Kinetic modelling and inverse treatment of the radical mechanism of the liquid-phase autoxidation of 1,2,3,4-tetrahydronaphthalene

Abstract

A kinetic model to account for the overall radical mechanism of the self-initiated liquid-phase autoxidation of 1,2,3,4-tetrahydronaphthalene (tetralin: RH2) without solvent and without catalyst at 60°C under normal pressure is presented. The model was constructed following a heuristic approach and it consists of 16 elementary steps and a species space including 6 radical and 7 nonradical compounds. Applying inverse computations, the model can be assigned to experimentally observed data within very good agreement where most of the computed values for the kinetic parameters are found to be close to literature values. The respective experimental data is characterized by an autocatalytic time-evolution of tetralin hydroperoxide (HROOH) and of α-tetralone (RO) where in the first stage of autoxidation a remarkable very slow rate of the RO formation in respect to that of the HROOH formation can be observed. This effect is explicitly expressed by the transformation process of tetralyloxy radicals (HRO·) into α-tetralol radicals (·ROH) which react with oxygen yielding RO. The transformation is suggested to be catalyzed by α-tetralol (HROH) which accumulates autocatalytically during the radical chain process. Hence, the nonlinear formation of HROH leads to a switching in the transformation of HRO· into ·ROH radicals which causes a growing rate of RO evolution. The autocatalytic nature of the overall process is explained by degenerate branching of one molecule of HROOH yielding radical species finally resulting into 2 tetralyl radicals (RH·). This process leads to a growing number of chain propagating reaction sequences involving RH· and tetralylperoxy radicals (HROO·) in which RH2 and O2 are consumed and HROOH is formed. Based on the fitting of experimental data the suggested general dynamic structure of the model is validated by computing the reaction fluxes vs. time of several mechanistic key steps. By computer simulations the model is also shown to predict the increase of product accumulation which can be observed experimentally if the autoxidation of RH2 is started after an initial addition of HROH. © 1996 John Wiley & Sons, Inc.