Characterisation of Porous Materials by FTIR Spectroscopy of Isotopically Labelled Probe Molecules

In this review, some of the advantages of the use of isotopically labelled molecules for FTIR characterisation of porous materials are summarised. The most important sites determining the properties of these materials are the hydroxyl groups and the incorporated cations. D → H exchange is very useful for characterisation of hydroxyls and here some new possibilities of the technique are shown. The OH → OD isotopic shift factor, i, of isolated hydroxyls is about 0.737-0.738, a value higher than the theoretical one (0.728). These experimental deviations are mainly due to anharmonicity. The anharmonicity slightly decreases when the OH groups participate in weak H-bonding with adsorbed molecules, which should lead to a decrease of i. However, contrary to the expectations, i additionally increases. This is attributed to the lower acidity of the OD groups as compared to the respective OH groups. It is shown that i of hydroxyls that are H-bonded to framework oxygen is also higher as compared to isolated ones. The use of D → H exchange also allows reassignment of some phenomena, as occurrence of Fermi resonance. Another important characteristic of mesoporous materials is the coordination state of guest cations. The number of coordinative vacancies is well determined by the utilisation of CO isotopic mixtures. Here the principles of this technique are considered and the advantages of CO-13C18O isotopic mixtures as compared to CO-13CO are demonstrated. Finally, other applications of labelled molecules for determination of the structures of species formed on porous materials are briefly reviewed.