A Strong Hint on Building Synthetic Hydrocarbons

Colleagues at Northwestern University and the University of Virginia are working to invent novel ways and catalytic materials to activate methane to produce ethylene.  The collaborating team published a paper in the online edition of the journal Nature Chemistry detailing the use of sulfur as a possible “soft” oxidant for catalytically converting methane into ethylene, a key “intermediate” for making chemicals, polymers, fuels and, ultimately, products such as films, surfactants, detergents, antifreeze, textiles and others.

Matthew Neurock, a chemical engineering professor in the University of Virginia’s School of Engineering and Applied Science explains, “. . . petroleum, in addition to being used to make fuels, is also used to make ethylene, propylene and other building blocks used in the production of a wide range of other chemicals. We need to develop innovative processes that can readily make these chemical intermediates from natural gas.”

The problem currently is there are no cost-effective ways to do this. Methane, the principal component of natural gas, is chemically quite stable and requires high temperatures to activate its strong chemical bonds; therefore the practical and successful conversion of methane to useful chemical intermediates has thus far eluded chemists and engineers.

Today the U.S. is tapping into its natural gas reserves with new recovery technology and producing enough to support most of its current needs for heating and power generation, and is beginning to export natural gas to other countries.  The trend is expected to continue, as new methods are developed to extract natural gas from vast unrecovered reserves embedded in shale. Natural gas can be used to generate electricity, and it burns cleaner than coal.  But one unexploited value is building larger molecules.

Chemists and engineers have diligently tried to develop catalysts and catalytic processes that use oxygen to make ethylene, methanol and other intermediates, but have had little success as oxygen is too reactive and tends to over-oxidize methane to common carbon dioxide.

Neurock starts by explaining the team’s work, “We show, through both theory using quantum mechanical calculations and laboratory experiments, that sulfur can be used together with novel sulfide catalysts to convert methane to ethylene, an important intermediate in the production of a wide range of materials.”

Neurock said that sulfur or other “softer” oxidants that have weaker affinities for hydrogen may be the answer, in that they can help to limit the over-reaction of methane to carbon disulfide. In the team’s process, methane is reacted with sulfur over sulfide catalysts used in petroleum processes. Sulfur is used to remove hydrogen from the methane to form hydrocarbon fragments, which subsequently react together on the catalyst to form ethylene.

Theoretical and experimental results indicate that how strong the sulfur bonds to the catalyst control the conversion of methane and the selectivity to produce ethylene. Using these concepts, the team explored different metal sulfide catalysts to ultimately tune the metal-sulfur bond strength in order to control the conversion of methane to ethylene.

Chemical companies consider methane a particularly attractive raw material because of the large reserves of natural gas in the U.S. and other parts of the world.

Back in 2007 Dow Chemical Company issued a “Methane Challenge,” seeking revolutionary chemical processes to facilitate the conversion of methane to ethylene and other useful chemicals. The company received about 100 proposals from universities, institutes and companies around the world.

Then in 2008 Dow awarded major research grants to Cardiff University and Northwestern University to advance the quest. Neurock is a member of the Northwestern University team. He is using theoretical methods and high-performance computing to understand the processes that control catalysis and to guide the experimental research at Northwestern.

Neurock picks up the explanation again with,  “The abundance of natural gas, along with the development of new methods to extract it from hidden reserves, offers unique opportunities for the development of catalytic processes that can convert methane to chemicals. Our finding of using sulfur to catalyze the conversion of methane to ethylene shows initial promise for the development of new catalytic processes that can potentially take full advantage of these reserves. The research, however, is really just in its infancy.”

“Infancy” might be optimistic, the hunt for building molecules in the hydrocarbon family has been going on for decades.  While the oil industry has extreme expertise in taking hydrocarbon molecules apart, the solutions of building molecules to specification have been elusive.

Neurock’s group is very likely on to something.  Nature has been building natural gas, larger hydrocarbon molecules and other plant life molecules for billions of years.  Without much surprise sulfur is commonly found in oil and gas deposits and it is an essential metabolic nutrient of plants.

Neurock’s co-corresponding author on the Nature Chemistry paper is Tobin Marks of Northwestern University with lead author Qingjun Zhu, Staci Wegener, Chao and University of Virginia colleague Obioma Uche.

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