Superhydrophobic/Superoleophilic MOF Composites for Oil-Water Separation

Bioinspired multi-scale interface design for wet gas sensing based on rational water management

Natural organisms have evolved multi-scale wet gas sensing interfaces with optimized mass transport pathways in biological fluid environments, which sheds light on developing artificial counterparts with improved wet gas sensing abilities and practical applications. Herein, we highlighted current advances in wet gas sensing taking advantage of optimized mass transport pathways endowed by multi-scale interface design. Common moisture resistance (e.g., employing moisture resistant sensing materials, post-modifying moisture resistant coatings, physical heating for moisture resistance, and self-removing hydroxyl groups) and moisture absorption (e.g., employing moisture absorption sensing materials and post-modifying moisture absorption coatings) strategies for wet gas sensing were discussed. Then, the design principles of bioinspired multi-scale wet gas sensing interfaces were provided, including macro-level condensation mediation, micro/nano-level transport pathway adjustment and molecular level moisture-proof design. Finally, perspectives on constructing bioinspired multi-scale wet gas sensing interfaces were presented, which will not only deepen our understanding of the underlying principles, but also promote practical applications.

Superhydrophobic MOF/polymer composite with hierarchical porosity for boosting catalytic performance in an humid environment

The poor hydrostability of most reported metal-organic frameworks (MOFs) has become a daunting challenge in their practical applications. Recently, MOFs combined with multifunctional polymers can act as a functional platform and exhibit unique catalytic performance; they can not only inherit the outstanding properties of the two components but also offer unique synergistic effects. Herein, an original porous polymer-confined strategy has been developed to prepare a superhydrophobic MOF composite to significantly enhance its moisture or water resistance. The selective nucleation and growth of MOF nanocrystals confined in the pore of PDVB-vim are closely related to the structure-directing and coordination-modulating properties of PDVB-vim. The resultant MOF/PDVB-vim composite not only produces superior superhydrophobicity without significantly disturbing the original features but also exhibits a novel catalytic activity in the Friedel-Crafts alkylation reaction of indoles with trans-β-nitrostyrene because of the accessible sites and synergistic effects.

Robust and stable superhydrophilic MIL-101 (Cr)-coated copper mesh for highly efficient oil/water emulsion separation

In this study, dip coating method was investigated to prepare superhydrophilic MIL-101 (Cr)-coated copper mesh for highly efficient oil/water emulsion separation. To increase the surface area of synthesized MIL-101 (Cr), a purification procedure was developed to remove unreacted H2BDC crystals present in the channel of the initial MIL-101 (Cr) sample synthesized. After that, a dispersing solution of MIL-101 (Cr) was needed to coat on the copper mesh. Thermoplastic polyurethane (TPU) was used as a binder in this procedure. The prepared membranes of M1 (once coated mesh) to M6 (six times coated mesh) were performed to separate oil/water emulsion effectively. Contact angle tests showed the superhydrophilic/underwater superoleophobic wettability behavior of MIL-101 (Cr)-coated copper meshes. The wetting mechanism of the prepared membranes is mostly relevant to the surface functional groups of purified MIL-101 (Cr). Also, the roughness of the nanostructured coated membranes was improved because of the uniform coating of MIL-101 (Cr) which is integrated into hydrophilic TPU. Oil/water separation results showed that M2 (twice coated mesh) showed the maximum amount of water flux (83076 L m-2 h-1) in oil/water separation and M3 (three times coated mesh) had the best performance of oil/water emulsion with 99.99% separation efficiency.

Fluorine-Functionalized Covalent Organic Framework Superhydrophobic Modified Melamine Sponge for Efficient oil-water Separation

Hydrophobic sponges have attracted significant interest in oil spills and water-oil separation as potential absorption materials due to their desirable absorptivity, selectivity, and elasticity. In this paper, a hydrophilic melamine sponge (MS) is transferred into a superhydrophobic sponge via polydimethylsiloxane (PDMS) modification followed by in situ growth of fluorine-functionalized covalent organic framework (denoted as TFA-COF) nanoparticles. Therefore, the PDMS@TFA-COF@MS sponge was successfully prepared for efficient oil-water separation. The resultant PDMS@TFA-COF@MS exhibits superhydrophobic properties with a water contact angle of 156.7°. The superhydrophobic sponge has selectivity adsorption for different organic solvents and oils from water as well as oil-water separation efficiency (96% after 30 cycles) and oil absorption capacity (12 646% after 30 cycles). Meanwhile, the PDMS@TFA-COF@MS sponge exhibits strong thermal stability and flame retardancy in addition to having exceptional resistance to chemical corrosion in acidic, alkaline, and salt solutions. Moreover, the surfactant-stabilized oil-in-water emulsion could be efficiently separated by the sponge. Therefore, the prepared superhydrophobic PDMS@TFA-COF@MS sponge demonstrates possible uses for long-life oil-water separation applications.

