Water Stable Zr-Benzenedicarboxylate Metal-Organic Frameworks as Photocatalysts for Hydrogen Generation

From UV to Near-Infrared Light-Responsive Metal-Organic Framework Composites: Plasmon and Upconversion Enhanced Photocatalysis

The exploitation of photocatalysts that harvest solar spectrum as broad as possible remains a high-priority target yet grand challenge. In this work, for the first time, metal-organic framework (MOF) composites are rationally fabricated to achieve broadband spectral response from UV to near-infrared (NIR) region. In the core-shell structured upconversion nanoparticles (UCNPs)-Pt@MOF/Au composites, the MOF is responsive to UV and a bit visible light, the plasmonic Au nanoparticles (NPs) accept visible light, whereas the UCNPs absorb NIR light to emit UV and visible light that are harvested by the MOF and Au once again. Moreover, the MOF not only facilitates the generation of "bare and clean" Au NPs on its surface and realizes the spatial separation for the Au and Pt NPs, but also provides necessary access for catalytic substrates/products to Pt active sites. As a result, the optimized composite exhibits excellent photocatalytic hydrogen production activity (280 µmol g^-1 h^-1 ) under simulated solar light, and the involved mechanism of photocatalytic H₂ production under UV, visible, and NIR irradiation is elucidated. Reportedly, this is an extremely rare study on photocatalytic H₂ production by light harvesting in all UV, visible, and NIR regions.

Fast and Sustained Degradation of Chemical Warfare Agent Simulants Using Flexible Self-Supported Metal-Organic Framework Filters

Self-detoxification filters against lethal chemical warfare agents (CWAs) are highly desirable for the protection of human beings and the environment. In this report, flexible self-supported filters of a series of Zr(IV)-based metal-organic frameworks (MOFs) including UiO-66, UiO-67, and UiO-66-NH₂ were successfully prepared and exhibited fast and sustained degradation of CWA simulants. A half-life as short as 2.4 min was obtained for the catalytic hydrolysis of dimethyl 4-nitrophenyl phosphate, and the percent conversion remained above 90% over a long-term exposure of 120 min, well exceeding those of the previously reported composite MOF filters and the corresponding MOF powders. The outstanding detoxification performance of the self-supported fibrous filter comes from the exceptionally high surface area, excellent pore accessibility, and hierarchical structure from the nano- to macroscale. This work demonstrates, for the first time, MOF-only filters as efficient self-detoxification media, which will offer new opportunities for the design and fabrication of functional materials for toxic chemical protection.

Constructing Cd0.5Zn0.5S@ZIF-8 nanocomposites through self-assembly strategy to enhance Cr(VI) photocatalytic reduction

A novel and highly efficient photocatalyst of Cd_0.5Zn_0.5S@ZIF-8 nanocomposite has been developed by a facile self-assembly strategy. This is the first report on the application of Cd_xZn_1-xS and metal-organic framework (MOF) nanocomposite as photocatalysts for the reduction of Cr(VI). The resulting Cd_0.5Zn_0.5S@ZIF-8 exhibited higher photocatalytic activity than that of pristine Cd_0.5Zn_0.5S and ZIF-8. Particularly, the CZS@Z60 composite with 60 wt% of ZIF-8 exhibited a photocatalytic activity that is about 1.6 times as high as that of Cd_0.5Zn_0.5S. The dominant reason for the improved photocatalytic reduction potential is proved to be the newly-formed interfacial SZn bonds that firmly connect Cd_0.5Zn_0.5S and ZIF-8 and substantially improve the separation efficiency of photo-excited electrons and holes. The newly-formed chemical bonds are confirmed by XPS analyses, and the prolonged lifetime of photo-excited electrons is evidenced by the electrochemical measurement of photocurrent, which shows that the photocurrent on Cd_0.5Zn_0.5S@ZIF-8 is much higher than that of Cd_0.5Zn_0.5S and ZIF-8. This study clearly demonstrates that the MOF-based composite nanomaterials hold great promises for applications in the field of environmental remediation and for design of novel photocatalytic materials.

