The formaldehyde decomposition chain mechanism

Abstract

A kinetic mechanism for the chain decomposition of formaldehyde consistent with recent theoretical and experimental results is presented. This includes new calculations and measurements of the rate constant for the abstraction reactionThe calculation uses a multi-reference configuration interaction wavefunction to construct the potential energy surface which is used in a tunnelingcorrected TST calculation of the rate constant. The rate constant for the bond fission C H 2 0 + M -HCO + H + M at high temperatures was determined by an RRKM extrapolation of direct low temperature measurements. This mechanism has been successfully tested against laser-schlieren measurements covering the temperature range 2200-3200 K. These measurements are insensitive to all but the above two reactions and they confirm the large, non-Arrhenius rate for the abstraction reaction derived here from theory. Modeling of previous experiments using IR emission, ARAS, and CO laser absorption with this mechanism is quite satisfactory. The branching ratio of the rate of the faster molecular dissociation (CH20 + (M) -C O + Hz + (M)), to that of the bond fission reaction, was estimated to be no more than 2 or 3 over 2000 to 3000 K. Such a ratio is consistent with one recent theoretical estimate and most of the experimental observations.

Thermal unimolecular decomposition of 1,3,5‐trioxane: Comparison of theory and experiment

“…Using a simple kinetic simulation and RRKM computer programs, the decomposition reaction was found to be totally free from the effect of the reverse reaction and only slightly pressure-dependent below 500 torr. The latter result is in full agreement with the conclusion reached by Irdam and Kiefer [3], who had performed a similar RRKM calculation.…”

“…The result of this quantum chemical calculation, as depicted schematically in Figure 1, indicates that the fragmentation of (CHz0)3 clearly occurs via the concerted mechanism with a relatively loose cyclic transition state. This theoretical result, although close to the extrapolated first-order rate constant obtained at high temperatures [3], differs noticeably from the reported low-temperature values [1,2]: K 1 (Exp) = (6 x 1014 -1 x 1015) exp(-47,400/RT) s-', where R = 1.987 cal/mol * deg.…”

High-Temperature Experimental and Theoretical Study of the Unimolecular Dissociation of 1,3,5-Trioxane

“…It is formed during the oxidation of alkanes, natural gas, alcohols, aldehydes, ethers and others. [1][2][3] Its importance in combustion led to a number of high temperature kinetic studies for its pyrolytic dissociation [3][4][5][6] and bimolecular reactions. 1,[7][8][9][10] A substantial number of earlier studies 5,7,9,[11][12][13][14][15][16] consisted of utilizing 1,3,5-trioxane (C 3 H 6 O 3 ) as a clean thermolytic source of formaldehyde to avoid the problem of formaldehyde polymerization.…”

“…to be an instantaneous process at high temperatures (above 1300 K τ < 0.5 µs) 5 to yield three formaldehyde molecules via a simple unimolecular and probably a concerted mechanism 12,17 .…”

“…13 Due to the existing discrepancies in literature, an additional study rendering to detailed kinetic analysis is warranted to fully ascertain the rate coefficients of the thermal dissociation of 1,3,5-trioxane. 5 Here, we carried out high-level quantum chemical and statistical rate theory calculations to unravel the temperature and pressure dependence of the rate coefficient for reaction (1).…”