Decarboxylation of tetrafluorophthalic acid in NH3-enriched high temperature liquid water

• NH3 was effective for accelerating decarboxylation of tetrafluorophthalic acid in HTLW.
• The rate constants and activation energy for decarboxylation were determined.
• The relationship between pH value and reaction activity was demonstrated.
• The mechanism for the decarboxylation of tetrafluorophthalic acid was proposed.

2,3,4,5-Tetrafluorobenzoic acid is an important pharmaceutical intermediate. A green technology for the decarboxylation of tetrafluorophthalic acid to 2,3,4,5-tetrafluorobenzoic acid in NH3-enriched high temperature liquid water (HTLW) was developed. Tetrafluorophthalic acid could undergo the decarboxylation with the catalysis of NH3 to 2,3,4,5-tetrafluorobenzoic acid in HTLW. A conversion of 98.1% and selectivity of 96.5% were achieved at 453.15 K for 1.5 h with an ammonia concentration of 13.4 mmol/L. The effects of ammonia concentration, initial concentration of reactant and reaction temperature were then investigated and correlated using a derivated equation. The results indicated that adequate addition of ammonia could adjust the pH of the reaction medium into 2.2–2.4, where the dominant form of the decarboxylation is the monovalent anion. Moreover, the kinetic data at different temperatures were fitted by a first-order equation, and the rate constants at each temperature and an activation energy of 190 kJ/mol were obtained.

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