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Siloxyaluminate and Siloxygallate Complexes as Models for Framework and Partially Hydrolyzed Framework Sites in Zeolites and Zeotypes.

  • Author(s): Dombrowski, James P;
  • Ziegler, Micah S;
  • Phadke, Neelay M;
  • Mansoor, Erum;
  • Levine, Daniel S;
  • Witzke, Ryan J;
  • Head-Gordon, Martin;
  • Bell, Alexis T;
  • Tilley, T Don
  • et al.
Abstract

Anionic molecular models for nonhydrolyzed and partially hydrolyzed aluminum and gallium framework sites on silica, M[OSi(OtBu)3 ]4 - and HOM[OSi(OtBu)3 ]3 - (where M=Al or Ga), were synthesized from anionic chlorides Li{M[OSi(OtBu)3 ]3 Cl} in salt metathesis reactions. Sequestration of lithium cations with [12]crown-4 afforded charge-separated ion pairs composed of monomeric anions M[OSi(OtBu)3 ]4 - with outer-sphere [([12]crown-4)2 Li]+ cations, and hydroxides {HOM[OSi(OtBu)3 ]3 } with pendant [([12]crown-4)Li]+ cations. These molecular models were characterized by single-crystal X-ray diffraction, vibrational spectroscopy, mass spectrometry and NMR spectroscopy. Upon treatment of monomeric [([12]crown-4)Li]{HOM[OSi(OtBu)3 ]3 } complexes with benzyl alcohol, benzyloxide complexes were formed, modeling a possible pathway for the formation of active sites for Meerwin-Ponndorf-Verley (MPV) transfer hydrogenations with Al/Ga-doped silica catalysts.

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