Study On The Preparation Of Salicylic Acid (sodium Salicylate) In Supercritical Carobn Dioxide

Supercritical CO₂ has been successfully applied in extraction and chemical reactions as an alternative solvent and reaction medium. In addition, CO₂ as one of the greenhouse gases, the fixation and utilization of CO₂ has been received great attention at home and abroad. Study on organic reactions involved supercritical CO₂ (CO₂ could act not only as a reagent but also as a solvent), which could expand the possible ways of CO₂ utilization and is the direction of green synthesis.In this paper the utilization of CO₂ and the production of salicylic acid, especially the utilization of CO₂ in organic synthesis based on supercritical fluid reaction are reviewed. The study on the carboxylation reaction in supercritical CO₂——the preparation of salicylic acid(sodium salicylate)in supercritical CO₂ is carried out.Sodium salicylate is synthesized in supercritical CO₂ from self-made sodium phenoxide. The yield of sodium salicylate is investigated in different preparation conditions of sodium phenoxide. Under the conditions of reaction temperature 105℃ 180℃, reaction time 0.5h 4h and reaction pressure 2 MPa 13 MPa, the carboxylation of sodium phenoxide in supercritical CO₂ has been studied. The results showed that the ideal molar ratio and temperature for sodium phenoxide preparation are 1.05 and 75⁸5℃repectively. the main product of the carboxylation is sodium salicylate and the selectivity is high. With temperature and pressure increased, the yield of sodium salicylate is increased and then decreased. With reaction time extended, the yield of sodium salicylate increased first and then tended to balance, and finally decreased slightly. Under the conditions of reaction temperature 150℃, reaction time 2 h and reaction pressure 8MPa, the yield of sodium salicylate can reach 54.6%. Based on the analysis of carboxylated product and experimental observations, a possible reaction mechanism is proposed. By the study on kinetics , the activation energy is 34.8 kJ·mol^-1.Selecting K₂CO₃ as catalyst, using phenol as raw material, the traditional two-step synthesis (Kolbe-Schimitt method) is replayed an one-step synthesis of salicylic acid from supercritical CO₂ and phenol. The carboxylation of phenol in supercritical CO₂ has been studied under the conditions of temperature 160℃²20℃, pressure 4 MPa¹2 MPa, the relative catalyst ammounts n_K2CO3/n_phenol=0.25-1.5, reaction time 1h 10h. The results showed that the main carboxylated product is salicylic acid with high selectivity, by-product is p-hydroxybenzoic acid of a very small amount. With the reaction temperature or pressure increased, the yield of salicylic acid is increased, then decreased. With the catalyst amount increased or reaction time extended, the yield of salicylic acid increased first and then became stable. The optimum conditions of the reaction are as followed:reaction temperature 190℃,reaction pressure 8MPa,reaction time 3-5h,amount of catalyst 5mmol(n_K2CO3/n_phenol=1/2),the yield could reach 46.6⁵1.2%. Based on the analysis of carboxylated product and experimental observations, a possible reaction mechanism is proposed. By the study on kinetics, the activation energy is 24.8 kJ·mol^-1.