Zr Metal-organic Frameworks Research Articles

Exploring the impact of Anions, Alkyl group and confinement effect in ionic liquid @ UiO-66(Zr) MOF: A DFT study

Metal organic frameworks (MOFs) have emerged as a strong research material due to their exceptional porosity and tunable functional nature. MOF structures consist of metal nodes with organic linker, which can be functionalized depending on the required application. With the tunable properties, it has wide range of applications in energy, environmental and therapeutic approaches. Nevertheless there are several factors which can affect their performance such as lower affinity towards target molecules, chemical and thermal stability etc. To counter these issues several studies have been devoted on functionalizing the MOF. In this work, we have considered UiO-66 (University of Oslo-66) MOF because of its exceptional thermal and chemical stability with a series of versatile ionic liquids (ILs). The selected ILs are 1-ethyl-3-methylimidazolium ([C2MIm]+) and 1‑butyl‑3-methylimidazolium ([C4MIm]+) cations with various anions [X]−, where X = acetate (OAc), tetrafluroborate (BF4), dicyanamide (DCA), nitrite (NO2) and chloride (Cl) anions. These ILs have been impregnated in tetrahedral (∼10 Å) and octahedral (∼16 Å) pores of UiO-66 structure. First principle density functional theory (DFT) approach is used to study the structure, stability, spectral properties and charge transfer between IL/UiO-66 interfaces. Our calculations suggest that, IL does not affect the structure of UiO-66 surface. The anion and the alkyl group in the cations play a vital role in the stability of the complexes. Furthermore, confinement effect influence the stability of the complexes, irrespective nature of anion and cation. Electron density and charge transfer analyses reveal that anions have profound liking towards MOF because of having high electronegative atoms. Our study will help to understand the molecular level interaction between IL@MOF interface, and these composite can be utilized for the energy and environment related applications.

Microfluidic Synthesis of Defective and Hierarchical Pore Zr Metal–Organic Framework Materials and CO2 Adsorption Performance Study

Microfluidic Synthesis of Defective and Hierarchical Pore Zr Metal–Organic Framework Materials and CO₂ Adsorption Performance Study

Modulation of the Photophysics and Internal Dynamics in a Zr Metal Organic Framework by the Inclusion of Fluorinated Guests

Modulation of the Photophysics and Internal Dynamics in a Zr Metal Organic Framework by the Inclusion of Fluorinated Guests

Effect of Functionalized Carbon Nanotubes and Graphene Oxide Nanoribbons on the Supercapacitive Behavior of Zr-Based Metal-Organic Frameworks in Acid Media

