Fabrication Of Polyimide And Covalent Organic Frameworks Mixed Matrix Membranes For Research Of CO₂/CH₄ Gas Permeability And Selectivity

Excessive carbon dioxide levels in natural gas will not only reduce the calorific value of natural gas,but also corrode transport pipelines.On the other hand,due to global warming,gas separation is also crucial to the environment.The dramatic increase in carbon dioxide emissions poses a great challenge to the sustainable development of modern society.With the development of new technologies and the discovery of new materials,membrane-based gas separation has evolved from a narrow scientific discipline and limited practical applications to a wider range of fields.The polymer material has the advantages of excellent membrane forming ability,strong machinability,and good mechanical properties.Also,in the industry,the cost of processing a polymer into a film and assembling it into a high-capacity module is relatively low,which makes the polymer film the most competitive film material for industrial gas separation applications.Polyimide?PI?membranes have also been widely used in gas separation,liquid purification,fuel cells and other fields.However,for most PI membranes,there is an inherent trade-off effect that materials with high permeability generally exhibit lower selectivity and vice versa.In addition,the plasticizing and aging effects of the membrane are also usually present in the polymer membrane.Thus,the current challenge for PI membranes is to achieve good gas selectivity without sacrificing gas permeability and increase their resistance to plasticization.To solve the above problems,the strategy of mixing matrix membranes has gradually attracted people's attention.The main research content of this paper was the preparation of Schiff-base covalent organic frameworks?COFs?SNW-1,which were used as nanofillers to prepare PI and co-PI composite matrix membrane with different filler content.The gas permeabilities of CO₂ and CH₄ of the PI/SNW-1 membrane were tested.The main findings are as follows:1.The Schiff-base COFs nanoparticles SNW-1 were synthesized using an improved preparation scheme.The structure and properties of the prepared compounds were identified by FT-IR,¹³C NMR,FESEM,nitrogen adsorption and desorption,and thermogravimetry.2.PI and various filler content PI/SNW-1 mixed matrix membranes were prepared.The thickness of the membranes are about 30?m.The structure,appearance and properties were characterized by FT-IR,FESEM,XRD and thermogravimetry.The results showed that SNW-1 was well dispersed in the polymer matrix.The addition of fillers had essentially no effect on the structure and thermal properties of the polymer,but increased the chain spacing of the PI matrix and increased the free volume.The TG and DTG curves showed that all membrane materials have a T_d 5%temperature around 500?.3.Gas permeation performances of CO₂ and CH₄ in PI and PI/SNW-1mixed matrix membranes were studied.With the increase of SNW-1,the permeability coefficients of CO₂ and CH₄ gases increase.Compared with the pure PI membrane,PI/SNW-1?5%?showed an increment of 48.7%in ideal selectivity of CO₂/CH₄ and an enhancement of 106.4%in CO₂ permeability at 5wt%SNW-1.4.Copolymerized PI/SNW-1 mixed matrix membrane were prepared and its structure and properties were characterized.The results showed that the polymer was more stable due to thermal cross-linking reactions and therefore had higher thermal stability.