The selectivity-conversion tradeoff in selective oxidation of methane with non-equilibrium plasmas

The direct of methane to methanol (DMtM) is a utilization pathway that enables feasible methane conversion at low temperatures, pressures, and over distributed scales. However, the poor selectivity toward methanol, and unwanted formation of CO and CO₂, continue to limit its broader adoption. This is referred to as selectivity-conversion limit. We show that, much like homogeneous and heterogeneous catalysis, plasma also suffers from the limit due to unwanted conversion of methanol to CO₂. We show scaling relations relating the physical parameter of a reactor to a flow property to achieve the maximum attainable yield of a transport-limited reactor (a reactor with constrained maximum yield due to flow limitations). We also show that active removal of methanol involving purge (mechanical non-equilibrium) and trap (phase non-equilibrium) can delay the transport-limiting regime, and thereby pushing the maximum attainable yield even higher. In other words, we break the selectivity – conversion limit of the DMtM conversion. The active removal makes the gradient with which the yield increases with the number of pulses stay linear for longer. Therefore, it pushes the yield beyond the maximum attainable yield of a transport-limited reactor by increasing the characteristic reactor flow timescale. We use water as our methanol trapping entity to get > 12% methanol selectivity (> 90% liquid oxygenate selectivity) with ~ 1% methanol yield as opposed to 0.1% in the absence of active removal. Thus, breaking the selectivity – conversion limit via active removal of methanol is a pathway to enable energy-efficient distributed utilization of methane with high yields.Aerospace Engineerin