Multiscale reaction pathway analysis of methyl tert-butyl ether hydrolysis under hydrothermal conditions

Two decomposition mechanisms for methyl tert-butyl ether (MTBE) under hydrothermal conditions were analyzed: a unimolecular decomposition pathway and an acid-catalyzed hydrolysis pathway. Ab initio quantum chemistry methods were employed to account for solvent effects on the activation energy of the unimolecular decomposition pathway over a range of compressed fluid conditions from 150 to 600 °C at 250 bar. Applying this correction resulted in a local minimum in the reaction rate below the critical temperature followed by a local maximum above the critical temperature. However, this reaction pathway contributed minimally to the overall decomposition, except at temperatures above 550 °C. Experiments were conducted under acidic and basic conditions to determine the effect of pH on the reaction rate. These results confirmed that the primary reaction path is acid-catalyzed hydrolysis. Experimentally determined reaction orders for the concentration of H⁺ ranged from 0.57 to 0.83 (±0.05) from 200 to 450 °C, below the reaction order of 1.0 predicted by the proposed mechanism. The discrepancy between measured and predicted values was most likely caused by the increasing importance of the reverse reaction in the acid-catalyzed hydrolysis pathway under more acidic conditions.