Orwik

Reduction of Se(IV) in Boom Clay: XAS Solid Phase Speciation.

The geochemical fate of selenium is of key importance for today's society due to its role as a highly toxic essential micronutrient and as a significant component of high level radioactive waste (HLRW) originating from the operation of nuclear reacto... expand abstractrs. Understanding and prediction of the long-term behavior of Se in natural environments requires identification of the in situ speciation of selenium. This article describes an XAS-based investigation into the solid phase speciation of Se upon interaction of Se(IV) with Boom Clay, a reducing, complex sediment selected as model host rock for clay-based deep geological disposal of HLRW in Belgium and Europe. Using a combination of long-term batch sorption experiments, linear combination XANES analysis and ITFA-based EXAFS analysis allowed for the first time to identify Se(0) as the dominant solid phase speciation of Se in Boom Clay systems equilibrated with Se(IV). collapse abstract

Investigation of Nanoparticles Occurring in the Colloidal Silicalite-1 Zeolite Crystallization Process Using Dissolution Experiments

In concentrated clear sol prepared from tetraethylorthosilicate, tetrapropylammonium hydroxide, and water suitable for crystallization of Silicalite-1 zeolite, the main part of the silica is present in nanoparticles. The nature of these nanoparticles... expand abstract and their evolution during the induction period and the stage of early crystal growth was investigated via dissolution experiments in the presence of excess TPAOH. The dissolution process was monitored in situ using static and dynamic light scattering (SLS/DLS) and synchrotron small-angle X-ray scattering (SAXS). The complete dissolution of an individual nanoparticle was observed to occur in one step. Dissolution transformed a nanoparticle into a cluster of silicate oligomers. Larger grown nanoparticles dissolved slower. Exponential dissolution rate constants scaled inversely proportional with the volume of the nanoparticle’s silica core. This experimentally observed dissolution behavior was modeled by assuming that a nanoparticle dissolved to oligomers via a... Silicalite-1 zeolite precursor nanoparticles dissolve to open clusters of silicate oligomers following a peculiar mechanistic pathway. Nanoparticles growing larger during the induction stage of Silicalite-1 crystallization exhibit slower dissolution kinetics. The observed dissolution behavior was modeled by assuming that a dissolving nanoparticle has to surpass a free energy barrier corresponding to a sequence of partially dissolved nanoparticles before complete conversion to oligomers. collapse abstract

A Rational Approach to the Ionothermal Synthesis of an AlPO(4) Molecular Sieve with an LTA-Type Framework.

Kinetics of intermediate-mediated self-assembly in nanosized materials: a generic model.

We propose in this paper a generic model of a nonstandard aggregation mechanism for self-assembly processes of a class of materials involving the mediation of intermediates consisting of a polydisperse population of nanosized particles. The model acc... expand abstractounts for a long induction period in the process. The proposed mechanism also gives insight on future experiments aiming at a more comprehensive picture of the role of self-organization in self-assembly processes. collapse abstract

Direct observation of molecular-level template action leading to self-assembly of a porous framework.

The molecular steps involved in the self-assembly of Cu(3)(BTC)(2) (BTC=1,3,5-benzenetricarboxylic acid) metal-organic frameworks that enclose Keggin-type H(3)PW(12)O(40) heteropolyacid molecules were unraveled by using solution (17)O, (31)P, and (18... expand abstract3)W NMR spectroscopy, small-angle X-ray scattering, near-IR spectroscopy, and dynamic light scattering. In aqueous solution, complexation of Cu(2+) ions with Keggin-type heteropolyacids was observed. Cu(2+) ions are arranged around the Keggin structure so that linking through benzenetricarboxylate groups results in the formation of the Cu(3)(BTC)(2) MOF structure HKUST-1. This is a unique instance in which a templating mechanism that relies on specific molecular-level matching and leads to explicit nanoscale building units can be observed in situ during formation of the synthetic nanoporous material. collapse abstract

Investigation of the Mechanism of Colloidal Silicalite-1 Crystallization by Using DLS, SAXS, and (29)Si NMR Spectroscopy.

