-
[show abstract]
[hide abstract]
ABSTRACT: The conversion of methanol to olefins (MTO) over a heterogeneous nanoporous catalyst material is a highly complex process involving a cascade of elementary reactions. The elucidation of the reaction mechanisms leading to either the desired production of ethene and/or propene or undesired deactivation has challenged researchers for many decades. Clearly, catalyst choice, in particular topology and acidity, as well as the specific process conditions determine the overall MTO activity and selectivity; however, the subtle balances between these factors remain not fully understood. In this review, an overview of proposed reaction mechanisms for the MTO process is given, focusing on the archetypal MTO catalysts, H-ZSM-5 and H-SAPO-34. The presence of organic species, that is, the so-called hydrocarbon pool, in the inorganic framework forms the starting point for the majority of the mechanistic routes. The combination of theory and experiment enables a detailed description of reaction mechanisms and corresponding reaction intermediates. The identification of such intermediates occurs by different spectroscopic techniques, for which theory and experiment also complement each other. Depending on the catalyst topology, reaction mechanisms proposed thus far involve aromatic or aliphatic intermediates. Ab initio simulations taking into account the zeolitic environment can nowadays be used to obtain reliable reaction barriers and chemical kinetics of individual reactions. As a result, computational chemistry and by extension computational spectroscopy have matured to the level at which reliable theoretical data can be obtained, supplying information that is very hard to acquire experimentally. Special emphasis is given to theoretical developments that open new perspectives and possibilities that aid to unravel a process as complex as methanol conversion over an acidic porous material.
ChemPhysChem 04/2013; · 3.41 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The reactivity of 3-hydroxy-4-(1,2-dihydroxyethyl)-β-lactams with regard to the oxidant sodium periodate was evaluated, unexpectedly resulting in the exclusive formation of new 2-hydroxy-1,4-oxazin-3-ones through a C3C4 bond cleavage of the intermediate 4-formyl-3-hydroxy-β-lactams followed by a ring expansion. This peculiar transformation stands in sharp contrast with the known NaIO(4) -mediated oxidation of 3-alkoxy- and 3-phenoxy-4-(1,2-dihydroxyethyl)-β-lactams, which exclusively leads to the corresponding 4-formyl-β-lactams without a subsequent ring enlargement. In addition, this new class of functionalized oxazin-3-ones was further evaluated for its potential use as building blocks in the synthesis of a variety of differently substituted oxazin-3-ones, morpholin-3-ones and pyrazinones. Furthermore, additional insights into the mechanism and the factors governing this new ring-expansion reaction were provided by means of density functional theory calculations.
Chemistry 01/2013; · 5.93 Impact Factor
-
E. Verheyen,
Lennart Joos,
Kristof Van Havenbergh,
Eric Breynaert,
Nataliia Kasian,
Elena Gobechiya,
Kristof Houthoofd,
Charlotte Martineau,
Manuel Hinterstein,
Francis Taulelle, Veronique Van Speybroeck,
Michel Waroquier,
Sara Bals,
Gustaaf Van Tendeloo,
Christine E. A. Kirschhock,
Johan A. Martens
-
[show abstract]
[hide abstract]
ABSTRACT: Nanofibres functionalised with pH-sensitive dyes could greatly contribute to the development of stimuli-responsive materials. However, the application of biocompatible polymers is vital to allow for their use in (bio)medical applications. Therefore, this paper focuses on the development and characterisation of pH-sensitive polycaprolactone (PCL) nanofibrous structures and PCL/chitosan nanofibrous blends with 20% chitosan. Electrospinning with added Nitrazine Yellow molecules proved to be an excellent method resulting in pH-responsive non-wovens. Unlike the slow and broad response of PCL nanofibres (time lag of more than 3h), the use of blends with chitosan led to an increased sensitivity and significantly reduced response time (time lag of 5min). These important effects are attributed to the increased hydrophilic nature of the nanofibres containing chitosan. Computational calculations indicated stronger interactions, mainly based on electrostatic interactions, of the dye with chitosan (ΔG of -132.3kJ/mol) compared to the long-range interactions with PCL (ΔG of -35.6kJ/mol), thus underpinning our experimental observations. In conclusion, because of the unique characteristics of chitosan, the use of PCL/chitosan blends in pH-sensitive biocompatible nanofibrous sensors is crucial.
Carbohydrate polymers. 01/2013; 91(1):284-93.
