V. Van Speybroeck

Bimetallic–Organic Framework as a Zero-Leaching Catalyst in the Aerobic Oxidation of Cyclohexene

Y-Y Liu, K. Leus, T. Bogaerts, K. Hemelsoet, E. Bruneel, V. Van Speybroeck, P. Van der Voort
ChemCatChem
5 (12), 3657–3664
2013
A1

Abstract 

A gallium 2,2′-bipyridine-5,5′-dicarboxylate metal–organic framework (MOF), denoted as COMOC-4, has been synthesized by solvothermal synthesis. This MOF exhibits the same topology as MOF-253. CuCl2 was incorporated into COMOC-4 by a post-synthetic modification (PSM). The spectroscopic absorption properties of the MOF framework before and after PSM were compared with theoretical data obtained by employing molecular dynamics combined with time-dependent DFT calculations on both the as-synthesized and functionalized linker. The catalytic behavior of the resulting Cu2+@COMOC-4 material was evaluated in the aerobic oxidation of cyclohexene with isobutyraldehyde as a co-oxidant. In addition, the catalytic performance of Cu2+@COMOC-4 was compared with that of the commercially available Cu-BTC (BTC=benzene-1,3,5-tricarboxylate) MOF. Cu2+@COMOC-4 exhibits a good cyclohexene conversion and an excellent selectivity towards cyclohexene oxide in comparison to the Cu-based reference catalyst. Furthermore, no leaching of the active Cu sites was observed during at least four consecutive runs.

Solution Enthalpy of Po and Te in solid Lead-Bismuth Eutectic

K. Rijpstra, A. Van Yperen-De Deyne, J. Neuhausen, V. Van Speybroeck, S. Cottenier
Journal of Nuclear Materials
450 (1–3), 287–291
2014
A1

Abstract 

It is examined to which extent first-principles calculations can be used to collect a priori information on the solution enthalpy and solubility of Po in solid lead-bismuth eutectic (LBE). Such information can be helpful to limit the number of complicated experiments that are required to measure these properties. It is found that in the thermodynamic limit and at 0 K, Po does not dissolve in solid LBE. Its solution enthalpy is negative, in particular in Pb-rich environments, but competing compound-forming reactions are more exothermic. A clear correlation is found between the calculated solution enthalpies for Te in LBE and for Po in LBE, suggesting that Te-experiments can be used to map the expected behaviour for Po. The role of spin-orbit coupling as the major relativistic effect on the solution enthalpies of these heavy atoms is inspected.

Ranking the stars: A refined Pareto approach to computational materials design

K. Lejaeghere, S. Cottenier, V. Van Speybroeck
Physical Review Letters
111 (7), 075501
2013
A1

Abstract 

We propose a procedure to rank the most interesting solutions from high-throughput materials design studies. Such a tool is becoming indispensable due to the growing size of computational screening studies and the large number of criteria involved in realistic materials design. As a proof of principle, the binary tungsten alloys are screened for both large-weight and high-impact materials, as well as for fusion reactor applications. Moreover, the concept is generally applicable to any design problem where multiple competing criteria have to be optimized.

Open Access version available at UGent repository

Cationic ring-opening polymerization of 2-propyl-2-oxazolines: Understanding structural effects on polymerization behavior based on molecular modeling

H. Goossens, S. Catak, M. Glassner, V. De La Rosa, B. Monnery, F. De Proft, V. Van Speybroeck, R. Hoogenboom
ACS Macro Letters
2, 651-654
2013
A1

Abstract 

The surprising difference in the cationic ring-opening polymerization rate of 2-cyclopropyl-2-oxazoline versus 2-n-propyl-2-oxazoline and 2-isopropyl-2-oxazoline was investigated both experimentally and theoretically. The polymerization kinetics of all three oxazolines were experimentally measured in acetonitrile at 140 °C, and the polymerization rate constant (kp) was found to decrease in the order c-PropOx > n-PropOx > i-PropOx. Theoretical free energy calculations confirmed the trend for kp, and a set of DFT-based reactivity descriptors, electrostatics, and frontier molecular orbitals were studied to detect the factors controlling this peculiar behavior. Our results show that the observed reactivity is dictated by electrostatic effects. More in particular, the charge on the nitrogen atom of the monomer, used to measure its nucleophilicity, was the most negative for c-PropOx. Furthermore, the electrophilicity of the cations does not change substantially, and thus, the nucleophilicity of the monomers is the driving factor for kp.

