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The conversion of greenhouse gases, such as CO2 and CH4, to value chemicals is a major challenge, because of the high stability of both molecules. In this study, density functional theory (DFT) calculations with long-range corrections and ONIOM were used to analyze the reaction mechanism for the conversion of CO2 and CH4 to acetic acid with MFI zeolite exchanged with Be, Co, Cu, Mg, Mn, and Zn cations. Our results demonstrate that (a) the highest reaction barrier on the reaction mechanism is CH4 dissociation, and the transition state energy in that step is directly related to the energy of the lowest unoccupied molecular orbital and the electronegativity of the metal exchanged zeolites; (b) a charge transfer between CH4 and the metal cation occurs simultaneously to CH4 dissociation; (c) CO2 insertion has a low energy barrier, and the protonation of the acetate species is spontaneous; (d) dispersion interactions are the main contributions to CH4 adsorption energies, whereas, in the rest of the steps of the reaction mechanism, the contribution of dispersion to the energies of reaction is almost negligible; (e) desorption of acetic acid could be promoted by the coadsorption of water; and (f) CH4 dissociation on Cu-MFI has an apparent activation energy of 11.5 kcal/mol, and a forward rate constant of 1.1 s–1 at 398 K.

Metal-substituted beta-zeolites have proven to be effective catalysts for various important reactions involving the transformation of biomass-derived molecules. In this study, a combination of quantum mechanical calculations and integrated quantum mechanics−molecular mechanics along with a polarizable continuum model were used to determine the preferred substitution site of Sn, Ti, and Ge metals in zeolite beta (BEA) and in the polymorphism C of zeolite beta (BEC), as well as the Lewis acidity and the hydrothermal stability of the metalsubstituted zeolites. Our results demonstrate (1) the most favorable substitution of Ti, Ge, and Sn in BEC is in the T1 site; (2) Ti-BEC has Lewis acidity similar to that of Sn-BEA; (3) the hydrolysis of Ge-BEC is energetically favorable when the Ge/Si ratio is 1/13; and (4) Ti-substituted zeolites show the highest hydrothermal stability of the zeolites studied.

DFT with long-range corrections and ONIOM along with a polarizable-continuum model were used to analyze zeolites BEA, FAU, MFI, and BEC substituted with Sn and Ti. The preferential substitution sites for Ti and Sn in the different frameworks are reported. The Lewis acidities were measured through the NH 3 binding energies and through the charge transfer of NH 3 upon adsorption. The deprotonation energies of the open sites, which are proportional to the Brønsted acidities, and the hydrolysis energies are also reported. We also present the properties of BEA with a single and a double Sn-substitution to compare the active sites obtained with two methods commonly employed for the synthesis of Sn−BEA. Among the zeolites analyzed in this study, Sn−BEA with a double Sn-substitution has the highest Lewis acidity. The formation of open sites through the hydrolysis of Sn−BEA, Sn−FAU, and Ti−FAU is energetically favorable, but it is not favorable in MFI or Ti−BEA. On the basis of the deprotonation energies, the open sites of Sn−BEA have a strong Brønsted acidity, comparable to Al−BEA or Al−MFI. We also demonstrate that the VDW forces in the binding energies of NH 3 on MFI are more significant than in the other zeolite frameworks and that these forces decrease with increasing pore size.

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