Solvent-assisted Linker Exchange Research Articles

Highly porous metal-organic framework liquids and glasses via a solvent-assisted linker exchange strategy of ZIF-8

By combining the porosity of crystalline metal-organic frameworks (MOFs) with the unique processability of the liquid state, melt-quenched MOF glasses offer exciting opportunities for molecular separation. However, progress in this field is limited by two factors. Firstly, only very few MOFs melt at elevated temperatures and transform into stable glasses upon cooling the corresponding MOF liquid. Secondly, the MOF glasses obtained thus far feature only very small porosities and very small pore sizes. Here, we demonstrate solvent-assisted linker exchange (SALE) as a versatile method to prepare highly porous melt-quenched MOF glasses from the canonical ZIF-8. Two additional organic linkers are incorporated into the non-meltable ZIF-8, yielding high-entropy, linker-exchanged ZIF-8 derivatives undergoing crystal-to-liquid-to-glass phase transitions by thermal treatment. The ZIF-8 glasses demonstrate specific pore volumes of about 0.2 cm3g–1, adsorb large amounts of technologically relevant C3 and C4 hydrocarbons, and feature high kinetic sorption selectivities for the separation of propylene from propane.

Tuning Main Group Element-based Metal-Organic Framework to Boost Electrocatalytic Nitrogen Reduction Under Ambient Conditions.

Main group element-based materials are emerging catalysts for ammonia (NH3 ) production via a sustainable electrochemical nitrogen reduction reaction (N2 RR) pathway under ambient conditions. However, their N2 RR performances are less explored due to the limited active behavior and unclear mechanism. Here, an aluminum-based defective metal-organic framework (MOF), aluminum-fumarate (Al-Fum), is investigated. As a proof of concept, the pristine Al-Fum MOF is synthesized by the solvothermal reaction process, and the defect engineering method namely solvent-assisted linker exchange, is applied to create the defective Al sites. The defective Al sites play an important role in ensuring the N2 RR activity for defective Al-Fum. It is found that only the defective Al-Fum enables stable and effective electrochemical N2 RR, in terms of the highest production rate of 53.9µg(NH3 ) h-1 mgcat -1 (in 0.4m K2 SO4 ) and the Faradaic efficiency of 73.8% (in 0.1m K2 SO4 ) at -0.15V vs reversible hydrogen electrode) under ambient conditions. Density functional theory calculations confirm that the N2 activation can be achieved on the defective Al sites. Such sites also allow the subsequent protonation process via the alternating associative mechanism. This defect characteristic gives the main group Al-based MOFs the ability to serve as promising electrocatalysts for N2 RR and other attractive applications.

Effect of Solvent Assisted Linker Exchange (SALE) and De Novo Synthetic Routes on CO2 Uptake and Fixation by Mixed-Linker Zeolitic Imidazolate Frameworks

Direct mixing (de novo) and Solvent Assisted Ligand Exchange (SALE) are the main methods used for the synthesis of Mixed-Linker Zeolitic Imidazolate Frameworks (ML-ZIFs). ML-ZIFs with combined –NO2 and –Br/–Cl functionalities were prepared via both synthetic routes. Thereafter the CO2 uptake of the ML-ZIFs were compared, as well as their abilities to fixate CO2 with epoxide substrates. The de novo synthesis resulted in ML-ZIFs with SOD topologies, 60: 40 (–NO2: –Br/–Cl) functionality ratios, higher porosities, better thermal stability and higher CO2 uptake than equivalent SALE products. SALE resulted in smaller ML-ZIF crystallites, only ~ 10% incorporation of –Br/–Cl functionalized imidazolate linkers, and phase change during activation. ML-ZIF-7Cl, obtained from direct mixing, resulted in the highest CO2 uptake (90 cm3 g−1), in line with its higher porosity. ML-ZIF-7Cl, in combination with tetrabutylammonium bromide (TBAB), showed a high catalytic activity (TOF of 446 h−1) for the fixation of CO2 with propylene oxide and was reusable for up to 4 cycles without loss in activity.Graphical

Synthesis, characterization, and gas adsorption performance of an efficient hierarchical ZIF-11@ZIF-8 core–shell metal–organic framework (MOF)

