Homogeneous catalysts overview

Although zeolite-encapsulated metal complexes were known for some time, the principle that such complexes could act as a new type of immobilized homogeneous catalysts was probably first demonstrated only in 1985 [37]. This achievement opened a new and fruitful area of research in immobilizing homogeneous catalysts. These catalysts are named appropriately ship-in-the-bottle (SIB) catalysts (see Fig. 42.5). General overviews of zeolite-encapsulated metal complexes were given [38, 39]. 

In this chapter, a brief overview of small-molecule activation by biological active sites and related small-molecule homogeneous catalysts has been provided. The six primary reactions, outlined in the introduction of this chapter (Section 4.5.1), are all carried out by enzyme active sites with remarkable efficiency and specificity. Importantly, all of the enzymes discussed in the preceding sections utilize readily available base metals in order to activate small molecules. This is a key requirement as we look to the needs and challenges of our energy future, which will require not only efficient catalysts, but also scalable and sustainable catalysts. Here, it is clear that there is a gap between what biology can enable and what has been achieved with small-molecule catalysts. While remarkable transformations have been reported for small-molecule catalysts that use noble metals, the documented examples of... 

Another approach towards asymmetric heterogeneous catalysts is the immobilization of chiral homogeneous complexes via different methods. In this way the advantages of homogeneous catalysts (high activity and selectivity) and heterogeneous catalysts (easy recovery) can be combined. For a complete overview of this active research field the reader is referred to several reviews on this topic [50, 51]. The practical applicability of these catalysts is hampered by the fact that severe demands of recyclability and stability need to be obeyed. In certain cases promising results have been obtained as outlined here. 

Enantioselective synthesis is a topic of undisputable importance in current chemical research and there is a steady flow of articles, reviews and books on almost every aspect involved. The present overview will concentrate on the application of solid chiral catalysts for the enantioselective synthesis of chiral molecules which are a special class of fine chemicals. Included is an account on our own work with the cinchona-modified Pt catalysts. Excluded is the wide field of immobilized versions of active homogeneous complexes or of bio-catalysts. During the preparation of this survey, several reviews have been found to be very informative [1-14]. 

Heterometal alkoxide precursors, for ceramics, 12, 60-61 Heterometal chalcogenides, synthesis, 12, 62 Heterometal cubanes, as metal-organic precursor, 12, 39 Heterometallic alkenes, with platinum, 8, 639 Heterometallic alkynes, with platinum, models, 8, 650 Heterometallic clusters as heterogeneous catalyst precursors, 12, 767 in homogeneous catalysis, 12, 761 with Ni—M and Ni-C cr-bonded complexes, 8, 115 Heterometallic complexes with arene chromium carbonyls, 5, 259 bridged chromium isonitriles, 5, 274 with cyclopentadienyl hydride niobium moieties, 5, 72 with ruthenium—osmium, overview, 6, 1045—1116 with tungsten carbonyls, 5, 702 Heterometallic dimers, palladium complexes, 8, 210 Heterometallic iron-containing compounds cluster compounds, 6, 331 dinuclear compounds, 6, 319 overview, 6, 319-352... 

The above literature review gives a comparison of different ways to control selectivity for both homogeneous and heterogeneous catalytic reactions. There are several common features for the four areas of stereoselectivity metal clusters, alloys and poisoning shape selectivity and reaction pathway control. In fact, many times more than one of these areas may be involved in a catalytic system. Some common features for all of these areas include precise control of the structural and compositional properties of the catalysts. This paper serves as an overview for the other manuscripts in this book. Specific review chapters on each of the four areas can be found in reviews that follow by D. Forster et al., K. J. Klabunde et al., M. E. Davis et al., and H. C. Foley and M. Klein et al. 

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