Ce-based UiO-66 metal-organic frameworks as a new redox catalyst for atomic spectrometric determination of Se(VI) and colorimetric sensing of Hg(II)

Tailoring Catalysts for CO2 Hydrogenation: Synthesis and Characterization of NH2-MIL-125 Frameworks

Copper nanoparticles have been integrated onto the framework of modified NH2-MIL-125(Ti), a metal-organic framework (MOF), and evaluated as catalysts for converting CO2 into valuable products. The modified MOF was achieved through a post-synthetic modification process involving the partial replacement of titanium with zirconium or cerium within the MOF's structure. The objective behind this alteration is to create a synergistic effect between the MOF, serving as a support matrix, and the embedded copper nanoparticles, thereby enhancing the performance of the catalyst. The obtained catalysts were characterized and evaluated in the hydrogenation of CO2 to methanol under different experimental conditions, reaching CO2 conversions of up to 5%, with a selectivity towards methanol that reached values of up to 60%. According to the obtained results, the catalyst composed of Ti, Zr and Cu stood out for having the highest CO2 conversion and selectivity towards methanol, in addition to practically inhibiting the production of methane. These results demonstrate that the interaction of the framework with the Cu nanoparticles, and thus its catalytic properties, can be changed by modifying the properties of the MOF.

Investigation the effect of exchange solvents on the adsorption performances of Ce-MOFs towards organic dyes

Cerium-based MOFs (Ce-MOFs) are regarded as attractive porous materials showing various structures, excellent thermal and chemical stability, tunable porous properties, and simple synthetic methods that are useful for wastewater treatment applications. Hence, in the present work, we synthesized a series of Ce-MOFs through a fast and green synthetic method at room temperature using water as a green solvent. Four different solvents including ethanol, chloroform, acetone, and methanol were used in the solvent-exchange process to engineer the properties of prepared Ce-MOFs. The influence of different exchange solvents on the crystalline structure, porous structure, thermal stability, and surface morphology of Ce-MOFs was studied systematically. It was found that exchange solvents can significantly affect the chemical and physical properties of prepared Ce-MOFs. Using ethanol as an exchange solvent results in the production of highly crystalline MOF that has the highest surface area (843 m2/g) and pore volume (0.7518 cm3/g) compared to other prepared Ce-MOFs. The dye adsorption experiments revealed that the activated sample by acetone (Ce-MOF-4) exhibited the highest adsorption capacities toward both anionic (270.27 mg/g for Congo Red (CR)) and cationic (227.27 mg/g for Malachite Green (MG)) dyes. This MOF adsorbs both organic dyes via different mechanisms including hydrogen bonding, pore-filling, π-π stacking, coordination, and electrostatic interactions. Moreover, it exhibited good structural stability in acidic solution, neutral solution, and during consecutive adsorption-desorption cycles, confirming its potential to be applied as a stable adsorbent for simultaneous removal of cationic and anionic organic dyes from water.

Mixed-valence metal-organic frameworks: concepts, opportunities, and prospects

Owing to increasing global demand for the development of multifunctional advanced materials with various practical applications, great attention has been paid to metal-organic frameworks due to their unique properties, such as structural, chemical, and functional diversity. Several strategies have been developed to promote the applicability of these materials in practical fields. The induction of mixed-valency is a promising strategy, contributing to exceptional features in these porous materials such as enhanced charge delocalization, conductivity, magnetism, etc. The current review provides a detailed study of mixed-valence MOFs, including their fundamental properties, synthesis challenges, and characterization methods. The outstanding applicability of these materials in diverse fields such as energy storage, catalysis, sensing, gas sorption, separation, etc. is also discussed, providing a roadmap for future design strategies to exploit mixed valency in advanced materials. Interestingly, mixed-valence MOFs have demonstrated fascinating features in practical fields compared to their homo-valence MOFs, resulting from an enhanced synergy between mixed-valence states within the framework.

