V. Van Speybroeck

A comparative study on the photophysical properties of anthocyanins and pyranoanthocyanins

K.T. Phan, S. De Meester, K. Raes, K. De Clerck, V. Van Speybroeck
Chemistry - A European Journal
2021
A1

Abstract 

Anthocyanins and pyranoanthocyanins are flavonoids that are present in various food products (e.g. fruit, vegetables, wine, etc.). The large chemical diversity amongst these molecules leads to compound specific properties such as color and stability towards external conditions. These properties are also attractive for food and non‐food applications. The photophysical experimental characterization is not easy as this generally demands advanced analytical techniques along with optimized separation procedures. Molecular modeling can provide insights into the fundamental understanding of the photophysical properties of these compounds in a uniform way for a broad set of compounds. However, the current literature is quite fragmented on this topic. Herein, a large set of 140 naturally derived anthocyanins was evaluated in a systematic way with three functionals (B3LYP, PBE0 and CAM‐B3LYP). The accuracy of these functionals was determined with experimental literature λ max,vis ‐values. Next to λ max,vis ‐values, TD‐DFT calculations also provided oscillator strengths, molar absorption coefficients and orbital energies, which define whether specific natural anthocyanin‐based compounds can be deployed in food and non‐food applications such as food additives/colorants, textile dyeing, analytical standards and dye sensitized solar cells (DSSCs).

Texture Formation in Polycrystalline Thin Films of All-Inorganic Lead Halide Perovskite

J.A. Steele, E. Solano, H. Jin, V. Prakasam, T. Braeckevelt, H. Yuan, Z. Lin, R. de Kloe, Q. Wang, S.M.J. Rogge, V. Van Speybroeck, D. Chernyshov, J. Hofkens, M. Roeffaers
Advanced Materials
2021
A1

Abstract 

Controlling grain orientations within polycrystalline all-inorganic halide perovskite solar cells can help increase conversion efficiencies toward their thermodynamic limits, however the forces governing texture formation are ambiguous. Using synchrotron X-ray diffraction, we report meso-structure formation within polycrystalline CsPbI2.85Br0.15 powders as they cool from a high-temperature cubic perovskite (α-phase). Tetragonal distortions (β-phase) trigger preferential crystallographic alignment within polycrystalline ensembles, a feature we suggest is coordinated across multiple neighboring grains via interfacial forces that select for certain lattice distortions over others. External anisotropy is then imposed on polycrystalline thin films of orthorhombic (γ-phase) CsPbI3-xBrx perovskite via substrate clamping, revealing two fundamental uniaxial texture formations; (i) I-rich films possess orthorhombic-like texture (<100> out-of-plane; <010> and <001> in-plane), while (ii) Br-rich films form tetragonal-like texture (<110> out-of-plane; <1-10> and <001> in-plane). In contrast to relatively uninfluential factors like the choice of substrate, film thickness and annealing temperature, Br incorporation modifies the γ-CsPbI3-xBrx crystal structure by reducing the orthorhombic lattice distortion (making it more tetragonal-like) and governs the formation of the different, energetically favored textures within polycrystalline thin films.

Theoretical and Spectroscopic Evidence of the Dynamic Nature of Copper Active Sites in Cu-CHA Catalysts under Selective Catalytic Reduction (NH3–SCR–NOx) Conditions

R. Millan, P. Cnudde, A.E.J. Hoffman, C.W. Lopes, P. Concepcion, V. Van Speybroeck, M. Boronat
Journal of Physical Chemistry Letters
11, 23, 10060-10066
2020
A1

Abstract 

The dynamic nature of the copper cations acting as active sites for selective catalytic reduction of nitrogen oxides with ammonia is investigated using a combined theoretical and spectroscopic approach. Ab initio molecular dynamics simulations of Cu-CHA catalysts in contact with reactants and intermediates at realistic operating conditions show that only ammonia is able to release Cu+ and Cu2+ cations from their positions coordinated to the zeolite framework, forming mobile Cu+(NH3)2 and Cu2+(NH3)4 complexes that migrate to the center of the cavity. Herein, we give evidence that such mobilization of copper cations modifies the vibrational fingerprint in the 800–1000 cm–1 region of the IR spectra. Bands associated with the lattice asymmetric T-O-T vibrations are perturbed by the presence of coordinated cations, and allow one to experimentally follow the dynamic reorganization of the active sites at operating conditions.

