Although supported Rh nanoparticles (NPs) are one of the best catalysts for the selective hydrogenation of phenol to cyclohexanol, they still suffer from poor selectivity problems. In this study, we promoted the selectivity of Rh NPs with CoOx through an in situ transformation of the bimetallic RhCo NPs on SiO2. Among the obtained Rh–CoOx/SiO2 catalysts, the one with a Co/Rh molar ratio of 1/3 exhibited the highest phenol conversion (i.e., 98.1%) and the highest selectivity to cyclohexanol (i.e., 99.4%). Our study showed that CoOx promoters modified the electronic states of Rh and created new active sites at their interface, which are beneficial to the activation of phenol and the desorption of cyclohexanol. Excessive coverage of Rh by CoOx, however, would cause deactivation due to the less exposure of the Rh–CoOx interface.
Hong Zhang,Wenlong Lu,Chen Li,Xinyu Chen,Lei Wang,Hui Wan,Guofeng Guan
In this study, Rh supported on SiO2 was modified with Zr for the first time to investigate its catalytic performance in propylene hydroformylation. The RhZr/SiO2 catalyst exhibits a butyraldehyde selectivity of over 90% along with a 2.7-fold enhancement in propylene conversion compared to the unmodified catalyst, outperforming previously reported Co- and K-promoted systems. XPS characterization, combined with differential charge density calculations and Bader charge analysis, revealed that the introduction of Zr reduced the electron density around Rh, which facilitates CO adsorption on Rh and thereby promotes the formation of more active HRh(CO)x species. This conclusion is further supported by CO adsorption energy calculations. Under optimized reaction conditions (20 mL toluene solvent, 0.4 g RhZr/SiO2 catalyst with ultralow Rh loading of 0.02 wt%, 1.5 g propene, 100 °C, 4 MPa syngas pressure, 4 h), the catalytic system achieved remarkable performance: 75.1% propene conversion, 96.7% butyraldehyde selectivity, and a normal/iso (n/i) ratio of 0.9. Notably, the system exhibited an exceptional intrinsic activity (TOF = 8838 h−1), surpassing all reported ligand-free heterogeneous Rh-based catalysts in recent years. The present work provides an effective strategy for designing modified Rh-based heterogeneous catalysts with ultralow metal loading for hydroformylation applications.
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