V. Van Speybroeck

Engineering of Phenylpyridine- and Bipyridine-Based Covalent Organic Frameworks for Photocatalytic Tandem Aerobic Oxidation/Povarov Cyclization

M. Debruyne, S. Borgmans, S. Radhakrishnan, E. Breynaert, H. Vrielinck, K. Leus, A. Laemont, J. De Vos, K. S. Rawat, S. Vanlommel, H. Rijckaert, H. Salemi, J. Everaert, F. Vanden Bussche, D. Poelman, R. Morent, N. De Geyter, P. Van der Voort, V. Van Speybroeck, C.V. Stevens
ACS Applied Materials & Interfaces
15, 29, 35092–35106
2023
A1

Abstract 

Covalent organic frameworks (COFs) are emerging as a new class of photoactive organic semiconductors, which possess crystalline ordered structures and high surface areas. COFs can be tailor-made toward specific (photocatalytic) applications, and the size and position of their band gaps can be tuned by the choice of building blocks and linkages. However, many types of building blocks are still unexplored as photocatalytic moieties and the scope of reactions photocatalyzed by COFs remains quite limited. In this work, we report the synthesis and application of two bipyridine- or phenylpyridine-based COFs: TpBpyCOF and TpPpyCOF. Due to their good photocatalytic properties, both materials were applied as metal-free photocatalysts for the tandem aerobic oxidation/Povarov cyclization and α-oxidation of N-aryl glycine derivatives, with the bipyridine-based TpBpyCOF exhibiting the highest activity. By expanding the range of reactions that can be photocatalyzed by COFs, this work paves the way toward the more widespread application of COFs as metal-free heterogeneous photocatalysts as a convenient alternative for commonly used homogeneous (metal-based) photocatalysts.

Open Access version available at UGent repository

Phosphorous Covalent Triazine Framework based Nanomaterials for the Electrocatalytic Hydrogen Evolution Reaction

H. S. Jena, C. Krishnaraj, B. K. Satpathy, K. S. Rawat, K. Leus, S. Veerapandian, R. Morent, N. De Geyter, V. Van Speybroeck, D. Pradhan, P. Van der Voort
ACS Applied Nano Materials
2023
A1

Abstract 

The production of hydrogen via electrocatalytic reduction of water using metal-free nanomaterials as the catalyst is a promising and ultimate green approach. Graphitic carbon nitride, covalent organic frameworks, and covalent triazine frameworks (CTFs) are some of the nanostructured materials that are investigated for this purpose. Currently, these materials still lack the efficiency to compete with other techniques (electrolysis). This is because the reaction mechanism and active sites are, in many cases, still poorly understood. In this work, we report a set of metal-free nanostructure-based electrocatalysts, phosphorus covalent triazine frameworks (PCTFs), for electrocatalytic hydrogen production. The hydrogen evolution reaction (HER) performance of PCTF-based nanomaterials is ascribed to the synergistic effect of isolated single nitrogen and phosphorus sites on the large surface area. By combining both experimental and theoretical studies, we found that especially the pyridinic-nitrogen species are the most active sites for the HER. The presence of phosphorus next to the pyridinic-N enhances the HERs. The present results provide a better understanding of the importance of different heteroatoms in nanomaterials as active sites in HERs. Theoretical studies confirmed that phosphorus, being electron rich, creates high electron densities on the nearby N atoms of the CTF materials and intensifies the HER process.

Additivity of atomic strain fields as a tool to strain-engineering phase-stabilized CsPbI3 perovskites

J. Teunissen, T. Braeckevelt, I. Skvortsova, J. Guo, B. Pradhan, E. Debroye, M. Roeffaers, J. Hofkens, S. Van Aert, S. Bals, S.M.J. Rogge, V. Van Speybroeck
The Journal of Physical Chemistry C
127, 48, 23400-23411
2023
A1

