Terpene speciation: Analytical insights into the oxidation and pyrolysis of limonene and 1,8-cineole via molecular-beam mass spectrometry

Abstract

Comprehensive speciation datasets for the stoichiometric oxidation and pyrolysis of the two monoterpenes limonene (C₁₀H₁₆) and 1,8-cineole (C₁₀H₁₈O) are measured in an atmospheric laminar flow reactor using electron-ionization molecular-beam mass spectrometry. This setup allows direct sampling from the reactive flow and preserves the actual gas composition. Furthermore, clear determination of the exact elemental composition of the formed species is possible with the used time-of-flight mass spectrometer. Limonene is a monocyclic terpene and 1,8-cineole is a saturated bicyclic terpene ether and both terpenes might be potential biofuel candidates. Focus in this study is the intermediate temperature region between 673 and 1173 K to obtain insights into the first fuel decomposition steps and the formation of typical soot precursors. The obtained mole fraction profiles for over 40 species in each of the investigated terpenes are a first step for future development and validation of chemical kinetic combustion mechanisms. While the overall species pool is similar, significant concentration differences can be observed for certain combustion intermediates. For limonene, larger quantities of C₈–C₁₀ hydrocarbons are detected and most of them are probably substituted benzenes or cyclohexadienes formed from hydrogen abstraction. Some reaction steps in the decomposition of limonene may also involve initial isomerization of the fuel molecule. In contrast, direct formation of C₇H₁₁ radicals and acetone (C₃H₆O) is identified as an important decomposition step of 1,8-cineole. C₇H₁₁ is then a source of toluene (C₇H₈) and cyclohexadienes (C₆H₈). Generally, a higher sooting propensity of limonene compared to 1,8-cineole can be expected due to the higher concentrations of polycyclic aromatic hydrocarbons (PAHs) in the investigated temperature range. During limonene oxidation, formation of oxygenated species larger than the fuel molecule are observed and might represent carbonyls or cyclic ethers from the first oxygen addition due to low-temperature chemistry.