A Titanium(IV)-Based Metal-Organic Framework Featuring Defect-Rich Ti-O Sheets as an Oxidative Desulfurization Catalyst.

While titanium-based metal-organic frameworks (MOFs) have been widely studied for their (photo)catalytic potential, only a few TiIV MOFs have been reported owing to the high reactivity of the employed titanium precursors. The synthesis of COK-47 is now presented, the first Ti carboxylate MOF based on sheets of TiIV O6 octahedra, which can be synthesized with a range of different linkers. COK-47 can be synthesized as an inherently defective nanoparticulate material, rendering it a highly efficient catalyst for the oxidation of thiophenes. Its structure was determined by continuous rotation electron diffraction and studied in depth by X-ray total scattering, EXAFS, and solid-state NMR. Furthermore, its photoactivity was investigated by electron paramagnetic resonance and demonstrated by catalytic photodegradation of rhodamine 6G.

Superactivity of MOF-808 toward Peptide Bond Hydrolysis.

MOF-808, a Zr(IV)-based metal-organic framework, has been proven to be a very effective heterogeneous catalyst for the hydrolysis of the peptide bond in a wide range of peptides and in hen egg white lysozyme protein. The kinetic experiments with a series of Gly-X dipeptides with varying nature of amino acid side chain have shown that MOF-808 exhibits selectivity depending on the size and chemical nature of the X side chain. Dipeptides with smaller or hydrophilic residues were hydrolyzed faster than those with bulky and hydrophobic residues that lack electron rich functionalities which could engage in favorable intermolecular interactions with the btc linkers. Detailed kinetic studies performed by 1H NMR spectroscopy revealed that the rate of glycylglycine (Gly-Gly) hydrolysis at pD 7.4 and 60 °C was 2.69 × 10-4 s-1 ( t1/2 = 0.72 h), which is more than 4 orders of magnitude faster compared to the uncatalyzed reaction. Importantly, MOF-808 can be recycled several times without significantly compromising the catalytic activity. A detailed quantum-chemical study combined with experimental data allowed to unravel the role of the {Zr6O8} core of MOF-808 in accelerating Gly-Gly hydrolysis. A mechanism for the hydrolysis of Gly-Gly by MOF-808 is proposed in which Gly-Gly binds to two Zr(IV) centers of the {Zr6O8} core via the oxygen atom of the amide group and the N-terminus. The activity of MOF-808 was also demonstrated toward the hydrolysis of hen egg white lysozyme, a protein consisting of 129 amino acids. Selective fragmentation of the protein was observed with 55% yield after 25 h under physiological pH.

Unravelling the Redox-catalytic Behavior of Ce4+ Metal-Organic Frameworks by X-ray Absorption Spectroscopy.

The introduction of Ce4+ as a structural cation has been shown to be a promising route to redox active metal-organic frameworks (MOFs). However, the mechanism by which these MOFs act as redox catalysts remains unclear. Herein, we present a detailed study of the active site in [Ce6 O4 (OH)4 ]-based MOFs such as Ce-UiO-66, involved in the aerobic oxidation of benzyl alcohol, chosen as a model redox reaction. X-ray absorption spectroscopy (XAS) data confirm the reduction of up to one Ce4+ ion per Ce6 cluster with a corresponding outwards radial shift due to the larger radius of the Ce3+ cation, while not compromising the structural integrity of the framework, as evidenced by powder X-ray diffraction. This unambiguously demonstrates the involvement of the metal node in the catalytic cycle and explains the need for 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) as a redox mediator to bridge the gap between the one-electron oxidation of the Ce4+ /Ce3+ couple and the two-electron alcohol oxidation. Finally, an improved catalytic system with Ce-MOF-808 and TEMPO was developed which outperformed all other tested Ce4+ -MOFs.

A Breathing Zirconium Metal-Organic Framework with Reversible Loss of Crystallinity by Correlated Nanodomain Formation.

The isoreticular analogue of the metal-organic framework UiO-66(Zr), synthesized with the flexible trans-1,4-cyclohexanedicarboxylic acid as linker, shows a peculiar breathing behavior by reversibly losing long-range crystalline order upon evacuation. The underlying flexibility is attributed to a concerted conformational contraction of up to two thirds of the linkers, which breaks the local lattice symmetry. X-ray scattering data are described well by a nanodomain model in which differently oriented tetragonal-type distortions propagate over about 7-10 unit cells.

