Researchers led by Matthias Beller at the Leibniz Institute for Catalysis in Rostock (Germany) report in the journal Angewandte Chemie on a new catalyst that allows for the use of bioalcohols, including ethanol, for the production of hydrogen. Their novel process proceeds efficiently under particularly mild conditions.
Ethanol and other alcohols do not willingly give up their hydrogen atoms; this type of reaction requires highly active catalysts. Previous catalytic processes require drastic reaction conditions: temperatures above 200 °C and the presence of strong bases. The Rostock researchers thus aimed to develop a catalyst that would also work efficiently at significantly milder temperatures.
The new catalyst demonstrates previously unachievable high efficiency in the extraction of hydrogen from alcohols under mild reaction conditions.
This is the first catalytic system that is capable of obtaining hydrogen from readily available ethanol at temperatures under 100 °C without the use of bases or other additives.—Matthias Beller
After initial successful tests with a relatively easily converted model alcohol (isopropanol), the researchers turned their attention to ethanol. Ethanol has taken on increasing importance as a renewable resource but is significantly harder to convert.
Employing ethanol instead of isopropyl alcohol leads to a TOF(2 h) of 1483 h-1...which is more than a sevenfold improvement compared to the previous state-of-the-art. Furthermore, no base is needed and much milder reaction conditions are employed. For the first time it is possible to dehydrogenate the thermodynamically less-favorable primary aliphatic alcohols below 100 °C efficiently.—Nielsen et al.
The active catalyst consists of a ruthenium complex that is formed in situ. The starting point is a central ruthenium atom that is surrounded by a special ligand that grasps it from three sides. The other ligands are a carbon monoxide molecule and two hydrogen atoms.
Upon heating, a hydrogen molecule (H2) is released from the complex. When the remaining complex comes into contact with ethanol or isopropanol it grabs two replacement hydrogen atoms, allowing the cycle to begin again.
Martin Nielsen, Anja Kammer, Daniela Cozzula, Henrik Junge, Serafino Gladiali, and Matthias Beller (2011) Efficient Hydrogen Production from Alcohols under Mild Reaction Conditions. Angewandte Chemie International Edition 50, No. 41, 9593–9597, DOI: 10.1002/anie.201104722