Highly Selective H2O2-Based Oxidation of Alkylphenols top-Benzoquinones Over MIL-125 Metal-Organic Frameworks

Visible light-driven oxidation of non-native substrate by laccase attached on Ru-based metal-organic frameworks

Light-induced electron transfer can broaden the substrate range of metalloenzyme. However, the efficiency of photo-enzyme coupling is limited by the poor combination of photosensitizer or photocatalyst with enzyme. Herein, we prepared the nano-photocatalyst MIL-125-NH2@Ru(bpy) by in site embedding ruthenium pyridine-diimine complex [Ru(bpy)3]2+ into metal organic frameworks MIL-125-NH2 and associated it with multicopper oxidase (MCO) laccase. Compared to [Ru(bpy)3]2+, the coupling efficiency of MIL-125-NH2@Ru(bpy)3 for enzymatic oxygen reduction increased by 35.7%. A series of characterizations confirmed that the amino group of laccase formed chemical bonds with the surface defects or hydrophobic groups of MIL-125-NH2@Ru(bpy)3. Consequently, the tight binding accelerated the quenching process and electron transfer between laccase and the immobilized ruthenium pyridine-diimine complex. This work would open an avenue for the synthesis of MOFs photocatalyst towards photo-enzyme coupling.

Building Porous Ni(Salen)-Based Catalysts from Waste Styrofoam via Autocatalytic Coupling Chemistry for Heterogeneous Oxidation with Molecular Oxygen

The development of robust and industrially viable catalysts from plastic waste is of great significance, and the facile construction of high performance heterogeneous catalyst systems for phenol-quinone conversions remains a grand challenge. Herein, a feasible strategy is demonstrated to reclaim Styrofoam into hierarchically porous nickel-salen-loaded hypercrosslinked polystyrene (PS@Ni-salen) catalysts with high activities through an unusual autocatalytic coupling route. The salen is immobilized onto PS chain by Friedel-Crafts alkylation of benzyl chloride derivatives, and the generated hydrogen chloride coordinately promotes the simultaneous crosslinking and bridge formation between aromatic rings via a Scholl coupling route, leading to hierarchically porous networks. After the metallization with Ni, the resultant networks exhibit high catalytic activity for the oxidation of 2,3,6-trimethylphenol to 2,3,5-trimethyl-1,4-benzoquinone under mild conditions (303 K, 1 bar of O2 ). This catalyst also demonstrates attractive recycling performance without an obvious loss of catalytic efficiency over five consecutive cycles. This methodology might provide a potential sustainable alternative to construct environmentally benign and cost-effective catalysts for specific organic transformation.

A protocol of carbonized on-column enrichment for urinary exosomal N-glycans profiling

As a widely present vesicle, exosome plays an important role in lots of biological processes due to its inclusive cargos. In particular, exosome glycan cargo is attracting attentions since its aberrant alteration is closely related to many progressions in diseases. In this work, a novel carbonized packing capillary trap column for urinary exosomal N-glycan enrichment was proposed. The carbonized packing exhibited large specific surface area, mesoporous structure with narrow pore size distribution and abundant carbon for specially interacting with oligosaccharides. Benefitting from all these advantages, the N-glycans deriving from standard glycoproteins or complex human urine exosomes could be identified with high sensitivity and selectivity. Finally, from the glycans identified in healthy volunteers and patients with bladder carcinoma, we observed that 10 of glycans shared by two groups were obvious downregulation and the 18 were upregulation. These results show great potential of capillary trap column as a tool for the enrichment and detection of glycans in exosomal, attracting more attention on disease progression monitoring and biomarker discovery.

