Multifunctional catalysts, applications

The multifunctional catalysts constitute a new class of widely applicable and... 

For a review about the applications of the multifunctional catalysts developed by the Shibasaki group, see M. Shibasaki, H. Sasai, T. Abai, Angew. Chem. 1997, 109, 1290-1310 Angew. Chem. Int. Ed. Engl. 1997, 36, 1236-1256. 

The unique features of zeolites, and the possibility of tuning acidic and basic sites, as well as the creation of multifunctional catalysts, open a wide field of applications in the production of fine chemicals. In this article we present new heterogeneously catalyzed processes for the synthesis of industrially relevant fragrances, flavors and aromas. The emphasis of this review article will remain mainly on solid acids. 

Multifunctional catalysts offer important opportunities for scientific advances and industrial applications since they are able to activate simultaneously different molecular species such as CO and H2. Of critical interest is the molecular structures of the catalyst responsible for such multiple activations, how the activated species interact, and how the reaction dynamics control activity and selectivity. 

Recent patents for HT use cover their application in fine chemicals productions, either as base or multifunctional catalysts. Figure 2.26 gives an example of the latter. 

In this area, Yang and colleagues have designed a bilayer stracture by combining platinum and cerium oxide nanocube into sub-lOnm layers on silica substrate [7]. Two distinct reactions can be utilized on a bilayer structure with two metal-metal oxide interfaces (CeO -Pt and Pt-SiO ). The CeO -Pt interface is used to catalyze the decomposition of methanol into CO and H, while ethylene hydroformylation happens on the Pt-SiO interface. This is an intriguing concept on making nanocrystal bilayer structures designed for the application of multifunctional catalysts, the so-called tandem catalyst. 

To date, only a few iridium catalysts have been applied to industrially relevant targets, especially on the larger scale. It is likely that several types of Ir catalyst are, in principle, feasible for technical applications in the pharmaceutical and agrochemical industries. At present, the most important problems are the relatively low catalytic activities of many highly selective systems and the fact, that relatively few catalysts have been applied to multifunctional substrates. For this reason, the scope and limitations of most catalysts known today have not yet been explored. For those in academic research, the lesson might be to employ new catalysts not only with monofunctional model compounds but also to test functional group tolerance and-as has already been done in some cases-to apply the catalysts to the total synthesis of relevant target molecules. 

We focus attention here on titania (Ti02) for the following reasons. The first is that titania is a widely used oxide support for both metal particles and metal oxides, and used in some cases also directly as catalyst (Claus reaction, for example). The second is that it possesses multifunctional properties, such as Lewis and Bronsted sites, redox centres, etc. The third is that it has several applications both as a catalyst and an advanced material for coating, sensors, functional films, etc. The fourth is its high photocatalytic activity which make titania unique materials. 

Conceptually new multifunctional asymmetric two-center catalysts, such as the Ln-BINOL derivative, LnMB, AMB, and GaMB have been developed. These catalysts function both as Brpnsted bases and as Lewis acids, making possible various catalytic, asymmetric reactions in a manner analogous to enzyme catalysis. Several such catalytic asymmetric reactions are now being investigated for potential industrial applications. Recently, the catalytic enantioselective opening of meso epoxides with thiols in the presence of a heterobimetallic complex has... 

Monoliths allow the efficient use of small catalyst particles, such as zeolites, and are remarkably flexible with respect to their catalyst inventory. Multifunctional reactor operations such as reactive stripping and distillation are challenging applications that are not far away. They have several potential applications in oil refineries, in fhe chemical process industry, and for consumers. The industrial application of the monolithic stirrer reactor as alternatives to many slurry-t)q5e reactors in fine chemisfry has the greatest potential as a new practice involving monolithic catalysts. 

Even though many of these principles appear to be applicable to for multifunctional carboxylates and alkoxides, it is important to recognize that more complex molecules may be more or less influenced by differences in surface conditions than others. Small, monofunctional molecules ably serve to highlight site requirements for some reactions, but there is no reason to expect that a large, multifunctional molecule will necessarily interact with a metal oxide catalyst as a mere combination of its functionalities. Consequently, it is important to characterize the adsorption and synthesis of larger molecules where possible, to determine the limitations of the principles explored here, and to develop an understanding of adsorption and reaction characteristics that will lead to more selective catalysts. 

The cleaning of flue gases from stationary sources is another field in which the application of monolithic catalysts will certainly rise. There will be no versatile catalyst for cleaning all off-gases. Therefore tailor-made catalysts with zeoliths of various types for specific applications will be developed. Incorporated-type monolithic catalysts are likely to prevail in this field. Since cleaning usually requires a set of equipment items in series (e.g., converter, heat exchangers), multifunctional reactors (reverse-flow reactors, rotating monoliths) will become more common. 

In addition to practical applications, metal cluster-derived catalysts, particularly intrazeolite metal cluster compounds, may aid in the identification of catalytically important bonding and structural patterns and thereby further our molecular understanding of surface science and heterogeneous catalysis. The ship-in-bottle technique for the synthesis of bulky metal-mixed metal cluster compounds inside zeolites and/or interlayered minerals has gained growing attention for the purpose of obtaining catalytic precursors surrounded by the interior constraint, imposing molecular shape selectivity. Such approaches may pave the way to offer the molecular architecture of hybrid (multifunctional) tailored catalysts to achieve the desired selectivity and stability for industrial processes. 

Cinchona alkaloids are readily available natural chiral compounds and have a long history to be utilized as organocatalysts in asymmetric catalysis [3, 4]. They are multifunctional, tunable, and more importantly, they could promote a diversity of reactions through different catalytic mechanisms, which make them privileged catalysts in organocatalysis. In this chapter, the applications of cinchona alkaloids and their derivatives for asymmetric cydoaddition reactions after 2000, especially for the construction of a variety of five- and six-membered cyclic compounds, are discussed. 

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