THE EFFECT OF CHLORIDE ION ON THE METAL INTERACTION AND REACTION SELECTIVITY OF Ft-Sn-Al_2O_3 CATALYSTS
Wang Chengyu, Wu Rongan and Lin Liwu(Dalian Institute of Chemical Physics, Academia Sinica)
A series of Pt-Sn-Al2O3-Cl catalysts with different Sn/Pt atomic ratios and chloride contents were prepared by impregnating r-A12O3 support with various Pt-Sn-Cl complexes. Some of these catalysts after removi g Cl-were ground to particles of different sizes, and then mechanically mixed with the chlorided A12O3 powder to make another series of bifunctional catalysts with defi lite mean distance between metallic sites and acidic sites. The activity, selectivity and stability of these catalysts were determined in a flow reactor for the reforming of n-heptane. Selectivities of the bifunctional catalysts influenced by the intimacy of metallic and acidic components were evaluated under reaction conditions of pressure 6 kg/cm2, temperature 500℃, LHSV 3.0 and hydrogen/n-heptane 6-6(mole). The results show that the catalyst prepared by incorporating Cl- as ligand of Pt-Sn complex is very stable and exhibits better selectivity for aromatization (13.0%) and isomerization (74.9%) of n-heptane,while those incorporated by Cl- through the addition of HC1 to the acidic component give much higher hydrocracking activity (37.2-72%) and lower selectivity for aromatization (7.5-10%) and isomerization (20-52%). This implies the larger the distance between Pt-Sn sites and Cl- sites, the lower the selectivity for aromatization and isomerization of n-heptane.Catalysts with stronger interaction between Pt and Sn atoms retain larger amount of Cl- after calcination and reduction, it means that Cl-plays an important role in the enhancement of rnetal-metal interactions of Pt-Sn-Al2O3 bimetallic catalysts.