Methyl formate as a hydrogen energy carrier

The global shift towards renewable energy sources is crucial for a sustainable future. Hydrogen is a promising energy carrier, but its storage and transportation present challenges due to its low volumetric energy density, flammability, and explosive nature. Ideal chemical hydrogen sources require high gravimetric and volumetric energy density, low toxicity, compatibility with existing infrastructure, ease of handling and transportation, and cost-effective storage and release. Current carriers like methanol and ammonia have high hydrogen content but are toxic and flammable. Liquid organic hydrogen carriers (LOHCs) offer ease of handling but have lower hydrogen density and potential toxicity issues. Formic acid has favorable thermodynamics and easy dehydrogenation but low hydrogen content and corrosiveness. Methyl formate (MF), surprisingly overlooked, offers a compelling alternative. It possesses a hydrogen storage capacity between methanol and formic acid, comparable to LOHCs, and its volumetric energy density is equivalent to pressurized hydrogen at 1200 bar. Importantly, MF's dehydrogenation is thermodynamically favored, it is non-toxic, non-irritating, and non-corrosive, and it benefits from existing industrial production infrastructure. This study investigates MF as a viable hydrogen carrier, exploring its dehydrogenation and the underlying catalytic mechanism.

The literature review section extensively cites previous research on various hydrogen storage materials, including methanol, ammonia, liquid organic hydrogen carriers (LOHCs), and formic acid. It highlights the advantages and disadvantages of each, emphasizing the need for a new hydrogen vector that overcomes the limitations of existing options. The authors note the existing industrial production of MF, highlighting its potential for sustainable synthesis via CO2 hydrogenation. However, a notable gap in the literature is the absence of prior research on MF as a hydrogen carrier or its dehydrogenation, making this study a pioneering effort.

The study investigated the catalytic dehydrogenation of MF using various ruthenium pincer catalysts. Initial screening identified several complexes (C1-C5) exhibiting high hydrogen turnover numbers (TONs) and turnover frequencies (TOFs), with CO content below 10 ppm. Complex C2 was selected for optimization studies, varying parameters such as the amount of water, solvent, base type and concentration, and reaction temperature. The optimal conditions were determined through systematic experimentation detailed in supplementary tables. The rate of MF dehydrogenation was compared with methanol and formic acid under identical mild conditions, demonstrating a significantly faster initial gas evolution rate for MF. Mechanistic insights were gained through kinetic isotope effect (KIE) measurements using D2O and DCOOCH3, revealing that the formyl C-H group in MF is more easily activated than C-H bonds in other hydrogen carriers. Density functional theory (DFT) calculations were employed to elucidate the reaction mechanism, proposing a direct MF dehydrogenation pathway with a lower energy barrier than the hydrolysis pathway. DFT calculations were validated by X-ray crystallography analysis of an intermediate complex and comprehensive NMR studies using 13C-labelled MF, which detected key intermediates consistent with the proposed mechanism. A time-resolved analysis tracked the concentration changes of reactants and products throughout the reaction. The catalyst system's stability was tested through consecutive runs of MF dehydrogenation, demonstrating its capacity for prolonged use with high TON and TOF. Solvent-free dehydrogenation was also explored, achieving high pressures of hydrogen generation.

The key findings highlight the superior performance of MF as a hydrogen carrier compared to established alternatives. The optimized Ru-pincer catalyst system achieved remarkably high activity, with a maximum turnover frequency (TOFmax) exceeding 44,000 h−1 and a turnover number (TON) exceeding 100,000. The dehydrogenation reaction exhibited high selectivity, with undetectable levels of CO in the produced gas. The initial gas evolution rate from MF was five times faster than formic acid and twenty times faster than methanol under identical mild conditions. Mechanistic studies, encompassing KIE measurements, DFT calculations, X-ray crystallography, and NMR spectroscopy, consistently supported a direct dehydrogenation pathway for MF, providing a detailed understanding of the catalytic cycle. The catalyst demonstrated exceptional stability, maintaining high activity over numerous consecutive runs. Solvent-free dehydrogenation further underscored the practicality of this approach.

The findings directly address the research question of identifying and characterizing a superior hydrogen storage material. MF's exceptional performance, particularly its speed of hydrogen release, high activity and selectivity of the catalyst, and stability over multiple cycles, makes it a compelling alternative to existing technologies. The detailed mechanistic understanding gained through multiple techniques enhances the potential for future catalyst optimization and process improvement. The success of solvent-free dehydrogenation significantly improves the feasibility of practical implementation. This work opens exciting avenues for sustainable hydrogen production and storage, contributing significantly to the development of a carbon-neutral energy system.

This research successfully establishes methyl formate (MF) as a promising hydrogen storage material. The remarkably high activity and selectivity of the developed Ru-pincer catalyst system, combined with the favorable properties of MF, demonstrate its significant potential for application in hydrogen storage and release. The detailed mechanistic understanding lays the groundwork for future optimization efforts, potentially leading to even higher efficiency and broader applicability. Future research could focus on exploring alternative catalyst systems, investigating different reaction conditions, and scaling up the process for industrial applications.

While the study demonstrates exceptional performance under optimized conditions, further investigation is needed to assess the long-term stability and durability of the catalyst under various operating conditions. The economic viability of large-scale MF production and hydrogen generation using this method needs to be assessed in detail, considering factors such as catalyst cost, energy consumption, and infrastructure requirements. The current study primarily focuses on laboratory-scale experiments, so upscaling and process engineering considerations require additional research.