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A prerequisite for the development or optimization of heterogeneous catalysts is a comprehensive understanding of the reaction mechanism and kinetics at the level as elementary as possible. In comparison with steady‐state catalytic tests typically used for this purpose, non‐steady‐state (transient) studies provide deeper insights due to higher time resolution. For several decades, temporal analysis of products (TAP) and steady‐state isotopic transient kinetic analysis (SSITKA) have been successfully applied to the analysis of various heterogeneous reactions. In this review, we highlight the contributions made by TAP and SSITKA studies between 2017 and 2023 to the understanding of reaction mechanisms and to the derivation of microkinetics. The versatility of these methods is illustrated by the examples of oxidative coupling of methane, CO/CO2 hydrogenation to hydrocarbons or methanol, metathesis of ethylene with 2‐butenes to propene, acrolein oxidation to acrylic acid, methanol conversion to olefins and non‐oxidative propane dehydrogenation to propene. We also provide our personal views on possible developments in this area. [ABSTRACT FROM AUTHOR]

Ethylene dimerization is an industrial process that is currently carried out using homogeneous catalysts. Here we present a highly active heterogeneous catalyst containing minute amounts of atomically dispersed Pd. It requires no co‐catalyst(s) or activator(s) and significantly outperforms previously reported catalysts tested under similar reaction conditions. The selectivity to C4‐ and C6‐hydrocarbons was about 80 % and 10 % at 42 % ethylene conversion at 200 °C using an industrially relevant feed containing 50 vol % ethylene, respectively. Our kinetic and catalyst characterization experiments complemented by density functional theory calculations provide molecular insights into the local environment of isolated Pd(II)Ox species and their role in achieving high activity in the target reaction. When the developed catalyst was rationally integrated with a Mo‐containing olefin metathesis catalyst in the same reactor, the formed butenes reacted with ethylene to propylene with a selectivity of 98 % at about 24 % ethylene conversion. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Conversion of propane or butanes from natural/shale gas into propene or butenes, which are indispensable for the synthesis of commodity chemicals, is an important environmentally friendly alternative to oil-based cracking processes. Herein, we critically analyse recent developments in the non-oxidative, oxidative, and CO2-mediated dehydrogenation of propane and isobutane to the corresponding olefins over metal oxide catalysts. Particular attention is paid to (i) comparing the developed catalysts in terms of their application potential, (ii) structure–activity–selectivity relationships for tailored catalyst design, and (iii) reaction-engineering aspects for improving product selectivity and overall process efficiency. On this basis, possible directions for further research aimed at the development of inexpensive and environmentally friendly catalysts with industrially relevant performance were identified. In addition, we provide general information regarding catalyst preparation and characterization as well as some recommendations for carrying out non-oxidative and CO2-mediated dehydrogenation reactions to ensure unambiguous comparison of catalysts developed in different studies. [ABSTRACT FROM AUTHOR]

Frontispiece: Performance Descriptors for Catalysts Based on Molybdenum, Tungsten, or Rhenium Oxides for Metathesis of Ethylene with 2-Butenes to Propene Keywords: 2-Butene; Ethylene; Metathesis; Propene; SSITKA EN 2-Butene Ethylene Metathesis Propene SSITKA 1 1 1 09/27/23 20231002 NES 231002 B Heterogeneous Catalysis b . In their Communication (e202308872), Tatiana Otroshchenko, Evgenii V. Kondratenko et al. report performance descriptors for catalysts based on molybdenum, tungsten, or rhenium oxides for ethylene metathesis with 2-butenes to propene. [Extracted from the article]

Keywords: 2-Buten; Ethen; Metathese; Propen; SSITKA DE 2-Buten Ethen Metathese Propen SSITKA 1 1 1 09/27/23 20231002 NES 231002 B Heterogeneous Catalysis b . In their Communication (e202308872), Tatiana Otroshchenko, Evgenii V. Kondratenko et al. report performance descriptors for catalysts based on molybdenum, tungsten, or rhenium oxides for ethylene metathesis with 2-butenes to propene. [Extracted from the article]

