V. Van Speybroeck

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

The Monomer Electron Density Force Field (MEDFF): A Physically Inspired Model for Non-Covalent Interactions

S. Vandenbrande, M. Waroquier, V. Van Speybroeck, T. Verstraelen
Journal of Chemical Theory and Computation (JCTC)
13 (1), 161–179
2017
A1

Abstract 

We propose a methodology to derive pairwise-additive noncovalent force fields from monomer electron densities without any empirical input. Energy expressions are based on the symmetry-adapted perturbation theory (SAPT) decomposition of interaction energies. This ensures a physically motivated force field featuring an electrostatic, exchange repulsion, dispersion, and induction contribution, which contains two types of parameters. First, each contribution depends on several fixed atomic parameters, resulting from a partitioning of the monomer electron density. Second, each of the last three contributions (exchange-repulsion, dispersion, and induction) contains exactly one linear fitting parameter. These three so-called interaction parameters in the model are initially estimated separately using SAPT reference calculations for the S66x8 database of noncovalent dimers. In a second step, the three interaction parameters are further refined simultaneously to reproduce CCSD(T)/CBS interaction energies for the same database. The limited number of parameters that are fitted to dimer interaction energies (only three) avoids ill-conditioned fits that plague conventional parameter optimizations. For the exchange repulsion and dispersion component, good results are obtained for all dimers in the S66x8 database using one single value for the associated interaction parameters. The values of those parameters can be considered universal and can also be used for dimers not present in the original database used for fitting. For the induction component such an approach is only viable for the dispersion dominated dimers in the S66x8 database. For other dimers (such as hydrogen-bonded complexes), we show that our methodology remains applicable. However, the interaction parameter needs to be determined on a case-specific basis. As an external validation:, the force field predicts interaction energies in good agreement with CCSD(T)/CBS values for dispersion dominated dimers extracted from an HIV-II protease crystal structure with a bound ligand (indinavir). Furthermore, experimental second virial coefficients of small alkanes and alkenes are well reproduced.

Open Access version available at UGent repository
Green Open Access

Benzene co-reaction with methanol and dimethyl ether over zeolite and zeotype catalysts: Evidence of parallel reaction paths to toluene and diphenylmethane

J. S. Martínez-Espín, K. De Wispelaere, M. Westgård Erichsen, S. Svelle, T. V. Janssens, V. Van Speybroeck, P. Beato, U. Olsbye
Journal of Catalysis
349, 136-148
2017
A1

Abstract 

The reactivity of methanol (MeOH) and dimethyl ether (DME) toward benzene was studied over zeolitic materials with different topology and acid strength (H-ZSM-5, H-SSZ-24, and H-SAPO-5) at 250–350 °C. Higher rates of methylation, and subsequent de-alkylation reactions, were observed with DME compared to MeOH. In addition, significant differences in product distribution based on the choice of methylating agent were observed. For reactions between MeOH and benzene a fraction of diphenylmethanes (DPMs) was formed, while this product group was nearly absent during reactions between DME and benzene. A range of co-feed and isotopic labeling experiments was performed, mainly over H-ZSM-5, in order to elucidate mechanistic information on the pathway from methanol and benzene to DPMs. Overall, these studies revealed that DPM formation involves the dehydrogenation of methanol to formaldehyde on the Brønsted acid site, followed by subsequent reaction with two benzene molecules. Theoretical calculations confirmed the higher reactivity of DME compared to MeOH toward benzene methylation and suggested a plausible route from formaldehyde and benzene to DPM.

Open Access version available at UGent repository
Green Open Access

Thermodynamic Insight in the High-Pressure Behavior of UiO-66: Effect of Linker Defects and Linker Expansion

S.M.J. Rogge, J. Wieme, L. Vanduyfhuys, S. Vandenbrande, G. Maurin, T. Verstraelen, M. Waroquier, V. Van Speybroeck
Chemistry of Materials
28 (16), 5721-5732
2016
A1

Abstract 

In this Article, we present a molecular-level understanding of the experimentally observed loss of crystallinity in UiO-66-type metal–organic frameworks, including the pristine UiO-66 to -68 as well as defect-containing UiO-66 materials, under the influence of external pressure. This goal is achieved by constructing pressure-versus-volume profiles at finite temperatures using a thermodynamic approach relying on ab initio derived force fields. On the atomic level, the phenomenon is reflected in a sudden drop in the number of symmetry operators for the crystallographic unit cell because of the disordered displacement of the organic linkers with respect to the inorganic bricks. For the defect-containing samples, a reduced mechanical stability is observed, however, critically depending on the distribution of these defects throughout the material, hence demonstrating the importance of judiciously characterizing defects in these materials.

This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.
http://pubs.acs.org/doi/abs/10.1021/acs.chemmater.6b01956

Open Access version available at UGent repository
Gold Open Access

Tandem addition of phosphite nucleophiles across unsaturated nitrogen-containing systems: mechanistic insights on regioselectivity

W. Debrouwer, D. Hertsen, T.S.A Heugebaert, E. Birsen Boydas, V. Van Speybroeck, S. Catak, C.V. Stevens
Journal of Organic Chemistry
82 (1), 188–201
2017
A1

Abstract 

The addition of phosphite nucleophiles across linear unsaturated imines is a powerful and atom-economical methodology for the synthesis of aminophosphonates. These products are of interest from both a biological and a synthetic point of view: they act as amino acid transition state analogs and Horner–Wadsworth–Emmons reagents, respectively. In this work the reaction between dialkyl trimethylsilyl phosphites and α,β,γ,δ-diunsaturated imines was evaluated as a continuation of our previous efforts in the field. As such, the first conjugate 1,6-addition of a phosphite nucleophile across a linear unsaturated N-containing system is reported herein. Theoretical calculations were performed to rationalize the observed regioselectivites and to shed light on the proposed mechanism.

