Hydrophobic Metal-organic Frameworks Research Articles

Mild-Temperature Supercritical Water Confined in Hydrophobic Metal-Organic Frameworks.

Fluids under extreme confinement show characteristics significantly different from those of their bulk counterpart. This work focuses on water confined within the complex cavities of highly hydrophobic metal-organic frameworks (MOFs) at high pressures. A combination of high-pressure intrusion-extrusion experiments with molecular dynamic simulations and synchrotron data reveals that supercritical transition for MOF-confined water takes place at a much lower temperature than in bulk water, ∼250 K below the reference values. This large shifting of the critical temperature (Tc) is attributed to the very large density of confined water vapor in the peculiar geometry and chemistry of the cavities of Cu2tebpz (tebpz = 3,3',5,5'-tetraethyl-4,4'-bipyrazolate) hydrophobic MOF. This is the first time the shift of Tc is investigated for water confined within highly hydrophobic nanoporous materials, which explains why such a large reduction of the critical temperature was never reported before, neither experimentally nor computationally.

Multifunctional hydrophobic MOFs modified by rosin acid with oil–water separation and room temperature catalysis

Metal-organic frameworks (MOFs) are high surface area porous materials with uniform pores. MOFs are among the widely investigated porous materials because of their potential applications in adsorption, oil–water separation, catalysis, and other such applications. However, MOFs are not stable enough in aqueous environments, which restricts their application in practice. We converted hydrophilic UiO-66-SH into hydrophobic UiO-66-RA by introducing rosin acid into organic linkers. The material was characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The superhydrophobic UiO-66-RA was successfully constructed and the water droplet contacts angle reached 157.2°. Importantly, the obtained UiO-66-RA demonstrated stable oil–water separation performance, with separation efficiencies of up to 96.1% and 96.8% for both light and heavy oils. The separation efficiencies also reached more than 95.1% after five repetitions. For stable oil–water emulsions, UiO-66-RA also achieved effective separation. In addition, the catalyst Pd@UiO-66-RA was successfully prepared by hybridization of palladium chloride with UiO-66-RA. The catalytic yields of Pd@UiO-66-RA was 95.4%. And the yield could also be kept above 60% after five cycles of Sonogashira reaction catalyzed. This work offers a new type of superhydrophobic MOFs, the multifunctional MOFs has the potential for environmentally benign catalysis and purification of wastewater.

Topological Data Analysis Combined with High-Throughput Computational Screening of Hydrophobic Metal–Organic Frameworks: Application to the Adsorptive Separation of C3 Components

The shape and topology of pores have significant impacts on the gas storage properties of nanoporous materials. Metal–organic frameworks (MOFs) are ideal materials with which to tailor to the needs of specific applications, due to properties such as their tunable structure and high specific surface area. It is, therefore, particularly important to develop descriptors that accurately identify the topological features of MOF pores. In this work, a topological data analysis method was used to develop a topological descriptor, based on the pore topology, which was combined with the Extreme Gradient Boosting (XGBoost) algorithm to predict the adsorption performance of MOFs for methane/ethane/propane. The final results show that this descriptor can accurately predict the performance of MOFs, and the introduction of the topological descriptor also significantly improves the accuracy of the model, resulting in an increase of up to 17.55% in the R2 value of the model and a decrease of up to 46.1% in the RMSE, compared to commonly used models that are based on the structural descriptor. The results of this study contribute to a deeper understanding of the relationship between the performance and structure of MOFs and provide useful guidelines and strategies for the design of high-performance separation materials.

Alcohol-based adsorption heat pumps using hydrophobic metal–organic frameworks

Alcohol adsorption using hydrophobic MOFs is a promising strategy for heating and cooling applications. Finding optimal conditions for each adsorbent-working fluid pair is key to maximize the performance of the devices.

Effect of Crystallite Size on the Flexibility and Negative Compressibility of Hydrophobic Metal-Organic Frameworks.

