A Combined Pulsed Electron Paramagnetic Resonance Spectroscopic and DFT Analysis of the13CO2and13CO Adsorption on the Metal–Organic Framework Cu2.97Zn0.03(btc)2

Abstract

Cu3(btc)2 (btc = 1,3,5-benzenetricarboxylate), also called HKUST-1, is one of the well-known representatives of the metal–organic framework (MOF) compounds. It exhibits a large surface area and a high pore volume. Due to the coordinatively unsaturated metal centers as preferential adsorption sites, Cu3(btc)2 is particularly interesting for the separation of CO2 and CO in gaseous mixtures. We studied the interactions of 13C-enriched carbon dioxide (13CO2) and carbon monoxide (13CO) with the Cu2+ centers in the zinc-substituted homologue Cu2.97Zn0.03(btc)2 using continuous wave (cw) and pulsed electron paramagnetic resonance (EPR) spectroscopy (Davies or Mims electron nuclear double resonance (ENDOR) and hyperfine sublevel correlation (HYSCORE)). Upon adsorption of 13CO2 and 13CO, the coordination geometry of the Cu2+ centers changed from square planar to square pyramidal. The cupric ion g-tensor and the 63/65Cu hyperfine coupling tensor ACu show the changes in the ligand field of Cu2+. Moreover, the interaction with the 13C nuclei of the gas molecules is reflected in the isotropic coupling constant AisoC and the dipolar coupling parameter T⊥C which are derived from the 13C hyperfine coupling tensor AC obtained by the pulsed EPR experiments. From the experimentally obtained parameters, we derived a geometrical model for the adsorption of 13CO2 and 13CO at the Cu2+ ions that is consistent with our DFT calculations. The 13CO molecule is found to coordinate linearly at the Cu2+ center via the 13C atom and perpendicular to the CuO4 plane with a Cu–C distance of rCuC = 2.57(10) Å (DFT, 2.42 Å). The 13CO2 molecule is coordinated slightly tilted via the O atom with a Cu–C distance of rCuC = 3.34(10) Å (DFT, 3.27 Å). The Cu–O distance for adsorbed 13CO2 is not directly accessible to EPR measurements but could be estimated from geometrical considerations in the range of rCuO = 2.53–2.73 Å (DFT, 2.39 Å). The results provide detailed insight into the geometry of adsorbed CO2 and CO in porous materials and show the potential of EPR spectroscopy for analyzing adsorption complexes.