Recyclable hydrogenation catalysts

Jacinto, M.J., Landers, R. and Rossi, L.M. (2009) Preparation of supported Pt(0) nanopartides as efficient recyclable catalysts for hydrogenation of alkenes... 

The current research areas with ruthenium chemistry include the effective asymmetric hydrogenation of other substrates such as imines and epoxides, the synthesis of more chemoselective and enantioselective catalysts, COz hydrogenation and utilization, new methods for recovering and recycling homogeneous catalysts, new solvent systems, catalysis in two or three phases, and the replace-... 

Finally, these particles generated in ionic liquids are efficient nanocatalysts for the hydrogenation of arenes, although the best performances were not obtained in biphasic liquid-liquid conditions. The main importance of this system should be seen in terms of product separation and catalyst recycling. An interesting alternative is proposed by Kou and coworkers [107], who described the synthesis of a rhodium colloidal suspension in BMI BF4 in the presence of the ionic copolymer poly[(N-vinyl-2-pyrrolidone)-co-(l-vinyl-3-butylimidazolium chloride)] as protective agent. The authors reported nanoparticles with a mean diameter of ca. 2.9 nm and a TOF of 250 h-1 in the hydrogenation of benzene at 75 °C and under 40 bar H2. An impressive TTO of 20 000 is claimed after five total recycles. 

Water-soluble complexes constitute an important class of rhodium catalysts as they permit hydrogenation using either molecular hydrogen or transfer hydrogenation with formic acid or propan-2-ol. The advantages of these catalysts are that they combine high reactivity and selectivity with an ability to perform the reactions in a biphasic system. This allows the product to be kept separate from the catalyst and allows for an ease of work-up and cost-effective catalyst recycling. The water-soluble Rh-TPPTS catalysts can easily be prepared in situ from the reaction of [RhCl(COD)]2 with the sulfonated phosphine (Fig. 15.4) in water [17]. 

The water-soluble ligand (TPPTS) was discussed earlier with regard to aldehyde reduction [17]. Similarly, in ketone transfer hydrogenation, high yields are obtained for a variety of substrates with the ability for efficient catalyst recycling at no expense of activity or selectivity (Fig. 15.10). 

An effective catalyst recycling with no loss of catalytic activity was accomplished by removing the liquid phase via the liquid sampling valve and re-charging the autoclave with a solution containing the substrate. In all cases, no rhodium leaching occurred. Remarkably, the hydrogenation activity of the 1,3-bis-... 

Brown, R.A., Pollet, R, McKoon, E., Eckert, C.A., Liotta, C.L., and Jessop, P.G., Asymmetric hydrogenation and catalyst recycling using ionic liquid and supercritical carbon dioxide, /. Am. Chem. Soc., 123,1254-1255,2001. 

Fig. 2.1.6.4 Hot filtration test and catalyst recycling for hydrogenation of dimethyl itaconate over RhDuphos/ AI-SBA-15.

A second process employing complex as the catalyst carrier was independently developed by the Standard Oil Co. (Indiana) and by The Texas Co. In this process (19,20), liquid butane containing make-up aluminum chloride and recycled hydrogen chloride is bubbled upward through a bed of preformed liquid complex about 20 feet in depth. Because the aluminum chloride in the feed is effectively transferred to the complex, catalyst carry-over in the reactor effluent is low and no recovery tower is needed. 

In this process, the liquid butane feed is employed first to recover aluminum chloride and antimony chloride from spent catalyst. This is accomplished in a scrubber, from which insoluble complex is continuously discarded. The butane stream then picks up recycled hydrogen chloride and enters the reactor, where mechanical agitation causes intimate contacting with an equal volume of catalyst. The undesirable complex formed in... 

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