Zeolite thicknesses

Synthetic calcium alumino-silicate (zeolite) having an effective pore diameter of 0,5 nm CO is adsorbed strongly 5A usually gives the best results of all synthetic zeolites thick and thin film columns are available. 

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. 

O2, and hydrocarbons, leading to a NH3 selective sensor. Arguments in favor of using a zeolite-based sensor in NH3 detection for automotive apphcations are low cost, high temperature stability, and suitability for use in thick-film technology, of common use in the automotive industry [72]. The sensors were tested on an engine test bank and the authors claim that the sensor itself meets aU the technological and economical demands of the automotive industry [73]. 

Reactor type Single-channel chip micro reactor Zeolite catalyst layer thickness 3 pm... 

Fig. 7.12 SEM images ofthe diatomite substrate through sequential assembly of positively-at low (A) and high (B) magnification. The di- charged PDDA and negatively-charged zeolite atomite has a disk-like shape about 1.2 pm in nanocrystals (C). The inset in (C) shows an thickness and 20-40 pm in diameterwith a nearly enlarged view of the nanoparticle-modified sur-regular array of submicrometer-sized pores face. (Reprinted from [129] with permission of (about 300-500 nm). SEM images of diatomite Wiley-VCH.) coated with three layers of zeolite ( nanocrystals...

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.

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. 

Reports on mixed-matrix membranes in the Hterature mainly focus on dense films. Mixed-matrix dense film has a symmetric structure and a thickness of more than 20 tm for most studies. Although dense films are not commercially attractive, they are used to measure the intrinsic separation properties including selectivity and permeability of the mixed-matrix membranes. Therefore, promising polymer and zeolite materials for making asymmetric mixed-matrix membranes for a particular separation can be identified through dense film study. 

Epitaxial growth of overlayer on the plane is envisaged in the field of material physics. Epitaxy of a relatively thick layer measurable In microns Is explained primarily by the matching of crystal planes. We assume a similar epitaxy for the deposition of silica on the external surface of zeolites, although direct observation by electron microscopy is extremely difficult. 

In many layer structures, such as clay minerals, the extent of lattice adjustment on entry of guest molecules is intermediate between the behaviors of zeolites and of clathrates. The layers remain intact, but the distance between them changes substantially (8). For water-free smectite crystals the d(001) distance is 9.4 A. The van der Waals diameter of a water molecule is 2.8 A so that, in batavite, for example, the water layer in the Na form is about 14.8 — 9.4 = 5.4 A thick, corresponding with two monolayers. 

Thick clastogenic series with zonal distribution of zeolites and clay minerals... 

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