Connecting defects and amorphization in UiO-66 and MIL-140 metal­organic frameworks: a combined experimental and computational study.

The mechanism and products of the structural collapse of the metal­organic frameworks (MOFs) UiO-66, MIL-140B and MIL-140C upon ball-milling are investigated through solid state 13C NMR and pair distribution function (PDF) studies, finding amorphization to proceed by the breaking of a fraction of metal­ligand bonding in each case. The amorphous products contain inorganic­organic bonding motifs reminiscent of the crystalline phases. Whilst the inorganic Zr6O4(OH)4 clusters of UiO-66 remain intact upon structural collapse, the ZrO backbone of the MIL-140 frameworks undergoes substantial distortion. Density functional theory calculations have been performed to investigate defective models of MIL-140B and show, through comparison of calculated and experimental 13C NMR spectra, that amorphization and defects in the materials are linked.

Three Series of Sulfo-Functionalized Mixed-Linker CAU-10 Analogues: Sorption Properties, Proton Conductivity, and Catalytic Activity.

Ten mixed-linker metal-organic frameworks [Al(OH)(m-BDC-X)(1-y)(m-BDC-SO3H)y] (H2BDC = 1,3-benzenedicarboxylic acid; X = H, NO2, OH) exhibiting the CAU-10-type structure were synthesized. The compounds can be grouped into three series according to the combination of ligands employed. The three series of compounds were obtained by employing different ratios of m-H2 BDC-X and m-H2BDC-SO3Li. The resulting compounds, which are denoted CAU-10-H/Sx, -N/Sx and -O/Sx, show exceptionally high thermal stability for sulfonated materials of up to 350 °C. Detailed characterization with special focus on polarity and acidity was performed, and the impact of the additional SO3H groups is clearly demonstrated by changes in the sorption affinities/capacities towards several gases and water vapor. In addition, selected samples were evaluated for proton conductivity and as catalysts for the gas-phase dehydration of ethanol to ethylene. While only very low proton conductivities were observed, a pronounced increase in catalytic activity was achieved. Although reactions were performed at temperatures of 250 and 300 °C for more than 40 h, no desulfonation and no loss of crystallinity were observed, and stable ethanol conversion resulted. This demonstrates the high stability of this material.

Adsorptive separation on metal-organic frameworks in the liquid phase.

While much attention of the MOF community has been devoted to adsorption and purification of gases, there is now also a vast body of data on the capability of MOFs to separate and purify liquid mixtures. Initial studies focused on separation of petrochemicals in apolar backgrounds, but the attention has moved now to the separation of complex, e.g. chiral compounds, and to the isolation of biobased compounds from aqueous media. We here give an overview of most of the existing literature, with an accent on separation mechanisms and structure-selectivity relationships.

Defect-Engineered Metal-Organic Frameworks.

Defect engineering in metal-organic frameworks (MOFs) is an exciting concept for tailoring material properties, which opens up novel opportunities not only in sorption and catalysis, but also in controlling more challenging physical characteristics such as band gap as well as magnetic and electrical/conductive properties. It is challenging to structurally characterize the inherent or intentionally created defects of various types, and there have so far been few efforts to comprehensively discuss these issues. Based on selected reports spanning the last decades, this Review closes that gap by providing both a concise overview of defects in MOFs, or more broadly coordination network compounds (CNCs), including their classification and characterization, together with the (potential) applications of defective CNCs/MOFs. Moreover, we will highlight important aspects of "defect-engineering" concepts applied for CNCs, also in comparison with relevant solid materials such as zeolites or COFs. Finally, we discuss the future potential of defect-engineered CNCs.

A Flexible Photoactive Titanium Metal-Organic Framework Based on a [Ti(IV)3(µ3-O)(O)2(COO)6] Cluster.

