Synthesis, Characterization, and Catalytic Activity of Mesostructured Titanosilicates Assembled from Polymer Surfactants with Preformed Titanosilicate Precursors in Strongly Acidic Media

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Abstract

Novel mesostructured titanosilicates designated as MTS-9 have been successfully synthesized from assembly of preformed nanosized titanosilicate precursors with polymer surfactants. Mesoporous MTS-9 shows highly hydrothermal stability in boiling water (over 120 h) as compared with that of Ti−MCM-41 and SBA-15. In phenol hydroxylation, Ti−MCM-41 shows very low catalytic activity (2.5%), but MTS-9 exhibits very high catalytic activity, with phenol conversion of 26%, which is comparable with TS-1. In styrene epoxidation, MTS-9 shows high activity and selectivity similar to those of TS-1, which are much different from those of Ti−MCM-41. In 2,3,6-trimethylphenol hydroxylation, Ti−MCM-41 is inactive because of the relatively low oxidation ability of Ti species in the amorphous wall of Ti−MCM-41, and TS-1 is also inactive because of the inaccessibility of the small micropores of TS-1 to the large diameter of a bulky molecule like 2,3,6-trimethylphenol. However, MTS-9 is very active for this reaction with conversion of 18.8% indicating that MTS-9 is an effective catalyst for the oxidation of bulky molecules. The MTS-9 samples were characterized with infrared, UV−visible, UV−Raman, and numerous other techniques. The results suggest that the titanium species in MTS-9 are TS-1-like, and that the pore walls of MTS-9 contains primary and secondary structural building units, similar to those of microporous zeolites. Such unique structural features might be responsible for the observed strong oxidation ability and high hydrothermal stability of the mesostructured titanosilicates. Heating MTS-9 at 500 °C leads to the transformation of titanium species, giving relatively low catalytic conversion in phenol hydroxylation, which suggests that increasing thermal stability of titanium sites like TS-1 species in the mesoporous wall is still a great task for preparation of mesostructured titanosilicates.

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