Synthesis And Properties Of Porous Aromatic Framework Membranes

Porous materials have attracted great attention due to their diverse porousstructures, high surface areas and adjustable nature of their inner walls. Gas separationvia porous membrane is one of the most interested applications and a series ofsystematic studies have been conducted. The aim of this thesis is to synthesizecontinuous defect-free porous aromatic framework (PAF) membranes and investigatetheir CO2/N2gas separation properties.Porous aromatic frameworks (PAFs) are porous materials build up from aromaticrigid building blocks. They are considered as suitable candidates for fabricatinggas-separation membranes because of their porous nature and processability. InChapter2, super acid of trifluoromethanesulfonic acid (CF3SO3H) was used as thecatalyst to polymerize aromatic nitriles. Characterizations of FTIR and NMR showedthat triazine rings were formed via coupling nitrile groups. SEM images showed thatthe membrane was continuous and smooth, which is suitable for selective gasseparation. Gas separation experiments showed an unusual performance of gaspermeations: as the feed pressure increased, the permeability of CO2decreased whilethe permeability of N2was increasing, which resulted in an increase of the idealseparation factor. The separation results of mixed gases showed the selectivity ofN2/CO2increased from1.7to4.5when the feed pressure increased from0.1MPa to0.5MPa. Moreover, the permeance of N2increased from4.26×10-7mol m-2s-1Pa-1to7.67×10-7mol m-2s-1Pa-1and CO2permeance kept almost constant around2.37×10-7mol m-2s-1Pa-1. This unique behavior of increased selectivity versus permeance in gasseparation is resulted from constant CO2permeance upon the increase of feedpressure, which is in contrast to the common trade-off relationship between selectivityand permeability of most porous membranes. Thus, a superb gas separation would beexpected at high pressures for PAF-57membrane.In Chapter3, a dendritic porous aromatic framework membrane (PAF-58) wassynthesized using trifluoromethanesulfonic acid (CF3SO3H) as catalyst as well. Thebasic properties of the membrane were initially investigated by general characterizations. NMR and FTIR studies reveal the formation of triazine from nitrilegroups. The gas sorption isotherms of N2and CO2on the PAF-58membrane weremeasured at273K and298K. N2uptake of the membrane was much lower than theCO2uptake. The CO2/N2adsorption selectivity was extraordinally high (119.3).Althrough the introduction of alkyl chain poseed a steric effect on gas molecules, ahigh CO2permeance was still observed due to the strong acid-base interactionbetween CO2and amine groups in nitrogen-modified heterogeneous pore. Whenbinary gas mixture (CO2/N2=1:1) was fed to the membrane, permeation of N2wasundetectable because of the low N2concentration in the permeate side. The CO2permeability (PCO2) was measured to be5.21×10-8mol m-2s-1Pa-1. The PAF-58membrane was proved to be robust by several cycles under the same conditions,evidenced by little loss in CO2/N2selectivity.