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Rutger A van Santen

Publications and source records attributed to Rutger A van Santen.

15 recordsLinked to original sources
PubMed2007

An efficient hybrid, nanostructured, epoxidation catalyst: titanium silsesquioxane-polystyrene copolymer supported on SBA-15.

A novel interfacial hybrid epoxidation catalyst was designed with a new immobilization method for homogeneous catalysts by coating an inorganic support with an organic polymer film containing active sites. The titanium silsesquioxane (TiPOSS) complex, which contains a single-site titanium active center, was immobilized successfully by in-situ copolymerization on a mesoporous SBA-15-supported polystyrene polymer. The resulting hybrid materials exhibit attractive textural properties (highly ordered mesostructure, large specific surface area (>380 m2 g-1) and pore volume (>or==0.46 cm3 g-1)), and high activity in the epoxidation of alkenes. In the epoxidation of cyclooctene with tert-butyl hydrogen peroxide (TBHP), the hybrid catalysts have rate constants comparable with that of their homogeneous counterpart, and can be recycled at least seven times. They can also catalyze the epoxidation of cyclooctene with aqueous H2O2 as the oxidant. In two-phase reaction media, the catalysts show much higher activity than their homogeneous counterpart due to the hydrophobic environment around the active centers. They behave as interfacial catalysts due to their multifunctionality, that is, the hydrophobicity of polystyrene and the polyhedral oligomeric silsesquioxanes (POSS), and the hydrophilicity of the silica and the mesoporous structure. Combination of the immobilization of homogeneous catalysts on two conventional supports, inorganic solid and organic polymer, is demonstrated to achieve novel heterogeneous catalytic ensembles with the merits of attractive textural properties, tunable surface properties, and optimized environments around the active sites.

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PubMed2006

Effect of aluminum on the nature of the iron species in Fe-SBA-15.

We report the preparation of highly ordered mesoporous Fe-Al-SBA-15 with isolated extraframework Fe species under acidic conditions. The materials were characterized by means of UV resonance Raman spectroscopy, in conjunction with BET, XRD, TEM, UV-vis, H2-TPR, FT-IR, and 27Al MAS NMR spectroscopy. The addition of both Fe and Al to the synthesis gel of SBA-15 results in the formation of isolated extraframework Fe species located close to the framework Al ions and the Fe content an order of magnitude higher than that in Fe-SBA-15 synthesized without Al. The existence of anchored extraframework Fe species was confirmed by the presence of a strong absorption band at 270 nm, hydrogen reduction at relatively low temperature, and the presence of a resonance Raman band at 1140 cm(-1). The location of Fe in close proximity to framework Al nuclei is further supported by 27Al MAS NMR measurements. Two characteristic UV Raman bands at 510 cm(-1) and 1090 cm(-1) excited by 244-nm laser are assigned to Fe-O-Si symmetric and asymmetric stretching modes of isolated tetrahedral Fe ions in the silica framework for Fe-SBA-15. The resonance Raman band at 1140 cm(-1) excited by 325-nm laser is attributed to the asymmetric stretching mode of the isolated extraframework iron species in Fe-Al-SBA-15. The isolated Fe species close to framework Al species are stable in acidic HCl solution, whereas the majority of Fe species in Fe-SBA-15 can be easily removed.

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PubMed2006

Mechanism of oligomerization reactions of silica.

The mechanism of the oligomerization reaction of silica, the initial step of silica formation, has been studied by quantum chemical techniques. The solvent effect is included by using the COSMO model. The formation of various oligomers (from dimer to tetramer) was investigated. The calculations show that the anionic pathway is kinetically preferred over the neutral route. The first step in the anionic mechanism is the formation of the SiO-Si linkage between the reactants to form a five-coordinated silicon complex, which is an essential intermediate in the condensation reaction. The rate-limiting step is water removal leading to the oligomer product. The activation energies for dimer and trimer formation ( approximately 80 kJ/mol) are significantly higher than those of the subsequent oligermerization. The activation energies for the ring closure reaction ( approximately 100 kJ/mol) are even higher. The differences in activation energies can be related to the details in intra- and intermolecular hydrogen bonding of the oligomeric complexes.

