Zeolite layers

Three different ways in which a zeolite membrane can contribute to a better sensor performance can be distinguished (i) the add-on selective adsorption or molecular sieving layer to the sensor improves selectivity and sensitivity, (ii) the zeolite layer acts as active sensing material and adds the selective adsorption and molecular sieving properties to this, and (iii) the zeohte membrane adds a catalytically active layer to the sensor, improving the selectivity by specific reactions. 

When NOj levels are measured electrochemicaUy, NO and NO2 can lead to opposing signals because NO is oxidized and NO2 tends to be reduced. Moreover, it is preferred to obtain a total NO, measurement instead of only one of the constituents. The latter can be achieved by catalytically equilibrating the feed with oxygen before contact with the sensor by coating an active zeolite layer on top or placing a active catalyst bed in front of the sensor. Both approaches have been demonstrated successfully with a Pt-Y zeohte as active catalyst [74, 75]. The additional advantage of the filter bed is a reduction in the cross-sensitivity with CO due to CO oxidation above 673 K. 

In a similar manner, Sahner et al. [76, 77] utilized a Pt-ZSM-5 layer to reduce the cross-sensitivity of a hydrocarbon (propane) sensor toward CO, propene, H2, and NO at 673 K. The zeolite layer was put on the sensor as a paste. The improved cross-sensitivity is attributed to selective oxidation of aU considered components except propane. Trimboli et al. [78] demonstrated the same concept by using a Pt-Y zeohte for the CO oxidation, maintaining the sensitivity for propane. 

The alkylation of phenol investigated over H-MCM-22, H-ITQ-2 and H-MCM-36 showed that the delamelation and pillaring did not improve the catalytic activity and this was explained on the secondary processes taking place during the preparation of the corresponding materials, and which strongly affect the total acidity and the acidity on the external surface. Also, the composition of the reaction products is not influenced to a considerable extent by product shape selectivity effects. This seems to show that the tert-butylation reaction preferentially proceed at (or close to) the external surface of the zeolite layers. 

Synthesis of zeolite Y in the presence of Gd(III) complexes of 18-crown-6 resulted not only in the encapsulation of the complex but the complex also served as a template for EMT polytype zeolite Y (Fig. 22b) (86). Feijen et al. described how the two different polytypes (the cubic FAU and the hexagonal EMT) can be formed (87). In the absence of an organic template, the FAU structure will form. If Na" "-18-crown-6 is present, it can be absorbed on the surface of the growing zeolite layer. This will influence the interconnection of the layers and, therefore, in the presence of this crown ether, the formation of the EMT framework may be favored. The difference between the pore window sizes is that in the EMT there are two different types 7.3 x 7.3 A in the [001] direction and 7.5 x 6.5 A perpendicular to the [001] direction. (The FAU has pore windows with 7.4 x 7.4 A in the [111] direction.)... 

A. Sayari, V.R. Karra and J. Sudhakar Reddy, Symposium on Synthesis of Zeolites, Layered compounds and other Microporous Solids, 209 National Meeting, Am. Chem. Soc. Anaheim (1995). 

The properties of the product depend on the bed geometry during treatment, and three distinct types of treatment are usually distinguished shallow bed (SB), the zeolite layer is less than 3 mm thick and is slowly heated to the activation temperature under vacuum normal bed (NB), the zeolite layer is thicker, but is also heated under vacuum deep bed (DB), a thick layer of zeolite is gradually heated under atmospheric pressure. The DB process gives the most stable product. 

During the last few years, ceramic- and zeolite-based membranes have begun to be used for a few commercial separations. These membranes are all multilayer composite structures formed by coating a thin selective ceramic or zeolite layer onto a microporous ceramic support. Ceramic membranes are prepared by the sol-gel technique described in Chapter 3 zeolite membranes are prepared by direct crystallization, in which the thin zeolite layer is crystallized at high pressure and temperature directly onto the microporous support [24,25],... 

It is evident that the ceramic membrane, which is represented in the XRD pattern (see Figure 10.6) by the amorphous component of the XRD profile, was covered by the AlP04-5 molecular sieve, since the crystalline component of the obtained XRD pattern fairly well coincides with the standards reported in the literature [107]. Consequently, the porous support was successfully coated with a zeolite layer, which was shaped by the hydrothermal process as previously described. Thus, a composite membrane, that is, an AlP04-5 molecular sieve thin film zeolite-based ceramic, was produced. 

