A.E.J. Hoffman

Understanding the phase transition mechanism in the lead halide perovskite CsPbBr₃ via theoretical and experimental GIWAXS and Raman spectroscopy

A.E.J. Hoffman, R.A. Saha, S. Borgmans, P. Puech, T. Braeckevelt, M.B.J. Roeffaers, J.A. Steele, J. Hofkens, V. Van Speybroeck
APL Materials
2023
A1

Abstract 

Metal-halide perovskites (MHPs) exhibit excellent properties for application in optoelectronic devices. The bottleneck for their incorporation is the lack of long-term stability such as degradation due to external conditions (heat, light, oxygen, moisture, and mechanical stress), but the occurrence of phase transitions also affects their performance. Structural phase transitions are often influenced by phonon modes. Hence, an insight into both the structure and lattice dynamics is vital to assess the potential of MHPs. In this study, GIWAXS and Raman spectroscopy are applied, supported by density functional theory calculations, to investigate the apparent manifestation of structural phase transitions in the MHP CsPbBr3. Macroscopically, CsPbBr3 undergoes phase transitions between a cubic (α), tetragonal (β), and orthorhombic (γ) phase with decreasing temperature. However, microscopically, it has been argued that only the γ phase exists, while the other phases exist as averages over length and time scales within distinct temperature ranges. Here, direct proof is provided for this conjecture by analyzing both theoretical diffraction patterns and the evolution of the tilting angle of the PbBr6 octahedra from molecular dynamics simulations. Moreover, sound agreement between experimental and theoretical Raman spectra allowed to identify the Raman active phonon modes and to investigate their frequency as a function of temperature. As such, this work increases the understanding of the structure and lattice dynamics of CsPbBr3 and similar MHPs.

Microscopic Linker Distribution in Mixed-Linker Zeolitic Imidazolate Frameworks via Computational Raman Spectroscopy: Implications for Gas Separation

A.E.J. Hoffman, J. Marreiros, S.M.J. Rogge, R. Ameloot, V. Van Speybroeck
ACS Applied Nano Materials
2023
A1

Abstract 

Mixed-linker zeolitic imidazolate frameworks (ZIFs) are important candidate materials for gas separation. By changing the linker content, their pore size can be tuned, offering the potential to regulate diffusion and adsorption. An important factor affecting these properties in mixed-linker ZIFs is the linker distribution, which is difficult to characterize. In this study, the microscopic linker distribution in mixed-linker ZIF-8/ZIF-90, with respectively methyl and carboxaldehyde functionalization, is elucidated via computational Raman spectroscopy. It is shown that the typical Raman band associated with the carboxaldehyde linker is shifted due to a change in hydrogen-bonding behavior. This insight allows one to explain the microscopic linker distribution in experimental mixed-linker structures.

How water and ion mobility affect the NMR fingerprints of the hydrated JBW zeolite: a combined computational-experimental investigation

S. Vanlommel, A.E.J. Hoffman, S. Smet, S. Radhakrishnan, K. Asselman, C. V. Chandran, E. Breynaert, C. Kirschhock, J.A. Martens, V. Van Speybroeck
Chemistry - A European Journal
28, 68, e202202621
2022
A1

Abstract 

An important aspect within zeolite synthesis is to make fully tunable framework materials with controlled aluminium distribution. A major challenge in characterising these zeolites at operating conditions is the presence of water. In this work, we investigate the effect of hydration on the 27 Al NMR parameters of the ultracrystalline K,Na-compensated aluminosilicate JBW zeolite using experimental and computational techniques. The JBW framework, with Si/Al ratio of 1, is an ideal benchmark system as a stepping stone towards more complicated zeolites. The presence and mobility of water and extraframework species directly affect NMR fingerprints. Excellent agreement between theoretical and experimental spectra is obtained provided dynamic methods are employed with hydrated structural models. This work shows how NMR is instrumental in characterising aluminium distributions in zeolites at operating conditions.

Gold Open Access

Unfolding the terahertz spectrum of soft porous crystals: rigid unit modes and their impact on phase transitions

A.E.J. Hoffman, I. Senkovska, J. Wieme, A. Krylov, S. Kaskel, V. Van Speybroeck
Journal of Materials Chemistry A
10 (33), 17254-17266
2022
A1

Abstract 

Phase transitions in exible metal-organic frameworks or soft porous crystals are mediated by low-frequency phonons or rigid-unit modes. The alteration of specic building blocks may change the lattice dynamics of these frameworks, which can inuence the phase transition mechanism. In this work, the impact of building block substitution on the rigid-unit modes in exible MIL-53 analogs with a winerack topology will be investigated via ab initio lattice dynamics calculations. First, the accuracy of the theoretical simulations is veried via experimental Raman measurements, which provide unique ngerprint vibrations in the terahertz range to characterize the phase transition. Following analysis of the low-frequency vibrations shows that there exists a set of universal rigid-unit modes inducing translations and/or rotations of the building blocks. The theoretical results demonstrate that linker substitutions have a large eect on the rigid-unit mode frequencies, whereas this is less so for inorganic chain substitutions. These ndings may help to rationally tune the phonon frequencies in soft porous crystals.

Gold Open Access

Towards modeling spatiotemporal processes in metal–organic frameworks

V. Van Speybroeck, S. Vandenhaute, A.E.J. Hoffman, S.M.J. Rogge
Trends in Chemistry
3 (8): 605-619
2021
A1

Abstract 

Metal–organic frameworks (MOFs) are hybrid materials constructed from metal clusters linked by organic linkers, which can be engineered for target functional applications in, for example, catalysis, sensing, and storage. The dynamic response of MOFs on external stimuli can be tuned by spatial heterogeneities such as defects and crystal size as well as by operating conditions such as temperature, pressure, moisture, and external fields. Modeling the spatiotemporal evolution of MOFs under operating conditions and at length and time scales comparable with experimental observations is extremely challenging. Herein, we give a status on the modeling of spatiotemporal processes in MOFs under working conditions and reflect on how modeling can be reconciled with in situ spectroscopy measurements.

