Abstract
Methane, the primary component of natural gas, is a potent greenhouse gas whose impact on climate change is substantially greater than carbon dioxide in shorter timeframes. With its rapidly increasing atmospheric levels due to activities such as fossil fuel extraction and use, effective strategies for mitigating methane emissions are critically needed to combat global warming. One promising approach is valorizing it into value-added products, such as through direct conversion at mild conditions into liquid fuels like methanol, offering a route to more sustainable fuel production and resource utilization. In this work, methane and ubiquitous water were exploited as reactants in a continuous flow nonthermal plasma system to convert 69.4% of feed methane into an array of gas and liquid products including methanol at a rate of 21.3 mg/h with a specific yield of 1.46 mgMeOH/gCH4 and liquid product selectivity of 90.76%. The performance of various perovskite catalysts, primarily bimetallic composites of lanthanum with one of cerium, nickel, iron, copper, or cobalt was assessed, with lanthanum cerium oxide achieving the highest methanol production rate (15.8 mg/h) and specific yield (2.96 mgMeOH/gCH4). A systematic investigation of process parameters through factorial design screened five 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 most significant factors affecting both production rate and specific yield, though often 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 gas flow rate, and 51 mL/min liquid flow rate for maximizing methanol production rate while maintaining high selectivity. Temperature studies revealed unexpected benefits of higher temperatures (92-96°C) despite methanol synthesis traditionally being favored at lower temperatures. The gas-phase reactions dominated, showing nine times faster production rates than liquid-phase reactions. Compared to conventional methods, this approach offers many advantages including lower reaction temperature and pressure, decent energy efficiency, high space time yield, and high selectivity towards methanol formation. This process showed much promise for cleaner fuel production and reduced greenhouse gas emissions. Future investigations will focus on reactor modifications to better integrate catalysts and optimize the balance between production rate and specific yield.