Effect Of Precursor Structure On The Gas Separation Performance Of Carbon Molecular Sieve Membranes

Carbon molecular sieve membranes are obtained by pyrolysis of carbon-containing materials in an inert atmosphere,and their unique bimodal pore structure gives them excellent gas separation properties.They are also widely studied in the field of membrane separation because of their excellent thermochemical stability compared to conventional polymer membranes.However,the wide pore size distribution caused by the thermal contraction of carbon structure leads to the poor gas separation performance of carbon molecular sieve membranes,which severely limits the large-scale application of carbon molecular sieve membranes.Therefore,in this paper,the pore structure of CMS membranes was modified by monomer molecular design to change the precursor structure.The specific studies are as follows:（1）In this chapter,we prepared four polyimide-based carbon molecular sieve membranes containing different phenylene structures and investigated the effect of the remote structures of the precursors on the microstructure and gas separation properties of the carbon molecular sieve membranes.The four polyimide precursors were successfully prepared as shown by ATR-FTIR and ¹HNMR and the remote structures of the precursors were characterised in terms of polymer size,internal rotational conformation and chain flexibility,showing that the p-phenylene structure increases the d-spacing and rigidity of the polyimide.Raman spectroscopy shows that the inclusion of a p-phenylene structure reduces the graphitisation of the derived carbon molecular sieve membrane.At carbonization temperature of 550°C,the carbon molecular sieve membrane（CM4）derived from a polyimide with a p-phenylene structure in the main chain had a CO₂ permeability of 6314 Barrer and an ideal CO₂/CH₄ selectivity of 58.6,which was much higher than that of the carbon molecular sieve membrane derived from a p-phenylenemethane,p-phenylene ether and m-phenylene polyimide in the main chain,with CO₂ permeabilities of3291 Barrer,3575 Barrer and 5435 Barrer,with ideal selectivity of 28.4,31.4 and 45.6 for CO₂/CH₄,respectively.Remarkably,the excellent a N₂ permeability of 446 Barrer with a N₂/CH₄ selectivity of 4.1 was obtained,which was higher than that of the state-of-the-art CMS membranes.The excellent gas separation performance of CM4 is due to the ability of the p-phenylene structure containing planar sp² hybridization to effectively suppress the stacking degree of the 6FDA-based polyimide helical structure,which gives CM4 a larger layer spacing,as confirmed by XRD.In addition,the planar structure of the rigid p-phenylene drives the carbon molecular sieve membrane to have a smaller shrinkage and a narrower distribution of ultramicropores,due to the better thermal stability of the rigid polyimide,which facilitates the slow release of volatile compounds from the matrix and the smaller shrinkage,allowing the carbon molecular sieve membrane to form a uniform pore structure.In addition,CM4 still has the highest gas separation performance when stored under air atmosphere for 30 days,which exceeds most carbon molecular sieve membranes.（2）In this chapter,a series of polyimides containing rigid large free volumes were prepared by condensation with 6FDA dianhydride monomer at low temperature using FFDA monomer containing fluorenyl groups and flexible ODA monomer containing ether oxygen groups as common diamine monomers and pyrolysed into carbon molecular sieve membranes.Infrared spectroscopy indicates successful introduction of FFDA into the polyimide backbone and XRD and FFV tests show that FFDA inhibits stacking of the polymer chains,resulting in larger d-spacing and free volume of the polymer chains.In addition,BET test data showed that the larger the free volume fraction of the precursor,the richer the pore structure of the derived carbon molecular sieve membrane.The effects of synthesis conditions（condensation time,condensation temperature and diamine ratio）and pyrolysis conditions（pyrolysis temperature and constant temperature time）on the gas separation performance of the derived carbon molecular sieve membranes were investigated in detail;The separation mechanism of the carbon molecular sieve membranes at different pyrolysis temperatures was investigated;The ageing properties of carbon molecular sieve membranes and their stability under mixed gas feed were also investigated.The results showed that the 6FDA/ODA:FFDA（7:3）derived carbon molecular sieve membrane had the best gas separation performance when the condensation time was 24 h and the condensation temperature was 0°C.The CO₂ permeability was 9984 Barrer and the ideal selectivity of CO₂/CH₄ was 33.4,which was much higher than the 2019 Robeson upper bound This is due to the relatively high molecular weight of the polyamide acid and the large free volume fraction of the resulting polyimide under these synthetic conditions.In addition,the CO₂ permeability of the 6FDA/ODA:FFDA（7:3）derived carbon molecular sieve membranes was 802 Barrer and the CO₂/CH₄ selectivity was 46.5 after 30 days of storage in air.