Aspects of Catalyst Recycling

Silica-supported homogeneous catalysts, especially phosphino-iridium compounds, appear more promising in the hydrogenation of a,y9-unsaturated aldehydes, provided that their productivity can be improved and catalyst deactivation is avoided so that recycling of these materials could be meaningful [35]. 

In order to synthesize a desired chemical compound, it is often necessary to pass through a sequence of different reaction steps because side reactions can occur, making the composition of the medium more and more complex and, thus, opening additional undesired reaction pathways. The presence of a large number of different compounds decreases the concentration of the desired intermediates by dilution processes, leading to lower reaction rates of the necessary reactions. In most of these cases, the consequence is an extremely low yield of the final product in question, a result which cannot be accepted from a practical point of view. 

in order to obtain a product in high yield, normally it is not synthesized in a one-pot reaction, starting from a certain number of raw materials, but it is produced by buying the necessary intermediates from the chemical market and using them in a one-step reaction. 

Even though monofunctional contacts work very sucessfully in the chemical industry (see, e. g., [4]), this type of catalyst shows a general disadvantage since it cannot shift a thermodynamic equilibrium state to obtain a higher yield. As a consequence, in a thermodynamically controlled reaction the educt utilization can be relatively low, even if an active catalyst is used. 

If methanol can be considered to be the intermediate in a multistep reaction to form a final species, e. g., gasoline as in the Mobil MTG process [4], then a bifunctional catalyst ought markedly to increase the utilization of the syngas. Since in the MTG process dimethyl ether (DME) is a key intermediate, Sofianos et al. [9] proposed for this purpose a bifunctional catalyst, prepared by intimate mixing of finely milled samples of the methanol catalyst and of y-alumina, the acid catalyst which dehydrates methanol to DME. The results obtained show that the CO conversion using the bifunctional catalyst is nearly four times higher than that obtained with the monofunctional catalyst. 

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In contrast, we intend to demonstrate the principle aspects of catalyst recycling and regeneration using the ionic liquid methodology. These aspects will be explored in more detail for the example of Rh-catalysed hydroformylation (see Section 7.2). First, however, we will briefly introduce important general facts concerning transition metal catalysis in ionic liquids (see Section 7.1.2). This will be followed by a consideration of liquid-liquid biphasic reactions in these media from an engineering point of view (see Section 7.1.3). 

Here we review recent progress and breakthroughs in research with promising, novel transition metal-functionalized dendrimer catalysts and discuss aspects of catalyst recycling and unique dendritic effects in catalysis. 

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