Overview of the Reactivity of Organics in Superheated Water: Geochemical and Technology Implications

The reactivity of organic molecules in hot water is a field of chemistry developing from studies aimed at understanding how organic matter (kerogen) forms in natural environments and then breaks down into energy source materials. In natural systems where kerogens are depolymerized, water is ubiquitous and hot and usually contains salt and minerals. Reactions such as cleavages and hydrolyses in these media are faciliated by changes in the chemical and physical properties of water as temperature increases. These changes make water more compatible with the reactions of organics. We will present a brief geochemical background, discuss chemical and physical properties of water, key features of known kerogen structural models and the aqueous chemistry of cleavage and hydrolysis reactions involved in kerogen depolymerization. Based on the understanding of the roles of water as a solvent, reagent and catalyst, potential applications in areas such as plastics recycling and synthesis of chemicals will be described. INTRODUCTION This article describes an emerging area of chemistry: the reactivity of organic compounds in superheated water. We begin with a brief geochemical background and then discuss the implications of the aqueous chemisq to understanding the formation of oil from the solid, insoluble, organic material (kerogen) in resources such as shale and coal. Finally, we point out potential implications to more technological areas such as plastics recycling. Common organic molecules that were previously considered unreactive in liquid water undergo many chemical reactions when the temperature is increased to 250-350 T ; these reactions were previously expected only in the presence of strong acid or base. For example, ethers and esters, which are unreactive to heat alone, undergo facile cleavage and hydrolysis, respectively, in water at 250-350 OC.' Ethers and esters are major cross-links in several oil shale kerogens and are illustrated in a portion of the detailed structural model of a Rundle Ramsay Crossing Type I kerogen in Figure 1. Analogously, polyethylene terephthalate polymers (found in plastic soft drink bottles) can be hydrolyzed quantitatively back to their starting materials in superheated water in less than an hour.' Other polyesters, and also polyamides (like nylon), are equally susceptible to hydrolysis. A major analogy to such polymer degradation reactions in nature is catagenesis: the process by which solid petroleum source rock kerogens, which are cross-linked macromolecular structures, are converted into liquid petroleum. Natural catagenesis takes place at temperatures below 200 "C over millions of years in aqueous environments at pressures of about 600 atmospheres. Because of the relatively low temperatures, it has been hypothesized that some of the chemistry by which petroleum is formed is catalyzed by clay minerals in the formations? Recent studies in our laboratories have made it clear that two additional factors can affect and catalyze the kerogen depolymerization chemistry that leads to petroleum formation. One factor is that simple aqueous chemistry generates water-soluble products that are acidic or basic, or have redox properties. The other factor involves the salts present in sea water or aqueous en~ironments.4.~ High temperature water under autogenic pressure provides a significantly more favorable reaction medium for ionic reactions of non-polar organic compounds than does water up to its boiling temperature (Table I). At 300 "C, water exhibits a density and polarity similar to those of acetone at room temperaturc6 The dielectric constant of water drops rapidly with temperature, and at 300 "C has fallen from 80 (at 20 "C) to 2' and its solubility parameter decreases from 23.4 to 14.5 caI/cm.* This means that, as the water temperature is increased, the solubility of organic compounds increases much more than expected for the natural effect of temperature. Furthermore, the negative logarithmic ionic product of water at 250 O C is 11, and of deuterium oxide is 12, as compared to 14 and 15, respectively, at 20 ' C 9 This means that water becomes both a stronger acid and a stronger base as the temperature increases. Therefore, in addition to the natural increase in kinetic rates with temperature, both acid and base catalysis by water are enhanced at higher temperatures. ,