Theoretical Study of the Mechanism of Surface Methoxy and Dimethyl Ether Formation from Methanol Catalyzed by Zeolitic Protons

Abstract

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.

Mechanistic Aspects of the Zeolite Catalyzed Methylation of Alkenes and Aromatics with Methanol: A Review

“…An obvious complication to this relatively simple, dualistic situation is that methanol always will be dehydrated to DME, quite often to the equilibrium ratio [20]. The mechanism of DME formation is well studied, and this reaction is also suggested to proceed via a direct or stepwise mechanism [21][22][23][24][25][26]. However, the mechanism of DME formation will not be discussed further, as its formation is quick compared to methylation [27] and several studies, both experimental and theoretical have shown that DME may undergo quite analogous reactions to those described above for methanol [12,27,28].…”

An Improved Catalytic Cracking of n-hexane via Methanol Coupling Reaction Over HZSM-5 Zeolite Catalysts

“…methanol that affect the route of n-hexane activation. Indeed, many mechanism studies of methanol conversion over zeolite have suggested that the possible intermediate species, such as methoxy groups or carbenium ion, display ionic character to some extent [7][8][20][21][22][23][24][25][26][27][28][29][30]. It seems that the intermediate species can more easily attack n-hexane molecules than Brønsted site of the zeolite surface.…”

Mechanistic Insights into the Desorption of Methanol and Dimethyl Ether Over ZSM-5 Catalysts

“…Density functional theory (DFT) calculations of the adsorption of one methanol molecule onto an active site give activation energies in the range of 104-139 kJ mol −1 [5,8,9]. Blaszkowski and van Santen [10][11][12] showed that the simultaneous adsorption and activation of two methanol molecules towards the formation of DME and H 2 O excluding surface methoxy group formation is the preferred pathway. However, surface methoxy groups have been readily observed with stopped flow NMR studies over ZSM-5 catalysts [13].…”