Toward the Balance between the Reductionist and Systems Approaches in Computational Catalysis: Model versus Method Accuracy for the Description of Catalytic Systems

Toward the Balance between the Reductionist and Systems Approaches in Computational Catalysis: Model versus Method Accuracy for the Description of Catalytic System

A Periodic DFT Study of Glucose to Fructose Isomerization on Tungstite (WO₃·H₂O): Influence of Group IV–VI Dopants and Cooperativity with Hydroxyl Groups

Periodic density functional theory (DFT) calculations were carried out to investigate the mechanism of glucose to fructose isomerization over tungstite (WO₃·H₂O). The isomerization reaction is catalyzed by undercoordinated W⁶⁺ sites. The reaction mechanism proceeds through an H-shift from C2 to C1 and involves a cooperative action of Lewis acidic tungsten sites with neighboring proton donors, which form a hydrogen-bonding surface network. Dopants of group IV–VI transition metals stabilize the preactivated complex, which is the deprotonated open form of glucose adsorbed to the surface. In particular, calculations reveal that doping the tungstite structure with Nb⁵⁺ and Ti⁴⁺ ions is effective in lowering the overall barrier for glucose isomerization

Structure, Stability, and Lewis Acidity of Mono and Double Ti, Zr, and Sn Framework Substitutions in BEA Zeolites: A Periodic Density Functional Theory Study

The siting of Ti, Sn, and Zr framework heteroatoms in zeolite BEA and the resulting Lewis acidity were systematically investigated by periodic density functional theory (DFT) calculations. Mono as well as double substitutions were considered. Substitution of Si by Ti, Sn, or Zr in the lattice of BEA zeolite is not random. For all substituted zeolites, the introduction of heteroatoms at the T2 crystallographic position is preferred. Water adsorption was used as a probe for Lewis acidity of lattice sites in these substituted zeolites. Although the Lewis acidity of Sn- and Zr-substituted BEA zeolites is generally quite similar, it is substantially higher than that of Ti–BEA. The Lewis acidity of substituted zeolites strongly depends on the crystallographic location of the heteroatoms. For those lattice sites that can be approached from two different directions, interaction with water will be favored from the more accessible direction. Stable structures containing double lattice substitutions at distances below 5.0 Å are found in Sn–BEA but not in Ti– or Zr–BEA. It is argued that substituted heteroatoms play an important role during the activation of reactants, with their ability to activate them depending on the type of heteroatom. The presence of paired lattice sites in Sn–BEA zeolite substantially enhances Lewis acidity of the zeolites, which clearly distinguishes Sn–BEA from its Ti- and Zr-substituted analogues

HiREX: High-Throughput Reactivity Exploration for Extended Databases of Transition-Metal Catalysts

A method is introduced for the automated analysis of reactivity exploration for extended in silico databases of transition-metal catalysts. The proposed workflow is designed to tackle two key challenges for bias-free mechanistic explorations on large databases of catalysts: (1) automated exploration of the chemical space around each catalyst with unique structural and chemical features and (2) automated analysis of the resulting large chemical data sets. To address these challenges, we have extended the application of our previously developed ReNeGate method for bias-free reactivity exploration and implemented an automated analysis procedure to identify the classes of reactivity patterns within specific catalyst groups. Our procedure applied to an extended series of representative Mn(I) pincer complexes revealed correlations between structural and reactive features, pointing to new channels for catalyst transformation under the reaction conditions. Such an automated high-throughput virtual screening of systematically generated hypothetical catalyst data sets opens new opportunities for the design of high-performance catalysts as well as an accelerated method for expert bias-free high-throughput in silico reactivity exploration

Mechanistic Complexity of Methane Oxidation with H₂O₂ by Single-Site Fe/ZSM‑5 Catalyst

