Tuning the Wettability of Metal-Organic Frameworks via Defect Engineering for Efficient Oil/Water Separation

Development of Underwater Oleophobic and Underoil Hydrophobic Strontium(II)-Cyclotriphosphazene Hexacarboxylate Framework with Prewetting-Induced Switchable Wettability and Self-Cleanability for Continuous Oil-Water Mixture and Emulsion Separations

Oil spill management presents significant challenges, particularly when addressing spills that occur beneath the water's surface. In this context, Sr-HCPCP (SRMIST-2) is an innovative MOF with underwater oleophobic and underoil hydrophobic properties, incorporating enhanced coordination, a strong affinity for water and hydrophilic strontium, and a nontoxic, eco-friendly, biocompatible, and hydrophobic cyclotriphosphazene. It is designed with switchable wetting properties and exceptional chemical and thermal stability. SRMIST-2 is synthesized via a hydrothermal reaction between strontium nitrate and hexakis(4-carboxylatophenoxy)-cyclotriphosphazene. Its structure consists of edge-sharing {Sr3(COO)6(H2O)3} polyhedra that form 1-D chains, which pair to create 2-D networks that further interact with HCPCP ligands to construct a three-dimensional framework. When coated onto cotton fiber using polydopamine, the resulting CF-PDA-SRMIST-2 demonstrates excellent oil-water separation. Depending on whether it is prewetted with water or oil, it achieves separation efficiencies of 88-99%, with high flux rates (3409 Lm2-h-1 for water and 2840 Lm2-h-1 for oil) and remains effective over 15 cycles. It effectively separates oil-in-water and water-in-oil emulsions with 98% and 95% efficiency, respectively. CF-PDA-SRMIST-2 remains stable under acidic, alkaline, saline, and extreme temperature conditions. Its self-cleaning, amphiphobic properties ensure durability and reusability. With its low-cost, scalability, and eco-friendly nature, CF-PDA-SRMIST-2 is a promising material for sustainable oil spill remediation and environmental protection.

Symbiotic defect-reinforced bimetallic MOF-derived fiber components for solar-assisted atmospheric water collection

Conversion of atmospheric water to sustainable and clean freshwater resources through MOF-based adsorbent has great potential for the renewable environmental industry. However, its daily water production is hampered by susceptibility to agglomeration, slow water evaporation efficiency, and limited water-harvesting capacity. Herein, a solar-assisted bimetallic MOF (BMOF)-derived fiber component that surmounts these limitations and exhibits both optimized water-collect capacity and short adsorption-desorption period is proposed. The proposed strategy involves utilizing bottom-up interface-induced assembly between carboxylated multi-walled carbon nanotube and hygroscopic BMOF on a multi-ply glass fiber support. The designed BMOF (MIL-100(Fe,Al)-3) skeleton constructed using bimetallic-node defect engineering exhibits a high specific surface area (1,535.28 m2/g) and pore volume (0.76 cm3/g), thereby surpassing the parent MOFs and other reported MOFs in capturing moisture. Benefiting from the hierarchical structure of fiber rods and the solar-driven self-heating interface of photothermal layer, the customized BMOF crystals realize efficient loading and optimized water adsorption-desorption kinetics. As a result, the resultant fiber components achieve six adsorption-desorption cycles per day and an impressive water collection of 1.45 g/g/day under medium-high humidity outdoor conditions. Therefore, this work will provide new ideas for optimizing the daily yield of atmospheric water harvesting techniques.

Defect-enabling zirconium-based metal-organic frameworks for energy and environmental remediation applications

