Ti-bridged silsesquioxanes as precursors of silica-supported titanium oxide catalysts for the epoxidation of cyclooctene

Abstract

ABSTRACT:Silica-supported titanium oxide catalysts were prepared by the controlled calcination of Ti-bridged polyhedral oligomeric silsesquioxanes (POSS) bearing four Si-O-Ti bonds, Ti[(c-C 5 H 9 ) 7 Si 7 O 11 (OSiMe 2 R)] 2 (1a; R = vinyl, 1b; R = methyl) and Ti[(c-C 5 H 9 ) 8 Si 8 O 13 )] 2 (2), supported on various silicas, and these catalysts showed excellent activities for the epoxidation of cyclooctene by t BuOOH as an oxidant to selectively give cyclooctene oxide in high yields. On the other hand, catalysts prepared using a corner-capped-type Ti-containing POSS, CpTi[(c-C 5 H 9 ) 7 Si 7 O 12 ] (3, Cp = cyclopentadienyl), or tetraisopropoxytitanium (4) showed lower activities. The 2 catalysts prepared from Ti-bridged POSS can be easily separated from the reaction mixture and reused without a significant loss of activity. These catalysts were also applicable for the epoxidation of limonene. A spectroscopic study revealed that calcination of Ti-bridged POSS supported on silica gave isolated tetrahedral Ti(IV) species, which would be responsible for the high activity, while calcination of silicasupported 3 or 4 gave aggregated surface titanium oxide species.

Titanosilicate Epoxidation Catalysts: A Review of Challenges and Opportunities

“…The formed single‐site catalysts (Scheme 4b) showed excellent catalytic performance in the epoxidation of cyclohexene with CHP (TOF values up to 1.8×10 −1 s −1 at 65 °C), confirming again the superiority of isolated Ti species as active sites for olefin epoxidation [110] . Along this line, Ti‐bridged silsesquioxanes were also used as precursors to produce silica supported Ti catalysts with isolated Ti species [111] …”

Ionic Complexes of Tetra‐ and Nonanuclear Cage Copper(II) Phenylsilsesquioxanes: Synthesis and High Activity in Oxidative Catalysis

“…Among the huge variety of 3 D polycyclic (cage) metal‐containing complexes, those based on structural silsesquioxane (RSiO 1.5 ) fragments attract special attention as systems that simulate metal‐center‐grafting to silica surfaces perfectly in heterogeneous catalytic applications . Such compounds (cage‐like metallasilsesquioxanes, CLMSs) are also (i) active in homogeneous catalysis, (ii) demonstrate specific (spin glass) magnetic properties, or (iii) provide opportunities for unusual reactivity and structural reorganization governed by thermal treatment . The most demanded synthetic approaches to CLMSs include reactions using cubane, cyclic, and trisilanol synthons as well as self‐assembly or transmetalation procedures.…”

Synthesis, characterization and catalytic application of a new organometallic oligomer based on polyhedral oligomeric silsesquioxane

“…POSS has been widely used to support transition metal oxides for applications in catalytic epoxidation . This support is attractive for its high specific surface area, mechanical stability and promotion of well‐dispersed active sites.…”

“…The epoxidation of organic substrates is especially interesting because of the industrial importance of this type of reaction, the products of which are widely used as starting materials in the synthesis of many organic compounds . Transition metal complexes, such as those of tungsten and molybdenum, are considered superior catalysts for the selective epoxidation of olefins by alkyl hydroperoxides . Molybdenum and tungsten are well‐known catalysts and have been widely investigated.…”