Prediction of C6–C12 Interconversion Rates Using DFT and Zeolite-specific kinetic Monte Carlo Simulation Methods

DeLuca, Mykela; Hibbitts, David

doi:10.26434/chemrxiv.8035565.v1

Cited by 4 publications

(4 citation statements)

References 56 publications

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“…Analogously, DFT calculations for reactions on 2D zeolite models and on similar external surface models and bulk models could be used for similar investigations. Ultimately, such data could be used to parameterize continuum-based kinetic models or kinetic Monte Carlo (KMC) , models that rigorously predict the effects of changing crystal habits on observed rate.…”

Section: Resultsmentioning

confidence: 99%

Brønsted Acid Strength Does Not Change for Bulk and External Sites of MFI Except for Al Substitution Where Silanol Groups Form

et al. 2023

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Zeolite synthesis efforts have recently focused on tuning diffusion path lengths by modifying the crystal habit, incorporating mesopores, and using pillared and finned materials to decrease diffusion path lengths, increase rates, and limit zeolite deactivation. These modifications also increase the surface-area-tovolume ratio, increasing the relevance of external Brønsted acid sites. The behaviors of such sites, however, are difficult to study by conventional kinetic studies. Here, we study the acid strength of sites that form on the surfaces of MFI zeolites using density functional theory and compare them to sites within the crystal. We determine acid strength by calculating heterolytic and homolytic O−H bond cleavage energies [deprotonation energy (DPE) and dehydrogenation energy (DHE), respectively] and the adsorption energy of ammonia (ΔE NHd 3 ), a common base titrant. These metrics indicate that most sites on the outer (010) surfaces of MFI have similar acid strength to those at internal or bulk positions. Ensemble average DPE values on external surface sites (1486−1543 kJ mol −1 ) are mostly similar to those at internal sites (1477−1506 kJ mol −1 ) on the MFI surface model, and ensemble averaged DHE values (457−483 kJ mol −1 ) are also similar to their bulk counterparts (457−476 kJ mol −1 ). Al substitution at terminal silanol groups (SiOH) on the outer zeolite surface yields Lewis acidic Al, resulting in different acid strengths at those sites, but such sites represent a minority of surface T-sites. Unlike DPE and DHE, ΔE NHd 3 probes a combination of acid strength and confinement because a gas-phase species enters the zeolite pores. Ensemble average ΔE NHd 3 values indicate that most sites bind NH 3 as strongly on the surface (−160 to −113 kJ mol −1 ) as bulk sites (−160 to −133 kJ mol −1 ), despite the reduced confinement on surface sites. These ΔE NHd 3 change very little on the MFI surface because most surface sites are within pockets that retain enough of the zeolite pore to solvate the NH 4+ cations that form when NH 3 binds. Some sites, however, bind NH 3 more weakly if the solvating pore is absent or if NH 3 binds and deprotonates the H 2 O on a Lewis acidic Al. Together, these data suggest that fully coordinated Brønsted acidic Al sites on the outside of zeolite crystals possess similar acid strength to those within the crystal and that many surface sites effectively confine an NH 4 + cation. Such partial confinement on external sites indicates that NH 3 adsorption experiments are ineffective for broadly distinguishing between internal and external sites in MFI materials.

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Section: Resultsmentioning

confidence: 99%

Brønsted Acid Strength Does Not Change for Bulk and External Sites of MFI Except for Al Substitution Where Silanol Groups Form

et al. 2023

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“…For example, the current methodology to study phenomena with vastly different times scales is the usage of kinetic Monte Carlo methods, where a series of stochastic trajectories is generated that propagate the system from one state to the other, and afterward the correct dynamics is retrieved by ensemble averaging over all of these trajectories . There are only a few examples of kMC approaches within the field of nanoporous frameworks, and the combination of diffusional, reaction, and adsorption sites has thus far not been performed. ,− As mentioned earlier in this section, kMC methods and also first-principles microkinetic models currently rely on kinetic and thermodynamics information from static-based simulations. However, it may be envisaged that in the future kMC models could be constructed for complex zeolitic systems, where input from first-principles MD simulations is used.…”

Section: Discussionmentioning

confidence: 99%

Operando Modeling of Zeolite-Catalyzed Reactions Using First-Principles Molecular Dynamics Simulations

Van Speybroeck,

Bocus,

Cnudde

et al. 2023

ACS Catal.

