The characterization of catalytically active sites on lanthanum sesquioxide for olefin isomerization and hydrogenation

Abstract

The active sites on La(,2)O(,3) that are responsible for 1-butene isomerization are "acid-base" pair sites which are generated by the thermal removal of surface hydroxyls over the pretreatment temperatue range 400(DEGREES)C to 650(DEGREES)C. These sites are present as an energetically heterogeneous array that is characterized by a composite activation energy for 1-butene isomerization of 6.5 Kcal/mole. Activation energies for individual sites within the array vary from less than 6.5 Kcal/mole to greater than 12.8 Kcal/mole. At pretreatment temperatures above 650(DEGREES)C, this active site array gradually transforms into an array with a composite activation energy of 12.8 Kcal/mole. La(,2)O(,3) activated at 600(DEGREES)C to 650(DEGREES)C exhibits similar activities for the isomerization of 1-butene (1.2 x 10('19) molecules/m('2)/sec) and for ethylene hydrogenation (4.0 x 10('18) molecules/m('2)/sec) at 0(DEGREES)C. The hydrogenations of propylene (1.1 x 10('15) molecules/m('2)/sec) and 1-butene (4.5 x 10('14) molecules/m('2)/sec) under the same conditions are considerably slower due to formation of energetically favorable (pi)-allylic species that occupy surface sites which could otherwise be utilized for hydrogenation. It has been demonstrated by various isomerization/hydrogenation co-reaction studies and extensive selective poisoning measurements that ethylene hydrogenation can occur on 1-butene isomerization sites, while propylene and 1-butene hydrogenations involve an additional active site array.