Just Soaping Them: The Simplest Method for Converting Metal Organic Frameworks into Superhydrophobic Materials

The incorporation of superhydrophobic properties into metal organic framework (MOF) materials is highly desirable to enhance their hydrolytic stability, gas capture selectivity in the presence of humidity and efficiency in oil-water separations, among others. The existing strategies for inducing superhydrophobicity into MOFs have several weaknesses, such as increased cost, utilization of toxic reagents and solvents, applicability for limited MOFs, etc. Here, we report the simplest, most eco-friendly, and cost-effective process to impart superhydrophobicity to MOFs, involving a rapid (90 min) treatment of MOF materials with solutions of sodium oleate, a main component of soap. The method can be applied to both hydrolytically stable and unstable MOFs, with the porosity of modified MOFs approaching, in most cases, that of the pristine materials. Interestingly, this approach was used to isolate superhydrophobic magnetic MOF composites, and one of these materials formed stable liquid marbles, whose motion could be easily guided using an external magnetic field. We also successfully fabricated superhydrophobic MOF-coated cotton fabric and fiber composites. These composites exhibited exceptional oil sorption properties achieving rapid removal of floating crude oil from water, as well as efficient purification of oil-in-water emulsions. They are also regenerable and reusable for multiple sorption processes. Overall, the results described here pave the way for an unprecedented expansion of the family of MOF-based superhydrophobic materials, as virtually any MOF could be converted into a superhydrophobic compound by applying the new synthetic approach.

Facilitate the preparation of naturally modified and self-healing superhydrophobic/superoleophilic biochar-based foams for efficient oil-water separation

Oil spills are sudden, complex, and long-term hazardous, and the existing adsorption materials still have the disadvantages of small selective adsorption capacity, easy secondary contamination, and difficult to repair after breakage in practical applications. Herein, melamine foam (MF) coated by ball milled biochar (BMBC) and natural beeswax (Wax@BMBC@MF) was prepared by a bio-inspired functionalization method and further added with self-healing function (SH-Wax@BMBC@MF) to cope with complex environments, and applied to oil-water separation for oil adsorption. SEM and FTIR results showed that BMBC and natural beeswax nanoparticles successfully encapsulated the smooth surface of the melamine foam skeleton. The loading of natural beeswax increased the foam's ability to absorb oil and organic solvents from 0.6108-1.134 g to 0.850-1.391 g, and the oil-absorbing capacity of the foam remained at 0. 758-1.263 g after being cut by a knife and self-healing. The oil-absorbing capacity of SH-Wax@BMBC@MF remained in the range of 0.936-1.336 g under acid/alkali environment (pH =1-13). The surface functional groups of BMBC improved the surface roughness of the material and strengthen the MF skeleton to adsorb oils and organic solvents by capillary action. The generation of the di-coordinated structure by Fe3+ and catechol group contributed the restoration of SH-Wax@BMBC@MF structure and oil absorption capacity. SH-Wax@BMBC@MF has superiority of superhydrophobic, superoleophilic, self-healing after damage, and environmental friendliness, which provides a promising solution for the treatment of oil spills at sea.

Fine Tuning the Hydrophobicity of a New Three-Dimensional Cu2+ MOF through Single Crystal Coordinating Ligand Exchange Transformations

The synthesis, characterization, and single-crystal-to-single-crystal (SCSC) exchange reactions of a new 3D Cu2+ MOF based on 5-aminoisophthalic acid (H2AIP), [Cu6(μ3-ΟΗ)3(ΑΙΡ)4(HΑΙΡ)]n·6nDMF·nH2O - UCY-16·6nDMF·nH2O, are reported. It exhibits a 3D structure based on two [Cu4(μ3-OH)2]6+ butterfly-like secondary building units, differing in their peripheral ligation, bridged through HAIP-/AIP2- ligands. This compound displays the capability to exchange the coordinating ligand(s) and/or guest solvent molecules through SCSC reactions. Interestingly, heterogeneous reactions of single crystals of UCY-16·6nDMF·nH2O with primary alcohols resulted not only in the removal of the lattice DMF molecules but also in an unprecedented structural alteration that involved the complete or partial replacement of the monoatomic bridging μ3-OH- anion(s) of the [Cu4(μ3-OH)2]6+ butterfly structural core by various alkoxy groups. Similar crystal-to-crystal exchange reactions of UCY-16·6nDMF·nH2O with long-chain aliphatic alcohols (CxH2x+1OH, x = 8-10, 12, 14, and 16) led to analogues containing fatty alcohols. Notably, the exchanged products with the bulkier alcohols UCY-16/n-CxH2x+1OH·S' (x = 6-10, 12, 14, and 16) do not mix with H2O being quite stable in this solvent, in contrast to the pristine MOF, and exhibit a hydrophobic/superhydrophobic surface as confirmed from the investigation of their water contact angles and capability to remove hydrophobic pollutants from aqueous media.

Macrophages-Cancer Membrane-Encapsulated Metal-Organic Frameworks with Copper-Depleting Moiety for Mitochondria-Targeted Therapeutics

Mitochondria-targeted therapeutics are an attractive approach against energy-dependent cancer. However, effective mitochondria organelle therapeutics agents are still highly desirable. Herein, a mitochondria-targeted therapeutics platform, termed CDM@MUiO-DP@MCHM, consisting of macrophages-cancer hybrid membrane (MCHM) encapsulated MUiO-66 metal-organic frameworks (MOFs) is reported, which is loaded with microRNA (miRNA) biomarker detection probe (DP) for cancer diagnosis and copper-depleting moiety (CDM) for mitochondrial copper depletion to suppress cancer growth. Using nude mice bearing MCF-7 as model, after injecting intravenously via the caudal vein of mice, the encapsulation of MCHM can not only greatly enhance the cancer homing-targeting ability of the nanoparticles (NPs) but also endows the NPs the immune escape capacity to extend the circulation time. The miRNA-21 biomarker can be detected by the fluorescence signal for diagnosis, while the CDM induced energy deficiency and compromised mitochondria membrane potential, leading to apoptosis of the cancer cell. The good performance of CDM@MUiO-DP@MCHM suggest the great potential mitochondria organelle therapeutics.

