A theoretical investigation into gallic acid pyrolysis.

Thermodynamic and kinetic information on the first two steps of gallic acid pyrolysis, a decarboxylation followed by a dehydrogenation, is obtained based on density functional theory and quantum chemistry. For the kinetics, transition states are identified with the help of the climbing image nudged elastic band method. Both reactions exhibit two transition states. One of them is related to the rotation of OH groups, and the other one is related to the breaking and forming of bonds. The gallic acid pyrolysis as a whole is judged to be endothermal, and it changes from endergonic to exergonic between 500 and 750 K. The second reaction, the dehydrogenation of pyrogallol, is identified as the rate-determining step of gallic acid pyrolysis, with reaction rate constants below 1 s -1 for temperatures below 1250 K.