Cyclic gas-phase heterogeneous process in a metal-organic framework involving a nickel nitrosyl complex

The cubic metal-organic framework MFU-4l ([Zn₅Cl₄(BTDD)₃], H₂-BTDD = bis(1H-1,2,3-triazolo[4,5-b],[4',5'-i])dibenzo[1,4]dioxin) featuring large pore apertures can be modified post-synthetically via partial or complete substitution of peripheral metal sites and chloride side-ligands, thus opening a route towards a large variety of functionalized MOFs. In this way, Ni-MFU-4l-nitrite (or Ni-MFU-4l-NO₂) with an analytically determined chemical composition [Zn_2.6Ni_2.4(NO₂)_2.9Cl_1.1(BTDD)₃], containing accessible Ni-NO₂ units, was prepared. Ni-MFU-4l-NO₂ undergoes selective heterogeneous gas-phase reduction by carbon monoxide at 350 °C, leading to formation of Ni-NO units at the peripheral sites of the MFU-4l framework (Ni-MFU-4l-NO). The crystallinity and porosity of the MFU-4l framework are completely retained upon this transformation. The so-formed nickel nitrosyl complex, showing high thermal stability, readily reacts with nitrogen monoxide at room temperature, producing Ni-NO₂ units and dinitrogen monoxide (N₂O). Hence, the reaction of Ni-MFU-4l-NO₂ with CO followed by NO represents a cyclic process with an overall stoichiometry 2NO + CO → N₂O + CO₂, in which the Ni-MFU-4l framework serves as a catalyst. It can be considered as a model process for the removal of highly toxic NO and CO gases, which are converted to non-toxic CO₂ and N₂O. Diffuse reflectance infrared Fourier transform spectroscopic studies show that at least 10 cycles can be repeated. The framework's reactivity drops down by ca. 50% after 10 cycles, which is most likely due to the accumulation of highly reactive NO₂ and N₂O₄ contaminants. Therefore, further investigations on characterizing reaction intermediates should be done in order to improve the catalyst's performance. Our results confirm the potential of MFU-4l frameworks as selective single-site catalysts for heterogeneous gas-phase transformations and provide a motivation for further studies.