| Gas separation is critical in many industrial processes,such as biogas purification,natural gas desulfurization,helium recovery,flue gas treatment,hydrogen purification,nitrogen production,etc.Natural gas decarbonization and pre-combustion carbon capture have become hot environmental and energy issues.The concentration of atmospheric carbon dioxide is increasing year by year,and it has been reported that increased concentrations of CO2 from anthropogenic emissions in the post-industrial era have caused global warming of about 1.0 ℃.Therefore,it is necessary to reduce CO2 emissions in order to avoid irreversible environmental damage.Sustainable energy sources such as biogas and hydrogen play a vital role on the path to CO2 reduction.Biogas is a clean energy source consisting mainly of methane,carbon dioxide and other trace gases.The presence of CO2 in biogas will corrode and destroy pipelines and affect the calorific value of combustion.Hydrogen energy plays an important role in solving the energy crisis,global warming and environmental pollution and other problems,fossil raw materials or biomass by reforming vaporization and water gas conversion reaction to obtain the conversion gas accounted for more than 70%of the entire hydrogen energy source,reforming hydrogen production after changing gas often contains a lot of H2 and CO2,the conversion gas need to be separated to remove CO2 to meet the demand of hydrogen.Therefore,it is necessary to develop an effective method to separate CO2 in order to alleviate the environmental and energy problems caused by CO2 emissions.Currently,the most widely adapted technologies in commercial gas separation include cryogenic distillation,solvent absorption and pressure swing adsorption,and gas separation membrane.Among the variety of methods,hollow fiber membranes have emerged as a preferred membrane based configuration due to their high packing density,operational flexibility,and high productivity of each unit volume.Typically composed of a porous supporting layer,a transition layer,and a separation layer,hollow fiber gas separation membranes can obtain high separation performance,and the dense separation layer is usually located on the outer surface of the membrane.However,during practical gas separation operations,fluctuations of the operating conditions can often lead to mutual squeezing and friction of the outer surface layer of the membrane,resulting in wear of the membrane separation layer and subsequent deterioration of the gas separation performance of the hollow fiber membranes.In addition,for hollow fiber membranes with the selective layer on the outer surface,the gas separation performance is constrained by the inlet mode of the feed mixture,for which a core inlet is typically used.While using a core inlet can improve membrane separation efficiency,it also results in a higher pressure drop,reduces the permeance,and deteriorates the pressure resistance of membranes.Moreover,polymer hollow fiber membranes are subject to challenges such as plasticization.Furthermore,large-scale implementation of polymer hollow fiber membranes is limited by the inherent trade-off between permeability and selectivity,defined by the well-known Robeson upper bound.In order to improve the pressure drop from the core layer inlet of the hollow fiber membrane and to mitigate the wear of the separation layer,a new type of aromatic copolyimide was designed based on the theory of phase transition.This co-polyimide was then prepared as the selective layer at the inner surface of the hollow fiber membrane using a non-solvent induced phase transition method.The resulting hollow fiber membrane with an inner surface selective layer exhibited a helium permeance of 110 GPU,CO2 permeance of 12.87 GPU,CO2/CH4 selectivity of 39,and a He/CH4 selectivity of 330.A strategy was proposed for preparing a separation layer on the inner layer of the hollow fiber membrane using non-solvent induced solidification,while the porous structure is induced by the coagulation bath.The thickness of the separation layer was controlled by adjusting the air gap and the ratio of water to N-methyl-2pyrrolidone(NMP)in the core fluid.The shear rate was controlled by changing the flow rate of the bore fluid and feed solution to obtain the appropriate molecular orientation.The composition of the coagulation bath was adjusted to regulate the precipitation and phase transition rate of the outer layer to obtain a porous outer layer.To prevent plasticization of the hollow fiber membrane and loss of selectivity for gas pairs,thermal crosslinking treatment was employed to further modify the selective layer on the inner surface of the hollow fiber membrane.After thermal crosslinking,the permeance of all gases increased,and the selectivity of He/CH4,CO2/CH4 and O2/N2 are improved compared with the uncrosslinked membrane.The permeance of He and H2 is more than 2 times higher,and the permeance of CO2 and O2 is 3 times higher than that of the uncrosslinked membrane.The selectivity of He/CH4,CO2/CH4 and O2/N2 was 8%,51%and 76%higher than that of uncrosslinked membrane,respectively,and showed good plasticization resistance at 4 MPa.In order to overcome the trade-off effect of polymer membranes in gas separation,a novel strategy was developed using amino functionalized MOF hollow fiber composite membranes with the dense selective surface layer on the inner surface,where the zeolite imidazole skeleton(ZIF)was used as the MOF material to prepare the hollow fiber composite membrane.In this experiment,UV induced surface aminografting was carried out on the porous shell side of the flexible PES hollow fiber,followed by in-situ microfluidic injection to form the ZIF layer.By adjusting the density of amino-grafting and microfluidic injection conditions,the orientation and gas separation performance of the NH2-ZIF-8@HF membrane were regulated.A highly(222)oriented NH2-ZIF-8@HF membrane exhibited superior H2/CO2 separation performance far beyond the upper bound reported in 2008.The orientation control of the ZIF-8 membrane was achieved by changing the amino-grafting density,which originate from the changes of adhesion energy between crystal faces.The highly(222)oriented NH2-ZIF-8@HF membrane exhibited excellent H2/CO2 separation performance at 150℃,with an H2 permeance of 75711±610 GPU,among one of the highest values reported for ZIF membranes supported by hollow fibers,and a H2/CO2 selectivity of 21.2±0.11.The ultra-permeable membrane showed high flexibility,and the deterioration of gas separation performance can be neglected after folding and unfolding 20 times at 180° and multiple temperature fluctuations.The NH2-ZIF-7@HF membrane,the NH2-ZIF-67@HF membrane,and the NH2-ZIF-8-La@HF membrane based on Rare Earth coordination also exhibited excellent H2/CO2 separation performance,the permeance of H2 and the H2/CO2 selectivity increased with the increase of testing temperature.The excellent gas separation performance of the NH2ZIF@HF membrane thus has great potential in industrial hydrogen production.In summary,this paper focuses on the separation of carbon dioxide from natural gas and pre-combustion carbon capture,hollow fiber membrane with dense and defectfree inner surface separation layer was prepared by non-solvent-induced phase transition method in this paper.Thermal cross-linking treatment was further carried out,which resulted in more efficient separation CO2/CH4,and the plasticization resistance of hollow fiber membrane with inner surface was improved through decarboxylated crosslinking.A high(222)orientation NH2-ZIF-8@HF membrane induced by surface amino groups was prepared using ZIF-8 functionalized modification on the substrate of the inner surface selective layer with excellent H2/CO2 separation performance.The proposed preparation method for the membrane is simple with excellent gas separation performance,paving the way for future industrialization,and it is of great significance for the promotion of environmental protection and the realization of dual-carbon goals. |
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