In situ growth of Zr-based metal-organic framework UiO-66-NH2 for open-tubular capillary electrochromatography

The high stability and other properties of Zr(IV)-based metal organic frameworks(MOFs) make it a promising choice for chromatographic separation, while the application in open-tubular capillary electrochromatography (OT-CEC) separation has not been explored yet. Herein, we report the first example of the in-situ growth of UiO-66-NH₂ onto the capillary for open-tubular capillary electrochromatography. UiO-66-NH₂ consists of ZrCl₄ and 2-amino-1,4-benzenedicarboxylic acid, which is highly porous and stable in a variety of solvents. The prepared UiO-66-NH₂ modified capillary was characterized by scanning electron microscopy (SEM) and Fourier transform infrared spectra (FT-IR), and the results confirmed the successful growth of the UiO-66-NH₂ . The baseline separation of chlorobenzenes, phenoxyacids and two groups of phenols was achieved owing to the combined interaction of π-π interaction, hydrophobic interaction, molecular sieve effect, electrophoretic migration and hydrogen-bonding interaction etc. Besides, the prepared capillaries showed good reproducibility, with relative standard deviations (RSDs) for intra-day, inter-day and column-to-column runs in the range of 1.38-2.60%, 3.39-4.05%, and 3.47-5.03%, respectively. Our work indicates Zr(IV)-based MOFs are promising materials as stationary phase in CEC separation.

The development of MOFs-based nanomaterials in heterogeneous organocatalysis

Metal-organic framework (MOF) is a class of inorganic-organic hybrid material assembled periodically with metal ions and organic ligands. MOFs have always been the focuses in a variety of frontier fields owing to the advantageous properties, such as large BET surface areas, tunable porosity and easy-functionalized surface structure. Among the various application areas, catalysis is one of the earliest application fields of MOFs-based materials and is one of the fastest-growing topics. In this review, the main roles of MOFs in heterogeneous organocatalysis have been systematically summarized, including used as support materials (or hosts), independent catalysts, and sacrificial templates. Moreover, the application prospects of MOFs in photocatalysis and electrocatalysis frontiers were also mentioned. Finally, the key issues that should be conquered in future were briefly sketched in the final parts of each item. We hope our perspectives could be beneficial for the readers to better understand these topics and issues, and could also provide a direction for the future exploration of some novel types of MOFs-based nanocatalysts with stable structures and functions for heterogeneous catalysis.

Stable Metal-Organic Frameworks with Group 4 Metals: Current Status and Trends

Group 4 metal-based metal-organic frameworks (M^IV-MOFs), including Ti-, Zr-, and Hf-based MOFs, are one of the most attractive classes of MOF materials owing to their superior chemical stability and structural tunability. Despite being a relatively new field, M^IV-MOFs have attracted significant research attention in the past few years, leading to exciting advances in syntheses and applications. In this outlook, we start with a brief overview of the history and current status of M^IV-MOFs, emphasizing the challenges encountered in their syntheses. The unique properties of M^IV-MOFs are discussed, including their high chemical stability and strong tolerance toward defects. Particular emphasis is placed on defect engineering in Zr-MOFs which offers additional routes to tailor their functions. Photocatalysis of M^IV-MOF is introduced as a representative example of their emerging applications. Finally, we conclude with the perspective of new opportunities in synthesis and defect engineering.

Electron Injection from Photoexcited Metal-Organic Framework Ligands to Ru2 Secondary Building Units for Visible-Light-Driven Hydrogen Evolution

We report the design of two new metal-organic frameworks (MOFs), Ru-TBP and Ru-TBP-Zn, based on Ru₂ secondary building units (SBUs) and porphyrin-derived tetracarboxylate ligands. The proximity of Ru₂ SBUs to porphyrin ligands (∼1.1 nm) facilitates multielectron transfer from excited porphyrins to Ru₂ SBUs to enable efficient visible-light-driven hydrogen evolution reaction (HER) in neutral water. Photophysical and electrochemical studies revealed oxidative quenching of excited porphyrin by Ru₂ SBUs as the initial step of the HER process and the energetics of key intermediates in the catalytic cycle. Our work provides a new strategy to building multifunctional MOFs with synergistic ligands and SBUs for efficient photocatalysis.

