The Preparation And Modification Of Chromium-based Catalysts And Research For Isobutane Dehydrogenation

Isobutene is a significant feedstock for a variety of industrial applications,such as producing methyl tert-buty ether?MTBE?,methl methacrylatr?MMA?,tert butyl alcohol?TBA?and others important chemicals intermediates.Isobutene are increasingly demanded as octane number boosters for unleaded gasoline in the worldwide recently.The development of Isobutane dehydrogenation technology is very important for China's petroleum and chemical industry.Therefore,the research and development of efficient dehydrogenation catalyst become one of the important problems in the ehydrogenation technology.This paper has used the industrial chromium catalyst from the modification of carrier,additives for many studies to develop the new modified catalyst with higher activity,excellent selectivity and stability.Firstly,from the perspective of the carrier,?-Al₂O₃ carriers were modified by hydrothermal treatment with two hydrothermal solvents and applied in isobutaue dehydrogention catalyst,the effect of catalytic dehydrogenation performance was studied using modified carriers.Modified?-Al₂O₃ carriers were characterized.The results showed that the crystallinity of?-Al₂O₃ was increased after hydration and calcination process,Hydrothermal treatment enhanced the interaction of active component and carriers,changed pore structure of?-Al₂O₃ and improved the performance of CrOx/Al₂O₃.By hydrothermal treatment with hydrothermal solvent?HTT-1?,CrOx/Al₂O₃ catalyst exhibited the optimal isobutane dehydrogenation performance when supports were treated at 140?for 4 h,isobutane dehydrogenation average conversion,average selectivity and deactivation ratio were 43.04%,86.34%and 13.7%,respectively,for reaction 10 h.A series of M modified CrO_X-M/?-Al₂O₃ catalysts were prepared by incipient wetness impregnation method.The resulting catalysts with different M content were investigatedinafixed-bedreactorforitsperformanceinisobutane dehydrogenation.The M modified CrO_X-M/?-Al₂O₃ catalysts were characterized by XRD?BET?H₂-TPR?SEM and UV-vis.The results showed that,the addition of M influenced dispersion of active species,altered interaction of active species and supports,maintained the higher surface of the catalysts and inhibited the formation of?-Cr₂O₃,which thereby decreased the side of cracking and carbon formation;but too much addition of M would lead to aggregation of MO₂,reduce the surface and pore volume of catalyst,that is not conducive to the dehydrogenation reaction.The catalyst exhibited the optimal isobutane dehydrogenation performance when the M content is around 2%,isobutane dehydrogenation average conversion were 49.86%respectively,for reaction 10 h.The chromia/alumia?CrM/?-Al₂O₃?catalysts with addition of chelating agents ?citric acid or oxalic acid?were prepared by the incipient impregnation method.The resulting catalysts with different citric acid?CA?or oxalic acid?OA?content were applied to the dehydrogenation of isobutane to isobutene.The results showed that the CrM/?-Al₂O₃ catalysts with addition of CA or OA exerted sligthly increase on specific surface area.The addition of the chelating agents as expected,determined a general decrease in the surface acidity.The addition of CA or OA for preparing these catalysts resulted in a beneficial effect on the reducibility of the Cr species to diminish the reduction temperature.The appropriate content of chelating agents could improve dispersion of metal species in the?-Al₂O₃ support.The catalytic activity showed an important enhancement when the metal species impregnated in presence of CA or OA.