V. Van Speybroeck

Effect of Lewis acids on the stereoregularity of N,N-dimethyl acrylamide: A computational approach

T. Furuncuoğlu, B. Kura, S. Catak, H. Goossens, V. Van Speybroeck, M. Waroquier, V. Aviyente
European Polymer Journal
83, 67–76
2016
A1

Abstract 

In this study, the effect of Lewis acid coordination (ScCl3) in controlling the stereoregularity during the free radical polymerization of N,N-dimethyl acrylamide (DMAM) has been investigated by Density Functional Theory (DFT). Experimentally, ScCl3, Sc(OTf)3 and Yb(OTf)3 have been used to increase the isotactic percentage in the polymerization of another acrylamide derivative, N-isopropyl acrylamide (NIPAM) (Habaue et al., 2002). The relative orientation of the terminal and penultimate side chains is expected to determine the stereoregularity in free radical polymerization reactions (Noble et al., 2014). We have analyzed the mechanistic details of the propagation reaction by considering all coordination types of the Lewis acid to the propagating species. Calculations have shown the bridging of the Lewis acid between the terminal side chain and the monomer to be the most probable pathway, which is in favor of the pro-meso propagation during the free radical polymerization of DMAM. In this case, it is the bridging capacity of the catalyst along the less crowded direction that dictates the preference for isotacticity. Overall, the strategy suggested in this study can be easily used by experimentalists in their endeavour of choosing the catalysts in order to end-up with the desired stereoregulation of the polymer chain.

Heterogeneous Ru(III) oxidation catalysts via ‘click’ bidentate ligands on a Periodic Mesoporous Organosilica support

S. Clerick, E. De Canck, K. Hendrickx, V. Van Speybroeck, P. Van der Voort
Green Chemistry
18, 6035–6045
2016
A1

Abstract 

A 100% monoallyl ring-type Periodic Mesoporous Organosilica (PMO) is prepared as a novel, versatile and exceptionally stable catalytic support with a high internal surface area and 5.0 nm pores. Thiol-ene ‘click’ chemistry allows straightforward attachment of bifunctional thiols (-NH2, -OH, -SH) which, exploiting the thioether functionality formed, give rise to ‘solid’ bidentate ligands. [Ru(acac)2(CH3CN)2]PF6 is attached and complex formation on the solid is studied via theoretical calculations. All resulting solid catalysts show high activity and selectivity in alcohol oxidation reactions performed in green conditions (25°C/ water). The PMO catalysts do not leach Ru during reaction and are thus easily recuperated and re-used for several runs. Furthermore, oxidation of poorly water-soluble (±)-menthol illustrates the benefits of using hydrophobic PMOs as catalytic supports.

DMRG-CASPT2 study of the longitudinal static second hyperpolarizability of all-trans polyenes

S. Wouters, V. Van Speybroeck, D. Van Neck
Journal of Chemical Physics
145 (5), 054120
2016
A1

Abstract 

We have implemented internally contracted complete active space second order perturbation theory (CASPT2) with the density matrix renormalization group (DMRG) as active space solver [Y. Kurashige and T. Yanai, J. Chem. Phys. 135, 094104 (2011)]. Internally contracted CASPT2 requires to contract the generalized Fock matrix with the 4-particle reduced density matrix (4-RDM) of the reference wavefunction. The required 4-RDM elements can be obtained from 3-particle reduced density matrices (3-RDM) of different wavefunctions, formed by symmetry-conserving single-particle excitations op top of the reference wavefunction. In our spin-adapted DMRG code chemps2 [https://github.com/sebwouters/chemps2], we decompose these excited wavefunctions as spin-adapted matrix product states, and calculate their 3-RDM in order to obtain the required contraction of the generalized Fock matrix with the 4-RDM of the reference wavefunction. In this work, we study the longitudinal static second hyperpolarizability of all-trans polyenes C_{2n}H_{2n+2} [n = 4 - 12] in the cc-pVDZ basis set. DMRG-SCF and DMRG-CASPT2 yield substantially lower values and scaling with system size compared to RHF and MP2, respectively.

