Mixed-linker Approach Research Articles

Exploring Defect-Engineered Metal-Organic Frameworks with 1,2,4-Triazolyl Isophthalate and Benzoate Linkers.

Synthesis and characterization of DEMOFs (defect-engineered metal-organic frameworks) with coordinatively unsaturated sites (CUSs) for gas adsorption, catalysis, and separation are reported. We use the mixed-linker approach to introduce defects in Cu2-paddle wheel units of MOFs [Cu2(Me-trz-ia)2] by replacing up to 7% of the 3-methyl-triazolyl isophthalate linker (1L2-) with the "defective linker" 3-methyl-triazolyl m-benzoate (2L-), causing uncoordinated equatorial sites. PXRD of DEMOFs shows broadened reflections; IR and Raman analysis demonstrates only marginal changes as compared to the regular MOF (ReMOF, without a defective linker). The concentration of the integrated defective linker in DEMOFs is determined by 1H NMR and HPLC, while PXRD patterns reveal that DEMOFs maintain phase purity and crystallinity. Combined XPS (X-ray photoelectron spectroscopy) and cw EPR (continuous wave electron paramagnetic resonance) spectroscopy analyses provide insights into the local structure of defective sites and charge balance, suggesting the presence of two types of defects. Notably, an increase in CuI concentration is observed with incorporation of defective linkers, correlating with the elevated isosteric heat of adsorption (ΔHads). Overall, this approach offers valuable insights into the creation and evolution of CUSs within MOFs through the integration of defective linkers.

Exploring the Influence of Linker Substitution and Ratios on Cooperative Framework Flexibility through the Mixed-Linker Approach

Exploring the Influence of Linker Substitution and Ratios on Cooperative Framework Flexibility through the Mixed-Linker Approach

A sulfonate ligand-defected Zr-based metal-organic framework for the enhanced selective removal of anionic dyes.

In this work, we introduce a novel defective analogue of the representative 6-connected zirconium-based metal-organic framework (MOF-808), by employing 5-sulfoisophthalic acid monosodium salt (H2BTC-SO3Na) as a defect inducer via a mixed-linker approach. The structural integrity and different physicochemical properties were investigated by various characterization techniques, including powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and nitrogen physisorption at 77 K. Additionally, proton nuclear magnetic resonance (1H-NMR), energy-dispersive X-ray (EDX), and inductively coupled plasma optical emission spectroscopy (ICP-OES) were employed to confirm the presence of 6.9 mol% of the 5-sulfoisophthalate ligand within the highly crystalline MOF-808 structure. The defective material exhibited significant enhancements in the removal efficiency of various organic dyes, including approximately 64% and 77% for quinoline yellow and sunset yellow, and 56% and 13% for rhodamine B and malachite green, compared to its pristine counterpart. Importantly, the defective MOF-808 showed a remarkable selectivity toward anionic species in binary-component dyes comprising both anionic and cationic dyes.

Mixed-linker MOF-303 membranes for pervaporation

Metal-organic frameworks (MOFs) hold great promise as porous materials for pervaporation applications. However, the exploration of MOF membranes in this field is still in its early stages. One of the main challenges is the relatively low mass flux and stability of pure MOF membranes compared to other materials used in pervaporation. In this study, we propose a novel approach to enhance the separation performance of MOF membranes for water and ethanol separation. Our strategy involves incorporating the 2,5-thiophenedicarboxylic acid (TDC) linker into the MOF-303 structure, partially replacing the 3,5-pyrazoledicarboxylic acid (PDC) linker. The goal is to increase the aperture size of the microporous channels in the pristine MOF-303 membrane, thereby improving the mass flux. X-ray diffraction characterization, combined with Rietveld refinement, confirmed that the partial substitution of PDC with TDC resulted in an increased pore-limiting diameter (PLD) of MOF-303. For instance, the pristine MOF-303 exhibited a PLD of 5.78 Å, while MOF-303(70/30) with 70% TDC replacement displayed a PLD of 6.02 Å. To fabricate the mixed-linker MOF-303 membranes, we utilized a seeded growth method, which yielded membranes with dense layers, as confirmed by scanning electron microscopy and air permeation characterization. The prepared membranes were subjected to pervaporation tests to evaluate their performance in separating 90 wt.% ethanol at 60 °C. The pristine MOF-303 membrane exhibited notable separation capabilities, with an average flux of 0.071 kg·m−2·hr−1 and a water/ethanol separation factor of 5371. Surpassing the unmodified MOF-303, the mixed-linker MOF-303(50/50) membrane demonstrated improved mass flux and water/ethanol separation factor. Specifically, the MOF-303(50/50) membrane displayed an average flux of 0.092 kg·m−2·hr−1 and a water/ethanol separation factor of 8500. Importantly, the unmodified MOF-303 membrane exhibited instability during prolonged pervaporation operation, whereas the mixed-linker MOF-303(50/50) membrane effectively addressed this issue. Further analysis using in situ Fourier transform infrared spectroscopy and water adsorption characterization revealed that MOF-303(50/50) possessed a strong affinity for water, comparable to the pristine MOF-303. Overall, our study highlights the potential of the mixed-linker approach to optimize the separation performance and stability of MOF-based membranes for pervaporation application.

