Heterogeneous ethylene hydroformylation on polymer supported Rh-based catalysts: a DFT analysis of the mechanism and Rh–P effect

Abstract

The loss of the precious metal Rh exists in the homogeneous catalytic formation of propionaldehyde by ethylene hydroformylation. Recently, Rh-based catalysts supported on porous vinyl triphenylphosphine (3V-PPh₃) polymers have been recommended for their high catalytic activity and stability. However, the effect of the carrier-ligand bifunctional interaction of 3V-PPh₃ on the mechanism of heterogeneous ethylene hydroformylation and why Rh is not easy to lose are less understood. Therefore, in this work, based on two kinds of crosslinked 3V-PPh₃ supported Rh-based catalysts, the formation process of catalyst active species after CO insertion and the reaction mechanism of ethylene hydroformylation to propionaldehyde were studied by the density functional theory (DFT) method. Meanwhile, the relationship between Rh–P bond strength and Rh activity in the reaction process was also revealed. It is found that in the step of forming a new active species, HRh(CO)₂(P-frame)₂, after CO insertion, the Rh and P coplanar configuration of crosslinking mode 2 is the most stable, and the Rh and P non-coplanar configuration of crosslinking mode 1 has the highest Rh activity (E_a = 11.99 kcal mol⁻¹). In the R1–R4 main and side reactions of propionaldehyde synthesis, the polymer catalyst of crosslinking mode 2 is more favorable for the CO coordination of the main reaction, while that of crosslinking mode 1 can effectively improve the selectivity of ethylene hydroformyl to propionaldehyde. The Rh activity and Rh–P bond strength changed alternately in the step of propionaldehyde synthesis, and the change trend of the two is inversely proportional. It is suggested that the main goal of designing new Rh-based heterogeneous catalysts is to adjust the electron-donating ability of organic phosphine ligands to obtain moderate Rh–P strength.