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 m² 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.     

Enhancing CO2 Separation Ability of a Metal–Organic Framework by Post‐Synthetic Ligand Exchange with Flexible Aliphatic Carboxylates

A series of porous metal–organic frameworks having flexible carboxylic acid pendants in their pores (UiO‐66‐ADn: n=4, 6, 8, and 10, where n denotes the number of carbons in a pendant) has been synthesized by post‐synthetic ligand exchange of terephthalate in UiO‐66 with a series of alkanedioic acids (HO₂C(CH₂)_n?2CO₂H). NMR, IR, PXRD, TEM, and mass spectral data have suggested that a terephthalate linker in UiO‐66 was substituted by two alkanedioate moieties, resulting in free carboxyl pendants in the pores. When post‐synthetically modified UiO‐66 was partially digested by adjusting the amount of added HF/sample, NMR spectra indicated that the ratio of alkanedioic acid/terephthalic acid was increased with smaller amounts of acid, implying that the ligand substitution proceeded from the outer layer of the particles. Gas sorption studies indicated that the surface areas and the pore volumes of all UiO‐66‐ADns were decreased compared to those of UiO‐66, and that the CO₂ adsorption capacities of UiO‐66‐ADn (n=4, 8) were similar to that of UiO‐66. In the case of UiO‐66‐AD6, the CO₂ uptake capacity was 34 % higher at 298 K and 58 % higher at 323 K compared to those of UiO‐66. It was elucidated by thermodynamic calculations that the introduction of flexible carboxyl pendants of appropriate length has two effects: 1) it increases the interaction enthalpy between the host framework and CO₂ molecules, and 2) it mitigates the entropy loss upon CO₂ adsorption due to the formation of multiple configurations for the interactions between carboxyl groups and CO₂ molecules. The ideal adsorption solution theory (IAST) selectivity for CO₂ adsorption over that of CH₄ was enhanced for all of the UiO‐66‐ADns compared to that of UiO‐66 at 298 K. In particular, UiO‐66‐AD6 showed the most strongly enhanced CO₂ uptake capacity and significantly increased selectivity for CO₂ adsorption over that of CH₄ at ambient temperature, suggesting that it is a promising material for sequestering CO₂ from landfill gas.     

Metal–Organic Gel Material Based on UiO‐66‐NH2 Nanoparticles for Improved Adsorption and Conversion of Carbon Dioxide

Metal–organic frameworks (MOFs) including the UiO‐66 series show potential application in the adsorption and conversion of CO₂. Herein, we report the first tetravalent metal‐based metal–organic gels constructed from Zr^IV and 2‐aminoterephthalic acid (H₂BDC‐NH₂). The ZrBDC‐NH₂ gel materials are based on UiO‐66‐NH₂ nanoparticles and were easily prepared under mild conditions (80 °C for 4.5 h). The ZrBDC‐NH₂‐1:1‐0.2 gel material has a high surface area (up to 1040 m² g^?1) and showed outstanding performance in CO₂ adsorption (by using the dried material) and conversion (by using the wet gel) arising from the combined advantages of the gel and the UiO‐66‐NH₂ MOF. The ZrBDC‐NH₂‐1:1‐0.2 dried material showed 38 % higher capture capacity for CO₂ at 298 K than microcrystalline UiO‐66‐NH₂. It showed high ideal adsorbed solution theory selectivity (71.6 at 298 K) for a CO₂/N₂ gas mixture (molar ratio 15:85). Furthermore, the ZrBDC‐NH₂‐1:1‐0.2 gel showed activity as a heterogeneous catalyst in the chemical fixation of CO₂ and an excellent catalytic performance was achieved for the cycloaddition of atmospheric pressure of CO₂ to epoxides at 373 K. In addition, the gel catalyst could be reused over multiple cycles with no considerable loss of catalytic activity.     

