Reactions of vanillic acid in sub- and supercritical water

The hydrothermal reactions of vanillic acid in water near the critical point have been investigated in a continuous tubular reactor at temperatures between 280 and 500 °C and pressures between 225 and 300 bar. Formation of 2-methoxy-phenol as primary product through decarboxylation of vanillic acid is the main decomposition reaction. Decarboxylation kinetics is first-order on vanillic acid concentration. The reaction rate constant is independent of pressure below the critical temperature and decreases with pressure at supercritical temperatures. For instance, at 350 °C it was 0.127±0.006 s−1 at 250 bar and 0.126±0.014 s−1 at 300 bar, while at 425 °C it was 0.694±0.042 and 0.505±0.024 s−1 at the same pressures, respectively. The effect of pressure on reaction rates above the critical temperature is discussed on the basis of a reaction mechanism that involves an activated state with a positive volume of activation, which corresponds to an unimolecular dissociation. The secondary products formed by decomposition of 2-methoxy-phenol are also considered.