Carbon–Carbon Bond Formation by Activation of CH₃F on Alumina

The reaction of CH₃F on partially dehydroxylated Al₂O₃ yields isobutene and higher hydrocarbons along with dimethyl ether. In a combined experimental and computational study, we show that the reaction starts through the initial formation of a methoxy intermediate observed experimentally and formed via an SN₂ type process between surface O atoms and the CH₃F molecule, which undergoes C–F activation on the most acidic Lewis site of the 110 termination of the Al₂O₃ surface. Coordination of an additional CH₃F molecule on adjacent Al Lewis acid sites generates methyl cation-like species, which abstract a hydride from the adjacent initially generated surface methoxy species, yielding a “AlOCH₂⁺” oxonium-like transient structure and CH₄. Both species can further react, forming the first carbon–carbon bond by producing an ethoxy intermediate in a process very exoergic and with accessible energy barriers. This ethoxy intermediate can further react in successive exoergic steps with additional “activated” CH₃F on Al sites leading to higher alkoxy species, which are finally liberated as isobutene and higher hydrocarbons.