Laser Photolysis Kinetic Study of OH Radical Reactions with Methyl tert-Butyl Ether and Trimethyl Orthoformate under Conditions Relevant to Low Temperature Combustion: Measurements of Rate Coefficients and OH Recycling.

Methyl tertiary butyl ether (MTBE) and trimethyl orthoformate (TMOF) are potential biofuel ethers and could replace conventional fossil fuels, or act as additives to aid combustion. Laser flash photolysis with laser-induced fluorescence detection of the OH radical has been used to measure the rate coefficients of the OH reaction with these ethers, from 298 K to approximately 740 K. The temperature dependence of the rate coefficients is parametrized as kOH+MTBE(298-680 K) = 9.8 × 10-13× ( T/298)2.7 × exp(2500/R T) cm3 molecule-1 s-1 and kOH+TMOF(298-744 K) = 8.0 × 10-13 × [( T/298)2.6 + ( T/298)-8.1] × exp[2650/R T] cm3 molecule-1 s-1. The room temperature (298 K) bimolecular rate coefficients were measured as kOH+MTBE = (2.81 ± 0.32) × 10-12 cm3 molecule-1 s-1 and kOH+TMOF = (4.65 ± 0.50) × 10-12 cm3 molecule-1 s-1 where the errors represent statistical uncertainties at the 2σ level in combination with an estimated 10% systematic error. Regeneration of OH radicals was observed for both reactions at higher temperatures in the presence of O2 via biexponential OH decays, which were observed above 489 K and 568 K, for TMOF and MTBE respectively. The OH yield from MTBE/O2, between 620 and 700 K, was invariant with the concentration of oxygen (1015-1018 molecules cm-3) at (36 ± 5)%. Mechanisms for OH regeneration from MTBE are briefly discussed and compared with those in the literature and from dimethyl and diethyl ether. The lower OH yield from MTBE, compared to these other ethers, is most likely due to competition with an HO2 formation channel.