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The present study illustrates the importance of the oxidation state of iron within the mesoporous iron trimesate Fe (3) O (H (2) O) (2) F (0. 81) (OH) (0. 19) C (6) H (3) (CO (2) ) (3) (2) denoted MIL-100 (Fe) (MIL= Material from Institut Lavoisier) during adsorption of molecules that can interact with the accessible metal sites through π-back donation. Adsorption of CO has been first followed by FTIR spectroscopy to quantify the Lewis acid sites in the dehydrated Fe (III) sample, outgassed at 150 °C, and on the partially reduced Fe (II/III), outgassed at 250 °C. The exposure of MIL-100 (Fe) to CO (2), propane, propene and propyne has then been studied by FTIR spectroscopy and microcalorimetry. It appears that π-back donating molecules are strongly adsorbed on reduced iron (II) sites despite the weaker Lewis acidity of cus Fe (2+) sites compared to that of Fe (3+) ones, as shown by pyridine adsorption.
Leclerc et al. (2011) studied this question.