Immobilization with the Help of Liquid Supports

In the Shell case, the classical homogeneous catalysis, an organic compound, i.e. a real liquid phase (a solvent) that dissolves aU reactants, catalysts, and products, is used. The role of the solvent is underlined by the fact that it has to be separated from the reaction products by an additional and costly step, for example by distillation. 

Olivier-Bourbigou and Hugues define the importance of solvents as follows  

The role of the solvent in organic reactions is of the utmost unportance. Its effect can Just be limited to a physical effect in making possible the solubilization of the reactants, with no direct interaction with the active center. More interesting are the cases in which the solvent interacts through specific forces, such as hydrogen bonds, thus altering the mechanism, the rate, and eventually the selectivity of the reaction in stabilizing certain reaction intermediates. 

In homogeneous catalysis, the solvent effect is often difficult to ejq)lain due to the large number of reaction intermediates involved. However, for catalytic applications involving coordinatively unsaturated cationic metal centers, the ideal solvent should be able to solubilize the metal ion, while maintaining its ionic character, and should create weak and labile metal-solvent bonds. 

It should be highly polar and non-coordinating for the active metal center. Most ofthe classical organic solvents are covalent and do not satisfy the above requirements. 

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