| 1. |
- Ravishankar, Raman, et al.
(författare)
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Characterization of Nanosized Material Extracted from Clear Suspensions for MFI Zeolite Synthesis
- 1999
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 103:24, s. 4960-4964
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Tidskriftsartikel (refereegranskat)abstract
- The silica species contained in an aged clear suspension, which upon heating gives rise to the crystallization of Silicalite-1, were extracted with 80% efficiency using a sequence of acidification, salting out, phase transfer into organic solvent, and freeze-drying methods. This silica powder was characterized by X-ray scattering, transmission electron microscopy, atomic force microscopy, and 29Si magic angle spinning nuclear magnetic resonance. These techniques gave evidence for the presence of a very specific morphology, corresponding to slab shaped particles, with dimensions of 1.3 × 4.0 × 4.0 nm. The nanoslabs have the MFI structure with nine channel intersections per particle, each containing a TPA cation. The identity of the extracted nanoslabs with the species in suspension is evidenced with in situ and ex situ X-ray scattering.
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| 2. |
- Ravishankar, Raman, et al.
(författare)
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Physicochemical characterization of silicalite-1 nanophase material
- 1998
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 102:15, s. 2633-2639
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Tidskriftsartikel (refereegranskat)abstract
- A silicalite-1 nanophase material with an elementary particle size of 18-100 nm is synthesized from clear solution and isolated and purified using supercentrifugation. The nanopowder is characterized in detail using scanning electron microscopy, high-resolution transmission electron microscopy, attenuated force microscopy, 29Si magic angle spinning NMR, 13C cross polarization magic angle spinning NMR, X-ray diffraction, dinitrogen physisorption, and thermogravimetric analysis and compared with micrometer-sized silicalite-1. The nanosized and micrometer-sized materials have many common properties including the refined structure and the nature and concentrations of tetrapropylammonium species incorporated during the synthesis. Unique properties of the nanophase are a splitting of the characteristic framework vibration at 550 cm-1 into a doublet at 555 and 570 cm-1, a high concentration of defect sites, and a strain in the crystallites along the "a" crystallographic direction. The nanophase exhibits a two-stage dinitrogen physisorption in the low-pressure region, ascribed to adsorptions in micropores created by the stacking of the nanoparticles in addition to adsorptions in the intracrystalline micropores.
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| 3. |
- Schoeman, Brian J., et al.
(författare)
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Dynamic light scattering applied to the synthesis of colloidal zeolite
- 1995
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Ingår i: Journal of porous materials. - 1380-2224 .- 1573-4854. ; 1:2, s. 185-198
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Tidskriftsartikel (refereegranskat)abstract
- The use of dynamic light scattering as an analysis method within the field of zeolite synthesis has proved to be a powerful and robust tool with which the particle size and the corresponding particle size distribution can be determined. The method has been employed in the evaluation of the crystallization of several types of colloidal zeolite from 'clear homogeneous' solutions. Examples of such zeolites are zeolite N-Y, hydroxysodalite, and TPA-silicalite-1. The fact that the particle size can be determined in as-synthesized zeolite sols as a function of synthesis time enables one to follow, for example, the crystallization process in terms of particle size increase, the process of particle size tailoring as well as to obtain information on the growth mechanism in zeolite synthesis. The colloidal nature of sols following redispersion of zeolite powders and colloidal zeolite organosols has been assessed using dynamic light scattering. The advantages as well as problems associated with the use of dynamic light scattering for characterization of colloidal zeolites are discussed. © 1995 Kluwer Academic Publishers.
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| 4. |
- Mintova, Svetlana, et al.
(författare)
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Preparation of ZSM-5 films from template free precursors
- 1997
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Ingår i: Journal of Materials Chemistry. - 0959-9428 .- 1364-5501. ; 7:12, s. 2341-2342
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Tidskriftsartikel (refereegranskat)abstract
- Thin films of zeolite ZSM-5 on quartz substrates have been prepared in the absence of organic templates by growth of adsorbed seed crystals attached to a polymer-modified substrate surface.
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| 5. |
- Mintova, Svetlana, et al.
(författare)
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ZSM-5 films prepared from template free precursors
- 1998
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Ingår i: Journal of Materials Chemistry. - 0959-9428 .- 1364-5501. ; 8:10, s. 2217-2221
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Tidskriftsartikel (refereegranskat)abstract
- Thin continuous films of zeolite ZSM-5 were synthesized on quartz substrates. The substrates were first surface modified and covered by a monolayer of colloidal silicalite-1 seed crystals. These crystals were grown into continuous films with thicknesses in the range 230-3500 nm by hydrothermal treatment in a synthesis gel free from organic templates. The preferential orientation of the crystals constituting the film was initially one with thec-axis close to parallel to the substrate surface. During the course of crystallization this orientation changed to one with most of the crystals having the c-axes directed approximately 35° from perpendicular to the substrate surface. A mechanism explaining this behavior is proposed. The final thickness of the film was controlled by the synthesis time but also by the addition of seed crystals to the synthesis gel. Films prepared according to this method may be of great value for the development of zeolite based membranes.
