Activation of CH4 and C2H6 by Al2O2+ from 300 to 600 K

Abstract

The kinetics of Al₂O₂⁺ + CH₄ and Al₂O₂⁺ + C₂H₆ are measured from 300 to 600 K at pressures near ∼0.35 Torr in a Selected-Ion Flow Tube apparatus. The reaction with CH₄ proceeds by hydrogen abstraction to yield an open chain AlOAlOH⁺ +CH₃. The rate constant is nearly temperature-independent at 7 ± 2 × 10⁻¹¹ cm³ s⁻¹. This competes with association, which decreases sharply with temperature (k = 3.3 ± 0.8 × 10⁻¹⁰ (T/300 K)^−3.1±0.2 cm³ s⁻¹). The reaction with C₂H₆ primarily yields Al₂O₂H₂⁺ + C₂H₄. The rate constant for this channel k_300K = 4 ± 1 × 10⁻¹⁰ cm³ s⁻¹ with a very slight negative temperature dependence. A second channel producing C₂H₅ + Al₂O₂H⁺ rises steeply with temperature (k = 10 ± 2 × 10⁻¹⁰ e^{−0.12 eV/kT} cm³ s⁻¹), and finally association decreases steeply with temperature (k = 6 ± 1.5 × 10⁻¹⁰ (T/300 K)^−3.2±0.2 cm³ s⁻¹). The reaction with methane is well-described using statistical theory based on reaction coordinates calculated using density functional theory. The total rate constant for the ethane reaction is also well-described using statistical theory, but the product branching is not, suggesting post-transition state non-statistical dynamics. One possibility is that the ethane reaction unexpectedly produces a higher energy C_2v isomer of Al₂O₂H⁺. The results support the prior interpretation that Al₂O₂⁺ activates hydrocarbons via a proton-coupled electron transfer (PCET) mechanism and not a hydrogen-atom transfer (HAT) mechanism.