Recycling of homogeneous catalysts

In general, heterogeneous catalysts are preferred to homogeneous catalysts because the separation and recycling of homogeneous catalysts often can be very difficult. Loss of homogeneous catalyst not only creates a direct expense through loss of material but also creates an environmental problem. 

For the production of chemicals, food additives and pharmaceutical products, homogeneous catalysis offers some attractive features such as a high selectivity and activity, e.g. in asymmetric synthesis. However, since most homogeneous catalysts are relatively expensive, their current industrial application is limited [3]. On the other hand, heterogeneous catalysts can easily be separated from the products and can be recycled efficiently. Membrane separations with emphasis on nanofil-tration and ultrafiltration will allow for a similar recyclability of homogeneous catalysts, which is important both from an environmental as well as a commercial... 

Heldebrant DJ, Jessop PG (2003) Liquid poly(ethylene glycol) and supercritical carbon dioxide a benign biphasic solvent system for use and recycling of homogeneous catalysts. J Am Chem Soc 125(19) 5600-5601... 

REVERSE FLOW ADSORPTION TECHNOLOGY FOR THE RECYCLING OF HOMOGENEOUS CATALYSTS SELECTION OF SUITABLE ADSORBENTS... 

As discussed later regarding the role of multiphase operations for homogeneous catalysis, research has been focused mainly on the issue of recyclability of homogeneous catalysts. Relevant progress has been made in this aspect, but the other problem, which could be indicated as intensification of homogeneous catalysis processes, is the issue that should be solved, at least for medium-large scale industrial productions. 

We conclude here by indicating that heterogeneous catalysts are still preferable in developing sustainable chemical processes, but that the notable recent progress in multiphase operations to allow improved catalyst separation, recovery and recycle of homogeneous catalysts [63-67] indicates that the use of homogeneous catalysis in industrial processes will certainly increase in the near future. 

From above, it is evident that ionic liquids used as unconventional green solvents may promote alkylation reactions and allow the recycling of homogeneous catalysts. Ionic liquids have exhibited wide prospects on their applications in alkylation. 

Ionic liquids in combination with supercritical fluids are a versatile tool for the immobilization and recycling of homogeneous catalysts, allowing continuous Friedel-Crafts acylation reactions to be realized. The acylation of anisole with acetic anhydride is carried out in a flow system using a metal triflate immobilized in the ionic liquid 1 -butyl-4-methylpyridinium bis(trifluoro-methylsulfonyl)imide as catalyst and scCO as continuous extraction phase [22]. Different metal triflates are utilized under continuous flow conditions using high pressure yttrium triflate possesses the best balance between sufficient acidity for catalytic activity and softness to release the product and so permits a good catalyst reuse (TONs up to 190). 

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