In-situ construction of Zr-based metal-organic framework core-shell heterostructure for photocatalytic degradation of organic pollutants

Photocatalysis is an eco-friendly promising approach to the degradation of textile dyes. The majority of reported studies involved remediation of dyes with an initial concentration ≤50 mg/L, which was away from the existing values in textile wastewater. Herein, a simple solvothermal route was utilized to synthesize CoFe₂O₄@UiO-66 core-shell heterojunction photocatalyst for the first time. The photocatalytic performance of the as-synthesized catalysts was assessed through the photodegradation of methylene blue (MB) and methyl orange (MO) dyes at an initial concentration (100 mg/L). Under simulated solar irradiation, improved photocatalytic performance was accomplished by as-obtained CoFe₂O₄@UiO-66 heterojunction compared to bare UiO-66 and CoFe₂O₄. The overall removal efficiency of dyes (100 mg/L) over CoFe₂O₄@UiO-66 (50 mg/L) reached >60% within 180 min. The optical and photoelectrochemical measurements showed an enhanced visible light absorption capacity as well as effective interfacial charge separation and transfer over CoFe₂O₄@UiO-66, emphasizing the successful construction of heterojunction. The degradation mechanism was further explored, which revealed the contribution of holes (h⁺), superoxide (•O₂ ^-), and hydroxyl (•OH) radicals in the degradation process, however, h⁺ were the predominant reactive species. This work might open up new insights for designing MOF-based core-shell heterostructured photocatalysts for the remediation of industrial organic pollutants.

Highly Efficient Synthesis of p-Benzoquinones Catalyzed by Robust Two-Dimensional POM-Based Coordination Polymers

Selective oxidation of alkyl-substituted phenols offers efficient access to p-benzoquinones (BQs) that serve as key components for synthesizing biologically active compounds, but rational manufacture of efficient recyclable catalysts for such a reaction remains a severe challenge. Herein, two crystalline 2D polyoxometalate-based coordination polymers (POMCPs), formulated as H₃[Cu^I₃(L)₃]₂[PM₁₂O₄₀]·xH₂O (M = Mo, x = 4 for 1; M = W, x = 6 for 2; and HL = 4-(1H-tetazol-5-yl)pyridine), are prepared by a mineralizer-assisted one-step synthesis strategy and explored as heterogeneous catalysts for p-BQs synthesis. Both compounds have been characterized through elemental analysis, EDS analysis, infrared spectroscopy, UV-vis diffuse reflectance spectrum, EPR, XPS, BET, single-crystal, and powder X-ray diffraction. Single-crystal X-ray diffraction analysis indicates that both 1 and 2 exhibit an interesting 2D sheet structure composed of 2-connected Keggin type anions [PM₁₂O₄₀]^3- and hexa-nuclear {Cu^I₆(HL)₆} cluster-based metal-organic chains via Cu···O interactions. When used as catalysts, POMCPs 1 and 2 have excellent catalytic activities in the selective oxidation of substituted phenols to p-BQs with H₂O₂. Notedly, in the model reaction from 2,3,6-trimethylphenol (TMP) to the vitamin E key intermediate trimethyl-p-benzoquinone (TMBQ), the catalytic activities expressed by turnover frequency (TOF) of 1 and 2 can reach an unprecedented 2400 and 2000 h^-1, respectively, at close to 100% TMBQ yield. The truly heterogeneous nature, stability, and structural integrity of both catalysts were ascertained by FTIR, PXRD techniques, and the following cycles. Mechanism studies reveal that both catalysts can involve a dual reaction pathway through a heterolytic oxygen atom transfer mechanism and homolytic radical mechanism. Moreover, the 2D POMCPs with highly accessible bilateral active sites and efficient mass transfer efficiency possess superior catalytic performance to their analogous 3D species.

Metal-Organic Frameworks in Oxidation Catalysis with Hydrogen Peroxide

In recent years, metal–organic frameworks (MOFs) have received increasing attention as selective oxidation catalysts and supports for their construction. In this short review paper, we survey recent findings concerning use of MOFs in heterogeneous liquid-phase selective oxidation catalysis with the green oxidant–aqueous hydrogen peroxide. MOFs having outstanding thermal and chemical stability, such as Cr(III)-based MIL-101, Ti(IV)-based MIL-125, Zr(IV)-based UiO-66(67), Zn(II)-based ZIF-8, and some others, will be in the main focus of this work. The effects of the metal nature and MOF structure on catalytic activity and oxidation selectivity are analyzed and the mechanisms of hydrogen peroxide activation are discussed. In some cases, we also make an attempt to analyze relationships between liquid-phase adsorption properties of MOFs and peculiarities of their catalytic performance. Attempts of using MOFs as supports for construction of single-site catalysts through their modification with heterometals will be also addressed in relation to the use of such catalysts for activation of H2O2. Special attention is given to the critical issues of catalyst stability and reusability. The scope and limitations of MOF catalysts in H2O2-based selective oxidation are discussed.

