Water-stable Metal-organic Frameworks Research Articles

Combination of Optimization and Metalated‐Ligand Exchange: An Effective Approach to Functionalize UiO‐66(Zr) MOFs for CO2 Separation

The strategy to functionalize water-stable metal-organic frameworks (MOFs) in order to improve their CO2 uptake capacities for efficient CO2 separation remains limited and challenging. We herein present an effective approach to functionalize a prominent water-stable MOF, UiO-66(Zr), by a combination of optimization and metalated-ligand exchange. In particular, by systematic optimization, we have successfully obtained UiO-66(Zr) of the highest BET surface area reported so far (1730 m(2) g(-1) ). Moreover, it shows a hybrid Type I/IV N2 isotherm at 77 K and a mesopore size of 3.9 nm for the first time. The UiO-66 MOF underwent a metalated-ligand-exchange (MLE) process to yield a series of new UiO-66-type MOFs, among which UiO-66-(COONa)2 -EX and UiO-66-(COOLi)4 -EX MOFs have both enhanced CO2 working capacity and IAST CO2 /N2 selectivity. Our approach has thus suggested an alternative design to achieve water-stable MOFs with high crystallinity and gas uptake for efficient CO2 separation.

A Modulated Hydrothermal (MHT) Approach for the Facile Synthesis of UiO-66-Type MOFs.

Developing a general and economically viable approach for the large-scale synthesis of water-stable metal-organic frameworks (MOFs) with repeatable quality remains the key step for their massive production and commercialization. We herein report a green (aqueous solutions), mild (100 °C, 1 atm), and scalable (can be up to kilograms) modulated hydrothermal (MHT) synthesis of UiO-66, an iconic MOF that has been widely studied recently for its high water stability. More importantly, the MHT synthetic approach can be applied to synthesize other water-stable MOFs with structures identical to UiO-66, such as UiO-66-(F)4, UiO-66-(OCH2CH3)2, and UiO-66-(COOH)4, which cannot be obtained via the traditional solvothermal method. Their performance in postcombustion CO2 capture has also been evaluated. Our MHT approach has clearly depicted a roadmap for the facile synthesis of zirconium-based water stable MOFs to facilitate their massive production and commercialization.

Synthesis of cobalt-, nickel-, copper-, and zinc-based, water-stable, pillared metal-organic frameworks.

The performance of metal-organic frameworks (MOFs) in humid or aqueous environments is a topic of great significance for a variety of applications ranging from adsorption separations to gas storage. While a number of water-stable MOFs have emerged recently in the literature, the majority of MOFs are known to have poor water stability compared to zeolites and activated carbons, and there is therefore a critical need to perform systematic water-stability studies and characterize MOFs comprehensively after water exposure. Using these studies we can isolate the specific factors governing the structural stability of MOFs and direct the future synthesis efforts toward the construction of new, water-stable MOFs. In this work, we have extended our previous work on the systematic water-stability studies of MOFs and synthesized new, cobalt-, nickel-, copper-, and zinc-based, water-stable, pillared MOFs by incorporating structural factors such as ligand sterics and catenation into the framework. Stability is assessed by using water vapor adsorption isotherms along with powder X-ray diffraction patterns and results from BET modeling of N2 adsorption isotherms before and after water exposure. As expected, our study demonstrates that unlike the parent DMOF structures (based on Co, Ni, Cu, and Zn metals), which all collapse under 60% relative humidity (RH), their corresponding tetramethyl-functionalized variations (DMOF-TM) are remarkably stable, even when adsorbing more than 20 mmol of H2O/g of MOF at 80% RH. This behavior is due to steric factors provided by the methyl groups grafted on the BDC (benzenedicarboxylic acid) ligand, as shown previously for the Zn-based DMOF-TM. Moreover, 4,4',4″,4‴-benzene-1,2,4,5-tetrayltetrabenzoic acid based, pillared MOFs (based on Co and Zn metals) are also found to be stable after 90% RH exposure, even when the basicity of the bipyridyl-based pillar ligand is low. This is due to the presence of catenation in their frameworks, similar to MOF-508 (Zn-BDC-BPY), which has also been reported to be stable after exposure to 90% RH.

