Studies On The Preparation And Performance Of Pt-Sn/γ-Al₂O₃Catalysts For The Dehydrogenation Of Isobutane

The supply of isobutene cannot meet the increasing demand from the world market due to the rapid development of isobutene downstream products and the lack of new isobutene sources. Therefore, it has an important significance for petrochemical field to develop good dehydrogenation catalysts and fully utilize C4resources.Alumina supports were prepared by hydrochloric acid reflux and ammonia precipitation methods, and effect of the alumina supports on the dehydrogenation performance of the Pt-based catalysts was investigated. The results showed that the catalysts supported on the alumina from ammonia precipitation were provided with smaller platinum particle size, weaker acidic distribution and stronger resistance against coke, thus exhibiting better catalytic performance during the dehydrogenation of isobutane. Over the catalysts with alumina synthesized by ammonia precipitation as the support, the conversion of isobutane was initially56.7%and then decreased to34.7%after316h. Moreover, the initial selectivity to isobutene was80.0%and it remained approximate93.5%after168h. Carbon-covered alumina supports were synthesized by the pyrolysis of sucrose and it was found that the new supports could obviously improve the dehydrogenation selectivity and stability of the catalysts, providing a new path for the development of dehydrogenation catalysts.Pt-Sn-K/y-Al2O3catalysts were prepared by complex impregnation method under vacuum, and effect of Sn and K promoters on the dehydrogenation performance of the catalysts was studied. The results showed that the appropriate Sn favored "Pt-SnOx-Al2O3" structure on the catalyst surface, which promoted the dehydrogenation reaction and isomerization reaction, but inhibited the cracking reaction. Moreover, the K promoter could neutralize the acidic centers on the catalysts, and thus improve the dehydrogenation selectivity and stability of the catalysts. When the concentration of Sn and K was respectively1.5wt.%and0.4wt.%, the catalysts exhibited the best dehydrogenation performance of isobutane. Over the catalysts, the conversion of isobutane was initially76.2%and the selectivity to isobutene was83.4%. After the reaction of24h, the conversion and selectivity was decreased to57.2%and79.3%, respectively.Deactivation origins of the catalysts during the dehydrogenation-regeneration cycles were analyzed by in-situ regeneration method. The results showed that the regeneration process had very slight influence on the pore structure of the catalysts. Coking was the main deactivation origin during the dehydrogenation process, and Pt sintering led to the deactivation of the regenerated catalysts.Effect of the operation conditions on the dehydrogenation performance of the catalysts was investigated by the single factor method and orthogonal method. The results showed that the reaction temperature and H2/i-C4ratio had major influences on the catalyst performance, however, the space velocity of feedgas had no remarkable influences. The appropriate conditions were as follows:the reaction temperature was580℃, the ratio of hydrogen to paraffins was0.5:1and the space velocity of feedgas was dependent on the circumstance.Cr2O3-based catalysts were characterized by XRD, UV-Vis DRS, H2-TPR, NH3-TPD, Pyridine-FTIR, XPS and TG, and effect of K promoter on the dehydrogenation performance of the catalysts was discussed. The results showed that the K promoter favored the formation of CrⅥ on the catalyst surface and decreased isolated CrⅢ species accordingly. Moreover, it could also diminish the acidic centers and increase the resistance against coke, thus improving the dehydrogenation selectivity and stability of the catalysts. When the concentration of K was1wt.%, the catalysts had the best dehydrogenation performance of isobutane. During the reaction of42h, the average yield of isobutene was29.1%and the selectivity remained above90%.