Hydrolytically Stable and Trifunctional Zirconium-Based Organic Frameworks toward Cr2O72- Detection, Capture, and Photoreduction

Efficient removal of Cr (VI) by Bifunction zinc porphyrin COF: Coupling adsorption with Photocatalysis, performance Evaluation, and mechanism analysis

HYPOTHESIS

Hexavalent chromium, recognized as one of the most toxic heavy metals, demands the development of advanced materials capable of both adsorption and photocatalysis for effective Cr (VI) removal.

EXPERIMENTS

This study successfully synthesized a two-dimensional zinc porphyrin covalent organic framework (ZnPor-COF) via a solvent-based method. Performance evaluations have demonstrated that the ZnPor-COF possesses outstanding capabilities for the adsorptive and/or photocatalytic elimination of Cr (VI). Particularly noteworthy is the observation that when adsorption and photocatalysis are coupled, the ZnPor-COF attains an exceptional 99.7 % removal rate for a Cr (VI) concentration of 30 mg/L within just 60 min, with minimal susceptibility to coexisting ions. After five consecutive cycles, the material sustains a removal efficiency of 90 %, indicative of its robust cyclability.

FINDINGS

Theoretical calculations, as well as experimental validations, have indicated that the integration of Zn ions into the porphyrin COF not only results in an expanded specific surface area and an increased count of adsorption sites but also significantly improves the COF's photosensitivity and the capability for charge carrier separation. Furthermore, the core of the synergistic effect between adsorption and photocatalysis lies in the ability of photocatalysis to substantially augment the adsorption process.

Multifunctional In-MOF and Its S-Scheme Heterojunction toward Pollutant Decontamination via Fluorescence Detection, Physical Adsorption, and Photocatalytic REDOX

A novel doubly interpenetrated indium-organic framework of 1 has been assembled by In3+ ions and highly conjugated biquinoline carboxylate-based bitopic connectors (H2L). The isolated 1 exhibits an anionic framework possessing channel-type apertures repleted with exposed quinoline N atoms and carboxyl O atoms. Owing to the unique architecture, 1 displays a durable photoluminescence effect and fluorescence quenching sensing toward CrO42-, Cr2O72-, and Cu2+ ions with reliable selectivity and anti-interference properties, fairly high detection sensitivity, and rather low detection limits. Ligand-to-ligand charge transition (LLCT) was identified as the essential cause of luminescence by modeling the ground state and excited states of 1 using DFT and TD-DFT. In addition, the negatively charged framework has the ability to rapidly capture single cationic MB, BR14, or BY24 and their mixture, including the talent to trap MB from the (MB + MO) system with high selectivity. Moreover, intrinsic light absorption capacity and band structure feature endow 1 with effective photocatalytic decomposition ability toward reactive dyes RR2 and RB13 under ultraviolet light. Notably, after further polishing the band structure state of 1 by constructing the S-scheme heterojunction of In2S3/1, highly efficient photocatalytic detoxification of Cr(VI) and degradation of reactive dyes have been fully achieved under visible light. This finding may open a new avenue for designing novel multifunctional MOF-based platforms to address some intractable environmental issues, i.e., detection of heavy metal ions, physical capture of pony-sized dyes, and photochemical decontamination of ultrastubborn reactive dyes and highly toxic Cr(VI) ions from water.

Simultaneous Detection and Decontamination of Dichromate Ions: The Fluorescence Response and Photocatalysis of Thiadiazole-Modified Zr-Metal-Organic Frameworks

The simultaneous analysis and removal of highly toxic hexavalent chromium (Cr (VI)) in contaminated water via an easy method remain a serious task. Based on the guidance of bibliometric analysis, a thiadiazole ligand-modified zirconium metal-organic framework (Zr-MOF) heralds a new and simple approach to Cr (VI) treatment. The concentration can be determined by fluorescence quenching with a low detection limit of 1.4 μM and a high quenching constant of 6.88 × 103 M-1. For the sensing mechanism, the fluorescence intensity of the Zr-MOF decreased rapidly due to the competition of Cr (VI) with the Zr-MOF for absorption excitation energy and the induction of Zr-MOF aggregation. The analysis system also displayed satisfactory stability and applicability. Apart from sensing application, Zr-MOF can convert Cr (VI) to Cr (III), and the reduction rate constant was 0.004 min-1 under irradiation. Therefore, the bifunctional Zr-MOF provided a potential application method for controlling the pollution caused by Cr (VI) in wastewater.

