| C8 aromatics is a basic raw material in petrochemical industry.Adsorption is an effective separation method to produce some high purity aromatics monomers.The performance of adsorbents is the main factor affecting the efficiency of C8 aromatics separation process.However,similar structures of C8 aromatic isomers make their separation difficult.Some C8-selective adsorbents remain the problems of low adsorption capacity and selectivity,which is not conducive to the efficient separation and comprehensive utilization of C8 aromatics.More efficient adsorbents and separation strategies are urgently needed.In this paper,based on the multi-dimensional shape difference between the molecules of C8 aromatics,a series of microporous framework materials with the molecular recognition ability of specific C8 aromatics isomers were synthesized through the fine regulation of pore structure,and the efficient separation of complex C8 aromatics systems was realized.On this basis,we systematically studied the law of interactions between microporous framework materials and C8 aromatics isomers,and summarized the molecular recognition mechanisms applicable to different C8 aromatics isomers.The main research contents include:In order to solve the problem of selective adsorption of ethylbenzene in C8 aromatics,two kinds of microporous framework materials[Ni3(HCOO)6]and[Co3(HCOO)6]with zigzag channels and fine-regulated pore sizes were designed and synthesized based on the size and configuration differences between ethylbenzene and other C8 aromatics,and the preferential adsorption of ethylbenzene was realized.Their adsorption and separation properties of C8 aromatics isomers were investigated by static adsorption experiments in both vapor and liquid phase.The dynamic separation properties of ethylbenzene and other isomers were studied by breakthrough experiments.The study showed that[Ni3(HCOO)6]and[Co3(HCOO)6]can screen guest molecules of different sizes due to the differentiated pore sizes obtained by metal ion regulation,wherein[Ni3(HCOO)6]with smaller pore size can simultaneously exclude ortho-xylene and meta-xylene,while[Co3(HCOO)6]with relatively larger pore size only screened ortho-xylene.The zigzag channel has strong shape recognition ability for V-shaped ethylbenzene molecules,but weak adsorption selectivity for linear para-xylene molecules.The ratio of ethylbenzene/p-xylene saturation adsorption capacity and Henry selectivity of[Ni3(HCOO)6]are 6.5 and 16.5,respectively,which are the highest values reported so far.The two materials still maintained high adsorption selectivity for C8 aromatics in liquid phase with high concentration.The breakthrough results for quartary C8 aromatics confirmed the excellent dynamic separation performance of the materials for ethylbenzene.In order to realize the efficient separation of styrene and ethylbenzene with similar properties,a new anionic hybrid microporous material NBOFFIVE-bpy-Ni(ZU-61)was constructed by introducing ligand with rotational flexibility and anion functional sites,which can identify styrene and ethylbenzene molecules with different molecular thickness.The static separation properties of styrene and ethylbenzene were investigated by static adsorption experiments in vapor and liquid phase.The dynamic separation and cycling properties of styrene/ethylbenzene mixtures were studied by dynamic breakthrough experiments under vapor and liquid phase conditions.The results showed that the styrene molecule could induce the rotation of the ligand pyridine to control the opening and closing of the adsorption site and realize the shape recognition of styrene.The material exhibited the highest static styrene adsorption capacity(5.2 mmol/g)reported in the literature.In the liquid phase,ZU-61 keeps high styrene adsorption capacity(3.3 mmol/g)and separation selectivity(6.6).The breakthrough experiments showed that the dynamic separation selectivity of ZU-61 was up to 10.8,and the material had good cycling performance.The introduction of para-xylene impurities had little effect on the separation performance of styrene/ethylbenzene mixture,and the separation selectivity of styrene/ethylbenzene and styrene/para-xylene were 11.0 and 4.3,respectively.Aiming at the selective separation of meta-xylene and ortho-xylene from C8 aromatics,we constructed limited adsorption spaces with molecular recognition ability of meta-xylene and ortho-xylene through the introduction and distribution regulation of multiple adsorption sites.Two meta-xylene selective microporous framework materials MIL-160 and CAU-10H and two ortho-xylene selective materials CAU-23 and ZU-100 were obtained.The static adsorption and dynamic separation properties of C8 aromatics were investigated by adsorption and breakthrough experiments in both vapor and liquid phase,and the competitive adsorption behavior of the molecules was studied by simulation calculation.The results showed that the suitable distance between ligand O sites significantly affected the meta-xylene selectivity of MIL-160.The competitive adsorption capacity and meta-xylene/para-xylene selectivity in the liquid phase were 1.3 mmol/g and 5.3,respectively.The simulation results showed that the binding energies of MIL-160 were-123 kJ/mol,-117 kJ/mol,and-118 kJ/mol,respectively.The order of binding energies was consistent with that of separation selectivity obtained by experiments.In addition,the F site with suitable distance in ZU-100 framework can realize the selective recognition of ortho-xylene,which can realize the efficient separation of ortho-xylene,meta-xylene and para-xylene.The static adsorption capacity of the material is up to 4.5 mmol/g,and the dynamic separation selectivity of the material is 3.6.In order to reveal the internal mechanism of efficient separation of C8 aromatics isomers by microporous framework materials,the adsorption and separation mechanism of C8 aromatics by microporous framework materials was studied by using single crystal X-ray diffraction and molecular simulation techniques,and the structure-property relationship between pore structure characteristics and separation performance was discussed.The results showed that in the selective separation process of ethylbenzene,the suitable pore size and zigzag pore shape of[Ni3(HCOO)6]made V-shaped ethylbenzene and linear para-xylene have different adsorption sites in the pores.Although CAU-23 had similar pore structure,larger pore size made the channel conducive to the stacking of paraxylene,showing the preferential adsorption of para-xylene.MIL-160 and ZU-100 constructed the adsorption space suitable for the shape of target molecules by finely regulating the distances between O and F sites within the framework,respectively,and realized the shape recognition of meta-xylene and ortho-xylene molecules by hydrogen bonding interactions and π-π interaction between host and guest.The introduction of framework flexibility can further improve the molecular recognition ability of the material.ZU-61 used the rotational flexibility of the ligand to significantly improve the adsorption performance of styrene through responsive deformation,while ethylbenzene showed weak adsorption capacity due to the large molecular thickness restricting its entry into the flexible pore space. |
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