker molecule. This signature is hypothesized to be independent of the topology in which the defects appear. Missing linker defects in Zr-UiO-66 are characterized by infrared absorption spectroscopy by the quenching of C–C and C–H vibrational modes confined to the linker molecule. Together, Raman and infrared vibrational spectroscopies coupled with standard characterization techniques are employed to directly probe the nature of defects as well as offer new characterization tools for missing cluster defects in UiO-66.

Adsorption and Degradation of Multiple Micropollutants with Trace Concentrations by Porous Zr-Porphyrin Metal-Organic Frameworks with Missing Linker Defects in Water

Adsorption and Degradation of Multiple Micropollutants with Trace Concentrations by Porous Zr-Porphyrin Metal-Organic Frameworks with Missing Linker Defects in Water

Creation of Exclusive Artificial Cluster Defects by Selective Metal Removal in the (Zn, Zr) Mixed-Metal UiO-66.

The differentiation between missing linker defects and missing cluster defects in MOFs is difficult, thereby limiting the ability to correlate materials properties to a specific type of defects. Herein, we present a novel and easy synthesis strategy for the creation of solely "missing cluster defects" by preparing mixed-metal (Zn, Zr)-UiO-66 followed by a gentle acid wash to remove the Zn nodes. The resulting material has the reo UiO-66 structure, typical for well-defined missing cluster defects. The missing clusters are thoroughly characterized, including low-pressure Ar-sorption, iDPC-STEM at a low dose (1.5 pA), and XANES/EXAFS analysis. We show that the missing cluster UiO-66 has a negligible number of missing linkers. We show the performance of the missing cluster UiO-66 in CO2 sorption and heterogeneous catalysis.

Dual Role of Missing-Linker Defects Terminated by Acetate Ligands in a Zirconium-Based MOF in Promoting Photocatalytic Hydrogen Peroxide Production

Dual Role of Missing-Linker Defects Terminated by Acetate Ligands in a Zirconium-Based MOF in Promoting Photocatalytic Hydrogen Peroxide Production

Enhanced Cr(VI) sorption capacity of the mechanochemically synthesized defective UiO-66 and UiO-66-NH2

Sorption of hexavalent chromium has become a growing global concern due to its high aqueous solubility and good mobility in underground environments. Among other Metal-Organic Frameworks (MOFs), water-stable Zr-MOFs have been recently investigated for the efficient decontamination of aqueous solution from Cr(VI). Herein, Zr-benzene dicarboxylate (UiO-66) and its amino derivative, Zr-NH2-benzene dicarboxylate (UiO-66-NH2), were prepared via a facile, benign mechanochemical method (Liquid Assisted Grinding, LAG) and for the first time applied to sorption studies for Cr(VI) removal. Through the comparative examination of the mechanochemically isolated UiO-66 (LAG) and UiO-66-NH2 (LAG) with the same Zr-MOFs made via conventional solvothermal synthesis (solvothermal, SOL) UiO-66 (SOL) and UiO-66-NH2 (SOL), we demonstrate better performance of the former towards Cr(VI) sorption. The Cr(VI) sorption kinetics for all compounds were well described by the pseudo second-order kinetic model, while the calculated sorption capacities of UiO-66 (LAG) and UiO-66-NH2 (LAG) for Cr(VI) (at pH 3) by the Langmuir model were 28.8 and 36.6 mg/g, respectively, overperforming those of UiO-66 (SOL) and UiO-66-NH2 (SOL), which were found to be 24.9 mg/g and 26.1 mg/g, respectively. Our findings point out that the increased missing-linker defects, generated through milling, boost Cr(VI) sorption. In addition, this work opens the perspective for the implementation of mechanochemical synthesis in order to examine the performance of other known MOF sorbents towards water remediation applications.