Colloidal silicalite-1 zeolite was crystallized from a concentrated clear sol prepared from tetraethylorthosilicate (TEOS) and aqueous tetrapropylammonium hydroxide (TPAOH) solution at 95 degrees C. The silicate speciation was monitored by using dyna... expand abstractmic light scattering (DLS), synchrotron small-angle X-ray scattering (SAXS), and quantitative liquid-state (29)Si NMR spectroscopy. The silicon atoms were present in dissolved oligomers, two discrete nanoparticle populations approximately 2 and 6 nm in size, and crystals. On the basis of new insight into the evolution of the different nanoparticle populations and of the silicate connectivity in the nanoparticles, a refined crystallization mechanism was derived. Upon combining the reagents, different types of nanoparticles (ca. 2 nm) are formed. A fraction of these nanoparticles with the least condensed silicate structure does not participate in the crystallization process. After completion of the crystallization, they represent the residual silicon atoms. Nanoparticles with a more condensed silicate network grow until approximately 6 nm and evolve into building blocks for nucleation and growth of the silicalite-1 crystals. The silicate network connectivity of nanoparticles suitable for nucleation and growth increasingly resembles that of the final zeolite. This new insight into the two classes of nanoparticles will be useful to tune the syntheses of silicalite-1 for maximum yield. collapse abstract

Synthesis and characterization of stable monodisperse silica nanoparticle sols for in vitro cytotoxicity testing.

For the investigation of the interaction of nanoparticles with biomolecules, cells, organs, and animal models there is a need for well-characterized nanoparticle suspensions. In this paper we report the preparation of monodisperse dense amorphous sil... expand abstractica nanoparticles (SNP) suspended in physiological media that are sterile and sufficiently stable against aggregation. SNP sols with various particle sizes (2-335 nm) were prepared via base-catalyzed hydrolysis and polymerization of tetraethyl orthosilicate under sterile conditions using either ammonia (Stober process (1) ) or lysine catalyst (Lys-Sil process (2) ). The series was complemented with commercial silica sols (Ludox). Silica nanoparticle suspensions were purified by dialysis and dispersed without using any dispersing agent into cell culture media (Dulbecco's Modified Eagle's medium) containing antibiotics. Particle sizes were determined by dynamic light scattering. SNP morphology, surface area, and porosity were characterized using electron microscopy and nitrogen adsorption. The SNP sols in cell culture medium were stable for several days. The catalytic activity of the SNP in the conversion of hydrogen peroxide into hydroxyl radicals was investigated using electron paramagnetic resonance. The catalytic activity per square meter of exposed silica surface area was found to be independent of particle size and preparation method. Using this unique series of nanoparticle suspensions, the relationship between cytotoxicity and particle size was investigated using human endothelial and mouse monocyte-macrophage cells. The cytotoxicity of the SNP was strongly dependent on particle size and cell type. This unique methodology and the collection of well-characterized SNP will be useful for further in vitro studies exploring the physicochemical determinants of nanoparticle toxicity. collapse abstract

Connectivity Analysis of the Clear Sol Precursor of Silicalite: Are Nanoparticles Aggregated Oligomers or Silica Particles?

The very first stages of the incorporation of tetraethoxysilane (TEOS) into aqueous tetrapropylammonium hydroxide (TPAOH) by hydrolysis are investigated to clarify the formation of silicate species in solution: oligomers and nanoparticles. Silicate s... expand abstractpeciation of both oligomers and nanoparticles were characterized using quantitative 29Si NMR, electrospray ionization mass spectrometry (ESI-MS), dynamic light scattering (DLS), and 1H diffusion-ordered NMR spectroscopy (DOSY). The main parameter measured for following this formation with the advancement of hydrolysis of TEOS is the evolution of silicon connectivity, in oligomers as well as in nanoparticles. At the beginning of TEOS hydrolysis, small oligomers are formed, which grow in number and size as the reaction progresses, with an average connectivity going from 0 to 2.1. At Si/TPAOH ratio of 1.0 and above, nanoparticles form through aggregation of oligomers with some additional condensation. Their connectivity varies from 2.4 to 3.1. These nanoparticles... collapse abstract

EXAFS and DFT: evidence for the TcO2+ core.

A structure elucidation strategy combining X-ray Absorption Spectroscopy (XAS) and Density Functional Theory (DFT) is demonstrated and reveals the existence of the [Tc=O](2+) moiety.

Framework breathing in the vapour-phase adsorption and separation of xylene isomers with the metal-organic framework MIL-53.