A critical review on metal organic frameworks (MOFs)-based sensors for foodborne pathogenic bacteria detection

The pervasive threat of foodborne pathogenic bacteria necessitates advancements in rapid and reliable detection methods. Traditional approaches suffer from significant limitations including prolonged processing times, limited sensitivity and specificity. This review comprehensively examines the integration of metal organic frameworks (MOFs) with sensor technologies for the enhanced detection of foodborne pathogens. MOFs, with their unique properties such as high porosity, tunable pore sizes, and ease of functionalization, offer new avenues for sensor enhancement. This paper provides a comprehensive analysis of recent developments in MOFs-based sensors, particularly focusing on electrochemical, fluorescence, colorimetric, and surface-enhanced Raman spectroscopy sensors. We have provided a detailed introduction for the operational principles of these sensors, highlighting the role of MOFs play in enhancing their performance. Comparative analyses demonstrate MOFs' superior capabilities in enhancing signal response, reducing response time, and expanding detection limits. This review culminates in presenting MOFs as transformative materials in the detection of foodborne pathogens, paving the way for their broader application in ensuring food safety.

Removal of Moxifloxacin from Aqueous Solutions Using GO/Cr-MOFs

The composite material, consisting of graphene oxide (GO) and chromium metal-organic frameworks (Cr-MOFs), was successfully synthesized by using a solvothermal method. The organic ligand employed was 2,5-dihydroxyterephthalic acid, while chromium acetate served as the source of the metal. The resulting material underwent characterization through Fourier transform infrared, scanning electron microscopy, and X-ray diffraction techniques. Subsequently, the adsorption capacity of the composite material toward moxifloxacin was evaluated. The results indicated a gradual increase in the moxifloxacin removal rate from GO/Cr-MOFs over time until reaching an equilibrium with a maximum removal rate of 90.4%. Additionally, it was observed that higher temperatures led to a decrease in the adsorption capacity. By incorporating 30 mg of GO/Cr-MOFs into a solution containing 40 ppm of moxifloxacin, the adsorption capacity could be maximized at 222.25 mg/g. Experimental data on MOF adsorption of moxifloxacin were analyzed using pseudo-first-order kinetics (PFO), pseudo-second-order kinetics (PSO), and Langmuir, Freundlich, and Temkin isotherm models for theoretical research purposes. Results showed that the PSO model exhibited a better correlation than the PFO model did. Furthermore, experimental data demonstrated good agreement with the Freundlich isothermal model, suggesting its effectiveness in accurately describing the adsorption process. Henceforth, it can be concluded that chemisorption plays a significant role in removing moxifloxacin by GO/Cr-MOFs. The van't Hoff equation analysis revealed an exothermic and spontaneous nature of moxifloxacin adsorption onto GO/Cr-MOFs. Compared to other materials, the GO/Cr-MOF composite exhibited high potential for applications such as drug removal or related fields.

Mn-modified porphyrin metal-organic framework mediated colorimetric and photothermal dual-channel probe for sensitive detection of organophosphorus pesticides

Herein, a novel dual-mode probe for organophosphorus pesticides (OPs) colorimetric and photothermal detection was developed based on manganese modified porphyrin metal-organic framework (PCN-224-Mn). PCN-224-Mn had excellent oxidase-like activity and oxidized colorless 3,3,5,5-tetramethylbenzidine (TMB) to blue-green oxidation state TMB (oxTMB), which exhibited high temperature under near-infrared irradiation. l-ascorbate-2-phosphate was hydrolyzed by acid phosphatase to produce ascorbic acid, which weakened colorimetric and photothermal signals by impacting oxTMB generation. The presence of OPs blocked the production of ascorbic acid by irreversibly inhibiting the activity of acid phosphatase, causing the restoration of chromogenic reaction and the increase of temperature. Under the optimal conditions, the probe showed a good linear response to OPs in the concentration range of 5 ∼ 10000 ng/mL, using glyphosate as the analog. The detection limits of glyphosate in colorimetric mode and photothermal mode were 1.47 ng/mL and 2.00 ng/mL, respectively. The probe was successfully used for sensitive identification of OPs residues in tea, brown rice, and wheat flour. This work proposes a simple and reliable colorimetric/photothermal platform for OPs identification, which overcomes the problem that single-mode detection probes are susceptible to external factors, and has broad application potential in the field of food safety.

