Thermolytic transformation of tris(alkoxy)siloxychromium(IV) single-source molecular precursors to catalytic chromia-silica materials

Reactions of 1 and 2 equiv of HOSi((OBu)-Bu-t)(3) with Cr((OBu)-Bu-t)(4) afforded the first Cr(IV) alkoxysiloxy complexes ((BuO)-Bu-t)(3)CrOSi((OBu)-Bu-t)(3) (1) and ((BuO)-Bu-t)(2)Cr[OSi((OBu)-Bu-t)(3)](2) (2), respectively. Both 1 and 2 contain pseudotetrahedral d2 chromium centers and crystallize in the monoclinic space group P2(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 (less than or equal to 180 degreesC), 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 m(2)/g that was slightly reduced to 275 m(2)/g after calcination at 500 degreesC in Oz. The xerogel obtained by the thermolysis of an n-octane solution of 1 (CrOSixg) had a surface area of 315 m(2)/g that was reduced to 205 m(2)/g upon calcination at 500 degreesC. 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 degreesC 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 degreesC. In the oxidative dehydrogenation of propane, CrOSixg (calcined at 500 degreesC) displayed intrinsic activities for propene formation that were up to 3 times higher than those for CrOSixg (calcined at 500 degreesC). In addition, the selectivity for propene formation was higher for the more active CrOSixg catalyst.