Effect of Sodium Ions on Catalytic Performance of TS-1 in Gas-Phase Epoxidation of Propylene with Hydrogen Peroxide Vapor

Na+ ions in TS-1 influence the results of liquid-phase oxidations of hydrocarbons seriously, and the avoiding of Na+ ion impurity in TS-1 synthesis is crucial for its catalytic application. In this paper, however, the solvent-free gas-phase epoxidation of propylene with H2O2 vapor (G-HPPO) was investigated over TS-1 zeolites with different Na+ ion content. Significant improvement in the performance of G-HPPO process was observed with NaOH solution hydrothermally modified TS-1 which had a Na/Ti ratio of 0.68. The performance of G-HPPO process was further enhanced when the Na/Ti ratio of hydrothermally modified TS-1 was increased to 1.0 via subsequent Na+ ion impregnation. The catalyst showed 16.9% propylene conversion, 97.5% PO selectivity and 79.3% H2O2 utility at a propylene to H2O2 ratio of around 5. On the other hand, when the Na+ ion content of the hydrothermally modified TS-1 was reduced via subsequent NH4+-exchange, the resulted catalyst exhibited a remarkably deteriorated G-HPPO process performance. By Combining the characterizations of UV–Raman, UV–vis and FT-IR with DFT calculation, it is concluded that in the NaOH solution hydrothermally modified TS-1 the Na+ ions served as counter cations of the silicon hydroxyls adjacent to “open” tetra-coordinated framework Ti sites. As a result, the local environment of the “open” Ti sites (with titanium hydroxyls) was adjusted and the Ti sites were properly activated. Whereas, in the case of excess Na+ ions were introduced into the TS-1 (for example Na/Ti ratio more than 1.0), the titanium hydroxyl of the “open” Ti sites would be occupied, to which the deteriorated G-HPPO process performance was ascribed.