Single Site Catalysis for Direct Conversion of Methane to Ethylene and Aromatics: Design, Test and Operando Characterization

乙烯、丙烯和苯等低碳烯烃和芳烃是化妆品、润滑剂、洗涤剂和聚合物等广泛化学商品的重要组成部分。甲烷直接转化为这些化学品，从而节省能源和成本，仍然是化学领域的重大挑战之一，因为甲烷分子非常稳定，很难激活。最近发现了一种嵌入在二氧化硅基质中的铁单中心的Fe？SiO_2催化剂，能够以极高的效率将甲烷直接转化为乙烯和芳烃。本项目的目标是确定Fe-SiO_2催化剂的活性中心的结构和组成以及甲烷直接转化的反应机理，以便全面了解这种新型催化剂的性质。这将使其进一步合理改进，并有可能产生更好的催化剂，用于生产特定的烯烃和芳烃。为此，将通过不同的制备方法来制备单位金属催化剂，包括先进的方法，如火焰喷雾热解。后者是一种一步合成纳米晶体材料的通用方法。除铁外，还将探索以其他金属(如钼)为基础的单位催化剂。一方面，这种替代的单活性中心催化剂可以表现出更高的甲烷直接转化活性，或者能够更好地调节产物的选择性。另一方面，对不同单位金属催化剂的研究将有助于更好地从根本上了解甲烷活化的结构要求，为优化甲烷转化率和产物对特定烯烃和芳烃的选择性铺平道路。为了实现这一目标，将使用扫描电子显微镜(STEM)、拉曼和漫反射红外傅里叶变换光谱(DRIFTS)和扩展X射线吸收精细结构(EXAFS)等辅助技术对催化剂进行全面表征。将详细研究其催化活性，包括反应动力学和失活研究。利用先进的同步辐射X射线方法，如高能量分辨荧光检测X射线近边吸收光谱(HERFD-XANES)和X射线发射光谱(XES)，将在现场研究具有应用前景的催化剂体系。最后，基于OPANDO X-射线的研究将用于在1250K以下的反应条件下监测所选催化剂的结构。我们预计，从这些原位和OPANDO光谱分析方法的结果中，将出现关于单位金属催化剂的结构及其在甲烷活化中的作用的详细图景。此外，结合反应动力学分析，将有助于更好地理解甲烷在负载型单位金属催化剂上转化为烯烃和芳烃的机理。

Light olefins and aromatics like ethylene, propylene and benzene are important building blocks for a wide range of chemical commodities such as cosmetics, lubricants, detergents and polymers. The direct and thus energy and cost-saving conversion of methane to these chemicals remains one of the grand challenges for chemistry because the methane molecule is very stable and difficult to activate. A Fe©SiO2 catalyst with iron single sites embedded within a silica matrix has recently been found to enable direct conversion of methane to ethylene and aromatics with extraordinary efficiency. The goal of this project is to identify the structure and composition of the active sites of the Fe©SiO2 catalyst and the reaction mechanism of direct methane conversion in order to gain comprehensive insight into the nature of this novel type of catalyst. This will allow its further rational improvement and potentially lead to even better catalysts for production of specific olefins and aromatics. For this purpose, single site metal catalysts will be prepared via different preparation methods including advanced routes such as flame spray pyrolysis. The latter is a versatile method for one step synthesis of nano-sized crystalline materials. Apart from iron, single site catalysts based on other metals (e.g. Mo) will be explored. On the one hand such alternative single site catalysts may exhibit even higher activity for direct conversion of methane or enable better tuning of the product selectivity. On the other hand, the investigation of different single site metal catalysts will lead to a better fundamental understanding of the structural requirements for methane activation, paving the way for optimizing both methane conversion and product selectivity towards specific olefins and aromatics.To reach this target, catalysts will be thoroughly characterized using complementary techniques including scanning transmission electron microscopy (STEM), Raman and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and extended X-ray absorption fine structure (EXAFS). The catalytic activity will be investigated in detail including reaction kinetic and deactivation studies. Promising catalyst systems will be studied in situ using advanced synchrotron X-ray based methods such as high energy-resolution fluorescence detected X-ray absorption near edge spectroscopy (HERFD-XANES) and X-ray emission spectroscopy (XES). Finally, operando X-ray based studies will be used to monitor the structure of selected catalysts under reaction conditions up to1250 K.We anticipate that a detailed picture will emerge of both the structure of single site metal catalysts and their role in methane activation from the results of these in situ and operando spectroscopic analysis methods. Moreover, the combination with reaction kinetic analyses will lead to a better mechanistic understanding of methane conversion into olefins and aromatics over supported single site metal catalysts.