Green Preparation Of Multi-fluo Robenzenes In NH₃-enriched High Temperature Liquid Water

Multi-fluorobenzenes and multi-fiuorobenzoic acids are both important fluorinated fine chemicals which are widely used in the synthesis of pharmaceutical, pesticide and liquid crystal material intermediates. Multi-fluorobenzenes can be prepared by decarboxylation of multi-fluorobenzoic acids. Organic amines with high boiling point, such as tri-n-butylamine, were commonly used as the traditional solvents. As the reaction medium for decarboxylation, organic solvents (eg. organic amines) have the issues such as high toxicity, difficulty recovery and environmental pollutionHigh temperature liquid water (HTLW) is an environmental benign medium. Along with the increases in temperature and pressure, the physical and chemical properties of HTLW shift dramatically and make it exhibit excellent performances for reaction. Hydrogen and hydroxyl ion concentrations in HTLW, however, are so low that the capacity for acid-/base-catalysis is weak and problems associated with using the HTLW include slow reaction rate, high reaction temperature, poor selectivity and abundance of by-products. In this thesis, multi-fluorobenzoic acids were used as the raw materials to prepare multi-fluorobenzenes. Pointing at these problems such as slow reaction rate and poor selectivity in HTLW, NH3-enriched HTLW was proposed to be the reaction medium for decarboxylation. Furthermore, for the decarboxylation requiring tougher reaction condition, metal catalysts were added into the NH3-Enriched HTLW to improve the decarboxylation rate and selectivity. Finally, a consecutive tubular reactor is designed for the decarboxylation of pentafluorobenzoic acid to produce pentafluorobenzene. The main research work was summarized as follows:Firstly, the decarboxylation of tetrafluorophthalic acid to 2,3,4,5-tetrafluorobenzoic acid in NH3-enriched high temperature liquid water (HTLW) was conducted. The effects of ammonia concentration, initial concentration of reactant and reaction temperature were investigated. The presence of ammonia could remarkably accelerate the decarboxylation rate, but had no significant effect on the selectivity of decarboxylation, which was mostly more than 90%. The kinetics data at different temperatures were fitted by the first-order equation, and the apparent activation energy of the reaction obtained by Arrhenius plot was 190±11kJ/mol. Based on the experiment results, the assumption that ammonia concentration and initial concentration of reactant changed the distribution fraction of various fcrms of tetrafluorophthalic acid by affecting the pH value of the reaction system was proposed. Based on the obtained dissociation constants of tetrafluorophthalic acid, the distribution fraction of various forms were determined. The correlation for the rate constant of decarboxylation (k(T)), pH value and reaction temperature was derivated, and possible reaction mechanism for decarboxylation was proposed.Next,1,2,3,4-tetraflaorobenzene and 1,2,4-trifluorobenzene were prepared from the decarboxylation of 2,3,4,5-tetrafluorobenzoic acid and 2,4,5-trifluorobenzoic acid respectively by using metal catalysts in NH3-enriched HTLW. The effects of ammonia concentration, initial concentration of reactant, reaction temperature and different metal catalysts were then investigated. The results showed that the addition of ammonia and copper based catalysts could improve the reaction rates and selectivities, and the selectivities to pentafluorobenzenes were around 90%. Nano copper powder and cuprous oxide were proved to be the most effective for the decarboxylation of these fluorobenzoic acids. The reusability of cuprous oxide was evaluated, and the catalyst kept good selectivity for the decarboxylation of 2,4,5-trifluorobenzoic acid after reused by two times. Furthermore, the effects of catalyst loading and initial reactant concentration on the 2,4,5-trifluorobenzoic acid were investigated. Only a very small quantity of cuprous oxide possessed excellent catalytic activity.Finally, a consecutive tubular reactor is designed for study on the consecutive preparation of pentafluorobenzene in HTLW. The effects of reaction temperature and residence time on the decarboxylation of pentafluorobenzoic acid were investigated. The conversion increased as the the reaction temperature and residence time increased. Nearly complete decarboxylation occurred after a residence time of 11.6 min at a temperature of 170 ℃. The results of the decarboxylation of pentafluorobenzoic acid in the batch reactor and consecutive reactor were compared, showing that rate constants for decarboxylation in the consecutive reactor was smaller than those in the batch reactor at the same temperature.