Geminal Coordinatively Unsaturated Sites on MOF-808 for the Selective Uptake of Phenolics from a Real Bio-Oil Mixture

A zirconium metal-organic framework functionalized with a S/N containing carboxylic acid (MOF-808(Zr)-Tz) as an efficient sorbent for the ultrafast and selective dispersive solid phase micro extraction of chromium, silver, and rhodium in water samples

Metal-organic frameworks (MOFs) are good adsorbents for targeted chemicals with their adjustable properties. Herein, we prepared a zirconium based MOF (MOF-808(Zr)) and functionalized it employing 2-mercapto-4-methyl-5-thiazolacetic acid (MOF-808(Zr)-Tz). The prepared MOFs were characterized by XRD, FTIR, SEM-EDX, TGA, N2 sorption, zeta potential measurements, and elemental analysis. The surface area of MOF-808(Zr)-Tz was 1348 m2/g. Dispersive solid-phase micro-extraction (D-SPµE) method based on MOF-808(Zr)-Tz was firstly developed and applied to the extraction of chromium, silver, and rhodium in waters. The determination of the analytes was done by FAAS. The optimal pH and eluent for analytes were 7.0 and 3 mL of 2 mol L-1 HCl, respectively. The contact times were 1 min for adsorption and 3 min for elution. The LOD and PFs of the D-SPμE for analytes were 2.3 μg L-1 and 13.3 for chromium, 2.1 μg L-1 and 13.3 for silver, and 3.1 μg L-1 and 13.3 rhodium, respectively. The D-SPμE method was verified with analyses of NW-TMDA-54.6 Lake water and SPS-WW1 Batch 114 Wastewater and with spiked dam water, river water, well water, sea water, and wastewater. The recoveries of the analytes changed from 89 to 108 %. The results indicated that the method is selective, simple, effective, and rapid for extracting chromium(III), silver(I) and rhodium(III) in waters.

Porous Core-membrane Microstructured Nanomaterial Composed of Deep Eutectic Solvents and MOF-808 for CO2 Capture

A series of porous core-membrane microstructured nanomaterials, constructed of a deep eutectic solvent (DES) membrane and porous MOF-808 core via liquid surface tensions and electrostatic interactions, are introduced for carbon dioxide capture with the sorption mechanism coupling diffusion, physisorption, and chemisorption. MOF-808 as the porous core considerably improves the diffusion interactions for DES membranes, hence significantly enhancing the sorption performance of DESs. Although the DES consisted by monoethanolamine and tetrapropylammonium chloride (MEA-TPAC-7) has the highest sorption capacity among all DESs, it is only 4.39 mmol g-1 at 2.4 bar and further attenuates by fastidious diffusion interactions when increasing viscosity or dose. The sorption capacities of DES@MOF-120 are 5.18 mmol g-1 at 3.0 bar and 4.78 mmol g-1 at 2.4 bar without apparent sorption hysteresis in pressure swing sorption, which are substantially improved contrasted to MEA-TPAC-7. The sorption isotherms are reconstructed via Sips models considering surface heterogeneity with regression correlation coefficients over 0.9454 to forecast maximum sorption capacity over 6.33 mmol g-1 .

Preparation of formate-free PMA@MOF-808 catalysts for deep oxidative desulfurization of model fuels

Due to the increasingly serious environmental problems caused by the combustion of sulfides in fuel, deep desulfurization of fuel became particularly urgent. Herein, the catalyst (PMA@MOF-808) of the Zr-based metal-organic framework (MOF-808) encapsulating phosphomolybdic acid (PMA) was prepared via a one-pot hydrothermal method. Besides, the formate ions of PMA@MOF-808 were removed by posttreatment with methanol, resulting in formate-free PMA@MOF-808-H catalysts with unsaturated open metal sites. The as-synthesized catalysts were systematically characterized by XRD, FT-IR, SEM, BET, TGA, ¹H NMR and XPS. The catalysts were also applied in catalytic oxidation desulfurization of fuel. The results indicated that the introduction of PMA and the removal of formate ions can improve the desulfurization performance of catalysts. Formate-free 0.2-PMA@MOF-808-H catalyst can reach 100% desulfurization rate for DBT. Besides, the kinetic properties were studied, and the apparent activation energy was 29.34 kJ/mol.