Cooperative Multifunctional Catalysts for Nitrone Synthesis: Platinum Nanoclusters in Amine‐Functionalized Metal–Organic Frameworks

Nitrones are key intermediates in organic synthesis and the pharmaceutical industry. The heterogeneous synthesis of nitrones with multifunctional catalysts is extremely attractive but rarely explored. Herein, we report ultrasmall platinum nanoclusters (PtNCs) encapsulated in amine‐functionalized Zr metal–organic framework (MOF), UiO‐66‐NH₂ (Pt@UiO‐66‐NH₂) as a multifunctional catalyst in the one‐pot tandem synthesis of nitrones. By virtue of the cooperative interplay among the selective hydrogenation activity provided by the ultrasmall PtNCs and Lewis acidity/basicity/nanoconfinement endowed by UiO‐66‐NH₂, Pt@UiO‐66‐NH₂ exhibits remarkable activity and selectivity, in comparison to Pt/carbon, Pt@UiO‐66, and Pd@UiO‐66‐NH₂. Pt@UiO‐66‐NH₂ also outperforms Pt nanoparticles supported on the external surface of the same MOF (Pt/UiO‐66‐NH₂). To our knowledge, this work demonstrates the first examples of one‐pot synthesis of nitrones using recyclable multifunctional heterogeneous catalysts.     

Turning Redundant Ligands into Treasure: A New Strategy for Constructing MIL‐53(Al)@Nanoscale TiO2 Layers

A strategy for in situ fabrication of nanoscale‐thin layers of anatase TiO₂ coated on the metal–organic framework (MOF) material, MIL‐53(Al), is developed. The preparation conditions for crystallized TiO₂ are normally incompatible with the thermal and chemical stability of MOFs. Based on our strategy, we found that the redundant organic ligands (1,4‐benzenedicarboxylic acid, H₂BDC) within the pores of the as‐synthesized MOF play a key function in the protection and support of the framework during hydrothermal loading of the TiO₂ precursor, as well as in preventing the infiltration of the precursor into the pores. After annealing, a nanoscale‐thin layer of highly crystalline anatase TiO₂, with a thickness of 6–10 nm, was successfully attached to the external surface of the MIL‐53(Al) crystals, while the porous framework remains intact. The core–shell structure of the MOF@TiO₂ nanocomposite endows the resulting materials with additional optical response and enhanced moisture and chemical stability.     

Post‐Synthetic Ligand Exchange in Zirconium‐Based Metal–Organic Frameworks: Beware of The Defects!

Post‐synthetic ligand exchange in the prototypical zirconium‐based metal–organic framework (MOF) UiO‐66 was investigated by in situ solution ¹H NMR spectroscopy. Samples of UiO‐66 having different degrees of defectivity were exchanged using solutions of several terephthalic acid analogues in a range of conditions. Linker exchange only occurred in defect‐free UiO‐66, whereas monocarboxylates grafted at defect sites were found to be preferentially exchanged with respect to terephthalic acid over the whole range of conditions investigated. A 1:1 exchange ratio between the terephthalic acid analogue and modulator was observed, providing evidence that the defects had missing‐cluster nature. Ex situ characterisation of the MOF powders after exchange corroborated these findings and showed that the physical‐chemical properties of the MOF depend on whether the functionalisation occurs at defective sites or on the framework.     

Boosting Photocatalytic Hydrogen Production of a Metal–Organic Framework Decorated with Platinum Nanoparticles: The Platinum Location Matters

Improving the efficiency of electron–hole separation and charge‐carrier utilization plays a central role in photocatalysis. Herein, Pt nanoparticles of ca. 3 nm are incorporated inside or supported on a representative metal–organic framework (MOF), UiO‐66‐NH₂, denoted as Pt@UiO‐66‐NH₂ and Pt/UiO‐66‐NH₂, respectively, for photocatalytic hydrogen production via water splitting. Compared with the pristine MOF, both Pt‐decorated MOF nanocomposites exhibit significantly improved yet distinctly different hydrogen‐production activities, highlighting that the photocatalytic efficiency strongly correlates with the Pt location relative to the MOF. The Pt@UiO‐66‐NH₂ greatly shortens the electron‐transport distance, which favors the electron–hole separation and thereby yields much higher efficiency than Pt/UiO‐66‐NH₂. The involved mechanism has been further unveiled by means of ultrafast transient absorption and photoluminescence spectroscopy.     