Metal-organic frameworks (MOFs) as a kind of crystalline porous materials have attracted much attention due to their high surface area and high electrochemical performance as electrode materials in supercapacitors 1. Recently, there has been a lot of interest in functionalized multiwalled carbon nanotubes (F-MWCNTs) based materials and especially graphene oxide nanoribbons (GONRs), which have unusual characteristics of ultrathin two-dimensional structures and, as promising materials for supercapacitors and other electrochemical energy storage devices. The F-MWCNTs and GONRs' high electrical conductivity, highly changeable surface area, strong chemical stability, excellent mechanical behavior, and capacity to modify attributes for the desired application are behind the reasons for choosing to make a composite of these materials with Zr-MOF 2.In this work, Zr-MOF was synthesized using the solvothermal method 3, MWCNTs were functionalized using acid washing (3 M H2SO4: 1 M HNO3), and GONRs were fabricated using the oxidative unzipping method 4. The Zr-MOF/F-MWCNTs and Zr-MOF/GONRs composites were prepared through physical mixing. The physical and chemical structures of the synthesized Zr-MOF, F-MWCNTs, and GONRs were evaluated using XRD, SEM, TEM, FT-IR, Raman spectroscopy, and XPS. The supercapactive behavior of the prepared composites was evaluated in comparison to their components (Zr-MOF, F-MWCNTs, and GONRs) in 1M H2SO4 utilizing various evaluation systems (three and two-electrode systems). Zr-MOFs showed a high specific capacitance (248 F/g at 1 A/g), making it a promising supercapacitor electrode material. That performance is dramatically improved by the addition of the nanocarbon materials (F-MWCNTs and GONRs). Zr-MOF-GONRs showed the highest specific capacitance (448 F/g at 1 A/g), with a large potential window (1.7 V) in the three-electrode system extended to (2V) in the two-electrode system, and good stability over 10,000 cycles at a high current density of 10 A/g. Reference: (1) Tan, Y.; Zhang, W.; Gao, Y.; Wu, J.; Tang, B. Facile Synthesis and Supercapacitive Properties of Zr-Metal Organic Frameworks (UiO-66). RSC Adv. 2015, 5 (23), 17601–17605. https://doi.org/10.1039/c4ra11896k.(2) Shrivastav, V.; Sundriyal, S.; Tiwari, U. K.; Kim, K. H.; Deep, A. Metal-Organic Framework Derived Zirconium Oxide/Carbon Composite as an Improved Supercapacitor Electrode. Energy 2021, 235, 121351. https://doi.org/10.1016/j.energy.2021.121351.(3) Abdelhamid, H. N. UiO-66 as a Catalyst for Hydrogen Production: Via the Hydrolysis of Sodium Borohydride. Dalton Trans. 2020, 49 (31), 10851–10857. https://doi.org/10.1039/d0dt01688h.(4) Kosynkin, D. V.; Higginbotham, A. L.; Sinitskii, A.; Lomeda, J. R.; Dimiev, A.; Price, B. K.; Tour, J. M. Longitudinal Unzipping of Carbon Nanotubes to Form Graphene Nanoribbons. Nature 2009, 458 (7240), 872–876. https://doi.org/10.1038/nature07872.

Leveraging Isoreticular Principle to Elucidate the Key Role of Inherent Hydrogen-Bonding Anchoring Sites in Enhancing Water Sorption Cyclability of Zr(IV) Metal-Organic Frameworks.

Water adsorption/desorption cyclability of porous materials is a prerequisite for diverse applications, including atmospheric water harvesting (AWH), humidity autocontrol (HAC), heat pumps and chillers, and hydrolytic catalysis. However, unambiguous molecular insights into the correlation between underlying building blocks and the cyclability are still highly elusive. In this work, by taking advantage of the well-established isoreticular synthetic principle in Zr(IV) metal-organic frameworks (Zr-MOFs), we show that the inherent density of hydrogen atoms in the organic skeleton can play a key role in regulating the water sorption cyclability of MOFs. The ease of isoreticular practice of Zr-MOFs enables the successful syntheses of two pairs of isostructural Zr-MOFs (NU-901 and NU-903, NU-950 and SJTU-9) from pyrene- or benzene-cored carboxylate linkers, which feature scu and sqc topological nets, respectively. NU-901 and NU-950 comprised of pyrene skeletons carrying more hydrogen-bonding anchoring sites show distinctly inferior cyclability as compared with NU-903 and SJTU-9 built of benzene units. Single-crystal X-ray crystallography analysis of the hydrated structure clearly unveils the water molecule-involved interactions with the hydrogen-bonding donors of benzene moieties. Remarkably, NU-903 and SJTU-9 isomers exhibit outstanding water vapor sorption capacities as well as working capacities at the desired humidity range with potential implementations covering indoor humidity control and water harvesting. Our findings uncover the importance of hydrogen-bonding anchoring site engineering of organic scaffold in manipulating the framework durability toward water sorption cycle and will also likely facilitate the rational design and development of highly robust porous materials.

Electrochemiluminescent quenching of luminol–doped Zr–MOFs for ATP detection through scavenging ROS using gallic acid–capped Au nanoparticles

In the present study, the highly efficient electrochemiluminescence (ECL) luminophores of Zr-based metal–organic frameworks (MOFs) coordinating with the amino group of luminol (Zr–Lu–MOFs) were developed for fabricating a sensing platform. The ECL emission of a Zr–Lu–MOF/H2O2 system with strong and stable ECL signals was triggered through cyclic voltammetry. Adenosine triphosphate (ATP) was firmly assembled on Zr–Lu–MOF because of the high affinity of Zr4+ toward phosphate groups. ATP was detected using gallic acid (GA)-capped Au (GA@Au) nanoparticles combined with an ATP aptamer. The GA@Au nanoparticles quenched the ECL behavior of the luminol/H2O2 system, and the quenching mechanism was ascribed to the GA scavenging superoxide radical (O2•−) that was generated via the electron redox reaction in H2O2. Under optimal experiments, an ECL biosensor with a linear range of 0.100–1000 nM and a detection limit of 0.0353 nM (S/N = 3) was designed for the selective and sensitive detection of ATP. Thus, the proposed ECL biosensor exhibited good performance with high stability and acceptable fabrication reproducibility, providing a valuable strategy for clinical diagnostics and therapeutics.

