Hydrophobic Metal-organic Framework Research Articles

Flat-Histogram Monte Carlo Simulation of Water Adsorption in Metal-Organic Frameworks.

Molecular simulations of water adsorption in porous materials often converge slowly due to sampling bottlenecks that follow from hydrogen bonding and, in many cases, the formation of water clusters. These effects may be exacerbated in metal-organic framework (MOF) adsorbents, due to the presence of pore spaces (cages) that promote the formation of discrete-size clusters and hydrophobic effects (if present), among other reasons. In Grand Canonical Monte Carlo (MC) simulations, these sampling challenges are typically manifested by low MC acceptance ratios, a tendency for the simulation to become stuck in a particular loading state (i.e., macrostates), and the persistence of specific clusters for long periods of the simulation. We present simulation strategies to address these sampling challenges, by applying flat-histogram MC (FHMC) methods and specialized MC move types to simulations of water adsorption. FHMC, in both Transition-matrix and Wang-Landau forms, drives the simulation to sample relevant macrostates by incorporating weights that are self-consistently adjusted throughout the simulation and generate the macrostate probability distribution (MPD). Specialized MC moves, based on aggregation-volume bias and configurational bias methods, separately address low acceptance ratios for basic MC trial moves and specifically target water molecules in clusters; in turn, the specialized MC moves improve the efficiency of generating new configurations which is ultimately reflected in improved statistics collected by FHMC. The combined strategies are applied to study the adsorption of water in CuBTC and ZIF-8 at 300 K, through examination of the MPD and the adsorption isotherm generated by histogram reweighting. A key result is the appearance of nontrivial oscillations in the MPD, which we show to be associated with water clusters in the adsorption system. Additionally, we show that the probabilities of certain clusters become similar in value near the boundaries of the isotherm hysteresis loop, indicating a strong connection between cluster formation/destruction and the thermodynamic limits of stability. For a hydrophobic MOF, the FHMC results show that the phase transition from low density to high density is suppressed to water pressure far above the bulk-fluid saturation pressure; this is consistent with results presented elsewhere. We also compare our FHMC simulation isotherm to one measured by a different technique but with ostensibly the same molecular interactions and comment on observed differences and the need for follow-up work. The simulation strategies presented here can be applied to the simulation of water in other MOFs using heuristic guidelines laid out in our text, which should facilitate the more consistent and efficient simulation of water adsorption in porous materials in future applications.

Surface modification of HKUST-1 for enhanced activity of immobilized formate dehydrogenase used in CO2 hydrogenation

Post synthetic modification of a hydrophilic metal–organic framework (MOF), HKUST-1, with stearic acid (SA) was carried out to enhance the stability of HKUST-1 in aqueous solution to be used as a support for formate dehydrogenase (FDH) used for CO2 conversion to formate. SA modification improved the hydrophobicity without affecting the morphology and crystal structure of MOF. Adsorption of FDH on the modified MOF (SA@HKUST-1) was compared to that of the native HKUST-1 and ZIF-L. The adsorption kinetics on all MOFs was found to follow pseudo-second order kinetics and the isotherm was best described by Freundlich model. The high stability of SA@HKUST-1 and enhanced hydrophobic interaction between support and CO2 resulted in high catalytic efficiency and stability of FDH@SA@HKUST-1. The immobilized enzyme retained 95.1% of its initial activity after 4 cycles of repeated use. It was also shown that FDH@SA@HKUST-1 retained morphology and crystal structure after repeated use. Results of the present work provide novel insight into the influence of hydrophobic MOFs on the activity and stability of immobilized FDH. These findings are expected to assist in developing highly active and stable biocatalysts for CO2 hydrogenation at commercial level.

Kinetic study of methane storage in hydrophobic ZIF-8 by adsorption-hydration hybrid technology

Adsorption-hydration hybrid (AHH) technology combining methane adsorption and hydrate formation has attracted extensive attention for methane storage/transportation. This work explored the AHH kinetic properties in the hydrophobic metal-organic framework ZIF-8, using a visualization platform, to elucidate the relationship between adsorption and hydrate formation. A two-stage adsorption process of methane physisorption and hydrate formation-adsorption occurred, in which hydrate fibers and hydrate chunks were formed successively. To investigate the effects of water content (Rw) and pressure on AHH kinetic properties, the mass transfer coefficient of methane and kinetic constant of hydrate formation in two stages were calculated, respectively. Increasing Rw from 0.3 to 1.0 resulted in a significant decrease of 68.2% and 89.8% in the mass transfer coefficient of methane in the first stage and the kinetic constant of hydrate formation in the second stage, respectively. The higher Rw hindered hydrate fiber formation by impeding methane diffusion within the pore space, while raising the initial pressure promoted methane diffusion. Specifically, when the pressure was increased from 5.3 MPa to 7.0 MPa, the methane physisorption rate and hydrate formation rate could increase by up to 1.77 and 1.35 times, respectively. Furthermore, it was found that the decrease of physisorption rate further reduced the hydrate formation rate, revealing the relationship between physisorption and hydrate formation. A combined analysis of initial pressure and water content suggested that the inhibitory effect of Rw on mass transfer outweighed the promoting effect of initial pressure.

