Direct Observation of Cyclic Carbenium Ions and Their Role in the Catalytic Cycle of the Methanol-to-Olefin Reaction over Chabazite Zeolites
Direct Observation of Cyclic Carbenium Ions and Their Role in the Catalytic Cycle of the Methanol-to-Olefin Reaction over Chabazite Zeolites
复制标题
环状碳正离子的直接观察及其在菱沸石上甲醇制烯烃反应催化循环中的作用
DOI：
10.1002/anie.201303586
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发表时间：
2013-10-25
影响因子：
16.6
通讯作者：
Liu, Zhongmin
中科院分区：
文献类型：
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作者：
Xu, Shutao;Zheng, Anmin;Liu, Zhongmin
原油储量的减少和乙烯、丙烯等基础化学品需求的快速增长，为探索替代化工原料、发展非石化工艺提供了强劲动力。迄今为止，甲醇制烯烃（MTO）工艺已成为利用丰富的天然气或煤炭资源生产低碳烯烃的最成功的非石化路线之一。[1-3]考虑到MTO反应产物在全球能源链和化学工业中的重要作用，[4]MTO反应机理的研究对于基础科学和工业应用都至关重要。 MTO工艺中使用的催化剂通常为微孔固体酸，包括沸石和沸石型分子筛，其中骨架型MFI的H-ZSM-5沸石和骨架型CHA的SAPO-34在MTO反应中具有最佳的催化性能。[1-4]尽管过去30年进行了大量的研究工作，[4-8]第一个CÀC键的反应机理 MTO 过程中的问题仍然难以捉摸。一般来说，烃池（HCP）机制[9]，即限制在沸石笼或通道交叉点中的环状有机物质充当助催化剂，已被普遍认为是由C1反应物甲醇生产烯烃的合理解释。 [2]根据HCP机理，提出了两条反应路线来解释MTO反应途径，即侧链甲基化路线和配对路线。在这两条路线中，碳正离子都参与其中，并充当生产烯烃的重要中间体。[10-15]具体而言，配对机制涉及六元环阳离子（聚甲基苯正离子）的收缩和五元环阳离子（聚甲基环戊烯基阳离子）的膨胀。相比之下，侧链甲基化路线通过聚甲基苯正离子上的甲醇甲基化，然后消除侧链基团来产生烯烃。在实际条件下直接观察所提出的机理中涉及的这些碳正离子对于理解MTO反应的反应机理和结构-性能相关性具有重要意义。尽管 MTO 反应中的一些碳正离子，例如阳离子 1-4（参见方案 1）已使用原位固态 NMR 进行了鉴定，但这些碳正离子的形成
The diminishing reserve of crude oil and the rapidly increasing demands for the base chemicals, such as ethene and propene, provide strong driving force for exploring alternative chemical feedstocks and developing non-petrochemical processes. To date, the methanol-to-olefins (MTO) process has been one of the most successful non-petrochemical routes for the production of light olefins from abundant resources of natural gas or coal.[1–3] Considering the essential role of the products of the MTO reaction in the global energy chain and chemical industry,[4] investigations of the MTO reaction mechanism are crucial for both of the fundamental science and industrial application. Catalysts used in the MTO process are typically microporous solid acids, including zeolites and zeotype molecular sieves, among which the H-ZSM-5 zeolite with framework type MFI and SAPO-34 with framework type CHA deliver the best catalytic performance in the MTO reaction.[1–4] Despite the tremendous research efforts over the past 30 years,[4–8] the reaction mechanism of the first CÀC bond in the MTO process remains elusive. In general, the hydrocarbon pool (HCP) mechanism,[9] that is, that cyclic organic species confined in the zeolite cage or intersection of channels act as co-catalysts, has been generally accepted as a rational explanation for the olefins production from the C1 reactant, methanol.[2] Two reaction routes have been proposed to explain the MTO reaction pathway according to the HCP mechanism, namely the side-chain methylation route and the paring route. In both of the two routes, carbenium ions are involved and act as the important intermediates to produce olefins.[10–15] Specifically, the paring mechanism involves the contraction of six-membered ring cations (polymethylbenzenium cations) and the expansion of five-membered ring cations (polymethylcyclopentenyl cations). In contrast, the side-chain methylation route proceeds via the methanol methylation on polymethylbenzenium cations and subsequent elimination of side-chain groups to produce olefins.Direct observation of these carbenium ions involved in the proposed mechanism under real conditions is of great significance to understand the reaction mechanism and the structure–performance correlation of the MTO reaction. Even though some carbenium ions in the MTO reaction, such as cations 1–4 (see Scheme 1) have been identified using in situ solid-state NMR, the formation of these carbenium