Biofuels have been proposed as a method to mitigate the effects of climate change as a result of fossil fuel combustion. Furans, a group of heterocyclic ethers, have been suggested as potential biofuels, in particular 2,5-dimethylfuran, which has a similar energy density to gasoline. In order to assess the feasibility for use as vehicle fuels, knowledge of the kinetics and reaction mechanisms occurring under combustion conditions is required.
A kinetic study of reactions relevant to low temperature combustion has been performed using laser flash photolysis-laser induced fluorescence spectroscopy. Chapters 4-6 describe a detailed experimental study of the reaction between the OH radical and furan and its alkylated derivatives over a range of temperatures (298-770 K) and pressures (4-150 Torr). Experiments were performed under pseudo-first-order conditions, using flash photolysis of H2O2/furan/N2 or (CH3)3COOH/furan/N2 gas mixtures with monitoring of OH radicals by laser-induced fluorescence (LIF) spectroscopy.
For the reaction of OH + furan, the room temperature rate coefficient was determined be independent of pressure, with an average value of k = (3.34 ± 0.48) × 10-11 cm3 molecule-1 s-1. Over all temperatures investigated (298-595 K) the reaction appears to be at, or close to, the high-pressure limit. Results demonstrate a negative temperature dependence over temperatures between 298 K and 595 K, with -Ea/R = (510 ± 71) K. This temperature dependence is consistent with the behaviour observed in previous studies. Results have been analysed in combination with high temperature shock tube data to provide a parameterisation of the kinetics that describes a temperature range relevant to both low and high temperature combustion.
The OH + 2-methylfuran (2-MF) room temperature rate coefficient was measured to be k = (7.38 ± 0.37) × 10-11 cm3 molecule-1 s-1. Results demonstrate a negative temperature dependence for the reaction of OH + 2-MF over temperatures between 298 K and 770 K, with -Ea/R = (716 ± 79) K, and no significant dependence on pressure over this temperature range. Results were combined with shock tube data to provide a parameterisation for use in combustion models over a wide temperature range.
For the reaction of OH + 2,5-DMF, the rate coefficient at 298 K, k = (1.10 ± 0.10) × 10-10 cm3 molecule-1 s-1, was found to be at, or close to, the high-pressure limit over the pressure range studied. Above 298 K the OH + 2,5-DMF reaction exhibits pressure-dependent kinetics with a negative temperature dependence observed over the temperature and pressure ranges investigated. Fits to the data indicate a low-pressure limit of k0 = (2.80 ± 1.60) × 10-29 exp((1930 ± 220)/T) cm6 molecule-1 s-1 and a high-pressure limit of k∞ = (1.50 ± 0.10) × 10-11 exp((640 ± 20)/T) cm3 molecule-1 s-1. The high pressure limiting rate coefficients were analysed in conjunction with previous shock tube data to describe the kinetics over a wide temperature range.
For OH + 2,5-dimethylfuran (2,5-DMF), the potential energy surface of the reaction was investigated at the M06-2X/cc-pVTZ level of theory. Master equation analyses have been performed for each reaction system using the Master Equation Solver for Multi-Energy well Reactions (MESMER) to complement the results obtained experimentally. The MESMER calculations indicate that addition of OH to the furan ring is dominant under the experimental conditions of this study, with addition to the C1 site dominant for furan, to the C2 and C5 sites, approximately equally, for 2-MF, and to the C2 site for 2,5-DMF.
Chapter 8 changes focus to the kinetics of the QOOH radical. QOOH radicals are key species in autoignition, produced by internal isomerisations of RO2 radicals, and are central to chain branching reactions in low temperature combustion. The kinetics of QOOH radical decomposition and reaction with O2 have been determined as a function of temperature and pressure, for the first time, using observations of OH radical production and decay following H-atom abstraction from tertiary-butyl hydroperoxide ((CH3)3COOH) by Cl atoms to produce QOOH (CH2(CH3)2COOH) radicals. The kinetics of QOOH decomposition have been investigated as a function of temperature, in the range 251 to 298 K, and pressure, in the range 10 to 350 Torr, in helium and nitrogen bath gases, and those of the reaction between QOOH and O2 have been investigated as a function of temperature, in the range 251 to 304 K, and pressure, in the range 10 to 100 Torr. Decomposition of the QOOH radicals was observed to display temperature and pressure dependence, with a barrier height for decomposition of (44.7 ± 4.0) kJ mol-1 determined by master equation fitting to the experimental data. The rate coefficient for the reaction between QOOH and O2 was determined to be (5.6 ± 1.7) × 10-13 cm3 s-1 at 298 K, with no significant dependence on pressure, and can be described by the Arrhenius parameters A = (7.3 ± 6.8) × 10-14 cm3 s 1 and Ea = -(5.4 ± 2.1) kJ mol-1 in the temperature range 251 to 304 K.
