V. Van Speybroeck

Asymmetric synthesis of 3,4-disubstituted 2-(trifluoromethyl)pyrrolidines through rearrangement of chiral 2-(2,2,2-trifluoro-1-hydroxyethyl)azetidines

J. Dolfen, E. Birsen Boydas, V. Van Speybroeck, S. Catak, K. Van Hecke, M. D'Hooghe
Journal of Organic Chemistry
82 (19), 10092–10109
2017
A1

Abstract 

Enantiopure 4-formyl-β-lactams were deployed as synthons for the diastereoselective formation of chiral 2-(2,2,2-trifluoro-1-hydroxyethyl)azetidines via trifluoromethylation through aldehyde modification followed by reductive removal of the β-lactam carbonyl moiety. Subsequent treatment of the (in situ) activated 2-trifluoroethylated azetidines with a variety of nitrogen, oxygen, sulfur and fluorine nucleophiles afforded chiral 3,4-disubstituted 2-(trifluoromethyl)pyrrolidines in good to excellent yields (45-99%) and high diastereoselectivities (dr > 99/1, 1H NMR) via interception of bicyclic aziridinium intermediates. Furthermore, representative pyrrolidines were N,O-debenzylated in a selective way and used for further synthetic elaboration to produce e.g. a CF3-substituted 2-oxa-4,7-diazabicyclo[3.3.0]octan-3-one system.

Hydrogen transfer versus methylation: on the genesis of aromatics formation in the Methanol-To-Hydrocarbons over H-ZSM-5

J. S. Martínez-Espín, K. De Wispelaere, T. V. Janssens, S. Svelle, K. P. Lillerud, P. Beato, V. Van Speybroeck, U. Olsbye
ACS Catalysis
7, 5773–5780
2017
A1

Abstract 

The catalytic conversion of methanol (MeOH) and dimethyl ether (DME) into fuels and chemicals over zeolites (MTH process) is industrially emerging as an alternative route to conventional oil-derived processes. After 40 years of research, a detailed mechanistic understanding of the intricate reaction network is still not fully accomplished. The overall reaction is described as two competitive catalytic cycles, dominated by alkenes and arenes, which are methylated and cracked or dealkylated to form effluent products. Herein, we present the reaction of isobutene with methanol and DME as an efficient tool for measuring the relative formation rates of alkenes and arenes, and we provide detailed mechanistic insight into the hydrogen-transfer reaction. We provide experimental and theoretical evidence that manifest a strong competition of methylation and hydrogen transfer of isobutene by methanol, while methylation is substantially favored by DME. Experiments performed at higher conversion facilitate projection of the results to the product distribution obtained when using MeOH or DME as feedstock during the MTH reaction.

Mechanical properties of a gallium fumarate metal-organic framework: a joint experimental-modelling exploration

P. Ramaswamy, J. Wieme, E. Alvarez, L. Vanduyfhuys, J.-P. Itié, P. Fabry, V. Van Speybroeck, C. Serre, P.G. Yot, G. Maurin
Journal of Materials Chemistry A
5 (22), 11047-11054
2017
A1

Abstract 

A gallium analogue of the commercially available Al-fumarate MOF A520 - recently identified as isotypic to MIL-53(Al)-BDC - has been synthesized and further characterized in its hydrated and dehydrated forms. The structural response under applied mechanical pressure of this MIL-53(Ga)-FA solid was investigated using advanced experimental techniques coupled with computational tools. Hg porosimetry and high-pressure X-Ray Powder Diffraction (XRPD) experiments evidenced that the pristine dehydrated large pore form undergoes an irreversible structure contraction upon an applied pressure of 85 MPa with an associated volume change of ca. 14% which makes this material promising for mechanical energy storage applications, in particular as a shock absorber. The breathing behavior was further rationalized performing a series of periodic Density Functional Theory (DFT) calculations with the construction of an energy profile as a function of volume for both MIL-53(Ga)-FA and its Aluminum analogue. As such we could fully unravel the microscopic origin of the difference in pressure-induced behavior for the aluminum and gallium fumarate based materials.