Two-Dimensional Lanthanide Metal-Organic Frameworks as a Platform for Sensing Pollutant and Nitrophenols Reduction

The discharge of harmful and toxic pollutants in water is destroying the ecosystem balance and human being health at an alarming rate. Therefore, the detection and removal of water pollutants by using stable and efficient materials are significant but challenging. Herein, three novel lanthanide metal-organic frameworks (Ln-MOFs), [La(L)(DMF)2(H2O)2]·H2O (LCUH-104), [Nd(L)(DMF)2(H2O)2]·H2O (LCUH-105), and [Pr(L)(DMF)2(H2O)2]·H2O (LCUH-106) [H3L = 5-(4-(tetrazol-5-yl)phenyl)isophthalic acid (H3TZI)] were solvothermally constructed and structurally characterized. In the three Ln-MOFs, dinuclear metallic clusters {Ln2} were connected by deprotonated tetrazol-containing dicarboxylate TZI3- to obtain a 2D layered framework with a point symbol of {42·84}·{46}. Their excellent chemical and thermal stabilities were beneficial to carry out fluorescence sensing and achieve the catalytic nitrophenols (NPs) reduction. Especially, the incorporation of the nitrogen-rich tetrazole ring into their 2D layered frameworks enables the fabrication of Pd nanocatalysts (Pd NPs@LCUH-104/105/106) and have dramatically enhanced catalytic activity by using the unique metal-support interactions between three Ln-MOFs and the encapsulating palladium nanoparticles (Pd NPs). Specifically, the reduction of NPs (2-NP, 3-NP, and 4-NP) in aqueous solution by Pd NPs@LCUH-104 exhibits exceptional conversion efficiency, remarkable rate constants (k), and outstanding cycling stability. The catalytic rate of Pd NPs@LCUH-104 for 4-NP is nearly 8.5 times more than that of Pd/C (wt 5%) and its turnover frequency value is 0.051 s-1, which indicate its excellent catalytic activity. Meanwhile, LCUH-105, as a multifunctional fluorescence sensor, exhibited excellent fluorescence detection of norfloxacin (NFX) (turn on) and Cr2O72- (turn off) with high selectivity and sensitivity at a low concentration, and the corresponding fluorescence enhancement/quenching mechanism has also been systematically investigated through various detection means and theoretical calculations.

Fluorine extraction from organofluorine molecules to make fluorinated clusters in yttrium MOFs

A new rare earth based two-dimensional coordination network and a three-dimensional metal-organic framework (MOF) have been synthesized using bicinchoninic acid (BCA) and yttrium(iii) ions. Yttrium dimer nodes are formed in the absence of a modulator, resulting in a 2D layered coordination network (Y-BCA-2D). The presence of fluorinating agents, e.g., 2-fluorobenzoic acid (2-FBA), 2,6-difluorobenzoic acid (2,6-DFBA), and perfluorohexanoic acid (PFHxA) result in μ3-F bridged metal hexaclusters (Y6F8) that form a three-dimensional MOF (Y-BCA-3D). It was found that Y3+ can break highly stable C-F bonds in aromatic and aliphatic fluorinated compounds. Single-crystal X-ray diffraction (SC-XRD) shows the presence of fluorine in the metal cluster which was confirmed by energy dispersive X-ray spectroscopy (EDS). High resolution X-ray photoelectron spectroscopy (XPS) and 19F Nuclear Magnetic Resonance (NMR) also verify the presence of metal-fluorine bonds in the cluster. The Y-BCA-3D MOF selectively adsorbs CO2 but not N2.

Exploring novel heterojunctions based on the cerium metal-organic framework family and CAU-1, as dissimilar structures, for the sake of photocatalytic activity enhancement

Ce-based metal-organic frameworks (Ce-MOFs) are excellent photocatalysts due to their high efficiency in charge transportation. The integration of this family with CAU-1 (CAU standing for Christian-Albrechts-University), as a MOF benefiting from its ultra-high surface area, can remarkably enhance the properties of the structure. This research includes four new heterojunctions, namely CAU-1/Ce-BDC-NH2, CAU-1/Ce-UiO-66, CAU-1/Ce-MOF-808, and CAU-1/Ce-BDC, prepared by an innovative method, and several characterization techniques were employed to study the structural features of the frameworks. Their high surface area and low bandgap energy seemed appropriate for catalytic applications. Therefore, CAU-1/Ce-BDC was chosen for the photocatalytic removal of Cr(vi), a dangerous heavy metal, from aqueous systems. According to the results, a 96% reduction of Cr(vi) to Cr(iii) within 75 min was observed, and the catalyst retained its stability after four runs of reactions under acidic conditions.

