The intermittency of renewable energy sources means that we need efficient technologies to store energy. One way to accomplish this is to use this energy to drive a reaction that affords a fuel such as H2. As this energy carrier is difficult to store at room temperature, researchers turn to latent H2 sources, such as methanol (CH3OH). Although CH3OH reacts with H2O to afford CO2 and the desired H2, this steam-reforming process requires high temperatures. By contrast, the reversible conversion of CH3OH and an amine to a formamide and H2 — termed ‘amine reforming’ — is a mild and practical means of H2 storage according to the group of Surya Prakash, who report their findings in the Journal of the American Chemical Society.

Credit: Rachael Tremlett/Macmillan Publishers Limited

Potentially, CH3OH/NH3 mixtures could reversibly store up to 9.5 wt% H2 while trapping carbon in the form of urea

Reversing steam reforming is problematic because the oxidized and dehydrogenated product of CH3OH (namely, CO2) is volatile and requires separation from H2. “An attractive alternative is to avoid the release of CO2 altogether,” states Prakash, a goal well met by their amine reforming reaction, in which a ruthenium catalyst dehydrogenates CH3OH to formaldehyde (CH2O), which then condenses with a secondary amine to form a carbinolamine. Subsequent dehydrogenation of this product affords a formamide — a non-volatile store of CH3OH-derived CO. The secondary diamine N,Nʹ-dimethylethylenediamine reacts reversibly with two equivalents of CH3OH to liberate a bis(formamide) and four equivalents of H2; the weight of H2 is 5.3% of the total. The conversion of ethylenediamine to the corresponding cyclic urea and H2 would seem a better choice (6.6 wt% H2), but the primary amines tested were found to react sluggishly.

Just as crucial as the choice of substrate is the type of catalyst used. The dehydrogenative coupling of CH3OH with N,Nʹ-dimethylethylenediamine proceeds smoothly (toluene, 5 mol% K3PO4, 120 °C, 90% yield) with the secondary amine complex [Ru{HN(CH2CH2PiPr2)2}H(CO)Cl]. The catalyst is bidirectional as it also hydrogenates the bis(formamide) back to the precursors (>95% yield). The mechanism involves interconversion of a hydridoruthenium amine with a ruthenium amide and H2; the complex of the methylated amine analogue, MeN(CH2CH2PiPr2)2, is inactive. Other catalysts mediate deleterious dehydrogenation of CH2O to CO, and the presence of the latter in the H2 produced — even in trace quantities — poisons H2 oxidation catalysts in fuel cells.

It is promising that a commercially available catalyst can reversibly dehydrocouple CH3OH and amines to give CO- and CO2-free H2. This study compares favourably with work in which elaborate catalysts mediated a similar reaction with CH3CH2OH, a heavier H2 carrier with a CH3 group that is inert under the reaction conditions. However, although Prakash's team can drive CH3OH dehydrocoupling in either direction by controlling H2 pressure, it needs heat and is slow in the absence of solvent, the weight of which (not to mention that of the catalyst) makes the total H2 weight percentage much lower than the US Department of Energy target (5.5 wt% H2 by 2020). Acknowledging this, Prakash is also looking for bidirectional catalysts for steam reforming under mild conditions. “Potentially, CH3OH/NH3 mixtures could reversibly store up to 9.5 wt% H2 while trapping carbon in the form of urea,” notes Prakash. Once developed, these technologies would boost the utility of a methanol economy, something that our environment will no doubt welcome.

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US Department of Energy Office of Energy Efficiency & Renewable Energy: https://energy.gov/eere/fuelcells/materials-based-hydrogen-storage