Identification of intermediates in zeolite-catalyzed reactions using in-situ UV/Vis micro-spectroscopy and a complementary set of molecular simulations

K. Hemelsoet, Q. Qian, T. De Meyer, K. De Wispelaere, B. De Sterck, B.M. Weckhuysen, M. Waroquier, V. Van Speybroeck
Chemistry - A European Journal
19, 49, 16595-16606
2013
A1

Abstract 

The optical absorption properties of (poly)aromatic hydrocarbons occluded in a nanoporous environment were investigated by theoretical and experimental methods. The carbonaceous species are an essential part of a working catalyst for the methanol-to-olefins (MTO) process. In situ UV/Vis microscopy measurements on methanol conversion over the acidic solid catalysts H-SAPO-34 and H-SSZ-13 revealed the growth of various broad absorption bands around 400, 480, and 580 nm. The cationic nature of the involved species was determined by interaction of ammonia with the methanol-treated samples. To determine which organic species contribute to the various bands, a systematic series of aromatics was analyzed by means of time-dependent density functional theory (TDDFT) calculations. Static gas-phase simulations revealed the influence of structurally different hydrocarbons on the absorption spectra, whereas the influence of the zeolitic framework was examined by using supramolecular models within a quantum mechanics/molecular mechanics framework. To fully understand the origin of the main absorption peaks, a molecular dynamics (MD) study on the organic species trapped in the inorganic host was essential. During such simulation the flexibility is fully taken into account and the effect on the UV/Vis spectra is determined by performing TDDFT calculations on various snapshots of the MD run. This procedure allows an energy absorption scale to be provided and the various absorption bands determined from in situ UV/Vis spectra to be assigned to structurally different species.

Synthesis modulation as a tool to increase the catalytic activity of MOFs: the unique case of UiO-66(Zr)

F. Vermoortele, B. Bueken, G. Le Bars, B. Van de Voorde, M. Vandichel, K. Houthoofd, A. Vimont, M. Daturi, M. Waroquier, V. Van Speybroeck, C. Kirschhock, D. De Vos
JACS (Journal of the American Chemical Society)
135 (31), 11465–11468
2013
A1

Abstract 

The catalytic activity of the zirconium terephthalate UiO-66(Zr) can be drastically increased by using a modulation approach. The combined use of trifluoroacetic acid and HCl during the synthesis results in a highly crystalline material, with partial substitution of terephthalates by trifluoroacetate. Thermal activation of the material leads not only to dehydroxylation of the hexanuclear Zr cluster but also to post-synthetic removal of the trifluoroacetate groups, resulting in a more open framework with a large number of open sites. Consequently, the material is a highly active catalyst for several Lewis acid catalyzed reactions.

Trans Effect and Trans Influence: Repulsion, rather than Competition for Donation

B. Pinter, V. Van Speybroeck, M. Waroquier, P. Geerlings, F. De Proft
Physical Chemistry Chemical Physics (PCCP)
15 (40), 17354-17365
2013
A1

Abstract 

The trans effect and trans influence were investigated and rationalized in the aminolysis, a typical nucleophilic substitution reaction, of trans-TPtCl2NH3 complexes (T = NH3, PH3, CO and C2H4) using energy decomposition analysis, both along the reaction paths and on the stationary points, and Natural Orbital for Chemical Valence analysis. In order to scrutinize the underlying principles and the origin of the kinetic trans effect, plausible structural constraints were introduced in the decomposition analysis, which allowed eliminating the distance dependence of the interaction energy components. It was established that the trans effect can be rationalized with the interaction of the TPtCl2 and NH3 fragments in the reactant state and TPtCl2 and (NH3)2 fragments in the transition state. It was evinced quantitatively that the σ-donor ability of T indeed controls the stability of the reactant, whereas in the case of π-acids, backdonation stabilizes the transition state, for which conceptually two mechanisms are available: intrinsic and induced π-backdonation. In the destabilization of the reactant and also in the labilization of the leaving group (trans influence) repulsion plays a more important role than orbital sharing effects, which are the cornerstones of the widely accepted interpretations of the trans influence, such as competition for donation or limitation of the donation of the leaving group by the trans ligand T. This repulsive interaction was rationalized both in terms of donated electron density and also in the molecular orbital framework. NOCV orbitals indeed clearly show that the σ-trans effect can be envisioned as a donation from the trans ligand not only to the metal but also to the σ* orbital of the metal-leaving group bond, which manifests as a repulsion between the metal and the leaving group.

