Computational Assessment of the Dominant Factors Governing the Mechanism of Methanol Dehydration over H-ZSM-5 with Heterogeneous Aluminum Distribution

Abstract

A van der Waals (vdW) corrected density functional theory (DFT) study of the methanol-to-DME reaction on H-ZSM-5 is conducted for both the associative and dissociative pathways. Calculations are performed for four different active site locations corresponding to Al sitings in sinusoidal and straight channels, and their intersections in the MFI zeolite framework. The Gibbs free energy landscape along the reaction paths computed for a typical set of conditions shows that the associative route is preferred, regardless of Al siting, but a transition in the mechanism from associative to dissociative is observed at higher temperatures. The crossover temperature, however, is not identical for the various active site locations, resulting in a temperature range over which both mechanisms are active. This observation may explain why methoxy, which is the key intermediate along the dissociative pathway, has been observed spectroscopically, whereas kinetic analysis points to dominant contributions of the associative pathway under similar conditions. Pore confinement effects largely contribute to transition state stabilization and have a significant impact on the reaction mechanism. The effect of acidity on kinetic performance is also tested by the substitution of three different heteroatom dopants (Al, Ga, In) at the active sites, but only a minor transition-state energy variation was observed. The fundamental information obtained in this study contributes to a better understanding of the complex interplay between pore confinement, acidity, and reaction conditions, and their effect on pathway selectivity. This knowledge can be utilized to either optimize DME production from methanol or facilitate the production of desired hydrocarbons in the methanol-to-hydrocarbon (MTH) process, which requires DME formation to initiate the conversion.

Methylation of olefins with ketene in zeotypes and its implications for the direct conversion of syngas to light olefins: a periodic DFT study

“…30,32,46 From the Gibbs free energy diagram at 0 K, we found that the decarbonylation is rate-determining in the sequential dissociative pathway, the overall Gibbs free energy barrier (or energetic span) is 185 kJ/mol at 0 K (M2 TS2-3/D), higher than that in the associative pathway (M4 TS4-5/D, 151 kJ/mol). The following is the breaking of C-C bond (decarbonylation) in CH3CO + ion leading to CO and methoxide (TS2-3/D).…”

“…44 The harmonic frequency calculations employed a partial Hessian vibrational analysis (PHVA), including H atom of acidic site, and organic species part of involved states. [30][31][32] The calculated Gibbs free energy diagram for the methylation of tetramethylethene (TME) with ketene is shown in Figure 1 and the optimized geometries of the reaction intermediates and transition states are shown in Figure 2. 46 are similar to the methylation of olefins with methanol or DME.…”

“…[24][25][26][27] The hydrocarbon pool mechanism mainly comprises (side) chain propagation and (side) chain elimination. [30][31][32] Interestingly, Rasmussen et al theoretically and experimentally verified that ketene is an important reaction intermediate in the carbonylation of DME to methyl acetate catalyzed in mordenite (H-MOR) zeolite. Using methanol as reactant, it was evidenced that the methylation reactions, such as olefin methylation, DME (dimethyl ester) formation, may involve the direct associative/concerted pathway or the sequential dissociative/stepwise pathway.…”

Quantification and Statistical Analysis of Errors Related to the Approximate Description of Active Site Models in Metal‐Exchanged Zeolites

“…1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 Among the monovalent species, protons are most commonly encountered in zeolites. Brønsted acid zeolites have been subject to many experimental and computational studies for various reactions such as methanol to hydrocarbons (MTH) [66][67][68][69][70][71][72] and methanol to dimethyl ether (DME), [73,74] and serve as reference for CÀ H bond activation on other zeolitic active sites. As seen in Table 1, the predominant dissociation pathway for CH 4 on a Brønsted acid site forms a frameworkbound methoxy species and H 2 gas.…”

Ethanol as a Binder to Fabricate a Highly‐Efficient Capsule‐Structured CuO−ZnO−Al2O3@HZSM‐5 Catalyst for Direct Production of Dimethyl Ether from Syngas

“…Syngas-to-methanol reaction is performed over copper-based catalysts (mainly, CuÀ ZnOÀ Al 2 O 3 ) in the first step, which is thermodynamic equilibrium limitation at high reaction temperature. [4][5][6] Methanol-to-DME reaction is carried out over solid acid catalysts such as acidic zeolites (HZSM-5, HZSM-22, HY, and H-SAPO), [7][8][9][10][11][12] γ-alumina, [13][14][15] heteropolyacid, [16,17] and mixed metal oxides [18][19][20] in the second step. However, methanol, as a relatively high-cost chemical, increases the cost for producing DME, besides the two-step DME synthesis is limited by the availability of methanol.…”