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p-Hydroxybenzaldehyde (PHB) is an important intermediate for a large variety of chemical products used in consumables such as soaps, fragrances, Pharmaceuticals, preservatives, pesticides, etc. The p-Hydroxybenzaldehyde can be synthesized either from phenols or from cresols. In the past decade, considerable effort has been paid on the development of green methods for the oxidation of substituted methylphenols to phenolie aldehydes. However, conventional methods adopted in industry have suffered from several disadvantages, such as high energy consumption, usage of heavy metal catalysts, high amount of NaOH used, and large amount of inorganic salts which may arise high separation cost and serious effluent problems to the environment, especially using methanolic solutions in this oxidation process, makes it virtually impossible to operate this oxidation on a large scale, unless specialized equipment is designed to overcome the safety issues. These issues existed because methanol is a toxic, flammable liquid and has a low flashpoint (11℃) and a wide flammability limit range (6to35%per volume).To solve the problems mentioned above, a method of p-Hydroxybenzalde-hyde synthesis from p-cresol was investigated. In this method, Co3O4was used as the catalyst, green and low-cost molecular oxygen was applied as oxidant instead of chemical oxidants, a mixture of ethylene glycol (EG) and water as the solvent, which can maintain the advantages gained by using methanolic solvent and overcome the drawbacks. Moreover, high yield of p-Hydroxybenzaldehyde was obtained. As a result, this method can reduce the environmental pollutions and the cost of salt separation in industry.In our research, besides different kinds of catalysts, other reaction parameters such as reaction time, temperature, the molar ratio of sodium hydroxide to p-cresol, type of solvent were also systematically studied. The optimized reaction conditions are as follows:0.7g Co3O4as catalyst, reaction time26h, reaction temperature90℃, p-cresol initial200mmol,3.3molar ratio of sodium hydroxide to p-cresol, a mixture of ethylene glycol (EG)(85ml) and water (15ml) as the solutions.Under this optimum reaction condition, the conversion of p-cresol and yield of p-Hydroxyblenzaldehyde reached100%and89.7%, respectively, as a result.Finally, the main product p-Hydroxybenzaldehyde could be simply separated from reaction mixtures through two-step crystallization. p-Hydroxybenzaldehyde, the desired product, was thus obtained with the purities>99.5%inspected by GC.
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