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

Abstract

The chemistry and applications of porous materials, particularly for gas adsorption and separation technologies, have advanced dramatically with the recent development of metal–organic frameworks (MOFs). These materials can mitigate air pollution, global warming, and the demand for renewable energy through their tunable pore sizes and pore chemistry, a feature that is not easily manipulated in conventional porous materials like zeolites and activated carbons. In the present study, two series of three-dimensional (3D) lanthanide-based MOFs (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 engineered with meticulous ultramicropore regulation to fit the kinetic diameters of polarized CO₂ molecules. The coordination geometries {LnO₉} in 1Ln and {LnO₈} in 2Ln are interconnected by oxalate (C₂O₄²⁻) bidentate ligands, forming diamond (dia)-like anionic frameworks that contain charge-balancing dimethyl ammonium (Me₂NH₂⁺) cations within ultramicropore channels. The removal and re-adsorption of water facilitate the transition between the crystal structures of 1Ln and 2Ln, leading to changes in the coordination number and geometry of the metal centres. At cryogenic temperatures and ambient pressure, the tailored ultramicropore facilitates a sieving effect in LnMOFs, yielding better CO₂ adsorption compared to N₂, CH₄, and Ar as shown by the instances of 1Gd, 1Er, and 2Yb. Furthermore, CO₂ sorption investigations at higher pressure for 1Gd and 2Yb exhibit steep S-shaped adsorption–desorption isotherms characterized by hysteresis. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) studies indicate that this phenomenon is due to the formation of dimethyl carbamic acid resulting from the chemisorption of guest Me₂NH₂⁺ cations and CO₂. The findings demonstrate that ultramicroporous anionic frameworks containing guest Me₂NH₂⁺ cations significantly improve CO₂ adsorption capacity at pressures exceeding ambient conditions. This research will drive the future logical design of ultramicroporous materials for the efficient separation of CO₂ through pressure or temperature swing adsorption methods. Compounds 1Eu and 1Tb exhibit red- and green-light emission in the solid state at room temperature.