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submitted on 28.05.2020 and posted on 29.05.2020by Mark Goldman, Jesse Kroll, William H. Green
Bio-derived isobutanol has been approved as a gasoline additive in the
U.S., but our understanding of its combustion chemistry still has
significant uncertainties. Detailed quantum calculations could improve
model accuracy leading to better estimation of isobutanol’s combustion
properties and its environmental impacts. This work examines 47
molecules and 38 reactions involved in the first oxygen addition to
isobutanol’s three alkyl radicals located α, β, and γ to the hydroxide.
Quantum calculations were mostly done at
CCSD(T)-F12/cc-pVTZ-F12//B3LYP/CBSB7, with 1-D hindered rotor
corrections obtained at B3LYP/6-31G(d). The resulting potential energy
surfaces are the most comprehensive isobutanol peroxy networks published
to date. Canonical transition state theory and a 1-D microcanonical
master equation are used to derive high-pressure-limit and
pressure-dependent rate coefficients, respectively. At all conditions
studied, the recombination of α- isobutanol radical with O2 forms HO2
and isobutanal. The recombination of γ-isobutanol radical with O2 forms
a stabilized hydroperoxy alkyl radical below 400 K, water and an alkoxy
radical at higher temperatures, and HO2 and an alkene above 1200 K. The
recombination of β-isobutanol radical with O2 results in a mixture of
products between 700-1100 K, forming acetone, formaldehyde and OH at
lower temperatures and forming HO2 and alkenes at higher temperatures.
The barrier heights, high-pressure-limit rates, and pressure-dependent
kinetics generally agree with the results from previous quantum
chemistry calculations. Six reaction rates in this work deviate by over
three orders of magnitude from kinetics in detailed models of isobutanol
combustion, suggesting the rates calculated here can help improve
modeling of isobutanol combustion and its environmental fate.
This fundamental research was funded by grants to MIT from the NSF and from BP. We note that BP is a part owner of BUTAMAX, a company commercializing the biofuel isobutanol. The authors declare that they have no financial interest in isobutanol commercialization and no conflict of interest.