The synthesis of titanium-carboxylate metal-organic frameworks (MOFs) is hampered by the high reactivity of the commonly employed alkoxide precursors. Herein, we present an innovative approach to titanium-based MOFs by the use of titanocene dichloride to synthesize COK-69, the first breathing Ti MOF, which is built up from trans-1,4-cyclohexanedicarboxylate linkers and an unprecedented [Ti(IV)3(µ3-O)(O)2(COO)6] cluster. The photoactive properties of COK-69 were investigated in depth by proton-coupled electron-transfer experiments, which revealed that up to one Ti(IV) center per cluster can be photoreduced to Ti(III) while preserving the structural integrity of the framework. The electronic structure of COK-69 was determined by molecular modeling, and a band gap of 3.77 eV was found.

The structure of the aluminum fumarate metal-organic framework A520.

The synthesis of the commercially available aluminum fumarate sample A520 has been optimized and its structure analyzed through a combination of powder diffraction, solid-state NMR spectroscopy, molecular simulation, IR spectroscopy, and thermal analysis. A520 is an analogue of the MIL-53(Al)-BDC solid, but with a more rigid behavior. The differences between the commercial and the optimized samples in terms of defects have been investigated by in situ IR spectroscopy and correlated to their catalytic activity for ethanol dehydration.

Synthesis modulation as a tool to increase the catalytic activity of metal-organic frameworks: the unique case of UiO-66(Zr).

The catalytic activity of the zirconium terephthalate UiO-66(Zr) can be drastically increased by using a modulation approach. The combined use of trifluoroacetic acid and HCl during the synthesis results in a highly crystalline material, with partial substitution of terephthalates by trifluoroacetate. Thermal activation of the material leads not only to dehydroxylation of the hexanuclear Zr cluster but also to post-synthetic removal of the trifluoroacetate groups, resulting in a more open framework with a large number of open sites. Consequently, the material is a highly active catalyst for several Lewis acid catalyzed reactions.

Metal-biomolecule frameworks (BioMOFs): a novel approach for "green" optoelectronic applications.

In this study, a water-stable microcrystalline bioMOF was synthesized, characterized, and loaded with silver ions or highly emissive rare earth (RE) metals such as Eu3+/Tb3+. The obtained materials were used as active layers in a proof-of-concept sustainable light-emitting device, highlighting the potential of bioMOFs in optoelectronic applications.

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

By using flexible metal organic frameworks such as MIL-53(Al), the selective uptake of 4-methylguaiacol was achieved from a simulated bio-oil (40 wt%). Similar high uptake capacity of phenolics (27 wt%) was observed from a real pyrolysis bio-oil, with good selectivity towards a variety of phenolics, e.g. guaiacol, 4-methylguaiacol and catechol.

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

The capping formate anions of the metal-organic framework (MOF) zirconium benzene-1,3,5-tricarboxylate (MOF-808) were removed by a solvent exchange procedure, resulting in a formate-free MOF-808 sample containing "geminal" defects consisting of six coordinatively unsaturated sites (CUSs) on each of the Zr6 nodes. Adsorption experiments with this material showed that the uptake of 4-methylguaiacol from a bio-oil mixture was proportional to the number of defects and amounted to one mole adsorbed per mole of zirconium. The selective uptake behavior of MOF-808 towards phenolic compounds was further evident from competitive adsorption experiments between furfuryl alcohol and 4-methylguaiacol as well as from the excellent (20 wt % for phenolic compounds and <7 wt % for other compounds) uptake performance for real bio-oil mixtures containing a large concentration and diversity of molecules.

Sodium-coupled electron transfer reactivity of metal-organic frameworks containing titanium clusters: the importance of cations in redox chemistry.

Stoichiometric reduction reactions of two metal-organic frameworks (MOFs) by the solution reagents (M = Cr, Co) are described. The two MOFs contain clusters with Ti8O8 rings: Ti8O8(OH)4(bdc)6; bdc = terephthalate (MIL-125) and Ti8O8(OH)4(bdc-NH2)6; bdc-NH2 = 2-aminoterephthalate (NH2-MIL-125). The stoichiometry of the redox reactions was probed using solution NMR methods. The extent of reduction is greatly enhanced by the presence of Na+, which is incorporated into the bulk of the material. The roughly 1 : 1 stoichiometry of electrons and cations indicates that the storage of e- in the MOF is tightly coupled to a cation within the architecture, for charge balance.

Layered Zn2[Co(CN)6](CH3COO) double metal cyanide: a two-dimensional DMC phase with excellent catalytic performance.

