Preparation and applications of metal–organic frameworks composed of sulfonic acid

High proton conduction in a series of three-dimensional lanthanide(III)-organic frameworks constructed by 2,5-dihydroxyterephthalic acid

In designing and preparing new proton-conductive materials, using cheap and easily available raw materials to efficiently prepare metal-organic frameworks (MOFs) with high stability and excellent proton conductivity is still a huge challenge. Herein, six lanthanide(III)-MOFs, {[Ln2(DHBDC)3(DMF)4](DMF)2}n [(Ln III = Pr III (1), Nd III (2), Sm III (3), Eu III (4), Gd III (5), Tb III (6))] with high stability were solvothermally synthesized utilizing 2,5-dihydroxy-1,4-benzenedicarboxylic acid (H4-DHBDC) as a bridging ligand. These isostructural MOFs all possess a three-dimensional framework and a dense H-bond network formed by the carbonyl groups in the framework, the non-coordinated hydroxyl groups, and the coordinated and free DMF molecules, which ensure efficient proton conduction. Their good water and thermal stability were verified using various characterization techniques (powder X-ray diffraction, thermogravimetric analysis, and infrared). Then, their proton conductivity was investigated in detail concerning temperature and relative humidity (RH). At 100 °C and 97 % RH, their optimum proton conductivity can reach up to 0.96 × 10-2, 0.67 × 10-2, 0.85 × 10-2, 1.03 × 10-2, 0.53 × 10-2, and 0.93 × 10-2 S/cm for 1-6, respectively. Finally, their proton-transport processes were thoroughly examined through detailed structural analyses, adsorption-property determinations, and activation energy values. Notably, these MOF materials have the advantages of easy preparation and relatively low cost, which paves the way for their practical applications.

Functionalization of Defective Zr-MOFs for Water Decontamination: Mechanistic Insight into the Competitive Roles of -NH2 and -SH Sites in the Removal of Hg(II) Ions

Functional metal-organic frameworks (MOFs), especially those based on sulfur and nitrogen atoms, were frequently applied for the removal of Hg(II) ions. However, a systematic study on the cooperative or competitive roles of -SH and -NH2 functions in the presence of secondary mechanisms (proton transfer and redox) is still rare. In this work, the UiO-66 framework (Zr6(OH)4O4(BDC)6, BDC2- = benzene-1,4-dicarboxylate) was decorated with functional monocarboxylate linkers including glycine (Gly), mercaptopropionic acid (Mer), and cysteine (Cys). Due to the molecular similarity of these functional linkers, the coordination affinity between the amine and thiol sites with Hg(II) ions can be compared, and the effect of proton transfer and redox mechanisms on the possible thiol···Hg(II) and amine···Hg(II) interactions can be investigated. The results show that the Cys@UiO-66 framework can adsorb 1288 mg g-1 of Hg(II), while Mer@UiO-66 and Gly@UiO-66 can adsorb 593 and 313 mg g-1 at pH = 7 and 500 ppm, respectively. This is due to the facts that both the amine and the thiol functions of the Cys@UiO-66 framework show synergism in Hg(II) removal, and the secondary mechanisms reduce the affinity of thiol in Mer@UiO-66 and amine in Gly@UiO-66 frameworks in the removal process of Hg(II) ions. Free -SH sites in Mer@UiO-66 undergo a redox convert to -SO3H groups, and free protonated -NH2 sites in Gly@UiO-66 do not fully deprotonate during Hg(II) removal. Yet, in the case of Cys@UiO-66, free protonated -NH2 sites are fully deprotonated, and free SH sites did not convert to -SO3H groups during Hg(II) removal. These observations show that the redox and proton transfer mechanisms can negatively affect the adsorption capacity of functional MOFs containing free -SH and -NH2 groups. So, not only the functionalization but also control over secondary mechanisms in the removal process are necessary parameters to improve the affinity between functional MOFs and Hg(II) ions.