Enhanced CO2 adsorption performance on amino-defective UiO-66 with 4-amino benzoic acid as the defective linker

UiO-66(Zr), one of the Zr-based metal–organic frameworks (MOFs), is a potential adsorbent for gas separation owing to its large surface area, easily tunable pore structure, and high chemical/thermal stability. Nevertheless, its CO2 adsorption amount is somewhat modest in comparison to that of various MOFs. In this study, 4-aminobenzoic acid (PABA) as amino-defective linker was mixed to terephthalic acid at various compositions in a one-step synthesis of UiO-66(Zr) framework. This new restructuring strategy produced defective UiO-66(Zr) with enhanced porosity owing to the missing-linker defects, simultaneously formed –NH2 groups in the framework, leading to the improvement of the CO2 capture capacity. At 298 K and 100 kPa, the defective UiO-66 modified with 10% PABA exhibited the highest CO2 uptake capacity of ~ 2.47 mmol g−1 and ideal adsorbed solution theory (IAST)-based CO2/N2 selectivity of ~ 46, which surpass many other CO2 benchmark adsorbents. This opens a new perspective for developing defective UiO-66(Zr) adsorbent contained amine functional groups, which can improve CO2 separation performance.

Defective Metal–Organic Framework Assisted with Nitrogen Doping Enhances the Photoelectrochemical Performance of BiVO4

BiVO4 is a promising n-type semiconductor for photoelectrochemical (PEC) water splitting, which can serve as a photoanode. However, severe surface recombination and slow water oxidation kinetics hinder the realization of its highly theoretical PEC performance. Single-atom catalyst-like metal–organic frameworks (MOFs) and their derived metal oxides have been broadly investigated to enhance the kinetics of BiVO4 photoanodes. According to the principle of catalysis, only coordinatively unsaturated atoms can participate in the catalytic reaction. Herein, a defective cobalt-based MOF (d-CoMOF) with missing linker defects is modified onto the surface of N-doped BiVO4 (N:BVO) photoanodes. The photocurrent density of d-CoMOF/N:BVO is 3.59 times higher than that of pristine BVO (0.56 mA/cm2) at 1.23 VRHE. The onset potential of d-CoMOF/N:BVO shows a cathodic shift of 300 mV relative to BVO. The roles of the d-CoMOF overlayer and N doping are experimentally revealed. The d-CoMOF modification and N doping could increase the electron density, passivate the surface states, and promote the catalysis kinetics of BVO. Thus, the kinetic parameters, such as the charge separation/injection efficiency, onset potential, and photocurrent density, are significantly enhanced. This work provides a way to use MOFs for PEC water splitting by the introduction of defects.

Hydrogenative Ring-Rearrangement of Furfural to Cyclopentanone over Pd/UiO-66-NO2 with Tunable Missing-Linker Defects.

Upgrading furfural (FAL) to cyclopentanone (CPO) is of great importance for the synthesis of high-value chemicals and biomass utilization. The hydrogenative ring-rearrangement of FAL is catalyzed by metal-acid bifunctional catalysts. The Lewis acidity is a key factor in promoting the rearrangement of furan rings and achieving a high selectivity to CPO. In this work, highly dispersed Pd nanoparticles were successfully encapsulated into the cavities of a Zr based MOF, UiO-66-NO2, by impregnation using a double-solvent method (DSM) followed by H2 reduction. The obtained Pd/UiO-66-NO2 catalyst showed a significantly better catalytic performance in the aforementioned reaction than the Pd/UiO-66 catalyst due to the higher Lewis acidity of the support. Moreover, by using a thermal treatment. The Lewis acidity can be further increased through the creating of missing-linker defects. The resulting defective Pd/UiO-66-NO2 exhibited the highest CPO selectivity and FAL conversion of 96.6% and 98.9%, respectively. In addition, the catalyst was able to maintain a high activity and stability after four consecutive runs. The current study not only provides an efficient catalytic reaction system for the hydrogenative ring-rearrangement of furfural to cyclopentanone but also emphasizes the importance of defect sites.

Defects controlled by acid-modulators and water molecules enabled UiO-67 for exceptional toluene uptakes: An experimental and theoretical study

A facile defect-engineering approach enabled by introducing monocarboxylic acid modulators (acetic acid, formic acid and benzoic acid) was conducted in this paper. The greatest toluene uptake (480 mg g−1) was obtained on defective UiO-67 synthesized with benzoic acid due to highest missing linker defects and higher compensating linker defects. It is worth mentioning that water molecule exhibited creative promoting effect on the adsorption behavior of defective UiO-67. The original structure of defective UiO-67 was inevitably destroyed under water-containing environment due to the coordination between water molecule and metal cluster. 1H NMR analysis demonstrated that slight loss of BPDC linkers and benzoate linkers due to occupied binding sites, more extra adsorption sites were generated in this process. Furthermore, adsorption mechanisms were studied via density functional theory (DFT) calculations. The synergic effects between π-π, OH-CH3 and OH-π hydrogen bonding strength obviously enforced toluene adsorption, and OH-π exhibited strongest interaction. Moreover, under the circumstance of structural changes due to water molecules, abundant μ3-OH groups were exposed and hydrogen bond interaction was strengthened, which greatly promoted toluene adsorption. Finally, experimental results as well as computational studies forecasted the potentials of defect-tunable MOFs as promising materials in gas adsorption.

