Suppressing Metal Leaching in a Supported Co/SiO2 Catalyst with Effective Protectants in the Hydroformylation Reaction

For the hydroformylation of alkenes, developing ligand-free heterogeneous catalysts is a research focus because of both fundamental research interests and potential commercial applications. However, the leaching of active metals (e.g., Co, Rh) during the hydroformylation reaction seriously hinders the development of heterogeneous catalysts. Suppressing metal leaching with the use of an effective protectant is a possible solution to this problem. In this work, Co leaching was suppressed by the addition of suitable protectants (e.g., citric acid, oxalic acid, and formic acid), thus providing a stable heterogeneous catalytic system. In situ Fourier transform infrared proved that toluene and 1-hexene can promote the solvation of Co carbonyls because of the solvent effect. In contrast, formic acid can be converted to formate species (HCOO) on the surface of Co and effectively suppress the formation of Co carbonyls. Theoretical calculations further proved that high CO coverage on the surface of Co leads to the formation of Co carbonyl species. Furthermore, carboxyl radicals resulting from the dissociation of the O-H bonds of organic acids exhibit strong adsorption on the surface of Co. The intrinsic role of the protectants in suppressing metal leaching is attributed to the decreased CO coverage on the surface of Co by competitive adsorption.