Gold Open Access

Identification of vanadium dopant sites in the metal–organic framework DUT-5(Al)

K. Maes, L.I.D.J. Martin, S. Khelifi, A.E.J. Hoffman, K. Leus, P. Van der Voort, E. Goovaerts, P.F. Smet, V. Van Speybroeck, F. Callens, H. Vrielinck
Physical Chemistry Chemical Physics (PCCP)
23, 7088-7100
2021
A1

Abstract 

Studying the structural environment of the VIV ions doped in the metal–organic framework (MOF) DUT-5(Al) ((AlIIIOH)BPDC) with electron paramagnetic resonance (EPR) reveals four different vanadium-related spectral components. The spin-Hamiltonian parameters are derived by analysis of X-, Q- and W-band powder EPR spectra. Complementary Q-band Electron Nuclear DOuble Resonance (ENDOR) experiments, Scanning Electron Microscopy (SEM), Energy Dispersive X-ray spectroscopy (EDX), X-Ray Diffraction (XRD) and Fourier Transform InfraRed (FTIR) measurements are performed to investigate the origin of these spectral components. Two spectral components with well resolved 51V hyperfine structure are visible, one corresponding to VIV=O substitution in a large (or open) pore and one to a narrow (or closed) pore variant of this MOF. Furthermore, a broad structureless Lorentzian line assigned to interacting vanadyl centers in each other's close neighborhood grows with increasing V-concentration. The last spectral component is best visible at low V-concentrations. We tentatively attribute it to (VIV=O)2+ linked with DMF or dimethylamine in the pores of the MOF. Simulations using these four spectral components convincingly reproduce the experimental spectra and allow to estimate the contribution of each vanadyl species as a function of V-concentration.

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.

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.

Atomistic insight in the flexibility and heat transport properties of the stimuli-responsive metal-organic framework MIL-53(Al) for water-adsorption applications using molecular simulations

A. Lamaire, J. Wieme, A.E.J. Hoffman, V. Van Speybroeck
Faraday Discussions
225, 301-323
2021
A1

Abstract 

To exploit the full potential of metal-organic frameworks as solid adsorbents in water-adsorption applications, many challenges remain to be solved. A more fundamental insight into the properties of the host material and the influence water exerts on them can be obtained by performing molecular simulations. In this work, the prototypical flexible MIL-53(Al) framework is modelled using advanced molecular dynamics simulations. For different water loadings, the presence of water is shown to affect the relative stability of MIL-53(Al), triggering a phase transition from the narrow-pore to the large-pore phase at the highest considered loading. Furthermore, the effect of confinement on the structural organisation of the water molecules is also examined for different pore volumes of MIL-53(Al). For the framework itself, we focus on the thermal conductivity, as this property plays a decisive role in the efficiency of adsorption-based technologies, due to the energy-intensive adsorption and desorption cycles. To this end, the heat transfer characteristics of both phases of MIL-53(Al) are studied, demonstrating a strong directional dependence for the thermal conductivity.

Gold Open Access

Engineering a highly defective stable UiO-66 with tunable Lewis-Brønsted acidity - The role of the hemilabile linker

X. Feng, J. Hajek, H. S. Jena, G. Wang, S. K. P. Veerapandian, R. Morent, N. De Geyter, K. Leyssens, A.E.J. Hoffman, V. Meynen, C. Marquez, D. De Vos, V. Van Speybroeck, K. Leus, P. Van der Voort
JACS (Journal of the American Chemical Society)
142 (6), 3174-3183
2020
A1

Abstract 

The stability of metal-organic frameworks (MOFs) typically decreases with an increasing number of defects, limiting the number of defects that can be created and limiting catalytic and other applications. Herein, we use a hemilabile (Hl) linker to create up to maximum 6 defects per cluster in UiO-66. We have synthesized hemilabile UiO-66 (Hl-UiO-66) using benzene dicarboxylate (BDC) as linker and 4-sulfonatobenzoate (PSBA) as the hemilabile linker. The PSBA acts not only as a modulator to create defects, but also as a co-ligand that enhances the stability of the resulting defective framework. Furthermore, upon a post-synthetic treatment in H2SO4, the average number of defects increases to the optimum of six missing BDC linkers per cluster (3 per formula unit), leaving the Zr-nodes on average 6-fold coordinated. Remarkably, the thermal stability of the materials further increases upon this treatment. Periodic density functional theory calculations confirm that the hemilabile ligands strengthen this highly defective structure by several stabilizing interactions. Finally, the catalytic activity of the obtained materials is evaluated in the acid-catalyzed isomerization of α-pinene oxide. This reaction is particularly sensitive to the Brønsted or Lewis acid sites in the catalyst. In comparison to the pristine UiO-66, which mainly possesses Brønsted acid sites, the Hl-UiO-66 and the post-synthetically treated Hl-UiO-66 structures exhibited a higher Lewis acidity and an enhanced activity and selectivity. This is further explored by CD3CN spectroscopic sorption experiments. We have shown that by tuning the number of defects in UiO-66 using PSBA as the hemilabile linker, one can achieve highly defective and stable MOFs and easily control the Brønsted to Lewis acid ratio in the materials, and thus their catalytic activity and selectivity.

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