Mixed-linker strategy for suppressing structural flexibility of metal-organic framework membranes for gas separation

Structural flexibility is a critical issue that limits the application of metal-organic framework (MOF) membranes for gas separation. Herein we propose a mixed-linker approach to suppress the structural flexibility of the CAU-10-based (CAU = Christian-Albrechts-University) membranes. Specifically, pure CAU-10-PDC membranes display high separation performance but at the same time are highly unstable for the separation of CO2/CH4. A partial substitution (30 mol.%) of the linker PDC with BDC significantly improves its stability. Such an approach also allows for decreasing the aperture size of MOFs. The optimized CAU-10-PDC-H (70/30) membrane possesses a high separation performance for CO2/CH4 (separation factor of 74.2 and CO2 permeability of 1,111.1 Barrer under 2 bar of feed pressure at 35°C). A combination of in situ characterization with X-ray diffraction (XRD) and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, as well as periodic density functional theory (DFT) calculations, unveils the origin of the mixed-linker approach to enhancing the structural stability of the mixed-linker CAU-10-based membranes during the gas permeation tests.

Adenine-mediated amide-containing metal-organic framework toward one-step ethylene purification from a ternary mixture

Simultaneous removal of acetylene (C2H2) and ethane (C2H6) from a ternary gas mixture of acetylene/ethylene/ethane via a one-step process to achieve high purity of ethylene (C2H4) is challenging but highly desirable. Herein, an amide-containing adenine-mediated rod metal-organic framework (MOF) of 1 was solvothermally constructed by means of mixed-linker approach. Structural analysis illustrates that 1 displays 1D microporous channels and features open metal site-free concomitant with uncoordinated carboxylate oxygen atoms and amide functional groups decorated interior pore walls. Interestingly, both single-component gas adsorption and ideal adsorbed solution theory (IAST) prediction reveal that 1 is C2H2 and C2H6 selective, which were eventually confirmed by dynamic column breakthrough experiments through not only binary gas mixtures of C2H2/C2H4 and C2H6/C2H4, but also a ternary mixture of C2H2/C2H4/C2H6 to validate one-step ethylene purification.

Defect-engineered metal–organic frameworks (MOF-808) towards the improved adsorptive removal of organic dyes and chromium (vi) species from water

Two defective MOF-808 materials synthesized by a facile mixed-linker approach show significantly higher adsorption capacity compared to standard MOF-808.

Defective Homojunction Porphyrin‐Based Metal‐Organic Frameworks for Highly Efficient Sonodynamic Therapy

Sonodynamic therapy (SDT) with non-invasiveness and high tissue-penetrating ability has attracted widespread interest in treating deep-seated tumors or infections. To enhance the treatment efficacy of SDT, the development of high-efficiency and stable sonosensitizers are still needed. Herein, a defective homojunction porphyrin-based metal-organic framework (MOF) with greatly enhanced sonocatalytic ability is easily prepared and used for SDT of osteomyelitis infected by methicillin-resistant Staphylococcus aureus (MRSA). Acetic acid and benzoic acid are chosen as modulators during the hydrothermal synthesis of porphyrin-based MOF. It is found that the crystal structure of MOF shifts from PCN-222 to PCN-224 as the amount of acetic acid increases. Interestingly, the defective PCN (D-PCN) contains a two-phase homojunction structure of PCN-222/PCN-224. The sonocatalytic reactive oxygen species production presents a volcano-type trend with increased acetic acid, among which D-PCN-2 with more content of PCN-224 has the best sonocatalytic antibacterial ability. The reduced band gap introduced a defect, and type-II homojunction structures of D-PCN-2 improve the separation of the ultrasound-triggered electron hole, which significantly enhances the SDT effect. Through a mixed linker approach, this work develops a new defect-induced homojunction MOF with great performance for SDT of MRSA-infected osteomyelitis.