Periodic density functional theory (DFT) calculations were carried out to investigate the mechanism of methane oxidation with H₂O₂ over the defined Fe sites in Fe/ZSM-5 zeolite. The initial Fe site is modeled as a [(H₂O)₂–Fe­(III)–(μO)₂–Fe­(III)–(H₂O)₂]²⁺ extraframework cluster deposited in the zeolite pore and charge-compensated by two anionic lattice sites. The activation of this cluster with H₂O₂ gives rise to the formation of a variety of Fe­(III)-oxo and Fe­(IV)-oxo complexes potentially reactive toward methane dissociation. These sites are all able to promote the first C–H bond cleavage in methane by following three possible reaction mechanisms: namely, (a) heterolytic and (b) homolytic methane dissociation as well as (c) Fenton-type reaction involving free OH radicals as the catalytic species. The C–H activation step is followed by formation of MeOH and MeOOH and regeneration of the active site. The Fenton-type path is found to proceed with the lowest activation barrier. Although the barriers for the alternative heterolytic and homolytic pathways are found to be somewhat higher, they are still quite favorable and are expected to be feasible under reaction conditions, resulting ultimately in MeOH and MeOOH products. H₂O₂ oxidant competes with CH₄ substrate for the same sites. Since the oxidation of H₂O₂ to O₂ and two [H⁺] is energetically more favorable than the C–H oxofunctionalization, the overall efficiency of the latter target process remains low

HiREX: High-Throughput Reactivity Exploration for Extended Databases of Transition-Metal Catalysts

A method is introduced for the automated analysis of reactivity exploration for extended in silico databases of transition-metal catalysts. The proposed workflow is designed to tackle two key challenges for bias-free mechanistic explorations on large databases of catalysts: (1) automated exploration of the chemical space around each catalyst with unique structural and chemical features and (2) automated analysis of the resulting large chemical data sets. To address these challenges, we have extended the application of our previously developed ReNeGate method for bias-free reactivity exploration and implemented an automated analysis procedure to identify the classes of reactivity patterns within specific catalyst groups. Our procedure applied to an extended series of representative Mn(I) pincer complexes revealed correlations between structural and reactive features, pointing to new channels for catalyst transformation under the reaction conditions. Such an automated high-throughput virtual screening of systematically generated hypothetical catalyst data sets opens new opportunities for the design of high-performance catalysts as well as an accelerated method for expert bias-free high-throughput in silico reactivity exploration

Nature and Catalytic Role of Extraframework Aluminum in Faujasite Zeolite: A Theoretical Perspective

A comprehensive periodic DFT study complemented by ab initio thermodynamic analysis was carried out to determine the speciation of extraframework aluminum (EFAl) in faujasite zeolite. The structure and stability of a wide range of mono- bi-, tri-, and tetranuclear EFAl complexes stabilized at different locations in faujasite were investigated. The thermodynamic cycles connecting these complexes were constructed involving such elementary steps as hydration/dehydration, proton transfer, and condensation reactions. Using ab initio thermodynamics analysis it was predicted that, during high-temperature zeolite activation, the EFAl species self-organize into cationic clusters with more than one Al center. The resulting tri- and tetranuclear clusters are preferentially stabilized inside the small sodalite cages of faujasite that provide a favorable coordination and charge-compensation environment for the large multiply charged cationic clusters. The presence of such cationic EFAl clusters inside the inaccessible sodalite cages strongly enhances the protolytic propane cracking activity of vicinal supercage Brønsted acid sites

Electronic Structure of the [Cu₃(μ-O)₃]²⁺ Cluster in Mordenite Zeolite and Its Effects on the Methane to Methanol Oxidation

