Amino-modified Cyclodextrin Metal Organic Framework Based Mixed Matrix Membranes For CO₂/N₂ Separation

The greenhouse effect will trigger a range of phenomena,including rising sea levels and an increase in severe weather events,with strong impact on life on Earth.As a major component of greenhouse gases,the separation and efficient utilization of CO₂ naturally become the main content of current research.Membrane separation technology is regarded as an extremely important new technology for industrial technological transformation in the 21st century due to its many advantages such as low cost and high efficiency.Mixed matrix membranes（MMMs）combine the easy processability of polymer membranes with the high permeability of inorganic materials,which have attracted much attention in recent years.However,there are significant challenges in terms of material dispersion and organic-inorganic interfacial compatibility for mixed matrix membranes.In this paper,an amino-modifiedβ-cyclodextrin metal organic backbone（β-CD MOF）was chosen to improve the interfacial interaction between the filler and the sulfonated poly（ether ether ketone）（SPEEK）matrix while providing a facilitated transport channel for CO₂ permeation through the membrane,and further mixed with carboxylated multi-walled carbon nanotubes（CNT）of large specific surface area and pore volume to develop a series of high performance SPEEK-based mixed matrix membranes.The details of the study are as follows:（1）Environmental friendly porousβ-CD MOF with hydroxyl-rich surface was selected as the filler and then was modified by silane coupling agents（APTES）for introducing amino carriers to its surface and pores.After that,the SPEEK/AS-CD-MOF（β）MMMs were designed by dispersing amino-functionalized MOF（AS-CD-MOF（β））into the hydrophilic SPEEK matrix.FTIR characterisation revealed N-H and C-N absorption peaks in the spectrum of the modifiedβ-CD MOF,indicating the successful attachment of the amino groups to the MOF.AS-CD-MOF（β）still retained mesoporous properties confirmed by BET results.Cross-sectional SEM and thermogravimetric analysis showed good compatibility between AS-CD-MOF（β）and SPEEK and excellent thermal stability of the MMMs,respectively.The effects of content,feed pressure,operating temperature and gas mixture（CO₂/N₂ volume ratio 15/85）on the performance of the MMMs were investigated in detail.At 1 bar and 25℃,the CO₂permeability of the SPEEK/AS-CD-MOF（β）MMMs with 10 wt%filler content was as high as727 Barrer with CO₂/N₂ selectivity of 70.Compared to the pure SPEEK membrane,the performance enhancement was 139%and 119%,respectively.The improved performance is mainly related to the amino-modified porous filler which enhances the hydrophilicity of the MMMs and the adhesion at the filler-matrix interface,while the pore channels and movable amino groups of the MOF provide facilitated CO₂ transport channels.The SPEEK/AS-CD-MOF（β）MMM maintained excellent separation performance without a significant downward trend over up to 360 h of mixed gas testing,with an average CO₂ permeability of 719 Barrer and CO₂/N₂ selectivity of 68,exceeding the 2019 upper bound.（2）To increase the CO₂ transport channels in the membranes,carboxylated CNT was further introduced to prepare a carboxylated CNT/amino-β-CD MOF dual-filler based MMMs,denoted as SPEEK/CM MMMs.FTIR tests indicated successful carboxylation of CNT and certain interaction between the carboxyl groups on the CNT and the amino or hydroxyl groups on the AS-CD-MOF（β）,improving their dispersion in the membranes.The BET tests showed that the carboxylated CNT increased the specific surface area and total pore volume of the dual-filler.The hydrogen bonding interactions and good compatibility of the dual-filler and the SPEEK matrix were proved by FTIR and SEM characterization of the membranes,respectively.The effects of the mass ratio of the mixed fillers,content,pressure,temperature and gas mixture（CO₂/N₂ volume ratio 15/85）on the membrane performance were studied.When the mass ratio of carboxylated CNT to AS-CD-MOF（β）was 5:5,the CO₂ permeability and CO₂/N₂ selectivity of the MMM with 7 wt%dual-filler content were 844 Barrer and 84 at 1 bar and 25℃,respectively,which increased by 178%and 163%relative to the pristine SPEEK membrane.Moreover,the membranes loaded with dual-filler had preferable separation performance than those loaded with the same amount of carboxyl modified CNT or amino modified MOF alone.It was shown that carboxylated CNT and AS-CD-MOF（β）had a synergistic effect in enhancing separation performance,with the former shortening the CO₂ transport pathway and increasing CO₂ permeability,and the latter selectively adsorbing CO₂ through reversible reactions and improving CO₂/N₂ selectivity.Different from pure gas condition,the separation performance of the membranes under mixed gas condition was mildly lower.In addition,the permeability and selectivity of the MMM doped with 7 wt%mixed fillers were approximately 837 Barrer and 81 during the 360 h stability test,far above the 2019 upper bound.