Separation of CO2/CH4 gas mixtures using nanoporous graphdiyne and boron-graphdiyne membranes: influence of the pore size

Abstract

Nanoporous carbon-based membranes have garnered significant interest in gas separation processes owing to their distinct structure and properties. We have investigated the permeation and separation of the mixture of CO₂ and CH₄ gases through membranes formed by thin layers of porous graphdiyne (GDY) and boron graphdiyne (BGDY) using Density Functional Theory. The main goal is to investigate the effect of the pore size. The interaction of CO₂ and CH₄ with GDY and BGDY is weak, and this guarantees that those molecules will not be chemically trapped on the surface of the porous membranes. The permeation and separation of CO₂ and CH₄ through the membranes are significantly influenced by the size of the pores in the layers. The size of the hexagonal pores in BGDY is large in comparison to the size of the two molecules, and the passing of these molecules through the pores is easy because there is no barrier. Then, BGDY is not able to separate CO₂ and CH₄. In sharp contrast, the size of the triangular pores in GDY is smaller, comparable to the diameter of the two molecules, and this raises an activation barrier for the crossing of the molecules. The height of the barrier for CO₂ is one half of that for CH₄, the reason being that CO₂ is a linear molecule which adopts an orientation perpendicular to the GDY layer to cross the pores, while CH₄ has a spherical-like shape, and cannot profit from a favorable orientation. The calculated permeances favor the passing of CO₂ through the GDY membrane, and the calculated selectivity for CO₂/CH₄ mixtures is large. This makes GDY a very promising membrane material for the purification of commercial gases and for the capture of the CO₂ component in those gases.