Self-Healing, Robust, Liquid-Repellent Coatings Exploiting the Donor-Acceptor Self-Assembly.

Liquid-repellent coatings with rapid self-healing and strong substrate adhesion have tremendous potential for industrial applications, but their formulation is challenging. We exploit synergistic chemistry between donor-acceptor self-assembly units of polyurethane and hydrophobic metal-organic framework (MOF) nanoparticles to overcome this challenge. The nanocomposite features a nanohierarchical morphology with excellent liquid repellence. Using polyurethane as a base polymer, the incorporated donor-acceptor self-assembly enables high strength, excellent self-healing property, and strong adhesion strength on multiple substrates. The interaction mechanism of donor-acceptor self-assembly was revealed via density functional theory and infrared spectroscopy. The superhydrophobicity of polyurethane was achieved by introducing alkyl-functionalized MOF nanoparticles and post-application silanization. The combination of the self-healing polymer and nanohierarchical MOF nanoparticles results in self-cleaning capability, resistance to tape peel and high-speed liquid jet impacts, recoverable liquid repellence over a self-healed notch, and low ice adhesion up to 50 icing/deicing cycles. By exploiting the porosity of MOF nanoparticles in our nanocomposites, fluorine-free, slippery liquid-infused porous surfaces with stable, low ice adhesion strengths were also achieved by infusing silicone oil into the coatings.

Highly Sensitive Ammonia Sensor Based on Modified Nanostructured Polypyrrole Decorated With MAF-6 to Reduce the Effect of Humidity

An ammonia gas sensor based on nanostructured polypyrrole functionalized by carboxylic groups (PPy-COOH) was prepared and characterized. The active layer was deposited on a ceramic substrate with gold interdigital electrodes by a low-cost airbrush-spray coating technique, and the sensor exhibited a strong response to ammonia ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${Z}/{Z}_{{0}}\,\,=$ </tex-math></inline-formula> 5.48 at 100 ppm) in comparison with pure PPy ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${Z}/{Z}_{{0}}\,\,=$ </tex-math></inline-formula> 1.5 at 100 ppm). Cross-sensitivity measurement showed a more significant response to relative humidity (RH) and so two approaches were used to reduce this undesirable humidity dependence: 1) the creation of a hydrophobic metal-organic framework (so-called MAF-6) on the PPy-COOH layer and 2) the fabrication of a heterostructure of PPy-COOH with multiwall carbon nanotubes (MWCNTs). Both approaches resulted in a significant reduction in the humidity dependence of the sensor, from the value <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${Z}/{Z}_{{0}}\,\,=$ </tex-math></inline-formula> 0.02 (PPy-COOH) to 0.404 and 0.427 for PPy-COOH-MAF-6 and PPy-COOH/MWCNT, respectively. However, the samples coated with MAF-6 showed a smaller decrease in response to ammonia. In addition, the proposed approach with MAF-6 functionalization allowed ammonia detection within a broader RH range (up to 60% RH) compared to the PPy-COOH sensing layer. The presence of MAF-6 on the surface of PPy-COOH was further verified by infrared spectroscopy (FTIR), X-ray diffraction (XRD) spectroscopy, and scanning electron microscopy together with energy-dispersive X-ray spectroscopy (SEM-EDX). The sensing mechanisms of ammonia and RH of the PPy-COOH material are also discussed in this article.

Adsorptive Separation of CO2 by a Hydrophobic Carborane-Based Metal-Organic Framework under Humid Conditions.

We report that the carborane-based metal-organic framework (MOF) mCB-MOF-1 can achieve high adsorptive selectivity for CO2:N2 mixtures. This hydrophobic MOF presenting open metal sites shows high CO2 adsorption capacity and remarkable selectivity values that are maintained even under extremely humid conditions. The comparison of mCB-MOF-1' with MOF-74(Ni) demonstrates the superior performance of the former under challenging moisture operation conditions.