Abstract 

CsPbI3 is a promising perovskite material for photovoltaic applications in its photoactive perovskite or black phase. However, the material degrades to a photovoltaically inactive or yellow phase at room temperature. Various mitigation strategies are currently being developed to increase the lifetime of the black phase, many of which rely on inducing strains in the material that hinder the black-to-yellow phase transition. Physical insight into how these strategies exactly induce strain as well as knowledge of the spatial extent over which these strains impact the material is crucial to optimize these approaches but is still lacking. Herein, we combine machine learning potential-based molecular dynamics simulations with our in silico strain engineering approach to accurately quantify strained large-scale atomic structures on a nanosecond time scale. To this end, we first model the strain fields introduced by atomic substitutions as they form the most elementary strain sources. We demonstrate that the magnitude of the induced strain fields decays exponentially with the distance from the strain source, following a decay rate that is largely independent of the specific substitution. Second, we show that the total strain field induced by multiple strain sources can be predicted to an excellent approximation by summing the strain fields of each individual source. Finally, through a case study, we illustrate how this additive character allows us to explain how complex strain fields, induced by spatially extended strain sources, can be predicted by adequately combining the strain fields caused by local strain sources. Hence, the strain additivity proposed here can be adopted to further our insight into the complex strain behavior in perovskites and to design strain from the atomic level onward to enhance their sought-after phase stability.

Open Access version available at UGent repository
Gold Open Access

Super-Oxidizing Covalent Triazine Framework Electrocatalyst for Two-Electron Water Oxidation to H2O2

R. Khan, J. Chakraborty, K. S. Rawat, R. Morent, N. De Geyter, V. Van Speybroeck, P. Van der Voort
Angewandte Chemie int. Ed.
Volume: 62, Issue: 47
2023
A1

Abstract 

Electrochemical two-electron water oxidation (2e WOR) is gaining surging research traction for sustainable hydrogen peroxide production. However, the strong oxidizing environment and thermodynamically competitive side-reaction (4e WOR) posit as thresholds for the 2e WOR. We herein report a custom-crafted covalent triazine network possessing strong oxidizing properties as a proof-of-concept metal-free functional organic network electrocatalyst for catalyzing 2e WOR. As the first-of-its-kind, the material shows a maximum of 89.9% Faradaic Efficiency and 1428 μmol/h/cm2 H2O2 production rate at 3.0 V bias potential (vs reversible hydrogen electrode, RHE), which are either better or comparable to the stateof-the-art electrocatalysts. We have experimentally confirmed a stepwise 2e WOR mechanism which was further computationally endorsed by density functional theory studies.

The Gradient Curves Method: An improved strategy for the derivation of molecular mechanics valence force fields from ab initio data

T. Verstraelen, D. Van Neck, P.W. Ayers, V. Van Speybroeck, M. Waroquier
LECTURE SERIES ON COMPUTER AND COMPUTATIONAL SCIENCES
Volume 7A-B, page 576 -+
2006
P1

Abstract 

A novel force-field parameterization procedure[1] is proposed that surmounts well-known difficulties of the conventional least squares parameterization. The multidimensional ab initio training data are first transformed into individual one-dimensional data sets, each associated with one term in the force-field model. In the second step conventional methods call be used to fit each energy term separately to its corresponding data set. The first step call be completed without any knowledge of the analytical expressions for the energy terms. Moreover the transformed data sets dictate the form of these expressions, which makes the method very suitable for deriving valence force fields. During the transformation in the first step, continuity and least-norm criteria are imposed. The latter facilitate the intuitive physical interpretation of the energy terms that are fitted to the transformed data sets, a prerequisite for transferable force fields. Benchmark parameterizations have been performed oil three small molecules, showing that the new method results in physically intuitive energy terms, exactly when a conventional parameterization would suffer from parameter correlations, i.e. when the number of redundant internal coordinates in the force-field model increases.

