Ti cluster-alkylated hydrophobic MOFs for photocatalytic production of hydrogen peroxide in two-phase systems

Molecular Engineering of Metal-Organic Frameworks for Boosting Photocatalytic Hydrogen Peroxide Production

The development of novel metal-organic frameworks (MOFs) as efficient photocatalysts for hydrogen peroxide production from water and oxygen is particularly interesting, yet remains a challenge. Herein, we have prepared four cyclic trinuclear units (CTUs) based MOFs, exhibiting good light absorption ability and suitable band gaps for photosynthesis of H2O2. However, Cu-CTU-based MOFs are not able to photocatalyzed the formation of H2O2, while the alteration of metal nodes from Cu-CTU to Ag-CTU dramatically enhances the photocatalytic performance for H2O2 production and the production rates can reach as high as 17476 μmol g-1 h-1 with an apparent quantum yield of 4.72 %, at 420 nm, which is much higher than most reported MOFs. The photocatalytic mechanism is comprehensively studied by combining the isotope labeling experiments and DFT calculation. This study provides new insights into the preparation of MOF photocatalysts with high activity for H2O2 production through molecular engineering.

Catalyst-free contact-electro-catalytic H2O2 synthesis via simple combination of a poly(tetrafluoroethylene) stir bar and ultrasound

Herein, we present a catalyst-free contact-electro-catalytic method for synthesizing hydrogen peroxide (H2O2) by combining continuous agitation with a poly(tetrafluoroethylene) (PTFE) stir bar and ultrasonication. A high H2O2 production rate of 256.6 μM h-1 is achieved under ambient conditions without adding particle catalysts and sacrificial agents, which is competitive with recent advancements in redox technology. Eco-friendliness, convenience and efficiency make this process a promising alternative method for H2O2 synthesis.

Highly efficient photosynthesis of hydrogen peroxide by a stable Zr(IV)-based MOF with a diamino-functionalized ligand

Metal-organic frameworks (MOFs) have emerged as a promising class of materials for solar-driven hydrogen peroxide (H2O2) generation due to their porosity, large surface area and designable molecular building blocks; however, producing H2O2 from oxygen and water without sacrificial agents remains a major challenge. Herein, we have constructed two UiO-67-type MOFs, UiO-67-NH2 and UiO-67-(NH2)2, by a solvothermal method using 2-amino-4,4'-biphenyldicarboxylic acid and 2,2'-diamino-4,4'-biphenyldicarboxylic acid as ligands, respectively. A variety of photochemical measurements have shown that the introduction of diamino groups into UiO-67-(NH2)2 not only enhances its absorption ability for visible light, but also facilitates the separation of photogenerated electron/hole pairs. Consequently, compared to monoamino-functionalized UiO-67-NH2, UiO-67-(NH2)2 exhibits a 5.5 times higher H2O2 production rate in pure water for 1 h. A two-step one-electron oxygen reduction reaction pathway for photocatalytic H2O2 production was suggested based on a series of control experiments and active species trapping tests by electron paramagnetic resonance spectra. This work provides new insights into the regulation of functionalized MOF ligands at the molecular level and a catalytic mechanism towards MOF-based photocatalysts for H2O2 production with high activity.

Hydrophobic Porphyrin Titanium-Based MOFs for Visible-Light-Driven CO2 Reduction to Formate

Three hydrophobic porphyrin titanium-based metal-organic frameworks (MOFs) (HPA/DGIST-1, DPA/DGIST-1, and OPA/DGIST-1) were synthesized through a postsynthetic coordination reaction by using alkylphosphonic acid of different lengths (HPA, hexylphosphonic acid; DPA, dodecylphosphonic acid; OPA, octadecylphosphonic acid). Compared with the hydrophilic DGIST-1, modified DGIST-1 exhibits excellent hydrophobicity and presents good stability in humid atmospheres. Due to the introduction of porphyrin ligands, HPA/DGIST-1, DPA/DGIST-1, and OPA/DGIST-1 showed good visible-light absorption (380-700 nm) and sensitive photogenerated charge responses. When acted as catalysts, these hydrophobic Ti-MOFs can selectively reduce CO2 to HCOO- under visible-light irradiation with average reaction rates of 150.9, 178.5, and 228.3 μmol·h-1·g-1, where these values are 1.3-2.0 times higher than the system mediated by the initial porphyrin Ti-MOF catalyst. 13C NMR spectroscopy demonstrates that the catalytic product HCOO- anion originates from the reactant CO2. The photocatalytic experiments, electron paramagnetic resonance, and photoluminescence spectra tests showed that porphyrin ligands and Ti-O units can act as catalytic activity centers to realize the conversion of CO2 to HCOO-. This work demonstrated that the combination of porphyrin titanium-based MOF and alkyl hydrophobic groups is an effective way to enhance the stability of titanium-based MOFs and maintain their high photocatalytic performance.

