一步法合成吲哚用Ag/SiO_2催化剂的失活及其抑制

Deactivation and Its Inhibition of Ag/SiO_2 Catalyst for Direct Synthesis of Indole

用简单浸渍法制备了Ag/SiO2 催化剂 .该催化剂对苯胺和乙二醇一步合成吲哚表现出很高的催化活性 ,但同时存在明显的失活 .TG ,XRD和TEM表征结果表明 ,催化剂失活的主要原因是反应过程中催化剂表面的积碳和银粒子的烧结 .向反应体系中通入氢和水蒸气能够减少催化剂表面的积碳 ;向催化剂体系加入ZnO助剂能较大程度地提高银的分散度 。

For direct synthesis of indole from aniline and ethylene glycol, Ag/SiO 2 catalyst prepared by incipient impregnation exhibits very high catalytic activity. However, its activity declines with time on stream in the reaction. The deactivation of Ag/SiO 2 catalyst was investigated by means of TG, XRD and TEM techniques. The Ag/SiO 2 catalyst (Ag loading 0 625 mmol/g ) was prepared by impregnation of SiO 2 with appropriate amounts of aqueous solution of silver nitrate. Prior to activity tests, the calcined sample was reduced in 50%H 2 50%N 2 mixture at 150 ℃ for 3 h. The Ag/SiO 2 MO catalysts (MOmetal oxide such as Li 2O, Na 2O, K 2O, MgO, CaO, SrO, BaO, CeO 2 and ZnO) were prepared by separate step impregnation. The direct synthesis of indole from aniline and ethylene glycol was carried out in a continuous flow fixed bed glass reactor with inside diameter of 12 mm. The reactants and products were analyzed by a Shimadzu GC 8A gas chromatograph with OV 17 column. According to the characterization results by TG, XRD and TEM, the used Ag/SiO 2 catalyst had obvious weightlessness and the diffraction peaks of silver crystal remarkably strengthened, meanwhile the silver particles grew up from 10～12 nm to 20～30 nm during the reaction. These indicated that the main reasons which resulted in the deactivation of Ag/SiO 2 catalyst are the carbon deposition on the catalyst surface and the sintering of silver particles during the reaction. Adding hydrogen and steam to the reaction system can efficiently decrease carbon deposition, and improve the catalytic performance and the stability of Ag/SiO 2 catalyst. Doping ZnO promoter into the Ag/SiO 2 catalyst can greatly improve the dispersion of silver on the support and inhibit the sintering of silver crystallite during the reaction, and thus greatly enhance the stability of the catalyst.