Hydrogenation of Hydrocarbons through Partial Oxidation in Supercritical Water

Abstract

We propose a new process for the hydrotreatment of heavy oils in supercritical water (SCW). The discussion in this paper is composed of three parts: (1) hydrogenation through water-gas shift reaction in supercritical water, (2) selective formation of carbon monoxide by partial oxidation in supercritical water and through combinations of these two, and (3) hydrogenation of hydrocarbons through their partial oxidation in supercritical water. In the experiments involving hydrogenation of dibenzothiophene, carbazole, and naphthalene, faster hydrogenation rates could be obtained in a CO−SCW atmosphere than in a H2−SCW atmosphere. Even in the case of a H2−CO2−SCW atmosphere, similarly faster reaction rates were obtained, which suggests that an intermediate species of the water-gas shift reaction is the actual reason for the high hydrogenation rates. Partial oxidation experiments were conducted for hexylbenzene and n-hexadecane. The selectivity of CO increased with increasing density of water, while CO2 was the main product of the gas-phase reaction. Partial oxidation of dibenzothiophene (with a sulfur-treated NiMO/Al2O3 catalyst) and n-hexadecane (without catalysts) were also examined. Hydrodesulfurization of dibenzothiophene proceeds effectively in water. For n-hexadecane oxidation, the alkane/alkene ratio increased with increasing water density. The observed product distribution is probably due to the production of carbon monoxide, which is followed by the formation of active hydrogenating species via the water-gas shift reaction. This work demonstrates that hydrogenation reactions can be greatly accelerated in supercritical water through the use of either direct introduction of carbon monoxide or in situ formation of carbon monoxide through the partial oxidation of hydrocarbons.