Researchers have developed a novel dual hydrogen atom transfer catalyst system allowing the complete dehydrogenation of cycloalkanes at room temperature under visible light irradiation, showcasing potential for efficient hydrogen storage and transport via liquid organic hydrogen carriers.
The advancement of effective hydrogen liberation technology from cycloalkanes, particularly through catalytic acceptorless dehydrogenation, is becoming increasingly significant for realizing sustainable hydrogen economies. The recent research published on January 23, 2025, demonstrates the first successful achievement of complete dehydrogenation of methylcyclohexane and cyclohexane to their aromatic equivalents without requiring hydrogen acceptors.
This innovative approach was led by researchers from The University of Tokyo, including R.A. Jagtap and Y. Nishioka, alongside collaborators supported by various grants for green catalysis research. Their work presents a method for overcoming traditional limitations associated with dehydrogenation—the release of hydrogen from stable cycloalkanes requires considerable heat and complex infrastructure.
Historically, liquid organic hydrogen carriers (LOHCs) have provided safe and effective means for hydrogen storage. Common LOHCs such as cyclohexane and methylcyclohexane can achieve impressive hydrogen storage capacities yet pose challenges for efficient, selective dehydrogenation processes. The sp3 C–H bonds found within these molecules are particularly stable, requiring significant energy input to break and release hydrogen, typically demanding high temperatures exceeding 150 °C and leading to efficiency concerns for practical applications.
Prior studies primarily relied on expensive metal catalysts under elevated temperatures, often resulting in low selectivity and hindered catalytic performance. The team’s innovation resides within the use of dual hydrogen atom transfer (HAT) catalysts, namely tetrabutylammonium chloride (TBACl) and thiophosphoric acid (TPA), which facilitate light-driven dehydrogenation reactions under conveniently mild conditions, effectively retaining high yield and selectivity.
By employing visible light irradiation, the researchers managed to achieve effective cleavage of stable C–H bonds without raising the reaction temperatures. Initially, chlorinated radicals derived from TBACl were shown to efficiently abstract hydrogen from cycloalkanes, creating reactive alkyl intermediates. Simultaneously, the TPA catalyst promoted the subsequent conversions, leading toward the formation of aromatic compounds and doubling the overall hydrogen yield.
This methodology has not only ushered in the first homogeneous complete catalytic acceptorless dehydrogenation of cyclohexane derivatives but has also demonstrated the capability for scalability and integration within existing hydrogen transport frameworks. During various experimental trials, the combination of TBACl and TPA produced yields as high as 68% for benzene from cyclohexane, with additional promising results for cycloalkane derivatives.
To optimize yields, the research team explored continuous-flow methods, enhancing reaction efficiency through improved light exposure and hydrogen gas management. The findings validate the practicality of implementing these methodologies for future applications.
Looking forward, the synergistic effect of employing two different HAT catalysts highlights exciting opportunities for future studies on reaction mechanisms and efficiencies. This work contributes to the evolution of cleaner, safer hydrogen storage technologies necessary for addressing global energy needs.
Arthur E. Fleischer, one of the study's contributors, remarked: "We have achieved the first homogeneous complete CAD of promising LOHC candidates, using visible light irradiation at room temperature." The team concludes with optimism, emphasizing the potential for this dual catalyst system to advance hydrogen technology significantly.