G-quadruplex DNA-based colorimetric biosensor for the ultrasensitive visual detection of strontium ions using MnO2 nanorods as oxidase mimetics

Strontium-90 (90Sr) is a major radioactive component that has attracted great attention, but its detection remains challenging since there are no specific energy rays indicative of its presence. Herein, a biosensor that is capable of rapidly detecting Sr2+ ions is demonstrated. Simple colorimetric method for sensitive detection of Sr2+ with the help of single-stranded DNA was developed by preparing MnO2 nanorods as oxidase mimic catalysis 3,3',5,5'-tetramethylbenzidine (TMB). Under weakly acidic conditions, MnO2 exhibited a strong oxidase-mimicking activity to oxidize colorless TMB into blue oxidation products (oxTMB) with discernible absorbance signals. Nevertheless, the introduction of a guanine-rich DNA aptamer inhibited MnO2-mediated TMB oxidation and reduced oxTMB formation, resulting in blue fading and diminished absorbance. Upon the addition of strontium ions to the system, the aptamers formed a stable G-quadruplex structure with strontium ions, thereby restoring the oxidase-mimicking activity of MnO2. Under the best experimental conditions, the absorbance exhibits a linear relationship with the Sr2+ concentration within the range 0.01-200 μM, with a limit of detection of 0.0028 µM. When the concentration of Sr2+ from 10-8 to 10-6 mol L-1, a distinct color change gradient could be observed in paper-based sensor. We successfully applied this approach to determine Sr2+ in natural water samples, obtaining recoveries ranging from 97.6 to 103% with a relative standard deviation of less than 5%. By providing technical solutions for detection, our work contributed to the effective monitoring of transportation of radioactive Sr in the environment.

Green Synthesis of Cerium-Based Metal-Organic Framework (Ce-UiO-66 MOF) for Wastewater Treatment

Green synthesis of metal-organic frameworks (MOFs) in aqueous solutions under ambient conditions with reduced production costs and environmental effects is an efficient technique to transfer lab-scale production to industrial large scale. Hence, this work proposes a green, low-cost, sustainable, rapid, and innovative synthetic strategy to produce cerium-based (Ce-UiO-66) MOFs under ambient conditions in the presence of water as a green solvent. This synthetic strategy exhibits great potential compared to conventional solvothermal synthetic techniques, and it does not need external activation energy and organic solvents, which can achieve the standards of green chemistry. Ce-UiO-66 MOF was synthesized successfully and utilized as a green adsorbent to efficiently eliminate anionic Congo Red (CR) dye from dye-containing wastewater. The experimental adsorption results were well matched to the pseudo-second-order kinetic and Langmuir isotherm models, in which the maximum CR adsorption capacity was measured to be about 285.71 mg/g. To evidence the applicability of Ce-UiO-66 MOFs in CR adsorption, the CR adsorption reaction was performed in the presence of interfering pollutants [e.g., salts (NaCl, KCl, and MgCl2) and cationic organic dyes (Malachite Green (MG) and Methylene Blue (MB)], where the results prove the promising adsorption performances of Ce-UiO-66 MOFs toward CR dye. Interestingly, the synthesized adsorbent exhibited high structural stability during repeated adsorption-desorption cycles, where the surface area of MOFs decreased from 555 to 376 m2/g after three cycles, while its CR adsorption capacity decreased by only 10% compared to that of the fresh adsorbent. All these outstanding properties indicate that the Ce-UiO-66 MOFs will be an effective adsorbent for water and wastewater treatment applications.

