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Ultraviolet light could provide a powerful new source of green fuel


A forest of gallium nitride nanowires harnesses energy in light to convert methanol to ethanol.

Sheng Chu/McGill University

By Robert F. Service

Methanol—a colorless liquid that can be made from agricultural waste—has long been touted as a green alternative to fossil fuels. But it’s toxic and only has half the energy as the same volume of gasoline. Now, researchers report they’ve created a potentially cheap way to use sunlight to convert methanol to ethanol, a more popular alternative fuel that’s less harmful and carries more energy.

The new report is “great work” says Zhongmin Liu, a chemist at the Dalian Institute of Chemical Physics in China who was not involved with the research. If the process can be optimized and scaled up, he says, “It has the potential to change the world.”

The notion of converting methanol to ethanol isn’t new. Companies already have a trio of chemical processes that do so. But these require adding heat, pressure, and toxic additives, such as carbon monoxide. Companies can also make ethanol directly by fermenting corn kernels or sugarcane. But growing those crops requires precious farmland that could otherwise grow food. Researchers and companies have also come up with ways to convert agricultural wastes into ethanol. So far, however, these have proved too costly to be competitive.

Chao-Jun Li, a chemist at McGill University in Montreal, Canada, thought there might be a better way. In 2014, he and his colleagues showed a tiny forest of nanowires made from the semiconductor gallium nitride (GaN) could act as a catalyst to convert methane gas into benzene, a commodity chemical used to make dozens of other industrial compounds such as plastics, solvents, and adhesives. Catalysts are compounds that encourage other chemicals to react but aren’t themselves used up in the job. As a result, they can do their job over and over. In this case, the nanowires rearranged chemical bonds between carbon atoms, which is also what’s needed to convert methanol to ethanol. So, the McGill researchers decided to see whether GaN nanowires could work their magic on methanol.

The researchers grew and tested several different nanowire compositions. As they report online last week in the journal Chem, they found a forest of long, thin GaN nanowires spiked with magnesium worked best to absorb ultraviolet (UV) light and use that energy to convert methanol to ethanol. The absorbed UV light caused the nanowires’ surfaces to become more negatively charged than their cores, the team found. The charge rips a water molecule from an individual methanol molecule sitting on a nanowire surface, leaving behind a reactive compound called methyl carbene. While the water molecule floats away, the carbene reacts with a neighboring methanol molecule to make ethanol.

Liu notes that the process is just a proof of concept. Converting methanol to ethanol requires UV light, which is just a sliver of the sunlight that reaches Earth. For the process to be economical, he says, it may require tweaking the nanowires to get them to work with visible light, which has less energy than UV light, but is far more abundant in the sun’s rays.

In addition to producing ethanol, the nanowire catalyst can also generate other valuable hydrocarbons like 1-propanol, an alcohol used in pharmaceutical manufacturing, from methanol, Li notes. This ability raises hopes for tailoring nanowires to convert low-value commodity chemicals into a range of higher value ones, using only light.

Source: Science Mag