Silicalite layer thickness

Finally Smith/Keizer et al. [99] reported the S3mthesis of a continuous silicalite layer (thickness 1 pm) on top of hollow-fibre carbon supports using a... 

Defect-free zeolite membranes have so far only been produced for membranes of the MFI (silicalite type) with thicknesses of about 50 im on stainless steel supports and 3-10 pm on alumina and carbon supports. They are produced by in situ methods of zeolite crystals grown directly on the support system. There are some reports of formation of defective membranes with, e.g., zeolite A. Much more research is needed to widen the range of available zeolite membrane types especially small and wide pore systems. The permeance values of the defect-free membranes is lower than that of the amorphous membranes (see Chapter 6) and to improve this the layer thickness must be decreased together with improving the crystal quality (no impurities, no surface layers, high crystallinity, crystal orientation) and microstructure (grain boundary engineering). 

As described in the previous section, the silica-alumina catalyst covered with the silicalite membrane showed exceUent p-xylene selectivity in disproportionation of toluene [37] at the expense of activity, because the thickness of the sihcahte-1 membrane was large (40 pm), limiting the diffusion of the products. In addition, the catalytic activity of silica-alumina was not so high. To solve these problems, Miyamoto et al. [41 -43] have developed a novel composite zeohte catalyst consisting of a zeolite crystal with an inactive thin layer. In Miyamoto s study [41], a sihcahte-1 layer was grown on proton-exchanged ZSM-5 crystals (silicalite/H-ZSM-5) [42]. The silicalite/H-ZSM-5 catalysts showed excellent para-selectivity of >99.9%, compared to the 63.1% for the uncoated sample, and independent of the toluene conversion. 

The permeation examples that are given here are based on own work and have been published in different articles [61-66]. The membrane used is of the asymmetric type and consists of a 40 pm thick layer of intergrown silicalite-1 crystals on a 3 mm thick layer of highly porous sintered stainless steel. The geometric surface area amounts to 3 cm. Stainless steel has the advantage of easy mounting in all types of equipment which facilitates practical application compared to ceramic supports. 

The synthesis and characterisation of silicalite-1 membranes on porous alumina ceramic supports have been described here. The growth of the silicalite-1 membrane could be optimised by controlling the hydrothermal synthesis conditions. It has been shown that by controlling the synthesis conditions it is possible to optimise the growth and structure of silicalite-1 membranes. Thus at lower synthesis temperatures (150 °C), the growth of silicalite inside the macro-pores of the ceramic support is favoured. At higher temperatures (190 °C), thick, well crystallised zeolite layers develop from the surface of the support. A more stable membrane is... 

Jia/Noble and coworkers [87,88] reported the successful synthesis of silicalite membranes on y-alumina composite supports using an interesting modification of the in situ crystallisation method. The support consisted of a short a-alumina tube coated on the inside with a 5 pm thick y-alumina film with an average pore diameter of 5 nm, commercially available from US Filter. The precursor solution was put into the support tube after plugging both ends with teflon and the filled tube was then placed in a teflon-lined autoclave. Hydrothermal treatment was carried out at 180°C for 12 h. After removal from the autoclave and washing the formed zeolite layer with water, the procedure was repeated with the tube inverted from its previous orientation to obtain a uniform coating. As reported by Vroon et al. [82,84,98], Jia/Noble [88] also concluded that at least two synthesis steps are necessary to obtain defect-free membranes. 

Jia and Noble and co-workers et al. reported a 10 pm thick silicalite membrane on a composite support of a-Al203 [27,77]. Finally, Xiang and Ma [76] partially filled the pores of a microporous a-alumina support with ZSM5 crystals. All the authors used an in situ hydrothermal crystallisation method to grow directly polycrystalline zeolite layers. 

Coating the support with the seeds is a critical task. Different strategies are proposed in the literature. The supports can be seeded by simple contact (deep coating for a few minutes) with a suspension of zeolite crystals at an appropriate pH, and subsequent washing to keep only a surface monolayer [51], Fig. 2 shows a thick layer of silicalite-1 seeds on (XAI2O3 support after 3 h contact with the seed suspension. 

Fig. 2. aAl203 support (200 nm pore size) covered with a thick and close pack layer of silicalite-1 seeds (3 h contact time) and dried for 6 hours at 155 °C [101 ]. 

In order to avoid the infiltration of seeds in the support and to develop ultra-thin membranes (typically 500 nm thick) with a high permeability, a masking techniques has been recently developed in Lulea University [111]. A solution of poly methyl methacrylate (PMMA) in acetone was applied and dried on the support top surface. The interior of the support was subsequently filled with wax and the protective PMMA layer was dissolved in acetone. The masked support was then seeded with a monolayer of silicalite-1 crystals before being submitted to the classical hydrothermal and calcination steps. 

A detailed study of the growth process and the structural evolution of silicalite-1 (MFI) films was undertaken with the aid of grazing incidence synchrotron X-ray diffraction. [65] The diffraction data of the adsorbed and grown zeolite films at different incident and exit angles reflect the distribution of the crystal orientation along the film thickness. The films were prepared via assisted adsorption of nanoscale MFI seed crystals, followed by calcination and subsequent hydrothermal synthesis on the seed layers. The adsorbed (multi-) layer of seed crystals consists of randomly oriented crystals. With progressing hydrothermal growth, the film surface becomes smoother and a preferred crystal orientation with the b-axis close to vertical to the substrate develops. 

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