Functionalization of Metal-Organic Frameworks for Photoactive Materials

Metal-organic frameworks (MOFs) are intriguing platforms with multiple functionalities. Additional functionalization of MOFs generates novel materials for various applications. Here, three main topics are examined regarding the functionalization of MOFs for use as photoactive materials. The first is chemical approaches for postsynthetic modification of the metal clusters and organic linkers in MOFs; that is, sites on pore surfaces and chemical trapping of photoactive moieties within the pores, which create materials with chemical functionalities for water splitting and CO₂ reduction by light. The second topic focuses on the functionalization of MOFs for photochemical response and the versatile applications of such materials. State-of-the-art research on functionalizing MOFs through photochemical reactions on the pore surface and within the pores as guests is also summarized. The third topic introduces the functionalization of MOFs for photofunctional materials, including photoluminescent tuning and integration, photoluminescent LED devices and barcodes, and photophysical applications for chemical sensing. Finally, conclusions and perspectives on the fields are given.

Ligand modification of UiO-66 with an unusual visible light photocatalytic behavior for RhB degradation

A series of isostructural UiO-66-X (X = H, NH₂, Br, (OH)₂, (SH)₂) catalysts have been successfully synthesized by modifying different functional groups on the ligand. The effects of the ligand modification of UiO-66 were investigated for their photocatalytic activity of Rhodamine B degradation under visible light. Surprisingly, UiO-66-NH₂ and UiO-66-(OH)₂ which have narrow bandgaps and excellent visible light absorption do not show outstanding photocatalytic performances compared to UiO-66 and UiO-66-Br. Electrochemical test results indicated that the conduction band potential of UiO-66-X and the separation efficiency of electrons were quite important in these photocatalytic reactions, other than the electronic effect as reported. Similar photocatalytic degradation behaviors were found for Congo red and methyl orange. Herein, we firstly reported different mechanisms of selective degradation in the case of UiO-66, which subverted the previous understanding of photodegradation behavior.

A Strategy to Boost H2 Generation Ability of Metal-Organic Frameworks: Inside-Outside Decoration for the Separation of Electrons and Holes

Inhibiting the recombination of electron and holes plays an essential role in photocatalytic process, particularly for metal-organic frameworks (MOFs), which had long been anticipated as high-efficient photocatalysts. Herein, we introduce a new strategy to make efficient separation of electrons and holes for the MOF-based photocatalyst, UiO-66-NH₂ . At first, encapsulation of Pt nanoparticles (NPs) into UiO-66-NH₂ (Pt@U6N) to shorten the electrons transport distance inside of MOF crystals, then using graphene oxide to wrap the external surface of Pt@U6N to facilitate the electrons transfer on the surface. The designed structure was found to possess superior H₂ -generation ability compared to only inside or outside decorated samples, highlighting that the enhanced property strongly correlates with the inhibited recombination of electrons and holes by the inside/outside modification strategy. These findings suggest a synergistic effect of Pt NPs and graphene oxide on UiO-66-NH₂ and reveal a new modification strategy to enhance the catalytic activity of the photocatalysts.

Synthesis, Characterization, and Photoelectrochemical Catalytic Studies of a Water-Stable Zinc-Based Metal-Organic Framework

Metal-organic frameworks (MOFs) are class of porous materials that can be assembled in a modular manner by using different metal ions and organic linkers. Owing to their tunable structural properties, these materials are found to be useful for gas storage and separation technologies, as well as for catalytic applications. A cost-effective zinc-based MOF ([Zn(bpcda)(bdc)]_n ) is prepared by using N,N'-bis(pyridin-4-ylmethylene)cyclohexane-1,4-diamine [N,N'-bis(pyridin-4-ylmethylene)cyclohexane-1,4-diamine] and benzenedicarboxylic acid (bdc) linkers. This new material exhibits remarkable photoelectrochemical (PEC) catalytic activity in water splitting for the evolution of oxygen. Notably, this non-noble metal-based MOF, without requiring immobilization on other supports or containing metal particles, produced a highest photocurrent density of 31 μA cm^-2 at 0.9 V, with appreciable stability and negligible photocorrosion. Advantageously for the oxygen evolution process, no external reagents or sacrificial agents are required in the aqueous electrolyte solution.

Photochemistry of Zr-based MOFs: ligand-to-cluster charge transfer, energy transfer and excimer formation, what else is there?