Design of a thermally controlled sequence of triazolinedione-based click and transclick reactions

H.A. Houck, K. De Bruycker, S. Billiet, B. Dhanis, H. Goossens, S. Catak, V. Van Speybroeck, J.M. Winne, F. Du Prez
Chemical Science
8 (4), 3098-3108
2017
A1

Abstract 

The reaction of triazolinediones (TADs) and indoles is of particular interest for polymer chemistry applications, as it is a very fast and irreversible additive-free process at room temperature, but can be turned into a dynamic covalent bond forming process at elevated temperatures, giving a reliable bond exchange or ‘transclick’ reaction. In this paper, we report an in-depth study aimed at controlling the TAD – indole reversible click reactions through rational design of modified indole reaction partners. This has resulted in the identification of a novel class of easily accessible indole derivatives that give dynamic TAD-adduct formation at significantly lower temperatures. We further demonstrate that these new substrates can be used to design a directed cascade of click reactions of a functionalized TAD moiety from an initial indole reaction partner to a second indole, and finally to an irreversible reaction partner. This controlled sequence of click and transclick reactions of a single TAD reagent between three different substrates has been demonstrated both on small molecule and macromolecular level, and the factors that control the reversibility profiles have been rationalized and guided by mechanistic considerations supported by theoretical calculations.

Open Access version available at UGent repository
Green Open Access

Nature of active sites on UiO-66 and beneficial influence of water in the catalysis of Fischer esterification

C. Caratelli, J. Hajek, F. G. Cirujano, M. Waroquier, F. X. Llabres i Xamena, V. Van Speybroeck
Journal of Catalysis
352, 401-414
2017
A1

Abstract 

Zirconium terephthalate UiO-66 type metal organic frameworks (MOFs) are known to be highly active, stable and reusable catalysts for the esterification of carboxylic acids with alcohols. Moreover, when defects are present in the structure of these MOFs, coordinatively unsaturated Zr ions with Lewis acid properties are created, which increase the catalytic activity of the resulting defective solids. In the present work, molecular modeling techniques combined with new experimental data on various defective hydrated and dehydrated materials allow to unravel the nature and role of defective active sites in the Fischer esterification and the role of coordinated water molecules to provide additional Brønsted sites. Periodic models of UiO-66 and UiO-66-NH2 catalysts have been used to unravel the reaction mechanism on hydrated and dehydrated materials. Various adsorption modes of water and methanol are investigated. The proposed mechanisms are in line with experimental observations that amino groups yield a reduction in the reaction barriers, although they have a passive role in modulating the electronic structure of the material. Water has a beneficial role on the reaction cycle by providing extra Brønsted sites and by providing stabilization for various intermediates through hydrogen bonds.

Open Access version available at UGent repository
Gold Open Access

Missing linkers: an alternative pathway to UiO-66 electronic structure engineering

A. De Vos, K. Hendrickx, P. Van der Voort, V. Van Speybroeck, K. Lejaeghere
Chemistry of Materials
29 (7), 3006–3019
2017
A1

Abstract 

UiO-66 is a promising metal-organic framework for photocatalytic applications. However, the ligand-to-metal charge transfer of an excited electron is inefficient in the pristine material. Herein we assess the influence of missing linker defects on the electronic structure of UiO-66 and discuss their ability to improve ligand-to-metal charge transfer. Using a new defect classification system, which is transparent and easily extendable, we identify the most promising photocatalysts by considering both relative stability and electronic structure. We find the properties of UiO-66 defect structures largely to depend on the coordination of the constituent nodes, and the nodes with the strongest local distortions to alter the electronic structure most. Defects hence provide an alternative pathway to tune UiO-66 for photocatalytic purposes, besides linker modification and node metal substitution. In addition, the decomposition of MOF properties into node- and linker-based behavior is more generally valid, so we propose orthogonal electronic structure tuning as a paradigm in MOF electronic structure engineering.

Open Access version available at UGent repository
Gold Open Access

Metal-organic and covalent organic frameworks as single-site catalysts

S.M.J. Rogge, A. Bavykina, J. Hajek, H. Garcia, A.I. Olivos-Suarez, A. Sepúlveda-Escribano, A. Vimont, G. Clet, P. Bazin, F. Kapteijn, M. Daturi, E.V. Ramos-Fernandez, F.X. Llabres i Xamena, V. Van Speybroeck, J. Gascon
Chemical Society Reviews
46 (11), 3134-3184
2017
A1

Abstract 

Heterogeneous single-site catalysts consist of isolated, well-defined, active sites that are spatially separated in a given solid and, ideally, structurally identical. In this review, the potential of metal–organic frameworks (MOFs) and covalent organic frameworks (COFs) as platforms for the development of heterogeneous single-site catalysts is reviewed thoroughly. In the first part of this article, synthetic strategies and progress in the implementation of such sites in these two classes of materials are discussed. Because these solids are excellent playgrounds to allow a better understanding of catalytic functions, we highlight the most important recent advances in the modelling and spectroscopic characterization of single-site catalysts based on these materials. Finally, we discuss the potential of MOFs as materials in which several single-site catalytic functions can be combined within one framework along with their potential as powerful enzyme-mimicking materials. The review is wrapped up with our personal vision on future research directions.

