Hydrothermal reactions of formaldehyde and formic acid: free-energy analysis of equilibrium.

The chemical equilibria concerning formaldehyde and formic acid are computationally investigated in water over a wide range of thermodynamic conditions. The free energy is evaluated in the method of energy representation for the solvent effect on the decomposition processes of these two compounds. The solvation is found to suppress the production of nonpolar species from a polar. In the two competitive decomposition reactions 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. The reaction equilibrium is further treated for aldehyde disproportionation reactions involving formaldehyde and formic acid. The disproportionation reactions are found to be subject to relatively weak solvent effects and to be dominated by the electronic contribution.