Hydroxylation of UiO-66 Metal-Organic Frameworks for High Arsenic(III) Removal Efficiency

High Capacity Arsenate Removal from Real Samples Using Dihydrotetrazine Decorated Zirconium-Based Metal-Organic Frameworks

Zirconium metal-organic frameworks (Zr-MOFs) are potential candidates for decontamination of water resources from harmful pollutants due to their modulable porosity and chemical stability in aqueous solutions. Linker functionalization is an approach for tuning the host-guest chemistry of Zr-MOFs and extends their applications in environmental monitoring. In this work, the structure of UiO-66(Zr) (formulated Zr6(OH)4O4(BDC)6, BDC2- = benzene-1,4-dicarboxylate) was functionalized with dihydrotetrazine group via postsynthesis linker exchange (PSLE) method. The functionalized framework, UiO-66(Zr)-DHTZ, was applied for the removal of arsenate ions from aqueous solutions. The results show that UiO-66(Zr)-DHTZ can adsorb 583 mg g-1 of As(V) at pH = 7 after 2 h, which is significantly higher than that of the UiO-66(Zr). According to X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared (FTIR), the removal mechanism is based on possible hydrogen bindings between free -C-NH and -C═N- sites of dihydrotetrazine function with -O- and -OH sites of As(V) species. Removal tests in real samples show that UiO-66(Zr)-DHTZ still has a high capacity (220 mg g-1) to As(V) ions in complex matrixes and also can decrease the concentration of As(V) below the detection limit (0.05 ppm) of the inductively coupled plasma optical emission spectroscopy (ICP-OES) method. Since the dihydrotetrazine-decorated UiO-66(Zr)-DHTZ reaches one the highest adsorption capacities to As(V) species, it can be considered a potential candidate for water treatment in real-life applications.

New sulfonated covalent organic framework for highly effective As(III) removal from water

The goal of taking out As(III) from water is to reduce the detriment that poisonous metals can do to people and nature. A substance that can absorb As(III), TFPOTDB-SO3H, was made by combining 2,5-diaminobenzenesulfonic acid and 2,4,6-tris-(4-formylphenoxy)-1,3,5-triazine in a reaction that joins molecules together. This substance can adsorb As(III) very well and has excellent qualities like being easy to use again, separate substances, and filter out liquids. At pH = 8 and at room temperature, TFPOTDB-SO3H adsorbed a lot of As(III). It achieved a removal rate of 97.1 % within 10 min and could adsorb up to 344.8 mg/g. A research was conducted to investigate the effect of co-existing anions on the elimination of arsenic. The findings indicated that the presence of anions had a minimal adverse impact, reducing As(III) uptake by approximately 1-7 %. The kinetics of the uptake process were found to be controlled by the quasi-second order kinetic model, while the Langmuir isotherm model validated that the mechanism for As(III) removal was monolayer chemisorption. According to the thermodynamic analysis, the adsorption process was endothermic and occurred spontaneously. Moreover, even after 4 successive adsorption-desorption cycles, the adsorbent preserved a substantial uptake productivity of 88.86 % for As(III). The results collectively indicate that TFPOTDB-SO3H holds considerable promise for the efficient adsorption and elimination of As(III) ions from wastewater.

Reduction-Induced Uptake of Cs+ in Metal-Organic Frameworks Loaded with Polyoxometalates

Materials for Cs+ adsorption continue to be important for the treatment of various solutions. Metal-organic frameworks (MOFs) with large specific surface areas promise adsorption properties for various gases, vapors, and ions. However, the utilization of MOFs for alkali ion capture, specifically, Cs+ capture is still in its infancy. Herein, MOFs are hybridized with polyoxometalates (POMs) to study the effect of i) MOF type, ii) POM type, and iii) POM loading amounts on Cs+ capture. In particular, the composite of ZIF-8 and [α-PMo12 O40 ]3- (PMo12 /ZIF-8) adsorbed Cs+ ions effectively when compared to pristine ZIF-8. In addition, the reduction of Mo within the POM from MoVI to MoV by ascorbic acid during the Cs+ uptake process doubled the Cs+ uptake capacity of PMo12 /ZIF-8. This observation can be attributed to the increased overall negative charge of the POM facilitating Cs+ uptake to compensate for the charge imbalance. Hybridization with other MOFs (MIL-101 and UiO-66) largely suppresses the Cs+ uptake, highlighting the importance of hydrophobicity in Cs+ capture. Furthermore, PMo12 /ZIF-8 led to an outstanding Cs+ uptake (291.5 mg g-1 ) with high selectivity (79.6%) from quinary mixtures of alkali metal cations even among other representative porous materials (Prussian blue and zeolites).