-
ChemCatChem 01/2013; 5(1):173-184. · 5.21 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: In this paper, we describe the post-functionalization of a V-containing Metal-organic framework with
TiO(acac)2 to create a bimetallic oxidation catalyst. The catalytic performance of this V/Ti-MOF was
examined for the oxidation of cyclohexene using molecular oxygen as oxidant in combination with cyclohexanecarboxaldehyde
as co-oxidant. A significantly higher cyclohexene conversion was observed for the
bimetallic catalyst compared to the non-functionalized material. Moreover, the catalyst could be recycled
at least 3 times without loss of activity and stability. No detectable leaching of V or Ti was noted. Electron
paramagnetic resonance measurements were performed to monitor the fraction of V-ions in the catalyst
in the +IV valence state. A reduction of this fraction by
∼17% after oxidation catalysis is observed, in
agreement with the generally accepted mechanism for this type of reaction.
Catalysis Today 12/2012; · 3.41 Impact Factor
-
Ying-Ya Liu,
Sarah Couck,
Matthias Vandichel,
Maciej Grzywa,
Karen Leus,
Shyam Biswas,
Dirk Volkmer,
Jorge Gascon,
Freek Kapteijn,
Joeri F M Denayer,
Michel Waroquier, Veronique Van Speybroeck,
Pascal Van Der Voort
[show abstract]
[hide abstract]
ABSTRACT: A vanadium based metal-organic framework (MOF), VO(BPDC) (BPDC(2-) = biphenyl-4,4'-dicarboxylate), adopting an expanded MIL-47 structure type, has been synthesized via solvothermal and microwave methods. Its structural and gas/vapor sorption properties have been studied. This compound displays a distinct breathing effect toward certain adsorptives at workable temperatures. The sorption isotherms of CO(2) and CH(4) indicate a different sorption behavior at specific temperatures. In situ synchrotron X-ray powder diffraction measurements and molecular simulations have been utilized to characterize the structural transition. The experimental measurements clearly suggest the existence of both narrow pore and large pore forms. A free energy profile along the pore angle was computationally determined for the empty host framework. Apart from a regular large pore and a regular narrow pore form, an overstretched narrow pore form has also been found. Additionally, a variety of spectroscopic techniques combined with N(2) adsorption/desorption isotherms measured at 77 K demonstrate that the existence of the mixed oxidation states V(III)/V(IV) in the titled MOF structure compared to pure V(IV) increases the difficulty in triggering the flexibility of the framework.
Inorganic Chemistry 12/2012; · 4.60 Impact Factor
-
Karen Leus,
Sarah Couck,
Matthias Vandichel,
Gauthier Vanhaelewyn,
Ying-Ya Liu,
Guy B Marin,
Isabel Van Driessche,
Diederik Depla,
Michel Waroquier, Veronique Van Speybroeck,
Joeri F M Denayer,
Pascal Van Der Voort
[show abstract]
[hide abstract]
ABSTRACT: An amino functionalized vanadium-containing Metal Organic Framework, NH(2)-MIL-47, has been synthesized by a hydrothermal reaction in an autoclave. Alternatively, a synthesis route via microwave enhanced irradiation has been optimized to accelerate the synthesis. The NH(2)-MIL-47 exhibits the same topology as MIL-47, in which the V center is octahedrally coordinated. After an exchange procedure in DMF the V(+III) center is oxidized to V(+IV), which is confirmed by EPR and XPS measurements. The CO(2) and CH(4) adsorption properties have been evaluated and compared to MIL-47, showing that both MOFs have an almost similar adsorption capacity and affinity for CO(2). DFT-based molecular modeling calculations were performed to obtain more insight into the adsorption positions for CO(2) in NH(2)-MIL-47. Furthermore our calculated adsorption enthalpies agree well with the experimental values.
Physical Chemistry Chemical Physics 10/2012; 14(44):15562-70. · 3.57 Impact Factor
-
E. Verheyen,
Lennart Joos,
Kristof Van Havenbergh,
Eric Breynaert,
Nataliia Kasian,
Elena Gobechiya,
Kristof Houthoofd,
Charlotte Martineau,
Manuel Hinterstein,
Francis Taulelle, Veronique Van Speybroeck,
Michel Waroquier,
Sara Bals,
Gustaaf Van Tendeloo,
Christine E. A. Kirschhock,
Johan A. Martens
[show abstract]
[hide abstract]
ABSTRACT: Although the search for new zeolites has traditionally been based on trial and error, more rational methods are now available. The theoretical concept of inverse σ transformation of a zeolite framework to generate a new structure by removal of a layer of framework atoms and contraction has for the first time been achieved experimentally. The reactivity of framework germanium atoms in strong mineral acid was exploited to selectively remove germanium-containing four-ring units from an UTL type germanosilicate zeolite. Annealing of the leached framework through calcination led to the new all-silica COK-14 zeolite with intersecting 12- and 10-membered ring channel systems. An intermediate stage of this inverse σ transformation with dislodged germanate four-rings still residing in the pores could be demonstrated. Inverse σ transformation involving elimination of germanium-containing structural units opens perspectives for the synthesis of many more zeolites.
