Common Mineral Capable of Making and Breaking Bonds

Previously, the team had found they could react organic molecules in hot pressurized water to produce many different types of products, but reactions were slow and conversions low. This work, however, shows that in the presence of sphalerite, hydrothermal reaction rates increased dramatically, the reaction approached equilibrium, and only one product formed. This very clean, very simple reaction was unexpected.

"We chose sphalerite because we had been working with iron sulfides and realized that we couldn't isolate the effects of iron from the effects of sulfur. So we tried a mineral with sulfur but not iron. Sphalerite is a common mineral in hydrothermal systems so it was a pretty good choice. We really didn't expect it to behave so differently from the iron sulfides," says Hartnett, an associate professor in the School of Earth and Space Exploration, and in the Department of Chemistry and Biochemistry at ASU.
This research provides information about exactly how the sphalerite mineral surface affects the breaking and making of the C-H bond. Sphalerite is present in marine hydrothermal systems i.e., black smokers, and has been the focus of recent origins-of-life investigations.

For their experiments, the team needed high pressures (1000 bar -- nearly 1000 atm) and high temperatures (300°C) in a chemically inert container. To get these conditions, the reactants (sphalerite, water, and an organic molecule) are welded into a pure gold capsule and placed in a pressure vessel, inside a furnace. When an experiment is done, the gold capsule is frozen in liquid nitrogen to stop the reaction, opened and allowed to thaw while submerged in dichloromethane to extract the organic products.

"This research is a unique collaboration because Dr. Gould is an organic chemist and you combine him with Dr. Hartnett who studies carbon cycles and environmental geochemistry, Dr. Shock who thinks in terms of thermodynamics and about high temperature environments, and Dr. Williams who is the mineral expert, and you get a diverse set of brains thinking about the same problems," says Shipp.

Hydrothermal organic reactions affect the formation, degradation, and composition of petroleum, and provide energy and carbon sources for microbial communities in deep sedimentary systems. The results have implications for the carbon cycle, astrobiology, prebiotic organic chemistry, and perhaps even more importantly for Green Chemistry (a philosophy that encourages the design of products and processes that minimize the use and generation of hazardous substances).

"This C-H bond activation is a fundamental step that is ultimately necessary to produce more complex molecules -- in the environment those molecules could be food for the deep biosphere -- or involved in the production of petroleum fuels," says Hartnett. "The green chemistry side is potentially really cool -- since we can conduct reactions in just hot water with a common mineral that ordinarily would require expensive or toxic catalysts or extremely harsh -- acidic or oxidizing -- conditions."