Zeolites oriented crystals

Figure 4.14 Image of cluster of oriented zeolite crystals. Because the TEM image shows a projection through the entire thickness of the cluster, it can be seen that the crystals terminate within the cluster leaving internal mesoporosity.

Activation of zeolites is a dehydration process aceomplished by the application of heat in a high vacuum. Some zeolite crystals show behavior opposite to that of activated carbon in that they selectively adsorb water in the presence of nonpolar solvents. Zeolites can be made to have specifie pore sizes that will increase their seleetive nature due to the size and orientation of the molecules to be adsorbed. Moleeules above a specific size could not enter the pores and therefore would not be adsorbed. 

Large zeolite crystals with dimensions of tens and hundreds of micrometers have proven to be irreplaceable as model materials for reactivity and diffusion studies in the field of zeolite science and heterogeneous catalysis [1-3], These large crystallites often possesses complex structures consisting of several intergrown subunits and since the pore orientations of the different elements are not always aligned, this phenomenon can have a considerable effect on the accessibility of the pores in different crystallite regions [4]. 

After embedding the zeolite crystals thus oriented into a gas-tight matrix (e.g., glass, metal, epoxy resin), by careful abrasion selective crystal faces can be opened for selective sorption uptake kinetics. Abrasion of the crystal arrangement shown in Fig. 22 gives the sample shown in Fig. 23. The sorption uptake kinetics on this sample is exclusively controlled by the mobility in the length direction of the ZSM-5 crystal, D,. For ZSM-5 crystals hori-... 

It will be clear that in case of onedimensional zeolites the orientation of the zeolite crystals should be such that the channel direction is perpendicular to the membrane layer configuration. Figure 2 shows in a schematic way several configurations in which zeolites govern - or contribute to -membrane permeation. 

Silicalite-1, are known to grow relatively easy on various surfaces, Moreover, some principles have been developed by Jansen et al. [8] how to govern the zeolite crystal orientation with respect to the support. Two articles with a reviewing character [7,44] as to MFI-based membranes are recommended to the reader. 

FIGURE 10.8 (See color insert following page 588.) Schematic representation of a c-oriented MFl zeolite crystal. 

In general, the properties and separation abilities of the resulting membranes depend on the synthesis procedure. The amount of zeolitic material, support composition, penetration and adhesion to the support, orientation of the zeolite crystals, the density and distribution of nonzeolitic pores (i.e., intercrystalline voids), crystal boundaries, and the thickness of the zeolite layer are the main variables which affect the quality of the obtained membrane. 

Wang Z and Yan Y. Oriented zeolite MFI monolayer films on metal substrates by in situ crystallization. Micropor Mesopor Mater 2001 48 229-238. 

Single-layer zinc-phosphate zeolite crystals were grown with more than 90% of their (111) faces oriented to a gold-coated silicon surface. Sudi oriented zeolite films might find application as membrane catalysts or as specific chemical sensors [66]. 

Z. Wang and Y. Yan, Oriented Zeolite MFI Monolayer Films on Metal Substrates by In Situ Crystalization. Microporous Mesoporous Mater., 2001, 48, 229-238. 

Figure 9.12 Schematic representation for loading of donor and acceptor dye molecules in zeolite L crystal structure. The dark rectangular squares at the two ends of the channel indicate the presence of acceptor dye, whereas the blank rectangular squares in the middle indicate the presence of donor dye. This diagram also shows the dye molecules and their electron transition moment orientation after the central part of the zeolite crystal is enlarged. Reproduced with permission from [68], Copyright (2000) NOVA Science Publishers...

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). 

Chemical sensors are important for industrial process control and environmental monitoring. They also find critical use in medical and defense applications. Two of the critical performance parameters of a sensor are selectivity and sensitivity. When integrated with an appropriate sensor platform (e.g., quartz crystal microbalance or electrochemical quartz crystal microbalance), a zeolite film can improve the selectivity and sensitivity of the sensor due to its ability to selectively adsorb a component out of a mixture. A large number of publications have appeared in this area, and an in-depth review on zeolite films for chemical sensor applications up to 1996 was provided as a section in a recent review by Bein. " Here, we highlight a recent development of selective chemical sensor based on continuous oriented zeolite films. 

The zeolite membranes quality and the development of methods able to rapidly identify the defects are key-factors for large scale applications. A large number of static and dynamic methods can be used to evaluate the quality of membranes [125J. Static methods allow to study the physico-chemical characteristics of the membrane material. Dynamic methods allow the detection of defects affecting the transport properties and consequently the membrane performance. For zeolite membranes the quality of the adhesion between the zeolite layer and the support, the orientation of zeolite crystals, the layer thickness, and the number and quality of... 

So far, only few studies have been reported on the use of porous zeolite films for optical applications. While significant efforts have been extended towards the encapsulation of various dye molecules into zeolite crystals or powders (see section 2.2), integration of such systems into thin films has not been pursued by many groups yet. As one of the few examples reported so far, we discuss the inclusion of oriented hemicyanine dyes into thin zeolite films aimed at Second Harmonic Generation (SHG).[106] The zeolite film plays the important role... 

As an alternative way to study diffusion anisotropy in zeolite crystallites, it is possible to analyze the shape of the NMR signal attenuation with increasing field gradient intensity. Since in a powder sample all orientations of the zeolite crystals with respect to the field gradient direction are possible, the signal attenuation results as a superposition of exponentials of the type of Eq. 9 with diffusivities determined by the orientation of any individual crystallite. All information about the diffusion tensor must be contained, therefore, in the shape of the echo attenuation. 

Transport inside the zeolite crystals occurs through diffusion inside the zeolite pores. Although it is known that diffusion in zeoHtes is generally anisotropic [40], the random orientation of the crystals inside the reactor justifies the approximation that micropore diffusion can be described as an isotropic process. A mass balance for the zeohte crystals yields for the adsorbed phase concentration Cx in the crystals... 

The aim of using the ex situ techniques is to obtain a better control of the microstructure and a preferential orientation of the crystals in the membrane with a shortened crystallization time rendering highly selective and permeable membranes. Preferential orientation is needed, not only for separation purposes, when high fluxes are required, but also for size-selective chemical sensors (see Section 11.6.6.2). Due to the anisotropy in the pore geometry of the zeolite crystals, an orientation that shows the widest channels in the direction of the flux is... 

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