Harnessing Particle Size‐Control and DNA‐Oligo Functionalization in ZIF‐76 for Biological Applications

AbstractAdvanced therapeutics require novel nanocarriers to ensure their functionality is preserved during transit. Zeolitic imidazolate frameworks (ZIFs) have emerged as promising materials in this field owing to their combined biocompatibility, high porosity, and tunable chemistry. While a diverse family of ZIFs has been reported, few have been explored beyond the prototypical ZIF‐8. Herein, the size‐controlled synthesis of three distinct ZIF‐76 analogs is demonstrated, overcoming the unique synthetic challenges intrinsic to the lta topology and complex crystallization kinetics associated with the mixed linker approach. This assesses the materials’ platform effectiveness for intracellular delivery first by exploring the structural and colloidal stability in biologically relevant media. To circumvent particle aggregation, fluorescently labeled DNA oligonucleotides are post‐synthetically attached to the ZIF surface. This modification significantly improves the colloidal stability in media and facilitates particle internalization tracking. Finally, the particle‐cell interactions are assessed, revealing rapid cell membrane association with macrophages, but not lung epithelial cells, and ZIF accumulation within macrophages which increased over time. Importantly, this study outlines a generalized approach toward expanding the available library of ZIFs for biological applications, enabling the potential for targeted therapeutic delivery for intracellular infections treatment.

Mixed-linker synthesis of L-histidine@zeolitic imidazole framework-8 on amyloid nanofibrils-modified polyacrylonitrile membrane with high separation and antifouling properties

Zeolitic imidazole framework-8 (ZIF-8) has been demonstrated significant potential as a support material for the fabrication of separation membranes for water treatment applications. However, the separation performance of a ZIF-8-containing membrane is limited by the poor surface and molecular sieving properties of ZIF-8. Herein, a continuous mixed-linker L-histidine@ZIF-8 (His@ZIF-8) layer was synthesized on the amyloid nanofibrils (ANFs)-modified hydrolyzed polyacrylonitrile (HPAN) membrane through a combination of self-assembly and in situ growth methods. The mixed-linker approach facilitated the tuning of surface and molecular sieving properties of ZIF-8 on the membranes, which effectively improved the overall separation and antifouling performance of the His@ZIF-8/ANFs membrane. Compared with the ZIF-8/ANFs membrane, the resultant His@ZIF-8/ANFs membrane exhibits higher water flux (42.6 vs 27.9 L/m2·h·bar) and superior heavy metal ion (Cu2+, Ni2+, Pb2+, and Cd2+) rejection (>98.6 vs 91.2%). Moreover, the His@ZIF-8/ANF membrane also exhibited a higher antibacterial efficiency of 82.9% against Escherichia coli than ANFs/HPAN membrane (16.7%). Therefore, through the mixed-linker approach, considerable modifications of the properties of ZIF-8 were realized, making it a suitable material for the preparation of membranes for wastewater treatment applications.

Large Cages of Zeolitic Imidazolate Frameworks.

