Unraveling the Mechanism of the Initiation Reaction of the Methanol to Olefins Process Using ab Initio and DFT Calculations

We report a theoretical investigation of the initiation of the methanol to olefin process, where we study the full reaction mechanism from methanol to propylene. The zeolite H-SSZ-13 is investigated with periodic density functional theory (DFT) calculations. These calculations are corrected with MP2-calculations on large (46T) cluster models, which is found to be crucial for sufficient accuracy. Our calculations clearly demonstrate that initiation via the formation of carbon monoxide is a realistic mechanism and is more likely than the methane–formaldehyde mechanism or variants thereof. A kinetic model of the autocatalytic carbon pool mechanism is employed to investigate the initiation kinetics in more detail, demonstrating that an assessment of the feasibility of an initiation reaction needs to be based on kinetic modeling of both the initiation reaction and autocatalysis. This model gives further evidence that initiation proceeds via oxidation of methanol to carbon monoxide, which subsequently forms the first carbon–carbon bond via carbonylation of methanol. The kinetic model also shows that only extremely small amounts of an olefin need to be formed for autocatalysis to start, implying that small impurities will dominate over initiation mechanisms

Kinetic Monte Carlo Model for Gas Phase Diffusion in Nanoscopic Systems

Transport of atoms and molecules via the gas phase plays an important role in many processes in heterogeneous catalysis. Macroscopic diffusion, for example, in reactors, is typically modeled with continuum models. Much smaller length scales are involved if diffusion occurs between nanoparticles. One such example is a sintering mechanism, where volatile PtO₂ mediates mass transfer between Pt particles. We developed a kinetic Monte Carlo model that explicitly simulates the kinetics of single atoms or molecules in the gas phase that result from collisions with a background gas. This model accurately reproduces ideal gas properties such as the diffusion constant. In model applications, we study gas-phase-mediated mass transfer as a function of the distance between the involved surfaces. If these distances are within the mean free path, typically a micrometer or lower, continuum models based on Fick’s laws deviate from the explicit simulation. This can be explained by the low number of collisions that occur if the length scale of diffusion is not significantly larger than the mean free path in the gas phase

Anharmonic Correction to Free Energy Barriers from DFT-Based Molecular Dynamics Using Constrained Thermodynamic Integration

For the calculation of anharmonic contributions to free energy barriers, constrained thermodynamic λ-path integration (λ-TI) from a harmonic reference force field to density functional theory is presented as an alternative to the established Blue Moon ensemble method (ξ-TI), in which free energy gradients along the reaction coordinate ξ are integrated. With good agreement in all cases, the λ-TI method is benchmarked against the ξ-TI method for several reactions, including the internal CH3 group rotation in ethane, a nucleophilic substitution of CH3Cl, a retro-Diels–Alder reaction, and a proton transfer in zeolite H-SSZ-13. An advantage of λ-TI is that one can use virtually any reference state to compute anharmonic contributions to reaction free energies or free energy barriers. This is particularly relevant for catalysis, where it is now possible to compute anharmonic corrections to the free energy of a transition state relative to any reference, for example, the most stable state of the active site and the reactants in the gas phase. This is in contrast to ξ-TI, where free energy barriers can only be computed relative to an initial state with all reactants coadsorbed. Finally, the Bennett acceptance ratio method combined with λ-TI is demonstrated to reduce the number of required integration grid points with tolerable accuracy, favoring thus λ-TI over ξ-TI in terms of computational efficiency

Theoretical Investigation of the Acid Catalyzed Formation of Oxymethylene Dimethyl Ethers from Trioxane and Dimethoxymethane

Oxymethylene dimethyl ethers (OMEs) are promising fuel additives that are accessible from renewable resources. We present a mechanistic investigation of the zeolite-catalyzed synthesis of OMEs from trioxane and OME1 based on first-principles calculations. The most favorable mechanism proceeds via direct incorporation of trioxane rather than formaldehyde incorporation after prior trioxane decomposition to formaldehyde. The computed mechanism is in agreement with the experimental kinetics as well as the selectivity after short and long reaction times. The competition between direct incorporation of trioxane and formaldehyde is predicted to depend on the acidity of the catalyst as well as the concentration of protic species in solution

Unexpected Reactivity Patterns of Ruthenium Alkylidenes with N‑Phosphino-Functionalized N‑Heterocyclic Carbene Ligands (NHCPs)

