M. Waroquier

How zeolitic acid strength and composition alter the reactivity of alkenes and aromatics towards methanol

M.W. Erichsen, K. De Wispelaere, K. Hemelsoet, S.L. Moors, T. Deconinck, M. Waroquier, S. Svelle, V. Van Speybroeck, U. Olsbye
Journal of Catalysis
328, 186-196
2015
A1

Abstract 

This work encompasses a combined experimental and theoretical assessment of how zeolitic acid strength and composition affects acid-catalysed methylation reactions. Overall, higher methylation rates were observed over the material with higher acid strength. Co-reactions of methanol with benzene at 250 degrees C over the two isostructural AFI materials H-SSZ-24 and H-SAPO-5 revealed large differences in selectivity. While the stronger acidic H-SSZ-24 mainly produced toluene and polymethylbenzenes, high yields of C4+ aliphatics were observed over H-SAPO-5. These results strongly suggest that alkene methylation was preferred over H-SAPO-5 even at very low conversion during methanol/benzene co-reactions. Furthermore, a comparison of benzene and propene methylation at 350-400 degrees C revealed a significantly faster rate of benzene than propene methylation in H-SSZ-24, whereas the rates of benzene and propene methylation were similar in H-SAPO-5. The observed difference in reactivity of the two hydrocarbons in both catalysts could be understood by careful analysis of various molecular dynamics simulations of the co-adsorbed complexes. The probability to form protonated methanol was, as expected, higher in the more acidic material. However, in H-SSZ-24, the probability for methanol protonation was higher when co-adsorbed with benzene than when co-adsorbed with propene, while the same was not observed in H-SAPO-5. Furthermore, it was found that benzene and methanol are more likely to form a reactive co-adsorbed complex in H-SSZ-24 compared to propene and methanol, while the opposite was observed for H-SAPO-5. This work shows that molecular dynamics simulations provide insights into the adsorption behaviour of guest molecules in large pore AFI materials. The obtained insights correlate with the experimentally observed reactivities. (C) 2015 Elsevier Inc. All rights reserved.

Open Access version available at UGent repository

Active site engineering in UiO-66 type Metal Organic Frameworks by intentional creation of defects : A theoretical rationalization

M. Vandichel, J. Hajek, F. Vermoortele, M. Waroquier, D. De Vos, V. Van Speybroeck
CrystEngComm
First published online 15 Sep 2014
2014
A1
Published while none of the authors were employed at the CMM

Abstract 

The catalytic activity of the Zr-benzenedicarboxylate (Zr-BDC) UiO-66 can be drastically increased in the absence of part of the linkers, as it removes the full coordination of the framework metal ions and leads to open sites. As a result, metal centers become more accessible and thus more active for Lewis acid catalysed reactions. Addition of modulators (MDL) to the synthesis mixture can create more linker deficiencies (Vermoortele et al., J Am Chem Soc, 2013, 135, 11465) and can lead to a significant increase of the catalytic activity due to the creation of a larger number of open sites. In this paper, we rationalize the function of the modulators under real synthesis conditions by the construction of free energy diagrams. The UiO-66 type materials form a very appropriate test case as the effect of addition of modulators HCl and trifluoroacetate (TFA) has been intensively investigated experimentally for the synthesis process and post-synthetic thermal activation. In synthesis conditions, direct removal of BDC linkers requires a high free energy but replacement of such linker by one or more TFA species might occur especially at high TFA:BDC ratios in the reaction mixture. The presence of HCl furthermore enhances the creation of defect structures. Post-synthesis activation procedures at higher temperatures lead to a substantial removal of the species coordinated to the Zr bricks creating open metal sites. A mechanistic pathway is presented for the dehydroxylation process of the hexanuclear Zr cluster. For the citronellal cyclization, we show that the presence of some residual TFA in the structure may lead to faster reactions in complete agreement with experiment. Hirshfeld-e partial charges for the Zr ions have been computed to investigate their sensitivity to substituent effects; a strong correlation with the experimental Hammett parameters, and with the rates of the citronellal cyclization is found. The theoretical rationalization may serve as a basis for detailed active site engineering studies.

Binary and Ternary Po-containing Molecules Relevant for LBE Cooled Reactors at Operating Temperature

A. Van Yperen-De Deyne, K. Rijpstra, M. Waroquier, V. Van Speybroeck, S. Cottenier
Journal of Nuclear Materials
458, 288-295
2015
A1

Abstract 

Quantum-chemical calculations at several levels of theory were used to assess the stability at different temperatures of a set of 13 binary and ternary Po-containing molecules that could possibly be formed in an environment with lead, bismuth, oxygen and water. The conclusions are that especially PoPb, PbPoO and PoOH and to a lesser extent Po2 and PoO are stable. These small molecules are therefore likely to be found near the Lead-Bismuth eutectic (LBE) coolant at operational temperatures in a heavy liquid metal cooled fission reactor. In contrast, Po3 and PoBi are unlikely to be present under the assumed conditions. Several stability criteria, such as the dissociation into free atoms or into molecular fragments at realistic Po-concentrations or in the thermodynamic limit are discussed at different temperatures. The results obtained with a medium level of theory (Density Functional Theory, PBE0 with relativistic effective core potentials) show good qualitative correspondence with calculations performed at a much higher level of theory (Multi Reference Configuration Interaction, with spin–orbit coupling and scalar relativistic Hamiltonian).