Post-Synthetic Surface Modification of Metal-Organic Frameworks and Their Potential Applications

Metal-organic frameworks (MOFs) are porous hybrid materials with countless potential applications. Most of these rely on their porous structure, tunable composition, and the possibility of incorporating and expanding their functions. Although functionalization of the inner surface of MOF crystals has received considerable attention in recent years, methods to functionalize selectively the outer crystal surface of MOFs are developed to a lesser extent, despite their importance. This article summarizes different types of post-synthetic modifications and possible applications of modified materials such as: catalysis, adsorption, drug delivery, mixed matrix membranes, and stabilization of porous liquids.

Robust Superhydrophobic PDMS@SiO2@UiO66-OSiR Sponge for Efficient Water-in-Oil Emulsion Separation

A major challenge in oil/water separation is the processing of surfactant-stabilized emulsions from the water medium. One of the feasible schemes of emulsion separation is the porous melamine sponge coupled with functional particles. Here, we proposed a novel superhydrophobic metal-organic framework (MOF)-based sponge for water-in-oil emulsion separation. The porous melamine sponge was combined with poly(dimethylsiloxane) (PDMS)-coated hydrophobic SiO₂ and UiO66-OSiR particles were prepared for demulsification via the one-step dipping method for the first time. The PDMS@SiO₂@UiO66-OSiR sponge revealed excellent superhydrophobicity at a water contact angle of 160.7° and superlipophilicity at an oil contact angle of 0°. Compared with the pristine melamine sponge, the size-controllable PDMS@SiO₂@UiO66-OSiR sponge could separate stabilized water-in-oil emulsions with ultrahigh separation efficiency (>98.64%) and high flux (e.g., 970 L·m^-2·h^-1). Meanwhile, the PDMS@SiO₂@UiO66-OSiR sponge exhibited superior durability and mechanical reusability. Under harsh conditions such as strong acid and alkali, organic solvent corrosion, etc., all water contact angles of the PDMS@SiO₂@UiO66-OSiR sponge were over 152°. Furthermore, the stress decreased by 5% when the sponge was subjected to 10 loading/unloading compression cycles at a constant strain of 60%. These results demonstrate that the PDMS@SiO₂@UiO66-OSiR sponge can efficiently separate water-in-oil emulsions through its adjustable porous structure coupled with demulsification and hydrophobic particles. This study provides a step forward in developing a feasible strategy for the MOF-based sponge for emulsion separation.

Eco-Friendly Fluorine Functionalized Superhydrophobic/Superoleophilic Zeolitic Imidazolate Frameworks-Based Composite for Continuous Oil-Water Separation

Superhydrophobic metal-organic framework (MOF)-based sponges have received increasing attention in terms of treating oil-water mixtures. However, highly fluorinated substances, commonly used as modifiers to improve the hydrophobicity of MOFs, have aroused much environmental concern. Developing a green hydrophobic modification is crucial in order to prepare superhydrophobic MOF-sponge composites. Herein, we report the preparation of a porous composite sponge via a polydopamine (PDA)-assisted growth of zeolitic imidazolate frameworks (ZIF-90) and eco-friendly hydrophobic short-chain fluorinated substances (trifluoroethylamine) on a melamine formaldehyde (MF) sponge. The composite sponge (F-ZIF-90@PDA-MF) exhibited superhydrophobicity (water contact angle, 153°) and superoleophilicity (oil contact angle, 0°), which is likely due to the combination of the low surface energy brought on by the grafted CF₃ groups, as well as the rough surface structures that were derived from the in situ growth of ZIF-90 nanoparticles. F-ZIF-90@PDA-MF showed an excellent adsorption capacity of 39.4-130.4 g g^-1 for the different organic compounds. The adsorbed organic compounds were easily recovered by physical squeezing. Continuous and selective separation for the different oil-water mixtures was realized by employing the composite sponge as an absorbent or a filter. The separation efficiency and flux reached above 99.5% and went up to 7.1 ×10⁵ L m^-2 h^-1, respectively. The results illustrate that the superhydrophobic and superoleophilic F-ZIF-90@PDA-MF sponge has potential in the field of water-oil separation, especially for the purposes of large-scale oil recovery in a water environment.

Metal-Organic Framework-Based Colloidal Particle Synthesis, Assembly, and Application

Metal-organic frameworks (MOFs) assembled from metal nodes and organic ligands have received significant attention over the past two decades for their fascinating porous properties and broad applications. Colloidal MOFs (CMOFs) not only inherit the intrinsic properties of MOFs, but can also serve as building blocks for self-assembly to make functional materials. Compared to bulk MOFs, the colloidal size of CMOFs facilitates further manipulation of CMOF particles in a single or collective state in a liquid medium. The resulting crystalline order obtained by self-assembly in position and orientation can effectively improve performance. In this review, we summarize the latest developments of CMOFs in synthesis strategies, self-assembly methods, and related applications. Finally, we discuss future challenges and opportunities of CMOFs in synthesis and assembly, by which we hope that CMOFs can be further developed into new areas for a wider range of applications.