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Within this Perspective, we critically reflect on the role of first-principles molecular dynamics (MD) simulations in unraveling the catalytic function within zeolites under operating conditions. First-principles MD simulations refer to methods where the dynamics of the nuclei is followed in time by integrating the Newtonian equations of motion on a potential energy surface that is determined by solving the quantum-mechanical many-body problem for the electrons. Catalytic solids used in industrial applications show an intriguing high degree of complexity, with phenomena taking place at a broad range of length and time scales. Additionally, the state and function of a catalyst critically depend on the operating conditions, such as temperature, moisture, presence of water, etc. Herein we show by means of a series of exemplary cases how first-principles MD simulations are instrumental to unravel the catalyst complexity at the molecular scale.Examples show how the nature of reactive species at higher catalytic temperatures may drastically change compared to species at lower temperatures and how the nature of active sites may dynamically change upon exposure to water. To simulate rare events, firstprinciples MD simulations need to be used in combination with enhanced sampling techniques to efficiently sample low-probability regions of phase space. Using these techniques, it is shown how competitive pathways at operating conditions can be discovered and how broad transition state regions can be explored. Interestingly, such simulations can also be used to study hindered diffusion under operating conditions. The cases shown clearly illustrate how first-principles MD simulations reveal insights into the catalytic function at operating conditions, which could not be discovered using static or local approaches where only a few points are considered on the potential energy surface (PES). Despite these advantages, some major hurdles still exist to fully integrate first-principles MD methods in a standard computational catalytic workflow or to use the output of MD simulations as input for multiple length/time scale methods that aim to bridge to the reactor scale. First of all, methods are needed that allow us to evaluate the interatomic forces with quantum-mechanical accuracy, albeit at a much lower computational cost compared to currently used density functional theory (DFT) methods. The use of DFT limits the currently attainable length/time scales to hundreds of picoseconds and a few nanometers, which are much smaller than realistic catalyst particle dimensions and time scales encountered in the catalysis process. One solution could be to construct machine learning potentials (MLPs), where a numerical potential is derived from underlying quantum-mechanical data, which could be used in subsequent MD simulations. As such, much longer length and time scales could be reached; however, quite some research is still necessary to construct MLPs for the complex systems encountered in industrially used catalysts. Second, most current...

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“…While there are no applications of kMC in catalysis in MOFs that we know of, kMC has recently been applied to zeolite catalysts. 49,50…”

Section: Coupling Diffusion To Reactionmentioning

confidence: 99%

“…However, if diffusion and reaction occur over different time scales, kinetic Monte Carlo (kMC) may be useful. While there are no applications of kMC in catalysis in MOFs that we know of, kMC has recently been applied to zeolite catalysts. , …”

Section: Coupling Diffusion To Reactionmentioning

confidence: 99%

Role of Molecular Simulations in the Design of Metal–Organic Frameworks for Gas-Phase Thermocatalysis: A Perspective

et al. 2022

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Metal organic frameworks (MOFs) are highly tunable porous crystalline solids with spatially and electronically isolated catalytically active sites that have been demonstrated for a variety of thermo-, redox-, and photocatalytic reactions. Their tunable natures and relatively well-defined active sites make them advantageous for catalyst design, and molecular simulations have proven highly valuable in this endeavor. However, complexities in the MOF structure require advanced simulation strategies that accurately capture quantum chemistry at the strongly correlated transition metal cation active sites, compositional and structural changes caused by finite temperature and pressure reaction conditions, and transport effects in variously sized pore environments. Luckily, several groups have started using such simulation strategies, paving the way for rich opportunities to design MOF catalysts. Herein, we highlight such examples and provide a perspective on the needs for molecular simulations in the design of MOF catalysts moving forward.

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Prediction of C6–C12 Interconversion Rates Using DFT and Zeolite-specific kinetic Monte Carlo Simulation Methods

Cited by 4 publications

References 56 publications

Brønsted Acid Strength Does Not Change for Bulk and External Sites of MFI Except for Al Substitution Where Silanol Groups Form

Brønsted Acid Strength Does Not Change for Bulk and External Sites of MFI Except for Al Substitution Where Silanol Groups Form

Operando Modeling of Zeolite-Catalyzed Reactions Using First-Principles Molecular Dynamics Simulations

Role of Molecular Simulations in the Design of Metal–Organic Frameworks for Gas-Phase Thermocatalysis: A Perspective

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