We elucidated the effects of H 2 and the kind (Cu or Ru) of supported metal on product selectivity and on-stream stability of MZrO x -based catalysts (M = La or Y) in the non-oxidative dehydrogenation of propane to propene. Coke formation was hindered in hydrogen presence. However, the selectivity to C 1 -C 2 hydrocarbons increased over Ru/MZrO x but did not change over Cu/LaZrO x. This difference was related to the low activity of Cu for cracking and hydrogenolysis reactions. Using feed with 40 vol% C 3 H 8 and 30 vol% H 2 in N 2 , Cu/LaZrO x showed stable conversion (20%) and propene selectivity (88%) within 4.5 h on stream at 550 °C. Unlabelled Image • C 3 H 8 dehydrogenation was studied over (Cu/Ru)/MZrO x (M = La/Y) in H 2 presence. • H 2 improves catalyst on-stream stability due to suppressing coke formation. • Selectivity to cracking products is affected by H 2 content and supported metal. • Cu reveals lower activity for cracking/hydrogenolysis than Ru. [ABSTRACT FROM AUTHOR]

The effect of Cs-, Ca- and P-containing promoters for binary Cr-Zr-O x on propene selectivity and catalyst stability with time-on-stream in the non-oxidative propane dehydrogenation was elucidated. Large improvements in this catalytic performance were achieved when combining Cs and P promoters. They were suggested to partially block dehydrogenation sites resulting in the isolation of the latter. As a reason thereof, coke formation through propene oligomerization is inhibited in favor of propene desorption from the catalyst surface. The present catalysts achieved ~30% conversion of propane at WHSV (C 3 H 8) of 5.89 h-1 at 550 °C in comparison with 20% over K-CrO x /Al 2 O 3. The selectivity to propene was determined to be ~94%. Unlabelled Image • Promoted CrZrOx materials were tested for non-oxidative propane dehydrogenation. • Catalytic performance was improved significantly by promoting with Cs, Ca and/or P. • Coke formation is the main reason for catalyst deactivation. • Isolating active sites by Cs and/or P inhibits coke formation. [ABSTRACT FROM AUTHOR]

• CrZrO x /SiO 2 with different degree of ZrO 2 crystallinity were prepared and tested in non-oxidative propane dehydrogenation. • Their activity increases with the crystallinity, while the selectivity to propene is not affected. • Coke formation is the main reason for catalyst deactivation. • CrZrO x /SiO 2 showed good durability in 10 PDH/regeneration cycles under industrial relevant conditions. A series of SiO 2 -supported CrZrO x -based catalysts with same Cr content but different degrees of ZrO 2 crystallinity were synthesized by a simple impregnation-calcination method. The catalysts were studied for their activity, selectivity and on-stream stability in non-oxidative propane dehydrogenation (PDH) under industrially relevant conditions. They were characterized by complementary techniques such as XRD, BET, NH 3 -TPD, H 2 -TPR, operando UV–vis, EPR, Raman spectroscopy and TPO. The results demonstrate that ZrO 2 crystallinity plays an important role in propene formation. Although catalyst activity can be significantly augmented upon increasing the degree of crystallinity of ZrO 2 , no decrease in the selectivity could be observed at the same degree of propane conversion, i.e. 92 % at about 20 % conversion. The catalysts showed good durability in 10 PDH/oxidative regeneration cycles at 550 °C. [ABSTRACT FROM AUTHOR]

• Differently prepared bare ZrO 2 materials were tested in non-oxidative C 3 H 8 dehydrogenation. • Their activity and selectivity depends on the ability of ZrO 2 to release lattice oxygen under reducing conditions. • This catalyst property can be tuned by preparation method. • Controlled decomposition of ZrO(NO 3) 2 results in ZrO 2 with improved reducibility and high activity and selectivity. A detailed study was carried out on the factors affecting the activity and, particularly, selectivity of bare ZrO 2 in the non-oxidative propane dehydrogenation (PDH) to propene. To this end, a series of ZrO 2 samples differing in their redox and acidic properties, morphology and phase composition were synthesized by precipitation, sol-gel, glycine-nitrate combustion, template, digestion and calcination methods as well as through controlled thermal decomposition of zirconium oxonitrate. The ability of ZrO 2 to release its lattice oxygen in reducing atmosphere was identified as the main factor affecting PDH activity and propene selectivity. This is due to the fact that so generated surface zirconium cations (Zr cus) with lower coordination than their regular counterparts show higher activity for propene formation. As the latter sites strongly contribute to coke formation, the selectivity to this undesired product can be hindered through improving the ability of ZrO 2 to generate Zr cus sites under reducing conditions. [ABSTRACT FROM AUTHOR]

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, CrOx, GaOx or VOx species. Identifying active sites and designing rationally heterogeneous catalysts are not inherently straightforward due to their complexity. Here, the authors reveal the nature of active sites for efficient C-H bond activation in C1-C4 alkanes over bare ZrO2 and provide fundamentals for controlling their concentration. [ABSTRACT FROM AUTHOR]