This comprehensive review explores the diverse applications of defective zirconium-based metal-organic frameworks (Zr-MOFs) in energy and environmental remediation. Zr-MOFs have gained significant attention due to their unique properties, and deliberate introduction of defects further enhances their functionality. The review encompasses several areas where defective Zr-MOFs exhibit promise, including environmental remediation, detoxification of chemical warfare agents, photocatalytic energy conversions, and electrochemical applications. Defects play a pivotal role by creating open sites within the framework, facilitating effective adsorption and remediation of pollutants. They also contribute to the catalytic activity of Zr-MOFs, enabling efficient energy conversion processes such as hydrogen production and CO2 reduction. The review underscores the importance of defect manipulation, including control over their distribution and type, to optimize the performance of Zr-MOFs. Through tailored defect engineering and precise selection of functional groups, researchers can enhance the selectivity and efficiency of Zr-MOFs for specific applications. Additionally, pore size manipulation influences the adsorption capacity and transport properties of Zr-MOFs, further expanding their potential in environmental remediation and energy conversion. Defective Zr-MOFs exhibit remarkable stability and synthetic versatility, making them suitable for diverse environmental conditions and allowing for the introduction of missing linkers, cluster defects, or post-synthetic modifications to precisely tailor their properties. Overall, this review highlights the promising prospects of defective Zr-MOFs in addressing energy and environmental challenges, positioning them as versatile tools for sustainable solutions and paving the way for advancements in various sectors toward a cleaner and more sustainable future.

Assembling 99% MOFs into Bioinspired Rigid-Flexible Coupled Membrane with Significant Permeability: The Impacts of Defects

Assembling metal-organic frameworks (MOFs) into high-performance macroscopic membranes is crucial but still challenging. MOF-containing hybrid membranes can effectively integrate the advantages of flexible guest materials and MOFs. Nevertheless, the inherent limitations in fully harnessing the distinct characteristics of MOFs persist due to the substantial guest material content necessitated in membrane fabrication. Herein, inspired by the rigid and flexible structures in biological systems, rigid MIP-202(Zr) and defective MIP-202(Zr) (D-MIP-202(Zr)) modified flexible graphene oxide (GO) sheets are synthesized in situ and then assembled into a rigid-flexible coupled MOF-based membrane. The defects in D-MIP-202(Zr) are introduced by using acetic acid as the modulation agent. The obtained GO@MIP-202(Zr) membrane possesses a hierarchical porous structure with a 99 wt% MOF proportion, which is higher than the GO@D-MIP-202(Zr) (75 wt%) membrane with a compact bulge-structured surface. The water permeability of the GO@MIP-202(Zr) membrane attains remarkedly 5762.92 L h-1 m-2 bar-1 , which is 960 and 2.6 times higher than that of the GO membrane and GO@D-MIP-202(Zr) membrane. Additionally, benefiting from the superhydrophilicity and underwater superoleophobicity, the resultant membrane not only demonstrates high rejection for oil-water emulsions but also exhibits exceptional recyclability and anti-fouling ability. These findings provide valuable insights into the assembly of MOFs into high-performance membranes.

Robust PVA/GO@MOF membrane with fast photothermal self-cleaning property for oily wastewater purification

The poor mechanical durability and weak fouling resistance of oil/water separation membranes severely restrict their applications in industry. Herein, a robust PVA/GO@MOF membrane with fast photothermal self-cleaning capability was developed through facile chemical crosslinking and suction-filtration strategies. Attributed to the powerful underwater superoleophobicity, the PVA/GO@MOF membrane exhibited extraordinary anti-oil adhesion even for high-viscosity crude oil and continuous crude oil emulsion purification capability with stable flux (1020 L m-2 h-1 bar-1) and exceptional efficiency (> 99.3%) even after 60 min. Most importantly, in comparison to reported photocatalytic self-cleaning oil/water separation membranes, the PVA/GO@MOF membrane can degrade organic contaminants more rapidly with a higher degradation rate (99.9%) in 50 min due to the superior photothermal conversion capacity. The synergistic photothermal and photocatalytic effects significantly enhanced photodegradation efficiency, which created opportunities for in-depth treatment of complex oily wastewater. Besides, the obtained membrane displayed excellent chemical and mechanical durability with underwater oil contact angle (UWOCA) above 150° even in harsh environments, such as corrosive solutions, UV irradiation, ultrasound treatment, abrasion experiment and bending test. Therefore, the developed PVA/GO@MOF membrane with robust durability and fast photocatalytic self-cleaning property is highly expected to purify oily wastewater and degrade organic pollutants.

