Preparation Of IL/ZIF-8 Composites And Their CO₂ Trapping Ability

Carbon dioxide(CO2),as a greenhouse gas,is mainly derived from human activities.A large number of CO2 is emitted into the atmosphere,causing a series of climate problems.Carbon capture and storage(CCS)technology plays an important role in reducing CO2 emissions.Among many CO2 capture technologies,solid adsorption technology has received extensive attention due to its high efficiency and low energy consumption.Metal-organic frameworks(MOF)has attracted many scholars’ attention in the field of CO2 capture and separation due to their regular crystal structure,large specific surface area and tunable size.In this paper,ZIF-8 was selected as the MOF base,and different types of ionic liquids(IL)were introduced into ZIF-8 to study the effect of IL on the structure and CO2 trapping ability of ZIF-8.Using zinc nitrate hexahydrate as zinc salt and 2-methylimidazole(HMIM)as ligand,the effects of synthesis conditions(solvent,reaction time,temperature,solvent dosage,molar ratio of raw materials)on ZIF-8 were studied by direct precipitation method.The optimal synthesis conditions of ZIF-8 were obtained by evaluating the properties of the samples within the scope of investigation:reaction time is 1h,solvent is methanol,reaction temperature is 25℃,solvent dosage is 90mL,molar ratio of zinc salt to HMIM is 3:18.ZIF-8 synthesized under the optimal conditions was characterized and its trapping ability of CO2 was evaluated.The results show that ZIF-8 has complete structure,high thermal stability,specific surface area,and normal CO2 adsorption capacity,indicating that the process optimization is successful.In order to explore the effect of the structure of anions and cations of IL on the structure and CO2 trapping ability of IL/ZIF-8 composites,five kinds of IL with the same cation[BMIM]+,different anions([BF4]-,[PF6]-,[Tf2N]-,[Cl]-)and the same anion[Tf2N]-and different cations([BMIM]+,[EMIM]+)were introduced into ZIF-8.The test results show that ZIF-8 maintains the original crystal structure and morphology after the introduction of IL,and there is an interaction between them.Compared with ZIF-8,the CO2 adsorption capacity of all IL/ZIF-8 composites decreased to the same extent,indicating that the change of anion and cation didn’t affect the adsorption capacity of CO2 when the adsorption pressure was greater than lbar.The main reason for this phenomenon may be that the specific surface area and pore volume of IL/ZIF-8 composites play an important role in the adsorption process when the adsorption pressure is greater than lbar.The introduction of IL led to the decrease of both,so the adsorption capacity of CO2 decreased.In order to explore the effect of poly ionic liquids(PIL)on the structure and CO2 trapping ability of ZIF-8,two kinds of IL with the same cation[VBIM]+ and different anions([Tf2N]-,[Br]-)were introduced into ZIF-8 and polymerized into PIL.The test results show that after the introduction of PIL,ZIF-8 maintains the original crystal structure and morphology,and there is an interaction between them.The CO2 adsorption capacity of PIL/ZIF-8 composites is lower than that of ZIF-8.There may be two main reasons for this phenomenon:PIL isn’t completely polymerized into the pores of ZIF-8 but partly deposited on the surface,resulting in the decrease of specific surface area and pore volume of PIL/ZIF-8 composites;the total pore volume of ZIF-8 is small,and PIL can’t form hierarchical pores to boost the confined pore size effect after entering the pores.