K. Leus

Elucidating the promotional effect of a covalent triazine framework in aerobic oxidation

S. Abednatanzi, P. Gohari Derakhshandeh, P. Tack, F. Muniz-Miranda, Y-Y Liu, J. Everaert, M. Meledina, F. Vanden Bussche, L. Vincze, C. Stevens, V. Van Speybroeck, H. Vrielinck, F. Callens, K. Leus, P. Van der Voort
Applied Catalysis B: Environmental
269, 118769
2020
A1

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.

Immobilization of Ir(I) complex on Covalent Triazine Frameworks for C-H Borylation Reactions: A Combined Experimental and Computational Study

N. Tahir, F. Muniz-Miranda, J. Everaert, P. Tack, T.S.A Heugebaert, K. Leus, L. Vincze, C. Stevens, V. Van Speybroeck, P. Van der Voort
Journal of Catalysis
371, 135-143
2019
A1

Abstract 

Metal-modified covalent triazine frameworks (CTFs) have attracted considerable attention in heterogeneous catalysis due to their strong nitrogen-metal interactions exhibiting superior activity, stability and hence recyclability. Herein, we report on a post-metalation of a bipyridine-based CTFs with an Ir(I) complex for CH borylation of aromatic compounds. Physical characterization of the Ir(I)-based bipyCTF catalyst in combination with density functional theory (DFT) calculations exhibit a high stabilization energy of the Ir-bipy moiety in the frameworks in the presence of B2Pin2. By using B2Pin2 as a boron source, Ir(I)@bipyCTF efficiently catalyzed the CH borylation of various aromatic compounds with excellent activity and good recyclability. In addition, XAS analysis of the Ir(I)@bipyCTF gave clear evidence for the coordination environment of the Ir.

A series of sulfonic acid functionalized mixed-linker DUT-4 analogues: synthesis, gas sorption properties and catalytic performance

G. Wang, K. Leus, K. Hendrickx, J. Wieme, H. Depauw, Y-Y Liu, V. Van Speybroeck, P. Van der Voort
Dalton Transactions
46, 14356
2017
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Abstract 

In this work, we present the successful synthesis of a series of sulfonic acid functionalized mixed-linker metal–organic frameworks (MOFs) having the DUT-4 topology by using different ratios of 2,6-naphthalenedicarboxylic acid (H2-NDC) and 4,8-disulfonaphthalene-2,6-dicarboxylic acid (H2-NDC-2SO3H) in one-pot reactions. The obtained materials were fully characterized and their CO2 adsorption properties at low and high pressures were studied and compared with those of the pristine DUT-4 material. Generally, the CO2 adsorption capacities range from 3.28 and 1.36 mmol g−1 for DUT-4 to 1.54 and 0.78 mmol g−1 for DUT-4-SO3H (50) up to 1 bar at 273 K and 303 K, respectively. Computational calculations corroborated the structural changes of the material in function of the loading of sulfonic acid groups. Furthermore, due to the strong Brønsted acid character, the resulting sulfonic acid based MOF material was evaluated as a catalyst for the ring opening of styrene oxide with methanol as a nucleophile under mild conditions, showing almost full conversion (99%) after 5 hours of reaction. A hot filtration experiment demonstrated that the catalysis occurred heterogeneously and the catalyst could be recovered and reused for multiple runs without significant loss in activity and crystallinity.

Biocompatible Zr-based nanoscale MOFs coated with modified poly (epsilon-caprolactone) as anticancer drug carriers

M. Filippousi, S. Turner, K. Leus, P.I. Siafaka, E.D. Tseligka, M. Vandichel, S.G. Nanaki, I.S. Vizirianakis, D.N. Bikiaris, P. Van der Voort, G. Van Tendeloo
International Journal of Pharmaceutics
509, 1-2, 208-218
2016
A1

