多级结构ZSM-5沸石分子筛的合成及其Mo基催化剂在甲烷无氧脱氢芳构化中的应用

以经盐酸预处理的碳纳米管为第二模板,在不添加其它有机溶剂的情况下,仅通过控制晶化条件,即采用变温水热晶化法合成具有多级结构的ZSM-5分子筛.通过x射线衍射、红外光谱测试、透射电镜和N2吸附对合成的分子筛进行了表征,结果表明,该合成分子筛呈近球形,是由纳米棒自组装形成的具有多级结构的亚微米球.该分子筛改性后用于甲烷无氧脱氢芳构化反应,显示出良好的催化性能,甲烷转化率最初达到19%,反应至24 h时甲烷转化率仍保持在10%左右,并且保持了较高的芳香物选择性(达到50%以上).

Synthesis of Hierarchical ZSM-5 Zeolite and Performance of Its Mo-Based Catalyst for Methane Dehydroaromatization

A hierarchical spherical-like ZSM-5 zeolite was synthesized using multiwall carbon nanotubes as secondary template. First, the nanotubes were purified by reflux with concentrated hydrochloric acid for 24 h. This purification step was repeated until the hydrochloric acid was no longer colored by impurities from the nanotubes (typically 3-4 times). Then the disposed nanotubes were added into the synthesis solution of ZSM-5, which was obtained by mixing H_2O, tetraethylorthosilicate, Al(C_3H_8O)_3, and tetrapropylammonium hydroxide (TPAOH). Afterward, this mixture was subjected to crystallization at varying temperatures. The obtained products were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscopy, and nitrogen adsorption analysis. The results indicated that ZSM-5 zeolite with uniform size of 300-400 nm was synthesized, and it was spherical-like aggregates and has hierarchical structure constructed by many ZSM-5 nanotubes of 20-40 nm. The hierarchical ZSM-5 zeolite was modified to form a Mo/HZSM-5 catalyst, and its catalytic performance for methane dehydroaromatization was investigated. The modified Mo/HZSM-5 catalyst exhibited CH_4 conversion as high as 19% and better life time. The CH_4 conversion was kept at 10% even when the reaction time was 24 h. It is suggested that hierarchical structure was favorable for the enhancement of catalyst performance, which is due to the presence of both micropores and mesopores.