| Hydrogen is a kind of ideal energy and important industrial raw material,which has many advantages such as cleanliness and efficiency.The demand for hydrogen is increasing due to population growth,fossil fuel reduction,and environmental pollution.As one of the hydrogen production industries,recycling hydrogen from by-product gas can improve the utilization of resources,reduce atmospheric pollution,and protect the environment.Ammonia synthesis is a large-scale chemical basic industry,about 150-250 m3 of purge gas was released from every ton of ammonia production.The difficulty and importance of hydrogen recovery from purge gas is the separation of H2/N2.The adsorption technology is a promising method for mixed gas separation but the reports about the adsorptive separation of H2/N2are limited,which prevents the design and modification of adsorbents.Herein,a combination of separation excess adsorption experiments,GCMC,MD,AIMD,and DFT was used to discuss the separation capacity and adsorption nature for H2/N2in different kinds of F-MOFs with different F densities.These works provide useful references for the design and modification of adsorbents.The main researches are as follows:1.The adsorption isotherms,adsorption selectivity,adsorption heat,self-diffusion coefficients,and adsorption sites of the H2/N2 in SIFSIX-2-Cu-i were investigated with a combination of excess adsorption experiments and GCMC simulations to reveal the adsorption performance and mechanism for H2/N2 mixture in SIFSIX-2-Cu-i.The polarization forcefield was considered to better describe the effect of polarization on the simulations.The results of the simulations are in good agreement with excess adsorption experiments,which verified the accuracy of the polarization forcefield.Results showed that the maximum selectivity of N2/H2in SIFSIX-2-Cu-i is more than 3.In theory,an economical PSA process can be designed in the industry to achieve the separation of H2/N2.The adsorption heat of N2 and H2 is about 17 kJ/mol and 10 kJ/mol,respectively,which indicates that SIFSIX-2-Cu-i has a greater thermodynamic adsorption selectivity for N2 than for H2.The main adsorption sites of H2 and N2 are very similar,both of them mainly interact with F atoms in SIFSIX-2-Cu-i or situate between two F1 atoms that are on opposite walls in the channels.2.The distribution of H2 or N2 in the pores of SIFSIX-2-Cu-i and the charge density redistribution of the system after adsorption was calculated with AIMD and DFT methods.As shown in RDF,the F2 atoms in SIFSIX-2-Cu-i interact preferentially with N2 and H2 but interact more strongly with N2.The redistribution of charge density showed that due to the strong electron-withdrawing ability of the F atoms,the electrons of the H atoms of H2molecules migrated to F atoms,which leads to increased charge density of F2 atoms and decreased charge density of H2or N2 molecules.Compared with H2,the charge transfer between N2 and F atoms is more obvious,which is mainly because of the denser electron cloud distribution of N2.3.In GCMC,the adsorption isotherms,adsorption selectivity,adsorption heat,and adsorption sites of N2/H2 in SIFSIX-3-Cu,SIFSIX-1-Cu,and SIFSIX-2-Cu were simulated to predict the separation capacities for H2/N2 in F-MOFs with different F densities.The results showed that SIFSIX-3-Cu with the highest F densities exhibited the best H2/N2separation capacities and its maximum N2/H2selectivity is 12.84 in 273K.SIFSIX-3-Cu exhibited the best N2/H2 selectivity in the F-MOFs studied.The adsorption heat of N2 or H2in SIFSIX-3-Cu is about 21 kJ/mol and 10 kJ/mol respectively.The adsorption heat of N2 is2.1 times that of H2,it is obvious that SIFSIX-3-Cu has higher thermodynamic selectivity for N2.In SIFSIX-2-Cu-i,the main sites of N2are near SiF62-while the main sites of H2 are near pyrazine. |