UiO-66(Zr) as drug delivery system for non-steroidal anti-inflammatory drugs

The toxicity for the human body of non-steroidal anti-inflammatory drugs (NSAIDs) overdoses is a consequence of their low water solubility, high doses, and facile accessibility to the population. New drug delivery systems (DDS) are necessary to overcome the bioavailability and toxicity related to NSAIDs. In this context, UiO-66(Zr) metal-organic framework (MOF) shows high porosity, stability, and load capacity, thus being a promising DDS. However, the adsorption and release capability for different NSAIDs is scarcely described. In this work, the biocompatible UiO-66(Zr) MOF was used to study the adsorption and release conditions of ibuprofen, naproxen, and diclofenac using a theoretical and experimental approximation. DFT results showed that the MOF-drug interaction was due to an intermolecular hydrogen bond between protons of the groups in the defect sites, (μ3 − OH, and − OH2) and a lone pair of oxygen carboxyl functional group of the NSAIDs. Also, the experimental results suggest that the solvent where the drug is dissolved affects the adsorption process. The adsorption kinetics are similar between the drugs, but the maximum load capacity differs for each drug. The release kinetics assay showed a solvent dependence kinetics whose maximum liberation capacity is affected by the interaction between the drug and the material. Finally, the biological assays show that none of the systems studied are cytotoxic for HMVEC. Additionally, the wound healing assay suggests that the UiO-66(Zr) material has potential application on the wound healing process. However, further studies should be done.

Shaping of metal–organic framework using chitosan and triphosphate cross-linker

Abstract The Zr metal–organic framework (MOF) UiO-66 was shaped into beads using chitosan and P3O105− as a cross-linker. The obtained UiO-66–chitosan beads showed decreases in pore size and Brunauer–Emmett–Teller (BET) surface area but enhanced N2 adsorption energy with retention of Zr-ligand connectivity. This finding will provide an important insight into the practical application of MOFs.

Functionalization of Defective Zr Metal-Organic Frameworks for Water Decontamination: Mechanistic Insight into the Competitive Roles of -NH2 and -SH Sites in Removal of As(III) Species.

Direct removal of trivalent arsenic, As(III), arsenite, or H3AsO3, is a great challenge in accessing clean sources of water. Different methodologies and materials were applied in this regard, but among them, direct removal of As(III) species using a metal-organic framework (MOF)-based adsorbent shows a great deal of potential. Although some studies were conducted on As(III) removal using MOFs, studies of functional groups are still quite rare. For this purpose, three novel functionalized defective Zr-MOFs, using UiO-66 [Zr6(OH)4O4(BDC)6, where BDC2- = benzene-1,4-dicarboxylate], were fabricated to investigate the competitive or cooperative roles of the free -NH2 and/or -SH site in the removal of As(III). UiO-66 was functionalized with monocarboxylate linkers, including glycine (Gly, NH2-CH2-COOH), cysteine [Cys, SH(CH2)-NH2(CH)-COOH], and mercaptopropionic acid [Mer, SH-(CH2)2-COOH]. Gly@UiO-66, Cys@UiO-66, and Mer@UiO-66 were applied for the direct removal of As(III) species. Although Cys@UiO-66 is functionalized with both amine and thiol functional groups, Gly@UiO-66 has a higher adsorption capacity (301.4 mg g-1) with respect to As(III), which is among the best reported values. This is due to the fact that (1) the affinity of amine sites in Gly@UiO-66 for As(III) is higher than that of thiol sites in Mer@UiO-66 and (2) Cys@UiO-66 has a very small surface area compared to that of Gly@UiO-66. Mechanistic studies using X-ray photoelectron spectroscopy and vibrational spectroscopy reveal that not only the functionalization and chemical nature of the function but also other parameters such as the protonation-deprotonation mechanisms and chemical state of the function are other critical factors for designing a functional MOF-based adsorbent with high affinity for and maximum capacity with respect to the target analyte.

