Synthesis Of Zeolitic Imidazolate Frameworks And Their Application In CO₂ Capture

CO₂ is one of the major anthropogenic greenhouse gases,it becomes more and more important to capture CO₂ effectively.ZIFs are a class of porous materials with similar structure to zeolites,belong to one branch of MOFs.They not only have structural diversity,high thermal stability and chemical stability,but also high adsorption capacity and good affinity for CO₂,which has great research value and application prospect in CO₂ capture and separation.This papaer is focused on the controllable synthesis and functionalization of ZIFs in order to enhance the performance of CO₂ capture.The main contents and results are as follows:1.The water-based synthesis of ZIF-8 was controlled by changing parameters and the effects on the relative crystallinity,specific surface area and pore volume,morphology,thermal stability,CO₂ adsorption capacity and cycle performance were analysized.The results showed that,when the ratio of metal and ligand was 1:20,water content was 27 m L,Zinc source was Zn（NO3）2·6H2O,temperature was 85 ℃,p H was 11.42,time was 1440 min,ZIF-8 that obtained had higher relative crystallinity.When Triethylamine or concentrated ammonia were the tempalate,the reation can perform under the stoichiometric ratio of metal source and ligand（1:2）,and obtained complete crystals,which reduced the usage of ligand to a great extent.When using the combination of OP-10 and concentrated ammonia,concentrated ammonia played the substantive role to reduce the usage of ligand,while OP-10 helped to change the crystal type of ZIF-8.ZIF-8 with higher specific surface area（1146.35 m2·g-1）and pore volume（0.65 m3·g-1）had higher CO₂ adsorption capacity（up to 3.04 mmol·g-1 at 35 ℃）.ZIF-8 synthsized by hydrothermal synthesis had a good recycle regeneration performance,and maintained above 97 % of the adsorption performance after 10 times of recycling.2.Six kinds of ligands,namely,imidazole,2-methylimidazole,1-acetylimidazole,benzimidazole,2-aminebenzimidazole,5-nitrobenzilimizole were used to synthsize ZIFs and the CO₂ adsorption capactiy of them was investigated.The results showed that,in view of imidazole,the CO₂ adsorption capacity of ZIFs decreased with the electron donating group（-CH3）while increases with the electron withdrawing group（-Ac）.In view of benzimidazole,the introduction of basic group（-NH2）enhanced the chemical adsorption effect,the introduction of strong polar group（-NO₂）informed a strong dipole moment with CO₂,both of them made ZIFs a higher CO₂ adsorption capacity.By screening,ZIFs synthesized with 5-nitrobenzimidazole had the highest CO₂ adsorption capacity（up to 2.95 mmol·g-1 at 35 ℃）.3.Imidazole,2-methylimidazole,benzimidazole were pairwise combined to synthesize mixed-ligand ZIF-4-7,ZIF-4-8 and ZIF-7-8 respectively.The results showed that,XRD diffraction peak and TGA curves showed regular changes with the change of the mole ratio of one ligand,and their thermal stable temperature in the air atomosphere were all above 500 ℃.CO₂ adsorption exiperiments showed that,when the mole ratio of Imidazole,Imidazole,benzilimidazole of ZIF-4-7,ZIF-4-8,ZIF-7-8 was 25 % respectively,ZIFs that obtained had a higher CO₂ adsorption capacity.