Electrostatic Sieving Mechanisms Study Of Porous Graphene （Graphene-like Material） For The Separation Of Flue Gases With Low Concentrations Of CO₂

The growing CO₂ emissions from human activities have been the main cause of global warming. The separation and carpture of CO₂ is significantly important in slowing global warming. Because fossil fuel-fired power plants account for about 40% of total CO₂ emissions, numerous studies have been conducted on CO₂ separation and carpture from flue gases（mainly CO₂/ N₂ separation） emitted by these point sources. Compared with traditional systems for gas separation, membrane separation of gases is desirable because of its low energy cost, small footprint, and easy maintenance. With respect to the CO₂-selective membranes, one main problem related to their practical applications is the low CO₂ concentrations in flue gases. This means that the small driving force for mass transfer across membranes will require significant energy cost to create pressure differences enough for separation. In contrast, considering high N₂ concentrations in flue gases, one advantage is that the driving force for mass transfer can be much larger than that across the CO₂-selective ones. However, as far as we know, few studies at the moment have been conducted to investigate such types of N₂-selective membranes for flue gas separation.Porous graphene（graphene-like material） can be considered ultrathin membrane because they are one atom thick, which make them promising for potential application as a membrane-separation material, because its infinitesimal thickness promises transport resistance minimization and flux maximization. In addtion, the considerable mechanical strength and chemical stability make them a promising candidate for potential applications in membrane materials. However, experimental study of two-dimensional porous material in gas separation areas is still in its infancy. At the same time, theoretical method can be used to understand and explore the inherent principle in the properties of materisls. Hence, in this study, density functional theory（DFT） calculations and molecular dynamic（MD） simulations were performed to investigate the gas separation performance and underlying gas transport mechanisms in the two-dimensional porous material. The knowledge obtained in the current study may provide useful guidance for future efforts to synthesize promising N₂-selective membranes for post-combustion CO₂ capture. The main contents and conclusions are as follow:（1） A systematic study is conducted in this study to explore the performance of H-passivated porous graphene membranes for CO₂/N₂ separation by DFT calculations and MD simulations. We found that the graphene membrane, H-pore-13, with its appropriate pore size of 4.06 ?, can efficiently separate N₂ from CO₂. Different from the previously reported preferential permeation of CO₂ over N₂ resulting from size sieving, H-pore-13 can exhibit high N₂ selectivity over CO₂ with a N₂ permeance of 105 GPU（gas permeation unit）, and no CO₂ was found to pass through the pore. It was further revealed that electrostatic sieving plays a cruical role in hindering the passage of CO₂ molecules through H-pore-13.（2） The interaction energy between gas molecules and graphene, which can be tailored by the chemical functionalization of graphene, could favor or hinder the passage of gas molecules. Hence, in this study, fluorinated graphene was chosen as a model material, to explore the gas separation performance and underlying gas transport mechanisms by taking CO₂/N₂ separation. It was revealed that the position of the attractive well for gas molecules in graphene is an important influencing factor for gaseous separation performance of graphene membranes. And by varying the chemical functional groups, one can change the position of attractive wells for gas molecules in graphene membranes, which can be considered as an effective regulation and control strategy for achieving reversible and selective gas separation in graphene membranes. The knowledge obtained in current study may provide a helpful guidance for the development of graphene membranes on the gas separation in the industries.（3） Poly（triazine imide）（PTI） was chosen as a H-pore-13 like two-dimensional porous material, to explore the CO₂/N₂ mixtures separation performance. Using DFT calculations and MD simulations, we demonstrated in this work that the poly（triazine imide）（PTI） membrane can be efficiently employed to separate N₂ from CO₂ with a selectivity of 530 and a N₂ permeance of 106 GPU, superior to those of most conventional membranes. This experimentally available N₂-selective ultimate membrane may be expected to find practical applications in post-combustion CO₂ capture.（4） As an extensive research, we also investigate the diffusion properties of gas through a porous monolayer covalent triazine framework（CTF-0） membrane. The study demonstrated that a monolayer CTF-0 membrane can exhibit exceptionally high He and H2 selectivities, superior to those of conventional carbon and silica membranes. For H2, the permeance exceeds the industrial standard by around 2 orders of magnitude at 300 K. For H2, the permeance is also higher than the industrially accepted standard above 355 K and exceeds the industrial standard by around 2 orders of magnitude at 500 K. The CTF-0 membrane may be potentially useful for helium separation and hydrogen purification.（5） The oxyfuel combustion CO₂ capture was also investigated. Separation of O₂ from air is an indispensable step in oxyfuel combustion CO₂ capture, which is currently carried out on a large scale using an energyintensive cryogenic distillation process. Metal–organic frameworks containing coordinatively unsaturated metal sites（CUS） have emerged as competitive new adsorbents for such targets. In this study, dispersion-corrected DFT calculations were performed to investigate the influence of framework metal ions on the adsorption behavior of O₂ in M3（BTC）₂-type materials（M= Cr, Mn, Fe, Co, Ni and Cu）. The results show that the magnitudes of charge transfer from CUS metals to O₂ correlate very well with the interaction energies of O₂ with M3（BTC）₂. Furthermore, this work suggests that Ni3（BTC）₂ can be potentially considered as promising oxygen adsorbent with relatively easier deoxygenation than Cr3（BTC）₂.