In this paper, the solvent-assisted linker exchange (SALE) technique was successfully applied for the synthesis of porous ZIF-11@ZIF-8 core–shell composite structure metal–organic framework (MOF). The RHO-type ZIF-11 owing to larger cavities along with higher freedom in linker rotation, acts as the core and contributes in adsorption capacity enhancement. On the other hand, the SOD-type ZIF-8 provides greater molecular sieving features due to its lower degree of linker flexibility and thereby results in an improvement of the adsorption selectivity. The morphology and structure of the fabricated MOFs were characterized by XRD, FTIR, FESEM, EDS, TEM, BET, and TGA analyses and formation of the core–shell structure was confirmed. The BET surface area and micropore volume of the synthesized ZIF-11@ZIF-8 MOF were obtained as high as 1023.4 m2/g and 0.435 cm3 g−1, respectively. Gas adsorption measurements were carried out for CO2, N2, CH4, C2H6, and C2H4 gases at 298 and 328 K and equilibrium pressures up to 4 bar. The results revealed a remarkable rise (∼100 %) in CO2 adsorption capacity of ZIF-11@ZIF-8 nanoparticles (8.21 mmol g−1), compared to the pristine ZIF-11 (4.35 mmol g−1) at 298 K. The CO2/N2 and CO2/CH4 adsorption selectivity values were also augmented by 131 % and 92 %, respectively. In addition, the fabricated core–shell MOF, exhibited greater ethane (C2H6) adsorption capacity by ∼ 65 %, along with ethane to ethylene (C2H4) adsorption selectivity enhancement by ∼ 50 %. The higher adsorption capacity values without sacrificing adsorption selectivity suggests that the controlled core–shell MOF structures would be promising candidates for effective separation of CO2 from CH4 and N2 as well as C2H6 from C2H4.

Fast-Response Low Humidity Sensor Based on Ionic Liquid Functionalized MOF-801 by Solvent-Assisted Linker Exchange

In this work, strongly hydrophilic ionic liquid (IL) was introduced into the metal organic frameworks (MOFs) by the solvent-assisted linker exchange (SALE) method. Low humidity sensors based on IL functionalized MOF-801 materials were prepared. The as-prepared sensor shows a high response of 20.3 (in the humidity range of 0% RH to 35% RH, 0 ppmv to 8138 ppmv), small hysteresis (~1% RH) and good long-term stability (>100 days). In particular, the optimized sensor shows fast response (~400 ms) and high resolution (1% RH). This work provides a potential humidity sensitive material for preparing high performance low humidity sensors.

Mechanochemistry-assisted linker exchange of metal-organic framework for efficient kinetic separation of propene and propane

Solvent-assisted linker exchange (SALE) is a powerful tool to increase the structural complexity of metal–organic frameworks (MOF) and expand their functionality, while amounts of organic solvent and energy-intensive heating are needed in this process. In this work, we propose a solvent-free and environment-friendly mechanochemistry-assisted linker exchange strategy (MALE) for MOF post-synthetic modification in a green and sustainable manner at room temperature, which is demonstrated by the efficient linker exchange in ZIF-8 (ZIF = Zeolitic imidazolate frameworks) with imidazole-2-formaldehyde (Im-CHO) ligand. Importantly, contrast experiments show that the linker exchange reaction rate of the MALE method is much faster and more efficient than the conventional SALE method. In addition, the versatility of the MALE method is certified by successfully extending to robust UiO-66 (UiO = University of Oslo) and other functional ZIF-8 analogues, including ZIF-8/Br, ZIF-8/CF3, ZIF-8/CH2OH, ZIF-8/Cl, and ZIF-8/Et. Besides, the obtained ZIF-8/CF3 shows a boosting kinetic separation performance for C3H6/C3H8 gas mixtures compared with pristine ZIF-8. This work not only proposes a green, efficient, and solvent-free linker exchange method of MOF, but also develops a potential adsorbent for efficient kinetic separation of C3H6/C3H8 mixtures.

Converting a Non-Porous Rare-Earth Metal-Organic Framework into a Porous Yttrium-Based NH2UiO-66 Network via a Linker Exchange Approach.

