Pressure-Dependent Competition among Reaction Pathways from First- and Second‑O₂ Additions in the Low-Temperature Oxidation of Tetrahydrofuran

Abstract

We report a combined experimental and quantum chemistry study of the initial reactions in low-temperature oxidation of tetrahydrofuran (THF). Using synchrotron-based time-resolved VUV photoionization mass spectrometry, we probe numerous transient intermediates and products at <i>P</i> = 10–2000 Torr and <i>T</i> = 400–700 K. A key reaction sequence, revealed by our experiments, is the conversion of THF-yl peroxy to hydroperoxy-THF-yl radicals (QOOH), followed by a second O₂ addition and subsequent decomposition to dihydrofuranyl hydroperoxide + HO₂ or to γ-butyrolactone hydroperoxide + OH. The competition between these two pathways affects the degree of radical chain-branching and is likely of central importance in modeling the autoignition of THF. We interpret our data with the aid of quantum chemical calculations of the THF-yl + O₂ and QOOH + O₂ potential energy surfaces. On the basis of our results, we propose a simplified THF oxidation mechanism below 700 K, which involves the competition among unimolecular decomposition and oxidation pathways of QOOH