| Platinum supported on alumna is an important petroleum reforming catalyst. Because of its large specific pore volume, high surface area, and ordered mesopores, MCM-41 is considered a promising catalyst support. The addition of Sn moderates coke deposition and platinum cluster sintering. Therefore, Pt/Sn-MCM-41 catalysts are considered a potential replacement for current petroleum reforming catalysts.; By using different Pt precursors and trying different preparation methods, we have developed a new method to introduce Pt onto (Sn-)MCM-41 support, which is called vacuum-evaporation-impregnation (VEI). With this new VEI preparation method, structured, high loading and high dispersion Pt/MCM-41 catalysts have been obtained.; The second cation, Sn, has been successfully incorporated into MCM-41 and tetrahedrally coordinated with neighboring oxygen atoms when K2SnO 3 is used as a precursor and co-precipitates with silica with the procedure described above. During the reduction of the Sn-MCM-41 support, Sn is partially reduced, close to Sn2+; after the reoxidation, Sn is oxidized, but does not fully recover to the initial oxidation level. This may be due to the protection of the neighboring MCM-41 framework atoms. After the introduction of Pt, Sn is more completely reduced and further reoxidized due to the Pt catalytic effect.; With increasing Pt cluster size, the selectivities toward hexenes and benzene decrease but the selectivities toward paraffins and iso-hexanes (P&IH) increase, owing to the different extents of structure sensitivity for different reactions. Potassium is added and functions as an electron donor; consequently, the n-hexane dehydrocyclization reactivity decreases but the selectivity toward benzene increases. After addition of Sn, Pt clusters become more electron rich because of the probable Pt-Sn alloy formation or at least strong interaction between Pt and Sn. As a result, the catalytic activity decreases but the selectivity toward hexenes increases significantly. After regeneration by oxidation, the apparent dispersion of Pt clusters on Pt/MCM-41 catalyst increases. The short-term deactivation, corresponding to a sharp activity decrease in the initial period, is caused by coke deposition on Pt clusters and/or MCM-41 support. Long-term deactivation, corresponding to the slow activity slide afterwards, is attributed to Pt cluster sintering. (Abstract shortened by UMI.)... |