Defective UiO-66(Zr) as an efficient catalyst for the synthesis of bio jet-fuel precursors via aldol condensation of furfural and MIBK

The production of jet-fuel precursors from furfural (FUR) via aldol-condensation with methyl-isobutyl ketone (MIBK) over a defective UiO-66(Zr) catalyst is presented. The resultant C11 adduct (FuMe) would allow the selective production of branched alkanes in the range of jet fuel via a subsequent hydrogenation/hydrodeoxygenation process. The catalyst is prepared using formic acid as modulator, leading to the incorporation of defects on the microcrystalline structure of the metalorganic framework (MOF) material, which dramatically boosts the catalytic performance in this transformation. Thus, the benchmarking with different commercial solid acid catalysts and Zr-based heterogeneous catalysts has identified the defective MOF, UiO-66(Zr)-FA, as clearly superior. An extensive characterization of the modified catalyst by means of X-ray diffraction (XRD), argon adsorption isotherm, thermogravimetry (TGA), acid titration, X-ray photoelectron spectroscopy (XPS), and diffuse reflectance infrared Fourier transform (DRIFT) of adsorbed deuterated acetonitrile, has confirmed the incorporation of missing-linker and missing-node defects within the structure, enabling to explain the enhancement in the catalytic process. The analysis of the reaction kinetics, together with the optimization of the reaction conditions by means of a response surface methodology (allowing predicting the behaviour of the catalytic system under very different conditions) have identified the temperature as the most relevant parameter affecting the selectivity to FuMe. Thus, under the optimized reaction conditions (130 °C; 4 h; FUR/Cat = 2; MIBK/FUR = 4), outstanding total FUR conversion and FuMe selectivity (~100%) can be achieved. However, the catalyst gets progressively deactivated in successive catalytic runs under the studied reaction conditions, which is attributed to the formation of organic deposits coming from furfural side reactions.

Linker Exchange via Migration along the Backbone in Metal-Organic Frameworks.

In metal-organic frameworks (MOFs), organic linkers are subject to postsynthetic exchange (PSE) when new linkers reach sites of PSE by diffusion. Here, we show that during PSE, a bulky organic linker is able to penetrate narrow-window MOF crystals. The bulky linker migrates by continuously replacing the linkers gating the otherwise impassable windows and serially occupying an array of backbone sites, a mechanism we term through-backbone diffusion. A necessary consequence of this process is the accumulation of missing-linker defects along the diffusion trajectories. Using fluorescence intensity and lifetime imaging microscopy, we found a gradient of missing-linker defects from the crystal surface to the interior, consistent with the spatial progression of PSE. Our success in incorporating bulky functional groups via PSE extends the scope of MOFs that can be used to host sizable, sophisticated guest species, including large catalysts or biomolecules, which were previously deemed only incorporable into MOFs of very large windows.

Ligand‐Conformer‐Induced Formation of Zirconium–Organic Framework for Methane Storage and MTO Product Separation

In pursuit of novel adsorbents with efficient adsorptive gas storage and separation capabilities remains highly desired and challenging. Although the documented zirconium-tricarboxylate-based metal-organic frameworks (MOFs) have displayed a variety of topologies encompassing underlying and geometry mismatch ones, the employed organic linkers are exclusively rigid and poorly presenting one type of conformation in the resultant structures. Herein, a used and semirigid tricarboxylate ligand of H3 TATAB was judiciously selected to isolate a zirconium-based spe-MOF after the preliminary discovery of srl-MOF. Single-crystal X-ray diffraction reveals that the fully deprotonated TATAB linker in spe-MOF exhibits two distinct conformers, concomitant with popular Oh and rare S6 symmetrical Zr6 molecular building blocks, generating an unprecedented (3,3,12,12)-c nondefault topology. Specifically, the spe-MOF exhibits structurally higher complexity, hierarchical micropores, open metal sites free and rich electronegative groups on the pore surfaces, leading to relatively high methane storage capacity without considering the missing-linker defects and efficient MTO product separation performance.