Immobilization of volatile and corrosive iodine monochloride (ICl) and I(2) reagents in a stable metal-organic framework.

The major discovery here is a robust and water-stable metal-organic framework (MOF) material capable of reversible binding of the volatile and reactive molecules of ICl and I2. The immobilization of I2 and ICl, as well as their controllable release thus achieved, is to facilitate the wide-ranging applications of these volatile species as catalysts and reagents in chemical and industrial processes. The framework material TMBP·CuI (hereafter TCuI) can be conveniently prepared in quantitative yields by heating CuI and the organic linker TMBP (3,3',5,5'-tetramethyl-4,4'-bipyrazol) in acetonitrile. The microporous three-dimensional net of TCuI features CuI chains that contribute to efficient and reversible binding of ICl and I2 molecules, to result in the stoichiometrically well-defined adducts of TCuI·ICl and TCuI·I2, respectively. Moreover, the confinement of a volatile compound like ICl within the MOF medium provides unique opportunities to enhance its reactivity and selectivity as a chemical reagent, as is exemplified by the iodination reactions examined herein. With this exemplary study, we intend to stimulate interest in further exploring MOFs and other porous media (e.g., porous polymers) for entrapping ICl and other volatile reagents (e.g., Br2, SCl2, S2Cl2, and SOCl2) and for potentially novel reactivity associated with the porous medium.

Water-stable metal–organic frameworks with intrinsic peroxidase-like catalytic activity as a colorimetric biosensing platform

Two iron(III)-based metal-organic frameworks (MOFs) are found to behave as efficient peroxidase mimics and catalyze the oxidation of different peroxidase substrates by H2O2 accompanied with significant color change in the solution. With these findings, a simple and sensitive colorimetric assay to detect H2O2 and ascorbic acid has been established.

The stability and defluoridation performance of MOFs in fluoride solutions

The stability of metal–organic frameworks (MOFs) in fluoride solutions was studied based on 11 water-stable MOFs (MIL-53(Fe, Cr, Al), MIL-68(Al), CAU-1, CAU-6, UiO-66(Zr, Hf) and ZIFs-7, -8, -9), and several factors were found to have large influence, such as the central metal activity, pore topology and coordination number. Furthermore, the defluoridation performance of UiO-66(Zr) was investigated comprehensively, which shows an adsorption capacity of 41.36mgg−1 that is higher than most of the reported adsorbents. The initial pH of fluoride solution has little effect in neutral range, while shows a negative effect when pH >9.0; the adsorption data of UiO-66(Zr) fit well with the Langmuir model, and pseudo-second-order kinetic model was found to be suitable for depicting the F− adsorption process, and finally, it was observed that the extent of the effect of co-existing ions is largely ion-dependent. On the basis of the systemic study, this work suggests that increasing the number of –OH groups is an efficient strategy to improve the defluoridation performance of MOFs, and MOFs are promising adsorbents for defluoridation in waste water.

A Water Stable Metal–Organic Framework with Optimal Features for CO2 Capture

One flue over the cuckoo's nest: A novel porous Zr-based MOF combining a high chemical stability, easy “green” synthesis and scalability is prepared. This material incorporating carboxylic functions on its organic linkers has thermodynamically and kinetically very promising properties for CO2 capture from post-combustion flue gas under real working conditions.

A Water Stable Metal–Organic Framework with Optimal Features for CO2 Capture

A Water Stable Metal–Organic Framework with Optimal Features for CO₂ Capture

Water stable metal-organic framework packed microcolumn for online sorptive extraction and direct analysis of naproxen and its metabolite from urine sample

