| Kolbe-Schmitt reaction is an important industrial method of preparing hydroxybenzoic acid, but the disadvantages in the present production process are obvious, including low reaction yield, long reaction time, the large amount usage of catalyst which can not be recovered. The reaction mechanism, plus the catalytic mechanism are still unknown, which is meaningful to understand the carboxylation more clearly. In this paper, we carried a systematic study on the reaction characteristics of the Kolbe-Schmitt reaction by synthesizing3,6-dichloro salicylic acid (3,6-DCSA), verified the possible side effects, and suggested a new technology to suppress the side reaction and finally improved the yield. Molecular simulation was used to investigate the reaction mechanism, also the reaction thermodynamics and kinetics were calculated theoretically, strengthening an in-depth understanding of the Kolbe-Schmitt reaction. Experimental and computational methods were used to explore the hybrid catalytic mechanism of K2CO3. At last, large particles of potassium carbonate with the micropores were utilized in the carboxylation reaction, to solve the problem of recovery caused by the conventional powdered catalyst.The influencing factors of synthesizing3,6-dichloro salicylic acid from2,5-dichloro phenol and CO2in organic solvent were studied in a batch autoclave. By screening the raw material and catalyst from the ones reported in the literature, Potassium2,5-dichloro phenoixde is proved to be the best carboxylated raw material and the potassium carbonate shows higher catalytic activity. The optimum conditions for the reaction is a temperature of160℃, a pressure of6MPa, a stirring rate of600rpm, a reaction time from4to6h, a molar ratio of potassium carbonate to phenol of1:1. An aprotic solvent with low dielectric constant such as xylene is the suitable solvent for the Kolbe-Schmitt reaction. Maintaining anhydrous state of system is a critical factor to obtain a high yield of3,6-DCSA, because trace moisture will cause a serious decline in the yield. The particle size distribution of2,5-dichloro phenol potassium in suspension was determined by a wet laser particle size analyzer, and an average particle diameter of the potassium salt is obtained around in8μm. A shrinking core model based on2,5-dichloro phenoxide as the reacting nuclear indicated that the reaction is ash layer controlled reaction under higher temperature, while is more likely surface reaction controlled under lower temperature.Liquid chromatography-electrospray ionization-tandem mass spectrometry (LC-ESI-MS) was first used to analysis the composition of aqueous production, to find the existing form of3,6-dichloro salicylate. The positive ESI mass spectra of3,6-dichloro salicylate showed only one ion peak [M+K]+at320.8m/z, which means di-potassium salt is the unique existing form of3,6-dichloro salicylate. HPLC analysis indicated that equivalent2,5-DCP as3,6-DCSA is also generated as the by-product. These discoveries provided explicit evidences to prove the existence of side reactions, causing a theoretical maximum yield of50%, which is thought to be the primary reason for low yields of3,6-DCSA. In addition,2,5-DCP not only inhibits a positive reaction balance but also deteriorates the mass transfer, eventually led to a seriously decline in yield. A secondary addition of alkali is suggested to dehydrate the phenol formed by side reaction to phenoxide, which significantly improves the yield of3,6-DCSA from47%to75%.The detailed thermodynamics of synthesizing3,6-dichlorosalicylic acid by Kolbe-Schmitt reaction was studied by using the B3LYP method of density functional theory. All of the structures of the reactants and products were optimized under6-311++g (3df,3pd) level. The changes in reaction enthalpy, Gibbs free energy and equilibrium constant under reaction temperature (300K-600K) for the primary and side reaction were respectively calculated and analyzed with the basic thermodynamic data obtained from vibration frequency analysis. The result shows that Kolbe-Schmitt reaction processes are exothermic but the primary reaction can’t spontaneously occur at lower temperature and lower pressure while the side reaction is entirely feasible on thermodynamic which can proceed easily under mild conditions. In addition, the Kolbe-Schmitt reaction is reversible that the equilibrium conversion could be promoted by increasing the pressure of CO2while crucially inhibited by increased phenol forming in side reaction.Density functional theory based molecular modeling method was also used to study the mechanism of both the primary and side reaction. The reaction route was optimized using B3LYP/6-311+G (d, p) basis set, showing a path including an electrophilic attack by CO2, and then followed by a proton transfer. The chlorine atoms substituted on benzene ring as electron-attracting groups cause higher activation energy barriers for electrophilic attack than the one in the case of typical unsubstituted phenol. The in-situ Fourier Transform Infrared (FTIR) spectroscopy technology under reaction conditions was employed to confirm the validity of the calculated mechanism. The results revealed the forming path of carboxyl, by showing firstly a carboxyl stretching vibration absorption bands at1739cm-1, which then splits into two bands at1584cm-1and1472cm-1assigned to the carbonate as the final product. The calculated vibrational spectra and experimental spectra showed a good agreement, giving a hypostatic evidence for the reliability of the calculated3-intermediates and3-transiton states mechanism for Kolbe-Schmitt reaction. Theoretical investigation by DFT revealed that the formation of2,5-DCP via the side reaction can undergo easily through a Brnsted-Lowry proton transfer mechanism, which is characterized by the rotation of carboxyl with a favorable thermodynamic potential. The conventional transition state theory (CTST) was employed to study the dynamics of synthesis of3,6-dichloro salicylic under the reaction condition (160℃,6MPa) based on the calculated mechanism. It was found that the reaction rate is fast in the initial5h, and then8h later reaching a maximum equilibrium yield of about42%, which agree well with the experiments.Hybrid promoting effects of potassium carbonate were found in the Kolbe-Schmitt reaction, which can both enhance the reaction rate and increase the equilibrium yield. On the one hand, CO2-TPD of K2CO3showed that there are two weak base center on the surface of K2CO3. Through DFT theoretical calculations, CO2is adsorbed and activated on (001) surface of potassium carbonate, which has the lowest surface energy. The CO2adsorbed on the surface is converted into monodantate carbonate structure, with an adsorption energy of-0.35eV, The C-O bond is elongated and C atom is highly activated with a high nucleophilicity, which can easily electrophilic attack the benzene. On the other hand, K2CO3acts like an alkali, which dehydrates2,5-dichloro phenol formed by the side reaction and produces2,5-dichloro phenoxide which can be further converted to3,6-dichloro salicylate, thus improve the final equilibrium yield. The conversion of phenol was11%in the case of K2CO3/2,5-DCP molar ratio is1.Large particle K2CO3(Φ3mm) with microporous was first used in the Kolbe-Schmitt reaction. The particles have one third specific surface of the powdery catalyst, but the same catalytic activity. After wash and calcination, the recovered catalyst can be regenerated with the same crystal texture as the fresh K2CO3, and shows the catalytic activity. After three repeated use of the catalyst, the yield of3,6-DCSA decreased from49%to42%. The use of large particle size of potassium carbonate makes recovery and reuse of the catalyst possible, also provides the foundation for the fixed bed continuous process of the industrial production of3,6-DCSA. |
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