Direct synthesis of Fe(III) immobilized Zr‐based metal–organic framework for aerobic oxidation reaction

A Zr‐based metal–organic framework with bipyridine units (UiO‐67) has been utilized for the immobilization of catalytically active iron species via a post‐synthetic metalation method. UiO‐67 bipyridine MOF was synthesized through a simple solvothermal method and was shown to have a UiO‐type structure. Post‐synthetic metalation of UiO‐67 MOF was performed for the immobilization of the catalytically active FeCl₃. FT‐IR and EDX element map suggested that FeCl₃ is coordinately bonded to the UiO‐67 bipyridine framework. The synthesized UiO‐67‐FeCl₃ catalyst was used for the aerobic oxidation of alcohols and benzylic compounds in the presence of molecular oxygen. In addition, the UiO‐67‐FeCl₃ catalyst can be reused as a solid heterogeneous catalyst without compromising its activity and selectivity.     

Defect‐Controlled Preparation of UiO‐66 Metal–Organic Framework Thin Films with Molecular Sieving Capability

Metal–organic framework (MOF) UiO‐66 thin films are solvothermally grown on conducting substrates. The as‐synthesized MOF thin films are subsequently dried by a supercritical process or treated with polydimethylsiloxane (PDMS). The obtained UiO‐66 thin films show excellent molecular sieving capability as confirmed by the electrochemical studies for redox‐active species with different sizes.     

Phosphonate‐Modified UiO‐66 Brønsted Acid Catalyst and Its Use in Dehydra‐Decyclization of 2‐Methyltetrahydrofuran to Pentadienes

Phosphorus‐modified all‐silica zeolites exhibit activity and selectivity in certain Brønsted acid catalyzed reactions for biomass conversion. In an effort to achieve similar performance with catalysts having well‐defined sites, we report the incorporation of Brønsted acidity to metal–organic frameworks with the UiO‐66 topology, achieved by attaching phosphonic acid to the 1,4‐benzenedicarboxylate ligand and using it to form UiO‐66‐PO₃H₂ by post‐synthesis modification. Characterization reveals that UiO‐66‐PO₃H₂ retains stability similar to UiO‐66, and exhibits weak Brønsted acidity, as demonstrated by titrations, alcohol dehydration, and dehydra‐decyclization of 2‐methyltetrahydrofuran (2‐MTHF). For the later reaction, the reported catalyst exhibits site‐time yields and selectivity approaching that of phosphoric acid on all‐silica zeolites. Using solid‐state NMR and deprotonation energy calculations, the chemical environments of P and the corresponding acidities are determined.     

Spray‐Coating of Catalytically Active MOF–Polythiourea through Postsynthetic Polymerization

A UiO‐66‐NCS MOF was formed by postsynthetic modification of UiO‐66‐NH₂. The UiO‐66‐NCS MOFs displays a circa 20‐fold increase in activity against the chemical warfare agent simulant dimethyl‐4‐nitrophenyl phosphate (DMNP) compared to UiO‐66‐NH₂, making it the most active MOF materials using a validated high‐throughput screening. The ?NCS functional groups provide reactive handles for postsynthetic polymerization of the MOFs into functional materials. These MOFs can be tethered to amine‐terminated polypropylene polymers (Jeffamines) through a facile room‐temperature synthesis with no byproducts. The MOFs are then crosslinked into a MOF–polythiourea (MOF–PTU) composite material, maintaining the catalytic properties of the MOF and the flexibility of the polymer. This MOF–PTU hybrid material was spray‐coated onto Nyco textile fibers, displaying excellent adhesion to the fiber surface. The spray‐coated fibers were screened for the degradation of DMNP and showed durable catalytic reactivity.     

