Preparation And Modification Of Chromium Catalyst For Propane Dehydrogenation

At present, CrO_x and Pt supported catalysts are generally considered two industrialized catalysts in the dehydrogenation of propane. Among various reported catalysts, the supported chromium catalysts were intensively attracted since they showed excellent activity and low cost.Firstly, a series of mixed Cr/Al₂O₃ oxide catalysts with various compositions were prepared through a previously developed alcoholic co-precipitation pathway. The physico-chemical structure was analyzed through several characterization methods and the activity of Cr/Al₂O₃ catalysts was investigated to obtain the suitable chromium content. The results show that the samples exhibit a common feature for γ-alumina, and they also have a larger specific surface area. Besides, there exists interaction between Cr species and support which could be proved by H2-TPR and XPS. The ratio of Cr⁶⁺/Cr³⁺ on the sample surface decreases with increasing Cr content and the ratio of Cr⁶⁺/Cr³⁺ is about 0.83 for 7.5Cr/Al catalyst. The propane conversion for 7.5Cr/Al sample is about 62% with high propylene selectivity（91%）. However, it is difficult to prepare same samples having the same or approximate activity by alcoholic co-precipitation pathway. It has too many influence factors during the preparation process, and it is easy to waste a lot of chemicals which will contaminate the environment.Secondly, Im-Cr/Al catalysts were prepared by incipient wetness impregnation and the optimal impregnating Cr content（5%） and aging temperature（25℃） was determined. The C3H8 conversion and the C3H6 selectivity is about 43% and 94-95% separately. Compared to Co-Cr/Al catalyst, Im-Cr/Al catalyst has smaller acid-basic sites, and the interaction between Cr species and support is weaker which maybe facilitate the formation of polymer chromium species. These are helpful to improve the catalytic activity of Im-Cr/Al samples. In case of Co-Cr/Al catalyst, the dehydrogenation activity is superior to that for Im-Cr/Al samples. It can be indicated that Co-Cr/Al catalyst possesses a larger special surface area which is favourable to the formation of active chromium sites. In spite of that, incipient wetness impregnation was chosen owing that it has a good reproducibility during the preparation of the samples.Finally, based on the chapter 4, several chemical elements（Ca、La、Fe、Zn、Y、Cu、Ni and so on） are introduced into Im-Cr/Al catalyst in order that the activity of catalyst would be improved. The result suggests the nickel modified Cr/Al catalysts exhibit much better catalytic activity for the propane dehydrogenation reaction. The activity of 2.5Cr-Ni/Al catalyst（47%） has the most significant improvement that the conversion of propane for it is about 18% percent higher than that for2.5Cr/Al catalyst（29%）. The enhanced catalytic activity of Cr-Ni/Al catalysts is attributed to the interaction between nickel and chromium. The H2-TPR, Uv-vis, and XPS characterizations show that the addition of nickel not only leads to the formation of oligomeric chromium species, but also decreases the Cr⁶⁺ content resulting in the forming of Cr³⁺active sites. These would have important influences on the dehydrogenation activity. The synergistic effects between nickel and chromium for Cr-Ni/Al catalysts greatly facilitate the propane dehydrogenation reaction.