Abstract
We report detailed density functional theory (DFT) calculations of important mechanisms in the methanol to gasoline (MTG) process in a zeolite catalyst. Various reaction paths and energy barriers involving C-O bond cleavage and the first C-C bond formation are investigated in detail using all-electron periodic supercell calculations and recently developed geometry optimization and transition state search algorithms. We have further investigated the formation of ethanol and have identified a different mechanism than previously reported [1], a reaction where water does not play any visible role. Contrary to recent cluster calculations, we were not able to find a stable surface ylide structure. However, a stable ylide structure built into the zeolite framework was found to be possible, albeit a very high reaction barrier.
Highlights
Zeolites are water-containing crystalline, porous aluminosilicates composed of SiO4 and AlO4-edge-sharing tetrahedra interlinked through common oxygen atoms giving rise to three-dimensional networks of channels, cages and rings
To test the accuracy of DMol3 in describing H-bond strengths, the interaction energies between two methanol molecules, and between two water molecules were computed using the PBE/DNP/Rcut = 4.0Å settings described in the previous section
Periodic calculations are necessary to highlight the significant reorganization of the zeolite cage that is likely to occur, as we have shown above, if certain adsorbed species get built into the zeolite framework
Summary
Zeolites are water-containing crystalline, porous aluminosilicates composed of SiO4 and AlO4-edge-sharing tetrahedra interlinked through common oxygen atoms giving rise to three-dimensional networks of channels, cages and rings. Yet another mechanism for the C-C bond formation has been proposed: this involves the formation of a surface ylide neighboring a Brønsted acid site [9,13]. First principles studies involving zeolites have commonly employed cluster models to represent the neighborhood of the Brønsted acid site [4,6 and 9-13].
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