An advanced approach for measuring acidity of hydroxyls in confined space: a FTIR study of low-temperature CO and 15N2 adsorption on MOF samples from the MIL-53(Al) series

Acidity of solids is decisive for their interaction with guest molecules. One of the most used methods for measuring the acidity of surface hydroxyl groups is the hydrogen bond method based on the spectral shift of the OH stretching modes induced by the adsorption of weak bases. However, many materials of practical interest (e.g. metal organic frameworks, zeolites, etc.) are porous and the OH groups are involved in H-bonding with framework basic sites. Here we show that MIL-53(Al) and NH2-MIL-53(Al) samples are characterized by one type of structural hydroxyl but three IR bands are detected at 100 K with these materials (at 3721, 3711 and 3683 cm−1). These bands are assigned to structural hydroxyls involved in H-bonding with different strengths. There is no correlation between the acidities of the hydroxyls, as measured by low-temperature CO or 15N2 adsorption, and the main reason for this is the pre-existing H-bond. A method for the estimation of the intrinsic frequency of the OH groups (i.e. if not participating in H-bonds), based on the analysis of the spectral data obtained with two molecular probes, is proposed. According to this method, the OH stretching frequency of the structural hydroxyls of MIL-53(Al) samples is determined to be 3727 cm−1. The formation of 1 : 1 adducts between the hydroxyls and strong bases leads to breaking of the pre-existing H-bonds. When the base is weak, bifurcated complexes are formed which slightly affects the spectral shift. The conclusions derived here considerably broaden the applicability of the H-bond method for assessing protonic acidity of materials and systems where the OH groups are preliminarily involved in H-bonding.