Multi-Catalyst Systems

Scheme 43.27 Asymmetric synthesis of 4,5-disubstituted isoxazoline N-oxides promoted by a multi-catalyst system.

In these (and other) solid superacid catalyst systems, bi- or multi-dentate interactions are thns possible, forming highly reactive intermediates. This amounts to the solid-state equivalent of protosolvation resulting in superelectrophilic activation. 

Examples of multi-disciplinary innovation can also be found in the field of environmental catalysis such as a newly developed catalyst system for exhaust emission control in lean burn automobiles. Japanese workers [17] have successfully merged the disciplines of catalysis, adsorption and process control to develop a so-called NOx-Storage-Reduction (NSR) lean burn emission control system. This NSR catalyst employs barium oxide as an adsorbent which stores NOx as a nitrate under lean burn conditions. The adsorbent is regenerated in a very short fuel rich cycle during which the released NOx is reduced to nitrogen over a conventional three-way catalyst. A process control system ensures for the correct cycle times and minimizes the effect on motor performance. 

Speed-up of mixing is known not only for mixing of miscible liquids, but also for multi-phase systems the mass-transfer efficiency can be improved. As an example, for a gas/liquid micro reactor, a mini packed-bed, values of the mass-transfer coefficient K a were determined to be 5-15 s [2]. This is two orders of magnitude larger than for typical conventional reactors having K a of 0.01-0.08 s . Using the same reactor filled with 50 pm catalyst particles for gas/Hquid/solid reactions, a 100-fold increase in the surface-to-volume ratio compared with the dimensions of laboratory trickle-bed catalyst particles (4-8 mm) is foimd. 

Manufacture of ruthenium precatalysts for asymmetric hydrogenation. The technology in-licensed from the JST for the asymmetric reduction of ketones originally employed BINAP as the diphosphine and an expensive diamine, DAIPEN." Owing to the presence of several patents surrounding ruthenium complexes of BINAP and Xylyl-BINAP, [HexaPHEMP-RuCl2-diamine] and [PhanePHOS-RuCl2-diamine] were introduced as alternative catalyst systems in which a cheaper diamine is used. Compared to the BINAP-based systems both of these can offer superior performance in terms of activity and selectivity and have been used in commercial manufacture of chiral alcohols on multi-100 Kg scales. 

The multi-component systems developed quite recently have allowed the efficient metal-catalyzed stereoselective reactions with synthetic potential [75-77]. Multi-components including a catalyst, a co-reductant, and additives cooperate with each other to construct the catalytic systems for efficient reduction. It is essential that the active catalyst is effectively regenerated by redox interaction with the co-reductant. The selection of the co-reductant is important. The oxidized form of the co-reductant should not interfere with, but assist the reduction reaction or at least, be tolerant under the conditions. Additives, which are considered to contribute to the redox cycle directly, possibly facilitate the electron transfer and liberate the catalyst from the reaction adduct. Co-reductants like Al, Zn, and Mg are used in the catalytic reactions, but from the viewpoint of green chemistry, an electron source should be environmentally harmonious, such as H2. 

Although valence band spectra probe those electrons that are involved in chemical bond formation, they are rarely used in studying catalysts. One reason is that all elements have valence electrons, which makes valence band spectra of multi-component systems difficult to sort out. A second reason is that the mean free path of photoelectrons from the valence band is at its maximum, implying that the chemical effects of for example chemisorption, which are limited to the outer surface layer, can hardly be distinguished from the dominating substrate signal. In this respect UPS, discussed later in this chapter, is much more surface sensitive and therefore better suited for adsorption studies. 

The known catalyst systems for olefin metathesis can be grouped into heterogeneous catalysts, homogeneous multi-component catalysts, and homogeneous single-component catalysts. 

The use of SCCO2 either alone [135-137] or in multi-phase systems [138-140] offers interesting process alternatives to other methods of catalyst immobilization such as heterogenization on solid supports [126-128], aqueous... 

Domino reactions attract significant attention due to their ability to allow for multi-step operations in one pot. Two classes of domino transformations are known for the PK reaction. The first involves the use of a dual-catalyst system, while the second uses a single catalyst with modification of the reaction parameters to control reactivity. 

In this paper, a few examples of Raney catalysts produced by metastable processes and their catalytic properties are discussed. Then, some examples of multi alloy systems, their microstructures and general properties will be shown. Finally, we will discuss the possibility of forming large particle materials with high specific surface area. 

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