Experimental studies of mineral-catalyzed dehydrogenation of C6 cyclic hydrocarbons under hydrothermal conditions

Hydrothermal gases are characterized by a complex mixture of organic volatiles. Patterns in the relative abundances of the main classes of organics are found in gases from natural systems (e.g. volcanoes, geothermal sites) characterized by similar T and redox, which suggests that the compositions of the gas mixtures are sensitive to physical and chemical conditions. The attainment of metastable equilibrium between organic compounds has been demonstrated experimentally (Seewald, 1994). Studies on fumarolic gases (Taran & Giggenbach, 2004) have shown that alkane/alkene interconversion can approach equilibrium in natural environments. Reversible catalytic reforming may be responsible for the high abundance of benzene observed in hydrothermal gases relative to saturated hydrocarbons (Capaccioni et al., 1993). Cyclic hydrocarbons are enriched in gases originating at T <150 °C (Tassi et al., 2012), which suggests that benzene could be produced via dehydrogenation of n-hexane, with C6 cyclic hydrocarbons as intermediates. We are investigating the mechanisms for formation of benzene from cyclic hydrocarbons under laboratory conditions. The reactions of cyclohexane, cyclohexene and cyclohexadiene have been studied at 300°C and 85 bar. Benzene production is observed, together with various oxidation, hydration and isomerization products, depending upon the conditions. Dehydrogenation of cyclohexadiene to form benzene is very efficient in water alone, but the corresponding reaction of cyclohexane gave negligible benzene even after reaction for 10 days. However, formation of both benzene and cyclohexene from cyclohexane was enhanced in the presence of minerals, in particular, sphalerite. Experiments with varying ratios of cyclohexane and sphalerite showed that dehydrogenation is a surface catalyzed reaction. Experiments in D2O showed that sphalerite very efficiently catalyzes breaking C-H bonds, even when conversion to benzene is low. For cyclohexene, benzene formation is negligible compared to oxygenation products in the absence of minerals, but sphalerite catalyzes formation of both benzene and cyclohexane. Finally, solution phase oxidation using Cu(II) forms primarily oxygenation products, suggesting that formation of benzene requires surface catalysis by minerals. Capaccioni B., Martini M., Mangani F., Giannini L., Nappi G. & Prati F., 1993. Light hydrocarbons in gas-emissions from volcanic areas and geothermal fields. Geochem J, 27, 7-17. Seewald J.S., 1994. Evidence for metastable equilibrium between hydrocarbons under hydrothermal conditions. Nature, 370, 285–287. Taran Y. & Giggenbach W.F., 2004. Evidence for metastable equilibrium between hydrocarbons in volcanic gases. In: R.B. Wanty and R.R. Seal Eds., Water-Rock Interaction, 193-195.A.A. Balkema, Leiden. Tassi F., Bonini M., Montegrossi G., Capecchiacci F., Capaccioni B. & Vaselli O., 2012. Origin of light hydrocarbons in gases from mud volcanoes and CH4-rich emissions. Chem Geol, 294-295, 113-126.