Theoretical study of the mechanism of surface methoxy and dimethyl ether formation from methanol catalyzed by zeolitic protons

Density functional theory is used to study the zeolite acid catalyzed methanol dehydration to dimethyl ether. Three different reaction pathways are proposed. In the first, methanol adsorption and surface methoxy species formation are the initial elementary steps for this reaction. Subsequent dimethyl ether formation by reaction of a new methanol molecule with the surface methoxy species takes place. The second path involves the simultaneous adsorption and activation of two methanol molecules with formation of dimethyl ether and water in one step. The third path involves also the simultaneous adsorption and activation of two methanol molecules. The difference is that, like in the first path, initially a methoxy surface species will be formed from dehydration of one of the methanol molecules, and this will be followed by dimethyl ether formation. The second path appears to be the preferred route for dimethyl ether formation, since its activation barrier is lower than the other two paths. The effect of making the zeolitic cluster slightly more acidic (by lengthening the Si-H bond distances) over the activation barriers of dimethyl ether formation has been studied. Changes on the order of 5 kJ/mol are observed. An analysis of the reaction rate constants for the three reaction paths of methanol dehydration is also presented.