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 (CH 3 COOH) formation from methane (CH 4 ) and carbon dioxide (CO 2 ) 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 CH 3 COOH formation under periodic operation. Herein, excess CO 2 reductions were found to be the unfavorable downhill reactions possibly occurring in the reaction aiming at CH 3 COOH formation under periodically operated CH 4 and CO 2 feeds. The reaction using an isotopic reactant (i. e., 13 CH 4 ) unveiled that excess CO 2 reductions to CO and even to CH 3 moiety could occur, indicating importance of catalyst development. Furthermore, it was proposed that H 2 O vapor introduction into the CO 2 feed, which increased the CH 3 COOH product, most likely facilitated the reverse reaction of the excess CO 2 reductions and thereby is effective to hamper the unfavorable side reaction.