Determination of the catalytically active oxidation Lewis acid sites in Sn-beta zeolites, and their optimisation by the combination of theoretical and experimental studies

Two types of Lewis acid sites are shown to exist in Sn-beta zeolite: partially hydrolysed framework tin centers (SiO)3SnOH (sites A) and fully framework coordinated tin atoms Sn(SiO)4 (sites B). ONIOM and DFT calculations for adsorbed acetonitrile, which is a probe molecule for Lewis acid sites, show that acetonitrile coordinates more strongly to partially hydrolysed framework Sn (sites A) in Sn-beta zeolite than to nonhydrolysed framework Sn (sites B). The corresponding IR bands associated with the ν(CN) stretching vibration were also calculated. Adsorption–desorption experiments of deuterated acetonitrile on Sn-beta give two IR bands at 2316 and 2308 cm−1 with IR shifts of 43 and 51 cm−1 with respect to gas-phase acetonitrile. These values are in excellent agreement with theoretical calculations, making it possible then to associate the Lewis sites related to 2316 and 2308 cm−1 with partially hydrolysed and nonhydrolysed framework Sn sites, respectively. Catalytic results confirm that partially hydrolysed Sn (sites A) is much more active than fully framework-integrated Sn (sites B) for Baeyer–Villiger oxidation of cyclic ketones. Finally, the ratio between these two types of sites could be influenced by postsynthesis treatments.