Carboxylation Of 2-furoic Acid/Benzenecarboxylic Acid With CO₂ Catalyzed By Cs₂CO₃

The chemical application of CO₂ is increasingly concerned,attracting researchers to explore continuously.The formation of C-C bond involving CO₂has the advantages of simplicity and high efficiency compared with the traditional rout.Many high-value carboxylic acid chemicals can be synthesized on the basis of fully utilizing CO₂,making the research for direct carboxylation of CO₂ become a very active scientific research field.Therefore,continuously expanding the range of substrates for carboxylation reactions involved in CO₂and optimizing the reaction conditions will be of great significance for future carboxylation applications from CO₂.At present,there are few studies on the carboxylation between weakly acidic C-H containing aromatic acids(such as2-furoic acid or benzenecarboxylic acid)and CO₂,which is not conducive to the expansion of the application of carboxylation reaction for CO₂.Direct carboxylation by 2-furoic acid or benzenecarboxylic acid to produce high-value chemicals is also limited.Therefore,this article focuses on 2-furoic acid and benzene carboxylic acid as research objects,and conducting a preliminary exploration of the carboxylation reaction for them catalyzed by Cs₂CO₃ through the combination of quantum chemistry and experimental operation,in order to provide scientific guidance and potential support for the research of chemicals synthesis from CO₂,as well as to provide basic data for a green and efficient preparation of aromatic acid chemicals.In this paper,the structure of related substances in the process of carboxylation of 2-furoic acid or benzenecarboxylic acid was optimized by quantum chemical calculation,and the frequency,energy,molecular orbit,Mulliken charge and bond level of each substance were calculated according to the needs of inquiry.The thermodynamic properties,product selectivity and reaction mechanism of carboxylation of 2-furoic acid with CO₂ were analyzed.The thermodynamic properties,theoretical reactivity and the relationship between carboxylation product distribution and structural properties of carboxylation of benzenecarboxylic acid with CO₂ were explored.Then comparing and verifying the above calculation results through specific experimental operations.Besides,analyzing and discussing the environmental requirements of the carboxylation reaction of 2-furoic acid or benzenecarboxylic acid with CO₂,respectively.Preliminary conclusions were obtained as following:(1)When Cs₂CO₃ catalyzes the carboxylation of 2-furoic acid with CO₂,the product is single because there is no isomer of the target product2,5-furandicarboxylic acid(FDCA).The reaction can be spontaneously carried out and it is an exothermic reaction in the range of 200?600 K,when the pressure is close to 0.5 MPa.Theoretically,low temperature is conducive to the reaction,but it is found through experiments that the reaction requires a certain temperature to activate the reaction system,as a result,it is necessary to consider the two factors of thermodynamic stability and kinetic rate in the chemical reaction.At a reaction temperature of 553 K,a reaction pressure of 0.8MPa and a reaction time of 3 h,the yield of FDCA is about 60%.The continuous increase of temperature and pressure is not conducive to the reaction.The apparent activation energy of this reaction is 40.43 kJ?mol^-1,and the pre-finger factor is 2401.41 h^-1 calculated by kinetic exploration.(2)Through quantum chemical calculations,it was found that the mechanism of the carboxylation reaction between cesium 2-furoate and CO₂ can be divided into two major reaction steps.First of all,the Cs₂CO₃ with strong basicity interacts with the active H on cesium 2-furoate in the process of hydrogen abstraction.There is a transition state,and the reaction energy barrier is 4.47 kJ?mol^-1;Then there is attacking from CO₂ with certain electrophilic to the C atom which loses the H atom in cesium 2-furoate,causing the product is produced after an electrophilic adsorption.This process is the decisive step of the entire reaction path.(3)Quantum chemistry calculation results show that in the range of200-600 K when the pressure is close to 2 MPa,the carboxylation of benzenecarboxylic acid with CO₂ catalyzed by Cs₂CO₃ is spontaneous and exothermic.CO₂ is mostly an electrophilic reagent in the reaction of C-C bond formation,so it is speculated that the reaction activity of benzenecarboxylic acid in the carboxylation reaction with CO₂ is related to the Mulliken charge of the reaction site on the benzenecarboxylic acid substrate.The speculation was verified by a combination of theoretical calculations and experiments.According to the Mulliken charge,it can be speculated that the activity sequence of the carboxylation reaction for low carboxyl number benzene carboxylic acid,which is trimellitic acid>trimesic acid>hemimellitic acid>isophthalic acid>phthalic acid>benzoic acid>terephthalic acid.In addition,the Mulliken charge of the carbon atom at the reaction site of benzenecarboxylic acid can also determine the reactivity of different carbon atoms in the same substrate molecule.that is,the more negative the Mulliken charge at the carbon site,the easier it is to generate a carboxyl group at that site,and the higher the selectivity of the corresponding carboxylation product,which can effectively infer the distribution of carboxylation products of benzenecarboxylic acid.(4)The experimental results show that the carboxylation of benzenecarboxylic acid and CO₂ catalyzed by Cs₂CO₃ is effective.Even benzenepentanoic with steric hindered acid can also generate mellitic acid by carboxylation.The formation of salt hydrolyzed and mixed from substrate and Cs₂CO₃ facilitates the reaction.In the process of carboxylation of benzene carboxylic acid,CO₂ gradually adsorbs each reaction site according to the activity sequence of different carbon sites of the substrate molecule,so that carboxyl groups are formed sequentially.