Nature Material 10/2012; · 32.84 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Free-energy calculations indicated that the NO(x) adsorption process on heteropolyacids is entropy-driven, as more gas molecules are released than adsorbed by substitution of H(5) O(2) (+) with NO(+) species. P yellow, W light blue, O red, H pink, N small dark blue spheres.
Angewandte Chemie International Edition 09/2012; 51(44):11010-3. · 13.45 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: An efficient protocol is presented to compensate for the basis set superposition error (BSSE) in DFT molecular dynamics (MD) simulations using localized Gaussian basis sets. We propose a classical correction term that can be added a posteriori to account for BSSE. It is tested to what extension this term will improve radial distribution functions (RDFs). The proposed term is pairwise between certain atoms in different molecules and was calibrated by fitting reference BSSE data points computed with the counterpoise method. It is verified that the proposed exponential decaying functional form of the model is valid. This work focuses on hydrogen-bonded liquids, i.e., methanol, and more specific on the intermolecular hydrogen bond, but in principle the method is generally applicable on any type of interaction where BSSE is significant. We evaluated the relative importance of the Grimme-dispersion versus BSSE and found that they are of the same order of magnitude, but with an opposite sign. Upon introduction of the correction, the relevant RDFs, obtained from MD, have amplitudes equal to experiment.
The Journal of chemical physics 09/2012; 137(10):104506. · 3.09 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The mechanism of ring-chain-ring tautomerization and the prominent effect of the solvent environment have been computationally investigated in an effort to explain the enantiomeric interconversion observed in 2-oxazolidinone derivatives, heterocyclic analogues of biphenyl atropisomers, which were isolated as single stable enantiomers and have the potential to be used as axially chiral catalysts. This study has shed light on the identity of the intermediate species involved in the ring-chain-ring tautomerization process as well as the catalytic effect of polar protic solvents. These mechanistic details will prove very useful in predicting and understanding ring-chain tautomeric equilibria in similar heterocyclic systems and will further enable experimentalists to devise appropriate experimental conditions in which axially chiral catalysts remain stable as single enantiomers.
Chemistry 09/2012; 18(40):12725-32. · 5.93 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Benzene methylation by methanol is studied on acidic zeolites H-ZSM-5 (MFI) and H-beta (BEA) to investigate the influence of the catalyst topology on the reaction rate. Experimental kinetic measurements at 350 °C using extremely high feed rates to suppress side reactions show that methylation occurs considerably faster on H-ZSM-5 than on H-beta. Theoretical rate constants, obtained from first-principles simulations on extended zeolite clusters, reproduce a higher methylation rate on H-ZSM-5 and provide additional insight into the various molecular effects that contribute to the overall differences between the two catalysts. The calculations indicate this higher methylation rate is primarily due to an optimal confinement of the reacting species in the medium pore material. Co-adsorption of methanol and benzene is energetically favored in H-ZSM-5 compared with H-beta, to the extent that the stabilizing host–guest interactions outweigh the greater entropy loss upon benzene adsorption in H-ZSM-5 vs
Journal of Catalysis 08/2012; 292:201-212. · 6.00 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The halochromism in solution of a prototypical example of an azo dye, ethyl orange, was investigated by using a combined theoretical and experimental approach. Experimental UV/Vis and Raman spectroscopy pointed towards a structural change of the azo dye with changing pH value (in the range pH 5-3). The pH-sensitive behavior was modeled through a series of ab initio computations on the neutral and various singly and doubly protonated structures. For this purpose, contemporary DFT functionals (B3LYP, CAM-B3LYP, and M06) were used in combination with implicit modeling of the water solvent environment. Static calculations were successful in assigning the most-probable protonation site. However, to fully understand the origin of the main absorption peaks, a molecular dynamics simulation study in a water molecular environment was used in combination with time-dependent DFT (TD-DFT) calculations to deduce average UV/Vis spectra that take into account the flexibility of the dye and the explicit interactions with the surrounding water molecules. This procedure allowed us to achieve a remarkable agreement between the theoretical and experimental UV/Vis spectrum and enabled us to fully unravel the pH-sensitive behavior of ethyl orange in aqueous environment.