Shape-selective C–H activation of aromatics to biarylic compounds using molecular palladium in zeolites

J. Vercammen, M. Bocus, S. E. Neale, A. Bugaev, P. Tomkins, J. Hajek, S. Van Minnebruggen, A. Soldatov, A. Krajnc, G. Mali, V. Van Speybroeck, D. De Vos
Nature Catalysis
3, 1002-1009
2020
A1

Abstract 

The selective activation of inert C–H bonds has emerged as a promising tool for avoiding the use of wasteful traditional coupling reactions. Oxidative coupling of simple aromatics allows for a cost-effective synthesis of biaryls. However, utilization of this technology is severely hampered by poor regioselectivity and by the limited stability of state-of-the-art homogeneous Pd catalysts. Here, we show that confinement of cationic Pd in the pores of a zeolite allows for the shape-selective C–H activation of simple aromatics without a functional handle or electronic bias. For instance, out of six possible isomers, 4,4′-bitolyl is produced with high shape selectivity (80%) in oxidative toluene coupling on Pd-Beta. Not only is a robust, heterogeneous catalytic system obtained, but this concept is also set to control the selectivity in transition-metal-catalysed arene C–H activation through spatial confinement in zeolite pores.

Strongly Reducing (Diarylamino)benzene Based Covalent Organic Framework for Metal-Free Visible Light Photocatalytic H2O2 Generation

C. Krishnaraj, H. S. Jena, L. Bourda, A. Laemont, P. Pachfule, J. Roeser, C. V. Chandran, S. Borgmans, S.M.J. Rogge, K. Leus, C.V. Stevens, J.A. Martens, V. Van Speybroeck, E. Breynaert, A. Thomas, P. Van der Voort
JACS (Journal of the American Chemical Society)
142 (47), 20107-20116
2020
A1

Abstract 

Photocatalytic reduction of molecular oxygen is a promising route toward sustainable production of hydrogen peroxide (H2O2). This challenging process requires photoactive semiconductors enabling solar energy driven generation and separation of electrons and holes with high charge transfer kinetics. Covalent organic frameworks (COFs) are an emerging class of photoactive semiconductors, tunable at a molecular level for high charge carrier generation and transfer. Herein, we report two newly designed two-dimensional COFs based on a (diarylamino)benzene linker that forms a Kagome (kgm) lattice and shows strong visible light absorption. Their high crystallinity and large surface areas (up to 1165 m2·g−1) allow efficient charge transfer and diffusion. The diarylamine (donor) unit promotes strong reduction properties, enabling these COFs to efficiently reduce oxygen to form H2O2. Overall, the use of a metal-free, recyclable photocatalytic system allows efficient photocatalytic solar transformations.

Gold Open Access

Access to bio-renewable and CO2-based polycarbonates from exovinylene cyclic carbonates

F. Siragusa, E. Van den Broeck, C. Ocando, A. Müller, G. De Smet, B. U. W. Maes, J. De Winter, V. Van Speybroeck, B. Grignard, C. Detrembleur
ACS Sustainable Chemistry & Engineering
2021
A1

Abstract 

We investigate the scope of the organocatalyzed step-growth copolymerization of CO2-sourced exovinylene bicyclic carbonates with bio-based diols into polycarbonates. A series of regioregular poly(oxo-carbonate)s were prepared from sugar- (1,4-butanediol and isosorbide) or lignin-derived (1,4-benzenedimethanol and 1,4-cyclohexanediol) diols at 25 °C with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as a catalyst, and their defect-free structure was confirmed by nuclear magnetic resonance spectroscopy studies. Their characterization by differential scanning calorimetry and wide-angle X-ray scattering showed that most of them were able to crystallize. When the polymerizations were carried out at 80 °C, some structural defects were introduced within the polycarbonate chains, which limited the polymer molar mass. Model reactions were carried out to understand the influence of the structure of alcohols, the temperature (25 or 80 °C), and the use of DBU on the rate of alcoholysis of the carbonate and on the product/linkage selectivity. A full mechanistic understanding was given by means of static- and dynamic-based density functional theory (DFT) calculations showing the determining role of DBU in the stability of intermediates, and its important role in the rate-determining steps is revealed. Furthermore, the origin of side reactions observed at 80 °C was discussed and rationalized by DFT modeling. As impressive diversified bio-based diols are accessible on a large scale and at low cost, this process of valorization of carbon dioxide gives new perspectives on the sustainable production of bioplastics under mild conditions.

