Performance Descriptors for Catalysts Based on Molybdenum, Tungsten, or Rhenium Oxides for Metathesis of Ethylene with 2-Butenes to Propene

The metathesis of ethylene with 2-butenes to propene is an established large-scale process. However, the fundamentals behind in situ transformation of supported WOx , MoOx , or ReOx species into catalytically active metal-carbenes and the intrinsic activity of the latter as well as the role of metathesis-inactive cocatalysts are still unsolved. This is detrimental for catalyst development and process optimization. In this study, we provide the required essentials derived from steady-state isotopic transient kinetic analysis. For the first time, the steady-state concentration, the lifetime, and the intrinsic reactivity of metal carbenes were determined. The obtained results can be directly used for the design and the preparation of metathesis-active catalysts and cocatalysts, thereby opening up possibilities for optimizing propene productivity.

Highly efficient Bi-promoted ZrO2-based materials for non-oxidative propane dehydrogenation

Surface or bulk promotion of ZrO₂-based catalysts with Bi₂O₃ facilitates the removal of lattice oxygen from ZrO₂ under reductive conditions resulting in the formation of coordinatively unsaturated Zr cations. The catalysts demonstrated an industrially relevant propene yield at 600 °C. The results highlight the importance of the usage of suitable promoters for controlling catalyst performance.

Fundamentals and application potential of the synergy effect between ZnO and Mo/SiO2 for propene production in the metathesis of ethylene and trans-2-butene

The enhancing effect of ZnO on the activity of Mo/SiO2 in the metathesis of ethylene with 2-butene is investigated. The strength of the effect depends on the reaction temperature and the manner in which ZnO and Mo/SiO2 are located in the reactor.

Effect of AlSiOx support modification by alkali or alkaline earth metals on propene formation in the metathesis of C2H4 and 2-C4H8 over MoOx-based catalysts

The metathesis of ethylene with 2-butenes to propene is currently applied for large-scale on-purpose production of this building block of the chemical industry. MoOx-based supported catalysts are promising alternatives to...

Room-Temperature Metathesis of Ethylene with 2-Butene to Propene Over MoOx-Based Catalysts: Mixed Oxides as Perspective Support Materials

We investigated the effect of supports based on ZrO2, TiO2, Al2O3, and SiO2 on the rate of propene formation in the metathesis of ethylene with 2-butene at 50 °C over Mo-containing catalysts possessing highly dispersed MoOx. Large improvements in this rate were achieved when using supports composed of mixed oxides (ZrO2–SiO2, ZrO2–PO4, TiO2–SiO2; Al2O3–SiO2) rather than of individual oxides (ZrO2, TiO2, Al2O3, SiO2). Although previous literature studies dealing with the metathesis reaction over Al2O3- or SiO2-suppported catalysts at higher temperatures suggest the importance of redox or acidic properties of supported MoOx species for catalyst activity, we were not able to establish any general direct correlation in this regard. Contrarily, the rate of propene formation can be significantly enhanced when promoting supports with an oxide promoter. We suggest that the created support lattice defects may facilitate the transformation of MoOx to Mo carbenes under reaction conditions or improve the intrinsic activity of the latter.

Graphic Abstract

Current status and perspectives in oxidative, non-oxidative and CO2-mediated dehydrogenation of propane and isobutane over metal oxide catalysts

Conversion of propane or isobutane from natural/shale gas into propene or isobutene, which are indispensable for the synthesis of commodity chemicals, is an important environmentally friendly alternative to oil-based cracking processes.

A chemical titration method for quantification of carbenes in Mo- or W-containing catalysts for metathesis of ethylene with 2-butenes: verification and application potential

We introduce and experimentally validate a simple titration method for quantifying the number of carbenes acting as active sites in the metathesis of ethylene with 2-butenes over Mo- or W-containing catalysts.

Mechanism for O-O Bond Formation via Radical Coupling of Metal and Ligand Based Radicals: A New Pathway

Artificial photosynthesis carries promise to deliver abundant clean energy for the needs of a growing population. Deep mechanistic understanding is required to achieve rational design of fast and durable water oxidation catalysts. Here we provided first evidence for a new mechanism of the O-O bond formation via radical coupling of the oxidized metal═oxo of radicaloid character (Ru^IV═O) and ligand based radical ([ligand-NO]^+• cation radical). O-O bond formation is facilitated via spin alignment and takes place via a virtually barrier less pathway inside the single metal complex. In situ reactive intermediate conversion was monitored by mass spectrometry, resonance Raman (RR) and EPR. Computational analysis have shown that the formation of [ligand-NO]^+• happens at a lower overpotential than the formation of the [Ru^V═O(ligand)]³⁺ intermediate. Overall, the presented paradigm for O-O bond formation opens new opportunities for rational catalyst design.

Control of coordinatively unsaturated Zr sites in ZrO2 for efficient C-H bond activation

Due to the complexity of heterogeneous catalysts, identification of active sites and the ways for their experimental design are not inherently straightforward but important for tailored catalyst preparation. The present study reveals the active sites for efficient C-H bond activation in C₁-C₄ alkanes over ZrO₂ free of any metals or metal oxides usually catalysing this reaction. Quantum chemical calculations suggest that two Zr cations located at an oxygen vacancy are responsible for the homolytic C-H bond dissociation. This pathway differs from that reported for other metal oxides used for alkane activation, where metal cation and neighbouring lattice oxygen form the active site. The concentration of anion vacancies in ZrO₂ can be controlled through adjusting the crystallite size. Accordingly designed ZrO₂ shows industrially relevant activity and durability in non-oxidative propane dehydrogenation and performs superior to state-of-the-art catalysts possessing Pt, CrO_x, GaO_x or VO_x species.