(376f) Defect Engineering of Mg-MOF-74 for Enhanced CO2 Uptake

One of the promising materials for CO₂ capture is metal-organic frameworks (MOFs), a type of porous crystalline materials built by coordination bonds between metal ions and organic linkers.¹ Though the vast microporous structure of MOFs provides attractive high surface area and pore volume for various separation technologies,² elevated diffusion resistance to molecules, such as CO₂, arising from the tortuous pathways within the vast microporous structures have limited its practical applications.³ Defect engineering is an effective and versatile approach that allows the pore accessibility of MOFs to be tailored.^4,5 However, successful defect engineering has largely been demonstrated only for MOFs that possess metal nodes with high-degree connection, like the UiO-66 family (12 connected nodes).^6,7 For MOFs with a low-coordinated environment, such as MOF-74 and CALF20 (6 connected nodes),^8,9 the weaker structural stability of these low coordinated MOFs has limited the success of defect engineering. Generally, the defect-rich structure of low-coordinated MOFs is more prone to collapse as more defects are introduced.

In this work, defect-rich Mg-MOF-74 is synthesized using graphene oxide (GO) as a solid modulator and controlling crystallization time. GO concentration (0 to 3.23 mg/mL) and reaction duration (5 to 40 h) were systematically controlled and the as-synthesized MOFs were characterized using XRD, SEM, BET and TGA. The SEM results were used to evaluate the crystal growth and morphology of the MOFs in the presence of the GO modulators, whilst Rietveld refinement of the XRD patterns were used to validate the presence of the defects. Pore dimension, architecture and accessibility were evaluated based on the results from BET under different measurement conditions as shown in Figure 1 (e.g., CO₂ capacity, CO₂/N₂ selectivity, temperature effects, adsorption enthalpy). The structural stability is studied by decomposition curve tested by TGA. In summary, the best performing sample (MOF@GO 40 h) showed an increased pore accessibility to CO₂, with an improvement of 18% in surface area and a 15% improvement in pore volume, compared to that of pristine Mg-MOF-74. Consequently, the active sites are more accessible, resulting in an increase of 19.29 % and 16.37 % in CO₂ capacity at 0.1 bar and 1 bar, respectively, at room temperature. Our study presents a versatile tool to incorporate defects into low-coordinated MOFs while maintaining the stability of the structure at the same time, so as to enhance the separation performance of MOFs.