Investigation of grafting silane coupling agents on superhydrophobicity of carbonyl iron/SiO2 particles for efficient oil/water mixture and emulsion separation

The present study demonstrated the wettability properties of grafting silane coupling agents on carbonyl iron (CI)/SiO₂ particles for efficient oil/water mixture and emulsion separation. CI particles were first reacted with Tetraethoxysilane (TEOS) to create a magnetic component. Then, CI/SiO₂ particles were altered by 1H,1H,2H,2H-perfluorodecyltriethoxysilane (FAS) and Hexamethyldisilazane (HDMS) to create magnetic superhydrophobic/superoleophilic, recyclable, and reusable sorbent powders. The water contact angle (WCA) values of the as-prepared particles, CI, CI/SiO₂, CI/SiO₂@FAS, and CI/SiO₂@HMDS, were 5.4° ± 1.3°, 6.4° ± 1.4°, 151.9° ± 2.1°, and 170.1° ± 1.1°, respectively. In addition, the oil contact angles (OCAs) of a variety of oils were found to be equivalent to 0°. Hence, superhydrophobic/superoleophilic particles for kind of different oils were shown sorption capacities of 1.7-3.1 g/g and 2.5-4.3 g/g for CI/SiO₂@FAS, and CI/SiO₂@HMDS, respectively. Besides, for 1%w/w hexane/water emulsion separation efficiency higher than 99%, the lowest mass was obtained at 50 and 200 mg for CI/SiO₂@HDMS and CI/SiO₂@HDMS, respectively, suggesting a new effective material for separating tiny oil droplets. Also, the reusability and chemical durability of the superhydrophobic samples made them a prime candidate for use in different harsh conditions.

Special Superwetting Materials from Bioinspired to Intelligent Surface for On-Demand Oil/Water Separation: A Comprehensive Review

Since superwetting surfaces have emerged, on-demand oil/water separation materials serve as a new direction for meeting practical needs. This new separation mode uses a single porous material to allow oil-removing and water-removing to be achieved alternately. In this review, the fundamentals of wettability are systematically summarized in oil/water separation. Most importantly, the two states, bioinspired surface and intelligent surface, are summarized for on-demand oil/water separation. Specifically, bioinspired surfaces include micro/nanostructures, bioinspired chemistry, Janus-featured surfaces, and dual-superlyophobic surfaces that these superwetting materials can possess asymmetric wettability in one structure system or opposite underliquid wettability by prewetting. Furthermore, an intelligent surface can be adopted by various triggers such as pH, thermal and photo stimuli, etc., to control wettability for switchable oil/water separation reversibly, expressing a thought beyond nature to realize innovative oil/water separation by external stimuli. Remarkably, this review also discusses the advantages of all the materials mentioned above, expanding the separation scope from the on-demand oil/water mixtures to the multiphase immiscible liquid-liquid mixtures. Finally, the prospects of on-demand oil/water separation materials are also concluded.

High-flux nanofiltration membrane with modified highly dispersed MOF particles as nano filler

The synthesis of optimized thin film nanocomposite (TFN) membrane with no or few defects is an efficacious method which can improve nanofiltration performance. However, poor dispersion of fillers in the organic phase and wrong compatibility between fillers and polymerizate are still a serious problem. In this study, the particle size of metal organic framework (MOF), aluminum-based metal-organic frameworks (CAU-1) was modulated and for the first time, dodecyl aldehyde was used to modify the surface hydrophobicity of CAU-1, which improved the dispersibility and inhibited the aggregation in the trimesoyl chloride (TMC)/n-hexane solution; later CAU-1 and modified CAU-1 were incorporated into the polyamide (PA) selective layer to synthesize TFN membrane by interfacial polymerization (IP). The particle size modulation and modification of the CAU-1 were demonstrated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) characterization. The characterization showed that PA selective layer was synthesized on the top layer of polysulfone (PSF) substrate. The pure water flux of the TFN membrane was increased to 79.89 ± 1.24 L·m^-2·h^-1·bar^-1 compared to the original thin film composite (TFC) membrane, which was due to the polymerization of 100 nm modified CAU-1 on the PA layer to form a new water molecular channel, thus increasing the water flux by about 70%.

Temperature-Dependent Superhydrophobic Functionalized Coordination Polymers (SFCPs) for Selective Adsorption of C2H4 over C2H6

We prepared two new superhydrophobic functionalized coordination polymers (SFCPs) [Zn₄(OH)₂(BTMB)₂(4,4'-Bipy)₂]_∞ ⊃ solvent, 1, and [Cd₄(OH)₂(BTMB)₂(4,4'-Bipy)₃]_∞ ⊃ solvent, 2, by solvothermal methods. For 1, the single-crystal XRD structure revealed that it contains two crystallographically distinct Zn²⁺ ions with two different types of coordination geometries of 4 and 6, exhibiting a unique superhydrophobic behavior with microporosity. Compound 1 exhibits superhydrophobicity with a contact angle of 155.5° (at 30 °C), which is stable even at high temperatures, whereas for the SFCP 2, all of the Cd²⁺ ions have only 6-coordination and exhibit a superhydrophobic character at room temperature with a contact angle of 156.7°(at 30 °C). However, surprisingly, this superhydrophobic character is stable only up to 60 °C, above which it is converted to hydrophilic nature, in contrast to the SFCP 1. Moreover, in this study, we also report a selective gas adsorption study of two C2 gases with similar kinetic diameters (∼3.9 Å) of ethylene over ethane.

