Metal Node Control of Brønsted Acidity in Heterobimetallic Titanium-Organic Frameworks.

Compared to indirect framework modification, synthetic control of cluster composition can be used to gain direct access to catalytic activities exclusive of specific metal combinations. We demonstrate this concept by testing the aminolysis of epoxides with a family of isostructural mesoporous frameworks featuring five combinations of homometallic and heterobimetallic metal-oxo trimers (Fe3, Ti3, TiFe2, TiCo2, and TiNi2). Only TiFe2 nodes display activities comparable to benchmark catalysts based on grafting of strong acids, which here originate from the combination of Lewis Ti4+ and Brønsted Fe3+-OH acid sites. The applicability of MUV-101(Fe) to the synthesis of ß-amino alcohols is demonstrated with a scope that also includes the gram scale synthesis of propranolol, a natural ß-blocker listed as an essential medicine by the World Health Organization, with excellent yield and selectivity.

Metal-Organic Frameworks as Versatile Heterogeneous Solid Catalysts for Henry Reactions.

Metal-organic frameworks (MOFs) have become one of the versatile solid materials used for a wide range of applications, such as gas storage, gas separation, proton conductivity, sensors and catalysis. Among these fields, one of the more well-studied areas is the use of MOFs as heterogeneous catalysts for a broad range of organic reactions. In the present review, the employment of MOFs as solid catalysts for the Henry reaction is discussed, and the available literature data from the last decade are grouped. The review is organized with a brief introduction of the importance of Henry reactions and structural properties of MOFs that are suitable for catalysis. The second part of the review discusses the use of MOFs as solid catalysts for the Henry reaction involving metal nodes as active sites, while the third section provides data utilizing basic sites (primary amine, secondary amine, amides and urea-donating sites). While commenting on the catalytic results in these two sections, the advantage of MOFs over other solid catalysts is compared in terms of activity by providing turnover number (TON) values and the structural stability of MOFs during the course of the reaction. The final section provides our views on further directions in this field.

Selective Implantation of Diamines for Cooperative Catalysis in Isoreticular Heterometallic Titanium-Organic Frameworks.

We introduce the first example of isoreticular titanium-organic frameworks, MUV-10 and MUV-12, to show how the different affinity of hard Ti(IV) and soft Ca(II) metal sites can be used to direct selective grafting of amines. This enables the combination of Lewis acid titanium centers and available -NH2 sites in two sizeable pores for cooperative cycloaddition of CO2 to epoxides at room temperature and atmospheric pressure. The selective grafting of molecules to heterometallic clusters adds up to the pool of methodologies available for controlling the positioning and distribution of chemical functions in precise positions of the framework required for definitive control of pore chemistry.

Diffusion Control in Single-Site Zinc Reticular Amination Catalysts.

Zn-containing metal-organic frameworks have been used for the first time as heterogeneous catalysts in the amination of C-Cl bonds. The use of extended bis(pyrazolate) linkers can generate highly porous architectures, which favor the diffusion of amines to the confined spaces with respect to other imidazolate frameworks with narrower pore windows. The N4Zn nodes of the Zn-reticular framework show comparable activity to state-of-the-art homogeneous Zn amination catalysts, avoiding the use of basic conditions, precious metals, or other additives. This is combined with long-term activity and stability upon several reaction cycles, without contamination of the reaction product.

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.

Boosting the Catalytic Performance of Metal-Organic Frameworks for Steroid Transformations by Confinement within a Mesoporous Scaffold.

Solid-state crystallization achieves selective confinement of metal-organic framework (MOF) nanocrystals within mesoporous materials, thereby rendering active sites more accessible compared to the bulk-MOF and enhancing the chemical and mechanical stability of MOF nanocrystals. (Zr)UiO-66(NH2 )/SiO2 hybrid materials were tested as efficient and reusable heterogeneous catalysts for the synthesis of steroid derivatives, outperforming the bulk (Zr)UiO-66(NH2 ) MOF. A clear correlation between the catalytic activity of the dispersed Zr sites present in the confined MOF, and the loading of the mesoporous SiO2 , is demonstrated for steroid transformations.

Pd-imidate@SBA-15: a multifunctional heterogeneous catalyst for the aqueous room temperature hydrogenation of CO2 to formic acid.

