Thermolytic Transformation of Tris(alkoxy)siloxychromium(IV) Single-Source Molecular Precursors to Catalytic Chromia−Silica Materials

Reactions of 1 and 2 equiv of HOSi(OtBu)3 with Cr(OtBu)4 afforded the first Cr(IV) alkoxysiloxy complexes (tBuO)3CrOSi(OtBu)3 (1) and (tBuO)2Cr[OSi(OtBu)3]2 (2), respectively. Both 1 and 2 contain pseudotetrahedral d2 chromium centers and crystallize in the monoclinic space group P21/n with four molecules in the unit cell. The high-yielding, convenient synthesis of 1 makes this complex a useful single-source molecular precursor, via the thermolytic molecular precursor method, to Cr/Si/O materials. The thermal transformations of 1 and 2 to chromia−silica materials occurred at low temperatures (≤180 °C), to give isobutene as the major carbon-containing product. The material generated from the solid-state conversion of 1 (CrOSiss) contained both micro- and mesoporosity with an unexpectedly high surface area of 315 m2/g that was slightly reduced to 275 m2/g after calcination at 500 °C in O2. The xerogel obtained by the thermolysis of an n-octane solution of 1 (CrOSixg) had a surface area of 315 m2/g that was reduced to 205 m2/g upon calcination at 500 °C. The nitrogen adsorption−desorption isotherm for the xerogel had characteristics indicating both microporosity and textural mesoporosity. Powder X-ray diffraction (PXRD) analysis was used to demonstrate that Cr2O3 was the only species that crystallized in CrOSiss and CrOSixg after calcination at temperatures up to 1200 °C in O2. Elemental analyses of these materials revealed that the Cr:Si ratio was very close to 1:1, indicating that the original stoichiometry of the precursor is retained upon thermal conversion to Cr/Si/O materials. Both materials contained very low amounts of carbon after calcination in O2 at 500 °C. In the oxidative dehydrogenation of propane, CrOSixg (calcined at 500 °C) displayed intrinsic activities for propene formation that were up to 3 times higher than those for CrOSiss (calcined at 500 °C). In addition, the selectivity for propene formation was higher for the more active CrOSixg catalyst.