Understanding of the photocatalytic behaviour of Zr-based MOFs is fundamental for the improved design of new and more efficient photocatalysts. The present work describes steady-state and photodynamical studies on the behavior of two MOFs: Zr-2,6-naphthalene dicarboxylate (Zr-NDC) and Zr-4-amino-2,6-naphthalene dicarboxylate (Zr-NADC, 65% NDC/35% NADC) in dichloromethane (DCM) and N,N-dimethylformamide (DMF) suspensions. In the DMF suspension, the Zr-NDC MOF exhibits excimer formation in 280 ps due to the interactions between neighboring linkers. Using the femtosecond transient absorption (fs-TA) technique we have found that the Zr-NADC MOF exhibits a ligand-to-cluster charge transfer (LCCT) event in ∼170 and <100 fs in DMF and DCM, respectively. The Zr-NDC MOF shows similar LCCT in <100 fs in both solvents. The Zr-NADC MOF in DMF suspension shows an energy transfer (ET) from the excited NDC linkers to the NADC ones in 1.1 ps. Flash photolysis experiments demonstrate dominant radiative electron-hole (e^--h⁺) recombination from two different trap states in 310 ns and 1.3 μs, and 32 ns and 1.0 μs in the photoexcited Zr-NADC and Zr-NDC MOFs in DMF, respectively. Excitation of the NDC linkers in the Zr-NADC and Zr-NDC MOFs in DCM allows for the characterization of the dynamics associated with the charge-separated state and the non-radiative recombination from the trap states (45 ns and 0.3 μs). Direct excitation of the NADC linkers (410 nm) in the Zr-NADC MOF in DCM suspension produces a radiative (e^--h⁺) recombination in 3.5 μs. Further experiments in the presence of electron donor (N,N,N',N'-tetramethyl-p-phenylenediamine, TMPD) and acceptor (methyl viologen, MV²⁺) molecules corroborate the formation of charge separated states in these MOFs. These findings are relevant for understanding the photocatalytic and photonic behaviours of these MOFs.

[Ti8Zr2O12(COO)16] Cluster: An Ideal Inorganic Building Unit for Photoactive Metal-Organic Frameworks

Metal-organic frameworks (MOFs) based on Ti-oxo clusters (Ti-MOFs) represent a naturally self-assembled superlattice of TiO₂ nanoparticles separated by designable organic linkers as antenna chromophores, epitomizing a promising platform for solar energy conversion. However, despite the vast, diverse, and well-developed Ti-cluster chemistry, only a scarce number of Ti-MOFs have been documented. The synthetic conditions of most Ti-based clusters are incompatible with those required for MOF crystallization, which has severely limited the development of Ti-MOFs. This challenge has been met herein by the discovery of the [Ti₈Zr₂O₁₂(COO)₁₆] cluster as a nearly ideal building unit for photoactive MOFs. A family of isoreticular photoactive MOFs were assembled, and their orbital alignments were fine-tuned by rational functionalization of organic linkers under computational guidance. These MOFs demonstrate high porosity, excellent chemical stability, tunable photoresponse, and good activity toward photocatalytic hydrogen evolution reactions. The discovery of the [Ti₈Zr₂O₁₂(COO)₁₆] cluster and the facile construction of photoactive MOFs from this cluster shall pave the way for the development of future Ti-MOF-based photocatalysts.

Novel stable metal–organic framework photocatalyst for light-driven hydrogen production

A novel stable Dye-based Dy-MOF exhibits efficient photocatalytic activity for hydrogen generation.

A new Zn(ii) metal–organic framework having 3D CdSO4 topology as luminescent sensor and photocatalyst for degradation of organic dyes

A new Zn(ii) metal–organic framework having 3D CdSO₄ topology synthesized and deployed as a dual channel luminescent sensor for ferric ion and nitroaromatics, and a photocatalyst for the degradation of organic dyes.

Boosting visible-light photocatalytic H2 evolution via UiO-66-NH2 octahedrons decorated with ultrasmall NiO nanoparticles

Noble metal-free UiO-66-NH₂-10 wt% NiO composites (U6N-NiO-2) with eosin Y sensitizer exhibit high visible-light hydrogen evolution activity.

The effect of functional groups in the aqueous-phase selective sensing of Fe(iii) ions by thienothiophene-based zirconium metal–organic frameworks and the design of molecular logic gates

The effect of functional groups in the fluorescence sensing of Fe(iii) ions in aqueous medium by four thienothiophene-based Zr MOFs is discussed.