Concentration- and Self-Catalysis-Dominated Rapid Synthesis of Multifunctional UiO-66(Ce) for Dual-Mode Sensing of Tetracycline

Simple and rapid synthesis of multifunctional metal-organic frameworks (MOFs) at room temperature (RT) with their multifunction controllable is still appealing for further expansion of the practical applications of MOFs. Herein, in this work, rapid RT synthesis of a multifunctional UiO-66(Ce) [M-UiO-66(Ce)] with both oxidase-like activity and fluorescence emission properties was facilely achieved within 15 min through a straightforward reactant concentration modulation and self-catalytic postmodification strategy. Appropriate concentrations of cerium ammonium nitrate or 1,4-benzenedicarboxylic acid (BDC) were beneficial for the synthesis of UiO-66(Ce) with better crystallization. During the postmodification process, through regulation of the self-photocatalysis of UiO-66(Ce), a high conversion rate from BDC to BDC-OH of up to 14% can be obtained, resulting in a significantly enhanced fluorescence signal of M-UiO-66(Ce) within 2 min. Moreover, M-UiO-66(Ce) enabled the accurate and reliable detection of tetracycline (TC) in real samples. Besides, the colorimetric and fluorescence modes complemented each other, expanding the linear range of TC detection and exhibiting its great potential for practical applications. This work provides new insights for the convenient and rapid synthesis of multifunctional materials based on MOFs, which is favorable for promoting the large-scale preparation of MOFs and their practical application in on-site environmental pollutant sensing.

Efficient removal of organic dyes using electrospun nanofibers with Ce-based UiO-66 MOFs

Cerium-based UiO-66 (Ce-UiO-66) metal-organic frameworks (MOFs) were synthesized via a facile solvothermal method and fully characterized using FTIR, XRD, BET, SEM, EDX, and zeta potential techniques. The synthesized Ce-UiO-66 particles were embedded into an electrospun cross-linked polyvinyl alcohol (PVA)/chitosan (CTS) nanofiber (EPCNF), and then employed to remove organic dyes from water. The adsorption results demonstrated that the adsorption capacities of both anionic (Congo Red (CR), Methyl Orange (MO) and Methyl Red (MR)) and cationic (Methylene Blue (MB)) dyes over the fabricated electrospun nanofibers (ENFs) increased with increasing the loadings of Ce-UiO-66 MOFs. Accordingly, the adsorption performance of EPCNF-10 (containing 10 wt% of Ce-UiO-66 MOFs) adsorbent toward these organic dyes is in the order of CR (102.04 mg/g) > MO (87.71 mg/g) > MR (65.35 mg/g) > MB (34.24 mg/g). Moreover, it was found that the Freundlich isotherm model and the pseudo-second-order kinetic model were appropriate for describing the adsorption behaviors of EPCNF-10 adsorbent toward both anionic and cationic dyes. Thus, it can be proposed that the fabricated EPCNF-10 adsorbent would be effective adsorbent materials for the removal of anionic and cationic dyes from water due to its excellent adsorption performance, facile preparation, good regeneration, and simple separation from aqueous solutions.

Advances in the application of metal-organic framework nanozymes in colorimetric sensing of heavy metal ions

Nanozymes, which can be defined as nanomaterials with excellent catalytic function, are well known to the scientific community due to their distinct merits, such as low cost and high stability, which render them preferable to natural enzymes. As porous organic-inorganic coordination materials, metal-organic frameworks (MOFs) possess a large number of active sites and thus can effectively mimic the properties of natural enzymes. Recently, MOF-based nanozymes have also exhibited good application potential for the analysis of heavy metal ions. In comparison to the traditional detection methods for heavy metal ions, nanozyme-based colorimetric sensing permits intuitive visual analysis by using relatively simple instruments, facilitating rapid and simple on-site screening. In this minireview, the preparation of MOF-based nanozymes and the different nanozyme activity types are briefly described, such as peroxidase-like and oxidase-like, and the relevant catalytic mechanisms are elaborated. Based on this, different response mechanisms of MOF-based colorimetric methods to heavy metal ions, such as turn-off, turn-on, and turn-off-on, are discussed. In addition, the colorimetric sensing applications of MOF-based nanozymes for the detection of heavy metal ions are summarized. Finally, the current research status of MOF-based nanozymes and the future development direction are discussed.