Chemistry 05/2012; 18(26):8120-9. · 5.93 Impact Factor
-
Toon Verstraelen,
Bartłomiej M. Szyja,
David Lesthaeghe,
Reinout Declerck, Veronique Van Speybroeck,
Michel Waroquier,
Antonius P. J. Jansen,
Alexander Aerts,
Lana R. A. Follens,
Johan A. Martens,
Christine E. A. Kirschhock,
Rutger A. van Santen
[show abstract]
[hide abstract]
ABSTRACT: Zeolite synthesis is driven by structure-directing agents, such as tetrapropyl ammonium ions (TPA+) for Silicalite-1 and ZSM-5. However, the guiding role of these organic templates in the complex assembly to highly ordered
frameworks remains unclear, limiting the prospects for advanced material synthesis. In this work, both static ab initio and
dynamic classical modeling techniques are employed to provide insight into the interactions between TPA+ and Silicalite-1 precursors. We find that as soon as the typical straight 10-ring channel of Silicalite-1 or ZSM-5 is formed
from smaller oligomers, the TPA+ template is partially squeezed out of the resulting cavity. Partial retention of the template in the cavity is, however,
indispensable to prevent collapse of the channel and subsequent hydrolysis.
Topics in Catalysis 04/2012; 52(9):1261-1271. · 2.62 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Functionalized linkers can greatly increase the activity of metal-organic framework (MOF) catalysts with coordinatively unsaturated sites. A clear linear free-energy relationship (LFER) was found between Hammett σ(m) values of the linker substituents X and the rate k(X) of a carbonyl-ene reaction. This is the first LFER ever observed for MOF catalysts. A 56-fold increase in rate was found when the substituent is a nitro group.
Angewandte Chemie International Edition 04/2012; 51(20):4887-90. · 13.45 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Zincated 3-chloro-3-methyl-1-azaallylic anions undergo a stereoselective aldol addition across aromatic aldehydes and subsequent mesylation to produce syn α-chloro-β-mesyloxyketimines, which were isolated in 80-84% yield and high diastereomeric excess (dr > 97/3) after purification via flash chromatography. The syn α-chloro-β-mesyloxyketimines were further stereoselectively reduced to give stereochemically defined 3-aminopropyl mesylates, which were cyclized to 1,2,3,4-tetrasubstituted 3-chloroazetidines containing three contiguous stereogenic centers. DFT calculations on the key aldol addition revealed the presence of a highly ordered bimetallic six-membered twist-boat-like transition state structure with a tetra-coordinated metal cyclic structure. DFT calculations revealed that chelation of both zinc and lithium cations in the transition state structure leads to the experimentally observed high syn diastereoselectivity of aldol reactions.
The Journal of Organic Chemistry 03/2012; 77(7):3415-25. · 4.45 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The reactivity of 2-bromomethyl-2-methylaziridines toward oxygen, sulfur, and carbon nucleophiles in different solvent systems was investigated. Remarkably, the choice of the solvent has a profound influence on the reaction outcome, enabling the selective formation of either functionalized aziridines in dimethylformamide (through direct bromide displacement) or azetidines in acetonitrile (through rearrangement via a bicyclic aziridinium intermediate). In addition, the experimentally observed solvent-dependent behavior of 2-bromomethyl-2-methylaziridines was further supported by means of DFT calculations.
The Journal of Organic Chemistry 02/2012; 77(7):3181-90. · 4.45 Impact Factor
-
The Journal of Physical Chemistry C. 02/2012; 116(9):5499-5508.
-
Karen Leus,
Matthias Vandichel,
Ying-Ya Liu,
Ilke Muylaert,
Jan Musschoot,
Steven Pyl,
Henk Vrielinck,
Freddy Callens,
Guy B. Marin,
Christophe Detavernier,
Paul V. Wiper,
Yaroslav Z. Khimyak,
Michel Waroquier, Veronique Van Speybroeck,
Pascal Van Der Voort
[show abstract]
[hide abstract]
ABSTRACT: A Metal Organic Framework, containing coordinatively saturated V+IV sites linked together by terephthalic
linkers (V-MIL-47), is evaluated as a catalyst in the epoxidation of cyclohexene. Different solvents
and conditions are tested and compared. If the oxidant TBHP is dissolved in water, a significant
leaching of V-species into the solution is observed, and also radical pathways are prominently operative
leading to the formation of an adduct between the peroxide and cyclohexene. If, however, the oxidant is
dissolved in decane, leaching is negligible and the structural integrity of the V-MIL-47 is maintained during
successive runs. The selectivity toward the epoxide is very high in these circumstances. Extensive
computational modeling is performed to show that several reaction cycles are possible. EPR and NMR
measurements confirm that at least two parallel catalytic cycles are co-existing: one with V+IV sites
and one with pre-oxidized V+V sites, and this is in complete agreement with the theoretical predictions.
Journal of Catalysis 01/2012; · 6.00 Impact Factor
