Tpm Theo Beelen

Diatom silicon biomineralization as an inspirational source of new approaches to silica production

The demand for new materials and products is still growing and the interest in naturally formed biopolymers and biominerals, such as chitin, calcium precipitates and silica is increasing. Photosynthesizing microalgae of the family Bacillariophyceae (diatoms) produce silica exoskeletons with a potential to be used in specific industrial or technological processes, they also are an excellent model in studies of silicon biomineralization. In contrast to geologically aged diatomaceous earth, the freshly prepared silica of cultured or harvested natural diatoms has been characterized insufficiently with respect to the properties (e.g. purity, specific surface area, porosity) required for technological and industrial application. In this contribution we summarize aspects of cellular processes that are involved in silicon biomineralization of diatoms and the current knowledge of the characterization of diatomaceous silica, following methods used for synthetically derived silica-based materials.

The hydrothermal synthesis of silicalite—an in situ small-angle neutron scattering study

Abstract Small-angle neutron scattering has been used to study in situ the gel transformations occurring in the amorphous zeolite reaction mixture during the so-called induction period and subsequent crystallization. The synthesis of silicalite was studied at various temperatures (130, 150, 170, 190°C). The size of the primary units in the precursor reaction mixture is approximately 45 A. These units grow during the synthesis. The largest particle sizes were obtained at the lowest temperatures owing to the smaller nucleation rate. Before crystallization can occur, a reorganization of the gel phase occurs. During the induction period as well as during the crystallization the gel units have a surface fractal structure. It is concluded that crystallization starts in or on the growing amorphous gel particles.

Simultaneous monitoring of amorphous and crystalline phases in silicalite precursor gels. An in situ hydrothermal and time-resolved small- and wide-angle X-ray scattering study

The gel transformations and subsequent crystallization that occur in the precursor reaction mixture of silicalite were investigated using simultaneous small- and wide-angle X-ray scattering (SAXS–WAXS). The SAXS–WAXS measurements, together with the use of a high flux of synchrotron radiation and a newly developed high-pressure reaction cell, provide the possibility of in situ hydrothermal and time-resolved monitoring of amorphous gel transformations and crystallization.

The role of hydrogen peroxide during the autoxidation of thiols promoted by bifunctional polymer-bonded cobaltphthalocyanine catalysts

Abstract Bifunctional catalysts composed of a polymeric base and a cobaltphthalocyanine were applied to the oxidation of thiols (RSH) to disulphides (RSSR). During this reaction hydrogen peroxide (H 2 O 2 ) is produced and consumed simultaneously. A mechanistic interpretation of the production of H 2 O 2 is presented and it is furthermore demonstrated that the base catalyzed reaction of RSH with H 2 O 2 can account for the measured conversion of H 2 O 2 . The consumption of H 2 O 2 by the reaction of H 2 O 2 with the polymeric amine groups and the disproportionation of H 2 O 2 into O 2 and H 2 O (catalyzed by cobaltphthalocyanines) is found to be of minor importance in the investigated reaction system. The reaction of RSH with H 2 O 2 results mainly in the formation of RSSR, but is has been shown by IR measurements that also small amounts of sulphur-containing oxy-acids are formed. The observed deactivation of the bifunctional catalytic systems results from the poisoning of the essential basic sites of the catalysts by these sulphur-acids. Although some decomposition of the cobaltphthalocyanine by H 2 O 2 occurs, it is proved that this oxidative destruction is not the primary cause of the deactivation.

Silica gel from water glass: a SAXS study of the formation and ageing of fractal aggregates

The influence of pH, cations, F- and temperature on the aggregation, gelation and ageing of aqueous silica gels has been investigated using small-angle X-ray scattering (SAXS). At both low and intermediate pH (pH--4 and 6-8, respectively), fractal aggregates are formed during gelation from silicic monomers or small oligomers. The fractal dimensionality is 2-20 (5), to be expected for reactionlimited cluster-cluster aggregates. During ageing silicic ions dissolve from the more soluble peripheral primary particles and condense in the crevices of the core of the aggregates. This process results in an increase in the size of the scattering primary particles and in a decrease in the fractal dimensionality of the silica aggregates from D--2-25 to D--2.0 or D1.85. The ageing processes are strongly enhanced by a slight increase in the pH of the solution, addition of fluorine anions and a rise in temperature. The aged silica gel is constructed of an ensemble of densified aggregates of which the density variation within the aggregates is larger than that of the freshly prepared aggregates. The primary particles of which the aggregates are built grow during ageing from molecular size to dense particles in the nanometer range, the ultimate radius depending on the type and duration of the ageing process.

Volume fraction dependence and reorganization in cluster-cluster aggregation processes

Off‐lattice diffusion limited cluster aggregation simulations in two dimensions have been performed in a wide volume fraction range between 0.001 and 0.60. Starting from a system of 10 000 monomers with radius 0.5, that follow Brownian trajectories, larger aggregates are generated by bond formation between overlapping aggregates. No rings are present in the nonaged structures. The influence of the initial monomer volume fraction on the fractal properties of the gels is studied and interpreted by calculation of small angle scattering structure factor patterns to find the fractal dimension. It is found that an increase of the volume fraction leads to the development of two distinct fractal regions. The fractal dimension at short length scale shows the diffusion limited cluster aggregation value of 1.45 up to the correlation length, while the long range fractal dimension gradually increases from 1.45 to 2.00, the Euclidean dimension of the simulation space. It is shown that high volume fractions lead to chan...

SIMULATION OF SMALL-ANGLE SCATTERING FROM LARGE ASSEMBLIES OF MULTI-TYPE SCATTERER PARTICLES

Abstract We describe a central processing unit (CPU)-efficient expansion of the Debye scattering formula for the calculation of small-angle scattering patterns of model systems composed of different types of scatterers. The algorithm permits the use of atomic scattering factors or form factors of hard spheres of variable radius and scattering density. We apply the algorithm to the computation of partial small-angle scattering profiles in biological multi-type systems and examine the relative importance of particles with different connectivities in determining the fractal dimension of large particle networks.

Extraction of Cu(II) and Ni(II) by camphorquinone dioxime

The extraction properties of three geometrical isomers (α-, β- and δ-) of D-camphorquinone dioxime (H2CQD) with copper and nickel are describ

Aggregation, gelation and aging in silica

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Diffusion- and reaction-limited aggregation of aqueous silicate solutions

Abstract Two different aggregation processes (diffusion- and reaction-limited cluster aggregation) are observed in aqueous silicate solutions which are acidified. The type of aggregation depends on total silica concentration of the starting silicate water glass and the presence of oligomeric silicate anions in the starting silicate solution. If Q43-silicate anions are present in solution and the water glass is low in concentration (≈ 0.2 wt%), fast, diffusion-limited cluster-cluster aggregation (DL-CCA) is observed, whereas if no oligomers are present, slow, reaction limited cluster-cluster aggregation (RL-CCA) is observed. Diffusion-limited cluster-cluster aggregation is induced by fast reaction of reactive oligomers with monomers and other oligomers. The low silica concentration induces diffusion to become rate determining in the aggregation process. The fractal dimensionality of gels formed by RL-CCA does not depend on total silica concentration, whereas the size of the aggregates strongly depends on total silica concentration.