Theoretical investigation of the side-chain mechanism of the MTO process over H-SSZ-13 using DFT and calculations

The side-chain mechanism of the methanol-to-olefins process over the H-SSZ-13 acidic zeolite was investigated using periodic density functional theory with corrections from highly accurate ab intio calculations on large cluster models. Hexa-, penta- and tetramethylbenzene are studied as co-catalysts for the production of ethene and propene. The highest barrier, both of ethene and propene formation, is found for the methylation of the side-chain towards the formation of an ethyl or isopropyl group. All other barriers are found to be substantially lower. This leads to a clear selectivity for ethene since the elimination of ethene with a rather low barrier competes with methylation towards propene which requires a barrier that is more than 100 kJ mol$^{-1}$ higher

Can Single Metal Atoms Trapped in Defective h-BN/Cu (111) Improve Electrocatalysis of the H2 Evolution Reaction?

Metal-supported hexagonal boron nitride monolayers (h-BN/M) are emerging as new potential electrocatalysts for various energy-related oxidation or reduction process. So far, several preparation methods have been developed to introduce, in a controlled way, defects such as vacancies or substitutional heteroatoms. Herein, we investigate by dispersion-corrected density functional theory (DFT) calculations, defective and metal-doped h-BN/Cu(111) systems as electrocatalysts for the hydrogen evolution reaction (HER). By calculating the hydrogen binding energy (ΔG*H) at different coverage conditions, we observe how the interaction between the defective/metal-doped h-BN layer and the Cu(111) substrate plays a key role in tuning the reactivity, leading to a thermoneutral hydrogen adsorption step (i.e., ΔG*H ≈ 0). These results could be generalized to other h-BN/M interfaces and may help their rational design for an improved H2-evolving electrocatalysis

Influence of Confinement on Barriers for Alkoxide Formation in Acidic Zeolites

The influence of the confinement imposed by eight different zeotypes on the formation of the alkoxides of 13 primary alcohols is studied using dispersion corrected density functional theory calculations with the PBE‐D3 functional. Adsorption energies of the alcohols are computed along with barriers for formation of the alkoxides, which is the first step of the stepwise dehydration mechanism. We find that variations in the adsorption and transition state energies are largely governed by van der Waals interactions between substrates and the zeolite framework. Trends between different reactants, on the other hand, are largely due to the size of the molecules, which can be described quantitatively by the number of atoms constituting them. We find that the stabilization of adsorbates is largest for frameworks that are neither too small, leading to repulsive interaction, nor too spacious leading only to weak interaction

Effect of Impurities on the Initiation of the Methanol-to-Olefins Process: Kinetic Modeling Based on Ab Initio Rate Constants

The relevance of a selection of organic impurities for the initiation of the MTO process was quantified in a kinetic model comprising 107 elementary steps with ab initio computed reaction barriers (MP2:DFT). This model includes a representative part of the autocatalytic olefin cycle as well as a direct initiation mechanism starting from methanol through CO-mediated direct C–C bond formation. We find that the effect of different impurities on the olefin evolution varies with the type of impurity and their partial pressures. The reactivity of the considered impurities for initiating the olefin cycle increases in the order formaldehyde < di-methoxy methane < CO < methyl acetate < ethanol < ethene < propene. In our kinetic model, already extremely low quantities of impurities such as ethanol lead to faster initiation than through direct C–C bond formation which only matters in complete absence of impurities. Graphic Abstract: [Figure not available: see fulltext.

Toward Computing Accurate Free Energies in Heterogeneous Catalysis: a Case Study for Adsorbed Isobutene in H-ZSM-5

Herein, we propose a novel computational protocol that enables calculating free energies with improved accuracy by combining the best available techniques for enthalpy and entropy calculation. While the entropy is described by enhanced sampling molecular dynamics techniques, the energy is calculated using ab initio methods. We apply the method to assess the stability of isobutene adsorption intermediates in the zeolite H-SSZ-13, a prototypical problem that is computationally extremely challenging in terms of calculating enthalpy and entropy. We find that at typical operating conditions for zeolite catalysis (400 °C), the physisorbed π-complex, and not the tertiary carbenium ion as often reported, is the most stable intermediate. This method paves the way for sampling-based techniques to calculate the accurate free energies in a broad range of chemistry-related disciplines, thus presenting a big step forward toward predictive modeling