Molecular Simulation Study On The Gas Transport Behavior And Separation Performance In New Highly Efficient Polymeric Membranes With Intrinsic Microporosity

Gas separation membrane technology has been applied to food, medicine, biology, environmental protection, chemistry, metallurgy, energy, oil, bionics and many other fields, which is attributed to the prominent advantages of “green” purification, high efficiency, low cost, large separation capability, easy regulation and control. Polymeric membranes possess good separation performance, mechanical properties, physical and chemical properties, thus become common and extensive tools in separation of gas mixtures. It is usually compromised by the trade-off restriction between permeability and selectivity of polymeric membrane illustrated by Robeson upper-bound relationship. The preparation of gas separation membranes with both high permeability and high selectivity to break the trade-off restriction has a very far-reaching impact on improving the gas separation efficiency and expanding the scope of applications.Polymers of intrinsic microporosity(PIMs) have ladder-like backbones with a “spiro-centre” that is a common atom shared by two adjacent rings. The spiro-centre and rigid rings provide the contorted sites which make the molecular structure be nonplanar and inflexible. Therefore, the polymer backbone cannot rotate freely, which leads to PIMs membranes packing space not efficiently and forming more continuous pores. Consequently, PIMs membranes are hopeful to increase gas permeability and maintain selectivity simultaneously to exceed Robeson upper bound.In this work, we have selected two novel PIMs, PIM-6FDA-OH and its thermally rearranged product PIM-PBO, and four different gases(N2, O2, CO2, CH4) as the research objectives. Molecular dynamics(MD) and grand canonical ensemble Monte Carlo(GCMC) methods have been employed to study the sorption, diffusion and permeation behaviors of four gases in both membranes, respectively. The effects of PIMs membrane structures on the micro process, mechanism and main controlling factors of gas transport are elucidated intrinsically and systematically. The structure-property relationship and the essential rules are summarized to optimize the adsorption, permeability and selectivity parameters of gas separation membranes. The simulation data would be expected to provide the reliable theoretical basis and guidance for the design, synthesis and application of new gas separation membranes of excellent performance.First of all, we have calculated and analyzed the fractional free volume, radial distribution function, wide angle X-ray diffraction, radius of gyration etc. to characterize the polymeric membrane morphology qualitatively and quantitatively. The results suggest that the PIM-6FDA-OH and PIM-PBO membranes have the larger d-spacing values than the commonly used polyimide membranes. It is due to the existence of the spiro-centre, high rigidity and distortion in PIMs molecular structures, which leads to more pores and looser membrane. Moreover, PIM-PBO membrane has the larger fractional free volume, higher porosity and more gas transport channels than PIM-6FDA-OH membrane. It is mainly because more intramolecular hydrogen bonds of the latter enable the structure arrangement to be more compact and the free volume to be smaller. Thus, it can be proved that thermal rearrangement is one of the effective means of modifying the polymeric membrane materials.Secondly, we have analyzed and compared the solubility coefficients and diffusion coefficients of four gases in two PIMs membranes, respectively. The calculated solubility coefficients agree well with the experimental results, and the relationship between solubility and critical temperature is also in line with the accepted empirical formula. These indicate that the simulation systems are similar with the actual systems, and the simulation results are credible. Solubility coefficients are in the order CO2 > CH4 > O2 > N2 for both membranes. CO2 has the largest solubility coefficient, because there is the maximum interaction strength between CO2 and polymer among four gas species. In consideration of PIM-6FDA-OH compared with PIM-PBO membrane, the gas molecules have the larger solubilities in the former, although the free volume of the latter is higher. It is because that the gas solubility is influenced by the free volume of membrane and the gas-polymer interaction collectively. The two factors compete mutually, and the latter accounts for dominance in our studied systems. The diffusion coefficients of four gases in two PIMs membranes are in the order O2 > CO2 > N2 > CH4. The diffusion coefficient is inversely proportional to gas molecular diameter except CO2. Because CO2 has the strong interaction with polymer, leading to its diffusion rate decreasing. Furthermore, the linear structure of CO2 molecule may also hinder other gas molecules from entering into the membrane matrix simultaneously. For the individual gas, the gas diffusion coefficient of PIM-PBO membrane is larger than that of PIM-6FDA-OH. Because PIM-PBO membrane possesses the greater free volume and looser structure, and provides more gas transport pathways.Finally, we have calculated the gas permeability coefficient and permeability selectivity according to the solubility and diffusion coefficients. CO2 has high permeability, while the permeability of CH4 and N2 is relatively low. Therefore, PIM-6FDA-OH and PIM-PBO membranes could be considered as the ideal promising membrane materials with high selectivity for CO2/CH4 and CO2/N2 separation.