RhB-Embedded Zirconium-Biquinoline-Based MOF Composite for Highly Sensitive Probing Cr(VI) and Photochemical Removal of CrO42-, Cr2O72-, and MO

How to accurately detect and efficiently sweep Cr(VI) from contaminated water has come into focus. Zirconium-based metal-organic frameworks (MOFs) play vital roles in water environmental chemistry due to excellent hydrolysis-resistant stability. However, as photochemical probes and photocatalysts, poor performances in detection sensitivity, selectivity, and photosensitiveness limit sole Zr-MOFs' applications. So, it is urgent to quest valid strategies to break through the dilemmas. Embedding luminous dyes into MOFs has been considered one of the most feasible avenues. Herein, a dual-emissive RhB@Zr-MOF with orange-yellow fluorescence has been assembled by in situ-encapsulating rhodamine B (RhB) into a zirconium-biquinoline-based MOF. Actually, within RhB@Zr-MOF, the aggregation fluorescence quenching (ACQ) effect of RhB molecules was effectively avoided. Notably, RhB@Zr-MOF exhibits a rapid fluorescence quenching response toward Cr(VI) ions with high selectivity, sensitivity, and anti-interference abilities. More interestingly, unlike the most widely reported fluorescence resonance energy transfer (FRET) between MOFs and encapsulated guest modules, photoinduced electron transfer from RhB to Zr-MOF has been confirmed by modeling the ground state and excited states of RhB@Zr-MOF using density functional theory (DFT) and time-dependent DFT (TD-DFT). The effective electron transfer makes RhB@Zr-MOF more sensitive in probing Cr₂O₇^2- and CrO₄^2- ions with ultralow detection limit (DL) values of 6.27 and 5.26 ppb, respectively. Prominently, the detection sensitivity based on DL values has been increased about 6 and 9 times, respectively, compared with pristine Zr-MOF. Moreover, rather negative CB and positive VB potentials make RhB@Zr-MOF have excellent photochemical scavenging ability toward Cr(VI) and MO.

Bifunctional Sensors Based on Phosphomolybdates for Detection of Inorganic Hexavalent Chromium and Organic Tetracycline

Exploring effective sensors for detecting possible hazards in a water system are greatly significant. This work proposed a strategy for stable and effective bifunctional sensors via incorporating hourglass-type phosphomolybdates into metal-organic fragments to construct a high-dimensional framework. Two hourglass-type phosphomolybdate-based electrochemical sensors toward heavy metal ion Cr(VI) and tetracycline (TC) detection were designed with the formula [Co^II₂(H₂O)₄Na^I₂][Co^II(Hbpe)][Na^I(bpe)_1.5]{Co^II[P^V₄Mo^V₆O₃₁H₆]₂}·9H₂O (1) and [Co^II(H₂O)₄Na^I₃][Co^II(Hbpe)][Co^II(bpe)]{Co^II[P^V₄Mo^V₆O₃₁H₆]₂}·9H₂O (2) [bpe = 1,2-di(4-pyridyl)ethylene]. Structural analysis showed that hybrids 1 and 2 possess three-dimensional POM-supported network features with favorable stability and exhibit reversible redox properties. Experiments found that this kind of hybrids as efficient sensors have excellent electrochemical performance toward Cr(VI) detection with high sensitivities of 0.111 μA·μM^-1 for 1 and 0.141 μA·μM^-1 for 2, fast response time of 1 s, and low detection limits of 30 nM for 1 and 27 nM for 2, which far meet the standard of WHO for drinking water. Moreover, hybrids 1-2 also exhibit fast responses to TC detection with sensitivities of 0.0073 and 0.022 μA·mM^-1 and detection limits of 0.426 and 0.084 mM. This work offers a novel strategy for the purposeful design of efficient POM-based electrochemical sensors for accurate determination of contaminants in a practical water system.

