Journal of Physical Chemistry C

Reactivity of CO on carbon covered cobalt surfaces in Fischer-Tropsch Synthesis

L. Joos, I. Filot, S. Cottenier, E. Hensen, M. Waroquier, V. Van Speybroeck, R.A. van Santen
Journal of Physical Chemistry C
118 (10), 5317–5327
2014
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Abstract 

Fischer–Tropsch synthesis is an attractive process to convert alternative carbon sources, such as biomass, natural gas, or coal, to fuels and chemicals. Deactivation of the catalyst is obviously undesirable, and for a commercial plant it is of high importance to keep the catalyst active as long as possible during operating conditions. In this study, the reactivity of CO on carbon-covered cobalt surfaces has been investigated by means of density functional theory (DFT). An attempt is made to provide insight into the role of carbon deposition on the deactivation of two cobalt surfaces: the closed-packed Co(0001) surface and the corrugated Co(112̅1) surface. We also analyzed the adsorption and diffusion of carbon atoms on both surfaces and compared the mobility. Finally, the results for Co(0001) and Co(112̅1) are compared, and the influence of the surface topology is assessed.

New Functionalized Metal–Organic Frameworks MIL-47-X (X = −Cl, −Br, −CH3, −CF3, −OH, −OCH3): Synthesis, Characterization, and CO2 Adsorption Properties

S. Biswas, D.E.P. Vanpoucke, T. Verstraelen, M. Vandichel, S. Couck, K. Leus, Y-Y Liu, M. Waroquier, V. Van Speybroeck, J.F.M. Denayer, P. Van der Voort
Journal of Physical Chemistry C
117 (44), 22784–22796
2013
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Abstract 

Six new functionalized vanadium hydroxo terephthalates [VIII(OH)(BDC-X)]•n(guests) (MIL-47(VIII)-X-AS) (BDC = 1,4-benzenedicarboxylate; X = -Cl; -Br, -CH3, -CF3, -OH, -OCH3; AS = as-synthesized) along with the parent MIL-47 were synthesized under rapid microwave-assisted hydrothermal conditions (170 ºC, 30 min, 150 W). The unreacted H2BDC-X and/or occluded solvent molecules can be removed by thermal activation under vacuum leading to the empty-pore forms of the title compounds (MIL-47(VIV)-X). Except pristine MIL-47 (+III oxidation state), the vanadium atoms in all the evacuated functionalized solids stayed in +IV oxidation state. The phase purity of the compounds was ascertained by X-ray powder diffraction (XRPD), diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, Raman, thermogravimetric (TG), and elemental analysis. The structural similarity of the filled and empty-pore forms of the functionalized compounds with the respective forms of parent MIL-47 was verified by cell parameter determination from XRPD data. TGA and temperature-dependent XRPD (TDXRPD) experiments in air atmosphere indicate high thermal stability in the range 330-385 ºC. All the thermally activated compounds exhibit significant microporosity (SLangmuir in the range 418-1104 m2 g-1), as verified by the N2 and CO2 sorption analysis. Among the six functionalized compounds, MIL-47(VIV)-OCH3 shows the highest CO2 uptake, demonstrating the determining role of functional groups on the CO2 sorption behaviour. For this compound and pristine MIL-47(VIV), Widom particle insertion simulations were performed based on ab initio calculated crystal structures. The theoretical Henry coefficients show a good agreement with the experimental values, and calculated isosurfaces for the local excess chemical potential indicate the enhanced CO2 affinity is due to two effects: (i) the interaction between the methoxy group and CO2 and (ii) the collapse of the MIL-47(VIV)-OCH3 framework.

Bipyridine-Based Nanosized Metal–Organic Framework with Tunable Luminescence by a Postmodification with Eu(III): An Experimental and Theoretical Study

Y-Y Liu, R. Decadt, T. Bogaerts, K. Hemelsoet, A. M. Kaczmarek, D. Poelman, M. Waroquier, V. Van Speybroeck, R. Van Deun, P. Van der Voort
Journal of Physical Chemistry C
117 (21), 11302–11310
2013
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Abstract 

A gallium 2,2′-bipyridine-5,5′-dicarboxylate metal-organic framework, Ga(OH)(bpydc), denoted as COMOC-4 (COMOC = Center for Ordered Materials, Organometallics and Catalysis, Ghent University) has been synthesized via solvothermal synthesis procedure. The structure has the topology of an aluminum 2,2′-bipyridine-5,5′-dicarboxylate, the so-called MOF-253. TEM and SEM micrographs show the COMOC-4 crystals are formed in nanoplates with uniform size of 30-50 nm. The UV-Vis spectra of COMOC-4 in methanol solution show maximal electronic absorption at 307 nm. This results from linker to linker transitions as elucidated by time-dependent density functional theory simulations on the linker and COMOC-4 cluster models. When excited at 400 nm, COMOC-4 displays an emission band centered at 542 nm. Upon immersion in different solvents, the emission band for the framework is shifted in the range of 525~548 nm, depending on the solvent. After incorporating Eu3+ cations, the emission band of the framework is shifted to even shorter wavelengths (505 nm). By varying the excitation wavelengths from 250 to 400 nm, we can fine-tune the emission from red to yellowish green in the CIE diagram. The luminescence behavior of Eu3+ cations is well preserved and the solid state luminescence lifetimes of λ1 = 45 µs (35.4 %) and λ2 = 162 µs (64.6 %) are observed.

