NMR Studies on the Adsorption of CO2 in Porous Metal-Organic Frameworks

Abstract

Since CO2 is believed to cause global warming and climate change, the separation and storage of CO2 have become a key challenge for sustainable development of the future world economy. Porous metal-organic frameworks (MOFs), inorganic-organic hybrid crystalline materials, are promising candidates for these purposes because of their higher uptake and selectivity for CO2 over other gas molecules such as CH4, N2, CO, and O2. This thesis describes the first applications of solid-state 13C NMR spectroscopy to the studies of structure and dynamics of CO2 molecules physisorbed in various MOFs, including a flexible, surface-functionalized, open-metal site, and interpenetrated framework. MIL-53 (Al) is a typical example of the flexible MOFs, which exhibits a remarkable feature of the reversible structural transitions between the narrow-pore (NP) and large-pore (LP) structures upon the adsorption of guest molecules such as H2O, CO2, Xe, and small hydrocarbons. The CO2 molecules adsorbed in MIL-53 (Al) at different pressures were examined directly for the first time by 13C magic angle spinning (MAS) NMR, demonstrating that solid-state 13C NMR can give valuable information on the structural and dynamic behavior of CO2 in MOFs. The results clearly show the coexistence of NP and LP structures at pressures close to the transition pressure. From the chemical shift anisotropy (CSA) pattern analysis of 13C MAS NMR, it was also confirmed that the interaction of CO2 molecules in the NP structures is quite strong with a restricted motion of CO2 in the channel, while this interaction becomes less important in the LP structures, rendering CO2 more mobile. The presence of basic amino groups (-NH2) on the surface of MOFs is expected to enhance the adsorption capacity and separation efficiency of CO2 due to the interaction between the frameworks and acidic CO2 molecules. The dynamic behavior of CO2 and its interaction with -NH2 groups in amino-MIL-53 (Al) were examined by 13C NMR spectroscopy. The results revealed that the CO2 molecules are physically adsorbed without strong interaction of amine with CO2, suggesting a similar adsorption mode of CO2 in both amino-MIL-53 (Al) and regular MIL-53 (Al). Mg-MOF-74, the best-known MOF containing open-metal sites, has 1D hexagonal channels of 11 ~12 Å in diameter and exhibits higher storage capacity and selectivity for CO2 molecules. Variable-temperature 13C NMR experiments at the CO2 pressures of 0.4 and 1.1 atm revealed that there are two sites available for CO2 adsorption in the frameworks, the primary adsorption sites being on the open Mg2+ sites. The results also indicate that the relative orientations of adsorbed CO2 molecules on two adsorption sites change with temperature and pressure, which may lead to a greater CO2 uptake by stabilizing the CO2 packing. The interpenetrated MOFs have potential applications in gas separation processes because their pore sizes often lie in the range for industrially important gas separation. Zn5(BTA)6(TDA)2, a robust and highly interpenetrated MOF, has two 1D channel systems in its frameworks with relatively higher selectivity for CO2 over CH4 at low pressures, mainly due to the presence of open-metal sites in the small channel. Variable-temperature 13C NMR experiments at the pressures of 0.3 and 1.1 atm revealed that the CO2 molecules in this MOF may not undergo a uniaxial rotation, but a hopping motion between different sites, yielding completely asymmetric 13C CSA patterns rather than axially symmetric ones.