Study In Dehydrogenation Reaction Of Isobutane Over Modified CrO_x/Al₂O₃Catalysts

In this paper, the CrOx/Al2O3catalyst, containing different alkaline additives, wereprepared by incipient impregnation. The catalytic behaviour of the samples was investigatedin the isobutane dehydrogenation reaction and the effect of alkaline additives and theirloadings, with some emphasis, was examined. Meanwhile, the chemical and textural featuresof the catalysts were also investigated by means of several techniques, such as XRD, SEM,NH3-TPD and chemical analysis, aiming to find correlations with the catalytic performanceand to clarify the role of alkali metals in affecting activity and selectivity.The results indicated that the formation of α-Cr2O3, which is the least active species, wasinhibited and the content of Cr6+was found to be increased after adding the alkaline additives,except for Mg, thus alkaline additives was supposed to have the positive effect on thedehydrogenation activity. XRD showed the formation of MgAl2O4和MgCr2O4, whichincreases the amount of crystalline Cr3+species at expense of the Cr6+species, was consideredto affecting the activity negatively. The results of NH3-TPD revealed that alkali promoters cancause the decrease in surface acidity, enhancing the catalytic selectivity. Whereas, alkalipromoter, MgO acting as poision promoting the formation of strong acid sites, lead to adecrease on the product selectivity. In addition, the characteristic of structure anddehydrogenation performance was also influenced by the amount of alkaline additives. At lowalkali content, the more the amount of alkaline additives, the more the content of Cr6+specieswhile the weaker the surface acid strength and the less the amount of acid center. As aconsequence, the catalyst showed better dehydrogenation performance. When alkalineadditives content was higher, the Cr6+species content existed in chromate crystal form whichowns low catalytic activity, so the additive amount of alkaline promoters should be moderate.On the other hand, the stronger the basicity of equal amount of alkali promoter, the more the content of Cr6+species. However, the product selectivity of the catalyst was not improvedwhen added by stronger basicity of alkali promoter, for that the alkali promoter playeddifferent roles on the strong and weak acid sites respectively. The performance evaluation ofmodified CrOx/Al2O3catalysts showed that potassium can behaviour more excellently thanother alkaline metals as a catalyst promoter.Moreover, the K2O-CrOx/Al2O3catalyst was further modified with silicon component.Compared with industrial catalyst FBD-4, the self-made Si-K2O-CrOx/Al2O3catalyst hadsimilar properties of crystal structure, surface appearance and dehydrogenation behavior,while the anti-carbon deposition ability was a bit poor, in need of improvement. Theregeneration capability of modified CrOx/Al2O3catalysts was discussed as well, thus theresults were: the K2O-CrOx/Al2O3catalyst had long one-way life and its isobutene yield wasstill maintained at30%after the catalyst on stream for6h. Due to part of Cr6+species the wasirreversibly reduced to less active Cr3+species after several dehydrogenation-regenerationcycles, so the catalytic activity of K2O-CrOx/Al2O3was found to be declined, while itsimproving isobutene selectivity was attributed to a migration of the potassium from chromateto the alumina. It was also found that regenerated Si-K2O-CrOx/Al2O3catalyst exhibitedalmost the same activity and selectivity as the corresponding fresh material.