The solvent-assisted linker exchange (SALE) method was used to produce amino-functionalized yttrium-based UiO-66 [NH2UiO-66(Y)], which is not obtainable via a direct synthetic method. Remarkably, SALE not only produced relatively highly porous NH2UiO-66(Y) from completely non-porous 3,3-bpdc-Y but also changed the network topology from 8-connected bcu in 3,3-bpdc-Y to 12-connected fcu in NH2UiO-66(Y). Based on our knowledge, this is one of the rare cases where SALE changes the whole network topology of the resulting metal-organic framework. NH2UiO-66(Y) also showed promising ability for selective detection of Cu2+ at a low concentration.

Synthesis of benzotriazole functionalized ZIF-8 by postsynthetic modification for enhanced CH4 and CO2 uptakes

In this work, a series of functionalized ZIF-8 were synthesized via incorporation of benzotriazole ligands into the framework with a post-synthetic method. The crystal structure and porosity were preserved for all functionalized samples. Although a relatively low percentage of ligand exchange (approximately 10–22%) was observed due to steric and kinetic effects, a remarkable improvement was found in CO2 and CH4 uptake capacities due to the incorporation of more polar N sites into the structure and the change in pore size of the frameworks. The resulting ZIF-8-S5 exhibited 45.17(CO2) and 15.08 (CH4) cm3 g−1 at 273 K under 1.2 bar, which corresponds to an enhancement of 20 and 35% compared to pristine ZIF-8. Further, all functionalized samples showed the significant improvement of selective CO2 over N2.

Exchange of coordinated carboxylates with azolates as a route to obtain a microporous zinc-azolate framework.

Metal-organic frameworks (MOFs) containing open metal sites are advantageous for wide applications. Here, carboxylate linkers are replaced with triazolate coordination in pre-formed Zn-MOF-74 via solvent-assisted linker exchange (SALE) to prepare the novel NU-250, within the known hexagonal channel-based MAF-X25 series that has not previously been synthesized de novo.

Substituent Controlled Framework Transformation Based on Solvent-Assisted Linker Exchange

The ligand exchange technique is a common method for postsynthesis isoreticular structures of metal–organic frameworks (MOFs). Here, an interesting framework transformation after linker exchange wa...

Synthesis and environmental applications of defective metal organic frameworks

Metal organic frameworks (MOFs) are attracting increasing attention in the field of pollution control due to their inherent characteristics, such as high porosity, large specific surface areas, and tunable structure and composition. Though more than 20000 kinds of MOFs structures have been reported so far, the majority of them are regular and composed of different kinds of building units. Defect engineering in MOFs is an exciting concept for tailoring material properties, which opens up novel opportunities in practical application. Defects are usually present in MOFs, which would alter the performance of MOFs by changing surface properties, pore structure, and the number of active sites. Since defects can enhance the performance of MOFs in adsorption and catalytic degradation of pollutants, various methods for designing and generating defects have been employed. Based on selected reports spanning the last decades, this review focuses on the most recent and significant developments in the classification of defects in MOFs and methods of introducing defects into MOFs. According to the composition of MOFs, defects can be divided into missing-linkers defect and missing-clusters defect. The main methods of introducing defects into MOFs include de novo synthesis and post-synthetic treatment. De novo synthesis method includes mixed linker approach, modulation approach and fast crystal growth approach. Post-synthetic treatment method involves acid/base treatment approach, harsh activation approach, solvent-assisted linker exchange and mechanical treatment approach. The use of defective MOFs as adsorbent and catalyst in pollutant removal is also highlighted. Because single physical-chemical characterization has its limitations, researchers adopt various techniques to characterize the existence and concentration of defects, such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), etc. Reasonable defects could be controlled by adjusting the synthesis conditions in an attempt to obtain MOFs with excellent performance in adsorption and catalysis. At the hand of adsorption, the adsorption of pollutants in water and gases onto defective MOFs is mainly affected by pore distribution, specific surface area and surface charge. The introduction of missing-linkers defect or missing-clusters defect would make the regular pore structure into locally disordered structure, and create new micropores, mesopores even macropores. Thereby, the pore volume and specific surface area of MOFs are increased, and then enhancing the adsorption performance. In view of photocatalysis, introducing defects into photo-responsive MOFs can not only promote the transfer of photo-generated electrons, but also inhibit electron-hole recombination, and even optimize the energy band structure, which synergistically results in the improvement of photocatalytic performance. From the perspective of chemical catalysis, the pores formed by missing-linkers defect or missing-clusters defect in MOFs would expose the interior surface, which creates additional catalytic sites for pollutants reaching, and then enhances the catalysis performance of MOFs. In short, defective MOFs have a wide range of applications in the fields of adsorption and catalysis. After summarizing the recent progress of the field, the advantages and shortcomings of present defective MOFs in pollution control are presented. On the basis of this, it is apparent that further deepening the basic theoretical study of defective MOFs will be helpful for its wide applications. The defect engineering of MOFs is beneficial for further industrialization. Finally, some potential research topics in construction of defects in MOFs for environmental application have also been proposed in the review.

