Studies On Hydrodesulfurization Properties Of Pd/ZrO₂-Al₂O₃ Catalysts

In this article, the Pd based catalysts for hydrodesulfurization (HDS) reaction of the thiophene were investigated. The effects of zirconium content, calcination temperature and preparing methods of ZrO₂-Al₂O₃ mixed supports on HDS activities of Pd/ZrO₂-Al₂O₃ catalysts were studied. The prepared samples were characterized by X-ray diffraction (XRD), temperature-programmed reduction (TPR), temperature -programmed desorption (TPD) and nitrogen adsorption experiment techniques.A series of ZrO₂-Al₂O₃ mixed oxides and Pd/ZrO₂-Al₂O₃ catalysts with various zirconium contents were prepared by the impregnation method. HDS reaction of thiophene over Pd/ZrO₂-Al₂O₃ catalysts was carded out in a flow reactor. The results showed that introducing suitable zirconium into Pd/Al₂O₃ resulted in relatively a higher dispersion, more acid amount and H₂ adsorbed amount, which led to the increase of the adsorption sites of thiophene and active centers, and the decrease of the apparent activation energy. All of above mentioned factors were favorable for the HDS of thiophene. The zirconia phase dispersed into the alumina phase strengthened electron- donated effect of Zr, which was beneficial to the dispersion of Pd particle in the ZrO₂-Al₂O₃(S) mixed oxide. The imbalance of charges of Zr--O--Al bonds caused the formation of Zr--O(H)--Al bonds, which were new Brφnsted acid sites. The more acid sites were believed to be the predominant reasons leading to their enhanced HDS activity of thiophene. The maximum HDS activity was observed for Pd/ZrO₂-Al₂O₃ catalysts with zirconium content (w) as 9 %.Influences of preparation methods on properties of Pd based catalysts for HDS of thiophene were studied. ZrO₂-Al₂O₃ mixed supports were prepared by impregnation (I) and sol-gel (S) methods separately. ZrO₂-Al₂O₃ (I) mixed oxide provided a lower S_BET than that of ZrO₂-Al₂O₃(S), which was due to the different particle size of ZrO₂ in the pores of Al₂O₃. The zirconia phase dispersed into the alumina phase in a homogenous way in ZrO₂-Al₂O₃(S)mixed oxide, and led to a significant change of the textural properties of the oxide precursor and structural stabilization, which was beneficial to strengthen electron-donated effect of zr and increase the dispersion of Pd particle in the ZrO₂-Al₂O₃(S) mixed oxide. Creation of the lattice defects in the ZrO₂-Al₂O₃(S) was mainly associated to the addition of the cationic surfactant during the synthesis of mixed oxide. The raise of acid sites in the ZrO₂-Al₂O₃(S) mixed oxide was due to the high dispersion of ZrO₂ in Al₂O₃, which created more interface between ZrO₂ and Al₂O₃, leading to the local charge imbalance associated with the zirconia chemically mixing with the alumina matrix. As compared with Pd/ZrO₂-Al₂O₃ (I) catalyst, Pd/ZrO₂-Al₂O₃(S) catalyst had a higher dispersion of palladium, stronger interaction between palladium and mixed supports and more acid sites, which presented an increasing driving-force toward the catalyzed conversion of thiophene.ZrO₂-Al₂O₃ (S) mixed supports were calcined at 773 K, 873K and 973K, respectively. The influence of calcination temperature of the mixed supports on the catalytic performance of Pd/ZrO₂-Al₂O₃ catalysts for the HDS of thiophene was studied. Our results showed that the ZrO₂-Al₂O₃ (773K) support was mainly mesoporous materials. The surface area and pore volume of the samples decreased as increasing calcination temperature. The ZrO₂-Al₂O₃ calcined at 773K exhibited larger BET surface area, pore volume and more acid sites than that of the samples calcined at 873 and 973K. The more the acid sites formed, the higher catalytic activities for HDS. The activity of the catalysts decreased in the following order: Pd/ZrO₂-Al₂O₃ (773K)＞Pd/ ZrO₂-Al₂O₃ (873K)＞Pd/ ZrO₂-Al₂O₃ (973K). Compared to Pd/ZrO₂-Al₂O₃ (873K)and Pd/ZrO₂-Al₂O₃ (973K)catalysts, Pd/ZrO₂-Al₂O₃ (773K) catalyst had the higher activity because of the stronger metal-support interaction, the more acid sites, the higher metal dispersion and the lower apparent activation energy.