Efficient Generation of Large Collections of Metal-Organic Framework Structures Containing Well-Defined Point Defects

High-throughput molecular simulations of metal-organic frameworks (MOFs) are a useful complement to experiments to identify candidates for chemical separation and storage. All previous efforts of this kind have used simulations in which MOFs are approximated as defect-free. We introduce a tool to readily generate missing-linker defects in MOFs and demonstrate this tool with a collection of 507 defective MOFs. We introduce the concept of the maximum possible defect concentration; at higher defect concentrations, deviations from the defect-free crystal structure would be readily evident experimentally. We studied the impact of defects on molecular adsorption as a function of defect concentrations. Defects have a slightly negative or negligible influence on adsorption at low pressures for ethene, ethane, and CO₂ but a strong positive influence for methanol due to hydrogen bonding with defects. Defective structures tend to have loadings slightly higher than those of defect-free structures for all adsorbates at elevated pressures.

Controlling polyHIPE Surface Properties by Tuning the Hydrophobicity of MOF Particles Stabilizing a Pickering Emulsion

Metal-organic frameworks (MOFs) show promise for the capture of greenhouse gases. To be used at a large scale in fixed-bed processes, their shaping under a hierarchical structure is mandatory and remains a major challenge, while keeping available their high specific surface area. For that purpose, we propose herein an original method based on the stabilization of a paraffin-in-water Pickering emulsion by a fluorinated Zr MOF (UiO-66(F4)) with polyHIPEs (polymers from high internal phase emulsions) strategy consisting of the polymerization of monomers in the external phase. After polymerization of the continuous phase and elimination of the paraffin, a hierarchically structured monolith is obtained with the UiO-66(F4) particles embedded in the polymer wall and covering the internal porosity. To avoid the pore blocking induced by the embedment of the MOF particles, our strategy was to modify their hydrophilic/hydrophobic balance with a controlled adsorption of hydrophobic molecules (perfluorooctanoic acid, PFOA) on the UiO-66(F4) particles. This will induce a displacement of the MOF position at the paraffin-water interface in the emulsion and then make the particles less embedded into the polymer wall. This leads to the formation of hierarchically structured monoliths integrating UiO-66(F4) particles with higher accessibility, maintaining their original properties and allowing their application in fixed-bed processes. This strategy was demonstrated by N₂ and CO₂ capture, and we believe that such original strategy could be applied to other MOF materials.

Dimensionality effects of g-C3N4 from wettability to solar light assisted self-cleaning and electrocatalytic oxygen evolution reaction

Unique interfacial properties of 2D materials make them more functional than their bulk counterparts in a catalytic application. In the present study, bulk and 2D graphitic carbon nitride nanosheet (bulk g-C₃N₄ and 2D-g-C₃N₄ NS) coated cotton fabrics and nickel foam electrode interfaces have been applied for solar light-driven self-cleaning of methyl orange (MO) dye and electrocatalytic oxygen evolution reaction (OER), respectively. Compared to bulk, 2D-g-C₃N₄ coated interfaces show higher surface roughness (1.094 > 0.803) and enhanced hydrophilicity (θ ∼ 32° < 62° for cotton fabric and θ ∼ 25° < 54° for Ni foam substrate) due to oxygen defect induction as confirmed from morphological (HR-TEM and AFM) and interfacial (XPS) characterizations. The self-remediation efficiencies for blank and bulk/2D-g-C₃N₄ coated cotton fabrics are estimated through colorimetric absorbance and average intensity changes. The self-cleaning efficiency for 2D-g-C₃N₄ NS coated cotton fabric is 87%, whereas the blank and bulk-coated fabric show 31% and 52% efficiency. Liquid Chromatography-Mass Spectrometry (LC-MS) analysis determines the reaction intermediates for MO cleaning. 2D-g-C₃N₄ shows lower overpotential (108 mV) and onset potential (1.30 V) vs. RHE for 10 mA cm^-2 OER current density in 0.1 M KOH. Also, the decreased charge transfer resistance (R_CT = 12 Ω) and lower Tafel's slope (24 mV dec^-1) of 2D-g-C₃N₄ make it the most efficient OER catalyst over bulk-g-C₃N₄ and state-of-the-art material RuO₂. The pseudocapacitance behavior of OER governs the kinetics of electrode-electrolyte interaction through the electrical double layer (EDL) mechanism. The 2D electrocatalyst demonstrates long-term stability (retention ∼94%) and efficacy compared to commercial electrocatalysts.