Open Access version available at UGent repository

QuickFF: toward a generally applicable methodology to quickly derive accurate force fields for Metal-Organic Frameworks from ab initio input

L. Vanduyfhuys, S. Vandenbrande, T. Verstraelen, R. Schmid, M. Waroquier, V. Van Speybroeck
Journal of Computational Chemistry
2015
A1
Published while none of the authors were employed at the CMM

Base catalytic activity of alkaline earth MOFs: a (micro)spectroscopic study of active site formation by the controlled transformation of structural anions

P. Valvekens, D. Jonckheere, T. De Baerdemaeker, A. Kubarev, M. Vandichel, K. Hemelsoet, M. Waroquier, V. Van Speybroeck, E. Smolders, D. Depla, D. Roeffaers, D. De Vos
Chemical Science
5 (11), 4517-4524
2014
A1

Abstract 

A new concept has been developed for generating highly dispersed base sites on metal-organic framework (MOF) lattices. The base catalytic activity of two alkaline earth MOFs, M2(BTC)(NO3)(DMF) (M = Ba or Sr, H3BTC = 1,3,5-benzenetricarboxylic acid, DMF = N,N-dimethylformamide) was studied as a function of their activation procedure. The catalytic activity in Knoevenagel condensation and Michael addition reactions was found to increase strongly with activation temperature. Physicochemical characterization using FTIR, 13C CP MAS NMR, PXRD, XPS, TGA-MS, SEM, EPR, N2 physisorption and nitrate content analysis shows that during activation, up to 85 % of the nitrate anions are selectively removed from the structure and replaced with other charge compensating anions such as O2-. The defect sites generated via this activation act as new strong basic sites within the catalyst structure. A fluorescence microscopic visualization of the activity convincingly proves that the activity is exclusively associated with the hexagonal crystals, and that reaction proceeds inside the crystal’s interior. Theoretical analysis of the Ba-material shows that the basicity of the proposed Ba2+-O2--Ba2+ motives is close to that of edge sites in BaO.

First principle chemical kinetics in zeolites: The Methanol-to-Olefin process as a case study

V. Van Speybroeck, K. De Wispelaere, J. Van der Mynsbrugge, M. Vandichel, K. Hemelsoet, M. Waroquier
Chemical Society Reviews
43 (21), 7326-7357
2014
A1

Abstract 

To optimally design next generation catalysts a thorough understanding of the chemical phenomena at the molecular scale is a prerequisite. Apart from qualitative knowledge on the reaction mechanism, it is also essential to be able to predict accurate rate constants. Molecular modeling has become a ubiquitous tool within the field of heterogeneous catalysis. Herein, we review current computational procedures to determine chemical kinetics from first principles, thus by using no experimental input and by modeling the catalyst and reacting species at the molecular level. Therefore, we use the methanol-to-olefin (MTO) process as a case study to illustrate the various theoretical concepts. This process is a showcase example where rational design of the catalyst was for a long time performed on the basis of trial and error, due to insufficient knowledge of the mechanism. For theoreticians the MTO process is particularly challenging as the catalyst has an inherent supramolecular nature, for which not only the Brønsted acidic site is important but also organic species, trapped in the zeolite pores, must be essentially present during active catalyst operation. All these aspects give rise to specific challenges for theoretical modeling. It is shown that present computational techniques have matured to a level where accurate enthalpy barriers and rate constants can be predicted for reactions occurring at a single active site. The comparison with experimental data such as apparent kinetic data for well-defined elementary reactions has become feasible as current computational techniques also allow predicting adsorption enthalpies with reasonable accuracy. Real catalysts are truly heterogeneous in a space- and time-like manner. Future theory developments should focus on extending our view towards phenomena occurring at longer length and time scales and integrating information from various scales towards a unified understanding of the catalyst. Within this respect molecular dynamics methods complemented with additional techniques to simulate rare events are now gradually making their entrance within zeolite catalysis. Recent applications have already given a flavor of the benefit of such techniques to simulate chemical reactions in complex molecular environments.

