Shock Tube Studies on the Decomposition of 2-Butanol

The thermal decomposition of 2-butanol have been studied at temperatures of 1045-1221 K and pressures of 1.5-6 bar using the single pulse shock tube technique. Dilute concentrations of 2-butanol have been decomposed in the presence of large quantities of a radical inhibitor. The mechanism for decomposition involves direct elimination of water producing cis- and trans-2-butene, and 1-butene, and C-C bond fission producing ethylene. Acetaldehyde, propionaldehyde, and propene were also observed in much smaller yields from C-C bond fission. The respective unimolecular rate expressions are as follows: k(C3H6(OH)-CH3 -> cis-CH3CH=CHCH3 + H2O) = 10(13.1 +/- 0.3) exp(-33414 +/- 755 K/T) s(-1); k(C3H6(OH)CH3 -> trans-CH3CH= CHCH3 + H2O) = 10(13.5 +/- 0.3) exp(-33820 +/- 755 K/T) s(-1); k(C3H6(OH)CH3 -> CH3CH2CH=CH2 + H2O) = 10(13.6 +/- 0.3) exp(-33002 +/- 755 K/T) s(-1); k(C3H6(OH)CH3 -> C2H5 center dot + (CH)-C-center dot(OH)CH3) = 10(15.9 +/- 0.3) exp(-39252 +/- 755 K/T) s(-1). These rate expressions are compared with analogous reactions for primary and tertiary butanols. They form a basis for the prediction of those for related systems. Comparison with estimated values used in the simulation of butanol combustion is indicative of the uncertainties in the rate constants that are used in such models:The activation energy of 326 kJ/mol leads to a bond dissociation energy of the CH(OH)CH3 radical (H-CH(OH)CH3) of 400 kJ/mol, in excellent agreement with earlier calculated results from theory and disagreement with the experimental results from iodination studies in the expected range.