In Situ Neutron Diffraction of Zn-MOF-74 Reveals Nanoconfinement-Induced Effects on Adsorbed Propene.

Even though confinement was identified as a common element of selective catalysis and simulations predicted enhanced properties of adsorbates within microporous materials, experimental results on the characterization of the adsorbed phase are still rare. In this study, we provide experimental evidence of the increase of propene density in the channels of Zn-MOF-74 by 16(2)% compared to the liquid phase. The ordered propene molecules adsorbed within the pores of the MOF have been localized by in situ neutron powder diffraction, and the results are supported by adsorption studies. The formation of a second adsorbate layer, paired with nanoconfinement-induced short intermolecular distances, causes the efficient packing of the propene molecules and results in an increase of olefin density.

Deciphering the Mechanism of Crystallization of UiO-66 Metal-Organic Framework.

Zirconium-containing metal-organic framework (MOF) with UiO-66 topology is an extremely versatile material, which finds applications beyond gas separation and catalysis. However, after more than 10 years after the first reports introducing this MOF, understanding of the molecular-level mechanism of its nucleation and growth is still lacking. By means of in situ time-resolved high-resolution mass spectrometry, Zr K-edge X-ray absorption spectroscopy, magic-angle spinning nuclear magnetic resonance spectroscopy, and X-ray diffraction it is showed that the nucleation of UiO-66 occurs via a solution-mediated hydrolysis of zirconium chloroterephthalates, whose formation appears to be autocatalytic. Zirconium-oxo nodes form directly and rapidly during the synthesis, the formation of pre-formed clusters and stable non-stoichiometric intermediates are not observed. The nuclei of UiO-66 possess identical to the crystals local environment, however, they lack long-range order, which is gained during the crystallization. Crystal growth is the rate-determining step, while fast nucleation controls the formation of the small crystals of UiO-66 with a narrow size distribution of about 200 nanometers.

Tandem Hydroformylation-Aldol Condensation Reaction Enabled by Zn-MOF-74.

The tandem hydroformylation-aldol condensation (tandem HF-AC) reaction offers an efficient synthetic route to the synthesis of industrially relevant products. The addition of Zn-MOF-74 to the cobalt-catalyzed hydroformylation of 1-hexene enables tandem HF-AC under milder pressure and temperature conditions than the aldox process, where zinc salts are added to cobalt-catalyzed hydroformylation reactions to promote aldol condensation. The yield of the aldol condensation products increases by up to 17 times compared to that of the homogeneous reaction without MOF and up to 5 times compared to the aldox catalytic system. Both Co2 (CO)8 and Zn-MOF-74 are required to significantly enhance the activity of the catalytic system. Density functional theory simulations and Fourier-transform infrared experiments show that heptanal, the product of hydroformylation, adsorbs on the open metal site (OMS) of Zn-MOF-74, thereby increasing the electrophilic character of the carbonyl carbon atom and facilitating the condensation.

Unraveling the molecular mechanism of MIL-53(Al) crystallization.

The vast structural and chemical diversity of metal-organic frameworks (MOFs) provides the exciting possibility of material's design with tailored properties for gas separation, storage and catalysis. However, after more than twenty years after first reports introducing MOFs, the discovery and control of their synthesis remains extremely challenging due to the lack of understanding of mechanisms of their nucleation and growth. Progress in deciphering crystallization pathways depends on the possibility to follow conversion of initial reagents to products at the molecular level, which is a particular challenge under solvothermal conditions. The present work introduces a detailed molecular-level mechanism of the formation of MIL-53(Al), unraveled by combining in situ time-resolved high-resolution mass-spectrometry, magic angle spinning nuclear magnetic resonance spectroscopy and X-ray diffraction. In contrast to the general belief, the crystallization of MIL-53 occurs via a solid-solid transformation mechanism, associated with the spontaneous release of monomeric aluminum. The role of DMF hydrolysis products, formate and dimethylamine, is established. Our study emphasizes the complexity of MOF crystallization chemistry, which requires case-by-case investigation using a combination of advanced in situ methods for following the induction period, the nucleation and growth across the time domain.

Catalyst overcoating engineering towards high-performance electrocatalysis.

Clean and sustainable energy needs the development of advanced heterogeneous catalysts as they are of vital importance for electrochemical transformation reactions in renewable energy conversion and storage devices. Advances in nanoscience and material chemistry have afforded great opportunities for the design and optimization of nanostructured electrocatalysts with high efficiency and practical durability. In this review article, we specifically emphasize the synthetic methodologies for the versatile surface overcoating engineering reported to date for optimal electrocatalysts. We discuss the recent progress in the development of surface overcoating-derived electrocatalysts potentially applied in polymer electrolyte fuel cells and water electrolyzers by correlating catalyst intrinsic structures with electrocatalytic properties. Finally, we present the opportunities and perspectives of surface overcoating engineering for the design of advanced (electro)catalysts and their deep exploitation in a broad scope of applications.

