Investigation of surface facilitation for upgrading natural gas to value-added liquids under ambient conditions

Abstract

The increasing production of low-cost natural gas, with methane (CH4) as its major component, provides a clear motivation for CH4 upgrading to value-added liquids. Single-step catalytic approaches have long been studied to provide environmentally-friendly and energy-efficient upgrading technologies to eliminate the extremely high operating temperatures and pressures. The use of hydrogen peroxide (H2O2) in aqueous solution has shown tremendous success in lowering the required reaction temperatures for CH4 activation, however the necessity of high reaction pressure still needs to be obviated. Built upon findings from our earlier study of “surface facilitation” on metal surfaces, in this research program, we have significantly advanced our understanding of such enabling phenomenon by 1) carefully conducting upgrading experiments with accurate and reliable quantifications on all three upgrading products; 2) conclusively establishing the mathematical kinetics model, capable of explaining upgrading results, regardless of metal surfaces or reaction conditions; 3) consistently reasoning the observed carbon efficiency by theoretical prediction developed from the kinetics model; and 4) comprehensively examining the impact of medium pH on the upgrading reactions with two representative metal surfaces (Pt and Cu). Our research has revealed the unprecedentedly high activity (two orders of magnitude higher than the state-of-the-art once normalized to standard condition) along with high carbon efficiency for CH4 upgrading with H2O2 in aqueous solution. The correlation between CH3−M binding energy of different metal surfaces (Pt, Pd, Cu, Re, Au and Ag) and kinetics of reaction was shown to be the determining factor for the performance of the system. The concept of “limiting-product concentration” is introduced to describe the contradicting relation between the product concentration and the carbon efficiency. We also observed the strong effect of medium pH on tuning the “limiting-product concentration”.