DOI: 10.14457/tu.the.2025.106
title: Synthesis of ultramicroporous metal-organic frameworks as adsorbents for carbon dioxide capture at high pressures and temperatures

description: The environmental impact caused by CO2 emissions has become a serious concern worldwide. Metal-organic frameworks (MOFs) have recently attracted interest as solid adsorbents due to their considerable potential for CO2 adsorption. In this dissertation, new lanthanide-based MOFs with ultramicropore were designed and synthesized. The materials have been characterized using single-crystal X-ray diffraction, powder X-ray diffraction, elemental analysis, Fourier transform infrared spectroscopy, thermogravimetric analysis, photoluminescence spectroscopy, and vapour and gas sorption properties. The reactions of hydrated Ln(NO3)3 with oxalic acid (H2ox) under the same solvothermal conditions yielded two series of threedimensional (3D) oxalate-bridged lanthanide MOFs, [Me2NH2][Ln(ox)2(H2O)]·3H2O (1Ln = Pr (1Pr), Nd (1Nd), Sm (1Sm), Eu (1Eu), Gd (1Gd), Tb (1Tb), Dy (1Dy), Er (1Er)) and [Me2NH2][Lu(ox)2]·H2O (2Ln = Tm (2Tm), Yb (2Yb) and Lu (2Lu)]. Structural analysis reveals that compounds in series 1Ln and 2Ln crystallize in the monoclinic system with the space group P21/n and I2/a, respectively, due to the influence of lanthanide contraction. Both series of 1Ln and 2Ln compounds display a three-dimensional framework characterised by a diamondoid (dia) topological network, with a pore diameter of approximately 5 Å, accommodating the cationic Me2NH2 molecules. The reversible crystal transformation between the 1Ln and 2Ln(2) phases was observed during thermal activation and water adsorption studies, accompanied by a change in the local coordination sphere surrounding the Ln(III) ions from 9 (1Ln) to 8 (2Ln). The adsorption-desorption isotherms of CO2, CH4, Ar, and N2 gases on representative samples of 1Gd, 1Er, and 2Yb were examined at cryogenic temperatures up to 1 bar, revealing significant uptake and selective CO2 adsorption compared to the other gases. The high-pressure sorption analyses (up to 50 bar) of typical samples 1Gd and 2Yb exhibit S-shaped isotherms with hysteresis that varies with temperature. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) studies revealed that the S-shaped sorption isotherms result from chemisorption between CO2 and Me2NH2 cations, leading to the formation of dimethyl carbamic acid. In addition, 1Eu and 1Tb displayed brilliant red and green colours in the solid state at ambient temperature. Furthermore, a new series of ultramicroporous 3D oxalate-bridged MOFs with the formula [N2H5][Ln(ox)₂]·4H2O (3Ln = Er (3Er), Tm (3Tm), Yb (3Yb), Lu (3Lu) were successfully synthesized using heavily Ln(III) ions through in situ generated oxalate ligands and hydrazinium cations derived from the hydrolysis of oxalyl hydrazide under hydrothermal conditions. These isostructural 3Ln crystallize in the orthorhombic system with space group Fddd and display 3D dia frameworks similar to those of 1Ln and 2Ln. A typical sample of 3Yb demonstrates a CO2 absorption capacity of 80 cm³·g⁻¹ at 195 K and 1 bar, whereas at an increased pressure of 20 bar and 318 K, the CO2 adsorption reaches 70 cm³·g⁻¹. Furthermore, 3Erdemonstrates a remarkable water vapor adsorption capability of 13.8 and 6.8 mmol·g-1 at 283 and 293 K, respectively. The isotherms with S-shaped inflections are significant for gas sorption and separation applications, as they can enhance operational efficiency under appropriate pressure and temperature circumstances. Consequently, the findings of this study may advance the creation of novel CO2 sorbents for low-concentration applications, providing an energy-efficient approach to industrial-scale separations in CO2/CH4/N2/Ar gas mixtures by pressure or temperature swing adsorption methods (PSA, TSA).