Enhanced Bimolecular Reaction in a Two-Component Fluid under Pyrolytic Conditions: In Situ Probing of the Pyrolysis of Jet Fuel Surrogates Using a Supersonic Expansion Molecular Beam Mass Spectrometer

Abstract

In situ mass spectrometry is demonstrated as a technique to study the pyrolysis of neat fluids at supercritical conditions. These fluids included pure hexane, benzene, and binary mixtures of the two, which were sampled in a supersonic expansion to cool and trap reactants, intermediates, and products in a molecular beam. To identify the reacting species, the molecular beam was subjected to electron impact ionization prior to analysis in a quadrupole mass filter. In addition to the previously reported gas-phase pyrolysis products for hexane and benzene, we observe the enhanced production of biphenyl in the binary mixture, which can be attributed to an energetically favored pathway by which the initial production of alkyl radicals seeds the formation of phenyl radicals via H atom abstraction reactions. These phenyl radicals quickly react to form biphenyl because of the proximity of solvent reaction partners and high collision frequency in the fluid. Our results illustrate a simple model system that highlights contemporary difficulties associated with multicomponent fuels due to the new pathways that become available even in simple mixtures.

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

“…Based on the results of our previous and current study, the chemical structures of substituted aromatics clearly differ when they are synthesized by catalysis versus high-temperature pyrolysis in the absence of a catalyst. In the former case, we have recorded the in situ CID spectra of m / z = 120 species, which can only be explained by a mixture that contains alkyl chain-substituted benzenes.…”

“…Further evidence that β C–C alkyl bonds do not survive as easily as α C–C alkyl bonds under the extreme temperature conditions comes from our previous study, which probed the pyrolysis of a neat binary mixture of benzene and hexane. In this example, we observed the enhanced production of biphenyl, which was attributed to the lower threshold for phenyl radical production via H-atom abstraction by alkyl radicals in the mixture (3.7–4.3 eV) when compared to direct benzene C–H fission (4.9 eV).…”

“…To provide in situ chemical analysis for fuels and fuel surrogates under extreme conditions by mass spectrometry (MS), we previously constructed an apparatus that samples the neat fluid via a supersonic expansion into the gas phase. − After skimming the expansion, a molecular beam is formed, which is subjected to electron-impact (EI) ionization, followed by MS using a quadrupole mass filter. This technique allows us to obtain a “snapshot” of the chemistry occurring in the condensed phase because bimolecular chemistry becomes unlikely as the collision frequency diminishes after the first few millimeters of the supersonic expansion.…”

In Situ Diagnostic of Supercritical Fuel Surrogates: Probing Heterogeneous Catalysis by Collision-Induced Dissociation in a Molecular Beam Tandem Mass Spectrometer

“…In Figure , a diagram of the instrument is given, which shows the supercritical pyrolysis source and the two mass spectrometers. In our previous studies, , we demonstrated the single quadrupole mass analyzer combined with 10 eV EI ionization to interrogate the molecular beam formed from the pyrolysis source. In this setup, the ions that are generated by EI are turned 90° into a quadrupole mass filter, which is scanned at a rate of 0.53 amu/ms.…”

“…Mass spectrometry is an alternative analytical method to optical spectrometry and is well suited for detailed chemical analysis of in situ pyrolysis reactions because species can be distinguished by their mass-to-charge ratios (m/z) with unit mass resolution using conventional quadrupole mass analyzers. Previously, we have shown that pyrolysis intermediates and products of neat fluids (n-hexane, cyclohexane, n-dodecane, and decalin), 16 binary mixtures (hexane and benzene), 17 and real fuels (Jet A) 16 can be interrogated by forming a supersonic expansion of the reacting fluid into the gas phase. The advantage of our approach when compared to standard analytical methods (e.g., LC-MS, GC-MS, etc.)…”