Enthalpy and entropy barriers explain the effects of topology on the kinetics of zeolite-catalyzed reactions

J. Van der Mynsbrugge, J. De Ridder, K. Hemelsoet, M. Waroquier, V. Van Speybroeck
Chemistry - A European Journal
19 (35), 11568-11576
2013
A1

Abstract 

The methylation of ethene, propene, and trans-2-butene on zeolites H-ZSM-58 (DDR), H-ZSM-22 (TON), and H-ZSM-5 (MFI) is studied to elucidate the particular influence of topology on the kinetics of zeolite-catalyzed reactions. H-ZSM-58 and H-ZSM-22 are found to display overall lower methylation rates compared to H-ZSM-5 and also different trends in methylation rates with increasing alkene size. These variations may be rationalized based on a decomposition of the free-energy barriers into enthalpic and entropic contributions, which reveals that the lower methylation rates on H-ZSM-58 and H-ZSM-22 have virtually opposite reasons. On H-ZSM-58, the lower methylation rates are caused by higher enthalpy barriers, owing to inefficient stabilization of the reaction intermediates in the large cage-like pores. On the other hand, on H-ZSM-22, the methylation rates mostly suffer from higher entropy barriers, because excessive entropy losses are incurred inside the narrow-channel structure. These results show that the kinetics of crucial elementary steps hinge on the balance between proper stabilization of the reaction intermediates inside the zeolite pores and the resulting entropy losses. These fundamental insights into their inner workings are indispensable for ultimately selecting or designing better zeolite catalysts.

Insight in the activity and diastereoselectivity of various Lewis acid catalysts for the citronellal cyclization

M. Vandichel, F. Vermoortele, S. Cottenie, D. De Vos, M. Waroquier, V. Van Speybroeck
Journal of Catalysis
305, 118-129
2013
A1

Abstract 

Industrial (-)-menthol production generally relies on the hydrogenation of (-)-isopulegol, which is in turn produced with high selectivity by cyclization of (+)-citronellal. This paper uses a combined theoretical and experimental approach to study the activity and selectivity of three Lewis acid catalysts for this reaction, namely ZnBr2, aluminum tris(2,6-diphenylphenoxide) (ATPH) and the heterogeneous metal-organic framework Cu3BTC2 (BTC = benzene-1,3,5-tricarboxylate). ATPH is a strong Lewis acid homogeneous catalyst with bulky ligands which provides very high selectivities for the desired stereo-isomer (> 99 %). The performance of the catalysts was evaluated as a function of temperature, which revealed that higher catalyst activity allows working at lower temperatures and improves the selectivity for isopulegol. The selectivity distribution is kinetically driven for ZnBr2 and ATPH. The theoretical selectivity distributions rely on the determination of an extensive set of diastereomeric transition states, for which the differences in free energy have been calculated using a complementary set of ab initio techniques. Given the sensitivity of the selectivity to small Gibbs free energy differences, the agreement between experimental and theoretical selectivities is satisfactory. On basis of the obtained insights rational design of new catalysts may be obtained. As proof of concept, the hypothetical Cu3(BTC-(NO2)3)2 Lewis catalyst – in which each phenyl hydrogen of the BTC ligand is replaced by a nitro group - is predicted to be very selective.

Open Access version available at UGent repository

Determining the Storage, Availability and Reactivity of NH3 within Cu-Chabazite-based Ammonia Selective Catalytic Reduction Systems

I. Lezcano-Gonzalez, U. Deka, A. Van Yperen-De Deyne, K. Hemelsoet, M. Waroquier, V. Van Speybroeck, B.M. Weckhuysen, A.M. Beale
Physical Chemistry Chemical Physics (PCCP)
16, 1639-1650
2014
A1

Abstract 

Three different types of NH3 species can be simultaneously present on Cu2+-exchanged CHA-type zeolites, commonly used in Ammonia Selective Catalytic Reduction (NH3-SCR) systems. These include ammonium ions (NH4+), formed on the Bronsted acid sites, [Cu(NH3)(4)](2+) complexes, resulting from NH3 coordination with the Cu2+ Lewis sites, and NH3 adsorbed on extra-framework Al ( EFAl) species, in contrast to the only two reacting NH3 species recently reported on Cu-SSZ-13 zeolite. The NH4+ ions react very slowly in comparison to NH3 coordinated to Cu2+ ions and are likely to contribute little to the standard NH3-SCR process, with the Bronsted groups acting primarily as NH3 storage sites. The availability/ reactivity of NH4+ ions can be however, notably improved by submitting the zeolite to repeated exchanges with Cu2+, accompanied by a remarkable enhancement in the low temperature activity. Moreover, the presence of EFAl species could also have a positive influence on the reaction rate of the available NH4+ ions. These results have important implications for NH3 storage and availability in Cu-Chabazite-based NH3-SCR systems.

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