Study of the role of alkaline sodium additive in selective hydrogenation of phenol

The selective hydrogenation of phenol to cyclohexanone is an important process in the chemical industry. However, achieving high selectivity at high conversion rates is highly challenging, particularly under continuous reaction conditions. Here, we found that the presence of Na alkaline additives (NaX, X = CO32–, HCO3–, or OH–) on Pd/Al2O3 not only promoted the phenol conversion from 8.3% to >99% but also increased the cyclohexanone selectivity from 89% to >97% during the continuous hydrogenation of phenol on a fixed bed reactor. After 1200 h of continuous reaction, no activity or selectivity attenuation was observed and the turnover number was approximately 2.9 × 105. Density functional theory calculations, spectroscopic, and dynamics studies demonstrated that the addition of NaX greatly promoted phenol adsorption and hydrogen activation, thereby improving catalytic activity. Simultaneously, the formation of a "-C=O-Na-" intermediate inhibited the excessive hydrogenation and intermolecular coupling of cyclohexanone, leading to high selectivity.