Open Access version available at UGent repository

Flexibility versus rigidity: what determines the stability of zeolite frameworks? A case study

E. Verheyen, L. Joos, C. Martineau, C.J. Dawson, C. Weidenthaler, W. Schmidt, R. Yuan, E. Breynaerts, V. Van Speybroeck, M. Waroquier, F. Taulelle, M.M.J. Treacy, J.A. Martens, C. Kirschhock
Materials Horizons
Vol. 1 , 582 - 587
2014
A1

Abstract 

All silica COK-14/-COK-14 with OKO topology is the first case of a zeolite which reversibly transforms from a systematically interrupted to a fully connected state and back. Analysis of the opening/closing behavior allowed the study of entropy and framework flexibility as determinants for the stability of zeolite topologies, which, until now, has been experimentally inaccessible. Interconversion of the all-silica COK-14 zeolite with fully connected OKO topology and its -COK-14 variant with systematic framework interruption was investigated using high-temperature XRD, thermogravimetric analysis, Si-29 MAS NMR, nitrogen adsorption and a range of modelling techniques. Specific framework bonds in the OKO framework can be reversibly hydrolyzed and condensed. Structural silanols of the parent -COK-14, prepared by degermanation of the IM-12 zeolite, were condensed by heating at 923 K, and hydrolyzed again to the initial state by contacting the zeolite with warm water. Molecular modelling revealed an inversion of the relative stabilities for both variants depending on temperature and hydration. Condensation of the structural silanols in -COK-14 to COK-14 is entropy driven, mainly resulting from the release of water molecules. Framework reopening in the presence of water is spontaneous due to the high rigidity of the fully connected OKO framework. Isomorphous substitution was demonstrated as a viable option for stabilization of the fully connected OKO framework as this renders the closed framework flexible.

Metal-dioxidoterephthalate MOFs of the MOF-74 type: microporous basic catalysts with well-defined active sites

P. Valvekens, M. Vandichel, M. Waroquier, V. Van Speybroeck, D. De Vos
Journal of Catalysis
317, 1–10
2014
A1

Abstract 

The hybrid frameworks M2dobdc (dobdc4− = 2,5-dioxidoterephthalate, M2+ = Mg2+, Co2+, Ni2+, Cu2+ and Zn2+), commonly known as CPO-27 or MOF-74, are shown to be active catalysts in base-catalyzed reactions such as Knoevenagel condensations or Michael additions. Rather than utilizing N-functionalized linkers as a source of basicity, the intrinsic basicity of these materials arises from the presence of the phenolate oxygen atoms coordinated to the metal ions. The overall activity is due to a complex interplay of the basic properties of these structural phenolates and the reactant binding characteristics of the coordinatively unsaturated sites. The nature of the active site and the order of activity between the different M2dobdc materials were rationalized via computational efforts; the most active material, both in theory and in experiment, is the Ni-containing variant. The basicity of Ni2dobdc was experimentally proven by chemisorption of pyrrole and observation by IR spectroscopy.

Open Access version available at UGent repository

Nucleophile-Dependent Regio- and Stereoselective Ring Opening of 1-Azoniabicyclo-[3.1.0]hexane Tosylate

Mi-Kyung Ji, D. Hertsen, D.-H. Yoon, H. Eum, H. Goossens, M. Waroquier, V. Van Speybroeck, M. D'Hooghe, N. De Kimpe, H.-J. Ha
Chemistry - An Asian Journal
2014 (9), 1060-1067
2014
A1

Abstract 

1-[(1R)-(1-Phenylethyl)]-1-azoniabicyclo[3.1.0]hexane tosylate was generated as a stable bicyclic aziridinium salt from the corresponding 2-(3-hydroxypropyl)aziridine upon reaction with p-toluenesulfonyl anhydride. This bicyclic aziridinium ion was then treated with various nucleophiles including halides, azide, acetate, and cyanide in CH3CN to afford either piperidines or pyrrolidines through regio- and stereoselective ring opening, mediated by the characteristics of the applied nucleophile. On the basis of DFT calculations, ring-opening reactions under thermodynamic control yield piperidines, whereas reactions under kinetic control can yield both piperidines and pyrrolidines depending on the activation energies for both pathways.

Critical analysis of the accuracy of models predicting or extracting liquid structure information

M. Van Houteghem, A. Ghysels, T. Verstraelen, W. Poelmans, M. Waroquier, V. Van Speybroeck
Journal of Physical Chemistry B
118 (9), 2451–2470
2014
A1

Abstract 

This work aims at a critical assessment of properties predicting or extracting information on the density and structure of liquids. State-of-the-art NVT and NpT molecular dynamics (MD) simulations have been performed on five liquids: methanol, chloroform, acetonitrile, tetrahydrofuran and ethanol. These simulations allow the computation of properties based on first principles, including the equilibrium density and radial distribution functions (RDFs), characterizing the liquid structure. Refinements have been incorporated in the MD simulations by taking into account Basis Set Superposition Errors (BSSE). An extended BSSE model for an instantaneous evaluation of the BSSE corrections has been proposed, and their impact on the liquid properties has been assessed. If available, the theoretical RDFs have been compared with the experimentally derived RDFs. For some liquids significant discrepancies have been observed and a profound but critical investigation is presented to unravel the origin of these deficiencies. This discussion is focused on tetrahydrofuran where the experiment reveals some prominent peaks completely missing in any MD simulation. Experiments providing information on liquid structure consist mainly of neutron diffraction measurements offering total structure factors as the primary observables. The splitting of these factors in reciprocal space into intra- and intermolecular contributions is extensively discussed, together with their sensitivity in reproducing correct RDFs in coordinate space.

Pages

Subscribe to RSS - M. Waroquier