Water-hydrocarbon radical interactions

This scheme is similar to the water hydrolysis mechanisms proposed by Siskin and Katritzky (1995) for kerogen-water interaction at elevated temperature, and is also supported by the ab initio results obtained from modeling the ether and ester hydrolysis reactions. 

The second scheme that Lewan (1997) proposed involves the direct interaction of dissolved water molecule with free radicals. These reactions may involve either the oxygen or hydrogen of the water molecule reacting with unpaired electrons to form aldehydes, alcohols, alkanes, hydrogen, or oxygen. The model reactions include the following  

Lewan (1997) also calculated the Gibbs free energy for the above reactions and the results indicate that these reactions are thermodynamically favorable. However, he also stated that these model reactions may have kinetic energy barriers to overcome. 

If water does interact with hydrocarbon radicals, one of the most important reactions has to be a water molecule reacting with a methyl radical to form a methane and hydroxyl radical. 

The ab initio results suggest that it is unlikely that water-kerogen interactions occur by a hydrocarbon thermal radical reaction pathway. This conclusion is supported by experimental and natural observations. For example, at 330°C, P-scission of an alkyl radical is 300 times faster than hydrogen abstraction from water so olefin formation will greatly exceed saturates formation (Ross 1992). However, formation of large amounts of olefin in hydrous pyrolysis has not been reported (Larson 1999) and olefins are rare components of crude oils (Hunt 1996). 

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In the following sections, we will discuss several proposed mechanisms of water-organic (kerogen) interactions. They include the direct interactions (hydrolysis) of water-kerogen at elevated temperature by Siskin and Katritzky (1995) water-hydrocarbon radical interactions by Lewan (1997) and hydrolytic disproportionation and kerogen oxidation by Helgeson et al. (1993). We will use ab initio method to test the feasibility of these mechanisms and and to find out how likely they can proceed under geological conditions. 

The radical anion of molecular oxygen (O ) has been prepared and trapped in a range of alcohols, water and benzene but not in aliphatic hydrocarbons (Bennett et al., 1968a). In contrast to COg the e.s.r. spectrum shows that 0 interacts strongly with its immediate environment. This interaction which alters the separation of the upper molecular orbitals of the anion is strongly dependent on the nature of the matrix. Previously, the Oj" radical ion has been stabilized only in ionic materials such as the alkali halides thus it is of particular interest to find that this anion can be trapped successfully in a non-polar matrix (benzene). There is some evidence (Evans, 1961), from optical spectroscopic studies that molecular oxygen can form a weak charge transfer complex with the 77-electron system in benzene and it seems probable that O2 is stabilized in benzene by the formation of a similar complex. 

Let us consider three types of interaction between (1) adsorbed hydrocarbon ion-radicals and oxygen of the gas phase (2) adsorbed oxygen and hydrocarbon of the gas phase (oxygen adsorbed both with and without dissociation) (3) adsorbed oxygen and adsorbed hydrocarbon. Saturated aldehydes and acids containing less carbon atoms than in the molecule of initial hydrocarbon, as well as carbon dioxide and water, are formed in the first case. The second type of interaction yields unsaturated aldehydes, olefine oxides, carbon monoxide, carbon dioxide, and water for the oxidation of unsaturated hydrocarbons and saturated aldehydes, carbon dioxide, carbon monoxide, and water for the oxidation of saturated hydrocarbons. The third type of reaction gives... 

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