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PubMed2006

Interplay of bonding and geometry of the adsorption complexes of light alkanes within cationic faujasites. Combined spectroscopic and computational study.

A FT-IR spectroscopic study of methane, ethane, and propane adsorption on magnesium and calcium forms of zeolite Y reveals different vibrational properties of the adsorbed molecules depending on the exchanged cation. This is attributed to different adsorption conformations of the hydrocarbons. Two-fold eta(2) coordination of light alkanes is realized for MgY, whereas in case of CaY zeolite quite different adsorption modes are found, involving more C-H bonds in the interaction with the cation. The topological analysis of the electron density distribution function of the adsorption complexes shows that when a hydrocarbon coordinates to the exchanged Mg(2+) ions, van der Waals bonds between H atoms of the alkane and basic zeolitic oxygens significantly contribute to the overall adsorption energy, whereas in case of CaY zeolite such interactions play only an indirect role. It is found that, due to the much smaller ionic radius of the Mg(2+) ion as compared to that of Ca(2+), the former ions are significantly shielded with the surrounding oxygens of the zeolitic cation site. This results in a small electrostatic contribution to the stabilization of the adsorbed molecules. In contrast, for CaY zeolite the stabilization of alkanes in the electrostatic field of the partially shielded Ca(2+) cation significantly contributes to the adsorption energy. This is in agreement with the experimentally observed lower overall absorption of C-H stretching vibrations of alkanes loaded to MgY as compared to those for CaY zeolite. The preferred conformation of the adsorbed alkanes is controlled by the bonding within the adsorption complexes that, in turn, strongly depends on the size and location of the cations in the zeolite cavity.

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PubMed2006

Confined space-controlled olefin-oxygen charge transfer in zeolites.

Density functional theory (DFT) calculations on the initial charge-transfer step in the photo-oxidation of alkenes in cationic zeolites are presented. The model system used represents a part of the Y-zeolite supercage containing two II sites with alkali-earth cations and coadsorbed 2,3-dimethylbutene-2 (DMB) and O(2) on them. It is found that the electrostatic field of the zeolite cavity plays only a minor role for the stabilization of a charge-transfer state, whereas the relative orientation and the distance between the DMB and O(2) molecules are the most important factors. On the basis of these results, the photo-oxidation considered is due to a confinement effect in which the reagents are oriented in a suitable "pre-transition state" configuration.

Letter↗
PubMed2006

Hollow silica spheres with an ordered pore structure and their application in controlled release studies.

Herein we report the synthesis and characterization of hollow silica spheres with a narrow size distribution, uniform wall thickness, and a worm-like pore structure. The formation of these spheres was monitored by confocal laser scanning microscopy and dynamic light scattering. A model for the molecular build-up of these silica hollow spheres is derived from these data in combination with studies of the as-made particles by transmission electron microscopy, scanning electron microscopy, pore size analysis, thermogravimetric analysis, and solid-state nuclear magnetic resonance. We further demonstrate that these spheres can be used for the encapsulation and subsequent release of different dye molecules.

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PubMed2005

Template-aluminosilicate structures at the early stages of zeolite ZSM-5 formation. A combined preparative, solid-state NMR, and computational study.

