Carbon chain growth from methanol to ethanol: A review of thermocatalytic mechanisms and strategies

Ethanol as a vital fuel and chemical platform faces sustainability and resource constrains in its conventional production from food crops. Thermocatalytic conversion of methanol, particularly green methanol derived from CO₂ or biomass to ethanol (MTE) offers a promising alternative. This review provides a critical and systematic analysis of four predominant thermocatalytic MTE routes in which dimethyl oxalate (DMO), methyl acetate (MA), acetic acid (AA), and formaldehyde (FA) are the main intermediate product, respectively. The carbonylation-dependent routes (DMO, MA, AA) achieve high ethanol selectivity (>90 %) in the downstream hydrogenation step, predominantly over Cu-based catalysts where Cu⁰/Cu⁺ synergy is crucial. However, their practical potential is primarily constrained by rapid catalyst deactivation (e.g., coking in zeolites) or reliance on corrosive promoters in the upstream carbonylation step. In contrast, the FA-mediated route follows a distinct, atom-economical pathway but presents a more complex catalytic landscape. It suffers from low intermediate selectivity (e.g., to glycolaldehyde) during the C-C coupling and faces further selectivity losses in the hydrogenation of highly oxygenated intermediates, typically resulting in ethanol yields below 50 %. A comparative technological assessment reveals that the MA route with its integrated three-step process currently represents the most commercially advanced and industrially demonstrated MTE technology. The DMO route is mature for ethylene glycol production while the AA route leverages established unit operations. By elucidating the underlying catalytic mechanisms and structure-performance relationships, this review establishes a foundational framework for the rational design of stable and selective catalysts in sustainable ethanol synthesis.