Effect of Linker Distribution in the Photocatalytic Activity of Multivariate Mesoporous Crystals.

The use of Metal-Organic Frameworks as crystalline matrices for the synthesis of multiple component or multivariate solids by the combination of different linkers into a single material has emerged as a versatile route to tailor the properties of single-component phases or even access new functions. This approach is particularly relevant for Zr6-MOFs due to the synthetic flexibility of this inorganic node. However, the majority of materials are isolated as polycrystalline solids, which are not ideal to decipher the spatial arrangement of parent and exchanged linkers for the formation of homogeneous structures or heterogeneous domains across the solid. Here we use high-throughput methodologies to optimize the synthesis of single crystals of UiO-68 and UiO-68-TZDC, a photoactive analogue based on a tetrazine dicarboxylic derivative. The analysis of the single linker phases reveals the necessity of combining both linkers to produce multivariate frameworks that combine efficient light sensitization, chemical stability, and porosity, all relevant to photocatalysis. We use solvent-assisted linker exchange reactions to produce a family of UiO-68-TZDC% binary frameworks, which respect the integrity and morphology of the original crystals. Our results suggest that the concentration of TZDC in solution and the reaction time control the distribution of this linker in the sibling crystals for a uniform mixture or the formation of core-shell domains. We also demonstrate how the possibility of generating an asymmetric distribution of both linkers has a negligible effect on the electronic structure and optical band gap of the solids but controls their performance for drastic changes in the photocatalytic activity toward proton or methyl viologen reduction.

Controllable Strategy for Metal–Organic Framework Light-Driven [2 + 2] Cycloaddition Reactions via Solvent-Assisted Linker Exchange

Flexible olefinic trans-1,2-bis(4-pyridyl)ethene linkers were postsynthetically introduced into the metal-organic frameworks (MOFs) containing parallel rigid 4,4'-bipyridine linkers with a spacing of less than 4.2 Å by the linker exchange strategy, and then, the MOF satisfied Schmidt criteria could be obtained. Eventually, MOF products connected by cyclobutane derivatives were formed by the photochemical [2 + 2] cycloaddition reaction under UV irradiation.

Coupling the Individual Motions of the Machine-like Components of Zirconium(IV) Organic Frameworks

Controlling the internal molecular dynamics in reticular materials is a major challenge for the development of nanotechnological applications. In this issue of Chem, Schurko, Loeb, and co-workers show how the ring motion of the [2]rotaxane-based linker and that of the surrounding triphenylene-based struts influence each other within a series of robust zirconium(IV) organic frameworks.

Guest-Responsive Reversible Electron Transfer in a Crystalline Porous Framework Supported by a Dynamic Building Node.

We demonstrate a redox-active, crystalline donor-acceptor (D-A) assembly in which the electron transfer (ET) process can be reversibly switched. This ET process, induced by a guest-responsive structural transformation at room temperature, is realized in a porous, metal-organic framework (MOF), having anthracene (D)-naphthalenediimide (A) as struts. A control MOF structure obtained by a solvent-assisted linker exchange (SALE) method, replacing an acceptor strut with a neutral one, supported the switchable electronic states in the D-A MOF. Combined investigations with X-ray diffraction, spectroscopy, and theoretical analyses revealed the dynamic metal paddle-wheel node as a critical unit for controlling structural flexibility and the corresponding unprecedented ET process.

Guest‐Responsive Reversible Electron Transfer in a Crystalline Porous Framework Supported by a Dynamic Building Node

AbstractWe demonstrate a redox‐active, crystalline donor–acceptor (D‐A) assembly in which the electron transfer (ET) process can be reversibly switched. This ET process, induced by a guest‐responsive structural transformation at room temperature, is realized in a porous, metal–organic framework (MOF), having anthracene (D)–naphthalenediimide (A) as struts. A control MOF structure obtained by a solvent‐assisted linker exchange (SALE) method, replacing an acceptor strut with a neutral one, supported the switchable electronic states in the D‐A MOF. Combined investigations with X‐ray diffraction, spectroscopy, and theoretical analyses revealed the dynamic metal paddle‐wheel node as a critical unit for controlling structural flexibility and the corresponding unprecedented ET process.

