Titanium molecular sieve TS-1 is a well-known powerful and selective redox catalyst. It has been found to catalyze many useful hydrocarbons with hydrogen peroxide under mild conditions. The framework Ti of the zeolite, most likely in a tetrahedral coordination, is usually considered as the active titanium species. In our previous investigation, the oxidation of trimethylphosphine (TMP) over TS-1 zeolite was used as a model reaction to detect the active centers of TS-1, in which oxygen was the oxidant. Since hydrogen peroxide is usually acted as oxidant in most of the oxidation reaction catalyzed by TS-1 zeolite, in this paper, we monitored the in situ oxidation process of TMP adsorbed on TS-1 zeolite, using andydrous solid urea-hydrogen peroxide (UHP) as oxidant. Similar to the oxidation process with oxygen, the TMP adsorbed on the Lewis acid sites of TS-1 can be easily oxidized by hydrogen peroxide. At the same time, no changes happened on either physisorbed TMP or those adsorbed on the Bronsted acid sites of TS-1 zeolite. As a control experiment, the TMP adsorbed on ZSM-5 couldn't be oxidized by hydrogen peroxide, which is different with the results when oxygen acted as oxidant. It indicates that only titanium-containing zeolite has active sites to oxide TMP to trimethylphosphine oxide (TMPO) with hydrogen peroxide. UHP is an excellent solid oxidant, which can release hydrogen peroxide gradually. When TS-1 is mixed with UHP, Ti-superoxo complex is formed, which shows absorption band in the range of 300~500 nm in DR UV-Vis spectrum. After solid solid interaction, Ti-O-Si bond is broken to form TiOOH and Si(OSi)3OH, which can also be detected in 29Si MAS NMR spectrum. Thus, the Ti-superoxo species produced by framework titanium and hydrogen peroxide are the active centers of TS-1 zeolite.

Probing the active sites of TS-1 zeolite by using <i>in situ</i> solid state NMR

247th National Meeting (2014)

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Probing the active sites of TS-1 zeolite by using in situ solid state NMR