CO2/N2 separation by vacuum swing adsorption using a metal–organic framework, CALF-20: Multi-objective optimization and experimental validation

• CALF-20 is a hydrophobic MOF commercialized for industrial CO 2 capture. • Experimental PVSA based CO 2 /N 2 separation on a MOF yielding 95% CO 2 purity. • Vacuum swing adsorption (VSA) experiments with 70 g of CALF-20. • Modelling & optimization of 2 VSA cycles to achieve US-DOE purity/recovery targets. • Experimental validation of VSA cycles to produce 95.5% CO2 purity and 88.1 %recovery. In this paper, the ability of CALF-20, a hydrophobic metal-organic framework (MOF), to capture CO 2 from dry flue gas (15/85 mol% of CO 2 /N 2 ) using two different adsorption configurations, basic four-step vacuum swing adsorption (VSA), and four-step with light-product pressurization (LPP) was evaluated. Pareto curves, for the simultaneous maximization of CO 2 purity and recovery, were generated. Five points from each Pareto curve representing five different process conditions were chosen to experimentally validate the model prediction. The experiments resulted in a CO 2 purity and recovery of 95%, and 70%, respectively for the basic four-step cycle, while the four-step LPP resulted in 95.5% purity and 88.1% recovery. The temperature history, the pressure transient, and the flow rate of different steps were in good agreement with the model predictions. The results of this study confirmed that CALF-20 is capable of separating CO 2 from dry flue gas, demonstrating the potential of the MOF for practical separations.

Dispersive solid phase extraction of several pesticides from fruit juices using a hydrophobic metal organic framework prior to HPLC-MS/MS determination

A metal organic framework (MOF)-based dispersive solid phase extraction procedure was introduced using a hydrophobic sorbent for the extraction of multi class pesticides from fruit juice samples prior to their determination by high performance liquid chromatography-tandem mass spectrometry. For this purpose, NH2-UiO-66(Zr) MOF was synthesized by a hydrothermal approach. Then the sorbent was silylated and used in the extraction procedure. The extraction procedure was done by using 7 mg of the sorbent from 5 mL of a sample solution containing the analytes. After that, the extracted analytes were eluted and preconcentrated by vaporizing the elution solvent. The method was linear within the range of 0.29–500 ng mL−1. The limits of detection, intra- and inter-day relative standard deviations, and extraction recoveries were in the ranges of 0.02–0.1 ng mL−1, 2.9–8.8%, 6.8–14.4%, and 74–89%, respectively. Performing the method on the selected samples showed that they were free of the analytes. This method can help to monitor the studied pesticides in juice samples as well as improving the safety of foods.

Separation of CO2 and N2 on a hydrophobic metal organic framework CALF-20

Adsorption equilibrium of pure CO2 and N2 on calgary framework-20 (CALF-20), a hydrophobic metal-organic framework (MOF), at various temperatures was measured using volumetric and gravimetric methods. The dual-site Langmuir isotherm was used to describe the pure CO2 and N2 equilibria. The competitive CO2/N2 adsorption and the column dynamics were measured using the dynamic column breakthrough technique. Various compositions of CO2/N2 were examined to obtain the competitive CO2/N2 isotherm. A one-dimensional column model was used to simulate the dynamic column breakthrough experiments. Both composition and temperature histories showed a good agreement between the experiments and simulations. A four-step vacuum swing adsorption (VSA) with a light product pressurization configuration was used to evaluate the ability of CALF-20 to capture CO2 from the flue gas containing 15 mol% CO2 and 85 mol% N2 under dry conditions. The results show that a 95% CO2 purity and 90% recovery could be obtained using CALF-20, and its performance was comparable with the benchmark materials zeolite 13X, UTSA-16, under these VSA conditions.

Molecular insights into hybrid CH4 physisorption-hydrate growth in hydrophobic metal–organic framework ZIF-8: Implications for CH4 storage

Adsorption-hydration hybrid method has been widely concerned due to its great application potential in CH4 storage/transportation. Monte Carlo and molecular dynamics simulations were performed to investigate the hybrid CH4 physisorption-hydrate growth in ZIF-8. The two-way migration phenomenon between water molecules inside the cavities and CH4 molecules outside the cavities first occurs in the physisorption process, which increases the CH4 adsorption capacity to a level close to that of dry ZIF-8. Due to the competitive adsorption for CH4 between ZIF-8 and hydrate surface, it is not conducive to the stability of intergranular CH4 hydrate before the CH4 adsorption capacity of ZIF-8 attains the maximum. After the maximum, intergranular hydrate could exist stably and grow continuously with the dissolution of CH4. Meanwhile, partially adsorbed CH4 molecules in the cavities migrate outwards and promote hydrate growth. These constitute the link between physisorption and hydrate growth. Because of the phase separation of CH4 and water molecules, hydrate cannot form in the ZIF-8 cavities. As a result, under the synergistic effect of physisorption and intergranular hydrates, CH4 storage capacity is significantly increased in H2O-oversaturated ZIF-8 compared with that in dry ZIF-8. Further, hydrophobic ZIF-8 surface facilitates hydrogen bonds formation in interfacial water by inducing ordered structures, and interfacial water as a medium promotes the contact between hydrate cage and ZIF-8 surface. An analysis of the diffusion properties of CH4/water molecules reveals that ZIF-8 cavities, hydrate cages, and intergranular liquid water exert inhibition on their self-diffusion coefficients in different degree. This study provides important microscopic insight into CH4 storage by adsorption-hydration hybrid in ZIF-8.