Fabrication of robust superhydrophobic magnetic multifunctional coatings and liquid marbles

To obtain durable and multi-function superhydrophobic surfaces, we reported a facial method to prepare a multifunctional suspension (γ-Fe₂O₃@SiO₂@PDMS suspension) named as FSP suspension, in which γ-Fe₂O₃ was coated by the silica shell and PDMS was used as the outer layer. Superhydrophobic magnetic polyurethane (SMPU) sponge was prepared by immersing the polyurethane (PU) sponge into the FSP suspension, exhibiting the superior ability to absorb oil. In addition, it could also move directionally by the attraction of magnets and absorb the oil along the fixed path. The heated superhydrophobic magnetic stainless steel (H-SMSS) mesh was acquired by spraying FSP suspension onto the stainless steel mesh and then heating at 400 °C, which demonstrated superior superhydrophobicity and resistance to abrasion and chemical corrosion. Besides, the H-SMSS mesh displayed excellent flux and efficiency to separate the oil/water mixture. Rolling droplets on FSP particles, the superhydrophobic magnetic liquid marbles (SMLMs) were fabricated, in which liquids with different volumes were encapsulated and transported directionally. Further, it was convenient to inject liquid into the SMLM and withdraw liquid from it. Thus, the prepared FSP suspension has promising applications in constructing large-area, robust, and multifunctional surfaces and microreactors.

Engineering Metal-Organic Framework Hybrid AIEgens with Tumor-Activated Accumulation and Emission for the Image-Guided GSH Depletion ROS Therapy

Aggregation-induced emission (AIE)-active luminogens (AIEgens) have demonstrated exciting potential for the application in cancer phototheranostics. However, simultaneously achieving tumor-activated bright emission, enhanced reactive oxygen species (ROS) generation, high tumor accumulation, and minimized ROS depletion remains challenging. Here, a metal-organic framework (MOF) hybrid AIEgen theranostic platform is designed, termed A-NUiO@DCDA@ZIF-Cu, composed of an AIEgen-loaded hydrophobic UiO-66 (A-NUiO@DCDA) core and a Cu-doped hydrophilic ZIF-8 (ZIF-Cu) shell. The fluorescence emission and therapeutic ROS activity of AIEgens are restrained during delivery. After uptake by tumor tissues, ZIF-Cu decomposition occurs in response to an acidic tumor microenvironment (TME), and the hydrophobic A-NUiO@DCDA cores self-assemble into large particles, extremely increasing the tumor accumulation of AIEgens. This results in enhanced fluorescence imaging (FLI) and highly improved ¹O₂ generation ability during photodynamic therapy (PDT). Meanwhile, the released Cu²⁺ reacts to glutathione (GSH) to generate Cu⁺, which provides an extra chemodynamic therapy (CDT) function through Fenton-like reactions with overexpressed H₂O₂, resulting in the GSH depletion-enhanced ROS therapy. As a result of these characteristics, the MOF hybrid AIEgens can selectively kill tumors with excellent efficacy.

The Dielectric Behavior of Protected HKUST-1

We investigated the adsorption properties and the dielectric behavior of a very well-known metal-organic framework (MOF), namely Cu3(BTC)2 (known as HKUST-1; BTC = 1,3,5-benzenetricarboxylate), before and after protection with some amines. This treatment has the purpose of reducing the inherent hygroscopic nature of HKUST-1, which is a serious drawback in its application of as low-dielectric-constant (low-κ) material. Moreover, we investigated the structure of HKUST-1 under a strong electric field, confirming the robustness of the framework. Even under dielectric perturbation, the water molecules adsorbed by the MOF remained almost invisible to X-ray diffraction, apart from those directly bound to the metal ions. However, the replacement of H2O with a more visible guest molecule such as CH2Br2 made the cavity that traps the guest more visible. Finally, in this work we demonstrate that impedance spectroscopy is a valuable tool for identifying water sorption in porous materials, providing information that is complementary to that of adsorption isotherms.

Structure and dynamics of a water/methanol mixture confined in zeolitic imidazolate framework ZIF-8 from atomistic simulations

A classical atomistic simulation study is reported for the microscopic structure and dynamics of a water/methanol mixture confined in flexible nanoporous zeolitic imidazolate framework ZIF-8. Both the radial density distribution and vivid two-dimensional density profile demonstrate that methanol molecules can roughly be viewed as "embedded" between two layers of water molecules to form a "sandwich" structure. The reason for the formation of such a specific structure is explained based on the hydrogen-bonding state and the strength of various hydrogen bonds. The investigation of guest molecular diffusion shows that the self-diffusion coefficient of confined water is generally one to two orders of magnitude smaller than that of bulk water. In addition, the dependence of the self-diffusion coefficient on loading is non-monotonic: the self-diffusion coefficient firstly shows a significant increase and then decreases at higher loading. Moreover, both the structure and dynamics of the hydrogen bond (HB) network of confined water molecules are investigated in a spatially resolved manner. The results indicate that both the HB structure and dynamics of water molecules near the ZIF-8 surface deviate significantly from those of bulk water. However, while water molecules located at the pore center are relatively similar to bulk water molecules with respect to the HB structure, they exhibit strong slowdown in HB dynamics when compared with bulk water. This simulation study elucidates in detail the structural and dynamical properties of a water/methanol mixture in nanoscopic ZIF-8 confinement, which is expected to provide a deep insight into the role of porous fillers, such as ZIF-8, in improving the performance of the dehydration of alcohols via pervaporation and other related processes.

