Synthesis, Structures And Energy-environment Related Gases Sorption Performances Of Porous Coordination Polymers Inserted By Functional Groups

Emerged as a new kind of inorganic-organic hybrid porous materials, porous coordination polymers (also known as Metal-Organic Frameworks, MOFs) have been proved to be promising candidates for H2, CH4and CO2gases storage and separation because of their large pore volumes/surface areas, tunable pore sizes and chemically-designable surface properties. Currently MOFs hold several records in porous materials including the highest surface areas, hydrogen uptake based on physical adsorption, methane and CO2storage. However, the storage capacities and the stabilities of the MOFs still need to be improved for some practical applications. To obtain high-efficiency gases capture and storage/separation materials, the impact on gases adsorption in MOFs by various factors has also been explored.In this dissertation, we are interested in the construction of novel porous materials from highly symmetric multidentate carboxylate ligands with bridging amide/acylamide/alkyne groups, focusing our attention on the merit of the functionalities for the materials’CO2uptake abilities. This dissertation mainly covers the following four parts:Firstly, four porous rht-type MOFs (1-4) have been successfully designed from two flexible hexacarboxylate ligands with amide/acylamide linking groups. Unlike4, attempts to activate1-3by solvent-exchange strategy led to complete framework collapse.4possesses the same pore sizes, surface area, pore volume and numbers of open Cu(II) sites as the isostructural analogue PCN-61(the only difference between4and PCN-61is the substitution of the acetylene moiety in PCN-61with an acylamide moiety), but surprisingly exhibits an enhanced heat of adsorption, larger CO2uptake and higher selectivity toward CO2/N2. These observations indicate that decoration of a MOF with polar acylamide groups can significantly enhance the CO2binding ability and selectivity of MOFs.Secondly, to further improve the CO2storage capacities of these acylamide-functionalized porous materials, we designed three expanded isoreticular rht-type MOFs from nanosized triangular acylamide-bridging hexacarboxylate linkers,(7).4,5,6and7exhibit a high apparent Brunauer-Emmett-Teller (BET) surface area of ca.3160,3011,3288and3360m2·g-1(the total pore volume obtained from N2(or87K Ar) isotherms is1.27,1.20,1.77and1.91cm3·g-1, respectively), which are among the highest values for porous materials reported to date. Interestingly, the CO2sorption isotherms of6and7demonstrate type-Ⅳ-like profile with marked breathing effect. To the best of our knowledge,6and7represent the first flexible MOF based on metal-organic polyhedra (MOPs) as supermolecular building blocks (SBBs). As expected,4-7are good CO2adsorbents and the exceptionally high unsaturation excess CO2uptake capacity of ca.121.4wt%.110.8wt%、157wt%and157wt%are observed respectively at20bar and273K. These values are larger than those of the corresponding isoreticular PCN-6X series MOFs with alkyne groups at the same conditions (except for5because its structure partly collapses after removal of guest molecules). Furthermore,4-7also exhibit high hydrogen and methane uptakes at20bar, their absolute storage capacities reach6.89wt%、4.58wt%、6.10wt%and6.14wt%, respectively for hydrogen at77K and15.0wt%、8.2wt%、9.3wt%and9.4wt%, respectively for methane at273K.Thirdly, to further probe the advantages of acylamide groups upon CO2adsorption at the molecular level,6is chosen as a benchmark structure to perform GCMC (Grand Canonical Monte Carlo) and First-principles calculations (DFT). The results unambiguously verify that, similar to the open Cu(II) sites, the acylamide groups within the framework can act as strong interaction sites and play an important role in the high CO2uptake of acylamide-functionalized MOFs. Additionally,7shows almost the same gases sorption behavior as6, despite that the surface is decorated by nitrogen-containing triazine rings. The introduction of N-heteroaryl moieties into MOFs may generally lead to the improvement of their CO2storage abilities which was confirmed by theoretical studies. However, in our case, it is almost useless.Fourthly, two new expanded microporous NbO-type MOFs (9-10) have been realized using elonged rectangular tetracarboxylate ligands with linking acylamide/alkyne groups.10exhibits high BET surface area of3097m2·g-1(the largest value for reported NbO-type MOFs up to data) and large total gravimetric H2adsorption capacity of6.3wt%at77K and20bar. Meanwhile,10can adsorb about104.9wt%、7.1wt%and12.9wt%(excess gravimetric uptake) of CO2、CH4and N2at273K and20bar, respectively, suggesting10a good CO2storage and separation material. As for9, due to the framework structure partly collapses after removal of guest molecules,9shows a lower BET surface area (1400m2·g-1) and pore volume (0.57cm3·g-1), and subsequently lower gases uptake (3.90wt%for H2at77K, and47.8wt%for CO2and9.4wt%for CH4at273K) in comparison with what is observed for10at the same conditions. However, at low pressures,9exhibits higher gases adsorption resulted from its larger gases heat of adsorption.To sum up, in the premise that the framework structure can retain its crystallinity after removal of solvent molecules, insertion bridging acylamide groups into the frameworks of MOFs may significantly enhance the CO2adsorption behaviors; this observation may have significant implications for future design of new high-performance CO2porous MOF materials.