Vapor-Phase Transport Method

The vapor-phase transfer (VPT) synthesis method consists of two steps covering the support with a synthesis gel and crystallization of the dried gel in saturated steam at a similar temperature or a mixture of steam and a structure-directing agent (SDA) under autogenous pressure [22-24]. Since one of the nutrient sources is limited to the support surface, zeolite membrane formation will be limited to the support surface. With this method, the membrane thickness can be well controlled since the amount of nutrient for growing zeolite is controlled directly by the amount of gel applied. However, the crystals produced are most likely randomly oriented. Furthermore, since the density of gel is much lower than that of zeolite crystals, the crystallization process 

Cioss-section at the location of the four exit holes 

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Gradient sublimation has been used for many years as a means to purify materials and many variations exist and have been proven effective for organic purification. A detailed description of the vapor phase transport method applied to organic materials was given by Ch. Kloc,... 

Supported zeolite membranes have been prepared using numerous procedures [4] such as alignment of crystals in electrical fields, electroplating, self-assembly, growth on organic molecular layers, covalent linkages, hydrothermal synthesis (in situ and ex situ), hydrothermal method microwave heating assisted, dry gel method (vapor-phase transport method and steam-assisted crystallization), synthesis at the interface between two fluid phases, etc. 

The so-called dry gel method [20-22] is another alternative for membrane synthesis where vapors containing templates (i.e., amines) and water are employed to crystallize silica or silica-alumina layers previously deposited onto the support (vapor-phase transport method) or where steam is used to crystallize silica and template or sUica-alumina and template dry layers previously deposited onto the support (steam-assisted crystallization). Using this approach, the reactant consumption is clearly diminished, an important issue for scale-up purposes on the other hand, synthesis time is delayed due to transport-controlled phenomena. 

Matsufuji T, Nishiyama N, Matsukata M, and Uyama K. Separation of butane and xylene isomers with MFI-t5fpe zeolitic membrane synthesized by a vapor-phase transport method. J Membr Sci 2000 178 25-34. 

N. Nishiyama, K. Ueyama, and M. Matsukata, Synthesis of Defect-free Zeolite-alumina Composite Membranes by a Vapor-phase Transport Method. Microporous Mater., 1996, 7, 299-308. 

Specifically for the preparation of zeolite films, zeolite nanoparticles could be used directly to form zeolite films by self-assembly. They could also be combined nicely with the seeded growth and vapor phase transport method to produce high quality polycrystalline zeolite films. There is clear evidence that small zeolite nanoparticles are preferred for producing compact continuous films. 

Nishiyama. N. Ueyama, K. Matsukata. M. Synthesis of defect-free zeolite-alumina composite membranes by a vapor—Phase transport method. Microporous Mat. 1996. 

Titanium dioxide occurs in nature in three crystalline forms, rutile, anatase, and brookite. The crystal structures are shown in Fig. 1 [24]. Some of the physical properties are given in Table 1. Much of the fundamental work on Ti02 has been carried out with rutile single crystals because these are relatively straightforward to produce. Anatase single crystals are more difficult to produce, and some work has been carried out with natural crystals but it is possible to grow such crystals with the use of the vapor-phase transport method [25]. Photocatalytic reactions... 

Bulk ZnO grown by vapor-phase transport method n/a 6.0 xlO 205 [159]... 

In the phases of composition ZrsSbsZ, obtained either by arc melting, vapor-phase transport, or metal flux methods,... 

To avoid pinholes and cracks, Matsukata et al. [46] optimized the density of the precursor phase by using a dry gel instead of a solution. This was achieved through the use of a slipcasting method. A dry porous alimina plate of 2.2 cm was dipped into a gel whereby the support surface was covered with an amorphous aluminosilicate phase. After drying, the sample was exposed to template vapor, triethylamine, ethylenediamine and steam (the Vapour-phase Transport Method). However, the zeolite layer (20 pm thick) consisted of a mkture of ZSM-5 and Ferrierite. Nitrogen and oxygen permeation were studied. 

In 1990, Xu et al. first reported the transformation of a dry aluminosilicate gel to crystalline MFI by contact with vapors of water and volatile amines, which was named dry gel conversion (DGC).[99] Since then, this method has been extensively studied and a large number of microporous materials with new compositions and structures were prepared. Generally, DGC can be divided into vapor-phase transport (VPT) and steam-assisted conversion (SAC) according to the volatility of the SDAs. For volatile SDAs such as ethylenediamine, a mixture of water and SDA was poured into the bottom of the autoclave and then a dry gel, which does not contain any SDAs, was placed over the liquid surface. Water and SDAs were vaporized at elevated temperature (150 200 °C), reached the dry gel, and initiated the crystallization, which was called VPT. Less volatile SDAs such as tetrapropylammonium hydroxide were usually involved in the dry gel. Only water steam is supplied during the reaction, which was called SAC. 

The invention of the so-called vapor-phase transport (VPT) method [164] is another argument for the existence of a solid-phase transformation mechanism (Figure 12.11). A dried aluminosilicate gel (423-473 K) was contacted only with vapors of an aqueous SDA phase. The solution is placed at the bottom of an autoclave, while the dry gel was on top of a supporting device half-way up the autoclave [165]. In a variation of the method denoted as steam-assisted crystallization (SAC), the dry SDAactivated with steam [166]. The method gives high yields of crystals and is useful for several zeolite phases such as Al-and Ti-Beta (BEA), TS-1 (MFI), TS-2 (MEL), and EU-1... 

Other methods of preparation include chemical vapor deposition (CVD) and electrochemical methods. The latter are used to form thin films and protective coatings as well as battery electrodes. Since this is generally a low-temperature method, the structure can differ from that of the same material prepared at a higher temperature. It can be disordered or amorphous. Chemical vapor deposition involves vapor phase transport of volatile organometallics... 

Chemical vapor-phase transport is one of the earliest methods developed for the synthesis of new materials [19-26]. The method can also be used for the growth of high-quality single crystals or for the purification of solid-state compounds. The method has been used to synthesize a wide range of LMCs. 

There are basically two approaches to synthesize supported zeolite membranes liquid-phase synthesis and vapor-phase transport (or dry-gel conversion) methods [2,12], The liquid-phase-synthesis approach is to bring the surface of a porous support in contact with a zeolite synthesis solution (sol or gel) and keep the system under controlled conditions so that the zeolite can nucleate and grow to a continuous film on the support surface. 

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