Large molecules, ions, radicals, and small soot particles in fuel-rich hydrocarbon flames: Part VI: positive ions of aliphatic and aromatic hydrocarbons in a low-pressure premixed flame of n-butane and oxygen

Abstract

Previous studies of large hydrocarbons in fuel-rich hydrocarbon flames have shown that the hydrogen content of intermediate large polycyclic aromatic hydrocarbons (PAH), which is correlated with their molecular structure, plays an important role in their growth pathways. For this reason it was investigated whether n-butane as fuel which is rich in hydrogen would have influence on the formation of H-rich PAHs. Because large, positive PAH ions rather closely simulate the fate of their uncharged counterparts, the method of sampling flame ions was chosen for this study. A low-pressure premixed n-butane-oxygen flame (C/O = 0.60, p = 2.66 kPa, unburned gas velocity, v u = 50 cm/s) was analyzed for large ions using molecular beam sampling combined with a time-of-flight mass spectrometer equipped with a reflectron. In contrast to flames of acetylene or benzene a large number of aliphatic protonated ions is formed in the first part of the oxidation zone. These ions contain as many as about 12 to 18 carbon atoms. In the early stages their average H/C is > 1, and some aliphatic ions are nearly saturated. Besides the prominent C 13H 9 + and C 19H 11 + which belong to the so-called ‘standard PAH’ there is a great number of larger hydrogen-rich PAH + in the oxidation zone which are interpreted as PAH + with aliphatic side groups and pericondensed PAH with open structures. The largest PAH + have about 50 carbon atoms. At the end of the oxidation zone (the region of maximum temperature) the C-distributions of pericondensed PAH have their maxima with the formulae of standard structures found in acetylene and benzene flames. However, a large fraction of the intermediate, hydrogen-rich PAH has decomposed upstream of this point. PAH + concentrations increase again slightly in the burned gas where their C-distributions indicate growth by addition of acetylene.