Space- and Time-Resolved In-situ Spectroscopy on the Coke Formation in Molecular Sieves: Methanol-to-Olefin Conversion over H-ZSM-5 and H-SAPO-34
Space- and Time-Resolved In-situ Spectroscopy on the Coke Formation in Molecular Sieves: Methanol-to-Olefin Conversion over H-ZSM-5 and H-SAPO-34
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DOI：
10.1002/chem.200801293
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发表时间：
2008-01-01
影响因子：
4.3
通讯作者：
Weckhuysen, Bert M.
中科院分区：
文献类型：
--
作者：
Mores, Davide;Stavitski, Eli;Weckhuysen, Bert M.
用空间分辨和时间分辨方法研究了甲醇制烯烃（MTO）反应中大尺寸H-ZSM-5和H-SAPO-34晶体中焦炭的生成。这已经成为可能，通过应用高温原位细胞结合显微光谱技术。光学活性碳质物质的积累允许用UV/维斯显微镜检测，而直立配置的共聚焦荧光显微镜可视化催化剂颗粒内焦炭分子及其前体的形成。在H-ZSM-5中，焦炭最初在三角形晶体边缘处形成，其中直通道开口直接到达外部晶体表面。在530至745 K的反应温度范围内，由于焦炭或其前体，检测到位于415和550 nm左右的两个吸收带。共聚焦荧光显微镜揭示了荧光碳物种，最初形成在近表面区域，并逐渐扩散向内的晶体中，内部共生边界阻碍了更庞大的芳香族化合物的一个容易渗透。在H-SAPO-34晶体的情况下，在反应期间在400 nm附近产生吸收带。如果反应在530和575 K之间进行，则该谱带的强度随时间增长，然后降低，而在较高温度下，其强度随运行时间保持稳定。在反应过程中荧光物质的形成被限制在H-SAPO-34晶体的近表面区域，从而在MTO反应过程中对向晶体中间移动的焦炭前沿产生扩散限制。这两个应用的显微光谱技术介绍允许LIS区分石墨状焦炭沉积在外部晶体表面和芳香族物种内形成的沸石通道。这些方法的使用可以扩展到各种各样的催化反应和其中形成碳质沉积物的材料。
Formation of coke in large H-ZSM-5 and H-SAPO-34 crystals during the methanol-to-olefin (MTO) reaction has been studied in a space- and time-resolved manner. This has been made possible by applying a high-temperature in-situ cell in combination with micro-spectroscopic techniques. The buildup of optically active carbonaceous species allows detection with UV/Vis microscopy, while a confocal fluorescence microscope in an upright configuration visualises the formation of coke molecules and their precursors inside the catalyst grains. In H-ZSM-5, coke is initially formed at the triangular crystal edges, in which straight channel openings reach directly the external crystal surface. At reaction temperatures ranging from 530 to 745 K, two absorption bands at around 415 and 550 nm were detected due to coke or its precursors. Confocal fluorescence microscopy reveals fluorescent carbonaceous species that initially form in the near-surface area and gradually diffuse inwards the crystal in which internal intergrowth boundaries hinder a facile penetration for the more bulky aromatic compounds. In the case of H-SAPO-34 crystals, an absorption band at around 400 nm arises during the reaction. This band grows in intensity with time and then decreases if the reaction is carried out between 530 and 575 K, whereas at higher temperatures its intensity remains steady with time on stream. Formation of the fluorescent species during the course of the reaction is limited to the near-surface region of the H-SAPO-34 crystals, thereby creating diffusion limitations for the coke front moving towards the middle of the crystal during the MTO reaction. The two applied micro-spectroscopic techniques introduced allow LIS to distinguish between graphite-like coke deposited on the external crystal surface and aromatic species formed inside the zeolite channels. The use of the methods can he extended to a wide variety of catalytic reactions and materials in which carbonaceous deposits are formed.