2 research outputs found
Shock Tube Study on the Thermal Decomposition of <i>n</i>‑Butanol
Dilute concentrations of normal-butanol has been decomposed in single pulse shock tube studies in the presence of large quantities of a chemical inhibitor that suppresses contributions from chain decomposition. Reaction temperatures and pressures are in the range of [1126–1231] K and [1.3–6.5] bar. Ethylene and 1-butene are the only products. The mechanism of the initial decomposition steps involves direct elimination of water and C–C bond cleavage. The fundamental high pressure unimolecular decomposition rate expressions are <i>k</i>(C<sub>4</sub>H<sub>9</sub>OH → CH<sub>3</sub> + CH<sub>2</sub>CH<sub>2</sub>CH<sub>2</sub>OH) = 10<sup>16.4±0.4</sup> expÂ(42410 ± 800 [K]/<i>T</i>) s<sup>–1</sup>; <i>k</i>(C<sub>4</sub>H<sub>9</sub>OH → CH<sub>3</sub>CH<sub>2</sub> + CH<sub>2</sub>CH<sub>2</sub>OH) = 10<sup>16.4±0.4</sup> expÂ(−41150 ± 800 [K]/<i>T</i>) s<sup>–1</sup>; <i>k</i>(C<sub>4</sub>H<sub>9</sub>OH → CH<sub>3</sub>CH<sub>2</sub>CH<sub>2</sub> + CH<sub>2</sub>OH) = 10<sup>16.4±0.4</sup> expÂ(−41150 ± 800 [K]/<i>T</i>) s<sup>–1</sup>; and <i>k</i>(C<sub>4</sub>H<sub>9</sub>OH → CH<sub>3</sub>CH<sub>2</sub>CHî—»CH<sub>2</sub> + H<sub>2</sub>O) = 10<sup>14.0±0.4</sup> expÂ(−35089 ± 800 [K]/<i>T</i>) s<sup>–1</sup>, where the rate expressions for C–C bond cleavage are based on assumptions regarding the relative rates of the three processes derived from earlier studies on the effect of an OH group on rate expressions. All reactions are in the high pressure limit and suggest that the step size down in the presence of argon is at least 1300 cm<sup>–1</sup>. These rate expressions are consistent with the following H–C bond dissociation energies: BDEÂ(H–CH<sub>2</sub>CH<sub>2</sub>CH<sub>2</sub>OH) = 417.2 ± 7 kJ/mol, BDEÂ(H–CH<sub>2</sub>CH<sub>2</sub>OH) = 419.2 ± 7 kJ/mol, and BDEÂ(H–CH<sub>2</sub>OH) = 401.7 ± 9 kJ/mol, with an estimated uncertainty of 6 kJ/mol. The kinetics and thermodynamic results are compared with estimates used in the building of combustion kinetics databases
Dehydration of Isobutanol and the Elimination of Water from Fuel Alcohols
Rate coefficients for the dehydration
of isobutanol have been determined
experimentally from comparative rate single pulse shock tube measurements
and calculated via multistructural transition state theory (MS-TST).
They are represented by the Arrhenius expression, <i>k</i>(isobutanol → isobutene + H<sub>2</sub>O)<sub>experimental</sub> = 7.2 × 10<sup>13</sup> expÂ(−35300 K/<i>T</i>) s<sup>–1</sup>. The theoretical work leads to
the high pressure rate expression, <i>k</i>(isobutanol →
isobutene + H<sub>2</sub>O)<sub>theory</sub> = 3.5 × 10<sup>13</sup> expÂ(−35400 K/<i>T</i>) s<sup>–1</sup>. Results are thus within a factor of 2 of each other. The experimental
results cover the temperature range 1090–1240 K and pressure
range 1.5–6 atm, with no discernible pressure effects. Analysis
of these results, in combination with earlier single pulse shock tube
work, made it possible to derive the governing factors that control
the rate coefficients for alcohol dehydration in general. Alcohol
dehydration rate constants depend on the location of the hydroxyl
group (primary, secondary, and tertiary) and the number of available
H-atoms adjacent to the OH group for water elimination. The position
of the H-atoms in the hydrocarbon backbone appears to be unimportant
except for highly substituted molecules. From these correlations,
we have derived <i>k</i>(isopropanol → propene +
H<sub>2</sub>O) = 7.2 × 10<sup>13</sup> expÂ(−33000
K/<i>T</i>) s<sup>–1</sup>. Comparison of experimental
determination with theoretical calculations for this dehydration,
and those for ethanol show deviations of the same magnitude as for
isobutanol. Systematic differences between experiments and theoretical
calculations are common