Flexible nanoporous materials are of great interest for applications in many fields such as sensors, catalysis, material separation, and energy storage. Of these, metal-organic frameworks (MOFs) are the most explored thus far. However, tuning their flexibility for a particular application remains challenging. In this work, we explore the effect of the exogenous property of crystallite size on the flexibility of the ZIF-8 MOF. By subjecting hydrophobic ZIF-8 to hydrostatic compression with water, the flexibility of its empty framework and the giant negative compressibility it experiences during water intrusion were recorded via in operando synchrotron irradiation. It was observed that as the crystallite size is reduced to the nanoscale, both flexibility and the negative compressibility of the framework are reduced by ∼25% and ∼15%, respectively. These results pave the way for exogenous tuning of flexibility in MOFs without altering their chemistries.

Dual positive charging sites for MIL-101 enhanced adsorption of toluene under high humidity conditions: Experimental and theoretical studies

Highly efficient capture of volatile organic compounds (VOCs) under humid conditions is a significant yet formidable task for metal–organic frameworks (MOFs). Based on the material simulation technique, we proposed implanting a significantly positive-density ligand (DABA) to modify MIL-101 and constructing dual positive-charging centers to improve MIL-101 competitive adsorption of benzene VOCs over H2O vapors both thermodynamically and kinetically. The introduction of DABA containing two –NH2 groups with electrostatically positive charges into MIL-101 resulted in the formation of an open tetrahedral cage with a dual positive charging center. This facilitated strong N-H…π interactions with toluene molecules, leading them to accumulate firmly in MOF pores, and thus dramatically enhanced its adsorption capacity from 17 to 31 toluene molecules per unit cell compared to pristine MIL-101. Moreover, the open tetrahedral cage in D-MIL-101 preferentially captured toluene and occupied the strongest adsorption sites first, thus effectively preventing H2O entry into MOF pores to contact with hydrophilic metal sites. This realized high water resistance and toluene adsorption uptake under high humidity conditions. Results showed that D-MIL-101 had a high specific surface area of 2558 m2/g and a new microporous structure of 9.5 Å size. It presented higher toluene uptake of 4.69 mmol/g at lower vapor pressure of P/P0 = 0.013 and 298 K temperature, as well as 3.3 folds higher working capacity of toluene compared to pristine MIL-101 under 80 % RH. This study presents a novel approach for constructing hydrophobic MOFs using new positive charging ligands and delves deeper into the competitive adsorption of VOCs/H2O in MOF composites for large-scale applications.

Design Principles for Using Amphiphilic Polymers To Create Microporous Water.

Aqueous dispersions of microporous nanocrystals with dry, gas-accessible pores─referred to as "microporous water"─enable high densities of gas molecules to be transported through water. For many applications of microporous water, generalizable strategies are required to functionalize the external surface of microporous particles to control their dispersibility, stability, and interactions with other solution-phase components─including catalysts, proteins, and cells─while retaining as much of their internal pore volume as possible. Here, we establish design principles for the noncovalent surface functionalization of hydrophobic metal-organic frameworks with amphiphilic polymers that render the particles dispersible in water and enhance their hydrolytic stability. Specifically, we show that block co-polymers with persistence lengths that exceed the micropore aperture size of zeolitic imidazolate frameworks (ZIFs) can dramatically enhance ZIF particle dispersibility and stability while preserving porosity and >80% of the theoretical O2 carrying capacity. Moreover, enhancements in hydrolytic stability are greatest when the polymer can form strong bonds to exposed metal sites on the external particle surface. More broadly, our insights provide guidelines for controlling the interface between polymers and metal-organic framework particles in aqueous environments to augment the properties of microporous water.

Automatic Machine Learning Combined with High-Throughput Computational Screening of Hydrophobic Metal–Organic Frameworks for Capture of Methanol and Ethanol from the Air

Automatic Machine Learning Combined with High-Throughput Computational Screening of Hydrophobic Metal–Organic Frameworks for Capture of Methanol and Ethanol from the Air

Parametric simulations of hierarchical core–shell MOF materials for direct air capture

