Secondary Growth Method

Since the location and density of nucleation sites can be well controlled in the secondary growth method, the nature of the support is less important for membrane formation than in the in-situ crystallization synthesis method. Furthermore, the elimination of the in-situ nucleation 

In order to obtain high-quality zeolite membranes, a variety of techniques have been developed to facilitate nucleation and crystal growth on the substrate and tailor the interactions between the zeolite layer and the substrate [1]. 

Good adherence of the zeolite seed crystals as well as the zeolite film to the support is important to guarantee the mechanical, thermal, and chemical stability of the composite membrane. The adherence of the zeolite crystals to the support surface is to a large extent determined by the hydrophilicity of the support surface. Treatment of the support with NaOH may increase the number of surface hydroxyl groups, and therefore will impart the support with more nucleation points as well as sites where crystals could adhere by means of Van der Waals interactions and H-bonding. 

The application of a thin, imiform, and continuous layer of seed crystals on the support determines the successful synthesis of a thin, defect-free zeolite film. The simplest and most often used method is to apply seed crystals to the substrate with mechanical rubbing [14]. Slip-coating [15] and dip-coating [16, 17] the substrate in a suspension of zeolite particles, followed by drying and calcination, are also used to seed the support surface, but the process often has to be repeated a few times in order to ensure a sufficiently high coverage of the support with zeolite seed crystals. Electrostatic deposition involves chaise modification of the substrate surface by 

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

Ex Situ Hydrothermal Synthesis or Secondary Growth Methods.274... 

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.

High-quality, thin (14 4m) MOF-5 membrane was prepared by Zhao et al. [166] through a secondary growth method. The MOF-5 membrane was composed of one to two layers of MOF-5 crystals of about 5-20 tm in size. The MOF-5 membranes were permselective for CO2 over N2 or H2 under experimental conditions. The MOF-5 membranes showed a separation factor for CO2/H2 of close to 5 with a feed CO2 composition of 82% and a separation factor for CO2/N2 greater than 60 with a feed CO2 composition of 88% at 445 kPa and 298 K. 

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. 

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