A theoretical study on decomposition of formic acid in sub- and supercritical water

The mechanisms of the dissociation of formic acid in subcritical and supercritical water are investigated theoretically. In this dissociation, water molecules around a formic acid play a role of a catalyst by transferring a proton along their locally formed hydrogen bond network. There are two channels of the dissociation, that is, the dehydration $(HCOOH→CO+H2O)$ starting from the trans-formed formic acid and the decarboxylation $(HCOOH→CO2+H2)$ from the cis-formed formic acid. The effects of hydration on these channels in sub- and supercritical water are analyzed by calculating the free energy and analyzing the water molecular coordination by the Monte Carlo method and molecular dynamics calculations. It is found that the hydration is stronger in the decarboxylation (via the cis-path) than in the dehydration (via the trans-path). The number of “catalytic” water molecules coordinated to the cis-formed formic acid, leading to decarboxylation, in supercritical is almost the same as that in subcritical water. On the other hand, the catalytic water molecular coordination on the trans-formed formic acid, leading to the dehydration, is found to be much more reduced in supercritical water than that in subcritical water. These facts manifest how the decarboxylation becomes more favorable than the dehydration in supercritical water, whereas both dissociation channels are equally probable in subcritical water.