Metal–organic frameworks (MOFs) can separate and adsorb specific gases from a gas mixture, making them an ideal candidate for direct air carbon capture. However, MOFs that adsorb CO2 well often competitively adsorb the more abundant H2O molecules in the atmosphere, degrading their adsorptive performance and operational longevity. MOF-contained-in-MOF (MOF⊂MOF) core–shell materials may resolve this issue if a shell can be synthesized to reject water while the core receives CO2. This paper builds on our team’s prior computational screening and experimental synthesis of individual and core–shell MOFs by performing multiphysics simulations on a spherical MOF⊂MOF core–shell pellet exposed to atmospheric air. This core–shell pellet is endowed with our previously calculated and experimentally measured constituent MOF properties to demonstrate a high throughput methodology for scale-up analysis on composite adsorbent structures. We simulate and screen five UiO-67(Zr) MOF variants for use as either the core or shell materials that lead to 25 MOF⊂MOF core–shell pellet combinations, five of which are pellets whose core and shell are the same MOF. The resulting rankings suggest that adding a shell MOF to a pellet does not necessarily improve the adsorptive performance of that pellet when using these UiO-67(Zr) variants. The highest-ranked core–shell pellet combination was a pellet whose core and shell were the same MOF material. However, when considering only the core domain of the pellet as a frame of reference, the ranking results suggest that using hydrophobic MOFs as the pellet core and hydrophilic MOFs as the pellet shell show improvement compared to without the shell MOF. Using this second frame of reference, we identify the MOF⊂MOF core–shell combination UiO-67(Zr)⊂2(NH2)-UiO-67(Zr) as the highest-performing pellet of those that show improvement over their homogeneous pellet counterparts. A set of parametric simulations are performed comparing the core–shell pellet UiO-67(Zr)⊂UiO-67(Zr) against that of UiO-67(Zr)⊂2(NH2)-UiO-67(Zr) to investigate the impact of air CO2 concentration, relative humidity, and shell thickness on each respective core MOF’s performance. From these studies, the largest improvement in the core MOF’s performance was observed when using elevated CO2 concentrations of 1000 ppm, reduced relative humidity levels of 10%, and an increased shell thickness of 0.03 cm on a pellet core 0.12 cm in radius. Together, these results suggest that additional UiO-67(Zr) MOF variants will need to be screened to determine the use cases where the use of a shell is of water-rejecting benefit to the pellet in the DAC setting. Further adsorption and porosimetry experimentation is necessary to replicate the proposed pellet under DAC conditions and establish a means of comparative analysis between the simulated and measured pellet properties.

Room Temperature Hydroxyl Group-Assisted Preparation of Hydrophobicity-Adjustable Metal-Organic Framework UiO-66 Composites: Towards Continuous Oil Collection and Emulsion Separation.

Developing a straightforward and effective hydrophobic modification for metal-organic frameworks (MOFs) under mild conditions is meaningful for MOF applications. Here, a post-synthetic modification approach assisted with metal hydroxyl groups at room temperature is reported to induce hydrophobicity in the hydrophilic UiO-66. The bonding between Zr-OH in UiO-66 and n-tetradecylphosphonic acid (TPA) is the essential force for the modifier TPA. Superhydrophobic and superoleophilic composites were constructed for efficient oil-water separation by coating TPA-modified UiO-66 (P-UiO-66) on commercial melamine sponges (MS) and filter papers (FP). The P-UiO-66/MS composite could quickly and selectively absorb oily liquids up to 43 times its weight from water. P-UiO-66/FP and P-UiO-66/MS showed high separation efficiencies for water-in-oil emulsions and oil-in-water emulsions, respectively, with high resistance to low/high temperatures and acid/base conditions. The metal hydroxyl group-assisted post-synthetic modification strategy offers a facile and broad way to prepare hydrophobic MOFs for promising applications in environmental fields.

Optimization of the wetting-drying characteristics of hydrophobic metal organic frameworks via crystallite size: The role of hydrogen bonding between intruded and bulk liquid

Hypothesis: The behavior of Heterogeneous Lyophobic Systems (HLSs) comprised of a lyophobic porous material and a corresponding non-wetting liquid is affected by a variety of different structural parameters of the porous material. Dependence on exogenic properties such as crystallite size is desirable for system tuning as they are much more facilely modified. We explore the dependence of intrusion pressure and intruded volume on crystallite size, testing the hypothesis that the connection between internal cavities and bulk water facilitates intrusion via hydrogen bonding, a phenomenon that is magnified in smaller crystallites with a larger surface/volume ratio.Experiments: Water intrusion/extrusion pressures and intrusion volume were experimentally measured for ZIF-8 samples of various crystallite sizes and compared to previously reported values. Alongside the practical research, molecular dynamics simulations and stochastic modeling were performed to illustrate the effect of crystallite size on the properties of the HLSs and uncover the important role of hydrogen bonding within this phenomenon.Findings: A reduction in crystallite size led to a significant decrease of intrusion and extrusion pressures below 100 nm. Simulations indicate that this behavior is due to a greater number of cages being in proximity to bulk water for smaller crystallites, allowing cross-cage hydrogen bonds to stabilize the intruded state and lower the threshold pressure of intrusion and extrusion. This is accompanied by a reduction in the overall intruded volume. Simulations demonstrate that this phenomenon is linked to ZIF-8 surface half-cages exposed to water being occupied by water due to non-trivial termination of the crystallites, even at atmospheric pressure.