Superhydrophilic Modification of Polyvinylidene Fluoride Membrane via a Highly Compatible Covalent Organic Framework-COOH/Dopamine-Integrated Hierarchical Assembly Strategy for Oil-Water Separation

The integration of membranes with additives such as functionalized nanomaterials can be recognized as an effective method to enhance membrane performance. However, to obtain an efficient nanoparticle-decorated membrane, the compatibility of nanomaterials remains a challenge. Hydrophilic carboxylated covalent organic frameworks (COF-COOH) might be expected to avoid the drawbacks of aggregation and easy shedding of inorganic materials caused by the poor interfacial compatibility. Herein, a highly compatible dip-coating strategy was proposed for the superhydrophilic modification of polyvinylidene fluoride membrane via COF-COOH integrated with dopamine. COF-COOH together with polydopamine nanoparticles were uniformly and stably attached to the membrane due to the high interfacial compatibility, constructing a coating with rough hierarchical nanostructures and abundant carboxyl groups. The synergistic effects of multiscale structures and chemical groups endow the membrane with superhydrophilicity and underwater superoleophobicity, the water contact angle decreased from 123 to 15°, and the underwater oil contact angle increased from 132 to 162°. Accordingly, the modified membrane exhibits an ultrahigh oil rejection ratio (>98%), a high flux (the maximum reaches 1843.48 L m^-2 h^-1 bar^-1), attractive antifouling ability, and impregnable stability. This work would provide a momentous reference for the application of COF-COOH in practical oily wastewater treatment.

Defect-induced tuning of polarity-dependent adsorption in hydrophobic-hydrophilic UiO-66

Structural defects in metal-organic frameworks can be exploited to tune material properties. In the case of UiO-66 material, they may change its nature from hydrophobic to hydrophilic and therefore affect the mechanism of adsorption of polar and non-polar molecules. In this work, we focused on understanding this mechanism during adsorption of molecules with different dipole moments, using the standard volumetric adsorption measurements, IR spectroscopy, DFT + D calculations, and Monte Carlo calculations. Average occupation profiles showed that polar and nonpolar molecules change their preferences for adsorption sites. Hence, defects in the structure can be used to tune the adsorption properties of the MOF as well as to control the position of the adsorbates within the micropores of UiO-66.

Superprotonic Conductivity of UiO-66 with Missing-Linker Defects in Aqua-Ammonia Vapor

The design and preparation of proton-conducting metal-organic frameworks (MOFs) with superconductivity are of significance for the proton-exchange membrane fuel cell (PEMFC). Introducing functional structural defects to enhance proton conductivity is a good approach. Here, we synthesized a series of UiO-66 (first synthesized in the University of Oslo) with missing-linker defects and investigated the effect of defect numbers on the proton conductivity of the samples. Among them, 60-UiO-66-1.8 (60 represents the synthesis temperature and 1.8 the number of defects) prepared with 3-mercaptopropionic acid as a modulator has the best proton conductivity, which is 3 × 10^-2 S cm^-1 at 100 °C and under 98% relative humidity (RH). The acidic sites induced by missing-linker defects further promote the chemisorption of ammonia molecules, resulting in the formation of a richer hydrogen-bond network and hence boosting the proton conductivity to 1.04 × 10^-1 S cm^-1 at 80 °C, which is one of the highest values among the reported MOF-based proton conductor. Therefore, this work provides a new strategy for enhancing proton conduction in MOF-based materials.

A mixed strategy to fabricate two bifunctional ligand-based Ag-complexes with high proton conductivity

High proton conductivity materials BAg-1 and BAg-2 were obtained using a mixed strategy with the same main bifunctional ligand.

Strategies for induced defects in metal-organic frameworks for enhancing adsorption and catalytic performance

Metal-organic frameworks (MOFs) have emerged among porous materials. The designable structure and specific functionality make them stand out for diverse applications. In conceptual MOF, the metal ions/clusters and organic ligands...