Tuning the Properties of MOF-808 via Defect Engineering and Metal Nanoparticle Encapsulation.

Defect engineering and metal encapsulation are considered as valuable approaches to fine‐tune the reactivity of metal–organic frameworks. In this work, various MOF‐808 (Zr) samples are synthesized and characterized with the final aim to understand how defects and/or platinum nanoparticle encapsulation act on the intrinsic and reactive properties of these MOFs. The reactivity of the pristine, defective and Pt encapsulated MOF‐808 is quantified with water adsorption and CO2 adsorption calorimetry. The results reveal strong competitive effects between crystal morphology and missing linker defects which in turn affect the crystal morphology, porosity, stability, and reactivity. In spite of leading to a loss in porosity, the introduction of defects (missing linkers or Pt nanoparticles) is beneficial to the stability of the MOF‐808 towards water and could also be advantageously used to tune adsorption properties of this MOF family.

Metal‐Organic Cages with Missing Linker Defects

AbstractWe present here the controlled synthesis of defective coordination cages by employing steric hindrance of organic linkers to manipulate coordination modes of the assembled metal ions. Three chiral 1,1′‐bi‐2‐naphthol (BINOL) derived bis‐tridentate ligands L1–L3 with pyridine‐2,6‐dicarboxamides (pcam) chelating moieties are therefore designed and synthesized, among which L3 has a smaller steric hindrance on the coordinating sites relative to the other two linkers. Complexes of L1 and L2 with lanthanides afford the irregular Ln8(L1)10 hexahedra with two missing edges and Ln4(L2)5 tetrahedra with one missing edge, respectively, both of which contain a 1:1 mixture of Ln(pcam)2 and Ln(pcam)3. In contrast, complex of L3 produces the regular twisted Ln6(L3)9 trigonal prisms without missing edges that contain only Ln(pcam)3 vertices. The defective cage has more freedom to adjust its structural conformation, affording adaptable cavity to accommodate a range of guest molecules with sizes comparable or much larger than the cavity portals.

Metal-Organic Cages with Missing Linker Defects.

We present here the controlled synthesis of defective coordination cages by employing steric hindrance of organic linkers to manipulate coordination modes of the assembled metal ions. Three chiral 1,1'-bi-2-naphthol (BINOL) derived bis-tridentate ligands L1 -L3 with pyridine-2,6-dicarboxamides (pcam) chelating moieties are therefore designed and synthesized, among which L3 has a smaller steric hindrance on the coordinating sites relative to the other two linkers. Complexes of L1 and L2 with lanthanides afford the irregular Ln8 (L1 )10 hexahedra with two missing edges and Ln4 (L2 )5 tetrahedra with one missing edge, respectively, both of which contain a 1:1 mixture of Ln(pcam)2 and Ln(pcam)3 . In contrast, complex of L3 produces the regular twisted Ln6 (L3 )9 trigonal prisms without missing edges that contain only Ln(pcam)3 vertices. The defective cage has more freedom to adjust its structural conformation, affording adaptable cavity to accommodate a range of guest molecules with sizes comparable or much larger than the cavity portals.

Impact of defects on the decomposition of chemical warfare agent simulants in Zr‐based metal organic frameworks

AbstractMetal organic frameworks (MOFs) containing zirconium secondary building units (SBUs) in UiO‐67 and related MOFs, are highly active for neutralizing both the chemical warfare agents and simulants, such as dimethyl methylphosphonate (DMMP). However, two recent publications gave conflicting reports of DMMP reaction with UiO‐67 under ultra high vacuum (UHV) conditions, with one reporting chemisorption and reaction (Wang et al., J Phys Chem C, 2017, 121, 11261–11272) and the other reporting only physisorption and reversible desorption (Ruffley et al., J Phys Chem C, 2019, 123, 19748–19758) from very similar temperature programmed desorption experiments. We show that the discrepancy between these experiments may be explained by different levels of missing linker defects in the UiO‐67 samples. We present density functional theory calculations showing that SBU sites having two‐adjacent missing linkers exhibit reaction barriers that are about 30 kJ/mol lower than SBU sites having a single missing linker. We also show that topology of the undercoordinated sites plays an important role in the reaction barrier under UHV conditions.