Detection of trace antimony by vanadium (IV) ion assisted photochemical vapor generation with inductively coupled plasma mass spectrometry measurement

In this work, a method for sensitive detection of trace antimony (Sb) was developed by inductively coupled plasma mass spectrometry (ICP MS) coupled with photochemical vapor generation (PVG). V(IV) ions were used as new "sensitizers" for improving the PVG efficiency of Sb. Factors influenced the PVG and the detection of Sb by ICP MS were investigated, including the type and concentration of low molecular weight organic acids, the UV irradiation time, the concentration of V(IV) ions, the air-liquid interface, the flow rate of Ar carrier gas, and interferences from co-existing ions. It was found that efficient reduction of Sb was obtained in the medium of 10% (v/v) formic acid (FA), 10% (v/v) acetic acid (AA), and 80 mg L^-1 of V(IV) with 100 s UV irradiation. Under the selected conditions, there was no significant difference in analytical sensitivity between Sb(III) and Sb(V). The limit of detection (LOD, 3σ) was 4.7 ng L^-1 for Sb with ICP MS measurement. Compared to traditional direct solution nebulization, the analytical sensitivity obtained in this work was enhanced about 19-fold. Relative standard deviations (RSDs, n = 7) were 1.9% and 2.3% for replicate measurement of 0.5 μg L^-1 Sb(III) and Sb(V) standard solutions, respectively. The proposed method was applied for the determination of trace Sb in water samples and two certified reference materials (CRMs) of sediments with satisfactory results. Moreover, the generated volatile species of Sb in this work was found to be (CH₃)₃Sb.

Visual detection of vitamin C in fruits and vegetables using UiO-66 loaded Ce-MnO2 mimetic oxidase

A nanocomposite (UiO-66/Ce-MnO₂) was fabricated by combining UiO-66 with cerium-doped manganese dioxide (Ce-MnO₂) for colorimetric detecting vitamin C (Vc). Compared with traditional artificial enzymes, the as-synthesized UiO-66/Ce-MnO₂ were simple to prepare and did not require the participation of other active substances. The doping of cerium increased the oxygen vacancies and the UiO-66 as a carrier improved the dispersibility. The formation of superoxide anion (O₂^-) and the inside Ce⁴⁺/Ce³⁺ and Mn⁴⁺/Mn³⁺ redox couples of UiO-66/Ce-MnO₂ endowed UiO-66/Ce-MnO₂ with a high catalytic capability, which could catalytically oxidize 3, 3', 5, 5'-tetramethylbenzidine (TMB) into oxidation state TMB (oxTMB) without H₂O₂, accompanying with color change and a prominent peak at 652 nm in UV-vis spectra. Based on the inhibitory effects of Vc on catalytic oxidation of TMB, detection of Vc can be achieved, exhibiting a linear relationship in the concentration of 1.13-17.01 μmol L^-1 with a low detection limit of 65.82 nmol L^-1. This system can also be detected by smartphone, the linear detection range is 12.47-22.67 μmol L^-1. Vc contents in fruits and vegetables detected by the sensor were in good agreement with the 2, 4-Dinitrophenylhydrazine colorimetry method (P > 0.05), indicating a reliable sensor for Vc detection.

