The quest for cleaner sources of energy has often turned towards efforts to harness the potential of hydrogen, one of the most abundant elements. Hydrogen has one of the highest energy densities by weight (33.3 kWh/kg) among all available fuel sources. But it is also one of the most difficult to tame.
Hydrogen does not occur independently. It occurs as part of compounds from which it can be chemically extracted in gaseous form. While the production of hydrogen from sources such as coal, oil, gas or water is highly expensive, storage and handling of the gas is also dangerous. It is easily flammable and can explode.
One way of getting around this is liquefaction. Liquid hydrogen can be easily managed and is already being used for a number of purposes, like as rocket fuel. The problem is that hydrogen liquefies at around -250°C, and it is often not possible or feasible to maintain such low temperatures. It also rules out the use of hydrogen for common uses like driving automobiles.
Another way of achieving the desired result is through the process of ‘hydrogenation’ and ‘dehydrogenation’ of a liquid compound. Hydrogen can be added to a liquid, through hydrogenation, and extracted when required, through the reverse process.
Among many liquid compounds used for this, ‘imines’ are a promising class containing unsaturated bonds between carbon and nitrogen. When hydrogenated, imine gets converted to an amine compound. This happens due to the conversion of a C=N double bond into a C-N single bond, and formation of new C-H and N-H bonds from the hydrogen molecule that is consumed during the process. In the reverse process, carbon and nitrogen get back to a double bond and hydrogen gets released which can then be burnt to produce energy. In this process, however, hydrogenation step takes place at high pressures, and the cycle needs two different catalysts.
We at IISER Bhopal have now managed to get a similar hydrogenation-dehydrogenation process done based on imine/amine compounds with a single ruthenium catalyst by varying the pH (an indicator of acidity) of the medium at near normal atmospheric pressures. The key to our breakthrough was the use of a special molecular design of the ruthenium catalyst in this process. Our ruthenium-based catalyst (a coordination compound), when exposed to hydrogen in basic pH, cleaves the H-H bond to let it form a hydride. This metal hydride interacts with the imine in a facile way to convert to amine even at atmospheric pressure and 40°C. Reverse dehydrogenation was achieved by the same catalyst in acidic pH at 100°C.
We published our finding recently in Angewandte Chemie, a weekly peer-reviewed scientific journal published by Wiley-VCH on behalf of the German Chemical Society (Angew. Chem. Int. Ed. 2017, 56, 5556–5560) and have now gone ahead to improve our results.
Using our newly developed catalyst, we are in the process of achieving a practical “chemical hydrogen-storage-and delivery-system” based on hydrogenation-dehydrogenation processes with multiple C=N containing organic liquid compounds that have high hydrogen storage capacity. Moreover, we are trying to hydrogenate carbon dioxide and convert it into formic acid, another liquid. Success with carbon dioxide can result in a variety of benefits.
Formic acid is a smaller compound and its hydrogen content per molecule is very high compared to more complex compounds like organic imines. The carbon dioxide formed upon dehydrogenation of formic acid can be recycled. Thus, harmful emissions can also be prevented.
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