Abstract
To generate purposely defects that can increase the catalytic activity, cobalt-based metal–organic framework (MOF) TMU-10 has been subjected to thermal treatment under an air atmosphere at temperatures between 100 and 700 °C. This process causes partial ligand removal, generating structural defects and additional hierarchical porosity in a convenient way. The resulting materials, denoted as quasi-MOFs, were subsequently employed as catalysts for the room-temperature borohydride reduction of 4-nitrophenol (4-NP). The quasi TMU-10 framework obtained at 300 °C (QT-300) exhibits excellent catalytic performance with an apparent rate constant, activity factor, and half-life time of 2.8 × 10–2 s– 1, 282 s–1 g–1, and 24.8 s, respectively, much better values than those of parent TMU-10. Coexistence of micro and mesopores, coordinatively unsaturated cobalt nodes, tetrahedral Co(II) ions, and Co(III) in QT-300 are responsible for this enhanced activity. Kinetic studies in the range of 25–40 °C varying the 4-NP and BH4– concentrations agree with the Langmuir–Hinshelwood model in which both reactants are adsorbed on the catalyst surface. Reduction of 4-NP by the surface-hydrogen species is the rate-determining step.
Highlights
Industrial activities can generally result in the generation of significant amounts of harmful substances and pollutants, which are detrimental to the environment and living ecosystems. 4-Nitrophenol (4-NP) is one of the most hazardous and toxic organic contaminants for humans and the environment. 4-NP is generated as a byproduct in a large number of industrial processes; its effects are complicated because of its high solubility and stability in the aquatic environment
Our findings demonstrate that QTMU-10 offers a porous structure in which substrate diffusion becomes easier and multisite catalytic centers derived from coordinatively unsaturated metal positions, which are not present in the parent metal−organic framework (MOF) or metal oxide nanoparticles
TMU-10, [Co6(oba)5(OH)2(H2O)2(DMF)4]n·2DMF, is a cobalt-based MOF prepared via refluxing of its precursors as reported in the Experimental Section
Summary
Industrial activities can generally result in the generation of significant amounts of harmful substances and pollutants, which are detrimental to the environment and living ecosystems. 4-Nitrophenol (4-NP) is one of the most hazardous and toxic organic contaminants for humans and the environment. 4-NP is generated as a byproduct in a large number of industrial processes; its effects are complicated because of its high solubility and stability in the aquatic environment. We report the improved catalytic activity for 4-NP reduction to 4-AP of a cobalt Q-MOF catalyst, obtained from TMU-10 This improved performance of the Q-MOF arises from the creation of super-high density unsaturated metal centers and the simultaneous generation of both micro and mesopores after partial thermal ligand removal treatment. TMU-10 is a suitable MOF to be subjected to deligandation considering its high thermal stability and the existence of hexanuclear secondary building units (SBUs) of cobalt centers.[23] In the adjusted Q-MOF, the porosity and structure of the parent TMU-10 are partially preserved, with the generation of defects that increase diffusion through the material, while creating a coordinatively unsaturated position around the Co ions that exhibit a general catalytic activity in oxidations and reductions. Our findings demonstrate that QTMU-10 offers a porous structure in which substrate diffusion becomes easier and multisite catalytic centers derived from coordinatively unsaturated metal positions, which are not present in the parent MOF or metal oxide nanoparticles
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