Effects of electron-donating groups on the photocatalytic reaction of MOFs

Regulating the synthesis of photocatalytic materials at the molecular level could affect the absorption of light and guide the synthesis of highly efficient photocatalysts for the photocatalytic degradation organic pollutants.

Catalysis and photocatalysis by metal organic frameworks

This review aims to provide different strategies employed to use MOFs as solid catalysts and photocatalysts in organic transformations.

A mechanistic approach towards the photocatalytic organic transformations over functionalised metal organic frameworks: a review

This review focuses on the possible mechanisms involved in the organic transformations occurring through photocatalysis over functionalised metal–organic frameworks.

Unexpected lability of the [RuIII(phtpy)Cl3] complex

Ruthenium(iii) complexes are known for their high stability and inertness. To the best of our knowledge, the only well-characterized example of a labile Ru(iii) complex is [Ru^III(edta)(H₂O)] as a consequence of an intramolecular hydrogen bonding leading to the formation of a large opening in the molecule front, thus changing the mechanism from dissociative to associative. Compelling experimental evidence is presented demonstrating that the [Ru^III(phtpy)Cl₃] complex is labile, also indicating that the Ru(iii)-phtpy bond is much weaker than expected, in contrast to the strongly π-back-bonding stabilized Ru(ii)-phtpy bond.

Electrochemical aptasensor for multi-antibiotics detection based on endonuclease and exonuclease assisted dual recycling amplification strategy

An ultrasensitive electrochemical aptasensor for multiplex antibiotics detection based on endonuclease and exonuclease assisted dual recycling amplification strategy was proposed. Kanamycin and chloramphenicol were selected as candidates. Firstly, aptamers of the antibiotics were immobilized on bar A and then binding with their endonuclease labeled complementary DNA strands to construct enzyme-cleavage probes. Secondly, The nano zirconium-metal organic framework (NMOF) particles with 1,4-benzene-dicarboxylate (BDC) as linker was defined as UiO-66. And its updated version, hierarchically porous UiO-66 (HP-UIO-66) decorated with different electroactive materials as signal tags were synthesized. Then they were immobilized on bar B linked by two duplex DNA strands which can be specifically cleaved by corresponding enzyme-cleavage probes in bar A. Once targets were introduced into system, aptamers can capture them and then release enzyme-cleavage probes. In the presence of exonuclease-I, exonuclease assisted target recycling amplification was triggered and more enzyme-cleavage probes were released into solution. The probes can trigger endonuclease assisted recycles and repeatedly cleave their corresponding duplex DNA strands on bar B then released numerous signal tags into supernatant. Thus two recycling amplification was performed in the system. Finally, MB and Fc in the signal tags were detected by square wave voltammetry after removing bar A/B and the current intensities were correspondent with the concentration of KANA and CAP respectively. Under the optimum condition, the limits of detection for the KANA and CAP were 35fM and 21fM respectively with a wide linear range from 1 × 10^-4 to 50nM. This dual recycling amplification detection system exhibited high sensitivities and specificity. It can be easily extended to detect other targets if changing the corresponding aptamers and has potential application values for screening of multiplex antibiotics residues in food safety.

Role of Modulators in Controlling the Colloidal Stability and Polydispersity of the UiO-66 Metal-Organic Framework

Nanoscale UiO-66 Zr₆(OH)₄O₄(C₈O₄H₄)₆ has been synthesized with a series of carboxylic acid modulators, R-COOH (where R = H, CH₃, CF₃, and CHCl₂). The phase purity and size of each MOF was confirmed by powder X-ray diffraction, BET surface area analysis, and scanning transmission electron microscopy (STEM). Size control of UiO-66 crystals from 20 nm to over 1 μm was achieved, and confirmed by STEM. The colloidal stability of each MOF was evaluated by dynamic light scattering and was found to be highly dependent on the modulator conditions utilized in the synthesis, with both lower pKa and higher acid concentration resulting in more stable structures. Furthermore, STEM was carried out on both colloidally stable samples and those that exhibited a large degree of aggregation, which allowed for visualization of the different degrees of dispersion of the samples. The use of modulators at higher concentrations and with lower pK_as leads to the formation of more defects, as a consequence of terephthalic acid ligands being replaced by modulator molecules, thereby enhancing the colloidal stability of the UiO-66 nanoparticles. These findings could have a significant impact on nanoscale MOF material syntheses and applications, especially in the areas of catalysis and drug delivery.

