In-situ 31P MAS NMR probing of the active centers in Ti silicalite molecular sieve

• Different sites of TS-1 zeolite were distinguished by 31P solid-state MAS NMR.
• Active centers in TS-1 were investigated by detecting the oxidation of TMP.
• Only the TMP bound to Lewis acid sites of TS-1 was oxidized.
• It was confirmed that Ti-superoxo species could be the active centers in TS-1.

The investigation of the active centers in titanium zeolite was conducted by a probe reaction, the oxidation of trimethylphosphine. The different sites of TS-1 zeolite can be clearly distinguished through 31P solid-state MAS NMR spectra of adsorbed trimethylphosphine. The results showed that only the trimethylphosphine bound to the coordinate-unsaturated framework Ti sites and the distorted tetrahedral Ti sites of TS-1 zeolite could be easily oxidized by urea hydrogen peroxide. Diffuse Reflectance UV–Vis and 29Si MAS NMR spectra further confirmed that the titanium peroxide species (TiOOH), which is formed through the solid–solid interaction between urea–H2O2 and TS-1 zeolite, could be catalytic active centers during the oxidation of trimethylphosphine.

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