(271a) Rate Constants and Branching Ratios for Hydrogen Abstractions By OH, H and CH3 From Methanol and Its Deuterated Isomers

Methanol (CH₃OH), the simplest alcohol, is an important molecule in combustion not only because of its potential to be a neat alternative transportation fuel but it also is a prominent intermediate in the combustion of a variety of oxygenated fuels. Due to its implications in combustion, methanol has been the subject of numerous high temperature studies over the past 30-40 years. Despite this, only recently experimental [1] and theoretical studies [2] (from our laboratory) have been instrumental in obtaining a complete understanding of the mechanistic features related to its high temperature decomposition. While these thermal decompositions act as initiations, subsequent propagation steps that involve H-atom abstractions are often the dominant channels for fuel destruction and also appear as sensitive steps for ignition. While the experimental studies of Srinivasan et al. [1] measured total rate constants for OH + CH₃OH, there is a severe lack of high temperature measurements for H/CH₃ + CH₃OH.  Consequently, the present studies were initiated with two specific aims; 1. Measure total rate constants for X + CH₃OH (X= H, CH₃) and 2. Use deuterated methanol isomers (CH₃OD, CD₃OH) to determine unambiguous abstraction branching ratios from the CH₃ site and the OH site by selectively monitoring H- and D-atoms. In summary, experimental rate constants were measured for,

            D + CH₃OH       → HD + CH₂OH                                                                 (1A)

                                       → HD + CH₃O                                                                    (1B)

            CH₃ + CH₃OH   → CH₄ + CH₂OH                                                                (2A)

                                       → CH₄ + CH₃O                                                                   (3B)

Reaction rate constants were also measured for OH + CH₃OD/CD₃OH → products. The present experiments were conducted using a reflected shock wave heating with Kr as the diluent. Tert-butyl hydroperoxide (TBH) was used as a thermal source of OH, deuterated iodo-ethane, C₂D₅I, was used as precursor for D-atoms, with diacetyl (butane-2,3-dione), C₄H₆O₂, used as a precursor for CH₃ radicals. H- and D- atom ARAS  (atomic resonance absorption spectrometry) detection were used to measure H- and D-atom signals to determine total rate constants for reactions (1) and (2). Experiments were conducted over a temperature range of T= 1173 – 1361 K and reflected shock pressures of P­₅­ = 0.4-0.5 atm. Initial fuel concentrations were approximately 50 ppm. Additionally, similar to the OH + methanol, experiments using the partially deuterated methanol isomers (CH₃OD and CD₃OH) were utilized (in reactions 1 and 2) to directly determine branching ratios between the two possible hydrogen abstractions routes: abstraction from the hydroxyl site and from the methyl site.

To complement the experimental work, theoretical studies were also initiated. These theoretical studies provided the necessary insight for simulating the H- and D-atom profiles and for determining the site specific branching ratios. The present efforts represent the first such direct measurements of branching ratios in high temperature abstractions. The experiments, theory and modeling will be discussed at length in this study.

This work was performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, U.S. Department of Energy, under contract number DE-AC02-06CH11357.

References:

1)      Srinivasan, N. K.; Su, M.-C.; Michael, J. V. J. Phys. Chem. A 2007, 111, 3951-3958.

2)      Jasper A. W.; Klippenstein, S. J.; Harding, L. B.; Ruscic, B. J. Phys. Chem. A 2007, 111, 3932-3950.