Revisiting Vibrational Spectroscopy to Tackle the Chemistry of Zr6O8 Metal-Organic Framework Nodes

The metal-organic framework MOF-808 contains Zr6O8 nodes with a high density of vacancy sites, which can incorporate carboxylate-containing functional groups to tune chemical reactivity. Although the postsynthetic methods to modify the chemistry of the Zr6O8 nodes in MOFs are well known, tackling these alterations from a structural perspective is still a challenge. We have combined infrared spectroscopy experiments and first-principles calculations to identify the presence of node vacancies accessible for chemical modifications within the MOF-808. We demonstrate the potential of our approach to assess the decoration of MOF-808 nodes with different catechol-benzoate ligands. Furthermore, we have applied advanced synchrotron characterization tools, such as pair distribution function analyses and X-ray absorption spectroscopy, to resolve the atomic structure of single metal sites incorporated into the catechol groups postsynthetically. Finally, we demonstrate the catalytic activity of these MOF-808 materials decorated with single copper sites for 1,3-dipolar cycloadditions.

Recent advances in metal-organic frameworks for gas adsorption/separation

The unique structural advantage of metal-organic frameworks (MOFs) determines the great prospect and developability in gas adsorption and separation. Both ligand design and microporous engineering based on crystal structure are significant lever for coping with new application exploration and requirements. Focusing on the designable pore and modifiable frameworks of MOFs, this review discussed the recent advances in the field of gas adsorption and separation, and analyzed the host-guest interaction, structure-performance relations, and the adsorption/separation mechanism from ligand design, skeleton optimization, metal node regulation, and active sites construction. Based on the function-oriented perspective, we summarized the main research recently, and made an outlook based on the focus of microporous MOFs that require further attention in the structure design and industrial application.

Controlling the molecular diffusion in MOFs with the acidity of monocarboxylate modulators

The catalytic performance of metal-organic frameworks (MOFs) is related to their physicochemical properties, such as particle size, defect chemistry and porosity, which can be potentially controlled by coordination modulation. By combining PXRD, ¹HNMR, FT-IR, and N₂ uptake measurements we have gained insights into the control of different types of defects (missing linker or missing cluster consequence of the spatial distribution of missing linkers, and a combination of both) by the type of modulator employed. We show that the molar percent of defects, either as missing linkers or as a part of missing cluster defects, is related to the acidity of a modulator and its subsequent incorporation into the UiO-66 structure. Modulators with strong acidity and small size result in a considerable defect induction that causes an increase in the external surface area and mesopore volume, which is beneficial for the ring-opening of epoxides with amines, using UiO-66 defect-modulated MOFs as heterogeneous catalysts.

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.

Millimetre-resolution mapping of citrate exuded from soil-grown roots using a novel, low-invasive sampling technique

The reliable sampling of root exudates in soil-grown plants is experimentally challenging. This study aimed at developing a citrate sampling and mapping technique with millimetre-resolution using DGT (diffusive gradients in thin films) ZrOH-binding gels. Citrate adsorption kinetics, DGT capacity, and stability of ZrOH gels were evaluated. ZrOH gels were applied to generate 2D maps of citrate exuded by white lupin roots grown in a rhizotron in a phosphorus-deficient soil. Citrate was adsorbed quantitatively and rapidly by the ZrOH gels; these gels can be stored after sampling for several weeks prior to analysis. The DGT capacity of the ZrOH gel for citrate depends on the ionic strength and the pH of the soil solution, but was suitable for citrate sampling. We generated for the first time 2D citrate maps of rhizotron-grown plants at a millimetre resolution to measure an illustrated plant response to phosphorus fertilization, demonstrating that DGT-based citrate sampling is suitable for studying root exudation in soil environments, at high spatial resolution. The change of binding material would also allow sampling of other exudate classes and exudation profiles of entire root systems. These aspects are crucial in cultivar breeding and selection.