Species at three stages in the self-assembly of zeolite ZSM-5 have been studied with one- and two-dimensional magic-angle-spinning 13C, 27Al, 29Si, and 1H NMR spectroscopy and compared with the earlier proposed structures: (1) precursor species containing 33-36 T sites around a tetrapropylammonium (TPA) cation, (2) nanoslabs consisting of a flat 4 x 3 array of such precursors, and (3) the final TPA-ZSM-5 zeolite. Synthesis was carried out in D2O to suppress the water and silanol protons. Under such conditions, the effective Si-H and Al-H distances measured with 29Si-{1H} and 27Al-{1H} rotational echo double resonance (REDOR) reflect the interactions between TPA cations and the surrounding aluminosilica. The 29Si-{1H} REDOR curves for Q4-type silicon atoms at the three mentioned stages are closely similar, as well as the observed 27Al-1H REDOR curve for the precursor species compared to that for the TPA-ZSM-5. This indicates that in addition to externally attached TPA, there is also internal TPA already incorporated at an early stage into the aluminosilicate in a similar way as in the final zeolite, in accordance with the earlier proposed MFI self-assembly pathway (Kirschhock et al. Angew. Chem. Int. Ed. 2001, 40, 2637). However, the effective distances extracted from the initial REDOR curvatures are significantly (10-15%) larger than those computed for the model. Since there is no temperature effect, we tentatively assign this difference to a reduction of the 29Si-1H and 27Al-1H interactions by multispin decoherence effects or self-decoupling caused by proton spin diffusion. By assuming the computed model distances and fitting Anderson-Weiss curves to the observed REDOR data, we obtain similar "decoherence times" in the order of 0.1 ms. The observed 29Si-{1H} REDOR dephasing for the Q3 sites in the precursors is significantly faster than that for the Q4 sites. This is tentatively ascribed to a partial deuteron-proton back exchange at the silanol positions.

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PubMed2005

Ab initio studies of a water layer at transition metal surfaces.

This paper presents a detailed study of a water adlayer adsorbed on Pt(111) and Rh(111) surfaces using periodic density functional theory methods. The interaction between the metal surface and the water molecules is assessed from molecular dynamics simulation data and single point electronic structure calculations of selected configurations. It is argued that the electron bands around the Fermi level of the metal substrate extend over the water adlayer. As a consequence in the presence of the water layer the surface as a whole still maintains its metallic conductivity-a result of a crucial importance for understanding the process of electron transfer through the water/metal interface and electrochemical reactions in particular. Our results also indicate that there exists a weak bond between the hydrogen of the water and the Rh metal atoms as opposed to the widespread (classical) models based on purely repulsive interaction. This suggests that the commonly used classical interactions potentials adopted for large scale molecular dynamics simulations of water/metal interfaces may need revision. Two adsorption models of water on transition metals with the OH bonds pointing towards or away of the surface are also examined. It is shown that due to the very close values of their adsorption energies one should consider the real structure of water on the surface as a mixture of these simple "up" and "down" models. A model for the structure of the adsorbed water layer on Rh(111) is proposed in terms of statistical averages from molecular dynamics simulations.

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PubMed2005

Controlled silica synthesis inspired by diatom silicon biomineralization.

Silica becomes increasingly used in chemical, pharmaceutical, and (nano)technological processes, resulting in an increased demand for well-defined silicas and silica-based materials. The production of highly structured silica from cheap starting materials and under ambient conditions, which is a target for many researchers, is already realized in the formation of diatom biosilica, producing highly hierarchical ordered meso- and macropores silica structures. This notion formed the starting point in our integrative biomolecular and biomimetic study on diatom silicon biomineralization in which we have analyzed silica transformations and structure-direction in polymer-mediated silica syntheses using a combination of (ultra)small-angle X-ray scattering and (cryo)electron microscopy. Using bio-analogous reaction conditions and reagents, such as waterglass and (combinations of) polyethylene oxide (PEO) based polymers, we demonstrate in this review the synthesis of tailor-made mesoporous silicas in which we can, as in biosilica synthesis, control the morphological features of the resulting materials on the nanometer level as well as on the micrometer level.

Biomimetics↗
PubMed2004

Silsesquioxane models for silica surface silanol sites with adjacent siloxide functionalites and olefin polymerization catalysts thereof.

