Design And Preparation Of Carbon Molecular Sieve Membranes And Gas Separation Performance Evaluation

Membrane separation technology has been paid much attention owing to high energy–efficiency,small foot-print and low pollution.However,the membrane material with excellent separation performance is the key to realize the further development of membrane separation technology.Polymer membranes have a trade-off relationship between gas permeability and selectivity due to intrinsic properties.Inorganic membranes are difficult to be scaled up due to their complicated preparation process and high cost.Therefore,it is critical to develop alternative high performance membrane material.Carbon molecular sieve（CMS）membranes based on polymer membrane precursors hold great promise to replace traditional industrial separation membranes.They have outstanding performance such as high permeability and high selectivity.The gas separation performance is highly dependent on the precursor materials and preparation process of CMS membranes.For large-scale practical applications,the properties of CMS membranes need to be elucidated.Our work focused on development of high-performance CMS membranes with industrial application prospects.Several precursors were selected and further functionalized to optimize the resulting CMS membranes’s separation performance.In this work,carbon molecular sieve membranes were fabricated based on carboxylated polyimides with various 6Fp DA:DABA molar ratios.The as-prepared6FDA-6Fp DA:DABA,thermally cross-linked ones,and CMS membranes were characterized for CO₂/CH₄ and C₂H₄/C₂H₆ separation performance.The thermally cross-linked precursor membranes exhibited increased gas permeability over non-cross-linked ones.All CMS membranes displayed the enhanced performance over precursor polymers.The representative CMS membranes pyrolyzed at 576℃had CO₂and C₂H₄ permeability of 3573 and 244.6 Barrer,respectively,with CO₂/CH₄ and C₂H₄/C₂H₆ ideal selectivity of 51.5 and 4.80.The increasing pyrolysis temperature enabled a decrease in permeability and increase in selectivity.The gas selectivity of C₂H₄/C₂H₆can reach 24.1 with C₂H₄permeability of 10.4 Barrer for CMS pyrolyzed at800℃.The Zn²⁺functionalized CMS membranes exhibited enhanced gas selectivity with a slight reduction of gas permeability.The CMS membranes had excellent gas separation performances surpassing upper-bound,holding great potential in CO₂separation and olefin recovery.We also selected 6FBI/ZIF-8 mixed matrix membranes（MMMs）as precursors for CMS membranes.It has been found that thermal annealing on precursor membranes enhanced gas permeability and ideal selectivity.The gas separation performance of CMS membranes derived from 6FBI/ZIF-8 was significantly improved after pyrolysis at 500℃.CMS membrane had CO₂ and C₂H₄ permeability of 1438 and 57.2 Barrer,respectively,with CO₂/CH₄ and C₂H₄/C₂H₆ ideal selectivity of 63 and 6.43.In a word,CMS membranes prepared in this work surpassed the polymer upper-bound,which are promising candidates for ethylene/ethane separation.