A novel strategy of post defect modification (PDM) for synthesizing hydrophobic FA-UiO-66-CF3 with enhanced n-hexane vapor adsorption capacity under humidity

Volatile organic compounds (VOCs) are hard to recover due to the existence of large amounts of water vapor in practical conditions by adsorption method. Given the abundant functional groups of metal-organic frameworks (MOFs), especially –NH2 group, which can be easily functionalized to solve this problem. However, their pore apertures of the modified MOFs are constricted after post synthesis modification (PSM), leading to unsatisfied adsorption capacity under high humidity. In this work, we proposed a novel strategy of post defect modification (PDM) to synthesize high-performance hydrophobic MOFs for improving adsorption performance under humidity. Originally, the defect-rich UiO-66-NH2 was prepared by using different monocarboxylic acids (MA). In particular, the sample FA-UiO-66-NH2, obtained by the addition of formic acid (FA), exhibited strong n-hexane vapor adsorption ability (198 mg·g−1), which was 97% larger than the non-defective UiO-66-NH2. Afterwards, the defect-engineered FA-UiO-66-NH2 was decorated with –CF3 groups for improving its hydrophobicity, which showed superhydrophobicity with a water contact angle (WCA) of 151.6° and decreased (60%) water vapor adsorption capacity. Dynamic competitive adsorption experiments also indicated that the FA-UiO-66-CF3 sample is more superior competitive adsorption performances for n-hexane vapor than water vapor. Furthermore, we provide a novel strategy for constructing defect-engineered and functionalized MOFs, which will expand their application fields.

Diazomethane mediated sealing of carboxylic moieties in MOFs: Opening doors for increased hydrophobicity

The toxic gas deactivation and liquid phase separation through the metal–organic frameworks (MOFs) have attracted rapid advances in this burgeoning field. However, MOF stability and its sensitivity to water are still in the developmental stages. Herein, we report a novel strategy to seal the moisture-sensitive functionality using scalable and safely generated continuous flow diazomethane, demonstrating enhanced hydrophobicity and its multifaceted application in organic/aqueous phase separation, and self-cleaning. This work demonstrates the first attempt at sealing sensitive functional groups with diazomethane for hydrophobic MOFs, exploring the new proof of concept in the design of stable MOFs with controlled wettability application. Taking advantage of these characteristics of HKUST-1-60 M as strengthening aqueous/ organic solution separation filters. MOF diazomethane treatment concept may be also widened to afford valuable insights in designing and developing advanced membranes for effective separation of aqueous organic solution.

Computational screening and machine learning of hydrophobic metal-organic frameworks for removal of chemical warfare agents from air

The removal of chemical warfare agents from humid air is particularly important for the battlefield and warfare agent leakage scenarios. In this work, the capture efficiency of hydrophobic Computation-Ready, Experimental-metal–organic frameworks (CORE-MOFs) toward four warfare agents (mustard gas (HD), 2-chloroethyl ethyl sulfide (2-CEES), Soman, and Dimethyl methanephosphonate (DMMP)) in air were obtained by combining high-throughput computational screening (HTCS) and machine learning (ML). First, the relationships between the structures and performance indicators (adsorption capacity (N), selectivity (S), and comprehensive trade-off between S and N (TSN)) of the MOFs were established through HTCS. After comparison, there was a strong correlation between the porosity, Henry's coefficient (KH), and the capture performance. Then, three machine learning algorithms, decision tree (DT), random forest (RF), and extreme gradient boosting (XGB), were used to predict the performances of the MOFs. Compared with N and S, TSN had a better prediction effect (average correlation coefficient (R) > 0.95) for the four warfare agents, and the XGB model was suitable for the prediction of the MOF–warfare agent system behavior. KH had the highest relative importance based on big data analysis. Finally, based on TSN, 10 MOFs with the highest performances were screened for the adsorption of four warfare agents, showing that MOF materials with open metal sites had excellent adsorption performances toward the four warfare agents. HTCS and ML were used to understand the relationships between the MOFs and four warfare agents from different perspectives, which provides a new direction for the efficient screening of MOFs to capture these four warfare agents in air.

