Scheme 3.

CO₂ can be converted to oxalic acid via four main pathways: (a) through direct conversion of CO₂ to alkali oxalate; (b) through a metal formate intermediate, which can be obtained from the electrocatalytic or photocatalytic reduction of CO₂; (c) via CO and the dialkyl oxalate process; and (d) via ethylene glycol and subsequent oxidation (in practice not done because ethylene glycol would be obtained from oxalic acid, not vice versa).