The metal-organic framework MIL-101 was fabricated in a polyetheretherketone (PEEK) tube as micro-trapping device, and applied to sorptive extraction of naproxen and its metabolite in urine samples. The remarkable water stability of the MIL-101 characterizes the material as being different from other moisture sensitive metal-organic framework. It is therefore suitable for extraction of pharmaceuticals from biological fluids. The adsorption isotherms in aqueous solution showed that the adsorption of naproxen on MIL-101 is endothermic. Additionally, MIL-101 exhibited higher extraction capacity to naproxen than that of C18-bonded silica and multi-walled nanotube. A specially designed in-tube sorptive extraction (ITSE) device endows the extraction process with the characteristic of rapidness, convenience, and easy of conjunction with high performance liquid chromatography (HPLC). Finally the MIL-101 based ITSE method coupled with HPLC and fluorescence detection was applied to analysis of naproxen and 6-O-desmethylnaproxen in urine samples. Parameters that influence the online extraction procedure, including pH of the sample solution, flow rate of extraction, sample volume, desorption solvents and time were investigated. The method is proved to be highly sensitive with the linear range of 0.05–6.0μgL−1 and the limits of detection of 0.034 and 0.011μgL−1 for naproxen and 6-O-desmethylnaproxen, respectively. The recoveries in urine samples were 85.3–98.3% for naproxen and 94.0–97.3% for 6-O-desmethylnaproxen with intra- and inter-day RSDs of 2.7–5.2% and 7.1–8.1%, respectively. Urine samples could be directly subjected to analysis without any additional sample pretreatment. The proposed method was demonstrated an efficient, flexible and versatile extraction tool which is ideally suitable for online conjunction with chromatographic methods.

A Water-Stable Metal–Organic Framework with Highly Acidic Pores for Proton-Conducting Applications

Metal-organic framework (MOF) materials are a nontraditional route to ion conductors, but their crystallinity can give insight into molecular-level transport mechanisms. However, some MOFs can be structurally compromised in humid environments. A new 3D metal-organic framework, PCMOF-5, is reported which conducts protons above 10(-3) S/cm at 60 °C and 98% relative humidity. The MOF contains free phosphonic acid groups, shows high humidity stability, and resists swelling in the presence of hydration. Channels filled with crystallographically located water and acidic groups are also observed.

Kinetic Water Stability of an Isostructural Family of Zinc-Based Pillared Metal–Organic Frameworks

The rational design of metal-organic frameworks (MOFs) with structural stability in the presence of humid conditions is critical to the commercialization of this class of materials. However, the systematic water stability studies required to develop design criteria for the construction of water-stable MOFs are still scarce. In this work, we show that by varying the functional groups on the 1,4-benzenedicarboxylic acid (BDC) linker of DMOF [Zn(BDC)(DABCO)(0.5)], we can systematically tune the kinetic water stability of this isostructural, pillared family of MOFs. To illustrate this concept, we have performed water adsorption studies on four novel, methyl-functionalized DMOF variations along with a number of already reported functionalized analogues containing polar (fluorine) and nonpolar (methyl) functional groups on the BDC ligand. These results are distinctly different from previous reports where the apparent water stability is improved through the inclusion of functional groups such as -CH(3), -C(2)H(5), and -CF(3) which only serve to prevent significant amounts of water from adsorbing into the pores. In this study, we present the first demonstration of tuning the inherent kinetic stability of MOF structures in the presence of large amounts of adsorbed water. Notably, we demonstrate that while the parent DMOF structure is unstable, the DMOF variation containing the tetramethyl BDC ligand remains fully stable after adsorbing large amounts of water vapor during cyclic water adsorption cycles. These trends cannot be rationalized in terms of hydrophobicity alone; experimental water isotherms show that MOFs containing the same number of methyl groups per unit cell will have different kinetic stabilities and that the precise placements of the methyl groups on the BDC ligand are therefore critically important in determining their stability in the presence of water. We present the water adsorption isotherms, PXRD (powder X-ray diffraction) patterns, and BET surface areas before and after water exposure to illustrate these trends. Furthermore, we shed light on the important distinction between kinetic and thermodynamic stability in MOFs. Molecular simulations are also used to provide insight into the structural characteristics governing these trends in kinetic water stability.

