Products of the Phenyl Radical (C6H5, X2A1) - Acetylene (C2H2, X1Σg+) Reaction in the 800-1,200 K Temperature Range.

Abstract

A pulsed flow high-temperature microreactor in tandem with vacuum ultraviolet photoionization (VUV PI) reflectron time-of-flight mass spectrometry (Re-TOF-MS) was employed to determine branching ratios of key product channels of the reaction of the phenyl radical (C₆H₅) with acetylene (C₂H₂) highlighting the role of the Hydrogen Abstraction - C₂H₂ Addition (HACA) mechanism at temperatures of 800-1,200 K. Temporal profiles of m/z = 26 (C₂H₂ ⁺), 30 (NO⁺), 52 (C₄H₄ ⁺), 77 (C₆H₅ ⁺), 78 (C₆H₆ ⁺), 102 (C₆H₅C₂H⁺), 103 (C₆H₅C₂H₂ ⁺), 107 (C₆H₅NO⁺), 128 (C₁₀H₈ ⁺), and 154 (C₁₂H₁₀ ⁺) were recorded and characterized within a molecular beam. From the time profiles of the ion signals and Computational Fluid Dynamics (CFD) calculations in the transient mode, zones of the molecular beam corresponding to the open valve duration time were identified. The gas velocity along the microreactor tube axis with the opened valve is strongly influenced by the backing pressure (p) in front of the nozzle and appears to be subsonic for p = 100 Torr and supersonic for p ≥ 300 Torr. The main quantified finding is that phenylacetylene (C₆H₅C₂H) constitutes the dominant product, accounting for up to 91% of consumed phenyl radicals at 1,200 K, whereas naphthalene (C₁₀H₈) formation is favored at lower temperatures and higher pressures, peaking at the yield of 53% (800 K, 300 Torr). The observed temperature and pressure dependence of naphthalene and phenylacetylene yields are consistent with the HACA mechanism.