V. Van Speybroeck

Reaching quantum accuracy in predicting adsorption properties for ethane/ethene in ZIF-8 at the low pressure regime

S. Ravichandran, M. Najafi, R. Goeminne, J. F. M. Denayer, V. Van Speybroeck, L. Vanduyfhuys
Journal of Chemical Theory and Computation
2024
A1

Abstract 

Nanoporous materials in the form of metal−organicframeworks such as zeolitic imidazolate framework-8 (ZIF-8) arepromising membrane materials for the separation of hydrocarbonmixtures. To compute the adsorption isotherms in suchadsorbents, grand canonical Monte Carlo simulations have provento be very useful. The quality of these isotherms depends on theaccuracy of adsorbate−adsorbent interactions, which are mostlydescribed using force fields owing to their low computational cost.However, force field predictions of adsorption uptake often showdiscrepancies from experiments at low pressures, providing theneed for methods that are more accurate. Hence, in this work, wepropose and validate two novel methodologies for the ZIF-8/ethane and ethene systems; a benchmarking methodology toevaluate the performance of any given force field in describing adsorption in the low-pressure regime and a refinement procedure torescale the parameters of a force field to better describe the host−guest interactions and provide for simulation isotherms with closeagreement to experimental isotherms. Both methodologies were developed based on a reference Henry coefficient, computed withthe PBE-MBD functional using the importance sampling technique. The force field rankings predicted by the benchmarkingmethodology involve the comparison of force field derived Henry coefficients with the reference Henry coefficients and ranking theforce fields based on the disparities between these Henry coefficients. The ranking from this methodology matches the rankingsmade based on uptake disparities by comparing force field derived simulation isotherms to experimental isotherms in the low-pressure regime. The force field rescaling methodology was proven to refine even the worst performing force field in UFF/TraPPE.The uptake disparities of UFF/TraPPE improved from 197% and 194% to 11% and 21% for ethane and ethene, respectively. Theproposed methodology is applicable to predict adsorption across nanoporous materials and allows for rescaled force fields to reachquantum accuracy without the need for experimental input.

High-Throughput Screening of Covalent Organic Frameworks for Carbon Capture Using Machine Learning

J. De Vos, S. Ravichandran, S. Borgmans, L. Vanduyfhuys, P. Van der Voort, S.M.J. Rogge, V. Van Speybroeck
Chemistry of Materials
36, 9, 4315-4330
2024
A1

Abstract 

Postcombustion carbon capture provides a high-potential pathway to reduce anthropogenic CO2 emissions in the short term. In this respect, nanoporous materials, such as covalent organic frameworks (COFs), offer a promising platform as adsorbent beds. However, due to the modular nature of COFs, an almost unlimited number of structures can possibly be synthesized. To efficiently identify promising materials and reveal performance trends within the COF material space, we present a computational high-throughput screening of 268,687 COFs for their ability to efficiently and selectively separate CO2 from the flue gas of power plants using a pressure swing adsorption process. Furthermore, we demonstrate that our screening can be significantly accelerated using machine learning to identify a set of promising materials. These are subsequently characterized with high accuracy, taking into account the effects of competitive adsorption and coadsorption. Our screening reveals that imide, (keto)enamine, and (acyl)hydrazone COFs are particularly interesting for carbon capture. Additionally, the best-performing materials are 3D COFs possessing strong CO2 adsorption sites between aromatic rings at opposite sides of pores with a diameter of 1.0 nm. In 2D COFs, a significant influence of the framework chemistry, such as imide linkages or fluoro groups, is observed. Our design rules can guide experimental researchers to construct high-performing COFs for CO2 capture.

Gold Open Access

Gas adsorption and framework flexibility of CALF-20 explored via experiments and simulations

R. Oktavian, R. Goeminne, L.T. Glasby, P. Song, R. Huynh, O. T. Qazvini, O. Ghaffari-Nik, N. Masoumifard, J. L. Cordiner, P. Hovington, V. Van Speybroeck, P. Z. Moghadam
Nature Communications
15, 3898
2024
A1

Abstract 

In 2021, Svante, in collaboration with BASF, reported successful scale up of CALF-20 production, a stable MOF with high capacity for post-combustion CO2 capture which exhibits remarkable stability towards water. CALF-20’s success story in the MOF commercialisation space provides new thinking about appropriate structural and adsorptive metrics important for CO2 capture. Here, we combine atomistic-level simulations with experiments to study adsorptive properties of CALF-20 and shed light on its flexible crystal structure. We compare measured and predicted CO2 and water adsorption isotherms and explain the role of water-framework interactions and hydrogen bonding networks in CALF-20’s hydrophobic behaviour. Furthermore, regular and enhanced sampling molecular dynamics simulations are performed with both density-functional theory (DFT) and machine learning potentials (MLPs) trained to DFT energies and forces. From these simulations, the effects of adsorption-induced flexibility in CALF-20 are uncovered. We envisage this work would encourage development of other MOF materials useful for CO2 capture applications in humid conditions.