Drug release and solubility properties of two zeolitic metal-organic frameworks influenced by their hydrophobicity/hydrophilicity.

Metal-organic frameworks (MOFs) have shown significant potential for drug delivery applications. However, there remains a scarcity of comprehensive research addressing the influence of surface properties of MOFs on drug release kinetics and drug solubility. This study focuses on examining the influence of MOFs hydrophilicity and hydrophobicity on the controlled release and solubility of drugs. To achieve this, we prepared drug-loaded nanoparticles through in situ synthesis and created a drug-MOF co-amorphous system using the ball milling technique. Under neutral conditions, the hydrophilic MOF-based drug delivery system demonstrated a comparatively slower drug release profile than its hydrophobic counterpart. This observation suggests that the hydrophilic system holds promise in mitigating drug side effects by enabling improved control over drug release. The implementation of hydrophobic MOFs in co-amorphous systems yields a more pronounced effect on enhancing solubility compared to hydrophilic MOFs. This study offers valuable insights for achieving optimal drug release kinetics and solubility by delicately manipulating surface properties of MOFs.

MOFs with Open Metal(III) Sites for the Environmental Capture of Polar Volatile Organic Compounds.

Metal-Organic Frameworks (MOFs) with open metal sites (OMS) interact strongly with a range of polar gases/vapors. However, under ambient conditions, their selective adsorption is generally impaired due to a high OMS affinity to water. This led previously to the privilege selection of hydrophobic MOFs for the selective capture/detection of volatile organic compounds (VOCs). Herein, we show that this paradigm is challenged by metal(III) polycarboxylates MOFs, bearing a high concentration of OMS, as MIL-100(Fe), enabling the selective capture of polar VOCs even in the presence of water. With experimental and computational tools, including single-component gravimetric and dynamic mixture adsorption measurements, in situ infrared (IR) spectroscopy and Density Functional Theory calculations we reveal that this adsorption mechanism involves a direct coordination of the VOC on the OMS, associated with an interaction energy that exceeds that of water. Hence, MOFs with OMS are demonstrated to be of interest for air purification purposes.

Molecular insights into CH4/H2O transport and hydrate formation in hydrophobic metal-organic frameworks ZIF-8: Implication for CH4 storage by adsorption-hydration hybrid method

The method of adsorption-hydration hybrid has been considered to have great application potential for future CH4 storage. However, the mechanism of CH4 /H2O transport and hydrate formation in metal–organic frameworks (MOFs) systems is not clear. In this work, the hydrophobicity of ZIF-8 was demonstrated by measuring the equilibrium contact angle (115.8 ± 1.2°) of the substrate at molecular level. The interaction between H2O molecules promotes the hydrogen bonds network formation on ZIF-8 surface. Because of the strong physisorption of CH4 by ZIF-8 cavities, excess CH4 accelerates migration and forms larger flat nanobubbles on ZIF-8 surface to cause separation of gas and liquid. On the basis of CH4 /H2O transport results, the whole process of hydrate nucleation and growth in ZIF-8 system was first investigated to reveal the microscopic mechanism of promoting CH4 storage by adsorption-hydration hybrid method. For one thing, a preferential direction may exist during the hydrate nucleation. For another, the migration of intergranular CH4 and H2O is enhanced to promote hydrate nucleation at early stage. Finally, the link between transportation of CH4 /H2O and hydrate formation were discussed in three ZIF-8 systems, elucidating that the larger gas–liquid ratio can accelerate the transportation of intergranular CH4 and H2O at early stage to promote hydrate nucleation and growth. However, because of the competitive adsorption between ZIF-8 inner cavities and hydrate cages, the flat nanobubbles aggregates and the solubility of intergranular CH4 decreases, which accelerate the diffusion of CH4 and H2O to hinder the hydrate continues growth and nucleation, respectively. This study provides crucial microscopic insight into the transportation of CH4 /H2O promotes hydrate formation, which facilitates application for natural gas storage in MOFs by adsorption-hydration hybrid method.

