The Preparation Of Pd-SiW₁₂-Based Catalyst And Its Study In The Direct Oxidation Of Ethylene To Acetic Acid

In this dissertation, the preparation of supported Pd-H4SiW12O40 catalyst and the catalytic performance in the direct oxidation of ethylene to acetic acid was examined.Our study indicated that the palladium precursor and its reduction method are the key factors for the activity of the catalyst. The activities of the catalyst using Pd(NH3)4Cl2 as the precursor are lower than those using H2PdCl4 as the precursor. The gas-phase reduction method is obviously better than the liquid-phase reduction method. Characterization of the catalyst showed that the palladium has higher dispersion using the hydrogen reduction method than that using the hydrazine reduction method, which caused the large discrepancy of the catalytic activities.The activity of the catalyst is strongly dependent upon the loading amount of palladium and tungstosilic acid. In the range of 0-3.5wt.%, with the increase of palladium loading, the conversion of ethylene and the space time yield of acetic acid(STYAcoH) increase; in the range of 0-40wt.%, with the increase of tungstosilic acid, the selectivity of dioxide decreases monotonously, while the STYAcoH attains a maximum at a tungstosilic acid content of about 30wt.% and then declined.A series of the third components was investigated based on Pd-SiW12/SiO2 catalyst. It was found that the promoter such as Se and Te, especially the incorporation of selenium promoter could improve the selectivity of acetic acid and remarkably enhance the space-time yield of acetic acid. At the same time, we first found that the 1,2-dichloroethane added into the feed gas can also improve the selectivity and space-time yield of acetic acid.The reaction conditions have a great influence on the direct oxidation of ethylene to acetic acid. Using the Pd-Se-SiW12/SiO2 catalyst, a selectivity to acetic acid of 88.1% and STYAcoH of 417.6g/h.L at 11.8% conversion of ethylene has been obtained at T=160℃, P=0.7MPa, GHSV=3000h-1 and a feed gas composition: C2H4/O2/N2/H2O=50/7/13/30(by volume).The stability of the Pd-Se-SiWi2/Si02 catalyst was investigated during a consecutive reaction time of 206h and the reason of the catalyst deactivation was initially examined. XRD analysis revealed that the "sintering" of palladium particle and the "spillage" of tungstosilic acid cause the decay of the catalyst.