Mechanism and kinetics of the autocatalytic dehydration of tert-butanol in near-critical water

Abstract

This work describes studies of the kinetics and mechanism for the autocatalytic dehydration of tert-butanol in near-critical water at 225°C, 34.5 MPa and 250°C, 34.5 MPa. Two tasks were performed in this study. First, to identify the mechanism for the autocatalytic dehydration of tert-butanol; second, to study the temperature effect to verify the mechanism. Experiments were conducted on two flow reactors which perform as plug flow reactors. At 225°C, 34.5 MPa and 250°C, 34.5 MPa, tert-butanol undergoes rapid and selective dehydration to isobutene without any catalyst in near-critical water. Four different models were postulated and tested. The kinetics of the dehydration reaction are consistent with an autocatalytic E1 mechanism with ether formation and decomposition. In this mechanism, tert-butanol plays three different roles: 1) dissociates as Arrhenius acid with catalytic acid/conjugate base pair H3O+/H2O; 2) acts as Bronsted acid with catalytic acid/conjugate base pair (CH3)3COH/(CH3)3CO;3) acts as alcohol which suffers dehydration. The autocatalytic E2 model with ether formation and decomposition, water catalytic E1 model with ether formation and decomposition, and a simplified autocatalytic E1 model without ether formation and decomposition are not consistent with the experiment measurements. The dissociation of tert-butanol as an acid in water is an endothermic reaction. The standard enthalpy and entropy associated with this reaction were found to be: ΔH°=142 kJ/mol, ΔS°=82 J/moI.K. The dehydration of tert-butanol is also an endothermic reaction, high temperature favors the conversion of tert-butanol to isobutene. The values of the standard entropy of activation associated with the elementary rate constant of the autocatalytic E1 model with ether formation and decomposition are also presented.