(10e) Van Der Waals Corrected First-Principles Study of the Methanol-to-DME Reaction Mechanism on H-ZSM-5

Conversion of methanol to olefins (MTO) or hydrocarbons (MTH) over acidic catalysts, in particular H-ZSM-5 (a 3-D channel zeolite with MFI framework), is a feasible route for the production of commodity chemicals (e.g. propylene) and liquid fuels, and has been a popular subject of catalytic studies. Although the overall process is very complicated, the first step is accepted to be methanol dehydration to dimethyl ether (DME). The formation of DME from methanol may proceed through two competing pathways. The associative pathway is characterized by the initial adsorption of a methanol dimer followed by water elimination, while in the dissociative pathway water formation occurs from a single methanol molecule followed by the DME formation step after a second methanol molecule adsorbs.

The active sites of H-ZSM-5 are Brønsted acid sites that can be found in 12 distinguishable locations, which may be grouped as belonging to the sinusoidal channel, the straight channel, or the intersection of each. We have recently demonstrated that the geometric and chemical properties of these 12 sites span a wide range of values, and that these variations may favor different reaction pathways [1]. Hence, H-ZSM-5 is ideally suited as a model zeolite to gain a thorough understanding of the mechanism of methanol dehydration, the effect of pore confinement, and the role of acidity. This information can be used to derive structure-function relationships for improved catalyst design. 

A theoretical study of the reaction mechanism for DME formation at the DFT-GGA level has been conducted recently over H-ZSM-22 (a 1-D channel zeolite with TON framework), where the dominant mechanism under industrially relevant conditions was determined to be the dissociative pathway. The effect of active site acidity on reaction barriers was also reported [2]. Here, we use periodic van der Waals density functional theory (vdW-DF) calculations to study the reaction mechanism of methanol dehydration over H-ZSM-5, a zeolite catalyst with known activity for this reaction that is frequently used for MTO/MTH. We show for representative active sites in the straight and sinusoidal channels, and at the channel intersection that the associative pathway possesses a lower reaction barrier at 0 K than the two steps of the dissociative pathway. Entropy calculations are carried out to compute the Gibbs free energy change of transition states with respect to the reference state (i.e. zeolite + gaseous reactants), from which we determine the dominant mechanism at realistic reaction temperatures.  

The use of the vdW-DF exchange-correlation functional enables us to investigate the significance of transition state stabilization through pore confinement effects. Large differences in reaction barriers are obtained when transition states are calculated in the absence and presence of vdW interactions. Moreover, vdW interactions prove to have a significant influence on the dominant reaction mechanism. Similar to the results of the study by Moses and Nørskov [2], exclusion of vdW interactions suggests dominance of the dissociative pathway for all practical temperature conditions. However, when vdW interactions are included the transition from the associative pathway at low temperatures to the dissociative pathway occurs at higher temperatures. These results imply that for practical conditions both pathways are feasible. The significance of geometry-dependent confinement effects on the reaction mechanism is investigated by calculating reaction barriers at T10, T11, and T12 sites representing sinusoidal, straight, and intersection positions, respectively.

To investigate the sensitivity to the strength and number of acid sites, we first tuned the active site acidity at specific locations in the MFI unit cell by using three different heteroatom substitutions, i.e. aluminum, gallium, and indium [2]. The resulting activation barriers are used as the site acidity descriptor and the relationship with other site properties is investigated. The number of acid sites is in practice altered by varying the Si/Al ratio. At lower Si/Al ratios the probability of finding two nearby acid sites increases, and we estimated the acidity and the respective kinetic parameters for DME formation for a Brønsted acid site pair. Characterization of the site pair acidity using the calculated adsorption energies of probe molecules (CH₃OH, NH₃) revealed that the average acidity of two neighboring acid sites in H-ZSM-5 is similar to that of a single site model. However, the stabilization of a methanol dimer and the transition state of the associative pathway shows distinct differences when a site pair is present. In summary, the results of our detailed and accurate DFT simulations can illuminate the underlying factors influencing the mechanism of the methanol-to-DME reaction over H-ZSM-5 and provide valuable insight into the design of improved catalysts.   

[1] Arian Ghorbanpour, Jeffrey D. Rimer, Lars C. Grabow, Periodic, vdW-corrected density functional theory investigation of the effect of Al siting in H-ZSM-5 on chemisorption properties and site-specific acidity, Catalysis Communications (2014), doi: 10.1016/j.catcom.2014.04.005.
[2] Poul Georg Moses, Jens K. Nørskov, Methanol to dimethyl ether over ZSM-22: a periodic density functional theory study, ACS Catalysis 3 (2013) 735-745.