Heterogeneous Metal-Organic Framework Catalysts for Suzuki-Miyaura Cross Coupling in the Pharma Industry.

The synthesis of drug substances (DS) requires the continuous effort of the pharma industry to ensure high sustainability standards. The Suzuki-Miyaura cross coupling is a fundamental C-C bond-forming reaction to produce complex DS intermediates. The present contribution points out the way in which the synthesis of DS intermediates by C-C cross coupling can be economically competitive, while minimizing waste by selecting the appropriate heterogeneous catalyst. By comparing homogeneous, immobilized heterogeneous catalysts on silica and metal-organic framework (MOF) catalysts, while considering the perspectives of academia and industry, the critical parameters for a successful industrial application of heterogeneous catalytic Suzuki-Miyaura cross coupling reactions were identified. Heterogeneous catalysts, such as MOFs, may provide a complementary platform for reducing waste and the costs of production related to such transformations.

Spatio-Chemical Heterogeneity of Defect-Engineered Metal-Organic Framework Crystals Revealed by Full-Field Tomographic X-ray Absorption Spectroscopy.

The introduction of structural defects in metal-organic frameworks (MOFs), often achieved through the fractional use of defective linkers, is emerging as a means to refine the properties of existing MOFs. These linkers, missing coordination fragments, create unsaturated framework nodes that may alter the properties of the MOF. A property-targeted utilization of this approach demands an understanding of the structure of the defect-engineered MOF. We demonstrate that full-field X-ray absorption near-edge structure computed tomography can help to improve our understanding. This was demonstrated by visualizing the chemical heterogeneity found in defect-engineered HKUST-1 MOF crystals. A non-uniform incorporation and zonation of the defective linker was discovered, leading to the presence of clusters of a second coordination polymer within HKUST-1. The former is suggested to be responsible, in part, for altered MOF properties; thereby, advocating for a spatio-chemically resolved characterization of MOFs.

Thermal degradation of defective high-surface-area UiO-66 in different gaseous environments.

UiO-66 is a versatile zirconium-based MOF, which is thermally stable up to 500 °C. In the present work, the thermal degradation of UiO-66 with a high number of defects has been studied in inert, oxidative and reductive environments. A sample of UiO-66 with a high BET surface area of 1827 m2 g-1 was prepared, which contains 2.3 missing linkers per hexa-zirconium node, as calculated by the thermogravimetric curve. The crystalline framework of this UiO-66 sample collapses at 250 °C, while thermal decomposition starts at 450 °C in the oxidative environment and at 500 °C in the reductive and inert environments. The BET surface area of the MOF is affected variably by heating under different gaseous conditions. Under inert conditions, porosity is maintained up to 711 m2 g-1, which is quite high when compared to that under reductive (527 m2 g-1) or oxidative (489 m2 g-1) conditions. Upon complete thermal decomposition at 600 °C, the MOF produces predominantly tetragonal zirconia. TEM images of the thermally decomposed samples show that the shape of the original MOF crystal is maintained during the heating process in the inert and reductive environments, whereas under oxidative conditions, all of the carbon is burnt to carbon dioxide, leaving no carbon matrix as the support.

Influence of Water in the Synthesis of the Zirconium-Based Metal-Organic Framework UiO-66: Isolation and Reactivity of [ZrCl(OH)2(DMF)2]Cl.

We recently discovered that aging a solution of zirconium(IV) tetrachloride (ZrCl4) in N,N-dimethylformamide (DMF) in the presence of water, followed by the addition of a terephthalic acid linker, reduces the crystallite size of the metal-organic framework UiO-66 (Chem. Commun. 2016, 52, 6411-6414). In an effort to shed light on the nature of the aging effect and on its relationship with the crystallite size of UiO-66, we report here the isolation and structural characterization of a microcrystalline zirconium-based compound of the formula [ZrCl(OH)2(DMF)2]Cl, which is formed during the aging process. The Zr(IV) ions are coordinated by hydroxide, DMF, and chloride to produce a one-dimensional polymer. Thanks to the presence of two -OH groups per zirconium atom, [ZrCl(OH)2(DMF)2]Cl is a suitable precursor for the synthesis of UiO-66 in dry DMF, affording a product having a smaller crystallite size than that obtained from a reaction mixture having the same chemical composition but using ZrCl4 as the Zr(IV) source. By starting from ZrCl4 and generating [ZrCl(OH)2(DMF)2]Cl in situ in solution through aging, we obtained smaller crystallites as the aging time increased, proving that [ZrCl(OH)2(DMF)2]Cl plays a role in the aging process. The possible role of [ZrCl(OH)2(DMF)2]Cl in the crystallization mechanism of UiO-66 is also discussed, with emphasis on its relationship with the amount of water in the reaction mixture.

