正戊烷芳构化反应的ZSM-5催化剂的锌改性研究

STUDY ON ZINC-MODIFIED ZSM-5 CATALYST FOR THE AROMATIZATION OF n-PENTANE

本文考察了正戊烷的不同锌改性催化剂上的芳构化反应。实验结果表明,对于以硝酸锌为锌源用离子交换法、浸渍法和机械混合法制备的催化剂,它们的芳构化性能非常相近。与这类催化剂相比,氧化锌机械混合法催化剂有明显不同的芳构化性能,前者有较高的芳烃选择性,但活性较低,并有明显的氢解性能。氨化学吸附法和吡啶吸附激光喇曼光谱法分析结果表明,以硝酸锌改性的ZSM-5催化剂的酸性低于氧化锌改性的催化剂的酸性,后者的酸性与HZSM-5的相近。通过考察锌含量对催化剂芳构化性能及酸性的影响,并分析催化剂的吡啶吸附红外光谱以及X射线衍射光谱,提出以下的观点:以硝酸锌为锌源的催化剂中,锌与分子筛表面酸性羟基发生作用生成新基团,而在以氧化锌为锌源的催化剂中,锌仍以氧化锌形式独立存在。

The aromatization performance of Zn-modified ZSM-5 catalyst has been studied in a continuous flow microreactor at atmospheric pressure with n-pentane as its feedstock. The results show that the cat- alysts modified with identical zinc precursor, such as zinc nitrate, exhibit about the same aromatization selectivity, although the preparation mothods are different. However, changing the zinc precursor from zinc nitrate to zinc oxide results in obvious differences in aromatization performance. Compared to zinc oxide-modified catalysts, the zinc nitrate-modified catalysts show lower activity, higher selectivity for aromatics and significantly stronger capability in hydrogenolysis. The acidity of these catalysts men- tioned as above were measured by ammonia chemical adsorption and Laser Raman spectroscopy and IR spectroscopy. The results indicate that the acidity of zinc nitrate-modified catalyst is lower than that of the zinc oxide-modified catalyst, and the latter is almost equal to that of HZSM-5. Furthermore, a series of ZSM-5 catalysts with various zinc contents were also studied. It has been found that, in zinc nitrate- modified catalysts, th+e interaction between zinc species and acidic hydroxyl group may take place on ZSM-5 zeolite, and results in the formation of a new chemical group as the active species, whereas, in zinc oxide-modified catalysts, the zinc species remains its original ZnO form.