Hydrothermal Reactions of Formic Acid: Free-Energy Analysis of Equilibrium

The chemical equilibria concerning formic acid are computationally investigated in water over a wide range of thermodynamic conditions. The solvation free energies for relevant C1 compounds are evaluated in the method of energy representation, and are used to assess the solvent effect on the formic acid reactions. In the two competitive decomposition processes of formic acid, the solvent strongly inhibits the decarboxylation (HCOOH CO2 + H2), and its effect is relatively weak for the decarbonylation (HCOOH CO + H2O). The equilibrium weights for the two decomposition pathways of formic acid are determined by the equilibrium constant of the water-gas-shift reaction (CO + H2O CO2 + H2), which is an essential and useful process in fuel technology. The reaction control by the solvent is then examined for the water-gas-shift reaction. Through the comparison of the equilibrium constants in the absence and presence of solvent, even the favorable side of the reaction is shown to be tuned by the solvent density and temperature.