Iron Mineral Effects on Ketone Reactions in Hydrothermal Fluids

Interactions in hydrothermal environments suggest that minerals participate in and alter organic compounds transformations at high temperatures and pressures [1]. Our previous experimental studies of a model ketone (dibenzyl ketone, DBK) in aqueous media under hydrothermal conditions (700 bars, 300 °C) indicate low conversion but multiple reaction pathways yielding diverse products. In the absence of minerals, DBK not only reversibly interconverts into 1,3-diphenyl-2-propanol, 1,3-diphenylpropene and 1,3-diphenylpropane along a reduction pathway, but also yields products including toluene, bibenzyl, stilbene and conjugated, dehydrogenated three- and four-ring coupling products from carbon-carbon (C-C) and carbon-hydrogen (C-H) bond-breaking pathways. Experiments involving oxide minerals that are not sensitive to redox process, such as quartz and corundum, show no effect when compared with H₂O alone in changing DBK hydrothermal reactions and product distributions. In the presence of iron bearing minerals, however, we observe that the overall reaction conversion of DBK increases by orders of magnitude, and that reaction pathways are controlled or favored differently if hematite (Fe₂O₃), magnetite (Fe₃O₄) or ferrous sulfide (FeS) is present. As an example, with the same mineral surface area, Fe₂O₃ expedites DBK conversion from 6.4% (H₂O only) to 26.4% after 168 hours, while Fe₃O₄ increases conversion up to 46.8%. Although more products are formed with introduction of iron oxide minerals, the major products are identical to those found in H₂O alone, such as toluene, bibenzyl and a few large coupling products from the bond-breaking pathways. Hydrothermal experiments using a synthesized asymmetrical p-methyl-DBK under the same conditions conducted with Fe₂O₃ and Fe₃O₄ are consistent with those for DBK, showing higher conversion than in H₂O, and more bond-breaking products like toluene, p-xylene, and three kinds of bibenzyls. This suggests that both Fe₂O₃ and Fe₃O₄ assist in breaking C-C bonds homolytically, generating freely diffusing radicals as intermediates at our experimental conditions. In contrast, FeS increases DBK conversion mainly by following the reduction pathway, from ketone to alcohol, alkene and alkane. As an example, the concentration of 1,3-diphenylpropane is orders of magnitude higher with FeS than with H₂O alone. One major difference is that, 1,3-diphenyl-2-propanol, which is very short-lived in H₂O alone, accumulates and predominates over the corresponding alkene and alkane in the first 70 hours, which suggests that the rate of transformation of DBK to the alcohol is greatly facilitated by the presence of FeS. [1] McCollom and Seewald (2007), Chem. Rev.107, 382-401.