Methanol coupling in the zeolite chabazite studied via Car–Parrinello molecular dynamics

Abstract

We have used Car–Parrinello constrained molecular dynamics to study the coupling of two molecules of methanol in the zeolite chabazite to form ethanol and water. We have chosen to study this reaction because it represents the formation of the first C–C bond, which is thought to be the rate limiting step for the MTO and MTG processes. We have elucidated a new mechanism for this reaction that does not require the prior formation of surface methoxy groups or dimethyl ether intermediates. The mechanism involves stable intermediates of methane and protonated formaldehyde. We have also calculated an upper bound of the free energy barrier for the overall reaction, and found that it compares favourably with the rough experimental measurements available. Finally, we consider what are the natural reaction coordinates for the methanol–methanol coupling process.