Investigation Of Oxidation Promoters In FCC Sulfur Transfer Additives

The cheapest and green method for removal of SO_x in FCC is the sulfur transfer additive technology. Sulfur transfer additives contain two kinds of active sites, one is sulfur absorption site which is profoundly studied, magnesium aluminate spinel and HTLcs are the typical examples, the other is oxidation promotion site whose action mechanism is still needed to understand deeply and the invention of new types of oxidation promoter is still needed. Especially, the oxidation promoters used at present are single transition metal oxides and always have a non-ideal desulfurization under the condition of poor oxygen. Hence, developing a new type of oxidation promoter which has an excellent desulfurization under poor oxygen condition is important.Firstly, the action mechanisms of lattice oxygen and oxygen in gas phase were discussed. Moreover, the optimum content and oxygen storage capacity of CeO₂ were investigated. Lattice oxygen was proved to be active oxygen species and supplied by oxygen in gas phase. Sulfur transfer additives had a better desulfurization when cerium content was 10 wt% due to their competitive and synergistic effect with sulfur absorption sites. Oxygen storage and release capacity of CeO₂ is inferior at low temperature and this is the reason why sulfur transfer additives have a worse desulfurization under the condition of poor oxygen.According to the concept of three-way catalysts design, zirconia was doped into cerium on a nanometer scale using a modified sol-gel method. Preparation conditions, proportion of cerium and zirconium, type of solvent, citric acid content, types of reducing medium and reducing temperatures were discussed. When the drying temperature and calcination temperature were 140℃ and 800℃ respectively, cerium-zirconia solid solutions had a better oxygen storage and release capacity at low temperature and SO_x uptake capacity. Citric acid had no significant effect on the number and strength of alkaline sites. Ce0.9Zr0.1O2 solid solution accompanied by the proper amount of citric acid offered the largest SO_x uptake capacity. Distilled water was cheap but efficient as solvent. Sulfur transfer additives had an excellent regenerability when H₂ was passed through the deactivated catalysts at 600℃.Other metal elements, such as Al, Mn, Cu, Co, Cr and so on, were doped in order to improve the oxygen storage and release capacity. The results showed that all of them could perfect the number and mobility of the active oxygen species. The order of SO_x uptake capacity were Co>Mn>Cr≈Al>Cu.