Schiff base MOFs and their derivatives for sequestration and degradation of pollutants: present and future

Removal of contaminants via adsorption and catalysis have received a significant interest as energy and money-saving solutions for treating the world's wastewater. Metal-organic frameworks (MOFs), a newly discovered class of porous crystalline materials, have demonstrated tremendous promise in the removal and destruction of contaminants for water purification. In order to improve the interactions of MOFs with the target pollutants for their selective removal and degradation, the Schiff base functionalities emerged as promising active sites. Through pre- and post-synthetic alterations, Schiff base functionalities are integrated into the pore cages of MOF adsorbent materials. To understand the adsorptive/catalytic mechanism, potential interactions between the Schiff base sites and the target pollutants are discussed. Based on cutting-edge techniques for their synthesis, this paper examines current developments in the creation of Schiff base-functionalized MOFs as innovative materials for adsorptive removal and catalytic degradation of contaminants for water remediation.

Construction of ZnIn2S4/MOF-525 heterojunction system to enhance photocatalytic degradation of tetracycline

Zirconium-based porphyrin metal organic frameworks (Zr-PMOFs) had attracted attention in the field of photocatalysis in recent years. However, the recombination of photogenerated carriers of monomer PMOF limits its performance of photocatalytic organic pollutants degradation. Metal sulfide has a suitable visible band gap, which can form a heterojunction with MOF materials to enhance the photocatalytic efficiency of MOF. Therefore, a typical metal sulfide semiconductor ZnIn₂S₄ (ZIS) was introduced into a Zr-MOF (MOF-525) by solvothermal method to prepare a series of ZIS/MOF-525 (ZIS/MOF-525-1, ZIS/MOF-525-2, ZIS/MOF-525-3 and ZIS/MOF-525-4) composite photocatalysts in this work. The results of characterization analysis, optical analysis and electrochemical analysis showed that the interface of ZIS/MOF-525 formed a typical type-II heterojunction, which accelerated the electron transport rate and effectively inhibited the recombination of photogenerated e^- and h⁺ in MOF-525. The optimal removal efficiency of tetracycline (TC) by ZIS/MOF-525-3 (the mass of MOF-525 is 30 mg) reached 93.8% under 60 min visible light illumination, which was greater than that of pure MOF-525 (37.2%) and ZnIn₂S₄ (70.0%), and it still maintained good stability after five cycles reusing experiment. This work provides feasible insight for the preparation of novel and efficient PMOF-based photocatalysts in the future.

Recent advances in application of metal-organic frameworks (MOFs) as adsorbent and catalyst in removal of persistent organic pollutants (POPs)

The presence of persistent organic pollutants (POPs) in the aquatic environment is causing widespread concern due to their bioaccumulation, toxicity, and possible environmental risk. These contaminants are produced daily in large quantities and released into water bodies. Traditional wastewater treatment plants are ineffective at degrading these pollutants. As a result, the development of long-term and effective POP removal techniques is critical. In water, adsorption removal and photocatalytic degradation of POPs have been identified as energy and cost-efficient solutions. Both technologies have received a lot of attention for their efforts to treat the world's wastewater. Photocatalytic removal of POPs is a promising, effective, and long-lasting method, while adsorption removal of persistent POPs represents a simple, practical method, particularly in decentralized systems and isolated areas. It is critical to develop new adsorbents/photocatalysts with the desired structure, tunable chemistry, and maximum adsorption sites for highly efficient removal of POPs. As a class of recently created multifunctional porous materials, Metal-organic frameworks (MOFs) offer tremendous prospects in adsorptive removal and photocatalytic degradation of POPs for water remediation. This review defines POPs and discusses current research on adsorptive and photocatalytic POP removal using emerging MOFs for each type of POPs.

Synthesis of metal–organic frameworks (MOFs) MIL-100(Fe) functionalized with thioglycolic acid and ethylenediamine for removal of eosin B dye from aqueous solution

The interaction of eosin B dye from aqueous solution with MIL-100(Fe) and functionalized MIL-100(Fe) metal–organic frameworks (MOFs) is reported in this study. MIL-100(Fe) was prepared and functionalized with thioglycolic acid (TH) and ethylenediammine (ED) separately by incorporating the thiol (–SH) and the amine (–NH2) group of the functionalizing agents into the open metal sites of the MIL-100(Fe) to obtain the acidic (TH-MIL-100) and basic (ED-MIL-100) forms of the MOF respectively. Characterization of the MOFs was done by melting point analysis, elemental analysis, spectroscopic techniques, scanning electron microscopy (SEM), and powdered X-ray diffraction (PXRD) analysis. The adsorption experiments were carried out at different conditions such as pH, adsorbent dosage, contact time, temperature, and initial concentration of the dye to estimate the optimum conditions and the maximum adsorption capacities. Adsorption capacities were observed to increase in the order of ED-MIL-100 < MIL-100 < TH-MIL-100, while the TH-MIL-100 was the most effective in the removal process due to acid–base interaction between the acidic thiol group (–SH) and the alkaline medium of eosin B dye solution. The Langmuir Isotherm was seen to fit well to adsorption data obtained for all three adsorbent materials studied, and adsorption processes followed the pseudo-second order kinetics. This study, therefore, indicates the suitability of functionalization of MIL-100(Fe) towards improving its adsorption capacity.

