High Temperature Measurements of the Reaction of Heptanone Isomers with Hydroxyl Radicals

Ketones are considered potential biofuel candidates and may be produced using lignocellulosic biomass. In the current study, the reaction rate coefficients of three heptanone isomers (4-heptanone, 2-methyl-3-hexanone, 2,4-dimethyl-3-pentanone) with hydroxyl (OH) radicals are investigated experimentally. During the combustion and oxidation of hydrocarbon fuels, the highly reactive OH radicals oxidize the fuel molecules representing the major fuel consumption pathway under combustion conditions. A shock tube facility at King Abdullah University of Science and Technology (KAUST) was used as a batch reactor and a ring-dye laser system was applied to create a narrow line-width laser beam at 307 nm to follow the concentration of OH radicals as the reaction progresses. Since OH radicals are unstable species, tert-butyl hydroperoxide (TBHP) is employed as an OH radical thermal precursor. A mixture of ~300 ppm of the ketone and ~15 ppm of TBHP is prepared in argon diluent, and the mixture is then introduced into the shock tube. Reaction rate coefficients are then measured under pseudo-first-order kinetics over a temperature range of 851-1328 K and pressures near 1.5 atm. Available detailed chemical kinetic model for 3-pentanone is upgraded with sub-mechanisms of TBHP and the three studied ketones. The modified chemical kinetic model is used in conjunction with the CHEMKIN-PRO commercial program to simulate OH profiles. Furthermore, a fitting procedure is used to obtain the best match between the simulation and experimental OH time-history profiles in order to extract the reaction rate coefficients of the following reactions: 4-heptanone + OH --> Products (R1) 2-methyl-3-hexanone + OH --> Products (R2) 2, 4-dimethyl-3-pentanone + OH --> Products (R3)

The Arrhenius forms of the obtained rate coefficients are:

k₁ = 3.15x10^-10 exp(-2949 K/T) cm³ molecule^-1 s^-1,

k₂ = 7.92x10^-11 exp(-1552 K/T) cm³ molecule^-1 s^-1,

k₃ = 7.67x10^-11 exp(-1883 K/T) cm³ molecule^-1 s^-1.

Finally, the experimentally measured reaction rate coefficients are compared with a well-known rate estimation method, known as Structure-Activity Relationship (SAR).