Precision-Engineered Construction of Proton-Conducting Metal-Organic Frameworks

Proton-conducting materials have attracted considerable interest because of their extensive application in energy storage and conversion devices. Among them, metal-organic frameworks (MOFs) present tremendous development potential and possibilities for constructing novel advanced proton conductors due to their special advantages in crystallinity, designability, and porosity. In particular, several special design strategies for the structure of MOFs have opened new doors for the advancement of MOF proton conductors, such as charged network construction, ligand functionalization, metal-center manipulation, defective engineering, guest molecule incorporation, and pore-space manipulation. With the implementation of these strategies, proton-conducting MOFs have developed significantly and profoundly within the last decade. Therefore, in this review, we critically discuss and analyze the fundamental principles, design strategies, and implementation methods targeted at improving the proton conductivity of MOFs through representative examples. Besides, the structural features, the proton conduction mechanism and the behavior of MOFs are discussed thoroughly and meticulously. Future endeavors are also proposed to address the challenges of proton-conducting MOFs in practical research. We sincerely expect that this review will bring guidance and inspiration for the design of proton-conducting MOFs and further motivate the research enthusiasm for novel proton-conducting materials.

Symmetry related proton conductivity tunability via aliovalent metal substitution in imidazolium templated stable metal-organic framework hybrid membranes

Proton-conducting materials have gained popularity owing to their extensive applications in biologic/chemical sensors, supercapacitors, proton sieving, and proton-exchange-membrane fuel cells. To date, the most commercially used polymer membrane has been the Nafion series that exhibits conductivity exceeding 0.1 S cm-1, however, this series is expensive, has poor dimensional stability, and requires a complex synthesis process. The key criterion for selecting Nafion alternatives is to achieve the systematic integration of high proton conductivity with high stability through a simple and efficient approach. In this study, we used an aliovalent metal substitution strategy to design serial metal-organic frameworks (MOFs), including tetragonal T-Cd-BTC (CH3NH2CH3)2[Cd(BTC)](H2O) and quasi-cubic quasi-C-In-BTC (C4H7N2)[In(BTC)] and Im@quasi-C-In-BTC (C3H5N2)2[In(BTC)] frameworks, with 2-methylimidazolium and imidazolium cations as templates, respectively. Because of the aliovalent substitution of In(III) for Cd(II), both the metal-oxygen bond strength and unit cell symmetry gradually increased, resulting in an increase in the thermal stability of quasi-C-In-BTC and Im@quasi-C-In-BTC at temperatures of up to 700 K. Compared with in situ loaded 2-methylimidazolium quasi-C-In-BTC, Im@quasi-C-In-BTC prepared by incorporating the imidazolium cation into the pores of activated quasi-C-In-BTC exhibited a higher proton conductivity of 7.1 × 10-2 S cm-1 at 338 K and 95 % relative humidity. Thus, Im@quasi-C-In-BTC demonstrated real-life application. This result was confirmed by integrating Im@quasi-C-In-BTC with a poly(vinyl pyrrolidone)-poly(vinylidene fluoride) polymer matrix. Density functional theory simulations indicated that Im@quasi-C-In-BTC was strongly acidic and had high water-adsorption capacities, which contributed to extensive hydrogen-bond networks and strong host-guest interactions, in accordance with the experimental finding.

Efficient Photocatalytic Desulfurization in Air through Improved Photogenerated Carriers Separation in MOF MIL101/Carbon Dots-g-C3N4 Nanocomposites