Hydrophobic metal organic framework for enhancing performance of acoustic wave formaldehyde sensor based on polyethyleneimine and bacterial cellulose nanofilms

A surface acoustic wave (SAW) formaldehyde gas sensor was fabricated on a 42° 75′ ST-cut quartz substrate, with a composite sensing layer of zeolitic imidazolate framework (ZIF)-8 on polyethyleneimine (PEI)/bacterial cellulose (BC) nanofilms. The addition of snowflake-like ZIF-8 structure on the PEI/BC sensitive film significantly improves the hydrophobicity of the SAW sensor and increases its sensitivity to formaldehyde gas. It also significantly increases surface roughness of the sensitive film. The hydrophobic nature of ZIF-8 prevents water molecules from entering into the internal pores of the BC film, thereby avoiding a significant mass loading caused by humidity when the sensor is used to detect low-concentration formaldehyde gas. The Zn2+ sites at the surface of ZIF-8 improves the sensor's response to formaldehyde gas through enhanced physical adsorptions of gas molecules. Experimental results show that the ZIF-8@PEI/BC SAW sensor has a response (e.g., frequency shift) of 40.3 kHz to 10 ppm formaldehyde gas at 25 °C and 30% relative humidity (RH). When the relative humidity is increased from 30 to 93%, the response of the sensor only varies ~ 5%, and the change in response is negligible at medium humidity levels (~ 50 to 60% RH).

Enhancement of the catalytic efficiency of Candida antarctica lipase A in enantioselective hydrolysis through immobilization onto a hydrophobic MOF support

Immobilization of Candida antarctica lipase A (CAL-A) onto MOF (metal-organic framework) supports were investigated to enhance the efficiency of CAL-A in catalysis of an enantioselective hydrolysis reaction. CAL-A@ZIF-8 prepared through physical adsorption of CAL-A onto ZIF-8 was characterized and further employed as the catalyzer in kinetic resolution of carprofen (CP) enantiomers through enantioselective hydrolysis of (R,S)-CP methyl ester. Results show that CAL-A is immobilized on ZIF-8 with a loading amount of 227 mg/g. Efficiency of CAL-A is significantly improved after immobilization, which is evaluated by the yield (YS) and enantiomeric excess (eep) of (S)-CP in the hydrolysis products, and the enhancement is ascribed for the hydrophobic nature of ZIF-8. The YS of 90.62 % and eep of 97.42 % are achieved with CAL-A@ZIF-8, while only 59.79 % of YS and 92.61 % of eep are obtained with free CAL-A under the identical conditions. Moreover, CAL-A@ZIF-8 possesses superior thermal stability and pH-tolerance than free CAL-A. Immobilization of the lipase onto a hydrophobic MOF is therefore proposed for enhancement of a reaction of lipase-catalyzed ester hydrolysis.

Highly hydrophobic metal–organic framework for self-protecting gate dielectrics

A high-κ copper-based metal–organic framework with unusual dielectric behaviour and outstanding water resistant properties was successfully designed for self-protecting microelectronic devices as gate dielectrics.

Self-Generation of Surface Roughness by Low-Surface-Energy Alkyl Chains for Highly Stable Superhydrophobic/Superoleophilic MOFs with Multiple Functionalities.

We transformed the hydrophilic metal-organic framework (MOF) UiO-67 into hydrophobic UiO-67-Rs (R=alkyl) by introducing alkyl chains into organic linkers, which not only protected hydrophilic Zr6 O8 clusters to make the MOF interspace superoleophilic, but also led to a rough crystal surface beneficial for superhydrophobicity. The UiO-67-Rs displayed high acid, base, and water stability, and long alkyl chains offered better hydrophobicity. Good hydrophobicity/oleophilicity were also possible with mixed-ligand MOFs containing metal-binding ligands. Thus, a (super)hydrophobic MOF catalyst loaded with Pd centers efficiently catalyzed Sonogashira reactions in water at ambient temperature. Studies of the hydrophobic effects of the coordination interspace and the outer surface suggest a simple de novo strategy for the synthesis of superhydrophobic MOFs that combine surface roughness and low surface energy. Such MOFs have potential for environmentally friendly catalysis and water purification.

