G. Vanhaelewyn

Ti-functionalized NH2-MIL-47: an effective and stable epoxidation catalyst

K. Leus, G. Vanhaelewyn, T. Bogaerts, Y-Y Liu, F. Esquivel, F. Callens, G.B. Marin, V. Van Speybroeck, H. Vrielinck, P. Van der Voort
Catalysis Today
208, 97-105
2013
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Abstract 

In this paper, we describe the post-functionalization of a V-containing Metal-organic framework with TiO(acac)2 to create a bimetallic oxidation catalyst. The catalytic performance of this V/Ti-MOF was examined for the oxidation of cyclohexene using molecular oxygen as oxidant in combination with cyclohexanecarboxaldehyde as co-oxidant. A significantly higher cyclohexene conversion was observed for the bimetallic catalyst compared to the non-functionalized material. Moreover, the catalyst could be recycled at least 3 times without loss of activity and stability. No detectable leaching of V or Ti was noted. Electron paramagnetic resonance measurements were performed to monitor the fraction of V-ions in the catalyst in the +IV valence state. A reduction of this fraction by ∼17% after oxidation catalysis is observed, in agreement with the generally accepted mechanism for this type of reaction.

Synthesis, characterization and sorption properties of NH2-MIL-47

K. Leus, S. Couck, M. Vandichel, G. Vanhaelewyn, Y-Y Liu, G.B. Marin, I. Van Driessche, D. Depla, M. Waroquier, V. Van Speybroeck, J.F.M. Denayer, P. Van der Voort
Physical Chemistry Chemical Physics (PCCP)
14, 15562–15570
2012
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Abstract 

An amino functionalized vanadium-containing Metal Organic Framework, NH2-MIL-47 has been synthesized by a hydrothermal reaction in an autoclave. Alternatively, a synthesis route via microwave enhanced irradiation has been optimized to accelerate the synthesis. The NH2-MIL-47 exhibits the same topology as MIL-47, in which the V center is octahedrally coordinated. After an exchange procedure in DMF the V+III center is oxidized to V+IV, which is confirmed by EPR and XPS measurements. The CO2 and CH4 adsorption properties have been evaluated and compared to MIL-47, showing that both MOFs have an almost similar adsorption capacity and affinity for CO2. DFT- based molecular modeling calculations were performed to obtain more insight into the adsorption positions for CO2 in NH2-MIL-47. Furthermore our calculated adsorption enthalpies agree well with the experimental values.

Radiation-Induced Radicals in Glucose-1-phosphate. I. Electron Paramagnetic Resonance and Electron Nuclear Double Resonance Analysis of in situ X-Irradiated Single Crystals at 77 K

H. De Cooman, G. Vanhaelewyn, E. Pauwels, E. Sagstuen, M. Waroquier
Journal of Physical Chemistry B
112 (47), 15045-15053
2008
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Abstract 

Electron magnetic resonance analysis of radiation-induced defects in dipotassium glucose-1-phosphate dihydrate single crystals in situ X-irradiated and measured at 77 K shows that at least seven different carbon-centered radical species are trapped. Four of these (R1−R4) can be fully or partly characterized in terms of proton hyperfine coupling tensors. The dominant radical (R2) is identified as a C1-centered species, assumedly formed by a scission of the sugar−phosphate junction and the concerted formation of a carbonyl group at the neighboring C2 carbon. This structure is chemically identical to a radical recently identified in irradiated sucrose single crystals. Radical species R1 and R4 most likely are C3- and C6-centered species, respectively, both formed by a net hydrogen abstraction. R3 is suggested to be chemically similar to but geometrically different from R4. Knowledge of the identity of the sugar radicals present at 77 K provides a first step in elucidating the formation mechanism of the phosphoryl radicals previously detected after X-irradiation at 280 K. In paper II, the chemical identity, precise conformation, and possible formation mechanisms of these radical species are investigated by means of DFT calculations and elementary insight into the radiation chemistry of sugar and sugar derivatives is obtained.

Radiation-Induced Radicals in Glucose-1-phosphate. II. DFT Analysis of Structures and Possible Formation Mechanisms

E. Pauwels, H. De Cooman, G. Vanhaelewyn, E. Sagstuen, F. Callens, M. Waroquier
Journal of Physical Chemistry B
112 (47), 15054-15063
2008
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Abstract 

Four radiation-induced carbon-centered radicals in dipotassium glucose-1-phosphate dihydrate single crystals are examined with DFT methods, consistently relying on a periodic computational scheme. Starting from a set of plausible radical models, EPR hyperfine coupling tensors are calculated for optimized structures and compared with data obtained from EPR/ENDOR measurements, which are described in part I of this work. In this way, an independent structural identification is made of all the radicals that were observed in the experiments (R1−R4) and tentative reaction schemes are proposed. Also, the first strong evidence for conformational freedom in sugar radicals is established: two species are found to have the same chemical composition but different conformations and consequently different hyperfine coupling tensors. Analysis of the calculated energies for all model compounds suggests that the radiation chemistry of sugars, in general, is kinetically and not necessarily thermodynamically controlled.