Combined Experimental and Theoretical Study of the Synthesis of 5,7-Dihydroxy-4-methylcoumarin via a Pechmann Condensation in the Presence of UiO-66-SO3H Catalysts

An efficient synthesis of 5,7-dihydroxy-4-methylcoumarin from phloroglucinol with ethyl acetoacetate in the UiO-66-SO3H metal-organic framework is reported. The potential of UiO-66-SO3H as a solid catalyst was determined through optimized-condition experiments and quantum molecular calculations. The optimal conditions for the synthesis of 5,7-dihydroxy-4-methylcoumarin with UiO-66-SO3H were as follows: phloroglucinol/ethyl acetoacetate molar ratio = 1:1.6, reaction time = 4 h, and temperature = 140 °C, for which the reaction yield reached 66.0%. The reusability of UiO-66-SO3H catalysts for Pechmann condensation was examined. The activation energy of the reaction occurring on a sulfonic group of the UiO-66-SO3H catalyst was 12.6 kcal/mol, which was significantly lower than 22.6 kcal/mol of the same reaction on the UiO-66 catalyst. To comprehend the reaction mechanism, density functional theory with the ONIOM approach was applied for the synthesis of coumarin on the UiO-66-SO3H and UiO-66 clusters. A possible reaction mechanism was proposed involving three steps: a trans-esterification step, an intramolecular hydroxyalkylation step, and a dehydration step. The rate-determining step was suggested to be the first step which acquired an activation energy of 15.7 and 29.5 kcal/mol, respectively. Information from this study can be used as guidelines to develop more efficient catalytic metal-organic frameworks for various organic syntheses.

Improvement of water adsorption performance of UiO-66 by post-synthetic modification

Post-synthetic modification can be used for structural replacement or functional modification of materials after they have been formed or assembled. It can effectively combine various modification methods for metal-organic frameworks (MOFs) such as defect control, replacement of metal sites, or functionalization of ligands. In this work, organic ligands that incorporate N-functionalities or amino groups were introduced into defective UiO-66 through post-synthetic ligand exchange (PSE) to improve its water adsorption performance. Parameters such as water adsorption capacity, half adsorption value (α), and Henry constant KH were used to characterize the water adsorption performance. After PSE, new ligands in different molar ratios entered the skeleton of UiO-66. The N sites or amino groups on the ligands provided new sites for the adsorption of water molecules. The water adsorption capacity and hydrophilicity of all samples were significantly superior to those of LD-UiO-66, which had almost no defects. H-UiO-66-PyDC samples exhibited the highest ligand replacement ratio and a significant enhancement of water adsorption performance. Compared to the unchanged H-UiO-66, the water uptake of H-UiO-66-PyDC increased from 0.08 g g-1 to 0.23 g g-1 at P/P0 = 0.30 and α decreased from 0.36 to 0.28. After 20 water adsorption/desorption tests, the water uptake of all samples did not decrease, showing excellent cycling stability. These results suggest that the combination of defect modulation and PSE is a potential tool to make UiO-66 more appropriate for applications based on reversible adsorption.

Enhanced arsenite removal from water using zirconium-ferrocene MOFs coupled with peroxymonosulfate:oxidation and multi-sites adsorption mechanism

The efficient removal of arsenite (As(III)) poses a significant challenge to traditional water treatment technologies due to its high toxicity and mobility. In this work, multifunctional Zirconium-Ferrocene Metal Organic Framework (ZrFc-MOF) fabricated with redox-active 1,1-ferrocene dicarboxylic acid ligands and Zr⁴⁺ precursors were elaborated to achieve remarkably enhanced As(III) removal via activation by peroxymonosulfate (PMS). The adsorption affinity coefficient increased from 0.097 to 2.035 L mg^-1 and the maximum adsorption capacity increased from 59.79 to 111.34 mg g^-1 compared with that without PMS. Besides the conventional homogeneous PMS oxidation and the following adsorption through Zr-O clusters of ZrFc-MOFs, the enhanced As(III) removal synergistic combines the oxidation mechanism of As(III) by reactive oxygen species (•OH, SO₄^•-, O₂^•- and ¹O₂) formed in Ferrocene (Fc) activating PMS process with the simultaneous formed extra adsorption sites of Ferrocenium (Fc⁺). PMS also help ZrFc-MOF to avoid destruction in harsh alkaline condition, making the effluent in this advanced treatment meet the World Health Organization (WHO) threshold of 10 μg L^-1 over a wide range of initial pH (2-11) with high selectivity and durability. These results indicate that this novel Fc-based MOFs activating PMS system has potential applicability for As(III) in oxidation and selectively capturing in the water environment.