Study On Adsorptive Separation Of Xylene Isomers By Metal-organic Framework Materials

Catalytic reforming of crude oil brings a large amount of mixed xylene,and each xylene isomer is an important chemical raw material with increasing demand year by year.So,separation of mixed xylene is of great significance.At present,the industrial xylenes separation is through adsorption method,but due to the low adsorption capacity and selectivity of the X and Y zeolite,adsorption efficient is low.Therefore,finding a superior adsorbent has become the focus.Metal-organic frameworks(MOFs)is a new type of porous material with extremely high surface area and has crucial applications in the field of adsorptive separation.In this paper,we studied the adsorption effect of xylene isomers on MOFs.Six kinds of MOFs were selected and synthesized including ZIF-8,MIL-53(Cr),UiO-66,MIL-96(Al),MIL-100(Fe)and MIL-101(Cr).Static competitive adsorption experiment was carried out to investigate the separation effect and reached the conclusion that MIL-53(Cr)was the best both on adsorption capacity and selectivity of the isomers.Further investigation on adsorption kinetics and adsorption thermodynamic of MIL-53(Cr)were studied in detail.The results showed that the adsorption equilibrium reached within 1 h and the capacities were all over 86% within 6 minutes.With the elevation of initial concentration,the adsorption amount of xylenes on MIL-53(Cr)increased and gradually reached equilibrium.While the capacity gradually decreased with temperature elevation.Different kinetic models and adsorption isotherm models were fitted to the experiment results and found that the adsorption processes of MIL-53(Cr)adsorbing three xylene isomers all conformed to the pseudo-second-order kinetic model,and the adsorption isotherms fit well to the Langmuir model with saturated adsorption capacity of 8.2 mol/kg for o-xylene,which was three times higher than industrial zeolite.The calculation of thermodynamic parameters put forward the adsorption of xylenes on MIL-53(Cr)a spontaneous exothermic process.The selective separation performance of MIL-53(Cr)was then studied.Static competitive experiments manifested the co-adsorption of xylenes on MIL-53(Cr).Among the three xylene isomers,MIL-53(Cr)selective adsorbed in the order of oxylene > m-xylene > p-xylene,with the selectivity for o-xylene/p-xylene about 14,o-xylene/m-xylene exceeding 8 and m-xylene/p-xylene around 1.5-1.9.Dynamic breakthrough experiment verified the ability of MIL-53(Cr)on effective separating xylene isomers.The adsorbed MIL-53(Cr)was washed with n-hexane and regenerated.After three cycles,the adsorption capacity slightly reduced,while the selectivity slightly increased.In summary,MIL-53(Cr)is an excellent adsorptive separation material with short equilibration time,high capacity and selectivity,it has broad perspective and application potential.