Gold Open Access

Computational Protocol for the Spectral Assignment of NMR Resonances in Covalent Organic Frameworks

S. Vanlommel, S. Borgmans, C. V. Chandran, S. Radhakrishnan, P. Van der Voort, E. Breynaert, V. Van Speybroeck
Journal of Chemical Theory and Computation (JCTC)
20, 9, 3823–3838
2024
A1

Abstract 

Solid-state nuclear magnetic resonance spectroscopy is routinely used in the field of covalent organic frameworks to elucidate or confirm the structure of the synthesized samples and to understand dynamic phenomena. Typically this involves the interpretation and simulation of the spectra through the assumption of symmetry elements of the building units, hinging on the correct assignment of each line shape. To avoid misinterpretation resulting from library-based assignment without a theoretical basis incorporating the impact of the framework, this work proposes a first-principles computational protocol for the assignment of experimental spectra, which exploits the symmetry of the underlying building blocks for computational feasibility. In this way, this protocol accommodates the validation of previous experimental assignments and can serve to complement new NMR measurements.

The application of porous organic polymers as metal free photocatalysts in organic synthesis

M. Debruyne, P. Van der Voort, V. Van Speybroeck, C. Stevens
Chemistry - A European Journal
2024
A1

Abstract 

Concerns about increasing greenhouse gas emissions and their effect on our environment highlight the urgent need for new sustainable technologies. Visible light photocatalysis allows the clean and selective generation of reactive intermediates under mild conditions. The more widespread adoption of the current generation of photocatalysts, particularly those using precious metals, is hampered by drawbacks such as their cost, toxicity, difficult separation, and limited recyclability. This is driving the search for alternatives, such as porous organic polymers (POPs). This new class of materials is made entirely from organic building blocks, can possess high surface area and stability, and has a controllable composition and functionality. This review focuses on the application of POPs as photocatalysts in organic synthesis. For each reaction type, a representative material is discussed, with special attention to the mechanism of the reaction. Additionally, an overview is given, comparing POPs with other classes of photocatalysts, and critical conclusions and future perspectives are provided on this important field.

Following the dynamics of industrial catalysts under operando conditions

V. Van Speybroeck
PNAS
Volume: 121, Issue: 2, Article number: e2319800121
2024
A1

Abstract 

Catalytic reactions taking place in industrial processes are often performed under extreme conditions of temperatures and pressures. A typical example is the Haber–Bosch process to industrially synthesize ammonia from nitrogen and hydrogen which operates under reaction pressures from 10 to 15 MPa and temperatures higher than 400 °C (1). Following the dynamics of heterogeneous catalysts under such extreme conditions is highly challenging both from experimental and theoretical point of view. In their paper, Bonati et al. give unique molecular insights into the dynamics, adsorption, and bond breakage of the N2 molecule when interacting with the (111) iron surface at high temperatures relevant for the Haber–Bosch catalytic system (2). The simulations reveal that the surface is much more dynamic than anticipated from low-temperature experiments or simulations. Active sites are continuously formed and disrupted, and this behavior is instrumental for driving the catalytic process. To follow the weakening of the nitrogen–nitrogen bond, the degree of charge transfer from the metallic surface to the triple bond was followed during various steps of the catalytic process.

The study of Bonati et al. is an important proof-of-concept study, showcasing that reaction mechanisms may be highly dependent on the reaction conditions and that an evaluation of the dynamics of the system at industrially relevant conditions is of utmost importance to obtain molecular insights. Such insights can not be obtained from low-temperature investigations. It is notoriously difficult to simulate the dynamics of industrially relevant catalytic reactions under operating conditions. Hence, Bonati et al. had to combine various innovations in the field of machine learning and enhanced sampling molecular dynamics to follow the adsorption and reaction on the fly at operating conditions during sufficiently long time scales. A summary of some essential ingredients of their workflow is schematically shown in Fig. 1 and discussed further below. The impact of the methodological advances presented in their study is of much greater importance than the specific case study discussed in the PNAS paper and opens perspectives to follow industrially relevant catalytic reactions on the fly at the conditions where the catalyst does the work.

A Critical Assessment on Calculating Vibrational Spectra in Nanostructured Materials

A.E.J. Hoffman, W. Temmerman, E. Campbell, A. A. Damin, I. Lezcano-Gonzalez, A.M. Beale, S. Bordiga, J. Hofkens, V. Van Speybroeck
Journal of Chemical Theory and Computation
Volume: 20, Issue: 2, Pages: 513-531
2023
A1