Abstract 

Nanoscale Zr-based metal organic frameworks (MOFs) UiO-66 and UiO-67 were studied as potential anticancer drug delivery vehicles. Two model drugs were used, hydrophobic paclitaxel and hydrophilic cisplatin, and were adsorbed onto/into the nano MOFs (NMOFs). The drug loaded MOFs were further encapsulated inside a modified poly(epsilon-caprolactone) with D-alpha-tocopheryl polyethylene glycol succinate polymeric matrix, in the form of microparticles, in order to prepare sustained release formulations and to reduce the drug toxicity. The drugs physical state and release rate was studied at 37 degrees C using Simulated Body Fluid. It was found that the drug release depends on the interaction between the MOFs and the drugs while the controlled release rates can be attributed to the microencapsulated formulations. The in vitro antitumor activity was assessed using HSC-3 (human oral squamous carcinoma; head and neck) and U-87 MG (human glioblastoma grade IV; astrocytoma) cancer cells. Cytotoxicity studies for both cell lines showed that the polymer coated, drug loaded MOFs exhibited better anticancer activity compared to free paclitaxel and cisplatin solutions at different concentrations. (C) 2016 Elsevier B.V. All rights reserved.

Systematic study of the chemical and hydrothermal stability of selected "stable" Metal Organic Frameworks

K. Leus, T. Bogaerts, J. De Decker, H. Depauw, K. Hendrickx, H. Vrielinck, V. Van Speybroeck, P. Van der Voort
Microporous and Mesoporous Materials
226, 110-116
2016
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Abstract 

In this work, the hydrothermal and chemical stability towards acids, bases, air, water and peroxides of Metal Organic Frameworks, that are commonly considered to be stable, is presented. As a proof of stability both the crystallinity and porosity are measured before and after exposure to the stress test. The major part of the MOFs examined in this study showed a good hydrothermal stability except for the UiO-67, NH2-MIL-101 (Al) and CuBTC material. The chemical stability towards acids and bases show a similar tendency and an ordering can be proposed as: MIL-101(Cr)>NH2-UiO-66>UiO-66>UiO-67>NH2-MIL-53>MIL-53(Al)>ZIF-8>CuBTC>NH2-MIL-101(Al). In the tests with the H2O2 solution most materials behaved poorly, only the UiO-66 and NH2-UiO-66 framework showed a good stability.

Understanding Intrinsic Light Absorption Properties of UiO- 66 Frameworks: A Combined Theoretical and Experimental Study

K. Hendrickx, D.E.P. Vanpoucke, K. Leus, K. Lejaeghere, A. Van Yperen-De Deyne, V. Van Speybroeck, P. Van der Voort, K. Hemelsoet
Inorganic Chemistry
54, 22, 10701-10710
2015
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Abstract 

A combined theoretical and experimental study is performed in order to elucidate the effects of linker functional groups on the photoabsorption properties of UiO-66-type materials. This study, in which both mono- and di-functionalized linkers (with X= -OH, -NH2, -SH) are studied, aims to obtain a more complete picture on the choice of functionalization. Static Time-Dependent Density Functional Theory (TD-DFT) calculations combined with Molecular Dynamics simulations are performed on the linkers and compared to experimental UV/VIS spectra, in order to understand the electronic effects governing the absorption spectra. Di-substituted linkers show larger shifts compared to mono-substituted variants, making them promising candidates for further study as photocatalysts. Next, the interaction between the linker and the inorganic part of the framework is theoretically investigated using a cluster model. The proposed Ligand-to-Metal-Charge Transfer (LMCT) is theoretically observed and is influenced by the differences in functionalization. Finally, computed electronic properties of the periodic UiO-66 materials reveal that the band gap can be altered by linker functionalization and ranges from 4.0 down to 2.2 eV. Study of the periodic Density of States (DOS) allows to explain the band gap modulations of the framework in terms of a functionalization-induced band in the band gap of the original UiO-66 host.

