Abstract:

Developing highly efficient stable Au-Ti bifunctional catalysts is scientific and technological challenge for direct propylene epoxidation with H₂ and O₂. This work describes a novel strategy of employing tetrapropylammonium hydroxide (TPAOH) to modify uncalcined titanium silicalite-1 (TS-1-B) by secondary crystallization. Urea is used as the precipitation agent to immobilize gold nanoparticles onto the modified TS-1-B by deposition-precipitation method, aiming at allowing high gold capture efficiency as well as avoiding the introduction of residual alkaline cations to the catalyst. Compared with the reference Au/TS-1-B catalyst, the nano-gold particles immobilized on the modified TS-1 delivers significantly enhanced stability (> 36 h), propylene oxide (PO) formation rate (about 120 g/(h·kg)), PO selectivity (about 92.1%) and hydrogen efficiency (about 33.3%). The Fourier transform infrared spectroscopy (FT-IR) spectra demonstrates that the strength of the hydrogen-bonded silanols and trapped water in the TS-1-B decreased remarkably after secondary crystallization, indicating that the hydrophobic of TS-1-B is increased, which can be responsible for the improved stability and activity of the modified TS-1-B immobilized nano-gold catalyst. Moreover, the analysis of kinetic characteristic demonstrates that the propylene conversion and PO formation rate increase with increasing reaction temperature, while the PO selectivity and hydrogen efficiency decrease monotonously, and the apparent activation energies of by-products are much higher than the PO, indicating that higher reaction temperature favours the formation of by-products. In addition, the kinetic behavior of the catalyst was also studied, and the activation energy for the formation of main/by-products was calculated.

Key words: catalysis, propylene oxide, propylene epoxidation, stability, modification, kinetics

摘要：

采用四丙基氢氧化铵（TPAOH）溶液对未焙烧钛硅分子筛TS-1（TS-1-B）进行二次晶化改性，以尿素为沉淀剂通过沉积-沉淀法制备改性和非改性的TS-1-B固载Au纳米催化剂，对比研究这两种催化剂丙烯氢氧环氧化反应性能的差异，阐明二次晶化改性对TS-1-B表面结构及催化行为的影响。结果表明：二次晶化改性提高了TS-1-B结晶度，降低了缺陷位硅羟基数量；改性的TS-1-B表面疏水性的提高有助于抑制产物环氧丙烷（PO）在羟基位点上吸附，其固载的金催化剂表现出显著提高的稳定性和活性。此外，对该催化剂的动力学行为也进行了研究，并计算了主/副产物生成活化能。

关键词: 催化, 环氧丙烷, 丙烯环氧化, 稳定性, 改性, 动力学