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 C-13 NMR techniques are used to investigate the interaction of CO and CO2 molecules with the host structure of the MOFs Cu-3(btc)(2) and Cu2.97Zn0.03(btc)(2). A defined amount of C-13-enriched molecules per copper atom was adsorbed. The C-13 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 CO2 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 CO2. 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 CO2 molecules (measured by means of C-13 MAS NMR between 213 and 353 K) and from the deconvolution of the overlapping C-13 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.