Preparation And Performance Study Of Zeolitic Imidazolate Frameworks Mixed Matrix Membranes For CO₂/N₂ Separation

The development of the industrial era has released large quantities of carbon dioxide into the atmosphere,and thus caused the greenhouse effect.Therefore,it is urgent to separate and capture CO₂.Membrane-based technology has attracted intensive attention in the field of CO₂separation due to its unique advantages,e.g.simple operation process and energy efficiency.Traditional polymeric membranes are suffering from the trade-off effect of permeability and selectivity,and the performance is below the Robeson upper bond.Mixed matrix membranes（MMMs）,which combine the advantages of inorganic and organic membranes,are demonstrated to be promising membranes to overcome the trade-off effect.Zeolite imidazolate frameworks（ZIFs）,as a subclass of metal organic frameworks（MOFs）,have been widely used as inorganic fillers in MMMs due to their tunable pore size and functionality.In this paper,aiming at the post-combustion carbon dioxide capture and separation（mainly CO₂/N₂separation）,two kinds of MMMs based on ZIF-7and ZIF-90 inorganic fillers have been developed.The effect of the surface property and particle size of the MOF fillers on the performance of MMMs was studied.1.In this work,amino（NH₂-）,hydroxyl（OH-）and hydroxymethyl（CH₂OH-）functionalized ZIF-7 were successfully synthesized.The equilibrium CO₂adsorption amount of each functionalized ZIF-7 was higher than that of parent ZIF-7,indicating that the introduction of functional group in ZIF-7 framework was indeed beneficial for CO₂adsorption.The ideal selectivities of CO₂over N₂estimated by the ideal adsorbed solution theory（IAST）method on the four materials increased in the following order:ZIF-7-OH>ZIF-7-CH₂OH>ZIF-7-NH₂>ZIF-7.The four ZIF-7 materials were incorporated into polyether-block-amide（Pebax^?2533）polymer to form mixed-matrix membranes（MMMs）.Single gas permeation results showed that both CO₂permeability and CO₂/N₂selectivity of MMMs increased as compared to the neat Pebax membrane,suggesting the positive effect of MOF fillers.Moreover,the ZIF-7-OH-Pebax MMMs exhibited the best performance for CO₂/N₂separation,which might be attributed to the highest CO₂/N₂adsorption selectivity of ZIF-7-OH（70%）fillers.The ZIF-7-OH-Pebax MMMs exhibited CO₂permeability of 273 Barrer and CO₂/N₂separation factor of 38,respectively,which increased by60%and 145%as compared with the neat Pebax^?2533 membrane.In addition,with the increase of pressure,the ZIF-7-OH-Pebax MMMs exhibited increased CO₂/N₂separation factor with the basically unaltered CO₂permeability,which is very suitable for high-pressure CO₂/N₂separation.2.The ZIF-90 crystals with adjustable particle size were obtained by optimizing the synthesis parameters such as reactant molar ratio,reaction time,etc.In order to investigate the effect of particle size of the ZIF-90 filler on its MMMs performance,two kinds of ZIF-90 crystals with the particle size of 798 nm and 115 nm were selected as the fillers to fabricate the polyether-block-amide（Pebax^?2533）based MMMs.The ZIF-90-Pebax MMM with the ZIF-90 particle size of 115nm showed high CO₂permeability of 307 Barrer and CO₂/N₂separation factor of 29.4,which increased by 80%and 90%as compared with the neat Pebax membrane.The result demonstrated that the addition of ZIF-90 with small particle size significantly improved both the CO₂permeability and CO₂/N₂separation selectivity,which might be due to the good compatibility between the ZIF-90 and polymer.However,The ZIF-90-Pebax MMM with the ZIF-90 particle size of 798 nm showed CO₂permeability of 188 Barrer and CO₂/N₂separation factor of 12.5,which is lower than that of pure Pebax membranes.This may be due to the formation of non-selective interface void in the polymer matrix,which has an adverse effect on the performance of the MMMs.