Synthesis of Stable Hierarchical MIL-101(Cr) with Enhanced Catalytic Activity in the Oxidation of Indene

Copper-Based Metal–Organic Frameworks (MOFs) as an Emerging Catalytic Framework for Click Chemistry

In the extensive terrain of catalytic procedures for the synthesis of organic molecules, metal–organic frameworks (MOFs) as heterogenous catalysts have been investigated in a variety of chemical processes, including Friedel–Crafts reactions, condensation reactions, oxidations, and coupling reactions, and utilized owing to their specific properties such as high porosity, tuneability, extraordinary catalytic activity, and recyclability. The eminent copper-tailored MOF materials can be exceptionally dynamic and regioselective catalysts for click reactions (1,3-dipolar cycloaddition reaction). Considering the fact that Cu(I)-catalyzed alkyne–azide cycloaddition (CuAAC) reactions can be catalyzed by several other copper catalysts such as Cu (II)-β-cyclodextrin, Cu(OAc)2, Fe3O4@SiO2, picolinimidoamide–Cu(II) complex, and Cu(II) porphyrin graphene, the properties of sorption and reusability, as well as the high density of copper-MOFs, open an efficient and robust pathway for regimented catalysis of this reaction. This review provides a comprehensive description and analysis of the relevant literature on the utilization of Cu-MOFs as catalysts for CuAAC ‘click’ reactions published in the past decade.

Precise regulation of defect concentration in MOF and its influence on photocatalytic overall water splitting

In this work, the defective Cu-BDC with different defect concentration and Cu¹⁺/Cu²⁺ coordinatively unsaturated sites (CUS) content were designed and synthesized by introducing defective linkers with different pK_a values. The low-concentration defects in Metal-organic frameworks (MOFs) structure act as the active sites to enhance their photocatalytic activity. In contrast, the high concentration defects serve as the recombination centers of photogenerated electrons and holes to decrease the transfer efficiency of charge carriers. Cu-BDC-FBA shows an excellent bifunctional photocatalytic performance for overall water splitting due to the suitable defect concentration, which gives an oxygen production rate of 3114 μmol g^-1 h^-1 and hydrogen production rate of 16 829 μmol g^-1 h^-1, respectively. It is expected that this study can deepen the understanding of the relationship between defects and photocatalytic activity, and provide a new idea for the design and synthesis of defective MOFs photocatalysts with excellent performance.

Advances in Metal-Organic Frameworks MIL-101(Cr)

MIL-101(Cr) is one of the most well-studied chromium-based metal-organic frameworks, which consists of metal chromium ion and terephthalic acid ligand. It has an ultra-high specific surface area, large pore size, good thermal/chemical/water stability, and contains unsaturated Lewis acid sites in its structure. Due to the physicochemical properties and structural characteristics, MIL-101(Cr) has a wide range of applications in aqueous phase adsorption, gas storage and separation, and catalysis. In this review, the latest synthesis of MIL-101(Cr) and its research progress in adsorption and catalysis are reviewed.

Selectivity Enhancement for Uric Acid Detection via In Situ Preparation of Blue Emissive Carbon Dots Entrapped in Chromium Metal-Organic Frameworks

In the present work, for the first time, the in situ formation of blue emissive carbon dots (bCDs) and encapsulation into the pores of chromium-based metal-organic frameworks (Cr-MOFs) are described. The luminescent bCDs via in situ process are formed and entrapped inside the pores of Cr-MOFs to form a nanocomposite of bCDs@Cr-MOFs. The bCDs@Cr-MOFs showed a strong broad blue emission at 420 nm (excited at 310 nm), which corresponds to both, the ligand (2-aminoterephthalic acid) in the Cr-MOF and the entrapped bCDs. This is assigned for the entrapping of bCDs in the pores of the MOFs. Additionally, transmission electron microscopy (TEM) images showed two types of particles, 150 rod-like shapes for Cr-MOF and 5-10 nm spherical shapes assigned for the presence of bCDs. The bCDs alone (without Cr-MOF) showed no selectivity, and their emission was quenched by different biomolecules and ions, such as ascorbic acid, uric acid, Fe³⁺, Cu²⁺, and Hg²⁺. The selectivity of bCDs toward uric acid was increased dramatically when they were encapsulated in the Cr-MOF. The linear range for uric acid was 20-50 μM, and the LOD was measured as 1.3 μM. Spike recoveries for the detection of uric acid in serum samples were between 94 and 108%. The relative standard deviation (RSD, n = 3) at each concentration value was less than 2%. The results showed high ruggedness and robustness of the assay due to its high shelf-life stability of probe (four weeks), water stability, and long working pH range. Validation experiments showed that the established MOF-based sensing system is appropriate for uric acid detection in real samples.

