Studies On Catalysts For Hydroxylation Of Phenol Using Hydrogen Peroxide As Oxidant

Diphenol (catechol and hydroquinone) is an important fine chemical intermediate product, which can be widely used as pharmaceutical applications, pesticides and dyestuff industry etc. The process of synthesize diphenol with hydrogen peroxide oxidation method, which is recognized as environmental friendly and raw materials cheap, would become one of the most promising production technology in 21st century. The catalysts for hydroxylation of phenol might be classified into types as follows: Heteroatom-substituted molecular sieves, heteropolyacid salts, metal oxides, composite metal oxide and kinds of others. However, there are no ideal catalysts for this reaction. The synthesis of functional molecular sieves is usually sophistic, highly costs, undergo with harsh synthetic condition, unstable activity and hardly reduplicate with different labs. Although the heteropolyacid compounds suitable for preparing single and highly selectivity diphenol isomers, partially reduce the separate energy-consuming. The disadvantage also exists in catalyst like highly preparation costs, difficultly recycle, strong solvents toxicity and large dosage hydrogen peroxide required and serious decomposition inevitable. The catalysts made from metal oxides or composite metal oxides are lower costs, simple preparation and stable. Expectation is focus on its relative lower activity and selectivity improving. In this paper, we will discuss the factors may have an impact on the synthesis process and optimize the synthetic conditions, especially on the activity and selectivity properties of catalysts.Catalytic performance for the hydroxylation of phenol with hydrogen peroxide using Fe₂O₃ reduced by KBH4 as catalyst had been studied. The effects of reaction temperature, the molar ratio of H₂O₂ to phenol and reaction time on the hydroxylation of phenol were investigated. The products are analyzed by High Performance Liquid Chromatography. The experimental results show that phenol conversion can reach18%～62% and diphenol selectivity above 85% as various molar ratio of hydrogen peroxide to phenol. With the increase of molar ratio of hydrogen peroxide to phenol, the selectivity of diphenol decreased slowly but changes very little. With water as the solvent, phenol/H₂O₂ =1.5 (molar ration), reaction temperature, catalyst/ phenol (mass ratio)= 5%, reaction temperature 40～80℃, giving diphenol selectivity of 85% and diphenol yield of 53%.Fe(OH)₃ was prepared with precipitation method, which was the best catalytic activity catalyst for phenol hydroxylation. The effects of the amount of catalyst, the molar ratio of H₂O₂ to phenol, reaction temperature and reaction time on the hydroxylation of phenol were investigated. With water as the solvent, reaction temperature 40～90℃, catalyst/ phenol (mass ratio)=0.5%～5%, phenol/ H₂O₂ = 0.8 (molar ration), giving phenolconversion of above 40% and diphenol selectivity of 88.0%.In this work, induction period and mechanism are also discussed preliminarily combined with the reactive characteristics of Fe₂O₃ and Fe (OH)₃, trying to shed some light on the nature of this system.