Penn Researchers Develop New Method for Clean and Efficient Usage of Methane

Methane is key component of shale and natural gas, and it is the most abundant hydrocarbon in the world. When methane is burned, it serves as an effective fuel. However, methane is a powerful greenhouse gas, and contributes to climate change. In fact, it has 24 times greater potency than that of carbon dioxide.

Researchers, headed by chemists at the University of Pennsylvania, have developed a new method that shows how methane can be effectively used as a versatile chemical building block, and not as a fossil fuel, to create complex molecules, like value-added substances and pharmaceuticals. The unique reaction also provides a new way to manipulate methane’s properties without emitting any greenhouse gases. The results of the study have been reported in the Science journal.

Finding ways to use methane besides burning it as a fuel constitutes a practical approach to using this abundant gas. 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.

Daniel J. Mindiola, Presidential Professor, Department of Chemistry in the School of Arts & Sciences, Penn University

For the study, Mindiola partnered with Kyle T. Smith the paper’s lead author and a graduate student in Mindiola’s lab; Mariano González-Moreiras, a visiting scholar; Simon Berritt, director of Penn’s High Throughput Screening Center based in the Department of Chemistry; Milton R. Smith III, a professor at Michigan State University; and Mu-Hyun Baik and Seihwan Ahn of Korea’s Advanced Institute of Science and Technology. Smith III along with Robert Maleczka, initially figured out the chemical reaction called carbon-hydrogen borylation on which the current study is based upon.

Methane contains a single carbon atom, which is bound to four hydrogen atoms. On burning, all the four carbon-hydrogen bonds break down, leading to the formation of water and carbon dioxide, which is a greenhouse gas.

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. 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.

Milton R. Smith III, Professor, Michigan State University

It has been quite difficult to selectively control the carbon-hydrogen bonds. As a result, chemists believed that methane remains relatively inert when it is not burned. Since methane also exists as a gas at ambient pressures and temperatures, it cannot be manipulated easily.

However, Mindiola came up with a new idea where methane could possibly be used to perform the borylation reaction while changing the pressure conditions. Smith and colleagues developed the carbon-hydrogen borylation process, where a single boron-containing compound reacts with a single hydrocarbon in the presence of a metal as a catalyst. In this reaction, a carbon-hydrogen bond on the hydrocarbon is substituted with a carbon-boron bond. This carbon-boron bond can be exchanged to bond the carbon with other chemical groups. While borylation was discovered years ago, no one has attempted it with methane.

The Penn researchers decided to try this reaction. For this, they leveraged the known conditions that were described in the literature for other types of substrates, and eventually established the right ratio of catalysts and compounds that could possibly work. A computational approach was then used to assess varied reagents and conditions that could enhance the efficiency of the reaction. To execute the experiment, the team used Penn’s High Throughput Screening Center to find out optimized conditions for the reaction. The Penn facility is one of a few facilities available in the country. It is equipped for testing 96 different types of reactions directly, and reactions can be performed under high-pressure conditions. This allowed the research team to utilize gaseous methane, rather than working under ambient conditions.

Using iridium as a catalyst, an optimum reaction was carried out under mild conditions of 150^oC and 500 pound per square inch of methane. This resulted in 52% of borylated methane, with greater selectivity for the carbon-hydrogen borylation of a single C-H bond relative to multiple bonds.

It turns out methane is not as inert as one would have expected. We were able to borylate it using off-the-shelf reagents, which is very convenient.

Daniel J. Mindiola, Presidential Professor, Department of Chemistry in the School of Arts & Sciences, Penn University

The researchers are now assessing other types of reagents to find alternative catalysts and perform an analogous reaction. This is because commercially available iridium is not only rare, but expensive. Cobalt however,  presents a potential alternative. The team is also analyzing silicon compounds as an alternative option to those comprising boron, which is also a rare element.

The petrochemical sector releases more than $50 million of methane each year in the form of gas flares, making methane an abundantly available hydrocarbon. This is partly due to lack of storage capacity. Some methane is utilized for steam reforming, a process that produces hydrogen and carbon monoxide, which can be utilized in fuel cells or can be used for producing ammonia for fertilizers. Nevertheless, the borylation reaction can provide an alternative usage of methane, according to the researchers.

I think this work is going to inspire a lot of chemistry and get people thinking about methane in a different way. That doesn’t mean that the natural gas industry is going to borylate all the methane they’re extracting — there is a lot out there and boron is rare — but it’s another valuable option.

Daniel J. Mindiola, Presidential Professor, Department of Chemistry in the School of Arts & Sciences, Penn University

Mindiola also observed that the new study complements another work that was reported in the same issue of Science, headed by Melanie Sanford of the University of Michigan. That study discovered a way to perform selective borylation of the carbon-hydrogen bonds in methane, borylating one or two bonds and expanding this technique to ethane, which happens to be the second most abundant hydrocarbon.

Implementation of both set of reactions could provide a practical approach for using methane in the pharmaceutical industry, and also in other industries to make designer molecules that could have a variety of applications.

The University of Pennsylvania, Korea’s Institute for Basic Science, the Ministry of Education of Spain, and the National Institutes of Health supported the study.