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

Abstract

The thermal decomposition of formic acid (HCOOH) and acetic acid (CH 3 COOH), 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:were measured using simultaneous infrared laser absorption of CO, CO 2 and H 2 O at wavelengths of 4.56, 4.18 and 2.93 microns, respectively. Reaction test conditions covered temperatures from 1230 to 1821 K and pressures from 1.0 to 6.5 atm 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, CO 2 and H 2 O time-history profiles. These two decomposition channels were found to be in the fall-off region and have a branching ratio, ! ! , of approximately 20 over the range of pressures studied here. The best-fit Arrhenius expressions of the first-order rates of R1 and R2were found to be:! 6.5 atm = 9.12 ×10 !" exp −30275 s -1 (±32%)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, ! ! , 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:! 1 atm = 3.18×10 !! ×exp −28679 s -1 (±30%)! 6 atm = 3.51×10 !" ×exp −31330 s -1 (±26%)! 1 atm = 7.9 ×10 !! exp −29056 s -1 (±34%).! 6 atm = 6.34×10 !" ×exp −31330 s -1 (±31%).

Accurate Calculations of Rate Constants for the Forward and Reverse H2O + CO ↔ HCOOH Reactions

“…Thus, the experimental values available so far cannot be taken as comparative data for this study since they all have been carried out at lower pressures ,,,,,. Nevertheless, the experimental activation energies obtained by fitting first‐order rates increase from 51.0 to 60.2 kcal mol –1 for diluted HCOOH mixtures in argon with total pressures of 1.0 and 6.5 atm, respectively . Hence, these values suggest that the high‐pressure limit will result in even larger activation energies.…”

“…Several experimental and theoretical values can be found for activation energies of the reverse reaction, but there are large discrepancies among them. However, Elwardany et al . showed that the high‐pressure limit is still not achieved for the HCOOH decomposition direction at pressures as high as 6.5 atm.…”

A study of the chemical structure of laminar premixed HC(O)OH/O2/Ar flames at 1 atm

“…The reactions of formic acid monomolecular decomposition were studied experimentally under low temperature in works [7,8] and under high temperatures in works [9][10][11][12]. According to the obtained results formic acid mostly decomposes via reaction HC(O)OH=CO+H2O whereas the reaction HC(O)OH=CO2+H2 has lower contribution.…”

Theoretical investigation of the formic acid decomposition kinetics

“…These channels are shown below: The decomposition reactions should be competitive, from comparison of their activation energies . However, experimental works have indicated that the dehydration is the preferred channel, and this fact still remains without a theoretical kinetic microscopic foundation. Takahashi et al have theoretically investigated this preference at the MP4/cc‐pVQZ//B4LYP/cc‐pVTZ level of theory and affirmed that canonical transition state theory cannot explain the dehydration preference.…”