Unraveling the Relationship between Zeolite Structure and MTO Product Distribution by Theoretical Study of the Reaction Mechanism

Abstract

Determination of the relative contributions of aromatic-based and alkene-based cycles in the MTO process is essential for understanding the reaction mechanism and estimating the product selectivity over various zeolites. However, it is a great challenge due to the high complexity of the reaction network. Thus, the olefin formation mechanisms are systematically investigated here by the density functional theory with van der Waals dispersive corrections over structurally different zeolites. It is shown that the aromatic-based cycle dominates the MTO process over H-SAPO-34 with a side-chain route as the major reaction pathway, while the alkene-based cycle plays a major role on H-BEA and H-ZSM-5. Over H-ZSM-22, the alkene-based cycle is the main route with C 5 + alkenes as primary products. The energy barriers of the aromatic-based cycle are similar over the above four types of zeolites. A linear relationship is found between the ethene/propene molar ratios and the free energy barriers of alkene-based cycles over these zeolites, indicating that the ethene and propene selectivities are mainly determined by the propagation of the alkene-based cycle. This suggests that the ultimate free energy barrier of the alkene-based cycle can be used as a measure to predict the C 2 = /C 3 = molar ratios obtained on topologically different zeolites.

A new mechanistic proposal for the aromatic cycle of the MTO process based on a computational investigation for H-SSZ-13

“…While our focus in this study has been on H-SSZ-13, we expect that the mechanism shown here is also the lowest energy mechanism for other zeotypes. Previous investigations have indeed shown that the original paring mechanism is unfavorable for H-ZSM-5 and H-SAPO-34 21 as well as H-BEA and H-ZSM-22, 58 thus strongly indicating that there has to be a more advantageous pathway for propylene formation with lower overall free energies.…”

“…Previous computational investigations on H-SAPO-34, 19,27,46 H-ZSM-5 21 and H-SSZ-13 49 have shown that the side-chain mechanism has a clear kinetic selectivity for ethylene. These investigations, however, have also shown that the rate-limiting step, the methylation of the side chain, is relatively high with computed activation free energies on the order of 190 kJ mol −1 or higher as determined by us for H-SSZ-13 57 and by others for H-SAPO-34, 49,58 H-ZSM-5, 58 H-BEA 58 and H-ZSM-22. 58 We note here that in our qualitative discussion of barrier heights, we always refer to Gibbs free energies and to the highest barrier encountered in the catalytic cycle relative to the lowest preceding intermediate, thus giving the largest 'span' as described nicely in the energetic span model.…”

“…These investigations, however, have also shown that the rate-limiting step, the methylation of the side chain, is relatively high with computed activation free energies on the order of 190 kJ mol −1 or higher as determined by us for H-SSZ-13 57 and by others for H-SAPO-34, 49,58 H-ZSM-5, 58 H-BEA 58 and H-ZSM-22. 58 We note here that in our qualitative discussion of barrier heights, we always refer to Gibbs free energies and to the highest barrier encountered in the catalytic cycle relative to the lowest preceding intermediate, thus giving the largest 'span' as described nicely in the energetic span model. 59 Individual barrier heights of elementary reaction steps, although often quoted in discussions, allow no immediate conclusion regarding the feasibility without determining the concentration of intermediates through full kinetic modeling.…”

“…21 This is in agreement with a recent investigation for H-SAPO-34, H-ZSM-5, H-BEA and H-ZSM-22, where barriers for the paring mechanism were always >200 kJ mol −1 relative to the most stable preceding adsorbate. 58 Herein, we propose an alternative paring mechanism that avoids unfavorable antiaromatic intermediates (see Scheme 2). In our proposed mechanism, propene is eliminated from a neutral isopropyl group leading to tetramethylfulvene.…”

“…21 This is in agreement with a recent investigation for H-SAPO-34, H-ZSM-5, H-BEA and H-ZSM-22, where barriers for the paring mechanism were always >200 kJ mol −1 relative to the most stable preceding adsorbate. 58…”