Atomistic modelling insight into the structure of lignite-based activated carbon and benzene sorption behavior

Improved structure-property relationships for activated carbon were obtained by devising realistic, large-scale, structural models. Herein, an improved approach was employed to construct atomistic models of a lignite precursor of activated carbon, based on the high resolution transmission electron micrographs (HRTEM) of pyrolyzed lignite coal, in combination with experimental pore size distribution analysis of tailored lignite-based activated carbon. Benzene sorption was experimentally characterized at 303 and 318 K and resulted in 13-18% mass gain. To model the carbon structure and benzene sorption, we have devised two structures, including either micropores (4-20 Å) or micro/mesopores (4-40 Å). For the 303 K conditions, the predictions of the two models are consistent with experimental observations. For the micro/mesoporous model, benzene molecules sorbed in both micropores and mesopores, as the mesopores provide access to the internal part of the carbon structure, and benzene molecules would pass readily through these small mesopores to the final sorption sites in micropores of 14-18 Å in size. The most favored sorption energy was -37.45 kJ mol^-1, with a preferred rotation angle from 20-30°, and a second favored angle from 30-40° relative to the graphene surface. These benzene molecules were aggregated in T-shaped and parallel-displaced configurations, with a separation distance of 5.75 Å from the benzene centers of mass to the carbon surface in a monolayer state. The most favored position was found to be parallel to and between two carbon surfaces, especially close to 5- or 7-membered rings.