Heterogeneous catalyst monolithic

Most industrial catalysts are heterogeneous catalysts consisting of solid active components dispersed on the internal surface of an inorganic porous support. The active phases may consist of metals or oxides, and the support (also denoted the carrier) is typically composed of small oxidic structures with a surface area ranging from a few to several hundred m2/g. Catalysts for fixed bed reactors are typically produced as shaped pellets of mm to cm size or as monoliths with mm large gas channels. A catalyst may be useful for its activity referring to the rate at which it causes the reaction to approach chemical equilibrium, and for its selectivity which is a measure of the extent to which it accelerates the reaction to form the desired product when multiple products are possible [1],... 

Then, a survey of micro reactors for heterogeneous catalyst screening introduces the technological methods used for screening. The description of microstructured reactors will be supplemented by other, conventional small-scale equipment such as mini-batch and fixed-bed reactors and small monoliths. For each of these reactors, exemplary applications will be given in order to demonstrate the properties of small-scale operation. Among a number of examples, methane oxidation as a sample reaction will be considered in detail. In a detailed case study, some intrinsic theoretical aspects of micro devices are discussed with respect to reactor design and experimental evaluation under the transient mode of reactor operation. It will be shown that, as soon as fluid dynamic information is added to the pure experimental data, more complex aspects of catalysis are derivable from overall conversion data, such as the intrinsic reaction kinetics. 

FIGURE 2 Some physical forms of heterogeneous catalysts. 1, Particulates 2, extrudates, 3, powders 4, rings 5, monoliths 6, tablets 7, spheres 8, carbon powders and particulates. 

A wide variety of methods has been reported for preparing monolithic catalysts. In fact, the methods stem from preparation methods developed in heterogeneous catalysts and methods for the manufacture of thin-layer materials. The latter are developed in the... 

Supported metal clusters play an important role in nanoscience and nanotechnology for a variety of reasons [1-6]. Yet, the most immediate applications are related to catalysis. The heterogeneous catalyst, installed in automobiles to reduce the amount of harmful car exhaust, is quite typical it consists of a monolithic backbone covered internally with a porous ceramic material like alumina. Small particles of noble metals such as palladium, platinum, and rhodium are deposited on the surface of the ceramic. Other pertinent examples are transition metal clusters and atomic species in zeolites which may react even with such inert compounds as saturated hydrocarbons activating their catalytic transformations [7-9]. Dehydrogenation of alkanes to the alkenes is an important initial step in the transformation of ethane or propane to aromatics [8-11]. This conversion via nonoxidative routes augments the type of feedstocks available for the synthesis of these valuable products. 

There have been many approaches published to "immobilize" a homogeneous catalyst. These heterogeneous catalysts must have better activity, selectivity, lifetime or some other property to warrant their use in an industrial chemical process. This talk will draw upon the author s experiences at DuPont and Air Products with Nation , Nafion on carbon, anionic attached [Rh(CO)2l2] to Reillex-425, carbons as catalysts for the hydrolysis of esters, heterogeneous catalyst to activate dimethyl ether as a methyl transfer reagent, and monoliths as hydrogenation catalysts. These individual experiences will show the complexities of a simple problem - "immobilization". 

Most heterogeneous catalysts exist in the form of microporous solids. The catalysts are usually produced in the shape of spheres, cylinders, or monoliths, such as those shown in Figure 1. The internal surface area is typically 10-10 m /g. Catalysis occurs either on the surface of the microporous solid, as in the case of zeolites, or on the surface of microdomains of active material dispersed inside the microporous solid, as in supported metals, oxides, sulfides, etc. In either case, the high internal surface area of the microporous solid is used to obtain a high concentration per unit volume of catalytically active centers. 

Zwinkels, M.F.M., Jaras, S.G., and Menon, P.G. (1994) Preparation of combustion catalysts by washcoating alumina-whiskers-covered metal monoliths using a sol-gel method. Proc. 6th Int. Symp. on Scientific Bases for the Preparation of Heterogeneous Catalysts, Louvain-la-Neuve, Belgium, Sep. 5-9, Vol. I, pp. 85-94. 

