Separation of Carbon Dioxide and Methane in Biogas by Adsorption: Computational Study of the Effect of Surface Functionalization of Activated Carbon

The decomposition of organic matter results in production of biogas, a renewable source that is composed primarily of CO2 and CH4. The CH4 concentration, acceptable for low efficiency combustion, is insufficient for high-performance applications. It is important to develop efficient and low-cost methods to separate CO2 and CH4. Adsorption/desorption cycles is one of the possible alternatives. Here we analyze computationally the effect of surface functionalization by polar groups on the adsorption of CO2 and CH4 in activated carbon (AC). The AC is modeled as slit-shaped pores formed by graphene fragments separated by distances between 8 and 20 Å [1-4] for different concentrations of hydroxyl and epoxy groups added randomly. The addition of polar groups significantly improves the selectivity of CO2 and CH4, especially at low (< 1 atm) and intermediate (1-10 atm) pressures and ca. room temperature or above. The mechanism of such added selectivity is the strong electrostatic interaction between the surface groups and the electric quadrupole of CO2.
[1] Z. Valleroy, et al., Langmuir 36, 3690−3702 (2020).
[2] M. Golebiowska, et al., Carbon 50, 225−234 (2012).
[3] L. Ortiz, et al., Mater. Res. Express 3, 055011 (2016).
[4] A. Albesa, et al., Langmuir 24, 3836−3840 (2008).