Oxidation of Isobutane to Methacrolein over Ga_2O_3/Bi_2Mo_3O_(12) Catalysis

摘要

Catalytic performance and the surface character of the Ga_2O_3 supported Bi-Mo complex oxides were studied to achieve direct formation of methacrolein from isobutane.Bi_2Mo_3O_(12) (alpha phase) and Bi_2Mo_1O_6 (gamma phase) showed higher catalytic activity than Bi_2Mo_2O_9 (beta phase) for isobutane partial oxidation.Supporting Ga_2O_3,which is an active catalyst for dehydrogenation of hydrocarbons,onto the oxides,ehanced the catalytic activity.The optimum amount of supported Ga_2O_3 on Bi_2Mo_3O_(12) was about 3 wt% for methacrolein formation.In the presentce of oxygen,a remarkable amounts of hydrogen over Ga_2O_3 during the isobutane oxidation but no hydrogen was formed over Ga_2O_3/Bi_2Mo_3O_(12).It is confirmed from TPR that Ga_2O_3 and Bi_2Mo_3O_(12) were not reduced until 550 deg C but the reduction of Ga_2O_3/Bi_2Mo_3O_(12) started at 350-380 deg C.The on-set temperature in TPR of the Bi-Mo complex oxides decreased to 350-380 deg C from 500 deg C by the supporting Ga_2O_3 onto the oxides,and the catalysts after TPR measurement are composed of BiO,Bi,and MoO_2 in addition to Bi_2Mo_3O_(12).These results suggest that the hydrogen spillover took place over supported catalyst.Ga_2O_3/Bi_2Mo_3O_(12) catalyst showed higher activity and high selectivity for methacrolein at 450 deg C.The improvement in the selectivity for methacrolein of the Ga_2O_3/Bi_2Mo_3O_(12) may be explained as following.Isobutane is adsorbed on the surface of Ga_2O_3 to form hydrogen atom and t-butyl fragment and both formed species migrates to Bi_2Mo_3O_(12) surface.Migrated hydrogen may modify the Bi_2Mo_3O_(12) surace 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 Ga_2O_3/Bi_2Mo_3O_(12) catalyst,the formation rate of CO_x signifiantly reduced,and methacrolein and isobutane were selectively obtained when the reduction degree of the catalyst was lower than 0.3% at 450 deg C.