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

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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.