Synthesis Of Size-tunable Nano-ZIF-8 And The CO₂ Separation Performance Of Its Mixed Matrix Membranes

In recent years,the CO₂ capture has been paid special attentions due to the increasingly prominent greenhouse effect.Membrane separation technology is a promising alternative and has drawn lot attentions in CO₂ separation,due to the advantages of high efficiency,simple,energy saving and easy to combine with other separation processes.However,for the polymer membrane,the permeability and selectivity of membrane are restricted by the inherent properties and can not contain at the same time.For the inorganic membrane,it is relativity and expensive.Mixed matrix membranes,which combine the advantages of polymer membrane and inorganic membrane,can enhance the separation performance of membrane and exceed the Robeson limit.Metal–organic frameworks?MOFs?are considered to be an attractive inorganic filler in terms of tunable pore size,large surface area and high porosity.However,for the MOFs based MMMs,micron-sized MOFs have poor compatibility with polymer due to their large size,and interfacial defects between MOFs and polymer result in decrease separation performance of membranes,on the other hands,the agglomeration of the nano-sized MOFs which caused by the larger surface free energy during the process of membrane fabrication also causes a decrease in membrane separation performance.Therefore,this paper proposes the synthesis of nano-sized ZIF-8 and amino-functionalized ZIF-8 with high dispersion by microemulsion method to enhance the compatibility between ZIF-8 and polymer,thereby improving the CO₂separation performance of membranes.In this paper,ZIF-8 nanoparticles with tunable morphology and size were synthesized via microemulsion,and the effects of reaction time,the ratio of metal to ligand and water contain?W?on the morphology and size of ZIF-8 were studied.Finally,the result shows that ZIF-8nanoparticles?ZIF-8-110,ZIF-8-90,ZIF-8-60,ZIF-8-40?with tunable morphology and different size could be obtained by adjusting the ratio of metal to ligand?1:2,1:5,1:8,1:16?under the condition of W=20 and 30 min.With the constant concentration of Zn²⁺,the nucleation sites of ZIF-8 increased with the increase of the ligand 2-methylimidazole,so the size of ZIF-8 nanoparticles decreased with the increase of the imidazole addition amount.Meanwhile,in order to enhance the affinity of carbon dioxide,NH₂-ZIF-8 with different morphology and size were synthesized by using mixed ligands under the same method and condition.With the use of 2-aminobenzimidazole range from 0 to 15%,the morphologies of ZIF-8 vary from cube to tetrahedron and the average sizes increase from 90 nm to 250 nm,which is probably due to the benzene ring in the 2-aminobenzimidazole.With the benzene ring,2-aminobenzimidazole has more space resistance than 2-methylimidazole.It may has influence on the coordination of ligands with zinc ions,and leading the change of sizes and morphologies of ZIF-8.Poly?ether-block-amide??Pebax?based mixed matrix membranes were fabricated by using the as-synthesized ZIF-8 and NH₂-ZIF-8 nanoparticles as fillers.The CO₂/N₂ separation performance of mixed-matrix membranes with different loadings of ZIF-8 and NH₂-ZIF-8 was studied.The results show that the ZIF-8/Pebax mixed matrix membrane has no obvious interface phase separation,which proves that the nano-sized ZIF-8 are dispersed in the polymer phase uniformly.The incorporation of ZIF-8 resulted in a significant improvement in CO₂permeability.At 20 wt.%ZIF-8 loading,the CO₂ permeability of MMM enhanced 70%by comparison to the pure Pebax membrane,with a slight decrease selectivity of CO ₂/N₂.Besides,at 5 wt.%loading of ZIF-8-90,both the permeability and the selectivity of ZIF-8/Pebax MMMs are increased 25%and 22%,respectively.For the NH₂-ZIF-8/Pebax MMMs,when there are 12 wt.%NH₂-ZIF-8?15?loading,the CO₂ permeability enhanced125%and 30%by comparison to the pure Pebax membrane and ZIF-8/Pebax MMMs,respectively,along with a degree of improving CO₂/N₂ selectivity.Particularly,compared with pure Pebax membrane and ZIF-8/Pebax membrane,the CO₂/N₂ selectivity of the 6 wt.%NH₂-ZIF-8?5?/Pebax MMM increased 30%and 17%,respectively.The results shows that the incorporation of nano-sized ZIF-8 synthesized by microemulsion can enhance the compatibility between inorganic particles and polymer and improve the permeability and selectivity of MMMs,thereby improving the performance of membranes.