Catalysts For Full-range FCC Gasoline Selective Hydrodesulfurization

FCC gasoline, accounting for more than 80% of commercial gasoline in China, containsvery high levels of sulfur compounds and olefins. More than 90% of sulfur compounds incommercial gasoline come from FCC gasoline. The removal of sulfur compounds bytraditional hydrodesulfurization (HDS) techniques will inevitably result in the saturation ofmany olefins and cause serious octane number lost. Therefore, the application of thetraditional HDS techniques in producing ultra-low sulfur containing gasoline is limited.Currently, most of the selective hydrodesulfurization processes rely in fact on cutting thefull-range FCC gasoline into light- and heavy- parts at first, and then applying the traditionalHDS treatment to the heavy part that contains more sulfur compounds and less olefins.However, the pre-fractionation of the FCC gasoline before HDS treatment is an energy-consumption step.The main purpose of this paper is to create sulfur compounds discriminating active centersfor selectively catalytic hydrodesulfurization. In order to characterize the catalysts, methodslike XRD, pyridine FT-IR, N₂ physisorption, NH₃-TPD and H₂-TPR were employed. Theperformances of the catalysts in the selective HDS of full-range. FCC gasoline wereevaluated on small fixed-bed reactors. Following main results and findings were obtained:(1)θ-alumina is more selective thanγ-alumina in the HDS of a full range FCC gasoline, itis therefore a better supporter for highly selective HDS catalyst.(2) Modification by ZnO is an effective way for the enhancement of the selectivity ofaluminas. The positive role of ZnO modification might be attributed to the formation ofspinel Zn(AlO₂)₂ structure in the supporters, and the adjustment of Lewis acidity.(3) Modifications by other mono-metal oxide like NiO, MgO and La₂O₃, as well asmodifications by di-metal oxides like ZnO-Cu, ZnO-Fe and ZnO-La₂O₃, and tri-ruetal oxideslike ZnO-Ni-La₂O₃ and ZnO-Ni-K₂O, are also helpful for increase the selectivity ofaluminas. The uses of multi-metal oxides have the effect of increasing the HDS activity ofthe modified supporters, but their selectivities are lower than ZnO modified aluminas'except ZnO-Cu.(4) Based on ZnO modified alumina, a kind of supported CoO-MoO₃ catalyst wasdeveloped (denoted DLS-1). By using small fixed-bed reactors, the selectivity, activity andstability of DLS-1 was compared with a commercial gasoline HDS catalyst (codedRef-catal). Results show that, under the conditions of 250℃, 2.0 Mpa, WHSV 3.0 h^-1, the hydrogen/oil(v) 600, 72% sulfur was removed from the full range FCC gasoline thatcontaining 580μg/g total sulfur and 29%(w) total olefins. DLS-1 catalyst exhibited the sameHDS activity as commercial Ref-catal catalyst, but the olefins loss of the product oilobtained on DLS-1 catalyst was 50% lower than that on the commercial Ref-catal catalystunder the same reaction conditions and the same sulfur removal rate. In a more than 400hours' time-on-stream, no performance decay was observed with DLS-1 catalyst. The highselectivity and the very well integrated performance of DLS-1 show a bright applicationforeground.