Conversion of methanol to hydrocarbons over zeolite H-ZSM-5: On the origin of the olefinic species

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• 作者：Morten Bjø ; rgen ; Stian Svelle ; Finn Joensen ; Jesper Nerlov ; Stein Kolboe ; Francesca Bonino ; Luisa Palumbo ; Silvia Bordiga ; Unni Olsbye
• 关键词：ZSM-5 ; MFI ; MTO ; MTH ; MTG ; Methanol ; Zeolite ; Mechanism ; Hydrocarbon pool ; Coking ; Deactivation
• 刊名：Journal of Catalysis
• 出版年：2007
• 期刊代码：97_00219517
• 类别：ce
• 出版时间：25 July 2007
• 卷：249
• 期：2
• 页码：195-207
• 文件大小：866 K

摘要

This study examined the reaction mechanism with respect to both catalyst deactivation and product formation in the conversion of methanol to hydrocarbons over zeolite H-ZSM-5. The reactivity of the organics residing in the zeolite voids during the reaction was assessed by transient ¹²C/¹³C methanol-switching experiments. In contrast to previously investigated catalysts (H-SAPO-34 and H-beta), hexamethylbenzene is virtually unreactive in H-ZSM-5 and is thus not a relevant reaction intermediate for alkene formation. However, the lower methylbenzenes are reaction intermediates in a hydrocarbon pool-type mechanistic cycle and are responsible for the formation of ethene and propene. An additional reaction cycle not applicable for ethene also must be taken into account. The C₃₊ alkenes are to formed through rapid alkene methylation and cracking steps to a considerable extent; thus, methanol is converted to hydrocarbons according to two catalytic cycles over H-ZSM-5. Moreover, in contrast to what occurs for large-pore zeolites/zeotypes, molecules larger than hexamethylbenzenes are not built up inside the H-ZSM-5 channels during deactivation. Thus, deactivation is explained by coke formation on the external surface of the zeolite crystallites only. This is a plausible rationale for the superior lifetime properties of H-ZSM-5 in the methanol-to-hydrocarbon reaction.