Silica immobilized ruthenium catalyst for formic acid synthesis from carbon dioxide hydrogenation (III): Effect of ligand on the catalyst performance

The effect of ligand on the activity of immobilized ruthenium complexes synthesized in situ was investigated. The activity of the catalyst containing ZPh(3) ligand changed as follows: PPh3>AsPh3>NPh3. When PPh3 was used as ligand, the corresponding TOF of formic acid was 656 h(-1) over the catalyst. In addition, using bidentate phenyl phosphine as ligand instead of PPh3 resulted in an even higher TOF, the value was 1190 h(-1) when dppe was used as ligand. The results of quantum chemistry theoretic calculation indicated that the PPh3 ligand which had the moderate capability of sigma-donor and pi-acceptor, less steric hindrance and preferable electron delocalization, acted as a better ligand than the other ZPh(3) ligand. The bidentate phenyl phosphine ligands which had good electron delocalization and chelation acted as a better ligand than monodentate phosphine ligand.