The design and synthesis of permanently porous materials with extended cage structures is a long-standing challenge in chemistry. In this Account, we highlight the unique role of zeolitic imidazolate frameworks (ZIFs), a class of framework materials built from tetrahedral nodes connected through imidazolate linkers, in meeting this challenge and illustrate specific features that set ZIFs apart from other porous materials. The structures of ZIFs are characteristic of a variety of large, zeolite-like cages that are covalently connected with neighboring cages and fused in three-dimensional space. In contrast to molecular cages, the fusion of cages results in extraordinary architectural and chemical stability for the passage of gases and molecules through cages and for carrying out chemical reactions within these cages while keeping the cages intact. The combination of the advantages from both cage chemistry and extended structures allows uniquely interconnected yet compartmentalized void spaces inside ZIF solids, rendering their wide range of applications in catalysis, gas storage, and gas separation.While the field of ZIFs has seen rapid development over the past decade, with hundreds of ZIF structures built from dozens of different cages of varying composition, size, and shapes reported, rational approaches to their design are largely unknown. In this Account, we summarize a vast number of cages formed in reported ZIFs and then review how the thermodynamic factors and traditional guest-templating strategies from zeolites influence the formation of cages. We highlight how the link-link interactions perform in the ZIF formation mechanism and serve as a means to target the formation of frameworks containing cages of specific sizes with structures exhibiting a level of complexity as yet unachieved in discrete coordination cages. For example, the giant ucb cage features a dimension of 46 Å and the complex moz cage is constructed from as many as 660 components.With the finding of these large and complex cages in ZIFs, we envision that the collection of cage structures will further be diversified by a mixed-linker approach utilizing a more complex combination of link-link interactions or by creating multivariant (MTV) systems that have been realized in other framework materials yet not widely employed in ZIFs. The more complicated cage structures can provide extra variations in chemical environments, and in addition to that, MTV systems can generate inhomogeneity inside each type of cage structure. The fused cages at such complexity that are difficult to be realized in solution environments will potentially enable more complex materials for smart applications.

Tunable hydrocarbon adsorption based on a zeolitic imidazolate framework in the sodalite topology

The structural flexibility of ZIF-7 is controlled via a mixed-linker approach resulting in modified adsorption properties tunable across 2 orders of magnitude of step pressure while maintaining the ZIF-7 structure throughout the compositional range.

Tuning Open Metal Site-Free ncb Type of Metal-Organic Frameworks for Simultaneously High Gravimetric and Volumetric Methane Storage Working Capacities.

The design and synthesis of a single metal-organic framework (MOF) material with simultaneously high gravimetric and volumetric methane storage working capacities are still a great challenge. The open metal site (OMS) in MOFs is generally regarded as an advantage to enhance host-guest affinity. However, it is detrimental to the methane storage working capacity to some extent due to the resulting high low-pressure uptake. Moreover, the reported methane storage MOFs are predominately focusing on edge-transitive or low-connected mixed-linker networks. In contrast, high-connected mixed-linker MOFs have been less investigated for methane storage. Herein, three isoreticular nine-connected trinuclear iron-based Fe-ncb-MOFs without OMSs have been judiciously designed and successfully constructed by means of the mixed-linker approach associated with the fixing amide-functionalized pyridyl-carboxylate ligand LP (4-(pyridin-4-ylcarbamoyl)benzoate) and three differing sized dicarboxylate ligands. High-pressure methane adsorption measurements show that, with the isoreticular extension from BDC (1,4-benzenedicarboxylate) to BPDC (4,4'-biphenyldicarboxylate) and ABDC (azobenzene-4,4'-dicarboxylate), three Fe-ncb-MOFs exhibit gradually increasing not only gravimetric but also volumetric storage capacities because of their balancing gravimetric surface area and volumetric surface area, hierarchical pore system, and modest CH4 heats of adsorption. Among them, the Fe-ncb-ABDC demonstrates a rare combination of simultaneously high gravimetric and volumetric CH4 storage working capacities of 0.302/0.37 g g-1 and 196/240 cm3 (STP) cm-3 at 298/273 K and between 80 and 5 bar, respectively, which outperform the 8-c Fe-8T18-ABDC assembled from a shorter pyridyl-carboxylate ligand IN (isonicotinate) and ABDC, due to its limited pore volume, the presence of OMSs, and more confined pore spaces, and place Fe-ncb-ABDC among the best performing MOFs.

Bandgap Modulation in Zr-Based Metal-Organic Frameworks by Mixed-Linker Approach.

