CH4/N2 Adsorption-Separation Performance of Activated Carbons Treated by Stepwise Modification Using Sodium Dodecyl Sulfate and Sulfuric Acid

Abstract Adsorption-based gas separation is recognized as a promising and efficient method for the purification of low-concentration coalbed methane (LCCBM). However, the performance of activated carbons (AC) is limited by their strong surface hydrophobicity and the micropore blockage of mineral ashes. Conventional acid modification exhibits a low ash-removal efficiency due to the poor penetration of acid solutions. In this work, a stepwise modification method using sodium dodecyl sulfate (SDS) and H2SO4 was proposed to treat AC. Experiments including low-temperature liquid nitrogen adsorption, Fourier transform infrared spectroscopy (FT-IR), and dynamic breakthrough adsorption were carried out to investigate the effects of the stepwise modification on the pore structure, surface functional groups, and CH4/N2 adsorption-separation performance of the modified AC. The results indicate that SDS reduced the contact angle of the acid solution on the AC surface and promoted the penetration of H2SO4 into deep micropores, thereby enhancing the ash removal rate. At an H2SO4 concentration of 0.05 mol/L, the stepwise modification increased the pore volume and specific surface area, thereby providing more active sites for CH4 adsorption. The CH4 saturation adsorption capacity of the modified AC showed a 22.6% increase compared to that of the raw AC, while the N2 adsorption capacity decreased. Compared with the AC treated by pure water, SDS solution, or H2SO4 solution, the AC modified by the stepwise method exhibited a higher CH4/N2 separation factor and a longer CH4 breakthrough time. However, as the H2SO4 concentration was further increased, the CH4/N2 separation performance of the modified AC gradually declined. This study provides a theoretical reference for the efficient LCCBM purification using the SDS and H2SO4 stepwise modification method.