碱性添加剂在钛硅分子筛/H2O2催化丙烯环氧化反应中的作用

Effect of basic additive in feedstock on propylene epoxidation catalyzed by TS-1/H2O2

考察了反应溶液中碱性添加剂{氨水(NH4OH)、氢氧化钠(NaOH)、碳酸钠(Na2CO3)、碳酸铵[(NH4)2CO3]和四甲基氢氧化铵(TMAOH)}及其浓度对钛硅分子筛(TS-1)催化丙烯环氧化反应性能的影响,并采用紫外拉曼光谱(UV-Raman)与气相色谱(GC)联用原位分析(Raman-GC)碱性添加剂的作用机理。结果表明:反应体系中添加碱性添加剂可有效改善TS-1催化丙烯环氧化反应的选择性。不同碱性添加剂对TS-1催化丙烯环氧化活性和稳定性影响不同,NaOH、Na2CO3和TMAOH的添加造成催化活性和稳定性降低。NH4OH或(NH4)2CO3作为添加剂改善环氧化反应选择性的同时提高了反应稳定性,其中以(NH4)2CO3效果最佳。添加(NH4)2CO3浓度为8.0×10-4mol/L时,TS-1催化丙烯环氧化在固定床反应器上连续运行336h时,X(H2O2)仍保持在90%以上。原位Raman-GC分析发现,反应体系中NH4OH和(NH4)2CO3添加可加快反应活性中间体Ti-OOH(η2)基团的生成,促进反应产物从活性中心快速扩散,从而提高丙烯环氧化反应选择性和稳定性。

NH4 OH, NaOH, Na2 CO3,(NH4)2 CO3 and TMAOH were selected as the basic additive in the feedstock of propylene epoxidation reaction and their effects on the performance of TS-1 catalysis were evaluated by in-situ Raman spectrometry-gas chromatography. The experimental results suggested that the performance of the TS-1 catalyst was influenced significantly by the composition of feedstock.However, the presence of NH4 OH or(NH4)2 CO3 at a suitable concentration could improve the catalytic activity, selectivity and the stability of the TS-1 catalyst. When(NH4)2 CO3 concentration in feedstock was 8.0×10-4 mol/L, the best catalytic performance was obtained. The X(H2 O2) still remained above 90% after a long-term run of 336 h in a fixed-bed reactor. In-situ Raman-GC showed that the basic additive promoted the formation of the reactive intermediates Ti-OOH(η2) and accelerated the diffusion of reaction products from active centers.