Mechanism for the Formaldehyde to Formic Acid and the Formic Acid to Carbon Dioxide Conversions Mediated by an Iron-Oxo Species

The mechanism and energetics for the formaldehyde to formic acid and the formic acid to carbon dioxide conversions are investigated using the FeO+ complex as an oxidant from theoretical calculations at the B3LYP DFT level. In the oxidation processes, there are a lot of reaction branches which are comparable in energy. The elementary processes can be viewed as C−H and O−H cleavage reactions by oxo and hydroxo ligands as well as OH group migrations. In formaldehyde oxidation, the initially formed complex OFe+−OCH2 is converted by a C−H bond cleavage to intermediate HO−Fe+−OCH, which is next transformed to the formic acid complex Fe+−OCHOH by an OH ligand migration and to the carbon monoxide complex H2O−Fe+−CO by a C−H bond cleavage. There are two possible reaction pathways for the formic acid to carbon dioxide conversion, both reaction pathways being downhill and highly exothermic. In an energetically favorable reaction pathway, complex OFe+−OCHOH is first converted to intermediate HO−Fe+−OCHO by an O−H bond cleavage, and after that, HO−Fe+−OCHO is transformed to the product complex H2O−Fe+−O2C by a C−H bond cleavage. Formaldehyde and formic acid are easily oxidized by an excess of oxidants to carbon dioxide and carbon monoxide. The overall reaction from methane to carbon dioxide is exothermic by 117.9 kcal/mol, and there is no high barrier after methanol formation, which can cause the well-known overoxidation problem in methane and alkane oxidation. Our calculations are in good agreement with previous experiments on methane oxidation over Fe−ZSM-5 zeolite with respect to the product branching ratio, suggesting that a surface iron-oxo species should have relevance to the interesting catalytic functions of Fe−ZSM-5 zeolite.