The combination of well-defined acid sites, shape-selective properties and outstanding stability places zeolites among the most practically relevant heterogeneous catalysts. The development of structure–performance descriptors for processes that they catalyse has been a matter of intense debate, both in industry and academia, and the direct conversion of methanol to olefins is a prototypical system in which various catalytic functions contribute to the overall performance. Propylene selectivity and resistance to coking are the two most important parameters in developing new methanol-to-olefin catalysts. Here, we present a systematic investigation on the effect of acidity on the performance of the zeolite ‘ZSM-5’ for the production of propylene. Our results demonstrate that the isolation of Brønsted acid sites is key to the selective formation of propylene. Also, the introduction of Lewis acid sites prevents the formation of coke, hence drastically increasing catalyst lifetime.

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The alkylation of isobutane with light alkenes plays an essential role in modern petrochemical processes for the production of high-octane gasoline. In this study we have employed periodic DFT calculations combined with microkinetic simulations to investigate the complex reaction mechanism of isobutane–propene alkylation catalyzed by zeolitic solid acids. Particular emphasis was given to addressing the selectivity of the alkylate formation versus alkene formation, which requires a high rate of hydride transfer in comparison to the competitive oligomerization and deprotonation reactions resulting in catalyst deactivation. Our calculations reveal that hydride transfer from isobutane to a carbenium ion occurs via a concerted C–C bond formation between a tert-butyl fragment and an additional olefin, or via deprotonation of the tert-butyl fragment to generate isobutene. A combination of high isobutane concentration and low propene concentration at the reaction center favor the selective alkylation. The key reaction step that has to be suppressed to increase the catalyst lifetime is the deprotonation of carbenium intermediates that are part of the hydride transfer reaction cycle.@en

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Identifying Cu-exchanged zeolites able to activate C-H bonds and selectively convert methane to methanol is a challenge in the field of biomimetic heterogeneous catalysis. Recent experiments point to the importance of trinuclear [Cu3(μ-O)3]2+ complexes inside the micropores of mordenite (MOR) zeolite for selective oxo-functionalization of methane. The electronic structures of these species, namely, the oxidation state of Cu ions and the reactive character of the oxygen centers, are not yet fully understood. In this study, we performed a detailed analysis of the electronic structure of the [Cu3(μ-O)3]2+ site using multiconfigurational wave-function-based methods and density functional theory. The calculations reveal that all Cu sites in the cluster are predominantly present in the Cu(II) formal oxidation state with a minor contribution from Cu(III), whereas two out of three oxygen anions possess a radical character. These electronic properties, along with the high accessibility of the out-of-plane oxygen center, make this oxygen the preferred site for the homolytic C-H activation of methane by [Cu3(μ-O)3]2+. These new insights aid in the construction of a theoretical framework for the design of novel catalysts for oxyfunctionalization of natural gas and suggest further spectroscopic examination.@en

Liu et al. recently reported their results on coconversion of methane and methanol at 973 K over a typical methane dehydroaromatization (MDA) catalysts, Mo/HZSM-5.1 In this work, the authors claimed that adding a small amount of methanol to a methane feed led to more than two times higher methane conversion, substantially higher xylene and toluene selectivities (i.e., combined ca. 80%, nearly an order of magnitude increase as compared to experiments without methanol), and improved catalyst stability to such an extent that no deactivation was observed during 60 h on stream. If reproducible, this result would be a significant achievement, because formation of coke in the MDA reaction has been considered inevitable hitherto. To support their experimental data, Liu et al. carried out a thermodynamic analysis, whose results were in good agreement with their experimental findings.

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In applying a multi-scale spectroscopic and computational approach, we demonstrate that the synthesis of stacked zeolite silicalite-1 nanosheets, in the presence of a long-tail diquaternary ammonium salt surfactant, proceeds through a pre-organised phase in the condensed state. In situ small-angle X-ray scattering, coupled to paracrystalline theory, and backed by electron microscopy, shows that this phase establishes its meso-scale order within the first five hours of hydrothermal synthesis. Quasi in situ vibrational and solid-state NMR spectroscopy reveal that this meso-shaped architecture already contains some elementary zeolitic features. The key to this coupled organisation at both micro- and meso-scale, is a structure-directing agent that is ambifunctional in shaping silica at the meso-scale whilst involved in molecular recognition at the micro-scale. The latter feature is particularly important and requires the structure-directing agent to reside within the silica matrix already at early stages of the synthesis. From here, molecular recognition directs stabilization of precursor species and their specific embedding into a lattice, as shown by force-field molecular dynamics calculations. These calculations, in line with experiment, further show how it is possible to subtly tune both the zeolite topology and aspect ratio of the condensating crystals, by modifying the headgroup of the structure-directing agent.