Metal–organic frameworks derived from a semi-rigid anthracene-based ligand and sulfonates: proton conductivity and dye degradation studies

The MOFs were constructed by ligand AHP and sulfonate analogues. MOF4 exhibits a high proton conductivity of 1.95 × 10⁻³ S cm⁻¹ at 95 °C and 98% relative humidity. MOFs 1–5 also serve as photocatalysts for methylene blue degradation.

Six CoII coordination polymers exhibiting UV-light-driven photocatalysis for the degradation of organic dyes

Six different Co^II coordination polymers based on a new bis-pyridyl-bis-amide and polycarboxylates were obtained, showing photocatalytic activity for organic dyes.

Mixed-Metal MOFs: Unique Opportunities in Metal-Organic Framework (MOF) Functionality and Design

Mixed-metal metal-organic frameworks (MM-MOFs) can be considered to be those MOFs having two different metals anywhere in the structure. Herein we summarize the various strategies for the preparation of MM-MOFs and some of their applications in adsorption, gas separation, and catalysis. It is shown that compared to homometallic MOFs, MM-MOFs bring about the opportunity to take advantage of the complexity and the synergism derived from the presence of different metal ions in the structure of MOFs. This is reflected in a superior performance and even stability of MM-MOFs respect to related single-metal MOFs. Emphasis is made on the use of MM-MOFs as catalysts for tandem reactions.

A Review on the Synthesis and Characterization of Metal Organic Frameworks for Photocatalytic Water Purification

This review analyzes the preparation and characterization of metal organic frameworks (MOFs) and their application as photocatalysts for water purification. The study begins by highlighting the problem of water scarcity and the different solutions for purification, including photocatalysis with semiconductors, such as MOFs. It also describes the different methodologies that can be used for the synthesis of MOFs, paying attention to the purification and activation steps. The characterization of MOFs and the different approaches that can be followed to learn the photocatalytic processes are also detailed. Finally, the work reviews literature focused on the degradation of contaminants from water using MOF-based photocatalysts under light irradiation.

Morphology-dependent sensing performance of dihydro-tetrazine functionalized MOF toward Al(III)

A pillared MOF, [Zn(OBA)(H₂DPT)_0.5].DMF (TMU-34), based on dihydro tetrazine functionalized pillar spacer (H₂DPT=3,6-di(pyridin-4-yl)-1,4-dihydro-1,2,4,5-tetrazine) and V-shape dicarboxylate linker (H₂OBA=4,4'-oxybis(benzoic acid)) was synthesized by reflux and ultrasonic methods. The effects of sonication time, initial concentration of reagents and sonication power on size and morphology have been optimized. This MOF has been characterized by scanning electron microscopy, FT-IR spectra, X-ray powder diffraction and N₂ adsorption at 77K. Bulk and nano samples of TMU-34 have been applied in cation sensing for detection of Al(III) in presence of other cations (Na(I), Mg(II), Sr(II), Al(III), Co(II), Ni(II), Cu(II), Zn(II), Cd(II), Pb(II), Hg(II),Cr(III), Li(I), Fe(III), K(I)). The results show that nano powder of TMU-34 with uniform separated plate-like morphology (TMU-34-F) has higher detection limit and short response time compared to bulk material. So, in this work we show the application of luminescent metal-organic frameworks synthesized by sonochemistry approach in effective cation detection.

High organic sulfur removal performance of a cobalt based metal-organic framework

Synthesis of a new porous cobalt based metal-organic framework, [Co₆(oba)₆(CH₃O)₄(O)₂]n·3DMF (TMU-11) has been carried out to introduce a new and highly efficient adsorbent of dibenzothiophene (DBT). This compound has been synthesized by solvothermal method using a nonlinear dicarboxylate ligand and characterized by single-crystal X-ray crystallography. To study the adsorption properties of the synthesized compound, TMU-11, for DBT removal, various factors, such as amount of adsorbent, contact time and temperature were examined. On the basis of the results, maximum efficiency and reusability in DBT removal occur under the mild reaction conditions. Furthermore, the DBT removal follows the pseudo-second order reaction kinetic. The maximum adsorption value is 825mg/g. The selectivity test of DBT over naphthalene (NA) clearly shows that π-π interactions between organic linkers of TMU-11 and the aromatic ring of DBT are not responsible for the adsorption desulfurization (ADS) process and the main part of adsorption takes place on unsaturated site around Co centres. Our findings may provide some insight into the preparation of the adsorbent with superior performance in practical applications.

Enhancement of photocatalytic performance in two zinc-based metal–organic frameworks by solvent assisted linker exchange

Solvent-assisted linker exchange (SALE) was performed on two pillared metal–organic frameworks (MOFs), [Zn₂(oba)₂(4-bpdb)]_n·(DMF)₂ (TMU-4) and [Zn(oba)(4-bpmb)_0.5]_n·(DMF)_1.5 (TMU-6), to tune their photocatalytic properties.

High photodegradation efficiency of phenol by mixed-metal–organic frameworks

Four MOFs, TMU-5, TMU-5(Cd 15%), TMU-5(Cd 30%) and (TMU-7) were synthesized and studied for phenol degradation from aqueous solutions under UV and/or visible light irradiation without auxiliary oxidants such as H₂O₂.

Template assisted fast photocatalytic degradation of azo dye using ferric oxide–gallia nanostructures

Photocatalytic activity of Fe₂O₃–Ga₂O₃ nanostructures, synthesized via two routes (with and without glycoluril as structure directing agent), containing various proportions of Fe₂O₃ in presence of 15 ppm CR was investigated.