DEHYDRATION AND DEOXYGENATION OF ORGANIC COMPOUNDS UNDER HYDROTHERMAL CONDITIONS

Hydrothermal organic geochemistry experiments can reveal how minerals affect organic transformations in the Earth and other planets. The goal of complementary geomimetic experiments is to learn from geochemical processes to use minerals and control hydrothermal organic reactivity. Our interest is in mechanistic analyses of the reaction pathways of common organic structures under hydrothermal conditions (250˚-300˚C, 4-100MPa). The hydrothermal dehydration alcohols is of particular interest because dehydration represents the fastest de-oxygenation reaction of organics at geologic conditions, and de-oxygenation is a major problem in the transformation of biomass into fuels and novel organic products.

Cellulose is a biopolymer of glucose in biomass that has potential as a fuel source, if de-oxygenation and reduction can be accomplished. Understanding alcohol dehydration mechanisms, particularly in cyclic systems, is crucial to the development of reaction conditions that enhance dehydration over other reactions of polyols. We have performed a kinetic and mechanistic study of alcohol dehydration under hydrothermal conditions, and have characterized the mechanisms by which water is eliminated. Alcohols in hydrothermal conditions tend to favor a unimolecular mechanism, which involves a positively charged carbocation intermediate, unless a favorable conformation is maintained that allows a bimolecular process that avoids the intermediate. The presence of a carbocation can result in rearrangement and isomerization of the structure. We have also developed reaction conditions that facilitate reduction, using elemental iron powder (10µm) as an electron source and 100 nm nickel powder as a solid-state catalyst, in water at 250˚C and 4MPa. Using this geomimetic hydrothermal approach, we have performed combined deoxygenation and reduction of a number of organic structures with various functional groups into higher energy hydrocarbons, using only water and first-row transition metals.