Multilevel quantum mechanical calculations show the role of promoter molecules in the dehydration of methanol to dimethyl ether in H-ZSM-5

Methyl carboxylate esters promote the formation of dimethyl ether (DME) from the dehydration of methanol in HZSM-5 zeolite. We employ a multilevel quantum method to explore the possible associative and dissociative mechanisms in the presence, and absence, of six methyl ester promoters. This hybrid method combines densityfunctional theory, with dispersion corrections (DFT-D3), for the full periodic system, with second-order Møller–Plesset perturbation theory (MP2) for small clusters representing the reaction site, and coupled cluster with single, double, andperturbative triple substitution (CCSD(T)) for the reacting molecules. The calculated adsorption enthalpy of methanol,and reaction enthalpies of the dehydration of methanol to DME within H-ZSM-5, agree with experiment to withinchemical accuracy (∼4 kJ mol−1). For the promoters, a reaction pathway via an associative mechanism gives loweroverall reaction enthalpies and barriers compared to the reaction with methanol only. Each stage of this mechanism isexplored and related to experimental data. We provide evidence that suggests the promoter's adsorption to the Brønstedacid site is the most important factor dictating its efficiency.