Defects engineering simultaneously enhances activity and recyclability of MOFs in selective hydrogenation of biomass

The development of synthetic methodologies towards enhanced performance in biomass conversion is desirable due to the growing energy demand. Here we design two types of Ru impregnated MIL-100-Cr defect engineered metal-organic frameworks (Ru@DEMOFs) by incorporating defective ligands (DLs), aiming at highly efficient catalysts for biomass hydrogenation. Our results show that Ru@DEMOFs simultaneously exhibit boosted recyclability, selectivity and activity with the turnover frequency being about 10 times higher than the reported values of polymer supported Ru towards D-glucose hydrogenation. This work provides in-depth insights into (i) the evolution of various defects in the cationic framework upon DLs incorporation and Ru impregnation, (ii) the special effect of each type of defects on the electron density of Ru nanoparticles and activation of reactants, and (iii) the respective role of defects, confined Ru particles and metal single active sites in the catalytic performance of Ru@DEMOFs for D-glucose selective hydrogenation as well as their synergistic catalytic mechanism.

The catalytic performance of metal‒organic frameworks can be tuned by introducing defects in their structure. Here, the authors introduce defects and impregnate ruthenium nanoparticles in cationic metal-organic frameworks, which enables enhanced recyclability and catalytic performance in D-glucose hydrogenation.

Development of a SPE/GC-MS method for the determination of organophosphorus pesticides in food samples using syringe filters packed by GNP/MIL-101(Cr) nanocomposite

In this study, we report the synthesis and application of a nanocomposite comprising metal-organic framework MIL-101(Cr) and graphene nanopowder (GNP) as a promising sorbent for the extraction of organophosphorus pesticides (OPPs) in juices, water, vegetables and honey samples. A syringe filter, for the first time, was used to host the synthesized nanocomposite and extract the OPPs followed by GC-MS analysis. Different characterization methods including XRD, FTIR, TGA, BET and SEM were employed to confirm the formation of studied nanocomposite. The results indicated that the GNP/MIL-101(Cr) could provide higher capacity for adsorption of OPPs and lower detection limit compared to pristine MIL-101(Cr). The detection limits were 0.005 to 15.0 µg/Kg and the linear range found between 0.05 and 400 µg/Kg. The proposed method showed very good repeatability with the RSD values ranging from 2.9% to 7.1%. The recoveries were between 84% -110% with the spiked levels of 2.0-100.0 µg/Kg.

A Polyoxometalate Composite Based on Hierarchical MIL-101 with Enhanced Catalytic Activity in Methanolysis of Styrene Oxide

A new efficient polyoxometalate composite catalyst of hierarchical MIL-101 and phosphotungstic acid (PTA) was facilely prepared by the immersion method. The material was thoroughly characterized by powder x-ray diffraction (PXRD), scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDX) and inductively coupled plasma‒optical emission spectrometry (ICP-OES). Compared to the pristine nonhierarchical MIL-101 composite, the hierarchical composite demonstrated much higher catalytic performance in methanolysis of styrene oxide, such as catalytic activity and reusability.

Comparison of Catalytic Activity of Chromium–Benzenedicarboxylate Metal–Organic Framework Based on Various Synthetic Approach

MIL-101(Cr), as a prototypical mesoporous metal–organic framework (MOF), can be facially prepared by involving different modulators to fit various demands. In this paper, a range of MIL-101(Cr) products were prepared under similar conditions. It was found that one of the additives, phenylphosphonic acid (PPOA), could give a stable hierarchical structure material. Compared to other MIL-101(Cr)s, though hierarchical MIL-101(Cr) showed less porosity, it gave a better catalytic performance in the oxidation of indene and 1-dodecene.

Benzoic acid as a selector-modulator in the synthesis of MIL-88B(Cr) and nano-MIL-101(Cr)

The concentration of benzoic acid was found to exercise efficient control over the formation of either MIL-101(Cr) or MIL-88B(Cr) under otherwise similar hydrothermal synthetic conditions. Nanocrystals of MIL-101(Cr) with ∼100 nm average size and excellent SBET = 3467 m2 g-1 are obtained at lower concentrations of benzoic acid, while at higher concentrations the microparticulated MIL-88B(Cr) product is formed. Hereby a new efficient synthetic method towards the elusive MIL-88B(Cr), yet reported only once without synthetic details, is proposed. The obtained MIL-88B(Cr) has an interesting and potentially valuable property of retaining its high-volume form (Vcell ∼ 2000 Å3) after thermal activation. The degassing of MIL-88B(Cr) in a vacuum at 250 °C yields a porous material with a SBET area of 1136 m2 g-1, which is around the theoretical maximum. The transition to the denser 'closed' form (Vcell ∼ 1500 Å3) occurs only at 350 °C, when all of the benzoate/benzoic acid, hindering the process, is removed.

Bio-related applications of porous organic frameworks (POFs)

Porous organic frameworks (POFs) are promising candidates for bio-related applications. This review highlights the recent progress in POF-based bioapplications, including drug delivery, bioimaging, biosensing, therapeutics, and artificial shells. These encouraging performances suggest that POFs used for bioapplications deserve more attention in the future.