Production of Formic Acid via Hydrogenation of CO2 over a Copper-Alkoxide-Functionalized MOF: A Mechanistic Study

Conversion of greenhouse gases to more valuable chemicals is important from both the environmental and industrial points of view. Herein, the reaction mechanisms of the hydrogenation of carbon dioxide (CO2) to formic acid (HCOOH) over Cu-alkoxide-functionalized metal organic framework (MOF) have been investigated by means of calculations with the M06-L density functional. The reaction can proceed via two different pathways, namely, concerted and stepwise mechanisms. In the concerted mechanism, the hydrogenation of CO2 to formic acid occurs in a single step. It requires a high activation energy of 67.2 kcal/mol. For the stepwise mechanism, the reaction begins with the hydrogen atom abstraction by CO2 to form a formate intermediate. The intermediate then takes another hydrogen atom to form formic acid. The activation energies are calculated to be 24.2 and 18.3 kcal/mol for the first and second steps, respectively. Because of the smaller activation barriers associated with this pathway, it therefore seems to be more favored than the concerted one. The catalytic effect of Cu-MOF-5 is also highlighted by comparing it with the gas-phase uncatalyzed reaction in which the reaction takes place in one step with a barrier of 73.0 kcal/mol. This study also demonstrates that the metal-functionalized MOF can be utilized for the greenhouse gas catalysis in addition to using it to capture and activate CO2.