| 1. |
- Schoeman, Brian J., et al.
(author)
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A study of the initial stage in the crystallization of TPA-silicalite-1
- 1996
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In: Zeolites. - 0144-2449. ; 17:5-6, s. 447-456
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Journal article (peer-reviewed)abstract
- The initial stage in the crystallization of discrete colloidal crystals of tetrapropylammonium (TPA)-silicalite-1 has been studied with high effect laser light scattering and cryo-transmission electron microscopy (cryo-TEM). The apparently clear TPA-silicate precursor solutions contain discrete subcolloidal particles prior to hydrothermal treatment with an average size of 3.8 nm measured by dynamic light scattering (DLS) and about 5 nm when observed with cryo-TEM. These subcolloidal particles are present in the hydrothermally treated sol throughout the course of crystallization. Following the onset of hydrothermal treatment, the average DLS particle size of the subcolloidal fraction increases from 3.8 to 5.0 nm because of redistribution of silica via an Ostwald ripening mechanism. The first sampling point was at 1.5 h at which time the particle size distribution is monomodal. The apparent absence of growing particles at this point is thought to be a result of the inability of the light scattering technique to resolve two particle populations of similar average size. The first indication of a second growing particle fraction, shown to be TPA-silicalite-1, is after 2.5 h, the second sampling time, at which stage the crystal size is 26 nm. The crystal size increase as a function of time, calculated as 18.5 nm/h, is linear from this time onward. The number of subcolloidal particles prior to hydrothermal treatment is estimated to be of the order of 1017/g of sol, whereas the number of growing crystals is only 4.6 · 1010/g of sol. The possible role of the subcolloidal particles is discussed.
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| 2. |
- Valtchev, Valentin, et al.
(author)
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Preparation and characterization of hollow fibers of silicalite-1
- 1996
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In: Zeolites. - 0144-2449. ; 17:5-6, s. 408-415
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Journal article (peer-reviewed)abstract
- Hollow fibers consisting solely of silicalite-1 were prepared by a novel method consisting of four basic steps. Carbon fibers were first surface modified to facilitate adsorption of colloidal seed crystals of silicalite-1. Such particles were then adsorbed as a monolayer on the fiber surface and induced to grow into a continuous film of intergrown crystals of silicalite-1. Finally, the carbon fiber used as a template was removed by calcination in air, yielding hollow fibers of silicalite-1. Hollow fibers were prepared in two experimental series employing different synthesis conditions resulting in two different modes of growth. The first experimental series was performed with a clear homogeneous TPAOH/silica solution crystallizing at 100°C, and the second series with a NaOH-TPABr/silica gel crystallizing at 150°C. Materials produced in the first series showed a smooth and homogeneous surface and a relatively uniform fiber wall with a final thickness of 1.1 μm. In these materials the film was formed by crystals growing outward from the surface of the carbon fiber. Materials produced in the second series were less uniform with a final thickness of the fiber wall of about 3.2 μm. The crystal growth in these materials appeared to take place both in an inward and outward direction starting from the seed crystals adsorbed on the carbon fiber. The hollow fibers prepared were characterized by SEM, XRD, FT i.r., and nitrogen adsorption measurements.
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