Selective hydrogenation of phenol by phosphotungstic acid modified Pd/Ce-AlOx catalyst in high-temperature water system

Hydrogenation of phenol was investigated in a high-temperature water (HTW) system. The characterization of the catalyst used in the system was investigated by BET surface area, XRD, XPS, and TEM testing methods. The results indicated that phosphotungstic acid (PTA) modified Pd/Ce-AlOx catalyst, i.e., Pd/W-Ce-AlOx, could not only increase the values of binding energy of the major elements and thus enhance its ability in attracting electrons, but also enhance the phenol degradation and selectivity in HTW when comparing with non-PTA modified catalysts. The Pd/W-Ce-AlOx catalyst also showed a high activity for the phenol hydrogenation and high selectivity at a moderate temperature (180 °C) in HTW system, because cyclohexanone is the major products (more than 95%) from the reaction. However, at the temperatures greater than 260 °C, the changes of transport properties would increase the dissociation of cyclohexanone from the acidic sites, which resulted in the future reactions with hydrogen to form cyclohexanol rather than cyclohexane. Based on the observation, a kinetic model of detail pathways to give the insight views was proposed. The model confirmed that at higher temperatures over 260 °C, the reactions to form cyclohexanol, benzene and cyclohexane increased dramatically. In the presence of Pd/W-Ce-AlOx catalyst, the formation of benzene can be effectively suppressed. Therefore, PTA modified catalysts have a great potential to improve the hydrogen selectivity of phenol in the HTW system.