Decomposition of Formic Acid under Hydrothermal Conditions

The thermal decomposition of formic acid was studied in dilute aqueous solutions and in the absence of added oxygen at temperatures between 320 and 500°C and pressures between 178 and 303 atm for residence times between 1.4 and 80 s. Under these conditions, the formic acid conversion ranged from 38% to 100%, and the major products were always CO₂ and H₂, which indicates that decarboxylation is the preferred reaction path for formic acid decomposition under hydrothermal conditions. CO also appeared as a product, which shows that a dehydration path is available, but the CO yield was always at least an order of magnitude lower than the yields of CO₂ and H₂. The kinetics of formic acid disappearance and product formation at temperatures above 320°C are consistent with a reaction rate law that is first order in formic acid. The implications of the present results to the generally accepted molecular decomposition mechanism are discussed, as are the alternative free-radical, ionic, and surface-catalyzed reaction mechanisms proposed in the literature.