Conversion of Methanol to Olefins over H-ZSM-5 Zeolite: Reaction Pathway Is Related to the Framework Aluminum Siting
Conversion of Methanol to Olefins over H-ZSM-5 Zeolite: Reaction Pathway Is Related to the Framework Aluminum Siting
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H-ZSM-5 沸石上甲醇转化为烯烃：反应路径与骨架铝选址相关
DOI：
10.1021/acscatal.6b01771
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发表时间：
2016-11-01
期刊：
影响因子：
12.9
通讯作者：
Wang, Jianguo
中科院分区：
文献类型：
--
作者：
Liang, Tingyu;Chen, Jialing;Wang, Jianguo
由于甲醇在沸石催化剂上转化为烯烃(MTO)是在骨架铝(Al-F)衍生的酸性中心上进行的，如果人们知道位于特定中心的特定Al-F的催化行为，就有可能通过改变Al-F的位置来提高催化性能。本文分别以硅溶胶和正硅酸乙酯为硅源合成了两个系列的H-ZSM-5沸石，即S-HZ-m和T-HZ-m。这两个系列的H-ZSM-5沸石具有相似的酸性、形貌和织构性质。然而，Co(11)离子的UV-Vis-DRS和Al-27MAS核磁共振结果表明，它们在Al-F位置上有很大的不同，S-HZ-m的Al-F在正弦和直通道中富集，而T-HZ-m的Al-F则集中在通道交叉处。与S-HZ-m相比，T-HZ-m具有更高的乙烯和芳烃选择性和较大的氢转移指数，而S-HZ-m对丙烯和高碳烯烃具有更高的选择性。C-13/C-12-甲醇交换实验表明，在T-HZ-m催化剂上，C-12与五甲苯和六甲基苯的反应较快，而在S-HZ-m催化剂上，C-12对C-3-C-5烯烃的争夺较快。这说明H-ZSM-5分子筛通道交叉处的Al可能更有利于芳香族环的传播，而正弦和直线型通道中的AlP更有利于烯烃环的传播。这些结果有助于阐明H-ZSM-5的骨架酸中心在MTO中的催化行为，进而提出了通过调节骨架铝的位置来提高催化性能的有效途径。
As the conversion of methanol to olefins (MTO) over a zeolite catalyst is conducted on acid sites derived from framework aluminum (Al-F), it is possible to enhance the catalytic performance by altering the siting of Al-F if one knows the catalytic behavior of specified Al-F located at certain sites. In this work, two series of H-ZSM-5 zeolites, viz., S-HZ-m and T-HZ-m, were synthesized with silica sol and tetraethyl orthosilicate, respectively, as the silicon source. Both series of H-ZSM-5 zeolites exhibit similar acidity, morphology, and textual properties. However, they are quite different with respect to Al-F siting, as determined by UV-vis-DRS of Co(11) ions and Al-27 MAS NMR; Al-F of S-HZ-m is enriched in the sinusoidal and straight channels, whereas Al-F of T-HZ-m is concentrated in the channel intersections. When they are used as the catalyst in MTO, T-HZ-m gives higher selectivity to ethene and aromatics and a larger hydrogen transfer index (HTI) than S-HZ-m, whereas S-HZ-m exhibits higher selectivity to propene and higher olefins. Moreover, the C-13/C-12-methanol-switching experiments indicate that the incorporation of C-12 into pentamethylbenzene and hexarnethylbenzene is faster on T-HZ-m, whereas the scramble of C-12 for C-3-C-5 olefins is speedier on S-HZ-m. All of these illustrate that Al, in the channel intersections of H-ZSM-5 is probably more favorable to the propagation of the aromatic-based cycle, whereas Alp in the sinusoidal and straight channels is more encouraging for the alkene-based cycle. These results help to clarify the catalytic behavior of given framework acid sites of H-ZSM-5 in MTO and then bring forward an effective approach to improving the catalytic performance by regulating the framework aluminum siting.