Photocatalytic and Electrocatalytic Generation of Hydrogen Peroxide: Principles, Catalyst Design and Performance

Hydrogen peroxide (H2O2) is a high-demand organic chemical reagent and has been widely used in various modern industrial applications. Currently, the prominent method for the preparation of H2O2 is the anthraquinone oxidation. Unfortunately, it is not conducive to economic and sustainable development since it is a complex process and involves unfriendly environment and potential hazards. In this context, numerous approaches have been developed to synthesize H2O2. Among them, photo/electro-catalytic ones are considered as two of the most promising manners for on-site synthesis of H2O2. These alternatives are sustainable in that only water or O2 is required. Namely, water oxidation (WOR) or oxygen reduction (ORR) reactions can be further coupled with clean and sustainable energy. For photo/electro-catalytic reactions for H2O2 generation, the design of the catalysts is extremely important and has been extensively conducted with an aim to obtain ultimate catalytic performance. This article overviews the basic principles of WOR and ORR, followed by the summary of recent progresses and achievements on the design and performance of various photo/electro-catalysts for H2O2 generation. The related mechanisms for these approaches are highlighted from theoretical and experimental aspects. Scientific challenges and opportunities of engineering photo/electro-catalysts for H2O2 generation are also outlined and discussed.

Integrating Bifunctionality and Chemical Stability in Covalent Organic Frameworks via One-Pot Multicomponent Reactions for Solar-Driven H2O2 Production

Multicomponent reactions (MCRs) can be used to introduce different functionalities into highly stable covalent organic frameworks (COFs). In this work, the irreversible three-component Doebner reaction is utilized to synthesize four chemically stable quinoline-4-carboxylic acid DMCR-COFs (DMCR-1-3 and DMCR-1NH) equipped with an acid-base bifunctionality. These DMCR-COFs show superior photocatalytic H₂O₂ evolution (one of the most important industrial oxidants) compared to the imine COF analogue (Imine-1). This is achieved with sacrificial oxidants but also in pure water and under an oxygen or air atmosphere. Furthermore, the DMCR-COFs show high photostability, durability, and recyclability. MCR-COFs thus provide a viable materials' platform for solar to chemical energy conversion.

Recent Advances in Tetra- (Ti, Sn, Zr, Hf) and Pentavalent (Nb, V, Ta) Metal-Substituted Molecular Sieve Catalysis

Metal substitution of molecular sieve systems is a major driving force in developing novel catalytic processes to meet current demands of green chemistry concepts and to achieve sustainability in the chemical industry and in other aspects of our everyday life. The advantages of metal-substituted molecular sieves include high surface areas, molecular sieving effects, confinement effects, and active site and morphology variability and stability. The present review aims to comprehensively and critically assess recent advances in the area of tetra- (Ti, Sn, Zr, Hf) and pentavalent (V, Nb, Ta) metal-substituted molecular sieves, which are mainly characterized for their Lewis acidic active sites. Metal oxide molecular sieve materials with properties similar to those of zeolites and siliceous molecular sieve systems are also discussed, in addition to relevant studies on metal-organic frameworks (MOFs) and some composite MOF systems. In particular, this review focuses on (i) synthesis aspects determining active site accessibility and local environment; (ii) advances in active site characterization and, importantly, quantification; (iii) selective redox and isomerization reaction applications; and (iv) photoelectrocatalytic applications.

A comprehensive review on water remediation using UiO-66 MOFs and their derivatives

Metal-organic frameworks (MOFs) are a versatile class of porous materials offering unprecedented scope for chemical and structural tunability. On account of their synthetic versatility, tunable and exceptional host-guest chemistry they are widely utilized in many prominent water remediation techniques. However, some of the MOFs present low structural stabilities specifically in aqueous and harsh chemical conditions which impedes their potential application in the field. Among the currently explored MOFs, UiO-66 exhibits structural robustness and has gained immense scientific popularity. Built with a zirconium-terephthalate framework, the strong Zr-O bond coordination contributes to its stability in aqueous, chemical, and thermal conditions. Moreover, other exceptional features such as high surface area and uniform pore size add to the grand arena of porous nanomaterials. As a result of its stable nature, UiO-66 offers relaxed admittance towards various functionalization, including synthetic and post-synthetic modifications. Consequently, the adsorptive properties of these highly stable frameworks have been modulated by the addition of various functionalities. Moreover, due to the presence of catalytically active sites, the use of UiO-66 has also been extended towards the degradation of pollutants. Furthermore, to solve the practical handling issues of the crystalline powdered forms, UiO-66 has been incorporated into various membrane supports. The incorporation of UiO-66 in various matrices has enhanced the rejection, permeate flux, and anti-fouling properties of membranes. The combination of such exceptional characteristics of UiO-66 MOF has expanded its scope in targeted purification techniques. Subsequently, this review highlights the role of UiO-66 in major water purification techniques such as adsorption, photocatalytic degradation, and membrane separation. This comprehensive review is expected to shed light on the existing developments and guide the inexhaustible futuristic scope of UiO-66 MOF.

