Evaluation of functionalized silicaʹs for the adsorptive recovery of homogenous catalysts through interaction with the metal centre

The goal of this paper is the evaluation of functionalized silicaʹs for the recovery of homogeneous catalysts by adsorption via its metal centre. As model catalysts, we selected bis(triphenylphosphine)cobalt(II)dichloride (CoCl2(PPh3)2), bis(triphenylphosphine)palladium(II)dichloride (PdCl2(PPh3)2) and tris(triphenylphosphine)rhodium(I)dichloride (RhCl(PPh3)3). Twelve functionalized groups selected from four classes containing one or more N-, O-, P- or S-atoms were evaluated. A preliminary selection of the adsorbents was done by investigating the adsorption of the metal salts for the cobalt and the palladium complex. The results could be explained by the Hard and Soft Acid Base (HSAB) theory. For the most suitable functionalized adsorbents, these experiments were extended by introducing the ligand in the system which promoted the competition of the functionalized groups on adsorbent and the ligands present in solution. These experiments demonstrated that different complex species are adsorbed. 2-(2pyridyl)ethyl-functionalized silica is selected as a promising adsorbent for adsorption of the CoCl2(PPh3)2 from acetonitrile, while 3-(mercapto)propyl-functionalized silica is selected as a promising adsorbent for adsorption of the PdCl2(PPh3)2 and RhCl(PPh3)3 from DMF. The presence of a ligand, an increase of the temperature and the presence of a solvent with the donor properties can decrease the adsorption equilibrium and need to be taken into account.