INFLUENCE OF PRESSURE ON THE ACID-CATALYZED RATE-CONSTANT FOR 1-PROPANOL DEHYDRATION IN SUPERCRITICAL WATER

The acid-catalyzed rate of dehydration of 1-propanol in supercritical water is first orderin 1-propanol at low reactant concentrations. Studies of the reaction rate in acidic and buffered solutions lead to values of the pKa of the sulfuric acid catalyst ranging from 2.1 to 1.5 at 375 °C as pressure increases from 22.1 MPa (Pr = 1.002) to 34.5 MPa (Pr = 1.563). The bisulfate anion dissociates to a negligible extent in supercritical water. Because the sulfuric acid is largely dissociated under these conditions, the rate of disappearance of 1-propanol is given by kH[H+][C3H7OH], which is an example of specific-acid catalysis. These findings are consistent with an acid-catalyzed, concerted E2 dehydration mechanism. The measured value of kH is linearly dependent on the reciprocal of the dielectric constant of water. The measured decrease in kH between 22.1 and 34.5 MPa at 375 °C (Tr= 1.003) is less than a factor of 3. This dependence can be used to predict accurately the rate of reaction in the close vicinity of water's critical point by the use of kinetic data obtained under supercritical conditions. From this perspective, the reaction rate exhibits no abnormal behavior in the close vicinity of water's critical point. © 1990, American Chemical Society. All rights reserved.