Adsorption Forms of CO₂ on MIL-53(Al) and NH₂-MIL-53(Al) As Revealed by FTIR Spectroscopy

Adsorption of CO2 on MIL-53(Al) and NH2-MIL-53(Al) has been studied by Fourier transform infrared (FTIR) spectroscopy at different temperatures and equilibrium pressures. For better interpretation of the spectra 13CO2 was also utilized. It is established that with both samples at low coverages CO2 forms O-bonded complexes with the structural OH groups (OH⋯O12CO). These species are characterized by μ3(12CO2) at 2337-2338 cm-1 and two μ2(12CO2) modes around 662 and 650 cm-1. Simultaneously, the μ(OH) modes of the hydroxyl groups are red-shifted, while the δ(OH) modes are blue-shifted. At higher coverages (OH⋯O12CO)2 dimeric species are formed and this leads to a decrease of the μ3(CO2) frequency by 2-4 cm-1. This change is due to vibrational interaction as proven by the observation that the frequency remains practically unaffected for (OH⋯O12CO) (OH⋯O13CO) dimeric species. Interaction between dimers leads to additional slight decrease of the value of μ3(CO2). In parallel with the CO2 adsorption a partial transformation of the material from large-pore to narrow-pore form occurs. Far before CO2 interacts with all hydroxyl groups, polymeric CO2 species are produced within the MIL-53(Al) sample. They are characterized by a split μ3(CO2) mode with a pronounced component at 2340 cm-1. The formation of these species involves some of the dimers and is accompanied by a reopening of the MIL-53 structure. Analysis of the shift of the OH modes led to the conclusion that the polymeric moiety interacts strongly with one OH group and more weakly with several other hydroxyls. No polymeric species were observed with the NH2-MIL-53(Al) sample which is associated with the more stable narrow-pore structure of this material. However, evidence of interaction between CO2 and the hydroxyls H-bonded to amino groups was found.ChemE/Catalysis Engineerin