Superhydrophilic and Photothermal Fe-TCPP Nanofibrous Membrane for Efficient Oil-in-Water Nanoemulsion Separation

Separation and purification of surfactant-stabilized oil-in-water nanoemulsions is a great environmental challenge. Membrane-based separation strategies are more effective over conventional methods in the treatment of nanoemulsion waste water. In this paper, we construct a superhydrophilic membrane by coating a thin photothermal-responsive iron tetrakis(4-carboxyphenyl)porphyrin (Fe-TCPP) nanofibrous metal organic framework (MOF) selective layer on a macroporous polyethersulfone membrane. The as-prepared membrane exhibits high separation performance of oil-in-water nanoemulsions with permeance of 46.4 L·m^-2·h^-1·bar^-1 and separation efficiency of 99%. It also demonstrates nice anti-oil/ionic-fouling property, good recyclability, and desirable stability. The high separation performance is accredited to the superhydrophilicity, highly charged surface, and nanometer pore sizes of the Fe-TCPP nanofibrous membrane. Due to the unique photothermal property of Fe-TCPP nanofibers, the permeance can be enhanced more than 50% by visible light without deteriorating the rejection. This photo-stimuli MOF-based thin-layer membrane offers great potential for the generation of point-of-use water treatment devices.

Multifunctional starch-based material for contaminated emulsions separation and purification

Stable water-in-oil (W/O) emulsions can produce at many industrial production occasions. However, most materials for its separation have serious fouling problems. To overcome this shortcoming, we fabricated an easy cleaning multifunctional starch-based material with unique wetting behavior which could realize efficient separation and purification of W/O emulsions. This material has a hierarchical structure and superoleophilic and under oil superhydrophobic surfaces which could separate various W/O emulsions in a high separation efficiency and flux without external pressure. In addition, the decrease of separation flux was not observed for this material, which can be reused more than 10 times after washing with ethanol and drying after each separation cycle. Furthermore, this material also could realize efficient removal of dyes and heavy-metal and rare-earth ions simultaneously during a separation process. The material shows great potential for separating and purifying stable W/O emulsions produced during the industrial production.

Hyper-Cross-Linked Polymer-Decorated Surfaces with Ultrahigh Efficiency for Oil/Water Emulsion Separation and Recovery

A novel superhydrophobic/superoleophilic surface has been developed by direct surface condensation of dichloroxylene that results in a controlled coating of hyper-cross-linked polymers. Specifically, the coating was successfully applied to a melamine formaldehyde sponge and optimized by fine-tuning the reaction variables. The resulting hierarchical porous sorbents stabilized by polydimethylsiloxane exhibited an increased surface area, good physiochemical stability, high selectivity, and adsorption capacities for a variety of oils and solvents. The composite can separate oil in water emulsions with ultrahigh separation efficiency >99% over 10 cycles in liter-scale experiments, wherein the highest separation efficiency was as low as 2 ppm even with a short period of filtration, suggesting strong potential for oil/water separation and recovery.

MOF based engineered materials in water remediation: Recent trends

The significant upsurge in the demand for freshwater has prompted various developments towards water sustainability. In this context, several materials have gained remarkable interest for the removal of emerging contaminants from various freshwater sources. Among the currently investigated materials for water treatment, metal organic frameworks (MOFs), a developing class of porous materials, have provided excellent platforms for the separation of several pollutants from water. The structural modularity and the striking chemical/physical properties of MOFs have provided more room for target-specific environmental applications. However, MOFs limit their practical applications in water treatment due to poor processability issues of the intrinsically fragile and powdered crystalline forms. Nevertheless, growing efforts are recognized to impart macroscopic shapability to render easy handling shapes for real-time industrial applications. Furthermore, efforts have been devoted to improve the stabilities of MOFs that are subjected to fragile collapse in aqueous environments expanding their use in water treatment. Advances made in MOF based material design have headed towards the use of MOF based aerogels/hydrogels, MOF derived carbons (MDCs), hydrophobic MOFs and magnetic framework composites (MFCs) to remediate water from contaminants and for the separation of oils from water. This review is intended to highlight some of the recent trends followed in MOF based material engineering towards effective water regeneration.

Adsorptive removal of hazardous organics from water and fuel with functionalized metal-organic frameworks: Contribution of functional groups

The purification of contaminated water and fuel is very important for our sustainability. Adsorptive removal has attracted significant attention because of possible applications in industry and the rapid development of metal-organic frameworks (MOFs), which can be competitive adsorbents. In this review, the possible/competitive purification of water (contaminated with organics) and fuel (composed of S- and N-Containing compounds) via adsorption using MOFs, especially those with various functional groups (FGs), will be discussed. The contribution of FGs such as -OH, -COOH, -SO₃H, -NH₂, and -NH₃⁺ to adsorption/purification will be analyzed in detail, not only to understand the plausible adsorption mechanism but also to utilize specific FGs in adsorption. Moreover, methods for introducing FGs onto MOFs will be summarized. Finally, the prospects for both adsorption/removal and emerging fields will be suggested. Studies for practical applications in industry with shaped MOFs from inexpensive route will be important. The solution pH should be considered for the adsorption of aqueous solution. Applications of MOFs in other fields like storage/delivery and enrichment of analytes might be deeply studied.