Visible-Light-Driven Rotation of Molecular Motors in a Dual-Function Metal-Organic Framework Enabled by Energy Transfer.

The visible-light-driven rotation of an overcrowded alkene-based molecular motor strut in a dual-function metal–organic framework (MOF) is reported. Two types of functional linkers, a palladium–porphyrin photosensitizer and a bispyridine-derived molecular motor, were used to construct the framework capable of harvesting low-energy green light to power the rotary motion. The molecular motor was introduced in the framework using the postsynthetic solvent-assisted linker exchange (SALE) method, and the structure of the material was confirmed by powder (PXRD) and single-crystal X-ray (SC-XRD) diffraction. The large decrease in the phosphorescence lifetime and intensity of the porphyrin in the MOFs upon introduction of the molecular motor pillars confirms efficient triplet-to-triplet energy transfer between the porphyrin linkers and the molecular motor. Near-infrared Raman spectroscopy revealed that the visible light-driven rotation of the molecular motor proceeds in the solid state at rates similar to those observed in solution.

Effect of ZIF-71 ligand-exchange surface modification on biofuel recovery through pervaporation

In this paper, zeolitic imidazolate framework-71 (ZIF-71) particles (~500 nm diameter) were modified with four different ligands through the solvent-assisted linker exchange (SALE) method. The objective of this was to assess the effect of ligand exchanged ZIF-71 on the performance of ZIF-71/polydimethylsiloxane (PDMS) mixed matrix membranes (MMMs) for removal of alcohol from dilute aqueous solutions. Both Fourier-transform infrared spectroscopy (ATR-FTIR) and proton nuclear magnetic resonance (1H NMR) spectroscopy confirmed the exchange of the new ligands into the ZIF-71 material. The 1H NMR spectra demonstrated the molar ratios of the ligand-exchange into the ZIF-71 particles. X-ray diffraction and scanning electron microscopy results showed that the crystal structure and the morphology of ZIF-71 were maintained after the ligand-exchange modification process. Modified ZIF-71/PDMS composite membranes were synthesized to study the effect of ligand modification on alcohol/water separation performance. Ligand-modified ZIFs dispersed more evenly in the condensation cure PDMS polymer matrix than the unmodified ZIFs. The alcohol permeability and alcohol/water selectivity for ethanol and 1-butanol remained the same or decreased for membranes made with ligand-exchanged ZIF-71 particles.

Rational synthesis of interpenetrated 3D covalent organic frameworks for asymmetric photocatalysis.

Covalent organic frameworks (COFs) show great promise as heterogeneous photocatalysts, but they have not yet been explored for asymmetric photocatalysis, which is important for the sustainable production of pharmaceuticals and fine chemicals. We report here a pair of twofold interpenetrated 3D COFs adopting a rare (3,4)-connected ffc topology for photocatalytic asymmetric reactions by imine condensation of rectangular and trigonal building blocks. Both COFs containing a photoredox triphenylamine moiety are efficient photocatalysts for the cross-dehydrogenative coupling reactions and asymmetric α-alkylation of aldehydes integrated with a chiral imidazolidinone catalyst. Under visible-light irradiation, the targeted chiral products are produced in satisfactory yields with up to 94% enantiomeric excess, which are comparable to those of reported reactions using molecular metal complexes or organic dyes as photosensitizers. Whereas the COFs became amorphous after catalysis, they can be recrystallized through solvent-assisted linker exchange and reused without performance loss. This is the first report utilizing COFs as photocatalysts to promote enantioselective photochemical reactions.

Solvent-assisted linker exchange as a tool for the design of mixed-linker MIL-140D structured MOFs for highly selective detection of gaseous H2S.

A MIL-140D-sdc framework has been used as a highly stable backbone for the introduction of 4,4′-azobenzene dicarboxylic acid (H2abdc) via solvent-assisted ligand exchange (SALE). The implemented azo groups can serve as coordination sites for copper ions. These can exchange ligands with different gases, but show a high selectivity against H2S, which makes this material promising for potential sensor applications.