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 (single bondSisingle bondOsingle bond)3Snsingle bondOH (sites A) and fully framework coordinated tin atoms Sn(single bondSisingle bondOsingle bond)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(Ctriple bond; length of mdashN) 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.