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, k(isobutanol → isobutene + H2O)experimental = 7.2 × 1013 exp(−35300 K/T) s–1. The theoretical work leads to the high pressure rate expression, k(isobutanol → isobutene + H2O)theory = 3.5 × 1013 exp(−35400 K/T) s–1. 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 k(isopropanol → propene + H2O) = 7.2 × 1013 exp(−33000 K/T) s–1. 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.