CAREER: Tailoring the Synergy between Catalyst Design and Reaction Engineering for Direct Conversion of Methane to Aromatics

甲烷在自然资源中的巨大储量使其成为一种有吸引力的化学原料，可转化为高碳氢化合物燃料和工业用途的散装化学品。然而，由于高昂的运输和加工成本，许多天然气被发现在搁浅的地方，并被燃烧。直接将甲烷转化为液态化学品可以降低加工成本，但由于尚未解决的科学挑战，目前在经济上不可行。该提案的目标是催化甲烷脱氢芳构化（MDA），将甲烷直接转化为苯和氢，这两种物质都是化工行业的主要原料。提出的研究旨在整合催化和反应工程，以克服在MDA过程中出现的技术挑战。研究结果将与教育和外联活动结合起来。目前阻碍MDA在工业水平上实施的两个主要挑战：(1)热力学限制导致低甲烷平衡转化；(2)催化剂快速失活。虽然从反应介质中去除氢产物的过程成功地将平衡向右转移并增加了甲烷向苯的转化，但它们也加速了焦化。因此，催化剂失活在MDA是一个普遍存在的问题，必须解决。在HZSM-5分子筛上负载的氧化钼是研究最多的丙二醛反应催化剂。人们一致认为，在诱导期，氧化Mo转变为碳化Mo，负责甲烷转化为芳烃。添加氧化共反应物有助于过程热力学，然而，只能使用低浓度的氧化剂，否则活性碳化钼相将被氧化破坏。在低氧化剂浓度下，苯的产率有所提高，但填料床反应器中氧化剂的分布并不均匀，因此动力学测量不能代表整个催化剂床。分散式进料膜反应器（DFMR）可以克服反应器非均质性问题，提高苯收率，但不能完全防止催化剂结焦。PI已经发现了一种方法，通过在第二种金属X （X= Fe， Co或Ni）存在的情况下制备非原位碳化Mo相来增加催化剂的抗结焦性，但Mo-X相互作用的性质仍然未知。拟议的项目包括研究稳定的Mo-X/HZSM-5催化剂在DFMR中与氧化剂（氧或二氧化碳）作为共反应物，以确定Mo-X- c相互作用在反应和催化剂失活途径中的作用。模型催化剂的制备将通过阻断沸石外表面的锚定位点来确保金属（Mo和X）位于沸石通道内。Mo-X相的结构、位置和演变将通过现场和Operando实验进行监测，使用先进的表征技术，包括x射线吸收和高分辨率粉末衍射。DFMR操作条件将调整，以确保均匀的轴向分布的氧化剂和最大限度地提高苯的产量。动力学测试将使用DFMR进行，并与PBR作为参考进行比较。动力学测试、原位结构表征和理论计算的结合将确定MDA在氧化剂存在下的反应和失活途径，并将为合理设计适合DFMR的催化剂提供基础。PI将开发两门新的研究生课程，重点是综合跨学科的催化方法，以及与德克萨斯理工大学戏剧和舞蹈学院合作的跨学科科学、技术、工程、艺术、数学（STEAM）项目，利用讲故事的方式提高公众对科学和工程重要性的认识，吸引学生进入STEM职业。这些故事将以各种形式传播，包括在当地学校的戏剧表演、数字故事、在YouTube频道上发布的播客。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The vast abundance of methane in natural resources makes it an attractive chemical feedstock for conversion into higher hydrocarbon fuels and bulk chemicals of industrial interest. However much of the natural gas is found in stranded locations and is flared due to high transportation and processing costs. Direct transformation routes of methane into liquid chemicals can reduce processing costs but are currently not economically viable due to unanswered scientific challenges. The proposal targets catalytic methane dehydroaromatization (MDA) for the direct conversion of methane to benzene and hydrogen, both mainstays in the chemicals industry. The proposed study seeks to integrate catalysis and reaction engineering to overcome technological challenges arising during MDA. The research results will be integrated with educational and outreach activities.Two major challenges currently prevent the implementation of MDA at an industrial level: (1) thermodynamic limitations leading to low methane equilibrium conversion; (2) rapid catalyst deactivation. While processes involving removal of the hydrogen product from the reaction medium are successful in shifting the equilibrium to the right and increasing methane conversion to benzene, they also accelerate coking. Thus, catalyst deactivation in MDA is a ubiquitous issue that must be addressed. Molybdenum (Mo) oxide supported on HZSM-5 zeolite is the most commonly studied catalyst for the MDA reaction. It is agreed that during an induction period Mo oxide species transform to Mo carbide species that are responsible for the conversion of methane to aromatics. Adding oxidant co-reactants can help the process thermodynamics, however, only low oxidant concentrations can be used since the active Mo carbide phases will otherwise be oxidatively destroyed. At low oxidant concentrations benzene yields are improved but the oxidant distribution in a packed-bed reactor (PBR) is not even, thus kinetic measurements are not representative of the entire catalyst bed. Distributed-feed membrane reactors (DFMR) can overcome the reactor heterogeneity problem and increase benzene yields, but catalyst coking is not fully prevented. The PI has discovered a method to increase catalyst resistance to coking by preparing Mo carbide phases ex situ in the presence of a second metal X (X= Fe, Co, or Ni), but the nature of the Mo-X interaction remains unknown. The proposed project includes studies of stable Mo-X/HZSM-5 catalysts in a DFMR with an oxidant (oxygen or carbon dioxide) as co-reactant to determine the role of Mo-X-C interactions on the reaction and catalyst deactivation pathways. Model catalysts will be prepared by ensuring that the metals (Mo and X) are located inside the zeolite channels by blocking the anchoring sites on the external surface. The structure, location and evolution of the Mo-X phases will be monitored by in situ and Operando experiments using advanced characterization techniques, including X-ray absorption and high-resolution powder diffraction. DFMR operating conditions will be tuned to ensure even axial distribution of the oxidant and maximum enhancement in benzene yield. Kinetic testing will be performed with the DFMR and compared to a PBR as reference. The combination of the kinetic tests, in situ structural characterization and theoretical calculations will result in the determination of the reaction and deactivation pathways of MDA in the presence of oxidants and will provide the basis for the rational design of catalysts tailored for a DFMR. The PI will develop two new graduate courses focused on an integrated interdisciplinary approach to catalysis and an interdisciplinary Science, Technology, Engineering, Art, Mathematics (STEAM) project with the School of Theatre and Dance at Texas Tech University to use storytelling to increase the awareness of the public on the importance of science and engineering and attract students into STEM careers. The stories will be propagated in various formats including theatre performances at local schools, digital stories and podcasts posted on a YouTube channel.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.