Influence of solvent structure and hydrogen bonding on catalysis at solid-liquid interfaces

Solvent molecules interact with reactive species and alter the rates and selectivities of catalytic reactions by orders of magnitude. Specifically, solvent molecules can modify the free energies of liquid phase and surface species via solvation, participating directly as a reactant or co-catalyst, or competitively binding to active sites. These effects carry consequences for reactions relevant for the conversion of renewable or recyclable feedstocks, the development of distributed chemical manufacturing, and the utilization of renewable energy to drive chemical reactions. First, we describe the quantitative impact of these effects on steady-state catalytic turnover rates through a rate expression derived for a generic catalytic reaction (A → B), which illustrates the functional dependence of rates on each category of solvent interaction. Second, we connect these concepts to recent investigations of the effects of solvents on catalysis to show how interactions between solvent and reactant molecules at solid-liquid interfaces influence catalytic reactions. This discussion demonstrates that the design of effective liquid phase catalytic processes benefits from a clear understanding of these intermolecular interactions and their implications for rates and selectivities.

Multiple Strategies to Fabricate a Highly Stable 2D CuIICuI-Organic Framework with High Proton Conductivity

Using multifunctional organic ligands with multiple acidic groups (carboxylate and sulfonate groups) to synthesize metal-organic frameworks (MOFs) bearing effective H-bond networks is a promising strategy to obtain highly proton conductive materials. In this work, a highly stable two-dimensional MOF, [Cu^II₅Cu^I₂(μ₃-OH)₄(H₂O)₆(L)₂(H₂L)₂]·3H₂O (denoted as YCu161; H₃L = 6-sulfonaphthalene-1,4-dicarboxylic acid) containing mixed-valence [Cu^II₅Cu^I₂(μ₃-OH)₄]⁸⁺ subunits, was successfully prepared. It exhibited excellent stability and temperature- and humidity-dependent proton conduction properties. Its optimal proton conductivity reached 1.84 × 10^-3 S cm^-1 at 90 °C and 98% relative humidity. On the basis of a crystal structure analysis, water vapor adsorption test results, and activation energy calculations, we deduced the proton conduction pathway and mechanism. Apparently, uncoordinated sulfonic and carboxyl groups and a network of abundant H-bonds inside the framework were responsible for the efficient proton transfer. Therefore, the strategy of selecting suitable bifunctional ligands to construct two-dimensional Cu-cluster-based MOFs with excellent proton conductivity is feasible.

Formulating Multiphase Medium Anti-wetting States in an Air-Water-Oil System: Engineering Defects for Interface Chemical Evolutions

Studies which regulate macroscopic wetting states on determined surfaces in multiphase media are of far-reaching significance but are still in the preliminary stage. Herein, inspired by the wettability subassembly of fish scales, Namib desert beetle shell, and lotus leaf upper side, interfaces in the air-water-oil system are programmed by defect engineering to tailor the anti-wetting evolution from double to triple liquid repellency states. By controlling the visible light irradiation and plasma treatment, surface oxygen vacancies on Cu_xO@TiO₂ nanowires (NWs) can be healed or reconstructed. The original membrane or the membrane after plasma treatment possesses abundant surface oxygen vacancies, and the homogeneous hydrophilic membrane shows only double anti-wetting states in the water-oil system. By the unsaturated visible light irradiation time, the surface oxygen vacancy partially healed, the heterogeneous hydrophilic-hydrophobic components occupied the membrane surface, and the anti-wetting state finally changed from double to triple in the air-water-oil system. After the illumination time reaches saturation, it promotes the healing of all surface oxygen vacancies, and the membrane surface only contains uniform hydrophobic components and only maintains double anti-wetting state in the air-oil system. The mechanism of the triple anti-wetting state on a heterogeneous surface is expounded by establishing a wetting model. The wetting state and the adhesion state of the Cu_xO@TiO₂ NW membrane show regional specificity by controlling the illumination time and region. The underwater oil droplets exhibit the "non-adhesive" and "adhesive" state in a region with unsaturated irradiation time or in an unirradiated region, respectively. Underwater oil droplet manipulation can be accomplished easily based on switchable wettability and adhesion. Current studies reveal that defect engineering can be extended to anti-wetting evolution in the air-water-oil system. Constructing an anti-wetting interface by heterogeneous components provides reference for designing the novel anti-wetting interface.

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.

A methylation-inspired mesoporous coordination polymer for identification and removal of organic pollutants in aqueous solutions

Tuning the interfacial chemistry of a metal–biomolecule coordination hybrid via bio-inspired methylation for structure enhancement and selective hazard adsorption/identification.