Promotion of diphosphine ligands (PPh2(CH2) PPh2, n = 1, 2, 3, 5, 6) for supported Rh/SiO2 catalysts in heterogeneous ethene hydroformylation

Hydroformylation is applied in industry to convert olefin and syngas to value-added products. Rh promoted by phosphine ligands are highly efficient catalysts for this reaction. The promotion mechanism of phosphines has been well studied in homogeneous system. It is important to extend the use of these ligands to heterogeneous system. The dpp* (PPh2(CH2)nPPh2, n = 1, 2, 3, 5 and 6) ligands were introduced to Rh/SiO2 and the ethene gaseous hydroformylation reaction was tested. Compared to unmodified Rh/SiO2, all the catalysts modified by phosphine showed enhanced activity (~100 times higher for the best ligand) and oxygenate selectivity (from 65% to ~97%). Among the investigation, L5 and L6 ligands exhibited the best performance while L1 and L2 only gave slight promotion. CO cutoff transient experiments were designed to identify the rate-limiting step (RLS) of hydroformylation reaction over the present catalysts. A mechanism was proposed for the change of RLS over the phosphine modified catalysts. Combined with in situ FTIR results, the relationship between electronic state and catalytic activity of the catalyst was verified.