| 1. |
- 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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| 2. |
- Schoeman, B.J., et al.
(författare)
-
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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| 3. |
- Schoeman, B.J., et al.
(författare)
-
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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| 4. |
- 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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