Moecular Simulation Of Gases Adsorption,Diffusion,and Separation In Porous Organic Framework Materials

Recently, the concern of the most severe environment issues is the global warming and climate change, which are mainly induced by the sharply growing of the anthropogenic CO2emission. Thus, the CO2capture and sequestration (CCS) technology, which can efficiently reduce the CO2emissions from existing sources such as coal-based power plants, will play an important role in slowing the global warming and climate change. Meanwhile, with the huge combustion and the increasing prices of coal and oil, it is urgent to search for an efficient and clean alternative energy.Hydrogen and methane are the most promising candidates for the replacement of the current energy sources due to their high energy density and cleaning. However, some technological problems, such as optimal conditions for their storage and transport, still remain unsolved. The common storage method for hydrogen and methane is the high pressure compression, which is energy-intensive and costly. The adsorption storage is revisable and simple, which is becoming an intense subject of scientific research. Porous covalent organic materials (COMs), a new class of porous solid materials, emerged approximately one decades ago and have quickly developed into a promising candidate for gas storage and separation due to their potential advantages, such as low densities, high surface areas, tuning pores, and high thermal stabilities.We have systematically investigated the storage, diffusion, and separation of H2, CH4and CO2in COMs by molecular simulation, including porous aromatic frameworks (PAFs), and covalent organic frameworks (COFs) and their Li-doped counterparts. Moreover, we proposed an efficient strategy to fast screen porous materials for gas separation. The main contents are summarized as follows.1. We investigated the adsorption of CO2, CH4, H2, and N2storage in a recently designed new class of porous aromatic frameworks (PAFs). The results show that PAFs are promising candidate for gas storage. At p=35bar, the gravimetric uptake of PAF-302is238mg/g. The uptakes of CO2in PAF-303and-304reach3432and3124mg/g at298K and50bar, respectively, which are larger than MOF-200(2437mg/g) and MOF-210(2396mg/g). Due to the small pore size of5.2A, the isosteric heat for PAF-301is much higher than the other three materials. For PAF-301, the isosteric heat increases slowly to a maximum and then decreases quickly, while for the other three materials, the isosteric heat increases monotonously due to the increase of cooperative interactions between adsorbate molecules.2. We investigated the diffusion behavior of hydrogen and methane in covalent organic frameworks (COFs) and Li-doped counterparts. The results indicate that the self-diffusivities of hydrogen and methane in COFs decrease monotonically with the increase of pressure. After Li doping into COFs, the self-diffusivities first increase at low pressure, and then reach a plateau, and finally decrease slightly at high pressure. This phenomenon stems from the fact that Li atom has a strong affinity to the gas molecules. In addition, it is also found that the COFs show a larger self-diffusivity than most of the metal-organic frameworks (MOFs). To further understand the effect of Li-doping on diffusion of gases, the isosurface, the contour plots of the center of mass, and radial distribution functions of gases are also explored.3.We studied the separation of hydrogen and methane in covalent organic frameworks (COFs) and Li-doped counterparts.The adsorption selectivity of Li-doped COFs for CH4/H2gets a significant improvement, compared to undoped ones. The permselectivities of the undoped COFs for H2/CH4are one order of magnitude higher than the Li-doped COFs. Especially for COF-108and COF-105, the permeabilities are higher than most of the MOFs and zeolites, and reach1.15×106and1.00×106Barrer, respectively, indicating the COF-based membranes might be very promising materials for the kinetic separation process.4.The CO2separation were explored in the PAFs. PAF-301exhibits much higher selectivities for the CO2/H2, CO2/N2, CO2/CH4, and CH4/H2mixtures than all the other three PAFs. The trends of the difference of the isosteric heat (DIH) are consistent with the selectivity trends, especially at low pressure. We derived the relation between the selectivity and the isosteric heats based on Langmuir adsorption theory, and validated the model (DIH equation) by comparing the predicted selectivity with the simulated and experimental data. The predicted selectivity reproduces well with the experimental data. The derived DIH equation can be used to quickly screen out the promising porous materials (including PAFs, COFs, MOFs, etc.) for separation of gas mixtures.