Methane is the world’s most abundant hydrocarbon. It’s the major component of natural gas and shale gas and, when burned, is an effective fuel. But it’s also a major contributor to climate change, with 24 times greater potency as a greenhouse gas than carbon dioxide.
With a new method, a research team of University of Pennsylvania and Michigan State University chemists has demonstrated the potential to use methane not as a fossil fuel but as a versatile chemical building block with which to make more complex molecules, such as pharmaceuticals and other value-added substances.
The study, published in Science, shows that the reaction also offers a way of taking advantage of the properties of methane without releasing greenhouse gases.
Methane is composed of a carbon atom bonded to four hydrogen atoms. When it is burned, all four of the carbon-hydrogen bonds are broken, resulting in the production of carbon dioxide and water, both of which are greenhouse gases.
“If only one or two hydrogen bonds could be broken efficiently, then it might be possible to connect carbon atoms from two or more methane molecules to make larger hydrocarbons,” said Milton Smith III, MSU chemist. “For example, gasoline is a mixture of hydrocarbons containing between four and 12 carbon atoms. The polyethylene used to make garbage bags and milk jugs is composed of millions of carbon atoms.”
Methane is so plentiful that excesses of the gas are commonly treated as waste and burned off. But is this the best use of this fuel?
“Finding ways to use methane besides burning it as a fuel constitutes a practical approach to using this abundant gas,” said Daniel Mindiola, senior author on the paper and a Presidential Professor in Penn’s Department of Chemistry in the School of Arts and Sciences. “Our method will hopefully provide inspiration to move away from burning our resources and instead using them more as a carbon building block to prepare more valuable materials.”
Mindiola collaborated on the work with Kyle Smith, a graduate student in Mindiola’s lab and the paper’s lead author; Simon Berritt, director of Penn’s High Throughput Screening Center based in the Department of Chemistry; Mariano González-Moreiras, a visiting scholar; Seihwan Ahn and Mu-Hyun Baik of Korea’s Advanced Institute of Science and Technology; and Smith, who, with MSU’s Rob Maleczka, first discovered the chemical reaction known as carbon-hydrogen borylation, upon which the current work builds.
Selectively controlling the carbon-hydrogen bonds has been difficult, however. Chemists have therefore considered methane relatively inert unless burned. In addition, because methane is a gas at ambient temperatures and pressures, it is not the easiest chemical to manipulate.
But Mindiola had a brainstorm: What if he tried a borylation reaction using methane while varying pressure conditions? Carbon-hydrogen borylation is a process developed by Smith and colleagues in which a hydrocarbon reacts with a boron-containing compound, catalyzed by a metal. The reaction results in the replacement of a carbon-hydrogen bond on the hydrocarbon with a carbon-boron bond. This bond can then later be easily swapped to bond the carbon to any number of other chemical groups. Though borylation was discovered more than a decade ago, no one had tried it using methane, the simplest of hydrocarbons.
The researchers decided to attempt this. Taking advantage of known conditions reported in the literature for other substrates, they determined the right combination of compounds and catalysts that might work, then used a computational approach to evaluate different conditions and reagents that might improve the reaction’s efficiency. Finally, they used Penn’s High Throughput Screening Center, one of only a handful of such facilities in the country, which allows for the testing of 96 different reactions at once, to identify the most efficient conditions for the reaction.
The petrochemical industry burns approximately $50 million of methane each year, in part due to a lack of storage capacity. Some methane is also used for steam reforming, a process that forms carbon monoxide and hydrogen that can be used in fuel cells or to make ammonia for fertilizers. But the researchers believe the borylation reaction can offer a meaningful alternative use for methane.
The study was supported by the University of Pennsylvania, the Ministry of Education of Spain, Korea’s Institute for Basic Science and the National Institutes of Health.