Investigation of acid-catalytic effect of hot compressed water–methanol mixture using dehydration of tertiary alcohol as a probe reaction

Abstract

The acid-catalytic effect of a hot compressed water–methanol mixture was investigated by analyzing the reaction kinetics of α-cumyl alcohol at 250 $℃$ and 30 MPa, with methanol mole fractions ranging from 0 to 0.4. By varying the HCl concentration to control proton content and measuring the corresponding reaction rate, the proton concentration associated with the dissociation of water and/or methanol in the absence of HCl was determined. The acid-catalytic effect of the reaction medium was evaluated using the stoichiometric autoprotolysis constant, because it can be used to calculate the amount of protons generated through solvent dissociation. This parameter reflects not only the polarity of the solvent but also its acidic and basic properties. The acid-catalytic effect attributable to autoprotolysis gradually diminished as the methanol content increased. However, the highest reaction rate of α-cumyl alcohol was observed when the methanol mole fraction was 0.2. This is because, while the autoprotolysis was suppressed with increasing methanol content, the rate constants increased with the methanol mole fraction and became nearly constant when it exceeded 0.2.