Towards metal–organic framework based field effect chemical sensors: UiO-66-NH2 for nerve agent detection

L. Stassen, B. Bueken, H. Reinsch, J.F.M. Oudenhoven, D. Wouters, J. Hajek, V. Van Speybroeck, N. Stock, P.M. Vereecken, R. Van Schajik, D. De Vos, R. Ameloot
Chemical Science
7, 5827-5832
2016
A1

Abstract 

We present a highly sensitive gas detection approach for the infamous ‘nerve agent’ group of alkyl phosphonate compounds. Signal transduction is achieved by monitoring the work function shift of metal–organic framework UiO-66-NH2 coated electrodes upon exposure to ppb-level concentrations of a target simulant. Using the Kelvin probe technique, we demonstrate the potential of electrically insulating MOFs for integration in field effect devices such as ChemFETs: a three orders of magnitude improvement over previous work function-based detection of nerve agent simulants. Moreover, the signal is fully reversible both in dry and humid conditions, down to low ppb concentrations. Comprehensive investigation of the interactions that lead towards this high sensitivity points towards a series of confined interactions between the analyte and the pore interior of UiO-66-NH2.

Open Access version available at UGent repository

Acidity constant (pKa) calculation of large solvated dye molecules: evaluation of two advanced molecular dynamics methods

T. De Meyer, B. Ensing, S.M.J. Rogge, K. De Clerck, E.J. Meijer, V. Van Speybroeck
ChemPhysChem
17 (21), 3447–3459
2016
A1

Abstract 

pH-sensitive dyes are increasingly applied onto polymer substrates for the creation of novel sensor materials. Recently, these dye molecules have been modified to form a covalent bond with the polymer host. This can have a large influence on the pH-sensitive properties, in particular on the acidity constant (pKa). Obtaining molecular control over the factors that influence the pK$_a$ value is mandatory for future intelligent design of sensor materials. Herein, we show that advanced molecular dynamics (MD) methods have reached the level where pKa values of large solvated dye molecules can be predicted with high accuracy. Two MD methods are used in this work: steered or restrained MD and the insertion/deletion scheme. Both are first calibrated on a set of phenol derivatives and afterwards applied to the dye molecule Bromothymol Blue. Excellent agreement with experimental values is obtained, which opens perspectives for using these methods for designing dye molecules.

Open Access version available at UGent repository

Effect of temperature and branching on the nature and stability of alkene cracking intermediates in H-ZSM-5

P. Cnudde, K. De Wispelaere, J. Van der Mynsbrugge, M. Waroquier, V. Van Speybroeck
Journal of Catalysis
345, 53-69
2017
A1

Abstract 

Catalytic cracking of alkenes takes place at elevated temperatures in the order of 773–833 K. In this work, the nature of the reactive intermediates at typical reaction conditions is studied in H-ZSM-5 using a complementary set of modeling tools. Ab initio static and molecular dynamics simulations are performed on different C4single bond C5 alkene cracking intermediates to identify the reactive species in terms of temperature. At 323 K, the prevalent intermediates are linear alkoxides, alkene π-complexes and tertiary carbenium ions. At a typical cracking temperature of 773 K, however, both secondary and tertiary alkoxides are unlikely to exist in the zeolite channels. Instead, more stable carbenium ion intermediates are found. Branched tertiary carbenium ions are very stable, while linear carbenium ions are predicted to be metastable at high temperature. Our findings confirm that carbenium ions, rather than alkoxides, are reactive intermediates in catalytic alkene cracking at 773 K.

Open Access version available at UGent repository

Water coordination and dehydration processes in defective UiO-66 type metal organic frameworks

M. Vandichel, J. Hajek, A. Ghysels, A. De Vos, M. Waroquier, V. Van Speybroeck
CrystEngComm
18 (37), 7056-7069
2016
A1

Abstract 

The UiO-66 metal organic framework is one of the most thermally and chemically stable hybrid materials reported to date. However, it is also accepted that the material contains structurally embedded defects, which may be engineered to enhance properties towards specific applications such as catalysis, sensing, etc. The synthesis conditions determine to a large extent the level of perfection of the material and additionally the catalytic activity may be enhanced by post-synthesis activation at high temperature under vacuum, in which defect coordinating species (H2O, HCl, monocarboxylic modulators, etc.) evaporate. The molecular level characterization of defects is extremely challenging from both theoretical and experimental points of view. Such experimental endeavor was recently proposed via experimental SXRD measurements, also unraveling the coordination of water on the Zr–O–Zr defect sites [Angew. Chem., Int. Ed., 2015, 54, 11162–11167]. The present work provides a theoretical understanding of defect structures in UiO-66(Zr) by means of periodic density functional theory calculations and ab initio molecular dynamics simulations. A range of defect structures are generated with different numbers of missing linkers. For each of the defects, the free energetic and mechanical stability is discussed and the coordination of water and charge balancing hydroxide ions is studied. For catalysis applications, the material is mostly pretreated to remove water by dehydration reactions. For each of the proposed defect structures, mechanistic pathways for dehydration reactions of the Zr-bricks are determined employing nudged elastic band (NEB) calculations. During the dehydroxylation trajectory, loose hydroxyl groups and terephthalate decoordinations are observed. Furthermore, dehydration reactions are lower activated if terephthalate linkers are missing in the immediate environment of the inorganic brick. The creation of defects and the dehydration processes have a large impact on the mechanical properties of the material, which is evidenced by lower bulk moduli and elastic constants for structures with more defects.

DOI 

10.1039/C6CE01027J

Pages

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