A Highly Efficient Heterogeneous Catalyst of Bimetal-Organic Frameworks for the Epoxidation of Olefin with H2O2

A series of bimetel organic framework Mn_xCu_1-x-MOF were prepared. The MOFs was characterized and analyzed by powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The catalytic activity of the developed catalyst was tested on various olefins by H₂O₂ as oxidant. The MOFs catalyst exhibits excellent catalytic activity for the epoxidations of various aromatic and cyclic olefins. Particularly, Mn_0.1Cu_0.9-MOF can achieve 90.2% conversion of styrene with 94.3% selectivity of styrene oxide at 0 °C after reaction 6 h. The MOF exhibited the catalytic activity of inverse temperature effect on epoxidation of styrene. The introduction of copper component can stabilize H₂O₂ and inhibit its decomposition to a certain extent. The catalyst can be reused at least five cycles without significant loss in activity towards epoxidation.

Stable Non-Covalent Co(Salphen)-Based Polymeric Catalyst for Highly Efficient and Selective Oxidation of 2,3,6-Trimethylphenol

Developing highly efficient catalyst systems for phenol-quinone transformation is of great significance in the chemical/biological industries. Herein, we reported a novel heterogenous catalytic system based on Co(Salphen) supramolecular polymers (CSP), which delivered an excellent catalytic performance in the oxidation of 2,3,6-trimethylphenol (TMP) under mild conditions. The CSP were constructed through a simple self-assembled process between BiCo(Salphen) complex and 4,4-dipyridine. By applying BiCo-BiPy1:1 CSP as the catalyst, 2,3,5-trimethyl-1,4-benzoquinone (TMBQ) could be obtained with an excellent conversion (>99%) and selectivity over 99% under mild reaction conditions (30 °C, 0.1 MPa). In addition, it can be recycled at least five times without substantial decline in catalytic activities (conversion and selectivity), suggesting its excellent stability and recyclability. This work may provide guidance on designing and building valuable catalysts for environmentally friendly and cost-effective oxidation reactions.

Multiunit Catalysts with Synergistic Reactivity: Three-Dimensional Polyoxometalate-Based Coordination Polymers for Highly Efficient Synthesis of Functionalized p-Benzoquinones

The rational design of highly efficient catalysts for the synthesis of functionalized p-benzoquinones (p-BQs) is of great significance for the manufacture of bioactive compounds. Herein, two 3D crystalline polyoxometalate-based coordination polymers (POMCPs) are used as heterogeneous catalysts for the synthesis of p-BQs, which are H[Cu^II(ttb)(H₂O)₃]₂[Cu^II(ttb)Cl]₂[PW₁₂O₄₀]·4H₂O (1) (Httb = 1-(tetrazol-5-yl)-4-(triazol-1-yl)benzene) and [ClCu₆^I(trz₎₄][ClCu₅^I(trz)₄]₂[Cu^II(H₂O)][PW₁₂O₄₀] (2) (trz = 1,2,4-triazole). Both compounds were characterized by elemental analysis, IR, XPS, solid diffuse reflective spectroscopy, TG analysis, and single-crystal X-ray diffraction. In 1, Keggin anions [PW₁₂O₄₀]^3- locate in 1D square channels constructed from wave-like Cu-ttb layers to form a 3D POMCP by coordinating to Cu ions, and in 2, [PW₁₂O₄₀]^3- anions situate in eight-membered Cu-trz channels via Cu···O interactions to yield a 3D POMCP structure. The catalytic activities of 1 and 2 have been evaluated in the selective oxidation of alkylphenols/alkoxybenzenes/methylnaphthalene, especially in the oxidation reaction of 2,3,6-trimethylphenol (TMP) to 2,3,5-trimethyl-p-benzoquinone (TMBQ, vitamin E key intermediate), with H₂O₂ as oxidant. By using catalysts 1 and 2 under optimal conditions, the yield of TMBQ can reach 99% and 96% within 10-20 min, respectively. Both catalysts demonstrated high turnover frequencies (300 h^-1 for 1 and 600 h^-1 for 2) and the truly heterogeneous nature. 1 and 2 catalyzed the synthesis of p-BQs on the basis of effective cooperative catalytic activities by POMs and metal nodes.

