Kinetic study of the aqueous Kolbe-Schmitt synthesis of 2,4- and 2,6-dihydroxybenzoic acids

• Published in: Chemical engineering science 2019-02, Vol.195, p.107-119
• Main Authors: Zhang, Xi-Bao, Liu, Yuan-Xing, Luo, Zheng-Hong
• Format: Article
• Language: English
• Subjects: 2,4-Dihydroxybenzoic acid; 2,6-Dihydroxybenzoic acid; Activation energy; Kinetic model; Kolbe-Schmitt reaction; Rate-determining step
• ISSN: 0009-2509; 1873-4405
• DOI: 10.1016/j.ces.2018.11.045

•Kinetic models for the Kolbe-Schmitt synthesis of 2,4- and 2,6-DHBA were developed.•The average relative error between calculated and experimental results is 2.89%.•Effects of various variables on the yields of 2,4- and 2,6-DHBA were investigated.•The rate-determining step of 2,6-DHBA formation was studied. The Kolbe-Schmitt reaction is the traditional method for preparing 2,4- and 2,6- dihydroxybenzoic acid (2,4- and 2,6-DHBA). In this work, accurate kinetic models for the Kolbe-Schmitt synthesis of 2,4- and 2,6-DHBA were successfully developed. The relative errors between the theoretical and experimental 2,4-DHBA equilibrium yields are less than 3.7% as T = 348–473 K, [C6H6O2] = 0.4–0.8 M and [KHCO3] = 1.2–4.0 M. The effects of reaction temperature, reaction time, KHCO3 and resorcinol (C6H6O2) concentrations on the formation rates and the yields of 2,4- and 2,6-DHBA were investigated by the developed models. Results show that the 2,4-DHBA equilibrium yield exhibits a strong dependence on the KHCO3 concentration, and the formation rate of 2,4-DHBA is strongly dependent on the reaction temperature. Furthermore, the reaction time is a critical factor in controlling the ratio of 2,4-DHBA to 2,6-DHBA as the reaction temperature varies from 433 to 473 K. Additionally, the results of the present work deepen the understanding of the reaction mechanism, e.g. the rate-determining step and main reaction pathway of 2,6-DHBA formation.