The products of the dehydrogenation reaction—N-formamide and N,N′- diformamide—are hydrogenated back to the free amine and methanol by a simple hydrogen pressure swing. Both H₂ “loading” and “unloading” are performed in the presence of the same Ru-pincer catalysts.

As a hydrogen carrier, liquid organic hydrogen carriers (LOHC) have gained significant attention recently as they are safe to store and transport, have high wt % H₂ storage capacities and can offer fully reversible H₂ loading and unloading. They can also enable a relatively easy transition by allowing the utilization of existing fuel infrastructures. Formic acid (HCO₂H), over the years, has been explored thoroughly as a potential LOHC, and highly efficient catalysts for both H₂ loading and unloading have been designed by us and others.

However, a maximum H₂ storage of only 4.4 wt % is feasible in HCO₂H with the emission of stoichiometric amount of CO₂ for each H₂. Methanol (CH₃OH) is a good alternative because of its 12.6 wt % H₂ content, ease of handling and convenient production. Steam reforming of CH₃OH is generally the preferred method to obtain H₂ and is performed at high temperatures (240−260 °C) and high pressures over heterogeneous catalysts. Recently, it was discovered that the use of homogeneous catalysts, mainly Ru13 and Fe14 pincer complexes, could also enable aqueous CH₃OH dehydrogenation at much lower temperatures (< 100 °C). Strongly basic conditions are nevertheless required in most cases to achieve high TON. In addition, CO₂ reduction to CH₃OH has also been reported using similar pincer catalysts. However, to the best of our knowledge, aqueous reforming of CH₃OH and the reverse reaction (CO₂ hydrogenation to CH₃OH) in the presence of same homogeneous catalytic system has not yet been demonstrated.

—Kothandaraman et al.

The authors said that their process has three main advantages over traditional methanol steam reforming in the context of sustainable H₂ storage and transportation:

1. It is reversible in the presence of the same catalytic system;

2. The dehydrogenative coupling products formamide (or urea), unlike CO₂ from steam reforming, do not need to be recaptured as they remain in solution and are readily available for the subsequent H₂ loading step; and

3. Pure H₂ gas is produced, which can be potentially used in H₂/Air fuel cells without purification.

The USC process is essentially a carbon-neutral cycle—the carbon is trapped in the form of formamide (or urea in the case of primary amine). In theory, the team said, a hydrogen storage capacity as high as 6.6 wt % is achievable. Dehydrogenative coupling and the subsequent amide hydrogenation proceed with good yields (90% and >95% respectively, with methanol and N,N′-dimethylethylenedi- amine as dehydrogenative coupling partners).

The paper describing the method was the the last major paper co-authored by USC’s first Nobel laureate, the late George Olah.

“The Methanol Economy” is a concept that the Olah-Prakash team first began refining in the mid-1990s, right after Olah was awarded a Nobel Prize in Chemistry in 1194 for his contributions to carbocations—the name that Olah himself coined for ions that have a positively charged carbon atom.

According to Olah and Prakash, the goal of a methanol-based economy would be to develop renewable sources of energy, led by methanol, that could mitigate the problem of climate change caused by carbon emissions, as well as the US dependence on other countries for energy, particularly oil.

Prakash, Olah and their team have been focused on finding a way to extract hydrogen fuel from methanol in ways that are not only carbon-neutral, but can even be carbon-positive.

The research was supported by the USC Loker Hydrocarbon Research Institute.

Resources

• Jotheeswari Kothandaraman, Sayan Kar, Raktim Sen, Alain Goeppert, George A. Olah, and G. K. Surya Prakash (2017) “Efficient Reversible Hydrogen Carrier System Based on Amine Reforming of Methanol” Journal of the American Chemical Society 139 (7), 2549-2552 doi: 10.1021/jacs.6b11637