| Light olefins are important raw materials in petrochemical industry.It is very essential to separate the mixture containing olefins/alkanes economically and effectively.MOFs porous materials have been extensively studied in the field of selective adsorption and separation of olefins due to their ordered internal microchannels,extremely high specific surface area and highly adjustable pore structure.In this paper,MOFs with different structures were synthesized by solvothermal method.MOF materials with good performance for the selective adsorption of 1-hexene/n-hexane were obtained,and the synthesis conditions were optimized.Furthermore,the adsorption separation performance of olefins/alkanes was improved by introducing and regulating bimetallic composition.Furthermore,the adsorption kinetics,adsorption thermodynamics and dynamic adsorption properties were evaluated in order to lay a foundation for the design and development of high-selectivity adsorption materials for the separation of olefins/alkanes.At first,based on the evaluation of 1-hexene/n-hexane adsorption on different MOFs materials,Mg-MOF-74 and Co-MOF-74 were selected as suitable materials for the separation of alkanes/olefins.A variety of techniques were adopted to characterize the synthesized samples.The optimal synthesis conditions were observed as follows:the ratio of raw materials of n(Co(N03)2·6H20):n(DHTP):n(THF):n(H20)= 2;1:165:750,crystallization temperature of 100 ℃,crystallization time of 24 h,activation temperature of 150 ℃,activation time of 2 h.Under optimal conditions,the synthesized Co-MOF-74 shows a specific surface area of 1209 m2/g,and micropore volume of 0.41 cm3/g.At 298K,the static adsorption capacity of Co-MOF-74 for 1-hexene and n-hexane are 128.3 mg/g and 79.1 mg/g,respectively.Subsequently,Cu2+,N2+ and Mg2+ were introduced to Co-MOF-74 strucure by one-step isomolar synthesis and ion exchange method in order to obtained enhanced performance.The results show that the adsorption selectivity of 1-hexene over n-hexane on the modified bimetallic CoMg-MOF-74 and CoNi-MOF-74 is higher than that of Co-MOF-74.Co0.30Mg0.70-MOF-74 has the best adsorption performance for 1-hexene.At 298 K,the static adsorption capacity of Co0.30Mg0.70-MOF-74 for 1-hexene and n-hexane are found to be 152.7 mg/g and 15.7 mg/g,respectively.In addition,the olefin/alkane selectivity is 9.74.As compared with monometallic Co-MOF-74 and Mg-MOF-74,the olefin adsorption capacity and selectivity are increased by 1.2-2.3 times and 4.7-6.0 times,respectively.The simulation results show that the adsorption of 1-hexene on Co0.12Mg0.88-MOF-74 releases the lowest energy.At last,the adsorption equilibrium behavior,kinetic characteristics and dynamic adsorption properties of 1-hexene and n-hexane on Co-MOF-74 and Co0.30Mg0.70-MOF-74 materials were studied.The results show that Langmuir-Freundlich adsorption model can describe the adsorption isotherms of 1-hexene on Co-MOF-74 and Co0.30Mg0.70-MOF-74 very well.The thermodynamic parameters indicate that the adsorption of 1-hexene on Co-MOF-74 and Co0.30Mg0.70-MOF-74 is a spontaneous,exothermic,and entropy reduced process.Within the temperature range of 283-313K,the apparent diffusion coefficients of liquid-phase adsorption of 1-hexene on Co-MOF-74 and Co0.30Mg0.70-MOF-74 are(2.3 1-5.61)×10-12 cm2/s and(1.91-4.97)×10-12 cm2/s,respectively.At 298K,the dynamic saturated adsorption capacities of 1-hexene on Co-MOF-74 and Co0.30Mg0.70-MOF-74 are 73.5 mg/g and 119.4 mg/g,respectively.Co0.30Mg0.70-MOF-74 shows 1.62 times higher adsorption capacity than that of Co-MOF-74. |
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