The synergistic effect of oxygen-containing functional groups and MnxOy for efficient CH4/N2 separation on coconut-shell activated carbon

Abstract

A hydrothermal-modification strategy was adopted to anchor manganese oxide (Mn_xO_y) nanoparticles over the coconut shell activated carbon (CSAC) for CH₄/N₂ separation. Systematic investigations were carried out to determine the effects of the type and amount of manganese salts on the adsorption performance. Dynamic breakthrough experiments showed that CSAC-KMnO₄ prepared by modification with KMnO₄ exhibited the optimal CH₄ adsorption performance with a breakthrough time of 131 s, which was approximately 40% longer than that of pristine CSAC. At 273 K and 100 kPa, its saturated CH₄ uptake was as high as 110.7 cm³ g⁻¹, 2.92 times that of pristine CSAC. Comprehensive characterization techniques revealed that highly dispersed Mn_xO_y nanoparticles exhibited the mixed valence states of Mn²⁺/Mn³⁺, and their abundant unsaturated coordination adsorption sites induced polarization of CH₄ molecules, leading to enhanced van der Waals interaction and high adsorption capacity. Moreover, the strong oxidizing properties of Mn⁷⁺ could etch pores of the activated carbon skeleton, introducing more COOH, CO and C–O–C groups, thereby further enhancing the polarity with increased CH₄ uptake. This work provides a new design pathway for highly efficient separation of CH₄/N₂.