| Cyclohexanone is an important industrial raw material and its synthetic products are widely used in the fields of textiles,electronic appliances,plastics,petrochemicals and pharmaceuticals.Selective hydrogenation of phenol is considered to be the most promising process in the industry to produce cyclohexanone.Noble-metal catalysts such as Pd,Pt and Ru,are intensively applied for the transformation of phenol to cyclohexanone.It is the key point and difficulty to improve the cyclohexanone selectivity under the premise of maintaining high activity since cyclohexanone is easily hydrogenated to cyclohexanol.Although many studies have focused on this field,developing catalysts with higher activity and selectivity and systematically studying the reaction mechanism are still great challenges.In spite of considerable works reveal that alkaline additives are benefit for improving the catalytic performance of transition-metal catalysts,a comprehensive understanding on the function mechanism of sodium additive is still unavailable.Here in this article,a series of Na-Pd/Al2O3 catalysts using different Na alkaline additives(Na2CO3,NaHCO3 and NaOH)were prepared by step impregnation method and studied in detail for the continuous hydrogenation of phenol in gas phase.Preliminary kinetics,density functional theory calculations,spectroscopic studies demonstrated that the adding of Na alkaline additives promoted phenol adsorption and H2 activation,thereby improving the catalytic activity.The formation of "-C=O-Na-" between carbonyl in cyclohexanone and Na species in Na-Pd/Al2O3 catalyst effectively inhibited the excessive hydrogenation and coupling reaction of cyclohexanone,leading to an increased selectivity.Thanks to the Na alkaline promoters,the phenol conversion was boosted from 8.3%to greater than 99%and the cyclohexanone selectivity increased from 89%to more than 99%.No activity or selectivity decay was observed even after a continuous 1200 h of reaction.It showed that the catalytic system had excellent catalytic activity,selectivity and stability.During the catalyst preparation or reaction processes,the metal particles in the metal supported catalysts are easily agglomerated which cause the catalyst deactivation.In recent years,cerium oxide has received more and more attentions in the heterogeneous catalysis due to its unique physicochemical properties.Here,a series of Pd/CeO2 catalysts were prepared by air calcination followed with hydrogen reduction.XRD,H2-TPR,HRTEM,XPS characterizations indicated that the Pd metal sites were re-dispersed,resulting to the highly dispersed Pd/CeO2 catalysts.When the prepared Pd/CeO2 catalysts were applied to the liquid-phase hydrogenation of phenol,the catalytic activity increased with the increase of calcination temperature,achieving the best performance in the Pd/CeO2-500 systems(98%conversion,500? Cair calcination).The series of Pd/CeO2 catalysts all exhibited excellent cyclohexanone selectivity(>99%).The increase in catalytic activity was attributed to the redispersion of the metal sites by air calcination.The DFT calculations indicated that in the Pd/CeO2 systems with smaller metal particle size,phenol was dissociated into phenoxyl and then adsorbed on the catalyst surface.The carbonyl oxygen was bonded with the Ce sites of the support which could protect the carbonyl group from hydrogenation.The hydrogen activated in Pd sites hydrogenated the benzene ring to cyclohexanone,and the produced cyclohexanone subsequently migrated on the CeO2 surface.As the reaction proceeds,the new phenol molecules occupied the active sites,and cyclohexanone was desorbed from the catalyst,thereby resuting in an excellent selectivity.In summary,studies concerning the selective hydrogenation of phenol have been carried out to produce cyclohexanone efficiently in our work.A series of catalysts with excellent catalytic performance were designed and prepared.The exploration of reaction mechanisms can provide guidance for the catalyst design and subsequent research. |
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