Oxidation of Isobutane to Methacrolein over Ga₂O₃/Bi₂Mo₃O₁₂ Catalysts

Abstract

Catalytic performance and the surface character of the Ga₂O₃ supported Bi-Mo complex oxides were studied to achieve direct formation of methacrolein from isobutane.
Bi₂Mo₃O₁₂ (α phase) and Bi₂Mo₁O₆ (γ phase) showed higher catalytic activity than Bi₂Mo₂O₉ (β phase) for isobutane partial oxidation. Supporting Ga₂O₃, which is an active catalyst for dehydrogenation of hydrocarbons, onto the oxides, enhanced the catalytic activity.
The optimum amount of supported Ga₂O₃ on Bi₂Mo₃O₁₂ was about 3 wt% for methacrolein formation. In the presence of oxygen, a remarkable amounts of hydrogen over Ga₂O₃ during the isobutane oxidation but no hydrogen was formed over Ga₂O₃⁄Bi₂Mo₃O₁₂. It is confirmed from TPR that Ga₂O₃ and Bi₂Mo₃O₁₂ were not reduced until 550°C but the reduction of Ga₂O₃/Bi₂Mo₃O₁₂ started at 350-380°C. The on-set temperature in TPR of the Bi-Mo complex oxides decreased to 350-380°C from 500°C by the supporting Ga₂O₃ onto the oxides, and the catalysts after TPR measurement are composed of BiO, Bi, and MoO₂ in addition to Bi₂Mo₃O₁₂. These results suggest that the hydrogen spillover took place over supported catalyst.
Ga₂O₃/Bi₂Mo₃O₁₂ catalyst showed higher activity and high selectivity for methacrolein at 450°C. The improvement in the selectivity for methacrolein of the Ga₂O₃/Bi₂Mo₃O₁₂ may be explained as following. Isobutane is adsorbed on the surface of Ga₂O₃ to form hydrogen atom and t-butyl fragment and both formed species migrates to Bi₂Mo₃O₁₂ surface. Migrated hydrogen may modify the Bi₂Mo₃O₁₂ surface property by the reaction with oxide ions, which is active for the deep oxidation resulting in high selectivity for methacrolein.
In the non-aerobic oxidation of isobutane over the Ga₂O₃/Bi₂Mo₃O₁₂ catalyst, the formation rate of CO_x significantly reduced, and methacrolein and isobutene were selectively obtained when the reduction degree of the catalyst was lower than 0.3% at 450°C.