Metal-Organic Frameworks (MOFs) as methane adsorbents: From storage to diluted coal mining streams concentration

Ventilation Air Methane emissions (VAM) from coal mines lead to environmental concern because their high global warming potential and the loss of methane resources. VAM upgrading requires pre-concentration processes dealing with high flow rates of very diluted streams (<1% methane). Therefore, methane separation and concentration is technically challenging and has important environmental and safety concerns. Among the alternatives, adsorption on Metal-Organic Frameworks (MOFs) could be an interesting option to methane selective separation, due to its tuneable character and outstanding physical properties. Most of the works devoted to the methane adsorption on MOFs deal with methane storage. Therefore, these works were reviewed to determine the properties governing methane-MOF interactions. In addition, the metallic ions and organic linkers roles have been identified. With these premises, decisive effects in the methane adsorption selectivity in nitrogen/methane lean mixtures have been discussed, since nitrogen is the most concentrated gas in the VAM stream, and it is very similar to methane molecule. In order to fulfill this overview, the effect of other aspects, such as the presence of polar compounds (moisture and carbon dioxide), was also considered. In addition, engineering considerations in the operation of fixed bed adsorption units and the main challenges associated to MOFs as adsorbents were also discussed.

Coordinatively unsaturated metal sites (open metal sites) in metal–organic frameworks: design and applications

The defined synthesis of OMS in MOFs is the basis for targeted functionalization through grafting, the coordination of weakly binding species and increased (supramolecular) interactions with guest molecules.

Simulations of hydrogen, carbon dioxide, and small hydrocarbon sorption in a nitrogen-rich rht-metal–organic framework

Detailed theoretical insights into the gas-sorption mechanism of Cu-TDPAH are presented for the first time.

Investigating gas sorption in an rht-metal-organic framework with 1,2,3-triazole groups

Simulations of CO₂ and H₂ sorption were performed in an rht-metal-organic framework (MOF) that consists of Cu²⁺ ions coordinated to 5,5',5''-(4,4',4''-(benzene-1,3,5-triyl)tris(1H-1,2,3-triazole-4,1-diyl))triisophthalate (BTTI) linkers; it is referred to as Cu-BTTI herein. This MOF was previously synthesized and reported by three different experimental groups [Zhao et al., Sci. Rep., 2013, 3, 1149; Schröder et al., Chem. Sci., 2013, 4, 1731-1736; Hupp et al., Energy Environ. Sci., 2013, 6, 1158-1163]. This MOF is notable for the presence of open-metal sites and nitrogen-rich regions through the copper paddlewheel ([Cu₂(O₂CR)₄]) clusters and 1,2,3-triazole groups, respectively, which allows this material to display remarkable CO₂ and H₂ sorption properties. All three groups report distinct experimental and theoretical gas sorption results for the MOF. In contrast to the force fields utilized in the aforementioned studies, our simulations include explicit many-body polarization interactions, which was important to reproduce sorption onto the open-metal sites. Simulations using polarizable potentials for the MOF and sorbates generated sorption isotherms and isosteric heat of adsorption (Q_st) values that are outstanding agreement with the corresponding experimental data for all three groups; this is in contrast to the theoretical results presented in the respective original references. The simulations carried out in the previous studies often looked reasonable but they missed a key feature of the sorption process that lead to unreliable results. Analysis of the radial distribution function (g(r)) about the open-metal sites and examination of the modeled structure reveal that the CO₂ and H₂ molecules prefer to sorb onto two unique types of Cu²⁺ ions that exhibit the highest partial positive charges. Sorption was also observed within the corners of the truncated tetrahedral (T-T_d) cages and onto the 1,2,3-triazole groups of the linkers for both sorbates. Overall, this study demonstrates how utilizing a classical polarizable force field led to the reproduction of experimental observables and allowed for an accurate description of the sorption mechanism in this MOF that is an important member of the rht-MOF family.