Biomass-derived hydrophobic metal-organic frameworks solid acid for green efficient catalytic esterification of oleic acid at low temperatures

Biodiesel, of great significance, is a promising kind of non-polluting renewable energy that can meet world energy needs. However, the by-product water of esterification reaction used for biodiesel production is not conducive to the positive reaction, and even destroys the activity and structure of the used catalyst. In this study, a renewable solid acid catalyst FDCA/SA-Hf with a degree of hydrophobicity was simply prepared by a solvent-free assembly manner using biomass-based materials. FDCA/SA-Hf was found to be efficient for producing biodiesel from the abundant free fatty acid and nonedible raw oils at a low temperature, and its reaction mechanism was also studied. FDCA/SA-Hf showed stable structural properties and excellent hydrophobic network (131.5°), wherein the chemical grafting of stearic acid improved its Lewis acidity and promoted the adsorption of free fatty acid and the desorption of water. Importantly, as promoted by FDCA/SA-Hf in esterification, a high biodiesel yield reached 98.6% (49 °C, 4.1 wt%, 19.5:1 and 9.5 h). After being easily reused, biodiesel yield still achieved 90% even in the presence of 6 wt% water or after six cycles, which was due to the high hydrophobicity and stability. FDCA/SA-Hf shows great potential in heterogeneous acid-mediated catalytic fields, especially for biodiesel production.

Synthesis and Characterization of the Metal-Organic Framework CIM-80 for Organic Compounds Adsorption.

Metal–organic frameworks (MOF) are a new type of porous materials that have great potential for adsorption of voltaic organic compounds (VOCs). These types of materials composed of metal ions and organic ligands are easy to synthesize, have high surface areas, their surface chemistry can be adjusted to the desired application, and they can also have good chemical and thermal stability. Therefore, this work focuses on the synthesis of a highly hydrophobic MOF material called CIM-80, a porous material that is made up of the Al3+ cation and the mesaconate linker. This MOF has a B.E.T. of approximately 800 m2/g and has potential applications for the adsorption of hydrophobic organic compounds. However, its synthesis is expensive and very dirty. Therefore, we have studied the synthesis conditions necessary to achieve high synthesis yields (85%) and materials with high crystallinity and accessible porosity. To achieve these results, we have used urea as a mild deprotonation reagent and modulator as an alternative to NaOH, which is traditionally used for the synthesis of this MOF. Once the synthesis of this material was controlled, its adsorption/desorption behavior of water and organic compounds such as toluene, cyclohexane and m-xylene was studied by means of vapor adsorption isotherms. The results show the hydrophobic character of the material and the greater affinity the material has toward aliphatic compounds than toward aromatic ones, with toluene being the most adsorbed compound, followed by cyclohexane and m-xylene.

Molecular-fingerprint machine-learning-assisted design and prediction for high-performance MOFs for capture of NMHCs from air

The capture of trace amounts of non-methane hydrocarbons (NMHCs) from air due to the toxicity of volatile organic compounds is a significant challenge. A total of 31399 hydrophobic metal–organic frameworks (MOFs) were first screened from 137953 hypothetical MOFs using high-throughput computational screening (HTCS), and their performance indices (adsorption capacity and selectivity) for the adsorption of NMHCs (C 3 –C 6 ) were obtained by molecular simulations. The discovery of a “second peak” near twice the kinetic diameter of the corresponding NMHC provided more choices for excellent MOFs that adsorb NMHCs. Four machine learning (ML) classification and regression algorithms predicted the performance of MOFs, and the relative importance values of the six descriptors were determined. The combination of the Random Forests algorithm and Molecular ACCess Systems molecular fingerprint (MF) had an excellent predictive ability for MOFs. According to the performance, the fingerprint commonalities of the 100 top-performing MOFs were counted, and the excellent bits (EBs) that could promote the performance were defined. Finally, new substructures containing all of the EBs were designed for each NMHC to build a new MOF database. This work combined the HTCS, ML, and MF to provide a detailed insight into the design of efficient MOFs for adsorbing NMHCs.