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。 虽然大多数天然气燃烧是为了其热值或发电，但构成天然气的轻质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。 催化在天然气上转换过程中起着重要的作用，可以加快反应速率，提高能源效率。 然而，即使有催化剂的帮助，将天然气转化为更高价值的液体产品仍然具有挑战性。 该项目旨在通过研究能够将甲烷氧化为甲醇的新催化剂来应对这一挑战，从而支持美国能源安全和化工行业的经济竞争力。 该项目包括教育活动，重点是鼓励本科生从事研究，以及旨在与当地社区进行公共科学接触的外联活动。 低碳烷烃的低温转化为有用的燃料和化学品是催化界的一个持续挑战。 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，其动机是在环境温度下将甲烷氧化成甲醇的金属酶。 含金属的沸石和金属有机框架对于模拟金属酶中的反应环境是特别有吸引力的，因为存在具有容纳在分子尺寸的反应袋中的必要局部构型的初级结合位点。 然而，异质性的原子在结合位点的安排，以及限制它们的空腔的大小和形状，复杂的物理化学表征和描述的普遍反应途径。 规避这些并发症是可能的，如果当地的协调和二次限制的空隙，包括活性中心是明确的。 实现这一目标的一个有前途的途径是将分子复合物封装在八面沸石的超笼内。 具体而言，该项目探索了水热合成的金属酞菁（MPC）封装在八面沸石中，以控制金属中心原子的身份和配体，并进行耦合的轻烷烃氧化和氮氧化物还原反应的光谱和动力学研究。 一个过度扩展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中固有的复杂性，同时创建一个材料平台，该平台允许控制水热稳定的单中心非均相催化剂中活性中心的电子学。 这些实验将允许直接量化的周转频率和内在的动力学和热力学参数，同时还评估水热稳定性，时间上的流稳定性，以及更广泛的实用性，这些材料在气相氧化反应。 本科生将积极参与研究，并将获得宝贵的技能，为他们在化学或化学工程研究生学习做好准备。 公共科学推广将通过实验示范向当地社区宣传研究结果和研究目标。 该项目的成果还将作为一个新的研究生选修课的讲座材料，该选修课是关于无机固体的光谱表征。该奖项反映了NSF的法定使命，并被认为值得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来支持。

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.