Adsorption of Small Molecules on Cu3(btc)2 and Cu3–xZnx(btc)2 Metal–Organic Frameworks (MOF) As Studied by Solid-State NMR

Static and MAS 13 C NMR techniques are used to investigate the interaction of CO and CO 2 molecules with the host structure of the MOFs Cu 3 (btc) 2 and Cu 2.97 Zn 0.03 (btc) 2 . A defined amount of 13 C-enriched molecules per copper atom was adsorbed. The 13 C chemical shift anisotropy and isotropic chemical shift were studied over a temperature range from 10 to 353 K. Already above 30 K an isotropic line for CO is found superimposed to the solidlike spectra belonging to the majority of adsorbed CO molecules. For adsorbed CO 2 an isotropic line can be detected above 70 K. This observation reflects differences in the local motion of both molecules. At high temperatures it is found that CO is desorbed more easily from the MOF framework in comparison to CO 2 . This is in agreement with conclusions derived from desorption measurements on Cu 3 (btc) 2 . From the temperature dependence of the chemical shift for adsorbed CO 2 molecules (measured by means of 13 C MAS NMR between 213 and 353 K) and from the deconvolution of the overlapping 13 C NMR lines for adsorbed CO molecules (between 180 and 323 K), the activation energy for the local motion of the adsorbed molecules was determined as 3.3 and 6.1 kJ/mol, respectively. Additionally, the motion is accompanied by a partial desorption of the adsorbed species.