Study On Preparation And Gas Separation Performance Of Functionalized Carbon Nanotubes Mixed Matrix Membranes

With the development of economy,enrichment of atmospheric greenhouse gases,mainly carbon dioxide,resulted from human activities has increased the global mean temperature.Because most of the carbon dioxide is emitted from coal-fired power plants and global energy supply will still rely on the fossil fuels in the future,it is very important for the researchers to separtate CO₂ from the flue gas.To date,the methods for the capture of flue gas CO₂ including physical adsorption,chemical adsorption and the separation at low temperature have some shortages.The membrane separation,which has the advantage of high efficiency,energy-saving,environmental protection,easy to control has been the most potential method for the CO₂ capture.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 an uniform dispersion of the nano-materials,the multi-walled carbon nanotubes?MWCNTs?was functionalized by chemical modification in this paper.The ethyl Cellulose?EC?/functionalized MWCNTs and Cardo polymers of intrinsic microporosity?Cardo-PIM-1?/functionalized MWCNTs composite membranes were prepared with the mixing method,and the gas separation performance of the membranes was studied.In the part of EC/MWCNTs nano-composite membranes,the MWCNTs were functionalized via acid and ethanediamine 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 in the matrix.FTIR and XPS confirmed that the functionalization had introduced carboxylic acid groups?-COOH?,hydroxyl groups?-OH?and amino groups?-NH₂?on the surface of MWCNTs.Then the EC/MWCNTs nano-composite membranes were synthesized with the mixing method.The morphology and structure of the membranes were characterized by SEM and XRD.The result indicated that the addition of MWCNTs had not changed the basic structure of EC matrix.However,the interaction between EC and MWCNTs might disrupt the chain packing.The d-spacing of the EC polymer increased with the addition of MWCNTs,which was anticipated to create more fractional free volume.This was beneficial to improve the gas permeability of membrane materials.The mechanical test showed that the mechanical properties of composite membrane were improved significantly.At last,the optimal test conditions were selected for the gas separation tests.The EC/n-MWCNTs MMMs showed better CO₂ separation performance compared to the p-EC membrane and EC/p-MWCNTs MMMs.In specific,the permeability of CO₂ increased from 112.5 Barrer?p-EC membrane?and 134.5 Barrer?EC/p-MWCNTs MMMs?to 153.9 Barrer?EC/n-MWCNTs MMMs?with 0.66 wt.%n-MWCNTs,respectively.Correspondingly the selectivity of CO₂/N₂ increased from 16.04 and 23.3 to 27.In the part of Cardo-PIM-1/MWCNTs nano-composite membranes,Cardo-PIM-1 was prepared by means of nucleophilic reaction in the manner of solution polymerization.Then Cardo-PIM-1 blended with MWCNTs to prepare the Cardo-PIM-1/MWCNTs MMMs.NMR and GPC analysis confirmed that the optimal reaction conditions for the synthesis of Cardo-PIM-1 powder were at 65? for 72h and N,N-dimethylformamide as the solvent with the mole ratio of TFTPN,TTSBI and BDPF was 2:1:1.The Cardo-PIM-1 powder had the specific surface area of 533 m²/g,a total pore volume of 0.38 cm³·g^-1,the pore size mainly of0.4 nm.The Cardo-PIM-1/MWCNTs nano-composite membranes were also synthesized with the mixing method.The morphology and structure of the membranes were characterized by SEM.The result showed that the MWCNTs were merged into the Cardo-PIM-1 matrix well,especially the n-MWCNTs showed an excellent dispersion in the matrix.Gas separation tests showed that the addition of n-MWCNTs improved the CO₂ separation performance of the menbranes compared to the p-PIM-1 membrane and Cardo-PIM-1/p-MWCNTs MMMs.In specific,the permeability of CO₂ increased from 13160 Barrer?p-Cardo-PIM-1?and 5536Barrer?Cardo-PIM-1/p-MWCNTs?to 28624 Barrer?Cardo-PIM-1/n-MWCNTs?with 7.5wt.%MWCNTs,respectively.Correspondingly the separation selectivity of CO₂/N₂ increased from 14.7 and 11 to 23.Therefore,Cardo-PIM-1 mixed matrix membrane will have a good prospect in the field of gas separation.