Vapour-phase adsorption and separation of the C8 alkyl aromatic compounds p-xylene, m-xylene, o-xylene, and ethylbenzene has been studied on the metal-organic framework MIL-53. Adsorption and desorption isotherms of the pure components at 110 degrees... expand abstract C were determined using the gravimetric technique. The adsorption isotherms show two well-defined steps and hysteresis, corresponding to the opening or breathing of the framework, as induced by the presence of the adsorbing molecules. In the first isotherm plateau, an adsorption capacity of about 18 wt % is observed. After the breathing phenomenon, the adsorption capacity increases to about 40 wt %. Breakthrough separation experiments with equimolar o-xylene/ethylbenzene mixtures were performed at 110 degrees C with varying hydrocarbon pressures. The separation mechanism is related to the state of the pore structure, as dictated by framework breathing. At low pressure, below the "pore-opening" pressure, MIL-53 shows no preference for any isomer. At pressures high enough to induce pore opening, separation of the C8 alkyl aromatic isomers becomes possible and separation factors as high as 6.5 are observed. The separation at a high degree of pore filling in the open form is a result of differences in the packing modes of the C8 alkyl aromatic components in the pores of MIL-53. collapse abstract

Synthesis of highly stable pure-silica thin-walled hexagonally ordered mesoporous material.

Hexagonally ordered mesoporous silica with a very narrow mesopore size distribution and exceptionally high stability paired with unusually thin pore walls was prepared using piperidine and cetyltrimethylammonium bromide.

Quantitative three-dimensional modeling of zeotile through discrete electron tomography.

Discrete electron tomography is a new approach for three-dimensional reconstruction of nanoscale objects. The technique exploits prior knowledge of the object to be reconstructed, which results in an improvement of the quality of the reconstructions.... expand abstract Through the combination of conventional transmission electron microscopy and discrete electron tomography with a model-based approach, quantitative structure determination becomes possible. In the present work, this approach is used to unravel the building scheme of Zeotile-4, a silica material with two levels of structural order. The layer sequence of slab-shaped building units could be identified. Successive layers were found to be related by a rotation of 120 degrees, resulting in a hexagonal space group. The Zeotile-4 material is a demonstration of the concept of successive structuring of silica at two levels. At the first level, the colloid chemical properties of Silicalite-1 precursors are exploited to create building units with a slablike geometry. At the second level, the slablike units are tiled using a triblock copolymer to serve as a mesoscale structuring agent. collapse abstract

Ordered End-Member of ZSM-48 Zeolite Family

ZSM-48 and related zeolites are considered to be highly disordered structures. Different polytypes can be clearly distinguished by simulation of high-resolution electron microscopy images. Synthesis of phase-pure polytypes was attempted. One of the i... expand abstractnvestigated samples crystallized via seeding designated as COK-8 consisted of nanoscopic, needlelike crystals with a very large length/width ratio, growing along the pore direction. These specimens are phase-pure polytype 6 (PT6, numbering according to Lobo and van Koningsveld). Aggregates of these nanoneedles occasionally contained a second polytype: PT1. The latter polytype occurred more abundantly in larger crystal rods in an IZM-1 sample crystallized in ethylene glycol. Here too, the isolated crystallites mainly consist of large, defect-free regions of PT6. A simulation of polytype lattice energies offers a rational explanation for the observed polytypical intergrowth formation. TEM revealed shear can locally transform the thermodynamically most stable end member polytype 6 of the disorder zeolite family into polytype 1. Shear forces result fom aggregation of phase-pure polytype 6 needles during synthesis. The study shows that in addition to chemical, mechanical parameters have to be considered. collapse abstract

Zeolite-inspired low-k dielectrics overcoming limitations of zeolite films.

Spin-on zeolite films deposited from Silicalite-1 nanocrystal suspensions prepared by hydrothermal treatment of clear solutions have the required properties for insulating media in microelectronics. However, on the scale of the feature sizes in on-ch... expand abstractip interconnects of a few tens of nanometers, their homogeneity is still insufficient. We discovered a way to overcome this problem by combining the advantages of the clear solution approach of Silicalite-1 synthesis with a sol-gel approach. A combination of tetraethyl orthosilicate and methyltrimethoxysilane silica sources was hydrolyzed and cocondensed in the presence of an aqueous tetraalkylammonium hydroxide template. The resulting suspension of nanoparticles of a few nanometers in size together with residual oligomeric silica species were spun onto support. The final zeolite-inspired low-k films (ZLK) with respect to pore size and homogeneity satisfied all requirements and presented excellent hydrophobicity, stiffness, and dielectric constant. The size and content of initially formed nanoparticles and the spatial hindrance promoted by occluded tetraalkylammonium molecules were found to be crucial elements in the definition of the final pore network. collapse abstract

Viscosity sensing in heated alkaline zeolite synthesis media.

