Wh Wim Dokter

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.

Microporous SiO2/MOx (M=Ti,Zr,Al) for ceramic membrane applications: A microstructural study of the sol-stage and the consolidated state

Microporous SiO2 and SiO2/MO2 (M=Ti, Zr, Al; 10 mol% MOx) materials for gas separation membrane applications have been prepared from polymeric sols. Characterization of these sols with SAXS showed that the mean fractal dimension of the SiO2 sols is 1.3–1.4 with a radius of gyration of approximately 2.5 nm. The dried and calcined films are microporous and the pore size distribution was bimodal with maxima at diameters of 0.5 nm and 0.75 nm. For the SiO2/TiO2, SiO2/ZrO2 and SiO2/Al2O3 systems, much milder reaction conditions proved to be necessary to obtain sols with comparable fractal dimensions due to the high reactivity of the Ti/Zr/Al-alkoxides. Microporous supported membranes with molecular sieve-like gas transport properties can be prepared from a relatively wide range of sol structures: from polymers too small to characterize with SAXS to structures with fractal dimensions: 1

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...

Gel transformations in silicas: A combined NMR and SAXS study

Abstract 1H NMR transverse spin—spin relaxation times (T2) of water have been measured to monitor the aggregation and aging processes of low concentration silica gels. Along with1H NMR, small-angle X-ray scattering (SAXS) and physisorption measurements were used to obtain additional information on aging. After acidification of water-glass (silica solution) the primary building units combine to form aggregates with fractal dimensionalities (decreasing T2). Gelation occurs when these growing aggregates form a percolating network, resulting in a minimum value of the T2 relaxation time. During and after these processes, rearrangements of silica take place (aging), a crucial step in the formation of pores. The aging of silica gel can be monitored by changing the relaxation behaviour (increasing T2). With SAXS a densification of the structure is indicated by an observed decrease in fractal dimensionalities and an increase in the radius of the primary building units, indicating a migration of silica from the tips of the aggregates towards the centre of the aggregates. Aging is a slow process compared with the aggregation of the primary particles, but it can be accelerated by adding catalytic quantities of fluorine or by preparation at higher pH or at higher temperatures. By using spin-spin relaxation measurements, reacting aqueous silica systems with a low solid content could be studied in situ. It is shown that hydroxyl and fluoride ions have different impacts on the silica systems, leading to different mechanisms. For aggregation and aging we observed (different) optimal fluorine concentrations.

Aggregation, gelation and aging in silica

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Aggregation and aging in silica gel

Abstract Aggregation and aging of silica gels, prepared by controlled addition of water glass to hydrochloric acid, and the transformation of an aged gel to a crystalline phase have been studied in-situ using high-brilliance synchrotron radiation. Small angle X-ray scattering (SAXS) has proved to be very informative in readily detecting transformations in the silica gels which can be described using the concepts of mass and surface fractality. The interpretation of SAXS spectra of aged silica gels has been accompanied by computer simulations of aggregation and aging based on a model for the aging mechanism and the calculation of the corresponding structure-factor patterns. Comparison with experimental spectra of aged silica highlights the important role of particle growth on the fractal dimension. Gel transformations during the transition from an amorphous gel to the crystalline phase of silicalite have been successfully monitored, in-situ, both for heterogeneous and for homogeneous preparations, using the combination of small and wide angle X-ray scattering (SAXS-WAXS).

Cluster-cluster aggregation and calculated SAXS patterns: application to concentration dependence of fractal parameters

Two new computer simulation programs are presented, GRASP and DALAI. GRASP is an off-lattice implementation of the cluster-cluster aggregation model, including diffusion and reaction-limited aggregation as well as aggregate reorganization. The aggregate systems produced by GRASP are characterized by DALAI, which calculates the X-ray scattering pattern using a CPU-efficient simplified version of the Debye formula. We describe the scope and potential of the two programs and we show how they can be used together to follow the time evolution of aggregation processes in large systems. We also present results of DLCA calculations for representative 2D and a series of 3D systems in the volume concentration range 0.25-30.0% with the purpose of investigating the influence of concentration on fractal parameters.

Growth and aging phenomena in silica gels

Growth and aging of silica aggregates are influenced both by temperature and by catalyzing fluorine ions as shown by SAXS and BET. It was found that both fluorine and increased temperature slightly increased the fractal dimension Df during aging, but the fluorine catalyzed system showed a lower BET surface area.To understand the effect of fluorine and increased temperature on the aggregates, 2D aggregations and SAXS simulations were carried out using two new programs GRASP and DALAI. In agreement with experiments it was found that binary RLCCA aggregates have a slightly higher Df value compared to DLCCA aggregates and that branch-flexibility during aging increases Df even further.

USAXS and x-ray microscopy investigation on silica and precursors of zeolites

Combination of SAXS and USAXS measurements provide an extended q-range (0.006–3.0 nm -1 ) to study fractal growth of both aging silica gel as well as precursors of zeolite-A. Mass (silica) and surface (zeolite) fractals are observed. Scanning transmission X-ray microscopy (STXM) proves to be an extremely useful technique to obtain direct images of wet samples in the 0.1–100 micron range, confirming the SAXS/USAXS results on even larger length scales.