Lignin-Supported Heterogeneous Photocatalyst for the Direct Generation of H2O2 from Seawater

The development of smart and sustainable photocatalysts is in high priority for the synthesis of H₂O₂ because the global demand for H₂O₂ is sharply rising. Currently, the global market share for H₂O₂ is around 4 billion US$ and is expected to grow by about 5.2 billion US$ by 2026. Traditional synthesis of H₂O₂ via the anthraquinone method is associated with the generation of substantial chemical waste as well as the requirement of a high energy input. In this respect, the oxidative transformation of pure water is a sustainable solution to meet the global demand. In fact, several photocatalysts have been developed to achieve this chemistry. However, 97% of the water on our planet is seawater, and it contains 3.0-5.0% of salts. The presence of salts in water deactivates the existing photocatalysts, and therefore, the existing photocatalysts have rarely shown reactivity toward seawater. Considering this, a sustainable heterogeneous photocatalyst, derived from hydrolysis lignin, has been developed, showing an excellent reactivity toward generating H₂O₂ directly from seawater under air. In fact, in the presence of this catalyst, we have been able to achieve 4085 μM of H₂O₂. Expediently, the catalyst has shown longer durability and can be recycled more than five times to generate H₂O₂ from seawater. Finally, full characterizations of this smart photocatalyst and a detailed mechanism have been proposed on the basis of the experimental evidence and multiscale/level calculations.

The covalent Coordination-driven Bi2S3@NH2-MIL-125(Ti)-SH heterojunction with boosting photocatalytic CO2 reduction and dye degradation performance

Herein, the optional and controllable growth of Bi₂S₃ onto NH₂-MIL-125 via covalent conjunction strategy was reported. The experimental results demonstrate that the obtained heterojunction exhibits boosting photocatalytic reduction CO₂ and organic dye degradation. The 18-Bi₂S₃@NH₂-MIL-125-SH displays the highest yield of 12.46 μmol g^-1h^-1 of CO, >13 times that of pure NH₂-MIL-125. Meanwhile, the reaction kinetic of 18-Bi₂S₃@NH₂-MIL-125-SH in the degradation of methylene blue is uppermost, which is 160 times than that of the commercial P25. The enhancement of photocatalytic performance could be ascribed to the covalent coordination-driven intimate interfacial interaction in n-scheme heterojunction. Meanwhile, the plausible mechanism was also investigated by UV-vis diffuse reflectance (UV-vis), photoluminescence (PL), electrochemical photocurrent, electron spin resonance (ESR) and electrochemical impedance spectroscopy (EIS).

The applications and prospects of hydrophobic metal-organic frameworks in catalysis

In recent years, large numbers of hydrophobic/superhydrophobic metal-organic frameworks (MOFs) have been developed. These hydrophobic MOFs not only retain rich structural variety, highly crystalline frameworks, and uniform micropores, but they also have lower affinity towards water and boosted hydrolytic stability. Until now, there were two main strategies to prepare hydrophobic MOFs, including a one-step method and post-synthesis modification (PSM). PSM was an often-used strategy for preparing hydrophobic MOFs. Hydrophobic MOFs showed unique advantages when used as catalysts for various categories of reactions. Herein, recent research advances relating to hydrophobic MOFs in the catalytic field are presented. The catalytic activities of hydrophobic MOFs and corresponding hydrophilic ones are also compared, and the superiority of hydrophobic MOFs or MOF materials as catalysts in 10 reactions is discussed. Finally, the advantages of hydrophobic MOFs as catalysts or auxiliary materials are summarized and promising future developments of hydrophobic MOFs are highlighted.

Hydrophobic Metal-Organic Frameworks: Assessment, Construction, and Diverse Applications

Tens of thousands of metal-organic frameworks (MOFs) have been developed in the past two decades, and only ≈100 of them have been demonstrated as porous and hydrophobic. These hydrophobic MOFs feature not only a rich structural variety, highly crystalline frameworks, and uniform micropores, but also a low affinity toward water and superior hydrolytic stability, which make them promising adsorbents for diverse applications, including humid CO2 capture, alcohol/water separation, pollutant removal from air or water, substrate-selective catalysis, energy storage, anticorrosion, and self-cleaning. Herein, the recent research advancements in hydrophobic MOFs are presented. The existing techniques for qualitatively or quantitatively assessing the hydrophobicity of MOFs are first introduced. The reported experimental methods for the preparation of hydrophobic MOFs are then categorized. The concept that hydrophobic MOFs normally synthesized from predesigned organic ligands can also be prepared by the postsynthetic modification of the internal pore surface and/or external crystal surface of hydrophilic or less hydrophobic MOFs is highlighted. Finally, an overview of the recent studies on hydrophobic MOFs for various applications is provided and suggests the high versatility of this unique class of materials for practical use as either adsorbents or nanomaterials.

The direct synthesis of hydrogen peroxide using a combination of a hydrophobic solvent and water

The use of a hydrophobic solvent in combination with water leads to significant suppression of H₂O₂ degradation pathways over a AuPd/C catalyst.

Metal–organic framework-based nanomaterials for photocatalytic hydrogen peroxide production

Metal–organic frameworks act as efficient photocatalysts for visible-light driven hydrogen peroxide production in a single-phase system and two-phase system.