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

Metal–organic frameworks are promising materials for CO2 capture and their CO2 adsorption properties can be tuned by appropriate functionalization. We have recently shown that hydroxyl functionalized MIL-53(Al) materials (where part of the linkers are 2,5-dihydroxy terephthalate ligands) possess enhanced CO2 adsorption capacity at relatively low pressures. In order to acquire a deeper insight of the process, we studied three samples with IR spectroscopy: MIL-53(Al), used as a reference; MIL-53(Al)–OH25, demonstrating improved CO2 adsorption capacity; and MIL-53(Al)–OH75, with a poor CO2 adsorption performance. MIL-53-Al is characterized by μ2-OH groups detected at 3708–03 cm–1. With the MIL-53(Al)–OH25 sample, part of these hydroxyls (band at 3686 cm–1) participate in weak H-bonding. The introduced hydroxyl groups (Ar–OH) are involved in medium-strength H-bonding and are characterized by ν(OH) at 3307 cm–1 and δ(OH) at 1120 cm–1. With the MIL-53(Al)–OH75 sample the μ2-OH groups are detected at 3689 and 3681 cm–1 and the Ar–OH at 3338 cm–1. In addition, some strongly H-bonded hydroxyls were observed. Low-temperature CO adsorption experiments with the MIL-53(Al) and MIL-53(Al)–OH25 samples revealed weak acidity of the μ2-OH and even weaker of the Ar–OH groups. The adsorption process was accompanied by some pore expanding. The MIL-53(Al)–OH75 sample remained in the NP form and did not adsorb CO. Adsorption of CO2 on all samples was performed at ambient temperature, and the process was followed by the changes in the ν3(12CO2) band at 2337 cm–1 (at pressures up to 50 mbar) and the ν3(13CO2) band at 2272–2271 cm–1 (for higher pressures up to 500 mbar). Concerning the μ2-OH groups the spectra indicate the strongest interaction with CO2 for the MIL-53(Al)–OH25 sample. It was also found that CO2 interacts with the Ar–OH groups through breaking (or weakening) the preexisting H-bond thus increasing the adsorption capacity. Because the process is not thermodynamically favored, Ar–OH···OCO complexes are appreciably formed only at high CO2 equilibrium pressures. Although CO2 adsorption leads to expanding the pores of the MIL-53(Al)–OH25 material, MIL-53(Al)–OH75 remains in the narrow-pore form even in the presence of 500 mbar CO2, which affects negatively the adsorption process.