Structural and photophysical properties of various polypyridyl ligands: A combined experimental and computational study

L. De Bruecker, J. Everaert, P. Van der Voort, C.V. Stevens, M. Waroquier, V. Van Speybroeck
ChemPhysChem
21 (22), 2489–2505
2020
A1

Abstract 

Covalent triazine frameworks (CTFs) with polypyridyl ligands are very promising supports to anchor photocatalytic complexes. Herein, we investigate the photophysical properties of a series of ligands which vary by the extent of the aromatic system, the nitrogen content and their topologies to aid in selecting interesting building blocks for CTFs. Interestingly, some linkers have a rotational degree of freedom, allowing both a trans and cis structure, where only the latter allows anchoring. Therefore, the influence of the dihedral angle on the UV‐Vis spectrum is studied . The photophysical properties are investigated by a combined computational and experimental study. Theoretically, both static and molecular dynamics simulations are performed to deduce ground‐ and excited state properties based on density functional theory (DFT) and time‐dependent DFT. The position of the main absorption peak shifts towards higher wavelengths for an increased size of the π‐system and a higher π‐electron deficiency. We found that the position of the main absorption peak among the different ligands studied in this work can amount to 271 nm; which has a significant impact on the photophysical properties of the ligands. This broad range of shifts allows modulation of the electronic structure by varying the ligands and may help in a rational design of efficient photocatalysts.

Gold Open Access

Unravelling thermal stress due to thermal expansion mismatch in metal-organic frameworks for methane storage

J. Wieme, V. Van Speybroeck
Journal of Materials Chemistry A
2020
A1

Abstract 

Thermal stress is present in all systems undergoing temperature changes during their operation. Metal-organic frameworks (MOFs) are a class of porous, crystalline materials ideally suited for a wide range of adsorption-based technologies. The release and consumption of the heat of adsorption instigate temperature fluctuations and thermal stress in these materials that could induce disruptive volume changes. To bring these materials to engineering applications, it is of utmost importance to understand their thermal expansion behavior and the overall induced thermal stress due to thermal expansion mismatch with other components. In this work, we focus on a large group of MOFs known to have promising methane adsorption properties and predict their thermal expansion coefficients based on force field molecular dynamics simulations. Negative thermal expansion (NTE) behavior is predicted for all studied MOFs, and the magnitude of the NTE coefficients is found to be positively correlated with the degree of porosity of the frameworks. Finally, as a proxy for the thermal stress, the thermal pressure coefficient is calculated, which is found to be in the range between polymers and ceramics. Variations within the operating temperature range of MOFs are therefore expected to result in a relatively low thermal stress.

Insight into the effects of confined hydrocarbon species on the lifetime of methanol conversion catalysts

I. Lezcano-Gonzalez, E. Campbell, A.E.J. Hoffman, M. Bocus, I.V. Sazanovich, M. Towrie, M. Agote-Aran, E.K. Gibson, A. Greenaway, K. De Wispelaere, V. Van Speybroeck, A.M. Beale
Nature Materials
19, 1081–1087
2020
A1

Abstract 

The methanol-to-hydrocarbons reaction refers collectively to a series of important industrial catalytic processes to produce either olefins or gasoline. Mechanistically, methanol conversion proceeds through a ‘pool’ of hydrocarbon species. For the methanol-to-olefins process, these species can be delineated broadly into ‘desired’ lighter olefins and ‘undesired’ heavier fractions that cause deactivation in a matter of hours. The crux in further catalyst optimization is the ability to follow the formation of carbonaceous species during operation. Here, we report the combined results of an operando Kerr-gated Raman spectroscopic study with state-of-the-art operando molecular simulations, which allowed us to follow the formation of hydrocarbon species at various stages of methanol conversion. Polyenes are identified as crucial intermediates towards formation of polycyclic aromatic hydrocarbons, with their fate determined largely by the zeolite topology. Notably, we provide the missing link between active and deactivating species, which allows us to propose potential design rules for future-generation catalysts.

Mechanistic insight into the framework methylation ofH-ZSM-5 for varying methanol loading and Si/Al ratiousing first principles molecular dynamics simulations

S. A. F. Nastase, P. Cnudde, L. Vanduyfhuys, K. De Wispelaere, V. Van Speybroeck, C.R.A. Catlow, A. J. Logsdail
ACS Catalysis
10, 15, 8904-8915
2020
A1
Gold Open Access

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