The extensive industrial applications of fuel oil, a critical strategic resource, are accompanied by significant environmental and health concerns due to the presence of sulfur-containing compounds in its composition, which result in hazardous combustion waste. Extensive research has been conducted to develop technologies for low-vulcanization fuel production to address this issue. Consequently, the investigation of catalysts for environmentally friendly and safe photocatalytic desulfurization becomes imperative. To that end, we have designed efficient MIL-101(Fe)/CQDs@g-C3N4 (MIL101/CDs-C3N4) Z-scheme heterojunction photocatalysts with high carrier separation and mobility through a thermal polymerization-hydrothermal strategy. The high concentration of photogenerated carriers facilitates the activation of oxygen and H2O2, leading to increased production of ROS (⋅O2 -, ⋅OH, h+), thereby enhancing the photocatalytic desulfurization (PODS). Additionally, DFT (Density functional theory) calculations were utilized to determine the electron migration pathways of the catalysts and adsorption energies of DBT (dibenzothiophene). Moreover, Gibbs free energy calculations indicated that MIL101/CDs-C3N4 exhibited the lowest activation energy for oxygen and H2O2. The mechanism of photocatalytic desulfurization was proposed through a combination of theoretical calculations and experimental studies. This study provides guidance for the development of MOF-based Z-scheme systems and their practical application in desulfurization processes.

Reductive dissolution of As-bearing iron oxides: Mediating mechanism of fulvic acid and dissimilated iron reducing bacteria

Fulvic acid (FA) and iron oxides often play regulating roles in the geochemical behavior and ecological risk of arsenic (As) in terrestrial ecosystems. FA can act as electron shuttles to facilitate the reductive dissolution of As-bearing iron (hydr)oxides. However, the influence of FA from different sources on the sequential conversion of Fe/As in As-bearing iron oxides under biotic and abiotic conditions remains unclear. In this work, we exposed prepared As-bearing iron oxides to FAs derived from lignite (FAL) and plant peat (FAP) under anaerobic conditions, tracked the fate of Fe and As in the aqueous phase, and investigated the reduction transformation of Fe(III)/As(V) with or without the presence of Shewanella oneidensis MR-1. The results showed that the reduction efficiency of Fe(III)/As(V) was increased by MR-1, through its metabolic activity and using FAs as electron shuttles. The reduction of Fe(III)/As(V) was closely associated with goethite being more conducive to Fe/As reduction compared to hematite. It is determined that functional groups such as hydroxy, carboxy, aromatic, aldehyde, ketone and aliphatic groups are the primary electron donors. Their reductive capacities rank in the following sequence: hydroxy> carboxy, aromatic, aldehyde, ketone> aliphatic group. Notably, our findings suggest that in the biotic reduction, Fe significantly reduction precedes As reduction, thereby influencing the latter's reduction process across all incubation systems. This work provides empirical support for understanding iron's role in modulating the geochemical cycling of As and is of significant importance for assessing the release risk of arsenic in natural environments.

A dual-functional cuprum coordination framework for high proton conduction and electrochemical dopamine detection

The present study selected 5, 5'-((6-(ethylamino)-1, 3, 5-triazine-2, 4-diyl) bis(azanediyl))diisophthalic acid (H4EATDIA) as ligand and an amino-functionalized cuprum-based MOF (EA-JUC-1000), successfully synthesized by microwave-assisted method, for proton conduction and dopamine sensing applications. In order to enhance the proton-conducting potential of EA-JUC-1000, the Brönsted acid (BA) encapsulated composites (BA@EA-JUC-1000) are dopped into chitosan (CS) to form a series of hybrid membranes (BA@EA-JUC-1000/CS). The impedance results display that the best proton conductivity of CF3SO3H@EA-JUC-1000/CS-8% reaches up to 1.23 × 10-3 S∙cm-1 at 338 K and ~ 98% RH, 2.6-fold than that of CS. Moreover, the EA-JUC-1000 is in-situ combined with reduced graphene oxide (rGO) (rGO/EA-JUC-1000), which makes EA-JUC-1000 have a wide detection range (0.1 ~ 500 μM) and a low limit of detection (50 nM), together with good anti-interference performance, reproducibility and repeatability. In addition, the electrochemical sensing method has been successfully applied to detect DA in bovine serum samples. The dual-functional MOF-based hybrid membrane and composites including proton conduction and DA sensing would provide an example of practical application for MOFs.