Exploration of Zr-Metal-Organic Framework as Efficient Photocatalyst for Hydrogen Production

The application of metal-organic framework (MOF) in the field of photocatalysis is limited, because of its unstable chemical properties and failure to respond in visible light. Herein, the Pd/MOF catalysts were prepared by impregnation reduction. It is important that we have reasonably constructed the dye-sensitized system of Pd/MOF and successfully extended the application of MOF to the visible range. It exhibited maximal photocatalytic activity (9.43 mmol/g) under visible-light irradiation (λ ≥ 420 nm) with eosin Y as a photo-sensitizer, which was enhanced twice the order of magnitude compared with the pure MOF (0.03 mmol/g). The activation process with an exchangeable guest solvent produced the Zr-MOF with the high surface area and the high stability, which provide a great electronic transmission capacity. The Pd nanoparticles provide an electronic outlet, and the dye broadens the spectral absorption range. The synergistic effect of the various components contributes to the high hydrogen production activity. This work provides a reference for the application of MOF in the field of photocatalysis.

Ultrahigh adsorption and singlet-oxygen mediated degradation for efficient synergetic removal of bisphenol A by a stable zirconium-porphyrin metal-organic framework

Bisphenol A (BPA), one of 23 most important endocrine disrupting chemicals, was efficiently removed and sequentially photodegraded by a zirconium-porphyrin metal–organic framework (MOF) catalyst under visible light for water treatment. Well control of photodegradation allows the kinetic separation of adsorption step and photodegradation step. Ultrahigh adsorption uptake of 487.69 ± 8.37 mg g⁻¹ is observed, while efficient photodegradation could be observed within 20 min at the rate of 0.004 mg min⁻¹. The synergetic effect boosts the photocatalytic efficiency and confirms that the catalysis happens inside the MOF pores other than in the solution phase. Furthermore, the mechanism was elucidated by diverse control experiments, such as in the conditions of ¹O₂ scavenger, in darkness and with the changes of light sensitizing ligands. It confirmed that BPA was oxidized by the ¹O₂ which was generated from porphyrin ligand within MOFs under visible-light. The excellent reusability and wide range of suitable pH range make the Zr-porphyrin MOFs practical for the photocatalytic water treatment processes.

Metal–Organic Frameworks and Their Derivatives for Photocatalytic Water Splitting

Amongst many strategies for renewable energy conversion, light-driven water splitting to produce clean H2 represents a promising approach and has attracted increasing attention in recent years. Owing to the multi-electron/multi-proton transfer nature of water splitting, low-cost and competent catalysts are needed. Along the rapid development of metal–organic frameworks (MOFs) during the last two decades or so, MOFs have been recognized as an interesting group of catalysts or catalyst supports for photocatalytic water splitting. The modular synthesis, intrinsically high surface area, tunable porosity, and diverse metal nodes and organic struts of MOFs render them excellent catalyst candidates for photocatalytic water splitting. To date, the application of MOFs and their derivatives as photocatalysts for water splitting has become a burgeoning field. Herein, we showcase several representative MOF-based photocatalytic systems for both H2 and O2 evolution reactions (HER, OER). The design principle of each catalytic system is specifically discussed. The current challenges and opportunities of utilizing MOFs for photocatalytic water splitting are discussed in the end.

An efficient combination of Zr-MOF and microwave irradiation in catalytic Lewis acid Friedel-Crafts benzoylation

A zirconium-based metal-organic framework, an effective heterogeneous catalyst, has been developed for the Friedel-Crafts benzoylation of aromatic compounds under microwave irradiation. Constructed by a Zr(iv) cluster and a linker 1,4-bis(2-[4-carboxyphenyl]ethynyl)benzene (H2CPEB), the MOF, possessing large pores and high chemical stability, was appropriate for the enhancement of Lewis acid activity under microwave irradiation. The reaction studies demonstrated that the material could give high yields for a few minutes and maintain its reactivity and structure over several cycles.

