Distinguishing Condensed-Phase Cracking Products with the Same Mass-To-Charge Ratio Using a Triple Quadrupole Mass Spectrometer in Product Scan Mode

In situ chemical analysis of aviation fuels at high temperatures and pressures is needed to optimize endothermic chemistry while mitigating coke deposits during high-speed flight applications. Toward this aim, we directly sample neat fluids at extreme temperatures (200-1000 degrees C) and pressures (400-1000 psi) by a supersonic expansion into the gas phase. The resulting molecular beam is ionized by electron-impact and analyzed by tandem mass spectrometry. Using this technique, we can distinguish cracking products not only by their different mass-to-charge ratios (m/z) but also by their distinct fragmentation patterns. In the current study, we have probed mass-degenerate aromatic products (m/z = 120), which were formed by catalytic cracking of a neat n-hexane fuel surrogate over a H-ZSM-5 catalyst. Tandem mass spectrometry was carried out using a triple quadrupole instrument via collision-induced dissociation. By comparing the fragmentation spectrum of the unknown catalytic product to the spectra of pure standards of m/z = 120 isomers, we have revealed structural insights about the catalytic product. Based on this comparison, alkyl chain-substituted and methyl-substituted benzenes are both likely present in the reaction mixture. This result contrasts our previous observations for pyrolysis products in the absence of a catalyst, which are predominantly methyl-substituted aromatics. Therefore, we have shown that alkyl chain-substituted aromatics are more stable at the lower temperatures associated with catalytic cracking than at the extreme temperatures associated with pyrolysis. Such a detailed structural insight confirms that in situ mass spectrometry is a powerful tool for chemical diagnostics of neat fuel surrogates operating under extreme conditions. Therefore, similar characterization experiments applied to real fuels that are relevant to the Air Force are now tractable.