Cyclohexene Oxidation with H2O2 over Metal-Organic Framework MIL-125(Ti): The Effect of Protons on Reactivity

The catalytic performance of the titanium-based metal–organic framework MIL-125 was evaluated in the selective oxidation of cyclohexene (CyH) with environmentally friendly oxidants, H2O2 and tBuOOH. The catalytic activity of MIL-125 as well as the oxidant utilization efficiency and selectivity toward epoxide and epoxide-derived products can be greatly improved by acid additives (HClO4 or CF3SO3H). In the presence of 1 molar equivalent (relative to Ti) of a proton source, the total selectivity toward CyH epoxide and trans-cyclohexane-1,2-diol reached 75–80% at 38–43% alkene conversion after 45 min of reaction with 1 equivalent of 30% H2O2 at 50 °C. With 50% H2O2 as the oxidant, the total selectivity toward heterolytic oxidation products increased up to 92% at the same level of alkene conversion. N2 adsorption, powder X-ray diffraction (PXRD), and infrared (IR) spectroscopy studies before and after the catalytic oxidations confirmed the absence of structural changes in the Metal–organic framework (MOF) structure. MIL-125 was stable toward titanium leaching, behaved as a truly heterogeneous catalyst, and could easily be recovered and reused several times without any loss of the catalytic properties.

Cu(II)-Schiff base covalently anchored to MIL-125(Ti)-NH2 as heterogeneous catalyst for oxidation reactions

MIL-125(Ti)-NH₂ has been modified by reaction of salicylaldehyde with the terephthalate amino groups to form a salicylideneimine that act as ligand of Cu²⁺. The success of the postsynthetic modification was assessed by FTIR spectroscopy of the MIL-125(Ti)-NH₂-Sal-Cu and by analysis by ¹H NMR spectroscopy of the organic linkers upon dissolution of MIL-125(Ti)-NH₂-Sal-Cu. In comparison with parent MIL-125(Ti)-NH₂ and MIL-125(Ti)-NH₂-Sal, that exhibit a poor activity, the presence of the Cu-Schiff base complex in MIL-125(Ti)-NH₂-Sal-Cu catalyst for the oxidation of 1-phenylethanol by tert-butylhydroperoxyde (TBHP, 3 eq.) increases notably the catalytic activity. Hot filtration test and reusability experiments confirm that the process is heterogeneous and that MIL-125(Ti)-NH₂-Sal-Cu is stable under the reaction conditions. Quenching studies and EPR spectra using N-^tbutylphenylnitrone indicate the generation of ^tBuOO and ^tBuO under the reaction conditions. The scope of MIL-125(Ti)-NH₂-Sal-Cu as oxidation catalyst by ^tBuOOH was studied for benzyl alcohol as well as alicyclic and aliphatic alcohols and ethylbenzene.

