A Rational Design of Microporous Nitrogen-Rich Lanthanide Metal–Organic Frameworks for CO₂/CH₄ Separation

Abstract

Three new lanthanide metal–organic frameworks IRHs-(1–3) supported by cyamelurate linkers have been synthesized and structurally characterized. The incorporation of numerous heteroatoms (N and O) into the pore walls and the relatively small microchannels of these porous solids enhance bonding force of the host–guest interactions, thus promoting the adsorption of carbon dioxide (CO2) over methane (CH4). The nonpolar covalent bonds in methane also favor the less uptake due to the hydrophilic walls of these frameworks. Grand canonical Monte Carlo simulations were performed to determine the origin of the adsorption. The density isocontour surfaces show that CO2 is mainly adsorbed on the walls composed of organic linkers and around the metal sites, whereas no specific adsorption site is observed for CH4, which indicates weak interactions between the framework and the adsorbed gas. As expected, the simulations show that CH4 is not observed around the metal center due to the presence of H2O molecules. The excellent selectivity of CO2/CH4 binary mixture was predicted by the ideal adsorbed solution theory (IAST) via correlating pure component adsorption isotherms with the Toth model. At 25 °C and 1 bar, the CO2 and CH4 uptakes for IRH-3 were 2.7 and 0.07 mol/kg, respectively, and the IAST predicated selectivity for CO2/CH4 (1:1) reached 27, which is among the best value for MOF materials.

Pyrazine-interior-embodied MOF-74 for selective CO2 adsorption

“…It suggests that the molecular sieve effect dominates the CO 2 selectivity in this case. Furthermore, py-MOF-74c shows overwhelming advantages over other MOFs in adsorptive CO 2 /CH 4 selectivity, for instance, ZU-66 with selectivity of 136 27 , Co-btz-ht with selectivity of 63 28 and IRH-3 with selectivity of 27 29 at 298 K. Pyrazine-bonded into the MOF-74 framework can also significantly widen the adsorption gaps between CO 2 and N 2 as a result of their differences in boiling point and polarizability besides molecular size (Table 2). The three modified materials show significantly higher adsorption selectivity than the original material for CO 2 /N 2 (Figure 6b and 6c).…”

Borate Anion Dopant Inducing Oxygen Vacancies over Co3O4 Nanocages for Enhanced Oxygen Evolution

“…Metal-organic frameworks (MOFs), consisting of inorganic metal ions/clusters coordinated with organic ligands, are distinctive organic-inorganic hybrid porous materials with controllable porous structures, high specific surface areas and well-tunable structures [34,35]. Because of these unique structural merits, MOFs and their derivatives have been widely applied in energy storage and conversion [36,37], gas adsorption and separation [38,39] and other research fields. Generally, MOFs are used as sacrificial templates for the fabrication of various nanostructured materials that inherit the advantages of their precursors [8].…”

Kinetic and Thermodynamic Control in Rare Earth Cyamelurates Synthesis

“…Also noteworthy are compounds with monomeric analogs of heptazine, for example, with 2,5,8-trihydroxy-sheptazine or cyameluric acid. Recently obtained metal-organic frameworks (MOFs) based on lanthanide ions and cyamelurate anion linkers showed high selectivity in the separation of a CO 2 /CH 4 gas mixture [5]. In addition, cyameluric acid derivatives exhibit reversible chromic behaviour [6] and have thermal stability up to 500°C [7].…”

“…Another example of thermodynamic control in synthesis of cyameluric acid salts is preparation of lanthanide MOFs [5]. Keeping the reaction mixture at 80°C for 24 hours followed by slow cooling resulted in a single-phase sample containing crystals of identical morphology, 50-150 µm in size.…”

“…As in the case of lanthanide melonates, metastable cyamelurates are likely to exist, which can be isolated under kinetic control using room temperature and short reaction times. It is also interesting whether compounds similar to MOFs [5] can be obtained using increased temperatures but with the same reaction time as in kinetic control synthesis. Therefore the aim of this work is synthesis and structural study of lanthanide cyamelurates obtained under kinetic and thermodynamic control.…”