Natural gas is great at heating our houses, but it’s not so good at fueling our cars—at least not yet. Researchers in the United States have discovered a new and more efficient method for converting the main components in natural gas into liquids that can be further refined into either common commodity chemicals or fuels. The work opens the door to displacing oil with abundant natural gas—and reducing both carbon emissions and society’s dependence on petroleum in the process.
Over the past several years, the United States and other countries have undergone an energy revolution as new drilling techniques and a process called hydraulic fracturing have made it possible to recover vast amounts of natural gas. Today, most of that gas is burned, either for heating homes or to drive electricity-generating turbines. But chemical companies have also long had the technology to convert the primary hydrocarbons in natural gas—methane, ethane, and propane—into alcohols, the liquid starting materials for plastics, fuels, and other commodities made by the train load. However, this technology has never been adopted on a wide scale, because it requires complex and expensive chemical plants that must run at temperatures greater than 800°C in order to carry out the transformation. Converting petroleum into those commodities has always been cheaper, which is why we’ve grown so dependent on oil.
Two decades ago, Roy Periana, a chemist at the Scripps Research Institute in Jupiter, Florida, started looking for metal catalysts that could transform natural gas into alcohols at lower temperatures. He knew he needed to find metals that were deft at breaking the carbon-hydrogen bonds that are at the heart of methane, ethane, and propane, short hydrocarbons known as alkanes, and then add in oxygen atoms that would transform the alkanes into alcohols. But all the catalysts he discovered—including platinum, rhodium, and iridium—are rare and expensive, and the technique was never commercialized.
Periana says that what he didn’t appreciate at the time was that to be a good catalyst, the metals need to do another job in addition to transforming C-H bonds into C-O bonds. That’s because in a reactor, these catalysts are surrounded by solvent molecules. So before a metal can break an alkane’s bond, the alkane must first nudge a solvent molecule aside. It turns out that the expensive metals Periana was using aren’t so good at that part of the process: They require extra energy to push the solvent molecules out of their midst. Periana’s team realized that the different electronic structure of more abundant “main group” metals means that they wouldn’t have to pay this energetic price, and, therefore, might be able to carry out the C-H to C-O transformation more efficiently.
It worked better than he expected, Periana says. When he and his colleagues at Scripps and Brigham Young University ran a methane reaction with thallium—a main group metal—alkanes pushed the solvent molecules aside 22 orders of magnitude faster than when the reaction was run with iridium, reducing the overall energy required by about one-third, they report online today in Science. The success brought other benefits as well. The reaction runs at 180°C, and works on all alkanes at the same time, unlike the conventional natural gas conversion technology that works on only one species of alkane at a time. That could make it far easier, and thus potentially cheaper, to build chemical plants to convert natural gas to liquids using the new approach.
“This is a highly novel piece of work that opens the way to upgrading of natural gas to useful chemicals with simple materials and moderate conditions,” says Robert Crabtree, a chemist at Yale University. But that way is not entirely clear yet, Periana cautions. For now, the chemistry works one batch at a time. To succeed as an industrial technology, researchers must work out the conditions to get it to work on a continuous basis, he says. If they do, it may one day make it cheaper to derive commodity chemicals and fuels from natural gas than from petroleum. And that would be an energy revolution indeed.