Research On The Synthesis And Properties Of Zeolitic Imidazolate Frameworks Based On The CH₄/N₂ Adsorption Separation

The key to recycle and utilize CH₄ in low concentration coalbed methane is the separation of CH₄/N₂.The key factor for efficient energy-saving adsorption separation method is the preparation of a CH₄/N₂ high-performance adsorbent.Zeolitic imidazolate frameworks?ZIFs?are a class of metal organic frameworks?MOFs?with zeolite topologies.ZIFs combine the advantages of both zeolites and MOFs: high stability and structural and functional tunability,and have been widely used in the study of adsorption separation,catalysis and other fields.The changes of the types and positions of substituted groups in the imidazole ring of the ZIFs structure may improve the selectivity of ZIFs for the adsorption separation of CH₄/N₂.In this paper,we try to improve the adsorption separation performance of CH₄/N₂ by changing the types and positions of substituted groups in the imidazole ligands in the synthesized ZIFs.Then we will select the high performance of the ZIFs material and imidazolate ligand.After obtained relatively high adsorption selectivity materials,CH₄/N₂ binary mixed gas separation tests will be carried out.The main research contents and conclusions are as follows:1.In this paper,five typical ZIFs materials were synthesized by introducing imidazole ligands into different groups?electron-donating-CH₃,electron-donated-CHO?and substituted positions?2 and 4,5 substitutions?: ZIF-8,ZIF-90,SIM-1,ZIF-93,ZIF-25,where ZIF-8,ZIF-90 and SIM-1 are SOD topologies,ZIF-93 and ZIF-25 are RHO topologies.All of the synthesized materials were characterized by PXRD,SEM,TG,FT-IR and adsorption and desorption isotherms of N₂.2.ZIF-8?2-methylimidazole / Im-2CH₃?,ZIF-90?imidazole-2-carbaldehyde / Im-2CHO?,SIM-1?5-methylimidazole-4-carbaldehyde / Im-4CHO-5CH₃?have the SOD Topologies which were investigated on CH₄ adsorption and CH₄/N₂ adsorption separation selectivity.The results show that the electron-donated group?ZIF-90,-CHO?is slightly superior to the electron-donating group?ZIF-8,-CH₃?in the 2-substituted ZIFs.SIM-1 which has asymmetric substituents was superior to the 2-substituted ZIFs?ZIF-8 and ZIF-90?,and the adsorption of CH₄ on the SIM-1 material at room temperature and atmospheric pressure was 27.7 cm3/g,the ratio of CH₄/N₂ adsorption separation is 4.2,the CH₄/N₂ adsorption selectivity calculated by IAST theory is 6.6.It has showed high CH₄/N₂ adsorption separation performance.3.With the high CH₄/N₂ adsorption separation performance of SIM-1,the ZIFs with asymmetric substituents?Im-4CHO-5CH₃?may have the high adsorption performance of CH₄ and adsorption selectivity of CH₄/N₂.Then we investigate the ZIFs with asymmetric substituents?Im-4CHO-5CH₃?but have different topologies?SIM-1 and ZIF-93?.The results showed that SIM-1 material with 6-membered ring cage?SOD?was superior to ZIF-93 material with 8-membered ring cage structure?RHO?.4.In order to test the immobilized separation performance of CH₄/N₂ binary mixed gas at room temperature and atmospheric pressure because SIM-1 material has a high CH₄/N₂ adsorption and separation performance.The results showed that N₂ was passed at 360 s,CH₄ was passed at 780 s,the interval of breakthrough time was 420 s,which showed a high CH₄/N₂ separation performance.We also compared it with the classical ZIF-8 material.In the ZIF-8 material penetration experiment,N₂ was passed at 70 s,CH₄ was revealed at 80 s,the interval of breakthrough time was only 10 s.5.In view of the excellent CH₄/N₂ adsorption and separation performance of SIM-1 material,2-methylimidazole in ZIF-8 structure was replaced by the ligand of 5-methylimidazole-4-formaldehyde formed SIM-1 material.Three kinds of synthetic materials were obtained.We also test the adsorption capacity of CH₄ and CH₄/N₂ adsorption separation selectivity.The results showed that the relative content of the target ligand?5-methylimidazole-4-carbaldehyde?was increased with the increase of the exchange time,and the adsorption of CH₄/N₂ Separation performance is also increasing,its performance is better than ZIF-8,but not more than SIM-1.