Gas Adsorption And Separation Properties Of Metal-Organic Frameworks

Metal-organic frameworks (MOFs) are of great interest and importance because of their novel co-ordination structures and diverse topologies with extremely high surface area, porosity, adjustable pore size, and varied chemical composition, leading to potential applications in gas adsorption, separation, and catalysis, and so on. It is of utmost importance to study the gas adsorption and separation properties of MOFs, which can lead to the design of new MOFs and their applications in gas storge, adsorption-based separation, and catalysis. The main results are summarized as follows:1. The adsorption and desorption of several adsorptives such as CO2, CH4, C2H2, C2H4, and C2H6 on MIL-53(Al) were systematically investigated by means of a volumetric method. The measured isotherms can be appropriately described by the Sips-Langmuir model, from which the adsorption thermodynamic properties are derived. The breathing effects of MIL-53(Al) upon the adsorption at low pressures are for the first time observed experimentally. After the adsorption at the sub-critical temperatures of the adsorptives, the host-guest interactions force the MIL-53(Al) framework to close and its cell to shrink, resulting in the transition from the large-pore (lp) form into the narrow-pore (np) one. However, the isotherms at the super-critical temperatures of the adsorptives show no sign of the structure transition in the pressure range investigated, and the framework thus remains in the lp form. This is plausibly attributed to the fact that above the critical temperatures of the adsorptives in the pressure range investigated, the interactions between adsorptive and adsorbent are too weak to force the framework to close.2. MIL-101 was synthesized hydrothermally and purified by a two-step method using ethanol and aqueous NH4F solution. The single-component adsorption equilibra of CO2, CH4, C2H4, C2H6, n-C4H10, and iso-C4H10 were systematically investigated by means of a volumetric method. The adsorption thermodynamic properties such as the isosteric heat as a function of loading were derived from their adsorption isotherms by Clausius-Clapeyron equation, indicating that the isosteric heat has a different loading dependence among the adsorptives investigated. The isosteric heats for CH4 and CO2 gradually decreases with increasing loading for CH4 and CO2. Although the isosteric heat for C2H6 increases with increasing loading at first, then it will decreases with increasing loading, but it doesn't change significantly with loading. For C2H4, however, the isosteric heat significantly decreases with an increase in loading. The derived values of the isosteric heat for C2H4 are much higher than that for C2H6 in the whole range of the adsorption loadings investigated, revealing a strong specific interaction between C2H4 and MIL-101. It also implies that MIL-101 is an energetically heterogeneous adsorbent. Furthermore, the separation of CO2/CH4, C2H4/C2H6, and n-C4H10/iso-C4H10 mixtures MIL-101 as adsorbent was predicted. The high selectivity towards CO2 over CH4 as well as C2H4 over C2H6 suggests that MIL-101 could be an effective absorbent for the separation of these mixtures. Although the predicted selectivity for n-C4H10 over iso-C4H10 on MIL-101 is not so high, the difference in the adsorption behavior between these two adsorptives indicates the shape selectivity due to a significant difference in the molecular shape between n-C4H10 over iso-C4H10.3. The adsorption properties of CO2, CH4, C2H2, C2H4 and C2H6 on CuBTC have been systematically investigated by means of a volumetric method. The dual-site Langmuir (DSL) adsorption equilibrium model appropriately describes the equilibrium adsorption data for CO2, CH4, C2H2, C2H4 and C2H6 over the whole range of the applied conditions. The derived thermodynamic properties such as isosteric heat associated with adsorption have been derived to characterize the interactions between adsorptive and adsorbent. The results indicate that as the molecular sizes of the adsorptives smaller than the window side of a tetrahedron-shaped side pocket in CuBTC, these adsorptives first occupy the side pockets or coordinatively unsaturated Cu2+ sites, in which there are stronger electrostatic and dispersive interactions with the adsorbent. After saturated in these side pockets, with further increasing pressure, the adsorptive molecules start occupying the larger square-shaped channels, in which there are weaker electrostatic and dispersive interactions. The derived total pore volume based on the measured CO2 isotherms model is slightly higher than that from N2 adsorption at 77 K, indicating that CO2 could be a better probe to determine the textual properties of microporous materials. Further work has been conducted on predicting the separation of CO2/CH4 and C2H4/C2H6 mixtures and C2H2 storage by CuBTC, implying that CuBTC might be an effective adsorbent for the mixture separation or C2H2 storage.