Spectroscopic and kinetic interrogation of organometallic complexes encapsulated in zeolites for gas-phase alkane oxidation catalysis

美国拥有丰富的天然气储量。虽然大多数天然气都是为了其热值或发电而燃烧，但组成天然气的轻烃也是制造易于运输的液体燃料和化学品的有吸引力的原料。催化在天然气上转化过程中起着重要的作用，可以加快反应速度，提高能源效率。然而，即使在催化剂的帮助下，将天然气转化为更高价值的液体产品仍然具有挑战性。该项目通过研究能够将甲烷氧化为甲醇的新型催化剂来应对这一挑战，从而支持美国在化工领域的能源安全和经济竞争力。该项目包括侧重于激励本科生从事研究的教育活动，以及旨在与当地社区进行公共科学接触的外联活动。在催化领域，将低碳烷烃低温转化为有用的燃料和化学品是一个长期的挑战。具有单核Fe活性中心的多孔催化剂已被广泛研究用于烷烃氧化反应，这是由常温下将甲烷氧化为甲醇的金属酶所驱动的。金属沸石和金属有机骨架在模拟金属酶中的反应环境方面特别有吸引力，因为存在具有必要局部构型的初级结合位点，这些结合位点位于分子尺寸的反应口袋中。然而，结合位原子排列的异质性，以及限制它们的空穴的大小和形状，使物理化学表征和普遍反应路径的描述变得复杂。如果构成活性中心的局部配位和二次限制空隙都被很好地定义，则有可能避免这些并发症。实现这一目标的一个有希望的途径是将分子络合物包裹在八面沸石分子筛的超笼中。具体地说，该项目探索水热合成包裹在八面沸石中的金属酞菁(MPC)，以控制金属中心原子的身份和配体，并对这些材料上耦合的低碳烷氧化和氮氧化物还原反应进行联合光谱和动力学研究。一个最重要的目标是将光谱特征和化学事件分配给已知结构的活性中心，从而克服现有含金属多孔催化剂固有的复杂性，同时创建一个材料平台，允许控制水热稳定的单位多相催化剂中活性中心的电子。这些实验将允许直接量化周转频率和本征动力学和热力学参数，同时还可以评估水热稳定性、投产时间稳定性以及这些材料在气相氧化反应中的更广泛用途。本科生将积极参与研究，并将获得宝贵的技能，为化学或化学工程的研究生学习做准备。公共科学外展将通过实验演示向当地社区传达研究结果和研究目标。该项目的结果还将作为一门新的研究生级别的无机固体光谱表征选修课的授课材料。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

The U.S. has vast reserves of natural gas. While most natural gas is burned for its heating value or to generate electricity, the light hydrocarbons that comprise natural gas are also attractive feedstocks for manufacturing readily-transportable liquid fuels and chemicals. Catalysis plays an important role in natural gas up-conversion processes by accelerating reaction rates and improving energy efficiency. Even with catalytic assistance, however, the conversion of natural gas to higher-value liquid products remains challenging. The project addresses that challenge through research aimed at new catalysts capable of oxidizing methane to methanol, thus supporting U.S. energy security and economic competitiveness in the chemical sector. The project includes educational activities focused on inspiring undergraduate students to pursue research, and outreach activities aimed at public science engagement with the local community. Low-temperature transformation of light alkanes to useful fuels and chemicals is a persistent challenge in the catalysis community. Porous catalysts with mononuclear Fe active sites have been studied extensively for alkane oxidation reactions, motivated by metalloenzymes that oxidize methane to methanol at ambient temperature. Metal-containing zeolites and metal organic frameworks are particularly attractive for emulating the reaction environment in metalloenzymes, given the presence of primary binding sites with the requisite local configurations that are housed in reaction pockets of molecular dimensions. However, heterogeneity in the arrangements of atoms in binding sites, and the size and shape of the cavities that confine them, complicate both physicochemical characterization and description of prevalent reaction pathways. Circumvention of these complications is possible if both the local coordination and the secondary confining voids that comprise the active centers are well-defined. A promising route towards this aim is encapsulation of molecular complexes within the supercages of faujasite zeolites. Specifically, the project explores hydrothermal synthesis of metal phthalocyanines (MPCs) encapsulated in faujasite to control the identities and ligands of metal-central atoms, and to perform a combined spectroscopic and kinetic study of coupled light alkane oxidation and nitrogen oxide reduction reactions over these materials. An over-arching goal is to assign spectroscopic signatures and chemical events to active centers of known structures, thereby overcoming complications inherent in existing metal-containing porous catalysts, while simultaneously creating a materials platform that allows for control over the electronics of active centers in hydrothermally stable, single-site heterogeneous catalysts. These experiments will allow straightforward quantification of turnover frequencies and intrinsic kinetic and thermodynamic parameters, while also evaluating hydrothermal stability, time-on-stream stability, and broader utility of these materials in gas-phase oxidation reactions. Undergraduate students will be heavily involved with the research and will gain valuable skills to prepare them for graduate studies in Chemistry or Chemical Engineering. Public science outreach will communicate findings and research goals to the local community through experimental demonstrations. Results from the project will also serve as lecture material in a new graduate-level elective on spectroscopic characterization of inorganic solids.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.