Secondary growth method, zeolite

Another recent modification to the secondary growth method as applied to MFI-type zeolite membranes is to perform the hydrothermal growth on the seeded... 

Huang AS, Lin YS, and Yang WS. Synthesis and properties of A-type zeolite membranes by secondary growth method with vacuum seeding. J Membr Sci 2004 245 41-51. 

Li G, Kikuchi E, and Matsukata M. The control of phase and orientation in zeolite membranes by the secondary growth method. 

These experimental results indicate that hierarchical pore zeolite materials can be prepared through a secondary growth method by using wood tissue as the template. Because the woods used are easily obtained, and the product overcomes the limitation of the single micropore of zeolites on mass transfer, there are potential applications in the adsorption and catalysis fields. It also provides a precedence for the further exploration of this kind of material. 

All the above mentioned high perm-selectivity of zeolite membranes can be attributed to the selective sorption into the membranes. Satisfactory performance can be obtained by defective zeolite membranes. Xylene isomers separation by zeolite membranes compared with polymeric membranes are summarized in Table 15.4. As shown, zeolite membranes showed much higher isomer separation performances than that of polymeric membranes. Specially, Lai et al. [41] prepared b-oriented silicalite-1 zeolite membrane by a secondary growth method with a b-oriented seed layer and use of trimer-TPA as a template in the secondary growth step. The membrane offers p-xylene permeance of 34.3 x 10 kg/m. h with p- to o-xylene separation factor of up to 500. Recently, Yuan et al. [42] prepared siUcalite-1 zeolite membrane by a template-free secondary growth method. The synthesized membrane showed excellent performance for pervaporation separation of xylene isomers at low temperature (50°C). 

Figure 3.2 Secondary growth method for the synthesis of zeolite membranes.

Abstract Zeolite membranes, with their weU-defmed pore size, and high thermal and chemical stability, can be used as membrane reactors. However, the development of zeohte membrane reactors requires the preparation of defect-free membranes. Different methods for their preparation have been develof>ed, including in situ and secondary growth methods. The secondary growth method has several advantages over the in situ method, such as easier operation, higher controllability in crystal orientation, a thicker microstructnre and a higher level of reproducibility. 

Zeolite membranes are generally synthesized as a thin, continuous film about 2-20 xm thick on either metallic or ceramic porous supports (e.g., alumina, zirco-nia, quartz, siHcon, stainless steel) to enhance their mechanical strength. Typical supported membrane synthesis follows one of two common growth methods (i) in situ crystallization or (ii) secondary growth. Figure 10.2 shows the general experimental procedure for both approaches. 

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... 

There are three main routes to synthesize continuous supported zeolite membranes the in-situ direct crystallization , the dry or wet gel conversion method and the seeding and secondary growth . 

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. 

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