Publications

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Increasing energy costs, the dependence on foreign oil supplies, and environmental concerns have emphasized the need to produce sustainable renewable fuels and chemicals. The strategy for producing next-generation biofuels must include efficient processes for biomass conversion to liquid fuels and the fuels must be compatible with current and future engines. Unfortunately, biofuel development generally takes place without any consideration of combustion characteristics, and combustion scientists typically measure biofuels properties without any feedback to the production design. We seek to optimize the fuel/engine system by bringing combustion performance, specifically for advanced next-generation engines, into the development of novel biosynthetic fuel pathways. Here we report an innovative coupling of combustion chemistry, from fundamentals to engine measurements, to the optimization of fuel production using metabolic engineering. We have established the necessary connections among the fundamental chemistry, engine science, and synthetic biology for fuel production, building a powerful framework for co-development of engines and biofuels.

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The pulsed photolytic chlorine-initiated oxidation of methyl-tert-butyl ketone (MTbuK), di-tert-butyl ketone (DTbuK), and a series of partially deuterated diethyl ketones (DEK) is studied in the gas phase at 8 Torr and 550-650 K. Products are monitored as a function of reaction time, mass, and photoionization energy using multiplexed photoionization mass spectrometry with tunable synchrotron ionizing radiation. The results establish that the primary 3-oxoalkyl radicals of those ketones, formed by abstraction of a hydrogen atom from the carbon atom in γ-position relative to the carbonyl oxygen, undergo a rapid rearrangement resulting in an effective 1,2-acyl group migration, similar to that in a Dowd-Beckwith ring expansion. Without this rearrangement, peroxy radicals derived from MTbuK and DTbuK cannot undergo HO2 elimination to yield a closed-shell unsaturated hydrocarbon coproduct. However, not only are these coproducts observed, but they represent the dominant oxidation channels of these ketones under the conditions of this study. For MTbuK and DTbuK, the rearrangement yields a more stable tertiary radical, which provides the thermodynamic driving force for this reaction. Even in the absence of such a driving force in the oxidation of partially deuterated DEK, the 1,2-acyl group migration is observed. Quantum chemical (CBS-QB3) calculations show the barrier for gas-phase rearrangement to be on the order of 10 kcal mol-1. The MTbuK oxidation experiments also show several minor channels, including β-scission of the initial radicals and cyclic ether formation. © 2013 American Chemical Society.

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This work measures the absolute photoionization cross-section of the vinyl radical (σvinyl(E)) between 8.1 and 11.0 eV. Two different methods were used to obtain absolute cross-section measurements: 193 nm photodissociation of methyl vinyl ketone (MVK) and 248 nm photodissociation of vinyl iodide (VI). The values of the photoionization cross-section for the vinyl radical using MVK, σvinyl(10.224 eV) = (6.1 ± 1.4) Mb and σvinyl(10.424 eV) = (8.3 ± 1.9) Mb, and using VI, σvinyl(10.013 eV) = (4.7 ± 1.1) Mb, σ vinyl(10.513 eV) = (9.0 ± 2.1) Mb, and σ vinyl(10.813 eV) = (12.1 ± 2.9) Mb, define a photoionization cross-section that is ∼1.7 times smaller than a previous determination of this value. © 2013 AIP Publishing LLC.

The CH(X2Π) + propene reaction is studied in the gas phase at 298 K and 4 Torr (533.3 Pa) using VUV synchrotron photoionization mass spectrometry. The dominant product channel is the formation of C 4H6 (m/z 54) + H. By fitting experimental photoionization spectra to measured spectra of known C4H6 isomers, the following relative branching fractions are obtained: 1,3-butadiene (0.63 ± 0.13), 1,2-butadiene (0.25 ± 0.05), and 1-butyne (0.12 ± 0.03) with no detectable contribution from 2-butyne. The CD + propene reaction is also studied and two product channels are observed that correspond to C 4H6 (m/z 54) + D and C4H5D (m/z 55) + H, formed at a ratio of 0.4 (m/z 54) to 1.0 (m/z 55). The D elimination channel forms almost exclusively 1,2-butadiene (0.97 ± 0.20) whereas the H elimination channel leads to the formation of deuterated 1,3-butadiene (0.89 ± 0.18) and 1-butyne (0.11 ± 0.02); photoionization spectra of undeuterated species are used in the fitting of the measured m/z 55 (C 4H5D) spectrum. The results are generally consistent with a CH cycloaddition mechanism to the C-C bond of propene, forming 1-methylallyl followed by elimination of a H atom via several competing processes. The direct detection of 1,3-butadiene as a reaction product is an important validation of molecular weight growth schemes implicating the CH + propene reaction, for example, those reported recently for the formation of benzene in the interstellar medium (Jones, B. M. Proc. Natl. Acad. Sci. U.S.A. 2011, 108, 452-457). © 2013 American Chemical Society.

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Hydrocarbon autoignition has long been an area of intense fundamental chemical interest, and is a key technological process for emerging clean and efficient combustion strategies. Carbon-centered radicals containing an -OOH group, commonly denoted QOOH radicals, are produced by isomerization of the alkylperoxy radicals that are formed in the first stages of oxidation. These QOOH radicals are among the most critical species for modeling autoignition, as their reactions with O2 are responsible for chain branching below 1000 K. Despite their importance, no QOOH radicals have ever been observed by any means, and only computational and indirect experimental evidence has been available on their reactivity. Here, we directly produce a QOOH radical, 2-hydroperoxy-2-methylprop-1-yl, and experimentally determine rate coefficients for its unimolecular decomposition and its association reaction with O 2. The results are supported by high-level theoretical kinetics calculations. Our experimental strategy opens up a new avenue to study the chemistry of QOOH radicals in isolation. © 2013 the Owner Societies.

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