The applications and prospects of hydrophobic metal-organic frameworks in catalysis

In recent years, large numbers of hydrophobic/superhydrophobic metal-organic frameworks (MOFs) have been developed. These hydrophobic MOFs not only retain rich structural variety, highly crystalline frameworks, and uniform micropores, but they also have lower affinity towards water and boosted hydrolytic stability. Until now, there were two main strategies to prepare hydrophobic MOFs, including a one-step method and post-synthesis modification (PSM). PSM was an often-used strategy for preparing hydrophobic MOFs. Hydrophobic MOFs showed unique advantages when used as catalysts for various categories of reactions. Herein, recent research advances relating to hydrophobic MOFs in the catalytic field are presented. The catalytic activities of hydrophobic MOFs and corresponding hydrophilic ones are also compared, and the superiority of hydrophobic MOFs or MOF materials as catalysts in 10 reactions is discussed. Finally, the advantages of hydrophobic MOFs as catalysts or auxiliary materials are summarized and promising future developments of hydrophobic MOFs are highlighted.

Modified-MOF-808-Loaded Polyacrylonitrile Membrane for Highly Efficient, Simultaneous Emulsion Separation and Heavy Metal Ion Removal

Oily wastewater, often containing heavy metal ions, is a common source of water pollution. In this study, a modified-MOF-loaded polyacrylonitrile membrane was designed and prepared via solvothermal reaction and electrospinning to simultaneously separate oil-in-water emulsions and adsorb heavy metal ions. The membrane shows superhydrophilicity and superoleophilicity in air and underwater superoleophobicity, which enables the membrane to efficiently separate various oil-in-water emulsions. The strong adsorption capacity of MOF-808 allows this membrane to adsorb heavy metal ions at various concentrations in a short time. The separation efficiency reached 99.97%, and the highest removal efficiency of heavy metal ions reached 97.7%. Additionally, the membrane demonstrated excellent recyclability and corrosion resistance. Overall, the membrane is highly efficient in treating wastewater, and it has great potential for practical applications.

Substitution Effects Regulate the Formation of Butterfly-Shaped Tetranuclear Dy(III) Cluster and Dy-Based Hydrogen-Bonded Helix Frameworks: Structure and Magnetic Properties

The generation of two types of complexes with different topological connections and completely different structural types merely via the substitution effect is extremely rare, especially for -CH₃ and -C₂H₅ substituents with similar physical and chemical properties. Herein, we used 3-methoxysalicylaldehyde, 1,2-cyclohexanediamine, and Dy(NO₃)₃·6H₂O to react under solvothermal conditions (CH₃OH:CH₃CN = 1:1) at 80 °C to obtain the butterfly-shaped tetranuclear Dy^III cluster [Dy₄(L¹)₄(μ₃-O)₂(NO₃)₂] (Dy₄, H₂L¹ = 6,6'-((1E,1'E)-(cyclohexane-1,3-diylbis(azanylylidene))bis(methanylylidene))bis(2-methoxyphenol)). The ligand H₂L¹ was obtained by the Schiff base in situ reaction of 3-methoxysalicylaldehyde and 1,2-cyclohexanediamine. In the Dy₄ structure, (L¹)^2- has two different coordination modes: μ₂-η¹:η²:η¹:η¹ and μ₄-η¹:η²:η¹:η¹:η²:η¹. The four Dy^III ions are in two coordination environments: N₂O₆ (Dy1) and O₉ (Dy2). The magnetic testing of cluster Dy₄ without the addition of an external field revealed that it exhibited a clear frequency-dependent behavior. We changed 3-methoxysalicylaldehyde to 3-ethoxysalicylaldehyde and obtained one case of a hydrogen-bonded helix framework, [DyL²(NO₃)₃]_n·2CH₃CN (Dy-HHFs, H₂L² = 6,6'-((1E,1'E)-(cyclohexane-1,3-diylbis(azanylylidene))bis(methanylylidene))bis(2-ethoxyphenol)), under the same reaction conditions. The ligand H₂L² was formed by the Schiff base in situ reaction of 3-ethoxysalicylaldehyde and 1,2-cyclohexanediamine. All Dy^III ions in the Dy-HHFs structure are in the same coordination environment (O₉). The twisted S-shaped (L²)^2- ligand is linked by a Dy(III) ion to form a spiral chain. The spiral chain is one of the independent units that is interconnected to form Dy-HHFs through three strong hydrogen-bonding interactions. Magnetic studies show that Dy-HHFs exhibits single-ion-magnet behavior (U_eff = 68.59 K and τ₀ = 1.10 × 10^-7 s, 0 Oe DC field; U_eff = 131.5 K and τ₀ = 1.22 × 10^-7 s, 800 Oe DC field). Ab initio calculations were performed to interpret the dynamic magnetic performance of Dy-HHFs, and a satisfactory consistency between theory and experiment exists.