A quartz disc resonator operating in thickness shear mode was used for the in situ monitoring of the viscosity during zeolite crystal formation.

Selective adsorption and separation of ortho-substituted alkylaromatics with the microporous aluminum terephthalate MIL-53.

The metal-organic framework MIL-53(Al) was tested for selective adsorption and separation of xylenes and ethylbenzene, ethyltoluenes, and cymenes using batch, pulse chromatographic, and breakthrough experiments. In all conditions tested, MIL-53 has t... expand abstracthe largest affinity for the ortho-isomer among each group of alkylaromatic compounds. Separations of the ortho-compounds from the other isomers can be realized using a column packed with MIL-53 crystallites. As evidenced by Rietveld refinements, specific interactions of the xylenes with the pore walls of MIL-53 determine selectivity. In comparison with the structurally similar metal-organic framework MIL-47, the selectivities among alkylaromatics found for MIL-53 are different. Separation of ethyltoluene and cymene isomers is more effective on MIL-53 than on MIL-47; the pores of MIL-53 seem to be a more suitable environment for hosting the larger ethyltoluene and cymene isomers than those of MIL-47. collapse abstract

Characterization of nanoparticles in diluted clear solutions for Silicalite-1 zeolite synthesis using liquid 29Si NMR, SAXS and DLS.

Clear solutions for colloidal Silicalite-1 synthesis were prepared by reacting tetraethylorthosilicate in aqueous tetrapropylammonium hydroxide solution. A dilution series with water resulting in clear solutions with a TEOS ratio TPAOH ratio H2O mola... expand abstractr ratio of 25 : 9 : 152 up to 25 : 9 : 15,000 was analysed using liquid 29Si nuclear magnetic resonance (NMR), synchrotron small angle X-ray scattering (SAXS) and dynamic light scattering (DLS). Particle sizes were derived independently from DLS and from the combination of SAXS and NMR. NMR allowed quantitative characterization of silicon distributed over nanoparticles and dissolved oligomeric silicate polyanions. In all samples studied, the majority of silicon (78-90%) was incorporated in the nanoparticle fraction. In concentrated suspensions, silicate oligomers were mostly double-ring species (D3R, D4R, D5R, D6R). Dilution with water caused their depolymerisation. Contrarily, the internal condensation and size of nanoparticles increased with increasing dilution. SAXS revealed a decrease of effective nanoparticle surface charge upon dilution, reducing the effective particle interactions. With DLS, the reduction of nanoparticle interactions could be confirmed monitoring the collective diffusion mode. The observed evolution of nanoparticle characteristics provides insight in the acceleration of the Silicalite-1 crystallization upon dilution, in view of different crystallization models proposed in the literature. collapse abstract

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

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 dep... expand abstractend 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... collapse abstract

Nominal and effective dosimetry of silica nanoparticles in cytotoxicity assays.

Because of their small size and large specific surface area (SA), insoluble nanoparticles are almost not affected by the gravitational force and are generally formulated in stable suspensions or sols. This raises, however, a potential difficulty in i... expand abstractn vitro assay systems in which cells adhering to the bottom of a culture vessel may not be exposed to the majority of nanoparticles in suspension. J. G. Teeguarden et al., 2007, Toxicol. Sci. 95, 300-312 have recently addressed this issue theoretically, emphasizing the need to characterize the effective dose (mass or number or SA dose of particles that affect the cells) which, according to their model based on sedimentation and gravitation forces, might only represent a very small fraction of the nominal dose. We hypothesized, in contrast, that because of convection forces that usually develop in sols, the majority of the particles may reach the target cells and exert their potential toxicity. To address this issue, we exposed three different cell lines (A549 epithelial cells, EAHY926 endothelial cells, and J774 monocyte-macrophages) to a monodisperse suspension of Stöber silica nanoparticles (SNP) in three different laboratories. Four different end points (lacticodehydrogenase [LDH] release, LDH cell content, tetrazolium salt (MTT), and crystal violet staining) were used to assess the cell response to nanoparticles. We found, in all cell lines and for all end points, that the cellular response was determined by the total mass/number/SA of particles as well as their concentration. Practically, for a given volume of dispersion, both parameters are of course intimately interdependent. We conclude that the nominal dose remains the most appropriate metric for in vitro toxicity testing of insoluble SNP dispersed in aqueous medium. This observation has important bearings on the experimental design and the interpretation of in vitro toxicological studies with nanoparticles. collapse abstract