Metal-organic frameworks (MOFs) have been a promising material for many applications, e.g., photocatalysis, luminescence-based sensing, optoelectronics, and electrochemical devices, due to their tunable electronic properties through linker functionalization. In this work, we investigate the effect of mixed organic linkers on the bandgap modulation of polymorphic zirconium-based MOFs, UiO-66 and MIL-140A using density functional theory (DFT) calculations. We show that the electronic properties of both MOFs are in contrast to Vegard's law for semiconductors, that is, mixed-linker systems exhibit bandgaps not intermediate within the range of single-linker systems. Calculations of the total and partial density of states revealed the formation of mid-gap states in mixed-linker MOFs, causing the bandgap reduction. Interestingly, although both MOFs have similar composition, the effect is more significant in MIL-140A than in UiO-66. This is due to the presence of π-π stacking interactions in MIL-140A, which does not occur in UiO-66. The simulation results reveal a direct relationship between the strength of π-π interactions and the bandgap. This illustrates that distinct structural features, particularly the orientation of organic linkers can give rise to different consequences in bandgap modulation. Moreover, this computational work highlights the possibility to engineer the electronic properties of MOFs through a mixed-linker approach.

Defect Engineering of Copper Paddlewheel-Based Metal–Organic Frameworks of Type NOTT-100: Implementing Truncated Linkers and Its Effect on Catalytic Properties

A series of new defect-engineered metal-organic frameworks (DEMOFs) were synthesized by framework doping with truncated linkers employing the mixed-linker approach. Two tritopic defective (truncated) linkers, biphenyl-3,3',5-tricarboxylates (LH) lacking a ligating group and 5-(5-carboxypyridin-3-yl)isophthalates (LPy) bearing a weaker interacting ligator site, were integrated into the framework of Cu2(BPTC) (NOTT-100, BPTC = biphenyl-3,3',5,5'-tetracarboxylates). Incorporating LH into the framework mainly generates missing metal node defects, thereby obtaining dangling COOH groups in the framework. However, introducing LPy forms more modified metal nodes featuring reduced and more accessible Cu sites. In comparison with the pristine NOTT-100, the defect-engineered NOTT-100 (DE-NOTT-100) samples show two unique features: (i) functional groups (the protonated carboxylate groups as the Brønsted acid sites or the pyridyl N atoms as the Lewis basic sites), which can act as second active sites, are incorporated into the MOF frameworks, and (ii) more modified paddlewheels, which provided extra coordinatively unsaturated sites, are generated. The cooperative functioning of the above characteristics enhances the catalytic performance of certain types of reactions. For a proof of concept, two exemplary reactions, namely, the cycloaddition of CO2 with propylene oxide to propylene carbonate and the cyclopropanation of styrene, were carried out to evaluate the catalytic activities of those DE-NOTT-100 materials depending on the defect structure.

In Situ Synthesis of Defect-Engineered MOFs as a Photoregenerable Catalytic Adsorbent: Understanding the Effect of LML, Adsorption Behavior, and Photoreaction Process.

Defect-engineering is an exciting strategy for the modification of metal-organic frameworks (MOFs), which can go beyond the limit of conventional MOFs, tailor material properties, and incorporate multiple functionalities. Herein, based on the large mixed-linker approach, we successfully integrated tetrakis(4-carboxyphenyl)porphyrin (TCPP) into stable UiO-66 via an in situ one-pot synthetic method and used the obtained material for the removal of diclofenac (DF). TCPP@UiO-66 maintained the structure, excellent stability, and porosity of UiO-66. The defect density significantly affected the phase purity, crystallite morphology, and properties of TCPP@UiO-66s. Owing to the delicate balance between defects, stability, and porosity, TCPP@UiO-66(25%) was the optimal material in our system. The pseudo-second-order kinetic model and the Sips isothermal model described the adsorption of DF onto defect-engineered MOFs, and the adsorption capacity was 590 mg/g. Electrostatic interaction, Lewis acid-base interaction, π-π interaction, hydrogen bonding, and anion-π interaction were possible adsorption mechanisms. Moreover, under simulated sunlight irradiation, TCPP@UiO-66(25%) was catalytically active for the degradation of DF with a removal efficiency of 99%. It displayed good recyclability during three reaction cycles. The result of electron spin resonance revealed the generation of 1O2, implying the occurrence of type II photosensitization reaction. Meanwhile, the first-order rate constants of DF photodegradation after the addition of scavengers confirmed that h+ is also a key reactive species. Both the energy transfer from TCPP to triplet oxygen and the electron transfer from TCPP to Zr clusters contributed to the degradation of DF. The degradation byproducts of DF were monitored by three-dimensional excitation-emission matrix (3D EEM). Therefore, TCPP@UiO-66(25%) was an attractive photoregenerable catalytic adsorbent for the effective removal of DF. Combining the advantages of the parent framework and the functional linker, our strategy expands the functionality of the stable MOFs for potential applications in environmental remediation.

