| 1. |
- Persson, A.E., et al.
(författare)
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Synthesis of stable suspensions of discrete colloidal zeolite (Na, TPA)ZSM-5 crystals
- 1995
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Ingår i: Zeolites. - 0144-2449. ; 15:7, s. 611-619
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Tidskriftsartikel (refereegranskat)abstract
- Synthesis of zeolite ZSM-5 in the form of stable suspensions of colloidal crystals from clear homogeneous solutions has been achieved from synthesis mixtures with low sodium and high TPAOH concentrations. The ultimate size of the discrete crystals is in the range 130-230 nm with a narrow particle size distribution. The crystal growth was monitored by the direct method of dynamic light scattering. An increasing alumina concentration was found to decrease the crystal growth rate, the number of crystals produced, and the ZSM-5 yield. The final crystal size was found to increase with increasing alumina concentration in the synthesis solution. The number of crystals produced decreased with an increased alkalinity in contradiction to what has been found for colloidal TPA-silicalite-1. This together with the presence of the two competing processes, crystal growth and crystal dissolution, which are both catalyzed by hydroxide ions, suggests the presence of an optimum alkalinity (depending on the alumina concentration).
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| 2. |
- Persson, A-E, et al.
(författare)
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The synthesis of discrete colloidal particles of TPA-silicalite-1
- 1994
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Ingår i: Zeolites. - : Elsevier BV. - 0144-2449. ; 14:7, s. 557-567
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Tidskriftsartikel (refereegranskat)abstract
- Discrete colloidal particles of TPA-silicalite-1 with an average particle size of less than 100 nm and with a narrow particle-size distribution have been synthesized in clear homogeneous solutions. The factors influencing particle size have been investigated. High silica contents in conjunction with high alkalinities favor the synthesis of colloidal silicalite suspensions. Polymeric silica sources result in larger particles compared to those synthesized with tetraethoxy silane-presumably due to the different nucleation kinetics. It was found that the base concentration influences nucleation behavior in that high alkalinities result in the nucleation of a relatively large number of particles. The growth rates are constant for a given silica content irrespective of the alkalinity, at least within the alkalinity range studied. This result is ascribed to a rate-limiting surface reaction step that is a consequence of the synthesis mixture composition. The linear growth rates recorded are lower than those typically reported in the literature.
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| 3. |
- 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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| 4. |
- 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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| 5. |
- Schoeman, B.J., et al.
(författare)
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Analysis of the crystal growth mechanism of TPA-silicalite-1
- 1994
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Ingår i: Zeolites. - : Elsevier BV. - 0144-2449. ; 14:7, s. 568-575
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Tidskriftsartikel (refereegranskat)abstract
- The method of chronomal analyses (dimensionless time analyses) according to Nielsen has been applied to the growth of discrete colloidal particles (particle sizes of less than 100 nm) of TPA-silicalite-1 to gain information on the crystal growth mechanism. The increase in particle size in the range 30-95 nm was monitored by dynamic light scattering and yielded the result that the increase in crystal size is a linear function of synthesis time. The linear growth rate of the almost spherical particles at 100°C is 3.79 nm/h - a low value that is ascribed to the synthesis conditions employed. Furthermore, the particle number concentration was shown to be constant, indicating that no secondary nucleation event occurs during the growth process. The crystallization kinetics recorded in the temperature interval 80-100°C correlate with a first-order surface reaction controlled growth mechanism in which the corresponding apparent energy of activation is 42 kJ/mol. A diffusional mechanism as well as a compound growth mechanism in which both surface reaction and diffusion compete for rate control can be ruled out as being operative.
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| 6. |
- Schoeman, B.J., et al.
(författare)
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Colloidal zeolite suspensions
- 1994
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Ingår i: Zeolites. - : Elsevier BV. - 0144-2449. ; 14:2, s. 110-116
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Tidskriftsartikel (refereegranskat)abstract
- A method is presented whereby it is possible to synthesize colloidal zeolite sols containing discrete zeolite particles with an average particle size of less than 150 nm and with a narrow particle-size distribution. The zeolite sols exhibit typical colloidal characteristics such as Tyndall light scattering and a low rate of sedimentation and coagulation due to electrolyte concentrations in excess of the critical coagulation concentration. The effect of sodium on the crystallization kinetics, product distribution, and particle size has been investigated. Relatively high Na2O AL2O3 ratios favor the formation of large crystals of zeolite A. Low Na2O AI2O3 ratios result in colloidal crystals of zeolite Y, whereas intermediate ratios yield zeolite Y and/or zeolite A with particle sizes of ca. 100 nm. Midsynthesis addition of sodium allows for a fivefold increase in zeolite yield.
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| 7. |
- 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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| 8. |
- Schoeman, B.J., et al.
(författare)
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The synthesis of colloidal zeolite hydroxysodalite sols by homogeneous nucleation
- 1994
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Ingår i: Zeolites. - : Elsevier BV. - 0144-2449. ; 14:3, s. 208-216
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Tidskriftsartikel (refereegranskat)abstract
- Colloidal suspensions of discrete particles of microcrystalline hydroxysodalite have been synthesized from clear homogeneous solutions wherein the average particle size determined by dynamic light scattering is 37 nm with a narrow particle-size distribution. Particle-size analysis performed with various methods yield results that are in agreement. The growth-limiting nutrient has been identified as alumina. As a result, the midsynthesis addition of alumina results in further particle growth to 48 nm with the corresponding increase in zeolite yield. Hence, the fine control of particle size and thereby zeolite yield without the formation of a secondary particle population is shown to be possible. Direct measurements of zeolite yield and particle size allow for a detailed evaluation of the events that occur during crystallization.
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| 9. |
- 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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| 10. |
- 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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