A Washable MOF

Chemistry![Figure][1] CREDIT: MAJUMDER ET AL., CHEM. MATER. 24 , 4647 (2012) Metal organic framework (MOF) materials are highly porous materials that consist of metal atoms linked together with organic ligands. They are useful for capturing, storing, or filtering gases, as well as for catalysis and sensing applications. Most MOF materials are unstable when exposed to water, thus preventing their use as aqueous sieves, where their uniform porosity with pores of just the right size would otherwise be very helpful. Majumder et al. have developed a family of water-stable MOF materials based on either magnesium ions or certain first-row transition metal ions—namely, nickel, cobalt, or manganese—that are linked together with perylene tetracarboxylate (PTC) ligands. The reaction between the potassium salt of PTC and the specific metal acetate was performed in water and could readily be done on multigram scales. The authors noted that if either the acetate or potassium salts were varied, only an amorphous material was obtained. Ni-PTC was able to extract methyl viologen, a known toxic herbicide, from parts per million aqueous solutions. The material also absorbed methylene blue, a dye with a similar hydrodynamic radius, although two larger molecules were clearly excluded from the MOF. Ni-PTC also showed 300:1 selectivity of CO2 versus N2, with a high binding enthalpy for the CO2, suggesting possible uses for gas capture applications. Chem. Mater. 24 , 4647 (2012). [1]: pending:yes

Tuning the Adsorption Properties of UiO-66 via Ligand Functionalization

UiO-66 is one of the few known water-stable MOFs that are readily amenable to direct ligand substitution. In this work, UiO-66 has been synthesized with amino-, nitro-, methoxy-, and naphthyl-substituted ligands to impart polar, basic, and hydrophobic characteristics. Pure-component CO(2), CH(4), N(2), and water vapor adsorption isotherms were measured in the materials to study the effect of the functional group on the adsorption behavior. Heats of adsorption were calculated for each pure gas on each material. The results indicate that the amino-functionalized material possesses the best adsorption properties for each pure gas due to a combination of polarity and small functional group size. The naphthyl-functionalized material exhibits a good combination of inhibited water vapor adsorption and high selectivity for CO(2) over CH(4) and N(2).

Low‐Energy Selective Capture of Carbon Dioxide by a Pre‐designed Elastic Single‐Molecule Trap

Single-molecule trap: Easy activation of the water-stable metal-organic framework PCN-200 provides a new route to low-energy selective CO(2) capture through stimuli-responsive adsorption behavior. This elastic CO(2) trapping effect was confirmed by single-component and binary gas-adsorption isotherms and crystallographic determination.

Zirconium‐Metalloporphyrin PCN‐222: Mesoporous Metal–Organic Frameworks with Ultrahigh Stability as Biomimetic Catalysts

In nature, metalloporphyrins are well known for performing many biological functions in aqueous media, such as light harvesting, oxygen transportation, and catalysis. Heme, the iron–porphyrin derivative, is the cofactor for many enzyme/ protein families, including peroxidases, cytochromes, hemoglobins, and myoglobins. Using synthetic systems to mimic natural enzymes with high catalytic activity and substrate selectivity has been a sought-after goal in the last decade. Direct application of a heme as an oxidation catalyst in aqueous solution is usually challenging due to the formation of catalytically inactive dimers and catalyst self-destruction in the oxidizing reaction media. One promising approach is to load heme on supports, such as zeolites, clays, nanoparticles, hydrogels, or carbon materials, a practice which inevitably dilutes the density of active sites. An alternative approach is to protect the heme center by modifying the porphyrin to produce dendrimers or molecular crystals, which is a synthetically demanding method. Herein, we propose a unique strategy employing heme-like active centers as structural motifs for the assembly of highly stable porous materials, which should possess well-defined mesochannels and ultrahigh stability in aqueous solution. Metal-organic frameworks (MOFs) are a new class of crystalline porous materials with fascinating structures and intriguing properties, such as permanent porosity, high surface area, and uniform open cavities. The availability of various building blocks consisting of metals and organic linkers makes it possible to construct MOFs with unique properties for diverse applications. However, these desirable features of MOFs have rarely been applied to an enzymatic mimic, especially for catalysis in an aqueous medium, despite the fact that the assembly of ligands bearing high-density active sites into 3D frameworks may provide an ideal system to both enhance the catalytic activity and protect the cofactors. One of the main reasons is the lack of water-stable MOFs containing redox-active metal centers. Furthermore, most MOFs are microporous (pore size< 2 nm). Although they are suitable for gas storage, the small pore size slows down diffusion and limits the access of large substrate molecules to the active sites inside a MOF. Therefore, MOFs with mesopores, accessible redox sites, and ultrahigh stability, especially in aqueous media, are indispensible for any successful biomimetic attempt. Herein we have employed Fe-TCPP (TCPP= tetrakis(4carboxyphenyl)porphyrin) as a heme-like ligand and chosen highly stable Zr6 clusters as nodes for the assembly of stable Zr-MOFs. With carefully selected starting materials, we have successfully constructed a 3D heme-like MOF, designated as PCN-222(Fe) (Figure 1; PCN= porous coordination net-

