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

Reactions of 1 and 2 equiv of HOSi(O t Bu) 3 with Cr(O t Bu) 4 afforded the first Cr(IV) alkoxysiloxy complexes ( t BuO) 3 CrOSi(O t Bu) 3 ( 1 ) and ( t BuO) 2 Cr[OSi(O t Bu) 3 ] 2 ( 2 ), respectively. Both 1 and 2 contain pseudotetrahedral d 2 chromium centers and crystallize in the monoclinic space group P 2 1 / 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 ( CrOSi ss ) contained both micro- and mesoporosity with an unexpectedly high surface area of 315 m 2 /g that was slightly reduced to 275 m 2 /g after calcination at 500 °C in O 2 . The xerogel obtained by the thermolysis of an n -octane solution of 1 ( CrOSi xg ) had a surface area of 315 m 2 /g that was reduced to 205 m 2 /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 Cr 2 O 3 was the only species that crystallized in CrOSi ss and CrOSi xg after calcination at temperatures up to 1200 °C in O 2 . 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 O 2 at 500 °C. In the oxidative dehydrogenation of propane, CrOSi xg (calcined at 500 °C) displayed intrinsic activities for propene formation that were up to 3 times higher than those for CrOSi ss (calcined at 500 °C). In addition, the selectivity for propene formation was higher for the more active CrOSi xg catalyst.