Facile Preparation of Metal-Organic Framework (MIL-125)/Chitosan Beads for Adsorption of Pb(II) from Aqueous Solutions

In this study, novel composite titanium-based metal-organic framework (MOF) beads were synthesized from titanium based metal organic framework MIL-125 and chitosan (CS) and used to remove Pb(II) from wastewater. The MIL-125-CS beads were prepared by combining the titanium-based MIL-125 MOF and chitosan using a template-free solvothermal approach under ambient conditions. The surface and elemental properties of these beads were analyzed using scanning electron microscopy, Fourier transform infrared and X-ray photoelectron spectroscopies, as well as thermal gravimetric analysis. Moreover, a series of experiments designed to determine the influences of factors such as initial Pb(II) concentration, pH, reaction time and adsorption temperature was conducted. Notably, it was found that the adsorption of Pb(II) onto the MIL-125-CS beads reached equilibrium in 180 min to a level of 407.50 mg/g at ambient temperature. In addition, kinetic and equilibrium experiments provided data that were fit to the Langmuir isotherm model and pseudo-second-order kinetics. Furthermore, reusability tests showed that MIL-125-CS retained 85% of its Pb(II)-removal capacity after five reuse cycles. All in all, we believe that the developed MIL-125-CS beads are a promising adsorbent material for the remediation of environmental water polluted by heavy metal ions.

MIL-Ti metal-organic frameworks (MOFs) nanomaterials as superior adsorbents: Synthesis and ultrasound-aided dye adsorption from multicomponent wastewater systems

Herein, 1,4-benzenedicarboxylate (BDC) and 2-amino-1,4-benzenedicarboxylate (NH₂-BDC) as organic linkers and tetraisopropyl orthotitanate as a metal source were used to synthesize several metal-organic frameworks (MOFs) nanomaterials. Five Materials Institut Lavoisiers (MILs) as MOFs include MIL-125(Ti), NH₂-MIL-125(Ti) and three MILs with different organic linkers molar ratios (BDC/NH₂-BDC: 75/25, 50/50 and 25/75 denoted as MIL-X1, MIL-X2 and MIL-X3, respectively). The synthesized nanomaterials were used for ultrasound-aided adsorption of cationic dyes (Basic Red 46 (BR46), Basic Blue 41 (BB41) and Methylene Blue (MB)) from single and multicomponent (binary) systems. The BET, XRD, FTIR, SEM, TEM, TGA and zeta potential were used for characterizing the MILs. Dye removal followed pseudo-second order kinetics with constant rate of 0.20833, 0.00481 and 0.00051 mg/g min for BR46, BB41 and MB, respectively. In addition dye adsorption obeyed the Langmuir isotherm model and the experimental dye adsorption capacity for BR46, BB41 and MB was 1296, 1257 and 862 mg/g, respectively. The synthesized MIL showed high reusability and stability over three cycles. The adsorption thermodynamics data presented that dye removal was a spontaneous, endothermic and physical reaction. The free Gibbs energy for dye removal by the NH₂-MIL-125(Ti) at 308K was -19.424, -15.721 and -17.413 kJ/mol for BR46, BB41 and MB, respectively.

Chelating agent-free, vapor-assisted crystallization method to synthesize hierarchical microporous/mesoporous MIL-125 (Ti)

Titanium-based microporous heterogeneous catalysts are widely studied but are often limited by the accessibility of reactants to active sites. Metal-organic frameworks (MOFs), such as MIL-125 (Ti), exhibit enhanced surface areas due to their high intrinsic microporosity, but the pore diameters of most microporous MOFs are often too small to allow for the diffusion of larger reactants (>7 Å) relevant to petroleum and biomass upgrading. In this work, hierarchical microporous MIL-125 exhibiting significantly enhanced interparticle mesoporosity was synthesized using a chelating-free, vapor-assisted crystallization method. The resulting hierarchical MOF was examined as an active catalyst for the oxidation of dibenzothiophene (DBT) with tert-butyl hydroperoxide and outperformed the solely microporous analogue. This was attributed to greater access of the substrate to surface active sites, as the pores in the microporous analogues were of inadequate size to accommodate DBT. Moreover, thiophene adsorption studies suggested the mesoporous MOF contained larger amounts of unsaturated metal sites that could enhance the observed catalytic activity.

High valence 3p and transition metal based MOFs

This article focuses on high valence 3p and transition metal based metal organic frameworks.