Separation selectivity and structural flexibility of graphene-like 2-dimensional membranes

Separation of C1/C3 binary organic gas mixtures via flexible nanoporous carbon molecular sieves: DFT calculations and MD simulations

The C₁/C₃ hydrocarbon gas separation characteristics of nanoporous carbon molecular sieves (CMS) are studied using DFT calculations and MD simulations. To efficiently separate the equimolar CH₄/C₃H₈ and CH₄/C₃H₆ gas mixtures, CNT gas transport channels are embed between the polyphenylene membranes which created structural deformation for both CNT and membrane. The adsorption and permeation of gas molecules via CMS and the effect of nanochannel density and electric field on gas selectivity are analyzed. In addition to the direct permeation, gas molecules that adsorbed on the NPG surface also making a significant contribution to the gas permeability comes from a surface mechanism. Results also uncovered that the gas selectivity is enhanced by the electric field along the + x and +y axes, whereas reduced by the electric field along the + z and -z axes. Plainly, this CMS provides a feasible way for the efficient separation of the C₁/C₃ organic gas mixtures.

Multicomponent gas separation and purification using advanced 2D carbonaceous nanomaterials

Multicomponent gas separation and purification is an important pre- or post-processing step in industry. Herein, we employed a multiscale computational approach to investigate the possibility of multicomponent low-weight gas (H₂, O₂, N₂, CO₂, CH₄) separation and purification using novel porous 2D carbonaceous nanomaterials, namely Graphdiyne (GD), Graphenylene (GN), and Rhombic-Graphyne (RG). The dispersion-corrected plane-wave density functional theory (DFT) calculation combined with the Climbing Image Nudged Elastic Band (CI-NEB) method was employed to study the gas/membrane interaction energy and diffusion barrier of different gases passing through the geometrically optimized membranes. The results from CI-NEB calculations were then fitted to the Morse potential function to construct a bridge between quantum mechanics calculations and non-equilibrium molecular dynamics (NEMD) simulation. The selectivity of each membrane for all binary mixtures was calculated using the estimated diffusion energy barriers based on the Arrhenius equation. Finally, a series of extensive NEMD simulations were carried out to evaluate the real word and time dependent separation process. According to the results, CH₄ molecules can be completely separated from the other gases using a GD membrane, O₂ molecules from CH₄, N₂, and CO₂ by a GN membrane, and H₂ molecules from all other gases using a RG membrane.

Molecular Dynamics of CH4/N2 Mixtures on a Flexible Graphene Layer: Adsorption and Selectivity Case Study

We theoretically investigate graphene layers, proposing them as membranes of subnanometer size suitable for CH₄/N₂ separation and gas uptake. The observed potential energy surfaces, representing the intermolecular interactions within the CH₄/N₂ gaseous mixtures and between these and the graphene layers, have been formulated by adopting the so-called Improved Lennard-Jones (ILJ) potential, which is far more accurate than the traditional Lennard-Jones potential. Previously derived ILJ force fields are used to perform extensive molecular dynamics simulations on graphene's ability to separate and adsorb the CH₄/N₂ mixture. Furthermore, the intramolecular interactions within graphene were explicitly considered since they are responsible for its flexibility and the consequent out-of-plane movements of the constituting carbon atoms. The effects on the adsorption capacity of graphene caused by introducing its flexibility in the simulations are assessed via comparison of different intramolecular force fields giving account of flexibility against a simplified less realistic model that considers graphene to be rigid. The accuracy of the potentials guarantees a quantitative description of the interactions and trustable results for the dynamics, as long as the appropriate set of intramolecular and intermolecular force fields is chosen. In particular it is shown that only if the flexibility of graphene is explicitly taken into account, a simple united-atom interaction potential can provide correct predictions. Conversely, when using an atomistic model, neglecting in the simulations the intrinsic flexibility of the graphene sheet has a minor effect. From a practical point of view, the global analysis of the whole set of results proves that these nanostructures are versatile materials competitive with other carbon-based adsorbing membranes suitable to cope with CH₄ and N₂ adsorption.