Zinc ore catalyzes Earth’s organic chemistry

Carbon compounds are ubiquitous, even in deep ocean sediments, in subduction zones, and at mid-ocean ridges. Although the essential ingredients of the chemical reactions in such environments are well known—organic molecules, hot pressurized water, and minerals—studies of the ways in which individual minerals influence reactivity are virtually nonexistent. That’s partly because geochemical organic reactions tend to generate complex product mixtures, which can obscure the mechanism. Hilairy Hartnett and a team of researchers at Arizona State University have now presented the first description of mineral catalytic effects on the most fundamental aspect of an organic reaction: the breaking and making of a covalent bond. In the group’s experiments, when the model alkane cis-1,2-dimethylcyclohexane was placed in a chamber at 300 °C and 1000 atmospheres and allowed to react with water alone, little happened. A mere 5% of the isomer converted into the trans- form of the alkane over a two-week time span, as shown by the blue curve. But when the hydrothermal reaction occurred in the presence of sphalerite, a mineral form of zinc sulfide commonly found on the sea floor among black smoker vents, the conversion rate rose dramatically (red curve). The trans- structure was primarily produced until an equilibrium between the stereoisomers was reached. The result is consistent with a mechanism in which sphalerite breaks a C–H bond to form an intermediate that can then reform the bond as either the cis- or trans- isomer. The catalysis of C–H bonding is not new, but the Arizona State experiments may interest industrial chemists. Unlike most organometallic catalysts, minerals are inexpensive, are robust, and require no synthesis. (J. A. Shipp et al., Proc. Natl. Acad. Sci. USA 111, 11642, 2014, doi:10.1073/pnas.1324222111 .)