Reaction of trimethylchlorosilane in spin-on Silicalite-1 zeolite film.

We present a study on the hydrophobization of spin-on Silicalite-1 zeolite films through silylation with trimethylchlorosilane. Microporous and micro-mesoporous Silicalite-1 films were synthesized by spin coating of suspensions of Silicalite-1 nanoze... expand abstractolite crystallized for different times. Ellipsometric porosimetry with toluene and water adsorbates reveals that silylation decreases the porosity and makes the films hydrophobic. The decrease in porosity depends on the exposed surface area in the pores. Water contact angle measurements confirm the hydrophobicity. Fourier transform infrared spectroscopy reveals that the trimethylsilyl groups are chemisorbed selectively on isolated silanols and less on geminal and vicinal silanols due to steric limitations. Time-of-flight secondary-ion mass spectroscopy and in situ ellipsometry analysis of the reaction kinetics show that the silylation is a bulk process occurring in the absence of diffusion limitation. Electrical current leakage on films decreases upon silylation. Silylation with trimethylchlorosilane is shown to be an effective hydrophobization method for spin-on Silicalite-1 zeolite films. collapse abstract

Formation of ZSM-22 zeolite catalytic particles by fusion of elementary nanorods.

An ZSM-22 aluminosilicate zeolite was synthesized using the hydrothermal gel method at 150 degrees C. Products obtained after different synthesis times were characterized using various techniques and catalytic testing. Massive formation of ZSM-22 nan... expand abstractocrystals occurs after only a short synthesis time, appearing as isolated rods with a cross section of 12+/-4 nm. Nanorods have aluminum enriched at their external surface. Later in the crystallization process nanorods align and fuse sideways, whereby the external surface is systematically converted into an internal micropore surface. The formation of aluminum bearing micropores by the joining of nanorod surfaces is responsible for the enhanced catalytic activity. For this, the zeolite synthesis of nanoscale crystallites is ineffective for enhancing catalytic activity. collapse abstract

Characterization of a molecular sieve coating using ellipsometric porosimetry.

Ellipsometric porosimetry was used to determine the adsorption isotherms of toluene, methanol, and water on b-oriented Silicalite-1 coatings with a thickness of less than ca. 250 nm and to obtain adsorption kinetics. The adsorption isotherms are of s... expand abstractufficient quality to reveal several aspects of the pore structure such as the adsorbate capacity and the adsorbate/framework affinity. The use of a combination of different molecular probes in ellipsometric porosimetry to elucidate the molecular accessibility of Silicalite-1 pores is demonstrated. It is shown that ellipsometric porosimetry is an appropriate technique for probing the influence of aging of the Silicalite-1 coating and of planarization polishing on the porosity, pore accessibility, and adsorbate/framework affinity. collapse abstract

Selective adsorption and separation of xylene isomers and ethylbenzene with the microporous vanadium(IV) terephthalate MIL-47.

Ultraviolet-Assisted Curing of Polycrystalline Pure-Silica Zeolites:  Hydrophobization, Functionalization, and Cross-Linking of Grains

We demonstrate the advantageous use of an ultraviolet-assisted curing for the preparation of polycrystalline zeolites. The ultraviolet-assisted curing effectively hydrophobizes the porous structure of a spin-on Silicalite-1 film by means of silanol c... expand abstractondensation and grafting of desorbing organic template fragments. Moreover, it enhances the packing of the grains, so the intergrain voids are substantially shrunk and the mechanical properties improved. collapse abstract

Ultraviolet-assisted curing of polycrystalline pure-silica zeolites: hydrophobization, functionalization, and cross-linking of grains.