V. Van Speybroeck

Totally conjugated and coplanar covalent organic frameworks as photocatalysts for water purification: Reduction of Cr (VI) while oxidizing water borne organic pollutants

L. Wang, J. Chakraborty, K. S. Rawat, M. Deng, J. Sun, Y. Wang, V. Van Speybroeck, P. Van der Voort
Separation and Purification Technology
359, 1, 130368
2025
A1

Abstract 

Covalent organic frameworks (COFs) have emerged as photocatalytic materials with bandgaps in the visible region. Imine-based COFs, which have been extensively explored, often suffer from limited stability and poor conjugation, hindering their photocatalytic activities. The chemical and hydrolytic stability and the photo catalytic performance of COFs is drastically enhanced by constructing 2D COFs that are fully conjugated in the x, y plane, that have alternating donor–acceptor (D-A) units for better charge separation and that have enhanced conjugation in the z-axis by p-orbital overlap by using highly planar building blocks. In this study, we introduce three highly crystalline sp 2 COFs that are able to photocatalyticlly reduce highly toxic Cr (VI) species to much less toxic and easily removable Cr (III) residues, while simultaneously oxidizing water borne organic pollutants. One of them, the TEB-COF, with the integration of the acetylene group, exhibited excellent photocatalytic ac tivity due to its superior planarity and extended conjugation. TEB-COF is able to completely remove the model dye Rhodamine B and Cr (VI) (10 mg/L) in less than 30 min. This research provides valuable insights into the development of recyclable metal-free photocatalysts for wastewater treatment.

In-Depth Thermodynamic and Kinetic Analysis of Ethane Diffusion in ZIF-8

B. Schmidt, P. Cnudde, V. Van Speybroeck, L. Vanduyfhuys
Journal of Physical Chemistry C
128, 43, 18509-18523
2024
A1

Abstract 

Flexible microporous ZIF-8 crystals show excellent separation behavior of small molecules such as ethaneand ethene. As such, hydrocarbon diffusion plays an essential role in the performance of these materials, yet determining accurate diffusion constants is nontrivial. Both ab initio and force-field based molecular dynamics simulations, coupled with umbrella sampling are applied in this work to characterize the diffusion of ethane in ZIF-8. Diffusion constants are extracted from the simulations by a combination of transition state theory and a random-walk hopping model, and are compared against experimentally measured values from literature. Ethane diffusion is a hindered process characterized by a transition state corresponding to an ethane molecule crossing the gate in between two neighboring cages formed by methylimidazole linkers. Free energy profiles of the diffusion process are derived and analyzed revealing the entropic nature of the barrier due to a counteracting of covalent host deformation energy and nonbonding host–guest interaction. A temperature analysis further confirms the entropic nature of the barrier and reveals an increased gate opening at increasing temperature. Finally, the loading dependency of diffusion is investigated revealing that increasing the ethane loading of the cages slightly slows down diffusion as a result of beneficial guest–guest interactions in the cages. Our findings yield essential elementary insight into how different molecular interactions influence the diffusion path of hydrocarbons throughout ZIF-8 crystals.

Investigation of the Octahedral Network Structure in Formamidinium Lead Bromide Nanocrystals by Low-Dose Scanning Transmission Electron Microscopy

N. J. Schrenker, T. Braeckevelt, A. De Backer, N. Livakas, C.-P. Yu, T. Friedrich, M.B.J. Roeffaers, J. Hofkens, J. Verbeeck, L. Manna, V. Van Speybroeck, S. Van Aert, S. Bals
Nano Letters
24, 35, 10936-10942
2024
A1

Abstract 

Metal halide perovskites (MHP) are highly promising semiconductors. In this study, we focus on FAPbBr3 nanocrystals, which are of great interest for green light-emitting diodes. Structural parameters significantly impact the properties of MHPs and are linked to phase instability, which hampers long-term applications. Clearly, there is a need for local and precise characterization techniques at the atomic scale, such as transmission electron microscopy. Because of the high electron beam sensitivity of MHPs, these investigations are extremely challenging. Here, we applied a low-dose method based on four-dimensional scanning transmission electron microscopy. We quantified the observed elongation of the projections of the Br atomic columns, suggesting an alternation in the position of the Br atoms perpendicular to the Pb–Br–Pb bonds. Together with molecular dynamics simulations, these results remarkably reveal local distortions in an on-average cubic structure. Additionally, this study provides an approach to prospectively investigating the fundamental degradation mechanisms of MHPs.

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Turning carbon dioxide into dialkyl carbonates through guanidinium-assisted SN2 ion-pair process

J. Delcorps, K. S. Rawat, M. Wells, E. B. Ayed, B. Grignard, C. Detrembleur, B. Blankert, P. Gerbaux, V. Van Speybroeck, O. Coulembier
Cell Reports Physical Science
5, 7, 102057
2024
A1

Abstract 

The synthesis of dialkyl carbonates, versatile compounds with applications in organic synthesis, pharmaceuticals, and polymers, has attracted considerable attention due to their environmentally benign nature. Here, we describe the selective bimolecular nucleophilic substitution (SN2) reaction between primary and secondary alkyl iodides with 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD)-based carbon dioxide-binding organic liquids. We show that TBD is a great candidate for bulk carbon dioxide and alcohol binding at 100C. TBDbased carbonate salts are selective for SN2 processes, allowing them to work with highly reactive alkyl iodide while eliminating unwanted base quaternization either in acetonitrile or in bulk at both 21C and 65C. The high reactivity of these TBD-based carbon dioxide-binding organic liquids toward backside SN2 processes at low temperature is explained by the presence of the TBD.H+ guanidinium, revealing a unique metal-free cation-assisted SN2 ion-pair process.

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
20, 12, 5225-5240
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.

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