Minimal Basis Iterative Stockholder: Atoms-in-Molecules for Force-Field Development

T. Verstraelen, S. Vandenbrande, F. Heidar-Zadeh, L. Vanduyfhuys, V. Van Speybroeck, M. Waroquier, P.W. Ayers
Journal of Chemical Theory and Computation (JCTC)
12(8), 3894-3912
2016
A1

Abstract 

Atomic partial charges appear in the Coulomb term of many force-field models and can be derived from electronic structure calculations with a myriad of atoms-in-molecules (AIM) methods. More advanced models have also been proposed, using the distributed nature of the electron cloud and atomic multipoles. In this work, an electrostatic force field is defined through a concise approximation of the electron density, for which the Coulomb interaction is trivially evaluated. This approximate "pro-density" is expanded in a minimal basis of atom-centered s-type Slater density functions, whose parameters are optimized by minimizing the Kullback-Leibler divergence of the pro-density from a reference electron density, e.g. obtained from an electronic structure calculation. The proposed method, Minimal Basis Iterative Stockholder (MBIS), is a variant of the Hirshfeld AIM method but it can also be used as a density-fitting technique. An iterative algorithm to refine the pro-density is easily implemented with a linear-scaling computational cost, enabling applications to supramolecular systems. The benefits of the MBIS method are demonstrated with systematic applications to molecular databases and extended models of condensed phases. A comparison to 14 other AIM methods shows its effectiveness when modeling electrostatic interactions. MBIS is also suitable for rescaling atomic polarizabilities in the Tkatchenko-Sheffler scheme for dispersion interactions.

Exploring the flexibility of MIL-47(V)-type materials using force field molecular dynamics simulations

J. Wieme, L. Vanduyfhuys, S.M.J. Rogge, M. Waroquier, V. Van Speybroeck
Journal of Physical Chemistry C
120 (27), 14934-14947
2016
A1

Abstract 

The flexibility of three MIL-47(V)-type materials (MIL-47, COMOC-2 and COMOC-3) has been explored by constructing the pressure-versus-volume and free energy-versus-volume curves at various temperatures ranging from 100 K to 400 K. This is done with first-principles based force fields using the recently proposed QuickFF parameterization protocol. Specific terms were added for the materials at hand to describe the asymmetry of the one-dimensional vanadium-oxide chain and to account for the flexibility of the organic linkers. The force fields are used in a series of molecular dynamics simulations at fixed volumes, but varying unit cell shapes. The three materials show a distinct pressure-versus-volume behavior, which underlines the ability to tune the mechanical properties by varying the linkers towards different applications such as nanosprings, dampers and shock absorbers.

Open Access version available at UGent repository

Cutting the cost of carbon capture: a case for carbon capture and utilization

L. Joos, J. Huck, V. Van Speybroeck, B. Smit
Faraday Discussions
192, 391-414
2016
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Abstract 

A significant part of the cost for Carbon Capture and Storage (CCS) is related to the compression of the captured CO2 to its supercritical state, at 150 bar and typically 99% purity. These stringent conditions may however not always be necessary for specific cases of Carbon Capture and Utilization (CCU). In this manuscript, we investigate how much the parasitic energy of an adsorbent-based carbon capture process may be lowered by utilizing CO2 at 1 bar and adapting the final purity requirement for CO2 from 99% to 70% or 50%. We compare different CO2 sources: the flue gases of coal-fired or natural gas-fired power plants and ambient air. We evaluate the carbon capture performance of over 60 nanoporous materials and determine the influence of the initial and final purity on the parasitic energy of the carbon capture process. Moreover, we demonstrate the underlying principles of the parasitic energy minimization in more detail using the commercially available NaX zeolite. Finally, the calculated utilization cost of CO2 is compared with reported prices for CO2 and published costs for CCS.

Open Access version available at UGent repository

A breathing zirconium metal-organic framework with reversible loss of crystallinity by correlated nanodomain formation

B. Bueken, F. Vermoortele, M.J. Cliffe, M.T. Wharmby, D. Foucher, J. Wieme, L. Vanduyfhuys, C. Martineau, N. Stock, F. Taulelle, V. Van Speybroeck, A.L. Goodwin, D. De Vos
Chemistry - A European Journal
2016, 22, 1-5
2016
A1

Abstract 

The isoreticular analogue of the metal–organic framework UiO-66(Zr), synthesized with the flexible trans-1,4-cyclohexanedicarboxylic acid as linker, shows a peculiar breathing behavior by reversibly losing long-range crystalline order upon evacuation. The underlying flexibility is attributed to a concerted conformational contraction of up to two thirds of the linkers, which breaks the local lattice symmetry. X-ray scattering data are described well by a nanodomain model in which differently oriented tetragonal-type distortions propagate over about 7–10 unit cells.

