Experimental Study On The Synthesis Of3-Methyl Salicylic Acid From Carboxylated O-Cresol In Supercritical Or Subcritical Carbon Dioxide

The organic synthetic reactions participated with supercritical orsubcritical carbon dioxide are not only consistent with the green synthesisconcept, but also expand the utilization of carbon dioxide. Thus, it hasbecome a research hotspot in recent years. As an important dye andpharmaceutical intermediates,3-methyl salicylic acid is widely used inrelated fields and it has aroused many scholars’ interestes in it’s newexploration of the synthetic methods based on the green chemistry.Therefore, one-step direct synthesis of3-methyl salicylic acid insupercritical or subcritical carbon dioxide has an important significance forboth the green synthesis of the3-methyl salicylic acid and the utilization ofcarbon dioxide.We compared the catalytic activity of anhydrous potash, anhydroussodium carbonate and so on. Finally, anhydrous potash was chosen to bethe best catalyst to synthesize3-methyl salicylic acid directly from o-cresolin supercritical or subcritical carbon dioxide by one-step method. Theo-cresol′s carboxylic reaction was studied in supercritical or subcriticalcarbon dioxide under conditions that catalyst′s relative dosage（nK2CO₃/nO Cresol）range of0～1.25（mmol/mmol），reaction time range of1～7hours，reaction temperature range of403⁵93K and reaction pressurerange of4～14MPa.The results showed that the conversion of o-cresol and the yield of3-methyl salicylic acid increased rapidly at the very start and then increasedslowly with the increase of catalyst′s relatively dosage（mmol/mmol）. Both the conversion of o-cresol and the yield of3-methyl salicylic acid displayeda growing trend with the extension of time，however the conversion ofo-cresol became stable at last，while the yield of the3-methyl salicylic aciddecreased slightly. With the rise of tempreture，both the conversion ofo-cresol and the yield of3-methyl salicylic acid increased at first and thendecreased，however the conversion of o-cresol rised again at last，while theyield of the3-methyl salicylic acid became stable. With the increase ofreaction pressure, both the conversion of o-cresol and the yield of3-methylsalicylic acid increased initially and then decreased. The optimalexperiment condition needs a catalyst’s relatively dosage of0.5（mmol/mmol），a reaction time of3hours，a reaction temperature of463K and a reaction pressure of8MPa, under this condition the conversionof o-cresol would come up to19.7%and the yield of3-methyl salicylicacid17.5%. In addition，a possible reaction mechanism of the o-cresolcarboxylation reaction in supercritical or subcritical carbon dioxide wasproposed and we obtained the value of activation energy Ea+=47.63kJ/molby the reaction kinetics research.