Effect of sequential desilication and dealumination on catalytic performance of ZSM-5 catalyst for pyridine and 3-picoli
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Yugang Cui, Zebao Rui, and Yongdan Lib) Tianjin Key Laboratory of Applied Catalysis Science and Technology, and State Key Laboratory for Chemical Engineering, School of Chemical Engineering, Tianjin University, Tianjin 300072, China (Received 5 May 2009; accepted 15 October 2009)
A sequential procedure of alkaline treatment followed by SiCl4 or hydrothermal treatment has been investigated to obtain a tailored ZSM-5 catalyst for the synthesis of pyridine bases. The major function of alkaline treatment is desilication; however, it is accompanied by the extraction of framework aluminum, which formed the extra-framework alumina and amorphous alumina by realumination. Moreover, a large number of intracrystalline meso- and macropores and silanol groups are created. The desilication and realumination cause the ratio of Lewis acid sites to Bro¨nsted acid sites (L/B) to increase. The subsequent SiCl4 or hydrothermal dealumination increases both L/B ratio and acid strength. The introduced hierarchical pores and changed acid strength distribution by the alkaline treatment improve the stability of the catalysts; the generated stronger acid sites by dealumination benefits the yields of pyridine bases; and the increased L/B by the sequential treatment promotes the selectivity of pyridine over 3-picoline. I. INTRODUCTION
The Chichibabin condensations of aldehydes or ketones with ammonia are an important category of reactions for the production of pyridine bases, which are key intermediates for pharmaceutical, agricultural, and other fine chemicals. This type of reaction has been the focus of recent works with H-ZSM-5 as a promising catalyst.1–4 This reaction is essentially acid catalyzed. However, deactivation of the catalyst due to coking has been encountered as a major obstacle for commercialization. It has been proposed that the acid site and strength distribution have a strong influence on the activity.4 Furthermore, the pore distribution was also found as a major effect on the reaction.5 It has been proposed that micropores may facilitate the fast deactivation due to coke formation, and the creation of mesopores can reduce the deactivation rate.6 The framework Si/Al ratio (Si/AlFrame) exerts a strong influence on the acid strength distribution and the activity. Postsynthesis modification techniques have often been used for creating mesopores in microporous zeolites and changing the Si/AlFrame.7 Dealumination with
SiCl4 leads to the substitution of framework aluminum (Alf) by silicon, without generating secondary pores8–10; whereas hydrothermal dealumination, achieved by steaming at elevated temperatures, could dislodge the Alf, create extra-framework aluminum (Alef), and make mesopores.11–14 However, dealumination of siliceous zeolites, such as ZSM-5, by hydrothermal treatment induces rather limited mesopores.15,16 More recently, many researcher reported6,17–23 that desilication, i.e., removal of framework silicon by leaching with diluted alkaline solution under a mild temperature, was an effective approach to create extr
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