Unimolecular decomposition of formic and acetic acids: A shock tube/laser absorption study

Abstract

The thermal decomposition of formic acid (HCOOH) and acetic acid (CH3COOH), two carboxylic acids which play an important role in oxygenate combustion chemistry, were investigated behind reflected shock waves using laser absorption. The rate constants of the primary decomposition pathways of these acids:HCOOH→CO+H2O(R1)HCOOH→CO2+H2(R2)CH3COOH→CH4+CO2(R3)CH3COOH→CH2CO+H2O(R4)were measured using simultaneous infrared laser absorption of CO, CO2 and H2O at wavelengths of 4.56, 4.18 and 2.93 microns, respectively. Reaction test conditions covered temperatures from 1230 to 1821K and pressures from 1.0 to 6.5atm for dilute mixtures of acids (0.25–0.6%) in argon. The rate constants of dehydration (R1) and decarboxylation (R2) reactions of formic acid were calculated by fitting exponential functions to the measured CO, CO2 and H2O time-history profiles. These two decomposition channels were found to be in the fall-off region and have a branching ratio, k1/k2, of approximately 20 over the range of pressures studied here. The best-fit Arrhenius expressions of the first-order rates of R1 and R2 were found to be:k1(1atm)=1.03×1011exp(-25651/T)s-1(±37%)k1(6.5atm)=9.12×1012exp(-30275/T)s-1(±32%)k2(1atm)=1.79×108exp(-21133/T)s-1(±41%)k2(6.5atm)=2.73×108exp(-20074/T)s-1(±37%)The rate constants for acetic acid decomposition were obtained by fitting simulated profiles, using an acetic acid pyrolysis mechanism, to the measured species time-histories. The branching ratio, k4/k3, was found to be approximately 2. The decarboxylation and dehydration reactions of acetic acid appear to be in the falloff region over the tested pressure range:k3(1atm)=3.18×1011exp(-28679/T)s-1(±30%)k3(6atm)=3.51×1012exp(-31330/T)s-1(±26%)k4(1atm)=7.9×1011exp(-29056/T)s-1(±34%)k4(6atm)=6.34×1012exp(-31330/T)s-1(±31%).