Hydrocarbon formation from methylating agents over the zeolite catalyst ZSM-5. Comments on the mechanism of carbon–carbon bond and methane formation

Abstract

The mechanism of hydrocarbon formation from methanol has been studied by reacting methylating agents MeX (X = OH, I, OSO₃Me) over the acidic form of the pentasil zeolite ZSM-5 and the sodium form of its conjugate base. At >0.1% conversions the product distributions obtained are similar, indicating that similar mechanisms for carbon–carbon bond formation operate with the three reagents. Hydrocarbon formation (CH₄ and C₂H₄) from Me₂SO₄ over Na-ZSM-5 was observed and is considered to be strong evidence against the involvement of dimethyloxonium methylide as an intermediate since such a species cannot be formed with this reactant under basic conditions. Further evidence against the involvement of a dimethyloxonium methylide is given using reactivity comparisons of oxygen and sulphur-containing reagents. MeI and Me₂SO₄ were found to be less reactive than MeOH and at low conversion the primary hydrocarbon products observed, CH₄and C₂H₄, suggest that these are the primary products formed in methanol conversion. Under conditions where conversion increased with reaction time, the methane/alkenes ratio decreased markedly and this is considered to be inconsistent with a mechanism involving a surface carbene intermediate formed in a one-step process from the reagent MeX. Instead it is proposed that the crucial first step in the mechanism is the formation of a surface methoxyl species which is primarily responsible for CH₄ formation at low conversions. Deprotonation of the surface methoxyl species generates a surface-bonded oxonium methylide which subsequently reacts to form the initial carbon–carbon bond. This mechanistic proposal is both consistent with and unifies the available experimental data obtained for this reaction to date.