Effect of pendant isophthalic acid moieties on the adsorption properties of light hydrocarbons in HKUST-1-like tbo-MOFs: application to methane purification and storage

Abstract

Equilibrium adsorption of methane (CH₄), C₂₊ gases (ethane (C₂H₆), ethylene (C₂H₄), propane (C₃H₈), and propylene (C₃H₆)), and carbon dioxide (CO₂) was measured on a series of tbo-MOFs (topological analogues of the prototypical MOF, HKUST-1, correspondingly dubbed tbo-MOF-1), which were developed via the supermolecular building layer (SBL) pillaring strategy. Specifically, tbo-MOF-2 and its isoreticular, functionalized analogue, tbo-MOF-2-{CH₂O[Ph(CO₂H)₂]}₂ (or tbo-MOF-3), which is characterized by pendant isophthalic acid moieties freely pointing into the cavities, were evaluated on the basis of potential use in methane storage and C₂₊/CH₄ separation. The parent, tbo-MOF-2, showed high gravimetric and volumetric CH₄ uptake, close to the U.S. Department of Energy (DOE) target for methane storage at 35 bar and room temperature. Though the presence of the pendant isophthalic acid moiety in the analogous compound, tbo-MOF-3, led to a decrease in total CH₄ uptake, due mainly to the reduced size of the cavities, interestingly, it increased the affinity of the SBL-based tbo-MOF platform for propane, propene, ethane, and ethylene at low pressures compared with CH₄, due additionally to the enhanced interactions of the highly polarizable light hydrocarbons with the isophthalic acid moiety. Using Ideal Adsorption Solution Theory (IAST), the predicted mixture adsorption equilibria for the C₃H₈/CH₄, C₃H₆/CH₄, C₂H₆/CH₄, C₂H₄/CH₄, and C₃H₈/CO₂ systems showed high adsorption selectivity for C₂₊ over methane for tbo-MOF-3 compared with tbo-MOF-2. The high working storage capacity of tbo-MOF-2 and the high affinity of tbo-MOF-3 for C₂₊ over CH₄ and CO₂ make tbo-MOF an ideal platform for studies in gas storage and separation.