The introduction of functional side groups and the application of the mixed-linker concept in divalent MIL-53(Ni) materials.

Metal-organic framework materials with functional side groups are commonly used for various purposes like post-synthetic modification reactions or the tuning of pore geometries. Additionally, the mixed-linker concept, in which different linkers are statistically distributed over equivalent lattice positions, is a versatile approach to adjust the number of functional groups within the framework. However, neither functional side groups nor the mixed-linker approach have been used in MIL-53 materials with divalent metal ions yet. In the present work, we report on the synthesis of MIL-53(Ni)-Br(100), which contains only 2-bromoterephthalate as a linker molecule, and mixed-linker MIL-53(Ni)-Br(50) containing both terephthalate and 2-bromoterephthalate. These two materials represent the first functionalized and mixed-linker MIL-53 materials in combination with a divalent metal. Consequently, the possibilities to tailor the properties of the still rarely used divalent MIL-53 materials were expanded.

Solid-Solution Mixed-Linker Synthesis of Isoreticular Al-Based MOFs for an Easy Hydrophilicity Tuning in Water-Sorption Heat Transformations

The solid-solution mixed-linker approach, where a linker is partially replaced by a similar one under retention of the isoreticular metal–organic framework (MOF) structure, offers an easy and inexpensive way to fine-tune MOF properties to design tailored compounds. A total of 10 aluminum mixed-linker MOFs, [Al(OH)(X)a(Y)1–a] (X = IPA, isophthalate; Y = FDC, 2,5-furandicarboxylate) spanning between the isostructural MOFs CAU-10-H (a = 1) and MIL-160 (a = 0), were synthesized by employing different ratios of the aforementioned linkers. CAU-10-H and MIL-160 have been reported as highly promising materials for cycling water sorption for heat transformation applications. A detailed characterization with a focus on the changes in the sorption properties for water vapor showed that the hydrophilicity is readily and easily tuned through the mixed-linker approach between the limits of MIL-160 and CAU-10-H. An increasing fraction of IPA shifts the steep increase in the S-shaped water adsorption isotherm in small steps from p/p0 = ∼0.05 for MIL-160 to p/p0 = ∼0.18 for CAU-10-H. Higher coefficient of performance (COPH) values for the mixed-linker materials over MIL-160 illustrate the well-balanced hydrophobicity/hydrophilicity of the former under the exemplary calculation conditions.

Studies on catalytic activity of MIL-53(Al) and structure analogue DUT-5(Al) using bdc- and bpdc-ligands functionalized with l-proline in a solid-solution mixed-linker approach

Proline-functionalized MOFs MIL-53(Al) (based on the benzene-1,4-dicarboxylate, bdc linker) and DUT-5(Al) (based on the biphenyl-4,4′-dicarboxylate, bpdc linker) were synthesized using a mixed-linker approach. The proline-functionalized bdc and bpdc ligand was mixed with both the non-functionalized ligand as well as the amino functionalized one. The influence of the length of the linker on the catalytic activity showed a counterintuitive confinement effect and advantageous increase of ee with longer linker and higher porosity, that is a change of ee values from no selectivity in the MIL-53(Al) MOFs up to 16% ee for 27-L-Pro/H2N-DUT-5(Al) and 19% ee for 26-L-Pro/H-DUT-5(Al). The conversion of 4-nitrobenzaldehyde remains over 85% after three catalytic runs for l-proline functionalized MIL-53(Al) MOFs. For the DUT-5(Al) MOFs with l-proline groups this value even increases up to 97%.

An insight into the effect of azobenzene functionalities studied in UiO-66 frameworks for low energy CO2 capture and CO2/N2 membrane separation

Tailoring the content of the light-responsive ligand in UiO-66 topology through a mixed-linker approach for CO2 adsorbent and mixed matrix membrane application.