Computational Studies And High-throughput Screening Of Metal-Organic Frameworks For Adsorptive Separation Of C_1-3 Light Hydrocarbons

Light hydrocarbon separation is one of the most important processes in petrochemical industry.Cryogenic distillation,which is the most mature technology,shows high energy consumption.Metal-organic frameworks（MOFs）can realize adsorptive separation at ambient temperature and have been considered as a potential platform of light hydrocarbon separation for the sake of reducing energy consumption.However,only a small amount of MOFs show great performance in adsorptive separation of light hydrocarbons.It is significant to find suitable modification methods to improve the separation performance.It has become a challenging task to efficiently find the target materials owing to the diversity of MOFs.Database-driven high-throughput screening is benificial to the discovery of target materials and has become a new method to facilitate the development of new materials.The adsorption configurations of 22 heterocycles with CH₄,C₂H₆,C₂H₄and C₂H₂were investigated by ab initio method.The energy component of different configurations was analyzed by energy decomposition.The main interaction is dispersion force in the systems of heterocycle interacting with CH₄or C₂H₆,while electrostatic force in the systems of heterocycle interacting with C₂H₄or C₂H₂.GCMC simulations were used to evaluate the adsorptive separations of C₂/C₁mixtures in modified Ui O-67 and compared with the parent material.The results indicate that 1,2,3-triazole contributes the most in improving the C₂H₂separation performance of MOFs,while imidazole is the best candidate in accelerating the separations of C₂H₄/CH₄and C₂H₆/CH₄.In addition,the interaction between light hydrocarbon and nitrogen-containing heterocycles is stronger than most of the others,indicating that nitrogen-rich MOFs are more promising in adsorptive separation of light hydrocarbons.The interaction between heterocycle and light hydrocarbon is moderate,leading to the low selectivity.Therefore,MOFs modified by heterocycles are not suitable for producing high-purity light hydrocarbons.To solve this problem,we proposed the modification by chelating zinc fluoride to the linkers of MOFs.The zinc fluoride clusters show strong interaction with C₂H₂,boosting the efficient separations of C₂H₂/C₂H₄,C₂H₂/C₂H₆,C₂H₂/CH₄,C₂H₂/CO and C₂H₂/H₂.The selectivity can be enhanced by 10-100 times.Through adsorption configuration analysis,it is confirmed that the preferential adsorption site of light hydrocarbon locates around organic linkers in modified MOFs.This is different from parent MOFs in which the metal cluster is the first adsorption site.In addition,MOFs chelated by zinc fluorides are more suitable for capturing a small amount of C₂H₂from C₂H₂/CO and C₂H₂/H₂mixtures which can benefit the full utilization of C₂H₂.The anion-pillared MOFs are promising in the separation of light hydrocrbons.An anion-pillared MOF database was constructed on the basis of experimentally reported structures.We compared the physical parameters of all MOFs in the database,and found that the physical parameters of anion-pillared MOFs can be tuned at different level by choosing different structural units.In addition,we calculated the adsorption Henry constants of CH₄,C₂H₄,C₂H₆,C₃H₆and C₃H₈at room temperature and found the Henry constants follow the order of C₃H₆≈C₃H₈>C₂H₄≈C₂H₆>CH₄.The value of Henry constant change a lot with hydrocarbon types classified by the carbon numbers,indicating that MOFs are easier to realize the separation of light hydrocarbons with different carbon numbers.Based on the anion-pillared MOF database,high-throughput screening for C₂H₆/C₂H₄,C₃H₈/C₃H₆,C₂H₄/CH₄,C₃H₆/CH₄,C₃H₆/C₂H₄and C₃H₈/C₂H₄was carried out with metrics of working capacity and Henry selectivity.The structure-property relationship of the separation processes was investigated.There exist critical values for the largest cavity diameter,accessible surface area and occupiable void fraction of MOFs.The selectivity decreases with the increase of the parameter when the value is greater than the critical value,while the molecular sieve effect may occur when the value is less than the critical value.GCMC simulation indicates that the materials SIFSIX-2-Ni-i（C₃H₆/C₃H₈）,ALFFIVE-3-Cd（C₂H₄/CH₄）,SIFSIX-6-Cd-i（C₂H₆/CH₄）,In FFIVE-5-Zn-i（C₃H₆/C₂H₄）and Ga FFIVE-5-Cd-i（C₃H₈/C₂H₄）show great separation performance,better than those of many previously reported materials.