Design Of Thermally Crosslinked Membranes And Carbon Molecular Sieve Membranes From 6FDA-based Polyimide For Gas Separations

Gas separation membrane technology has the advantages of low energy consumption,high separation efficiency,small footprint,no pollution,and simple operation,and is known as the third-generation gas separation technology.At present,commercial polymer membranes cannot meet the requirements of industrial separation due to their low permeability and easy plasticization under high CO₂pressure.The thermal oxidation cross-linked film is a facile technique to prepare high performance membranes.During the cross-linking process,some functional groups on the chain undergo cross-linking reaction,which reduces the mobility of the segments,improves the anti-plasticization performance and the gas separation performance.The method to further significantly improve the gas separation performance is to pyrolyze the polyimide membrane to prepare the carbon molecular sieve membrane which follows the molecular sieving mechanism,enabling its high permeability and high selectivity.In this paper,three kinds of polyimides were prepared by copolymerizing the same dianhydride 6FDA with BI with imidazole ring and DAM with methyl groups group respectively:6FDA-BI,6FDA-DAM and 6FDA-BI:DAM（1:1）.The cross-linking reaction between polymer chains occurs through thermal treatment in air,and a cross-linked membrane for efficient separation of H₂/CH₄and CO₂/CH₄is prepared.During heat treatment in air atmosphere,the methyl groups on the adjacent DAM groups undergo oxidative cross-linking to form a C-O-C oxygen bridge cross-linking structure;the NH on the imidazole ring of the BI group undergoes dehydrogenation and cross-linking to form an N-N cross-linking structure.The optimal conditions for thermal crosslinking of 6FDA type polyimide membrane are:375-400℃,crosslinking in air for 10 h.In this work,the effect of cross-linking time and cross-linking temperature on the separation performance of 6FDA-BI:DAM（1:1）membrane was investigated first.The results show that the degree of crosslinking reaction is low at low temperature（≤35℃）,and the chain segment relaxation mainly occurs,which reduces the permeability and increases the selectivity.At high temperature（≥375℃）,the cross-linking reaction is dominant,and at the same time,partial oxidative decomposition occurs in the chain segment,which increases the permeability and reduces the selectivity.Short-time（10 h）cross-linking can reduce the chain decomposition,and O₂is required to participate in the cross-linking reaction.Secondly,the gas separation performance of crosslinked polymers was compared.The permeability of H₂and CO₂increased from 32.6 Barrer and 10.6 Barrer to 47.3 Barrer and 14.9 Barrer,and the ideal selectivity of H₂/CH₄and CO₂/CH₄increased from 208.4 and 67.9 to 407.8 and129.7,respectively,breaking the Robeson upper in 2008.The main reason is that the NH on the imidazole ring of the BI group is dehydrogenated and cross-linked,resulting in an N-N cross-linked structure,which increases the chain spacing,significantly improves the diffusion coefficient,and at the same time improves the permeability and selectivity.Finally,the CO₂/CH₄（1:1）mixed gas separation performance of 6FDA-BI:DAM（1:1）and the crosslinked membrane in air for 10 h at375℃was compared at 35℃.The results showed that 6FDA-BI:DAM（1:1）plasticized at about 30 bar,but no plasticization was observed in the cross-linked film,indicating that the anti-plasticization performance was improved after oxidative cross-linking.In order to further improve the gas separation performance,this work further used 6FDA-BI,6FDA-DAM and 6FDA-BI:DAM（1:1）as precursor materials to prepare carbon molecular sieve membranes to explore the effect of precursor types,pyrolysis temperature on gas separation and the effect of groups in the polymer on entropy selectivity and energy selectivity.By comparing the permeability of the CMS membranes prepared from the three precursors pyrolyzed at 550℃,it was found that the carbon molecular sieve membrane based on the high free volume precursor polymer had higher permeability.For example,the carbon molecular sieve membrane prepared by 6FDA-DAM with the highest free volume has a high permeability of9091 Barrer and 4241 Barrer for H₂and CO₂,while the selectivity difference between H₂/CH₄and CO₂/CH₄is small.In this work,the gas separation performance of three different carbon molecular sieve membranes was tested at 35-65℃and 4 bar to investigate the influence of the precursor structure on the energy selectivity and entropy selectivity of carbon molecular sieve membranes.The results show that the carbon molecular sieve membrane prepared by the presence of BI groups in the precursor membrane requires the gas to have a higher diffusion activation energy to pass through,which improves the selectivity.The DAM group with high steric hindrance leads to more open pore structure in the carbon sieve membrane,which reduces the entropy selectivity and facilitates the rapid passage of gas.The 6FDA-BI:DAM（1:1）was used as the precursor to further investigate the effect of the final pyrolysis temperature on the structure and gas separation performance of carbon molecular sieve membranes.It was found that with the increase of the final pyrolysis temperature,the pores in the carbon molecular sieve membrane merged and shrunk.As the pyrolysis temperature increased from 550℃to700℃,the permeability of H₂and CO₂decreased from 5128 Barrer and 2351 Barrer to 1818 Barrer and 315 Barrer,and the ideal selectivity of H₂/CH₄and CO₂/CH₄increased from 118 and 54 to 423 and 73,which improve by 258%and 35%,respectively.Therefore,an appropriate increase in the pyrolysis temperature is beneficial to the separation of gases with large differences in kinetic diameters.The gas separation performance of all carbon molecular sieve membranes in this work exceeds the upper-bound of polymers.