Storing intermittent renewable energy sources is key their continued widespread adoption. One potential use case for renewable sources is microgrids – an energy network serving a small community alone, or as a component of the broader energy grid. In this microgrid, one can envision a closed loop system wherin energy is stored and consumed on a seasonal basis. Liquid organic hydrogen carriers (LOHCs) are potential candidates for the medium in which energy is stored. In this system, the catalyst completing the storage and harvest must be stable, selective, and able to operate at mild conditions so as to keep capital and operating costs low. Additionally, since the overall time to store energy occurs over an entire season, they need not be particularly fast. Homogeneous catalysts like Ru(MACHO) are well suited to meet these requirements. In this talk, we use a combination of theory (density functional theory) and experiment to probe the mechanism for acceptorless dehydrogenative coupling (ADC) by Ru(MACHO) to evolve hydrogen from two LOHC candidates, ethanol and 1,4-butanediol (BDO). The mechanism for ADC of ethanol to form ethyl acetate is discussed and H<sub>2</sub> evolution is shown to be rate determining, both experimentally and theoretically. We focus also on the mechanism for deactivation, which via water binding and coupling to form an acetyl ligand which is strongly bound. We show that this pathway is energetically competitive with the favored ADC pathway, but leads to a deep energetic sink. To avoid this state, modifications to the catalyst are investigated but eventually show that water binding and subsequent deactivation will be competitive with ethanol ADC despite changes to the electronic structure, thus requiring engineering solutions to overcome this issue. Alternatively unimolecular ADC is proposed with BDO. The mechanism for this process is discerned. Finally the two mechanisms and their differences in rate and selectivity are discussed.

Acceptorless dehydrogenative coupling of ethanol and 1,4-butanediol by Ru(MACHO) for use in microgrid applications

ACS Fall 2021

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