[ { "id": "thesis:18603", "collection": "thesis", "collection_id": "18603", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05202026-000057271", "type": "thesis", "title": "Development and Exploration of Electrochemical Cascades for Titanium-Mediated Nitrogen Reduction to Ammonia", "author": [ { "family_name": "Klein", "given_name": "Channing K.", "orcid": "0000-0002-1593-3896", "clpid": "Klein-Channing-K" } ], "thesis_advisor": [ { "family_name": "Manthiram", "given_name": "Karthish", "orcid": "0000-0001-9260-3391", "clpid": "Manthiram-Karthish" } ], "thesis_committee": [ { "family_name": "Wang", "given_name": "Zhen-Gang", "orcid": "0000-0002-3361-6114", "clpid": "Wang-Zhen-Gang" }, { "family_name": "Greer", "given_name": "Julia R.", "orcid": "0000-0002-9675-1508", "clpid": "Greer-J-R" }, { "family_name": "See", "given_name": "Kimberly", "orcid": "0000-0002-0133-9693", "clpid": "See-Kimberly" }, { "family_name": "Manthiram", "given_name": "Karthish", "orcid": "0000-0001-9260-3391", "clpid": "Manthiram-Karthish" } ], "local_group": [ { "literal": "div_chem" } ], "abstract": "The Haber-Bosch process, invented at the turn of the 19th century, revolutionized agriculture by providing a way to make ammonia\u2014an important component of fertilizer\u2014from nitrogen and hydrogen. It's been estimated that without artificial ammonia synthesis, up to half the population would starve. However, this miracle chemistry comes with costly downsides, responsible for the consumption of an estimated 2% of global energy and for up to 1.6% of global carbon dioxide emissions. Therefore, the development of new methods of ammonia synthesis that are compatible with a greener society is a task of paramount importance. In this thesis, one such method for doing so\u2014an electrochemical cascade reaction involving a plated alkali metal and a homogeneous titanium compound\u2014is developed and explored. Electrochemically-generated sodium naphthalenide in conjunction with titanium isopropoxide is shown to enable a high rate of ammonia synthesis, but low selectivity. By contrast, potassium metal with titanium isopropoxide allows for much higher selectivity at the cost of rate. It is found that the rate-determining step of these reactions is usually electrochemical reductant generation, a powerful discovery that allows for precise tuning of rate via changing applied current density. This spatially-decoupled reaction paradigm is also applied to other alkali metal and transition metal compounds in order to open up a wider chemical space. While much remains unknown about the mechanism of this system, the novel reactivity demonstrated in this work will have immense impacts on the field of sustainable ammonia synthesis.", "doi": "10.7907/vs6d-n280", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" }, { "id": "thesis:18630", "collection": "thesis", "collection_id": "18630", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05262026-184400266", "type": "thesis", "title": "Electrochemical Carboxylation of Aldehydes with CO\u2082: Mechanistic Insights and Enantioselective Synthesis", "author": [ { "family_name": "Ton", "given_name": "Thu Nu Minh", "orcid": "0000-0003-0134-2435", "clpid": "Ton-Thu-Nu-Minh" } ], "thesis_advisor": [ { "family_name": "Manthiram", "given_name": "Karthish", "orcid": "0000-0001-9260-3391", "clpid": "Manthiram-Karthish" } ], "thesis_committee": [ { "family_name": "See", "given_name": "Kimberly", "orcid": "0000-0002-0133-9693", "clpid": "See-Kimberly" }, { "family_name": "Stoltz", "given_name": "Brian M.", "orcid": "0000-0001-9837-1528", "clpid": "Stoltz-B-M" }, { "family_name": "Cushing", "given_name": "Scott K.", "orcid": "0000-0003-3538-2259", "clpid": "Cushing-Scott-K" }, { "family_name": "Manthiram", "given_name": "Karthish", "orcid": "0000-0001-9260-3391", "clpid": "Manthiram-Karthish" } ], "local_group": [ { "literal": "div_chem" } ], "abstract": "

The electrochemical carboxylation of organic substrates with CO\u2082 is a promising strategy for the sustainable synthesis of value-added carboxylic acids, offering mild operating conditions and avoiding the need for highly reactive organometallic reagents. As the electrical grid transitions toward renewable energy sources, electrochemical activation of CO\u2082 is poised to become an increasingly carbon-negative process, further improving the sustainability of chemical manufacturing. Despite these advantages, the scope of electrochemical carboxylation remains limited, and a deeper mechanistic understanding is needed to guide rational improvements in selectivity and efficiency. This thesis addresses these gaps by investigating the electrochemical carboxylation of aldehydes, an underexplored but industrially relevant substrate class, and demonstrating for the first time that this transformation can be performed enantioselectively.

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For aromatic aldehydes, electroanalytical and spectroscopic studies support a mechanism in which a substrate-derived ketyl radical couples with CO\u2082 to form the \u03b1-hydroxy carboxylic acid product. EPR spectroscopy provided direct evidence for formation of the ketyl radical intermediate under electrolytic conditions, and kinetic studies identified the coupling step as the rate-determining step. Lewis acidic additives such as MgCl2 were found to play a key role in promoting ketyl radical formation. Extending this chemistry to aliphatic aldehydes revealed that their more negative reduction potentials open an alternative CO\u2082 reduction pathway towards electrochemical carboxylation. However, it was also found that polymerization of the substrate under reductive conditions limits carboxylation selectivity, prompting future works to understand and develop strategies towards mitigating the polymerization pathway.

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Building on this mechanistic foundation, the first enantioselective electrochemical carboxylation of an aldehyde was demonstrated using chiral metal salen complexes as homogeneous catalysts, affording optically active mandelic acid with up to 43% enantiomeric excess. Both the applied potential and metal center identity were found to strongly influence enantioselectivity, pointing to an electronic basis for stereocontrol. Cyclic voltammetry studies further suggest that the most effective catalysts readily undergo reductive activation under the applied conditions, enabling coordination with CO\u2082 to initiate the enantioselective pathway. Taken together, the insights developed in this thesis provide a foundation for the rational design of more selective, efficient, and enantiocontrolled electrochemical CO\u2082 incorporation strategies.

", "doi": "10.7907/5f9j-mn04", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" } ]