| C6 alkanes are one of the main components of gasoline.The research octane numbers of hexane isomers increase with the degree of branching.Therefore,increasing the content of di-branched hexane in gasoline can effectively promote the research octane number of gasoline.In industry,high-octane gasoline is produced by adsorption separation of hexane isomers with different degrees of branching.However,the current industrial adsorbent Zeolite 5 A can only separate linear and branched hexane due to the restriction of its pore size.In order to achieve the highly selective adsorption separation of mono-and di-branched hexanes and further improve the gasoline octane number,eight CAU-10 isostructural metal-organic frameworks with different pore sizes and surface properties were synthesized based on two pore size control strategies of functionalization and multivariable metal-organic frameworks in this paper,to study their adsorption separation performance of n-Hexane(nHex),3-methylpentane(3-MP)and 2,2-dimethylbutane(2,2-DMB).CAU-10-H,CAU-10-CH3,CAU-10-OCH3,CAU-10-NO2,CAU-10-NH2,CAU-10-OH,CAU-10-Br and CAU-10-H/Br were synthesized by hydrothermal reactions,whoes pore sizes are restricted by the demensions and structures of different functional groups.The physical and chemical properties of materials like crystal structure,thermal stability,and surface area were characterized by experimental methods such as scanning electron microscopy,powder X-ray diffraction,thermogravimetric analysis,and N2/CO2 adsorption-desorption.Elemental analysis and hydrogen nuclear magnetic resonance spectroscopy experiments were used to verify the ligand ratio in CAU-10-H/Br.The single-component static adsorption isotherms of nHex,3-MP and 2,2-DMB show that among the eight materials,CAU-10-CH3,CAU-10-NO2,CAU-10-Br and CAU-10-H/Br with suitable pore size and structural flexibility can adsorb nHex and 3-MP while basically exclude 2,2-DMB,possessing high selectivities for adsorption separation of mono-and di-branched hexane,in which 3-MP/2,2-DMB henry coefficient selectivity of CAU-10-Br under 393 K is up to 13956.The kinetic diffusion experiments show that the order of magnitudes of the diffusion time constants of nHex and 3-MP are basically between 10-4 and 10-3 and the diffusion time constants increase with the rise of temperature.The diffusion rates of 3-MP with larger kinetic diameter are lower than nHex under the same conditions,and the diffusion rates of adsorbate molecules in different materials conform to the change law of materials' pore size.The three-component fixed bed breakthrough experiments show that the breakthrough time intervals of 2,2-DMB and 3-MP on CAU-10-CH3,CAU-10-NO2,CAU-10-Br and CAU-10-H/Br are 161,132,140 and 164 min/g,indicating the ability of these four materials to separate mono-and di-branched hexane by size seiving is excellent,which proves the successful application of the above two pore size control strategies in the separation of hexane isomers.In the study of the adsorption mechanism,the theory of the instantaneous change of the framework structure induced by temperature and guest molecules was used to explain the anti-thermodynamic phenomenon that the adsorption capacities of hexane isomers in the CAU-10 isostructural material increase with the rise of temperature,and the experimental method of the high-temperature X-ray diffraction was used to prove the theory.The calculation results of diffusion activation energy show that the diffusion resistance of guest molecules increases with the decrease of pore size.The configuration-bias Monte Carlo simulation results show that the adsorbed guest molecules are located in the large cavity of the channels and the orientation distribution of nHex is restricted by the ligand molecules,while the orientation distribution of 3-MP is more random.Isosteric heat of adsorption and dispersion-corrected density functional theory calculation show that the introduction of highly electronegative atoms such as N,O and Br will strengthen the host-guest interaction. |
PDF Full Text Request