Chemistry of Simple Organic Peroxy Radicals under Atmospheric through Combustion Conditions: Role of Temperature, Pressure, and NOx Level.

Organic peroxy radicals (RO2) are key intermediates in the oxidation of organic compounds in both combustion systems and the atmosphere. While many studies have focused on reactions of RO2 in specific applications, spanning a relatively limited range of reaction conditions, the generalized behavior of RO2 radicals across the full range of reaction conditions (temperatures, pressures, and NO levels) has, to our knowledge, never been explored. In this work, two simple model systems, n-propyl peroxy radical and γ-isobutanol peroxy radical, are used to evaluate RO2 fate using pressure-dependent kinetics. The fate of these radicals was modeled based on literature data over 250-1250 K, 0.01-100 bar, and 1 ppt to 100 ppm of NO, which spans the typical range of atmospheric and combustion conditions. Covering this entire range provides a broad overview of the reactivity of these species under both atmospheric and combustion conditions, as well as under conditions intermediate to the two. A particular focus is on the importance of reactions that were traditionally considered to occur in only one of the two sets of conditions: RO2 unimolecular isomerization reactions (long known to occur in combustion systems but only recently appreciated in atmospheric systems) and RO2 bimolecular reactions of RO2 with NO (thought to occur mainly in atmospheric systems and rarely considered in combustion chemistry).

Pressure-dependent kinetics of peroxy radicals formed in isobutanol combustion.

Bio-derived isobutanol has been approved as a gasoline additive in the US, 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, ß, and γ to the hydroxide. Quantum calculations are 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 γ-isobutanol radical with O2 forms HO2 + isobutanal. The recombination of ß-isobutanol radical with O2 forms a stabilized hydroperoxy alkyl radical below 400 K, water + an alkoxy radical at higher temperatures, and HO2 + an alkene above 1200 K. The recombination of ß-isobutanol radical with O2 results in a mixture of products between 700-1100 K, forming acetone + formaldehyde + OH at lower temperatures and forming HO2 + 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.

Correct Symmetry Treatment for X + X Reactions Prevents Large Errors in Predicted Isotope Enrichment.

Confusion over how to account for symmetry numbers when reactants are identical can cause significant errors in isotopic studies. An extraneous factor of 2 in the reaction symmetry number, as proposed in the literature, violates reaction equilibrium and causes huge enrichment errors in isotopic analysis. In actuality, no extra symmetry factor is needed with identical reactants.

Modeling of aromatics formation in fuel-rich methane oxy-combustion with an automatically generated pressure-dependent mechanism.

With the rise in production of natural gas, there is increased interest in homogeneous partial oxidation (POX) to convert methane to syngas (CO + H2), ethene (C2H4) and acetylene (C2H2). In POX, polycyclic aromatic hydrocarbons (PAH) are important undesired byproducts. To improve the productivity of such POX processes, it is necessary to have an accurate chemical mechanism for methane-rich combustion including PAH. A new mechanism was created to capture the chemistry from C0 to C12, incorporating new information derived from recent quantum chemistry calculations, with help from the Reaction Mechanism Generator (RMG) software. For better estimation of kinetics and thermochemistry of aromatic species, including reactions through carbene intermediates, new reaction families and additional data from quantum chemistry calculations were added to RMG-database. Many of the rate coefficients in the new mechanism are significantly pressure-dependent at POX conditions. The new mechanism was validated against electron-ionization molecular beam mass spectrometry (EI-MBMS) data from a high-temperature flow reactor reported by Kohler et al. In this work quantification of additional species from those experiments is reported including phenylacetylene (C8H6), indene (C9H8), naphthalene (C10H8) and acenaphthylene (C12H8) at many temperatures for several feed compositions. Comparison of the experimental species concentration data and the new kinetic model is satisfactory; the new mechanism is generally more accurate than other published mechanisms. Moreover, because the new mechanism is composed of elementary chemical reaction steps instead of global fitted kinetics, pathway analysis of species could be investigated step-by-step to understand PAH formation. For methane-rich combustion, the most important routes to key aromatics are propargyl recombination for benzene, reactions of the propargyl radical with the phenyl radical for indene, and hydrogen abstraction acetylene addition (HACA) for naphthalene.