Q-Band EPR and ENDOR of Low Temperature X-Irradiated β-d-Fructose Single Crystals

G. Vanhaelewyn, E. Pauwels, F. Callens, M. Waroquier, E. Sagstuen, P. Matthys
Journal of Physical Chemistry A
110 (6), 2147–2156
2006
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Abstract 

β-d-Fructose single crystals were in situ X-irradiated at 80 K and measured using electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR) and ENDOR-induced EPR (EIE) techniques at Q-band (34 GHz) microwave frequencies. The measurements revealed the presence of at least four carbon-centered radicals stable at 80 K. By means of ENDOR angular variations in the three principal crystallographic planes, six proton hyperfine coupling tensors could be determined and were assigned to four different radicals by the aid of EIE. Two of the radicals exhibit only β-proton hyperfine couplings and reveal almost identical EIE spectra. For the other two radicals, the major hyperfine splitting originates from a single α-proton hyperfine coupling and their EIE spectra were also quite similar. The similarity of the EIE spectra and hyperfine tensors led to the assumption that there are only two essentially different radical structures. The radical exhibiting only β-proton hyperfine couplings was assigned to a C3 centered radical arising from H3 abstraction and the other radical suggested to be an open-ring species with a disrupted C2−C3 bond and a double C2−O2 bond. A possible formation mechanism for the latter open-ring radical is presented. By means of cluster density functional theory (DFT) calculations, the structures of the two radicals were determined and a fairly good agreement between the calculated and experimental hyperfine tensors was found.

Article Experimental and Theoretical Electron Magnetic Resonance Study on Radiation-Induced Radicals in α-l-Sorbose Single Crystals

G. Vanhaelewyn, B. Jansen, E. Pauwels, E. Sagstuen, M. Waroquier, F. Callens
Journal of Physical Chemistry A
108 (16), 3308-3314
2004
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Abstract 

α-l-Sorbose single crystals were X-irradiated at 295 K (room temperature). A combined electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), and ENDOR-induced EPR (EI-EPR) study at 120 K revealed a realm of radiation-induced free radicals in this sugar system. In the present work, a pair of closely related radicals is focused on, being dominant immediately after irradiation, but unstable with respect to long time storage or upon warming the samples. A density functional theory (DFT) study was carried out considering the complete hyperfine coupling tensors (principal axes and anisotropic and isotropic couplings) in comparison with the observed electron−proton interactions. This combined approach yielded very plausible models for both radicals, which are formed by a net hydrogen-abstraction from the C3 position of the six-membered sorbose ring. It appears that the difference between the two species is linked to the molecular disorder in the sorbose crystal structure. In addition, DFT calculations of the g tensors were performed for the plausible radical conformations.

Tentative Structures for the Radiation-Induced Radicals in Crystalline β-d-Fructose Using Density Functional Theory

E. Pauwels, P. Lahorte, G. Vanhaelewyn, F. Callens, F. De Proft, P. Geerlings, M. Waroquier
Journal of Physical Chemistry A
106 (51), 12370-12375
2002
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Abstract 

In this study, density functional theory calculations were used to identify the structure of the radiation-induced radicals in solid state β-d-fructose, using a single molecule approach. Four model radicals were proposed, and the electron paramagnetic resonance (EPR) parameters were calculated for the optimized geometries. These calculated parameters were subsequently compared with those of two radical species, observed in an experimental EPR and electron nuclear double resonance study on irradiated fructose (Vanhaelewyn, G.; Lahorte, P.; De Proft, F.; Mondelaers, W.; Geerlings, P.; Callens, F. Phys. Chem. Chem. Phys. 2001, 3, 1729). On the basis of this preliminary comparison, three model structures were rejected. By varying the main degree of freedom of the remaining model, a number of conformations were obtained that yielded isotropic and anisotropic hyperfine tensor components in close agreement with experimental results. To disentangle between these possible conformers, a detailed study was made of the hyperfine tensor eigenvectors. One conformation was found to be in close agreement with the experimental measurement of the hyperfine tensor of the two observed radical species. It was concluded that these experimental species are in fact manifestations of one and the same radical, with a structure conforming to our model but with slightly altered conformations.

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