Development of a Mixed Multinuclear Cluster Strategy in Metal-Organic Frameworks for Methane Purification and Storage

Metal-organic frameworks (MOFs) have attracted extensive attention in methane (CH₄) purification and storage. Specially, multinuclear cluster-based MOFs usually have prominent performance because of large cluster size and abundant open metal sites. However, compared to diverse combinations of organic linkers, one MOF with two or more multinuclear clusters is difficult to achieve. In this paper, we demonstrate a mixed multinuclear cluster strategy, which successfully led to three new heterometallic MOFs (SNNU-328-330) with the same common H₃TATB [2,4,6-tris(4-carboxyphenyl)-1,3,5-triazine] tritopic linker and six types of multinuclear clusters ([YCd(COO)₄(μ₂-H₂O)], [YCd₂(COO)₈], [In₃(COO)₆(μ₃-OH)], [In₃Eu₂(COO)₉(μ₃-OH)₃(μ₄-O)], [Y₉(COO)₁₂(μ₃-OH)₁₄] and [Y₂Cd₈(COO)₁₆(μ₂-H₂O)₄(μ₃-OH)₈]). Three MOF adsorbents all show great potentials to remove the impurities (CO₂ and C₂-hydrocarbons) in natural gas and show prominent high-pressure methane storage capacity. Among them, the ideal adsorbed solution theory separation ratios of equimolar C₂H₂/CH₄, C₂H₄/CH₄, C₂H₆/CH₄, and CO₂/CH₄ at 298 K for SNNU-328 reach to 29.7-16.0, 19.1-8.2, 33.2-10.3, and 74.3-8.5, which have surpassed many famous MOF adsorbents. Dynamic breakthrough experiments conducted at 273 and 298 K showed that SNNU-330 can separate CH₄ from C₂H₂/CH₄, C₂H₄/CH₄, C₂H₆/CH₄, and CO₂/CH₄ mixtures with the breakthrough interval times of about 48.2, 17.9, 37.2, and 17.1 min g^-1 (273 K, 1 bar, v/v = 50/50, 2 mL min^-1), respectively. Remarkably, SNNU-329 exhibits extremely high methane storage performance at 298 K with the total uptake and working capacity of 192 cm³ cm^-3 (95 bar) and 171 cm³ cm^-3 (65 bar) due to the synergistic effects of high surface area, suitable pore sizes, and multiple open metal sites.