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| 6. |
- Schoeman, Brian J.
(författare)
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A high temperature in situ laser light-scattering study of the initial stage in the crystallization of TPA-silicalite-1
- 1997
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Ingår i: Zeolites. - 0144-2449. ; 18:2-3, s. 97-105
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Tidskriftsartikel (refereegranskat)abstract
- The events occurring in a clear TPA-silicalite-1 precursor solution, hydrothermally treated at 70°C, were monitored in situ by dynamic light scattering using a high-effect laser light source. Subcolloidal silicate particles with an average diameter of 3.3 nm are detected in the greater part of the crystallization. An initial decrease in the scattered light intensity upon hydrothermal treatment indicates a reduction in the particle number concentration at the expense of an increase in the particle size. This is interpreted as being due to an Ostwald ripening mechanism. A second discrete particle population (which coexists with the subcolloidal particles) is detected after ca. 9.5 h of hydrothermal treatment, at which time the average particle size is ca. 12 nm. The two particle populations (subcolloidal particles and the growing crystals) present in suspension before sampling at 9.5 h cannot be distinguished due to the resolution limitations inherent in the light-scattering technique. Deconvolution of intensity data collected during the time interval, corresponding to particle sizes of ca. 3 to 10 nm, indicates that the large-sized particles (crystalline silicalite-1) have as their origin subcolloidal particles initially present in solution before hydrothermal treatment. There are indications therefore that certain subcolloidal particles may possess a short-range structure such that they increase in size upon hydrothermal treatment and may thus be termed zeolitic nuclei. The initial growth of the growing crystals may be described as being a non-linear function of time until an average particle size of ca. 20 nm is attained, after which linear growth at a rate of 0.72 nm/h was recorded.
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| 7. |
- Schoeman, Brian J., et al.
(författare)
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A study of the initial stage in the crystallization of TPA-silicalite-1
- 1996
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Ingår i: Zeolites. - 0144-2449. ; 17:5-6, s. 447-456
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Tidskriftsartikel (refereegranskat)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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| 8. |
- Schoeman, Brian J.
(författare)
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Analysis of the nucleation and growth of TPA-silicalite-1 at elevated temperatures with the emphasis on colloidal stability
- 1998
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Ingår i: Microporous and Mesoporous Materials. - 1387-1811 .- 1873-3093. ; 22:1-3, s. 9-22
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Tidskriftsartikel (refereegranskat)abstract
- The question as to whether silicalite-1 grows via an aggregation of smaller particles or similar sized particles has been addressed by considering the fundamentals governing colloidal stability - the quantitative theory underlying colloidal stability being given by the extended Derjaguin-Landau and Verwey-Overbeek (DLVO) theory. Application of the extended DLVO theory to discrete colloidal particles in silicalite-1 precursor sols shows that a net repulsive interactive energy exists between the negatively charged particles. The thermal energy of the colloidal particles (1/2kT at 373 K, the crystallization temperature) is not sufficient to overcome the net repulsive energy barrier. The extended DLVO theory has been applied to two growth scenarios with similar results: growth by aggregation of particles of very different sizes and growth by aggregation of similar sized particles. The significance of these conclusions is that a proposed growth mechanism of MFI type zeolite (growth by aggregation of subcolloidal particles) is deemed not to be a reasonable description of molecular sieve growth. The conclusion that the colloidal crystals are stable with respect to aggregation is supported by experimental observations. The ideas presented in this study are based upon crystallization of molecular sieves from clear solutions in the presence of quaternary ammonium cations that play a significant role in the stabilization of the colloidal crystals. Extension of the ideas presented shows that the extended DLVO theory is equally applicable to wholly inorganic heterogeneous systems.
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| 9. |
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| 10. |
- Schoeman, Brian J.
(författare)
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Homogeneous nature of clear TPA-silicalite-1 precursor solutions
- 1997
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Ingår i: Microporous Materials. - 0927-6513 .- 1873-3107. ; 9:5-6, s. 267-271
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Tidskriftsartikel (refereegranskat)abstract
- The polymeric silica present in clear tetrapropylammonium (TPA) silicate solutions, with the molar composition 9TPAOH:25SiO2:480H2O:100ethanol and from which discrete colloidal crystals of TPA-silicalite-1 may be synthesized, has been characterized with respect to particle size using a modified method in which the reaction rate of monomeric silica with molybdic acid was recorded. The estimated particle size of 2.8 nm obtained with this method agrees well with previous studies of the sub-colloidal fraction wherein cryo-TEM and high effect laser light scattering techniques were employed.
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| 11. |
- Schoeman, Brian J., et al.