On the thermodynamics of framework breathing: A free energy model for gas adsorption in MIL-53

A. Ghysels, L. Vanduyfhuys, M. Vandichel, M. Waroquier, V. Van Speybroeck, B. Smit
Journal of Physical Chemistry C
117, 11540-11554
2013
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Abstract 

When adsorbing guest molecules, the porous metal-organic framework MIL-53(Cr) may vary its cell parameters drastically while retaining its crystallinity. A first approach to the thermodynamic analysis of this 'framework breathing' consists of comparing the osmotic potential in two distinct shapes only (large-pore and narrow-pore). In this paper, we propose a generic parametrized free energy model including three contributions: host free energy, guest-guest interactions, and host-guest interaction. Free energy landscapes may now be constructed scanning all shapes and any adsorbed amount of guest molecules. This allows to determine which shapes are the most stable states for arbitrary combinations of experimental control parameters, such as the adsorbing gas chemical potential, the external pressure, and the temperature. The new model correctly reproduces the structural transitions along the CO2 and CH4 isotherms. Moreover, our model successfully explains the adsorption versus desorption hysteresis as a consequence of the creation, stabilization, destabilization, and disappearance of a second free energy minimum under the assumptions of a first order phase transition and collective behavior. Our general thermodynamic description allows to decouple the gas chemical potential μ and mechanical pressure P as two independent thermodynamic variables and predict the complete (μ,P) phase diagram for CO2 adsorption in MIL-53(Cr). The free energy model proposed here is an important step towards a general thermodynamics description of flexible metal-organic frameworks.

Efficient Approach for the Computational Study of Alcohol and Nitrile Adsorption in H-ZSM-5

J. Van der Mynsbrugge, K. Hemelsoet, M. Vandichel, M. Waroquier, V. Van Speybroeck
Journal of Physical Chemistry C
116 (9), 5499-5508
2012
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Abstract 

Since many industrially important processes start with the adsorption of guest molecules inside the pores of an acidic zeolite catalyst, a proper estimate of the adsorption enthalpy is of paramount importance. In this contribution, we report ab initio calculations on the adsorption of water, alcohols and nitriles at the bridging Brønsted sites of H-ZSM-5, using both cluster and periodic models to account for the zeolite environment. Stabilization of the adsorption complexes results from hydrogen bonding between the guest molecule and the framework, as well as from embedding, i.e. van der Waals interactions with the pore walls. Large-cluster calculations with different DFT-methods, in particular B3LYP(-D), PBE(-D) M062X(-D) and ωB97X-D, are tested for their ability to reproduce the experimental heats of adsorption available in literature. (J. Phys. Chem. B 1997, 101, 3811-3817) A proper account of dispersion interactions is found to be crucial to describe the experimental trend across a series of adsorbates of increasing size, i.e. an increase in adsorption enthalpy by 10-15 kJ/mol for each additional carbon atom. The extended-cluster model is shown to offer an attractive alternative to periodic simulations on the entire H-ZSM-5 unit cell, resulting in virtually identical results for the final adsorption enthalpies. Comparing calculated stretch frequencies of the zeolite acid sites and the adsorbate functional groups with experimental IR-data additionally confirms the cluster approach provides an appropriate representation of the adsorption complexes.

Open Access version available at UGent repository

Computation of charge distribution and electrostatic potential in silicates with the use of chemical potential equalization models

T. Verstraelen, S.V. Sukhomlinov, V. Van Speybroeck, M. Waroquier, K. Smirnov
Journal of Physical Chemistry C
116 (1), 490–504
2012
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Abstract 

New parameters for the electronegativity equalization model (EEM) and the split-charge equilibration (SQE) model are calibrated for silicate materials, based on an extensive training set of representative isolated systems. In total, four calibrations are carried out, two for each model, either using iterative Hirshfeld (HI) charges or ESP grid data computed with Density Functional Theory (DFT) as a reference. Both the static (ground state) reference quantities and their responses to uniform electric fields are included in the fitting procedure. The EEM model fails to describe the response data, while the SQE model quantitatively reproduces all the training data. For the ESP-based parameters, we found that the reference ESP data are only useful at those grid points where the electron density is lower than 10-3 a.u. The density value correlates with a distance criterion used for selecting grid points in common ESP fitting schemes. All parameters are validated with DFT computations on an independent set of isolated systems (similar to the training set), and on a set of periodic systems including dense and microporous crystalline silica structures, zirconia, and zirconium silicate. Although the transferability of the parameters to new isolated systems poses no difficulties, the atomic hardness parameters in the HI-based models must be corrected to obtain accurate results for periodic systems. The SQE/ESP model permits the calculation of the ESP with similar accuracy in both isolated and periodic systems.