Incompletely condensed silsesquioxanes of the type R7Si7O9(O[SiR'2O]n)OH (R = c-C5H9, c-C6H11; R' = Me, Ph; n = 1-4), containing a siloxane ring of variable size and rigidity and a remaining silanol, are described. Compared with a truly isolated silanol [R7Si8O12(OH)], solution and solid state FT-IR spectra of these compounds show a nu(OH) shift of approximately 150 cm(-1) to lower frequency, which suggests hydrogen bonding of the silanol with the internal siloxane ring. In agreement with this, the relative ion pair acidities of the silanols in THF, determined by UV/Vis, were lowered by 0.8-1.2 compared with a truly isolated silanol. Density functional theory (DFT) calculations on these systems confirm the presence of intramolecular hydrogen bonding. Possible interaction of the silyl ether functionalities with Lewis acidic metal sites was studied for the neutral gallium-substituted systems and cationic titanium silsesquioxane complexes, models for an immobilized titanium olefin polymerization catalyst. The electron donating capability of the siloxide functionalities in 1, 6, and 7 is not sufficient to satisfy the electron deficiency of the corresponding gallium silsesquioxane species, which form dimeric structures with a bridging siloxide unit rather than Lewis base adducts with coordinated siloxide functionalities. Metallation of 1 and 4 with [Cp"Ti(CH2Ph)3] (Cp" = eta5-1,3-C5H3(SiMe3)2) in a 1:1 ratio afforded monomeric titanasilsesquioxanes. To probe the effect of the neighboring siloxane ring on the highly Lewis acidic titanium center, the catalytic activities of the corresponding cationic half-sandwich complexes were tested in 1-hexene polymerization. Compared with the catalyst system based on the isolated silanol [(c-C5H9)7Si8O12OH], the presence of a neighboring siloxane ring causes considerable retardation of the polymerization process but also improves the stability of the catalyst.

Alkenes↗
PubMed2004

Synthesis and characterization of tin containing polyhedral oligometallasilsesquioxanes (POMSS).

Tin silicate species have shown good catalytic activity in various oxidation reactions. In an attempt to mimic surface tin species, several tin containing silsesquioxanes have been synthesized. Incompletely condensed silsesquioxanes (c-C5H9)7Si7O9(OH)3 and (c-C5H9)7Si7O9(OSiMe3)(OH)2 were reacted with common tin-precursors, which afforded several silsesquioxane ligated tin compounds. Divalent stannasilsesquioxanes form dimers of the type [(c-C5H9)7Si7O11(OX)Sn]2(X=H, SiMe3) with three-coordinated tin centers. The three-coordinated tin(II) are hydrolytically unstable whereas the octahedrally surrounded tetravalent stannasilsesquioxanes [(c-C5H9)7Si7O11(OX)]Sn(acac)2(X=H, OSiMe3) are hydrolytically robust. An unprecedented anionic trimeric cluster, [[(c-C5H9)7Si7O12Sn]3(mu2-OH)3(mu3-OH)]-[HNEt3]+, stabilized by bridging hydroxyl groups was formed when the product formed upon reacting (c-C5H9)7Si7O9(OH)3 with SnCl4 was slowly hydrolyzed. The stannasilsesquioxanes showed no catalytic activity in oxidation reactions.

Journal Article↗
PubMed2002

Hydrogen activation on Mo-based sulfide catalysts, a periodic DFT study.

Hydrogen adsorption on Mo[bond]S, Co[bond]Mo[bond]S, and Ni[bond]Mo[bond]S (10 1 macro 0) surfaces has been modeled by means of periodic DFT calculations taking into account the gaseous surrounding of these catalysts in working conditions. On the stable Mo[bond]S surface, only six-fold coordinated Mo cations are present, whereas substitution by Co or Ni leads to the creation of stable coordinatively unsaturated sites. On the stable MoS(2) surface, hydrogen dissociation is always endothermic and presents a high activation barrier. On Co[bond]Mo[bond]S surfaces, the ability to dissociate H(2) depends on the nature of the metal atom and the sulfur coordination environment. As an adsorption center, Co strongly favors molecular hydrogen activation as compared to the Mo atoms. Co also increases the ability of its sulfur atom ligands to bind hydrogen. Investigation of surface acidity using ammonia as a probe molecule confirms the crucial role of sulfur basicity on hydrogen activation on these surfaces. As a result, Co[bond]Mo[bond]S surfaces present Co[bond]S sites for which the dissociation of hydrogen is exothermic and weakly activated. On Ni[bond]Mo[bond]S surfaces, Ni[bond]S pairs are not stable and do not provide for an efficient way for hydrogen activation. These theoretical results are in good agreement with recent experimental studies of H(2)[bond]D(2) exchange reactions.

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