Maximizing the Photocatalytic Activity of Metal-Organic Frameworks with Aminated-Functionalized Linkers: Substoichiometric Effects in MIL-125-NH2

Despite the promise of utilizing metal-organic frameworks (MOFs) as highly tunable photocatalytic materials, systematic studies that interrogate the relationship between their catalytic performances and the amount of functionalized linkers are lacking. Aminated linkers are known to enhance the absorption of light and afford photocatalysis with MOFs under visible-light irradiation. However, the manner in which the photocatalytic performances are impacted by the amount of such linkers is poorly understood. Here, we assess the photocatalytic activity of MIL-125, a TiO₂/1,4-benzenedicarboxylate (bdc) MOF for the oxidation of benzyl alcohol to benzaldehyde when increasing amounts of bdc-NH₂ linkers (0%, 20%, 46%, 70%, and 100%) are incorporated in the framework. Analytical TEM allowed assessing the homogeneous localization of bdc-NH₂ in these mixed-linker MOFs. Steady state reaction rates reveal two regimes of catalytic performances: a first linear regime up to ∼50% bdc-NH₂ into the hybrid framework whereby increased amounts of bdc-NH₂ yielded increased photocatalytic rates, followed by a plateau up to 100% bdc-NH₂. This unexpected "saturation" of the catalytic activity above ∼50% bdc-NH₂ content in the framework whatever the wavelength filters used demonstrates that amination of all linkers of the MOF is not required to obtain the maximum photocatalytic activity. This is rationalized on the basis of mixed-valence Ti³⁺/Ti⁴⁺ intermediate catalytic centers revealed by electron spin resonance (ESR) measurements and recent knowledge of lifetime excited states in MIL-125-type of solids.

Missing Linkers: An Alternative Pathway to UiO-66 Electronic Structure Engineering

UiO-66 is a promising metal-organic framework for photocatalytic applications. However, the ligand-to-metal charge transfer of an excited electron is inefficient in the pristine material. Herein, we assess the influence of missing linker defects on the electronic structure of UiO-66 and discuss their ability to improve ligand-to-metal charge transfer. Using a new defect classification system, which is transparent and easily extendable, we identify the most promising photocatalysts by considering both relative stability and electronic structure. We find that the properties of UiO-66 defect structures largely depend on the coordination of the constituent nodes and that the nodes with the strongest local distortions alter the electronic structure most. Defects hence provide an alternative pathway to tune UiO-66 for photocatalytic purposes, besides linker modification and node metal substitution. In addition, the decomposition of MOF properties into node- and linker-based behavior is more generally valid, so we propose orthogonal electronic structure tuning as a paradigm in MOF design.

Fixation of carbon dioxide into dimethyl carbonate over titanium-based zeolitic thiophene-benzimidazolate framework

A titanium-based zeolitic thiophene-benzimidazolate framework has been designed for the direct synthesis of dimethyl carbonate (DMC) from methanol and carbon dioxide. The developed catalyst activates carbon dioxide and delivers over 16% yield of DMC without the use of any dehydrating agent or requirement for azeotropic distillation.

Recent Progress in Metal-Organic Frameworks for Applications in Electrocatalytic and Photocatalytic Water Splitting

The development of clean and renewable energy materials as alternatives to fossil fuels is foreseen as a potential solution to the crucial problems of environmental pollution and energy shortages. Hydrogen is an ideal energy material for the future, and water splitting using solar/electrical energy is one way to generate hydrogen. Metal-organic frameworks (MOFs) are a class of porous materials with unique properties that have received rapidly growing attention in recent years for applications in water splitting due to their remarkable design flexibility, ultra-large surface-to-volume ratios and tunable pore channels. This review focuses on recent progress in the application of MOFs in electrocatalytic and photocatalytic water splitting for hydrogen generation, including both oxygen and hydrogen evolution. It starts with the fundamentals of electrocatalytic and photocatalytic water splitting and the related factors to determine the catalytic activity. The recent progress in the exploitation of MOFs for water splitting is then summarized, and strategies for designing MOF-based catalysts for electrocatalytic and photocatalytic water splitting are presented. Finally, major challenges in the field of water splitting are highlighted, and some perspectives of MOF-based catalysts for water splitting are proposed.