Identifying Cu-exchanged zeolites able to activate C–H bonds and selectively convert methane to methanol is a challenge in the field of biomimetic heterogeneous catalysis. Recent experiments point to the importance of trinuclear [Cu₃(μ-O)₃]²⁺ complexes inside the micropores of mordenite (MOR) zeolite for selective oxo-functionalization of methane. The electronic structures of these species, namely, the oxidation state of Cu ions and the reactive character of the oxygen centers, are not yet fully understood. In this study, we performed a detailed analysis of the electronic structure of the [Cu₃(μ-O)₃]²⁺ site using multiconfigurational wave-function-based methods and density functional theory. The calculations reveal that all Cu sites in the cluster are predominantly present in the Cu­(II) formal oxidation state with a minor contribution from Cu­(III), whereas two out of three oxygen anions possess a radical character. These electronic properties, along with the high accessibility of the out-of-plane oxygen center, make this oxygen the preferred site for the homolytic C–H activation of methane by [Cu₃(μ-O)₃]²⁺. These new insights aid in the construction of a theoretical framework for the design of novel catalysts for oxyfunctionalization of natural gas and suggest further spectroscopic examination

Stability of Extraframework Iron-Containing Complexes in ZSM‑5 Zeolite

The stability of oxygenated and hydroxylated iron complexes in Fe/ZSM-5 is studied by periodic DFT calculations. The reaction paths for the interconversion of various potential iron-containing complexes confined in the zeolite matrix are discussed. It is demonstrated that the distribution of mononuclear [FeO]⁺ species depends only slightly on the specific local zeolite environment. For all binuclear complexes considered, a notable preference for the location at the larger eight-membered ring γ site in the sinusoidal channel is observed. Nevertheless, the formation of the mononuclear species [FeO]⁺ in realistic systems is very unlikely. Irrespective of their location inside the zeolite matrix, such species show a strong tendency toward self-organization into binuclear oxygen-bridged [Fe­(μ-O)₂Fe]²⁺ complexes. Using ab initio thermodynamic analysis of the stability of different Fe complexes in ZSM-5, it is demonstrated that two distinct extraframework cationic complexes can be present in the Fe/ZSM-5 catalyst, namely, [Fe^III(μ-O)₂Fe^III]²⁺ and [Fe^II(μ-O)­Fe^II]²⁺. The [Fe^II(μ-O)­Fe^II]²⁺ complexes containing bivalent iron centers are mainly present in the Fe/ZSM-5 catalyst activated at low oxygen chemical potential and H₂O-free conditions, whereas the formation of its Fe³⁺-containing counterpart [Fe^III(μ-O)₂Fe^III]²⁺ is favored upon the high-temperature calcination in an O₂-rich environment

Scaling Relations for Acidity and Reactivity of Zeolites

Zeolites are widely applied as solid acid catalysts in various technological processes. In this work we have computationally investigated how catalytic reactivity scales with acidity for a range of zeolites with different topologies and chemical compositions. We found that straightforward correlations are limited to zeolites with the same topology. The adsorption energies of bases such as carbon monoxide (CO), acetonitrile (CH₃CN), ammonia (NH₃), trimethylamine (N­(CH₃)₃), and pyridine (C₅H₅N) give the same trend of acid strength for FAU zeolites with varying composition. Crystal orbital Hamilton populations (COHP) analysis provides a detailed molecular orbital picture of adsorbed base molecules on the Brønsted acid sites (BAS). Bonding is dominated by strong σ donation from guest molecules to the BAS for the adsorbed CO and CH₃CN complexes. An electronic descriptor of acid strength is constructed based on the bond order calculations, which is an intrinsic parameter rather than adsorption energy that contains additional contributions due to secondary effects such as van der Waals interactions with the zeolite walls. The bond order parameter derived for the CH₃CN adsorption complex represents a useful descriptor for the intrinsic acid strength of FAU zeolites. For FAU zeolites the activation energy for the conversion of π-adsorbed isobutene into alkoxy species correlates well with the acid strength determined by the NH₃ adsorption energies. Other zeolites such as MFI and CHA do not follow the scaling relations obtained for FAU; we ascribe this to the different van der Waals interactions and steric effects induced by zeolite framework topology