Probing oxygen exchange between UiO-66(Zr) MOF and water using 17 O solid-state NMR.

The Zr-based Metal Organic Framework (MOF) UiO-66(Zr) is widely employed owing to its good thermal and chemical stabilities. Although the long-range structure of this MOF is preserved in the presence of water during several days, little is known about the formation of defects, which cannot be detected using diffraction techniques. We apply here 17 O solid-state NMR spectroscopy at 18.8 T to investigate the reactivity of UiO-66, through the exchange of oxygen atoms between the different sites of the MOF and water. For that purpose, we have selectively enriched in 17 O isotope the carboxylate groups of UiO-66(Zr) by using it with 17 O-labeled terephthalic acid prepared using mechanochemistry. In the presence of water at 50 °C and a following dehydration at 150 °C, we observe an overall exchange of O atoms between COO- and μ3 -O2- sites. Furthermore, we demonstrate that the three distinct oxygen sites, μ3 -OH, μ3 -O2- and COO- , of UiO-66(Zr) MOF can be enriched in 17 O isotope by post-synthetic hydrothermal treatment in the presence of 17 O-enriched water. These results demonstrate the lability of Zr-O bonds and the reactivity of UiO-66(Zr) with water.

Uranyl uptake into metal-organic frameworks: a detailed X-ray structural analysis.

Metal-organic frameworks (MOF) are a subclass of porous framework materials that have been used for a wide variety of applications in sensing, catalysis, and remediation. Among these myriad applications is their remarkable ability to capture substances in a variety of environments ranging from benign to extreme. Among the most common and problematic substances found throughout the world's oceans and water supplies is [UO2]2+, a common mobile ion of uranium, which is found both naturally and as a result of anthropogenic activities, leading to problematic environmental contamination. While some MOFs possess high capability for the uptake of [UO2]2+, many more of the thousands of MOFs and their modifications that have been produced over the years have yet to be studied for their ability to uptake [UO2]2+. However, studying the thousands of MOFs and their modifications presents an incredibly difficult task. As such, a way to narrow down the numbers seems imperative. Herein, we evaluate the binding behaviors as well as identify the specific binding sites of [UO2]2+ incorporated into six different Zr MOFs to elucidate specific features that improve [UO2]2+ uptake. In doing so, we also present a method for the determination and verification of these binding sites by Anomalous wide-angle X-ray scattering, X-ray fluorescence, and X-ray absorption spectroscopy. This research not only presents a way for future research into the uptake of [UO2]2+ into MOFs to be conducted but also a means to evaluate MOFs more generally for the uptake of other compounds to be applied for environmental remediation and improvement of ecosystems globally.

Development of ligandless dispersive micro-solid-phase extraction method based on NH2-UiO-66 (Zr) MOF using DES eluent in determination of Cd(II) and Cu(II) ions in water and fruit juice samples

ABSTRACTA metal–organic framework (MOF) was synthesised and used for dispersive micro-solid-phase extraction of heavy metal ions from fruit juice and water sample without adding an additional complexing agent. In this work, 9 mg of the synthesised adsorbent (NH2-UiO-66 (Zr) MOF) was dispersed into 6 mL of sample solution containing Cu(II) and Cd(II) ions. Then, a synthesised deep eutectic solvent was used to elute and desorb the analytes from the surface of the sorbent. The effect of important parameters such as sorbent amount, sample solution volume, extraction time, elution solvent type and volume, desorption time, pH and salt addition on the extraction efficiency of the method was investigated. The developed procedure had linear ranges of 1–100 and 0.5–150 µg L−1 with the limits of detection of 0.37 and 0.19 µg L−1, for Cu(II) and Cd(II) ions, respectively, with flame atomic absorption spectrometry determination. To study the validity of the procedure, the concentration of the analytes was determined in a certified reference material (SPS-WW2) by the proposed method. Finally, the proposed procedure was used for the determination of the concentrations of Cu(II) and Cd(II) ions in different fruit juice and water samples.

