Study On Structure And CO₂ Separation Performance Of Polyimide/Carbon Nano-materials Composite Membranes

Carbon dioxide?CO₂?is one of the primary greenhouse gases that contributes to global warming and climate change,thus energy-efficient and scalable carbon capture stands as one of the greatest challenges.Compared with traditional separation technologies,membrane separation has evolved as a green and affordable alternative,owing to its intrinsic advantages such as high efficiency,easy intensification,simple operation and low capital and operating costs.However,membranes designed for gas separations have been known to have a near-universal trade-off between permeability and selectivity as shown in upper bound curves developed by Robeson.Therefore,mixed matrix membranes?MMMs?have been studied as an alternative approach to solve the trade-off phenomena of polymeric membranes in gas separation as well as water treatment.For the gas separation,the interfacial interaction between nanoparticles and polymer matrix,the properties of separated gases and the preparation method of membranes are very important factors to the final separation performance.To get a uniform dispersion of the nano-materials,MWCNTs and GO were functionalized by chemical modification in this paper.The PI/Carbon nano-materials composite membranes were prepared by in-situ polymerization approach?i.e.,solution-casting followed by subsequent imidization?,and the gas separation performance of the membranes were studied.In the part of PI/MWCNTs nano-composite membranes,the MWCNTs were functionalized via acid and GPTMS at first.TEM showed that the entanglements of MWCNTs were disappeared and MWCNTs were cut into short ropes,which would improve the dispersion of MWCNTs.FTIR and XPS confirmed that the functionalization had introduced carboxylic acid groups?-COOH?,hydroxyl groups?-OH?and GPTMS on the surface of MWCNTs.Then the PI/MWCNTs nano-composite membranes were synthesized by in-situ polymerization approach.The morphology and structure of the membranes were characterized by SEM,XRD and FTIR.The result suggested that the addition of MWCNTs had not change the basic structure of PI matrix.However,the interaction between PI and MWCNTs might disrupt the chain packing.And the d-Spacing of the PI polymer increased with the addition of MWCNTs,which was anticipated to create more free volume.This was beneficial to improve the gas permeability of membrane materials.At last,the optimal test conditions were selected for the gas separation tests.The o-MWCNTs and Si-MWCNTs showed better CO₂ separate performance compared to the pure PI membrane and pristine MWCNTs.In specific,the permeability of CO₂ increased from 2.31 Barrer to 9.06 Barrer and11.14 Barrer with 3 wt.%o-MWCNTs and 4 wt.%Si-MWCNTs,respectively.The selectivity of CO₂/N₂ increased from 15.55 to 38.39and 38.01,the selectivity of CO₂/CH₄ increased from10.03 to 24.20 and 25.53.In the part of PI/GO nano-composite membranes,the GO were functionalized via ethylenediamine?EDA?.FTIR and XPS confirmed that the functionalization had introduced-NH₂ to the GO.SEM showed that the lamellar structure of the GO did not undergo any destructive process during the functionalization,but the specific area and dispersity were improved.The PI/GO nano-composite membranes were also synthesized by in-situ polymerization approach.The morphology and structure of the membranes were characterized by SEM,XRD and FTIR.The result showed that the GO were merged into the PI matrix well,especially the NGO showed an excellent dispersion in PI.Gas separation tests showed that the addition of GO can improved the CO₂ separation performance of the menbranes.In specific,the CO₂ permeability and CO₂/N₂ selectivity were 4.69 times and 2.67 times compared to pure PI with 3 wt.%NGO,respectively.Finally,the comparison of GO and MWCNTs on gas separation performance were carried out.The result indicated that the MWCNTs mainly improved the gas diffusion rate,whereas the GO mainly improved the solubility of gas in the membrane materials.In conclusion,both MWCNTs and GO were the ideal choice for improving the gas separation performance of membrane materials.