Starch-based nanospheres modified filter paper for o/w emulsions separation and contaminants removal

There is a pressing need around the world to develop novel functional biodegradable materials to separate oil/water mixtures and emulsions completely. Recently, superhydrophilicity and underwater superoleophobicity materials have been attracted attention due to their high efficiency in oil/water separation. However, it is still a challenge to prepare materials that combine oil/water separation and water purification in an environment-friendly way. In this work, biodegradable starch-based nanospheres (SNPs) coated filter paper was prepared in a low-cost, simple, and environmentally friendly manner. The SNPs coating could not only help to change the wettability of the substrate material but also build the hierarchical micro and nano structures which are conducive to separation and purification process. After modification by coating SNPs, the filter paper exhibited excellent performance in a wide range of oil/water mixtures or emulsions separation and the wettability of the filter paper could be regulated by adjusting the pH value. The modified filter paper presented good recyclability after several separation process. Furthermore, the as-prepared filter paper could also remove water-soluble contaminants during the oil/water separation process, thus realizing to combine separation and purification process in one single step. This biodegradable starch-based separating material with good separation performance, stability and recyclability has significant application potential in practical separation and purification process.

Robust super-hydrophobic/super-oleophilic sandwich-like UIO-66-F4@rGO composites for efficient and multitasking oil/water separation applications

Super-wetting MOFs@graphene hybrid has shown promising application for oil/water separation, due to high porosity, low density, and controllable wettability, however, achieving excellent stability and recyclability are found to be still challenging. In this study, sandwich-like UIO-66-F₄@rGO hybrid was synthesized by immobilization of UIO-66-F₄ nanoparticles on rGO matrix, which featured the unique micro/nano hierarchy with hydrophobic characteristics. In order to realize the oil/water separation, as-prepared sandwich-like UIO-66-F₄@rGO hybrid was applied as a potential candidate for constructing robust super-hydrophobic/super-oleophilic interfaces by using filter paper (FP) and melamine sponge (MS) as substrates. Typically, the surface modification of substrates can be easily achieved by simple dip-coating method, and interfacial adhesion between substrates and UIO-66-F₄@rGO was enhanced by cross-linking of hydroxyl-fluoropolysiloxane (FPSO). Consequently, the super-hydrophobic/oleophilic UIO-66-F₄@rGO/FP exhibited high contact angle of 169.3 ± 0.6° and was capable of separating various water-in-oil emulsions effectively. The flux and separation efficiency were 990.45 ± 36.28 Lm^-2 h^-1 and 99.73 ± 0.19 % driven by gravity, respectively. The super-hydrophobic/super-oleophilic UIO-66-F₄@rGO/MS possessed selective oil absorption with absorption capacity of 26∼61 g/g depending on the viscosity of oils and continuous cleaning of oil spill. Furthermore, the UIO-66-F₄@rGO composite could tolerate high/low temperature, corrosive solutions, and physical damage, displaying robust and stable super-hydrophobic/super-oleophilic interfaces for treating oily wastewater in harsh environments.

Rational design and synthesis of a stable pillar-layer NaI-organic framework as a multi-responsive luminescent sensor in aqueous solutions

A stable pillar-layer Na^I-organic framework, [Na₂(DCPB)∙(H₂O)₂]_n (namely 1), was rationally designed and synthesized by the assemble process of Na^I and 1,3-di(4'-carboxyl-phenyl)benzene (H₂DCPB). Benefiting from the luminescent property and high stability in water, the as-synthesized 1 is a potential multi-responsive luminescent sensor material toward Cr₂O₇^2-, Fe³⁺, and nitrofurazone (NFZ) in water. Ground 1 not only has the excellent detectability and selectivity but also possesses outstanding stability and circularity. The calculated K_sv values of 1 are 8.8×10³ for Fe³⁺, 9.9×10³ for Cr₂O₇^2-, and 2.7×10⁴ M^-1 for NFZ in aqueous solutions, respectively. Furthermore, 1 is able to accurately detect NFZ in real bovine serum samples through luminescence detection technology.

Triggering Lewis Acidic Nature through the Variation of Coordination Environment of Cd-Centers in 2D-Coordination Polymers

The rational design and successful synthesis of novel functional metal-organic frameworks relies on careful selection of metals and versatile organic ligands. A newly designed pyrazole-based dicarboxylate ligand, 5-(3,5-dimethyl-1H-pyrazol-1-yl) 1,3-benzenedicarboxylic acid (H₂L), was utilized to obtain two new Cd-based coordination polymers I [Cd(L)(H₂O)]·H₂O and II [Cd(L)] under similar reaction conditions via solvothermal strategy. Single-crystal X-ray data confirmed that compound I exhibits a two-dimensional (2D) skeleton comprising pentagonal bipyramidal Cd-ions and an organic ligand moiety. Compound II has also formed a two-dimensional layer arrangement with the connectivity between trigonal bipyramidal Cd-ions and the organic ligand. Topological analysis revealed that compound I has formed unique 4³.6³ net topology while compound II has displayed a 4⁴.6² sql net topology with 2D frameworks. The Lewis acidic nature of both I and II containing a Cd²⁺ metal center has been correlated with the coordination number through dye adsorption-desorption and catalysis studies. The selective adsorption of anionic dye and the extent of adsorption are interrelated with the Cd-ion geometry. For the first time, the role of coordinated water molecule has been analyzed through heterogeneous catalysis reaction (i.e., cyanosilylation) with Cd-based 2D-coordination polymers (CPs). The plausible mechanisms have been proposed to explain the subsequent role of coordination number and environment in CPs.