Boosting the Catalytic Performance of Metal–Organic Frameworks for Steroid Transformations by Confinement within a Mesoporous Scaffold

Solid‐state crystallization achieves selective confinement of metal–organic framework (MOF) nanocrystals within mesoporous materials, thereby rendering active sites more accessible compared to the bulk‐MOF and enhancing the chemical and mechanical stability of MOF nanocrystals. (Zr)UiO‐66(NH₂)/SiO₂ hybrid materials were tested as efficient and reusable heterogeneous catalysts for the synthesis of steroid derivatives, outperforming the bulk (Zr)UiO‐66(NH₂) MOF. A clear correlation between the catalytic activity of the dispersed Zr sites present in the confined MOF, and the loading of the mesoporous SiO₂, is demonstrated for steroid transformations.     

An Evaluation of UiO‐66 for Gas‐Based Applications

In addition to its high thermal stability, repetitive hydration/dehydration tests have revealed that the porous zirconium terephthalate UiO‐66 switches reversibly between its dehydroxylated and hydroxylated versions. The structure of its dehydroxylated form has thus been elucidated by coupling molecular simulations and X‐ray powder diffraction data. Infrared measurements have shown that relatively weak acid sites are available while microcalorimetry combined with Monte Carlo simulations emphasize moderate interactions between the UiO‐66 surface and a wide range of guest molecules including CH₄, CO, and CO₂. These properties, in conjunction with its significant adsorption capacity, make UiO‐66 of interest for its further evaluation for CO₂ recovery in industrial applications. This global approach suggests a strategy for the evaluation of metal–organic frameworks for gas‐based applications.     

Molybdenum (VI)‐functionalized UiO‐66 provides an efficient heterogeneous nanocatalyst in oxidation reactions

A novel heterogeneous nanocatalyst was established by supporting molybdenum (VI) on Zr₆ nodes in the structure of the well‐known UiO‐66 metal–organic framework (MOF). The structure of the UiO‐66 before and after Mo (VI) immobilization was confirmed with XRD, DR‐FTIR and UV–vis spectroscopy, and the presence and amount of Mo (VI) was identified by X‐ray photoelectron spectroscopy and inductively coupled plasma atomic emission spectroscopy. TEM imaging confirmed the absence of Mo clusters on the MOF surface, while SEM confirmed that the appearance of the MOF has not changed upon immobilizing the Mo (VI) catalyst. BET adsorption measurements were used to confirm the porosity of the catalyst. The catalytic activity of this heterogeneous catalyst was investigated in oxidation of sulfides with H₂O₂ in acetonitrile and oxidative desulfurization of dibenzothiophene. Easy work up, convenient and steady reuse and high activity and selectivity are prominent properties of this new hybrid material.     

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

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

Imparting Multifunctionality by Utilizing Biporosity in a Zirconium‐Based Metal–Organic Framework

In this work, we have synthesized nanocomposites made up of a metal–organic framework (MOF) and conducting polymers by polymerization of specialty monomers such as pyrrole (Py) and 3,4‐ethylenedioxythiophene (EDOT) in the voids of a stable and biporous Zr‐based MOF ( UiO‐66 ). FTIR and Raman data confirmed the presence of polypyrrole ( PPy ) and poly3,4‐ethylenedioxythiophene ( PEDOT ) in UiO‐66‐PPy and UiO‐66‐PEDOT nanocomposites, respectively, and PXRD data revealed successful retention of the structure of the MOF. HRTEM images showed successful incorporation of polymer fibers inside the voids of the framework. Owing to the intrinsic biporosity of UiO‐66 , polymer chains were observed to selectively occupy only one of the voids. This resulted in a remarkable enhancement (million‐fold) of the electrical conductivity while the nanocomposites retain 60–70 % of the porosity of the original MOF. These semiconducting yet significantly porous MOF nanocomposite systems exhibited ultralow thermal conductivity. Enhanced electrical conductivity with lowered thermal conductivity could qualify such MOF nanocomposites for thermoelectric applications.     