Water Stable Metal–Organic Framework Evolutionally Formed from a Flexible Multidentate Ligand with Acylamide Groups for Selective CO2 Adsorption

A water stable porous metal–organic framework has been synthesized through an evolution approach in terms of both structures and properties. This interesting pillar-layered MOF ({[Cu2(TCMBT)(bpp)(μ3-OH)]·6H2O}n (1)) constructed from a flexible ligand possesses a BET surface area of 808.5 m2 g–1 and exhibits good selectivities of CO2/N2 (20.1:1) and CO2/CH4(4.0:1), which are comparable to those of the ZIF materials reported by Yaghi’s group. The excellent water stability of complex 1 would be quite preferable in practical application.

Metal–organic framework MIL-53(Al) as a solid-phase microextraction adsorbent for the determination of 16 polycyclic aromatic hydrocarbons in water samples by gas chromatography–tandem mass spectrometry

In this paper, the potential applications of metal-organic framework (MOF) materials as fiber coatings for the solid-phase microextraction (SPME) of polycyclic aromatic hydrocarbons (PAHs) in water samples were explored. Fibers coated with MIL-53(Al, Cr, Fe) materials were fabricated by an adhesive method for SPME. The quantitation was performed by gas chromatography-tandem mass spectrometry (GC-MS/MS) using the multiple reaction monitoring mode. Among the three MIL-53(M) coatings, MIL-53(Al) showed the highest extraction efficiency towards PAHs under the current fabrication procedure. Under optimized conditions, the MIL-53(Al)-coated fiber showed good precision (relative standard deviation <12.5%), low detection limits (0.10 ng L(-1) to 0.73 ng L(-1), S/N = 3), and good linearity (R(2) > 0.98) for aqueous solutions containing 16 PAH . The fiber also offered high thermal and chemical stability. The method developed based on MIL-53(Al) SPME-GC-MS/MS was successfully applied in the analysis of real water samples. Based on the simulation results, the PAHs were adsorbed on MIL-53(Al) primarily through the hydrophobic and π-π interactions between PAHs and the organic linker of the material. The results presented in this paper indicate that water-stable MOF materials have great potential for the SPME of aromatic compounds in water samples.

Extracting organic contaminants from water using the metal–organic framework CrIII(OH)·{O2C–C6H4–CO2}

The water-stable metal-organic framework MIL-53(Cr) is able to adsorb phenol and p-cresol from contaminated water as well as the monomeric sugar D-(-)-fructose. Based on the isotherm for phenol uptake from the liquid phase, it is proposed that the framework breathes to maximize the uptake.

MOFs as Adsorbents for Low Temperature Heating and Cooling Applications

The 3D metal-organic framework (MOF) (3)(infinity){[Ni(3)(mu(3)-btc)(2)(mu(4)-btre)(2)(mu-H(2)O)(2)]. approximately 22H(2)O} is found to be a reversibly dehydratable-hydratable water-stable MOF material with a large loading spread of 210 g/kg as a candidate for solid adsorbents in heat transformation cycles for refrigeration, heat pumping, and heat storage.