Surface-Modified Activated Carbon with a Superior CH4/CO2Adsorption Selectivity for the Biogas Upgrading Process

Abstract

The conversion of biogas to biomethane is an interesting alternative for clean energy production. Although biogas separation by physical adsorption using activated carbons has many advantages, the selectivity for CH4/CO2adsorption is fairly low. In this work, the surface chemistry of a commercial activated carbon (CNR-115) was modified by a two-step process: oxidative thermal treatment followed by ammonia modification. The activated carbons resulting from each modification step (CNR-115oxand CNR-115am, respectively) were characterized texturally and chemically, and their CH4/CO2equimolar mixture adsorption behaviors were measured. The results showed a significant loss of both surface area and porosity after the modification steps. However, the increased amount of polar surface functionalities leads to a remarkable increase of the selectivity factor (max. selectivity: 2.7 for CNR-115 < 23.4 for CNR-115ox< 129.0 for CNR-115am). The reasons behind the significantly enhanced selectivity are discussed on the basis of the surface chemistries and textural properties of the materials, in relation with molecule characteristics. We demonstrate herein a very efficient surface modification strategy, which allowed to obtain activated carbon adsorbents with oxygen and nitrogen groups that favor a superior CH4/CO2separation capacity. Furthermore, the stability of the added surface groups and the adsorption behavior was tested, proving the maintenance of the chemical characteristics and the adsorption performance after 10 adsorption/desorption cycles.