Recent Progress in the Development of MOF-Based Photocatalysts for the Photoreduction of Cr(VI)

There has been a direct correlation between the rate of industrial development and the spread of pollution on Earth, particularly in the last century. The organic and inorganic pollutants generated from industrial activities have created serious risks to human life and the environment. The concept of sustainability has emerged to tackle the environmental issues in developing chemical-based industries. However, pollutants have continued to be discharged to water resources, and finding appropriate techniques for the removal and remedy of wastewater is in high demand. Chromium is one of the high-risk heavy metals in industrial wastewaters that should be removed via physical adsorption and/or transformed into less hazardous chemicals. Photocatalysis as a sustainable process has received considerable attention as it utilizes sunlight irradiation to remedy Cr^(VI) via a cost-effective process. Numerous photocatalytic systems have been developed up to now, but metal-organic frameworks (MOFs) have gained growing attention because of their unique versatilities and facile structural modulations. A variety of MOF-based photocatalysts have been widely employed for the photoreduction of Cr^(VI). Here, we review the recent progress in the design of MOF photocatalysts and summarize their performance in photoreduction reactions.

Visible-Light-Driven Sonophotocatalysis for the Rapid Reduction of Aqueous Cr(VI) Based on Zirconium-Porphyrin Metal-Organic Frameworks with csq Topology

Photochemical treatment of highly toxic Cr(VI) is a desirable and ecofriendly method to protect the environment and human beings. In this study, a MOF-based sonophotocatalytic system is established, in which visible-light-driven sonophotocatalytic reduction of toxic Cr(VI) to Cr(III) in water is investigated using zirconium-porphyrin metal-organic frameworks (MOFs) structured as PCN-222(M) [M = H₂, Zn(II), Fe(III), Co(II)]. In the view of the synergistic effect of sonochemistry and photocatalysis, PCN-222(M) exhibited enhanced activities for Cr(VI) reduction compared with the photocatalytic process. Kinetic studies showed that apparent reaction rate constants in the sonophotocatalytic system of PCN-222(M) are 1.5-3.3 times higher than those in photocatalysis. Fluorescence and UV-vis absorption spectra measurements demonstrate that the sonophotocatalytic process promotes the transfer of photoinduced electrons from PCN-222(M) to Cr(VI), thus enhancing the catalytic performance. The innovative combination of porous MOFs and sonophotocatalytic technology might become a feasible strategy to improve the existing MOF-based photocatalytic systems.

Multifunctional Zr-MOF Based on Bisimidazole Tetracarboxylic Acid for pH Sensing and Photoreduction of Cr(VI)

Herein, a new luminescent zirconium MOF [Zr-BBI, BBI = 4,4',4″,4‴-(1,4-phenylenebis(1H-imidazole-2,4,5-triyl))tetrabenzoic acid] was successfully constructed by a rationally designed functionalized bisimidazole tetracarboxylic acid ligand. Zr-BBI consists of eight-connected Zr₆ clusters and four-connected BBI ligands. The high connection mode must be responsible for the high stabilities of Zr-BBI in both acidic and basic systems. Apart from the high stability, the inherent bisimidazole units endow Zr-BBI with the traits of intense fluorescent emission as well as the protonation/deprotonation behavior. Therefore, Zr-BBI could display a highly sensitive fluorescence response along with the varying pH values in the aqueous solutions and act as a pH sensing scaffold, especially in the pH value range from 4.6 to 7.12. Zr-BBI also shows good fluorescence detection performance toward Cr₂O₇^2- at a low concentration with a high K_SV value up to 6.49 × 10⁴ M^-1. Moreover, by the utilization of Zr-BBI as a catalyst, Cr(VI) could be effectively photoreduced to Cr(III) in aqueous solution under visible light irradiation, in which the introduction of a hole scavenger (benzyl alcohol) could further significantly enhance the photocatalytic efficiency. Compared to that of the recently representative MOFs, the k value of the photocatalytic reaction over Zr-BBI is as high as 0.073 min^-1. With the consideration of the presented results, Zr-BBI can serve as a multifunctional platform for efficiently sensing and photoreducing Cr₂O₇^2- in an aqueous system, which fully illustrates the feasibility that introducing specific functional groups in the framework of MOFs would enhance the related photocatalytic activity.

A series of novel Co(ii)-based MOFs: syntheses, structural diversity, and various properties

Three novel Co(ii)-based MOFs, having structural diversities and various properties are successfully synthesized.