Zeolite-coated crystals

A second option is to apply the membrane on the particle level (millimeter scale) by coating catalyst particles with a selective layer. As a third option, application at the microlevel (submicrometer scale) is distinguished. This option encompasses, for example, zeolite-coated crystals or active clusters (e.g., metal nanoparticles). Advantages of the latter two ways of application are that there are no sealing issues, it is easy to scale-up, the membrane area is large per unit volume, and, if there is a defect in the membrane, this will have a very limited effect on the overall reactor performance. Because of these advantages, it is believed that using a zeolite... 

Rebrov et al. [158] synthesized ZSM-5 zeolite in micro channels. The main focus of this work was to assess the performance benefits of zeolitic coatings in micro channels compared with conventional zeolite-based pellets and powders. The coatings were performed in a sandwich of two plates of 1 cm length and width at a thickness of 2 mm. The plates carried seven channels each, which were 1 cm long and had a diameter of500 pm. The plates were positioned at a distance of280 pm from each other in the housing of the reactor. A zeolitic film of one crystal thickness was formed under the optimum synthesis conditions, which were determined as a water/silicon ratio of 130 and a template/aluminum ratio of 2 at a temperature of 130 °C after 35 h on a flat plate. The Si/Al ratio of the zeolite, which also lowers the crystal size when decreased, was optimized to a value of 28. 

V. Valtchev and S. Mintova, The effect of the metal substrate composition on the crystallization of zeolite coatings. Zeolites 75 171 (1995). 

Valtchev V and Mintova S. Layer-by-layer preparation of zeolite coatings of nanosized crystals. Micropor Mesopor Mater 2001 43(1) 41 9. 

Sasaki I, Tsuchiya H, Nishioka M, Sadakata M, and Okubo T. Gas sensing with zeolite-coated quartz crystal microbalances—principal component analysis approach. Sens Actuators B 2002 86(l) 26-33. 

V. Valtchev and S. Mintova, Layer-by-layer Preparation of Zeolite Coatings of Nanosized Crystals. Microporous Mesoporous Mater., 2001, 43, 41-49. 

X.L. Cheng, Z.B. Wang, and Y.S. Yan, Corrosion resistant Zeolite Coatings by In-situ Crystallization. Electrochem. Solid State Lett., 2001, 4, B23-B26. 

Cheng, X. Wang, Z. Yan, Y. Corrosion-resistant zeolite coatings by in-situ crystallization. Electro-chem. Solid-State Letts. 2001, 4 (5), B23-B26. 

Zeolite membranes and films have been employed to modify the surface of conventional chemical electrodes, or to conform different types of zeolite-based physical sensors [49]. In quartz crystal microbalances, zeolites are used to sense ethanol, NO, SO2 and water. Cantilever-based sensors can also be fabricated with zeolites as humidity sensors. The modification of the dielectric constant of zeolites by gas adsorption is also used in zeolite-coated interdigitaled capacitors for sensing n-butane, NH3, NO and CO. Finally, zeolite films can be used as barriers (for ethanol, alkanes,...) for increasing the selectivity of both semiconductor gas sensors (e.g. to CO, NO2, H2) and optical chemical sensors. 

The polymerization reaction is partially diffurion controlled. As soon as the first polymer is formed the surface of each catalyst crystal becomes coated with a film of the pdymer which increases in thickness as the reaction proceeds. The monmner reaches the active caitres on the zeolite surfa or within the zeolite by disscdving in the pdymer and diffusing through it to the catalyst. The relaticmdiip... 

Subsequently, a ZSM-5 zeolite coating was prepared with a zeolitic layer thickness of a single crystal, particularly suitable for the considered catalytic reaction. 

Kimura M, Sugawara M, Sato S, Fukawa T, Mihara T (2010) Volatile otganic compound sensing by quartz crystal microbalances coated with nanostructured macromolecular metal complexes. Chem Asian J 5 869-876 Kosal ME, Chou J-H, Wilson SR, SusUck KS (2002) A functional zeolite analogue assembled from metaUoporphyrins. Nat Mater 1 118-121... 

Apart from the above described core-shell catalysts, it is also possible to coat active phases other than zeolite crystals, like metal nanoparticles, as demonstrated by van der Puil et al. [46]. More examples of applications on the micro level are given in Section 10.5, where microreactors and sensor apphcations are discussed. 

Cationic and neutral dyes have the tendency to adsorb at the inner and at the outer surface of the zeolite crystals. It is to be expected that the affinity of molecules to the coat and the base area differs. The coat and the base area of a good zeolite L material are nicely illustrated on the left and right side, respectively, of Figure 1.7. The number of molecules needed to form a monolayer nD on a cylinder of surface Az is... 

The CgQ surface coverage was determined to be 2.0 10 mol cm . The monolayer can be further modified with monomeric amine reagents, which demonstrates the potential of the self-assembly process for growing three-dimensional fullerene structures. Different surfaces such as quartz. Si-oxide [105] or ITO [102] were coated with multilayers of fullerene up to stacks of 9 layers. An imidirectional electron transfer is possible across the fullerene mulhlayers [102]. Not only can multiple layers of fullerenes be connected to a certain surface but amino-functionalized can also serve as a linker between two different surfaces. 3-Aminopropyl-tethered glass plates could be linked via a Cgg layer to 3-aminopropyl covered zeolite crystals [106]. 

Bentone-34 has commonly been used in packed columns (138—139). The retention indices of many benzene homologues on squalane have been determined (140). Gas chromatography of C —C aromatic compounds using a Ucon B550X-coated capillary column is discussed in Reference 141. A variety of other separation media have also been used, including phthalic acids (142), liquid crystals (143), and Werner complexes (144). Gel permeation chromatography of alkylbenzenes and the separation of the Cg aromatics treated with zeolites are described in References 145—148. 

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],... 

To obtain a uniform distribution of the zeolite in the micro channels, a two-step procedure was developed, including nucleation growth at high temperature at the horizontally oriented plates followed by a growth period at the vertically oriented plates, which was performed at lower temperature and water/silicon ratio. The crystals were oriented parallel to the surface of the carrier. Nitrogen adsorption revealed the typical micropore distribution of ZSM-5 for the coating. Thermal cycling... 

The H-ZSM-5 coatings were tested for the one-step oxidation of benzene by nitrous oxide to phenol. The grids had a total area of 9 cm2, a wire diameter of 250 pm and a mesh size of 800 pm. Fifteen grids formed a stack separated by steel rings. By acid pretreatment of the grids, defects were generated which are known to become crystallization centers during the synthesis of the zeolite. 

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