(författare)
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Nanoparticles of microporous materials
- 1999
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Ingår i: Nanostructured materials. - 0965-9773 .- 1872-9150. ; 12:1, s. 49-54
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Tidskriftsartikel (refereegranskat)abstract
- The hydrolysis and condensation of tetraethoxy silane in the presence of structure directing agents, such as the tetraalkylammonium cations, results in the formation of microporous siliceous particles with average sizes in the range 2 to 5 nm, the size being a function of the hydrolysis conditions and the composition of the mixture. Increased dilution favors larger particles while increased pH results in smaller particles. The option of isomorphous substitution of silicon for either alumina or titania is also shown to result in nano-sized particles were the substituted metals exist in the tetrahedral coordination
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| 12. |
- Schoeman, Brian J., 1961
(författare)
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Synthesis and Properties of Colloidal Zeolite
- 1994
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Several zeolite phases, zeolite N-Y, N-A, ZSM-2, (hydroxy)sodalite and TPA-silicalite- 1, have been synthesized in the form of stable suspensions of discrete colloidal crystals wherein the average particle size is less than 150 nanometers and in certain cases, less than 50 nanometers, with a narrow particle size distribution, PSD. Clear homogeneous tetraalkylammonium (alumino)silicate solutions containing controlled amounts of alkali metal cations are hydrothermally treated at 100 °C without stirring and under reflux. The zeolite sols display the typical colloidal characteristics of Tyndall light scattering, susceptibility to elevated electrolyte contents and a low rate of sedimentation. The absence of a solid amorphous aluminosilicate in the synthesis solution throughout the course of the crystallization permits the use of analysis techniques common within the field of colloidal science, i.e. dynamic light scattering and ultracentrifugation. Growth rates, particle sizes and the corresponding PSD, could be determined with a high degree of precision - the two latter parameters were confirmed by electron microscopy and XRD peak broadening. The colloidal zeolite crystals are XRD crystalline and possess a high degree of stability under prolonged hydrothermal treatment. The colloidal nature of the zeolite is illustrated by the fact that the particles in aqueous suspension are coagulated by excess amounts of electrolyte and are not compatible with organic solvents. The ion-exchange of colloidal zeolite may be achieved however by the use of ion-exchange resins and the sols may be transformed to organosols. The topics of crystallinity, structural order and solubility are adressed. The method of direct analysis has been employed to monitor the course of crystallization as well as the process of particle size tailoring. Furthermore, the mechanism of zeolite crystal growth has been investigated by applying experimental data to the classical equations describing crystal growth.
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| 13. |
- Schoeman, Brian J., et al.
(författare)
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The growth of sub-micron films of TPA-silicalite-1 on single crystal silicon wafers from low-temperature clear solutions
- 1997
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Ingår i: Zeolites. - 0144-2449. ; 19:1, s. 21-28
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Tidskriftsartikel (refereegranskat)abstract
- The direct synthesis of thin films of crystalline silicalite-1 upon single crystal silicon wafers at a crystallization temperature of 100°C has been investigated by varying the composition of the clear tetrapropylammonium (TPA) silicate synthesis solutions. Synthesis mixture compositions known to yield monodisperse colloidal crystals of TPA-silicalite-1 upon hydrothermal treatment as well as those reported to yield silicalite-1 films at higher temperatures have been found not suitable for the preparation of silicalite-1 films at 100°C. Lower crystal growth rates and smaller thicknesses of the gel film that forms on the wafer at this temperature decrease the tolerance to alkalinity, resulting in etching via the consumption of the gel layer before the growing crystals succeed in forming a closed film followed by the removal of the protective silicon oxide film on the wafer. Thin oriented silicalite-1 films with thicknesses in the range of 180 nm to 1 μm have been obtained by varying the alkalinity and water, the TPA, and the silica contents of the reaction mixture. Lower alkalinities and higher silica concentrations favor the formation of a thicker amorphous gel layer. Although increased TPA+ concentrations at constant alkalinity increase the number of nuclei that form on this layer, higher TPA+ concentrations have been observed to be required at higher alkalinities to achieve similar rates of nucleation. Rinsing the wafer surfaces initially with a 0.025 M TPAOH solution before rinsing with water and acetone produces cleaner surfaces free of post-treatment artifacts
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| 14. |
- Sterte, Johan, et al.
(författare)
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Thin molecular sieve films on noble metal substrates
- 1997
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Ingår i: Zeolites. - 0144-2449. ; 18:5-6, s. 387-390
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Tidskriftsartikel (refereegranskat)abstract
- A novel method whereby dense thin microporous films less than 300 nm thick may be prepared on noble metal substrates is exemplified by the deposition of silicalite-1 on a gold surface in a three-step process. The gold surface is modified with a silane coupling agent, gamma-mercaptopropyltrimethoxysilane, after which the silane is hydrolyzed to yield a negatively charged interface. Positively charged colloidal silicalite-1 crystals are readily adsorbed as a monolayer upon this surface and are induced to crystallize further in a separate hydrothermal treatment step to yield a dense thin silicalite-1 film.
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| 15. |
- Valtchev, Valentin, et al.
(författare)
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Preparation and characterization of hollow fibers of silicalite-1
- 1996
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Ingår i: Zeolites. - 0144-2449. ; 17:5-6, s. 408-415
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Tidskriftsartikel (refereegranskat)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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