Characterization of catalytic lanthanum oxide for double bond isomerization of n-butenes
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Lanthanum sesquioxide, when properly activated, is an extremely active catalyst for double bond migration in the n-butenes. The initial rate of 1-butene conversion on rehydrated La2O3 varies with final calcination temperature, and attains a maximum of 4.2 1020 molecules/ m2/min at 0 C, following catalyst evacuation at 650 C. In the temperature range 0 to 50 C, the reaction is zero order in 1-butene for initial reactant pressures of 25 to 200 torr. A cumulative self-poisoning of the catalyst occurs at reaction temperatures due to adsorption of a strongly held, unreactive form of n-butene, but the latter is completely removed and activity restored upon subsequent rehydration and calcination. Initial cis-2-butene/trans-2-butene ratios are large (7 to 8 at 0 C), and the rates of reaction of both 2-butene isomers are much lower than that of 1-butene. Direct interconversion of cis- and trans-2-butene occurs very slowly or not at all on La2O3, and a reaction scheme for the three-component system is proposed. Active sites for 1-butene isomerization involve surface O2- ions in conjunction with adjacent defect structures such as anion vacancies. A difference of 4.2 kcal/mol between the apparent activation energies of cis- and trans-2-butene appearances suggests that formations of the two 2-butene isomers from the 1-butene reactant may occur on energetically and/or structurally dissimilar sites on La2O3. 1977.
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