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首页> 外文期刊>Chemistry: A European journal >Coke formation during the methanol-to-olefin conversion: In situ microspectroscopy on individual H-ZSM-5 crystals with different br?nsted acidity
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Coke formation during the methanol-to-olefin conversion: In situ microspectroscopy on individual H-ZSM-5 crystals with different br?nsted acidity

机译：甲醇到烯烃转化过程中的焦炭形成：原位光谱用于不同布朗斯台德酸度的单个H-ZSM-5晶体

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摘要

Coke formation during the methanol-to-olefin (MTO) conversion has been studied at the single-particle level with in situ UV/Vis and confocal fluorescence microscopy. For this purpose, large H-ZSM-5 crystals differing in their Si/Al molar ratio have been investigated. During MTO, performed at 623 and 773 K, three major UV/Vis bands assigned to different carbonaceous deposits and their precursors are observed. The absorption at 420 nm, assigned to methyl-substituted aromatic compounds, initiates the buildup of the optically active coke species. With time-on-stream, these carbonaceous compounds expand in size, resulting in the gradual development of a second absorption band at around 500 nm. An additional broad absorption band in the 600 nm region indicates the enhanced formation of extended carbonaceous compounds that form as the reaction temperature is raised. Overall, the rate of coke formation decreases with decreasing aluminum content. Analysis of the reaction kinetics indicates that an increased Br?nsted acid site density facilitates the formation of larger coke species and enhances their formation rate. The use of multiple excitation wavelengths in confocal fluorescence microscopy enables the localization of coke compounds with different molecular dimensions in an individual H-ZSM-5 crystal. It demonstrates that small coke species evenly spread throughout the entire H-ZSM-5 crystal, whereas extended coke deposits primarily form near the crystal edges and surfaces. Polarization-dependent UV/Vis spectroscopy measurements illustrate that extended coke species are predominantly formed in the straight channels of H-ZSM-5. In addition, at higher temperatures, fast deactivation leads to the formation of large aromatic compounds within channel intersections and at the external zeolite surface, where the lack of spatial restrictions allows the formation of graphite-like coke.
机译：甲醇到烯烃（MTO）转化过程中的焦炭形成已通过原位UV / Vis和共聚焦荧光显微镜在单颗粒水平进行了研究。为此，已经研究了Si / Al摩尔比不同的大H-ZSM-5晶体。在623和773 K进行的MTO期间，观察到分配给不同含碳沉积物及其前驱体的三个主要UV / Vis带。归属于甲基取代的芳族化合物的在420 nm处的吸收引发了光学活性焦炭物种的积累。随着时间的流逝，这些碳质化合物的尺寸会扩大，从而导致大约500 nm处的第二吸收带逐渐形成。在600 nm区域有一个额外的宽吸收带，表明随着反应温度的升高，形成的延伸碳质化合物的形成得以增强。总体而言，焦炭形成速率随铝含量的降低而降低。反应动力学分析表明，布朗斯台德酸位点密度的增加促进了较大焦炭种类的形成并提高了它们的形成速率。在共聚焦荧光显微镜中使用多个激发波长可以在单个H-ZSM-5晶体中定位具有不同分子尺寸的焦炭化合物。结果表明，小的焦炭种类均匀地分布在整个H-ZSM-5晶体中，而扩展的焦炭沉积物主要形成在晶体边缘和表面附近。偏振相关的UV / Vis光谱测量表明，扩展的焦炭种类主要在H-ZSM-5的直通道中形成。另外，在较高的温度下，快速失活导致通道交叉点内和外部沸石表面形成大量的芳族化合物，而缺乏空间限制则允许形成类似石墨的焦炭。

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