DENSITY EFFECTS IN THE REACTION OF 2-PROPANOL IN SUPERCRITICAL WATER

Abstract

ABSTRACT The effect of density on the reactivity of 2-propanol in supercritical water (SCW) has been investigated in a stirred batch reactor featuring direct injection of small amounts of organic liquids directly into the supercritical environment. The substrate is taken from ambient temperatures to supercritical conditions in ≤200 ms, preventing interference from subcritical hot-water chemistry and allowing one to isolate reactivity in the SCW region. On-line microscale sampling allows product analysis without significantly perturbing the reaction conditions. At densities of 0.24 g cm−3 to 0.58 g cm−3 the main reaction of 2-propanol is found to be dehydration to give propene and water. The reaction reaches equilibrium in times of a few minutes to tens of hours, with the rate strongly dependent on the water density. This phenomenon was investigated at water densities between 0.24 g cm−3 and 0.58 g cm−3 and temperatures between 654 K and 686 K. It is postulated that much of the rate increase with density is due to the increasing importance of ionic processes and acid catalysis due to increasing dissociation of water that occurs at higher densities. Stabilization of charge-separated transition states due to the change in water dielectric constant with density could also be important. Some implications with regard to scale-up of SCW technology based on data from small laboratory-scale studies are discussed.