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The performance of a range of nanoparticulate gold catalysts in the hydrogenation of carbon dioxide to formates was investigated and a superior performance of Au/Al₂O₃ was revealed. The comparative studies with unsupported gold nanoparticles pointed to the crucial role of the metal–support interaction for the CO₂ hydrogenation activity. Cyanide leaching tests complemented by XPS and STEM studies point to the importance of metallic Au⁰ species for the catalytic activity. The Au/Al₂O₃ catalyst shows stable activity that allows reaching equilibrium formate yields in a broad temperature range. A kinetic study revealed that a near-zero apparent activation barrier for the hydrogenation reaction is an intrinsic property of the catalytic system. The reaction mechanism is proposed on the basis of the obtained reactivity and characterization data.

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In this Perspective, we highlight the main challenges to be addressed in the development of heterogeneous catalysts for the direct functionalization of methane. Along with our personal view on current developments in this field, we outline the main mechanistic, engineering, and catalyst design issues that have hampered implementation of new technologies and highlight possible paths to overcome these problems.

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Non-oxidative methane dehydroaromatization is a promising reaction to directly convert natural gas into aromatic hydrocarbons and hydrogen. Commercialization of this technology is hampered by rapid catalyst deactivation because of coking. A novel approach is presented involving selective oxidation of coke during methane dehydroaromatization at 700 °C. Periodic pulsing of oxygen into the methane feed results in substantially higher cumulative product yield with synthesis gas; a H₂/CO ratio close to two is the main side-product of coke combustion. Using ¹³C isotope labeling of methane it is demonstrated that oxygen predominantly reacts with molybdenum carbide species. The resulting molybdenum oxides catalyze coke oxidation. Less than one-fifth of the available oxygen reacts with gaseous methane. Combined with periodic regeneration at 550 °C, this strategy is a significant step forward, towards a process for converting methane into liquid hydrocarbons.

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The effect of exposure to ambient air of MoS₂-based, γ-Al₂O₃-supported, hydrodesulfurization (HDS) catalysts has been studied using high-resolution transmission electron microscopy (HRTEM). Analysis of unpromoted as well as Ni- and Co-promoted MoS₂ samples showed that the number of MoS₂ slabs and the average slab length decreased as a function of air exposure time. A parallel X-ray photoelectron spectroscopy (XPS) study showed this effect to be due to oxidation. During the first 24 h of exposure to air, all 1 bar sulfided (Ni/Co)MoS₂ samples showed an initial slab length decrease of approximately 20%. After an additional month in air, the slabs had deteriorated significantly further. A sample of CoMoS₂ sulfided at 30 bar showed a slightly enhanced effect of oxidation, particularly after the first 5 min in air. The combined HRTEM and XPS results lead to the proposal of the formation of a protective oxide ring around the remaining sulfidic species inside the MoS₂ slabs to explain the mechanism of this oxidation process. The data obtained in this study emphasize the general necessity of shielding vulnerable catalyst samples from air during preparation and characterization, a message relevant in all fields of research related to catalysis.

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Metal-ligand cooperative properties of a bis-N-heterocyclic carbene ruthenium CNC pincer catalyst and its activity in CO₂ hydrogenation to formates were studied by DFT calculations complemented by NMR spectroscopy and kinetic measurements. The dearomatized Ru-CNC∗ pincer (1∗) is significantly more reactive toward metal-ligand cooperative activation of H₂ and CO₂ than the structurally related phosphine-based Ru-PNP complex. The enhanced reactivity of Ru-CNC∗ stems from the combination of electronic properties of this system and the reduced geometric constraints imposed onto the Ru center by the large and flexible CNC chelate. Heterolytic dissociation of H₂ by 1∗ results in the bis-hydrido complex 2 that is active in hydrogenation of CO₂. However, under commonly applied reaction conditions, the catalyst rapidly deactivates via metal-ligand cooperative paths. The transient formation of the dearomatized complex Ru-CNC∗ (1∗) in the course of the reaction leads to the irreversible cooperative activation of CO₂, resulting in the stable adduct 3 that is not catalytically competent. By an increase in the H₂/CO₂ ratio, this deactivation path can be effectively suppressed, resulting in a stable and rather high catalytic performance of Ru-CNC. (Chemical Equation Presented).

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The catalytic reduction of carboxylic acid derivatives has witnessed a rapid development in recent years. These reactions, involving molecular hydrogen as the reducing agent, can be promoted by heterogeneous and homogeneous catalysts. The milestone achievements and recent results by both approaches are discussed in this Review. In particular, we focus on the mechanistic aspects of the catalytic hydrogenation and highlight the bifunctional nature of the mechanism that is preferred for supported metal catalysts as well as homogeneous transition metal complexes.

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