Insights into the key to highly selective synthesis of oxime via ammoximation over titanosilicates

A thorough investigation was carried out to demonstrate the key to highly selective synthesis of oxime via ammoximation over titanosilicates. The oxidation pathways of H₂O₂ between the formation of NH₂OH and deep oxidation of oxime stayed competitive, which could control product selectivity. These pathways were influenced by oxime chemical activity, NH₃ concentration, and the diffusional restriction and intrinsic catalytic activity of titanosilicates. Diffusional restriction could lower the selectivity of highly active oxime to some extent through intensifying deep oxidation. However, it was not the decisive factor in achieving high oxime selectivity, since the pathway of deep oxidation proceeded without involvement of Ti active sites. The positive role of high NH₃ concentration in promoting the pathway of generating NH₂OH was relatively limited, as the crucial step of activating H₂O₂ into Ti–OOH species was primarily determined by the intrinsic catalytic ability of titanosilicates, which was related to their Lewis acidity. It is found that titanosilicates with strong Lewis acidity of Ti active sites contribute to highly selective synthesis of oxime by promoting H₂O₂ to participate into the highly efficient formation of NH₂OH, and thus the corresponding side reaction of consecutive homogeneous oxidation of oxime with free H₂O₂ is fundamentally suppressed. By regulating the strength of Lewis acidity of titanosilicates, selectivity of oxime with various chemical activities could be improved significantly.