Influence of pressure on the acid-catalyzed rate constant for 1-propanol dehydration in supercritical water

Article abstract of DOI:10.1021/ja00161a043

The acid-catalyzed rate of dehydration of 1-propanol in supercritical water is first order in 1-propanol at low reactant concentrations. Studies of the reaction rate in acidic and buffered solutions lead to values of the pK(a) of the sulfuric acid catalyst ranging from 2.1 to 1.5 at 375 °C as pressure increases from 22.1 MPa (P(r) = 1.002) to 34.5 MPa (P(r) = 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 k(H)[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 k(H) is linearly dependent on the reciprocal of the dielectric constant of water. The measured decrease in k(H) between 22.1 and 34.5 MPa at 375 °C (T(r) = 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.