Molecular Structure and Reactivity of Model Mn/Na2WO4/SiO2 Oxidative Coupling of Methane Catalyst under Operating Conditions

该研究项目解决了天然气的主要成分甲烷直接催化转化为增值化学原料的问题。特别是，这项研究为两个甲烷分子如何结合在一起形成两个碳原子的各种分子，即OCM反应的机制提供了基本的见解。后一类分子是化学工业感兴趣的化学品和燃料。OCM反应作为一种比大规模的、资本密集型的甲烷转化途径更经济的替代方法，特别是对于无法经济地将甲烷输送到集中化工厂的偏远甲烷储层，是目前工业感兴趣的。该项目将研究、教学和推广计划相结合，重点是了解性能最好的OCM催化剂的局限性，并探索在现有水平上提高其活性和选择性的方法。这项研究将在利哈伊谷达芬奇科学中心一年一度的夏季研讨会上展示，在那里，各个年龄段的游客都将接触到正在开发的将甲烷转化为化学物质的新技术。研讨会、课程开发和研究整合到高级化学工程项目设计顶点课程中，每年将影响大约180名来自不同社会经济背景的中学生、高中生和本科生。这项研究是与国际研究人员合作进行的，这将使学生能够使用里海大学没有的尖端研究设施，提高研究的知名度，并使参与研究的学生接触到领先的国际实验室的研究文化。该项目建立在假设负载Mn/Na2WO4/SiO2催化剂的OCM活性位点是锚定在二氧化硅载体上的孤立表面WOx物质。研究人员的初步研究首次确定了OCM过程中催化该反应的未促进和促进的分离表面WOx位点的存在。该项目的目标是：(1)结合动力学研究和密度泛函理论（DFT），在OCM过程中应用现代原位和operando光谱建立基本的催化剂结构-活性关系；(2)应用新的基本见解来指导合理设计先进的活性和选择性OCM催化剂，在较低的温度下发挥作用。各种非促进和Na-， Mn-和Na/Mn-促进的sio2负载的WOx位点的排列正在被合成。二氧化硅载体也正在用金属钠和纳米连接剂（TiOx, ZrO2, Al2O3）进行表面改性，这将增加表面孤立的WOx位点的数量，并调整其还原性和酸碱特性。系统地改变启动子与w的比例，以探索它们对WOx位点的数量和结构以及OCM活性/选择性的影响。用原位和操作光谱（拉曼光谱、紫外-可见光谱和napp - xps）对制备的催化剂和在OCM反应条件下的催化剂进行了表征，以确定二氧化硅上WOx位点的分子和电子结构。实验结果正在与分子水平的DFT计算相补充，以提供对结构-活性关系的额外见解。相应的CH4/CD4和1602 / 18o2同位素动力学研究分别解决了不同WOx位点的速率决定步骤和相对参与，后者通过时间分辨拉曼-质谱。通过与欧洲伙伴的合作，利用分子束质谱技术获得了有关气相自由基及其与特定催化剂结构关系的信息。新的见解将导致概念上新的和现实的催化剂模型，这将有可能使一步OCM催化过程的发展。这项研究的成功实施有可能导致从廉价、丰富、但运输困难和昂贵的甲烷中生产经济上可行的C2碳氢化合物，从而使美国更接近能源独立，同时也为向可持续化学品过渡提供了更长的时间框架。该项目将涉及与Siluria和SABIC两家公司的合作，这将促进新催化剂向化学工业的转移。

This research project addresses the direct catalytic conversion of methane, the primary component of natural gas, to value-added chemical feedstocks. In particular, the research provides fundamental insight into the mechanisms of how two methane molecules couple together to form various molecules with two carbon atoms, the OCM reaction. The latter types of molecules are chemicals and fuels of interest to chemical industries. The OCM reaction is of current industrial interest as a more economical alternative to large-scale, capital-intensive routes for methane conversion, especially for remotely located methane reservoirs whose methane cannot be economically transported to centralized chemical plants. The project combines research, teaching and an outreach plan focused on understanding limitations of the best performing OCM catalyst, and exploring options for improving its activity and selectivity beyond current levels. This research is being showcased in annual summer workshops at the Lehigh Valley Da Vinci Science Center where visitors of all ages will be exposed to the new technologies being developed that convert methane to chemicals. Workshops, curricula development, and research integration into a senior Chemical Engineering Project Design capstone course will impact approximately180 middle-, high-school and undergraduate students of diverse socioeconomic backgrounds per year. The research is being conducted in collaboration with international researchers, which will allow access to cutting edge research facilities not available at Lehigh, enhance visibility of the research, and expose the participating students to the research culture in leading international laboratories.The project is built on the hypothesis that the active site for OCM by supported Mn/Na2WO4/SiO2 catalysts is an isolated surface WOx species anchored to the silica support. The investigators' preliminary studies have identified, for the first time, the presence of unpromoted and promoted isolated surface WOx sites during OCM that catalyze this reaction. The objectives of the project are to (1) establish the fundamental catalyst structure-activity relationships by application of modern in situ and operando spectroscopy during OCM combined with kinetic studies and density functional theory (DFT), and (2) apply the new fundamental insights to guide rational design of advanced active and selective OCM catalysts functioning at lower temperatures. Various permutations of unpromoted and Na-, Mn- and Na/Mn-promoted SiO2-supported WOx sites are being synthesized. The silica support is also being surface-modified with sodium metal and nanolinkers (TiOx, ZrO2, Al2O3) that will increase the number of isolated surface WOx sites and tune their reducibility and acid-base characteristics. The promoters-to-W ratios is being systematically varied to explore their effects on both the number and structure of the WOx sites and OCM activity/selectivity. The catalysts are being characterized as prepared and during OCM reaction conditions with in situ and operando spectroscopy (Raman, UV-vis and NAP-XPS) to determine the molecular and electronic structures of the WOx sites on silica. Experimental findings are being complemented with molecular level DFT calculations to provide additional insights into structure-activity relationships. Corresponding kinetic studies with isotopic CH4/CD4 and 16O2/18O2 are addressing the rate-determining-step and relative participation of different WOx sites, respectively, - the latter via time-resolved Raman-Mass Spectroscopy. Information about gas phase radicals and their relationship to specific catalyst structures is being obtained with Molecular Beam Mass Spectroscopy through collaboration with European partners. The new insights will lead to conceptually new and realistic catalyst models that will have the potential to enable development of one-step OCM catalytic processes. Successful execution of the proposed research has the potential to lead to economically viable production of C2 hydrocarbons from cheap, abundant, yet difficult and costly to transport methane, and thus move the US closer to energy independence while also providing a longer timeframe to transition into sustainable chemicals. The project will involve collaborations with two companies, Siluria and SABIC, that will facilitate transfer of new catalysts to the chemical industry.