Experimental and kinetic modeling study of n-pentane oxidation at 10 atm, Detection of complex low-temperature products by Q-Exactive Orbitrap

Abstract

Renewable feedstock such as biomass derivatives (hemicellulose, furfural) can be used to produce synthetic fuels, e.g., n-pentane, of interest for enhancing performance of diesel and gasoline engines. Whereas numerous studies of n-pentane have been published, its low temperature oxidation is not fully characterized and even recent kinetic models do not incorporate extended oxidation pathways which still need to be observed for n-pentane. In this context, a continuous flow fused-silica jet-stirred reactor (JSR) was used for studying the oxidation of 2500 ppm of n-pentane at 520‒800 K, 10 atm (representative of piston engines operating conditions), an equivalence ratio of 0.5, and a residence time of 1.5 s. Oxidation products were analyzed in the gas phase using gas chromatography (thermal conductivity, TCD and flame ionization, FID), Fourier-transform infrared spectrometry (FTIR), and electron impact ionization-quadrupole mass spectrometry (EI-qMS). Gaseous products were also dissolved in acetonitrile for characterization using flow injection analysis (FIA), high-pressure and ultra-high-pressure liquid chromatography (HPLC and UHPLC) coupled to atmospheric pressure chemical ionization (APCI) and Q-Exactive®-Orbitrap high resolution mass spectrometry (HRMS). This allowed detecting lower and higher mass oxygenated molecules such as methyl vinyl ketone (MVK) and 2,5-dihydrofuran (C4H6O), 2-butanone (C4H8O), 3-pentene-2-one (C5H8O), pentanediones (C5H8O2), cyclic ethers and pentanones (C5H10O), C3‒C5 alkylhydroperoxides (C3H8O2, C4H10O2, C5H12O2), C2‒C5 alkenylhydroperoxides (C2H4O2, C3H6O2, C4H8O2, C5H10O2), C3‒C5 keto-hydroperoxides (C3H6O3, C4H8O3, C5H10O3), and highly oxygenated molecules (C5H8O4, C5H12O4, C5H10O4, C5H10O5, C5H10O7) produced through multiply O2 addition on fuel's radicals and internal H-shifts. Among these products 15 had not been reported before. Simulation of the present experiments showed discrepancies between experimental results and predictions.