N-phosphino-functionalized N-heterocyclic carbene (NHCP) ligands have been evaluated as potential supporting ligands in ruthenium-catalyzed olefin metathesis. Initial density functional theory (DFT) calculations suggested that these NHCP ligands may allow access to neutral 14 valence electron (VE) speciesequivalents of the active 14 VE species formed by phosphine dissociation from Grubbs II precatalystsvia facile decoordination of the NHCP phosphino donor of the strained four-membered [RuPNC] chelate systems. Their attempted synthesis from NHCPs and Grubbs-type Ru carbenes revealed addition of an NHCP donor atom (P or C) to the alkylidene fragment, forming a new C–P or C–C bond in five-membered chelate structures. DFT investigations showed that these reactions are controlled kinetically and must not be neglected as important possible deactivation routes in olefin metathesis

Ru(II)-Triphos Catalyzed Amination of Alcohols with Ammonia via Ionic Species

An active and selective system for the amination of primary alcohols to primary amines with ammonia based on ruthenium and triphos as the tridentate phosphine ligand was developed. On the basis of detailed mechanistic studies, we propose that the active catalyst is, unlike the previously reported systems on this reaction, a cationic ruthenium complex. The experimental findings are supported by detailed density functional theory (DFT) calculations on the catalytic cycle. Because of the cationic nature of the active catalyst, strong anion and solvent effects were observed in the catalytic amination reaction when using the ruthenium triphos complexes. Therefore, a higher activity could be achieved when the nonpolar solvent toluene is used in this amination instead of tetrahydrofuran. Our findings can help to develop and optimize the system systematically for an application to relevant target molecules

Mechanistic Details of the Nickel-Mediated Formation of Acrylates from CO₂, Ethylene and Methyl Iodide

Methyl iodide induces the stoichiometric cleavage of nickelalactones, which are key intermediates in the nickel-mediated reaction of CO₂ and alkenes to acrylates. Herein, we propose a modified and extended mechanism for this reaction on the basis of theoretical and experimental investigations for the bidentate P ligand 1,2-bis­(di-<i>tert</i>-butylphosphino)­ethane (dtbpe). The calculated elementary steps agree well with experimental findings: reaction barriers are reasonable and explain the facile liberation of acrylate from a nickelalactone by methyl iodide. We were able to isolate reactive intermediates and to verify the existence of proposed reaction pathways. Additionally, we have identified unproductive pathways leading to byproducts (e.g., propionates and catalytically inactive organometallic species). Although those side reactions can be suppressed to a certain extent, the strong binding of acrylate to nickel prevents a catalytic reaction, at least for the chosen ligand

Mechanistic Details of the Nickel-Mediated Formation of Acrylates from CO₂, Ethylene and Methyl Iodide

Methyl iodide induces the stoichiometric cleavage of nickelalactones, which are key intermediates in the nickel-mediated reaction of CO₂ and alkenes to acrylates. Herein, we propose a modified and extended mechanism for this reaction on the basis of theoretical and experimental investigations for the bidentate P ligand 1,2-bis­(di-<i>tert</i>-butylphosphino)­ethane (dtbpe). The calculated elementary steps agree well with experimental findings: reaction barriers are reasonable and explain the facile liberation of acrylate from a nickelalactone by methyl iodide. We were able to isolate reactive intermediates and to verify the existence of proposed reaction pathways. Additionally, we have identified unproductive pathways leading to byproducts (e.g., propionates and catalytically inactive organometallic species). Although those side reactions can be suppressed to a certain extent, the strong binding of acrylate to nickel prevents a catalytic reaction, at least for the chosen ligand

Modeling the Migration of Platinum Nanoparticles on Surfaces Using a Kinetic Monte Carlo Approach

We propose a kinetic Monte Carlo (kMC) model for simulating the movement of platinum particles on supports, based on atom-by-atom diffusion on the surface of the particle. The proposed model was able to reproduce equilibrium cluster shapes predicted using Wulff-construction. The diffusivity of platinum particles was simulated both purely based on random motion and assisted using an external field that causes a drift velocity. The overall particle diffusivity increases with temperature; however, the extracted activation barrier appears to be temperature independent. In addition, this barrier was found to increase with particle size, as well as, with the adhesion between the particle and the support

Alcohol Amination with Ammonia Catalyzed by an Acridine-Based Ruthenium Pincer Complex: A Mechanistic Study

The mechanistic course of the amination of alcohols with ammonia catalyzed by a structurally modified congener of Milstein’s well-defined acridine-based PNP-pincer Ru complex has been investigated both experimentally and by DFT calculations. Several key Ru intermediates have been isolated and characterized. The detailed analysis of a series of possible catalytic pathways (e.g., with and without metal–ligand cooperation, inner- and outer-sphere mechanisms) leads us to conclude that the most favorable pathway for this catalyst does not require metal–ligand cooperation