Review: Synthesis of metal organic framework-based composites for application as immunosensors in food safety

Ensuring food safety continues to be one of the major global challenges. For effective food safety monitoring, fast, sensitive, portable, and efficient food safety detection strategies must be devised. Metal organic frameworks (MOFs) are porous crystalline materials that have attracted attention for use in high-performance sensors for food safety detection owing to their advantages such as high porosity, large specific surface area, adjustable structure, and easy surface functional modification. Immunoassay strategies based on antigen-antibody specific binding are one of the important means for accurate and rapid detection of trace contaminants in food. Emerging MOFs and their composites with excellent properties are being synthesized, providing new ideas for immunoassays. This article summarizes the synthesis strategies of MOFs and MOF-based composites and their applications in the immunoassays of food contaminants. The challenges and prospects of the preparation and immunoassay applications of MOF-based composites are also presented. The findings of this study will contribute to the development and application of novel MOF-based composites with excellent properties and provide insights into advanced and efficient strategies for developing immunoassays.

Syntheses and High Proton Conductivities of Two 3D Zr(IV)/Hf(IV)-MOFs from Furandicarboxylic Acid

With the gradual progress of research on proton-conducting metal-organic framework (MOFs), it has become a challenging task to find MOF materials that are easy to prepare and have low toxicity, high stability, and splendid proton conductivity. With the abovementioned objectives in mind, we selected the non-toxic organic ligand 2,5-furandicarboxylic acid and the low toxic quadrivalent metals zirconium(IV) or hafnium(IV) as starting materials and successfully obtained 2 three-dimensional porous MOFs, [M6O4(OH)4(FDC)4(OH)4(H2O)4] [M = ZrIV (1) and HfIV (2)], with ultrahigh water stability using a rapid and green synthesis approach. Their proton conductive ability is remarkable, thanks to the large number of Lewis acidic sites contained in their porous frameworks and the abundant H-bonding network, hydroxyl groups, as well as coordination and crystalline water molecules. The positive correlation of their proton conductivity with relative humidity (RH) and the temperature was observed. Notably, their optimized proton conductivities are 2.80 × 10-3 S·cm-1 of 1 and 3.38 × 10-3 S·cm-1 of 2 under 100 °C/98% RH, which are at the forefront of Zr(IV)/Hf(IV) MOFs with prominent proton conductivity. Logically, their framework features, nitrogen/water adsorption/desorption data, and activation energy values are integrated to deduce their proton conductivity and conducting mechanism differences.

Defective SO3H-MIL-101(Cr) for capturing different cationic metal ions: Performances and mechanisms

For broad-spectrum adsorption and capture toward cationic metal ions, a facile strategy was adopted to fabricate defective SO₃H-MIL-101(Cr) (SS-SO₃H-MIL-101(Cr)-X, X = 2, 3, 4) with enhanced vacancies using seignette salt (SS) as the modulating agent. The boosted adsorption performances of SS-SO₃H-MIL-101(Cr)-X toward eight different ions, including Ag⁺, Cs⁺, Pb²⁺, Cd²⁺, Ba²⁺, Sr²⁺, Eu³⁺ and La³⁺ in both individual component and mixed component systems, could be ascribed to the effective mass transfer resulting from the exposure of defective sites. Especially, the optimal SS-SO₃H-MIL-101(Cr)-3 could remove all the selected metal cations to below the permissible limits required by the World Health Organization (WHO) in the continuous-flow water treatment system. Furthermore, SS-SO₃H-MIL-101(Cr)-3 exhibited good adsorption capacity (189.6 mg·g^-1) toward Pb²⁺ under neutral condition and excellent desorption recirculation performance (removal efficiency > 95% after 5 cycles). Moreover, the adsorption mechanism involved the electrostatic adsorption and coordinative interactions resulting from complexation between the adsorption active sites and targeted cations (like Cr-O-M and S-O-M), which were explored systematically via both X-ray photoelectron spectroscopy (XPS) determination and density functional theory (DFT) calculations. Overall, this work provided guidance for modulating SS-SO₃H-MIL-101(Cr)-X to promote its potential application in widespread metal cations removal from wastewater.