Highly Efficient Phenol Hydrogenation to Cyclohexanone over Pd/MIL-100 in Aqueous Phase: Promotion of Lewis Acidity

Abstract

Selective hydrogenation of phenol to cyclohexanone is of great significance for the production of value-added chemical intermediates and products. Pd/MIL-100(Cr), featuring highly dispersed Pd nanoparticles (∼2.5 nm), were synthesized using a double-solvent impregnation method followed by NaBH₄ reduction. During aqueous-phase phenol hydrogenation, the catalyst achieved full conversion with 98.3% selectivity toward cyclohexanone within 1 h at 0.1 MPa and 100 °C. The catalyst exhibited high stability, maintaining performance up to 5 cycles without deactivation. The Lewis acidity of MIL-100(Cr) significantly influenced the phenol hydrogenation selectivity. By incorporation of Al, V, and Ce species into the MIL-100(Cr) framework, cyclohexanone selectivity in phenol hydrogenation was found to increase with the amount of Lewis acid sites (LAS). The acid properties of the Pd/MIL-100 catalysts were investigated using NH₃-TPD, pyridine-adsorbed FT-IR, and EPR probe molecules. The interaction between Pd nanoparticles and the coordinatively unsaturated sites from the trimeric clusters of MIL-100 was found to be crucial in determining the Lewis acidity of the Pd/MIL-100 catalysts. Demonstrating a synergistic effect between the Pd and LAS, Pd/MIL-100(Cr) emerges as a promising candidate for the selective hydrogenation of phenol to cyclohexanone.