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.

Periodic density functional theory calculations with long-range corrections were used to analyze the opening of fructose and glucose rings catalyzed by metal-substituted β zeolites (M-BEA). The reaction mechanisms were systematically analyzed on BEA substituted with tin (Sn), titanium (Ti), zirconium (Zr), and hafnium (Hf). Here, we proposed a mechanism for the conversion of fructose to dihydroxyacetone and glyceraldehyde and a novel mechanism for the glucose ring opening. The preferential site of substitution of the metals in BEA was reported. The adsorption energies of fructose and glucose through their different oxygen atoms on M-BEA were also reported. The transition state energies were calculated using the nudged elastic band and dimmer methods. Among the zeolites studied, Sn-BEA displays the lowest energies barriers for the conversion of the fructose to its trioses and for the glucose ring opening.

Methane gas reserves (CH4) have been growing exponentially in the United States due to the discovery of new shale gas deposits. CH4 is currently burned to produce synthesis gas, but new alternatives are being sought for its use. This paper discusses the feasibility of Mg-MFI catalyst synthesis for CH4 conversion. A DFT analysis was performed to quantify the effect of the distribution of aluminum atoms in the ring of zeolite MFI exchanged with magnesium (Mg). Our results indicate that the most stable substitution of Al in Mg-MFI is achieved at the T11-T2 crystallographic sites of zeolite. Furthermore, the adsorption energies of CH4 in Mg-MFI reached values of up to-111 kJ/mol, being the strongest adsorption energies reported so far for a zeolite. After the adsorption, the activation of CH4 occurs leading to the formation of an acid Brønsted site in the α ring and the formation of a bond between the methyl (CH3) and Mg. Our results show that the activation of CH4 requires high reaction energies that are in the range between 154 to 266 kJ/mol, suggesting that Mg-MFI would not be a good catalyst for reactions that require CH4 activation.