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The metathesis of ethylene with 2-butene 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 the industrially applied WOx-containing systems. The activity of such catalysts depends on the amount of active Mo–carbene (MoCHR) sites formed in situupon reaction of supported MoOxspecies with olefins. Herein, we demonstrate a way to control the ability of such species on the surface of AlSiOxto be transformed into Mo–carbenes by promoting the support with tiny amounts of alkali or alkaline earth metal (Li, Sr, Ca, Mg, Na, K, Ba, Rb, or Cs) oxides. The electrophilic properties of these additives were established to be relevant for in situtransformation of supported MoOxspecies into Mo–carbenes. The additive electronegativity (Pauling scale) affects olefin adsorption and therefore the formation of carbenes.

The metathesis of ethylene with 2-butene to propene is a 100%-carbon-efficient process for on-purpose production of this important building block in the chemical industry. Herein, we demonstrate that the activity of Mo/SiO2can be increased up to 270-fold when using ZnO as a cocatalyst, which is inert for the target reaction. An intimate physical contact between ZnO and Mo/SiO2is required to efficiently use the enhancing effect of the cocatalyst. H/D isotopic exchange tests with 2-C4H8and 2-C4D8suggest that the enhancing effect depends on the ability of ZnO to activate 2-butene. A gas-phase promoter (not 1-butene but probably butene radical) formed in this reaction is suggested to contribute to the formation of catalytically active Mo–carbenes under reaction conditions and/or hinder their transformation into inactive ones. This fundamental knowledge opens the possibility for further improvements through catalyst or cocatalyst design and reactor operation. The obtained results are also attractive from a practical point of view. Both ZnO and Mo/SiO2can be regenerated without changing their initial performance as proven in several metathesis/regeneration cycles.

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. This method is based on the experimentally proven mechanistic sequence of carbene formation through the reaction of ethylene with oxidized metal oxide species: (i) reduction of M(vi) to M(iv) by ethylene, (ii) addition of ethylene to M(iv) to yield MC2H4carbene and (iii) transformation of the latter to MCH2upon reaction with C2H4yielding C3H6. As MCH2cannot produce C3H6when reacting with C2H4, the amount of C3H6formed in step (iii) corresponds to the number of catalytically active carbenes. The applicability of this method was supported by establishing a correlation between this number and the steady-state rate of propene formation in the metathesis of ethylene with 2-butenes.

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 (RuIV═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 situreactive 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 [RuV═O(ligand)]3+intermediate. Overall, the presented paradigm for O—O bond formation opens new opportunities for rational catalyst design.

Non-oxidative dehydrogenation (DH) of propane, n-butane, and isobutane was investigated over bare ZrO2and binary MZrOx(M = Li, Ca, Mg, Y, Sm or La) materials. Selected samples were characterized by XRD, Raman spectroscopy, NH3temperature-programmed desorption, IR spectroscopy and O2titration experiments. A correlation between the rate of olefin formation and the concentration of anion vacancies formed upon reductive catalyst treatment was established. Such defects represent the catalytically active sites, which are coordinatively unsaturated Zr4+(Zrcus) cations. From a structural viewpoint, the active Zrcussites should have a lower coordination number than the Zrcussites located on the top of an ideal flat surface, thus highlighting the importance of surface defects in the target reactions. However, the intrinsic activity of such sites and/or their concentration depends on the kind of dopant for ZrO2. The dopant was also shown to inhibit side reactions especially those leading to coke due to reducing the concentration of strong acidic sites related to ZrO2. Similar to an analogue of industrial K-CrOx/Al2O3catalyst, all binary catalysts exhibited low activity for consecutive reactions of the target olefins; the desired selectivity was higher than 85% at an alkane conversion of about 30%. With respect to the feed alkane, primary selectivity (at a zero degree of alkane conversion) to the target olefin decreases in the order: C3H8, iso-C4H10, and n-C4H10. This was explained by the higher stability of surface intermediates formed from the latter two contributing to the formation of coke and 1,3-butadiene, respectively.