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Infrared spectra of MIL-100(Al) have been recorded after evacuation from room temperature up to 623 K. In addition to adsorbed water molecules characterized by specific (ν+δ)H2O combination bands at about 5300 cm–1, spectra analysis shows the presence of impurities like carboxylic acid and nitrates resulting from the synthesis step, explaining the low amount of Al–OH groups detected. The Lewis acidity has been characterized by CO ν(CO) at 2183 cm–1, pyridine ν8a band estimated at 1618 cm–1, and CD3CN ν(CN) at 2326 cm–1 adsorption on the activated sample. The acidity is strong as revealed by the ν(CN) wavenumber. Interestingly, CO gives rise to an interaction weaker than that expected from pyridine and CD3CN results. Quantitative results relative to the number of Al3+5c sites are in full agreement with those reported elsewhere from 27Al NMR experiments. The Brønsted acidity mainly results from the presence of coordinated water species in the nonfully dehydrated sample and not from the structural Al–OH groups.
Volkringer et al. (2012) studied this question.