Carbon dioxide removing with ionic liquids

Compressed liquid or supercritical carbon dioxide has been recognized as a useful alternative reaction medium for radical and ionic polymerization reactions (see Chapter 4.5). Many of the benefits associated with the use of SCCO2 in these processes apply equally well to polymerizations relying on a metal complex as the chain-carrying species. However, the solubility of the metal catalyst and hence the controlled initiation of chain growth add to the complexity of the systems under study. Furthermore, many of the environmental benefits would be diminished if subsequent conventional purification steps were needed to remove the metal from the polymer. Nevertheless, the interest in metal-catalyzed polymerizations is increasing, and some promising systems have been described. 

One of the main efforts towards the development of sustainable chemistiy is the reduction in the generation of waste. The largest amount of waste produced by a reaction is typically associated with the solvent employed as it is the component added in the largest quantity and, usually not incorporated into the final product but rather removed and disposed of at the end of the process. For these reasons, many efforts are being devoted to the development of catalytic systems that can operate under solvent-free conditions. When the use of a solvent is mandatory, the efforts are directed towards the use of sustainable solvents. In this context, water, which is the only natural solvent, is the preferred choice. Other green alternatives are ionic liquids or supercritical carbon dioxide. 

The products - mainly aldehydes and unreacted or isomerized alkene - are simply removed by decantation and filtration, extraction with the aid of an organic solvent immiscible with the ionic liquid or using supercritical carbon dioxide, and distillation. 

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