Kinetics Of Phenol Selective Hydrogenation Over Pd/C Catalyst

As an important basic chemical raw material,cyclohexanone is widely used in chemical industry and other fields.As a part of fine chemical industry,the development of phenol selective hydrogenation has huge economic benefits.The Pd/C catalyst was prepared by impregnation method using activated carbon as support.The selective hydrogenation of phenol over Pd/C catalyst in fixed-bed reactor was studied.The effects of reaction conditions on the conversion of phenol and cyclohexanone selectivity were investigated.Changes in temperature not only reduce the conversion of phenol,but also cause new side reactions in reaction system.The increase of pressure will increase the conversion of phenol,while the selectivity will decrease sharply.This may be related to the phenomenon of hydrogen spillover on the surface of the Pd/C catalyst.The selective hydrogenation of phenol is a gas-solid heterogeneous reaction,so the effect of the molar ratio of hydrogen to phenol and space velocity of phenol on reaction can be explained by the diffusion-reaction model.The optimal conditions for this reaction were obtained through experiments:the temperature is between 160 and 175?,the pressure is slightly higher than normal pressure but less than 0.4MPa,the molar ratio of hydrogen to phenol is among 4 and 6,and the space velocity of phenol is among 0.4 and 0.8h-1.This condition guarantees maximum yield of cyclohexanone.The kinetic model was obtained based on the Langmuir-Hinshelwood-Hougen-Watsont priciple.Hydrogenation step was used as rate-determing step.The activation energy of the cyclohexanone formation was approximately 59kJ·mol-1 while activation energy of cyclohexanol was nearly 3 9kJ·mol-1.It was proved by residual analysis and statistical test that the kinetic models are reliable and acceptable.The results of catalyst characterization showed that the main reasons for the deactivation of Pd/C catalyst in this system were the loss of metal Pd,the aggregation of Pd crystallites and the blockage of support micropores in the catalyst.