Open Access version available at UGent repository
Gold Open Access

High-Throughput Screening of Extrinsic Point Defect Properties in Si and Ge: Database and Applications

M. Sluydts, M. Pieters, J. Vanhellemont, V. Van Speybroeck, S. Cottenier
Chemistry of Materials
29 (3), pp 975–984
2016
A1

Abstract 

Increased computational resources now make it possible to generate large data sets solely from first principles. Such “high-throughput” screening is employed to create a database of embedding enthalpies for extrinsic point defects and their vacancy complexes in Si and Ge for 73 impurities from H to Rn. Calculations are performed both at the PBE and HSE06 levels of theory. The data set is verified by comparison of the predicted lowest-enthalpy positions with experimental observations. The effect of temperature on the relative occupation of defect sites is estimated through configurational entropy. Potential applications are demonstrated by selecting optimal vacancy traps, directly relevant to industrial processes such as Czochralski growth as a means to suppress void formation.

Theoretical Analysis of the Influence of Pore Geometry on Monomolecular Cracking and Dehydrogenation of n-Butane in Brønsted-Acid Zeolites

J. Van der Mynsbrugge, A. Janda, S. Mallikarjun Sharada, L.-C. Lin, V. Van Speybroeck, M. Head-Gordon, A.T. Bell
ACS Catalysis
7, 2685-2697
2017
A1

Abstract 

Recent experimental work has shown that variations in the confinement of n-butane at Brønsted-acid sites due to changes in zeolite framework structure strongly affect the apparent and intrinsic enthalpy and entropy of activation for cracking and dehydrogenation. Quantum chemical calculations have provided good estimates of the intrinsic enthalpies and entropies of activation extracted from experimental rate data for MFI, but extending these calculations to less confining zeolites has proven challenging, particularly for activation entropies. Herein, we report our efforts to develop a theoretical model for the cracking and dehydrogenation of n-butane occurring in a series of zeolites containing 10-membered ring channels and differing in cavity size (TON, FER, SVR, MFI, MEL, STF and MWW). We combine a QM/MM approach to calculate intrinsic and apparent activation parameters, with thermal corrections to the apparent barriers obtained from configurational-bias Monte Carlo simulations, to account for configurational contributions due to global motions of the transition state. We obtain good agreement between theory and experiment for all activation parameters for central cracking. For terminal cracking and dehydrogenation, good agreement between theory and experiment is found only at the highest confinements. Experimental activation parameters, especially those for dehydrogenation, tend to increase with decreasing confinement. This trend is not captured by the theoretical calculations, such that deviations between theory and experiment increase as confinement decreases. We propose that because transition states for dehydrogenation are later than those for cracking, relative movements between the fragments produced in the reaction become increasingly important in the less confining zeolites.

The remarkable amphoteric nature of defective UiO-66 in catalytic reactions

J. Hajek, B. Bueken, M. Waroquier, D. De Vos, V. Van Speybroeck
ChemCatChem
9 (12), 2203-2210
2017
A1

Abstract 

One of the major requirements in solid acids and bases catalyzed reactions is that the reactants, intermediates or activated complexes cooperate with several functions of catalyst support. In this work the remarkable bifunctional behavior of the defective UiO-66(Zr) metal organic framework is shown for acid-base pair catalysis. The active site relies on the presence of undercoordinated zirconium sites, which may be tuned by removing framework linkers and by removal of water from the inorganic bricks using a dehydration treatment. To elucidate the amphoteric nature of defective UiO-66, the Oppenauer oxidation of primary alcohols has been theoretically investigated using density functional theory (DFT) and the periodic approach. The presence of acid and basic centers within molecular distances has been shown crucial for determining the catalytic activity of the material. Hydrated and dehydrated bricks have a distinct influence on modulation of the acidity and basicity of the active sites. In any case both functions need to cooperate in a concerted way to enable the chemical transformation.

Open Access version available at UGent repository
Gold Open Access

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