Science: Molecules Offer Cleaner, Milder Way to Break Down Natural Gas

Scientists have discovered inexpensive materials that can convert natural gas into useful chemicals under mild conditions, a new study in the 14 March issue of the journal Science reports. This approach may eventually compete with technologies for generating the same chemicals from petroleum, a fossil fuel that emits a lot of carbon dioxide into the atmosphere when it burns.

Like petroleum, natural gas is a fossil fuel that can be refined and separated into a number of widely-used consumer products including paints, fuel and fertilizer. "Almost anything that can be made from petroleum can be made from natural gas," explained Science co-author and Scripps Research Institute scientist Roy Periana.

Natural gas is a cleaner fuel than petroleum, however, emitting no soot or ash and less carbon dioxide when burned. In a recent Science Live Chat, Environmental Defense Fund Chief Scientist Steve Hamburg emphasized the benefit of burning natural gas over petroleum. "Compared to other fossil fuels, you get a lot more work per molecule of carbon dioxide emitted into the air with natural gas," he said. "This is a real plus, because if we have any hope of reducing climate change, we have to quickly, radically reduce the carbon emissions from our economy, and natural gas is an important way to do this from the fossil fuels side."

Despite the cleaner status of natural gas and its availability, the current technology for converting it into chemicals and fuels is very expensive. Thus, the United States and many other countries rely more heavily on petroleum. It also requires multiple steps and temperatures in excess of 800 degrees Celsius. "If we could utilize lower temperatures and create technology that generates these products in one step, not only would we save billions of dollars but we would also reduce the carbon footprint of these essential processes," said Periana. "It could change the petrochemical industry."

Unfortunately, the processes to convert natural gases in this way haven't been developed. "This is considered one of the holy grails of chemistry," Periana explained.

Now, Periana and colleagues have identified new materials that can successfully break down natural gas into its constituent goods at lower temperatures and with as few process steps as possible.

 
Roy Periana describes a pathway for cleaner natural gas products. | Video courtesy of The Scripps Energy and Materials Center (SEMC)

Natural gas is composed of three alkanes—methane, ethane and propane. These materials are composed of carbon and hydrogen atoms held together by some of the strongest bonds in chemistry, which prevent reactivity with other materials and require high temperatures to break apart.

Current technology also uses solids, which are mixed with the alkanes in acid solution, to facilitate the production of commodity chemicals. "Even though these materials have been around since the nineteenth century," Periana said, "we still can't engineer them to allow reactions at lower temperatures, or with high specificity for selected products like alcohols."

Periana and his colleagues believed the solution lay in molecules, which, unlike solids, can be modified with almost complete control. Scientists also have the tools to comprehensively understand molecular reactivity with other materials. In the 1990s, the researchers were successful in developing molecules that allowed reactions with alkanes at low temperatures. "But unfortunately," Periana explained, "these systems were based on precious metals such as platinum, palladium and gold, materials that are impractical for large-scale industrial processes."

Now, the team has found that other, more common materials called main group compounds can be used to break down natural gas. These include lead and thallium salts. Scientists hadn't focused on these compounds before because of questions about their reactivity. "The idea of working on these main group compounds was thrown out before it could be fully investigated," said Science co-author Brian Hashiguchi, also of Scripps.

But Hashiguchi, Periana and the rest of the team gave main group compounds another look and showed they worked to convert natural gas. "The advantage of these materials," said Periana, "is that they can react with the alkanes at temperatures below 200 degrees Celsius and importantly, they lead directly to alcohol products with very high specificity."

"What's also exciting is that these materials have never been used in this way before," Periana continued. "We expect that other non-precious metals could be utilized for these conversions."

Periana noted that it's important to understand that the systems they report in Science are not yet ready for commercialization. "However, because of the simplicity, the fast reactions and the high selectivity, we are optimistic that through further research, these could be utilized in practical applications."