Development of realistic models for Double Metal Cyanide catalyst active sites.

Realistic molecular models of one and two-centre catalytic active sites originating from the cleavage of a precursor material known to give rise to an active double metal cyanide catalyst are described. Via periodic density functional calculations the structure of the proposed catalytic sites are shown to be dependent on electrostatic and structural relaxation processes occurring at the surfaces of the precursor material. It is shown how these effects may be adequately captured by small molecular models of the active sites. The general methodology proposed should provide a computationally efficient basis for detailed future studies into catalytic reactions over double metal cyanide materials.

High-density storage of H2 in microporous crystalline silica at ambient conditions.

Molecular hydrogen was encapsulated in the cages of clathrasil decadodecasil 3R (DD3R) during the hydrothermal synthesis of this microporous silicate. The crystalline structure of DD3R facilitates high-density hydrogen storage at ambient conditions. Prompt gamma activation analysis (PGAA) revealed that on average about one molecule of H2 is trapped in each (5(12)) cage of DD3R. The presence of molecular hydrogen inside the DD3R framework was confirmed by solid-state 1H NMR spectroscopy. Temperature-programmed decomposition (TPD) in combination with mass spectrometry showed that the encapsulated hydrogen is released upon decomposition of the clathrasil structure. This release can be promoted by the presence of water.

Molecular dynamics-based approach to study the anisotropic self-diffusion of molecules in porous materials with multiple cage types: application to H2 in losod.

The anisotropic self-diffusion of molecular hydrogen in the multiple cage clathrasil losod (LOS) is modeled by means of molecular dynamics (MD) simulations of up to 1 micros for the temperature range 900-1200 K while treating the framework as fully flexible. The LOS diffusion tensor is calculated employing an analytical method based on hopping rates. The diffusion in the c-direction of the unit cell is found to be approximately two times more rapid than in the a- and the b-directions, a characteristic of importance for the application of LOS as a membrane. The overall diffusion is based on five different hop types for which the individual hopping rates and diffusion barriers are calculated separately. We show explicitly that the shape and volume of the cages have a significant effect on the hopping rates and further that even small deformations of the circular Si6O6 apertures have a large influence on the energetic barrier for hydrogen diffusion. Compared to the single cage clathrasils dodecasil 3C (MTN) and sodalite (SOD), LOS has a lower diffusion rate. However, from a technical point of view this rate (at 573 K) is still fast enough for LOS to be interesting as a size-selective membrane or as a hydrogen-adsorption medium.

Co-TUD-1: a ketone-selective catalyst for cyclohexane oxidation.

Cobalt-containing mesoporous TUD-1 with different Si/Co ratios (100, 50, 20, and 10) was synthesized by the direct hydrothermal treatment method (DHT) using triethanolamine (TEA) as a template. The prepared samples (denoted as Co-TUD-1) were characterized by XRD, UV/Vis spectroscopy, elemental analysis, N2 sorption measurements, 29Si magic-angle spinning (MAS)-NMR, Raman spectroscopy, and HRTEM. The catalytic performance of Co-TUD-1 samples was tested in the liquid-phase oxidation of cyclohexane with tert-butylhydroperoxide (TBHP) as oxidant under solventless conditions. Higher catalytic efficiency was observed in samples with low Co-loading and thus, most likely isolated CoII active sites, than in samples with high Co-loading and nanoparticles or bulk Co3O4 embedded in the TUD-1 silica.

TiO2 nanoparticles in mesoporous TUD-1: synthesis, characterization and photocatalytic performance in propane oxidation.

