Kinetics and mechanism of n-butene isomerization over activated lanthanum oxide
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The catalytic behavior of lanthanum sesquioxide for double-bond isomerization of the n-butenes has been studied in the temperature range 0-50 C. Initial activity for individual conversions of all three n-butene reactants increases with increasing pretreatment temperature, due to removal of surface hydroxyls, attains a maximum following evacuation at ~650 C, and then declines with further increase in pretreatment temperature, due to decreases in surface anion disorder. Initial reaction rates of each isomer at 0 C are described by a Langmuirian rate equation of the form Rate = kbP/(1 + bP), which reduces to a zero-order expression in the case of 1-butene reactant. Double bond migration in the n-butenes occurs very rapidly over La2O3, but the rates of direct cis trans interconversion are, by comparison, negligibly slow throughout the temperature range investigated, due possibly to the absence of surface sites that permit the necessary anti--allyl syn--allyl rotation. As a result, although conversion of 1-butene involves characteristically high initial cis/trans product ratios, the sole primary product observed for both 2-butene reactants is 1-butene. Tracer studies employing the three perdeuterated n-butenes indicate that initial proton abstraction is the rate-determining step for each interconversion, and that the hydrogen transfer process involved in double-bond migration occurs via an intramolecular mechanism. Both directions of the one cis interconversion are accompanied by a much larger initial H-D isotope effect (9 to 10) than those observed for the one trans reaction (1 to 3), suggesting that the two reversible processes may proceed by dissimilar mechanisms or occur on different types of surface sites. 1981.
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