Abstract:  Hydrogen (H2) is considered a promising energy carrier due to its high energy density of 33.3 kWh/kg and its potential applications in combustion engines and fuel cells. However, efficient methods for H2 storage and transportation remain critical for its widespread use. Formic acid (FA), with a volumetric hydrogen capacity of 53 g H2/L, is a viable liquid organic hydrogen carrier (LOHC). FA can be easily stored in vehicles and catalytically decomposed into H2 and carbon dioxide (CO2), making it ideal for H2 storage and utilization in automotive applications.

The objective of this dissertation is to develop a catalytic system that functions under mild conditions, selectively facilitates the dehydrogenation of FA, exhibits both high catalytic activity and stability, as indicated by high turnover numbers (TON) and is compatible with hydrogen fuel cell integration. State-of-the-art homogeneous catalysts for FA dehydrogenation typically rely on the presence of a base and solvent. However, the efficiency of the process was compromised by the inclusion of solvents and other volatile substances, which were prone to evaporation.

Thus, achieving optimal efficiency requires conducting the reaction without solvents or volatile additives. In this context, FA dehydrogenation performed under neat conditions would be advantageous for achieving a higher H2 carrying capacity. In this dissertation, a series of iridium complexes were introduced featuring an azocarboxamide ligand backbone, which efficiently decompose FA into H2 and CO2 without the need for solvents or volatile additives. Under optimized conditions, a turnover frequency (TOF) of 12500 h−1 was achieved, with no detectable CO formation. In addition, the high catalytic efficiency of a non-symmetrical ruthenium pincer system was investigated for the dehydrogenation of FA under mild conditions. This innovative catalytic system benefits from the synergistic interaction between non-symmetrical Ru-PN3P and sodium formate (SF), enabling FA dehydrogenation with an average TOF reaching up to 51400 h -1 under heat-integrated conditions suitable for proton-exchange membrane fuel cell applications (<100 °C). Over a period of 36 days, the Ru−PN3P/SF system successfully converted 1.2 L of FA, achieving a TON upto 32 million.

Carbon dioxide (CO2) represents a plentiful, sustainable, and economically viable C1 feedstock with considerable promise in synthetic organic chemistry. Its utilization as a renewable resource aligns with efforts to mitigate climate change and promote a circular carbon economy. However, the inherent thermodynamic stability and kinetic inertness of CO2 present significant obstacles, necessitating the development of advanced catalytic strategies. Recent progress in reductive functionalization has enabled the efficient transformation of CO2 into high-value products, such as formamides, under mild reaction conditions. These advancements underscore the pivotal role of CO2 in advancing sustainable and innovative chemical processes. The additive-free N-formylation of amines through CO2 hydrogenation is a significant method in organic synthesis. We introduced a ruthenium pincer complex that efficiently facilitates the selective conversion of various amines into formamides under additive-free conditions, achieving an exceptional turnover number (TON) of 980,000 in a single batch. Mechanistic studies indicate that the reaction pathway involves the initial reduction of CO2 to ammonium formate, followed by its dehydration to formamide. NMR experiments identified potential intermediates and revealed the critical role of metal-ligand cooperativity in activating the substrate.

Additionally, this approach was explored as a means to establish a carbon-neutral hydrogen storage cycle, utilizing the reversible conversion between formamides and their precursors. By integrating insights from these catalytic systems, this dissertation demonstrates the compatibility of FA dehydrogenation with CO2 utilization processes. This integration not only supports renewable hydrogen production but also contributes to global efforts aimed at reducing greenhouse gas emissions. The findings of this study, in conjunction with recent advancements in iridium- and ruthenium-based catalysis, provide a solid foundation for the development of scalable, carbon-neutral hydrogen technologies utilizing formic acid