A series of TiO2-TUD-1 samples was synthesized with a variable Ti loading in the range Si/Ti = 100, 20, 2.5, and 1.6, by using a one-pot surfactant-free procedure. The materials obtained were characterized by elemental analysis; X-ray diffraction (XRD); N2 sorption measurements; high-resolution TEM (HR-TEM); 29Si NMR, UV-visible and Raman spectroscopy. As a function of increasing metal loading either isolated Ti atoms, or (above a Ti loading of approximately 2.5 wt- %) combinations of isolated Ti atoms and anatase (TiO2) nanoparticles were obtained; both were incorporated in the highly porous siliceous matrix. The photocatalytic performance of these materials was tested by studying the propane oxidation process following irradiation at lambda = 365 nm, selectively activating the anatase nanoparticles. In comparison to commercial anatase powder, TiO2 nanoparticles in TUD-1 showed high photochemical selectivity towards acetone, the sample with a Si/Ti ratio of 1.6 being the most selective. Size and confinement effects are consistent with the difference in performance of the TUD-1 materials and TiO2, limiting the number of electron transfers available for each propane molecule.

The effect of Ge incorporation on the Brønsted acidity of ZSM-5.

The effect of the isomorphous substitution of some of the Si atoms in ZSM-5 by Ge atoms on the Brønsted acid strength has been investigated by i) DFT calculations on cluster models of the formula ((HO)3SiO)3-Al-O(H)-T-(OSi(OH)3)3, with T=Si or Ge, and ((HO)3SiO)3-Al-O(H)-Si-(OGe(OH)3)(OSi(OH)3)2, ii) a 31P NMR study of zeolite samples contacted with trimethyl phosphine oxide probe molecules and iii) a X-ray photoelectron spectroscopy (XPS) study of ZSM-5 and Ge-ZSM-5 samples. The calculations reveal that the effect of Ge incorporation on the framework acidity strongly depends on the degree of substitution and on the exact T-atom positions that are occupied by Ge. High Ge concentrations allow for enhanced stabilisation of the deprotonated Ge-ZSM-5 through structural relaxation, resulting in a slightly higher acidity as compared to ZSM-5. This structural relaxation is not achievable in Ge-ZSM-5 with a low Ge content, which therefore has a slightly lower acidity than ZSM-5. The NMR study indicates no difference between the Brønsted acidity of ZSM-5(47) and Ge(0.09)ZSM-5(36). Instead, evidence for the presence of a substantial amount of Ge-OH groups in the Ge-containing samples was obtained from the NMR results, which is consistent with earlier FTIR studies. The XPS results do not point to an effect of Ge on the framework acidity of ZSM-5(47), instead, the results can be best interpreted by assuming the presence of additional Ge-OH and Si-OH groups near the surface of the Ge(0.08)ZSM-5(47) sample.

Zeolite nanocrystals inside mesoporous TUD-1: a high-performance catalytic composite.

A hierarchically structured composite material with interconnecting meso- and micropores has been developed with the aim to optimize zeolite performance. A general synthetic method has been developed that, in a controlled manner, allows for various types of nanosized zeolite to be incorporated into a three-dimensional mesoporous matrix. Nanosized zeolite Beta was used to exemplify this new approach, resulting in a system in which zeolite Beta shows a higher cracking activity per gram of zeolite than pure nanosized zeolite Beta for the model feed n-hexane. Additionally, FTIR studies of CO and NH3 adsorption revealed that the nature of the acid sites in the nanozeolite has been partially modified due to the interactions with the mesoporous matrix, TUD-1.

Synthesis, structural characterization, and thermal stability of a new layered germanate structure, Na4Ge16O28(OH)12.

The new layered germanate structure Na4Ge16O28(OH)12 has been synthesized under hydrothermal conditions and characterized by single-crystal X-ray diffraction, FTIR, and SEM. The crystal lattice parameters are a = 7.3216(6) A, b = 14.3986(9) A, c = 7.7437(6) A, alpha = 90.0 degrees, beta = 100.179(7) degrees, gamma = 90.0 degrees, and V = 803.5(1) A3. The space group is C2/m with Z = 1. The germanium oxide sheets are connected non-covalently via electrostatic interactions with the sodium cations and H-bridging. At temperatures above 400 degrees C, the structure starts decomposing into sodium enneagermanate (Na4Ge9O20), germanium dioxide, and water as determined by powder X-ray diffraction, TGA, DTA, DSC, and GCMS.