Investigation of the preparation and reactivity of metal-organic frameworks of cerium and pyridine-2,4,6-tricarboxylate

The synthesis of three coordination polymers of cerium(III) and the ligand pyridine-2,4,6-tricarboxylate (PTC) is reported. Two of the materials crystallise under hydrothermal conditions at 180 °C, with [Ce(PTC)(H₂O)₂]·1.5H₂O, (1), being formed on extended periods of reaction time, 3 days or longer, and Ce(PTC)(H₂O)₃, (2), crystallising after 1 day. Both phases contain Ce(III) but are prepared using the Ce(IV) salt Ce(SO₄)₂·4H₂O as reagent. Under solvothermal conditions (mixed water-N,N-dimethylformamide (DMF)), the phase [Ce(PTC)(H₂O)(DMF)]·H₂O (3) is crystallised. The structures of the three materials are resolved by single crystal X-ray diffraction, with the phase purity of the samples determined by powder X-ray diffraction and thermogravimetric analysis. (1) is constructed from helical chains cross-linked by the PTC linkers to give a three-dimensional structure that contains clusters of water molecules in channels that are hydrogen-bonded to each other and to additional waters that are coordinated to cerium. (2) also contains nine-coordinate cerium but these are linked to give a dense framework, in which water is directly coordinated to cerium. (3) contains corner-shared nine-coordinate cerium centres, linked to give a framework in which Ce-coordinated DMF fills space. Upon heating the material (1) in air all water is irreversibly lost to give a poorly crystalline anhydrous phase Ce(PTC), as deduced from X-ray thermodiffractometry and thermogravimetric analysis. The material (1), however, is hydrothermally stable, and is also stable under oxidising conditions, where immersion in 30% H₂O₂ gives no loss in crystallinity. Oxidation of around 50% of surface Ce to the +4 oxidation state is thus possible, as evidenced by X-ray photoelectron spectroscopy, which is accompanied by a colour change from yellow to orange. Photocatalytic activity of (1) is screened and the material shows effective degradation of methyl orange.

Cerium based UiO-66 MOF as a multipollutant adsorbent for universal water purification

Herein, we demonstrate the use of cerium (Ce)-UiO-66 metal organic framework (MOF) for the removal of a variety of potentially toxic pollutants. The Ce-UiO-66 MOF, with similar framework topologies to Zr-UiO-66, has not been explored for its adsorptive properties in water remediation. The replacement of Zr metal center with Ce yields a MOF that can be synthesized in shorter durations with lesser energy consumptions and with excellent multipollutant adsorption properties. Further, the Ce-UiO-66 MOF was also studied for its adsorption abilities in the binary component system. Interestingly, the adsorbent showed higher adsorption capacities in the presence of other pollutants. Removal studies for other potentially toxic anionic and cationic dyes showed that the Ce-UiO-66 MOF has a wide range of contaminant removal abilities. Investigations of individual adsorption capacities revealed that the Ce-UiO-66 MOF has a maximum adsorption capacity of 793.7 mg/g for congo red (CR), 110 mg/g for methylene blue (MB), 66.1 mg/g for fluoride (F^-), 30 mg/g for Cr⁶⁺ and 485.4 mg/g for the pharmaceutical waste diclofenac sodium (DCF). To imply the practical applications of the Ce-UiO-66 MOF we have also demonstrated an adaptable filter that could separate all the potentially toxic pollutants.

Metalloenzyme-Inspired Ce-MOF Catalyst for Oxidative Halogenation Reactions

The structure of UiO-66(Ce) is formed by CeO_2-x defective nanoclusters connected by terephthalate ligands. The initial presence of accessible Ce³⁺ sites in the as-synthesized UiO-66(Ce) has been determined by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR)-CO analyses. Moreover, linear scan voltammetric measurements reveal a reversible Ce⁴⁺/Ce³⁺ interconversion within the UiO-66(Ce) material, while nanocrystalline ceria shows an irreversible voltammetric response. This suggests that terephthalic acid ligands facilitate charge transfer between subnanometric metallic nodes, explaining the higher oxidase-like activity of UiO-66(Ce) compared to nanoceria for the mild oxidation of organic dyes under aerobic conditions. Based on these results, we propose the use of Ce-based metal-organic frameworks (MOFs) as efficient catalysts for the halogenation of activated arenes, as 1,3,5-trimethoxybenzene (TMB), using oxygen as a green oxidant. Kinetic studies demonstrate that UiO-66(Ce) is at least three times more active than nanoceria under the same reaction conditions. In addition, the UiO-66(Ce) catalyst shows an excellent stability and can be reused after proper washing treatments. Finally, a general mechanism for the oxidative halogenation reaction is proposed when using Ce-MOF as a catalyst, which mimics the mechanistic pathway described for metalloenzymes. The superb control in the generation of subnanometric CeO_2-x defective clusters connected by adequate organic ligands in MOFs offers exciting opportunities in the design of Ce-based redox catalysts.

