Plasma-enabled methane upgrading in a gas-liquid reactor: Optimizing a one-step methanol production process

In this work, methane (CH₄) and ubiquitous water (H₂O) were exploited as reactants in a continuous flow, catalyst-free, nonthermal plasma process to produce methanol at a rate of 21.28 mg/h with a specific yield of 1.46 mg_MeOH/g_CH4 and selectivity of 90.8 % among other liquid products. A systematic investigation of process parameters through factorial design screened five process factors, i.e., applied power, gas flow rate, liquid flow rate, catalyst loading, and pH. While catalyst loading and pH showed minimal significance, gas flow rate, liquid flow rate, and applied power emerged as the significant factors affecting both production rate and specific yield, though with competing effects where higher gas flow rates enhanced production rates but reduced specific yields. Subsequent optimization using Box-Behnken design determined optimal conditions of 368 W applied power, 273 mL/min methane flow rate, and 51 mL/min water flow rate for maximizing methanol production rate while maintaining high selectivity. OES and NMR analyses revealed a radical-mediated pathway primarily involving methyl and hydroxyl radical coupling for methanol formation. This catalyst-free process showed great promise for cleaner fuel production, reduced greenhouse gas emissions, and efficient utilization of natural gas and biogas resources.