Increasing Exposed Metal Site Accessibility in a Co-MOF-74 Material With Induced Structure-Defects

Abstract

Metal-organic frameworks (MOFs) are promising nanoporous materials with many practical applications. This owes largely to their remarkable porosity and the presence of specific chemical functionalities, such as exposed metal sites (EMS). The MOF-74 structure is known for exhibiting one of the highest EMS densities among porous materials. Moreover, the inclusion of structural defects has been proposed to enhance activity further. This was previously achieved by mixing the original linker together with a second one, having lower topology. The presence of structural defects was evidenced by the resulting crystalline properties and thermal stability. In this work, different mixtures of tetratopic 2,5-dihydroxyterephthalic acid with up to 60% of the tritopic hydroxyterephtalic acid were used to synthesize crystalline Co-MOF-74-like materials. Materials synthesized from higher proportions than 30% of hydroxyterephtalic acid in the synthesis media collapse upon partial removal of the solvent molecules. This indicates the presence of structural defects and the importance of the solvent molecules in stabilizing the crystalline structures. Electron microscope images show that crystal size reduces with inclusion of hydroxyterephtalic acid as the second linker. The presence of coordinated solvent molecules at the EMS was evaluated by Fourier-transform infrared spectra (FTIR) spectroscopy, so that a higher degree of solvent-exchange was observed during washing for defective structures. Furthermore, TG analysis suggests defective structures exhibit lower desolvation temperatures than the defect-free structures. Finally, N 2 adsorption-desorption analyses at −196 • C showed an enhanced accessibility of the gas to the inner porosity of the defective structures and therefore, the EMS of the material. All these finding make this pathway interesting to enhance the potential interest of these materials for an industrial application because of both a facilitated activation and a better access to the active sites.

Rational incorporation of defects within metal–organic frameworks generates highly active electrocatalytic sites

“…We proceeded to synthesize our target materials with a simple one-pot solution phase approach, adapting a previously established recipe in which missing hydroxyl linker defects were generated in Co-MOF-74. 39 The nominally defect-free MOF, Ni-MOF-74, was synthesized with only 2,5-dihydroxyterepthalic acid while the defect-containing MOF, Ni-MOF-74D, was synthesized using a 3 : 1 ratio of 2,5-dihydroxyterepthalic acid : 2-hydroxyterepthalic acid ( Fig. 2a ).…”

Synthesis of MoS x /Ni‐metal‐organic framework‐74 composites as efficient electrocatalysts for hydrogen evolution reactions

“…The as‐synthesized Ni‐MOF‐74 was activated using the thermal method that was confirmed by FTIR spectra, as shown in Figure 1B. A low signal at 1662 cm −1 implied that the solvent molecules were significantly eliminated during the Ni site activation process 57 . In addition, the asymmetric and symmetric stretching vibrational frequencies of CO (COOH) were located at 1557 and 1417 cm −1 , respectively, while the vibrational frequency at 1193 cm −1 was assigned to CO (H) bonding.…”

“…A low signal at 1662 cm −1 implied that the solvent molecules were significantly eliminated during the Ni site activation process. 57 In addition, the asymmetric and symmetric stretching vibrational frequencies of C O ( COOH) were located at 1557 and 1417 cm −1 , respectively, while the vibrational frequency at 1193 cm −1 was assigned to C O (H) bonding. The porous structure of the materials was analyzed using the N 2 sorption isotherm, as displayed in Figure 1C 3D,F).…”

Rational Incorporation of Missing Linker Defects Within Metal-Organic Frameworks Generates Highly Active Electrocatalytic Sites

“…We proceeded to synthesize our target materials with a simple one-pot solution phase approach, adapting a previously established recipe in which missing linker defects were generated in Co-MOF-74. 38 The nominally defect-free MOF, Ni74, was synthesized with only 2,5-dihydroxyterepthalic acid while the defect-containing MOF, Ni74D, was synthesized using a 3:1 ratio of 2,5-dihydroxyterepthalic acid: 2hydroxyterepthalic acid (Fig. 2a).…”