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Fundamental origins of different activity and time-on-stream stability of supported Mo-, W-, and Re-based materials in metathesis of ethylene and 2-butene to propene

Subject Area Technical Chemistry
Term from 2011 to 2015
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 188582486
 
Final Report Year 2014

Final Report Abstract

To fulfill the growing demand for propylene, metathesis of ethylene with 2-butenes has been commercialized using WOx/SiO2 catalysts. Importantly, all earlier mechanistic knowledge was mainly obtained for the reverse reaction and cannot be simply applied to propylene production because 2-butene can participate in cis-/trans- and 2-/1- isomerization which may influence the target reaction. Therefore, we elucidated mechanistic and kinetic aspects of propylene formation and isomerization reactions over catalysts with well-defined surface MeOx (Me=Mo (potential alterative) or W) species. Steady-state and transient kinetic tests combined with thorough characterization of catalysts by complementary ex and in situ techniques enabled us to derive the following main results. In contrast to previous knowledge, we established that the degree of polymerization of MeOX species and their acidity cannot be separately discussed to explain the activity in the metathesis of ethylene and 2-butenes to propylene. The propylene production over highly dispersed tetrahedral and polymerized octahedral MeOX species depends on their Brønsted acidity. Brønsted OH groups are required to generate the metathesis active carbene species. The acidity can be tuned by metal loading or/and by the kind of support with Brønsted acidity; the higher the loading (avoiding formation of bulk metal oxides) or/and the support acidity, the higher the Brønsted acidity of the MeOX species. Support Lewis acidity determines the degree of polymerization of MeOX species. This knowledge allows to design the desired surface structures, which influence both metathesis and isomerization reaction pathways: We demonstrated for the first time that polymerized octahedral MeOx species generate propylene directly form ethylene and 2-butenes, while propylene formation over their isolated tetrahedral counterparts mainly occurs through metathesis of 1- and 2-butenes. This difference is due to the fact that isolated tetrahedral MeOx species i) show significantly higher activity for isomerization of 2- to 1-butene and ii) exhibit lower ability to engage ethylene in propene formation. The latter property is related to the strong ethylene adsorption as concluded from kinetic analysis of transient experiments in the temporal analysis of products reactor. In addition, we established that propylene formation is strongly accelerated by using CaO, Al2O3 or Al2O3-SiO2 as a pre-bed upstream to a Mo-containing catalyst. The strength of the enhancing effect increases with a decreasing degree of polymerization of MoOx species. Moreover, the enhancing effect decreases with an increase in Al2O3 content in the support for a series of supported MoOx/SiO2-Al2O3 catalysts with similarly structured MoOx species. From a mechanistic point of view, we suggested that an active gas-phase compound is formed over the pre-beds, which then participates in the generation/stabilization of catalytically active Mo-carbenes. These additional carbenes lose their high metathesis activity in absence of such compound. The chemical nature of this enhancing compound is not known at the moment. However, we proved that it is formed from 2-butene and not from ethylene.

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