Unraveling the Mechanisms of Zirconium Metal–Organic Frameworks-Based Mixed-Matrix Membranes Preventing Polysulfide Shuttling

W. Lu, Z. Pang, A. Lamaire, F. Liu, S.Dai, M. L. Pinto, R. Demir-Cakan, K. O. Tan, V. Van Speybroeck, V. Pimenta, C. Serre
Small Science
4, 2300339
2024
A1

Abstract 

Lithium-sulfur batteries are considered as promising candidates for next-generation energy storage devices for grid applications due to their high theoretical energy density. However, the inevitable shuttle effect of lithium polysulfides and/or dendrite growth of Li metal anodes hinder their commercial viability. Here, the microporous Zr fumarate MOF-801(Zr) was considered to produce thin (~15.6 µm, ~1mg cm²) mixed matrix membranes (MMM) as a novel interlayer for Li-S batteries. It was found that the MOF-801(Zr)/C/PVDF-HFP composite interlayer facilitates Li+ ions diffusion, and anchors polysulfides while promoting their redox conversion effectively. We demonstrated that MOF-801 effectively trapped polysulfides at the cathode side, and confirmed for the first time the nature of the interaction between the adsorbed polysulfides and the host framework, through a combination of solid-state NMR and molecular dynamics simulations. The incorporation of MOF-801(Zr)/C/PVDF-HFP MMM interlayer resulted in a notable enhancement in the initial capacity of Li-S batteries up to 1110 mA h g-1. Moreover, even after 50 cycles, a specific capacity of 880 mA h g-1 was delivered.

OGRe: Optimal grid refinement protocol for accurate free energy surfaces and its application to proton hopping in zeolites and 2D COF stacking

S. Borgmans, S.M.J. Rogge, L. Vanduyfhuys, V. Van Speybroeck
Journal of Chemical Theory and Computation
19, 24, 9032-9048
2024
A1

Abstract 

While free energy surfaces are the crux of our understanding in many chemical and biological processes, their accuracy is generally unknown. Moreover, many developments to improve their accuracy are often complicated, impeding their general use. Luckily, several tools and guidelines are already in place to identify these shortcomings, but they are typically lacking in flexibility or fail to systematically determine how to improve the accuracy of the free energy calculation. To overcome these limitations, this work introduces OGRe--a python package for optimal grid refinement in an arbitrary number of dimensions. OGRe is based on three metrics which gauge the confinement, consistency, and overlap of each simulation in a series of umbrella sampling (US) simulations, an enhanced sampling technique ubiquitously adopted to construct free energy surfaces for hindered processes. As these three metrics are fundamentally linked to the accuracy of the weighted histogram analysis method, adopted to generate free energy surfaces from US simulations, they facilitate a systematic construction of accurate free energy profiles, where each metric is driven by a specific umbrella parameter. This allows for the derivation of a consistent and optimal collection of umbrellas for each simulation, largely independent of the initial values, thereby dramatically increasing the ease-of-use towards accurate free energy surfaces. As such, OGRe is particularly suited to determined complex free energy surfaces, with large activation barriers and shallow minima, which underpin many physical and chemical transformations, and hence to further our fundamental understanding of these processes.

Gold Open Access

On the Prediction of Spectroscopic Fingerprints of Co2+ Complexes Relevant for the ZIF Nucleation Process

L. De Bruecker, M. Filez, V. Van Speybroeck
Inorganic Chemistry
Volume: 62, Issue: 40, Pages: 16304-16322
2023
A1

Abstract 

The nucleation process of zeolitic imidazolate frameworks (ZIFs) is to date not completely understood. Recently, it has been found that, during the formation of Co-ZIF-67, after mixing imidazole-type ligands with octahedral precursors containing oxygen-coordinated ligands, a metal–organic pool with a diversity of transition metal complexes (TMCs) is formed showing fingerprints of octahedral and tetrahedral Co2+ complexes with both types of ligands [Filez, M. Cell Rep. Phys. Sci. 2021, 2, 100680]. In order to further unravel this process, we aim to characterize the d–d transitions of the TMCs and focus on their number, intensity, and position, which change during the process and can thus serve as a fingerprint for the formed species. It was previously shown that the number of ligands and symmetry has a detrimental influence on the ground state properties of Co2+ TMCs. Herein, we investigate how far excited state properties of TMCs relevant during nanoporous formation processes can be predicted by time-dependent density functional theory (TDDFT) and ligand field density functional theory (LFDFT). As TMCs are known to be challenging systems with possibly degenerate ground states and double excitations, we first investigate the performance of both techniques on first-row octahedral aqua-complexes. With this knowledge, we then focus on tetrahedral Co2+ complexes with aqua and imidazole-type ligands in order to investigate in how far we can propose a spectroscopic fingerprint that allows us to follow the Co2+ complexes during the formation of Co-ZIF-67. The results of TDDFT and LFDFT are qualitatively in agreement and provide complementary information. We found that various features can be used to distinguish between the species. However, as LFDFT is not suited for TMCs possessing the extended imidazole-type ligands and double and spin-flip states are not included in TDDFT, both techniques need to be complemented with more advanced methods to obtain complete insight into the d–d excitations of TMCs with imidazole ligands. Therefore, we particularly explored ab initio ligand field theory, which is capable of describing double excitations and is, in contrast to LFDFT, suitable for TMCs with extended ligands.