Abstract 

Vibrational spectroscopy is an omnipresent spectroscopic technique to characterize functional nanostructured materials such as zeolites, metal–organic frameworks (MOFs), and metal–halide perovskites (MHPs). The resulting experimental spectra are usually complex, with both low-frequency framework modes and high-frequency functional group vibrations. Therefore, theoretically calculated spectra are often an essential element to elucidate the vibrational fingerprint. In principle, there are two possible approaches to calculate vibrational spectra: (i) a static approach that approximates the potential energy surface (PES) as a set of independent harmonic oscillators and (ii) a dynamic approach that explicitly samples the PES around equilibrium by integrating Newton’s equations of motions. The dynamic approach considers anharmonic and temperature effects and provides a more genuine representation of materials at true operating conditions; however, such simulations come at a substantially increased computational cost. This is certainly true when forces and energy evaluations are performed at the quantum mechanical level. Molecular dynamics (MD) techniques have become more established within the field of computational chemistry. Yet, for the prediction of infrared (IR) and Raman spectra of nanostructured materials, their usage has been less explored and remain restricted to some isolated successes. Therefore, it is currently not a priori clear which methodology should be used to accurately predict vibrational spectra for a given system. A comprehensive comparative study between various theoretical methods and experimental spectra for a broad set of nanostructured materials is so far lacking. To fill this gap, we herein present a concise overview on which methodology is suited to accurately predict vibrational spectra for a broad range of nanostructured materials and formulate a series of theoretical guidelines to this purpose. To this end, four different case studies are considered, each treating a particular material aspect, namely breathing in flexible MOFs, characterization of defects in the rigid MOF UiO-66, anharmonic vibrations in the metal–halide perovskite CsPbBr3, and guest adsorption on the pores of the zeolite H-SSZ-13. For all four materials, in their guest- and defect-free state and at sufficiently low temperatures, both the static and dynamic approach yield qualitatively similar spectra in agreement with experimental results. When the temperature is increased, the harmonic approximation starts to fail for CsPbBr3 due to the presence of anharmonic phonon modes. Also, the spectroscopic fingerprints of defects and guest species are insufficiently well predicted by a simple harmonic model. Both phenomena flatten the potential energy surface (PES), which facilitates the transitions between metastable states, necessitating dynamic sampling. On the basis of the four case studies treated in this Review, we can propose the following theoretical guidelines to simulate accurate vibrational spectra of functional solid-state materials: (i) For nanostructured crystalline framework materials at low temperature, insights into the lattice dynamics can be obtained using a static approach relying on a few points on the PES and an independent set of harmonic oscillators. (ii) When the material is evaluated at higher temperatures or when additional complexity enters the system, e.g., strong anharmonicity, defects, or guest species, the harmonic regime breaks down and dynamic sampling is required for a correct prediction of the phonon spectrum. These guidelines and their illustrations for prototype material classes can help experimental and theoretical researchers to enhance the knowledge obtained from a lattice dynamics study.

The nature of extraframework aluminum species and Brønsted acid site interactions under catalytic operating conditions

J. L. Mancuso, V. Van Speybroeck
Journal of Catalysis
429, 115211
2024
A1

Abstract 

A systematic investigation of hydrated extraframework aluminum (EFAl) species interacting with Brønsted acid sites (BAS) in H-ZSM-5 is presented to understand the cooperative active site structure under catalytic operating conditions. Static models of EFAl species confined in the H-ZSM-5 unit cell show that isolated BAS protonate neutral EFAl species to form cations. Ab-initio molecular dynamics (AIMD) simulations and enhanced sampling performed at the temperature for methanol-to-hydrocarbon conversion reveal two regimes of stable EFAl species, namely the [Al(OH)2]+ ion existing with two bonds to the zeolite scaffold or as a pore-guest in the form of [Al(OH)2(H2O)2]+. Our results indicate that hydrogen-bonding plays a significant role in BAS-EFAl active site structure, especially at higher BAS density and that EFAl species can function as both Bronsted and Lewis acidic components to alter proton transfer kinetics as well as shape selectivity within these microporous solids.

Gold Open Access

Development of porous organic polymers as metal free photocatalysts for the aromatization of N-heterocycles

M. Debruyne, N. Raeymackers, H. Vrielinck, S. Radhakrishnan, E. Breynaert, M. Delaey, A. Laemont, K. Leus, J. Everaert, H. Rijckaert, D. Poelman, R. Morent, N. De Geyter, P. Van der Voort, V. Van Speybroeck, C. Stevens, T.S.A Heugebaert
ChemCatChem
2024
A1

Abstract 

Porous organic polymers (POPs), and especially covalent triazine frameworks (CTFs), are being developed as the next generation of metal-free heterogeneous photocatalysts. However, many of the current synthetic routes to obtain these photoactive POPs require expensive monomers and rely on precious metal catalysts, thus hindering their widespread implementation. In this work, a range of POPs was synthesized from simple unfunctionalized aromatic building blocks, through Lewis acidcatalyzed polymerization. The obtained materials were applied, for the first time, as heterogeneous photocatalysts for the aromatization of N-heterocycles. With the use of the most active material, denoted as CTF-Pyr, which consists of photoactive pyrene and triazine moieties, a wide range of pyridines, dihydroquinoline-5-ones, tetrahydroacridine-1,8-diones and pyrazoles were obtained in excellent yields (70-99%). Moreover, these reactions were carried out under very mild conditions using air and at room temperature, highlighting the potential of these materials as catalysts for green transformations.

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

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