Metathese von Ethylen mit 2-Butenen an WOx-, MoOx- und ReOx-basierten Katalysatoren: Verständnis der Grundlagen der Katalysatorwirkungsweise
Zusammenfassung der Projektergebnisse
The metathesis of ethylene with 2-butenes is one of the large-scale processes for onpurpose production of propene. Several catalytic systems containing WOx , MoOx , and ReOx supported species are applied for this reaction. From a mechanistic viewpoint, the reaction occurs with participation of metal-carbenes, which are formed in situ upon a reaction of feed olefins with supported metal oxide species. Therefore, the catalyst ability to generate the desired metal-carbenes is a key parameter affecting its activity. Another important activitydetermining factor is the intrinsic activity of metal-carbenes. The current project was focused on elucidating the fundamentals influencing the rate of propene formation over Mo-, W-, or Re-containing catalysts. Particularly, the effects of the kind of support, the kind and the content of supported metal, the presence of co-catalyst, and the presence of hydrogen in the reaction mixture on the activity were investigated. In order to quantify the number of catalytically active metal-carbenes, two methods were developed and validated. Unsteady-state kinetic modelling of catalytic data was performed to understand activation/deactivation mechanisms occurring in the course of metathesis reaction. It has been demonstrated that the nature of the support strongly influences catalyst metathesis activity. In this context, the presence of defects and hydroxyls as well as electrophilic promoters on the surface of support material facilitates the transformation of supported metal oxide species into active metal-carbenes and might help in further activation of olefin molecules required for metathesis. A quantitative estimation of the number of carbenes formed from MO x (M=Mo, W) species was performed by means of titration experiments with ethylene. Such estimation allowed us to conclude that different activity of W- and Mo-containing catalysts is related to the different ability of WOx and MoOx species to be transformed into carbenes, but not to the different intrinsic activity of carbenes formed from these species. It has been shown that surface density of MOx species influences their structure (isolated/polymerized) and affects their ability to be transformed into carbenes. The enhancing effect of co-catalyst (ZnO or ZrO2) on catalyst activity was mostly investigated for MoOx-based catalysts. The effect was related to the ability of surface defect sites of the co-catalyst to activate C-H bond in 2-butene with the formation of an “excited species”, e.g., allyl radicals or allyl-oxo radicals, which facilitate the formation of active metalcarbene sites on the surface of the catalyst. Steady state isotopic transient kinetic analysis experiments performed with selected catalysts confirmed the increase in the number of carbenes in the presence of ZrO2 co-catalyst. The efficiency of the co-catalyst can be controlled through the adjustment of ZrO2 crystallite size: the smaller the crystallites are, the higher the concentration of coordinatively unsaturated Zr cations responsible for the activation of 2-butene is and accordingly the stronger the enhancing effect of the co-catalyst is. The application potential of a physical mixture of Mo/SiO 2 (main catalyst) with ZnO or ZrO 2 (cocatalyst) was validated in a series of metathesis/regeneration cycles at 200°C under industrially relevant conditions. The increase in the rate of propene formation in the presence of small amounts of co-fed hydrogen was observed at close to room temperature for the catalysts on the basis of Al2O3- containing supports. The strength of the enhancing effect strongly depends on the kind of supported metal (W, Mo; Re), its content, as well as on the content of Al 2O3 in the support material. Moreover, the effect becomes more pronounced at elevated temperature (higher than 100°C) and could be also observed for the catalysts on the basis of Al 2O3-free supports (SiO2, ZrO2, TiO2, etc.). The effect of hydrogen on catalyst activity was related to the ability of hydrogen to prevent/inhibit the formation of “inactive” complex on the surface of the catalyst. Unsteady-state kinetic modelling of catalytic data allowed us to determine optimal reaction conditions required for high production of propene. It has been demonstrated that the usage of high ethylene/2-butene ratio leads to a hindered formation of “inactive” complex and to an increase in the steady-state metathesis activity.
Projektbezogene Publikationen (Auswahl)
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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. Catalysis Science & Technology, 9(20), 5660-5667.
Otroshchenko, Tatiana; Reinsdorf, Ole; Linke, David & Kondratenko, Evgenii V.
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Enhancing Propene Formation in the Metathesis of Ethylene with 2-Butene at Close to Room Temperature over MoOx/SiO2 through Support Promotion with P, Cl, or S. ACS Catalysis, 11(22), 14159-14167.
Otroshchenko, Tatiana; Zhang, Qiyang & Kondratenko, Evgenii V.
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Room-Temperature Metathesis of Ethylene with 2-Butene to Propene Over MoOx-Based Catalysts: Mixed Oxides as Perspective Support Materials. Catalysis Letters, 152(8), 2366-2374.
Otroshchenko, Tatiana; Zhang, Qiyang & Kondratenko, Evgenii V.
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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. Catalysis Science & Technology, 12(16), 5210-5216.
Zhang, Qiyang; Otroshchenko, Tatiana & Kondratenko, Evgenii V.