Suppression of the Aromatic Cycle in Methanol-to-Olefins Reaction over ZSM-5 by Post-Synthetic Modification Using Calcium

I. Yarulina, S. Bailleul, A. Pustovarenko, J. Ruiz-Martinez, K. De Wispelaere, J. Hajek, B.M. Weckhuysen, K. Houben, M. Baldus, V. Van Speybroeck, F. Kapteijn, J. Gascon
ChemCatChem
8 (19) 3057–3063
2016
A1

Abstract 

Incorporation of Ca in ZSM-5 results in a twofold increase of propylene selectivity (53 %), a total light-olefin selectivity of 90 %, and a nine times longer catalyst lifetime (throughput 792 gMeOH gcatalyst−1) in the methanol-to-olefins (MTO) reaction. Analysis of the product distribution and theoretical calculations reveal that post-synthetic modification with Ca2+ leads to the formation of CaOCaOH+ that strongly weaken the acid strength of the zeolite. As a result, the rate of hydride transfer and oligomerization reactions on these sites is greatly reduced, resulting in the suppression of the aromatic cycle. Our results further highlight the importance of acid strength on product selectivity and zeolite lifetime in MTO chemistry.

Ligand Addition Energies and the Stoichiometry of Colloidal Nanocrystals

M. Sluydts, K. De Nolf, V. Van Speybroeck, S. Cottenier, Z. Hens
ACS Nano
10 (1), 1462-1474
2016
A1

Abstract 

Experimental non-stoichiometries of colloidal nanocrystals such as CdSe and PbS are accounted for by attributing to each constituent atom and capping ligand a formal charge equal to its most common oxidation state to obtain an overall neutral nanocrystal. In spite of its apparent simplicity, little theoretical support of this approach - called here the oxidation-number sum rule - is present in the current literature. Here, we introduce the ligand addition energy, which we define as the energy gained or expended upon the transfer of one ligand from a reference state to a (metal-rich) solid surface. For the combination of CdSe, ZnSe and InP with either chalcogen, halogen or hydrochalcogen ligands, we compute successive ligand addition energies using ab initio methods and determine the thermodynamically stable surface composition as that composition where ligand addition turns endothermic. We find that the oxidation-number sum rule is valid in many situations, although exceptions occur for each material studied most notably when exposed to small oxidative ligands. In the case of InP violations are more severe, extending towards the entire chalcogen ligand family. In addition, we find that electronegativity rather than chemical hardness is a reasonable predictor for ligand addition energies, with the most electronegative ligands yielding the most exothermic addition energies. Finally, we argue that the ligand addition energy will be a most useful quantity for future computational studies on the structure, stability and reactivity of nanocrystal surfaces.

Towards molecular control of elementary reactions in zeolite catalysis by advanced molecular simulations mimicking operating conditions

K. De Wispelaere, S. Bailleul, V. Van Speybroeck
Catalysis Science & Technology
6, 2686 – 2705
2016
A1

Abstract 

Zeolites are the workhorses of today’s chemical industry. For decades they have been successfully applied, however many features of zeolite catalysis are only superficially understood and in particular the kinetics and mechanism of individual reaction steps at operating conditions. Herein we use state-of-the-art advanced ab initio molecular dynamics techniques to study the influence of catalyst topology and acidity, reaction temperature and the presence of additional guest molecules on elementary reactions. Such advanced modeling techniques provide complementary insight to experimental knowledge as the impact of individual factors on the reaction mechanism and kinetics of zeolite-catalyzed reactions may be unraveled. We study key reaction steps in the conversion of methanol to hydrocarbons, namely benzene and propene methylation. These reactions may occur either in a concerted or stepwise fashion, i.e. methanol directly transfers its methyl group to a hydrocarbon or the reaction goes through a framework-bound methoxide intermediate. The DFT-based dynamical approach enables mimicking reaction conditions as close as possible and studying the competition between two methylation mechanisms in an integrated fashion. The reactions are studied in the unidirectional AFI-structured H-SSZ-24, H-SAPO-5 and TON-structured H-ZSM-22 materials. We show that varying the temperature, topology, acidity and number of protic molecules surrounding the active site may tune the reaction mechanism at the molecular level. Obtaining molecular control is crucial in optimizing current zeolite processes and designing emerging new technologies bearing alternative feedstocks.

Open Access version available at UGent repository

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