NSF-BSF: CAS: Green Hydrogen from Liquid-Organic Hydrogen Carriers Using Supported Molecular Catalysts

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In order to address climate change, global efforts to reduce carbon dioxide (CO2) emissions are needed. To that end, sustainable production of hydrogen – as both an energy source and critical component of many chemicals – will play a key role in displacing fossil resources and their associated CO2 emissions. Because of its gaseous nature, both the transport and storage of hydrogen are challenging. The project addresses those challenges through research focused on the efficient design and utilization of Liquid Organic Hydrogen Carriers (LOHCs) – chemicals that are rich in hydrogen, manufactured at commodity scale, and transported/stored in liquid form. LOHCs must also be designed such that they readily release the hydrogen as needed and can be efficiently recycled to their hydrogen-rich state. The project focuses on catalyst technology enabling the more difficult reaction involving hydrogen release, for several promising LOHC systems, through an international collaboration with researchers at the Weizmann Institute of Science in Israel. LOHCs represent a viable approach for large-scale renewable energy storage that has the potential to obviate the inherent limitations of competing technologies based on batteries, liquid hydrogen, and liquid ammonia. The project operates at the interface of soluble- and solid-catalysis methodologies to advance the design, synthesis, characterization, and performance of supported molecular catalysts (SMCs). The targeted LOHCs build on pioneering developments in the Milstein group for dehydrogenative polymerization of ethylene glycol (EG) to form polyesters and hydrogen, and the reverse hydrogenation to regenerate EG; they also leverage promising recent catalysts designed by the Israeli group that function with formic acid (FA) as a hydrogen carrier. The project combines that group's progress with the U.S. team's solid-catalyst design and synthesis approaches, which enable a degree of preservation of organic-ligand control over catalysis typically associated with homogeneous systems, while offering advantages of solid catalysts that include ease of catalyst recovery and enhanced stability and activity. Achieving those goals involves fusing the tunability imparted by organic ligands in soluble catalysts to the decoupling of hydrogen storage and release enabled by solid catalysts. The collaboration relies on two emerging strategies for supporting molecular sites on a solid catalyst surface: (i) electrostatic anchoring on zeolite external-surface pockets and (ii) mechanical encapsulation of active sites on porous amorphous supports. These approaches enable control over the outer-sphere environment experienced by the catalyst, while inner-sphere control of the active site is achieved by anchoring well-defined metal complexes. The project will advance the state-of-the-art in several areas, including synthesis of SMCs, characterization via X-ray absorption spectroscopy, and molecular modeling. Beyond the technical aspects, the NSF project will provide mentorship opportunities for graduate and undergraduate students, with an emphasis on underrepresented groups in STEM. Members of the Katz lab will form teams in the programs BASIS and BEAM; these teams will volunteer to teach science lessons in local elementary and middle schools in economically-disadvantaged areas in the East Bay. In addition, the project will provide research opportunities for undergraduate students in underrepresented groups in STEM through the Cal NERDS programs UC LEADS and NSF CAMP, which seek to increase diversity within STEM, and stress participation from African American, LatinX, and Native American students. Furthermore, results from this research will be disseminated to a broad audience via integration into an elective course on catalyst design. The project is co-funded by the CBET Division Catalysis program and the Chemistry Division Chemical Catalysis program.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.