Computational Investigation of Adsorptive Removal of Pb

Adsorption using metal–organic frameworks (MOFs) such as UiO-66 has shown great promise in remediating water sources contaminated with toxic heavy metals such as Pb2+, but detailed information about the adsorption process remains limited. In this article, we gained mechanistic insights into Pb2+ adsorption using both functionalised and defective UiO-66 by performing density functional theory calculations using cluster models. Our benchmarked approach led to a computational model of solvated Pb2+ (a hemidirected Pb(H2O)62+ complex) fully consistent with experimental reports. The analysis of Pb2+ adsorption using functionalised UiO-66 determined that factors such as electrostatic attraction, chelation, and limited constraints on the Pb2+ coordination geometry lead to enhanced binding affinity. For these reasons, UiO-66-COO– was identified as the most promising functionalised MOF, consistent with experimental literature. We additionally explored a novel aspect of Pb2+ adsorption by UiO-66: the role of missing linker defects that often characterise this MOF. We found that the defects expected to form in an aqueous environment can act as excellent adsorption sites for Pb2+ and the preferred adsorption geometry is again determined by electrostatic attraction, chelation, and constraints on the Pb2+ coordination geometry. Overall, we conclude that functional groups and defect sites can both contribute to Pb2+ adsorption and our study provides crucial design principles for improving the UiO-66 MOF performance in toxic Pb2+ removal from water.

The Effects of Functional Groups and Missing Linkers on the Adsorption Capacity of Aromatic Hydrocarbons in UiO-66 Thin Films

The adsorption of benzene, toluene, ethylbenzene, and xylene isomers, also known as BTEX, from the gas phase into porous thin films of the metal–organic framework UiO-66-X, where X = H, NH2, and NO2, was measured to quantify adsorption capacity. The thin films were grown by a vapor-conversion method onto Au-coated quartz microbalance crystals. The MOF thin films were characterized by IR and Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy. The thin films were activated by heating under high vacuum and exposed to each gas to calculate the Henry’s constant. The results demonstrate that the functional groups in the organic linker and missing-linkers both play important roles in the adsorption capacity. Several trends can be observed in the data. First, all the compounds in the BTEX family have lower Henry’s constants in the UiO-66-H films compared to the UiO-66-NH2 and UiO-66-NO2 films, which can largely be attributed to the absence of a functional group on the linker. Second, at 25 °C, the Henry’s constants for all the BTEX compounds in UiO-66-NO2 films are larger than UiO-66-NH2 films. Third, the role of missing linkers is addressed by comparing the measured adsorption capacity to ideal pore filling. The results show that the UiO-66-H films are the most defect-free and the UiO-66-NO2 films have the most missing linker defects.

Influence of Missing Linker Defects on the Thermal Conductivity of Metal-Organic Framework HKUST-1.

Although metal-organic frameworks (MOFs) are promising materials for gas storage and separation applications, the heat released during the exothermic adsorption process can potentially negatively impact their practical utility. Thermal transport in MOFs has not been widely studied, and among the relatively few reports on the topic, MOFs have either been assumed to be defect free or the presence of defects was not discussed. However, defects naturally exist in MOFs and can also be introduced intentionally. Here, we investigate the effect of missing linker defects on the thermal conductivity of HKUST-1 using molecular dynamics (MDs) simulation and the Green-Kubo method. We found that missing linker defects, even at low concentrations, substantially reduce the thermal conductivity of HKUST-1. If not taken into account, the presence of defects could lead to significant discrepancies between experimentally measured and computationally predicted thermal conductivities.

Copper-zirconia interfaces in UiO-66 enable selective catalytic hydrogenation of CO2 to methanol

Molecular interactions with both oxides and metals are essential for heterogenous catalysis, leading to remarkable synergistic impacts on activity and selectivity. Here, we show that the direct link between the two phases (and not merely being together) is required to selectively hydrogenate CO2 to methanol on catalysts containing Cu and ZrO2. Materials consisting of isolated Cu particles or atomically dispersed Cu–O–Zr sites only catalyze the reverse water-gas shift reaction. In contrast, a metal organic framework structure (UiO-66) with Cu nanoparticles occupying missing-linker defects maximizes the fraction of metallic Cu interfaced to ZrO2 nodes leading to a material with high adsorption capacity for CO2 and high activity and selectivity for low-temperature methanol synthesis.