Pyrolysis of ethanol: A shock-tube/TOF-MS and modeling study

The pyrolysis of ethanol was studied in a shock tube behind reflected shock waves in the temperature range 1300–1510K at pressures of about 1.1bar by using time-of-flight mass spectrometry for detection. For the first time, concentration–time profiles of C2H5OH, C2H4O, (C2H2+C2H4+C2H6), H2O, and CH4 were simultaneously recorded. It could be shown that under our experimental conditions, the major products are H2O and C2H4, which are mainly formed in the reaction C2H5OH→C2H4+H2O (R1). The rate coefficient of reaction (R1) could be determined for the first time from directly measured concentration–time profiles of H2O in a shock tube. The results can be expressed by the Arrhenius equation k1(T, P ∼1.1bar)=(2.5±1.0)×1010 exp (−23320K/T)s−1 with an estimated uncertainty of ±40%. The multiple-species concentration–time profiles from our experiments can be used for a most rigorous test of ethanol pyrolysis mechanisms. As an exemplary case, we employed the mechanism from Marinov (1999) and found a very good agreement between the experimental and simulation results. We also calculated rate coefficients for the most important decomposition steps of ethanol from statistical rate theory by solving a thermal multichannel master equation. Reaction (R1) as well as the bond fission channels C2H5OH→CH3+CH2OH (R2) and C2H5OH→C2H5+OH (R3) were considered. The specific rate coefficients were obtained from RRKM theory (R1) and the Statistical Adiabatic Channel Model (R2 and R3) with molecular and transition state data from quantum chemical calculations at CCSD(T)-F12/def2-QZVPP//MP2/cc-pVQZ level of theory. The rate coefficient obtained from the ab initio data were slightly fitted to recently published experimental values of k1, k2, and k3 and inserted into Marinov’s mechanism. A similar good representation of the concentration–time profiles as with the original mechanism was obtained.