Hydrogenation using supported

SELECTIVE HYDROGENATION USING SUPPORTED RHODIUM CATALYSTS... 

This section will review some latest developments in the area of asymmetric hydrogenation using supported metal catalysts. It will not cover homogeneous or biocatalysis. The field has been extensively reviewed by Blaser and Muller... 

Many workers (5,6,7,87) have compared various metals for the selective hydrogenation of lower acetylenes to olefins, and it was always found that palladium was by far the most selective. This conclusion concurs with the usual synthetic experience, but under special circumstances other metals, such as platinum, may prove more useful (35,63). The catalyst support may also have an influence (21,65). Carbon, calcium carbonate, and barium sulfate are frequently used supports. Examples of some differences are noted later,... 

Hydrogenolysis. Hydrogenation of polycyclopropanone was carried out using supported catalysts. Before hydrogenation was attempted the polymers were end-capped in a refluxing mixture of acetic anhydride and pyridine. This was done to convert thermally unstable hydroxyl end groups, which may have been present in the polymer, to more thermally stable acetyl end groups as shown in Equation 8. This was... 

The disadvantages of the general method using supported palladium and hydrogen are a lack of selectivity and overreduction. Where selectivity is not an important requirement, Pd/C is a widely used catalyst for the hydrogenolysis of the C-O bond in benzyl ethers. 

Scheme 12.14 Microwave-mediated hydrogenation using a polymer-supported hydrogen donor.

Lin, Y.M. and M.H. Rei, Separation of hydrogen from the gas mixture out of a catalytic reformer by using supported palladium membrane, Sep. Purif. Technol., 25,87-95,2001a. 

Some general reviews on hydrogenation using transition metal complexes that have appeared within the last five years are listed (4-7), as well as general reviews on asymmetric hydrogenation (8-10) and some dealing specifically with chiral rhodium-phosphine catalysts (11-13). The topic of catalysis by supported transition metal complexes has also been well reviewed (6, 14-29), and reviews on molecular metal cluster systems, that include aspects of catalytic hydrogenations, have appeared (30-34). 

Table 41.17 Comparative hydrogenation studies using supported ionic liquid catalysts, biphasic catalyst systems and the classical homogeneous catalyst systems [116].a) ... 

To our knowledge, there is to date only one report concerning asymmetric hydrogenation using a SILP catalyst [117]. Methyl acetoacetate was hydrogenated using Ru-BINAP dissolved in [BMIM][PF6] and immobilized on the poly(diallyl-dimethy-lammonium chloride) support (Fig. 41.14). 

Ultimately, the ligand itself can be polymerized and used as both ligand and support in one [110]. Pu and coworkers have prepared the rigid chiral poly-BI-NAP ligand below (Fig. 42.17) and used it after treatment with Rh and Ru in asymmetric hydrogenations. The supported catalysts showed similar activities and enantioselectivities to their homogeneous analogues, with the benefit of... 

Lin, Y.-M., Rei, M.-Fl. 2000. Proeess development for generating high purity hydrogen by using supported palladium membrane reaetor as steam reformer. Int J Flydrogen Energy 25 211-219. 

The hydrogenation of nitrobenzene and nitrosobenzene are complex and a range of factors can influence by-product reactions, e.g. hydrogen availability, support acid/base properties (13,14). In this study we have examine competitive hydrogenation between nitrobenzene, nitrosobenzene and azobenzene. This methodology coupled with the use of deuterium has further elucidated the mechanism of these reactions. 

Supported metal catalysts are much easier to employ and have obvious attractions for industrial use from their ease of handling and economic considerations of obtaining maximum utilisation of the catalytically active metal, by using very small particles with a high surface-to-volume ratio, which are stable on the support and not susceptible to sintering. In spite of the inherent difficulties of variable activity, kinetics and activation energies [11] associated with their use, supported metals have been extensively used as hydrogenation catalysts. 

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