Study On Green Synthesis Of Aromatic Acids By One-Step Carboxylation Of Aromatic Phenols

The intensification of human activities and heavy combustion of fossil fuels produce a large amount of CO2, which leads to the greenhouse effect becoming more deteriorating. However, as an significant carbon source, how to enhance the CO2 utilization as well as reduce the CO2 emission are becoming a pressing problem to be solve urgently.The Koble-Schmitt Reaction is the reaction to give ortho hydroxyl aromatic formic acid from phenols and CO2 under pressure. It is an important way to utilize CO2 directly to synthesis high valued chemicals and has already find wide applications in industry. However, it is still faced some problems. The traditional procedure is normally divided into two steps: i) the formation of phenoxide; ii) the carboxylation of phenoxide with CO2. In the first step, phenoxide must be strictly dry because it is very deliquescent and readily oxidized. Besides, two steps are also needed. These problems mentioned above result in huge energy consumption and do not comply with the green chemistry philosophy. Recently, researchers focus on developing one-step procedure to synthesize aromatic formic acid.The first part of the work focuses on the synthesis of 5-aminosalicylic acid(5-ASA). Several parameters such as temperature, CO2 pressure, reaction time and filling factor of the substance have been investigated. The experiments show that 99.2% GC yield is obtained after 1 h reaction at 180 °C under 2.5 MPa CO2 pressure using K2CO3 as the base. After simple treatment, the product purity can reach the EP 8.0 standard( > 99.95%). Nevertheless, the procedure requires large amount of K2CO3 under the optimal condition, which leads to heavy CO2 emission during the following acidification process. Therefore, the whole process is practically CO2 emission increase, but not CO2 emission reduce through calculation.In order to achieve the CO2 net absorption goal, the work aims to modify the process above. The second part of the work is based on one-step carboxylation of o-cresol as a model reaction. Various reaction parameters such as temperature, pressure, and the ratio of the base to substrate have been tested. The results showed that 95.3% total yield of all carboxylated products in this Koble-Schmitt reaction are obtained using K3PO4 as a neutralized base under 3.0 MPa CO2 and 150 °C. Additionally, no CO2 is released during the neutralization process following the synthesis and the K3PO4 could be recovered and recycled with the neutralization of KH2PO4 with KOH. Therefore, compared to the traditional route, this can truly realize the CO2 net absorption with high total yield, which makes it a green process for CO2 fixation.