The incorporation of hydrophilic and basic sites from phosphotriazaadamantane and saccharine in a water-soluble Pd complex and the subsequent confinement into mesoporous silica support increases the activity and stability of the palladium catalytic species for the room-temperature aqueous hydrogenation of either bicarbonate or CO2 into formic acid. The use of low Pd loadings (<0.1mmolPd·g-1) of the well-dispersed complex on SBA-15 mesoporous silica allows performing the reaction under room temperature, and aqueous conditions, exhibiting TON of ca. 40-100, for the CO2 or bicarbonate hydrogenation, respectively, which is one order of magnitude higher than the homogeneous case, allowing the easy isolation and recycling of the solid catalyst.

Tuning Ni-Pyrazolate Frameworks by Post-Synthetic Fe-Incorporation for Oxidase-Mimicking H2 O2 Activation.

The introduction of iron ionic sites by metal exchange of defective homometallic nickel pyrazolate frameworks generates non-precious, Earth-abundant, first-row heterometallic Fe/Ni-pyrazolate frameworks. The Fe incorporation at the Ni nodes of the framework allows to control the hydrogen peroxide activation, minimizing its decomposition and O2 liberation, occurring at the homometallic Ni nodes. The generation of Fe-OH reactive oxygen species at the heterometallic Fe/Ni nodes is demonstrated by the higher activity in the proof-of-concept oxidation of 1-phenylethanol to acetophenone in an aqueous medium.

MOF nanoparticles as heterogeneous catalysts for direct amide bond formations.

The influence of composition and textural characteristics of a family of ultra-small isoreticular UiO-type metal-organic frameworks (MOFs) with different functionalized and extended linkers on their catalytic performance is evaluated. Two direct amide bond formations across four different substrates (benzylamine + phenylacetic acid and aniline + formic acid) are employed as proof-of-concept reactions to test the activity of the Zr-MOF nanoparticles. The reaction rates of amide bond formation are evaluated against physico-chemical properties such as crystallinity, porosity, particle size or linker functionality, alongside the Lewis acid and hydrophobic properties of the MOFs, in order to gain insights into the catalytic mechanism and optimal properties for its enhancement.

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.

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.

Surfactant-templated zeolites for the production of active pharmaceutical intermediates.

A hierarchical USY zeolite has been produced using the surfactant-templating method and used as a catalyst for the production of two important active pharmaceutical ingredients. The presence of intracrystalline mesoporosity in the zeolite results in a significant increase in both the activity (up to 30 fold increase in TOF) and reusability for Friedel-Crafts alkylation and aldol condensation steps.

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.

Metal ion exchange in Prussian blue analogues: Cu(ii)-exchanged Zn-Co PBAs as highly selective catalysts for A3 coupling.

The occurrence of metal ion exchange in Zn3[Co(CN)6]2 and Cu3[Co(CN)6]2 Prussian blue analogues (Zn-Co and Cu-Co PBAs) was demonstrated for the first time. While Cu(ii) ion exchange easily occurs in Zn-Co PBA, the exchange of Cu(ii) atoms in Cu-Co PBA by Zn(ii) proved to be more difficult. At low to medium Cu(ii) loadings, the catalytic activity of the exchanged PBAs for the A3 coupling reaction of benzaldehyde, piperidine and phenylacetylene was higher than that of the bimetallic PBAs and that of multi metal PBAs of similar composition prepared by co-precipitation. This result showcases the benefits of the ion exchange process as a preparation method of PBA catalysts, since it is believed to lead to the incorporation of the desired metal in a more accessible position for reactant molecules. At higher Cu(ii) loadings, ion exchange with Cu(CH3COO)2·H2O also resulted in co-incorporation of CH3COO-. This incorporation considerably boosted the catalytic activity of the PBAs by providing a basic function that facilitates the C-H activation of phenylacetylene. The most active of the studied PBAs, catallytically outperforms other Cu(ii) based A3 coupling catalysts and completely suppresses the activity for the homocoupling of phenylacetylene, even under oxidative conditions. Furthermore, the basicity of the PBAs was investigated in the nitroaldol (Henry) reaction, where a clear effect of the presence of CH3COO- was observed. The CH3COO- containing PBAs exhibited an activity three times higher than the rest of the PBAs. The presence of the basic CH3COO- groups represents the first case of basic functionalization of PBAs.

Improved MOF nanoparticle recovery and purification using crosslinked PVDF membranes.

Crosslinked PVDF-membranes are demonstrated to offer a viable alternative for centrifugation in the preparation of MOF-particles, thus realising new opportunities at lab-scale and continuous production at large-scale. The membranes combine extreme-pH with solvent stability, thus enabling application in any MOF synthesis, demonstrated here for ZIF-8, ZIF-67, HKUST-1, UiO-66 and MIL-53(Al).