Study On Air-promoted Phenol Hydrogenation To Cyclohexanone Over Pd-based Catalyst

Cyclohexanone is an important industry intermediate in the synthesis of nylons and resins.Oxidation of cyclohexane is a traditional method for cyclohexanone production,which usually requires high energy consumption and exhibits low conversion and selectivity.With the increasing global crisis of environment and energy,there is an urgent need to find a low-cost and high efficient method to replace the traditional process.Selective hydrogenation of phenol is therefore considered as a promising route and Pd based catalyst which shows low deeply hydrogenation rate of cyclohexanone is thought to be the best choice for this process.The conversion and selectivity are exceeding 99%over Pd based catalyst at relate low temperature(<100?).More importantly,organic solvent also can be replaced with water,which conforms to the concept of "green solvent".However,when using the Pd catalysts,the relate high apparent activate energy(55-70 kJ/mol)of phenol hydrogenation to cyclohexanone usually led to a low reaction rate.Although the increase of reaction temperature could improve the reaction activity of phenol to cyclohexanone,the low selectivity caused by the deeply hydrogenation of cyclohexanone to cyclohexanol and high energy consumption are inevitable.Consequently,a new and effective method for the selective hydrogenation of phenol to cyclohexanone at low temperature is highly desired.In this thesis,a new strategy to activate substrate and enhance the hydrogenation rate of phenol to cyclohexanone is successfully developed by introducing air.The possible mechanism for the activation process is proposed.Major results have been summarized as follow:(1)Pd nanocatalyst with exposed(111)planes supported on Al2O3(Pd/Al2O3)was in-suit synthesized as a model catalyst to investigate the catalytic performance of phenol hydrogenation to cyclohexanone with the presence of air.By using water as the solvent,the conversion and selectivity of phenol hydrogenation at 30 ? are higher than 99%,and the corresponding TOF is about 80 times higher than the state-of-art Pd catalysts.(2)The mechanism research reveals that hydroxyl radicals derived from hydrogen,oxygen and protic solvent on the surface of Pd(111)can activate the phenol and generate phenoxyl radicals,resulting in the decreased of apparent activation energy and enhanced hydrogenation rate of phenol to cyclohexanone dramatically.(3)The strategy of air-promoted phenol selective hydrogenation to cyclohexanone can be extended to other Pd based catalysts.