The metathesis of propene to ethene and 2-butene has been discovered in the middle of the last century by R. L. Banks and G. C. Bailey. Currently, this reaction is industrially performed in the reverse direction, i.e. metathesis of ethene and 2-butene to propene, to fulfill the continuously increasing demand for propene. This reaction is catalyzed by supported transition metal oxides, e.g. ReOx, MoOx or WOx. Importantly, all earlier mechanistic knowledge was mainly obtained for the reverse reaction and cannot be simply applied to propene production since 2-butene can participate in 2/1-isomerization, which may influence the target reaction. Therefore, the main aims of the present study were to (i) understand structure-reactivity/selectivity relationships and (ii) elucidate mechanistic and kinetic aspects of ethene and 2-butene metathesis to propene as well as the role of 2-butene isomerization reactions over catalysts with well-defined surface MOx (M = W or Mo) species. Steady-state and transient kinetic tests combined with thorough characterization of catalysts by complementary ex and in situ techniques enabled to obtain the following main results.
Compared with previous knowledge, not only the degree of polymerization of MOx but also their Brønsted acidity were found to affect catalyst performance. Brønsted acidic OH groups on MOx species or in its vicinity are required to generate metathesis active metal alkylidene species. The acidity can be tuned by metal loading and by the acidic character of the support; the higher the metal loading (avoiding formation of bulk metal oxides) and support Brønsted acidity, the higher the Brønsted acidity of the MOx species, and, consequently, the higher the metathesis activity. Besides, support Lewis acidity was established to determine the degree of polymerization of MOx species and their reducibility. The formation of metal alkylidenes from ethene and 2-butene was investigated by transient experiments and in situ FTIR spectroscopic studies. Irrespective of the degree of polymerization of MOx species, the first step is reaction of such species with gas-phase olefins to yield oxygenates and reduced MOx sites. Metal alkylidenes are formed from the latter species upon oxidative addition of new gas-phase alkenes. It was demonstrated, for the first time, that metal alkylidenes formed from octahedral MOx species generate propene directly from ethene and 2-butene, while propene formation over their tetrahedral counterparts mainly occurs through metathesis of 1- and 2-butenes. This difference is due to the fact that the latter species show significantly higher activity for 2- to 1-butene isomerization and exhibit lower ability to engage ethene in propene formation. The latter property is related to the strong ethene adsorption as concluded from kinetic analysis of transient experiments in the temporal analysis of products (TAP) reactor.
|Commitee:||Schomäcker, Evgenii V., Kondratenko, Reinhard, Grünert, Wolfgang|
|School:||Technische Universitaet Berlin (Germany)|
|Source:||DAI-C 81/1(E), Dissertation Abstracts International|
|Keywords:||Ethene, 2-butene, Propene, MoOx- und WOx-cased catalysts|
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