Au@UiO-66: a base free oxidation catalyst

K. Leus, P. Concepcion, M. Vandichel, M. Meledina, A. Grirrane, D. Esquivel, S. Turner, D. Poelman, M. Waroquier, V. Van Speybroeck, G. Van Tendeloo, H. Garcia, P. Van der Voort
RSC Advances
5 (29), 22334–22342
2015
A1

Abstract 

We present the in situ synthesis of Au nanoparticles within the Zr based Metal Organic Framework, UiO-66. The resulting Au@UiO-66 materials were characterized by means of N2 sorption, XRPD, UV-Vis, XRF, XPS and TEM analysis. The Au nanoparticles (NP) are homogeneously distributed along the UiO-66 host matrix when using NaBH4 or H2 as reducing agents. The Au@UiO-66 materials were evaluated as catalysts in the oxidation of benzyl alcohol and benzyl amine employing O2 as oxidant. The Au@MOF materials exhibit a very high selectivity towards the ketone (up to 100 %). Regenerability and stability tests demonstrate that the Au@UiO-66 catalyst can be recycled with a negligible loss of Au species and no loss of crystallinity. In situ IR measurements of UiO-66 and Au@UiO-66-NaBH4, before and after treatment with alcohol, showed an increase in IR bands that can be assigned to a combination of physisorbed and chemisorbed alcohol species. This was confirmed by velocity power spectra obtained from the molecular dynamics simulations. Active peroxo and oxo species on Au could be visualized with Raman analysis.

Open Access version available at UGent repository

Vanadium Metal-Organic Frameworks: Structures and Applications

P. Van der Voort, K. Leus, Y-Y Liu, M. Vandichel, V. Van Speybroeck, M. Waroquier, S. Biswas
New Journal of Chemistry
38, 1853-1867
2014
A1

Abstract 

This perspective review paper describes the V-containing Metal-Organic Framworks that have been developed since the first systematic reports on MOFs almost 15 years ago. These hybrid crystalline materials, containing V(III) or V(IV) as metal nodes show interesting behaviour in oxidation catalysis and gas sorption. A significant amount of papers has appeared on the use of these structures in gas (hydrocarbon, CO2) separation. Promising future research and development of V-MOFs is suggested.

Open Access version available at UGent repository

New Functionalized Metal–Organic Frameworks MIL-47-X (X = −Cl, −Br, −CH3, −CF3, −OH, −OCH3): Synthesis, Characterization, and CO2 Adsorption Properties

S. Biswas, D.E.P. Vanpoucke, T. Verstraelen, M. Vandichel, S. Couck, K. Leus, Y-Y Liu, M. Waroquier, V. Van Speybroeck, J.F.M. Denayer, P. Van der Voort
Journal of Physical Chemistry C
117 (44), 22784–22796
2013
A1

Abstract 

Six new functionalized vanadium hydroxo terephthalates [VIII(OH)(BDC-X)]•n(guests) (MIL-47(VIII)-X-AS) (BDC = 1,4-benzenedicarboxylate; X = -Cl; -Br, -CH3, -CF3, -OH, -OCH3; AS = as-synthesized) along with the parent MIL-47 were synthesized under rapid microwave-assisted hydrothermal conditions (170 ºC, 30 min, 150 W). The unreacted H2BDC-X and/or occluded solvent molecules can be removed by thermal activation under vacuum leading to the empty-pore forms of the title compounds (MIL-47(VIV)-X). Except pristine MIL-47 (+III oxidation state), the vanadium atoms in all the evacuated functionalized solids stayed in +IV oxidation state. The phase purity of the compounds was ascertained by X-ray powder diffraction (XRPD), diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, Raman, thermogravimetric (TG), and elemental analysis. The structural similarity of the filled and empty-pore forms of the functionalized compounds with the respective forms of parent MIL-47 was verified by cell parameter determination from XRPD data. TGA and temperature-dependent XRPD (TDXRPD) experiments in air atmosphere indicate high thermal stability in the range 330-385 ºC. All the thermally activated compounds exhibit significant microporosity (SLangmuir in the range 418-1104 m2 g-1), as verified by the N2 and CO2 sorption analysis. Among the six functionalized compounds, MIL-47(VIV)-OCH3 shows the highest CO2 uptake, demonstrating the determining role of functional groups on the CO2 sorption behaviour. For this compound and pristine MIL-47(VIV), Widom particle insertion simulations were performed based on ab initio calculated crystal structures. The theoretical Henry coefficients show a good agreement with the experimental values, and calculated isosurfaces for the local excess chemical potential indicate the enhanced CO2 affinity is due to two effects: (i) the interaction between the methoxy group and CO2 and (ii) the collapse of the MIL-47(VIV)-OCH3 framework.

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