Zeolite nucleation and growth

02-0-01 - Small angle X-ray scattering on TPA-Silicalite-1 precursors in clear solutions influence of silica source and cations 

02-0-02 - Nucleation processes in zeolite synthesis revealed through the use of different temperature-time profiles 

Centre for Microporous Materials, UMIST, colin.cundy umist.ac.uk, UK 

A comparison between thermal and microwave syntheses of colloidal silicalite-1 has provided a clear demonstration of the separate contributions of room-temperature ageing, heating rate and synthesis temperature to the nucleation process. At ageing times up to about 50 days, the product crystal size obtainable from a single synthesis composition is sensitive to reaction temperature and heating rate. After this time, ageing-generated proto-nuclei are so numerous that the normal self-nucleation of the reaction mixture is suppressed and the product crystal size is independent of reaction conditions. There is a limit to the number of crystals which can be nucleated and this is an intrinsic property of the system. 

02-0-03 - High yield synthesis of colloidal crystals of faujasite zeolites 

---

SUMMARIES of POSTER PRESENTATIONS 02 - Zeolite Nucleation and Growth (Monday)... 

NOx reduction by propene NOx reduction with hydrocarbons NOx sorption, metal zeolites NOx storage, metal zeolites Noble metal zeolite Noble metals/MCM-4I Nonasil, synthesis Non-ionic surfactant Nu-86 synthesis NU-88, cracking catalyst NU-88, hydroconversion catalyst NU-88, pore topology nuclear waste Nucleation agent, zeolitic Nucleation and growth Nucleation, FAU Nucleation, LTA Nucleation, zeolites Nucleophilic Substitution... 

The first hypothesis, proposed by Breck and Flanigen [52,55], to account for the crystallization of aluminosilicate zeolites affirms that it proceeds through the formation of the aluminosilicate gel or reaction mixture, and the nucleation and growth of zeolite crystals from the reaction mixture. This initial model has been almost abandoned, and replaced by the hypothesis of Barrer and others [53,55], In the framework of this hypothesis, it is assumed that the formation of zeolite crystals occurs in solution. Accordingly, in this model, the nucleation and growth of crystalline nuclei are a consequence of condensation reactions between soluble species, where the gel plays a limited role as a reservoir of matter. 

In this section, we detail our results on the nucleation and growth of zeolite crystals with Si/Al ratios between 1 and 2. Various perturbations, including the effects of reaction time, D20, CH30H and C2Hs0H on the zeolite process are examined. A narrow range of starting compositions and reaction conditions are chosen, so that the effects of the perturbations can be evaluated with a minimum set of variables. These results are discussed in the context of present theories of zeolite growth in the next section. 

Factorial experiments can successfully serve to determine significant synthesis parameters for aluminophosphates and zeolites. Future studies will also focus on the underlying mechanisms of the nucleation and growth of high-silica zeolites out of alkaline-free ammonia containing reaction mixtures. 

Methods. The crystallization of silicoaluminate mixtures into zeolite omega in the temperature range 105-130°C was performed in the presence of a structure-directing mixture (SDM) (10,11). The method gives minimum overlap between the nucleation and growth steps as indicated by the very homogeneous distribution of size of the crystals in the final product. The use of kaolinite as the aluminium source presents two main advantages (10). First, the low solubility of the clay under the crystallization conditions prevents the formation of a gel. Second, under the low supersaturation levels achieved, secondary nucleation is hindered. 

The metastable form which preferentially crystallizes could then transform to a more stable phase (i.e. zeolite Y - zeolite X or zeolite A - hydroxysodalite). Nucleation and growth rates would then become the limiting factors in determining how long this would take. For example, when synthesis conditions are chosen to produce zeolite A, the rate of hydroxysodalite formation is dependent on five variables. These variables and their effect on the conversion of zeolite A to hydroxysodalite are as follows ... 

The most successful approach to control membrane formation involves segregation of the processes of crystal nucleation and growth [24]. The so-called ex situ or secondary (seeded) growth methods, unlike the direct synthesis procedures just discussed, include a first step in which a closely packed layer of colloidal zeolite crystals, synthesized homogenously, is deposited onto... 

---