Open Access version available at UGent repository

MFI Fingerprint: How Pentasil-Induced IR Bands Shift during Zeolite Nanogrowth

D. Lesthaeghe, P. Vansteenkiste, T. Verstraelen, A. Ghysels, C. Kirschhock, J.A. Martens, V. Van Speybroeck, M. Waroquier
Journal of Physical Chemistry C
112 (25), 9186-9191
2008
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Abstract 

Silicalite-1 zeolite exhibits a characteristic pentasil framework vibration around 540−550 cm−1. In the initial stages of zeolite synthesis, however, this band is observed at much higher wavenumbers: literature shows this vibration to depend on particle size and to shift over 100 cm−1 with increasing condensation. In this work, the pentasil vibration frequency was derived from theoretical molecular dynamics simulations to obtain the correct IR band assignments for important nanoparticles. The IR spectroscopic fingerprint of oligomeric five-ring containing precursors proposed in the literature was computed and compared with experimental data. Our theoretical results show that, while isolated five-membered rings show characteristic vibrational bands around 650 cm−1, the combination of five-membered rings in the full MFI-type structure readily generates the bathochromic shift to the typical pentasil vibration around 550 cm−1. As opposed to what was previously believed, the IR band does not shift gradually as nanoparticle size increases, but it is highly dependent on the specific way structural units are added. The most important feature is the appearance of an additional band when double five-membered rings are included, which allows for a clear distinction between the key stages of early zeolite nucleation. Furthermore, the combination of the simulated spectra with the experimental observation of this spectral feature in nanoparticles extracted from silicalite-1 clear solutions supports their structured nature. The theoretical insights on the dependency of pentasil vibrations with the degree of condensation offer valuable support toward future investigations on the genesis of a zeolite crystal.

Global DFT-Based Reactivity Indicators:  An Assessment of Theoretical Procedures in Zeolite Catalysis

K. Hemelsoet, D. Lesthaeghe, V. Van Speybroeck, M. Waroquier
Journal of Physical Chemistry C
111 (7), 3028-3037
2007
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Abstract 

The dependence of global reactivity descriptors on electronic structure method as well as basis set is investigated for typical reactions in zeolite catalysis. This research is especially focused on hard−hard interactions between small probe molecules (such as chloromethane, methanol, ethylene, and propene) and different zeolite clusters containing both oxygen and amine functionalities. The performance of novel hybrid metafunctionals (such as BMK and MPWB1K) on crucial reactivity predictors is assessed through comparison with both Hartree−Fock and B3-LYP results. For the complex bifunctional zeolite systems, we find accurate results using any of the DFT functionals, in conjunction with a basis set of at least double-ζ quality further augmented with both polarization and diffuse functions. Reactivity sequences, based on global softness differences as well as activation hardness values, are generally found to be independent of the level of theory whenever a DFT functional is used.

29Si NMR and UV-Raman Investigation of Initial Oligomerization Reaction Pathways in Acid-Catalyzed Silica Sol-Gel Chemistry

A. Depla, D. Lesthaeghe, T.S. van Erp, A. Aerts, K. Houthoofd, F. Fan, C. Li, V. Van Speybroeck, M. Waroquier, C. Kirschhock, J.A. Martens
Journal of Physical Chemistry C
115 (9), 3562–3571
2011
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Abstract 

The initial molecular steps of the acid-catalyzed silica sol−gel process de-parting from tetraethylorthosilicate (TEOS) were investigated by in situ 29Si NMR and UV−Raman spectroscopy. The use of a substoichiometric H2O:TEOS molar ratio (r-value 0.2−1.2) slowed the silicate oligomerization reaction and allowed unraveling the initial steps of silica condensation. Molecular modeling confirmed Raman signal and 29Si NMR shift assignment. A comprehensive listing of all Raman and 29Si NMR assignments is provided, including unique Raman assignments of cyclosilicates and the linear tetramer. The combination of experiment and modeling allowed an analysis of the reaction kinetics. The derived kinetic model and the experimental observation both revealed that the H2O:TEOS molar ratio had a strong influence on the reaction kinetics but not on the reaction pathways. The multianalytical approach led to development of an oligomerization scheme. As dominant oligomerizations, chain growth, cyclodimerization, and branching were identified. Under the investigated conditions, chains did not grow longer than pentamer, and ring sizes were limited to 6-rings. Chains of 4 Si atoms and 4-rings were abundant species. Branched rings and chains were formed by attachment of dimers and trimers. Gelation proceeded from branched 4-rings and branched chains with limited hydroxyl functionalities.

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