Hierarchical Pore Development by Plasma Etching of Zr‐Based Metal–Organic Frameworks

The typically stable Zr‐based metal–organic frameworks (MOFs) UiO‐66 and UiO‐66‐NH₂ were treated with tetrafluoromethane (CF₄) and hexafluoroethane (C₂F₆) plasmas. Through interactions between fluoride radicals from the perfluoroalkane plasma and the zirconium–oxygen bonds of the MOF, the resulting materials showed the development of mesoporosity, creating a hierarchical pore structure. It is anticipated that this strategy can be used as a post‐synthetic technique for developing hierarchical networks in a variety of MOFs.     

Ultra‐Fast Degradation of Chemical Warfare Agents Using MOF–Nanofiber Kebabs

The threat associated with chemical warfare agents (CWAs) motivates the development of new materials to provide enhanced protection with a reduced burden. Metal–organic frame‐works (MOFs) have recently been shown as highly effective catalysts for detoxifying CWAs, but challenges still remain for integrating MOFs into functional filter media and/or protective garments. Herein, we report a series of MOF–nanofiber kebab structures for fast degradation of CWAs. We found TiO₂ coatings deposited via atomic layer deposition (ALD) onto polyamide‐6 nanofibers enable the formation of conformal Zr‐based MOF thin films including UiO‐66, UiO‐66‐NH₂, and UiO‐67. Cross‐sectional TEM images show that these MOF crystals nucleate and grow directly on and around the nanofibers, with strong attachment to the substrates. These MOF‐functionalized nanofibers exhibit excellent reactivity for detoxifying CWAs. The half‐lives of a CWA simulant compound and nerve agent soman (GD) are as short as 7.3 min and 2.3 min, respectively. These results therefore provide the earliest report of MOF–nanofiber textile composites capable of ultra‐fast degradation of CWAs.     

Monodentate AIEgen Anchored on Metal‐Organic Framework for Fast Fluorescence Sensing of Phosphate

Herein, a novel sensor (TPE‐UiO‐66) was designed via anchoring monodentate tetraphenylethylene (TPE) onto UiO‐66 framework. The combination of the distinct aggregation‐induced emission (AIE) of TPE and the easy replacement of monodentate linker by guest phosphate, makes TPE‐UiO‐66 an ideal platform for sensing HPO₄^2–. Experimental results indicate that TPE‐UiO‐66 can selectively sense HPO₄^2– from other common anions. The limit of detection (LOD) can reach to 5.56 μmol·L^–1 and more importantly, TPE‐UiO‐66 also exhibits an ultra‐fast equilibrium response of 2 min, far faster than those of other sensors especially for UiO‐66‐NH₂. The combination of experimental analysis and density functional theory (DFT) calculations demonstrates that the high selectivity, high sensitivity and fast response of HPO₄^2– detection by TPE‐UiO‐66 can be attributed to the stronger coordination interactions of HPO₄^2– with Zr‐O cluster of UiO‐66 than that of TPE molecule. This study not only provides a potential probe for phosphate, but also represents a novel strategy to design stimuli‐responsive fluorescent MOF‐based sensors via using monodentate AIEgens.     

Studies on Photocatalytic CO2 Reduction over NH2‐Uio‐66(Zr) and Its Derivatives: Towards a Better Understanding of Photocatalysis on Metal–Organic Frameworks

Metal–organic framework (MOF) NH₂‐Uio‐66(Zr) exhibits photocatalytic activity for CO₂ reduction in the presence of triethanolamine as sacrificial agent under visible‐light irradiation. Photoinduced electron transfer from the excited 2‐aminoterephthalate (ATA) to Zr oxo clusters in NH₂‐Uio‐66(Zr) was for the first time revealed by photoluminescence studies. Generation of Zr^III and its involvement in photocatalytic CO₂ reduction was confirmed by ESR analysis. Moreover, NH₂‐Uio‐66(Zr) with mixed ATA and 2,5‐diaminoterephthalate (DTA) ligands was prepared and shown to exhibit higher performance for photocatalytic CO₂ reduction due to its enhanced light adsorption and increased adsorption of CO₂. This study provides a better understanding of photocatalytic CO₂ reduction over MOF‐based photocatalysts and also demonstrates the great potential of using MOFs as highly stable, molecularly tunable, and recyclable photocatalysts in CO₂ reduction.