Excess CO 2 Reductions during CH 3 COOH Formation from CH 4 and CO 2 under Periodic Operation: Downhill Side Reactions in an Uphill Target Reaction under Unsteady Conditions

Acetic acid (CH3COOH) formation from methane (CH4) and carbon dioxide (CO2) is an ideal reaction for chemical production, whereas this reaction possesses a severe thermodynamic limitation. To address this issue, it has been reported that periodic operation allowing a non-equilibrium condition can overcome the thermodynamic limitation. However, although an intrinsic issue of uphill reactions in non-equilibrium conditions generally is occurrence of unfavorable downhill reactions, this issue has seldom been discussed for the CH3COOH formation under periodic operation. Herein, excess CO2 reductions were found to be the unfavorable downhill reactions possibly occurring in the reaction aiming at CH3COOH formation under periodically operated CH4 and CO2 feeds. The reaction using an isotopic reactant (i.e., 13CH4 ) unveiled that excess CO2 reductions to CO and even to CH3 moiety could occur, indicating importance of catalyst development. Furthermore, it was proposed that H2 O vapor introduction into the CO2 feed, which increased the CH3COOH product, most likely facilitated the reverse reaction of the excess CO2 reductions and thereby is effective to hamper the unfavorable side reaction.