Modulator-Dependent Dynamics Synergistically Enabled Record SO2 Uptake in Zr(IV) Metal-Organic Frameworks Based on Pyrene-Cored Molecular Quadripod Ligand.

Developing innovative porous solid sorbents for the capture and storage of toxic SO2 is crucial for energy-efficient transportation and subsequent processing. Nonetheless, the quest for high-performance SO2 sorbents, characterized by exceptional uptake capacity, minimal regeneration energy requirements, and outstanding recyclability under ambient conditions, remains a significant challenge. In this study, we present the design of a unique tertiary amine-embedded, pyrene-based quadripod-shaped ligand. This ligand is then assembled into a highly porous Zr-metal-organic framework (MOF) denoted as Zr-TPA, which exhibits a newly discovered 3,4,8-c woy net structure. Remarkably, our Zr-TPA MOF achieved an unprecedented SO2 sorption capacity of 22.7 mmol g-1 at 298 K and 1 bar, surpassing those of all previously reported solid sorbents. We elucidated the distinct SO2 sorption behaviors observed in isostructural Zr-TPA variants synthesized with different capping modulators (formate, acetate, benzoate, and trifluoroacetate, abbreviated as FA, HAc, BA, and TFA, respectively) through computational analyses. These analyses revealed unexpected SO2-induced modulator-node dynamics, resulting in transient chemisorption that enhanced synergistic SO2 sorption. Additionally, we conducted a proof-of-concept experiment demonstrating that the captured SO2 in Zr-TPA-FA can be converted in situ into a valuable pharmaceutical intermediate known as aryl N-aminosulfonamide, with a high yield and excellent recyclability. This highlights the potential of robust Zr-MOFs for storing SO2 in catalytic applications. In summary, this work contributes significantly to the development of efficient SO2 solid sorbents and advances our understanding of the molecular mechanisms underlying SO2 sorption in Zr-MOF materials.

Microwave-assisted synthesis of metal–organic frameworks UiO-66 and MOF-808 for enhanced CO2/CH4 separation in PIM-1 mixed matrix membranes

This study presents a sustainable microwave-assisted synthesis, based on the use of acetone and a water/acetic acid mixture instead of typical harmful DMF, to fabricate Zr-metal–organic frameworks (MOFs) UiO-66 and MOF-808 as fillers in a PIM-1 matrix for gas separation application. The mixed matrix membranes (MMMs) were prepared with varying loadings (2.5–10 wt%) of MOFs. The physicochemical properties (1H NMR, FTIR, XRD, N2 adsorption, TGA and SEM) of the resulting PIM-1, MOFs and MMMs were analyzed. The CO2/CH4 separation performance and membrane aging characteristics of the MMMs were evaluated. The incorporation of MOF fillers significantly improved CO2 permeability and CO2/CH4 selectivity, attributed to their CO2-philicity and narrow pore size (UiO-66 ≈ 0.6 nm and MOF-808 ≈ 1.8 nm). The MMMs with higher filler loadings (7.5 and 10 wt%) exhibited the most favorable separation performance. Due to the better crystallinity and textural properties, MOF-808 produced the best separation results at 10 wt% filler loading (a CO2/CH4 separation selectivity of 16.2 at 9090 Barrer of CO2 permeability). Aging led to a decrease in CO2 permeability but a slight increase in CO2/CH4 selectivity for all MMMs. Overall, the study highlights the potential of PIM-1/UiO-66 and PIM-1/MOF-808 MMMs as efficient materials for (CO2/CH4) separation comparing with the pristine PIM-1.

