Catalysts coated

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... 

As an example of the selective removal of products, Foley et al. [36] anticipated a selective formation of dimethylamine over a catalyst coated with a carbon molecular sieve layer. Nishiyama et al. [37] demonstrated the concept of the selective removal of products. A silica-alumina catalyst coated with a silicalite membrane was used for disproportionation and alkylation of toluene to produce p-xylene. The product fraction of p-xylene in xylene isomers (para-selectivity) for the silicalite-coated catalyst largely exceeded the equilibrium value of about 22%. 

As is obvious, many potential hurdles discussed in the previous sections do not apply to appHcation of zeolite membranes at the micro- and particle levels. Issues Hke scale-up and high-temperature sealing do not play a role here. Additionally, coated catalyst particles do not require a change of reactor, but only replacement of the catalyst. Application of zeoHte membranes at these levels is therefore considered to be easier and their implementation will probably occur earlier. 

Figure 2.66 Cross-section of a micro channel coated with a catalyst layer (left) (source INM, Saarbrucken, Germany) and typical surface morphology of wash-coat catalyst carriers (right).

Uses. Coating screens of television picture tubes mold binders corrosion-resistant coatings catalyst preparation silicone intermediate... 

Vinyl acetate is produced by the oxidation of ethylene and acetic acid (4,5). Catalysts for the gas phase oxidation are made from palladium compounds with additional metal compounds on a porous support (6). Catalysts, preferably coated catalysts, can be used for many heterogeneously catalyzed reactions such as hydrogenations and oxidations. 

Among other things, Pd-Au coated catalysts are extremely well suited to the catalysis of the gas phase oxidation of ethylene and acetic acid to give vinyl acetate. The catalytically active metals are deposited in the form of a shell on or in the outermost layer of the support. They are often produced by penetration of the support with metal salts into a surface region and subsequent precipitation by alkalis to form water insoluble Pd-Au compounds (5). 

Major applications could He in nanotechnology, paints, surface coatings, catalysts, film formation, optoelectronics, gelators, diagnostics. 

The inhibition by hydrogen was obviously more pronounced in the micro channels. Without hydrogen in the feed, the reaction rate was on average 34% higher for the coated catalysts. The kinetic expression described the reaction rate experimentally observed with an error of < 15% for the packed bed and < 20% for the micro channels (see Figure 2.6) [24]. 

Pt/y-alumina wash-coat catalyst introduced by co-impregnation was used for butane combustion. After brief heating for 10 s, the reaction ignited and proceeded... 

Kolb, G., Zapf, R., Pennemann, H., Hessel, V., Lowe, H., Wash-coat catalysts applied for the water-gas shift reaction in micro channels, in Proceedings of the AIChE Spring Meeting (26-29 April 2004), New Orleans, 2004, 2d, published on CD. 

Sputtering technologies offer an even faster procedure at the cost of the versatility of the catalysts produced. Sputtering produces catalysts with a dense surface layer. These catalysts are different from industrially used catalysts, which usually have a larger surface area. The BET surface area of sputtered catalysts is below 1 m2 g 1 and thus much lower than the surface area of the wash-coated catalysts (usually above 60 m2 g1). However, if catalysts for a fast reaction have to be screened, the gas components will mainly react at the surface of the catalyst and the porosity of the catalyst is not important. 

Also an increase in volume flow in these reactors is inevitable. Micro structured reactors for high-throughput applications can consist of hundreds of plates all of them have to be of a uniform size with a uniform coating and, of course, should be inexpensive [158], especially as the micro structured plates sometimes will have to be exchanged in case of fouling or deactivation of the coated catalyst. 

The hydrogenation of p-nitrotoluene in the presence of a supported Pd catalyst was carried out in a microreactor with stacked plates with complete selectivity [283,320]. The Pd catalyst was prepared in three different ways. Conversions were 58-98% for an impregnated aluminum oxide wash-coat catalyst, depending on the process conditions. The conversions for an electrodeposited catalyst and an impregnated catalyst on electrooxidized nanoporous substrate were 58 and 89%, respectively. The best latter result is similar to that of a conventional fixed-bed reactor (85%), while the maximum yield of 30% in a microreactor was superior because of the high selectivity. 

On the other hand, in addition to adsorption properties, nanoporous materials are a group of advanced materials with other excellent properties and applications in many fields, for example, optics, electronics, ionic conduction, ionic exchange, gas separation, membranes, coatings, catalysts, catalyst supports, sensors, pollution abatement, detergency, and biology [1-42],... 

Note that, despite the typically high operating temperatures of fuel cells, radiative heat transfer was neglected. Lee and Aris (16) have discussed such effects in parallel-channel monoliths. The importance of radiative transport depends on the emissivity of the surface for the low (about 0.1) emissivity of Pt-coated catalyst-electrodes, their analysis suggests that radiative effects can be neglected. 

Table 4 shows a conqrarison of the performance of a coated FCC zeolite and a standard FCC catalyst in a micro simulation test both after Ni inqrregnation and steaming and after cyclic deactivation with vanadium [19]. The coated catalyst shows a better steam stability after Ni in regnation and a much better stability after cyclic deactivation with V in the feed. The hi er conversion points to a better stabi of the zeolite under deactivation conditions. The main advantage of the coated zeolite is the better protection against V attack. 

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