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

• Selectivity in various ketones ammoximation over Ti-MWW, TS-1 and TS-2 are compared.
• Diffusional restriction of catalyst reinforces deep oxidation of oxime with H2O2.
• Competitive oxidation pathways of H2O2 control selectivity in ammoximation.
• High-efficient formation of NH2OH is crucial to achieve high oxime selectivity.
• Strong Lewis acidity of titanosilicates strengthens the pathway of forming NH2OH.

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.

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