| In recent years, Metal-Organic Frameworks (MOFs) have attracted great attentions from chemists worldwidely, due to their fascinating topologies and potential applications, particularly in gas storage and separation, drug delievery and catalysis. The rational design and assembly of MOFs, aiming at achieving excellent properties, is a hot research topic induced by the perspective of "structure determines property". In the early 1990s, Robson initially introduced the "node and spacer" approach. Later on, another breakthrough was made by O’Keeffe and Yaghi by using inorganic clusters as the augments of vertices in a given net to construct MOFs, and reticular chemistry was formed. Zaworotko and Yaghi further developed the idea of metal-organic polyhedras (MOPs), as supermolecular building blocks (SBBs) to extend MOFs. By these strategies, numerous MOFs have been synthesized and reported. However, how to rationally and efficiently synthesize MOFs with high surface area and excellent properties is still of great interest.Although recently some important breakthroughs have been made in the gas adsorption properties of MOFs, the study of its stability is still an extremely challengeable and practical field. Normally, we mean its water stability and thermal stability when the stability of MOFs is referred. For water stability, besides the traditional approach of strengthening hydrophobicity of channels or pores by introducing hydrophobic groups, incooperating nitrogen atoms as coordination participants is also regarded to be an effective way of defending the attack of water molecules as it strengthened coordination bonds. While for thermal stability, it is practicable by enhancing coordination bonds as well as increasing the connectivity of structures.In this dissertation, we have synthesized several V-shaped pyridyl-tetracarboxylic acids and obtained a series of high porous MOFs by assembling with cupric paddle-wheel clusters, under the guidance of crystal engineering and the strategies of SBBs and reticular chemistry. Subsequently, the influence of polar acylamide groups, nonpolar alkyne groups, the repeating unit of para-amino carboxylic benzene as well as the hanging polar groups on CO2 adsorption and structural stability was investigated via structure determination and gas adsorption measurement, which may provide new concept for the design of MOFs with great performance in gas separation and storage.Firstly, inspired by the strategy of SBBs, to construct MOFs with high degree of connectivity, V-shaped pyridyl-tetracarboxylic ligands with acylamide groups and alkyne groups, respectively, were synthesized and introduced to assembly with cupric nitrate, and then the expected iso-structural (3,36)-connected MOFs, compound 1 and compound 2, were successfully obtained. Due to the tiny distinctions in their structures, the surface area and pore volume of them show a few differences, with the values of 1979 m2 g-1 and 0.785 cm3 g-1 for compound 1, and 2221 m2 g-1 and 0.871 cm3 g-1 for compound 2. Meanwhile, compound 1 exhibits a better affinity with CO2 than compound 2, attributed to the difference of polarity in their inner surface. Compound 1 possesses not only good CO2 adsorption capacity (206.5 cm3 g"1 at 1 bar and 273 K), but also good CO2 selectivity over N2 (99) and CH4 (11). Owing to the high degree of connectivity, both compound 1 and compound 2 exhibit good thermal stability, and as well as better water stability than many well-known MOFs, despite the weaker water stability of compound 1, compared with compound 2, caused by the polar acylamide groups.Secondly, enlightened by the good stability of the kind of MOFs as compound 1 and 2, and the unique promotion of acylamide groups on CO2 adsorption, the new way of extending the ligands by para-amino carboxylic benzene moieties was further conducted, and consequently iso-structural high porous compound 3 and meso-porous compound 4 were successfully synthesized. The BET surface area and Langmuir surface area of compound 3 and 4 are 2177 m2 g-1,3299 m2 g-1 and 2519 m2 g-1,5019 m2 g-1, respectively. Compared with traditional means of ligands expansion with alkyne groups or benzene groups, such a way of extending ligands by para-amino carboxylic benzene moieties can retard the sharp decrease of multifunctional sites per unit volume caused by expansion of pore volume. Therefore, the zero-coverage CO2 adsorption enthalpies of compound 3 and 4 can keep very close to that of compound 1 (compound 3:25.2 kJ mol-1, compound 4:25.1 kJ mol"1, compound 1:25.9 kJ mol-1), and as well as the CO2 selectivity (compound 3:CO2/N2=98, CO2/CH4=9; compound 4:CO2/N2= 83, CO2/CH4=7; compound 1:CO2/N2= 99, CO2/CH4= 11). Meanwhile, both compound 3 and compound 4 show great capacities in CO2 storage, with the values of 1046 mg g-1 and 1430 mg g-1 at 298 K and 40 bar, respectively. In addition, compound 3 exhibits better water stability than compound 1, but the water stability of compound 4 is unsatisfactory. In general, expanding the ligands with para-amino carboxylic benzene moieties can provide an easy and cheap way of synthesizing micro-meso porous MOFs with high surface area and high CO2 selectivity and storage capacity.Thirdly, due to steric hindrance in the V-shaped pyridyl-tetracarboxylic ligand with smaller size, an expected (3,36)-connected structure was not successfully synthesized, but a rare (3,3,4,4)-connected MOF was obtained instead, namely compound 5, of which the nitrogen atoms are uncoordinated. Compound 5 exhibits a high BET surface area of 2883 m2 g"1 and a pore volume of 1.11 cm3 g-1. In contrast with the iso-structural compound 6, synthesized by the ligand of H4BDPB, although compound 5 possesses larger BET surface area and pore volume, its H2 and CH4 uptake capacities are weaker than that of compound 6, which indicates that benzene rings is more preferred by hydrogen and methane molecules than pyridine rings. Specifically, the H2 uptake amount of compound 6 at 77 K and 1 bar is 2.61 wt%, ranking in the top 10 MOFs by far with high H2 uptake capacity in low pressure range. Meanwhile, compound 6 exhibits a high CH4 total volumetric uptake amount of 199.1 cm3 cm-3, surpassing the DOE’s former target for vehicular methane storage. Moreover, the CH4 uptake amount of compound 6 at 50 bar reaches up to 244.7 cm3 cm-3, which is quite close to the new DOE target updated in 2013.Inspired by the exchange of nitrogen atoms and carbon atoms in compound 5 and 6, this kind of structure can be served as a platform for ligand functionalization. Then compound 7 and compound 8 were successfully synthesized under similar synthetic condition, embedded with amino groups and carboxylic groups, respectively. After functionalization, the BET surface area and pore volume of compound 7 and compound 8 decrease, with the value of 2082 m2 g-1 and 0.85 cm3 g-1 for compound 7 and 2353 m2 g"1 and 0.92 cm3 g-1 for compound 8. Transparently, the bigger the functional group is, the smaller the BET surface area and pore volume are. The corresponding changes in CO2 adsorption capacity and selectivity among compound 6,7, and 8 reveals that carboxylic groups are helpful for improving CO2 adsorption enthalpy but causing a loss in uptake amount, while amino groups benefit both CO2 adsorption enthalpy and CO2 uptake amount. Additionally, both carboxylic group and amino group are useful for improving the CO2 selectivity. Although it has been recognized as a common sense that amino can promote the affinity of CO2 molecules with MOFs and as well as the CO2 selectivity, relevant reports on carboxylic group are still unseen. Therefore, compound 7 is the first practical example that confirms the theoretical results. |
PDF Full Text Request