One key obstacle in CO2 reduction that is often overlooked is the reduction of formic acid (HCOOH), a common intermediate en route to CH3OH production. In fact, most CO2 reduction catalysts successfully convert CO2 to HCOOH or its deprotonated form formate (HCOO-), but fail to achieve the subsequent reduction of HCOOH to ultimately produce CH3OH. We address this challenge by showing how competent catalysts, when synergistically combined with optimum reaction conditions, can accomplish HCOOH reduction. We employ high-level CCSD(T) quantum chemical calculations to demonstrate a viable concerted proton-coupled electron transfer (PCET) mechanism for the homogeneous reduction of formic acid (HCOOH) by pyridinium radical (PyH0) and hydronium ion (H3O+). The predicted barrier (?H0act) for concerted PCET to convert HCOOH to PyH+�CH(OH)20 is 10.5 kcal/mol. PyH+�CH(OH)20 can then dissociate and complex with pyridine to form Py�CH(OH)20, which undergoes condensation and ultimately forms Py�OCH0, a proposed intermediate in formaldehyde formation. The highest barrier step for HCOOH reduction is the condensation reaction which results in C-O bond cleavage with ?H0act = 14.9 kcal/mol. The electron affinity and pKa quantitatively show that concerted PCET is favored over sequential PCET in the PyH0 catalyzed reduction of HCOOH due to the high cost of localizing electron density on HCOOH and deprotonating PyH0.

Homogeneous reduction of formic acid by pyridine: A key step in CO<sub>2</sub> reduction to CH<sub>3</sub>OH

246th National Meeting (2013)

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Homogeneous reduction of formic acid by pyridine: A key step in CO2 reduction to CH3OH

Homogeneous reduction of formic acid by pyridine: A key step in CO₂ reduction to CH₃OH