Most of the gas-liquid applications of monoliths have used a heterogeneous catalyst (be it supported noble metals or immobihzed enzymes) on the channel walls. Here, we also consider the use of monohths without a catalyst on the walls in gas-liquid applications, i.e. homogeneously catalyzed liquid-phase reactions. The fluid mechanics of the system do not change appreciably by lethng the reaction take place in the liquid bulk instead of in a washcoat layer, and it is interesting to consider such reactions in a discussion of mass transfer and power-input requirement. Of course, the mass-transfer behavior does change by changing the locahon where the reaction takes place, and we will discuss gas-hquid reactors and gas-liquid-solid reactors separately. 

Combinatorial chemistry can perhaps help discover new catalyst formulations for reactions presently of particular interest, such as oxidations or ammoxidation, and generally all reactions of alkanes. Reactions traditionally made in different kinds of processes are frequently shown to be also activated by heterogeneous catalysts (e.g., epoxidations). Reactors of unexpected design allow surprisingly selective reactions (e.g., monoliths for the oxidative dehydrogenation of light alkanes). However, the distance often remains long between these discoveries and the manufacture of active and selective catalysts adequately structured for particular use in an industrial reactor inserted in an industrial plant. 

In conventional industrial multi-phase reactors, the heterogeneous catalyst can be organized as a packed (or fixed) bed of catalyst particles (e.g., in trickle-bed reactors or in submerged up-flow reactors), as catalyst particles suspended or fluidized in one of the two phases (in the hquid phase of a three-phase reactor, as for example in a slurry-stirred reactor and a slurry-bubbling reactor) or finally as a structured catalyst (e.g., monolith and membrane reactors). Structured catalysts are regular solid structures which reduce randomness through a well-defined structure and shape at a reactor level. The selection of the most appropriate traditional multi-phase... 

Structured catalysts, including monoliths, are very promising as far as pressure drop and high performance for selective reactions are concerned. The perspectives for the use of monolithic catalysts in heterogeneous catalysis have been analysed by Cybulski and Moulijn (1994) and are further discus.sed in Section 5.4.7.7. 

Heterogeneous catalytic processes can often be intensified by the use of monolithic catalysts (39). These are metallic or nonmetallic bodies forming a multitude of straight, narrow channels of defined uniform cross-sectional shapes (Figure 13). In order to ensure sufficient porosity and to enhance the catalytically active surface, the inner walls of the monolith channels are usually covered with a thin layer of washcoat, which acts as the support for the catalytically active species. 

It has been observed that enantioselective polymer-bound catalysts prepared by copolymerization produce in some cases better asymmetric inductions than systems prepared by grafting [175]. After much optimization, a monolithic polymer catalyst 51 suitable for a titanium-TADDOLate catalyzed Diels-Alder reaction was developed (Scheme 4.77). The monolith was applied in a flow system both under one pass and 24 h recirculation conditions, the latter producing the best yield (55%) and ee (23%) however, this contrasts poorly with the homogeneous batch reaction although the ee is comparable with the heterogeneous batch process. The reversal of topicity was also... 

The various aspects that are to be considered to achieve a proper and efficient catalyst testing approach are presented. This applies to heterogeneous systems in which the catalyst is the solid phase and the reactants are in the gaseous and/or the liquid phase. The presence of a solid phase introduces complicating phenomena on which this chapter focuses. In this respect, homogeneous catalysis is a limiting case and does not need separate treatment. The solid catalyst can be present as either a packed bed of particles, a wash-coated monolith, a fluid bed, an entrained bed, or in a liquid-phase slurry. 

Wanker R., Berg M., Raupenstrauch H. and Staudinger G. Numerical Simulation of Monolithic Catalysts with a Heterogeneous Model and Comparison with Experimental Results from a Wood-fired Domestic Boiler. Accepted for publication in Chemical Engineering Journal. 

The most common polymer supports used for chiral catalyst immobilization are polystyrene-based crosslinked polymers, although poly(ethylene glycol) (PEG) represents an alternative choice of support. In fact, soluble PEG-supported catalysts show relatively high reactivities (in certain asymmetric reactions) [le] which can on occasion be used in aqueous media [le]. Methacrylates, polyethylene fibers, polymeric monoliths and polynorbornenes have been also utilized as efficient polymer supports for the heterogenization of a variety of homogeneous asymmetric catalysts. 

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