Magnetic S-Functionalized Nanoporous Zr-Metal Organic Frameworks with Defect-Induced Performance in Hg(II) Removal

Magnetic S-Functionalized Nanoporous Zr-Metal Organic Frameworks with Defect-Induced Performance in Hg(II) Removal

Dispersive solid phase extraction of noble metal nanoparticles from environmental samples on a thiol-functionalized Zirconium(IV) metal organic framework and determination with atomic absorption spectrometry

This work reports for the first time, the utility of a thiol-functionalized Zr-metal organic framework (MOF-SH) for the extraction of noble metal nanoparticles from environmental water samples. Due to the presence of a free thiol terminal group, extraction of metal nanoparticles is performed on the external surface area of the MOF, instead of its pores, thus significantly decreasing the extraction time. Both gold nanoparticles and gold ions, as model species, could be quantitatively extracted from aqueous samples using dispersive solid phase extraction and determined with atomic absorption spectrometry. The MOF-SH material could effectively retain AuNPs of variable sizes and coatings thus enabling the determination of the total concentration of AuNPs in the sample. Due to the strong affinity of gold species for the thiol group, the separation of AuNPs from gold ions could be achieved prior to extraction with ultracentrifugation, followed by the independent extraction and determination of each species. Under the optimum experimental conditions, the method could effectively extract and discriminate gold nanoparticles and gold ions from freshwater samples with detection limits for AuNPs as low as 650 femto-mol/L (by extracting 25 mL of water sample), recoveries between 73 and 132% and precision between 11.8 and −13.4%, which are comparable to other methods.

Decoration of Defective Sites in Metal–Organic Frameworks to Construct Tight Heterojunction Photocatalyst for Hydrogen Production

Zr–metal–organic frameworks (MOFs) have received much interest for their ultrahigh stability and are considered an up‐and‐coming class of catalysts for photocatalytic water splitting. However, their activity still needs to be improved. In this work, a series of defective UiO‐66‐NH2‐x@CdS nanoparticles (NPs) (x = 0, 50, 100, 150, 200, denotes the molar equivalent of the defect modulator) heterostructure photocatalysts is constructed for water splitting using defect engineering followed by a postmodification strategy. Defective structures are introduced to improve the photocatalytic activity of heterojunctions in the following ways: 1) modulating the energy band structure of UiO‐66‐NH2‐x, 2) providing chelate binding sites for modifying the bridging molecules and thus building strong interactions between UiO‐66‐NH2‐x and CdS NPs, and 3) serving as trapping sites to separate the photogenerated electron–hole pairs. Hence, this constructed series of UiO‐66‐NH2‐xCdS NPs exhibit ultrahigh photocatalytic water splitting for H2 production, especially UiO‐66‐NH2‐150@CdS NPs with moderately defective levels showing catalytic activity up to 2303 μmol g−1 h−1, which is 2.36 times higher than pure CdS NPs. Furthermore, the heterojunction catalysts with different defect levels exhibit a volcano‐type trend, demonstrating the feasibility of defect engineering. This work provides novel insights for developing advanced defect‐based MOF‐constructed composite photocatalysts.

Highly selective ratiometric fluorescent sensing of fleroxacin via functionalized Zr metal–organic frameworks

The fluoroquinolone antibacterial agent fleroxacin (FLO) has been widely used for the treatment of urinary infections. Quantitatively analyzing the amount of FLO is vital for human health and the ecological system, but still proves challenging due to the complex environmental interference. In this work a ratiometric fluorescence sensor MOF-808-TPY is successfully constructed by incorporating a functional ligand, 2,2′:6′,2′'-terpyridine-4′-carboxylic acid (TPY), to the zirconium node of MOF-808 via post-synthetic modification. With the addition of fleroxacin, the prepared MOF-808-TPY sensor exhibits decreased fluorescence intensity at 370 nm, while the intrinsic fluorescence from fleroxacin at 440 nm is greatly enhanced. The yielded ratiometric response can effectively eliminate interference from environmental factors, and is able to discriminate FLO from other fluoroquinolone antibiotics efficiently, demonstrating high selectivity towards FLO analysis. A linear relationship of the fluorescence ratio F370/F440 against the concentration of FLO is observed in the range of 0 ∼ 13.50 μM with the detection of limit down to 0.01 μM. Excellent recoveries of FLO levels in real water samples for practical application are also achieved. Additionally, MOF-808-TPY could be easily integrated onto nanofibers for FLO sensing in aspect of potential sensing device. This work represents a promising strategy for constructing MOFs-based ratiometric sensors in monitoring fluorescent antibiotics.

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

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

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.