X-ray and aluminium MAS NMR measurements were carried out on samples rehydrated in a desiccator over an aqueous NH,C1 solution. A portion of the zeolites synthesized with organic templates was heated for 5 h at 600 °C to remove organic compounds. The Na /H ion exchange was carried out at room temperature with an aqueous solution of 0.5 N HC1. The preparation of 1H MAS NMR samples was performed under shallow bed activation conditions in a glass tube of 5.5 mm inner diameter and 10 mm height of the zeolite layer. The temperature was increased at a rate of 10 K/hr. After maintaining the samples at the final activation temperature of 400 °C under a pressure below 10 L Pa for 24 hrs., they were cooled and sealed. 

Hydrogenation of 2-butene to butane Ft or CaA-zeolite layer Stainless steel (disk) Only trans-2 butene is selectively separated hydrogenated Suzuki, 1987... 

Zaspalis et al. [1991b] and Bitter [1988] utilized alumina membrane reactors containing Pt catalysts to examine dehydrogenation of n-butane to butene and 2-methylbutenes to isoprene, respectively. Both the conversion and selectivity improved by using the membrane reactors. The increase of conversion is about 50% in both cases. Moreover, Suzuki [1987] used stainless steel membranes and Pi or CaA-zeolite layer catalysts to perform dehydrogenation of isobutene and propene to produce propane. 

Zeolite layer mesoporous layer — porous support... 

Ad Figure 2.1,2. Zeolite layers can be grown by hydrothermal synthesis onto porous supports (clay, alumina, sintered metal). Especially layers of MFI-type zeolite have been studied [e.g. 5-7]. Such MFI-layers were shown to survive template removal and subsequent thermal cycles up to 350 °C, which is taken as a strong indication for chemical bonding [8] at the support interface. To understand chemical attachment to metals one has to take into consideration that metals - by exposure to air - will be covered with a thin (1-2 nm) oxide film. Sometimes an intermediate mesoporous layer has been applied, e.g. a metakaolin film on clay or on zirconia [5] or metal wool on sintered metal [6]. 

No adherence exists when a zeolite layer is deposited onto a teflon [9] or a carbon support. Here, binding possibilities between the zeolite layer and the support are lacking. 

PiA Figure 2.3,4. Another approach is to apply a dense support (e.g. stainless steel) equipped with a regular perforation. The in situ growing of zeolite then aims at a zeolite layer covering the whole support or at zeolite growth in the openings of the support. For an example see Section 3 of this chapter,... 

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. 

Gavalas et al. [7] prepared ZSM-5 membranes onto porous a-alumina disks by in-situ hydrothermal synthesis at 175°C. The zeolite layers were formed on the bottom face of disks placed horizontally near the air-liquid interface of clear synthesis solutions. The films grown at the optimized conditions were about 10 pm thick and consisted of well-intergrown crystals of about 2 pm in size Pure gas permeation measurements of the best preparations yielded hydrogen isobutane and butane isobutane ratios of 151 and 18 at room temperature and of 54 and 31 at 185°C, respectively. 

Figure 9 Schematic view of a zeolite layer on top of a porous/non-porous substrate...

In type a., the separating zeolite layer is equipped with catalytic sites (Bronsted add sites, Lewis acid sites (cations, special Al-sites), metal clusters, catalytic complexes). In type b., the non-supported side of the zeolite layer serves as a support for catalytic entities, e.g. metal crystallites. In type c., zeolite crystals with catalytic power are embedded in a matrix, e.g. a polymer membrane. 

Here, we also mention a composite system developed by Van der Puil et al. [90] in which an MFI (Silicalite-1) layer is completely covering a platinum-on-flat support catalyst. The zeolite layer is governing the acces to the platinum and this leads in the competitive hydrogenation of 1-heptene and 3,3-dimethyl-l-butene seeFigure 31 + 32) to highly selective conversion of 1-... 

Without the zeolite layer, the two olefins are converted with the same rate. 

It was pointed out that diffusion effects are less severe for thin zeolite layers on monolithic substrates than for pellets, such as were used for performance comparison. Methanol partial pressure variations led to results similar to other work in which it has been found that lower pressures favor olefin formation. As the temperature is raised, light... 

Suzuki K., Kiyozumi Y. and Sekine T, Preparation and characterization of a zeolite layer. Chemistry Express 5 793 (1990). 

In the preparation of zeolite membranes, in situ crystallization in an autoclave is used to deposit a zeolite layer on a macroporous metallic membrane substrate. For example, Geus [96] reported the preparation of ZSM-5 zeolite on a macroporous metal substrate, which can be used in separations at high temperatures. 

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. 

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