Double metal cyanides (DMCs) are well known, industrially applied catalysts for ring opening polymerization reactions. In recent years, they have been studied for a variety of catalytic reactions, as well as other applications, such as energy storage and Cs sorption. Herein, a new, layered DMC phase (L-DMC), Zn2[Co(CN)6](CH3COO)·4H2O, was synthesized. The structure, which crystallizes in the monoclinic space group P21/m, consists of positively charged {Zn2Co(CN)6}+ DMC layers linked through acetate groups and presents a new layered structure to the family of double metal cyanides. L-DMC proved to be a reusable and stable catalyst that exhibited a higher activity than the benchmark DMC catalyst in two important applications: hydroamination of phenylacetylene with 4-isopropylaniline and polymerization of 1,2-epoxyhexane.

Single-site metal-organic framework catalysts for the oxidative coupling of arenes via C-H/C-H activation.

C-H activation reactions are generally associated with relatively low turnover numbers (TONs) and high catalyst concentrations due to a combination of low catalyst stability and activity, highlighting the need for recyclable heterogeneous catalysts with stable single-atom active sites. In this work, several palladium loaded metal-organic frameworks (MOFs) were tested as single-site catalysts for the oxidative coupling of arenes (e.g. o-xylene) via C-H/C-H activation. Isolation of the palladium active sites on the MOF supports reduced Pd(0) aggregate formation and thus catalyst deactivation, resulting in higher turnover numbers (TONs) compared to the homogeneous benchmark reaction. Notably, a threefold higher TON could be achieved for palladium loaded MOF-808 due to increased catalyst stability and the heterogeneous catalyst could efficiently be reused, resulting in a cumulative TON of 1218 after three runs. Additionally, the palladium single-atom active sites on MOF-808 were successfully identified by Fourier transform infrared (FTIR) and extended X-ray absorption fine structure (EXAFS) spectroscopy.

Highly stable and porous porphyrin-based zirconium and hafnium phosphonates - electron crystallography as an important tool for structure elucidation.

The Ni-metallated porphyrin-based tetraphosphonic acid (Ni-tetra(4-phosphonophenyl)porphyrin, Ni-H8TPPP) was used for the synthesis of highly porous metal phosphonates containing the tetravalent cations Zr4+ and Hf4+. The compounds were thoroughly characterized regarding their sorption properties towards N2 and H2O as well as thermal and chemical stability. During the synthesis optimization the reaction time could be substantially decreased under stirring from 24 to 3 h in glass vials. M-CAU-30, [M2(Ni-H2TPPP)(OH/F)2]·H2O (M = Zr, Hf) shows exceptionally high specific surface areas for metal phosphonates of aBET = 1070 and 1030 m2 g-1 for Zr- and Hf-CAU-30, respectively, which are very close/correspond to the theoretical values of 1180 and 1030 m2 g-1. CAU-30 is always obtained as mixtures with one mol ZrO2/HfO2 per formula unit as proven by TEM, electron diffraction, TG and elemental analysis. Hence experimentally derived specific surface areas are 970 and 910 m2 g-1, respectively. M-CAU-30 is chemically stable in the pH range 0 to 12 in HCl/NaOH and thermally up to 420 °C in air as determined by variable-temperature powder X-ray diffraction (VT-PXRD). The crystal structure of M-CAU-30 was determined by combining electron diffraction tomography for structure solution and powder X-ray diffraction data for the structure refinement. The crystal structure consists of chains of corner sharing MO6 octahedra interconnected by the partly deprotonated linker molecules Ni-H2TPPP6-. Thus 1D channels with pore diameters of 1.3 × 2.0 nm are formed. The redox activity of Zr-CAU-30 was investigated by cyclic voltammetry resulting in a reversible redox process at a half-wave potential of E1/2 = -0.649 V.

A precursor method for the synthesis of new Ce(iv) MOFs with reactive tetracarboxylate linkers.

A precursor method has been developed to synthesize Ce(iv) MOFs that could not be prepared directly from Ce(iv) salts. Starting from Ce6 clusters, two Ce-UiO-66 analogues and four tetracarboxylate-based Ce(iv) MOFs could be synthesized. The applied method facilitates framework formation by evading reactive individual Ce(iv)-ions thereby paving the way for further development of Ce-MOFs.