Metal-organic frameworks as kinetic modulators for branched selectivity in hydroformylation.

Finding heterogeneous catalysts that are superior to homogeneous ones for selective catalytic transformations is a major challenge in catalysis. Here, we show how micropores in metal-organic frameworks (MOFs) push homogeneous catalytic reactions into kinetic regimes inaccessible under standard conditions. Such property allows branched selectivity up to 90% in the Co-catalysed hydroformylation of olefins without directing groups, not achievable with existing catalysts. This finding has a big potential in the production of aldehydes for the fine chemical industry. Monte Carlo and density functional theory simulations combined with kinetic models show that the micropores of MOFs with UMCM-1 and MOF-74 topologies increase the olefins density beyond neat conditions while partially preventing the adsorption of syngas leading to high branched selectivity. The easy experimental protocol and the chemical and structural flexibility of MOFs will attract the interest of the fine chemical industries towards the design of heterogeneous processes with exceptional selectivity.

The catalytic and radical mechanism for ethanol oxidation to acetic acid.

Au/TiO2 is a much-used catalyst for the conversion of ethanol to acetic acid. The proposed mechanism speaks of two essential reaction steps on the catalytic surface. The first is the ethanol to acetaldehyde and the second the acetaldehyde to acetic acid. When operating in the gas phase, acetic acid is usually absent. This work focuses on determining what triggers the second step by comparing the ethanol with acetaldehyde oxidation and the liquid with gas-phase reaction. We propose an updated reaction mechanism: acetaldehyde autoxidises non-catalytically to acetic acid, likely driven by radicals. The requirement for the autoxidation is the presence of oxygen and water in the liquid-phase. The understanding of the interplay between the catalytic ethanol to acetaldehyde and the following non-catalytic reaction step provides guiding principles for the design of new and more selective alcohol oxidation catalysts.

Response to Comment on "Selective anaerobic oxidation of methane enables direct synthesis of methanol".

Labinger argues that stepwise reaction of methane with water to produce methanol and hydrogen will never be commercially feasible because of its substoichiometric basis with respect to the active site and the requirement of a large temperature swing. This comment is not touching any new ground, beyond describing the thermodynamic feasibility, thermal cycling, and the role of water as discussed previously. Most important, it does not have a solid numerical basis.

Metal-Organic Frameworks Invert Molecular Reactivity: Lewis Acidic Phosphonium Zwitterions Catalyze the Aldol-Tishchenko Reaction.

The influence of metal-organic frameworks (MOFs) as additives is herein described for the reaction of n-alkyl aldehydes in the presence of methylvinylketone and triphenylphosphine. In the absence of a MOF, the expected Morita-Baylis-Hillman product, a ß-hydroxy enone, is observed. In the presence of MOFs with UMCM-1 and MOF-5 topologies, the reaction is selective to Aldol-Tishchenko products, the 1 and 3 n-alkylesters of 2-alkyl-1,3-diols, which is unprecedented in organocatalysis. The (3-oxo-2-butenyl)triphenylphosphonium zwitterion, a commonly known nucleophile, is identified as the catalytic active species. This zwitterion favors nucleophilic character in solution, whereas once confined within the framework, it becomes an electrophile yielding Aldol-Tishchenko selectivity. Computational investigations reveal a structural change in the phosphonium moiety induced by the steric confinement of the framework that makes it accessible and an electrophile.

Response to Comment on "Selective anaerobic oxidation of methane enables direct synthesis of methanol".

Periana argues that the stepwise reaction of methane with water is thermodynamically unfavorable and therefore impractical. We reply by presenting an in-depth thermodynamic analysis of each step in the process and show that the surface concentrations of the reactants and products as well as the stabilizing effect of additional water molecules, as discussed in the original paper, fully support the feasibility of the proposed reaction.

The Direct Catalytic Oxidation of Methane to Methanol-A Critical Assessment.

Despite the large number of disparate approaches for the direct selective partial oxidation of methane, none of them has translated into an industrial process. The oxidation of methane to methanol is a difficult, but intriguing and rewarding, task as it has the potential to eliminate the prevalent natural gas flaring by providing novel routes to its valorization. This Review considers the synthesis of methanol and methanol derivatives from methane by homogeneous and heterogeneous pathways. By establishing the severe limitations related to the direct catalytic synthesis of methanol from methane, we highlight the vastly superior performance of systems which produce methanol derivatives or incorporate specific measures, such as the use of multicomponent catalysts to stabilize methanol. We thereby identify methanol protection as being indispensable for future research on homogeneous and heterogeneous catalysis.