The Electrophilic Aromatic Bromination of Benzenes: Mechanistic and Regioselective Insights from Density Functional Theory

X. Deraet, E. Desmedt, R. Van Lommel, V. Van Speybroeck, F. De Proft
Physical Chemistry Chemical Physics (PCCP)
25, 28581 - 28594
2023
A1

Abstract 

The electrophilic aromatic substitution of benzenes is part of any undergraduate organic chemistry textbook, yet the mechanism, and more precisely the Wheland intermediate, remains a matter of debate. In this paper, we have computed different reaction paths for the bromination of benzene, anisole and nitrobenzene at the B97X-D/cc-pVTZ level of theory. This revealed, independently of the considered benzenes, a clear kinetic preference for an addition-elimination mechanism, rather than a substitution. Moreover, both mechanisms do not involve a charged Wheland-like intermediate, not in the gas phase nor in the investigated solvents (CCl4 and acetonitrile). Insight into the regioselectivity of the bromination was provided using a combination of conceptual DFT reactivity indices, aromaticity indices, Wiberg bond indices and the non-covalent interaction index. The ortho/para directing effect of the electron-donating methoxy-group in anisole was retrieved and ascribed to a synergy between strong electron delocalisation and attractive interactions. In contrast, the preferred meta-addition on nitrobenzene could not be traced back to any of these effects, nor to the intrinsic reactivity property of the reactant. In this case, an electrostatic clash between the ipso-carbon of the ring and the nitrogen atom resulting from the later nature of the rate-determining step, with respect to anisole, appeared to play a crucial role.

DFT-Quality Adsorption Simulations in Metal–Organic Frameworks Enabled by Machine Learning Potentials

R. Goeminne, L. Vanduyfhuys, V. Van Speybroeck, T. Verstraelen
Journal of Chemical Theory and Computation (JCTC)
19, 18, 6313-6325
2023
A1

Abstract 

Nanoporous materials such as metal–organic frameworks (MOFs) have been extensively studied for their potential for adsorption and separation applications. In this respect, grand canonical Monte Carlo (GCMC) simulations have become a well-established tool for computational screenings of the adsorption properties of large sets of MOFs. However, their reliance on empirical force field potentials has limited the accuracy with which this tool can be applied to MOFs with challenging chemical environments such as open-metal sites. On the other hand, density-functional theory (DFT) is too computationally demanding to be routinely employed in GCMC simulations due to the excessive number of required function evaluations. Therefore, we propose in this paper a protocol for training machine learning potentials (MLPs) on a limited set of DFT intermolecular interaction energies (and forces) of CO2 in ZIF-8 and the open-metal site containing Mg-MOF-74, and use the MLPs to derive adsorption isotherms from first principles. We make use of the equivariant NequIP model which has demonstrated excellent data efficiency, and as such an error on the interaction energies below 0.2 kJ mol–1 per adsorbate in ZIF-8 was attained. Its use in GCMC simulations results in highly accurate adsorption isotherms and heats of adsorption. For Mg-MOF-74, a large dependence of the obtained results on the used dispersion correction was observed, where PBE-MBD performs the best. Lastly, to test the transferability of the MLP trained on ZIF-8, it was applied to ZIF-3, ZIF-4, and ZIF-6, which resulted in large deviations in the predicted adsorption isotherms and heats of adsorption. Only when explicitly training on data for all ZIFs, accurate adsorption properties were obtained. As the proposed methodology is widely applicable to guest adsorption in nanoporous materials, it opens up the possibility for training general-purpose MLPs to perform highly accurate investigations of guest adsorption.

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