Novel methane activation by sulfur dioxide and molecular oxygen via trifluoroacetylsulfuric acid

Abstract

Sulfur trioxide and sulfuric acid have been used for the conversion of methane (CH₄) to methanesulfonic acid (MSA), whereas their precursor sulfur dioxide (SO₂) has rarely been used. Herein, we report a novel methane sulfonation method using SO₂ and its newly explored free radical mechanism. We developed practical conditions to transform SO₂ to trifluoroacetyl sulfuric acid (TFAOSO₃H) by reacting with molecular oxygen in trifluoroacetic acid. At 50 °C, the resulting TFAOSO₃H facilitates the carbon–hydrogen bond activation of methane to efficiently generate MSA via a radical mechanism with hydrogen peroxide as the radical initiator. As limiting reagents, SO₂ and CH₄ were selectively converted to MSA in 74 and 95% yields, respectively. Since the greenhouse gas CH₄ and the toxic gas SO₂ can be fully used to produce MSA as a value-added transportable liquid product, this pragmatic one-pot two-step protocol would have great significance in environmental and economical applications.