Zr-MOF-808 as Catalyst for Amide Esterification

In this work, zirconium-based metal-organic framework Zr-MOF-808-P has been found to be an efficient and versatile catalyst for amide esterification. Comparing with previously reported homogeneous and heterogeneous catalysts, Zr-MOF-808-P can promote the reaction for a wide range of primary, secondary and tertiary amides with n-butanol as nucleophilic agent. Different alcohols have been employed in amide esterification with quantitative yields. Moreover, the catalyst acts as a heterogeneous catalyst and could be reused for at least five consecutive cycles. The amide esterification mechanism has been studied on the Zr-MOF-808 at molecular level by in situ FTIR spectroscopic technique and kinetic study.

Cooperative acid–base bifunctional ordered porous solids in sequential multi-step reactions: MOF vs. mesoporous silica

Two different catalytic platforms, MOF and mesoporous silica, were compared as porous support for basic amino groups to promote sequential multi-step reactions.

Organic synthesis of high added value molecules with MOF catalysts

Recent examples of organic synthesis of fine chemicals and pharmaceuticals in confined spaces of MOFs are highlighted and compared with silica-based ordered porous solids, such as zeolites or mesoporous (organo)silica. These heterogeneous catalysts offer the possibility of stabilizing the desired transition states and/or intermediates during organic transformations of functional groups and (C-C/C-N) bond forming steps towards the desired functional high added value molecular scaffolds. A short introduction on zeolites, mesoporous silica and metal-organic frameworks is followed by relevant applications in which confined active sites in the pores promote single or multi-step organic synthesis of industrially relevant molecules. A critical discussion on the catalytic performances of the different types of hybrid inorganic-organic catalysts in the synthesis of O- and N-containing acyclic and heterocyclic molecules has been presented.

Modulator-Mediated Functionalization of MOF-808 as a Platform Tool to Create High-Performance Mixed-Matrix Membranes

Modulator-mediated functionalization (MoFu) is introduced as a new and versatile platform tool to improve the separation performance of metal-organic framework (MOF)-based membranes, exemplified here by the creation of mixed-matrix membranes (MMMs) with enhanced CO₂ separation efficiency. The unique structure of MOF-808 allows incorporation of CO₂-philic modulators in the MOF framework during a one-pot synthesis procedure in water, thus creating a straightforward way to functionalize both MOF and corresponding MMM. As a proof of concept, a series of fluorinated carboxylic acids [trifluoroacetic acid (TFA), pentafluoropropionic acid (PFPA), and heptafluorobutyric acid (HFBA)] and nonfluorinated alkyl carboxylic acids (acetic acid (AA), propionic acid (PA), and butyric acid (BA)) were used as a modulator during MOF-808 synthesis. Two of the best MMMs prepared with 30 wt % MOF-TFA (100% increase in CO₂/CH₄ separation factor, 350% increase in CO₂ permeability) and 10 wt % MOF-PFPA (140% increase in CO₂/CH₄ separation factor, 100% increase in CO₂ permeability) scored very close to or even crossed the 2008 and 2018 upper bound limits for CO₂/CH₄. Because of its facile functionalization (and its subsequent excellent performance), MOF-808 is proposed as an alternative for widely used UiO-66, which is, from a functionalization point-of-view and despite its widespread use, a rather limited MOF.

Phenolics isolation from bio-oil using the metal–organic framework MIL-53(Al) as a highly selective adsorbent

The selective uptake of phenolic compounds, a key step of bio-refining, was achieved on MIL-53(Al) and Basolite A100 both from a simulated bio-oil and a real pyrolysis bio-oil.