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

Abstract

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 [Zn2.6Ni2.4(NO2)2.9Cl1.1(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.