Activity Promotion of Rh8–xCoxP4 Bimetallic Phosphides in Styrene Hydroformylation: Dual Influence of Adsorption and Surface Reaction
The development of a highly active heterogeneous catalyst for styrene hydroformylation is a major challenge, and we previously found that Rh2P was a good candidate. By doping a second metal of Co, we further improved the activity by 1.7 times, and the highest TOF reached 2563 h–1 on Rh7Co1P4/SiO2. The addition of Co tuned the electronic environment of surface Rh atoms without changing the crystal structure, and the reaction activity showed a volcano relationship with the Co ratio. DFT calculations showed that Co doping decreased the overall activation energy of surface reactions and destabilized H2 adsorption simultaneously. When x ≤ 2 for Rh8–xCoxP4/SiO2 catalysts, surface Rh atoms had positive valence and H2 adsorption was exothermic. Therefore, the hydroformylation rate was mainly determined by surface reaction with a first order of styrene concentration. When x > 2, however, surface Rh atoms were negatively charged and H2 adsorption became thermodynamically unfavorable. The reaction activity was then determined by H2 adsorption and was independent of styrene concentration. We introduced δE, which was the sum of overall activation energy and H2 coadsorption energy with styrene, to predict activity. Moderate addition of Co decreased the value of δE and promoted the activity, while excessive doping would increase δE.
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