Carbon dioxide adsorption properties in 3D ultramicroporous diamondoid lanthanide-oxalate frameworks

Abstract

Two series of ultramicroporous lanthanide-based metal-organic frameworks (LnMOFs) viz., (Me2NH2)[Ln(C2O4)2(H2O)]·3H2O (1Ln; Ln = Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Er) and (Me2NH2)[Ln(C2O4)2]·H2O (2Ln; Ln = Tm, Yb, Lu), were synthesized using one-pot solvothermal crystallization, exhibiting scalability from milligramme to gramme quantities. Structural investigations revealed that the isostructural compounds in series 1Ln and 2Ln crystallize in the monoclinic system with space groups P21/n and I2/a, respectively. Both series of compounds exhibit similar three-dimensional (3D) anionic diamond (dia) topological frameworks, formed by the interconnection of coordination geometries {LnO9} in 1Ln and {LnO8} in 2Ln, utilizing C2O42− two-connecting linkers. The charge-balancing Me2NH2+ cations reside within ultramicropore channels (~6 Å) of dia frameworks, markedly affecting volumetric CO2 uptake isotherms in high-pressure studies. A reversible transformation between the crystal structures of 1Ln and 2Ln takes place through the removal and re-adsorption of water, resulting in a modification of the coordination number and geometry of the metal centres. The narrow pore size distribution in 3D dia frameworks coincides more effectively with the kinetic diameter of polarized CO2 molecules in comparison to N2, CH4, and Ar. At low pressures (up to 1 bar) and at 195 K, the representative activated samples 1Gd, 1Er, and 2Yb demonstrate high CO2 uptake capacities of 59 cm3/g (2.63 mmol/g), 61 cm3/g (2.72 mmol/g), and 40 cm3/g (1.79 mmol/g), respectively. High-pressure CO2 sorption investigations of 1Gd and 2Yb revealed that the profiles exhibit steep S-shaped isotherms with hysteresis that fluctuate with temperature and pressure. This phenomenon arises from the chemisorption of CO2 and Me2NH2+, resulting in the formation of dimethyl carbamic acid, which is further corroborated by DRIFTS studies. Inflection points and selective features in sorption isotherms are essential for the advancement of innovative CO2 sorbents for energy-efficient separation processes utilising pressure or temperature swing adsorption methods. Additionally, the photoluminescence properties of 1Eu and 1Tb were studied in the solid state at room temperature.