Self‐Generation of Surface Roughness by Low‐Surface‐Energy Alkyl Chains for Highly Stable Superhydrophobic/Superoleophilic MOFs with Multiple Functionalities

AbstractWe transformed the hydrophilic metal–organic framework (MOF) UiO‐67 into hydrophobic UiO‐67‐Rs (R=alkyl) by introducing alkyl chains into organic linkers, which not only protected hydrophilic Zr6O8 clusters to make the MOF interspace superoleophilic, but also led to a rough crystal surface beneficial for superhydrophobicity. The UiO‐67‐Rs displayed high acid, base, and water stability, and long alkyl chains offered better hydrophobicity. Good hydrophobicity/oleophilicity were also possible with mixed‐ligand MOFs containing metal‐binding ligands. Thus, a (super)hydrophobic MOF catalyst loaded with Pd centers efficiently catalyzed Sonogashira reactions in water at ambient temperature. Studies of the hydrophobic effects of the coordination interspace and the outer surface suggest a simple de novo strategy for the synthesis of superhydrophobic MOFs that combine surface roughness and low surface energy. Such MOFs have potential for environmentally friendly catalysis and water purification.

Discovery of an Optimal Porous Crystalline Material for the Capture of Chemical Warfare Agents

Chemical warfare agents (CWAs) are regarded as a critical challenge in our society. Here, we use a high-throughput computational screening strategy backed up by experimental validation to identify and synthesize a promising porous material for CWA removal under humid conditions. Starting with a database of 2,932 existing metal–organic framework (MOF) structures, we selected those possessing cavities big enough to adsorb well-known CWAs such as sarin, soman, and mustard gas as well as their nontoxic simulants. We used Widom method to reduce significantly the simulation time of water adsorption, allowing us to shortlist 156 hydrophobic MOFs where water will not compete with the CWAs to get adsorbed. We then moved to grand canonical Monte Carlo (GCMC) simulations to assess the removal capacity of CWAs. We selected the best candidates in terms of performance but also in terms of chemical stability and moved to synthesis and experimental breakthrough adsorption to probe the predicted, excellent performance. Th...

Metal–organic framework sorbents for the removal of perfluorinated compounds in an aqueous environment

A hydrophobic metal–organic framework has revealed high and fast sorption capacity for fluorine pollutants.

Computational Study of Water Adsorption in the Hydrophobic Metal–Organic Framework ZIF-8: Adsorption Mechanism and Acceleration of the Simulations

Adsorption of water can have an important effect on chemical and physical processes in porous materials, such as zeolites and metal–organic frameworks (MOFs). However, the molecular simulation of water adsorption brings many challenges, especially the slow simulation speed. In this study, we examined the hydrophobic MOF ZIF-8 as a representative adsorbent to discover the adsorption mechanism of water in hydrophobic MOFs. Based on the mechanistic insights obtained, we proposed and investigated several advanced Monte Carlo algorithms including energy-bias moves and continuous fractional component Monte Carlo (CFC MC) and were able to accelerate the simulation speed by a factor of 6.7 over the conventional grand canonical Monte Carlo algorithm. The insights obtained from this work may also help improve the molecular simulation efficiency for studies of water adsorption in other hydrophobic materials.

A multi-aromatic hydrocarbon unit induced hydrophobic metal–organic framework for efficient C2/C1 hydrocarbon and oil/water separation

A highly hydrophobic Cu(ii) MOF (UPC-21) was synthesized through a newly developed “diauxic growth” strategy. The multi-aromatic hydrocarbon units in the framework are responsible for hydrophobic character of UPC-21. Its application on water purification and oil/water separation was studied for the first time.

A Highly Stable Nonhysteretic {Cu2 (tebpz) MOF+water} Molecular Spring.

A molecular spring formed by a hydrophobic metal-organic framework Cu2 (tebpz) (tebpz=3,3',5,5'-tetraethyl-4,4'-bipyrazolate) and water is presented. This nanoporous heterogeneous lyophobic system (HLS) has exceptional properties compared to numerous reported systems of such type in terms of stability, efficiency, and operating pressure. Mechanical and thermal energetic characteristics as well as stability of the system are discussed and compared in detail with those of other previously reported HLS.