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

Abstract

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 ν3(12CO2) at 2337–2338 cm–1 and two ν2(12CO2) modes around 662 and 650 cm–1. Simultaneously, the ν(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 ν3(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 ν3(CO2). In parallel with the CO2 adsorption a pa...