Construction Of Microporous Polymers And Their Adsorption Properties Of CO₂Gas And Organic Vapors

For dealing with the escalating greenhouse effect, resource depletion and chemical pollution, a series of novel microporous polymers, such as polyimides, poly(Schiff-base)s and poly-aminals, were synthesized and used for CO2capture and storage as well as volatile organic compounds adsorption. Their adsorption properties of CO2gas and organic vapors were evaluated and investigated in term of the chemical compositions and porosity parameters. The main contents and results are described as follows:Phenyl-and naphthalene-based microporous polyimides (MPIs and NPIs) were synthesized via one-pot polycondensation using tetrakis(4-aminophenyl)methane, tris(4-aminophenyl)-amine and1,3,5-tris(4-aminophenyl)benzene reacted with pyromellitic dianhydride and1,4,5,8-naphthalenetetracarboxylic dianhydride, respectively. The uniform micropore structures were successfully built up in amorphous polyimide skeletons through elaborately optimizing the polymerization process, and their pore sizes locate at0.52-0.60nm. The three-dimensional network with tetraphenylmethanes linked by rigid imide rings is liable to generate large amount of micropores. Thus, MPI-1exhibits the largest BET surface area up to1454m2/g. Furthermore, the high contents of oxygen and nitrogen atoms from the imide heterocycles in the polyimide skeletons can effectively enhance the affinity for CO2molecule, leading to the CO2uptake as high as16.8wt%at273K and1bar. Meanwhile, the narrow pore size distributions and ultra small pores (ca.6.0A) in MPIs and NPIs also can promote the molecule size selectivity effect. The separation factors of CO2/N2and CO2/CH4lie in the regions of34～102and8～13, respectively. In addition, these aromatic microporous polyimide skeletons exhibit excellent benzene vapor adsorption capacities (41.5～119.8wt%), and the conjugated naphthalene units in NPIs are more favorable for the selective adsorption of benzene vapor over nitrogen, cyclohexane and water vapors.Incorporation polar groups onto microporous networks can significantly enhance the CO2adsorption capacity and selectivity. The carbonylated and fluorinated microporous polyimides (MPI-4s and MPI-5s) were synthesized from tetrakis(4-aminophenyl)methane and1,3,5,7-tetrakis(4-aminophenyl)adamantane building blocks with the respective dianhydride linking struts. The resultant MPI-4s and MPI-5s show the moderate high BET surface areas of368-783m2/g. Their pore size distributions are also quite uniform, centering at0.48～0.71nm. Compared to the aromatic MPI-4and MPI-5, the polar groups played a critical role in improving CO2affinity, and their CO2adsorption enthalpies show a obviously increasing from28.7kJ/mol to33.3kJ/mol. At0.15bar, the CO2uptakes of carbonylated and fluorinated samples are higher than that of aromatic MPI-4and MPI-5, even though MPI-4C and MPI-5C having lower surface areas. The separation factors of CO2/N2and CO2/CH4also exhibit significant improvement from20.1up to55.1, due to the introduction of carbonyl and trifluoromethyl on the pore wall. On the other hand, the fluorinated polyimide skeletons have excellent hydrophobicity, e.g. MPI-4F with the largest surface area show the lower water vapor uptake of8.5wt%.A series of tetraphenyladamantane-based nitrogen-rich microporous poly(Schiff-base)s (PSNs) were synthesized via one-pot Schiff-base condensation from1,3,5,7-tetrakis-(4-aldehydephenyl)adamantane A4building block reacted with a series of B2, B3and B4linking struts. The directing effect of linking struts is a determining factor on building microporous architectures. For example, PSN-1with meta-linking struts has a distorted cross-linked skeletons and shows a quite narrow pore size distribution of around0.72nm. The BET surface area of PSN-1is up to1045m2/g. However, the network of PSN-2with para-linking struts is more stretching, and its major pore size locates at around2.20nm in mesoporous region. Thus, PSN-2has a much lower BET surface area of376m2/g. Their characteristic of pore structures lead to a significant difference in CO2adsorption capacity. At273K and1bar, the CO2uptake of PSN-1is as high as15.0wt%, whereas the CO2uptake for PSN-2is only6.7wt%. The other samples of PSN-3-PSN-6linked by B3/B4-type struts show the BET surface area of419-865m2/g. At273K and1bar, their CO2uptakes are as high as7.4-13.6wt%, and their highest CO2/N2and CO2/CH4separation factors are up to89.4and13.6, respectively. Additionally, due to the presence of aromatic and cycloaliphatic units in poly(Schiff-base) skeletons, they exhibit the good adsorption capacities for both benzene vapor and cyclohexane vapor.The nitrogen-rich microporous polyaminals (PAN-1and PAN-2) were synthesized via one-pot aminal polycondensation from pyrimidine-2,4,6-triamine and triazine-2,4,6-triamine with l,3,5,7-tetrakis(4-aldehydephenyl)adamantane, respectively. The BET surface area for microporous PAN-2is up to1242m2/g, and PAN-1contained some mesopores show a lower BET surface area of925m2/g. The molecule size and reactivity of pyrimidine-2,4,6-triamine and triazine-2,4,6-triamine result a significant difference in bonding manner, crosslinked density and pore structure in PAN-1and PAN-2. The CO2uptake for PAN-2at273K and1bar is up to17.6wt%, and the CO2/N2and CO2/CH4separation factors are as high as104.3and23.8, respectively, superior to many other microporous polymers. Then, triazine-2,4,6-triamine and a class of commercial benzaldehyde,4-methyl benzaldehyde,4-fluoro benzaldehyde and4-trifluoromethyl benzaldehyde,2-furanaldehyde and2-thenaldehyde were used to building up microporous polyaminals (PAN-3-PAN-6) with BET surface areas of615～907m2/g. Compared to the phenyl-based PAN-3and toluene-based P AN-4, the incorporation of fluorine atom and thiophene unit can effectively enhance the CO2adsorption capacity and selectivity. At273K and1bar, thiophene-based PAN-6can uptake14.8wt%of CO2gas. Moreover, the polyaminal skeletons also exhibit the good adsorption capacities for both benzene vapor and cyclohexane vapor.