New Vanadium Tris(<i>tert</i>-butoxy)siloxy Complexes and Their Thermolytic Conversions to Vanadia−Silica Materials

2002-02-13T00:00:00Z (GMT) by Kyle L. Fujdala T. Don Tilley
The V(IV) alkoxysiloxy complexes (<i><sup>t</sup></i><sup></sup>BuO)<sub>3</sub>VOSi(O<i><sup>t</sup></i><sup></sup>Bu)<sub>3</sub> (<b>1</b>) and (<i><sup>t</sup></i><sup></sup>BuO)<sub>2</sub>V[OSi(O<i><sup>t</sup></i><sup></sup>Bu)<sub>3</sub>]<sub>2</sub> (<b>2</b>) were synthesized via the silanolysis of V(O<i><sup>t</sup></i><sup></sup>Bu)<sub>4</sub> with 1 and 2 equiv of HOSi(O<i><sup>t</sup></i><sup></sup>Bu)<sub>3</sub>, respectively. Complexes <b>1</b> and <b>2</b> are efficient single-source molecular precursors to homogeneous V/Si/O materials via the <i>thermolytic molecular </i><i>precursor </i>method. The thermal transformations of these complexes occurred at low temperatures (≤160 °C), via the elimination of isobutene as the major carbon-containing product. Thermolyses of <b>1</b> in the solid state and in solution (forming a xerogel in the latter case) yielded V/Si/O materials (<b>VOSi1</b><b><sub>ss</sub></b> and <b>VOSi1</b><b><sub>xg</sub></b>, respectively) with low surface areas (30−40 m<sup>2</sup> g<sup>-1</sup>). After calcination at 400 °C in O<sub>2</sub>, these materials exhibited powder X-ray diffraction (PXRD) patterns consistent with the presence of V<sub>2</sub>O<sub>5</sub>. Similar thermolyses of <b>2</b> yielded V/Si/O materials (<b>VOSi2</b><b><sub>ss</sub></b> and <b>VOSi2</b><b><sub>xg</sub></b>) that exhibited higher surface areas (up to 170 and 70 m<sup>2</sup> g<sup>-1</sup>, respectively) and V<sub>2</sub>O<sub>5</sub> crystallite formation after calcination at 400 °C. Comparisons of <b>VOSi1</b><b><sub>xg</sub></b> and <b>VOSi2</b><b><sub>xg</sub></b> with a previously reported V/Si/O xerogel (<b>VOSi3</b><b><sub>xg</sub></b>), generated from the V(V) precursor OV[OSi(O<i><sup>t</sup></i><sup></sup>Bu)<sub>3</sub>]<sub>3</sub>, revealed interesting differences. Crystalline V<sub>2</sub>O<sub>5</sub> was first observed in <b>VOSi3</b><b><sub>xg</sub></b> after calcination at only 300 °C in O<sub>2</sub>. Transmission electron microscopy (TEM) and PXRD were used to determine that the average size of the V<sub>2</sub>O<sub>5</sub> crystallites in the V/Si/O xerogels, after calcination at a given temperature, increased with increasing silicon content of the precursor and was highest for the V(V) tris(siloxide). The precursors containing vanadium in the lower oxidation state (IV) appear to initially provide more homogeneous V/Si/O materials. Higher Si content for the precursor leads to a greater surface area for the resultant material, but also to earlier phase separation during the calcination process.