The chemistry and applications of hafnium and cerium(iv) metal-organic frameworks

The coordination connection of organic linkers to the metal clusters leads to the formation of metal-organic frameworks (MOFs), where the metal clusters and ligands are spatially entangled in a periodic manner. The immense availability of tuneable ligands of different length and functionalities gives rise to robust molecular porosity ranging from several angstroms to nanometres. Among the large family of MOFs, hafnium (Hf) based MOFs have been demonstrated to be highly promising for practical applications due to their unique and outstanding characteristics such as chemical, thermal, and mechanical stability, and acidic nature. Since the report of UiO-66(Hf) and DUT-51(Hf) in 2012, less than 200 Hf-MOFs (ca. 50 types of structures) have been reported. Besides, tetravalent cerium [Ce(iv)] has been proven to be capable of forming similar topological MOF structures to Zr and Hf since its first discovery in 2015. So far, ca. 40 Ce(iv) MOFs with 60% having UiO-66-type structure have been reported. This review will offer a holistic summary of the chemistry, uniqueness, synthesis, and applications of Hf/Ce(iv)-MOFs with a focus on presenting the development in the Hf/Ce(iv)-clusters, topologies, ligand structures, synthetic strategies, and practical applications of Hf/Ce(iv)-MOFs. In the end, we will present the research outlook for the development of Hf/Ce(iv)-MOFs in the future, including fundamental design of Hf/Ce(iv)-clusters, defect engineering, and various applications including membrane development, diversified types of catalytic reactions, irradiation absorption in nuclear waste treatment, water production and wastewater treatment, etc. We will also present the emerging computational approaches coupled with machine-learning algorithms that can be applied in screening Hf and Ce(iv) based MOF structures and identifying the best-performing MOFs for tailor-made applications in future practice.

The chemistry of Ce-based metal-organic frameworks

Metal-organic frameworks (MOFs) have gained widespread attention due to their modular construction that allows the tuning of their properties. Within this vast class of compounds, metal carboxylates containing tri- and tetravalent metal ions have been in the focus of many studies due to their often high thermal and chemical stabilities. Cerium has a rich chemistry, which depends strongly on its oxidation state. Ce(iii) exhibits properties typically observed for rare earth elements, while Ce(iv) is mostly known for its oxidation behaviour. In MOF chemistry this is reflected in their unique optical and catalytic properties. The synthetic parameters for Ce(iii)- and Ce(iv)-MOFs also differ substantially and conditions must be chosen to prevent reduction of Ce(iv) for the formation of the latter. Ce(iii)-MOFs are usually reported in comprehensive studies together with those constructed with other RE elements and normally they are isostructural. They exhibit a greater structural diversity, which is reflected in the larger variety of inorganic building units. In contrast, the synthesis conditions of Ce(iv)-MOFs were only recently (2015) established. These lead selectively to hexanuclear Ce-O clusters that are well-known for Zr-MOFs and therefore very similar structural and isoreticluar chemistry is found. Hence Ce(iv)-MOFs exhibit often high porosity, while only a few porous Ce(iii)-MOFs have been described. Some of these show structural flexibility which makes them interesting for separation processes. For Ce(iv)-MOFs the redox properties are most relevant. Thus, they are intensively discussed for catalytic, photocatalytic and sensing applications. In this perspective, the synthesis, structural chemistry and properties of Ce-MOFs are summarized.