AbdelDayem Hany M, Al-Shihry Shar S, Hassan Salah A
Faculty of Science, Chemistry Department, Ain Shams University, Abassia, Cairo 11566, Egypt.
College of Science, Chemistry Department, King Faisal University, Al-Hasa, Al-Hofuf 31982/380, Eastern Province, Saudi Arabia.
ACS Omega. 2020 Feb 25;5(9):4469-4481. doi: 10.1021/acsomega.9b03645. eCollection 2020 Mar 10.
In this work, direct gas-phase epoxidation of propylene (DPO) to propylene oxide by molecular oxygen has been studied by using Ag-MoO supported on titanium-containing hexagonal mesoporous silica (Ti-HMS ) of different Si/Ti molar ratios. The promotion effect of NaCl on the synthesized catalysts has also been investigated. Among the studied supports, the hexagonal mesoporous silica (HMS) with a Si/Ti ratio of 10 was the most suitable one for production of propylene oxide (PO). The optimal performance of the AgMo/Ti-HMS catalyst in DPO exhibited a selectivity to PO of 43.2% with a propylene conversion of 14.1%, at 400 °C, 0.1 MPa, and a space velocity of 12,000 h. The catalyst verified good stability over at least 20 h on stream. Only 2.7% PO selectivity with a propylene conversion of 10.1% was achieved over the AgMo/HMS sample. The incorporation of Ti into the HMS frame could optimize the particle size distribution of Ag, producing Ag nanoparticles with an average size of 6.8 nm compared with that of Ag/HMS (24.3 nm). The in situ Raman spectrum of AgMo/Ti-HMS heated in a stream of CH/He at 400 °C showed new bands at 616, 390, and 210 cm, characteristic of the Ag Mo O intermediate phase. The obtained results suggested that this formed AgMo/Ti-HMS phase could most likely be relevant for selective epoxidation of propylene. However, during the reaction of CH with AgMo/HMS, the formation of this intermediate was hardly detected. On the other hand, the hydrogen temperature-programmed reduction measurements indicated improved reducibility of MoO in the AgMo/Ti-HMS catalyst, which acknowledged the role of Mo in gaining electrons from silver to form positively charged Ag. This could reduce the effective charge of the adsorbed oxygen on silver sites and in turn favor the epoxidation path of propylene rather than the combustion route. Also, during the reaction of CH with NaAgMo/Ti-HMS at 400 °C, two new Raman bands were detected at 277 and 350 cm, characteristic of AgMoO.
在本工作中，通过使用负载在不同Si/Ti摩尔比的含钛六角形介孔二氧化硅（Ti-HMS）上的Ag-MoO，研究了丙烯直接气相环氧化（DPO）制环氧丙烷的反应。还研究了NaCl对合成催化剂的促进作用。在所研究的载体中，Si/Ti比为10的六角形介孔二氧化硅（HMS）是最适合用于生产环氧丙烷（PO）的载体。在400℃、0.1MPa和空速12000h⁻¹条件下，AgMo/Ti-HMS催化剂在DPO反应中的最佳性能表现为环氧丙烷选择性为43.2%，丙烯转化率为14.1%。该催化剂在至少20h的连续运行中表现出良好的稳定性。在AgMo/HMS样品上，丙烯转化率为10.1%时，环氧丙烷选择性仅为2.7%。将Ti引入HMS骨架可以优化Ag的粒径分布，与Ag/HMS（24.3nm）相比，生成平均粒径为6.8nm的Ag纳米颗粒。在400℃的CH₄/He气流中加热的AgMo/Ti-HMS的原位拉曼光谱显示在616、390和210cm⁻¹处出现新峰，这是Ag₂Mo₂O₅中间相的特征峰。所得结果表明，这种形成的AgMo/Ti-HMS相很可能与丙烯的选择性环氧化有关。然而，在CH₄与AgMo/HMS的反应过程中，几乎检测不到这种中间相的形成。另一方面，氢气程序升温还原测量表明，AgMo/Ti-HMS催化剂中MoO₃的还原性得到改善，这证实了Mo从银获取电子形成带正电的Ag⁺的作用。这可以降低银位点上吸附氧的有效电荷，进而有利于丙烯的环氧化路径而非燃烧路径。此外，在400℃下CH₄与NaAgMo/Ti-HMS的反应过程中，在277和350cm⁻¹处检测到两个新的拉曼峰，这是AgMoO₂的特征峰。
