| Utilization efficiency of naphtha resource can be enhanced largely by using adsorption separation technology based on molecular management strategy.MOF materials are receiving extensive attention in the field of selective adsorption and separation of hydrocarbon mixtures due to their changeable microporous structure,extremely high specific surface area and tunable internal pore size.Here,the hybrid-ligand ZIF-8-90 materials were synthesized by solvothermal method.Meanwhile,the regulation of structrure and performance of ZIFs with template added was explored.Furthermore,the adsorption performance of n-alkanes on synthesized ZIFs was studied by means of both experiments and molecular simulation,in order to develop high-performance adsorption materials for improving the adsorption and separation efficiency of naphtha.The hybrid-ligand ZIF-8-90 materials with 2-MeIM and OHC-IM were synthesized by solventhermal method and characterized by applying multiple techniques.The results show that as the OHC-IM proportion in the framework rises from 0.31 for ZIF-8-90-S1 to 0.88 for ZIF-8-90-S5,the specific surface area decreases from 1374 m2/g to 1208 m2/g,the pore volume decreases from 0.649 cm3/g to 0.596 cm3/g,the average micropore diameter shrinks from 0.72 nm to 0.65 nm while the mesopore width monotonically increases from 2.18 nm to 3.29 nm.In addition,the thermal stability of ZIF-8-90 degrades and the DSC peak temperature decreases from 372 ? to 308 ? with increasing OHC-IM content.According to the liquid phase adsorption experiment,the ZIF-8-90-S4 sample having OHC-IM ratio of 0.82 has the highest static adsorption performance for n-hexane at 298K,and its adsorption capacity is 221 mg/g.Subsequently,template was introduced into the synthesis system to modify the ZIF-8-90 materials.The results show that ZIF-8-90-M with microporous and mesoporous structure can be synthesized by using CTAB as template.Compared with template-free ZIF-8-90-S4 sample,with the increase of the template,the crystal size decreases from 10 ?m to 200 nm,both the specific surface area and pore volume decrease while the thermal stability increases.When the template dosage n(CTAB)/n(Zn(NO3)2·6H2O)=0.25,the ZIF-8-90-M2 sample has the highest specific surface area and pore volume,reaching 1380 m2/g and 0.982 cm3/g,respectively.On this basis,gas phase adsorption equilibrium performance and dynamic adsorption performance of different ZIF materials were investigaed.According to the Langmuir-Frendlich model,the saturated adsorption amount of n-hexane on ZIF-8-90 at 298K are 243,233,275,295 and 267 mg/g,respectively.The dynamic saturated adsorption amount of ZIF-8-90 at 298K are 231.2,225.1,229.4,243.2 and 230.1 mg/g,respectively.The Thomas rate constants are 0.00516,0.00449,0.00416,0.00549 and 0.00422 mL/(min·mg),respectively,which were both higher than those of ZIF-8.The saturated gas phase adsorption of n-hexane at 298K on ZIF-8-90-M2,ZIF-8-90-M3,ZIF-8-90M4 were 343,332 and 326 mg/g,respectively,which are more than 33 percentage points higher than that of ZIF-8.Finally,GCMC method was used to calculate the process of adsorbing n-alkanes on ZIFs.According to the simulation,the adsorption sites of n-hexane at ZIF-8 are mainly distributed around the 2-MeIM ligands,n-hexane tends to be adsorbed near the OHC-IM ligands while 2-MeIM and OHC-IM ligands coexist in the cell structure of ZIF-8-90,which indicate that organic ligands play a crucial role in the adsorption of n-hexane by ZIFs.Additionally,the vdW force energy of ZIF-8-90 adsorbing n-hexane was computed,the highest energy can be 5.505 kcal/mol at the ligand ratio of 19:5(very close to ZIF-8-90-S4 sample).The adsorption capacity of the model structure constructed based on the three samples of ZIF-8-90-M2,ZIF-8-90-S3 and ZIF-8-90-S4 modified by CTAB are higher than that of the corredsponding model structure of the unmodified ZIF-8-90-S4.The simulation results are in good agreement with the experimental results. |
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