Biogas upgrading by Grazynes

Abstract

The present study focuses on the possible use of grazynes as membranes to separate biogas methane (CH4) from carbon dioxide (CO2), an impurity found when biogas is gained from the anaerobic digestion of organic matter. This is assessed by theoretical simulations using Density Functional Theory (DFT) in conjunction with the Perdew-Burke-Ernzerhof (PBE) exchangecorrelation functional, and including a description of dispersive forces by means of Grimme D3 method (PBE-D3). The analysis focused on the CO2 and CH4 diffusions through the pores of [1],[1]-, [2],[1]-, and [3],[1]-grazynes. To this end, adsorption energies for both molecules on two possible conformations were acquired as a matter of coverage and pore size, revealing that low-coverages are needed for a molecular diffusion, otherwise high-steric repulsions and diffusion energy barriers are found. As far as pore size is concerned, the larger the pore, the smaller the energy barriers. Aside, the interaction of the molecules with the grazynes is weak, mostly due to dispersive forces, which lower the adsorption energies and diffusion energy barriers. The study found that such grazynes are impermeable to both molecules, and only when achieving acetylenic defects on c(2×2) supercells, one could effectively expect a biogas upgrading. There, the CO2 molecule is found to trespass the membrane when overcoming energy barriers of ca. 0.5-0.6 eV when approaching with its molecular axis perpendicular to the grazyne pore. Diffusion is found to be impossible for other conformations, e.g. parallel CO2, or any conformation of CH4 molecule, revealing mechanisms of grazyne bulging out the impinging molecules, or, when forcing it, the observation of large deformations of the structure and/or the molecule. Thus, CO2 could be 100% selectively separated when using defective [1],[1,2]{0,1}-, [2],[1,2]{0,1}-, and [3],[1,2]{0,1}-grazynes, enabling them to be used to get a purified methane stream in biogas upgrading